In this letter we study the effect of time-reversal symmetric impurities on the Josephson supercurrent through two dimensional helical metals such as on topological insulator surface state. We show that contrary to the usual superconducting-normal me
tal-superconducting junctions, the suppression of supercurrent in superconducting-helical metal-superconducting junction is mainly due to fluctuations of impurities in the junctions. Our results, which is a condensed matter realization of a part of the MSW effect for neutrinos, shows that the relationship between normal state conductance and critical current of Josephson junctions is significantly modified for Josephson junctions on the surface of topological insulators. We also study the temperature-dependence of supercurrent and present a two fluid model which can explain some of recent experimental results in Josephson junctions on the edge of topological insulators.
Topological insulators (TIs) hold great promise for realizing zero-energy Majorana states in solid-state systems. Recently, several groups reported experimental data suggesting that signatures of Majorana modes in topological insulator Josephson junc
tions (TIJJs) have -- indeed -- been observed. To verify this claim, one needs to study the topological properties of low-energy Andreev-bound states (ABS) in TIs of which the Majorana modes are a special case. It has been shown theoretically that topologically non-trivial low-energy ABS are also present in TIJJs with doped topological insulators up to some critical level of doping at which the system undergoes a topological phase transition. Here, we present first experimental evidence for this topological transition in the bulk band of a doped TI. Our theoretical calculations, and numerical modeling link abrupt changes in the critical current of top-gated TIJJs to moving the chemical potential in the charge-accumulation region on the surface of the doped TI across a band-inversion point. We demonstrate that the critical-current changes originate from a shift of the spatial location of low-energy ABS from the surface to the boundary between topologically-trivial and band-inverted regions after the transition. The appearance of a decay channel for surface ABS is related to the vanishing of the band effective mass in the bulk and thus exemplifies the topological character of surface ABS as boundary modes. Importantly, the mechanism suggest a means of manipulating Majorana modes in future experiments.
We study thermoelectric transport at low temperatures in correlated Kondo insulators, motivated by the recent observation of a high thermoelectric figure of merit(ZT) in $FeSb_2$ at $T sim 10 K$. Even at room temperature, correlations have the potent
ial to lead to high ZT, as in $YbAl_3$, one of the most widely used thermoelectric metals. At low temperature correlation effects are especially worthy of study because fixed band structures are unlikely to give rise to the very small energy gaps $E_g sim 5 kT$ necessary for a weakly correlated material to function efficiently at low temperature. We explore the possibility of improving the thermoelectric properties of correlated Kondo insulators through tuning of crystal field and spin-orbit coupling and present a framework to design more efficient low-temperature thermoelectrics based on our results.
Spatially inhomogeneous strains in graphene can simulate the effects of valley-dependent magnetic fields. As demonstrated in recent experiments, the realizable magnetic fields are large enough to give rise to well-defined flat pseudo-Landau levels, p
otentially having counter-propagating edge modes. In the present work we address the conditions under which such edge modes are visible. We find that, whereas armchair edges do not support counter-propagating edge modes, zigzag edges do so, through a novel selective-hybridization mechanism. We then discuss effects of interactions on the stability of counter-propagating edge modes, and find that, for the experimentally relevant case of Coulomb interactions, interactions typically decrease the stability of the edge modes. Finally, we generalize our analysis to address the case of spontaneous valley polarization, which is expected to occur in charge-neutral strained graphene.
If superconductivity is induced in the metallic surface states of topological insulators via proximity, Majorana modes will be trapped on the vortex cores. The same effects hold for doped topological insulators which become bulk s-wave superconductor
s as long as the doping does not exceed a critical values $ mu^{pm}_c.$ It is this critical chemical potential at which the material forgets it arose from a band-inverted topological insulator; it loses its topological emph{imprint.} For the most common classes of topological insulators, which can be modeled with a minimal 4-band Dirac model the values of $mu^{pm}_c$ can be easily calculated, but for materials with more complicated electronic structures such as HgTe or ScPtBi the result is unknown. We show that due to the hybridization with an additional Kramers pair of topologically trivial bands the topological imprint of HgTe-like electronic structures (which includes the ternary Heusler compounds) can be widely extended for p-doping. As a practical consequence we consider the effects of such hybridization on the range of doping over which Majorana modes will be bound to vortices in superconducting topological insulators and show that the range is strongly extended for p-doping, and reduced for n-doping. This leaves open the possibility that other topological phenomena may be stabilized over a wider range of doping.
Vortices in the simplest superconducting state of graphene contain very low energy excitations, whose existence is connected to an index theorem that applies strictly to an approximate form of the relevant Bogoliubov-deGennes equations. When Zeeman i
nteractions are taken into account, the zero modes required by the index theorem are (slightly) displaced. Thus the vortices acquire internal structure, that plausibly supports interesting dynamical phenomena.
The Dirac-like electronic structure can host a large number of competing orders in the form of mass terms. In particular, two different order parameters can be said to be dual to each other, when a static defect in one of them traps a quantum number
(or charge) of the other. We discuss that such complementary nature of the pair of the order parameters shows up in their correlation functions and dynamical properties when a quantum phase transition is driven by fluctuations of the one of the order parameters. Approaching the transition from the disordered (paramagnetic) side, the order parameter correlation function at the critical point is reduced, while such fluctuations enhance the correlation of the dual order parameter. Such complementary behaviors in the correlation function can be used to diagnose the nature of quantum fluctuations that is the driving force of the quantum phase transition.
In several members of the ferro-pnictides, spin density wave (SDW) order coexists with superconductivity over a range of dopings. In this letter we study the anomalous magnetic Zeeman response of this coexistence state and show that it can be used to
confirm the extended s-wave gap structure as well as structure of superconducting (SC) gap in coexisting phase. On increasing the field, a strongly anisotropic reduction of SC gap is found. The anisotropy is directly connected to the gap structure of superconducting phase. The signature of this effect in quasiparticle interference measured by STM, as well as heat transport in magnetic field is discussed. For the compounds with the nodal SC gap we show that the nodes are removed upon formation of SDW. Interestingly the size of the generated gap in the originally nodal areas is anisotropic in the position of the nodes over the Fermi surface in direct connection with the form of SC pairing.
Several small-bandgap semiconductors are now known to have protected metallic surface states as a consequence of the topology of the bulk electron wavefunctions. The known topological insulators with this behavior include the important thermoelectric
materials Bi_2Te_3 and Bi_2Se_3, whose surfaces are observed in photoemission experiments to have an unusual electronic structure with a single Dirac cone. We study in-plane (i.e., horizontal) transport in thin films made of these materials. The surface states from top and bottom surfaces hybridize, and conventional diffusive transport predicts that the tunable hybridization-induced band gap leads to increased thermoelectric performance at low temperatures. Beyond simple diffusive transport, the conductivity shows a crossover from the spin-orbit induced anti-localization at a single surface to ordinary localization.
We argue that by inducing superconductivity in graphene via the proximity effect, it is possible to observes the quantum valley Hall effect. In the presence of magnetic field, supercurrent causes valley pseudospin to accumulate at the edges of the su
perconducting strip. This, and the structure of the superconducting vortex core, provide possibilities to experimentally observe aspects of the deconfined quantum criticality.
