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We prepared a new two-dimensional oxyantimonide, BaTi 2 Sb 2 O, which shows a superconducting transition at 1.2 K, representing the first superconductivity in a system with Ti 3þ (d 1 ) in a square lattice. The TiO 2 Sb 4 mixed anionic coordination stabilizes a unique half-filled Ti d xy orbital configuration in Ti 2 O plane, which is analogous to Cu 2þ (d 9 ) in the high-T c superconductors. A charge density wave (CDW)-or spin density wave (SDW)-like anomaly appears at 50 K, which is significantly reduced compared with 200 K for the isostructural and non-superconducting BaTi 2 As 2 O.Since the discovery of high-T c superconductivity in cuprates, 1) there has been a longstanding quest to find novel superconductors. Although several classes of materials such as MgB 2 , iron pnictides, and fullerides show high T c 's, 2-4) the cuprates still hold the highest T c record. Yet, the mechanism for its occurrence is unclear and still under debate despite intense investigation. To clarify the mechanism of high-T c superconductivity, there have been a variety of attempts to find a novel superconductor that is isostructural and isoelectric with the high-T c cuprates. One plausible approach is carrier doping into a perovskite oxide with a 3d 1 electron configuration, such as AE 2 V 4þ O 4 (AE = Sr, Ba). Here, the electronic configuration is complementary with respect to the 3d 9 cuprates; although La 2 Cu 2þ O 4 has one hole per Cu 2þ , AE 2 V 4þ O 4 has one electron per V 4þ . However, as will be discussed later, this view can be seen as an oversimplified description, since it neglects orbital degeneracy derived from the octahedral crystal field around the transition metal. In fact, experimentally, carrier-doped AE 2 V 4þ O 4 does not show superconductivity, but shows metallic conductivity. 5,6) As shown in Fig. 1, the titanium oxypnictides Na 2 Ti 3þ 2 Pn 2 O (Pn = As, Sb) 7) and La 2 CuO 4 are somewhat similar in structure. Na 2 Ti 2 Pn 2 O has a Ti 2 O square net that adopts the anticonfiguration to the CuO 2 square net in La 2 CuO 4 . In this net, Ti 3þ (3d 1 ) is coordinated octahedrally by two oxide anions and four pnictide anions, and these TiO 2 Pn 4 octahedra share edges to form the square lattice. BaTi 2 As 2 O [ Fig. 1(c)], where the two sodium cations have been replaced with one barium cation, has the same square lattice framework and has recently been reported. 8) The mixed anionic coordination of TiO 2 Pn 4 and the octahedral connectivity in the ab plane, as shown in Fig. 1(f ), provide a unique opportunity for the t 2g orbitals to split to a greater extent (relative to pure oxide coordination), owing to the anions having different valences, electronegativities, and ionic radii.Unfortunately, none of these compounds show superconductivity. 9,10) Interestingly, the susceptibility and resistivity showed an anomaly at T a ¼ 330 K for Na 2 Ti 2 As 2 O, 120 K for Na 2 Ti 2 Sb 2 O, and 200 K for BaTi 2 As 2 O, which is ascribed to a CDW or SDW (CDW/SDW) transition. Given that CDW/SDW instabilities are also commonly ...

Nanoscale molecular rotors that can be driven in the solid state have been realized in Cs2([18]crown-6)3[Ni(dmit)2]2 crystals. To provide interactions between the molecular motion of the rotor and the electronic system, [Ni(dmit)2]- ions, which bear one S=1/2 spin on each molecule, were introduced into the crystal. Rotation of the [18]crown-6 molecules within a Cs2([18]crown-6)3 supramolecule above 220 K was confirmed using X-ray diffraction, NMR, and specific heat measurements. Strong correlations were observed between the magnetic behavior of the [Ni(dmit)2]- ions and molecular rotation. Furthermore, braking of the molecular rotation within the crystal was achieved by the application of hydrostatic pressure.

We have succeeded in synthesizing single crystals of a new organic radical 3-Cl-4-F-V [3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl]. Through the ab initio molecular orbital calculation and the analysis of the magnetic properties, this compound was confirmed to be the first experimental realization of an S=1/2 spin-ladder system with ferromagnetic leg interactions. The field-temperature phase diagram indicated that the ground state is situated very close to the quantum critical point. Furthermore, we found an unexpected field-induced successive phase transition, which possibly originates from the interplay of low dimensionality and frustration.

1H-imidazol-1-oxyl.͖ PNNNO and PIMNO crystallize to form quasi-onedimensional lattices, but F 2 PNNNO to form a quasi-two-dimensional lattice. The temperature dependences of the susceptibility and the high-field magnetization process up to 34 T were measured down to 0.5 K. The results are analyzed by comparing with the theoretical calculations based on the crystal structures. PNNNO and PIMNO are considered to be antiferromagnetic Heisenberg spin chains consisting of Sϭ1/2 spin pairs ͑dimers͒ in which the two spins are coupled ferromagnetically. At low temperatures, an antiferromagnetic ordering occurs in these crystals, which is confirmed by the thermodynamic discussion through specific heat measurements. On the other hand, F 2 PNNNO is thought to be a two-dimensional Heisenberg system, in which the spin pairs are connected by two types of antiferromagnetic interactions. The ground state is singlet. The high-field magnetization process shows a two-step saturation with a plateau of the half value of the saturation magnetization. ͓S0163-1829͑99͒00342-2͔