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Register a new userTuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering
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We reduce the dimensionless interaction strength in graphene by adding a water overlayer in ultra-high vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30 percent, due to the background dielectric constant enhancement leading to reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short range impurities is decreased by almost 40 percent, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron/hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.
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Electrons in graphene behave like Dirac fermions, permitting phenomena from high energy physics to be studied in a solid state setting. A key question is whether or not these Fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite, and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding, graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine structure constant, alpha *(k,omega), whose value approaches alpha * ~ 1/7 at low energy and large distances. This value is substantially smaller than the nominal alpha = 2.2, suggesting that, on the whole, graphene is more weakly interacting than previously believed.
Exciton dissociation at heterojunctions in photovoltaic devices is not completely understood despite being fundamentally necessary to generate electrical current. One of the fundamental issues for ab initio calculations is that hybrid interfaces combining materials with Wannier-Mott excitons and those with Frenkel excitons can easily require thousands of atoms to encompass the exciton-wave function. The problem is further exacerbated by a large permittivity difference at the interface, which requires meso-scale boundary conditions to accurately predict electrostatic potentials. For these reasons, we have constructed a model of excited states at hybrid interfaces based on an effective mass Schroedinger equation. In this continuum model, carrier wave functions are represented by their envelope function rather than resolving the atomic scale variations. Electrostatic interactions are accounted for using the Poisson equation. For our model system, we use a pentacene/silicon interface. Because carrier mobility is low in pentacene relative to silicon, the hole is frozen such that it only interacts with the electron though an immobile positive charge density. The inputs to this model are as follows: dielectric permittivities, electron effective masses, interfacial width, band alignment, and the hole wave function. We find that the energetic favorability of charge transfer states relative to bulk excitons is most easily controlled by band alignment. However, when both states have similar energies, interface proximity and electrostatics become important secondary means of tuning the relative stability of these states.
Using transmission electron microscopy (TEM) we studied CaCu3Ti4O12, an intriguing material that exhibits a huge dielectric response, up to kilohertz frequencies, over a wide range of temperature. Neither in single crystals, nor in polycrystalline samples, including sintered bulk- and thin-films, did we observe the twin domains suggested in the literature. Nevertheless, in the single crystals, we saw a very high density of dislocations with a Burger vector of [110], as well as regions with cation disorder and planar defects with a displacement vector 1/4[110]. In the polycrystalline samples, we observed many grain boundaries with oxygen deficiency, in comparison with the grain interior. The defect-related structural disorders and inhomogeneity, serving as an internal barrier layer capacitance (IBLC) in a semiconducting matrix, might explain the very large dielectric response of the material. Our TEM study of the structure defects in CaCu3Ti4O12 supports a recently proposed morphological model with percolating conducting regions and blocking regions.
The structural and magnetic properties of the face-centered cubic double perovskite Ba2MnWO6 were investigated using neutron powder diffraction, DC-magnetometry, muon spin relaxation and inelastic neutron scattering. Ba2MnWO6 undergoes Type II long-range antiferromagnetic ordering at a Neel temperature of 8(1) K with a frustration index, f = 8. Inelastic neutron scattering was used to identify the magnetic coupling constants J1 and J2, which were found to equal -0.080 meV and -0.076 meV respectively. This indicated that both of the magnetic coupling constants were antiferromagnetic with similar magnitudes, which is in contrast to other known 3d metal double perovskites Ba2MWO6. Above the Neel temperature, muon spin relaxation measurements and inelastic neutron scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered cubic lattice antiferromagnet MnO.
We have studied the dielectric screening of electric field which is induced by a gate voltage in twisted double bilayer graphene by using a sample with a mismatch angle of about 5 degrees. In low temperature magnetotransport measurements, quantum oscillations of magnetoresistance originating from two bands with different carrier density were observed. The behavior of the carrier densities with respect to the total carrier density were distinct from that of the AB-stacked tetralayer graphene. The carrier density ratio was theoretically analyzed in terms of the model that the induced charge decays exponentially with distance with a screening length {lambda}. The estimated {lambda} was slightly larger than that of AB-stacked graphene, which would possibly reflect the difference in the inter-plane distribution of probability of the wave function.
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