arXiv subjects

Daniel G. Barci

Publications and source records attributed to Daniel G. Barci.

At least 55 records · Page 3Linked to original sources

arXiv2010

Multipolar interactions and complex phases in ferromagnetic ultrathin films

We present a model to describe complex phases observed at mesoscopic scales in ultrathin magnetic films. The model is based on the interaction between dipolar as well as quadrupolar magnetic moments. In the special case of strong perpendicular crystal anisotropy, we show that quadrupolar degrees of freedom associated with orientation of domain walls are essential in order to correctly describe the observed phenomenology of domain formation. A nematic phase characterized by orientational order of domain walls (stripe-like) but without translational order is predicted. This isotropic-nematic transition belongs to the Kosterlitz-Thouless type in the thermodynamic limit. However, we found that in actual experimental scales, the fluctuations of the nematic order parameter are regularized by the sample size, and orientational long range order, as predicted by mean field, should be observable. The transition may be completely characterized experimentally from measurements of the magnetic structure factor as well as from the non-linear in-plane magnetic susceptibility.

cond-mat.stat-mech↗

arXiv2010

Functional integral approach for multiplicative stochastic processes

We present a functional formalism to derive a generating functional for correlation functions of a multiplicative stochastic process represented by a Langevin equation. We deduce a path integral over a set of fermionic and bosonic variables without performing any time discretization. The usual prescriptions to define the Wiener integral appear in our formalism in the definition of Green functions in the Grassman sector of the theory. We also study non-perturbative constraints imposed by BRS symmetry and supersymmetry on correlation functions. We show that the specific prescription to define the stochastic process is wholly contained in tadpole diagrams. Therefore, in a supersymmetric theory the stochastic process is uniquely defined since tadpole contributions cancels at all order of perturbation theory.

cond-mat.stat-mech↗

arXiv2008

Orientational order in two dimensions from competing interactions at different scales

We discuss orientational order in two dimensions in the context of systems with competing isotropic interactions at different scales. We consider an extension of the Brazovskii model for stripe phases including explicitly quartic terms with nematic symmetry in the energy. We show that leading fluctuations of the mean field nematic solution drive the isotropic-nematic transition into the Kosterlitz-Thouless universality class, i.e. these systems have a thermodynamic phase with orientational quasi-long-range order.

cond-mat.soft↗

arXiv2008

Metastable anisotropy orientation of nematic quantum Hall fluids

We analyze the experimental observation of metastable anisotropy resistance orientation at half filled quantum Hall fluids by means of a model of a quantum nematic liquid in an explicit symmetry breaking potential. We interpret the observed ``rotation'' of the anisotropy axis as a process of nucleation of nematic domains and compute the nucleation rate within this model. By comparing with experiment, we are able to predict the critical radius of nematic bubbles, $R_c\sim 2.6 μm $. Each domain contains about $10^4$ electrons.

cond-mat.mes-hall↗

arXiv2008

Competition between Pomeranchuk instabilities in the nematic and hexatic channels in a two-dimensional spinless Fermi fluid

We study the competition between the nematic and the hexatic phases of a two-dimensional spinless Fermi fluid near Pomeranchuk instabilities. We show that the general phase diagram of this theory contains a bicritical point where two second order lines and a first order nematic/hexatic phase transition meet together. We found that at criticality, and deep inside the associated symmetry broken phases, the low energy theory is governed by a dissipative cubic mode, even near the bicritical point where nematic and hexatic fluctuations cannot be distinguished due to very strong dynamical couplings.

cond-mat.str-el↗

arXiv2007

Universal Landauer conductance in chiral symmetric 2d systems

We study transport properties of an arbitrarily shaped ultraclean graphene sheet, adiabatically connected to leads,composed by the same material. If the localized interactions do not destroy chiral symmetry, we show that the conductance is quantized, since it is dominated by the quasi one-dimensional leads. As an example, we show that smooth structural deformations of the graphene plane do not modify the conductance quantization.

cond-mat.mes-hall↗

arXiv2007

Reply to comment on ``Competing Interactions, the Renormalization Group and the Isotropic-Nematic Phase Transition''

