Influence Of Superconductivity Research Articles

Структурные свойства сверхрешеток Nb/Dy и Nb/Ho

Proximity effects in structures with helimagnetic ordering are investigated. Structures with sharp boundaries were prepared by magnetron sputtering. Results indicating a change in the magnetic state of a helimagnet under the influence of superconductivity were obtained by polarized neutron reflectometry. A new type of structure with alternating layers of a superconductor and a rare-earth helimagnet is proposed for studying magnetic proximity effects. The results of studies using X-ray methods and atomic force microscopy demonstrate the high quality of the obtained structures.

Effects of disorder on magnetotransport oscillations in a two-dimensional electron gas terminated by superconductors

The coupling of superconductivity in a two-dimensional electron gas (2DEG) generates a number of magnetotransport oscillations. For instance, an Aharonov–Bohm-type oscillation at intermediate magnetic fields and an Altshuler–Aronov–Spivak-like oscillation around zero magnetic field appear under the circumstance of the coexistence of Andreev and normal reflections from the interface between the normal-conductor and the superconductor. The presence and the characteristics of such magnetotransport oscillations are investigated in this work by carrying out fully quantum-mechanical simulations. The significant role of the quantum interference is thereby demonstrated. It is also shown how the oscillations are affected by the presence of a potential disorder and finite excitation biases. Shubnikow–de Haas oscillations of the 2DEG are not always suppressed by the disorder under the influence of superconductivity, making their distinction from the Aharonov–Bohm-type oscillation possibly ambiguous.

Magnetism in structures with ferromagnetic and superconducting layers

The influence of superconductivity on ferromagnetism in the layered Ta/V/Fe1–x V x /V/Fe1–x V x /Nb/Si structures consisting of ferromagnetic and superconducting layers is studied using polarized neutron reflection and scattering. It is experimentally shown that magnetic structures with linear sizes from 5 nm to 30 μm are formed in these layered structures at low temperatures. The magnetization of the magnetic structures is suppressed by superconductivity at temperatures below the superconducting transition temperatures in the V and Nb layers. The magnetic states of the structures are shown to undergo relaxation over a wide magnetic-field range, which is caused by changes in the states of clusters, domains, and Abrikosov vortices.

Theoretical studies of the crystal–field energy levels and EPR g factors for CeFeAsO1−xFx

The ion-based superconductor [Formula: see text] has received much attention because this rare earth oxypnictide belongs to a new class [Formula: see text] of high-[Formula: see text] superconductors with critical temperatures surpassed by high-[Formula: see text] copper oxides. Since the [Formula: see text] is situated near the superconducting FeAs plane, it can be used as a probe to study the influence of superconductivity on spin dynamic properties of the FeAs layer. It is important to determine the optical and magnetic properties of rare earth ion in oxypnictides. Here, based on the crystal–field theory, the crystal–field (CF) energy levels and electron paramagnetic resonance parameters [Formula: see text] factors of [Formula: see text] in [Formula: see text] are studied by diagonalizing the full energy matrix. In the calculations, the contributions of various admixtures and interactions are all considered. The results are discussed.

Inelastic contribution of the resistivity in the hidden order in URu2Si2

In the hidden order of URu2Si2 the resistivity at very low temperature shows no T2 behavior above the transition to superconductivity. However, when entering the antiferromagnetic phase, the Fermi liquid behavior is recovered. We discuss the change of the inelastic term when entering the AF phase with pressure considering the temperature dependence of the Grüneisen parameter at ambient pressure and the influence of superconductivity by an extrapolation of high field data.

Local triplet superconductivity ofLa0.65Ca0.35MnO3−Xpoint contacts(X=Pb,MgB2)

Microconstrictions between spin-singlet superconductor, Pb or $\mathrm{Mg}{\mathrm{B}}_{2}$, tip and half-metallic ferromagnet, ${\mathrm{La}}_{0.65}{\mathrm{Ca}}_{0.35}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO), plate have been realized by means of a point-contact inset. Measurements of the current-voltage characteristics and of the dynamic conductance $G(V)$ versus bias have been performed to probe mutual influence of superconductivity and half-metallic ferromagnetism. In the contacts, which we distinguish as proximity affected ones, a few principal effects have been observed. Namely, with decreasing temperature, a spectacular drop of the contact's resistance has been detected with an onset of the Pb or $\mathrm{Mg}{\mathrm{B}}_{2}$ superconductivity; for small voltages, an excess current and doubling of the normal-state conductance have also been found. We conclude that the underlying physical explanations for these results are the conversion from spin-singlet pairs to spin-triplet ones at the Pb $(\mathrm{Mg}{\mathrm{B}}_{2})∕\mathrm{LCMO}$ interface and a long-range proximity induced superconductivity of LCMO. Superconducting state of LCMO is also supported by the observation of the coherent multiple Andreev reflections (subharmonic gap resonances). We also found that the character of $G(V)$ vs voltage dependence corresponds to that for the induced superconducting energy gap of LCMO much larger than that of Pb or $\mathrm{Mg}{\mathrm{B}}_{2}$. All specific characteristics of proximity affected contacts suggest that the local triplet superconducting fluctuations are essentially sustained in LCMO and the singlet superconductors only fix the phase coherency of a superconducting state.

