Browsing UA Faculty Research by Authors
Anisotropic attosecond charge carrier dynamics and layer decoupling in quasi-2D layered SnS2Eads, Calley N.; Bandak, Dmytro; Neupane, Mahesh R.; Nordlund, Dennis; Monti, Oliver L. A.; Univ Arizona, Dept Chem & Biochem; Univ Arizona, Dept Phys (NATURE PUBLISHING GROUP, 2017-11-08)Strong quantum confinement effects lead to striking new physics in two-dimensional materials such as graphene or transition metal dichalcogenides. While spectroscopic fingerprints of such quantum confinement have been demonstrated widely, the consequences for carrier dynamics are at present less clear, particularly on ultrafast timescales. This is important for tailoring, probing, and understanding spin and electron dynamics in layered and two-dimensional materials even in cases where the desired bandgap engineering has been achieved. Here we show by means of core-hole clock spectroscopy that SnS2 exhibits spin-dependent attosecond charge delocalization times (tau(deloc)) for carriers confined within a layer, tau(deloc) < 400 as, whereas interlayer charge delocalization is dynamically quenched in excess of a factor of 10, tau(deloc) > 2.7 fs. These layer decoupling dynamics are a direct consequence of strongly anisotropic screening established within attoseconds, and demonstrate that important two-dimensional characteristics are also present in bulk crystals of van der Waals-layered materials, at least on ultrafast timescales.
Eigenstate-specific temperatures in two-level paramagnetic spin latticesMasthay, Mark B.; Eads, Calley N.; Johnson, Amber N.; Keil, Robert G.; Miller, Philip; Jones, Ross E.; Mashburn, Joe D.; Fannin, Harry B.; Univ Arizona, Dept Chem & Biochem; Department of Chemistry, University of Dayton, 300 College Park, Dayton, Ohio 45469-2357, USA; et al. (AMER INST PHYSICS, 2017-12-07)Increasing interest in the thermodynamics of small and/or isolated systems, in combination with recent observations of negative temperatures of atoms in ultracold optical lattices, has stimulated the need for estimating the conventional, canonical temperature T-c(conv) of systems in equilibrium with heat baths using eigenstate-specific temperatures (ESTs). Four distinct ESTs-continuous canonical, discrete canonical, continuous microcanonical, and discrete microcanonical-are accordingly derived for two-level paramagnetic spin lattices (PSLs) in external magnetic fields. At large N, the four ESTs are intensive, equal to T-c(conv), and obey all four laws of thermodynamics. In contrast, for N < 1000, the ESTs of most PSL eigenstates are non-intensive, differ from T-c(conv), and violate each of the thermodynamic laws. Hence, in spite of their similarities to T-c(conv) at large N, the ESTs are not true thermodynamic temperatures. Even so, each of the ESTs manifests a unique functional dependence on energy which clearly specifies the magnitude and direction of their deviation from T-c(conv); the ESTs are thus good temperature estimators for small PSLs. The thermodynamic uncertainty relation is obeyed only by the ESTs of small canonical PSLs; it is violated by large canonical PSLs and by microcanonical PSLs of any size. The ESTs of population-inverted eigenstates are negative (positive) when calculated using Boltzmann (Gibbs) entropies; the thermodynamic implications of these entropically induced differences in sign are discussed in light of adiabatic invariance of the entropies. Potential applications of the four ESTs to nanothermometers and to systems with long-range interactions are discussed. Published by AIP Publishing.