arXiv subjects

Fernando Dominguez

Publications and source records attributed to Fernando Dominguez.

12 recordsLinked to original sources

arXiv2026

Interedge backscattering in quantum spin Hall-based NS and SNS junctions

We investigate the microscopic conditions that allow for the coupling between opposite quantum spin Hall (QSH) edges in hybrid junctions with superconductors. Using a microscopic Bernevig--Hughes--Zhang model and the Bogoliubov--de Gennes formalism, we model a potential barrier along the NS interface and identify the parameter regimes in which the QSH edges are coupled. In normal--superconductor junctions, such coupling manifests as deviations from the quantized zero-bias Andreev conductance $G=4e^2/h$. These deviations are controlled by the induced gap in the barrier, the barrier geometry, the interface transparency, orbital and Fermi-velocity mismatch, and disorder strength as well as the bias voltage leading to a zero-bias peak. We then analyze the impact of this interedge-coupling mechanism in Josephson junctions at equilibrium and show that it hybridizes the edge-resolved Andreev branches, opens gaps at the time-reversal-invariant phase differences $\varphi=0$ and $\varphi=\pi$, and modifies the superconducting quantum interference pattern. In a reflection-symmetric geometry, the relative sizes of the two gap openings provide complementary information about the interedge dynamical phase, which also determines the parity of the suppressed lobes in the magnetic interference pattern. This investigation sheds light on the microscopic details that control the coupling of helical edge states in actual devices and the resulting consequences for superconducting hybrid systems.

cond-mat.mes-hall↗

arXiv2026

Interfering trajectories in a ballistic Andreev cavity

The conventional description of transport through the interface between a normal conductor and a superconductor reduces the system to a one-dimensional problem treating Andreev reflection based on a zero-dimensional Sharvin type point-contact model, and effectively neglects all considerations of device geometry. While this has been successful in systems where conductance in the normal material is in the diffusive transport regime, such an over-simplification of the problem fails in other transport regimes. In particular, when transport is ballistic as in a typical semiconductor-superconductor hybrid structure, geometrical effects are inherently important, and a proper description must consider a one-dimension contact injecting into a two-dimensional ballistic cavity. We present the first study of this regime and explore the bias-voltage dependence of Andreev transport in a cavity-type device comprised of a high mobility HgTe quantum well side-contacted by one superconducting and one normal contact, each creating a one-dimensional interface. The enhanced conductance from Andreev transport features two finite bias conductance peaks, observed at energies within the energy gap of the superconductor. Interestingly, these two peaks respond differently to the application of a perpendicular-to-plane magnetic field. Using a semi-classical model for the quantum transport within the cavity, we are able to attribute each peak to a different class of ballistic trajectories. One class is dominated by normal reflection, and its interference condition is independent of magnetic field, whereas the other one contains retro-reflected Andreev processes at the superconductor interface. These create closed trajectories that are strongly suppressed by magnetic field due to Aharonov-Bohm and Doppler shift effects.

cond-mat.mes-hall↗

arXiv2026

Limits of Thermal Conductance Quantization in Chiral Topological Josephson Junctions

We investigate thermal and non-local electrical transport in four-terminal Josephson junctions formed by a normal region coupled to two transverse chiral superconducting leads, supporting phases characterized by Chern numbers ${\cal C}=0,\,1$\,and\,2. We identify the conditions under which a single chiral Majorana mode (${\cal C}=1$) produces a robust half-quantized thermal conductance, while non-local electrical conductance remains strongly suppressed by particle-hole symmetry. Thermal conductance quantization occurs near a superconducting phase difference $\pi$, but only in the low-doping regime of the central region and in the intermediate- to long-junction limits. At finite Zeeman fields, the thermal response broadly follows the topology of the isolated superconducting leads for the $C=1$ phase while, in the ${\cal C}=2$ phase, the thermal conductance generally deviates from quantization, depending on the momentum-space location of the Majorana modes. Our results establish clear criteria for probing chiral Majorana modes in Josephson junctions and highlight the essential role of momentum-space structure, finite-size geometry, and sample parameters in thermal transport.

