arXiv subjects

Martin Albrecht

Publications and source records attributed to Martin Albrecht.

36 records · Page 2Linked to original sources

arXiv2021

Fingerprints of optical absorption in the perovskite LaInO$_{3}$: Insight from many-body theory and experiment

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite LaInO$_{3}$. Employing density-functional theory and many-body perturbation theory, we predict a direct electronic quasiparticle band gap of about 5 eV and an effective electron (hole) mass of 0.31 (0.48) m$_{0}$. We find the lowest-energy excitation at 0.2 eV below the fundamental gap, reflecting a sizeable electron-hole attraction. Since the transition from the valence band maximum (VBM, $Γ$ point) is, however, dipole forbidden the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not resolved by the ellipsometry measurement. The absorption edge shows a strong dependency on the light polarization, reflecting the character of the involved valence states. Temperature-dependent measurements show a redshift of the optical gap by about 120 meV by increasing the temperature from 5 to 300 K. Renormalization due to zero-point vibrations is extrapolated from the latter measurement to amount to 150 meV. By adding the excitonic binding energy of 0.2 eV obtained theoretically to the experimental optical absorption onset, we determine the fundamental band gap at room temperature to be 4.55 eV.

cond-mat.mtrl-sci↗

arXiv2020

The role of the interface in controlling the epitaxial relationship between orthorhombic $\text{LaInO}_\text{3}$ and cubic $\text{BaSnO}_\text{3}$

Epitaxial perovskite oxide interfaces with different symmetry of the epitaxial layers have attracted considerable attention due to the emergence of novel behaviors and phenomena. In this paper, we show by aberration corrected transmission electron microscopy that orthorhombic $\text{LaInO}_\text{3}$ films grow in form of three different types of domains on the cubic $\text{BaSnO}_\text{3}$ pseudosubstrate. Quantitative evaluation of our TEM data shows that $c_{pc}$-oriented and $a_{pc}/b_{pc}$-oriented domains are present with similar probability. While continuum elasticity theory suggests that $c_{pc}$-oriented domains should exhibit a significantly higher strain energy density than $a_{pc}/b_{pc}$-oriented domains, density functional calculations confirm that $c_{pc}$- and $a_{pc}$-oriented domains on $\text{BaSnO}_\text{3}$ have similar energies.

cond-mat.mtrl-sci↗

arXiv2020

The role of 2s Bloch wave state excitations on STEM-HAADF intensity in quantitative analysis of alloys

In this work, we emphasize the important contribution of the 2s Bloch wave state to the properties of a STEM electron probe propagating on an atomic column. For a strong enough column potential, the confinement of the 2s state leads to a long-period oscillation of the electron wave function, which is reflected in the resulting STEM-HAADF intensity. We show how this influences STEM composition quantification even at large thicknesses. We found additionally that the excitation of the 2s state affects the intensity of alloys where long-range order phenomena are present, which in turn provides a way to probe the degree of order in alloys.

cond-mat.mtrl-sci↗

arXiv2020

Plasma-assisted molecular beam epitaxy of NiO on GaN(00.1)

The growth of NiO on GaN(00.1) substrates by plasma-assisted molecular beam epitaxy under oxygen rich conditions was investigated at growth temperatures between 100 $^{\circ}$C and 850 $^{\circ}$C. Epitaxial growth of NiO(111) with two rotational domains, with epitaxial relation $\normalsize{}\mathrm{\mathrm{\mathrm{NiO}(1\bar{\mathrm{1}}0)}\:||\:\mathrm{\mathrm{GaN}(11.0)}}$ and $\mathrm{\mathrm{\mathrm{NiO}\mathrm{(10\bar{\mathrm{1}})}\:||\:\mathrm{GaN(11.0)}}}$, was observed by X-ray diffraction (XRD) and confirmed by in-situ reflection high-energy electron diffraction as well as transmission electron microscopy (TEM) and electron backscatter diffraction. With respect to the high lattice mismatch of 8.1 % and a measured low residual tensile layer strain, growth by lattice matching epitaxy or domain matching epitaxy is discussed. The morphology measured by atomic force microscopy showed a grainy surface, probably arising from the growth by the columnar rotational domains visible in TEM micrographs. The domain sizes measured by AFM and TEM increase with the growth temperature, indicating an increasing surface diffusion length. Growth at 850 $^{\circ}$C, however, involved local decomposition of the GaN substrate that lead to an interfacial $\mathrmβ$-Ga$\mathrm{_{2}}$O$\mathrm{_{3}}$($\bar{\mathrm{2}}$01) layer and a high NiO surface roughness. Raman mesurements of the quasi-forbidden one-phonon peak indicate increasing layer quality (decreasing defect density) with increasing growth temperature. The results above suggest optimum growth temperatures around 700 $^{\circ}$C for high layer and interface quality.

