arXiv subjects

Keivan Esfarjani

Publications and source records attributed to Keivan Esfarjani.

35 records · Page 2Linked to original sources

arXiv2014

Thermal Interface Conductance between Aluminum and Silicon by Molecular Dynamics Simulations

The thermal interface conductance between Al and Si was simulated by a non-equilibrium molecular dynamics method. In the simulations, the coupling between electrons and phonons in Al are considered by using a stochastic force. The results show the size dependence of the interface thermal conductance and the effect of electron-phonon coupling on the interface thermal conductance. To understand the mechanism of interface resistance, the vibration power spectra are calculated. We find that the atomic level disorder near the interface is an important aspect of interfacial phonon transport, which leads to a modification of the phonon states near the interface. There, the vibrational spectrum near the interface greatly differs from the bulk. This change in the vibrational spectrum affects the results predicted by AMM and DMM theories and indicates new physics is involved with phonon transport across interfaces. Keywords:

cond-mat.mtrl-sci↗

arXiv2013

Gallium Arsenide Thermal Conductivity and Optical Phonon Relaxation Times from First-Principles Calculations

In this paper, thermal conductivity of crystalline GaAs is calculated using first-principles lattice dynamics. The harmonic and cubic force constants are obtained by fitting them to the force-displacement data from density functional theory calculations. Phonon dispersion is calculated from dynamical matrix constructed using the harmonic force constants and phonon relaxation times are calculated using Fermi Golden rule. The calculated GaAs thermal conductivity agrees well with experimental data. Thermal conductivity accumulations as a function of phonon mean free path and as a function of wavelength are obtained. Our results predict significant size effect on the GaAs thermal conductivity in the nanoscale. Relaxation times of optical phonons and their contributions from different scattering channels are also studied. Such information will help understanding hot phonon effects in GaAs-based devices.

cond-mat.mtrl-sci↗

arXiv2012

Lifetime of sub-THz coherent acoustic phonons in a GaAs-AlAs superlattice

We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses. By comparing measurements conducted at room temperature and liquid nitrogen temperature we separate the intrinsic (phonon-phonon scattering) and extrinsic contributions to phonon relaxation. The estimated room temperature intrinsic lifetime of 0.95 ns is compared to available calculations and experimental data for bulk GaAs. We conclude that ~0.3 THz phonons are in the transition zone between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room temperature.

cond-mat.mes-hall↗

arXiv2012

Microscopic mechanism of low thermal conductivity in lead-telluride

The microscopic physics behind low lattice thermal conductivity of single crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by the first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity, and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carrier in bulk materials.

cond-mat.mtrl-sci↗

arXiv2011

Heat transport in silicon from first principles calculations

Using harmonic and anharmonic force constants extracted from density-functional calculations within a supercell, we have developed a relatively simple but general method to compute thermodynamic and thermal properties of any crystal. First, from the harmonic, cubic, and quartic force constants we construct a force field for molecular dynamics (MD). It is exact in the limit of small atomic displacements and thus does not suffer from inaccuracies inherent in semi-empirical potentials such as Stillinger-Weber's. By using the Green-Kubo (GK) formula and molecular dynamics simulations, we extract the bulk thermal conductivity. This method is accurate at high temperatures where three-phonon processes need to be included to higher orders, but may suffer from size scaling issues. Next, we use perturbation theory (Fermi Golden rule) to extract the phonon lifetimes and compute the thermal conductivity $κ$ from the relaxation time approximation. This method is valid at most temperatures, but will overestimate $κ$ at very high temperatures, where higher order processes neglected in our calculations, also contribute. As a test, these methods are applied to bulk crystalline silicon, and the results are compared and differences discussed in more detail. The presented methodology paves the way for a systematic approach to model heat transport in solids using multiscale modeling, in which the relaxation time due to anharmonic 3-phonon processes is calculated quantitatively, in addition to the usual harmonic properties such as phonon frequencies and group velocities. It also allows the construction of accurate bulk interatomic potentials database.

