31 research outputs found
Orbital effects in manganites
In this paper I give a short review of some properties of the colossal
magnetoresistance manganites, connected with the orbital degrees of freedom.
Ions Mn{3+}, present in most of these compounds, have double orbital degeneracy
and are strong Jahn-Teller ions, causing structural distortions and orbital
ordering. Mechanisms leading to such ordering are shortly discussed, and the
role of orbital degrees of freedom in different parts of the phase diagram of
manganites is described. Special attention is paid to the properties of
low-doped systems (doping 0.1 - 0.25), to overdoped systems (x > 0.5), and to
the possibility of a novel type of orbital ordering in optimally doped
ferromagnetic metallic manganites.Comment: 28 pages, 7 figures, to be published in J. Mod. Phys.
Role of Orbitals in the Physics of Correlated Electron Systems
Rich properties of systems with strongly correlated electrons, such as
transition metal oxides, is largely connected with an interplay of different
degrees of freedom in them: charge, spin, orbital ones, as well as crystal
lattice. Specific and often very important role is played by orbital degrees of
freedom. In this comment I will shortly summarize the main concepts and discuss
some of the well-known manifestations of orbital degrees of freedom, but will
mostly concentrate on a recent development in this field.Comment: To be published in "Comments on Solid State Physics", part of
"Physica Scripta
Orbital order in classical models of transition-metal compounds
We study the classical 120-degree and related orbital models. These are the
classical limits of quantum models which describe the interactions among
orbitals of transition-metal compounds. We demonstrate that at low temperatures
these models exhibit a long-range order which arises via an "order by disorder"
mechanism. This strongly indicates that there is orbital ordering in the
quantum version of these models, notwithstanding recent rigorous results on the
absence of spin order in these systems.Comment: 7 pages, 1 eps fi
Orbital ordering in manganites in the band approach
We consider the orbital ordering in LaMnO3 and similar systems, proceeding
from the band picture. We show that for the realistic magnetic structure of
A-type there exists a complete nesting betweeen two e_g-bands. As a result
there occurs an instability towards an excitonic insulator-like state -- an
electron-hole pairing with the wave vector Q=(\pi,\pi), which opens a gap in
the spectrum and makes the system insulating. In the resulting state there
appeasr an orbital ordering -- orbital density wave (ODW), the type of which
coincides with those existing in LaMnO3.Comment: 4 pages, 2 figure
Local structural changes in paramagnetic and charge ordered phases of Sm0.2Pr0.3Sr0.5MnO3: An EXAFS Study
Sm{0.5-x}Pr{x}Sr{0.5}MnO{3} exhibits variety of ground states as x is varied
from 0 to 0.5. At an intermediate doping of x = 0.3 a charge-ordered CE type
antiferromagnetic insulating (AFI) ground state is seen. The transition to this
ground state is from a paramagnetic insulating (PMI) phase through a
ferromagnetic metallic phase (FMM). Local structures in PMI and AFI phases of x
= 0.3 sample have been investigated using Pr K-edge and Sm K-edge Extended
X-ray Absorption Fine Structure (EXAFS). It can be seen that the tilting and
rotation of the MnO6 octahedra about the b-axis are responsible for the charge
ordered CE-type antiferromagnetic ground state at low temperatures. In addition
a shift in the position of the rare earth ion along the c-axis has to be
considered to account for observed distribution of bond distances around the
rare earth ion
Dielectric anomaly at the orbital order-disorder transition in LaMnO_(3+delta)
We report a novel dielectric anomaly around the Jahn-Teller orbital
order-disorder transition temperature T_JT in LaMnO_(3+delta). The transition
has been characterized by resistivity (rho)versus temperature (T), calorimetry,
and temperature-dependent X-ray diffraction studies. Measurements of complex
dielectric permittivity epsilon* (= epsilon'-i.epsilon'') over a low-frequency
range (1 Hz - 10 MHz)across T_JT reveal a distinct anomaly. This observation,
and the reported relatively high static dielectric constant at T = 0 (epsilon0
\~18-20), possibly indicate that the orbital order gives rise to intrinsic
polarization that undergoes transition at T_JT. The frequency dispersion of the
dielectric response at any given temperature, however, reveals that the
