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 Q22: Superconductivity: Proximity & Related Effects |
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Sponsoring Units: DCMP Chair: Aranya Goswami, University of California, Santa Barbara Room: Room 214 |
Wednesday, March 8, 2023 3:00PM - 3:12PM |
Q22.00001: Equilibrium supercurrent in proximity-induced superconductors due to band asymmetry Pavan R Hosur We theoretically investigate the consequences of proximity-induced conventional superconductivity in metals that break time-reversal and inversion symmetries through their energy dispersion but may not harbor Berry phases in their bands. We discover behaviors impossible in an isolated superconductor, such as an equilibrium supercurrent that apparently violates Bloch's theorem and, at suitable topological defects, non-conservation of electric charge reminiscent of the chiral anomaly. We distinguish two cases, type-A and type-B, based on whether band asymmetry and equilibrium supercurrent onset at different temperatures or the same temperature, and sketch their Landau phase diagrams. For both types, the equilibrium supercurrent can be trained by a helical electromagnetic field. |
Wednesday, March 8, 2023 3:12PM - 3:24PM |
Q22.00002: Electromagnetic proximity effect in superconducting spin valves heterostrucutres in the clean limit Jian-Lin Li, Chien-Te Wu, Klaus B Halterman We theoretically study the magnetic field effect produced by the exchange coupling in ferromagnet-superconductor junctions. The inverse proximity effect induces magnetism inside the superconductor which then generates a spontaneous current. The spontaneous current in turn affects the vector potential via Maxwell's equations, thereby adjusting the Bogoliubov-de Gennes (BdG) Hamiltonian. To self-consistently determine physical quantities such as the currents and superconducting gap, we simultaneously find solutions to the BdG equations and Maxwell's equations iteratively. An alternative approach to solve this problem is the quasi-classical Green's function technique, but it requires a strong boundary condition on the vector potential to fix the gauge freedom in order to obtain a unique solution. In addition, this technique is not adequate for a strong ferromagnet. In the literature, the dirty limit is usually assumed to reduce the Eilenberger equation into a simpler Usadel equation. Still, it is difficult to self-consistently solve the Usadel equation along with Maxwell's equation. In contrast, we demonstrate that our BdG approach is natural to achieve self-consistency from a step-by-step analysis with Hamiltonain, symmetry and free energy. Finally, our work also offers a direction to find the relation between the odd-frequency triplet pairing and electromagnetism which can be controlled in the spin valve structure. |
Wednesday, March 8, 2023 3:24PM - 3:36PM |
Q22.00003: cQED detection of anisotropic pairing in a mesoscopic superconductor-ferromagnet bilayer: part I Nicholas R Poniatowski, Charlotte Bøttcher, Andrey Grankin, Marie E Wesson, Uri Vool, Victor M Galitski, Amir Yacoby Mesoscopic and low-dimensional materials represent one of the frontiers in the study of unconventional superconductivity. Owing to their small size, these materials are challenging to probe using conventional measurement techniques and require new experimental probes to successfully characterize them. In this talk, we introduce one such probe which enables us to measure the superfluid density of micron-size superconductors using microwave techniques drawn from circuit quantum electrodynamics (cQED). We apply this technique to a well-studied system, the superconductor-ferromagnet bilayer, where we find evidence for anisotropic induced superconductivity. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q22.00004: Scanning Tunneling Spectroscopy Study of Superconducting Proximity Effect in Cuprate/Perovskite Heterostructures Rainni K Chen, Charles Zhang, Young-June Kim, John Y Wei Recent studies have reported an anomalously long-range superconducting proximity effect (PE) in c-axis heterostructures of ferromagnetic La2/3Ca1/3MnO3 (LCMO) and superconducting YBa2Cu3O7-δ (YBCO) [1]. Although this long-range PE has been attributed to spin-triplet pairing with d-wave symmetry, it remains controversial since scanning tunneling spectroscopy (STS) has failed to observe any direct and microscopic evidence [2]. Our work extends prior STS studies to epitaxial {110}-oriented LCMO/YBCO bilayer films to take advantage of both the longer in-plane coherence