arXiv subjects

Yuriko Baba

Publications and source records attributed to Yuriko Baba.

18 recordsLinked to original sources

arXiv2026

Multiple chiral Majorana states in proximitized magnetic topological insulator heterostructures

Achieving robust topological superconductivity with multiple Majorana channels is a key step for scalable topological quantum computing. To this end, we investigate magnetically doped three-dimensional topological insulator heterostructures proximitized by an s-wave superconductor within a fully three-dimensional extended Bernevig-Hughes-Zhang framework that explicitly accounts for vertical confinement. We show that magnetic exchange coupling, orbital mixing, and bulk band inversion cooperate to generate effective equal-spin p-wave pairing channels supporting multiple chiral Majorana modes. The number of Majorana channels is determined by the confined modes in the vertical direction and the high-Chern-number phases of the normal state, which allows us to derive an analytical criterion for the emergence of the multiple-Majorana topological superconducting phases. The chiral Majorana modes remain robust against strong disorder and moderate symmetry-breaking perturbations. We also demonstrate that a superconducting phase difference defining a vertical Josephson junction acts as a tunable parameter that controls the hybridization, minigap, and effective multiplicity of the low energy Majorana channels. Our results establish magnetic three-dimensional topological- insulator heterostructures as a promising platform for engineering multiple chiral Majorana modes.

cond-mat.mes-hall↗

arXiv2026

Cubic edge dispersion in a semi-Dirac Chern insulator

Topological edge states in Chern insulators are typically characterized by a linear dispersion relation inherited from the Dirac structure of the bulk Hamiltonian. Here we show that this paradigm can be fundamentally altered in systems with anisotropic semi-Dirac band structures. We introduce a minimal two-band lattice model realizing a semi-Dirac Chern insulator and determine its topological phase diagram analytically. Using a mass-domain-wall approach in a semi-infinite geometry, we derive an explicit expression for the chiral edge states and find that their low-energy dispersion scales cubically with momentum, $E(k)\propto k^3$. Numerical diagonalization of the corresponding tight-binding ribbon confirms the analytical prediction. Our results demonstrate that unconventional bulk band structures can produce qualitatively different boundary excitations, providing a route to engineering nonstandard chiral edge dynamics in topological materials and synthetic quantum systems.

cond-mat.mes-hall↗

arXiv2025

Unconventional Floquet topological phases in the SSH lattice

Topological materials, known for their edge states robust against local perturbations, hold promise for next-generation quantum technologies, but remain scarce in nature and challenging to realize in static systems. The Su-Schrieffer-Heeger chain is a one-dimensional system for topological phases, although its static control is limited. To overcome these limitations, we propose to use high-frequency monochromatic driving and modulated amplitude pulses to dynamically induce and switch the Floquet topological phases. Using a Kramers-Henneberger-like transformation, we encode all Floquet sidebands into a single effective Hamiltonian. We demonstrate that both monochromatic and experimental pulse protocols (Gaussian and fast-beating envelopes) can induce topological edge states, enabling dynamic phase switching. Notably, fast-beating modulations require significantly lower field strengths than monochromatic ones, especially with larger inter-dimer separations. Our findings offer an experimentally feasible route for Floquet engineering, paving the way for ultrafast and energy efficient control of topological phases in quantum platforms, opening up new possibilities in the field of dynamic quantum materials.

quant-ph↗

arXiv2025

Emergent topology by Landau level mixing in quantum Hall-superconductor nanostructures

We demonstrate the emergence of novel topological phases in quantum Hall-superconductor hybrid systems driven by Landau level mixing and spin-orbit interactions. Focusing on a narrow superconducting stripe atop a two-dimensional electron gas, we identify regimes where the hybridization of the chiral Andreev states at each side of the stripe leads to different phases beyond the long sought $p$-wave superconducting one. These topological phases exhibit distinctive transport signatures, including quantized nonlocal conductance arising from electron cotunneling at filling factor $\nu=1$, which can coexist with quantized crossed Andreev reflection at $\nu=2$. A combination of numerical simulations and effective modelling reveals the role of spin-orbit coupling and stripe geometry in controlling these transitions. Our findings suggest new strategies for realizing and detecting topology in proximized quantum Hall devices.

cond-mat.mes-hall↗

arXiv2025

Spin-dependent transport through edge states in 2D semi-Dirac materials with Rashba spin-orbit coupling and band inversion