dielectric response consists of Maxwell-Wagner component, due to interfaces,
within such a low frequency range. The T_JT and the nature of the anomaly in
epsilon'(omega,T), epsilon''(omega,T) at T_JT, of course, vary - from a sharp
upward feature to a smeared plateau and then a downward trend - depending on
the Mn^4+ concentration of the sample. The observation of an intrinsic
dielectric response due to long-range orbital order in LaMnO_3 - where no
ferroelectric order is possible due to the absence of off-centre distortion in
MnO_6 octahedra - may throw a new light onto these classes of materials
vis-a-vis multiferroic materials.Comment: 22 pages including 7 figures; pdf only; accepted for publication in
J.Phys.:Condens. Matte
Doped orbitally-ordered systems: another case of phase separation
A possible mechanism of electronic phase separation in the systems with
orbital ordering is analyzed. We suggest a simple model taking into account an
interplay between the delocalization of charge carriers introduced by doping
and the cooperative ordering of local lattice distortions. The proposed
mechanism is quite similar to the double exchange usually invoked for
interpretation of phase separation in doped magnetic oxides like manganites,
but can be efficient even in the absence of any magnetic ordering. It is
demonstrated that the delocalized charge carriers favor the formation of
nanoscale inhomogeneities with the orbital structure different from that in the
undoped material. The directional character of orbitals leads to
inhomogeneities of different shapes and sizes.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn2O5 and BiMn2O5
The origin of magnetic frustration was stated and the ions whose shift is
accompanied by emerging magnetic ordering and ferroelectricity in TbMn2O5 and
BiMn2O5 were determined on the basis of calculation of magnetic coupling
parameters by using the structural data. The displacements accompanying the
magnetic ordering are not polar, they just induce changes of bond valence
(charge disordering) of Mn1 and Mn2, thus creating instability of the crystal
structure. To approximate again the bond valence to the initial value (charge
ordering) under magnetic ordering conditions is possible only due to polar
displacement of Mn2 (or O1) and O4 ions along the b axis that is the cause of
ferroelectric transition.Comment: 17 pages, 3 figures, 5 table
Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices
We demonstrate that delta-doping can be used to create a dimensionally
confined region of metallic ferromagnetism in an antiferromagnetic (AF)
manganite host, without introducing any explicit disorder due to dopants or
frustration of spins. Delta-doped carriers are inserted into a manganite
superlattice (SL) by a digital-synthesis technique. Theoretical consideration
of these additional carriers show that they cause a local enhancement of
ferromagnetic (F) double-exchange with respect to AF superexchange, resulting
in local canting of the AF spins. This leads to a highly modulated
magnetization, as measured by polarized neutron reflectometry. The spatial
modulation of the canting is related to the spreading of charge from the doped
layer, and establishes a fundamental length scale for charge transfer,
transformation of orbital occupancy and magnetic order in these manganites.
Furthermore, we confirm the existence of the canted, AF state as was predicted
by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained
elusive
RIXS interferometry and the role of disorder in the quantum magnet BaTiIrO
Motivated by several claims of spin-orbit driven spin-liquid physics in
hexagonal BaTiIrO hosting Ir2O9 dimers, we report on
resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge for different x.
We demonstrate that magnetism in BaTiIrO is governed by an
unconventional realization of strong disorder, where cation disorder affects
the character of the local moments. RIXS interferometry, studying the RIXS
intensity over a broad range of transferred momentum q, is ideally suited to
assign different excitations to different Ir sites. We find pronounced Ir-Ti
site mixing. Both ions are distributed over two crystallographically
inequivalent sites, giving rise to a coexistence of quasimolecular singlet
states on Ir2O9 dimers and spin-orbit entangled j=1/2 moments of 5d
Ir ions. RIXS reveals different kinds of strong magnetic couplings for
different bonding geometries, highlighting the role of cation disorder for the
suppression of long-range magnetic order in this family of compounds.Comment: 12 pages, 9 figure