Optical phonons in twisted bilayer graphene with gate-induced asymmetric doping.

Twisted bilayer graphene (tBLG) devices with ion gel gate dielectrics are studied using Raman spectroscopy in the twist angle regime where a resonantly enhanced G band can be observed. We observe prominent splitting and intensity quenching on the G Raman band when the carrier density is tuned away from charge neutrality. This G peak splitting is attributed to asymmetric charge doping in the two graphene layers, which reveals individual phonon self-energy renormalization of the two weakly coupled layers of graphene. We estimate the effective interlayer capacitance at low doping density of tBLG using an interlayer screening model. The anomalous intensity quenching of both G peaks is ascribed to the suppression of resonant interband transitions between the two saddle points (van Hove singularities) that are displaced in the momentum space by gate-tuning. In addition, we observe a softening (hardening) of the R Raman band, a superlattice-induced phonon mode in tBLG, in electron (hole) doping. Our results demonstrate that gate modulation can be used to control the optoelectronic and vibrational properties in tBLG devices.

Intermediate metallic phase in VO2 observed with scanning tunneling spectroscopy.

This investigation focuses on the formation of nanoscale puddles of an intermediate metallic phase (IMP) in the metal-insulator transition (MIT) temperature regime of single-crystalline vanadium dioxide (VO2) nanowires. The electronic structure of VO2 nanowires was examined with scanning tunneling spectroscopy. The evolution of the local density of states of individual nanowires throughout the MIT regime is presented with differential tunneling conductance spectra and images measured as the temperature was increased. Our results show that the formation of an IMP plays an important role in the MIT of intrinsic VO2.

Synthesis, characterization, and finite size effects on electrical transport of nanoribbons of the charge density wave conductor NbSe3.

NbSe(3) exhibits remarkable anisotropy in most of its physical properties and has been a model system for studies of quasi-one-dimensional charge density wave (CDW) phenomena. Herein, we report the synthesis, characterization, and electrical transport of single-crystalline NbSe(3) nanoribbons by a facile one-step vapour transport process involving the transport of selenium powder onto a niobium foil substrate. Our investigations aid the understanding of the CDW nature of NbSe(3) and the growth process of the material. They also indicate that NbSe(3) nanoribbons have enhanced CDW properties compared to those of the bulk phase due to size confinement effects, thus expanding the search for new mesoscopic phenomena at the nanoscale level. Single nanoribbon measurements of the electrical resistance as a function of temperature show charge density wave transitions at 59 and 141 K. We also demonstrate significant enhancement in the depinning effect and sliding regimes mainly attributed to finite size effects.

Single-nanowire raman microprobe studies of doping-, temperature-, and voltage-induced metal-insulator transitions of W(x)V(1-x)O2 nanowires.

Considerable recent research interest has focused on mapping the structural phase diagrams of anisotropic VO(2) nanobeams as model systems for elucidating single-domain behavior within strongly correlated electronic materials, to examine in particular the coupling of lattice and orbital degrees of freedom. Nevertheless, the role of substitutional doping in altering the phase stabilities of competing ground states of VO(2) remains underexplored. In this study, we use individual nanowire Raman microprobe mapping to examine the structural phase progressions underlying the metal-insulator transitions of solution-grown W(x)V(1-x)O(2) nanowires. The structural phase progressions have been monitored for three distinctive modes of inducing the electronic metal-insulator phase transition: as a function of (a) W doping at constant temperature, (b) varying temperature for specific W dopant concentrations, and (c) varying applied voltage for specific W dopant concentrations. Our results suggest the establishment of a coexistence regime within individual nanowires wherein M1 and R phases simultaneously exist before the percolation threshold is reached and the nanowire becomes entirely metallic. Such a coexistence regime has been found to exist during both temperature- and voltage-induced transitions. No evidence of an M2 phase is observed upon inducing the electronic phase transition by any of the three distinctive methods (temperature, doping, and applied voltage), suggesting that substitutional tungsten doping stabilizes the M1 phase over its M2 counterpart and further corroborating that the latter phase is not required to mediate M1→R transformations.

Colossal above-room-temperature metal-insulator switching of a Wadsley-type tunnel bronze.

We show evidence of the manifestation of enormous metal-insulator switching ranging up to almost six orders of magnitude in individual nanowires of ß'-Cu(x)V(2)O(5). The magnitude and temperature of the phase transition is in strong contrast to data reported over several decades for other Wadsley-type mixed valence tunnel bronze structures.

Synthesis, spectroscopic characterization, and observation of massive metal-insulator transitions in nanowires of a nonstoichiometric vanadium oxide bronze.

Metal-insulator transitions in strongly correlated materials, induced by varying either temperature or dopant concentration, remain a topic of enduring interest in solid-state chemistry and physics owing to their fundamental importance in answering longstanding questions regarding correlation effects. We note here the unprecedented observation of a four-orders-of-magnitude metal-insulator transition in single nanowires of delta-K(x)V(2)O(5), when temperature is varied, which thus represents a rare new addition to the pantheon of materials exhibiting pronounced metal-insulator transitions in proximity to room temperature.