25 research outputs found
Localized itinerant electrons and unique magnetic properties of SrRu2O6
SrRu2O6 has unique magnetic properties. It is characterized by a very high
N\'eel temperature, despite its quasi-two-dimensional structure, and has a
magnetic moment more than twice reduced compared to the formal ionic count.
First principles calculations show that only an ideal Neel ordering in the Ru
plane is possible, with no other metastable magnetic solutions, and, highly
unusually, yield dielectric gaps for both antiferromagnetic and nonmagnetic
states. We demonstrate that this strange behavior is the result of the
formation of very specific electronic objects, recently suggested for a
geometrically similar Na2IrO3 compound, whereby each electron is well localized
on a particular Ru6 hexagon, and completely delocalized over the corresponding
six Ru sites, thus making the compound strongly localized and highly
itinerant
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
Na2IrO3 as a molecular orbital crystal
Contrary to previous studies that classify Na2IrO3 as a realization of the
Heisenberg-Kitaev model with dominant spin-orbit coupling, we show that this
system represents a highly unusual case in which the electronic structure is
dominated by the formation of quasi-molecular orbitals (QMOs), with substantial
quenching of the orbital moments. The QMOs consist of six atomic orbitals on an
Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of
such QMOs in solids invokes very different physics compared to the models
considered previously. Employing density functional theory calculations and
model considerations we find that both the insulating behavior and the
experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow
from the QMO model.Comment: Final version, accepted by PR
Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet CsCuClBr
We report on a systematic study of the magnetic properties on single crystals
of the solid solution CsCuClBr (0 x 4), which
include the two known end-member compounds CsCuCl and CsCuBr,
classified as quasi-two-dimensional quantum antiferromagnets with different
degrees of magnetic frustration. By comparative measurements of the magnetic
susceptibility () on as many as eighteen different Br concentrations,
we found that the inplane and out-of-plane magnetic correlations, probed by the
position and height of a maximum in the magnetic susceptibility, respectively,
do not show a smooth variation with x. Instead three distinct concentration
regimes can be identified, which are separated by critical concentrations
x = 1 and x = 2. This unusual magnetic behavior can be explained
by considering the structural peculiarities of the materials, especially the
distorted Cu-halide tetrahedra, which support a site-selective replacement of
Cl- by Br- ions. Consequently, the critical concentrations x (x)
mark particularly interesting systems, where one (two) halidesublattice
positions are fully occupied.Comment: 15 pages, 4 figure
Fermionic response from fractionalization in an insulating two-dimensional magnet
Conventionally ordered magnets possess bosonic elementary excitations, called
magnons. By contrast, no magnetic insulators in more than one dimension are
known whose excitations are not bosons but fermions. Theoretically, some
quantum spin liquids (QSLs) -- new topological phases which can occur when
quantum fluctuations preclude an ordered state -- are known to exhibit Majorana
fermions as quasiparticles arising from fractionalization of spins. Alas,
despite much searching, their experimental observation remains elusive. Here,
we show that fermionic excitations are remarkably directly evident in
experimental Raman scattering data across a broad energy and temperature range
in the two-dimensional material -RuCl. This shows the importance of
magnetic materials as hosts of Majorana fermions. In turn, this first
systematic evaluation of the dynamics of a QSL at finite temperature emphasizes
the role of excited states for detecting such exotic properties associated with
otherwise hard-to-identify topological QSLs.Comment: 5 pages, 3 figure
Recommended from our members
Correlation induced electron-hole asymmetry in quasi- two-dimensional iridates
The resemblance of crystallographic and magnetic structures of the quasi-two-dimensional iridates Ba2IrO4 and Sr2IrO4 to La2CuO4 points at an analogy to cuprate high-Tc superconductors, even if spin-orbit coupling is very strong in iridates. Here we examine this analogy for the motion of a charge (hole or electron) added to the antiferromagnetic ground state. We show that correlation effects render the hole and electron case in iridates very different. An added electron forms a spin polaron, similar to the cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplet states of total angular momentum that strongly affect the hole motion. This not only has ramifications for the interpretation of (inverse-)photoemission experiments but also demonstrates that correlation physics renders electron- and hole-doped iridates fundamentally different