Investigation of the Spin-Peierls transition in CuGeO_3 by Raman scattering

Raman experiments on the spin-Peierls compound CuGeO$_3$ and the substituted (Cu$_{1- x}$,Zn$_x$)GeO$_3$ and Cu(Ge$_{1-x}$,Ga$_x$)O$_3$ compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO$_3$, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below T$_{SP}$=14 K a singlet-triplet excitation at 30 cm$^{-1}$, two-magnon scattering from 30 to 227 cm$^{-1}$ and folded phonon modes at 369 and 819 cm$^{-1}$ were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition T$_{SP}$ and approximately 100 K a broad intensity maximum centered at 300 cm$^{-1}$ is observed.Comment: 7 pages, LaTex2e, including 3 figures (eps) to be published in Physica B (1996

Interference of a first-order transition with the formation of a spin-Peierls state in alpha'-NaV2O5?

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline alpha'-NaV2O5. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33K: A sharp first-order transition at T1=(33+-0.1)K slightly below the onset of a second-order transition at T2onset around (34+-0.1)K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at T1 induced by the incipient spin-Peierls (SP) order parameter above T2=TSP.Comment: 5 pages, 7 figure

Effectiveness of lurasidone in schizophrenia or schizoaffective patients switched from other antipsychotics: a 6-month, open-label, extension study

Objective. To evaluate the long-term safety and tolerability of lurasidone in schizophrenia and schizoaffective disorder patients switched to lurasidone. Method. Patients in this multicenter, 6-month open-label, flexible-dose, extension study had completed a core 6-week randomized trial in which clinically stable, but symptomatic, outpatients with schizophrenia or schizoaffective disorder were switched to lurasidone. Patients started the extension study on treatment with the same dose of lurasidone taken at study endpoint of the 6-week core study; following this, lurasidone was flexibly dosed (40-120 mg/day), if clinically indicated, starting on Day 7 of the extension study. The primary safety endpoints were the proportion of patients with treatment emergent adverse events (AEs), serious AEs, or who discontinued due to AEs. Secondary endpoints included metabolic variables and measures of extrapyramidal symptoms and akathisia, as well as the Positive and Negative Syndrome Scale (PANSS), Clinical Global Impressions-Severity (CGI-S), and the Calgary Depression Scale for Schizophrenia (CDSS). The study was conducted from August 2010 to November 2011. Results. Of the 198 patients who completed the 6-week core study, 149 (75.3%) entered the extension study and 148 received study medication. A total of 98 patients (65.8%) completed the 6-month extension study. Lurasidone 40, 80, and 120 mg were the modal daily doses for 19 (12.8%), 65 (43.9%), and 64 (43.2%) of patients, respectively. Overall mean (SD) daily lurasidone dose was 102.0 mg (77.1). The most commonly reported AEs were insomnia (13 patients [8.8%]), nausea (13 patients [8.8%]), akathisia (12 patients [8.1%]), and anxiety (9 patients [6.1%]). A total of 16 patients (10.8%) had at least one AE leading to discontinuation from the study. Consistent with prior studies of lurasidone, there was no signal for clinically relevant adverse changes in body weight, lipids, glucose, insulin, or prolactin. Movement disorder rating scales did not demonstrate meaningful changes. Treatment failure (defined as any occurrence of discontinuation due to insufficient clinical response, exacerbation of underlying disease, or AE) was observed for 19 patients (12.8% of patients entering) and median time to treatment failure was 58 days (95% CI 22-86). The discontinuation rate due to any cause was 50/148 (33.8%), and median time to discontinuation was 62 days (95% CI 30-75). The mean PANSS total score, mean CGI-S score, and mean CDSS score decreased consistently from core study baseline across extension visits, indicating an improvement in overall condition. Conclusions. In this 6-month, open-label extension study, treatment with lurasidone was generally well-tolerated with sustained improvement in efficacy measures observed in outpatients with schizophrenia or schizoaffective disorder who had switched to lurasidone from a broad range of antipsychotic agents

NaV_2O_5 as a quarter-filled ladder compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound \alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.Comment: Final version for PRL, value of U correcte

Charge Order Driven spin-Peierls Transition in NaV2O5

We conclude from 23Na and 51V NMR measurements in NaxV2O5(x=0.996) a charge ordering transition starting at T=37 K and preceding the lattice distortion and the formation of a spin gap Delta=106 K at Tc=34.7 K. Above Tc, only a single Na site is observed in agreement with the Pmmn space group of this first 1/4-filled ladder system. Below Tc=34.7 K, this line evolves into eight distinct 23Na quadrupolar split lines, which evidences a lattice distortion with, at least, a doubling of the unit cell in the (a,b) plane. A model for this unique transition implying both charge density wave and spin-Peierls order is discussed.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Frustration induced Raman scattering in CuGeO_3

