Heat capacity of the highly frustrated triangulated Kagome lattice Cu9Cl2(cpa)6

William M Farmer,Samuel F Skinner,Leonard W Ter Haar

doi:10.1063/1.5042777

Abstract

Magnetic field-dependent heat capacity measurements on the tunable molecular magnet Cu9Cl2(cpa)6, a metal organic framework (MOF) based on copper(II) ions and carboxypentonic acid, suggest that this molecular magnetic material could serve as a candidate for quantum spin liquid (QSL) studies. The 2-D framework has been described as a ‘triangles-in-triangles’ (TIT) Kagomé or triangulated-Kagomé-lattice (TKL), and previous magnetic experiments and theoretical studies have suggested this Kagomé Heisenberg antiferromagnet (KHAF) is highly frustrated. Here we report heat capacity data down to 50mK that confirms this frustration. Above 5K the material exhibits field-independent heat capacity that largely fits C = αT3+βT behavior with parameters that vary with the temperature window of analysis. Below 5K, the zero-field heat capacity exhibits no obvious phase transition down to 0.050K and levels off at a nonzero value. Upon the application of magnetic field, a peak in the heat capacity (Cmax) emerges from below 0.050K. The temperature at which (Cmag/T)max occurs, Tmax, scales linearly with applied field. As Tmax increases, the magnitude of (Cmag/T)max decreases and the peak broadens, eventually subsumed into the αT3 behavior above 3T. Below Cmax, the heat capacity decreases to zero at finite temperatures in large fields. These data corroborate magnetic data and confirm that this frustrated TKL material does not exhibit a phase transition in low-to-zero field down to 0.050 K. Further, the temperature Tmax nominally extrapolates to 0K in zero-field, yielding a T-H phase diagram similar to Herbertsmithite; one that suggests CPA could be a possible candidate to develop additional chemical variants for QSL studies.

Highlights

Macroscopic quantum states, such as quantum spin liquids (QSL), are of fundamental interest in low-dimensional frustrated magnetism.[1,2,3,4,5,6] Frustrated systems have application potential because of the magnetocaloric effect (MCE)[7,8] for adiabatic demagnetization refrigeration (ADR), especially space-based[9] applications
Frustrated materials can lead to enhanced MCE and ADR at ultralow temperatures because any increase in entropy due to frustration helps resist the onset of magnetic ordering.[8,9]
The exotic nature of QSLs can lead to potential applications in data storage and even topological quantum computation.[10,11]

Summary

INTRODUCTION

Macroscopic quantum states, such as quantum spin liquids (QSL), are of fundamental interest in low-dimensional frustrated magnetism.[1,2,3,4,5,6] Frustrated systems have application potential because of the magnetocaloric effect (MCE)[7,8] for adiabatic demagnetization refrigeration (ADR), especially space-based[9] applications. We recently reported[20] synthetic strategies and magnetic data that demonstrate Cu9X2(cpa)6 ·42H2O (X = Cl,[21] Br, F;23 cpa = anion of 2-carboxy-pentonic acid) – abbreviated CPA – can serve as a promising class of tunable magnetic MOFs. Initial reports[23,24,25] of novel magnetism identified CPA as a topologically unique 2-D triangulated-Kagome lattice (TKL) with a remarkably large frustration ratio, θCW/Tc (a key signature of extremely frustrated systems[2]). Our synthetic goal has been to chemically alter CPA to manipulate the TKL ground state and its concomitant physics. We report magnetic field dependent heat capacity of Cu9Cl2(cpa)[6] to gain insight into any residual entropy from the spin frustration that might be subject to chemical manipulation

EXPERIMENTAL

RESULTS AND DISCUSSION

CONCLUSION

Introduction to Frustrated Magnetism