Excitonic resonances in thin films of WSe2: From monolayer to bulk material

We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated.Comment: 12 pages, 5 figure

Herziening Besluit Psychotherapeut: kans voor sprong voorwaarts.

Het Besluit Psychotherapeut (bp) 1998 regelt de toegang tot het beroep van psychotherapeut in het big-register. Er klinken geluiden om de definitie van het beroep psychotherapeut opnieuw onder de loep te nemen via een herziening van het bp. Een herziening van dit besluit binnen afzienbare tijd is nuttig en noodzakelijk. De bijdrage van psychotherapieonderzoekers aan de herziening is essentieel, omdat de kwaliteitseisen moeten stoelen op onderzoeksgegevens over de werkzaamheid en kosten)effectiviteit van therapieën in plaats van op theoretische stromingen. Van herziening van het besluit valt een belangrijk effect op de praktijk van de psychotherapeutische hulpverlening te verwachten

Thermodynamics of the vortex liquid in heavy ion-irradiated superconductors

It is shown that the large effect of heavy ion-irradiation on the thermodynamical properties of the anisotropic superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ extends well into the superconducting fluctuation regime. The presence of the induced amorphous columnar defects shifts the specific heat maximum at the normal-to-superconducting transition. This effect is similar to that recently put into evidence in cubic K$_{x}$Ba$_{1-x}$BiO$_{3}$ ($x \simeq 0.35$). In both compounds, vortex pinning manifests itself as a sharp angular dependence of the \em equilibrium \rm torque. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, pinning by the defects appears at the temperature $T_{C_{p}}^{max}$ of the specific heat maximum, well above the magnetic irreversibility line $T_{irr}(H)$. In isotropic K$_{x}$Ba$_{1-x}$BiO$_{3}$, the onset of the pinning-related torque anomaly tracks the onset of the specific heat anomaly and the irreversibility line. In YBa$_{2}$Cu$_{3}$O$_{7-\delta}$, fluctuations of the amplitude of the order parameter (and not vortex line wandering) are ultimately responsible for the vanishing of pinning. In K$_{x}$Ba$_{1-x}$BiO$_{3}$, vortex pinning disappears only at the superconducting-to-normal transition. The results indicate that in both compounds, the pinning energy at the ``Bose glass'' transition is large with respect to the total free energy gain in the superconducting state. By implication, the mechanism of this latter transition should be reconsidered.Comment: 9 pages, 9 figures, resubmitted to PRB 23-09-200

Observation of correlations up to the micrometer scale in sliding charge-density waves

High-resolution coherent x-ray diffraction experiment has been performed on the charge density wave (CDW) system K$_{0.3}$MoO$_3$. The $2k_F$ satellite reflection associated with the CDW has been measured with respect to external dc currents. In the sliding regime, the $2k_F$ satellite reflection displays secondary satellites along the chain axis which corresponds to correlations up to the micrometer scale. This super long range order is 1500 times larger than the CDW period itself. This new type of electronic correlation seems inherent to the collective dynamics of electrons in charge density wave systems. Several scenarios are discussed.Comment: 4 pages, 3 figures Typos added, references remove

Superconductivity in doped sp3 semiconductors: The case of the clathrates

We present a joint experimental and theoretical study of the superconductivity in doped silicon clathrates. The critical temperature in Ba-8@Si-46 is shown to strongly decrease with applied pressure. These results are corroborated by ab initio calculations using MacMillan's formulation of the BCS theory with the electron-phonon coupling constant lambda calculated from perturbative density functional theory. Further, the study of I-8@Si-46 and of gedanken pure silicon diamond and clathrate phases doped within a rigid-band approach show that the superconductivity is an intrinsic property of the sp(3) silicon network. As a consequence, carbon clathrates are predicted to yield large critical temperatures with an effective electron-phonon interaction much larger than in C-60

Dependence of the structural, transport and magnetic properties of Tl1-yFe2-z(Se1-xSx)2 with isovalent substitution of Se by S

