Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

The mechanisms of yield and plastic flow in HgTe and Cd x Hg1−x Te

Single crystals of HgTe and Cd _x Hg_1–x (0.18<x<0.30), oriented for single slip, have been deformed in four-point bending at strain rates 10^–4 sec^–1 and temperatures from –11 to +84° C for HgTe, and 20 to 195° C for Cd _x Hg_1–x Te. At the lowest temperatures, the stress-strain curve exhibits a sharp yield relaxation and subsequent zero work hardening regime, as commonly observed for other semiconductors. Experiments show that the yielding mechanism is that proposed by Johnston and Gilman for LiF. Possible explanations for the post-yield zero work hardening phenomenon are discussed. The influence of composition, temperature and strain rate on the stress-strain behaviour are reported. At 20° C, the upper and lower yield stresses ( _uy and _1y ) increase with increasingx in qualitative agreement with our earlier hardness results. For Cd_0.2Hg_0.8Te, _1y varies with temperature,T, at a strain rate of 10^–4 sec^–1, according to _1y exp (Q/kT) whereQ is 0.16 eV. For HgTe the comparable value is 0.11 eV. Atx=0.25 and constant temperature, _1y depends on strain rate as _1y ^1/n wheren is 4. The stress level for deformation of Cd_0.2Hg_0.8Te at 10^–4 sec^–1 and 20° C is 2–3 kg mm^–2, comparable with that for InSb at 300° C or Si at 1000° C. Strain rate cycling tests on Cd _x Hg_1–x Te give values of activation volumeV* around 10b³ at 20° C, independent of plastic strain (up to 2–3%), suggesting that deformation in these alloys is controlled by the Peierls mechanism, as observed in other II–VI compounds.     

Finite-Wave-Vector Phonon Coupling to Degenerate Electronic States in La2?xSrxCuO4

We consider finite-k electron–phonon coupling appropriate for a superconductor with a very short coherence length and local intersite pairs. From group theoretical analysis we find that k 0 phonons can couple to degenerate electronic states in La_{2– x} Sr _x CuO₄ and a Jahn-Teller-like distortion is found to be operative for E _g- and E _u-symmetry O states. Experimentally observed anomalies in inelastic neutron scattering, ESR, and EXAFS are found to be consistent with such an interaction of the ₁ symmetry -point mode with the planar O P _x and P _y states. The proposed interaction is found to naturally facilitate symmetry breaking associated with pairing and/or stripe formation.     

Spin diffusion coefficient of the A and B phases of liquid3He

The spin diffusion coefficient tensors are calculated in the A and B phases of liquid ³ He in the _{L D} <1 regime by using the Kubo formula approach. The spin wave dispersions are also obtained in the presence of diffusive flow of the normal component. The results agree with Combescot's results qualitatively in the _{L D} <1 regime.     

Simultaneous determination of the thermal conductivity and diffusivity of metals using an electronic heater

A nonstationary method allowing simultaneous determination of the thermal conductivity and diffusivity of metals using an electronic heater is described.Notation q_e heat flux density - t temperature - x distance to thermocouple - heating time - t₀ temperature at time zero - thermal conductivity of material examined - a thermal diffusivity of material - c coefficient describing the energy losses - X₁, x₂, x₃ distance to first, second, and third thermocouples, respectively - ₁, ₂, ₃ times during which specimen is heated to temperature t at points X₁, x₂, X₃, respectively     

