arXiv subjects

Javier Osca

Publications and source records attributed to Javier Osca.

At least 19 recordsLinked to original sources

arXiv2026

Transport in magnetic-topological-insulator nanoribbons containing multiple superconductor-proximitized sectors

Transport in devices with multiple proximitized sectors depends heavily on the complex phases of the pairing gaps of those sectors. We investigate magnetic topological insulator nanoribbons in two- and three-terminal setups with proximitized sectors and asymptotic normal leads. Our focus is on the regime of single chiral Majoranas. The characteristic electric and thermal interferometries of chiral Majoranas can be controlled by the complex phases of the pairing. We predict an AC Majorana effect, in which phase dynamics induced by a voltage bias generate measurable time-dependent conductance oscillations. A three-terminal junction with superconducting islands can be used as a Majorana router when the relative complex phases are configured.

cond-mat.mes-hall↗

arXiv2025

Electrostatic gating and the interference of chiral Majoranas in thin slabs of magnetic topological insulators

We study the interference of chiral Majoranas in a magnetic topological insulator thin slab having a grounded section proximity coupled to a superconductor and another section under the influence of top-bottom electrostatic gating. The gated section locally widens an energy gap and mediates the coupling between the quantum anomalous Hall states of the leads and the chiral Majorana states of the proximitized sector. Local and non-local conductances offer measurable hints of the existence of transport mediated by chiral Majorana modes. Local conductances on the two leads reveal characteristic oscillatory patterns as a function of the gating strength, with peculiar correlations depending on the distance between gated and proximitized sectors. A gate tunable Majorana diode effect on nonlocal conductances emerges when the chemical potential deviates from zero. We suggest a protocol to identify chiral Majorana physics based on a sequence of electrostatic gates that allows the tuning of chiral Majorana interference.

cond-mat.mes-hall↗

arXiv2023

SOQCS: A Stochastic Optical Quantum Circuit Simulator

We present Stochastic Optical Quantum Circuit Simulator (SOQCS) C++/Python library for the simulation of quantum optical circuits, and we provide its implementation details. SOQCS offers a framework to define, simulate and study quantum linear optical circuits in the presence of various imperfections. These come from partial distinguishability of photons, lossy propagation media, unbalanced beamsplitters and non-ideal emitters and detectors for example. SOQCS is developed as a series of different modules which provide quantum circuits, different simulator cores and tools to analyze the output. Quantum circuits can be defined from basic components, including emitters, linear optical elements, delays and detectors. Post-selection can be configured straightforwardly as part of detector definitions. An important attribute of SOQCS is its modularity which allows for its further development in the future.

quant-ph↗

arXiv2023

Implementation of photon partial distinguishability in a quantum optical circuit simulation

We are concerned with numerical simulations of quantum optical circuits under certain realistic conditions, specifically that photon quantum states are not perfectly indistinguishable. The partial photon distinguishability presents a serious limitation in implementation of optical quantum information processing. In order to properly assess its effect on quantum information protocols, accurate numerical simulations, which closely emulate quantum circuit operations, are essential. Our specific objective is to provide a computer implementation of the partial photon distinguishability which is in principle applicable to existing simulation techniques used for ideal quantum circuits and which avoids a need for their significant modification. Our approach is based on the Gram-Schmidt orthonormalization process, which is well suited for our purpose. Photonic quantum states are represented by wavepackets which contain information on their time and frequency distributions. In order to account for the partial photon distinguishability, we expand the number of degrees of freedom associated with the circuit operation extending the definition of the photon channels to incorporate wavepacket degrees of freedom. This strategy allows to define delay operations in the same footing as the linear optical elements.

quant-ph↗

arXiv2021

Fabry-Pérot interferometry with gate-tunable 3D topological insulator nanowires

Three-dimensional topological insulator (3D TI) nanowires display various interesting magnetotransport properties that can be attributed to their spin-momentum-locked surface states such as quasiballistic transport and Aharonov-Bohm oscillations. Here, we focus on the transport properties of a 3D TI nanowire with a gated section that forms an electronic Fabry-Pérot (FP) interferometer that can be tuned to act as a surface-state filter or energy barrier. By tuning the carrier density and length of the gated section of the wire, the interference pattern can be controlled and the nanowire can become fully transparent for certain topological surface-state input modes while completely filtering out others. We also consider the interplay of FP interference with an external magnetic field, with which Klein tunneling can be induced, and transverse asymmetry of the gated section, e.g., due to a top-gated structure, which displays an interesting analogy with Rashba nanowires. Due to its rich conductance phenomenology, we propose a 3D TI nanowire with gated section as an ideal setup for a detailed transport-based characterization of 3D TI nanowire surface states near the Dirac point, which could be useful towards realizing 3D TI nanowire-based topological superconductivity and Majorana bound states.

