Nuclear spin-lattice relaxation in a Kondo superconductor

The nuclear spin-lattice relaxation rate R _s in a Kondo superconductor has been calculated numerically in the framework of the Müller-Hartmann-Zittartz depairing theory. The calculations have been performed in the case of the usual Korringa mechanism for certain concentrations of magnetic impurities and some typical values of T _K/T _c0, where T _K is the Kondo temperature and T _c0 is the superconducting transition temperature in the absence of magnetic impurities. The temperature variation of R _s shows very interesting behavior for values of T _K/T _c0corresponding to the reentrance behavior of superconductivity. Results are compared with the experimental data on the La(Ce)Al₂ system obtained by MacLaughlin, Alloul, and Daugherty. Other possible relaxation mechanisms in the La(Gd, Ce)Al₂ systems are also discussed.     

Nuclear spin-lattice relaxation in superlattices

The nuclear spin-lattice relaxation rate in a superlattice subjected to a strong magnetic field H parallel to its axis is studied theoretically. The energy conservation law allows flip-flop spin-reversal processes due to the hyperfine interaction between the nuclear and electron spins only in some intervals of H. In particular, the width of the highest occupied superlattice subband /spl Delta/ must exceed the spin splitting of Landau levels, which results in the relaxation offset at some critical magnetic field H/sub 2/. At H相似文献   

Thermal conductivity of a Kondo superconductor in the gapless state

The thermal conductivity coefficient for a Kondo superconductor in the gapless state is obtained within the framework of the Matsuura, Ichinose, and Nagaoka theory. By expanding the heat current correlation function appearing in the Kubo formula for the thermal conductivity in powers of the order parameter, analytical expressions for the thermal conductivity to second power in the order parameter are obtained. The thermal conductivities for bothT _KT _c 0 andT _KT _c 0 superconductors are obtained.     

Electron spin-lattice relaxation in oxide superconductors and fullerides

Using the original technique of measuring very short spin-lattice relaxation times T₁, the electron-spin relaxation has been investigated in a number of HTSC materials and related phases. The relaxation of Gd³⁺ ions in GdBa₂Cu₃O_6+x is shown to be affected by both quasiparticle and antiferromagnetic fluctuations, the latter being increased at lower oxigen content. The pseudo-spin gap of about 200 K is found for both contributions. For Cu²⁺ EPR in YBaCuO, the T₁ values reveal several types of paramagnetic centers including the green phase Y₂BaCuO₅. In the fulleride RbC₆₀, the measuring of T₁ and T₂ yielded additional support for 1D conductivity and allowed to monitor the metal-insulator phase.     

Nuclear spin lattice relaxation rate in the excitonic state

The nuclear spin lattice relaxation rate in the excitonic state is studied. Use is made of the two-band model, where the Fermi radii of the electron band and the hole band are assumed to be of the same size. It is important to distinguish two cases; the spin singlet case (i.e., the nonmagnetic excitonic state) and the spin triplet case (i.e., the antiferromagnetic state). In the case of the spin singlet excitonic state the nuclear spin lattice relaxation rate first increases in the excitonic state just below the transition temperature and then decreases rapidly as the temperature decreases. In the case of the spin triplet excitonic state the relaxation rate depends on whether the nuclear spin is polarized parallel or perpendicular to the spin-density wave describing the excitonic state. In the parallel case the relaxation rate decreases monotonically in the excitonic state as the temperature decreases, while in the transverse case it has a small peak just below the transition temperature.     

Nuclear spin-lattice relaxation in metals at low temperatures

In NMR experiments performed on metals at low temperatures, the analysis to obtain the nuclear spin-lattice relaxation time ₁ can be quite complicated. The electronic heat capacity ultimately becomes less than that of the nuclear spins, and the characteristic times for heat flow through the various resistances between the nuclear spins and the thermal bath can become long relative to ₁. This makes it necessary to solve the full set of coupled differential equations describing the thermal relaxation. This problem has been examined because ₁ is used as a thermometric parameter, and we consider the cases when the radiofrequency field penetrates the sample completely (powders and foils) and when only the skin-depth nuclei are excited (bulk specimens). Using our theoretical model, we analyze experimental results for platinum powder and a thallium bulk sample and obtain consistent values for =₁ T over a wide range of experimental conditions, even when thermal bottlenecks occur and the apparent relaxation time is much longer than ₁. These values are (Pt)=(34.2±0.6)×10^–3 sec K and (Tl)=(4.37±0.08)×10^–3 sec K.On leave from Department of Physics, University of British Columbia, Vancouver, British Columbia, Canada.     

