Criticality-Enhanced Magnetocaloric Effect in Quantum Spin Chain Material Copper Nitrate

TL;DR

This study combines advanced simulations, first-principles calculations, and experiments to reveal a criticality-enhanced magnetocaloric effect in copper nitrate, a quantum spin chain material, highlighting its potential as a quantum critical coolant.

Contribution

The paper introduces a new TTN method for accurately modeling Cu(NO$_3$)$_2$ $ullet$ 2.5H$_2$O and demonstrates its significant magnetocaloric effect near quantum critical points.

Findings

Identified key magnetic couplings and Landé factors in copper nitrate.

Visualized superexchange pathways via electron density calculations.

Discovered prominent quantum-criticality-enhanced MCE near critical fields.

Abstract

Low-dimensional quantum magnets, due to the existence of abundant exotic quantum phases therein and experimental feasibilities in laboratories, continues intriguing people in condensed matter physics. In this work, a comprehensive study of Cu(NO$_3$)$_2$ $\cdot$ 2.5H$_2$O (copper nitrate hemipentahydrate, CN), a spin chain material, is performed with multi-technique approach including thermal tensor network (TTN) simulations, first-principles calculations, as well as magnetization measurements in experiments. Employing a cutting-edge TTN method developed in the present work, we determine the couplings $J=5.13$ K, $\alpha=0.23(1)$ and Land\'e factors $g_{\parallel}=2.31$, $g_{\perp}=2.14$ in an alternating Heisenberg antiferromagnetic chain model, with which one can fit strikingly well the magnetothermodynamic properties. Part of the fitted experimental data are measured on the single-crystal CN specimens synthesized by us. Based on first-principles calculations, we reveal explicitly the spin chain scenario in CN by displaying the calculated electron density distributions, from which the distinct superexchange paths are visualized. On top of that, we investigated the magnetocaloric effect (MCE) in CN by calculating its isentropes and magnetic Gr\"ueisen parameter (GP). Prominent quantum-criticality-enhanced MCE was uncovered, the TTN simulations are in good agreements with measured isentropic lines in the sub-Kelvin region. We propose that CN is potentially a very promising quantum critical coolant, due to the remarkably enhanced MCE near both critical fields of moderate strengths as 2.87 and 4.08 T, respectively.