M. R. Trunin

Microwave response of V3Si single crystals: Evidence for two-gap superconductivity

The investigation of the temperature dependences of microwave surface impedance and complex conductivity of V3Si single crystals with different stoichiometry allowed to observe a number of peculiarities which are in remarkable contradiction with single-gap Bardeen-Cooper-Schrieffer theory. At the same time, they can be well described by two-band model of superconductivity, thus strongly evidencing the existence of two distinct energy gaps with zero-temperature values Delta1~1.8Tc and Delta2~0.95Tc in V3Si.

Superfluid density in the underdoped YBa2Cu3O7-x: evidence for d-density-wave order of the pseudogap.

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.

Dielectric constant, loss tangent, and surface resistance of PCB materials at K-band frequencies

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 10-4 to 10-2, 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

Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB2 samples

The real Rs(T) and imaginary Xs(T) parts of the surface impedance Zs(T) = Rs(T) + iXs(T) in polycrystalline MgB2 samples of different density with the critical temperature Tc ≈ 38 K are measured at the frequency of 9.4 GHz and in the temperature range 5 ≤ T < 200 K. The normal skin-effect condition Rs(T) = Xs(T) at T ≥ Tc holds only for the samples of the highest density with roughness sizes not more than 0.1 μm. For such samples extrapolation T → 0 of the linear at T < Tc/2 temperature dependences λL(T) = Xs(T)/ωμ0 and Rs(T) results in values of the London penetration depth λL(0) ≈ 600 A and residual surface resistance Rres ≈ 0.8 mΩ. In the entire temperature range the dependences Rs(T) and Xs(T) are well described by the modified two-fluid model.

Characteristic features of the temperature dependence of the surface impedance of YBa2Cu3O6.95 single crystals

The real and imaginary parts of the surface impedance Zs=Rs+iXs of YBa2Cu3O6.95 single crystals are measured at a frequency ω/π=9.4 GHz. The quantities Rs(T) and Xs(T) are linear functions of temperature for T<0.3Tc (Tc=93.5 K). A maximum of Rs(T) and a plateau of Xs(T) are observed in the interval 35<T<65 K. Our experimental data, just as all recent measurements of Rs(T) in YBa2Cu3O6.95 single crystals in the temperature range 0<T<1.3Tc, are described well in a two-fluid model which assumes electron-phonon scattering of quasiparticles.

Phenomenological description of the microwave surface impedance and complex conductivity of high-T c single crystals

Measurements of the microwave surface impedance Zs(T) = Rs(T) + iXs(T) and the complex conductivity σs(T) in the ab-plane of high-quality high-Tc YBCO, BSCCO, TBCCO, and TBCO single crystals are analyzed. Experimental data of Zs(T) and σs(T) are compared with calculations based on a modified two-fluid model that includes a temperature-dependent quasiparticle scattering and a unique temperature variation of the density of superconducting carriers. We describe the agreement and disagreement of our analysis with the salient features of the experimental data. We review the existing microscopic models based on unconventional symmetry types of the order parameter and on novel quasiparticle relaxation mechanisms.

Measuring microwave properties of laminated dielectric substrates

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 (e) of the dielectric substrate in the range from 2 to 10, (ii) loss tangent (tan δ) of the substrate in the range from 10−4 to 10−2, and (iii) ranging from 0.03 to 0.3 Ωsurface resistance of the laminating metal layer adjoining the dielectric material.

Microwave response of high-Tc superconducting crystals: Results, problems, and prospects

AbstractThe results of studying temperature behavior of the microwave surface impedance Zs(T) and conductivity tensor

c-axis penetration depth in Bi2Sr2CaCu2O8+δ single crystals measured by ac-susceptibility and cavity perturbation technique

\hat \sigma (T)