Dynamics of ethylene groups and hyperfine interactions between donor and anion molecules in $\lambda$-type organic conductors studied by $^{69,71}$Ga-NMR spectroscopy

We present the results of $^{69,71}$Ga-NMR measurements on an organic antiferromagnet $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$ [BEDSe-TTF=bis(ethylenediseleno)tetrathiafulvalene], with comparison to reports on $\lambda$-(BETS)$_2$GaCl$_4$ [BETS=bis(ethylenedithio)tetraselenafulvalene] [T. Kobayashi et al., Phys. Rev. B 102, 235131 (2020)]. We found that the dynamics of two crystallographically independent ethylene groups induce two types of quadrupolar relaxation in the high-temperature region. As the ethylene motion freezes, hyperfine (HF) interactions develop between $\pi$ spin and Ga nuclear spin below 100 K, and thereby magnetic fluctuations of the $\pi$-spin system are detected even from the Ga site. The HF interaction in $\lambda$-(BETS)$_2$GaCl$_4$ was more than twice as large as in $\lambda$-(BEDSe-TTF)$_2$GaCl$_4$, implying that the short contacts between Cl atoms and the chalcogens of fulvalene part are essential for the transferred HF interaction. We propose that NMR using nuclei in anion layers is useful for studying interlayer interactions in organic conductors, which have not been studied experimentally. In addition, because the mechanism of the transferred HF interaction is considered to be the same as $\pi$-$d$ interaction in isostructural Fe-containing $\lambda$-type salts, our findings aid in the understanding of their physical properties.


I. INTRODUCTION
Most ET-based organic conductors are regarded as quasi-two-dimensional (Q2D) electronic systems, as well as cuprate and iron-based superconductors, where ET denotes bis(ethylenedithio)tetrathiafulvalene. They often exhibit unconventional superconductivity in the vicinity of magnetically ordered phases [1]. To further understand these Q2D electronic systems, the role of the interlayer interaction has been discussed since long-range ordered states, such as superconductivity and magnetic ordering, require three-dimensional interaction. Electron spin resonance measurements in representative κtype ET salts suggest that the Fermi liquid and Mott insulating states of these materials are formed below the temperature at which interlayer electron hopping and exchange interaction develop, respectively [2,3]. The magnetic structures of two κ-type salts have recently been revealed. Although they had been considered to be located in the same antiferromagnetic (AF) phase, their magnetic structures were found to be different each other [4,5]. It is concluded that the difference in magnetic structure is due to difference in the sign of the interlayer magnetic interaction.
Interlayer interactions not only act between the conducting layers but also act between the conducting and insulating layers when magnetic ions are introduced into the insulating layer. λ-(BETS) 2 FeCl 4 (BETS represents bis(ethylenedithio)tetraselenafulvalene [ Fig. 1(a)]) exhibits a field-induced superconductivity above 17 T by the exchange interaction between the conductive * tkobayashi@phy.saitama-u.ac.jp π electron and the localized 3d spin of Fe 3+ [6][7][8], which is known as π-d interaction. This interaction gives rise to the metal-insulator transition accompanied by AF ordering at zero magnetic fields [9,10]. In addition, π-d interaction causes unusual multistep magnetization processes in λ-(STF) 2 FeCl 4 and λ-(BEDSe-TTF) 2 FeCl 4 [11,12], where STF and BEDSe-TTF (noted BEST in Ref. [12]) represent unsymmetricalbis(ethylenedithio)diselenadithiafulvalene and bis(ethylenediseleno)tetrathiafulvalene [ Fig.  1(a)], respectively. These behaviors suggest that interlayer interactions play a crucial role for the various phenomena appearing in Q2D organic systems. However, there are few experimental approaches to evaluate the strength or path of the interlayer interaction.
Meanwhile, in ET-based organic conductors, the dynamics of the ethylene end groups on donor molecules affect the electronic states. In κ-type organic conductors, it has been proposed that a bad metal state is realized at high temperatures because of its dynamics [13,14]. Particularly in κ-(ET) 2 Cu[N(CN) 2 ]I, the conformation of the ethylene groups changes the ground state [15,16]. Moreover, hydrogen bonding between protons of the ethylene groups and the anion has been discussed [17,18]; therefore, the relationships between the dynamics of ethylene groups and the interlayer interactions should be addressed.
