Impurity Scattering Research Articles

Thermoelectric power of Dirac fermions in graphene

On the basis of self-consistent Born approximation for Dirac fermions under charged impurity scatterings in graphene, we study the thermoelectric power using the heat current-current correlation function. The advantage of the present approach is its ability to effectively treat the low-doping case where the coherence process involving carriers in both upper and lower bands becomes important. We show that the low-temperature behavior of the thermoelectric power as a function of the carrier concentration and the temperature observed by the experiments can be successfully explained by our calculation.

Hall coefficient of Dirac fermions in graphene under charged impurity scatterings

With a conserving formalism within the self-consistent Born approximation, we study the Hall conductivity of Dirac fermions in graphene under charged impurity scatterings. The calculated inverse Hall coefficient is compared with the experimental data. It is shown that the present calculations for the Hall coefficient and the electric conductivity are in good agreement with the experimental measurements.

Magnetoconductivity of Dirac fermions in graphene under charged impurity scatterings

On the basis of the self-consistent Born approximation, we solve the Bethe–Salpeter matrix equations for a Cooperon propagator of Dirac fermions in graphene under charged impurity scatterings and a weak external magnetic field. In the absence of the magnetic field, the quantum interference effect in the electric conductivity from the contribution of the Cooperon propagator is studied and a possible weak localization in the system is discussed in terms of the sample length and temperature. The magnetoconductivity stemming from the quantum interference effect is calculated and the obtained results are in good agreement with experimental measurements.

Effect of Magnetic Impurity Scattering on Superconducting Transition in Low Dimensions: Effect of Long-Time-Tail of Magnetization Diffusion

On the basis of de Gennes' formalism, it is shown that the effect of the long-time-tail in the “force correlation function”, due to the diffusion process of the magnetization of conduction electrons (for the correlation function of the time-reversal operator K ), gives rise to an excess suppression of the superconducting transition temperature T c of conventional superconductors with magnetic impurities. The suppression of T c is prominent in the one-dimensional case, while those in two- and three-dimensional cases are almost invisible for realistic values of parameter for the quantum correction.

Dissipationless spin-Hall current contribution in the extrinsic spin-Hall effect

This paper shows that a substantial amount of dissipationless spin-Hall current contribution may exist in the extrinsic spin-Hall effect, which originates from the spin-orbit coupling induced by the applied external electric field itself that drives the extrinsic spin-Hall effect in a nonmagnetic semiconductor (or metal). By assuming that the impurity density is in a moderate range such that the total scattering potential due to all randomly distributed impurities is a smooth function of the space coordinate, it is shown that this dissipationless contribution shall be of the same orders of magnitude as the usual extrinsic contribution from spin-orbit dependent impurity scatterings (or may even be larger than the latter one). The theoretical results obtained are in good agreement with recent relevant experimental results.

Temperature dependence of microwave-induced magnetoresistance oscillation in two-dimensional electron systems

Microwave-induced magnetoresistance oscillations in two-dimensional electron systems are examined from a photon-assisted transport scheme with short-range impurity scatterings. Analytical results at high filling factors strongly support early considerations and the recent experimental observation that the temperature dependence of the oscillation amplitude is exponential and originates primarily from the single-particle lifetime.

Doping and energy evolution of quasiparticle transport in cuprate superconductors with extended impurities

Within the nodal approximation of the quasiparticle excitations and scattering processes, we study the effect of the extended impurity scatterers on the quasiparticle transport of cuprate superconductors based on the framework of the kinetic energy driven superconducting mechanism. It is shown that there is a cusplike shape of the energy dependent microwave conductivity spectrum. In contrast with the dome shape of the doping dependent superconducting gap parameter, the minimum microwave conductivity occurs around the optimal doping, and then increases in both underdoped and overdoped regimes.

Interband Contributions from the Magnetic Field on Hall Effects for Dirac Electrons in Bismuth

The Hall conductivity sigma_{xy} of Dirac electrons with a spin-orbit interaction is examined. It is shown that there is an unconventional contribution to sigma_{xy} generated by the interband effects of a magnetic field, which is remarkable near the band edges and does not depend on impurity scatterings so much, suggesting the same origin as the known large diamagnetism. Correspondingly, the Hall coefficient exhibits unexpected peaks at around the band edges. Implications of the present results to bismuth alloys are discussed.

Effects of magnetic impurity scattering on superfluid 3He in aerogel

We investigate impurity effects on superfluid 3He in aerogel whose surface is not coated with 4He, different from most experimental situations. In systems with no 4He coating, spins of solid 3He absorbed on the aerogel surface are active and interact with spins of quasiparticles relevant to superfluidity and, for this reason, such an aerogel is treated as magnetic scatterers. It is found that, in the ABM pairing state affected by magnetic scatterings, not only the l-vector but also the d-vector has no long-ranged orientational order, and that the strong-coupling correction due to impurity scatterings is less suppressed than that in the nonmagnetic case, implying an expansion of the A-like phase region.

