arXiv subjects

Uwe Gerstmann

Publications and source records attributed to Uwe Gerstmann.

13 recordsLinked to original sources

arXiv2026

Second harmonic study of thermally oxidized mono- and few-layer 2H-MoS2

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

cond-mat.mtrl-sci↗

arXiv2024

Topological Defects in Semiconducting Carbon Nanotubes as Triplet Exciton Traps and Single-Photon Emitters

We investigate the role of topological defects in exciton behavior in (6,5) semiconducting single-walled carbon nanotubes using density functional theory. Our study identifies the helical Stone-Wales defect as a prominent trap for triplet excitons, characterized by a large zero-field splitting consistent with experimental data and a small singlet-triplet gap. The weak electron-phonon coupling, as evidenced by a Huang-Rhys factor of 0.74, renders it a promising single-photon emitter, with the zero-phonon line predicted at 1.6 $\mu$m, within the telecom range. These insights into defect-engineered electronic structure and exciton dynamics offer promising opportunities for improving the performance of carbon nanotube-based quantum light sources and optoelectronic devices.

cond-mat.mes-hall↗

arXiv2023

DFT-Assisted Investigation of the Electric Field and Charge Density Distribution of Pristine and Defective 2D WSe$_2$ by Differential Phase Contrast Imaging

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe$_2$ bi- and trilayers are revealed by using an emerging microscopy technique, differential phase contrast (DPC) imaging in the scanning transmission electron microscope (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

cond-mat.mtrl-sci↗

arXiv2021

Electron-nuclear coherent coupling and nuclear spin readout through optically polarized VB- spin states in hBN

Coherent coupling of defect spins with surrounding nuclei along with the endowment to read out the latter, are basic requirements for an application in quantum technologies. We show that negatively charged boron vacancies (VB-) in electron-irradiated hexagonal boron nitride (hBN) meet these prerequisites. We demonstrate Hahn-echo coherence of the VB- electron spin with a characteristic decay time Tcoh = 15 us, close to the theoretically predicted limit of 18 us for spin defects in hBN. Modulation in the MHz range superimposed on the Hahn-echo decay curve are shown to be induced by coherent coupling of the VB- spin with the three nearest 14N nuclei through a nuclear quadrupole interaction of 2.11 MHz. Supporting DFT calculation confirm that the electron-nuclear coupling is confined to the defective layer. Our findings allow an in-depth understanding of the electron-nuclear interactions of the VB- defect in hBN and demonstrate its strong potential in quantum technologies.

cond-mat.mes-hall↗

arXiv2021

Polaronic Enhancement of Second-Harmonic Generation in Lithium Niobate

Density-functional theory within a Berry-phase formulation of the dynamical polarization is used to determine the second-order susceptibility $\chi^{(2)}$ of lithium niobate (LiNbO$_3$). Defect trapped polarons and bipolarons are found to strongly enhance the nonlinear susceptibility of the material, in particular if localized at Nb$_\mathrm{V}$-V$_{\mathrm{Li}}$ defect pairs. This is essentially a consequence of the polaronic excitation resulting in relaxation-induced gap states. The occupation of these levels leads to strongly enhanced $\chi^{(2)}$ coefficients and allows for the spatial and transient modification of the second-harmonic generation of macroscopic samples.

