Suppression of superconductivity in granular metals

We investigate the suppression of the superconducting transition temperature due to Coulomb repulsion in granular metallic systems at large tunneling conductance between the grains, g(T)>1. We find the correction to the superconducting transition temperature for 3D granular samples and films. We demonstrate that, depending on the parameters of superconducting grains, the corresponding granular samples can be divided into two groups: (i). the granular samples that belong to the first group may have only insulating or superconducting states at zero temperature depending on the bare intergranular tunneling conductance g(T), while (ii). the granular samples that belong to the second group in addition have an intermediate metallic phase where superconductivity is suppressed while the effects of the Coulomb blockade are not yet strong.     

Suppression of superconductivity in mesoscopic superconductors

We propose a new boundary-driven phase transition associated with vortex nucleation in mesoscopic superconductors (of size of the order of, or larger than, the penetration depth). We derive the rescaling equations and we show that boundary effects associated with vortex nucleation lower the conventional transition temperature in mesoscopic superconductors by an amount which is a function of the size of the superconductor. This result explains recent experiments in small superconductors where it was found that the transition temperature depends on the size of the system and is lower than the critical Berezinsk?-Kosterlitz-Thouless temperature.     

Suppression of superconductivity in disordered interacting wires

We study superconductivity suppression due to thermal fluctuations in disordered wires using the replica nonlinear sigma-model (NLsigmaM). We show that in addition to the thermal phase slips there is another type of fluctuations that result in a finite resistivity. These fluctuations are described by saddle points in NLsigmaM and cannot be treated within the Ginzburg-Landau approach. The contribution of such fluctuations to the wire resistivity is evaluated with exponential accuracy. The magnetoresistance associated with this contribution is negative.     

Ferromagnetism versus superconductivity in ramdom alloys

Magnetic, superconducting narrow-band alloys are studied within a random lattice model. Coulomb repulsion and contact-type attraction of the itinerant electrons, and the indirect exchange between local magnetic moments are taken into account. Averaging the free energy expansion in coherent potential approximation, the Ginzburg-Landau functional is derived with two biquadratically coupled order parameters. Coherent orderings and concentration-dependent critical temperatures are determined. The coefficients of theq ²-terms, i.e. the stiffness and diffusion constants, involve vertex corrections due to impurity scattering. The coexistence criterion resulting from the 4th-order couplings cannot be fulfilled, referring to a first order transition between ferromagnetism and superconductivity. Tetracritical behaviour is found by including the direct exchanges between the localized spins.     

Suppression of superconductivity in YBCO/LCMO superlattices

The competition of superconductivity and magnetism in superlattices composed of alternating YBa₂Cu₃O_7−d and La_0.67Ca_0.33MnO₃ thin films is investigated using low-energy optical spectroscopy. The thickness of the superconducting YBCO layers is varied from 30 to 20 nm while the thickness of the magnetic LCMO layers is kept constant at 20 nm. We clearly observe that the superconducting condensate density in the superconducting state of super lattice is drastically reduced by the magnetic subsystem which may be connected with proximity effects that distort the gap symmetry and thus suppress superconductivity.     

Theory of superconductivity for transition-metal alloys

A theory of superconductivity for transition-metal alloys is presented by using the coherent potential approximation. Expressions for the superconducting order parameter and transition temperature of transition-metal alloys A_xB_1?x in terms of the parameters characterizing the pure metals are given.     

Structural instability and superconductivity in niobium-titanium alloys

Niobium-titanium is the most widely used technical superconductor. Titaniumrich transition metal alloys, quenched from high temperatures, can generally be retained in the bccβ phase. This phase is metastable and the instability is relieved by a variety of low temperature structural transformations. This aspect has been investigated using x-ray, TEM, low temperature resistivity,T _c and dH _c2/dT studies, in a series of Nb-Ti alloys. The instability has been characterized by the normal state resistivityρ _n and dρ/dT. The commercially used Nb-Ti alloys are Ti rich per atom-wise. This stems basically from the anomalous increase in the normal state resistivityρ _n as the Ti concentration is increased. This is a consequence of a dynamical process through which theβ phase instability tends to be relieved leading to athermal ω precipitation. The resulting anomalous resistivity behaviour can be understood in terms of a ‘two-level system’ model generally invoked for amorphous materials. It has also been possible to induce instability towards athermal ω precipitation in a system spontaneously undergoing a martensitic transformation to become stable. Thus in an alloy of Nb-83 at % Ti, addition of 1% nitrogen has suppressed the martensitic transformation, giving a three-fold increase inρ _n (about 150µΘ cm), the highest known in Nb-Ti so far. The increase in the normal state resistivity has beneficial effects on the upper critical field. From studies on several Nb-Ti alloys, it is inferred that a peak inH _c2(0) occurs at 17–18 tesla at aρ _n value of 100µΘ cm. It is pointed out that in the present commercial alloys, the sequence of thermo-mechanical treatments given to optimizeJ _c, restrictsρ _n, perhaps owing to the partial relieving of the metastability of theβ phase. They are therefore non-optimized with respect toH _c2.     

