Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.     

电子迴旋激射不稳定性

本文分析了下述情况下的电子迴旋波的激射不稳定性:当相对性的单能高能电子斜向注入具有背景等离子体的磁场区域内,并且在电子等离子体频率与电子迴旋频率可以比拟时,考虑了背景等离子体密度远大于单能的高能电子的密度,以及与前者相反的两种情况。当单能的高能电子具有弱相对论性效应时,在电子迴旋频率的基频和二次谐波附近,分别有ο模和χ模的不稳定性存在。文中计算了这两种模的增长率,并作了讨论。 关键词：     

Cyclotron masses in n-GaAs/Ga 1−xAl xAs heterojunctions

Electron cyclotron resonance is studied in single layer GaAs/Ga _1−xAl _xAs heterostructures. The cyclotron mass is measured over a wide range of the two-dimensional electron density N _s and the magnetic field strength B to enable detailed comparison with self-consistent theoretical results that take into account the effects of band nonparabolicity. The calculations are performed using an effective 2 × 2 subband Hamiltonian, which is derived from a five-level k·p-model by fourth order perturbation theory and includes remote band contributions. Close agreement between experimental and theoretical cyclotron masses is achieved and the importance of band nonparabolicity in these systems is demonstrated.     

Surface cyclotron resonance in Si under uniaxial stress

The cyclotron resonance of inversion-layer electrons on (100)p-type Si is found to depend sensitively on an externally applied compressive stress. At low temperatures (T ? 10 K) we observe a considerable increase of the cyclotron mass m¹_c with stress S along the [001] direction. The effect is most strongly observed at low electron densities n_s. For $\text{S～1.5 × 10}{}^9\text{dyne}\text{cm}{}^2$ and n_s～2 × 10¹¹cm^-2 we obtain $\text{m}{}^1{}_{\mathrm{c}}\text{～0.4 m}{}_0$ instead of the expected 0.2m₀. Along with this change of $\text{m}{}^1{}_{\mathrm{c}}$ a strong narrowing of the resonance is noted. Raising the temperature gives an additional n_s- dependent increase of $\text{m}{}^1{}_{\mathrm{c}}$.     

Magnetoabsorption in n-type InSb from 8 to 15 μm

Magnetoabsorption measurements in n-type InSb at T?30K have been made between ～ 8 and 15 μm using magnetic fields up to 150 kOe. The observed absorption peaks are identified as due to combined resonance, harmonics of cyclotron resonance and the corresponding LO phonon-assisted resonances. These resonant absorptions are considered to be important in the interpretation of the observed magnetic field behavior of the threshold and output power of the InSb spin-flip Raman laser pumped with 10.6 μm CO₂ laser.     

Shubnikov-de Haas effect of n-inversion layers in InSb grain boundaries

Hall effect and Shubnikov-de Haas effect have been investigated in a n-inversion layer adjacent to the grain boundary in p-InSb bicrystals. The observed quantum oscillations of the magnetoconductivity result from a superposition of the Shubnikov-de Haas effect in two occupied subbands. The electron densities of the subbands n_si(n_s1=3.5 × 10¹¹cm^-2;n_s0= 1.1 × 10¹²cm^-2) and the effective cyclotron mass of the lower subband m_c0=(0.023 ± 0.002)m₀ have been evaluated.     

Plasmon dispersion and intersubband resonance at high wavevectors in Si(100) inversion layers

Deviations from the classical two-dimensional plasmon dispersion are observed at high wavevectors q in electron inversion layers on Si(100) MOS-capacitors with periodically structured gate electrodes. For high inversion electron densities (n_s> 6 × 10¹²cm^?2) an unexpected mass enhancement is extracted from the plasmon dispersion. In addition the plasmon linewidth is found to be significantly larger than predicted from transport experiments. On the same samples non-vertical intersubband resonance transitions are observed with radiation incident normal to the interface. They are excited by an electric field component normal to the surface that is induced by the structured gate electrode.     

Surface demagnetizing field as a source of uniaxial surface anisotropy in GdCoMo thin amorphous films

Surface magnetic anisotropy energy was studied for (Gd_0.26Co_0.74)_0.96Mo_0.04 and (Gd_0.29Co_0.71)_0.96Mo_0.04 thin amorphous films by means of microwave spectroscopy at the X-band within the temperature range 4–295 K. Excitations of surface spin waves were observed in the spin wave resonance spectra. The experiment was performed in a rotating external magnetic field. The angular dependence of the resonance field for the uniform mode (spin wave vector k=0) and the surface mode made it possible to determine the surface uniaxial anisotropy constant K_s and its temperature dependence. An inhomogeneity of the saturation magnetization M_s within a close-to-surface layer of thickness d can generate the surface anisotropy energy with anisotropy constant K_s given by the formula: K_s=4πM^b_s (M^b_s−M^surf_s)d, where the indexes b and surf correspond to the bulk and surface values, respectively. The temperature dependence of K_s calculated by means of the formula agrees qualitatively with temperature dependence of K_s found in the experiment.     

