Measurements of the nuclear spin-lattice relaxation times in BCC3He for high magnetic fields,high molar volumes,and low temperatures

Experimental studies of the nuclear spin-lattice relaxation times in BCC helium three are reported for high molar volumes and low temperatures close to the minimum in the melting curve. The results show the existence of a new relaxation mechanism at low temperatures with an approximately linear temperature dependence (T ₁ T ^–1). An analysis of the vacancy-induced relaxation shows that both the vacancy formation energy and the vacancy mobility decrease dramatically as a function of density on approaching the maximum possible molar volume (minimum density) of the solid. A possible interpretation of both the new relaxation mechanism and the vacancy properties is outlined.     

Correlation between structural relaxation enthalpy and superconducting properties of amorphous Zr70Cu30 and Zr70Ni30 alloys

The anneal-induced change in the superconducting properties together with the irrecoverable relaxation enthalpy (H _i,exo) and recoverable relaxation enthalpy (H _r,endo) of amorphous Zr₇₀Cu₃₀ and Zr₇₀Ni₃₀ alloys was examined. The increase in _i,exo and the degradation ofT _c progress logarithmically with annealing timet _a in a temperature range of 373 to 523 K. The activation energy and the attempted frequency were respectively estimated to be 1.5eV and 6.6 × 10¹³ sec^–1 for the increase in H _i,exo and 1.5eV and 1.9×10¹⁴ sec^–1 for the degradation ofT _c. The recoverable structure relaxation exerts little effect onT _c. Based on the agreement between the kinetic parameters for the changes of H _i,exo andT _c, it appears that the degradation ofT _c on annealing is associated with the irrecoverable structural relaxation as a result of the annihilation of frozen-in defects and the topological and compositional atomic rearrangement. The values of the attempted frequency being of the order of Debye frequency suggest that the irrecoverable structural relaxation processes occur more or less independently from each other. The dressed density of electronic states at the Fermi level,N(E _f)(1+), determined from the measured values of _n and -(dH _c2/dT)_{T c} using GLAG (theory), was found to have a similar annealing dependence to that ofT _c. The degradation ofT _c by the irrecoverable relaxation was thus inferred as resulting from the decrease in due to the decrease inN(E _f) and the increases inM and . Furthermore, the irrecoverable structural relaxation resulted in a significant depression of fluxoid pinning force and was interpreted as due to an enhanced structural homogeneity on the scale of coherence length.     

Effect of annealing on the superconducting properties of two amorphous alloys: Nb70Zr15Si15 and Zr85Si15

The changes in the superconducting and electronic properties of amorphous Nb₇₀Zr₁₅Si₁₅ and Zr₈₅Si₁₅ alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys.T _c rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature,t _d, lowers monotonically to a final relaxed value (3.15 K for Nb₇₀Zr₁₅Si₁₅ and 2.49 K for Zr₈₅Si₁₅), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering ofT _c occurs exponentially witht _d, and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×10¹⁴ sec^–1 for Nb₇₀Zr₁₅Si₁₅ and about 1.28 eV and 1.2×10¹¹ sec^–1 for Zr₈₅Si₁₅, respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level,N(E _f) (1+), which was calculated from the measured values of _n and (dH _c2dT)T_c, exhibited a similar annealing temperature dependence to that ofT _c. From this the change inT _c on thermal relaxation was interpreted as due to the changes in and/orN(E _f). From the depressions ofJ _c(H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (7.5 nm) during the annealing.     

Pinning of vortices and critical current in the presence of parallel twinning planes

Magnetic and transport properties of superconductors with a periodic array of twinning planes (TP's) are studied for the case of an increase of the critical temperature in the neighborhood of a TP. Based on the modified London model, the critical magnetic field H_c1, the barrier field H_s, and the critical current j_c(H) are calculated for low temperatures much less than T_c. For resonant vortex configurations, a peak in the curve j_c(H) is predicted with a peak value which cannot be larger than 10⁶ A/cm². Furthermore, the interplay of the bulk and surface pinning for TT_c as well as the magnetic and transport properties for T_c0c are studied.On leave from the Institute of Physics, Belgrade University, Belgrade, Yugoslavia. Also affiliated with Fachbereich Physik WE 5, Freie Universität Berlin, D-1000 Berlin 33.     

