Pressure-induced metamagnetic transition in the Cd<Subscript>0.7</Subscript>Mn<Subscript>0.3</Subscript>GeAs<Subscript>2</Subscript> ferromagnetic semiconductor

The magnetic susceptibility χ/χ₀ and the longitudinal Δρ _zz /ρ₀ and transverse Δρ _xx /ρ₀ magnetoresistances have been measured as functions of the hydrostatic pressure P ≤ 7 GPa at room temperature in the high-temperature ferromagnetic semiconductor Cd_0.7Mn_0.3GeAs₂ with a chalcopyrite structure and the Curie temperature T _c = 355 K. A pressure-induced metamagnetic transition from the low-magnetization state to the high-magnetization state has been observed in Cd_0.7Mn_0.3GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of the magnetic susceptibility and magnetoresistance.     

Magnetoresistance and magnetic ordering in praseodymium and neodymium hexaborides

The magnetoresistance Δρ/ρ of single-crystal samples of praseodymium and neodymium hexaborides (PrB₆ and NdB₆) has been measured at temperatures ranging from 2 to 20 K in a magnetic field of up to 80 kOe. The results obtained have revealed a crossover of the regime from a small negative magnetoresistance in the paramagnetic state to a large positive magnetoresistive effect in magnetically ordered phases of the PrB₆ and NdB₆ compounds. An analysis of the dependences Δρ(H)/ρ has made it possible to separate three contributions to the magnetoresistance for the compounds under investigation. In addition to the main negative contribution, which is quadratic in the magnetic field (−Δρ/ρ ∝ H ²), a linear positive contribution (Δρ/ρ ∝ H) and a nonlinear ferromagnetic contribution have been found. Upon transition to a magnetically ordered state, the linear positive component in the magnetoresistance of the PrB₆ and NdB₆ compounds becomes dominant, whereas the quadratic contribution to the negative magnetoresistance is completely suppressed in the commensurate magnetic phase of these compounds. The presence of several components in the magnetoresistance has been explained by assuming that, in the antiferromagnetic phases of PrB₆ and NdB₆, ferromagnetic nanoregions (ferrons) are formed in the 5d band in the vicinity of the rareearth ions. The origin of the quadratic contribution to the negative magnetoresistance is interpreted in terms of the Yosida model, which takes into account scattering of conduction electrons by localized magnetic moments of rare-earth ions. Within the approach used, the local magnetic susceptibility χ_loc has been estimated. It has been demonstrated that, in the temperature range T _N < T < 20 K, the behavior of the local magnetic susceptibility χ_loc for the compounds under investigation can be described with good accuracy by the Curie-Weiss dependence χ_loc ∝ (T − Θ _p )⁻¹.     

Normal-state electrical resistivity and superconducting magnetic penetration depth in Eu<Subscript>0.5</Subscript>K<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> polycrystals

We report measurements of the temperature dependence of the electrical resistivity, ρ(T), and magnetic pen-etration depth, λ(T), for polycrystalline samples of Eu_0.5K_0.5Fe₂As₂ with T _c = 31 K. ρ(T) follows a linear temperature dependence above T _c and bends over to a weaker temperature dependence around 150 K. The magnetic penetration depth, determined by radio frequency technique displays an unusual minimum around 4 K which is associated with short-range ordering of localized Eu³⁺ moments. The article is published in the original.     

Ion conduction and space-charge polarization processes in Li<Subscript>2</Subscript>Ge<Subscript>7</Subscript>O<Subscript>15</Subscript> crystals

The electrical properties of a lithium heptagermanate (Li₂Ge₇O₁₅) crystal have been studied in DC and AC measuring fields at temperatures from 500 to 700 K. In a DC field, a substantial decrease of electrical conductivity σ with time has been detected. On the basis of kinetic dependences σ(t), estimates of the charge carrier diffusion coefficient D have been obtained. In the frequency range 10¹–10⁵ Hz, the spectra of complex impedance ρ*(f) have been measured. The analysis of diagrams in the complex plane (ρ″–ρ′) has been performed within the equivalent circuit approach. It has been shown that, in the considered temperature and frequency intervals, the electrical properties of Li₂Ge₇O₁₅ crystals have been determined by the hopping conduction of interstitial lithium ions A _Li and accumulation of charge carriers near the blocking Pt electrodes.     

