arXiv subjects

Roderick Melnik

Publications and source records attributed to Roderick Melnik.

63 records · Page 4Linked to original sources

arXiv2013

Electrical control of phonon mediated spin relaxation rate in semiconductor quantum dots: the Rashba vs the Dresselhaus spin-orbit couplings

In symmetric quantum dots (QDs), it is well known that the spin-hot spot (i.e., the cusp-like structure due to the presence of degeneracy near the level or anticrossing point) is present for the pure Rashba case but is absent for the pure Dresselhaus case [Phys. Rev. Lett. 95, 076805 (2005)]. Since the Dresselhaus spin-orbit coupling dominates over the Rashba spin-orbit coupling in GaAs and GaSb QDs, it is important to find the exact location of the spin-hot spot or the cusp-like structure even for the pure Dresselhaus case. In this paper, for the first time, we present analytical and numerical results that show that the spin-hot spot can also be seen for the pure Dresselhaus spin-orbit coupling case by inducing large anisotropy through external gates. At or nearby the spin-hot spot, the spin transition rate enhances and the decoherence time reduces by several orders of magnitude compared to the case with no spin-hot spot. Thus one should avoid such locations when designing QD spin based transistors for the possible implementation in quantum logic gates, solid state quantum computing and quantum information processing. It is also possible to extract the exact experimental data (Phys. Rev. Lett. 100, 046803 (2008)) for the phonon mediated spin-flip rates from our developed theoretical model.

cond-mat.mes-hall↗

arXiv2012

Temperature dependent elastic constants and ultimate strength of graphene and graphyne

Based on the first principles calculation combined with quasi-harmonic approximation, in this work we focus on the analysis of temperature dependent lattice geometries, thermal expansion coefficients, elastic constants and ultimate strength of graphene and graphyne. For the linear thermal expansion coefficient, both graphene and graphyne show a negative region in the low temperature regime. This coefficient increases up to be positive at high temperatures. Graphene has superior mechanical properties, with Young modulus E11=371.0 N/m, E22=378.2 N/m and ultimate tensile strength of 119.2 GPa at room temperature. Based on our analysis, it is found that graphene's mechanical properties have strong resistance against temperature increase up to 1200 K. Graphyne also shows good mechanical properties, with Young modulus E11=224.7 N/m, E22=223.9 N/m and ultimate tensile strength of 81.2 GPa at room temperature, but graphyne's mechanical properties have a weaker resistance with respect to the increase of temperature than that of graphene.

cond-mat.mtrl-sci↗

arXiv2012

Anisotropic effects and phonon induced spin relaxation in gate-controlled semiconductor quantum dots

In this paper, a detailed analysis of anisotropic effects on the phonon induced spin relaxation rate in III-V semiconductor quantum dots (QDs) is carried out. We show that the accidental degeneracy due to level crossing between the first and second excited states of opposite electron spin states in both isotropic and anisotropic QDs can be manipulated with the application of externally applied gate potentials. In particular, anisotropic gate potentials enhance the phonon mediated spin-flip rate and reduce the cusp-like structure to lower magnetic fields, in addition to the lower QDs radii in III-V semiconductor QDs. In InAs QDs, only the Rashba spin-orbit coupling contributes to the phonon induced spin relaxation rate. However, for GaAs QDs, the Rashba spin-orbit coupling has a contribution near the accidental degeneracy point and the Dresselhaus spin-orbit coupling has a contribution below and above the accidental degeneracy point in the manipulation of phonon induced spin relaxation rates in QDs.

cond-mat.mes-hall↗

arXiv2012

Temperature dependent elastic constants for crystals with arbitrary symmetry: combined first principles and continuum elasticity theory

To study temperature dependent elastic constants, a new computational method is proposed by combining continuum elasticity theory and first principles calculations. A Gibbs free energy function with one variable with respect to strain at given temperature and pressure was derived, hence the full minimization of the Gibbs free energy with respect to temperature and lattice parameters can be put into effective operation by using first principles. Therefore, with this new theory, anisotropic thermal expansion and temperature dependent elastic constants can be obtained for crystals with arbitrary symmetry. In addition, we apply our method to hexagonal beryllium, hexagonal diamond and cubic diamond to illustrate its general applicability.

cond-mat.mtrl-sci↗

arXiv2011

Hydrogen-doped cubic diamond and the crystal structure of n-diamond

To understand the crystal structure of n-diamond, a hydrogen-doped (H-doped) diamond model has been investigated using first principles calculations. In particular, hydrogen concentration dependent elastic constants and lattice parameters for the H-doped diamond have been analyzed. Our results indicate that when the hydrogen concentration is less than 19 at.%, the H-doped diamond is mechanically stable. When the hydrogen concentration is about 4 at.%, the optimized lattice parameter, simulated XRD pattern and electronic properties for the H-doped diamond all agree well with the corresponding experimental values of n-diamond. The results imply that the n-diamond is likely to be an H-doped diamond.

