Session A47: Surface and Thin Film Phase Transitions and Electronic Properties

High $T_{\mathrm{IMT}}$ insulator-to-metal transition of the VO$_{\mathrm{2}}$ films on AlN/Si substrate.

Electronical and structural properties of the VO$_{\mathrm{2}}$ thin films are strongly affected by growth conditions and underlying substrate providing a flexibility of their functional parameters. We present a new VO$_{\mathrm{2}}$/AlN/Si heterostructure, where VO$_{\mathrm{2}}$ is characterized by an excellent insulator-to-metal transition (IMT) occurred at a higher temperature $T_{\mathrm{IMT}}$ than that typical for single crystals. Mentioned characteristics are associated with growth mechanism of the film and its epitaxial alignment with respect to the substrate. In particular, the $T_{\mathrm{IMT}}$ upshift in VO$_{\mathrm{2}}$/AlN/Si is explained by a stable crystallographic configuration in the plane of the VO$_{\mathrm{2}}$ film as well as a tensile deformation of a monoclinic $a$-axis formed by tilted and dimerized V$^{\mathrm{4+}}$-V$^{\mathrm{4+}}$, responsible for strong electron correlations. Moreover, proposed synergy of VO$_{\mathrm{2}}$ and Si is able to make new results for advanced materials fabrication and development of switching devices of new generation.   

Ultrafast dynamics of VO$_2$ thin films measured in pump-probe configuration

The semiconductor-metal transition of VO$_2$ continues to be a vigorously studied phenomenon due to complicated interplay between the structural change and the electronic bands. It is also potentially a very useful material, particularly because of its ultrafast transition to the metallic state excited with a femtosecond pulse. We have been exploring the effects of polarization of the pump in relation to the probe affects the sub-picosecond response of VO$_2$ thin films, which will be important in designing ultrafast switches. We have also been looking at pumping our VO$_2$ films with a THz source that directly pumps the lattice, and have found the film responds optically on a slower scale than when pumped with 800 nm, suggesting that there is an electronic response from disturbing the lattice.   

Investigation of the Effect of Crystal Thickness on Free-Standing Vanadium Dioxide Nanocrystals

The first-order metal-insulator transition (MIT) that vanadium dioxide exhibits at 65 \textordmasculine C has been extensively studied in the last decade thanks to the growth of single crystal nanobeams/plates smaller than characteristic domain size as well as the advances in epitaxial film growth techniques. The effect of crystal thickness has been studied extensively in epitaxially grown VO$_{2}$ films yet not in free-standing nanocrystals[1]. This is mainly due to lack of control over the crystal thickness in physical vapor transport growth of the nanocrystals. Here, we report first observations on the MIT of VO$_{2}$ nanocrystals grown on oxidized silicon substrate thinned using argon-ions. Among these observations AFM measurements reveal an etch rate of 4 nm/min for 1keV Ar-ion energy. Two terminal suspended nanobeam measurements reveal an intriguing phase transition properties below a threshold thickness. [1]Aetukuri, N.B. et al. Nature Phys. 9, 661-666 (2013).   

Cryogenic optical nano-imaging of phase coexistence in correlated oxides

Correlated transition metal oxides exhibit a bevy of textbook electronic phases characterized by richly interacting lattice, spin, and orbital degrees of freedom. A broad array of accessible thermodynamic phases, ranging from Mott insulator to superconductor, enables abrupt transitions in physical and electronic properties under modest external stimuli, accompanied by spontaneous phase coexistence at the nano-scale. We present a novel near-field optical scanning probe capable of resolving the electronic character of such ``switched'' phases in the coexistent regime, even insulators, at 10 nm resolution and down to liquid helium temperatures. We demonstrate variable-temperature optical, structural, and magnetic imaging functionalities through studies of the insulator-metal transition in two prototypic correlated oxides under epitaxial strain. Structural and electronic attributes of the Mott transition are distinguished in a V$_{\mathrm{2}}$O$_{\mathrm{3}}$ thin film, whereas metastable electronic and magnetic phase coexistence is revealed across a 200K range in the strained manganite La$_{\mathrm{0.67}}$Ca$_{\mathrm{0.33}}$MnO$_{\mathrm{3}}$   

Collapse of the low temperature insulating state in Cr-doped V$_{2}$O$_{3}$ thin films

We have grown epitaxial Cr-doped V$_{2}$O$_{3}$ thin films with Cr concentrations between $0$ and $20$\% on ($0001$)-Al$_{2}$O$_{3}$ by oxygen-assisted molecular beam epitaxy. For the highly doped samples ($>$ $3$\%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between $1$\% and $3$\%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below $3$\% and increases the room temperature resistivity towards the values of Cr-doped V$_{2}$O$_{3}$ single crystals. It is well known that oxygen excess stabilizes a metallic state in V$_{2}$O$_{3}$ single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V$_{2}$O$_{3}$ films can be interesting candidates for field effect devices.   

