Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q61: Superconductivity: Low dimensional systemsIRecordings Available

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Sponsoring Units: DCMP Chair: Lei Wang, Yale Room: Hyatt Regency Hotel Field 
Wednesday, March 16, 2022 3:00PM  3:12PM 
Q61.00001: Anomalously strong nearneighbor attraction in doped 1D cuprate chains Zhuoyu Chen, Yao Wang, Slavko Rebec, Tao Jia, Makoto Hashimoto, Donghui Lu, Brian Moritz, Robert G Moore, Thomas P Devereaux, ZhiXun Shen In the cuprates, onedimensional (1D) chain compounds provide a distinctive opportunity to understand the microscopic physics, owing to the availability of reliable theories. However, progress has been limited by the challenge of controllably doping these materials. We report the synthesis and spectroscopic analysis of the 1D cuprate Ba_{2x}Sr_{x}CuO_{3+δ} over a wide range of hole doping. Our angleresolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong nearneighbor attraction, which may arise from coupling to phonons, quantitatively explains experiments for all accessible doping levels. Considering structural and quantum chemistry similarities among cuprates, this attraction may play a similarly important role in hightemperature cuprate superconductors. 
Wednesday, March 16, 2022 3:12PM  3:24PM 
Q61.00002: Tunable Superconductivity in Bilayer T_{d}MoTe_{2} Apoorv Jindal, Zizhong Li, Amartyajyoti Saha, Kenji Watanabe, Takashi Taniguchi, James C Hone, Cory R Dean, Turan Birol, Rafael M Fernandes, Daniel Rhodes, Abhay N Pasupathy Van der Waals heterostructures of encapsulated, lowcarrier density superconductors offer a unique platform to study 2D superconductivity in the clean limit. Furthermore, their properties can be tuned with an electrostatic gate, enabling the observation of phase transitions. In this talk, we will present recent results on superconductivity in bilayer T_{d} – MoTe_{2}. Bilayer T_{d} – MoTe_{2 }features small electron and hole pockets of nearly equal size. We show that superconductivity in this system is maximized when the hole and electron densities are equal. Eliminating either one via electrostatic gating renders the material ‘normal’, giving us a domeshaped superconducting phase diagram visavis doping and temperature. These results show that the likely scenario is one where superconductivity is mediated by interband pairing between the electron and hole pockets. We confirm this scenario with DFT and tightbinding calculations of the electronic structure and charge susceptibility, which shows strong nesting near charge compensation. 
Wednesday, March 16, 2022 3:24PM  3:36PM 
Q61.00003: Tunable superconductivity at KTaO_{3} (111) interface Changjiang Liu, Xianjing Zhou, Deshun Hong, Brandon Fisher, John Pearson, Dafei Jin, MR Norman, Anand Bhattacharya The discovery of superconductivity at the oxideinsulator/KTaO_{3} interface provides a new platform for investigating interfacial superconductivity. Exploring how the superconductivity in this system may be tuned is of great importance to both fundamental research and scientific applications. Here, we report on a strong tunability of superconductivity at the KTaO3 (111) interface by varying the carrier density via chemical doping, as well as through electrostatic gating. We show that the superconducting transition temperature T_{c} is strongly dependent on carrier density over a range of nearly one order of magnitude, while it is insensitive to the disorder level present in the as grown samples. The measured GinzburgLandau coherence length show a scaling law in these samples over the entire doping range. Our electrostatic gating measurements on samples with different doping levels show a monotonic dependence of the meanfield T_{c} on eigher the applied electric field or on the electrostatically induced carriers. 
Wednesday, March 16, 2022 3:36PM  3:48PM 
Q61.00004: Toggling superconductivity at the LaAlO_{3}/SrTiO_{3} interface via uniaxial strain Xinyi Wu, Megan K Briggeman, Joseph A Albro, Jianan Li, Jungwoo Lee, KiTae Eom, ChangBeom Eom, Patrick R Irvin, Jeremy Levy Oxide interfaces offer new ways to probe superconductivity in reduced spatial dimensions. Using conductive atomic force microscope (cAFM) lithography [1], conductive nanostructures can be reversibly created at the LaAlO_{3}/SrTiO_{3} interface, showing superconductivity at subKelvin temperatures. Here we focus on the effects of uniaxial stress on the transport of superconducting quantum wires formed at the LaAlO_{3}/SrTiO_{3} interface. We find that superconductivity is strongly affected by uniaxial strain applied parallel to the nanowire direction and in the transverse directions. Under certain conditions, we find that minute changes in uniaxial strain can abruptly and reversibly switch LaAlO_{3}/SrTiO_{3} nanowires from a superconducting state to a normal state. We will discuss the implications of these results on possible electronpairing mechanisms. 
