Screening Cloud Research Articles

Kondo effect in the quantum dot embedded in the Aharonov-Bohm ring

The properties of the ground state of a closed dot-ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the Slave-Boson mean-field theory. It is shown that at T＝０, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature; the physical quantities of this system depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviors of this system can attribute to the coexistence of both the f inite-size effect and the Kondo screening effect in this system. It is also poss ible to detect the Kondo screening cloud by measuring the persistent current or the zero_field impurity susceptibility χimp directly in future exp eriments.

Finite-size effect and Kondo screening effect in an A-B ring with a quantum dot

The properties of the ground state of a closed dot–ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. It is shown that at T=0, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature. The physical quantities depend sensitively on both the parity of the system and the size of the ring; the rich physical behaviour can be attributed to the coexistence of both the finite-size effect and the Kondo screening effect. It is also possible to detect the Kondo screening cloud by measuring the persistent current or the zero field impurity susceptibility χimp directly in future experiments.

The persistent current in an Aharonov–Bohm ring with a side-coupled quantum dot

We have investigated the persistent current in a mesoscopic ring with a side-coupled quantum dot. The problems are probed by using the one-impurity Anderson Hamiltonian and are treated with the slave boson mean field theory. It is shown that the persistent current in this system has the spin fluctuations and the charge transfers between the two subsystems are suppressed in the limit of Δ/T K 0<<1. The minimum value of the persistent current for ξK/L = 5 of the odd parity system provides an opportunity to detect the Kondo screening cloud.

Understanding the Screening of Nuclear Reaction in Stellar Cores

The screening of nuclear reactions in stars takes place under the special conditions that the total number of particles in the screening cloud is of the order of unity. Hence, standard mean field theory is not expected tobe valid. We demonstrate by the use of Molecular dynamics that fluctuations dominate the screening and lead to a smaller screening compared with the predictions of the classical mean field theory.

Universality in the screening cloud of dislocations surrounding a disclination

A detailed analytical and numerical analysis for the dislocation cloud surrounding a disclination is presented. The analytical results show that the combined system behaves as a single disclination with an effective fractional charge which can be computed from the properties of the grain boundaries forming the dislocation cloud. Expressions are also given when the crystal is subjected to an external two-dimensional pressure. The analytical results are generalized to a scaling form for the energy which up to core energies is given by the Young modulus of the crystal times a universal function. The accuracy of the universality hypothesis is numerically checked to high accuracy. The numerical approach, based on a generalization from previous work by S. Seung and D.R. Nelson ({\em Phys. Rev A 38:1005 (1988)}), is interesting on its own and allows to compute the energy for an {\em arbitrary} distribution of defects, on an {\em arbitrary geometry} with an arbitrary elastic {\em energy} with very minor additional computational effort. Some implications for recent experimental, computational and theoretical work are also discussed.

Kondo screening cloud effects in mesoscopic devices

We study how finite size effects may appear when a quantum dot in the Kondo Coulomb blockade regime is embedded into a mesoscopic device with finite wires. These finite size effects appear when the size of the mesoscopic device containing the quantum dot is of the order of the size of Kondo cloud and affect all thermodynamic and transport properties of the Kondo quantum dot. We also generalize our results to the experimentally relevant case where the wires contain several transverse modes/channels. Our results are based on perturbation theory, Fermi liquid theory and slave boson mean field theory.

Kondo resonances in an Aharonov–Bohm ring with an in‐line quantum dot

AbstractThe properties of the ground state of a closed dot‐ring system with a magnetic flux in the Kondo regime are studied theoretically by means of a one‐impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave‐boson mean‐field theory. It is shown that at T = 0, a suppressed Kondo effect exists in this system even when the mean level spacing of electrons in the ring is larger than the bulk Kondo temperature; the physical quantities of this system depend sensitively on both the parity of the system and the size of the ring; the rich physical behavior of this system can attributed to the coexistence of both the finite‐size effect and the Kondo screening effect in this system. It is also possible to detect the Kondo screening cloud by measuring the PC or the zero field impurity susceptibility χimp directly in future experiments.

Core-hole effects on the ELNES of absorption edges in SrTiO 3

Near-edge structures of absorption edges in electron energy-loss spectra (ELNES) of SrTiO 3 were calculated and compared to experimental inelastic electron scattering data. The goal of this study was to investigate final-state effects on the electronic structure. Two theoretical approaches were applied: density-functional theory with a band-structure supercell method and a real-space multiple-scattering cluster approach. Within both techniques, the Z+1 approximation was used to model the core hole generated by the inelastic scattering process. For the band-structure calculations, supercells of ( SrTiO 3) n (n=1,4,8,16) composition with three-dimensional periodic boundary conditions were applied. The influence of supercell size and shape on calculated site- and symmetry-projected local densities of unoccupied states is assessed quantitatively. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole, and the interaction energy of neighbouring core hole centres. For a sufficiently large supercell size, the Z+1 approximation yields a reasonable description of the local densities of unoccupied states probed by the energy losses of inelastically scattered electrons of the Ti L 3-, O K- and Sr L 3-absorption edges. The quantitative equivalence of ELNES information extracted from the multiple-scattering cluster calculations and the band-structure supercell calculations is demonstrated. Discrepancies between theoretical and experimental results are discussed.

