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arXiv:1909.00117 · Physical review. B./Physical review. B

Bose-Einstein condensation of triplons close to the quantum critical point in the quasi-one-dimensional spin- 1 2 antiferromagnet NaVOPO 4

Prashanta K. Mukharjee, K. M. Ranjith, Bonho Koo, J. Sichelschmidt, M. Baenitz, Y. Skourski, Yuji Inagaki, Yuji Furukawa, Alexander A. Tsirlin, R. Nath

Published Oct 25, 2019• 17 citations
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Abstract

Structural and magnetic properties of a quasi-one-dimensional spin-$\frac{1}{2}$ compound ${\mathrm{NaVOPO}}_{4}$ are explored by x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, electron spin resonance, and $^{31}\mathrm{P}$ nuclear magnetic resonance measurements, as well as complementary ab initio calculations. Whereas magnetic susceptibility of ${\mathrm{NaVOPO}}_{4}$ 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 $\ensuremath{\alpha}\ensuremath{\simeq}0.98$ and the ensuing zero-field spin gap ${\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}\ensuremath{\simeq}2.4\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ 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 ${H}_{c1}\ensuremath{\simeq}1.6\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ give rise to a magnetic ordering transition with the phase boundary ${T}_{N}\ensuremath{\propto}{(H\ensuremath{-}{H}_{c1})}^{\frac{1}{\ensuremath{\phi}}}$, where $\ensuremath{\phi}\ensuremath{\simeq}1.8$ is close to the value expected for Bose-Einstein condensation of triplons. With its weak alternation of the exchange couplings and small spin gap, ${\mathrm{NaVOPO}}_{4}$ lies close to the quantum critical point.

DOI
10.1103/physrevb.100.144433

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