The electronic structure of an isostructural series of molecular conductors, β'-X[Pd(dmit)_2]_2 is systematically studied by a first-principles method based on the density-functional theory. The calculated band structures are fitted to the tight-binding model based on Pd(dmit)_2 dimers on the triangular lattice. We find systematic variation in the anisotropy of the transfer integrals along the three directions of the triangular lattice taking different values. The transfer integral along the face-to-face stacking direction of Pd(dmit)_2 dimers is always the largest. Around the quantum spin liquid, X = EtMe_3Sb, the other two transfer integrals become comparable. We also report sensible differences in the distribution of wavefunctions near the Fermi level between the two dmit ligands of the Pd(dmit)_2 molecule.
Nuclear spin-lattice (1/T1) and spin-spin (1/T2) relaxation rates of the cation sites of a quantum spin-liquid candidate b'-EtMe3Sb[Pd(dmit)2]2 and its deuterated sample are presented. The enhanced 1/T1 of 1H and 2D are well analyzed considering the rotations of methyl- and ethyl-groups of the cation with the activation energies of 200K and 1200K respectively. The 1/T1 and 1/T2 at the Sb site that is located on the 2-fold rotation axis remain active down to the lowest temperature with an algebraic temperature dependence of the correlation time as has been observed in the ac response of the die…
Organic charge transfer salts based on the molecule Pd(dmit)2 display strong electronic correlations and geometrical frustration, leading to spin liquid, valence bond solid, and superconducting states, amongst other interesting phases. The low energy electronic degrees of freedom of these materials are often described by a single band model; a triangular lattice with a molecular orbital representing a Pd(dmit)2 dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low energy effective model Hamiltonians for a class of Me(4-n) EtnX[Pd(dmit)2]2 (X=As…
We systematically derive low-energy effective Hamiltonians for molecular solids $β^\prime$-$X$[Pd(dmit)$_{2}$]$_{2}$ ($X$ represents a cation) using ab initio density functional theory calculations and clarify how the cation controls the inter-dimer transfer integrals and the interaction parameters. The effective models are solved using the exact diagonalization method and the antiferromagnetic ordered moment is shown to be significantly suppressed around the spin-liquid candidate of $X$=EtMe$_{3}$Sb, which is reported in experiments. We also show that both the geometrical frustration and the …
We show that the electronic structures of the title compounds predicted by density functional theory (DFT) are well described by tight binding models. We determine the frustration ratio, J'/J, of the Heisenberg model on the anisotropic triangular lattice, which describes the spin degrees of freedom in the Mott insulating phase for a range of Pd(dmit)2 salts. All of the antiferromagnetic materials studied have J'/J < 0.5 or J'/J > 0.9, consistent with predictions for the Heisenberg model. All salts with 0.5 < J'/J < 0.9, where many-body theories find a number of competing ground states, are kno…
We parameterize Hubbard and spin models for EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ from broken symmetry density functional calculations. This gives a scalene triangular model where the largest net exchange interaction is three times larger than the mean interchain coupling. The chain random phase approximation shows that the difference in the interchain couplings is equivalent to a bipartite interchain coupling, favoring long-range magnetic order. This competes with ring exchange, which favors quantum disorder. Ring exchange wins. We predict that the thermal conductivity, $κ$, along the chain direction …
Electronic properties of quasi-two-dimensional molecular conductors $X$[Pd(dmit)$_2$]$_2$ are studied theoretically. We construct an effective model based on the fragment molecular orbital scheme developed recently, which can describe the multi-orbital degree of freedom in this system. The tight-binding parameters for a series of $β'$-type compounds with different cations $X$ are evaluated by fitting to first-principles band calculations. We find that the transfer integrals within the dimers of Pd(dmit)$_2$ molecules, along the intramolecular and intermolecular bonds including the diagonal one…
The Mott insulator $β'$-EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ belongs to a class of charge transfer solids with highly-frustrated triangular lattice of $S=1/2$ molecular dimers and a quantum-spin-liquid ground state. Our experimental and ab initio theoretical studies show the fingerprints of strong correlations and disorder, important role of cation-dimer bonding in charge redistribution, no sign of intra- and inter-dimer dipoles, and the decisive van der Waals contribution to inter-dimer interactions and the ground state structure. The latter consists of quasi-degenerate electronic states related to t…
