Electron-phonon Mass Enhancement Factor Research Articles

Interstitial disorder in iron-based superconductor: A first-principles study

FeSe in bulk, film or layer form as well as its alloys have shown many intriguing magnetic and superconducting properties. In this paper, we examine the role of interstitial disorder in changing the various electronic, magnetic and superconducting properties of FeSe using ab-initio method. We have studied the electronic properties of FeSe doped interstitially with the transition metal (TM) impurities, Fe(TM) x Se (where TM ≡ V, Mn, Co, Ni, and x = 0.0, 0.01, 0.04 and 0.10). Our results, analyzed in terms of density of states (DOS), band structure, Fermi surface (FS), and local and total magnetic moments, show that the excess of the transition-metal atoms substantially modify the Fermi surface of FeSe. We have also estimated the value of the electron-phonon mass enhancement factor, the Hopfield parameter and the superconducting transition temperature for these alloys.

Ab initio study of the electron-phonon coupling at the Cr(001) surface

It is experimentally well established that the Cr(001)-surface exhibits a sharp resonance around the Fermi level. However, there is no consensus about its physical origin. It is proposed to be either due to a single particle dz2 surface state renormalised by electron-phonon coupling or the orbital Kondo effect involving the degenerate dxz/dyz states. In this work we examine the electron-phonon coupling of the Cr(001)-surface by means of ab-initio calculations in the form of density functional perturbation theory. More precisely, the electron-phonon mass-enhancement factor of the surface layer is investigated for the 3d states. For the majority and minority spin dz2 surface states we find values of 0.19 and 0.16. We show that these calculated electron-phonon mass-enhancement factors are not in agreement with the experimental data even if we use realistic values for the temperature range and surface Debye frequency for the fit of the experimental data. More precisely, then experimentally an electron-phonon mass-enhancement factor of 0.70~0.10 is obtained, which is not in agreement with our calculated values of 0.19 and 0.16. Therefore, we conclude that the experimentally observed resonance at the Cr(001)-surface is not due to polaronic effects, but due to electron-electron correlation effects.

Excess of manganese in an iron-based superconductor: a first-principles study of its electronic, magnetic and superconducting properties

Alloying and doping have always been an effective way of improving the properties of materials—be it electronic, magnetic or superconducting. In this direction, we present a first-principles investigation of the electronic, magnetic and superconducting properties of the systems (where x = 0.0, 0.01, 0.02 and 0.03). The calculations were performed using the Korringa–Kohn–Rostoker (KKR) atomic sphere approximation (ASA). In order to treat the disorder, we employed the coherent-potential approximation (CPA) into the KKR method. We performed both unpolarized and spin-polarized calculations and found that these systems are close to being magnetic. The results have been analyzed in terms of the density of states (DOS), electronic band structures, Fermi surfaces and the superconducting transition temperature Tc. We find that there are no significant changes in the shape of the Fermi surfaces in unpolarized calculations with respect to pure FeSe, but in spin-polarized calculations, the excess of the transition-metal Mn atoms substantially modifies the Fermi surface compared with the parent FeSe and compounds. We have also estimated the value of electron–phonon mass enhancement factor λ which is the sum of the electron–phonon coupling constant as well as the electron–electron interaction. Using λ, we have calculated the superconducting transition temperature Tc for the Mn doped superconductor. The calculated Tc are in good agreement with the experimental results. Additionally, Hopfield parameters are also calculated from the charge self-consistent potential using the KKR–ASA–CPA method for understanding the trend of Tc and DOS at Fermi energy.

First-principles study of electronic and magnetic properties of Fe1−xTMxSe0.5Te0.5 alloys (TM ≡ Cr, Mn, Co, Ni)

We have studied the effects of transition metal (TM) substitution on Fe sub-lattice in Fe1−xTMxSe0.5Te0.5 (TM ≡ Cr, Mn, Co, Ni and x = 0, 0.01, 0.05, 0.10) in terms of changes in its electronic and magnetic properties. The calculations were performed with the Korringa-Kohn-Rostoker (KKR) Atomic Sphere Approximation (ASA) method within the coherent-potential approximation (CPA). Our spin-polarized calculations show these alloys to be magnetic in nature. We discuss the calculated density of states (DOS), band structures and Fermi surfaces (FS) in terms of the contributions of Fe3d64s2, Cr3d54s1, Mn3d54s2, Co3d74s2, Ni3d84s2 and Se4s24p4 outer most electrons. Our results indicate that the effects of doping on the electronic properties are significant with respect to parent FeSe and FeSe0.5Te0.5 superconductor. We have also estimated the value of electron-phonon mass enhancement factor λ (which is the sum of electron–phonon coupling constant and the electron–electron interaction present in the material) to calculate the superconducting transition temperature Tc for the TM-doped FeSe0.5Te0.5. Hopfield parameters are also calculated from the charge self-consistent potentials obtained from KKR-ASA CPA method.

