374 research outputs found
Three-particle Complexes in Two-Dimensional Semiconductors
We map the three-body problem in two dimensions onto one particle in a three
dimensional potential treatable by a purposely-developed
boundary-matching-matrix method. We evaluate binding energies of trions
, excitons bound by a donor/acceptor charge , and overcharged
acceptors/donors in two-dimensional atomic crystals of transition metal
dichalcogenides, where interaction between charges features logarithmic
behavior at intermediate distances. We find that dissociation energy of
is, typically, much larger than that of localised exciton complexes,
so that trions are more resilient to heating, despite that their recombination
line in optics is much less red-shifted from the exciton line, as compared to
Comment: 5.1 pages, 3 figures,+ supplementary material (5 pages); Improved
numerics; Monte Carlo data added; Published versio
The physics of single-side fluorination of graphene: DFT and DFT+U studies
We present density functional theory (DFT) calculations of the electronic and magnetic properties of fluorine adatoms on a single side of a graphene monolayer. By extrapolating the results, the binding energy of a single fluorine adatom on graphene in the dilute limit is calculated. Our results confirm that the finite-size error in the binding energy scales inversely with the cube of the linear size of the simulation cell. We establish relationships between stability and C–F bond nature, diffusion of fluorine adatoms and total magnetization in different configurations of adatoms. For single-side fluorination, sp 2.33 is the maximum p-content re-hybridization found in the C–F bond. We show that semilocal DFT cannot predict correctly the magnetic properties of fluorinated graphene and a higher level theory, such as DFT+U is needed. The results indicate a tendency of graphene to reduce the imbalance between adsorption on the two sublattices, and therefore total magnetization, through low-energy-barrier pathways on a time scale of ~10 ps at room temperature. The thermodynamically favored arrangements are those with the smallest total magnetization. Indeed, the electronic structure is intimately related to the magnetic properties and changes from semi-metallic to p-type half-metallic or semiconducting features, depending on the adatoms arrangement
Trail-Needs pseudopotentials in quantum Monte Carlo calculations with plane-wave/blip basis sets
We report a systematic analysis of the performance of a widely used set of Dirac-Fock pseudopotentials for quantum Monte Carlo (QMC) calculations. We study each atom in the periodic table from hydrogen (Z = 1) to mercury (Z = 80), with the exception of the 4f elements (57 ≤ Z ≤ 70). We demonstrate that ghost states are a potentially serious problem when plane-wave basis sets are used in density functional theory (DFT) orbital-generation calculations, but that this problem can be almost entirely eliminated by choosing the s channel to be local in the DFT calculation; the d channel can then be chosen to be local in subsequent QMC calculations, which generally leads to more accurate results. We investigate the achievable energy variance per electron with different levels of trial wave function and we determine appropriate plane-wave cutoff energies for DFT calculations for each pseudopotential. We demonstrate that the so-called “T-move” scheme in diffusion Monte Carlo is essential for many elements. We investigate the optimal choice of spherical integration rule for pseudopotential projectors in QMC calculations. The information reported here will prove crucial in the planning and execution of QMC projects involving beyond-first-row elements
Importance of high-angular-momentum channels in pseudopotentials for quantum Monte Carlo
Quantum Monte Carlo methods provide in principle a highly accurate treatment of the many-body problem of calculating the ground and excited states of condensed systems. In practice, however, uncontrolled errors, such as those arising from the fixed-node and pseudopotential approximations can be problematic. We show that the accuracy of some quantum Monte Carlo calculations is limited by the properties of currently available pseudopotentials. The use of pseudopotentials involves several approximations, and we will focus on one that is relatively simple to correct during the pseudopotential design phase. It is necessary to include angular-momentum channels in the pseudopotential for excited angular-momentum states and to choose the local channel appropriately to obtain accurate results. Variational and diffusion Monte Carlo calculations for Zn, O, and Si atoms and ions demonstrate these issues. Adding higher-angular-momentum channels into the pseudopotential description reduces such errors without a significant increase in computational cost
Nature of the metallization transition in solid hydrogen
We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the C 2 / c , P c , P b c n , and P 6 3 / m structures at pressures of 250, 300, and 350 GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body gap in the thermodynamic limit. By comparing our static-nucleus DMC energy gaps with available experimental results, we demonstrate the important role played by nuclear quantum effects in the electronic structure of solid hydrogen
Off-shell selfenergy for 1-D Fermi liquids
The selfenergy in Born approximation including exchange of interacting
one-dimensional systems is expressed in terms of a single integral about the
potential which allows a fast and precise calculation for any potential
analytically. The imaginary part of the self energy as damping of
single-particle excitations shows a rich structure of different areas limited
by single-particle and collective excitation lines. The corresponding spectral
function reveals a pseudogap, a splitting of excitation into holons and
antiholons as well as bound states
Quantum Monte Carlo study of Doppler broadening of positron annihilation radiation in semiconductors and insulators
The measurement of the momentum distribution of positron annihilation
radiation is a powerful method to detect and identify open-volume defects in
crystalline solids. The Doppler broadening of the 511 keV line of the
electron-positron annihilation event reflects the momentum density of
annihilating pairs and local electron momenta at positron annihilation sites.
It can provide information on the chemical surroundings of vacancies, such as
the impurity atoms around them. Accurate methods based on first-principles
calculations are crucial for interpreting measured Doppler spectra. In this
work we will validate such a method based on variational quantum Monte Carlo by
benchmarking results in aluminium nitride and silicon against experimental data
measured from defect-free reference samples. The method directly models
electron-positron correlations using variational wave functions. We achieve
better agreement with experiments for our test set than conventional
state-of-the-art methods. We show that normalized Doppler broadening spectra
calculated with quantum Monte Carlo converge rapidly as a function of
simulation cell size, and backflow transformations have only a minor effect.
This makes the method robust and practical to support positron-based
spectroscopies.Comment: 10 pages, 4 figure
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