The molecular solids $β^\prime$-$X$[Pd(dmit)$_2$]$_2$ (where $X$ represents a cation) are typical compounds whose electronic structures are described by single-orbital Hubbard-type Hamiltonians with geometrical frustration. Using the $ab$ $initio$ downfolding method, we derive the low-energy effective Hamiltonians for $β^\prime$-$X$[Pd(dmit)$_2$]$_2$ with available room- and low-temperature structures. We find that the amplitudes of the Coulomb interactions and the anisotropy of the hopping parameters in the effective Hamiltonians are sensitive to the changes in the lattice constants induced b…
In molecular-based quantum-spin-liquid candidate EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ with two-dimensional $S$=1/2 triangular lattice, a finite residual linear term in the thermal conductivity, $κ_0/T\equivκ/T (T \rightarrow 0)$, has been observed and attributed to the presence of itinerant gapless excitations. Here we show that the data of $κ$ measured in several single crystals are divided into two groups with and without the residual linear term. In the first group with finite $κ_0/T$, the phonon thermal conductivity $κ_{ph}$ is comparable to that of other organic compounds. In these crystals, the …
The behavior in magnetic field of a paramagnetic center is characterized by its $g$ tensor. An anisotropic form of the latter implies different kind of response along different crystallographic directions. Here we shed light on the anisotropy of the $g$ tensor of Yb$^{3+}$ 4$f^{13}$ ions in NaYbS$_2$ and NaYbO$_2$, layered triangular-lattice materials suggested to host spin-liquid ground states. Using quantum chemical calculations we show that, even if the ligand-cage trigonal distortions are significant in these compounds, the crucial role in realizing strongly anisotropic, `noncubic' $g$ fac…
The organic charge-transfer salt EtMe$_3$P[Pd(dmit)$_2$]$_2$ is a quasi-two-dimensional Mott insulator with localized spins $S$ = 1/2 residing on a distorted triangular lattice. Here we report measurements of the uniaxial thermal expansion coefficients $α_i$ along the in-plane $i$ = $a$- and $c$-axis as well as along the out-of-plane $b$-axis for temperatures 1.4\,K $\leq $T$ \leq$ 200\,K. Particular attention is paid to the lattice effects around the phase transition at $T_{VBS}$ = 25\,K into a low-temperature valence-bond-solid phase and the paramagnetic regime above where effects of short-r…
Using large-scale dynamical cluster quantum Monte Carlo simulations, we explore the unconventional superconductivity in the hole-doped Hubbard model on the triangular lattice. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, we find a doubly degenerate singlet pairing state at an interaction strength close to the bare bandwidth. Such an unconventional superconducting state is mediated by antiferromagnetic spin fluctuations along the $Γ$-$K$ direction, where the Fermi surface is nested. An exact decomposition of the irreducible particle-particl…
We study the effect of Dzyaloshinskii-Moriya (DM) interaction on the triangular lattice $U(1)$ quantum spin liquid (QSL) which is stabilized by ring-exchange interactions. A weak DM interaction introduces a staggered flux to the $U(1)$ QSL state and changes the density of states at the spinon fermi surface. If the DM vector contains in-plane components, then the spinons gain nonzero Berry phase. The resultant thermal conductances $κ_{xx}$ and $κ_{xy}$ qualitatively agree with the experimental results on the material EtMe$_3$Sb[Pd(dmit)$_2]_2$. Furthermore, owing to perfect nesting of the fermi…
Motivated by Mo$_3$S$_7$(dmit)$_3$, we investigate the Hubbard model on the triangular necklace lattice at two-thirds filling. We show, using second order perturbation theory, that in the molecular limit, the ground state and the low energy excitations of this model are identical to those of the spin-one Heisenberg chain. The latter model is known to be in the symmetry protected topological Haldane phase. Away from this limit we show, on the basis of symmetry arguments and density matrix renormalization group (DMRG) calculations, that the low-energy physics of the Hubbard model on the triangul…
Based on dynamical cluster approximation (DCA) quantum Monte Carlo simulations, we study the interaction-driven Mott metal-insulator transition (MIT) in the half-filled Hubbard model on the anisotropic two-dimensional triangular lattice, where the degree of frustration is varied between the unfrustrated case and the fully frustrated, isotropic triangular lattice. Upon increasing the DCA cluster size, we analyze the evolution of the MIT phase boundary as a function of frustration in the phase diagram spanned by the interaction strength and temperature, and provide a quantitative description of …
Linear spin wave theory shows that ring exchange induces a quantum disordered region in the phase diagram of the title model. Spin wave spectra show that this is a direct manifestation of competing classical orders. A spin liquid is found in the `Goldilocks zone' of frustration, where the quantum fluctuations are large enough to cause strong competition between different classical orderings but not strong enough to stabilize spiral order. We note that the spin liquid phases of $κ$-(BEDT-TTF)${_2}X$ and $Y$[Pd(dmit)$_2$]$_2$ are found in this Goldilocks zone.
