Research

General introduction

Our research focuses on 'quantum complexity', that is, the quantum properties of systems with many degrees of freedom. The number of 'degrees of freedom' in a system is roughly the same as the number of independent sub-units - bees in a bee-hive, neurons in the brain, atoms in a crystal... Systems with many degrees of freedom abound in nature - indeed, any macroscopic object fits the description; many of these systems, such as the human body, the brain, or the modern economy, exhibit complex behaviour in the form of 'emergent' properties, by which we mean properties that you would not easily predict based on the study of the behaviour of an isolated sub-unit. For example, from the study of one neuron, one cannot easily predict the phenomenon of human conciousness. We know that this is true, because neurons are quite well understood, but how the human mind works is not well understood at all, because it has something to do with the interaction of enormous numbers of neurons. A good place to read further about his topic is P.W. Anderson's introduction to the book More is Different.

The special twist in our approach to complexity is the quantum aspect. In order to see quantum effects in many-body systems, they have to be very cold because temperature is really a kind of random noise that wipes out subtle quantum effects. It also helps a lot if the objects under study are very light, so we study electrons in metals, oxides, and insulators.

The closest thing in our field to the human consciousness problem is the question of the origin of high-temperature superconductivity. This is a very important scientific and technical issue. It is clear that the superconductivity originates from some novel interaction between the electrons, but while the behaviour of isolated electrons is extremely well understood, despite intense experimental and theoretical research since 1986 (the year in which high-Tc superconductivity was discovered), the way that the superconductivity emerges from the collective behaviour of huge numbers of electrons remains elusive.

But there are several other problems in this field that we are working on, including new ordered phases of metals at very low temperatues, and the limits of using a language appropriate to 'particles', to think about conduction in some metals.

Overview for experts

In our laboratory we are studying a number of crystalline metals and oxides, including ruthenates, cuprates, vanadates, and heavy fermion metals such as CeCoIn5 and YbRh2Si2. Our primary experimental methods are: quantum oscillations (in particular the de Haas van Alphen effect); magnetotransport; and high-pressure applied using clamp cells or anvil cells.

A particular focus of our research is on quantum phase transitions, but we are also interested in general issues of the electronic structure of metals, and incoherent electronic transport.

Construction of the laboratory was begun in September 2004; in June 2006 we began taking data on our 18 tesla magnet/10 mK dilution refrigerator system (pictured here). After a few teething problems, and installation of low temperature electronics and a rotation stage, we began our first proper measurements in 2008 (high pressure study of Sr3Ru2O7), followed by de Haas-van Alphen studies of some heavy fermion metals. Our pressure work employs clamp cells (up to 1.8 GPa in non-magnetic clamp cells), anvil cells (up to 15 GPa as of summer 2010) and, most recently, indenter cells (over 30 kbar as of summer 2010). You can read more about our high pressure work here.

Dilution refrigerator

Further details of our research can be inferred via the publications link, above left. Here is a list of selected publications on which Stephen Julian is a co-author:

Significant publications

  • P.M.C. Rourke, A. McCollam, I. Swainson, D.G. Rancourt and S.R. Julian, Magnetic field dependence of the YbRh2Si2 Fermi surface Physical Review Letters 101 237205 (2008)
  • S.R. Julian and M.R. Norman, High temperature superconductivity -- Local pairs and small surfaces Nature 447 537 (2007)
  • R. Daou, C. Bergemann and S.R. Julian, Continuous evolution of the Fermi surface of CeRu2Si2 across the metamagnetic transition Phys. Rev. Lett. 96, 026401 (2006)
  • A. McCollam, S.R. Julian, P.M.C. Rourke, J. Flouquet and D. Aoki, Anomalous quantum oscillations in CeCoIn5, Phys. Rev. Lett. 94, 186401 (2005)
  • C. Bergemann, A.P. Mackenzie, S.R. Julian, D. Forsythe and E. Omichi, Quasi-two-dimensional Fermi liquid properties of the unconventional superconductor Sr2RuO4, Advances in Physics 52 (2003) 639-725.
  • N. Doiron-Leyraud, I.R. Walker, L. Taillefer, M.J. Steiner, S.R. Julian and G.G. Lonzarich, Fermi-liquid breakdown in the paramagnetic phase of a pure metal, Nature 425 (2003) 595-599.
  • F. Nakamura, T. Goko, M. Ito, T. Fujita, S. Nakatsuji, H. Fukazawa, Y. Maeno, P. Alireza, D. Forsythe and S.R. Julian, From Mott insulator to ferromagnetic metal: A pressure study of Ca2RuO4, Phys. Rev. B 65 (2002) art. no. 220402.
  • I.H. Inoue, C. Bergemann, I. Hase and S.R. Julian, Fermi surface of 3d(1) perovskite CaVO3 near the Mott transition, Phys. Rev. Lett. 88 (2002) art. no. 236403.
  • C. Pfleiderer, S.R. Julian and G.G. Lonzarich, Non-Fermi-liquid nature of the normal state of itinerant-electron ferromagnets, Nature 414 (2001) 427-430.
  • S.A. Grigera, R.S. Perry, A.J. Schofield, M. Chiao, S.R. Julian, G.G. Lonzarich, S.I. Ikeda, Y. Maeno, A.J. Millis and A.P. Mackenzie, Magnetic field-tuned quantum criticality in the metallic ruthenate Sr3Ru2O7, Science 294 (2001) 329-332.
  • S.S. Saxena, P. Agarwal, K. Ahilan, F.M. Grosche, R.K.W. Haselwimmer, M.J. Steiner, E. Pugh, I.R. Walker, S.R. Julian, P. Monthoux, G.G. Lonzarich, A. Huxley, I. Sheikin, D. Braithweite and J. Flouquet, Superconductivity on the border of itinerant electron ferromagnetism in UGe2, J. Magn. Magn. Mater. 226 (2001) 45-47.
  • C. Bergemann, S.R. Julian, A.P. Mackenzie, S. Nishizaki and Y. Maeno, Detailed topography of the Fermi surface of Sr2RuO4, Phys. Rev. Lett. 84 (2000) 2662-2665.
  • N.D. Mathur, F.M. Grosche, S.R. Julian, I.R. Walker, D.M. Freye, R.K.W. Haselwimmer and G.G. Lonzarich, Magnetically mediated superconductivity in heavy fermion compounds, Nature 394 (1998) 39-43.
  • A. P. Mackenzie, S. R. Julian, A. J. Diver, G. J. McMullan, M. P. Ray, G. G. Lonzarich, Y. Maeno, S. Nishizaki and T. Fujita, Quantum oscillations in the layered perovskite superconductor Sr2RuO4, Phys. Rev. Lett. 76 (1996) 3786-3789.
  • A.P. Mackenzie, S.R. Julian, G.G. Lonzarich, A. Carrington, S.D. Hughes, R.S. Liu and D.C. Sinclair, Resistive upper critical field of Tl2Ba2CuO6 at low temperatures and high magnetic fields, Physical Review Letters 71 (1993) 1238-1241.

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