432 research outputs found
Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap Antiferromagnet
Magnetic susceptibility, high-field magnetization and inelastic neutron
scattering experiments are used to study the magnetic properties of a new S=1
quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are
shown to be almost, but not quite, strong enough to destroy the nonmagnetic
singlet ground state and the energy gap in the magnetic excitation spectrum.
Substituting nonmagnetic Mg (S=0) ions for Ni (S=1) induces a
magnetically ordered state at low temperatures. To our knowledge this is the
first observation of doping-induced long-range order in a Haldane-gap system.Comment: 5 pages including 4 figure
Field Induced Multiple Reentrant Quantum Phase Transitions in Randomly Dimerized Antiferromagnetic S=1/2 Heisenberg Chains
The multiple reentrant quantum phase transitions in the
antiferromagnetic Heisenberg chains with random bond alternation in the
magnetic field are investigated by the density matrix renormalization group
method combined with the interchain mean field approximation. It is assumed
that the odd-th bond is antiferromagnetic with strength and even-th bond
can take the values {\JS} and {\JW} ({\JS} > J > {\JW} > 0) randomly
with probability and , respectively. The pure version ( and
) of this model has a spin gap but exhibits a field induced
antiferromagnetism in the presence of interchain coupling if Zeeman energy due
to the magnetic field exceeds the spin gap. For , the
antiferromagnetism is induced by randomness at small field region where the
ground state is disordered due to the spin gap in the pure case. At the same
time, this model exhibits randomness induced plateaus at several values of
magnetization. The antiferromagnetism is destroyed on the plateaus. As a
consequence, we find a series of reentrant quantum phase transitions between
the transverse antiferromagnetic phases and disordered plateau phases with the
increase of the magnetic field for moderate strength of interchain coupling.
Above the main plateaus, the magnetization curve consists of a series of small
plateaus and the jumps between them, It is also found that the
antiferromagnetism is induced by infinitesimal interchain coupling at the jumps
between the small plateaus. We conclude that this antiferromagnetism is
supported by the mixing of low lying excited states by the staggered interchain
mean field even though the spin correlation function is short ranged in the
ground state of each chain.Comment: 5 pages, 8 figure
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.
Far-Infrared Spectroscopy in Spin-Peierls Compound CuGeO_3 under High Magnetic Fields
Polarized far-infrared (FIR) spectroscopic measurements and FIR
magneto-optical studies were performed on the inorganic spin-Peierls compound
CuGeO_3. An absorption line, which was found at 98 cm in the dimerized
phase (D phase), was assigned to a folded phonon mode of B symmetry. The
splitting of the folded mode into two components in the incommensurate phase
(IC phase) has been observed for the first time. A new broad absorption
centered at 63 cm was observed only in the axis
polarization, which was assigned to a magnetic excitation from singlet ground
state to a continuum state.Comment: 9 pages multicolREVTeX, 10 figure
Bond-randomness-induced Neel order in weakly coupled antiferromagnetic spin chains
Quasi-one-dimensional antiferromagnetic (AF) quantum spin systems show a wide
range of interesting phenomena such as the spin-Peierls transition and disorder
driven long range ordering. While there is no magnetic long range order in
strictly one-dimensional systems, in real systems some amount of interchain
coupling is always present and AF long range order may appear below a Neel
ordering temperature T_N. We study the effect of bond randomness on Neel
ordering in weakly coupled random AF S=1/2 chains both with and without
dimerization (or spin-Peierls order). We use the real space renormalization
group method to tackle the intrachain couplings, and a mean-field approximation
to treat the interchain coupling. We show that in the non-dimerized chain,
disorder (represented by bond randomness) enhances the Neel order parameter; in
the dimerized chain which shows no magnetic ordering in the weak interchain
coupling limit without randomness, disorder can actually lead to long range
order. Thus disorder is shown to lead to, or enhance the tendency toward long
range order, providing another example of the order-by-disorder phenomenon. We
make a qualitative comparison of our results with the observed phenomenon of
doping induced long range ordering in quasi-one-dimensional spin systems such
as CuGeO_3.Comment: 8 pages, 4 figure
Scaling Properties of Antiferromagnetic Transition in Coupled Spin Ladder Systems Doped with Nonmagnetic Impurities
We study effects of interladder coupling on critical magnetic properties of
spin ladder systems doped with small concentrations of nonmagnetic impurities,
using the scaling theory together with quantum Monte Carlo (QMC) calculations.
