175 research outputs found
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
Structure of end states for a Haldane Spin Chain
Inelastic neutron scattering was used to probe edge states in a quantum spin
liquid. The experiment was performed on finite length antiferromagnetic spin-1
chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance
below the Haldane gap. The wave vector dependence of its intensity provides
direct evidence for staggered magnetization at chain ends, which decays
exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions
to the chain end spectrum indicate inter-chain-segment interactions. We also
observe a finite size blue shift of the Haldane gap.Comment: 4 pages RevTex, 3 figure
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
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
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
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]
A primary cutaneous adenoid-cystic carcinoma in a young woman. Differential diagnosis and clinical implications
Primary cutaneous adenoid-cystic carcinoma (PCACC) is a rare slow-growing neoplasm of disputed histogenesis characterized by a cribriform pattern at histology and local aggressive behaviour. Up to date about 60 cases of PCACC have been reported in the literature. This tumour is most common in the scalp, affects middle-aged and older individuals (mean age 59) and has predilection for women. We describe an unexpected case of PCACC in a 32-years-old woman referred to our clinic for a subcutaneous nodule in the scalp showing a slow growth and indolent course. The differential diagnosis and the clinical management of this PCACC patient, successfully treated with a wide local excision, are presented and discussed
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]
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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