arXiv subjects

Miguel A. Cazalilla

Publications and source records attributed to Miguel A. Cazalilla.

32 records · Page 2Linked to original sources

arXiv2017

Anomalous Nonlocal Resistance and Spin-charge Conversion Mechanisms in Two-Dimensional Metals

We uncover two anomalous features in the nonlocal transport behavior of two-dimensional metallic materials with spin-orbit coupling. Firstly, the nonlocal resistance can have negative values and oscillate with distance, even in the absence of a magnetic field. Secondly, the oscillations of the nonlocal resistance under an applied in-plane magnetic field (Hanle effect) can be asymmetric under field reversal. Both features are produced by direct magnetoelectric coupling, which is possible in materials with broken inversion symmetry but was not included in previous spin diffusion theories of nonlocal transport. These effects can be used to identify the relative contributions of different spin-charge conversion mechanisms. They should be observable in adatom-functionalized graphene, and may provide the reason for discrepancies in recent nonlocal transport experiments on graphene.

cond-mat.mes-hall↗

arXiv2017

The Haldane model under nonuniform strain

We study the Haldane model under strain using a tight-binding approach, and compare the obtained results with the continuum-limit approximation. As in graphene, nonuniform strain leads to a time-reversal preserving pseudo-magnetic field that induces (pseudo) Landau levels. Unlike a real magnetic field, strain lifts the degeneracy of the zeroth pseudo Landau levels at different valleys. Moreover, for the zigzag edge under uniaxial strain, strain removes the degeneracy within the pseudo-Landau levels by inducing a tilt in their energy dispersion. The latter arises from next-to-leading order corrections to the continuum-limit Hamiltonian, which are absent for a real magnetic field. We show that, for the lowest pseudo-Landau levels in the Haldane model, the dominant contribution to the tilt is different from graphene. In addition, although strain does not strongly modify the dispersion of the edge states, their interplay with the pseudo-Landau levels is different for the armchair and zigzag ribbons. Finally, we study the effect of strain in the band structure of the Haldane model at the critical point of the topological transition, thus shedding light on the interplay between non-trivial topology and strain in quantum anomalous Hall systems.

cond-mat.mes-hall↗

arXiv2016

Valley Hall Effect and Nonlocal Transport in Strained Graphene

Graphene subject to high levels of shear strain leads to strong pseudo-magnetic fields resulting in the emergence of Landau levels. Here we show that, with modest levels of strain, graphene can also sustain a classical valley hall effect (VHE) that can be detected in nonlocal transport measurements. We provide a theory of the strain-induced VHE starting from the quantum Boltzmann equation. This allows us to show that, averaging over short-range impurity configurations destroys quantum coherence between valleys, leaving the elastic scattering time and inter-valley scattering rate as the only parameters characterizing the transport theory. Using the theory, we compute the nonlocal resistance of a Hall bar device in the diffusive regime. Our theory is also relevant for the study of moderate strain effects in the (nonlocal) transport properties of other two-dimensional materials and van der Walls heterostructures.

cond-mat.mes-hall↗

arXiv2016

Raise and collapse of strain-induced pseudo-Landau levels in graphene

Lattice deformations couple to the low energy electronic excitations of graphene as vector fields similar to the electromagnetic potential \cite{SA02b,VKG10}. The suggestion that certain strain configurations would be able to induce pseudo landau levels in the spectrum of graphene \cite{GKG10,GGKN10}, and the subsequent experimental observation of these \cite{LBetal10} has been one of the most exciting events in an already fascinating field. It opened a new field of research "straintronics" linked to new applications, and had a strong influence on the physics of the new Dirac materials in two and three dimensions \cite{Amorim16}. The experimental observation of pseudo landau levels with scanning tunnel microscopy presents nevertheless some ambiguities. Similar strain patterns show different images sometimes difficult to interpret. In this work we strain the analogy of the pseudo versus real electromagnetic fields, as well as the fact that graphene has a relativistic behavior and show that, for some strain configurations, the deformation potential acts like a parallel electric field able to destabilize the Landau level structure via a mechanism identical to that described in \cite{LSB07,PC07} for real electromagnetic fields. The underlying physics is the combination of Maxwell electrodynamics and special relativity implying that different reference frames observe different electromagnetic fields \cite{LSB07}. The mechanism applies equally if the electric field has an external origin, which opens the door to an electric control of the giant pseudomagnetic fields in graphene.

