Hysteresis phenomena in charging of Si Mosfet in quantizing magnetic field

We report the observation of nonequilibrium processes of charging in 2D electron layer in Si MOSFET. There were investigated the hysteresis variations of a 2D-layer charge Q_S with a gate voltage V_g or with a magnetic field H, and the hysteresis of a gate potential U_g vs magnetic field at Q_S = Const. At sufficiently low temperature T < 1K the relaxation time of a nonequilibrium state is much higher than 10³ sec. The observed phenomena may be explained qualitatively in frame of a surface potential randomness conception.     

Transport properties of conduction electrons in n-type inversion layers in (100) surfaces of silicon

The conductivities of n-type inversion layers in (100) surfaces of p-type silicon were measured extensively as functions of electron density in the inversion layer, the ambient temperature and the applied magnetic field. Measurements were made on the carefully fabricated four “classes” of MOS field-effect transistors whose maximum mobilities at 4·2K were 14,000, 8000, 6800 and 1500 cm²/V·sec, respectively. From the temperature dependence of the mobility, dominant momentum scattering was reasonably ascribed to surfon at 100 ～ 300 K. and degenerate or non-degenerate coulomb scattering at lower temperatures as treated by Stern and Howard. From the curves of conductivity vs temperature at low temperatures and low electron concentration for specimens with high mobilities, an activation energy of 1·2 meV, relating to the shallow bound states associated with the lowest electrin sub-band, was observed. The conductivity σ_xx of the inversion layer in a strong transverse magnetic field showed behaviors like those of completely free electrons without effects belonging to its material in its oscillation pattern. That is, the peak value of σ_xx as a function of the gate voltage V_R dependend only on the Landau index. The σ_xx as a function of the magnetic field H at a constant V_R showed a similar Shubnikov-de Haas (SdH) type oscillation to that of three dimensional one. The SdH oscillation gave an “apparent” g-value g* which ranges from 2 to 5 depending on the surface carrier density n_s, due to the change in the ratios of the widths of the Landau levels to the level separation. The “reasonable” g-value of the conduction electrons in the inversion layer has been determined using a modified tilted magnetic field method. The g-value at the fixed magnetic field was independent of surface carrier density n_s and tended to 2 in the extreme strong magnetic field.Discussion is made of the g-value relating to the Landau level width and the energy gaps in the density of states under strong magnetic field.     

High-pressure induced magnetoresistance in <Emphasis Type="Italic">p</Emphasis>-InAs:Mn and <Emphasis Type="Italic">p</Emphasis>-CdGeAs<Subscript>2</Subscript>:Mn

The baric (P ≤ 5GPa) and magnetic-field (H ≤ 5 kOe) dependences of the transverse magnetore-sistance Δρ _xx /ρ₀ have been measured for p-InAs (R _H = 22.5 cm³/C, ρ = 0.15 Ω cm) and the new ferromag-netic semiconductor p-CdGeAs₂ (R _H = 5 cm³/C, ρ = 0.62 Ω cm), doped with a magnetic impurity (Mn), near the temperature T = 297 K. The dependences Δρ _xx /ρ₀ (P, H) for p-InAs:Mn and p-CdGeAs₂:Mn exhibit a magnetoresistive effect with an increase in pressure, and a pressure-induced magnetoresistance hysteresis is observed in p-CdGeAs₂:Mn with a pressure drop.     

Magnetoresistance in dilute p‐Si/SiGe in parallel and tilted magnetic fields

We report the results of an experimental study of the magnetoresistance ρ_xx and ρ_xy in two samples of p‐Si/SiGe with low carrier concentrations p = 8.2 × 10¹⁰ cm^‐2 and p = 2 × 10¹¹ cm^‐2. The research was performed in the temperature range of 0.3–2 K and in the magnetic fields of up to 18 T, parallel or tilted with respect to the two‐dimensional (2D) channel plane. The large in‐plane magnetoresistance can be explained by the influence of the in‐plane magnetic field on the orbital motion of the charge carriers in the quasi‐2D system. The measurements of ρ_xx and ρ_xy in the tilted magnetic field showed that the anomaly in ρ_xx, observed at filling factor ν = 3/2 is practically nonexistent in the conductivity σ_xx. The anomaly in σ_xx at ν = 2 might be explained by overlapping of the levels with different spins 0 ↑ and 1 ↓ when the tilt angle of the applied magnetic field is changed. The dependence of g‐factor g^*(Θ)/g^*(0⁰) on the tilt angle Θ was determined.     

