Dynamic scaling for spin glasses near the de Almeida-Thouless line

We calculate the dynamic local susceptibility () of an Ising spin glass near the de Almeida-Thouless (AT) line within the soft spin dynamics for the Sherrington-Kirkpatrick model. We find a crossover from analytic behaviour of () at =0 above the AT line to a power law behaviour ()(–i) ^v on the AT line and discuss the analytic properties of the crossover function. The frequencyscale is proportional to ^1/v , where measures the distance from the AT line. We determine the spectrum of relaxation times which diverge as ^1–1/v . The average relaxation time diverges as ^–1/v wherev1/2. In addition we determine the absolute frequency scale and prove the consistency of the ansatz of Sompolinsky and Zippelius ()–(0)(–i) ^v at and below the AT line.     

Critical dynamics in Cd1-xMnxTe

We report a.c. susceptibility measurements in the diluted magnetic semiconductor Cd_1-xMn_xTe for x = 0.45 and 0.55 between 10 Hz and 6 MHz. The experimental data are interpreted in terms of dynamic scaling laws with a transition temperature T_c consistent with the static results.     

Activated scaling of classical and quantum spin glasses

A model for thermally activated dynamics in disordered systems shows that the linear and nonlinear susceptibility follows a generic exponential form with a "critical rounding," chi(1) proportional to chi(3) proportional to [T ln(t/tau(0)')/K](gamma/b phi) exp - [Tt(g)(phi b)ln(t/tau(0)'/K)](nu/b) (T=temperature, t=time, K=barrier constant, t(g) = 1 - T(SG)/T, and T(SG) = transition temperature; gamma>0 for chi(3) and <0 for chi(1)). This model, also valid in the presence of resonant tunneling states at energies K(0) < K [provided that K is replaced by K(0)+2T ln (1/Gamma(0)), where Gamma(0)(2) proportional, variant tunnel splitting of a spin S=1], is potentially applicable to a wide variety of systems opening the way for the study of thermally activated quantum phase transitions. The famous spin-glass system LiHo(x)Y(1-x) seems to follow this model.     

Validity and limitations of scaling laws in the theory of spin glasses

Current theories for the derivation of the internal-field distribution function for statistical models of spin-glasses are briefly reviewed, with reference to the form of the distribution function and the scaling behavior of the function itself and of related observable quantities.It is shown that deviations from scaling are inherent to the statistical theory itself and do not require separate mechanisms for their explanation, and the meaning of the currently-used Lorentzian-like forms of the distribution function is clarified.     

Strong universality and algebraic scaling in two-dimensional Ising spin glasses

At zero temperature, two-dimensional Ising spin glasses are known to fall into several universality classes. Here we consider the scaling at low but nonzero temperatures and provide numerical evidence that eta approximately equal 0 and nu approximately equal 3.5 in all cases, suggesting a unique universality class. This algebraic (as opposed to exponential) scaling holds, in particular, for the +/- J model, with or without dilutions, and for the plaquette diluted model. Such a picture, associated with an exceptional behavior at T = 0, is consistent with a real space renormalization group approach. We also explain how the scaling of the specific heat is compatible with the hyperscaling prediction.     

Aspect-ratio scaling and the stiffness exponent theta for Ising spin glasses

We introduce the technique of aspect-ratio scaling to study the scale dependence of interfacial energies in Ising spin glasses, and we show how one can use it to determine the stiffness exponent theta in a clean way, with results that are independent of the domain-wall-forcing boundary conditions imposed on the system. In space dimension d = 2 we obtain theta = -0.282(3) for a Gaussian distribution of exchange interactions.     

Studies of Spectroscopic Ellipsometry in Cd1-xMnxTe/CdTe Superlattices

Cd1-xMnxTe/CdTe superlattices and thin films were grown by molecular beam epitaxy on GaAs （001） substrates. Spectroscopic ellipsometry measurements were performed on Cd1-x Mnx Te/ CdTe superlattices with compositions x = 0.4, 0.8, and Cd1-xMnx Te thin films with x = 0.2, 0.4, 0.6 at room temperature in the photon energy range 1.4-5 eV. In superlattices the pseudodielectric functions measured by ellipsometry show specific features related to the exciton transition between quantized interbands. The exciton transitions related to the heavy holes of 11 H, 22H, and 33H are observed and identified. In thin films spectroscopic ellipsometry allows the clear identification of the energy gap Eo. Additionally, critical point transitions are observable in both the spectra of the superlattices and films. Photoreflectance spectra were also performed at room temperature in order to compare with our ellipsometry results. After taking into account the strain-induced and quantum confinement effects, the theoretical calculations are in good agreement with our experimental spectra. Ellipsometry appears to be a suited technique to monitor the MBE growth, ultimately also in situ, of diluted magnetic low-dimensional systems.     

Magnetization and photomagnetic effects in diluted magnetic microcrystalline Cd1-xMnxTe

We have investigated the magnetic and photomagnetic properties of microcrystalline Cd_1-xMn_xTe prepared by rf sputtering. Magnetization measurements were carried out using an rf SQUID magnetometer in the temperature range of 1.8 to 300 K at various magnetic fields up to 5.5 T. For temperatures above 40 K, the sample showed Curie-Weiss behaviour with a Curie temperature indicating predominantly antiferromagnetic interactions. A spin-glass phase transition was also observed. Photomagnetization measurements were performed using a fibre-optic system. The light was shone onto the sample utilizing an optical fibre and the subsequent change in the magnetization was sensed by the SQUID. Photo-induced magnetization was observed when the sample was illuminated by unpolarized light. Our results enable qualitative and quantitative conclusions to be drawn on the magnetic behaviour and the interplay between optical and magnetic properties of the diluted magnetic microcrystalline semiconductors.PACS: 68.55.Gi; 75.50.Pp.     

Corrections to scaling are large for droplets in two-dimensional spin glasses

The energy of a droplet of linear extent l in the droplet theory of spin glasses goes as l(theta) for large l. It is shown by numerical studies of large droplets in two-dimensional systems that this formula needs to be modified by the addition of a scaling correction l(-omega) in order to accurately describe droplet energies at the length scales currently probed in numerical simulations. Using this simple modification, it is now possible to explain many results which have been found in simulations of three-dimensional Ising spin glasses with the droplet model.     

Characterization of MBE grown Cd1-xMnxTe structures by SQUID magnetometry

The paper reports on the influence of a magnetic confinement on the formation of the spin glass phase in superlattices, and on the antiferromagnetic phase transition in zinc-blende MnTe. The effect of interface mixing on the magnetic properties is also discussed.