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Register a new userEmergence of charge degrees of freedom under high pressure in an organic dimer-Mott insulator ${beta}^{prime}$-(BEDT-TTF)$_2$ICl$_2$
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Added by
Kenichiro Hashimoto
Publication date
2015
fields
Physics
and research's language is
English
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To elucidate the pressure evolution of the electronic structure in an antiferromagnetic dimer-Mott (DM) insulator ${beta}^{prime}$-(BEDT-TTF)$_2$ICl$_2$, which exhibits superconductivity at 14.2 K under 8 GPa, we measured the polarized infrared (IR) optical spectra under high pressure. At ambient pressure, two characteristic bands due to intra- and interdimer charge transfers have been observed in the IR spectra, supporting that this salt is a typical half-filled DM insulator at ambient pressure. With increasing pressure, however, the intradimer charge transfer excitation shifts to much lower energies, indicating that the effective electronic state changes from half-filled to 3/4-filled as a result of weakening of dimerization. This implies that the system approaches a charge-ordered state under high pressure, in which charge degrees of freedom emerge as an important factor. The present results suggest that charge fluctuation inside of dimers plays an important role in the high-temperature superconductivity.
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We investigated the electronic states of the quasi-one-dimensional organic conductors $delta_{P}$-(BPDT-TTF)$_2$ICl$_2$ and $delta_{C}$-(BPDT-TTF)$_2$ICl$_2$, both of which are insulating at room temperature owing to strong electron correlations. Through measurements of electrical resistivity, optical conductivity, and magnetic susceptibility, as well as band-structure calculations, we have revealed that the two materials possess completely different ground states, even though they have the same chemical composition and stacking configuration of the donor molecules. We have found that the $delta_P$-type salt with an effective half-filled band behaves as a dimer-Mott (DM) insulator and exhibits a nonmagnetic transition at 25 K, whereas the $delta_C$-type salt with a 3/4-filled band shows a charge ordering (CO) transition just above room temperature and becomes nonmagnetic below 20 K. The optical spectra of the $delta_P$-type salt are composed of two characteristic bands due to intra- and interdimer charge transfers, supporting the DM insulating behavior arising from the strong on-site Coulomb interaction. By contrast, in the $delta_C$-type salt, a single band characterizing the formation of CO arising from the off-site Coulomb interactions is observed. Upon lowering the temperature, the shape of the optical spectra in the $delta_C$-type salt becomes asymmetric and shifts to much lower frequencies, suggesting the emergence of domain-wall excitations with fractional charges expected in a one-dimensional CO chain. The temperature dependence of the magnetic susceptibility of the $delta_P$-type salt is well described by a 2D spin-1/2 Heisenberg AFM model on an anisotropic square lattice in the dimerized picture, while in the $delta_C$-type salt, it can be explained by a 2D spin-1/2 Heisenberg AFM model on an anisotropic honeycomb lattice formed in the CO state.
We investigated the infrared optical spectra of an organic dimer Mott insulator $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl, which was irradiated with X-rays. We observed that the irradiation caused a large spectral weight transfer from the mid-infrared region, where interband transitions in the dimer and Mott-Hubbard bands take place, to a Drude part in a low-energy region; this caused the Mott gap to collapse. The increase of the Drude part indicates a carrier doping into the Mott insulator due to irradiation defects. The strong redistribution of the spectral weight demonstrates that the organic Mott insulator is very close to the phase border of the bandwidth-controlled Mott transition.
Inelastic neutron scattering measurements on the molecular dimer-Mott insulator $kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl reveal a phonon anomaly in a wide temperature range. Starting from $T_{rm ins}sim50$-$60$ K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature $T_mathrm{N} = 27$ K, where also a ferroelectric ordering at $T_{rm FE} approx T_{rm N}$ occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below $T_mathrm{N}$, while no change of the lattice symmetry is observed across $T_mathrm{N}$ in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.
The density of states of the organic superconductor $kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, measured by scanning tunneling spectroscopy on textit{in-situ} cleaved surfaces, reveals a logarithmic suppression near the Fermi edge persisting above the critical temperature $T_mathrm{c}$. A soft Hubbard gap as predicted by the Anderson-Hubbard model for systems with disorder exactly describes the experimentally observed suppression. The electronic disorder also explains the diminished coherence peaks of the quasiparticle density of states below $T_mathrm{c}$.
The organic charge-transfer salt $kappa$-(BEDT-TTF)$_{2}$Hg(SCN)$_{2}$Br is a quasi two-dimensional metal with a half-filled conduction band at ambient conditions. When cooled below $T=80$ K it undergoes a pronounced transition to an insulating phase where the resistivity increases many orders of magnitude. In order to elucidate the nature of this metal-insulator transition we have performed comprehensive transport, dielectric and optical investigations. The findings are compared with other dimerized $kappa$-(BEDT-TTF) salts, in particular the Cl-analogue, where a charge-order transition takes place at $T_{rm CO}=30$ K.
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