Random flux is commonly believed to be incapable of driving metal-insulator transitions. Surprisingly, we show that random flux can after all induce a metal-insulator transition in the two-dimensional Su-Schrieffer-Heeger model, thus reporting the fi
rst example of such a transition. Remarkably, we find that the resulting insulating phase can even be a higher-order topological insulator with zero-energy corner modes and fractional corner charges, rather than a conventional Anderson insulator. Employing both level statistics and finite-size scaling analysis, we characterize the metal-insulator transition and numerically extract its critical exponent as $ u=2.48pm0.08$. To reveal the physical mechanism underlying the transition, we present an effective band structure picture based on the random flux averaged Greens function.
We show that Cooper pairing can occur intrinsically away from the Fermi surface in $j=3/2$ superconductors with strong spin-orbit coupling and equally curved bands in the normal state. In contrast to conventional pairing between spin-$1/2$ electrons,
we derive that pairing can happen between inter-band electrons having different total angular momenta, i.e., $j=1/2$ with $j=3/2$ electrons. Such superconducting correlations manifest themselves by a pair of indirect gap-like structures at finite excitation energies. An observable signature of this exotic pairing is the emergence of a pair of symmetric superconducting coherence peaks in the density of states at finite energies. We argue that finite-energy pairing is a generic feature of high-spin superconductors, both in presence and absence of inversion symmetry.
Magnetic oscillations of Dirac surface states of topological insulators are expected to be associated with the formation of Landau levels or the Aharonov-Bohm effect. We instead study the conductance of Dirac surface states subjected to an in-plane m
agnetic field in presence of a barrier potential. Strikingly, we find that, in the case of large barrier potentials, the surface states exhibit pronounced oscillations in the conductance when varying the magnetic field, in the textit{absence} of Landau levels or the Aharonov-Bohm effect. These novel magnetic oscillations are attributed to the emergence of textit{super-resonant regimes} by tuning the magnetic field, in which almost all propagating electrons cross the barrier with perfect transmission. In the case of small and moderate barrier potentials, we also identify a positive magnetoconductance which is due to the increase of the Fermi surface by tilting the surface Dirac cone. Moreover, we show that for weak magnetic fields, the conductance displays a shifted sinusoidal dependence on the field direction with period $pi$ and phase shift determined by the tilting direction with respect to the field direction. Our predictions can be applied to many topological insulators, such as HgTe and Bi$_{2}$Se$_{3}$, and provide important insights into exploring and understanding exotic magnetotransport properties of topological surface states.
We propose an intrinsic 3D Fabry-Perot type interferometer, coined higher-order interferometer, that utilizes the chiral hinge states of second-order topological insulators and cannot be equivalently mapped to 2D space because of higher-order topolog
y. Quantum interference patterns in the two-terminal conductance of this interferometer are controllable not only by tuning the strength but also, particularly, by rotating the direction of the magnetic field applied perpendicularly to the transport direction. Remarkably, the conductance exhibits a characteristic beating pattern with multiple frequencies with respect to field strength or direction. Our novel interferometer provides feasible and robust magneto-transport signatures to probe the particular hinge states of higher-order topological insulators.
We evaluate the microscopically relevant parameters for electrical transport of hybrid superconductor-semiconductor interfaces. In contrast to the commonly used geometrically constricted metallic systems, we focus on materials with dissimilar electro
nic properties like low-carrier density semiconductors combined with superconductors, without imposing geometric confinement. We find an intrinsic mode-selectivity, a directional momentum-filter, due to the differences in electronic band-structure, which creates a separation of electron reservoirs each at the opposite sides of the semiconductor, while at the same time selecting modes propagating almost perpendicular to the interface. The electronic separation coexists with a transport current dominated by Andreev reflection and low elastic back-scattering, both dependent on the gate-controllable electronic properties of the semiconductor.