Quantum magneto-transport in two-dimensional GaAs electron gases and SiGe hole gases

We have measured the low-temperature transport properties of two-dimensional (2D) GaAs electron gases and 2D SiGe hole gases. Our experimental results fall into three categories. (i) Collapse of spin-splitting and an enhanced Landé g-factor at Landau level filling factors both ν=3 and ν=1 in a 2D GaAs electron gas are observed. Our experimental results show direct evidence that the effective disorder is stronger at ν=1 than that at ν=3 over approximately the same perpendicular magnetic field range. (ii) We present evidence for spin-polarisation of a dilute 2D GaAs electron gas. The Lande g-factor of the system is estimated to be 1.66. This enhanced g value is ascribed to electron–electron interactions at ultra low carrier density limit. (iii) In a high-quality SiGe hole gas, there is a temperature-independent point in the magnetoresistivity ρ_xx and ρ_xy which is ascribed to experimental evidence for a quantum phase transition between ν=3 and ν=5. We also present a study on the temperature(T)-driven flow lines in our system.     

Thermally activated flux flow,vortex-glass phase transition and the mixed-state Hall effect in 112-type iron pnictide superconductors

The transport properties in the mixed state of high-quality Ca_(0.8)La_(0.2)Fe_(0.98)Co_(0.02)As_2single crystal,a newly discovered 112-type iron pnictide superconductor,are comprehensively studied by magneto-resistivity measurement.The field-dependent activation energy,U_0,is derived in the framework of thermally activated flux flow(TAFF)theory,yielding a power law dependence U_0～H~αwith a crossover at a magnetic field around 2 T in both H⊥ab and H//ab,which is ascribed to the different pinning mechanisms.Moreover,we have clearly observed the vortex phase transition from vortex-glass to vortex-liquid according to the vortex-glass model,and vortex phase diagrams are constructed for both H⊥ab and H//ab.Finally,the results of mixed-state Hall effect show that no sign reversal of transverse resistivityρ_(xy)(H)is detected,indicating that the Hall component arising from the vortex flow is in theories or experiments previously reported on some high-T_ccuprates.     

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.     

Dependence on substrate bias of the effective mass and dingle temperature of electrons in the surface inversion layer of silicon

The amplitude of the quantum oscillations in the magnetoconductance of a silicon inversion layer has been studied as a function of gate voltage V_g, for different values of the temperature T, applied magnetic field strength H and substrate bias V_s. By analyzing the amplitude of the oscillations at fixed V_g and V_g as a function of T and H, the dependence of the cyclotron effective mass m¹ and the Dingle temperature T_D on V_g and V_s can be obtained. The dependence of m¹ on V_g for different values of V_s is compared with the prediction of theory.     

高温超导体霍尔电阻和霍尔角在涡旋玻璃相变附近的普适标度律及统一霍尔电阻方程

发现了一个钉扎效应影响霍尔电阻ρ_xy和霍尔角θ_H的普适标度律. 同时,根据纵向电阻ρ_xx的扩展幂律形式和对霍尔电导σ_xy的微观分析,给出了一个对有一次或多次霍尔反号的高温超导体都适用的霍尔电阻方程. 关键词： 高温超导体 涡旋玻璃相变 普适标度律 霍尔电阻方程     

Least squares adjustment of the coupling constants in nuclear beta-decay

Assuming theV-A theory, fully polarized neutrinos, and lepton conservation, best values of the vector and axial vector coupling constants are deduced from all available data on neutron decay and from theft- values of¹⁴O,^26mAl and³⁴C1. Special consideration is given to the treatment of errors and to radiative corrections. For the determination of the coupling constantratio, radiative corrections are, at present, unimportant. We find ¦G_A/G_V¦=1.250 + 0.009 with a phase angle φ=(181.1±1.3)⁰. From the weighted mean ?t-value, including charge dependent and “outer” radiative corrections, we find an effective coupling constant G′_V=(1.413±0.002)x 10⁴⁹erg cm³. The error is mainly due to the uncertainty of charge dependent corrections.     