Transformation of Magnetic State under the Influence of Superconductivity in Epitaxial V/Pd1−x Fe x Bilayers

We have performed ferromagnetic resonance (FMR) studies of epitaxial V/Pd1−xFex (001) bilayers with a V thickness of the order of 40 nm and with a Pd1−xFex thickness in the range from 0.8 to 4.4 nm. For a bilayer with a Pd1−xFex thickness of 1.2 nm and x = 0.03, the FMR measurements revealed a decrease in the effective magnetization 4πMeff of the ferromagnetic layer below the superconducting transition temperature of vanadium. As a possible explanation for this decrease, we suggest a spatial modulation of the ferromagnetic order in the Pd1−xFex layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.

Proximity effects in superconductor-ferromagnet heterostructures

The very special characteristic of the proximity effect in superconductor-ferromagnet systems is the damped oscillatory behavior of the Cooper pair wave function in a ferromagnet. In some sense, this is analogous to the inhomogeneous superconductivity, predicted long time ago by Larkin and Ovchinnikov (1964), and Fulde and Ferrell (1964), and constantly searched since that. After the qualitative analysis of the peculiarities of the proximity effect in the presence of the exchange field, the author provides a unified description of the properties of the superconductor-ferromagnet heterostructures. Special attention is paid to the striking non-monotonous dependance of the critical temperature of the multilayers and bilayers on the ferromagnetic layer thickness and conditions of the realization of the "Pi"- Josephson junctions. The recent progress in the preparation of the high quality hybrid systems permitted to observe on experiments many interesting effects, which are also discussed in the article. Finally, the author analyzes the phenomenon of the domain-wall superconductivity and the influence of superconductivity on the magnetic structure in superconductor-ferromagnet bilayers.

Possible reconstruction of the ferromagnetic state under the influence of superconductivity in epitaxial V/Pd1−x Fe x bilayers

Epitaxial V/Pd1−x Fe x (001) bilayers with a V thickness of the order of 40 nm and with a Pd1−x Fe x thickness in the range from 0.8 nm to 4.4 nm were prepared by molecular beam epitaxy techniques. The Curie temperature of the Pd1−x Fe x layers varies between 90 and 250 K. For a bilayer with a Pd1−x Fe x thickness of 1.2 nm the ferromagnetic resonance measurements revealed a decrease of the effective magnetization 4πM eff of the ferromagnetic layer below the superconducting transition temperature of V. As a possible explanation for this decrease we suggest a spatial modulation of the ferromagnetic order in the Pd1−x Fe x layer due to modifications of the indirect exchange interaction of magnetic ions via conduction electrons in the superconducting state. A comparison with a recent theoretical investigation supports this possibility.

Proximity effect in superconductor/ferromagnet layered system: Influence of superconductivity on magnetic properties of Nb/Fe epitaxial bilayers

Magnetic and superconducting properties of Fe/Nb epitaxial bilayers have been studied. Single crystal Fe/Nb (110) bilayers with a Nb thicknessd Nb in the range from 250 to 650 A and with a Fe thicknessd Fe in the range from 7 to 27 A were prepared using molecular beam epitaxy techniques. Magnetization measurements showed the existence of a magnetically “dead” Fe-interface layer with a thicknessd NM varying strongly with minor modifications of the growth conditions. For bilayers with a small magnetic layer thicknessd M and a smalld NM the FMR measurements revealed an anomalous decrease of the effective magnetization 4πM eff of the ferromagnetic Fe layer below the superconducting transition temperatureT c. The absolute change of 4πM eff belowT c depends strongly on the actual values ofd M andd NM: upon decreasing bothd M andd NM the effect increases considerably. As a possible mechanism for the anomalous temperature dependence of 4πM eff belowT c we discuss a spatial modulation of ferromagnetic order due to a modification of the RKKY inter-action in the superconducting state.

On the influence of superconductivity on the phonon frequencies in a two-band model

The influence of the onset of hole pairing onq=0 phonon frequency in a two-band model of high-Tc superconductivity is investigated. The estimated frequency shifts indicate a phonon softening with the lowering of temperature and agree with the experimental magnitude.

