Impurity Systems Research Articles

Crossover physics in the nonequilibrium dynamics of quenched quantum impurity systems.

A general framework is proposed to tackle analytically local quantum quenches in integrable impurity systems, combining a mapping onto a boundary problem with the form factor approach to boundary-condition-changing operators introduced by Lesage and Saleur [Phys. Rev. Lett. 80, 4370 (1998)]. We discuss how to compute exactly the following two central quantities of interest: the Loschmidt echo and the distribution of the work done during the quantum quench. Our results display an interesting crossover physics characterized by the energy scale T(b) of the impurity corresponding to the Kondo temperature. We discuss in detail the noninteracting case as a paradigm and benchmark for more complicated integrable impurity models and check our results using numerical methods.

Electron-electron correlations in a dynamical impurity system with a Fermi edge singularity

We study spatial correlations in the ground state of a one-dimensional electron gas coupled to a dynamic quantum impurity. The system displays a nontrivial many-body effect known as the Fermi edge singularity: transitions between discrete internal states of the impurity have a power-law dependence on the internal energies of the impurity states. We present compact formulas for the static current-current correlator and the pair-correlation function. These reveal that spatial correlations induced by the impurity decay slowly (as the third inverse power of distance) and have a power-law energy dependence, characteristic of the Fermi edge singularity.

Influence of alloying elements on phase stability and elastic properties of aluminum and magnesium studied by first principles

Influence mechanisms of alloying elements Al, Mg, Ti, V, Cr, Fe, Ni, Cu, Zr, Nb, Mo, Sn, Ta, and W on phase stability and elastic properties of magnesium and aluminum were studied by first principles total energy calculations. Calculations show that all systems with impurity satisfy the mechanical stability criteria. However, only Sn could enhance the stability of Mg, but decreases the stability of Al energetically. Mg and Cr also show the same effect as Sn on Al. The electronic structure analysis illustrates that an extra bonding peak appears in the low energy region in Mg–Sn, which increases the stability of Mg. While other alloying elements slightly move the Fermi energy level toward the conduction band causing a decrease on stability of Mg. In Al with impurity systems, the alloying elements except Sn, Mg, and Cr greatly increase the number of bonding peaks in low energy regions and consequently increase the stability of the systems.

Interference related corrections to classical gas diffusion in a system of small heterogeneities

Diffusion of an ideal gas in a system of small spherical heterogeneities (collective of particles and a system of small cavities in homogeneous medium) is considered. The effective diffusion coefficient is calculated using methods of multiple scattering theory. Some possible contribution to diffusion of multiple passing closed loops on the trajectory of a molecule is considered. The interest to such loops is related to constructive interference of amplitudes corresponding to two alternative ways of the loop passing (clockwise and counterclockwise). This interference always exists, irrespective to the medium disorder. In this paper, we show that the interference corrections to classical diffusion lead to appearance of low frequency macroscopic oscillations of the gas concentration and complete stagnation of the diffusion process. The latter resembles the Anderson localization of electrons in a system of impurities in an ideal lattice.

Hybrid NRG-DMRG approach to real-time dynamics of quantum impurity systems

A hybrid approach to nonequilibrium dynamics of quantum impurity systems is presented. The numerical renormalization group serves as a means to generate a suitable low-energy Hamiltonian, allowing for an accurate evaluation of the real-time dynamics of the problem up to exponentially long times using primarily the time-adaptive density-matrix renormalization group. We extract the decay time of the interaction-enhanced oscillations in the interacting resonant-level model and show their quadratic divergence with the interaction strength U. Our numerical analysis is in excellent agreement with analytic predictions based on an expansion in 1/U.

Hierarchical Liouville-Space Approach for Accurate and Universal Characterization of Quantum Impurity Systems

A hierarchical equations of motion based numerical approach is developed for accurate and efficient evaluation of dynamical observables of strongly correlated quantum impurity systems. This approach is capable of describing quantitatively Kondo resonance and Fermi-liquid characteristics, achieving the accuracy of the latest high-level numerical renormalization group approach, as demonstrated on single-impurity Anderson model systems. Its application to a two-impurity Anderson model results in differential conductance versus external bias, which correctly reproduces the continuous transition from Kondo states of individual impurity to singlet spin states formed between two impurities. The outstanding performance on characterizing both equilibrium and nonequilibrium properties of quantum impurity systems makes the hierarchical equations of motion approach potentially useful for addressing strongly correlated lattice systems in the framework of dynamical mean-field theory.

