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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6

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Full Text: YesSubject: search.filters.subject.Physics, condensed matter

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Now showing 1 - 5 of 5
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    PublicationOpen Access
    Weakening the strength of CO binding on subsurface alloyed Pt(111)
    (Elsevier, 2019) Ogasawara, Hirohito; Department of Chemistry; N/A; Kaya, Sarp; Gürlek, Sezen; Panahi, Mohammad; Solati, Navid; Faculty Member; PhD Student; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); College of Sciences; Graduate School of Sciences and Engineering; 116541; N/A; N/A; N/A
    The interaction of CO with Pt/3d/Pt(111) subsurface alloys (3d: Ni, Co, Fe) was investigated by combination of vibrational spectroscopy and temperature programmed desorption. The binding strength of CO is significantly reduced on Pt/Fe/Pt(111) and Pt/Co/Pt(111) and is sorted in the following order: Pt >Ni > Co approximate to Fe. Our analysis shows that the 3d admetal is stable in the subsurface region and CO is linearly bonded to the atop sites of the Pt(111) surface. At high CO coverage, compressed structures are obtained. At low CO coverage, the desorption activation energy of CO (similar to 143 kJ/mol for Pt(111)) drops to 124 kJ/mol for Pt/Ni/Pt(111), and goes further down to 111 and 110 kJ/mol for Pt/Co/Pt(111) and Pt/Fe/Pt(111), respectively. The enhancement in the surface activity is attributed to the modified backdonation to the antibonding states of adsorbed CO due to the downshifted 5d-band center of Pt upon embedding 3d metals in the subsurface region.
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    PublicationOpen Access
    Predicting new iron garnet thin films with perpendicular magnetic anisotropy
    (Elsevier, 2020) N/A; Department of Electrical and Electronics Engineering; Zanjani, Saeedeh Mokarian; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 258783
    Magnetic iron garnets are insulators with low Gilbert damping with many applications in spintronics. Many emerging spintronic applications require perpendicular magnetic anisotropy (PMA) although garnets have only a few PMA types (i.e. terbium and samarium garnet). More and stable PMA garnet options are needed for investigating new spintronic phenomena. In this study, we predict 20 new epitaxial magnetic iron garnet film/substrate pairs with stable PMA at room temperature. The effective anisotropy energies of 10 different garnet films that are lattice-matched to 5 different commercially available garnet substrates (total 50 film/substrate pairs) have been calculated using shape, magnetoelastic and magnetocrystalline anisotropy terms. Strain type, tensile or compressive depending on substrate choice, as well as the sign and the magnitude of the magnetostriction constants of garnets determine if a garnet film may possess PMA. We show the conditions in which Samarium, Gadolinium, Terbium, Holmium, Dysprosium and Thulium garnets may possess PMA on the investigated garnet substrate types. New PMA garnet films with tunable saturation moment and field may improve spin-orbit torque memory and compensated magnonic thin film devices.
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    PublicationOpen Access
    Spin-torque oscillation in a magnetic insulator probed by a single-spin sensor
    (American Physical Society (APS), 2020) Zhang, H.; Ku, M. J. H.; Casola, F.; Du, C. H.; van der Sar, T.; Ross, C. A.; Tserkovnyak, Y.; Yacoby, A.; Walsworth, R. L.; Department of Electrical and Electronics Engineering; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; 258783
    We locally probe the magnetic fields generated by a spin-torque oscillator (STO) in a microbar of ferrimagnetic insulator yttrium-iron-garnet using the spin of a single nitrogen-vacancy (NV) center in diamond. The combined spectral resolution and sensitivity of the NV sensor allows us to resolve multiple spin-wave modes and characterize their damping. When damping is decreased sufficiently via spin injection, the modes auto-oscillate, as indicated by a strongly reduced linewidth, a diverging magnetic power spectral density, and synchronization of the STO frequency to an external microwave source. These results open the way for quantitative, nanoscale mapping of the microwave signals generated by STOs, as well as harnessing STOs as local probes of mesoscopic spin systems.
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    PublicationOpen Access
    Quantum dot to nanorod transition for efficient white-light-emitting diodes with suppressed absorption losses
    (American Chemical Society (ACS), 2022) Melikov, Rustamzhon; N/A; Department of Electrical and Electronics Engineering; N/A; Önal, Asım; Sadeghi, Sadra; Karatüm, Onuralp; Nizamoğlu, Sedat; Eren, Güncem Özgün; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; N/A; N/A; N/A; 130295; N/A
    Colloidal nanocrystals have great potential for next-generation solid-state lighting due to their outstanding emission and absorption tunability via size and morphology, narrow emission linewidth, and high photoluminescence quantum yield (PLQY). However, the losses due to self-and interabsorption among multitudes of nanocrystals significantly decrease external quantum yield levels of light-emitting diodes (LEDs). Here, we demonstrate efficient white LEDs via CdSe/CdS dot to ""dot-in-rod"" transition that enabled a large Stokes shift of 780 meV and significantly reduced absorption losses when used in conjunction with near-unity PLQY ZnCdSe/ZnSe quantum dots (QDs) emitting at the green spectral range. The optimized incorporation of nanocrystals in a liquid state led to the white LEDs with an ultimate external quantum efficiency (EQE) of 42.9%, with a net increase of EQE of 10.3% in comparison with white LEDs using CdSe/CdS dots. Therefore, combinations of nanocrystals with different nanomorphologies hold high promise for efficient white LEDs.
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    PublicationOpen Access
    Unidirectional invisibility and PT symmetry with graphene
    (American Physical Society (APS), 2018) Taş, Murat; Department of Physics; Sarısaman, Mustafa; Researcher; Department of Physics; College of Sciences
    We investigate the reflectionlessness and invisibility properties of the transverse electric-mode solution of a linear homogeneous optical system which comprises PT-symmetric structures covered by graphene sheets. We derive analytic expressions, indicate the roles of each parameter governing the optical system with graphene, and prove that optimal conditions of these parameters give rise to broadband and wide-angle invisibility. The presence of graphene turns out to shift the invisible wavelength range and to reduce the required gain value considerably, based on its chemical potential and temperature. We substantiate that our results yield broadband reflectionless and invisible configurations for realistic materials of small refractive indices, usually around eta = 1, and of small thicknesses with graphene sheets of rather low temperatures and chemical potentials. Finally, we demonstrate that pure PT-symmetric graphene yields invisibility at low temperatures and chemical potentials.