Temperature-independent Relaxation Time Research Articles

Quantum annealing of a frustrated magnet

Quantum annealing, which involves quantum tunnelling among possible solutions, has state-of-the-art applications not only in quickly finding the lowest-energy configuration of a complex system, but also in quantum computing. Here we report a single-crystal study of the frustrated magnet α-CoV2O6, consisting of a triangular arrangement of ferromagnetic Ising spin chains without evident structural disorder. We observe quantum annealing phenomena resulting from time-reversal symmetry breaking in a tiny transverse field. Below ~ 1 K, the system exhibits no indication of approaching the lowest-energy state for at least 15 hours in zero transverse field, but quickly converges towards that configuration with a nearly temperature-independent relaxation time of ~ 10 seconds in a transverse field of ~ 3.5 mK. Our many-body simulations show qualitative agreement with the experimental results, and suggest that a tiny transverse field can profoundly enhance quantum spin fluctuations, triggering rapid quantum annealing process from topological metastable Kosterlitz-Thouless phases, at low temperatures.

Tuning electrical and thermal conductivities of the two-dimensional electron gas in AlN/GaN heterostructures by piezoelectricity

We investigate the electrical and thermal conductivities of the two-dimensional electron gas (2DEG) confined in the quantum well formed at the heterojunction between a thin GaN layer and an AlN layer strained by an Al x Ga1−x N capping layer in the temperature range from 10 to 360 K. The experimental protocol developed to deduce from calorimetric and Hall-effect measurements at a variable temperature the critical characteristics and transport properties of the confined 2DEG is presented. It is found that, in the measured temperature range (10–360 K), the electrical conductivity of the 2DEG is temperature-independent, due to the predominance of scattering processes by interface defects. However, the thermal conductivity shows a linear temperature dependence, mirroring the specific heat of free electrons. The temperature-independent relaxation time associated with the overall electron scattering means that the values obtained for electrical and thermal conductivities are in excellent agreement with those stipulated by the Weidemann–Franz law. It is also found that for weak strain fields in the AlN layer, both the electrical and thermal conductivities of the two-dimensional interfacial electrons increase exponentially with strain. The importance of 2DEG in AlN/GaN quantum wells lies in the fact that the strong piezoelectricity of AlN allows the transport properties of the 2DEG to be tuned or modulated by a weak electric field even with the high density of lattice mismatch induced defects at the AlN-GaN interface .

Unexpected rheological behavior of solutions of aromatic polyamide in transient physical states

The subject of this study was an aromatic polyamide in dimethylacetamide/LiCl solutions in a concentration range from 0.5 vol. % – 5 vol. %. Dilute and semi-dilute solutions of this polymer demonstrate a complex of unexpected time- and temperature-dependent rheological effects under shearing in a heating–cooling cycle. In a static state, all systems under study are transparent solutions and no temperature-dependent thermal or visual effects are observed. However, superposition of shearing radically changes the situation. Heating up to 100 °C–140 °C leads to the phase separation with the coexistence of the amorphous and LC phases. On cooling of low-concentrated solutions, a decrease in the temperature leads to a several-times increase in the viscosity, and the subsequent viscosity decrease takes place at further temperature decrease. Both changes are kinetic effects. The first one is treated as an order-to-disorder transition. The decrease in viscosity is accompanied by a heat release, which reflects the reverse process of the disorder-to-order state transition. The isothermal viscosity decline in time is described by the Maxwell relaxation law with temperature-independent relaxation time. Hence, this is a non-temperature-activated process. At higher concentrations, strong temperature thixotropic behavior with much lower viscosity values on cooling, in comparison with the heating, is characteristic of these solutions due to their tendency for undercooling. The shear-induced transition, conjugated with the heat excess, was observed in semi-dilute solutions at the same temperature similar to that observed for dilute solutions. So during cooling under shearing, the solutions under study display a rather unusual phenomenon of a first-order transition. This type of phenomenon has not been described before. The phase transitions become quite evident in the polarized light. The observed kinetic effects in transient physical states are discussed on the basis of the concept of delayed and latent structure and phase transitions, including the formation of the LC state, which are initiated by the shearing.

Yb2Sn2O7: A magnetic Coulomb liquid at a quantum critical point

We report neutron scattering and ac susceptibility measurements on the pyrochlore Yb${}_{2}$Sn${}_{2}$O${}_{7}$. The appearance of magnetic diffuse scattering, low-energy spin excitations, and a nearly temperature-independent relaxation time are signatures that the system enters a short-range-ordered regime with quantum dynamics beneath 2 K. ac susceptibility data further suggest that Yb${}_{2}$Sn${}_{2}$O${}_{7}$ enters a long-range magnetically ordered state below 0.11 K. These results suggest that Yb${}_{2}$Sn${}_{2}$O${}_{7}$ is a ferromagnet with persistent spin dynamics down to 0.05 K. Referencing the phase diagram for Yb pyrochlores, Yb${}_{2}$Sn${}_{2}$O${}_{7}$ is approaching a quantum phase transition near the ferromagnetic ordered critical point.

