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See detailDisorder-driven exceptional lines and Fermi ribbons in tilted nodal-line semimetals
Moors, Kristof; Zyuzin, Alexander A.; Zyuzin, Alexander Yu. et al

in Physical Review. B, Condensed Matter (2019), 99

We consider the impact of disorder on the spectrum of three-dimensional nodal-line semimetals. We show that the combination of disorder and a tilted spectrum naturally leads to a non-Hermitian self-energy ... [more ▼]

We consider the impact of disorder on the spectrum of three-dimensional nodal-line semimetals. We show that the combination of disorder and a tilted spectrum naturally leads to a non-Hermitian self-energy contribution that can split a nodal line into a pair of exceptional lines. These exceptional lines form the boundary of an open and orientable bulk Fermi ribbon in reciprocal space on which the energy gap vanishes. We find that the orientation and shape of such a disorder-induced bulk Fermi ribbon is controlled by the tilt direction and the disorder properties, which can also be exploited to realize a twisted bulk Fermi ribbon with nontrivial winding number. Our results put forward a paradigm for the exploration of non-Hermitian topological phases of matter. [less ▲]

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See detailMicrostructural-defect-induced Dzyaloshinskii-Moriya interaction
Michels, Andreas UL; Mettus, Denis; Titov, Ivan UL et al

in Physical Review. B, Condensed Matter (2019), 99

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See detailZ4 parafermions in one-dimensional fermionic lattices
Calzona, Alessio UL; Meng, Tobias; Sassetti, Maura et al

in Physical Review. B, Condensed Matter (2018), 98

Parafermions are emergent excitations which generalize Majorana fermions and are potentially relevant to topological quantum computation. Using the concept of Fock parafermions, we present a mapping ... [more ▼]

Parafermions are emergent excitations which generalize Majorana fermions and are potentially relevant to topological quantum computation. Using the concept of Fock parafermions, we present a mapping between lattice Z4-parafermions and lattice spin-1/2 fermions which preserves the locality of operators with Z4 symmetry. Based on this mapping, we construct an exactly solvable, local one-dimensional fermionic Hamiltonian which hosts parafermionic edge states. We numerically show that the parafermionic phase remains stable in a wide range of parameters, and discuss its signatures in the fermionic spectral function. [less ▲]

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See detailDirect and indirect excitons in boron nitride polymorphs: A story of atomic configuration and electronic correlation
Sponza, Lorenzo; Amara, Hakim; Attaccalite, Claudio et al

in Physical Review. B, Condensed Matter (2018), 98(12), 125206

We present a detailed discussion of the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA′ ... [more ▼]

We present a detailed discussion of the electronic band structure and excitonic dispersion of hexagonal boron nitride (hBN) in the single layer configuration and in three bulk polymorphs (usual AA′ stacking, Bernal AB, and rhombohedral ABC). We focus on the changes in the electronic band structure and the exciton dispersion induced by the atomic configuration and the electron-hole interaction. Calculations are carried out at the level of ab initio many-body perturbation theory (GW and Bethe Salpeter equation) and of a purposely developed tight-binding model. We confirm the change from direct to indirect electronic gap when going from single layer to bulk systems and we give a detailed account of its origin by comparing the effect of different stacking sequences. We emphasize that the inclusion of the electron-hole interaction is crucial for the correct description of the momentum-dependent dispersion of the excitations. As a result the electron-hole dispersion is flatter than the one obtained from the band structure. In the AB stacking this effect is particularly important as the lowest-lying exciton is predicted to be direct despite the indirect electronic band gap. [less ▲]

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See detailAb initio and semiempirical modeling of excitons and trions in monolayer TiS3
Torun, Engin UL; Sahin, H.; Chavez, A. et al

in Physical Review. B, Condensed Matter (2018), 98

We explore the electronic and the optical properties of monolayer TiS3, which shows in-plane anisotropy and is composed of a chain-like structure along one of the lattice directions. Together with its ... [more ▼]

