Materials: Cuprates, Pnictides, Chalcogenides, Infinite-layer nickelates, Ruthenates, Heavy fermions, Hydrides, Conventional superconductors, etc.
Platforms: Bulk, interface, heterostructure, superlattice, Kagome lattice, Twisted bilayers, disorder, strain, non-equilibrium, etc.
Symmetry breakings: Translation, spatial rotation, spatial inversion, spatial Mirror, time-reversal, Ising, etc.
Tunability: Carrier density, electromagnetic fields, pressure, strain, pulse, non-equilibrium, non-recipricity, etc.
Superconducting phases: Conventional, unconventional, triplet, chiral, topological, pair density waves, non-equilibrium, etc.
Competing States: Strange metals, non-Fermi liquid, density waves, nematicity, Mott physics, vortices, topology, etc.
Theory: Pairing mechanism, Pairing symmetry, SYK, non-equilibrium superconductors, pair density waves, etc.
Spectroscpies: ARPES, STM, Neutron, Raman, Ultrafast, THz, X-ray, etc.
Infinite layer nickelates
Hydrides and Conventional Superconductors
Twisted bilayer superconductors
Novel theory of superconductivity
Topological and Kagome superconductors
Strange metals and superconductivity
Models of superconductivity
Novel Spectroscopic techniques
Broken inversion symmetry
Broken time-reversal symmetry
Broken rotational symmetry
Pair density waves
Invited Talks: 25+5 minutes
Invited Early Career Talks: 12+3 minutes
11 December 2022 18.00: Registration & Welcome Dinner at Main Guest House
12 December 2022 (Monday)
08.30 - 09.45
09.45 - 10.00
Session I A: Cuprates
Moderator: Arun Bansil, Northeastern University, Boston
10.00 - 10.30
University of Tokyo, Japan, and National Tsing Hua University, Taiwan
Spectroscopic manifestations of electron fractionalization in the pseudogap state of cuprates
The origin of the pseudogap in cuprates has been debated for decades, but still remains enigmatic. In this talk, we demonstrate that the concept of electron fractionalization based on Mott physics explains recent spectroscopic data related to the pseudogap. ARPES studies have shown that, upon electron doping, the upper Hubbard band (UHB) is fractionalized into an UHB and an in-gap band (IGB) with a pseudogap in between . Upon hole doping, the lower Hubbard band (LHB) is fractionalized into a LHB and an IGB with a pseudogap in between. Excitonic excitation across the LHB and IGB using RIXS exhibits a temperature dependence characteristic of the pseudogap origninating from electron fractionalization .  M. Horio et al., arXiv:1801.04247;  A. Singh et al., arXiv:2207.12352.
10.30 - 11.00
Mark S. Golden
University of Amsterdam, The Netherlands
Momentum dependent scaling exponents of cuprate strange-metal self energies: ARPES meets semi-holography
ARPES enables the precise experimental determination of the electronic self-energy as we present here from the strange metal single-layer cuprate (Pb,Bi)2Sr2−xLaxCuO6+δ over a wide range in ω- and T for k along the nodal direction. Constant energy cuts through the spectral function (MDCs) show a non-Lorentzian lineshape as k increases away from kF: the nodal self-energy is k dependent. These experimental data provide a new test for aspiring theories. The self energy extracted from experiment is captured remarkably well by a power law that smoothly evolves with hole doping with - crucially - a k-dependent scaling exponent. In fact, this description emerges naturally from AdS/CFT-based semi-holography, putting a spotlight on holographic methods for the quantitative modelling of strongly interacting quantum materials like the cuprate strange metals .  S. Smit et al., arxiv.org/pdf/2112.06576.
11.00 - 11.30 Coffee break
Session I B: Infinite Layer Nickelates
Moderator: Awadesh Narayan, Indian Institute of Science, Bangalore
11.30 - 12.00
Hai Hu Wen
Nanjing University, China
Superconducting gap structure of Nd1-xSrxNiO2 thin films and exploration of superconductivity in some bulk nickelate materials
We report single particle tunneling measurements on the superconducting nickelate Nd1-xSrxNiO2 thin films. We find predominantly two types of tunneling spectra, one shows a V-shape feature which can be fitted well by a d-wave gap function with gap maximum of about 3.9 meV, another one exhibits a full gap of about 2.3 meV. Some spectra demonstrate mixed contributions of these two components. Combining with theoretical calculations, we attribute the d-wave gap to the orbital. Several possible reasons are given to explain the smaller full gap . We also found a dominant Pauli limit behavior of upper critical field . Meanwhile we have tried to explore superconductivity in other nickelate bulk samples, including Nd1-xSrxNiO2  and Sm1-xSrxNiO2. However, no success has been achieved so far. We will discuss the possible reasons for the absence of superconductivity in the nickelate bulk materials.  Q. Q. Gu, et al., Nat. Commun. 11, 6027(2020).  Y. Xiang, et al., Chin. Phys. Lett. 38, 047401(2021).  Q. Li, et al., Commun. Mat. 1, 16(2020).  C. P. He, et al., . Phys.: Condens. Matter 33, 265701(2021).
12.00 - 12.30
Indian Association for Cultivation of Science, India
The Nickel Age of Superconductivity
In this talk employing first-principles density functional theory calculations we shall analyze the electronic structure and derive a low energy tight-binding model Hamiltonian for undoped, infinite-layer nickelate compounds NdNiO2, PrNiO2, and LaNiO2. Our calculations reveal that the low energy degrees of freedom consists of two orbitals: Ni x2−y2, and an axial orbital. The axial orbital is constructed out of Nd/La d, Ni 3z2−r2, Ni s and other farther orbitals. Very similar to the cuprates [2,3] the axial orbital brings in the material dependence to the problem, making NdNiO2, PrNiO2 different from LaNiO2. In particular, the axial orbital reflects the nonzero f-ness of Nd and Pr atoms, as opposed to the f0 occupancy of La. The nonzero f-ness becomes effective in lowering the energy of the rare-earth 5d hybridized axial orbital, thereby enhancing the electron pockets and influencing the Fermi surface topology. The Fermi surface topology of NdNiO2 and PrNiO2 is strikingly similar, while differences are observed for LaNiO2. This difference shows up in computed doping-dependent superconducting properties of the three compounds within a weak coupling theory, which indicates two-gap superconductivity for NdNiO2 and PrNiO2, and the possibility of a single-gap superconductivity for LaNiO2 with the strength of superconductivity suppressed by almost a factor of 2, compared to the Nd or Pr compound [1,4]. Finally, we shall discuss our results in the light of recent experiments. * work done in collaboration with P Adhikary (IISc, Bangalore), S Bandyopadhyay (IACS, Kolkata), T Das (IISc, Bangalore) and T Saha-Dasgupta ( S.N Bose Centre, Kolkata)   P Adhikary, S Bandyopadhyay, T Das, I Dasgupta, T Saha-Dasgupta Physical Review B 102 (Rapid Communication), 100501 (2020)  E Pavarini, I Dasgupta, T Saha-Dasgupta, O Jepsen, OK Andersen Physical Review letters 87, 047003 (2001)  E Pavarini, I Dasgupta, T Saha-Dasgupta, OK Andersen Physical Review letters 124, 109701 (2020)  S Bandyopadhyay, P Adhikary, T Das, I Dasgupta, T Saha-Dasgupta Physical Review B 102 (Rapid Communication), 220502 (2020)
12.30 - 12.45
Chungnam National University, South Korea
Nature and origin of electronic correlations in the infinite-layer nickelate superconductors
