Transition temperature of ferromagnetic semiconductors: a dynamical mean field study

We formulate a theory of doped magnetic semiconductors such as Ga$_{1-x}$Mn$_x$As which have attracted recent attention for their possible use in spintronic applications. We solve the theory in the dynamical mean field approximation to find the magnetic transition temperature $T_c$ as a function of magnetic coupling strength $J$ and carrier density $n$. We find that $T_c$ is determined by a subtle interplay between carrier density and magnetic coupling.Comment: 4 pages, 4 figure

Activity ageing in growing networks

We present a model for growing information networks where the ageing of a node depends on the time at which it entered the network and on the last time it was cited. The model is shown to undergo a transition from a small-world to large-world network. The degree distribution may exhibit very different shapes depending on the model parameters, e.g. delta-peaked, exponential or power-law tailed distributions.Comment: 9 pages, 2 figure

Magnetization Reversal in Elongated Fe Nanoparticles

Magnetization reversal of individual, isolated high-aspect-ratio Fe nanoparticles with diameters comparable to the magnetic exchange length is studied by high-sensitivity submicron Hall magnetometry. For a Fe nanoparticle with diameter of 5 nm, the magnetization reversal is found to be an incoherent process with localized nucleation assisted by thermal activation, even though the particle has a single-domain static state. For a larger elongated Fe nanoparticle with a diameter greater than 10 nm, the inhomogeneous magnetic structure of the particle plays important role in the reversal process.Comment: 6 pages, 6 figures, to appear in Phys. Rev. B (2005

Spectroscopic determination of hole density in the ferromagnetic semiconductor Ga1−x_{1-x}Mnx_{x}As

The measurement of the hole density in the ferromagnetic semiconductor Ga$_{1-x}$Mn$_{x}$As is notoriously difficult using standard transport techniques due to the dominance of the anomalous Hall effect. Here, we report the first spectroscopic measurement of the hole density in four Ga$_{1-x}$Mn$_{x}$As samples ($x=0, 0.038, 0.061, 0.083$) at room temperature using Raman scattering intensity analysis of the coupled plasmon-LO-phonon mode and the unscreened LO phonon. The unscreened LO phonon frequency linearly decreases as the Mn concentration increases up to 8.3%. The hole density determined from the Raman scattering shows a monotonic increase with increasing $x$ for $x\leq0.083$, exhibiting a direct correlation to the observed $T_c$. The optical technique reported here provides an unambiguous means of determining the hole density in this important new class of ``spintronic'' semiconductor materials.Comment: two-column format 5 pages, 4 figures, to appear in Physical Review

Star copolymers in porous environments: scaling and its manifestations

We consider star polymers, consisting of two different polymer species, in a solvent subject to quenched correlated structural obstacles. We assume that the disorder is correlated with a power-law decay of the pair correlation function g(x)\sim x^{-a}. Applying the field-theoretical renormalization group approach in d dimensions, we analyze different scenarios of scaling behavior working to first order of a double \epsilon=4-d, \delta=4-a expansion. We discuss the influence of the correlated disorder on the resulting scaling laws and possible manifestations such as diffusion controlled reactions in the vicinity of absorbing traps placed on polymers as well as the effective short-distance interaction between star copolymers.Comment: 13 pages, 3 figure

Duplication-divergence model of protein interaction network

We show that the protein-protein interaction networks can be surprisingly well described by a very simple evolution model of duplication and divergence. The model exhibits a remarkably rich behavior depending on a single parameter, the probability to retain a duplicated link during divergence. When this parameter is large, the network growth is not self-averaging and an average vertex degree increases algebraically. The lack of self-averaging results in a great diversity of networks grown out of the same initial condition. For small values of the link retention probability, the growth is self-averaging, the average degree increases very slowly or tends to a constant, and a degree distribution has a power-law tail.Comment: 8 pages, 13 figure

Exchange interactions and Curie temperature in (GaMn)As

We use supercell and frozen-magnon approaches to study the dependence of the magnetic interactions in (Ga,Mn)As on the Mn concentration. We report the parameters of the exchange interaction between Mn spins and the estimates of the Curie temperature within the mean-field and random-phase approximations. In agreement with experiment we obtain a nonmonotonous dependence of the Curie temperature on the Mn concentration. We estimate the dependence of the Curie temperature on the concentration of the carries in the system and show that the decrease of the number of holes in the valence band leads to fast decrease of the Curie temperature. We show that the hole states of the valence band are more efficient in mediating the exchange interaction between Mn spins than the electron states of the conduction band

Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement

The dynamics of magnetization under femtosecond optical excitation is studied in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved magneto-optical Kerr effect measurement with two color probe beams. The transient reflectivity change indicates the rapid rise of the carrier temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a very slow rise of the spin temperature of the order of 500ps is observed. This anomalous behavior originates from the thermal isolation between the charge and spin systems due to the spin polarization of carriers (holes) contributing to ferromagnetism. This constitutes experimental proof of the half-metallic nature of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type mechanism originates from the d-band character of holes