Wednesday, March 16, 2022 3:48PM  4:00PM 
Q61.00005: Superconfucting cuprate ladder Sr(14x)Ca(x)Cu(24)O(41): The need to go beyond both one and multiband ladder models Sumitendra Mazumdar, R. Torsten Clay, JeongPil Song Attempts to explain unconventional superconductivity (SC) in the cuprates have focused overwhelmingly on the layered materials. Significantly less effort has gone into into understanding the SC the ladder compounds (Sr,Ca)(14)Cu(24)O(41). The assumption has been that singleband Hubbard modelbased theories of 2leg ladders suffice for these systems. We discuss the failures of both the simplest one and multiband Hubbard models to explain SC in these systems. Experimentally, the appearance of SC under pressure is accompanied by a 1Dto2D dimensional crossover [1] that has no theoretical explanation. NMR studies for the x=12 compound have found lowlying spin excitations [2,3]. Theoretically, we have recently shown from DMRG calculations for the 2leg cuprate ladder containing both Cu and O that superconducting pair correlations decay extremely rapidly for realistic parameters [4]. We argue that taken together these observations indicate that neither the one nor the multiband weakly doped Hubbard model is sufficient to understand the SC even in these simplest of superconducting cuprates. 
Wednesday, March 16, 2022 4:00PM  4:12PM 
Q61.00006: The Valence Transition model for the pressureinduced 1Dto2D dimensionality crossover in Sr(14x)Ca(x)Cu(24)O(41). JeongPil Song, R. Torsten Clay, Sumitendra Mazumdar We investigate theoretically the pressureinduced 1Dto2D dimensionality crossover [1] in (Sr,Ca)[14]Cu[24]O[41] using DMRG for coupled multiband ladders containing both Cu and O. We report numerical results for two coupled ladders of 8 and 12 CuOCu rungs, using realistic hopping and Hubbard interaction parameters. Dimensionality effects are measured through computations of intra and interladder OO bond orders (chargetransfers). The ratio of intraversus interladder bond orders is very large for realistic parameters and carrier densities, in agreement with the experimentally observed onedimensional behavior under ambient pressure [1]. We show that neither the assumption of increasing interladder hopping nor carrier density can explain the dimensionality crossover. We then show that the recently proposed valence transition model [2], within which there occurs a discrete jump in Cu(2+)toCu(1+) ionicity, leading to negative chargetransfer gap and a very large concomitant increase in the number of charge carriers on the Osites, aptly explains the dimensionality crossover. We argue that a similar valence transition explains the very large dopinginduced carrier densities in hole[3] and electrondoped layered cuprates [4]. 
Wednesday, March 16, 2022 4:12PM  4:24PM 
Q61.00007: PhononMediated LongRange Attractive Interaction in 1D Cuprates Brian Moritz, Yao Wang, Zhuoyu Chen, Tao Shi, Zhixun Shen, Thomas P Devereaux Establishing a minimal microscopic model for cuprates is a key step toward understanding the highT_{c} mechanism. Through quantitative comparison with recent in situ angleresolved photoemission spectroscopy measurements on doped 1D cuprate chains, our simulations highlight a significant contribution from longrange electronphonon coupling, beyond standard Hubbard model descriptions. Reasonable values for the coupling strength and phonon energy produce a strong attractive interaction between neighboring electrons, consistent with the effective interaction extracted from experiment. Considering the structural and chemical similarity between 1D and 2D cuprate materials, models that include longrange electronphonon coupling will provide valuable insights on cuprates, unconventional highT_{c} superconductivity in general, and related quantum phases. 