served. We suggested that this molecular motion could be induced by the ethylene motion; however, the study from a structural viewpoint is needed to prove this. At low temperatures where the molecular motion freezes, magnetic fluctuation of the electronic system was observed via the transferred hyperfine (HF) interaction between the π spins and Ga nuclei. This fact suggests that anion NMR is useful for evaluating interlayer interactions. While λ-type salts exhibit physical properties derived from interlayer interactions as seen in λ-D 2 FeCl 4 (D: donor molecule), their physical properties change by donor molecular substitutions due to the electron correlation effect. In λ-D 2 GaCl 4 , D = ET and BEDSe-TTF salts are antiferromagnetic insulators [20,22]. D = STF salt is a paramagnetic insulator without long-range ordering [23] and exhibits superconductivity under pressure [24]. While D = BETS salt exhibits semiconducting behavior above 100 K, it becomes a metal and a superconductor below ∼ 6 K [25]. These properties can be understood by the universal phase diagram shown in Fig. 1 (b) [20,21]. Note that D = BETS and other salts exhibit the same semiconducting behavior near room temperature, but the electrical resistivities of them significantly differ at room temperature, e.g., 0.03 Ω cm for D = BETS [26] and 10 Ω cm for D = BEDSe-TTF [27]. The difference in U/W can explain their electrical conductivity [20], where U and W are on-site Coulomb repulsion and bandwidth, respectively. How molecular substitutions change the interlayer interactions along with the electronic correlations is also important for a comprehensive understanding of λ-type salts, including λ-D 2 FeCl 4 . To investigate this, 69,71 Ga NMR is an effective probe because it can observe the magnetism of λ-D 2 GaCl 4 from the same Ga site regardless of donor molecule.
In this study, we report 69,71 Ga-NMR measurements on a AF insulator λ-(BEDSe-TTF) 2 GaCl 4 (hereafter λ-BEDSe) and compare the HF coupling constant of λ-BEDSe with that of a superconductor λ-BETS. We can discuss the path of the interlayer interaction since S and Se atoms in the BETS molecule are exchanged in the BEDSe-TTF molecule, as shown in Fig. 1(a). In addition, x-ray diffraction measurements were conducted to discuss the relationship between the quadrupolar relaxation at high temperatures and the ethylene motion.

II. EXPERIMENTS
Single crystals of λ-BEDSe were synthesized electrochemically [20]. NMR measurements of 69 Ga (nuclear spin I = 3 2 , gyromagnetic ratio 69 γ/2π = 10.219 MHz/T, nuclear quadrupole moment 69 Q = 0.171 barns) and 71 Ga (I = 3 2 , 71 γ/2π = 12.984 MHz/T, 71 Q = 0.107 barns) were performed on the central ( 1 2 ↔ − 1 2 ) transition under the magnetic fields of 6.083 T. We used a moderately crushed polycrystalline sample of about 20 mg, which was randomly oriented with respect to the magnetic field. The spectra were obtained via fast Fourier transformation of the echo signal with a π 2 -π pulse sequence, where the π 2 pulse length was typically 3 µs. The spin-lattice relaxation time, T 1 , was measured using the conventional saturation-recovery method. Single-crystal x-ray diffraction data of λ-BEDSe were collected using a Bruker SMART APEX II ULTRA diffractometer with Mo-Kα radiation (λ = 0.71073Å) at the Comprehensive Analysis Center for Science, Saitama University. The crystal structures were solved and refined by SHELXT [28] and SHELXL [29], respectively.

III. RESULTS AND DISCUSSION
A. Spectra Figure 2 shows 69,71 Ga-NMR spectra of λ-BEDSe and λ-BETS [19] at 210 K originating from the central ( 1 2 ↔ − 1 2 ) transition. The spectra with I = 3 2 can be described by the nuclear spin Hamiltonian as follows: where n = 69, 71. H Z is the Zeeman interaction: and H are the reduced Planck constant and external magnetic field, respectively. H Q is the quadrupolar interaction: η and ω Q denotes the asymmetry parameter of electric field gradient (EFG) and the nuclear quadrupolar frequency, respectively. n ω Q is defined as n ω Q = e n QV ZZ / 2 , where e and V ZZ are the elementary charge and principal axis of the EFG, respectively. The EFG at the Ga site is almost zero because the Ga nucleus in these materials is tetrahedrally coordinated by four Cl − ions. Thus,  Table I. H Q is sufficiently smaller than H Z . Two peaks were observed in 69,71 Ga-NMR spectra of λ-BETS, and the spectral shapes are determined by the powder pattern due to the second-order perturbation effect when H Q is treated as a perturbation to H Z [19,30]. The slightly distorted tetrahedral coordination of GaCl − 4 causes the finite EFG at the Ga site.