We investigate the optical properties of the normal state of the infinite-layer nickelates using density-functional theory plus dynamical mean-field theory. We find a correlated metal which exhibits substantial transfer of spectral weight to high energies relative to the density functional theory. The correlations are not due to Mott physics, which would suppress the charge fluctuations and the integrated optical spectral weight as we approach a putative insulating state. Instead we find the unusual situation, that the integrated optical spectral weight decreases with doping and increases with increasing temperature. We contrast this with the coherent component of the optical conductivity, which decreases with increasing temperature as a result of a coherence-incoherence crossover. Our studies reveal that the effective crystal field splitting is dynamical and increases strongly at low frequency. This leads to a picture of a Hund's metallic state, where dynamical orbital fluctuations are visible at intermediate energies, while at low energies a Fermi surface with primarily dx2-y2 character emerges. The nickelates are thus in an intermediate position between the iron based high temperature superconductors where multiorbital Hund's physics dominates, and a one band system such as the cuprates. To capture this physics we propose a low-energy two-band model with atom centered eg states.  C.-J. Kang and G. Kotliar, Phys. Rev. Lett. 126, 127401 (2021)
13.00 - 14.30 Lunch break
Session I C: Pnictides/Chalcogenides
Moderator: Kalobaran Maiti, Tata Institute of Fundamental Research, Mumbai
14.30 - 15.00
Oxford University, UK
Fermi surfaces and quasiparticle effective masses in the high-pressure phase of superconducting iron-chalcogenides, FeSe1-xSx
Electronic nematic and spin-density wave phases of FeSe family of iron-chalcogenides superconductors can be intertwined and difficult to assess their relevance on superconductivity. A tuning parameter, like applied pressure and chemical pressure [1,2], are versatile tuning parameters that can be used to explore their relative importance. In my talk, I will present quantum oscillations studies under high applied hydrostatic pressure in FeSe1-xSx using magnetotransport and tunnel diode oscillator experiments [3,4]. I will discuss the evolution of the Fermi surface and electronic correlations away from the nematic phases inside the high-pressure phase where superconductivity is enhanced. I will also present the changes in the upper critical field as a function of applied chemical and applied pressure and discuss the nature of the high-pressure phase of FeSe1-xSx. References . A. I. Coldea, Frontiers in Phys. 8, 594500 (2021). . A. I. Coldea et al., npj Quantum Materials, 4, 2 (2019). . P. Reiss et al, Nature Physics, 16, 89 (2020). . Z. Zajicek et al., in preparation (2021).
15.00 - 15.30
BCS-BEC crossover in FeSe-based
The physics of the crossover between weak-coupling BCS and strong-coupling BEC limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems but has been challenging to find a solid-state realization. Here we show that superconducting Fe(Se1-xSx) offers the possibility to enter the previously unexplored realm where the two energies, Fermi energy εF, and superconducting gap Δ, become comparable, indicating that this system is deep inside the BCS–BEC crossover regime. Through scanning tunneling microscopy and laser-excited angle-resolved photoemission spectroscopy, we demonstrate that εF of Fe(Se1-xSx) is extremely small, with the ratio Δ/εF∼0.3-1 for all bands. We discuss several unusual superconducting properties associated with the crossover, including non-Gaussian superconducting fluctuations, pseudogap, quantum vortex core, FFLO phase, and unusual Bogoliubov quasiparticle band dispersions. Some of these properties are not expected for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in the multiband system.
15.30 - 16.00
Hidden Symmetry Breaking and High
We will report our recent angle-resolved photoemission (ARPES) studies FeSe-based superconductors [1-3]. We observed highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor . We identified two hole-like Fermi surface sheets around the Brillouin zone center, and the splitting of the associated bands, in bulk FeSe which indicate that, in addition to the nematic order and spin-orbit coupling, there is an additional order in FeSe that breaks either inversion or time-reversal symmetries. Finally, we present spectroscopic evidence of superconductivity pairing at 83 K in single-layer FeSe/SrTiO3 films . We find that the superconductivity pairing state can be further divided into two temperature regions of 64-83 K and below 64 K. These results indicate that either Tc as high as 83 K is achievable in iron-based superconductors, or there is a pseudogap formation in single-layer FeSe/SrTiO3 films.  Defa Liu, Cong Li, Jianwei Huang, X. J. Zhou et al., Phys. Rev. X 8, 031033 (2018).  Cong Li, L. Zhao, X. J. Zhou et al., Phys. Rev. X 10, 031033 (2020).  Yu Xu, L. Zhao, X. J. Zhou et al., Nature Communications 2, 2840 (2021).
16.00 - 16.30 Coffee break
Session I D: Hydride and Conventional Superconductors
Moderator: Uwe Bovensiepen, University of Duisburg-Essen, Germany
16.30 - 17.00
First-principles study of
superconductivity in superhydrides
Recently, high Tc superhydrides are attracting broad interest. Among them, LaH10 is a prototypical binary compound for which Tc~250K was observed. Under extremely high pressure, LaH10 takes a symmetric cubic structure with a large density of states around the Fermi level. It was theoretically shown that this structure is particularly favorable for superconductivity. In this talk, we first show that both the effect of the zero-point motion of hydrogen atoms and that of finite temperature which are usually neglected in conventional calculations are critically important to stabilize the cubic structure. Second, based on the newly developed efficient Eliashberg scheme in which the retardation effect and mass enhancement effect are treated from first principles, we discuss whether we can enhance the superconducting Tc and realize room-temperature superconductivity.
17.00 - 17.30
Osaka University, Japan
High-temperature superconductivity in hydrogen-rich compounds and the experimental background
Recent intensive theoretical and experimental studies under high pressure have led to the synthesis of many hydrogen-rich superconductors. Synthesis of RTS, a room-temperature superconductor is one of the goals of material science and technology. Here we present that Pressure is one of the powerful tools to improve the superconducting property and synthesize new superconductors. Almost hydrogen-rich superconductors have been synthesized from metals and elemental hydrogen H2 or hydrogen source by laser heating at high pressure, combined with the crystal structure analysis by using the synchrotron x-rays. The detail of the recent experimental investigations for synthesis and observation of the hydrogen-rich superconductors will be reviewed.
17.30 - 17.45
Superconductivity in (Ba,K)SbO3
Hole-doped perovskite bismuthates, (Ba,K)BiO3 (BKBO), have attracted particular attention due to their high Tc (30 K), the second highest amongst all oxide superconductors except the cuprates. However, the microscopic mechanism of superconductivity remains unclear. Here, We present the discovery of superconductivity in (Ba,K)SbO3 (BKSO), sibling compounds of BKBO. As the potassium concentration is varied, the Tc of BKSO rises up to 15 K. Lower than the maximum Tc of BKBO, this Tc value is still higher than the Tc of BKBO by more than a factor of two at a comparable potassium concentration. The discovery enables us to perform a detailed comparative study of the two superconductors and address some long-standing questions on the unexpectedly high Tc of BKBO.
