Monte Carlo Investigation of Shot-noise Suppression in Nondegenerate Ballistic and Diffusive Transport Regimes

We review recent theoretical investigations of shot-noise suppression in nondegenerate semiconductor structures surrounded by two contacts acting as thermal reservoirs. Calculations make use of an ensemble Monte Carlo simulator self-consistently coupled with a one-dimensional Poisson solver. By taking the doping of the injecting contacts and the applied voltage as variable parameters, the influence of elastic and inelastic scattering as well as of tunneling between heterostructures in the active region is investigated. In the case of a homogeneous structure at T = 300 K the transition from ballistic to diffusive transport regimes under different contact injecting statistics is analysed and discussed. Provided significant space-charge effects take place inside the active region, long-range Coulomb interaction is found to play an essential role in suppressing shot noise at applied voltages much higher than the thermal value. In the elastic diffusive regime, momentum space dimensionality is found to modify the suppression factor γ, which within numerical uncertainty takes values respectively of about ⅓, ½ and 0·7 in the 3D, 2D and 1D cases. In the inelastic diffusive regime, shot noise is suppressed to the thermal value. In the case of single and multiple barrier non-resonant heterostructures made by GaAs/AlGaAs at 77 K, the mechanism of suppression is identified in the carrier inhibition to come back to the emitter contact after having been reflected from a barrier. This condition is realised in the presence of strong inelastic scattering associated with emission of optical phonons. At increasing applied voltages for a two-barrier structure, shot noise is suppressed up to about a factor of 0·50 in close analogy with the corresponding resonant barrier-diode. For an increasing number of barriers, shot noise is found to be systematically suppressed to a more significant level by following approximately a 1/(N + 1) behaviour, N being the number of barriers. This mechanism of suppression is expected to conveniently improve the signal-to-noise ratio of these devices.

Position Indeterminacy

Calculations assuming position indeterminacy in the Dirac equation are reported.Energy shift contributions for low-lying states of hydrogen-like atoms are calculated by treating the position indeterminacy as a perturbation additional to standard quantum mechanics. The results are consistent with the current discrepancy between theory and experiment for Lamb shifts in hydrogen, deuterium and the helium ion.

Effect of Interfaces on Dielectric Relaxation in La0.35Sr0.6MnO3 -YSZ

The perovskite structure material La 0.35 Sr 0.6 MnO 3 was sintered in air and the mixed ceramic of a 0.35 Sr 0.6 MnO 3 and 8 mol % Y 2 O 3 +ZrO 2 with a 1:1 mass rate was also sintered in air, both at 1530 K for 12 hour. The AC complex impedance spectra have been measured and the effect of the interface between La 0.35 Sr 0.6 MnO 3 and YSZ on dielectric relaxation properties of La 0.35 Sr 0.6 MnO 3 has been investigated. A schematic of diffusion phase formation and the equivalent circuits are proposed for explaining the results. In the complex impedance spectra, the first and second semicircles correspond to the grain-boundary and interface respectively. The results show that the dielectric relaxation properties and structure of the mixed ceramic are different to that of single phase La 0.35 Sr 0.6 MnO 3 . The interface has an important effect on the electric properties of the mixed ceramic and produces a dielectric relaxation in low frequency.

Some properties of the world crystal in fractal spacetime theory.

We prove that the wave-particle duality, inertia and the Heisenberg uncertainty relation are properties of a fractal spacetime, self-structured by a gravitomagnetic background field, in the world crystal.

Electromagnetic Absorption in Two-dimensional Systems under a Magnetic Field and a Unidirectional Periodic Potential

The absorption of electromagnetic waves by a high mobility two-dimensional electron gas subjected to a magnetic field and a weak periodic potential is investigated. We show that the periodic modulation on the Laudau states has a profound effect on the absorption of electromagnetic waves. We develop a formalism which treats the electron–electron interaction beyond the random-phase-approximation (RPA) and includes the electron-impurity scattering in the lowest order. A RPA dielectric function was employed to study the electromagnetic absorption in modulated systems. Simultaneous excitation of an electron–hole pair with finite momentum contributes significantly to the absorption around and below the cyclotron frequency. Such a process is absent for a uniform electron gas under a magnetic field.