Wednesday, March 16, 2022 4:24PM  4:36PM 
Q61.00008: Upper Critical Field and Shubnikovde Haas (SdH) Quantum Oscillation Studies in a New quasi 2D Organic Superconductor Raju Ghimire, Brett Laramee, David E Graf, Scott Hannahs, Lee Martin, Charles C Agosta The upper critical field and SdH quantum oscillations have been studied in the new layered organic superconductor β″(BEDTTTF)_{2}[(H_{2}O)Cr(C_{2}O_{4})_{3}]18crown6 by measuring the inplane rfpenetration depth with the tunnel diode oscillator technique. The reported upper critical field is 8T, above the calculated ClogstonChandrasekhar limit (Hp). This, along with the strong 2D nature of this crystal & its wide anion layers suggest that there could be a possibility of an inhomogeneous FFLO superconducting state in this material. We will highlight data when the applied magnetic field is parallel to the conducting layers at low temperature using a 32T superconducting magnet at the National High Magnetic Field Laboratory. We will also present SdH quantum oscillation data when the applied field is perpendicular to the conducting layers and to compare the Fermi surface with the related compounds, λ(BETS)_{2}GaCl_{4} and К(ET)_{2}Cu[N(CN)_{2}]Br, other layered organics. The difference in Fermi surface topology will be discussed in relation to the existence of the FFLO state. 
Wednesday, March 16, 2022 4:36PM  4:48PM 
Q61.00009: Probing the FFLO State in Quasi2D Organic Superconductors using Sample Angular Rotations Brett Laramee, Raju Ghimire, Charles C Agosta, John A Schlueter, William A Coniglio, Akiko Kobayashi Organic superconductors can be prime candidates to investigate the FFLO state, an exotic, inhomogeneous superconducting state existing at high magnetic fields above the paramagnetic limit, H_{p}, where superconductivity is normally destroyed. Our layered organic crystals are highly anisotropic and have quasi2D structures, so the angle that their conduction planes make with an external Bfield greatly affects features such as H_{c2}, the FFLO phase transition, and vortex dynamics such as the lockin effect. Thus, we merge traditional Field and Temperatureanalysis with sample rotations to explore these features as a function of angle. We will present rfpenetration depth measurements made using a tunnel diode oscillator circuit on λ(BETS)_{2}GaCl_{4} and κ(ET)_{2}Cu(NCS)_{2} and compare our angular data to theoretical modeling. Combining results from field sweeps and angle rotations in pulsed and DCfields, respectively, we will show a preliminary 3D phase diagram of the FFLO state in FieldAngleTemperature phase space. 
Wednesday, March 16, 2022 4:48PM  5:00PM 
Q61.00010: Pseudospin magnetism and superconductivity in rhombohedral trilayer graphene Igor Blinov, Chunli Huang, Allan H MacDonald Rhombohedral (ABC) graphene trilayers consist of three layers of graphene stacked on top of each other with a relative shift. A lowenergy description of the system involves 4 flavors of fermions, corresponding to valley and spin degrees of freedom (pseudospins). Recent experiments [1,2] performed in this system without a moir\'e potential uncovered a cascade of phase transitions as a function of carrier doping and an external displacement field. I will explain how these transitions can be understood as generalized magnetic phase transitions in the 4dimensional pseudospin space, and focus on the valleyXY ordered phase. Using a simple phenomenological model supported by microscopic meanfield calculations, we show that a strongly trigonal warped Fermisurface like those in ABC trilayer graphene is unstable towards valleyXY formation at even weak Coulomb interactions. I will discuss superconductivity mediated by valleyXY paramagnons and comment on similarities and differences between ABCtrilayers and ABbilayers. 
Wednesday, March 16, 2022 5:00PM  5:12PM 
Q61.00011: Spatially Inhomogeneous Magnetic Multiband Superconductors Maxim Dzero, Alex Levchenko We present the solution to a problem of superconductivity coexistence with the spindensitywave order in disordered multiband metals. It is assumed that random variations of the disorder potential on short length scales render the interactions between electrons to become spatially correlated. As a consequence, both superconducting and magnetic order parameters become spatially inhomogeneous and are described by the universal phenomenological quantities, whereas all the microscopic details are encoded in the correlation function of the coupling strength fluctuations. We use a minimal model with two nested twodimensional Fermi surfaces and disorder potentials which include both intra and interband scattering. We use the quasiclassical approach to show that shortscale pairingpotential disorder leads to a broadening of the coexistence region. We also derive the expression for the exact Tmatrix for our model and analyze the structure of the subgap states across the coexistence region. 
