arXiv subjects

Hernan L. Calvo

Publications and source records attributed to Hernan L. Calvo.

17 recordsLinked to original sources

arXiv2026

Position-dependent tight-binding model for Li impurities in monolayer and bilayer graphene

Lithium adsorption and intercalation can significantly modify the low-energy electronic properties of graphene-based materials, making their characterization relevant for understanding Li-ion transport and storage in graphitic electrodes. In this work, we investigate the electronic structure of a Li ion adsorbed on monolayer graphene (MLG) and intercalated within AB-stacked bilayer graphene (BLG). To this end, we develop a semi-empirical tight-binding model that incorporates Li-position dependence. Its parameters are determined by fitting to density-functional-theory calculations for different configurations, heights, and supercell sizes. The obtained model accurately reproduces the electronic bands near the Fermi level for both MLG and BLG and provides a transparent interpretation of the impurity-induced modifications in terms of symmetry breakings, intervalley mixing, and band-gap openings. We find that the perturbation introduced by the Li ion is strongly localized and that its effect decreases with increasing supercell size. The fitted parameters further reveal systematic differences between MLG and BLG in the spatial profile of the impurity potential. The obtained results provide an efficient framework for studying dilute Li impurities and constitute a useful starting point for future investigations of Li diffusion and impurity-induced transport phenomena in graphene-based materials.

cond-mat.mes-hall↗

arXiv2025

Engineering Floquet moiré patterns for scalable photocurrents

While intense laser irradiation and moiré engineering have independently proven powerful for tuning material properties on demand in condensed matter physics, their combination remains unexplored. Here we exploit tilted laser illumination to create spatially modulated light-matter interactions, leading to two striking phenomena in graphene. First, using two lasers tilted along the same axis, we create a quasi-1D supercell hosting a network of Floquet topological states that generate controllable and scalable photocurrents spanning the entire irradiated region. Second, by tilting lasers along orthogonal axes, we establish a 2D polarization moiré pattern giving rise to closed orbital propagation of Floquet states, reminiscent of bulk Landau states. These features, imprinted in the bulk of the irradiated region and controlled through laser wavelength and tilt angles, establish a new way for engineering quantum states through spatially modulated light-matter coupling.

cond-mat.mes-hall↗

arXiv2021

Role of coherence in quantum-dot-based nanomachines within the Coulomb blockade regime

During the last decades, quantum dots within the Coulomb blockade regime of transport have been proposed as essential building blocks for a wide variety of nanomachines. This includes thermoelectric devices, quantum shuttles, quantum pumps, and even quantum motors. However, in this regime, the role of quantum mechanics is commonly limited to provide energy quantization while the working principle of the devices is ultimately the same as their classic counterparts. Here, we study quantum-dot-based nanomachines in the Coulomb blockade regime, but in a configuration where the coherent superpositions of the dots' states plays a crucial role. We show that the studied system can be used as the basis for different forms of "true" quantum machines that should only work in the presence of these coherent superpositions. We analyze the efficiency of these machines against different nonequilibrium sources (bias voltage, temperature gradient, and external driving) and the factors that limit it, including decoherence and the role of the different orders appearing in the adiabatic expansion of the charge/heat currents.

cond-mat.mes-hall↗

arXiv2020

Floquet boundary states in AB-stacked graphite

We report on the effect of laser illumination with circularly polarized light on the electronic structure of AB-stacked graphite samples. By using Floquet theory in combination with Green's function techniques, we find that the polarized light induces band-gap openings at the Floquet zone edge $\hbarΩ/2$, bridged by chiral boundary states. These states propagate mainly along the borders of the constituting layers as evidenced by the time-averaged local density of states and the probability current density in several geometries. Semianalytic calculations of the Chern number suggest that these states are of topological nature, similar to those found in illuminated 2D samples like monolayer and bilayer graphene. These states are promising candidates for the realization of a three-dimensional version of the quantum Hall effect for Floquet systems.

cond-mat.mes-hall↗

arXiv2019

Thermodynamics and steady state of quantum motors and pumps far from equilibrium

In this article, we briefly review dynamical and thermodynamical aspects of different forms of quantum motors and quantum pumps. We then extend previous results to provide new theoretical tools for a systematic study of those phenomena at far-from-equilibrium conditions. We mainly focus on two key topics: (1) The steady-state regime of quantum motors and pumps, paying particular attention to the role of higher-order terms in the nonadiabatic expansion of the current-induced forces. (2) The thermodynamical properties of such systems, emphasizing systematic ways of studying the relationship between different energy fluxes (charge and heat currents, and mechanical power) passing through the system when beyond-first-order expansions are required. We derive a general order-by-order scheme based on energy conservation to rationalize how every order of the expansion of one form of energy flux is connected with the others. We use this approach to give a physical interpretation of the leading terms of the expansion. Finally, we illustrate the above-discussed topics in a double quantum dot within the Coulomb-blockade regime and capacitively coupled to a mechanical rotor. We find many exciting features of this system for arbitrary nonequilibrium conditions: A definite parity of the expansion coefficients with respect to the voltage or temperature biases; negative friction coefficients; and the fact that, under fixed parameters, the device can exhibit multiple steady states where it may operate as a quantum motor or as a quantum pump depending on the initial conditions.

