A. F. Shevchun

Russian Academy of Sciences, Moskva, Moscow, Russia

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Publications (17)26.65 Total impact

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    ABSTRACT: Mass transfer during the electrolysis of melts of Y0.02Ba0.30Cu0.70Oy and Y0.02Ba0.25Cu0.75Oy samples was studied at 950°C (for 0.5 h) and currents of 5–1050 mA. YBa2Cu3O6 + δ (123) tetragonal oxide crystal boules were grown, and their cationic composition and structure were studied by X-ray powder diffraction and by electron diffraction and elemental analysis in a transmission electron microscope (ED/TEM and EA/TEM). The 123 oxide was found to have cationic off-stoichiometry and to have a domain structure with domain sizes of 20–50 Å. Magnetic susceptibility versus temperature curves measured in the crystals after oxygen annealing (450°C, 1 h) feature four kinks, which indicate the occurrence of four superconducting phases with T c = 45, 52, 75, and 86 K. Electrolysis byproducts are platinum-containing oxides Ba9Pt4Cu3Oy and Ba50Pt15Y16Al13Cu7Oy unknown hitherto; we report structure data for them.
    Russian Journal of Inorganic Chemistry 01/2014; 59(3). · 0.42 Impact Factor
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    ABSTRACT: Precision measurements of the real and imaginary parts of the microwave surface impedance Z ac(T) = R ac(T) + iX ac(T) of the conducting ac layers of the k-(BEDT-TTF)2Cu[N(CN)2]Br crystals in the temperature interval of 0.5 < T < 100 K have demonstrated a series of features: (i) the temperature course of the field penetration depth is close to linear Δλac(T)∞ΔX ac(T) in the superconducting state at T T c ∼ 11.5 K; (ii) the curves R ac(T) = X ac(T) coincide at T c < T < 40 K; (iii) the X ac(T) value at T > 40 K increases in comparison with R ac(T); (iv) the dependence R ac(T) at T > 40 K is nonmonotonic in thin crystals. These features of the impedance Z ac(T) with increasing T are interpreted in terms of (i) the d-type symmetry of the superconducting order parameter, (ii) normal skin effect, (iii) manifestations of the antiferromagnetic fluctuations, and (iv) the size effect. The electrodynamic parameters of k-(BEDT-TTF)2Cu[N(CN)2]Br have been determined.
    JETP Letters 10/2012; 96(3). · 1.52 Impact Factor
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    ABSTRACT: The phase composition of Yx Ba1−x CuOy (x = 0.29−0.40) samples annealed in air (at 930–990°C) and in an oxygen atmosphere (450–800°C, P(O2) = 101 kPa) was studied by X-ray powder diffraction, chemical analysis, electron diffraction, and elemental analysis in a transmission electron microscope. A considerable cation nonstoichiometry was discovered in particles having the tetragonal and orthorhombic structures of YBa2Cu3O6 + δ. The variation range of particle compositions comprises matrix oxides of the Bam Cum + n Oy series with (Ba: Cu) 3: 5, 5: 8, 2: 3, and 5: 7, which in the presence of yttrium form the Yn Bam Cum + n Oy series. Tetragonal oxides Y2Ba3Cu5Oy (235), Y3Ba5Cu8Oy (358), YBa2Cu3Oy (123), and Y2Ba5Cu7Oy (257) are formed at the primary synthesis step in air and are preserved in an orthorhombic structure during short-term (1 h) oxygen annealing. Most particles of the 3: 5 and 5: 8 oxides are undersaturated with yttrium relative to the stoichiometry of the Yn Bam Cum + n Oy series, those of the 2: 3 oxide correspond to this stoichiometry, and those of the 5: 7 oxide are supersaturated with yttrium over the stoichiometry. A trend is observed for the fractions of these oxides to change during long-term (5–51 h) annealing in an oxygen atmosphere at 450°C and to the alternation of the dominant role of one of the four phases with the superconducting transition temperature T c = 82, 85, 86, and 91 K. Each orthorhombic oxide undergoes structural transformations during oxygen annealing with a change in T c. The coexistence of these oxides in the form of nanometer-sized domains does not allow their individual superstructures to be recognized.
