Joule-Thomson temperature of a triplon system of dimerized quantum magnets

It is well known that, for a system of atomic (molecular) gases both kinds of processes, isentropic as well as isenthalpic are realizable and widely used in refrigeration technique. Particularly, magnetic refrigeration exploits always isentropic process, characterized by Grüneisen parameter Γ_H =_{(∂ T / ∂ H) S} / T. We propose that, for quantum magnets an isenthalpic (Joule-Thomson) process, characterized by Joule-Thomson coefficient κ_T =_{(∂ T / ∂ H) W} may be also available. We considered this effect for a simple paramagnetic and dimerized spin-gapped quantum magnets at low temperatures. We have shown that for both kind of materials refrigeration by using Joule-Thomson effect is more effective than by using ordinary isentropic process, i.e. κ_T > TΓ_H at low temperatures. For dimerized spin-gapped magnets, where Bose-Einstein condensation of triplon gas may take place, the Joule-Thomson temperature corresponds to the maximal temperature of liquefaction of the triplon system, which is compared with recent experimental observations performed by Dresden group (Wang et al. (2016) [21]). The inversion temperature, where reverse of cooling and heating up regimes takes place, found to be finite for triplons, but infinite for magnons in a simple paramagnetic.