Quantum tunneling of vortices in single crystal Tl2CaBa2Cu2O8 superconductors

Data are presented on the temperature-dependent time-logarithmic magnetic relaxation rate S(T) = ¦dM/d_in t¦ of the high-T_c superconductor Tl₂CaBa₂ Cu₂O₈. It is found that at low temperatures the relaxation rate has the form S(T) = A(H) exp[(T/T^*)²], which does not extrapolate to zero at T = 0, thus excluding conventional thermally activated flux creep and providing evidence of quantum vortex tunneling. From a quantum flux tunneling theory, it is shown that S(T) I/_er _p ² . The measurements of the relaxation rate thus provide information about the effective viscosity _e of fluxons.     

Two-stage enthalpy relaxation behaviour of (Fe0.5Ni0.5)83P17 and (Fe0.5Ni0.5)83B17 amorphous alloys upon annealing

The anneal-induced enthalpy relaxation behaviour for (Fe_0.5Ni_0.5)₈₃P₁₇ and (Fe_0.5Ni_0.5)₈₃B₁₇ amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures belowT _g, an excess endothermic reaction (enthalpy relaxation) occurs aboveT _a. The C _p,endo evolves reversibly in a continuous manner with Int _a. The changes in the magnitude of C _p,endo and H _endo withT _a show clearly two distinct stages; a low-temperature one which peaks at aboutT _g-200 K and a high-temperature peak just belowT _g. The activation energy,Q _m, increases with the peak temperature of C _P,T _m, from 1.7 to 2.5 eV for the Fe-Ni-P and from 1.8 to 2.0 eV for the Fe-Ni-B for the low-temperature peak, and from 2.6 to 5.0 eV for the Fe-Ni-P for the high-temperature peak. The reversible change inT _c for the Fe-Ni-P alloy pre-annealed for 1 min at 640 K as a function ofT _a was also found to show two stages; a low-temperature stage ranging from 400 to 550 K whereT _c rises, and a high-temperature one above 550 K whereT _c lowers. From the almost complete agreement of the temperature region and the peak temperature for each stage as well as the marked contrast in the change ofT _c, it was proposed that the low-temperature endothermic peak is attributed to local and medium range rearrangements of metallic (iron and nickel) atoms and the high-temperature reaction to the long-range cooperative regroupings of metallic and metalloid atoms. The mechanism for the appearance of the two-stage enthalpy relaxation was investigated based on the new concept of two-stage distribution in relaxation times (or glass transitions) centring atT _g1 andT _g2 which arise respectively from the metal atoms and from the metal-metalloid atoms and the distinct two-stage splitting was interpreted to generate by the distinctly distinguishable difference in the easiness of structural relaxation between metal-metal and metal-metalloid. It was further found that the present irreversible and reversible enthalpy relaxation results are fairly well interpreted by a possible mechanism of two-stage relaxation processes consisting of as-quenched amorphous local and medium range relaxation of metal atoms cooperative relaxation of metal and metalloid atoms.     

Effects of superconducting fluctuations on the NMR relaxation rateT 1 −1 of two-dimensional systems aboveT c

Fluctuation-induced corrections to the NMR relaxation rate T ₁ ^–1 in dirty two-dimensional superconductors above the superconducting transition (T>T_c) are re-investigated including dynamic fluctuations. Static fluctuations, as discussed by Kuboki and Fukuyama,¹ dominate the fluctuation terms near T_c. We find that dynamic fluctuations become important for T/T_c1.05, and might even reverse the sign of the total fluctuation-induced contributions to T ₁ ^–1 . We observe that (T₁T)^–1 has a minimum at T=T_min (·) for ·/T_c0.1 which depends on the pair-breaking parameter ·. Furthermore, we show that the fluctuation contributions to T ₁ ^–1 can be obtained numerically with very high accuracy from multipoint Padé approximants.     