Leading-twist contribution to χ<Subscript><Emphasis Type="Italic">c</Emphasis>0,2</Subscript> → ωω decays

The leading twist contribution to χ _c0,2 → ωω decays in the color-singlet approximation is considered. It is shown that the prediction for Br(χ _c0 → ωω) is in good agreement with the experimental data, while Br(χ _c2 → ωω) differs from the experiment significantly. The text was submitted by the author in English.     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Covariant tensor formalism for partial-wave analyses of ψ decays into γB¯, γγV and ψ(2s)↦γχ<Subscript>c0,1,2</Subscript> with χ<Subscript>c0,1,2</Subscript>↦K¯πand 2π<Superscript>+</Superscript>2π<Superscript>-</Superscript>

With accumulation of high statistics data at BES and CLEO-c, many new interesting channels can get enough statistics for partial-wave analysis (PWA). Among them, ψ↦γpˉ,γΛˉ,γΣˉ,γΞˉ channels provide a good place for studying baryon-antibaryon interactions; the double radiative decays ψ↦γγV with V ≡ ρ,ω,φ have a potential to provide information on the flavor content of any meson resonances (R) with positive charge parity (C = +) and mass above 1 GeV through ψ↦γR↦γγV; ψ(2s)↦γχ_c0,1,2 with χ_c0,1,2↦Kˉπ⁺π^- and 2π⁺2π^- decays are good processes to study χ_cJ charmonium decays. Using the covariant tensor formalism, here we provide theoretical PWA formulae for these channels.     

Magnetic and transport properties of monocrystalline<Emphasis Type="Italic">Fe</Emphasis><Subscript>3</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>4</Subscript>

A monocrystal ofFe ₃ O ₄ is characterized by resistance, magnetoresistance and magnetic measurements in a temperature range from 4.2 K to 350 K and magnetic field-cycling from −9 T to 9 T. The resistance measurements revealed a metal-insulator Verwey transition (VT) atT _v =123.76 K with activation energy E=92.5 meV at T >T _v and temperature-substitute for the activation energy below the VT,T ₀=E/k _B ≈3800 K within 70 K–110K. The magnetotransport results independently verified the VT at 123.70 K, with discontinuous change in the magnetic moment ΔM≈0.21 ΔM≈0.21μ _B and resistance hysteresis, dependent on the magnetic field in a narrow temperature range of 0.4° around theT _v . The magnetic characterization established self consistentlyT _v as ≈123.67 K, the jump in the magnetization at the VT≈0.25μ _B and confirmed, that the magnetocrystalline anisotropy is the main microscopic mechanism responsible for the magnetization of the monocrystal (88%) with additional natural and imposed defects contributing as 12%.     

Magnetocaloric effect in the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript> manganite

The magnetocaloric effect ΔT has been studied by a direct method in two samples of the manganite Sm_0.55Sr_0.45MnO₃, namely, a single crystal (sample A) and a ceramic sample (sample C). The temperature dependences of the ΔT effect of both samples exhibit a maximum at T _max = 143.3 K for the sample A and T _max = 143 K for the sample C. In these maxima, the values of the ΔT effect are 0.8 and 0.4 K in the magnetic field H = 14.2 kOe for the samples A and C, respectively. In addition, the ΔT(T) curve of the sample A has a minimum at T _min = 120 K, in which ΔT = −0.1 K. The maximum value of the ΔT effect increases with an increase in the magnetic field H in the range of magnetic fields up to 14.2 kOe, and the rate of this increase at H > 8 kOe is higher than that at H < 8 kOe. These features of the ΔT effect are explained by the presence of ferromagnetic and antiferromagnetic A- and CE-type clusters in the samples.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Kinetic phenomena in zero-gap semiconductors CuFeS<Subscript>2</Subscript> and CuFeTe<Subscript>2</Subscript>: Effect of pressure and heat treatment

