Vortex Phase Transition Research Articles

Dynamics of transient disordered vortex states inBi2Sr2CaCu2O8+δ

The dynamics of transient disordered vortex states in BSCCO was magneto-optically traced in three experiments: (i) during continuous injection of transient vortex states while ramping up the external magnetic field, (ii) during annealing of injected transient states while keeping the external field constant, and (iii) during annealing of transient 'supercooled' disordered states while ramping down the external field. The results reveal front-like propagation (experiment i) or retreat (experiments ii and iii) of the transient vortex states, at a rate governed by the rate of change of the external field, the annealing time tau of the transient states and the creep rate. The experimental results are theoretically analyzed in terms of competition between generation and annealing of transient disordered vortex states. Extraction of the annealing time tau from the above three experiments, yields the same results for tau as a function of the induction, B, and temperature T. Knowledge of tau(B,T) allows for correct determination of the thermodynamic order-disorder vortex phase transition line.

Annealing of transient vortex states near the order-disorder phase transition inBi2Sr2CaCu2O8+δ

High-temporal resolution magneto-optical measurements in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ demonstrate that transient vortex states govern its magnetic behavior near the order-disorder vortex phase transition. These data enable direct measurement of the annealing time $\ensuremath{\tau}$ of the transient states as a function of the induction, temperature, and electric field. The behavior of $\ensuremath{\tau}$ reveals fundamental limitations of common experimental procedures in characterizing the disorder-driven vortex phase transition.

Time evolution of the second magnetization peak in Bi2Sr2CaCu2O8+δ

Local magnetization curves at different times, extracted from high-temporal resolution magneto-optical measurements in Bi2Sr2CaCu2O8+δ, demonstrate the absence of the second magnetization peak at short times, its appearance at longer times, and the movement of its onset toward higher-induction fields approaching the thermodynamic vortex order–disorder transition field. We relate this phenomena to metastable disordered states induced by edge contamination for inductions in the vicinity of the order–disorder vortex phase transition. We show that the time evolution of the second magnetization peak is governed by the relaxation times of the metastable disordered vortex states.

Nonequilibrium order-disorder vortex transitions inBi2Sr2CaCu2O8+δ

Magneto-optical snapshots of the induction distribution across a ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ crystal subjected to ramped magnetic fields reveal strong effects of transient disordered vortex states near the disorder-driven vortex phase transition. These transient states, predominantly generated by edge contamination during field ascending and by supercooling of the disordered vortex lattice during field descending, cause smearing of the transition and shift of its onset towards lower inductions. With decreasing temperature, these effects are more pronounced as the lifetime of the transient vortex states increases. Similar effects are observed with increasing field sweep rates, as transient states with shorter lifetime play a role. These observations clarify previously reported puzzles, e.g., the absence of the second magnetization peak at low temperatures, its appearance at longer times, and the shift of its onset to lower inductions for higher field sweeping rates.

Vortex phase transitions in Bi 2Sr 2CaCu 2O 8+ y in fields nearly parallel and perpendicular to the CuO 2 plane

The angular dependence of vortex phase transitions is studied in Bi 2Sr 2CaCu 2O 8+ y through detailed magnetization measurements using micro-Hall probes, including the two limiting cases, H almost parallel ( θ=0°) and perpendicular ( θ=90°) to the CuO 2 plane. The crossing-lattices ground state is found to be taken over by a homogeneously tilted vortex lattice state at small θ (<10°). At larger θ close to 90°, the linear in-plane field dependence of the vortex phase transition fields breaks down, suggesting the presence of another phase transition. At the transition point, the spacing of pancake vortices becomes nearly equal to the spacing of Josephson vortices along the plane, suggesting a matching effect as a possible origin for this transition. Several other anomalies in the hysteresis curve are also found. We discuss possible origins of these new anomalies.

Josephson plasma resonance in solid and glass phases of Bi 2Sr 2CaCu 2O 8+ δ

Vortex matter phases and phase transitions are investigated by means of Josephson plasma resonance in under-doped Bi 2Sr 2CaCu 2O 8+ δ single crystals in a microwave frequency range between 19 and 70 GHz. Accompanied by the vortex lattice melting transition, a jump of the interlayer phase coherence extracted from the field dependence of the plasma frequency was observed. In the solid phase, the interlayer coherence little depends on field at a temperature region well below T c while it gradually decreases as field increases toward the melting line up to just below T c. As a result, the magnitude of the jump decreases with increasing temperature and is gradually lost in the vicinity of T c. This indicates that the vortex lines formed in the vortex solid phase are thermally meandering and the phase transition becomes weak especially just below T c.