Influence of superconductivity on quantum diffusion of the positive muon in aluminium

We present low temperature studies of muon diffusion and trapping in normal and superconducting aluminium doped with 75 at.ppm Li. The trapping rate at the lithium impurities, and thus the deduced diffusion rate, increases far more rapidly with decreasing temperature in the superconducting state compared with the normal state. This establishes the importance of conduction electrons at the Fermi surface in limiting long-range quantum diffusion in a metal.

Influence of superconductivity on quantum diffusion of the positive muon in aluminium

We present low temperature studies of muon diffusion and trapping in normal and superconducting aluminium doped with 75 at.ppm Li. The trapping rate at the lithium impurities, and thus the deduced diffusion rate, increases far more rapidly with decreasing temperature in the superconducting state compared with the normal state. This establishes the importance of conduction electrons at the Fermi surface in limiting long-range quantum diffusion in a metal.

Preparation, characterization, and far-infrared study of ceramic high-Tc superconductors

We report on a study of single-phase (superconducting) YBa2Cu3O7 and (nonconducting) Y2BaCuO5 and mixed phases of these compounds in bulk and film samples and on an investigation of ceramic bulk material of the homologous series Tl2Ba2CanCun+1O2n+6. We describe preparation and structural characterization by x-ray diffraction analyses and further characterization by electric, magnetic, tunnel junction, and Raman scattering studies, and we present results of far-infrared reflectivity measurements. In a theoretical analysis we relate the phonon structure of reflectivity spectra to geometric structure of a metallic network, with holes filled with nonconducting material. The influence of superconductivity on reflectivity is also investigated.

Evidence of Successive Magnetic Transition in La2-xBaxCuO4 –NQR Studies of 139La–

Nuclear quadrupole resonance (NQR) of 139 La has been performed to study the electronic and magnetic properties of La 2- x Ba x CuO 4 between 1.5 and 240 K. The temperature dependence of the separation of the double peaks in spectra clearly indicates the magnetic ordering in the small x region. The internal fields at La sites due to magnetic order largely decrease with Ba concentration. The divergence of 1/ T 2 indicates that another phase appears at low temperatures. The electric field gradient changes on passing from the normal to the superconducting states, indicating the apparent influence of superconductivity on the electronic state at La sites in this system.

Electron structure and phase diagrams of low‐temperature rare‐earth magnets

AbstractThe transition ion states are described by mixed many‐electron operators. The hybridization coefficients are calculated by the variational principle. Low‐temperature magnets are considered within the weak‐exchange energy limit. The destruction of the magnetic order is studied at the increase of the single‐ion anisotropy and (Ising's) exchange anisotropic energy. The magnetic phase diagrams (MPD) of the uniaxial and cubic magnets are calculated. The restrictions of the moment value J and anisotropy constant k at which these critical phenomena occur are determined here. The influence of superconductivity (SC) on the MPD is studied. An enhancement of SC is shown in the antiferromagnetic state. The intermediate helix phase is studied between the SC and ferromagnetic states.

Influence of superconductivity on magnetic exchange interactions

Using the formulation due to Fedro and Robinson of magnetic exchange in the presence of superconductivity, the mean field magnetic interaction energy has been calculated for rare earth (RE) ions in the RERh4B4 lattice. It is shown that increasing the strength of the superconducting interaction relative to the magnetic interaction results in a decrease of ferromagnetic coupling, and in some cases may drive ferromagnetic systems into antiferromagnetism.

Influence of superconductivity on the sound propagation in disordered metals

It is believed that in disordered metals low energy excitations, similar to those in insulating glasses, are present. The interaction of these excitations with electrons and phonons can be studied by ultrasonic methods at low temperatures. A brief review of the ultrasonic properties of disordered metals is presented. Special attention is given to the recent experiments on superconducting disordered metals, which are best suited to study the interaction of electrons with the low energy excitations.

Influence of superconductivity on the sound propagation in disordered metals

Abstract It is believed that in disordered metals low energy excitations, similar to those in insulating glasses, are present. The interaction of these excitations with electrons and phonons can be studied by ultrasonic methods at low temperatures. A brief review of the ultrasonic properties of disordered metals is presented. Special attention is given to the recent experiments on superconducting disordered metals, which are best suited to study the interaction of electrons with the low energy excitations.

Influence of superconductivity on the sound propagation in disordered metals

The velocity of sound has been investigated in the disordered metal Nb 20 Zr 80 which contains low energy excitations. From measurements in the superconducting as well as in the normal state the interaction between these excitations and conduction electrons has been studied in detail.