Electrodynamic response ofSr2RuO4

The surface impedance ${Z}_{s}={R}_{s}+i{X}_{s}$ of a number of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ crystals has been measured in the frequency range $f\ensuremath{\sim}2--15\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ using a dielectric resonator technique. ${Z}_{s}$ conforms to the classical skin effect only above $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, because the quasiparticle relaxation rate $1∕\ensuremath{\tau}$ approaches $2\ensuremath{\pi}f$ at lower temperatures. Below ${T}_{c}$, ${R}_{s}$ decreases while ${X}_{s}$ initially increases and peaks at a frequency-dependent temperature. The microwave properties are quite unlike those of conventional and high-${T}_{c}$ superconductors and imply a finite quasiparticle fraction at $T=0$ with a temperature-independent relaxation time throughout the superconducting state.

Anisotropy and field-dependence of the spin-density-wave dynamics in the quasi one-dimensional conductor (TMTSF)2PF6

The anisotropic and non-linear transport properties of the quasi one-dimensional organic conductor (TMTSF)_2PF_6 have been studied by dc, radiofrequency, and microwave methods. Microwave experiments along all three axes reveal that collective transport, which is considered to be the fingerprint of the spin-density-wave condensate, also occurs in the perpendicular b' direction. The pinned mode resonance is present in the $a$ and b'-axes response, but not along the least conducting c* direction. The ac-field threshold, above which the spin-density-wave response is non-linear, strongly decreases as the temperature drops below 4 K. With increasing strength of the microwave electric field and of the radiofrequency signal, the pinned mode and the screened phason loss-peak shift to lower frequencies. In the non-linear regime, in addition to the phason relaxation mode with Arrhenius-like resistive decay, an additional mode with very long and temperature-independent relaxation time appears below 4 K. We attribute the new process to short-wavelength spin-density-wave excitations associated with discommensurations in a random commensurate N=4 domain structure.

Temperature dependence of the absorption saturation relaxation time in light- and heavy-ion-irradiated bulk GaAs

The absorption saturation relaxation time in light- and heavy-ion-irradiated GaAs saturable absorbers has been measured as a function of the temperature in the range from 7 to 300 K. For both types of samples, the relaxation time is shorter than 4 ps at 7 K. A regular increase of this time with temperature is observed for light-ion-irradiated samples, a value of 9.5 ps being reached at room temperature. In contrast, an almost temperature-independent relaxation time is found for heavy-ion-irradiated samples. The results are interpreted on the basis of a simplified relaxation model accounting for capture and emission from defect levels. We suggest that light-ion irradiation creates shallow centers whereas heavy-ion irradiation creates deep centers.

Weak localization in InSb thin films heavily doped with lead

We study the weak localization (WL) in three-dimensional polycrystalline thin films of InSb. The films are closely compensated showing the electron concentration $n>{10}^{16} {\mathrm{cm}}^{\ensuremath{-}3}$ at the total concentration of the donor- and acceptor-type structural defects ${N}_{t}>{10}^{18} {\mathrm{cm}}^{\ensuremath{-}3}.$ Unless Pb doped, the InSb films do not show any measurable or show very small WL effect at 4.2 K. The Pb doping to the concentration of the order of ${10}^{18} {\mathrm{cm}}^{\ensuremath{-}3}$ leads to pronounced WL effects below 7 K. From the comparison of the experimental data on temperature dependence of the magnetoresistivity and sample resistance with the WL theory, the dependence of the phase destroying time ${\ensuremath{\tau}}_{\ensuremath{\varphi}}(T)$ is determined. It is concluded that the dephasing is connected to three separate processes. The first is due to the spin-orbit scatterings and is characterized by temperature-independent relaxation time ${\ensuremath{\tau}}_{\mathrm{so}}\ensuremath{\simeq}{10}^{\ensuremath{-}12} \mathrm{s}.$ The second is associated with the electron-phonon interaction $({\ensuremath{\tau}}_{i}\ensuremath{\sim}{T}^{\ensuremath{-}3}).$ The third dephasing process is characterized by temperature-independent relaxation time ${\ensuremath{\tau}}_{c}=(1--7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} \mathrm{s},$ which is tentatively ascribed to inelastic scattering at extended structural defects. The resulting time ${\ensuremath{\tau}}_{\ensuremath{\varphi}}$ shows saturation in its temperature dependence for $\stackrel{\ensuremath{\rightarrow}}{T}0.$ The temperature dependence of the resistance can be explained by the electron-electron interaction for $T<1 \mathrm{K},$ and by the WL effect for $T>2 \mathrm{K}.$

Flux-flow conductivity in high-Tc superconductors

Using an early theory by Houghton and Maki based on the BCS model, which assumes a temperature-independent relaxation time tau , we compute the flux-flow magnetoresistivity rho S(T,H) of layered superconductors near H(c2) in the clean limit. Then we use our results to analyse the magnetoresistivity curves of single-crystal YBaCuO. Extending the validity of the calculation to a temperature-dependent tau , we find that for constant tau , or for relaxation times obtained from experiments, the theory reproduces qualitatively the saturation-like behaviour of the observed magnetoresistance, and also the broadening of the resistive transition, as well as the changes in the slope d rho S(T)/dT with field. However, quantitative agreement with experiments is not achieved. This allows an estimate to be made of the contribution of the thermal vortex line fluctuations to the total dissipation; we find the former to be about 10% of the latter.

One-dimensional kinetic ising model with one impurity

The effect of one impurity with arbitrary temperature independent relaxation time on the one-dimensional kinetic Ising model is exactly studied. The dynamical susceptibility for the low concentration of defect is also obtained.