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| Cavendish Laboratory > Research > Quantum Matter > |
Quantum Matter Links
Quantum Matter Home |
 Research Themes in the Quantum Matter Group
OverviewOur work focusses on two major themes in condensed matter research. The first is understanding the nature of quantum order in itinerant-electron systems on the border of magnetism at low temperatures. In recent years this area of research has led to the study of novel metallic states not described by Fermi liquid theory, anisotropic types of electron-electron and electron-hole pair condensates, and quantum critical phenomena. The second theme is studying the physics of novel superconducting materials, such as the high-Tc superconductors, MgB2, graphite intercalates and the ruthenates. The following sections give a more in-depth look at some of these areas, as well an overview of our experimental infrastructure.
Exotic States of Matter on the Edge of MagnetismThe crossover from ferromagnetism or anti-ferromagnetism to paramagnetism as a function of a 'quantum' control parameter such as pressure has turned out to be a much more subtle and interesting problem than was originally imagined in the early days of our field.
On the border of long-range magnetic order one expects several quantum states of more or less exotic nature to be nearly degenerate. Minute metallurgical or environmental changes can induce transitions between these states and give rise to complex behaviour that, in extreme cases, could be called 'adaptive'. Our research has focused on the investigation of the temperature-pressure phase diagram on the border of magnetism in relatively simple stoichiometric compounds of high purity in which well-defined forms of quantum order might be observed. One such example is the ferromagnet UGe2, shown at left. Upon applying hydrostatic pressure in a pressure cell, the magnetic transition temperature TCurie, is suppressed towards zero, revealling a superconducting state at low temperatures. Superconductivity in a ferromagnetically ordered system is exotic and rare, and was thought impossible many decades. In this material there is also evidence that electrons pair up in an unconventional way, with spins aligned in the same direction forming an unusual 'spin-triplet' state.
The experimental infrastructure in the quantum matter group offers access to extreme environments. We have low temperature
probes capable of achieving base temperatures of 2 mK in an 18 Tesla magnetic field, and diamond anvil cells capable of pressures
higher that 100 kBar. We are actively involved in developing cryogenic technologies using the technique of adiabatic demagnetisation,
as well as developing methods for reliabley achieving high pressures.
An example of a diamond anvil cell constructed by Sam Brown in our
lab is shown at left.
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After almost a century of research into superconductivity, this fascinating phenomena
continues to yield surprises. One example is the recently discovered graphite based
superconductors C6Yb and C6Ca (see figure at right), which are formed by intercalating
Ytterbium or Calcium between sheets of Carbon atoms. These compounds superconduct
at temperatures an order of magnitude higher than previous generation of intercalated superconductors, and
have sparked debate as to the cause. Our group was involved in
the discovery of these new materials, and there is an ongoing effort to characterize and study
these and other graphite based superconductors.
The high Tc superconductors are the class of materials with the highest recorded transition temperatures,
on the order of 100 K and above. Discovered over 20 years ago, there remain a number of unanswered questions
about these compounds. One of the key mysteries surrounds the identity of the so-called 'pseudogap' region of the
phase diagram (see figure at right). In this regime there exists a gap like feature observed in many physical
quantities, yet there is no obvious signs of an ordered phase. Some of the research in the group has been
directed at studying the specific heat of the high Tc's in the pseudogap region of the phase diagram.
A second intriguing questions is surrounds how the high-Tc's evolve with charge carrier
concentration (doping), moving from
an antiferromagnetically ordered insulator at low doping, to an unconventional superconductor and eventually
a Fermi liquid metal at higher doping. At low dopings, there is evidence for a 'glassy' behaviour of electonic spins,
and part of our research effort is directed towards exploring this region of the phase diagram through
muon-spin relaxation, transport and dielectric measurements.
The simplest ferroelectric materials turn out to provide a 'text-book' example of quantum criticality in the solid state. Ferroelectrics are materials that when cooled below their phase transition temperature Tc, undergo breaking of lattice inversion symmetry and spontaneously develop a macroscopic polarization. Pressure can be used as a 'quantum tuning parameter', as shown in the first figure, to suppress Tc towards absolute zero. In the region where the temperature scale is much less than the Debye temperature of the lowest transverse-optic soft mode, quantum rather than classical statistical mechanics are required to understand the observed behaviour. In this (yellow) region, order-parameter fluctuations are typically found to exist in a four-dimensional space and the lattice is observed to fluctuate in a wildly unconventional form.