Anisotropy of the superconductive to normal transition in tantalum-nitrogen single crystals

Systematic experiments on the transition to the normal state in superconducting tantalum single crystals doped with interstitially dissolved nitrogen impurities are presented. On the low- side of the magnetic phase diagram, where at some low temperature 0 <T ^* <T _ca transition from type I to type II superconductivity occurs, a new effect has been observed, as reported briefly elsewhere: because of the correlation ofH _{c 2} with crystal directions, at fixed temperatures nearT ^*, whether the sample is a type I or a type II superconductor depends on which crystal direction is oriented parallel to the external field. In the type II superconducting range all samples display anH _{c 2} anisotropy effect, which is commonly observed in cubic superconductors, even in the extreme dirty limit (~12). However, the second anisotropy coefficient (l = 6 component) is found to be negligibly small in all tantalum samples.     

Flux dynamics and low-field magnetic properties of the heavy-fermion superconductor CeCu2Si2

Strong flux creep effects are observed in CeCu₂Si₂ at low magnetic inductions and temperatures in the range 6 mK<T<T _c . Measurements of flux creep in single crystal as well as polycrystalline specimens show that the magnetization in CeCu ₂ Si ₂ decays following a power law time dependence of the typeM(t)t ^– for 1<t<10 ⁵ sec, where the exponent a ranges between 0.015 and 0.06. The activation energy values obtained from the relaxation rates are around 2–3 meV. The flux creep data give clear evidence of the existence of different types of pinning centers in CeCu ₂ Si ₂ : a large number of shallow potential barriers that characterize the vortex dynamics at short times and few strong pinning centers responsible for the long-time relaxation behavior. Quantum tunneling of flux lines through weak pinning centers probably gives rise to the non-thermally activated relaxation observed at the lowest temperatures. The physical origin of weak and strong pinning centers in CeCu ₂ Si ₂ is discussed. From isothermal dc magnetization measurements we estimate the lower critical fieldH _c1 as well as the critical current densityJ _c . TheH _c1 versusT curve shows an anomalous positive curvature for temperatures close toT _c .     

Effect of magnetic field and temperature on the voltage-current characteristics of YBCO superconductor

Voltage-current characteristics of YBCO superconductor was studied under magnetic field up to 0.4 T at different temperatures below T_c. The critical temperature decreases and the transition width broadens under magnetic field. V-I data below T_c were fitted to a power law expression V I^(T,B) in which (T,B) is found to decrease with increase of magnetic field and temperature, gradually approaching unity as T approaches T_c, being independent of magnetic field. Similarly, (T,B) approaches unity as magnetic field increases being independent of temperature.     

Kondo effect and impurity interactions in the resistivity of diluteZnMn alloys

The electrical resistivity of diluteZnMn alloys (c=1.7–2400 ppm Mn) has been investigated in the temperature range from 0.05 to 14 K. For the most dilute sample, single-impurity Kondo behavior is found, well described by the Hamann formula withT _K=0.9 K,S=3/2. ForT<50 mK, aT ² law with _R =0.3 K is expected. In the dilute limit the Kondo slope is –(1/c)d()/d(logT)=3.7±0.2 µ-cm/at % dec. In the more highly concentrated alloys, the slope decreases with increasing c and the lnT-like variation of the Kondo resistivity roughly terminates near a temperatureT _W(c),T _W being related to the average Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction strength between the Mn impurities. For 20 ppm c 1000 ppm, the resistivity slightly decreases at low temperatures and a broad resistivity maximum is observed atT _m (c), withT _m c ^0.7. ForT 2 _m, the resistivity dependence is linear inT, and for the most concentrated alloy aT ^3/2 orT ² dependence is measured at the lowest temperatures attainable. The investigation of the transition temperatureT _c (c) to superconductivity ofZnMn results in a critical concentrationc _cr=18 ppm Mn. The concentration dependence ofT _c below 0.3 K suggests the presence of the Kondo effect, although impurity interactions may also influenceT _c in this temperature range.Supported by Deutsche Forschungsgemeinschaft.     

