115 research outputs found
Parasitic small-moment-antiferromagnetism and non-linear coupling of hidden order and antiferromagnetism in URu2Si2 observed by Larmor diffraction
We report simultaneous measurements of the distribution of lattice constants
and the antiferromagnetic moment in high-purity URu2Si2, using both Larmor and
conventional neutron diffraction, as a function of temperature and pressure up
to 18 kbar. We establish that the tiny moment in the hidden order (HO) state is
purely parasitic and quantitatively originates from the distribution of lattice
constants. Moreover, the HO and large-moment antiferromagnetism (LMAF) at high
pressure are separated by a line of first-order phase transitions, which ends
in a bicritical point. Thus the HO and LMAF are coupled non-linearly and must
have different symmetry, as expected of the HO being, e.g., incommensurate
orbital currents, helicity order, or multipolar order.Comment: 4 pages, 4 figure
Bose-Einstein condensation and entanglement in magnetic systems
We present a study of magnetic field induced quantum phase transitions in
insulating systems. A generalized scaling theory is used to obtain the
temperature dependence of several physical quantities along the quantum
critical trajectory (, ) where is a longitudinal external
magnetic field and the critical value at which the transition occurs.
We consider transitions from a spin liquid at a critical field and
from a fully polarized paramagnet, at , into phases with long range
order in the transverse components. The transitions at and
can be viewed as Bose-Einstein condensations of magnons which however belong to
different universality classes since they have different values of the dynamic
critical exponent . Finally, we use that the magnetic susceptibility is an
entanglement witness to discuss how this type of correlation sets in as the
system approaches the quantum critical point along the critical trajectory,
, .Comment: 7 pages, 1 Table; accepted version; changes in text and new
reference
High pressure study of BaFe2As2 - role of hydrostaticity and uniaxial stress
We investigate the evolution of the electrical resistivity of BaFe2As2 single
crystals with pressure. The samples used were from the same batch grown from
self flux and showed properties that were highly reproducible. Samples were
pressurised using three different pressure media: pentane-isopentane (in a
piston cylinder cell), Daphne oil (in an alumina anvil cell) and steatite (in a
Bridgman cell). Each pressure medium has its own intrinsic level of
hydrostaticity, which dramatically affects the phase diagram. An increasing
uniaxial pressure component in this system quickly reduces spin density wave
order and favours the appearance of superconductivity, similar to what is seen
in SrFe2As2.Comment: 11 page
Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URuSi
We report low-energy inelastic neutron scattering data of the paramagnetic
(PM) to hidden-order (HO) phase transition at in
URuSi. While confirming previous results for the HO and PM phases, our
data reveal a pronounced wavevector dependence of low-energy excitations across
the phase transition. To analyze the energy scans we employ a damped harmonic
oscillator model containing a fit parameter which is expected to
diverge at a second-order phase transition. Counter to expectations the
excitations at show an abrupt step-like suppression of
below , whereas excitations at , associated
with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement
and a pronounced peak of at . Therefore, at the critical HO
temperature , LMAF fluctuations become nearly critical as well. This is
the behavior expected of a super-vector order parameter with nearly degenerate
components for the HO and LMAF leading to nearly isotropic fluctuations in the
combined order-parameter space.Comment: 6 pages; v3 accepted journal version; minor modifications compared to
v
Unconventional resistivity at the border of metallic antiferromagnetism in NiS2
We report low-temperature and high-pressure measurements of the electrical
resistivity \rho(T) of the antiferromagnetic compound NiS_2 in its
high-pressure metallic state. The form of \rho(T) suggests that metallic
antiferromagnetism in NiS_2 is quenched at a critical pressure p_c=76+-5 kbar.
Near p_c the temperature variation of \rho(T) is similar to that observed in
NiS_{2-x}Se_x near the critical composition x=1 where the Neel temperature
vanishes at ambient pressure. In both cases \rho(T) varies approximately as
T^{1.5} over a wide range below 100 K. However, on closer analysis the
resistivity exponent in NiS_2 exhibits an undulating variation with temperature
not seen in NiSSe (x=1). This difference in behaviour may be due to the effects
of spin-fluctuation scattering of charge carriers on cold and hot spots of the
Fermi surface in the presence of quenched disorder, which is higher in NiSSe
than in stoichiometric NiS_2.Comment: 7 page
Topological Hall effect in the A-phase of MnSi
Recent small angle neutron scattering suggests, that the spin structure in
the A-phase of MnSi is a so-called triple- state, i.e., a superposition of
three helices under 120 degrees. Model calculations suggest that this structure
in fact is a lattice of so-called skyrmions, i.e., a lattice of topologically
stable knots in the spin structure. We report a distinct additional
contribution to the Hall effect in the temperature and magnetic field range of
the proposed skyrmion lattice, where such a contribution is neither seen nor
expected for a normal helical state. Our Hall effect measurements constitute a
direct observation of a topologically quantized Berry phase that identifies the
spin structure seen in neutron scattering as the proposed skyrmion lattice
Neutron Halo Isomers in Stable Nuclei and their Possible Application for the Production of Low Energy, Pulsed, Polarized Neutron Beams of High Intensity and High Brilliance
We propose to search for neutron halo isomers populated via -capture
in stable nuclei with mass numbers of about A=140-180 or A=40-60, where the
or neutron shell model state reaches zero binding energy.
