Disorder effects in the quantum Heisenberg model: An Extended Dynamical mean-field theory analysis

We investigate a quantum Heisenberg model with both antiferromagnetic and disordered nearest-neighbor couplings. We use an extended dynamical mean-field approach, which reduces the lattice problem to a self-consistent local impurity problem that we solve by using a quantum Monte Carlo algorithm. We consider both two- and three-dimensional antiferromagnetic spin fluctuations and systematically analyze the effect of disorder. We find that in three dimensions for any small amount of disorder a spin-glass phase is realized. In two dimensions, while clean systems display the properties of a highly correlated spin-liquid (where the local spin susceptibility has a non-integer power-low frequency and/or temperature dependence), in the present case this behavior is more elusive unless disorder is very small. This is because the spin-glass transition temperature leaves only an intermediate temperature regime where the system can display the spin-liquid behavior, which turns out to be more apparent in the static than in the dynamical susceptibility.Comment: 15 pages, 7 figure

Intrinsic susceptibility and bond defects in the novel 2D frustrated antiferromagnet Ba2_{2}Sn2_{2}ZnCr7p_{7p}Ga10−7p_{10-7p}O22_{22}

We present microscopic and macroscopic magnetic properties of the highly frustrated antiferromagnet Ba$_{2}$Sn$_{2}$ZnCr$_{7p}$Ga$_{10-7p}$O$_{22}$, respectively probed with NMR and SQUID experiments. The $T$-variation of the intrinsic susceptibility of the Cr$^{3+}$ frustrated kagom\'{e} bilayer, $\chi_{kag}$, displays a maximum around 45 K. The dilution of the magnetic lattice has been studied in detail for $0.29 \leq p \leq0.97$. Novel dilution independent defects, likely related with magnetic bond disorder, are evidenced and discussed. We compare our results to SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$. Both bond defects and spin vacancies do not affect the average susceptibility of the kagom\'{e} bilayers.Comment: Published in Phys. Rev. Lett. 92, 217202 (2004). Only minor changes as compared to previous version. 4 pages, 4 figure

A Systematic Review of Cognitive Function in First-Episode Psychosis, Including a Discussion on Childhood Trauma, Stress, and Inflammation

Objective: To carry out a systematic review of the literature addressing cognitive functions in first-episode psychosis (FEP), divided into domains. Although this is not a full "cognitive-genetics-in-schizophrenia review," we will also include putative ideas of mechanism(s) behind these impairments, focusing on how early stress, and genetic vulnerability may moderate cognitive function in psychosis. Method: Relevant studies were identified via computer literature searches for research published up to and including January 2013, only case-control studies were included for the neurocognitive meta-analysis. Results: Patients with FEP present global cognitive impairment compared to healthy controls. The largest effect size was observed for verbal memory (Cohen's d effect size = 2.10), followed by executive function (effect size = 1.86), and general IQ (effect size = 1.71). However, effect sizes varied between studies. Conclusion: Cognitive impairment across domains, up to severe level based on Cohen's effect size, is present already in FEP studies. However, differences in levels of impairment are observed between studies, as well as within domains, indicating that further consolidation of cognitive impairment over the course of illness may be present. Cognitive abnormalities may be linked to a neurodevelopmental model including increased sensitivity to the negative effect of stress, as well as genetic vulnerability. More research on this field is needed

Ga-NMR local susceptibility of the kagome-based magnet SrCr_9pGa_(12-9p)O_19. A high temperature study

We report a high-$T$ Ga-NMR study in the kagome-based antiferromagnetic compound SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ ($.81\leq p\leq .96$), and present a refined mean-field analysis of the high T local NMR susceptibility of Cr frustrated moments. We find that the intralayer kagome coupling is $J=86(6)$ K, and the interlayer coupling through non-kagome Cr moments is $J^{\prime }=69(7)$ K. The $J^{\prime}/J=0.80(1)$ ratio confirms the common belief that the frustrated entity is a pyrochlore slab.Comment: 8 pages, 4 figures Conference paper: Highly Frustrated Magnetism 2000, Waterloo (Canada) Submitted to Canadian Journal of Physic

A Survey of e-Biodiversity: Concepts, Practices, and Challenges

The unprecedented size of the human population, along with its associated economic activities, have an ever increasing impact on global environments. Across the world, countries are concerned about the growing resource consumption and the capacity of ecosystems to provide them. To effectively conserve biodiversity, it is essential to make indicators and knowledge openly available to decision-makers in ways that they can effectively use them. The development and deployment of mechanisms to produce these indicators depend on having access to trustworthy data from field surveys and automated sensors, biological collections, molecular data, and historic academic literature. The transformation of this raw data into synthesized information that is fit for use requires going through many refinement steps. The methodologies and techniques used to manage and analyze this data comprise an area often called biodiversity informatics (or e-Biodiversity). Biodiversity data follows a life cycle consisting of planning, collection, certification, description, preservation, discovery, integration, and analysis. Researchers, whether producers or consumers of biodiversity data, will likely perform activities related to at least one of these steps. This article explores each stage of the life cycle of biodiversity data, discussing its methodologies, tools, and challenges

Molecular Deuterion crystallitation under cuasi-1D confienment

ECNS 2015, Zaragoza (Spain), August 30th-September 4th 2015A particularly interesting aspect of Carbon Nanotubes is their use as nearly one-dimensional nano-containers. Apart of their possibilities for controlled chemistry in nano- fluidics devices new phenomena induced by confinement are also expected, such as liquid like ordered structures or exotic crystalline phases. Here, we present a series of neutron diffraction measurements (instrument D20, ILL, Grenoble) of molecular deuterium confined within Multiple Wall Carbon Nanotubes (MWCTNs). Bulk liquid D2 at its vapour pressure crystallises in an hcp structure at ~18.7 K. At low uptakes we have found a clear depression of the solidification temperature down to ~13.25 K. Interestingly, at the lowest uptake the diffraction pattern is consistent with the minimal fcc lattice compatible with a cylindrical symmetry.Peer Reviewe

