Cleavage of C<Subscript>60</Subscript> fullerite crystals

It is found that, under certain conditions, C₆₀ fullerite crystals can be cleaved along cleavage planes that are close-packed planes of the {111} type. Rigid gas-phase grown crystals exhibit good cleavage properties. In experiments with active compressive deformation, these crystals showed a high yield point τ_y = 2.65 MPa, a “parabolic” stress-strain curve, and brittle fracture after attaining a shear strain of about 8%. The fracture surface was clearly seen to have fragments parallel to the (111) plane. Typical microstructures observed in the cleavage plane are discussed: crystallographic cleavage steps, an indentation pattern, and a dislocation prick rosette. The fact that the activation volume V ? 60b³ is small (b is the Burgers vector of a dislocation) and strain-independent indicates the Peierls character of fullerite deformation or dislocation drag in a dense network of local defects.     

Internal friction of Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> single crystals

This paper reports on the results of measurements of the internal friction Q^?1 and the shear modulus G of Li₂B₄O₇ single crystals along the crystallographic directions [100] and [001] in the temperature range 300–550 K for strain amplitudes of (2–10)×10^?5 at infralow frequencies. The anomalies observed in Q^?1 and G in the temperature range 390–410 K are due to thermal activation of the mobility of lithium cations and their migration from one energetically equivalent position to another. A jump in the internal friction background is revealed in the vicinity of the Q^?1 and G anomalies for the Li₂B₄O₇ crystal. The magnitude of this jump depends on the crystallographic direction.     

Electrically stimulated motion of dislocations in a constant magnetic field

This paper reports on the results of investigations into the dislocation mobility in n-Si single crystals (N _d =5×10²⁴ m^?3) upon simultaneous exposure to electric (j=3×10⁵ A/m²) and magnetic (B≤1 T) fields. It is found that the introduction of dislocations (≈10⁹ m^?2) into dislocation-free silicon doped with phosphorus leads to the appearance of the paramagnetic component of the magnetic susceptibility. The paramagnetic component increases with an increase in the dopant concentration. Similar transformations in silicon account for the formation of magnetically sensitive impurity stoppers that respond to external magnetic perturbations. An analysis of the behavior of dislocations in electric and magnetic fields has revealed a parabolic dependence of the dislocation path length on the magnetic induction B. The effective charges and mobilities of dislocations are numerically calculated from the results obtained. A model is proposed according to which the observed increase in the dislocation mobility is associated with the decrease in the retarding power of magnetically sensitive stoppers due to a local change in the magnetic characteristics of the material and the spin-dependent reactions stimulated by a magnetic field.     

Dielectric constant anomalies of an improper Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript> ferroelastic near the phase-transition temperature

The temperature dependence of the permittivity of a Hg₂Cl₂ crystal has been investigated within the Landau phenomenological theory. At T < T _C, the linear permittivity has a singularity in the form ～(T _C?T)^1/2; however, this anomaly may disappear in a multidomain sample. The nonlinear permittivity also has an anomaly near T _C but of stronger type: ～(T _C ? T)^?1/2.     

Studies on structural and electrical properties of Mg<Subscript>0. 5+y</Subscript> (Zr<Subscript>2-y</Subscript>Fe<Subscript>y</Subscript>) <Subscript>2</Subscript> (PO<Subscript>4</Subscript>) <Subscript>3</Subscript> ceramic electrolytes

The sample of Mg_{0. 5+y} (Zr_1-y Fe_y) ₂ (PO₄) ₃ (0.0 ≤y ≤0.5) was synthesized using the sol-gel method. The structures of the samples were investigated using X-ray diffraction and Fourier transform infrared spectroscopy measurement. XRD studies showed that samples had a monoclinic structure which was iso-structured with the parent compound, Mg_0.5Zr (PO₄) ₃. The complex impedance spectroscopy was carried out in the frequency range 1–6 MHz and temperature range 303 to 773 K to study the electrical properties of the electrolytes. The substitutions of Fe³⁺ with Zr⁴⁺ in the Mg_0.5Zr (PO₄) ₃ structure was introduced as an extrainterstitial Mg²⁺ ion in the modified structured. The compound of Mg_0.5+y (Zr_1-y Fe_y)₂(PO₄)₃ with y?=?0.4 gives a maximum conductivity value of 1.25?×?10^?5 S cm^?1 at room temperature and 7.18?×?10^?5 S cm^?1 at 773 K. Charge carrier concentration, mobile ion concentration, and ion hopping rate are calculated by fitting the conductance spectra to power law variation, σ _ac (ω)?=?σ _o ? +?Aω ^α . The charge carrier concentration and mobile ion concentration increases with increase of Fe³⁺ inclusion. This implies the increase in conductivity of the compounds was due to extra interstitial Mg²⁺ ions.     

