Effect of impurities scattering potential on NMR relaxation rate in impure d-wave superconductors

The purpose of our research is to study the nuclear spin lattice relaxation rate of impure d-wave superconductors. We use the Green’s function method to derive the approximation equation of density of states including the impurity scattering potential. We can get the analytic equation of the nuclear spin lattice relaxation rate that contained the impurity scattering potential in case of weak scattering potential and strong scattering potential in the simple form as the power series of Δ(T) and T. The numerical calculations show that there is coherence peak in the weak impurity scattering potential but there is no peak in the strong impurity scattering potential.     

Pressure effects on heavy fermion alloys

In this Letter, the effects of pressure on heavy fermion alloys are studied in the framework of the Yoshimori–Kasai model under the coherent potential approximation. A unified picture is presented for both the electron-type heavy fermion systems and the hole-type heavy fermion systems. The density of states of f-electrons under the applied pressure and its variation with the alloy concentration are calculated self-consistently. The slave-boson parameter and the Kondo temperature are also obtained. These calculations show the incorporated results of alloying effects and pressure effects, in agreement with the experiments qualitatively.     

Low-frequency electronic resistivity of crystals due to the scattering from dipoles

The screened potential of an electrical dipole impurity in crystalline solids is obtained in the Debye-Hückel approximation. In the one-band effective-mass approximation and Born approximation of the scattering, the low-frequency transport relaxation time of current-carriers, electrical conductivity and the electronic part of permittivity of crystals are calculated when the scattering mechanism is connected with the randomly distributed screened dipole centres. It is shown that in some cases the scattering from dipole impurity centres may be essential and even more effective than the other scattering mechanisms.     

Electromagnetic response of unconventional superconductors

The magnetic field penetration depth, surface resistance and far infrared reflectivity are calculated for two anisotropic order parameters having either points or lines of nodes in the energy gap. Resonant impurity scattering is taken into account for a wide range of scattering rates. Comparison with experimental results on heavy Fermion superconductors shows that the order parameter cannot be deduced unambiguously from the temperature dependence of the penetration depth. Fits to surface resistance measurements on UBe₁₃ are best in the Born approximation, rather than the unitarity limit. Experiments on high-T _c materials are largely inconclusive, offering little support for the applicability of BCS theory and providing no evidence against novel pairing interactions leading to unusual order parameters.     

Evidence for anisotropic kondo behavior in Ce0.8La0.2Al3

We have performed an inelastic neutron scattering study of the low energy spin dynamics of the heavy fermion compound Ce0.8La0.2Al3 as a function of temperature and external pressure up to 5 kbar. At temperatures below 3 K, the magnetic response transforms from a quasielastic form, common to many heavy fermion systems, to a single well-defined inelastic peak, which is extremely sensitive to external pressure. The scaling of the spin dynamics and the thermodynamic properties are in agreement with the predictions of the anisotropic Kondo model.     

Conductivity of Wigner liquid

The conductivity of two-dimensional electron systems with low carrier concentration is considered on the basis of the previously suggested model (Fermi liquid with a soft mode) under the assumption that the equilibrium in each of the (fermion and boson) subsystems is established faster than the impurity relaxation and the relaxation between the subsystems (hydrodynamic approximation). The conductivity of the system depends on three characteristic times: τ₁(τ₂) is determined by the fermion (boson) impurity scattering and τ₁₂ is determined by the friction between the subsystems; the respective temperature dependences are obtained. The conductivity is related to the relaxation time τ in the usual way, and τ obeys the relationship τ^?1=τ ₁ ^?1 +(τ₂+τ₁₂)^?1. It follows from the results obtained that the resistivity of pure samples should increase with temperature and tend towards saturation.     

The bound one-dimensional “relativistic” polaron in a one-loop approximation

A quasirelativistic model of the narrow-gap semiconductor is considered. The interaction of the Dirac electron field with the non-dispersive phonon field in the short-range external potential (impurity) is taken into account. The classical solutions corresponding to the discrete spectrum are found. The fermion vacuum polarization effect is studied within the one-loop approximation.     

Integral equations for the dynamics of Anderson's magnetic impurity model

Integral equations for the Anderson model at finiteU and arbitrary degeneracyN are set up, which may be viewed as a generalization of the non-crossing approximation in the infiniteU case. In the Kondo (or heavy fermion) limit an analytic treatment is possible near the threshold, where the system is difficult to analyze numerically. Threshold exponents follow from a quartic equation.     

