Browsing by Subject "Metal insulator transition"

Now showing 1 - 4 of 4

Open Access
Ab-initio electron transport calculations of carbon based string structures
(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, Salim
The new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.
Open Access
Effects of thickness on the metal-insulator transition in free-standing vanadium dioxide nanocrystals
(American Chemical Society, 2017) Fadlelmula, M. M.; Sürmeli, E. C.; Ramezani, M.; Kasırga, T. S.
Controlling solid state phase transitions via external stimuli offers rich physics along with possibilities of unparalleled applications in electronics and optics. The well-known metal-insulator transition (MIT) in vanadium dioxide (VO2) is one instance of such phase transitions emerging from strong electronic correlations. Inducing the MIT using electric field has been investigated extensively for the applications in electrical and ultrafast optical switching. However, as the Thomas-Fermi screening length is very short, for considerable alteration in the material’s properties with electric field induced MIT, crystals below 10 nm are needed. So far, the only way to achieve thin crystals of VO2 has been via epitaxial growth techniques. Yet, stress due to lattice mismatch as well as interdiffusion with the substrate complicate the studies. Here, we show that free-standing vapor-phase grown crystals of VO2 can be milled down to the desired thickness using argon ion-beam milling without compromising their electronic and structural properties. Among our results, we show that even below 4 nm thickness the MIT persists and the transition temperature is lowered in two-terminal devices as the crystal gets thinner. The findings in this Letter can be applied to similar strongly correlated materials to study quantum confinement effects.
Open Access
Investigation of low-temperature electrical conduction mechanisms in highly resistive GaN bulk layers extracted with Simple Parallel Conduction Extraction Method
(Springer, 2009-12-03) Yildiz, A.; Lisesivdin, S. B.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel; Balkan, N.
The electrical conduction mechanisms in various highly resistive GaN layers of Al x Ga1-x N/AlN/GaN/AlN heterostructures are investigated in a temperature range between T=40 K and 185 K. Temperature-dependent conductivities of the bulk GaN layers are extracted from Hall measurements with implementing simple parallel conduction extraction method (SPCEM). It is observed that the resistivity (ρ) increases with decreasing carrier density in the insulating side of the metal-insulator transition for highly resistive GaN layers. Then the conduction mechanism of highly resistive GaN layers changes from an activated conduction to variable range hopping conduction (VRH). In the studied temperature range, ln∈(ρ) is proportional to T -1/4 for the insulating sample and proportional to T -1/2 for the more highly insulating sample, indicating that the transport mechanism is due to VRH.
Open Access
Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO2
(Institute of Physics Publishing, 2016-07) Kasirga, T. S.; Coy, J. M.; Park, J. H.; Cobden, D. H.
Hydrogen intercalation in solids is common, complicated, and very difficult to monitor. In a new approach to the problem, we have studied the profile of hydrogen diffusion in single-crystal nanobeams and plates of VO2, exploiting the fact that hydrogen doping in this material leads to visible darkening near room temperature connected with the metal-insulator transition at 65 ï¿½C. We observe hydrogen diffusion along the rutile c-axis but not perpendicular to it, making this a highly one-dimensional diffusion system. We obtain an activated diffusion coefficient ∼ 0.01 e-0.6eV/kBT cm2s-1, applicable in metallic phase. In addition, we observe dramatic supercooling of the hydrogen-induced metallic phase and spontaneous segregation of the hydrogen into stripes implying that the diffusion process is highly nonlinear, even in the absence of defects. Similar complications may occur in hydrogen motion in other materials but are not revealed by conventional measurement techniques.