Dynamic Relaxation Rate Research Articles

Critical current, magnetization relaxation and activation energies for YBa 2Cu 3O 7 and YBa 2Cu 4O 8 films

The supercurrent j s and the dynamical relaxation rate Q ≡ dln j s / dln( dB e/ dt) of YBa 2Cu 3O 7 and YBa 2Cu 4O 8 films of about 100 nm thickness have been measured by means of a torque magnetometer for temperatures up to T c and fields up to B e = 7 T. From these data the true critical current j c ( T, B e) and the activation energy function U( j, T, B e) for thermally activated flux motion have been calculated. For fields smaller than about 2 T, both samples are in the single vortex regime. The obtained U( j, T = 0, B e ) dependence is rather well described by the collective pinning interpolation formula.

Observation of quantum creep in oxygen deficient YBa 2Cu 3O 7−δ at fields up to 7 T

Superconducting current densities j s and dynamic relaxation rates Q ≡ d ln j s/d ln (dB e/dt) in an oxygen deficient YBa 2Cu 3O 7−δ epitaxial film were measured as a function of temperature (1.5K < T < T irr ) in magnetic fields up to 7 T by means of high sensitivity capacitance torquemeters. Below 5 K, the relaxation rate remains high and does not extrapolate to Q = 0 at T = 0 K. This is associated to the quantum tunneling of vortices through the energy barrier separating two adjacent vortex configurations. In the case of ohmic dissipation the quantum creep rate is proportional to the normal state resistivity ϱ n (0) at T = 0 K. In order to check this theoretical prediction we measured Q as a function of the oxygen content of the sample. The quantum creep rate increases systematically with increasing ϱ n (0). However, Q remains finite even when ϱ n = 0.

Vortex tunneling in thin films of YBa 2Cu 3O 7−δ and YBa 2Cu 4O 8 in fields up to 7 T

The superconducting current density j s of YBa 2Cu 3O 7−δ(123) and YBa 2Cu 4O 8 (Y124) epitaxial thin films is studied down to 100 mK as function of the applied magnetic field B e. Torque magnetometry makes it possible to measure j s in magnetic fields up to 7 T in a wide range of magnetic field sweep-rates dB e/dt. From these data we extract the dynamic relaxation rate Q >2.trbnd; dlnj s/d ln (dB e/dt) which extrapolates to Q(0) = 2.2% for Y123 and Q(0) = 2.6% for Y124 at T = 0 K. This provides evidence for vortex motion by tunneling. We calculate the normal state resistivity ϱ n(0) using Q(0) ≈ ϱ n(0) for the case of ohmic dissipation in the vortex cores. The values of ϱ n(0) are comparable for both compounds and similar to ϱ n(0) found for Bi 2Sr 2CaCu 2O 8+°. This indicates that CuO-chains play a minor role and dissipation is mainly taking place in the CuO-planes.

Non-Newtonian flow behaviour of glass melts as a consequence of viscoelasticity and anisotropic flow

A new flow equation is derived for glass forming melts starting from very high deformation rates, ε dot , where the fluid behaves totally unrelaxed like a brittle body with exclusively elastic components. With decreasing ϵ dot flow occurs with a finite ultimate viscosity connected with a dynamic relaxation process which is characterized by the ‘dynamic relaxation rate’. This partly relaxed condition turns to a totally relaxed fluid with decreasing ϵ dot resulting in a Newtonian flow regime. In order to get the real flow curve, stress σ versus ϵ dot , the experimentally obtained values have to be corrected by a heat belance between dissipated heat and heat loss during the mechanical deformation. From those thermally corrected flow curves, expressed by the new flow equation, the true and apparent non-Newtonian viscosity are obtained as a function of ϵ dot and of log ϵ dot , respectively. In this way it could be shown that the non-Newtonian flow behaviour is a consequence of viscoelasticity. This viscoelasticity is, however, not the only one origin of non-Newtonian flow behaviour because additionally the orientability or at least the anisotropy of the flow units play an important part, which is shown by the different character of the flow curves and of the true and apparent viscosity of a silicate glass melt with a three-dimensionally connected dynamic network and of an alkali metaphosphate glass melt with a one-dimensionally connected dynamic network. Additional evidence about the anisotropic flow behaviour is obtained directly from literature on flow birefringence of single-phase glass melts and on the frozen-in birefringence of drawn glass fibres, glass rods and compressed glass cylinders above T g .

