Squeeze electrons by diamond anvils: Discovery of a novel confined metal at high pressure - Dr. Yang Ding

MAY 25, 2016

Insulator-metal transition (IMT), where a system changes from insulator to metal with a largely enhanced electrical conductivity, represents an important topic in contemporary condensed matter physics. New study led by scientist Dr. Yang Ding from Center for High Pressure Science and Technology (HPSTAR) found that the material Sr3Ir2O7 undergoes an IMT and becomes a confined metal at high pressure, showing metallicity in the crystal ab-plane but insulating along the c-axis. Such unusual phenomenon resembles the strange metal phase in cuprate high-temperature superconductors. This novel discovery opens up a new field for synthesizing functional materials. The work is published this week in Physical Review Letters.

5d transition-metal oxides are new scientific frontiers for studying exotic states of matter emerging from the interplay of spin-orbit coupling, electron interaction, and crystal field effect. Sr3Ir2O7 is a compound belonging to the Ruddlesden-Popper perovskite iridates Srn+1IrnO3n+1 (where n is the number of SrIrO3 perovskite layers between extra SrO layers), and it is insulating at ambient condition. The relatively low electrical resistivity and small charge gap of Sr3Ir2O7 suggest that the material can undergo a potential IMT upon external perturbation, such as carrier doping, magnetic field, or external pressure. Among these, pressure has been found to be a unique and clean tool in tuning a system’s interatomic distances and atomic arrangements.

In a recent step towards this direction, Ding and co-workers have performed electric resistance and resonant inelastic x-ray scattering measurements on single-crystal Sr3Ir2O7 combined with diamond anvil cell techniques. The resistance measurements indicate an IMT occurring around 59 GPa, and the high-pressure phase of Sr3Ir2O7 exhibits a novel confinement phenomenon: a metallic behavior within the ab-plane but an insulating one along the c-axis.

“Our discovery of a novel high-pressure metallic phase with intriguing confinement phenomenon similar to that found in over-doped cuprate superconductors suggests that superconductivity could potentially be found in doped Sr3Ir2O7 under high pressure,” stated in the paper.

This intriguing but somewhat unexpected phenomenon is found for the first time under high pressure. Because the x-ray measurements show robust spin-orbit and electron Coulomb interactions within the investigated pressure range, this newly-discovered transition from insulator to confined metal is potentially driven by a first-order structure change at a nearby pressure as reported by recent diffraction measurements.

“Pressure shows a distinct effect compared to doping or dimensionality-change, thereby pointing to a new way of synthesizing or finding novel quantum states of matter that is inaccessible with other methods,” remarked Dr. Mao, the director of HPSTAR.

Caption: Schematics of the experiment.

“We will continue the study to even lower temperature and higher pressure conditions to investigate the possibility of metal-superconductor transition in perovskite iridates. We also will systematically study other 5d transition-metal compounds to investigate their electronic properties and discover exotic interesting physical properties,” said Dr. Yang Ding, the lead author and a staff scientist at HPSTAR and HPSynC, who was also a former staff scientist of Advanced Photon Source (APS), Argonne National Laboratory.

The other co-authors include L. Yang, Z. Zeng, and H. K. Mao of HPSTAR and Geophysical Laboratory; C.-C. Chen of APS and University of Alabama; H.-S. Kim and M. J. Han of Korea Advanced Institute of Science and Technology; W. Luo of University of Uppsala; Z. Feng, M. Upton, D. Casa, J. Kim, and T. Gog of APS; G. Cao of University of Kentucky; M. Veenendaal of APS and Northern Illinois University.