Nonlinear magnetotransport in a high-mobility quasi-two-dimensional electron system

The influence of a dc electric current I _dc on the low-temperature magnetotransport of high-mobility electrons in a GaAs double quantum well with two occupied size-quantization levels has been studied. The oscillations of the resistance ρ _xx , which are periodic in the inverse magnetic field, have been shown to appear in the quasitwo-dimensional system under consideration at a temperature of T = 4.2 K in magnetic fields B > 0.1 T; the oscillations are caused by isoenergetic resonance transitions of the electrons between the Landau levels of different subbands. The inversion of the oscillations with an increase in I _dc has been discovered. It has been found that the observed effect is due to the electron transport in a nonlinear regime.     

High-performance integrating infrared detector

Improvement in the performance of a Si: P integrating infrared (IR) detector are described. The detector has the structure of a p-i-n diode but the mode of IR detection involves photoemission of charge stored in localized impurity states in the i-region at low temperatures. After IR exposure, the remaining occupied localized states are field ionized. In order to measure the remaining charge, the transient field ionization current is digitized and integrated. Increased quantum efficiency is obtained by increasing the photosensitive i-region thickness and impurity concentration. With increasing impurity concentration, the quantum efficiency of a 110 μm thick detector can be increased from 0.5% to about 70% in the neighborhood of 27 μm while the maximum charge storage time (IR integration time) decreased from more than 12 h to about l s.     

Strongly correlated electron system in the magnetic field

In the range of hole concentrations 0.08<x<0.18$0.08<x<0.18$ the density of states of the two-dimensional t–J model reveals oscillations with changing the magnetic field. Oscillation frequencies correspond to large Fermi surfaces. However, the oscillations are modulated with a frequency which is smaller by an order of magnitude. The modulation is related to van Hove singularities in the Landau subbands, which traverse the Fermi level with changing the field. The singularities are connected with bending the subbands due to strong electron correlations. The frequency of the modulation is of the same order of magnitude as quantum oscillation frequencies in underdoped cuprates.     

Magneto-coulomb oscillations in GaAs-AlGaAs quantum dot structures

We report on experiments of the magnetotransport properties of GaAs-AlGaAs lateral quantum dots. At high magnetic fields for a 1 μm square dot structure, current flow occurred via edge states and, with the point contacts adjusted to allow transmission of one or more edge states, a strong backscattering resonance followed by short period oscillations were observed in the magnetoresistance, as B increased. At higher fields for a 2 μm dot, we observe a rapid rise in the magnetoresistance associated with the depopulation of the point contacts and the isolation of the dot from the leads. At still higher fields there occur periodic oscillations whose period was two orders of magnitude larger than would result from interference, or Aharonov-Bohm type effects.We analyze these phenomena using self-consistent electronic structure calculations for our devices. In particular, we show that the evolution of the terrace like structure of the potential profile profoundly affects the single particle spectrum within the dot when several Landau levels are occupied. For the large dot device, we expect that in the high field regime with the dot isolated from the leads, only a single Landau level is occupied in both the dot and the 2DEG region. In this regime, tunneling into and out of the dot is regulated by charging effects. We have introduced a "magneto-Coulomb oscillations" explanation of the periodic resonances that are observed.     

Two-dimensional electron gas in double quantum wells with tilted bands

When a voltage is applied to double quantum wells based on AlGaAs/GaAs heterostructures with contact regions (n-i-n structures), a two-dimensional (2D) electron gas appears in one of the quantum wells. Under illumination which generates electron-hole pairs, the photoexcited holes become localized in a neighboring quantum well and recombine radiatively with the 2D electrons (tunneling recombination through the barrier). The appearance, ground-state energy, and density of the degenerate 2D electron gas are determined from the structure of the Landau levels in the luminescence and luminescence excitation spectra as well as from the oscillations of the radiative recombination intensity in a magnetic field with detection directly at the Fermi level. The electron density is regulated by the voltage between the contact regions and increases with the slope of the bands. For a fixed slope of the bands the 2D-electron density has an upper limit determined by the resonance tunneling of electrons into a neighboring quantum well and subsequent direct recombination with photoexcited holes. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 11, 840–845 (10 June 1997)     

Manifestation of exitonic effects in the magneto-oscillations of the intensity of the recombination radiation of 2D electrons

A study is made of the temperature dependence of the magnetooscillations of the recombination radiation of 2D electrons from the photoexcited size-quantization subband in an isolated GaAs/AlGaAs quantum well. It is shown that at high temperatures (T>10 K) the period of the oscillations is determined by the ratio of the intersubband energy splitting and the sum of the electron and hole cyclotron energies. It is found that as the temperature decreases (T<5 K), a new series of oscillations (with the same period but with a larger phase shift), which are associated with the appearance of excitonic states under the Landau levels, appears. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 719–724 (25 November 1996)     

de Haas van Alphen (dHvA) effect in two-dimensional (2D) conductors: susceptibility oscillations

