Isotope effects in the Hubbard-Holstein model within dynamical mean-field theory

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P. Paci,¹ M. Capone,^2,3 E. Cappelluti,^2,3 S. Ciuchi,^4,2 and C. Grimaldi^5,6
¹International School for Advanced Studies (SISSA), via Beirut 2-4, 34014 Trieste, Italy
²SMC-INFM and Istituto dei Sistemi Complessi, CNR, via dei Taurini 19, 00185 Roma, Italy
³Dipartamento di Fisica, Università "La Sapienza," Piazzale A. Moro 2, 00185 Roma, Italy
⁴Dipartamento di Fisica, Università de L'Aquila and INFM UdR AQ, 67010 Coppito-L'Aquila, Italy
⁵Ecole Polytechnique Fédérale de Lausanne, LPM, Station 17, CH-1015 Lausanne, Switzerland
⁶DPMC, Université de Genève, 24 quai Ernest Ansermet, CH-1211 Genève 4, Switzerland

Received 6 May 2006; revised 7 July 2006; published 9 November 2006

Westudy the isotope effects arising from the coupling of correlatedelectrons with dispersionless phonons by considering the Hubbard-Holstein model athalf-filling within the dynamical mean-field theory. In particular we calculatethe isotope effects on the quasiparticle spectral weight Z, therenormalized phonon frequency, and the static charge and spin susceptibilities.In the weakly correlated regime U/t1.5, where U is theHubbard repulsion and t is the bare electron half-bandwidth, thephysical properties are qualitatively similar to those characterizing the Holsteinmodel in the absence of Coulomb repulsion, where the bipolaronicbinding takes place at large electron-phonon coupling and it isreflected in divergent isotope responses. On the contrary in thestrongly correlated regime U/t1.5, where the bipolaronic metal-insulator transition becomesof first order, the isotope effects are bounded, suggesting thatthe first-order transition is likely driven by an electronic mechanism,rather then by a lattice instability. These results point outhow the isotope responses are extremely sensitive to phase boundariesand they may be used to characterize the competition betweenthe electron-phonon coupling and the Hubbard repulsion.

URL:	http://link.aps.org/doi/10.1103/PhysRevB.74.205108
DOI:	10.1103/PhysRevB.74.205108
PACS:	71.38.Cn; 63.20.Kr; 71.38.Ht 71.38.Cn Mass renormalization in metals (polarons/electron–phonon interactions) 63.20.Kr Phonon–electron and phonon–phonon interactions 71.38.Ht Self-trapped or small polarons YEAR: 2006
KEYWORDS:	isotope effects, Hubbard model, quasiparticles, magnetic susceptibility, bipolarons, electron-phonon interactions, metal-insulator transition, strongly correlated electron systems, electron correlations

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