On the surface properties of a nanodiamond

The dependences of the specific surface energy, the surface tension, and the surface pressure on the size, the surface shape, and the temperature of a nanodiamond with a free surface have been investigated using the Mie-Lennard-Jones interatomic interaction potential. The nanocrystal has the form of a parallelepiped faceted by the (100) planes with a square base. The number of atoms N in the nanocrystal varies from 5 to ∞. The isothermal isomorphic dependences of the specific surface energy, its isochoric derivative with respect to the temperature, the surface tension, and the surface pressure on the nanodiamond size have been calculated at temperatures ranging from 20 to 4300 K. According to the results of the calculations, the surface energy under this conditions is positive, which indicates that the nanodiamond cannot be fragmented at temperatures up to 4300 K. The surface pressure for the nanodiamond P _sf (N) ∼ N ^−1/3 is considerably less than the Laplace pressure P _ls (N)^−1/3 ∼ N ^−1/3 for the same nanocrystal at the given values of the temperature, the density, and the number of atoms N. As the temperature increases from 20 to 4300 K, the lowering of the isotherm P _sf (N) is considerably more pronounced than that of the isotherm P _ls (N). At high temperatures, the isotherm P _sf (N) changes the shape of the size dependence and goes to the range of extension of small nanocrystals. It has been demonstrated that the lattice parameter of the nanodiamond can either decrease or increase with a decrease in the nanocrystal size. The most significant change in the lattice parameter of the nanodiamond is observed at temperatures below 1000 K.     

Relaxationserscheinungen im 63 P 2-Zustand des Quecksilber-Isotops199Hg bei Stößen mit Quecksilberatomen im Grundzustand

The decay ofrf resonance signals (Δm _F =± 1, ΔF=0) in the hfs-states (F=3/2, 5/2) of the 6³ P ₂-state of¹⁹⁹Hg has been observed by means of a sampling method. By comparing the relaxation times to those of the even isotopes, the cross section σ₂(F) for the destruction of an alignment in the hfs-states by collisions with ground state Hg-atoms could be measured. The following ratios were obtained: σ₂(F=3/2)/σ₂=1.04±0.06 and σ₂(F=5/2)/σ₂=0.90± 0.03. The cross section σ₂ for the even isotopes was found to be (2.620±0.265) 10^?14cm². Assuming total decoupling of nuclear spinI and electronic angular momentumJ during the collision, the cross sections for the destruction of an orientation (σ₁) and an “octupolarisation” (σ₃) could be calculated. For the even isotopes the following ratios were derived: σ₁/σ₂=0.76 ± 0.07 and σ₃/σ₂=1.08 ± 0.09.     

Surface energy and pressure of diamond and silicon nanocrystals

Based on the pair potential of interatomic interaction, we study the dependence of various properties of diamond and silicon nanocrystals with a free surface on size, surface shape, and temperature. A model nanocrystal has the form of a parallelepiped faceted by {100} planes with a square base. The number of atoms N in the nanocrystals is varied from 5 to infinity. The Debye temperature, Gruneisen parameter, specific surface energy, isochoric derivative of specific surface energy with respect to temperature, and surface pressure are calculated as a function of the size and shape of diamond and silicon nanocrystals at temperatures ranging from 20 K to the melting point. The surface pressure P _sf(N) ∼ N ^−1/3 is much lower than the pressure calculated by the Laplace formula for similar nanocrystals for given values of density, temperature, and number of atoms. As the temperature increases from 20 K to the melting point, the isotherm P _sf(N) lowers and changes the shape of the dependence on N; at high temperatures, it goes to the region of extension of small nanocrystals of diamond and silicon.     

Dielectric relaxation and ac conductivity of sodium tungsten phosphate glasses

Studies of dielectric relaxation and ac conductivity have been made on three samples of sodium tungsten phosphate glasses over a temperature range of 77–420 K. Complex relative permitivity data have been analyzed using dielectric modulus approach. Conductivity relaxation frequency increases with the increase of temperature. Activation energy for conductivity relaxation has also been evaluated. Measured ac conductivity (σ_m(ω)) has been found to be higher than σ_dc at low temperatures whereas at high temperature σ_m(ω) becomes equal to σ_dc at all frequencies. The ac conductivity obeys the relation σ_ac(ω)=Aω ^S over a considerable range of low temperatures. Values of exponent S are nearly equal to unity at about 78 K and the values decrease non-linearly with the increase of temperature. Values of the number density of states at Fermi level (N(E _F)) have been evaluated at 80 K assuming values of electron wave function decay constant α to be 0.5 (Å)^?1. Values of N(E _F) have the order 10²⁰ which are well within the range suggested for localized states. Present values of N(E _F) are smaller than those for tungsten phosphate glasses.     

