Is the term "type-1.5 superconductivity" warranted by Ginzburg-Landau theory?

It is shown that within the Ginzburg-Landau (GL) approximation the order parameters Delta1(r, T) and Delta2(r, T) in two-band superconductors vary on the same length scale, the difference in the zero-T coherence lengths xi0_i ~vF_i/Delta_i(0), i = 1, 2 notwithstanding. This amounts to a single physical GL parameter kappa and the classic GL dichotomy: kappa < 1/sqrt(2) for type-I and kappa > 1/sqrt(2) for type-II.Comment: 5 pages, revised and extended version; previous title "Two-band superconductors near Tc" change

NMR relaxation time in a clean two-band superconductor

We study the spin-lattice relaxation rate of nuclear magnetic resonance in a two-band superconductor. Both conventional and unconventional pairing symmetries for an arbitrary band structure in the clean limit are considered. The importance of the inter-band interference effects is emphasized. The calculations in the conventional case with two isotropic gaps are performed using a two-band generalization of Eliashberg theory.Comment: 9 pages, 3 figure

Electron-lattice interaction and its impact on high Tc superconductivity

In this Colloquium, the main features of the electron-lattice interaction are discussed and high values of the critical temperature up to room temperature could be provided. While the issue of the mechanism of superconductivity in the high Tc cuprates continues to be controversial, one can state that there have been many experimental results demonstrating that the lattice makes a strong impact on the pairing of electrons. The polaronic nature of the carriers is also a manifestation of strong electron-lattice interaction. One can propose an experiment that allows an unambiguous determination of the intermediate boson (phonon, magnon, exciton, etc.) which provides the pairing. The electron-lattice interaction increases for nanosystems, and this is due to an effective increase in the density of states

Superfluid density and specific heat within self-consistent scheme for two-band superconductor

The two gaps in a two-band clean s-wave superconductor are evaluated self-consistently within the quasiclassical Eilenberger weak-coupling formalism with two in-band and one inter-band pairing potentials. Superfluid density, free energy and specific heat are given in the form amenable for fitting the experimental data. Well-known two-band MgB$_2$ and V$_3$Si superconductors are used to test the developed approach. The pairing potentials obtained from the fit of the superfluid density data in MgB$_2$ crystal were used to calculate temperature-dependent specific heat, $C(T)$. The calculated $C(T)$ compares very well with the experimental data. Advantages and validity of this, what we call the "$\gamma$-model", are discussed and compared with the commonly used empirical (and \textit{not self-consistent}) "$\alpha$-model". Correlation between the sign of the inter-band coupling and the signs of the two order parameters is discussed. Suppression of the critical temperature by the inter-band scattering is evaluated and shown to be severe for the inter-band repulsion as compared to the attraction. The data on a strong $T_c$ suppression in MgB$_2$ crystals by impurities suggest that the order parameters on two effective bands of this material may have opposite signs, i.e., may have the $s_{\pm}$ structure similar to the current proposals in iron-based pnictide superconductors

Field dependence of the vortex core size probed by scanning tunneling microscopy

We study the spatial distribution of the density of states (DOS) at zero bias N(r) in the mixed state of single and multigap superconductors. We provide an analytic expression for N(r) based on deGennes' relationship between DOS and the order parameter that reproduces well scanning tunneling microscopy (STM) data in several superconducting materials. In the single gap superconductor β-Bi2Pd, we find that N(r) is governed by a length scale ξH = √φ0/2πH, which decreases in rising fields. The vortex core size C, defined via the slope of the order parameter at the vortex center, C ∝ (d%/dr|r→0)−1, differs from ξH by a material dependent numerical factor. The new data on the tunneling conductance and vortex lattice of the 2H-NbSe1.8S0.2 show the in-plane isotropic vortices, suggesting that substitutional scattering removes the in-plane anisotropy found in the two-gap superconductor 2H-NbSe2. We fit the tunneling conductance of 2H-NbSe1.8S0.2 to a two gap model and calculate the vortex core size C for each band. We find that C is field independent and has the same value for both bands. We also analyze the two-band superconductor 2H-NbS2 and find the same result. We conclude that, independently of the magnetic field induced variation of the order parameter values in both bands, the spatial variation of the order parameter close to the vortex core is the same for all bands