Motivated by experiments on non-magnetic triangular-lattice Mott insulators, we study one candidate paramagnetic phase, the columnar dimer (or valence-bond) phase. We apply variants of the bond-operator theory to a dimerized and spatially anisotropic spin-1/2 Heisenberg model and determine its zero-temperature phase diagram and the spectrum of elementary triplet excitations (triplons). Depending on model parameters, we find that the minimum of the triplon energy is located at either a commensurate or an incommensurate wavevector. Condensation of triplons at this commensurate-incommensurate tra…
Inspired by the recent theoretical suggestion that the random-bond $S=1/2$ antiferromagnetic Heisenberg model on the triangular and the kagome lattices might exhibit a randomness-induced quantum spin liquid (QSL) behavior when the strength of the randomness exceeds a critical value, and that this "random-singlet state" might be relevant to the QSL behaviors experimentally observed in triangular organic salts $κ{\rm -(ET)_2 Cu_2 (CN)_3}$and ${\rm EtMe_3 Sb[Pd(dmit)_2]_2}$ and in kagome herbertsmithite ${\rm CuZn_3(OH)_6Cl_2}$, we further investigate the nature of the static and the dynamical sp…
Experimental quest for the hypothetical "quantum spin liquid" state has recently met a few promising candidate materials including organic salts κ-(ET)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, S=1/2 triangular-lattice Heisenberg antiferromagnets consisting of molecular dimers. These compounds exhibit no magnetic ordering nor the spin freezing down to very low temperature, while various physical quantities exhibit gapless behaviors. Recent dielectric measurements revealed the glassy dielectric response suggesting the random freezing of the electric polarization degrees of freedom. Inspired by this obs…
This review summarizes more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last two decades. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the fir…
LiZn$_2$Mo$_3$O$_8$ is an electrically insulating geometrically frustrated antiferromagnet in which inorganic Mo$_3$O$_{13}$ clusters each behaves as a single $S = 1/2$ unit, with the clusters arranged on a two-dimensional triangular lattice. Prior results have shown that LiZn$_2$Mo$_3$O$_8$ does not exhibit static magnetic order down to at least $T = 0.05\,K$, and instead possesses a valence bond ground state. Here, we show that LiZn$_2$Mo$_3$O$_8$ can be hole doped by oxidation with $\mathrm{I}_2$ and subsequent removal of $\mathrm{Zn}^{2+}$ cations to access the entire range of electron cou…
The Johns Hopkins University（约翰斯·霍普金斯大学）；Institute for Quantum Matter, William H. Miller III Department of Physics and Astronomy（量子物质研究所，威廉·H·米勒三世物理与天文系）；Quantum Matter Institute, University of British Columbia（不列颠哥伦比亚大学量子物质研究所）；Neutron Scattering Division, Oak Ridge National Laboratory（橡树岭国家实验室中子散射部）；Department of Physics & Astronomy, University of British Columbia（不列颠哥伦比亚大学物理与天文系）；Centre for Molecular and Materials Science, TRIUMF（TRIUMF分子与材料科学中心）
