271 research outputs found
Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain
Measurements are reported of the magnetic field dependence of excitations in
the quantum critical state of the spin S=1/2 linear chain Heisenberg
antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was
measured at k_B T/J <= 0.025 for H=0 and H=8.7 Tesla where the system is ~30%
magnetized. At H=0, the results are in quantitative agreement with exact
calculations of the dynamic spin correlation function for a two-spinon
continuum. At high magnetic field, there are multiple overlapping continua with
incommensurate soft modes. The boundaries of these continua confirm
long-standing predictions, and the intensities are consistent with exact
diagonalization and Bethe Ansatz calculations.Comment: 4 pages, 4 figure
Frustration-Induced Two Dimensional Quantum Disordered Phase in Piperazinium Hexachlorodicuprate
Piperazinium Hexachlorodicuprate (PHCC) is shown to be a frustrated
quasi-two-dimensional quantum Heisenberg antiferromagnet with a gapped
spectrum. Zero-field inelastic neutron scattering and susceptibility and
specific heat measurements as a function of applied magnetic field are
presented. At T = 1.5 K, the magnetic excitation spectrum is dominated by a
single propagating mode with a gap, Delta = 1 meV, and bandwidth of
approximately 1.8 meV in the (h0l) plane. The mode has no dispersion along the
b* direction indicating that neighboring a-c planes of the triclinic structure
are magnetically decoupled. The heat capacity shows a reduction of the gap as a
function of applied magnetic field in agreement with a singlet-triplet
excitation spectrum. A field-induced ordered phase is observed in heat capacity
and magnetic susceptibility measurements for magnetic fields greater than H_c1
approximately equal to 7.5 Tesla. Analysis of the neutron scattering data
reveals the important exchange interactions and indicates that some of these
are highly frustrated.Comment: 13 pages with 14 figures, 7 pages of text, 6 pages of figures.
Submitted to Phys. Rev. B 4/7/2001. email comments to [email protected] or
[email protected]
Less than 50% sublattice polarization in an insulating S=3/2 kagome' antiferromagnet at low T
We have found weak long range antiferromagnetic order in the
quasi-two-dimensional insulating oxide which contains
Cr S=3/2 ions on a kagom\'{e} lattice. In a sample with 76%
occupancy of the chromium sites the ordered moment is 1.1(3) per
chromium ion which is only one third of the N\'{e}el value .
The magnetic unit cell equals the chemical unit cell, a situation which is
favored by inter-plane interactions. Gapless quantum spin-fluctuations
(T_NS(Q,\omega)$ in the ordered
phase.Comment: 18 pages, RevTex/Latex, with 6 figure
Unconventional ferromagnetic and spin-glass states of the reentrant spin glass Fe0.7Al0.3
Spin excitations of single crystal Fe0.7Al0.3 were investigated over a wide
range in energy and reciprocal space with inelastic neutron scattering. In the
ferromagnetic phase, propagating spin wave modes become paramagnon-like
diffusive modes beyond a critical wave vector q0, indicating substantial
disorder in the long-range ordered state. In the spin glass phase, spin
dynamics is strongly q-dependent, suggesting remnant short-range spin
correlations. Quantitative model for S(energy,q) in the ``ferromagnetic'' phase
is determined.Comment: 4 pages, 5 figure
Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet
We report magnetic susceptibility, specific heat, and neutron scattering
measurements as a function of applied magnetic field and temperature to
characterize the quasi-two-dimensional frustrated magnet piperazinium
hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low
temperatures and fields the material forms a quantum paramagnet with a 1 meV
singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state
involves multiple spin pairs some of which have negative ground state bond
energies. Increasing the field at low temperatures induces three dimensional
long range antiferromagnetic order at 7.5 Tesla through a continuous phase
transition that can be described as magnon Bose-Einstein condensation. The
phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla.
