Quantum criticality among entangled spin chains

An important challenge in magnetism is the unambiguous identification of a quantum spin liquid 1 , 2 , of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems 3 , 4 wherein classical order is suppressed by a frustrating lattice 5 , an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point—a region of highly fluctuating spins—by an applied magnetic field. Such systems suffer from a weak specific-heat peak at the quantum critical point, with little entropy available for quantum fluctuations 6 . Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K 2 PbCu(NO 2 ) 6 . Across the temperature–magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point. Exploiting the magnetic field-induced shift of entropy in certain molecular salts when going from 1D short-range ordering to a 3D quantum critical point could provide a route for producing strongly fluctuating quantum materials.