length of YBCO and the presence of resonant d-wave Andreev bound states on {110} surfaces. We directly gauged the PE length scale, by probing these states and varying the LCMO layer thickness. To probe the effect of ferromagnetism on this d-wave PE, we substituted LCMO with LaNiO3 as a paramagnetic control. Additionally, we substituted LCMO with Sr2IrO4 to explore the effect of strong spin-orbit coupling on this d-wave PE. Our results are discussed in terms of various mechanisms for singlet-to-triplet conversion. |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q22.00005: Induced Superconducting States in Proximitized Weyl Semimetals Robert Dawson, Vivek Aji We explore the properties of induced superconducting states in proximitized Weyl Semimetals. Conventionally, an effective tunneling approach is employed where the host superconductor is treated as a passive source of Cooper Pairs. To better characterize the physics across the interface, we solve the BdG equation written in a basis that spans the entire heterostructure. A numerical approach is employed for the resultant Bogoliubov equations to obtain the pairing amplitude throughout the device. This allows us to determine the coherence length, symmetry, and conditions, if any, under which a topological superconductor is realized. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q22.00006: Supercurrent through single electron transverse mode in 1D Josephson junctions QPC Bomin Zhang, Zhuan Li, Victor Aguilar, Mihir Pendharkar, Connor P Dempsey, Joon Sue Lee, Sean Harrington, Chris J Palmstrom, Sergey M Frolov We study Josephson Junctions (JJs) in InSb nanowires with 15nm Tin shells [1]. We observe quantized conductance plateaus. This phenomenon is the evidence of transparent transmission and ballistic transport, which is known as Quantum Point Contacts (QPC). In the gate voltage range where conductance is corresponding to the 1st electron sub-band, we obtain supercurrent carried by single transverse mode. We study gate voltage and magnetic field dependence of supercurrent in the 1st electron mode. In a previous paper, it has been demonstrated that supercurrent in an external parallel field is affected by the interference between transverse modes. In the contrast, a numerical simulation based on the tight-binding model predicts that the supercurrent carried by the single mode is less affected by the interference [2]. Our measurement results have good agreement with this theoretical prediction. |
Wednesday, March 8, 2023 4:12PM - 4:24PM |
Q22.00007: Superconducting via contacts to graphene Cequn Li, Ke Huang, Kenji Watanabe, Takashi Taniguchi, Jun Zhu Highly transparent superconducting contacts is of great interest to research exploring the superconducting proximity effect in two-dimensional materials. In prior studies on NbSe2/graphene heterostructures [Li et al, Phys. Rev. B 101, 195405 (2020)], Andreev reflections with efficiencies of 80% were demonstrated. This work suggests that a clean van der Waals interface facilitates the superconducting proximity coupling. Here we report on the fabrication and performance of superconducting NbN contacts to graphene, realized through transferred via contacts. The NbN via contacts are sputtered into etched pits of a h-BN sheet, and then transferred to graphene to form a van der Waals-like interface. Our NbN films have a superconducting transition temperature of Tc ~7 K, and an upper critical field of Hc2 > 9 T at 2K. Differential conductance measurements on NbN/graphene junctions show enhancement at small DC voltage biases, as a result of Andreev reflection occurring at the NbN/graphene interface. We characterize the performance of junctions fabricated using different processes. The large Hc2 of NbN will facilitate the studies of the superconducting proximity effect in the quantum Hall regime. |
Wednesday, March 8, 2023 4:24PM - 4:36PM |
