Thermodynamic properties and elementary excitations in S ¼ 1=2 one-dimensional Heisenberg antiferromagnet KCuGaF 6 were investigated by magnetic susceptibility, specific heat and ESR measurements. Due to the Dzyaloshinsky-Moriya interaction with alternating D-vectors and/or the staggered g-tensor, the staggered magnetic field is induced when subjected to external magnetic field. Specific heat in magnetic field clearly shows the formation of excitation gap, which is attributed to the staggered magnetic field. The specific heat data was analyzed on the basis of the quantum sine-Gordon (SG) model. We observed many ESR modes including one soliton and three breather excitations characteristic of the quantum SG model. Studies of antiferromagnetic Heisenberg chain (AFHC) have long history. For S ¼ 1=2 uniform AFHC, the ground state, thermodynamic properties and magnetic excitations are well understood with the help of exact solutions [1][2][3][4] and accurate analytical and numerical calculations.5-8) These theoretical results demonstrate the importance of the quantum fluctuation characteristic of low-dimensional systems. In particular, exact result of the magnetic excitations in an external magnetic field H is qualitatively different from the results of the linear spin wave theory. The gapless excitations occur at incommensurate wave vectors q ¼ AE2mðHÞ and AE 2mðHÞ in addition to at q ¼ 0 and , where mðHÞ is the magnetization per site in the unit of g B . 4)Recently, the physics of S ¼ 1=2 AFHC in staggered magnetic field induced by the external magnetic field has been attracting considerable attention.9,10) The model Hamiltonian of such system is written aswhere h is the staggered field perpendicular to the external field H and is given by h ¼ c s H. This magnetic model is actualized in some S ¼ 1=2 AFHC systems such as (H 2 O) 2 (PM = pyrimidine). 15,16) The staggered field originates from the alternating g-tensor and the antisymmetric interaction of the Dzyaloshinsky-Moriya (DM) type with the alternating D vector. In these compounds, the fieldinduced gap ÁðHÞ almost proportional to H 2=3 was commonly observed. This field dependence of the gap cannot be explained by the linear spin wave theory, which derives ÁðHÞ / H 1=2 . Using the field theoretical approach, Oshikawa and Affleck 9,10) argued that the model (1) can be mapped onto the quantum sine-Gordon (SG) model with Lagrangian density L ¼ ð1=2Þð@ Þ þ hC cosð2R Þ, where is a canonical Bose field, is the dual field, R is the compactification radius and C is a coupling constant, and that the gap is expressed as ÁðhÞ ' 1:85Jðg B h=JÞ 2=3 lnðJ=g B hÞ 1=6 . Their result is in agreement with experimental results. [11][12][13][14][15][16] In the above-mentioned compounds, the exchange interaction is order of 10 K and the proportional coefficient is rather small, c s ¼ 0:08.13,16) For the comprehensive understanding of the systems described by the model (1), new compounds having different interaction constants are necessary. In this letter, we show that the static and d...

Magnetic susceptibility and specific heat measurements in magnetic fields were performed on an S = 1/2 one-dimensional antiferromagnet KCuGaF6. Exchange interaction was evaluated as J/kB ≃ 100 K. However, no magnetic ordering was observed down to 0.46 K. It was found that an applied magnetic field induces a staggered magnetic susceptibility obeying the Curie law and an excitation gap, both of which should be attributed to the antisymmetric interaction of the Dzyaloshinsky-Moriya type and/or the staggered g-tensor. With increasing magnetic field H, the gap increases almost in proportion to H 2/3 .

We have observed the magnetic resonant mode in the superconducting state of the ironbased superconductor K x Fe 2−y Se 2. It is centred at 12.7 ± 0.2 meV at Q = (0.25, 0.5, 0.5), using the unfolded Fe-sublattice scheme. By studying the temperature dependence of the resonance, we have shown that the mode is linked to the superconducting state and has an associated change in the low energy spectral weight at T c. Despite the resonance meeting a well defined nesting condition that indicates a superconductor with homogeneous phase, it appears the resonance does not disappear sharply at T c , unlike in the analogous iron arsenide superconductors.