Some remarks on the equation of state for hard repulsive potentials

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32291/1/0000358.pd

Point-contact spectroscopy of the antiferromagnetic superconductor HoNi2B2C in the normal and superconducting state

Point-contact (PC) spectroscopy measurements on antiferromagnetic (AF) (T_N=5.2K) HoNi2B2C single crystals in the normal and two different superconducting (SC) states (T_c=8.5K and $T_c^*=5.6K) are reported. The PC study of the electron-boson(phonon) interaction (EB(P)I) spectral function reveals pronounced phonon maxima at 16, 22 and 34meV. For the first time the high energy maxima at about 50meV and 100meV are resolved. Additionally, an admixture of a crystalline-electric-field (CEF) excitations with a maximum near 10meV and a `magnetic` peak near 3meV are observed. The contribution of the 10-meV peak in PC EPI constant \lambda_PC is evaluated as 20-30contribution of the high energy modes at 50 and 100meV amounts about 10each maxima, so the superconductivity might be affected by CEF excitations. The SC gap in HoNi2B2C exhibits a standard single-band BCS-like dependence, but vanishes at$T_c^*=5.6K<T_c, with 2\Delta/kT_c^*=3.9. The strong coupling Eliashberg analysis of the low-temperature SC phase with T_c^*=5.6K =T_N, coexisting with the commensurate AF structure, suggests a sizable value of the EPI constant \lambda_s=0.93. We also provide strong support for the recently proposed by us ''Fermi surface (FS) separation'' scenario for the coexistence of magnetism and superconductivity in magnetic borocarbides, namely, that the superconductivity in the commensurate AF phase survives at a special (nearly isotropic) FS sheet without an admixture of Ho 5d states. Above T_c^* the SC features in the PC characteristics are strongly suppressed pointing to a specific weakened SC state between T_c* and T_c.Comment: 11 pages, 8 figs, to be published in PRB, Vol.75, Iss.2

Electronic Collective Modes and Superconductivity in Layered Conductors

A distinctive feature of layered conductors is the presence of low-energy electronic collective modes of the conduction electrons. This affects the dynamic screening properties of the Coulomb interaction in a layered material. We study the consequences of the existence of these collective modes for superconductivity. General equations for the superconducting order parameter are derived within the strong-coupling phonon-plasmon scheme that account for the screened Coulomb interaction. Specifically, we calculate the superconducting critical temperature Tc taking into account the full temperature, frequency and wave-vector dependence of the dielectric function. We show that low-energy plasmons may contribute constructively to superconductivity. Three classes of layered superconductors are discussed within our model: metal-intercalated halide nitrides, layered organic materials and high-Tc oxides. In particular, we demonstrate that the plasmon contribution (electronic mechanism) is dominant in the first class of layered materials. The theory shows that the description of so-called ``quasi-two-dimensional superconductors'' cannot be reduced to a purely 2D model, as commonly assumed. While the transport properties are strongly anisotropic, it remains essential to take into account the screened interlayer Coulomb interaction to describe the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure

Breakdown of the Migdal-Eliashberg theory in the strong-coupling adiabatic regime

In view of some recent works on the role of vertex corrections in the electron-phonon system we readress an important question of the validity of the Migdal-Eliashberg theory. Based on the solution of the Holstein model and inverse coupling constant expansion, we argue that the standard Feynman-Dyson perturbation theory by Migdal and Eliashberg with or without vertex corrections cannot be applied if the electron-phonon coupling constant $\lambda$ is larger than 1 for any ratio of the phonon and Fermi energies. In the extreme adiabatic limit of the Holstein model electrons collapse into self-trapped small polarons or bipolarons due to spontaneous translational-symmetry breaking when $\lambda$ is between 0.5 and 1.3 (depending on the lattice dimensionality). With the increasing phonon frequency the region of the applicability of the theory shrinks to lower values of the coupling constant.Comment: 4 pages, 1 figur