Magnetic states of the surface and bulk of ferrites near a phase transition at the Curie temperature

Investigations of the magnetic state of a surface layer ~200 nm thick and of the bulk in macroscopic ferrite crystals of the type Ba-M (BaFe₁₂O₁₉) are performed in the phase transition region around the Curie temperature (T _c). The method of simultaneous gamma, x-ray, and electron Mössbauer spectroscopy, which made it possible to compare directly the phase states of the surface and bulk of the sample, is used for the measurements. It is observed experimentally that in BaFe₁₂O₁₉ the transition of a surface layer ~200 nm thick to the paramagnetic state occurs at temperatures below T _c. It is established that the transition temperature T _c(L) of a thin layer localized at depth L from the surface of the crystal increases with distance from the surface and reaches the value T _c at the lower boundary of the “critical” surface layer. Therefore, near T _c a nonuniform state in which the crystal is magnetically ordered in the bulk but disordered at the surface is observed. A phase diagram of the states of the surface and of the bulk of macroscopic magnets near the Curie (or Néel) point is proposed on the basis of all the experimental results obtained in the present work as well as previously published results.     

Electronic state on the surface in the strongly correlated electron systems

In order to clarify the tunneling spectroscopy in high-T_c cuprates, we study electronic state of the surface in the strongly correlated electron systems. First, we obtain Green's function of strongly correlated normal bulk system using the fluctuation exchange (FLEX) approximation. Next, we insert infinite potential into the bulk system and obtain Green's function of surface. We find that the density of states (DOS) in strongly correlated bulk systems are different from that on the surface, and the difference decreases as the magnitude of Coulomb interaction (U) increases.     

Mössbauer investigations of the surface state of hexagonal ferrites Sr-M near the curie point

The temperature dependence of the parameters of the hyperfine interaction in the surface layers and in the bulk of macroscopic crystals of hexagonal ferrites of the type Sr-M (SrFe₁₂O₁₉) is investigated by the method of simultaneous gamma-, x-ray, and electron Mössbauer spectroscopy. It is shown experimentally that the transition of an ≈ 200 nm thick surface layer of macroscopic ferromagnets to the paramagnetic state occurs at a temperature 3° below the Curie point (T _c) for the bulk of the crystal. It was established that the transition temperatureT _c(L) of a thin layer localized at a depthL from the surface of the crystal increases away from the surface and reaches the valueT _c at the lower (away from the surface) boundary of the so-called “critical” surface layer. A nonuniform state in which the bulk region of the crystal is magnetically ordered while the surface region is disordered is observed nearT _N.     

Specific heat of A-15 Nb3Si produced by explosive compression

The specific heat from 1.2 to 23 K has been measured on a new high T_c superconductor, A-15 Nb₃Si. The sample was prepared by explosive compression and has an onset of bulk superconductivity at 18.0 K, with a transition width of 0.7 K. The density of states for pure A-15 Nb₃Si implied from the specific heat data is 0.94 ± 0.20 states/eV-atom, ΔC/γ T_c is 2.0 ± 0.2.     

Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.     

Superconducting energy gap in Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals

Current-voltage characteristics of S-I-S tunnel break junctions fabricated from pure undoped Bi2223 single crystals (T _c =110 K) were measured. High quality of the crystals enabled production of good tunnel junctions with a low or almost zero leakage current and well developed gap structure in the tunneling spectra. The peak-to-peak energy gap values 2Δ_p-p in different crystals and the tunnel junctions ranged from 80 to 105 meV. The tunneling conductance in the superconducting state was normalized to that in the normal state and compared to a smeared BCS density of states. A simple fit of the data gave the average value of Δ=38.5 meV and reduced gap 2Δ/kT _c ?8, consistent with a very strong coupling mechanism.     

Vortex core shapes measured by STM

We have studied vortex core shapes in superconducting NbSe₂ by STM, as function of temperature and bias voltage. The experimentally measured tunnel current profiles are compared with the results of calculations using microscopic theory. We find that, at low temperatures (T/T _c ? 0.25), the apparent vortex core radius strongly depends on the bias voltage, which demonstrates the energy dependence of the scale for spatial variation of the quasiparticle density of states. Good quantitative agreement between measured and calculated profiles is found by using the accepted value for the superconducting coherence length Ξ_S, without further adjustable parameters. This shows that the bias dependence is a useful extra tool in the interpretation of local density of states measurements.     

