Electronic properties across metal-insulator transition in \beta-pyrochlore-type CsW2O6 epitaxial films

In CsW2O6, which undergoes a metal-insulator transition (MIT) at 213 K, the emergence of exotic properties associated with rattling motion of Cs is expected owing to its characteristic \beta-pyrochlore-type structure. However, a hurdle for crystal growth hampers elucidation of detailed properties and mechanisms of the MIT. Here we report on the epitaxial growth of \beta-pyrochlore-type CsW2O6 films and their electronic properties across the MIT. Using pulsed-laser deposi-tion technique, we grew single-crystalline CsW2O6 films exhibiting remarkably lower resistivity compared with a poly-crystalline bulk and sharp MIT around 200 K. Negative magnetoresistance and positive Hall coefficient were found, which became pronounced below 200 K. The valence-band and core-levels photoemission spectra indicated the drastic changes across the MIT. In the valence band photoemission spectrum, the finite density of states was observed at the Fermi level in the metallic phase. In contrast, an energy gap appeared in the insulating phase. The split of W 4f core-level spectrum suggested the charge disproportionation of W5+ and W6+ in the insulating phase. The change of spectral shape in the Cs 4d core levels reflected the rattling motion of Cs+ cations. These results strongly suggest that CsW2O6 is a novel material, in which MIT is driven by the charge disproportionation associated with the rattling motion.Comment: 8 pages, 6 figure

Growth of Antiperovskite Oxide Ca3SnO Films by Pulsed Laser Deposition

We report the epitaxial growth of Ca3SnO antiperovskite oxide films on (001)-oriented cubic yttria-stabilized zirconia (YSZ) substrates by using a conventional pulsed laser deposition (PLD) technique. In this work, a sintered Ca3SnO pellet is used as the ablation target. X-ray diffraction measurements demonstrate the (001) growth of Ca3SnO films with the antiperovskite structure and a cube-on-cube orientation relationship to the YSZ substrate. The successful synthesis of the antiperovskite phase is further confirmed by x-ray photoemission spectroscopy. These results strongly suggest that antiperovskite-oxide films can be directly grown on substrates from the target material using a PLD technique

Origin of the anomalous mass renormalization in metallic quantum well states of correlated oxide SrVO3_3

$In$ $situ$ angle-resolved photoemission spectroscopy (ARPES) has been performed on SrVO$_3$ ultrathin films, which show metallic quantum well (QW) states, to unveil the origin of the anomalous mass enhancement in the QW subbands. The line-shape analysis of the ARPES spectra reveals that the strength of the electron correlation increases as the subband bottom energy approaches the Fermi level. These results indicate that the anomalous subband-dependent mass enhancement mainly arises from the quasi-one-dimensional character of confined V $3d$ states as a result of their orbital-selective quantization.Comment: 6 pages, 3 figure

Emergence of quantum critical behavior in metallic quantum-well states of strongly correlated oxides

Controlling quantum critical phenomena in strongly correlated electron systems, which emerge in the neighborhood of a quantum phase transition, is a major challenge in modern condensed matter physics. Quantum critical phenomena are generated from the delicate balance between long-range order and its quantum fluctuation. So far, the nature of quantum phase transitions has been investigated by changing a limited number of external parameters such as pressure and magnetic field. We propose a new approach for investigating quantum criticality by changing the strength of quantum fluctuation that is controlled by the dimensional crossover in metallic quantum well (QW) structures of strongly correlated oxides. With reducing layer thickness to the critical thickness of metal-insulator transition, crossover from a Fermi liquid to a non-Fermi liquid has clearly been observed in the metallic QW of SrVO$_3$ by \textit{in situ} angle-resolved photoemission spectroscopy. Non-Fermi liquid behavior with the critical exponent ${\alpha} = 1$ is found to emerge in the two-dimensional limit of the metallic QW states, indicating that a quantum critical point exists in the neighborhood of the thickness-dependent Mott transition. These results suggest that artificial QW structures provide a unique platform for investigating novel quantum phenomena in strongly correlated oxides in a controllable fashion.Comment: 6 pages, 3 figure

Observation of a Dirac nodal line in AlB2

We have performed angle-resolved photoemission spectroscopy of AlB2 which is isostructural to high-temperature superconductor MgB2. Using soft-x-ray photons, we accurately determined the three-dimensional bulk band structure and found a highly anisotropic Dirac-cone band at the K point in the bulk hexagonal Brillouin zone. This band disperses downward on approaching the H point while keeping its degeneracy at the Dirac point, producing a characteristic Dirac nodal line along the KH line. We also found that the band structure of AlB2 is regarded as a heavily electron-doped version of MgB2 and is therefore well suited for fully visualizing the predicted Dirac nodal line. The present results suggest that (Al,Mg)B2 system is a promising platform for studying the interplay among Dirac nodal line, carrier doping, and possible topological superconducting properties.Comment: 6 pages, 3 figure

Natural van der Waals heterostructural single crystals with both magnetic and topological properties

Heterostructures having both magnetism and topology are promising materials for the realization of exotic topological quantum states while challenging in synthesis and engineering. Here, we report natural magnetic van der Waals heterostructures of (MnBi2Te4)m(Bi2Te3)n that exhibit controllable magnetic properties while maintaining their topological surface states. The interlayer antiferromagnetic exchange coupling is gradually weakened as the separation of magnetic layers increases, and an anomalous Hall effect that is well coupled with magnetization and shows ferromagnetic hysteresis was observed below 5 K. The obtained homogeneous heterostructure with atomically sharp interface and intrinsic magnetic properties will be an ideal platform for studying the quantum anomalous Hall effect, axion insulator states, and the topological magnetoelectric effect.Comment: 40 pages, 15 figure