Pseudogap In High-temperature Superconductors Research Articles

Pseudogap in high-temperature superconductors from realistic Fröhlich and Coulomb interactions

It has been recently shown that the competition between unscreened Coulomb and Fröhlich electron-phonon interactions can be described in terms of a short-range spin exchange Jp and an effective on-site interaction in the framework of the polaronic t-Jp- model. This model, that provides an explanation for high-temperature superconductivity in terms of Bose-Einstein condensation (BEC) of small and light bipolarons, is now studied as a charged Bose-Fermi mixture. Within this approximation, we show that a gap between bipolaron and unpaired polaron bands results in a strong suppression of low-temperature spin susceptibility, specific heat and tunnelling conductance, signalling the presence of a pseudogap regime in the normal state without any assumptions on pre-existing orders or broken symmetries in the normal state of the model.

Fluctuation conductivity and pseudogap in YBCO high-temperature superconductors (Review)

A considerable part of the theoretical and experimental works reflecting the current status of research on high-temperature superconductivity and the unusual phenomenon of a pseudogap in high-temperature superconductors (HTSCs) is reviewed. The concept of local pairs in systems with low and intermediate charge-carrier density, which can include HTSCs, is examined. The experimental part is primarily based on the study of excess and fluctuation conductivity (FC) in YBa2Cu3O7−y (TBCO) and Y1−xPrxBa2Cu3O7−y (YPrBCO) thin epitaxial films. A new approach to analyzing FC and the pseudogap in such high-temperature systems is proposed and checked experimentally. The approach is based on the idea that excess conductivity σ′(T) forms in HTSCs at temperatures substantially above critical temperature Tc as a result of the formation of pair states in the form of noninteracting strongly bound bosons, demonstrating with decreasing temperature a transition from a regime with localized pairs conforming to the Bose–Einstein condensation theory into a regime with fluctuation Cooper pairs which conform to the BCS theory.

Pseudogap in High Temperature Superconductors Investigated by High-Field Interlayer Transport(Current Topic)

Pseudogap in High Temperature Superconductors Investigated by High-Field Interlayer Transport(Current Topic)

The mysterious pseudogap in high temperature superconductors: an infrared view

We review the contribution of infrared spectroscopy to the study of the pseudogap in high temperature superconductors. The pseudogap appears as a depression of the frequency dependent conductivity in the c-axis direction and seems to be related to a real gap in the density of states. It can also be seen in the Knight shift, photoemission and tunneling experiments. In underdoped samples it appears near room temperature and does not close with temperature. Another related phenomenon that has been studied by infrared is the depression in the ab-plane scattering rate. Two separate effects can be discerned. At high temperatures there is broad depression of scattering below 1000 cm −1 which may be related to the gap in the density of states. At a lower temperature a sharper structure is seen, which appears to be associated with scattering from a mode at 300 cm −1, and which governs the carrier life time at low temperatures. This mode shows up in a number of other experiments, as a kink in angle resolved photoemission dispersion, and a resonance at 41 meV in magnetic neutron scattering. Since the infrared technique can be used on a wide range of samples it has provided evidence that the scattering mode is present in all high temperature cuprates and that its frequency in optimally doped materials scales with the superconducting transition temperature. The lanthanum and neodymium based cuprates do not follow this scaling and appear to have depressed transition temperatures.

Isotope effects on the pseudogap in high-temperature superconductors

Inelastic neutron scattering studies of the relaxation rate of crystal-field excitations have provided evidence for large oxygen and copper isotope effects on the pseudogap in high-temperature superconductors. Existing experiments in the slightly underdoped HoBa2Cu4O8 compound will be reviewed, as well as possible implications of such findings on the microscopic mechanism of high-temperature superconductivity. New relevant experiments performed in the La1-xSrxCuO4 family will also be presented.

Psevdoshchel' v vysokotemperaturnykh sverkhprovodnikakh

Psevdoshchel' v vysokotemperaturnykh sverkhprovodnikakh

Is the Asymptotic Scenario of Precursor r space Bosons Consistent with Observation for Electron (hole) Liquids in Underdoped High-T cCuprates?

The motivation for this study is the review by Timusk and Statt [Rep. Prog. Phys. 62, 61 (1999)] on the pseudogap in high-temperature superconductors. Here, we confront the experimental data they present, plus that of Norman et al. [Nature 392, 157 (1998)] on the Fermi surface in underdoped high-Tc materials with the theoretical asymptotic scenario of precursor r space Boson formation. This reveals no inconsistencies with available observations concerning electron (hole) liquids flowing through such underdoped cuprates with their antiferromagnetic spin fluctuations.

Fluctuation-induced pseudogap in high-temperature superconductors

We study the effect of fluctuations on the ac conductivity of a layered superconductor for c-axis electromagnetic wave polarization. The fluctuation contributions of different physical nature and sign (paraconductivity, Maki-Thompson anomalous contribution, one-electron density-of-states renormalization) are found to be suppressed by the external field at different characteristic frequencies (ωAL∼T-Tc, ωMT∼max{T-Tc,τϕ−1}, ωDOS∼min{T, τ−1}). As a result, the appearance of a nonmonotonic frequency dependence (pseudogap) in the infrared optical conductivity of high-temperature superconductor is predicted.