cond-mat.mtrl-sci↗

arXiv2011

Manipulation of single electron spin in a GaAs quantum dot through the application of geometric phases: The Feynman disentangling technique

The spin of a single electron in an electrically defined quantum dot in a 2DEG can be manipulated by moving the quantum dot adiabatically in a closed loop in the 2D plane under the influence of applied gate potentials. In this paper we present analytical expressions and numerical simulations for the spin-flip probabilities during the adiabatic evolution in the presence of the Rashba and Dresselhaus linear spin-orbit interactions. We use the Feynman disentanglement technique to determine the non-Abelian Berry phase and we find exact analytical expressions for three special cases: (i) the pure Rashba spin-orbit coupling, (ii) the pure Dresselhause linear spin-orbit coupling, and (iii) the mixture of the Rashba and Dresselhaus spin-orbit couplings with equal strength. For a mixture of the Rashba and Dresselhaus spin-orbit couplings with unequal strengths, we obtain simulation results by solving numerically the Riccati equation originating from the disentangling procedure. We find that the spin-flip probability in the presence of the mixed spin-orbit couplings is generally larger than those for the pure Rashba case and for the pure Dresselhaus case, and that the complete spin-flip takes place only when the Rashba and Dresselhaus spin-orbit couplings are mixed symmetrically.

cond-mat.mes-hall↗

arXiv2011

Manipulation of the Land$\acute{\text{e}}$ g-factor in InAs quantum dots through the application of anisotropic gate potentials: Exact diagonalization, numerical and perturbation methods

We study the variation in the Land$\acute{\text{e}}$ g-factor of electron spins induced by both anisotropic gate potentials and magnetic fields in InAs quantum dots for possible implementation towards solid state quantum computing. In this paper, we present analytical expressions and numerical simulations of the variation in the Land$\acute{\text{e}}$ g-factor for both isotropic and anisotropic quantum dots. Using both analytical techniques and numerical simulations, we show that the Rashba spin-orbit coupling has a major contribution in the variation of the g-factor with electric fields before the regime, where level crossing or anticrossing occurs. In particular, the electric field tunability is shown to cover a wide range of g-factor through strong Rashba spin-orbit interaction. Another major result of this paper is that the anisotropic gate potential gives quenching effect in the orbital angular momentum that reduces the variation in the E-field and B-field tunability of the g-factor if the area of the symmetric and asymmetric quantum dots is held constant. We identify level crossings and anticrossings of the electron states in the variation of the Land$\acute{\text{e}}$ g-factor. We model the wavefunctions of electron spins and estimate the size of the anticrossing for the spin states $|0,-1,+1/2>$ and $|0,0,-1/2>$ corresponding to a quantum dot that has been recently studied experimentally (Phys. Rev. Lett. \textbf{104}, 246801 (2010)).

cond-mat.mes-hall↗

arXiv2010

Multiband Hamiltonians of the Luttinger-Kohn Theory and Ellipticity Requirements

Modern applications require a robust and theoretically strong tool for the realistic modeling of electronic states in low dimensional nanostructures. The $k \cdot p$ theory has fruitfully served this role for the long time since its creation. During last two decades several problems have been detected in connection with the application of the $k \cdot p$ approach to such nanostructures. These problems are closely related to the violation of the ellipticity conditions for the underlying system, the fact that until recently has been largely overlooked. We demonstrate that in many cases the models derived by a formal application of the Luttinger-Kohn theory fail to satisfy the ellipticity requirements. The detailed analysis, presented here on an example of the $6 \times 6$ Hamiltonians, shows that this failure has a strong impact on the physically important properties conventionally studied with these models.

cond-mat.mtrl-sci↗

arXiv2010

Coarse Graining RNA Nanostructures for Molecular Dynamics Simulations

A series of coarse-grained models have been developed for the study of the molecular dynamics of RNA nanostructures. The models in the series have one to three beads per nucleotide and include different amounts of detailed structural information. Such a treatment allows us to reach, for the systems of thousands of nucleotides, a time scale of microseconds (i.e. by three orders of magnitude longer than in the full atomistic modelling) and thus to enable simulations of large RNA polymers in the context of bionanotechnology. We find that the 3-beads-per-nucleotide models, described by a set of just a few universal parameters, are able to describe different RNA conformations and are comparable in structural precision to the models where detailed values of the backbone P-C4' dihedrals taken from a reference structure are included. These findings are discussed in the context of the RNA conformation classes.

q-bio.QM↗