Although the Ga site of λ-BEDSe is in the same situation as that of λ-BETS, spectral splitting due to secondorder perturbation was observed only in 69 Ga NMR, implying that the EFG at the Ga site in λ-BEDSe is smaller than that in λ-BETS because the splitting interval is proportional to ω 2 Q [31]. To discuss the difference in the spectra of the two salts quantitatively, we carried out numerical simulation to reproduce them, as shown by the dashed lines in Fig. 2. 69,71 Ga-NMR spectra of each salt can be reasonably reproduced by the parameters shown in Table I, where n ω Q ∝ n Q. In terms of the results of λ-BETS, the parameters are consistent with those obtained from simulation of the overall 69 Ga-NMR spec- trum including satellite peaks at 80 K [19]. When the results of both salts are compared, n ω Q of λ-BEDSe is smaller than that of λ-BETS, resulting in no splitting in the 71 Ga-NMR spectra with smaller Q. Reference [32] suggested that the EFG tensor at the tetrahedrally coordinated central atom is sensitive to the arrangement of the surrounding ions. The difference in n ω Q between λ-BETS and λ-BEDSe is only ∼ 20 % despite the sensitivity to Cl − ion arrangement, suggesting that the GaCl − 4 in both salts is almost in the same environment. Figure 3 shows the temperature evolution of 69,71 Ga-NMR spectra of λ-BEDSe. Whereas the spectral shapes are almost unchanged above 40 K, significant line broadening was observed below 20 K. This behavior is understood as the development of the internal magnetic field due to the AF ordering of the π-spin system at T N = 22 K [20]. This observation suggests that 69,71 Ga-NMR spectra can detect the static magnetic properties of the π spin system of λ-BEDSe, although the Ga site is far from the π spins.
Gaussian-shaped spectra were observed below 20 K, whereas the spectral shapes above 40 K are dominated by the powder pattern due to the second-order perturbation. To phenomenologically evaluate the linewidth, we estimated the square root of the second moment:  increase sharply below approximately 40 K owing to the magnetic fluctuation of the π spins. In addition, 69 f 2nd 1/2 is larger than 71 f 2nd 1/2 at high temperatures; however, the relationship is reversed at low temperatures. This indicates that the second moment is dominated by the quadrupolar interaction at high temperatures ( 69 Q > 71 Q) and the magnetic interaction at low temperatures ( 71 γ > 69 γ). The inset of Fig. 4 shows that the isotopic ratio of f 2nd 1/2 below 18 K is almost identical to 71 γ/ 69 γ.
In 69,71 Ga-NMR spectral measurements, we observed spectra with the typical shapes generated by the finite EFG at the Ga site in the high-temperature region as well as λ-BETS and confirmed the line broadening due to the AF ordering of the π spin system despite the Ga site being far from the π spins. Thus, the spectral study enable us to discuss the static properties of both the EFG and magnetic field at the Ga site.
B. Dynamics of molecular motion in high-temperature region T 1 measurements allow us to evaluate the magnetic and EFG fluctuations and to distinguish which is dominant using the difference in the properties of the two isotopes. The recovery curves for the central ( 1 2 ↔ − 1 2 ) transition were fitted using the following function; 1 − where M (t) is the nuclear magnetization at time t after the saturation, M (∞) is the nuclear magnetization at equilibrium (t → ∞), and β is the stretch exponent. The recovery curves above 100 K were well fitted by the function with β = 1, whereas β decreased towards T N at of λ-BETS [19]. Solid lines are the fitting curves by Eq. (2) with the fitting parameters listed in Table II.  (2). (12) 353(8) 215(4) 348(4) 71 ω 2 Q,i 1/2 /2π (kHz) 176 (7) 220 (5) 134 (2) (12) 0.98 (14) low temperatures as discussed in Sec. III C. Figure 5 shows the temperature dependence of 69,71 T −1 1 (T −1 1 of 69,71 Ga NMR) of λ-BEDSe above 100 K. 69 T −1 1 is greater than 71 T −1 1 in the high-temperature range and the ratio corresponds to the ratio of Q 2 , indicating that the quadrupolar relaxation mechanism is dominant. Above 150 K, 69,71 T −1 1 strongly depend on temperature and exhibit shoulder-like anomalies at around 230 K. Similar behavior was also observed in 69,71 T −1 1 of λ-BETS (inset of Fig. 5) [19], which was explained by the Bloembergen-Purcell-Pound (BPP) formula [33]. We conducted the same analysis with slight modification [34]  well, including the reported results of λ-BETS [19]. The obtained fitting parameters for both salts shown in Table II are similar each other, indicating that the quadrupolar relaxation in the high-temperature region can be explained by the same mechanism. This mechanism has been interpreted as the translational motion of GaCl − 4 ions, resulting in the EFG fluctuation at the Ga site [19]. In addition, the nearly identical E A,i and τ 0,i between λ-BETS and λ-BEDSe show that the donor molecular substitution effect on the time scale of the molecular motion is negligible.
