Graphite to diamond transition induced by photoelectric absorption of ultraviolet photons.

The phase transition from graphite to diamond is an appealing object of study because of many fundamental and also, practical reasons. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. They induce it by increasing either pressure or temperature on graphite. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. Electron energy loss spectroscopy of the islands show that the sp2 to sp3 hybridation transition is a surface effect. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands.

Disorder induced power-law gaps in an insulator-metal Mott transition.

A correlated material in the vicinity of an insulator-metal transition (IMT) exhibits rich phenomenology and a variety of interesting phases. A common avenue to induce IMTs in Mott insulators is doping, which inevitably leads to disorder. While disorder is well known to create electronic inhomogeneity, recent theoretical studies have indicated that it may play an unexpected and much more profound role in controlling the properties of Mott systems. Theory predicts that disorder might play a role in driving a Mott insulator across an IMT, with the emergent metallic state hosting a power-law suppression of the density of states (with exponent close to 1; V-shaped gap) centered at the Fermi energy. Such V-shaped gaps have been observed in Mott systems, but their origins are as-yet unknown. To investigate this, we use scanning tunneling microscopy and spectroscopy to study isovalent Ru substitutions in Sr3(Ir1-xRux)2O7 (0 ≤ x ≤ 0.5) which drive the system into an antiferromagnetic, metallic state. Our experiments reveal that many core features of the IMT, such as power-law density of states, pinning of the Fermi energy with increasing disorder, and persistence of antiferromagnetism, can be understood as universal features of a disordered Mott system near an IMT and suggest that V-shaped gaps may be an inevitable consequence of disorder in doped Mott insulators.

Responses to drugs of abuse and non-drug rewards in leptin deficient ob/ob mice.

RATIONALE: Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice. OBJECTIVES: We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse. METHODS: Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement. RESULTS: In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking. CONCLUSIONS: In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.

Peripheral visual field thresholds using Humphrey Field Analyzer program 60-4 in normal eyes.

PURPOSE: Various methods have been used for testing peripheral visual field disturbances such as defects caused by drug toxicity. Static threshold perimetry with Humphrey Field Analyzer (HFA) is widely available. The aim of this study was to better define the normal thresholds for peripheral visual field (PVF) sensitivity and to refine analysis strategies. METHODS: Automated PVF testing was performed with HFA 60-4 program in 33 normal subjects. Test locations were organized into inner, middle, and outer eccentricity rings and divided into 4 zones: nasal, temporal, superior, and inferior. The threshold visual sensitivity (TVS) in decibels was established for each point. RESULTS: The majority of points with the lowest TVS and highest between-subject variability were located within the nasal area of the outer ring. Points with the highest TVS and least variability were detected in the inner ring and in the temporal area of the middle and outer rings. Mean zone TVS decreased and variability increased with increasing eccentricity. CONCLUSIONS: The areas that demonstrate the highest between-subject consistency and thus might best reveal peripheral visual abnormalities with HFA 60-4 are the inner ring, inferior and temporal zone of the middle ring, and temporal zone of the outer ring. These observations may be useful for developing strategies to detect peripheral field loss at an early stage when central vision is not yet affected.