    Russian Journal of Inorganic Chemistry 09/2012; 57(9). · 0.42 Impact Factor
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    ABSTRACT: The normal state resistivity in multiband superconductors has been analyzed in the framework of Eliashberg theory. The results are compared with measurements of the temperature dependence of normal state resistivity of high-purity Ba0.68K0.32Fe2As2 single crystals with the highest reported transition temperature T c = 38.5 K. The experimental data demonstrate strong deviations from the Bloch-Grüneisen behavior, namely the tendency to saturation of the resistivity at high temperatures. The observed behavior of the resistivity is explained within the two band scenario when the first band is strongly coupled and relatively clean, while the second band is weakly coupled and is characterized by much stronger impurity scattering.
    JETP Letters 01/2011; 94:333-337. · 1.52 Impact Factor
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    ABSTRACT: We present theoretical analysis of the normal state resistivity in multiband superconductors in the framework of Eliashberg theory. The results are compared with measurements of the temperature dependence of normal state resistivity of high-purity Ba$_{0.68}$K$_{0.32}$Fe$_{2}$As$_{2}$ single crystals with the highest reported transition temperature $T_c$ = 38.5 K. The experimental data demonstrate strong deviations from the Bloch-Gr\"{u}neisen behavior, namely the tendency to saturation of the resistivity at high temperatures. The observed behavior of the resistivity is explained within the two band scenario when the first band is strongly coupled and relatively clean, while the second band is weakly coupled and is characterized by much stronger impurity scattering.
    11/2010;
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    ABSTRACT: Common and distinctive features of the temperature dependences of microwave surface impedance Z(T)=R(T)+iX(T) and conductivity σ(T) in the ab-plane and along the c-axis of high-T c single crystals (HTSC) are discussed. The main attention is focused on an evolution of these dependences in YBaCuO crystal with the oxygen deficiency. Comparison of YBaCuO with other HTSC crystals reveals a number of peculiarities, namely, the linear dependence Rab(T) ∞ T up to T c/2 in HTSC single crystals with tetragonal lattice and up to T c/3 in YBaCuO where R ab (T) shows also a broad peak at T∼ Tc/2; breakdown of normal skin effect in some HTSC crystals; dramatic effect of pseudogap on the superfluid density in the heavily underdoped YBCO; several orders of magnitude higher residual surface resistance in HTSC when compared to conventional superconductors; etc. Possible explanations are discussed in the context of the specific features of HTSC structure and in the framework of recent models of quasiparticles’ c-transport and pseudogap state in HTSC.
    Journal of Superconductivity and Novel Magnetism 10/2006; 19(7):579-590. · 0.93 Impact Factor
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    A. F. Shevchun, M. R. Trunin
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    ABSTRACT: A method and a setup for making precision measurements of the temperature dependences of components of surface impedance Z(T) = R(T) + iX(T) of small superconductor crystals in the temperature range 0.4–120 K have been developed. The setup combines a high-quality-factor resonance system for measuring the microwave response of a sample at a frequency of 28 GHz and a refrigerating unit with the use of evaporation of 3He vapors. Measurements of Z(T) of optimally doped YBa2Cu3O6.93 single crystals in the superconducting and normal states were performed to illustrate the operation of the setup.
    Instruments and Experimental Techniques 10/2006; 49(5):669-675. · 0.33 Impact Factor
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    ABSTRACT: The temperature dependences of the upper critical field B c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba1 − x KxBiO3 single crystals with transition temperatures 6 ≤ T c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B c2(T) curves of the crystals with T c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T c R(T) and X(T) dependences of the crystals with T c ≈ 32 K and T c ≈ 11 K in the temperature range T ≪ T c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.
    JETP Letters 01/2006; 83(9):405-409. · 1.52 Impact Factor
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    ABSTRACT: This paper develops the theoretical approach and describes the design of a practical test rig for measuring the microwave parameters of unclad and laminated dielectric substrates. The test rig is based on a sapphire whispering-gallery resonator and allows the measurement of the following parameters: dielectric constant (epsiv) of the dielectric substrate in the range from 2 to 10, loss tangent (tandelta) of the dielectric substrate in the range from 10<sup>-4</sup> to 10<sup>-2</sup>, and microwave losses of copper coating of the substrate in the range from 0.03 to 0.3 Omega. Measurements of numerous commonly used microwave printed-circuit-board materials were performed at frequencies between 30-40 GHz and over a temperature range of -50degC to +70degC
    IEEE Transactions on Microwave Theory and Techniques 03/2005; · 2.23 Impact Factor
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    ABSTRACT: The theoretical approach is developed and the test rig is designed for measuring the microwave parameters of dielectric substrates at frequencies between 30 and 40 GHz and over a temperature range from -50 to +70° C . The test rig is based on sapphire resonators and allows the measurements of the following parameters of commonly used PCB materials: (i) dielectric constant (ε) of the dielectric substrate in the range from 2 to 10, (ii) loss tangent ( tan δ) of the substrate in the range from 10<sup>-4</sup> to 10<sup>-2</sup> , and (iii) ranging from 0.03 to 0.3 Ω surface resistance of the laminating metal layer adjoining the dielectric material.
