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 T42: Topological Superconductivity: TheoryFocus
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Sponsoring Units: DMP Chair: Sheng-Jie Huang, Max Planck Institute for the Physics of Complex Systems Room: Room 318 |
Thursday, March 9, 2023 11:30AM - 12:06PM |
T42.00001: Topological superconductivity by phase tuning Invited Speaker: Yuval Oreg Topological superconductivity in one dimension requires time-reversal symmetry breaking, but at the same time, it is hindered by the necessity for large external magnetic fields. We introduce a scheme in which a systematic manipulation of the phases of three superconducting pads tunes the system into a topological state. This scheme requires a tiny field that could be even less than a microtesla or current bias of the phases of the superconductors. The scheme is operational for several platforms and materials, including wires with strong spin-orbit coupling, planar 2D, certain transition metal dichalcogenides, and two layers of 2D systems. Our platform relies on two key ingredients: the three parallel superconductors form two SNS junctions with phase winding, and the Fermi velocities for the two spin branches transverse to the junction must be different from one another. The two phase differences between the three superconductors define a parameter plane that includes large topological regions. We analytically derive the critical curves where the topological phase transitions occur and corroborate the results with a numerical calculation based on a tight-binding model. We further propose material platforms with unequal Fermi velocities, establishing the experimental feasibility of our approach |
Thursday, March 9, 2023 12:06PM - 12:18PM |
T42.00002: DFT calculations for singlet and triplet superconductivity Chih-Kai Yang FeTe1-xSex is a family of iron-based superconductors with its critical temperature (Tc) dependent on the composition of Se. A well-known Tc is 14.5 K for x = 0.45, which exhibits an s-wave superconducting gap associated with topological surfaces states. Exchange interaction between the electrons has been proposed as the mechanism behind the formation of singlet Cooper pairs for the sample of FeTe0.5Se0.5. We provide further proof that exchange interaction, and hence the associated Tc, depends on the applied pressure on FeTe0.5Se0.5. Using DFT calculation for electrons and phonons and the Bardeen-Cooper-Schrieffer (BCS) theory for superconductivity, we found that Tc and superconducting gap for FeTe0.5Se0.5 soars under increasing compression, consistent with the results of experiment. UTe2 is a typical triplet superconductor. We will discuss how DFT calculation of exchange integrals and phonon bands affects its Tc and superconducting gap. |
Thursday, March 9, 2023 12:18PM - 12:30PM |
T42.00003: Majorana corner modes in three-dimensional transition metal dichalcogenides Yi-Ting Hsu, Kyungwha Park, Sheng-Jie Huang One key challenge in the field of topological superconductivity (Tsc) has been the rareness of material realization. This is true not only for the first-order Tsc featuring Majorana surface modes, but also the higher-order Tsc, which host Majorana hinge and corner modes. In this work, we first propose a general material-searching "recipe" for higher-order Tsc with Majorana corner modes. This recipe is obtained from a set of strong, weak, and mixed symmetry indicators we derived in the presence of inversion symmetry. Following this recipe, we propose that centrosymmetric MoTe2, which was found superconducting experimentally, is a higher-order Tsc. By performing DFT calculation and mean-field analysis for a 44-band realistic tight-binding model, we find that MoTe2 could host Majorana corner and hinge modes. Our recipe and proposed candidate could provide general guiding principle and example for material searching and designing of higher-order Tsc. |
Thursday, March 9, 2023 12:30PM - 12:42PM |
T42.00004: Identifying true Majorana zero modes through their non-Abelian statistics Poliana H Penteado, Rodrigo A Dourado, J. Carlos Egues, J. Carlos Egues Establishing a superconducting topological phase is the first step in realizing Majorana-based topological quantum computation, which relies on the braiding and fusion of Majorana zero modes. In this work, we numerically investigate the parameter space of the Kitaev model and find regions where the hybridization energy between the Majorana modes, detrimental, e.g., to qubit operations, is zero even in the presence of moderate disorder. We show that the operator γ2, associated with this low-energy mode, as a function of the parameters of the system presents a plateau pinned to 1 as opposed to ordinary fermions that have γ2 = 0. We use the operator γ to identify true Majorana zero-energy states with non-Abelian statistics. In addition, we propose a way of probing these zero-energy solutions by coupling a quantum dot to the Kitaev wire [1]. The Majorana signature emerges as a e2/2h peak in the conductance of the dot when the dot level is far detuned from the Fermi energy. Our findings show an optimal parameter regime of finite Majorana wires to suppress the hybridization energy, which might guide the choice of parameters in future experiments. |