cond-mat.mes-hall↗

arXiv2015

Graphene Electrodynamics in the presence of the Extrinsic Spin Hall Effect

We extend the electrodynamics of two dimensional electron gases to account for the extrinsic spin Hall effect (SHE). The theory is applied to doped graphene decorated with a random distribution of absorbates that induce spin-orbit coupling (SOC) by proximity. The formalism extends previous semiclassical treatments of the SHE to the non-local dynamical regime. Within a particle-number conserving approximation, we compute the conductivity, dielectric function, and spin Hall angle in the small frequency and wave vector limit. The spin Hall angle is found to decrease with frequency and wave number, but it remains comparable to its zero-frequency value around the frequency corresponding to the Drude peak. The plasmon dispersion and linewidth are also obtained. The extrinsic SHE affects the plasmon dispersion in the long wavelength limit, but not at large values of the wave number. This result suggests an explanation for the rather similar plasmonic response measured in exfoliated graphene, which does not exhibit the SHE, and graphene grown by chemical vapor deposition, for which a large SHE has been recently reported. Our theory also lays the foundation for future experimental searches of SOC effects in the electrodynamic response of two-dimensional electron gases with SOC disorder.

cond-mat.mes-hall↗

arXiv2015

Extrinsic Spin Hall Effect from Anisotropic Rashba Spin-Orbit Coupling in Graphene

We study the effect of anisotropy of the Rashba coupling on the extrinsic spin Hall effect due to spin-orbit active adatoms on graphene. In addition to the intrinsic spin-orbit coupling, a generalized anisotropic Rashba coupling arising from the breakdown of both mirror and hexagonal symmetries of pristine graphene is considered. We find that Rashba anisotropy can strongly modify the dependence of the spin Hall angle on carrier concentration. Our model provides a simple and general description of the skew scattering mechanism due to the spin-orbit coupling that is induced by proximity to large adatom clusters.

cond-mat.mes-hall↗

arXiv2015

Electron-spin to Phonon Coupling in Graphene Decorated with Heavy Adatoms

The naturally weak spin-orbit coupling in Graphene can be largely enhanced by adatom deposition (e.g. Weeks et al. Phys. Rev. X 1, 021001 (2011)). However, the dynamics of the adatoms also induces a coupling between phonons and the electron spin. Using group theory and a tight-binding model, we systematically investigate the coupling between the low-energy in-plane phonons and the electron spin in single-layer graphene uniformly decorated with heavy adatoms. Our results provide the foundation for future investigations of spin transport and superconductivity in this system. In order to quantify the effect of the coupling to the lattice on the electronic spin dynamics, we compute the spin-flip rate of electrons and holes. We show that the latter exhibits a strong dependence on the quasi-particle energy and system temperature.

cond-mat.mes-hall↗

arXiv2014

Destruction of Long-range Order by Quenching the Hopping Range in One Dimension

We study the dynamics in a one dimensional hard-core Bose gas with power-law hopping after an abrupt reduction of the hopping range using the time-dependent density-matrix renormalization group (t-DMRG) and bosonization techniques. In particular, we focus on the destruction of the Bose-Einstein condensate (BEC), which is present in the initial state in the thermodynamic limit. We argue that this type of quench is akin to a sudden reduction in the effective dimensionality $d$ of the system (from $d > 1$ to $d = 1$). We identify two regimes in the evolution of the BEC fraction. For short times the decay of the BEC fraction is Gaussian while for intermediate to long times, it is well described by a stretched exponential with an exponent that depends on the initial effective dimensionality of the system. These results are potentially relevant for cold trapped-ion experiments which can simulate an equivalent of hard-core bosons, i.e. spins, with tunable long-range interactions.

