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Register a new userEffect of microstructure on the electronic transport properties of epitaxial CaRuO$_3$ thin films
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We have carried out extensive comparative studies of the structural and transport properties of CaRuO$_3$ thin films grown under various oxygen pressure. We find that the preferred orientation and surface roughness of the films are strongly affected by the oxygen partial pressure during growth. This in turn affects the electrical and magnetic properties of the films. Films grown under high oxygen pressure have the least surface roughness and show transport characteristics of a good metal down to the lowest temperature measured. On the other hand, films grown under low oxygen pressures have high degree of surface roughness and show signatures of ferromagnetism. We could verify that the low frequency resistance fluctuations (noise) in these films arise due to thermally activated fluctuations of local defects and that the defect density matches with the level of disorder seen in the films through structural characterizations.
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We study the magneto-optical Kerr effect (MOKE) in SrRuO$_3$ thin films, uncovering wide regimes of wavelength, temperature, and magnetic field where the Kerr rotation is not simply proportional to the magnetization but instead displays two-component behavior. One component of the MOKE signal tracks the average magnetization, while the second anomalous component bears a resemblance to anomalies in the Hall resistivity which have been previously reported in skyrmion materials. We present a theory showing that the MOKE anomalies arise from the non-monotonic relation between the Kerr angle and the magnetization, when we average over magnetic domains which proliferate near the coercive field. Our results suggest that inhomogeneous domain formation, rather than skyrmions, may provide a common origin for the observed MOKE and Hall resistivity anomalies.
We have prepared high-quality epitaxial thin films of CaRuO$_3$ with residual resistivity ratios up to 55. Shubnikov-de Haas oscillations in the magnetoresistance and a $T^2$ temperature dependence in the electrical resistivity only below 1.5 K, whose coefficient is substantially suppressed in large magnetic fields, establish CaRuO$_3$ as a Fermi liquid (FL) with anomalously low coherence scale. Non-Fermi liquid (NFL) $T^{3/2}$ dependence is found between 2 and 25 K. The high sample quality allows access to the intrinsic electronic properties via THz spectroscopy. For frequencies below 0.6 THz, the conductivity is Drude-like and can be modeled by FL concepts, while for higher frequencies non-Drude behavior, inconsistent with FL predictions, is found. This establishes CaRuO$_3$ as a prime example of optical NFL behavior in the THz range.
The anomalous charge transport observed in some strongly correlated metals raises questions as to the universal applicability of Landau Fermi liquid theory. The coherence temperature $T_{FL}$ for normal metals is usually taken to be the temperature below which $T^2$ is observed in the resistivity. Below this temperature, a Fermi liquid with well-defined quasiparticles is expected. However, metallic ruthenates in the Ruddlesden-Popper family, frequently show non-Drude low-energy optical conductivity and unusual $omega/T$ scaling, despite the frequent observation of $T^2$ dc resistivity. Herein we report time-domain THz spectroscopy measurements of several different high-quality metallic ruthenate thin films and show that the optical conductivity can be interpreted in more conventional terms. In all materials, the conductivity has a two-Drude peak lineshape at low temperature and a crossover to a one-Drude peak lineshape at higher temperatures. The two-component low-temperature conductivity is indicative of two well-separated current relaxation rates for different conduction channels. We discuss three particular possibilities for the separation of rates: (a) Strongly energy-dependent inelastic scattering; (b) an almost-conserved pseudomomentum operator that overlaps with the current, giving rise to the narrower Drude peak; (c) the presence of multiple conduction channels that undergoes a crossover to stronger interband scattering at higher temperatures. None of these scenarios require the existence of exotic quasiparticles. The results may give insight into the possible significance of Hunds coupling in determining interband coupling in these materials. Our results also show a route towards understanding the violation of Matthiessens rule in this class of materials and deviations from the Gurzhi scaling relations in Fermi liquids.
We report molecular beam epitaxy growth of Sr-doped Bi$_2$Se$_3$ films on (111) BaF$_2$ substrate, aimed to realize unusual superconducting properties inherent to Sr$_x$Bi$_2$Se$_3$ single crystals. Despite wide range of the compositions, we do not achieve superconductivity. To explore the reason for that we study structural, morphological and electronic properties of the films and compare them to the corresponding properties of the single crystals. The dependence of the c-lattice constant in the films on Sr content appears to be more than an order of magnitude stronger than in the crystals. Correspondingly, all other properties also differ substantially, indicating that Sr atoms get different positions in lattices. We argue that these structural discrepancies come from essential differences in growth conditions. Our research calls for more detailed structural studies and novel growth approaches for design of superconducting Sr$_x$Bi$_2$Se$_3$ thin films.
We report that the {pi}-electrons of graphene can be spin-polarized to create a phase with a significant spin-orbit gap at the Dirac point (DP) using a graphene-interfaced topological insulator hybrid material. We have grown epitaxial Bi2Te2Se (BTS) films on a chemical vapor deposition (CVD) graphene. We observe two linear surface bands both from the CVD graphene notably flattened and BTS coexisting with their DPs separated by 0.53 eV in the photoemission data measured with synchrotron photons. We further demonstrate that the separation between the two DPs, {Delta}D-D, can be artificially fine-tuned by adjusting the amount of Cs atoms adsorbed on the graphene to a value as small as {Delta}D-D = 0.12 eV to find any proximity effect induced by the DPs. Our density functional theory calculation shows a spin-orbit gap of ~20 meV in the {pi}-band enhanced by three orders of magnitude from that of a pristine graphene, and a concomitant phase transition from a semi-metallic to a quantum spin Hall phase when {Delta}D-D $leq$ 0.20 eV. We thus present a practical means of spin-polarizing the {pi}-band of graphene, which can be pivotal to advance the graphene-based spintronics.
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