Transport poperties of a GaAs/AlGaAs ring interferometer in the tunneling regime

The transport properties of GaAs/AlGaAs submicron rings with split gates in the conditions corresponding to the ring resistance R _SD τ;h/e ² are studied. Oscillations of R _SD as a function of the gate voltage V _G are experimentally observed. The oscillations are caused by the single-electron charging of two triangular conducting regions into which the ring is divided in the tunneling regime.     

Electron-positron bremsstrahlung at ADONE. A new limit for the existence of a heavy electron

A new measurement of the cross section for the reaction e⁺e^? → e⁺e^?γ has been performed at the ADONE storage ring, using a wide angle electromagnetic detector together with tagging counters for detecting small-angle electrons. The results are in agreement with the QED predictons. New limits for the mass and coupling constant (e¹, eγ) of a heavy electron, e¹, have been established.     

Coulomb blockade in a lateral triangular small quantum dot

An AlGaAs/GaAs lateral quantum dot of triangular shape with a characteristic size L<100 nm (the smallest size for dots of this type) containing less than ten electrons was studied theoretically and experimentally. Single-electron oscillations of the conductance G of this dot were measured at G<e ₂/h. When going from G?e ₂/h to G≈0.5e ₂/h, a decrease was found not only in the amplitude but also in the period of oscillations. A calculation of the electrostatics demonstrated that this effect is due to a change in the dot size produced by control voltages.     

Transmission in symmetrical GaAs/AlxGa1-xAs double-barriers with compositionally nonabrupt interfaces

Transmission properties of electrons through GaAs/Al_xGa_1-xAs symmetrical double-barriers with abrupt and nonabrupt interfaces are calculated and compared. The interface potential and carrier effective-mass are obtained assuming a linear variation of the aluminium molar fraction in the transition regions GaAs ⇔ Al_xGa_1-xAs. When the electron energy E_0,-1,e is smaller than the double-barrier height V_x0 , changes in the internal interfaces widths shift tunneling resonances, while changes in the external interfaces increase the energy widths of the resonant transmission peaks. When E_0,-1,e > V_x0, both external and internal interfaces modify remarkably the transmission of nonabrupt GaAs/Al_xGa_1-xAs double-barriers when compared with the abrupt double-barrier, even if their widths are as small as two GaAs lattice parameters. However, the first transmission peaks of abrupt and nonabrupt GaAs/Al_xGa_1-xAs double-barriers are very similar, except when the interface widths are greater than four lattice parameters.     

Mesoscopic fluctuations of thermopower in a periodic antidot lattice

The mesoscopic fluctuations of thermopower (MFT) were experimentally observed in an AlGaAs/GaAs heterojunction with a low-resistance (～0.02h/e²) periodic antidot lattice in the situation where the mesoscopic fluctuations of conductance (MFC) were absent to within the experimental accuracy. The MFT spectrum contained a periodic component associated with the Aharonov-Bohm h/e oscillations in the area occupied by one antidot, whereas the h/2e oscillations were not observed. It is shown that a sizable contribution to the MFT comes from the interference of electron trajectories localized inside billiards formed by four neighboring antidots. Contrary to MFC, the MFT autocorrelation function in single billiards deviates from the Lorentzian form.     

Magneto-transport in a binary alloy ring

Magneto-transport properties are investigated in a binary alloy ring subjected to an Aharonov–Bohm (AB) flux ?   within a single-band non-interacting tight-binding framework. In the first part, we expose analytically the behavior of persistent current in an isolated ordered binary alloy ring as functions of electron concentration _Ne$N_e$ and AB flux ?. While, in the second part of the Letter, we discuss electron transport properties through a binary alloy ring attached to two semi-infinite one-dimensional metallic electrodes. The effect of impurities is also analyzed. From our study we propose that under suitable choices of the parameter values the system can act as a p-type or an n-type semiconductor.     

Improvement of stability and pulse energy in a diode-pumped dual-loss-modulated QML Nd:Lu0.2Y0.8VO4 laser with EO modulator and GaAs

The Q-switched and mode-locked (QML) performance in a diode-pumped Nd:Lu_0.2Y_0.8VO₄ laser with electro-optic (EO) modulator and GaAs saturaber absorber is investigated. In comparison with the solely passively QML laser with GaAs, the dual-loss-modulated QML laser with EO and GaAs can generate pulses with higher stability and shorter pulse width of Q-switched envelope, as well as higher pulse energy. At the repetition rate 1 kHz of EO, the pulse width of Q-switched pulse envelope has a compression of 89% and the pulse energy has an improvement of 24 times. The QML laser characteristics such as the pulse width, pulse peak power etc. have been measured for different small-signal transmittance (T₀) of GaAs, different reflectivity (R) of output coupler and modulation frequencies of the EO modulator (f_e). The highest peak power and the shortest pulse width of mode-locked pulses are obtained at f_e = 1 kHz, R = 90% and T₀ = 92.6%. By considering the influences of EO modulator, a developed rate equation model for the dual-loss-modulated QML laser with EO modulator and GaAs is proposed. The numerical solutions of the equations are in good agreement with the experimental results.     