Generalized inversion scheme for the determination of activation energies from flux-creep experiments in high-Tc superconductors.

For thermally activated flux creep, which can be characterized by a temperature-magnetic-field-, and current-density-dependent activation energy U(j,T;${\mathit{B}}_{\mathit{e}}$), we show that this function can be determined from a combination of magnetization and magnetization dynamical relaxation rate data. As an illustration the method is applied to creep data on epitaxial ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ thin films. In contrast to previously proposed procedures, the present inversion scheme is much more general as it does not require a priori assumptions about the explicit temperature or field dependence of U.

Thermally activated flux motion and quantum creep in YBa 2Cu 3O 7 and Y 2Ba 4Cu 8O 16 films

The dynamical relaxation rate Q  dln j s/dln(dB/dt) where j s is the current induced in a superconductor when the external field is swept at a rate dB/dt, has been measured in YBa 2Cu 3O 7 and Y 2Ba 4Cu 8O 16 films of 100 nm and 180 nm thickness as a function of magnetic field at temperatures between 2 K and T c. The data show i) a non zero relaxation at low T due to quantum creep, ii) a plateau between 15 and ∼ 50 K and iii) a divergence when T approaches the irreversibility line. The data are analyzed within the collective pinning model. At low temperatures it is shown that both j s and Q are influenced by quantum creep.

Experimental observation of quantum creep in high-T c superconducting YBCO-PrBCO superlattices

Measurements of the dynamical relaxation rate of the magnetic moment of YBCO/PrBCO multilayers with decoupled YBCO layers of 1.2, 2.4, 3.6, 4.8 and 9.6 nm thickness have been carried out with a high sensitivity capacitance torque magnetometer. The relaxation at low temperature increases with decreasing YBCO layer thickness and reaches ∼ 7% at 2 T in the thinnest sample. Deviations from thermally activated flux motion, due to thermally assisted vortex tunneling are observed at temperatures up to 8 K.

A Green functions approach to the orientational behaviour of pairs of coupled two-level tunnelling units

The equation of motion method for the Green's functions is applied to study the behaviour of pairs of interacting two-level tunnelling units, coupled with a phonon field to calculate, within the framework of the small-polaron theory, the dynamic susceptibilities and relaxation rates of the system. Explicit expressions for these quantities are obtained in the Debye approximation.

Dynamic susceptibility of the Anderson model: A quantum Monte Carlo study.

Using a highly accurate method of analytic continuation, we calculated the dynamic susceptibility and NMR relaxation rate 1/{ital T}{sub 1} of magnetic (Anderson or Kondo) impurities over the {ital entire} range {ital T}{much lt}{ital T}{sub {ital K}} to {ital T}{much gt}{ital T}{sub {ital K}}. We find that the susceptibility and NMR relaxation rate are universal functions when properly scaled and that the NMR relaxation rate is directly proportional to the universal Kondo resistivity.

Charge-imbalance fluctuations in superconductors

We calculate that the mean-square amplitude of the fluctuations of the condensate chemical potential ..mu../sub s/ due to charge-imbalance fluctuations in the limit ..delta../k/sub B/T = 2(k/sub B/T)/sup 2//..pi..delta..cap omega..N(0) in a volume ..cap omega.. of superconductor. We relate these fluctuations via Nyquist's theorem to measured values of the contribution of self-injected charge imbalance to the dc resistance of SIN tunnel junctions. In this relation the dynamic charge-imbalance relaxation rate is 1/tau/sub E/, the electron-phonon scattering rate.