232 research outputs found
Random Bond Effect in the Quantum Spin System (TlK)CuCl
The effect of exchange bond randomness on the ground state and the
field-induced magnetic ordering was investigated through magnetization
measurements in the spin-1/2 mixed quantum spin system
(TlK)CuCl for . Both parent compounds TlCuCl and
KCuCl are coupled spin dimer systems, which have the singlet ground state
with excitation gaps K and 31 K, respectively. Due to
bond randomness, the singlet ground state turns into the magnetic state with
finite susceptibility, nevertheless, the excitation gap remains. Field-induced
magnetic ordering, which can be described by the Bose condensation of excited
triplets, magnons, was observed as in the parent systems. The phase transition
temperature is suppressed by the bond randomness. This behavior may be
attributed to the localization effect.Comment: 19 pages, 7 figures, 12 eps files, revtex, will appear in PR
US adolescent food intake trends from 1965 to 1996
AIM—To examine adolescent food consumption trends in the United States with important chronic disease implications.METHODS—Analysis of dietary intake data from four nationally representative United States Department of Agriculture surveys of individuals 11-18 years of age (n = 12 498).RESULTS—From 1965 to 1996, a considerable shift in the adolescent diet occurred. Total energy intake decreased as did the proportion of energy from total fat (39% to 32%) and saturated fat (15% to 12%). Concurrent increases occurred in the consumption of higher fat potatoes and mixed dishes (pizza, macaroni cheese). Lower fat milks replaced higher fat milks but total milk consumption decreased by 36%. This decrease was accompanied by an increase in consumption of soft drinks and non-citrus juices. An increase in high fat potato consumption led to an increase in vegetable intake but the number of servings for fruits and vegetables is still below the recommended five per day. Iron, folate, and calcium intakes continue to be below recommendations for girls.CONCLUSIONS—These trends, far greater than for US adults, may compromise health of the future US population
Dispersive magnetic excitations in the S=1 antiferromagnet BaMnO
We present powder inelastic neutron scattering measurements of the S=1
dimerized antiferromagnet BaMnO. The K magnetic spectrum
exhibits a spin-gap of meV and a dispersive spectrum with
a bandwidth of approximately 1.5 meV. Comparison to coupled dimer models
describe the dispersion and scattering intensity accurately and determine the
exchange constants in BaMnO. The wave vector dependent scattering
intensity confirms the proposed S=1 dimer bond. Temperature dependent
measurements of the magnetic excitations indicate the presence of both
singlet-triplet and thermally activated triplet-quintet excitations.Comment: 8 pages, 8 figures, Submitted to Physical Review B, Resubmited
versio
Neutron Scattering Study of Magnetic Ordering and Excitations in the Doped Spin Gap System Tl(CuMg)Cl
Neutron elastic and inelastic scattering measurements have been performed in
order to investigate the spin structure and the magnetic excitations in the
impurity-induced antiferromagnetic ordered phase of the doped spin gap system
Tl(CuMg)Cl with . The magnetic Bragg reflections
indicative of the ordering were observed at with integer
and odd below K. It was found that the spin structure
of the impurity-induced antiferromagnetic ordered phase on average in
Tl(CuMg)Cl with is the same as that of the
field-induced magnetic ordered phase for in the parent
compound TlCuCl. The triplet magnetic excitation was clearly observed in
the - plane and the dispersion relations of the triplet excitation
were determined along four different directions. The lowest triplet excitation
corresponding to the spin gap was observed at with integer
and odd , as observed in TlCuCl. It was also found that the spin gap
increases steeply below upon decreasing temperature. This strongly
indicates that the impurity-induced antiferromagnetic ordering coexists with
the spin gap state in Tl(CuMg)Cl with .Comment: 24 pages, 7 figures, 11 eps files, revtex style, will appear in Phys.
Rev.
Field- and pressure-induced magnetic quantum phase transitions in TlCuCl_3
Thallium copper chloride is a quantum spin liquid of S = 1/2 Cu^2+ dimers.