Giant thermal hysteresis of sound velocity and internal friction in a La0.8Sr0.2MnO3 single crystal

Temperature dependences of the velocity of longitudinal sound V ₁ and the internal friction Q ^?1 are studied for a La_0.8Sr_0.2MnO₃ single crystal in the temperature range 5–350 K. The latter includes the temperature of the structural phase transition T _s ≈95 K (from the Pnma orthorhombic low-temperature phase to the $R\bar 3c$ rhombohedral high-temperature one) and the Curie point T _c =308 K. Near the temperatures T _s and T _c , the curves V ₁(T) and Q ^?1(T) exhibit pronounced singularities. Outside the vicinities of T _s and T _c , the velocity of sound monotonically decreases with increasing temperature. A thermal hysteresis of giant width is observed in the aforementioned dependences. The hysteresis is attributed to the following mechanism: when the crystal under study is heated starting at temperatures T<T _s , some regions occupied by the Pnma low-temperature phase are retained in the $R\bar 3c$ matrix up to the temperature T=350 K.     

The ground state of two-dimensional electrons in a nonuniform magnetic field

An exact solution to the Schrödinger equation for the ground state of two-dimensional Pauli electrons in a nonuniform transverse magnetic field H is presented for two cases. In the first case, the field H depends on a single variable, H = H(y), while in the second case, the field is axially symmetric, H = H(ρ), ρ²=x ²+y ². The electron density distributions n = n(y) and n = n(ρ) that correspond to a completely filled lower level are found. For quasiuniform fields of fixed sign, the functions n(y) and n(ρ) are locally related to the magnetic field: n(y) = H(y)/?₀ and n(ρ) = H(ρ)/?₀, where ?₀ = hc/|e| is a magnetic flux quantum. Magnetic fields are considered that are periodic, singular, and bounded in the plane xy. Finite electron objects in a nonuniform magnetic field are analyzed.     

Colossal room-temperature magnetoresistance in thin La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ag<Subscript>y</Subscript>MnO<Subscript>3</Subscript> epitaxial films

The first thin La_1?xAg_yMnO₃ epitaxial films (_y ≤ x) were grown on SrTiO₃ (110) substrates with silver present in the ionized state (Ag⁺) only. The Curie temperatures T_C of the compositions with x = y = 0.05, x = y = 0.1, and x = 0.3 and y = 0.27 crystallizing in the hexagonal structure \(R\bar 3c\) above or close to room temperature. The temperature dependences of electrical resistivity ρ and of magnetoresistance ¦Δρ/ρ/¦ = ¦(ρ_H ? ρ _H = 0)/ρ_H=0¦ pass through maxima near T_C, with the magnetoresistance being negative and reaching colossal values of ～7–20% in a magnetic field H = 8.2 kOe not only at T_C but also at room temperature. The magnetic moment per formula unit as derived from the saturation magnetization at T = 5 K is substantially smaller than expected for complete ferromagnetic ordering. The magnetization in fields of up to 6 kOe depends on the actual sample cooling conditions, and the hysteresis loop of a field-cooled sample is displaced along the H axis by ΔH. The above properties can be accounted for by the fact that the films are in a two-phase magnetic (ferromagnetic-antiferromagnetic) state induced by strong s-d exchange. The maximum value of Δ H was used to calculate the energy of exchange coupling between the ferromagnetic and antiferromagnetic parts of a sample.     

Spectroscopic investigation of structure transformation in wavellite

The Electron Spin Resonance of VO²⁺, and the Infrared and Nuclear Magnetic Resonance of protons were studied in wavellite. Interpretation of¹H NMR by shape function calculation allows us to determine the distribution of protons between molecular water, OH-groups, the three-spin magnetic configuration H₃O⁺ and to ascertain the characteristic distances between protons. The spin-Hamiltonian parameters, Fermi contact interaction parameters and dipolar hyperfine coupling parameterPK were calculated. Three types of VO₂₊ complexes were detected in wavellite. It is found that VO²⁺(1)- and VO²⁺(2)-centres disappear and a new VO²⁺(3)-centre appears during annealing. VO²⁺(1) is more stable than VO²⁺(2). The parameters of the VO²⁺(3) complex change in the process of annealing. The transformation of VO²⁺(1) is connected with the destruction of acidic H₃O⁺-groups, and the transformation of VO²⁺(2) correlates with the loss of weakly-bonded water molecules in the wavellite structure. ThePK parameter and Δg?Δg _⊥ ratio can be used as markers of disorder in the wavellite structure.     