The focus of our work is to identify conditions for the presence of an isotropic-nematic phase transition in the context of a generic system with isotropic competing interactions. The comment 0709.4205 criticizes our results by showing that the low temperature fluctuations of a stripe phase in 2d diverge linearly in the thermodynamic limit. The analysis is restricted to the stripe phase and does not apply to the central result of our letter. We show, contrary to what is suggested in the comment, that the model introduced by D. G. Barci and D. A. Stariolo in PRL98, 200604 (2007) undergoes an isotropic-nematic phase transition in the Kosterlitz-Thouless universality class.

cond-mat.soft↗

arXiv2007

Competing Interactions, the Renormalization Group and the Isotropic-Nematic Phase Transition

We discuss 2D systems with Ising symmetry and competing interactions at different scales. In the framework of the Renormalization Group, we study the effect of relevant quartic interactions. In addition to the usual constant interaction term, we analyze the effect of quadrupole interactions in the self consistent Hartree approximation. We show that in the case of repulsive quadrupole interaction, there is a first order phase transition to a stripe phase in agreement with the well known Brazovskii result. However, in the case of attractive quadrupole interactions there is an isotropic-nematic second order transition with higher critical temperature.

cond-mat.soft↗

arXiv2007

Condensation of Vortex-Strings: Effective Potential Contribution Through Dual Actions

Topological excitations are believed to play an important role in different areas of physics. For example, one case of topical interest is the use of dual models of quantum cromodynamics to understand properties of its vacuum and confinement through the condensation of magnetic monopoles and vortices. Other applications are related to the role of these topological excitations, nonhomogeneous solutions of the field equations, in phase transitions associated to spontaneous symmetry breaking in gauge theories, whose study is of importance in phase transitions in the early universe, for instance. Here we show a derivation of a model dual to the scalar Abelian Higgs model where its topological excitations, namely vortex-strings, become manifest and can be treated in a quantum field theory way. The derivation of the nontrivial contribution of these vacuum excitations to phase transitions and its analogy with superconductivity is then made possible and they are studied here.

hep-th↗

arXiv2006

Long range interactions and the 3D asymptotic fermion spectrum

In this article, we study the stability of the space of asymptotic fermion states in (2+1)D, when long range interparticle interactions are present. This is done in the framework of bosonization, where the fermion propagator can be represented in terms of a vortex correlator. In particular, we discuss possible instabilities in the large distance behavior of the induced action for the vortex worldline.

hep-th↗

arXiv2005

Abelian Higgs Model Effective Potential in the Presence of Vortices

We determine the contribution of nontrivial vacuum (topological) excitations, more specifically vortex--strings of the Abelian Higgs model in 3+1 dimensions, to the functional partition function. By expressing the original action in terms of dual transformed fields we make explicit in the equivalent action the contribution of the vortex--strings excitations of the model. The effective potential of an appropriately defined local vacuum expectation value of the vortex--string field in the dual transformed action is then evaluated both at zero and finite temperatures and its properties discussed in the context of the finite temperature phase transition.

hep-th↗

arXiv2005

Non-perturbative approach to backscattering off a dynamical impurity in 1D Fermi systems

We investigate the problem of backscattering off a time-dependent impurity in a one-dimensional electron gas. By combining the Schwinger-Keldysh method with an adiabatic approximation in order to deal with the corresponding out of equilibrium Dirac equation, we compute the total energy density (TED) of the system. We show how the free fermion TED is distorted by the backscattering amplitude and the geometry of the impurity.

cond-mat.mes-hall↗

arXiv2005

Non-perturbative behavior of the quantum phase transition to a nematic Fermi fluid

