arXiv subjects

Rodolfo A. Borzi

Publications and source records attributed to Rodolfo A. Borzi.

7 recordsLinked to original sources

arXiv2025

Perfectly hidden order and Z2 confinement transition in a fully packed monopole liquid

We investigate a variant of spin ice whose degenerate ground states are densely packed monopole configurations. An applied field drives this model through a Z2 confinement transition. This phase change is a variant of the U(1) Kasteleyn transition, but instead of saturated order the system has fluctuations in the confined phase, and shows critical scaling on both sides of the transition. Remarkably, the magnetic response scales with the critical exponent of the specific heat in the 3D Ising universality class. We prove this universality using a Kramers--Wannier duality to map to an Ising model. The dual order parameter maps back to a non-local string order parameter in the original model, invisible to any local probe. Further, we describe the transition in terms of a bosonic field theory including a pairing term.

cond-mat.str-el↗

arXiv2020

Anomalous out-of-equilibrium dynamics in the spin-ice material Dy$_2$Ti$_2$O$_7$ under moderate magnetic fields

We study experimentally and numerically the dynamics of the spin ice material Dy$_2$Ti$_2$O$_7$ in the low temperature ($T$) and moderate magnetic field ($\mathbf{B}$) regime ($T\in[0.1,1.7]K$, $B\in[0,0.3]T$). Our objective is to understand the main physics shaping the out-of-equilibrium magnetisation vs. temperature curves in two different regimes. Very far from equilibrium, turning on the magnetic field after having cooled the system in zero field (ZFC) can increase the concentration of magnetic monopoles; this accelerates the dynamics. Similarly to electrolytes, this occurs through dissociation of bound monopole pairs. However, for spin ices the polarisation of the vacuum out of which the monopole pairs are created is a key factor shaping the magnetisation curves, with no analog. We observe a threshold field near $0.2T$ for this fast dynamics to take place, linked to the maximum magnetic force between the attracting pairs. Surprisingly, within a regime of low temperatures and moderate fields, an extended Ohm's law can be used to describe the ZFC magnetisation curve obtained with the dipolar spin-ice model. However, in real samples the acceleration of the dynamics appears even sharper than in simulations, possibly due to the presence of avalanches. On the other hand, the effect of the field nearer equilibrium can be just the opposite to that at very low temperatures. Single crystals, as noted before for powders, abandon equilibrium at a blocking temperature $T_B$ which increases with field. Curiously, this behaviour is present in numerical simulations even within the nearest-neighbours interactions model. Simulations and experiments show that the increasing trend in $T_B$ is stronger for $\mathbf{B}\parallel[100]$. This suggests that the field plays a part in the dynamical arrest through monopole suppression, which is quite manifest for this field orientation.

cond-mat.stat-mech↗

arXiv2019

Monte Carlo study on the detection of classical order by disorder in real antiferromagnetic Ising pyrochlores

We use Monte Carlo simulations to evaluate the feasibility of detecting thermal order by disorder in real antiferromagnetic Ising pyrochlores, frustrated by a magnetic field applied in the $[110]$ direction. Building on an ideal system with only nearest-neighbour exchange interactions and a perfectly oriented field, we consider the effects of dipolar interactions and field misalignment. Our approach is special in that it relies more in the possibility to switch on and off the entropic drive towards order than in the absence of (or immunity to) a particular perturbation. It can then be applied, in principle, to other uncontrolled interactions expected to be naturally present in real {magnetic} materials. We establish the conditions under which entropic effects can be discerned from an interaction drive towards order, show how to use neutron scattering as a means to unveil this mechanism, and discuss possible materials where to test these ideas.

cond-mat.stat-mech↗

arXiv2016

Field-tuned order by disorder in Ising frustrated magnets with antiferromagnetic interactions

