Dynamical Spin Response Functions for Heisenberg Ladders

We present the results of a numerical study of the 2 by L spin 1/2 Heisenberg ladder. Ground state energies and the singlet-triplet energy gaps for L = (4-14) and equal rung and leg interaction strengths were obtained in a Lanczos calculation and checked against earlier calculations by Barnes et al. (even L up to 12). A related moments technique is then employed to evaluate the dynamical spin response for L=12 and a range of rung to leg interaction strength ratios (0 - 5). We comment on two issues, the need for reorthogonalization and the rate of convergence, that affect the numerical utility of the moments treatment of response functions.Comment: Revtex, 3 figure

Area evolution, bulk viscosity and entropy principles for dynamical horizons

We derive from Einstein equation an evolution law for the area of a trapping or dynamical horizon. The solutions to this differential equation show a causal behavior. Moreover, in a viscous fluid analogy, the equation can be interpreted as an energy balance law, yielding to a positive bulk viscosity. These two features contrast with the event horizon case, where the non-causal evolution of the area and the negative bulk viscosity require teleological boundary conditions. This reflects the local character of trapping horizons as opposed to event horizons. Interpreting the area as the entropy, we propose to use an area/entropy evolution principle to select a unique dynamical horizon and time slicing in the Cauchy evolution of an initial marginally trapped surface.Comment: Some references added, 5 pages, Phys. Rev. D, in pres

Topological spin excitations of Heisenberg antiferromagnets in two dimensions

In this paper we discuss the construction and the dynamics of vortex-like topological spin excitations in the Schwinger-boson description of Heisenberg antiferromagnets in two dimensions. The topological spin excitations are Dirac fermions (with gap) when spin value $S$ is a half-integer. Experimental and theoretical implications of these excitations are being investigated.Comment: Latex file, no figur

Doping-Dependent Raman Resonance in the Model High-Temperature Superconductor HgBa2CuO4+d

We study the model high-temperature superconductor HgBa2CuO4+d with electronic Raman scattering and optical ellipsometry over a wide doping range. The resonant Raman condition which enhances the scattering cross section of "two-magnon" excitations is found to change strongly with doping, and it corresponds to a rearrangement of inter-band optical transitions in the 1-3 eV range seen by ellipsometry. This unexpected change of the resonance condition allows us to reconcile the apparent discrepancy between Raman and x-ray detection of magnetic fluctuations in superconducting cuprates. Intriguingly, the strongest variation occurs across the doping level where the antinodal superconducting gap reaches its maximum.Comment: 4 pages, 4 figures, contact authors for Supplemental Materia

Charge-Transfer Excitations in the Model Superconductor HgBa2_2CuO4+δ_{\bf 4+\delta}

We report a Cu $K$-edge resonant inelastic x-ray scattering (RIXS) study of charge-transfer excitations in the 2-8 eV range in the structurally simple compound HgBa$_2$CuO$_{4+\delta}$ at optimal doping ($T_{\rm c} = 96.5$ K). The spectra exhibit a significant dependence on the incident photon energy which we carefully utilize to resolve a multiplet of weakly-dispersive ($< 0.5$ eV) electron-hole excitations, including a mode at 2 eV. The observation of this 2 eV excitation suggests the existence of a charge-transfer pseudogap deep in the superconducting phase. Quite generally, our data demonstrate the importance of exploring the incident photon energy dependence of the RIXS cross section.Comment: 5 pages, 3 figure

Unraveling the Nature of Charge Excitations in La2_2CuO4_4 with Momentum-Resolved Cu KK-edge Resonant Inelastic X-ray Scattering

Results of model calculations using exact diagonalization reveal the orbital character of states associated with different Raman loss peaks in Cu $K$-edge resonant inelastic X-ray scattering (RIXS) from La$_{2}$CuO$_{4}$. The model includes electronic orbitals necessary to highlight non-local Zhang-Rice singlet, charge transfer and $d$-$d$ excitations, as well as states with apical oxygen 2$p_z$ character. The dispersion of these excitations is discussed with prospects for resonant final state wave-function mapping. A good agreement with experiments emphasizes the substantial multi-orbital character of RIXS profiles in the energy transfer range 1-6 eV.Comment: Original: 4.5 pages. Replaced: 4 pages and 4 figures with updated content and reference

A randomised controlled trial (MindChamp) of a mindfulness-based intervention for children with ADHD and their parents

BACKGROUND: Family mindfulness‐based intervention (MBI) for child attention‐deficit/hyperactivity disorder (ADHD) targets child self‐control, parenting and parental mental health, but its effectiveness is still unclear. METHODS: MindChamp is a pre‐registered randomised controlled trial comparing an 8‐week family MBI (called ‘MYmind’) in addition to care‐as‐usual (CAU) (n = 55) with CAU‐only (n = 48). Children aged 8–16 years with remaining ADHD symptoms after CAU were enrolled together with a parent. Primary outcome was post‐treatment parent‐rated child self‐control deficits (BRIEF); post hoc, Reliable Change Indexes were explored. Secondary child outcomes included ADHD symptoms (parent/teacher‐rated Conners’ and SWAN; teacher‐rated BRIEF), other psychological symptoms (parent/teacher‐rated), well‐being (parent‐rated) and mindfulness (self‐rated). Secondary parent outcomes included self‐ratings of ADHD symptoms, other psychological symptoms, well‐being, self‐compassion and mindful parenting. Assessments were conducted at post‐treatment, 2‐ and 6‐month follow‐up. RESULTS: Relative to CAU‐only, MBI+CAU resulted in a small, statistically non‐significant post‐treatment improvement on the BRIEF (intention‐to‐treat: d = 0.27, p = .18; per protocol: d = 0.33, p = .11). Significantly more children showed reliable post‐treatment improvement following MBI+CAU versus CAU‐only (32% versus 11%, p < .05, Number‐Needed‐to‐Treat = 4.7). ADHD symptoms significantly reduced post‐treatment according to parent (Conners’ and SWAN) and teacher ratings (BRIEF) per protocol. Only parent‐rated hyperactivity impulsivity (SWAN) remained significantly reduced at 6‐month follow‐up. Post‐treatment group differences on other secondary child outcomes were consistently favour of MBI+CAU, but mostly non‐significant; no significant differences were found at follow‐ups. Regarding parent outcomes, significant post‐treatment improvements were found for their own ADHD symptoms, well‐being and mindful parenting. At follow‐ups, some significant effects remained (ADHD symptoms, mindful parenting), some additional significant effects appeared (other psychological symptoms, self‐compassion) and others disappeared/remained non‐significant. CONCLUSIONS: Family MBI+CAU did not outperform CAU‐only in reducing child self‐control deficits on a group level but more children reliably improved. Effects on parents were larger and more durable. When CAU for ADHD is insufficient, family MBI could be a valuable addition