Heat flow model of rapid solidification with nonhomogeneous thermal contact

A heat flow model is presented of the solidification process of a thin melt layer on a heat conducting substrate. The model is based on the two-dimensional heat conduction equation, which was solved numerically. The effect of coexisting regions of good and bad thermal contact between foil and substrate is considered. The numerical results for thermal parameters of the Al-Cu eutectic alloy show considerable deviations from one-dimensional solidification models. Except for drastic differences in the magnitude of the solidification rate near the foil-substrate interface, the solidification direction deviates from being perpendicular to the substrate and large lateral temperature gradients occur. Interruption of the thermal contact may lead to back-melting effects. A new quantity, the effective diffusion length, is introduced which allows some conclusions to be drawn concerning the behaviour of the frozen microstructure during subsequent cooling.Nomenclature _i ,a _i Thermal diffusivity _i = _i /c _{i i} ,a _i = _i / ₁ - c _i Specific heat capacity - d Foil thickness - D Solid state diffusion coefficient - e_x, e_z Unit vectors - H Latent heat of fusion - h ,h Foil-substrate heat transfer coefficients - i Index: 1, melt; 2, solidified foil; 3, substrate - _i ,k _i Thermal conductivityk _i = _i / ₁ - n Normal unit vector - Nu ,Nu Nusselt numbers for regions of badNu(x,) and good thermal contact, respectivelyNu =h Nu _d / ₁,,Nu(x, )=h(x,)d/ ₁ - R Universal gas constant - , s Position of the liquid-solid interface ¯s/d=s=s _xe_x+s _ze_z - Local solidification rate /d = s =s _xe_x +s _ze_z - t Real time - T _i Temperature field - T ₀ Ambient temperature - T _f Melting temperature - u _i Dimensionless temperature fieldu _i (x, z,)=T _i (x,z,)/T _f - u ₀ Dimensionless ambient temperatureu ₀=T ₀/T _f - _i Local cooling rate within the foil _i = du _i /d - W Stefan numberW=H/c ₁ T _f - ,x Cartesian coordinate parallel to the foil-substrate interfacex= /d - ₀,x ₀ Lateral extension of foil sectionx ₀= ₀/d - ₁,x ₁ Lateral contact lengthx ₁= ₁/d - ,z Cartesian coordinate perpendicular to the foil-substrate interfacez= /d - ₀,z ₀ Substrate thicknessz ₀= ₀/d - E Activation energy of diffusion - T Initial superheat of the melt - u Dimensionless initial superheat u=T/T _f - (x) Step function - _eff Dimensionless effective diffusion length - _i Mass density - Dimensionless time=t ₁/d ² - _f, _f(x, z) Total and local dimensionless freezing time, respectively     

Jeener-Broekaert three-pulse NMR sequences in solid hydrogen using the packer analysis

Jeener-Broekaert 90 _x --45 _y -T-45 _y nuclear magnetic resonance pulse sequences have been applied to solid hydrogen for ortho concentrations <0.5. Below 1 K for longer than the free induction decay time we have observed multiple echoes quantitatively obeying the theory due to K. J. Packer.Supported in part by the National Science Foundation.     

Thermodynamic properties of antiferromagnetic superconductors containing nonmagnetic,magnetic, and spin-orbit impurities

Effects of all types of impurities (nonmagnetic, magnetic, and spin-orbit) on an antiferromagnetic superconductor (AFSC) have been investigated by studying the transition temperatureT _c and the specific heat jump. We have assumed a one-dimensional electron band. The impurity scattering is treated within the self-consistent Born approximation. We find that: (a) the molecular fieldH _Q and the magnetic impurities depress superconductivity of AFSC and their pair-breaking effect is additive; (b) the effect of spin-orbit impurities is the same as that of nonmagnetic impurities—these enhance superconductivity by screening the molecular field; and (c) in the extreme dirty limit, the AFSC is described in terms of an effective pair-breaking parameter given by 1/_eff=1/₂+H _Q ² where 1/=1/₁+2/3_so(1/₁, 1/₂, and 1/_so, respectively, are the scattering rates from nonmagnetic, magnetic and spin-orbit impurities).     

A thermally-stimulated-current study of ethylene-methyl-methacrylate copolymers

Two ethylene-methyl methacrylate (EMMA) copolymers were studied by infrared (i.r.) spectroscopy, Differential Scanning Calorimetry (DSC) and Thermally Stimulated Currents (TSCs). The critical temperature, T _c, relaxation time, _c (measured in seconds), and Degree of Disorder (DOD) of the polymers were also measured by Relaxation Map Analysis (RMA). The i.r. and DSC results showed that these two copolymers are random copolymers with a melting point, T _m, at 82.6 °C for a sample containing 18 wt % methyl methacrylate (EMMA18), and at 53.0 °C for a sample having a MMA content of 38 wt % (EMMA38). From TSC spectra analysis, EMMA18 showed a glass-transition point, T _g, at – 41.0 °C, whereas EMMA38 had a T _g at – 36.0 °C together with a -relaxation at – 120 °C. Both samples have their space charges in the range 5–30 °C. The RMA showed that EMMA18 has T _c = – 5.74 °C, log _c = – 2.90 ( in seconds) and DOD = 54.65. EMMA38, however, has T _c = – 1 5.71 °C, log _c = – 1.59 and DOD = 48.69. The absence of -relaxation in EMMA18 was explained by evidence that it has a higher DOD. While the low mechanical properties of EMMA38 were mainly attributed to the contribution from the short chain segments, demonstrated by its low T _c.     