cond-mat.mes-hall↗

arXiv2018

Robust edge states induced by electron-phonon interaction in graphene nanoribbons

The search of new means of generating and controlling topological states of matter is at the front of many joint efforts, including bandgap engineering by doping and light-induced topological states. Most of our understading, however, is based on a single particle picture. Topological states in systems including interaction effects, such as electron-electron and electron-phonon, remain less explored. By exploiting a non-perturbative and non-adiabatic picture, here we show how the interaction between electrons and a coherent phonon mode can lead to a bandgap hosting edge states of topological origin. Further numerical simulations witness the robustness of these states against different types of disorder. Our results contribute to the search of topological states, in this case in a minimal Fock space.

cond-mat.mes-hall↗

arXiv2017

Real-time diagrammatic approach to current-induced forces: Application to quantum-dot based nanomotors

During the last years there has been an increasing excitement in nanomotors and particularly in current-driven nanomotors. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. Here we consider nanoelectromechanical devices composed by a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. We use a real-time diagrammatic approach to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We prove our expressions accomplish with Onsager's reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes. The obtained results are illustrated in a nanomotor consisting of a double quantum dot capacitively coupled to some rotating charges. We analyze the dynamics and performance of the motor as function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

cond-mat.mes-hall↗

arXiv2016

Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds

We study the spin chain behavior, a transition to 3D magnetic order and the magnitudes of the exchange interactions for the metal-amino acid complex Cu(D,L-alanine)2.H2O, a model compound to investigate exchange couplings supported by chemical paths characteristic of biomolecules. Thermal and magnetic data were obtained as a function of temperature (T) and magnetic field (B0). The magnetic contribution to the specific heat, measured between 0.48 and 30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an intrachain antiferromagnetic (AFM) exchange coupling constant 2J0 = (-2.12 $\pm$ 0.08) cm$^{-1}$, between neighbor coppers at 4.49 Å along chains connected by non-covalent and H-bonds. We also observe a narrow specific heat peak at 0.89 K indicating a phase transition to a 3D magnetically ordered phase. Magnetization curves at fixed T = 2, 4 and 7 K with B0 between 0 and 9 T, and at T between 2 and 300 K with several fixed values of B0 were globally fitted by an intrachain AFM exchange coupling constant 2J0 = (-2.27 $\pm$ 0.02) cm$^{-1}$ and g = 2.091 $\pm$ 0.005. Interchain interactions J1 between coppers in neighbor chains connected through long chemical paths with total length of 9.51 Å are estimated within the range 0.1 < |2J1| < 0.4 cm$^{-1}$, covering the predictions of various approximations. We analyze the magnitudes of 2J0 and 2J1 in terms of the structure of the corresponding chemical paths. The main contribution in supporting the intrachain interaction is assigned to H-bonds while the interchain interactions are supported by paths containing H-bonds and carboxylate bridges, with the role of the H-bonds being predominant. We compare the obtained intrachain coupling with studies of compounds showing similar behavior and discuss the validity of the approximations allowing to calculate the interchain interactions.

cond-mat.mes-hall↗

arXiv2013

Non-perturbative laser effects on the electrical properties of graphene nanoribbons

The use of Floquet theory combined with a realistic description of the electronic structure of illuminated graphene and graphene nanoribbons is developed to assess the emergence of non-adiabatic and non-perturbative effects on the electronic properties. Here, we introduce an efficient computational scheme and illustrate its use by applying it to graphene nanoribbons in the presence of both linear and circular polarization. The interplay between confinement due to the finite sample size and laser-induced transitions is shown to lead to sharp features on the average conductance and density of states. Particular emphasis is given to the emergence of the bulk limit response.

cond-mat.mes-hall↗

arXiv2012

Laser-induced effects on the electronic features of graphene nanoribbons

We study the interplay between lateral confinement and photon-induced processes on the electronic properties of illuminated graphene nanoribbons. We find that by tuning the device setup (edges geometries, ribbon width and polarization direction), a laser with frequency Ω may either not affect the electronic structure, or induce bandgaps or depletions at \hbar Ω/2, and/or at other energies not commensurate with half the photon energy. Similar features are also observed in the dc conductance, suggesting the use of the polarization direction to switch on and off the graphene device. Our results could guide the design of novel types of optoelectronic nano-devices.