Nuclear spin-lattice relaxation measurements in small superconducting aluminum particles

The ²⁷Al nuclear spin-lattice relaxation times T ₁ have been measured in the normal and superconducting states of small aluminum particles with average particle diameters ranging from 170 to 750 ». Experiments were performed in magnetic fields between 0 and 1200 Oe and at temperatures between 0.5 and 4.2 K. The nuclear spin-lattice relaxation rate 1/T ₁ at zero and low magnetic fields is characterized by a sum of a Korringa rate (due to conduction-electron spin-flip scattering) and an additional rate, which is attributed to a cross-relaxation process involving nuclei near the particle surfaces. At the highest field (1200 Oe), where the Zeeman energy either dominates in larger particles or plays an important role in smaller particles, measured rates for all three samples are in good agreement with the microscopic theory of Sone. At low fields (H 150 Oe) our T ₁ results, which show a strong field dependence and less effect of superconducting fluctuations near T _c, are in disagreement with Sone. We suggest that the theory has overestimated the effects of fluctuations on quasiparticle excitations at these low fields.     

Nuclear spin-lattice relaxation of isotopic impurities in solid hydrogen

We report calculations of the nuclear spin-lattice relaxation of isotopic impurities (HD) in solid hydrogen. The temperature-independent relaxation observed at low temperatures is attributed to the quantum tunneling of the impurities, and the pronounced minimum observed at intermediate temperatures is associated with scattering from thermally excited vacancies.     

Nuclear spin-lattice relaxation times of metallic antimony at low temperatures

We have used pulsed Nuclear Quadrupole Resonance (NQR) techniques to measure the nuclear spin-lattice relaxation times in antimony at low temperatures. High quality echoes with strong signal/noise ratios were only observed for finely powdered samples of high purity (99.9999%). The samples were carefully annealed and diluted with fine silica to below the percolation limit to minimize RF heating. The powder mixture was immersed in liquid³He to ensure good thermal contact to a sintered silver heat exchanger attached to a copper nuclear demagnetization refrigerator. We report the temperature dependence of the nuclear spin-lattice relaxation times for the multiple level nuclear spin system.     

Collective modes in charged,gapless superconductors

The collective modes of charged, gapless superconductors are discussed within the framework of the Gor'kov-Eliashberg theory, and the dynamical pair-field susceptibility is calculated. The effect of the charge is to replace the characteristic oscillations (fourth sound) associated with the uncharged system by a diffusive behavior qualitatively similar to that predicted by time-dependent Ginzburg-Landau theory. For sufficiently small pair breaking, 10^–3, and sufficiently close toT _c, (T_c–T)/T_c4×10^–6, a new oscillating mode appears, with (T _c–T)^1/3 q ^4/3, in both charged and uncharged systems.     

Kondo impurities in strong coupling superconductors

Generalized Eliashberg equations including the effects of Kondo impurities in strong coupling superconductors are given. The Kondo effect is treated using the Matsuura, Ichinose, Nagaoka (MIN) theory. The transition temperatureT _c is calculated from the general equations by using the square well model for the electron-phonon interaction. Deviations of the present results from those of MIN are significant. In general, the depression ofT _c by the impurities is softened.On leave of absence from Department of Physics, Srinakharinwirot University, Prasarnmitr, Bangkok, Thailand.     

Investigation of the gapless state in bismuth-antimony alloys

The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi _1–x Sb _x by pressure-induced band inversion. The magnetoresistance properties of the alloys have been investigated both in weak magnetic fields (H « 1) and in strong fields (H75 kOe) at liquidhelium temperatures in the Sb concentration interval 0.06x0.15 and pressure interval 1 bar p<20 kbar. At pressuresp close to the pressurep _k at which the gapless state is realized a semiconductor-semimetal-semiconductor transition is detected inBi _1–x Sb _x alloys withx=0.070 and 0.071. The rates of change of the gap _gL before and after inversion are determined: –(2.5±0.5)×10^–6 eV/bar and (1.5±0.5)×10^–6 eV/bar, respectively. A reduction in the carrier effective mass as _gL 0 is observed down to values of 10^–4 m ₀. It is shown that as _gL 0 the carrier mobilities in the alloys increase abruptly, the effect being a maximum in the purest alloys, where forT=4.2 K the mobility along the binary axis attains the record-high value of 3×10⁸ cm²/V · sec.     

