3,681 research outputs found

    Carrier-density effects in many-polaron systems

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    Many-polaron systems with finite charge-carrier density are often encountered experimentally. However, until recently, no satisfactory theoretical description of these systems was available even in the framework of simple models such as the one-dimensional spinless Holstein model considered here. In this work, previous results obtained using numerical as well as analytical approaches are reviewed from a unified perspective, focussing on spectral properties which reveal the nature of the quasiparticles in the system. In the adiabatic regime and for intermediate electron-phonon coupling, a carrier-density driven crossover from a polaronic to a rather metallic system takes place. Further insight into the effects due to changes in density is gained by calculating the phonon spectral function, and the fermion-fermion and fermion-lattice correlation functions. Finally, we provide strong evidence against the possibility of phase separation.Comment: 13 pages, 6 figures, accepted for publication in J. Phys.: Condens. Matter; final versio

    Optical conductivity of polaronic charge carriers

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    The optical conductivity of charge carriers coupled to quantum phonons is studied in the framework of the one-dimensional spinless Holstein model. For one electron, variational diagonalisation yields exact results in the thermodynamic limit, whereas at finite carrier density analytical approximations based on previous work on single-particle spectral functions are obtained. Particular emphasis is put on deviations from weak-coupling, small-polaron or one-electron theories occurring at intermediate coupling and/or finite carrier density. The analytical results are in surprisingly good agreement with exact data, and exhibit the characteristic polaronic excitations observed in experiments on manganites.Comment: 23 pages, 11 figure

    Two electrons on a hypersphere: a quasi-exactly solvable model

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    We show that the exact wave function for two electrons, interacting through a Coulomb potential but constrained to remain on the surface of a D\mathcal{D}-sphere (D1\mathcal{D} \ge 1), is a polynomial in the interelectronic distance uu for a countably infinite set of values of the radius RR. A selection of these radii, and the associated energies, are reported for ground and excited states on the singlet and triplet manifolds. We conclude that the D=3\mathcal{D}=3 model bears the greatest similarity to normal physical systems.Comment: 4 pages, 0 figur

    Interplay of charge and spin correlations in nickel perovskites

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    Analyzing the motion of low--spin (s=1/2)(s=1/2) holes in a high--spin (S=1)(S=1) background, we derive a sort of generalized t--J Hamiltonian for the NiO2\rm NiO_2 planes of Sr--doped nickelates. In addition to the rather complex carrier--spin and spin--spin couplings we take into account the coupling of the doped holes to in--plane oxygen breathing modes by a Holstein--type interaction term. Because of strong magnetic confinement effects the holes are nearly entirely prelocalized and the electron--phonon coupling becomes much more effective in forming polarons than in the isostructural cuprates. In the light of recent experiments on La2xSrxNiO4\rm La_{2-x}Sr_xNiO_4 we discuss how the variety of the observed transport and charge/spin--ordering phenomena can be qualitatively understood in terms of our model Hamiltonian.Comment: 2 pages, LTpaper.sty, Proc. XXI Int. Conf. on Low Temp. Phys. Prague 9

    Invariance of the correlation energy at high density and large dimension in two-electron systems

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    We prove that, in the large-dimension limit, the high-density correlation energy \Ec of two opposite-spin electrons confined in a DD-dimensional space and interacting {\em via} a Coulomb potential is given by \Ec \sim -1/(8D^2) for any radial confining potential V(r)V(r). This result explains the observed similarity of \Ec in a variety of two-electron systems in three-dimensional space.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let

    Phonon affected transport through molecular quantum dots

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    To describe the interaction of molecular vibrations with electrons at a quantum dot contacted to metallic leads, we extend an analytical approach that we previously developed for the many-polaron problem. Our scheme is based on an incomplete variational Lang-Firsov transformation, combined with a perturbative calculation of the electron-phonon self-energy in the framework of generalised Matsubara functions. This allows us to describe the system at weak to strong coupling and intermediate to large phonon frequencies. We present results for the quantum dot spectral function and for the kinetic coefficient that characterises the electron transport through the dot. With these results we critically examine the strengths and limitations of our approach, and discuss the properties of the molecular quantum dot in the context of polaron physics. We place particular emphasis on the importance of corrections to the concept of an antiadiabatic dot polaron suggested by the complete Lang-Firsov transformation.Comment: 30 pages, 15 figures, revised version including new figure

    Sub-picosecond compression by velocity bunching in a photo-injector

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    We present an experimental evidence of a bunch compression scheme that uses a traveling wave accelerating structure as a compressor. The bunch length issued from a laser-driven radio-frequency electron source was compressed by a factor >3 using an S-band traveling wave structure located immediately downstream from the electron source. Experimental data are found to be in good agreement with particle tracking simulations.Comment: 19 pages, 9 figures, submitted to Phys. Rev. Spec. Topics A&

    Far Infrared Spectroscopy

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    Contains reports on five research projects

    An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator

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    Narrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs

    Photoemission spectra of many-polaron systems

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    The cross over from low to high carrier densities in a many-polaron system is studied in the framework of the one-dimensional spinless Holstein model, using unbiased numerical methods. Combining a novel quantum Monte Carlo approach and exact diagonalization, accurate results for the single-particle spectrum and the electronic kinetic energy on fairly large systems are obtained. A detailed investigation of the quality of the Monte Carlo data is presented. In the physically most important adiabatic intermediate electron-phonon coupling regime, for which no analytical results are available, we observe a dissociation of polarons with increasing band filling, leading to normal metallic behavior, while for parameters favoring small polarons, no such density-driven changes occur. The present work points towards the inadequacy of single-polaron theories for a number of polaronic materials such as the manganites.Comment: 15 pages, 13 figures; final version, accepted for publication in Phys. Rev.
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