85 research outputs found
Microwave response of bulk MgB2 samples of different granularity
The microwave response of three high-density bulk MgB2 samples has been
investigated in the linear and nonlinear regimes. The three samples,
characterized by different mean size of grains, have been obtained by reactive
infiltration of liquid Mg in powdered B preforms. The linear response has been
studied by measuring the microwave surface impedance; the nonlinear response by
detecting the power radiated by the sample at the second-harmonic frequency of
the driving field. Our results suggest that bulk MgB2 prepared by the liquid Mg
infiltration technique is particularly promising for manufacturing resonant
cavities operating at microwave frequencies.Comment: 4 pages, 2 embedded figures; Proceedings of 7th EUCAS Conference
(11-15 September 2005, Vienna - Austria
Microwave surface impedance anisotropy of YBaCuO single crystals with different oxygen content
The linear microwave response of ultra high-quality YBaCuO single
crystals grown in BaZrO crucibles is measured at 9.4 GHz in rf magnetic
fields parallel and perpendicular to the -plane in the temperature range
5 K. Having found the analytic solution for the magnetic field
distribution on the sample surface we determine both the surface impedance
in the -plane and along c-axis of
the crystals. For the first time the evolution of the and
dependences on the same sample and in a wide range of oxygen content is
obtained. For (optimum oxygen content) the temperature dependence of
the imaginary part of the c-axis conductivity
is found to be strikingly similar to that of
and becomes more convex with lowering.Comment: 2 pages, 2 figures. To be published in Physica C as LT23 Proceeding
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.Comment: Submitted to Europhysics Letter
Superfluid density in the underdoped YBa_2Cu_3O_{7-x}: Evidence for d-density wave order of 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.Comment: 4 pages, 5 figures. To be published in Phys.Rev.Let
Pseudogap in the microwave response of YBa_2Cu_3O_{7-x}
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 SUS
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