Browsing by Subject "Monte Carlo method"

Now showing 1 - 4 of 4

Open Access
Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime
(The American Physical Society, 2003) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.
Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1-2 MV/ cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission., However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.
Open Access
Exploring the electronic and magnetic properties of new metal halides from bulk to two-dimensional monolayer: RuX3 (X = Br, I)
(Elsevier, 2018) Ersan, Fatih; Vatansever, E.; Sarikurt, S.; Yüksel, Y.; Kadıoğlu, Yelda; Ozaydin, H.; Aktürk, O. Ü.; Akıncı, Ü.; Aktürk, E.
Theoretical and experimental studies present that metal halogens in MX3 forms can show very interesting electronic and magnetic properties in their bulk and monolayer phases. Many MX3 materials have layered structures in their bulk phases, while RuBr3 and RuI3 have one-dimensional chains in plane. In this paper, we show that these metal halogens can also form two-dimensional layered structures in the bulk phase similar to other metal halogens, and cleavage energy values confirm that the monolayers of RuX3 can be possible to be synthesized. We also find that monolayers of RuX3 prefer ferromagnetic spin orientation in the plane for Ru atoms. Their ferromagnetic ground state, however, changes to antiferromagnetic zigzag state after U is included. Calculations using PBE + U with SOC predict indirect band gap of 0.70 eV and 0.32 eV for the optimized structure of RuBr3 and RuI3, respectively. Calculation based on the Monte Carlo simulations reveal interesting magnetic properties of RuBr3, such as large Curie temperature against RuI3, both in bulk and monolayer cases. Moreover, as a result of varying exchange couplings between neighboring magnetic moments, magnetic properties of RuBr3 and RuI3 can undergo drastic changes from bulk to monolayer. We hope our findings can be useful to attempt to fabricate the bulk and monolayer of RuBr3 and RuI3.
Open Access
Influence of phase function on modeled optical response of nanoparticle-labeled epithelial tissues
(2011) Cihan, C.; Arifler, D.
Metal nanoparticles can be functionalized with biomolecules to selectively localize in precancerous tissues and can act as optical contrast enhancers for reflectance-based diagnosis of epithelial precancer. We carry out Monte Carlo (MC) simulations to analyze photon propagation through nanoparticle-labeled tissues and to reveal the importance of using a proper form of phase function for modeling purposes. We first employ modified phase functions generated with a weighting scheme that accounts for the relative scattering strengths of unlabeled tissue and nanoparticles. To present a comparative analysis, we repeat ourMCsimulations with simplified functions that only approximate the angular scattering properties of labeled tissues. The results obtained for common optical sensor geometries and biologically relevant labeling schemes indicate that the exact form of the phase function used as model input plays an important role in determining the reflectance response and approximating functions often prove inadequate in predicting the extent of contrast enhancement due to labeling. Detected reflectance intensities computed with different phase functions can differ up to ̃60% and such a significant deviation may even alter the perceived contrast profile. These results need to be taken into account when developing photon propagation models to assess the diagnostic potential of nanoparticle-enhanced optical measurements. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).
Open Access
Spin-dependent analysis of two-dimensional electron liquids
(The American Physical Society, 2002) Bulutay, C.; Tanatar, Bilal
Two-dimensional electron liquid (2D EL) at full Fermi degeneracy is revisited, giving special attention to the spin-polarization effects. First, we extend the recently proposed classical-map hypernetted-chain (CHNC) technique to the 2D EL, while preserving the simplicity of the original proposal. An efficient implementation of CHNC is given utilizing Lado's quadrature expressions for the isotropic Fourier transforms. Our results indicate that the paramagnetic phase stays to be the ground state until the Wigner crystallization density, even though the energy separation with the ferromagnetic and other partially polarized states become minute. We analyze compressibility and spin stiffness variations with respect to density and spin polarization, the latter being overlooked until now. Spin-dependent static structure factor and pair-distribution functions are computed; agreement with the available quantum Monte Carlo data persists even in the strong-coupling regime of the 2D EL.