Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W57: Superconductivity: TMC & Other SC materials IVRecordings Available

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Sponsoring Units: DCMP Chair: John Wei, University of Toronto Room: Hyatt Regency Hotel Clark 
Thursday, March 17, 2022 3:00PM  3:12PM 
W57.00001: Anisotropic superconducting properties of Pd_{3}Bi_{2}Se_{2} single crystals Ramakanta Chapai, Matthew P Smylie, WaiKwong Kwok, Ulrich Welp, John F Mitchell The Parkeritetype transitionmetal chalcogenides have been known for their intriguing properties such as charge density wave, superconductivity, and intercalation reactions. Here, we report electrical transport down to 1.8 K and tunnel diode oscillator (TDO) measurements down to 450 mK on high quality single crystalline Pd_{3}Bi_{2}Se_{2} adopting the Parkerite structure. The temperature dependence of the electrical resistivity shows typical metallic behavior. Superconductivity is observed with an onset temperature of ~0.82 K via the TDO measurements. By measuring the temperature dependence of the TDO resonant frequency in various applied fields, the phase diagrams are constructed for both inplane and out of plane directions. The ratio of the upper critical field H_{c2}(inplane)/H_{c2}(out of plane) is found to be 1.75, indicating a weak anisotropic nature of the superconductivity in Pd_{3}Bi_{2}Se_{2}. This low value of the anisotropy is unexpected considering the layered monoclinic structure of Pd_{3}Bi_{2}Se_{2} suggesting that the bonding along the out of plane direction is reasonably strong in this material. 
Thursday, March 17, 2022 3:12PM  3:24PM 
W57.00002: Enhanced superconductivity, new polymorphic phase, and rich phase diagram in the PdSe_{2x}Te_{x} system wenhao liu, Mehrdad R Osanloo, Sheng Li, Xiqu Wang, Nikhil Dhale, Hanlin Wu, Maarten L Van de Put, Sabyasachi Tiwari, William G. Vandenberghe, Pawan Koirala, Bing Lv We report a combined experimental and theoretical study of the PdSe_{2−x}Te_{x} system. With increasing Te fraction, structural evolutions, first from an orthorhombic phase (space group Pbca) to a new monoclinic phase (space group C2/c) and then to a trigonal phase (space group P−3m1), are observed accompanied with clearly distinct electrical transport behaviors. This new monoclinic phase (C2/c) belongs to a very rare Verbeekite polymorphism and is discovered within a narrow range of Te composition (0.3≤x≤0.8). Interestingly, in the trigonal phase region, superconductivity with significantly enhanced critical temperature is observed within a narrow range of Te content (1.0≤x≤1.2) in comparison with that of 1TPdTe_{2} phase. The superconductivity has been unambiguously confirmed by the magnetization, resistivity, and specific heat measurements. The detailed electrical transport analysis and electronic calculations across the rich phase diagram will be presented. The unusual superconductivity properties, the new polymorphic structure, and the rich phase diagram in this system could further stimulate interest to explore the new superconducting mechanism and new types of polymorphs in the large family of transition metal dichalcogenides. 
Thursday, March 17, 2022 3:24PM  3:36PM 
W57.00003: Role of magnetic and nonmagnetic impurities in unconventional behavior of Ising superconductors Igor Mazin, Darshana Wickramaratne, Maxim Khodas, Menashe Haim Apart from their signature giant anistotropy of the critical field, Ising superconductors (IS) exhibit other rather unexpected properties: (1) in tunneling through a ferromagnetic barrier, NbSe_{2} IS demonstrates anticorrelaton between the tunneling gap value and gap broadening, internal hysteretic behavior that sets 2 K below T_{c}, spontaneous breaking of the C_{6} symmetry (interpreted originally as"nematic superconductivity") and (2) NbSe_{2} IS shows nonmonotonic behavior of Tc and Δ as a function of Se⇒S (initially interpreted as "multifractal superconductivity") and Nb⇒Mo doping . We show, utilizing results of first principle and model calculations, that all these effects are consequence of highly nontrivial interaction of IS with magnetic and nonmagnetic impurity. In this talk we will discuss (1), while (2) will be presented in the following talk by D. Wickramaratne. Finally, we will briefly discuss some nontrivial predictions/suggestions for new experiments that follow from the presented theory. 