Mapping Dirac quasiparticles near a single Coulomb impurity on graphene

The response of Dirac fermions to a Coulomb potential is predicted to differ significantly from the behavior of non-relativistic electrons seen in traditional atomic and impurity systems. Surprisingly, many key theoretical predictions for this ultra-relativistic regime have yet to be tested in a laboratory. Graphene, a 2D material in which electrons behave like massless Dirac fermions, provides a unique opportunity to experimentally test such predictions. The response of Dirac fermions to a Coulomb potential in graphene is central to a wide range of electronic phenomena and can serve as a sensitive probe of graphene's intrinsic dielectric constant, the primary factor determining the strength of electron-electron interactions in this material. Here we present a direct measurement of the nanoscale response of Dirac fermions to a single Coulomb potential placed on a gated graphene device. Scanning tunneling microscopy and spectroscopy were used to fabricate tunable charge impurities on graphene and to measure how they are screened by Dirac fermions for a Q = +1|e| impurity charge state. Electron-like and hole-like Dirac fermions were observed to respond very differently to tunable Coulomb potentials. Comparison of this electron-hole asymmetry to theoretical simulations has allowed us to test basic predictions for the behavior of Dirac fermions near a Coulomb potential and to extract the intrinsic dielectric constant of graphene: {\epsilon}_g= 3.0 \pm 1.0. This small value of {\epsilon}_g indicates that microscopic electron-electron interactions can contribute significantly to graphene properties.

Repulsive polarons found

Quasiparticles known as repulsive polarons are predicted to occur when 'impurity' fermionic particles interact repulsively with a fermionic environment. They have now been detected in two widely differing systems. See Letters p.615 & p.619 Metastable states in Fermi gases with strong repulsive interactions are of fundamental interest, but the realization of such systems is challenging because they are intrinsically unstable against decay. Two groups have overcome this obstacle and report the detection of the theoretically predicted repulsive Fermi polarons. Kohstall et al. study a three-dimensional system of ultracold potassium impurities resonantly interacting with a Fermi sea of lithium atoms. The character of the interaction stabilizes the repulsive regime, enabling the authors to detect long-lived, metastable repulsive polarons. Koschorreck et al. study both attractive and repulsive Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas of potassium atoms, and find evidence for a pairing transition. The results from these two studies hold promise for the creation of exotic states with ultracold fermionic atoms, such as ferromagnetic quantum phases.

Atomistic modeling of an impurity element and a metal–impurity system: pure P and Fe–P system

An interatomic potential for pure phosphorus, an element that has van der Waals, covalent and metallic bonding character, simultaneously, has been developed for the purpose of application to metal–phosphorus systems. As a simplification, the van der Waals interaction, which is less important in metal–phosphorus systems, was omitted in the parameterization process and potential formulation. On the basis of the second-nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential formalism applicable to both covalent and metallic materials, a potential that can describe various fundamental physical properties of a wide range of allotropic or transformed crystalline structures of pure phosphorus could be developed. The potential was then extended to the Fe–P binary system describing various physical properties of intermetallic compounds, bcc and liquid alloys, and also the segregation tendency of phosphorus on grain boundaries of bcc iron, in good agreement with experimental information. The suitability of the present potential and the parameterization process for atomic scale investigations about the effects of various non-metallic impurity elements on metal properties is demonstrated.

