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Materials Science (cond-mat.mtrl-sci)

Mon, 17 Apr 2023

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1.Morphology of buried interfaces in ion-assisted magnetron sputter deposited 11B4C-containing Ni/Ti multilayer neutron optics investigated by grazing incidence small angle scattering

Authors:Sjoerd Broekhuijsen, Naureen Ghafoor, Mattias Schwartzkopf, Anton Zubayer, Jens Birch, Fredrik Eriksson

Abstract: The achieved interface width in multilayers is crucial for the performance of different optical components used in neutron beamlines. In this work we investigate how different growth conditions affect the interface morphology of Ni/Ti based multilayers, giving a crucial insight into the optimization of multilayer growth. Specifically, the effects of incorporating low-neutron-absorbing 11B4C into Ni/Ti multilayers have been investigated, as well as the effects of different ion assistance schemes. Coupled fits on combined X-ray and neutron reflectivity data reveal achieved interface widths in the multilayers of 2.7 {\AA} using multilayers where the growth parameters were optimized to the found conditions.

2.Atomistic Wear Mechanisms in Diamond: Effects of Surface Orientation, Stress, and Interaction with Adsorbed Molecules

Authors:Huong T. T. Ta, Nam V. Tran, M. C. Righi

Abstract: Despite its unrivaled hardness, diamond can be severely worn during the interaction with others, even softer materials. In this work, we calculate from first-principles the energy and forces necessary to induce the atomistic wear of diamond, and compare them for different surface orientations and passivation by oxygen, hydrogen, and water fragments. The primary mechanism of wear is identified as the detachment of carbon chains. This is particularly true for oxidized diamond and diamond interacting with silica. A very interesting result concerns the role of stress, which reveals that compressive stresses can highly favor wear, making it even energetically favorable.

3.Ionization Potentials and Fundamental Gaps in Atomic Systems from the Ensemble-DFT Approach

Authors:Sharon Lavie, Yuli Goshen, Eli Kraisler

Abstract: Calculations in Kohn-Sham density functional theory crucially rely on high-quality approximations for the exchange-correlation (xc) functional. Standard local and semi-local approximations fail to predict the ionization potential (IP) and the fundamental gap, departing from the Kohn-Sham orbital energies, due to the deviation of the total energy from piecewise-linearity and the absence of the derivative discontinuity. The ensemble generalization procedure introduced in Phys. Rev. Lett. 110, 126403 (2013) restores, to a large extent, these features in any approximate xc functional and improves its ability to predict the IP and the fundamental gap with negligible additional computational effort. In this work we perform an extensive study of atoms and first ions across the Periodic Table, generalizing the local spin-density and the Perdew-Burke-Ernzerhof approximations. By applying the ensemble generalization to a variety of systems, with s-, p- and d-character, we assess the accuracy of the method and identify important trends. In particular, we find that the accuracy of our approach heavily depends on the character of the frontier orbitals: when d-orbitals are involved, the performance is far less accurate. Possible sources of error are discussed and ways for further improvement are outlined.

4.Mechanical behavior of ion-irradiated ODS RAF steels strengthened with different types of refractory oxides

Authors:M. Frelek-Kozak, Ł. Kurpaska, K. Mulewska, M. Zieliński, R. Diduszko, A. Kosińska, D. Kalita, W. Chromiński, M. Turek, K. Kaszyca, A. Zaborowska, J. Jagielski

Abstract: In the present work, authors focused on verifying structural and mechanical properties of Oxide Dispersed Strengthening (ODS) steels strengthened by three different types of refractory oxides submitted to ion-irradiation. Three materials strengthened with Y2O3 or Al2O3 or ZrO2 were produced by mechanical alloying and Spark Plasma Sintering technique. Specimens have been submitted to high energy Ar-ion irradiation at room temperature with three fluences. This procedure allowed to generate strongly damaged zone with a thickness of 230nm. SEM/EBSD and TEM observations, GIXRD analysis, and nanoindentation tests have been included in examination of modified layers. Investigation revealed alteration of structural and mechanical features as a result of Ar-irradiation. Obtained results showed a strong correlation between the strengthening oxide and materials' behavior under radiation damage. It has been proved that below 1x1015ions/cm2 mechanical properties in the modified layer of all materials are very similar. Reported behavior may be related to the efficient annealing of the radiation defect process. Above this limit, significant differences between the materials are visible. It is believed that described phenomenon is directly related to the presence of the structural features and their capacity to act as defect sinks. Consequently, type of dominant mechanisms occurring in modified layer is proposed.

