Coherence properties of a radiating electron-hole condensate

We analyze both the spatial as well as the temporal coherence of an electron-hole condensate and the radiation emitted from it. These coherences evolve from being full for the low density Bose-Einstein condensate to a chaotic behavior for a high density Bardeen-Cooper-Schrieffer-like state. Time coherence is transferred, to the emitted radiation in the ultrafast regime, in a damped oscillatory way.     

Two-dimensional electron-hole system in a strong magnetic field

A theory of a 2-D e-h-system in a strong magnetic field is presented. Its properties drastically depend on the existence of the e-h-symmetry in it. In an asymmetric system excitons interact with each other and as a result can from magnetic biexcitons. The system is unstable to the formation of a charge-density waves (CDW) at low temperatures.     

Instability of dipole magnetoexcitons in quantum wells' and graphene superlattices

We propose the Bose-Einstein condensation and superfluidity of quasi-two-dimensional spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field. For this system the instability of the ground state of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is found. The density of superfluid component ns(T) and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two-dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both QW and graphene realizations.     

Ground state energy of the electron-hole liquid in type-II (GaAs)m/(AlAs)m quantum wells

The ground state energy of quasi-two-dimensional electron-hole liquid (EHL) at zero temperature is calculated for type-II (GaAs)_m/(AlAs)_m (5≤m≤10) quantum wells (QWs). The correlation effects of Coulomb interaction are taken into account by a random phase approximation of Hubbard. Our EHL ground state energy per electron-hole pair is lower than the exciton energy calculated recently for superlattices, so we expected that EHL is more stable state than excitons at high excitation density. It is also demonstrated that the equilibrium density of EHL in type-II GaAs/AlAs QWs is of one order of magnitude larger than that in type-I GaAs/AlAs QWs.     

Mesoscopics in a strong perpendicular magnetic field

Properties of the diffusive motion in a strong magnetic field are discussed. We describe two possible phase memory breaking mechanisms. We present a high magnetic field form of the discussion propagator which can be used to describe the conductance fluctuations of a two-dimensional electron gas in the regime of well developed Shubnikov-de Haas oscillations.     

Secondary peak in the optical absorption spectra: A possible criterion for Bose condensation of excitons

By solving the BCS and Bethe-Salpeter equations, we confirm the result by Chu and Chang that a secondary peak appears in the optical absorption spectrum immediately after the 1S-exciton peak in the presence of a condensed phase. The observation of the secondary peak indicates the presence of exciton condensate.     

Low-temperature photoluminescence spectra of highly excited quantum wires

Optical spectra of highly excited quantum wires at low temperatures have been studied within the dynamical screening approximation. We found a strong Fermi-edge singularity (FES) in the photoluminescence spectra. The spectral shape and FES intensity strongly depend on temperature in agreement with recent experimental results.     

Development of a magnetic measurement device for thin ribbon samples

This paper presents a magnetic measurement device for thin ribbon samples, which are produced by rapid cooling technique. This device enables us to measure magnetic properties easily by only inserting a ribbon sample into a sample holder. The sample holder was made by bakelite to fix any width sample. A long solenoid coil was used to generate a uniform magnetic field and the sample holder was placed at the mid part of the solenoid. The magnetic field strength was measured using a shunt resistor and the magnetic flux density and magnetization in sample ribbons were evaluated by using search coils. The accuracy of measurement was verified with an amorphous metal ribbon sample. Next, we have measured magnetic properties of some magnetic shape memory alloys, which have different compositions. The measured results are compared and we clarified the effect of Sm contents on the magnetic properties.     

Cold and ultracold Rydberg atoms in strong magnetic fields

Cold Rydberg atoms exposed to strong magnetic fields possess unique properties which open the pathway for an intriguing many-body dynamics taking place in Rydberg gases, consisting of either matter or anti-matter systems. We review both the foundations and recent developments of the field in the cold and ultracold regime where trapping and cooling of Rydberg atoms have become possible. Exotic states of moving Rydberg atoms, such as giant dipole states, are discussed in detail, including their formation mechanisms in a strongly magnetized cold plasma. Inhomogeneous field configurations influence the electronic structure of Rydberg atoms, and we describe the utility of corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We review recent work on large, extended cold Rydberg gases in magnetic fields and their formation in strongly magnetized ultracold plasmas through collisional recombination. Implications of these results for current antihydrogen production experiments are pointed out, and techniques for the trapping and cooling of such atoms are investigated.     

