Hidden reentrant superconducting phase in a magnetic field in (TMTSF)<Subscript>2</Subscript>ClO<Subscript>4</Subscript>

We suggest explanation of the high upper critical magnetic field, perpendicular to conducting chains and parallel to conducting layers H _c2 ^b′ ≃ 6 T, experimentally observed in the organic superconductor (TMTSF)₂ClO₄. In particular, we show that H _c2 ^b′ can be higher than both the quasiclassical upper critical field and Clogston-Chandrasekhar paramagnetic limit in a singlet quasi-one-dimensional superconductor. We predict the coexistence of the hidden Reentrant and Larkin-Ovchinnikov-Fulde-Ferrell phases in a magnetic field. Our results are compared to the recent experimental data and shown to be in a good agreement with the experiments.     

Surface critical magnetic field H<Subscript>c3</Subscript>(T) of a bulk superconductor MgB<Subscript>2</Subscript> using two-band Ginzburg-Landau theory

Two-band Ginzburg-Landau (TB G–L) equations for a bulk MgB₂ were solved analytically to determine the temperature dependence of surface critical magnetic fieldH _c3 (T). It is shown thatH _c3 (T) has the same temperature dependence with H_c2 (T), similar to the case of a single-band superconductor,H _c3 (T) = 166H _c2 (T). We use an elimination procedure for the decoupling of G–L equations of two-band superconductivity, which eases the calculations. It is expected that the temperature dependence forH _c3 (T) gives positive curvature nearT _c .     

Effect of surface free energy on critical field <Emphasis Type="Italic">H</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis>3</Subscript>

The emergence of surface superconductivity in a type I superconductor is considered taking into account the surface free energy of the superconducting phase. It is shown that the disregard of the surface energy leads to a substantial error in determining the Ginzburg-Landau parameter from the measurements of the H _c3 field.     

Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)2ClO4

Using various transport and magnetotransport probes, we study the coexistence of spin-density wave and superconductor states in (TMTSF)₂ClO₄ at various degrees of ClO₄ anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor and spin-density wave regions overlap on the temperature—dimerization gap V phase diagram, where V is varied by anion ordering. The effect of anion ordering on (TMTSF)₂ClO₄ properties is thus analogous to that of pressure on (TMTSF)₂X (X = PF₆ or AsF₆), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.     

Magnetic state of the structural separated anion-deficient La<Subscript>0.70</Subscript>Sr<Subscript>0.30</Subscript>MnO<Subscript>2.85</Subscript> manganite

The results of neutron diffraction studies of the La_0.70Sr_0.30MnO_2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups R[`3]cR\bar 3c and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La_0.70Sr_0.30MnO_2.85 is a cluster spin glass, which is the result of frustration of Mn³⁺-O-Mn³⁺ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.     

Fluctuation-induced conductivity and dimensionality in the new Y-based Y<Subscript>3</Subscript>Ba<Subscript>5</Subscript>Cu<Subscript>8</Subscript>O<Subscript>18-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductor

The fluctuation conductivity measurement on the new Y-based Y₃Ba₅Cu₈O_{18- x} superconductor is presented. The dimensional crossovers between different temperature regimes were analyzed with Aslamazov-Larkin (AL) theory and a good quantitative agreement was achieved for the experimental data. For our data, the mean field regime is dominated by 2D AL fluctuations. Our results reveal the occurrence of critical fluctuation regime in consistent with the prediction of the full dynamic 3D XY model. We found the dynamical critical exponent to be z = 3.4 for our data. We analyzed also the excess conductivity data by Hikami-Larkin theory and estimated the phase relaxation time.     

Photostimulated electron transfer and its action on paramagnetism of Sp<Subscript>3</Subscript>Cr(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals

The effect of irradiation by ultraviolet light on the effective magnetic moment of a paramagnetic single crystal based on photochrome spiropyran (Sp) and chromium oxalates Sp₃Cr(C₂O₄)₃ molecules is detected. It is shown that the deviation of the temperature dependence of the magnetic moment from the Curie law is caused not by the exchange interaction, but by electron redistribution between Cr³⁺ and Cr⁴⁺ ions and spiropyran molecules Sp⁰ and Sp⁺. Analysis of the angular dependence of EPR spectra makes it possible to determine the contribution of Cr³⁺ ions to the magnetic properties of the crystals and to determine the crystal field parameters D = 0.619 cm⁻¹ and E = 0.024 cm⁻¹. Irradiation of hydrated samples by ultraviolet light leads to intensity redistribution of EPR lines attributed to Cr³⁺ and Sp⁰. Thermally stimulated paramagnetism of triplet states of spiropyran ions Sp⁺ and the SpI salt is observed.     

