Intermediate Metallic State Research Articles

The Critical Role of Initial/Operando Oxygen Loading in General Bismuth-Based Catalysts for Electroreduction of Carbon Dioxide.

Operando reconstruction of solid catalyst into a distinct active state frequently occurs during electrocatalytic processes. The correlation between initial and operando states, if ever existing, is critical for the understanding and precise design of a catalytic system. Inspired by recently established intermediate metallic state of Bi-based catalysts during electrocatalytic carbon dioxide reduction (CO2RR), here we investigate a series of Bi oxide catalysts (Bi, Bi2O3, BiO2) and demonstrate that the operando surface/subsurface oxygen loading, positively correlated to the initial oxygen content, plays a critical role in determining Bi-based CO2RR performance. Higher initial oxygen loading indicates a better electrocatalytic efficiency. Further analysis shows that this conclusion generally applies to all Bi-based electrocatalysts reported up to date. Following this principle, cost-effective BiO2 nanocrystals demonstrated the highest formate Faradaic efficiency (FE) and current density compared to Bi/Bi2O3, further allowing a pair-electrolysis system with 800 mA/cm2 current density and an overall 175% FE for formate production.

Optical Control of Multistage Phase Transition via Phonon Coupling in MoTe_{2}.

The temporal characters of laser-driven phase transition from 2H to 1T^{'} has been investigated in the prototype MoTe_{2} monolayer. This process is found to be induced by fundamental electron-phonon interactions, with an unexpected phonon excitation and coupling pathway closely related to the nonequilibrium relaxation of photoexcited electrons. The order-to-order phase transformation is dissected into three substages, involving energy and momentum scattering processes from optical (A_{1}^{'} and E^{'}) to acoustic phonon modes [LA(M)] in subpicosecond timescale. An intermediate metallic state along the nonadiabatic transition pathway is also identified. These results have profound implications on nonequilibrium phase engineering strategies.

A patterned look into a mysterious phase

Solid-State Physics A thin superconducting film can become insulating by, for example, exposure to a sufficiently large magnetic field. In between the superconducting and insulating regimes, an intermediate metallic state has been observed whose nature remains unresolved. To study the superconductor–metal insulator transition, C. Yang et al. patterned a film of the high-temperature superconductor yttrium barium copper oxide (YBCO) into a network of triangular superconducting islands connected by bridges (see the Perspective by Phillips). The reactive ion-etching process used for patterning reduced the quality of the film in a controlled manner. By increasing the etching time, the film's transport properties could be tuned from superconducting, through metallic, to insulating. The metallic phase exhibited a bosonic character. Science , this issue p. [1505][1]; see also p. [1450][2] [1]: /lookup/doi/10.1126/science.aax5798 [2]: /lookup/doi/10.1126/science.aaz9902

Intermediate bosonic metallic state in the superconductor-insulator transition.

Whether a metallic ground state exists in a two-dimensional system beyond Anderson localization remains an unresolved question. We studied how quantum phase coherence evolves across superconductor-metal-insulator transitions through magnetoconductance quantum oscillations in nanopatterned high-temperature superconducting films. We tuned the degree of phase coherence by varying the etching time of our films. Between the superconducting and insulating regimes, we detected a robust intervening anomalous metallic state characterized by saturating resistance and oscillation amplitude at low temperatures. Our measurements suggest that the anomalous metallic state is bosonic and that the saturation of phase coherence plays a prominent role in its formation.

Superconductivity in thallium double atomic layer and transition into an insulating phase intermediated by a quantum metal state

We report on the first observation of superconductivity in a double atomic layer of Tl on Si(1 1 1) using in situ electrical resistivity measurements in ultrahigh vacuum. The structure of the Tl bilayer was characterized by a set of techniques, including scanning tunneling microscopy, electron diffraction and photoemission spectroscopy, which confirmed the metastability and metallic nature of the Tl bilayer. The epitaxial growth of atomically thin ‘soft’ metallic film over the entire surface of substrate enabled us to find a macroscopic superconducting transition at 0.96 K, accompanied by thermal and quantum fluctuations of order parameter. The system also demonstrates a perpendicular-magnetic-field-induced superconductor-insulator transition, together with an intermediate metallic state. We have found that the magnetoresitivity at the lowest temperature is consistent with the Bose metal picture, which is a consequence of strong quantum fluctuations.

Nature of the quantum metal in a two-dimensional crystalline superconductor

Two-dimensional (2D) materials are not expected to be metals at low temperature due to electron localization. Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field. However, more recent works have revealed the presence of an intermediate metallic state occupying a substantial region of the phase diagram whose nature is intensely debated. Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe$_2$ in inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the intermediate metallic state. We find a new power law scaling with field in this phase, which is consistent with the Bose metal model where metallic behavior arises from strong phase fluctuations caused by the magnetic field.

Non-congruence of thermally driven structural and electronic transitions in VO2

The multifunctional properties of vanadium dioxide (VO2) arise from coupled first-order phase transitions: an insulator-to-metal transition (IMT) and a structural phase transition (SPT) from monoclinic to tetragonal. The characteristic signatures of the IMT and SPT are the hysteresis loops that track the phase transition from nucleation to stabilization of a new phase and back. A long-standing question about the mechanism of the VO2 phase transition is whether and how the almost-simultaneous electronic and structural transitions are related. Here, we report independent measurements of the IMT and SPT hystereses in epitaxial VO2 films on c-sapphire with distinct morphologies. The measurements show that the IMT and the SPT are not congruent, in that the structural phase transition requires more energy to reach completion than the electronic, insulator-to-metal transition. This result is independent of nanoscale film morphology and grain orientation on the substrate, so that the non-congruence is an intrinsic property of the VO2 phase transition. Our conclusion is supported by effective-medium calculations of the dielectric function incorporating the measured volume fractions of the monoclinic and tetragonal states. The results are consistent with the existence of an intermediate metallic state in which the electron-electron correlations characteristic of the monoclinic state begin to disappear before the transition to the tetragonal structural state.

Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide

We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO${}_{2}$) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO${}_{2}$ is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.