Exciton Optical Absorption in Self-Similar Aperiodic Lattices

Exciton optical absorption in self-similar aperiodic one-dimensional systems is considered, focusing our attention on Thue-Morse and Fibonacci lattices as canonical examples. The absorption line shape is evaluated by solving the microscopic equations of motion of the Frenkel-exciton problem on the lattice, in which on-site energies take on two values, according to the Thue-Morse or Fibonacci sequences. Results are compared to those obtained in random lattices with the same stechiometry and size. We find that aperiodic order causes the occurrence of well-defined characteristic features in the absorption spectra which clearly differ from the case of random systems, indicating a most peculiar exciton dynamics. We successfully explain the obtained spectra in terms of the two-center problem. This allows us to establish the origin of all the absorption lines by considering the self-similar aperiodic lattices as composed of two-center blocks, within the same spirit of the renormalization group ideas.Comment: 16 pages in REVTeX 3.0. 2 figures on request to F. D-A ([email protected]

Nonequilibrium functional RG with frequency dependent vertex function: A study of the single impurity Anderson model

We investigate nonequilibrium properties of the single impurity Anderson model by means of the functional renormalization group (fRG) within Keldysh formalism. We present how the level broadening Gamma/2 can be used as flow parameter for the fRG. This choice preserves important aspects of the Fermi liquid behaviour that the model exhibits in case of particle-hole symmetry. An approximation scheme for the Keldysh fRG is developed which accounts for the frequency dependence of the two-particle vertex in a way similar but not equivalent to a recently published approximation to the equilibrium Matsubara fRG. Our method turns out to be a flexible tool for the study of weak to intermediate on-site interactions U <= 3 Gamma. In equilibrium we find excellent agreement with NRG results for the linear conductance at finite gate voltage, magnetic field, and temperature. In nonequilibrium, our results for the current agree well with TD-DMRG. For the nonlinear conductance as function of the bias voltage, we propose reliable results at finite magnetic field and finite temperature. Furthermore, we demonstrate the exponentially small scale of the Kondo temperature to appear in the second order derivative of the self-energy. We show that the approximation is, however, not able to reproduce the scaling of the effective mass at large interactions.Comment: [v2] - minor changes throughout the text; added new Fig. 3; corrected pert.-theory data in Figs. 10, 11; published versio

Physical nature of critical wave functions in Fibonacci systems

We report on a new class of critical states in the energy spectrum of general Fibonacci systems. By introducing a transfer matrix renormalization technique, we prove that the charge distribution of these states spreads over the whole system, showing transport properties characteristic of electronic extended states. Our analytical method is a first step to find out the link between the spatial structure of these critical wave functions and the quasiperiodic order of the underlying lattice.Comment: REVTEX 3.0, 11 pages, 2 figures available upon request. To appear in Phys. Rev. Let

Band-theoretical prediction of magnetic anisotropy in uranium monochalcogenides

Magnetic anisotropy of uranium monochalcogenides, US, USe and UTe, is studied by means of fully-relativistic spin-polarized band structure calculations within the local spin-density approximation. It is found that the size of the magnetic anisotropy is fairly large (about 10 meV/unit formula), which is comparable with experiment. This strong anisotropy is discussed in view of a pseudo-gap formation, of which crucial ingredients are the exchange splitting of U 5f states and their hybridization with chalcogen p states (f-p hybridization). An anomalous trend in the anisotropy is found in the series (US>>USe<UTe) and interpreted in terms of competition between localization of the U 5f states and the f-p hybridization. It is the spin-orbit interaction on the chalcogen p states that plays an essential role in enlarging the strength of the f-p hybridization in UTe, leading to an anomalous systematic trend in the magnetic anisotropy.Comment: 4 pages, 5 figure

