arXiv subjects

Steven T. Bramwell

Publications and source records attributed to Steven T. Bramwell.

24 records · Page 2Linked to original sources

arXiv2012

Topological Sector Fluctuations and Curie Law Crossover in Spin Ice

At low temperatures, a spin ice enters a Coulomb phase - a state with algebraic correlations and topologically constrained spin configurations. In Ho2Ti2O7, we have observed experimentally that this process is accompanied by a non-standard temperature evolution of the wave vector dependent magnetic susceptibility, as measured by neutron scattering. Analytical and numerical approaches reveal signatures of a crossover between two Curie laws, one characterizing the high temperature paramagnetic regime, and the other the low temperature topologically constrained regime, which we call the spin liquid Curie law. The theory is shown to be in excellent agreement with neutron scattering experiments. On a more general footing, i) the existence of two Curie laws appears to be a general property of the emergent gauge field for a classical spin liquid, and ii) sheds light on the experimental difficulty of measuring a precise Curie-Weiss temperature in frustrated materials; iii) the mapping between gauge and spin degrees of freedom means that the susceptibility at finite wave vector can be used as a local probe of fluctuations among topological sectors.

cond-mat.stat-mech↗

arXiv2011

Melting artificial spin ice

Artificial spin ice arrays of micromagnetic islands are a means of engineering additional energy scales and frustration into magnetic materials. Despite much progress in elucidating the properties of such arrays, the `spins' in the systems studied so far have no thermal dynamics as the kinetic constraints are too high. Here we address this problem by using a material with an ordering temperature near room temperature. By measuring the temperature dependent magnetization in different principal directions, and comparing with simulations of idealized statistical mechanical models, we confirm a dynamical `pre-melting' of the artificial spin ice structure at a temperature well below the intrinsic ordering temperature of the island material. We thus create a spin ice array that has real thermal dynamics of the artificial spins over an extended temperature range.

cond-mat.mes-hall↗

arXiv2008

Dy2Ti2O7 Spin Ice: a Test Case for Emergent Clusters in a Frustrated Magnet

Dy2Ti2O7 is a geometrically frustrated magnetic material with a strongly correlated spin ice regime that extends from 1 K down to as low as 60 mK. The diffuse elastic neutron scattering intensities in the spin ice regime can be remarkably well described by a phenomenological model of weakly interacting hexagonal spin clusters, as invoked in other geometrically frustrated magnets. We present a highly refined microscopic theory of Dy2Ti2O7 that includes long-range dipolar and exchange interactions to third nearest neighbors and which demonstrates that the clusters are purely fictitious in this material. The seeming emergence of composite spin clusters and their associated scattering pattern is instead an indicator of fine-tuning of ancillary correlations within a strongly correlated state.

cond-mat.stat-mech↗

arXiv2008

Universal Window for Two Dimensional Critical Exponents

Two dimensional condensed matter is realised in increasingly diverse forms that are accessible to experiment and of potential technological value. The properties of these systems are influenced by many length scales and reflect both generic physics and chemical detail. To unify their physical description is therefore a complex and important challenge. Here we investigate the distribution of experimentally estimated critical exponents, $β$, that characterize the evolution of the order parameter through the ordering transition. The distribution is found to be bimodal and bounded within a window $\sim 0.1 \le β\le 0.25$, facts that are only in partial agreement with the established theory of critical phenomena. In particular, the bounded nature of the distribution is impossible to reconcile with existing theory for one of the major universality classes of two dimensional behaviour - the XY model with four fold crystal field - which predicts a spectrum of non-universal exponents bounded only from below. Through a combination of numerical and renormalization group arguments we resolve the contradiction between theory and experiment and demonstrate how the "universal window" for critical exponents observed in experiment arises from a competition between marginal operators.

cond-mat.stat-mech↗

arXiv2002

Spin Ice State in Frustrated Magnetic Pyrochlore Materials

A frustrated system is one whose symmetry precludes the possibility that every pairwise interaction (``bond'') in the system can be satisfied at the same time. Such systems are common in all areas of physical and biological science. In the most extreme cases they can have a disordered ground state with ``macroscopic'' degeneracy, that is, one that comprises a huge number of equivalent states of the same energy. Pauling's description of the low temperature proton disorder in water ice was perhaps the first recognition of this phenomenon, and remains the paradigm. In recent years a new class of magnetic substance has been characterised, in which the disorder of the magnetic moments at low temperatures is precisely analogous to the proton disorder in water ice. These substances, known as spin ice materials, are perhaps the ``cleanest'' examples of such highly frustrated systems yet discovered. They offer an unparalleled opportunity for the study of frustration in magnetic systems at both an experimental and a theoretical level. This article describes the essential physics of spin ice, as it is currently understood, and identifies new avenues for future research on related materials and models.

cond-mat.dis-nn↗

arXiv1999

Comment on `Ising Pyrochlore Magnets: Low Temperature Properties, ``Ice Rules,'' and Beyond' by R. Siddharthan et al., Phys. Rev. Lett. 83, 1854 (1999)

In this Comment we argue that Ho$_2$Ti$_2$O$_7$ does not exhibit a transition to a partially ordered state unlike what is argued in PRL 83, 1854 (1999), that it exhibits spin ice behavior, and that the experimental specific heat data by Siddharthan et al. can be accounted for in terms of a "dipolar spin ice" model by including an expected contibution from the nuclear Ho spins to the appropriate long-range treatment of the dipole-dipole interactions.

cond-mat.dis-nn↗