We discuss shape (Pomeranchuk) instabilities of the Fermi surface of a two-dimensional Fermi system using bosonization. We consider in detail the quantum critical behavior of the transition of a two dimensional Fermi fluid to a nematic state which breaks spontaneously the rotational invariance of the Fermi liquid. We show that higher dimensional bosonization reproduces the quantum critical behavior expected from the Hertz-Millis analysis, and verify that this theory has dynamic critical exponent $z=3$. Going beyond this framework, we study the behavior of the fermion degrees of freedom directly, and show that at quantum criticality as well as in the the quantum nematic phase (except along a set of measure zero of symmetry-dictated directions) the quasi-particles of the normal Fermi liquid are generally wiped out. Instead, they exhibit short ranged spatial correlations that decay faster than any power-law, with the law $|x|^{-1} \exp(-\textrm{const.} |x|^{1/3})$ and we verify explicitely the vanishing of the fermion residue utilizing this expression. In contrast, the fermion auto-correlation function has the behavior $|t|^{-1} \exp(-{\rm const}. |t|^{-2/3})$. In this regime we also find that, at low frequency, the single-particle fermion density-of-states behaves as $N^*(ω)=N^*(0)+ B ω^{2/3} \logω+...$, where $N^*(0)$ is larger than the free Fermi value, N(0), and $B$ is a constant. These results confirm the non-Fermi liquid nature of both the quantum critical theory and of the nematic phase.

cond-mat.str-el↗

arXiv2004

Single Superfield Representation for Mixed Retarded and Advanced Correlators in Disordered Systems

We propose a new single superfield representation for mixed retarded and advanced correlators for noninteracting disordered systems. The method is tested in the simpler context of Random Matrix theory, by comparing with well known universal behavior for level spacing correlations. Our method is general and could be especially interesting to study localization properties encoded in the mixed correlators of Quantum Hall systems.

cond-mat.dis-nn↗

arXiv2003

Strongly correlated fermions with nonlinear energy dispersion and spontaneous generation of anisotropic phases

Using the bosonization approach we study fermionic systems with a nonlinear dispersion relation in dimension d>2. We explicitly show how the band curvature gives rise to interaction terms in the bosonic version of the model. Although these terms are perturbatively irrelevant in relation to the Landau Fermi liquid fixed point, they become relevant perturbations when instabilities take place. Using a coherent state path integral technique we built up the effective action that governs the dynamics of the Fermi surface fluctuations. We consider the combined effect of fermionic interactions and band curvature on possible anisotropic phases triggered by negative Landau parameters. In particular we study in some detail the phase diagram for the isotropic/nematic/hexatic quantum phase transition.

cond-mat.str-el↗

arXiv2002

Equilibration Time Scales in Homogeneous Bose-Einstein Condensate Dynamics

We study the nonequilibrium growth of a weakly interacting homogeneous Bose gas after a quench from a high-temperature state to a temperature below the Bose-Einstein critical condensation temperature. We quantitatively characterize the departure from thermal equilibrium and observe the presence of two equilibration time scales. The equilibration times are shown to be inversely proportional to the density.

cond-mat.soft↗

arXiv2002

Theory of the Quantum Hall Smectic Phase, I: Low Energy Properties of the Quantum Hall Smectic Fixed Point

We develop an effective low energy theory of the Quantum Hall (QH) Smectic or stripe phase of a two-dimensional electron gas in a large magnetic field in terms of its Goldstone modes and of the charge fluctuations on each stripe. This liquid crystal phase corresponds to a fixed point which is explicitly demonstrated to be stable against quantum fluctuations at long wavelengths. This fixed point theory also allows an unambiguous reconstruction of the electron operator. We find that quantum fluctuations are so severe that the electron Green function decays faster than any power-law, although slower than exponentially, and that consequently there is a deep pseudo-gap in the quasiparticle spectrum. We discuss, but do not resolve the stability of the quantum Hall smectic to crystallization. Finally, the role of Coulomb interactions and the low temperature thermodynamics of the QH smectic state are analyzed.

cond-mat.mes-hall↗

arXiv2002

Theory of the Quantum Hall Smectic Phase II: Microscopic Theory

We present a microscopic derivation of the hydrodynamic theory of the Quantum Hall smectic or stripe phase of a two-dimensional electron gas in a large magnetic field. The effective action of the low energy is derived here from a microscopic picture by integrating out high energy excitations with a scale of the order the cyclotron energy.The remaining low-energy theory can be expressed in terms of two canonically conjugate sets of degrees of freedom: the displacement field, that describes the fluctuations of the shapes of the stripes, and the local charge fluctuations on each stripe.

cond-mat.mes-hall↗