Spin-lattice decoupling in a triangular-lattice quantum spin liquid

Takayuki Isono,Taichi Terashima,Kazushi Kanoda,Shiori Sugiura,Shinya Uji,Kazuya Miyagawa

doi:10.1038/s41467-018-04005-1

Takayuki Isono, Taichi Terashima + Show 4 more

Open Access

https://doi.org/10.1038/s41467-018-04005-1

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Abstract

A quantum spin liquid (QSL) is an exotic state of matter in condensed-matter systems, where the electron spins are strongly correlated, but conventional magnetic orders are suppressed down to zero temperature because of strong quantum fluctuations. One of the most prominent features of a QSL is the presence of fractionalized spin excitations, called spinons. Despite extensive studies, the nature of the spinons is still highly controversial. Here we report magnetocaloric-effect measurements on an organic spin-1/2 triangular-lattice antiferromagnet, showing that electron spins are decoupled from a lattice in a QSL state. The decoupling phenomena support the gapless nature of spin excitations. We further find that as a magnetic field is applied away from a quantum critical point, the number of spin states that interact with lattice vibrations is strongly reduced, leading to weak spin–lattice coupling. The results are compared with a model of a strongly correlated QSL near a quantum critical point.

Highlights

A quantum spin liquid (QSL) is an exotic state of matter in condensed-matter systems, where the electron spins are strongly correlated, but conventional magnetic orders are suppressed down to zero temperature because of strong quantum fluctuations
A quantum spin liquid (QSL) is an intriguing exception for the Landau theory of phase transitions; at sufficiently low temperatures, condensed-matter systems form an ordered state characterized by broken symmetries and corresponding order parameters
The ground state of the triangular-lattice Heisenberg AF system is known to be 120° AF order[5], its ordered state can be suppressed by ring-exchange interactions[6], NN interactions[7], or a spatial distribution of an exchange coupling constant[8], leading to QSL

Summary

Introduction

A quantum spin liquid (QSL) is an exotic state of matter in condensed-matter systems, where the electron spins are strongly correlated, but conventional magnetic orders are suppressed down to zero temperature because of strong quantum fluctuations. One of the most fundamental properties of a QSL is the presence of charge neutral excitations carrying spin-1/2 quantum number, spinons. These fractional excitations are clearly distinct from spin-1 magnon excitations in magnetically ordered states. Despite the large NN AF interactions, range order happens down to JT/kB∼~3205m0 KK,12n–o14,mwahgnicehticis longfour orders of magnitude lower than J/kB This suggests that the QSL state is realized in κ-(BEDT-TTF)2Cu2(CN)[3]. A finite value of the specific heat divided by temperature C/T for T → 0, and Pauli-like magnetic susceptibility are hallmarks of gapless spin excitations[15,16]

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