We explore the electronic and the optical properties of monolayer TiS3, which shows in-plane anisotropy and is composed of a chain-like structure along one of the lattice directions. Together with its robust direct band gap, which changes very slightly with stacking order and with the thickness of the sample, the anisotropic physical prop- erties of TiS3 make the material very attractive for various device applications. In this study, we present a detailed investigation on the effect of the crystal anisotropy on the excitons and the trions of the TiS3 monolayer. We use many-body perturbation theory to calculate the absorption spectrum of anisotropic TiS3 monolayer by solving the Bethe-Salpeter equation. In parallel, we implement and use a Wannier-Mott model for the excitons that takes into account the anisotropic effective masses and Coulomb screening, which are obtained from ab initio calculations. This model is then extended for the investigation of trion states of monolayer TiS3. Our calculations indicate that the absorption spectrum of monolayer TiS3 drastically depends on the polarization of the incoming light, which excites different excitons with distinct binding energies. In addition, the binding energies of positively and the negatively charged trions are observed to be distinct and they exhibit an anisotropic probability density distribution. [less ▲]

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See detailFundamental limits to helical edge conductivity due to sphin-phonon scattering
Groenendijk, Solofo UL; Dolcetto; Schmidt, Thomas UL

in Physical Review. B, Condensed Matter (2018), 97

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See detailInterlayer and intralayer excitons in MoS2/WS2 and MoSe2/WSe2 heterobilayers
Torun, Engin UL; Miranda, Henrique P.C.; Molina-Sánchez, Alejandro et al

in Physical Review. B, Condensed Matter (2018), 97

Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in ... [more ▼]

Accurately described excitonic properties of transition metal dichalcogenide heterobilayers (HBLs) are crucial to comprehend the optical response and the charge carrier dynamics of them. Excitons in multilayer systems possess an inter- or intralayer character whose spectral positions depend on their binding energy and the band alignment of the constituent single layers. In this paper, we report the electronic structure and the absorption spectra of MoS2/WS2 and MoSe2/WSe2 HBLs from first-principles calculations. We explore the spectral positions, binding energies, and the origins of inter- and intralayer excitons and compare our results with experimental observations. The absorption spectra of the systems are obtained by solving the Bethe-Salpeter equation on top of a G0W0 calculation, which corrects the independent-particle eigenvalues obtained from density-functional theory. Our calculations reveal that the lowest energy exciton in both HBLs possess an interlayer character which is decisive regarding their possible device applications. Due to the spatially separated nature of the charge carriers, the binding energy of interlayer excitons might be expected to be considerably smaller than that of intralayer ones. However, according to our calculations, the binding energy of lowest energy interlayer excitons is only ∼20% lower due to the weaker screening of the Coulomb interaction between layers of the HBLs. Therefore, it can be deduced that the spectral positions of the interlayer excitons with respect to intralayer ones are mostly determined by the band offset of the constituent single layers. By comparing oscillator strengths and thermal occupation factors, we show that in luminescence at low temperature, the interlayer exciton peak becomes dominant, while in absorption it is almost invisible. [less ▲]

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See detailQuantum thermodynamics of the resonant-level model with driven system-bath coupling
Haughian, Patrick; Esposito, Massimiliano UL; Schmidt, Thomas UL

in Physical Review. B, Condensed Matter (2018), 97

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See detailRevealing dipolar-coupled moment correlations in clusters of superparamagnetic nanoparticles by polarized SANS
Bender, Philipp Florian UL; Wetterskog, E.; Honecker, D. et al

in Physical Review. B, Condensed Matter (2018), 98

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See detailHelical gaps in interacting {Rashba} wires at low electron densities
Schmidt, Thomas UL; Pedder, Christopher

in Physical Review. B, Condensed Matter (2016), 94

Rashba spin-orbit coupling and a magnetic field perpendicular to the Rashba axis have been predicted to open a partial gap (“helical gap”) in the energy spectrum of noninteracting or weakly interacting ... [more ▼]

Rashba spin-orbit coupling and a magnetic field perpendicular to the Rashba axis have been predicted to open a partial gap (“helical gap”) in the energy spectrum of noninteracting or weakly interacting one-dimensional quantum wires. By comparing kinetic energy and Coulomb energy we show that this gap opening typically occurs at low electron densities where the Coulomb energy dominates. To address this strongly correlated limit, we investigate Rashba wires using Wigner crystal theory. We find that the helical gap exists even in the limit of strong interactions but its dependence on electron density differs significantly from the weakly interacting case. In particular, we find that the critical magnetic field for opening the gap becomes an oscillatory function of electron density. This changes strongly the expected signature of the helical gap in conductance measurements. [less ▲]

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