Origin of the Anomalous Mass Renormalization in Metallic Quantum Well States of Strongly Correlated Oxide SrVO_{3}

Respective Roles of Electron-Phonon and Electron-Electron Interactions in the Transport and Quasiparticle Properties of SrVO_{3}

The spectral and transport properties of strongly correlated metals, such as SrVO_{3} (SVO), are widely attributed to electron-electron (e-e) interactions, with lattice vibrations (phonons) playing a secondary role. Here, using first-principles electron-phonon (e-ph) and dynamical mean field theory calculations, we show that e-ph interactions play an essential role in SVO: they govern the electron scattering and resistivity in a wide temperature range down to 30 K, and induce an experimentally observed kink in the spectral function. In contrast, the e-e interactions control quasiparticle renormalization and low temperature transport, and enhance the e-ph coupling. We clarify the origin of the near T^{2} temperature dependence of the resistivity by analyzing the e-e and e-ph limited transport regimes. Our work disentangles the electronic and lattice degrees of freedom in a prototypical correlated metal, revealing the dominant role of e-ph interactions in SVO.

Mechanisms of metal-insulator transitions in ultrathin SrMoO3films

As the thickness of a transition metal oxide thin film is reduced to several unit cells, dimensional and interfacial effects modulate its structure and properties, and initiate low-dimension quantum phase transitions different from its bulk counterparts. To check if a metal-insulator transition (MIT) occurs to a low-dimensional 4d2electron systems, we investigated SrMoO₃thin films by characterizing and analyzing their lattice structures, electric transport properties and electronic states. Among various dimensional effects and interfacial effects, quantum confinement effect (QCE) was discerned as the dominating mechanism of the thickness-driven MIT. Surface/interface scattering contributes to the residual resistivity while the competition of several interactions modulated by QCE governs the temperature dependence of the resistivity of SrMoO₃ultrathin films.

Resonant tunneling driven metal-insulator transition in double quantum-well structures of strongly correlated oxide

The metal-insulator transition (MIT), a fascinating phenomenon occurring in some strongly correlated materials, is of central interest in modern condensed-matter physics. Controlling the MIT by external stimuli is a key technological goal for applications in future electronic devices. However, the standard control by means of the field effect, which works extremely well for semiconductor transistors, faces severe difficulties when applied to the MIT. Hence, a radically different approach is needed. Here, we report an MIT induced by resonant tunneling (RT) in double quantum well (QW) structures of strongly correlated oxides. In our structures, two layers of the strongly correlated conductive oxide SrVO₃ (SVO) sandwich a barrier layer of the band insulator SrTiO₃. The top QW is a marginal Mott-insulating SVO layer, while the bottom QW is a metallic SVO layer. Angle-resolved photoemission spectroscopy experiments reveal that the top QW layer becomes metallized when the thickness of the tunneling barrier layer is reduced. An analysis based on band structure calculations indicates that RT between the quantized states of the double QW induces the MIT. Our work opens avenues for realizing the Mott-transistor based on the wave-function engineering of strongly correlated electrons.

Terahertz spectroscopic evidence of electron correlations in SrVO3epitaxial thin films

Electron correlation in transition metal oxides (TMOs) is an intriguing topic in condensed matter physics, revealing a wide variety of exotic physical properties. Investigating low-energy carrier dynamics by terahertz (THz) spectroscopy is an efficient route to obtain the essential insights into electron correlation. In the present study, THz-time-domain spectroscopy is employed to probe electron correlation in SrVO₃epitaxial thin films. The low energy carrier dynamics of SrVO₃in the range of 0.2-6.0 meV shows a typical metallic behavior as overserved in dc transport measurements. The obtained temperature-dependent optical parameters provide evidence of mass renormalization in the low energy regime and carrier momentum relaxation happens via the electron-electron scattering mechanism. Overall, the frequency and temperature-dependent optical parameters indicate the Fermi liquid ground state in a Mott-Hubbard type correlated metal SrVO₃thin film. Our results provide significant insight on low energy carrier dynamics in the correlated electron system, particularly perovskite-basedd¹TMOs.

Electron-Phonon Coupling and Electron-Phonon Scattering in SrVO3

Understanding the physics of strongly correlated electronic systems has been a central issue in condensed matter physics for decades. In transition metal oxides, strong correlations characteristic of narrow d bands are at the origin of remarkable properties such as the opening of Mott gap, enhanced effective mass, and anomalous vibronic coupling, to mention a few. SrVO3 with V4+  in a 3d1  electronic configuration is the simplest example of a 3D correlated metallic electronic system. Here, the authors' focus on the observation of a (roughly) quadratic temperature dependence of the inverse electron mobility of this seemingly simple system, which is an intriguing property shared by other metallic oxides. The systematic analysis of electronic transport in SrVO3  thin films discloses the limitations of the simplest picture of e-e correlations in a Fermi liquid (FL); instead, it is shown show that the quasi-2D topology of the Fermi surface (FS) and a strong electron-phonon coupling, contributing to dress carriers with a phonon cloud, play a pivotal role on the reported electron spectroscopic, optical, thermodynamic, and transport data. The picture that emerges is not restricted to SrVO3  but can be shared with other 3d and 4d metallic oxides.

Exploring itinerant states in divalent hexaborides using rare-earth L edge resonant inelastic x-ray scattering

We present a study of resonant inelastic x-ray scattering (RIXS) spectra collected at the rare-earth L edges of divalent hexaborides YbB₆ and EuB₆. In both systems, RIXS-active features are observed at two distinct resonances separated by [Formula: see text] eV in incident energy, with angle-dependence suggestive of distinct photon scattering processes. RIXS spectra collected at the divalent absorption peak resemble the unoccupied 5d density of states calculated using density functional theory. We discuss possible origins of this correspondence including a scenario which changes the 4f  valence. In addition, anomalous resonant scattering is observed at higher incident energy, where no corresponding absorption feature is present. Our results demonstrate the potential for L-edge RIXS to assess the itinerant-state properties of f -electron materials.

Engineering Carrier Effective Masses in Ultrathin Quantum Wells of IrO_{2}

The carrier effective mass plays a crucial role in modern electronic, optical, and catalytic devices and is fundamentally related to key properties of solids such as the mobility and density of states. Here we demonstrate a method to deterministically engineer the effective mass using spatial confinement in metallic quantum wells of the transition metal oxide IrO_{2}. Using a combination of in situ angle-resolved photoemission spectroscopy measurements in conjunction with precise synthesis by oxide molecular-beam epitaxy, we show that the low-energy electronic subbands in ultrathin films of rutile IrO_{2} have their effective masses enhanced by up to a factor of 6 with respect to the bulk. The origin of this strikingly large mass enhancement is the confinement-induced quantization of the highly nonparabolic, three-dimensional electronic structure of IrO_{2} in the ultrathin limit. This mechanism lies in contrast to that observed in other transition metal oxides, in which mass enhancement tends to result from complex electron-electron interactions and is difficult to control. Our results demonstrate a general route towards the deterministic enhancement and engineering of carrier effective masses in spatially confined systems, based on an understanding of the three-dimensional bulk electronic structure.

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 SrVO₃ by in situ angle-resolved photoemission spectroscopy. Non-Fermi liquid behavior with the critical exponent α = 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.

Quasiparticle Interference on Cubic Perovskite Oxide Surfaces

We report the observation of coherent surface states on cubic perovskite oxide SrVO_{3}(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d_{xy}-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO_{2} plane play a critical role in controlling the coherent surface state via modulating orbital state.