Cyclotron resonance of electrons and holes in electric surface subbands of tellurium

We present first measurements of the submillimeter-cyclotron resonance of electrons and holes in electric surface subbands of tellurium. From the resonance in the inversion layer we have obtained for the magnetic field paralled to the trigonal axis, the cyclotron mass of the surface electron m_ce = (0.117 ± 0.002)m₀. The resonance of the accumulation layer splits and suggests the contribution of different nonparabolic subbands.     

Cyclotron resonance of hot electrons and nonlinear transport phenomena in n-type indium antimonide

Impurity states and nonlinear transport phenomena in n-type indium antimonide under strong magnetic fields have been extensively studied at liquid helium temperatures through H₂O laser cyclotron resonance combined with d.c. measurements. A new type cyclotron resonance with modulation by pulsed electric field, or PEM-CR, has been utilized throughout. Origin of several weak transitions so far indefinite has been identified. Existence of the donor binding state in a magnetic field as low as 2·85 kOe for the excess donor concentration N_D ? N_A = 2 × 10¹³ cm^?3 is experimentally confirmed. Joint determination of resistivity and carrier distribution in the energy space has yielded a fair success in separating the mechanisms for the nonlinear transport behavior.     

Charge accumulation layers and surface states in ultrathin Cs,Ba/n-GaN(0001) interfaces

It is found that ultrathin cesium and barium coatings radically change the electronic properties of the surface and the near-surface region of epitaxial n-GaN(0001) layers. A charge accumulation layer serving as a quasi-two-dimensional electronic channel is first formed by adsorption on the surface of a semiconductor. It is revealed that photoemission from the accumulation layer is excited by visible light from the transparency region of GaN and is characterized by a high quantum yield. It is found that the photoemission thresholds hν _s and hν _p for s-and p-polarized excitation are equal to each other and correspond to the work function. The lowest work function for Cs,Ba/n-GaN interfaces is observed at Cs or Ba coverages close to 0.5 monolayer. Two bands induced by the local interaction of cesium (barium) adatoms with gallium dangling bonds are detected in the electronic spectrum of surface states of Cs,Ba/n-GaN interfaces. An oscillation structure is observed in spectral dependences of the photoyield. This effect is new for photoemission. A model of the effect is proposed. It is found that electronic and photoemission properties of the interfaces correlate with the structural perfectness of the epitaxial n-GaN(0001) layers.     

Three-photon resonance enhanced excitation spectrum of atomic thallium

We present first measurements on the resonance enhanced three-photon excitation in thallium, using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. The even-parity 6s²ns²S_1/2 (15 ? n ? 31) and nd ²D_5/2 (13 ? n ? 42) Rydberg states have been observed. The measured level energies reveal a dynamic shift from the photoabsorption values, which is decreasing with increasing n, while the asymmetry in the line profile is observed to be increasing with increasing n. In addition, an autoionising level (sp²⁴P_3/2) adjacent to the ionization threshold has been observed and quantitatively analyzed using the Fano’s photoionization cross-section relation for an isolated autoionising resonance.     

Surface states of charge carriers in epitaxial films of the topological insulator Bi2Te3

The galvanomagnetic properties of p-type bismuth telluride heteroepitaxial films grown by the hot wall epitaxy method on oriented muscovite mica substrates have been investigated. Quantum oscillations of the magnetoresistance associated with surface electronic states in three-dimensional topological insulators have been studied in strong magnetic fields ranging from 6 to 14 T at low temperatures. The cyclotron effective mass, charge carrier mobility, and parameters of the Fermi surface have been determined based on the results of analyzing the magnetoresistance oscillations. The dependences of the cross-sectional area of the Fermi surface S(k _F), the wave vector k _F, and the surface concentration of charge carriers n _s on the frequency of magnetoresistance oscillations in p-type Bi₂Te₃ heteroepitaxial films have been obtained. The experimentally observed shift of the Landau level index is consistent with the value of the Berry phase, which is characteristic of topological surface states of Dirac fermions in the films. The properties of topological surface states of charge carriers in p-type Bi₂Te₃ films obtained by analyzing the magnetoresistance oscillations significantly expand fields of practical application and stimulate the investigation of transport properties of chalcogenide films.     

Surface modes in magnetic thin amorphous films of GdCoMo alloys

Surface modes in spin wave resonance in thin amorphous films of (Gd_1-xCo_x)_1-yMo_y alloys were studied. The samples were obtained with rf sputtering technique and a bias voltage was applied. Technological conditions were carefully pre-determined for which surface modes were excited in the resonance experiment. One surface mode was present for samples just after deposition and two modes could be observed in some cases for the same samples kept at room temperature for two or three months. The surface anisotropy constant K_s was determined from the surface inhomogeneity (SI) model with symmetrical or non-symmetrical boundary conditions for one or two surface modes, respectively. The fitted K_s values agree with theoretically predicted ones and they are also compatible with numbers found by experimentalists for monocrystalline, polycrystalline or amorphous films.For all samples we also determined the critical angles θ_c's between the external magnetic field and the normal to the film plane for which the position of the surface mode coincides with the position of the first volume mode. The corresponding critical angles ?_c's for the magnetization differ from π/4 which suggests the presence of surface inhomogeneities of the magnetization distribution.     