Relaxation phenomena in TlGa<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript>Se<Subscript>2</Subscript> single crystals

The relaxation electronic phenomena occurring in TlGa_0.99Fe_0.01Se₂ single crystals in an external dc electric field are investigated. It is established that these phenomena are caused by electric charges accumulated in the single crystals. The charge relaxation at different electric field strengths and temperatures, the hysteresis of the current-voltage characteristic, and the electric charge accumulated in the TlGa_0.99Fe_0.01Se₂ single crystals are consistent with the relay-race mechanism of transfer of a charge generated at deep-lying energy levels in the band gap due to the injection of charge carriers from the electric contact into the crystal. The parameters characterizing the electronic phenomena observed in the TlGa_0.99Fe_0.01Se₂ single crystals are determined to be as follows: the effective mobility of charge carriers transferred by deep-lying centers μ_f=5.6×10^?2 cm²/(V s) at 300 K and the activation energy of charge transfer ΔE=0.54 eV, the contact capacitance of the sample C _c =5×10^?8 F, the localization length of charge carriers in the crystal d _c =1.17×10^?6 cm, the electric charge time constant of the contact τ=15 s, the time a charge carrier takes to travel through the sample t _t =1.8×10^?3 s, and the activation energy of traps responsible for charge relaxation ΔE _σ = ΔE _Q = 0.58 eV.     

Structural and magnetic phase transformations in the BiFeO<Subscript>3</Subscript>-CaFe<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> system

The crystal structure and magnetic properties of the Bi_{1 ? x} Ca _x Fe_{1 ? x/2}Nb _x/2O₃ system were studied. It is shown that, at x ≤ 0.15, the unit-cell symmetry of solid solutions is rhombohedral (space group R3c). Solid solutions with x ≥ 0.3 have an orthorhombic unit cell (space group Pbnm). The rhombohedral compositions are antiferromagnetic, while the orthorhombic compositions exhibit a small spontaneous magnetization due to Dzyaloshinski?-Moriya interaction. In CaFe_0.5Nb_0.5O₃, the Fe³⁺ and Nb⁵⁺ ions are partially ordered and the unit cell is monoclinic (space group P2₁/n). In the concentration range 0.15 < x < 0.30, a two-phase state (R3c + Pbnm) is revealed.     

Effect of correlation in the impurity defect distribution on the micromechanical properties of GaAs: Te single crystals

The micromechanical properties of Te-doped GaAs single crystals with free carrier density n ₀ = 10¹⁷?5 × 10¹⁸ cm^?3 were studied. The obtained data are as follows: the nonmonotonic concentration dependences of the microhardness, the lengths of dislocation rosette rays, the densities of dislocations, and the position and half-width of the Raman line of a transverse optical phonon. The data are interpreted in terms of spatial correlation in the impurity defect distribution.     

Comparative study of solid-state reaction and sol-gel process for synthesis of Zr-doped Li<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> solid electrolytes

We investigated the synergistic influences of synthesis methods (solid-state reaction vs. sol-gel process) and Zr⁴⁺ doping on the structure and ionic conductivity of perovskite-structured Li_0.5La_0.5TiO₃ (LLTO) solid electrolytes. The lithium-ion conductivity of Li_0.5La_0.5Ti_1???x Zr _x O₃ ceramic specimens was evaluated as a function of x value and compared carefully between those two synthesis methods. Regarding the conductivity, sol-gel process is better for the synthesis of LLTO than solid-state reaction. As a result, the highest grain conductivity is obtained in the sol-gel-derived pure LLTO sample with x?=?0, reaching 1.10?×?10^?3 S?·?cm^?1. Partial substitution of Zr⁴⁺ enlarges the LLTO’s grain aggregate size and increases the total superficial area of aggregates. Consequently, Zr⁴⁺ substitution not only affects the grain (bulk) conductivity, but more importantly, also improves the grain boundary conductivity and the total conductivity. The highest total conductivity is 5.84?×?10^?5 S?·?cm^?1 with x?=?0.04 by sol-gel process.     