physics.app-ph↗

arXiv2016

Atomic signatures of local environment from core-level spectroscopy in $β$-Ga$_2$O$_3$

We present a joint theoretical and experimental study on core-level excitations from the oxygen $K$ edge of $β$-Ga$_2$O$_3$. A detailed analysis of the electronic structure reveals the importance of O-Ga hybridization effects in the conduction region. The spectrum from O 1$s$ core electrons is dominated by excitonic effects, which overall redshift the absorption onset by 0.5 eV, and significantly redistribute the intensity to lower energies. Analysis of the spectra obtained within many-body perturbation theory reveals atomic fingerprints of the inequivalent O atoms. From the comparison of energy-loss near-edge fine-structure (ELNES) spectra computed with respect to different crystal planes, with measurements recorded under the corresponding diffraction conditions, we show how the spectral contributions of specific O atoms can be enhanced while quenching others. These results suggest ELNES, combined with ab initio many-body theory, as a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and to their local environment.

cond-mat.mtrl-sci↗

arXiv2015

Comparison of the luminous efficiency of Ga- and N-polar In$_{x}$Ga$_{1-x}$N/In$_{y}$Ga$_{1-y}$N quantum wells grown by plasma-assisted molecular beam epitaxy

We investigate the luminescence of Ga- and N-polar In$_{x}$Ga$_{1-x}$N/In$_{y}$Ga$_{1-y}$N quantum wells (QWs) grown by plasma-assisted molecular beam epitaxy on freestanding GaN as well as 6H-SiC substrates. In striking contrast to their Ga-polar counterparts, the N-polar QWs prepared on freestanding GaN do not exhibit any detectable photoluminescence. Theoretical simulations of the band profiles combined with resonant excitation of the QWs allow us to rule out carrier escape and subsequent surface recombination as the reason for the absence of luminescence. To explore the hypothesis of a high concentration of nonradiative defects at the interfaces between wells and barriers, we analyze Ga- and N-polar QWs prepared on 6H-SiC as a function of the well width. Intense luminescence is observed for both Ga- and N polar samples. As expected, the luminescence of the Ga-polar QWs quenches and red-shifts with increasing well width due to the quantum confined Stark effect. In contrast, both the intensity and the energy of the luminescence from the N-polar samples are essentially independent of well width. Transmission electron microscopy reveals that the N-polar QWs exhibit abrupt interfaces and homogeneous composition, excluding emission from In-rich clusters as the reason for this anomalous behavior. The microscopic origin of the luminescence in the N-polar QWs is elucidated using spatially resolved cathodoluminescence spectroscopy. Regardless of well width, the luminescence is found to not originate from the N-polar QWs, but from the semipolar facets of v-pit defects. These results cast serious doubts on the potential of N-polar QWs grown by plasma-assisted molecular beam epitaxy for the development of long-wavelength light emitting diodes. What remains to be seen is whether unconventional growth conditions may enable a significant reduction in the concentration of nonradiative defects.

cond-mat.mtrl-sci↗

arXiv2014

Temperature-Dependent Thermoelectric Properties of Individual Silver Nanowires

Individual highly pure single crystalline silver nanowires (Ag NWs) were investigated with regard to the electrical conductivity $σ$, the thermal conductivity $λ$ and the Seebeck coefficient $S$ as function of the temperature $T$ between $1.4\,\mathrm{K}$ and room temperature (RT). Transmission electron microscopy was performed subsequently to the thermoelectric characterization of the Ag NWs, so that their transport properties can be correlated with the structural data. The crystal structure, surface morphology and the rare occurrence of kinks and twinning were identified. The thermoelectric properties of the Ag NWs are discussed in comparison to the bulk: $S_{\mathrm{Ag,Pt}}(T)$ was measured with respect to platinum and is in agreement with the bulk, $σ(T)$ and $λ(T)$ showed reduced values with respect to the bulk. The latter are both notably dominated by surface scattering caused by an increased surface-to-volume ratio. By lowering $T$ the electron mean free path strongly exceeds the NW's diameter of $150\,\mathrm{nm}$ so that the transition from diffusive transport to quasi ballistic one dimensional transport is observed. An important result of this work is that the Lorenz number $L(T)$ turns out to be independent of surface scattering. Instead the characteristic of $L(T)$ is determined by the material's purity. Moreover, $σ(T)$ and $L(T)$ can be described by the bulk Debye temperature of silver.