cond-mat.mtrl-sci↗

arXiv2008

Negative differential resistance in molecular junctions: application to graphene ribbon junctions

Using self-consistent calculations based on Non-Equilibrium Green's Function (NEGF) formalism, the origin of negative differential resistance (NDR) in molecular junctions and quantum wires is investigated. Coupling of the molecule to electrodes becomes asymmetric at high bias due to asymmetry between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. This causes appearance of an asymmetric potential profile due to a depletion of charge and reduction of screening near the source electrode. With increasing bias, this sharp potential drop leads to an enhanced localization of the HOMO and LUMO states in different parts of the system. The reduction in overlap, caused by localization, results in a significant reduction in the transmission coefficient and current with increasing bias. An atomic chain connected to two Graphene ribbons was investigated to illustrate these effects. For a chain substituting a molecule, an even-odd effect is also observed in the NDR characteristics.

cond-mat.mes-hall↗

arXiv2007

A new methodology for the extraction of anharmonic force constants from first principles density functional calculations

A new method for extracting force constants (FC) from first principles is introduced. It requires small supercells but very accurate forces. In principle, provided that forces are accurate enough, it can extract harmonic as well as anharmonic FCs up to any neighbor shell. Symmetries of the FCs as well as those of the lattice are used to reduce the number of parameters to be calculated. Results are illustrated for the case of Lennard-Jones potential where forces are exact and FCs can be calculated analytically, and Si in the diamond structure. The latter are compared to previously calculated harmonic FCs.

cond-mat.mtrl-sci↗

arXiv2007

Divergence of effective mass in 'Uncorrelated State Percolation' Model

We want to answer the question of whether the divergence in the effective mass in metal-insulator transition (MIT) in 2DEG is in the same universality class as percolation. We use a model to make Percolated state in 2D and then calculate the effective mass in a super-cell and the Bloch Theorem. It is seen that the effective mass, m*, scales as m*~(P-Pc)^a with a=1 and Pc being the (classical) percolation threshold.

cond-mat.mes-hall↗

arXiv2006

Monte Carlo Simulation of Electron Transport in Degenerate Semiconductors

A modified algorithm is proposed to include Pauli exclusion principle in Monte-Carlo simulations. This algorithm has significant advantages to implement in terms of simplicity, speed and memory storage. We show that even in moderately high applied fields, one can estimate electronic distribution with a shifted Fermi sphere without introducing significant errors. Furthermore, the free-flights must be coupled to state availability constraints; this leads to substantial decrease in carrier heating at high fields. We give the correct definition for electronic temperature and show that in high applied fields, the quasi Fermi level is valley dependent. The effect of including Pauli exclusion principle on the band profile; electronic temperature and quasi Fermi level for inhomogeneous case of a single barrier heterostructure is illustrated.

cond-mat.mtrl-sci↗

arXiv2006

Thermoelectric transport perpendicular to thin film heterostructures using Monte Carlo technique

The Monte Carlo technique is used to calculate electrical as well as thermoelectric transport properties across thin film heterostructures. We study a thin InGaAsP barrier layer sandwiched between two InGaAs contact layers, when the barrier thickness is in 50nm-2000nm range. We found that with decreasing size, the effective Seebeck coefficient is increased substantially. The transition between pure ballistic thermionic transport and fully diffusive thermoelectric transport is also described.

cond-mat.mtrl-sci↗

arXiv2006

Spin dynamics characterization in magnetic dots

The spin structure in a magnetic dot, which is an example of a quantum few-body system, is studied as a function of exchange coupling strength and dot size with in the semiclassical approximation on a discrete lattice. As the exchange coupli ng is decreased or the size is increased, the ground state undergoes a phase cha nge from a single domain ferromagnet to a spin vortex. The line separating these two phases has been calculated numerically for small system sizes. %, and analytically for larger dots. The dipolar interaction has been fully included in our calculations. Magnon frequencies in such a dot have also been calculated in both phases by the linearized equation of motion method. These results have also been reproduced f rom the Fourier transform of the spin autocorrelation function. From the magnon Density Of States (DOS), it is possible to identify the magnetic phase of the dot. Furthermore, the magnon modes have been characterized for both the ferromagnetic and the vortex phase, and the magnon instability mechanism leading to the vortex-ferro transition has also been identified. The results can also be used to compute finite temperature magnetization or vort icity of magnetic dots.

