Analysis of the relationship between the electron-capture and electron-loss cross sections for carbon ions

The electron-loss cross sections σ _{i, i + 1} and the electron-capture cross sections σ _{i, i ? 1} for carbon ions with energies of 35–330 keV/nucleon in hydrogen and neon are determined from experimental data. It is demonstrated that, for particle energies which satisfy the condition σ _{i, i + 1} = σ _{i, i ? 1} or σ _{i, i ? 1} = σ _{i ? 1, i} , the average equilibrium ion charge can be evaluated without solving the system of differential equations for charge exchange. The dependence of the average equilibrium ion charge on the ion energy is investigated for carbon ions.     

First principles calculations of Sr-based fluoroperovskite compounds under applied pressure

The structural, electronic and thermoelectric properties of SrXF₃ (X?=?Li, Na, K, Rb) compounds are performed using first principle calculations. The mBJ-GGA method has been considered to obtain accurate band gaps. The present compounds are found to be thermodynamically stable under 0?GPa and 10?GPa. This stability has been determined using the standard enthalypy of formation. The band structures of the compounds display direct band-gap (Γ-Γ). The band gap has slightly increased for almost studied compounds under 10?GPa. The Boltzmann transport calculations are used to calculate and explain the thermoelectric properties as a function of temperature within the range 20–1500?K. The majority charge carriers of SrXF₃ compounds are holes rather than electrons. Under 10 GP pressure the SrLiF₃ compound is shifted from n-type to p-type doping, whereas SrKF₃ and SrRbF₃ are shifted from p-type to n-type. SrNaF₃ has p-type doping character under 0?GPa and 10?GPa. The Seebeck coeffiecient is found to decrease, whereas σ/τ and S² σ/τ increase for higher temperature. According to the figure of merit and the high S² σ/τ values for SrXF₃, promising thermoelectric applications are expected for the present compounds.     

Ionic conductivity of polycrystalline samples of KBrKI solid solutions

Ionic conductivity of KBr_xI_1?x(0?x?1) mixed crystals, σ_x, has been measured as a function of temperature in the range of 370°C to close to their melting points. The variation in conductivity, σ_x, with composition in the intrinsic region was found to be non-linear, having a maximum value at x=0.3. The maximum conductivity of KBr_xI_1?x mixed crystals was never far outside the range of conductivity of the component crystals. Several expressions of the relative conductivity, σ_x/σ₁ (σ₁ refers to KBr) have been suggested.     

Hopping conduction in the Landau level tails in GaAs-AlxGa1-xAs heterostructures at low temperatures

We have investigated the temperature dependence of the transverse conductivity σ_xx in the two-dimensional Shubnikov—de Haas oscillation minima in GaAs-Al_xGa_1-xAs heterostructures at temperatures between 10mK and 1 K. We found that for σ_xx ? σ_xx (max), where σ_xx (max) is the conductivity in the corresponding oscillation maxima, the results agree qualitatively with a theory recently published by Ono, predicting $\text{σ}{}_{\mathrm{xx}}\text{∝ (1/T) exp - (T}{}_0\text{/T)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, but are not consistent with the estimated T₀-values.     

Probing of energy gap distribution in superconducting tunnel junctions by low temperature scanning electron microscopy

A two-dimensional voltage image of the energy gap distribution of a superconducting tunnel junction was obtained by scanning the current biased junction with an electron beam and detecting the voltage change δV. The value of the energy gap at the point of irradiation was determined quantitatively from the δV σ(V) curves, where σ(V) is the electric conductance of the junction. Further the quasiparticle diffusion length was found by measuring the length of the transition between a high- and low-gap region generated by a double tunnel junction configuration. The theoretical predictions could be verified by investigating a double tunnel junction configuration, where the energy gap could be changed deliberately by quasiparticle injection.     

On the fermi velocity and static conductivity of epitaxial graphene

The models of the energy density of states of a metallic or semiconductor substrate, which does not further lead to divergences, have been proposed to calculate the characteristics of epitaxial graphene. The Fermi velocity of epitaxial graphene formed on a metal has been shown to be greater than that in free-standing graphene irrespective of the position of the Fermi level. On the contrary, the Fermi velocity of graphene formed on a semiconductor is lower so that the lower is the Fermi velocity, the closer is the Fermi level to the center of the band gap of the semiconductor. The zero-temperature static conductivity σ of epitaxial graphene has been calculated according to the Kubo-Greenwood formula. The quantity σ_m of undoped graphene on metal has been shown to decrease with an increase in the deviation of the Dirac point ?_D (which coincides with the Fermi level of the system) from the center of the conduction band of the substrate. In the case of the semiconductor substrate, the static conductivity σ_sc turns out to be nonzero and amounts to σ_sc = 2e ²/π?-only under the condition ?_F =?′_D, where ?′_D is the Dirac-point energy renormalized by the interaction with the substrate.     