5.Development of Nb-GaAs based superconductor semiconductor hybrid platform by combining in-situ dc magnetron sputtering and molecular beam epitaxy

Authors:Clemens Todt, Sjoerd Telkamp, Filip Krizek, Christian Reichl, Mihai Gabureac, Rüdiger Schott, Erik Cheah, Peng Zeng, Thomas Weber, Arnold Müller, Christof Vockenhuber, Mohsen Bahrami Panah, Andreas Wallraff, Werner Wegscheider

Abstract: We present Nb thin films deposited in-situ on GaAs by combining molecular beam epitaxy and magnetron sputtering within an ultra-high vacuum cluster. Nb films deposited at varying power, and a reference film from a commercial system, are compared. The results show clear variation between the in-situ and ex-situ deposition which we relate to differences in magnetron sputtering conditions and chamber geometry. The Nb films have critical temperatures of around $9 \textrm{K}$. and critical perpendicular magnetic fields of up to $B_{c2} = 1.4 \textrm{T}$ at $4.2 \textrm{K}$. From STEM images of the GaAs-Nb interface we find the formation of an amorphous interlayer between the GaAs and the Nb for both the ex-situ and in-situ deposited material.

6.Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2

Authors:Sikandar Aftab, Muhammad Arslan Shehzad, Muhammad Salman Ajmal, Fahmid Kabir, Muhammad Zahir Iqbal

Abstract: In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional material (2D), MoTe2, caused by phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semi-metallic 1T-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 meV to 63 meV when phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semi-metallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 uA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30 %, respectively. Our findings unveil distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials, and a novel approach to reach high efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

7.Evolution of dislocation loops in irradiated α-Uranium: An atomistically-informed cluster dynamics investigation

Authors:Sanjoy Kumar Mazumder, Tiankai Yao, Anter El-Azab

Abstract: An atomistically informed mean field cluster dynamics model has been presented to investigate the nucleation and growth of defect loops in irradiated {\alpha}-U. TEM analysis of neutron irradiated {\alpha}-U shows the evolution of SIA and vacancy loops on (010) and (100) crystallographic planes respectively, resulting in an anisotropic swelling of the face-centered orthorhombic crystal. The accumulation of such loops, on irradiation, has been closely estimated using the cluster dynamics model. Parameters of the model, namely, the binding energy of point defects, i.e., Ui and VU, to SIA and vacancy loops respectively and the diffusivity of point defects govern the energetics and kinetics of the defect clustering phenomenon. We have studied the crystallography of defect loops and computed the binding energy of point defects to such loops using an angular dependent EAM potential in classical MD simulations. Using bond-boost hyperdynamics in LAMMPS, the anisotropic diffusion of Ui and VU in {\alpha}-U has been investigated. The mechanisms of point defect diffusion and the associated migration energies have also been reported and compared with previous DFT studies. Our CD model uses the computed parameters, within their error ranges, to predict the population of defect clusters with a dose-rate and temperature similar to the neutron irradiation experiments. The predictions show an accumulation of small sized vacancy loops along with a population of large and growing SIA loops which closely corresponds to the TEM observations.

8.Ferroelastic twin angles at the surface of CaTiO$_\mathrm{3}$ quantified by PhotoEmission Electron Microscopy

Authors:G. Magagnin, C. Lubin, M. Escher, N. Weber, L. Tortech, N. Barrett

Abstract: We use photoemission electron microscopy to measure the ferroelastic twin wall angles at the surface of CaTiO$_\mathrm{3}$(001) and deduce the strain ordering. We analyze the angular dependence of the photoelectron emission from different domain surfaces, each with their own characteristic tilt angle in the factory roof-like topography. By considering the surface topography as a field perturbation, the offset in the photoemission threshold can be directly related to the tilt angles. With knowledge of the symmetry allowed twin walls we quantify twin topography between 179.1{\deg} to 180.8{\deg}.