Thursday, March 17, 2022 3:36PM  3:48PM 
W57.00004: Ising superconductivity in NbSe_{2} alloys Darshana Wickramaratne, Igor Mazin Monolayer NbSe_{2} and NbS_{2} are isostructural twodimensional materials that exhibit contrasting superconducting properties. Monolayer NbSe_{2} is an Ising superconductor,while there are no reports of superconductivity in monolayer NbS_{2}.Recent experiments on NbSe_{2} alloyed with sulfur have shown T_{c} varies nonmonotonically with sulfur content.This was interpreted as a manifestation of fractal superconductivity. In this talk we will show that the nonmonotonic dependence of T_{c} is insufficient evidence for fractal superconductivity. Using firstprinciples calculations we highlight how this nonmonotonic change in T_{c }can arise due to magnetic point defects such as chalcogen vacancies, which we expect to be present in appreciable concentrations, along with changes in the density of states and proximity to magnetism as a function of alloying.b We use these insights to show how alloying on the Nb site with molybdenum may also lead to a nonmonotonic change of T_{c} in NbSe_{2}. 
Thursday, March 17, 2022 3:48PM  4:00PM 
W57.00005: Multiband s^{++} superconductivity in V_{3}Si determined from the response to a controlled disorder Ruslan Prozorov, Kyuil Cho, Marcin Konczykowski, Sunil Ghimire, Makariy A Tanatar, LinLin Wang, Vladimir G Kogan Superfluid density, ρ(T), of a highquality V_{3}Si single crystal shows fullygapped isotropic BCS superconductivity with an additional distinct feature of two almost decoupled superconducting gaps. However, this ρ(T) can be obtained if these two order parameters have different (s_{±}) or the same (s_{++}) sign. To investigate, the sample was irradiated at 20 K by 2.5 MeV electrons three times, repeating the measurements between the irradiation runs. A large total dose of 6×10^{19} cm^{2} was accumulated, after which the superconducting transition temperature, T_{c}, decreased from 16.4 K in pristine state to 14.7 K. This substantial suppression is impossible for a single isotropic gap, yet it is not large enough for a signchanging s_{±} pairing state. The electronic band structure calculations show how five bands crossing the Fermi level can naturally support two effective gaps, not dissimilar from the iron pnictides. The twogap selfconsistent theories for both, ρ(T) and △T_{c},_{ }describe the data very well. Thus, the experimental results and theoretical analysis provide strong support for s_{++ }superconductivity with two unequal gaps, △_{1} =2.5 meV and △_{2} =1.4 meV, and a very weak interband coupling in the V_{3}Si superconductor. 
Thursday, March 17, 2022 4:00PM  4:12PM 
W57.00006: Crossover of Ising to RashbaType Superconductivity in Epitaxial Bi_{2}Se_{3}/Monolayer NbSe_{2} Heterostructures Hemian Yi, Lunhui Hu, Run Xiao, Yuanxi Wang, Jiaqi Cai, Denielle R. Hickey, Chengye Dong, Yifan Zhao, Lingjie Zhou, Ruoxi Zhang, Anthony R Richardella, Joshua A Robinson, Moses H Chan, Xiaodong Xu, Nitin Samarth, Chaoxing Liu, CuiZu Chang When two different materials are brought together, the resultant interface between them often shows unexpected quantum phenomena. For example, the interface between a topological insulator (TI) and an swave superconductor can host an unusual form of superconductivity known as topological superconductivity. In this work, we grew the Bi_{2}Se_{3}/monolayer NbSe_{2} heterostructures with different Bi_{2}Se_{3} thicknesses using molecular beam epitaxy. We found that the gapless Dirac surface states are formed when the Bi_{2}Se_{3} thickness is greater than 3 quintuple layers (QLs). Moreover, we observed the Rashbatype bulk conduction bands for the Bi_{2}Se_{3 }thickness greater than 2 QLs. Our observations are well interpreted by the firstprinciples calculations. By performing magnetotransport measurements, we found that the inplane upper critical magnetic field of the superconductivity in Bi_{2}Se_{3}/monolayer NbSe_{2} heterostructures is greatly suppressed when the Rashba bands emerge, indicating the occurrence of a crossover from Ising to Rashbatype pairings. Our success in the synthesis of Bi_{2}Se_{3}/monolayer NbSe_{2} heterostructures and the demonstration of the crossover from Ising to Rashbatype superconductivity in these heterostructures provide more impetus for exploring the topological superconductivity in TI/superconductor heterostructures. 
