Raman-gain, linewidth, and effective g -value with spin-flip-Raman scattering in inSb

In this paper we report on measurements of spin-flip-Raman gain inn-InSb as a function of the magnetic field. The measurements were carried out at temperatures of 1.8 K and 4.2 K and at a carrier concentration of 1.35×10¹⁵ cm⁻³. The Raman cross sections obtained from these results, e.g. 1.25×10⁻²⁰ cm²/sr at a magnetic field of 10 kG and a pump frequency of 1884.35 cm⁻¹, agree very well with those theoretically predicted by Wherrett and Wolland. Furthermore, these measurements yield line shapes and linewidths of the spontaneous scattering (100–1500 MHz) and allow the determination of the effectiveg-value with an accuracy known from ESR-investigations. These results are discussed in terms of already published theoretical investigations.     

Ferromagnetism of Mn-implanted silicon: Magnetization and magnetooptic faraday effect

Manganese-implanted silicon plates of both n and p types have been obtained by implanting 195-keV manganese ions with doses from 1 × 10¹⁵ to 2 × 10¹⁶ cm^?2. According to magnetic measurements by a vibrating sample magnetometer and a SQUID magnetometer, all of the samples exhibit a ferromagnetic ordering at room temperature. The magnetooptic Faraday effect is manifested in the spectral region 1–6 μm in the temperature interval 80–305 K. The characteristic features of the field and temperature dependences of magnetization and the spectrum of the Faraday effect indicate a percolation type of magnetic ordering at low temperatures and a crucial role of the exchange between delocalized p-type carriers and Mn ions at temperatures above 100 K.     

Interactions of μAl acceptor impurity in weakly and heavily doped silicon

The interactions of the aluminum acceptor impurity in silicon are investigated using polarized negative muons. The polarization of negative muons is studied as a function of temperature on crystalline silicon samples with phosphorus (1.6×10¹³ cm^?3) and boron (4.1×10¹⁸ cm^?3) impurities. The measurements are performed in a magnetic field of 4.1 kG perpendicular to the muon spin, in the temperature range from 4 to 300 K. The experimental results show that, in phosphorus-doped n-type silicon, an _μAl acceptor center is ionized in the temperature range T>50 K. For boron-doped silicon, the temperature dependence of the shift of the muon spin precession frequency is found to deviate from the 1/T Curie law in the temperature range T ? 50 K. The interactions of a _μAl acceptor that may be responsible for the effects observed in the experiment are analyzed.     

Raman gain,effectiveg-value and spontaneous spin-flip Raman linewidth in Hg0.77Cd0.23Te

We report on measurements of spin-flip Raman gain inn-Hg_1?x Cd _x Te (x=0.23, carrier density 1.0×10¹⁵cm^?3) as a function of the magnetic field up to 1.6T. The measurements were carried out by a small signal gain technique at a temperature of 1.8 K. Furthermore, the measurements yield lineshapes and linewidths of the spontaneous scattering and allow a precise determination of the effectiveg-value. The highest gain observed is 0.2 cm/W. The band edge value of the effectiveg-value is ?93.2 and the widths of the symmetric lines are between 18 and 120 G, depending on the magnetic field.     

The transport properties of Dirac fermions in chemical vapour-deposited single-layer graphene

Abstract

The electronic transport properties of Dirac fermions in chemical vapour-deposited single-layer epitaxial graphene on anSiO₂/Si substrate have been investigated using the Shubnikov–de Haas (SdH) oscillations technique. The magnetoresistance measurements were performed in the temperature range between 1.8 and 43 K and at magnetic fields up to 11 T. The 2D carrier density and the Fermi energy have been determined from the period of the SdH oscillations. In addition, the in-plane effective mass as well as the quantum lifetime of 2D carriers have been calculated from the temperature and magnetic field dependences of the SdH oscillation amplitude. The sheet carrier density (1.42 × 10¹³ cm^?2 at 1.8 K), obtained from the low-field Hall Effect measurements, is larger than that of 2D carrier density (8.13 × 10¹² cm^?2). On the other hand, the magnetoresistance includes strong magnetic field dependent positive, non-oscillatory background magnetoresistance. The strong magnetic field dependence of the magnetoresistance and the differences between sheet carrier and 2D carrier density can be attributed to the 3D carriers between the graphene sheet and the SiO₂/Si substrate.     

