2,645 research outputs found
Spin-lattice coupling in frustrated antiferromagnets
We review the mechanism of spin-lattice coupling in relieving the geometrical
frustration of pyrochlore antiferromagnets, in particular spinel oxides. The
tetrahedral unit, which is the building block of the pyrochlore lattice,
undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom
are coupled to the antiferromagnetism. By restricting our considerations to
distortions which preserve the translational symmetries of the lattice, we
present a general theory of the collective spin-Jahn-Teller effect in the
pyrochlore lattice. One of the predicted lattice distortions breaks the
inversion symmetry and gives rise to a chiral pyrochlore lattice, in which
frustrated bonds form helices with a definite handedness. The chirality is
transferred to the spin system through spin-orbit coupling, resulting in a
long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit
models of spin-lattice coupling using local phonon modes, and their
applications in other frustrated magnets.Comment: 23 pages, 6 figures. Lecture notes for Trieste Summer School, August
2007. To appear as a chapter in "Highly Frustrated Magnetism", Eds. C.
Lacroix, P. Mendels, F. Mil
Quantum Effects and Broken Symmetries in Frustrated Antiferromagnets
We investigate the interplay between frustration and zero-point quantum
fluctuations in the ground state of the triangular and Heisenberg
antiferromagnets, using finite-size spin-wave theory, exact diagonalization,
and quantum Monte Carlo methods. In the triangular Heisenberg antiferromagnet,
by performing a systematic size-scaling analysis, we have obtained strong
evidences for a gapless spectrum and a finite value of the thermodynamic order
parameter, thus confirming the existence of long-range N\'eel order.The good
agreement between the finite-size spin-wave results and the exact and quantum
Monte Carlo data also supports the reliability of the spin-wave expansion to
describe both the ground state and the low-energy spin excitations of the
triangular Heisenberg antiferromagnet. In the Heisenberg model, our
results indicate the opening of a finite gap in the thermodynamic excitation
spectrum at , marking the melting of the antiferromagnetic
N\'eel order and the onset of a non-magnetic ground state. In order to
characterize the nature of the latter quantum-disordered phase we have computed
the susceptibilities for the most important crystal symmetry breaking
operators. In the ordered phase the effectiveness of the spin-wave theory in
reproducing the low-energy excitation spectrum suggests that the uniform spin
susceptibility of the model is very close to the linear spin-wave prediction.Comment: Review article, 44 pages, 18 figures. See also PRL 87, 097201 (2001
Suppression of Dimer Correlations in the Two-Dimensional - Heisenberg Model: an Exact Diagonalization Study
We present an exact diagonalization study of the ground state of the
spin-half model. Dimer correlation functions and the susceptibility
associated to the breaking of the translational invariance are calculated for
the and the clusters. These results -- especially when
compared to the one dimensional case, where the occurrence of a dimerized phase
for large enough frustration is well established -- suggest either a
homogeneous spin liquid or, possibly, a dimerized state with a rather small
order parameter
Thermodynamics of the quantum easy-plane antiferromagnet on the triangular lattice
The classical XXZ triangular-lattice antiferromagnet (TAF) shows both an
Ising and a BKT transition, related to the chirality and the in-plane spin
components, respectively. In this paper the quantum effects on the
thermodynamic quantities are evaluated by means of the pure-quantum
self-consistent harmonic approximation (PQSCHA), that allows one to deal with
any spin value through classical MC simulations. We report the internal energy,
the specific heat, and the in-plane correlation length of the quantum XX0 TAF,
for S=1/2, 1, 5/2. The quantum transition temperatures turn out to be smaller
the smaller the spin, and agree with the few available theoretical and
numerical estimates.Comment: 4 pages,3 postscript figure
Treatment responses to antiangiogenetic therapy and chemotherapy in nonsecreting paraganglioma (PGL4) of urinary bladder with SDHB mutation: a case report
Paraganglioma (PGL) is a rare neuroendocrine tumor. Currently, the malignancy is defined as the presence of metastatic spread at presentation or during follow-up. Several gene mutations are listed in the pathogenesis of PGL, among which succinate dehydrogenase (SDHX), particularly the SDHB isoform, is the main gene involved in malignancy. A 55-year-old male without evidence of catecholamine secretion had surgery for PGL of the urinary bladder. After 1 year, he showed a relapse of disease and demonstrated malignant PGL without evidence of catecholamine secretion with a germline heterozygous mutation of succinate dehydrogenase B (SDHB). After failure of a second surgery for relapse, he started medical treatment with sunitinib daily but discontinued due to serious side effects. Cyclophosphamide, vincristine, and dacarbazine (CVD) chemotherapeutic regimen stopped the disease progression for 7 months.
