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Register a new userStrong charge and spin fluctuations in La$_2$O$_3$Fe$_2$Se$_2$
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The electronic structure and magnetic properties of the strongly correlated material La$_2$O$_3$Fe$_2$Se$_2$ are studied by using both the density function theory plus $U$ (DFT+$U$) method and the DFT plus Gutzwiller (DFT+G) variational method. The ground-state magnetic structure of this material obtained with DFT+$U$ is consistent with recent experiments, but its band gap is significantly overestimated by DFT+$U$, even with a small Hubbard $U$ value. In contrast, the DFT+G method yields a band gap of 0.1 - 0.2 eV, in excellent agreement with experiment. Detailed analysis shows that the electronic and magnetic properties of of La$_2$O$_3$Fe$_2$Se$_2$ are strongly affected by charge and spin fluctuations which are missing in the DFT+$U$ method.
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La$_2$O$_3$Fe$_2$Se$_2$ can be explained in terms of Mott localization in sharp contrast with the metallic behavior of FeSe and other parent parent compounds of iron superconductors. We demonstrate that the key ingredient that makes La$_2$O$_3$Fe$_2$Se$_2$ a Mott insulator, rather than a correlated metal dominated by the Hunds coupling is the enhanced crystal-field splitting, accompanied by a smaller orbital-resolved kinetic energy. The strong deviation from orbital degeneracy introduced by the crystal-field splitting also pushes this materials close to an orbital-selective Mott transition. We predict that either doping or uniaxial external pressure can drive the material into an orbital-selective Mott state, where only one or few orbitals are metallized while the others remain insulating.
By using solid-state reactions, we successfully synthesize new oxyselenides CsV$_2$Se$_{2-x}$O (x = 0, 0.5). These compounds containing V$_2$O planar layers with a square lattice crystallize in the CeCr$_2$Si$_2$C structure with the space group of $P4/mmm$. Another new compound V$_2$Se$_2$O which crystallizes in space group $I4/mmm$ is fabricated by topochemical deintercalation of cesium from CsV$_2$Se$_2$O powder with iodine in tetrahydrofuran(THF). Resistivity measurements show a semiconducting behavior for CsV$_2$Se$_2$O, while a metallic behavior for CsV$_2$Se$_{1.5}$O, and an insulating feature for V$_2$Se$_2$O. A charge- or spin-density wave-like anomaly has been observed at 168 K for CsV$_2$Se$_2$O and 150 K for CsV$_2$Se$_{1.5}$O, respectively. And these anomalies are also confirmed by the magnetic susceptibility measurements. The resistivity in V$_2$Se$_2$O exhibits an anomalous log(1/$T$) temperature dependence, which is similar to the case in parent phase or very underdoped cuprates indicating the involvement of strong correlation. Magnetic susceptibility measurements show that the magnetic moment per V-site in V$_2$Se$_2$O is much larger than that of CsV$_2$Se$_{2-x}$O, which again suggests the correlation induced localization effect in the former.
Semiconductor Bi$_2$O$_2$Se nanolayers of high crystal quality have been realized via epitaxial growth. These two-dimensional (2D) materials possess excellent electron transport properties with potential application in nanoelectronics. It is also strongly expected that the 2D Bi$_2$O$_2$Se nanolayers could be of an excellent material platform for developing spintronic and topological quantum devices, if the presence of strong spin-orbit interaction in the 2D materials can be experimentally demonstrated. Here, we report on experimental determination of the strength of spin-orbit interaction in Bi$_2$O$_2$Se nanoplates through magnetotransport measurements. The nanoplates are epitaxially grown by chemical vapor deposition and the magnetotransport measurements are performed at low temperatures. The measured magnetoconductance exhibits a crossover behavior from weak antilocalization to weak localization at low magnetic fields with increasing temperature or decreasing back gate voltage. We have analyzed this transition behavior of the magnetoconductance based on an interference theory which describes the quantum correction to the magnetoconductance of a 2D system in the presence of spin-orbit interaction. Dephasing length and spin relaxation length are extracted from the magnetoconductance measurements. Comparing to other semiconductor nanostructures, the extracted relatively short spin relaxation length of ~150 nm indicates the existence of strong spin-orbit interaction in Bi$_2$O$_2$Se nanolayers.
I use first principles calculations to investigate the thermal conductivity of $beta$-In$_2$O$_3$ and compare the results with that of $alpha$-Al$_2$O$_3$, $beta$-Ga$_2$O$_3$, and KTaO$_3$. The calculated thermal conductivity of $beta$-In$_2$O$_3$ agrees well with the experimental data obtain recently, which found that the low-temperature thermal conductivity in this material can reach values above 1000 W/mK. I find that the calculated thermal conductivity of $beta$-Ga$_2$O$_3$ is larger than that of $beta$-In$_2$O$_3$ at all temperatures, which implies that $beta$-Ga$_2$O$_3$ should also exhibit high values of thermal conductivity at low temperatures. The thermal conductivity of KTaO$_3$ calculated ignoring the temperature-dependent phonon softening of low-frequency modes give high-temperature values similar that of $beta$-Ga$_2$O$_3$. However, the calculated thermal conductivity of KTaO$_3$ does not increase as steeply as that of the binary compounds at low temperatures, which results in KTaO$_3$ having the lowest low-temperature thermal conductivity despite having acoustic phonon velocities larger than that of $beta$-Ga$_2$O$_3$ and $beta$-In$_2$O$_3$. I attribute this to the fact that the acoustic phonon velocities at low frequencies in KTaO$_3$ is less uniformly distributed because its acoustic phonon branches are more dispersive compared to the binary oxides, which causes enhanced momentum loss even during the normal phonon-phonon scattering processes. I also calculate thermal diffusivity using the theoretically obtained thermal conductivity and heat capacity and find that all four materials exhibit the expected $T^{-1}$ behavior at high temperatures. Additionally, the calculated ratio of the average phonon scattering time to Planckian time is larger than the lower bound of 1 that has been observed empirically in numerous other materials.
Bi$_2$O$_2$Se is a promising material for next-generation semiconducting electronics. It exhibits premature metallicity on the introduction of a tiny amount of electrons, the physics behind which remains elusive. Here we report on transport and dielectric measurements in Bi$_2$O$_2$Se single crystals at various carrier densities. The temperature-dependent resistivity ($rho$) indicates a smooth evolution from the semiconducting to the metallic state. The critical concentration for the metal-insulator transition (MIT) to occur is extraordinarily low ($n_textrm{c}sim10^{16}$ cm$^{-3}$). The relative permittivity of the insulating sample is huge ($epsilon_textrm{r}approx155(10)$) and varies slowly with temperature. Combined with the light effective mass, a long effective Bohr radius ($a_textrm{B}^*approx36(2)$ $textrm{nm}$) is derived, which provides a reasonable interpretation of the metallic prematurity according to Motts criterion for MITs. The high electron mobility ($mu$) at low temperatures may result from the screening of ionized scattering centers due to the huge $epsilon_textrm{r}$. Our findings shed light on the electron dynamics in two dimensional (2D) Bi$_2$O$_2$Se devices.
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