New experimental results for electron transport in weak accumulation layers of ZnO crystals

Previous Hall measurements on (0001) and (0001&#x0304;) faces of ZnO have shown a Hall mobility oscillating as a function of Hall surface electron density in the range between N_SH=10⁶ and 10¹¹ cm^?2. Here we report on new results obtained by a field effect arrangement for free surfaces in UHV. With donors from H exposure or by illumination weak accumulation layers (n_sh <10¹¹ cm^?2) are established. The field effect shows oscillations in surface conductivity as a function of gate voltage. Also the combination of a field effect with a Hall effect measurement reveals discrete values of Hall surface electron density n_sh. Various pretreatments do not change the periodicity of these oscillations. Necessary preconditions are a temperature below 130 K, a Hall surface electron density below 3 × 10¹² cm^?2 and a source-drain field of a few V/cm. A model regarding impurity levels in the space charge layer relates the results of the field effect measurements to the results of the Hall effect measurements.     

Effect of the mutual dragging of electrons and phonons on the thermomagnetic effects in HgSe

The longitudinal and transverse Nernst-Ettingshausen (NE) effects were measured in HgSe samples doped with various concentrations of gallium, 0.6×10¹⁸<N(Ga)<4.7×10¹⁸ cm^?3. The NE measurements were performed for the classical interval of magnetic field strengths (0–40 kOe) in the temperature range from 20 to 60 K. It was established that the low-temperature NE coefficients change sign with increasing Ga concentration or the applied magnetic field strength. It is shown that all unusual features of the NE effects observed in HgSe crystals can be attributed to the effect of mutual dragging of electrons and phonons. This implies that the effect of mutual electron-phonon dragging can be experimentally detected in semiconductors with high concentration of conduction electrons.     

Electrical properties of n-type vapor-grown gallium nitride

Hall measurements are reported for undoped and Zn-doped vapor-grown single crystal GaN on (0001) Al₂O₃ layers with 298 K carrier concentrations (n-type) between 1·4×10¹⁷cm^?3 and 9×10¹⁹ cm^?3. Then n～10¹⁷ cm^?3 crystals (undoped) have mobilities up to μ～440 cm²/V sec at 298 K. Their conduction behavior can be described by a two-donor model between 150 and 1225 K and by impurity band transport below 150 K. Crystals with n≥8×10¹⁸ cm^?3 show metallic conduction with no appreciable variation in n or μ between 10 and 298 K.Results of mass spectrographic analyses indicate that the total level of impurities detected is too low to account for the observed electron concentration at the n～10¹⁹ cm^?3 level, and suggest the presence of a high concentration of native donors in these crystals. No significant reduction in carrier concentration was achieved with Zn doping up to concentrations of ～10²⁰ cm^?3 under the growth conditions of the present work, and no evidence was found to indicate that high conductivity p-type behavior may be achieved in GaN. The influence of factors such as growth rate, crystalline perfection and vapor phase composition during growth on the properties of the layers is described.     

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Magnetoresistivity in a tilted magnetic field in p-Si/SiGe/Si heterostructures with an anisotropic g-factor. Part II

The magnetoresistance components ??xx and ??xy are measured in two p-Si/SiGe/Si quantum wells that have an anisotropic g-factor in a tilted magnetic field as a function of the temperature, field, and tilt angle. Activation energy measurements demonstrate the existence of a ferromagnetic-paramagnetic (F-P) transition for the sample with the hole density p = 2 × 10¹¹ cm^?2. This transition is due to the crossing of the 0?? and 1?? Landau levels. However, in another sample with p = 7.2 × 10¹⁰ cm^?2, the 0?? and 1?? Landau levels coincide for angles ?? = 0?C70°. Only for ?? > 70° do the levels start to diverge which, in turn, results in the energy gap opening.     

Hall coefficient and conduction mechanism in intermediate concentration n-Ge at helium temperatures

Hall coefficient measurements for intermediate concentration n-type Ge were carried out at liquid helium temperatures. The measurements show that the Hall coefficient and mobility increase with decreasing temperature down to 1.7 K and with increasing magnetic field up to 25 KG. These behaviours are opposite to what was observed in low concentration samples. We conclude that the thermal activated localised hopping motion does not exist in our concentration level, 6 × 10¹⁶ cm^?3, but rather the delocalised quasi-free carriers still dominate the overall conduction for temperature as low as 1.7 K. A model is suggested to explain the Hall mobility behaviour. The model based on the decrease of the dominant scattering mechanism, ionised impurity scattering in our case, as the temperature is lowered and when the magnetic field is increased. From the Hall coefficient behaviour at 4.2 and 1.7 K as well as the resistivity measurements, we found no effect of magnetic field on the unique activation energy existing in this concentration level.     