13 December 2022 (Tuesday)
09.30 - 10.00 Lecture by G. Baskaran
(Superconductivity and Spectroscopy - Light from Indian Subcontinent)
Bose statistics (1924) and Bose-Einstein condensation underlie a wealth of macroscopic quantum phenomena, including superconductivity. Raman effect (1928) and Raman Spectroscopy pioneered modern inelastic spectroscopy. K.S. Singwi (1952) and Abdus Salam (1953) provided a little known, field theory approach to superconductivity. Kumar and Sinha (1968) predicted photon mediated room temperature superconductivity. With the advent of HTSC in cuprates (1986), superconductivity research got a boost across India. Ganguly-Rao's, earlier measurements (1984) on Mott insulating La2CuO4 helped P.W. Anderson in his proposal (at IISc, January 1987) of RVB theory of HTSC in cuprates. Superconductivity in Nickel based layered material YNi2Bi2C (1994, Tc ~ 12 K) and elemental Bi (2016, ultra low Tc ~ 0.53 mK) were discovered at TIFR. As a theorist, I see a `Silver lining for a Golden era' of room temperature superconductivity, in signals from Ag-Au nano-structures at IISc (2018 to 2022).
Session II A: Twisted Bilayer superconductors
Moderator: Sumilan Banerjee, Indian Institute of Science, Bangalore
10.00 - 10.30
The Ohio State
Bounds on Tc for strongly correlated
I will first describe exact upper bounds  on the BKT Tc in 2D superconductors that are expressed in terms of the optical spectral weight sum rule. These general results are valid for multi-band systems independent of pairing strength or mechanism, with the only assumption that the vector potential couples to the kinetic energy and not the interactions. I will show that the bounds are particularly useful for strongly correlated superconductors where mean field theory fails, and then discuss applications to a variety of systems including the BCS-BEC crossover , Li:ZrNCl, monolayer FeSe/STO, and magic angle twisted bilayer graphene. I will then generalize these results  to flat-band models, where we need to bound the low-energy optical spectral weight for which we cannot use the usual Peierls’ substitution. I will present bounds on Tc for both trivial and topological flat bands that are related to the quantum geometry of the flat band Wannier functions. The challenges in establishing Tc bounds in 3D systems will be discussed.  T. Hazra, N. Verma, M. Randeria, Phys. Rev. X 9, 031049 (2019)  M. Randeria, Science 372,132 (2021)  N. Verma, T. Hazra, M. Randeria, Proc. Nat. Acad. Sci. 118, e2106744118 (2021)
10.30 - 10.45
Indian Institute of Science, India
Electric field tunable superconductivity in near magic-angle twisted bilayer graphene
Superconductivity (SC) has been previously reported in magic-angle twisted bilayer graphene (MATBLG) with the variation of carrier concentrations, twist angles and the screening strength to shed insights into its origin, and shown no necessary concomitance between the SC and the correlation. Here we report the electric field tunable superconductivity in a near magic-angle twisted bilayer graphene with twist angle of 0.95◦. At zero displacement field (D), we observe the superconductivity at ν = 2.7 around the van Hove singularities (vHS), which shifts to higher filling with the application of displacement field, and the optimal doping for the SC phase closely follows the vHS. At higher field (D>0.25 V/nm), the superconducting transition temperature (Tc) starts to diminish quite rapidly together with the prominent appearance of the Fermi surface resetting. The displacement field induced band structure and its role on SC in the present study has potential to shed light on the possible origin of SC in MATBLG.
10.45 - 11.00
Jedediah H. Pixley
Rutgers University, USA
Twisting Nodal Superconductors
11.00 - 11.30 Coffee break
Session II B: Dilute superconductors
Moderator: Srimanta Middey, Indian Institute of Science, Bangalore
11.30 - 12.00
Tunable superconductivity at KTaO3 interfaces and its possible origins
In this talk I will discuss the recently discovered superconducting electron gas  at interfaces between KTaO3 (KTO) and insulating overlayers. The superconducting Tc can be as high as 2.2 K, about an order of magnitude higher than in the LaAlO3/SrTiO3 system, and is two-dimensional. Using transmission electron microscopy and resonant x-ray scattering, we establish the presence of substitutional defects and oxygen vacancies at the KTO interface that can act as donors of electrons and may lead to the formation of the interfacial electron gas. Furthermore, we have uncovered several unusual properties of the superconducting state, including its orientation selectivity, dependence on chemical doping and electric field-effect gating . These properties point to a highly tunable two-dimensional interfacial superconductor. I will also discuss a proposed mechanism for superconductivity at KTaO3 interfaces involving TO1 phonons that accounts for several key properties of the superconducting state. *This work was primarily supported by the US Department of Energy, Basic Energy Sciences.  C. Liu et al., Science 2021. DOI: 10.1126/science.aba5511  C. Liu et al., arXiv:2203.05867 (2022).
12.00 - 12.15
Field effect experiments at
12.15 - 12.30
Indian Institute of Science, India
Ising superconductivity in single-layer MoS2
I will report our study of superconductivity in single-layer MoS2 due to orbital coupling with a proximate few-layer NbSe2 flake. A primary feature of our discovery is the achievement of interfacial transparency that is more than three orders of magnitude better than ever reported in two-dimensional semiconductor-superconductor heterostructures. Through electronic transport measurements and first principle-based electronic-structure calculations, we establish that the superconductivity is induced in two steps – forming a metallic phase at high temperatures (due to hole doping from NbSe2) followed by the appearance of intrinsic superconducting correlations in single-layer MoS2 below 7 K. Further, we show that the superconducting phase has strong Ising spin-orbit correlations which make the holes spin non-degenerate. Our system thus forms a novel platform that can potentially host topological chiral superconductors with non-abelian excitations as well as finite momentum-paired superconductors per the predictions of Fulde and Ferrell and of Larkin and Ovchinnikov.
12.30 - 13.00
IISER Kolkata, India
New paradigm for superconducting vortices with non-metallic cores
We carry out microscopic study of vortex lattice in a strongly correlated, type-II, d-wave superconductor (SC), addressing two independent questions: (a) Does a vortex get pinned by an impurity in accordance with conventional wisdom derived for conventional weakly coupled s-wave SCs? (b) What are the manifestation of competing subdominant orders in a vortex core of such SCs? Commonly accepted truism is that a vortex binds to impurity. Both an impurity and a magnetic field depletes SC pairing amplitude, and this depletion comes for an energy cost. Therefore, a vortex gets pinned to an impurity to minimize the energy cost. Early study based on a weak-coupling theory showed that the local density profile of charge carrier remains largely unaltered at the vortex core. On the other hand in strongly correlated materials, most often responsible for an unconventional superconductivity, the nature of vortex core undergoes a changes with doping . Proximity to a Mott insulator of the parent compound of such SCs reflects in the fact that the vortex core turns into a Mott insulating region at underdoping with a fully formed gap, while the optimal to overdoped regime still shows a metallic-like Abrikosov vortex core. This contrast is associated self-consistently with the charge accumulation and depletion (respectively) in the core region. Integrating these considerations, we demonstrate that the binding of a vortex to an impurity depends on several parameters. In particular, we illustrate such dependency on the disorder strength as well as on the doping. We emphasize that this seemingly unanticipated behavior arises from strong correlation effects, and is absent otherwise. We have established that by contrasting these results with those from a weakly correlated description. In addition, motivated by the experiments by Hoffman et al. , where they observed spatial modulation of Local Density of States (LDOS) in the vortex core with a period about four lattice spacing, we simulate the microscopic details of vortex cores in a clean and strongly correlated d-wave superconductor to show the emergence of subdominant charge and Bond-density wave order at the vortex core where superconducting pairing amplitude vanishes.  A. Datta, H. J. Changlani, K. Yang and A. Ghosal, arXiv preprint arXiv:2206.07752 (2022).  J. E. Hoffman, E. W. Hudson, K. M. Lang, V. Madhavan, H. Eisaki, S. Uchida and J. C. Davis, Science, 295, 466 (2002).
