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. 2011:2:275.
doi: 10.1038/ncomms1274.

Gapless spin liquid of an organic triangular compound evidenced by thermodynamic measurements

Satoshi Yamashita  1 , Takashi Yamamoto, Yasuhiro Nakazawa, Masafumi Tamura, Reizo Kato
Affiliations
Free PMC article

Gapless spin liquid of an organic triangular compound evidenced by thermodynamic measurements

Satoshi Yamashita et al. Nat Commun. 2011.
Free PMC article

Abstract

In frustrated magnetic systems, long-range ordering is forbidden and degeneracy of energy states persists, even at extremely low temperatures. Under certain conditions, these systems form an exotic quantum spin-liquid ground state, in which strongly correlated spins fluctuate in the spin lattices. Here we investigate the thermodynamic properties of an anion radical spin liquid of EtMe(3)Sb[Pd(dmit)(2)](2), where dmit represents 1,3-dithiole-2-thione-4,5-dithiolate. This compound is an organic dimer-based Mott insulator with a two-dimensional triangular lattice structure. We present distinct evidence for the formation of a gapless spin liquid by examining the T-linear heat capacity coefficient, γ , in the low-temperature heat capacity. Using comparative analyses with κ-(BEDT-TTF)(2)Cu(2)(CN)(3), a generalized picture of the new spin liquid in dimer-based organic systems is discussed. We also report anomalous enhancement of γ, produced by a kind of criticality inherent to the Pd(dmit)(2) phase diagram.