We investigate the bulk-boundary correspondence in two-dimensional type-I semi-Dirac materials with band inversion and Rashba spin-orbit coupling. Employing a dimensional reduction framework, we identify the Zak phase along the quadratically dispersing direction as a topological invariant that captures the presence of edge states. In the non-trivial topological regime, systems with finite width exhibit energy-dependent edge states that are topologically protected only at specific momenta. At kx equal to zero, symmetry-protected edge states emerge, analogous to the Rashba-free case. At finite kx, the interplay of spin-orbit coupling and band structure gives rise to spin-dependent edge states, localized on specific edges based on its spin and particle-hole character. We compute spin-resolved conductance through these edge channels and observe robust, tunable oscillations attributable to spin precession induced by the effective Rashba magnetic field. These results reveal how spin-orbit interactions enrich the edge physics of semi-Dirac systems and provide a platform for spintronic control in anisotropic topological materials.

cond-mat.mes-hall↗

arXiv2025

Monolayer transition metal dichalcogenides under finite-pulse polarized radiation

Recent advances in time-resolved angle-resolved photoemission spectroscopy have enabled access to ultrafast electron states and their spin dynamics in solids. Atomically thin transition metal dichalcogenides are paradigmatic two-dimensional materials where electron momentum and spin degrees of freedom are coupled, being suitable candidates for time-resolved spectroscopy studies. In this work, we present a thorough study of the electron dynamics when these materials are subject to an intense finite-pulse driving radiation. We extend the scope of the conventional Floquet engineering and rely of the so-called $t-t^{\prime}$ formalism to deal with driving fields described with two distinct time scales, namely the envelope amplitude timescale and the time period of the external field. The interplay between the finite-pulse timescales and the intrinsic properties of the electrons gives rise to transient valley polarization and dynamical modifications of band structures, revealed by the time-dependent circular dichroism of the sample.

cond-mat.mes-hall↗

arXiv2025

Influence of carrier density and disorder on the Quantum Hall plateau widths in epitaxial graphene

Since its discovery, graphene has been one of the most prominent 2D materials due to its unique properties and broad range of possible applications. In particular, the half-integer Quantum Hall Effect (HI-QHE) characterized by the quantization of Hall resistivity as a function of applied magnetic field, offers opportunities for advancements in quantum metrology and the understanding of topological quantum states in this 2D material. While the role of disorder in stabilizing quantum Hall plateaus (QHPs) is widely recognized, the precise interplay between the plateaus width, disorder, mobility and carrier density remains less explored. In this work, we investigate the width of the $\nu=6$ QHP in epitaxial graphene Hall bars, focusing on two distinct regions of the device with markedly different electronic mobilities. Depending on the storage conditions, it is possible to modify the carrier density of graphene QHE devices and consequently increase or reduce the mobility. Our experiments reveal mobility variations of up to 200$\%$ from their initial value. In particular, the sample storage time and ambient conditions cause also noticeable changes in the positions and extension of the QHPs. Our results show that the QHP extension for $\nu=6$ differs significantly between the two regions, influenced by both mobility and disorder, rather than solely by carrier density. Transport simulations based on the Landauer-B\"uttiker formalism with Anderson disorder in a scaled model reveal the critical role of impurities in shaping graphene transport properties defining the extension of the QHPs. This study provides valuable insights into the interplay between mobility, disorder, and quantum transport in graphene systems.

cond-mat.mes-hall↗

arXiv2024

Radiation-induced dynamical formation of Floquet-Bloch bands in Dirac Hamiltonians

Recent experiments, combing ultrafast strong-field irradiation of surfaces with time- and angle-resolved photoemission spectroscopy, allow for monitoring the time-dependent charge carrier dynamics and the build-up of transient sidebands due to the radiation pulses. While these structures are reminiscent of Floquet-Bloch bands, standard Floquet theory is not applicable since it requires a strictly time-periodic driving field. To study the emergence and formation of such sidebands, i.e. to provide a link between common Floquet physics and dynamical mechanisms underlying short driving pulses, we consider a generalization of Floquet theory, the so-called $t-t^{\prime}$ formalism. This approach naturally extents Floquet theory to driving field amplitudes with a superimposed envelope shape. Motivated by experiments we study 2D Dirac Hamiltonians subject to linearly and circularly polarised light waves with a Gaussian field envelope of a few cycles. For these Floquet-Bloch Hamiltonians we study the evolution of their Floquet-Bloch spectra, accompanied by a systematic analysis of the time-dependent (sideband) transitions. We show that sideband occupation requires circularly polarized light for linear Dirac systems such as graphene, while for Dirac models with trigonal warping, describing surface states of topological insulators such as Bi$_2$ Se$_3$, both linearly and circularly polarised pulses induce sideband excitations.