Charge transport in PbMoO<Subscript>4</Subscript> crystals

Dc and ac electrical conductivity of lead molybdate crystals is studied in the temperature range 300–550 K. The electrical conductivity was shown to have electronic (hole) impurity character. The I–V characteristics are typical of a space charge-limited current. The carrier mobility was estimated to be 10^?5 cm² V^?1 sat T=300 K. The results of the study suggest the hopping mechanism of conduction in PbMoO₄ crystals.     

Effective pairing interaction for the Argonne nucleon-nucleon potential v<Subscript>18</Subscript>

The effective pairing interaction corresponding to the Argonne nucleon-nucleon potential v₁₈ is found within the local potential approximation for several values of the boundary energy E ₀ that specifies the model subspace S ₀. A detailed comparison with the analogous effective interaction calculated previously for the Paris nucleon-nucleon potential is performed. It is shown that the effective interactions obtained for the two different nucleon-nucleon potentials at the same value of E ₀ are very close to each other. In the case of the Paris potential, a very wide subspace S′ complementary to S ₀ was required in calculating the effective interaction (the corresponding cutoff momentum being k _max = 160?180 fm^?1), whereas a much narrower subspace S′ corresponding to k _max = 10?12 fm^?1 could be used in the case of the Argonne potential.     

Optical and x-ray diffraction studies of the symmetry of distorted phases of the (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>ZrF<Subscript>7</Subscript> crystal

(NH₄)₃ZrF₇ single crystals were grown, and polarization-optical and x-ray diffraction studies were performed on powders and crystalline plates of various cuts over a wide temperature range. Phase transitions are revealed at temperatures T _1↑ = 280 K, T _2↑ = 279.6 K, T _3↑ = 260–265 K, and T _4↑ = 238 K on heating and at T _1↓ = 280 K, T _2↓ = 269–270 K, T _3↓ = 246 K, and T _4↓ = 235 K on cooling. The sequence of changes in symmetry is established to be as follows: O _h ⁵ (Z = 4) ? D _2h ²⁵ (Z = 2) ? C _2h ³ (Z = 2) ? C _i ¹ (Z = 108) ? monoclinic²(Z = 216).     

Mechanisms of low-temperature high-frequency conductivity in systems with a dense array of Ge<Subscript>0.7</Subscript>Si<Subscript>0.3</Subscript> quantum dots in silicon

High-frequency (HF) conductivity in systems with a dense (with a density of n = 3 × 10¹¹ cm^?2) array of self-organized Ge_0.7Si_0.3 quantum dots in silicon with different boron concentrations n_B is determined by acoustic methods. The measurements of the absorption coefficient and the velocity of surface acoustic waves (SAWs) with frequencies of 30–300 MHz that interact with holes localized in quantum dots are carried out in magnetic fields of up to 18 T in the temperature interval from 1 to 20 K. Using one of the samples (n_B = 8.2 × 10¹¹ cm^?2), it is shown that, at temperatures T ≤ 4 K, the HF conductivity is realized by the hopping of holes between the states localized in different quantum dots and can be explained within a two-site model in the case of

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, where ω is the SAW frequency and τ₀ is the relaxation time of the populations of the sites (quantum dots). For T > 7 K, the HF conductivity has an activation character associated with the diffusion over the states at the mobility threshold. In the interval 4 K < T < 7 K, the HF conductivity is determined by a combination of the hopping and activation mechanisms. The contributions of these mechanisms are distinguished; it is found that the temperature dependence of the hopping HF conductivity approaches saturation at T* ≈ 4.5 K, which points to a τ₀ ≤ 1. A value of τ₀(T*) ≈ 5 × 10^?9 s is determined from the condition ωτ₀(T*) ≈ 1.

     

Frustrated antiferromagnetism in the Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript> double perovskite

The spins of Ru⁵⁺ ions in Sr₂YRuO₆ form a face-centered cubic lattice with antiferromagnetic nearest neighbor interaction J≈25 meV. The antiferromagnetic structure of the first type experimentally observed below the Néel temperature T _N =26 K corresponds to four frustrated spins of 12 nearest neighbors. In the Heisenberg model in the spin-wave approximation, the frustrations already cause instability of the antiferromagnetic state at T=0 K. This state is stabilized by weak anisotropy D or exchange interaction I with the next-nearest neighbors. Low D/J～I/J～10^?3 values correspond to the experimental T _N and sublattice magnetic moment values.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

Dielectric response of a (1000 nm)SrTiO<Subscript>3</Subscript> layer epitaxially grown on (001)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> to temperature variation and electric field