Interdimer superexchange interactions give a three-dimensional magnon
dispersion and a spin gap significantly smaller than the dimer coupling. This
gap is closed by an applied hydrostatic pressure of approximately 2kbar or by a
magnetic field of 5.6T, offering a unique opportunity to explore the both types
of quantum phase transition and their associated critical phenomena. We use a
bond-operator formulation to obtain a continuous description of all disordered
and ordered phases, and thus of the transitions separating these. Both
pressure- and field-induced transitions may be considered as the Bose-Einstein
condensation of triplet magnon excitations, and the respective phases of
staggered magnetic order as linear combinations of dimer singlet and triplet
modes. We focus on the evolution with applied pressure and field of the
magnetic excitations in each phase, and in particular on the gapless
(Goldstone) modes in the ordered regimes which correspond to phase fluctuations
of the ordered moment. The bond-operator description yields a good account of
the magnetization curves and of magnon dispersion relations observed by
inelastic neutron scattering under applied fields, and a variety of
experimental predictions for pressure-dependent measurements.Comment: 20 pages, 17 figure
Drastic Change of Magnetic Phase Diagram in Doped Quantum Antiferromagnet TlCuMgCl
TlCuCl is a coupled spin dimer system, which has a singlet ground state
with an excitation gap of = 5.5 T.
TlCuMgCl doped with nonmagnetic Mg ions undergoes
impurity-induced magnetic ordering. Because triplet excitation with a finite
gap still remains, this doped system can also undergo magnetic-field-induced
magnetic ordering. By specific heat measurements and neutron scattering
experiments under a magnetic field, we investigated the phase diagram in
TlCuMgCl with , and found that impurity- and
field-induced ordered phases are the same. The gapped spin liquid state
observed in pure TlCuCl is completely wiped out by the small amount of
doping.Comment: 9 pages, 5 figures, jpsj2 class file, to be published in J. Phy. Soc.
Jpn. Vol.75 No.3 (2006); layout changed, unrelated figure remove
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]
Impurity-Induced Antiferromagnetic Ordering in the Spin Gap System TlCuCl_3
The magnetization measurements have been performed on the doped spin gap
system TlCu_{1-x}Mg_xCl_3 with x <= 0.025. The parent compound TlCuCl_3 is a
three-dimensional coupled spin dimer system with the excitation gap Delta/k_B =
7.7 K. The impurity-induced antiferromagnetic ordering was clearly observed.
The easy axis lies in the (0,1,0) plane. It was found that the transition
temperature increases with increasing Mg^{2+} concentration x, while the
spin-flop transition field is almost independent of x. The magnetization curve
suggests that the impurity-induced antiferromagnetic ordering coexists with the
spin gap for x <= 0.017.Comment: 5 pages, 6 figures, revtex styl
Three-Dimensional Ordering in Weakly Coupled Antiferromagnetic Ladders and Chains
A theoretical description is presented for low-temperature magnetic-field
induced three-dimensional (3D) ordering transitions in strongly anisotropic
quantum antiferromagnets, consisting of weakly coupled antiferromagnetic
spin-1/2 chains and ladders. First, effective continuum field theories are
derived for the one-dimensional subsystems. Then the Luttinger parameters,
which determine the low-temperature susceptibilities of the chains and ladders,
are calculated from the Bethe ansatz solution for these effective models. The
3D ordering transition line is obtained using a random phase approximation for
the weak inter-chain (inter-ladder) coupling. Finally, considering a Ginzburg
criterion, the fluctuation corrections to this approach are shown to be small.
The nature of the 3D ordered phase resembles a Bose condensate of integer-spin
magnons. It is proposed that for systems with higher spin degrees of freedom,
e.g. N-leg spin-1/2 ladders, multi-component condensates can occur at high
magnetic fields.Comment: RevTex, 18 pages with 7 figure
Magnetization plateaux in dimerized spin ladder arrays
We investigate the ground state magnetization plateaux appearing in spin 1/2
two-leg ladders built up from dimerized antiferromagnetic Heisenberg chains and
dimerized zig-zag interchain couplings. Using both Abelian bosonization and
Lanczos methods we find that the system yields rather unusual plateaux and
exhibits massive and massless phases for specific choices or ``tuning'' of
exchange interactions. The relevance of this behavior in the study of
NH_4CuCl_3 is discussed.Comment: 9 pages, RevTeX, 11 postscript figure
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