arXiv subjects

Alejandro Lopez-Bezanilla

Publications and source records attributed to Alejandro Lopez-Bezanilla.

At least 19 recordsLinked to original sources

arXiv2026

Disorder signatures in coherent electronic waveguides

Microscopic disorder in a coherent conductor is encoded in the magnitude of the transmitted current and in the energy- and channel-resolved structure of the scattering response. We develop a conductance-balanced learning framework for identifying microscopic disorder mechanisms from coherent quantum-transport spectra. Using armchair graphene nanoribbons as controlled multichannel tight-binding waveguides, we generate ensembles in which distinct disorder classes occupy the same intervals of integrated transmission, removing the dominant shortcut associated with average conductance. Within this constrained setting, we compare scalar transmission spectra with transmission-eigenvalue spectra, and use supervised classification, scale-normalizing controls, plateau-resolved tests, and principal-component analysis to identify the spectral structures that remain informative. The learned distinctions persist after normalization of the conductance envelope and are strongest in multichannel energy windows, where mode mixing and channel redistribution shape the scattering response. Principal components provide interpretable transport coordinates whose loadings identify the energy and eigenchannel sectors responsible for the dominant spectral deformations. The results establish a controlled AI-assisted protocol for learning disorder signatures from nonlinear quantum-scattering data.

cond-mat.mes-hall↗

arXiv2026

Moire Control of Alterelectric Quadrupolar Order

Alterelectricity is a compensated ferroic state in which quadrupolar electronic order reshapes low-energy electronic structure without producing a net polarization. Here we show that a moiré superlattice can turn such order into a controllable phase. Within a Bloch-periodic two-orbital theory, the slowly varying interlayer registry is coarse-grained into an effective moiré field acting on a self-consistent two-component alterelectric quadrupole. The resulting phase develops above a strongly filling-dependent instability threshold and crosses over from a weakly selected regime into a robust axial-dominated ground state, while the diagonal-dominated branch remains only a weak competitor. A registry-phase sweep supplies an explicit continuous path through internal quadrupole space, demonstrating that the moiré superlattice does more than stabilize alterelectricity: it steers its internal orientation. This orientational selection is encoded directly in the redistribution of low-energy spectral weight across the moiré Brillouin zone. These results identify moiré superlattices as a generic route to controllable alterelectric order and to programmable anisotropic electronic functionality.

cond-mat.mtrl-sci↗

arXiv2026

Identifying quantum coherence in quantum annealers

Demonstrating genuine many-body quantum coherence in large-scale quantum processors remains a central challenge for near-term quantum technologies. Recent experiments on D-Wave quantum annealers have investigated quenches of Ising chains and observed defect densities that show Kibble-Zurek scaling, consistent with coherent quantum dynamics. However, identical scaling can arise from classical or thermal processes. Here we propose the use of many-body coherent oscillations (MBCO) as a diagnostic for the identification of system-wide coherence in analog quantum simulators. Solving the time-dependent Schrodinger equation, we show that quenches of a staggered one-dimensional Ising chain across a quantum critical point produce oscillatory signatures in defect observables. We implement this model on the D-Wave Advantage quantum annealer. Using fast-anneal protocols, we find that, although defect densities follow Kibble-Zurek scaling, the expected oscillatory behavior is absent. We demonstrate that static disorder associated with individual qubits is not likely responsible for the absence of MBCO. Modest modifications to annealing schedules can dramatically enhance oscillation visibility. This work gives a general roadmap for the search for quantum coherence in noisy, large-scale quantum platforms.

quant-ph↗

arXiv2025

Programmable Exploration of Magnetic States in Lieb-Kagome Interpolated Lattices

We investigate a hybrid modeling framework in which a quantum annealer is used to simulate magnetic interactions in molecular qubit lattices inspired by experimentally realizable systems. Using phthalocyanine assemblies as a structurally constrained prototype, we model a continuous deformation from a Lieb to a kagome lattice, revealing frustration-driven disorder and magnetic field-induced reordering in the spin structure. The annealer provides access to observables such as the static structure factor and magnetization over a wide parameter space, enabling the characterization of magnetic arrangements beyond the reach of current molecular architectures. This surrogate modeling approach supports a feedback loop between experiment and programmable quantum hardware, offering a pathway to explore and iteratively design tunable magnetic states in synthetic quantum materials. The synthetic design, structural characterization, and quantum simulation framework established here defines a modular and scalable paradigm for probing the limits of engineered quantum matter across chemistry, condensed matter, and quantum information science.

