arXiv subjects

Rafael A. Molina

Publications and source records attributed to Rafael A. Molina.

At least 19 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

Exceptional-Point Geometry of Weak Topological Boundary States

In this article, we demonstrate that weak topology can be formulated geometrically in terms of exceptional singularities of an analytically continued Bloch Hamiltonian. A general plaquette chiral model in two dimensions serves as a minimal realization of dual weak topology, possessing two independent families of weak topological invariants, one for each spatial direction. The weak-topological edge states correspond to exceptional points in complex momentum space, while corner zero modes emerge from exceptional curves obtained by complexifying both momenta. Compact localized states arise when the exceptional roots collapse to the origin. This framework provides a unified complex-momentum description of edge, corner, and compact localization.

cond-mat.mes-hall↗

arXiv2026

Many-body quantum chaos in excitonic spectra from first principles

We demonstrate that realistic excitonic many-body Hamiltonians obtained from first-principles GW-Bethe-Salpeter equation calculations can exhibit quantum chaos governed by random-matrix universality. Considering a prototypical van der Waals heterostructure (WS$_2$-graphene), with and without lattice disorder, we analyze their energy-resolved spectral correlations and identify a disorder-driven crossover from regular to complete chaotic dynamics. We show that while pristine samples exhibit incomplete chaos (non-ergodicity) due to an approximate valley symmetry that restricts excitonic mixing, the presence of disorder-induced electronic flat bands act as a catalyst for valley mixing to drive the system into a fully developed chaotic (ergodic) regime with reduced symmetry. Crucially, fluctuations in many-body oscillator strengths are shown to follow universal Porter-Thomas statistics, directly linking the underlying quantum chaos and experimentally accessible optical observables. Finally, by examining long-range spectral correlations, we estimate the Thouless time associated to excitonic mixing across the entire many-body bandwidth. Our results establish excitons as a highly tunable platform for probing many-body ergodicity and its spectroscopic signatures in realistic interacting 2D materials.

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↗

arXiv2026

Spectroscopic Signatures of a Liouvillian Exceptional Spectral Phase in a Collective Spin

Non-Hermitian degeneracies of Lindblad generators (Liouvillian exceptional points) can induce non-exponential relaxation and higher-order poles in dynamical response functions. A collective spin coupled to a polarized Markovian bath exhibits an \emph{exceptional spectral phase} in which defective Liouvillian modes imprint super-Lorentzian features in frequency-resolved spectra. We compute the emission spectrum via the Liouvillian resolvent, identify symmetry-sector selection rules, and demonstrate that exceptional-point signatures are strongly state-dependent: they are suppressed in steady-state fluorescence yet become unambiguous for generic (infinite-temperature or random) initial states. Our results provide an experimentally accessible spectroscopic diagnostic of many-body Liouvillian exceptional phases and clarify when steady-state emission can (and cannot) reveal them.

quant-ph↗

arXiv2025

Role of exceptional points in the dynamics of the Lindblad Sachdev-Ye-Kitaev model

The out of equilibrium dynamics of the Sachdev-Ye-Kitaev model (SYK), comprising $N$ Majoranas with random all-to-all four-body interactions, minimally coupled to a Markovian bath modeled by the Lindblad formalism, displays intriguing nontrivial features. In particular, the decay rate towards the steady state is a non-monotonic function of the bath coupling $μ$, and an analogue of the Loschmidt echo for dissipative quantum systems undergoes a first order dynamical phase transitions that eventually becomes a crossover for sufficiently large $μ$. We provide evidence that these features have their origin in the presence of exceptional points in the purely real eigenvalues of the SYK Liouvillian closest to the zero eigenvalue associated with the steady state. An analytic calculation at small $N$, supported by numerical results for larger $N$, reveals that the value of $μ\sim 0.1$ at which the exceptional point corresponding to the longest living modes occurs is close to a local maximum of the decay rate. This value marks the start of a region of anomalous equilibration where the relaxation rate diminishes as the coupling to the bath becomes stronger. Moreover, the mentioned change from transition to crossover in the Loschmidt echo occurs at a larger $μ\sim 0.3$ corresponding with a proliferation of exceptional points in the low energy limit of the Liouvillian spectrum. We expect these features to be generic in the approach to equilibrium in quantum strongly interacting many-body Liouvillians.

