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A. S. T. Pires

Publications and source records attributed to A. S. T. Pires.

16 recordsLinked to original sources
arXiv2024

Hall transport in the topological non-Hermitian checkerboard lattice

The checkerboard lattice is a two-dimensional non-trivial structure usually seen as a planar version of the pyrochlore lattice. This geometry supports a two-band insulating electronic system with Chern topology induced by a complex hopping parameter. Inspired by the recent advances in the topology of non-Hermitian systems, in this work we study a non-Hermitian version of the topological checkerboard lattice. The complex band structure and Berry curvature are calculated. In the insulating phase, the Chern number is the same as in the Hermitian version, but the Hall conductivity is no longer quantized. The dependence of the Hall conductivity with the non-Hermitian parameters is investigated. The non-Hermiticity can be seen as a result of dissipation caused by coupling the system to the environment, so this study casts light on the topology of open systems in condensed matter physics.

cond-mat.mes-hall↗
arXiv2024

Magnon bands and transverse transport in a proposed two-dimensional $Cu_2F_5$ ferrimagnet

The copper fluoride $Cu_2F_5$ is a proposed stable compound that can be seen as a layered lattice of $S=1$ and $S=1/2$ sites, corresponding to copper ions. Intending to cast light on the transport properties of ferrimagnetic magnons, we use the linear spin wave approach to study the magnon band structure of the 2D lattice in a ferrimagnetic off-plane order, as well as the transverse transport of magnons in the crystal bulk. A magnetic field or temperature gradient can induce transverse (Hall-like) transport within the linear response theory generated by the Berry curvature of the eigenstates. As in most cases involving magnons, the Berry curvature is related to Dzyaloshinskii-Moriya interactions between next-near-neighbors. The band structure of the system is non-degenerate and the transport coefficients are non-null. The novel and interesting feature of the system is that, within certain conditions, the transport coefficients versus temperature curves are non-monotonic and present a sign change, opening the exciting possibility of controlling the transverse magnon flow direction and magnitude with the temperature change.

cond-mat.mtrl-sci↗
arXiv2023

Magnon Hall effect in antiferromagnetic lattices

Topology applied to condensed matter is an important area of research and technology, and topological magnetic excitations have recently become an active field of study. This paper presents a general discussion of magnon Hall transport in two-dimensional antiferromagnets. Although the Chern number is zero for a collinear antiferromagnet, we offer a general discussion that can be used in the more general case. First, we study the Union Jack lattice, where an effective time-reversal symmetry is broken, making the system display the magnon Hall effect. Then, we investigate the brick-wall lattice where such symmetry is present. Consequently, we have a phenomenon similar to the quantum spin Hall effect in electronic systems. Both lattices have not yet been studied from the topological point of view. The coexistence of opposite spin polarization in an antiferromagnet resembles the electron spin in various transport phenomena. We study magnon transport in the lattices mentioned above with Dzyaloshinskii-Moriya interaction and easy-axis single-ion anisotropy. We calculate the Berry curvature from the eigenvalues of the Hamiltonian. From that, we plot the spin Hall and thermal Hall conductivities, as well as the spin Nernst coefficient, as functions of the temperature. In the Union Jack lattice, we treat the effect of anharmonic interactions using a mean-field spin wave theory where the Hamiltonian becomes implicitly temperature-dependent. We determine self-consistently the renormalized dispersion and the staggered magnetization as a function of temperature. Our calculations can be applied to other antiferromagnetic lattices.

cond-mat.str-el↗
arXiv2023

Transversal transport of magnons in a modified Lieb lattice

We studied a two-band magnon insulating model whose geometry is that of a modified Lieb lattice in which one of the sites was removed. Anisotropic ferromagnetic exchange interactions exist between the three nearest neighbors, and the anisotropy opens a gap in the magnon energy band structure. A non-vanishing Berry curvature is induced by a Dzyaloshinskii-Moriya interaction (DMI). The topology of the bands is trivial (in the sense of a null Chern number), but the finite Berry curvature induces Hall-like transport effects whose coefficients were calculated. Their dependence on temperature was studied and shows a resemblance with other magnon insulating systems found in the literature. The dependence on exchange couplings, DMI parameter, and external magnetic field was also investigated.

cond-mat.mtrl-sci↗
arXiv2015

Spin-$1$ $J_1-J_2-J_3$ ferromagnetic Heisenberg model with an easy-plane crystal field on the cubic lattice: A bosonic approach

