Control of condensation and evaporation of electron–hole liquid in diamond by femtosecond laser pulses

We demonstrate the ultrafast control of condensation and evaporation of an electron–hole liquid in diamond prepared by chemical vapor deposition. The electron–hole liquid dynamics was measured using a three‐pulse pump and probe experiment. The transient transmission changes in the presence of electron–hole drops were assigned to the increase in the Drude scattering rate in the model of free carrier absorption due to a high carrier density. The fast (～1 ps) liquid evaporation was induced by infrared femtosecond pulses and its dynamics was investigated under different photon energies and fluences. The value of the electron–hole liquid binding energy per electron–hole pair 80 ± 40 meV determined from the measured transient transmission signal agrees well with previously published values. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Composite fermion picture for multi-component plasmas in 2D electron–hole systems in a strong magnetic field

Low-lying states of a 2D electron–hole system contain electrons and one or more types of charged excitonic complexes. Binding energies and angular momenta of these excitonic ions, and the pseudopotentials describing their interactions with electrons and with one another are obtained from numerical studies of small systems. Incompressible fluid ground states of such multi-component plasmas are found in exact numerical diagonalizations. A generalized composite fermion (CF) picture involving Chern–Simons charges and fluxes of different types is proposed and shown to predict the low-lying states at any value of the magnetic field.     

Observations of nascent superfluidity in a bilayer two-dimensional electron system at

Single-layer longitudinal and Hall resistances have been measured in a bilayer two-dimensional electron system at ν_T=1 with equal but oppositely directed currents flowing in the two layers. At small effective layer separation and low temperature, the bilayer system enters an interlayer coherent state expected to exhibit superfluid properties. We detect this nascent superfluidity through the vanishing of both resistances as the temperature is reduced. This corresponds to the counterflow conductivity rising rapidly as the temperature falls, reaching by . This supports the prediction that the ground state of this system is an excitonic superfluid.     

Correlated electron states at level crossings of bilayer two-dimensional electron systems in tilted magnetic fields

We have measured the energy-level structure of high mobility, strongly coupled bilayer two-dimensional electron systems in tilted magnetic fields by means of magnetotransport experiments. At tilt angles where single-particle levels with opposite spin and symmetry cross, we observe a surprising sudden broadening of the quantum Hall plateaus and a deepening of the Shubnikov–de Haas minima. This observation is explained by an interaction-induced rearrangement of the energy level structure which strongly increases the energetic splitting of two (anti-)crossing levels.     

Josephson tunneling in bilayer quantum Hall system

A Bose–Einstein condensation is formed by composite bosons in the quantum Hall state. A composite boson carries the fundamental charge (−e  ). We investigate Josephson tunneling of such charges in the bilayer quantum Hall system at the total filling ν=1$\nu =1$. We show the existence of the critical current for the tunneling current to be coherent and dissipationless. Our results explain recent experiments due to [L. Tiemann, Y. Yoon, W. Dietsche, K. von Klitzing, W. Wegscheider, Phys. Rev. B 80 (2009) 165120] and due to [Y. Yoon, L. Tiemann, S. Schmult, W. Dietsche, K. von Klitzing, Phys. Rev. Lett. 104 (2010) 116802]. We predict also how the critical current changes as the sample is tilted in the magnetic field.     

Thermopower evidence for Wigner crystallization in the insulating phase of two-dimensional GaAs bilayer hole systems

We report on low-temperature thermopower measurements of interacting GaAs bilayer hole systems in the limit of no interlayer tunneling. These systems exhibit a reentrant insulating phase near the many-body quantum Hall state (QHS) at total filling factor ν=1, when both layers have the same density. The diffusion thermopower is expected to diverge as T^-1 in the presence of an energy gap (Wigner crystal) or to vanish in the case of a disordered induced mobility gap. Our results show that, as the temperature is decreased, the diffusion thermopower exhibits a T^-1 dependence in the insulating phase around ν=1. This behavior clearly indicates the opening of an energy gap at low temperature, in agreement with the formation of a pinned Wigner solid. Finally, we report on the T-dependence of the thermopower at ν=1.     

The electric field of an electron in an electron–hole plasma with degenerate electrons: Possibility of formation of a superconductivity state

We consider the possibility of the formation of a superconductivity state either in a semiconductor or in an electron–hole plasma with degenerate electrons due to the attractive forces between the electrons as a result of the exchange effects through the electron–hole sound wave by an analogy to the phonon waves in a solid state. We have determined an interaction potential between two electrons in a degenerate electron–hole plasma. The potential appears to be attractive at distances much larger than the Debye radius and decreases as 1/r³. We discuss the conditions in which the bound electron state, the so‐called “Cooper Pair,” in a such field can be formed.     