Phonon anomalies in YNi2 11B2C

Recently we have discovered a new peak with phonon character which appears only below T_c in an intermetallic superconductor, YNi₂ ¹¹B₂C (Tc14-2K) by neutron inelastic scattering measurements. The peak intensity of the new peak shows the order-parameter-like temperature dependence of the superconductivity. The new peak grows by absorbing the spectral weight from the above-lying soft phonon mode. In the present study we investigate the magnetic field dependence of the new peak. With increasing magnetic field, the new peak is gradually suppressed and disappears by H=H_c2 (4.7T at T=5.5K). Beyond H_c2, the profile of the scattering function recovers that of the non-superconducting state above T_c These results strongly indicate that this new peak is undoubtedly associated with the superconducting state in YNi₂ ¹¹B₂C.     

Quantum effects and the resistive transition in superconducting granular films

Static and dynamic properties of an array of Josephson junctions shunted by Ohmic resistors are discussed within a quantum Ginzburg-Landau theory. The phase diagram at zero temperature is calculated in mean field approximation. It shows that global superconductivity atT=0 is possible only if the normal-state film resistanceR _n is smaller than a critical valueR _n ^c which depends only logarithmically on the Josephson coupling and charging energies. The particular valueR _n ^c =6.5 k found in recent experiments on granular films is in reasonable agreement with estimates for these parameters. A phenomenological order parameter relaxation mechanism is introduced and the associated fluctuation-induced conductivity and diamagnetic susceptibility aboveT _c are determined. The resulting precursor conductivity does not explain the observed exponential decrease withR _n-R _n ^c of the residual resistance at low temperature. However, a very simple model for the resistance due to vortex flow, generalizing the classical Kosterlitz-Thousless picture, is in good agreement with the experimental data.     

Nuclear magnetic relaxation of3He gas. II.3He-4He mixtures

Longitudinal relaxation timesT ₁ have been measured in³He-⁴He gas mixtures, using pulsed NMR, in the temperature range 0.6–15 K. Helium-3 number densities of the order of 10²⁴ atoms m^–3 were used. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements ofT ₁ give information about the surface phases. A cryogenic precoating of solid molecular hydrogen was used to reduce the helium-substrate binding energy from 100 K on Pyrex to 13 K for³He and 15 K for⁴He. TheT ₁ data at high temperatures were similar to those observed previously in the pure³He-H₂ system. The presence of⁴He generally causedT ₁ to rise on cooling below 2 K due to the preferential adsorption of⁴He over³He at the surface. However,³He atoms that go into quasiparticle states in the superfluid helium film can be an extra source of relaxation. In uncleaned cells, relaxation probably takes place in quasiparticle states at the free surface of the superfluid film, which are bound with an energy of 5.1±0.3 K. Baking the Pyrex cells under vacuum and rf discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gasT ₁ by two or three orders of magnitude. In a cleaned, sealed cell aT ₁ of 8 h was measured at 7.7 MHz and 0.8 K. In this case relaxation is probably occurring two or three helium layers away from the helium-hydrogen interface. It may be possible to observe a predicted minimum in the intrinsic dipolarT ₁ of the bulk gas by using a⁴He wall coating to suppress wall relaxation effects (which usually dominate the nuclear relaxation of the bulk gas).     