Theoretical study of electronic structure and superconductivity in FeSe0.9−x(Si/Sb)x alloys

We have studied the effects of doping Si and Sb at the Se site in FeSe0.9 (i.e., FeSe0.9−x(Si/Sb)x for x = 0, 0.05, 0.10,0.15 and 0.20) using the Korringa-Kohn-Rostoker Atomic Sphere Approximation method within the coherent-potential approximation. Our calculations show that the ground state of FeSe0.9 is nonmagnetic while FeSe0.9−x(Si/Sb)x alloys tend to be magnetic. We present the results of unpolarized and spin-polarized, charge self-consistent, calculations for FeSe0.9−x(Si/Sb)x alloys in terms of density of states (DOS), band structure and Fermi surface (FS). In particular, we find that the doped FeSe0.9 exhibits the same topology of the FS as that of FeSe, differing only in subtle FS nesting features in case of unpolarized calculations but in case of spin-polarized calculations, the topology of the FS is very different from that of FeSe. We have also estimated the values of electron–phonon mass enhancement factor λ (which is the sum of electron–phonon coupling constant as well as the electron–electron interaction) to calculate the superconducting transition temperature Tc for the FeSe0.9−x(Si/Sb)x alloys. We find that the calculated Tc are in good agreement with experimental results.

BAND STRUCTURE, METALLIZATION AND SUPERCONDUCTIVITY OF InP AND InN UNDER HIGH PRESSURE

The electronic band structure, structural phase transition, metallization and superconducting transition of cubic zinc blende type indium phosphide (InP) and indium nitride (InN), under pressure, are studied using FP-LMTO method. These indium compounds become metals and superconductors under high pressure but before that they undergo structural phase transition from ZnS to NaCl structure. The ground state properties and band gap values are compared with the experimental and previous theoretical results. From our analysis, it is found that the metallization pressure increases with increase of lattice constant. The superconducting transition temperatures (Tc) of InP and InN are obtained as a function of pressure for both the ZnS and NaCl structures and these compounds are identified as pressure induced superconductors. When pressure is increased Tc increases in both the normal ( ZnS ) and high pressure ( NaCl ) structures. The dependence of Tc on electron–phonon mass enhancement factor λ shows that InP and InN are electron–phonon mediated superconductors. The non-occurrence of metallization, phase transition and onset of superconductivity simultaneously in InP and InN is confirmed.

PRESSURE-INDUCED METALLIZATION AND SUPERCONDUCTIVITY IN InP AND InN

The electronic band structure, structural phase transition, metallization and superconducting transition of cubic zinc blende-type indium phosphide ( InP ) and indium nitride ( InN ), under pressure, are studied using TB-LMTO method. These indium compounds become metals and superconductors under high pressure but before that they undergo structural phase transition from ZnS to NaCl structure. The ground-state properties and band gap values are compared with the experimental and previous theoretical results. From our analysis, it is found that the metallization pressure increases with increase of lattice constant. The superconducting transition temperatures (Tc) of InP and InN are obtained as a function of pressure for both the ZnS and NaCl structures and these compounds are identified as pressure-induced superconductors. When pressure is increased Tc increases in both the normal ( ZnS ) and high pressure ( NaCl ) structures. The dependence of Tc on electron–phonon mass enhancement factor λ shows that InP and InN are electron–phonon mediated superconductors. The non-occurrence of metallization, phase transition and onset of superconductivity simultaneously in InP and InN are confirmed.

BAND STRUCTURE, METALLIZATION, AND SUPERCONDUCTIVITY OF GaAs AND InAs UNDER HIGH PRESSURE

The electronic band structure, metallization, structural phase transition, and superconductivity of cubic zinc blende type GaAs and InAs are investigated. The equilibrium lattice constant, bulk modulus, and the phase transition pressure at which the compounds undergo structural phase transition from ZnS to NaCl are predicted from the total energy calculations. The density of states at the Fermi level (N(E F )) get enhanced after metallization, which leads to the superconductivity in GaAs and InAs . The superconducting transition temperatures (T c ) of GaAs and InAs are obtained as a function of pressure for both the ZnS and NaCl structures. GaAs and InAs come under the class of pressure-induced superconductors. When pressure is increased T c increases in both the normal and high pressure-structures. The dependence of T c on electron–phonon mass enhancement factor λ shows that GaAs and InAs are electron–phonon-mediated superconductors. Also, it is found that GaAs and InAs retained in their normal structure under high pressure give appreciably high T c .

