Low Temperature Crystal Structures and Band Parameters inβ'-(Me4-xEtxZ)[Pd(dmit)2]2
Kohei Ueda1, Takao Tsumuraya1,2 and Reizo Kato1
12Condensed Molecular Materials Laboratory, RIKEN

Computational Materials Science Unit, National Institute for Materials Science

β'-EtMe3Sb[Pd(dmit2)]2 (Pd(dmit)2: Figure 1) forms a spin-1/2 isosceles triangular lattice of the dimerunit [Pd(dmit2)]2, characterized with J = 200-300 K. A quantum spin liquid (QSL) state is realized as amagnetic ground state in the EtMe3Sb salt. Many experimental and theoretical studies were performed toreveal the nature of QSL realized in the EtMe3Sb salt [1-3]. The triangular lattice in this system is not a resultof crystal symmetry, but of the combination of interdimer transfer integrals, tS, tB and tr, (tS ~ tB = t, tr = t' inFigure 2) [1]. The series of β'-Me4-xEtxZ salts are isostructural with different anisotropy parameters t'/t. The
magnetic ground state of these salts can be divided into three types, QSL, anti-ferromagnetic long-rangeorder (AFLO) and charge order (CO). The difference in the ground state is understood by the anisotropy ofisosceles triangular lattice. The geometric frustration of the EtMe3Sb salt, realizing the QSL state, is strong(t'/t = 0.91 at room temperature), but the triangular lattice of the EtMe3Sb salt is slightly distorted from theregular triangular lattice. The QSL state is reported in several molecular conductors with slightly distorted
triangular lattices, including the EtMe3Sb salt [5-6]. The reason
why the QSL is observed in the distorted triangular lattice systeminmolecularconductors

remains

anopenquestion.

Intermolecular lengths and molecular arrangements of molecularFigure 1. Pd(dmit)2 molecule.

crystals are easily changed with temperature. The values of tS, tB
and tr at the temperature where the QSL state is realized should
be different from those at room temperature due to the thermalcontraction. In order to understand the nature of the QSL state,knowledges of the low temperature crystal structure andelectronic structure need to be examined in detail. We
determined crystal structures of the EtMe3Sb salt (QSL), andMe4Sb, Et2Me2As and Me4As salts (TN = 12-35K) from room
temperature to 5 K. Then we calculated electric structures withtight-binding approximation method and the first-principle DFT

Figure 2. Interdimer transfer integrals.

method at each temperature based on experimentally obtained
crystal structures [7].
Figure 3 shows temperature dependence of tS, tB and tr in the EtMe3Sb salt. They linearly increased withlowering temperature. Since tS and tB increased keeping the same ratio and tr slightly increased, theanisotropy of the triangular lattice was enhanced with lowering temperature. The situation is the same inother salts. At 5 K, the ratio t'/t decreased by 15% compared with the room temperature value in every salt
(Fig. 4).

The intradimer transfer integral TA is approximately related to the effective on-site Coulomb

energy on the dimer [Pd(dmit)2]2. Therefore, the ratio TA/W (W: band width) shows the strength of thecorrelation effect. With lowering temperature, W increased more rapidly than TA. The TA/W was decreasedby 10% at 5 K from the room temperature value, that is, the correlation effect decreased with loweringtemperature. Due to the tight dimerization, the intradimer S···S distances show smaller changes than theinterdimer S···S distances. Therefore, the temperature dependence of TA/W should be the result of thedifference between intra- and interdimer thermal contraction. This is common feature of all β’-Pd(dmit)2salts.
We performed the first-principles DFT calculations. The anisotropy of the Fermi surface is enhanced withlowering temperature, which is very similar to the result of the tight-binding calculation. This temperaturedependence is consistent with the enhancement of the anisotropy of triangular lattice. The conducting bandis composed of the anti-bonding HOMO and is isolated from other bands at each temperature. This indicatesthat the dimer model using only HOMO-HOMO interactions can well describe the low-temperature electric
structure of the EtMe3Sb salts. The partial density of states (PDOS) for the conduction band indicates that themajority of the conduction band is composed of the p-orbital in S. Although some short S···HC contacts
exist between cation and Pd(dmit)2 molecule, the contribution of C atoms in the EtMe3Sb cation to the PDOS
is less than 0.015 /eV and thus the cation orbitals cannot hybridize to the conduction band. The arch-shaped
distortion of the Pd(dmit)2 molecule plays a key role in the cation dependence of the t'/t value. It wasrevealed that the short S···HC contacts controls the arch-shaped distortion, and the resulting steric effects
have an impact on the electric structure.

2.01.040

Et2Me2As

Me4Sb0.9EtMe3SbtB

Et2Me2As

0.8ts

30

TA / W

EtMe3Sbt' / tt / meV

35

0.7

25

200100

Me4Sb

Me4As

1.6Me4As

tr

1.8

0.6200

300

0.5
0200

300

T/K

T/K
Figure 3. Temperature dependence ofinter dimer transfer integrals in theEtMe3Sb salt.

100

1.4
0100

200

300

T/K

Figure 4. Temperature dependence of t'/t Figure 5. Temperature dependence ofin the EtMe3Sb, Me4Sb, Et2Me2As and TA/W in the EtMe3Sb, Me4Sb, Et2Me2As
and Me4As salts.
Me4As salts.

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