Flux-dependent transport through an Aharonov-Bohm interferometer with embedded multiple coupled quantum dots

The electron transport through an Aharonov-Bohm (AB) interferometer with embedded four coupled quantum dots (QDs) is studied with the Green's function technique. The QDs are coupled to each other by the hopping integral tc. Two among them connect with the left lead and the other two with the right lead by the tunneling matrix element T which incorporates the effects of the applied magnetic field φ. The linear conductance spectra swap between the molecular levels and the atomic states by adjusting tc and T. Fano effect appears when the electrons tunnel through different channels contributed by different QDs energy levels, and the Fano resonance peaks split for large tc. The Fano factor can be manipulated by tc, T, φ, and the QD energy levels.     

Aging effects in the nonequilibrium behavior of multilayer magnetic superstructures

A numerical Monte Carlo study of the nonequilibrium behavior of multilayer magnetic superstructures consisting of alternating magnetic and nonmagnetic nanolayers is performed. The calculated two-time autocorrelation function and the staggered magnetization of the structure at its evolution starting from various initial states are analyzed. The analysis reveals aging effects characterized by a slowing down of the relaxation and correlation characteristics in the system with the waiting time. It is shown that, in contrast to bulk magnetic systems, the aging effects in magnetic superstructures arise not only near the ferromagnetic ordering temperature T _c in the films but also within a wide temperature range at T ≤ T _c.     

Theory of magnetic impurities in superconductors. I

The Nagaoka Green's functions equations for a superconductor are solved exactly and the question of bound states within the gap is answered unambiguously. It is shown that there are always two bound states symmetrically located with respect to the center of the gap. For ferromagnetic coupling,γ>0, and for antiferromagnetic coupling, butT _K ?T _c0 orT _K ?T _c0, these bound states are very close to the gap edges. ForT _K ～T _c 0, however, the bound states move towards the center of the gap. It is pointed out that in the latter case important deviations from the Abrikosov-Gorkov theory, i.e. manifestations of the Kondo effect, should occur which could be tested experimentally by investigating the density of states.     

Critical temperature of metallic hydrogen sulfide at 225-GPa pressure

The Eliashberg theory generalized for electron—phonon systems with a nonconstant density of electron states and with allowance made for the frequency behavior of the electron mass and chemical potential renormalizations is used to study T _c in the SH₃ phase of hydrogen sulfide under pressure. The phonon contribution to the anomalous electron Green’s function is considered. The pairing within the total width of the electron band and not only in a narrow layer near the Fermi surface is taken into account. The frequency and temperature dependences of the complex mass renormalization ReZ(ω), the density of states N(ε) renormalized by the electron—phonon interactions, and the electron—phonon spectral function obtained computationally are used to calculate the anomalous electron Green’s function. A generalized Eliashberg equation with a variable density of electron states has been solved. The frequency dependence of the real and imaginary parts of the order parameter in the SH₃ phase has been obtained. The value of T _c ≈ 177 K in the SH₃ phase of hydrogen sulfide at pressure P = 225 GPa has been determined by solving the system of Eliashberg equations.     

High-temperature superconductivity in two-band materials with interband pairing

Eliashberg theory is generalized with account of the specific properties of two-band electron-phonon (EP) systems. The superconducting transition temperature T _c is examined in two-band materials, one of the representatives of which are pnictides. The strong EP coupling and the pairing within the full width of the electron band and not only in a narrow region near the Fermi surface are taken into account. It is found that the effect of the pairing of electrons belonging to different bands is crucial to the appearance of high T _c in these materials. It is shown that the high T _c value in the materials, such as pnictides, is displayed with the use of a two-band spectral function of the EP interaction derived from calculations and from tunnel experiments. The existence of specific conditions for the high T _c appearance in the two-band materials is established.     

