Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high temperature superconductor YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering has thus the potential to explore the evolution of this state under extreme conditions.Comment: 12 pages, 11 figure

Thermodynamic evidence for nematic phase transition at the onset of pseudogap in YBa2_2Cu3_3Oy_y

A central issue in the quest to understand the superconductivity in cuprates is the nature and origin of the pseudogap state, which harbours anomalous electronic states such as Fermi arc, charge density wave (CDW), and $d$-wave superconductivity. A fundamentally important, but long-standing controversial problem has been whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature $T^*$. Electronic nematicity, a fourfold ($C_4$) rotational symmetry breaking, has emerged as a key feature inside the pseudogap regime, but the presence or absence of a nematic phase transition and its relationship to the pseudogap remain unresolved. Here we report thermodynamic measurements of magnetic torque in the underdoped regime of orthorhombic YBa$_2$Cu$_3$O$_y$ with a field rotating in the CuO$_2$ plane, which allow us to quantify magnetic anisotropy with exceptionally high precision. Upon entering the pseudogap regime, the in-plane anisotropy of magnetic susceptibility increases after exhibiting a distinct kink at $T^*$. Our doping dependence analysis reveals that this anisotropy is preserved below $T^*$ even in the limit where the effect of orthorhombicity is eliminated. In addition, the excess in-plane anisotropy data show a remarkable scaling behaviour with respect to $T/T^*$ in a wide doping range. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which is distinct from the CDW transition that accompanies translational symmetry breaking. This suggests that nematic fluctuations near the pseudogap phase boundary have a potential link to the strange metallic behaviour in the normal state, out of which high-$T_c$ superconductivity emerges.Comment: 6 pages, 4 figures. An updated manuscript with SI will appear in Nature Physic

Bullying girls - Changes after brief strategic family therapy: A randomized, prospective, controlled trial with one-year follow-up

Background: Many girls bully others. They are conspicuous because of their risk-taking behavior, increased anger, problematic interpersonal relationships and poor quality of life. Our aim was to determine the efficacy of brief strategic family therapy (BSFT) for bullying-related behavior, anger reduction, improvement of interpersonal relationships, and improvement of health-related quality of life in girls who bully, and to find out whether their expressive aggression correlates with their distinctive psychological features. Methods: 40 bullying girls were recruited from the general population: 20 were randomly selected for 3 months of BSFT. Follow-up took place 12 months after the therapy had ended. The results of treatment were examined using the Adolescents' Risk-taking Behavior Scale (ARBS), the State-Trait Anger Expression Inventory (STAXI), the Inventory of Interpersonal Problems (IIP-D), and the SF-36 Health Survey (SF-36). Results: In comparison with the control group (CG) (according to the intent-to-treat principle), bullying behavior in the BSFT group was reduced (BSFT-G from n = 20 to n = 6; CG from n = 20 to n = 18, p = 0.05) and statistically significant changes in all risk-taking behaviors (ARBS), on most STAXI, IIP-D, and SF-36 scales were observed after BSFT. The reduction in expressive aggression (Anger-Out scale of the STAXI) correlated with the reduction on several scales of the ARBS, IIP-D, and SF-36. Follow-up a year later showed relatively stable events. Conclusions: Our findings suggest that bullying girls suffer from psychological and social problems which may be reduced by the use of BSFT. Expressive aggression in girls appears to correlate with several types of risk-taking behavior and interpersonal problems, as well as with health-related quality of life. Copyright (c) 2006 S. Karger AG, Basel

SLAC/CERN high gradient tests on an X-band accelerating section

High frequency linear collider schemes envisage the use of rather high accelerating gradients: 50 to 100 MV/m for X-band and 80 MV/m for CLIC. Because these gradients are well above those commonly used in accelerators, high gradient studies of high frequency structures have been initiated and test facilities have been constructed at KEK [1], SLAC [2] and CERN [3]. The studies seek to demonstrate that the above mentioned gradients are both achievable and practical. There is no well-defined criterion for the maximum acceptable level of dark current but it must be low enough not to generate unacceptable transverse wakefields, disturb beam position monitor readings or cause RF power losses. Because there are of the order of 10,000 accelerating sections in a high frequency linear collider, the conditioning process should not be too long or difficult. The test facilities have been instrumented to allow investigation of field emission and RF breakdown mechanisms. With an understanding of these effects, the high gradient performance of accelerating sections may be improved through modifications in geometry, fabrication methods and surface finish. These high gradient test facilities also allow the ultimate performance of high frequency/short pulse length accelerating structures to be probed. This report describes the high gradient test at SLAC of an X-band accelerating section built at CERN using technology developed for CLIC

Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5

THz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structure. In complex oxides, this method has been used to melt electronic orders, drive insulator to metal transitions or induce superconductivity. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab initio density functional theory calculations, we determine here the crystal structure of this exotic non-equilibrium state. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered dilation/contraction of the Cu-O2 intra/inter- bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause dramatic changes in the electronic structure. Amongst these, the enhancement in the dx2-y2 character of the in-plane electronic structure is likely to favor superconductivity.Comment: 28 pages, including Supplemen

Similar zone-center gaps in the low-energy spin-wave spectra of NaFeAs and BaFe2As2

We report results of inelastic-neutron-scattering measurements of low-energy spin-wave excitations in two structurally distinct families of iron-pnictide parent compounds: Na(1-{\delta})FeAs and BaFe2As2. Despite their very different values of the ordered magnetic moment and N\'eel temperatures, T_N, in the antiferromagnetic state both compounds exhibit similar spin gaps of the order of 10 meV at the magnetic Brillouin-zone center. The gap opens sharply below T_N, with no signatures of a precursor gap at temperatures between the orthorhombic and magnetic phase transitions in Na(1-{\delta})FeAs. We also find a relatively weak dispersion of the spin-wave gap in BaFe2As2 along the out-of-plane momentum component, q_z. At the magnetic zone boundary (q_z = 0), spin excitations in the ordered state persist down to 20 meV, which implies a much smaller value of the effective out-of-plane exchange interaction, J_c, as compared to previous estimates based on fitting the high-energy spin-wave dispersion to a Heisenberg-type model.Comment: 5 pages, 4 figures, 1 tabl

Evolution of the eyes of vipers with and without infrared-sensing pit organs

We examined lens and brille transmittance, photoreceptors, visual pigments, and visual opsin gene sequences of viperid snakes with and without infrared-sensing pit organs. Ocular media transmittance is high in both groups. Contrary to previous reports, small as well as large single cones occur in pit vipers. Non-pit vipers differ from pit vipers in having a twotiered retina, but few taxa have been examined for this poorly understood feature. All vipers sampled express rh1, sws1 and lws visual opsin genes. Opsin spectral tuning varies but not in accordance with the presence/absence of pit organs, and not always as predicted from gene sequences. The visual opsin genes were generally under purifying selection, with positive selection at spectral tuning amino acids in RH1 and SWS1 opsins, and at retinal pocket stabilization sites in RH1 or LWS (and without substantial differences between pit and nonpit vipers). Lack of evidence for sensory trade-off between viperid eyes (in the aspects examined) and pit organs might be explained by the high degree of neural integration of vision and infrared detection; the latter representing an elaboration of an existing sense with addition of a novel sense organ, rather than involving the evolution of a wholly novel sensory system