Electrical resistivity ρ and Hal coefficient R are measured as a function of the temperature (T = 1.7−310 K) and the magnetic field (up to H = 28 kOe) in zero-gap semiconductor CuFeS₂ samples subjected to hydrostatic compression and under various heat-treatment conditions. At low temperatures, anomalies are observed in the kinetic effects related to the presence of ferromagnetic clusters: the magnetoresistance at T = 4.2 K and T = 20.4 K acquires a hysteretic character and thermopower α changes its sign at T < 15 K. The temperature dependence of conduction-electron concentration n in CuFeS₂ has a power form in the temperature range T = 14−300 K, which is characteristic of the intrinsic conductivity in zero-gap semiconductors. In CuFeS₂, we have n(T) ∝ T ^1.2; in isoelectron compound Cu_1.13Fe_1.22Te₂, we have n(T) ∝ T ^1.93. Heat treatment is found to affect the intrinsic conductivity of CuFeS₂, as the action of hydrostatic compression (carrier concentration changes); that is, the carrier concentration changes. However, a power form of the n(T) and ρ(T) dependences is retained.     

Radiation damages under irradiation of the BCS superconductor MgB<Subscript>2</Subscript> by high-energy electrons

This paper reports on the results of investigations of the influence of irradiation of the two-band BCS superconductor MgB₂ by electrons with an average energy $ \bar {\rm E} $ \bar {\rm E} ∼ 10 MeV at high doses (0 ≤ ϕt ≤ ∼2.5 × 10¹⁸ cm⁻²) on the temperature and width of the transition to the superconducting state, the temperature dependence of the electrical resistivity in the normal state, the crystal lattice parameters, and the diffraction line intensity. An increase in the electron irradiation dose ϕt leads to the following effects: a decrease in the critical temperature T _c ; an increase in the width of the superconducting transition ΔT _c ; and a decrease in the “residual electrical resistivity” ρ_{273 K}/ρ_{40 K}, in the parameters a and c of the hexagonal crystal lattice, and in the ratio between the diffraction line intensities I ₁₁₀/I ₁₀₀. From analyzing the results obtained, it has been established that the main type of radiation damages under irradiation of the BCS superconductor MgB₂ by high-energy electrons is the formation of vacancies in the B sublattice, which leads to a narrowing of the large band gap Δ_σ on the Fermi surface.     

Photoluminescence of nanoparticles of (Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>)<Subscript>0.95</Subscript>(Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.05</Subscript> solid solution

We have ground bulk samples to obtain nanoparticles of (Ga₂S₃)_1–x (Eu₂O₃) _x solid solutions, the sizes of which were determined using an atomic force microscope. The photoluminescence spectra of the nanoparticles were studied in the temperature interval 77–300 K. We have established the mechanisms for emission and transfer of energy from the matrix to the rare-earth ion, and we determined the Stokes shift (ΔS = 0.7 eV), the Huang–Rhys parameter (S = 16), and the optical phonon energy (ħ−ω = 23 meV).     

Observation of multiple superconducting gaps in the infrared reflectivity spectra of Ba(Fe<Subscript>0.9</Subscript>Co<Subscript>0.1</Subscript>)<Subscript>2</Subscript>As<Subscript>2</Subscript>

The results of infrared reflectivity measurements for the iron-based high-temperature superconductor Ba(Fe_0.9Co_0.1)₂As₂ are reported. The reflectivity is found to be close to unity at frequencies ω lower than 2Δ/h (2Δ is the superconducting gap and h is Planck’s constant). This is evidence for the s ^+/− or s ^+/+ symmetry of the superconducting order parameter in the studied compound. The infrared reflectivity spectra of Ba(Fe_0.9Co_0.1)₂As₂ manifest opening of several superconducting gaps at temperatures lower than critical T _c .     

Anisotropy of the vortex structure and resistivity in the basal plane of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript> single crystals

The vortex lattice of the YBa₂Cu₄O₈ high-temperature superconductor is studied in the basal plane of monocrystalline samples using the decoration technique in a field interval of 40–600 Oe. Vortex lattice anisotropy (field-independent “compression” of a regular hexagonal vortex cell in the poorly conducting direction by a factor of about 1.3) is detected. Resistivity anisotropy ρ _a /ρ _b measured at temperatures from T _c to room temperature is 16–9. Possible reasons for the discrepancy between our results and the available data are discussed.     

