9 research outputs found
Cubic symmetry and magnetic frustration on the spin lattice in KIrCl
Cubic crystal structure and regular octahedral environment of Ir
render antifluorite-type KIrCl a model fcc antiferromagnet with a
combination of Heisenberg and Kitaev exchange interactions. High-resolution
synchrotron powder diffraction confirms cubic symmetry down to at least 20 K,
with a low-energy rotary mode gradually suppressed upon cooling. Using
thermodynamic and transport measurements, we estimate the activation energy of
eV for charge transport, the antiferromagnetic Curie-Weiss
temperature of K, and the extrapolated saturation
field of T. All these parameters are well reproduced \textit{ab
initio} using eV as the effective Coulomb repulsion
parameter. The antiferromagnetic Kitaev exchange term of K is about
one half of the Heisenberg term K. While this combination removes
a large part of the classical ground-state degeneracy, the selection of the
unique magnetic ground state additionally requires a weak second-neighbor
exchange coupling K. Our results suggest that KIrCl may
offer the best possible cubic conditions for Ir and demonstrates the
interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page
Toward cubic symmetry for Ir: structure and magnetism of antifluorite KIrBr
Crystal structure, electronic state of Ir, and magnetic properties of
the antifluorite compound KIrBr are studied using high-resolution
synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS),
thermodynamic and transport measurements, and ab initio calculations. The
crystal symmetry is reduced from cubic at room temperature to tetragonal below
170 K and eventually to monoclinic below 122 K. These changes are tracked by
the evolution of the non-cubic crystal-field splitting measured by
RIXS. Non-monotonic changes in are ascribed to the competing effects
of the tilt, rotation, and deformation of the IrBr octahedra as well as
tetragonal strain on the electronic levels of Ir. The N\'eel temperature
of K exceeds that of the isostructural KIrCl, and the
magnitude of frustration on the fcc spin lattice decreases. We argue that the
replacement of Cl by Br weakens electronic correlations and enhances magnetic
couplings.Comment: published version: 13 pages + Supplemental Materia
Pressure-induced dimerization and collapse of antiferromagnetism in the Kitaev material α−Li2IrO3
We present magnetization measurements carried out on polycrystalline and
single-crystalline samples of -LiIrO under hydrostatic
pressures up to 2 GPa and establish the temperature-pressure phase diagram of
this material. The N\'eel temperature () of -LiIrO
is slightly enhanced upon compression with = 1.5 K/GPa. Above
1.2 GPa, -LiIrO undergoes a first-order phase transition toward
a nonmagnetic dimerized phase, with no traces of the magnetic phase observed
above 1.8 GPa at low temperatures. The critical pressure of the structural
dimerization is strongly temperature-dependent. This temperature dependence is
well reproduced on the ab initio level by taking into account lower phonon
entropy in the nonmagnetic phase. We further show that the initial increase in
of the magnetic phase is due to a weakening of the Kitaev
interaction along with the enhancement of the Heisenberg term and
off-diagonal anisotropy . Our study reveals a common thread in the
interplay of magnetism and dimerization in pressured Kitaev materials.Comment: 8 pages, 7 figure
Antiferroelectric instability in the kagome francisites Cu3Bi(SeO3)2O2X (X=Cl,Br)
Density-functional calculations of lattice dynamics and high-resolution
synchrotron powder diffraction uncover antiferroelectric distortion in the
kagome francisite CuBi(SeO)OCl below 115K. Its Br-containing
analogue is stable in the room-temperature crystal structure down to at least
10K, although the Br compound is on the verge of a similar antiferroelectric
instability and reveals local displacements of Cu and Br atoms. The
I-containing compound is stable in its room-temperature structure according to
density-functional calculations. We show that the distortion involves
cooperative displacements of Cu and Cl atoms, and originates from the
optimization of interatomic distances for weakly bonded halogen atoms. The
distortion introduces a tangible deformation of the kagome spin lattice and may
be responsible for the reduced net magnetization of the Cl compound compared to
the Br one. The polar structure of CuBi(SeO)OCl is only
slightly higher in energy than the non-polar antiferroelectric structure, but
no convincing evidence of its formation could be obtained.Comment: 11 pages, 7 figure
Acoustic phonon dispersion of α−RuCl3
Acoustic phonons have recently been posited as playing an integral role in
explaining the half-quantized thermal Hall effect in -RuCl.
Therefore, we present much needed inelastic x-ray scattering measurements of
its acoustic phonon dispersion, along with calculations using the frozen-phonon
method. We also discuss a temperature study which conclusively shows a
first-order structural transition to a non- space group at low
temperature. Together these results are an important backbone for future
theoretical and experimental studies of -RuCl