arXiv subjects

Jose A. Riera

Publications and source records attributed to Jose A. Riera.

13 recordsLinked to original sources

arXiv2018

Spin Hall effect in Rashba-Dresselhaus planar strips in the presence of electron correlations

A model with both Rashba and Dresselhaus spin orbit (SO) couplings and Hubbard electron-electron interaction is studied on planar strips at quarter filling at zero temperature in the clean limit. In the absence of Hubbard repulsion and at equilibrium, within linear response theory, a nonmonotonic behavior of the spin Hall conductivity as a function of the ratio of the Rashba (alpha) and Dresselhaus (beta) strengths was found for large enough SO strengths. This behavior is signalled by a peak or a cusp, depending on the strip width, at intermediate values of beta/alpha in the interval [0,1]. This behavior of the spin Hall conductivity was correlated with the one for the longitudinal spin conductivity. This study was then extended to the out-of-equilibrium regime that arises by imposing a finite voltage bias between the two ends of an open strip. This system, in the presence of a Hubbard term with coupling U, was treated with the density matrix renormalization group technique and with the Landauer-Buttiker formalism. It was found that relevant properties to the spin Hall effect, such as the transversal spin current and the spin accumulation, present a similar nonmonotonic behavior as the one found for the spin conductivities. More importantly, it was also found that these properties are enhanced by the repulsive Hubbard interaction up to a moderate value of U.

cond-mat.str-el↗

arXiv2017

Transport properties of Rashba conducting strips coupled to magnetic moments with spiral order

Magnetic and transport properties of a conducting layer with Rashba spin-orbit coupling (RSOC) magnetically coupled to a layer of localized magnetic moments are studied on strips of varying width. The localized moments are free to rotate and they acquire an order that results from the competition between the magnetic exchange energy and the kinetic energy of the conduction electrons. By minimizing the total Hamiltonian within the manifold of variational spiral orders of the magnetic moments, the phase diagram in the space of the interlayer exchange J_{sd}, and the ratio of the Rashba coupling to the hopping integral, lambda/t was determined. Two main phases with longitudinal spiral order were found, one at large interlayer coupling J_{sd} with uniform order in the transversal direction, and the other at small J_{sd} showing a transversal staggered order. This staggered spiral order is unstable against an antiferromagnetic (AFM) for large values of lambda/t. In both spiral phases, the longitudinal spiral momentum that departs from the expected linear dependence with the RSOC for large values of lambda/t. Then, various transport properties, including the longitudinal Drude weight and the spin Hall conductivity, inside these two phases are computed in linear response, and their behavior is compared with the ones for the more well-studied cases of a fixed ferromagnetic (FM) and AFM localized magnetic orders.

cond-mat.str-el↗

arXiv2017

Longitudinal optical and spin-Hall conductivities of Rashba conducting strips coupled to ferro- and antiferromagnetic layers

A system composed of a conducting planar strip with Rashba spin-orbit coupling (RSOC), magnetically coupled to a layer of localized magnetic moments, at equilibrium, is studied within a microscopic Hamiltonian with numerical techniques at zero temperature in the clean limit. In particular, transport properties for the cases of ferromagnetic (FM) and antiferromagnetic (AFM) coupled layers are computed in linear response on strips of varying width. Some behaviors observed for these properties are consistent with the ones observed for the corresponding Rashba helical currents. The case of uncoupled Rashba strips is also studied for comparison. In the case of Rashba strips coupled to an AFM localized order, results for the longitudinal dc conductivity, for small strip widths, suggest the proximity to a metal-insulator transition. More interesting, in the proximity of this transition, and in general at intermediate values of the RSOC, it is observed a large spin-Hall conductivity that is two orders of magnitude larger than the one for the FM order for the same values of the RSOC and strip widths. There are clearly two different regimes for small and for large RSOC, which is also present in the behavior of Rashba helical currents. Different contributions to the optical and the spin-Hall conductivities, according to a new classification of inter- or intra-band origin proposed for planar strips in the clean limit, or coming from the hopping or spin-orbit terms of the Hamiltonian, are examined. Finally, the effects of different orientation of the coupled magnetic moments will be also studied.