Electrical and magnetic properties of single crystal [NEt4]2[CuII(mnt)2]

We report electrical and magnetic studies of [NEt₄]₂[Cu^II(mnt)₂]. This crystal is composed of chains of theplanar [Cu^II(mnt)₂]^?2 anions (space group $\text{P}\text{1}$ and z = 1) which exhibit only weak magnetic interactions. The material behaves as a semiconductor; from 300–400°K the conductivity increases by six orders of magnitude and the resistivity values above 300°K are comparable to those of some of the better known wide band-gap inorganic semiconductors. In contrast with the behavior of other linear chain systems, at room temperature the conductivity along the chain (σ_∥) is less than that perpendicular to the chain (σ_⊥). As the temperature is increase, the anisotropy ratio, σ_∥/σ_⊥, becomes greater than unity and increases to 1.6 × 10² at 400°K.     

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).     

Quantitative Measurement of Transverse and Longitudinal Cross-Correlation between13C–1H Dipolar Interaction and13C Chemical Shift Anisotropy: Application to a13C-Labeled DNA Duplex

Measurement of both longitudinal and transverse relaxation interference (cross-correlation) between¹³C chemical shift anisotropy and¹³C–¹H dipolar interactions is described. The ratio of the transverse to longitudinal cross-correlation rates readily yields the ratio of spectral densitiesJ(0)/J(ω_C), independent of any structural attributes such as internuclear distance or chemical shift tensor. The spectral density at zero frequencyJ(0) is also independent of chemical exchange effects. With limited internal motions, the ratio also enables an accurate evaluation of the correlation time for overall molecular tumbling. Applicability of this approach to investigating dynamics has been demonstrated by measurements made at three temperatures using a DNA decamer duplex with purines randomly enriched to 15% in¹³C.     

On the surface properties and baric fragmentation of iron

The dependence of the specific surface energy (??) on the normalized volume (V/V ₀) and temperature (T) for the body-centered cubic (BCC) lattice of iron has been studied on the basis of the Mie-Lennard-Jones potential of interatomic interaction. It is shown that below the definite value of the normalized volume (V/V ₀)_fr the specific surface energy of the BCC-Fe lattice passes to the negative range of values: ??(V/V ₀)_fr = 0, and the (V/V ₀)_fr value increases almost linearly with temperature. In the case of compression, when (V/V ₀) < (V/V ₀)_fr, the exothermal process of crystal fragmentation into dendritic domains with the maximum possible specific intercrystalline surface takes place. In the case of nanofragmentation, surface pressure (P _sf) emerges, leading to self-packing of the formed nanocrystals. The dependence of the ??(V/V ₀) and P _sf (V/V ₀) functions on the size and shape of the BCC-Fe nanocrystals has been studied at different temperature values T = 1500?C3500 K. It has been shown that the P _sf function increases with a decrease in the size; this occurs more strongly, the more the nanocrystal deviates from the most thermodynamically stable cubic shape. The dimensional compression of the lattice parameter of BCC-Fe nanocrystals has been studied. The specific (per volume unit) amount of heat released during fragmentation of the BCC-Fe lattice at high pressures and temperatures has been estimated.     

Surface tension of pure liquid helium isotopes

The effects caused by vapor inhomogeneity over liquid helium are considered. Both pure isotopes have surface levels, whose population increases with temperature T. We separated their contribution to the temperature dependence of surface tension σ₃(T) and σ₄(T) and compared our theoretical results with the results of Japanese experimental works [1–3]. For liquid He³, one has σ₃(T)=σ₃(0)?σ ₃ ^∞ T² at 0.2 K<T<1 K and σ₃(T)=σ₃(0)?α ₃ ⁰ T²exp(?Δ₃/T) at T<0.2 K, with Δ₃≈0.25 K. For liquid He⁴, σ₄(T)=σ₄(0)?AT^7/3? α ₄ ⁰ T²exp(?Δ₄/T) at T<2 K, where A is the Atkins constant and Δ₄≈4 K. The parameters α ₃ ⁰ , α ₃ ^∞ , and α ₄ ⁰ depend on the fluid properties.     

