Spin dynamics and magnetic correlation length in two-dimensional quantum heisenberg antiferromagnets

The correlated spin dynamics and temperature dependence of the correlation length xi(T) in two-dimensional quantum (S = 1/2) Heisenberg antiferromagnets (2DQHAF) on a square lattice are discussed in light of experimental results of proton spin lattice relaxation in copper formiate tetradeuterate. In this compound the exchange constant is much smaller than the one in recently studied 2DQHAF, such as La2CuO4 and Sr2CuO2Cl2. Thus the spin dynamics can be probed in detail over a wider temperature range. The NMR relaxation rates turn out to be in excellent agreement with a theoretical mode-coupling calculation. The deduced temperature behavior of xi(T) is in agreement with high-temperature expansions, quantum Monte Carlo simulations, and the pure quantum self-consistent harmonic approximation. Contrary to the predictions of the theories based on the nonlinear sigma model, no evidence of crossover between different quantum regimes is observed.

A novel field for possible use in medical impedance tomography.

A single-phase electrical supply is normally used to energize the electrodes of the belt worn by the subject for medical impedance tomography. In the process, the supply is switched from one adjacent pair of electrodes to the next, so that all such pairs are energized in turn. However, if a three-phase voltage of constant amplitude were to be employed, with electrodes uninterruptedly connected to the lines of the supply in phase sequence around the periphery, and the current flowing through each electrode were to be simultaneously measured, it will be shown that, in principle, data of higher utility would result. This paper examines that possibility, and considers the field spatial harmonics arising in consequence of the electrode geometry. Expressions are obtained for the magnitude of those harmonics in close proximity to the surface of the belt, from which the spatial harmonic components of the applied field at deeper levels can be calculated. It is shown that if bar electrodes are employed, the spatial harmonic composition of the field is critically dependent on the ratio of the width of the electrodes to the width of the gaps between them.

Electrical impedance tomography. The provision of an electric field of controllable penetration into a cylinder.

If a cylinder carries on its surface an electric charge of a density which is constant in the axial direction, but which varies sinusoidally with an integral number of wavelengths around the circumference, then inside the cylinder the electric field has its maximum value at points adjacent to the surface, and reduces continuously as the axis is approached. The rate of reduction increases with the number of wavelengths. A rotating cylinder of such charge may be simulated by a number (divisable by three) of equally spaced conducting bars mounted longitudinally on the surface of a stationary cylinder, the bars being connected in sequence to the lines of a three-phase voltage supply. The number of wavelengths of charge around the circumference can be varied by the connection together of a chosen number of adjacent bars effectively to form a single bar, and so fields of varying penetration depths can be produced from a fixed frequency supply. The variation of the resulting currents would be related to the corresponding variation in the complex conductivity of the cylinder material.