Berücksichtigung des Schutzgutes Boden bei Trassenplanungen

Die Energiewende ist eines der zentralen Themen unserer Zeit. Zu ihrer Umsetzung sind zahlreiche neue Stromtrassen erforderlich, die unterirdisch als Erdkabel geplant und errichtet werden. Der Erdkabelvorrang ist im Bundesbedarfsplangesetz (BBPlG) verankert und ist mit erheblichen Auswirkungen auf den Boden verbunden. Im Vergleich zum Bau einer Freileitung, ist der Boden hierbei durch baubedingte Eingriffe mit dem 10- bis 20fachen Volumen betroffen. In Abhängigkeit von den Bodeneigenschaften können durch eine Erdverkabelung umfangreiche Beeinträchtigungen des Bodens und seiner Bodenfunktionen sowie der Bodennutzung verursacht werden. In Niedersachsen sind v.a. die Trassen SüdLink und A-Nord relevant, die das Land jeweils von der Küste bis zur südlichen Ländergrenze durchqueren. Es erfolgt eine intensive Begleitung der Vorhaben durch zwei niedersächsische ressortübergreifende AGs. Der Bodenschutz ist in beiden Ressort-AGs vertreten. Aufgrund der hohen Betroffenheit der Böden, der Bodenfunktionen und der Bodennutzung ist es aus fachlicher Sicht erforderlich, das Schutzgut Boden frühzeitig und umfassend auf allen Planungsebenen sowie in der Bau- und Nachbauphase zu berücksichtigen. Dabei kommt Maßnahmen zur Vermeidung und Minimierung eine besondere Bedeutung zu. Ein nachhaltiger und schonender Umgang mit dem Boden trägt dem im BBodSchG und BNatSchG verankerten Vorsorgegedanken Rechnung und fördert die Akzeptanz bei den Flächennutzern. Gleichzeitig verringert sich der naturschutzfachliche Kompensationsbedarf in Hinblick auf das Schutzgut Boden und Aufwand und Kosten für Rekultivierung und Entschädigung können reduziert werden. Eine fachgerechte Berücksichtigung des Bodens kann somit zu einer Vereinfachung und Beschleunigung des Verfahrens beitragen. Der Beitrag zeigt die Betroffenheit des Bodens und der Bodenfunktionen bei der Erdverkabelung und die Besonderheiten bei Trassenplanungen in Hinblick auf den Boden auf, stellt die relevanten fachlichen Anforderungen des Bodenschutzes vor und erläutert die Aktivitäten zum Bodenschutz in Niedersachsen bei Trassenplanungen und Trassenbau

Resonance Energy Transfer from Monolayer WS2 to Organic Dye Molecules Conversion of Faint Visible Red into Bright Near Infrared Luminescence

The synergetic combination of transition metal dichalcogenides TMDCs with organic dye molecules in functional heterostructures is promising for various optoelectronic applications. Here resonance energy transfer RET from a red emitting WS2 monolayer 1L WS2 to a layer of near infrared NIR emitting organic dye molecules is demonstrated. It is found that the total photoluminescence PL yield of the heterostructures is up to a factor of eight higher as compared to the PL yield of pristine 1L WS2. This is attributed to the efficient conversion of the mostly non radiative excitons in 1L WS2 into radiative excitons in the dye layer. A type I energy level alignment of the 1L WS2 dye interface assures the emission of bright PL. From excitation density dependent PL experiments, it is concluded that RET prevails against defect assisted non radiative recombination as well as Auger type exciton exciton annihilation in 1L WS2. The work paves the way for employing organic dye molecules in heterostructures with TMDCs in nanoscale light emitting devices with improved efficiency and tunable colo

Band Calculations for Ce Compounds with AuCu3_{3}-type Crystal Structure on the basis of Dynamical Mean Field Theory I. CePd3_{3} and CeRh3_{3}

Band calculations for Ce compounds with the AuCu$_{3}$-type crystal structure were carried out on the basis of dynamical mean field theory (DMFT). The auxiliary impurity problem was solved by a method named NCA$f^{2}$vc (noncrossing approximation including the $f^{2}$ state as a vertex correction). The calculations take into account the crystal-field splitting, the spin-orbit interaction, and the correct exchange process of the $f^{1} \rightarrow f^{0},f^{2}$ virtual excitation. These are necessary features in the quantitative band theory for Ce compounds and in the calculation of their excitation spectra. The results of applying the calculation to CePd$_{3}$ and CeRh$_{3}$ are presented as the first in a series of papers. The experimental results of the photoemission spectrum (PES), the inverse PES, the angle-resolved PES, and the magnetic excitation spectra were reasonably reproduced by the first-principles DMFT band calculation. At low temperatures, the Fermi surface (FS) structure of CePd$_{3}$ is similar to that of the band obtained by the local density approximation. It gradually changes into a form that is similar to the FS of LaPd$_{3}$ as the temperature increases, since the $4f$ band shifts to the high-energy side and the lifetime broadening becomes large.}Comment: 12 pasges, 13 figure