Charge-inhomogeneity doping relations inYBa2Cu3Oydetected by angle-dependent nuclear quadrupole resonance

Abstract

The origin of charge inhomogeneity in YBa2Cu3Oy is investigated using a new experimental method designed to determine the nuclear quadrupole resonance (NQR) asymmetry parameter η for very wide NQR lines at different positions on the line. The method is based on the measurement of the echo intensity as a function of the angle between the radio frequency field H1 and the principal axis of the electric field gradient. Static charge inhomogeneity deduced from η > 0 are found in this compound, but only in conjunction with oxygen deficiency. This limits considerably the possible forms of charge inhomogeneity in bulk YBa2Cu3Oy.The discussion on the mechanism for high temperature superconductivity (HTSC) is focused these days on the presence or absence of charge and spin inhomogeneity in the CuO 2 planes. Such inhomogeneity can lead to a one dimensional boundary, possibly in the form of stripes, between "hole-rich" and "hole-poor" regions and, allegedly, to superconductivity.[1] Indeed, it is now established by both surface and bulk techniques that most of the underdoped cuprates phase separate into "hole-rich" and "hole-poor" regions. [LSCO] where the evidence for phase separation is derived from experiments sensitive to both magnetic fluctuation, such as muon spin relaxation (µSR) [3], and charge fluctuations such as nuclear quadrupole resonance (NQR).[4] Even in very underdoped YBa 2 Cu 3 O y (YBCO y ) phase separation is observed; neutron scattering from phonons related to charge inhomogeneity is found in doping levels up to YBCO 6.35 [5], and µSR detects a spin glass phase for a similar doping range. [6] However, the origin of this phase separation is still not clear; is it coming from competing phases in the CuO 2 plane, or does it simply stem from the quenched disorder introduced by doping?A possible way to address this question is to apply NQR to YBCO y since in this compound one can distinguish between different in-plane coppers [Cu(2)] resonance lines, and associate each line with a local environment. However, a standard NQR based determination of charge homogeneity is not available to date due to the complicated and very wide spectrum of underdoped YBCO (which will be reviewed below). The purpose of this work is to overcome the NQR problems and to shed light on the evolution of charge homogeneity in the bulk of YBCO close to optimal doping. For this purpose we developed a new experimental technique based on the measurement of the Cu(2) NQR echo intensity as a function of the angle between the radio frequency (RF) field, H 1 , and the principal axis of the electric field gradient (EFG) at the copper site. We call this angle-dependent NQR technique ADNQR. Standard NQR is based on the fact that nuclei with spin I > 1/2 can be viewed as positively charged oval objects. As a result, their energy inside a solid depends on their orientation in the electrostatic potential V (r) generated by the other nuclear and electronic charges. When the nuclear poles are close to positive charges their energy is high, and when th...

Perspective on the phase diagram of cuprate high-temperature superconductors

“…However, the nuclear electric quadrupole moment for nuclei with spin I >1/2 ( I =3/2 for 63,65 Cu, I =5/2 for 17 O) interacts with the local electric field gradient (EFG) causing a quadrupole splitting ( ν Q ) of the NMR lines in high magnetic fields. The EFG at the nuclear site is very sensitive to the local charge symmetry, and has been useful for assigning NMR signals to the various lattice positions or detecting inhomogeneous charge distributions in the CuO 2 plane1314. Since the quadrupole splittings of planar Cu and O depend on doping, models have been put forward that attempted to understand these changes in terms of the hole content of certain orbitals, see for example, refs 15, 16, 17, 18 and works cited therein.…”

An Intrinsic Bond-Centered Electronic Glass with Unidirectional Domains in Underdoped Cuprates

“…The superconductivity usually does not appear until n ~ 5 to 10%. Therefore, another low-temperature state intervenes; it is usually thought of as an electronic "cluster glass" (ECG) state (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21). At higher dopings, the ECG signatures coexist with diminishing intensity with the strengthening superconductivity, until they disappear somewhere near n ~ 15%.…”

“…Nuclear magnetic/quadrupole resonance measurements (14)(15)(16)(17)(18)(19)(20) reveal static charge heterogeneity at the nm scale: The hole-density component of the ECG "cluster". Powder neutron diffraction reveals related local lattice distortions (21) and optical spectroscopy the anomalous anisotropic conductivity and electron-phonon couplings (27 …”

NMR and µSR in Highly Frustrated Magnets

“…This term is non-zero when the nuclear spin I > 1/2 and is, of course, absent in the muon interaction Hamiltonian. The quadrupole interaction is very sensitive to the modifications in the local configuration and allows to evidence distortions induced by the spin-lattice coupling [4] or the presence of a non-homogeneous charge distribution induced by charge ordering, for instance [5]. The third term is the most relevant one to probe frustrated magnetism, as it describes the hyperfine interaction with the electron spins S. As most of the systems we shall be dealing with in the following are insulators, one can write…”