Electrophysical properties and the structure of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> compound thermally restored after low-temperature decomposition

The electrophysical properties and structure of the nonstoichiometric high-temperature superconductor YBa₂Cu₃O _y restored at T = 930–950°C after low-temperature decomposition (T = 200°C) into phases different in the oxygen content have been studied. It has been shown that, unlike heat treatments at T ≤ 900°C, the superconducting properties are almost completely restored for 3–5 h during grain recrystallization, which is impossible at lower temperatures. After short-term annealing at T = 930–950°C (for 1–2 h), the ceramic material still contains a significant number of structural defects, most likely, in cation sublattices. These defects can contribute to the pinning of magnetic vortices, which substantially increases the critical current density in magnetic fields up to 2 T as compared to ceramic materials produced by the conventional technology.     

First-principle study of the electronic structure and magnetism in RuSr<Subscript>2</Subscript>GdCu<Subscript>2</Subscript>O<Subscript>8</Subscript> under pressure

We have performed a first-principle calculation of the structural, electronic and high pressure properties of RuSr₂GdCu₂O₈, a ferromagnetic superconductor, by employing a full-potential linearized augmented plane-wave method within the density-functional theory. The effect of pressure was achieved by varying the volume of the unit cell with constant a:b:c ratio. The experimentally observed anti-phase rotation of RuO₆ octahedra has been attributed to the residual forces on O_Ru which results in shear strain in the RuO₂ layer. Partial charge analysis shows that applying pressure up to 6 GPa leads to hole creation in the CuO₂ sheets which causes increase in the superconducting transition temperature. We have estimated the Curie temperature T _M of this compound in the mean-field approximation using Heisenberg model with first-nearest neighbor exchange interactions determined from DFT calculations for parallel and anti-parallel spin configurations of Ru moment in RuO₂ planes. The effect of pressure causes the magnetic moment of Ru atoms to decrease due to the increase of hybridization between the adjacent Ru atoms. The calculated exchange splitting in Cu d _{x 2 - y 2} states increases slightly with pressure but it is still very small that it does not affect superconductivity, and the hole doping mechanism is dominant.     

Influence of the interplay between helicoidal magnetic ordering and superconductivity on the differential conductance in HoNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C/Ag junctions

The point contact spectra of magnetic superconductor HoNi₂B₂C/Ag-based junctions is analysed in the framework of Blonder-Tinkham-Klapwijk (BTK) theory. The anomalous behavior in the dI/dV curves above the Neel temperature(T _N ∼ 5 K) is attempted to be explained by the partial suppression of superconducting gap parameter of the prevailing helical incommensurate structure     

Electronic structure of novel multiple-band superconductor SrPt<Subscript>2</Subscript>As<Subscript>2</Subscript>

The electronic structure of the recently discovered superconductor SrPt₂As₂ with T _c = 5.2 K has been calculated in the local-density approximation. Despite its chemical composition and crystal structure are somehow similar to FeAs-based high-temperature superconductors, the electronic structure of SrPt₂As₂ is very much different. The crystal structure is orthorhombic (or tetragonal if idealized) and has layered nature with alternating PtAs₄ and AsPt₄ tetrahedra slabs sandwiched with Sr ions. The Fermi level is crossed by Pt-5d states with rather strong admixture of As-4p states. Fermi surface of SrPt₂As₂ is essentially three-dimensional, with complicated sheets corresponding to multiple bands. We compare SrPt₂As₂ with 1111 and 122 representatives of FeAs-class of superconductors, as well as with isovalent (Ba,Sr)Ni₂As₂ superconductors. Brief discussion of superconductivity in SrPt₂As₂ is also presented.     