Modified Young's equation for equilibrium dihedral angles of grain boundary grooves in thin films at the nanoscale

We derive the modified Young’s equation for the equilibrium dihedral angle at the triple junction of the grain boundary groove by taking into account the discrete structure of the low angle grain boundary. For low angle grain boundaries, the geometric relation that the misorientation of the bicrystal is inversely proportional to the dislocation spacing naturally gives rise to the variation in the misorientation when the grain boundary length changes (holding the number of dislocations constant). The fact that the grain boundary energy increases as the grain boundary length decreases due to a smaller dislocation spacing leads to a larger dihedral angle compared to that of the classical theory. Two atomistic continuum modelling tools, namely the phase field crystal model and the amplitude equations, are used to simulate the equilibrium dihedral angle. The numerical results are in quantitatively good agreement with the derived modified Young’s equation.   

First measurements of bulk and shear mechanical loss in optical thin film materials

As advanced gravitational wave detectors come online, and the possibility of the first gravitational wave detection nears, plans for the next generation of gravitational wave detectors are already in the works. These new detectors, and those already planned for the future, are expected to be limited in their most sensitive frequency bands by the Brownian thermal noise generated within the optical thin films used to produce the interferometer mirrors. In order to fully predict the level of this Brownian noise, it is necessary to know the two independent mechanical moduli (Young modulus and Poisson ratio, Bulk and Shear moduli, etc.) as well as their associated mechanical loss parameters. Traditional measurements of the mechanical loss of thin films has measured only one linear combination of these two loss parameters. Here, we present measurements of the bulk and shear mechanical loss of tantalum pentoxide (tantala) thin films made by taking advantage of the differing ratios of elastic deformation in the various resonant modes of a coated silica disc. These results may have immediate implications for the ultimate sensitivity of currently operated gravitational wave detectors.   

Between Crystal and Glass: Thermal Transport in C60 Molecular Crystals

Molecular crystals of the fullerene C60 and its derivatives [e.g., phenyl-C61-butyric acid methyl ester (PCBM)] are candidate materials for use in photovoltaics and thermoelectrics. In thermoelectrics, their usefulness is due in part to their exceptionally low thermal conductivities (~0.4 W/m-K for C60 and ~0.05 W/m-K for PCBM) at room temperature. Little is known regarding the microscopic physics underlying these low thermal conductivities. An important question is whether thermal transport in the C60 molecular crystal is (i) crystal-like, where energy is transported as collective vibrations of the centers of mass of the molecules, or (ii) amorphous-like, where energy diffuses from molecule to molecule. We use molecular dynamics (MD) simulations and the Green-Kubo method to probe this question by predicting the relative contributions of crystal-like and amorphous-like transport to the thermal conductivity of the C60 molecular crystal. To isolate crystal-like transport, we perform simulations on C60 crystals where molecular rotations and intra-molecular vibrations are prohibited. To isolate amorphous-like transport, we fix the centers of mass of the molecules. We compare the MD results to predictions from a fully diffusive network resistance model.   

C$_{60}$-Induced Devil's Staircase Transformation on Pb/Si(111) Wetting Layer

Density functional theory is used to study structural energetics of Pb vacancy cluster formation on C$_{60}$/Pb/Si(111) to explain the unusually fast and error-free transformations between the ``Devil's Staircase'' (DS) phases on the Pb/Si(111) wetting layer at low temperature (\textasciitilde 110 K). The formation energies of vacancy clusters are calculated in C$_{60}$/Pb/Si(111) as Pb atoms are progressively ejected from the initial dense Pb wetting layer. Vacancy clusters larger than 5 Pb atoms are found to be stable with 7 being the most stable, while vacancy clusters smaller than 5 are highly unstable, which agrees well with the observed ejection rate of \textasciitilde 5 Pb atoms per C$_{60}$. The high energy cost (\textasciitilde 0.8 eV) for the small vacancy clusters to form indicates convincingly that the unusually fast transformation observed experimentally between the DS phases, upon C$_{60}$ adsorption at low temperature, cannot be the result of single-atom random walk diffusion but correlated multi-atom processes.   