cond-mat.other↗

arXiv2005

Thermoelectric properties of a nanocontact

Thermoelectric properties of a nanocontact made of two capped single wall nanotubes (SWNT) are calculated within the tight-binding approximation and Green's function method. It is found that semiconducting nanotubes can have high Seebeck coefficient very near the actual Fermi energy. This in turn leads to very high figures of merit easily exceeding one. Modifying the properties by shifting the chemical potential can be achieved by doping or application of a (nano-)gate voltage. The presence of impurities near the contact also strongly modify the properties and this can be taken advantage of, in order to design devices with high thermopower and figures of merit.

cond-mat.mtrl-sci↗

arXiv2004

Analytical results in coherent quantum transport for periodic quantum dot

In this paper we have calculated electron transport coefficient in ballistic regime through a periodic dot sandwiched between uniform leads. We have calculated the Green's function (GF), density of states (Dos) and the coherent transmission coefficient (conductance) fully analytically. The quasi gap, bound states energies, the energy and wire-length dependence of the GF and conductance for this system are also derived.

cond-mat.mtrl-sci↗

arXiv2004

Some analytic results in coherent quantum transport

A quantum wire of uniform cross section (but with eventual disorder) with three regions: dot, left lead, and right lead, is considered. Assuming that the same unitary transformation diagonalizes all unit cells of this wire, we propose a new formula for the calculation of the Greens function (GF) and the coherent transmission coefficient. This formula allows to calculate these quantitites much faster than the standard methods. In particular, the problem of a uniform dot (simple cubic uniform dot attached to the simple cubic wire), with all onsites equal and all hoppings equal is solved fully analytically. The energy and dot-length dependence of the GF, local density of states (LDOS), the transmission coefficient and bound state energies are also derived.

cond-mat.mes-hall↗

arXiv2003

Nonlinear charging, and transport times in doped nanotubes junctions

The nonlinear capacitance in doped nanotube junctions is calculated self consistently. It decreases as a function of the applied bias when the latter becomes larger than the pseudogap of the nanotube. For this device, one can deduce a relaxation time of about 0.1 femtosecond. Because of its negative differential resistance (NDR), a switching time of less than a fs can also be deduced.

cond-mat.mtrl-sci↗

arXiv2003

Optical Transitions at the Neutral and Charged Vacancies in Diamond

We used the exact eigenvectors of the generalized Hubbard Hamiltonian solution to predict the transition intensities of the well known $GR1$ and $ND1$ transitions at the neutral and charged vacancies in diamond. In addition to using exact eigenvectors, the method of the calculation is more precise than already reported calculations. The quantitative results can exlain recent experimental data very good.

cond-mat.mtrl-sci↗

arXiv2003

Exact solutions of generalized Hubbard Hamiltonian for diamond vacancies

A new formalism to calculate electronic states of vacancies in diamond has been developed using many-body techniques.This model is based on prevoius molecular models but does not use configuration interaction and molecular orbital techniques. A generalized Hubbard Hamiltonian which consists of all electron-electron interaction terms is calculated on the atomic orbital bases. Spatial symmetry $T_{d}$ and spin information of system are included in the form of Hamiltonian, so the eigenstates have automatically the correct spin and symmetry properties. Optimizing two key parameters of the model that justifies already reported semi-empirical values can predict accurate values of the famous absorption lines in neutral and charged vacancies i.e. $GR1$ and $ND1$. With these parameters the location of the low lying $^{3}T_{1}$ state is 113 mev above the ground state. In addition to these levels whole of the energy states of the system is predicted. Since the results are obtained without configuration interaction the model can gives the exact contribution of electronic configurations in the ground and excited states of the neutral and charged vacancies. The new results of the model for the ground and excited states of GR1 are reported.

cond-mat.mtrl-sci↗