Interference of ballistic electrons in an open quantum dot at high temperatures

The Aharonov-Bohm-type oscillations in the ballistic magnetoconductivity of an open quantum dot ～1 μm in size created in the GaAs/AlGaAs heterostructure by local anode oxidation have been studied. The measurements have been performed at temperatures of 4.2 and 1.5 K, which are high enough to expect a considerable suppression of the oscillations with period h/e (in magnetic flux units). The magnetoconductivity oscillations with a period less than the quantum h/e with respect to the magnetic field are observed at 1.5 K. The explanation is proposed on the basis of the interference of the electrons moving along the time-reversed paths inscribed into the quantum dot that have their initial and final points at one of the contacts, i.e., corresponding to a period of h/2e.     

Wide angle electron-positron bremsstrahlung at adone a new limit for the existence of a heavy electron

The cross section for the reaction e⁺+e^?→e⁺+e^?+γ has been measured at the Adone storage ring. The results in agreement with the QED predictions, establish a new limit for the mass and the coupling constant $\text{e}{}^1\text{e}\text{γ}$ of a heavy electron e¹.     

Semiconductor artificial graphene: Effects in weak magnetic fields

Two-dimensional quantum transport through the stripe of the hexagonal lattice of antidots built in the multimode channel in the GaAs/AlGaAs structure has been studied numerically. It has been found that the low perpendicular magnetic fields (～3 mT) suppress the bulk currents and cause the appearance of the edge Landau states and high positive magnetic resistance on both sides of the Dirac point. Tamm edge states are present in some energy intervals; as a result, the 4e ²/h-amplitude oscillations caused by the quantization of these states on the lattice length are added to the steps of the conductance quantization G _n = (2|n| + 1)2e ²/h.     

Observation of a narrow resonance at 9.46 GeV in electron - positron annihilations

We observe a narrow resonance in the reaction e⁺e^? → hadrons using the DASP detector at the DORIS storage ring. The mass is found to be (9.46 ± 0.01) GeV and the observed width is compatible with the storage ring resolution of ±8 MeV. The energy-integrated cross section results in an electronic width Γ_ee = (1.3 ± 0.4) keV.     

Magnetic field dependence of the thermoelectric power of n - type gallium arsenide in the extreme quantum limit

The effect of the quantization of the electron energy levels in a strong transverse magnetic field H on the low-temperature thermoelectric power (TEP) S of a high-purity isotropic semiconductor ($\text{n}$ - type gallium arsenide GaAs) is investigated theoretically. The “electron-diffusion” (S_e) and “phonon-drag” (S_p) components of S( = S_e + S_p) are calculated in the extreme quantum limit, when all the electrons in the conduction band are concentrated in the lowest Landau level. The transition to nondegeneracy, which takes place when the bottom of the lowest Landau level is driven through the Fermi level, has a large effect on the variations of S_e and S_p with magnetic field. The results are illustrated with numerical calculations for $\text{n}$ - type GaAs at 4.2 K with 1.2 × 10¹⁶ cm^-3 electrons.     

Thermoelectric power of semiconductors in the extreme quantum limit—I: The ‘electron-diffusion’ contribution

A study is made of the effect of the quantization of the electron energy levels in a strong magnetic field on the ‘electron-diffusion’ contribution, S_e, of the transverse thermoelectric power of a high-purity isotropic semiconductor (n-type gallium arsenide GaAs) in the extreme quantum limit, when all the carriers in the conduction band are in the lowest Landau level. A theoretical expression for S_e is derived, taking into account of spin splitting. The sign of S_e is either positive or negative, depending on the sign of the conducting carriers. The transition to nondegeneracy, which takes place when the lowest Landau level is driven through the Fermi level, has a large effect on the variation of S_e with magnetic field. This effect is characterized by a large and rapid increase in |S_e|. Spin splitting effects, even with a small effective mass ($\text{m1 = 0·07 m}{}_0$, where m₀ is the free-electron mass) and a very small g-factor (g ≈ 0·32), greatly modify and reduce |S_e| a a function of field. For large fields, |S_e| increases monotonically with H. Calculations are carried out for n-type GaAs at T = 0·5°K with N = 1·2 × 10¹⁶cm^?3 carriers.     

