Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 2Citation - Scopus: 2Cosmological States in Loop Quantum Gravity on Homogeneous Graphs(American Physical Society, 2023) Baytaş, Bekir; Yokomizo, N.We introduce a class of states characterized by proposed conditions of homogeneity and isotropy in loop quantum gravity and construct concrete examples given by Bell-network states on a special class of homogeneous graphs. Such states provide new representations of cosmological spaces that can be explored for the formulation of cosmological models in the context of loop quantum gravity. We show that their local geometry is described in an automorphism-invariant manner by one-node observables analogous to the one-body observables used in many-body quantum mechanics, and compute the density matrix representing the restriction of global states to the algebra of one-node observables. The von Neumann entropy of this density matrix provides a notion of entanglement entropy of a local region that is invariant under automorphisms and can be applied to states involving superpositions of distinct graphs. © 2023 American Physical Society.Article Citation - WoS: 1Citation - Scopus: 1Type-3/2 Seesaw Mechanism(American Physical Society, 2021) Demir, Durmuş Ali; Karahan, Canan; Sargın, OzanThe type-I seesaw mechanism provides a natural explanation for tiny neutrino masses. The right-handed neutrino masses it requires are, however, too large to keep the Higgs boson mass at its measured value. We show that vector spinors, singlet leptons that are like right-handed neutrinos, generate tiny neutrino masses naturally through the exchange of spin-1/2 and spin-3/2 components. This one-step seesaw mechanism, which we call the type-3/2 seesaw, keeps the Higgs boson mass unchanged at one loop and gives cause therefore to no fine-tuning problem. If the on-shell vector spinor is a pure spin-3/2 particle, then it becomes a potential candidate for hidden dark matter which gets diluted due only to the expansion of the Universe. The type-3/2 seesaw provides a natural framework for the neutrino, Higgs boson, and dark matter sectors, with overall agreement with current experiments and observations.Article Citation - WoS: 14Citation - Scopus: 17Enhancement of Thermoelectric Efficiency of T-Hfse2 Via Nanostructuring(American Physical Society, 2021) Ünsal, Elif; Senger, Ramazan Tuğrul; Sevinçli, HaldunIn this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of T-HfSe2 nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the n-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the p-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The p-type Seebeck coefficient and the power factor for T-phase HfSe(2 )are enhanced in NRs. We report that the p-type ZT value of HfSe2 NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.Article Citation - WoS: 16Citation - Scopus: 16Riemann-Eddington Theory: Incorporating Matter, Degravitating the Cosmological Constant(American Physical Society, 2014) Demir, Durmuş AliHere we show that Eddington's pure affine gravity, when extended with Riemann curvature, leads to gravitational field equations that incorporate matter. This Riemanned Eddington gravity outfits a setup in which matter gravitates normally with Newton's constant but vacuum gravitates differently with an independent gravitational constant. This novel setup enables degravitation of the vacuum to observed level not by any fine-tuning but by a large hierarchy between its gravitational constant and its energy density. Remarkably, degravitation of the cosmological constant is local, causal and natural yet only empirical because the requisite degravitation condition is not predicted by the theory.Article Citation - WoS: 100Citation - Scopus: 81Search for Physics Beyond the Standard Model in Final States With a Lepton and Missing Transverse Energy in Proton-Proton Collisions at Root S = 8 Tev(American Physical Society, 2015) Demir, Durmuş Ali; Karapınar, GülerA search for new physics in proton-proton collisions having final states with an electron or muon and missing transverse energy is presented. The analysis uses data collected in 2012 with the CMS detector, at an LHC center-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of 19.7 fb(-1). No significant deviation of the transverse mass distribution of the charged lepton-neutrino system from the standard model prediction is found. Mass exclusion limits of up to 3.28 TeVat 95% confidence level for a W0-boson with the same couplings as that of the standard model W-boson are determined. Results are also derived in the framework of split universal extra dimensions, and exclusion limits on Kaluza-Klein Wd(KK)((2)) states are found. The final state with large missing transverse energy also enables a search for dark matter production with a recoiling W-boson, with limits set on the mass and the production cross section of potential candidates. Finally, limits are established for a model including interference between a left-handed W'-boson and the standard model W-boson and for a compositeness model.Article Citation - WoS: 3Citation - Scopus: 3Atomic Collapse in Disordered Graphene Quantum Dots(American Physical Society, 2020) Polat, Mustafa; Güçlü, Alev DevrimIn this paper, we numerically study a Coulomb impurity problem for interacting Dirac fermions restricted in disordered graphene quantum dots. In the presence of randomly distributed lattice defects and spatial potential fluctuations, the response of the critical coupling constant for atomic collapse is mainly investigated by local density of states calculations within the extended mean-field Hubbard model. We find that both types of