Vortex Melting Line Research Articles

Vortex phase diagram of YBa 2Cu 4O 8

We have investigated the vortex phase diagram of YBa 2Cu 4O 8 single crystals of intermediate c-axis anisotropy by means of the magnetic torque, the DC magnetization, and the AC susceptibility. The single crystals have been grown by a self-flux method under high-pressure. The magnetization curve shows a second peak. The torque also shows the multiple peak effects. The central peak is due to the intrinsic pinning. The first torque peak at θ ca ≃90±10° requires the existence of the field component parallel to the CuO 2 planes. The second torque peak at θ ca ∼90±30° roughly correlates with the second peak in magnetization. We summarize the vortex melting line and the lines for the various peaks in the HT diagram. We find that vortex phase diagram of YBa 2Cu 4O 8 in H ∥ c is very different from those for the YBa 2Cu 3O 7−δ and Bi 2Sr 2CaCu 2O 8+δ.

Unified order-disorder vortex phase transition in high-Tcsuperconductors

The diversity of vortex melting and solid-solid transition lines measured in different high-${T}_{c}$ superconductors is explained, postulating a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of monotonic and nonmonotonic transition lines similar to those measured in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8},$ ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4\ensuremath{-}\ensuremath{\delta}},$ ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}},$ and $({\mathrm{La}}_{0.937}{\mathrm{Sr}}_{0.063}{)}_{2}{\mathrm{CuO}}_{4}.$

Critical points in heavy ion irradiated untwinned YBa(2)Cu3O(7-delta) crystals

The critical points in untwinned YBa(2)Cu(3)O(7-delta) crystals with dilute columnar defects are investigated. We find a convergence of a first order vortex melting line with an irreversibility line associated with the onset of the Bose glass critical regime at the lower critical point. In addition, we find that columnar defects raise the upper critical point, implying that vortex line meandering is a basic feature controlling its position.

Specific heat of a DyBa 2Cu 3O 7− y single crystal in magnetic fields up to 8 T

The specific heat of a twinned DyBa 2Cu 3O 7− y (DyBCO) single crystal has been measured in magnetic fields up to 8 T near T c. The measurements were performed on a small single crystal with an a.c. calorimetry technique. We have scaled the specific heat both by the method of removing a non-singular background and by the method of scaling the magnetic field derivative of the specific heat. Both lowest Landau level (LLL) scaling and 3D XY scaling have been tested by these methods. We find that by the method of field derivative scaling, the two models give similar results, but by the method of removing a non-singular background, the LLL scaling fails in fields below 2 T. On the low temperature side of the transition, small steps are observed in low fields which turn into small peaks (∼0.3% of C/ T) for higher fields. The position of these peaks is consistent with the vortex melting line and gives a melting entropy of 0.3 K B per vortex per CuO 2 bi-layer in an applied magnetic field of 5 T.

Discrete temperatures in high-temperature superconductors

The low-amplitude AC susceptibility on intact and deformed Bi2223/Ag tapes has been measured as a function of temperature, frequency, AC amplitude and DC magnetic field. The deformation resulted in the splitting of the χ″( T) peak into three peaks situated near 30, 58 and 90 K. In zero magnetic field, these temperatures were identified as the Kosterlitz–Thouless transition temperatures of low number stacks of superconducting layers. An external magnetic field redistributed the dissipation among the peaks, and moved them to lower temperatures (and suppressed the highest temperature peak). In a finite field, each peak corresponds to the stack melting temperature T m. The melting temperature in each stack was found to be a field-dependent parameter, with a minimum value= T KT of a stack of thickness that is less by one layer. The T m decreases exponentially with the field, and the rate of decrease depends on the interstack Josephson and magnetic interactions. With a universal set of T KT, the vortex melting line of a tape is a linear combination of the T m( H) for the low-number stacks.

Origin of vortex melting line in Bi2223/Ag tapes

Using AC diamagnetic and transport measurements we have derived a three-dimensional diagram for high-quality Bi2223/Ag tapes. The diagram shows an abrupt decrease of critical current caused by the melting of a vortex glass. The melting line was defined as the contour line for the surface at , where showed the steepest decrease with H and T. This line coincided with the locus of the maximum, at a frequency Hz and an AC field amplitude Oe. The melting line reflects the process of the separation of unit-cell stacks. The most important are the low-number stacks that contain one to three unit cells. These stacks have the lowest Kosterlitz-Thouless transition temperatures and the lowest corresponding vortex melting temperatures. A magnetic field causes the separation of the stacks and reduces in each stack to the melting temperature of the lower stack. At the universal , the vortex melting line is the combination of for different, mainly low-number, stacks.

