Crossover between Cooper-Pair Condensation and Bose-Einstein Condensation of “Di-Electronic Molecules” in Two-Dimensional Superconductors

Tokumitu, Akio; Miyake, Kazumasa; Yamada, Kazuo

doi:10.1143/ptps.106.63

Cited by 15 publications

(8 citation statements)

References 15 publications

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“…The cross-over of two regions was first formulated at T = 0 by Leggett [ 330] and extended to the finite temperature by Nozières and Schmitt-Rink [ 331]. After that, the application to the two-dimensional system has been investigated [ 332,333,334,335]. These two regions are continuously described by adjusting the chemical potential.…”

Section: Mf Cmentioning

confidence: 99%

Theory of superconductivity in strongly correlated electron systems

et al. 2003

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In this article we review essential natures of superconductivity in strongly correlated electron systems (SCES) from a universal point of view. First we summarize experimental results on SCES by focusing on typical materials such as cuprates, BEDT-TTF organic superconductors, and ruthenate Sr 2 RuO 4 . Experimental results on other important SCES, heavy-fermion systems, will be reviewed separately. The formalism to discuss superconducting properties of SCES is shown based on the Dyson-Gor'kov equations. Here two typical methods to evaluate the vertex function are introduced: One is the perturbation calculation up to the third-order terms with respect to electron correlation. Another is the fluctuationexchange (FLEX) method based on the Baym-Kadanoff conserving approximation. The results obtained by the FLEX method are in good agreement with those obtained by the perturbation calculation. In fact, a reasonable value of T c for spin-singlet d-wave superconductivity is successfully reproduced by using both methods for SCES such as cuprates and BEDT-TTF organic superconductors. As for Sr 2 RuO 4 exhibiting spin-triplet superconductivity, it is quite difficult to describe the superconducting transition by using the FLEX calculation. However, the superconductivity can be naturally explained by the perturbation calculation, since the third-order terms in the anomalous self-energy play the essential role to realize the triplet superconductivity. Another important purpose of this article is to review anomalous electronic properties of SCES near the Mott transition, since the nature of the normal state in SCES has been one of main issues to be discussed. Especially, we focus on pseudogap phenomena observed in under-doped cuprates and organic superconductors. A variety of scenarios to explain the pseudogap phenomena based on the superconducting and/or spin fluctuations are critically reviewed and examined in comparison with experimental results. According to the recent theory, superconducting fluctuations, inherent in the quasi-two-dimensional and strong-coupling superconductors, are the origin of the pseudogap formation. In these compounds, superconducting fluctuations induce a kind of resonance between the Fermi-liquid quasi-particle and the Cooper-pairing states. This resonance gives rise to a large damping effect of quasi-particles and reduces the spectral weight near the Fermi energy. We discuss the magnetic and transport properties as well as the single-particle spectra in the pseudogap state by the microscopic theory of the superconducting fluctuations. As for heavy-fermion superconductors, experimental results are reviewed and several theoretical analyses on the mechanism are provided based on the same viewpoint as explained above.

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Section: Mf Cmentioning

confidence: 99%

Theory of superconductivity in strongly correlated electron systems

et al. 2003

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“…In 2D there is, however, a more fundamental problem with the T -matrix approximation used in any, including the fully self-consistent, form. The essence of the problem is rather well-known: since the superconducting fluctuations are treated in an RPA-like or Gaussian approximation the BKT transition is not recovered by a T -matrix approach [95,92,135] (see also [200,201], where the effect of interaction between fluctuations was considered). One may hope however to derive BKT physics, or to be more exact the properties of the system for T < T BKT 21 only going beyond the T -matrix approximation, e.g.…”