Wednesday, March 16, 2022 5:12PM  5:24PM 
Q61.00012: Geometric Higgs spectroscopy Kristian Hauser A Villegas, Bo Yang Higgs spectroscopy is an emerging field in studying superconductivity. It is used to probe superconductor properties such as the pairing symmetry of the Cooper pairs. In the conventional theory, the Higgs mode is generated via a nonlinear coupling with the external electromagnetic field and mediated by the band curvature. This coupling vanishes for flat band superconductors and is expected to be weak for narrowband superconductors such as in twisted bilayer graphene. We have recently shown that geometric quantities such as the quantum metric and Berry curvature also mediates the coupling of the external field to the Higgs mode. More importantly, this geometric Higgs mode dominates over the conventional one for flat and narrow band superconductors. In this work, we use this result to study the geometric Higgs mode for various pairing symmetries. We consider the HarperHubbard model with p wave pairing and the tenband model of twisted bilayer graphene with s and d+id pairings. We show that Higgs spectroscopy can be a useful experimental tool in probing the pairing symmetries, and thus the microscopic nature of the narrow band superconductors. 
Wednesday, March 16, 2022 5:24PM  5:36PM 
Q61.00013: Gatetunable unconventional superconductivity in 2D oxide interfaces nanodevices Gyanendra Singh In conventional BardeenCooperSchrieffer superconductors, inversion and timereversal symmetries are preserved. The breaking of these symmetries is expected to lead to the formation of unconventional superconducting pairing. In this respect, the twodimensional (2D) electron systems in SrTiO_{3} based heterostructures, such as LaAlO_{3}/SrTiO_{3}, is a paradigmatic noncentrosymmetric superconductor due to the coexistence of gate tunable large Rashba spinorbit coupling and ability to modulate multiorbital 2D superconductivity. We present experimental evidence of unconventional superconductivity in the LaAlO_{3}/SrTiO_{3} interface nanodevices. The central observations are the giant anomalous enhancement of the critical current by small outofplane magnetic fields and the asymmetric response with respect to the magnetic field direction. These features have a unique trend in intensity and sign upon electrostatic gating that, together with their dependence on temperature and nanowire dimensions, cannot be accommodated within a scenario of spinsinglet superconductivity. We theoretically demonstrate that the hallmarks of the experimental observations unambiguously indicate the coexistence of Josephson channels with intrinsic phase shifts due to different sign of the order parameter. 
Wednesday, March 16, 2022 5:36PM  5:48PM 
Q61.00014: Electric fieldinduced superconductivity modulation in WC nanowires Rosa Córdoba, Vladimir M Fomin, Pablo Orús, JOSE MARIA DE TERESA In recent years, the electric fieldinduced control of superconductivity has been observed mostly in metallic superconducting materials [1]. However, so far the occurrence of this phenomenon is not fully understood in the framework of the microscopic BCS theory and it is still under debate. Here, we present a study of the electric fieldinduced modulation of superconductivity in WC nanowires fabricated using focused ion beam induced deposition [2], a single step nanolithography method. It is experimentally found by electrical transport characterization that an increasing sidegate voltage applied in the vicinity of a 45 nmwide WC nanowire progressively decreases its superconducting critical current, up to a full suppression of the superconducting state. A theoretical model based on the GL theory explains this modulation by the squeezing of the superconducting state by the electric field. Given their broadened working temperature range and the smaller gate voltages required to modulate superconductivity compared to other materials, WC nanostructures are promising candidates to serve as components in future superconducting devices with electric fieldinduced modulation. 
Wednesday, March 16, 2022 5:48PM  6:00PM 
Q61.00015: Gatetunability of the superconducting state in EuO/KTaO_{3} (111) interface Weiliang Qiao, Yang Ma, Jiaojie Yan, Wenyu Xing, YUNYAN YAO, Ranran Cai, Boning Li, Richen Xiong, X.C. Xie, Xi Lin, Wei Han The recent discovery of superconducting interfaces in the KTaO_{3} (111)based heterostructures is intriguing, since a much higher superconducting critical temperature (T_{C}; ~ 2 K) is achieved compared to that in the SrTiO_{3} heterostructures (~ 300 mK). In this work, we report the superconducting properties of EuO/KTaO_{3} (111) interface as a function of the interface carrier density (n_{s}) tuned by varying the EuO growth condition and by applying a back gate on the KTaO_{3} crystals. The T_{C} reaches its maximum of 2 K with n_{s} ~ 1×10^{14} cm^{2}. As n_{s} varies, the critical current density (J_{C}) exhibits a trend similar to that of T_{C}, while the upper critical magnetic field (B_{C2}) exhibits the opposite trend. The opposite trends between B_{C2} and T_{C} in the underdoped region are similar to the characteristics of high temperature superconducting cuprates, suggesting a relatively stronger Cooper pairing potential at lower n_{s}. Our results could pave the way for the further understanding the pairing mechanisms of superconductivity interfaces in the KTaO_{3} (111) heterostructures. 
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