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 Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} single crystals measured by ac-susceptibility and cavity perturbation technique

The $c$-axis penetration depth $\Delta\lambda_c$ in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (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 $\Delta\lambda_c(T)\propto T$ in the temperature range T<0.5 T_c. Electrodynamic analysis of the impedance anisotropy has allowed us to estimate $\lambda_c(0)\approx 50 \mu$m in BSCCO crystals overdoped with oxygen ($T_c\approx 84$ K) and $\lambda_c(0)\approx 150 \mu$m at the optimal doping level ($T_c\approx 90$ 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 ${\vec B}$ 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 YBa2_2Cu3_3O6.95_{6.95} 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 $b\gg a>c$ in a microwave magnetic field ${\bf H_{\omega}}\perp ab$. To obtain the c-axis complex conductivity from measurements at ${\bf H_{\omega}}\parallel ab$ 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 YBa$_2$Cu$_3$O$_{6.95}$ single crystal, grown in BaZrO$_3$ crucible, are determined at a frequency of $f=9.4$ GHz in its normal and superconducting states. All of them proved to be linear at $T<T_c/2$, and their extrapolation to zero temperature gives the values of residual surface resistance $R_{ab}(0)\approx 40$ $\mu\Omega$ and $R_c(0)\approx 0.8$ m$\Omega$ and magnetic field penetration depth $\lambda_{ab}(0)\approx 150$ nm and $\lambda_c(0)\approx 1.55$ $\mu$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 MgB2_2 samples

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 publicatio