Summary of Workshop talks
Prof. M McMahon (The University of Edinburgh) Low Temperature Powder and Single Crystal Studies of Elements
This talk presented structural data on (mainly single crystalline) light elements in the temperature range above 15 K to 293 K up 100 GPa. The work on sulphur and nitrogen are recent examples which highlight the importance of the availability of a cryostat at the beam line – to grow the crystals and to perform the experiment in the required T-p area of the phase diagram.
The measurements of the structure factor of amorphous sulphur (a-S), a high pressure modification of sulphur, were performed in the temperature and pressure range of 40 K to 175 K and 50GPa to 100 GPa, respectively. These data yielded the structure factor, the radial distribution functions and the densities and showed that two regions of two amorphous forms of a-S exist. The access to low temperature was not only needed for the experiment but also for growing the starting phase at low temperature.
The study of the disordered state of single crystalline nitrogen in the delta-phase, which exists at temperatures of the order of 100 K and at moderate pressures (approx 1 GPa – 20 GPa) revealed a new delta*-phase which is partially ordered.
Dr M de Vries, (University of St. Andrews), Resonant x-ray diffraction from electronic phase separations in quantum phase transitions
Pressure can tune strongly-correlated electron systems to a quantum phase transition. Thus, at a critical pressure the long-range magnetic order is suppressed to zero temperature and quantum fluctuations can lead to novel phases, like non-BCS superconductivity or a partially ordered state. UGe2 and MnSi were presented as examples for these low-temperature phases induced by pressure.
UGe2 is a ferromagnetically ordered heavy Fermion compound where pressure induces superconductivity in a narrow pressure range just before the Curie temperature (~53 K at p=0) is suppressed in a first order phase transition at 1.6GPa. µ-SR-data under pressure revealed that the pressure-induced maximum of the critical temperature (~1K at about 1GPa) is related to a change of the ferromagnetic order. The phase line Tx(p) between the two ordered states might be related to a structural change. Structural data taken at 4 K or below for pressures up to 2 GPa could clarify the relation between magnetism and structure and give further insight in the unconventional superconducting state.
A partial ordered state in MnSi is established at a first order transition at 1.4 GPa when the magnetic order (TC=29K at p=0) is pressure driven to zero temperature. As in the case of UGe2 the type of the phase transition (first order) allows a phase separation to take place in the vicinity of the critical pressure. Therefore, it is highly desirable to investigate if the structural parameters are affected by this.
Dr E Pugh (University of Cambridge) High Pressure, Low Temperature Studies of Unconventional Superconductivity and Novel Quantum Order
This talk gave an overview of the equipment (various types pressure cells) used in detecting of unconventional superconductivity (Tc of the order of 2 K) and novel quantum order in Ce-based heavy Fermion compounds. The results on ZrZn2 showed clearly that the sample preparation (growth and cutting) is crucial for the development of unconventional superconductivity. The need of structural data in the T-p field of the phase diagram on preselected samples for this type of compounds was emphasised. A combined measurement using x-rays and e.g. a magnetic probe could achieve this within one experiment.
Prof. M Abd-Elmeguid (University Cologne, Germany)
Recent examples of pressure-induced phase transitions in strongly correlated electron systems It was shown that the interplay between charge-, spin-, and orbital- degrees of freedom in Mott-insulators can lead to unconventional metallic ground states. Charge ordering and low-spin to high-spin transitions are coupled to the lattice and can be tuned by external pressure. The series of intermetallic compounds RNiO3 (R=Rare Earth element) provides the possibility to investigate the pressure- induced transition from an insulating charge-and magnetically state to a metallic state. In the case of LuNiO3, one observes a pressure-induced metallic magnetic state coexists with charge ordering. Other examples like EuNiO3 showed that magnetism was suppressed at a first order transition at about 11 GPa. It is of high importance to investigate if these dramatic changes in the electronic ground state are drive by a structural transition a low temperature.
Unconventional superconductivity was induced by applying pressure to a new class of Mott-insulating systems GaNb4S8, GaNb4Se8, and GaTa4Se8 (Tc ~5K at p>10GPa). In these compounds the electronic conduction originates from hopping of localized unpaired electrons among widely separated Nb4 or Ta4 clusters. High-pressure single-crystal X-ray studies of GaTa4Se8 at ambient temperature reveal that the superconducting transition is connected with a gradual decrease of the MX6 (X=Se, S) octahedral distortion and a simultaneous softening of the phonon associated with M-X bonds. Furthermore, low-temperature diffraction data at ambient pressure reveal a structural phase transition (cubic to tetragonal) at 31K. It is assumed that this leads to a non-collinear antiferromagnetic spin structure. This results in a compensating of the spin components, which is in agreement with the loss in magnetic susceptibility below 30K. It was speculated that this structural transition could be suppressed by pressure to zero temperature, being a unique example zero-temperature structural transition, which is intimately connected with enhanced electronic correlations in the proximity of the pressure-induced superconductivity.