Bose-Einstein condensation of triplons close to the quantum critical point in the quasi-one-dimensional spin-12 antiferromagnet NaVOPO4

• Published in: Physical review. B 2019-10, Vol.100 (14)
• Main Authors: Mukharjee, Prashanta K, Ranjith, K M, Koo, B, Sichelschmidt, J, Baenitz, M, Skourski, Y, Inagaki, Y, Furukawa, Y, Tsirlin, A A, Nath, R
• Format: Article
• Language: English
• Subjects: Antiferromagnetism; Bose-Einstein condensates; Condensation; Couplings; Critical field (superconductivity); Critical point; Crystal structure; Electron paramagnetic resonance; Electron spin; Heat exchange; Long range order; Magnetic permeability; Magnetic properties; Magnetism; Magnetization; NMR; Nuclear magnetic resonance; Phase transitions; Spin resonance
• ISSN: 2469-9950; 2469-9969
• DOI: 10.1103/PhysRevB.100.144433

Structural and magnetic properties of a quasi-one-dimensional spin-12 compound NaVOPO4 are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and P31 nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of NaVOPO4 may be compatible with the gapless uniform spin chain model, detailed examination of the crystal structure reveals a weak alternation of the exchange couplings with the alternation ratio α≃0.98 and the ensuing zero-field spin gap Δ0/kB≃2.4K directly probed by field-dependent magnetization measurements. No long-range order is observed down to 50 mK in zero field. However, applied fields above the critical field Hc1≃1.6T give rise to a magnetic ordering transition with the phase boundary TN∝(H−Hc1)1ϕ, where ϕ≃1.8 is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, NaVOPO4 lies close to the quantum critical point.