Browsing by Subject "Spin dynamics"
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Item Open Access Band Structure and Optical Properties of Kesterite Type Compounds: First principle calculations(Institute of Physics Publishing, 2017) Palaz S.; Unver H.; Ugur G.; Mamedov, Amirullah; Özbay, EkmelIn present work, our research is mainly focused on the electronic structures, optical and magnetic properties of Cu2FeSnZ4 (Z = S, Se) compounds by using ab initio calculations within the generalized gradient approximation (GGA). The calculations are performed by using the Vienna ab-initio simulation package (VASP) based on the density functional theory. The band structure of the Cu2FeSnZ4 ( Z = S, Se) compounds for majority spin (spin-up) and minority spin (spin-down) were calculated. It is seen that for these compounds, the majority spin states cross the Fermi level and thus have the metallic character, while the minority spin states open the band gaps around the Fermi level and thus have the narrow-band semiconducting nature. For better understanding of the electronic states, the total and partial density of states were calculated, too. The real and imaginary parts of dielectric functions and hence the optical functions such as energy-loss function, the effective number of valance electrons and the effective optical dielectric constant for Cu2FeSnZ4 (Z = S, Se) compounds were also calculated. © Published under licence by IOP Publishing Ltd.Item Open Access A controllable spin prism(IOP Institute of Physics Publishing, 2009) Hakiolu, T.Based on Khodas et al (2004 Phys. Rev. Lett. 92 086602), we propose a device acting like a controllable prism for an incident spin. The device is a large quantum well where Rashba and Dresselhaus spin-orbit interactions are present and controlled by the plunger gate potential, the electric field and the barrier height. A totally destructive interference can be manipulated externally between the Rashba and Dresselhaus couplings. The spin-dependent transmission/reflection amplitudes are calculated as the control parameters are changed. The device operates as a spin prism/converter/filter in different regimes and may stimulate research in promising directions in spintronics in analogy with linear optics. © 2009 IOP Publishing Ltd.Item Open Access Electronic structure of half-metallic ferromagnet Co2MnSi at high-pressure(Springer New York LLC, 2010) Gökoǧlu, G.; Gülseren, O.In this study, first principles calculation results of the half-metallic ferromagnetic Heusler compound Co2MnSi are presented. All calculations are based on the spin-polarized generalized gradient approximation (σ-GGA) of the density functional theory and ultrasoft pseudopotentials with plane wave basis. Electronic structure of related compound in cubic L21 structure is investigated up to 95 GPa uniform hydrostatic pressure. The half-metal to metal transition was observed around ∼70 GPa together with downward shift of the conduction band minimum (CBM) and a linear increase of direct band gap of minority spins at Γ-point with increasing pressure. The electronic density of states of minority spins at Fermi level, which are mainly due to the cobalt atoms, become remarkable with increasing pressure resulting a sharp decrease in spin polarization ratio. It can be stated that the pressure affects minority spin states rather than that of majority spins and lead to a slight reconstruction of minority spin states which lie below the Fermi level. In particular, energy band gap of minority spin states in equilibrium structure is obviously not destroyed, but the Fermi level is shifted outside the gap.Item Open Access Excitonic condensation under spin-orbit coupling and BEC-BCS crossover(The American Physical Society, 2007) Hakioǧlu T.; Şahin, M.The condensation of electron-hole pairs is studied at zero temperature and in the presence of a weak spin-orbit coupling (SOC) in coupled quantum wells. Under realistic conditions, a perturbative SOC can have observable effects in the order parameter of the condensate. First, the fermion exchange symmetry is absent. As a result, the condensate spin has no definite parity. Additionally, the excitonic SOC breaks the rotational symmetry yielding a complex order parameter in an unconventional way; i.e., the phase pattern of the order parameter is a function of the condensate density. This is manifested through finite off-diagonal components of the static spin susceptibility, suggesting a new experimental method to confirm an excitonic condensate.Item Open Access Magnetization of graphane by dehydrogenation(AIP Publishing, 2009) Şahin, H.; Ataca, C.; Çıracı, SalimUsing first principles calculations, we show that each hydrogen vacancy created at graphane surface results in a local unpaired spin. For domains of hydrogen vacancies the situation is, however, complex and depends on the size and geometry of domains, as well as whether the domains are single or double sided. In single-sided domains, hydrogen atoms at the other side are relocated to pair the spins of adjacent carbon atoms by forming ππ-bonds. Owing to the different characters of exchange coupling in different ranges and interplay between unpaired spin and the binding geometry of hydrogen, vacancy domains can attain sizable net magnetic moments.Item Open Access Nuclear spin squeezing via electric quadrupole interaction(American Physical Society, 2016) Korkmaz, Y. A.; Bulutay, C.Control over nuclear-spin fluctuations is essential for processes that rely on preserving the quantum state of an embedded system. For this purpose, squeezing is a viable alternative, so far that has not been properly exploited for the nuclear spins. Of particular relevance in solids is the electric quadrupole interaction (QI), which operates on nuclei having spin higher