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See detailFrom hybrid polariton to dipolariton using non-Hermitian Hamiltonians to handle particle lifetimes
Chenu, Aurélia UL; Shiau, Shiue-Yuan; Chien, Ching-Hang et al

in Physical Review. B, Condensed Matter (2022)

We consider photons strongly coupled to the excitonic excitations of a coupled quantum well, in the presence of an electric field. We show how, under a field increase, the hybrid polariton made of a ... [more ▼]

We consider photons strongly coupled to the excitonic excitations of a coupled quantum well, in the presence of an electric field. We show how, under a field increase, the hybrid polariton made of a photon coupled to hybrid carriers lying in the two wells transforms into a dipolariton made of a photon coupled to direct and indirect excitons. We also show how the cavity photon lifetime and the coherence time of the carrier wave vectors that we analytically handle through non-hermitian Hamiltonians affect these polaritonic states. While the hybrid polaritons display a spectral singularity where the eigenvalues coalesce, known as an exceptional point, that depends on detuning and lifetimes, we find that the three dipolaritonic states display an anticrossing without exceptional point due to the interaction between photons, direct, and indirect excitons. [less ▲]

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See detailObservation of surface magnons and crystalline electric field shifts in superantiferromagnetic NdCu2 nanoparticles
Jefremovas, E. M.; de la Fuente Rodríguez, M.; Damay, F. et al

in Physical Review. B, Condensed Matter (2021), 104

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See detailActive control of ultrafast electron dynamics in plasmonic gaps using an applied bias
Ludwig, Markus; K. Kazansky, Andrey; Aguirregabiria, Garikoitz et al

in Physical Review. B, Condensed Matter (2020)

In this joint experimental and theoretical study we demonstrate coherent control of the optical field emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our ... [more ▼]

In this joint experimental and theoretical study we demonstrate coherent control of the optical field emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that an external THz field or a minor dc bias, orders of magnitude smaller than the optical bias owing to the laser field, allows one to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultrafast nanoscale vacuum diode for lightwave electronics. Using time-dependent density functional theory calculations we elucidate the main physical mechanisms behind the observed effects and show that an applied dc field significantly modifies the optical field emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device, which allows us to establish a paradigm for controlling nanocircuits at petahertz frequencies [less ▲]

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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 detailSpin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi studied by small-angle neutron scattering
Grigoriev; Pschenichnyi; Altynbaev et al

in Physical Review. B, Condensed Matter (2019), 100

The spin wave stiffness was measured by small-angle neutron scattering method in the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi with x=0.25,0.30,0.50. It has been shown that the spin wave ... [more ▼]

The spin wave stiffness was measured by small-angle neutron scattering method in the Dzyaloshinskii-Moriya helimagnet compounds Fe1−xCoxSi with x=0.25,0.30,0.50. It has been shown that the spin wave dispersion in the fully polarized state is anisotropic due to Dzyaloshinskii-Moriya interaction. It is reflected in the neutron scattering pattern as two circles for neutrons obtaining and losing the magnon energy, respectively. The centers of the circles are shifted by the momentum transfer oriented along the applied magnetic field H and equal to the wave vector of the spiral ±ks. The radius of the circles is directly related to the stiffness of spin waves and depends on the magnetic field. We have found that the spin-wave stiffness A change weakly with temperature for each individual compound. On the other hand, the spin-wave stiffness A increases linearly with x in contrast to the x dependences of the critical temperature Tc and the low-temperature ordered moment. Experimentally obtained values of the stiffness A approve quantitative applicability of the Bak-Jensen model for the compounds under study. [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 UL; Esposito, Massimiliano UL; Schmidt, Thomas UL

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

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See detailDipolar-coupled moment correlations in clusters of magnetic nanoparticles
Bender, Philipp Florian UL; Wetterskog, E.; Honecker, Dirk UL et al

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

Here, we resolve the nature of the moment coupling between 10-nm dimercaptosuccinic acid–coated magnetic <br />nanoparticles. The individual iron oxide cores were composed of >95% maghemite and ... [more ▼]

Here, we resolve the nature of the moment coupling between 10-nm dimercaptosuccinic acid–coated magnetic <br />nanoparticles. The individual iron oxide cores were composed of >95% maghemite and agglomerated to <br />clusters. At room temperature the ensemble behaved as a superparamagnet according to Mössbauer and magnetization <br />measurements, however, with clear signs of dipolar interactions. Analysis of temperature-dependent <br />ac susceptibility data in the superparamagnetic regime indicates a tendency for dipolar-coupled anticorrelations <br />of the core moments within the clusters. To resolve the directional correlations between the particle moments <br />we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross section <br />of the powder in low magnetic field at 300 K. We extract the underlying magnetic correlation function of <br />the magnetization vector field by an indirect Fourier transform of the cross section. The correlation function <br />suggests nonstochastic preferential alignment between neighboring moments despite thermal fluctuations, with <br />anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo <br />simulations of such core clusters. [less ▲]

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See detailPhysical adsorption at the nanoscale: Towards controllable scaling of the substrate-adsorbate van der Waals interaction
Ambrosetti, Alberto; Silvestrelli, Pier Luigi; Tkatchenko, Alexandre UL

in Physical Review. B, Condensed Matter (2017), 95(1), 235417

The Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct large-distance limit for the van der Waals (vdW) interaction of adsorbates (atoms, molecules, or nanoparticles) with solid ... [more ▼]

