Electronic Transport Properties in Magnetic Field and under Pressure

Prof. Dr. Siegfried Horn, Dr. Stefan Klimm

In this project electronic transport properties of systems close to a metal insulator transition (MIT) are examined as a function of temperature, magnetic field, external pressure and doping. The properties of such correlated systems strongly depend on the ratio U/W (U = Coulomb interaction, W = bandwidth). The application of pressure reduces the interatomic distances thereby increasing the bandwidth without changing the band filling and without introducing disorder, which, in contrast to doping, represents a "clean" method for tuning U/W.

Phase diagrams of electronic, structural and magnetic transitions are investigated within the available parameter range (0.3 K < T < 400 K, B < 14.5 T, p < 120 kbar). Special attention is paid to the evolution of characteristic features like Hall-constant, T²-coefficient of resistivity, thermopower and semiconducting gap on approaching the MIT. Furthermore at low temperatures Fermi surfaces and cyclotron masses of metallic samples are measured via Shubnikov-de Haas (SdH) oscillations and compared to electronic structure calculations.

In semiconducting systems the transport mechanism and respective activation energies are investigated by means of temperature dependent resistivity measurements up to 10¹⁵ Ohms.

Up to now the pressure and magnetic field dependent transport properties of several vanadium oxides and related compounds have been examined, for example:

• Measurements on stoichiometric V₂O₃ as a function of pressure confirmed the importance of electronic correlations for this system.
• Metamagnetic behavior was observed in the spin density wave (SDW) state of V-deficient V_2-yO₃.
• SdH measurements on MoO₂, which is isostructural to VO₂, are in excellent agreement with calculated Fermi surfaces showing that the local density approximation yields a good representation for the electronic structure of these oxides.
• The effect of external pressure on the semiconducting gap of low dimensional sodium vanadates was also examined. The strong pressure dependence for NaV₆O₁₅ is in contrast to a negligible variation of activation energy for NaV₂O₅.