Malcolm P. Kennett

Diluted Magnetic Semiconductors in the Low Carrier Density Regime

This paper, based on a presentation at the Spintronics 2001 conference, provides a review of our studies on II–VI and III–V Mn-doped Diluted Magnetic Semiconductors. We use simple models appropriate for the low carrier density (insulating) regime, although we believe that some of the unusual features of the magnetization curves should qualitatively be present at larger dopings (metallic regime) as well. Positional disorder of the magnetic impurities inside the host semiconductor is shown to have observable consequences for the shape of the magnetization curve. Below the critical temperature the magnetization is spatially inhomogeneous, leading to very unusual temperature dependence of the average magnetization as well as specific heat. Disorder is also found to enhance the ferromagnetic transition temperature. Unusual spin and charge transport is implied.

Anisotropic scattering and anomalous normal-state transport in a high-temperature superconductor

The metallic state of high-temperature copper-oxide superconductors, characterized by unusual and distinct temperature dependences in the transport properties1,2,3,4, is markedly different from that of textbook metals. Despite intense theoretical efforts5,6,7,8,9,10,11, our limited understanding is impaired by our inability to determine experimentally the temperature and momentum dependence of the transport scattering rate. Here, we use a powerful magnetotransport probe to show that the resistivity and the Hall coefficient in highly doped Tl2Ba2CuO6+δ originate from two distinct inelastic scattering channels. One channel is due to conventional electron–electron scattering; the other is highly anisotropic, has the same symmetry as the superconducting gap and a magnitude that grows approximately linearly with temperature. The observed form and anisotropy place tight constraints on theories of the metallic state. Moreover, in heavily doped non-superconducting La2−xSrxCuO4, this anisotropic scattering term is absent12, suggesting an intimate connection between the origin of this scattering and superconductivity itself.

Heterogeneous Aging in Spin Glasses

We introduce a set of theoretical ideas that form the basis for an analytical framework capable of describing nonequilibrium dynamics in glassy systems. We test the resulting scenario by comparing its predictions with numerical simulations of short-range spin glasses. Local fluctuations and responses are shown to be connected by a generalized local out-of-equilibrium fluctuation-dissipation relation. Scaling relationships are uncovered for the slow evolution of heterogeneities at all time scales.

Monte Carlo simulations of an impurity-band model for III-V diluted magnetic semiconductors

We report the results of a Monte Carlo study of a model of (III,Mn)V diluted magnetic semiconductors which uses an impurity band description of carriers coupled to localized Mn spins and is applicable for carrier densities below and around the metal-insulator transition. In agreement with mean-field studies, we find a transition to a ferromagnetic phase at low temperatures. We compare our results for the magnetic properties with the mean-field approximation, as well as with experiments, and find favorable qualitative agreement with the latter. The local Mn magnetization below the Curie temperature is found to be spatially inhomogeneous, and strongly correlated with the local carrier charge density at the Mn sites. The model contains fermions and classical spins, and hence we introduce a perturbative Monte Carlo scheme to increase the speed of our simulations.

Dispersion in an intrinsically relativistic, one-dimensional, strongly magnetized pair plasma

The properties of a relativistic plasma dispersion function (RPDF) for an intrinsically extremely relativist, strongly magnetized, one-dimensional, electron-positron plasma are discussed in detail. For a plasma with a mean Lorentz factor 1 in its rest frame, the RPDF has a large peak > at a phase speed a fraction of order 1/ below the speed of light, and the asymptotic value (infinite phase speed) is ∼ 1/ . These features are not particularly sensitive to the choice of distribution function. The RPDF is used to discuss the properties of waves in such plasmas. Particular points discussed are the implications of the RPDF for the maximum frequency for parallel Langmuir waves, and for the reconnection between the Langmuir mode and the Alfven mode.

