Browsing by Subject "Exciton condensation"

Now showing 1 - 3 of 3

Open Access
Exciton condensate driven force in double layer systems
(2016-02) Özgün, Ege
Excitonic systems are challenging to deal with both theoretically and experimentally but in return, they offer a very rich physics and exotic features. We will investigate their properties under weak magnetic field and the resultant instabilities reminiscent of Sarma-I and Sarma-II phases. A new type of force in condensed matter physics, emerging due to the presence of the excitonic condensation will be demonstrated via semi-analytical and numerical calculations in two different systems of GaAs double quantum well geometries and layered transition metal dichalcogenide material 1T-TiSe2. Competition of charge-density wave and exciton condensate orders in layered systems will also be discussed in detail and an alternative explanation for the periodic lattice distortions observed in 1T-TiSe2 will be posed.
Open Access
Robust ground state and artificial gauge in DQW exciton condensates under weak magnetic field
(Elsevier B.V., 2014) Hakioǧlu, Tuğrul; Özgün, Ege; Günay, Mehmet
An exciton condensate is a vast playground in studying a number of symmetries that are of high interest in the recent developments in topological condensed matter physics. In double quantum wells (DQWs) they pose highly nonconventional properties due to the pairing of non-identical fermions with a spin dependent order parameter. Here, we demonstrate a new feature in these systems: the robustness of the ground state to weak external magnetic field and the appearance of the artificial spinor gauge fields beyond a critical field strength where negative energy pair-breaking quasi particle excitations, i.e. de-excitation pockets (DX-pockets), are created in certain k regions. The DX-pockets are the Kramers symmetry broken analogs of the negative energy pockets examined in the 1960s by Sarma. They respect a disk or a shell-topology in k-space or a mixture between them depending on the magnetic field strength and the electron-hole density mismatch. The Berry connection between the artificial spinor gauge field and the TKNN number is made. This field describes a collection of pure spin vortices in real space when the magnetic field has only inplane components.
Open Access
Superconducting systems of low dimensionality
(1992) Gedik, M. Zafer
It is possible to call the last five years as the golden age of superconductivity. The two most important developments in the field are the discovery of copper oxide and fullerene superconductors. In this work, some possible pairing mechanisms for these materials ai’e examined by giving emphasis on the reduced dimensionality. First, an older problem, spatially separated electron-hole system, is investigated to identify the possible phases in coupled double quantum well structures in electric field. Secondly, the superconducting transition temperature and response to external magnetic fields of layered systems with varying number of layers are studied by means of a microscopic model and its GinzburgLandau version. It is also shown that an interlayer pairing mechanism, phonon assisted tunneling, can induce superconductivity. Finally, effects of the spherical structure of fullerenes are examined by solving a two fermion problem on an isolated molecule where the particles interact via a short range attractive potential. As a possible mechanism of superconductivity in alkali metal doped fullerenes, coupling between electrons and the radial vibrations of the molecule is investigated.