First-order Vortex Lattice Melting Transition Research Articles

Tilted vortex lattice in irradiate Bi2Sr2CaCu2O8+δ single crystals

In order to enlighten the structure of vortex matter in irradiated layered Bi2Sr2CaCu2O8+δ single crystals, the interaction of Josephson vortices and pancake vortices in was investigated by means of the local ac-magnetic permeability measurements by using the miniature local coils, while vortex matter in pristine crystals was studied by in-plane resistivity measurements. The transition anomaly, separating the strong pinning phase and the weak pinning vortex phase was found by both techniques deep in the vortex solid phase solid near ab-plane, indicating crossover from the vortex chains + lattice phase to tilted vortex chains phase. While the columnar defects affect strongly the first-order vortex-lattice melting transition, the magnetic permeability anomaly, associated with the crossover from vortex chains + lattice phase to tilted lattice, is surprisingly still clear, deep in the vortex solid phase. However, the stronger columnar defects eventually affect the crossover anomaly that it disappears too.

Vortex states of Bi2Sr2CaCu2O8+y with antidot array probed by c-axis transport measurements

To study the vortex states in Bi2Sr2CaCu2O8+y (Bi2212) high- T c superconductor with periodic array of antidots, we have measured the c -axis critical current in a stack of the intrinsic Josephson junctions with a square lattice of antidots, which gives us information on the c -axis correlation of the pancake vortices. Without the antidot array, the c -axis critical current can probe the first-order vortex lattice melting transition. Additionally, a suppression of the critical current in the vicinity of zero magnetic field is found, which may be related to the instability of vortex lattice under the parallel current flow. With the square lattice of antidots, enhancements of the critical current due to the matching effect are observed in both the integer and fractional matching fields. By mapping the critical current on H - T diagram, the influence of the matching effect on the phase boundary of vortex solid and liquid is shown.

Vortex phase diagram in heavy ion irradiated untwinned YBa 2Cu 3O y single crystals

In order to study the effects of columnar disorder on the vortex phase diagram, we have measured the resistivity ρ( H, T) and magnetization M( H) in heavy ion irradiated untwinned YBa 2Cu 3O y single crystals. With increasing irradiation dose, the first-order vortex lattice melting transition H m( T) is suppressed near the upper and lower critical points. The first-order melting transition completely disappears when the irradiation dose has a value of 0.5 T < B Φ < 1.0 T and the transition becomes the second-order Bose glass transition when B Φ ⩾ 1.0 T. For B Φ = 0.5 T, the first-order melting transition exists in the very narrow field region between 4.5 T and 10 T without any characteristic features such as the order–disorder transition H ∗( T) line in the vortex solid phase.

Vortex phase diagram in untwinned YBa2Cu3Oy with columnar disorder

We have measured resistivity ρ(H, T) and magnetization M(H) up to 15T in heavy ion irradiated untwinned YBa2Cu3Oy single crystals in order to study effects of columnar disorder on the vortex phase diagram. With increasing irradiation dose B, the first-order vortex lattice melting transition Tm(H) is suppressed near the upper and lower critical points. For B = 0.5 T, the first-order transition exists in the very narrow field region between 4.5 T and 10 T without any characteristic features such as the field-driven order-disorder transition H*(T) line in the vortex solid phase. The first-order transition completely disappears and the transition becomes the second-order Bose glass transition when Bϕ ⩾ 1.0 T.

Vortex phase diagram in impurity doped YBa 2Cu 3O y single crystals

Impurity substituted single crystals of untwinned YBa 2(Cu 1− x M x ) 3O 6.95 (M = Zn, Ni) have been prepared and the Zn and Ni substitution effect on the vortex phase diagram has been studied as a function of impurity concentration x. In YBa 2(Cu 1− x M x ) 3O 6.95 with small amount of impurity [ x (Zn) = 0.0003 ( T c = 91 K), x (Ni) = 0.003 ( T c = 90.5 K)], although the first-order vortex lattice melting transition H m( T) disappears, the order–disorder transition line H*( T) between the Bragg glass and the vortex glass phases has been observed. The obtained results suggest that Zn and Ni impurities strongly affect the vortex system in comparison with the oxygen vacancy in pure YBa 2Cu 3O y .