Spin-flip Raman scattering from electrons in localized donor states in n-InSb

We report the first observation of spin-flip Raman scattering from electrons localized in shallow donor states in InSb. For a non-degenerate n-InSb sample (8×10¹³ cm^-3) measurements of the spin-flip Raman gain and the effective g-value as a function of the magnetic field show lineshapes and magnetic field dependences completely different to that of an InSb sample with the electron gas being in a degenerate regime (1.35×10¹⁵ cm^-3). For the 8×10¹³ cm^-3 InSb sample, at magnetic fields greater than 11.5 kG, a splitting of the spin-flip Raman line into two lines is observed which may be an indication that two shallow donor states with different effective g-values are concerned.     

Leed,aes and photoemission measurements of epitaxially grown GaAs(001), (111)A and (1̄1̄1̄)B surfaces and their behaviour upon cs adsorption

Epitaxially grown GaAs(001), (111) and (1?1?1?) surfaces and their behaviour on Cs adsorption are studied by LEED, AES and photoemission. Upon heat treatment the clean GaAs(001) surface shows all the structures of the As-stabilized to the Ga-stabilized surface. By careful annealing it is also possible to obtain the As-stabilized surface from the Ga-stabilized surface, which must be due to the diffusion of As from the bulk to the surface. The As-stabilized surface can be recovered from the Ga-stabilized surface by treating the surface at 400°C in an AsH₃ atmosphere. The Cs coverage of all these surfaces is linear with the dosage and shows a sharp breakpoint at 5.3 × 10¹⁴ atoms cm^?2. The photoemission reaches a maximum precisely at the dosage of this break point for the GaAs(001) and GaAs(1?1?1?) surface, whereas for the GaAs(111) surface the maximum in the photoemission is reached at a higher dosage of 6.5 × 10¹⁴ atoms cm^?2. The maximum photoemission from all surfaces is in the order of 50μA Im^?1 for white light (T = 2850 K). LEED measurements show that Cs adsorbs as an amorphous layer on these surfaces at room temperature. Heat treatment of the Cs-activated GaAs (001) surface shows a stability region of 4.7 × 10¹⁴ atoms cm^?2 at 260dgC and one of 2.7 × 10¹⁴ atoms cm^?2 at 340°C without any ordering of the Cs atoms. Heat treatment of the Cs-activated GaAs(111) crystal shows a gradual desorption of Cs up to a coverage of 1 × 10¹⁴ atoms cm^?2, which is stable at 360°C and where LEED shows the formation of the GaAs(111) (√7 × √7)Cs structure. Heat treatment of the Cs-activated GaAs(1?1?1?) crystal shows a stability region at 260°C with a coverage of 3.8 × 10¹⁴ atoms cm^?2 with ordering of the Cs atoms in a GaAs(1?1?1?) (4 × 4)Cs structure and at 340°C a further stability region with a coverage of 1 × 10¹⁴ at cm^?2 with the formation of a GaAs(1?1?1?) (√21 × √21)Cs structure. Possible models of the GaAs(1?1?1?) (4 × 4)Cs, GaAs(1?1?1?)(√21 × √21)Cs and GaAs(111) (√7 × √7)Cs structures are given.     

Study of the semi-conducting properties of pyrolusite

The electrical conductivity σ of the specimens is found to obey a relation of the type σ=C exp (?B/k T) over a temperature range 300 to 500° K whereC andB are constants. The experimentally determined values ofB and C are (1)B≈1490 to 2199 (2)C ≈ 0·72 × 10^?3 to 4·9 × 10^?3 ohms^?1 cms^?1. The value of the activation energy determined from the values ofB are ≈0·13 to 0·19 e.V. In A.C measurements σ is found to vary with voltage applied across the specimen at a given temperature. The i?V characteristics of metal point semi-conductor contacts are non-linear symmetrical curves and are strongly temperature dependant. The value of the Hall constant (?0·14 cm³/coul) yields carrier concentration as 4·3 × 10¹⁹/cm³, and mobility 1·2 cm² volt^?1 second^?1. Δ?/? for the specimen is found to vary asH ² where ? is the resistivity andH the value of magnetic field. The specimens develop a thermo-electric power of magnitude 200 μV/K to 500 μV/K which is fairly constant over the temperature range 300 to 800° K. The sign of the thermo-e.m.f. and of the Hall constant indicate that the specimens are “n” type.     

