Thermal-transport measurements in a quantum spin-liquid state of the frustrated triangular magnet κ-(BEDT-TTF)2Cu2(CN)3

Abstract

The notion of quantum spin-liquids (QSLs), antiferromagnets with quantum fluctuation-driven disordered ground states, is now firmly established in one-dimensional (1D) spin systems as well as in their ladder cousins. The spin-1/2 organic insulator κ-(bis(ethylenedithio)-tetrathiafulvalene) 2 Cu 2 (CN) 3 (κ-(BEDT-TTF) 2 Cu 2 (CN) 3 ; ref. 1) with a 2D triangular lattice structure is very likely to be the first experimental realization of this exotic state in D ≥ 2. Of crucial importance is to unveil the nature of the low-lying elementary spin excitations 2,3 , particularly the presence/absence of a 'spin gap', which will provide vital information on the universality class of this putative QSL. Here, we report on our thermal-transport measurements carried out down to 80 mK. We find, rather unexpectedly, unambiguous evidence for the absence of a gapless excitation, which sharply contradicts recent reports of heat capacity measurements 4 . The low-energy physics of this intriguing system needs be reinterpreted in light of the present results indicating a spin-gapped QSL phase.In antiferromagnetically coupled spin systems, geometrical frustrations enhance quantum fluctuations. Largely triggered by the proposal of the resonating-valence-bond theory for S = 1/2 degrees of freedom residing on a frustrated twodimensional (2D) triangular lattice 5-7 and its possible application to high-T c cuprates with a doped 2D square lattice 8,9 , realizing/detecting QSLs in 2D systems has been a longsought goal. Recently, discoveries of QSL states on S = 1/2 triangular lattices have been reported in organic compounds, κ-(BEDT-TTF) 2 Cu 2 (CN) 3 (Fig. 1, inset) 1,10,11 , C 2 H 5 (CH 3 ) 3 Sb

Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator

“…According to the slope of the inverse dielectric constant, C, the dipole moment parallel to the b axis is estimated to be 0.13ed, where e and d are the electron charge and distance between the BEDT-TTF molecules in a dimer, 3.6 A 17 , respectively. The Curie temperature T C was found to be 67 K. This T C is ten times larger than that for κ-(BEDT-TTF) 2 Cu 2 (CN) 3 in which large spin and charge frustrations have been expected 9,[14][15][16] .…”

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)Elsässer1, Wu2, Dressel3 et al. 2012Phys. Rev. B

“…At the same time, spin excitations remain gapped, as inferred from measurements of the thermal conductivity. 11 These mobile singlets of the fluctuating paired-electron liquid contribute to the optical conductivity differently than single electrons probed by dc transport. It is highly desirable to further investigate this scenario theoretically and extract possible power-laws in the optical conductivity.…”

“…9 Numerous theoretical and experimental work was performed during the last decade in order to explore mainly the thermodynamic and magnetic properties. [10][11][12][13][14][15] In the high-temperature range, the 1 H-NMR relaxation rate shows anomalies around 200 K, 8 the thermopower at 150 K, 16 microwave experiments exhibit a dielectric anomaly at 113 K, 17 while a peak in ǫ ′ (T ) occurs below 60 K in the radio-frequency range. 18 In addition, a low-temperature anomaly near 6 K has been observed in thermodynamic, 7,10 transport, 11 dielectric 17 and lattice 19 properties that has not been explained satisfactorily.…”

Quantum Spin-Liquid Behavior in the Spin-1/2 Random-Bond Heisenberg Antiferromagnet on the Kagome Lattice

“…Anderson of the resonating valence bond state [1], the quantum spin-liquid (QSL) state possibly realized in certain S = 1/2 frustrated magnets has attracted interest of researchers [2]. After a long experimental quest, several candidate materials were recently reported in geometrically frustrated magnets, including both the triangular-lattice and the kagome-lattice antiferromagnets (AFs).Examples of the triangular-lattice AF might be S = 1/2 organic salts such as κ-(ET) 2 Cu 2 (CN) 3 [3][4][5][6] and [7][8][9][10], which exhibit no magnetic ordering, neither regular nor random, down to a very low temperature. The QSL state of these compounds exhibits gapless behaviors characterized by, e.g., the lowtemperature specific heat [4,9] or the thermal conductivity [5,8] linear in the absolute temperature T .…”

“…The QSL state of these compounds exhibits gapless behaviors characterized by, e.g., the lowtemperature specific heat [4,9] or the thermal conductivity [5,8] linear in the absolute temperature T . An example of the second category, the kagome-lattice AF, might be herbertsmithite CuZn 3 (OH) 6 Cl 2 [11][12][13][14][15][16][17].…”