cond-mat.mes-hall↗

arXiv2013

High-Voltage Field Effect Transistors with Wide-Bandgap β-Ga2O3 Nanomembranes

Nanoscale semiconductor materials have been extensively investigated as the channel materials of transistors for energy-efficient low-power logic switches to enable scaling to smaller dimensions. On the opposite end of transistor applications is power electronics for which transistors capable of switching very high voltages are necessary. Miniaturization of energy-efficient power switches can enable the integration with various electronic systems and lead to substantial boosts in energy efficiency. Nanotechnology is yet to have an impact in this arena. In this work, it is demonstrated that nanomembranes of the wide-bandgap semiconductor gallium oxide can be used as channels of transistors capable of switching high voltages, and at the same time can be integrated on any platform. The findings mark a step towards using lessons learnt in nanomaterials and nanotechnology to address a challenge that yet remains untouched by the field.

cond-mat.mes-hall↗

arXiv2010

F4/5

We describe an algorithm to compute Gröbner bases which combines F4-style reduction with the F5 criteria. Both F4 and F5 originate in the work of Jean-Charles Faugère, who has successfully computed many Gröbner bases that were previously considered intractable. Another description of a similar algorithm already exists in Gwenole Ars' dissertation; unfortunately, this is only available in French, and although an implementation exists, it is not made available for study. We not only describe the algorithm, we also direct the reader to a study implementation for the free and open source Sage computer algebra system. We conclude with a short discussion of how the approach described here compares and contrasts with that of Ars' dissertation.

math.AC↗

arXiv2008

Efficient Multiplication of Dense Matrices over GF(2)

We describe an efficient implementation of a hierarchy of algorithms for multiplication of dense matrices over the field with two elements (GF(2)). In particular we present our implementation -- in the M4RI library -- of Strassen-Winograd matrix multiplication and the "Method of the Four Russians" multiplication (M4RM) and compare it against other available implementations. Good performance is demonstrated on on AMD's Opteron and particulary good performance on Intel's Core 2 Duo. The open-source M4RI library is available stand-alone as well as part of the Sage mathematics software. In machine terms, addition in GF(2) is logical-XOR, and multiplication is logical-AND, thus a machine word of 64-bits allows one to operate on 64 elements of GF(2) in parallel: at most one CPU cycle for 64 parallel additions or multiplications. As such, element-wise operations over GF(2) are relatively cheap. In fact, in this paper, we conclude that the actual bottlenecks are memory reads and writes and issues of data locality. We present our empirical findings in relation to minimizing these and give an analysis thereof.

cs.MS↗

arXiv2005

A wave function based ab initio non-equilibrium Green's function approach to charge transport

We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Green's function formalism. In addition we present an extension so as to include effects of the two-particle propagator. Our procedure is demonstrated for a dithiolbenzene molecule between silver electrodes. The full current-voltage characteristic is calculated. Specific conclusions for the contribution of correlation and two-particle effects are derived. The latter are found to contribute about 5% to the current. The order of magnitude of the current coincides with experiments.

cond-mat.mtrl-sci↗

arXiv2005

Charge Transport Properties of Molecular Junctions built from Dithiol Polyenes

We present a study of the charge transmission behavior of a series of dithiol polyenes in the context of molecular junctions. Using the Landauer theory and zero voltage approximation the Green's functions of the inserted molecules are calculated from a fully ab initio wave function based procedure. Various possibilities in approximating the correlation space are explored and quantitatively evaluated. Our results show that the transmission behavior of a molecular junction is not a monotonic function of the length of the employed molecule. Moreover we introduce the analytic solution of a suitable model system to countercheck the ab initio results and find a remarkable degree of correspondence.

cond-mat.mtrl-sci↗

arXiv2005

Ab initio Green's function formalism for band structures

Using the Green's function formalism, an ab initio theory for band structures of crystals is derived starting from the Hartree-Fock approximation. It is based on the algebraic diagrammatic construction scheme for the self-energy which is formulated for crystal orbitals (CO-ADC). In this approach, the poles of the Green's function are determined by solving a suitable Hermitian eigenvalue problem. The method is not only applicable to the outer valence and conduction bands, it is also stable for inner valence bands where strong electron correlations are effective. The key to the proposed scheme is to evaluate the self-energy in terms of Wannier orbitals before transforming it to a crystal momentum representation. Exploiting the fact that electron correlations are mainly local, one can truncate the lattice summations by an appropriate configuration selection scheme. This yields a flat configuration space; i.e., its size scales only linearly with the number of atoms per unit cell for large systems and, under certain conditions, the computational effort to determine band structures also scales linearly. As a first application of the new formalism, a lithium fluoride crystal has been chosen. A minimal basis set description is studied, and a satisfactory agreement with previous theoretical and experimental results for the fundamental band gap and the width of the F 2p valence band complex is obtained.

