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Magnetocaloric effect and magnetic cooling near a field-induced quantum-critical point

Abstract:

Phase transitions occurring at absolute zero temperature and governed by a critical value of a variable like pressure, magnetic field, doping etc. are called quantum phase transitions and the point of the phase diagram where it happens the quantum critical point (QCP). A huge accumulation of entropy at the QCP leads to a competition between different ground states and hence, unusual behaviour in the thermodynamics and transport properties of systems close/at the QCP. A one dimensional (1-d) spin ½ antiferromagnetic Heisenberg chain (AfHC) is expected to be quantum critical at a field H_s (saturation field) above which it undergoes a transition to a ferromagnetic state. Recently, it has been theoretically predicted that the divergence and sign change of Grüneisen parameter across the quantum critical point can be used as a novel technique to probe quantum criticality. For a field induced quantum critical point, the Grüneisen parameter is, in fact, magnetocaloric effect (MCE) which is the heating or cooling of a system adiabatically, in response to a changing magnetic field. In this talk, I will present measurements and theoretical calculations of the MCE in a metal organic polymer system built from Cu^2+ (S =½ ) ions, which is a very good realization of a 1-d AfHC. We verify unambiguously the theoretical predictions and demonstrate that the 1-d AfHC shows an extraordinarily large MCE and a pronounced magnetic cooling effect near the QCP. Our results suggest that quantum magnets near a H-induced QCP open up new possibilities for realizing very efficient low-temperature coolants. We, therefore, suggest the MCE experiments as a new means of exploring quantum criticality, one of the most interesting issues in modern condensed matter physics.