3,103 research outputs found
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
Dirty-boson physics with magnetic insulators
We review recent theoretical and experimental efforts aimed at the
investigation of the physics of interacting disordered bosons (so-called dirty
bosons) in the context of quantum magnetism. The physics of dirty bosons is
relevant to a wide variety of condensed matter systems, encompassing Helium in
porous media, granular superconductors and ultracold atoms in disordered
optical potentials, to cite a few. Nevertheless, the understanding of the
transition from a localized, Bose-glass phase to an ordered, superfluid
condensate phase still represents a fundamentally open problem. Still to be
constructed is also a quantitative description of the highly inhomogeneous and
strongly correlated phases connected by the transition. We discuss how
disordered magnetic insulators in a strong magnetic field can provide a well
controlled realization of the above transition. Combining numerical simulations
with experiments on real materials can shed light on some fundamental
properties of the critical behavior, such as the scaling of the critical
temperature to condensation close to the quantum critical point
Comment on "Transition from Bose glass to a condensate of triplons in TlKCuCl"
We argue that the interpretation of the calorimetric data for disordered
quantum antiferromagnets TlKCuCl 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 and its disordered derivatives.
Furthermore, we question the very existence of a proper field-induced
thermodynamic phase transition in TlKCuCl.Comment: 1 page, submitted to PRB as Comment on: Yamada et al., PRB 83,
020409(R) (2011
Coexistence of Haldane gap excitations and long-range order in R_2BaNiO_5 (=rare earth)
( 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 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
Reversality of optical interactions in noncentrosymmetric media
The interaction of an electromagnetic wave with a noncentrosymmetric crystal is not necessarily time reversible, and the departure from reversality may be seen in nonlocal (wave-vector linear) phenomena. However, relativistic symmetry with respect to simultaneous time and space inversion is always preserved in optics
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
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
Coherent control of Snell's law
We demonstrate coherent control of the generalized Snell's law in ultrathin gradient metasurfaces constructed by an array of V-shaped slot nanoantennas
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