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Victor Laliena

Publications and source records attributed to Victor Laliena.

At least 19 recordsLinked to original sources
arXiv2026

Helicity in monoaxial chiral magnets

The equilibrium state of a monoaxial chiral magnet surrounded by a non-magnetic medium (such as air or vacuum) is a helical texture characterized by a single, well-defined wave vector. No metastable states have ever been observed in such systems. Recently, however, it was demonstrated that when a chiral magnet is in close contact with two uniaxial ferromagnets, a large number of metastable helical states emerge in addition to the equilibrium state [Phys. Rev. B 109, 214424]. These helical states are distinguished by their wave number (helicity). In the present work, we elucidate the topological origin of the stabilization of these states --a mechanism we term dynamical topological protection-- and investigate their static and dynamic properties. We find that, as a consequence of this dynamical topological protection, the winding number of the helical states remains constant under the application of sufficiently weak magnetic fields and polarized electric currents. Furthermore, when a polarized current is applied to a metastable helical state, a static configuration is reached. This state retains the original winding number, but its winding number density becomes non-homogeneously distributed, concentrating near the interface with one of the ferromagnets. The dynamic response to sufficiently large magnetic fields and currents provides mechanisms to switch between different helical states. Since the magnetic properties depend on helicity, these metastable helical states are highly promising for applications in spintronics and magnonics.

cond-mat.mes-hall↗
arXiv2023

Continuum of Metastable Helical States in Monoaxial Chiral Magnets: Effect of the Boundary Conditions

In a recent publication we showed that a monoaxial chiral magnet has a continuum of metastable helical states differing by the helix wave number. This intringuing result was obtained for the case of an infinite magnet (or of a magnet with periodic boundary conditions). However, it has been pointed out that in a real magnet only one of these states is compatible with the boundary conditions, because the helix wave number is determined by the surface chiral twist. Thus, only one of the continuum of states is physically realizable. This is true for the case of a chiral magnet in contact with a non magnetic medium (vacuum or air, for instance), but the boundary conditions can be altered by setting the chiral magnet in contact with another magnetic medium, which may be able to absorb the surface chiral twist. We show here that this is indeed the case by studying a composite magnet system, which consists of one monoaxial chiral magnet of rectangular parallelepiped shape which has two similar uniaxial ferromagnets attached to each of the faces that are perpendicular to the chiral axis. We show that, in the case of zero applied field, this composite system has a number of metastable helical states which is proportional to the length L0 of the chiral magnet along the chiral axis, and that the results of our previous publication are recovered when L0 tends to infinity.

cond-mat.mes-hall↗
arXiv2022

Scattering of spin waves by a Bloch domain wall: effect of the dipolar interaction

It is known that a Bloch domain wall in an anisotropic ferromagnet is transparent to spin waves. This result is derived by approximating the dipolar interaction between magnetic moments by an effective anisotropy interaction. In this paper we study the the scattering of spin waves by a domain wall taking into account the full complexity of the dipolar interaction, treating it perturbatively in the distorted wave Born approximation. Due to the peculiarities of the dipolar interaction, the implementation of this approximation is not straightforward. The difficulties are circumvented here by realizing that the contribution of the dipolar interaction to the spin wave operator can be split into two terms: i) an operator that commutes with the spin wave operator in absence of dipolar interaction, and ii) a local operator suitable to be treated as a perturbation in the distorted wave Born approximaton. We analyze the scattering parameters obtained within this approach. It turns out that the reflection coefficient does not vanish in general, and that the transmitted waves suffer a lateral shift even at normal incidence. This lateral shift can be greatlty enhanced by making the spin wave go through an array of well separated domain walls. The outgoing spin wave will no be attenuated by the scattering at the domain walls since the reflection coefficient vanishes at normal incidence. This effect may be very useful to control the spin waves in magnonic devices.

cond-mat.mes-hall↗
arXiv2020

Magnonic Goos-Hanchen effect induced by one dimensional solitons

The magnon spectral problem is solved in terms of the spectrum of a diagonalizable operator for a generic class of magnetic states that includes several types of domain walls and the chiral solitons of monoaxial helimagnets. Focusing on the isolated solitons of monoaxial helimagnets, it is shown that the spin waves scattered (reflected and transmitted) by the soliton suffer a lateral displacement analogous to the Goos-Hanchen effect of optics. The displacement is a fraction of the wavelength, but can be greatly enhanced by using an array of well separated solitons. Contrarily to the Goos-Hanchen effect recently studied in some magnetic systems, which takes place at interfaces between different magnetic systems, the effect predicted here takes place at the soliton position, what it is interesting from the point of view of applications since solitons can be created at different places and moved across the material. This kind of Goos-Hanchen effect is not particular of monoaxial helimagnets, but it is generic of a class of magnetic states, including domain walls in systems with interfacial Dzyaloshinskii-Moriya interaction.