Vortex-melting and vortex-glass transitions in a high purity twinnedYBa2Cu3O7−δsingle crystal

We present experimental evidence for the coexistence of a first-order vortex-lattice melting transition and a continuous vortex-glass melting transition at a lower temperature in high purity twinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal samples grown in ${\mathrm{BaZrO}}_{3}$ crucibles. The correlation of temperature T and magnetic field H dependence of the electrical resistivity $\ensuremath{\rho}$ and electric field vs current density E vs J isotherms is consistent with a two-step phase transition from the normal to the superconducting state. Analysis of the data strongly indicates that the vortex matter in the single crystal is inhomogeneous. This scenario is supported by the observation of a vortex-lattice melting at a temperature separating two vortex-glass states with different critical exponents $\ensuremath{\nu}(z\ensuremath{-}1).$ In light of independently performed numerical simulations [Crabtree et al., Phys. Rev. B 61, 1446 (2000)], the data support a scenario where the intermediate phase crosses over from a state where vortex lines in the twin boundaries are pinned and those within the bulk undergo driven plastic vortex flow, to a state where the vortex lines in and near the twin boundaries undergo driven disordered motion and those within the bulk move as an ordered lattice.

Vortex Phase Diagram in Zn-Doped YBa 2 Cu 3 O y Crystals

Effects of Zn impurities on the vortex phase diagram of YBa2(Cu1−x Zn x )3O y single crystals with 0≤x≤0.064 have been investigated in magnetic field up to 18 T. The first-order vortex lattice melting transition and the field-driven disordering transition are extremely suppressed by small amount of Zn impurities. As for Zn-doped YBCO with x>0.001, the resistive transition shows a broad temperature dependence due to the second-order vortex glass melting transition, while the field-driven disordering transition survives up to 0.003. These behaviors suggest that disorderes induced by Zn impurities strongly affect the vortex system in comparison with the oxygen deficiency in YBCO.

Two-component vortex phase diagram inBi2Sr2CaCu2O8+yunder tilted fields studied by a micro-Hall probe

Angular dependence measurements of local magnetization in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+y}$ revealed the existence of multiple peak effects peculiar to the hysteresis loops in tilted fields. We discuss the origins of these peak effects in relation to changes of underlying vortex states, and propose a phase diagram for two-component vortex matter composed of pancake and Josephson vortices. Furthermore, systematic studies with changing temperature and oxygen concentration clarified the nature of the breakpoint of linear suppression of the first-order vortex-lattice melting transition, and the second peak field at low temperatures. It is also demonstrated that the two-component vortex matter shows two kinds of penetration field for pancake vortices.

Non-linear resistance behavior in parallel magnetic fields: indication of the vortex-smectic phase in Bi 2Sr 2CaCu 2O 8+ δ

The phase diagram of the vortex matter in single crystals of Bi 2Sr 2CaCu 2O 8+ δ is studied by the in-plane resistivity measurements in the Corbino geometry in magnetic fields tilted away from the c-axis. The resistivity kink attributed to the first-order vortex lattice melting transition is clearly detected across a wide angular range. However, very close to the ab-plane, the resistance kink disappears and the Ohmic resistance, observed in the vortex liquid for the lower angles, is replaced by the strong non-Ohmic behavior possibly indicating the vortex-smectic phase. At a certain temperature, close to T c, the resistance kink suddenly occurs in parallel magnetic fields H∥ ab which may be related to the change from continuous phase transition at low temperatures to the first-order phase transition at high temperatures.

Melting transition of vortex matter in YBa 2Cu 3O y with various oxygen contents

The effect of oxygen content on the vortex phase diagram of untwinned YBa 2Cu 3O y (YBCO) single crystals has been studied by magnetization and resistivity measurements in high magnetic fields up to 30 T. The cantilever magnetometry with a capacitance torque meter has been developed for precise magnetization measurement in high magnetic field. As for fully oxidized and optimally doped YBCO, the magnetic properties near the first-order vortex lattice melting transition line are discussed in comparison with the vortex phase diagram obtained from the resistivity measurement.

