arXiv subjects

Daniel Hashemi

Publications and source records attributed to Daniel Hashemi.

10 recordsLinked to original sources

arXiv2026

Theory of the charge response of the boron-vacancy spin in monolayer hexagonal boron nitride

The boron vacancy $V_{\mathrm{B}}^{-}$ in hexagonal boron nitride is a leading two-dimensional spin sensor, but the microscopic connection between nearby charge positions and its spin Hamiltonian remains poorly understood. Calculations for carbon and oxygen donors 3.85--8.08 $\mathring{\mathrm{A}}$ away show that direction matters, not only distance. In the frozen lattice, reversing a carbon donor's azimuth at 4.35 $\mathring{\mathrm{A}}$ (C1 to C1m) more than halves $|E|$ and turns its axis across the mirror line. The ratio is 0.467 with the Perdew--Burke--Ernzerhof (PBE) functional, against 1 for distance alone. A field-plus-gradient model fitted to six donor configurations, including C1 but excluding C1m, predicted a ratio of 0.443 before C1m was calculated. Heyd--Scuseria--Ernzerhof hybrid statics give 0.432 and PBE relaxation 0.600. Exploratory analysis suggests why direction matters: a charge's potential differs at the three vacancy nitrogens and moves the spin of the two $e'$ holes among them, more when the charge faces one nitrogen than the gap between two. $E$ follows this spin imbalance through one constant, $c\approx8$ GHz per $μ_{\mathrm{B}}$, across donors and applied fields, whereas in PBE $D$ is nearly insensitive to it. A three-site model with one fitted screening constant reproduces the frozen-lattice $E$ to 83 MHz rms. Results are for one charge state in a free-standing monolayer; absolute $|E|$ varies by tens of percent across approximations.

cond-mat.mtrl-sci↗

arXiv2026

A rhombic spin-1 interpretation of a room-temperature GaN emitter

Single defects in GaN show optically detected magnetic resonance at room temperature. In the more abundant class, a minimal $S=3/2$ model leaves a $g\approx 4$ line unexplained. For one defect, this line lies near the sum of the two outer lines, consistent with transitions among three shared levels. A rhombic $S=1$ model reproduces the three line positions with rms errors of 8.1-9.8 MHz. The tested second-order $S=3/2$ fits containing all three transitions and the published minimal model do not reproduce these positions. A ladder fitted to the strong lines remains viable if the $g\approx 4$ line is a separate transition near the sum of the outer two. An unresolved frequency-window shift prevents the existing data from establishing the $S=1$ assignment uniquely. We propose a zero-field sweep that directly tests the isolated $S=1$ assignment.

cond-mat.mtrl-sci↗

arXiv2026

Placement-driven reversal of preferred spin alignment in monolayer hBN vacancy pairs

The negatively charged boron vacancy is an optically addressable spin centre in hexagonal boron nitride (hBN), but bringing two vacancies together introduces electronic interactions absent from the isolated-defect picture. Here we use density-functional theory at total charge q = -2 to show that atomic-scale placement reverses the preferred collinear spin alignment in monolayer hBN. Among seven configurations, the closest pair, separated by 4.3 angstrom, favours parallel alignment by 27.6 meV within the semilocal approximation. Moving one vacancy by a single lattice vector reverses the preference, and a 7.5 angstrom pair favours antiparallel alignment by 50.7 meV. Numerical controls change these two reference splittings by less than 1 meV; hybrid-functional calculations retain their signs while substantially changing their magnitudes. Local distortions connect the antiparallel preference at 7.5 angstrom to changes in vacancy-state hybridization along an intervening atomic chain. The closest pair instead exhibits a contact-localized spin-density contrast and constrained angular energies that depart from the bilinear Heisenberg form. These results identify a placement-sensitive regime of interacting defects and provide a microscopic foundation for investigating coupled spin centres in atomically thin materials.

cond-mat.mtrl-sci↗

arXiv2023

A molecular dynamics simulation study of thermal conductivity of plumbene

We investigate the thermal conductivity of plumbene using molecular dynamics simulations, overcoming existing limitations by optimizing the parameters of Tersoff and Stillinger-Weber potentials via artificial neural networks. Our findings indicate that at room temperature, the thermal conductivity of a 1050 A* 300 A plumbene sheet is approximately 8 W/m.K, significantly lower (23%) than that of bulk lead. Our analysis elucidates that thermal conductivity is enhanced by increased sample length, while it is reduced by temperature. Moreover, plumbene samples with zigzag edges display superior thermal conductivity compared to those with armchair edges. In addition, the thermal conductivity of plumbene exhibits an increase at low tensile strains, whereas it decreases as the strains become larger. This investigation provides crucial insights into the thermal conductivity behavior of plumbene under varying conditions.

