V. Ta Phuoc

Thermal emittance changes at the charge ordering transition of (Sm0.35Ca0.65)MnO3

The reversible charge ordering transition of (Sm1−x,Cax)MnO3 is associated with strong changes in electrical, magnetic, and infrared properties. In this work, normal spectral emissivity is calculated from infrared reflectance of bulk polished ceramics. Emissivity versus temperature shows a sharp variation at the charge ordering transition near room temperature. By comparing the apparent temperature of the sample with the absolute temperature, we show that Sm0.35Ca0.65MnO3 is a good candidate for infrared furtivity around room temperature.

Ultrafast filling of an electronic pseudogap in an incommensurate crystal

We investigate the quasiperiodic crystal (LaS)1.196(VS2) by angle and time resolved photoemission spectroscopy. The dispersion of electronic states is in qualitative agreement with band structure calculated for the VS2 slab without the incommensurate distortion. Nonetheless, the spectra display a temperature dependent pseudogap instead of quasiparticles crossing. The sudden photoexcitation at 50 K induces a partial filling of the electronic pseudogap within less than 80 fs. The electronic energy flows into the lattice modes on a comparable timescale. We attribute this surprisingly short timescale to a very strong electron-phonon coupling to the incommensurate distortion. This result sheds light on the electronic localization arising in aperiodic structures and quasicrystals.

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.

ω/Tscaling of the optical conductivity in strongly correlated layered cobalt oxide

We report infrared spectroscopic properties of the strongly correlated layered cobalt oxide [BiBa

Bose-glass transition in irradiated Bi2Sr2Ca1-xYxCu2O8 crystals

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

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(Sm1-x,Cax)MnO3 Ceramics with Tunable Emissivity

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

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