Topological materials, such as the quintessential topological insulators in the Bi2X3 family (X = O, S, Se, Te), are extremely promising for beyond Moore’s Law computing applications where alternative state variables and energy efficiency are prized. It is essential to understand how the topological nature of these materials changes with growth conditions and, more specifically, chalcogen content. In this study, we investigate the evolution of the magnetoresistance of Bi2TexSe3−x for varying chalcogen ratios and constant growth conditions as a function of both temperature and angle of applied field. The contribution of 2D and 3D weak antilocalization are investigated by utilizing the Tkachov-Hankiewicz model and Hakami-Larkin-Nagaoka models of magnetoconductance.

Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light-matter interactions. The dispersion relations of elementary excitations such as phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO2 ). It is observed that the insulator-to-metal transition in VO2 has a profound impact on the polaritons in the proximal hBN layer. In effect, the real-space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO2 sublayer in the course of the temperature-tuned insulator-to-metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.

Yuhan Wang,1,2 Jiawei Zhang,3 Yue Ni,3 Xinzhong Chen,3 Ryan Mescall,3 Tamara Isaacs-Smith,4 Ryan B. Comes,4 Salinporn Kittiwatanakul,1,5 Stuart A. Wolf,1,6 Jiwei Lu,1,* and Mengkun Liu3,7,† 1Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904, USA 2School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China 3Department of Physics, Stony Brook University, Stony Brook, New York 11794, USA 4Department of Physics, Auburn University, Auburn, Alabama 36849, USA 5Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand 6Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA 7Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China

High energy gap Nb-based superconducting alloys with low normal state resistivity are fundamentally important for realization of low-loss high frequency circuits operating above the typical ~670 GHz gap frequency of elemental Nb. NbTiN has been shown to have a superconducting energy gap nearly twice that of elemental Nb, and is emerging as an important material for various detector and superconduc...

Epitaxial [(SrVO3)(7)/(SrTiO3)(4)]r (SVO/STO) superlattices were grown on (001)-oriented LSAT substrates using a pulsed electron-beam deposition technique. The transport properties of the superlattices were investigated by varying the number of repetitions of the SVO/STO bilayers r (1 <= r <= 9). A single SVO/STO bilayer (r = 1) was semiconducting, whereas an increase in the number of repetitions r resulted in metallic behavior in the superlattices with r >= 3. The transport phenomena in the SVO/STO superlattices can be regarded as conduction through parallel-coupled SVO layers, the SVO layer embedded in the superlattices showed a great enhancement in the conductivity compared with the single SVO layer. This work provides further evidence of electronic phase separation in the SVO ultrathin layer that has been recently discovered, the SVO ultrathin layer is considered as a 2D Mott insulator with metallic and insulating phases coexisting, the coupling between SVO layers embedded in the SVO/STO superlattices creates more conduction pathways with increasing number of repetitions r, resulting in a crossover from insulating to metallic behavior.

We report on a new mechanism capable of inducing the insulator metallic transition (IMT) in VO2 via surface plasmon polariton (SPP) excitation and the corresponding effect that the IMT will have on the surface plasmon resonance (SPR).

We report on a new mechanism capable of inducing the insulator-metal transition (IMT) in VO2 via surface plasmon polaritons (SPP). Our theoretical model predicts that for a bilayer Au-VO2 sample an enhanced electromagnetic energy density at the Au-VO2 interface will occur at 1064nm laser wavelength when SPPs are excited in the Au layer. This effect can assist the IMT in the VO2 layer and at the same time, the SPP absorption can be used to detect it. Changes in the optical properties of the VO2 thin layer in such structure can be observed in the reflected light in Kretschmann configuration, via a shift in the nadir location due to light absorption at resonance. This optical mechanism occurs at 2mW threshold transition energies and fully saturates at 5mW.

