Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session D27: Superconductivity:Low Dimensional-I |
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Sponsoring Units: DCMP Chair: Yafis Barlas, University of Nevada, Reno Room: Room 219 |
Monday, March 6, 2023 3:00PM - 3:12PM |
D27.00001: Spin-triplet superconductivity at the onset of isospin order in biased bilayer graphene Zhiyu Dong, Andrey V Chubukov, Leonid Levitov The quest for unconventional superconductivity governed by Coulomb repulsion between electrons rather than phonon attraction received new momentum with the advent of moiré graphene. Initially, delineating the phonon and Coulomb-repulsion-based pairing mechanisms has proven to be a challenging task, however, the situation has changed after the recent discovery of superconductivity in non-twisted graphene bilayers and trilayers. Superconductivity occurring at the phase boundaries of spin and valley polarized orders calls for non-phonon scenarios, yet the specific pairing mechanisms remain to be understood. Here we analyze a striking example --- superconductivity in graphene bilayers occurring at the onset of valley-polarized order. We describe the phenomenon of field-induced pairing by a nominally repulsive interaction mediated by a quantum-critical mode and argue that it fully explains the observed phenomenology. While it is usually notoriously difficult to infer the pairing mechanism without probing superconducting order, this case presents a rare exception, allowing for a fairly unambiguous identification of the origin of the pairing glue. A combination of factors such as the location of the superconducting phase at the onset of the isospin-polarized phase, a threshold in a magnetic field, above which superconductivity occurs, and its resilience at high magnetic fields paints a clear picture of triplet superconductivity driven by quantum-critical fluctuations. We show that the same mechanism holds at zero field in the presence of spin-orbit coupling and argue that it explains recently observed superconductivity in bilayer graphene on WSe2 monolayer |
Monday, March 6, 2023 3:12PM - 3:24PM |
D27.00002: Geometric Contribution to the Superfluid Density in Inversion Asymmetric Bilayer Graphene Yafis Barlas, Enrico Rossi, Xiang Hu We study superconductivity and exciton condensation in bilayer graphene-based two-dimensional crystal heterostructures in the presence of a broken $mathcal{C}_2$ symmetry. |
Monday, March 6, 2023 3:24PM - 3:36PM |
D27.00003: Temperature-dependence of superconducting transition of LaAlO3/SrTiO3 nanowires under uniaxial strain Sayanwita Biswas, Xinyi Wu, Ki-Tae Eom, Kyoungjun Lee, Chang-Beom Eom, Patrick R Irvin, Jeremy Levy Oxide interfaces provide new ways to probe superconductivity in lower dimensions. A high-mobility 2D electron gas that can be superconducting at sub-Kelvin temperatures exists at the interface of LaAlO3 and SrTiO3. We can create 1D and 2D nanostructures at the interface of LAO/STO using conductive atomic force microscope (c-AFM) lithography [1]. Here we focus on the effects of uniaxial strain on the transport properties of superconducting quantum wires formed at the LaAlO3/SrTiO3 interface. We find that the superconducting transition temperature is unaffected by uniaxial strain irrespective of the applied strain direction. Although we notice that above the superconducting transition temperature, tension lowers the four-terminal resistance of a quantum nanowire whereas compression increases it. We will discuss the implications of these results in the context of the ferroelastic domain hypothesis. |
Monday, March 6, 2023 3:36PM - 3:48PM |
D27.00004: Superconductivity in cuasi-1D crystals from the Boson-Fermion theory Israel Chávez, Patricia Salas, Miguel A Solís-Atala From the Boson-Fermion (BF) theory [1-2] where electrons coexist with two-electron (2eCPs) and explicitly two-hole (2hCP) Cooper pairs, both as composite bosons, we solve three coupled equations numerically: the particle number equation, the gap-like equations for 2eCPs and for 2hCPs, from where we obtain the superconducting thermodynamic properties. For a cuasi-1D crystalline superconductor, where its ions are modeled as Dirac’s delta potentials equally spaced, coexisting with the gaseous mixture of bosons and fermions, we report among other thermodynamic properties, the energy gap, the critical temperature, and the specific heat. By comparison with the experimental data [3] we find that by introducing the ions explicitly, our results are better than in the case of the mixture without external potential. Surprisingly, we find that the energy gap and the critical temperature present a disproportionate maximum for optimal values of the intensity and separation of the deltas, showing a resonance-like state. We thank partial support from grant DGAPA-PAPIIT-UNAM IN114523. |
Monday, March 6, 2023 3:48PM - 4:00PM |
D27.00005: Searching for Non-Bulk Superconductors: Estimating Substrate Tc Enhancement Via Electron-Phonon Coupling in 2D thin films Oliver A Dicks The demonstrated enhancement of the Tc in an iron selenide (FeSe) overlayer due to interactions with its SrTiO3 substrate opened up a whole new area of material combinations to explore in the decades old search for room temperature superconductivity. |
