Kevin G. West

Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial

Electron–electron interactions can render an otherwise conducting material insulating, with the insulator–metal phase transition in correlated-electron materials being the canonical macroscopic manifestation of the competition between charge-carrier itinerancy and localization. The transition can arise from underlying microscopic interactions among the charge, lattice, orbital and spin degrees of freedom, the complexity of which leads to multiple phase-transition pathways. For example, in many transition metal oxides, the insulator–metal transition has been achieved with external stimuli, including temperature, light, electric field, mechanical strain or magnetic field. Vanadium dioxide is particularly intriguing because both the lattice and on-site Coulomb repulsion contribute to the insulator-to-metal transition at 340 K (ref. 8). Thus, although the precise microscopic origin of the phase transition remains elusive, vanadium dioxide serves as a testbed for correlated-electron phase-transition dynamics. Here we report the observation of an insulator–metal transition in vanadium dioxide induced by a terahertz electric field. This is achieved using metamaterial-enhanced picosecond, high-field terahertz pulses to reduce the Coulomb-induced potential barrier for carrier transport. A nonlinear metamaterial response is observed through the phase transition, demonstrating that high-field terahertz pulses provide alternative pathways to induce collective electronic and structural rearrangements. The metamaterial resonators play a dual role, providing sub-wavelength field enhancement that locally drives the nonlinear response, and global sensitivity to the local changes, thereby enabling macroscopic observation of the dynamics. This methodology provides a powerful platform to investigate low-energy dynamics in condensed matter and, further, demonstrates that integration of metamaterials with complex matter is a viable pathway to realize functional nonlinear electromagnetic composites.

Growth and characterization of vanadium dioxide thin films prepared by reactive-biased target ion beam deposition

Using a novel growth technique called reactive bias target ion beam deposition, the authors have prepared highly oriented VO2 thin films on Al2O3 (0001) substrates at various growth temperatures ranging from 250to550°C. The influence of the growth parameters on the microstructure and transport properties of VO2 thin films was systematically investigated. A change in electrical conductivity of 103 was measured at 341K associated with the well known metal-insulator transition (MIT). It was observed that the MIT temperature can be tuned to higher temperatures by mixing VO2 and other vanadium oxide phases. In addition, a current/electric-field induced MIT was observed at room temperature with a drop in electrical conductivity by a factor of 8. The current densities required to induce the MIT in VO2 are about 3×104A∕cm2. The switching time of the MIT, as measured by voltage pulsed measurements, was determined to be no more than 10ns.

Very large anisotropy in the dc conductivity of epitaxial VO2 thin films grown on (011) rutile TiO2 substrates

In this letter, we reported a very large anisotropy in dc conductivity of epitaxial VO2 thin films deposited on a (011) TiO2 substrate. The VO2 film grew epitaxially on TiO2 and x-ray diffraction showed that VO2 had the tetragonal symmetry due to the substrate clamping effect at room temperature. There was a compressive strain of −1.2% along the c-axis of the rutile VO2. We observed a very strong angular dependence of in-plane dc conductivity. We calculated that σ1/σ3∼5.14, which was anomalously large. We attributed the drastic increase to the compressive strain along the c-axis of the rutile VO2 due to substrate clamping. This very large anisotropy disappeared above the metal-insulator transition.

Propagation of exchange bias in CoFe∕FeMn∕CoFe trilayers

CoFe∕FeMn, FeMn∕CoFe bilayers and CoFe∕FeMn∕CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field HEB depends strongly on the order of depositions and is much higher at CoFe∕FeMn than at FeMn∕CoFe interfaces. By combining the two bilayer structures into symmetric CoFe∕FeMn(tFeMn)∕CoFe trilayers, HEBt and HEBb of the top and bottom CoFe layers, respectively, are both enhanced. Reducing tFeMn of the trilayers also results in enhancements of both HEBb and HEBt. These results evidence the propagation of exchange bias between the two CoFe∕FeMn and FeMn∕CoFe interfaces mediated by the FeMn antiferromagnetic order.

