Neutron Scattering Study of Quantum Phase Transitions in Integral Spin Chains

Quite a few low-dimensional magnets are quantum-disordered ``spin liquids'' with a characteristic gap in the magnetic excitation spectrum. Among these are antiferromagnetic chains of integer quantum spins. Their generic feature are long-lived massive (gapped) excitations (magnons) that are subject to Zeeman splitting in external magnetic fields. The gap in one of the magnon branches decreases with field, driving a soft-mode quantum phase transition. The system then enters a qualitatively new high-field phase. The actual properties at high fields, particularly the spin dynamics, critically depend on the system under consideration. Recent neutron scattering studies of organometallic polymer crystals NDMAP (Haldane spin chains with anisotropy) and NTENP (dimerized S=1 chains) revealed rich and unique physics.Comment: 8 pages, 9 figures, semi-plenary lecture at the LT24 conference, Orlando, Florida, August 200

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

Magnetic anisotropy and low-energy spin waves in the Dzyaloshinskii-Moriya spiral magnet Ba_2 Cu Ge_2 O_7

New neutron diffraction and inelastic scattering experiments are used to investigate in detail the field dependence of the magnetic structure and low-energy spin wave spectrum of the Dzyaloshinskii-Moriya helimagnet Ba_2 Cu Ge_2 O_7. The results suggest that the previously proposed model for the magnetism of this compound (an ideal sinusoidal spin spiral, stabilized by isotropic exchange and Dzyaloshinskii-Moriya interactions) needs to be refined. Both new and previously published data can be quantitatively explained by taking into account the Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) term, a special magnetic anisotropy term that was predicted to always accompany Dzyaloshinskii-Moriya interactions in insulators.Comment: 30 pages, 10 figures, submitted to PR

Intermediate phase in the spiral antiferromagnet Ba_2CuGe_2O_7

The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear sigma model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H_1 and H_2 such that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.Comment: 15 pages, 11 figures, 2 table

Structure of the exotic spin-flop states in BaCu2Si2O7

The unusual 2-stage spin flop transition in BaCu2Si2O7 is studied by single-crystal neutron diffraction. The magnetic structures of the various spin-flop phases are determined. The results appear to be inconsistent with the previously proposed theoretical explanation of the 2-stage transition.Comment: 6 pages 5 figure

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

Coexistence of Haldane gap excitations and long-range order in R_2BaNiO_5 (RR=rare earth)

$R_2BaNiO_5$ ($R=$ rare earth) quasi-1-D antiferromagnets are structurally equivalent to the well-studied 1-D S=1 Haldane-gap compound Y_2BaNiO_5. Unlike the Y-nickelate though, these materials undergo 3-D magnetic ordering at finite temperatures. Recent inelastic neutron scattering studies of Pr_2BaNiO_5 and (Nd_{x}Y_{1-x})_2BaNiO_5 revealed purely 1-dimensional gap excitations that propagate exclusively on the Ni-chains and are strikingly similar to Haldane gap modes in Y_2BaNiO_5. In the ordered phase these excitations survive and actually coexist with conventional spin waves. Below $T_{N}$ the gap energy increases and scales as the square of the ordered moment on the Ni sites. The results suggest that the Haldane singlet ground state of the Ni-chains is not fully destroyed by N\'{e}el ordering.Comment: Invited paper for the International Conference on Neutron Scattering, Toronto, Canada, August 17-21, 199