    Review of Scientific Instruments 12/2004; · 1.60 Impact Factor
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    M R Trunin, Yu A Nefyodov, A F Shevchun
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    ABSTRACT: The in-plane and out-of-plane surface impedance and microwave conductivity components of YBa2Cu3O7−x () single crystal are determined in the wide ranges of temperature T and carrier concentration p in CuO2 planes. The following features of the superfluid density are observed at T<Tc/2 and : (i) ns(0,p) depends linearly on p, (ii) the derivative depends on p slightly in the optimally and moderately doped regions (); however, it rapidly increases with p decreasing, and (iii) the latter finding is accompanied by the linear low-temperature dependence changing to . For optimum oxygen content the temperature dependence of the normalized imaginary part of the c-axis conductivity λc2(0)/λc2(T) is found to be strikingly similar to that of λab2(0)/λab2(T) and becomes more convex with p decreasing. λc−2(0,p) values are roughly proportional to the normal state conductivities σc(Tc,p) along the c-axis. All these properties can be treated in the framework of d-density wave order of a pseudogap.
    Superconductor Science and Technology 06/2004; 17(8):1082. · 2.76 Impact Factor
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    M R Trunin, Yu A Nefyodov, A F Shevchun
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    ABSTRACT: The investigation of the penetration depth lambda(ab)(T,p) in YBa2Cu3O7-x crystals allowed one to observe the following features of the superfluid density n(s)(T,p) proportional, variant lambda(-2)(ab)(T,p) as a function of temperature T<T(c)/2 and carrier concentration 0.078< or p < or = 0.16 in CuO2 planes: (i) n(s)(0,p) depends linearly on p; (ii) the derivative absolute value of dn(s)(T,p)/dT (T-->0) depends on p slightly in the optimally and moderately doped regions (0.10<p< or =0.16), however, it rapidly increases with p further lowering; (iii) the latter finding is accompanied by the linear low-temperature dependence [-Delta n(s)(T)] proportional, variant T changing to [-Delta n(s)(T)] proportional, square root of T. All these peculiarities can be treated in the framework of the d-density-wave scenario of electronic processes in underdoped high-T(c) materials.
    Physical Review Letters 02/2004; 92(6):067006. · 7.73 Impact Factor
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    M. R. Trunin, Yu. A. Nefyodov, A. F. Shevchun
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    ABSTRACT: The investigation of the penetration depth \lambda_{ab}(T,p) in YBa_2Cu_3O_{7-x} crystals allowed to observe the following features of the superfluid density n_s(T,p)\propto \lambda_{ab}^{-2}(T,p) as a function of temperature T<Tc/2 and carrier concentration 0.078\le p\le 0.16 in CuO_2 planes: (i) n_s(0,p) depends linearly on p, (ii) the derivative |dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately doped regions (0.10<p\le 0.16); however, it rapidly increases with p further lowering and (iii) the latter finding is accompanied by the linear low-temperature dependence [-\Delta n_s(T)]\propto T changing to [-\Delta n_s(T)]\propto \sqrt{T}. All these peculiarities can be treated in the framework of d-density wave scenario of electronic processes in underdoped high-Tc materials.
    01/2004;
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    ABSTRACT: The theoretical model is fully developed and the test rig is designed for the measurements of microwave parameters of unclad and laminated dielectric substrates. The geometry of the electromagnetic field in the resonator allows dielectric measurements with electric field component orthogonal to the sample surface. The test rig was completely automatized for measurements of the following parameters: (i) dielectric constant of the dielectric substrate in the range from 2 to 10, (ii) loss tangent of the dielectric substrate in the range from 10^-4 to 10^-2, and (iii) microwave ohmic loss at the interface between the metal layer and the dielectric material in the range from 0.01 to 0.2 Ohm.Measurements for a number of commonly-used microwave PCB materials were performed in the frequency range from 30 to 40 GHz and over a temperature range from -50C to +70C.
