arXiv subjects

Daniel G. Barci

Publications and source records attributed to Daniel G. Barci.

At least 37 records · Page 2Linked to original sources

arXiv2018

Quantum corrections for the phase diagram of systems with competing order

We use the effective potential method of quantum field theory to obtain the quantum corrections to the zero temperature phase diagram of systems with competing order parameters. We are particularly interested in two different scenarios: regions of the phase diagram where there is a bicritical point, at which both phases vanish continuously, and the case where both phases coexist homogeneously. We consider different types of couplings between the order parameters, including a bilinear one. This kind of coupling breaks time-reversal symmetry and it is only allowed if both order parameters transform according to the same irreducible representation. This occurs in many physical systems of actual interest like competing spin density waves, different types of orbital antiferromagnetism, elastic instabilities of crystal lattices, vortices in a multigap SC and also applies to describe the unusual magnetism of the heavy fermion compound URu2Si2. Our results show that quantum corrections have an important effect on the phase diagram of systems with competing orders.

cond-mat.str-el↗

arXiv2017

Quantum and thermal melting of stripe forming systems with competing long ranged interactions

We study the quantum melting of stripe phases in models with competing short range and long range interactions decaying with distance as $1/r^σ$ in two space dimensions. At zero temperature we find a two step disordering of the stripe phases with the growth of quantum fluctuations. A quantum critical point separating a phase with long range positional order from a phase with long range orientational order is found when $σ\leq 4/3$, which includes the Coulomb interaction case $σ=1$. For $σ> 4/3$ the transition is first order, which includes the dipolar case $σ=3$. Another quantum critical point separates the orientationally ordered (nematic) phase from a quantum disordered phase for any value of $σ$. Critical exponents as a function of $σ$ are computed at one loop order in an $ε$ expansion and, whenever available, compared with known results. For finite temperatures it is found that for $σ\geq 2$ orientational order decays algebraically with distance until a critical Kosterlitz-Thouless line. Nevertheless, for $σ< 2$ it is found that long range orientational order can exist at finite temperatures until a critical line which terminates at the quantum critical point at $T=0$. The temperature dependence of the critical line near the quantum critical point is determined as a function of $σ$.

cond-mat.stat-mech↗

arXiv2016

Vortex and disclination structures in a nematic-superconductor state

The nematic-superconductor state is an example of a quantum liquid crystal that breaks gauge as well as rotation invariance. It was conjectured to exist in the pseudogap regime of the cuprates high $T_c$ superconductors. The nematic-superconductor state is characterized by two complex order parameters: one of them is related with superconductivity and the other one describes a nematic order. It supports two main classes of topological defects: half-vortices and disclinations. In this paper we present a Ginzburg-Landau approach to study the structure of these topological defects. Due to a geometrical coupling between the superconductor and the nematic order parameters, we show that vortices are strongly coupled with disclinations. We have found a restoring force between vortices and disclinations that produces harmonic excitations whose natural frequency depends on the geometrical coupling constant and the superconductor condensation energy. Moreover, in a regime with high density of defects, we have found a structural phase transition between vortex-disclination lattices with different symmetries.

cond-mat.supr-con↗

arXiv2016

Dynamical symmetries of Markov processes with multiplicative white noise

We analyse various properties of stochastic Markov processes with multiplicative white noise. We take a single-variable problem as a simple example, and we later extend the analysis to the Landau-Lifshitz-Gilbert equation for the stochastic dynamics of a magnetic moment. In particular, we focus on the non-equilibrium transfer of angular momentum to the magnetization from a spin-polarised current of electrons, a technique which is widely used in the context of spintronics to manipulate magnetic moments. We unveil two hidden dynamical symmetries of the generating functionals of these Markovian multiplicative white-noise processes. One symmetry only holds in equilibrium and we use it to prove generic relations such as the fluctuation-dissipation theorems. Out of equilibrium, we take profit of the symmetry-breaking terms to prove fluctuation theorems. The other symmetry yields strong dynamical relations between correlation and response functions which can notably simplify the numerical analysis of these problems. Our construction allows us to clarify some misconceptions on multiplicative white-noise stochastic processes that can be found in the literature. In particular, we show that a first-order differential equation with multiplicative white noise can be transformed into an additive-noise equation, but that the latter keeps a non-trivial memory of the discretisation prescription used to define the former.