Thursday, March 17, 2022 4:12PM  4:24PM 
W57.00007: Superconductivity mediated by polar modes in doped ferroelectrics Stephen E Rowley The occurrence of superconductivity in doped SrTiO_{3} and related materials at low carrier densities points to the presence of an unusually strong pairing interaction that has eluded understanding for several decades. We present experimental results showing the pressure dependence of the superconducting transition temperature, T_{c}, that sheds light on the nature of this interaction. We find that T_{c} increases dramatically when the energy gap of ferroelectric critical modes is suppressed, i.e., as the ferroelectric quantum critical point is approached, in a way reminiscent of behaviour observed in magnetic counterparts. However, in contrast to the latter, the coupling of itinerant electrons to the critical modes in ferroelectrics is predicted to be small. We present a superconductivity model that shows quantitative agreement with experiment without the use of adjustable parameters which involves electron pairing via the virtual exchange of longitudinal hybridpolarmodes. The model accounts for the domes of superconductivity observed as function of carrier density and pressure and the enhancement of T_{c} close to quantum criticality even in the absence of a direct coupling to the transverse critical modes. 
Thursday, March 17, 2022 4:24PM  4:36PM 
W57.00008: Enhanced superconductivity in 2MWS_{2} through doping Hari Paudyal, Elena R Margine A new member of transition metal dichalcogenide (TMD) family, 2MWS_{2}, has been recently discovered and shown to display superconductivity with a critical temperature (T_{c}) of 8.8 K, the highest T_{c} among superconducting TMDs at ambient pressure. Using firstprinciples calculations combined with the anisotropic MigdalEliashberg formalism, we explore how the superconducting properties of 2MWS_{2 }can be enhanced through doping. We find that compared to the parent compound the electronphonon coupling can be increased by ptype doping, leading to an enhancement of the T_{c} up to 50%. 
Thursday, March 17, 2022 4:36PM  4:48PM 
W57.00009: Suppression of electronphonon mediated superconductivity by strong spin fluctuation effects in the 2D Ising superconductor NbSe_{2} Suvadip Das, Hari Paudyal, Roxana Margine, Igor Mazin Twodimensional materials particularly transition metal dichalcogenides (TMDs) are promising candidates for atomically thin electronic applications, such as spintronic, valleytronic, and quantum hall devices. A later addition to the landscape pertains to the discovery of an unusual superconducting state in monolayers of NbSe_{2} dubbed ‘Ising superconductivity’. It is generally believed that the superconductivity of monolayer NbSe_{2} is dominated primarily by electronphonon effects. However, comparison with the experiment indicates that conventional firstprinciples calculation of the electronphonon coupling severely overestimates the critical temperature and superconducting gap. Since it was recently found that this system is close to a magnetic instability, and given that the first principles Eliashberg theory has been very successful in the prediction of the superconducting temperature in phonon superconductors, it is natural to assume that superconductivity in NbSe2 is partially suppressed by spin fluctuations. In our work, we include the latter on the level of BerkSchrieffer's theory. It appears crucially important to include retardation effects accounting for the different energy scales of phonons and paramagnons. Both momentum and frequency dependence of spinfluctuations were extracted from ab initio calculations, along with the full electronphonon momentumresolved Eliashberg function. After solving the full band and momentumresolved Eliashberg equations, we extracted the momentum and temperaturedependent order parameter, as well as the critical temperature. We then investigated the average order parameter averaged over each Fermi surface, as well as over the all electronic state, and after comparison with the tunneling data, discuss, as to what level the calculated anisotropy survives in real samples. 
Thursday, March 17, 2022 4:48PM  5:00PM 
W57.00010: Bogoliubov Fermi surfaces due to outofplane magnetic fields in monolayer superconducting NbSe$_2$ Saumitran Kasturirangan, Guannan Chen, Somesh C Ganguli, Nandini Trivedi, Rafael M Fernandes, Vidya Madhavan, Fiona J Burnell We use a lowenergy model to investigate the effect of an outofplane magnetic field on the superconducting properties of monolayer NbSe$_2$. We find that, even when the singletchannel interactions dominate, such that the zerofield superconducting state is fully gapped, there is a threshold Zeeman field beyond which the quasiparticle spectrum is no longer fully gapped, giving rise to Bogoliubov Fermi surfaces. We establish this by selfconsistently solving the gap equations for mixed singlet and triplet pairing on the $\Gamma$ pocket, in the presence of both Ising spinorbit coupling and Zeeman field. We find that the resulting evolution of the density of states with magnetic field qualitatively agrees with the scanning tunneling microscopy data in this material. 
