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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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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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f5: 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.

Mentions: Although the liquid-like states of the two organic salts exhibit similar thermodynamic properties, their phase diagrams surrounding the spin-liquid phase are different. Here, we discuss the difference in the γ terms of pristine and deuterated samples of EtMe3Sb[Pd(dmit)2]2 in terms of the electron-phase diagram. Figure 5 shows a schematic electronic phase diagram of the Pd(dmit)2 system, which reveals that the spin-liquid phase occurs in a narrow region. The AF phase and CO phase are both suppressed around the spin-liquid phase23. The AF phase is suppressed due to the increase in the frustration effects denoted by t′/t. The point at which TN is reduced to 0 is defined as a quantum critical point between spin-solid and liquid states. As mentioned above, the pristine EtMe3Sb[Pd(dmit)2]2 is located in the spin-liquid state with t′/t=0.92. Deuteration of the Me groups slightly reduces the cation size and moves the electronic state from the spin-liquid region toward the AF region, as Me4Sb[Pd(dmit)2]2, which has a smaller cation, is located in the AF phase17. A possible explanation of the enhancement in the γ term is that the deuterated compound is located very close to the critical point, which causes a critical divergence in its thermodynamic properties. In heavy-electron systems produced by strong correlations of f-electrons in Ce and U compounds, anomalous divergence of CpT−1 expressed by the −log T term is known as a non-Fermi liquid behaviour at the quantum critical point33, where the AF phase is suppressed to TN=0 and an anomalous Fermi liquid phase appears. A similar kind of criticality might be expected in the critical region of a spin-liquid phase. Precise tuning of electronic properties by chemical pressures generated by partial substitution of deuterons in molecules has been achieved in the (DMe-DCNQI)2Cu system34; this should also be possible by controlling the cations in Pd(dmit)2 systems. Another possible explanation of this γ enhancement is related to the nodal structure produced in the low-energy excitations in the gap. If a finite density of states at zero energy was detected as the γ term in thermodynamic experiments, like in d-wave superconductivity, as is suggested by a theoretical study by Grover et al.35 and the NMR experiments32, it might be sensitive to slight changes in the chemical pressure in the spin-liquid phase. A further possible explanation is a change in the disorder potential produced by the low symmetries of cations. Although these possibilities remain speculative, the low-energy excitations in this organic spin-liquid state contain important implications for electron correlation problems peculiar to Mott-insulator triangular systems. In contrast to this complicated situation in the Pd(dmit)2 system, the phase diagram around the spin-liquid phase of κ-(BEDT-TTF)2Cu2(CN)3 is relatively simple and it retains a spin-liquid ground state up to 0.36 GPa as reported by Kurosaki et al.36 Above this pressure, the spin-liquid phase undergoes a first-order transition and a superconductive phase appears36. The deuterated sample of κ-(d8; BEDT-TTF)2Cu2(CN)3 shows very similar thermodynamic behaviour as the hydrogenated samples, which is in fine contrast with the EtMe3Sb[Pd(dmit)2]2 system29. The presence of a quantum criticality in the spin-liquid state may have interesting implications for organic triangular 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)

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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f5: 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.
Mentions: Although the liquid-like states of the two organic salts exhibit similar thermodynamic properties, their phase diagrams surrounding the spin-liquid phase are different. Here, we discuss the difference in the γ terms of pristine and deuterated samples of EtMe3Sb[Pd(dmit)2]2 in terms of the electron-phase diagram. Figure 5 shows a schematic electronic phase diagram of the Pd(dmit)2 system, which reveals that the spin-liquid phase occurs in a narrow region. The AF phase and CO phase are both suppressed around the spin-liquid phase23. The AF phase is suppressed due to the increase in the frustration effects denoted by t′/t. The point at which TN is reduced to 0 is defined as a quantum critical point between spin-solid and liquid states. As mentioned above, the pristine EtMe3Sb[Pd(dmit)2]2 is located in the spin-liquid state with t′/t=0.92. Deuteration of the Me groups slightly reduces the cation size and moves the electronic state from the spin-liquid region toward the AF region, as Me4Sb[Pd(dmit)2]2, which has a smaller cation, is located in the AF phase17. A possible explanation of the enhancement in the γ term is that the deuterated compound is located very close to the critical point, which causes a critical divergence in its thermodynamic properties. In heavy-electron systems produced by strong correlations of f-electrons in Ce and U compounds, anomalous divergence of CpT−1 expressed by the −log T term is known as a non-Fermi liquid behaviour at the quantum critical point33, where the AF phase is suppressed to TN=0 and an anomalous Fermi liquid phase appears. A similar kind of criticality might be expected in the critical region of a spin-liquid phase. Precise tuning of electronic properties by chemical pressures generated by partial substitution of deuterons in molecules has been achieved in the (DMe-DCNQI)2Cu system34; this should also be possible by controlling the cations in Pd(dmit)2 systems. Another possible explanation of this γ enhancement is related to the nodal structure produced in the low-energy excitations in the gap. If a finite density of states at zero energy was detected as the γ term in thermodynamic experiments, like in d-wave superconductivity, as is suggested by a theoretical study by Grover et al.35 and the NMR experiments32, it might be sensitive to slight changes in the chemical pressure in the spin-liquid phase. A further possible explanation is a change in the disorder potential produced by the low symmetries of cations. Although these possibilities remain speculative, the low-energy excitations in this organic spin-liquid state contain important implications for electron correlation problems peculiar to Mott-insulator triangular systems. In contrast to this complicated situation in the Pd(dmit)2 system, the phase diagram around the spin-liquid phase of κ-(BEDT-TTF)2Cu2(CN)3 is relatively simple and it retains a spin-liquid ground state up to 0.36 GPa as reported by Kurosaki et al.36 Above this pressure, the spin-liquid phase undergoes a first-order transition and a superconductive phase appears36. The deuterated sample of κ-(d8; BEDT-TTF)2Cu2(CN)3 shows very similar thermodynamic behaviour as the hydrogenated samples, which is in fine contrast with the EtMe3Sb[Pd(dmit)2]2 system29. The presence of a quantum criticality in the spin-liquid state may have interesting implications for organic triangular 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