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Figures

Figure 1
Figure 1. Temperature dependences of heat capacities of EtMe3Sb[Pd(dmit)2]2 and its related salts.
(a) A schematic illustration of the molecular arrangement in the acceptor plane of EtMe3Sb[Pd(dmit)2]2 (upper) and molecular structure of Pd(dmit)2 (lower). The longer axis of the molecule is arranged perpendicular to the plane. Pd(dmit)2 molecules form a dimerized structure, which is indicated by the circle. The dimers form a triangular lattice structure. The definition of three transfer integrals between neighbouring dimers (tB, tr, ts ) of Pd(dmit)2 systems is shown in the upper figure. The magnitude of these transfers is (tr=) t′/t (=ts≈tB) and if t′/t=1, a regular triangular system is established. The stacking direction of Pd(dmit)2 molecules is shown by the arrow. (b) The data of EtMe3Sb[Pd(dmit)2]2 (red squares), EtMe3P[Pd(dmit)2]2 (P21/m: aqua blue crosses) and κ-(BEDT-TTF)2Cu2(CN)3 (blue diamonds) are plotted on a logarithmic scale. The heat capacities of spin-liquid compounds EtMe3Sb[Pd(dmit)2] and κ-(BEDT-TTF)2Cu2(CN)3 are larger than those of EtMe3P[Pd(dmit)2]2 with ordered ground state due to the spin entropy remains at low temperatures in the former two compounds. The realization of the spin-liquid state is suggested in EtMe3Sb[Pd(dmit)2]2.
Figure 2
Figure 2. Low-temperature heat capacities of EtMe3Sb[Pd(dmit)2]2.
(a) CpT−1 versus T2 plot of EtMe3Sb[Pd(dmit)2]2 (h9-EtMe3Sb) below 2 K obtained under 0 T (red squares), 1 T (green filled circles), 2 T (blue diamonds), 5 T (ocher crosses) and 8 T (purple filled circles). This figure contains the data of related Pd(dmit)2 salts of EtMe3As[Pd(dmit)2]2(EtMe3As red pluses), EtMe3P[Pd(dmit)2]2 (EtMe3P blue crosses) and Et2Me2Sb[Pd(dmit)2]2 (Et2Me2Sb green filled circles), which have ordered ground states for comparison. The fitting lines obtained by using the data of 0 T of each salt are shown by the same colours with the data. The existence of a T-linear contribution even in the insulating state of EtMe3Sb[Pd(dmit)2]2 is observed. A large upturn below 1 K that masks the information of the electron spins is probably attributable to the rotational tunnelling of Me groups. The inset figure shows CpT−1 versus T2 plot of EtMe3Sb[Pd(dmit)2]2 data below 0.7 K, where a large upturn with magnetic field dependence appears. The data obtained under 0 T (red squares), 1 T (green filled circles), 2 T (blue diamonds), 5 T (ocher crosses), 8 T (purple filled circles) and 10 T (orange squares) are plotted. (b) The overall behaviour of CpT−1 below 4 K of EtMe3Sb[Pd(dmit)2]2 (h9-EtMe3Sb) and its deuterated compound of d9-EtMe3Sb[Pd(dmit)2]2 (d9-EtMe3Sb) in a logarithmic plot. The data under 0 T (red squares), 1 T (green filled circles) and 2 T (blue diamonds) of EtMe3Sb[Pd(dmit)2]2 is shown by the same symbols as in (a). The data obtained under 0 T (purple crosses) and 2 T (ocher filled circles) of d9-EtMe3Sb[Pd(dmit)2]2 are compared in the same plot. The upturn has been reduced down to about few percent by deuteration. The origin of the upturn is extrinsic for the discussion of electronic spins and is attributed to the existence of rotational tunnelling levels of Me groups in the cation.
Figure 3
Figure 3. Comparison of electronic heat capacity coefficient γ between EtMe3Sb[Pd(dmit)2]2 and d9-EtMe3Sb[Pd(dmit)2]2.
(a) Low-temperature heat capacities of h9-EtMe3Sb[Pd(dmit)2]2 (h9-EtMe3Sb; 0 T red squares, 2 T blue diamonds) and d9-EtMe3Sb[Pd(dmit)2]2 (d9-EtMe3Sb; 0 T purple pluses, 2 T ocher filled circles) below 3.1 K. Upward deviation of heat capacities of d9-EtMe3Sb[Pd(dmit)2]2 is observed below 2 K. The enhancement of the electronic heat capacity of d9-EtMe3Sb[Pd(dmit)2]2 is realized in this temperature region. (b) Low-temperature heat capacities of d9-EtMe3Sb[Pd(dmit)2]2 0 T (purple crosses), 2 T (ocher filled circles), 5 T (aqua blue squares) and 9 T (orange triangles) below 0.65 K plotted in CpT−1 versus T2. The enhanced T-linear contribution in heat capacity does not have drastic magnetic field dependence.
Figure 4
Figure 4. Analyses of temperature dependences of the heat capacity of Pd(dmit)2 salts.
(a) CpT−1 versus T2 plots of the heat capacity of EtMe3Sb[Pd(dmit)2]2 (EtMe3Sb; 0 T red squares, 8 T ocher crosses), EtMe3As[Pd(dmit)2]2 (EtMe3As; 0 T purple pluses), Et2Me2Sb[Pd(dmit)2]2 (Et2Me2Sb; 0 T green filled circles) and EtMe3P[Pd(dmit)2]2 (EtMe3P; 0 T aqua blue crosses). The lines shown in the figure are βT3 terms determined by the low-temperature data below 2.0 K. Around 3–4 K, a broad hump structure is observed only in EtMe3Sb[Pd(dmit)2]2. The data obtained under 8 T of EtMe3Sb[Pd(dmit)2]2 also show the hump structure. (b) The temperature dependences of ΔCp=Cp−βT3 defined as a difference of the heat capacity data from the βT3 for each compound in (a) are shown in ΔCpT−1 vs T plot. The symbols of the data are the same as those shown in (a). The data clearly indicate that the broad hump structure exists only in EtMe3Sb[Pd(dmit)2]2 in the Pd(dmit)2 system. The result of similar analysis for κ-(BEDT-TTF)2Cu2(CN)3 (blue diamonds) is also presented in the figure.
Figure 5
Figure 5. A schematic view of electronic phase diagram of the Pd(dmit)2 system.
The electronic properties of Pd(dmit)2 system are dominated by t/t′ ratio. The spin-liquid phase is located in the narrow region between antiferromagnetic (AF) and charge order (CO) phases. The positions of EtMe3Sb[Pd(dmit)2]2(h9-EtMe3Sb), EtMe3As[Pd(dmit)2]2 (EtMe3As), Me4Sb[Pd(dmit)2]2 (Me4Sb) and Et2Me2Sb[Pd(dmit)2]2 (Et2Me2Sb) are indicated in the figure.
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