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.

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.

Effect of Cr doping on the electrical conductivity and Seebeck coefficient in the superconductors obtained from the Bi–Pb–Sr–Ca–Cu–Cr–O-type glassy precursors by annealing

2000 ◽  
Vol 15 (5) ◽  
pp. 1076-1082 ◽  
Author(s):  
H. K. Barik ◽  
S. K. Ghorai ◽  
S. Bhattacharya ◽  
D. Kilian ◽  
B. K. Chaudhuri
Keyword(s):  
Conduction Mechanism ◽  
Linear Part ◽  
Band Model ◽  
Phonon Drag ◽  
Temperature Dependent ◽  
Breaking Mechanism ◽  
Cr Concentration ◽  
Cr Doping ◽  
Two Band Model ◽  
Variable Range Hopping Conduction

Homogenous (Bi3Pb)Sr3Ca3 (Cu4−nCrn)Ox (n 4 0 to 0.20) type glassy precursors become high-Tc superconductors by annealing at 840 °C. The suppression of Tc with increase of Cr concentration supports the pair-breaking mechanism. The feeble semiconducting behavior shown by the doped samples above their respective Tc values followed Mott's variable range hopping conduction mechanism. Like Ti- and Fe-doped samples, studied earlier, the thermoelectric power (TEP) of the present Cr-containing sample showed small positive peak above Tc, which was considered to be associated with the phonon-drag effect. The linear part of the temperature-dependent TEP (above Tc) well fitted the two-band 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.

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.

scholarly journals Temperature dependent magnetization of the two band model for diluted magnetic semiconductors

2014 ◽  
Vol 24 (2) ◽  
pp. 141
Author(s):  
Vu Kim Thai ◽  
Hoang Anh Tuan
Keyword(s):  
Diluted Magnetic Semiconductors ◽  
Magnetic Semiconductors ◽  
Magnetic Coupling ◽  
Band Model ◽  
Coupling Constant ◽  
Temperature Dependent ◽  
Impurity Bands ◽  
Diluted Magnetic ◽  
Two Band Model ◽  
Temperature Dependent Magnetization

The temperature dependent magnetization of a two band model for diluted magnetic semiconductors as a function of magnetic coupling constant, hopping parameters and carrier densities is calculated by using the coherent potential approximation.  It is shown that the degree of overlapping of the impurity bands  and carrier density are crucial parameters determining the magnetization behavior of the system.

IDEAL RESISTIVITY OF PLATINUM BELOW 20 °K

1967 ◽  
Vol 45 (5) ◽  
pp. 1693-1708 ◽  
Author(s):  
R. J. Berry
Keyword(s):  
Electrical Resistivity ◽  
Impurity Scattering ◽  
Band Model ◽  
Temperature Dependency ◽  
Pure Platinum ◽  
Two Band Model

An improved estimate for the electrical resistivity of ideally pure platinum is derived for the 3–20 °K region from highly accurate measurements on a specimen of thermometric-grade Pt. The two-band model is used to correct for the small, but significant, impurity scattering. The temperature dependency of this "ideal" resistivity function is compared with theory and previous work. Certain discrepancies are noted.

(111) Monocrystalline Nickel Films

1972 ◽  
Vol 30 ◽  
pp. 512-513
Author(s):  
T.E. Pratt ◽  
R.W. Vook
Keyword(s):  
Film Thickness ◽  
Deposition Rate ◽  
Room Temperature ◽  
Narrow Range ◽  
High Vacuum ◽  
Residual Pressure ◽  
Temperature Dependent ◽  
Deposition Parameters ◽  
Transmission Electron ◽  
Substrate Temperatures

(111) oriented thin monocrystalline Ni films have been prepared by vacuum evaporation and examined by transmission electron microscopy and electron diffraction. In high vacuum, at room temperature, a layer of NaCl was first evaporated onto a freshly air-cleaved muscovite substrate clamped to a copper block with attached heater and thermocouple. Then, at various substrate temperatures, with other parameters held within a narrow range, Ni was evaporated from a tungsten filament. It had been shown previously that similar procedures would yield monocrystalline films of CU, Ag, and Au.For the films examined with respect to temperature dependent effects, typical deposition parameters were: Ni film thickness, 500-800 A; Ni deposition rate, 10 A/sec.; residual pressure, 10-6 torr; NaCl film thickness, 250 A; and NaCl deposition rate, 10 A/sec. Some additional evaporations involved higher deposition rates and lower film thicknesses.Monocrystalline films were obtained with substrate temperatures above 500° C. Below 450° C, the films were polycrystalline with a strong (111) preferred orientation.

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.

Sign in / Sign up

Export Citation Format

Share Document