We examine the phase diagram of the spin-$1$ $J_1-J_2-J_3$ ferromagnetic Heisenberg model with an easy-plane crystal field on the cubic lattice, in which $J_1$ is the ferromagnetic exchange interaction between nearest neighbors, $J_2$ is the antiferromagnetic exchange interaction between next-nearest neighbors and $J_3$ is the antiferromagnetic exchange interaction between next-next-nearest neighbors. Using the bond-operator formalism, we investigate the phase transitions between the disordered paramagnetic phase and the ordered ones. We show that the nature of the quantum phase transitions change as the frustration parameters ($\frac{J_2}{J_1}$, $\frac{J_3}{J_1}$) are varied. The zero-temperature phase diagram exhibits second- and first-order transitions, depending on the energy gap behavior. Remarkably, we find a disordered nonmagnetic phase, even in the absence of a crystal field, which is suggested to be a quantum spin liquid candidate. We also depict the phase diagram at finite temperature for some values of crystal field and frustration parameters.

cond-mat.str-el↗
arXiv2011

Vortex core dynamics induced by hole defects in antiferromagnetic nanodisks

Direct observation of vortex states in an antiferromagnetic layer have been recently reported [Wu, et al, Nature Phys. 7, 303 (2011)]. In contrast to their analogues in ferromagnetic systems, namely in nanomagnets, the vortex core of antiferromagnets are not expected (and have not been observed) to present gyrotropic or any other remarkable dynamics, even when external fields are applied. Using simulated annealing and spin dynamics techniques we have been able to describe a number of properties of such a vortex state. Besides of being in agreement with reported results, our results also indicate, whenever applied to antiferromagnetic nanodisks, that the presence of holes in the sample may induce two types of motions for this vortex. Its dynamics depends upon the relative separation between its core and the hole: when they are very apart the vortex core oscillates near the nanodisk center (its equilibrium position); while, if they are sufficiently close, the core moves towards the hole where it is captured and remains static.

cond-mat.mtrl-sci↗
arXiv2010

Phase diagram of the antiferromagnetic XY model in two dimensions in a magnetic field

The phase diagram of the quasi-two-dimensional easy-plane antiferromagnetic model, with a magnetic field applied in the easy plane, is studied using the self-consistent harmonic approximation. We found a linear dependence of the transition temperature as a function of the field for large values of the field. Our results are in agreement with experimental data for the spin-1 honeycomb compound BaNi_2V_2O_3

cond-mat.stat-mech↗
arXiv2006

Free energy variational approach for the classical anisotropic XY model in a crystal field

A variational approach for the free energy is used to study the three-dimensional anisotropic XY model in the presence of a crystal field. The magnetization and the phase diagrams as a function of the parameters of the Hamiltonian are obtained. Some limiting results for isotropic XY and planar rotator models in two and three dimensions are analyzed and compared to previous results obtained from analytical approximations as well as from those obtained from more reliable approaches such as series expansion and Monte Carlo simulations. It is also shown that from this general variational approach some simple assumptions can drastically simplify the self-consistent implicit equations. The validity of the low temperature region of this approach is analyzed and compared to Monte Carlo results as well.

cond-mat.stat-mech↗
arXiv2006

Geometrical pinning of magnetic vortices induced by a deficit angle on a surface: anisotropic spins on a conic space background

We study magnetic vortex-like excitations lying on a conic space background. Two types of them are obtained. Their energies appear to be linearly dependent on the conical aperture parameter, besides of being logarithmically divergent with the sample size. In addition, we realize a geometrical-like pinning of the vortex, say, it is energetically favorable for it to nucleate around the conical apex. We also study the problem of two vortices on the cone and obtain an interesting effect on such a geometry: excitations of the same charge, then repealing each other, may nucleate around the apex for suitable cone apertures. We also pay attention to the problem of the vortex pair and how its dissociation temperature depends upon conical geometry.

cond-mat.mtrl-sci↗
arXiv2005

Spin Dynamics in the Two-Dimensional Spin 1/2 Heisenberg Antiferromagnet

We present low-temperature dynamic properties of the quantum two-dimensional antiferromagnetic Heisenberg model with spin S=1/2. The calculation of the dynamic correlation function is performed by combining a projection operator formalism and the modified spin-wave theory (MSW), which gives a gap in the dispersion relation for finite temperatures. The so calculated dynamic correlation function shows a double peak structure.We also obtain the spin-wave damping and compare our results to experimental data and to theoretical results obtained by other authors using different approaches.