Electronic and magnetic structure of a possible iron based superconductor BaFe<Subscript>2</Subscript>Se<Subscript>3</Subscript>

We present results of LDA calculations (band structure, densities of states, Fermi surfaces) for possible iron based superconductor BaFe₂Se₃ (Ba123) in normal (paramagnetic) phase. Results are briefly compared with similar data on prototype BaFe₂As₂ and (K,Cs)Fe₂Se₂ superconductors. Without doping this system is anti-ferromagnetic with T _N^exp ∼ 250 K and rather complicated magnetic structure. Neutron diffraction experiments indicated the possibility of two possible spin structures (antiferromagnetically ordered “plaquettes” or “zigzags”), indistinguishable by neutron scattering. Using LSDA calculated exchange parameters we estimate Neel temperatures for both spin structures within the molecular field approximation and show τ₁ (plaquettes) spin configuration to be more favorable than τ₂ (zigzags).     

Dynamics of the magnetic flux component normal to the layers of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8</Subscript> in a tilted field

High-frequency losses in the strongly anisotropic layered superconductor Bi₂Sr₂CaCu₂O₈ are measured at 600 MHz under a magnetic field rocking about the ab plane. Anomalies in losses and hysteretic phenomena are found while performing periodic rocking, i.e., cycling the magnetic field component normal to the sample surface. Based on these observations, conclusions are drawn about the nature of magnetic-flux penetration into the superconductor. It is found that, in the range between 60 K and T _c , the dynamics of magnetic-flux vortex lines normal to the ab plane in the presence of a constant magnetic field applied parallel to this plane is governed by the critical penetration field H _c ^⊥* and the surface barrier in the presence of thermally activated vortex motion (giant flux creep). The dependences of H _c1 ^⊥* and the characteristic field of the surface barrier on the magnitude of the parallel magnetic field are measured.     

Colossal magnetodielectric effect in SmFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> multiferroic

The colossal (more than threefold) decrease in the dielectric constant ɛ in the easy-plane SmFe₃(BO₃)₄ ferroborate in a magnetic field of ∼5 kOe applied in the basal ab plane of the crystal has been found. A close relation of this effect to anomalies in the field dependence of the electric polarization has been established. It has been shown that this magnetodielectric effect is due to the contribution to ɛ from the electric susceptibility, which is related to the rotation of spins in the ab plane, arises in the region of the antiferromagnetic ordering T < T _N = 33 K, and is suppressed by the magnetic field. A theoretical model describing the main features of the behavior of ɛ and electric polarization in the magnetic field has been proposed, taking into account the additional anisotropy in the basal plane induced by the magnetoelastic stresses.     

Anisotropy of the vortex structure and resistivity in the basal plane of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript> single crystals

The vortex lattice of the YBa₂Cu₄O₈ high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ _a /ρ _b measured at temperatures from T _c to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.     

Field-induced confinement in (TMTSF)2ClO4 under accurately aligned magnetic fields

We present transport measurements along the least conducting c direction of the organic superconductor (TMTSF)₂ClO₄  performed under an accurately aligned magnetic field in the low temperature regime. The experimental results reveal a two-dimensional confinement of the carriers in the (a, b) planes which is governed by the magnetic field component along the b^′ direction. This 2-D confinement is accompanied by a metal-insulator transition for the c axis resistivity. These data are supported by a quantum mechanical calculation of the transverse transport taking into account in self consistent treatment the effect of the field on the interplane Green function and on the intraplane scattering time.     

Structure and Anisotropy of the Superconducting Order Parameter in Ba<Subscript>0.65</Subscript>K<Subscript>0.35</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Probed by Andreev Spectroscopy

The superconducting order parameters in optimally doped Ba_0.65K_0.35Fe₂As₂ single crystals have been directly measured using multiple Andreev reflection effect spectroscopy of superconductor–normal metal–superconductor break-junctions. We determine two superconducting gaps, which are nodeless in the k _x k _y -plane of the momentum space, and resolve a substantial in-plane anisotropy of the large gap. The temperature dependences of the gaps indicate a strong coupling within the bands where Δ_L develops, a weak coupling in the condensate with the small gap Δ_S, and a moderate interband interaction between the two condensates. The own critical temperatures of both condensates have been estimated (under the hypotherical assumption of zero interband interaction).     

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ with T _c = 31 K. ρ(T) follows a linear temperature dependence above T _c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu³⁺ moments. The article is published in the original.