The structure and dielectric characteristics of the (1000 nm)SrTiO₃ spacer in a (001)SrRuO₃ ‖ (001)SrTiO₃ ‖ (001)La_0.67Ca_0.33MnO₃ trilayer heterostructure grown on a (001)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrate have been studied. Both oxide electrodes, as well as the strontium titanate layer, were cube-on-cube epitaxially grown. The unit cell parameter in the SrTiO₃ layer measured in the substrate plane (3.908±0.003 Å) practically coincided with that determined along the normal to the substrate surface (3.909±0.003 Å). The temperature dependence of the real part of the permittivity ?′ of the SrTiO₃ layer in the range 70–180 K fits the relation (?′)^?1 ～ ? ₀ ^?1 C ₀ ^?1 (T-T _C ) well, where C₀ and T_C are the Curie constant and the Curie-Weiss temperature, respectively, for bulk strontium titanate crystals and ?₀ is the free-space permittivity. The data obtained on the temperature dependence of the permittivity of SrTiO₃ films enabled us to evaluate the effective depth of electric field penetration into the manganite electrode (L _e ≈ 0.5 nm) and the corresponding capacitance (C _e ≈1×10^?6 F/cm²) of the interface separating the (001)SrTiO₃ layer from the (001)La_0.67Ca_0.33MnO₃ bottom electrode.     

In-plane torque and gap symmetry of untwinned YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7</Subscript> crystals

The reversible magnetic torque of untwinned YBa₂Cu₃O₇ single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thed _x ²?y ² symmetry and rule out the possibility of thed _xy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\).     

Heat capacity of the La<Subscript>0.9</Subscript>Ag<Subscript>0.1</Subscript>MnO<Subscript>3</Subscript> manganite near the curie temperature

The heat capacity of the La_0.9Ag_0.1MnO₃ manganite is measured in the temperature range 77–350 K and studied in detail in the vicinity of the Curie temperature for the first time. The regularities of the variation in the universal critical parameters in the vicinity of the phase transition point are established. The critical exponent and the amplitude of the heat capacity are calculated to be α = ?0.127 and A ⁺/A ^? = 1.146 with due regard for the scaling corrections. These parameters correspond to the critical behavior within the three-dimensional Heisenberg model. The size of ferromagnetic droplets in the paramagnetic range at T > T _C is estimated as ξ ≈ 19 Å. The results obtained are analyzed thoroughly and compared with theoretical data for a number of model systems.     

Weak ferromagnetism in BiFeO<Subscript>3</Subscript>-based multiferroics

The magnetic properties of the Bi_{1 ? x} Ln _x FeO₃ (Ln is a rare-earth ion), Bi_{1 ? x} A _x FeO_{3 ? x/2} (A is an alkali earth ion), and BiFe_{1 ? x} Ti _x O_{3 + δ} solid solutions in magnetic fields up to 14 T have been studied. The concentration ranges of the existence of the ferroelectric phase described by the space group R3c have been determined. It is shown that the substitution of the rare-earth ions for the Bi³⁺ ions leads to a sharp decrease in the critical fields inducing the metamagnetic transition from a modulated antiferromagnetic state to a weakly ferromagnetic one; however, the modulated structure in the concentration range of the R3c phase is mainly retained. The substitution of the alkali earth ions (x ～ 0.1) for the bismuth ions leads to the total destruction of the modulated structure and to the implementation of the weakly ferromagnetic state within the R3c phase. A homogeneous weakly ferromagnetic state has been revealed when the Ti⁴⁺ ions (x = 0.1) are substituted for the Fe³⁺ ions in the ferroelectric R3c phase.     

Effect of cation substitution on the lattice dynamics and ferroelectric instability of cubic BaTiO<Subscript>3</Subscript> and BaZrO<Subscript>3</Subscript> doped with Bi and La ions

The spectra of lattice vibration frequencies of solid solutions Ba_{1 ? x} Bi_2x/3? _x/3Ti(Zr)O₃ and Ba_{1 ? x} La _x Ti(Zr)_{1 ? x/4}? _x/4O₃ are calculated in terms of a generalized Gordon-Kim model with inclusion of the dipole and quadrupole polarizabilities. Over the entire concentration range, the calculated phonon spectra contain a ferroelectric soft mode. The effect of various interactions on the ferroelectric instability of these solid solutions is studied. It is shown that the character of ferroelectric instability is largely determined by the mechanism of charge compensation.