Effect of boundaries and impurities on electron–phonon dephasing

Electron scattering from boundaries and impurities destroys the single-particle picture of the electron–phonon interaction. We show that quantum interference between ‘pure‘ electron–phonon and electron–boundary/impurity scattering may result in the reduction as well as to the significant enlargement of the electron dephasing rate. This effect crucially depends on the extent, to which electron scatterers, such as boundaries and impurities, are dragged by phonons. Static and vibrating scatterers are described by two dimensionless parametersq_Tl and q_TL, where q is the wavevector of the thermal phonon, l is the total electron mean-free path, L is the mean-free path due to scattering from static scatterers. According to the Pippard ineffectiveness condition , without static scatterers the dephasing rate at low temperatures is slower by the factor 1 / ql than the rate in a pure bulk material. However, in the presence of static potential the dephasing rate turns out to be 1 / qL times faster. Thus, at low temperatures electron dephasing and energy relaxation may be controlled by electron boundary/impurity scattering in a wide range.     

Photoionization and electron–ion recombination of Cr I

Using the unified method, the inverse processes of photoionization and electron–ion recombination are studied in detail for neutral chromium, (), for the ground and excited states. The unified method based on close-coupling approximation and R-matrix method (i) subsumes both the radiative recombination (RR) and dielectronic recombination (DR) for the total rate and (ii) provides self-consistent sets of photoionization cross sections σ_PI and recombination rates α_RC. The present results show in total photoionization of the ground and excited states an enhancement in the background at the first excited threshold, state of the core. One prominent phot-excitation-of-core (PEC) resonance due to one dipole allowed transition (⁶S-⁶P^o) in the core is found in the photoionization cross sections of most of the valence electron excited states. Structures in the total and partial photoionization, for ionization into various excited core states and ground state only, respectively, are demonstrated. Results are presented for the septet and quintet states with n≤10 and l≤9 of Cr I. These states couple to the core ground state ⁶S and contribute to the recombination rates. State-specific recombination rates are also presented for these states and their features are illustrated. The total recombination rate shows two DR peaks, one at a relatively low temperature, at 630 K, and the other around 40,000 K. This can explain existence of neutral Cr in interstellar medium. Calculations were carried out in LS coupling using a close-coupling wave function expansion of 40 core states. The results illustrate the features in the radiative processes of Cr I and provide photoionization cross sections and recombination rates with good approximation for this astrophysically important ion.     

Quantum transport study of canted antiferromagnetic phase in the bilayer quantum Hall state

We examine the ν=2 bilayer quantum Hall (QH) state in clean two-dimensional electron systems (2DESs) to study effects due to not only the layer degree of freedom called pseudospin but also the real spin degree of freedom. The novel canted antiferromagnetic phase (CAF phase) has been predicted to emerge from subtle many-body electron interactions between the singlet (S) and ferromagnet (F) phases. Though several experiments indicate an onset of the CAF phase, a systematic transport study is not yet to be demonstrated. We have carried out magnetotransport measurements of the ν=2 bilayer QH state using a sample with tunneling energy . Activation energy was precisely measured as a function of the total density of the 2DES and the density difference between the two layers. Results support an appearance of the CAF phase between the S and F phases.     

Effect of electron–ion coupling on the electric microfield distribution in plasmas

The electronic and ionic effective potential of a fully ionized hydrogen plasma containing an impurity of electric charge (+Z _m e ) are calculated in a two‐component plasma model under semiclassical conditions using classical statistical mechanics with a regularized electron–ion interaction. These effective potentials are coupled in a system of nonlinear integral equations (or coupled differential equations), which is solved numerically with two methods, namely the fixed‐point method and the Runge–Kutta method. The Baranger–Moser electric microfield distributions are calculated and compared with those from molecular dynamics simulation. Agreement between theory and simulation is satisfactory, in general.     

Electron–hole complexes in self-assembled quantum dots in strong magnetic fields

We present results of calculations and experiments on electron–hole complexes in InGaAs/GaAs self-assembled quantum dots in high magnetic field (B). Due to hidden symmetries, the chemical potential of an N-exciton system at special B fields becomes insensitive to the exciton number as well as the magnetic field. This results in plateau regions of high intensity in measured magneto-PL spectrum. Theoretical calculations using exact diagonalization techniques successfully explain the measured magneto-photoluminescence spectrum with B fields up to 28 T.     

