R. Orbach

Full aging in spin glasses.

The discovery of dynamic memory effects in the magnetization decays of spin glasses in 1983 marked a turning point in the study of the highly disordered spin glass state. Detailed studies of the memory effects have led to much progress in understanding the qualitative features of the phase space. Even so, the exact nature of the magnetization decay functions has remained elusive, causing confusion. In this Letter, we report strong evidence that the thermoremanent magnetization decays scale with the waiting time t(w). By employing a series of cooling protocols, we demonstrate that the rate at which the sample is cooled to the measuring temperature plays a major role in the determination of scaling. As the effective cooling time t(eff)(c) decreases, t/t(w) scaling improves and for t(eff)(c)<20 s we find almost perfect t/t(w) scaling, i.e., full aging.

EXTRACTION OF THE SPIN GLASS CORRELATION LENGTH

A change in magnetic field H quenches the barriers in a spin glass for heights less than an associated change in Zeeman energy, E_z. The shift of the peak of S(t)=-dM_{TRM}(t,t_w)/H/{d lnt} with H generates E_z={N_s}{X_fc}{H^2}, with X_fc the field cooled magnetic susceptibility per spin, and N_s the number of spins participating in barrier quenching (and barrier hopping). Experiments on Cu:Mn 6at.% and CdCr_{1.7}In_{0.3}S_4 for ranges of H, T, and waiting times t_w generate the correlation length, Ksi(t_w,T) ~ {N_s}^{1/3}, fitted by both the hierarchical model, Ksi(t_w,T)=0.635(t_w}/tau_0)^{0.169T/T_g}, in numerical accord with simulations, or the droplet model, Ksi(t_w,T)={10^{-5}}{{[(T/T_g)Ln(t_w/tau_0)]}^{1/0.21}}, with too small a prefactor for simulation time scales, and exponent 1/psi at the lower limit for psi.

Characteristics of Fractons: from Specific Realizations to Ensemble Averages

Specific realizations due to computer experiments show that fracton excitations possess a core in which the vibrational amplitudes fall off sharply at their edges. When constructing the ensemble average of the fracton wave functions, we find \(\langle\phi_{\text{fr}}\rangle{\sim}\exp[-(r/\varLambda(\omega))^{d_{\phi}}]\), where \(\varLambda(\omega){\sim}\omega^{-0.71}\). The exponent, -0.71, is in close agreement with the prediction of the fracton dispersion law for a two-dimensional percolating network, -0.705, and the geometrical exponent d φ is much larger than current theoretical limits for impurity electronic states. This suggests that the two problems are different.

From linear to nonlinear response in spin glasses: Importance of mean-field-theory predictions

The scaling of the magnetic field dependence of the remanent magnetization for different temperatures and different spin-glass samples is studied. Particular attention is paid to the effect of the de Almeida-Thouless (AT) critical line on spin-glass dynamics. It is shown that results of the mean-field theory of aging phenomena, with two additional experimentally justified assumptions, predict

On the increase of thermal conductivity in glasses above the plateau region

H/H_{AT}(T)

Transport and vibrational lifetimes in amorphous structures

scaling for remanent magnetization curves. Experiments on a single crystal Cu:Mn 1.5 at % sample in the temperature interval from

Effect of magnetic fields on the relaxation of the thermoremanent magnetization in spin glasses

0.7T_{g}

Magnetic Field Dependence of Spin Glass Free Energy Barriers

to

Spin-glass dynamics and the barrier model: Extraction of the Parisi physical order parameter

0.85T_{g}

MAGNETIC PROPERTIES OF WEAKLY DOPED ANTIFERROMAGNETS

give results consistent with this scaling. Magnetization vs. field curves for different Cu:Mn and thiospinel samples also scale together. These experimental results support the predictions of the mean-field theory of aging phenomena.