International audienceThe effect of selenium substitution by sulfur in the Tl$_{1-y}$Fe$_{2-z}$Se$_{2}$ antiferromagnet was studied by x-ray and electron diffraction, magnetization and transport measurements. Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ (nominal composition) solid solution was synthesized in the full x range (0~$\le x(S)\le$~1) using the sealed tube technique. No superconductivity was found down to 4.2K in the series despite the fact that the optimal crystallographic parameters, determined by Rietveld refinements, are reached in the series (i.e. the Fe-(Se,S) interplane height and (Se,S)-Fe-(Se,S) angle for which the critical superconducting transition T$_{c}$ is usually maximal in pnictides). Quasi~full Tl site ($y\sim0.05$) compared to significant alkaline deficiency ($y=0.2-0.3$) in analogous A$_{1-y}$Fe$_{2-z}$Se$_{2}$ (A~=~K,~Rb,~Cs), and the resulting differences in iron valency, density of states and doping, are suggested to explain this absence of superconductivity. Transmission electron microscopy confirmed the existence of ordered iron vacancies network in the samples of the Tl$_{0.8}$Fe$_{1.5}$(Se$_{1-x}$S$_{x}$)$_{2}$ series in the form of the tetragonal $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c superstructure (\textit{I4/m}). The N\'{e}el temperature (T$_{N}$) indicating the onset of antiferromagnetism order in this $\sqrt{5}$ a $\times \sqrt{5}$ a $\times$ c supercell is found to decrease from 450K in the selenide (x=0) to 330K in the sulfide (x=1). We finally demonstrate a direct linear relationship between $T_{N}$ and the Fe-(Se,S) bond length (or Fe-(Se,S) height)

Effect of pressure on the superconductivity of Rb0.19WO3

International audienceWe have performed electrical resistivity measurements under pressures up to 20GPa between 1 and 300K on monocrystalline hexagonal Rb0.19WO3. For pressures lower than ~5GPa, we observe a decrease of the metallic-like resistivity at room temperature as well as a small decrease of Tc. At this pressure, the resistivity starts to increase slowly up to 10GPa accompanied by a sharper decrease of Tc .The resistivity curves above 10GPa denote an activated behaviour and a Tc lower than 3K indicating that there is a phase transition that takes place gradually between 5 and 10GPa. We interpret our measurements as the result of structural transformations under high pressure

Influence of thermal fluctuations on the Nernst signal in superconducting (K,Ba)BiO3 single crystals

International audienceWe report on the Nernst effect, specific heat and transportmeasurements performed in high quality (K,Ba)BiO3 single crystals close to optimal doping (Tc ∼ 31 K). We show that a nonzero Nernst effect remains visible well above the upper critical field unambiguously deduced from the onset of the specific heat anomaly. This finite Nernst signal is attributed to fluctuations of the amplitude of the order parameter in a region where the free energy is smaller than kBT . Despite the absence of any vortex liquid phase (and hence of any significant phase fluctuations), the field and temperature dependence of the Nernst coefficient is very similar to the one obtained in electron-doped cuprates

Reduced Creatine Kinase B Activity in Multiple Sclerosis Normal Appearing White Matter

Background: Two studies using 31 P-magnetic resonance spectroscopy (MRS) reported enhanced phosphocreatine (PCr) levels in normal appearing white matter (NAWM) of subjects with multiple sclerosis (MS), but this finding could not be properly explained. Methodology/Principal Findings: We performed 31 P-MRS and 1 H-MRS in the NAWM in 36 subjects, including 17 with progressive MS, 9 with benign MS, and 10 healthy controls. Compared to controls, PCr/b-ATP and PCr/total 31 P ratios were significantly increased in subjects with progressive MS, but not with benign MS. There was no correlation between PCr ratios and the N-acetylaspartate/creatine ratio, suggesting that elevated PCr levels in NAWM were not secondary to axonal loss. In the central nervous system, PCr is degraded by creatine kinase B (CK-B), which in the white matter is confined to astrocytes. In homogenates of NAWM from 10 subjects with progressive MS and 10 controls without central nervous system disease, we measured CK-B levels with an ELISA, and measured its activity with an enzymatic assay kit. Compared to controls, both CK-B levels and activity were decreased in subjects with MS (22.41 versus 46.28 mg/ml; p = 0.0007, and 2.89 versus 7.76 U/l; p,0.0001). Conclusions/Significance: Our results suggest that PCr metabolism in the NAWM in MS is impaired due to decreased CK-B levels. Our findings raise the possibility that a defective PCr metabolism in astrocytes might contribute to the degeneratio

Intergroup time bias and racialized social relations

Within the framework of intergroup relations, the authors analyzed the time people spent evaluating ingroup and outgroup members. They hypothesized that White participants take longer to evaluate White targets than Black targets. In four experiments, White participants were slower to form impressions of White than of Black people; that is, they showed an intergroup time bias (ITB). In Study 1 (N = 60), the ITB correlated with implicit prejudice and homogeneity. Study 2 (N = 60) showed that the ITB was independent of the type of trait in question (nonstereotypical vs. stereotypical). Study 3 (N = 100) demonstrated that ITB correlates with racism measured 3 months beforehand, is independent of motivation to control prejudice, and is not an epiphenomenon of homogeneity. In Study 4 (N = 40) participants not only showed the ITB in a racialized social context but also displayed it following a minimal group manipulation