GAP MEASUREMENTS IN MgB2 BREAK-JUNCTION AND POINT-CONTACT HETEROSTRUCTURES: TEST OF THE TWO-BAND MODELS

2002 ◽  
Vol 16 (11n12) ◽  
pp. 1553-1561 ◽  
Author(s):  
R. S. GONNELLI ◽  
G. A. UMMARINO ◽  
D. DAGHERO ◽  
A. CALZOLARI ◽  
V. A. STEPANOV
Keyword(s):  
Critical Temperature ◽  
Point Contact ◽  
Temperature Evolution ◽  
Band Model ◽  
Break Junction ◽  
Dirty Limit ◽  
Clean Limit ◽  
Two Band Model ◽  
Contact Measurements

We present a series of results concerning the determination of the gap(s) in MgB2 by means of break-junction and point-contact measurements. We were able to observe the small 3D gap predicted by the two-band models for MgB2 in the clean limit as well as the gap in dirty limit. We were also able to follow the temperature evolution of the latter, from 4.2 K up to a reduced critical temperature. We show that all our results can be explained in the framework of the two-band model provided that the existence of a damaged surface in dirty limit proximized by the clean bulk as well as the occurrence of parallel contacts are admitted.

Critical Temperature and Isotope Exponent in a Two-band Model for Superconducting Fe-pnictides

2009 ◽  
Vol 22 (6) ◽  
pp. 539-542 ◽  
Author(s):  
J. J. Rodríguez-Núñez ◽  
A. A. Schmidt ◽  
R. Citro ◽  
C. Noce
Keyword(s):  
Critical Temperature ◽  
Band Model ◽  
Two Band Model

scholarly journals POINT-CONTACT SPECTROSCOPY OF MAGNESIUM DIBORIDE WITH DIFFERENT COUNTER-ELECTRODES

2003 ◽  
Vol 17 (27n28) ◽  
pp. 1405-1416
Author(s):  
A. I. D'YACHENKO ◽  
V. YU. TARENKOV ◽  
M. A. BELOGOLOVSKII ◽  
V. N. VARYUKHIN ◽  
A. V. ABAL'OSHEV ◽  
...  
Keyword(s):  
Andreev Reflection ◽  
Magnesium Diboride ◽  
Three Dimensional ◽  
Point Contact ◽  
Band Model ◽  
Counter Electrodes ◽  
Break Junctions ◽  
Point Contact Spectroscopy ◽  
Dc Current ◽  
Two Band Model

We report on tunneling and Andreev-reflection conductance spectra of 39 K superconducting magnesium diboride, obtained with Pb and Au counter-electrodes. Two distinct steps at close to 2.7 and 7.1 meV appear in a low-resistance metallic-type Au–MgB 2 junction characteristic, whereas a tunneling-like spectrum measured for the same junction, annealed by the application of DC current, exhibits only a rounded contribution of the larger gap. Junctions with a superconducting lead counter-electrode pressed into a bulk MgB 2 sample reveal two conductance peaks that are interpreted as the result of the formation of a highly-transmitting break junctions inside the magnesium diboride ceramic. Our results strongly support the two-band model with two different gap values on quasi-two-dimensional σ (7.1 meV) and three-dimensional π (2.7 meV) Fermi surface sheets of MgB 2.

BCS-LIKE APPROXIMATION IN A TWO-BAND MODEL FOR SUPERCONDUCTIVITY IN MgB2

2005 ◽  
Vol 19 (11) ◽  
pp. 503-516 ◽  
Author(s):  
EUGEN-VICTOR MACOCIAN
Keyword(s):  
Critical Temperature ◽  
Weak Coupling ◽  
Analytical Calculation ◽  
Gap Functions ◽  
Band Model ◽  
Phonon Interaction ◽  
Electron Phonon Interaction ◽  
Multiband Model ◽  
Two Band Model ◽  
Interband Coupling

The BCS limit of a two-band model for superconductivity in MgB 2 is discussed and the temperature dependence of the gap functions is calculated together with their values at T = 0. We have studied the effect of interband coupling on the order parameters and critical temperature. The validity of the approximation was investigated by direct comparison with more accurate Eliashberg calculation and it was found that the BCS-like model is appropriate for temperatures up to T ~ 10 K . The two-band superconductivity in MgB 2 has been extensively studied in the literature, in Eliashberg or weak-coupling calculations, as well as in ab initio evaluations of electron–phonon interaction. Here we find from an analytical calculation the well known result that the ratio 2Δ/Tc is no longer universal in the multiband model, in contrast with the classical single-band BCS model.