Competing length scales for the electronic structure of rings of C60

Recently, rings of C60 have been fabricated. This opens up the possibility of studying the electronic structure of complex nanosystems with competing length scales: here the length scale defined by individual C60 molecules, the length scale defined by moving along the inner edge of the ring of C60s, and the length scale for the outer edge. The effects of such competing length scales could be probed with a magnetic field B. We use a tight-binding model to study these effects theoretically. Noninteracting electrons are considered. B is included with a Peierls transformation. Calculated electronic spectrum for an isolated ring of carbons, here used as a simple model for C60, is compared with spectra for rings of carbon rings. Changes in spectra due to inter-ring hopping are identified. New structure in the density of states is correlated with the spatial distribution of states in rings of rings. A magnetic field is applied to access and couple different length scales. Calculated spectra for rings of full C60 molecules are compared with the model results to highlight the effects of competing length scales in C60 rings. Results are used to suggest possible experiments for rings of C60 molecules.   

The Size and Shape dependence of the Surface Free Energy of Nanocrystals

Based on many recent reports, it became possible to control the synthesis of nanomaterials with certain sizes and shapes. A theoretical model to investigate the effect of size and shape on the surface free energy of nanocrystals is worked out in this research. The model is applied to a general shape and size nanocrsytal designated by a shape factor. The model considers all nanocrystals with different morphologies (but with the same shape factor) to be the same. The results were tested for gold and silver. The surface free energy was found to decrease with size for spherical nanocrystals. On the other hand, the surface free energy is enhanced for non-spherical nanocrystals. These findings are in qualitative agreement with previous experimental and theoretical predictions. The results pave the road to manufacture controlled- mechanical properties materials.   

Theory of Space Charge Limited Current in Fractional Dimensional Space

The concept of fractional dimensional space has been effectively applied in many areas of physics to describe the fractional effects on the physical systems. We will present some recent developments of space charge limited (SCL) current in free space and solid in the framework of fractional dimensional space which may account for the effect of imperfectness or roughness of the electrode surface. For SCL current in free space, the governing law is known as the Child-Langmuir (CL) law. Its analogy in a trap-free solid (or dielectric) is known as Mott-Gurney (MG) law. This work extends the one-dimensional CL Law and MG Law for the case of a $D$-dimensional fractional space with $0< D \leq 1$; where parameter $D$ defines the degree of roughness of the electrode surface. Such a fractional dimensional space generalization of SCL current theory can be used to characterize the charge injection by the imperfectness or roughness of the surface in applications related to high current cathode (CL law), and organic electronics (MG law). In terms of operating regime, the model has included the quantum effects when the spacing between the electrodes is small.   

Significantly enhanced giant Rashba splitting in a thin film of binary alloy.

Dirac cones in a 2D environment have attracted much attention not only because of the massless Dirac fermions but also due to their capability to lock the spin direction with the momentum. Here we demonstrate that the Rashba effect within a single layer of a binary alloy composed of heavy atoms, Pb and Au, can be driven by and even tweaked with the adjacent top and bottom layers to yield cones-like structures and further enhance the Rashba coupling strength. Two cones are observed at the surface zone center $\bar{{\Gamma }}$with giant Rashba parameters 1.53 and 4.45 eVÅ; an anisotropic giant Rashba splitting at the surface zone boundary$\bar{{M}}$has a great value, 6.26 eVÅ, inferring the critical role of $p$-$d $hybridization between Pb and Au. Our results reveal not only an interesting natural phenomenon but also a feasible method of tweaking the Rashba effect of a 2D system.   

A new approach to measure spatially resolved thermovoltage

We have recorded spatial maps of the thermovoltage of a Au(111) surface with a scanning tunneling microscope using a novel approach. The novel approach relies a method were we record quasisimultaneously the normal topography as well as the thermovoltage by switching the feedback and sample bias on and off. The thermovoltage, which arises from a temperature difference between scanning tunneling microscope tip and sample, is very sensitive to small variations of the local electronic density of states in vicinity of the Fermi level. Near step edges and defects we have observed well-defined Friedel oscillations.