3DXYScaling of the Irreversibility Line ofYBa2Cu3O7Crystals

We find experimentally that the irreversibility line, H{sub irr}(T) , and the reversible magnetization of twinned YBa{sub 2}Cu {sub 3}O{sub 7} crystals both show 3D XY scaling, which suggests a common origin for both quantities. Analysis of this result and of a correlation between the condensation energy and H{sub irr}(T) of oxygen deficient YBa{sub 2}Cu {sub 3}O{sub 7{minus}{delta}} for 0{lt}{delta}{lt}0.6 , implies that H{sub irr}(T) is strongly influenced by critical fluctuations. It is possible that these values of H{sub irr}(T) and the recently discovered vortex melting line, H{sub m}(T) , both correspond to an upper critical field line which has been suppressed by thermodynamic fluctuations. {copyright} {ital 1997} {ital The American Physical Society}

Specific heat anomalies on the vortex melting line in YBa 2Cu 3O 7−δ up to B=23 teslas: Observation of first- and second-order transitions, effect of oxygen doping, field orientation, crystal purity and detwinning

Because strong fluctuations transform the upper critical field line B c2(T) into a smooth crossover, vortex melting is now believed to be the only true phase transition of high temperature superconductors in a magnetic field. We review recent work performed by the Geneva and Grenoble laboratories in the calorimetric determination of the melting line B m(T) in YBa 2Cu 3O x. This includes three crystals showing specific heat steps upon crossing B m(T), and three crystals showing specific heat peaks. Adiabatic calorimetry (up to 23 T) and oscillating dynamic methods (up to 7 T) are used. We investigate the effect of field orientation (B//c and B⊥c), oxygen concentration x (6.95 – 6.97), and detwinning. These experiments document both processes that may occur on the B m(T) line, depending on pinning properties: the melting of a vortex lattice with a first-order transition, and the melting of a vortex glass with a second-order or continuous transition.

Observation of the vortex melting line in YBa2Cu3O7 by specific heat measurements in very high magnetic field : H ≤ 23 Tesla

We have performed specific heat (C) measurements using a very sensitive adiabatic calorimeter on a twinned, high-purity single crystal (18mg) of YBa2Cu3O7 (referred thereafter as YBCO). In the same crystal, for 5T ≤ B ≤ 16T, very well defined peaks in C have been reported at the vortex lattice melting transition occurring at a characteristic temperature Tm[1]. We tracked this line B(Tm) up to 21.5 Tesla and, finally, no anomaly was detectable in 23 Tesla, suggesting the existence of an ending critical point for a characteristic magnetic field B*, 21.5T ≤ B* ≤ 23T. In these very strong magnetic fields, the shape of the anomaly in C associated with the freezing of the vortex-liquid state becomes strongly dependent on the experimental conditions.

Vortex pinning in YBa2Cu3O7−x films

Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa2Cu3O7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T c and magnetic field 0<B<6 T. Below the vortex lattice melting transition Bg(T) but above a threshold field Bp≈8(1-T/T c ) T, the inductance of vortices increases as B2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical scaling persists over the region Bg(T<B<B*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ*≡ξ(B=B*)≈450A. The value of ξ* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at Bg.

Discontinuity of reversible magnetization in untwinned YBCO single crystals at the first order vortex melting transition.

We report magnetization measurements on clean and untwinned YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{6.95}$ single crystals with extremely low pinning. For the first time, an irreversibility line well below the vortex melting line has been achieved. A discontinuity in the magnetization at the vortex melting transition is observed, indicating a first order transition. The entropy change at the phase transition is ${0.8k}_{B}$ per vortex per Cu${\mathrm{O}}_{2}$ double layer at 10 kOe and ${0.6k}_{B}$ per vortex per Cu${\mathrm{O}}_{2}$ double layer at 40 kOe, in reasonable agreement with theoretical calculations.

Effects of columnar defects on the superconducting properties of Y 1Ba 2Cu 3O 7−δ: Limits to the irreversibility line

A series of YBa 2Cu 3O 7−δ(YBCO) epitaxial films and crystals were irradiated with energetic heavy ions (Sn 30+ or Ag 29+ at energy 0.58 GeV) to produce columnar defects of highly disordered material. The irreversibility lines, B ∗(T) , show enhancement due to the increased pinning. However, while B ∗ does not exceed that observed in the highest- J c as-grown epitaxial thin films, the values approach the proposed vortex melting line determined by a number of experiments on nearly defect-free single crystals. These results are interpreted in terms of the predicted upper limit of pinning energies in anisotropic superconductors, and the thermally-induced decoupling of vortex segments.

XY-like critical behavior of the thermodynamic and transport properties of YBa2Cu3O7-x in magnetic fields near Tc.

The heat capacity, magnetization, and electrical conductivity of single-crystal samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{x}}$ have been measured and are shown to support the existence of an intermediate critical regime in the vicinity of ${\mathit{T}}_{\mathit{c}}$, governed by the XY-like critical exponent \ensuremath{\nu}\ensuremath{\simeq}2/3. Clear evidence is found for the divergence of the ohmic conductivity along the line ${\mathit{H}}_{\mathit{m}}$(T)\ensuremath{\propto}(1-T/${\mathit{T}}_{\mathit{c}}$${)}^{2\ensuremath{\nu}}$, the vortex melting line. The glass exponents along that line satisfy ${\mathit{z}}_{\mathit{g}}$(${\ensuremath{\nu}}_{\mathit{g}}$-1)\ensuremath{\simeq}6.