Calculation of Tc and the ratio 2Δ(0)/kBTc of MgB2 within a two-band model of superconductivity

Superconductivity in the MgB₂ superconductor is described within the framework of a two-band Eliashberg formalism. Different gaps are assumed to open on the different parts of the Fermi surface of this compound. Separation of the order parameter (OP) into two components is achieved by taking the Fourier transform of the OP using the momentum states of the σ- and π-bands of MgB₂. Expressions for the T_c and the ratio 2Δ_σ(0)/k_BT_c for this superconductor are obtained. Numerical values for these two properties are obtained for a range of values of the cut-off frequency of the phonons responsible for the superconductivity and for a range of values of the ratio between the two energy gaps. This was done for various values of the normalized partial densities of states on the σ-sheet of the Fermi surface.     

Characteristics of the Li<Superscript>+</Superscript>-Ion Conductivity of Li<Subscript>3</Subscript>R<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> Crystals (R = Fe,Sc) in the Superionic State

The characteristics of Li+-ion conductivity σdc of structural γ modifications of Li₃R₂(PO₄)₃ compounds (R = Fe, Sc) existing in the superionic state have been investigated by impedance spectroscopy. The type of structural framework [R₂P₃O₁₂]_∞^3- affects the σdc value and the σdc activation enthalpy in these compounds. The ion transport activation enthalpy in γ-Li₃R₂(PO₄)₃ (ΔH_σ = 0.31 ± 0.03 eV) is lower than in γ-Li₃Fe₂(PO₄)₃ (ΔH_σ = 0.36 ± 0.03 eV). The conductivity of γ-Li₃Fe₂(PO₄)₃ (σ_dc = 0.02 S/cm at 573 K) is twice as high as that of γ-Li₃R₂(PO₄)₃. A decrease in temperature causes a structural transformation of Li₃R₂(PO₄)₃ from the superionic γ modification (space group Pcan) through the intermediate metastable β modification (space group P2₁/n) into the “dielectric” α modification (space group P2₁/n). Upon cooling, σdc for both phosphates decreases by a factor of about 100 at the superionic TSIC transition. In Li₃Fe₂(PO₄)₃ σdc gradually decreases in the temperature range T_SIC = 430–540 K, whereas in Li₃R₂(PO₄)₃ σdc undergoes a jump at T_SIC = 540 ± 25 K. Possible crystallochemical factors responsible for the difference in the σdc and ΔH_σ values and the thermodynamics and kinetics of the superionic transition for Li₃R₂(PO₄)₃ are discussed.     

Atomic beam magnetic resonance investigations in the 2p 2 3 P ground multiplet of the stable carbon isotopes12C and13C

The hyperfine structure seperations Δv andg _J -factors have been measured in the 2p ^{2 3} P states of¹³C(I=1/2) and¹²C(I=0), respectively, using the atomic beam magnetic resonance method. The results are Δv(³ P ₁,¹³C)=4.200 (25) MHz, Δv(³ P ₂,¹³C)=372.593 (25) MHz,g _J (³ P ₁,¹²C)=1.501052 (13), andg _J (³ P ₂,¹²C)=1.501039 (15). After applying corrections due to perturbations by neighbouring fine structure levels one deduces the constants of the magnetic dipole interactionA(³ P ₁,¹³C)=+2.838 (17) MHz, A(³ P ₂,¹³C)=+149.055 (10) MHz. No signs of theA-factors were determined by the experiment; they follow from the known positive sign of the nuclear magnetic moment μ _I of¹³C. CombiningA(³ P ₂,¹³C) with the results of other measurements on¹¹C, yields μ _I (¹¹C)=?0.964 (1) nm.     

Observation of the (π−, π−n) and (π+, π+p) reactions at 165 MeV

The (π^?, π^?n) and the (π⁺, π⁺p) reactions were studied on C, Fe, and Bi at 165 MeV by coincidence measurement o outgoing particles. The nucleon-knockout reaction is the dominant inelastic process at backward pion-scattering angles at this energy. The quasifree (π^?, π^?n) and (π⁺, π⁺p) cross sections were determined from the π-N an correlations. Their σ_qf(π^?, π^?n)/σ_qf(π⁺, π⁺p) ratio is consistent with unity for C and is larger than the N/Z ratio for the heavier nuclei.     

The determination of hopping rates and carrier concentrations in ionic conductors by a new analysis of ac conductivity

The a.c. conductivity σ(ω) of ionic materials takes the form, σ(ω) = σ(0) + Aωⁿ. The carrier hopping rate, ω_p, is obtained from the new expression σ(0) = A ω_pⁿ and the carrier concentration is estimated from σ(0). The contribution of creation and migration terms to the activation energy for conduction may be determined from the thermal activation of σ(0) and ω_p and the corresponding entropy terms quantified. Data have been analyzed for four widely different ionic materials: single crystal Na β-alumina, polycrystalline Li₄SiO₄, Ag₇l₄AsO₄ glass and Ca(NO₃)₂/KNO₃ glass and melt. For each, the carrier concentration and hopping rates have been obtained.     