Temperature-dependent scanning tunneling spectroscopy of Cr(001): Orbital Kondo resonance versus surface state

The temperature-dependent surface electronic structure of Cr(001) is investigated by means of scanning tunneling spectroscopy (STS) in the temperature range between 22 and $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Consistent with earlier observations, a sharp peak is found in the tunneling spectra close to the Fermi level ${E}_{\mathrm{F}}$. While the binding energy remains unchanged, the peak broadens with increasing temperature. The experimental data are fitted within the framework of two physical models, i.e., in terms of a single-particle band theory and an orbital Kondo effect. Both models lead to a reasonable agreement between the fit and the experimental data. Using the Kondo model, a Kondo temperature ${T}_{\mathrm{K}}$ of $123\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is obtained. If interpreted in terms of a single-particle band theory, the fit results in an electron-phonon mass-enhancement factor $\ensuremath{\lambda}$, which is 5--10 times larger than the Cr bulk value.

Band structure, metallization and superconductivity of GaP and GaN under high pressure

AbstractThe electronic band structure, metallization, structural phase transition and superconductivity of cubic zinc blende‐type GaP and GaN are investigated. The equilibrium lattice constant, bulk modulus and pressure at which the compounds undergo a structural phase transition from ZnS structure to NaCl structure are predicted from the total energy calculations. The density of states at the Fermi level (N(EF)) is enhanced after metallization, which leads to the superconductivity in GaP and GaN. The superconducting transition temperatures (Tc) of GaP and GaN are obtained as a function of pressure for both the ZnS and NaCl structures and GaP and GaN come under the category of pressure‐induced superconductors. When pressure is increased Tc increases in both the normal and high‐pressure structures. The dependence of Tc on the electron–phonon mass enhancement factor λ shows that GaP and GaN are electron–phonon‐mediated superconductors. Also, it is found that GaP and GaN retained in their normal structure under high pressure give appreciably high Tc. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pressure dependence of metallization and superconducting transition in AgCl andAgBr

The electronic band structure, metallization, structural phase transition andsuperconducting transition of silver chloride (AgCl) and silver bromide (AgBr) underpressure are studied using the TB-LMTO method. The ground state properties andbandgap values are compared with the experimental and previous theoretical results.These silver halides become metals and superconductors under high pressure,but before this they undergo structural phase transition from NaCl phase toCsCl phase. It is found that the charge transfers between Ag 5s, 4d and Xnp,nd (X = Cl,Br; n = 3, 4) states cause metallization. It is found that the metallization pressure increaseswith decrease of lattice constant. The density of states at the Fermi level(N(EF)) is enhanced as the pressure is further increased, which leads to thesuperconductivity in AgCl and AgBr. Like CsI, AgCl and AgBr come underthe class of pressure induced superconductors. The pressure effects onλ (electron–phonon mass enhancement factor) andμ* (electron–electron interaction parameter) clearly suggest that AgCl and AgBr areelectron–phonon mediated superconductors. The non-occurrence of metallization, phasetransition and onset of superconductivity simultaneously in ionic solids is also confirmed.

Low-temperature specific heat of near-equiatomic Ni-rich B19′-TiNi-alloys

The total DOS has been calculated for B19′-TiNi-alloys using three compositions (50.0, 51.6 and 53.1 at.% Ni) by means of the linearized muffin-tin-orbital (LMTO) method. No drastic increase in the Fermi-level DOS has been found at 51.6 at.% Ni (with respect to the composition of 50.0 at.% Ni). There is a slight increase at 53.1 at.% Ni. The electron–phonon mass enhancement factor (λ ep ) has in this case only a small contribution to the electronic coefficient of the low-temperature specific heat (γ). According to our calculations, it is not the composition in itself which can cause the large experimental value of γ at 51.5 at.% Ni. Our results agree with the view that it is not the high Fermi-level DOS but an anomaly in the electron–phonon system which is responsible for the composition-dependent variation of the martenstitic transition temperature ( T Ms ). It is probable that the large experimental value of γ at 51.5 at.% Ni is caused by impurities: by precipitates or impurity phases.

Effect of thermal phonons and two-dimensional acoustic plasmons on superconductivity in YBa 2CuO 7

The superconducting transition temperature, T c, has been studied using the strongly coupled theory of superconductors in high- T c YBa 2Cu 3O 7 in the presence of realistic physical parameters, thermal phonons and two-dimensional acoustic plasmons. The calculated value of T c, using a realistic phonon distribution function, the electron-electron Coulomb interaction and the Stoner factor, comes out to be ≈ 65 K. When the finite temperature effect is taken into account through the modification of the temperature dependence of the electron-phonon mass enhancement factor, the value of T c increases significantly by ≈ 18 K to ≈ 83 K. Finally, when the two-dimensional acoustic plasmons are considered, a further increase of ≈ 17 K results, making the calculated T c ≈ 102 K, which is in reasonable agreement with the experimental results. The superconductivity in YBa 2Cu 3O 7 is, thus, marked by strong coupling effects, electron-phonon interaction, the finite temperature effect and electron-plasmon interaction, which contribute significantly towards its high T c.

