arXiv subjects

Peter A. Dowben

Publications and source records attributed to Peter A. Dowben.

10 recordsLinked to original sources

arXiv2025

In pursuit of the low-energy Solar neutron flux

Understanding the origin of low-energy solar neutrons flux is crucial for probing solar energetic processes and neutron transport mechanisms in interplanetary space. This study investigates the role of coronal mass ejections (CMEs) in modulating the low-energy solar neutrons. The neutron flux is modeled by incorporating a CME-width scaling factor into existing neutron fluence models. Our analysis, based on CME data from the SECHHI COR2 database during the DANSON experiment (2016-10-27 to 2017-03-17), identified narrow-width (20$^\circ$ $<$ $α$ $<$ 80$^\circ$) and fast (v $>$ 800 km/s) CMEs as key contributors to neutron production. The revised model predicts a neutron flux of 5 - 49 neutrons cm$^{-2}$ s$^{-1}$ at 1 AU for a neutron travel time of 1.66 hours, aligning with previous reports. Additionally, the estimated total flux of 2.45 MeV neutrons over a 4-hour period accounts for 10.23\% of DANSON's total flux. These findings suggest that CME-driven mechanisms may significantly influence the low-energy solar neutron flux. More observational efforts are needed to refine neutron flux estimates and improve background subtraction techniques for spaceborne neutron detectors.

astro-ph.SR↗

arXiv2024

The surface termination of a Fe (III) spin crossover molecular salt

From a comparison of energy dispersive analyses of X-rays (EDAX), the known molecular stoichiometry and X-ray photoemission spectroscopy (XPS), it is evident that the Fe(III) spin crossover salt [Fe(qsal)2Ni(dmit)2] has a preferential surface termination with the Ni(dmit)2 moiety. This preferential surface termination leads to a significant surface to bulk core level shift for the Ni 2p X-ray photoemission core level, not seen in the corresponding Fe 2p core level spectra. Inverse photoemission spectroscopy (IPES), thus provides some indication of the density of states resulting from the dmit2 = 1,3-dithiol-2-thione-4,5-dithiolato ligand unoccupied molecular orbitals.

cond-mat.mtrl-sci↗

arXiv2023

Post deposition interfacial Néel temperature tuning in magnetoelectric B:Cr2O3

Boron (B) alloying transforms the magnetoelectric antiferromagnet Cr2O3 into a multifunctional single-phase material which enables electric field driven π/2 rotation of the Néel vector. Nonvolatile, voltage-controlled Néel vector rotation is a much-desired material property in the context of antiferromagnetic spintronics enabling ultra-low power, ultra-fast, nonvolatile memory, and logic device applications. Néel vector rotation is detected with the help of heavy metal (Pt) Hall-bars in proximity of pulsed laser deposited B:Cr2O3 films. To facilitate operation of B:Cr2O3-based devices in CMOS environments, the Néel temperature, TN, of the functional film must be tunable to values significantly above room temperature. Cold neutron depth profiling and x-ray photoemission spectroscopy depth profiling reveal thermally activated B-accumulation at the B:Cr2O3/ vacuum interface in thin films deposited on Al2O3 substrates. We attribute the B-enrichment to surface segregation. Magnetotransport data confirm B-accumulation at the interface within a layer of about 50 nm thick where the device properties reside. Here TN enhances from 334 K prior to annealing, to 477 K after annealing for several hours. Scaling analysis determines TN as a function of the annealing temperature. Stability of post-annealing device properties is evident from reproducible Néel vector rotation at 370 K performed over the course of weeks.

cond-mat.mtrl-sci↗

arXiv2021

Colossal intrinsic exchange bias in epitaxial CoFe2O4/Al2O3 thin films

In this work, we demonstrate a massive intrinsic exchange bias (3 kOe) in epitaxial CoFe2O4(111) thin films deposited on Al2O3(0001) substrates. This exchange bias is indicative of intrinsic exchange or a ferromagnetic material combined with an antiferromagnet. The analysis of structure, magnetism and electronic states corroborate that there is an interfacial layer CoO between the CoFe2O4(111) thin film and the Al2O3(0001) substrate. The power-law thickness dependence of the intrinsic exchange bias verifies its interfacial origin. This work suggests interfacial engineering can be an effective route for achieving large exchange bias.