Emergent order at mesoscopic length scales in condensed matter can provide fundamental insight into the underlying competing interactions and their relationship with the order parameter. Using spectromicroscopy, we show that mesoscopic stripe order near the metal-insulator transition (MIT) of strained VO_{2} represents periodic modulations in both crystal symmetry and V-V dimerization. Above the MIT, we unexpectedly find the long-range order of V-V dimer strength and crystal symmetry become dissociated beyond ≈200  nm, whereas the conductivity transition proceeds homogeneously in a narrow temperature range.

We explored the effect of Cr dopant on the transport behaviors of polycrystalline VO2 thin films in order to suppress the sharp metal-insulator transition, and tune the temperature coefficient of resistivity (TCR) value. A reactive bias target ion beam deposition was used for combinational sputtering to Cr doped VO2 thin films (similar to 100 nm). The addition of Cr led a structural change in the semiconducting phase of VO2. With the Cr content > 7 at. %, the sharp metal-semiconductor transition and the hysteresis loop was suppressed in thin film VO2. A further increase of Cr content reduced the TCR. Separately the effect of the oxygen flow rate was investigated to modulate the TCR and the resistivity value of Cr doped VO2. We demonstrated the resistivity of Cr doped VO2 was modulated by 2 orders of magnitude with a very small change in the oxygen flow rate. We obtained TCR of similar to 4.5 %/K in Cr doped VO2 grown on single crystal sapphire substrate near room temperature.

The current state-of-the-art approach for superconductor-insulator-superconductor (SIS) junction fabrication is based on magnetron sputtering and the Gurvitch Al overlayer trilayer process, where an Al overlayer is deposited onto the Nb base electrode in order to subsequently grow a critical similar to 1-nm AlOx or AlN tunnel barrier. While the switch from AllO(x) to AlN barriers has significantly increased the achievable critical current density, and significant research has been performed to understand and optimize inductively coupled plasma AlN growth, the use of Nb electrodes provides an upper limit to the frequency range for low-noise operation when employed in terahertz (THz) mixer applications and has led to the study and use of alternative materials with higher transition temperatures (T-c) such as NbN and NbTiN. The Nb electrodes also impose stringent cooling requirements; replacement of both electrodes with higher T-c materials can increase the required device operating temperature to 10 K, thus reducing the power consumption of the refrigeration system, a building block for the realization of more energy efficient superconducting computing systems, such as those based on single-flux-quantum logic. In this work, we have departed from conventional SIS material growth techniques through the use of an alternative material deposition technology-reactive bias target ion beam deposition (RBTIBD)-that offers unique capabilities to tailor materials and interfaces. Using RBTIBD technology, we have realized the first ever NbTiN/AlN/NbTiN SIS junctions with highest yet reported sum-gap voltages exceeding 5.0 mV, potentially extending the theoretical limit of low-loss SIS mixing applications beyond 1.2 THz and relaxing the cooling requirements for superconducting device applications.

One of many challenges for niobium (Nb) based superconducting devices is the improvement over the surface morphology and superconducting properties as well as the reduction of defects. We employed a novel deposition technique, i.e. biased target ion beam deposition technique (BTIBD) to prepare Nb thin films with controlled crystallinity and surface morphology. We found that the target current (ITarget) and the target bias (VTarget) were critical to the crystallinity and surface morphology of Nb films. The high target current (ITarget >500 mA and VTarget = 400 V bias) during the deposition degraded the Nb crystallinity, and subsequently reduced the critical temperature for superconductivity (Tc). VTarget was critical to the surface morphology, i.e. grain size and shape and the surface roughness. The optimized growth condition yielded very smooth film with RMS roughness of 0.4 nm that was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was also close to the value of the bulk Nb. The quality of Nb film was evident in the presence of a very thin proximity layer (~ 0.8 nm). The experimental results demonstrated that the preparation of smooth Nb films with adequate superconductivity by BTIBD could serve as a base electrode for the in-situ magnetic layer or insulating layer for superconducting electronic devices.