physics.optics↗

arXiv2020

Tetracene ultrathin film growth on silicon

Inorganic-organic interfaces are important for enhancing the power conversion efficiency of silicon-based solar cells through singlet exciton fission (SF). We elucidated the structure of the first monolayers of tetracene (Tc), a SF molecule, on hydrogen-passivated Si(111) [H-Si(111)] and hydrogenated amorphous Si (a-Si:H) by combining near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) experiments with density functional theory (DFT) calculations. For samples grown at or below substrate temperatures of 265 K, the resulting ultrathin Tc films are dominated by almost upright-standing molecules. The molecular arrangement is very similar to the Tc bulk phase, with only slightly higher average angle between the conjugated molecular plane normal and the surface normal ($\alpha$) around 77{\deg}. Judging from carbon K-edge X-ray absorption spectra, the orientation of the Tc molecules are almost identical when grown on H-Si(111) and a-Si:H substrates as well as for (sub)mono- to several-monolayer coverages. Annealing to room temperature, however, changes the film structure towards a smaller $\alpha$ of about 63{\deg}. A detailed DFT-assisted analysis suggests that this structural transition is correlated with a lower packing density and requires a well-chosen amount of thermal energy. Therefore, we attribute the resulting structure to a distinct monolayer configuration that features less inclined, but still well-ordered molecules. The larger overlap with the substrate wavefunctions makes this arrangement attractive for an optimized interfacial electron transfer in SF-assisted silicon solar cells.

cond-mat.mtrl-sci↗

arXiv2019

Spin decontamination for magnetic dipolar coupling calculations: Application to high-spin molecules and solid-state spin qubits

An accurate description of the two-electron density, crucial for magnetic coupling in spin systems, provides in general a major challenge for density functional theory calculations. It affects, e.g., the calculated zero-field splitting (ZFS) energies of spin qubits in semiconductors that frequently deviate significantly from experiment. In the present work (i) we propose an efficient and robust strategy to correct for spin contamination in both extended periodic and finite-size systems, (ii) verify its accuracy using model high-spin molecules, and finally (iii) apply the methodology to calculate accurate ZFS of spin qubits (NV$^-$ centers, divacancies) in diamond and silicon carbide. The approach is shown to reduce the dependence on the used exchange-correlation functional to a minimum.

quant-ph↗

arXiv2019

Investigation of Near-Surface Defects of Nanodiamonds by High-Frequency EPR and DFT Calculation

Nanodiamond (ND) hosting nitrogen-vacancy (NV) centers is a promising platform for quantum sensing applications. Sensitivity of the applications using NV centers in NDs is often limited due to presence of paramagnetic impurity contents near the ND surface. Here, we investigate near-surface paramagnetic impurities in NDs. Using high-frequency (HF) electron paramagnetic resonance spectroscopy, the near-surface paramagnetic impurity within the shell of NDs is probed and its g-value is determined to be 2.0028(3). Furthermore, HF electron-electron double resonance-detected nuclear magnetic resonance spectroscopy and a first principle calculation show that a possible structure of the near-surface impurity is the negatively charged vacancy V-. The identification of the near-surface impurity by the present investigation provides a promising pathway to improve the NV properties in NDs and the NV-based sensing techniques.

cond-mat.mtrl-sci↗

arXiv2018

Proton Irradiation Induced Defects in \b{eta}-Ga2O3: a combined EPR and Theory Study

Proton irradiation of both n-type and semi-insulating bulk samples of \b{eta}-Ga2O3 leads to the formation of one paramagnetic defect with spin S=1/2, monoclinic point symmetry, a g-tensor with principal values of gb=2.0313, gc=2.0079, ga*= 2.0025 and quasi isotropic superhyperfine interaction of 13G with two equivalent Ga neigbours. Its high introduction rate indicates it to be a primary irradiation induced defect. At low temperature, photoexcitation transforms this defect into a different metastable S=1/2 center with principal g-values of gb=2.0064, gc=2.0464, ga*= 2.0024 and a reduced hyperfine interaction of 9G. This metastable defect is stable up to T=100K, when it switches back to the previous configuration. Density functional theory calculations of the Spin Hamiltonian parameters of various intrinsic defects are carried out using the Gauge Including Projector Augmented Wave method in order to determine the microscopic structure of these defects.Our results do not support the intuitive model of the isolated octahedral or tetrahedral gallium vacancy, VGa2-, but favor the model of a gallium vacancy complex VGa-Gai-VGa.

cond-mat.mtrl-sci↗

arXiv2018

Computational identification of Ga-vacancy related electron paramagnetic resonance centers in $\beta$-Ga$_2$O$_3$