cond-mat.mtrl-sci↗

arXiv2019

Probe Ferroelectricity by X-ray Absorption Spectroscopy in Molecular Crystal

We carry out X-ray absorption spectroscopy experiment at oxygen K-edge in croconic acid (C5H2O5) crystal as a prototype of ferroelectric organic molecular solid, whose electric polarization is generated by proton transfer. The experimental spectrum is well reproduced by the electron-hole excitation theory simulations from configuration generated by ab initio molecular dynamics simulation. When inversion symmetry is broken in ferroelectric state, the hydrogen bonding environment on the two bonded molecules become inequivalent. Such a difference is sensitively probed by the bound excitation in the pre-edge, which are strongly localized on the excited molecules. Our analysis shows that a satellite peak in the pre-edge will emerge at higher excitation energy which serves as a clear signature of ferroelectricity in the material.

physics.comp-ph↗

arXiv2016

Electronic Structure and Stability of the CH3NH3PbBr3 (001) Surface

The energetics and the electronic structure of methylammonium lead bromine (CH3NH3PbBr3) perovskite (001) surfaces are studied based on density functional theory. By examining the surface grand potential, we predict that the CH3NH3Br-terminated (001) surface is energetically more favorable than the PbBr2-terminated (001) surface, under thermodynamic equilibrium conditions of bulk CH3NH3PbBr3. The electronic structure of each of these two different surface terminations retains some of the characteristics of the bulk, while new surface states are found near band edges which may affect the photovoltaic performance in the solar cells based on CH3NH3PbBr3. The calculated electron affinity of CH3NH3PbBr3 reveals a sizable difference for the two surface terminations, indicating a possibility of tuning the band offset between the halide perovskite and adjacent electrode with proper interface engineering.

cond-mat.mtrl-sci↗

arXiv2016

Phase separation in LuFeO3 films

The structural transition at about 1000 °C, from the hexagonal to the orthorhombic phase of LuFeO3, has been investigated in thin films of LuFeO3. Separation of the two structural phases of LuFeO3 occurs on a length scale of micrometer, as visualized in real space using X-ray photoemission electron microscopy (X-PEEM). The results are consistent with X-ray diffraction and atomic force microscopy obtained from LuFeO3 thin films undergoing the irreversible structural transition from the hexagonal to the orthorhombic phase of LuFeO3, at elevated temperatures. The sharp phase boundaries between the structural phases are observed to align with the crystal planes of the hexagonal LuFeO3 phase. The coexistence of different structural domains indicates that the irreversible structural transition, from the hexagonal to the orthorhombic phase in LuFeO3, is a first order transition, for epitaxial hexagonal LuFeO3 films grown on Al2O3.

cond-mat.mtrl-sci↗

arXiv2016

On the Structural Origin of the Single-ion Magnetic Anisotropy in LuFeO3

Electronic structures for the conduction bands of both hexagonal and orthorhombic LuFeO3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectra shape and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states has not changed, the spectra are sensitive to the changes in the local environments of the Fe3+ and Lu3+ sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe3+, which are non-zero and uneven. The single ion anisotropy on Fe3+ sites is found to originate from these uneven occupancies of the spin minority states via spin-orbit coupling in LuFeO3.

cond-mat.mtrl-sci↗

arXiv2011

Surface state engineering of molecule-molecule interactions

Engineering the electronic structure of organics through interface manipulation, particularly the interface dipole and the barriers to charge carrier injection, is of essential importance to improved organic devices. This requires the meticulous fabrication of desired organic structures by precisely controlling the interactions between molecules. The well-known principles of organic coordination chemistry cannot be applied without proper consideration of extra molecular hybridization, charge transer and dipole formation at the interfaces. Here we identify the interplay between energy level alignment, charge transfer, surface dipole and charge pillow effect and show how these effects collectively determine the net force between adsorbed porphyrin 2H-TPP on Cu(111). We show that the forces between supported porphyrins can be altered by controlling the amount of charge transferred across the interface accurately through the relative alignment of molecular electronic levels with respect to the Shockley surface state of the metal substrate, and hence govern the self-assembly of the molecules.

cond-mat.mtrl-sci↗

arXiv2010

Robust isothermal electric switching of interface magnetization: A route to voltage-controlled spintronics

Roughness-insensitive and electrically controllable magnetization at the (0001) surface of antiferromagnetic chromia is observed using magnetometry and spin-resolved photoemission measurements and explained by the interplay of surface termination and magnetic ordering. Further, this surface in placed in proximity with a ferromagnetic Co/Pd multilayer film. Exchange coupling across the interface between chromia and Co/Pd induces an electrically controllable exchange bias in the Co/Pd film, which enables a reversible isothermal (at room temperature) shift of the global magnetic hysteresis loop of the Co/Pd film along the magnetic field axis between negative and positive values. These results reveal the potential of magnetoelectric chromia for spintronic applications requiring non-volatile electric control of magnetization.

cond-mat.mtrl-sci↗