Thursday, March 9, 2023 12:42PM - 12:54PM |
T42.00005: Topological Superconductivity From Forward Phonon Scatterings Shaozhi Li In this talk, I will present a new mechanism to attain topological superconductivity via forward phonon scatterings. The crucial message is that electron-phonon interactions with small momentum transfers favor spin-triplet Cooper pairing under an applied magnetic field. This process facilitates the formation of chiral topological superconductivity even without Rashba spin-orbit coupling. As a proof of concept, we propose an experimentally feasible heterostructure to systematically study the entangled relationship among forward-phonon scatterings, Rashba spin-orbit coupling, pairing symmetries, and the topological property of the superconducting state. This theory not only deepens our understanding of the superconductivity induced by the electron-phonon interaction but also sheds light on the critical role of the electron-phonon coupling in pursuing non-Abelian Majorana quasiparticles. |
Thursday, March 9, 2023 12:54PM - 1:06PM |
T42.00006: Can Caroli-de Gennes-Matricon and Majorana vortex states be distinguished in the presence of impurities? Bruna S. de Mendonça, Antonio L Manesco, Nancy P Sandler, Luis G Dias Da Silva Majorana zero modes states (MZMs) are predicted to appear as bound states in vortices of topological superconductors. MZMs are pinned at zero energy and have zero charge due to particle-hole symmetry. MZMs in vortices of topological superconductors tend to coexist with other subgap states, named Caroli-de Gennes-Matricon (CdGM) states. The distinction between MZMs and CdGM is limited since current experiments rely on zero-bias peak measurements obtained via scanning tunneling spectroscopy. In this work, we show that a local impurity potential can push CdGM states to zero energy. Furthermore, the finite charge in CdGM states can also drop to zero under the same mechanism. We establish, through exploration of the impurity parameter space, that these two phenomena generally happen in consonance. This means that energy and charge shifts in CdGM may be enough to imitate spectroscopic signatures of MZMs. |
Thursday, March 9, 2023 1:06PM - 1:18PM |
T42.00007: Majorana braiding without an exponential Hilbert space Eric Mascot, Themba Hodge, Daniel Crawford, Jasmin Bedow, Dirk K Morr, Stephan Rachel Qubits built with Majorana zero modes are the primary path towards topologically protected quantum computing. However, braiding errors can be introduced from sources such as quasiparticle poisoning, Majorana hybridization, and diabatic processes. It is not fully understood how these error rates behave in a full quasiparticle background, which is often included using the full exponential Hilbert space. We present a method to calculate expectation values, <ψ(t)|Â|ψ(t)>, and overlaps of time evolved many-body states from single-particle states in a superconductor. We calculate the fidelity, transition probabilities, and joint parities of Majorana pairs to quantify the braiding errors. We show how error rates depend on the timescale and smoothness of the time dependence, as well as the energy gap and distance between Majorana zero modes. This method is a powerful tool to test and analyze the many theoretical implementations of Majorana qubits. Moreover, this method can be used to study the dynamics of any superconducting system. |
Thursday, March 9, 2023 1:18PM - 1:30PM |
T42.00008: Signatures of parafermionic modes in superconductor-fractional quantum Hall heterostructures Ida E Nielsen, Karsten Flensberg, Reinhold Egger, Michele Burrello With recent developments in the experimental control of graphene-based fractional quantum Hall systems, it has now been demonstrated possible to induce superconducting pairing between counterpropagating fractional edge modes. Theoretical works predict that such systems could host parafermionic modes; non-Abelian anyons which share a fractional charge in a nonlocal way. These modes could be exploited for topological quantum computing. |
Thursday, March 9, 2023 1:30PM - 1:42PM |
T42.00009: A quantitative theory of Yu-Shiba-Rusinov states of magnetic atomic structures on different superconducting substrates Bendegúz Nyári, Andras Laszloffy, Kyungwha Park, Nóra Kucska, Laszlo Szunyogh, Balazs Ujfalussy Based on a fully relativistic multiple scattering approach combined with the solution of Bogoliubov--de Gennes equations it is possible to study the spectral properties of different magnetic impurities on various superconducting substrates in a material-specific framework. These systems have attracted considerable interest since they serve as building blocks of Kitaev chains playing an important role in the quest of finding Majorana states. The understanding of the characteristic features provided by even a single impurity is essential for the design of nanosystems that can host Majorana zero modes. |
