Raman modes and Grüneisen parameters of graphite under compressive biaxial stress

We study the behaviour of the most characteristic Raman contributions of highly oriented pyrolytic graphite at biaxial stresses up to 5 GPa. We use moissanite anvils to compress the samples, which allowed us to observe for the first time the evolution with stress of the disorder-induced D band along with all the overtones and combination bands in the Raman spectrum of graphite. Theoretical calculations at different pressures, performed within the density functional theory, are provided to confirm that the D band process involves iTO phonons along the K–C direction. A complete set of Grüneisen parameters is reported for all the features of the Raman spectrum of graphite; the analysis of the ratios of such parameters provides direct information on the intrinsic coupling and stress response of each phonon branch.</p

Self-pixelation through fracture in VO2 thin films

Vanadium dioxide (VO2) is an archetypal Mott material with a metal-insulator transition (MIT) near room temperature. In thin films, this transition is affected by substrate-induced strain but, as film thickness increases, the strain is gradually relaxed and the bulk properties are recovered. Epitaxial films of VO2 on (001)-oriented rutile titanium dioxide (TiO2) relax substrate strain by forming a network of fracture lines that crisscross the film along well-defined crystallographic directions. This work shows that the electronic properties associated with these lines result in a pattern that resembles a “street map” of fully strained metallic VO2 blocks separated by insulating VO2 stripes. Each block of VO2 is thus electronically self-insulated from its neighbors and its MIT can be locally induced optically with a laser, or electronically via the tip of a scanning probe microscope, so that the films behave functionally as self-patterned pixel arrays