In Eq.
(2), we assumed two components of the BPP formula (i = 1, 2), as shown in Table II. The components of i = 1 and 2 correspond to the shoulder-like structures at around 230 K and the further enhancement above 250 K of 69,71 T −1 1 (the latter corresponds to the peak at 300 K in 69,71 T −1 1 of λ-BETS shown in the inset of Fig. 5 [19]), respectively. These results indicate that there are two types of molecular motion with different parameters. Moreover, the translational motion of GaCl − 4 can be induced by the conformational motion of ethylene groups near anion layers, as mentioned in the previous 69,71 Ga-NMR study on λ-BETS [19]. To further clarify the relationship between the observed 69,71 T −1 1 and the ethylene motion, x-ray structural analysis was performed at room temperature (293 K) and 110 K [37]. Figure 6 shows the crystallographically inequivalent molecules I and II extracted from the crystal structure of λ-BEDSe. At room temperature, the ethylene groups at one end of both molecules I and II are disordered, occupying staggered and eclipsed conformations with a ratio of occupancy 0.636(6) : 0.364(6) (molecule I) and 0.131(11) : 0.869(11) (molecule II). At 110 K, molecule I still has disordered sites with the occupancy of 0.797(5) : 0.203(5), whereas molecule II is completely ordered in the eclipsed conformation. Therefore, the ethylene motion of molecule I that remains disordered at lower temperatures is assigned to the i = 1 component in Eq.
(2), contributing to quadrupolar relaxation on the low-temperature side, and that of molecule II is assigned to i = 2 component. This relationship between the two components in the quadrupolar relaxation and the ethylene groups on the two crystallographically independent molecules is considered to hold also in λ-BETS [19].
In addition to the fact that λ-BETS has two types of the ethylene motion [19], the x-ray structural analyses of λ-(BEDSe-TTF) 2 FeCl 4 show that the ethylene groups at the same sites are disordered [12], implying that a similar degree of ethylene motion commonly exists in the λ-type salts. These results will aid our understanding of the mechanisms that cause π-d spin correlations and dielectric anomalies that develop at low temperatures where ethylene motion freezes, as reported for λ-(BETS) 2 FeCl 4 [38, 39].

C. Hyperfine interaction between Ga nuclear spin
and π spin  can be attributed to the magnetic transition since the line broadening around T N is also magnetic, as shown in Fig. 4. In addition, the stretch exponent, β, depends on temperatures below 100 K, as shown in Fig. 8(a). With a decrease in temperature, β decreases from 1 to a minimum at T N = 22 K. β approaches 1 as the temperature further decreases.
The isotopic ratio of T −1 1 , 71 T −1 1 / 69 T −1 1 , also confirms that the low-temperature relaxation is due to magnetic fluctuations. Figure 8 The observed magnetic fluctuation originates from the spin fluctuation of the π spins in the donor layer since the Ga ion is nonmagnetic. The temperature dependence of T −1 1 of 13 C NMR, 13 T −1 1 , is also plotted in Fig. 7 for comparison [20]. Both 69,71 T −1 1 and 13 T −1 1 qualitatively have the same temperature dependence. Thus, the magnetic fluctuation of the π-spin system was probed by 69,71 Ga NMR in λ-BEDSe as well as in λ-BETS [19]. These magnetic fluctuations were observed by the suppression of EFG fluctuations as the ethylene motion freezes.