Anomalous angular dependence of vortex melting transition in single crystal Bi 2Sr 2CaCu 2O 8+δ

The first-order vortex lattice melting transition was studied in single crystal Bi 2Sr 2CaCu 2O 8+δ by in-plane resistivity measurements in oblique magnetic fields close to ab-plane. The linear-like in-plane field dependence of the c-axis melting field component H melt c , which indicated the crossed pancake and Josephson lattices, abruptly terminates and transforms into a plateau. The in-plane magnetic field value where this transformation occurs is substantially lower than that obtained from the crossing lattices model [1]. The possible explanation is proposed.

Vortex dynamics in low magnetic fields in single crystals Bi 2Sr 2CaCu 2O 8+δ

In-plane resistivity measurements were performed for two single crystals Bi 2Sr 2CaCu 2O 8+δ with electric contacts in the Corbino geometry in order to avoid the influence of the surface barriers and to probe the pure bulk properties of superconductors. The first-order vortex lattice melting transition was identified by the sharp resistance drop even as low as 1 Oe. It was found that the resistance level R T m where the melting transition occurs, does not depend on magnetic fields in the high temperature region. However, below appx. 55 K the resistance level R T m sharply drops while the resistance anomaly itself loses sharpness and gradually disappears indicating that the melting transition may change its character from first-order to second-order phase transition.

Vortex lattice melting transition in oblique magnetic fields in single crystal Bi 2Sr 2CaCu 2O 8+ δ

The vortex dynamics in single crystal of Bi 2Sr 2CaCu 2O 8+ δ was studied in oblique magnetic fields. The sharp drop of the resistance attributed to the first-order vortex lattice melting transition (VLMT) separates the vortex solid and vortex liquid phases in the wide range of magnetic field directions, even at angles very close to the ab-plane. It was observed that VLMT does not scale with the c-axis magnetic field component but is depressed linearly by the in-plane magnetic field in agreement with recent theoretical prediction by Koshelev. Additionally, it was found that VLMT becomes independent of the magnetic field orientation above the certain value H ab ∗ of the in-plane magnetic field component.

Superconducting properties of untwinned YBa 2Cu 3O y single crystals annealed in high-pressure oxygen

The effect of the oxygen content on the superconducting properties have been studied in untwinned YBa 2Cu 3O y single crystals. The oxygen content was controlled by annealing at several temperatures in 8 bar oxygen. The heavily overdoped sample ( T c∼88 K) annealed at T an=350°C shows the first-order vortex lattice melting transition up to 26 T. With decreasing oxygen content, the critical point of the melting transition line decreases to 4 T for the slightly underdoped sample ( T an=600°C, T c∼92.5 K). The magnetization hysteresis curves also show a remarkable change as a function of the oxygen content.

Josephson plasma resonance in Bi 2Sr 2CaCu 2O 8+ δ in vortex liquid and solid states

Both vortex liquid and solid states of high-temperature superconductor Bi 2Sr 2CaCu 2O 8+ δ have been studied by means of Josephson plasma resonance (JPR). It is found that although the JPR frequency changes at the first-order vortex lattice melting transition (VLMT), T M, by 20–30% at low temperatures, it crosses T M almost continuously at high temperatures near T c. Such a nearly continuous change from liquid to solid state strongly implies that the phase transition is so weak that the disappearance of the superconducting phase correlations is nearly continuous. This situation may be realized by considering the strong 2D superconducting fluctuations, resulting in the strong softening of the proper plasma mode in the solid phase.

