Masaaki Isobe

Orbital switching in a frustrated magnet

The orbital is one of the four fundamental degrees of freedom in a solid, besides spin, charge and lattice. In transition metal compounds, it is usually the d orbitals which play deciding roles in determining the crystallographic and physical properties. Here we report the discovery of a unique structural transition in single crystals of the spin-1/2 quasi-kagomé antiferromagnet volborthite, Cu(3)V(2)O(7)(OH)(2)·2H(2)O, whereby the unpaired electron switches from one d orbital to another upon cooling. This is not a conventional orbital order-disorder transition, but rather an orbital switching that has not previously been observed. The structural transition is found to profoundly affect the magnetic properties of volborthite, because magnetic interactions between Cu spins in the kagomé lattice are considerably modified by the orbital switching. This finding provides us with an interesting example to illustrate the intimate interplay between the orbital degree of freedom and competing magnetic interactions in a frustrated magnet.

The electronic structure of the high-symmetry perovskite iridate Ba2IrO4

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba2IrO4 (Ba-214), an antiferromagnetic (TNxa0=xa0230xa0K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO6 octahedra that is present in its sister compound Sr2IrO4 (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin–orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above TN, and favor a Mott over a Slater scenario for this compound.

β-Vesignieite BaCu3V2O8(OH)2: a structurally perfect S = 1/2 kagomé antiferromagnet

A novel copper mineral β-vesignieite BaCu3V2O8(OH)2 with trigonal symmetry has been synthesized for the first time using a hydrothermal technique. β-Vesignieite is an ideal spin-1/2 kagome antiferromagnet. Antiferromagnetic long-range order is found at 9 K, probably due to a finite Dzyaloshinski–Moriya interaction caused by the kagome symmetry.

Bulk nature of layered perovskite iridates beyond the Mott scenario: An approach from a bulk-sensitive photoemission study

We present genuine bulk Ir 5d jeff states of layered perovskite iridates obtained by hard-x-ray photoemission spectroscopy (HAXPES) with s- and p-polarized lights. HAXPES spectra of Sr2IrO4 and Ba2IrO4 are well reproduced by the quasi-particle densities of states calculated by the local density approximation with dynamical mean-field theory (LDA+DMFT). It is demonstrated that the insulating nature of the iridates is triggered by antiferromagnetic correlation (Slater type) combined with electron correlation (Mott type). The extremely-low-energy bulk-sensitive photoemission spectroscopy reveals bad metallic states in the paramagnetic phase of the iridates, suggesting strongly renormalized metallic states above the Neel temperature as predicted by the LDA+DMFT.

Partially disordered state and spin-lattice coupling in an S=3/2 triangular lattice antiferromagnet Ag2CrO2

Ag{sub 2}CrO{sub 2} is an S = 3/2 frustrated triangular lattice antiferromagnet without an orbital degree of freedom. With decreasing temperature, a four-sublattice spin state develops. However, a long-range partially disordered state with five sublattices abruptly appears at T{sub N} = 24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic.

Two-Magnon Scattering in Spin–Orbital Mott Insulator Ba2IrO4

A spin–orbit induced Mott insulator Ba2IrO4 with the pseudo-spin Jeff = 1/2, showing an antiferromagnetic order (TN = 240 K), has been investigated by Raman spectroscopy. A broad peak with the B1g symmetry is found in a wide temperature region up to 400 K, which is ascribed to the two-magnon scattering. From the peak position and width, the exchange coupling and the antiferromagnetic correlation length are estimated to be 590 cm−1 and 45 A at 90 K, respectively. The results are compared with the antiferromagnet La2CuO4 with the spin S = 1/2. We conclude that there is no significant difference in the short wavelength spin-excitation between the S = 1/2 and Jeff = 1/2 systems.

Spin-Orbit Mott State in the Novel Quasi-2D Antiferromagnet Ba2IrO4

The spin-orbit Mott state in the novel quasi-2D iridate Ba2IrO4 was studied on crystal structure, electrical resistivity, magnetic susceptibility, and μSR experiments. Ba2IrO4 crystallizes in a K2NiF4-type structure (I4/mmm, a = 4.030(1) A and c = 13.333(4) A) which includes IrO2 square planer lattices with straight Ir-O-Ir bonds. The minimal Mott-gap size is ~70 meV The magnetic ground state is antiferromagnetic long-range order (TN ~ 240 K) without spin canting. The magnetic moment (|μ| ~ 0.34 μB/Ir) is much reduced by a low-dimensional quantum spin fluctuation with a large intra-plane correlation |J|. The critical exponent (β ~ 0.18) suggests that the magnetic state is classified into 2-D X-Y or (anisotropic) Heisenberg spin systems with weak 3-D interlayer coupling |J|.