Alexandre I. Buzdin

Coexistence of superconductivity and magnetism theoretical predictions and experimental results

Abstract Superconductivity and ferromagnetic ordering are two antagonistic types of ordering, and their mutual influence leads to many interesting phenomena which have been studied recently in ternary compounds. Theoretical analysis of ferromagnetic materials which are type II superconductors near the superconducting transition point T cl shows that they become type I near the magnetic transition point T M. The proposed theory constructed for the case T M « T cl predicts the formation of a transverse domain-like (DS phase) magnetic structure below T M. The electronic spectrum appears to be gapless in the DS phase of clean compounds with a re-entrant transition. The change from type II to type I behaviour as the sample is cooled to T M has been observed in ErRh4B4. Experimental data for HoMo6S8, HoMo6Se8 and ErRh4B4 give evidence for the coexistence of super-conductivity and non-uniform magnetic ordering below T M. Mutual influence of superconducting and magnetic orderings is also studied.

Twinning-plane superconductivity

Abstract A new phenomenon has been discovered recently: twinning-plane superconductivity (TPS). The present paper is a survey of experimental and theoretical work on TPS. TPS arises at temperatures higher than the bulk critical temperature of a single crystal, and has a localized character. This phenomenon is caused by Cooper-pairing enhancement near the twinning plane. TPS is characterized by anomalously strong diamagnetism above the critical point. The phase (H, T) diagram of TPS is very specific and differs noticeably for type-I and type-II superconductors (e.g. tin and niobium). The interaction of closely spaced twinning planes is discussed, as well as the case of TPS in microscopic particles. The proximity effect is shown to be reduced under these conditions, and the critical temperature noticeably increased (by a factor of two to three times for tin). The prospects for further investigations of TPS are considered.

Giant superconductivity-induced modulation of the ferromagnetic magnetization in a cuprate–manganite superlattice

Artificial multilayers offer unique opportunities for combining materials with antagonistic orders such as superconductivity and ferromagnetism and thus to realize novel quantum states. In particular, oxide multilayers enable the utilization of the high superconducting transition temperature of the cuprates and the versatile magnetic properties of the colossal-magnetoresistance manganites. However, apart from exploratory work, the in-depth investigation of their unusual properties has only just begun. Here we present neutron reflectometry measurements of a [Y(0.6)Pr(0.4)Ba(2)Cu(3)O(7) (10 nm)/La(2/3)Ca(1/3)MnO(3) (10 nm)](10) superlattice, which reveal a surprisingly large superconductivity-induced modulation of the vertical ferromagnetic magnetization profile. Most surprisingly, this modulation seems to involve the density rather than the orientation of the magnetization and is highly susceptible to the strain, which is transmitted from the SrTiO(3) substrate. We outline a possible explanation of this unusual superconductivity-induced phenomenon in terms of a phase separation between ferromagnetic and non-ferromagnetic nanodomains in the La(2/3)Ca(1/3)MnO(3) layers.

Long range triplet Josephson effect through a ferromagnetic trilayer

The coexistence of superconductivity and ferromagnetism is very rare in bulk systems. However, it can be easily achieved in artificially fabricated superconductor/ ferromagnet S/F heterostructures. The S/F proximity effect is characterized by the damped oscillatory behavior of the Cooper pair wave function in the ferromagnet. This phenomenon leads to nonmonotonic dependence of the critical temperature of S/F multilayers on the F layer thickness and the realization of Josephson junctions for a review, see Refs. 1 and 2. In the diffusive limit, the proximity effect in a F metal is rather short ranged due to the large value of the ferromagnetic exchange field. This is related to the incompatibility between singlet superconductivity and ferromagnetism. Interestingly, nonuniform magnetization can induce triplet superconducting correlations which are long ranged on the same scale as for the superconductor/normal N metal proximity effect. 3 Several experimental indications exist for this triplet proximity effect. 4,5 However, the transition from the usual to the long range triplet proximity effect has not been observed in the same system. In the present work, we investigate the conditions for the observation of the Josephson current due to a long range triplet component under controllable conditions. Noncollinear magnetization may serve as a source of the long range triplet component. However, it is not possible to have a Josephson current due to the interference of the triplet and singlet components. Two sources of triplet components are needed to observe the long range triplet Josephson effect between them. Thus, the simplest experimental realization of such a situation may be a S/F/F/F/S system with the magnetic moments of the F and F layers noncollinear with the F interlayer see Fig. 1. The optimal condition for triplet Josephson current observation is when the thicknesses dL and dR of the layers F and F are of the order of the coherence length f in the ferromagnet. Indeed, for large dL and dR, the triplet component is exponentially small due to the short range proximity effect in the layers F and F, while for very thin dL and dR, it is also small. Thus, we predict that the magnitude of the Josephson current in a structure with F layer thickness much larger than f will be comparable to that of a S/N/S junction with the same length. A similar phenomenon could be observed in lateral Josephson junctions made of a nanostructured ferromagnetic film, allowing control of its magnetic domain structure.