Finite-size effects in conductance measurements on quantum dots.

The observation of the Kondo effect in quantum dots has provided new opportunities to finally observe the controversial Kondo screening cloud. Here we study the conductance of a quantum dot embedded in a finite length quantum wire, predicting a change in behavior when the length of the wire is comparable to the size of the screening cloud.

Nonlinear screening of the field of a dopant ion on the metal side of the Mott phase transition in semiconductors

The nonlinear screening of an ionized donor by the degenerate gas of conduction electrons in a crystalline semiconductor is analyzed. For nonlinear screening, the charge density of the electron cloud screening the ion is not proportional to the total electrostatic potential produced by the ion and cloud. As a result, the potential decreases with the distance from the ion more weakly than it does within the linear approximation and the energy of the electrostatic correlation between the ion and screening cloud is smaller.

How many-particle interactions develop after ultrafast excitation of an electron-hole plasma.

Electrostatic coupling between particles is important in many microscopic phenomena found in nature. The interaction between two isolated point charges is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the screening cloud surrounding each charge carrier. One such system involves ultrafast interactions between quasi-free electrons in semiconductors-which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static and dynamical screening models that assume the instantaneous formation of interparticle correlations. However, some quantum kinetic theories suggest that a regime of unscreened bare Coulomb collisions might exist on ultrashort timescales. Here we monitor directly the temporal evolution of the charge-charge interactions after ultrafast excitation of an electron-hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a distinct time delay of the order of the inverse plasma frequency, that is, several 10(-14) seconds.

Persistent currents through a quantum dot

We study the persistent currents induced by the Aharonov-Bohm effect in a closed ring which either embeds or is directly side coupled to a quantum dot at Kondo resonance. We predict that in both cases, the persistent current is very sensitive to the ratio between the length of the ring and the size of the Kondo screening cloud which appears as a fundamental prediction of scaling theories of the Kondo effect. Persistent current measurements provide therefore an opportunity to detect this cloud which has so far never been observed experimentally.

Detecting the Kondo screening cloud around a quantum dot.

A fundamental prediction of scaling theories of the Kondo effect is the screening of an impurity spin by a cloud of electrons spread out over a mesoscopic distance. This cloud has never been observed experimentally. Recently, aspects of the Kondo effect have been observed in experiments on quantum dots embedded in quantum wires. Since the length of the wire may be of order the size of the screening cloud, such systems provide an ideal opportunity to observe it. We point out that persistent current measurements in a closed ring provide a conceptually simple way of detecting this fundamental length scale.

Density-functional modelling of core-hole effects in electron energy-loss near-edge spectra

A series of (MgO) n supercells ( n=1,4,8,16,32) with three-dimensional periodic boundary conditions is investigated by density-functional band-structure calculations. The influence of supercell size and shape on calculated electron energy-loss near-edge spectra is assessed quantitatively, employing the Z+1 approximation for the representation of final-state effects. Relevant convergence criteria are the length scale set by the spatial extension of the valence-electron screening cloud around the core hole and the interaction energy of neighbouring core-hole centres. A sufficient supercell size provided, the Z+1 approximation yields a highly satisfactory description of excitations from the 1s shell of light elements, such as Mg and O, compared to experimental data. For comparison, pseudopotentials for excited states were generated for Mg, both with a large core (1s, 2s, 2p orbitals) and a small core (1s orbital only) included into the pseudopotential. The corresponding calculations with frozen core holes lead to very good agreement with the results from the Z+1 calculation for the 1s excitations. The explicit treatment of the subvalence shell (2s, 2p), however, is mandatory for the proper modelling of excitations from orbitals higher than 1s. This indicates that the core polarisability plays an important role in excitations from more extended shells.

Free energy of a screened ion pair

We calculate the effect of weak electrostatic screening of ions in a plasma. The original calculation by Salpeter is based on a linearization of the Poisson–Boltzmann equation for the screened electrostatic potential. This approximation is valid where the potential is small, but is formally invalid in the vicinity of the ions where the solutions for the potential and the associated charge distribution diverge. In reality, quantum exclusion must prevent the divergence of the charge density of the screening cloud. Nevertheless, in the limit in which screening is weak, Salpeter’s value for the total screening energy is essentially correct. Here we extend Salpeter’s calculation to account approximately for both quantum-mechanical exclusion in the vicinity of the ions, using what we call the Poisson–Boltzmann–Fermi–Dirac approximation, and the polarization of the screening cloud. By matching the solution onto an expression for the two-center Poisson–Boltzmann charge distribution far from the ions we are able to construct a consistent solution over all space. We obtain the first-order term in the expansion of that solution, from which we calculate the free-energy associated with the screened ion pair.

