Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 1Citation - Scopus: 1Modeling Cosmological Perturbations of Thermal Inflation(IOP Publishing, 2024) Bae, Jeong-Myeong; Hong, Sungwook E.; Zoe, HeeseungWe consider a simple system consisting of matter, radiation and vacuum components to model the impact of thermal inflation on the evolution of primordial perturbations. The vacuum energy magnifies the primordial modes entering the horizon before its domination, making them potentially observable, and the resulting transfer function reflects the phase changes and energy contents. To determine the transfer function, we follow the curvature perturbation from well outside the horizon during radiation domination to well outside the horizon during vacuum domination and evaluate it on a constant radiation density hypersurface, as is appropriate for the case of thermal inflation. The shape of the transfer function is determined by the ratio of vacuum energy to radiation at matter-radiation equality, which we denote by upsilon , and has two characteristic scales, ka and kb , corresponding to the horizon sizes at matter radiation equality and the beginning of the inflation, respectively. If upsilon MUCH LESS-THAN1 , the Universe experiences radiation, matter and vacuum domination eras and the transfer function is flat for kMUCH LESS-THANkb , oscillates with amplitude 1/5 for kbMUCH LESS-THANkMUCH LESS-THANka and oscillates with amplitude 1 for k >> ka . For upsilon >> 1 , the matter domination era disappears, and the transfer function reduces to being flat for kMUCH LESS-THANkb and oscillating with amplitude 1 for k >> kb .Article Citation - WoS: 11Citation - Scopus: 10Intercalation Leads To Inverse Layer Dependence of Friction on Chemically Doped Mos2(IOP Publishing, 2023) Açıkgöz, Oğulcan; Guerrero, Enrique; Yanılmaz, Alper; Dağdeviren, Ömür E.; Çelebi, Cem; Strubbe, David A.; Baykara, Mehmet Z.We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse, dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.Article Citation - WoS: 5Citation - Scopus: 4Electromagnetically Induced Transparency and Absorption Cross-Over With a Four-Level Rydberg System(IOP Publishing, 2022) Oyun, Yağız; Çakır, Özgür; Sevinçli, SevilayElectromagnetically induced transparency (EIT) and absorption (EIA) are quantum coherence phenomena which result from the interference of excitation pathways. Combining these with Rydberg atoms have opened up many possibilities for various applications. We introduce a theoretical model to study Rydberg-EIT and Rydberg-EIA effects in cold Cs and Rb atomic ensembles in a four-level ladder type scheme taking into account van der Waals type interactions between the atoms. The proposed many-body method for analysis of such systems involves a self-consistent mean field approach and it produces results which display a very good agreement with recent experiments. Our calculations also successfully demonstrate experimentally observed EIT-EIA cross-over in the Rb case. Being able to simulate the interaction effects in such systems has significant importance, especially for controlling the optical response of these.Article Echoes From the Event Horizon of a Superfluid Vortex(IOP Publishing, 2022) Güven, Kaan; Demirkaya, BetülA vortex formed in the superfluid state of a Bose-Einstein condensate may exhibit superradiance a la blackhole for radially propagating acoustic fluctuations. The analogy is usually based on the so-called draining bathtub model of the vortex, in which an event horizon and ergosphere emerges when the radial velocity of the superfluid exceeds the propagation speed of sound in the condensate. The acoustic fluctuations mimic a massless scalar field in the curved Lorentzian space-time of the vortex and are governed by the Klein-Gordon wave equation. One common main approximation is the constant background density of the superfluid even in the presence of the vortex. This sets a constant relativistic sound speed. However, the vortex state solution of the Gross-Pitaevskii equation clearly shows that both the density and the speed of sound vary radially near the vortex core, where the event horizon and thus the superradiance will take place. What changes would this complex interdependence bring to the formulation and to the outcomes of the superradiance based on constant density approximation? Here, we recount this question posed under the guidance of Prof. Tekin Dereli and present recent results. We show that the self-consistent density modifies the amplification dynamics near the event horizon significantly, thereby altering the temporal and spectral fingerprint of the superradiance of the vortex.Conference Object Citation - WoS: 3Citation - Scopus: 3Photometric Multi-Site Campaign on Massive B Stars in the Open Cluster Chi Persei (ngc 884)(IOP Publishing, 2008) Saesen, S.; Pigulski, A.; Carrier, F.; De Ridder, J.; Aerts, C.; Handler, G.; Drummond, R.; Kalomeni, BelindaIn 2005 a photometric observation campaign started on the open cluster chi Persei, involving 