arXiv subjects

R. Prozorov

Publications and source records attributed to R. Prozorov.

189 records · Page 11Linked to original sources

arXiv1999

Magnetic penetration depth in electron-doped cuprates - evidence for gap nodes

The in-plane penetration depth λ(T) is measured in electron-doped single crystals Nd1.85Ce0.15CuO4-x (NCCO) and Pr1.85Ce0.15CuO4-x (PCCO) using a 11 MHz LC resonator. In NCCO, λ(T) exhibits a minimum at 3.8 K and a pronounced upturn down to 0.4 K due to the paramagnetic contribution of Nd3+ ions. The London penetration depth contribution is linear in T. The paramagnetic contribution is absent in PCCO, where λ(T)~T^2 at low temperatures. Our results indicate the presence of nodes in the superconducting gap, i.e., non s-wave symmetry of the order parameter in electron-doped cuprates.

cond-mat.supr-con↗

arXiv1999

Low Temperature Penetration depth of κ-(ET)_2Cu[N(CN)_2]Br and κ-(ET)_2Cu(NCS)_2

We present high precision measurements of the penetration depth λof single crystals of κ-(ET)_2Cu[N(CN)2]Br and κ-(ET)_2Cu(NCS)_2 at temperature down to 0.4 K. We find that, at low temperatures, the in-plane penetration depth varies as a fractional power law, λ\sim T^1.5. W hilst this may be taken as evidence for novel bose excitation processes, we show that the data are also consistent with a quasi-linear variation of the superfluid density, as is expected for a d-wave superconductor with impurities. Our data for the interplane penetration depth show similar features and give a direct measurement of the absolute value, λ(0)=100\pm 20 μm.

cond-mat.supr-con↗

arXiv1999

Self-organization of vortices in type-II superconductors during magnetic relaxation

We revise the applicability of the theory of self-organized criticality (SOC) to the process of magnetic relaxation in type-II superconductors. The driving parameter of self-organization of vortices is the energy barrier for flux creep and not the current density. The power spectrum of the magnetic noise due to vortex avalanches is calculated and is predicted to vary with time during relaxation.

cond-mat.supr-con↗

arXiv1999

Magnetic irreversibility and relaxation in assembly of ferromagnetic nanoparticles

Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for three $Fe_{2}O_{3}$ samples composed of regular amorphous, acicular amorphous and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. We develop a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization and therefore on time. The time dependent barrier yields a natural explanation to the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and crystalline magnetic anisotropies define a new energy scale that controls the magnetic irreversibility. Introducing this energy scale yields a self-consistent explanation of the experimental data.

cond-mat↗

arXiv1998

Current density inhomogeneity throughout the thickness of superconducting films and its effect on their irreversible magnetic properties

We calculate the distribution of the current density $j$ in superconducting films along the direction of an external field applied perpendicular to the film plane. Our analysis reveals that in the presence of bulk pinning $j$ is inhomogeneous on a length scale of order the inter vortex distance. This inhomogeneity is significantly enhanced in the presence of surface pinning. We introduce new critical state model, which takes into account the current density variations throughout the film thickness, and show how these variations give rise to the experimentally observed thickness dependence of $% j$ and magnetic relaxation rate.

cond-mat.supr-con↗

arXiv1998

Collective flux creep: beyond the logarithmic solution

Numerical studies of the flux creep in superconductors show that the distribution of the magnetic field at any stage of the creep process can be well described by the condition of spatial constancy of the activation energy $U$ independently on the particular dependence of $U$ on the field B and current $j$. This results from a self-organization of the creep process in the undercritical state $j<j_{c}$ related to a strong non-linearity of the flux motion. Using the spatial constancy of $U$, one can find the field profiles $B(x)$, formulate a semi-analytical approach to the creep problem and generalize the logarithmic solution for flux creep, obtained for $U=U(j)$, to the case of essential dependence of $U$ on $B$. This approach is useful for the analysis of dynamic formation of an anomalous magnetization curve (''fishtail''). We analyze the quality of the logarithmic and generalized logarithmic approximations and show that the latter predicts a maximum in the creep rate at short times, which has been observed experimentally. The vortex annihilation lines (or the sample edge for the case of remanent state relaxation), where B=0, cause instabilities (flux-flow regions) and modify or even destroy the self-organization of flux creep in the whole sample.

cond-mat.supr-con↗

arXiv1996

On the origin of the irreversibility line in thin YBaCuO7 films with and without columnar defects

We report on measurements of the angular dependence of the irreversibility temperature $T_{irr}(θ) $ in $YBa_2Cu_3O_{7-δ}$ thin films, defined by the onset of a third harmonic signal and measured by a miniature Hall probe. From the functional form of $T_{irr}(θ)$ we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle.

supr-con↗

arXiv1996

Plastic vortex-creep in YBa$_2$Cu$_3$O$_{7-x}$ crystals

Local magnetic relaxation measurements in YBa$_2$Cu$_3$O$_{7-x}$ crystals show evidence for plastic vortex-creep associated with the motion of dislocations in the vortex lattice. This creep mechanism governs the vortex dynamics in a wide range of temperatures and fields below the melting line and above the field corresponding to the peak in the ''fishtail'' magnetization. In this range the activation energy $U_{pl}$, which decreases with field, drops below the elastic (collective) creep activation energy, $U_{el}$, which increases with field. A crossover in flux dynamics from elastic to plastic creep is shown to be the origin of the fishtail in YBa$_2$Cu$_3$O$_{7-x}$.

supr-con↗

arXiv1996

Irreversible magnetization in thin YBCO films rotated in external magnetic field

The magnetization M of a thin YBaCuO film is measured as a function of the angle $θ$ between the applied field H and the c-axis. For fields above the first critical field, but below the Bean's field for first penetration H*, M is symmetric with respect to $θ=π$ and the magnetization curves for forward and backward rotation coincide. For H>H* the curves are asymmetric and they do not coincide. These phenomena have a simple explanation in the framework of the Bean critical state model.

supr-con↗