Density Of Columnar Defects Research Articles

Transition from elastic to plastic vortex phase and its evolution with quenched disorder inYBa2Cu3Oysingle crystals

We report on a detailed investigation of history effects in the magnetic hysteresis of pure ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ single crystals with various types and densities of pinning sites. A partial magnetization loop technique is employed that enables us to detect the point where topological disorder first invades the vortex system. Accordingly, studies of detwinned single crystals with very low densities of point defects reveal a transition in the mixed state of the superconductor that separates a dislocation-free Bragg glass from a highly disordered vortex phase. The transition line is identified in the field-temperature phase diagram and found to lie in the vicinity of the onset of the second magnetization peak. The effect of decreasing oxygen stoichiometry in the studied region $(6.550l~yl~6.999)$ is the shift of this boundary to lower fields, and its final disappearance for high values of the oxygen deficiency in agreement with theoretical predictions. Above the transition metastable topological defects proliferate in the vortex lattice resulting in prominent history effects in the critical current. The last diminish at high enough fields for low oxygen concentrations near optimal doping, an effect not seen for samples close to the stoichiometric state. We also study the influence of extended defects and find that a low density of twin boundaries does not affect the Bragg glass significantly. However, a high concentration of twins as well as low densities of columnar defects are shown to suppress the transition and eliminate the memory effects.

FROZEN FLUX SUPERCONDUCTORS

We propose a new class of composite superconductors with embedded magnetic nanoparticles/nanorods. These nanomagnets can create a thermodynamically stable, frozen, dense flux line network. We discuss the methods of fabrication of these novel materials. We show that due to the frozen flux created by magnetic nanorods, the critical current in this composite superconductor can be higher than in the superconductor with a comparable density of columnar defects.

Low-temperature magnetic relaxation inHgBa2Ca2Cu3O8+δsingle crystals with columnar defects

Micrometer-sized Hall probes are used to measure the low-temperature magnetic properties of ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals with and without columnar defects. Introduction of columnar defects by heavy-ion irradiation suppresses the quasiexponential temperature dependence of the critical current, but only when the density of flux lines is less than the density of defects. The current-dependent creep activation energy $U(J)$ is found to be best described by the power law: ${U(J)=U}_{0}[{(J}_{c}{/J)}^{\ensuremath{\mu}}\ensuremath{-}1].$ At high current densities J, and low fields, there is a crossover to a quantum vortex creep process with a temperature-independent magnetic relaxation rate. On the low-J side of this crossover in the unirradiated sample, $\ensuremath{\mu}g~1,$ indicating that the thermally activated vortex creep process is consistent with collective creep. When the flux-line density exceeds the columnar defect density in the irradiated sample, the vortex creep process is characterized by $\ensuremath{\mu}\ensuremath{\sim}1.$ For flux-line densities less than the defect density, an anomalously large exponent is found $(\ensuremath{\mu}g~2),$ which is inconsistent with vortex transport occurring either by the nucleation and expansion of half-loop excitations or by variable-range hopping of vortex lines as described by the Bose-glass theory.

Phase coherence along the c-axis in Bi 2Sr 2CaCu 2O 8+ y with columnar defects probed by Josephson plasma resonance

We studied the c-axis phase coherence in Bi 2Sr 2CaCu 2O 8+ y with columnar defects using the Josephson plasma resonance as a probe. At temperatures close to T c, the Josephson plasma resonance field ( H p) shows a characteristic temperature dependence of decoupled pancake vortices similar to the pristine Bi 2Sr 2CaCu 2O 8+ y . The drastic increase in H p occurs at a frequency independent characteristic field even in the vortex liquid phase. We interpret this change as a manifestation of recoupling of decoupled vortices by the columnar defects. The increase in H p becomes larger with increasing the density of columnar defects. The temperature dependence of H p at lower temperatures but above the irreversibility line can be understood by considering two kinds of vortices, some trapped in the columnar defects and others located between them giving a decoupled liquid-like response.

Vortex matter phase transitions in Bi 2Sr 2CaCu 2O 8

Vortex-lattice phase transitions in Bi 2Sr 2CaCu 2O 8 crystals are studied using local magnetization measurements. The vortex matter is found to exhibit at least three distinct phases. A rather ordered quasilattice phase is present at low fields. At elevated temperatures the quasilattice melts (or sublimates) through a first-order phase transition, whereas at lower temperatures a transition to a highly disordered vortex solid occurs at the second magnetization peak. A very low density of columnar defects results in significant pinning of the quasilattice, demonstrating the finite shear modulus of this phase. Local ac measurements reveal that in the presence of columns the pinned quasilattice still melts through a first-order phase transition with an observable step in equilibrium magnetization at elevated temperatures.