cond-mat.str-el↗

arXiv2016

Helical currents in metallic Rashba strips

We study the texture of helical currents in metallic planar strips in the presence of Rashba spin-orbit coupling (RSOC) on the lattice at zero temperature. In the noninteracting case, and in the absence of external electromagnetic sources, we determine by exact numerical diagonalization of the single-particle Hamiltonian, the distribution across the strip section of these Rashba helical currents (RHC) as well as their sign oscillation, as a function of the RSOC strength, strip width, electron filling, and strip boundary conditions. Then, we study the effects of charge currents introduced into the system by an Aharonov-Bohm flux for the case of rings or by a voltage bias in the case of open strips. The former setup is studied by variational Monte Carlo, and the later, by the time-dependent density-matrix-renormalization group technique. Particularly for strips formed by two, three and four coupled chains, we show how these RHC vary in the presence of such induced charge current, and how their differences between spin-up and spin-down electron currents on each chain, help to explain the distribution across the strip of charge currents, both of the spin conserving and the spin flipping types. We also predict the appearance of polarized charge currents on each chain. Finally, we show that these Rashba helical currents and their derived features remain in the presence of an on-site Hubbard repulsion as long as the system remains metallic, at quarter filling, and even at half-filling where a Mott-Hubbard metal-insulator transition occurs for large Hubbard repulsion.

cond-mat.str-el↗

arXiv2014

Magnetic and transport signatures of Rashba spin-orbit coupling on the ferromagnetic Kondo lattice model in two dimensions

Motivated by emergent phenomena at oxide surfaces and interfaces, particularly those involving transition metal oxides with perovskite crystal structure such as LaTiO3/SrTiO3, we examine the ferromagnetic Kondo lattice model (FKLM) in the presence of a Rashba spin-orbit coupling (RSOC). Using numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin and transport properties on square clusters at zero temperature. We find that the main effect of the RSOC is the destruction of the ferromagnetic state present in the FKLM at low electron fillings, with the consequent suppression of conductivity. In addition, near half-filling the RSOC leads to a departure of the antiferromagnetic state of the FKLM with a consequent reduction to the intrinsic tendency to electronic phase separation. The interplay between phase separation on one side, and magnetic and transport properties on the other, is carefully analyzed as a function of the RSOC/hopping ratio.

cond-mat.str-el↗

arXiv2003

Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature

The t-J model on the square lattice, close to the t-J_z limit, is studied by quantum Monte Carlo techniques at finite temperature and in the underdoped regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify the antiferromagnetic domains on each Trotter slice. The results show that the model presents at high enough temperature finite antiferromagnetic (AF) domains which collapse at lower temperatures into a single ordered AF state. While there are domains, holes would tend to preferentially move along the domain walls. In this case, there are indications of hole pairing starting at a relatively high temperature. At lower temperatures, when the whole system becomes essentially fully AF ordered, at least in finite clusters, holes would likely tend to move within phase separated regions. The crossover between both states moves down in temperature as doping increases and/or as the off-diagonal exchange increases. The possibility of hole motion along AF domain walls at zero temperature in the fully isotropic t-J is discussed.

cond-mat.str-el↗

arXiv2002

Phase separation in effective hard-core boson and triplet models in one and two dimensions

Effective models of hard-core hole pair bosons and triplets are derived from the t-J model in ladders and on the square lattice by performing a change from site to dimer basis. The Hilbert space is truncated by projecting out single-occupied electron states and only nearest neighbor interactions are retained. The resulting effective models in one and two dimensions are studied by numerical techniques. In both spatial dimensions, the main result is that each hole pair is surrounded by a singlet cloud expelling triplet excitations from its vicinity. It is suggested an interpretation of this feature as a phase separated state between a hole-pair rich singlet phase and an undoped triplet phase with antiferromagnetic correlations. The possible relevance of this result to other theoretical scenarios and experimental results is discussed.

cond-mat.supr-con↗

arXiv2002

Superconductivity and antiferromagnetism in a hard-core boson spin-1 model in two dimensions