Effects of alloying in the dilute limit on the quantum states of electrons in magnesium

The electron quantum interference phenomenon was used to determine the effects of substitutional alloying upon the quantum-state lifetime and band gapsE _g at the Fermi energy in single crystals of pure Mg. Vapor-grown alloys containing either Zn or Cd in concentrationsC0.3–15 ppm were studied. The magnetic field dependence of the interference oscillation amplitudes for these samples indicates reductions in and significant increases inE _g relative to pure Mg (impurity concentration 10^–8). Within experimental accuracy the observed quantum-state lifetime satisfies the relation ^–1=C, with 2.7×10¹⁰ sec^–1 per ppm of Cd and 2×10¹¹ sec^–1 per ppm of Zn. The band gap corresponding to Bragg reflection from the (0001) plane of the hcp structure (which in pure Mg arises solely from spin-orbit coupling) was found to increase by more than a factor of two upon the addition of only 10 ppm Cd to Mg. These results are discussed within the framework of the pseudopotential theory of alloying. It is shown that there are discrepancies of more than three orders of magnitude between the experimental and theoretical values forE _g and of about one order of magnitude for in these dilute-limit alloys.Work supported by the National Science Foundation.Submitted to the Department of Physics, the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.Fannie and John Hertz Foundation Fellow.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Nonlinearity in the flux-flow behavior of thin-film superconductors

The nonlinear flux-flow behavior due to the nonequilibrium distribution of quasiparticles predicted by Larkin and Ovchinnikov has been observed in thin films of Al, In, and Sn. From the voltage-current characteristics the inelastic scattering time _E of the electrons was obtained. For Al and In the values of _E are in excellent agreement with the previous results. For Sn the value _E =5×10^–10 sec is slightly larger than the previous results.     

Spin Transport in Very Dilute 3He-4He at Very High B/T

We report preliminary results for pulsed-NMR measurements on a very dilute ³ He- ⁴ He mixture (x ₃ =900 ppm) in a 14.74 T field at temperatures down to 3 mK. The aim of the project is to investigate low temperature transverse spin relaxation and possible spin diffusion anisotropy. Spin-echo and T₁-type measurements were employed to measure the transverse and longitudinal spin diffusion coefficients. At high temperatures both spin diffusion coefficients are well explained assuming a quasiparticle interaction consistent with previous experiments. Conversely, at low temperatures the measured value of || is apparently limited to about ten by a poorly-understood process that accelerates spin-echo decay. This hampers an accurate determination of the transverse diffusion coefficient at the lowest temperatures reached.     

Orientation and temperature dependence of the flow stress in the intermetallic compound Ni3Ge single crystals

Tensile tests on Ni₃Ge single crystals were carried out to make clear the mechanism of the positive temperature dependence of the CRSS and the work-hardening rate as a function of the tensile axis orientation below room temperature. Both the CRSS () and the maximum work hardening rate ( _M) show positive temperature dependence even below room temperature. The increments of and _M are satisfactorily expressed by the Schmid factor ratio of the cube cross slip system to the primary octahedral one,N=(010) [¯101]/(111) [¯101 ]. Then, the positive temperature dependence of both the CRSS and the maximum work-hardening rate is thought to be governed by two mechanisms. One arises from the Kear—Wilsdorf mechanism, which depends on the orientation. The other seems to arise from the orientation-independent factor, although it is obscure at present.     

Study of La1−xMnO3−δ non-stoichiometry and defect structure using ESR and iodometry

The oxidation states of manganese in the La_1–xMnO_3– (x = 0.09–0.11) were investigated by electron spin resonance (ESR) and iodometry. The ESR analysis carried out at room temperature for the samples prepared in air revealed the presence of broad peaks at g = 2.0, considered to be relevant to Mn²⁺. It was also found that the intensity of the peak increased as lanthanum vacancy content increased. The average valence state of manganese, determined by iodometry, was approximately 3.2, and decreased by 1 as the lanthanum vacancy increased by 1. Similar trends were observed with the samples prepared at P _{o 2} = 1×10^–7. The results indicated that Mn²⁺ is stably present in the La_1–xMnO_3– having an average valence number exceeding 3.0. A series of experimental results with respect to the non-stoichiometry of La_1–xMnO_3– can be explained by assuming that Mn²⁺ is stabilized after forming a complex with a lanthanum vacancy and two oxygen vacancies.     