disorder cause an amplification of the critical threshold. As a result, up to a 34% increase in the critical coupling constant is reported. This numerical result may explain why the Coulomb impurities remain subcritical in experiments, even if they are supercritical in theory. Our results also point to the possibility that atomic collapse can be observed in defect-rich samples such as Ar+ ion bombarded, He+ ion irradiated, and hydrogenated graphene.Article Citation - WoS: 5Citation - Scopus: 5Enhanced Indirect Exchange Interactions in the Presence of Circular Potentials in Graphene(American Physical Society, 2019) Canbolat, Ahmet Utku; Çakır, ÖzgürWe calculate indirect exchange interaction between two magnetic impurities in pristine graphene in the presence of a circular potential. In bulk graphene structures indirect exchange interaction, also known as RKKY (Ruderman-Kittel-Kasuya-Yosida) interaction, shows a power-law decay with distance for both doped and undoped cases. Here we show that under a circular electric potential quasibound states lead to enhanced RKKY interactions between magnetic moments located in the vicinity of the potential well. It is shown that the strength of the potential well and Fermi energy can be tuned to create enhanced, nondecaying, long ranged RKKY interactions. We show that when the Fermi level lies at the quasibound state energy, the scattering processes between the states of the same chirality dominate over the other scattering channels and this leads to a predominantly ferromagnetic, nondecaying interaction between the impurities at long distances. The predicted effect can enable electrical control of RKKY interactions in graphene or other two-dimensional materials.Article Citation - WoS: 85Citation - Scopus: 91Ballistic Thermoelectric Properties of Monolayer Semiconducting Transition Metal Dichalcogenides and Oxides(American Physical Society, 2019) Özbal, Gözde; Senger, Ramazan Tuğrul; Sevik, Cem; Sevinçli, HaldunCombining first-principles calculations with Landauer-Mittiker formalism, ballistic thermoelectric transport properties of semiconducting two-dimensional transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are investigated in their 2H and 1T phases. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we report the thermoelectric properties of the semiconducting ones. We find that 2H phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T phases. The maximum room temperature p-type thermoelectric figure of merit (ZT) of 1.57 is obtained for 2H-HfSe2, which can be as high as 3.30 at T = 800 K. Additionally, 2H-ZrSe2, 2H-ZrTe2, and 2H-HfS2 have considerable ZT values (both nand p-type), that are above 1 at room temperature. The 1T phases of Zr and Hf-based oxides possess relatively high power factors, however their high lattice thermal conductance values limit their ZT values to below 1 at room temperature.Article Citation - WoS: 30Citation - Scopus: 29Stacking-Dependent Excitonic Properties of Bilayer Blue Phosphorene(American Physical Society, 2019) İyikanat, Fadıl; Torun, Engin; Senger, Ramazan Tuğrul; Şahin, HasanAb initio calculations in the framework of many-body perturbation theory (MBPT) are performed to calculate the electronic and optical properties of monolayer and bilayer blue phosphorene with different stacking configurations. It is found that the stacking configuration of bilayer blue phosphorene strongly affects the electronic band gap of the material. By solving the Bethe-Salpeter equation (BSE) on top of the G(0)W(0) calculation, the binding energies, spectral positions, and band decomposition of excitons of monolayer and bilayer configurations are investigated. The most prominent two excitonic peaks of bilayers are examined in detail. Our calculations show that different stacking configurations lead to distinct interlayer interaction characteristics which lead to substantial change in the optical spectrum of bilayer blue phosphorene. Mostly intralayer and mixed interlayer excitons with quite high binding energies are obtained in bilayer blue phosphorene. Our results show that excitonic properties of ultrathin materials play an important role in tuning and improving the optoelectronic performance of two-dimensional materials.Article Citation - WoS: 21Citation - Scopus: 20Mass Spectrum and Higgs Profile in B - L Symmetric Ssm(American Physical Society, 2016) Ün, Cem Salih; Özdal, ÖzerWe investigate the predictions on the mass spectrum and Higgs boson decays in the supersymmetric standard model extended by U(1)(B-L) symmetry (BLSSM). The model requires two singlet Higgs fields, which are responsible for the radiative breaking of U(1)(B-L) symmetry. It predicts degenerate right-handed neutrino masses (1.7-2.2 TeV) as well as the right-handed sneutrinos of mass less than or similar to 4 TeV. The presence of right-handed neutrinos and sneutrinos triggers the baryon and lepton number violation processes, until they decouple from the standard model particles. Besides, the model predicts rather heavy colored particles; m((t) over bar), m((h) over bar) greater than or similar to 1.5 TeV, while m((tau) over tilde) greater than or similar to 100 GeV and m((chi) over tilde1 +/-) greater than or similar to 600 GeV. Even though the implications are similar to the minimal supersymmetric standard model, BLSSM can predict another Higgs boson lighter than 150 GeV. We find that the second Higgs boson can be degenerate with the lightest charge parity (CP)-even Higgs boson of mass about 125 GeV and contribute to the Higgs decay into two photons. In addition, it can provide an explanation for the excess in h -> 4l at the mass scale similar to 145 GeV.