Conclusion: Malignant PGL is a very rare tumor, and SDHB mutations must be always considered in molecular diagnosis because they represent a critical event in the progression of the oncological disease. Currently, there are few therapeutic protocols, and it is often difficult, as this case demonstrates, to decide on a treatment option according to a reasoned set of choices.
Abbreviations: CVD = cyclophosphamide, vincristine and dacarbazine, HIF-1a = hypoxia inducible factor 1 alpha, PGL = paraganglioma, SDH = succinate dehydrogenase, VEGF = vasoendothelial growth factor
Absence of Translational Symmetry Breaking in Nonmagnetic Insulator Phase on Two-Dimensional Lattice with Geometrical Frustration
The ground-state properties of the two-dimensional Hubbard model with
nearest-neighbor and next-nearest-neighbor hoppings at half filling are studied
by the path-integral-renormalization-group method. The nonmagnetic-insulator
phase sandwiched by the the paramagnetic-metal phase and the
antiferromagnetic-insulator phase shows evidence against translational symmetry
breaking of the dimerized state, plaquette singlet state, staggered flux state,
and charge ordered state. These results support that the genuine Mott insulator
which cannot be adiabatically continued to the band insulator is realized
generically by Umklapp scattering through the effects of geometrical
frustration and quantum fluctuation in the two-dimensional system.Comment: 4 pages and 7 figure
The Rapid Measurement of Benzodiazepines in a Milk-Based Alcoholic Beverage Using QuEChERS Extraction and GC–MS Analysis
none5noBenzodiazepines (BDZs) are widely used as tranquilizers and antidepressive
drugs in common clinical practice. However, their ready
availability and their synergistic effects with alcohol make them
attractive for criminal intentions. To prove criminal action for legal
reasons, it is often necessary to analyze beverage residues from a
crime scene. Milk-based alcoholic drinks (whiskey creams) are gaining
popularity due to their lower alcohol content pleasant taste.
However, the complexity of this sample, containing proteins and
fatty acids, can mask the presence of drugs or other substances in
standard analysis methods. These characteristics make whiskey
creams highly suitable for illicit purposes. In this study, eight BDZs,
including diazepam, chlordiazepoxide, clobazam, flunitrazepam, bromazepam,
flurazepam, nitrazepam and clonazepam, were extracted
from whiskey cream using the Quick, Easy, Cheap, Effective,
Rugged and Safe (QuEChERS) method and analyzed using GC–MS.
The QuEChERS protocol can efficiently separate most of the matrix
from the target compounds while maintaining acceptable recoveries.
The presented method is simple and rapid and has been validated in
terms of precision, accuracy and recoveries. Limits of detection and
limits of quantitationwere in the range of 0.02–0.1 and 0.1–0.5 mg/mL,
respectively. Whiskey cream beverages, fortified with commercial
drugs at 20 mg/mL, were extracted and analyzed demonstrating the
applicability of the method in forensic analysis.openFamiglini, G.; Capriotti, F.; Palma, P.; Termopoli, V.; Cappiello, A.Famiglini, Giorgio; Capriotti, Fabiana; Palma, Pierangela; Termopoli, Veronica; Cappiello, Achill
Transcriptional alteration of gene biomarkers in hemocytes of wild ostrea edulis with molecular evidence of infections with bonamia spp. And/or marteilia refringens parasites
The European flat Ostrea edulis is highly susceptible to intracellular parasitic infections, particularly bonamiosis and marteiliosis. The defensive response of oyster to both bonamiosis and marteiliosis is typically mediated by hemocytes, which play a pivotal role in immune system homeostasis. In the present study, we first used a DNA-based tool in order to rapidly and specifically detect the presence of parasites in oysters from natural banks in the middle Adriatic Sea. In a second step, we used qRT-PCR to analyze the mRNA levels of a set of genes (i.e., superoxide dismutase (SOD), glutathione S-transferase (GST), metallothionein (MT), heat shock protein (HSP) 70 and 90, inhibitor of apoptosis (IAP), fas ligand (FAS), galectin (GAL) and extracellular superoxide dismutase (Ec-SOD)) expressed by hemocytes of flat oysters infected by the parasites, present singularly or in combination, compared to hemocytes from non-infected specimens. The results indicate that the presence of parasite DNA may be associated to a general upregulation of host genes related to apoptosis, detoxification and oxidative stress protection, with the exception of Ec-SOD, whose trend to a downregulation might reflect a mechanism for parasite escape before internalization
- …