Analysis of the spin splitting of maxima of quantum oscillations of the resistivity of n-Bi-Sb semiconductor alloys in a magnetic field parallel to the bisector axis

In samples of semiconductor alloys n-Bi_0.93Sb_0.07 with different electron concentrations (n ₁ = 8 × 10¹⁵ cm^?3, n ₂ = 1.2 × 10¹⁷ cm^?3, and n ₃ = 1.9 × 10¹⁸ cm^?3), dependences of the electrical resistivity on magnetic fields up to 45 T parallel to the current and the bisector axis (H ‖ C ₁ ‖ j) have been measured at temperatures of 1.5, 4.5, and 10 K. The obtained dependences ρ₂₂(H) demonstrate quantum oscillations of the resistivity (Shubnikov-de Haas effect), and, in high magnetic fields, there is a resistivity maximum far away from other maxima. On assumption that this maximum is related to the spin-split Landau level N = 0^? for electrons of the main ellipsoid, the spin-splitting parameters are calculated for electrons of the main ellipsoid: γ₁ = 0.87, γ₂ = 0.8, and γ₃ = 0.73. Using these values, the oscillation maxima can be reliably related to the numbers of split Landau levels for electrons of the main and secondary ellipsoids. The dependences of the resistivity ρ₁₁ and the Hall coefficient R _31.2 on magnetic field have been measured in a transverse magnetic field at H ‖ C ₁ and j ‖ C ₂ on the sample with the electron concentration n ₄ = 1.4 × 10¹⁷ cm^?3. Using similar analysis, the spin-splitting parameter is found to be γ₄ = 0.85, which is close to the value of γ₂ = 0.8 obtained for the sample with close electron concentration (n ₂ = 1.2 × 10¹⁷ cm^?3) during the measurements in a longitudinal magnetic field. The quantum oscillation maxima of Hall coefficient R _31.2 are shifted to the range of high magnetic fields as compared to the quantum oscillation maxima of resistivity ρ₁₁.     

Quasi-elastic neutron scattering of benzene in Na-mordenite

The rotational and translational dynamics of benzene adsorbed in Na-mordenite have been studied by incoherent quasi-elastic neutron scattering. The measurements were performed at two benzene coverages at 300, 400 and 450 K. The observed quasi-elastic broadenings are described by a uniaxial rotational model about the six-fold axis of benzene. The mean time between successive jumps, at 300 K, is τ=1.45 × 10^?12 s at low coverage and 2.05×10^?12 s at high coverage. The correlation times follow an Arrhenius law with E_A=4.51 kJ mol^?1, at both coverages. The translational diffusion coefficient has been measured at 300 K and was found to be 0.67 × 10^?6 cm²s^?1.     

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×10 ¹⁹ cm ^?3 ) and boron (4.1×10 ¹⁸ , 1.34×10 ¹⁹ , and 4.9×10 ¹⁹ cm ^?3 ) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ～10 ¹⁷ cm ^?3 , the relaxation rate ν of the magnetic moment of a _μ Al acceptor in silicon with a high impurity concentration of germanium (9×10 ¹⁹ cm ^?3 ) depends on temperature as ν～T ^q , q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T?30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σ _h ) and electrons (σ _e ) from an Al acceptor center in Si: σ _h ～10^?13 cm² and σ _e ～8×10^?15 cm² at the acceptor (donor) impurity concentration n _a (n _d )～4×10¹⁸ cm^?3.     