Thursday, March 9, 2023 1:42PM - 1:54PM |
T42.00010: Effects of Disorder in Planar Topological Josephson Junctions Baris Pekerten, Tong Zhou, Jong E Han, Alex Matos-Abiague, Javad Shabani, Igor Zutic Topological properties of planar Josephson junctions, which recently emerged as promising systems for topological superconductivity [1], depend sensitively on disorder and impurities in the materials. While planar Josephson junctions may be a viable platform to demonstrate fusion and non-Abelian statistics of Majorana bound states [2], the role of disorder in these proposals is unclear. We analytically and numerically investigate the effect of disorder in these systems and give limits of the amount of disorder on experimental setups in order to achieve nontrivial topological phases [3]. We demonstrate reentrant behavior of topological properties on planar Josephson junctions and conclude that an intermediate disorder may be beneficial to establishing topological phases [4-6]. |
Thursday, March 9, 2023 1:54PM - 2:06PM |
T42.00011: Chiral Majoranas morphing into corner states in ordinary QAH/SC systems Lucas Pupim, Matthew J Gilbert, J. Carlos Egues Typically higher-order topological superconductivity is a consequence of anisotropic mass gaps, e.g., d-wave or s±-wave pairings, that are responsible for gaping the edge states and the emergence of Majorana corner states. In this work, we investigate the spectrum and linear conductance G of a square-shaped quantum anomalous Hall (QAH) insulator in proximity to an ordinary s-wave superconductor (SC) via exact diagonalization and non-equilibrium Green functions. We find that a single extended chiral Majorana mode running along the perimeter of our QAH-SC square lattice evolves into four localized zero-energy MCSs, one at each corner, as the chemical potential μ and the pairing gap Δ are varied. These MCSs unveil an unexpected 2nd-order topological phase region with peak conductance G = e2/h — within the QAH-SC bulk (μ,Δ) phase diagram of Qi et. al. [1]. Our MCSs are protected by a combination of time-reversal and in-plane mirror symmetries. We conjecture that these MCSs arise from the emergence of pairing-gap phase domains around the corners of our system. A Zeeman field with varying in-plane directions can be used to manipulate these MCSs. |
Thursday, March 9, 2023 2:06PM - 2:18PM |
T42.00012: Fractional transconductance in a chain of Josephson junctions Hannes Weisbrich, Raffael Klees, Oded Zilberberg, Wolfgang Belzig Many robust physical phenomena in quantum physics are based on topological invariants, which are intriguing geometrical properties of quantum states. A prime example is the 2D quantum Hall effect with its quantized quantum Hall conductance in units of e2/h protected by the respective 2D topology. A comparable effect in superconducting systems is the appearance of a quantized transconductance in units of 4e2/h for topological Andreev bound states in multiterminal Josephson junctions [1]. |
Thursday, March 9, 2023 2:18PM - 2:30PM |
T42.00013: Nodal higher-order topological superconductivity from a $mathcal{C}_4$-symmetric Dirac semimetal Zhenfei Wu, Yuxuan Wang We analyze the topological properties of the possible superconducting states emerging from a Cd$_3$As$_2$-like, $mathcal{C}_4$-symmetric Dirac semimetal, with two four-fold degenerate Dirac points separated in the $k_z$ direction. Unlike the simplest Weyl semimetal for which all pairing orders are topologically obstructed and nodal, we show that the topological obstruction for pairing in Dirac semimetals crucially only exists for certain pairing symmetries. In particular, we focus on odd-parity $B_{1u}$ and $B_{2u}$ pairing states, both of which can be induced by Ising ferromagnetic fluctuations. The $B_{1u}$ and $B_{2u}$ pairing states inherit the topological obstruction from the normal state, which dictates that these states necessarily hosts four BdG Dirac point nodes protected by a $mathbb{Z}_2$ monopole charge. By a Wannier state analysis, we show that the topological obstruction in the superconducting states is of higher-order nature. As a result, in a rod geometry with gapped surfaces, arcs of higher-order Majorana zero modes exist in certain $k_z$ regions of the hinges between the BdG Dirac points. Unlike Fermi arcs in Weyl semimetals, the higher-order Majorana arcs are stable against self-annihilation due to an additional $mathbb{Z}$-valued monopole charge of the BdG Dirac points protected by $mathcal{C}_4$ symmetry. We find that the same $mathbb{Z}$-valued charge is also carried by $B_{1g}$ and $B_{2g}$ channels, where the BdG spectrum hosts bulk ``nodal cages", i.e., cages formed by nodal lines, that are stable against symmetry preserving perturbations. |
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