Lee-Modell mit Coulomb-Wechselwirkung

The Hamiltonian of the Lee model in the sector (1, 0) with an additional N Θ-interaction is diagonalized by means ofGreen's operator (E-H)^?1. In addition to a continuous spectrum of scattering states one gets a discrete spectrum consisting of a groundstate level determined essentially byLee's interaction H₁ and excited states generated by the additional interactionH ₂.     

Infrared contribution to the double-logarithmic small-x asymptotics of structure functions

The role playing by the “soft” (κ _T < 1 GeV/c) region in the small-x behaviour ofg ₁(x,Q ²) and the non-singlet structure functionf _{1, NS}(x, Q²) has been studied with the help of the effective QCD Lagrangian which takes into account the lightest degrees of freedom — the constituent quarks and the π-mesons (Goldstone bosons). It has been shown that the quark-quark interaction due to the pion exchange has a negative couplingg for the isovector component (I = 1 in thet-channel) off ₁(x,Q ²) and isosinglet component ofg ₁(x, Q²). Here the pion induced interaction changes mainly the normalization of the quark distribution (it decreasesf _1,NS ^{I = 1} (x,Q ²) two times atx < 3·10^?3) and changes slightly the effective exponents λ (?_1,NS ,g ₁ ～x ^?λ atx → 0). On the other hand due to a positive value of couplingg the value of λ increases by 15% for the isovector part ofg ₁ (x,Q ²) and up to λ ≈ 0.5 (instead of λ ≈ 0.2 without the pion contribution) for the isoscalar non-singlet structure functionf _1,NS ^{I = 0} (x,Q ²).     

t-channel factorization description of γγ→V 1 V 2

At-channel factorization model is used to estimate cross sections for the processes γγ→V ₁ V ₂. WheneverV=ρ, the width of the ρ has been included in the calculations. The channels γγ→ρ⁰ρ⁰, ρ⁰?, ??, ωω, ρ⁰ω and ρ⁺ρ^? are calculated for two quasi-real photons. Predictions are also given for the process γ^*γ→ρ⁰ρ⁰ for virtual photon mass squaredQ ²<5 GeV². Our results are consistent with all available experimental data.     

The electric field gradient at the nuclear site of177Lu in Zn and In

The electric quadrupole interaction frequencyν _Q =eQV _zz /h of¹⁷⁷Lu in single crystals of Zn and In has been measured by the method of low temperature nuclear orientation. The results are $$\begin{gathered} v_Q ({}^{177}Lu\underline {Zn} ) = - 180(5)MHz \hfill \\ v_Q ({}^{177}Lu\underline {In} ) = - 19(5)MHz. \hfill \\ \end{gathered} $$ With the known quadrupole moment of¹⁷⁷LuQ=3.39 (2) b we derive for the electric field gradientV _zz (Lu Zn)=?2.20 (5)×10¹⁷ V/cm² andV _zz (Lu In)=?0.23 (6)×10¹⁷ V/cm². The results are compared with magnetostriction measurements of silver single crystals doped with rare earth atoms.     

On the invariants of the complex Ginzburg-Landau equation

Using Lie group theory, both the invariants and the similarity variables of the complex Ginzburg—Landau equation V_xx = a(V_t + bV) + cV|V|^2kwherea,b,c?Iandk?I are constructed.     

Mößbauereffekt in157Gd und155Gd

Using the Mössbauer technique electric hyperfine interactions of the first excited non-rotational states at 64 keV in¹⁵⁷Gd and at 86.5 keV in¹⁵⁵Gd have been determined in GdF₃ and GdCl₃· 6H₂O. For the ratio of quadrupolmoments the ratiosQ ₆₄ ¹⁵⁷ /Q _g ¹⁵⁷ =1.74±0.04,Q _86.5 ¹⁵⁵ /Q _g ¹⁵⁷ =?0.07 ± 0.21 andQ _g ¹⁵⁵ /Q _g ¹⁵⁷ =0.78 ± 0.06 were found. In addition isomer shifts were observed from which a ratio δ〈r²〉 ₆₄ ¹⁵⁷ /δ〈r²〉 _86.5 ¹⁵⁵ =?2.6±0.15 can be inferred.