Theory for strong-coupling superconductivity in transition-metal alloys

Strong-coupling superconductivity in transition-metal alloys of the type A_xB_1?x is treated by applying the coherent-potential approximation to the Eliashberg theory of superconductivity. Expressions are given for the average electron-phonon coupling constant λ_eff, the average Coulomb-pseudopotential μ¹_eff, the superconducting transition temperature T_c, and the order parameter.     

Coexistence of superconductivity and itinerant-antiferromagnetism in CrRe alloys

Zero field Re NMR of CrRe alloy with more than 18%Re in superconducting state has been observed. The nuclear spin-lattice relaxation time (T₁) of Re in the vortex state increases exponentially with decreasing temperature as in the case of BCS superconductor. The result shows a coexistence of superconductivity and itinerant-electron antiferromagnetism.     

Suppression of superconductivity in zinc nanowires by bulk superconductors

Transport measurements were made on a system consisting of a zinc nanowire array sandwiched between two bulk superconducting electrodes (Sn or In). It was found that the superconductivity of Zn nanowires of 40 nm diameter is suppressed either completely or partially by the superconducting electrodes. When the electrodes are driven into their normal state by a magnetic field, the nanowires switch back to their superconducting state. This phenomenon is significantly weakened when one of the two superconducting electrodes is replaced by a normal metal. The phenomenon is not seen in wires with diameters equal to or thicker than 70 nm.     

Unconventional superconductivity in low density electron systems and conventional superconductivity in hydrogen metallic alloys

In this short review, we first discuss the results, which are mainly devoted to the generalizations of the famous Kohn–Luttinger mechanism of superconductivity in purely repulsive fermion systems at low electron densities. In the context of repulsive-U Hubbard model and Shubin–Vonsovsky model we consider briefly the superconducting phase diagrams and the symmetries of the order parameter in novel strongly correlated electron systems including idealized monolayer and bilayer graphene. We stress that purely repulsive fermion systems are mainly the subject of unconventional low-temperature superconductivity. To get the high temperature superconductivity in cuprates (with T_C of the order of 100 K) we should proceed to the t–J model with the van der Waals interaction potential and the competition between short-range repulsion and long-range attraction. Finally we note that to describe superconductivity in metallic hydrogen alloys under pressure (with T_C of the order of 200 K) it is reasonable to reexamine more conventional mechanisms connected with electron–phonon interaction. These mechanisms arise in the attractive-U Hubbard model with static onsite or intersite attractive potential or in more realistic theories (which include retardation effects) such as Migdal–Eliashberg strong coupling theory or even Fermi–Bose mixture theory of Ranninger et al. and its generalizations.     

Effect of pressure on superconductivity in transition metal alloys

Measurements on twelve alloys of the series Zr-Nb-Mo show a close correlation ofdT _c /dp with thed-band structure of these alloys and suggest that thed-band is virtually rigid with respect to pressure. The results are not compatible with an empirical observation of McMillan thatT _c is governed only by a phonon factor Mω ².     

体超导铟对一维锌超导纳米线超导电性的抑制效应

文章细致研究了超导体铟/一维锌超导纳米线阵列/超导体铟夹心结构的超导电性.实验发现, 当锌纳米线的长度在２—６μm、直径等于40nm时,　宏观尺寸的超导体铟电极对中间的锌纳米线的超导电性具有反常的抑制作用. 即当铟处在超导态时, 中间的锌纳米线则停留在正常态. 如果施加一个磁场,使超导体铟电极变为正常态, 锌纳米线则恢复其超导电性. 这种奇异的现象与超导电极材料的类型及锌纳米线的直径和长度有关.     