Multicritical behavior of the phase diagrams of ferromagnet/superconductor layered structures

For ferromagnet/superconductor (F/S) layered structures, new 3D Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states are predicted. In most cases, these states are characterized by a higher critical temperature T _c than the known 1D LOFF states. It is shown that the nonmonotonic behavior of T _c is determined by the oscillations of the Cooper pair flux through the F/S boundary, which occur as a result of the 3D-1D-3D phase transitions at the Lifshits triple points. The appearance of the new 3D LOFF states and the presence of nonmagnetic impurities leads to a strong damping of the 1D oscillations of the LOFF pair amplitude and to a considerable smoothing of the dependence of T _c on the F layer thickness d _f . An interpretation of the behavior of the experimental dependences T _c (d _f ) obtained for F/S structures is proposed.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

Temperature dependence of the energy gap in Bi2223 metal oxide superconductor

Spectroscopic studies of the silver-optimum-doped Bi2223 contacts show that the temperature dependence of the parameter Δ follows the BCS curve. However, the tunnel measurements performed for the same series of specimens did not reveal any temperature dependence of the energy gap Δ. The feature observed in the tunnel density of states was retained at temperatures T>T _c , manifesting the presence of the temperature-independent pseudogap E _p . The difference between the data obtained with tunnel spectroscopy and Andreev reflection spectroscopy is explained by the fact that the latter measures the true superconducting energy gap Δ_s(T), whereas the peaks of the tunneling conductivity are related to the total energy gap Δ of cuprates, which includes both the parameter Δ_s and the pseudogap $E_p :\Delta \approx \sqrt {\Delta _s^2 + E_p^2 } $ .     

Superconductivity of Li under pressure

The superconductivity of Li under pressure is studied by a density functional method. Structural and elastic properties, transition temperature (T_c), density of states (DOS), are considered for the material at ambient and at higher pressures. The calculations, particularly of T_c and DOS as a function of pressure, are compared with other available results. This is in view of the wide difference between the previously predicted maximum T_c-value and those observed by both magnetic susceptibility and electrical resistivity experiments. The present calculations yield better estimates of T_c and other parameters with respect to measured values.     

The superconducting energy gap measured by tunneling into quench-condensed germanium-gold

Ge_0.5Au_0.5 films, quenched on a substrate below 2K are superconducting with T_c = 3.10 ± 0.12K, and 2Δ(0)k_BT_c= 3.75 ± 0.1. No phonon structure is visible in the second derivative of the tunnel current.     

Spin-orbit effects in carbon nanotubes – Analytical results

We report a study of the normal and superconducting state properties of the Ti _x V_1?x alloys for x = 0.4, 0.6, 0.7 and 0.8 with the help of dc magnetization, electrical resistivity and heat capacity measurements along with the electronic structure calculation. The superconducting transition temperature T _c of these alloys is higher than that of elemental Ti and is also higher than elemental V for x ≤ 0.7. The roles of electron density of states, electron-phonon coupling and spin fluctuations in the normal and superconducting state properties of these alloys have been investigated in detail. The experimentally observed value of T _c is found to be considerably lower than that estimated on the basis of electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys. There is some evidence as well for the preformed Cooper pair in all these Ti-V alloys in the temperature regime well above T _c . Similar to x = 0.6 [Md. Matin, L.S. Sharath Chandra, R.K. Meena, M.K. Chattopadhyay, A.K. Sinha, M.N. Singh, S.B. Roy, Physica B 436, 20 (2014)], the normal state properties of the x = 0.4 alloy showed the signature of the presence of spin fluctuations. The difference between the experimentally observed T _c and that estimated by considering electron density of states and electron-phonon coupling in the x = 0.4, 0.6 and 0.7 alloys is attributed to the possible influence of these spin fluctuations. We show that the non-monotonous variation of T _c as a function of x in the Ti _x V_1?x alloys is due to the combined effects of the electron-phonon coupling and the spin fluctuations.     

NMR studies of singlet-ground-state rare-earth ions in high-T c superconductors

Many of presently known high-T _c superconductors contain rare-earth (RE) ions with an even number of electrons in an unfilled 4f-shell (Pr³⁺, Tb³⁺, Ho³⁺, Tm³⁺). If the ground state of 4f-electrons is non-degenerate and separated from excited states by high enough energy intervals, one can observe the so-called “enhanced NMR” of RE nuclei at low temperatures. In the present paper some aspects of the enhanced NMR are analyzed in applications to the crystal and electron structure of high-T _c superconductors.