Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition $T_{m}$. This latter is identified by the abrupt loss of pinning and by an unexpected step of $\rho_{ff}(T)$ at $T_{m}$.Comment: accepted for publication in EPJ

Saturation of electrical resistivity

Resistivity saturation is observed in many metallic systems with a large resistivity, i.e., when the resistivity has reached a critical value, its further increase with temperature is substantially reduced. This typically happens when the apparent mean free path is comparable to the interatomic separations - the Ioffe-Regel condition. Recently, several exceptions to this rule have been found. Here, we review experimental results and early theories of resistivity saturation. We then describe more recent theoretical work, addressing cases both where the Ioffe-Regel condition is satisfied and where it is violated. In particular we show how the (semiclassical) Ioffe-Regel condition can be derived quantum-mechanically under certain assumptions about the system and why these assumptions are violated for high-Tc cuprates and alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at http://www.mpi-stuttgart.mpg.de/andersen/saturation

Absence of dynamical crossover in the vortex creep near by the second peak effect in superconducting Hg-1201 single crystals

PACS. 74.60.Ge Flux pinning and creep, and flux-line lattice dynamics - 74.72.Gr Hg-based cuprates,

Electrodynamics of the vortex lattice in untwinned YBaCuO by complex impedance measurements

We report complex impedance measurements in an untwinned YBaCuO crystal. Our broad frequency range covers both the quasi static response and the resistive response of the vortex lattice. It allow us to characterize the irreversibility line without the need of any frequency dependent pinning parameters. We confirm the validity of the two modes model of vortex dynamic, and extract both the surface critical current and the flux flow resistivity around the first order transition $T_{m}$. This latter is identified by the abrupt loss of pinning and by an unexpected step of $\rho_{ff}(T)$ at $T_{m}$

Anomalous microwave conductivity due to collective transport in the pseudogap state of cuprate superconductors The microwave surface impedance Z

O 6.5 is found to be anomalous in that R s (TϾT c ) X s (TϾT c ) in the pseudogap state. This implies plasmonlike response and negative permittivities Ј()Ͻ0 at microwave frequencies indicating non-Fermi-liquid transport in the ab plane. The anomalous microwave response is shown to arise from a collective mode characterized by a plasma frequency pCM ϳ0.1 eV and extremely low damping ⌫ CM ϳ10 Ϫ5 -10 Ϫ4 eV, distinctly different from those observed at optical frequencies. The ''normal'' state above T c of the high-temperature cuprate superconductors is well known to be extremely abnormal. A wide variety of experimental techniques ͑photoemis-sion, optical conductivity, NMR, tunneling, neutron scattering, infrared, Raman, etc.͒ ͑Ref. 1͒ have been applied to its study and suggest that there is a common phenomenology for all high-temperature superconductors: the existence of a partial gap or a pseudogap meaning the suppression of the low-energy density of states. An important issue is the nature of the pseudogap, several alternative theoretical models of this having been proposed, such as superconducting fluctuations 2 or islands, 3 competing order parameter, 4 and stripes. Single crystals of Hg:1201 (T c ϭ94.4 K), Hg:1223 (T c ϭ122 K), Tl:2201 (T c ϭ91K), and underdoped YBa 2 Cu 3 O 6.5 (T c ϭ60 K) were prepared by appropriate methods for each material. The high quality of the crystals discussed here has been confirmed by a variety of other techniques. 8 Since Z s ϭR s ϩiX s ϭ ͱ i 0 / , from R s and X s it is possible to obtain 1 and 2 , the real and imaginary parts of the conductivity, using ϭ 1 Ϫi 2 ϭi 0 /(R s ϩiX s ) 2 . In all microwave measurements, R s (T) can be measured absolutely, while relative changes ⌬X s (T)ϵX s (T)ϪX s (0) are typically measured. We obtain X s (0)ϭ 0 ab (0) from estimates of the low-T penetration depth ab (0):130 nm (Hg:1223), 117 nm (Hg:1201), and 260 nm (YBCO6.5). It should be emphasized that because X s (Ṫ ϾT c )ӷX s (0), the results discussed in this paper are not sensitive to X s (0) or (0). The temperature dependences of X s and R s for Hg:1223 when the microwave magnetic field H ʈc axis and of ⌬X s and R s when H Ќc are shown i