13.00 - 14.30 Lunch break
Session IIC: Novel Theory of superconductivity
Moderator: Navinder Singh, Physical Reasearch Laboratory, Ahmedabad
14.30 - 15.00
University of Illinois at Urbana-Champaign, USA
Beyond BCS: An Exact Model for
The Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity described all superconductors until the 1986 discovery of the high-temperature counterpart in the cuprate ceramic materials. This discovery has challenged conventional wisdom as these materials are well known to violate the basic tenets of the Landau Fermi liquid theory of metals, crucial to the BCS solution. Precisely what should be used to replace Landau's theory remains an open question. The natural question arises: What is the simplest model for a non-Fermi liquid that yields tractable results. Our work builds on an overlooked symmetry that is broken in the normal state of generic models for the cuprates and hence serves as a fixed point. A surprise is that this fixed point also exhibits Cooper's instability[2,3]. However, the resultant superconducting state differs drastically from that of the standard BCS theory. For example the famous Hebel-Slichter peak is absent and the elementary excitations are no longer linear combinations of particles and holes but rather are superpositions of composite excitations. Our analysis here points a way forward in computing the superconducting properties of strongly correlated electron matter.  E. Huang, G. La Nave, P. Phillips, Discrete symmetry breaking defines the Mott quartic fixed point, Nat. Phys., 18, 511-516 (2022)  PWP, L. Yeo, E. Huang, Exact theory for superconductivity in a doped Mott insulator, Nature Physics, 16, 1175-1180 (2020). J. Zhao, L. Yeo, E. Huang, PWP, Thermodynamics of an exactly solvable model for superconductivity in a doped Mott insulator Phys. Rev. B 105, 184509 .
15.00 - 15.30
Tata Institute of Fundamental Research, India
Collective Modes of Disordered s-wave Superconductors
We study two particle spectral functions and collective modes of disordered s-wave superconductors using a negative U Hubbard modelon a square lattice. We show that the disorder induced scattering between the collective modes lead to finite subgap two particle spectral weight in the long wavelength limit. Near half-filling, the proximity to a CDW state leads to an effective two mode model which explains the systematic trends in the subgap spectral weight with density of the system.
15.30 - 17.00 Poster Session + Coffee break
Session II D: Topological and Kagome Superconductors
Moderator: Tanusri Saha-Dasgupta, S N Bose National Center for Basic Science, Kolkata
17.00 - 17.15
Tata Institute of
Superconductivity in multifold fermion chiral metal RhGe
Finding superconductivity in topological materials is one of the frontiers in condensed matter physics and materials science. Such topological materials can realize nontrivial superconductivity and long-sought Majorana fermions as the boundary modes. Despite significant efforts, finding topological materials with coexisting superconducting states remains a challenge. Here based on first-principles theoretical modelling and Migdal- Elliashberg theory, we predict electron-phonon interaction mediated bulk superconductivity in RhGe with a superconducting transition temperature of 2.8 K. RhGe belongs to CoSi class of Kramers Weyl metals where nonsymmorphic and chiral symmetries enforce multifold Weyl fermions to lie at the high-symmetry momentum points. It supports spin-polarized long topological Fermi arc states spanning the whole surface Brillouin zone. Our detailed analysis shows that the presence of high spin-orbit coupling, chiral multifold fermions, and nontrivial surface states coexisting with bulk superconductivity make RhGe a promising candidate to explore mixed-parity pairing and topologically protected zero-energy modes.
17.15 - 17.30
Charge density wave order in a magnetic Kagome system
Emergent phases often appear when the electronic kinetic energy is small compared to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional (2D) kagome lattice, including magnetic order, time-reversal symmetry breaking charge density wave (CDW), nematicity, and superconductivity. In this talk, I will present our recent experimental efforts in exploring the existence and nature of charge density wave order in the magnetic Kagome system FeGe.
17.30 - 18.00
Fermiology of AV3Sb5 kagome
The layered vanadium antimonides AV3Sb5 (A = K, Rb, Cs) are a new family of topological kagome metals that exhibit a range of strongly correlated electronic phases including charge order and superconductivity . If and how the distinctive electronic structure of the kagome lattice is linked to these broken-symmetry phases is a matter of active debate. I will discuss our observation of multiple kagome-derived van Hove singularities (vHS) coexisting near the Fermi level of CsV3Sb5 and examine their role toward electronic symmetry breaking . The vHS are characterized by two distinct sublattice flavors (p-type and m-type), with the m-type vHS (from V 3dxz/3dyz orbitals) and the p-type vHS (from V 3dxy/3dx2–y2 orbitals) being located very close to the Fermi level, setting the stage for electronic symmetry breaking. The former band exhibits pronounced Fermi surface nesting, however, it is the latter that exhibits the largest gap, revealing a strong-coupling origin of charge order. This work highlights the role of kagome-derived vHS for the collective phenomena realized in the AV3Sb5 family.  Ortiz, B. R., et al., Phys. Rev. Mater. 3, 94407 (2019); Ortiz, B. R., et al., Phys. Rev. Lett. 125, 247002 (2020).  M. Kang, et al., Nature Physics 18, 301 (2022).
