456 research outputs found

### Topological Quantum Phase Transition in 5$d$ Transition Metal Oxide Na$_2$IrO$_3$

We predict a quantum phase transition from normal to topological insulators
in the 5$d$ transition metal oxide Na$_2$IrO$_3$, where the transition can be
driven by the change of the long-range hopping and trigonal crystal field
terms. From the first-principles-derived tight-binding Hamiltonian we determine
the phase boundary through the parity analysis. In addition, our
first-principles calculations for Na$_2$IrO$_3$ model structures show that the
interlayer distance can be an important parameter for the existence of a
three-dimensional strong topological insulator phase. Na$_2$IrO$_3$ is
suggested to be a candidate material which can have both a nontrivial topology
of bands and strong electron correlations

### Interplay between spin-orbit coupling and van Hove singularity in the Hund's metallicity of Sr$_2$RuO$_4$

We investigate the dynamical properties of Sr$_2$RuO$_4$ at zero and very low
temperature using density functional theory plus dynamical mean-field theory
with an exact diagonalization solver. By considering rotationally invariant
local interaction, we examine how Hund's coupling and spin-orbit coupling
affect the correlated nature of the system. In the absence of Hund's coupling,
the system shows a Fermi liquid behavior over the entire range of temperatures
we consider. We confirm that the Fermi liquid persists at zero temperature even
with nonzero Hund's coupling; however, at sufficient temperatures Hund's
coupling significantly reduces the Fermi liquid regime and the system evolves
into a typical Hund's metal. At the bare electronic occupancy of Sr$_2$RuO$_4$
($t_{2g}^4$), a stronger Hund's metallicity accompanies a larger long-time
correlator. Remarkably, electron doping further destabilizes the Fermi liquid
even though the long-time correlator and magnetic fluctuations decrease upon
doping. This suppression of the Fermi liquid is driven by the van Hove
singularity above the Fermi level in Sr$_2$RuO$_4$, combined with an enhanced
Van Vleck susceptibility by spin-orbit coupling. Such findings point to the
important role that electronic structure plays in the behavior of Hund's
metals, in addition to magnetic fluctuations.Comment: 7 pages, 4 figure

### Fundamental thickness limit of itinerant ferromagnetic SrRuO$_3$ thin films

We report on a fundamental thickness limit of the itinerant ferromagnetic
oxide SrRuO$_3$ that might arise from the orbital-selective quantum confinement
effects. Experimentally, SrRuO$_3$ films remain metallic even for a thickness
of 2 unit cells (uc), but the Curie temperature, T$_C$, starts to decrease at 4
uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T$_C$
decrease to a decrease in the density of states (N$_o$). Namely, in the thin
film geometry, the hybridized Ru-d$_yz,zx$ orbitals are terminated by top and
bottom interfaces, resulting in quantum confinement and reduction of N$_o$.Comment: 20 pages, 4 figure

### Orbital-selective confinement effect of Ru $4d$ orbitals in SrRuO$_3$ ultrathin film

The electronic structure of SrRuO$_3$ thin film with thickness from 50 to 1
unit cell (u.c.) is investigated via the resonant inelastic x-ray scattering
(RIXS) technique at the O K-edge to unravel the intriguing interplay of orbital
and charge degrees of freedom. We found that orbital-selective quantum
confinement effect (QCE) induces the splitting of Ru $4d$ orbitals. At the same
time, we observed a clear suppression of the electron-hole continuum across the
metal-to-insulator transition (MIT) occurring at the 4 u.c. sample. From these
two clear observations we conclude that QCE gives rise to a Mott insulating
phase in ultrathin SrRuO$_3$ films. Our interpretation of the RIXS spectra is
supported by the configuration interaction calculations of RuO$_6$ clusters.Comment: 7 pages, 7 figure

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