Browsing by Subject "Temperature dependence"

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Electronic properties and electron-electron interaction effects in transition metal dichalcogenides
(2018-08-09) Larentis, Stefano; Tutuc, Emanuel, 1974-; Banerjee, Sanjay K; MacDonald, Allan H; Register, Leonard F; Shi, Li
Transition metal dichalcogenides (TMDs) are a new class of two-dimensional layered materials characterized by a MX₂ chemical formula, where M (X) stands for a transition metal (chalcogen). MoS₂, MoSe₂ and MoTe₂ are semiconducting TMDs, which at the monolayer limit possess bandgaps >1 eV, rendering them attractive as possible channel material for scaled transistors. The bandstructures of monolayers feature coupled spin and valley degrees of freedom, thanks to large spin-orbit interaction, and large effective masses (m*), suggesting that electron-electron interaction effects are expected to be important in these semiconductors. In this dissertation we discuss the fabrication and electrical characterization of TMD-based electronic devices, with a focus on their electronic properties, including scattering mechanisms contributing to the mobility, carriers' effective mass, band offset in heterostructures, electronic compressibility, and spin susceptibility. We begin studying the four-point field-effect mobilities of few-layers MoS₂, MoSe₂ and MoTe₂ field effect transistors (FETs), in top-contact, bottom-gate architectures. Using hexagonal boron-nitride dielectrics, we fabricate FETs with an improved bottom-contact, dual-gate architecture to probe transport at low temperatures in monolayer MoS₂, and mono- and bilayer MoSe₂. From conductivity and carrier density measurements we determine the Hall mobility, which shows strong temperature dependence, consistent with phonon scattering, and saturates at low temperatures because of impurity scattering. High mobility MoSe₂ samples probed in perpendicular magnetic field, at low temperatures show Shubnikov-de Haas oscillations. Using magnetotransport we probe carriers in spin split bands at the K point in the conduction band and extract their m* = 0.8m [subscript e]; m [subscript e] is the bare electron mass. Quantum Hall states emerging at either odd or even filling factors are explained by a density dependent, interaction enhanced Zeeman splitting. Gated graphene-MoS₂ heterostructures reveal a saturating electron branch conductivity at the onset of MoS₂ population. Magnetotransport measurements probe the graphene electron density, which saturates and decreases as MoS₂ populates, a finding associated with the negative compressibility of MoS₂ electrons, modeled by a decreasing chemical potential, where many-body contributions dominate. Using a multi-gate architecture in monolayer MoTe₂ FETs, that allows for independent contact resistance and threshold voltage tuning, we integrate reconfigurable n- and p-FETs, and demonstrate a complementary inverter.
Temperature dependence of Barkhausen Jumps in hysterisis loops of permalloy Ni₀Fe₂₀
(2004-12-18) Gao, Bingrong; Erskine, James L.
This thesis studies the temperature dependence of Barkhausen Jumps (BJ) using the Magneto-Optical Kerr effect (MOKE). Both positive and negative jumps are analyzed at different temperatures. Measurements and analysis of positive jumps and negative jumps at room temperature (300 K) and low temperature (110 K) are described. The jump-amplitude critical exponent of the negative jumps (about 2.1) is observed to be bigger than that of positive jumps (about 1.6) and for positive jumps the exponent does not depend on temperature. The negative jump exponent measured at low temperature is bigger than at room temperature. The coercive force Hc becomes bigger when the temperature is reduced. We also show that the negative jumps are probably not caused by thermal activation but are most likely a consequence of the limited area covered by the beam spot. A next Generation experiment, based on two laser beams that simultaneously probe small and large overlapping regions of the sample is proposed to resolve the origin of non-thermally activated negative BJs.
The effect of temperature on the transport properties of high performance polymers
(2016-12) Stevens, Kevin Anthony; Freeman, Benny D. (Benny D.); Paul, Donald R; Ellison, Christopher J; Sanchez, Isaac C; Riffle, Judy S
The transport properties of high-performance polymers are typically studied near ambient conditions. However, many separations are performed at temperatures either above or below ambient conditions. The permeability and solubility of light gases in thermally rearranged (TR) polymers were measured as a function of temperature. Solubilities decreased with increasing temperature for all samples. At low TR conversion, the sorption process initially becomes less exothermic. However, enthalpies of sorption do not significantly change with TR conversion after thermal rearrangement. Permeabilities increase with increasing temperature for all but CO2 at the highest TR conversion. As extent of thermal rearrangement increases, activation energies increase slightly before decreasing significantly at higher TR conversions. Activation energies of diffusion decreased with increasing TR conversion while enthalpies of sorption remained mostly constant. At the highest TR conversion, decreases in temperature move the polymer toward the upper right on the upper bound. Polybenzimidazoles have been the focus of increasing amounts of study due to their good H2/CO2 separation properties and high thermal stability. Gas transport properties of a novel series of polybenzimidazoles based on a new tetraaminodiphenylsulfone (TADPS) monomer have been characterized at temperatures from 35 to 190 °C. Permeability increases with increasing temperature for all gases. Separations with TADPS-based PBIs are strongly size selective, with CO2/N2, CO2/CH4, and N2/CH4 selectivities decreasing with increasing temperature. However, H2/CO2 selectivities increase with increasing temperature due to a lower activation energy of permeation for CO2 than for H2.