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Register a new userAnomalous Thermal Hall Effects in Chiral Superconductors
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We report theoretical results for the electronic contribution to thermal and electrical transport for chiral superconductors belonging to even or odd-parity E$_1$ and E$_2$ representations of the tetragonal and hexagonal point groups. Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk Fermionic excitations, and -- as we highlight -- the structure of the impurity potential. The anomalous thermal Hall effect is shown to be sensitive to the structure of the electron-impurity t-matrix, as well as the winding number, $ u$, of the chiral order parameter, $Delta(p)=|Delta(p)|,e^{i uphi_p}$. For heat transport in a chiral superconductor with isotropic impurity scattering, i.e., point-like impurities, a transverse heat current is obtained for $ u=pm 1$, but vanishes for $| u|>1$. This is not a universal result. For finite-size impurities with radii of order or greater than the Fermi wavelength, $Rgehbar/p_f$, the thermal Hall conductivity is finite for chiral order with $| u|ge2$, and determined by a specific Fermi-surface average of the differential cross-section for electron-impurity scattering. Our results also provide quantitative formulae for interpreting heat transport experiments for superconductors predicted to exhibit broken time-reversal and mirror symmetries.
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Chiral superconductors exhibit novel transport properties that depend on the topology of the order parameter, topology of the Fermi surface, the spectrum of bulk and edge Fermionic excitations, and the structure of the impurity potential. In the case of electronic heat transport, impurities induce an anomalous (zero-field) thermal Hall conductivity that is easily orders of magnitude larger than the quantized edge contribution. The effect originates from branch-conversion scattering of Bogoliubov quasiparticles by the chiral order parameter, induced by potential scattering. The former transfers angular momentum between the condensate and the excitations that transport heat. The anomalous thermal Hall conductivity is shown to depend to the structure of the electron-impurity potential, as well as the winding number, $ u$, of the chiral order parameter, $Delta(p)=|Delta(p)|,e^{i uphi_{p}}$. The results provide quantitative formulae for interpreting heat transport experiments seeking to identify broken T and P symmetries, as well as the topology of the order parameter for chiral superconductors.
Generic chiral superconductors with three-dimensional electronic structure have nodal gaps and are not strictly topological. Nevertheless, they exhibit a spontaneous thermal Hall effect (THE), i.e. a transverse temperature gradient in response to a heat current even in the absence of an external magnetic field. While in some cases this THE can be quantized analogous to the Quantum Hall effect, this is not the case for nodal superconductors in general. In this study we determine the spontaneous THE for tight binding models with tetragonal and hexagonal crystal symmetry with chiral $p$- and d-wave superconducting phase. At the zero-temperature limit, the thermal Hall conductivity $ kappa_{xy} $ provides information on the structure of the gap function on the Fermi surface and the Andreev bound states on the surface. The temperature dependence at very low temperatures is determined by the types of gap nodes, point or line nodes, leading to characteristic power law behaviors in the temperature, as known for other quantities such as specific heat or London penetration depth. The generic behavior is discussed on simple models analytically, while the analysis of the tight-binding models is given numerically.
We investigate the effect of thermal fluctuations on the two-particle spectral function for a disordered $s$-wave superconductor in two dimensions, focusing on the evolution of the collective amplitude and phase modes. We find three main effects of thermal fluctuations: (a) the phase mode is softened with increasing temperature reflecting the decrease of superfluid stiffness; (b) remarkably, the non-dispersive collective amplitude modes at finite energy near ${bf q}=[0,0]$ and ${bf q}=[pi,pi]$ survive even in presence of thermal fluctuations in the disordered superconductor; and (c) the scattering of the thermally excited fermionic quasiparticles leads to low energy incoherent spectral weight that forms a strongly momentum-dependent background halo around the phase and amplitude collective modes and broadens them. Due to momentum and energy conservation constraints, this halo has a boundary which disperses linearly at low momenta and shows a strong dip near the $[pi,pi]$ point in the Brillouin zone.
After the recognition of the possibility to implement Majorana fermions using the building blocks of solid-state matters, the detection of this peculiar particle has been an intense focus of research. Here we experimentally demonstrate a collection of Majorana fermions living in a one-dimensional transport channel at the boundary of a superconducting quantum anomalous Hall insulator thin film. A series of topological phase changes are controlled by the reversal of the magnetization, where a half-integer quantized conductance plateau (0.5e2/h) is observed as a clear signature of the Majorana phase. This transport signature can be well repeated during many magnetic reversal sweeps, and can be tracked at different temperatures, providing a promising evidence of the chiral Majorana edge modes in the system.
The Weyl semimetal is characterized by three-dimensional linear band touching points called Weyl nodes. These nodes come in pairs with opposite chiralities. We show that the coupling of circularly polarized photons with these chiral electrons generates a Hall conductivity without any applied magnetic field in the plane orthogonal to the light propagation. This phenomenon comes about because with all three Pauli matrices exhausted to form the three-dimensional linear dispersion, the Weyl nodes cannot be gapped. Rather, the net influence of chiral photons is to shift the positions of the Weyl nodes. Interestingly, the momentum shift is tightly correlated with the chirality of the node to produce a net anomalous Hall signal. Application of our proposal to the recently discovered TaAs family of Weyl semimetals leads to an order-of-magnitude estimate of the photoinduced Hall conductivity which is within the experimentally accessible range.
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