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Dr Vladimir Jovanović

Electrical transport and magnetotransport properties of cuprates

The work in this field of research is being done as a part of long-lasting collaboration with solid state physics laboratory in Orsay, LPS (Laboratoire de Physique des Solides, Universite Paris-Sud, Orsay, France) and especially with dr Helene Raffy. Experiments were conducted in LPS (dr H. Raffy, dr Z.Z. Li, dr L. Fruchter and dr F. Bouquet) and also in NHMFL in Grenoble, with the help of dr P. Monceau and dr G. Reményi.
Cuprates are copper-oxide-based materials with laminar structure, with CuO2 layer as the main component. Their properties strongly depend on electric doping (hole and electron doping) and vary so much that the cuprates can be normal metals, antiferromagnetic insulators and even superconductors with the highes critical temperatures of all materials!
We studied experimentally electrical properties, such as resistivity, critical current, magnetoresistance and Hall effect, of epitaxial thin films of electron-doped Sr1-xLaxCuO2, a cuprate with the simplest structure and of hole-doped Bi2Sr2Can-1CunOy, where n = 1 or 2.
The main published results for electron-doped compound concern:
• Synthesis of the Sr1-xLaxCuO2 infinite layer compound
• Magnetic field shifts superconducting transition to lower T without significant broadening, similarly to conventional superconductors
• The change of sign of Hall constant (existence of two types of charge carriers with different mobilities)
• Negative magnetoresistance in parallel magnetic field, which is anisotropic (two- and fourfold symmetric) when field rotates within CuO2 planes (antiferromagnetic fluctuations which coexist with superconductivity)
The results on hole-doped cuprates are in the process of writing and concern superconductor-to-insulator transition and magnetoresistance in high magnetic fields.

• V. P. Jovanović, Z. Z. Li and H. Raffy, Superconducting properties, anisotropy and critical currents of SrLaCuO e-doped epitaxial thin films, Superconductor Science & Technology 24, 055002 (2011).
• L. Fruchter, V. Jovanović, H. Raffy, S. Labdi, F. Bouquet, and Z. Z. Li, Penetration depth of electron-doped infinite-layer Sr0.88La0.12CuO2+x thin films, Physical Review B 82, 144529 (2010).
• V. P. Jovanović, L. Fruchter, Z. Z. Li and H. Raffy, Anisotropy of the in-plane angular magnetoresistance of electron-doped Sr1-xLaxCuO2 thin films, Physical Review B 81, 134520 (2010).
• V. P. Jovanović, Z. Z. Li, H. Raffy, J. Briatico, A. A. Sinchenko and P. Monceau, Resistive upper critical fields and anisotropy of an electron-doped infinite-layer cuprate, Physical Review B 80, 024501 (2009).
• Z. Z. Li, V. Jovanovic, H. Raffy and S. Megtert, Influence of oxygen reduction on the structural and electronic properties of electron-doped Sr1-xLaxCuO2 thin films, Physica C 469, 73 (2009).
• Z. Z. Li , F. Bouquet, L. Fruchter and H. Raffy, Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO thin films, V. Jovanović, Journal of Physics: Conference Series 150, 052086 (2009).

Figure 1: Temperature dependence of Hall constant of two Sr0.88La0.12CuO2+x thin films that differ in electron doping (oxygen content). Curves cross zero which implies the existence of two types of charge carriers with different mobilities.

Figure 2: The shift of superconducting transition of Sr0.88La0.12CuO2 thin film in parallel magnetic field.

Figure 3: Left: Negative magnetoresistance of Sr0.88La0.12CuO2 thin film in parallel field at temperatures indicated. Right: Anisotropy of the magnetoresistance in 6 T when the magnetic field rotates within CuO2 plane.

Organic quasi-2D superconductors

Some unconventional superconductors, such as heavy fermions, high Tc cuprates, borocarbide and organic superconductors follow so called Uemura trend – the proportionality between critical temperature and density of superconducting electrons. The question of the mechanism that leads to their superconductivity is still open, and studying one of the above mentioned compounds might give some idea about the mechanism of the others.
We studied theoretically some physical properties that depend on the order parameter symmetry of organic quasi-2D superconductors, κ-(ET)2X salts, which have many similarities with cuprates, particularly in structure and the vicinity of antiferromagnetic and superconducting region in the phase diagram. We obtained results for the electronic density of states, specific heat, spin susceptibility, magnetic penetration depth and tunneling conductance for three types of order parameter symmetry (d-wave, and dxy-like extended-s) of κ-(ET)2X salts with an elliptical Fermi surface by using generalized BCS theory without impurities.

• L. Dobrosavljević-Grujić, V. Jovanović and R. Zikic, Superfluid density and pairing in planar organic superconductors, Physica C 432, 140 (2005).
• V. Jovanović, R. Zikic and L. Dobrosavljević-Grujić, Pairing in planar organic superconductors, Physica C 423, 15 (2005).

Figure 1: Fermi surface (a) and polar plots of the superconducting gap functions: (b) d-wave, (c) dxy-like wave, and (d) extended s-wave.

Figure 2: Temperature dependence of the order parameter for three types of unconventional pairing: d-wave, dxy-like and extended s-wave. Full line represent an approximation to classical BCS parameter Δ(T)/Δ(0) = tanh[1.74√√(Tc/T – 1)].

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