The onset and the pinning of charge density wave in a quasi one-dimensional conductor

he recently proposed idea by the authors about the onset and the pinning of charge density wave around an impurity is applied to the case of a quasi one-dimensional conductor. The process is described in terns of the local Fermi wave- number and the local amplitude of charge density wave.     

Vacuum polarization and screening of supercritical impurities in graphene

Screening of charge impurities in graphene is analyzed using the exact solution for vacuum polarization obtained from the massless Dirac-Kepler problem. For the impurity charge below a certain critical value, no density perturbation is found away from the impurity, in agreement with perturbation theory. For the supercritical charge, however, the polarization distribution is shown to have a power law profile, leading to screening of the excess charge at large distances. The Dirac-Kepler scattering states give rise to standing wave oscillations in the local density of states which are prominent in the supercritical regime.     

Local density of states of one-dimensional Mott insulators and charge-density wave states with a boundary

We determine the local density of states of one-dimensional incommensurate charge-density wave states in the presence of a strong impurity potential, which is modeled by a boundary. We find that the charge-density wave gets pinned at the impurity, which results in a singularity in the Fourier transform of the local density of states at momentum 2k_{F}. At energies above the spin gap we observe dispersing features associated with the spin and charge degrees of freedom, respectively. In the presence of an impurity magnetic field we observe the formation of a bound state localized at the impurity. All of our results carry over to the case of 1D Mott insulators by exchanging the roles of spin and charge degrees of freedom. We discuss the implications of our result for scanning tunneling microscopy experiments on spin-gap systems such as two-leg ladder cuprates.     

Charge Transfer in the Lattice with an Impurity Site. Reflection and Transmission of the Wave Packet

The quantum dynamics of the propagation of the charge wave function in a uniform lattice containing a single impurity site is considered. A nonstationary problem is solved in the tight-binding approximation. The initial state is the wave function fully localized at one of the lattice sites. The coefficients of transmission of the wave packet through the impurity site and reflections from it are calculated as a function of the parameters of the problem, that is, the additional energy on the impurity and the distance between the impurity and the initial position of the charge. The problem is solved for two types of boundary conditions: an infinite and a semi-infinite lattice. Good agreement with numerical simulation is obtained.     

Local impurity phase pinning and pinning force in charge density waves

Starting from the static Fukuyama-Lee-Rice equation for a three-dimensional incommensurate charge density wave (CDW) in quasi one-dimensional conductors a solvable model for local phase pinning by impurities is defined and studied. We find that average CDW energy and average pinning force show critical behaviour with respect to the pinning parameter h. Specifically the pinning force exhibits a threshold at h=1 with exponent . Our model exemplifies a general concept of local impurity pinning in which the force exerted by the impurity on the periodic CDW structure becomes multivalued and metastable states appear beyond a threshold. It is found that local impurity pinning becomes less effective at low temperatures and may eventually cease completely. These results are independent of spatial dimensionality as expected for local impurity pinning. Comparison with Larkin's model is also made. Received 8 July 1998     

Imaging bond order near nonmagnetic impurities in square-lattice antiferromagnets

We study the textures of generalized "charge densities" (scalar objects invariant under time reversal), in the vicinity of nonmagnetic impurities in square-lattice quantum antiferromagnets, by order parameter field theories. Our central finding is the structure of the vortex in the generalized density wave order parameter centered at the nonmagnetic impurity. Using exact numerical data from quantum Monte Carlo simulations on an antiferromagnetic spin model, we are able to verify the results of our field theoretic study. We extend our phenomenological approach to the period-4 bond-centered density wave found in the underdoped cuprates.     

Impurity pinning studies in Nb1−xTaxSe3 alloys

We have measured the nonlinear conductivity of Nb_1−xTa_xSe₃ alloys in both charge density wave states. The threshold field E_T for the onset of nonlinear conduction increases linearly with x. A large scatter for relatively pure samples suggests other pinning mechanisms, such as grain boundaries, for small impurity concentrations.     