The Lifshitz-Zaremba-Kohn (LZK) theory is commonly considered as the correct large-distance limit for the van der Waals (vdW) interaction of adsorbates (atoms, molecules, or nanoparticles) with solid substrates. In the standard approximate form, implicitly based on local dielectric functions, the LZK approach predicts universal power laws for vdW interactions depending only on the dimensionality of the interacting objects. However, recent experimental findings are challenging the universality of this theoretical approach at finite distances of relevance for nanoscale assembly. Here, we present a combined analytical and numerical many-body study demonstrating that physical adsorption can be significantly enhanced at the nanoscale. Regardless of the band gap or the nature of the adsorbate specie, we find deviations from conventional LZK power laws that extend to separation distances of up to 10–20 nm. Comparison with recent experimental observations of ultra-long-ranged vdW interactions in the delamination of graphene from a silicon substrate reveals qualitative agreement with the present theory. The sensitivity of vdW interactions to the substrate response and to the adsorbate characteristic excitation frequency also suggests that adsorption strength can be effectively tuned in experiments, paving the way to an improved control of physical adsorption at the nanoscale. [less ▲]

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See detailLifting the Franck-Condon blockade in driven quantum dots
Haughian, Patrick UL; Walter, Stefan; Nunnenkamp, Andreas et al

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

Electron-vibron coupling in quantum dots can lead to a strong suppression of the average current in the sequential tunneling regime. This effect is known as Franck-Condon blockade and can be traced back ... [more ▼]

Electron-vibron coupling in quantum dots can lead to a strong suppression of the average current in the sequential tunneling regime. This effect is known as Franck-Condon blockade and can be traced back to an overlap integral between vibron states with different electron numbers which becomes exponentially small for large electron-vibron coupling strength. Here, we investigate the effect of a time-dependent drive on this phenomenon, in particular the effect of an oscillatory gate voltage acting on the electronic dot level. We employ two different approaches: perturbation theory based on nonequilibrium Keldysh Green's functions and a master equation in Born-Markov approximation. In both cases, we find that the drive can lift the blockade by exciting vibrons. As a consequence, the relative change in average current grows exponentially with the drive strength. [less ▲]

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See detailExcitons in boron nitride single layer
Galvani, Thomas; Paleari, Fulvio UL; Pereira Coutada Miranda, Henrique UL et al

in Physical Review. B, Condensed Matter (2016), 94(125303),

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed ... [more ▼]

Boron nitride single layer belongs to the family of two-dimensional materials whose optical properties are currently receiving considerable attention. Strong excitonic effects have already been observed in the bulk and still stronger effects are predicted for single layers. We present here a detailed study of these properties by combining ab initio calculations and a tight-binding Wannier analysis in both real and reciprocal space. Due to the simplicity of the band structure with single valence (π) and conduction (π∗) bands the tight-binding analysis becomes quasiquantitative with only two adjustable parameters and provides tools for a detailed analysis of the exciton properties. Strong deviations from the usual hydrogenic model are evidenced. The ground-state exciton is not a genuine Frenkel exciton, but a very localized tightly bound one. The other ones are similar to those found in transition-metal dichalcogenides and, although more localized, can be described within a Wannier-Mott scheme. [less ▲]

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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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See detailTemperature-dependent excitonic effects in the optical properties of single-layer MoS2
Molina-Sanchez, Alejandro UL; Palummo, Maurizia; Marini, Andrea et al

in Physical Review. B, Condensed Matter (2016), 93

The electron-phonon interaction alters substantially the conventional picture of the band structure. It also changes the properties of excitonic states, which are very pronounced in many 2D materials ... [more ▼]

The electron-phonon interaction alters substantially the conventional picture of the band structure. It also changes the properties of excitonic states, which are very pronounced in many 2D materials. Using many-body perturbation theory, the authors describe how the inclusion of temperature modifies the electronic bands of single-layer MoS2. Different bands and different regions in the Brillouin zone are affected in different ways by electron-phonon coupling. Using the temperature-broadened bands as input for the Bethe-Salpeter equation, the authors explain why, for the bound A and B excitons, the electron-phonon coupling changes mainly the position, and for the C exciton, only the width is affected by temperature, while the energy is rather constant. [less ▲]

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See detailCharacterization of competing distortions in YFe2O4
Blasco, J; Lafuerza, S; Garcia et al

in Physical Review. B, Condensed Matter (2016), 93

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See detailDipolar spin-misalignment correlations in inhomogeneous magnets: Comparison between neutron scattering and micromagnetic
Erokhin, Sergey; Berkov, Dmitry; Michels, Andreas UL

in Physical Review. B, Condensed Matter (2015), 92(1), 014427

In inhomogeneous bulk ferromagnets, the dominating sources of spin disorder are related to spatial variations of (i) the magnitude of the local saturation magnetization and of (ii) the magnitude and/or ... [more ▼]

In inhomogeneous bulk ferromagnets, the dominating sources of spin disorder are related to spatial variations of (i) the magnitude of the local saturation magnetization and of (ii) the magnitude and/or direction of the magnetic anisotropy field. For the particular example of a porous ferromagnet, where the magnetization inhomogeneity is at maximum, we demonstrate, by means of experimental neutron scattering data and micromagnetic simulations, the anisotropic character of magnetization fluctuations induced by the dipolar interaction. [less ▲]

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