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Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl 3 in magnetic fields parallel to the b-axis. The magnetic Bragg peaks which indicate the fieldinduced Néel ordering were observed for the magnetic field higher than the gap field H g ≈ 5.5 T at Q = (h, 0, l) with odd l in the a * − c * plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Néel ordering as a Bose-Einstein condensation of magnons.KEYWORDS: TlCuCl 3 , spin gap, field-induced magnetic ordering, spin structure, neutron elastic scattering, BoseEinstein condensation of magnonsThe singlet ground state with the excitation gap (spin gap) is a notable realization of the macroscopic quantum effect in quantum spin systems. When a magnetic field is applied in the spin gap system, the gap Δ is suppressed and closes completely at the gap field H g = Δ/gμ B . For H > H g the system can undergo magnetic ordering due to three-dimensional (3D) interactions. Such field-induced magnetic ordering was studied first for Cu(NO 3 ) 2 · 5 2 H 2 O. 1) However, the magnetic properties near H = H g have not been investigated because of the very low ordering temperature, the maximum of which is 0.18 K. Recently, the study of fieldinduced magnetic ordering has been revived, because the spin gap has been found in many quantum spin systems. Field-induced 3D ordering has been observed in several quasi-one-dimensional spin gap systems. [2][3][4][5][6] This paper is concerned with field-induced magnetic ordering in TlCuCl 3 . This compound has a monoclinic structure (space group P 2 1 /c). 7) TlCuCl 3 contains planar dimers of Cu 2 Cl 6 , in which Cu 2+ ions have spin-1 2 . These dimers are stacked on top of one another to form infinite double chains parallel to the crystallographic aaxis. These double chains are located at the corners and center of the unit cell in the b−c plane, and are separated by Tl + ions. The magnetic ground state is the spin singlet with the excitation gap Δ/k B ≈ 7.5 K. 8, 9) The magnetic excitations in TlCuCl 3 were investigated by Oosawa et al., 10) who found that the lowest excitation occurs at Q = (0, 0, 1) and its equivalent reciprocal points, as observed in KCuCl 3 . 11, 12) The origin of the gap is the strong antiferromagnetic interaction J = 5.26 meV on the planar dimer Cu 2 Cl 6 in the double chain. The * E-mail: tanaka@lee.phys.titech.ac.jp neighboring dimers couple magnetically along the chain and in the (1, 0, −2) plane.Our previous magnetic measurements revealed that TlCuCl 3 undergoes 3D magnetic ordering in magnetic fields higher than the gap field H g ≈ 5.5 T. 9) The magnetization exhibits a cusplike minimum at the ordering temperature T N . The phase boundary on the temperature vs field diagram is independent of the field direction when normalized by the g-factor, and can be represented by the power lawwith φ = 2.2...

The magnetic susceptibility and high-field magnetization process of NH 4 CuCl 3 with double chains of CuCl 3 have been measured using single crystals. No anomaly indicative of the threedimensional ordering is observed in susceptibility data above 1.7 K. It is found, by magnetization measurement down to 0.5 K, that in contrast to KCuCl 3 and TlCuCl 3 , NH 4 CuCl 3 has a gapless magnetic ground state at zero field. It is observed that the magnetization curve has two plateaus at one-quarter and three-quarters of the saturation magnetization, irrespective of the external field direction. The origin of the plateaus is attributed not to the magnetic anisotropy, but to the quantum effect. The relation between the plateaus and the period of the spin state is discussed in terms of a recent theory presented by Oshikawa et al. [Phys. Rev. Lett. 78 (1997) 1984.KEYWORDS: NH 4 CuCl 3 , double chain, high field, magnetization process, magnetization plateausThe magnetization processes of low-dimensional quantum spin systems with spatial structures such as spin ladders and exchange-alternating chains, are new problems in magnetism. In particular, magnetization plateaus are of great interest, because the magnetization is quantized at the plateaus. In a spin-1/2 Heisenberg chain with ferromagnetic (J F )-ferromagnetic (J F )-antiferromagnetic (J AF ) interactions, the magnetization curve has a plateau at 1/3 of the saturation magnetization M s , when J F /J AF < 5 ∼ 6.1, 2) For a spin-1/2 antiferromagnetic alternating Heisenberg chain with the next-nearestneighbor interaction, a plateau can appear at (1/2)M s in an appropriate parameter region. 3,4) In this case the next-nearest-neighbor interaction is essential for the existence of the plateau. The period of the spin state at the plateau is twice as large as the period of the Hamiltonian.4) Thus the plateau is caused by the quantum manybody effect. For a spin-1 antiferromagnetic Heisenberg chain, a plateau exists at (1/2)M s as long as the exchange interaction alternates.5, 6) The 1/2-plateau has been observed experimentally in the nickel compound8) investigated general Heisenberg chains in a magnetic field. They showed that the magnetization can have plateaus, and that it is quantized at the plateaus aswhere n is the period of the spin state, S the magnitude of spin and m the magnetization per site in the unit of gμ B . All of the above-mentioned plateaus satisfy this quantization condition. * Author to whom correspondence should be addressed. E-mail: tanaka@lee.cme.phys.titech.ac.jp 1548In this letter we report the magnetization plateaus observed in NH 4 CuCl 3 . At room temperature, NH 4 CuCl 3 is isostructural with KCuCl 3 which belongs to the monoclinic space group P 2 1 /c. 9, 10) The crystal structure is composed of double chains of edge-sharing CuCl 6 octahedra along the a-axis. The double chains are located at the corners and center of the unit cell in the bc-plane, and are separated by NH 4+ ions. There are three kinds of nearest-neighbor-interactions, J 1 , J 2 and J 3...