These halo nuclei can be produced for the first time with new -beams of
high intensity and small band width ( 0.1%) achievable via Compton
back-scattering off brilliant electron beams thus offering a promising
perspective to selectively populate these isomers with small separation
energies of 1 eV to a few keV. Similar to single-neutron halo states for very
light, extremely neutron-rich, radioactive nuclei
\cite{hansen95,tanihata96,aumann00}, the low neutron separation energy and
short-range nuclear force allows the neutron to tunnel far out into free space
much beyond the nuclear core radius. This results in prolonged half lives of
the isomers for the -decay back to the ground state in the 100
ps-s range. Similar to the treatment of photodisintegration of the
deuteron, the neutron release from the neutron halo isomer via a second,
low-energy, intense photon beam has a known much larger cross section with a
typical energy threshold behavior. In the second step, the neutrons can be
released as a low-energy, pulsed, polarized neutron beam of high intensity and
high brilliance, possibly being much superior to presently existing beams from
reactors or spallation neutron sources.Comment: accepted for publication in Applied Physics
On the Hidden Order in URuSi --- Antiferro Hexadecapole Order and its Consequences
An antiferro ordering of an electric hexadecapole moment is discussed as a
promising candidate for the long standing mystery of the hidden order phase in
URuSi. Based on localized -electron picture, we discuss the
rationale of the selected multipole and the consequences of the antiferro
hexadecapole order of symmetry. The mean-field solutions and
the collective excitations from them explain reasonably significant
experimental observations: the strong anisotropy in the magnetic
susceptibility, characteristic behavior of pressure versus magnetic field or
temperature phase diagrams, disappearance of inelastic neutron-scattering
intensity out of the hidden order phase, and insensitiveness of the NQR
frequency at Ru-sites upon ordering. A consistency with the strong anisotropy
in the magnetic responses excludes all the multipoles in two-dimensional
representations, such as . The expected azimuthal angle
dependences of the resonant X-ray scattering amplitude are given. The
-type antiferro quadrupole should be induced by an in-plane
magnetic field along , which is reflected in the thermal expansion and
the elastic constant of the transverse mode. The
-type [-type] antiferro quadrupole is also induced by
applying the uniaxial stress along direction [ direction]. A
detection of these induced antiferro quadrupoles under the in-plane magnetic
field or the uniaxial stress using the resonant X-ray scattering provides a
direct redundant test for the proposed order parameter.Comment: 10 pages, 10 figures, 5 table
Frustration and the Kondo effect in heavy fermion materials
The observation of a separation between the antiferromagnetic phase boundary
and the small-large Fermi surface transition in recent experiments has led to
the proposal that frustration is an important additional tuning parameter in
the Kondo lattice model of heavy fermion materials. The introduction of a Kondo
(K) and a frustration (Q) axis into the phase diagram permits us to discuss the
physics of heavy fermion materials in a broader perspective. The current
experimental situation is analysed in the context of this combined "QK" phase
diagram. We discuss various theoretical models for the frustrated Kondo
lattice, using general arguments to characterize the nature of the -electron
localization transition that occurs between the spin liquid and heavy Fermi
liquid ground-states. We concentrate in particular on the Shastry--Sutherland
Kondo lattice model, for which we establish the qualitative phase diagram using
strong coupling arguments and the large- expansion. The paper closes with
some brief remarks on promising future theoretical directions.Comment: To appear in a special issue of JLT
Fibrillization of Human Tau Is Accelerated by Exposure to Lead via Interaction with His-330 and His-362
and its mutants at physiological pH. interaction with His-330 and His-362, with sub-micromolar affinity. in the pathogenesis of Alzheimer disease and provide critical insights into the mechanism of lead toxicity
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