We explored the effect of Cr dopant on the transport behaviors of polycrystalline VO2 thin films in order to suppress the sharp metal-insulator transition, and tune the temperature coefficient of resistivity (TCR) value. A reactive bias target ion beam deposition was used for combinational sputtering to Cr doped VO2 thin films (~100 nm). The addition of Cr led a structural change in the semiconducting phase of VO2. With the Cr content >7 at. %, the sharp metal-semiconductor transition and the hysteresis loop was suppressed in thin film VO2. A further increase of Cr content reduced the TCR. Separately the effect of the oxygen flow rate was investigated to modulate the TCR and the resistivity value of Cr doped VO2. We demonstrated the resistivity of Cr doped VO2 was modulated by 2 orders of magnitude with a very small change in the oxygen flow rate. We obtained TCR of ~ 4.5 %/K in Cr doped VO2 grown on single crystal sapphire substrate near room temperature.

Ultrafast optical pump-optical probe and optical pump-terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the spectral weight oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in both materials. The low-energy Drude response of V2O3 appears more amenable than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four-to fivefold increase in VO2 and a three-to fivefold increase in V2O3 across the insulator-to-metal phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, paving the way to similar studies in other complex materials.

DC current induced magnetization reversal and magnetization oscillation was observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular external field enhanced the coercive field separation between the reference layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a large window of external magnetic field for exploring the free-layer reversal. The magnetization precession was manifested in terms of the multiple peaks on the differential resistance curves. Depending on the bias current and applied field, the regions of magnetic switching and magnetization precession on a dynamical stability diagram has been discussed in details. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight for synchronization of inhomogenieties in large sized device. The ability to manipulate spin-dynamics on large size devices could prove useful for increasing the output power of the spin-transfer nano-oscillators (STNOs).

While vanadium dioxide (VO2) is one of the most extensively studied highly correlated materials, there are intriguing similarities and differences worth exploring in another highly correlated oxide, niobium dioxide (NbO2). Both materials exhibit a thermally-induced first-order insulator-metal transition at a material-dependent critical temperature, which is considerably higher in NbO2 than in VO2 - approximately 1080 K and 340 K in bulk, respectively. This transition, evidenced by up to 6 orders of magnitude change in DC and optical conductivities, can also be induced in VO2 via photo-doping on a sub-picosecond timescale. Here, we present the first ultrafast pump-probe studies on the optically-induced transition of NbO2 thin films and the comparison with similar VO2 films. It is observed that NbO2 films transition faster and exhibit significantly faster recovery time than VO2 films of similar thickness and microstructure, showcasing that NbO2 is a promising material for next generation high-speed optoelectronic devices. (C) 2016 Optical Society of America

Oxide heterostructures and superlattices (SLs) have attracted a great deal of attention in recent years owing to the rich exotic properties encountered at their interfaces. We focus on the potential of tunable correlated oxides by investigating the spectral function of the prototypical correlated metal SrVO3, using soft x-ray absorption spectroscopy and resonant inelastic soft x-ray scattering to access both unoccupied and occupied electronic states, respectively. We demonstrate a remarkable level of tunability in the spectral function of SrVO3 by varying its thickness within the SrVO3/SrTiO3 SL, showing that the effects of electron correlation can be tuned from dominating the energy spectrum in a strongly correlated Mott–Hubbard insulator, towards a correlated metal. We show that the effects of dimensionality on the correlated properties of SrVO3 are augmented by interlayer coupling, yielding a highly flexible correlated oxide that may be readily married with other oxide systems.

The strong electron-lattice interactions in correlated electron systems provide unique opportunities for altering the material properties with relative ease and flexibility. In this Rapid Communication, we use localized strain control via a focused-ion-beam patterning of $\mathrm{Ti}{\mathrm{O}}_{2}$ substrates to demonstrate that one can selectively engineer the insulator-to-metal transition temperature, the fractional component of the insulating and metallic phases, and the degree of optical anisotropy down to the length scales of the intrinsic phase separation in $\mathrm{V}{\mathrm{O}}_{2}$ thin films without altering the quality of the films. The effects of localized strain control on the strongly correlated electron system are directly visualized by state-of-the-art IR near-field imaging and spectroscopy techniques and x-ray microdiffraction measurements.