Marlyse Roulin

Flux Line Lattice Melting Transition in YBa2Cu3O6.94 Observed in Specific Heat Experiments

When a magnetic field penetrates a type II superconductor, it forms a lattice of thin quantized filaments called magnetic vortices. Resistance, magnetization, and neutron diffraction experiments have shown that the vortex lattice of high-temperature superconductors can melt along a line in the field-temperature plane. The calorimetric signature of melting on this line was observed in a high-accuracy adiabatic specific heat experiment performed on YBa2Cu3O6.94. The specific heat of the vortex liquid was greater than that of the vortex solid.

Specific heat peaks observed up to 16 T on the melting line of the vortex lattice in YBa2Cu3O7

Abstract Adiabatic specific heat experiments on YBa2Cu3O7.00 are performed in magnetic fields from 0 to 16 T (B||c and B ⊥ c The twinned, 99.995% pure 18 mg crystal is grown in an inert BaZrO3 crucible, and oxidized in high oxygen pressure to suppress the magnetic “fishtail” effect. First-order-like specific heat peaks rising up to 1.1% above the background are observed on the melting line of the vortex lattice from 6 to 16 T (B||c). The entropy under these peaks is 0.6 ± 0.1 kB/vortex/layer. The fields Bm and peak temperatures Tm obey the relation Bm and peak temperatures Tm obey th relation B m = Bo(1− T n m T c ) 4 3 with B0 = 133 T. The order of the transition is investigated. Only faint anomalies are observed for B||ab. Thermodynamically incompatible specific heat steps and magnetization jumps are observed below 6 T (B||c), suggesting irreversible glassy behaviour. No such features are seen in an YBa2Cu3O6.92 crystal that is characterized by strong pinning.

SCALING BEHAVIOR OF THE DERIVATIVES OF THE SPECIFIC HEAT OF YBA2CU3O6.93 AT THE SUPERCONDUCTING TRANSITION UP TO 16 TESLA

Abstract The scaling properties of the specific heat of the high- T c superconductor YBa 2 Cu 3 O 6.93 (YBCO) are studied. We used a large crystal (0.29 g) at optimal doping. The measurements are performed with a high-resolution (∼10 −4 ) quasi-adiabatic continuous heating type calorimeter in magnetic fields up to 16 T. Rather than scaling the electronic specific heat, we scale the derivatives versus field and versus temperature of the total specific heat. This new approach is less sensitive to arbitrary background subtraction and/or normalization. It is found unexpectedly that the low-field 3D- XY model is valid at high fields for H > 1 T, and that the lowest Landau level (3D-LLL) approximation is a poor description if H c2 ( T ) is taken to be linear in T . But if we take into account the experimentally observed curvature of H c 2 (T) ∝ |T − T c | 4 3 , then 3D-LLL scaling is also satisfied for H > 1 T. With this modification, both descriptions are valid in the same range of fields. Measurements of the magnetization confirm this result. The shape of the scaling function agrees with that predicted in the modified 3D-LLL approximation.

Experimental survey of critical fluctuations in the specific heat of high-temperature superconductors

Thermal fluctuations are observed in the specific heat of high-temperature superconductors (HTS) near Tc. This is seen both in the shape of the zero-field anomaly, characterized by critical exponents and amplitude ratios, and in the broadening of the transition versus the magnetic field, which obeys scaling laws. 3D-XY fluctuations are found for YBa2Cu3Ox (Y-123) near optimal doping, whereas systematic deviations occur away from this point. The large deviations found for the most anisotropic HTS may be attributed to both reduced dimensionality and a crossover between BCS and Bose condensations. Low-temperature data are discussed in this context. The melting curve of vortex matter, now detected in the specific heat of Dy-123, Eu-123 and Y-123 up to very high fields, is described by a critical exponent.