Collective excitations in liquid D2 confined within the mesoscopic pores of a MCM-41 molecular sieve

We present a comparative study of the excitations in bulk and liquid D2 confined within the pores of MCM-41. The material (Mobile Crystalline Material-41) is a silicate obtained by means of a template that yields a partially crystalline structure composed by arrays of nonintersecting hexagonal channels of controlled width having walls made of amorphous SiO2. Its porosity was characterized by means of adsorption isotherms and found to be composed by a regular array of pores having a narrow distribution of sizes with a most probable value of 2.45 nm. The assessment of the precise location of the sample within the pores is carried out by means of pressure isotherms. The study was conducted at two pressures which correspond to pore fillings above the capillary condensation regime. Within the range of wave vectors where collective excitations can be followed up (0.3<Q<3.0 $\AA$&#8722;1), we found confinement brings forward a large shortening of the excitation lifetimes that shifts the characteristic frequencies to higher energies. In addition, the coherent quasielastic scattering shows signatures of reduced diffusivity.Comment: 6 page

Crystallization of para-Hydrogen: a quantum phase transition at finite temperature?

4 págs.; 3 figs.; Open Access funded by Creative Commons Atribution Licence 3.0; 7th Meeting of the Spanish Neutron Scattering Association (SETN)Recent neutron scattering measurements have revealed that para-Hydrogen prepared in its fundamental state solidifies in a crystal that do not show temperature response (temperature independent molecular displacement and first rotational level linewidth as well as negligible thermal expansion). Here, it is argued that such unique behaviour in the crystal phase can be understood if the crystallization is driven by pure quantum fluctuations.Work supported in part by grants MAT2007-65711-C04-01 and MAT2012-33633 from the Spanish Ministerio de Economía y Competitividad.Peer Reviewe

Evidence for two disparate spin dynamic regimes within Fe-substituted La0.7 Pb0.3 (Mn1-x Fex) O3 (0≤x≤0.2) colossal magnetoresistive manganites: Neutron spin-echo measurements

10 págs.; 7 figs.; 1 tab. ; PACS number s : 75.25. z, 75.30.Ds, 75.40.Gb, 75.47.GkThe spin dynamics of substituted colossal magnetoresistive (CMR) manganites of general formula La0.7 Pb0.3 (Mn1-x Fex) O3, 0≤x≤0.2 is investigated by means of neutron spin-echo measurements. Substitution of Mn by Fe leads to a strong decrease of the temperature of macroscopic magnetic long-range ordering with a concomitant enhancement of the CMR effect. For x=0.2, a long-range-ordered state is not achieved as a result of the increase in antiferromagnetic interactions brought forward by Fe+3 -Mn couplings. The results display two relaxations having well separated decay constants. A fast process with a relaxation time of about 10 ps within the paramagnetic phase is found for all compositions. It shows a remarkably strong dependence with temperature and sample composition as the apparent activation energy for spin diffusion as well as the preexponential term exemplify. The physical origin of such a fast relaxation is assigned to heavily damped or overdamped spin waves (spin diffusion) on the basis of some signatures of excitations having finite frequencies found for the parent compound La0.7 Pb0.3 Mn O3 at temperatures just below Tc, together with preliminary data on the effect of Fe doping on the stiffness constant. A slower relaxation is present for all compositions. Its temperature dependence follows the behavior of the macroscopic magnetization, and its intensity grows within the ordered ferromagnetic state. Its physical origin is ascribed to collective reorientation of nanoscale ferromagnetic domains on the basis of the wave-vector dependence of its relaxation rate and amplitude. © 2007 The American Physical Society.J.G. and J.M.B. thank the Spanish Ministerio de Educacion y Ciencia for financial support under research Grant No. MAT2005-0686-C04-03. F.J.B. and P.R. acknowledge financial support from the European Commission through NMI3 to carry out preliminary measurements at the FZJ facilities.Peer Reviewe

White matter integrity as a predictor of response to treatment in first episode psychosis

The integrity of brain white matter connections is central to a patient's ability to respond to pharmacological interventions. This study tested this hypothesis using a specific measure of white matter integrity, and examining its relationship to treatment response using a prospective design in patients within their first episode of psychosis. Diffusion tensor imaging data were acquired in 63 patients with first episode psychosis and 52 healthy control subjects (baseline). Response was assessed after 12 weeks and patients were classified as responders or non-responders according to treatment outcome. At this second time-point, they also underwent a second diffusion tensor imaging scan. Tract-based spatial statistics were used to assess fractional anisotropy as a marker of white matter integrity. At baseline, non-responders showed lower fractional anisotropy than both responders and healthy control subjects (P < 0.05; family-wise error-corrected), mainly in the uncinate, cingulum and corpus callosum, whereas responders were indistinguishable from healthy control subjects. After 12 weeks, there was an increase in fractional anisotropy in both responders and non-responders, positively correlated with antipsychotic exposure. This represents one of the largest, controlled investigations of white matter integrity and response to antipsychotic treatment early in psychosis. These data, together with earlier findings on cortical grey matter, suggest that grey and white matter integrity at the start of treatment is an important moderator of response to antipsychotics. These findings can inform patient stratification to anticipate care needs, and raise the possibility that antipsychotics may restore white matter integrity as part of the therapeutic response