9.Exciton band structure of V$_2$O$_5$

Authors:Vitaly Gorelov, Lucia Reining, Matteo Gatti

Abstract: Excitonic effects due to the correlation of electrons and holes in excited states of matter dominate the optical spectra of many interesting materials. They are usually studied in the long-wavelength limit. Here we investigate excitons at non-vanishing momentum transfer, corresponding to shorter wavelengths. We calculate the exciton dispersion in the prototypical layered oxide V$_2$O$_5$ by solving the Bethe-Salpeter equation of many-body perturbation theory. We discuss the change of excitation energy and intensity as a function of wavevector for bright and dark excitons, respectively, and we analyze the origin of the excitons along their dispersion. We highlight the important role of the electron-hole exchange with its impact on the exciton dispersion, the singlet-triplet splitting and the difference between the imaginary part of the macroscopic dielectric function and the loss function.

10.Pressure-control of non-ferroelastic ferroelectric domains in ErMnO3

Authors:O. W. Sandvik, A. M. Müller, H. W. Ånes, M. Zahn, J. He, M. Fiebig, Th. Lottermoser, T. Rojac, D. Meier, J. Schultheiß

Abstract: Mechanical pressure controls the structural, electric, and magnetic order in solid state systems, allowing to tailor and improve their physical properties. A well-established example is ferroelastic ferroelectrics, where the coupling between pressure and the primary symmetry breaking order parameter enables hysteretic switching of the strain state and ferroelectric domain engineering. Here, we study the pressure-driven response in a non-ferroelastic ferroelectric, ErMnO3, where the classical stress-strain coupling is absent, and the domain formation is governed by creation-annihilation processes of topological defects. By annealing ErMnO3 polycrystals under variable pressures in the MPa-regime, we transform non-ferroelastic vortex-like domains into stripe-like domains. The width of the stripe-like domains is determined by the applied pressure as we confirm by three-dimensional phase field simulations, showing that pressure leads to highly oriented layer-like periodic domains. Our work demonstrates the possibility to utilize mechanical pressure for domain engineering in non-ferroelastic ferroelectrics, providing a processing-accessible lever to control their dielectric, electromechanical, and piezoelectric response.

11.Ab-initio Simulations of Coherent Phonon-Induced Pumping of Carriers in ZrTe$_5$

Authors:Tao Jiang, Peter P. Orth, Liang Luo, Lin-Lin Wang, Feng Zhang, Cai-Zhuang Wang, Jin Zhao, Kai-Ming Ho, Jigang Wang, Yong-Xin Yao

Abstract: Laser-driven coherent phonons can act as modulated strain fields and modify the adiabatic ground state topology of quantum materials. We use time-dependent first-principles and effective model calculations to simulate the effect of a strong terahertz electric field on electronic carriers in the topological insulator ZrTe$_5$. We show that a coherent $A_\text{1g}$ Raman mode modulation can effectively pump carriers across the band gap, even though the phonon energy is about an order of magnitude smaller than the equilibrium band gap. We reveal the microscopic mechanism of this effect which occurs via Landau-Zener-St\"uckelberg tunneling of Bloch electrons in a narrow region in the Brillouin zone center where the transient energy gap closes when the system switches from strong to weak topological insulator. The quantum dynamics simulation results are in excellent agreement with recent pump-probe experiments in ZrTe$_5$ at low temperature.

12.Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Authors:Abhijit Biswas, Rui Xu, Joyce Christiansen-Salameh, Eugene Jeong, Gustavo A. Alvarez, Chenxi Li, Anand B. Puthirath, Bin Gao, Arushi Garg, Tia Gray, Harikishan Kannan, Xiang Zhang, Jacob Elkins, Tymofii S. Pieshkov, Robert Vajtai, A. Glen Birdwell, Mahesh R. Neupane, Bradford B. Pate, Tony Ivanov, Elias J. Garratt, Pengcheng Dai, Hanyu Zhu, Zhiting Tian, Pulickel M. Ajayan

Abstract: Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.