arXiv subjects

J. V. Alvarez

Publications and source records attributed to J. V. Alvarez.

At least 19 recordsLinked to original sources

arXiv2026

Topology of Nonequilibrium Currents Controls Active Transport

Structured environments repeatedly redirect active particles, producing transport pathways that cannot be readily inferred from individual trajectories. Here, we show that the large-scale organization of these transport pathways is governed by topological constraints. Hydrodynamic scattering generates nonequilibrium current fields whose defect structure, characterized by integer indices, constrain transport pathways and renders them robust to smooth perturbations. This principle is demonstrated with rotating colloids in obstacle arrays and extended to stokeslet and force-dipole flows, thereby linking microscale transport to the topology of hydrodynamically generated nonequilibrium currents.

cond-mat.soft↗

arXiv2026

Programmable transport of rotating particles in obstacle arrays

Rotating colloids, or spinners, in obstacle arrays exhibit frequency-set stationary orbits and currents set by the competition between an inertial, Magnus-like lift and short-range attraction. Fully resolved lattice-Boltzmann simulations reveal the hydrodynamic coupling and identify the lift mechanism, while a symmetry-based Langevin model captures the resulting balance. In periodic lattices, the superposition of scalar and vector potentials produces two robust orbital regimes: corner states, in which spinners orbit individual posts, and inner states, in which orbits couple across four neighboring obstacles. Slow frequency modulation toggles these states and produces directed, stepwise transport across the grid. This establishes a minimal hydrodynamic mechanism, controlled by a single driving parameter, for programmable guidance of active rotors in structured environments.

cond-mat.soft↗

arXiv2020

Generalized Elastodynamic Model for Nanophotonics

A self-consistent theory for the classical description of the interaction of light and matter at the nano-scale is presented, which takes into account spatial dispersion. Up to now, the Maxwell equations in nanostructured materials with spatial dispersion have been solved by the introduction of the so-called Additional Boundary Conditions. In this paper, we derive an approach where non-local effects are studied in a precise and uniquely defined way, thus allowing the treatment of all solid-solid interfaces (metals, semiconductors or insulators), as well as solid-vacuum interfaces in the same framework. The theory is based on the derivation of a potential energy for an ensemble of electrons in a given potential, where the deformation of the ensemble is treated as in a solid, including both shear and compressional deformations, instead of a fluid described only by a bulk compressibility like in the hydrodynamical approach. The derived classical equation of motion for the ensemble describes the deformation vector and the corresponding polarization vector as an elastodynamic field, including viscous forces, from which a generalized non-local constitutive equation for the dielectric constant is derived. Boundary conditions are identical to that of elastodynamics and they emerge in a natural way, without any physical hypothesis outside the current description, as it is commonly required in other non-local approaches. This description does not require the discontinuity of any component of the electric, magnetic or polarization fields and, consequently, no bounded currents or charges are present at the interface, which is a more suitable description from the microscopic point of view. It is shown that the method converges to the local boundary conditions in the low spatial dispersion limit for insulators and conductors, quantified by means of a parameter defined as the "characteristic length".

physics.optics↗

arXiv2018

Topological mechanical metamaterial with nonrectilinear constraints

In this paper we study Maxwell lattices with non-rectilinear constraints, where the elastic energy is determined by the collective motion of three or more particles, in contrast to a rectilinear spring whose elastic energy only relies on the displacement of two particles attached to the two ends of the spring. Utilizing polygon-shaped constraints, we found that the Maxwell counting argument and the topological construction, based on the compatibility matrix and the equilibrium matrix, can be generalized in our models, and our elastic systems follow the same topological classification. In addition, we also found that non-rectilinear constraints offers a natural pass towards topological states with higher topological indices and multiple edge states, which can be achieved even for a simple unit cell with one degree of freedom per unit cell without enlarging the unit cell in the bulk or at the edge.

physics.class-ph↗

arXiv2016

Gapped electron fractionalization in robustly one dimensional Li0.9Mo6O17

The angle resolved photoemission spectroscopy lineshapes of quasi one-dimensional (1d) Li0.9Mo6O17 display both agreement with and departures from the one-band Tomonaga-Luttinger model. We show that the departures can be understood by explicitly accounting for the four modes arising from the two quasi-1d bands known to cross the Fermi energy. The key assumption is that the antisymmetric charge mode is gapped with a magnitude near the temperature (T) of a mysterious 25K powerlaw resistivity upturn. The gap is consistent with the lack of a charge or spin density wave accompanying the upturn, is able to control the upturn T, and prevents crossover to a Fermi liquid (FL) down to the superconducting transition at 1.9K.

cond-mat.str-el↗

arXiv2016

Effective mass theory for the anisotropic exciton in 2D crystals: Application to phosphorene

