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Register a new userBoundary-limited and glassy-like phonon thermal conduction in EtMe$_3$Sb[Pd(dmit)$_2$]$_2$
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Minoru Yamashita Dr.
Publication date
2019
fields
Physics
and research's language is
English
Authors
Minoru Yamashita
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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, $kappa_0/Tequivkappa/T (T rightarrow 0)$, has been observed and attributed to the presence of itinerant gapless excitations. Here we show that the data of $kappa$ measured in several single crystals are divided into two groups with and without the residual linear term. In the first group with finite $kappa_0/T$, the phonon thermal conductivity $kappa_{ph}$ is comparable to that of other organic compounds. In these crystals, the phonon mean free path $ell_{ph}$ saturates at low temperatures, being limited by sample size. On the other hand, in the second group with zero $kappa_0/T$, $kappa_{ph}$ is one order of magnitude smaller than that in the first group, comparable to that of amorphous solids. In contrast to the first group, $ell_{ph}$ shows a glassy-like non-saturating behavior at low temperatures. These results suggest that the crystals with long $ell_{ph}$ are required to discuss the magnetic excitations by thermal conductivity measurements.
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EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, an organic Mott insulator with nearly isotropic triangular lattice, is a candidate material for a quantum spin liquid, in which the zero-point fluctuations do not allow the spins to order. The itinerant gapless excitations inferred from the thermal transport measurements in this system have been a hotly debated issue recently. While the presence of a finite linear residual thermal conductivity, $kappa_0/T equiv kappa/T (T rightarrow 0)$, has been shown [M. Yamashita {it et al.} Science {bf 328}, 1246 (2010)], recent experiments [P. Bourgeois-Hope {it et al.}, Phys. Rev. X {bf 9}, 041051 (2019); J. M. Ni {it et al.}, Phys. Rev. Lett. {bf 123}, 247204 (2019)] have reported the absence of $kappa_0/T$. Here we show that the low-temperature thermal conductivity strongly depends on the cooling process of the sample. When cooling down very slowly, a sizable $kappa_0/T$ is observed. In contrast, when cooling down rapidly, $kappa_0/T$ vanishes and, in addition, the phonon thermal conductivity is strongly suppressed. These results suggest that possible random scatterers introduced during the cooling process are responsible for the apparent discrepancy of the thermal conductivity data in this organic system. The present results provide evidence that the true ground state of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is likely to be a quantum spin liquid with itinerant gapless excitations.
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 $beta$-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 ones, are the same order, leading to hybridization between different molecular orbitals. This results in charge disproportionation within each molecule, as seen in our previous ab initio study [T. Tsumuraya et al, J. Phys. Soc. Jpn. 82, 033709 (2013)], and also to a revised picture of an effective dimer model. Furthermore, we discuss broken-symmetry insulating states triggered by interaction effects, which show characteristic features owing to the multi-orbital nature. The on-site Coulomb interaction induces antiferromagnetic states with intramolecular antiparallel spin pattern, while electron-lattice couplings stabilize non-magnetic charge-lattice ordered states where two kinds of dimers with different charge occupation arrange periodically. These states showing different spatial patterns compete with each other as well as with the paramagnetic metallic state.
Motivated by the rich interplay among electronic correlation, spin-orbit coupling (SOC), crystal-field splitting, and geometric frustrations in the honeycomb-like lattice, we systematically investigated the electronic and magnetic properties of Li$_2$RhO$_3$. The material is semiconducting with a narrow band gap of $Deltasim$78 meV, and its temperature dependence of resistivity conforms to 3D variable range hopping mechanism. No long-range magnetic ordering was found down to 0.5 K, due to the geometric frustrations. Instead, single atomic spin-glass behavior below the spin-freezing temperature ($sim$6 K) was observed and its spin dynamics obeys the universal critical slowing down scaling law. First principle calculations suggested it to be a relativistic Mott insulator mediated by both electronic correlation and SOC. With moderate strength of electronic correlation and SOC, our results shed new light to the research of Heisenberg-Kitaev model in realistic materials.
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Structural, magnetization and heat capacity studies were performed on Ce$_2$(Pd$_{1-x}$Ni$_x$)$_2$Sn ($0 leq x leq 1$) alloys. The substitution of Pd atoms by isoelectronic Ni leads to a change in the crystallographic structure from tetragonal (for $x leq 0.3$) to centered orthorhombic lattice (for $x geq 0.4$). The volume contraction thorough the series is comparable to the expected from the atomic size ratio between transition metal components. The consequent weak increase of the Kondo temperature drives the two transitions observed in Ce$_2$Pd$_2$Sn to merge at $x = 0.25$. After about a 1% of volume collapse at the structural modification, the system behaves as a weakly magnetic heavy fermion with an enhanced degenerate ground state. Notably, an incipient magnetic transition arises on the Ni-rich size. This unexpected behavior is discussed in terms of an enhancement of the density of states driven by the increase of the $4f$-conduction band hybridization and the incipient contribution of the first excited crystal field doublet on the ground state properties.
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