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

Yamashita S, Yamamoto T, Nakazawa Y, Tamura M, Kato R - Nat Commun (2011)

Bottom Line: In frustrated magnetic systems, long-range ordering is forbidden and degeneracy of energy states persists, even at extremely low temperatures.This compound is an organic dimer-based Mott insulator with a two-dimensional triangular lattice structure.We also report anomalous enhancement of γ, produced by a kind of criticality inherent to the Pd(dmit)(2) phase diagram.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan.

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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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.
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f4: 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.

Mentions: In the case of κ-(BEDT-TTF)2Cu2(CN)3, the low-temperature quantum liquid state is realized below a crossover temperature of 5.7 K, at which the heat capacity has a broad hump structure14. Recent thermal expansion measurements by Manna et al.31 indicate a clear anomaly at this temperature. Abdel-Jawad et al.16 have suggested that the unusual dielectric properties at higher temperatures are related to charge disproportionation in dimers. As shown in Figure 4a, a similar broad hump structure is observed in the CpT−1 vs T2 curve of EtMe3Sb[Pd(dmit)2]2. Comparison with the data for EtMe3P[Pd(dmit)2]2, EtMe3As[Pd(dmit)2]2 and Et2Me2Sb[Pd(dmit)2]2 in Figure 4a reveals the hump structure in the EtMe3Sb[Pd(dmit)2]2 data. The lines in Figure 4a represent the βT3 term determined from the low-temperature data below 2.0 K for these compounds. The β value is 24.1 mJ K−4 mol−1 for EtMe3Sb[Pd(dmit)2]2, 14.3 mJ K−4 mol−1 for EtMe3P[Pd(dmit)2]2, 19.5 mJ K−4 mol−1 for EtMe3As[Pd(dmit)2]2 and 13.2 mJ K−4 mol−1 for Et2Me2Sb[Pd(dmit)2]2. Deviations of the experimental data of Cp from βT3 are determined using the formula ΔCp=Cp−βT3, and ΔCpT−1 values of these compounds are plotted as a function of temperature in Figure 4b. The data of EtMe3Sb[Pd(dmit)2]2 clearly reveal the broad hump structure. Figure 4b also shows additional heat capacity due to the hump in the κ-(BEDT-TTF)2Cu2(CN)314 obtained by similar analysis. The hump temperature and the magnitude of the ΔCpT−1 peak of EtMe3Sb[Pd(dmit)2]2 are, respectively, about 3.7 K and 35 mJ K−1 mol−1, whereas they are, respectively, 5.7 K and 60 mJ K−2 mol−1 for κ-(BEDT-TTF)2Cu2(CN)3. As the lattice heat capacities of organic salts do not obey the simple βT3 term around the hump temperature of κ-(BEDT-TTF)2Cu2(CN)3, ΔCpT−1 does not reflect the correct magnetic heat capacity in this compound. The lattice heat capacity has been more accurately estimated in our previous work using the lattice heat capacities of similar κ-(BEDT-TTF)2X compounds, and the excess entropy of the hump was estimated to be 700–1,000 mJ K−1 mol−1 (ref. 14). The excess entropy related to the hump in the EtMe3Sb[Pd(dmit)2]2 data is approximately evaluated as 70–100 mJ K−1 mol−1, which is 7–14% of that of κ-(BEDT-TTF)2Cu2(CN)3. The hump temperatures of both salts seem to correspond to temperatures at which a small dip-like structure appears in the temperature dependences of T1−1 obtained by Itou et al. for EtMe3Sb[Pd(dmit)2]2 (ref. 18) and by Shimizu et al. for κ-(BEDT-TTF)2Cu2(CN)3 (ref. 7). Figure 4b also reveals that this hump structure of EtMe3Sb[Pd(dmit)2]2 does not have a magnetic field dependence up to 8 T.


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

Yamashita S, Yamamoto T, Nakazawa Y, Tamura M, Kato R - Nat Commun (2011)

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.
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f4: 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.
Mentions: In the case of κ-(BEDT-TTF)2Cu2(CN)3, the low-temperature quantum liquid state is realized below a crossover temperature of 5.7 K, at which the heat capacity has a broad hump structure14. Recent thermal expansion measurements by Manna et al.31 indicate a clear anomaly at this temperature. Abdel-Jawad et al.16 have suggested that the unusual dielectric properties at higher temperatures are related to charge disproportionation in dimers. As shown in Figure 4a, a similar broad hump structure is observed in the CpT−1 vs T2 curve of EtMe3Sb[Pd(dmit)2]2. Comparison with the data for EtMe3P[Pd(dmit)2]2, EtMe3As[Pd(dmit)2]2 and Et2Me2Sb[Pd(dmit)2]2 in Figure 4a reveals the hump structure in the EtMe3Sb[Pd(dmit)2]2 data. The lines in Figure 4a represent the βT3 term determined from the low-temperature data below 2.0 K for these compounds. The β value is 24.1 mJ K−4 mol−1 for EtMe3Sb[Pd(dmit)2]2, 14.3 mJ K−4 mol−1 for EtMe3P[Pd(dmit)2]2, 19.5 mJ K−4 mol−1 for EtMe3As[Pd(dmit)2]2 and 13.2 mJ K−4 mol−1 for Et2Me2Sb[Pd(dmit)2]2. Deviations of the experimental data of Cp from βT3 are determined using the formula ΔCp=Cp−βT3, and ΔCpT−1 values of these compounds are plotted as a function of temperature in Figure 4b. The data of EtMe3Sb[Pd(dmit)2]2 clearly reveal the broad hump structure. Figure 4b also shows additional heat capacity due to the hump in the κ-(BEDT-TTF)2Cu2(CN)314 obtained by similar analysis. The hump temperature and the magnitude of the ΔCpT−1 peak of EtMe3Sb[Pd(dmit)2]2 are, respectively, about 3.7 K and 35 mJ K−1 mol−1, whereas they are, respectively, 5.7 K and 60 mJ K−2 mol−1 for κ-(BEDT-TTF)2Cu2(CN)3. As the lattice heat capacities of organic salts do not obey the simple βT3 term around the hump temperature of κ-(BEDT-TTF)2Cu2(CN)3, ΔCpT−1 does not reflect the correct magnetic heat capacity in this compound. The lattice heat capacity has been more accurately estimated in our previous work using the lattice heat capacities of similar κ-(BEDT-TTF)2X compounds, and the excess entropy of the hump was estimated to be 700–1,000 mJ K−1 mol−1 (ref. 14). The excess entropy related to the hump in the EtMe3Sb[Pd(dmit)2]2 data is approximately evaluated as 70–100 mJ K−1 mol−1, which is 7–14% of that of κ-(BEDT-TTF)2Cu2(CN)3. The hump temperatures of both salts seem to correspond to temperatures at which a small dip-like structure appears in the temperature dependences of T1−1 obtained by Itou et al. for EtMe3Sb[Pd(dmit)2]2 (ref. 18) and by Shimizu et al. for κ-(BEDT-TTF)2Cu2(CN)3 (ref. 7). Figure 4b also reveals that this hump structure of EtMe3Sb[Pd(dmit)2]2 does not have a magnetic field dependence up to 8 T.

Bottom Line: In frustrated magnetic systems, long-range ordering is forbidden and degeneracy of energy states persists, even at extremely low temperatures.This compound is an organic dimer-based Mott insulator with a two-dimensional triangular lattice structure.We also report anomalous enhancement of γ, produced by a kind of criticality inherent to the Pd(dmit)(2) phase diagram.

View Article: PubMed Central - PubMed

Affiliation: Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan.

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.

Show MeSH
Related in: MedlinePlus