14 December 2022 (Wednesday)
Session III A: Strange metals and Superconductivity
Moderator: Philip Phillips, University of Illinois at Urbana-Champaign, USA
09.30 - 10.00
Nigel E. Hussey
University of Bristol, UK
Strange metallicity and superconductivity in high-Tc cuprates
Perfectly T-linear resistivity is observed in a variety of strongly correlated metals close to a quantum critical point and has been attributed to a scattering rate 1/τ of charge carriers that reaches the Planckian limit, with ℏ/𝜏 = α 𝑘𝐵𝑇 where α is of order unity. While this relationship is often inferred from simple estimates, a T-linear scattering rate has yet to be measured. To directly access the Planckian scattering rate, we measured the angle-dependent magnetoresistance (ADMR) of Nd-LSCO at p = 0.24: a cuprate that demonstrates T-linear resistivity over a wide temperature range at the pseudogap critical point p*. The ADMR reveals a well-defined Fermi surface that precisely agrees with ARPES. In addition, we extract a T-linear scattering rate that has the Planckian value, namely α = 1.2 ± 0.4 . Remarkably, this inelastic scattering rate is isotropic.  G. Grissonnanche et al., Nature 595, 667 (2021)
10.00 - 10.15
T-linear resistivity from an isotropic
10.15 - 10.45
Chalmers University of Technology,
Interplay between strange metal phase Charge Density Wave and
The “strange metal” phase of High Critical Temperature Superconductors (HTS) is one of the most striking manifestations of the strong electron-electron correlation correlations in these materials. At optimal doping the strange metal manifests as a linear temperature dependence of the resistivity that persists to the lowest T when superconductivity is suppressed. This behavior is fundamentally different from that observed in more conventional metals, where a T-linear dependence of the resistivity is found, only at high temperatures, where phonon scattering dominates the transport. For underdoped cuprates this behavior is lost below the pseudogap temperature T*, where Charge Density Waves (CDW) together with other intertwined local orders characterize the ground state. The association between the departure from the T-linear resistivity and the occurrence of the pseudogap phenomenon has long been speculated without a general consensus. To address this issue we have tuned the ground state of underdoped HTS by using the geometric modification of the unit cell under the strong strain induced by the substrate. We show that the T-linear resistivity of highly strained, ultrathin and underdoped YBa2Cu3O 7-δ (YBCO) films is restored when the CDW amplitude, detected by Resonant Inelastic X-ray scattering, is suppressed . This observation points towards an intimate connection between the onset of CDW and the departure from the strange metal behavior in underdoped cuprates. In addition we find that in ultrathin thin films the superconducting critical temperature onset and the upper critical field Hc2 are enhanced compared to single crystals which point towards a competition between CDW and superconductivity. Our results also illustrate the potential of using strain control to manipulate the ground state of quantum materials. References  E. Wahlberg,R. Arpaia, G. Seibold, E. Trabaldo, S. Caprara , U. Gran, L. Braichovich, G. Ghiringhelli , T. Bauch and F. Lombardi Science 373, 1506 (2021)
10.45 - 11.10
Two Best Poster (Tuesday) Presentations
11.00 - 11.30 Coffee break
Session III B: Models of Superconductivity
Moderator: Vidhyadhiraja N S, Jawaharlal Nehru Centre For Advanced Scientific Research
11.30 - 12.00
Superconductivity from local moment fluctuations
The generic phase diagram of unconventional superconductors exhibits a superconducting dome next to a magnetically ordered phase and below a metallic state with non-fermi-liquid properties. A possible mechanism which naturally explains this phenomenology is the spin-freezing crossover which occurs in the metallic regime of multi-orbital models with Hund coupling. Dynamical mean field theory simulations show that the local moment fluctuations associated with this crossover can induce spin-triplet superconductivity, which may be relevant for uranium based superconductors . An analogous mechanism, based on the freezing of orbital moments, explains the phase diagram of fulleride superconductors with effectively negative Hund coupling . A mapping of the two-dimensional Hubbard model to an effective multi-orbital system with large Hund coupling furthermore allows to interpret the phase diagram of cuprates  and nickelates  in terms of a spin freezing crossover.  S. Hoshino and P. Werner, Phys. Rev. Lett. 115, 247001 (2015).  S. Hoshino and P. Werner, Phys. Rev. Lett. 118, 177002 (2017).  P. Werner, S. Hoshino and H. Shinaoka, Phys. Rev. B 94, 245134 (2016).  P. Werner and S. Hoshino, Phys. Rev. B 101, 041104 (2020).
12.00 - 12.15
IISER Mohali, India
Topological phase separation in an
Topological phase transitions are usually believed to be accompanied by a closing and re-opening of the spectral gap at the transition. With the help of a toy model for topological superconductivity, we uncover a simple scenario for topological transitions without gap closing. Using a combination of the mean-field Bogoliubov-de Gennes (BdG) approach and the Density Matrix Renormalization Group (DMRG) method, we discover discontinuous topological transitions between topological superconducting and trivial insulating phases in a sawtooth lattice of inter-site attractive fermions. Topological characterization of different phases is achieved in terms of winding numbers, Majorana edge modes and entanglement spectra. By studying the effect of disorder on the discontinuous topological phase transitions, we establish the disorder-induced topological phase coexistence as a mechanism for generating a finite density of Majorana particles.
12.15 - 12.45
University of Michigan, USA
Mechanism of Superconductivity in the Hubbard Model at Intermediate Interaction Strength
We study the fluctuations responsible for pairing in the d-wave superconducting state of the two-dimensional Hubbard model at intermediate coupling within a cluster dynamical mean-field theory with a numerically exact quantum impurity solver. By analyzing how momentum and frequency dependent fluctuations generate the d−wave superconducting state in different representations, we identify antiferromagnetic fluctuations as the pairing glue of superconductivity both in the underdoped and the overdoped regime. Nevertheless, in the intermediate coupling regime, the predominant magnetic fluctuations may differ significantly from those described by conventional spin-fluctuation theory.
12.45 - 13.00
Indian Institute of
Modelling spectroscopic signatures of unconventional superconductivity in monolayer di-chalcogenide
Topological superconductivity and non-trivial edge state physics in quantum spin Hall insulator (QSHI) 1T’-WTe2 can be studied by inducing a superconducting gap by fabricating heterostructures with superconductors like 2H-NbSe2. However, the interface hybridization in such heterostructures can act as an important parameter for both the normal state and superconducting properties. In this work we discuss our theoretical calculations on the modelling of scanning tunnelling microscopy measurements on the heterostructure between monolayer 1T’-WTe2 and superconducting 2H-NbSe2.
13.00 - 14.30 Lunch break
Session III C: Strain/Pressure dependent Superconductors
Moderator: Floriana Lombardi, Chalmers University of Technology, Göteborg
14.30 - 15.00
St. Andrews, MPIDresden, Germany
The elastic effects of a Lifshitz transition studied in Sr2RuO4
I will describe two recent experiments studying the coupling between elastic and electronic degrees of freedom in Sr2RuO4 as it is tuned across a Lifshitz transition by the application of uniaxial pressure. In the first, we employ a new experimental technique, the ac elastocaloric effect, to deduce the entropy as a function of temperature and pressure across its phase diagram. In the second, we study its stress-strain relationship, and observe a strong softening of its Young’s modulus at the Lifshitz transition. We show that, counterintuitively, this large effect is due entirely to the conduction electrons in the material’s γ band.
15.00 - 15.30
Phenomena and findings in pressurized high-Tc superconductors
15.30 - 16.00 Coffee break
Session III D: Novel Spectroscopic Techniques
Moderator: Atsushi Fujimori, University of Tokyo, Japan,National Tsing Hua University, Taiwan
16.00 - 16.30
Institute for Solid State Research, Germany
Instant ARPES of superconductors an related materials
The relationship among unconventional superconductivity, antiferromagnetism, and nematic order in iron-based superconductors (FeSCs) is still highly debated. In many FeSCs superconductivity is in proximity of a nematically and magnetically ordered state. LiFeAs is an exceptional stoichiometric FeSC becoming superconducting below 18 K without undergoing a structural or magnetic transition.We apply the novel ARPES technique  to investigate the question of the possible influence of nematicity on the superconducting state [2, 3] in this and other materials.  S. Borisenko et al. Nat Commun 13, 4132 (2022).  Y. Kushnirenko et al. Phys. Rev. B 102, 184502 (2020).  M. Wissmann et al. Phys. Rev. B 106, 054508 (2022).