Superconducting virtual bound state alloys

Static and dynamic properties of superconducting alloys containing resonant impurity scattering centers are considered. The formation of bound states within the energy gap is described and connected with locald-level correlations induced by superconductivity. The effect of the bound states on the exactly solubled-spin dynamics is investigated by evaluating the impurity atom's magnetic excitation spectrum. Finite impurity concentrations are treated within a self-consistent approximation scheme. For increasing impurity content the bound states merge to impurity bands which tend to suppress superconductivity. The relevance of the simple extra orbital model is discussed in connection with the interpretation of pressure-induced variations on the properties of superconductors alloyed with ambivalent rare earth ions.     

Free-carrier absorption in a parabolic quantum well with consideration of scattering on ionized impurities

Intra-subband transitions caused by light absorption in a parabolic quantum well is considered taking into account the scattering by ionized impurity centers. To calculate the scattering matrix element, the Born approximation is used and the interaction with the impurity is described by the Coulomb potential. An analytical expression for the absorption coefficient of processes with the initial absorption of photon and further scattering by an ionized impurity center is obtained. For absorption coefficient the frequency characteristics and dependence on the width of quantum well are examined.     

Model calculations for electronic densities of states in heavy-fermion systems

The influence of the lattice on the density of states for conduction-band andf-electrons in heavy fermion and mixed valent systems has been calculated from an extension of the non-crossing approximation to the lattice. It is shown that the main features of such a calculation can be obtained by a simple numerical simulation.     

Spin diffusion and relaxation in dilute magnetic alloys: Electron-electron interaction

Diffusion and relaxation of conduction electrons is studied without making the usual approximation that the spin-orbit scattering be smaller than the impurity potential scattering. Renormalization of impurity vertices by the electron-electron interaction reduces the potential, enhances the s-d, but does not affect the spin-orbit scattering cross-section.     

Two-phonon bound states in imperfect crystals

The question of the occurrence of two-phonon bound states in imperfect crystals is investigated. It is shown that the anharmonicity mediated two-phonon bound state which is present in perfect crystals gets modified due to the presence of impurities. Moreover, the possibility of the occurrence of a purely impurity mediated two-phonon bound state is demonstrated. The bound state frequencies are calculated using the simple Einstein oscillator model for the host phonons. The two-phonon density of states for the imperfect crystal thus obtained has peaks at the combination and difference frequencies of two host phonons besides the peaks at the bound state frequencies. For a perfect crystal the theory predicts a single peak at the two-phonon bound state frequency in conformity with experimental observations and other theoretical calculations. Experimental data on the two-phonon infrared absorption and Raman scattering from mixed crystals of GA_1−c Al _c P and Ge_1−c Si _c are analysed to provide evidence in support of impurity-mediated two-phonon bound states. The relevance of the zero frequency (difference spectrum) peak to the central peak observed in structural phase transitions, is conjectured; This work is a part of the thesis to be submitted by one of the authors (SS) in partial fulfilment of the degree of Doctor of Philosophy to Utkal University, Bhubaneswar, India.     

电子-杂质和电子-声子相互作用对石墨烯磁光导的影响

在外加垂直磁场的石墨烯系统中,基于格林函数方法以自能的形式理论研究了电荷杂质散射和光学声子散射中心对朗道能谱的影响,采用久保(Kubo)公式研究了单层石墨烯的磁光电导谱以及跃迁选择定则。具体计算中电子-杂质库仑相互作用考虑了介电环境的屏蔽效应,对由散射引起的自能以及单粒子格林函数做自洽计算,另外在强磁场下单杂质散射是一个很好的近似模型。理论计算结果表明电荷杂质散射引起朗道能级对称展宽;同时考虑电荷杂质和光学声子两类散射后态密度表现为非对称的展宽。研究结果表明磁光电导谱的峰值和强度强烈依赖于填充因子和态密度。     

Influence of structural disorder on the current-voltage characteristic of a quasi-one-dimensional tunnel junction

The influence of the subbarrier impurity scattering of tunneling electrons on the current-voltage characteristic of a quasi-one-dimensional insulator layer with weak structural disorder (a small impurity concentration) is considered in the one-electron approximation at T=0. An expansion in powers of the impurity concentration gives the form of the current-voltage characteristic and the conditions for small mesoscopic fluctuations of the static tunneling conductance of such a layer in the cases of resonant and nonresonant tunneling. Zh. éksp. Teor. Fiz. 113, 1522–1530 (April 1998)     

