Non-analytical angular dependence of the upper critical magnetic field in a quasi-one-dimensional superconductor

We have derived the so-called gap equation, which determines the upper critical magnetic field, perpendicular to conducting chains of a quasi-one-dimensional superconductor. By analyzing this equation at low temperatures, we have found that the calculated angular dependence of the upper critical magnetic field is qualitatively different than that in the so-called effective mass model. In particular, our theory predicts a non-analytical angular dependence of the upper critical magnetic field, H _c2(0) − H _c2(α) ∼ α^3/2, when magnetic field is close to some special crystallographic axis and makes an angle α with it. We discuss possible experiments on the superconductor (DMET)₂I₃ to discover this non-analytical dependence.     

Effective crystal fields in the rare earth pnictides

Various contributions to the effective 4f electron crystal fields in the rare earth pnictides are estimated and compared. These include the direct interaction with the ligand electrostatic field and effects associated with the 5d band electrons.     

Influence of twinning planes on upper critical fields

Within the framework of a Ginzburg-Landau theory for a scalar order parameter, we discuss the effect of the presence of twinning planes on the behavior of the upper critical field as function of the temperature. In particular we study the case of a finite array of planes in the region of weak magnetic field, both from the analytical and the numerical point of view.     

Anisotropies of the lower and upper critical fields in MgB2 single crystals

The temperature dependence of the upper (H(c2)) and lower (H(c1)) critical fields has been deduced from Hall probe magnetization measurements of high quality MgB2 single crystals along the two main crystallographic directions. We show that Gamma(H(c2))=H(c2 axially ab)/H(c2 axially c) and Gamma(H(c1))=H(c1 axially c)/H(c1 axially ab) differ significantly at low temperature (being approximately 5 and approximately 1, respectively) and have opposite temperature dependencies. We suggest that MgB2 can be described by a single field dependent anisotropy parameter gamma(H) (=lambda(c)/lambda(ab)=xi(ab)/xi(c)) that increases from Gamma(H(c1)) at low field to Gamma(H(c2)) at high field.     

Temperature dependence of upper critical fields for clean superconductors of the second kind

TheD ₁₃ pion-nucleon scattering phase-shifts are calculated using the Khuri representation suitably modified in order to exhibit explicitly the isolated nucleon pole in theu-channel. The results of the calculation are compared with the phase-shift analysis ofRoper.     

Interlayer Aharonov-Bohm interference in tilted magnetic fields in quasi-one-dimensional organic conductors

Different types of angular magnetoresistance oscillations in quasi-one-dimensional layered materials, such as organic conductors (TMTSF)2X, are explained in terms of Aharonov-Bohm interference in interlayer electron tunneling. A two-parameter pattern of oscillations for generic orientations of a magnetic field is visualized and compared to the experimental data. Connections with angular magnetoresistance oscillations in other layered materials are discussed.     

Paramagnetism and reentrant behavior in quasi-one-dimensional superconductors at high magnetic fields

The thermodynamics of quasi-one-dimensional superconductors in the presence of large magnetic fields is studied. When the quantum effects of the magnetic field are taken into account, several reentrant superconducting phases persist at very high fields. In the last reentrant phase the free-energy change, the specific-heat jump and the excess magnetization are estimated near the critical temperature. In particular, the excess magnetization is found to be paramagnetic as opposed to diamagnetic in weak fields and its sign is controlled by the slope of H_c2(T). We further generalize this result to the entire phase diagram (including all quantum phases) and to different physical systems using general thermodynamic relations which show that the sign of the excess magnetization ΔM of the superconducting state near H_c2(T) follows dH_c2(T)/dT. These relations provide a scenario for the evolution of the sign of ΔM from weak fields to strong fields.     

Superconductivity in an organic insulator at very high magnetic fields

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor lambda-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe3+ ions. We further argue that the Fe3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state.     

The spin-peierls critical temperature in an XY quasi-one-dimensional system with nearest and next-nearest neighbour interactions

The mean field critical temperature for the spin-Peierls phase transition in the XY antiferromagnetic chain is obtained for nearest and next nearest neighbour exchange interaction. An increase in T_c is predicted for next nearest neighbour antiferromagnetic exchange and a decrease is obtained for ferromagnetic exchange. This model is applied to the alkali-TCNQ salts, which are treated in the framework of the highly correlated Hubbard model.     

Temperature dependence of upper critical fields for clean superconductors of the second kind. II

A numerical mistake made in a previous paper is corrected. For specular reflexion of electrons by the surface the result now agrees with a result obtained earlier byEbneth andTewordt. In order to facilitate a comparison with experimental data the calculation of the ratioH _c3(T)H_c2(T) is extended up to terms linear (T _c ?T)/T _c for arbitrary reflexion kernels.     

Theory of the upper critical field in layered superconductors with magnetic intercalates

We propose a model for the layered superconductors with electronically isolated magnetic intercalates in which the electrons propagate freely in the dirty superconducting layers with arbitrarily strong spin-orbit scattering, and spin-flip scatter off the magnetic ions during interlayer tunneling. We calculate the upper critical field H_c2 including demagnetization effects. In a parallel field below the dimensional-crossover temperature T¹, a new type of spin ordering is predicted.