Impurity Strength Research Articles

Thermal conductivity of disordered AA-stacked bilayer graphene in the presence of bias voltage

We study thermal conductivity of biased bilayer graphene doped with acceptor impurity atoms for AA-stacking in the context of tight binding model Hamiltonian. The effect of scattering by dilute charged impurities is discussed in terms of the self-consistent Born approximation. Green’s function approach has been implemented to find the behavior of thermal conductivity of carbon nanotubes within linear response theory. We have found the temperature dependence of thermal conductivity for different values of concentration and scattering strength of dopant impurity. Also the dependence of thermal conductivity on the chemical potential and bias voltage has been investigated in details. A peak appears in the plot of thermal conductivity versus chemical potential. Furthermore we find thermal transport rises with temperature for any impurity concentration and scattering strength.

Oscillator strength of impurity doped quantum dots: Influence of Gaussian white noise

We make a rigorous analysis of profiles of oscillator strength of a doped quantum dot in the presence and absence of noise. The noise employed here is a Gaussian white noise. The quantum dot is doped with repulsive Gaussian impurity. Noise has been administered additively and multiplicatively to the system. A perpendicular magnetic field is also present and a static external electric field has been applied. Profile of OS has been minutely monitored with variation of several important quantities such as confinement energy, electric field strength, dopant location, magnetic field strength, dopant potential, noise strength, Al concentration, and mode of application of noise. The profiles are enriched with significant subtleties and often reveal enhancement and maximization of oscillator strength in the presence of noise. These observations are indeed useful in the study of linear and nonlinear optical properties of doped QD systems which bear sufficient technological importance.

Symmetry-protected topological invariant and Majorana impurity states in time-reversal-invariant superconductors

We address the question of whether individual nonmagnetic impurities can induce zero-energy states in time-reversal-invariant topological superconductors, and define a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength. These symmetries allow the definition of a position-space topological Z_2 invariant, which is related to the standard bulk topological Z_2 invariant. Our general results are applied to the time-reversal-invariant p-wave phase of the doped Kitaev-Heisenberg model, where we demonstrate how a lattice of impurities can drive a topologically trivial system into the nontrivial phase.

Zeroth order phase transition in a holographic superconductor with single impurity

We investigate the single normal impurity effect in a superconductor by the holographic method. When the size of impurity is much smaller than the host superconductor, we can reproduce the Anderson theorem, which states that a conventional s-wave superconductor is robust to a normal (non-magnetic) impurity with small impurity strength. However, by increasing the size of the impurity in a fixed-size host superconductor, we find a decreasing critical temperature Tc of the host superconductor, which agrees with the results in condensed matter literatures. More importantly, the phase transition at the critical impurity strength (or the critical temperature) is of zeroth order.

Topological properties of helical Shiba chains with general impurity strength and hybridization

Recent experiments announced an observation of topological superconductivity and Majorana quasiparticles in Shiba chains, consisting of an array of magnetic atoms deposited on top of a superconductor. In this work we study helical Shiba chains and generalize the microscopic theory of subgap energy bands to a regime where the decoupled magnetic impurity energy and the hybridization of different impurity states can be significant compared to the superconducting gap of the host material. From exact solutions of the Bogoliubov-de Gennes equation we extract expressions for the topological phase boundaries for arbitrary values of the superconducting coherence length. The subgap spectral problem can be formulated as a nonlinear matrix eigenvalue problem from which we obtain an analytical solution for energy bands in the long coherence length limit. Physical consequences and departures from the previously obtained results in the deep-dilute impurity limit are discussed in detail.

Quantum transport in three-dimensional Weyl electron system in the presence of charged impurity scattering

We theoretically study the quantum transport in three-dimensional Weyl electron system in the presence of the charged impurity scattering using a self-consistent Born approximation (SCBA). The scattering strength is characterized by the effective fine structure constant $\alpha$, which depends on the dielectric constant and the Fermi velocity of the linear band. We find that the Boltzmann theory fails at the band touching point, where the conductivity takes a nearly constant value almost independent of $\alpha$, even though the density of states linearly increases with $\alpha$. There the magnitude of the conductivity only depends on the impurity density. The qualitative behavior is quite different from the case of the Gaussian impurities, where the minimum conductivity vanishes below a certain critical impurity strength.