We demonstrate the appearance of thermal order by disorder in Ising pyrochlores with staggered antiferromagnetic order frustrated by an applied magnetic field. We use a mean-field cluster variational method, a low-temperature expansion and Monte Carlo simulations to characterise the order-by-disorder transition. By direct evaluation of the density of states we quantitatively show how a symmetry-broken state is selected by thermal excitations. We discuss the relevance of our results to experiments in $2d$ and $3d$ samples, and evaluate how anomalous finite-size effects could be exploited to detect this phenomenon experimentally in two-dimensional artificial systems, or in antiferromagnetic all-in--all-out pyrochlores like Nd$_2$Hf$_2$O$_7$ or Nd$_2$Zr$_2$O$_7$, for the first time.

cond-mat.stat-mech↗

arXiv2014

Monopole ordered phases in dipolar and nearest-neighbours Ising pyrochlore: from spin ice to the "all-in--all-out" antiferromagnet

We study Ising pyrochlores by means of Monte Carlo simulations. We cover a set of exchange constants ranging from the frustrated ferromagnetic case (spin-ice) to the fully-ordered "all-in--all-out" antiferromagnet in the dipolar model, reinterpreting the results --as in an ionic system-- in terms of a temperature vs. magnetic charge density phase diagram. In spite of its spin nature and the presence of both double and single non-conserved magnetic charges, the dipolar model gives place to a phase diagram which is quite comparable with those previously obtained for on-lattice systems of electric charges, and on spin ice models with conserved number of single magnetic charges. The contrast between these systems, to which we add results from the nearest-neighbours model, put forward other features of our phase diagram --notably, a monopole fluid with charge order at high monopole densities that persists up to arbitrarily high temperatures-- that can only be explained taking into account construction constraints forced by the underlying spin degrees of freedom.

cond-mat.stat-mech↗

arXiv2003

On the fluctuations of jamming coverage upon random sequential adsorption on homogeneous and heterogeneous media

The fluctuations of the jamming coverage upon Random Sequential Adsorption (RSA) are studied using both analytical and numerical techniques. Our main result shows that these fluctuations (characterized by $σ_{θ_J}$) decay with the lattice size according to the power-law $σ_{θ_J} \propto L^{-1/ ν}$. The exponent $ν$ depends on the dimensionality $D$ of the substrate and the fractal dimension of the set where the RSA process actually takes place ($d_f$) according to $ν= 2 / (2D - d_f)$.This theoretical result is confirmed by means of extensive numerical simulations applied to the RSA of dimers on homogeneous and stochastic fractal substrates. Furthermore, our predictions are in excellent agreement with different previous numerical results. It is also shown that, studying correlated stochastic processes, one can define various fluctuating quantities designed to capture either the underlying physics of individual processes or that of the whole system. So, subtle differences in the definitions may lead to dramatically different physical interpretations of the results. Here, this statement is demonstrated for the case of RSA of dimers on binary alloys.

cond-mat.stat-mech↗

arXiv2003

Scaling behavior of jamming fluctuations upon random sequential adsorption

It is shown that the fluctuations of the jamming coverage upon Random Sequential Adsorption ($σ_{θ_J}$), decay with the lattice size according to the power-law $σ_{θ_J} \propto L^{-1 / ν_J}$, with $ν_{J} = 2 / (2D - d_f)$, where $D$ is the dimension of the substrate and $d_{\rm f}$ is the fractal dimension of the set of sites belonging to the substrate where the RSA process actually takes place. This result is in excellent agreement with the figure recently reported by Vandewalle {\it et al} ({\it Eur. Phys. J.} B. {\bf 14}, 407 (2000)), namely $ν_J = 1.0 (1)$ for the RSA of needles with $D = 2$ and $d_f = 2$, that gives $ν_J = 1$. Furthermore, our prediction is in excellent agreement with different previous numerical results. The derived relationships are also confirmed by means of extensive numerical simulations applied to the RSA of dimers on both stochastic and deterministic fractal substrates.

cond-mat.stat-mech↗