Two-gap Superconductivity Research Articles

Specific heat and thermal conductivity in the vortex state of the two-gap superconductorMgB2

The specific heat coefficient gamma_s(H) and the electronic thermal conductivity kappa_{es}(H) are calculated for Abrikosov's vortex lattice by taking into account the effects of supercurrent flow and Andreev scattering. First we solve the gap equation for the entire range of magnetic fields. We take into account vertex corrections due to impurity scattering calculated in the Born approximation. The function gamma_s(H)/gamma_n increases from zero and becomes approximately linear above H/H_{c2} \sim 0.1. The dependence on impurity scattering is substantially reduced by the vertex corrections. The upward curvature of kappa_{es}(H)/kappa_{en}, which is caused by decreasing Andreev scattering for increasing field, is reduced for increasing impurity scattering. We also calculate the temperature dependence of the scattering rates 1/tau_{ps}(H) of a phonon and 1/tau_{es}(H) of a quasiparticle due to quasiparticle and phonon scattering, respectively. At low temperatures the ratio tau_{pn}/tau_{ps}(H) increases rapidly to one as H tends to H_{c2} which yields a rapid drop in the phononic thermal conductivity kappa_{ph}. Our results are in qualitative agreement with the experiments on the two-gap superconductor MgB_2.

Specific heat of Mg11B2: evidence for two energy gaps

Measurements of the specific heat of Mg11B2, from 1 to 50 K in magnetic fields to 9 T, are reported. The superconducting-state electron contribution is very different from that of other superconductors, but the general features are consistent with predictions for a two-gap superconductor, and a two-gap model based on BCS thermodynamics can quantitatively represent it. Parameters characterizing the gaps are in agreement with theoretical calculations. An unusually strong magnetic field dependence of the temperature-proportional term in the electron contribution to the vortex-state specific heat is another manifestation of the two gaps.

TRANSPORT IN MgB2 IN HIGH MAGNETIC FIELDS

Magnetotransport measurements are presented on polycrystalline MgB 2 samples. The 'resistive' upper critical magnetic field reveals a temperature dependence with positive curvature from Tc = 39.3 K down to about 20 K, then changes to slightly negative curvature reaching 26 T at 1.5 K. The 26-Tesla upper critical field is much higher than what is known so far on polycrystals of MgB 2 but it is in agreement with the recent data obtained on epitaxial MgB 2 films.1 The deviation of Hc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound.2 Recently we have found evidence for the existence of two superconducting energy gaps in this system using Andreev reflection spectroscopy.3 The temperature dependence of the resistivity and the normal state magnetoresistance are analyzed.

Vortices with fractional flux in two-gap superconductors and in extended faddeev model.

We discuss linear topological defects allowed in two-gap superconductors and equivalent extended Faddeev model. We show that, in these systems, there exist vortices which carry an arbitrary fraction of magnetic flux quantum. Besides that, we discuss topological defects which do not carry magnetic flux and describe features of ordinary one-magnetic-flux-quantum vortices in the two-gap system. The results could be relevant for the newly discovered two-band superconductor MgB2.

TWO GAP SIGNATURE IN MAGNESIUM DIBORIDE

Scanning Tunneling Spectroscopy is used at low temperature in an extensive study of the recently discovered superconducting compound MgB2 . Both powder- and crystal-based samples show a very unusual double-gap structure in the tunneling conductance spectra. The spectral weights of the gaps may vary from one junction to another but their energy positions remain constant. Both gaps vanish at the critical temperature of the bulk material. These findings confirm the existence of two-gap superconductivity in MgB2 , which opens a new class of multiple-gap superconductors.

Angle-resolved magnetotransport studies in anisotropicMgB2single crystals

We report the angle-resolved magnetotransport measurements on ${\mathrm{MgB}}_{2}$ single crystals that exhibit moderate anisotropy (\ensuremath{\gamma}) in upper critical fields with $\ensuremath{\gamma}=2.6\ifmmode\pm\else\textpm\fi{}0.1.$ Unusual ``kink'' features in resistivity are observed, which appear most clearly for field parallel to the c axis. We discuss the origin of the ``kink'' features in relation with the vortex-lattice melting and the recently proposed model of two-gap superconductivity. The influences of anisotropy on superconducting properties including the kink features are also demonstrated.