Section: Limitations Of the T -Matrix Approximationmentioning

confidence: 99%

Phase fluctuations and pseudogap phenomena

2001

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This article reviews the current status of precursor superconducting phase fluctuations as a possible mechanism for pseudogap formation in high-temperature superconductors. In particular we compare this approach which relies on the twodimensional nature of the superconductivity to the often used T -matrix approach. Starting from simple pairing Hamiltonians we present a broad pedagogical introduction to the BCS-Bose crossover problem. The finite temperature extension of these models naturally leads to a discussion of the Berezinskii-Kosterlitz-Thouless superconducting transition and the related phase diagram including the effects of quantum phase fluctuations and impurities. We stress the differences between simple Bose-BCS crossover theories and the current approach where one can have a large pseudogap region even at high carrier density where the Fermi surface is welldefined. The Green's function and its associated spectral function, which explicitly show non-Fermi liquid behaviour, is constructed in the presence of vortices. Finally different mechanisms including quasi-particle-vortex and vortex-vortex interactions for the filling of the gap above T c are considered. 5The superconducting transition as a BKT transition and the temperature scale for the opening of the pseudogap 52 5.1 Phase diagram based on the classical phase fluctuations in the absence of Coulomb repulsion 52 5.2 The peculiarities of the phase diagram for the lattice model 67 5.3 The effects of Coulomb repulsion and quantum phase fluctuations 70 5.4 The effect of non-magnetic impurities 77 6 The Green's function in modulus-phase representation and non-Fermi liquid behaviour 86 6.1 The modulus-phase representation for the fermion Green function 87 6.2 The correlation function for the phase fluctuations 88 6.3 The Fourier transform of D(r) 92 6.4 The derivation of the fermion Green's function in Matsubara representation and its analytical continuation 93 6.5 The branch cut structure of G(ω, k) and non-Fermi liquid behaviour 97 7 The spectral function in the modulus-phase representation and filling of the gap 99 7.1 Absence of gap filling for the Green's function calculated for the static phase fluctuations in the absence of spin-charge coupling 99 7.2 Gap filling by static phase fluctuations due to quasi-particle vortex interactions. The phenomenology of ARPES 103 7.3 Gap filling due to dynamical phase fluctuations without quasi-particle vortex interactions 106 8 Concluding remarks 109 A Calculation of the effective potential 111 B Another representation for the retarded Green's function 113 References 114particle weight, or equivalently a pseudogap, below T * . Until recently most ARPES measurements were performed for the photon energy range between 19 and 25eV . The most recent data [10], which was taken with a higher photon energy of 33eV , is in contradiction with the previous data. This question is now under intensive investigation [11], particularly since ohmic losses may alter the ARPES spectra [12].An explanation of the pseudogap phenomen...

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“…11) The importance of the strong coupling superconductivity has been proposed 10,12,13,15,14) on the basis of the well-known Nozières and Schmitt-Rink (NSR) theory. 16,17) Furthermore, the strong coupling superconductivity has been phenomenologically proposed by Geshkenbein et al 18) with reference to the sign problem of the fluctuational Hall effect. 19) We have previously shown that the pseudogap phenomena are naturally understood by considering the resonance scattering due to the strong superconducting fluctuations.…”

Section: §1 Introductionmentioning

confidence: 99%

Theory on Superconducting Transition from Pseudogap State

2000

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The anomalous properties of High-T c cuprates are investigated both in the normal state and in the superconducting state. In particular, we pay attention to the pseudogap in the normal state and the phase transition from the pseudogap state to the superconducting state. The pseudogap phenomena observed in cuprates are naturally understood as a precursor of the strong coupling superconductivity. We have previously shown by using the self-consistent Tmatrix calculation that the pseudogap is a result of the strong superconducting fluctuations which are accompanied by the strong coupling superconductivity in quasi-two dimensional systems [J. Phys. Soc. Jpn. 68 (1999) 2999.]. We extend the scenario to the superconducting state. The close relation between the pseudogap state and the superconducting state is pointed out. Once the superconducting phase transition occurs, the superconducting order parameter rapidly grows rather than the result of BCS theory. With the rapid growth of the order parameter, the gap structure becomes sharp, while it is remarkably broad in the pseudogap state. The characteristic energy scale of the gap does not change. These results well explain the phase transition observed in the spectroscopic measurements. Further, we calculate the magnetic and transport properties which show the pseudogap phenomena. The comprehensive understanding of the NMR, the neutron scattering, the optical conductivity and the London penetration depth is obtained both in the pseudogap state and in the superconducting state. Many experiments such as the nuclear magnetic resonance (NMR), 1) optical conductivity, 2) transport, 3) electronic specific heat, 4) angle-resolved photo-emission spectroscopy (ARPES), 5) tunneling *

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Crossover between Cooper-Pair Condensation and Bose-Einstein Condensation of “Di-Electronic Molecules” in Two-Dimensional Superconductors

Cited by 15 publications

References 15 publications

Theory of superconductivity in strongly correlated electron systems

Theory of superconductivity in strongly correlated electron systems

Phase fluctuations and pseudogap phenomena

Theory on Superconducting Transition from Pseudogap State

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