Thursday, March 17, 2022 5:00PM  5:12PM 
W57.00011: Phononinduced rotation of the electronic nematic director in superconducting doped Bi_{2}Se_{3} Matthias Hecker, Rafael M Fernandes Nematic superconductivity has been directly observed in the doped topological insulator A_{x}Bi_{2}Se_{3}, with A={Cu, Nb, Sr}, as manifested by a twofold symmetric inplane critical field H_{c2}. The orientation of the ellipticallyshaped inplane H_{c2} is set by the electronic nematic director, which is parametrized by an angle α. Since the nematic order parameter follows the Z_{3}Potts model, the angle α naturally aligns with one of the three highsymmetry directions of the trigonal lattice of Bi_{2}Se_{3}. We find that the inclusion of acoustic phonons changes this phenomenology in a fundamental way in trigonal systems, but not in hexagonal ones. Trigonal systems have an additional elastic constant c_{14}, stemming from the fact that the inplane and outofplane shear strain doublets belong to the same irreducible representation. We show that, when c_{14} is large enough, the contribution to the nematic action coming from the coupling to the acoustic phonons makes the nematic director unlock from the highsymmetry directions. We discuss the implication of this rotation of the nematic director in H_{c2} measurements, as well as the applicability of these results to doped Bi_{2}Se_{3}. 
Thursday, March 17, 2022 5:12PM  5:24PM 
W57.00012: Investigation of superconducting properties of AuAgTe_{4} under pressure Gyanu P Kafle, Evgenia Komleva, Y. Amiel, A. Palevski, E. Greenberg, S. Chariton, Yurii Ponosov, Daniel I Khomskii, G. K Rozenberg, Sergey V Streltsov, Igor Mazin, Elena R Margine Materials containing gold have been seldom explored since gold is a very inert element forming relatively few compounds. In this work, we investigated the mineral sylvanite, AuAgTe_{4}, under compression using firstprinciples calculations and found a structural phase transition from monoclinic P2/c to P2/m structure around 5 GPa (in agreement with Xray diffraction and Raman results). We further examined the superconducting properties of the P2/m phase using the EPW package and identified that the main contribution to the electronphonon coupling arises from the lowenergy phonon modes. 
Thursday, March 17, 2022 5:24PM  5:36PM 
W57.00013: Investigation of pressuredriven superconductivity in TlInTe_{2} Christopher Renskers, Elena R Margine A domeshaped curve has been typically observed in many superconductors under compression. However, it has recently been found that the Zintl compound TlInTe_{2} exhibits a Vshaped phase diagram of superconducting critical temperature (T_{c}) upon increase in pressure (P). In this work, we have explored the superconducting properties of TlInTe_{2} using the MigdalEliashberg formalism to shed light on this T_{c}P behavior. 
Thursday, March 17, 2022 5:36PM  5:48PM 
W57.00014: Charge Density Wave Proximity Effect Induced in Graphene on 1TTaS_{2} Skandaprasad V Rao, Michael Altvater, Nikhil M Tilak, Guohong Li, Eva Y Andrei, SangWook Cheong, Choongjae Won Recent experiments have shown that the charge density wave (CDW) lattice of 1TTaS_{2} can be observed within a graphene layer placed on top using scanning tunneling microscopy and spectroscopy (STM/STS)^{13}. This observation suggests the possibility of an unexplored proximityinduced CDW within graphene when placed in contact with 1TTaS_{2}. The nature of such a novel CDW proximity effect is shown to involve shortrange Coulomb and exchange interactions between itinerant Dirac electrons in graphene and localized electrons in 1TTaS_{2}. We further explore the CDW proximity effect by probing the local electronic properties of graphene both on top of and in lateral contact with 1TTaS_{2} using STM and STS. Finally, we organize our results by relating the CDW proximity effect with wellknown contact proximity effects such as the spinorbit, magnetic, and superconducting proximity effects. 
Thursday, March 17, 2022 5:48PM  6:00PM 
W57.00015: Nematic, chiral, and topological superconductivity in transition metal dichalcogenides Constantin Schrade, Liang Fu We introduce and study a realistic model for superconductivity in twisted bilayer WSe2, where electron pairing arises from spinvalley fluctuations in the weakcoupling regime. Our model comprises both the full continuum model moiré bandstructure and a shortranged repulsive interaction that accounts for the Coulomb interaction projected onto the localized Wannier orbitals. By calculating the spinvalley susceptibility, we identify a Fermi surface nesting feature near halffilling of the topmost moiré band, which induces significantly enhanced spinvalley fluctuations. We then analyze the dominant KohnLuttinger pairing instabilities due to these spinvalley fluctuations and show that the leading instability can induce nematic, chiral and topological superconductivity. As our findings are asymptotically exact for small interaction strengths, they provide a viable starting point for future studies of superconductivity in twisted transition metal dichalcogenide bilayers. 
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