16.30 - 17.00
The cuprate phase diagram demystified
In this talk, I will review our collaborative efforts to understand the cuprate phase diagram, including x-ray measurements of charge order , transport results , as well as measurements of superconducting correlations  and of ‘hidden’ structural correlations . These results point to a surprisingly simple picture of the cuprates in which inherent inhomogeneity and charge (de)localization play a central role . Work funded by the US Department of Energy through the University of Minnesota Center for Quantum Materials, under grant number DE-SC-0016371.  W. Tabis et al., Nat. Commun. 5, 5875 (2014) and Phys. Rev. B 96, 134510 (2017); B. Yu et al., Phys. Rev. X 10, 021059 (2020)  N. Barišić et al., Proc. Nat. Acad. Sci. 110, 12235 (2013); S. I. Mirzai et al., Proc. Nat. Acad. Sci. 110, 5774 (2013); M. K. Chan et al., Phys. Rev. Lett. 113, 177005 (2014); N. Barišić et al., New J. Phys 21, 113007 (2019); W. Tabis et al., arXiv:2106.07457  D. Pelc et al., Nat. Commun. 9, 4327 (2018); P. Popcevic et al., npj Quant. Mater. 3, 42 (2018); G. Yu et al., Phys. Rev. B 99, 214502 (2019); D. Pelc et al., Nat. Commun. 10, 2729 (2019)  D. Pelc et al., arXiv:2103.05482; Z. Anderson et al., unpublished.  D. Pelc et al., Sci. Adv. 5, eaau4538 (2019) and Phys. Rev. B 102, 075114 (2020)
17.30 - 21.30 Banquet
15 December 2022 (Thursday)
Session IV A: Ultrafast spectroscopy
Moderator: Anshu Pandey, Indian Institute of Science, Bangalore
09.30 - 10.00
The University of
Coherent c-axis charge carrier responses in photoexcited cuprate
We have investigated the c-axis charge carrier responses of cuprate superconductors, YBa 2 Cu 3 O y , and La 2-x-y Nd y Sr x CuO 4 , in their photoexcited states above T c by using optical pump and terahertz probe spectroscopy. In both cases, Drude responses with unusually small scattering rates were observed after the photoexcitation. In YBa 2 Cu 3 O y , none of collective modes associated with the superconductivity was observed in the photoexcitd state, suggesting that the photoinduced state is distinct from the long-range ordered superconductivity. In La 2-x-y Nd y Sr x CuO 4 , a long-lived narrow Drude response was observed sustaining more than 1 ns in the spin-stripe ordered phase. Possible origins of those coherent charge carrier responses will be discussed.  Kota Katsumi , Morihiko Nishida , Stefan Kaiser , Shigeki Miyasaka , Setsuko Tajima , Ryo Shimano , arXiv:2209.01633
10.00 - 10.15
Tracking the low-energy electrodynamics of Bi-based cuprates by TR-ARPES
The phase diagram of cuprates hosts numerous intertwined phases as disparate as high-temperature superconductivity, charge order and the pseudogap. Here we employ time- and angle-resolved photoemission spectroscopy (TR-ARPES) to investigate the transient evolution of the low-energy electrodynamics of Bi-based cuprates, both along the gapless nodal direction and in the near-nodal superconducting gap region [1-3]. In particular, we first discuss the temperature-dependent breakdown of the nodal coherent spectral weight across single- and bi-layer Bi cuprates [1-2] and then move to the study of the superconducting gap in the near-nodal region confirming the dominant role played by phase coherence in the emergence of high-temperature superconductivity [2-3].  M. Zonno*, F. Boschini* et al., PRB 103, 155109 (2021)  M. Zonno et al., J. Electron Spectrosc. Relat. Phenom. 251, 147091 (2021)  F. Boschini et al., Nat. Mater. 17, 416 (2018)
10.15 - 10.45
Non-equilibrium energy transfer
An excited electron propagates in condensed matter with its momentum k at an energy Ek and experiences elastic and inelastic scattering processes, which lead to electronic relaxation and energy transfer to microscopic excitations of the lattice and spin systems on ultrafast timescales. In heterostructures energy and momentum conservation determine transfer of excitations across interfaces. In this talk, the analysis of non-local dynamics of excited electrons in femtosecond photoelectron spectroscopy and its sensitivity to buried media will be reported . Furthermore, the competition of e-e with e-ph scattering will be discussed in [Fe/MgO]n heterostructures. Pump-probe experiments using soft x-ray absorption spectroscopy provide element- and site-specific insights on the mechanism of energy transfer across interfaces . The importance of local electronic interactions on microscopic time scales will be discussed for Ni based on femtosecond laser induced electronic excitations and changes at the Ni L3 absorption edge.  Y. Beyazit J. Beckord, P. Zhou, J. Meyburg, F. Kühne, D. Diesing, M. Ligges, [Text Wrapping Break] U. Bovensiepen, Phys. Rev. Lett. 125, 076803 (2020).  N. Rothenbach et al., Phys. Rev. B 100, 174301 (2019).
10.45 - 11.00
Technological advancements over the past decade have enabled the generation of intense THz (ħω~meV) pulses, suitable for nonlinear spectroscopy. Using a THz two-dimensional coherent spectrometer, capable of phase-resolved measurements of higher-order spectroscopic responses, we have studied strongly disordered s-wave superconductors near metal insulator transition . While in “clean” conventional superconductors the third-order nonlinear response approximately follows the temperature dependence of the superfluid density, for highly disordered samples, the nonlinearity persists to much higher temperatures, well beyond the Tc. Possible origins of this remarkably large nonlinearity include an enhancement of the temperature scales of superconductivity close to localization.We have also measured high-Tc cuprates, where we observe an anomalous temperature and doping dependence of the nonlinear response. We further use THz 2D spectroscopy to identify the different underlying nonlinear processes that contribute to the overall response.  D. Chaudhuri et. al., arXiv:2204.04203 (2022).
11.00 - 11.30 Coffee break
Session IV B: Broken inversion symmetry
Moderator: Subhro Bhattacharjee, International Center for Theoretical Science, Bangalore
11.30 - 12.00
Superconductivity in crystals lacking local inversion symmetry has been investigated for over a decade. This topic was initiated as an extension of the noncentrosymmetric superconductivity to a broad class of centrosymmetric superconductors, in which the inversion symmetry at an atomic site is broken while the inversion center is present in the middle of atoms. This class of crystals is called locally noncentrosymmetric crystals. After studies in the decade, exotic phenomena unique to locally noncentrosymmetric superconductors have been predicted. An example is the parity transition, which means the parity of superconductivity changes from even to odd as increasing the magnetic field . I will discuss the relevance of the recently discovered multiple phases in CeRh 2 As 2  and predict topological superconductivity  and quantum critical phenomena .  T. Yoshida, M. Sigrist, and Y. Yanase, Phys. Rev. B 86, 134514 (2012).  S. Khim et al., Science 373, 1012 (2021).  K. Nogaki, A. Daido, J. Ishizuka, and Y. Yanase, Phys. Rev. Research 3, L032071 (2021).  K. Nogaki and Y. Yanase, Phys. Rev. B 106, L100504 (2022).
12.00 - 12.30
RIKEN Center for
Diode effect in superconductors
Nonreciprocal transport in noncentrosymmetric quantum materials is an issue of current great interest . It is even more enhanced once the superconductivity sets in, which introduces a small energy scale, i.e., the superconducting gap, below which the bosonic transport dominates. In this talk, I would discuss recent advances in this respect, i.e., the diode effect in superconductors especially in two-dimensions.It includes the resistive states where the fluctuation of the superconducting order parameter gives the paraconductivity , the nonreciprocal resistivity dominated by vortex motion , nonreciprocal Josephson junction , and asymmetric ctirical current for the break-down of the superconductivity .  Y.Tokura, and N. Nagaosa, Nat. Commun. 9 3740 (2018).  R. Wakatsuki at al., Sci. Adv. 3 e1602390 (2017)  K. Yasuda et al., Nat. Commun. 10 2734 (2019)  K. Misaki and N. Nagaosa, Rev. B 103 245302 (2021)  James Jun He, Yukio Tanaka, Naoto Nagaosa, New J. Phys. 24 053014 (2022)
12.30 - 13.00
Institute for Solid State and Material Physics, Technical University, Dresden, Germany
Interplay of T0 order and superconductivity in CeRh2As2
We report on recent experimental insight into the physics of the unconventional superconductor CeRh2As2. This fascinating compound has a Tc of 0.26 K, a large critical field of 14 T, and two superconducting states for magnetic field along the c axis of the tetragonal structure . The two superconducting states are interpreted as even- and odd-parity states in a locally non-centrosymmetric crystal structure. Because an additional ordered state to a possible quadrupole-density-wave order is observed below T0 = 0.4 K, an alternative interpretation of the phase transition within the superconducting state is that it originates in the suppression of this order happening near the transition field. So far, the resolution of the T0 transition did not permit to decide between the two scenarios. By measuring samples with higher quality than before, we establish that this transition meets the Tc line at roughly 6 T in c-axis fields suggesting that the suppression of T0 is not at the origin of the phase transition inside the superconducting state but rather the even- to odd-parity transition.  Khim et al. Science 373, 1012 (2021).