Effects of correlated disorder on vortex-lattice melting in BSCCO

The vortex-lattice phase transitions in Bi2Sr2CaCu2O8 crystals with very low doses of artificially induced correlated disorder are studied using local magnetization measurements. A low density of columnar defects shifts the first-order transition to higher fields, apparently transforming it into a continuous transition.

Upper Limit of the Bose-Glass Transition in YB2C3O7at High Density of Columnar Defects

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Importance of vortex interactions in Tl 2Ba 2CaCu 2O 8 epitaxial films with columnar defects

The effects of columnar defects on electrical transport measurements in Tl 2Ba 2CaCu 2O 8 epitaxial films were found to persist up to magnetic fields of at least forty times the vortex matching field, that is, the field at which the vortex density corresponds to the density of columnar defects. This demonstrates the importance of interactions between vortices and is in contrast to experiments that suggest that the matching field has sharp significance.

Phase diagram of Bi2Sr2CaCu2O8 in the mixed state: effects of anisotropy and disorder

The vortex-lattice phase transitions in Bi2Sr2CaCu2O8 crystals with various oxygen stoichiometry and artificially induced disorder are studied using local magnetization measurements. The first-order phase transition line at elevated temperatures shifts upward for more isotropic over-doped samples. At lower temperatures another sharp transition is observed at the onset of second magnetization peak. The two lines merge at a multicritical point at intermediate temperatures forming apparently a continuous phase transition line that is anisotropy dependent. Weak point disorder preserves the first-order transition and shifts it to lower fields. This shift is accompanied by a corresponding downward shift of the second magnetization peak. A low density of columnar defects shifts the first-order transition to higher fields, apparently transforming it into a continuous transition. The first-order transition in clean crystals is concluded to involve melting of the vortex-lattice. Schematic phase diagram based on this study is presented.

Vortex lock-in deep in the Bose glass.

We use a Bi gaussmeter of micron dimensions to explore the magnetic field dependence of the magnetization relaxation rate and the critical current down to millikelvin temperatures in untwinned single crystals of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ with columnar defects. The response separates into three regimes as a function of the ratio of vortex density to columnar defect density $\frac{B}{{B}_{\ensuremath{\varphi}}}$: enhancements in both critical current and quantum creep in the dilute limit, vanishing magnetization relaxation at the matching density (the proposed "Mott insulator" phase line), and the emergence of temperature-dependent vortex motion for $B\ensuremath{\gg}{B}_{\ensuremath{\varphi}}$.

Effects of transport current and columnar defects on the rf penetration depth of NbSe2.

We report on simultaneous measurements of the dc critical current and rf penetration depth of superconducting 2H-${\mathrm{NbSe}}_{2}$. We observed that the large peak in the dc critical current (the ``peak effect'') occurs at the same applied magnetic field H(T) as a maximum in the rf penetration depth, indicating that the anomaly in flux-line dynamics is associated with a change in flux-line pinning strength. The anomalies in the critical current and rf penetration depth disappear in a sample with a high density of columnar defects produced by Ag-ion bombardment; both superconducting and charge-density-wave phase transitions are substantially broadened by the defects.

Columnar defects induced by high energy heavy ions in HTSC. Their effect on irreversibility line and pinning properties

The defects induced by high energy heavy ions have been studied by high resolution electron microscopy. Columnar amorphous defects of 7 nm in diameter have thus been characterized in the case of 6 GeV-Pb ion irradiation of Y-123, Bi-2212 and Tl-2212 superconductors. Because of their diameter close to the coherence length ξ and their shape similar to the vortex core they have been demonstrated as the best pinning centers that can be introduced in HTSC. Drastic changes of the magnetic hysteresis and critical current density are thus reported. Volume pinning force modifications and strong shifts of the irreversibility line have been studied as a function of the temperature and the columnar defect density i.e. the ion dose. The magnetic field dependence of the critical current density has been discussed on the basis of a competition between pinning and vortex repulsion.

Irreversibility lines of Bi-2212 single crystals irradiated by 6 GeV-Pb ions

6 GeV-Pb ion irradiation effects on the irreversibility line (IL) of Bi-2212 single crystals have been studied as a function of the induced columnar defect density. The crystals were irradiated parallel to the c-axis and the magnetic study of the IL was carried out with the field applied along this same direction. In a broad range of intermediate temperatures, we observe that the IL position is mainly determined by the value of the track density. The irreversibility fields in this domain correspond to a vortex density close to the track density, and the temperature dependence is weak. At lower temperatures, the ILs tend to join the position prior to irradiation and, at higher temperatures, a net change in the B irr( T) dependence is observed. This crossover at high temperatures could be interpreted as the result of a 3D-2D transition in the vortex lattice.