Semiclassical theory of shot noise in ballistic n+-i-n+ semiconductor strucutres: relevance of Pauli and long range Coulomb correlations

We work out a semiclassical theory of shot noise in ballistic n+-i-n+ semiconductor structures aiming at studying two fundamental physical correlations coming from Pauli exclusion principle and long range Coulomb interaction. The theory provides a unifying scheme which, in addition to the current-voltage characteristics, describes the suppression of shot noise due to Pauli and Coulomb correlations in the whole range of system parameters and applied bias. The whole scenario is summarized by a phase diagram in the plane of two dimensionless variables related to the sample length and contact chemical potential. Here different regions of physical interest can be identified where only Coulomb or only Pauli correlations are active, or where both are present with different relevance. The predictions of the theory are proven to be fully corroborated by Monte Carlo simulations.Comment: 15 pages, 11 figures. Title changed and Introduction rewritten. Accepted for publication in Physical Review

Spin Injection in a Ballistic Two-Dimensional Electron Gas

We explore electrically injected, spin polarized transport in a ballistic two-dimensional electron gas. We augment the Buettiker-Landauer picture with a simple, but realistic model for spin-selective contacts to describe multimode reservoir-to-reservoir transport of ballistic spin 1/2 particles. Clear and unambiguous signatures of spin transport are established in this regime, for the simplest measurement configuration that demonstrates them directly. These new effects originate from spin precession of ballistic carriers; they exhibit strong dependence upon device geometry and vanish in the diffusive limit. Our results have important implications for prospective ``spin transistor'' devices.Comment: Submitted to Phys. Rev. Let

Energy Gap from Tunneling and Metallic Sharvin Contacts onto MgB2: Evidence for a Weakened Surface Layer

Point-contact tunnel junctions using a Au tip on sintered MgB2 pellets reveal a sharp superconducting energy gap that is confirmed by subsequent metallic Sharvin contacts made on the same sample. The peak in the tunneling conductance and the Sharvin contact conductance follow the BCS form, but the gap values of 4.3 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk Tc of 39 K. The low value of Delta compared to the BCS value for the bulk Tc is possibly due to chemical reactions at the surface.Comment: 3 pages, 3 figure

Unified description of ballistic and diffusive carrier transport in semiconductor structures

A unified theoretical description of ballistic and diffusive carrier transport in parallel-plane semiconductor structures is developed within the semiclassical model. The approach is based on the introduction of a thermo-ballistic current consisting of carriers which move ballistically in the electric field provided by the band edge potential, and are thermalized at certain randomly distributed equilibration points by coupling to the background of impurity atoms and carriers in equilibrium. The sum of the thermo-ballistic and background currents is conserved, and is identified with the physical current. The current-voltage characteristic for nondegenerate systems and the zero-bias conductance for degenerate systems are expressed in terms of a reduced resistance. For arbitrary mean free path and arbitrary shape of the band edge potential profile, this quantity is determined from the solution of an integral equation, which also provides the quasi-Fermi level and the thermo-ballistic current. To illustrate the formalism, a number of simple examples are considered explicitly. The present work is compared with previous attempts towards a unified description of ballistic and diffusive transport.Comment: 23 pages, 10 figures, REVTEX

Ballistic versus diffusive magnetoresistance of a magnetic point contact

The quasiclassical theory of a nanosize point contacts (PC) between two ferromagnets is developed. The maximum available magnetoresistance values in PC are calculated for ballistic versus diffusive transport through the area of a contact. In the ballistic regime the magnetoresistance in excess of few hundreds percents is obtained for the iron-group ferromagnets. The necessary conditions for realization of so large magnetoresistance in PC, and the experimental results by Garcia et al are discussedComment: 4 pages, TEX, 1 Figur

Resistivity of a Metal between the Boltzmann Transport Regime and the Anderson Transition

We study the transport properties of a finite three dimensional disordered conductor, for both weak and strong scattering on impurities, employing the real-space Green function technique and related Landauer-type formula. The dirty metal is described by a nearest neighbor tight-binding Hamiltonian with a single s-orbital per site and random on-site potential (Anderson model). We compute exactly the zero-temperature conductance of a finite size sample placed between two semi-infinite disorder-free leads. The resistivity is found from the coefficient of linear scaling of the disorder averaged resistance with sample length. This ``quantum'' resistivity is compared to the semiclassical Boltzmann expression computed in both Born approximation and multiple scattering approximation.Comment: 5 pages, 3 embedded EPS figure

Mesoscopic scattering in the half-plane: squeezing conductance through a small hole

We model the 2-probe conductance of a quantum point contact (QPC), in linear response. If the QPC is highly non-adiabatic or near to scatterers in the open reservoir regions, then the usual distinction between leads and reservoirs breaks down and a technique based on scattering theory in the full two-dimensional half-plane is more appropriate. Therefore we relate conductance to the transmission cross section for incident plane waves. This is equivalent to the usual Landauer formula using a radial partial-wave basis. We derive the result that an arbitrarily small (tunneling) QPC can reach a p-wave channel conductance of 2e^2/h when coupled to a suitable reflector. If two or more resonances coincide the total conductance can even exceed this. This relates to recent mesoscopic experiments in open geometries. We also discuss reciprocity of conductance, and the possibility of its breakdown in a proposed QPC for atom waves.Comment: 8 pages, 3 figures, REVTeX. Revised version (shortened), accepted for publication in PR

Spin injection into a ballistic semiconductor microstructure

A theory of spin injection across a ballistic ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann regime. Spin injection coefficient $\gamma$ is suppressed by the Sharvin resistance of the semiconductor $r_N^*=(h/e^2)(\pi^2/S_N)$, where $S_N$ is the Fermi-surface cross-section. It competes with the diffusion resistances of the ferromagnets $r_F$, and $\gamma\sim r_F/r_N^*\ll 1$ in the absence of contact barriers. Efficient spin injection can be ensured by contact barriers. Explicit formulae for the junction resistance and the spin-valve effect are presented.Comment: 5 pages, 2 column REVTeX. Explicit prescription relating the results of the ballistic and diffusive theories of spin injection is added. To this end, some notations are changed. Three references added, typos correcte