cond-mat.supr-con↗

arXiv2025

Backscattering in Topological Edge States Despite Time-Reversal Symmetry

Spin-momentum-locked edge states of quantum spin Hall insulators (QSHIs) provide a compelling platform for spintronic applications, owing to their intrinsic protection against backscattering from non-magnetic disorder. This protection emerges from time-reversal symmetry, which pairs Kramers partners of helical edge modes with opposite spin and momentum, thereby strictly forbidding elastic single-particle backscattering within the pair. Yet, contrary to the idealized notion of linear edge bands, the non-monotonic dispersions of realistic materials can host multiple Kramers pairs, reintroducing backscattering channels between them without violating time-reversal symmetry. Here, we investigate inter-Kramers pair backscattering in the non-linear edge bands of the QSHI indenene, highlighting a critical aspect of edge-state stability. Using quasiparticle interference in scanning tunneling spectroscopy -- a direct probe of backscattering -- we observe pairwise coupling between energy-degenerate Kramers pairs, while energy regions with only a single Kramers pair remain strictly protected. Supported by theoretical analysis, our findings provide an unprecedented experimental demonstration of edge state backscattering fully consistent with their underlying topological protection. This insight has profound implications for numerous QSHI candidates, emphasizing that the mere presence of gap-traversing edge modes does not inherently guarantee their protection against backscattering.

cond-mat.mes-hall↗

arXiv2024

Interedge backscattering in time-reversal symmetric quantum spin Hall Josephson junctions

Using standard tight-binding methods, we investigate a novel backscattering mechanism taking place on quantum spin Hall N'SNSN' Josephson junctions in the presence of time-reversal symmetry. This extended geometry allows for the interplay between two types of Andreev bound states (ABS): the usual phase-dependent ABS localized at the edges of the central SNS junction \emph{and} phase-independent ABS localized at the edges of the N'S regions. Crucially, the latter arise at discrete energies $E_n$ and mediate a backscattering process between opposite edges on the SNS junction, yielding gap openings when both types of ABS are coherently coupled. In this scenario, a 4$\pi$-periodic ABS decouples from the rest of the 2$\pi$-periodic spectrum, yielding several observable consequences: Firstly, we show that the $4\pi$-periodic spectrum can be probed by means of the Shapiro experiment even in the presence of dynamical transitions between the ABS and the quasicontinuum. Secondly, the presence of this backscattering mechanism distorts the superconducting quantum interference (SQI) pattern within the length scale, determined by the ratio between $4\pi$- and $2\pi$-periodic supercurrent contributions. Finally, we propose to use a magnetic flux to tune $E_n$ to zero, resulting in the selective lifting of the fractional Josephson effect.

cond-mat.mes-hall↗

arXiv2024

Confinement-induced Majorana modes in a nodal topological superconductor

We investigate the topological phase diagram of {an extension of the Haldane model with equal spin pairing superconductivity}. In two dimensions, we find a topological nodal superconducting phase, which exhibits a chiral Majorana mode propagating along the edges of nanoribbons with cylindrical boundary conditions. This phase is however unstable in a finite two-dimensional rectangular-shaped lattice, yielding corner states close to zero energy in a flake with alternating zigzag and armchair edges. When we reduce one of the dimensions, quantum confinement gaps out the bulk bands faster than the edge states. In this scenario, hybridization between the edge states can then result in Majorana zero modes. Our results hence suggest quantum confinement as a crucial ingredient in building quasi-one-dimensional topological superconducting phases out of two-dimensional nodal topological superconductors. Furthermore, we characterize the emergence of this novel topological phase by means of its topological invariant, coinciding with a quantized conductance of $2 e^2/h$ in a normal-superconducting junction.

cond-mat.mes-hall↗

arXiv2024

Photonic cross-noise spectroscopy of Majorana bound states

We propose a route to detect Majorana bound states (MBSs) by coupling a topological superconductor to quantum dots (QDs) in a $pnp$ junction. Here, two MBSs are coherently coupled to electrons on two QDs, which recombine with holes to photons. We focus on the spectroscopy of cross-correlated shot noise and the polarization of the emitted photons. Our detection scheme allows us to probe the necessary condition for the emergence of MBSs, specifically, the existence of nonlocal triplet superconducting correlations and also the fundamental property that two MBSs comprise a single complex fermion. We compare our results to the ones obtained from nontopological quasi-MBSs (qMBSs) and establish a correspondence between the number of peaks in the cross-correlation with the number of MBSs in the system. Here, we can identify a tunneling regime that facilitates differentiation between topological MBSs and trivial qMBSs. Additionally, we test the robustness of the detection scheme by the addition of uncorrelated quasiparticles.