Fluctuations of the Order Parameter''s Phase in Layered Superconductors

The influence of the quantum fluctuations of the order parameter's phase on the critical temperature T _c is studied for a Josephson coupled layered superconductor. Two characteristic critical temperatures exist for a system, namely the superconducting critical temperature T ⁽²⁾ _c for a single layer estimated by the mean-field theory and the transition temperature for the outset or the superconducting phase coherence T* _c . The true critical temperature T _c is shown to vary inside the intervals T* _c T _c T ⁽²⁾ _c . For a strong quantum phase fluctuation limit, the superconducting layers become decoupled.     

Modulus spectroscopy of lead potassium titanium niobate (Pb<Subscript>0.95</Subscript>K<Subscript>0.1</Subscript>Ti<Subscript>0.25</Subscript>Nb<Subscript>1.8</Subscript>O<Subscript>6</Subscript>) ceramics

The modulus Spectroscopy of Lead Potassium Titanium Niobate (Pb_0.95K_0.1Ti_0.25Nb_1.8O₆, PKTN) Ceramics was investigated in the frequency range from 45 Hz to 5 MHz and the temperature, from 30 to 600 °C. XRD analysis in PKTN indicated a orthorhombic structure with lattice parameters a = 18.0809 Å, b = 18.1909 Å and c = 3.6002 Å. The dielectric anomaly with a peak was observed at 510 °C. Variation of ε^I and ε^II with frequency at different temperatures exhibit high values, which reflects the effect of space charge polarization and/or conduction ion motion. The electrical relaxation in ionically conducting PKTN ceramic analyzed in terms of Impedance and Modulus formalism. The Cole–Cole plots of impedance were drawn at different temperatures. The dielectric modulus, which describes the dielectric relaxation behaviour is fitted to the Kohlrausch exponential function. Near the phase transition temperature, a stretched exponential parameter β indicating the degree of distribution of the relaxation time has a small value. From the AC conductivity measurements the activation energy near phase transition temperature (T _C°C) has been found to different from that of the above and below T _C. The temperature dependence of electrical modulus has been studied and results are discussed.     

Frequency dependence of superfluid onset temperature of4He films on oscillating substrates

We have studied the frequency dependence of the superfluid onset for⁴He films on Mylar substrates using a two-torsional-oscillators technique. A peak of the dissipation accompanied by the superfluid onset appears at higher temperatures for 2009 Hz than for 530 Hz. The temperature difference T_p between the two peaks is 2.0 ± 0.5 mKfor coverages with the onset temperature T_c of 1.4 K, and increases by a factor of three with decreasing T_c from 1.4 to 0.95 K. In the temperature dependence of the dissipation, the dissipation width also increases by a factor of three with decreasing T_c from 1.4 to 0.95 K. The T_c dependencies of T_p and the dissipation width indicate that an intrinsic parameter D/r₀ ² of a vortex decreases with the superfluid onset temperature, where D is a diffusion constant and r₀ a core radius.     

Sound Modes of Superfluid 3He in Aerogel

We present the results of experiments on sound propagation at audio frequencies in ³ He-filled aerogel. Sound modes were observed at temperatures of 0.8–100 mK in an aerogel sample of 98% porosity. We find that below T _c for superfluid ³ He in the aerogel matrix the speed of sound in the composite system increases by as much as 1.5%. Also below the aerogel T _c new modes appear which correspond to propagation speeds of up to 10 m/s.     

Low-temperature spin fluctuations in La2−xSrxCuO4 studied by Cu nuclear spin-lattice relaxation

Measurements of the temperature (T) dependence of the planar⁶³Cu nuclear spin-lattice relaxation rate (1/T₁)for the⁶³Cu-enriched high-T_c Cu oxides, La_2–xSr_xCuO₄ (x=0.13 and 0.18; T_c34K), are reported. 1/T₁T shows a maximum around 50 K, similar to that for V₂O₃ under a high pressure and for the bilayered high-T_c Cu oxides. The T-dependent anisotropy of 1/T₁ suggests that the T-dependent antiferromagnetic correlation length can explain the peaked T-dependence of 1/T₁T.