Ladders in a magnetic field: a strong coupling approach

We show that non-frustrated and frustrated ladders in a magnetic field can be systematically mapped onto an XXZ Heisenberg model in a longitudinal magnetic field in the limit where the rung coupling is the dominant one. This mapping is valid in the critical region where the magnetization goes from zero to saturation. It allows one to relate the properties of the critical phase (H _c ¹, H _c ², the critical exponents) to the exchange integrals and provide quantitative estimates of the frustration needed to create a plateau at half the saturation value for different models of frustration. Received: 7 May 1998 / Revised and Accepted: 10 July 1998     

Stimulated parametric scattering of excitonic polaritons in planar GaAs microcavities: Distinctive feature of QW electric field

Instabilities in the electric field at the QW in the active region of a GaAs based microcavity have been investigated under the resonance excitation near the inflection point of the low polariton (LP) dispersion curve with the use of four wave mixing technique. The electric field jump due to an LP bistability has been found to precede the development of a stimulated parametric scattering of LPs. The latter has been found to develop with a delay of a few hundreds ps from the beginning of the excitation pulse. These results are in qualitative agreement with hard regime of excitation of the stimulated parametric LP scattering predicted recently.     

Landau levels line widths in potential scattering in the presence of a strong magnetic field

Summary The FBA scattering cross-section in the presence of a strong magnetic field diverges at the Landau thresholds. Such divergences are eliminated by the introduction of a modified density of states, accounting for the finite Landau states lifetime of the electrons in a magnetized plasma.     

Field effect on the dynamics of the Ising spin glass near the percolation threshold

The effect of a uniform field H on the dynamics of the Ising spin glass Fe_xZn_1-xF₂, x=0.25, is studied through Monte Carlo simulations. The correlation function data are consistent with the droplets picture, with a single scaling variable t/t_w, where t_w is the waiting time under H, activation over logarithmic energy barriers near the percolation threshold, x_p=0.24, and considerable change of the glassy phase for intermediate and high H.     

Signatures of stimulated bosonic exciton-scattering in semiconductor luminescence

We observe signatures of stimulated bosonic scattering of excitons, a precursor of Bose-Einstein-Condensation (BEC), in the photoluminescence of semiconductor quantum wells. The optical decay of a spinless molecule of two excitons (biexciton) into an exciton and a photon with opposite angular momenta is subject to bosonic enhancement in the presence of other excitons. In a gas of biexcitons and spin polarized excitons the bosonic enhancement breaks the symmetry of two equivalent biexciton decay channels leading to circularly polarized luminescence of the biexciton with the sign opposite to the circularly polarized exciton luminescence. Comparison of experiment and many body theory clearly indicates the existence of stimulated exciton-scattering, but excludes the presence of a fully condensed BEC-like state.     

Finite size effects on the optical transitions in quantum rings under a magnetic field

We present a theoretical study of the energy spectrum of single electron and hole states in quantum dots of annular geometry under a high magnetic field along the ring axis in the frame of uncorrelated electron-hole theory. We predict the periodic disappearance of the optical emission of the electron-hole pair as the magnetic field increases, as a consequence of the finite height of the barriers. The model has been applied to semiconductor rings of various internal and external radii, giving as limiting cases the disk and antidot.     

The conductivity of a two-dimensional electronic system of finite width in the presence of a strong perpendicular magnetic field and a random potential

Starting from the Kubo formula the conductivity tensor of a two-dimensional electronic system in a perpendicular magnetic field is evaluated. It is shown that at zero temperature only the states at the Fermi level contribute. The Hall conductivity of a purely periodic system of finite width is calculated and compared with earlier suggestions by Thouless et al. For a system described by a periodic and a random potential the Hall conductivity is calculated as a function of the electron density. The results emphasize the importance of disorder independent current carrying states for the Quantum Hall effect which extend along the boundaries of the system. The plateaux values of the Hall conductivity are related to the number of these states, and are independent of the existence of extended bulk states below the Fermi energy.     

Valley splitting of AlAs two-dimensional electrons in a perpendicular magnetic field

By measuring the angles at which the Landau levels overlap in tilted magnetic fields (the coincidence method), we determine the splitting of the conduction-band valleys in high-mobility two-dimensional electrons confined to AlAs quantum wells. The data reveal that, while the valleys are nearly degenerate in the absence of magnetic field, they split as a function of perpendicular magnetic field. The splitting appears to depend primarily on the magnitude of the perpendicular component of the magnetic field, suggesting electron-electron interaction as its origin.     

Stabilization of electron-hole liquid in uniaxially strained germanium in a strong magnetic field

Luminescence spectra of uniaxially and uniformly strained high-purity germanium crystals at liquid-helium temperatures in a magnetic field of up to 14 T have been investigated. In strongly strained Ge crystals, a new line has been detected on the low-energy side of the excitonline in magnetic fields higher than 4 T. Studies of this line’s characteristics as functions of pressure, temperature, and magnetic field have led us to conclude that its presence is due to recombination of electron-hole pairs in an electron-hole liquid. The experimental data suggest that the metallic electron-hole liquid is stabilized in a strong magnetic field. By approximating the shape of the newly detected line using the model of metallic electron-hole liquid, we have obtained the electron-hole liquid density n _EHL(B) and Fermi energies E _Fe,h of electrons and holes. The liquid binding energy ø as a function of magnetic field has been estimated.