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 lattice⁵, 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 fluctuations⁶. Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K₂PbCu(NO₂)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.

磁性方面的一个重要挑战是对量子自旋液体^1,2的明确识别，这对于量子计算具有潜在的重要性。在这样的材料中，磁自旋应该在量子状态中波动，而不是冻结在经典的远距离有序状态中。尽管此要求决定了系统^3,4，其中经典顺序被令人沮丧的晶格⁵抑制了，理想的系统将允许通过外部参数调整量子涨落。常规的三维反铁磁体可以通过施加的磁场通过量子临界点（高度自旋的区域）进行调谐。这样的系统在量子临界点具有弱的比热峰，几乎没有熵可用于量子涨落⁶。在这里，我们研究了另一种类型的反铁磁体，它由分子盐K ₂ PbCu（NO ₂）_6中实现的弱耦合反铁自旋1/2链组成。在三维场和顺磁相之间的温度-磁场边界上，比热表现出一个大峰值，其峰值接近暗示孤立的量子自旋链的自旋Sommerfeld系数的值。这些结果证明了通过磁场引起的熵从一维短程到三维量子临界点的移动来产生量子物质的另一种方法。