We present a theoretical study of the exciton binding energy for anisotropic two-dimensional crystals. We obtain analytical expressions from variational wave functions in different limits of the screening length to exciton size ratio and compare them with numerical solutions, both variational and exact. As an example, we apply these results to phosphorene, a monolayer of black phosphorous. Aided by density functional theory calculations for the evaluation of the two-dimensional polarizability, our analytical solution for the exciton binding energy gives a result which is very close to the numerical ones and, in turn, is comparable to the experimental value, as recently reported.

cond-mat.mes-hall↗

arXiv2015

Triplet superconductivity in a model of Li$_{0.9}$Mo$_6$O$_{17}$

Superconductivity in the quasi-one-dimensional material Li$_{0.9}$Mo$_6$O$_{17}$ is analyzed based on a multiorbital extended Hubbard model. We found strong charge fluctuations at two different momenta ${\bf Q_1}$ and ${\bf Q_2}$ giving rise to two different charge ordered phases. Evaluating the superconducting vertex, we found superconductivity near strong charge fluctuations at ${\bf Q_1}$. The order parameter has the p-wave symmetry with nodes on the Fermi surface. The metallic state displays a characteristic charge collective mode ${\bf Q_1}$ due to nesting and for on-site Hubbard repulsion sufficiently large, a charge critical mode ${\bf Q_2}$ driven by Coulomb repulsion, which softens at the proximity to the transition. The results are quite robust for different coupling parametrizations. A phase diagram discussing the relevance of the model to the physics of the material is proposed.

cond-mat.supr-con↗

arXiv2014

Isolation and characterization of few-layer black phosphorus

Isolation and characterization of mechanically exfoliated black phosphorus flakes with a thickness down to two single-layers is presented. A modification of the mechanical exfoliation method, which provides higher yield of atomically thin flakes than conventional mechanical exfoliation, has been developed. We present general guidelines to determine the number of layers using optical microscopy, Raman spectroscopy and transmission electron microscopy in a fast and reliable way. Moreover, we demonstrate that the exfoliated flakes are highly crystalline and that they are stable even in free-standing form through Raman spectroscopy and transmission electron microscopy measurements. A strong thickness dependence of the band structure is found by density functional theory calculations. The exciton binding energy, within an effective mass approximation, is also calculated for different number of layers. Our computational results for the optical gap are consistent with preliminary photoluminescence results on thin flakes. Finally, we study the environmental stability of black phosphorus flakes finding that the flakes are very hydrophilic and that long term exposure to air moisture etches black phosphorus away. Nonetheless, we demonstrate that the aging of the flakes is slow enough to allow fabrication of field-effect transistors with strong ambipolar behavior. Density functional theory calculations also give us insight into the water-induced changes of the structural and electronic properties of black phosphorus.

cond-mat.mes-hall↗

arXiv2014

Charge fluctuations in the unconventional metallic state of Li0.9Mo6O17

Charge fluctuations in the quasi-one-dimensional material Li0.9Mo6O17 are analyzed based on a multi orbital extended Hubbard model. A charge ordering transition induced by Coulomb repulsion is found with a charge ordering pattern different from a conventional charge density wave driven by Fermi surface nesting. The metallic state displays a characteristic charge collective mode which softens signalling the proximity to the transition. We argue that the strong scattering between electrons generated by these charge order fluctuations can lead to the unconventional metallic state observed above the superconducting transition temperature in Li0.9Mo6O17.

cond-mat.str-el↗

arXiv2014

Hydrogenation-induced ferromagnetism on graphite surfaces

We calculate the electronic structure and magnetic properties of hydrogenated graphite surfaces using van der Waals density functional theory (DFT) and model Hamiltonians. We find, as previously reported, that the interaction between hydrogen atoms on graphene favors adsorption on different sublattices along with an antiferromagnetic coupling of the induced magnetic moments. On the contrary, when hydrogenation takes place on the surface of graphene multilayers or graphite (Bernal stacking), the interaction between hydrogen atoms competes with the different adsorption energies of the two sublattices. This competition may result in all hydrogen atoms adsorbed on the same sublattice and, thereby, in a ferromagnetic state for low concentrations. Based on the exchange couplings obtained from the DFT calculations, we have also evaluated the Curie temperature by mapping this system onto an Ising-like model with randomly located spins. Remarkably, the long-range nature of the magnetic coupling in these systems makes the Curie temperature size dependent and larger than room temperature for typical concentrations and sizes.