Theory of magnetic impurities in superconductors. III

The structure of the bound states in superconductors with magnetic impurities is investigated in detail following earlier work. It is shown that there is exactly one bound state per magnetic impurity with a wave function which decays exponentially on a characteristic scale. However, there is no accumulation of charge around the impurity as the bound state charge distribution is compensated by the contribution from band states apart from Ruderman-Kittel oscillations.     

Charge transfer in binary alloys and to impurities in iron

The energy density formalism in the Thomas-Fermi-Dirac approximation was used to study charge transfer in equiatomic binary ordered alloys and in dilute iron alloys. This method was applied outside of a core region around each nucleus; the inner region was treated by taking the electron density to be that given by free atom wave functions.The results for the equivolume charge transfer in binary alloys agreed satisfactorily with existing KKR-selfconsislent field calculations.For metalloid impurities in iron, the charge transfer to the impurity, and the variation of lattice parameter with concentration was computed. The latter compared fairly well with experiments.The theory explicitly displays the role of the electron density at the cell boundary in alloy formation and charge transfer effects. Alloy formation is visualized as a process in which atomic cells of the pure metals are expanded or compressed to the equilibrium cell volume of the alloy, bringing the cells together to form the alloy, and then allowing the electron distribution to relax.The general conclusion of this work is that modifying the TFD theory by using it only in regions where the electron density does not vary too rapidly, and using quantum theory results for the density close to the nuclei. provides a satisfactory method for studying charge transfer.     

Thermopower of doped and damaged NbSe3

We have measured the thermoelectric power of pure NbSe₃ as well as samples which have been substitutionally doped with isoelectronic Ta and the charged impurity Ti and separate samples which have been radiation damaged by 2.5 MeV protons. We find that 5% Ta doping supresses the lower temperature charge density wave transition. In contrast, the radiation damaged samples and 0.1% Ti samples with larger residual resistivities then the Ta doped samples retain the CDW transitions. A discussion is given of the difference between doping and radiation damage.     

Electronic structure of two-electron quantum dot with parabolic potential

In this study, we investigate the parabolic potential effects on the ground and excited energy states of two-electron quantum dot with impurity inside an infinite spherical confining potential well. The wave function and energy eigenvalues were calculated using a modified variational optimization procedure based mainly on quantum genetic algorithm and Hartree–Fock–Roothaan method. The results show that the parabolic potential and impurity charge have a strong effect on the energy states and ionization energies. It is worth pointing out that as impurity charge increases, the ionization energy rises, but the ionization dot radius decreases. On the other hand, as parabolic potential increases, the ionization energy decreases, but the ionization dot radius increases.     

Transition metal impurity effect on charge and spin density in iron: Ab initio calculations and comparison with Mössbauer data

Density functional theory was applied to study influence of the isolated impurity located on the regular site of the α-Fe crystal on the charge and spin density (hyperfine interactions) on the iron nucleus. Calculations were performed using both pseudopotential and the full potential methods. The scalar relativistic approximation was applied. Perturbations of the charge and spin density on iron were calculated for all d impurities soluble in iron and additionally for Ga impurity. It was found that impurities have measurable effect on the iron charge and spin density up to the second or third coordination shell depending on the impurity. Hyperfine parameters of iron adjacent to the impurity are affected by two intermixed physical mechanisms, i.e., the volume mismatch due to the impurity and electron band mixing caused by the electronic configuration of the impurity outer shells. Some correlations between ab initio calculations and Mössbauer experimental results are discussed. A table is provided with the parameters allowing calculate Mössbauer spectrum of the binary iron alloy with d impurity or Ga. On the other hand, provided parameters allow extraction from the Mössbauer data information about impurity concentration and eventual order.     

Charge density waves in a relaxation semiconductor

The response of a relaxation inhomogeneous semiconductor (specimen with an injecting contact and/or an inversion layer) containing an impurity with positive correlation energy, to a sudden increase in the electrical field and/or bias lighting is investigated. Under definite conditions the charge-carrier plasma turns out to be unstable relative to charge density wave excitation; their frequencies and damping decrements are found. Under the influence of waves of spatial overcharging of centers with positive correlation energy an increase is possible in the imaginary part of the impedance, as is the appearance of an oscillating frequency dependence of the imaginary and real parts of the impedance. The time dependence of the response is examined, and both exponential and power-law damping is possible depending on the kind of charge carrier forming the wave.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 54–62, June, 1987.     

