L. Ammor

Magnetoresistance in Bi-2212 single crystal

Abstract We report a thermally activated dissipation measured in Bi-Sr-Ca-Cu-O (2212) single crystal down to 45 K and up to 8 T. The magnetic field was slightly titled with respect to the ab planes. By an unusual way variant of the flux-creep approach, we show that the crossover between ohmic thermally activated flux-flow (TAFF) regime and non-ohmic thermally activated flux-motion (TAFM) regime is adequately described. It is found that the activation energy is well approximated by U ∼(1-( T / T c ) 2 ) 2 / H . The crossover line in the H - T plane between the TAFF and TAFM regimes is found to be the so-called “irreversibility line”.

Scaling behaviour and crossover of resistive transition in YBa2Cu3O7-x

Abstract Our resistivity measurements on YBa2Cu3O7-x ceramics, in the limit I=0, confirm a second-order phase transition at a critical temperature Tc(0) T∗(I) >Tc(0), σ∼ t -γ with γ⋍0.9 and t = (T-T c (I))/T c (0) where Tc(I) corresponds to the critical temperature shift due to I. This conductivity critical behaviour is identical to the one observed as T→Tc(0)+ and I=0. For the temperature range Tc(I) t −γ where the critical exponent value γ⋍1.3 appears to be in good agreement with the one computed elsewhere for the pure 3D XY system. We suggest that phase fluctuations account for the excess conductivity above the percolation threshold and we propose an intergranular coherence transition at Tc(I).

Comparative infrared study of optimally doped and underdoped La2−xSrxCuO4 single crystals

The temperature dependence of the optical spectra of two La2?xSrxCuO4 single crystals was investigated for both in-plane and out-of-plane directions. For the underdoped (x = 0.08) single crystal, the in-plane optical conductivity that was analysed by a generalized Drude formalism shows a suppression of the scattering rate 1/?(?) and an increased effective mass m* as the temperature decreases at low frequencies. Since this behaviour can be explained by the pseudogap effect, it is concluded that the pseudogap is not present at the optimal doping (x = 0.15). This result is confirmed by the c-axis optical conductivity, which decreases only for the underdoped single crystal. The absence of the pseudogap at the optimal composition is in accord with the quantum critical point model and it can explain the phase diagram of high-temperature superconductors.

Peak effect in an irradiated microbridge of Bi-2212 single crystal: evidence for a Bragg Bose glass? ☆

We have investigated I–V characteristics at very low temperature on a micro-bridge in a Bi-2212 single crystal with columnar defects patterned by laser ablation. We have shown that the critical current IC presents a maximum when increasing the applied magnetic field. This maximum is located below the matching field at a filling fraction f=BMax/BΦ=0.8. The transverse Meissner effect (TME) has also been checked by tilting the magnetic field away from the tracks. Contrary to the low field regime (f<0.8) where the vortex are strongly localized, the transverse component of the magnetic field is not screened anymore when the parallel one is higher than BMax. It suggests that the vortex system could be considered as a weak Bose glass with interstitial vortices. It suggests that the vortex system could be considered as a weak Bose glass with interstitial vortices. Finally, our results strongly support the existence of a typical vortex phase at low temperature and low magnetic field.

Bose-glass behaviour in Bi2Sr2Ca1−xYxCu2O8 crystals with columnar defects: experimental evidence for variable-range hopping

Abstract We report on vortex transport in Bi 2 Sr 2 Ca 1− x Y x Cu 2 O 8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur (Phys. Rev. B. 48 1993 13060).

Current density dependence of the activation energy in Tl-2212 thin films at weak magnetic fields

Abstract The flux motion has been investigated in a Tl 2 Ba 2 CaCu 2 O 8 thin film by transport measurements with a weak applied magnetic field perpendicular to the ( a , b ) plane ranging from 20 to 300 mT. We have found a well defined line T ∗ (H) separating ohmic from non ohmic behaviour in the H-T ∗ diagram. This line is well described by 1 - t ∗ ≈ H 2 3 where t ∗ = T ∗ /T c and T c is the zero magnetic field transition temperature. Above T ∗ (H) , a thermally activated resistance is observed. Our experimental results clearly indicate that the activation energy is well approximated by U(T, H) ∼ [l-(T/T c ] 3 2 /H both below and above the T ∗ (H) line in the H-T ∗ diagram. The nonlinear current dependence of the activation energy U ( T , H , I ) is extracted from I – V characteristics, and it is found that U ( T , H , I )= U ( T , H ) ln( I 0 / I ). Satisfactory agreement is found between our results and recent models which take into account a periodic logarithmic-like pinning potential.

Dimensional crossover vortex fluctuations in irreversibility line of high-quality Tl2Ba2CaCu2O8 thin film

Abstract The current-voltage (I–V) characteristics are investigated in a high quality Tl-2212 thin film in magnetic fields (up to 8 Tesla) applied perpendiculary to the layers. We find experimental support for the existence of a crossover from (3D) to (2D) vortex fluctuations occuring at magnetic field B cr . For B ≪ B cr , the scaling behaviour of the vortex glass transition is observed with critical exponents z ≈ 3 and ν ≈ 1.9, which are field independent. At sufficiently large fields B ≫ B cr , we show that the thereshold measurable ohmic resistance can be attributed to melting of a vortex lattice triggered by (2D) vortex fluctuations.

Bose-glass transition in irradiated Bi2Sr2Ca1-xYxCu2O8 crystals

Abstract Current-voltage characteristics are investigated in Bi2Sr2Ca1-xYxCu2O8 crystals irradiated parallel to the c-axis with 5.8 GeV Pb ions. Over a wide range of filling fractions 0.026 ≤ f ≤ 1, I–V isotherm curves near the superconducting transition are consistent with the Bose-glass scaling theory. The Bose glass line, derived from scaling analysis, is well described by the Lindemann criterion which accounts for the contribution of correlated disorder. Two characteristic temperatures t0≈0.72 (f≈0.8) and t1≈0.83 (f≈ 1 3 ), beyond which the vortex system is increasingly dominated by vortex-vortex interactions or thermal fluctuations, respectively, are identified.

Superconducting transition in Tl-2 2 1 2 thin films and polycrystals: Effect of the dimensionality of the sample

A former paper had shown that some Tl-2 2 1 2 thin films can be described as two-dimensional arrays of weakly coupled superconducting grains. We here report on a comparative study of the superconducting transition (R = 0) in a Tl-2 2 1 2 thin film and a Tl-2 2 1 2 polycrystal. We find in this way rather striking evidence for two kinds of voltage-current behaviours respectively which places emphasis on the dimensionality of samples. We show that both these dissipative behaviours can quantatively be explained in terms of phase-coherence transitions predicted in D-dimensional arrays of superconducting grains. In particular, universal scaling properties of the three-dimensional phase-coherence transition and Kosterlitz-Thouless-like phase transition are found for the polycrystal and the film respectively.

Bose-Glass Behavior in Bi2Sr2Ca1-xYxCu2O8 Crystals with Columnar Defects: Experimental Evidence for Variable-Range Hopping

We report on vortex transport in Bi2Sr2Ca1-xYxCu2O8 crystals irradiated at different doses of heavy ions. We show evidence of a flux-creep resistivity typical of a variable-range vortex hopping mechanism as predicted by Nelson and Vinokur.