NMR characterization of spin - 12 alternating antiferromagnetic chains in the high - pressure phase of ( VO ) 2 P 2 O 7

Abstract

Local-susceptibility measurements via the NMR shifts of P and V nuclei in the high-pressure phase of (VO)2P2O7 confirmed the existence of a unique alternating antiferromagnetic chain with a zero-field spin gap of 34 K. The P nuclear spinlattice relaxation rate scales with the uniform spin susceptibility below about 15 K which shows that the temperature dependence of both the static and dynamical spin susceptibilities becomes identical at temperatures not far below the spin-gap energy. Magnetic excitations of a low-dimensional quantum antiferromagnet have been one of the current topics among the condensed matter physicists. Vanadyl pyrophosphate (VO)2P2O7 had long been believed as a prototype of a spin1 2 two-leg ladder which has a magnetic lattice intermediate between one and two spatial dimensions [1, 2, 3, 4, 5]. The ladder model, however, has been rejected by an observation of a dominant magnetic interaction perpendicular to the supposed ladder axis via the inelastic neutron scattering (INS) measurements [6]. A dimerized (alternating) chain model has now been becoming accepted as an alternative starting point, although a mechanism of the major exchange interaction between distant pairs of V spins via PO4 tetrahedra is still under study [7, 8, 9]. The INS experiments has also revealed the existence of the mode with a gap nearly twice the gap of the lowest excited triplet which cannot be explained by a simple alternating-chain model. This mode has first been assigned as a bound state of two magnons possibly formed via interchain couplings [10], but it was difficult to account for the intensity comparable to the fundamental mode. Recent NMR [11] and highfield magnetization [12] studies have suggested on this issue that the two structurallydistinguishable chains of V atoms, which were thought to be magnetically identical, have different spin-gap energies. This gives a natural explanation for the existence of two distinct modes with almost equal spectral weight, and has been supported by § Present address: Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan (kikuchi@issp.u-tokyo.ac.jp) Letter to the Editor 2 S pi nE ch o A m pl itu de ( ar b. u ni ts ) 2.64 2.63 2.62 2.61 2.60 2.59 External Field (T) AP-(VO)2P2O7 HP-(VO)2P2O7 P NMR 45.2 MHz 20 K Figure 1. P NMR spectrum in the ambient-pressure (upper panel and the highpressure (lower panel) phases of (VO)2P2O7 at 20 K. The dotted line indicates the zero-shift position for P. the subsequent Raman-scattering experiments [13] and theoretical studies on relevant exchange interactions [7, 8, 9]. The above confusion concerning the modelling and interpretation of the experimental results of (VO)2P2O7 comes not only from the unexpectedly strong V-V exchange via PO4 tetrahedra, but also from the presence of structurally-inequivalent V chains [14, 15]. More recently, Azuma et al have found that (VO)2P2O7 transforms into another phase with different symmetry under pressure [16]. All the V atoms occupies a unique crystallographic site in the high-pressure (HP) phase, so that the magnetic chains made of V spins are all equivalent. Therefore, HP-(VO)2P2O7 will be a better example of the alternating antiferromagnetic chain with quantum spin 1 2 . In this letter, we report microscopic characterization of the magnetic chains in the HP phase of (VO)2P2O7 via NMR. A single spin component characterized by a zero-field gap of 34 K was found, presenting support for the double-chain scenario for the ambient-pressure (AP) phase. Single crystals of the HP phase of (VO)2P2O7 were grown as described in [17]. Since the crystals were too small to observe an NMR signal, they were crushed into powders and the NMR measurements were made on these powders. Standard spinecho pulse techniques were utilized for most of the experiments. An example of the field-swept P NMR spectrum in the HP phase of (VO)2P2O7 is shown in figure 1. The spectrum in the AP phase [11] is also shown for comparison. The spectrum in the HP phase consists of a single line as expected from the unique crystallographic site of phosphor in the unit cell. This is contrasted with the AP phase where the spectrum splits into two groups of lines owing to the presence of two kinds of V chains with different gap energies [11]. The line-shape analysis revealed that the symmetry of an NMR-shift tensor at the P site is almost uniaxial. Assuming the exact uniaxial symmetry, we determined the two independent principal values K‖ and Letter to the Editor 3

5 Figures and Tables

Cite this paper

@inproceedings{Kikuchi2004NMRCO, title={NMR characterization of spin - 12 alternating antiferromagnetic chains in the high - pressure phase of ( VO ) 2 P 2 O 7}, author={Jun Kikuchi and Kiyoichiro Motoya and T Saito and Mizutomo Azuma and Masashi Takano}, year={2004} }