cond-mat.other↗

arXiv2005

An ab initio non-equilibrium Green's function approach to charge transport: dithiolethine

We present a novel ab initio non-equilibrium approach to calculate the current across a molecular junction. The method rests on a wave function based full ab initio description of the central region of the junction combined with a tight binding approximation for the electrodes in the frame of the Keldysh Green's function formalism. Our procedure is demonstrated for a dithiolethine molecule between silver electrodes. The main conducting channel is identified and the full current-voltage characteristic is calculated.

cond-mat.mtrl-sci↗

arXiv2005

Implicit infinite lattice summations for real space ab initio correlation methods

We suggest a local wave function based ab initio correlation method for infinite periodic systems, which can describe both the near range as well as the long range correlation effects coherently in the same scheme. Specifically, this work introduces a formalism which allows to describe the long range polarization cloud around a quasi particle in a solid explicitly in the formalism of local wave function based ab initio descriptions. To this end we reformulate the infinite lattice summation underlying the quantum chemistry formula to second order in a closed analytic form employing the elliptic theta function of the third kind. All formulas and manipulations are developed explicitely in full detail and a first numeric example demonstrates the principle idea. Good results for the long range polarization effects in LiH and LiF are found in agreement with earlier estimates.

cond-mat.mtrl-sci↗

arXiv2004

Towards a frequency independent incremental ab initio scheme for the self energy

The frequency dependence of the self energy of a general many--body problem is identified as a main obstacle in correlation calculations based on local approaches. A frequency independent formulation is proposed instead and proven to yield exactly the same numerical results as the original common scheme. Our approach is embedded in a general local-orbital based ab initio frame to obtain the Green's function for large heterogenous systems. First a Green's function formalism is introduced. Then the self energy is constructed from an incremental scheme. Subsequently we apply the proposed frequency independent formulation. The theory is applied to para-ditholbenzene as a realistic system and the numerical accuracy of the correlation contributions obtained from our frequency independent access are carefully tested against the exact frequency dependent results. Perfect agreement is reached and a speed--up of a factor 50 is established for the incremental scheme.

cond-mat.mtrl-sci↗

arXiv1998

A Hartree-Fock ab initio band-structure calculation employing Wannier-type orbitals

An ab initio Wannier-function-based approach to electronic ground-state calculations for crystalline solids is outlined. In the framework of the linear combination of atomic orbitals method the infinite character of the solid is rigorously taken into account. The Hartree-Fock ground-state energy, cohesive energy, lattice constant and bulk modulus are calculated in a fully ab initio manner as it is demonstrated for sodium chloride, NaCl, using basis sets close to the Hartree-Fock limit. It is demonstrated that the Hartree-Fock band-structure can easily be recovered with the current approach and agrees with the one obtained from a more conventional Bloch-orbital-based calculation. It is argued that the advantage of the present approach lies in its capability to include electron correlation effects for crystalline insulators by means of well-established quantum chemical procedures.

cond-mat↗

arXiv1997

Correlated ab-initio calculations for ground-state properties of II-VI semiconductors

Correlated ab-initio ground-state calculations, using relativistic energy-consistent pseudopotentials, are performed for six II-VI semiconductors. Valence ($ns,np$) correlations are evaluated using the coupled cluster approach with single and double excitations. An incremental scheme is applied based on correlation contributions of localized bond orbitals and of pairs and triples of such bonds. In view of the high polarity of the bonds in II-VI compounds, we examine both, ionic and covalent embedding schemes for the calculation of individual bond increments. Also, a partitioning of the correlation energy according to local ionic increments is tested. Core-valence ($nsp,(n-1)d$) correlation effects are taken into account via a core-polarization potential. Combining the results at the correlated level with corresponding Hartree-Fock data we recover about 94% of the experimental cohesive energies; lattice constants are accurate to \sim 1%; bulk moduli are on average 10% too large compared with experiment.

cond-mat.mtrl-sci↗