Nuclear spin-lattice relaxation in bulk and divided samples of solid 3He at low temperatures

At temperatures in the region of 0.1 K, the nuclear spin-lattice relaxation in solid ³ He exhibits some unusual features which suggest that the mechanism might be diffusion in the spin system to surfaces within the crystal at which relaxation takes place rapidly. The results of a number of measurements of the diffusion of magnetization in solid ³ He specimens, including the influence of magnetic field, are presented. Relaxation time measurements on specimens formed in sintered glass give results consistent with the diffusion hypothesis, and for such specimens we can calculate the relaxation time and its magnetic field dependence with surprising accuracy. Similar calculations applied to bulk specimens, however, fail to account for the observations. The evidence suggests that the relaxation process in bulk specimens is not controlled by diffusion. An empirical formula is proposed which fits the relaxation data in bulk specimens over a range of parameters, although no physical process with the required properties can be identified.Work supported by research grants from the Science Research Council, who also provided financial support for RPG and JPS, and by Queen's College, Oxford, through a Junior Research Fellowship held by WST.     

Kondo effect in superconductors: The magnetic susceptibility

The vertex-part integral equation is adopted for the study of the Kondo problem in superconductors. The bound-states obtained are in good agreement with other results, but we have obtained two critical values for the bound states as a consequence of the two signs forJ (thes-d interaction). The temperature dependence of the magnetic susceptibility has been calculated using the result for the vertex function.     

Kondo effect in superconductors: Depression of the transition temperature

The influence of magnetic impurities on the transition temperature of superconducting alloys is investigated by means of the higher order parquet theory, which is essentially a self-consistent renormalization of the s-d interaction vertex and the quasi-fermion self-energy. The effective electron-electron interaction is evaluated beyond the logarithmic approximation, in terms of the renormalized spin-flip amplitude, which is responsible for the pair-breaking effect of the magnetic impurities. An expression for the maximal pair breaking, which is dependent on the spin and the superconducting coupling constant, is derived.     

Anomalous spin-lattice relaxation in solid3He

Nuclear spin-lattice relaxation measurements have been carried out in bcc and hcp solid³He at low temperatures. The observed relaxation rates are anomalously high and indicate that a typical sample has a substructure of small domains at whose boundaries rapid spin-lattice equilibrium takes place. Experiments on the sample anisotropy show that these domains cannot be crystallites and we believe that they may be determined by stacking faults or small-angle grain boundaries. The magnetic field dependence of the spin diffusion coefficient and the effects of dissolved⁴He are also investigated.     

Effect of superconducting fluctuations on the nmr relaxation rate of high-T c superconductors

The effect of superconducting order parameter fluctuations on the nuclear-spin relaxation rate, 1/T ₁, is studied for clean two-dimensional systems by calculating the three Maki-Thomson-type diagrams which represent the lowest-order fluctuation contributions to the transverse susceptibility. For Gaussian fluctuations and for temperatures near the mean field transition temperature,T _c0, we employ a weak-coupling theory in which the pair-fluctuation propagator can also include pair-breaking effects. We also go beyond the Gaussian theory and take into account the interactions between Cooper-pair fluctuations corresponding to the fourth-order Ginzburg-Landau fluctuation terms. We compare our results with previous results in the dirty limit and in 3D. We obtain a pronounced peak in 1/T₁ atT _c and briefly discuss possible reasons why this peak is not observed.     

Nuclear relaxation in the superconducting state of (MDT-TTF)2AuI2

We report the results of our attempt to measure the proton nuclear relaxation rate, 1/T ₁, in the superconducting state of the title material. The relaxation rate in the superconducting state at a field of 1 T was found much longer than that in the normal state, but it became clear that the dominant contribution came from the normal core region. The nuclear relaxation at zero field was examined by using the field cycling technique. An ln(t) term in the relaxation curve was observed at low temperatures, suggesting the contribution of the creeping motion of vortices. We discuss the possibility to determine the intrinsic temperature dependence of 1/T ₁ in the superconducting state.     

Retarded magnetic relaxation in levitated superconductors

The phenomenon of the magnetic relaxation slowing down in levitated superconductors has been observed. It is suggested that the decrease in the magnetic relaxation rate is related to a magnetic bias feedback in the sample, which restores a nonequilibrium magnetic structure broken by the magnetic flux creep.     

Effects of relaxation in levitating superconductors

The effect of the magnetic flux creep on the levitation stability of high-temperature superconductors was studied. It was shown experimentally that under a unipolar magnetization the levitation force decreased at a logarithmic velocity characteristic of the creep process. If the current structure was bi- or multi-polar one, which was formed in a sample exposed to a reversing external magnetic field, the force remained unchanged during a certain period of time. The theory of relaxation of magnetization and force for a partial and full penetration of the critical state was considered. It was shown that relaxation decelerated sharply if the region with a current producing the main magnetization was far from the superconductor surface. A concept of an open and internal magnetic relaxation was introduced. The time of the internal relaxation for different reverse depths was estimated. The calculated values approached the experimental values of the levitation stabilization time.