quant-ph↗

arXiv2025

Observation of Magnetic Devil's Staircase-Like Behavior in Quasiperiodic Qubit Lattices

The devil's staircase (DS) phenomenon is a fractal response of magnetization to external fields, traditionally observed in periodic ferromagnetic systems, where the commensurability between spin arrangements, lattice parameters, and external magnetic fields governs abrupt changes in magnetization. Its occurrence in aperiodic, fractal-type systems has remained largely unexplored, despite their natural compatibility with such phenomena. Using a quantum annealing device, we uncover a wealth of abrupt magnetic transitions between spin manifolds driven by increasing external magnetic fields within a simple yet effective Ising-model framework. In contrast to periodic systems, where DS arises from long-range competing interactions, our findings reveal that short-range, purely antiferromagnetic couplings in aperiodic geometries produce equally rich ground-state magnetization patterns. We demonstrate that while magnetic textures are determined by the lattice size, their formation remains remarkably robust and independent of scale, with commensurability emerging locally. Our results challenge the prevailing view that DS behavior is limited to periodic systems and establish quasiperiodic geometries as a natural host for this phenomenon.

cond-mat.mtrl-sci↗

arXiv2025

Quantum dynamics in frustrated Ising fullerenes

The complex energy landscapes exhibited by frustrated magnetic systems undergoing quantum fluctuations are a challenge to accurately simulate, and thus of great interest for testing diverse qubit platforms in the field of quantum simulation. This study experimentally demonstrates quantum fluctuations lifting the degenerate ground-state manifold of classical magnetic configurations in fullerene Ising models with resonating dimers. The interplay between degeneracy and quantum fluctuations makes these boundary-free models a suitable benchmark for quantum simulators. Indeed, we observe significant performance improvement across generations of superconducting quantum annealers, showing the potential of highly symmetric, frustrated systems for assessing the precision of quantum-simulation technologies.

quant-ph↗

arXiv2023

Kagome qubit ice

Topological phases of spin liquids with constrained disorder can host a kinetics of fractionalized excitations. However, spin-liquid phases with distinct kinetic regimes have proven difficult to observe experimentally. Here we present a realization of kagome spin ice in the superconducting qubits of a quantum annealer, and use it to demonstrate a field-induced kinetic crossover between spin-liquid phases. Employing fine control over local magnetic fields, we show evidence of both the Ice-I phase and an unconventional field-induced Ice-II phase. In the latter, a charge-ordered yet spin-disordered topological phase, the kinetics proceeds via pair creation and annihilation of strongly correlated, charge conserving, fractionalized excitations. As these kinetic regimes have resisted characterization in other artificial spin ice realizations, our results demonstrate the utility of quantum-driven kinetics in advancing the study of topological phases of spin liquids.

cond-mat.stat-mech↗

arXiv2022

Semi-metallicity and electron-hole liquid in two-dimensional C and BN based compounds

Insulating-metallic transition mediated by substitutional atoms is predicted in a series of two-dimensional carbon-based structures. Introducing Si atoms in selected sites of tetrahexcarbon [Carbon 137 (2018) 266] according to rational chemical rules, metallicity by trivial band inversion without band gap opening is induced. Additional substitution of remaining C atoms by BN dimers introduces no changes in the metallic properties. A series of isomorphous two-dimensional materials with isoelectronic structures derived by exchanging group IV elements exhibiting various band gaps is obtained. Dynamical stability is verified with phonon analysis and beyond the harmonic approximation with molecular dynamics up to room temperature. The semi-metallic compounds have well-nested pockets of carriers and are good candidates for the formation of an excitonic insulator.

cond-mat.mtrl-sci↗

arXiv2022

Design Principles and Physical Properties of Two-Dimensional Heterostructured Borides

Principles of design to create dynamically stable transition metal, lanthanide, and actinide based low-dimensional borides are presented. A charge transfer analysis of donor metal atoms to electron deficient honeycombed B lattices allows to predict complex covalent heterostructures hosting Dirac states. The applicable guidelines are supported with the analysis of phonon spectra computed with first-principles calculations to demonstrate the physical stability of nanometer-thick heterostructures. Similar or dissimilar layered borides can be stacked on top of each other in a layer-by-layer fashion creating an interface that can be fundamentally different from the individual layers, opening a rich playground to explore novel physical properties and new materials. Functionalities such as multiple Dirac states, highly dispersive electronic bands, and decoupled acoustic-optical phonon are studied. The combination of appealing electronic properties and physical realization make of predicted layered borides promising materials to integrate a new generation of two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2022

