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Takenaka, K.; Fukuzumi, Y.; Mizuhashi, K.; Uchida, S.; Asaoka, Hidehito; Takei, Hiroyuki

doi:10.1103/physrevb.56.5654

Cited by 27 publications

(29 citation statements)

References 38 publications

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“…When the electron-electron correlation becomes strong, L becomes smaller. For YBa 2 Cu 3 O 7−δ , L has been estimated to be 1.2 − 2.0 × 10 −8 WΩ/K 2 near T c by Hirschfeld and Putikka, 40 while Takenaka et al 41 estimated L to be 2.4 − 3.3 × 10 −8 WΩ/K 2 above T c . So there is still no consensus value of L for cuprates.…”

Section: B Magnetic Properties Of La2−yeuycuo4mentioning

confidence: 99%

Heat transport ofLa2−yEuyCuO

2003

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To study the rare-earth doping effect on the phonon heat transport of La-based cuprates and shed light on the mechanism of phonon scattering, both ab-plane and c-axis thermal conductivities (κ ab and κc) are measured for La2−yEuyCuO4 (y = 0, 0.02 and 0.2) and La1.88−yEuySr0.12CuO4 (y = 0 and 0.2) single crystals. It is found that the phonon peak (at 20 -25 K) in κ ab (T ) of La2−yEuyCuO4 shows an anomalous Eu-doping dependence: it completely disappears for y = 0.02, which is discussed to be due to the local lattice distortions around Eu dopants, and reappears for y = 0.2 with much smaller peak magnitude compared to y = 0 sample. In contrast to the strong suppression of the phonon peak in Eu-doped La2CuO4, Eu-doping to La1.88Sr0.12CuO4 enhances the low-T phonon heat transport that results in the reappearance of the phonon peak in this chargecarrier-doped system. The data clearly show that the establishment of static stripe phase rather than the structural change is responsible for the reappearance of phonon peak.

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Section: B Magnetic Properties Of La2−yeuycuo4mentioning

confidence: 99%

Heat transport ofLa2−yEuyCuO

2003

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confidence: 99%

“…Some experiments have attemped to get around this problem in a variety of methods [25,26,27]. In particular, Takenaka et al [25] found that κ e is constant or weakly T -dependent in the normal state of Y Ba 2 Cu 3 O 6+x . This approximately Tindependent κ e therefore implies the violation of the Wiedemann-Franz law (since the resistivity is found to be a non-linear function of temperature) in the underdoped region.…”

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confidence: 99%

Lorenz Number in HighTcSuperconductors: Evidence for Bipolarons

2003

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Strong electron-phonon interaction in the cuprates has gathered support over the last decade in a number of experiments. While phonons remain almost unrenormalised, electrons are transformed into itinerent bipolarons and thermally excited polarons when the electron-phonon interaction is strong. We calculate the Lorenz number of the system to show that the Wiedemann-Franz law breaks down because of the interference of polaron and bipolaron contributions in the heat flow. The model fits numerically the experimental Hall Lorenz number, which provides a direct evidence for bipolarons in the cuprates.The discovery of high-temperature superconductors [1,2] has broken constraints on the maximum T c predicted by the conventional theory of low-temperature superconducting metals and alloys. Understanding the pairing mechanism of carriers and the nature of the normal state in the cuprates and other novel superconductors has been a challenging problem of the Condensed Matter Physics. A number of theoretical models have been proposed, which rely on different non-phononic mechanisms of pairing (see, for example [3,4]). On the other hand, over the last decade, increasing evidence for the electron-phonon interaction has been provided by isotope effect measurements [5], infrared [6,7,8] and thermal conductivity [9], neutron scattering [10], and more recently by ARPES [11,12].To account for the high values of T c in the cuprates, one has to consider electron-phonon (e-ph) interactions, which are larger than those used in the intermediate coupling theory of superconductivity [13]. Regardless of the adiabatic ratio, the Migdal-Eliashberg theory of superconductivity and the Fermi-liquid have been shown to breakdown at the e-ph coupling constant λ ≈ 1 [14]. The many-electron system collapses into the small (bi)polaron regime at λ 1 with well separated vibration and chargecarrier degrees of freedom. Although it might have been thought that these carriers would have a mass too large to be mobile, the inclusion of the on-site Coulomb repulsion and the poor screening of the long-range e-ph interaction do lead to mobile intersite bipolarons [15,16]. Above T c the Bose gas of these bipolarons is non-degenerate and below T c their phase coherence sets in and superfluidity of the doubly-charged 2e bosons can occur. In this picture, the thermally excited single polarons co-exist with the Bose gas.There is much evidence for the crossover regime at T * and normal state charge and spin gaps in the cuprates [17]. These energy gaps could be understood as being half of the binding energy ∆ and the singlet-triplet gap of preformed bipolarons, respectively [18]. Many other experimental observations were explained using the bipolaron model [19]. These include the Hall ratio, the Hall angle, ab and c-axis resistivities, magnetic susceptibility, and angle-resolved photoemission. The bipolaron model provides parameter-free fits of critical temperatures, upper critical fields, explains a remarkable increase of the quasiparticle lifetime below T c [21], an...

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“…Particular interest within this paper lies in the conclusion that the Wiedemann-Franz law is violated in cuprates. This departure from the Fermi liquid picture is seen in both the superconducting and normal state and might be related by a common mechanism 44 . The extraction of the electronic thermal conductivity term has proven difficult as both the electronic term, κ el and the phononic term, κ ph make a comparable contribution.…”

Section: Introductionmentioning

confidence: 99%

“…The extraction of the electronic thermal conductivity term has proven difficult as both the electronic term, κ el and the phononic term, κ ph make a comparable contribution. However recent experiments appear to have got around this problem 42,44,45 . Takenaka et al 44 found that the insulating state thermal conductivity (therefore predominantly phononic) was approximately independent of doping except at y ∼ 1 − 0.75 in Y Ba 2 Cu 3 O 7−y .…”

Section: Introductionmentioning

confidence: 99%

Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates

2004

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With the superconducting cuprates in mind, a set of unitary transformations was used to decouple electrons and phonons in the strong-coupling limit. While phonons remain almost unrenormalised, electrons are transformed into itinerent singlet and triplet bipolarons and thermally excited polarons. The triplet/singlet exchange energy and the binding energy of the bipolarons are thought to account for the spin and charge pseudogaps in the cuprates, respectively. We calculated the Hall Lorenz number of the system to show that the Wiedemann-Franz law breaks down due to the interference of the polaron and bipolaron contributions to heat flow. The model provides a quantitative fit to magnetotransport data in the cuprates. Furthermore we are able to extract the phonon component of the thermal conductivity with the use of experimental data and the model. Our results further validate the use of a charged Bose gas model to describe normal and superconducting properties of unconventional superconductors.

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In-plane thermal conductivity and Lorenz number in YBa2Cu3O

Cited by 27 publications

References 38 publications

Heat transport ofLa2−yEuyCuO

Heat transport ofLa2−yEuyCuO

Lorenz Number in HighTcSuperconductors: Evidence for Bipolarons

Breakdown of the Wiedemann-Franz law in strongly-coupled electron-phonon system, application to the cuprates

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