cond-mat.mtrl-sci↗

arXiv2019

Vicinal metal surfaces as potential catalysts for phosphorene epitaxial growth

Phosphorene, a single layer of black phosphorous (BLK-P), has a significant potential for flexible and tunable electronics, but attempts to grow it epitaxially have been unsuccessful to date. Meanwhile, hexagonal blue phoshorous (BL-P) has been achieved on closed-packed (111) metal surfaces in special growth conditions of high vapor pressure and high reactivity of phosphorous. The (111) surfaces favors BL-P over BLK-P due to its hexagonal symmetry. Here, we investigate computationally the alternative offered by stepped substrates. Using the Cu(311) surface as a model, we find that surface steps can favor energetically BLK-P over BL-P. This can be rationalized in terms of surface density of states and orbital hybridization, which lead to a stronger surface bonding of the lower BLK-P half-layer. This work suggests that vicinal metal surfaces of metals can offer a viable path towards phosphorene synthesis.

cond-mat.mtrl-sci↗

arXiv2019

Substrate-Controlled Magnetism: Fe nanowires on vicinal Cu surfaces

Here we present a novel approach to control magnetic interactions in atomic-scale nanowires. Our ab initio calculations demonstrate the possibility to tune magnetic properties of Fe nanowires formed on vicinal Cu surfaces. Both intrawire and interwire magnetic exchange parameters are extracted from DFT calculations. This study suggests that the effective interwire magnetic exchange parameters exhibit Ruderman--Kittel--Kasuya--Yosida-like (RKKY) oscillations as a function of Fe interwire separation. The choice of vicinal Cu surface offers possibilities for controlling the magnetic coupling. Furthermore, an anisotropic Heisenberg model was used in Monte Carlo simulations to examine the stability of these magnetic configurations at finite temperature. The predicted critical temperatures of the Fe nanowires on Cu(422) and Cu(533) surfaces are well-above room temperature.

cond-mat.mtrl-sci↗

arXiv2017

Design principles for the energy level tuning in donor/acceptor conjugated polymers

To identify reliable molecular design principles for energy level tuning in donor/acceptor conjugated polymers (CPs), we studied the governing factors by means of ab initio calculations based on density-functional theory (DFT). We investigated a series of CPs in which we independently and systematically varied the electron withdrawing power of the acceptor unit and the electron donating power of the donor unit, while maintaining the same conjugated chain conformation. We observed that the introduction of a stronger acceptor unit, while keeping the same donor unit in the CP, lowers the LUMO level, but leaves the HOMO level almost unchanged. Conversely, enhancing the strength of the donor unit for the same acceptor unit raises the HOMO level, while maintaining the LUMO level. We identified strong correlations between the frontier orbital energy levels and the degree of orbital localization, which depends on the electron donating or withdrawing power of the molecular groups carrying the orbitals. Moreover, the HOMO/LUMO gap of the CP is directly proportional to the charge transfer between donating and accepting units, which provides a robust design criterion for CPs.

cond-mat.mtrl-sci↗

arXiv2017

Effect of Long-Range Dielectric Screening on Charge Transfer States

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.

cond-mat.mes-hall↗

arXiv2017

Predictive Simulations for Tuning Electronic and Optical Properties of SubPc Derivatives

Boron subphthalocyanine chloride is an electron donor material used in small molecule organic photovoltaics with an unusually large molecular dipole moment. Using first-principles calculations, we investigate enhancing the electronic and optical properties of boron subphthalocyanine chloride, by substituting the boron and chlorine atoms with other trivalent and halogen atoms in order to modify the molecular dipole moment. Gas phase molecular structures and properties are predicted with hybrid functionals. Using positions and orientations of the known compounds as the starting coordinates for these molecules, stable crystalline structures are derived following a procedure that involves perturbation and accurate total energy minimization. Electronic structure and photonic properties of the predicted crystals are computed using the GW method and the Bethe-Salpeter equation, respectively. Finally, a simple transport model is use to demonstrate the importance of molecular dipole moments on device performance.

cond-mat.mtrl-sci↗

arXiv2017

Semiclassical Model for Calculating Exciton and Polaron Pair Energetics at Interfaces

Exciton and polaron pair dissociation is a key functional aspect of photovoltaic devices. To improve upon the current state of interfacial transport models, we augment the existing classical models of dielectric interfaces by incorporating results from ab initio calculations, allowing us to calculate exciton and polaron binding energies more accurately. We demonstrate the predictive capabilities of this new model using two interfaces: (i) the boron subphthalocyanine chloride (SubPc) and C60 interface, which is an archetype for many organic photovoltaic devices; and (ii) pentacene and silicon (100), which represents a hybrid between organic and inorganic semiconductors. Our calculations predict that the insertion of molecular dipoles at interfaces can be used for improving polaron pair dissociation and that sharp transitions in dielectric permittivity can have a stronger effect on the polaron pair dissociation than even the electron-hole Coulomb interaction.

cond-mat.mtrl-sci↗