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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 04/2012; 49(5):669-675. · 0.36 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 04/2012; 94(4):333-337. · 1.35 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 < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The 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 04/2012; 83(9):405-409. · 1.35 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: We discuss on the electromagnetic response of a polycrystalline sample of LaO0.94F0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and high-frequency responses have been investigated by measuring the AC susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At low as well as high DC magnetic fields, the susceptibility strongly depends on the amplitude of the AC driving field, highlighting enhanced nonlinear effects. The field dependence of the AC susceptibility exhibits a magnetic hysteresis that can be justified considering the intragrain-field-penetration effects on the intergrain critical current density. The microwave surface resistance exhibits a clockwise magnetic hysteresis, which cannot be justified in the framework of the critical-state models of the Abrikosov-fluxon lattice; it may have the same origin as that detected in the susceptibility.
Journal of Physics Conference Series 07/2010; 234(1):012001.
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ABSTRACT: We discuss on the electromagnetic response of a polycrystalline sample of
LaO_0.94F_0.06FeAs exposed to DC magnetic fields up to 10 kOe. The low- and
high-frequency responses have been investigated by measuring the AC
susceptibility at 100 kHz and the microwave surface resistance at 9.6 GHz. At
low as well as high DC magnetic fields, the susceptibility strongly depends on
the amplitude of the AC driving field, highlighting enhanced nonlinear effects.
The field dependence of the AC susceptibility exhibits a magnetic hysteresis
that can be justified considering the intragrain-field-penetration effects on
the intergrain critical current density. The microwave surface resistance
exhibits a clockwise magnetic hysteresis, which cannot be justified in the
framework of the critical-state models of the Abrikosov-fluxon lattice; it may
have the same origin as that detected in the susceptibility.
10/2009;
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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. · 0.65 Impact Factor
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ABSTRACT: A study of temperature dependences of the upper critical field B_{c2}(T) and surface impedance Z(T)=R(T)+iX(T) in Ba_{1-x}K_xBiO_3 single crystals that have transition temperatures in the range 6 < T_c < 32 K (roughly 0.6>x>0.4) reveals a transition from BCS to unusual type of superconductivity. B_{c2}(T) curves corresponding to the crystals that have T_c>20 K have positive curvature (like in some HTSC), and those of the crystals with T_c<15 K fall on the usual Werthamer-Helfand-Hohenberg curve. R(T) and X(T) dependences of the crystals with T_c~30 K and T_c~11 K are respectively linear (like in HTSC) and exponential (BCS) in the temperature range T << T_c. The experimental results are discussed in connection with the extended saddle point model by Abrikosov.
07/2006;
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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; · 1.85 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; · 1.37 Impact Factor
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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. · 2.66 Impact Factor
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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. · 7.37 Impact Factor
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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 in-plane and out-of-plane surface impedance and microwave conductivity components of one and the same YBa_2Cu_3O_{7-x} (0.07\le x\le 0.47) single crystal are determined in the wide ranges of temperature T and carrier concentration p in CuO_2 planes. The following features of the superfluid density n_s(T,p)\propto\lambda_{ab}^{-2}(T,p) are observed at T<Tc/2 and 0.078\le p\le 0.16: (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}). For optimum oxygen content the temperature dependence of the normalized imaginary part of the c-axis conductivity \lambda_c^2(0)/\lambda_c^2(T) is found to be strikingly similar to that of \lambda_{ab}^2(0)/\lambda_{ab}^2(T) and becomes more convex with p lowering. \lambda_c^{-2}(0,p) values are roughly proportional to the normal state conductivities \sigma_c(T_c,p) along the c-axis. All these properties can be treated in the framework of d-density wave order of pseudogap. Comment: 7 pages, 9 figures, presented at EUCAS 2003 (September 14-18), submitted to SUST
12/2003;
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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
07/2001;