The ordered antiferromagnetic phase is surrounded by a renormalized classical
regime. The crossover to this phase from the quantum paramagnet is marked by a
distinct anomaly in the magnetic susceptibility which coincides with closure of
the finite temperature singlet-triplet pseudo gap. The phase boundary between
the quantum paramagnet and the Bose-Einstein condensate features a finite
temperature minimum at K, which may be associated with coupling to
nuclear spin or lattice degrees of freedom close to quantum criticality.Comment: Submitted to New Journal of Physic
Machine learning outperforms clinical experts in classification of hip fractures
Hip fractures are a major cause of morbidity and mortality in the elderly, and incur high health and social care costs. Given projected population ageing, the number of incident hip fractures is predicted to increase globally. As fracture classification strongly determines the chosen surgical treatment, differences in fracture classification influence patient outcomes and treatment costs. We aimed to create a machine learning method for identifying and classifying hip fractures, and to compare its performance to experienced human observers. We used 3659 hip radiographs, classified by at least two expert clinicians. The machine learning method was able to classify hip fractures with 19% greater accuracy than humans, achieving overall accuracy of 92%
Spin Pseudo Gap in La2-xSrxCuO4 Studied by Neutron Scattering
Spin excitations of La2-xSrxCuO have been studied using inelastic neutron
scattering techniques in the energy range of 2 meV =< w =< 12 meV and the
temperature range of 8 K =< T =< 150 K. We observed a signature of a spin
pseudo gap in the excitation spectrum above Tc for the slightly overdoped
sample with x = 0.18. On heating, the spin pseudo gap gradually collapses
between T = 80 K and 150 K. For the x = 0.15 and 0.20, although the visibility
of gap-like structure at T ~ Tc is lower compared to the x = 0.18 sample, the
broad bump of kai"(w) appears at w ~ 5 meV,close to the spin-gap energy at base
temperature, suggests the existence of the spin pseudo gap in the normal state.Comment: revtex, 7 pages, 8 eps figures, PRB (2003) in pres
Frustrated 3-Dimensional Quantum Spin Liquid in CuHpCl
Inelastic neutron scattering measurements are reported for the quantum
antiferromagnetic material Cu_2(C_5H_12N_2)_2Cl_4 (CuHpCl). The magnetic
excitation spectrum forms a band extending from 0.9 meV to 1.4 meV. The
spectrum contains two modes that disperse throughout the a-c plane of the
monoclinic unit cell with less dispersion along the unique b-axis. Simple
arguments based on the measured dispersion relations and the crystal structure
show that a spin ladder model is inappropriate for describing CuHpCl. Instead,
it is proposed that hydrogen bond mediated exchange interactions between the
bi-nuclear molecular units yield a three-dimensional interacting spin system
with a recurrent triangular motif similar to the Shastry-Sutherland Model
(SSM). Model independent analysis based on the first moment sum rule shows that
at least four distinct spin pairs are strongly correlated and that two of
these, including the dimer bond of the corresponding SSM, are magnetically
frustrated. These results show that CuHpCl should be classified as a
frustration induced three dimensional quantum spin liquid.Comment: 13 pages, 17 figures (Color) ReSubmitted to Phys. Rev. B 9/21/2001
resubmission has new content email comments to [email protected] or
[email protected]
Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties
We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review
Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V_2O_3
Magnetic correlations in all four phases of pure and doped vanadium
sesquioxide V_2O_3 have been examined by magnetic thermal neutron scattering.
While the antiferromagnetic insulator can be accounted for by a Heisenberg
localized spin model, the long range order in the antiferromagnetic metal is an
incommensurate spin-density-wave, resulting from a Fermi surface nesting
instability. Spin dynamics in the strongly correlated metal are dominated by
spin fluctuations in the Stoner electron-hole continuum. Furthermore, our
results in metallic V_2O_3 represent an unprecedentedly complete
characterization of the spin fluctuations near a metallic quantum critical
point, and provide quantitative support for the SCR theory for itinerant
antiferromagnets in the small moment limit. Dynamic magnetic correlations for
energy smaller than k_BT in the paramagnetic insulator carry substantial
magnetic spectral weight. However, the correlation length extends only to the
nearest neighbor distance. The phase transition to the antiferromagnetic
insulator introduces a sudden switching of magnetic correlations to a different
spatial periodicity which indicates a sudden change in the underlying spin
Hamiltonian. To describe this phase transition and also the unusual short range
order in the paramagnetic state, it seems necessary to take into account the
orbital degrees of freedom associated with the degenerate d-orbitals at the
Fermi level in V_2O_3.Comment: Postscript file, 24 pages, 26 figures, 2 tables, accepted by Phys.
Rev.
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