arXiv subjects

P. C. E. Stamp

Publications and source records attributed to P. C. E. Stamp.

60 records · Page 4Linked to original sources

arXiv1995

Quantum Coherence in Magnetic Grains

We consider a model of the dynamics of a magnetic grain, incorporating interactions with nuclear and paramagnetic spins, conduction electrons, and phonons. Decoherence comes both from the spins and the electrons, although electron effects can be neglected if the substrate is insulating, and the grain has spin $S \le 10^5$. A successful experiment will require both isotopic and chemical purification, if coherence is to be seen; we suggest a "spin echo" arrangement to do this.

cond-mat↗

arXiv1995

A pair of interacting spins coupled to an environmental sea: dissipation and mutual decoherence

We consider the quantum dynamics of two spin-1/2 systems, each coupled to a bath of oscillators, so that a bath-mediated coupling is generated between the spins. We find that the interactions destroys any coherent motion of the 2 spins, even if the coupling of each spin to the bath is quite weak, unless the interaction is extremely small. This is because the dynamic mutual bias between the spins blocks any coherent transitions between nearly degenerate states. In many quantum measurement operations this means that decoherence effects will be much stonger during the actual measurement

cond-mat↗

arXiv1995

Quantum Relaxation of Magnetisation in Magnetic Particles

At temperatures below the magnetic anisotropy energy, monodomain magnetic systems (small particles, nanomagnetic devices, etc.) must relax quantum mechanically. This quantum relaxation must be mediated by the coupling to both nuclear spins and phonons (and electrons if either particle or substrate is conducting. We analyze the effect of each of these couplings, and then combine them. Conducting systems can be modelled by a "giant Kondo" Hamiltonian, with nuclear spins added in as well. At low temperatures, even microscopic particles on a conducting substrate (containing only $10-50$ spins) will have their magnetisation frozen over millenia by a combination of electronic dissipation and the "degeneracy blocking" caused by nuclear spins. Raising the temperature leads to a sudden unblocking of the spin dynamics at a well defined temperature. Insulating systems are quite different. The relaxation is strongly enhanced by the coupling to nuclear spins. At short times the magnetisation of an ensemble of particles relaxes logarithmically in time, after an initial very fast decay; this relaxation proceeds entirely via the nuclear spins. At longer times phonons take over, but the decay rate is still governed by the temperature-dependent nuclear bias field acting on the particles - decay may be exponential or power-law depending on the temperature. The most surprising feature of the results is the pivotal role played by the nuclear spins. The results are relevant to any experiments on magnetic particles in which interparticle dipolar interactions are unimportant. They are also relevant to future magnetic device technology.

cond-mat↗

arXiv1995

Decoherence in the Spin Bath

We develop a mathematical description of the decoherence caused by "spin baths", such as nuclear spins or magnetic impurities. In contrast to the usual oscillator bath models of quantum environments, decoherence in the spin bath can occur without any dissipation. Given the almost ubiquitous presence of nuclear spins in nature, our results have important consequences for quantum measurement theory, particularly as the decoherence mechanisms in spin baths work very differently from those in oscillator baths.

cond-mat↗

arXiv1994

Influence of gauge-field fluctuations on composite fermions near the half-filled state

Taking into account the transverse gauge field fluctuations, which interact with composite fermions, we examine the finite temperature compressibility of the fermions as a function of an effective magnetic field $ΔB = B - 2 n_e hc/e$ ($n_e$ is the density of electrons) near the half-filled state. It is shown that, after including the lowest order gauge field correction, the compressibility goes as ${\partial n \over \partial μ} \propto e^{- Δω_c / 2 T} \left ( 1 + {A (η) \over η- 1} {(Δω_c)^{2 \over 1 + η} \over T} \right )$ for $T \ll Δω_c$, where $Δω_c = {e ΔB \over mc}$. Here we assume that the interaction between the fermions is given by $v ({\bf q}) = V_0 / q^{2 - η} \ (1 \le η\le 2)$, where $A (η)$ is a $η$ dependent constant. This result can be interpreted as a divergent correction to the activation energy gap and is consistent with the divergent renormalization of the effective mass of the composite fermions.

cond-mat↗

arXiv1993

The Superfluidity and Experimental Properties of Odd-Energy-Gap Superconductors

We consider the experimental properties of superconductors with a gap which is an odd function of energy $\bepsk=\epsk - μ$, i.e.\ , a gap which vanishes everywhere on the Fermi surface; this is done within a in a BCS framework. Apart from the standard phenomenology (density of states, penetration depth, NMR, $C_V(T)$, $B_{c2}(T)$), we also look at the stability of the superconducting state. It is stable to finite supercurrents (although with a normal fluid density which increases with the supercurrent density), and is also very weakly affected by non-magnetic impurities. We find two classes of odd-gap superconductors, which strongly differ in their low-energy properties. For a certain parameter range, most of the results resemble those for d-wave superconductors (except for the effect of impurities).

cond-mat↗