Comment on "Transition from Bose glass to a condensate of triplons in Tl1−x_{1-x}Kx_xCuCl3_3"

We argue that the interpretation of the calorimetric data for disordered quantum antiferromagnets Tl$_{1-x}$K$_x$CuCl$_3$ in terms of Bose Glass physics by F. Yamada {\it et al.} in [Phys. Rev. B {\bf 83}, 020409(R) (2011)] is not unambiguous. A consistent analysis shows no difference in the crossover critical index for the disorder-free TlCuCl$_3$ and its disordered derivatives. Furthermore, we question the very existence of a proper field-induced thermodynamic phase transition in Tl$_{1-x}$K$_x$CuCl$_3$.Comment: 1 page, submitted to PRB as Comment on: Yamada et al., PRB 83, 020409(R) (2011

Field-concentration phase diagram of a quantum spin liquid with bond defects

The magnetic susceptibility of the gapped quantum spin liquid compound (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$ and its chemically disordered derivatives (C$_4$H$_{12}$N$_2$)Cu$_2$(Cl$_{1-x}$Br$_{x}$)$_6$ are systematically studied in magnetic fields of up to 45 T, as a function of Br concentration. The corresponding field-temperature and field-concentration phase diagrams are determined. Measurements on the disorder-free parent compound are not fully consistent with previously published results by other authors [PRL{\bf 96}, 257203 (2006)]. The effect of Br/Cl substitution on the magnetic properties is superficially similar to that of finite temperature. However, important differences are identified and discussed with reference to the previously studied magnetic excitation spectra.Comment: 6 pages, 6 figures. Submitted to PRB as regular pape

Far-infrared spectroscopy of spin excitations and Dzyaloshinskii-Moriya interactions in a Shastry-Sutherland compound SrCu2_2(BO3_3)$_2

We have studied spin excitation spectra in the Shastry-Sutherland model compound SrCu$_2$(BO$_3$)$_2$ in magnetic fields using far-infrared Fourier spectroscopy. The transitions from the ground singlet state to the triplet state at 24 cm$^{-1}$ and to several bound triplet states are induced by the electric field component of the far-infrared light. To explain the light absorption in the spin system we invoke a dynamic Dzyaloshinskii-Moriya (DM) mechanism where light couples to a phonon mode, allowing the DM interaction. Two optical phonons couple light to the singlet to triplet transition in SrCu$_2$(BO$_3$)$_2$. One is $a$-polarized and creates an intra-dimer dynamic DM along the c axis. The other is $c$-polarized and creates an intra-dimer dynamic DM interaction, it is in the $(ab)$ plane and perpendicular to the dimer axis. Singlet levels at 21.5 and 28.6 cm$^{-1}$ anti-cross with the first triplet as is seen in far-infrared spectra. We used a cluster of two dimers with a periodic boundary condition to perform a model calculation with scaled intra- and inter-dimer exchange interactions. Two static DM interactions are sufficient to describe the observed triplet state spectra. The static inter-dimer DM in the c-direction $d_1=0.7$ cm$^{-1}$ splits the triplet state sub-levels in zero field [C\'{e}pas et al., Phys. Rev. Lett. \textbf{87}, 167205 (2001)]. The static intra-dimer DM in the $(ab)$ plane (perpendicular to the dimer axis) $d_2=1.8$ cm$^{-1}$, allowed by the buckling of CuBO$_3$ planes, couples the triplet state to the 28.6 cm$^{-1}$ singlet as is seen from the avoided crossing.Comment: 12 pages with 7 figures, some references correcte

Electron spin resonance study of anisotropic interactions in a two-dimensional spin gap magnet PHCC

Fine details of the excitation spectrum of the two-dimensional spin-gap magnet PHCC are revealed by electron spin resonance investigations. The values of anisotropy parameters and the orientations of the anisotropy axes are determined by accurate measurements of the angular, frequency-field and temperature dependences of the resonance absorption. The properties of a spin-gap magnet in the vicinity of critical field are discussed in terms of sublevel splittings and g-factor anisotropy.Comment: submitted to PR