Evidence of precursor superconductivity as high as 180 K from infrared spectroscopy (original) (raw)
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The European Physical Journal Special Topics, 2010
With the technique of infrared ellipsometry we performed a detailed study of the temperature-and doping dependence of the c-axis response of a series of YBa 2Cu3O7−δ single crystals. In particular, we explored the anomalous electronic properties at temperatures above the macroscopic superconducting transition temperature, T c, whose conflicting explanations range from a precursor superconducting state to electronic correlations that compete with superconductivity. We show that the c-axis spectra provide evidence that both kinds of correlations are present and that their contributions can be disentangled based on an analysis with a so-called multilayer-model. We find that the onset temperature, T * , and the energy scale, ΔPG, of the competing pseudogap increase rapidly towards the underdoped side whereas they vanish on the overdoped side. In addition, we provide evidence that in a strongly underdoped sample the precursor superconducting correlations develop below an onset temperature, T ons , that is considerably lower than T * but still much higher than Tc.
Physical Review B, 1990
A detailed study of infrared properties (reflectivity, conductivity, and dielectric response), emphasizing reproducible results from fully oxygenated YBa2Cu307 crystals (T, =93 K) and films, is presented. The extrapolated values of 0. &(co) at low frequency are roughly consistent with the measured temperature-dependent dc resistivity. Although not well understood, this infrared conductivity can be interpreted in terms of a frequency-dependent scattering rate of-kT+Ae, with a lowfrequency mass enhancement of roughly 2 to 4 associated with a carrier-spin related interaction. Infrared measurements polarized along the c axis suggest a conductivity anisotropy of roughly 40:1 near T, in the normal state. In the superconducting state an energy scale of 2h, = 3kT, is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2h, b-8kT, is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very 0 large gap, a reasonable estimate (=1700 A) for the (a-b)-plane penetration depth is obtained. Evidence for non-BCS temperature dependence, strong pair breaking scattering, and possible fluctuation effects is discussed. A comparison to infrared data from Bi2Sr,CaCu208 "shows a similarly large energy scale, 2b, b-8kT"' for the cubic Bap 6Ko 4Bi03 superconductor a more conventional energy scale, 2b =4kT, is observed. The unusually large energy scale obtained from the (a-b)-plane measurements of the layered cuprates lies far beyond the range of previously studied superconducting energy gaps (25 =3 to SkT,).
Doping-induced change of optical properties in underdoped cuprate superconductors
Journal of Physics: Condensed Matter, 1999
We report on the ab-plane optical reflectance measurements of single crystals of Y-doped Bi 2 Sr 2 CaCu 2 O 8+δ and Pr-doped YBa 2 Cu 3 O 7−δ over a wide frequency range from 80 to 40 000 cm −1 (10 meV-5 eV) and at temperatures between 20 and 300 K. Y and Pr doping both decrease the hole concentration in the CuO 2 planes. This has allowed us to investigate the evolution of ab-plane charge dynamics at doping levels ranging from heavily underdoped to nearly optimally doped. Our results of the low-frequency optical conductivity and spectral weight do not show any features associated with the normal-state pseudogap. Instead, one-component analysis for the optical conductivity shows the low-frequency depression in the scattering rate at T > T c , signalling entry into the pseudogap state. Alternatively, no clear indications of the normal-state pseudogap are detected in the temperature-dependent zero-frequency free-carrier scattering rate by using two-component analysis. In the superconducting state, there is also no convincing evidence of superconducting gap absorption in all spectra. We find that there is a 'universal correlation' between the numbers of carriers and the transition temperature. This correlation holds whether one considers the number of carriers in the superfluid or the total number of carriers.
Competition between the pseudogap and superconductivity in the high-Tc copper oxides
Nature, 2009
A pairing gap and coherence are the two hallmarks of superconductivity. In a classical BCS superconductor they are established simultaneously at T c . In the cuprates, however, an energy gap (pseudogap) extends above T c [1, 2, 3,. The origin of this gap is one of the central issues in high temperature superconductivity. Recent experimental evidence demonstrates that the pseudogap and the superconducting gap are associated with different energy scales . It is however not clear whether they coexist independently or compete . In order to understand the physics of cuprates and improve their superconducting properties it is vital to determine whether the pseudogap is friend or foe of high temperature supercondctivity . Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that the pseudogap and high temperature superconductivity represent two competing orders. We find that there is a direct correlation between a loss in the low energy spectral weight due to the pseudogap and a decrease of the coherent fraction of paired electrons. Therefore, the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space where the superconducting gap is largest. This leads to a very unusual state in the underdoped cuprates, where only part of the Fermi surface develops coherence.
Bulk superconductivity at 84 K in the strongly overdoped regime of cuprates
By means of magnetization, specific heat, and muon-spin relaxation measurements, we investigate newly synthesized high-pressure oxidized Cu 0.75 Mo 0.25 Sr 2 YCu 2 O 7.54 , in which overdoping is achieved up to p ∼ 0.46 hole/Cu, well beyond the T c-p superconducting dome of cuprates, where Fermi-liquid behavior is expected. Surprisingly, we find bulk superconductivity with T c = 84 K and superfluid density similar to those of optimally doped YBa 2 Cu 3 O 7−δ. On the other hand, specific heat data display a large electronic contribution at low temperature, comparable to that of nonsuperconducting overdoped La 2−x Sr x CuO 4. These results point at an unusual high-T c phase with a large fraction of unpaired holes. Further experiments may assess the Fermi-liquid properties of the present phase, which would put into question the paradigm that the high T c of cuprates originates from a non-Fermi-liquid ground state.
Physical Review Letters, 2001
The doping dependence of the superconducting condensate density, n o s , has been studied by muon-spin-rotation for Y 0.8 Ca 0.2 Ba 2 (Cu 1−z Zn z ) 3 O 7−δ and Tl 0.5−y Pb 0.5+y Sr 2 Ca 1−x Y x Cu 2 O 7 . We find that n o s exhibits a pronounced peak at a unique doping state in the slightly overdoped regime. Its position coincides with the critical doping state where the normal state pseudogap first appears depleting the electronic density of states. A surprising correlation between n o s and the condensation energy U o is observed which suggests unconventional behavior even in the overdoped region.
Nature Communications, 2011
In strongly-correlated systems the electronic properties at the Fermi energy (EF) are intertwined with those at high energy scales. One of the pivotal challenges in the field of high-temperature superconductivity (HTSC) is to understand whether and how the high energy scale physics associated with Mott-like excitations (|E-EF|>1 eV) is involved in the condensate formation. Here we show the interplay between the many-body high-energy CuO2 excitations at 1.5 and 2 eV and the onset of HTSC. This is revealed by a novel optical pump supercontinuum-probe technique, which provides access to the dynamics of the dielectric function in Bi2Sr2Ca0.92Y0.08Cu2O8+δ over an extended energy range, after the photoinduced partial suppression of the superconducting pairing. These results unveil an unconventional mechanism at the base of HTSC both below and above the optimal hole concentration required to attain the maximum critical temperature (Tc).