cond-mat.stat-mech↗

arXiv2016

Path Integral approach to nonequilibrium potentials in multiplicative Langevin dynamics

We present a path integral formalism to compute potentials for nonequilibrium steady states, reached by a multiplicative stochastic dynamics. We develop a weak-noise expansion, which allows the explicit evaluation of the potential in arbitrary dimensions and for any stochastic prescription. We apply this general formalism to study noise-induced phase transitions. We focus on a class of multiplicative stochastic lattice models and compute the steady state phase diagram in terms of the noise intensity and the lattice coupling. We obtain, under appropriate conditions, an ordered phase induced by noise. By computing entropy production, we show that microscopic irreversibility is a necessary condition to develop noise-induced phase transitions. This property of the nonequilibrium stationary state has no relation with the initial stages of the dynamical evolution, in contrast with previous interpretations, based on the short-time evolution of the order parameter.

cond-mat.stat-mech↗

arXiv2016

Bosonic binary mixtures with Josephson-type interactions

Motivated by experiments in bosonic mixtures composed of a single element in two different hyperfine states, we study bosonic binary mixtures in the presence of Josephson interactions between species. We focus on a particular model with $O(2)$ isospin symmetry, lifted by an imbalanced population parametrized by a Rabi frequency, $Ω_{R}$, and a detuning, $ν$, which couples the phases of both species. We have studied the model at mean-field approximation plus Gaussian fluctuations. We have found that both species simultaneously condensate below a critical temperature $T_{c}$ and the relative phases are locked by the applied laser phase, $α$. Moreover, the condensate fractions are strongly dependent on the ratio $Ω_{R}/|ν|$ that is not affected by thermal fluctuations.

cond-mat.quant-gas↗

arXiv2015

Langevin dynamics for vector variables driven by multiplicative white noise: a functional formalism

We discuss general multi-dimensional stochastic processes driven by a system of Langevin equations with multiplicative white noise. In particular, we address the problem of how time reversal diffusion processes are affected by the variety of conventions available to deal with stochastic integrals. We present a functional formalism to built up the generating functional of correlation functions without any type of discretization of the Langevin equations at any intermediate step. The generating functional is characterized by a functional integration over two sets of commuting variables as well as Grassmann variables. In this representation, time reversal transformation became a linear transformation in the extended variables, simplifying in this way the complexity introduced by the mixture of prescriptions and the associated calculus rules. The stochastic calculus is codified in our formalism in the structure of the Grassmann algebra. We study some examples such as higher order derivatives Langevin equations and the functional representation of the micromagnetic stochastic Landau-Lifshitz-Gilbert equation.

cond-mat.stat-mech↗

arXiv2014

Magnetization dynamics: path-integral formalism for the stochastic Landau-Lifshitz-Gilbert equation

We construct a path-integral representation of the generating functional for the dissipative dynamics of a classical magnetic moment as described by the stochastic generalization of the Landau-Lifshitz-Gilbert equation proposed by Brown, with the possible addition of spin-torque terms. In the process of constructing this functional in the Cartesian coordinate system, we critically revisit this stochastic equation. We present it in a form that accommodates for any discretization scheme thanks to the inclusion of a drift term. The generalized equation ensures the conservation of the magnetization modulus and the approach to the Gibbs-Boltzmann equilibrium in the absence of non-potential and time-dependent forces. The drift term vanishes only if the mid-point Stratonovich prescription is used. We next reset the problem in the more natural spherical coordinate system. We show that the noise transforms non-trivially to spherical coordinates acquiring a non-vanishing mean value in this coordinate system, a fact that has been often overlooked in the literature. We next construct the generating functional formalism in this system of coordinates for any discretization prescription. The functional formalism in Cartesian or spherical coordinates should serve as a starting point to study different aspects of the out-of-equilibrium dynamics of magnets. Extensions to colored noise, micro-magnetism and disordered problems are straightforward.

