High Pressure Lab

Group members: Wenlong Wu, Fazel Tafti, Dan Sun, Di Tian

We use high pressures to modify the properties of materials in a highly controlled way. By changing the lattice parameter of a solid we can gradually modify the electron wave-functions (normally increasing the hopping integrals). We use this in two ways: firstly to try to understand interesting states that have been discovered at ambient pressure, and secondly to search for interesting states (such as superconductivity) by 'tuning' the properties of a given material, for example across a metal-insulator transition, or a magnetic-non-magnetic phase boundary.

In the first category is our work on the bilayer ruthenate Sr₃Ru₂O₇, which shows a novel nematic state near its metamagnetic transition (Grigera 2004). We are both trying to induce this nematic state by tuning with pressure (Wu 2010, submitted to Physical Review B), and to modify the phase diagram of the known nematic state with pressure. We are also using pressure to try to understand the cross-over from incoherent to coherent transport in metals that are close to a Mott transition, such as FeCrAs and some frustrated spin systems, in collaboration with researchers at ISSP Tokyo.

This second application (searching for interesting new states) is a bit like poor-man's materials science: in contrast to the traditional condensed-matter approach of using chemistry to make new materials with interesting and useful new properties, we try to use pressure to discover interesting and useful properties in existing materials. In previous work we have used this approach to discover new superconducting states at very low temperatures, on the border of magnetic order (eg Julian 1996, Mathur 1998), but now we are looking for new physics associated with metal-insulator transitions in oxides, which requires much higher pressures.

Using moissanite (single crystal silicon-carbide) and diamond anvil cells (pictured above), we have so-far measured resistivity at pressures of up to 10 GPa (100 kbar), and our latest cells go to well above 15 GPa. This is sufficient to significantly modify the properties of many materials, and to induce metal-insulator transitions in some oxides with many-body insulating states. The picture on the right shows a sample of FeCrAs, with four wires attached, in the 0.6 mm hole in an anvil cell gasket, at a pressure about 6 GPa.

References:

Grigera 2006: S.A. Grigera, P. Gegenwart, R.A. Borzi, F. Weickert, A.J. Schofield, R.S. Perry, T. Tayama, T. Sakakibara, Y. Maeno, A.G. Green and A.P. Mackenzie, Science 206 (2004) 1154.

Julian 1996: S.R. Julian, C. Pfleiderer, F.M. Grosche, N.D. Mathur, G.J. McMullan, A.J. Diver, I.R. Walker and G.G. Lonzarich, J. Phys.-Cond. Mat. 8 (1996) 9675.

Mathur 1998: N.D. Mathur, F.M. Grosche, S.R. Julian, I.R. Walker, D.M. Freye, R.K.W. Haselwimmer and G.G. Lonzarich, Nature 394 (1998) 39.

Wu 2010: W. Wu, A. McCollam, S.A. Grigera, R.S. Perry, A.P. Mackenzie and S.R. Julian, subitted to Phys. Rev. B, arXiv: