Session E13: Non-centro Symmetric Materials Based Topological Superconductivity

Topological superconductivity in monolayer transition metal dichalcogenides

Theoretically it has been known that splitting the spin degeneracy and forming pairs with the resulting 'spinless' fermions is a promising path to topological superconductivity. Yet, topological superconductors are still rare to date. Here we propose to lightly hole-dope monolayer transition metal dichalcogenides, a family of noncentrosymmetric multivalley semiconductors, to achieve momentum-split spinless fermions on their spin-valley locked Fermi pockets. Performing a renormalization group analysis, we found that this spin-valley locking together with repulsive interactions selectively favors two topological superconducting states: an inter-pocket paired state with Chern number 2 and an intra-pocket paired state with finite pair-momentum. Besides the intrinsic topological superconductivity driven by repulsion, I will also discuss the induced nematic superconductivity in hole-doped monolayer TMDs proximitized by a cuprate.   

Molecular Beam Epitaxy Growth and Properties of Strained Superconducting Half-Heusler LaPtBi Film, a Candidate Topological Superconductor

LaPtBi is a Half-Heusler compound which is recently predicted to exhibit multi-functionalities: the superconductivity and topological edge states, namely topological superconductor. LaPtBi has been shown to superconduct in the bulk. However, and importantly, its topological property can only be stimulated by applying substantial uniaxial strain. In this talk, we report the observation of superconductivity in MBE grown epitaxial non-centrosymmetric LaPtBi film on MgO (001). Transport measurement shows T_c(onset) at 0.7 K and an upper critical field (B_c2 (0)) of 2.1 T. By optimizing the growth parameters, a compressive uniaxial strain of 17% was introduced into the film. In such a strained LaPtBi film, the predicted topological non-trivial gap at Γ point is expected to emerge. Characterization of these strained films, including ARPES, will be presented. The realization of superconducting phase in the strained LaPtBi films is an important step, as a topological superconductor, to seek other predicted exotic properties such as Majorana.   

Noncentrosymmetric superconductivity in epitaxial half-Huesler LaPtBi films

The lack of inversion symmetry and presence of superconductivity makes half-Huesler compound LaPtBi a noncentrosymmetric superconductor. The LaPtBi films we study are grown on MgO by molecular beam epitaxy with significant compressive strain in the films. Magneto-resistance in the normal state exhibits a cusp-like minima at low magnetic fields which only depends on the total magnetic field. This is attributed to electron-electron interaction effects in disordered systems. Transmission electron microscopy images also confirm the nanocrystalline film growth. We observe superconductivity at onset of 0.7 K. The critical magnetic field has a linear dependence on temperature down to 50 mK, a non BCS type behavior. The critical current decreases linearly with magnetic field. The IV characterestics indicate the presence of intrinsic Josephson effect in the nanocrystalline films.   

Nonreciprocal transport in a non-centrosymmetric bulk superconductor

Non-centrosymmetric superconductors have attracted considerable interest in view of rich exotic properties. Recently discovered nonlinear electric transport in superconducting states created the renewed interest in non-centrosymmetric superconductors, offering opportunities of investigating new properties and functionalities. Nonlinear superconducting transport has been investigated in superconducting chiral nanotubes [1] and 2D non-centrosymmetric superconductors [2] so far, but there has been no report of 3D non-centrosymmetric superconductors. In this presentation, we report the observation of the nonlinear superconducting transport in bulk superconductor PbTaSe₂ with three-fold rotational symmetry. Observed nonlinear resistance was enhanced in superconducting states and characteristic selection rule corresponding to trigonal crystal symmetry has been confirmed. [1] F. Qin et al., Nat. Comm. 8, 14465 (2017). [2] R. Wakatsuki et al., Sci. Adv. 3, e1602390 (2017).   

Enhancement of the Upper Critical Field in Disordered Transition Metal Dichalcogenide Monolayers

We calculate the effect of impurities on the superconducting phase diagram of transition metal dichalcogenide monolayers in the presence of an in-plane magnetic field. Because of strong intrinsic spin-orbit coupling, the upper critical field greatly surpasses the Pauli limit at low temperatures. We find that it is insensitive to intravalley scattering and, ultimately, limited by intervalley scattering. We use parameters extracted from the weak-localization signal of n-doped transition metal dichalcogenide monolayers in the normal state to compare our predictions for the upper critical line with the experimentally measured ones.   

Complex tensor order in half-Heusler superconductors

A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling of electrons, such as the half-Heusler compounds YPtBi or LuPdBi. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin-3/2, which allows for higher-spin Cooper pairing and potentially topological superconductivity. I will illuminate how this system may undergo a transition into a phase with complex tensor order, which is a superconducting state captured by a complex second-rank tensor valued order parameter describing Cooper pairs having spin-2. The interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the mean-field phase structure as a function of doping and temperature, and relate our finding to experiments in YPtBi. Further, the critical properties of this new paradigm for superconductivity will be addressed. (I. Boettcher and I. F. Herbut, arXiv:1707.03444)   

Quantum oscillations and a non-trivial Berry phase in the noncentrosymmetric topological superconductor candidate BiPd

The search for topological superconductors (TSCs) is now at the forefront of condensed matter problems. TSCs are characterized by a full superconducting gap in the bulk and topologically protected gapless surface or edge states at the surface, which are Andreeb bound states made of Majorana fermionic modes. In general the topological systems are investigated through their electronic structures via quantum oscillations. We report the measurements of the de Haas-van Alphen (dHvA) oscillations in the noncentrosymmetric superconductor BiPd. Several pieces of a complex multi-sheet Fermi surface have been identified. Among those, a 40 T pocket was found to be three dimensional and anisotropic in nature. From the temperature dependence of the amplitude of the oscillations, the cyclotron effective mass is (0.18 ± 0.1)m_e. Further analysis showed a non-trivial π-Berry phase is associated with the 40 T pocket, which strongly supports the presence of topological states in the interior of BiPd and may result in topological superconductivity due to proximity effect and or band coupling.   

