On the theory of localized superconductivity: Fluctuational diamagnetism

The diamagnetic susceptibility of a twinning plane at temperatures slightly higher than the localized superconductivity temperature T_c as well as the heat capacity jump at T = T_c are calculated. The possibility of an appreciable increase of the superconductivity temperature in small particles containing twinning planes is studied.     

Fluctuation enhanced diamagnetic susceptibility below Tc

The effect of fluctuating Cooper pairs on the diamagnetic susceptibility of dirty superconducting thin films is investigated, for temperature T < T_c. The susceptibility decreases with the decrease in temperature, approaching $\text{?}\text{1}\text{4}\text{π}$ at T = 0°K and remains finite at T = T_c.     

Fluctuations in superconductors belowT c

Thermal fluctuations of the order parameter in the superconducting state are investigated, particularly near the transition temperature, using the time dependent Ginzburg-Landau theory. These fluctuations give rise to a contribution of the dynamical conductivity, which strongly increases as the temperature is raised towardsT _c , in contrast to the temperature dependence of the electromagnetic response due to the static order parameter. At the transition temperature this contribution joins continuously to the extra response (conductivity, susceptibility) which has been calculated and observed in the temperature region aboveT _c , where it represents the onset of superconductivity in the normal state. Particularly the dynamical conductivity due to fluctuations is calculated and discussed for bulk material, thin films and thin wires belowT _c . The temperature and frequency dependence should be observable in microwave experiments.     

Structure,superconductivity, characterization and Fe Mössbauer studies on high temperature superconductor Y1Ba2(Cu3−x Fe x )O7−δ

Different samples of Y₁Ba₂(Cu_3?x Fe _x )O_7?δ compounds have been synthesized, the resultant bulk superconductors were found to show diamagnetic behaviour and they have been investigated from the point of view of superconductivity through the study of X-ray diffraction, A.C. magnetic susceptibility, electrical conductivity and Mössbauer effect.     

Effect of high pressures on exchange and hyperfine interactions in rare-earth orthoferrites

The effect of high pressures P on the Néel temperature T _N , the crystal lattice parameters, and the magnitude of the hyperfine magnetic field H ^Sn at a nucleus of a diamagnetic tin atom was studied in pure and tin-doped orthoferrites RFeO₃ (R = Nd, Lu). The dependence of these quantities on the geometry of the exchange bonds, specifically, the angle and length of the chemical bond Fe-O-Fe(Sn), was analyzed. It was established that under pressure the angular contribution decreases and the radial contribution increases T _N and H ^Sn, the radial contribution being greater than the angular contribution in absolute magnitude. Numerical estimates were obtained for the angular and radial contributions to T _N and H ^Sn. In lutecium orthoferrite, at P > 30 GPa anomalies indicating a possible phase transition were observed in the behavior of the lattice parameters and the value of H ^Sn.     

External electric field, hydrostatic pressure and conduction band non-parabolicity effects on the binding energy and the diamagnetic susceptibility of a hydrogenic impurity quantum dot

Electric field, hydrostatic pressure and conduction band non-parabolicity effects on the binding energies of the lower-lying states and the diamagnetic susceptibility of an on-center hydrogenic impurity confined in a typical GaAs/Al_xGa_1−xAs spherical quantum dot is theoretically investigated, by direct diagonalization of the Hamiltonian. To this end, the effect of band non-parabolicity has been performed, by means of the Luttinger-Kohn effective mass equation. Binding energies and diamagnetic susceptibility of the hydrogenic impurity are computed as a function of the dot size, external electric field strength and hydrostatic pressure, with considering the edge-band non-parabolicity. Results show that the external electric field and the hydrostatic pressure have an obvious influence on the binding energies and the diamagnetic susceptibility of the impurity.     

