Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Electrical conduction and critical magnetic field for superconductivity of a Nb3Se4 single crystal

With respect to the quasi-one dimensionality of single crystals of Nb₃Se₄, the electrical resistivity from 1.3 to 320 K and the critical magnetic field for superconductivity are measured. The resistivity along the Nb-chain direction is represented as a sum of a temperature independent and an intrinsic temperature dependent term. The temperature dependence of the intrinsic resistivity subjects to T³ form between 10 and 80 K above which it tends to a T linear form. The critical magnetic field is proportional to the temperature difference from the transition temperature. Its dependence is well fitted by the elliptical fluxoid model of Ginzburg-Landau theory. The ratio of the parallel and the perpendicular to the c-axis is 5.7.     

Critical magnetic field of surface superconductivity in lead

The critical superconductivity field H _c3 is measured on lead single crystals. It is shown that the temperature dependence of H _c3/H _c in the vicinity of superconducting transition temperature T _c is essentially nonlinear. Relative changes in the value of H _c3/H _c reach approximately 30%, which cannot be described by the Ginzburg-Landau theory. The experimental temperature dependences lead to the conclusion that the surface superconducting transition temperature noticeably exceeds the superconducting transition temperature in the bulk of the semiconductor. The differences in the critical temperatures and in the Ginzburg-Landau parameters for lead are estimated.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Superheating,supercooling, surface superconductivity and Ginzburg-Landau parameters of pure type-I superconductors and their alloys

We have measured the low-frequency differential susceptibility of several pure type 1 superconductors, viz. Sn, In, Tl, Pb and Hg, as a function of increasing or decreasing applied magnetic fields. Special emphasis was placed on the observation of metastable (superheated and supercooled) states. Experimental values of the Ginzburg-Landau parameter? at the transition temperatureT _c and of suitably defined temperature dependent parameters for superheating,? _sh(t), and for supercooling,? _sc(t), have been obtained and compared with theoretical estimates. The rapid drop in? _sh close toT _c observed in materials with??1 such as Sn, In, and Tl is interpreted as a manifestation of nonlocal electrodynamics. Supercooling of the normal state down to the bulk nucleation fieldH _c2, rather thanH _c3, has been achieved close toT _c in Cu-plated samples of Sn and In. The parameters? and?(t) have also been determined for a series of SnIn alloys spanning the range between type I and type II behavior. Supercooling of the superconducting surface sheath below the thermodynamic critical fieldH _c has been observed in these alloys as well as in pure Pb. The measured values of? are compared with estimates based on normal state parameters extracted from independent low temperature experiments. Corrections for energy gap anisotropy and electron-phonon retardation are typically below 10%. The strong temperature dependence of? _sc(t), observed in all the materials studied, is not explained by present theories, even in the concentrated alloys where anisotropy effects should be washed out.     

Enhancement of superconductivity in disordered films by parallel magnetic field

We show that the superconducting transition temperature T _c (H) of a very thin highly disordered film with strong spin-orbital scattering can be increased by a parallel magnetic field H. This effect is due to the polarization of magnetic impurity spins, which reduces the full exchange scattering rate of electrons; the largest effect is predicted for spin-1/2 impurities. Moreover, for some range of magnetic impurity concentrations, the phenomenon of superconductivity induced by magnetic field is predicted: the superconducting transition temperature T _c (H) is found to be nonzero in the range of magnetic fields 0 < H* ≤ H ≤ H _c .     

Suppression of superconductivity in YNi2B2C at the atomic disordering

The behavior of the electrical resistivity ρ(T), the superconducting transition temperature T _c , and the upper critical field H _c2(T) of a polycrystalline sample of YNi₂B₂C irradiated by thermal neutrons with the subsequent high-temperature isochronous annealing in the temperature interval T _ann = 100–1000°C has been studied. It has been found that the irradiation of YNi₂B₂C with a fluence of 10¹⁹cm^?2 leads to the suppression of the superconductivity. The final disordered state is reversible; i.e., the initial ρ(T), T _c , and H _c2(T) values are almost completely recovered upon annealing at up to T _ann = 1000°C. The quadratic dependence ρ(T) = ρ₀ + a ₂ T ² is observed for the sample in the superconducting state (T _c = 5.5?14.5 K). The coefficient a ₂ (proportional to the square of the electron mass m*) hardly changes. The form of the dependence of T _c on ρ₀ can be interpreted as the suppression of the two superconducting gaps, Δ₁ and Δ₂ (Δ₁ ～ 2Δ₂). The degradation rate of Δ₁ is about three times higher than that of Δ₂. The dependences dH _c2/dT on ρ₀ and T _c may be described by the relations for a superconductor in the intermediate limit (the coherence length ζ₀ is on the order of the electron mean free path l _tr) under the assumption of a nearly constant electron density of states on the Fermi level N(E _F). The observed behavior of T _c obviously does not agree with the widespread opinion about the purely electron-phonon mechanism of superconductivity in the compounds of this type supposing the anomalous type of superconducting pairing.     

