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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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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.
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f2: 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.

Mentions: To elucidate the detailed characteristics of low-energy excitations from the ground state, data in the low-temperature region are plotted as CpT−1 vs T2 in Figure 2a. Fitting the 0-T data between 0.9 and 2.0 K for EtMe3Sb[Pd(dmit)2]2 using the formula CpT−1=γ+βT2 gives γ=19.9 mJ K−2 mol−1 and β=24.1 mJ K−4 mol−1. The finite electronic heat capacity coefficient γ in a triangular S-1/2 spin system suggests that gapless excitations occur from a liquid-like ground state, similar to the case of κ-(BEDT-TTF)2Cu2(CN)314. As Figure 2a shows, γ is not seriously affected by magnetic fields up to 8 T. This excludes the possibility that paramagnetic impurity spins are the origin of the γ term. The measurement was also performed for over 50 pieces of microcrystals, and the temperature and magnetic field dependences obtained were almost the same as those in Figure 2a. Thus, the large heat capacity at low temperatures is attributed to the intrinsic properties of this material.


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)

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.
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f2: 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.
Mentions: To elucidate the detailed characteristics of low-energy excitations from the ground state, data in the low-temperature region are plotted as CpT−1 vs T2 in Figure 2a. Fitting the 0-T data between 0.9 and 2.0 K for EtMe3Sb[Pd(dmit)2]2 using the formula CpT−1=γ+βT2 gives γ=19.9 mJ K−2 mol−1 and β=24.1 mJ K−4 mol−1. The finite electronic heat capacity coefficient γ in a triangular S-1/2 spin system suggests that gapless excitations occur from a liquid-like ground state, similar to the case of κ-(BEDT-TTF)2Cu2(CN)314. As Figure 2a shows, γ is not seriously affected by magnetic fields up to 8 T. This excludes the possibility that paramagnetic impurity spins are the origin of the γ term. The measurement was also performed for over 50 pieces of microcrystals, and the temperature and magnetic field dependences obtained were almost the same as those in Figure 2a. Thus, the large heat capacity at low temperatures is attributed to the intrinsic properties of this material.

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