QUBO-based density matrix electronic structure method

Density matrix electronic structure theory is used in many quantum chemistry methods to "alleviate" the computational cost that arises from directly using wave functions. Although density matrix based methods are computationally more efficient than wave functions based methods, yet significant computational effort is involved. Since the Schrödinger equation needs to be solved as an eigenvalue problem, the time-to-solution scales cubically with the system size, and is solved as many times in order to reach charge or field self-consistency. We hereby propose and study a method to compute the density matrix by using a quadratic unconstrained binary optimization (QUBO) solver. This method could be useful to solve the problem with quantum computers, and more specifically, quantum annealers. The method hereby proposed is based on a direct construction of the density matrix using a QUBO eigensolver. We explore the main parameters of the algorithm focusing on precision and efficiency. We show that, while direct construction of the density matrix using a QUBO formulation is possible, the efficiency and precision have room for improvement. Moreover, calculations performing Quantum Annealing with the D-Wave's new Advantage quantum processing units is compared with classical Simulated annealing, further highlighting some problems of the proposed method. We also show some alternative methods that could lead to a better performance of the density matrix construction.

physics.chem-ph↗

arXiv2021

Exploring Metastable States in UO$_2$ using Hybrid Functionals and Dynamical Mean Field Theory

A detailed exploration of the $f$-atomic orbital occupancy space for UO$_2$ is performed using a first principles approach based on density functional theory (DFT), employing a full hybrid functional within a systematic basis set. Specifically, the PBE0 functional is combined with an occupancy biasing scheme implemented in a wavelet-based algorithm which is adapted to large supercells. The results are compared with previous DFT+U calculations reported in the literature, while dynamical mean field theory (DMFT) is also performed to provide a further base for comparison. This work shows that the computational complexity of the energy landscape of a correlated $f$-electron oxide is much richer than has previously been demonstrated. The resulting calculations provide evidence of the existence of multiple previously unexplored metastable electronic states of UO$_2$, including those with energies which are lower than previously reported ground states.

cond-mat.str-el↗

arXiv2021

Qubit spin ice

Artificial spin ices are frustrated spin systems that can be engineered, wherein fine tuning of geometry and topology has allowed the design and characterization of exotic emergent phenomena at the constituent level. Here we report a realization of spin ice in a lattice of superconducting qubits. Unlike conventional artificial spin ice, our system is disordered by both quantum and thermal fluctuations. The ground state is classically described by the ice rule, and we achieve control over a fragile degeneracy point leading to a Coulomb phase. The ability to pin individual spins allows us to demonstrate Gauss's law for emergent effective monopoles in two dimensions. The demonstrated qubit control lays the groundwork for potential future study of topologically protected artificial quantum spin liquids.

quant-ph↗

arXiv2020

Electrical band flattening, valley flux, and superconductivity in twisted trilayer graphene

Twisted graphene multilayers have demonstrated to yield a versatile playground to engineer controllable electronic states. Here, by combining first-principles calculations and low-energy models, we demonstrate that twisted graphene trilayers provide a tunable system where van Hove singularities can be controlled electrically. In particular, it is shown that besides the band flattening, bulk valley currents appear, which can be quenched by local chemical dopants. We finally show that in the presence of electronic interactions, a non-uniform superfluid density emerges, whose non-uniformity gives rise to spectroscopic signatures in dispersive higher energy bands. Our results put forward twisted trilayers as a tunable van der Waals heterostructure displaying electrically controllable flat bands and bulk valley currents.

cond-mat.mes-hall↗

arXiv2019

Defect-induced magnetism and Yu-Shiba-Rusinov states in twisted bilayer graphene

Atomic defects have a significant impact in the low-energy properties of graphene systems. By means of first-principles calculations and tight-binding models we provide evidence that chemical impurities modify both the normal and the superconducting states of twisted bilayer graphene. A single hydrogen atom attached to the bilayer surface yields a triple-point crossing, whereas self-doping and three-fold symmetry-breaking are created by a vacant site. Both types of defects lead to time-reversal symmetry-breaking and the creation of local magnetic moments. Hydrogen-induced magnetism is found to exist also at the doping levels where superconductivity appears in magic angle graphene superlattices. As a result, the coexistence of superconducting order and defect-induced magnetism yields in-gap Yu-Shiba-Rusinov excitations in magic angle twisted bilayer graphene.