Session IV C: Broken time-reversal superconductors
Moderator: Somesh Chandra Ganguli, Aalto University, Finland
14.30 - 15.00
The Royal Institute of Technology (KTH) Stockholm, Sweden
Theory of electron quadrupling condensates
Superconductivity is caused by the formation and condensation of pairs of electrons. A mechanism for forming fermion-quadrupling condensates in systems that break multiple symmetries was proposed in . The especially intersting class of fermion quadrupling order spontaneously breaks time-reversal symmetry due to four-fermion correlations . The recently reported set of experimental observations  in hole-doped Ba1−xKxFe2As2 are consistent with the formation of such fermion quadrupling condensate above superconducting phase transition. For x∼0.8, the experiments  observe a state with zero resistance and no detectable diamagnetic screening, but the spontaneous breakdown of time-reversal symmetry is indicated by spontaneous Nernst effect and muon spin rotation experiments. In that state long-range order exists between pairs of electron pairs in different bands, in the absence of a superconducting order parameter. While the long-wave length models for superconducting and superfluid states are Ginzburg-Landau and Gross-Pitaevskii functionals, the fermion quadrupling states of the kind reported in Ba1−xKxFe2As2 are described by a model related to the Faddeev-Skyrme model [3,4].  E. Babaev, A. Sudbø. N.W. Ashcroft Nature 431, 666–668 (2004). E Babaev Nuclear Physics B 686 (3), 397-412 for a review B.V. Svistunov, E.S. Babaev, N.V. Prokof'ev see Superfluid states of matter Crc Press (2015)  T. A. Bojesen, E. Babaev, A. Sudbø, Phys. Rev. B 88, 220511 (2013). T. A. Bojesen, E. Babaev, A. Sudbø, Rev. B 89, 104509 (2014).  Vadim Grinenko, Daniel Weston, Federico Caglieris, Christoph Wuttke, Christian Hess, Tino Gottschall, Ilaria Maccari, Denis Gorbunov, Sergei Zherlitsyn, Jochen Wosnitza, Andreas Rydh, Kunihiro Kihou, Chul-Ho Lee, Rajib Sarkar, Shanu Dengre, Julien Garaud, Aliaksei Charnukha, Ruben Hühne, Kornelius Nielsch, Bernd Büchner, Hans-Henning Klauss, Egor Babaev, Nature Physics volume 17 1254–1259 (2021) Julien Garaud, E. Babaev Physical Review Letters 129 (8), 087602 (2022)
15.00 - 15.15
BTRS superconductors and the related quartic phases
Any superconducting (SC) state breaks spontaneously U(1) gauge symmetry at the SC transition temperature (Tc). In some cases, an SC state breaks in addition time reversal symmetry (Z2). Usually, Z2 is broken at TcZ2 ≤ Tc. However, it was predicted that fluctuations could push Tc below TcZ2 . Here, we study the multiband Ba1-xKxFe2As2 system experimentally. At high K doping levels, Lifshitz transition triggers superconductivity that breaks time reversal symmetry (BTRS) [2,3]. The BTRS SC state exists at a narrow doping range of 0.7 ≤ x ≤ 0.85. The most intriguing is that at x ~ 0.8, we found the BTRS state at TcZ2 > Tc . Based on the theoretical analysis, we proposed that TcZ2 is associated with a quartic order - a correlated state between pairs of Cooper pairs.  T. A. Bojesen, E. Babaev, and A. Sudbø, Phys. Rev. B 89, 104509 (2014).  V. Grinenko, P. Materne, R. Sarkar et al., Phys. Rev. B 95, 214511 (2017).  V. Grinenko, R. Sarkar, K. Kihou et al., Nat. Phys. 16, 789–794 (2020).  V. Grinenko, D. Weston, F. Caglieris et al., Nat. Phys. 17, 1254–1259 (2021).
15.15 - 15.30
University of Geneva, Switzerland
Wang-MacDonald Vortex Core States Revealed in a
The electronic structure of the Abrikosov vortex cores is one among the outstanding puzzles in understanding high temperature superconductivity. We present here very recent scanning tunneling microscopy measurements of heavily overdoped Bi 2 Sr 2 CaCu 2 O 8+ (Bi2212) done at an unprecedented low magnetic field . Under these conditions, we found the characteristic vortex core signatures predicted by Wang and MacDonald in 1995 for a d-wave superconductor , with a clear zero-bias anomaly splitting into higher-energy levels at a distance from the flux line center. At higher magnetic fields, interestingly, we observe the checkerboard and subgap states previously reported in the vortex halos of optimally and underdoped crystals. While our low-field findings are consistent with the d-wave symmetry of the superconducting order parameter in Bi2212, the puzzling field dependence of the electronic vortex core structure remains to be understood.  T. Gazdić et al., Phys. Rev. X 11, 031040 (2021)  Y. Wang and A.H. MacDonald, Physical Review B 52, R3876 (1995)
15.30 - 17.00 Poster + Coffee break
Session IV D: Disordered superconductors
Moderator: Ivan Maggio-Aprile, University of Geneva, Switzerland
17.00 - 17.30
Tata Institute of
Cooper pairing without superconductivity: Phase fluctuations, Pseudogap state, Superinsulator and Bose Metal
Cooper pairing, where pairs of electrons with opposite momentum and opposite spins form a singlet bound state is usually associated with the onset of superconductivity. Within the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity spin-zero Cooper pairs condense into a phase coherent state, giving rise to the zero resistance. However, an increasing number of experiments suggest that superconductivity is just one of the exotic states made of Cooper pairs. For example, it is now observed that in many superconductors Cooper pairing continues to persist above Tc even though the zero resistance state is destroyed by phase fluctuations. On the other hand, even at very low temperatures in many systems under the application of magnetic field one observes transition to a “superinsulator” where the conductance instead of resistance appears to vanish at a finite temperature. It is believed that in a superinsulator the Cooper pairs are in an eigenstate of number instead of phase and hence localized. Even more interestingly, in some systems Cooper pairs appear to exist in a dissipative metallic state, called Bose metal. In this talk, I will talk about these various novel states in the backdrop of the work done over the last ten years in the Superconductivity Lab at TIFR.