First-order vortex phase transition inBi2Sr2CaCu2Oysingle crystals with different carrier concentrations studied by resistivity measurements

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Vortex phase transition and correlation in partially Au-ion irradiated YBa 2Cu 3O 7 films

Multiterminal transport measurements have been made on YBa 2Cu 3O 7 films with irradiation patterns, which were prepared by partially masked irradiation with 200 MeV Au ions. Two kinds of transitions, which correspond to the vortex glass transition and the Bose glass transition, are observed at T g and T g ′ for the unirradiated part and the irradiated part, respectively. In temperature regions with T g< T< T g ′, where flux flow in unirradiated part is confined between the Bose glass solids, we find the apparent increase of the glass exponents in the resistive transition. The phenomena can be ascribed to diverging development of the vortex shear viscosity.

Dimensionality and lock-in transition in Tl2Ba2CuO6 singlecrystal

The lock-in transition on good-quality over-dopedTl2Ba2CuO6 single crystal was investigated. The observed sharp drop,a dip and a peak together in the flux flow resistivity ρ(θ) as themagnetic field angle θ with the c-axis approaches 90° havebeen ascribed to vortex phase transitions of different types. Ademonstration of the inadequacy of the two models of the angular dependence ofthe upper critical fields to explain this phenomenon is presented here.

Vortex lock-in transition in Tl 2Ba 2CuO 6 superconductor and vortex phase diagram

Lock-in transition in good quality over-doped Tl 2Ba 2CuO 6 crystals has been investigated. As the c-axis angle θ with the magnetic field H approaches 90°, the flux flow resistivity ρ( θ) exhibits a sharp drop, a dip and a peak. These features were observed for a particular range of H and temperatures T and they have been assigned to the vortex phase transitions of different types. These enabled us to identify the associated vortex phase diagrams, viz. the θ– t and θ– H phase diagrams, where t is the reduced temperature T/ T c We made an attempt to fit the data with the available theories and to discuss the results.

New vortex-matter size effect observed in Bi(2)Sr(2)CaCu(2)O(8 + delta).

The vortex-matter 3D to 2D phase transition is studied in micron-sized Bi(2)Sr(2)CaCu(2)O(8 + delta) single crystals using local Hall magnetization measurements. At a given temperature, the second magnetization peak, the signature of a possible 3D--2D vortex phase transition, disappears for samples smaller than a critical length. We suggest that this critical length should be equated with the 2D vortex lattice ab-plane correlation length R(2D)(c). The magnitude and temperature dependence of R(2D)(c) agree well with Larkin-Ovchinnikov collective pinning theory.

Josephson plasma and vortex modes in layered superconductors

The Josephson-plasma and vortex modes in layered superconductors have been studied theoretically for low magnetic fields parallel and perpendicular to layers. The two modes belong to the same lowest-frequency branch of collective-mode spectrum localized near vortices. This is the Josephson-plasma resonance if pancakes are strongly pinned and cannot move. Otherwise the lowest-frequency mode is a vortex mode governed by pancake pinning and the vortex mass. The latter is strongly enhanced by wandering of a vortex line around its average direction. The recently observed jump of the magnetoabsorption-resonance frequency at the vortex phase transition line is interpreted as a transition from the Josephson-plasma to the vortex mode.

Crossover from elastic to plastic vortex creep across the second magnetization peak of high-temperature superconductors

We investigated the relaxation of the irreversible magnetization of YBa2Cu3O72d , Pb2Sr2Y0.53Ca0.47Cu3O8, and Bi2Sr2CaCu2O81d single crystals with significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic field between the onset field and the peak field the relevant current-density dependence of the activation energy exhibits a sudden change, which can be interpreted as a crossover from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier between the onset field and the peak field. The observed increase of the collective pinning barrier is limited by the plastic barrier at the peak field. This appears to be a general behavior, and may have important consequences on the interpretation of the thermally induced vortex phase transition at high magnetic fields.

Intrinsic Josephson junctions for studies of high- Tc superconductors

Pre-contacted, small mesas can be etched into a single crystal of Bi-2212 high- T c superconductor to study tunneling between superconducting planes in the c-axis direction, in so called intrinsic Josephson junctions. These can be applied, for example, to study current transport in the c-axis direction under magnetic fields in order to map vortex phase transitions and the irreversibility line, vortex pinning by columnar defects from heavy ion irradiation, or the effects on the pseudogap and the superconducting ininteraction by changing the separations of the copper oxygen planes by intercalation or pressure.