PLE studies of two-dimensional hole systems in the quantum Hall regime

We report a spectroscopic investigation of the densities of both occupied and unoccupied states of a high-quality two-dimensional hole system, in the regime of the quantum Hall effects (QHEs). Photoluminescence and photoluminescence excitation spectroscopies are used to elucidate the complicated valence band structure of the holes, and to establish their optical response to the QHEs.     

On the observation of electron–hole liquid luminescence under low excitation in Al2O3‐passivated c‐Si wafers

We report on low‐temperature photoluminescence (PL) from aluminum oxide (Al₂O₃)‐passivated c‐Si wafers, which surprisingly exhibits clear signature of the formation of the so‐called electron–hole liquid (EHL), despite the use of excitation powers for which the condensed phase is not usually observed in bulk Si. The elevated incident photon densities achieved with our micro‐PL setup together with the relatively long exciton lifetimes associated with a good quality, indirect band‐gap semiconductor such as our float‐zone c‐Si, are considered the key aspects promoting photogenerated carrier densities above threshold. Interestingly, we observe a good correlation between the intensity of the EHL feature in PL spectra and the passivation performance of the Al₂O₃ layer annealed at different temperatures. The change in the extension of the sub‐surface space‐charge region that results from the balance between the induced fixed charge in the Al₂O₃ and the defect states at the alumina/Si interface is at the origin of the observed correlation. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Temperature scaling of quantum Hall plateau transition in bilayer systems

We have investigated the scaling behavior of the quantum Hall plateau transition in double quantum well systems with different interlayer tunneling strengths. The scaling behavior of the localization property is found to be similar between the case when the relevant Landau level (LL) is non-degenerate and the case when two LLs associated with the two layers are accidentally degenerate. In both cases, the scaling exponent κ0.4 close to the canonical value is obtained, and it is unaffected by the in-plane magnetic field which changes the interlayer tunneling strength.     

Dissipative ion acoustic solitary waves in collisional,magneto‐rotating,non‐thermal electron–positron–ion plasma

The linear and non‐linear dynamics of ion acoustic waves are investigated in three‐component magnetized plasma consisting of cold inertial ions and non‐thermal electrons and positrons. The non‐thermal components are modelled by the hybrid distribution, representing the combination of two (kappa and Cairn's) non‐thermal distributions. The relevant processes, including the slow rotation of plasma along the magnetic field axis and collision between ions and neutrals, are taken into consideration. It is shown that the non‐linear dynamics of the considered system are governed by the Zakharov–Kuznetsov equation in modified form. In the general dissipation regime, the effects of the two non‐thermal distributions on the solitary waves are compared. The effects of other plasma parameters, such as collisional and rotational frequency, are also discussed in detail.     

Electric-field control of electron–hole wave functions in a wide quantum well

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-δ-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the first excited state has a dip in the electric-field dependence which reflects the node of the electron wave function.     

Quantum Hall effect in back-gated InAs/GaSb heterostructures under a tilted magnetic field

We investigate the quantum Hall effect (QHE) in the InAs/GaSb hybridized electron–hole system grown on a conductive InAs substrate which act as a back-gate. In these samples, the electron density is constant and the hole density is controlled by the gate-voltage. Under a magnetic field perpendicular to the sample plane, the QHE appears along integer Landau-level (LL) filling factors of the net-carriers, where the net-carrier density is the difference between the electron and hole densities. In addition, longitudinal resistance maxima corresponding to the crossing of the extended states of the original electron and hole LLs make the QHE regions along integer-ν_net discontinuous. Under tilted magnetic fields, these R_xx maxima disappear in the high magnetic field region. The results show that the in-plane magnetic field component enhances the electron–hole hybridization and the formation of minigaps at LL crossings.     

Electron–hole states in the fractional quantum Hall regime probed by photoluminescence

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling factors.     

Zeeman and spin–orbit effects on the spin-Hall conductance

We show that when a two-dimensional interacting electron gas is submitted to a perpendicular magnetic field, the application of an in-plane electric field E induces a spin current perpendicular to E whose conductivity is quantized. This current can lead to spin accumulation that might be detected by means of optical experiments. The appearance of this intrinsic spin-Hall effect is crucially based on the validity of Kohn's theorem and on the presence of the Zeeman term in the electron Hamiltonian. The possibility of resonant effects in the spin-Hall conductivity due to the combined effect of Rashba and Dresselhaus spin–orbit couplings is discussed.