cond-mat.mes-hall↗

arXiv2007

Superconductivity and magnetic order in CeRhIn$_{5}$; spectra of coexistence

We discuss the fixed-point Hamiltonian and the spectrum of excitations of a quasi-bidimensional electronic system supporting simultaneously antiferromamagnetic ordering and superconductivity. The coexistence of these two order parameters in a single phase is possible because the magnetic order is linked to the formation of a spin density wave, and its order parameter is not associated to a spectral gap but to an energy shift of the paramagnetic bands. This peculiarity entails several distinct features in the phase diagram and the spectral properties of the model, which may have been observed in CeRhIn$_5$. Apart from the coexistence, we find an abrupt suppression of the spin density wave when the superconducting and magnetic ordering temperatures are equal. The divergence of the cyclotron mass extracted from de Haas-van Alphen experiments is also analyzed in the same framework.

cond-mat.str-el↗

arXiv2006

New Luttinger liquid physics from photoemission on Li$_{0.9}$Mo$_6$O$_{17}$

Temperature dependent high resolution photoemission spectra of quasi-1 dimensional Li$_{0.9}$Mo$_6$O$_{17}$ evince a strong renormalization of its Luttinger liquid density-of-states anomalous exponent. We trace this new effect to interacting charge neutral critical modes that emerge naturally from the two-band nature of the material. Li$_{0.9}$Mo$_6$O$_{17}$ is shown thereby to be a paradigm material that is capable of revealing new Luttinger physics.

cond-mat.str-el↗

arXiv2005

The antiferromagnetic/paramagnetic transition in mixed-spin compounds R2BaNiO5

We present an extensive Quantum Monte Carlo study of the magnetic properties of the mixed-spin quantum systems R2BaNiO5 (R= magnetic rare earth) which show coexistence of 3-dimensional magnetic long-range order with 1-dimensional quantum gap excitations. We discuss the validity of the performed simulations in the critical region and show the excellent agreement with experimental results. We emphasize the importance of quantum fluctuations contained in our study which is absent in previous mean-field-like treatments.

cond-mat.str-el↗

arXiv2004

A model dielectric response function for metallic nanotube ropes

We propose a model dielectric function for ropes of single-walled nanotubes distributed in a glassy graphite host medium. We study the significance of the bosonic charge excitations arising in interacting quasi-one-dimensional systems in the screening processes. We also pay special attention to the role of the intertube Coulomb interactions. In order to compare with experiments, weak relaxation processes are also considered in the relaxation-time approximation.

cond-mat.str-el↗

arXiv2004

Disordered and Ordered States in a Frustrated Anisotropic Heisenberg Hamiltonian

We use a recently proposed perturbative numerical renormalization group algorithm to investigate ground-state properties of a frustrated three dimensional Heisenberg model on an anisotropic lattice. We analyze the ground state energy, the finite size spin gap and the static magnetic structure factor. We find in two dimensions a frustration-induced gapless spin liquid state which separates two magnetically ordered phases. In the spin liquid state, the magnetic structure factor shows evidence that this state is made of nearly disconnected chains. This spin liquid state is unstable against unfrustrated interplane couplings.

cond-mat.str-el↗

arXiv2004

Renormalization group method for weakly coupled quantum chains: comparison with exact diagonalization

We show that numerical quasi-one-dimensional renormalization group allows accurate study of weakly coupled chains with modest computational effort. We perform a systematic comparison with exact diagonalization results in two and three-leg spin ladders with a transverse Hamiltonian that can involve frustration. Due to the variational nature of the algorithm, the accuracy can be arbitrarily improved enlarging the basis of eigenstates of the density matrix defined in the transverse direction. We observe that the precision of the algorithm is directly correlated to the binding of the chains. We also show that the method performs especially well in frustrated systems.

cond-mat.str-el↗

arXiv2004

Anomalous percolation and quantum criticality in diluted rare-earth nickelates

A microscopic model for the diluted spin-mixed compounds (R_xY_(1-x))_2BaNiO_5 (R=magnetic rare-earth) is studied using Quantum Monte Carlo (QMC). The ordering temperature is shown to be a universal function of the impurity concentration x and the intrinsic Ni-chain correlation length. An effective model for the critical modes is derived. The possibility of a quantum critical point driven by the rare earth-concentration and the existence of a Griffiths phase in the high dilution limit is investigated. Several possible experimental approaches to verify the results are put forward.

cond-mat.str-el↗

arXiv2003

Insulating, superconducting and large-compressibility phases in nanotube ropes

The superconducting properties of carbon nanotube ropes are studied using a new computational framework that incorporates the renormalization of intratube interactions and the effect of intertube Coulomb screening. This method allows to study both the limits of thin and thick ropes ranging from purely one-dimensional physics to the setting of three-dimensional Cooper-pair coherence, providing good estimates of the critical temperature as a function of the rope physical parameters. We discuss the connection of our results with recent experiments.

cond-mat.str-el↗