Browsing by Subject "Phase transition"

Now showing 1 - 7 of 7

Open Access
Cumulants associated with geometric phases and their implementation in modern theory of crystalline polarization
(2022-09) Cengiz, Sertac
Many fields have been influenced by Berry's geometric phase because of its physical meaning and observable effects. One of the breakthroughs that stem from geometric phases is the modern theory of polarization. The expectation value of the position was not possible to calculate for crystalline structures because of ill-defined position operator. The modern theory of polarization showed that the geometric phase obtained by Zak phase, integral across the Brillouin zone, gives thefirst cumulants so that polarization is obtainable by the geometric phase. This indicates that cumulants are essential for studies such as polarization, charge transport, and electron localization. In the context of the modern theory of polarization, gauge-invariant cumulants are derived but they are not geometric even though they are physically well defined. In order to deal with this issue, a Binder cumulant associated with the adiabatic cycle is introduced, so called geometric Binder cumulant. Since the definition of Binder cumulants is based on a ratio of two cumulants, it is possible to eliminate factors that prevent the quantity to become geometric. An alternative way to extract cumulants associated with the adiabatic cycle is proposed as well. Error terms of the Cumulants are improved when they are extracted in an alternative way. Distortion around the transition points which modern theory of polarization has been reduced significantly. Geometric Binder cumulant is implemented to observe the difference between gapped and gapless band structures. One-dimensional and two-dimensional models are investigated and phase transition between metallic and insulating states is clearly observed. SSH model is investigated to make a comparison with the modern theory of polarization and development in the formalism is shown. Geometric Binder cumulant also lets us observe the correlated model and a method based on renormalization group theory is used to locate transition points in the correlated model. Results are in good agreement with each other. An alternative way to extract cumulants is also extended to two-dimensional systems and phase transition is observed in two-dimensional systems with the usage of geometric Binder cumulant. Regardless of whether the two-dimensional system has a zero-dimensional or one-dimensional Fermi surface, Geometric Binder cumulant is a quantity that is sensitive for the metallic and insulating cases. For the open gap case, geometric Binder cumulant is affected by the system size, and the effect of the system size is distinct. An increase in the system size improves the quantity.
Open Access
Exceptional adaptable MWIR thermal emission for ordinary objects covered with thin VO2 film
(Elsevier Ltd, 2021-01-25) Durna, Yılmaz; Kocer, Hasan; Aydın, Koray; Cakir, Mehmet Cihan; Soydan, Mahmut Can; Odabasi, Oguz; Işık, Halil; Ozbay, Ekmel
Monotonous thermal radiation emitted from an ordinary object can be brought into a dynamic and versatile form that can be shaped according to the application area with the ingenious design of the surface coatings. Building the coatings with phase change materials provides exceptional and surprising properties in terms of tunability, adaptability and multifunctionality. In this paper, we investigate the thermal radiation properties in the MWIR band through comprehensive thermographic measurements and theoretical methods while a thin (similar to 90 nm thick) vanadium dioxide (VO2) layer on the sapphire substrate (VO2 thin film) is placed on different ordinary objects under heating/cooling conditions. It is indicated that the emission of the metal object (low emittance) can be boosted and the emission of the blackbody-like object (high emittance) can be suppressed at the relevant temperatures. The thermal emission of the objects covered with thin VO2 film at high temperatures (>75 degrees C) is determined by only the VO2 thin film, since the VO2 layer is completely metallized and the MWIR radiation of the underlying object is masked. When the actual temperature of the object behaving like a blackbody rises up to 95 degrees C, the temperature detected in the MWIR thermal camera is reduced by more than 20% to approx. 75 degrees C due to the VO2 thin film on this object, providing thermal camouflage. It is experimentally and theoretically revealed that the underlying physical mechanism on these strange results is associated with the drastic change in the infrared optical parameters of the VO2 as a result of the applied temperature. (C) 2020 Elsevier Ltd. All rights reserved.
Open Access
Gibbs measures and phase transitions in various one-dimensional models
(2013) Şensoy, Ahmet
In the thesis, limiting Gibbs measures of some one dimensional models are investigated and various criterions for the uniqueness of limiting Gibbs states are considered. The criterion for models with unique ground state formulated in terms of percolation theory is presented and some applications of this criterion are discussed. A one-dimensional long range Widom-Rowlinson model with periodic and biased particle activities is explored. It is shown that if the spin interactions are sufficiently large versus particle activities then the Widom-Rowlinson model does not exhibit a phase transition at low temperatures. Finally, an interdisciplinary approach is followed. A financial application of the theory of phase transition is considered by applying the Ising model to understand the role of herd behavior on stock market crashes. Accordingly, model suggests a criteria to detect the existence of herd behavior in financial markets under certain assumptions.
Open Access
Ground states of one-dimensional long-range ferromagnetic ising model with external field
(World Scientific Publishing, 2012) Kerimov, A.
A zero-temperature phase-diagram of the one-dimensional ferromagnetic Ising model is investigated. It is shown that at zero temperature spins of any compact collection of lattice points with identically oriented external field are identically oriented. © 2012 World Scientific Publishing Company.
Open Access
One-dimensional long-range ferromagnetic ising model under weak and sparse external field
(2009) Kerimov, A.
We consider the one-dimensional ferromagnetic Ising model with very long range interaction under weak and sparse biased external field and prove that at sufficiently low temperatures, the model has a unique limiting Gibbs state. © 2009 World Scientific Publishing Company.
Open Access
The one-dimensional long-range ferromagnetic Ising model with a periodic external field
(Elsevier, 2012) Kerimov, A.
We consider the one-dimensional ferromagnetic Ising model with very long-range interaction under a periodic, biased and weak external field and prove that at sufficiently low temperatures the model has a unique limiting Gibbs state. © 2012 Elsevier B.V. All rights reserved.
Open Access
Strain-induced structural phase transition in GeN monolayer
(Elsevier BV, 2021-11-30) Abboud, Mohammad; Özbey, D.H.; Durgun, Engin
The recent synthesis of SiP, SiAs, GeP, and GeAs monolayers has brought two-dimensional (2D) group IV–V systems into the limelight. To date, all the fabricated structures of this class belong to the C2/m space group which has a low structural symmetry, while the class could exist in more symmetric phases (i.e., P3m1 and P6m2). The realization of more symmetric phases can enhance the intrinsic properties of these materials and increase their potential field of usage. In this study, the possibility of a structural phase transition in GeN monolayer by application of mechanical strain is investigated. Based on ab initio simulations, we first confirm the stability of the GeN monolayer in all phases, then demonstrate how a large enough compressive strain (~%12) can transform C2/m into P3m1 phase. The results are interpreted by analyzing the geometry, bond order, electron localization functions, and net atomic charges of the structures. Upon transition into the P3m1 phase, tensile strength and in-plane stiffness double, while the compressive strength quadruples. On the other hand, the effect of the phase transition on the electronic properties is not substantial and similar band structure profiles with narrowed band gap are obtained. Our study provides insight on how to experimentally achieve the P3m1 phase of the GeN monolayer, which is in principle applicable to other group IV–V monolayers under suitable conditions involving the optimization of pressure, temperature, and impurity concentration. These unique features of the GeN monolayer render them ideal candidates for a variety of high technological nanoscale applications.