We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO_3 and theoretical calculations from a one-dimensional frustrated spin model. The theory is based on (a) exact diagonalization and (b) a recently developed solitonic mean field theory. We find good agreement between the 1D-theory in the homogeneous phase and evidence for a novel dimerization of the Raman operator in the spin-Peierls state. Finally we present evidence for a coupling between the interchain exchange, the spin-Peierls order parameter and the magnetic excitations along the chains.Comment: Phys. Rev. B, Rapid Comm, in Pres

Magnetic Excitations in quasi two-dimensional Spin-Peierls Systems

A study is presented of a two-dimensional frustrated and dimerized quantum spin-system which models the effect of inter-chain coupling in a spin-Peierls compound. Employing a bond-boson method to account for quantum disorder in the ground state the elementary excitations are evaluated in terms of gapful triplet modes. Results for the ground state energy and the spin gap are discussed. The triplet dispersion is found to be in excellent agreement with inelastic neutron scattering data in the dimerized phase of the spin-Peierls compound CuGeO_3. Moreover, consistent with these neutron scattering experiments, the low-temperature dynamic structure factor exhibits a high-energy continuum split off from the elementary triplet mode.Comment: 8 pages, Revtex, 8 eps-figure

Spin gap behavior and charge ordering in \alpha^{\prime}-NaV_2O_5 probed by light scattering

We present a detailed analysis of light scattering experiments performed on the quarter-filled spin ladder compound $\alpha^\prime$-NaV$_{2}$O$_{5}$ for the temperature range 5 K$\le$T$\le$300 K. This system undergoes a phase transition into a singlet ground state at T=34 K accompanied by the formation of a super structure. For T$\leq$34 K several new modes were detected. Three of these modes are identified as magnetic bound states. Experimental evidence for charge ordering on the V sites is detected as an anomalous shift and splitting of a V-O vibration at 422 cm$^{-1}$ for temperatures above 34 K. The smooth and crossover-like onset of this ordering at T$_{\rm CO}$= 80 K is accompanied by pretransitional fluctuations both in magnetic and phononic Raman scattering. It resembles the effect of stripe order on the super structure intensities in La$_2$NiO$_{4+\delta}$.Comment: 36 pages, 11 figures, accepted for publication in PRB (sept.99

Statistical validation of the criteria for symptom remission in schizophrenia: Preliminary findings

<p>Abstract</p> <p>Background</p> <p>Published methods for assessing remission in schizophrenia are variable and none have been definitively validated or standardized. Andreasen et al (2005) suggest systematic operational criteria using eight PANSS items for which patients must score ≤ 3 (mild) for at least six months.</p> <p>Methods</p> <p>Using data from a one year, multi-site clinical trial (n = 675) remission criteria were compared to total PANSS scores and other endpoints and demonstrate excellent agreement with overall clinical status.</p> <p>Results</p> <p>Compared to total PANSS score of 60 points and other criteria, at time points > 6 months (8 and 12 months) the specificity of the remission criteria was 85%, i.e. of the patients who had a total score >60, 85% were classified as "not in remission." Sensitivity was also very high; 75% of patients with scores of <60 were classified as "in remission."Patients who dropped out of the trial were more likely not to be in remission prior to dropping out.</p> <p>Conclusion</p> <p>These findings indicate that the remission criteria are both sensitive and specific indicators of clinical status. Additional analyses are required to determine if remission status predicts other outcomes, such as employment, independent living, and prognosis.</p

Molecular-field approach to the spin-Peierls transition in CuGeO_3

We present a theory for the spin-Peierls transition in CuGeO_3. We map the elementary excitations of the dimerized chain (solitons) on an effective Ising model. Inter-chain coupling (or phonons) then introduce a linear binding potential between a pair of soliton and anti-soliton, leading to a finite transition temperature. We evaluate, as a function of temperature, the order parameter, the singlet-triplet gap, the specific heat, and the susceptibility and compare with experimental data on CuGeO_3. We find that CuGeO_3 is close to a first-order phase transition. We point out, that the famous scaling law \sim\delta^{2/3} of the triplet gap is a simple consequence of the linear binding potential between pairs of solitons and anti-solitons in dimerized spin chains.Comment: 7.1 pages, figures include