1,213 research outputs found
Effect of nonmagnetic dilution in honeycomb lattice iridates NaIrO and LiIrO
We have synthesized single crystals of Na(IrTi)O and
polycrystals of Li(IrTi)O and studied the effect of
magnetic depletion on the magnetic properties by measurements of the magnetic
susceptibility, specific heat and magnetocaloric effect at temperatures down to
0.1~K. In both systems, the non-magnetic substitution rapidly changes the
magnetically ordered ground state into a spin glass, indicating strong
frustration. While for the Li system the Weiss temperature
remains unchanged up to , a strong decrease is found
for the Na system. This suggests that only for the former system magnetic
exchange beyond nearest neighbors is dominating. This is also corroborated by
the observation of a smeared quantum phase transition in
Li(IrTi)O near , i.e. much beyond the site
percolation threshold of the honeycomb lattice.Comment: 8 pages including supplemental, 12 figure
Origin of the insulating state in honeycomb iridates and rhodates
A burning question in the emerging field of spin-orbit driven insulating
iridates, such as Na2IrO3 and Li2IrO3 is whether the observed insulating state
should be classified as a Mott-Hubbard insulator derived from a half-filled
relativistic j_eff=1/2 band or as a band insulator where the gap is assisted by
spin-orbit interaction, or Coulomb correlations, or both. The difference
between these two interpretations is that only for the former, strong
spin-orbit coupling (lambda >~ W, where W is the band width) is essential. We
have synthesized the isostructural and isoelectronic Li2RhO3 and report its
electrical resistivity and magnetic susceptibility. Remarkably it shows
insulating behavior together with fluctuating effective S=1/2 moments, similar
to Na2IrO3 and Li2IrO3, although in Rh4+ (4d5) the spin-orbit coupling is
greatly reduced. We show that this behavior has non-relativistic one-electron
origin (although Coulomb correlations assist in opening the gap), and can be
traced down to formation of quasi-molecular orbitals, similar to those in
Na2IrO3.Comment: 7 pages, 7 figure
Investigation of unconventional reconstruction and electronic properties on the Na2IrO3 surface
Na2IrO3 is an intriguing material for which spin-orbit coupling plays a key
role. Theoretical predictions, so far unverified, have been made that the
surface of Na2IrO3 should exhibit a clear signature of the quantum spin Hall
effect. We studied the surface of Na2IrO3 using scanning tunneling microscopy
and density-functional theory calculations. We observed atomic level resolution
of the surface and two types of terminations with different surface periodicity
and Na content. By comparing bias-dependent experimental topographic images to
simulated images, we determined the detailed atomistic structure of both
observed surfaces. One of these reveals a strong relaxation to the surface of
Na atoms from the subsurface region two atomic layers below. Such dramatic
structural changes at the surface cast doubt on any prediction of surface
properties based on bulk electronic structure. Indeed, using spatially resolved
tunneling spectroscopy we found no indication of the predicted quantum spin
Hall behavior
Unconventional magnetic order on the hyperhoneycomb Kitaev lattice in -Li2IrO3: full solution via magnetic resonant x-ray diffraction
The recently-synthesized iridate -LiIrO has been proposed as a
candidate to display novel magnetic behavior stabilized by frustration effects
from bond-dependent, anisotropic interactions (Kitaev model) on a
three-dimensional "hyperhoneycomb" lattice. Here we report a combined study
using neutron powder diffraction and magnetic resonant x-ray diffraction to
solve the complete magnetic structure. We find a complex, incommensurate
magnetic order with non-coplanar and counter-rotating Ir moments, which
surprisingly shares many of its features with the related structural polytype
"stripyhoneycomb" -LiIrO, where dominant Kitaev interactions
have been invoked to explain the stability of the observed magnetic structure.
The similarities of behavior between those two structural polytypes, which have
different global lattice topologies but the same local connectivity, is
strongly suggestive that the same magnetic interactions and the same underlying
mechanism governs the stability of the magnetic order in both materials,
indicating that both - and -LiIrO are strong candidates
to realize dominant Kitaev interactions in a solid state material.Comment: 14 pages, 9 figure
Classical theta constants vs. lattice theta series, and super string partition functions
Recently, various possible expressions for the vacuum-to-vacuum superstring
amplitudes has been proposed at genus . To compare the different
proposals, here we will present a careful analysis of the comparison between
the two main technical tools adopted to realize the proposals: the classical
theta constants and the lattice theta series. We compute the relevant Fourier
coefficients in order to relate the two spaces. We will prove the equivalence
up to genus 4. In genus five we will show that the solutions are equivalent
modulo the Schottky form and coincide if we impose the vanishing of the
cosmological constant.Comment: 21 page
Penrose-Onsager Criterion Validation in a One-Dimensional Polariton Condensate
We perform quantum tomography on one-dimensional polariton condensates,
spontaneously occurring in linear disorder valleys in a CdTe planar microcavity
sample. By the use of optical interferometric techniques, we determine the
first-order coherence function and the amplitude and phase of the order
parameter of the condensate, providing a full reconstruction of the single
particle density matrix for the polariton system. The experimental data are
used as input to theoretically test the consistency of Penrose-Onsager
criterion for Bose-Einstein condensation in the framework of nonequilibrium
polariton condensates. The results confirm the pertinence and validity of the
criterion for a non equilibrium condensed gas.Comment: 5 pages, 4 figure
Effect of isoelectronic doping on honeycomb lattice iridate A_2IrO_3
We have investigated experimentally and theoretically the series
(NaLi)IrO. Contrary to what has been believed so far,
only for the system forms uniform solid solutions. For larger Li
content, as evidenced by powder X-ray diffraction, scanning electron microscopy
and density functional theory calculations, the system shows a miscibility gap
and a phase separation into an ordered NaLiIrO phase with
alternating Na and LiIrO planes, and a Li-rich phase close to pure
LiIrO. For we observe (1) an increase of with Li
doping up to , despite the fact that in pure LiIrO is
smaller than in NaIrO, and (2) a gradual reduction of the
antiferromagnetic ordering temperature and ordered moment. The
previously proposed magnetic quantum phase transition at may
occur in a multiphase region and its nature needs to be re-evaluated.Comment: 8 pages, 9 figures including supplemental informatio
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