cond-mat.mes-hall↗

arXiv2021

Torque field and skyrmion movement by spin transfer torque in a quasi-2d interface in presence of strong spin-orbit interaction

We investigate the torque field and skyrmion movement at an interface between a ferromagnet hosting a skyrmion and a material with strong spin-orbit interaction. We analyze both semiconductor materials and topological insulators using a Hamiltonian model that includes a linear term. The spin torque inducing current is considered to flow in the single band limit therefore a quantum model of current is used. Skyrmion movement due spin transfer torque proves to be more difficult in presence of spin orbit interaction in the case where only interface in-plane currents are present. However, edge effects in narrow nanowires can be used to drive the skyrmion movement and to exert a limited control on its movement direction. We also show the differences and similarities between torque fields due to electric current in the many and in the single band limits.

cond-mat.mes-hall↗

arXiv2021

Skyrmion elongation, duplication and rotation by spin-transfer torque under spatially varying spin current

The effect of the spatially varying spin current on a skyrmion is numerically investigated. It is shown that an inhomogeneous current density induces an elongation of the skyrmion. This elongation can be controlled using current pulses of different strength and duration. Long current pulses lead to a splitting that forms two replicas of the initial skyrmion while for short pulses the elongated skyrmion relaxes back to its initial circular state through rotation in the MHz-GHz frequency range. The frequency is dependent on the strength of the damping coefficient.

cond-mat.mes-hall↗

arXiv2020

Skyrmion spin transfer torque due to current confined in a nanowire

In this work we compute the torque field present in a ferromagnet in contact with a metallic nanowire when a skyrmion is present. If the nanowire is narrow enough the current is carried by a single conduction band. In this regime the classical torque model breaks down and we show that a skyrmion driven by spin transfer torque moves in a different direction than predicted by the classical model. However, the amount of charge current required to move a skyrmion with a certain velocity in the single band regime is similar to a classical model of torque where it is implicitly assumed current transport by many conduction bands. The single band regime is more efficient creating spin current from charge current because of the perfect polarization of the single band but is less efficient creating torque from spin current. Nevertheless, it is possible to take profit of the single band regime to move skyrmions even with no net charge or spin current flowing between the device contacts. We have also been able to recover the classical limit considering an ensemble of only a few electronic states. In this limit we have discovered that electron diffusion needs to be considered even in ballistic nanowires due the effect of the skyrmion structure on the electron current.

cond-mat.mes-hall↗

arXiv2018

Magnetic orbital motion and 0.5$e^2/h$ conductance of quantum-anomalous-Hall hybrid strips

The magnetic-induced orbital motion of quasiparticles affects the conductance properties of a hybrid strip of a quantum-anomalous-Hall topological material with induced superconductivity. We elucidate the scenario of topological NSN ideal junctions in presence of orbital magnetic motion, showing how it leads to a halved quantized conductance $0.5e^2/h$ even in absence of Majorana modes. The magnetic orbital effect favours Fermionic charged modes with finite wave numbers, in contradistinction to Majorana zero modes which are chargeless zero-energy modes with vanishing wave number. The bias sensitivity of the 0.5 conductance plateau allows to distinguish the two cases. Conductance oscillations due to backscattering interference are absent in the charged Fermion case.

cond-mat.mes-hall↗

arXiv2018

Conductance oscillations and speed of chiral Majorana mode in a quantum-anomalous-Hall 2d strip

We predict conductance oscillations in a quantum-anomalous Hall 2d strip having a superconducting region of length $L_x$ with a chiral Majorana mode. These oscillations require a finite transverse extension of the strip $L_y$ of a few microns or less. Measuring the conductance periodicity with $L_x$ and a fixed bias, or with bias and a fixed $L_x$, yields the speed of the chiral Majorana mode. The physical mechanism behind the oscillations is the interference between backscattered chiral modes from second to first interface of the NSN double junction. The interferometer effect is enhanced by the presence of side barriers.

cond-mat.mes-hall↗

arXiv2018

Spatial coupling of quantum-anomalous-Hall and chiral-Majorana modes

We calculate density and current spatial distributions of a 2D model junction between a normal QAH contact and a superconducting QAH region hosting propagating (chiral) Majorana modes. We use a simplified Hamiltonian describing the spatial coupling of the modes on each side of the junction, as well as the related junction conductance. We study how this coupling is affected by orbital effects caused by an external magnetic field.

