Abstract
Phase transitions occurring at absolute zero temperature and governedby a critical value of a variable like pressure, magnetic field,doping etc. are called quantum phase transitions and the point of thephase diagram where it happens the quantum critical point (QCP). Ahuge accumulation of entropy at the QCP leads to a competition betweendifferent ground states and hence, unusual behaviour in thethermodynamics and transport properties of systems close/at the QCP. Aone 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 andsign change of Grüneisen parameter across the quantum critical pointcan be used as a novel technique to probe quantum criticality. For afield induced quantum critical point, the Grüneisen parameter is, infact, magnetocaloric effect (MCE) which is the heating or cooling of asystem adiabatically, in response to a changing magnetic field. Inthis talk, I will present measurements andtheoretical calculations of the MCE in a metal organic polymer systembuilt from Cu^2+ (S=½ ) ions, which is a very good realization of a1-d AfHC. We verify unambiguously the theoretical predictions anddemonstrate that the 1-d AfHC shows an extraordinarily large MCE and apronounced magnetic cooling effect near the QCP. Our results suggestthat quantum magnets near a H-induced QCP open up new possibilitiesfor realizing very efficient low-temperature coolants. We, therefore,suggest the MCE experiments as a new means of exploring quantumcriticality, one of the most interesting issues in modern condensedmatter physics.
