Page 45 - ILL Annual Report 2019

low
0.5 a)S-phase 0
0 1 2 xz/ yz
xy
b)S*-phase
0 1 2 xz/ yz
xy
−0.5XΓYM ΓZA()XΓYM ΓZA() high
0.5 c)L*-phase 0
0 1 2 xz/ yz
xy
d)T=400K
0 1 2
xy
xz/ yz ΓZ A()
−0.5X Γ Y M
Figure 2
DMFT-calculated intensity maps for electron spectral function as a function of energy and momentum along high-symmetry directions in the Brillouin zone. The orbitally resolved local density of states is shown on the right of each panel. The light grey lines overlay equivalent Density Functional Theory (DFT) calculations. The S-phase is insulating (a), while in the S*-phase the insulating gap is closed and small electron and hole pockets form at the Fermi surface, indicating semi-metallic behaviour (b). The overlap increases significantly in the L*-phase (c), showing similarity with the T = 400 K equilibrium state (d).
Rocking scans of the out-of-plane (006) reflection were performed at ~10 K intervals from 280 K through the magnetic transition TN = 110 K. The process was then repeated without applied current for comparison with
the equilibrium state. D9 uses an area detector; figure 1a and b plots scans conducted at T = 130 K as colour maps that sums the vertical detector range (perpendicular to the scattering plane). The single reflection in equilibrium splits into two reflections under current flow, indicating the presence of two phases. Using a 2D Gaussian least- squares fitting process (depicted as contour lines), the temperature evolution of the c-axis lattice parameter was calculated (figure 1c). It is apparent that the phases
in non-equilibrium are distinct from the equilibrium state (known as the S-phase) and are assigned the shorter c-axis S*-phase and the elongated L*-phase. The S*-phase expresses constant expansion over the equilibrium state that persists to the lowest measured temperatures, while the L*-phase follows a trend similar to the equilibrium L-phase that arises at high temperatures
(T = 400 K) [3].
A precise determination of the atomic positions of the S* and L* phases was achieved using an extensive range of reflections collected at T = 45 K and 130 K and
J = 10 A·cm−2. Compared with equilibrium, the S*-phase atomic positions reveal distinct structural distortions, including a marked decrease in orthorhombicity combined with a relatively small reduction in tetragonal compression;
ΓZ A() X Γ Y M
while the L*-phase continues to share behaviour with the equilibrium L-phase. At the same time, we observed no sign of superstructure reflections, indicating that the equilibrium antiferromagnetic state is fully suppressed under current flow.
To study the sensitivity of the electronic state to these crystallographic distortions, band structure calculations were performed using dynamical mean field theory (DMFT), taking into account spin–orbit coupling and electronic correlations. Figure 2 shows the calculated spectral function and the local density of states near
the Fermi level for the S, S* and L*-phases along with the high-temperature L-phase from the literature [3]. The equilibrium S-phase is Mott-insulating, with a clear gap between the lower and upper Hubbard bands. In the S*-phase, these bands broaden and overlap, releasing hole and electron charge carriers. The overlap is very small, suggesting a semi-metallic state with hole and electron pockets at the Fermi surface. On the other hand, the electronic states in the L*-phase resemble those of the fully metallic, high-temperature L-phase.
These results reveal that the electronic band structure is extremely sensitive to current density through the RuO6 distortions. Furthermore, the conspicuous deviation
of the S*-phase from the usual trend of other metallic phases in equilibrium indicates a unique mechanism for the current-induced state that underlies the strong anomalous diamagnetism.
SCIENTIFIC HIGHLIGHTS
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ω (eV) ω (eV)
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