123 research outputs found
Spin-driven Phonon Splitting in Bond-frustrated ZnCr2S4
Utilizing magnetic susceptibility, specific heat, thermal expansion and IR
spectroscopy we provide experimental evidence that the two subsequent
antiferromagnetic transitions in ZnCr_2S_4 at T_N1 = 15 K and T_N2= 8 K are
accompanied by significant thermal and phonon anomalies. The anomaly at T_N2
reveals a strong temperature hysteresis typical for a first-order
transformation. Due to strong spin-phonon coupling both magnetic phase
transitions induce a splitting of phonon modes, where at T_N1 the
high-frequency and at T_N2 the low-frequency modes split. The anomalies and
phonon splitting observed at T_N2 are strongly suppressed by magnetic field.
Regarding the small positive Curie-Weiss temperature Theta= 8 K, we argue that
this scenario of two different magnetic phases with concomitant different
magneto-elastic couplings results from the strong competition of ferromagnetic
and antiferromagnetic exchange of equal strength.Comment: 4 pages, 4 figure
Spin-driven Phase Transitions in ZnCrSe and ZnCrS Probed by High Resolution Synchrotron X-ray and Neutron Powder Diffraction
The crystal and magnetic structures of the spinel compounds ZnCrS and
ZnCrSe were investigated by high resolution powder synchrotron and
neutron diffraction. ZnCrSe exhibits a first order phase transition at
K into an incommensurate helical magnetic structure. Magnetic
fluctuations above are coupled to the crystal lattice as manifested by
negative thermal expansion. Both, the complex magnetic structure and the
anomalous structural behavior can be related to magnetic frustration.
Application of an external magnetic field shifts the ordering temperature and
the regime of negative thermal expansion towards lower temperatures. Thereby,
the spin ordering changes into a conical structure. ZnCrS shows two
magnetic transitions at K and K that are accompanied by
structural phase transitions. The crystal structure transforms from the cubic
spinel-type (space group \={3}) at high temperatures in the paramagnetic
state, via a tetragonally distorted intermediate phase (space group /
) for into a low temperature orthorhombic phase
(space group ) for . The cooperative displacement of
sulfur ions by exchange striction is the origin of these structural phase
transitions. The low temperature structure of ZnCrS is identical to the
orthorhombic structure of magnetite below the Verwey transition. When applying
a magnetic field of 5 T the system shows an induced negative thermal expansion
in the intermediate magnetic phase as observed in ZnCrSe.Comment: 11 pages, 13 figures, to be published in PR
Spin Driven Jahn-Teller Distortion in a Pyrochlore system
The ground-state properties of the spin-1 antiferromagnetic Heisenberg model
on the corner-sharing tetrahedra, pyrochlore lattice, is investigated. By
breaking up each spin into a pair of 1/2-spins, the problem is reduced to the
equivalent one of the spin-1/2 tetrahedral network in analogy with the valence
bond solid state in one dimension. The twofold degeneracy of the spin-singlets
of a tetrahedron is lifted by a Jahn-Teller mechanism, leading to a cubic to
tetragonal structural transition. It is proposed that the present mechanism is
responsible for the phase transition observed in the spin-1 spinel compounds
ZnVO and MgVO.Comment: 4 pages, 3 eps figures, REVTeX, to appear in Phys. Rev. Let
Magnetic frustration in the spinel compounds Ge Co_2 O_4 and Ge Ni_2 O_4
In both spinel compounds GeCoO and GeNiO which order
antiferromagnetically (at and , ) with different Curie Weiss temperatures (=80.5 K and -15 K),
the usual magnetic frustration criterion is not fulfilled.
Using neutron powder diffraction and magnetization measurements up to 55 T,
both compounds are found with a close magnetic ground state at low temperature
and a similar magnetic behavior (but with a different energy scale), even
though spin anisotropy and first neighbor exchange interactions are quite
different. This magnetic behavior can be understood when considering the main
four magnetic exchange interactions. Frustration mechanisms are then
enlightened.Comment: submitted to Phys.Rev.B (2006
A crystallographic phase transition within the magnetically ordered state of Ce_2Fe_17
X-ray diffraction experiments were performed on polycrystalline and
single-crystal specimens of CeFe at temperatures between 10 K and
300 K. Below = 1182 K, additional weak superstructure
reflections were observed in the antiferromagnetically ordered state. The
superstructure can be described by a doubling of the chemical unit cell along
the direction in hexagonal notation with the same space group as the room-temperature structure. The additional antiferromagnetic
satellite reflections observed in earlier neutron diffraction experiments can
be conclusively related to the appearance of this superstructure.Comment: 8 pages, figures, submitted for publication in Phys. Rev.