A new scheme for analyzing the de Haas van Alphen (dHvA) effect in nearly two dimensional (2D) metals (i.e. with nearly cylindrical Fermi surface) is presented. The envelope of the magnetic susceptibility oscillations is calculated in the entire range of magnetic fields and temperatures. The resulting envelope function is found to be proportional to a universal function of the dimensionless parameter Q=hω_c/k _B T. The upper (i.e. paramagnetic) branch of the susceptibility envelope has a maximum at a certain Q = 5.45. This universal value may be useful for determining the effective cyclotron mass and the Fermi energy of nearly 2D metals. A simple relation between magnetization oscillations amplitude and calculated susceptibility amplitudes is derived. The corresponding limiting formulae for the magnetization oscillations envelope are found to match smoothly around the value X = 2π²/Q?2 of the Lifshitz-Kosevich (LK) smearing parameter. The influence of Fermi surface sheets with open orbits on magneto-quantum oscillations is considered. Triangle-like rather than saw-tooth-like oscillations at ultralow temperatures are obtained and substantially diminished magnetization and susceptibility amplitudes are calculated. This suggests the possibility of estimating the band structure parameters of Fermi surface sheets from magneto-quantum oscillations measurements.     

Zener tunneling between Landau levels in a double quantum well at high filling factors

Differential resistance r _xx in a double GaAs quantum well with two occupied size-quantization subbands has been studied at a temperature of 4.2 K in magnetic fields B < 2 T. The oscillations of r _xx with a period in the inverse magnetic field determined by the value of a dc bias current I _dc have been discovered in the electron system under investigation at high filling factors in the presence of I _dc. The amplitude of magneto-intersubband oscillations has been shown to increase in the r _xx oscillation maxima, while the oscillation reversal has been observed in the minima. The discovered oscillations have been shown to be due to Zener tunneling of electrons between Landau levels tilted by a Hall electric field. The experimental data are qualitatively explained by the effect of intersubband transitions on the I _dc-dependent component of the electron distribution function.     

Zeeman splitting of interacting two-dimensional electrons with two effective masses

We have realized an AlAs two-dimensional electron system in which electrons occupy conduction-band valleys with different Fermi contours and effective masses. In the quantum Hall regime, we observe both resistivity spikes and persistent gaps at crossings between the Landau levels originating from these two valleys. From the positions of the spikes in tilted magnetic fields and measurements of the energy gaps away from the crossings, we find that, after occupation of the minority valley, the spin susceptibility drops rapidly, and the electrons possess a single interaction-enhanced g-factor, despite the dissimilarity of the two occupied valleys.     

Fisher Information of Free-Electron Landau States

An electron in a constant magnetic field has energy levels, known as the Landau levels. One can obtain the corresponding radial wavefunction of free-electron Landau states in cylindrical polar coordinates. However, this system has not been explored so far in terms of an information-theoretical viewpoint. Here, we focus on Fisher information associated with these Landau states specified by the two quantum numbers. Fisher information provides a useful measure of the electronic structure in quantum systems, such as hydrogen-like atoms and under some potentials. By numerically evaluating the generalized Laguerre polynomials in the radial densities, we report that Fisher information increases linearly with the principal quantum number that specifies energy levels, but decreases monotonically with the azimuthal quantum number m. We also present relative Fisher information of the Landau states against the reference density with m=0, which is proportional to the principal quantum number. We compare it with the case when the lowest Landau level state is set as the reference.     

Random magnetic impurities and the Landau problem

The 2-dimensional density of states of an electron is studied for a poissonian random distribution of point vortices carrying α flux in units of the quantum of flux. It is shown that, for any given density of impurities, there is a transition, when α ≅ 0.3–0.4, from an “almost free” density of state - with only a depletion of states at the bottom of the spectrum characterized by a Lifschitz tail - to a Landau density of state with sharp Landau level oscillations. Several evidences and arguments for this transition - numerical and analytical - are presented.     

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.     

Dephasing in the presence of a two-dimensional electron gas at high magnetic fields

By means of degenerate four-wave mixing (FWM), we investigate the quantum coherence of electron–hole pairs in the presence of a two-dimensional electron gas in modulation-doped GaAs–AlGaAs quantum wells in the regime of the integer quantum-Hall effect. We observe large jumps in the decay time of the FWM signal at even Landau level filling factors. The main features of the experimental observations can be qualitatively reproduced by a model which takes into account the number of unoccupied states within the highest partially occupied Landau level. Furthermore, we observe quantum beats between up to three different Landau level transitions.     