cond-mat.str-el↗
arXiv2002

Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets

Using a model of nonmagnetic impurity potential, we have examined the behavior of planar vortex solutions in the classical two-dimensional XY ferromagnets in the presence of a spin vacancy localized out of the vortex core. Our results show that a spinless atom impurity gives rise to an effective potential that repels the vortex structure.

cond-mat.str-el↗
arXiv2000

Soliton dynamics in 1D quantum antiferromagnets

The problem of dissipative motion of solitons in the case of tetramethyl ammonium manganese chloride (TMMC) is studied as a function of the external magnetic field and the temperature. Two specific situations are analyzed separately; the first, above the transition temperature $T_N$, in which the classical motion of the spin degree of freedom is described by a sine-Gordon (SG) equation of motion and, the second, below $T_N$, in which the system is described by a double sine-Gordon (2SG) equation of motion. The existence of a dissipative regime for the soliton motion and its influence on the dynamical structure factor -which might be experimentally detected- are reported.

cond-mat↗
arXiv2000

A Model to Describe Transport Properties in $Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$

A pseudo-spin model is proposed, as a means to describe some transport properties (resistivity and Hall mobility) in $Bi_2Sr_2(Ca_zPr_{1-z})Cu_2O_{8+y}$. Our model is based in a double-well potential where tunneling in a given site and interaction between different lattice sites are allowed only through the excited states. Doping of the pure system by the addition of $Pr$ increases the ratio between the activation energy and the tunneling constant. The model Hamiltonian displays some features which are present in the hydrogen-bonded ferroelectrics. Its dynamics is treated in the random phase approximation and the characteristic frequency (time) is used in a Drude formula in order to obtain some transport properties of the system, namely the electric resistivity and the Hall mobility. The quantities calculated in this work are compared with the experimental data of B. Beschoten, S. Sadewasser, G. Güntherodt and C. Quitmann [Phys. Rev. Lett.77, 1837(1996)].

cond-mat.supr-con↗
arXiv1999

Spinwave damping in the two-dimensional ferromagnetic XY model

The effect of damping of spinwaves in a two-dimensional classical ferromagnetic XY model is considered. The damping rate $Γ_{q}$ is calculated using the leading diagrams due to the quartic-order deviations from the harmonic spin Hamiltonian. The resulting four-dimensional integrals are evaluated by extending the techniques developed by Gilat and others for spectral density types of integrals. $Γ_{q}$ is included into the memory function formalism due to Reiter and Solander, and Menezes, to determine the dynamic structure function $S(q,ω)$. For the infinite sized system, the memory function approach is found to give non-divergent spinwave peaks, and a smooth nonzero background intensity (``plateau'' or distributed intensity) for the whole range of frequencies below the spinwave peak. The background amplitude relative to the spinwave peak rises with temperature, and eventually becomes higher than the spinwave peak, where it appears as a central peak. For finite-sized systems, there are multiple sequences of weak peaks on both sides of the spinwave peaks whose number and positions depend on the system size and wavevector in integer units of $2π/L$. These dynamical finite size effects are explained in the memory function analysis as due to either spinwave difference processes below the spinwave peak or sum processes above the spinwave peak. These features are also found in classical Monte Carlo -- Spin-Dynamics simulations.

cond-mat.mtrl-sci↗
arXiv1999

Low Temperature Static and Dynamic Behavior of the Two-Dimensional Easy-Axis Heisenberg Model

We apply the self-consistent harmonic approximation (SCHA) to study static and dynamic properties of the two-dimensional classical Heisenberg model with easy-axis anisotropy. The static properties obtained are magnetization and spin wave energy as functions of temperature, and the critical temperature as a function of the easy-axis anisotropy. We also calculate the dynamic correlation functions using the SCHA renormalized spin wave energy. Our analytical results, for both static properties and dynamic correlation functions, are compared to numerical simulation data combining cluster-Monte Carlo algorithms and Spin Dynamics. The comparison allows us to conclude that far below the transition temperature, where the SCHA is valid, spin waves are responsible for all relevant features observed in the numerical simulation data; topological excitations do not seem to contribute appreciably. For temperatures closer to the transition temperature, there are differences between the dynamic correlation functions from SCHA theory and Spin Dynamics; these may be due to the presence of domain walls and solitons.

cond-mat↗