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 a high-mobility two-dimensional electron gas between two insulating complex oxides, especially LaAlO3/SrTiO3, has enhanced the potential of oxides for electronics. The occurrence of this conductivity is believed to be driven by polarization discontinuity, leading to an electronic reconstruction. In this scenario, the crystal orientation has an important role and no conductivity would be expected, for example, for the interface between LaAlO3 and (110)-oriented SrTiO3, which should not have a polarization discontinuity. Here we report the observation of unexpected conductivity at the LaAlO3/SrTiO3 interface prepared on (110)-oriented SrTiO3, with a LaAlO3-layer thickness-dependent metal-insulator transition. Density functional theory calculation reveals that electronic reconstruction, and thus conductivity, is still possible at this (110) interface by considering the energetically favourable (110) interface structure, that is, buckled TiO2/LaO, in which the polarization discontinuity is still present. The conductivity was further found to be strongly anisotropic along the different crystallographic directions with potential for anisotropic superconductivity and magnetism, leading to possible new physics and applications.

We report optical, electrical and magnetotransport properties of oxygen deficient SrTiO(3) (SrTiO(3-x)) thin films fabricated by pulsed laser deposition technique. The oxygen vacancies (O(vac)) in the thin film are expected to be uniform. By comparing its electrical properties to those of bulk SrTiO(3-x), it was found that O(vac) in bulk SrTiO(3-x) is far from uniform over the whole material. The metal-insulator transition (MIT) observed in the SrTiO(3-x) film was found to be induced by the carrier freeze-out effect. The low temperature frozen state can be reexcited by Joule heating, electric and intriguingly magnetic field.

We demonstrate the growth of high quality single phase films of VO2(A, B, and M) on SrTiO3 substrate by controlling the vanadium arrival rate (laser frequency) and oxidation of the V atoms. A phase diagram has been developed (oxygen pressure versus laser frequency) for various phases of VO2 and their electronic properties are investigated. VO2(A) phase is insulating VO2(B) phase is semi-metallic, and VO2(M) phase exhibits a metal-insulator transition, corroborated by photo-electron spectroscopic studies. The ability to control the growth of various polymorphs opens up the possibility for novel (hetero)structures promising new device functionalities.

materials has been actively investigated for magnetic memory applications (as this requires much smaller current densities compared to direct current switching of magnetic domains) and a strong SOC in a layer adjacent to the soft magnetic layer is key to switching in-plane [ 5 ] or perpendicular magnetization. [ 6 ] SOC in atoms is usually signifi cant only in high-Z elements because the interaction increases as the fourth power of the atomic number. However, disruption of the periodic boundary conditions in a solid, particularly broken inversion symmetry at surfaces and interfaces, also leads to SOC. The classic examples are interfaces of GaAs/GaAlAs [ 7 ] and InAs/InGaAs [ 8 ] and more recently LaAlO 3 / SrTiO 3 (100) (LAO/STO). [9][10][11] Recently, it has been shown that at the (001) surface of SrTiO 3, the Ti 3 d orbital subband is spinsplit by 90 meV with spins of opposite chirality, using spin-and angle-resolved photoemission spectroscopy. [ 12 ] A large perpendicular electric fi eld is expected at the LAO/ STO interface due to broken structural inversion symmetry, which might produce a Rashba SOC. In the Rashba effect, the conduction electrons of wave vector k moving in a potential gradient normal to the interface ( ∂V / ∂z ), experience an effective in-plane magnetic fi eld B so where 1The Rashba interaction is represented by the Rashba Hamilto-where σ is the Pauli spin matrix, ẑ is a unit vector normal to the interface and α is the SOC strength. , which is enormous compared to metallic systems and will be an ideal spin-polarized source. In addition to the in-plane effect, there is an unexpected gate-tunable out-of-plane SOC at the LaAlO 3 /SrTiO 3 (110) interface when the spins lie outof-plane due to broken symmetry in the plane of the interface. It is demonstrated that this can be manipulated by varying the LaAlO 3 thickness showing that this interface can be engineered for spin-orbit torque devices.