Monday, March 6, 2023 4:00PM - 4:12PM |
D27.00006: Study the surface oxidation effect on superconducting transition in MoGe films and nanowires. Sam Feldman Suppression of superconductivity (SC) in nanowires remains to a large extent poorly understood. Even for the most comprehensively studied system, MoGe nanowires, existing experimental works conflict with each other. Thus, a bosonic type of SC suppression, via quantum phase slips (QPS), was claimed for nanowires covered with Ge layer (to protect from oxidation). On the other hand, in nanowires with no such layer, fabricated both by molecular templating technique and e-beam lithography, different behavior (no QPS) was observed. Moreover, in the later wires, the suppression of the critical temperature with the cross-sectional area was a hundred times faster than that predicted by existing theories. One of the possible explanations for these discrepancies is that they are caused by magnetic moments spontaneously formed on the surface of a wire in the process of oxidation. To check this, we carried out X-ray photoelectron spectroscopy (XPS) study and transport measurements on several series of intentionally oxidized and protected MoGe films and nanowires, made of two alloys, Mo78Ge22 and Mo50Ge50. From XPS we found that the oxidation leads to the formation of several Mo oxides. The differences between oxidized and protected samples were found in thin films (below about 3 nm) We will discuss if this effect can explain discrepancies observed in nanowires. |
Monday, March 6, 2023 4:12PM - 4:24PM |
D27.00007: Analogue viscous current flow near the onset of superconductivity Koushik Ganesan, Andrew Lucas Spatially resolved transport in two-dimensional quantum materials can reveal dynamics which is invisible in conventional bulk transport measurements. We predict striking patterns in spatially inhomogeneous transport just above the critical temperature in two-dimensional superconducting thin films, where electrical current will appear to flow as if it were a viscous fluid obeying the Navier-Stokes equations. Compared to viscous electron fluids in ultrapure metals such as graphene, this analogue viscous vortex fluid can exhibit a far more tunable crossover, as a function of temperature, from Ohmic to non-Ohmic transport, with the latter arising on increasingly large length scales close to the critical temperature. Experiments using nitrogen vacancy center magnetometry, or transport through patterned thin films, could reveal this analogue viscous flow in a wide variety of materials. |
Monday, March 6, 2023 4:24PM - 4:36PM |
D27.00008: Role of band geometry on the transition from BCS to a pair density wave state Guodong Jiang, Yafis Barlas The superfluid weight in multi-band superconductors contains a geometric contribution that is proportional to the quantum metric of the band for a uniform pairing of the orbitals. Here, we develop a band-projection formalism to calculate additional geometric contributions to the superfluid density in the presence of a non-trivial orbital-dependent pairing order parameter. We find that this band geometric contribution to the stiffness tensor can be locally non-positive-definite in some regions of the Brillouin zone. When these regions are large enough or include nodal zeroes, the total superfluid weight becomes non-positive-definite, indicating that the BCS $d{q}=0$ state is no longer stable. We identify this instability as a transition from the BCS state to a pair density wave, where the Cooper pairs have the same nonzero center-of-mass momentum $d{q}$. This pair density wave is stabilized by band geometry instead of a Zeeman field, so has no de-pairing in the momentum space. |
Monday, March 6, 2023 4:36PM - 4:48PM |
D27.00009: d-wave helical superconducting state in tricolor Kondo superlattices revealed by nonreciprocal transport Yuichi Kasahara, Hiroto Asaeda, Yuuki Kosuge, Toshiki Kiyosue, Shota Suetsugu, Masahiro Naristuka, Takahito Terashima, Tomoya Asaba, Yuji Matsuda In two-dimensional (2D) superconductors, strong Rashba spin-orbit interaction (SOI) under broken inversion symmetry gives rise to several fascinating superconducting properties, including the admixture of spin-singlet and triplet pairing, and topoogical superconductivity. Upon the application of a parallel magnetic field to the 2D plane, helical superconducting state characterized by the formation of Cooper pairs with nonzero total momentum is expected to emerge as a result of the shift of Fermi surfaces with the Rashba SOI. However, realization of helical superconductivity has remained largely unclear because of the lack of detection technique. Recently, it has been proposed that nonreciprocal transport can be a promising bulk probe to detect the helical superconductivity. |
Monday, March 6, 2023 4:48PM - 5:00PM |
