Pseudogap and Superconducting Fluctuation in High- T c Cuprates: Theory beyond 1-Loop Approximation

2009

Phys. Rev. B

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We study the angular Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in which the rotation symmetry is spontaneously broken, in population imbalanced fermion gases near the BCS-BEC crossover. We investigate the superfluid gases at low temperatures on the basis of the Bogoliubov-de Gennes equation, and examine the stability against thermal fluctuations using the T -matrix approach beyond the local-density approximation. We find that the angular FFLO state is stabilized in the gases confined in the toroidal trap but not in the harmonic trap. The angular FFLO state is stable near the BCS-BEC crossover owing to the formation of pseudogap. Spatial dependences of number density and local population imbalance are shown for an experimental test.PACS numbers: 71.10. Ca, 03.75.Hh, 03.75.Ss, 05.30.Fk Superfluidity in cold fermion gases provides vast opportunities to study novel quantum phenomena. 1 One of the goals of current studies is the realization of the FuldeFerrell-Larkin-Ovchinnikov (FFLO) state 2,3 in population imbalanced superfluid gases. 4,5 FFLO superfluidity/superconductivity is attracting growing interest in a variety of fields, such as condensed-matter physics, 6 astrophysics, and nuclear physics. 7 Since many parameters can be experimentally controlled, 1 cold fermion gases are promising candidates for the FFLO state. 8,9 Spontaneous breaking of space symmetry is a characteristic feature of the FFLO state. However, no firm evidence has been obtained for the space symmetry breaking in the condensed-matter physics. In contrast to the superconductors, the spatial structure of superfluid is directly measured in cold atom gases. Therefore, it is highly desired to observe the space symmetry breaking due to the FFLO superfluidity in cold fermion gases. While the translation symmetry plays a major role in superconductors, cold atom gases lack translation symmetry owing to the trap potential. Instead, the rotation symmetry is well defined in the latter. The purpose of this Rapid Communication is to investigate the FFLO state which spontaneously breaks the rotation symmetry.On the basis of the mean-field Bogoliubov-de Gennes (BdG) equations, some authors investigated the radial FFLO (R-FFLO) state 10,11,12,13,14 in which the order parameter changes its sign along the radial direction around the edge of the harmonic trap. However, no space symmetry is broken in the R-FFLO state, and therefore it is difficult to distinguish it from the phase separated state. 15,16,17 In this Rapid Communication we show that the angular FFLO (A-FFLO) state with broken rotation symmetry is stabilized in the toroidal trap. 18 Several experiments are proposed for an unambiguous evidence for the A-FFLO state.The superfluidity has been realized in the imbalanced fermion gases near the BCS-BEC crossover. 4,5,19,20,21 However, that is not achieved in the BCS limit since the transition temperature T c is too small. Because the mean-field theory breaks down near the BCS-BEC crossover, 1,22 a theoretical treatment beyond the BdG equations is d...

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confidence: 59%

“…25 Therefore, the RSTA is used to determine the instability to the superfluid state. The T c is determined by the Thouless criterion.…”

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confidence: 99%

Angular Fulde-Ferrell-Larkin-Ovchinnikov state in cold fermion gases in a toroidal trap

2009

Phys. Rev. B

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“…The expression of the RSTA is shown in [48]. The validity of the self-consistent 1-loop approximation has been examined in the clean limit [49].…”

Section: Formulationmentioning

confidence: 99%

Superconductivity in compensated and uncompensated semiconductors

Science and Technology of Advanced Materials

Yorozu

2008

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We investigate the localization and superconductivity in heavily doped semiconductors. The crossover from the superconductivity in the host band to that in the impurity band is described on the basis of the disordered three-dimensional attractive Hubbard model for binary alloys. The microscopic inhomogeneity and the thermal superconducting fluctuation are taken into account using the self-consistent 1-loop order theory. The superconductor-insulator transition accompanies the crossover from the host band to the impurity band. We point out an enhancement of the critical temperature T c around the crossover. Further localization of electron wave functions leads to the localization of Cooper pairs and induces the pseudogap. We find that both the doping compensation by additional donors and the carrier increase by additional acceptors suppress the superconductivity. A theoretical interpretation is proposed for the superconductivity in the boron-doped diamond, SiC, and Si.

“…The quantum dynamics of SC fluctuation, namely the imaginary time dependence of fluctuation propagator, is ignored in this approximation and only the thermal fluctuation is taken into account. [37][38][39][40][41][42][43][44][45][46] This approximation is valid at finite temperature around the critical point.…”

Section: -Loop Order Theory In Disordered Systemsmentioning

confidence: 99%

“…Then, the qualitative behaviors of T c do not depend on the phenomenological criterion. 46 However, the nature of long range order is not clear in the highly disordered system. It is expected from Figs.…”

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confidence: 99%

Superconducting Fluctuation and Pseudogap in Disordered Short Coherence Length Superconductor

2006

J. Phys. Soc. Jpn.

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We investigate the role of disorder on the superconducting (SC) fluctuation in short coherence length d-wave superconductors. The particular intetest is focused on the disorder-induced microscopic inhomogeneity of SC fluctuation and its effect on the pseudogap phenomena. We formulate the self-consistent 1-loop order theory for the SC fluctuation in inhomogeneous systems and analyze the disordered t-t ′ -V model. The SC correlation function, electronic DOS and the critical temperature are estimated. The SC fluctuation is localized like a nanoscale granular structure when the coherence length is short, namely the transition temperature is high. This is contrasted to the long coherence length superconductors where the order parameter is almost uniform in the microscopic scale. In the former case, the SC fluctuation is enhanced by the disorder in contrast to the Abrikosov-Gorkov theory. These results are consistent with the STM, NMR and transport measurements in high-Tc cuprates and illuminate the essential role of the microscopic inhomogeneity. We calculate the spacial dependence of DOS around the single impurity and discuss the consistency with the NMR measurements.