arXiv subjects

Ilaria Maccari

Publications and source records attributed to Ilaria Maccari.

23 records · Page 2Linked to original sources

arXiv2020

Interplay of spin waves and vortices in the 2D XY model at small vortex-core energy

The Berezinskii-Kosterlitz-Thouless (BKT) mechanism describes universal vortex unbinding in many two-dimensional systems, including the paradigmatic XY model. However, most of these systems present a complex interplay between excitations at different length scales that complicates theoretical calculations of nonuniversal thermodynamic quantities. These difficulties may be overcome by suitably modifying the initial conditions of the BKT flow equations to account for noncritical fluctuations at small length scales. In this work, we perform a systematic study of the validity and limits of this two-step approach by constructing optimised initial conditions for the BKT flow. We find that the two-step approach can accurately reproduce the results of Monte-Carlo simulations of the traditional XY model. In order to systematically study the interplay between vortices and spin-wave excitations, we introduce a modified XY model with increased vortex fugacity. We present large-scale Monte-Carlo simulations of the spin stiffness and vortex density for this modified XY model and show that even at large vortex fugacity, vortex unbinding is accurately described by the nonperturbative functional renormalisation group.

cond-mat.str-el↗

arXiv2018

Kane-Fisher weak link physics in the clean scratched-XY model

The nature of the superfluid-insulator transition in 1D has been much debated recently. In particular, to describe the strong disorder regime characterized by weak link proliferation, a scratched-XY model has been proposed [New J. Phys. \textbf{18}, 045018 (2016)], where the transport is dominated by a single anomalously weak link and is governed by Kane-Fisher weak link physics. In this article, we consider the simplest problem to which the scratched-XY model relates: a single weak link in an otherwise \textit{clean} system, with an intensity $J_W$ which decreases algebraically with the size of the system $J_W\sim L^{-α}$. Using a renormalization group approach and a vortex energy argument, we describe the Kane-Fisher physics in this model and show that it leads to a transition from a transparent regime for $K>K_c$ to a perfect cut for $K<K_c$, with an adjustable $K_c=1/(1-α)$ depending on $α$. We check our theoretical predictions with Monte Carlo numerical simulations complemented by finite-size scaling. Our results clarify two important assumptions at the basis of the scratched-XY scenario, the behaviors of the crossover length scale from weak link physics to transparency and of the superfluid stiffness.

cond-mat.stat-mech↗

arXiv2018

Disordered XY model: effective medium theory and beyond

We study the effect of uncorrelated random disorder on the temperature dependence of the superfluid stiffness in the two-dimensional classical XY model. By means of a perturbative expansion in the disorder potential, equivalent to the T-matrix approximation, we provide an extension of the effective-medium-theory result able to describe the low-temperature stiffness, and its separate diamagnetic and paramagnetic contributions. These analytical results provide an excellent description of the Monte Carlo simulations for two prototype examples of uncorrelated disorder. Our findings offer an interesting perspective on the effects of quenched disorder on longitudinal phase fluctuations in two-dimensional superfluid systems.

cond-mat.supr-con↗

arXiv2018

Melting of the vortex lattice through intermediate hexatic fluid in a-MoGe thin film

The hexatic fluid refers to a phase in between a solid and a liquid which has short range positional order but quasi-long range orientational order. In the celebrated theory of Berezinskii, Kosterlitz and Thouless and subsequently refined by Halperin, Nelson and Young, it was predicted that a 2-dimensional hexagonal solid can melt in two steps: first, through a transformation from a solid to a hexatic fluid which retains quasi long range orientational order and then from a hexatic fluid to an isotropic liquid. In this paper, using a combination of real space imaging and transport measurements we show that the 2-dimensional vortex lattice in a-MoGe thin film follows this sequence of melting as the magnetic field is increased. Identifying the signatures of various transitions on the bulk transport properties of the superconductor, we construct a vortex phase diagram for a two dimensional superconductor.

cond-mat.supr-con↗

arXiv2018

BKT universality class in the presence of correlated disorder

The correct detection of the Berezinskii-Kosterlitz-Thouless (BKT) transition in quasi-two-dimensional superconductors still remains a controversial issue. Its main signatures, indeed, are often at odds with the theoretical expectations. In a recent work\cite{me} we have shown that the presence of spatially correlated disorder plays a key role in this sense being it the reason underlying the experimentally-observed smearing of the universal superfluid-density jump. In the present paper, we closely investigate the effects of correlated disorder on the BKT transition, addressing specifically the issue whether it changes or not the BKT universality class.

cond-mat.supr-con↗