Generation of Multicavity Entangled States with a Single Three-Level Atom

A simple scheme is proposed for the generation of maximally entangled states for several separated cavities, in which each cavity is in a one-photon state or in the vacuum state. In the scheme a laddertype three-level atom is sent through the cavities and additional classical fields. The whole system finally evolves into a state, which is given by the product of the highly entangled field state with an atomic state.

Microwave Absorption in Layered Media: Application to Landmine Detection

The absorption of microwave radiation and subsequent thermal conduction by simple composite media, consisting of parallel layers with disparate thermal properties,is analysed.The solutions for a one-dimensional conduction model are used to investigate the time evolution and distribution of thermal energy within moisture-laden soils containing non-absorbing objects.The application of these results to the detection of landmines is discussed and evaluated.

‘Anomalous’ Resurgence of Shot Noise in Long Conductors

There has been renewed interest in the physics of the so-called ‘crossover’ for current fluctuations in mesoscopic conductors, most recently involving the possibility of its appearance in the passage to the macroscopic limit. Shot noise is normally absent from solid-state conductors in the large, and its anomalous resurgence there has been ascribed to a rich interplay of drift, diffusion, and Coulomb screening. We demonstrate that essentially the same rise in shot noise occurs in a much less complex system: the Boltzmann—Drude—Lorentz model of a macroscopic, uniform gas of strictly non-interacting carriers. We conclude that the ‘anomalous crossover’ is a manifestation of simple kinetics. Poissonian carriers, if driven by a high enough field, cross the sample faster than any scattering time, thus fulfilling Schottky’s condition for ideal shot noise.

Influence of Inversion Symmetry on the Metallic Behaviour in a Dilute Two-dimensional Hole System

In the past five years numerous experimental studies of a wide variety of low disorder two-dimensional (2D) semiconductor systems have revealed an unexpectedly large decrease in the resistance as the temperature is lowered from T ~ 1 K, suggesting the existence of a 2D metal. Although numerous theories have been put forward to explain this metallic-like behaviour (which contradicts the expectations of one parameter scaling theory), its origins, and the question of whether it persists to T = 0, are still subjects of great debate. We present a detailed study of the influence of inversion symmetry on the B = 0 metallic behaviour in a low density GaAs hole gas close to the apparent two-dimensional metal—insulator transition. The strength of the metallic behaviour (determined by the size of the drop in resistance as T→ 0) is found to be almost independent of the electric field across the hole gas, and is predominantly determined by the magnitude of k F l at low temperatures (i.e. by the low temperature resistivity). These results suggest that the shape of the potential well and spin—orbit effects alone cannot account for the existence of metallic behaviour in low density, strongly interacting 2D systems.

Acoustic Vibrational Properties and Fractal Bond Connectivity of Praseodymium Doped Glasses

The velocities of longitudinal and shear ultrasonic waves propagated in the (Pr2O3)x(P2O5)1-x glass system, where x is the mole fraction of Pr2O3 and (1 - x) is the mole fraction of P2O5, have been measured as functions of temperature and hydrostatic pressure. The temperature dependencies of the second order elastic stiffness tensor components (SOEC) CS IJ , which have been determined from the velocitydata between 10 and 300 K, show no evidence of phonon mode softening throughout the whole temperature range. The elastic stiffnesses increased monotonically, the usual behaviour associated with the effect of the phonon anharmonicityof atomic vibration. At low temperatures, strong phonon interactions with two-level systems have been observed. The ultrasonic wave attenuation of longitudinal and shear waves is dominated bya broad acoustic loss peak whose height and peak position are frequencydependent. This behaviour is consistent with the presence of thermally activated structural relaxation of the two-level systems in these glasses. The fractal bond connectivity of these glasses, obtained from the elastic stiffnesses determined from ultrasonic wave velocities, has a value between 2.32 to 2.55, indicating that their connectivitytends towards having a threedimensional character. The hydrostatic pressure dependencies of longitudinal ultrasonic waves show a slight increase with pressure. As a consequence, the hydrostatic pressure derivatives ( CS11/ P)P=0 of the elastic stiffness CS11/ and (BS/P)P=0 of the bulk modulus BS of (Pr2O3)x(P2O5)1-x glasses are positive. The bulk modulus increases with pressure, and thus these glasses stiffen under pressure, which is associated with the normal elastic behaviour. The GrÜneisen parameter approach has been used to quantifythe vibrational anharmonicityof the long-wavelength acoustic phonons in these glasses.