Hydrodynamics of a vortex ring

We considered the kinematics and dynamics of a vortex ring in an incompressible fluid in toroidal coordinates. We obtained the change in the pressure difference along the boundary between two flow regions in the case of a moving torus.Notation , , toroidal coordinates - (V ;V ;V ) velocity of a fluid particle and its projections in toroidal coordinates - g ,g ,g metric tensor components - the Jacobian of transition to curvilinear coordinates - V ₀ velocity at the center of a vortex ring on its symmetry axis - x, y, z Cartesian coordinates - z, y, cylindrical coordinates - a distance from the axis of a torus (V=0) to its axis of symmetry (Oz) - angle between the Oy axis and the line that connects a fluid particle on the streamline =const, which represents a circle [16], with the center of this circle - U _z,U _y velocities in the cylindrical system of coordinates - ₀ stream function of a stationary vortex ring - velocity circulation - U V ₁, velocity of a rectilinear flow at infinity - ₁ stream function of a rectilinear flow - = ₀ + ₁ superposition of two flows - n=k ⁴=V ₁/V ₀ velocity ratio coefficient - R radius of a vortical region - U velocity of fluid particles at the boundary in polar coordinates (r, ) with the center at the coordinate origin (point 0) - fluid density - p ₀,p pressure at infinity and at a certain point of flow - pressure difference Polotsk State University, Polotsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 4, pp. 531–536, July–August, 1995.     

Density,differential thermal analysis and direct-current conductivity of Sb10S90 −x Ge x chalcogenide glasses

The glass-transition temperature,T _g, and the crystallization temperature,T _c for glassy Sb₁₀S_{90 –x} Ge _x samples were increased by increasing the Ge content. This was attributed to the closeness of the network by bridging Sb and/or S atoms with additional Ge atoms to form harder Ge&#x2013;Sb and/or Ge&#x2013;S bonds. The direct current (d.c.) electrical conductivity for the system Sb₁₀S_{90 –x} Ge _x shows semiconducting behaviour with increasing temperature. At low temperatures, the conductivity was attributed to a hopping conduction mechanism, while at high temperatures it was due to regular band-type conduction. The activation energy, E and the conductivity, ₀ were increased at low Ge contents. This may be related to the entrance of Ge atoms into the S&#x2013;S chains. On the other hand, the increase of the Ge content above 20 at % lead to a conversion of the behaviour of the E and ₀ from the insulating character of S to the semiconducting character of Ge.     

Influence of Temperature on the Lifetime of Electronically Excited Uranyl Ion: II. Frozen Aqueous Solutions of H2SO4. Effect of Phase Transitions

The temperature dependences of the lifetime () of electronically excited uranyl ion (UO₂ ^{2 +})*, measured both on heating from 77 K and on cooling from 300 to 77 K of UO₂SO₄ solutions in 0.2-5.0 M of H₂SO₄, pass through extrema. The peaks of phase transitions in DTA curve and peaks in the temperature dependence of appear at the same temperature. The influence of the phase transitions on the temperature dependence of is due not only to a change in the molecular mobility upon a phase transition (this factor causes the temperature quenching) but also to the concentration quenching of excited states of (UO₂ ^{2 +})*.     

Electron mobility in selectively doped GaAs/Inx Ga1−xAs multiple quantum well structures

The photoluminescence, the temperature dependence of the electrical conductivity (0.4300 K), the magnetoresistance and the Hall effect were investigated in selectively doped GaAs/In _xGa_1–xAs multiple quantum well (MQW) structures. The dependencies of the electron mobility on the width of the quantum wells and on temperature were measured. It is shown that in narrow MQW structures the value of mobility is restricted by interface roughness scattering. In wider MQW structures, the alloy scattering has a detrimental effect on the low magnetic field transport properties. A negative magnetoresistance was observed in low magnetic fields. From a detailed comparison between theory of weak localisation and experiment the wave function relaxation time and its temperature dependence were evaluated. The quantum Hall effect was investigated in all samples in the temperature interval 0.4 K ÷ 4.2 K in magnetic fields up to 40 T.     

Motion of a thermal wave front in a nonlinear medium with absorption

We study the evolution of a thermal perturbation in a nonlinear medium whose thermal conductivity depends on the temperature and the temperature gradient according to a power law.Notation u temperature - k coefficient of thermal conductivity - t time - x spatial variable - x₊ a point on the thermal wave front - a ² generalized coefficient of thermal diffusivity - , , , and s parameters of the process - (x^s) Dirac delta-function - B[, ] a beta function - v(, x), (t) auxiliary functions - A, C, T_o, T_m, T*, R, r, p, and _m constants and parameters Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 4, pp. 728–731, October, 1980.