cond-mat.mes-hall↗

arXiv2017

Circular dichroism of chiral Majoranas

Majorana states in condensed matter devices may be of a localized nature, such as in hybrid semiconductor/superconductor nanowires, or chirally propagating along the edges such as in hybrid 2D quantum-anomalous-Hall/superconductor structures. We calculate the circular dichroism due to chiral Majorana states in a hybrid structure made of a quantum-anomalous-Hall-insulator and a superconductor. The optical absorption of chiral Majoranas is characterized by equally spaced absorption peaks of both positive and negative dichroism. In the limit of a very long structure (a 2D ribbon) peaks of a single sign are favored. Circular-dichroism spectroscopy of chiral Majoranas is suggested as a relevant probe for these peculiar states of topological matter.

cond-mat.mes-hall↗

arXiv2017

Majorana states in prismatic core-shell nanowires

We consider core-shell nanowires with conductive shell and insulating core, and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e. along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e. thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.

cond-mat.mes-hall↗

arXiv2017

Topological suppression of magnetoconductance oscillations in NS junctions

We show that the magnetoconductance oscillations of laterally-confined 2D NS junctions are completely suppressed when the superconductor side enters a topological phase. This suppression can be attributed to the modification of the vortex structure of local currents at the junction caused by the topological transition of the superconductor. The two regimes (with and without oscillations) could be seen in a semiconductor 2D junction with a cleaved-edge geometry, one of the junction arms having proximitized superconductivity. We predict similar oscillations and suppression as a function of the Rashba coupling. The oscillation suppression is robust against differences in chemical potential and phases of lateral superconductors.

cond-mat.mes-hall↗

arXiv2016

Current distributions in stripe Majorana junctions

We calculate current and density distributions in stripe (2D planar) junctions between normal and Majorana nanowires having a finite ($y$) transverse length. In presence of a magnetic field with vertical and in-plane components, the $y$-symmetry of the charge current distribution in the normal lead changes strongly across the Majorana phase transition: from center-symmetric if a Majorana mode is present to laterally-shifted (as expected by the Hall effect) if the field is tilted such as to destroy the Majorana mode due to the projection rule. We compare quasi-particle and charge distributions of current and density, as well as spin magnetizations. The Majorana mode causes opposite spin accumulations on the junction and the emergence of a spin current.

cond-mat.mes-hall↗

arXiv2015

Majorana mode stacking, robustness and size effect in cylindrical nanowires

We discuss the robustness of Majorana edge modes in a finite quantum nanowire of cylindrical shape. The nanowire is modeled as a bidimensional cylindrical shell of semiconductor material with proximity-induced superconductivity and an intrinsic Rashba spin-orbit interaction. The latter is characterized by effective electric and magnetic fields in transverse direction of the nanowire. An applied external magnetic field pointing in an arbitrary orientation is also assumed. The numerical diagonalization of the Hamiltonian allows us to study the spectrum of the nanowire for different experimental configurations. The Majorana modes prove robust against tilting of the magnetic field away from the cylinder longitudinal axis, if the tilt direction is perpendicular to the effective spin-orbit magnetic field, but fragile otherwise. On the other hand, we find an increasing number of Majorana modes in the same cylinder edge for increasing values of the nanowire radius. We refer to this phenomenon as "stacking effect" and it occurs due to the orthogonality between Majorana mode wave functions. In this manner, different Majoranas take complementary positions on the nanowire surface.

cond-mat.mes-hall↗

arXiv2015

Electromagnetic absorption of quasi-1D Majorana nanowires

We calculate the electromagnetic absorption cross section of long and narrow nanowires, in the so-called quasi-1D limit. We consider only two transverse bands and compute the dipole absorption cross section taking into account quasiparticle transitions from negative to positive energy eigenstates of the Bogoliubov-de Gennes Hamiltonian. The presence of the zero energy (Majorana) state manifests in the different absorption spectra for $x$ (parallel) and $y$ (transverse) polarizations of the electromagnetic field. In the $y$-polarized case, the Majorana state causes a low energy absorption plateau extending from mid-gap up to full-gap energy. Increasing further the energy, the plateau is followed by a region of enhanced absorption due to transitions across the full gap. For $x$ polarization the low energy absorption plateau is not observed.

cond-mat.mes-hall↗

arXiv2015

Quasi-particle current in planar Majorana nanowires

We calculate the local quasi-particle current of a Majorana state in a planar hybrid (superconductor-semiconductor) nanowire. In absence of perpendicular components of the magnetic field the current flows in circular trajectories without a preferred orientation. On the other hand, when a perpendicular component of the magnetic field is present the quasi-particle current circulates surrounding the Majorana density peak with an orientation established by the magnetic field.

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