Simultaneous effects of temperature and pressure on diamagnetic susceptibility of a shallow donor in a quantum antidot

The effect of temperature and pressure, simultaneously, on the diamagnetic susceptibility and binding energy of a hydrogenic donor impurity at the center of a GaAs/Ga_1−xAl_xAs quantum antidot is studied within the effective mass approximation. For this goal, we first analytically solve the Schrödinger equation to obtain wavefunctions and energy levels. Then, using the electronic states, we can calculate the diamagnetic susceptibility. The results obtained from the present work reveals that (i) the diamagnetic susceptibility increases with increasing pressure, (ii) the diamagnetic susceptibility decreases by increasing temperature, (iii) the value of 〈r²〉 decreases with increasing pressure due to the quantum confinement, and (iv) an increase in the pressure enhances the binding energy for a constant temperature.     

The a.c. susceptibility third harmonic component of NdO1−0.14F0.14FeAs: A flux dynamic magnetic analysis

In this contribution we present the analysis of the third harmonic susceptibility data of the new superconductor NdFeAs_1−0.14F_0.14. ‘Cole-Cole’ polar plots respect to the magnetic frequency of the exciting field are presented and discussed. Data show that NdFeAs_1−0.14F_0.14 exhibit a ‘bulk pinning’ with a 3D flux dynamic character. A comparison of the responses of high T_c materials and an evaluation of theoretical critical states is also presented.     

Studying the magnetic properties of CoSi single crystals

The magnetic properties of CoSi single crystals have been measured in a range of temperatures T = 5.5–450 K and magnetic field strengths H ≤ 11 kOe. A comparison of the results for crystals grown in various laboratories allowed the temperature dependence of magnetic susceptibility χ(T) = M(T)/H to be determined for a hypothetical “ideal” (free of magnetic impurities and defects) CoSi crystal. The susceptibility of this ideal crystal in the entire temperature range exhibits a diamagnetic character. The χ(T) value significantly increases in absolute value with decreasing temperature and exhibits saturation at the lowest temperatures studied. For real CoSi crystals of four types, paramagnetic contributions to the susceptibility have been evaluated and nonlinear (with respect to the field) contributions to the magnetization have been separated and taken into account in the calculations of χ(T).     

Temperature and concentration quenching of mid-IR photoluminescence in iron doped ZnSe and ZnS laser crystals

Concentration dependences of the mid-IR kinetic of luminescence at ⁵E?⁵T₂ transition in Fe:ZnSe and Fe:ZnS laser samples were studied in 14–300 K temperature ranges. Radiation lifetime in Fe:ZnSe samples measured using low doped samples with iron concentration 0.1×10¹⁸ cm^?3 was estimated to be 57 μs. The magnetic susceptibility for higher doped (C_Fe=38 and 112×10¹⁸ cm^?3) Fe:ZnSe samples was found to consist of a paramagnetic Curie–Weiss behavior arising from the weakly interacting Fe²⁺ ions and a diamagnetic ZnSe contribution plus a temperature-independent, field-dependent contribution possibly arising from very small amounts of aggregated Fe.     

Surface magnetism of nanocrystalline copper monoxide

The effect of the surface on the magnetic susceptibility of nanopowders of the CuO semiconducting antiferromagnet was studied. Single-phase nanopowders with nanoparticles 15, 45, and 60 nm in size were prepared through copper vapor condensation in an argon environment, with subsequent oxidation of the copper. The temperature dependences of the magnetic susceptibility of the nanopowders differ qualitatively from the χ (T) relations for bulk samples. In the region 80≤T≤600 K, the magnetic susceptibility of nanopowders is inversely proportional to temperature and is described by the sum of contributions due to the bulk part of CuO and to the Cu²⁺ paramagnetic ions localized in surface layers. The paramagnetic contribution to the total susceptibility is shown to increase with decreasing particle size and sample density. A comparison of the χ (T) relations is made for nanopowders and for a dense CuO nanoceramic with grain size 5≤d≤100 nm prepared using the shock wave technique.     

Effect of plastic deformation of V,Nb and Ta on the superconducting transition temperature and the specific heat coefficients

Specific heat measurements performed between 1.4 and 20 K on bulk and cold-worked V, Nb and Ta superconducting materials are presented. The plastic deformation produces an increase in the superconducting transition temperature T_c, an increase which is relatively less important for Ta than for Nb and less for Nb than for V. An increase is registered for the normal linear coefficient of specific heat γ whereas the Debye temperature decreases slightly. The apparent relation between the increase of T_c and γ suggests qualitatively that the vibrating mobile dislocation contribution is not the only origin of the increase of γ but that a band structure contribution is also to be taken into account.     