Vortex matter phase transitions under tilted fields in pristine and Pb-substituted Bi 2Sr 2CaCu 2O 8+ y

Vortex matter phase transitions in Bi 2Sr 2CaCu 2O 8+ y (BSCCO) under fields tilted from the c-axis is studied by using Hall probes. Vortex lattice melting ( H m (θ)) and peak effect ( H p (θ)) are found to follow a novel angular scaling; transition fields linearly depends on the in-plane component of the field. The novel scaling is valid in the vortex solid phase up to doping-level-dependent angle, θ*. New anomalies in the magnetization hysteresis curve, H* p and H* cp , appear only at large tilting angles. Above θ*, the angular dependence of H p , H* p , and H* cp changes drastically, suggesting the presence of vortex phase transition at this angle.

Effect of silver doping on the transport properties of epitaxial YBa 2Cu 3O 7−x thin films

Epitaxial YBa 2Cu 3O 7 (YBCO) thin films doped with 5–20 wt % Ag have been grown “ in situ” at high oxygen pressure by a dc sputtering technique on (001) SrTiO 3 substrates. Critical current densities, J c, were measured in 5 wt % Ag-doped YBCO films in magnetic fields up to 6 T, and were found to be greater than the values obtained for undoped samples. Current-voltage (I–V) characteristics were used to investigate the vortex phase transition, in magnetic field of 6 T applied parallel to the c-axis. The ϱ-j curves scale according to the 3D vortex-glass (VG) model with values of the critical exponents similar to those found for undoped YBCO samples (ν=1.4 and z=4.87).

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

DC magnetic relaxation measurements in HgBa2CuO4 single crystals are analyzed nearby the fishtail line. It is found that in this case, it is not necessary to introduce any crossover from plastic creep to elastic creep models at the fishtail line. This type of fishtail effect comes only from a competition between a critical current at low temperature which increases versus field and the activation energy, which decreases versus field. According to the doping level of the compound, the fishtail effect can be observed or not, without any correlation with a vortex phase transition. Moreover, in this type of fishtail effect, there is no history effects as recently observed in YBaCu2O3 by the partial magnetization loop technique, suggesting that the transition from plastic to elastic flow is here hidden by the disorder of these materials.

Change in the order of the melting transition with oxygen content inYBa2Cu3O7−δ

The vortex phase transition was systematically studied in twin-free ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for various \ensuremath{\delta}. For $7\ensuremath{-}\ensuremath{\delta}&gt;~6.94,$ the first order transition was evidenced by an abrupt jump and a hysteresis of magnetization in the field-cooled (FC) cooling-warming cycles, which indicates the coexistence of vortex crystallites and liquid. For $7\ensuremath{-}\ensuremath{\delta}&lt;6.89$ the jump was not sharp and FC magnetization was reversible, suggesting a second order transition. The first and the second order transitions merge at a critical point (CP) which goes to zero field for $7\ensuremath{-}\ensuremath{\delta}&lt;6.89.$ Three-dimensional XY scaling satisfactorily fits the data at 1--7 T for $7\ensuremath{-}\ensuremath{\delta}&lt;6.94$ and for $7\ensuremath{-}\ensuremath{\delta}&lt;6.89,$ but does not fit for $6.89&lt;7\ensuremath{-}\ensuremath{\delta}&lt;6.94,$ where the CP passes through this field range.

Vortex phase transition and vortex state in (La{sub 0.954}Sr{sub 0.046}){sub 2}CuO{sub 4} single crystals

The vortex state above and below the first order transition was examined in (La 0.954 Sr 0.046 ) 2 CuO 4 single crystals by means of magnetization and two types of transport measurements. The vortex fluctuations were found to have pronounced effects in the fluid region, which was observed as a smaller slope (∥M/∥lnH) of the reversible magnetization curve in the vortex fluid than that in the solid, and as an increase of the difference between them with temperature. In addition, transport measurements with the transformer-type electrode configuration revealed a drastic decrease of the vortex velocity correlation along the field in the fluid phase. Both results were consistent with a vortex lattice state in the solid phase and a highly fluctuating pancake-vortex state in the fluid phase.

Vortex States and Phase Transition of High-To Superconductors in a Magnetic Field Viewed by Computer Simulation

Vortex States and Phase Transition of High-To Superconductors in a Magnetic Field Viewed by Computer Simulation