Sb掺杂对透明SnO_2薄膜导电性能影响的第一性原理计算(英文)

本文根据密度泛函理论(DFT),采用第一性原理平面波赝势方法,计算了Sb掺杂对透明导电薄膜SnO2电子结构及导电性能的影响,讨论了掺杂下SnO2晶体的结构变化、能带结构、电子态密度.计算结果表明,Sb掺杂的SnO2具有高的电导率,且随着掺杂浓度的增加,能带简并化加剧,浅施主杂质能级向远离导带底方向移动.     

Anisotropy of impurity scattering and electrical conductivity in quasi-two-dimensional electronic systems

Scattering of charge carriers by impurity ions in quasi-two-dimensional electronic systems is investigated. The dependences of the longitudinal and transverse components of the relaxation time on the wave vector components and the screening length are analyzed. The anisotropy of electrical conductivity of a degenerate quasi-two-dimensional electron gas is studied as a function of the ratio between the energy at the Fermi level and the miniband width, the impurity concentration, and the lattice constant.     

一维自旋1/2电子系统的单杂质散射研究

应用高斯波泛函方法，研究了单个局域杂质对一维自旋1/2传导电子的散射问题．研究结果表明，双粒子散射与单粒子散射具有同样的重要性．由于这两种散射机制相互竞争，形成了拉亭格液体中电荷密度波和自旋密度波的各自的重整化质量．随着这些质量的消失，在参量空间中出现了相界，在不同相区中系统呈现不同的动力学行为 关键词：     

Specific features of the resonance interaction of a coherent wave with an ensemble of impurity atoms of a photonic crystal

Two-quantum relaxation transitions in an impurity atom that are activated by its resonance interaction with a monochromatic wave lead to hole burning in the distribution of atoms populating the lower energy level over detunings from the resonance. In this case, the levels of an impurity atom lying within the gap or the pseudogap in the density of photon states of a photonic crystal act as a trap. The impurity atoms change to these levels in the process of interaction of a monochromatic wave with the atomic transition that is not influenced by specific spectral features of a photonic crystal.     

Phonon drag conductivity in the Fröhlich-Peierls phase in quasi-one-dimensional electron phonon systems with impurities

The effect of impurities on the static conductivity in the Fröhlich-Peierls phase is discussed by using the technique of the temperature Green function. The diagrams of the phonon drag type especially are shown to give the most important contribution to the static conductivity at low temperatures. The temperature dependence of this contribution is of the activation type with an activation energy equal to the pinning frequency of the phase mode of the charge density wave. The dependence of the preexponential factor on the impurity concentrations is also discussed.On leave of absence     

金属/掺杂聚合物/金属结构中载流子的注入与输运

基于紧束缚的Su-Schrieffer-Heeger模型,利用非绝热的动力学方法,研究了金属/聚合物/金属三明治结构中载流子注入与输运的动力学过程,其中,聚合物中含有一个掺杂离子. 电子波函数随时间的演化满足含时Schrdinger方程,晶格运动满足经典的牛顿运动方程. 研究发现,掺杂离子对注入到聚合物中的载流子的作用类似一个控制阀门,阀门的状态与电场的强度和偏压的大小密切相关. 关键词： 载流子输运 波包 非绝热动力学     

Intraminiband absorption of a nonlinear electromagnetic wave in a semiconductor superlattice

The intraminiband absorption of a nonlinear electromagnetic wave is investigated taking into account the fact that impurity absorption occurs simultaneously. The impurity absorption leads to an increase in the concentration of charge carriers in the conduction miniband, which, in turn, changes the shape of the electromagnetic wave. Thus, this problem is self-consistent. The dependences of the intraminiband absorption coefficient of a cnoidal electromagnetic wave on the electric field strength of the wave taking into account the ionization processes and disregarding them are found numerically. It is shown that taking into account the ionization processes leads to the expansion of the strong absorption region to higher fields.