Q22.00008: Infinite magnetoresistance in superconducting spin switch with spin-orbit coupling Hisakazu Matsuki, Guang Yang, Greg Mazur, Nadia Stelmashenko, Lesley Cohen, Jason Robinson At a thin-film superconductor (S) interface with a ferromagnetic insulator (FI), the magnetic exchange field (MEF) of the FI can spin-split the density of states1,2 and suppress the critical temperature (Tc) of S3. In a FI/S/FI spin switch, the suppression of Tc is reduced for antiparallel (AP) magnetizations between the two FI layer due to a net cancellation effect of MEF acting on the S layer. Conversely, for parallel (P) magnetizations, the exchange fields add enhancing the suppression of Tc i.e., ΔTc=Tc(AP)–Tc(P)>0. For S materials with weak spin-orbit coupling (SOC) such as Al the spin splitting in S can exceed several Tesla1,2 and ΔTc can reach tens of mK3. It has also been shown that by dusting an atomic layer of Au at the Al/EuS interface, it is possible to quench the spin splitting in Al due to SOC in Au4. Here we report EuS/Nb/EuS superconducting spin switches in which the superconducting layer of Nb has strong SOC and a Tc down to a thickness of only 2 nm. By optimizing the Nb/EuS interface we obtain record-breaking values of ΔTc that exceed 1.5 K with a ΔTc/Tc(P) ratio of nearly 100%. The results indicate physics that goes beyond the standard quasiclassical picture of S/FI proximity effects in which superconducting spin-switch performance is boosted by the large values of Tc in the nearly nm-thick layer of Nb in conjunction with strong SOC. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q22.00009: Large scale 2D semiconductor MoS2 on superconducting YBa2Cu3O7 for proximity devices Kevin Seurre, Florian GODEL, Vincent Humbert, David Perconte, Bruno Dlubak, Pierre Seneor, Javier E Villegas Coupling two-dimensional (2D) materials like graphene with HTc superconductors allows for new physical properties, induced by the proximity effect and the intrinsic tunability of 2Ds[1]. In this context, transition metal dichalcogenides (TMD) such as MoS2 are interesting because they enable studying the proximity behavior in presence of spin-orbit coupling. While most of the existing experimental work is based on s-wave superconductors, the specificities of d-wave ones expectedly enrich the resulting physics. We will first discuss how stable CVD-grown graphene can be transferred onto YBa2Cu3O7 (YBCO) and lead to robust superconducting proximity effect probed in solid state devices[2-3]. We will then discuss how in order to realize similar TMD-based devices a different approach has to be tackled, which is the direct growth of MoS2 onto YBCO. This process, based on the growth of TMD materials by Pulsed-Laser Deposition (PLD) [4-5], preserves the superconducting properties of the cuprate and further allows us to limit the processing steps. We will discuss the requirements to both grow MoS2 and preserve high-Tc superconductivity as well as electronic transport characterizations of realized devices. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q22.00010: Loss and decoherence at the quantum Hall - superconductor interface Lingfei Zhao, Zubair Iftikhar, Trevyn Larson, Ethan G Arnault, Kenji Watanabe, Takashi Taniguchi, Francois Amet, Gleb Finkelstein High quality type-II superconducting contacts have recently been developed to a variety of 2D systems, allowing one to explore the superconducting proximity in the quantum Hall (QH) regime. Inducing superconducting correlations into a chiral system has long been viewed as a route for creating exotic topological states and excitations. However, it appears that before these exciting predictions could be realized, one should develop a better understanding of the limitations imposed by the physics of real materials. Here, we perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the QH regime. We find that the probability of Andreev conversion of electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our study may pave the road to engineering future generation of hybrid devices for exploiting superconductivity proximity in chiral channels. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q22.00011: Gate tunable superconductivity in Sn/InAs nanowires Amritesh Sharma, Victor Aguilar, Po Zhang, Bomin Zhang, An-Hsi Chen, Connor P Dempsey, Moira Hocevar, Chris J Palmstrom, Sergey M Frolov Ever-increasing interest in super-semiconducting hybrid nanowire systems, as building blocks for realizing Majorana zero modes and field-tunable qubits, solicits research on new material combinations and fabrication techniques. Sn has emerged as a promising alternative to Al, inducing hard-gap and large switching currents in these semiconducting 1D channels, also alleviating the need for epitaxially matched interfaces [1,2]. Here, we report our findings on induced superconductivity in Sn/InAs nanowires and gate tunability in accidentally defined shadow SNS junctions. We will also discuss our progress in deterministic etching of Sn that would enable definition of superconducting junctions and islands on these nanowires. