Seminarium Oddziału Badań Magnetyków
Microsoft Teams
Quantum and classical aspects of a low-temperature (~700 mK) phase transition in TbAl3(BO3)4
prof. dr hab. Andrzej Szewczyk
Instytut Fizyki PAN w Warszawie
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Streszczenie:
Specific heat, CB, of a TbAl3(BO3)4 single crystal was studied for temperatures, T, from 50 mK to 300 K, with emphasis on the T< 1 K range, where a phase transition was found at 0.68 K. Nuclear, non-phonon, and lattice contributions to CB were separated. Based on the CB and magnetization, M, studies, we found that: (i) the phase transition shifts to lower temperatures with increase in magnetic field B||, parallel to the easy magnetization axis, (ii) the critical, i.e., related to the phase transition, contribution to the specific heat, Ccr, shows an unusual dependence on T, ?cr~??0, where y0 is a positive exponent, and (iii) the Grüneisen ratio, Γ, defined as: Γ=−1?(??/??)?(??/??)?=−(??/??)???(?)=1?(????)?
where S denotes entropy, diverges as a function of B|| for B|| approaching a critical value of 0.6 T. The determined behaviors of both Ccr and Γ as a function of T (especially scaling of the latter for B|| ≥ 0.30 T), as well as dependence of Γ on B|| are characteristic of the systems, in which the classical phase transition line is influenced by quantum fluctuations, QF, and ends at a quantum critical point. Based on the determined y0 and Γ values, we assessed the dynamical critical exponent z to be 0.82 ≤ z ≤ 0.96. Taking into account all these results, we suppose that QF dominate the behavior of the system and destroy the long range order, i.e., we suppose the transition found to have a quantum character.
The physical nature of the transition is not clear. The interpretation that this is the transition to the ferromagnetic order of Tb3+ magnetic moments is the most natural, intuitive, and supported by the M studies. However, such a classical transition should be smeared and shifted to higher T by B||, while we observe the opposite effect. Such effect was observed in systems, in which not only the exchange interactions but also magnetic dipolar interactions are essential [1]. However, the possibility, that the transition is related to any other kind of ordering, e.g., a multipolar ordering, and the ordering of the Tb3+ moments is a “side effect” only can not be ruled out a priori.
* This work was supported partially by the National Science Centre (NCN), Poland, under Project No. 2018/31/B/ST3/03289.
[1] G. Mennenga, L. J. de Jongh, W. J. Huiskamp, J. Magn. Magn. Mater. 44, 59 (1984).