Temperature,injection level,and frequency dependences of the luminescence in lightly - and heavily-doped CdTe:In

The temperature dependence, injection level dependence, and modulation frequency response of cathodoluminescence have been measured in Te-rich CdTe:In for materials with In concentrations ranging from 3 × 10¹⁵cm^?3 to 1 × 10¹⁸cm^?3. In lightly-doped material, the 80 K luminescence shows sharp band-edge emission near 1.57 eV and a broad impurity-defect band near 1.4 eV. As temperature increases, the 1.4 eV band quenches out, leaving only the band-edge emission. In heavily-doped material, the band- edge emission is absent and the 80 K luminescence shows only the 1.4 eV band. As the temperature increases from 80 K to 300 K, the 1.4 eV band does not quench out but rather undergoes a complex evolution into a long tail on the band-edge emission which begins to appear at approximately 140 K. At a temperature of 200 K, where the luminescence of the heavily-doped material consists of a broad but structured band approximately 0.2 eV in width, frequency response measurements indicate that band-to-band transitions contribute to the high-energy part of the broad luminescence while the remainder of the band results from slower transitions. The frequency and temperature dependences suggest that the luminescence involves an impurity level that has merged with a band edge at an In concentration of $\text{1 × 10}{}^{18}\text{cm}{}^3$. We interpret this behavior as suggesting that the 1.4 eV luminescence in Te-rich CdTe:In results from a partially-forbidden transition between conduction band and a deep acceptor level rather than from an intracenter type of transition.     

Adsorption of CO on the (110) plane of tungsten electron impact,thermal desorption,and work function measurements

The adsorption of CO on the (110) plane of tungsten has been studied using electron impact desorption, thermal desorption, and work function measurements in a single apparatus combining these various techniques. It is concluded that a single molecular adsorption state exists at 20–250 K (virgin-CO). At 300–400 K, 60% of the low temperature layer desorbs, the remainder converting principally to a beta-1 state, which has very small electron impact cross section; in addition to beta-1 an O⁺ yielding state, which we call beta-precursor is formed. The beta-1 state is stable to 900 K, where some desorption and conversion of the remaineder to a beta-2 state occurs. The O⁺ yielding state decays with increasing T and is gone at 800 K. Readsorption on beta-1 leads to two types of adsorption states called alpha and gamma, which seem to be site specific. Electron impact desorption yields mostly CO⁺ and CO for virgin, O⁺ for beta-precursor, and CO⁺ and CO for the readsorption states. There is no isotopic mixing in virgin or in readsorbed CO, nor does readsorbed CO exchange with beta-1 or beta precursor. There is complete isotopic mixing in beta desorption. In addition, massive EID creates another state, characterized by a large dipole moment, also yielding O⁺ in EID. This state can be converted to beta-1 by heating to 400 K. The total disappearance cross sections for the various states are virgin-CO5 × 10^?17cm²; γ-CO 1.6 × 10^?16cm²; α-CO 5 × 10^?17cm²; β-precursor 6 × 10^?18cm²and 1.2 × 10^?19cm²; EID induced state 8 × 10^?18cm². In addition, cross sections for ion production are determined and found to be several orders of magnitude less than total disappearance cross sections. These results, and Leed and coverage data obtained in parallel investigations are used to formulate models of the various adsorption states. It is concluded that virgin and readsorbed CO are molecular and beta-precursor and beta dissociated, although strong interactions between C and O remain. The electron impact desorption of physisorbed CO was investigated and found to yield C⁺, O⁺, and neutral CO, but very little CO⁺. These results suggest primary dissociation of CO by electron impact, and desorption of neutral physisorbed CO by the energetic fragments. Physisorbed CO⁺, although undoubtedly created, lies on the attractive part of its potential curve relative to the surface, and thus does not desorb as CO⁺.     

The weight,shape, and speed of the universe

Present astronomical data indicate an unbound universe with density ～1.6 × 10^?31 g cm^?3 in which galaxies could not have formed gravitationally. We show how magnetohydrodynamic (MHD) processes allow galaxy formation in an open anisotropic MHD universe with shear, rotation, and fluid flow. The dipole anisotropy of the microwave background radiation sets their respective first-order values at 3.7×10^?15 yr^?1, 10^?14 yr^?1, and 5×10^?4 c. Second-order effects of Maxwell and Reynolds stresses require that the magnetic field, shear, and Hubble expansion be 10^?8 G, 3.7×10^?15 yr^?1, and 10^?10 yr^?1 (100 km sec^?1 Mpc^?1). The model is rigidly self-consistent, predicting both the recent value of the Hubble expansion above and of the shear (? 9×10^?15 yr^?1) given by the microwave background's recently measured quadrupole anisotropy.