Low noise field-effect transistor for integrated strain sensing in microelectromechanical systems at 77 K

We have fabricated a strain-sensing field-effect transistor (FET) from a GaAs/AlGaAs heterostructure containint a near surface two-dimensional electron gas. At 77 K the FET shows a typical transconductance of 30 μS and small signal drain-source resistance of 30 MO. The charge noise has a flat spectrum at high frequencies with magnitude 0.7eHz^−1/2and a 1/fnoise corner of approximately 1 kHz. The strain response, due to the piezoelectric effect, shows a noise limited strain sensitivity <4×10⁻⁹Hz^−1/2. Integrated strain sensing FETs offer advantages in small GaAs/AlGaAs microelectromechanical systems.     

Evidence for a resonant structure of the hadron production from e+e− annihilations in the 1700–1950 cm energy region

We present experimental evidence for a resonant behaviour of the hadron production from e⁺e^? annihilations at the e⁺e^? storage ring ADONE. A Breit-Wigner fit to the enhancement present between 1800 and 1850 MeV gives the following parameters M = 1812_?13⁺⁷ MeV, Γ = 34_?15⁺²¹ MeV.     

Energy relaxation of two-dimensional electrons in the quantum Hall effect regime

The mm-wave spectroscopy with high temporal resolution is used to measure the energy relaxation times τ_e of 2D electrons in GaAs/AlGaAs heterostructures in magnetic fields B=0–4 T under quasi-equilibrium conditions at T=4.2 K. With increasing B, a considerable increase in τe from 0.9 to 25 ns is observed. For high B and low values of the filling factor ν, the energy relaxation rate τ _e ^?1 oscillates. The depth of these oscillations and the positions of maxima depend on the filling factor ν. For ν>5, the relaxation rate τ _e ^?1 is maximum when the Fermi level lies in the region of the localized states between the Landau levels. For lower values of ν, the relaxation rate is maximum at half-integer values of τ _e ^?1 when the Fermi level is coincident with the Landau level. The characteristic features of the dependence τ _e ^?1 (B) are explained by different contributions of the intralevel and interlevel electron-phonon transitions to the process of the energy relaxation of 2D electrons.     

Investigation of the mesoscopic Aharonov–Bohm effect in low magnetic fields

We have investigated the Aharonov–Bohm effect in mesoscopic semiconductor GaAs/GaA1As rings in low magnetic fields. The oscillatory magnetoconductance of these systems is systematically studied as a function of electron density. We observe phase shifts of π in the magnetoconductance oscillations, and halving of the fundamental h/e period, as the density is varied. Theoretically, we find agreement with the experiment, by introducing an asymmetry between the two arms of the ring.     

Measurement of electron energy and density in an H. F. electrodeless ring discharge

The electron temperatureT _e and the electron density were measured as functions of the radial distance in a 10 Mc/s electrodeless ring discharge in hydrogen in the pressure range 0.1–0.3 Torr. It was found thatT _e remains nearly constant along the radius of the cylindrical vessel. The measured values ofT _e have been compared with those observed by other workers and an estimate of the effective electric field in the discharge has been made. From this estimate it has been inferred that even after the first stage of the ring discharge is well established the longitudinal component of the electric field remains of considerable importance. The radial density distribution of the electrons was found to be different from those in an uniform electric field. This deviation in the radial density distribution has been attributed to the influence of the azimuthal electric field.     

Search for the radiative η′(958)+γ decay of the J/Ψ (3100) resonance produced in e+e− annihilation

The decay J/Ψ (3100)→ η(958) + γ has been searched for at tthe Adone e⁺e^+- storage ring. No candidate event has been found. This allows to set an upper limit of 1.0 keV (90% C.L.) for this decay partial width.     

Influence of exciton energy on intersubband transition of chirped superlattice GaAs/AlGaAs quantum cascade laser

The influence of exciton energy on intersubband transition was simulated for a chirped supperlattice quantum cascade laser of GaAs/Al_xGa_1-xAs. Exciton energy was modelled as a function of QW width for alloys of various percentages of constituent elements. The results showed that the exciton energy decreased proportionally with increasing QW width. Models were also generated to study exciton energy as a function of the percent alloy contents of Al_xGa_1-xAs barriers for QWs of various widths. Exciton energy showed characteristics of higher discrete energy when QW width was narrower. Transition energy was also simulated from e₁ and e₂ to the 1s exciton state as functions of applied electric field at various QW widths. Our simulation results showed that the transition energy from e₂ to the 1s exciton state increased proportionally to the increasing strength of the electric field. This transition energy was indicative of THz range radiation.