Local Magnetization Nucleated by Non-magnetic Impurities in Fe-based Superconductors

We study impurity-induced magnetic order within a five-band Hubbard model relevant to the normal paramagnetic phase of iron-based superconductors. The existence of the local magnetic order is explained in terms of an impurity-enhancement of states near the Fermi level, and we map out the resulting phase diagram of the existence of magnetization as a function of impurity strength and Coulomb correlations. In particular, the presence of impurity-induced magnetism in only a certain range of potential scattering strengths can be understood from the specific behavior of the impurity resonant state.

Repetitive ‘Snakes’ and Their Damping Effect on Core Toroidal Rotation in EAST Plasmas with Multiple H—L—H Transitions

Repetitive impurity snake modes are observed after H—L mode transitions (high to low confinement modes) in EAST plasmas exhibiting multiple H—L—H transitions. Such snake modes are observed to lower the core plasma toroidal rotation. A critical impurity strength factor associated with snake-mode formation is estimated to be as high as αZ,c = nZ,cZ2/ne ∼ 0.75. These observations have implications for ITER H-mode sustainability when the heating power is only slightly above the H-mode power threshold.

Majorana modes in a topological insulator/s-wave superconductor heterostructure.

In a recent experiment, signatures of Majorana fermion (MF) were found in the vortex core threading a heterostructure composed of n layers of topological insulator (TI) deposited on a bulk s-wave superconductor. Here we provide strong theoretical support to the experiment. First, we demonstrate that MF modes appear on both top and bottom layers of TI, and are well separated for n ~ 6. The top MF becomes more extended with increasing n, in agrement with the experiment. Second, we show both analytically and numerically that right at the vortex core the MF mode is always accompanied by another low energy bound state, leading to a zero-bias peak plus a side peak in the local density of states (LDOS) therein. However, a local scalar impurity at the core can wipe out the accompanying side-peak state while leaving the zero-energy MF mode intact. Consequently the LDOS becomes symmetric about the fermi level, and the peak does not branch near the vortex core, in agreement with the experiment. Finally but unfortunately, while the MF is extremely stable against a single local impurity, the stability in terms of the critical impurity strength is reduced drastically for a moderate concentration (e.g., 10%) of impurities.

Density of states of interacting quantum wires with impurities: A Dyson equation approach

We calculate the density of states for an interacting quantum wire in the presence two impurities of arbitrary potential strength. To perform this calculation, we describe the Coulomb interactions in the wire within the Tomonaga-Luttinger liquid theory. After establishing and solving the Dyson equation for the fermionic retarded Green's functions, we study how the profile of the local density of states is affected by the interactions in the entire range of impurity potentials. Same as in the non-interacting case, when increasing the impurity strength, the central part of the wire becomes more and more disconnected from the semi-infinite leads, and discrete localized states begin to form; the width of the corresponding peaks in the spectrum depends on the interaction strength. As expected from the Luttinger liquid theory, impurities also induce a reduction of the local density of states at small energies. Two other important aspects are highlighted: the appearance of an extra-modulation in the density of states at non-zero Fermi momentum when interactions are present, and the fact that forward scattering must be taken into account in order to recover the Coulomb blockade regime for strong impurities.

Simultaneous determination of the impurity and radial tensile strength of reduced glutathione tablets by a high selective NIR-PLS method

