17 research outputs found
Anisotropic Small-Polaron Hopping In W:Bivo4 Single Crystals
DC electrical conductivity, Seebeck and Hall coefficients are measured between 300 and 450 K on single crystals of monoclinic bismuth vanadate that are doped n-type with 0.3% tungsten donors (W:BiVO4). Strongly activated small-polaron hopping is implied by the activation energies of the Arrhenius conductivities (about 300 meV) greatly exceeding the energies characterizing the falls of the Seebeck coefficients' magnitudes with increasing temperature (about 50 meV). Small-polaron hopping is further evidenced by the measured Hall mobility in the ab-plane (10(-1) cm(2) V-1 s(-1) at 300 K) being larger and much less strongly activated than the deduced drift mobility (about 5 x 10(-5) cm(2) V-1 s(-1) at 300 K). The conductivity and n-type Seebeck coefficient is found to be anisotropic with the conductivity larger and the Seebeck coefficient's magnitude smaller and less temperature dependent for motion within the ab-plane than that in the c-direction. These anisotropies are addressed by considering highly anisotropic next-nearest-neighbor (approximate to 5 angstrom) transfers in addition to the somewhat shorter (approximate to 4 angstrom), nearly isotropic nearest-neighbor transfers. (C) 2015 AIP Publishing LLC.U.S. Department of Energy (DOE), DE-FG02-09ER16119Welch Foundation Grant F-1436Hemphill-Gilmore Endowed FellowshipNSF MIRT DMR 1122603Chemical EngineeringTexas Materials InstituteChemistr
Spin-Phonon Interaction in Yttrium Iron Garnet
Spin-phonon interaction is an important channel for spin and energy
relaxation in magnetic insulators. Understanding this interaction is critical
for developing magnetic insulator-based spintronic devices. Quantifying this
interaction in yttrium iron garnet (YIG), one of the most extensively
investigated magnetic insulators, remains challenging because of the large
number of atoms in a unit cell. Here, we report temperature-dependent and
polarization-resolved Raman measurements in a YIG bulk crystal. We first
classify the phonon modes based on their symmetry. We then develop a modified
mean-field theory and define a symmetry-adapted parameter to quantify
spin-phonon interaction in a phonon-mode specific way for the first time in
YIG. Based on this improved mean-field theory, we discover a positive
correlation between the spin-phonon interaction strength and the phonon
frequency.Comment: 12 pages, 4 figures, 1 table; (Supp. Info. 10 pages, 5 figures, 2
tables
Pure Spin Current and Magnon Chemical Potential in a Nonequilibrium Magnetic Insulator
Nonequilibrium phenomena are ubiquitous in nature and in a wide range of systems, including cold atomic gases and solid-state materials. While these phenomena are challenging to describe both theoretically and experimentally, they are essential for the fundamental understanding of many-body systems and practical devices. In the context of spintronics, when a magnetic insulator (MI) is subjected to a thermal gradient, a pure spin current is generated in the form of magnons without the presence and dissipation of a charge current-attractive for reducing energy consumption and central to the emerging field of spin caloritronics. However, the experimental methods for directly quantifying a spin current in insulators and for probing local phonon-magnon nonequilibrium and the associated magnon chemical potential are largely missing. Here, we apply a heating laser to generate a thermal gradient in the MI yttrium iron garnet (YIG), Y3Fe5O12, and evaluate two components of the spin current, driven by temperature and chemical potential gradients, respectively. The experimental method and theory approach for evaluating quasiparticle chemical potential can be applied for analogous phenomena in other many-body systems
Combined Charge Carrier Transport and Photoelectrochemical Characterization of BiVO<sub>4</sub> Single Crystals: Intrinsic Behavior of a Complex Metal Oxide
Bismuth
vanadate (BiVO<sub>4</sub>) is a promising photoelectrode
material for the oxidation of water, but fundamental studies of this
material are lacking. To address this, we report electrical and photoelectrochemical
(PEC) properties of BiVO<sub>4</sub> single crystals (undoped, 0.6%
Mo, and 0.3% W:BiVO<sub>4</sub>) grown using the floating zone technique.
We demonstrate that a small polaron hopping conduction mechanism dominates
from 250 to 400 K, undergoing a transition to a variable-range hopping
mechanism at lower temperatures. An anisotropy ratio of ∼3
was observed along the <i>c</i> axis, attributed to the
layered structure of BiVO<sub>4</sub>. Measurements of the ac field
Hall effect yielded an electron mobility of ∼0.2 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> for Mo and W:BiVO<sub>4</sub> at 300 K. By application of the Gärtner model, a hole
diffusion length of ∼100 nm was estimated. As a result of low
carrier mobility, attempts to measure the dc Hall effect were unsuccessful.
Analyses of the Raman spectra showed that Mo and W substituted for
V and acted as donor impurities. Mott–Schottky analysis of
electrodes with the (001) face exposed yielded a flat band potential
of 0.03–0.08 V versus the reversible H<sub>2</sub> electrode,
while incident photon conversion efficiency tests showed that the
dark coloration of the doped single crystals did not result in additional
photocurrent. Comparison of these intrinsic properties to those of
other metal oxides for PEC applications gives valuable insight into
this material as a photoanode