Charge Degree of Freedom and the Single-Spin Fluid Model inYBa2Cu4O8

Abstract

We present a 17 O nuclear magnetic resonance study in the stoichiometric superconductor YBa2Cu4O8. A double irradiation method enables us to show that, below around 200 K, the spin-lattice relaxation rate of plane oxygen is not only driven by magnetic, but also significantly by quadrupolar fluctuations, i.e. low-frequency charge fluctuations. In the superconducting state, on lowering the temperature, the quadrupolar relaxation diminishes faster than the magnetic one. These findings show that, with the opening of the pseudo spin gap, a charge degree of freedom of mainly oxygen character is present in the electronic low-energy excitation spectrum.PACS numbers: 74.25.Nf, 74.72.Bk, One of the central issues in understanding cupratebased high-temperature superconductors (HTSC) has been to determine the minimal number of electronic degrees of freedom which are necessary to describe the physics at the atomic scale of these structures. Zhang and Rice proposed that doped holes, which go into oxygen states, form a spin-resonant singlet state with the quasilocalized holes at copper [1]. Therefore, only one spin degree of freedom would be necessary to describe the lowenergy dynamics of the electronic system in the normal state of the HTSC. This scenario is often called singlespin fluid (SSF) and is described within a t-J model. However, as was discussed by various authors [2], this model might be an oversimplification.Due to its local character, nuclear magnetic resonance (NMR) is a very attractive method to address these questions. NMR probes the low-energy excitations of the HTSC electronic system and it provides information about copper and oxygen independently. Studies of the uniform spin susceptibility, Re{χ(q = 0, ω = 0} [3], indicate that only one spin degree of freedom is involved thus supporting the SSF model. On the other hand, oxygen and copper reveal a very different temperature dependence of the spin-lattice relaxation [4] which is related to the dynamic spin susceptibility, Im{χ(q, ω L )} [5]. This could suggest independent spin degrees of freedom. However, as pointed out before [6], the hyperfine field, due to antiferromagnetically correlated copper spins, cancels at the oxygen site and this fact can explain the different temperature dependences within a SSF model.There is, however, evidence that this view is incomplete. The 17 O NMR in Y-Ba-Cu-O HTSC has extensively been studied by various groups. Especially the pronounced temperature dependence of the anisotropy of the experimentally determined effective spin-lattice relaxation rate, 17 W eff , of the plane oxygen is still puzzling [7][8][9] while the SSF model predicts an almost temperature independent rate anisotropy. (The meaning of effective will be discussed below.) As we pointed out in Ref.[8], it is also very difficult to explain, within a simple SSF model, the ratio of this rate and the yttrium rate, namely 17 W eff / 89 W (with the external magnetic field B 0 parallel to the c axis). Furthermore, in a detailed analysis, Walstedt et al.[10] compare...

Separation of Quadrupolar and Magnetic Contributions to Spin–Lattice Relaxation in the Case of a Single Isotope

“…We will discuss experimental details guided by our study of the high-temperature superconductor YBa 2 Cu 4 O 8 ; these investigations will be published elsewhere 18 . The experiment was performed by using a combination of two standard pulse spectrometers together with a magnetic field of B 0 = 8.9945 T, (B 0 c).…”

“…This clearly shows that the spin-lattice relaxation process contains a strong quadrupolar contribution which could not be detected otherwise so far. The discussion of the origin of this effect, which does not arise from phonons, as well as its temperature dependence will be given elsewhere 18 .…”

Developments in the negative-Umodelling of the cuprate HTSC systems

“…Working in such fields is however a strong perturbation on many factors and the electronic thermal conductance experiment on 124 is more definitive. Weak localization often arises through structural disorder but YBCO-124 is a stoichiometrically well-defined compound, with crystallographically no immediate register of its mixed valence; indeed NMR on 124 would indicate a single planar Cu site signal [137].…”

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

“…The temperature dependence of ν Q (Cu2) is very unconventional, particularly striking is the change of the course around 190 K where a broad minimum in the frequency appears [3]. 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].…”