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q22.00012: High transparency Josephson junctions on SiGe/Ge/SiGe heterostructures for quantum information Chotivut Tangchingchai, Zahra Sadre-Momtaz, Elyjah Kiyooka, Axel Leblanc, Jean-Michel Hartmann, Boris Brun-Barriere, Vivien Schmitt, Gonzalo Troncoso, Simon Zihlmann, Romain Maurand, Silvano De Franceschi, François Lefloch In recent years, there is increasing interest in hybrid superconductor-semiconductor (S-Sm) devices due to several possible applications in quantum information including gate-tunable transmons and parametric amplifiers. These realizations rely on S-Sm-S Josephson junctions for which the non-dissipative supercurrent can be tuned by an electrostatic gate creating a Josephson Field Effect Transistor (JoFET). Additionally, coherent double Cooper pairs transmission can occur in highly transparence S-Sm interface. The phenomenon corresponds to the cos(2φ) term in energy-phase relation which is necessary to implement the protected qubit. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q22.00013: Electrical conductivity and NMR relaxation rate of Eliashberg superconductors in the weak-coupling limit Rufus Boyack, Frank Marsiglio, Sepideh Mirabi Electrical conductivity is an important transport response in superconductors, enabling clear signatures of dynamical interactions to be observed. Of primary interest is to study signatures of the electron-phonon interaction in weak-coupling Eliashberg theory (Eth), and to note the distinctions from BCS theory. Recent analysis of weak-coupling Eth has shown that while there are modifications from the BCS results, certain dimensionless ratios are in agreement. Here we show that the conductivities in BCS theory and Eth fundamentally differ, with the latter having an imaginary gap component that damps a divergence. We focus on the dirty limit, and for both BCS theory and Eth we derive expressions for the small-frequency limit of the real conductivity. For Eth specifically, there are two limits to consider, depending on the relative size of the frequency and the imaginary part of the gap. In the case of identically zero frequency, we derive an analytical expression for the NMR relaxation rate. Our analysis of the conductivity complements the previous study of the Meissner response and provides a thorough understanding of weak-coupling Eth. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q22.00014: Eliashberg theory beyond Migdal’s approximation Hari Paudyal, Christopher Renskers, Roxana Margine The Migdal-Eliashberg formalism is based on the adiabatic approximation in which higher order vertex corrections to the electron-phonon vertices are neglected. While this approximation is valid for many phonon-mediated (conventional) superconductors, it is expected that higher order vertex corrections play a significant role in describing the superconducting properties in systems with a relatively large characteristic phonon frequency and small Fermi energy. In this talk, we provide a brief overview of the non-adiabatic formalism that has recently been implemented in the EPW code, which includes the first vertex correction in the electronic self-energy. Further, we present benchmark tests illustrating the scalability and performance of this approach. |
Wednesday, March 8, 2023 5:48PM - 6:00PM |
Q22.00015: Can unconventional pairing arise from a bare isotropic electron-phonon coupling? Philip M Dee, Benjamin Cohen-Stead, Steven S Johnston, Peter Hirschfeld Since its introduction over sixty years ago, the Migdal-Eliashberg formalism has become a standard tool for conventional superconductivity. Over that same period, many have worked to understand the role of corrections to the electron-phonon interaction vertex omitted under Migdal's approximation. Some have recently found that the lowest order vertex correction in Migdal-Eliashberg theory can lead to unconventional pairing symmetry in the superconducting order parameter, even when the bare vertex is isotropic. To better understand this result, we have used hybrid Monte Carlo to study the Holstein using identical model parameters as previous work. Contrary to the vertex-corrected Eliashberg theory, Monte Carlo simulations do not show evidence for a leading unconventional pairing symmetry. Instead, our simulations suggest that the highest two-particle correlations are due to the formations of a charge-density wave or s-wave pairing. We will also discuss the primary pathology leading to the unconventional pairing symmetry and the ramifications for further attempts to include vertex corrections. |
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