The size effect on transport properties of colossal magnetoresistance materials La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

High quality La_0.67Ca_0.33MnO₃ (LCMO) film was deposited via a novel pulsed electron deposition technique on SrTiO₃(100) single crystal substrate. The micro-bridge with different widths was fabricated by using electron beam lithography (EBL) technique and their transport properties were studied. For the micro-bridges with width of 2 and 1.5 μm, the insulator-to-metal transition temperature (T _P) keeps unchanged compared with the film. For the micro-bridges with width of 1 μm, the T _P shifts towards the lower temperature by 50 K. When the width decreases down to 500 nm, the insulator-to-metal transition disappears. The magnetoresistance behavior of these micro-bridges was studied, and the results show that the low field magnetoresistance (LFMR) decreases and the high field magnetoresistances (HFMR) keep almost unchanged as the width of micro-bridge is reduced. Supported by the Key Project of the Natural Science Foundation of Zhejiang Province of China (Grant No. Z605131), the National Natural Science Foundation of China (Grant No. 60571029), and W. H. Tang was supported by the Creative Research Group of National Natural Science Foundation of China (Grant No. 60321001)     

Optimized second-order optical nonlinearity in thermally poled GeS<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>S<Subscript>3</Subscript>-KI chalcohalide glass

0.56GeS₂-0.24Ga₂S₃-0.2KI (mol%) chalcohalide glass was prepared and second-harmonic generation was observed by the thermal poling process. Second-order optical nonlinearity in the glass was also investigated by different poling temperature, voltage and time to optimize the poling parameters to improve χ ⁽²⁾. The maximum χ ⁽²⁾ in our study as large as 3.74 pm/V was obtained under the optimized poling condition with 5.2 kV, 260°C and 120 minutes.     

Polarons in axial transport in single-layer high-T<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> superconductors

The temperatureT dependencies ρ(T) of normal state electric resistivitiesρ _c (axial) andρ _ab (in plane) of single-layer high-T _c superconductors show common trends: AsT is raised, the resistivity first drops steeply before it starts rising αT above an apparent semiconductor-to-metal crossoverT _cross . To analyze ρ(T) we plottT/ρ againstT at various dopingsx for bothρ _c andρ _ab .T/ρ is inversely proportional to the traversal time across a potential barrier as an ionic particle drifts in an electric field. We findT/ρ in good agreement with theT dependence of the quantum rate of migrating particles: AsT is raised, a zero-point rate at the lowestT is extended to a nearly flat plateau before a thermally activated branch sets in. We also find evidence for the admixture of 1- & 2-phonon absorptions below the Arrhenius range. These features shape the semiconductor-like branch below T _cross . AboveT _cross a metallic-like branch sets in, its αT character deriving from the field coupling of the migrating particle. Our analysis suggests that metal physics may not suffice if ionic features play a role in transport. We attribute our conclusions to the drift of strong-coupling polarons along Cu−O bonds. These “bond polarons” originate from carrier scattering by double-well potentials associated with the bonds. A bond polaron dissociates to a free hole as it passes onto a neighboring O-O link.     

Structural,magnetic, and magnetothermal properties of the Tb<Subscript>0.3</Subscript>Dy<Subscript>0.7</Subscript>Co<Subscript>2</Subscript> compound

A complex investigation of the structural, magnetic, and magnetothermal properties of the Tb_0.3Dy_0.7Co₂ compound synthesized with the use of high-purity rare-earth metals has been performed. The phase composition has been controlled using the X-ray structural analysis, and the topology of the alloy surface has been investigated using atomic-force microscopy. It has been established that the Tb_0.3Dy_0.7Co₂ compound is single-phase, while the samples selected for measurements possess a clearly pronounced texture. The magnetization has been measured using a vibrating-sample magnetometer in the fields up to 100 kOe in a temperature range from 4.2 to 200 K. The Curie temperature of the compound is 170 K. The data on the temperature dependence of heat capacity of Tb_0.3Dy_0.7Co₂ have been obtained. The magnetocaloric effect ΔT has been measured by a direct method in the fields up to 18 kOe applied both along and perpendicularly to the texture axis. The anisotropic behavior of the magnitude ΔT for this compound, which possesses the cubic structure, has been found. The maximum value of the magnetocaloric effect ΔT = 2.3 K (ΔH = 18 kOe) has been observed upon applying the magnetic field along the texture axis.