419 research outputs found
Two Experimental Tests of the Halperin-Lubensky-Ma Effect at the Nematic-Smectic-A Phase Transition
We have conducted two quantitative tests of predictions based on the
Halperin-Lubensky-Ma (HLM) theory of fluctuation-induced first-order phase
transitions. First, we explore the effect of an external magnetic field on the
nematic-smectic-A (NA) transition in a liquid crystal. Second, we examine the
dependence of the first-order discontinuity as a function of mixture
concentration in pure 8CB and three 8CB-10CB mixtures. We find the first
quantitative evidence for deviations from the HLM theory.Comment: 4 pages, 2 figure
Zener double exchange from local valence fluctuations in magnetite
Magnetite (FeO) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (80 meV) are
shifted upwards in energy above due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above .Comment: 4 pages, 5 figure
The Role of Grain Boundaries in Determining the Transport Properties in Magnetite
We present magnetoresistance and Hall-effect measurements on single crystal magnetite (Fe3O4) close to the Verwey transition TV = 123.8 K. We show that the formation of grain boundaries accompanying the reduction in crystal symmetry plays a significant role in the electron scattering mechanism, and that grain boundaries can account for the apparent change in sign of the charge
carriers below TV
Four-fold basal plane anisotropy of the nonlocal magnetization of YNi2B2C
Studies of single crystal YNi2B2C have revealed a four-fold anisotropy of the
equilibrium magnetization in the square crystallographic basal plane. This pi/2
periodicity occurs deep in the superconductive mixed state. In this crystal
symmetry, an ordinary superconductive mass anisotropy (as in usual London
theory) allows only a constant, isotropic response. In contrast, the
experimental results are well described by generalized London theory
incorporating non-local electrodynamics, as needed for this clean,
intermediate-k superconductor.Comment: 4 pages, 4 figure
Influence of nonlocal electrodynamics on the anisotropic vortex pinning in
We have studied the pinning force density Fp of YNi_2B_2C superconductors for
various field orientations. We observe anisotropies both between the c-axis and
the basal plane and within the plane, that cannot be explained by usual mass
anisotropy. For magnetic field , the reorientation structural
transition in the vortex lattice due to nonlocality, which occurs at a field
, manifests itself as a kink in Fp(H). When , Fp is
much larger and has a quite different H dependence, indicating that other
pinning mechanisms are present. In this case the signature of nonlocal effects
is the presence of a fourfold periodicity of Fp within the basal plane.Comment: 4 pages, 3 figure
External-field-induced tricritical point in a fluctuation-driven nematic-smectic-A transition
We study theoretically the effect of an external field on the
nematic-smectic-A (NA) transition close to the tricritical point, where
fluctuation effects govern the qualitative behavior of the transition. An
external field suppresses nematic director fluctuations, by making them
massive. For a fluctuation-driven first-order transition, we show that an
external field can drive the transition second-order. In an appropriate liquid
crystal system, we predict the required magnetic field to be of order 10 T. The
equivalent electric field is of order .Comment: revtex, 4 pages, 1 figure; revised version, some equations have been
modifie
Field induced magnetic order in the frustrated magnet Gadolinium Gallium Garnet
Gd3Ga5O12, (GGG), has an extraordinary magnetic phase diagram, where no long
range order is found down to 25 mK despite \Theta_CW \approx 2 K. However, long
range order is induced by an applied field of around 1 T. Motivated by recent
theoretical developments and the experimental results for a closely related
hyperkagome system, we have performed neutron diffraction measurements on a
single crystal sample of GGG in an applied magnetic field. The measurements
reveal that the H-T phase diagram of GGG is much more complicated than
previously assumed. The application of an external field at low T results in an
intensity change for most of the magnetic peaks which can be divided into three
distinct sets: ferromagnetic, commensurate antiferromagnetic, and
incommensurate antiferromagnetic. The ferromagnetic peaks (e.g. (112), (440)
and (220)) have intensities that increase with the field and saturate at high
field. The antiferromagnetic reflections have intensities that grow in low
fields, reach a maximum at an intermediate field (apart from the (002) peak
which shows two local maxima) and then decrease and disappear above 2 T. These
AFM peaks appear, disappear and reach maxima in different fields. We conclude
that the competition between magnetic interactions and alternative ground
states prevents GGG from ordering in zero field. It is, however, on the verge
of ordering and an applied magnetic field can be used to crystallise ordered
components. The range of ferromagnetic and antiferromagnetic propagation
vectors found reflects the complex frustration in GGG.Comment: 6 pages, 7 figures, HFM 2008 conference pape
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