Modeling MgB 2 two-gap superconductivity

A two-band model for the description of MgB 2 two-gap superconductivity is developed. Debye-layer electron–phonon σ intraband coupling with unbounded σ intraband and σ–π interband Coulombic interactions are incorporated. An adjustable parameter set is determined which reproduces consistently T c, the specific heat jump and the gap ratio. Absolute values of the gaps and the isotope effect exponent containing no further adjustable parameters agree with the experiment. T c vs doping and gaps vs temperature curves are given.

Manifestation of multiband optical properties of MgB 2

The optical conductivity of MgB 2 has been determined on a dense polycrystalline sample in the spectral range 6 meV–4.6 eV using a combination of ellipsometric and normal incidence reflectivity measurements. σ 1( ω) features a narrow Drude peak with anomalously small plasma frequency (1.4 eV) and a very broad ‘dome’ structure, which comprises the bulk of the low-energy spectral weight. This fact can be reconciled with the results of band structure calculations by assuming that charge carriers from the 2D σ-bands and the 3D π-bands have principally different impurity scattering rates and negligible interband scattering. This also explains a surprisingly small correlation between the defect concentration and T c, expected for a two-gap superconductor. The large 3D carrier scattering rate suggests their proximity to the localization limit.

Lower critical field of MgB 2 measured by Hall probe

We report on the precise determination of the lower critical field ( H c1) of the newly discovered superconductor MgB 2 measured by Hall probe. Two polycrystalline samples with different grain sizes and one single crystalline sample are compared. By setting a small criterion for the first flux penetration into the sample, H c1 was determined. Based on our measurements, MgB 2 seems more similar to borocarbides and low- T c superconductors, which follow BCS temperature dependence, than to high- T c superconductors. Comparison with two-gap superconductivity is also made.

Specific heat of Mg11B2, a two-gap superconductor

We report measurements of the specific heat of Mg11B2, from 1 to 50 K and in magnetic fields to 9 T, and give the values of parameters relevant to the superconductivity. The superconducting-state electron contribution is dramatically different from that of other superconductors, but the general features are consistent with predictions for a two-gap superconductor, and it can be quantitatively represented by a two-gap model based on BCS thermodynamics. Parameters characterizing the gaps are in good agreement with some spectroscopic determinations, and also with theoretical calculations. An unusually strong magnetic field dependence of the temperature-proportional term in the electron contribution to the vortex-state specific heat is evidently another manifestation of the two gaps.

Magnetotransport and the upper critical magnetic field in MgB 2

Magnetotransport measurements are presented on polycrystalline MgB 2 samples. The `resistive' upper critical magnetic field reveals a temperature dependence with a positive curvature from T c=39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25-T upper critical field is much higher than what is known so far on polycrystals of MgB 2 but it is in agreement with recent data obtained on epitaxial MgB 2 films. The deviation of B c2( T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.

Two-gap state density in MgB(2): a true bulk property or a proximity effect?

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound MgB(2) directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions, a small crystal of MgB(2) is used as a tip in the STM experiment. The "sample" is chosen to be a 2H- NbSe(2) single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages V(S) approximately +/-3.8 mV and V(L) approximately +/-7.8 mV. With rising temperature both peaks disappear at the T(C) of the bulk MgB(2), a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.

Two-gap superconductivity and microwave properties of YBCO

The YBa[sub 2]Cu[sub 3]O[sub 7[minus]x] compound contains two conductive subsystem and as a result displays a two-gap structure. The Cu-O[sub 2] planes are characterized by intrinsic pairing, whereas the superconducting state of the Cu-O chains is induced by two different charge-transfer channels (intrinsic proximity effect and phonon-mediated transfer). Oxygen ordering affects the value of the induced energy gap. Such spectrum along with a short coherence length lead to peculiar microwave properties of YBCO thin films. The large residual microwave losses are explained in the framework of the two-gap model. 14 refs.

Major parameters of high T/sub c/ oxides

The major normal and superconducting parameters of several cuprate superconductors are evaluated and discussed. The effects of the two-gap structure in Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ are especially emphasized. It is pointed out that, for the two-gap superconductor such as Y-Ba-Cu-O, the analysis of the strength of the coupling should be carried out with considerable care. This is connected with the presence of three coupling constants, whose values can be entirely different.