Addendum to “Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to Fermi Liquid Behavior in f2-Impurity Systems with Crystalline-Electric-Field Singlet State Competing with Kondo–Yosida Singlet State”

Addendum to “Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to Fermi Liquid Behavior in f²-Impurity Systems with Crystalline-Electric-Field Singlet State Competing with Kondo–Yosida Singlet State”

Entanglement of alkaline-earth-metal fermionic atoms confined in optical lattices

We calculate the entanglement of the ground state of alkaline-earth-metal fermionic atoms confined in one-dimensional optical lattices. This system can be described using the Kondo lattice model plus a harmonic confining potential, and we adopt the density-matrix renormalization group to study its ground state. We find that the local von Neumann entropy is constant in the insulating domains, and a one-to-one correspondence with the variance of the local density is observed. We show that the average entropy and its derivative are useful tools for identifying quantum transitions in impurity systems.

ChemInform Abstract: Polar States of the Impurity System KTaO3: Li

AbstractReview: 25 refs.

Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to Fermi Liquid Behavior in f2-Impurity Systems with Crystalline-Electric-Field Singlet State Competing with Kondo–Yosida Singlet State

We investigate the magnetic field dependence of the physical properties of f^2-configuration systems with a crystalline-electric field (CEF) singlet ground state, which gives rise to a non- Fermi liquid (NFL) fixed point due to the competition between the Kondo-Yosida singlet and CEF singlet states. On the basis of the numerical renormalization group method, we find that the magnetic field breaks this NFL fixed point via two mechanisms: one causing the polarization of f-electrons and the other giving the "channel" anisotropy. These two mechanisms induce a difference in the magnetic field dependence of the characteristic temperature T_F^{*}(H), the crossover temperature from NFL to Fermi-liquid behavior. While the polarization of f-electrons gives T_F^{*}(H) \propto H^x (x\sim2.0), the "channel" anisotropy gives the H-independent T_F^{*}(H). These two mechanisms cross over continuously at approximately the crossover magnetic field H_c, where an anomalous H-dependence of T_F^{*}(H) appears. Such T_F^{*}(H) well reproduces the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2. We also find that the H-dependence of the resistivity and the magnetic susceptibility are in good agreement with the experimental results of this material. These results suggest that the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2 can be understood if this material is located in the CEF singlet side near the critical phase boundary between the two singlet states.

Many-polaron description of impurities in a Bose-Einstein condensate in the weak-coupling regime

The weak-coupling many-polaron formalism is applied to the case of the polaronic system consisting of impurities in a Bose-Einstein condensate. This allows investigating the ground-state properties and the response of the system to Bragg spectroscopy. Then, this theory is applied to the system of spin-polarized fermionic lithium-6 impurities in a sodium condensate. The Bragg spectrum reveals a peak that corresponds to the emission of Bogoliubov excitations. Both the ground-state properties and the response spectrum show that the polaronic effect vanishes at high densities. We also look at two possibilities to define the polaronic effective mass and observe that this results in a different quantitative behavior if multiple impurities are involved.

Numerical study of Kondo impurity models with strong potential scattering: Reverse Kondo effect and antiresonance

Accurate numerical results are derived for transport properties of Kondo impurity systems with potential scattering and orbital degeneracy. Using the continuous-time quantum Monte Carlo (CT-QMC) method, static and dynamic physical quantities are derived in a wide temperature range across the Kondo temperature T_K. With strong potential scattering, the resistivity tends to decrease with decreasing temperature, in contrast to the ordinary Kondo effect. Correspondingly, the quasi-particle density of states obtains the antiresonance around the Fermi level. Thermopower also shows characteristic deviation from the standard Kondo behavior, while magnetic susceptibility follows the universal temperature dependence even with strong potential scattering. It is found that the t-matrix in the presence of potential scattering is not a relevant quantity for the Friedel sum rule, for which a proper limit of the f-electron Green's function is introduced. The optical theorem is also discussed in the context of Kondo impurity models with potential scattering. It is shown that optical theorem holds not only in the Fermi-liquid range but also for large energies, and therefore is less restrictive than the Friedel sum rule.

