27 research outputs found
Microscopic aspects of magnetic lattice demagnetizing factors
The demagnetizing factor N is of both conceptual interest and practical importance. Considering localized magnetic moments on a lattice, we show that for nonellipsoidal samples, N depends on the spin dimensionality (Ising, XY, or Heisenberg) and orientation, as well as the sample shape and susceptibility. The generality of this result is demonstrated by means of a recursive analytic calculation as well as detailed Monte Carlo simulations of realistic model spin Hamiltonians. As an important check and application, we also make an accurate experimental determination of N for a representative collective paramagnet (i.e., the Dy_2Ti_2O_7 spin ice
compound) and show that the temperature dependence of the experimentally determined N agrees closely with our theoretical calculations. Our conclusion is that the well-established practice of approximating the true sample shape with “corresponding ellipsoids” for systems with long-range interactions will in many cases overlook
important effects stemming from the microscopic aspects of the system under consideration
A Bragg glass phase in the vortex lattice of a type II superconductor
Although crystals are usually quite stable, they are sensitive to a
disordered environment: even an infinitesimal amount of impurities can lead to
the destruction of the crystalline order. The resulting state of matter has
been a longstanding puzzle. Until recently it was believed to be an amorphous
state in which the crystal would break into crystallites. But a different
theory predicts the existence of a novel phase of matter: the so-called Bragg
glass, which is a glass and yet nearly as ordered as a perfect crystal. The
lattice of vortices that can contain magnetic flux in type II superconductors
provide a good system to investigate these ideas. Here we show that neutron
diffraction data of the vortex lattice in type II superconductors provides
unambiguous evidence for a weak, power-law decay of the crystalline order
characteristic of a Bragg glass. The theory also predicts accurately the
electrical transport properties of superconductors; it naturally explains the
observed phase transition and the dramatic jumps in the critical current
associated with the melting of the Bragg glass. Moreover the model explains
experiments as diverse as X-ray scattering in disordered liquid crystals and
conductivity of electronic crystals.Comment: 9 pages, 4 figure
Experimental signatures of emergent quantum electrodynamics in PrHfO
In a quantum spin liquid, the magnetic moments of the constituent electron
spins evade classical long-range order to form an exotic state that is quantum
entangled and coherent over macroscopic length scales [1-2]. Such phases offer
promising perspectives for device applications in quantum information
technologies, and their study can reveal fundamentally novel physics in quantum
matter. Quantum spin ice is an appealing proposal of one such state, in which
the fundamental ground state properties and excitations are described by an
emergent U(1) lattice gauge theory [3-7]. This quantum-coherent regime has
quasiparticles that are predicted to behave like magnetic and electric
monopoles, along with a gauge boson playing the role of an artificial photon.
However, this emergent lattice quantum electrodynamics has proved elusive in
experiments. Here we report neutron scattering measurements of the rare-earth
pyrochlore magnet PrHfO that provide evidence for a quantum spin
ice ground state. We find a quasi-elastic structure factor with pinch points -
a signature of a classical spin ice - that are partially suppressed, as
expected in the quantum-coherent regime of the lattice field theory at finite
temperature. Our result allows an estimate for the speed of light associated
with magnetic photon excitations. We also reveal a continuum of inelastic spin
excitations, which resemble predictions for the fractionalized, topological
excitations of a quantum spin ice. Taken together, these two signatures suggest
that the low-energy physics of PrHfO can be described by emergent
quantum electrodynamics. If confirmed, the observation of a quantum spin ice
ground state would constitute a concrete example of a three-dimensional quantum
spin liquid - a topical state of matter which has so far mostly been explored
in lower dimensionalities.Comment: 15 pages, 3 figure
States in non-associative quantum mechanics: uncertainty relations and semiclassical evolution
Multiple Coulomb phase in the fluoride pyrochlore CsNiCrF6
The Coulomb phase is an idealized state of matter whose properties are determined by factors beyond conventional considerations of symmetry, including global topology, conservation laws and emergent order. Theoretically, Coulomb phases occur in ice-type systems such as water ice and spin ice; in dimer models; and in certain spin liquids. However, apart from ice-type systems, more general experimental examples are very scarce. Here we study the partly disordered material CsNiCrF6 and show that this material is a multiple Coulomb phase with signature correlations in three degrees of freedom: charge configurations, atom displacements and spin configurations. We use neutron and X-ray scattering to separate these correlations and to determine the magnetic excitation spectrum. Our results show how the structural and magnetic properties of apparently disordered materials may inherit, and be dictated by, a hidden symmetry—the local gauge symmetry of an underlying Coulomb phase
Refrustration and competing orders in the prototypical Dy₂ Ti₂ O₇ spin ice material
Spin ices, frustrated magnetic materials analogous to common water ice, have emerged over the past 15 years as exemplars of high frustration in three dimensions. Recent experimental developments aimed at interrogating anew the low-temperature properties of these systems, in particular whether the predicted transition to long-range order occurs, behoove researchers to scrutinize our current dipolar spin ice model description of these materials. In this work, we do so by combining extensive Monte Carlo simulations and mean-field theory calculations to analyze data from previous magnetization, diffuse neutron scattering, and specific-heat measurements on the paradigmatic Dy2 Ti2 O7 spin ice material. In this work, we also reconsider the possible importance of the nuclear specific heat C nuc in Dy 2 Ti 2 O 7 . We find that C nuc is not entirely negligible below a temperature ∼ 0.5 K and must therefore be taken into account in a quantitative analysis of the calorimetric data of this compound below that temperature. We find that in this material, small effective spin-spin exchange interactions compete with the magnetostatic dipolar interaction responsible for the main spin ice phenomenology. This causes an unexpected “refrustration” of the long-range order that would be expected from the incompletely self-screened dipolar interaction and which positions the material at the boundary between two competing classical long-range-ordered ground states. This allows for the manifestation of new physical low-temperature phenomena in Dy 2 Ti 2 O 7 , as exposed by recent specific-heat measurements. We show that among the four most likely causes for the observed upturn of the specific heat at low temperature [an exchange-induced transition to long-range order, quantum non-Ising (transverse) terms in the effective spin Hamiltonian, the nuclear hyperfine contribution, and random disorder], only the last appears to be reasonably able to explain the calorimetric data. </p