Carrier mobility and relaxation time in BiCuSeO

To obtain thermoelectric properties of materials, a constant relaxation time approximation is generally employed. By employing deformation potential theory, a derivation of relaxation time and carrier mobility of BiCuSeO system is proposed combining with density functional theory calculation. And the inter-valley scattering, acoustic phonon scattering and ionized impurity scattering were considered in the model. The calculated values of relaxation time and carrier mobility in BiCuSeO are in good agreement with the results of experiment. The results suggest that acoustic phonon scattering is in dominant and the constant relaxation time approximation is reasonable in lightly doped sample, and the ionized impurity scattering play a significant role in heavily doped system.     

Contribution of the electron-phonon interaction to the broadening of cyclotron resonance line of a two-dimensional electron gas in multiple quantum well structures

Summary The theory of cyclotron resonance (CR) lineshape of a two-dimensional electron gas (2 DEG) due to the electron-phonon interaction in multiple-quantum-well structures (MQWS) is investigated. The contribution of the deformation potential acoustic and piezoelectric phonon scattering to the broadening of the cyclotron resonance spectra (CRSB) of such a system is calculated fro GaAs/AlAs. The piezoelectric phonon scattering contribution to the linewidth is smaller as compared to the deformation potential acoustic phonon scattering but is significantly comparable. The magnetic-field dependence of CRSB due to the deformation potential acoustic and piezoelectric phonons isB ^1/2 andB ^1/4, respectively, and the frequency shift Δ _N,p =0 for both interactions in the elastic-scattering approximation. Observed numerical values of the CRSB indicate that at low temperatures acoustic and piezoelectric phonons are dominant scatterers and interact strongly with 2 DEG in MQWS where the impurity scattering is suppressed due to the modulation doping. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Computer simulation of the electronic structure and chemical bond in the ternary system Ti1−x AlxC

The electron energy structure of the ternary carbide system Ti-Al-C having an NaCl-type lattice is calculated by the local-coherent-potential method within the framework of multiple scattering theory. The cluster version of multiple scattering approximation is used to calculate the crystal potential. The electron energy structures of ternary and binary titanium carbide systems are compared in one approximation. The hybridized band of the ternary carbide system is 2.7 eV broader than that of the binary system, resulting from the formation of a covalent bond between Al and C. Metallic interaction takes place in the ternary system, its fraction increasing with the concentration of the 3p impurity. The energies of the chemical bond in the ternary and binary titanium carbide phases and in diamond are compared, and they are also compared with experiment. Fiz. Tverd. Tela (St. Petersburg) 39, 211–215 (February 1997)     

Electric field effect on the Raman scattering of a hydrogenic impurity in spherical quantum dot

Using the perturbation method and the effective mass approximation, we studied the combined effects of impurity and external electric field on Raman scattering in a spherical quantum dot with a parabolic potential. Based on the computed energies and wave functions, the differential cross-section involved in this process is investigated, and the selection rules are also calculated. Our results suggest that the scattering intensity is strongly affected by the impurity and external electric field considered in this work.     

Focusing of high-energy particles in the electrostatic field of a homogeneously charged sphere and the effective momentum approximation

The impact of the strongly attractive electromagnetic field of heavy nuclei on electrons in quasi-elastic (e, e') scattering is often accounted for by the effective momentum approximation. This method is a plane wave Born approximation which takes the twofold effect of the attractive nucleus on initial- and final-state electrons into account, namely the modification of the electron momentum in the vicinity of the nucleus, and the focusing of electrons towards the nuclear region leading to an enhancement of the corresponding wave function amplitudes. The focusing effect due to the attractive Coulomb field of a homogeneously charged sphere on a classical ensemble of charged particles incident on the field is calculated in the highly relativistic limit and compared to results obtained from exact solutions of the Dirac equation. The result is relevant for the theoretical foundation of the effective momentum approximation and describes the high-energy behavior of the amplitude of continuum Dirac waves in the potential of a homogeneously charged sphere. Our findings indicate that the effective momentum approximation is a useful approximation for the calculation of Coulomb corrections in (e, e') scattering off heavy nuclei for sufficiently high electron energies and momentum transfer.