cond-mat.mes-hall↗

arXiv2012

Interaction-induced charge and spin pumping through a quantum dot at finite bias

We investigate charge and spin transport through an adiabatically driven, strongly interacting quantum dot weakly coupled to two metallic contacts with finite bias voltage. Within a kinetic equation approach, we identify coefficients of response to the time-dependent external driving and relate these to the concepts of charge and spin emissivities previously discussed within the time-dependent scattering matrix approach. Expressed in terms of auxiliary vector fields, the response coefficients allow for a straightforward analysis of recently predicted interaction-induced pumping under periodic modulation of the gate and bias voltage [Phys. Rev. Lett. 104, 226803 (2010)]. We perform a detailed study of this effect and the related adiabatic Coulomb blockade spectroscopy, and, in particular, extend it to spin pumping. Analytic formulas for the pumped charge and spin in the regimes of small and large driving amplitude are provided for arbitrary bias. In the absence of a magnetic field, we obtain a striking, simple relation between the pumped charge at zero bias and at bias equal to the Coulomb charging energy. At finite magnetic field, there is a possibility to have interaction-induced pure spin pumping at this finite bias value, and generally, additional features appear in the pumped charge. For large-amplitude adiabatic driving, the magnitude of both the pumped charge and spin at the various resonances saturate at values which are independent of the specific shape of the pumping cycle. Each of these values provide an independent, quantitative measurement of the junction asymmetry.

cond-mat.mes-hall↗

arXiv2011

Tuning laser-induced bandgaps in graphene

Could a laser field lead to the much sought-after tunable bandgaps in graphene? By using Floquet theory combined with Green's functions techniques, we predict that a laser field in the mid-infrared range can produce observable bandgaps in the electronic structure of graphene. Furthermore, we show how they can be tuned by using the laser polarization. Our results could serve as a guidance to design opto-electronic nano-devices.

cond-mat.mes-hall↗

arXiv2010

Exact time-reversal focusing of acoustic and quantum excitations in open cavities: The perfect inverse filter

The time-reversal mirror (TRM) prescribes the reverse playback of a signal to focalize an acoustic excitation as a Loschmidt echo. In the quantum domain, the perfect inverse filter (PIF) processes this signal to ensure an exact reversion provided that the excitation originated outside the cavity delimited by the transducers. We show that PIF takes a simple form when the initial excitation is created inside this cavity. This also applies to the acoustical case, where it corrects the TRM and improves the design of an acoustic bazooka. We solve an open chaotic cavity modeling a quantum bazooka and a simple model for a Helmholtz resonator, showing that the PIF becomes decisive to compensate the group velocities involved in a highly localized excitation and to achieve subwavelength resolution.

cond-mat.mes-hall↗

arXiv2008

Semiclassical Theory of Time-Reversal Focusing

Time reversal mirrors have been successfully implemented for various kinds of waves propagating in complex media. In particular, acoustic waves in chaotic cavities exhibit a refocalization that is extremely robust against external perturbations or the partial use of the available information. We develop a semiclassical approach in order to quantitatively describe the refocusing signal resulting from an initially localized wave-packet. The time-dependent reconstructed signal grows linearly with the temporal window of injection, in agreement with the acoustic experiments, and reaches the same spatial extension of the original wave-packet. We explain the crucial role played by the chaotic dynamics for the reconstruction of the signal and its stability against external perturbations.

cond-mat.mes-hall↗

arXiv2008

Time-reversal of a wave packet emitted by an internal source: the perfect inverse filter

The time-reversal mirror (TRM) of acoustic waves is used as a focusing technique that is robust against disorder and chaos. The propagation of an ultrasonic pulse from the focal point is detected and recorded by a surrounding array of transducers that enclose the cavity region. The registered signals are then played-back in the reverse temporal sequence and an imperfect Loschmidt echo is obtained. A similar technique developed in the quantum domain, the perfect inverse filter (PIF), ensures an exact reversion. Applied to the acoustical case, it allows an improvement in the design of a minimally invasive inverse filter. Although that version of PIF accounts for external source condition, now we propose a generalization where the initial excitation is created inside the cavity. We present the theoretical principles for this form of PIF and perform a numerical test in a multichannel system that improves the focalization quality compared to the TRM version.

cond-mat.mes-hall↗

arXiv2007

Time Reversal Mirror and Perfect Inverse Filter in a Microscopic Model for Sound Propagation

Time reversal of quantum dynamics can be achieved by a global change of the Hamiltonian sign (a hasty Loschmidt daemon), as in the Loschmidt Echo experiments in NMR, or by a local but persistent procedure (a stubborn daemon) as in the Time Reversal Mirror (TRM) used in ultrasound acoustics. While the first is limited by chaos and disorder, the last procedure seems to benefit from it. As a first step to quantify such stability we develop a procedure, the Perfect Inverse Filter (PIF), that accounts for memory effects, and we apply it to a system of coupled oscillators. In order to ensure a many-body dynamics numerically intrinsically reversible, we develop an algorithm, the pair partitioning, based on the Trotter strategy used for quantum dynamics. We analyze situations where the PIF gives substantial improvements over the TRM.

cond-mat.mes-hall↗

arXiv2006

Towards a time-reversal mirror for quantum systems

The reversion of the time evolution of a quantum state can be achieved by changing the sign of the Hamiltonian as in the polarization echo experiment in NMR. In this work we describe an alternative mechanism inspired by the acoustic time reversal mirror. By solving the inverse time problem in a discrete space we develop a new procedure, the perfect inverse filter. It achieves the exact time reversion in a given region by reinjecting a prescribed wave function at its periphery.

quant-ph↗