Multiband Superconductivity in Heavy Fermion Compound CePt3Si without Inversion Symmetry: An NMR Study on a High-Quality Single Crystal

We report on novel superconducting characteristics of the heavy fermion (HF) superconductor CePt 3 Si without inversion symmetry through 195 Pt-NMR study on a single crystal with T c = 0.46 K that is lower than T c ∼0.75 K for polycrystals. We show that the intrinsic superconducting characteristics inherent to CePt 3 Si can be understood in terms of the unconventional strong-coupling state with a line–node gap below T c = 0.46 K. The mystery about the sample dependence of T c is explained by the fact that more or less polycrystals and single crystals inevitably contain some disordered domains, which exhibit a conventional BCS s -wave superconductivity (SC) below 0.8 K. In contrast, the Néel temperature T N ∼2.2 K is present regardless of the quality of samples, revealing that the Fermi surface responsible for SC differ from that for the antiferromagnetic order. These unusual characteristics of CePt 3 Si can be also described by a multiband model; in the homogeneous domains, the coherent HF bands are responsible for the unconventional SC, whereas in the disordered domains the conduction bands existing commonly in LaPt 3 Si may be responsible for the conventional s -wave SC. We remark that some impurity scatterings in the disordered domains break up the 4 f -electrons-derived coherent bands but not others. In this context, the small peak in 1/ T 1 just below T c reported before [Yogi et al. (2004)] is not due to a two-component order parameter composed of spin-singlet and spin-triplet Cooper pairing states, but due to the contamination of the disorder domains which are in the s -wave SC state.

Structural, electrical, and optical properties of InAsxSb1−x epitaxial films grown by liquid-phase epitaxy

The InAsxSb1−x films were grown on (100) GaSb substrates by liquid-phase epitaxy, and their structural, electrical, and optical properties were investigated. The high-resolution x-ray diffraction results reveal that the single crystalline InAsxSb1−x films with a midrange composition are epitaxially grown on the GaSb substrates. Temperature dependence of the Hall mobility was theoretically modeled by considering several predominant scattering mechanisms. The results indicate that ionized impurity and dislocation scatterings dominate at low temperatures, while polar optical phonon scattering is important at room temperature (RT). Furthermore, the InAsxSb1−x films with the higher As composition exhibit the better crystalline quality and the higher mobility. The InAs0.35Sb0.65 film exhibits a Hall mobility of 4.62×104 cm2 V−1 s−1. The cutoff wavelength of photoresponse is extended to about 12 μm with a maximum responsivity of 0.21 V/W at RT, showing great potential for RT long-wavelength infrared detection.

Effects of Nonmagnetic Impurity Scattering in a Ferromagnet with Inhomogeneous Magnetization

Effects of Nonmagnetic Impurity Scattering in a Ferromagnet with Inhomogeneous Magnetization

Doping and energy dependent microwave conductivity of kinetic energy driven superconductors with extended impurities

Within the framework of the kinetic energy driven superconducting mechanism, the effect of the extended impurity scatterers on the quasiparticle transport of cuprate superconductors in the superconducting state is studied based on the nodal approximation of the quasiparticle excitations and scattering processes. It is shown that there is a cusplike shape of the energy dependent microwave conductivity spectrum. At low temperatures, the microwave conductivity increases linearly with increasing temperatures, and reaches a maximum at intermediate temperature, then decreases with increasing temperatures at high temperatures. In contrast with the dome shape of the doping dependent superconducting gap parameter, the minimum microwave conductivity occurs around the optimal doping, and then increases in both underdoped and overdoped regimes.

Single Impurity Scattering in One-Channel Spinless Luttinger Liquid

Using bosonization and phase shift representation, we rigorously treat backward scattering of electrons on an impurity in a one-dimensional interacting electronic system, and demonstrate that correlation exponents of the system depend on a phase shift induced by the backward scattering, and usual exponent duality of the correlation functions between ultraviolet and infrared fixed points comes from the phase shift dependence of the correlation exponents. Finally, we study the tunnelling conductance of the system at zero temperature and obtain a modified Landauer-Bütiker formula.

Measuring dopant concentrations in compensated p-type crystalline silicon via iron-acceptor pairing

We present a method for measuring the concentrations of ionized acceptors and donors in compensated p-type silicon at room temperature. Carrier lifetime measurements on silicon wafers that contain minute traces of iron allow the iron-acceptor pair formation rate to be determined, which in turn allows the acceptor concentration to be calculated. Coupled with an independent measurement of the resistivity and a mobility model that accounts for majority and minority impurity scatterings of charge carriers, it is then possible to also estimate the total concentration of ionized donors. The method is valid for combinations of different acceptor and donor species.

