Showing papers by "Yoseph Imry published in 1981"

Scaling of Nonlinear Susceptibility in MnCu and GdAl Spin-Glasses

Abstract: SQUID measurements of susceptibility X versus field 0 are fitted by X= yp —p~H at a given temperature T, for two classic spin-glasses: 4.6' Mn in Cu (polycrystalline) and 37% Gd in Al (amorphous). X&(T) shows a much stronger anomaly than po(T), while a(T) has a minimum at the freezing temperature T~. The data can also be scaled according to y, —X=H' f((T Tf)—/H' ~) with 6= 5+1 and y = 4 5+p 5.

Lower Critical Dimension and the Roughening Transition of the Random-Field Ising Model

Abstract: It is shown by a calculation analogous to that carried out by Wallace and Zia for the pure Ising model that the lower critical dimension of the Ising model in a random field is 3 and not 2 as suggested by domain energy arguments. Further, the critical dimension for the roughening transition is shown to be 5 as compared to 3 for the pure Ising model.

Destruction of superconductivity in granular and highly disordered metals

Abstract: The destruction of superconductivity in granular and highly disordered metals is discussed in the light of the modern picture of the metal-insulator transition induced by Anderson localization. For small grains superconductivity is lost when localization sets in. Systems composed of large grains may phase lock at a temperature, ${T}_{C}$, much smaller than the ${T}_{C}^{0}$ of the clean metal. ${T}_{C}$ is estimated as a function of the appropriate parameters and the results are shown to be consistent with existing experiments.

Transport at low temperature in amorphous magnetic metals (invited)

Abstract: The scaling theory of conduction for a noninteracting electron gas in a disordered system at T = 0 is reviewed and the conductivity at various length scales obtained. The concepts of the mean free time between inelastic collisions and the corresponding inelastic length lin, are introduced. The conductivity for a dirty metal is obtained as a function of temperature at low temperatures in terms of lin(T). For the case of a disordered magnetic metal the effect of the temperature‐dependent magnetic disorder is introduced and several possible typical situations schematically analyzed. The effect of electron‐electron interactions is briefly discussed, as determining the effective inelastic length and a simple qualitative picture of conduction as a function of temperature suggested. Some cautionary remarks about the testing of these ideas by experiments are made.

Long-time cooling-rate dependence of spin-glass freezing

Abstract: Superconducting-quantum-interference-device magnetometry is used to study the long-time cooling-rate dependence of the susceptibility cusp of the spin-glasses, $\mathrm{Cu}\mathrm{Mn}$ 4.6 at.% and $Al\mathrm{Gd}$ 37 at.%. Over a range of time delay $\ensuremath{\Delta}T=6 \mathrm{to} 2400$ min per temperature change $\ensuremath{\Delta}T=0.2$ K, no change is observed in the position of the cusp. These long-time results do not fit an extrapolation from Tholence's earlier ac susceptibility data as a function of frequency ($\ensuremath{ u}=10 \mathrm{to} 30000$ Hz). Thus, they appear to be inconsistent with current nonequilibrium models for spin-glass freezing and rather suggest a phase-transition phenomenon.

Magnetoconductive effects in an effectively two-dimensional system. Weak Anderson localization

Abstract: Experimental results pertaining to the transport properties of thin indium oxide films are given and discussed. The data presented for the resistivity, the transverse magnetoresistance, the Hall constant, and their temperature dependences give consistent support to the basic ideas underlying weak-localization theories. In particular, the Hall constant, measured in small fields, is temperature independent.

Microwave‐induced ’’Devil’s Staircase’’ structure and ’’chaotic’’ behavior in current‐fed Josephson junctions

Abstract: We have obtained the various types of I‐V characteristics measured experimentally and in analog simulations, by merely changing the junction and the microwave parameters within the same resistively shunted junction model with purely sinusoidal current‐phase relation. It was found that the subharmonic steps do exist in the limit βc→0, though they can have finite rounding without thermal noise. The statistical properties of the ’’chaotic’’ solutions wer e discussed and their effective temperature was defined and calculated.

Dynamics of the DC‐SQUID

Abstract: The dynamics of two inductively‐coupled Josephson junctions, driven by a current source and by an external flux were studied. Approximate analytical arguments and numerical simulations were used. The various characteristics times of the DC‐SQUID were systematically obtained as functions of the parameters of the junctions and the superconducting loop. We have studied the effects of the external magnetic field and quasiparticle conductance on the structure of the I‐V characteristics. The transition times between the various branches of the I‐V characteristics were calculated using approximate analytical arguments as well as numerical simulations. The results, both for the I‐V characteristics and the transition times, were compared with existing experiments.

Field-dependence of DC susceptibility of spin glasses

Abstract: The field dependence of the DC susceptibility of CuMn and amorphous GdAl spin glasses is studied by SQUID magnetometry. The susceptibility cusp is found to move up in temperature with increasing field, and the dependence of the maximum susceptibility on field has an upward curvature down to low fields.

Current noise effects on the microwave induced steps in current fed Josephson junctions

Abstract: Harmonic and subharmonic microwave induced steps in the I-V characteristics of current fed Josephson junction were found and investigated by numerical and approximate analytical methods, for a purely sinusoidal Josephson current. The dependence of the steps on the various parameters is given and their local stability is discussed. Various types of I-V characteristics are shown. We have introduced thermal noise effects via a Langevin term in the equation of motion. The smaller steps are easily washed out by the thermal noise. The width of the fundamental step was found to be proportional to Txfor the range of parameters which we used, where x \gsim 1/2 .

Concentration-pressure-temperature phase diagram of PZT

Abstract: A simplified phenomenological model for PZT solid solutions is developed for the (x-p-T) concentration-pressure-temperature phase transition and compared with experimental data.