A model of hard-core bosons and spin-1 sites with single-ion anisotropy is proposed to approximately describe hole pairs moving in a background of singlets and triplets with the aim of exploring the relationship between superconductivity and antiferromagnetism. The properties of this model at zero temperature were investigated using quantum Monte Carlo techniques. The most important feature found is the suppression of superconductivity, as long range coherence of preformed pairs, due to the presence of both antiferromagnetism and $S^z=\pm 1$ excitations. Indications of charge ordered and other phases are also discussed.

cond-mat.supr-con↗

arXiv2001

Charge and spin ordering, and charge transport properties in a two-dimensional inhomogeneous t-J model

We study a two-dimensional t-J model close to the Ising limit in which charge inhomogeneity is stabilized by an on-site potential e_s, by using diagonalization in a restricted Hilbert space and finite temperature Quantum Monte Carlo. Both site and bond centered stripes are considered and their similitudes and differences are analyzed. The amplitude of charge inhomogeneity is studied as e_s -> 0. Moreover, we show that the anti-phase domain ordering occurs at a much lower temperature than the formation of charge inhomogeneities and charge localization. Hole-hole correlations indicate a metallic behavior of the stripes with no signs of hole attraction. Kinetic energies and current susceptibilities are computed and indications of charge localization are discussed. The study of the doping dependence in the range 0.083 < x < 0.167 suggests that these features are characteristic of the whole underdoped region.

cond-mat.supr-con↗

arXiv2001

Quantum Monte Carlo simulations of the t-Jz model with stripes on the square lattice

Finite temperature quantum Monte Carlo simulations are performed on the anisotropic t-J model and in particular on its Ising limit. Straight site-centered stripes are imposed by an on-site potential representing external mechanisms of stripe formation. In this model, we show that, even though charge inhomogeneity exists at a high temperature, the anti-phase ordering of the spin domains between stripes occurs at a much lower temperature. The magnetic correlations at this spin ordering crossover are analyzed. The stripes show metallicity, with absence of hole attraction. Comparison between this model and others that have been proposed to explain or describe stripes, as well as possible relations with experimental features on underdoped cuprates are discussed.

cond-mat.supr-con↗

arXiv2001

Effects of interladder couplings in the trellis lattice

Strongly correlated models on coupled ladders in the presence of frustration, in particular the trellis lattice, are studied by numerical techniques. For the undoped case, the possibility of incommensurate peaks in the magnetic structure factor at low temperatures is suggested. In the doped case, our main conclusion for the trellis lattice is that by increasing the interladder coupling, the balance between the magnetic energy in the ladders and the kinetic energy in the zig-zag chains is altered leading eventually to the destruction of the hole pairs initially formed and localized in the ladders.

cond-mat.str-el↗

arXiv1994

Optical conductivity of the Hubbard model at finite temperature

The optical conductivity, $σ(ω)$, of the two dimensional one-band Hubbard model is calculated at finite temperature using exact diagonalization techniques on finite clusters. The in-plane d.c. resistivity, $ρ_{ab}$, is also evaluated. We find that at large U/t and temperature T, $ρ_{ab}$ is approximately linear with temperature, in reasonable agreement with experiments on high-T$_c$ superconductors. Moreover, we note that $σ(ω)$ displays charge excitations, a mid-infrared (MIR) band and a Drude peak, also as observed experimentally. The combination of the Drude peak and the MIR oscillator strengths leads to a conductivity that decays slower than $1/ω^2$ at energies smaller than the insulator gap near half-filling.

cond-mat↗

arXiv1993

Phase separation and pairing in coupled chains and planes

A generalization of the $t-J$ model in a system of two coupled chains or planes is studied by numerical diagonalization of small clusters. In particular, the effect of density fluctuations between these one- or two-dimensional coupled layerson intralayer phase separation and pairing is analyzed. The most robust signals of superconductivity are found at quarter filling for couplings just before the fully interlayer phase separated regime. The possibility of an enhancement of the intralayer superconducting pairing correlations by the interlayer couplings is investigated.

cond-mat↗