Weyl superconductors feature Weyl points at zero energy in the three-dimensional (3D) Brillouin zone and arc states that connect the projections of these Weyl points on the surface. We report that higher-order Weyl superconductors can be realized in
odd-parity topological superconductors with time-reversal symmetry being broken by periodic driving. Different from conventional Weyl points, the higher-order Weyl points in the bulk separate 2D first- and second-order topological phases, while on the surface, their projections are connected not only by conventional surface Majorana arcs, but also by hinge Majorana arcs. We show that the Weyl-point connectivity via Majorana arcs is largely enriched by the underlying higher-order topology and becomes anisotropic with respect to surface orientations. We identify the anisotropic Weyl-point connectivity as a characteristic feature of higher-order Weyl materials. As each 2D subsystem can be singled out by fixing the periodic driving, we propose how the Majorana zero modes in the 2D higher-order topological phases can be detected and manipulated in experiments.
Recently, the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ has attracted great attention. It has an out-of-plane antiferromagnetic order, which is believed to open a sizable energy gap in the surface states. This gap, however, was not alwa
ys observable in the latest angle-resolved photoemission spectroscopy (ARPES) experiments. To address this issue, we analytically derive an effective model for the two-dimensional (2D) surface states by starting from a three-dimensional (3D) Hamiltonian for bulk MnBi$_2$Te$_4$ and taking into account the spatial profile of the bulk magnetization. Our calculations suggest that the diminished surface gap may be caused by a much smaller and more localized intralayer ferromagnetic order. In addition, we calculate the spatial distribution and penetration depth of the surface states, which indicates that the surface states are mainly embedded in the first two septuple layers from the terminating surface. From our analytical results, the influence of the bulk parameters on the surface states can be found explicitly. Furthermore, we derive a $bf{k}cdot bf{p}$ model for MnBi$_2$Te$_4$ thin films and show the oscillation of the Chern number between odd and even septuple layers. Our results will be helpful for the ongoing explorations of the MnBi$_x$Te$_y$ family.
We investigate the electronic properties of type-II superconducting Nb(110) in an external magnetic field. Scanning tunneling spectroscopy reveals a complex vortex shape which develops from circular via coffee bean-shaped to elliptical when decreasin
g the energy from the edge of the superconducting gap to the Fermi level. This anisotropy is traced back to the local density of states of Caroli-de-Gennes-Matricon states which exhibits a direction-dependent splitting. Oxidizing the Nb(110) surface triggers the transition from the clean to the dirty limit, quenches the vortex bound states, and leads to an isotropic appearance of the vortices. Density functional theory shows that the Nb(110) Fermi surface is stadium-shaped near the Gamma point. Calculations within the Bogoliubov-de-Gennes theory using these Fermi contours consistently reproduce the experimental results.
Second-order topological superconductors (SOTSs) host localized Majorana fermions and provide a new platform for topological quantum computation. We propose a remarkable and feasible way to realize networks based on SOTSs which allow to nucleate and
braid Majorana bound states (MBSs) in an all-electrical manner without fine-tuning. The proposed setups are scalable in a straightforward way and can accommodate any even number of MBSs. Moreover, the MBSs in the networks allow defining qubits whose states can be initialized and read out by measuring Josephson currents flowing between SOTS islands. Our proposal can be implemented in monolayers of $text{FeTe}{}_{1-x}text{Se}_{x}$, monolayers of 1T-WTe$_2$, and inverted Hg(Cd)Te quantum wells in proximity to conventional superconductors.
Majorana fermions feature non-Abelian exchange statistics and promise fascinating applications in topological quantum computation. Recently, second-order topological superconductors (SOTSs) have been proposed to host Majorana fermions as localized qu
asiparticles with zero excitation energy, pointing out a new avenue to facilitate topological quantum computation. We provide a minimal model for SOTSs and systematically analyze the features of Majorana zero modes with analytical and numerical methods. We further construct the fundamental fusion principles of zero modes stemming from a single or multiple SOTS islands. Finally, we propose concrete schemes in different setups formed by SOTSs, enabling us to exchange and fuse the zero modes for non-Abelian braiding and holonomic quantum gate operations.
