Spin Excitations in the Two-Leg Ladder Antiferromagnet CaV₂O₅

Abstract

The nuclear spin-lattice relaxation rate at the V sites in CaV2O5 was measured. The relaxation rate (1/T1) in the two-leg ladder was calculated with a strong-coupling expansion (J ⊥ ≫ J ) and the result was compared with the experimental data. The comparison of the temperature dependence shows a good agreement and provides an estimate of both J ⊥ and J as well as the energy gap ∆. The magnetic field dependence of 1/T1 is also reproduced with this approximation in the temperature range T ∆/4. It is found that the magnon-magnon collision, which leads to a diffusive behavior, is negligible in this temperature range. The effect of inter-ladder coupling is investigated and found to be weak in CaV2O5. Thus the strong-coupling expansion calculation agrees satisfactorily with the experimental data, but a discrepancy between the exchange parameters deduced from the magnetic susceptibility and 1/T1 is found.KEYWORDS: CaV 2 O 5 , NMR, spin ladder, spin diffusion, nuclear spin relaxation §1. IntroductionThe discovery of high-T C superconductivity in doped two-dimensional antiferromagnets renewed interest in quantum antiferromagnets. In particular, spin-1 2 two-leg ladder antiferromagnets have been extensively studied, being motivated by a theoretical prediction of the quantum disordered ground state with an energy gap and possible superconductivity.1) Actually, the energy gap was experimentally observed in SrCu 2 O 3 .2) Moreover, superconductivity was discovered in Sr 14−x Ca x Cu 24 O 41 under high pressure.3)The low-energy excitations in such gapful spin systems have not been fully understood yet. One problem is a significant discrepancy between the energy gaps determined from the magnetic susceptibility and the nuclear spinlattice relaxation rate 1/T 1 . For the two-leg ladder antiferromagnet SrCu 2 O 3 , 1/T 1 suggests a much larger gap ∆ ∼ 700 K than ∆ ∼ 420 K by the susceptibility.2, 4) Itoh et al. surveyed the data of the susceptibility and 1/T 1 of many gapful quantum paramagnets and pointed out that this discrepancy is observed widely in one-dimensional gapful systems such as Haldane-gap and two-leg ladder antiferromagnets.5) Kishine and Fukuyama calculated 1/T 1 with the Majorana fermion representation of the two-leg ladders. 6) They found a dominant contribution with three times larger energy gap than the lowest excitation and conclude that this is responsible for the discrepancy. Sachdev and Damle developed a semiclassical continuum theory for one-dimensional gapful systems and obtained diffusive behavior in the long-time spin correlation.7, 8) They showed that 1/T 1 is not proportional to e −∆/T but e −3∆/2T at sufficient but not too low temperatures. The diffusive behavior was actually observed * Present Address: Department of Physics, Faculty of Science, Chiba University, Chiba, 263-8522. in magnetic field dependence of 1/T 1 in the Haldane-gap antiferromagnet AgVP 2 S 6 10) and another gapful system (VO) 2 P 2 O 7 .11, 12) These theoretical arguments suggest that the intrinsic energy gap is unique and that ...

Magnetic properties of vanadium oxide nanotubes probed by static magnetization andV51NMR

“…Usually owing to a spin gap the huge slowing down of relaxation with decreasing temperature is observed, as, e.g. in ) the case of the low-dimensional vanadium oxides, it has been observed in the NMR relaxation measurements of a two-leg spin-ladder CaV 2 O 5 , 17,18 or a quarter-filled ladder compound NaV 2 O 5 . 19 The occurrence of a spin gap ∆ yields an exponential temperature dependence of the longitudinal relaxation rate T −1 1 for T < ∆: 20,21…”

Spin ladder compoundPb0.55Cd0.45V2O5: Synthesis and investigation

“…In CaV 2 O 5 the strongest interaction runs along the rung of the lad- der (J 1 ≈ 600 K, see Fig. 1) while other interactions (J 2 , J 3 , J 4 ) do not exceed 100 -150 K. 11,12,13,14 Thus, weakly coupled spin dimers are formed and CaV 2 O 5 reveals a spin gap ∆ ≈ J 1 [experimental values of ∆ range from 560 to 660 K (Refs. 14,15,16)].…”

“…1) while other interactions (J 2 , J 3 , J 4 ) do not exceed 100 -150 K. 11,12,13,14 Thus, weakly coupled spin dimers are formed and CaV 2 O 5 reveals a spin gap ∆ ≈ J 1 [experimental values of ∆ range from 560 to 660 K (Refs. 14,15,16)]. In contrast, MgV 2 O 5 reveals comparable nearest-neighbor interactions J 1 , J 2 , J 3 of about 100 K. 11,12 The presence of a spin gap in MgV 2 O 5 is still argued and the suggested values of ∆ are quite small (15 − 20 K).…”

S=12quantum critical spin ladders produced by orbital ordering inBa2CuTeO6

“…Very few materials have so far been found to be near to this QCP. Often the candidates turn out to be in the strong-leg limit, for example bis(2,3-dimethylpyridinium) tetrabromocuprate [6], or the strong-rung (dimer) limit such as (C 5 H 12 N) 2 CuBr 4 [7] and CaV 2 O 5 [8,9], which places those systems far from isotropic ladder physics. It is not easy to tune J inter close to J c .…”