Magnetic-Field Variations of the Pair-Breaking Effects of Superconductivity in (TMTSF)2ClO4

We have studied the onset temperature of the superconductivity Tc_onset of the organic superconductor (TMTSF)2ClO4, by precisely controlling the direction of the magnetic field H. We compare the results of two samples with nearly the same onset temperature but with different scattering relaxation time tau. We revealed a complicated interplay of a variety of pair-breaking effects and mechanisms that overcome these pair-breaking effects. In low fields, the linear temperature dependences of the onset curves in the H-T phase diagrams are governed by the orbital pair-breaking effect. The dips in the in-plane field-angle phi dependence of Tc_onset, which were only observed in the long-tau sample, provides definitive evidence that the field-induced dimensional crossover enhances the superconductivity if the field direction is more than about 19-degrees away from the a axis. In the high-field regime for H//a, the upturn of the onset curve for the long-tau sample indicates a new superconducting state that overcomes the Pauli pair-breaking effect but is easily suppressed by impurity scatterings. The Pauli effect is also overcome for H//b' by a realization of another state for which the maximum of Tc_onset(phi) occurs in a direction different from the crystalline axes. The effect on Tc_onset of tilting the applied field out of the conductive plane suggests that the Pauli effect plays a significant role in determining Tc_onset. The most plausible explanation of these results is that (TMTSF)2ClO4 is a singlet superconductor and exhibits Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in high fields.

Electron mobility in modulation doped AlGaN/GaN and InGaN/GaN quantum wells: A comparative study

The low field mobility characteristics of two dimensional electrons confined to modulation doped AlGaN/GaN and InGaN/GaN quantum wells are studied using ensemble Monte Carlo technique and the results are compared. Acoustic and optical phonon, ionized remote impurity and interface roughness scatterings are considered in mobility calculations. It is found that InGaN/GaN quantum wells have superior mobility characteristics compared to AlGaN/GaN quantum wells with regard to temperature, doping concentration and spacer length.

Electric transport theory of Dirac fermions in graphene

Using the self-consistent Born approximation to the Dirac fermions under finite-range impurity scatterings, we show that the current-current correlation function is determined by four-coupled integral equations. This is very different from the case of impurities with short-range potentials. As a test of the present approach, we calculate the electric conductivity in graphene for charged impurities with screened Coulomb potentials. The obtained conductivity at zero temperature varies linearly with the carrier concentration, and the minimum conductivity at zero doping is larger than the existing theoretical predictions, but still smaller than that of the experimental measurement. The overall behavior of the conductivity obtained by the present calculation at room temperature is similar to that at zero temperature except that the minimum conductivity is slightly larger.

Plasma Instability Noise in InP n+nn+Structures: Monte Carlo Simulation

The plasma instabilities (PI) are promising mechanisms for high frequency microwave generation in semiconductor nnn structures. The two types of PI at near collisionless (NCL) and optical phonon emission dominated (OPED) electron transport conditions in the nnn structures are considered by Ryzhij et al. [1]. They found that both types of PI’s can lead to current self-oscillations up to the plasma frequency, which depends on the carrier concentration in the n-region of the nnn structure (see Ref. [1] and references therein). The regularity of selfoscillations is sensitive to the spatial resonance conditions for space charge waves periodically growing and decaying due to the PI, i.e. there exists the optimum length of n-region at the given doping of this region. In real nnn structures the OPED and especially NCL plasma instabilities are suppressed by the lattice (mainly due to accoustic phonons) and impurity scatterings. However, the progress in manufacturing structures with remote doped conducting channels recently gave the second breath to PI investigations. Also the recent achievements in THz frequency noise measurements have opened new directions in PI investigation. It is well known that the noise spectrum contains integrated information about physical processes such as carrier scattering, various instabilities, including PI, etc. Therefore, the identification of noise spectrum pecularities such as the peaks in the noise spectra related to the certain process is important.

Strong-coupling effects in superfluidHe3in aerogel

Effects of impurity scatterings on the strong-coupling (SC) contribution, stabilizing the ABM (axial) pairing state, to the quartic term of the Ginzburg-Landau free energy of superfluid $^{3}\mathrm{He}$ are theoretically studied to examine recent observations suggestive of an anomalously small SC effect in superfluid $^{3}\mathrm{He}$ in aerogels. To study the SC corrections, two approaches are used. One is based on a perturbation in the short-range repulsive interaction, and the other is a phenomenological approach used previously for the bulk liquid by Sauls and Serene [Phys. Rev. B 24, 183 (1981)]. It is found that the impurity scattering favors the BW pairing state and shrinks the region of the ABM pairing state in the $T\text{\ensuremath{-}}P$ phase diagram. In the phenomenological approach, the resulting shrinkage of the ABM region is especially substantial and, if assuming an anisotropy over a large scale in aerogel, leads to justifying the phase diagrams determined experimentally.