Electric quadrupole effects and nuclear spin relaxation in aAl72.4Pd20.5Mn7.1icosahedral quasicrystal studied by NMR

The ${}^{27}\mathrm{Al}\mathrm{}\mathrm{NMR}$ spectrum and the spin-lattice relaxation rate of an icosahedral ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ quasicrystal were studied in the temperature range from 300 to 4 K at two magnetic fields. The results are compatible with the hypothesis that a screening electron cloud is formed around the Mn ions in an aluminum-rich environment, which together with the core electrons produces a large electric-field gradient (EFG) at the sites of the Al nuclei. The EFG is temperature dependent due to the polarization of the screening cloud and the core electrons by the magnetic moments of the Mn unpaired d electrons. The shift of the ${}^{27}\mathrm{Al}$ central line shows an inverse magnetic-field dependence and appears to be of an electric quadrupolar origin. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation rate shows an ${aT+bT}^{3}$ temperature dependence that can be explained by approximating the quasicrystalline electronic structure by a two-band model, consisting of a broad s and a narrow d conduction bands. The ${T}^{3}$ term originates from the narrowness of the d band. The ${}^{27}\mathrm{Al}$ spin-lattice relaxation in ${\mathrm{Al}}_{72.4}{\mathrm{Pd}}_{20.5}{\mathrm{Mn}}_{7.1}$ is governed by two relaxation mechanisms of similar strength, the electric quadrupole and the magnetic hyperfine. The quadrupole mechanism is stronger at high temperatures such as room temperature whereas the magnetic relaxation dominates at low temperatures.

Dynamic nonlinear screening of slow ions in an electron gas

We have used the ensemble Kohn-Sham method with the local-density approximation to determine self-consistently the dynamic screening cloud around an impurity moving adiabatically through a uniform electron gas with velocity below the Fermi velocity. The asymmetry of the induced field and electronic density caused by the impurity motion is fully taken into account. We discuss the forward-backward asymmetry for the case of bare positive and negative charges as well as for hydrogen atoms. We show that the stopping power of the electron gas varies linearly with the projectile velocity up to velocities close to the Fermi velocity. We study the asymmetry of the bound states, its contribution to the stopping power and the evolution of the binding energy as a function of projectile velocity. Finally, we give a complete derivation of the low energy behavior of the stopping power and the generalization of the Friedel sum rule and oscillations for the case of moving impurities.

Nonlinear Debye–Onsager relaxation effect

The quantum kinetic equation for charged particles in strong electric fields is used to derive the nonlinear particle flux. The relaxation field is calculated quantum mechanically up to any order in the applied field provided a given Maxwellian plasma. The classical limit is given in analytical form. In the range of weak fields the deformation of the screening cloud is responsible for the Debye–Onsager relaxation effect.

Screening in Thermonuclear Reaction Rates in the Sun

We evaluate the effect of electrostatic screening by ions and electrons on low-Z thermonuclear reactions in the Sun. We use a mean field formalism and calculate the electron density of the screening cloud using the appropriate density matrix equation of quantum statistical mechanics. Because of well-understood physical effects that are included for the first time in our treatment, the calculated enhancement of reaction rates does not agree with the frequently used interpolation formulae. Our result does agree, within small uncertainties, with Salpeter's weak screening formula. If weak screening is used instead of the commonly employed screening prescription of Graboske et al., the predicted 8B neutrino flux is increased by 7% and the predicted chlorine rate is increased by 0.4 SNU.

Screening cloud in thek-channel Kondo model: Perturbative and large-kresults

We demonstrate the existence of a large Kondo screening cloud in the k-channel Kondo model using both renormalization group improved perturbation theory and the large-k limit. We study position (r) dependent spin Green's functions in both static and equal time cases. The equal-time Green's function provides a natural definition of the screening cloud profile, in which the large Kondo scale appears. At large distances it consists of both a slowly varying piece and a piece which oscillates at twice the Fermi wave-vector. This function is calculated at all r in the large-k limit. Static Green's functions (Knight shift or susceptibility) consist only of a term oscillating at 2kF, and appear to factorize into a function of r times a function of T for rT << vF, in agreement with NMR experiments. Most of the integrated susceptibility comes from the impurity-impurity part with conduction electron contributions suppressed by powers of the bare Kondo coupling. The single-channel and overscreened multi-channel cases are rather similar although anomalous power-laws occur in the latter case at large r and low T due to irrelevant operator corrections.