cond-mat.stat-mech↗

arXiv2014

Nonlinear inhomogeneous Fokker-Planck equation within a generalized Stratonovich prescription

We deduce a nonlinear and inhomogeneous Fokker-Planck equation within a generalized Stratonovich, or stochastic $α$-, prescription ($α=0$, $1/2$ and $1$ respectively correspond to the Itô, Stratonovich and anti-Itô prescriptions). We obtain its stationary state $p_{st}(x)$ for a class of constitutive relations between drift and diffusion and show that it has a $q$-exponential form, $p_{st}(x) = N_q[1 - (1-q)βV(x)]^{1/(1-q)}$, with an index $q$ which does not depend on $α$ in the presence of any nonvanishing nonlinearity. This is in contrast with the linear case, for which the index $q$ is $α$-dependent.

cond-mat.stat-mech↗

arXiv2013

The nematic phase in stripe forming systems within the self consistent screening approximation

We show that in order to describe the isotropic-nematic transition in stripe forming systems with isotropic competing interactions of the Brazovskii class it is necessary to consider the next to leading order in a 1/N approximation for the effective Hamiltonian. This can be conveniently accomplished within the self-consistent screening approximation. We solve the relevant equations and show that the self-energy in this approximation is able to generate the essential wave vector dependence to account for the anisotropic character of two-point correlation function characteristic of a nematic phase.

cond-mat.str-el↗

arXiv2013

Nematic Phase in two-dimensional frustrated systems with power law decaying interactions

We address the problem of orientational order in frustrated interaction systems as a function of the relative range of the competing interactions. We study a spin model Hamiltonian with short range ferromagnetic interaction competing with an antiferromagnetic component that decays as a power law of the distance between spins, $1/r^α$. These systems may develop a nematic phase between the isotropic disordered and stripe phases. We evaluate the nematic order parameter using a self-consistent mean field calculation. Our main result indicates that the nematic phase exists, at mean-field level, provided $0<α<4$. We analytically compute the nematic critical temperature and show that it increases with the range of the interaction, reaching its maximum near $α\sim 0.5$. We also compute a corse-grained effective Hamiltonian for long wave-length fluctuations. For $0<α<4$ the inverse susceptibility develops a set of continuous minima at wave vectors $|\vec k|=k_0(α)$ which dictate the long distance physics of the system. For $α\to 4$, $k_0\to 0$, making the competition between interactions ineffective for greater values of $α$.

cond-mat.stat-mech↗

arXiv2013

Glassy behavior of two-dimensional stripe-forming systems

We study two dimensional frustrated but non-disordered systems applying a replica approach to a stripe forming model with competing interactions. The phenomenology of the model is representative of several well known systems, like high-Tc superconductors and ultrathin ferromagnetic films, which have been the subject of intense research. We establish the existence of a glass transition to a non-ergodic regime accompanied by an exponential number of long lived metastable states, responsible for slow dynamics and non-equilibrium effects.

cond-mat.stat-mech↗

arXiv2013

Superconductivity near Pomeranchuk instabilities in the spin channel

We study the competition between a Pomeranchuk instability in the spin channel with angular momentum $\ell=1$ and an attractive interaction, favoring Cooper-pair formation. We found that the superconducting gap strongly suppresses the phase space for the Pomeranchuk instability. We computed a mean-field phase diagram displaying a first order transition between two superconductor phases with different symmetries: p-wave (with spontaneously generated spin-orbit interaction) and s-wave for greater values of the coupling constant. Moreover, we have looked for a possible modulated superconducting phase. We have found that this phase appears only as a meta-stable state in the strong coupling regime.