Doppler-shifted cyclotron resonance in tungsten plates with atomic pure surface

The surface resistance of thin monocrystalline W plates as a function of the constant magnetic field H directed along the normal to the sample surface is studied in the r.f. spectrum region. The sample surface was cleaned in high vaccum (10^-11 torr) or coated with the monomolecular impurity film. The oscillating with the magnetic field part R_osc due to the Doppler-shifted cyclotron resonance is studied. The doppleron oscillation amplitude is found to depend on the surface state and increases with the crystal cleaning. The observed changes are caused by the increase of the specular reflection coefficient for resonance electrons. With the deviation of the magnetic field from the normal to the plate surface, the doppleron wave undergoes a collisionless magnetic Landau damping and the signal amplitude decreases down to values comparable with that of Gantmakher-Kaner oscillations. Cleaning of the surface (and related increase of specularity) gives rise to a further decrease of the doppleron amplitude and appearance of additional interference maxima induced by the Gantmakher-Kaner effect.     

Memory function for cyclotron resonance of two-dimensional electron systems

Coulomb effects on cyclotron resonance in two-dimensional electron systems are investigated based on a self-consistent approach which improves the random-phase approximation. The memory function in the dynamic magnetoconductivity depends on the electron density not only through the filling factor v but also on its combination with the dimensionless density parameter r_s in the form $\text{r}{}_{\mathrm{s}}\text{v}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$. The memory function reproduces the data of Wilson, Allen and Tsui for intermediate densities.     

Effective electron mass in high-mobility SiGe/Si/SiGe quantum wells

The effective mass m* of the electrons confined in high-mobility SiGe/Si/SiGe quantum wells has been measured by the analysis of the temperature dependence of the Shubnikov-de Haas oscillations. In the accessible range of electron densities, n _s , the effective mass has been found to grow with decreasing n _s , obeying the relation m*/m _b = n _s /(n _s ? n _c ), where m _b is the electron band mass and n _c ≈ 0.54 × 10¹¹ cm^?2. In samples with maximum mobilities ranging between 90 and 220 m²/(V s), the dependence of the effective mass on the electron density has been found to be identical suggesting that the effective mass is disorder-independent, at least in the most perfect samples.     

Electron effective mass determination in asymmetric modulation-doped field-effect transistor heterostructures using InxGa1 − xAs quantum well and InAs–GaAs superlattice channels

Cyclotron resonance and photoluminescence measurements have been performed on two types of modulation-doped field-effect transistor heterostructures having their bidimensional channel based, respectively, on an In_xGa_1 − xAs quantum well and an InAs–GaAs short-period superlattice. A linear dependence of the electron effective mass as a function of indium content of the channel was obtained from cyclotron resonance measurements. For a given average value of the indium content, the effective mass in the InAs–GaAs short-period superlattice channel is found to be systematically higher than that obtained in structures with an alloy-based channel. This is attributed to larger nonparabolic effects in the former case. In our theoretical model, the electron and heavy hole energy levels and the electron wavefunction are determined self-consistently and used to estimate the nonparabolic corrections that apply to the effective mass deduced from cyclotron resonance measurements.     

On the cyclotron resonance width of an electron impurity system in two dimensions

Using the proper connected diagram expansion which incorporates the quasi-particle effect naturally we calculated the cyclotron resonance width γ in the extreme quantum limit.(a) γ ∝n

$\text{1}\text{2}$

_s B

$\text{1}\text{2}$

for a short range interaction, and (b) γ = π

$\text{1}\text{2}$

ze² κ^-1 <?crossed h.c.h;^-1 n

$\text{1}\text{2}$

_s for Coulomb interaction, are obtained. The field (B) and concentration (n_s)-dependence is in satisfactory agreement with experimental data. The variation γ = (γ²₁ + γ²₂ + …)

$\text{1}\text{2}$

of Matthiessen's rule γ = γ₁ + γ₂ + …, holds when there exist scattering centers of different kinds in the system.     

Iron overload diseases: the chemical speciation of non-heme iron deposits in iron loaded mammalian tissues

⁵⁷Fe Mössbauer spectra of iron overloaded human spleen, rat spleen and rat liver tissue samples at 78 K were found to consist of a quadrupole doublet (major component) with magnetic sextet (minor component with fractional spectral area F _s). The distributions of F _s for spleen tissue from two different clinically identifiable groups (n = 7 and n = 12) of thalassemic patients were found to be significantly different. The value of F _s for dietary-iron loaded rat liver was found to rise significantly with age/duration (up to 24 months) of iron loading.