First-principles calculations on surface hydroxyl impurities in BaF 2

OH − impurities located near the (1 1 1) BaF 2 surface have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Twenty surface OH − configurations were studied, and the hydroxyls located on the first surface layer are the energetically most favorable configurations. For the (1 1 1) BaF 2 surface atomic layers, the surface hydroxyls lead to a remarkable XY-translation and a dilating effect in the Z-direction, overcoming the surface shrinking effect in the perfect slab. Bond population analysis shows that the surface effect strengthens the covalency of surface OH − impurities. The studies on band structures and density of states (DOS) of the surface OH −-impurity systems demonstrate that there are two defect levels induced by OH − impurities. The O p x and p y orbitals form two superposed occupied O bands, located above the valence bands (VB), and the H s orbitals do the major contribution to an empty H band, located below the conduction bands (CB). Because of the surface effect, the O bands move downward, toward the VB with respect to these bands in the bulk case, and this leads to narrowing of the VB → O gap and widening of the O → H gap which corresponds to the first optical absorption.

Spontaneous spin polarization of systems with impurity hybridized electron states in conduction band of crystals

A theoretical description of spontaneous spin polarization in systems of hybridized electron states on donor impurities with low concentration in a conduction band of a crystal is developed. On the base of general equations for the energy of spin splitting of the electron spectrum, formulated within the framework of the Fermi-liquid approach, it is shown that in systems of such a type the spin ordering mechanism of conduction electrons, which is more efficient than the usually considered mechanism of an exchange interaction with magnetic impurities, takes place. The possible realization of spontaneous spin ordering of electrons in hybridized states is substantiated for the case of total polarization of the system within the model of Fermi-liquid constants. Possibilities of finding manifestations of spontaneous spin polarization of an electron impurity system in thermodynamic properties (heat capacity and elastic moduli) are discussed. Evidences of such manifestations are presented from an analysis of available experimental data.

Real-space renormalization group flow in quantum impurity systems: Local moment formation and the Kondo screening cloud

The existence of a length-scale $\xi_K\sim 1/T_K$ (with $T_K$ the Kondo temperature) has long been predicted in quantum impurity systems. At low temperatures $T\ll T_K$, the standard interpretation is that a spin-$\tfrac{1}{2}$ impurity is screened by a surrounding `Kondo cloud' of spatial extent $\xi_K$. We argue that renormalization group (RG) flow between any two fixed points (FPs) results in a characteristic length-scale, observed in real-space as a crossover between physical behaviour typical of each FP. In the simplest example of the Anderson impurity model, three FPs arise; and we show that `free orbital', `local moment' and `strong coupling' regions of space can be identified at zero temperature. These regions are separated by two crossover length-scales $\xi_{\text{LM}}$ and $\xi_K$, with the latter diverging as the Kondo effect is destroyed on increasing temperature through $T_K$. One implication is that moment formation occurs inside the `Kondo cloud', while the screening process itself occurs on flowing to the strong coupling FP at distances $\sim \xi_K$. Generic aspects of the real-space physics are exemplified by the two-channel Kondo model, where $\xi_K$ now separates `local moment' and `overscreening' clouds.

Impurity entanglement entropy in Kondo systems from conformal field theory

The entanglement entropy in Kondo impurity systems is studied analytically using conformal field theory. From the impurity contribution to the scaling corrections of the entanglement entropy we extract information about the screening cloud profile for general non-Fermi-liquid fixed points. By also considering the finite-temperature corrections to scaling of the von Neumann entropy we point out a direct connection between the large-distance screening cloud profile and thermodynamic observables such as the specific heat.

Ab initio calculations of the hydroxyl impurities in BaF2

OH− impurities in BaF2 crystal have been studied by using density functional theory (DFT) with hybrid exchange potentials, namely DFT-B3PW. Three different configurations of OH− impurities were investigated and the (111)-oriented OH− configuration is the most stable one. Our calculations show that OH− as an atomic group has a steady geometrical structure instead of electronic properties in different materials. The studies on band structures and density of states (DOS) of the OH−-impurity systems indicate that there are two defect levels induced by OH− impurities. The two superposed occupied OH−-bands located 1.95eV above the valance bands (VB) at Γ point mainly consist of the O p orbitals, and the H s orbitals do the major contribution to the empty defect level located 0.78eV below the conduction bands (CB). The optical absorption due to the doped OH− is centered around 8.61eV.