32 research outputs found
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
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
c-axis penetration depth in BiSrCaCuO single crystals measured by ac-susceptibility and cavity perturbation technique
The -axis penetration depth in
BiSrCaCuO (BSCCO) single crystals as a function of
temperature has been determined using two techniques, namely, measurements of
the ac-susceptibility at a frequency of 100 kHz and the surface impedance at
9.4 GHz. Both techniques yield an almost linear function
in the temperature range T<0.5 T_c.
Electrodynamic analysis of the impedance anisotropy has allowed us to estimate
m in BSCCO crystals overdoped with oxygen
( K) and m at the optimal doping
level ( K).Comment: 5 pages, 4 figure
Observation of microwave induced resistance and photovoltage oscillations in MgZnO/ZnO heterostructures
Microwave induced resistance and photovoltage oscillations were investigated in Mg_xZn_(1−x)O/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35–120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m^∗ = (0.35 ± 0.01)m₀, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps
Goldstone Mode Relaxation in a Quantum Hall Ferromagnet due to Hyperfine Interaction with Nuclei
Spin relaxation in quantum Hall ferromagnet regimes is studied. As the
initial non-equilibrium state, a coherent deviation of the spin system from the
direction is considered and the breakdown of this Goldstone-mode
state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring
non-exponentially with time is studied in terms of annihilation processes in
the "Goldstone condensate" formed by "zero spin excitons". The relaxation rate
is calculated analytically even if the initial deviation is not small. This
relaxation channel competes with the relaxation mechanisms due to spin-orbit
coupling, and at strong magnetic fields it becomes dominating.Comment: 8 page
Observation of microwave induced resistance and photovoltage oscillations in MgZnO/ZnO heterostructures
Microwave induced resistance and photovoltage oscillations were investigated in Mg_xZn_(1−x)O/ZnO heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35–120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass m^∗ = (0.35 ± 0.01)m₀, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps
Surface impedance anisotropy of YBaCuO single crystals: electrodynamic basis of the measurements
An electrodynamic technique is developed for determining the components of
surface impedance and complex conductivity tensors of HTSC single crystals on
the basis of measured quantities of a quality factor and a resonator frequency
shift. A simple formula is obtained for a geometrical factor of a crystal in
the form of a plate with dimensions in a microwave magnetic field
. To obtain the c-axis complex conductivity from
measurements at we propose a procedure which
takes account of sample size effects. With the aid of the technique involved
temperature dependences of all impedance and conductivity tensors components of
YBaCuO single crystal, grown in BaZrO crucible, are
determined at a frequency of GHz in its normal and superconducting
states. All of them proved to be linear at , and their extrapolation
to zero temperature gives the values of residual surface resistance
and m and
magnetic field penetration depth nm and
m.Comment: 9 pages, 7 figures. Submitted to Phys.Rev.B 05Jun2002; accepted for
publication 21Febr200
Characteristic features of the temperature dependence of the surface impedance in polycrystalline MgB samples
The real and imaginary parts of the surface impedance
in polycrystalline MgB samples of different density
with the critical temperature K are measured at the frequency
of 9.4 GHz and in the temperature range K. The normal skin-effect
condition at holds only for the samples of the
highest density with roughness sizes not more than 0.1 m. For such samples
extrapolation of the linear at temperature dependences
and results in values of the London
penetration depth \AA and residual surface resistance
m. In the entire temperature range the dependences
and are well described by the modified two-fluid model.Comment: 7 pages, 3 figures. Europhysics Letters, accepted for publicatio