Fluctuation effects in underdoped cuprate superconductors under a magnetic field

Abstract

Fluctuation effects in underdoped cuprates under high fields are examined by trying to fit theoretical results to resistivity and Nernst data in vortex states. The superconducting (SC) fluctuation in underdoped cuprates includes not only the ordinary thermal contribution but also a large amount of quantum dynamical contributions. Together with this, the presence of a SC pseudogap region T0 − Tc0 increasing with underdoping is found to be the origin of the Nernst coefficient becoming anomalously smaller and the in-plane coherence length apparently increasing with underdoping.It has been well understood that the field-induced fanshaped broadenings of curves of resistivity and thermodynamic quantities, typically seen in optimally (hole-)doped high T c0 cuprate superconductors (HTS), are thermal superconducting (SC) fluctuation phenomena mainly in the vortex liquid region of the normal phase below the zero field (H = 0) transition point T c0 [1]. In applied magnetic fields of tesla range perpendicular to the SC planes, the in-plane resistivity and other physical quantities in these materials show familiar behaviors [2, 3] correlated with one another. For instance, the onset temperature of fluctuation effects suggested from resistive data is almost the same as the corresponding one of thermodynamic and thermomagnetic data. This familiar correlation is typically seen in much lower fields than H c2 (0), where the fluctuation is purely thermal [2], and the quantum fluctuation contribution is negligible.In contrast, the resistivity in other HTSs with lower T c0 often behaves in an uncorrelated manner with thermodynamic quantities. Typically, as the applied field is higher, resistivity data in electron-doped HTSs and some of over (hole-)doped materials show not the fan-shaped [1] but a flat curve [4,5] following the in-plane normal resistivity ρ n (T ) = (σ n (T )) −1 curve until a vortexglass (VG) transition field, lying much below [5,6] an effective H c2 (T ) determined thermodynamically, is approached from above. In over (hole)-doped materials with high σ n -value (≃ 10 2 (R q s) −1 [4]), such an absence of correlation is not surprising because the fluctuation conductivity σ f is negligible compared with σ n in the total conductivity σ = σ n + σ f over a wide temperature range, where R q = 6.45(kΩ) is resistance quantum, and s ≃ 10(A) is a typical size of the spacing between SC layers. However, the corresponding uncorrelated behavior seen in the electron-doped materials [5] with σ n of the same order as in the optimally doped YBCO [3] is intrinsic, and its origin needs to be attributed to a fluctuation property. Similar behaviors have been also found in overdoped La 2−x Sr x CuO 4 (LSCOx) [7] and κ-(ET) 2 organic superconductors [8,9]. As argued elsewhere [10] by fitting to data [9], the main origin is expected to consist in the quantum dynamical nature, enhanced with increasing the field, of the SC fluctuation. In general, σ f defined in a Ginzburg-Landau (GL) theory decreases [11]

Effects of proximity to an electronic topological transition on normal-state transport properties of thehigh−Tcsuperconductors

“…[9][10][11] Such findings have been interpreted as an effect of precursor pairing above T c in the pseudogap region, as well as of quantum superconducting fluctuations. 12 In the case of the Hall effect, superconducting fluctuations induce a characteristic deviation from the normal-state temperature dependence of the Hall conductivity above T c ͑Hall anomaly͒. 13 In particular, the fluctuation Hall conductivity ⌬ xy has been evaluated within the TDGL theory, 14,4 and it has been shown that a Hall sign reversal takes place below T c .…”

Decoherence, Entanglement and Information Protection in Complex Quantum Systems

“…This is of no consequence for optical spectroscopy techniques (infrared and Raman) that cannot probe masses, because of tiny momentum transfer values. Only recently, effective masses have been determined thanks to vibrational spectroscopy with neutrons [Ikeda 2002]. The existence of well-defined (of course) effective masses should be included in further theoretical developments.…”

“…Partial landscape views in three dimensions are shown in Fig. 6 [Ikeda 2002]. Energy transfer values correspond to proton modes and the momentum transfer vector spans various planes in reciprocal space, (Q i , Q j ), such as Q k = 0, with i, j or k = x, y or z.…”

“…It transpires that proton dynamics for bending modes correspond very closely to isolated harmonic oscillators with a mass of 1 amu [Ikeda 2002]. They are largely de-coupled from the lattice.…”

“…Planar centrosymmetric dimer entities (HCO 3 ) − 2 are parallel to the same direction, practically (301) planes, throughout the crystal. Thanks to the particular arrangement of hydrogen bonds, all virtually parallel to the same direction, one can probe the anisotropy of proton dynamics and determine directly the orientation of the normal modes [Kashida 1994;Ikeda 2002]. The center of symmetry gives rise to full quantum entanglement for pairs of protons.…”