This paper establishes a high-throughput and high selective method to determine the impurity named oxidized glutathione (GSSG) and radial tensile strength (RTS) of reduced glutathione (GSH) tablets based on near infrared (NIR) spectroscopy and partial least squares (PLS). In order to build and evaluate the calibration models, the NIR diffuse reflectance spectra (DRS) and transmittance spectra (TS) for 330 GSH tablets were accurately measured by using the optimized parameter values. For analyzing GSSG or RTS of GSH tablets, the NIR-DRS or NIR-TS were selected, subdivided reasonably into calibration and prediction sets, and processed appropriately with chemometric techniques. After selecting spectral sub-ranges and neglecting spectrum outliers, the PLS calibration models were built and the factor numbers were optimized. Then, the PLS models were evaluated by the root mean square errors of calibration (RMSEC), cross-validation (RMSECV) and prediction (RMSEP), and by the correlation coefficients of calibration (Rc) and prediction (Rp). The results indicate that the proposed models have good performances. It is thus clear that the NIR-PLS can simultaneously, selectively, nondestructively and rapidly analyze the GSSG and RTS of GSH tablets, although the contents of GSSG impurity were quite low while those of GSH active pharmaceutical ingredient (API) quite high. This strategy can be an important complement to the common NIR methods used in the on-line analysis of API in pharmaceutical preparations. And this work expands the NIR applications in the high-throughput and extraordinarily selective analysis.

Impurity scattering in Weyl semimetals and their stability classification

Weyl semimetals (WS) are a new class of Dirac-type materials exhibiting a phase with bulk energy nodes and an associated vanishing density of states (DOS). We investigate the stability of this nodal DOS suppression in the presence of local impurities and consider whether or not such a suppression can be lifted by impurity-induced resonances. We find that while a scalar (chemical potential type) impurity can always induce a resonance at arbitrary energy and hence lift the DOS suppression at Dirac/Weyl nodes, for many other impurity types (e.g. magnetic or orbital mixing), resonances are forbidden in a wide range of energy. We investigate a four-band tight-binding model of WS adapted from a physical heterostructure construction due to Burkov et al (2011 Phys. Rev. B 84 235126), and represent a local impurity potential by a strength g as well as a matrix structure Λ. A general framework is developed to analyze this resonance dichotomy and make connection with the phase shift picture in scattering theory, as well as to determine the relation between resonance energy and impurity strength g. A complete classification of impurities based on Λ, based on their effect on nodal DOS suppression, is tabulated. We also discuss the differences between continuum and lattice approaches.

Quantum Phase Slips of Trapped Superfluid Bose Gases in One Dimension

We discuss transport of trapped one-dimensional superfluids through a single impurity potential, in connection with quantum phase-slip nucleation rate Γ. We specifically consider damping of dipole oscillations induced by sudden displacement of the trapping potential, which has been investigated in previous experiments. Applying the time-evolving block decimation method to the 1D Bose-Hubbard model with an impurity potential in the hardcore limit, we calculate the dynamics of dipole oscillations and extract the damping rate from the oscillations. We show that there is a broad parameter region in which the damping rate G of the oscillation obeys the formula G∝Γ/v∝v2K−2 with the Tomonaga-Luttinger parameter K, regardless of whether the impurity potential is repulsive or attractive. We find that in that parameter region the damping rate is almost symmetric with respect to the change of the sign of the impurity strength.

Pseudo-Spin–Orbit Coupling in Graphene within Hydrogenic Impurity Context

In graphene, we report the first theoretical demonstration of how the intrinsic spin orbit interaction can be deduced from the theory and how it can be controlled by tuning a uniform magnetic field, and/or by changing the strength of a long range Coulomb like impurity (adatom), as well as gap parameter. In the impurity context, we find that intrinsic spin-orbit interaction energy may be enhanced by increasing the strength of magnetic field and/or by decreasing the band gap mass term. Additionally, it may be strongly enhanced by increasing the impurity strength. Furthermore, from the proposal of Kane and Mele [Phys. Rev. Lett. 95, 226801 (2005)], it was discussed that the pristine graphene has a quantized spin Hall effect regime where the Rashba type spin orbit interaction term is smaller than that of intrinsic one. Our analysis suggest the nonexistence of such a regime in the ground state of flat graphene.

Laser assisted bound-free transition of a polaron bound to an impurity center

The scattering of a bound Frohlich large polaron in the presence of a coulomb impurity center as well as a linearly polarized laser field is studied. The differential (DCS) as well as the momentum transfer cross sections (MTCS) and the conductivity depend remarkably on the electron–phonon coupling (αP) and the strength of the coulomb impurity (β). The strongly bound polaron is found to be reluctant to contribute to the conductivity which is justified physically. The curves for MTCS and conductivity verses αP and β appear to be complementary, indicating the possibility of controlling the mobility and thereby conductivity by varying one of the parameters. This behavior indicates the possibility of optimizing the composition of optoelectronic materials where laser is the dominant source of excitation.

