Metamaterials open up various exotic means to control electromagnetic waves and among them polarization manipulations with metamaterials have attracted intense attention. As of today, static responses of resonators in metamaterials lead to a narrow-band and single-function operation. Extension of the working frequency relies on multilayer metamaterials or different unit cells, which hinder the development of ultra-compact optical systems. In this work, we demonstrate a switchable ultrathin terahertz quarter-wave plate by hybridizing a phase change material, vanadium dioxide (VO2), with a metasurface. Before the phase transition, VO2 behaves as a semiconductor and the metasurface operates as a quarter-wave plate at 0.468 THz. After the transition to metal phase, the quarter-wave plate operates at 0.502 THz. At the corresponding operating frequencies, the metasurface converts a linearly polarized light into a circularly polarized light. This work reveals the feasibility to realize tunable/active and extremely low-profile polarization manipulation devices in the terahertz regime through the incorporation of such phase-change metasurfaces, enabling novel applications of ultrathin terahertz meta-devices.
The observation of magnetic interaction at the interface between nonmagnetic oxides has attracted much attention in recent years. In this report, we show that the Kondo-like scattering at the SrTiO3-based conducting interface is enhanced by increasing the lattice mismatch and growth oxygen pressure PO2. For the 26-unit-cell LaAlO3/SrTiO3 (LAO/STO) interface with lattice mismatch being 3.0%, the Kondo-like scattering is observed when PO2 is beyond 1 mTorr. By contrast, when the lattice mismatch is reduced to 1.0% at the (La0.3Sr0.7)(Al0.65Ta0.35)O3/SrTiO3 (LSAT/STO) interface, the metallic state is always preserved up to PO2 of 100 mTorr. The data from Hall measurement and X-ray absorption near edge structure (XANES) spectroscopy reveal that the larger amount of localized Ti3+ ions are formed at the LAO/STO interface compared to LSAT/STO. Those localized Ti3+ ions with unpaired electrons can be spin-polarized to scatter mobile electrons, responsible for the Kondo-like scattering observed at the LAO/STO interface.
Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO2, TiO2 and SrTiO3 originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-Tc cuprates, NdBa2Cu3O7-δ (NBCO) and Pr2-xCexCuO4 (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific.Keywords: electrolyte gating, cuprates, superconductors, ionic liquid, oxygen migration, electrostatic field effect 3 The electronic properties of strongly-correlated materials depend strongly on carrier density and experiments where the carrier density is modified systematically are improtant for understanding the fundamental physics behind the studied material phenomena. 1 Electric field effect induced charge modulation is preferred to chemical doping because the carrier density can be tuned quasi-continuously without introducing chemical or structural changes. [1][2][3][4][5][6][7][8][9][10] However, the electric field effect on the stronglycorrelated materials remains small in the field effect transistor (FET) devices using conventional solid dielectric layers. This is because most of the phenomena occur at carrier densities exceeding 10 14 cm -2 , which is not easy to achieve without extremely high K layers. For instance, only several Kelvin of Tc shift was realized for high-Tc cuprates using solid dielectrics. [2][3][4][5][6][7] The situation was changed recently when the technique of electrolyte gating using ionic liquids (ILs) as gate dielectrics was introduced. 11-13 Based on the principle of electrochemical capacitor, ions in the ILs are separated and accumulated on the solid surface of both electrodes under gate voltages. Meanwhile, opposite charges of equivalent density accumulate on the electrode side, creating an electric double-layer (EDL) effectively working as an interface capacitor. 14 The nanoscale separation of the EDL results in ultrahigh electric field of the order of 10 MV cm -1 with induced carrier density up to 10 15 cm -2 . This technique has been used to induce insulator-to-metal transition, 13, 15-17 superconductivity 18-22 and other electronic phase transitions. [23][24][25][26][27] Continuous superconductor...
Poor reversibility and high desorption temperature restricts the practical use of lithium borohydride (LiBH4) as an advanced hydrogen store. Herein, a LiBH4 composite confined in unique double‐layered carbon nanobowls prepared by a facile melt infiltration process is demonstrated, thanks to powerful capillary effect under 100 bar of H2 pressure. The gradual formation of double‐layered carbon nanobowls is witnessed by transmission electron microscopy (TEM) observation. Benefiting from the nanoconfinement effect and catalytic function of carbon, this composite releases hydrogen from 225 °C and peaks at 353 °C, with a hydrogen release amount up to 10.9 wt%. The peak temperature of dehydriding is lowered by 112 °C compared with bulk LiBH4. More importantly, the composite readily desorbs and absorbs ≈8.5 wt% of H2 at 300 °C and 100 bar H2, showing a significant reversibility of hydrogen storage. Such a high reversible capacity has not ever been observed under the identical conditions. The usable volumetric energy density reaches as high as 82.4 g L−1 with considerable dehydriding kinetics. The findings provide insights in the design and development of nanosized complex hydrides for on‐board applications.
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