766 research outputs found
High-field phase diagram of the Haldane-gap antiferromagnet
We have determined the magnetic phase diagram of the quasi-one-dimensional
1 Heisenberg antiferromagnet by
specific heat measurements to 150 mK in temperature and 32 T in magnetic field.
When field is applied along the spin-chain direction, a new phase appears at
T. For the previously known phases of field-induced order,
accurate determination is made of the power-law exponents of the ordering
temperature near the zero-temperature critical field , owing to the
four-fold improvement of the minimum temperature over the previous work. The
results are compared with the predictions based on the Bose-Einstein
condensation of triplet excitations. Substituting deuterium for hydrogen is
found to slightly reduce the interchain exchange.Comment: 6 pages, 6 figure
Specific heat of the spin-dimer antiferromagnet BaMnO in high magnetic fields
We have measured the specific heat of the coupled spin-dimer antiferromagnet
BaMnO to 50 mK in temperature and to 29 T in the magnetic field.
The experiment extends to the midpoint of the field region (25.9 T 32.3 T) of the magnetization plateau at 1/2 of the saturation
magnetization, and reveals the presence of three ordered phases in the field
region between that of the magnetization plateau and the low-field spin-liquid
region. The exponent of the phase boundary with the thermally disordered region
is smaller than the theoretical value based on the Bose-Einstein condensation
of spin triplets. At zero field and 29 T, the specific-heat data show gapped
behaviors characteristic of spin liquids. The zero-field data indicate that the
gapped triplet excitations form two levels whose energies differ by nearly a
factor of two. At least the lower level is well localized. The data at 29 T
reveal that the low-lying excitations at the magnetization plateau are weakly
delocalized.Comment: 6 pages, 5 figures, revised versio
Alterations in cellular adhesion and apoptosis in epithelial cells overexpressing prostaglandin endoperoxide synthase 2
AbstractProstaglandin endoperoxide synthase 2, also referred to as cyclooxygenase 2 (COX-2), is a key enzyme in the conversion of arachidonic acid to prostaglandins and other eicosanoids. Rat intestinal epithelial (RIE) cells were permanently transfected with a COX-2 expression vector oriented in the sense (RIE-S) or antisense (RIE-AS) direction. The RIE-S cells expressed elevated COX-2 protein levels and demonstrated increased adhesion to extracellular matrix (ECM) proteins. E-cadherin was undetectable in RIE-S cells, but was elevated in parental RIE (RIE-P) and RIE-AS cells. RIE-S cells were resistant to butyrate-induced apoptosis, had elevated BCL2 protein expression, and reduced transforming growth factor β2 receptor levels. The phenotypic changes involving both increased adhesion to ECM and inhibition of apoptosis were reversed by sulindac sulfide (a COX inhibitor). These studies demonstrate that overexpression of COX-2 leads to phenotypic changes in intestinal epithelial cells that could enhance their tumorigenic potential
Field-induced magnetic ordering in the Haldane system PbNi2V2O8
The Haldane system PbNi2V2O8 was investigated by the temperature dependent
magnetization M(T) measurements at fields higher than H_c, with H_c the
critical fields necessary to close the Haldane gap. It is revealed that M(T)
for H > H_c exhibits a cusp-like minimum at T_{min}, below which M(T) increases
with decreasing T having a convex curve. These features have been observed for
both and , with c-axis being parallel to the chain.
These data indicate the occurrence of field-induced magnetic ordering around
T_{min}. Phase boundaries for and do not cross each
other, consistent with the theoretical calculation for negative single-ion
anisotropy D.Comment: 3 figures, submitted to Phys. Rev.
Single crystal MgB2 with anisotropic superconducting properties
The discovery of superconductor in magnesium diboride MgB2 with high Tc (39
K) has raised some challenging issues; whether this new superconductor
resembles a high temperature cuprate superconductor(HTS) or a low temperature
metallic superconductor; which superconducting mechanism, a phonon- mediated
BCS or a hole superconducting mechanism or other new exotic mechanism may
account for this superconductivity; and how about its future for applications.
In order to clarify the above questions, experiments using the single crystal
sample are urgently required. Here we have first succeeded in obtaining the
single crystal of this new MgB2 superconductivity, and performed its electrical
resistance and magnetization measurements. Their experiments show that the
electronic and magnetic properties depend on the crystallographic direction.
Our results indicate that the single crystal MgB2 superconductor shows
anisotropic superconducting properties and thus can provide scientific basis
for the research of its superconducting mechanism and its applications.Comment: 7 pages pdf fil
Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet Observed by the Specific Heat
Specific heat experiments on single crystals of the S=1 quasi-one-dimensional
bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have
been performed in magnetic fields applied both parallel and perpendicular to
the spin chains. We have found for the parallel field configuration that the
magnetic specific heat (C_mag) is proportional to temperature (T) above a
critical field H_c, at which the energy gap vanishes, in a temperature region
above that of the long-range ordered state. The ratio C_mag/T increases as the
magnetic field approaches H_c from above. The data are in good quantitative
agreement with the prediction of the c=1 conformal field theory in conjunction
with the velocity of the excitations calculated by a numerical diagonalization,
providing a conclusive evidence for a Tomonaga-Luttinger liquid.Comment: 4 pages, 4 postscript figure
- …