1,720,979 research outputs found
A high-performance full-configuration-interaction study of Wigner crystallization in quantum dots
No full textWe study the transition between liquid and solid electron phases of a two-dimensional quantum dot by performing extensive full-configuration- interaction calculations. From the analysis of pair correlation functions we find evidence of localization for values of the dimensionless density parameter λ ≈ 4. © 2007 American Institute of Physics
Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality
Full text linkWe study the energy spectra of small three-dimensional (3D) and two-dimensional (2D) semiconductor quantum dots through different theoretical approaches (single-site Hubbard and Hartree-Fock Hamiltonians); in the smallest dots we also compare with exact results. We find that purely 2D models often lead to an inadequate description of the Coulomb interaction existing in realistic Structures, as a consequence: of the overestimated carrier localization. We show that the dimensionality of the dots has a crucial impact on (i) the accuracy of the predicted addition spectra, and (ii) the range of validity of approximate theoretical schemes. When applied to realistic 3D geometries, the latter are found to be much more accurate than in the corresponding 2D cases for a large class of quantum dots; the single-site Hubbard Hamiltonian is shown to provide a very effective and accurate scheme to describe quantum dot spectra, leading to good agreement with experiments. [S0163-1829(99)10211-X]
A monolayer transition-metal dichalcogenide as a topological excitonic insulator
Full text linkTopological insulators have been studied primarily with regard to the behaviour of electrons. A theoretical study now shows that a single layer of a metal dichalcogenide can become a topological insulator for excitons.Monolayer transition-metal dichalcogenides in the T ' phase could enable the realization of the quantum spin Hall effect(1) at room temperature, because they exhibit a prominent spin-orbit gap between inverted bands in the bulk(2,3). Here we show that the binding energy of electron-hole pairs excited through this gap is larger than the gap itself in the paradigmatic case of monolayer T ' MoS2, which we investigate from first principles using many-body perturbation theory(4). This paradoxical result hints at the instability of the T ' phase in the presence of spontaneous generation of excitons, and we predict that it will give rise to a reconstructed 'excitonic insulator' ground state(5-7). Importantly, we show that in this monolayer system, topological and excitonic order cooperatively enhance the bulk gap by breaking the crystal inversion symmetry, in contrast to the case of bilayers(8-16) where the frustration between the two orders is relieved by breaking time reversal symmetry(13,15,16). The excitonic topological insulator is distinct from the bare topological phase because it lifts the band spin degeneracy, which results in circular dichroism. A moderate biaxial strain applied to the system leads to two additional excitonic phases, different in their topological character but both ferroelectric(17,18) as an effect of electron-electron interaction
Response to "Comment on 'Field-controlled suppression of phonon-induced transitions in coupled quantum dots' [Appl. Phys. Lett. 88, 4729 (2006)]"
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Anomalous screening in narrow-gap carbon nanotubes
Full text linkThe screening of Coulomb interaction controls many-body physics in carbon nanotubes, as it tunes the range and strength of the force that acts on charge carriers and binds electron-hole pairs into excitons. In doped tubes, the effective Coulomb interaction drives the competition between Luttinger liquid and Wigner crystal, whereas in undoped narrow-gap tubes it dictates the Mott or excitonic nature of the correlated insulator observed at low temperature. Here, by computing the dielectric function of selected narrow- and zero-gap tubes from first principles, we show that the standard effective-mass model of screening systematically underestimates the interaction strength at long wavelength, hence missing the binding of low-energy excitons. The reason is that the model critically lacks the full three-dimensional topology of the tube, being adapted from graphene theory. As ab inito calculations are limited to small tubes, we develop a two-band model dielectric function based on the plane-wave expansion of Bloch states and the exact truncated Coulomb cutoff technique. We demonstrate that our - computationally cheap - approach provides the correct screening for narrow-gap tubes of any size and chirality. A striking result is that the screened interaction remains long-ranged even in gapless tubes, as an effect of the microscopic local fields generated by the electrons moving on the curved tube surface. As an application, we show that the effective electron-electron force that is felt at distances relevant to quantum transport experiments is super Coulombic
Magnetic field dependence of triplet-singlet relaxation in quantum dots with spin-orbit coupling
Full text linkWe estimate the triplet-singlet relaxation time due to spin-orbit coupling assisted by phonon emission in weakly confined quantum dots. Calculations are performed taking into account Coulomb and spin-orbit interactions exactly within the full configuration interaction method, and Fermi golden rule. Our results for two and four electrons show that different triplet-singlet relaxation trends observed in recent experiments under magnetic fields can be understood within a unified theoretical description, as the result of the competition between spin-orbit coupling and phonon emission efficiency. Moreover, we show that properly designed QD structures may give access to very long-lived triplet states as well as to selective population of the triplet Zeeman sublevels
Suppression of acoustic-phonon-induced electron transitions in coupled quantum dots
No full textWe calculate the longitudinal-acoustic phonon scattering rate for a vertical double quantum dot (DQD) system and show that a strong modulation of the single-electron excited states lifetime can be induced by an external magnetic field. The results are obtained for typical realistic devices using a Fermi golden rule approach and a three-dimensional description of the electronic quantum states. The DQDs considered are characterized by a weak lateral confinement and the longitudinal-phonon scattering represents the dominant source of decoherence, its tunable suppression can be a valuable tool for an experimental measure of electron-states lifetimes and serve as a signature for coherent delocalization of electrons in the DQD
Evidence of ideal excitonic insulator in bulk MoS2 under pressure
Full text linkSpontaneous condensation of excitons is a long-sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons—electron (e) and hole (h) pairs interacting by Coulomb force—overcomes the band gap, giving rise to a new phase: the “excitonic insulator” (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and demonstrate theoretically that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular. Starting from first-principles many-body perturbation theory, we also predict that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an antiferroelectric texture in the layer plane: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a unique Raman fingerprint for the EI formation. Finally, we identify such fingerprint in a Raman feature that was previously observed experimentally, thus providing direct spectroscopic confirmation of an ideal excitonic insulator phase in bulk MoS2 above 30 GPa
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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