dc ionic conductivity measurements on the mixed conductor Cu2−xSe

A brief review and some remarks on the theory and practice of the four point dc ionic conductivity measurement in a mixed conductor are presented. Measurements were made on Cu_2?xSe which has the electronic to ionic conductivity ratio of 2×10³–2×10⁵. The transients which appear on the voltage probes when a constant current is sent through the sample were found in complete agreement with the dependence predicted from the solution of the diffusion equation. Temperature dependence of σ_i of Cu_1.99Se in the range between 40°C and 180°C confirms the expected behaviour in the low- and high-temperature and intermediate phase. Composition dependence of σ_i of Cu_2?xSe in the high-temperature phase is not linear in x.     

The influence of phonons on the optical properties and the conductivity of quasi-one-dimensional metals

The frequency dependent conductivity σ(ω) completely taking into account the interaction between electrons is studied. The shape and the temperature dependence of optical absorption near the frequency of a molecular phonon activated due to the interaction with electrons are found. For a system with attractive sign of the e-e backward scattering amplitude g₁<0 an absorption edge near the gap 2Δ in the electronic spectrum is studied. A low frequency conductivity is discussed. The properties under consideration depend essentially on the magnitudes of e-e interactions and are critical to the sign of g₁.     

Domain growth and aging scaling in coarsening disordered systems

We explore the electronic and transport properties out of a biased multilayer hexagonal boron nitride (h-BN) by first-principles calculations. The band gaps of multilayer h-BN decrease almost linearly with increasing perpendicular electric field, irrespective of the layer number N and stacking manner. The critical electric filed (E ₀) required to close the band gap decreases with the increasing N and can be approximated by E ₀ = 3.2 / (N ? 1) (eV). We provide a quantum transport simulation of a dual-gated 4-layer h-BN with graphene electrodes. The transmission gap in this device can be effectively reduced by double gates, and a high on-off ratio of 3000 is obtained with relatively low voltage. This renders biased MLh-BN a promising channel in field effect transistor fabrication.     

Giant forced volume magnetostriction of Fe-rich iron-cobalt-zirconium amorphous alloys

The forced volume magnetostriction has been measured as a function of composition and temperature in amorphous (Fe_1?xCo_x)₉₀Zr₁₀ alloys. The maximum value of $\text{δω}\text{δH}$ (T = 0 K), equal to 285 × 10^-10 Oe^-1, has been observed for Fe₉₀Zr₁₀ alloys. The effect of pressure on the magnetization σ₀ at T = 0 K was calculated from $\text{δω}\text{δH}$ data. The pressure dependence of the Curie temperature T_c and σ₀ behaves in a similar way as those observed for crystalline NiPd and NiRh alloys.     

Fe deep level optical spectroscopy in InP

The optical cross-section σ_n⁰(hv) and σ_p⁰(hv) associated with the (Fe³⁺ ? Fe²⁺) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe²⁺ ground state to the Γ and L point minima of the conduction band for σ_n⁰(hv) and from the valence band to the ground and excited state for Fe²⁺ for σ_p(hv). A theoretical model which accounts for the main features of the experimental data is proposed.     

Evaluation of the modulation index of the standing wave Raman-Nath mode-locking device

The performance of an acoustooptic standing wave Raman-Nath deflector as active mode-locking switch is evaluated and compared with theory. The time averaged intensities of the diffracted beam up to the eight order are calculated for acoustic phase shifts up to Δ? = 4. With fused quartz the highest attainable value of σ_m was 6.4 at 633 nm. In the theory describing the pulse width of homogeneously broadened AM mode-locked laser oscillators the modulation index σ_m is a parameter, given by δ_m = 2 Δ?.     

Cross sections of electron capture by Boron ions in gaseous media

We study the cross sections σ_{i, i?1}, σ_{i, i?2}, and σ _{i, i?3} of capture of one, two, and three electrons by boron ions with charges i=1?5 and velocities V=(1.83?5.50)V₀ in gaseous media with atomic numbers Z_t varying from 1 to 54. The oscillatory form of the Z_t dependence of electron capture cross section by boron ions, which has been established for lighter ions, is confirmed.     