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The strain-promoted "double-click" (SPDC) reaction using Sondheimer diyne, a novel convergent method conjugating three molecules spontaneously, has enabled us to readily modify an azido-biomolecule with a small reporter azido-molecule.

We investigated the crystal structure of Rb 2 Cu 3 SnF 12 and its magnetic properties using single crystals. This compound is composed of Kagomé layers of corner-sharing CuF 6 octahedra with a 2a  2a enlarged cell as compared with the proper Kagomé layer. Rb 2 Cu 3 SnF 12 is magnetically described as an S ¼ 1=2 modified Kagomé antiferromagnet with four kinds of neighboring exchange interaction. From magnetic susceptibility and high-field magnetization measurements, it was found that the ground state is a disordered singlet with the spin gap, as predicted from a recent theory. Exact diagonalization for a 12-site Kagomé cluster was performed to analyze the magnetic susceptibility, and individual exchange interactions were evaluated. Antiferromagnets on highly frustrated lattices produce a rich variety of physics.1,2) In particular, a two-dimensional Heisenberg Kagomé antiferromagnet (2D HKAF) is of great interest from the viewpoint of the interplay of the frustration and quantum effects. There are many theoretical studies on the 2D HKAF. The spin wave theory for a large spin value predicted an ordered ground state with the so-called ffiffi ffi 3 p  ffiffi ffi 3 p structure, which is selected by quantum fluctuation from infinite classical ground states, 3,4) whereas for a small spin value, a disordered ground state was observed by various approaches.5-9) Recent careful analyses and numerical calculations for an S ¼ 1=2 case demonstrated that the ground state is a spin liquid state composed of singlet dimers only, and that the ground state is gapped for triplet excitations, but gapless for singlet excitations.10-12) Consequently, magnetic susceptibility has a rounded maximum at T $ ð1=6ÞJ=k B and decreases exponentially toward zero with decreasing temperature, while specific heat exhibits a power law behavior at low temperatures. 8,13) Specific heat also shows an additional structure, peak or shoulder at low temperatures after exhibiting a broad maximum at T $ ð2=3ÞJ=k B .The experimental studies of the S ¼ 1=2 HKAF have been limited, and the above-mentioned intriguing predictions have not been verified experimentally. The cupric com- 27) Unfortunately, these systems undergo structural phase transitions at T t ¼ 220 and 170 K, respectively, and also magnetic phase transitions at T N ' 24 K. 27) However, the magnetic susceptibilities observed at T > T t can be perfectly described using theoretical results for an S ¼ 1=2 HKAF with large exchange interactions J=k B $ 250 K. 28)In the present work, we synthesized the new hexagonal compound Rb 2 Cu 3 SnF 12 with a similar crystal structure as Cs 2 Cu 3 ZrF 12 and performed magnetic susceptibility and high-field magnetization measurements using single crystals. As shown below, we found that the ground state is a disordered singlet with a finite gap for magnetic excitations.Rb 2 Cu 3 SnF 12 crystals were synthesized via the chemical reaction 2RbF þ 3CuF 2 þ SnF 4 ! Rb 2 Cu 3 SnF 12 . RbF, CuF 2 , and SnF 4 were dehydrated by heating in vacuum at 60 -100 C for three days....

Four crystal structures of the title complexes have been analyzed by X-ray crystallography. 1:  C80H68N17Nd, a = 17.0545(6) Å, c = 22.807(3) Å, tetragonal, P4/ncc, Z = 4. 2:  C80H68N17Gd, a = 17.399(8) Å, c = 11.377(6) Å, tetragonal, P4212, Z = 2. 3:  C80H70N17OHo, a = 18.096(5) Å, c = 11.079(6) Å, tetragonal, P4212, Z = 4. 4:  C80H70N17OLu, a = 18.171(3) Å, c = 10.987(4) Å, tetragonal, P4/nmm, Z = 2. The [LnIIIPc2]- and [NBu n 4]+ ions in each crystal stack alternately in a column along the 4-fold axis. The Ln(III) cation of each [LnIIIPc2]- is eight-coordinated by two phthalocyanato dianions (Pc). The interplanar distances between the two Pc rings have a linear relationship to the ionic radii of the central Ln(III) cations. The skew angles between the two Pc rings are 6.2, 34.4, 43.2, and 45.0° for the Nd, Gd, Ho, and Lu complexes, respectively. The skew angle seems to depend on the strength of the π−π interaction between the two Pc rings.