THz spectroscopy of VO2 epitaxial films: controlling the anisotropic properties through strain engineering

We investigate far-infrared properties of strain engineered vanadium dioxide nanosheets through epitaxial growth on a (100)R TiO2 substrate. The nanosheets exhibit large uniaxial strain leading to highly uniform and oriented cracks along the rutile c-axis. Dramatic anisotropy arises for both the metal-insulator transition temperature, which is different from the structural transition temperature along the cR axis, and the metallic state conductivity. Detailed analysis reveals a Mott-Hubbard like behavior along the rutile cR axis. §contributed equally to this work

Role of surface electromagnetic waves in metamaterial absorbers

Metamaterial absorbers have been demonstrated across much of the electromagnetic spectrum and exhibit both broad and narrow-band absorption for normally incident radiation. Absorption diminishes for increasing angles of incidence and transverse electric polarization falls off much more rapidly than transverse magnetic. We unambiguously demonstrate that broad-angle TM behavior cannot be associated with periodicity, but rather is due to coupling with a surface electromagnetic mode that is both supported by, and well described via the effective optical constants of the metamaterial where we achieve a resonant wavelength that is 19.1 times larger than the unit cell. Experimental results are supported by simulations and we highlight the potential to modify the angular response of absorbers by tailoring the surface wave.

Symmetry breaking and geometric confinement in VO2: Results from a three-dimensional infrared nano-imaging

Epitaxial strain can play an important role in controlling the local phase dynamics of transition metal oxides. With scattering-type scanning near-field optical microscopy, we visualize the three dimensional landscape of phase inhomogeneity in strained VO2 films grown on [100]R TiO2 substrates. We demonstrate that three different symmetries are spontaneously broken in the vicinity of the VO2 phase transition: (1) Monoclinic-tetragonal (rutile) crystal symmetry breaking due to the structural phase transition, (2) in-plane (x-y plane) rotational symmetry breaking due to the formation of periodic strain domains, and (3) out-of-plane (z-axis) mirror symmetry breaking at the film cross-section due to substrate-induced epitaxial strain.

Bi-layer metamaterials as fully functional near-perfect infrared absorbers

In this letter, we discuss the design, fabrication, and experimental characterization of a bi-layer fully functional near-perfect metamaterial absorber (MMA) in the long-wavelength infrared (LWIR), which is broadband and generally insensitive to polarization up to a 60° incidence angle. A spectral absorptance of ≥99% was attained simultaneously at multiple LWIR wavelengths, with a bandwidth of 2 μm where the absorptance is ≥90%. This remarkable behavior is attributed to the strong mixing of coupling modes between the two resonators and the ground plane in the presence of a lossy dielectric, in which single layer structures do not exhibit. Furthermore, we show, by comparing two different MMA structures, how the absorption can be tailored by design within and across several IR subdivisions through a slight change in geometrical parameters. The bi-layer MMA has the immediate application of a functionally versatile, low-profile thermal sensor or emitter.

Magnetic properties and spin polarization of Ru doped half metallic CrO2

Chromium dioxide (CrO2) is a half metal that is of interest for spintronic devices. It has not been synthesized through traditional physical vapor deposition (PVD) techniques because of its thermodynamic instability in low oxygen pressures. Epitaxial thin films of Ru doped tetragonal rutile CrO2 were synthesized by a PVD technique. The as-deposited RuxCr1−xO2 was ferrimagnetic with the saturation magnetization moment showing a strong dependence on the Ru concentration. Curie temperature as high as 241 K has been obtained for ∼23 at. % Ru. The Ru substitution increased the electrical conductivity by increasing the minority spin concentration. The spin polarization was found to be as high as 70% for 9 at. % Ru and decreased to ∼60% with Ru concentrations up to ∼44 at. %, which is determined by the Fermi velocities of the majority and minority spins. First principle calculations were performed to understand the effect of Ru content on the properties of CrO2. The PVD processes of Ru doped CrO2 could lead to t...

Exchange bias induced by the Fe3O4 Verwey transition

We present a study of exchange bias in different configurations of V