Propagation-induced Circular Polarisation in Synchrotron Sources

The small degree of circular polarisation observed in some synchrotron sources has a frequency dependence that is not consistent with simple predictions based on the intrinsic circular polarisation of synchrotron emission. The suggestion is explored that the circular polarisation arises as a propagation effect within the source. The physical basis of this alternative mechanism is the fact that the natural wave modes of a synchrotron emitting gas are linearly polarised, allowing partial conversion of linear into circular polarisation as in a quarter-wave plate. A relativistic rotation measure (RRM) is defined to characterise the magnitude of this effect.

Separation of time scales and reparametrization invariance for aging systems

We show that the generating functional describing the slow dynamics of spin-glass systems is invariant under reparametrizations of the time. This result is general and applies for both infinite and short-range models. It follows simply from the assumption that a separation between short time scales and long time scales exists in the system, and from the constraints of causality and unitarity. Global-time reparametrization invariance suggests that the low action excitations in a spin-glass may be smoothly spatially varying time reparametrizations. These Goldstone modes may provide the basis for an analytic dynamical theory of short-range spin glasses.

Spatially heterogeneous ages in glassy dynamics

We construct a framework for the study of fluctuations in the nonequilibrium relaxation of glassy systems with and without quenched disorder. We study two types of two-time local correlators with the aim of characterizingthe heterogeneous evolution in these systems: in one case we average the local correlators over histories of the thermal noise, in the other case we simply coarse grain the local correlators obtained for a given noise realization. We explain why the noise-averaged correlators describe the fingerprint of quenched disorder when it exists, while the coarse-grained correlators are linked to noise-induced mesoscopic fluctuations. We predict constraints on the distribution of the fluctuations of the coarse-grained quantities. In particular, we show that locally defined correlations and responses are connected by a generalized local out-of-equilibrium fluctuation-dissipation relation. We argue that large-size heterogeneities in the age of the system survive in the long-time limit. A symmetry of the underlying theory, namely, invariance under reparametrizations of the time coordinates, underlies these results. We establish a connection between the probabilities of spatial distributions of local coarse-grained quantities and the theory of dynamic random manifolds. We define, and discuss the behavior of, a two-time dependent correlation length from the spatial decay of the fluctuations in the two-time local functions. We characterize the fluctuations in the system in terms of their fractal properties. For concreteness, we present numerical tests performed on disordered spin models in finite and infinite dimensions. Finally, we explain how these ideas can be applied to the analysis of the dynamics of other glassy systems that can be either spin models without disorder or atomic and molecular glassy systems.

Aging dynamics of quantum spin glasses of rotors

We study the long time dynamics of quantum spin glasses of rotors using the non-equilibrium Schwinger-Keldysh formalism. These models are known to have a quantum phase transition from a paramagnetic to a spin glass phase, which we approach by looking at the divergence of the spin relaxation rate at the transition point. In the aging regime, we determine the dynamical equations governing the time evolution of the spin response and correlation functions, and show that all terms in the equations that arise solely from quantum effects are irrelevant at long times under time reparametrization group (RpG) transformations. At long times, quantum effects enter only through the renormalization of the parameters in the dynamical equations for the classical counterpart of the rotor model. Consequently, quantum effects only modify the out of equilibrium fluctuation dissipation relation (OEFDR), i.e. the ratio X between the temperature and the effective temperature, but not the form of the classical OEFDR.

Sensitivity of the interlayer magnetoresistance of layered metals to intralayer anisotropies

Many of the most interesting and technologically important electronic materials discovered in the past two decades have both a layered crystal structure and strong interactions between electrons. Two fundamental questions about such layered metals concern the origin of intralayer anisotropies and the coherence of interlayer charge transport. We show that angle dependent magnetoresistance oscillations (AMROs) are sensitive to anisotropies around an intralayer Fermi surface and can hence be a complementary probe of such anisotropies to angle-resolved photoemission spectroscopy and scanning tunneling microscopy. However, AMROs are not very sensitive to the coherence of the interlayer transport which has implications for recent AMRO experiments on an overdoped cuprate.