cond-mat.quant-gas↗

arXiv2012

Easy-axis ferromagnetic chain on a metallic surface

The phases and excitation spectrum of an easy-axis ferromagnetic chain of S=1/2 magnetic impurities built on the top of a clean metallic surface are studied. As a function of the (Kondo) coupling to the metallic surface and at low temperatures, the spin chain exhibits a quantum phase transition from an Ising ferromagnetic phase with long-range order to a paramagnetic phase where quantum fluctuations destroy the magnetic order. In the paramagnetic phase, the system consists of a chain of Kondo-singlets where the impurities are completely screened by the metallic host. In the ferromagnetic phase, the excitations above the Ising gap are damped magnons, with a finite lifetime arising due to the coupling to the substrate. We discuss the experimental consequences of our results to spin-polarized electron energy loss spectroscopy (SPEELS), and we finally analyze possible extensions to spin chains with S>1/2.

cond-mat.mes-hall↗

arXiv2012

Magnetic phases in the one-dimensional Kondo chain on a metallic surface

We study the low-temperature properties of a one-dimensional spin-1/2 chain of magnetic impurities coupled to a (normal) metal environment by means of anisotropic Kondo exchange. In the case of easy-plane anisotropy, we obtain the phase diagram of this system at T=0. We show that the in-plane Kondo coupling destabilizes the Tomonaga-Luttinger phase of the spin-chain, and leads to two different phases: i) At strong Kondo coupling, the spins in the chain form Kondo singlets and become screened by the metallic environment, and ii) At weak and intermediate Kondo coupling, we find a novel dissipative phase characterized by diffusive gapless spin excitations. The two phases are separated by a quantum critical point of the Wilson-Fisher universality class with dynamical exponent $z\simeq2$.

cond-mat.mes-hall↗

arXiv2012

Tuning the Kosterlitz-Thouless transition to zero temperature in Anisotropic Boson Systems

We study the two-dimensional Bose-Hubbard model with anisotropic hopping. Focusing on the effects of anisotropy on the superfluid properties such like the helicity modulus and the normal-to-superfluid (Berezinskii-Kosterlitz-Thouless, BKT) transition temperature, two different approaches are compared: Large-scale Quantum Monte Carlo simulations and the self-consistent harmonic approximation (SCHA). For the latter, two different formulations are considered, one applying near the isotropic limit and the other applying in the extremely anisotropic limit. Thus we find that the SCHA provides a reasonable description of superfluid properties of this system provided the appropriate type of formulation is employed. The accuracy of the SCHA in the extremely anisotropic limit, where the BKT transition temperature is tuned to zero (i.e. into a Quantum critical point) and therefore quantum fluctuations play a dominant role, is particularly striking.

cond-mat.quant-gas↗

arXiv2011

Thermalization and Quantum Correlations in Exactly Solvable Models

The generalized Gibbs ensemble introduced for describing few body correlations in exactly solvable systems following a quantum quench is related to the nonergodic way in which operators sample, in the limit of infinite time after the quench, the quantum correlations present in the initial state. The nonergodicity of the correlations is thus shown \emph{analytically} to imply the equivalence with the generalized Gibbs ensemble for quantum Ising and XX spin chains as well as for the Luttinger model the thermodynamic limit, and for a broad class of initial states and correlation functions of both local and nonlocal operators.

cond-mat.stat-mech↗

arXiv2011

Dynamical theory of superfluidity in one dimension

A theory accounting for the dynamical aspects of the superfluid response of one dimensional (1D) quantum fluids is reported. In long 1D systems the onset of superfluidity is related to the dynamical suppression of quantum phase slips at low temperatures. The effect of this suppression as a function of frequency and temperature is discussed within the framework of the relevant correlation function that is accessible experimentally, namely the momentum response function. Application of these results to the understanding of the superfluid properties of helium confined in nanometer-size pores, edge dislocations in solid $^4$He, and ultra-cold atomic gases is also briefly discussed.

cond-mat.stat-mech↗

arXiv2001

Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects

We report ab initio calculation of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

cond-mat↗