Temperature dependence of single particle excitations in a S=1 chain: exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1 Heisenberg chains at finite temperatures are presented and compared to a recent inelastic neutron scattering experiment for temperatures T up to 7.5 times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point q=1 and at q=0.5 remain resonant up to at least T=2J, confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at q=1 and at q=0.5 is well predicted for the paramagnetic infinite-temperature limit. For T=2J, however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.Comment: 9 pages revtex, submitted to PR

Transfer-matrix scaling from disorder-averaged correlation lengths for diluted Ising systems

A transfer matrix scaling technique is developed for randomly diluted systems, and applied to the site-diluted Ising model on a square lattice in two dimensions. For each allowed disorder configuration between two adjacent columns, the contribution of the respective transfer matrix to the decay of correlations is considered only as far as the ratio of its two largest eigenvalues, allowing an economical calculation of a configuration-averaged correlation length. Standard phenomenological-renormalisation procedures are then used to analyse aspects of the phase boundary which are difficult to assess accurately by alternative methods. For magnetic site concentration $p$ close to $p_c$, the extent of exponential behaviour of the $T_c \times p$ curve is clearly seen for over two decades of variation of $p - p_c$. Close to the pure-system limit, the exactly-known reduced slope is reproduced to a very good approximation, though with non-monotonic convergence. The averaged correlation lengths are inserted into the exponent-amplitude relationship predicted by conformal invariance to hold at criticality. The resulting exponent $\eta$ remains near the pure value (1/4) for all intermediate concentrations until it crosses over to the percolation value at the threshold.Comment: RevTeX 3, 11 pages +5 figures, uuencoded, to appear in Phys. Rev. B (1994), PUC/RJ preprin

Critical temperature for the two-dimensional attractive Hubbard Model

The critical temperature for the attractive Hubbard model on a square lattice is determined from the analysis of two independent quantities, the helicity modulus, $\rho_s$, and the pairing correlation function, $P_s$. These quantities have been calculated through Quantum Monte Carlo simulations for lattices up to $18\times 18$, and for several densities, in the intermediate-coupling regime. Imposing the universal-jump condition for an accurately calculated $\rho_s$, together with thorough finite-size scaling analyses (in the spirit of the phenomenological renormalization group) of $P_s$, suggests that $T_c$ is considerably higher than hitherto assumed.Comment: 5 pages, 6 figures. Accepted for publication in Phys. Rev.

Situationally edited empathy: an effect of socio-economic structure on individual choice

Criminological theory still operates with deficient models of the offender as agent, and of social influences on the agent’s decision-making process. This paper takes one ‘emotion’, empathy, which is theoretically of considerable importance in influencing the choices made by agents; particularly those involving criminal or otherwise harmful action. Using a framework not of rational action, but of ‘rationalised action’, the paper considers some of the effects on individual psychology of social, economic, political and cultural structure. It is suggested that the climate-setting effects of these structures promote normative definitions of social situations which allow unempathic, harmful action to be rationalised through the situational editing of empathy. The ‘crime is normal’ argument can therefore be extended to include the recognition that the uncompassionate state of mind of the criminal actor is a reflection of the self-interested values which govern non-criminal action in wider society

Cosmological distance indicators

We review three distance measurement techniques beyond the local universe: (1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and (3) HI intensity mapping. We describe the principles and theory behind each method, the ingredients needed for measuring such distances, the current observational results, and future prospects. Time delays from strongly lensed quasars currently provide constraints on $H_0$ with < 4% uncertainty, and with 1% within reach from ongoing surveys and efforts. Recent exciting discoveries of strongly lensed supernovae hold great promise for time-delay cosmography. BAO features have been detected in redshift surveys up to z <~ 0.8 with galaxies and z ~ 2 with Ly-$\alpha$ forest, providing precise distance measurements and $H_0$ with < 2% uncertainty in flat $\Lambda$CDM. Future BAO surveys will probe the distance scale with percent-level precision. HI intensity mapping has great potential to map BAO distances at z ~ 0.8 and beyond with precisions of a few percent. The next years ahead will be exciting as various cosmological probes reach 1% uncertainty in determining $H_0$, to assess the current tension in $H_0$ measurements that could indicate new physics.Comment: Review article accepted for publication in Space Science Reviews (Springer), 45 pages, 10 figures. Chapter of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Ag

On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection

A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)

Velocity-space sensitivity of the time-of-flight neutron spectrometer at JET

The velocity-space sensitivities of fast-ion diagnostics are often described by so-called weight functions. Recently, we formulated weight functions showing the velocity-space sensitivity of the often dominant beam-target part of neutron energy spectra. These weight functions for neutron emission spectrometry (NES) are independent of the particular NES diagnostic. Here we apply these NES weight functions to the time-of-flight spectrometer TOFOR at JET. By taking the instrumental response function of TOFOR into account, we calculate time-of-flight NES weight functions that enable us to directly determine the velocity-space sensitivity of a given part of a measured time-of-flight spectrum from TOFOR