quant-ph↗

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↗

arXiv2024

Bound States in the Continuum in a Double Whispering Gallery Resonator

In this work, we investigate the single-photon transport through two whispering gallery resonators (WGRs) coupled to a one-dimensional waveguide. Using Green's function formalism, we compute the transmission spectra and the photonic density of states (DOS) for the stationary states. We also obtain the formation of two types of bound states in the continuum (BICs). The first kind is localized into the WGR and are symmetry-protected BICs. In contrast, the second depends on the distance between resonators through the waveguide and is of the Fabry-Perot kind. These BICs are represented as Dirac delta functions in the local density of states. Additionally, we show that quasi-BICs manifest as sharp resonances in photonic transmission due to small symmetry-breaking perturbations. Furthermore, we investigate the dynamics of a single-photon wave packet interacting with the WGRs and analyze the mechanism for storing the wave packet in the structure formed by the WGRs and the finite waveguide between them.

quant-ph↗

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↗

arXiv2024

Beyond symmetry-protected BICs: transmission through asymmetric crossbar junctions in one-dimensional waveguides

Over the last few decades, the study of Bound States in the Continuum, their formation, and properties has attracted lots of attention, especially in optics and photonics. It is particularly noticeable that most of these investigations base their studies on symmetric systems. In this article, we study the formation of bound states in the continuum in electronic and photonic transport systems consisting of crossbar junctions formed by one-dimensional waveguides, considering asymmetric junctions with commensurable lengths for the upper and lower arms. We also study how BICs form in linear junction arrays as a function of the distance between consecutive junctions and their commensurability with the upper and lower arms. We solve the Helmholtz equation for the crossbar junctions and calculate the transmission probability, probability density in the intersections, and quality factor. The presence of quasi-BICs is reflected in the transmission probability as a sharp resonance in the middle of a symmetric Fano resonance along with Dirac's delta functions in the probability density and divergence in the quality factors.

physics.optics↗

arXiv2023

Experimental observation of phase transitions of a deformed Dicke model using a reconfigurable, bi-parametric electronic platform

We experimentally study the infinite-size limit of the Dicke model of quantum optics with a parity-breaking deformation strength that couples the system to an external bosonic reservoir. We focus on the dynamical consequences of such symmetry-breaking, which makes the classical phase space asymmetric with non-equivalent energy wells. We present an experimental implementation of the classical version of the deformed Dicke model using a state-of-the-art bi-parametric electronic platform. Our platform constitutes a playground for studying representative phenomena of the deformed Dicke model in electrical circuits with the possibility of externally controlling parameters and initial conditions. In particular, we investigate the dynamics of the ground state, various phase transitions, and the asymmetry of the energy wells as a function of the coupling strength $γ$ and the deformation strength $α$ in the resonant case. Additionally, to characterize the various behavior regimes, we present a two-dimensional phase diagram as a function of the two intrinsic system parameters. The onset of chaos is also analyzed experimentally. Our findings provide a clear connection between theoretical predictions and experimental observations, demonstrating the usefulness of our bi-parametric electronic setup.

quant-ph↗

arXiv2023

Missing levels in intermediate spectra

We derive an expression for the nearest-neighbor spacing distribution $P(s)$ of the energy levels of quantum systems with intermediate dynamics between regularity and chaos and missing levels due to random experimental errors. The expression is based on the Brody distribution, the most widely used for fitting mixed spectra as a function of one parameter. By using Monte Carlo simulations of intermediate spectra based on the $β$-Hermite ensemble of Random Matrix Theory, we evaluate the quality of the formula and its suitability for fitting purposes. Estimations of the Brody parameter and the fraction of missing levels can be obtained by a least-square two-parameter fitting of the experimental $P(s)$. The results should be important to distinguish the origins of deviations from RMT in experimental spectra.

quant-ph↗

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

Exceptional Spectral Phase in a Dissipative Collective Spin Model

We study a model of a quantum collective spin weakly coupled to a spin-polarized Markovian environment and find that the spectrum is divided into two regions that we name normal and exceptional Liouvillian spectral phases. In the thermodynamic limit, the exceptional spectral phase displays the unique property of being made up exclusively of second order exceptional points. As a consequence, the evolution of any initial density matrix populating this region is slowed down and cannot be described by a linear combination of exponential decays. This phase is separated from the normal one by a critical line in which the density of Liouvillian eigenvalues diverges, a phenomenon analogous to that of excited-state quantum phase transitions observed in some closed quantum systems. In the limit of no bath polarization, this criticality is transferred onto the steady state, implying a dissipative quantum phase transition and the formation of a boundary time crystal.

quant-ph↗

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 moiré 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

From integrability to chaos in quantum Liouvillians

The dynamics of open quantum systems can be described by a Liouvillian, which in the Markovian approximation fulfills the Lindblad master equation. We present a family of integrable many-body Liouvillians based on Richardson-Gaudin models with a complex structure of the jump operators. Making use of this new region of integrability, we study the transition to chaos in terms of a two-parameter Liouvillian. The transition is characterized by the spectral statistics of the complex eigenvalues of the Liouvillian operators using the nearest neighbor spacing distribution and by the ratios between eigenvalue distances.

nlin.CD↗