Browsing by Subject "polarons"

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    Phonon anomalies in high temperature superconductors
    (1996) Türeci, E Hakan
    Anomalously large low temperature phonon anharmonicities can lead to static as well as dynamical changes in the low temperature properties of the vibrational system. In this work, we focus our attention on the low temperature lattice anharmonicity and its effect on the electron-phonon ground state. We are, in particular, motivated by certain high temperature superconductors. The third and fourth order anharmonic coupling constants for YBCO, LBCO and several other superconducting compounds are extracted from their measured elastic constants using the anharmonic elastic continuum model. The coupling constants are then used to extract the average anharmonic potential energy for the transverse and longitudinal modes in the Cu-0 planes. We find that, anharmonic contribution to the lattice potential relative to the harmonic one is unusually high for all examined high Tc compounds. The presence of anharmonic phonons elicits non-perturbative dynamical effects in the ground state of the electron-phonon system. Phonon correlations induced by anharmonic effects enhance the electron-phonon interaction which then create a self-consistent mechanism to act back on the ground state of the electron-phonón system. In result, strong momentum correlations are created and the ground state comprises fluctuating polarons. The zero point fluctuations and other ground state properties are obtained by self-consistent numerical calculations. The influence of low temperature phonon anharmonicity on the superconducting properties in the intermediate coupling range is also investigated. It is shown that, the otherwise bare electron-phonon coupling is strengthened in the presence of correlated polarons and the zero point fluctuations are enhanced. Within this frame, it is plausible to achieve superconducting transition temperatures as high as %20 of the characteristic vibrational energy scale. The non-perturbative, self-consistent formalism thus introduced also offers an account for the recently observed temperature anomalies near Tc in the Debye-Waller factor and dynamical pair correlations of certain high temperature superconductors.
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    Phonon renormalization effects in low dimensional electron-hole systems
    (1995) Güven, Kaan
    Ia)vv (linuMisioiial sciniromluclor st i iid iii4's lia\4‘ 1и‘(М1 an (‘Xt(Misiv(‘ i4\soa,rrh iwcix ill (4)iul(4iscd iiiatlcr physics. In j^aii iculai', iiindi (‘llorl has 1)(Ч'п dcvolnd to lhc‘ study ol ciuasi-one-diiiK'iisional siMniconductor sli-iiclures in гесчмП, yc'ars. The IK'W physic'al pluMioiiK'iia. in\'ol\4*d in tlu'sc* sysitMUs arisin,^ diu‘ to th(‘ Г(‘(1п(чм1 dimensionality point to various |)ot(Milial applicalions for iuture teclinologi(.‘s. y\ltliough the theory is lainiliar with tin* "oiK'-dimensionar’ pi’oblem for a long time, the realization of such structures (also known as (juanlum wires) extends onl}^ to ¿1 decade belonx Ilowciver, the по\ч'1 рго(1 исГкя1 t(‘chni([n('s led to a rapid increase in the experimental studies which, in turn, required a mor(' realistic and comi)rc*hensive theory to anal^'/c' and int('rpr('t tln^ obtaiiKnl data. This tlu'sis work intends to make a contribution in i1k‘ diiH’ctioii of th(‘S(' imj>rovem(‘nts. Our study is based on a (juasi-one-diiiKMisional eh'ctron-hole syst em as realized in pliotoexcit(‘d quantum wires, interacting with tlu' bulk bO-])honons. VVe investigate the polaronic corrections to the band gap and the c:arri(n* eilective mass, cirid the dependence of this correction itself to carrier density, temperature, and quantum finite size effects. VVe apply two different formal approaches; tho' p(n·Lurl.)гıtion theory and the vaiiational iiK'thod. 'I'he latter enables the investigation of dynamical screening elfects, thereby clarifying tin? (luestion of validity of the static screi'iiing approximation in one dimension. Our results have shown that dynamical screening is r<'levant in low dimensions. The dielectric function, which is a key quantity in describing tin' many-particle properties, is analyzf'd under diihn(*nt tip'proximations such as the 1 lartree-Fock ap[)roximation, random-|)hase api)roximation. and the more advaiuaxl local-field correction. Several confinement |)otentials (inlinit(‘ well, paralrolic, cj lindrical) are presented. I'kxplicit results iirc.' ol)tained hn' a (¡a.As (|uan(iim wiiv. VVe compare the results of tin' |)olar(niic corn'cticnis with tluit of tin? (‘xchangecorndalion induced corrections. W'e loiiinl that they are comi)arable in orrler of magnitudi', indicating that cari'i('r-|)lu)non int('iactions are more enhanced as the dimensionality reduces, and Ik.micc' should be treated on tin etpial footing along with the carrier-carri(‘r interact i(jiis. WV* make comparison with the pohironic corrections in two diiiK'iisiinial sysUnus. kdiially, we brielly discuss the renormalization due to confined phuiions as well.