cond-mat.mes-hall↗

arXiv2024

High order momentum topological insulator in 2D semi-Dirac materials

Semi-Dirac materials in 2D present an anisotropic dispersion relation, linear along one direction and quadratic along the perpendicular one. This study explores the topological properties and the influence of disorder in a 2D semi-Dirac Hamiltonian. Anisotropic edge states appear only in one direction. Their topological protection can be rigorously founded on the Zak phase of the one-dimensional reduction of the semi-Dirac Hamiltonian, parametrically depending on one of the momenta. In general, only a single value of the momentum is topologically protected so these systems can be considered as high order momentum topological insulators. We explore the dependence on the disorder of the edge states and the robustness of the topological protection in these materials. We also explore the consequences of the high order topological protection in momentum space for the transport properties in a two-terminal configuration.

cond-mat.mes-hall↗

arXiv2022

Effect of the external fields in high Chern number quantum anomalous Hall insulators

A quantum anomalous Hall state with high Chern number has so far been realized in multiplayer structures consisting of alternating magnetic and undoped topological insulator layers. However, in previous proposals, the Chern number can be only tuned by varying the doping concentration or the width of the magnetic topological insulator layers. This drawback largely restrict the applications of dissipationless chiral edge currents in electronics since the number of conducting channels remains fixed. In this work, we propose a way of varying the Chern number at will in these multilayered structures by means of an external electric field applied along the stacking direction. In the presence of an electric field in the stacking direction, the inverted bands of the unbiased structure coalesce and hybridize, generating new inverted bands and collapsing the previously inverted ones. In this way, the number of Chern states can be tuned externally in the sample, without the need of modifying the number and width of the layers or the doping level. We showed that this effect can be uncovered by the variation of the transverse conductance as a function of the electric field at constant injection energy at the Fermi level.

cond-mat.mes-hall↗

arXiv2022

Generation and control of non-local chiral currents in graphene superlattices by orbital Hall effect

Graphene-based superlattices offer a new materials playground to exploit and control a higher number of electronic degrees of freedom, such as charge, spin, or valley for disruptive technologies. Recently, orbital effects, emerging in multivalley band structure lacking inversion symmetry, have been discussed as possible mechanisms for developing orbitronics. Here, we report non-local transport measurements in small gap hBN/graphene/hBN moir\'e superlattices which reveal very strong magnetic field-induced chiral response which is stable up to room temperature. The measured sign dependence of the non-local signal with respect to the magnetic field orientation clearly indicates the manifestation of emerging orbital magnetic moments. The interpretation of experimental data is well supported by numerical simulations, and the reported phenomenon stands as a formidable way of in-situ manipulation of the transverse flow of orbital information, that could enable the design of orbitronic devices.

cond-mat.mes-hall↗

arXiv2022

Impact of electron-electron interactions on the thermoelectric efficiency of graphene quantum point contacts

Thermoelectric materials open a way to harness dissipated energy and make electronic devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. This goal is largely achieved with the help of nanostructured materials, even if the bulk counterpart is not highly efficient. In this work, we investigate how thermoelectric efficiency is enhanced by many-body effects in graphene nanoribbons at low temperature. To this end, starting from the Kane-Mele-Hubbard model within a mean-field approximation, we carry out an extensive numerical study of the impact of electron-electron interactions on the thermoelectric efficiency of graphene nanoribbons with armchair or zigzag edges. We consider two different regimes, namely trivial and topological insulator. We find that electron-electron interactions are crucial for the appearance of interference phenomena that give rise to an enhancement of the thermoelectric efficiency of the nanoribbons. Lastly, we also propose an experimental setup that would help to test the validity of our conclusions.

cond-mat.mes-hall↗

arXiv2021

Persistence of symmetry-protected Dirac points at the surface of the topological crystalline insulator SnTe upon impurity doping

We investigate the effect of a non-magnetic donor impurity located at the surface of the SnTe topological crystalline insulator. In particular, the changes on the surface states due to a Sb impurity atom are analyzed by means of ab initio simulations of pristine and impurity-doped SnTe. Both semi-infinite and slab geometries are considered within the first-principles approach. Furthermore, minimal and Green's function continuum models are proposed with the same goal. We find that the Dirac cones are shifted down in energy upon doping; this shift strongly depends on the position of the impurity with respect to the surface. In addition, we observe that the width of the impurity band presents an even-odd behavior by varying the position of the impurity. This behavior is related to the position of the nodes of the wave function with respect to the surface, and hence it is a manifestation of confinement effects. We compare slab and semi-infinite geometries within the ab initio approach, demonstrating that the surface states remain gapless and their spin textures are unaltered in the doped semi-infinite system. In the slab geometry, a gap opens due to hybridization of the states localized at opposite surfaces. Finally, by means of a continuum model, we extrapolate our results to arbitrary positions of the impurity, clearly showing a non-monotonic behavior of the Dirac cone.