Vortex-matter phase diagram in YBa2Cu3Oy

This paper reviews the progressive results of the vortex-matter phase diagram for the high-Tc superconductor YBa2Cu3Oy in magnetic fields up to 30 T. Evidence of the first-order vortex-lattice melting transition and the second-order vortex-glass transition is demonstrated by resistivity and magnetization measurements. The feature of the novel second magnetization peak is summarized and the origin of the new line H*(T), which is defined by the sharp onset of the second peak, is discussed on the basis of the field-driven disordering transition theory. Effects of the weak point disorder on the vortex-matter phase diagram are also studied by the irradiation of 2.5 MeV electrons. It is revealed that three transition lines of the vortex-lattice melting line Hm(T), the vortex-glass transition line Hg(T), and the field-driven disordering transition line H*(T) terminate at the critical point and divide the vortex-matter phase into three different phases: the vortex liquid, the Bragg glass, and the vortex glass. Interesting similarities of the vortex matter phase diagram between YBa2Cu3Oy and Bi2Sr2CaCu2Oy are also discussed.

Giant peak effect observed in an ultrapureYBa2Cu3O6.993crystal

A giant peak in the temperature dependence of the screening current is observed in the ac magnetic response of an ultra-pure YBa$_2$Cu$_3$O$_{6.993}$ crystal in a magnetic field. At H = 2.0 T ({\bf $H$}$||${\bf c}), the screening current density $J_c(T)$ exhibits a 35-fold rise with 0.5 K increase in temperature, indicating an abrupt $10^3$-fold collapse in the characteristic volume of ordered regions in the vortex array. The peak-effect anomaly is most pronounced for $H<$ 4.0 T, but detectable up to 7.0 T. The temperature dependence of the equilibrium magnetization exhibits a small discontinuous jump (for high fields) inside the peak-effect regime, suggesting that the underlying phase transition is a weak first-order vortex-lattice melting transition.

Phase transitions in isotropic extreme type-II superconductors

Using large scale Monte Carlo simulations on a uniformly frustrated 3DXY model, we report a first order vortex lattice melting transition in clean, isotropic extreme type-II $\kappa \to \infty$ superconductors. This work clarifies an important issue: the unpinned vortex liquid is always incoherent with no phase coherence in any directions for all anisotropies. Previous claims of a disentangled vortex liquid for isotropic superconductors based on simulations, are due to finite size effects. We explicitly show that the effective vortex-line tension vanishes precisely at the superconducting phase transition in zero magnetic field. This loss of line tension is accompanied by an abrupt change in the connectivity of the vortex tangle across the superconductor. We also obtain results indicating that the connectivity of the vortex tangle changes in a similar way even in finite magnetic field, and suggest that this could also be associated with a genuine phase-transition.

Magnetic phase diagram of untwinned YBa 2Cu 3O y single crystals annealed in high pressure oxygen atmosphere

Magnetization and resistivity for untwinned YBa 2Cu 3O y single crystals with controlled oxygen contents were measured at high fields ( H‖c) up to 27 T. With increasing the oxygen content above the optimum condition, the superconducting transition temperature T c changed from ≃93 K to ≃88 K and the hysteresis width of the magnetization M( H) curve decreased. The resistivity measurement for the heavily overdoped sample showed a discontinuous jump which results from the first-order vortex lattice melting transition (VMLT) even at 26 T. The result indicates that the vortex pinning is strongly reduced for heavily overdoped sample and the oxygen vacancy has great influence on its phase diagram in untwinned YBa 2Cu 3O y .

Moving Vortex States Studied by Current Flow in Single CrystalBi2Sr2CaCu2O8+δ

Nonequilibrium vortex states have been investigated in single crystalline ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ using in-plane resistance and magnetization measurements. It was found that the first-order vortex-lattice melting transition (VLMT) phase line persists even in the dynamic vortex states in a wide range of driving forces and sharply separates the vortex solid and the vortex liquid phases. The activation energy $U$ deduced from the relation ${R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}R}_{0}\mathrm{exp}(\ensuremath{-}U/{k}_{B}T)$ abruptly increases below VLMT, strongly depends on transport currents, and weakly depends on magnetic fields in the vortex solid state. The resistivity attains zero value sharply at VLMT in the low current limit.