Taro Kitano

Spin-Singlet Ground State in Two-Dimensional S=1/2 Frustrated Square Lattice: (CuCl)LaNb2O7

We report on the magnetic properties of double-layered perovskite (CuCl)LaNb 2 O 7 with a square lattice of S =1/2 Cu 2+ ions, prepared by topotactic ion-exchange reactions. Magnetic susceptibility exhibits a thermal-activated behavior, and the analysis of the data suggests competing ferro- and antiferromagnetic exchanges for first- and second-nearest-neighbor bonds, respectively. Neutron scattering experiments provide explicit evidence for a spin-singlet ground state with an energy gap of 2.3 meV. The well-defined low-lying triplet mode is almost independent of Q despite densely packed magnetic ions in the CuCl plane. In addition, on the basis of the isolated dimer model, we found that the intradimer distance is as long as 8.8 A, which corresponds to the fourth-nearest-neighbor bond. These results indicate a sizable geometrical frustration that should impose severe constraints on the propagation of the triplet excitation, as in the case of SrCu 2 (BO 3 ) 2 . This argument is further reinforced by the obs...

Collinear order in frustrated quantum antiferromagnet on square lattice (CuBr)LaNb2O7

Magnetic susceptibility, heat capacity, high-field magnetization and neutron diffraction measurements have been performed on a two-dimensional S ¼ 1=2 square-lattice system (CuBr)LaNb2O7, prepared by a topotactic ion-exchange reaction of a nonmagnetic double-layered perovskite RbLaNb2O7. (CuBr)LaNb2O7 exhibits a second-order magnetic transition at 32 K, in marked contrast to a spin-singlet nature for its Cl-based counterpart (CuCl)LaNb2O7, despite nearly identical structural parameters. The magnetic structure is a novel collinear antiferromagnetic (CAF) ordering characterized by a modulation vector q ¼ð �; 0 ;� Þ with a reduced moment of 0.6� B. Mixed ferromagnetic nearest-neighbor (J1) and antiferromagnetic second-nearest-neighbor (J2) interactions are of comparable strength (J1=kB ¼ � 35:6 K and J2=kB ¼ 41:3 K), placing the system in a more frustrated region of the CAF phase than ever reported.

Anomalous Magnetization of Two-Dimensional S ¼ 1=2 Frustrated Square-Lattice Antiferromagnet (CuCl)LaNb2O7

High-field magnetization measurements have been performed up to 56 T for a two-dimensional S = 1/2 frustrated square-lattice antiferromagnet (CuCl)LaNb 2 O 7 , a recently discovered spin gap system (Δ/ k B = 26.7 K). It is found that the spin gap closes at a surprisingly low field H c1 = 10.3 T compared with that expected from the zero-field spin gap (Δ/ g µ B = 18.4 T). For H > H c1 , the magnetization exhibits a linear increase without any trace of anomalies such as fractional plateaus until it saturates at H c2 = 30.1 T. This means that the gapless phase, where the field-induced magnetic ordering is expected to occur at low temperatures, is stable over a wide field region. These results suggest strong correlations of triplet excitations in the layer and the proximity of the spin-liquid phase to the magnetically ordered phase.

Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7

We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and transmission electron microscopy (TEM) studies on the quasi-two-dimensional spin-gap system (CuCl)...

Bose–Einstein Condensation of Quasi-Two-Dimensional Frustrated Quantum Magnet (CuCl)LaNb2O7

A quasi-two-dimensional S = 1/2 Heisenberg square-lattice antiferromagnet (CuCl)LaNb 2 O 7 is studied by specific heat and magnetic susceptibility measurements in external fields up to 14 T. The experimental results in low fields verify the absence of the order–disorder transition, as reported previously. By further application of magnetic fields, we obtain direct evidence for the phase transition, which can be interpreted as the Bose–Einstein condensation (BEC) of magnons. However, the critical field of 10 T significantly deviates from that estimated from the zero-field gap, indicating that, unlike known experimental examples, the one-magnon mode is not a primary source to drive the BEC.

Antiferromagnetic Nuclear Resonance in the Quasi-Two-Dimensional (CuBr)LaNb2O7

We report nuclear magnetic resonance (NMR) studies in the antiferromagnetic state of the quasi-two-dimensional (CuBr)LaNb2O7. The NMR spectra at zero magnetic field and 4.2 K indicate a unique Cu and Br sites with an internal field of 5.7 T (at Cu) and 16.4 T (at Br), confirming a magnetic order. For the large internal field at the Br sites to be compatible with the collinear antiferromagnetic order observed by neutron diffraction experiments (N. Oba et al., J. Phys. Soc. Jpn. 75, (2006) 113601), the Br atoms must move significantly off the center of the square of the Cu sublattice so that the Br nuclei couple predominantly to two parallel Cu moments. While invalidating the frustrated J1-J2 model defined on a C4-symmetric square lattice, our results are compatible with the structural model proposed for (CuCl)LaNb2O7 by Yoshida et al. (J. Phys. Soc. Jpn. 76 (2007) 104703).

S = 1/2 Square-Lattice Antiferromagnets: (CuX)LaB2O7 and (CuCl)A2B3O10 (X = Cl, Br; A = Ca, Sr; B = Nb, Ta)

A series of magnetic compounds with the formula (CuX)LaB 2 O 7 and (CuCl)A 2 B 3 O 10 (X = Cl, Br: A = Ca, Sr; B = Nb, Ta) have been prepared through a low-temperature topochemical route starting from nonmagnetic double- (n = 2) and triple- (n = 3) layered perovskites, respectively. The magnetic susceptibility of these compounds typically exhibits a broad maximum at low temperatures, characteristic of low-dimensional antiferromagnetic compounds. However, depending on the choice of the parameters. X, A, B and n, physical quantities such as the Weiss temperature and the temperature at a maximum susceptibility vary to a great extent, which enables us to study the phase diagram of the S = 1/2 frustrated square-lattice antiferromagnets (the so-called J 1 -J 2 model). In particular, (CuCl)LaNb 2 O 7 , possibly having a ferromagnetic J 1 and an antiferromagnetic J 2 , shows a spin-liquid behavior with the spill gap of 27 K.