Browsing by Author "Hetenyi, Balazs"

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Open Access
Az anyag Polarizaciojanak modern elmelete: A polarizáció teljes eloszlásának kiszámolása kristályos rendszerekben
(2020) Hetenyi, Balazs
Open Access
Extended Creutz ladder with spin-orbit coupling: a one-dimensional analog of the Kane-Mele model
(Institute of Physics Publishing, 2018) Gholizadeh, Sina; Yahyavi, M.; Hetenyi, Balazs
We construct a topological one-dimensional ladder model following the steps which lead to the Kane-Mele model in two dimensions. Starting with a Creutz ladder we modify it so that the gap closure points can occur at either or . We then couple two such models, one for each spin channel, in such a way that time-reversal invariance is restored. We also add a Rashba spin-orbit coupling term. The model falls in the CII symmetry class. We derive the relevant topological index, calculate the phase diagram and demonstrate the existence of edge states. We also give the thermodynamic derivation (Středa-Widom) of the quantum spin Hall conductance. Approximate implementation of this result indicates that this quantity is sensitive to the topological behavior of the model.
Open Access
Generalized Aubry-Andre-Harper model with modulated hopping and p-wave pairing
(American Physical Society, 2019) Yahyavi, Mohammad; Hetenyi, Balazs; Tanatar, Bilal
We study an extended Aubry-André-Harper model with simultaneous modulation of hopping on-site potential and p-wave superconducting pairing. For the case of commensurate modulation of β=1/2 it is shown that the model hosts four different types of topological states: Adiabatic cycles can be defined which pump particles two types of Majorana fermions or Cooper pairs. In the incommensurate case we calculate the phase diagram of the model in several regions. We characterize the phases by calculating the mean inverse participation ratio and perform multifractal analysis. In addition we characterize whether the phases found are topologically trivial or not. We find an interesting critical extended phase when incommensurate hopping modulation is present. The rise between the inverse participation ratio in regions separating localized and extended states is gradual rather than sharp. When in addition the on-site potential modulation is incommensurate we find several sharp rises and falls in the inverse participation ratio. In these two cases all different phases exhibit topological edge states. For the commensurate case we calculate the evolution of the Hofstadter butterfly and the band Chern numbers upon variation of the pairing parameter for zero and finite on-site potential. For zero on-site potential the butterflies are triangularlike near zero pairing when gap closure occurs they are squarelike and hexagonal-like for larger pairing but with the Chern numbers switched compared to the triangular case. For the finite case gaps at quarter and three-quarters filling close and lead to a switch in Chern numbers.
Open Access
Quantum phase transitions from analysis of the polarization amplitude
(American Physical Society, 2019) Hetenyi, Balazs; Dora, B.
In the modern theory of polarization, polarization itself is given by a geometric phase. In calculations for interacting systems the polarization and its variance are obtained from the polarization amplitude. We interpret this quantity as a discretized characteristic function and derive formulas for its cumulants and moments. In the case of a noninteracting system, our scheme leads to the gauge-invariant cumulants known from polarization theory. We study the behavior of such cumulants for several interacting models. In a one-dimensional system of spinless fermions with nearest neighbor interaction the transition at which gap closure occurs can be clearly identified from the finite size scaling exponent of the variance. When next nearest neighbor interactions are turned on a model with a richer phase diagram emerges, but the finite size scaling exponent is still an effective way to identify the localization transition.
Open Access
Superfluid weight and polarization amplitude in the one-dimensional bosonic Hubbard model
(American Physical Society, 2019) Hetenyi, Balazs; Martelo, L. M.; Tanatar, Bilal
We calculate the superfluid weight and the polarization amplitude for the one-dimensional bosonic Hubbard model with focus on the strong-coupling regime via variational, exact diagonalization, and strong coupling calculations. Our variational approach is based on the Baeriswyl wave function, implemented via Monte Carlo sampling. We derive the superfluid weight appropriately in a variational setting. We emphasize the importance of implementing the Peierls phase in position space and to allow for many-body interference effects, rather than implementing the Peierls phase as single particle momentum shifts. At integer filling, the Baeriswyl wave function gives zero superfluid response at any coupling. At half filling our variational superfluid weight is in reasonable agreement with exact diagonalization results. We also calculate the polarization amplitude, the variance of the total position, and the associated size scaling exponent, which corroborate that this variational approach produces an insulating state at integer filling. Our Baeriswyl based variational method is applicable to significantly larger system sizes than exact diagonalization or quantum Monte Carlo.