Magnetic properties of low-doped bulk Sr(Ti, Co)O3−δ perovskites

Magnetic properties of bulk SrTi_1−xCo_xO_3−δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO₃ matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.     

Diamagnetic susceptibility of hydrogenic donor impurity in low-dimensional semiconducting systems

The binding energies of a hydrogenic donor both in the parabolic and non-parabolic conduction band model within the effective mass approximation have been computed for the low-dimensional semiconducting systems (LDSS) like quantum well, quantum well wire and quantum dot taking GaAs/Al_xGa_1−xAs systems as an example. It is observed that the effect of non-parabolicity is not effective when the system goes to lower dimensionality. The diamagnetic susceptibility of a hydrogenic donor impurity has also been computed in these LDSS in the infinite barrier model. Since no theoretical or experimental works on the diamagnetic susceptibility of LDSS are available in the literature, as a realistic case the diamagnetic susceptibility has been computed in the finite barrier model (x=0.3) for a quantum well and the results are discussed in the light of semiconductor-metal transition.     

Pair-breaking in thin superconducting films — the effects on Tc and fluctuation conductivity

The thickness dependent pair breaking parameter, inherent in the Maki-Thompson contribution to the excess conductivity due to superconducting fluctuations, is found to be equal to the observed shift of T_c relative to the bulk value. Measurements were performed on thin amorphous Be_0.7Al_0.3 films produced by vapour quenching.     

Diamagnetic properties of tetrahedrally coordinated Ge1-xSx and Ge1-xSex amorphous alloys

Measurements of the diamagnetic susceptibility χ at room temperature are reported for Ge_1-xS_x and Ge_1-xSe_x amorphous alloys as a function of composition. The composition dependence of χ is shown to reflect the corresponding changes in the average gap E_g and it is concluded that the dia diamagnetic enhancement observed in tetrahedrally bonded amorphous semiconductors is caused by a decrease in the interband matrix element in the Van-Vleck paramagnetic susceptibility term with bond angle distortion rather than changes in E_g.     

Evidence for impurity phase superconductivity in EuMo6S8 under pressure

Magnetization measurements in a static field as a function of temperature (2.5–20K) and pressure to 14 kbar are reported for a series of well characterized samples of EuMo_6+xS_8?y. From these d.c. magnetization data, we find no evidence for bulk superconductivity in any of our samples, although some of them exhibit a strong diamagnetic anomaly in the a.c. susceptibility under pressure, similar to that reported in the literature. An explanation for this anomaly is presented in terms of the presence of a granular superconducting impurity on the grain boundaries of the Chevrel phase compound EuMo₆S₈.     

Incomplete Meissner effect of triplet superconductivity

The magnetic properties of a triplet superconductor are investigated using a phenomenological Ginzburg-Landau theory. Due to the presence of a paramagnetic term in the free energy arising from the energy required to flip the spins of a triplet pair by the magnetic field, the system does not exhibit a complete Meissner effect below T_c. This paramagnetic contribution to the magnetization is stabilized by the non-linear terms in the free energy, and for certain values of the parameters, can even cancel the diamagnetic term. The results are discussed in terms of the Anderson- Morel and Balian-Werthamer states.     

On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Investigations of the magnetic structure of the surface and volume of aluminum-substituted Sr-M type hexaferrites

The magnetic structure of the surface layer of single crystals of hexagonal ferrites of the type Sr-M (SrFe₁₂O₁₉) in which some iron ions are replaced by diamagnetic Al ions is investigated, in direct comparison with the magnetic structure in the bulk of the sample, by the method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy. It is found that under conditions of diamagnetic dilution of the magnetic lattice of hexagonal ferrites of the type Sr-M by Al ions, a layer ~200 nm thick in which the orientation of the magnetic moments is not collinear with the direction of the moments in the bulk of the sample is observed on the surface of SrFe_10.2Al_1.8O₁₉ crystals. Thus a “transitional” surface layer has been observed on macroscopic ferromagnetic crystals.