Spin-gap formation in bilayer cuprates due to enhanced interlayer pairing

Interlayer-Coupling-Driven High-Temperature Superconductivity in La_{3}Ni_{2}O_{7} under Pressure

The newly discovered high-temperature superconductivity in La_{3}Ni_{2}O_{7} under pressure has attracted a great deal of attention. The essential ingredient characterizing the electronic properties is the bilayer NiO_{2} planes coupled by the interlayer bonding of 3d_{z^{2}} orbitals through the intermediate oxygen atoms. In the strong coupling limit, the low-energy physics is described by an intralayer antiferromagnetic spin-exchange interaction J_{∥} between 3d_{x^{2}-y^{2}} orbitals and an interlayer one J_{⊥} between 3d_{z^{2}} orbitals. Taking into account Hund's rule on each site and integrating out the 3d_{z^{2}} spin degree of freedom, the system reduces to a single-orbital bilayer t-J model based on the 3d_{x^{2}-y^{2}} orbital. By employing the slave-boson approach, the self-consistent equations for the bonding and pairing order parameters are solved. Near the physically relevant 1/4-filling regime (doping δ=0.3∼0.5), the interlayer coupling J_{⊥} tunes the conventional single-layer d-wave superconducting state to the s-wave one. A strong J_{⊥} could enhance the interlayer superconducting order, leading to a dramatically increased T_{c}. Interestingly, there could exist a finite regime in which an s+id state emerges.

Origin of the enhanced copper spin echo decay rate in the pseudogap regime of the multilayer high-T(c) cuprates

We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate Hg(0.8)Re(0.2)Ba(2)Ca(2)Cu(3)O(8) (T(c)=126 K). The angular dependence of the second moment (T(-2)(2M) identical with <Deltaomega(2)>) deduced from the decay curves indicates that T(-2)(2M) for H0 parallel c is enhanced in the pseudogap regime below T(pg) approximately 170 K, as seen in bilayer systems. Comparison of T(-2)(2M) between H0 parallel c and H0 perpendicular c indicates that this enhancement is caused by electron spin correlations between the inner and the outer CuO2 layers. The results provide the answer to the long-standing controversy regarding the opposite T dependences of (T1T)(-1) and T(-2)(2G) (T(2G): Gaussian component) in the pseudogap regime of multilayer systems.