High Doping Regime Research Articles

Metal-insulator transition in doped bismuthate superconductors

We study the metal-insulator transition of BaBi 1- x Pb x O 3 and Ba 1- x K x BiO 3 within a Holstein molecular crystal model. The coherent potential approximation is used to calculate the density of states upon doping in both cases. It is found that in the Pb substituted compound the gap is filled up with impurity states while the pseudo-gap structure survives even in the high doping regime. In the K-doped compound we consider the formation of polaron states in the vicinity of the band edges leading to a rapid decrease of the CDW gap upon doping. The relation to the cuprate superconductors is discussed.

Temperature-dependent phase separation within the Emery model.

The variational principle of Bogoliubov, combined with a canonical transformation, is used to derive the thermodynamic functions for the hole-doped two-dimensional Emery model of the copper-oxygen planes in high-temperature-superconducting (HTSC) materials. A local ansatz and a transformation optimized for small values of the ratio of the hopping constant to the Coulomb correlation gives the lowest free energy in the neutral case and in the case of moderate doping. The regions of thermodynamic instability in the curves of the chemical potential versus doping, as coexistence regions of phase separation, are determined numerically with use of a Maxwell construction. In the high-doping regime, the coexistence regions are reduced considerably by the condition that the strong-coupling result is the lowest one, better than, e.g., the standard Hartree-Fock solution. The phase diagram of paramagnetic, ferromagnetic, antiferromagnetic, and separated phases is given. The parameter dependence of the maximum temperature of phase separation shows the same tendency as the empirical parameter dependence of the HTSC transition temperatures.

Tunneling spectroscopy on the GaAs(110) surface: Effect of dopant concentration

Scanning tunneling spectroscopy has been used to investigate the effect of doping concentration on the current-voltage characteristics of the GaAs(110) surface. For a fixed tip-sample separation, the conductivity gap is found to increase as the doping concentration is reduced. The results are compared with the predictions of a one-dimensional planar tunneling model which takes the tip-induced band-bending into account. Good agreement between the experiment and the calculations is achieved in the high-doping regime. The disagreement at lower doping levels suggests the absence of a complete equilibrium between the minority carriers at the surface and the majority carriers in the bulk, as well as the importance of dimensionality in describing the tip-sample interaction.

Superpolaron Model for Metallic Polyacetylene

We present a theory for the metallic phase of heavily doped polyacetylene. The Hartree-Fock (HF) ground state in this system is a charged soliton (S ± ) lattice that has a nearly periodic long range order in the charge density as well as the one in lattice distortion. We take account of electronic correlation effects by considering superpolarons (SP's) as local fluctuations in the electronic order parameter (OP). They are electrons and holes injected into a S ± lattice that make no lattice distortions but are selftrapped and make local defects in the electronic OP. They make intrachain quantum translation and interchain hopping through dopants. The formation energy of a SP pair is reduced by these quantum motions and become negative in a high doping regime. Therefore, the ground state in this regime is a S ± lattice with spontaneously produced SP's. The SP's may be regarded as Fermion quasi-particles. Since they partially occupy the translational bands of SP's, metallic properties appear in this phase.

Superpolaron Model of Metallic Polyacetylene

ABSTRACTWe develop a new model for heavily doped metallic polyacetylene. An electron and a hole injected in a doped chain make localized defects called superpolarons (SP's) in the electronic order parameters (OP) in a charged soliton lattice. The SP's can make quantum intrachain translational motion and interchain transfer via dopant ions. They are stabilized by their quantum motions and SP pairs are spontaneously produced in a high doping regime. The SP's are Fermions and can be a Fermi liquid in the doping regime. Some metallic properties of the Fermi liquid of the SP' s are studied.

Hole dynamics in a strongly correlated two-dimensional spin background.

In this paper we present results of finite-lattice studies of the t-J and the t-t'-J model for 18 (20) sites with up to 4 (2) holes. A modified Lanczos algorithm allowed for the classification of the ground state according to total spin S. In the one-hole sector we find evidence for the existence of hole pockets. We calculate the effective quasiparticle band and show qualitative differences between the t-J and t-t'-J model for J\ensuremath{\gtrsim}0.2. As U increases we notice a substantial band narrowing, which is maximal at the Nagaoka transition, accompanied by S-level crossings rendering a quasiparticle description difficult. In the high-doping regime we present hole binding energies for up to six holes for the t-J model. Evidence for phase separation is only found for J>t. We also studied the magnetic structure of the ground state on the 18-site lattice in the limit J=0. We present a complete ground-state classification for all numbers of holes ${\mathit{N}}_{\mathit{h}}$ and find S=${\mathit{S}}_{\mathrm{max}}$ at quarter filling in addition to the Nagaoka case.

Ab initio studies of soliton defects in trans-polyacetylene

We present results from geometry optimizations of charged soliton defects in segments of trans-polyacetylene. In particular, the effect of the counterion on the localization of the soliton defect is investigated. Using an ab initio Hartree-Fock method, we find that the soliton is considerably more localized in the presence of a single counterion than without the counterion present. We have taken the case of a single counterion to represent the situation at low doping levels. In order to simulate the high doping regime, we include the electrostatic interaction with soliton-counterion complexes on neighbouring chains. For this case we observe an increase of the soliton width, compared to the system with a single counterion. Finally, we investigate the effect of electron correlation on the dimerization of polyenes. Including second order Møller-Plesset corrections to the Hartree-Fock energy, we observe an increase in the length of the double bonds in the chain whereas the single bonds are unchanged. Thus, the dimerization is found to decrease due to electron correlation.

Fabrication and Characterization of Epitaxial Heavily Phosphorus‐Doped Silicon

Phosphorus‐doped silicon layers have been grown by epitaxy in a system. Doping levels from have been obtained. A combination of measurements of electrical resistivity, Hall mobility, and phosphorus concentration by secondary ion mass spectroscopy has yielded accurate values for the electron mobility and the Hall scattering factor. At the lowest doping levels, the phosphorus concentration is proportional to the partial pressure of phosphine in the reactor, while in the high doping regime the dependence is . The growth rate drops by about 20% at the highest doping levels. The defect densities increase quadratically with phosphorus concentration and reach a value of 3000 cm−2 at .

A three-dimensional model for the transport properties of polycrystalline silicon

Considered the transport properties of p-type polysilicon using a three-dimensional spherical model and compared the results with experimental data and a one-dimensional model commonly used. It was observed that the main differences between the two models exist after the crystal grains are totally depleted. This happens to be a region where reliable data (especially for carrier density) are scarce. From the theoretical analyses, the three-dimensional model seemed to be as good as the one-dimensional model in the high doping regime and for the resistivity data. Some significant differences existed between theory and experiment for the activation energies based on either of the two models and it is suspected that gross inhomogeneity could be the cause.

Models and method of calculation of doping and injection-dependent impurity density of states in GaAS

Using the solution to the problem of screened Coulomb potential for the low-doping regime, the theories of Morgan and of Halperin and Lax, respectively, for the medium- and high-doping regimes, the conduction-and valence-band impurity density of states of GaAs are expressed in explicit analytic forms and calculated self-consistently. The effects of injection and doping on the density of states are studied. To avoid the self-consistent iterative process but still take into account the effects of screening, a method of calculation for the density of states is obtained. It is shown that in many cases, the screening length can be determined using the Boltzmann approximation or the parabolic-band approximation. The region of validity for each approximation in the injection-doping space is found. Density of states calculated using these approximations are compared with density of states calculated self-consistently.