cond-mat.str-el↗
arXiv2020

Dynamics of chiral solitons driven by polarized currents in monoaxial helimagnets

Chiral solitons are one dimensional localized magnetic structures that are metastable in some ferromagnetic systems with Dzyaloshinskii-Moriya interactions and/or uniaxial magnetic anisotropy. Though topological textures in general provide a very interesting playground for new spintronics phenomena, how to properly create and control single chiral solitons is still unclear. We show here that chiral solitons in monoaxial helimagnets, characterized by a uniaxial Dzyaloshinskii-Moriya interaction, can be stabilized with external magnetic fields. Once created, the soliton moves steadily in response to a polarized electric current, provided the induced spin-transfer torque has a dissipative (nonadiabatic) component. The structure of the soliton depends on the applied current density in such a way that steady motion exists only if the applied current density is lower than a critical value, beyond which the soliton is no longer stable.

cond-mat.str-el↗
arXiv2018

New approximation to compute the incoherent scattering function of harmonic lattices

A new method to compute the incoherent scattering function of harmonic lattices is introduced. It is based in a saddle point approximation for each term of the phonon expansion, and is simple enough to be used in practice. The method gives very accurate results even for the tails of the scattering function, and is more accurate than the usual gaussian approximation, which can be derived from this saddle point approximation in the limit in which the order of the phonon expansion term becomes large. Numerical comparisons are provided using vanadium as a test case.

cond-mat.other↗
arXiv2018

Stability of the skyrmion lattice near the critical temperature in cubic helimagnets

The phase diagram of cubic helimagnets near the critical temperature is obtained from a Landau-Ginzburg model, including fluctuations to gaussian level. The free energy is evaluated via a saddle point expansion around the local minima of the Landau-Ginzburg functional. The local minima are computed by solving the Euler-Lagrange equations with appropriate boundary conditions, preserving manifestly the full nonlinearity that is characteristic of skyrmion states. It is shown that the fluctuations stabilize the skyrmion lattice in a region of the phase diagram close to the critical temperature, where it becomes the equilibrium state. A comparison of this approach with previous computations performed with a different approach (truncated Fourier expansion of magnetic states) is given.

cond-mat.str-el↗
arXiv2018

An improved discretization of Schrodinger-like radial equations

A new discretization of the radial equations that appear in the solution of separable second order partial differential equations with some rotational symmetry (as the Schrodinger equation in a central potential) is presented. It cures a pathology, related to the singular behaviour of the radial function at the origin, that suffers in some cases the discretization of the second derivative with respect to the radial coordinate. This pathology causes an enormous slowing down of the convergence to the continuum limit when the two point boundary value problem posed by the radial equation is solved as a discrete matrix eigenvalue problem. The proposed discretization is a simple solution to that problem. Some illustrative examples are discussed.

physics.comp-ph↗
arXiv2018

Thermal fluctuations in the conical state of monoaxial helimagnets

The effect of thermal fluctuations on the phase structure of monoaxial helimagnets with external magnetic field parallel to the chiral axis is analyzed by means of a saddle point expansion of the free energy. The phase transition that separates the conical and forced ferromagnetic phases is changed to first order by the thermal fluctuations. In a purely monoaxial system the pitch of the conical state remains independent of temperature and magnetic field, as in mean field theory, even when fluctuations are taken into account. However, in presence of weak Dzyaloshinskii-Moriya interactions in the plane perpendicular to the chiral axis, thermal fluctuations induce a dependence of the pitch on temperature and magnetic field. This may serve to determine the nature of magnetic interactions in such systems.

cond-mat.str-el↗
arXiv2017

Stability of skyrmion textures and the role of thermal fluctuations in cubic helimagnets: a new intermediate phase at low temperature

The stability of the four known stationary points of the cubic helimagnet energy functional: the ferromagnetic state, the conical helix, the conical helicoid, and the skyrmion lattice, is studied by solving the corresponding spectral problem. The only stable points are the ferromagnetic state at high magnetic field and the conical helix at low field, and there is no metastable state. Thermal fluctuations around the stationary point, included to quadratic order in the saddle point expansion, destabilize the conical helix in a region where the ferromagnetic state is unstable. Thus, a new intermediate phase appears which, in a region of the phase diagram, is a skyrmion lattice stabilized by thermal fluctuations. The skyrmion lattice lost the stability by lowering temperature, and a new intermediate phase of unknown type, presumably with three dimensional modulations, appears in the lower temperature region.

cond-mat.str-el↗
arXiv2016

Nucleation, instability, and discontinuous phase transitions in monoaxial helimagnets with oblique fields