Static and high-frequency hole transport in <Emphasis Type="Italic">p</Emphasis>-Si/SiGe heterostructures in the extreme quantum limit

Complex high-frequency (HF), σ^AC = σ₁ ? iσ₂, and static, σ^DC, conductivities, as well as current-voltage characteristics, have been measured in p-Si/SiGe heterostructures with a low hole density (p = 8.2 × 10¹⁰ cm^?2) at temperatures T = 0.3–4.2 K in the ultraquantum limit, when the filling factor is v < 1. In order to determine the components of the HF conductivity, the acoustic contactless method in the “hybrid configuration” is used, when the surface acoustic wave propagates on the surface of the LiNbO₃ piezoelectric and the heterostructure is pressed to the surface by a spring. The conductivities σ₁ and σ₂ are determined from the damping and velocity of the surface acoustic waves that are measured simultaneously with varying the magnetic field. The revealed HF conductivity features—σ₁ ? |σ₂|, the negative sign of σ₂, the threshold behavior of the current-voltage characteristic, and the dependence I ∝ exp(-A/V ^0.3) in the subthreshold region—indicate the formation of a pinned Wigner crystal (glass) in the ultraquantum limit (T = 0.3–0.8 K, B > 14 T).     

Observation of unfilled electron states in alkali graphite intercalation compounds by secondary electron spectroscopy

The energy position of distinct σ-electron energy bands above the Fermi level has been measured in pure graphite, in a variety of stage 1 alkali intercalation compounds and in several stages of C_xK. Changes of the σ-band gap between occupied and unoccupied states near the Λ-point by a nonuniform shift of the valence- and conduction-bands are small for the heavy alkali graphite intercalation compounds, whereas a change of 1 eV is observed for C₆Li.     

Magnetoreflectance of the Γ6 – Γ8 exciton ground state in cubic ZnSe

Magnetoreflectance measurements on the ground state of the Γ₆ – Γ₈ free exciton in cubic ZnSe in magnetic fields up to 18 T are reported. The splitting between the |1, ±1〉 states was derived from the measured difference spectrum between σ⁺ and σ^--polarized reflectance in Faraday configuration. The splitting between the two states corresponding to |2, 0〉 and |1, 0〉 at B = 0 was determined by means of a lineshape analysis. We derive an electron g-factor g = 1.48 ± 0.25, in reasonable agreement with existing k · p calculations, and obtain an effective hole g-value $\text{K}\text{?}\text{= -0.26±0.06}$. In addition, we find an upper limit for the short range electron-hole spin exchange energy Δ ? 0.1 meV, which is considerably smaller than values, which is considerably smaller than values reported in the literature, but agrees with recent results on ZnTe obtained by uniaxial stress and also magnetoreflectance measurements.     

Antihysteresis of the electrical resistivity of graphene on a ferroelectric Pb(Zr x Ti1 − x )O3 substrate

A quantitative model is developed to explain the antihysteretic behavior of the electrical resistivity of graphene on a ferroelectric Pb(Zr _x Ti_{1 ? x} )O₃ substrate as a function of the gate voltage. The model takes into account the trapping of the electrons from the graphene layer by the states related to the graphene-ferroelectric interface. The finite energy gap of impurity states is also taken into account, which makes it possible to describe the well-known experimental dependences, including an increase and the subsequent saturation of a “memory window” with the switching gate voltage. The obtained estimates can be important for creating next-generation nonvolatile memory elements, which use the two stable values of electrical resistivity (one of them is attributed to logical “0” and the other, to “1”) that result from the antihysteresis effect.     

Magnetooptical evidence of exchange interactions in zero-gap Hg1−xFexTe mixed crystals

Interband magnetoabsorption is carried out on zero gap Hg_1-xFe_xTe alloys of x ～ 0.015.Γ₆ → Γ₈ magnetooptical spectra for σ⁺, σ^-, γ 6 H polarization are quantitatively interpreted within the “quasi Ge” model modified by the inclusion of exchange contributions. The field dependence of the magnetization provides evidence of antiferromagnetic interactions between localized spins.     

Predictions on the couplings of f- and ϱ- trajectories to mesons from chiral symmetry

We employ a chiral saturation scheme involving 15, $\text{L = 0,}\text{and}\text{15}\text{, L = 1}$ mesons in a classification according to SU(4) ? O(3) (i.e. the mesonns π, ?, ω, A₁, A₂, f, A₀ (δ), B, D and σ) and calculate all f and ? Regge couplings between these mesons via a matrix version of finite-energy sum rules. In addition the size of exotic exchanges is evaluated and found to be small. The results are compared with experimental numbers and other models as far as available.     

Grain size, stress, and creep in polycrystalline solids

If a stress σ is applied to a polycrystal of grain size L, the mode of creep deformation depends on the answers to the following questions: (I) Does σ exceed the Peierls stress σ_p; (II) Does L exceed the dislocation spacing in a Taylor lattice stabilized by σ_p; (III) Does Lσ exceed the value required for a Frank-Read or Bardeen-Herring source to operate within the grain? (IV) Does L ^1/2σ exceed the Hall-Petch value required for slip to propagate across a grain boundary? The (L, σ) plane is thus partitioned into regions in which different creep modes predominate.