Page 44 - ILL Annual Report 2019

CHEMISTRY AND CRYSTALLOGRAPHY
Joel Bertinshaw. Australian
Max Planck Institute for Solid State Research, Stuttgart, Germany
‘ My research is focused upon the study of physics that underlie exotic ground states in complex oxides, using neutron and resonant X-ray scattering techniques. We collaborate
to combine detailed experimental studies with advanced theoretical approaches. Crystal and magnetic structural determination is a vital component of our research, making neutron diffraction an indispensable tool.’
A distinct crystallographic structure identified in a non-equilibrium state
Single-crystal diffractometer D9 with in situ applied DC voltage
The exploration of non-equilibrium phenomena in correlated-electron systems is a major frontier in condensed matter research. Recently, it was
shown that the Mott insulator Ca2RuO4 expresses a rare example of a non- equilibrium state stabilised by electric current, involving the transition to an electrically conducting phase with strong diamagnetism. Combining neutron diffraction with dynamical mean field theory, we establish that this phase assumes a distinct crystal structure,
which leads to a semi-metallic state with a partially gapped Fermi surface. This study provides a new basis for theoretical work on the origin of the anomalous diamagnetism and represents a new approach to tuning complex oxides.
Figure 1
a) Single-crystal neutron diffraction rocking scan around the (006) reflection at T = 130 K in the equilibrium state, shown as a map plotting the scattering angle versus the horizontal detector axis.
b) The same measurement under current flow reveals two separate peaks in the non-equilibrium state.
c) The fitted Q position of the reflections was used to calculate the temperature trend of the c-axis lattice parameters. The non-equilibrium S* and L*-phases display a behaviour distinct from the equilibrium S phase, shown in grey. The inset plots the in situ resistance of the two states.
AUTHORS
J. Bertinshaw and B. Keimer (MPI-FKF, Stuttgart, Germany) B.J. Kim (Postech, Pohang, South Korea)
P. Hansmann (MPI-CPFS, Dresden, Germany)
O. Fabelo (ILL)
ARTICLE FROM
Phys. Rev. Lett. (2019)—doi: 10.1103/PhysRevLett.123.137204
REFERENCES
[1] Jain et al., Nat. Phys. (2017) (10.1038/nphys4077)
[2] Sow et al., Science (2017) (10.1126/science.aah4297) [3] Friedt et al., Phys. Rev. B (2001) (10.1103/
PhysRevB.63.174432)
The 4d-electron transition metal oxide Ca2RuO4 represents a convergence of spin–orbit coupling and strongly correlated electron-interaction energies that leads to exciting quantum phenomena [1]. Recently, it was established that under current flow the insulating ground state transforms into a semi-metallic phase with high diamagnetic susceptibility [2]. An important next step in our understanding of this non-equilibrium state is an accurate knowledge of the atomic positions, which would allow ab initio calculations of the electronic structure to be performed. Such information is difficult to obtain in pump-probe experiments; however, here the non-equilibrium phase is maintained by the applied current, offering a rare opportunity to apply neutron diffraction, which is also a direct probe of magnetic structures.
Neutron diffraction measurements were conducted
with a high-quality Ca2RuO4 crystal, prepared using a floating-zone mirror furnace. The crystal was mounted
in such a way that the in situ DC voltage was applied along the c-axis in a two-probe circuit. At T = 280 K, the applied voltage was systematically increased through the transition to the metallic regime before maintaining the current density at J = 10 A·cm−2 for the experiment.
a) c)
b)
ANNUAL REPORT 2019