Nernst-Ettingshausen coefficient measurements on Al below 1 K.

A system has been constructed for high-sensitivity transport measurements with a dilution re- frigerator in magnetic fields up to 3 T and used to measure the high-field Nernst-Ettingshausen coefficient of Al below 1 K. For magnetic fields up to 2 T, the data are consistent with a simple electron-phonon mass-enhancement factor of 1+\ensuremath{\lambda}. Data above 2 T were not sufficiently reliable to analyze.

Mass Enhancement from Nernst-Ettingshausen Measurement on Al below 1K

Nernst-Ettingshausen coefficient measurements on Al at 0.21 K are reported for magnetic fields up to 3T. For fields up to 2T, the data are consistent with a simple electron-phonon mass-enhancement factor of 1+λ.

Band structure, Fermi surface, superconductivity, and resistivity of actinium under high pressure

The electronic band structures of fcc actinium (Ac) have been calculated for a wide range of pressures by reducing the unit-cell volume from $1.0{V}_{0}$ to $0.5{V}_{0}$ with use of the relativistic augmented-plane-wave method. The density of states and Fermi-surface cross sections corresponding to various volumes are obtained. Calculations for the band-structure-related quantities such as electron-phonon mass enhancement factor $\ensuremath{\lambda}$, superconducting transition temperature ${T}_{c}$, and resistivity $\ensuremath{\rho}$ corresponding to different volumes are performed. It is seen that ${T}_{c}$ increases with pressure, i.e., with decreasing volume. A new empirical relation for the volume dependence of ${T}_{c}$ is proposed and its validity is checked using the ${T}_{c}$ values obtained from the above band-structure results. The resistivity $\ensuremath{\rho}$ first increases with increasing pressure (i.e., with decreasing volume) and then decreases for higher pressures (i.e., for smaller volumes).

Transverse magnetoresistance and thermopower in metals

The transport equation is solved for a quantum-mechanical system in a transverse magnetic field. The solution gives the linear transport coefficients for electrostatic fields and temperature gradients for electrons in metals. In term of a small expansion parameter, the solution to lowest order confirms the existing semiclassical theory on electrical conductivity and thermal conductivity. We also found that the electron-phonon mass enhancement factor is present in the adiabatic thermopower. The next-higher-order approximation exhibits the anisotropic effect caused by the presence of magnetic fields and explains the experimentally observed linear magnetoresistance in simple metals under a high magnetic field.

Low temperature thermal properties of amorphous Mg 0.7Zn 0.3

Measurements of the specific heat (0.09 K ⩽ T ⩽ 1.7 K) and the thermal conductivity (0.5 K ⩽ T ⩽ 20 K) on the metallic glass Mg 0.7Zn 0.3 are reported. The electronic specific heat coefficient ψ can be described by the free electron density of states with an electron-phonon mass enhancement factor 1 + λ = 1.24. Bulk superconductivity is observed below T c = 0.11 K. From T c , λ is calculated to be 0.3 in good agreement with the value deduced from ψ. The small λ value suggests that amorphous Mg 0.7Zn 0.3 is a weak-coupling superconductor. This is supported by the size of the specific-heat jump at T c which is close to the BCS value. The thermal conductivity is mostly due to conduction electrons. The phonon heat transport is limited by scattering from conduction electrons which can be accounted for in the free electron model, and by scattering from low energy excitations (two-level systems) with roughly the same coupling strength to phonons as found in other amorphous metals.

Fermi surfaces and electron-phonon mass enhancement factor in Zr-Nb-Mo solid solutions

The Fermi surfaces for two BCC solid solutions in the ZrcNb1-c alloys have been investigated within the KKR CPA framework. The alloy compositions studied correspond to e/a ratios of 4.8 and 4.4 respectively. In this concentration range a change in the Fermi surface topology should be expected on the grounds of anomalies in the experimental elastic constants for such systems. The calculations do not give clear evidence that such a change does indeed occur. In fact the authors find that the influence of the disorder is more pronounced with respect to the Nb-Mo alloys and the claimed transition from the jungle-gym to the superjungle-gym structure involves heavily damped states. It turns out that such a smoothness is also reflected in the computed electronic contribution eta to the electron-phonon mass enhancement factor as a function of the concentration.

Electrical Resistivity of Transition Metals: APW or TBA

The electron-phonon mass enhancement factor λ and the electrical resistivity of Mo are calculated both by the APW rigid muffin-tin approximation (RMTA) and the tightbinding approximation. The detailed analysis of the results shows that RMTA disregards both the positive contribution from the outside of the inscribed sphere and the negative contribution due to the electron response to the perturbation. The value of λ or the resistivity at high temperatures given by RMTA seems to be fairly close to the true value owing to the fortunate cancellation of the errors.