Specific heat of high temperature superconductors in high magnetic fields

Abstract We review investigations of the mixed-state specific heat (C) of extreme type-II superconductors in magnetic fields B≤16 T, from 1 K to Tc. In YBa2Cu3Ox (Y123) at low temperature T, a scaling law Δ C ∞( TB 1 2 )ƒ( T/B 1 2 ) , which is a signature of d-wave symmetry, is observed for the background-free anisotropic component ΔCC(B//c)-C(B//ab). At intermediate T, a C(B)C(0)∞TnBlnB behaviour, which may be explained by the London model, smoothly sets in. Near Tc, superconducting fluctuations are investigated using the background-free derivative ∂(C/T)/∂B. 3D-XY scaling properties are verified up to the overdoped regime. Fluctuations obey different laws in Bi2Sr2CaCu2Ox (Bi2212), which is strongly affected by its 2D character. Finally, the melting line of the vortex solid, which is now believed to be the only true phase transition above Hc1 in the presence of fluctuations, is observed in Y123 either as a 1st- or 2nd-order transition, depending on pinning.

Melting of the flux line lattice observed by specific heat experiments in YBa2Cu3O7−δ

High resolution adiabatic specific heat experiments on YBa2Cu3O7−δ (0≤δ≤0.05) are performed in magnetic fields from 0 to 14 T (B∥c and B⊥c). In a 0.3 gram, twinned crystal with strong pinning, a step is consistently observed at the melting temperature Tm of the vortex solid up to a critical point that depends on δ. The field Bm and step temperature Tm obey the relation Bm=Bmo(δ)(1−Tm/Tc)≈4/3. The anisotropy of Bm and that of the upper critical field Bc2 are found to be equal. Alternatively, in a 18 mg, twinned crystal of high purity with low pinning, first-order-like specific heat peaks are observed on the melting line from 8 to 14 T. The entropy under these peaks is ≈0.5 kB /vortex/bilayer. These characteristic features are attributed to the melting of a vortex glass in the former case and that of a vortex lattice in the latter case.

Observation of second-order transitions below Tc in the specific heat of YBa2Cu3Ox: case for the melting of a vortex glass

Abstract High accuracy specific heat experiments in closely spaced fields from 0 to 14 T are reported for a 0.3 g crystal of YBa2Cu3Ox. Specific heat steps are consistently observed on a line Bm(T) that is well separated from the upper critical field cross-over Bc2(T). Such second-order transitions are predicted to occur when a vortex glass melts. The Bm(T) line is studied for different oxygen concentrations, using the same sample. The anisotropy of the melting field Bm and that of the upper critical field Bc2 are found to be equal. Bm is nearly proportional to the mass anisotropy ratio (mab/mc)1/2

A dimensional characteristic in the specific heat of high temperature superconductors

Abstract The specific heat C of superconductors is strongly depressed just below the transition temperature by an increase of the magnetic field ΔB . A measurement of the function | ∂ ( C / T )/ ∂B | for different magnetic fields at T ≈ T c , which can be approximated by a Lorentzian of half-height width ΔT ( B ) αB n , is found to be a direct way of determining the dimensionality of a superconducting system in the LLL (lowest Landau level) approximation, with n= 1 2 D and n= 2 3 for 3D. No baseline subtraction is required. Examples are given using high resolution specific heat measurements between 0 and 16 tesla on YBa 2 Cu 3 O 7−δ (YBCO) and Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) single crystals. The temperature T cB ( B ) of the minimum of | ∂ ( C / T )/ ∂B | can be related to the mean-field critical temperature T c ( B ) in the framework of the theory of Tesanovic at high fields.

Specific-heat measurements up to 14 T of the high-temperature superconductor (Ca0.4La0.6)(Ba1.35La0.65)Cu3Oδ for different oxygen concentrations

Abstract (Ca x La 1− x )(Ba 1.75− x La 0.25+ x )Cu 3 O δ belongs to the same 1:2:3 family as YBa 2 Cu 3 O δ (YBCO). We report the first measurement of the specific heat at high temperature (40–150 K) and high fields (0–14 T) for a series of superconducting ceramics with x = 0.4. The effect of the oxygen content is studied for δ = 7.09-7.19. The specific-heat anomaly at T c , which does not exceed 1% of the total heat capacity in the most favorable case, changes from a 3D YBCO-like behavior at optimal doping to a 2D Bi 2 Sr 2 CaCu 2 O 8 -like behavior in the underloop regime.

Specific heat anomalies on the vortex melting line in YBa2Cu3O7−δ up to B=23 teslas: Observation of first- and second-order transitions, effect of oxygen doping, field orientation, crystal purity and detwinning

Abstract 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 2 Cu 3 O 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.