cond-mat.mes-hall↗

arXiv2021

Lifting topological protection in a quantum spin Hall insulator by edge coupling

The scientific interest in two-dimensional topological insulators (2D TIs) is currently shifting from a more fundamental perspective to the exploration and design of novel functionalities. Key concepts for the use of 2D TIs in spintronics are based on the topological protection and spin-momentum locking of their helical edge states. In this study we present experimental evidence that topological protection can be (partially) lifted by pairwise coupling of 2D TI edges in close proximity. Using direct wave function mapping via scanning tunneling microscopy/spectroscopy (STM/STS) we compare isolated and coupled topological edges in the 2D TI bismuthene. The latter situation is realized by natural lattice line defects and reveals distinct quasi-particle interference (QPI) patterns, identified as electronic Fabry-P\'erot resonator modes. In contrast, free edges show no sign of any single-particle backscattering. These results pave the way for novel device concepts based on active control of topological protection through inter-edge hybridization for, e.g., electronic Fabry-P\'erot interferometry.

cond-mat.mes-hall↗

arXiv2021

Network model and four-terminal transport in minimally twisted bilayer graphene

We construct a two-channel scattering model for the triangular network of valley Hall states in interlayer-biased minimally twisted bilayer graphene from symmetry arguments and investigate electronic transport in a four-terminal setup. In the absence of forward scattering, a single phenomenological parameter tunes the network between a triplet of chiral zigzag modes and pseudo-Landau levels. Moreover, the chiral zigzag modes give rise to robust Aharonov-Bohm resonances in the longitudinal conductance in the presence of a perpendicular magnetic field or an in-plane electric field. Interestingly, we find that when both a magnetic field and an in-plane electric field are applied, the resonances of different zigzag branches split depending on their propagation direction relative to the in-plane electric field. We further demonstrate that while the Hall response vanishes in the chiral zigzag regime, a finite Hall response is obtained without destroying the Aharonov-Bohm resonances in the longitudinal response, by weakly coupling different zigzag branches, which also gives rise to Hofstadter physics at accessible magnetic fields.

cond-mat.mes-hall↗

arXiv2020

Effective Floquet model for minimally twisted bilayer graphene

We construct an effective Floquet lattice model for the triangular network that emerges in interlayer-biased minimally twisted bilayer graphene and which supports two chiral channels per link for a given valley and spin. We introduce the Floquet scheme with the one-channel triangular network and subsequently extend it to the two-channel case. From the bulk topological index (winding number) and finite system calculations, we find that both cases host anomalous Floquet insulators (AFIs) with a different gap-opening mechanism. In the one-channel network, either time-reversal or in-plane inversion symmetry has to be broken to open a gap. In contrast, in the two-channel network, interchannel coupling can open a gap without breaking these symmetries yielding a valley AFI with counterpropagating edge states. This phase is topologically trivial with respect to the total winding number but robust in the absence of intervalley scattering. Finally, we demonstrate the applicability of the Floquet model with magnetotransport calculations.

cond-mat.mes-hall↗

arXiv2019

Crystalline Weyl semimetal phase in Quantum Spin Hall systems under magnetic fields

We investigate an unconventional topological phase transition that occurs in quantum spin Hall (QSH) systems when applying an external in-plane magnetic field. We show that this transition between QSH and trivial insulator phases is separated by a stable topological gapless phase, which is protected by the combination of particle-hole and reflection symmetries, and thus, we dub it as crystalline Weyl semimetal. We explore the stability of this new phase when particle-hole symmetry breaking terms are present. Especially, we predict a robust unconventional topological phase transition to be visible for materials described by Kane and Mele model even if particle-hole symmetry is significantly broken.

cond-mat.mes-hall↗

arXiv2018

Testing Topological Protection of Edge States in Hexagonal Quantum Spin Hall Candidate Materials

We analyze the detailed structure of topological edge mode protection occuring in hexagonal quantum spin Hall (QSH) materials. We focus on bismuthene, antimonene, and arsenene on a SiC substrate, which, due to their large bulk gap, may offer new opportunities for room-temperature QSH applications. While time reversal symmetry is responsible for the principal symmetry protected character of QSH states, the hexagonal edge terminations yield further aspects of crystal symmetry which affect the topological protection. We show that armchair QSH edge states remain gapless under an in-plane magnetic field in the direction along the edge, a hallmark of their topological crystalline protection. In contrast, an out-of-plane magnetic field opens a gap of the order of a few meV within realistic ranges of parameters. We use these intriguing signatures of armchair QSH edge states to predict experimentally testable fingerprints of their additional topological crystalline character and their helicity emerging in tunneling spectroscopy and ballistic magnetotransport.

cond-mat.mes-hall↗