    01/2004;
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    ABSTRACT: YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub> (YBCO) film quality has reached a stage enabling rf applications at T≤80 K. Higher operating temperatures or better power handling are very much desired. Tl<sub>2</sub>Ba<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub> (Tl-2212) is a perfect candidate with its higher transition temperature T<sub>c</sub>>100 K. We studied the rf field dependence of the impedance Z<sub>s</sub> of epitaxial Tl-2212, which shows already surface resistances at low fields superior to YBCO and promising power handling with a nonlinearity onset above μ<sub>0</sub>H<sup>*</sup>&sime;3 mT. Defective Tl-2212 films show anomalous nonlinear behavior for μ<sub>0</sub>H<sub>rf</sub>≤μ<sub>0</sub>H<sup>*</sup>∼0.3 to 1 mT, extremely pronounced at T<40 K, indicating the presence of very weak-links yielding flux-flow, followed at higher fields μ<sub>0</sub>H<sub>rf</sub>≥0.5 mT by hysteresis losses with a linear field dependence δR<sub>s</sub>(H<sub>rf</sub>)∞H<sub>rf</sub>. Better quality Tl-2212 films show δR<sub>s</sub>(H<sub>rf</sub>)∞H<sub>rf</sub><sup>2</sup> up to fields of about 5 to 10 mT. The δR<sub>s</sub>(H<sub>rf</sub>)∞H<sub>rf</sub><sup>n</sup> (n=1,2) dependencies together with the ratios r(T)=δX<sub>s</sub>/δR<sub>s</sub>∼1 to 10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films.
    IEEE Transactions on Applied Superconductivity 07/2003; · 1.20 Impact Factor
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    ABSTRACT: YBa2Cu3O7 epitaxial film quality has reached a stage where rf applications at T⩽80 K are carried through. Higher-operating temperatures or better power handling are very much desired. Tl2Ba2CaCu2O8 (TBCCO) is an excellent candidate with its higher-transition temperature Tc≈100–105 K. We study the rf field dependence of surface impedance Zs of epitaxial TBCCO films using dielectric resonators with TE011-modes at 8.5 and 15.3 GHz at 4.2 K<T<100 K and at 0.01 mT⩽Hrf⩽10 mT in the frequency and time domain. High-quality films show surface resistances at low fields Rs(77 K) superior to YBCO and promising power handling with a nonlinearity onset above turn-up field μ0H∗⩾3 mT. Defective TBCCO films show anomalous nonlinear behavior δRs(Hrf)∝√Hrf for μ0Hrf⩽μ0H∗∼0.3–1 mT, extremely pronounced at T<40 K, indicating the presence of very weak links yielding flux flow, followed at higher fields μ0Hrf≳0.5 mT by hysteresis losses with a linear field dependence δRs(Hrf)∝Hrf. Better quality TBCCO films show δRs(Hrf)∝Hrf2 up to fields of about 5–10 mT. The δRs(Hrf)∝Hrfn (n=1,2) dependencies together with ratios r(T)=δXs/δRs∼1–10 hint to hysteresis losses of Josephson fluxons enhanced by slits or holes in the films. © 2003 American Institute of Physics.
    Journal of Applied Physics 03/2003; 93(7):4049-4054. · 2.21 Impact Factor
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    ABSTRACT: The real $R_s(T)$ and imaginary $X_s(T)$ parts of the surface impedance $Z_s(T)=R_s(T)+iX_s(T)$ in polycrystalline MgB$_2$ samples of different density with the critical temperature $T_c\approx 38$ K are measured at the frequency of 9.4 GHz and in the temperature range $5\le T<200$ K. The normal skin-effect condition $R_s(T)=X_s(T)$ at $T\ge T_c$ holds only for the samples of the highest density with roughness sizes not more than 0.1 $\mu$m. For such samples extrapolation $T\to 0$ of the linear at $T<T_c/2$ temperature dependences $\lambda_L(T)=X_s(T)/\omega\mu_0$ and $R_s(T)$ results in values of the London penetration depth $\lambda_L(0)\approx 600$ \AA and residual surface resistance $R_{res}\approx 0.8$ m$\Omega$. In the entire temperature range the dependences $R_s(T)$ and $X_s(T)$ are well described by the modified two-fluid model. Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publication
    EPL (Europhysics Letters) 07/2001; · 2.26 Impact Factor