13 telescopes spread over the whole northern hemisphere. After two years we gathered almost 1200 hours of data. We present here preliminary results on the variability search, especially from the 60-cm telescope in Bialkow (Poland), which show seven confirmed beta Cephei stars, four candidate B-type pulsators and other interesting variable stars.Article Citation - WoS: 6Citation - Scopus: 6Variation of the Light and Period of the Magnetic Cataclysmic Variable Am Her(IOP Publishing, 2008) Kalomeni, Belinda; Yakut, KadriGround-based, long-term optic variability of AM Her, covering the period between 2003-2008, has been conducted to study the features seen in both low and high states of the system. Low-state analysis shows the presence of short-term, low-amplitude light variations of about 0.02-0.03 mag with a mean power time between 16 s and 226 s. Brightness variations on the order of 0.7-2 mag, which could be due to the stellar activity of the component in the system, are also detected. A total of 30 years' times of minimum light given in the literature are combined with nine times of minima obtained in this study. We represented the (observed-calculated) diagram by a parabolic curve and also by two broken lines. Under the assumption of a parabolic variation, we estimate an increase in period, dP/dt = 7.5(1.2) x 10(-9) days yr(-1), with a mass transfer rate of. M = 8(2) x 10(-9) M(circle dot) yr(-1), in agreement with the previous findings by a different method.Article Citation - WoS: 6Citation - Scopus: 6Experimental and Density Functional Theory Study on Humidity Sensing Properties of Copper Phthalocyanine (cupc)(IOP Publishing, 2019) Farzaneh, Amir; Esrafili, Mehdi D.; Okur, SalihThe quartz crystal microbalance (QCM) technique was applied to investigate humidity sensing properties of a copper phthalocyanine (CuPc) thin film prepared by drop cast method. The humidity adsorption and desorption kinetics of (CuPc) thin film was evaluated. The QCM and electrical measurements results showed that humidity sensing properties of CuPc is very sensitive to humidity changes and reversible adsorption/desorption performance which is an indicative of a good humidity sensor even at room temperature. Reproducible experimental results indicated that CuPc thin films have an abundant potential for humidity sensing applications at ambient temperature. According to the first-principle density functional theory calculations, the promising humidity sensing properties of CuPc can be attributed to the considerable charge transfer from the water molecule into Cu atom.Article Citation - WoS: 6Citation - Scopus: 5A Mechanism for Formation of Bose-Einstein Condensation in Cosmology(IOP Publishing, 2019) Erdem, Recai; Gültekin, KemalWe introduce a toy model of scalar particles with a trilinear scalar coupling in cosmology. The trilinear coupling phi(2)chi causes production of non-relativistic phi particles through the process chi chi -> phi phi where, initially, only relativistic chi particles are present. We consider the initial times of chi chi -> phi phi and observe that the curved space effects promote formation of Bose-Einstein condensate of phi particles.Article Citation - WoS: 607Citation - Scopus: 736Black Holes, Gravitational Waves and Fundamental Physics: a Roadmap(IOP Publishing, 2019) Barack, Leor; Cardoso, Vitor; Nissanke, Samaya; Sotiriou, Thomas P.; Askar, Abbas; Belczynski, Chris; Erdem, RecaiThe grand challenges of contemporary fundamental physics dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on 'Black holes, Gravitational waves and Fundamental Physics'.Article Citation - WoS: 13Citation - Scopus: 11Geometric dark matter(IOP Publishing, 2020) Demir, Durmuş Ali; Puliçe, BeyhanThe dark matter, needed for various phenomena ranging from flat rotation curves to structure formation, seems to be not only neutral and long-living but also highly secluded from the ordinary matter. Here we show that, metric-affine gravity, which involves metric tensor and affine connection as two independent fields, dynamically reduces, in its minimal form, to the usual gravity plus a massive vector field. The vector, which interacts with only the quarks, leptons and gravity, is neutral and long-living (longer than the age of the Universe) when its mass range is 9.4 MeV < M-Y < 28.4 MeV. Its scattering cross section from nucleons, which is some 60 orders of magnitude below the current bounds, is too small to facilitate direct detection of the dark matter. This property provides an explanation for whys and hows of dark matter searches. We show that due to its geometrical origin the Y(mu )does not couple to scalars and gauge bosons. It couples only to fermions. This very feature of the Y-mu it makes it fundamentally different than all the other vector dark matter candidates in the literature. The geometrical dark matter we present is minimal and self-consistent not only theoretically but also astrophysically in that its feebly interacting nature is all that is needed for its longevity.