GENERALIZED GINZBURG-LANDAU THEORY FOR NONUNIFORM FFLO SUPERCONDUCTORS

Abstract We derive a generalized Ginzburg-Landau (GL) functional near the tricritical point in the ( T , H )-phase diagram for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state, in one, two, and three dimensions. We find that the transition from the normal to the FFLO state is of second order in one and two dimensions, and the order parameter with one-coordinate sine modulation corresponds to the lowest energy near the transition line. We also compute the jump of the specific heat and describe in the one-dimensional case the transformation of the sine modulation into the soliton-lattice state as the magnetic field decreases. In three dimensions however, we find that the transition into an FFLO state is of first order, and it is impossible to obtain an analytic expression for the critical temperature. In this case the generalized GL functional proposed here provides a suitable basis for a numerical study of the properties of the FFLO state, and in particular for computing the critical temperature, and for describing the transition into a uniform state.

Enhancement of the superconducting transition temperature in Nb/permalloy bilayers by controlling the domain state of the ferromagnet.

In (S/F) hybrids the suppression of superconductivity by the exchange field h(ex) of the ferromagnet can be partially lifted when different directions of h(ex) are sampled simultaneously by the Cooper pair. In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F layer yield a similar effect. We study the transport properties of Ni(0.80)Fe(0.20)/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples.

Direct coupling between magnetism and superconducting current in the Josephson phi0 junction.

We study the proximity effect between conventional superconductor and magnetic normal metal with a spin-orbit interaction of the Rashba type. Using the phenomenological Ginzburg-Landau theory and the quasiclassical Eilenberger approach it is demonstrated that the Josephson junction with such a metal as a weak link has a special nonsinusoidal current-phase relation. The ground state of this junction is characterized by the finite phase difference phi{0}, which is proportional to the strength of the spin-orbit interaction and the exchange field in the normal metal. The proposed mechanism of the phi{0} junction formation gives a direct coupling between the superconducting current and the magnetic moment in the weak link. Therefore the phi{0} junctions open interesting perspectives for the superconducting spintronics.

High quality ferromagnetic 0 and π Josephson tunnel junctions

The authors fabricated high quality Nb∕Al2O3∕Ni0.6Cu0.4∕Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11K. The critical current density in the π state was up to 5A∕cm2 at T=2.11K, resulting in a Josephson penetration depth λJ as low as 160μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces.

Josephson junctions with second harmonic in the current-phase relation : Properties of φ junctions

Several recent experiments revealed a change of the sign of the first harmonic in the current-phase relation of Josephson junctions JJs based on, e.g., d-wave superconductors or JJs with ferromagnetic barrier. In this situation, the role of the second harmonic can become dominant; in this case, it determines the scenario of a 0- transition. We discuss different mechanisms of the second harmonic generation and its sign. If the second harmonic is positive and the first harmonic changes sign as a function of any control parameter, e.g., temperature, the ground state phase changes abruptly between 0 and . If the second harmonic is negative, the ground state phase changes continuously passing all values between 0 and and the realization of a so-called -junction is possible in the vicinity of the point where the first harmonic vanishes. We study the unusual properties of this kind of Josephson junction such as critical currents, magnetic field penetration, plasma gap, microwave response, and zero field steps. We also analyze the possible experimental techniques for the observation of predicted effects.

The effect of a magnetic field on the inversion temperature of a spin crossover compound revisited

The compound MnIII[(pyrol)3tren] where (pyrol)3tren is the trianionic Schiff base resulting from the condensation of pyrolle-2-carboxaldehyde with 2,2′,2′′-tris(ethylamino)amine exhibits an abrupt LS (S=1) ↔ HS (S=2) spin conversion with an inversion temperature in low field of T1/2 = 43.7 K. The temperature dependence of the heat capacity has been measured. The enthalpy and entropy variations associated with the spin conversion have been found as ΔH=0.60 kJ mol−1 and ΔS=13.8 J K−1 mol−1. The temperature dependence of the magnetization has been measured under various magnetic fields up to 23 T. T1/2 has been found to decrease as the field increases, first in a smooth fashion, then more rapidly. Under 23 T, T1/2 is equal to 42.1 K. A theoretical model based on a mean field approximation of the spin crossover phenomenon has been compared with the experimental findings.