体超导铟对一维锌超导纳米线超导电性的抑制效应

文章细致研究了超导体铟／一维锌超导纳米线阵列／超导体铟夹心结构的超导电性．实验发现，当锌纳米线的长度在2—6μm、直径等于40nm时，宏观尺寸的超导体铟电极对中间的锌纳米线的超导电性具有反常的抑制作用,即当铟处在超导态时，中间的锌纳米线则停留在正常态．如果施加一个磁场，使超导体铟电极变为正常态，锌纳米线则恢复其超导电性，这种奇异的现象与超导电极材料的类型及锌纳米线的直径和长度有关。     

Suppression of superconductivity in Zn-doped SmFeAsO0.8F0.2 system

A series of SmFe_1?xZn_xAsO_0.8F_0.2 samples with x = 0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature T_c being reduced from 52.5 K to 23.3 K for the sample of x = 0.05, and to 18.2 K for the sample of x = 0.1. For the samples with x > 0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140 K. The metal to semiconductor transition is also observed in the SmFe_1?xZn_xAsO_0.8F_0.2 system. The behavior of SmFe_1?xZn_xAsO_0.8F_0.2 is very different from that of R_EFeAsO (R_E = rare earth metal), which reveals a very strong electron correlation in SmFe_1?xZn_xAsO_0.8F_0.2.     

Search for superconductivity in PdAg alloys to 0.1 mK

Pd_1?xAg_x alloys with 0.065 ≤ × ≤ 0.55 were examined for superconductivity down to temperatures of 0.1 mK. The theoretically predicted superconducting transition between 10 mK and 500 mK for 0.2 ≤ × ≤ 0.4 could not be detected.     

Enhancement of superconductivity by an external magnetic field in magnetic alloys

An infinite-volume limit solution of the thermodynamics of a BCS superconductor containing spin 1/2 and 7/2 magnetic impurities, obtained recently in [D. Borycki, J. Ma?kowiak, Supercond. Sci. Technol. 24, 035007 (2011)] is exploited to derive the expressions for critical magnetic field $\mathcal{H}_c$ (T). The credibility of the resulting thermodynamically limited theoretical equations, which depend on the magnetic coupling constant g and impurity concentration c, is verified on the experimental data for the following superconducting alloys: LaCe, ThGd and SmRh₄B₄. Good quantitative agreement with experimental data is found for sufficiently small values of c. The discrepancies between theoretical and experimental values of $\mathcal{H}_c$ (T) for larger values of c in case of LaCe and ThGd are reduced by introducing the concept of the effective temperature $\tilde T$ , which accounts for the Coulomb interactions between the electron gas and impurity ions. At low temperatures, the critical magnetic field is found to increase with decreasing temperature T. This enhancement of the critical magnetic field provides evidence of the Jaccarino-Peter effect, which was experimentally observed in the Kondo systems like LaCe, (La_{1 ? x} Ce _x )Al₂ and also in the pseudoternary compounds, including Sn_{1 ? x} Eu _x Mo₆S₈, Pb_{1 ? x} Eu _x Mo₆S₈ and La_{1.2 ? x} Eu _x Mo₆S₈. The effect of an external magnetic field $\mathcal{H}$ on a BCS superconductor perturbed by magnetic impurities was also studied. On these grounds, by analyzing the dependence of superconducting transition temperature T _c on $\mathcal{H}$ of (La_{1 ? x} Ce _x )Al₂, we have shown, that for certain parameter values, external magnetic field compensates the destructive effect of magnetic impurities.     

Enhanced superconductivity of the Ti-Zr alloys in the high-pressure BCC phase

A high-pressure study of the crystal structure and superconductivity of Ti-Zr alloys demonstrates an increase in the ω-β-transition pressure from about 30 to 43–57 GPa when the titanium content in the alloys increases from 0 to 50 at. %. The isobaric values of the BCC β-phase superconducting temperature (at 46 GPa) increase from 5.7 to more than 15 K between 0 and 50 at. % Ti, the latter value being the absolute maximum for BCC d-metal alloys. These data correlate with the earlier assumption of an s-d electron transfer in Zr under pressure.