D27.00010: Superconductivity in 2D and 3D lattice models of correlated fermions - combining matrix-product states with mean-field theory Per G Bollmark, Svenja Marten, Thomas Koehler, Lorenzo Pizzino, Yiqi Yang, Johannes S Hofmann, Hao Shi, Shiwei Zhang, Salvatore R Manmana, Thierry Giamarchi, Adrian Kantian Correlated electron states are at the root of many important phenomena including unconventional superconductivity (USC), where electron-pairing arises from repulsive interactions. Computing the properties of correlated electrons, such as the critical temperature Tc for the onset of USC, efficiently and unbiased remains a major challenge. Here, we combine matrix-product states (MPS) with static mean field (MF) to provide a solution to this challenge for 2D/3D materials comprised of weakly coupled correlated chains. This framework of Q1D fermions is developed and validated for attractive Hubbard systems and further enhanced via analytical field theory. Finally, we investigate the formation of transient non- quilibrum SC by a real-time evolution of a 3D extended Hubbard system out-of-equilibrium. |
Monday, March 6, 2023 5:00PM - 5:12PM |
D27.00011: Superconducting Fluctuations and Paraconductivity in Ultrathin a-Pb Films near Superconductor-Insulator Transition Haoyang Liu, Ashwani Kumar, Liuqi Yu, Richard P Barber, Peng Xiong The Aslamazov-Larkin (A-L) equation describes the extra conductance above TC of a superconductor due to superconducting fluctuations. While agreement with A-L model has been found in some conventional two-dimensional (2D) superconductors, its applicability in ultrathin films near the superconductor-insulator transitions (SITs) has not been fully ascertained. Here we report a detailed examination of superconducting fluctuation and paraconductivity in ultrathin 2D amorphous Pb films near the SITs tuned by in situ variation of the film thickness and incremental deposition of paramagnetic impurities (Cr) on the same sample [1]. The two sets of R(T) were fitted to the A-L equation, both in the form of paraconductance and total conductance [2], with TC, RN and magnitude of the paraconductivity, gexp, as the fitting parameters. In both cases, in the vicinity of the SIT (TC below ~ 1 K), the paraconductivity increases precipitously, reaching as high as 50 times that of the 2D A-L value. The significant enhancement of superconducting fluctuations above the A-L theory near the SITs suggests pronounced emergent electronic inhomogeneities in the morphologically uniform 2D amorphous films. |
Monday, March 6, 2023 5:12PM - 5:24PM |
D27.00012: Enhanced superconductivity by near-neighbor attraction in the doped extended Hubbard model beyond 1D Cheng Peng, Yao Wang, Jiajia Wen, Young S Lee, Thomas Devereaux, Hong-Chen Jiang A recent experiment has unveiled an anomalously strong electron-electron attraction in the one-dimensional copper-oxide chain Ba2-xSrxCuO3+δ. While the effect of the near-neighbor electron attraction V in the one-dimensional extended Hubbard chain has been examined recently, its effect in the Hubbard model beyond the one-dimensional chain remains unclear. We report a density-matrix renormalization group study of the extended Hubbard model on long four-leg cylinders on the square lattice. The near-neighbor electron attraction V can notably enhance the long-distance superconducting correlations while simultaneously suppressing the charge-density-wave correlations. Specifically, for a modestly strong electron attraction, the superconducting correlations become dominant over the CDW correlations with a Luttinger exponent Ksc~1 and strong divergent superconducting susceptibility. Our results provide a promising way to realize long-range superconductivity in the doped Hubbard model. The relevance of our numerical results to cuprate materials will also be discussed. |
Monday, March 6, 2023 5:24PM - 5:36PM |
D27.00013: Chiral to Nematic Crossover in the Superconducting State of 4Hb-TaS2 Itai Silber, Yoram Dagan, Mathimalar S, Ilay Mangel, Omer Green, Nurit Avraham, Haim Beidenkopf, Irena Feldman, Amit Kanigel, Avraham Klein, Moshe Goldstein, Anurag Banerjee, Eran Sela Most superconductors have an isotropic, single component order parameter and are well described by the standard (BCS) theory for superconductivity. Unconventional, multiple-component superconductors are exceptionally rare and are much less understood.Here, we combine scanning tunneling microscopy and angle-resolved macroscopic transport for studying the candidate chiral superconductor, 4Hb-TaS2. We reveal quasi-periodic one-dimensional modulations in the tunneling conductance accompanied by two-fold symmetric superconducting critical-field. The strong modulation of the in-plane critical field, Hc2, points to a nematic, unconventional order parameter. However, the imaged vortex core is nearly circular symmetric, suggesting an isotropic order parameter. We reconcile this apparent discrepancy by modeling competition between a dominating chiral superconducting order parameter and a nematic one. The latter emerges close to the normal phase. Our results strongly support the existence of two-component superconductivity in 4Hb-TaS2 and can provide valuable insights into other systems with coexistent charge order and superconductivity. |
Monday, March 6, 2023 5:36PM - 5:48PM |
D27.00014: Thickness-Dependent Tunneling Spectra of 2H-TaS2 Shahar Simon Transition metal dichalcogenide (TMD) superconductors have attracted much attention, in part due to the ease with which thin flakes can be mechanically exfoliated, and due to their strong Ising spin-orbit coupling (ISOC), expected unconventional electron pairing and topological states, making them a focus of interest in recent years. |
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