A conducting partially oxidized salt of TPP[Fe(Pc)(CN) 2 ] 2 , which contains paramagnetic iron(III), has been obtained by electrocrystallization of TPP[Fe(Pc)(CN) 2 ]. The crystal is composed of one-dimensional TPP arrays surrounded by slipped-stacked one-dimensional Fe(Pc)(CN) 2 chains. This structure is isomorphous with TPP[Co(Pc)(CN) 2 ] 2 , in which the central metal is non-magnetic cobalt(III). The electrical conductivity at room temperature, ca. 10 V 21 cm 21 , is about one order of magnitude lower than that of the Co analogue. On lowering the temperature, the conductivity shows a much steeper decline compared with the temperature dependence of the conductivity of TPP[Co(Pc)(CN) 2 ] 2 . The band width, estimated from both thermoelectric power measurements (which show apparent metallic behavior at high temperatures) and an extended Hu È ckel calculation, is found to be the same as that obtained for TPP[Co(Pc)(CN) 2 ] 2 . This is consistent with observations from the single-crystal re¯ectance spectra, in which both partially oxidized salts show almost the same plasma edge. On the other hand, the magnetic susceptibility of TPP[Fe(Pc)(CN) 2 ] 2 is quite different from that of TPP[Co(Pc)(CN) 2 ] 2 ; both of the magnetic moments originated from Fe III and the p-radical seem to be localized in TPP[Fe(Pc)(CN) 2 ] 2 , while the moments originated from the p-radical in TPP[Co(Pc)(CN) 2 ] 2 are interacting with each other to lead to temperature-independent Pauli-like susceptibility. At low temperatures, a decrease in the susceptibility has been observed, suggesting the existence of antiferromagnetic interactions.

We find a striking peak structure in the magnetoresistance of the quasi-two-dimensional conductors ␤-͑BEDT-TTF͒ 2 I 3 and ␣-͑BEDT-TTF͒ 2 NH 4 Hg͑SCN͒ 4 for magnetic fields nearly parallel to the conducting plane. The peak structure can be ascribed to a Fermi-surface topological effect due to the small closed orbits on the side of the warped cylindrical Fermi surface. This effect provides a clue about how to evaluate the interlayer transfer integral. ͓S0163-1829͑98͒06503-5͔

We have found giant negative magnetoresistance in the one-dimensional conductor TPP͓Fe(Pc)(CN) 2 ͔ 2 below 50 K. The reduction of the resistance is larger in the field perpendicular to the one-dimensional axis than parallel to the axis. The magnetic susceptibility shows anisotropic Curie-Weiss behavior. The experimental results suggest the interaction between the one-dimensional electron system and the local moments. The reduction of the spin scattering of the itinerant electrons by the local moments under the field is proposed as the origin of the giant negative magnetoresistance.The interplay between conduction electrons and local magnetic moments has provided a variety of interesting phenomena such as Kondo effect, Ruderman-Kittel-KasuyaYosida interaction, heavy fermion, giant magnetoresistance, and colossal magnetoresistance. 1-5 The giant negative magnetoresistance was reported in the manganese oxides 5 and -BETS 2 FeCl 4 , 6 and is accompanied or caused by the sharp metal-insulator transition.There have been very few reports on these phenomena in the one-dimensional system, although theoretical studies predict several attractive phenomena such as Kondo effects in a Luttinger liquid. 3,7,8 Quasi-one-dimensional molecular conductors are good candidates for this research. By introducing the local moments (d electron͒ into these conductors, the interaction (-d interaction͒ between the one-dimensional conduction electrons ( electron͒ and the local moments is expected to give different electronic states. In fact, interesting phenomena such as negative magnetoresistance were reported in the one-dimensional molecular conductor ͓Cu x Ni 1Ϫx (Pc)͔(I 3 ) 1/3 , 9,10 where electron in Pc ͑ϭphthalocyanine͒ forms the conduction band and d electron in Cu 2ϩ affords the local moment. However, owing to the experimental difficulty, the systematic studies on the anisotropy of this system have not been reported. The negative magnetoresistance was also reported in the one-dimensional Peierls compounds, 11 and the critical temperature of the negative magnetoresistance decreases on increasing the magnetic-field strength.The title compound TPP͓Fe(Pc)(CN) 2 ͔ 2 and the isostructural compound TPP͓Co(Pc)(CN) 2 ͔ 2 ͑TPPϭtetraphenyl-phosphonium͒ have a one-dimensional conduction band coming from the orbital of Pc, since the partially oxidized Pc units stack uniformly along the c axis. The Fe compound has Sϭ1/2 local moments of Fe 3ϩ in the conduction path, 12 while there are no local moments in the Co compound. 13 TPP molecules and CN ligands hold closed-shell orbitals. The striking difference in the resistance behavior of these compounds was reported. 12,13 This difference suggests the possibility of the interaction between the one-dimensional conduction electrons and the local moments.The Pc molecule has a structure with fourfold symmetry and one can expect the degeneracy in the molecular orbitals of Pc. It is of great interest how this degeneracy influences the physical properties.In order to obtain the clear-cut experimental evidenc...

Lifshitz transition, a change in Fermi surface topology, is likely to greatly influence exotic correlated phenomena in solids, such as high-temperature superconductivity and complex magnetism.However, since the observation of Fermi surfaces is generally difficult in the strongly correlated systems, a direct link between the Lifshitz transition and quantum phenomena has been elusive so far. Here, we report a marked impact of the pressure-induced Lifshitz transition on thermoelectric performance for SnSe, a promising thermoelectric material without strong electron correlation. By applying pressure up to 1.6 GPa, we have observed a large enhancement of thermoelectric power factor by more than 100% over a wide temperature range (10-300 K). Furthermore, the high carrier mobility enables the detection of quantum oscillations of resistivity, revealing the emergence of new Fermi pockets at ∼0.86 GPa. The observed thermoelectric properties linked to the multi-valley band structure are quantitatively reproduced by first-principles calculations, providing novel insight into designing the SnSe-related materials for potential valleytronic as well as thermoelectric applications.

The metal-insulator transition has been investigated for pyrochlore molybdates R(2)Mo(2)O(7) with nonmagnetic rare-earth ions R. The dynamical scaling analysis of ac susceptibility reveals that the geometrical frustration causes the atomic spin-glass state. The reentrant spin-glass phase exists below the ferromagnetic transition. The electronic specific heat is enhanced as compared to the band calculation result, perhaps due to the orbital fluctuation in the half-metallic ferromagnetic state. The large specific heat is rather reduced upon the transition, likely because the short-range antiferromagnetic fluctuation shrinks the Fermi surface.