On the problem of high temperature superconductivity

This article is a brief review of the present situation in theoretical investigations of high temperature superconductivity. The main subject is a discussion of the formula for a critical temperature T_c for a homogeneous and isotropic substance with arbitrary permeability ?(q, ω). The problem of the maximum possible value of T_c is discussed. It is concluded that at present there are no grounds to consider that values of T_c ? 100°K to be impossible. The paper ends with some remarks on the exciton mechanism of superconductivity for the case of superconductors with a plane geometry.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

The origin of the pseudogap phase: precursor superconductivity versus a competing energy gap scenario

In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature T_c is different from the energy gap onset temperature T^∗. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter Δ_sc is to be distinguished from the excitation gap Δ; in this way, pseudogap effects persist below T_c. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper, we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T^∗ and T_c on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self-consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics, and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter.     

Pinning mechanism in carbon nanotube doping of MgB2 superconductor

MgB_1.9C_0.1 samples are synthesized under the ambient pressure (AP) and high pressure (HP), respectively. The further studies demonstrate different field-dependence of the critical current density J_c(H) in each sample. In the view of two-gap superconductivity in these samples, δT_c pinning (resulting from the spatial fluctuations of the transition temperature) is dominant in the AP sample, while in the HP sample, both δT_c and δl pinning (due to the mean-free-path fluctuations) act together and their contributions vary with temperature. Besides the improvement of H_c2(0), due to the different pinning mechanism, J_c(H) of the HP sample shows a slower decay with the increasing fields than that of the AP sample in high fields, which suggests a possible method of retarding the rapid decay of J_c(H) under elevated fields.     

As-NMR/NQR study of pressure-induced superconductivity in CaFe2As2

⁷⁵As-zero-field nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements are performed on CaFe₂As₂ under pressure. At P=4.7 and 10.8 kbar, the temperature dependence of nuclear-spin-lattice relaxation rate (1/T₁) measured at tetragonal phase show no coherence peak just below T_c and decrease with decreasing temperature. The superconductivity is of gapless at P=4.7 kbar but evolves to multiple gaps at P=10.8 kbar. We find that the superconductivity appears near a quantum critical point. Both electron correlation and superconductivity disappear in the collapsed tetragonal phase. A systematic study under pressure indicates that electron correlations play a vital role in forming Cooper pairs in this compound.     

Magnetic and superconducting phase diagrams in ErNi2B2C

We present measurements of the superconducting upper critical field H_c2(T) and the magnetic phase diagram of the superconductor ErNi₂B₂C made with a scanning tunneling microscope (STM). The magnetic field was applied in the basal plane of the tetragonal crystal structure. We have found large gapless regions in the superconducting phase diagram of ErNi₂B₂C, extending between different magnetic transitions. A close correlation between magnetic transitions and H_c2(T) is found, showing that superconductivity is strongly linked to magnetism.     

Theory for superconductivity in Pd-H and Pd-D systems

A theory is presented explaining the recently observed superconductivity at 9 °K in Pd-H and at 11 °K in Pd-D as due to the quenching of the spin-fluctuations and as due to the decrease of the Coulomb pseudo-potential μ present in Pd. That one observes for PdD a larger superconducting transition temperatureT _c than for PdH is explained by the larger lattice expansion resulting for adding H to Pd. The observed non-monotonic dependence ofT _c on the Hydrogen concentration is explained by the fact that approximately λ ∝N(0) for smallN(0), whereN(0) is the electronic density of states at the Fermi-energy and where λ denotes the electron-phonon coupling constant. Superconducting transition temperatures are estimated for PtH and RhH.     

