arXiv subjects

Ali R. Koymen

Publications and source records attributed to Ali R. Koymen.

5 recordsLinked to original sources

arXiv2025

Temperature Dependent Optical Response Of High- Tc Yba2cu3o7-δ (Ybco) Thin Films

We report on the temperature-dependent optical response of thin films of YBa2Cu3O7-δ (YBCO) in the visible spectral range under cryogenic conditions. Specifically, we observe an increase in transmittance near the superconducting transition temperature (Tc), which saturates within a few kelvins below Tc. The increase in transmittance is accompanied by a corresponding decrease in reflectance as the temperature drops below Tc, and both quantities track the superconducting phase transition. Changes in transmittance are found to be wavelength dependent, with the maximum variation occurring at 633 nm and minimal at 450 nm. These observations establish a correlation between the variation in optical response and the superconducting phase transition, even in the visible regime. The results of our experiment highlight the potential for using non-contact optical measurements to determine Tc. The effect can be explained using the two-fluid model, which can account for the observed temperature and wavelength dependence of the transmittance of the superconducting thin films.

cond-mat.supr-con↗

arXiv2023

Design and fabrication guide for a new metamaterial as an absorber of visible light with exceptionally high absorption efficiency

We present a guide for the design and fabrication of a CMOS-compatible metamaterial microstructure as an absorber of visible light with exceptionally high absorption efficiency (~ 98%), for wavelengths 400nm-700nm. The structural parameters of the microstructure have been optimized by using Finite Element method (FEM) based COMSOL multyphysics software simulations. An optimized 2D unit cell of the structure consists of 4um by 160 nm TiN base on glass substrate covered with 70 nm thick silicon dioxide (SiO2). A periodic structure of titanium nitride (TiN) straps (each of 90 nm thick and 2-micron wide) is deposited over SiO2. The straps are capped with 40 nm thick layer of high-temperature dielectric hafnium dioxide (HfO2) with periodicity of 4-micron. This unit is symmetric along the other dimension and repeated periodically along the horizontal direction. Additionally, though this microstructure was optimized for visible light spectrum, it also shows almost similar absorption of ~96% intigrated over wavelenght spectrum from 400 nm to 1200 nm. This investigation shows good agreement between simulation and experimental results.

physics.optics↗

arXiv2022

Photoemission spectroscopy using virtual photons emitted by positron sticking: a new probe of the top layer surface electronic structure

We present a spectroscopic method which utilizes virtual photons to selectively measure the electronic structure of the top-most atomic layer. These virtual photons are created when incident positrons transition from vacuum states to bound surface states on the sample surface and can transfer sufficient energy to excite electrons into the vacuum. The short interaction range of the virtual photons restricts the penetration depth to approximately the Thomas-Fermi screening length. Measurements and analysis of the kinetic energies of the emitted electrons made on a single-layer of graphene deposited on Cu and on the clean Cu substrate shows that the ejected electrons originate exclusively from the top-most atomic layer. Moreover, we find that the kinetic energies of the emitted electrons reflect the density of states at the surface. These results demonstrate that this technique will be a complementary tool to existing spectroscopic techniques in determining the electronic structure of 2D materials and fragile systems due to the absence of subsurface contributions and probe-induced surface damage.

cond-mat.mes-hall↗

arXiv2020

Direct evidence for low-energy electron emission following O LVV Auger transitions at oxide surfaces

Oxygen, the third most abundant element in the universe, plays a key role in the chemistry of condensed matter and biological systems. Here, we report evidence for a hitherto unexplored Auger transition in oxides, where a valence band electron fills a vacancy in the 2s state of oxygen, transferring sufficient energy to allow electron emission. We used a beam of positrons with kinetic energies of $\sim$1 eV to create O 2s holes via matter-antimatter annihilation. This made possible the elimination of the large secondary electron background that has precluded definitive measurements of the low-energy electrons emitted through this process. Our experiments indicate that low-energy electron emission following the Auger decay of O 2s holes from adsorbed oxygen and oxide surfaces are very efficient. Specifically, our results indicate that the low energy electron emission following the Auger decay of O 2s hole is nearly as efficient as electron emission following the relaxation of O 1s holes in TiO$_2$. This has important implications for the understanding of Auger-stimulated ion desorption, Coulombic decay, photodynamic cancer therapies, and may yield important insights into the radiation-induced reactive sites for corrosion and catalysis.

cond-mat.mtrl-sci↗

arXiv2010

Giant magnetic broadening of ferromagnetic resonance in a GMR Co/Ag/Co/Gd quadlayer

Both magnetic-resonance damping and the giant magnetoresistance effect have been predicted to be strongly affected by the local density of states in thin ferromagnetic films. We employ the antiferromagnetic coupling between Co and Gd to provide a spontaneous change from parallel to antiparallel alignment of two Co films. A sharp increase in magnetic damping accompanies the change from parallel to antiparallel alignment, analogous to resistivity changes in giant magnetoresistance.

cond-mat.mtrl-sci↗