cond-mat.mtrl-sci↗

arXiv2019

Emergence of Flat-Band Magnetism and Half-Metallicity in Twisted Bilayer Graphene

Evidence of flat-band magnetism and half-metallicity in compressed twisted bilayer graphene is provided with first-principles calculations. We show that dynamic band-structure engineering in twisted bilayer graphene is possible by controlling the chemical composition with extrinsic doping, the interlayer coupling strength with pressure, and the magnetic ordering with external electric field. By varying the rotational order and reducing the interlayer separation an unbalanced distribution of charge density resulting in the spontaneous apparition of localized magnetic moments without disrupting the structural integrity of the bilayer. Weak exchange correlation between magnetic moments is estimated in large unit cells. External electric field switches the local magnetic ordering from ferromagnetic to anti-ferromagnetic. Substitutional doping shifts the chemical potential of one spin distribution and leads to half-metallicity. Flakes of compressed twisted bilayer graphene exhibit spontaneous magnetization, demonstrating that correlation between magnetic moments is not a necessary condition for their formation.

cond-mat.mtrl-sci↗

arXiv2018

Zirconia and hafnia polymorphs -- ground state structural properties from diffusion Monte Carlo

Zirconia (zirconium dioxide) and hafnia (hafnium dioxide) are binary oxides used in a range of applications. Because zirconium and hafnium are chemically equivalent, they have three similar polymorphs, and it is important to understand the properties and energetics of these polymorphs. However, while density functional theory calculations can get the correct energetic ordering, the energy differences between polymorphs depend very much on the specific density functional theory approach, as do other quantities such as lattice constants and bulk modulus. We have used highly accurate quantum Monte Carlo simulations to model the three zirconia and hafnia polymorphs. We compare our results for structural parameters, bulk modulus, and cohesive energy with results obtained from density functional theory calculations. We also discuss comparisons of our results with existing experimental data, in particular for structural parameters where extrapolation to zero temperature can be attempted. We hope our results of structural parameters as well as for cohesive energy and bulk modulus can serve as benchmarks for density-functional theory based calculations and as a guidance for future experiments.

cond-mat.mtrl-sci↗

arXiv2018

Interplay between p- and d- orbitals yields multiple Dirac states in one- and two-dimensional CrB4

Theoretical evidence of the existence of six inequivalent and six threefold degenerate pairs of Dirac cones in the low-spectrum diagram of monolayered hexagonal CrB4 is provided. The four d-electrons of the Cr atom are yielded to the B sublattices creating an isoelectronic structure to graphene where the interplay between p- and d- orbitals leads to the appearance of additional Dirac states on both one- and two-dimensional geometries. Ab initio calculations show that, although spin-orbit interaction splits the cone-shaped valence and conduction bands, CrB4 is a semimetal with compensated electron-hole pockets. As the two-dimensional layer is shaped into finite-width ribbons, one actual and one symmetry-frustrated Dirac point are observed at the Fermi level, yielding massless fermions in a one-dimensional nano-structure with no topological insulating features. A rational explanation in terms of periodic boundary conditions across the ribbon axis is given to unveil the origin of the Dirac point.

cond-mat.mtrl-sci↗

arXiv2018

Twelve Inequivalent Dirac Cones in Two-Dimensional ZrB2

Theoretical evidence of the existence of 12 inequivalent Dirac cones at the vicinity of the Fermi energy in monolayered ZrB$_2$ is presented. Two-dimensional ZrB$_2$ is a mechanically stable d- and p-orbital compound exhibiting a unique electronic structure with two Dirac cones out of high-symmetry points in the irreducible Brillouin zone with a small electron-pocket compensation. First-principles calculations demonstrate that while one of the cones is insensitive to lattice expansion, the second cone vanishes for small perturbation of the vertical Zr position. Internal symmetry breaking with external physical stimuli, along with the relativistic effect of SOC, is able to remove selectively the Dirac cones. A rational explanation in terms of d- and p-orbital mixing is provided to elucidate the origin of the infrequent amount of Dirac cones in a flat structure. The versatility of transition metal d-orbitals combined with the honeycomb lattice provided by the B atoms yields novel features never observed in a two-dimensional material.

cond-mat.mtrl-sci↗