The physics of helical electron beam in a uniform magnetic field as a testing ground of gauge principle

According to Bliokh et al., allowing free propagation along the direction of a uniform magnetic field, the familiar Landau electron state can be regarded as a non-diffracting version of the helical electron beam propagating along the magnetic field. Based on this observation, they argued that, while propagating along the magnetic field, the Landau electrons receive characteristic rotation with three different angular velocities, depending on the eigen-value m of the canonical OAM operator, which is generally gauge-variant, and this splitting was in fact experimentally confirmed. Through complete analyses of highly mysterious m-dependent rotational dynamics of the quantum Landau states, we try to make clear how and why their observation does not contradict the widely-believed gauge principle.     

Magnetotunneling relaxation of electrons in double quantum wells

Electron tunneling relaxation in double quantum wells subject to a transverse magnetic field is studied. The resonant peaks in the tunneling relaxation rate appear when the energy splitting Δ of the tunnel-coupled pair of the left- and right- well electron states is a multiple of the cyclotron energy ℏω_c and two series of the Landau levels coincide. The shape of such resonant oscillations of the relaxation rate is determined by the Landau levels' broadening (which is associated with the intrawell scattering in the case of small tunnel coupling), but it is not expressed through the electron density of states directly. The dependence of the tunneling relaxation rate on ℏω_c and Δ is calculated taking into account elastic scattering of the electrons by the inhomogeneities of the structure in the limit when the scattering potential is slowly changing on the magnetic length scale.     

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.     

Magnetic field dependence of charge density waves in PbTe

Previous work by the authors indicated the possible existence of charge density waves in PbTe in the presence of strong magnetic fields so that the extreme quantum limit conditions prevail. The present work follows the transition as the field is lowered so that successively higher Landau levels become occupied. The transition persists at increasingly lower temperatures, having a maximum value, T_cmax, identifiable with each value, N, of the last (partially) occupied Landau level.     

Zener Tunneling between the Landau Levels in a Two-Dimensional Electron System with One-Dimensional Periodic Modulation

Nonlinear magnetotransport in a two-dimensional electron gas in one-dimensional lateral lattices fabricated from a selectively doped GaAs/AlAs heterostructure is investigated. One-dimensional potential modulation is imposed on the two-dimensional electron gas by means of a set of metal strips formed on the planar surface of Hall bars. The dependences of the differential resistance r_xx on the magnetic field B < 0.5 T are studied at a temperature T = 1.6 K in lattices with a period of a ≈ 200nm. It is shown that periodic oscillations in r_xx(1/B) occur in such lattices under the action of a current-induced Hall field due to Zener tunneling between Landau levels. Interference is found between Zener oscillations and commensurability oscillations of r_xx in two-dimensional electron systems with one-dimensional periodic modulation. The experimental results are qualitatively explained by the role of Landau bands in nonlinear transport at large filling factors.     

Electronic properties of a Weyl semimetal in crossed magnetic and electric fields

The study of Weyl semimetals is one of the most challenging problems of condensed matter physics. These materials exhibit interesting properties in a magnetic field. In this work, we investigate the Landau bands and the density of states (DOS) oscillations in a Weyl semimetal in crossed magnetic and electric fields. An expression is obtained for the energy spectrum of the system using the following three different methods: an algebraic approach, a Lorentz shift-based approach, and a quasi-classical approach. It is interesting that the energy spectrum calculated in terms of the quasi-classical approach coincides with the spectrum obtained using the microscopic approaches. An electric field is shown to change the Landau bands radically. In addition, the classical motion of a three-dimensional Dirac fermion in crossed fields is studied. In the case of a Dirac spectrum, the longitudinal (with respect to magnetic field) component of momentum (p _z ∥ H) is shown to be an oscillating function of the magnetic field. When the electric field is v⊥H/c, the Landau levels collapse and the motion becomes fully linear in an unusual manner. In this case, the wavefunction of bulk states vanishes and only states with p _z = 0 are retained. An electric field affects the character of DOS oscillations. An analytical expression is obtained for the quantum capacitance in crossed fields in the cases of strong and weak electric fields. Thus, an electric field is an additional parameter for adjusting the diamagnetic properties of Weyl semimetals.     

The RPA nuclear continuum in16O(e,e′) reactions at low momentum transfer decay on (e,e′p) and (e,e′n) reaction channels

A theoretical investigation has been performed of¹⁶O(e, e′) and¹⁶O(e, e′x) reactions at low momentum transfer in the frame of a self-consistent HF and RPA theory with a SK3 interaction. Nuclear responses and their multipole components have been calculated in the whole energy-range for the two electron kinematicsi) ? _i =67 MeV and θ=40°,ii) ? _i =130 MeV and θ=50°. The microscopic structure of HF and RPA resonating states in the energy continuum has been inferred from the calculation. Decay properties in the reaction channels (e, e′p) and (e, e′n) have been discussed in the two cases of a semidirect and a knockout reaction process.