When the magnetic relaxation is due to the spin fluctuation of an electronic system, T −1 1 is generally written as follows [40]: where γ e is the gyromagnetic ratio of the electron, n A q is the wave vector q-dependent HF coupling constant, and χ ′′ (q) is the imaginary part of the dynamic susceptibility. Ga sites feel the transferred HF and dipole fields from Because the external field in the present experiment is larger than the spin-flop field as expected from the typical ET-based antiferromagnet [42], the line broadening of 69,71 Ga NMR is dominated by the dipole field. As can be seen in 71 f 2nd 1/2 (Fig. 4), the line broadening by AF ordering is ∼ 14 kHz, which corresponds to 11 Oe. This is consistent with the calculated value. Next, we compare the calculated HF interaction from the dipole field with experiments. For simplicity, the following discussion uses the results for 71 T −1 1 with larger γ. In 71 T −1 1 and 13 T −1 1 , the q χ ′′ (q) part in Eq. (3) should be the same since the same magnetic fluctuation appeared. Therefore, (4) Figure 9 shows 71 T −1 1 / 13 T −1 1 below 100 K in the paramagnetic state, where the magnetic fluctuations dominate. As depicted by the dashed line, an average value (2). 13 A is approximately ∼ 5 kOe/µ B , referring to the λ-BETS results [43]. Applying the same magnitude of 13 A to λ-BEDSe (details below) and neglecting the q dependence of the HF coupling constant, the observed HF coupling constant of 71 Ga NMR is estimated to be 5 kOe/µ B × 0.11 = 550 Oe/µ B . This is an order of magnitude larger than the value of the dipole field, indicating that the transferred HF interaction is the dominant as the HF mechanism.
We now discuss the magnitude of the transferred HF interaction for λ-BETS and λ-BEDSe from the ratio of the values of Eq. (4). All the data used in the following discussion were measured at the same condition, and values other than the HF coupling constant cancel out. We obtained the following equation: where the donor molecules are indicated as subscripts.
The donor molecular substitution effect on 13 A was ignored in the latter approximation since λ-D 2 GaCl 4 are isostructural and the local environments around 13 C nucleus are similar. Actually, 13 A STF and 13 A BETS are of the same order [23,43], where 13 A STF is the HF coupling constant in λ-(STF) 2 GaCl 4 . Therefore, Eq. (5) allows us to compare the magnitude of the HF interaction between λ-BETS and λ-BEDSe from T −1 1 measurements.
As shown in Fig. 9, the average values were obtained as ( 71 T −1 1 / 13 T −1 1 ) BETS = 0.098 (9) and ( 71 T −1 1 / 13 T −1 1 ) BEDSe = 0.015 (5), resulting in 71 A BETS / 71 A BEDSe = 2.6(4). This result implies that the magnitude of the transferred HF interaction in λ-BETS is more than two times larger than that in λ-BEDSe. The difference in the magnitudes between λ-BETS and λ-BEDSe suggests that the proton-Cl contacts are not dominant for the transferred HF interaction.
The variation in physical properties due to the donor molecular substitution in λ-D 2 GaCl 4 has been discussed in terms of chemical pressure effect [21], with λ-BEDSe compound located at the most negative position on the pressure axis ( Fig. 1(b)) [20]. Our findings indicate that replacing BETS or STF molecules with BEDSe-TTF molecule causes negative-pressure effect on not only intralayer but also interlayer interactions, which may be necessary for a comprehensive understanding of their physical properties.
Some Se/S-Cl contacts shorter than van der Waals distances (Se-Cl: 3.7Å, S-Cl: 3.6Å [44]) exist in λ-BETS [20,26] and λ-BEDSe, as shown in Fig. 10, which significantly contribute to the transferred HF interaction. The short Se-Cl contact would contribute more to the interaction than the S-Cl contact since the atomic radius of Se is larger than that of S. Although the number of short Se-Cl contacts in λ-BEDSe is greater than that in λ-BETS with an extremely short Se-Cl contact of d 2 = 3.2601(8)Å, this study demonstrated that the transferred HF interaction of λ-BEDSe is smaller than that of λ-BETS. Therefore, the paths through the inner chalcogens and Cl are more essential for the transferred HF interaction. The inner chalcogens in the ET molecule have larger coefficients of the highest occupied molecular orbital proportional to the electron density than the outer chalcogens [45], which could be why the paths through the inner chalcogen and Cl are crucial. As discussed above, the coupling constants between π electrons and the Ga nuclear spins were obtained by combining the results of 69,71 Ga-NMR and 13 C-NMR measurements. This enables us to explain the coupling between the conduction layers via the anion layer although such a discussion has been difficult until now. In addition to the difference in the electronic states between λ-BEDSe and λ-BETS, this study reveals that the interlayer interaction is smaller in λ-BEDSe. Thus, we demonstrate that anion NMR experiment is a powerful method for investigating the interlayer interactions. The contribution of such interactions has also been suggested for κ-type organic antiferromagnets. Since the κ-(ET) 2 Cu[N(CN) 2 ]Cl and deuterated κ-(ET) 2 Cu[N(CN) 2 ]Br salts are isostructural, the same magnetic structure is expected from the anisotropy resulting from dipole interactions. However, magnetization measurements and numerical simulations have recently revealed that the magnetic structures are different between them [4,5]. In organic magnets, which are similar to the Heisenberg spin systems, interlayer coupling could be particularly important for three-dimensional order. Anion NMR will be helpful for clarifying this issue and also the development of interlayer coupling upon cooling [2,3].