Scaling properties in a wide region in the parameter space of the impurity
concentration x and the interladder coupling are governed by the quantum
critical point (QCP) of the undoped system for the transition between
antiferromagnetically ordered and spin-gapped phases. This multi-dimensional
and strong-coupling region has characteristic power-law dependences on x for
magnetic properties such as the N\'eel temperature. The relevance of this
criticality for understanding experimental results of ladder compounds is
stressed.Comment: 4 pages LaTeX including 3 PS figure
Electronic structure and magnetic properties of the linear chain cuprates Sr_2CuO_3 and Ca_2CuO_3
Sr_2CuO_3 and Ca_2CuO_3 are considered to be model systems of strongly
anisotropic, spin-1/2 Heisenberg antiferromagnets. We report on the basis of a
band-structure analysis within the local density approximation and on the basis
of available experimental data a careful analysis of model parameters for
extended Hubbard and Heisenberg models. Both insulating compounds show
half-filled nearly one-dimensional antibonding bands within the LDA. That
indicates the importance of strong on-site correlation effects. The bonding
bands of Ca_2CuO_3 are shifted downwards by 0.7 eV compared with Sr_2CuO_3,
pointing to different Madelung fields and different on-site energies within the
standard pd-model. Both compounds differ also significantly in the magnitude of
the inter-chain dispersion along the crystallographical a-direction: \approx
100 meV and 250 meV, respectively. Using the band-structure and experimental
data we parameterize a one-band extended Hubbard model for both materials which
can be further mapped onto an anisotropic Heisenberg model. From the
inter-chain dispersion we estimate a corresponding inter-chain exchange
constant J_{\perp} \approx 0.8 and 3.6 meV for Sr_2CuO_3 and Ca_2CuO_3,
respectively. Comparing several approaches to anisotropic Heisenberg problems,
namely the random phase spin wave approximation and modern versions of coupled
quantum spin chains approaches, we observe the advantage of the latter in the
reproduction of reasonable values for the N\'eel temperature T_N and the
magnetization m_0 at zero temperature. Our estimate of gives the
right order of magnitude and the correct tendency going from Sr_2CuO_3 to
Ca_2CuO_3. In a comparative study we also include CuGeO_3.Comment: 23 pages, 5 figures, 1 tabl
Direct Evidence for the Localized Single-Triplet Excitations and the Dispersive Multi-Triplets Excitations in SrCu2(BO3)2
We performed inelastic neutron scattering on the 2D Shastry-Sutherland system
SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around
3, 5 and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is
almost dispersionless with a bandwidth of 0.2 meV at most, showing a
significant constraint on a single-triplet hopping owing to the orthogonality
of the neighboring dimers. In contrast, the correlated two-triplets excitations
at 5 meV exhibit a more dispersive behavior.Comment: 12 pages, with 4 figures, to appear in Phys. Rev. Let
Charge-ordering and optical transitions of LiV2O5 and NaV2O5
We present the measurements of the polarized optical spectra of NaV2O5 and
LiV2O5. In an energy range from 0.5 to 5.5 eV we observe similar peaks in the E
parallel a spectra of LiV2O5 and NaV2O5, which suggests similar electronic
structure along the a axis in both materials. On the other hand, we find an
almost complete suppression of the peaks in sigma_b of LiV2O5 around 1 and 5
eV. We attribute this suppression to the charge localization originating from
the existence of double-chain charge-ordering patterin in LiV2O5.Comment: 7 pages, 3 figures final version, to appear in PR
Activation-Induced Cytidine Deaminase Deficiency Causes Organ-Specific Autoimmune Disease
Activation-induced cytidine deaminase (AID) expressed by germinal center B cells is a central regulator of somatic hypermutation (SHM) and class switch recombination (CSR). Humans with AID mutations develop not only the autosomal recessive form of hyper-IgM syndrome (HIGM2) associated with B cell hyperplasia, but also autoimmune disorders by unknown mechanisms. We report here that AID−/− mice spontaneously develop tertiary lymphoid organs (TLOs) in non-lymphoid tissues including the stomach at around 6 months of age. At a later stage, AID−/− mice develop a severe gastritis characterized by loss of gastric glands and epithelial hyperplasia. The disease development was not attenuated even under germ-free (GF) conditions. Gastric autoantigen -specific serum IgM was elevated in AID−/− mice, and the serum levels correlated with the gastritis pathological score. Adoptive transfer experiments suggest that autoimmune CD4+ T cells mediate gastritis development as terminal effector cells. These results suggest that abnormal B-cell expansion due to AID deficiency can drive B-cell autoimmunity, and in turn promote TLO formation, which ultimately leads to the propagation of organ-specific autoimmune effector CD4+ T cells. Thus, AID plays an important role in the containment of autoimmune diseases by negative regulation of autoreactive B cells
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