ELECTRON-PHONON SPECTRAL FUNCTION AND TWO-BAND MODEL IN TUNNELING MEASUREMENTS ON MgB2

2003 ◽  
Vol 17 (04n06) ◽  
pp. 643-648
Author(s):  
G. A. UMMARION ◽  
R. S. GONNELLI ◽  
O. V. DOLGOV ◽  
S. V. SHULGA
Keyword(s):  
Spectral Function ◽  
Band Model ◽  
Eliashberg Equations ◽  
Spectral Functions ◽  
Principle Calculation ◽  
First Principle Calculation ◽  
Clean Limit ◽  
Densities Of States ◽  
First Time ◽  
Two Band Model

In the present work we calculate for the first time the density of states of MgB 2 for different tunneling directions by directly solving the two-band Eliashberg equations in the real-axis formulation starting from the first-principle calculation of the interband and intraband electron-phonon spectral functions. This complicated numeric procedure allows preserving the fine structures of the DOS in the phonon energy range. We show that the numeric inversion of the standard single-band Eliashberg equations when applied to the densities of states obtained by the two-band model may lead to artifacts in the extracted electron-phonon spectral function α2 F(ω). We also suggest that the small DOS structures produced by the two-band inter-action at energies between 20 and 100 meV can be observed only at very low temperature in junctions in perfect clean limit.

scholarly journals Study of the Electron-Phonon Interaction in Metal Diborides MeB2 (Me = Zr, Nb, Ta, Mg) by Point-Contact Spectroscopy

2003 ◽  
Vol 17 (10n12) ◽  
pp. 657-666 ◽  
Author(s):  
I. K. Yanson ◽  
Yu. G. Naidyuk ◽  
O. E. Kvitnitskaya ◽  
V. V. Fisun ◽  
N. L. Bobrov ◽  
...  
Keyword(s):  
Phonon Dispersion ◽  
Point Contact ◽  
Band Model ◽  
Phonon Interaction ◽  
Metal Compounds ◽  
Phonon Dispersion Curves ◽  
Electron Phonon Interaction ◽  
Excess Current ◽  
Metal Diborides ◽  
Two Band Model

We review investigations of the electron-phonon interaction (EPI) in metal diborides MeB2 (Me = Zr, Nb, Ta, Mg) by point-contact (PC) spectroscopy. For transition metal compounds the PC EPI functions were recovered and EPI parameter λ ≲ 0.1 were estimated. The data are consistent with the measured surface phonon dispersion curves. The low λ value questions some reports about superconductivity in these compounds. Contrary, EPI in superconducting MgB2 films manifests also in the PC spectra itself by virtue of an elastic EPI contribution to the excess current determined by the energy dependence of the superconducting order parameter. To analyse the phonon features in the PC spectra of MgB2 a two-band model is exploited and the proximity effect in the k-space is suggested.

scholarly journals Temperature-Dependent s± ↔ s++ Transitions in the Multiband Model for Fe-Based Superconductors with Impurities

Symmetry ◽  
2018 ◽  
Vol 10 (8) ◽  
pp. 323 ◽  
Author(s):  
V. Shestakov ◽  
M. Korshunov ◽  
O. Dolgov
Keyword(s):  
Narrow Range ◽  
Impurity Scattering ◽  
Band Model ◽  
Temperature Dependent ◽  
Clean Limit ◽  
Scattering Rate ◽  
Nonmagnetic Impurities ◽  
Multiband Model ◽  
Critical Temperature Tc ◽  
Two Band Model

We study the dependence of the superconducting gaps on both the disorder and the temperature within the two-band model for iron-based materials. In the clean limit, the system is in the s± state with sign-changing gaps. Scattering by nonmagnetic impurities leads to the change of the sign of the smaller gap, resulting in a transition from the s± to the s++ state with the sign-preserving gaps. We show here that the transition is temperature-dependent. Thus, there is a line of s±→s++ transition in the temperature–disorder phase diagram. There is a narrow range of impurity scattering rates, where the disorder-induced s±→s++ transition occurs at low temperatures, but then the low-temperature s++ state transforms back to the s± state at higher temperatures. With increasing impurity scattering rate, the temperature of such s++→s± transition shifts to the critical temperature Tc, and only the s++ state is left for higher amounts of disorder.

IONIZATION ENERGY AND IMPURITY BAND CONDUCTION OF SHALLOW DONORS IN n-GALLIUM ARSENIDE

1967 ◽  
Vol 45 (1) ◽  
pp. 119-126 ◽  
Author(s):  
J. Basinski ◽  
R. Olivier
Keyword(s):  
Ionization Energy ◽  
Impurity Band ◽  
Shallow Donor ◽  
Band Model ◽  
A Value ◽  
Transition Concentration ◽  
Temperature Electron ◽  
Two Band Model ◽  
Band Conduction ◽  
Made In

Hall effect and resistivity measurements have been made in the temperature range 4.2–360 °K on several samples of n-type GaAs grown under oxygen atmosphere and without any other intentional dopings. The principal shallow donor in this material is considered to be Si. All samples exhibited impurity-band conduction at low temperature. Electron concentrations in the conduction band were calculated, using a two-band model, and then fitted to the usual equation expressing charge neutrality. A value of 2.3 × 10−3 eV was obtained for the ionization energy of the donors, for donor concentration ranging from 5 × 1015 cm−3 to 2 × 1016 cm−3. The conduction in the impurity band was of the hopping type for these concentrations. A value of 3.5 × 1016 cm−3 was obtained for the critical transition concentration of the impurity-band conduction to the metallic type.

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