Finally, we discuss the relationship between the transferred HF interaction in λ-D 2 GaCl 4 and the π-d interaction in λ-D 2 FeCl 4 . The transferred HF interaction is between the π spin and the Ga nuclear spin, whereas the π-d interaction is between the π spin and the 3d spin of Fe 3+ ion. This study experimentally revealed the importance of the path through the inner chalcogen and Cl for the transferred HF interactions. Mori and Katsuhara also reported that the interaction via short Se-Cl contacts has the largest contribution to the π-d interaction in λ-(BETS) 2 FeCl 4 from the calculation of the intermolecular overlap integrals [46]. Therefore, the transferred HF and π-d interactions can be attributed to the same mechanism. The reported λ-D 2 FeCl 4 salts are D = BETS, STF, and BEDSe-TTF, which are antiferromagnets with T N = 8.5, 16, and 25 K, respectively [10][11][12]. Among them, λ-(BETS) 2 FeCl 4 is expected to have the largest π-d interaction, and λ-(BEDSe-TTF) 2 FeCl 4 is expected to have the smallest π-d interaction due to the weak interaction between inner S and Cl [12]. Magnetic susceptibility of λ-(BEDSe-TTF) 2 FeCl 4 does not show an anisotropy [12] as well as that of λ-(BETS) 2 FeBr x Cl 4−x for 0.3 < x < 0.5 [47], whereas those of λ-(BETS) 2 FeCl 4 and λ-(STF) 2 FeCl 4 show an anisotropy below 8 K [10,48]. Akutsu et al. claimed that the π-d interaction in λ-(BETS) 2 FeBr x Cl 4−x for 0.3 < x < 0.5 is weaker than that in λ-(BETS) 2 FeCl 4 [47]. Thus, λ-(BEDSe-TTF) 2 FeCl 4 has been referred to as a weaker π-d system, which is consistent with our findings.

IV. SUMMARY
We performed 69,71 Ga-NMR and x-ray diffraction measurements on an organic antiferromagnet, λ-BEDSe. The spectral measurements revealed a powder pattern due to finite EFG at high temperatures and a spectral broadening due to AF ordering at low temperatures, indicating that the static nature of EFG and magnetic fields can be detected even by NMR at Ga sites far from the electron system. In the T 1 measurements, we observed the quadrupolar relaxation described by the BPP formula with two components in the high-temperature re-gion. By x-ray structural analysis, it was revealed that the two components were assigned to the two crystallographically independent ethylene groups. In the lowtemperature region where the ethylene motion freezes, the magnetic fluctuation of the π-spin system was observed through the transferred HF interaction between the π spin and the Ga nuclear spin, as in the case of λ-BETS. We discovered that λ-BETS had a lager transferred HF interaction than λ-BEDSe. This finding enables us to conclude that the short contacts between the chalcogens of the fulvalene part of the donor molecules and the Cl of anion molecules are the most significant contributors to this interaction. The transferred HF interaction in λ-D 2 GaCl 4 and the π-d interaction in the isostructural λ-D 2 FeCl 4 can be derived from the same path. Therefore, we also evaluated the π-d interaction in the λ-type π-d system. Present study reveals that anion NMR measurement is an effective tool for studying the interlayer interaction for which experimental approaches are limited in Q2D organic conductors. [41] Assuming that there is a paramagnetic moment of 1µB/dimer, which is distributed to each atom by the square of the highest occupied molecular orbital coefficient of BEDSe-TTF [20], dipole field D at the Ga site can be calculated from D = i (µ/r 3 i −(µ·ri)ri/r 5 i ), with ri ≡ Ri − R0 and ri ≡ |ri|. Here, µ is the distributed moment on each atom of the BEDSe-TTF molecule, Ri and R0 are position vectors of dipole moment on each atom and Ga atom, respectively. As a result, the dipole field tensor for an arbitrary moment direction derived from BEDSe-TTF molecules in the range of ±10 unit cells is,