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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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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.
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f1: 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.

Mentions: Among these spin-liquid materials, an organic compound of κ-(BEDT-TTF)2Cu2(CN)3 offer unique insights into frustration physics. It is a dimer-based molecular Mott insulator with a two-dimensional structure. The electronic properties are dominated by strong magnetic interactions, /J//kB, larger than 200 K7. In this compound, spin-liquid nature has been observed by 13C-NMR7, heat capacity14 and thermal transport15 measurements. Although the gapless feature of the low-energy excitations is reported by the heat capacity measurement14, thermal transport result implies a possibility of a tiny gap opening with an order of 0.5 K15. A possible relation with the superconductivity of conductive π-electrons and coupling with charge degrees of freedom in the dimer unit16 are stimulating extensive interest in frustration problems and electron correlations. Recently, an anion radial compound of EtMe3Sb[Pd(dmit)2]2 has been reported as another candidate of organic quantum spin-liquid material by magnetic susceptibility17, 13C-NMR1819 and thermal conductivity20 measurements. EtMe3Sb[Pd(dmit)2]2 consists of an anion of an acceptor molecule Pd(dmit)2 and a countercation EtMe3Sb+. Dimerized [Pd(dmit)2]2− anions form a segregated stacking structure in a crystal lattice with space group C2/c and a quasi-two-dimensional π-electron system is established in it21. Figure 1a shows the molecular structure of Pd(dmit)2 and crystal packing in the 2D plane. As the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is small, an electron spin localized on each dimer originates from the antibonding HOMOs22. The d-orbital of the Pd ion makes a negligible contribution to the HOMO and thus does not contribute to the spin density. By changing the central pnictogen atom and the numbers of methyl (Me) and ethyl (Et) groups in countercations, the ratio of the transfer integrals between the dimers (tr=) t′/t (=ts≈tB) shown in the inset of Figure 1a can be tuned from the antiferromagnetic (AF) region to the charge-ordered (CO) region in the phase diagram23. Tight-binding calculations based on the crystallographic data at room temperature suggest that the compound of EtMe3Sb[Pd(dmit)2]2 is located at t′/t=0.92, which implies that a nearly ideal triangle structure is realized1824. This compound is similar to κ-(BEDT-TTF)2Cu2(CN)3; they are Mott insulators with a layered structure consisting of a triangular lattice of dimers. The remarkable structural feature of EtMe3Sb[Pd(dmit)2]2 is that it has a stacking direction of acceptor molecules, shown in Figure 1a, which is a characteristic factor in discussing the Pd(dmit)2 compounds.


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)

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.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC3104558&req=5

f1: 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.
Mentions: Among these spin-liquid materials, an organic compound of κ-(BEDT-TTF)2Cu2(CN)3 offer unique insights into frustration physics. It is a dimer-based molecular Mott insulator with a two-dimensional structure. The electronic properties are dominated by strong magnetic interactions, /J//kB, larger than 200 K7. In this compound, spin-liquid nature has been observed by 13C-NMR7, heat capacity14 and thermal transport15 measurements. Although the gapless feature of the low-energy excitations is reported by the heat capacity measurement14, thermal transport result implies a possibility of a tiny gap opening with an order of 0.5 K15. A possible relation with the superconductivity of conductive π-electrons and coupling with charge degrees of freedom in the dimer unit16 are stimulating extensive interest in frustration problems and electron correlations. Recently, an anion radial compound of EtMe3Sb[Pd(dmit)2]2 has been reported as another candidate of organic quantum spin-liquid material by magnetic susceptibility17, 13C-NMR1819 and thermal conductivity20 measurements. EtMe3Sb[Pd(dmit)2]2 consists of an anion of an acceptor molecule Pd(dmit)2 and a countercation EtMe3Sb+. Dimerized [Pd(dmit)2]2− anions form a segregated stacking structure in a crystal lattice with space group C2/c and a quasi-two-dimensional π-electron system is established in it21. Figure 1a shows the molecular structure of Pd(dmit)2 and crystal packing in the 2D plane. As the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is small, an electron spin localized on each dimer originates from the antibonding HOMOs22. The d-orbital of the Pd ion makes a negligible contribution to the HOMO and thus does not contribute to the spin density. By changing the central pnictogen atom and the numbers of methyl (Me) and ethyl (Et) groups in countercations, the ratio of the transfer integrals between the dimers (tr=) t′/t (=ts≈tB) shown in the inset of Figure 1a can be tuned from the antiferromagnetic (AF) region to the charge-ordered (CO) region in the phase diagram23. Tight-binding calculations based on the crystallographic data at room temperature suggest that the compound of EtMe3Sb[Pd(dmit)2]2 is located at t′/t=0.92, which implies that a nearly ideal triangle structure is realized1824. This compound is similar to κ-(BEDT-TTF)2Cu2(CN)3; they are Mott insulators with a layered structure consisting of a triangular lattice of dimers. The remarkable structural feature of EtMe3Sb[Pd(dmit)2]2 is that it has a stacking direction of acceptor molecules, shown in Figure 1a, which is a characteristic factor in discussing the Pd(dmit)2 compounds.

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