NMR Study of Carrier Doping Effects on Spin Gaps in the Spin LadderSr14−xAx

Abstract

The discovery of high-T c superconductivity in lightly doped antiferromagnets has provided much interest in low-dimensional quantum spin systems. In order to obtain insight into the physics of the high-T c superconductivity, the Heisenberg spin-ladder systems, which consist of two (or more) chains of magnetic coupled ions, have been investigated intensively, since novel quantum phenomena appear in spin-ladder system [1-6]. The spin degree of freedom in the single ladder is well described by the Heisenberg Hamiltonianwhere J and J 0 are exchange couplings along the legs and the rungs of the ladder, respectively. Recent theories predicted that the S 1 2 antiferromagnetic spin ladder forms a spin liquid state and has an energy gap in the spin excitation spectra (spin gap) for ladders with even numbers of leg, but has no gap for odd-numbered leg systems. Theories [3,4] Very recently, superconductivity of T c ϳ 10 K was observed in the highly doped sample of the Ca substitution for Sr, when high pressure of ϳ3 GPa was applied [18].In a previous paper [19], we have confirmed the existence of the energy gaps in spin excitation spectra in both Cu sites by NMR͞NQR measurements, as we have observed Cu-NMR͞NQR signals of the chain and ladder site separately. We assign the gap energy of D 140 K for the CuO 2 chains and D 470 K for the Cu 2 O 3 ladder in Sr 14 Cu 24 O 41 . The value of 140 K is close to the 11 meV peak and D 470 K is close to the 35 meV peak for inelastic neutron experiments [20,21]. The difference in the spin gap between the chain and the ladder is attributed to the large exchange coupling in the ladder and the small one in the chain, which may be owing to the different atomic bond configurations; namely 180 ± Cu-O-Cu bond for the ladder and the nearly 90 ± Cu-O-Cu bond for the chain [12,13].In this Letter, we report carrier doping effects on the spin gaps in both sites in Sr 142x A x Cu 24 O 41 (A Ca, Y, and La). The main results are as follows. The spin gap for the ladder increases with Y and La substitutions for Sr, decreases with increasing x in Sr 142x Ca x Cu 24 O 41 , and seems to collapse around x 13 of the Ca substitution. We will discuss hole-doping effects on the gap energy of spin excitation spectra in terms of magnon and holedepairing excitations.Polycrystalline samples were prepared by a conventional solid state reaction under ambient pressure [15 -17]. NMR and NQR were measured by a conventional phase coherent pulse method.

Absence of a Spin Gap in the Superconducting Ladder Compound Sr 2 Ca 12 Cu 24 O 41

“…2 reveals the emergence of a zero temperature susceptibility which amounts to 53 % of the room temperature value. Our data at ambient pressure provide a confirmation for the depression of the spin gap upon doping, as previously announced in the Knight shift study of a Sr 5 Ca 9 Cu 24 O 41 single crystal [25] and also from T 1 data in the whole series corresponding to 0 ≤ x ≤ 9 [26]. However, what is remarkable in Fig.…”

Spin-gap evolution upon Ca doping in the spin-ladder series Sr14−xCaxCu24

“…Up to now, the ground state of this spin-ladder superconductor in ambient pressure has not been well understood because of contradictive reports from different experimental methods. 9,10 Similar to the high temperature superconducting cuprates with 2D lattices, investigating the physics of the normal phase of Sr 14−x Ca x Cu 24 O 41 should provide a solid basis to understand the superconducting mechanism. Therefore, it is extremely important to reveal the physics of the normal state of this spin-ladder compound in order to find out the differences between Sr 14−x Ca x Cu 24 O 41 and other 2D superconductors, and further our understanding to their superconducting mechanisms.…”

Spin rotational symmetry breaking by orbital current patterns in two-leg ladders

“…16,17 On the theoretical side significant efforts were made to probe the possible existence of such a phase, either using numerical approaches [18][19][20] or starting from Mott insulating side. 21 As a way of investigating the physics of these states in a more controlled way and also in view of possible connections with experimental systems, [22][23][24][25][26] phases exhibiting orbital current patterns ͑OCP͒ were also considered in the context of two-leg ladders. 27,28 Indeed for this essentially onedimensional ͑1D͒ case, various analytical tools allow one to explore the consequences of such an unusual phase.…”