Weyl superconductivity between trivial superconductor and topological crystalline superconductor phases with mirror symmetry

Weyl superconductivity can appear in three-dimensional superconductors breaking time-reversal symmetry. On the other hand, mirror-symmetric superconductors can realize topological crystalline superconductivity characterized by nontrivial mirror Chern numbers. We investigate Weyl superconductivity in an odd-parity and a noncentrosymmetric superconductor with mirror symmetry when time-reversal symmetry is broken. We show that Weyl nodes emerge in the superconductors when the mirror Chern number changes. As a result, it is found that Weyl superconductivity generally appears between the trivial and the topological crystalline superconductor phases. We also study a relationship between the topological phase transition and trajectories of the Weyl nodes which realize the topological crystalline superconductor phase.   

Identifying Weyl Superconductivity in UCoGe via the Chiral-Magnetothermal effect.

The experimental identification of Weyl superconductivity (WSC) presents a more difficult problem than its Weyl semimetal cousin. The standard surface measurements such as ARPES are no longer useful as the relevant systems are at very low temperatures and therefore lie outside the sensitivity range. We show that one can identify WSC in UCoGe [2, 3] due to a new phenomenon we call the chiral-magnetothermal effect (CMTE). CMTE is analogous to the chiral anomaly in other condensed matter systems. In the CMTE the electric and magnetic field are replaced by a temperature gradient and an effective magnetic field that arises from a torsional field (such as that from the vortex texture of the superconducting order parameter). The effect, if present, should give rise to spin polarisation, an effect which cannot be attributed to Zeeman splitting due to the absence of a magnetic field in the bulk. Using non-equilibrium quantum field theory in the quasiclassical limit, we show that the spin polarisation that occurs due to this effect appears under the first order quantum correction to the quasiclassical propagator. Such an effect should be observable using standard Knight shift measurements.

[1] Mineev V P Phys. Rev. B 66 134504 (2002)
[2] Hattori T et al. Phys. Rev. Lett. 108 066403 (2012)   

Josephson effects of monopole superconductivity in Weyl semimetals

In this talk, we would like to propose experimental signatures of monopole superconductivity in time-reversal symmetry broken Weyl semimetals, where nontrivial Berry phases of Cooper pairs lead to novel nodal superconducting symmetry protected by topology. By calculating the energy-phase relation of a Josephson junction between superconducting Weyl semimetals, we show the zero-energy Majorana modes emerging at the junction interface that leads to a single electron tunneling with $4\pi$ fractional Josephson effect. Furthermore, we show a sharp signature of the interference pattern arise from a SQUID for probing the pairing symmetry of a Weyl superconductor.   

Enhancement of Odd-parity Pairing near Inversion-breaking Quantum Critical Points in Two-dimensional Systems

Microscopic mechanism for topological superconductivity in 2D has attracted many attentions in the past several years. It was proposed that the quantum fluctuation near an inversion-symmetry-breaking quantum critical point (QCP) may lead to the enhancement of odd-parity pairings. We study the effect of inversion-breaking quantum fluctuations in a microscopic model of electrons on a bilayer lattice by fermion-problem-free quantum Monte-Carlo (QMC) simulations. Our results show that pairing in odd-parity channels can be more enhanced than the even-parity channels in the vicinity of the inversion-breaking QCP, which is consistent with the renormalization-group analysis.   

Superconductivity provides access to the chiral magnetic effect of an unpaired Weyl cone

The massless fermions of a Weyl semimetal come in two species of opposite chirality, in two cones of the band structure. As a consequence, the current $j$ induced in one Weyl cone by a magnetic field $B$ (the chiral magnetic effect, CME) is cancelled in equilibrium by an opposite current in the other cone. Here we show that superconductivity offers a way to avoid this cancellation, by means of a flux bias that gaps out a Weyl cone jointly with its particle-hole conjugate. The remaining gapless Weyl cone and its particle-hole conjugate represent a single fermionic species, with renormalized charge $e^\ast$ and a single chirality $\pm$ set by the sign of the flux bias. As a consequence, the CME is no longer cancelled in equilibrium but appears as a supercurrent response $\partial j/\partial B=\pm(e^\ast e/h^2)\mu$ along the magnetic field at chemical potential $\mu$.   

Topological Andreev bands in multiterminal Josephson junctions: Weyl modes, Chern numbers, conductances, and supercurrents

We consider mesoscopic multiterminal Josephson junctions and study emergent topological properties of the Andreev subgap bands. We use symmetry-constrained analysis for Wigner-Dyson classes of scattering matrices to derive band dispersions. When scattering matrix of the normal region connecting superconducting leads is energy-independent, the determinant formula for Andreev spectrum can be reduced to a palindromic equation that admits a complete analytical solution. Band topology manifests with an appearance of the Weyl nodes which serve as monopoles of finite Berry curvature. The corresponding fluxes are quantified by Chern numbers that translate into a quantized nonlocal conductance that we compute explicitly for the time-reversal-symmetric scattering matrix. The topological regime can be also identified by supercurrents as Josephson current-phase relationships exhibit pronounced non-analytic behavior and discontinuities near Weyl points that can be controllably accessed in experiments.