The phase diagram of the monoaxial chiral helimagnet as a function of temperature (T ) and magnetic field with components perpendicular (H x ) and parallel (H z ) to the chiral axis is theoretically studied via the variational mean field approach in the continuum limit. A phase transition surface in the three dimensional thermodynamic space separates a chiral spatially modulated phase from a homogeneous forced ferromagnetic phase. The phase boundary is divided into three parts: two surfaces of second order transitions of instability and nucleation type, in De Gennes terminology, are separated by a surface of first order transitions. Two lines of tricritical points separate the first order surface from the second order surfaces. The divergence of the period of the modulated state on the nucleation transition surface has the logarithmic behavior typical of a chiral soliton lattice. The specific heat diverges on the nucleation surface as a power law with logarithmic corrections, while it shows a finite discontinuity on the other two surfaces. The soliton density curves are described by a universal function of H x if the values of T and H z determine a transition point lying on the nucleation surface; otherwise, they are not universal.

cond-mat.str-el↗
arXiv2016

Understanding the H-T phase diagram of the mono-axial helimagnet

Some unexpected features of the phase diagram of the monoaxial helimagnet in presence of an applied magnetic field perpendicular to the chiral axis are theoretically predicted. A rather general hamiltonian with long range Heisenberg exchange and Dzyaloshinskii--Moriya interactions is considered. The continuum limit simplifies the free energy, which contains only a few parameters which in principle are determined by the many parameters of the hamiltonian, although in practice they may be tuned to fit the experiments. The phase diagram contains a Chiral Soliton Lattice phase and a forced ferromagnetic phase separated by a line of phase transitions, which are of second order at low T and of first order in the vicinity of the zero-field ordering temperature, and are separated by a tricritical point. A highly non linear Chiral Soliton Lattice, in which many harmonics contribute appreciably to the spatial modulation of the local magnetic moment, develops only below the tricritical temperature, and in this case the scaling shows a logarithmic behaviour similar to that at T=0, which is a universal feature of the Chiral Soliton Lattice. Below the tricritical temperature, the normalized soliton density curves are found to be independent of T, in agreement with the experimental results of magnetorresistance curves, while above the tricritical temperature they show a noticeable temperature dependence. The implications in the interpretation of experimental results of CrNb3S6 are discussed.

cond-mat.str-el↗
arXiv2016

Incommensurate--commensurate transitions in the mono-axial chiral helimagnet driven by the magnetic field

The zero temperature phase diagram of the mono-axial chiral helimagnet in the magnetic field plane formed by the components parallel and perpendicular to the helical axis is thoroughly analyzed. The nature of the transition to the commensurate state depends on the angle between the field and the helical axis. For field directions close to the directions parallel or perpendicular to the helical axis the transition is continuous, while for intermediate angles the transition is discontinuous and the incommensurate and commensurate states coexist on the transition line. The continuous and discontinuous transition lines are separated by two tricritical points with specific singular behaviour. The location of the continuous and discontinuous lines and of the tricritical points depend strongly on the easy-plane anisotropy, the effect of which is analyzed. For large anisotropy the conical approximation locates the transition line very accurately, although it does not predict the continuous transitions nor the tricitical behaviour. It is shown that for large anisotropy, as in CrNb3S6, the form of the transition line is universal, that is, independent of the sample, and obeys a simple equation. The position of the tricritical points, which is not universal, is theoretically estimated for a sample of CrNb3S6

cond-mat.str-el↗
arXiv2007

Composite boson dominance in relativistic field theories

We apply a new bosonization technique to relativistic field theories of fermions whose partition function is dominated by bosonic composites, and derive the effective action for these bosons. The derivation respects all symmetries, including gauge invariance, with the exception of Euclidean invariance which must be checked a posteriori. We use a lattice regularization which should make applications to gauge theories easier. We test the method on a fermion field theory with quartic interaction in the limit when the number of flavours N_f is large, and show that it reproduces the exact results in the bosonic sector, namely condensation of a compositeboson with the right mass which breaks the discrete chiral invariance of the model. Moreover we determine the structure function of the condensed composite, whose spatial part turns out to be identical to that of the Cooper pairs of the BCS model of superconductivity.

hep-lat↗
arXiv2005

Phase diagram of QCD with four quark flavors at finite temperature and baryon density

We analyze the phase diagram of QCD with four staggered flavors in the (mu, T) plane using a method recently proposed by us. We explore the region T > 0.7 Tc and mu <1.4 Tc, where Tc is the transition temperature at zero baryon density, and find a first order transition line. Our results are quantitatively compatible with those obtained with the imaginary chemical potential approach and the double reweighting method, in the region where these approaches are reliable, T > 0.9 Tc and mu < Tc. But, in addition, our method allows us to extend the transition line to lower temperatures and higher chemical potentials.

hep-lat↗
arXiv2005

Phase structure of four flavor QCD in the T μplane from a new method for simulations of lattice gauge theories at non zero baryon density

We review a method for numerical simulations of lattice gauge theories at non-zero baryonic chemical potential we recently proposed. We first report on a test of the method using a solvable model and then present results for the phase structure of four flavour QCD. For the first time the region of chemical potential up to 1.4 T_C is explored, finding a first order transition line.

hep-lat↗