Electrode reaction at Pt,O2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

This study aims to make clear the reaction kinetics at Pt, O₂(g)/zirconia electrodes in the oxygen partial pressure, P_O2, of ∼ 10⁻⁴ − 1 atm at ∼ 400–∼800°C. By a critical review on the preceding studies, problems were pointed out in the application of the Langmuir adsorption isotherm to the P_O2 dependence of electrode conductance, in the assumption of electric double layer at the electrode interface, and of the inconsistency between the recent reaction model of surface diffusion controlled kinetics and the absolute rate theory. It was shown that the charge transfer kinetics cannot be the rate determining step (RDS) of the electrode reaction. The possible RDS was concluded to be either (i) dissociative adsorption of oxygen molecules on the Pt surface or (ii) surface diffusion of O_ad atoms on the Pt surface to the Pt/zirconia contact. The diffusion of O_ad atoms on the Pt surface was considered to be proportional to θ(1−θ)(∂μ_O/∂x), where θ is the occupancy of O_ad atoms on Pt and μ_O is the oxygen chemical potential on the Pt surface. The rate equation, current-potential relationship, and the electrode conductivity, σ_E, were calculated for the cases the RDS is (i) and (ii), respectively. By comparing the calculated σ_E versus log P_O2 relationship with the reported ones, it was shown that the RDS is (i) for T ≲ 500°C and is (ii) for T ≳ 600°C. In the former case, σ_E is essentially constant irrespective to P_O2, and in the latter case, σ_E maximum appears on the σ_E versus log P_O2 relations.     

Conductivity of a periodic two-component system of rhombic type

The conductivity and the distribution of electric field, current, and charge density in a periodic two-component system composed of rhombs with an arbitrary vertex angle of 2α are investigated. The effective conductivity of such a medium is represented by a tensor with components σ _eff ¹¹ (α) and σ _eff ²² (α) in the principal axes that satisfy the Dykhne relation σ _eff ¹¹ (α) σ _eff ²² (α)=σ ₁σ₂, where σ₁, σ₂ are the isotropic conductivities of media 1 and 2. In addition, the relation σ _eff ²² (α)=σ _eff ¹¹ (π/2?α) is satisfied. The principal axes are directed along the diagonals of the rhombs. It is shown that there are three lines in the rectangle 0<α ≤π/2,?1<Z<1((Z=σ₁?σ₂)/(σ ₁+σ₂)) on which the charge density is expressed in terms elliptic functions. An explicit expression is obtained for all physical quantities on these lines.     

Frequency Dependence of the Dielectric Loss Angle in Disordered Semiconductors in the Terahertz Frequency Range

Frequency dependence of the real part of the conductivity σ₁(ω) in the region of the transition from almost linear (s < 1) to quadratic (s ≈ 2) can indicate a change in the conduction mechanism (the transition from the variable-range to the fixed-range hopping with increasing frequency); in this case, the sharpness of the change in the slope of the frequency characteristic is related to the dependence of the preexponential factor of the resonance integral on the intercenter distance in the pair. The frequency dependence of the imaginary part of the conductivity σ₂(ω) has no kink in the vicinity of the transition frequency ωcr, remaining almost linear. A large dielectric loss angle |cotγ| = |σ₂|/σ₁ can indicate that the imaginary part of the conductivity at ω < ω_cr is defined by the larger zero-phonon contribution in σ₂^res the region of weak variation in the loss angle γ(ω), which significantly exceeds the relaxation contribution σ₂^res.     

Change in the lattice properties and melting temperature of a face-centered cubic iron under compression

All four parameters of the Mie–Lennard-Jones pair interatomic potential have been determined, and the state equation (P) and baric dependences of the lattice properties of an fcc iron are calculated using a previously proposed method. The dependences have been studied for the following properties: Debye temperature; the first, second, and third Gruneisen parameters; isothermal bulk modulus B _T and B′(P); isochoric specific heat C _v and C _v ′(P); isobaric specific heat C _p ; coefficient of thermal expansion α _p and α _p ′(P); specific surface energy σ and σ′(P). Calculations performed along two isotherms (1500 and 3000 K) have shown good agreement with the experimental data. Analytical approximations of the baric dependences for B′(P), α _p (P), C _p (P), and σ′(P) have been obtained, and it is shown that at P → ∞ the functions B _T (P) and σ(P) change linearly, while the functions α _p′(P) and C _p ′(P) tend to zero. The calculated baric dependence of the melting temperature shows good agreement with the experimental data.