Josephson junction on one edge of a two dimensional topological insulator affected by magnetic impurity

The current–phase relation in a Josephson junction formed by putting two s-wave superconductors on the same edge of a two dimensional topological insulator is investigated. We consider the case in which the junction length is finite and magnetic impurity exists. The similarities and differences with respect to a conventional Josephson junction are discussed. Both the 2π- and 4π-period current–phase relations (I2π(ϕ),I4π(ϕ)) are studied. There is a sharp jump at ϕ = π and ϕ = 2π for I2π and I4π, respectively, in the clean junction. For I2π, the sharp jump is robust against the impurity strength and distribution. However, for I4π, an impurity makes the jump at ϕ = 2π smooth. The critical (maximum) current Ic,2π of I2π is given and we find it will be increased by an asymmetrical distribution of the impurity.

Stability of Weyl metals under impurity scattering

We investigate the effects of bulk impurities on the electronic spectrum of Weyl semimetals, a recently identified class of Dirac-type materials. Using a $T$-matrix approach, we study resonant scattering due to a localized impurity in tight binding versions of the continuum models recently discussed by Burkov, Hook, and Balents, describing perturbed four-component Dirac fermions in the vicinity of a critical point. The impurity potential is described by a strength $g$ as well as a matrix structure $\Lambda$. Unlike the case in d-wave superconductors, where a zero energy resonance can always be induced by varying the impurity scalar and/or magnetic impurity strength, we find that for certain types of impurity ($\Lambda$), the Weyl node is protected, and that a scalar impurity will induce an intragap resonance over a wide range of scattering strength. A general framework is developed to address this question, as well as to determine the dependence of resonance energy on the impurity strength.

KLEIN TUNNELING IN THE PRESENCE OF RANDOM IMPURITIES

In this paper, we study Klein tunneling in random media. To this purpose, we simulate the propagation of a relativistic Gaussian wave packet through a disordered medium with randomly distributed potential barriers (impurities). The simulations, based on a relativistic quantum lattice Boltzmann (QLB) method, permit to compute the transmission coefficient across the sample, thereby providing an estimate for the conductivity (or permeability) as a function of impurity concentration and strength of the potentials. It is found that the conductivity loss due to impurities is significantly higher for wave packets of massive particles, as compared to massless ones. A general expression for the loss of conductivity as a function of the impurity percentage is presented and successfully compared with the Kozeny–Carman law for disordered media in classical fluid-dynamics.

Impurity-induced bound states in superconductors with topological order

The study of classical spins in topological insulators (Liu and Ma 2009 Phys. Rev. B 80 115216) is generalized to topological superconductors. Based on the characteristic features of the so-called F-function, the Bogoliubov–de Gennes Hamiltonian for superconductors is classified to positive, negative, and zero ‘gap’ categories for topologically trivial and nontrivial phases of a topological superconductor as well as a BCS superconductor, respectively. It is found that the F-function determines directly the presence or absence of localized excited states, induced by bulk classical spins and nonmagnetic impurities, in the superconducting gap and their persistence with respect to impurity strength. Our results provide an alternative way to identify topologically insulating and superconducting phases in experiments without resorting to the surface properties.

Subsurface impurities and vacancies in a three-dimensional topological insulator

Using a three-dimensional microscopic lattice model of a strong topological insulator (TI) we study potential impurities and vacancies in surface, subsurface, and bulk positions. For all impurity locations we find impurity-induced resonance states with energy proportional to the inverse of the impurity strength, although the impurity strength needed for a low-energy resonance state increases with the depth of the impurity. For strong impurities and vacancies as deep as 15 layers into the material, resonance peaks will appear at and around the Dirac point in the surface energy spectrum, splitting the original Dirac point into two nodes located off-center. Furthermore, we study vacancy clusters buried deep inside the bulk and find zero-energy resonance states for both single and multiple-site vacancies. Only fully symmetric multiple-site vacancy clusters show resonance states expelled from the bulk gap.