17.30 - 18.00
Delft University of
Direct evidence for Cooper pairing
The idea that preformed Cooper pairs could exist in a superconductor at temperatures higher than it zero-resistance critical temperature (Tc) has been explored for unconventional, interfacial and disordered superconductors, but direct experimental evidence is lacking. In this talk, I will present how we use scanning tunneling noise spectroscopy to show that preformed Cooper pairs exist up to temperatures much higher than Tc in the disordered superconductor titanium nitride (TiN). This is done by observing an enhancement in the shot noise that is equivalent to the change of the effective charge from one to two electron charges. We further show that the spectroscopic gap fills up rather than closes with increasing temperature. Our results  demonstrate the existence of a state above Tc that, much like an ordinary metal, has no (pseudo)gap but carries charge through paired electrons. *This work is supported by the European Research Council (ERC StG SpinMelt) and by the Netherlands Organization for Scientific Research (NWO) as part of the Frontiers of Nanoscience program, as well as through a Vidi grant (680-47-536). The presenting author is supported by the NWO Veni talent scheme (VI.Veni.212.019),  K.M. Bastiaans et al., Science 374, 608-611 (2021).
16 December 2022 (Friday)
Session V A: Broken rotational symmetry
Moderator:Kazuhiro Fujita, Brookhaven National Laboratory, USA
09.30 - 10.00
Karlsruhe Institute of Technology,
Lattice dynamics, nematicity and CDWs in superconducting BaNi2(As1-xPx)2
Understanding the organizing principles of interacting electrons and the emergence of novel electronic phases is a central endeavor of condensed matter physics. Electronic nematicity, in which the discrete rotational symmetry in the electron fluid is broken while the translational one remains unaffected, is a prominent example of such a phase. It has proven ubiquitous in correlated electron systems, and is of prime importance to understand Fe-based superconductors. Here, we find that fluctuations of such broken symmetry are exceptionally strong over an extended temperature range above the formation temperature of an incommensurate charge-density-wave order (I-CDW) in BaNi2(As1-xPx)2, the nickel homologue to the Fe-based systems. This lends support to a type of electronic nematicity, dynamical in nature, which exhibits a particularly strong coupling to the underlying crystal lattice. Fluctuations between degenerate nematic configurations cause a splitting of phonon lines, without lifting degeneracies nor breaking symmetries, akin to spin liquids in magnetic systems. We further used diffuse and inelastic x-ray scattering to study the formation of the I-CDW in BaNi2As2 and observed strong phonon anomalies associated with it. Its reciprocal space position is well captured by our ab initio calculations, which however indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.  Yao et al. arxiv:2207.03161, accepted in Nature Communications (2022)  Souliou et al. arxiv:2207.07191
10.00 - 10.30
Ruhr-Universität Bochum, Germany
Nematicity and Bardasis-Schriffer mode in nonequilibrium dynamics of unconventional superconductors
Motivated by the recent development of terahertz pump-probe experiments, I will discuss in my talk the short-time dynamics in superconductors with multiple attractive pairing channels and competing nematic instability. Studying a single-band and multiband superconductors, we analyze the signatures of collective excitations of the pairing symmetries (known as Bardasis-Schrieffer modes) as well as the order parameter amplitude (Higgs mode) in the short-time dynamics of the spectral gap and quasiparticle distribution after an excitation by a pump pulse. We show that the polarization and intensity of the pulse can be used to control the symmetry of the non-equilibrium state as well as frequencies and relative intensities of the contributions of different collective modes[1-2]. Finally, I address the question of whether pump-probe technique can be used to reveal an interplay between various collective modes visible in the superconducting state and to distinguish the Pomeranchuk nematic collective mode from the BS mode due to the subdominant Cooper-pairing channel[3-4].  Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 100, 140501(R) (2019)  Marvin A. Müller, Pavel A. Volkov, Indranil Paul, Ilya M. Eremin, Phys. Rev. B 103, 024519 (2021)  Marvin A. Müller and Ilya M. Eremin, Phys. Rev. B 104, 144508 (2021).  Luke C. Rhodes, Jakob Böker, Marvin A. Müller, Matthias Eschrig and Ilya M. Eremin, npj Quantum Mater. 6, 45 (2021)
10.30 - 10.45
University of Paris Diderot, France
Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Co-Ba122
Anisotropic strain is an external field capable of selectively addressing the role of nematic fluctuations in promoting superconductivity. We demonstrate this using polarization-resolved elasto-Raman scattering to probe the evolution of nematic fluctuations under strain in the normal and superconducting states of the paradigmatic iron-based superconductor Ba(Fe1−xCox)2As2 [1,2]. For the superconducting compound, the suppression of the nematic susceptibility correlates with the decrease of the superconducting critical temperature Tc. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarii of enhanced Tc near a nematic quantum critical point.  J. C. Philippe et al. Phys. Rev. B 105, 024518 (2022)  J. C. Philippe et al. arXiv:2204.12213 (2022)
10.45 - 11.10
Two Best Poster (Thursday) presentations
11:00 - 11.30 Coffee break
Session V A: Pair Density Waves
Moderator: Subroto Mukherjee, Indian Institute of Science, Bangalore
Brookhaven National Laboratory, USA
Visualizing the Cuprate Pair Density Wave State
The defining characteristic of Cooper pairs with finite center-of-mass momentum is a spatially modulating superconducting energy gap Δ(r), which is predicted to exisit in the cuprates as the pair density wave (PDW) state. Although Cooper-pair tunneling has detected an indirect evidence of a cuprate PDW, the definitive signature in single-electron tunneling of a periodic Δ(r) modulation has never been observed. Here, using a new approach, we discover strong Δ(r) modulations in Bi2Sr2CaCu2O8+δ that have eight-unit-cell periodicity or wavevectors Q~2π/a0(1/8,0); 2π/a0(0,1/8). We will also discuss how such a PDW impact on the local-density-of-states N(r,E). In addition, we will talk about a relationship of the N(r,E) modulation to the lattice structure and discuss how they are linked to the phonon anomalies detected in the recent X-ray measurements. This work was done in collaboration with Zengyi Du, Hui Li Sanghyun Joo, Elizabeth P. Donoway, Jinho Lee, J. C. Séamus Davis, Genda Gu, Peter D. Johnson, Abhay N. Pasupathy, John M. Tranquada.  Z. Du and H. Li et al., Nature 580 65-70 (2020).
12:00 - 12.30
Stanford University, USA
A controlled solution of a system with a pair-density wave ground state
12.30 - 13.00 Summary Lecture by T.V. Ramakrishnan
(High Temperature Superconductivity (mostly in the Cuprates) : Quo Vadis?)
AbstractThe defining characteristic of Cooper pairs with finite center-of-mass momentum is a spatially modulating superconducting energy gap Δ(r), which is predicted to exisit in the cuprates as the pair density wave (PDW) state. Although Cooper-pair tunneling has detected an indirect evidence of a cuprate PDW, the definitive signature in single-electron tunneling of a periodic Δ(r) modulation has never been observed. Here, using a new approach, we discover strong Δ(r) modulations in Bi2Sr2CaCu2O8+δ that have eight-unit-cell periodicity or wavevectors Q~2π/a0(1/8,0); 2π/a0(0,1/8). We will also discuss how such a PDW impact on the local-density-of-states N(r,E). In addition, we will talk about a relationship of the N(r,E) modulation to the lattice structure and discuss how they are linked to the phonon anomalies detected in the recent X-ray measurements. This work was done in collaboration with Zengyi Du, Hui Li Sanghyun Joo, Elizabeth P. Donoway, Jinho Lee, J. C. Séamus Davis, Genda Gu, Peter D. Johnson, Abhay N. Pasupathy, John M. Tranquada.  Z. Du and H. Li et al., Nature 580 65-70 (2020).
13.00 Lunch and Departure