Surface and volume superconductivity of Pb embedded in nanopores

The heat capacity of lead embedded in glass nanopores (7 nm in diameter) and bulk lead was studied in the temperature range 2–40 K without a magnetic field and in magnetic fields of 1–8 T. The properties of lead nanoparticles and bulk lead were compared. The results obtained allowed us to separate the surface superconductivity from the volume superconductivity. The temperature dependence of the heat capacity of lead nanoparticles was shown to exhibit two superconducting transitions above and below the transition temperature for bulk lead (T _c = 7.2 K), which are associated with the surface and volume superconductivity. The upper critical fields H _c3 for the surface superconductivity and H _c2 for the volume superconductivity were determined. It turned out that these fields for Pb nanoparticles are two orders of magnitude higher than those for bulk lead. The “superconductor-normal metal” phase diagrams were constructed for lead nanoparticles. The study established an increase in the density of low-frequency excitations in Pb nanocrystals as compared to bulk Pb and a difference in the electronic heat capacity of Pb nanoparticles as compared to bulk Pb.     

Annealing effect on the superconductivity of In2O3–ZnO thin films

We have investigated the relation among ρ–T characteristics, superconductivity, annealing conditions and the crystallinity of polycrystalline (In₂O₃)_1−x–(ZnO)_x films. We annealed as-grown amorphous films in air by changing annealing temperature and time. It is found that the films annealed at 200 °C or 300 °C for a time over 0.5 h shows the superconductivity. Transition temperature T_c and the carrier density n are T_c < 3.3 K and n ≈ 10²⁵–10²⁶ m⁻³, respectively. Investigations for films with x = 0.01 annealed at 200 °C have revealed that the T_c, n and crystallinity depend systematically on annealing time. Further, we consider that there is a suitable annealing time for sharp resistive transition because the transition width becomes wider with longer annealing times. We studied the upper critical magnetic field H_c2(T) for the film with different annealing time. From the slope of dH_c2/dT for all films, we have obtained the resistivity ρ dependence of the coherence length ξ(0) at T = 0 K.     

Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

Enhanced superconductivity in Sr2CuO4−v

A critical review of previous investigations of the superconductivity with enhanced T_c ∼ 95 K found in Sr₂CuO_4−v shows that new physics occurs in a highly overdoped region of the cuprate phase diagram. Moreover, evidence is adduced from the literature that 30% of the oxygen sites in the CuO₂ layers are vacant; a conclusion which is at odds with the universally made assumption that superconductivity originates in stoichiometric CuO₂ layers. While further research is needed in order to identify the pairing mechanism(s) responsible for the enhanced T_c, we suggest possible candidates.     

Negative U-centers and defect superconductivity

A connection between the formation of defect bipolarons (i.e. U-centers or U-bipolarons) and superconductivity in high-T_csuperconductors (HTSC) is considered in two pairing limits of carriers in realr- andk-spaces. The irrelevance ofr-space U-bipolarons to superconductivity is motivated. It is shown that the formation ofk-space U-bipolarons and their subsequent attractive single particle and pair condensation lead to depressed (in comparison with lattice bipolarons) superconductivity due to a large mass of such U-bipolarons. It is argued that the coexistence ofk-space lattice bipolarons andr-space U-bipolarons leads to the shift of the maximum of the concentration dependenceT_c(n)to higher carrier concentrations, in accordance with the observations in HTSC.     

Possible influence of two-band superconductivity on the behaviour of the second magnetization peak in La2−xSrxCuO4 single crystals

An anomalous temperature T dependence of the field H_on for the onset of the second magnetization peak (SMP) in La_1.81Sr_0.19CuO₄ single crystals (with the external magnetic field H oriented parallel to the c axis) is discussed. While the peak field H_p and the magnetization at H_p have a continuous variation with T, H_on exhibits a sudden decrease with increasing T for T  11 K–15 K. The analysis of the nature of the SMP in La_2−xSr_xCuO₄ single crystals in terms of a simple dynamic energy balance relation suggests that the observed behaviour could be related to the particular T dependence of the superfluid density in the case of two-band superconductivity, affecting the T variation of the elastic energy of the vortex system at low H.