The layered compound α-RuCl3 is composed of a honeycomb lattice of magnetic Ru 3+ ions with the 4d 5 electronic state. We have investigated the magnetic properties of α-RuCl3 via magnetization and specific heat measurements using single crystals. It was observed that α-RuCl3 undergoes a structural phase transition at Tt ≃ 150 K accompanied by fairly large hysteresis. This structural phase transition is expected to be similar to that observed in closely related CrCl3. The magnetizations and magnetic susceptibilities are strongly anisotropic, which mainly arise from the anisotropic g-factors, i.e., g ab ≃ 2.5 and gc ≃ 0.4 for magnetic fields parallel and perpendicular to the ab plane, respectively. These g-factors and the obtained entropy indicate that the effective spin of Ru 3+ is one-half, which results from the low-spin state. Specific heat data show that magnetic ordering occurs in four steps at zero magnetic field. The successive magnetic phase transitions should be ascribed to the competition among exchange interactions. The magnetic phase diagram for H ab is obtained. We discuss the strongly anisotropic g-factors in α-RuCl3 and deduce that the exchange interaction is strongly XY-like. α-RuCl3 is magnetically described as a three-dimensionally coupled XY-like frustrated magnet on a honeycomb lattice.

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Determining ground states of correlated electron systems is fundamental to understanding novel phenomena in condensed matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global topology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states 1 , potentially prompting the emergence of quantum spin states, such as resonating valence bond (RVB) and valence bond solid (VBS). Although theoretically proposed to exist in a kagome lattice -one of the most highly frustrated lattices in two dimensions (2D) being comprised of corner-sharing triangles -such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the "pinwheel" VBS ground state in the S = 1 2 deformed kagome lattice antiferromagnet Rb 2 Cu 3 SnF 12 . In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.

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 ...

Background: Dipeptidyl peptidases (DPPs) are required for protein metabolism in Porphyromonas gingivalis. Results: Asp/Glu-specific novel DPP (DPP11) was discovered and characterized. Conclusion: DPP11 ensures efficient degradation of oligopeptide substrates in Gram-negative anaerobic rods. Significance: This observation suggests further variation of substrate specificity in the DPP members.

We report magnetocaloric and magnetic-torque evidence that in Cs2CuBr4--a geometrically frustrated Heisenberg S=1/2 triangular-lattice antiferromagnet--quantum fluctuations stabilize a series of spin states at simple increasing fractions of the saturation magnetization Ms. Only the first of these states--at M=1/3Ms--has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. We argue that the first-order nature of the transitions into those states is due to strong lowering of the energies by quantum fluctuations, with implications for the general character of quantum phase transitions in geometrically frustrated systems.

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Magnetization measurements were performed to investigate the critical behavior of the field-induced magnetic ordering in gapped spin system TlCuCl 3 . The critical density of the magnons was obtained as a function of temperature and the magnon-magnon interaction constant was evaluated. The experimental phase boundary for T < 5 K agrees almost perfectly with the magnon Bose-Einstein condensation (BEC) theory based on the Hartree-Fock approximation with realistic dispersion relations. The phase boundary can be described by the power law ½H N ðTÞ À H c / T . With decreasing fitting temperature range, the critical exponent decreases and converges at BEC ¼ 3=2 predicted by the magnon BEC theory.KEYWORDS: TlCuCl 3 , spin dimer system, magnons, quantum phase transition, Bose-Einstein condensation, critical behavior DOI: 10.1143/JPSJ.77.013701Quantum spin system composed of antiferromagnetic spin dimer often shows a gapped singlet ground state. In an external magnetic field exceeding the energy gap Á, S z ¼ 1 component of the spin triplet is created in the system. The field-induced S z ¼ 1 component has the characteristics of boson and is called magnon or triplon. Magnons move to neighboring dimers and interact with one another due to the transverse and longitudinal components of the interdimer exchange interactions, respectively. Consequently, the spin dimer system in the magnetic field can be represented as an interacting boson system. 1) Magnons can undergo BoseEinstein condensation (BEC) in a magnetic field higher than the critical field H c ¼ Á=g B , which leads to field-induced magnetic ordering (FIMO).2,3) Nikuni et al.2) discussed the FIMO observed in TlCuCl 3 , 4) applying the Hartree-Fock (HF) approximation to a simplified modelwhere " k is the kinetic energy determined by the curvature of the dispersion around the lowest excitation, the chemical potential given by ¼ g B ðH À H c Þ, U the interaction constant and N the number of dimers. If a parabolic isotropic dispersion relation " k ¼ ðh " kÞ 2 =2m is used, then the critical chemical potential is given byThis relation leads to the phase boundary described by the power lawwith critical exponent BEC ¼ 3=2, 2) where H N ðTÞ is the transition field at temperature T. A point given by T ¼ 0 and H ¼ H c on the temperature vs field diagram denotes the quantum critical point (QCP). Equation (2) or (3) gives the critical behavior near the QCP characteristic of the magnon BEC. The BEC of magnons has been studied extensively in many gapped spin systems [5][6][7][8][9][10][11][12] and the power law behavior of the phase boundary was confirmed.TlCuCl 3 is an S ¼ 1=2 interacting spin dimer system in which a chemical dimer Cu 2 Cl 6 forms a antiferromagnetic spin dimer. The interactions between neighboring dimers are three-dimensional. The lowest excitation occurs at Q ¼ ð0; 0; 1Þ and its equivalent reciprocal points. [13][14][15] The magnitude of the excitation gap is Á=k B ¼ 7:5 K. 4,16) In the previous magnetization and specific heat measurements in magnetic fields on Tl...