A combined experimental/theoretical study of the EPR in irradiated $\beta$-Ga$_2$O$_3$ is presented. Four EPR spectra, two $S=1/2$ and two $S=1$, are observed after high-energy proton or electron irradiation. One of the S=1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters $g_b=2.0313$, $g_c=2.0079$, $g_{a*}=2.0025$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of $~\sim$14 G on $^{69}$Ga. The second (EPR2) is observed after photoexcitation (with threshold 2.8 eV) at low temperature and is characterized by $g_b=2.0064$, $g_c=2.0464$, $g_{a*}=2.0024$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of 10 G. A spin $S=1$ spectrum with a similar g-tensor and a 50\% reduced hyperfine splitting accompanies each of these, which is indicative of a defect of two weakly coupled $S=1/2$ centers. DFT calculations of the magnetic resonance fingerprint of a wide variety of native defect models are carried out to identify these EPR centers in terms of specific defect configurations. The EPR1 center is proposed to correspond to a complex of two tetrahedral $V_\mathrm{Ga1}$ with an interstitial Ga in between them. This model was previously shown to have lower energy than the simple tetrahedral Ga vacancy and has a $2-/3-$ transition level higher than other $V_\mathrm{Ga}$ related models, which would explain why the other ones are already in their diamagnetic $3-$ state and are thus not observed if the Fermi level is pinned approximately at this level. The EPR2 spectra are proposed to correspond to the octahedral $V_\mathrm{Ga2}$. Models based on self-trapped holes and oxygen interstitials are ruled out because they would have hyperfine interaction with more than two Ga nuclei and because they can not support a corresponding $S=1$ center.

cond-mat.mtrl-sci↗

arXiv2015

Mechanism for nuclear and electron spin excitation by radio frequency current

Recent radio frequency scanning tunneling spectroscopy (rf-STS) experiments have demonstrated nuclear and electron spin excitations up to $\pm12\hbar$ in a single molecular spin quantum dot (qudot). Despite the profound experimental evidence, the observed independence of the well-established dipole selection rules is not described by existing theory of magnetic resonance -- pointing to a new excitation mechanism. Here we solve the puzzle of the underlying mechanism by presenting all relevant mechanistic steps. At the heart of the mechanism, periodic transient charging and electric polarization due to the rf-modulated tunneling process cause a periodic asymmetric deformation of the qudot, enabling spin transitions via spin-phonon-like coupling. The mechanism has general relevance for a broad variety of different spin qudots exhibiting internal mechanical degrees of freedom (organic molecules, doped semiconductor qudots, nanocrystals, etc.).

cond-mat.mes-hall↗

arXiv2010

First-principles theory of the orbital magnetization

We compute the orbital magnetization in real materials by evaluating a recently discovered formula for periodic systems, within density functional theory. We obtain improved values of the orbital magnetization in the ferromagnetic metals Fe, Co, and Ni, by taking into account the contribution of the interstitial regions neglected so far in literature. We also use the orbital magnetization to compute the EPR $g$-tensor in molecules and solids. The present approach reproduces the $g$-tensor obtained by linear response (LR), when the spin-orbit can be treated as a perturbation. However, it can also be applied to radicals and defects with an orbital-degenerate ground-state or containing heavy atoms, that can not be properly described by LR.

cond-mat.mtrl-sci↗

arXiv2006

Phosphorus donors in highly strained silicon

The hyperfine interaction of phosphorus donors in fully strained Si thin films grown on virtual Si$_{1-x}$Ge$_x$ substrates with $x\leq 0.3$ is determined via electrically detected magnetic resonance. For highly strained epilayers, hyperfine interactions as low as 0.8 mT are observed, significantly below the limit predicted by valley repopulation. Within a Green's function approach, density functional theory (DFT) shows that the additional reduction is caused by the volume increase of the unit cell and a local relaxation of the Si ligands of the P donor.

cond-mat.mtrl-sci↗