Isotope Dependence of the Spin Gap inYBa2Cu4O8as Determined by Cu NQR Re

Abstract

We performed high accuracy 63 Cu NQR spin-lattice relaxation and SQUID magnetization measurements on 16 O and 18 O exchanged YBa 2 Cu 4 O 8 to determine the isotope shift of the temperature of the opening of the spin gap, T ‫ء‬ , and the superconducting transition temperature, T c . The corresponding isotope exponents are a T ‫ء‬ 0.061͑8͒ and a Tc 0.056͑12͒ which are the same within the error bars and suggest a common origin for the superconducting and the spin gap. [S0031-9007(98)08085-5] 74.25.Nf, 74.62.Dh One of the central and heavily debated questions in high-temperature superconductivity research concerns the origin of the so-called pseudogap occurring in the normal state of underdoped superconductors [1]. The pseudogap refers to the transfer to higher energy of the density of low-energy excited states. One may ask whether the pseudogap is caused by superconducting correlations, which develop above T c , or whether it is an independent phenomenon. In nuclear magnetic resonance (NMR) and neutron scattering experiments, the pseudogap reveals itself as a spin gap. For instance, the Cu spin-lattice "relaxation rate per temperature unit," ͑T 1 T ͒ 21 , increases with falling temperature and reaches a maximum at T ‫ء‬ , which is a proper scale for the temperature dependence of the spin gap. For YBa 2 Cu 4 O 8 (Y124), the corresponding values are T c 81 K and T ‫ء‬ ഠ 150 K.Recently [2], we detected anomalies in the temperature dependence of several NMR and NQR (nuclear quadrupole resonance) quantities measured in the normal state of Y124, for instance in NQR frequencies, Knight shifts, line widths, and relaxation times. These anomalies, which occur around T y 180 K, are the signature of an electronic crossover which involves enhanced charge fluctuations in planes and chains. Because of the proximity of T y and T ‫ء‬ , we have argued that the spin gap effect in Y124 is caused by a transition due to a charge density wave (CDW) instability [3]. Using the t-J model and including electron-phonon interaction, we could, among others, explain the strong temperature dependence of the magnetic shift of the planar Cu nuclei, which we had measured previously [4], and we predicted a dependence of T ‫ء‬ on the isotope mass. Thus, corresponding measurements allow one to check the consistency of the CDW model.NMR͞NQR are techniques that can determine the pseudogap with a precision allowing one to establish whether an isotope effect is present or not. Y124, because of its well-defined oxygen stoichiometry and its negligible oxygen diffusion, is the ideal compound for such a study that requires experimental results not hampered by reproducibility problems. Therefore, the NQR study of the isotope effect on the spin gap in Y124 and its comparison with the superconducting gap is a unique experiment to explore the relation between the two gaps.In this Letter, we report a high-accuracy NQR study of the planar 63 Cu nuclei, supplemented by susceptibility measurements, on 16 O and 18 O exchanged Y124 samples which revealed the presenc...

Unconventional isotope effects in the high-temperature cuprate superconductors

“…[52] attributed the characterisitc temperature T p to a pseudogap formation temperature, one cannot rule out the possibility that T p may be related to a dynamical charge ordering. We would like to mention that a small positive oxygen-isotope effect on the spin pseudogap has been observed in YBa 2 Cu 4 O 8 [54]. The Electron Paramagnetic Resonance (EPR) is a powerful technique to probe and understand various phenomena in solid state physics.…”

Relevance of electron–lattice coupling in cuprate superconductors

“…The magnetic field induced finite size effect revealed lower bounds ranging from L = 182 to 814A [9,10]. Sixth, since the change of the lattice parameters upon oxygen isotope exchange is negligibly small [11,12], the occurrence of a significant modification of the domains spatial extent, will provide clear evidence for superconductivity mediated by local lattice distortions.This letter concentrates on Y 1−x Pr x Ba 2 Cu 3 O 7−δ with 16 O and 18 O. It addresses these issues by providing clear evidence for a finite size effect on the in-plane London penetration depth, revealing the existence of superconducting domains with spatial nanoscale extent and its significant change upon oxygen isotope exchange.…”

“…The magnetic field induced finite size effect revealed lower bounds ranging from L = 182 to 814A [9,10]. Sixth, since the change of the lattice parameters upon oxygen isotope exchange is negligibly small [11,12], the occurrence of a significant modification of the domains spatial extent, will provide clear evidence for superconductivity mediated by local lattice distortions.…”

Oxygen isotope effect of the plane-copper NQR frequency inYBa2Cu4OMali1, Roos2, Keller3 et al. 2002Phys. Rev. BSelf Cite10060View full textAdd to dashboardCite

“…After subtracting this extrapolation from the original ν Q (Cu2) data we get a "rest" whose temperature dependence looks very familiar, namely like the Cu2 Knight shift temperature dependence turned on head [11]. Lacking more insight we decide to fit this "rest" by an empirical function B tanh 2 ( ∆ 2T ), that is very reminiscent of the empirical spin-pseudogap function used to fit Cu2 Knight shift and spin-lattice relaxation data [1,11]. In the actual ν Q (Cu2) vs. T fit we combine the above two steps and use the expression:…”

“…In the same temperature region other anomalies in nuclear magnetic resonance (NMR) and NQR quantities, for instance in Knight shifts, line widths, and relaxation times oc- * Electronic address: mali@physik.unizh.ch cur which signal an electronic crossover [4,5]. To learn more about the CuO 2 planes especially about a possible charge redistribution during the pseudogap formation as well as to have very precise ν Q (Cu2) values frequently demanded in other Cu2 NMR and NQR studies we decided to implement the Cu2 ν Q and linewidth data by additional measurements on the very same material on which we recently determined by Cu2 spin-lattice relaxation the oxygen isotope effect of the spin-pseudogap [1]. Therefore we performed between 5 K and 350 K very accurate measurements of the temperature dependence of the Cu2 NQR line frequency ν Q (Cu2) and the linewidth in normal and superconducting 16 O and 18 O exchanged YBa 2 Cu 4 O 8 .…”