cond-mat.mes-hall↗

arXiv2021

Tailoring topological states of core-shell nanoparticles

In this work we investigate novel spherical core-shell nanoparticles with band inversion. The core and the embedding medium are normal semiconductors while the shell material is assumed to be a topological insulator. The envelope functions are found to satisfy a Dirac-like equation that can be solved in a closed form. The core-shell nanoparticle supports midgap bound states located at both interfaces due to band inversion. These states are robust since they are topologically protected. The energy spectrum presents mirror symmetry due to the chiral symmetry of the Dirac-like Hamiltonian. As a major result, we show that the thickness of the shell acts as an additional parameter for the fine tuning of the energy levels, which paves the way for electronics and optoelectronics applications.

cond-mat.mes-hall↗

arXiv2020

Many-impurity scattering on the surface of a topological insulator

We theoretically address the impact of a random distribution of non-magnetic impurities on the surface states formed at the interface between a trivial and a topological insulator. The interaction of electrons with the impurities is accounted for by a separable pseudo-potential method that allows us to obtain closed expressions for the density of states. Spectral properties of surface states are assessed by means of the Green's function averaged over disorder realizations. For comparison purposes, the configurationally averaged Green's function is calculated by means of two different self-consistent methods, namely the self-consistent Born approximation (SCBA) and the coherent potential approximation (CPA). The latter is often regarded as the best single-site theory for the study of the spectral properties of disordered systems. However, although a large number of works employ the SCBA for the analysis of many-impurity scattering on the surface of a topological insulator, CPA studies of the same problem are scarce in the literature. In this work we find that the SCBA overestimates the impact of the random distribution of impurities on the spectral properties of surface states compared to the CPA predictions. The difference is more pronounced when increasing the magnitude of the disorder.

cond-mat.mes-hall↗

arXiv2020

Rashba coupling and spin switching through surface states of Dirac semimetals

We study the effect of the Rashba spin-orbit coupling on the Fermi arcs of topological Dirac semimetals. The Rashba coupling is induced by breaking the inversion symmetry at the surface. Remarkably, this coupling could be enhanced by the interaction with the substrate and controlled by an external electric field. We study analytically and numerically the rotation of the spin of the surface states as a function of the electron's momentum and the coupling strength. Furthermore, a detailed analysis of the spin-dependent two-terminal conductance is presented in the clean limit and with the addition of a random distribution of impurities. Depending on the magnitude of the quadratic terms in the Hamiltonian, the spin-flip conductance may become dominant, thus showing the potential of the system for spintronic applications, since the effect is robust even in the presence of disorder.

cond-mat.mes-hall↗

arXiv2019

Spin-dependent electronic lenses based on hybrid graphene nanostructures

We study electronic transport in graphene/ferromagnetic insulator hybrid devices. The system comprises an armchair graphene nanoribbon with a lens-shaped EuO ferromagnetic insulator layer deposited on top of it. When the device supports a large number of propagating modes, the proximity exchange interaction of electrons with the magnetic ions of the ferromagnetic insulator results in electrons being spatially localised at different spots depending on their spin. We found the spin-dependent electron focusing is robust under moderate edge disorder. A spin-polarised electric current can be generated by placing a third contact in the proper place. This opens the possibility to use these effects for fabricating tunable sources of polarized electrons.

cond-mat.mes-hall↗

arXiv2019

Electric field manipulation of surface states in topological semimetals

We investigate the consequences of applying electric fields perpendicularly to thin films of topological semimetals. In particular, we consider Weyl and Dirac semimetals in a configuration such that their surface Fermi arcs lie on opposite edges of the films. We develop an analytical approach based on perturbation theory and a single-surface approximation and we compare our analytical results with numerical calculations. The effect of the electric field on the dispersion is twofold: it shifts the dispersion relation and renormalizes the Fermi velocity, which would, in turn, have direct effects on quantum transport measurements. Additionally, it modifies the spatial decay properties of surface states which will impact the connection of the Fermi arcs in opposite sides of a narrow thin film.

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