Magnetic properties of the frustrated AFM spinel ZnCr_2O_4 and the spin-glass Zn_{1-x}Cd_xCr_2O_4 (x=0.05,0.10)
The -dependence (2- 400 K) of the electron paramagnetic resonance (EPR),
magnetic susceptibility, , and specific heat, , of the
antiferromagnetic (AFM) spinel ZnCrO and the spin-glass
(SG) ZnCdCrO () is reported. These
systems behave as a strongly frustrated AFM and SG with K and -400 K K. At high-
the EPR intensity follows the and the -value is -independent.
The linewidth broadens as the temperature is lowered, suggesting the existence
of short range AFM correlations in the paramagnetic phase. For
ZnCrO the EPR intensity and decreases below 90 K and 50
K, respectively. These results are discussed in terms of nearest-neighbor
Cr (S %) spin-coupled pairs with an exchange coupling of 50 K. The appearance of small resonance modes for K,
the observation of a sharp drop in and a strong peak in
at K confirms, as previously reported, the existence of long range
AFM correlations in the low- phase. A comparison with recent neutron
diffraction experiments that found a near dispersionless excitation at 4.5 meV
for and a continuous gapless spectrum for ,
is also given.Comment: 17 pages, 8 figures, 1 Table. Submitted to Physical Review
Identification of Ischemic Regions in a Rat Model of Stroke
Investigations following stroke first of all require information about the spatio-temporal dimension of the ischemic core as well as of perilesional and remote affected tissue. Here we systematically evaluated regions differently impaired by focal ischemia.Wistar rats underwent a transient 30 or 120 min suture-occlusion of the middle cerebral artery (MCAO) followed by various reperfusion times (2 h, 1 d, 7 d, 30 d) or a permanent MCAO (1 d survival). Brains were characterized by TTC, thionine, and immunohistochemistry using MAP2, HSP72, and HSP27. TTC staining reliably identifies the infarct core at 1 d of reperfusion after 30 min MCAO and at all investigated times following 120 min and permanent MCAO. Nissl histology denotes the infarct core from 2 h up to 30 d after transient as well as permanent MCAO. Absent and attenuated MAP2 staining clearly identifies the infarct core and perilesional affected regions at all investigated times, respectively. HSP72 denotes perilesional areas in a limited post-ischemic time (1 d). HSP27 detects perilesional and remote impaired tissue from post-ischemic day 1 on. Furthermore a simultaneous expression of HSP72 and HSP27 in perilesional neurons was revealed.TTC and Nissl staining can be applied to designate the infarct core. MAP2, HSP72, and HSP27 are excellent markers not only to identify perilesional and remote areas but also to discriminate affected neuronal and glial populations. Moreover markers vary in their confinement to different reperfusion times. The extent and consistency of infarcts increase with prolonged occlusion of the MCA. Therefore interindividual infarct dimension should be precisely assessed by the combined use of different markers as described in this study
Plasma antibodies against heat shock protein 70 correlate with the incidence and severity of asthma in a Chinese population
BACKGROUND: The heat shock proteins (Hsps) are induced by stresses such as allergic factors and inflammatory responses in bronchi epithelial cells and therefore may be detectable in patients with asthma. However, the etiologic link between anti-Hsps and asthma (its severity and related inflammatory responses such as interleukin-4 and immunoglobulin E) has not been established. We determined whether antibodies against Hsp60 and Hsp70 were present in patients with asthma and evaluated their associations with risk and severity of asthma. METHODS: We determined the levels of anti-Hsp60 and anti-Hsp70 by immunoblot and their associations with risk and symptom severity of asthma in 95 patients with asthma and 99 matched non-symptomatic controls using multivariate logistic regression analysis. RESULTS: Compared to the controls, asthma patients were more likely to have detectable anti-Hsp60 (17.2% vs 5.1%) and anti-Hsp70 (33.7% vs 8.1%) (p ≤ 0.001). In particular, the presence of anti-Hsp70 was associated with a greater than 2 fold risk for asthma (adjusted OR = 2.21; 95% CI = 1.35~3.59). Furthermore, both anti-Hsp60 and anti-Hsp70 levels were positively correlated with symptom severity (p < 0.05) as well as interleukin-4 and immunoglobulin E (p < 0.05). Individuals with antibodies against anti-Hsp60 and anti-Hsp70 were more likely to have a family history of asthma (p < 0.001) and higher plasma concentrations of total immunoglobulin E (p = 0.001) and interleukin-4 (p < 0.05) than those without antibodies. CONCLUSIONS: These data suggest that anti-Hsp60 and especially anti-Hsp70 correlate with the attacks and severity of asthma. The underlying molecular mechanisms linking antibodies to heat shock proteins and asthma remain to be investigated
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