Bulletin of the American Physical Society
Annual Meeting of the APS Four Corners Section
Volume 60, Number 11
Friday–Saturday, October 16–17, 2015; Tempe, Arizona
Session E4: Condensed Matter V |
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Chair: Weigang Wang, Arizona State University Room: MU202 |
Friday, October 16, 2015 3:17PM - 3:29PM |
E4.00001: \textbf{Optical Constants and Band Structure of NiO} Ayana Ghosh, Cayla Nelson, Lina Abdallah, Stefan Zollner Using spectroscopic ellipsometry and transmission measurements, the optical constants (absorption coefficient, complex refractive index, dielectric function) of bulk NiO from 0.08 to 6.5 eV are determined. To obtain an accurate dielectric function of NiO, elastic scattering by oxygen bubbles was ruled out by analyzing the data. The effects of surface roughness were also removed to calculate the same. A direct band gap of 0.85 eV was found from transmission and assigned to direct interband transitions from the Ni-O hybrid valence band states to the Ni 4s conduction band at the center of the Brilloun zone. At 4 eV, we find the well-known charge transfer gap from the lower to the upper Hubbard band. Several intermediate sharp peaks were also found. The temperature dependence of the NiO charge transfer gap is similar to the E1 gap of Si between 100 and 700 K. At higher temperatures, heating NiO in vacuum leads to sublimation, which has drastic irreversible consequences for the pseudodielectric function of the sample, including a strong Ni nanoparticle plasmon peak at 2 eV. [Preview Abstract] |
Friday, October 16, 2015 3:29PM - 3:41PM |
E4.00002: Dynamical transition in glycerol Salman Seyedi, Daniel Martin, Dmitry Matyushov Mean square displacements of hydrogens reported by incoherent elastic neutron scattering show a kink in their temperature dependence. This crossover connects two approximately linear regimes and is known as the dynamical transition temperature. The dynamical transition is widely observed for glass-formers, including molecular liquids and biopolymers. It is often assigned to the dynamical freezing of subsets of molecular modes at the point of equality between the relaxation time of the system and the instrumental observation window. The origin of the dynamical transition is studied here for glass-forming glycerol by extensive molecular dynamics simulations. We found that the dynamical transition occurs for center of mass translations and molecular rotations at the same temperature insensitive to changes of the observation window. In addition, both the translational and rotational dynamics of glycerol show a dynamic crossover from the structural to a secondary relaxation at the temperature of the dynamical transition. The simulation results consistently point to a structural alteration of glycerol responsible for both the kink in the mean square displacements and the dynamic crossover, instead of the effect of the finite observation window. [Preview Abstract] |
Friday, October 16, 2015 3:41PM - 4:17PM |
E4.00003: Ultrafast Broadband Optical Spectroscopy of Complex Materials. Invited Speaker: Dmitry Yarotski Remarkable properties of complex materials, such as multiferroics and topological insulators, often emerge from strong interactions among charge, lattice, spin and orbital degrees of freedom. Labyrinthine pattern of these competing interactions has prevented development of predictive theoretical frameworks and basic principles required to harness materials properties for technological applications. In this regard, the femtosecond temporal resolution, combined with spectral selectivity available with ultrafast optical spectroscopy offers an unmatched ability to temporally discriminate the dynamics of various degrees of freedom, and more importantly, the dynamics of the coupling among them. Here, we discuss an application of powerful ultrafast spectroscopic techniques to investigate the non-equilibrium behavior of several classes of complex materials. In actinide materials, hybridization between localized f-electrons and itinerant d-electrons result in a wide spectrum of exotic states. We probed the quasiparticle relaxation dynamics in URu$_{2}$Si$_{2}$ in a broad temperature range, and demonstrated an appearance of pseudogap state as a possible precursor to the enigmatic hidden-order phase. Topological insulators represent a new state of matter where insulating bulk is surrounded by a conducting surface. We applied terahertz spectroscopy at low temperatures to separate the bulk from the surface transient responses in Bi$_{2}$Se$_{3}$, and showed that short-lived bulk carriers co-exist with the long-lived surface carriers which feature significantly higher mobility. Finally, we revealed energy transfer pathways from electrons to magnons in multiferroic HoMnO3 compound where ferroelectricity and magnetism co-exist and are strongly coupled. Our measurements show that energy of photoexcitation is initially transferred from electrons to phonons and subsequently to magnons through spin-lattice relaxation. [Preview Abstract] |
Friday, October 16, 2015 4:17PM - 4:29PM |
E4.00004: A simplified Markov model for 1/f noise Andrew Shevchuk, Ralph Chamberlin, Bryce Davis, Priscilla Greenwood If a system is weakly coupled to an effectively infinite heat bath, standard fluctuation theory predicts that the power spectral density will exhibit frequency-independent (or ``white'') noise. However, at low frequencies most materials exhibit 1/f (or ``pink'') noise, which still has no widely accepted general explanation. A recent mechanism is that 1/f noise arises from direct coupling between the system and its bath. We investigate a Markov model containing (Ising-like) binary degrees of freedom that define a system, and unary degrees of freedom that reside in a bath. The model takes the form of a rectangular matrix for which the rows represent the thermodynamic macrostates and the columns represent the thermodynamic microstates. The macrostates are a function of the system’s binary degrees of freedom alone, so that the bath states cause the dynamics of the system to pause. Although these pause states simply delay the dynamics of the system, they must be included in the total number of microstates to maintain maximum entropy. The partitioning of microstates between the system and the bath affects the transition probabilities between macrostates. Including these pause states in a Markov process yields 1/f noise, qualitatively agreeing with the behavior observed in materials. [Preview Abstract] |
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