than 1/2. In its general form, QI involves an electric-field gradient (EFG) biaxiality term. Here, we show that as this EFG biaxiality increases, it enables continuous tuning of single-particle squeezing from the one-axis twisting to the two-axis countertwisting limits. A detailed analysis of QI squeezing is provided, exhibiting the intricate consequences of EFG biaxiality. The initial states over the Bloch sphere are mapped out to identify those favorable for fast initial squeezing, or for prolonged squeezings. Furthermore, the evolution of squeezing in the presence of a phase-damping channel and an external magnetic field are investigated. We observe that dephasing drives toward an antisqueezed terminal state, the degree of which increases with the spin angular momentum. Finally, QI squeezing in the limiting case of a two-dimensional EFG with a perpendicular magnetic field is discussed, which is of importance for two-dimensional materials, and the associated beat patterns in squeezing are revealed. © 2016 American Physical Society.Item Open Access Quantum correlations of spin-1 atoms in an optical lattice(Institute of Physics Publishing, 2009) Öztop, B.; Oktel, M. Ö.; Müstecaplioǧlu, Ö. E.In this work, we investigate the system of cold spin-1 atoms in a one dimensional optical lattice in relation with squeezing and entanglement. By using the corresponding Bose-Hubbard Hamiltonian, both superfluid and Mott-insulator phases are studied by using numerical methods in the mean-field approximation. To observe the presence of entanglement, we used a squeezing measure as a criterion for quantum correlations. We further investigate the two interaction regimes, namely ferromagnetic and antiferromagnetic in the case of zero and nonzero but very small angle between the counterpropagating laser beams that form the optical lattice. States in the superfluid phase are calculated analytically by using the perturbation theory.Item Open Access Quantum entanglement of spin-1 bosons with coupled ground states in optical lattices(IOP Institute of Physics Publishing, 2009) Öztop, B.; Oktel, M. Ö.; Müstecapliolu, Ö. E.; You, L.We examine particle entanglement, characterized by pseudo-spin squeezing, of spin-1 bosonic atoms with coupled ground states in a one-dimensional optical lattice. Both the superfluid and Mott-insulator phases are investigated separately for ferromagnetic and antiferromagnetic interactions. Mode entanglement is also discussed in the Mott-insulating phase. The role of a small but nonzero angle between the polarization vectors of counter-propagating lasers forming the optical lattice on quantum correlations is investigated as well.Item Open Access Simple test for hidden variables in spin-1 systems(2008) Klyachko, A. A.; Can, M. A.; Binicioǧlu, S.; Shumovsky, A. S.We resolve an old problem about the existence of hidden parameters in a three-dimensional quantum system by constructing an appropriate Bell's type inequality. This reveals the nonclassical nature of most spin-1 states. We shortly discuss some physical implications and an underlying cause of this nonclassical behavior, as well as a perspective of its experimental verification. © 2008 The American Physical Society.Item Open Access Spin filtering in a quantum ring with Rashba coupling(IEEE, 2010) Tanatar, Bilal; Moldoveanu V.We study the effect of Rashba spin-orbit coupling on the spin interference in a non-interacting one-dimensional ring connected to two lead theoretically within the non-equilibrium Greens' function formalism. We compute the charge and spin currents and analyze their Aharonov-Bohm oscillations. The geometry of the system is conveniently described by the angle δ between the two leads. We show that for δ=180° (i.e for symmetrically coupled leads) a good tutering of up or down spin orientation is obtained around half-integer multiples of Φ/Φ0. These particular flux values correspond to degeneracy points for clockwise and counter-clockwise propagating state related to the same spin orientation in the local spin frame of the ring. In contrast, for the asymmetric coupling, i.e., δ=135° the filter efficiency is maximum around integer multiples of Φ/Φ0. The numerical results suggest that the spin filtering is obtained when the clockwise or counter-clockwise states interfere destructively. The spin filtering regime is stable against variations of the bias applied on the system. ©2010 IEEE.Item Open Access Theory of the Pseudospin resonance in semiconductor bilayers(The American Physical Society, 2007) Abedinpour, S. H.; Polini, M.; MacDonald, A. H.; Tanatar, Bilal; Tosi, M. P.; Vignale, G.The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic-resonance mode of a ferromagnet. We present a many-body theory of the dependence of the energy and the damping of this mode on layer separation d. Based on these results, we discuss the possibilities of realizing transport-current driven pseudospin-transfer oscillators in semiconductors, and of using the pseudospin-transfer effect as an experimental probe of intersubband plasmons.Item Open Access Unconventional pairing in excitonic condensates under Spin-orbit coupling(The American Physical Society, 2009) Can, M. A.; Hakioǧlu T.It is shown that Rashba and Dresselhaus spin-orbit couplings enhance the conclusive power in the experiments on the excitonic condensate by at least three low temperature effects. First, spin-orbit coupling facilitates the photoluminescence measurements via enhancing the bright contribution in the otherwise dominantly dark ground state. The second is the presence of a low temperature power law dependence of the specific heat and weakening of the second order transition at the critical temperature. The third is the appearance of the nondiagonal elements in the static spin susceptibility.