Evolution of spectral function in a doped Mott insulator : surface vs. bulk contributions

We study the evolution of the spectral function with progressive hole doping in a Mott insulator, $La_{1-x}Ca_xVO_3$ with $x$ = 0.0 - 0.5. The spectral features indicate a bulk-to-surface metal-insulator transition in this system. Doping dependent changes in the bulk electronic structure are shown to be incompatible with existing theoretical predictions. An empirical description based on the single parameter, $U/W$, is shown to describe consistently the spectral evolution.Comment: Revtex, 4 pages, 3 postscript figures. To appear in Phys. Rev. Let

Quantum Coherence Oscillations in Antiferromagnetic Chains

Macroscopic quantum coherence oscillations in mesoscopic antiferromagnets may appear when the anisotropy potential creates a barrier between the antiferromagnetic states with opposite orientations of the Neel vector. This phenomenon is studied for the physical situation of the nuclear spin system of eight Xe atoms arranged on a magnetic surface along a chain. The oscillation period is calculated as a function of the chain constant. The environmental decoherence effects at finite temperature are accounted assuming a dipole coupling between the spin chain and the fluctuating magnetic field of the surface. The numerical calculations indicate that the oscillations are damped by a rate $\sim (N-1)/ \tau$, where $N$ is the number of spins and $\tau$ is the relaxation time of a single spin.Comment: 10 pages, Latex, two postscript figures; submitted to Phys. Rev.

Mode Transitions in Magnetically Shielded Hall Effect Thrusters

A mode transition study is conducted in magnetically shielded thrusters where the magnetic field magnitude is varied to induce mode transitions. Three different oscillatory modes are identified with the 20-kW NASA-300MS-2 and the 6-kW H6MS: Mode 1) global mode similar to unshielded thrusters at low magnetic fields, Mode 2) cathode oscillations at nominal magnetic fields, and Mode 3) combined spoke, cathode and breathing mode oscillations at high magnetic fields. Mode 1 exhibits large amplitude, low frequency (1-10 kHz), breathing mode type oscillations where discharge current mean value and oscillation amplitude peak. The mean discharge current is minimized while thrust-to-power and anode efficiency are maximized in Mode 2, where higher frequency (50-90 kHz), low amplitude, cathode oscillations dominate. Thrust is maximized in Mode 3 and decreases by 5-6% with decreasing magnetic field strength. The presence or absence of spokes and strong cathode oscillations do not affect each other or discharge current. Similar to unshielded thrusters, mode transitions and plasma oscillations affect magnetically shielded thruster performance and should be characterized during system development

Photoemission by Polarized X-Rays

The angular distribution of photoelectrons excited by X-rays is strongly correlated with the degree and type of polarization of incident photons. The angular and energy spectrum of signal photoelectrons leaving a solid is considerably modified as compared to the differential photoelectric cross-section owing to elastic and inelastic collisions the electrons suffer on their way out of a target. In this report an analytical expression for the angular distribution of photoelectrons escaping from the sample without being scattered inelastically was found in the transport approximation. The dependence of the angular distribution on the parameters characterizing optical orientation and polarization of incident X-rays was studied in detail

Mean Escape Depth of Signal Photoelectrons from Amorphous and Polycrystalline Solids

Escape depth of photoelectrons leaving a target without being scattered inelastically was submitted to extensive theoretical analysis. Dependence of the mean escape depth on the X-ray angle of incidence and the electron initial angular distribution inside the sample was considered. The Monte Carlo algorithm was developed basing on a differential elastic scattering cross-section calculated within the partial-wave expansion method and a realistic electron-atom interaction potential. The mean escape depth was evaluated by means of the depth distribution function found analytically by solving a kinetic equation and by the Monte Carlo technique. The agreement between the results obtained from two methods is excellent. Elastic scattering was found to reduce considerably the escape depth. This reduction may reach up to 25% in the case of heavy elements in the practical X-ray photoelectron spectroscopy analysis. It was shown that the mean escape depth expressed in units of the electron transport mean free path is a universal function of the ratio of the inelastic to the transport mean free paths, the asymmetry parameter and the X-ray angle of incidence. A simple explicit expression for this function is proposed

Signal Photoelectron Yield Dependence on the X-Ray Angle of Incidence

The photoelectron emission from solids irradiated by X-rays was described by the analytical theory of electron transport and simulated by the Monte Carlo technique. The medium energy electron transport problem is treated by means of a Boltzmann type kinetic equation satisfying appropriate boundary conditions. The solution of the transport equation was obtained in the transport approximation based on the generalized radiative field similarity principle. Simple and reliable formalism was derived for both the differential and the total photoelectron yields. The dependence of the photoelectron yield on the X-ray incidence angle and the "flattening" effect of multiple elastic scattering on the angular distribution of electrons leaving the target are analysed in detail. The photoelectron yields and angular distributions calculated by the Monte Carlo algorithm, based on a realistic differential elastic scattering cross-section, are in good agreement with the results found from analytical theory. It is shown that main characteristics of the photoelectron emission are determined primarily by two parameters: the inelastic and the transport mean free paths