cond-mat.supr-con↗

arXiv2013

Pomeranchuk-Nematic instability in the presence of a weak magnetic field

We analyze a two-dimensional Pomeranchuk-Nematic instability, trigger by the Landau parameter $F_2<0$, in the presence of a small magnetic field. Using Landau Fermi liquid theory in the isotropic phase, we analyze the collective modes near the quantum critical point $F_2=-1,ω_c=0$ (where $ω_c$ is the cyclotron frequency). We focus on the effects of parity symmetry breaking on the Fermi surface deformation. We show that, for studying the critical regime, the linear response approximation of the Landau-Silin equation is not sufficient and it is necessary to compute corrections at least of order $ω_c^2$. Identifying the slowest oscillation mode in the disordered phase, we compute the phase diagram for the isotropic/nematic phase transition in terms of $F_2$ and $ω_c$.

cond-mat.str-el↗

arXiv2012

Hidden symmetries and equilibrium properties of multiplicative white-noise stochastic processes

Multiplicative white-noise stochastic processes continuously attract the attention of a wide area of scientific research. The variety of prescriptions available to define it difficults the development of general tools for its characterization. In this work, we study equilibrium properties of Markovian multiplicative white-noise processes. For this, we define the time reversal transformation for this kind of processes, taking into account that the asymptotic stationary probability distribution depends on the prescription. Representing the stochastic process in a functional Grassman formalism, we avoid the necessity of fixing a particular prescription. In this framework, we analyze equilibrium properties and study hidden symmetries of the process. We show that, using a careful definition of equilibrium distribution and taken into account the appropriate time reversal transformation, usual equilibrium properties are satisfied for any prescription. Finally, we present a detailed deduction of a covariant supersymmetric formulation of a multiplicative Markovian white-noise process and study some of the constraints it imposes on correlation functions using Ward-Takahashi identities.

cond-mat.stat-mech↗

arXiv2012

Supersymmetric formulation of multiplicative white--noise stochastic processes

We present a supersymmetric formulation of Markov processes, represented by a family of Langevin equations with multiplicative white-noise. The hidden symmetry encodes equilibrium properties such as fluctuation-dissipation relations. The formulation does not depend on the particular prescription to define the Wiener integral. In this way, different equilibrium distributions, reached at long times for each prescription, can be formally treated on the same footing.

cond-mat.stat-mech↗

arXiv2011

Microscopic approach to orientational order of domain walls

We develop a fully microscopic, statistical mechanics approach to study phase transitions in Ising systems with competing interactions at different scales. Our aim is to consider orientational and positional order parameters in a unified framework. In this work we consider two dimensional stripe forming systems, where nematic, smectic and crystal phases are possible. We introduce a nematic order parameter in a lattice, which measures orientational order of interfaces. We develop a mean field approach which leads to a free energy which is a function of both the magnetization (density) and the orientational (nematic) order parameters. Self-consistent equations for the order parameters are obtained and the solutions are described for a particular system, the Dipolar Frustrated Ising Ferromagnet. We show that this system has an Ising-nematic phase at low temperatures in the square lattice, where positional order (staggered magnetization) is zero. At lower temperatures a crystal-stripe phase may appear. In the continuum limit the present approach connects to a Ginsburg-Landau theory, which has an isotropic-nematic phase transition with breaking of a continuous symmetry.

cond-mat.stat-mech↗

arXiv2011

Phase transitions in superconductor liquid crystals

We study properties of phase transitions of 2D superconductor liquid crystal phases, and analyze the competition between the recently proposed Pair Density Wave (PDW) and nematic $4e$ superconductor ($4e$SC). Nematic fluctuations enhance the $4e$SC and suppress the PDW phase. In the absence of lattice effects, the PDW state exists only at T=0 and the low temperature phase is a nematic $4e$SC with short ranged PDW order. A geometric description of the $4e$ SC is presented.

cond-mat.supr-con↗