Spin Gap Phenomenon Research Articles

Anomalous metallic phase in Cu oxides and other 2D Mott systems

Normal state properties of high- T c oxides measured up to very high temperatures are presented, where the formation of the anomalous metallic phase (AMP) can clearly be seen. Based on various kinds of experimental results, it is proposed that the AMP is a metallic and magnetically correlated phase without long-range spin order. The common existence of similar anomalous phases in 2D non-Cu Mott systems supports the above idea. The spin-gap behavior of CaV 4O 9, which has the S = 1 2 2 D spin system of V 4+, is also discussed in relation to the spin-gap phenomena of Cu oxides.

Superconductivity-induced B 2u phonon anomalies of YBa 2Cu 3O 6 + x and symmetry of the order parameter. Neutron inelastic scattering studies

Superconductivity-induced phonon anomalies have been observed by neutron scattering for a c-axis polarized B 2u mode with out-of-phase motions of O(2) and O(3) within a plane in YBa 2Cu 3O 6 + x ( x = 1.0 and 0.7). A mode which has in-phase motions of these oxygens with respect to Cu atoms within a CuO 2-plane does not show any appreciable anomaly. Based on these results and recent theoretical arguments, the symmetry of the superconducting order parameter is pointed out to be ( x 2 − y 2)-like. In YBa 2Cu 3O 6.7 the B 2u mode also exhibits phonon anomalies associated with the so-called spin-gap phenomena.

Theory of the spin gap in high-temperature superconductors.

We analyze pairing in two dimensional spin liquids. We argue that interplane pairing enhanced by magnetic correlations is the most plausible explanation of the spin gap phenomenon observed in underdoped cuprates. The details of the pairing theory depend on the in-plane antiferromagnetic correlations. We consider two models: $2p_F$ correlations induced by a strong gauge field interaction and undamped spin waves. We estimate the pairing temperature $T_s$ and the angular dependence of the gap function and discuss physical consequences.

Spin-phonon coupling in the single-layer extended t-J model.

We consider the implications of spin-phonon coupling within the slave-boson, mean-field treatment of the extended t-J model of a high-temperature superconductor. In bilayer cuprates such as YBaCuO, where the $CuO_2$ plane is buckled, this interaction is linear in $O$ displacement along the c-axis, and the coupling constant is found to be large. The formation of a spin singlet causes additional contributions to the phonon self-energy, and we calculate from these the superconductive phonon anomalies. The magnitude and sign of the frequency shift and linewidth broadening for various mode symmetries correspond well with Raman and infra-red light scattering experiments, and with neutron scattering studies. In the t-J model, spin singlet formation and superconductivity do not coincide in the low-doping regime, giving rise to spin-gap features and a variety of temperature scales in the spin response observed by NMR and neutron investigations. Phonon anomalies in underdoped YBaCuO compounds indeed show evidence of spin-gap phenomena with the same characteristic temperature, suggesting that the theory may offer the possibility of a unified understanding of the anomalies in magnetic and lattice properties. While the origin of the superconducting interaction is electronic, this spin-phonon coupling affords the possibility of a small isotope effect, and our estimate is in good agreement with recent site-selective $O$-substitution experiments.

Magnetic behavior of the cuprate superconductors

I review recent work on magnetic dynamics of the high temperature superconductors using a model that combines two weakly interacting species of low-energy excitations: the antiferromagnetic spin waves which carry spin-1 and no charge, and Fermi-liquid-like quasiparticles which carry spin- 1 2 and charge e. The model allows conversion of spin waves into electron-hole pairs; however, the low-energy spin waves are not collective modes of the quasiparticles near the Fermi surface, but rather are a separate branch of the low-energy spectrum. With certain experimentally justified assumptions, this theory is remarkably universal: the dependence on the detailed microscopic Hamiltonian and on doping can be absorbed into several experimentally measurable parameters. The z = 1 theory of the insulators and z = 2 theory of the overdoped materials, are both reproduced as limiting cases of the theory described here, which predicts that the underdoped materials remain in z = 1 universality class at sufficiently high temperature. This theory provides a framework for understanding both the experimental results and microscopic calculations, and in particular yields a possible explanation of the spin gap phenomenon. I also discuss some of the important unresolved issues.

Spin dynamics in high- Tc cuprates

Characteristic features of the spin excitations in high- T c cuprates revealed by neutron scattering and nuclear magnetic resonance experiments are summarised, and analysed on the basis of the slave-boson mean-field theory for the extended t— J model, placing special emphasis on the spin-gap phenomenon.

Spin excitations, phonons and phase diagram in high-T c cuprates

The implications of the slave-boson mean-field theory for the extended t-J model are summarised and compared with various experiments on spin fluctuations and certain phonon modes in high-Tc cuprates, placing special emphasis on the spin-gap phenomenon.