-
[show abstract]
[hide abstract]
ABSTRACT: The nematic state of the iron-based superconductors is studied in the undoped
limit of the three-orbital ($xz$, $yz$, $xy$) spin-fermion model via the
introduction of lattice degrees of freedom. Monte Carlo simulations show that
in order to stabilize the experimentally observed lattice distortion and
nematic order, and to reproduce photoemission experiments, {\it both} the
spin-lattice and orbital-lattice couplings are needed. The interplay between
their respective coupling strengths regulates the separation between the
structural and N\'eel transition temperatures. Experimental results for the
temperature dependence of the resistivity anisotropy and the angle-resolved
photoemission (ARPES) orbital spectral weight are reproduced by the present
numerical simulations.
05/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: The emergence of magnetic reconstructions at the interfaces of oxide
heterostructures are often explained via subtle modifications in the electronic
densities, exchange couplings, or strain. Here an additional possible route for
induced magnetism is studied in the context of the
(LaNiO$_3$)$_n$/(LaMnO$_3$)$_n$ superlattices using a hybrid tight-binding
model. In the LaNiO$_3$ region, the induced magnetizations decouple from the
intensity of charge leakage from Mn to Ni, but originate from the spin-filtered
quantum confinement present in these nanostructures. In general, the induced
magnetization is the largest for the (111)-stacking and the weakest for the
(001)-stacking superlattices, results compatible with the exchange bias effects
reported by Gibert et al. Nat. Mater. 11, 195 (2012).
02/2013;
-
[show abstract]
[hide abstract]
ABSTRACT: The random-phase approximation (RPA) is here applied to a two-orbital model
for the BiS2-based superconductors that was recently proposed by Usui et al.,
arXiv:1207.3888. Varying the density of doped electrons per Bi site, n, in the
range 0.46 < n < 1.0, the spin fluctuations promote competing A1g and B2g
superconducting states with similar pairing strengths, in analogy with the
A1g-B1g near degeneracy found also within RPA in models for pnictides. At these
band fillings, two hole-pockets centered at (0,0) and (\pi,\pi) display nearly
parallel Fermi Surface segments close to wavevector(\pi/2,\pi/2), whose
distance increases with n. After introducing electronic interactions treated in
the RPA, the inter-pocket nesting of these segments leads to pair scattering
with a rather "local" character in k-space. The similarity between the A1g and
B2g channels observed here should manifest in experiments on BiS2-based
superconductors if the pairing is caused by spin fluctuations.
12/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: The two-orbital double-exchange model is employed for the study of the
magnetic and orbital orders in ($R$MnO$_3$)$_n$/($A$MnO$_3$)$_{2n}$ ($R$: rare
earths; $A$: alkaline earths) superlattices. The A-type antiferromagnetic order
is observed in a broad region of parameter space for the case of SrTiO$_3$ as
substrate, in agreement with recent experiments and first-principles
calculations using these superlattices. In addition, also a C-type
antiferromagnetic state is predicted to be stabilized when using substrates
like LaAlO$_3$ with smaller lattice constants than SrTiO$_3$, again in
agreement with first principles results. The physical mechanism for the
stabilization of the A- and C- magnetic transitions is driven by the orbital
splitting of the $x^2-y^2$ and $3z^2-r^2$ orbitals. This splitting is induced
by the $Q_3$ mode of Jahn-Teller distortions created by the strain induced by
the substrates. In addition to the special example of
(LaMnO$_3$)$_n$/(SrMnO$_3$)$_{2n}$, our phase diagrams can be valuable for the
case where the superlattices are prepared employing narrow bandwidth
manganites. In particular, several non-homogenous magnetic profiles are
predicted to occur in narrow bandwidth superlattices, highlighting the
importance of carrying out investigations in this mostly unexplored area of
research.
11/2012;
-
Qinlong Luo,
Andrew Nicholson,
Julián Rincón,
Shuhua Liang,
José Riera,
Gonzalo Alvarez,
Limin Wang,
Wei Ku,
German D. Samolyuk,
Adriana Moreo, Elbio Dagotto
[show abstract]
[hide abstract]
ABSTRACT: Recent neutron scattering experiments addressing the magnetic state of the
two-leg ladder selenide compound BaFe$_2$Se$_3$ have unveiled a dominant spin
arrangement involving ferromagnetically ordered 2$\times$2 iron-superblocks,
that are antiferromagnetically coupled among them (the "block-AFM" state).
Using the electronic five-orbital Hubbard model, first principles techniques to
calculate the electronic hopping amplitudes between irons, and the real-space
Hartree-Fock approximation to handle the many-body effects, here it is shown
that the exotic block-AFM state is indeed stable at realistic electronic
densities close to $n \sim 6.0$. Another state (the "CX" state) with parallel
spins along the rungs and antiparallel along the legs of the ladders is close
in energy. This state becomes stable in other portions of the phase diagrams,
such as with hole doping, as also found experimentally via neutron scattering
applied to KFe$_2$Se$_3$. In addition, the present study unveils other
competing magnetic phases that could be experimentally stabilized varying
either $n$ chemically or the electronic bandwidth by pressure. Similar results
were obtained using two-orbital models, studied here via Lanczos and DMRG
techniques. A comparison of the results obtained with the realistic selenides
hoppings amplitudes for BaFe$_2$Se$_3$ against those found using the hopping
amplitudes for pnictides reveals several qualitative similarities, particularly
at intermediate and large Hubbard couplings.
05/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: In a recent publication [ S. Dong et al. Phys. Rev. Lett. 103 127201 (2009)], two (related) mechanisms were proposed to understand the intrinsic exchange bias present in oxides heterostructures involving G-type antiferromagnetic perovskites. The first mechanism is driven by the Dzyaloshinskii-Moriya interaction, which is a spin-orbit coupling effect. The second is induced by the ferroelectric polarization, and it is only active in heterostructures involving multiferroics. Using the SrRuO3/SrMnO3 superlattice as a model system, density-functional calculations are here performed to verify the two proposals. This proof-of-principle calculation provides convincing evidence that qualitatively supports both proposals.
Phys. Rev. B. 12/2011; 84(22).
-
[show abstract]
[hide abstract]
ABSTRACT: The undoped three-orbital spin fermion model for the Fe-based superconductors
is studied via Monte Carlo techniques in two-dimensional clusters. At low
temperatures, the magnetic and one-particle spectral properties are in good
agreement with neutron and photoemission experiments. Our most important
results are the resistance vs. temperature curves that display all the features
experimentally observed in BaFe$_2$As$_2$ detwinned single crystals (under
uniaxial stress), including a low-temperature anisotropy between the two
directions followed by a peak at the magnetic ordering temperature, here
induced by short-range spin order and concomitant Fermi Surface orbital order.
11/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The hole-doped ground state of a recently introduced extended "t-U-J"
two-orbital Hubbard model for the Fe-based superconductors is studied via exact
diagonalization methods on small clusters. Similarly as in the previously
studied case of electron doping, A. Nicholson et al., Phys. Rev. Lett. 106
21702 (2011), upon hole doping it is observed that there are several competing
pairing symmetries including A_{1g}, B_{1g}, and B_{2g}. However, contrary to
the electron-doped case, the ground state of the hole-doped state has
pseudocrystal momentum k=(\pi,\pi) in the unfolded Brillouin zone. In the two
Fe-atom per unit cell representation, this indicates that the ground state
involves anti-bonding, rather than bonding, combinations of the orbitals of the
two Fe atoms in the unit-cell. The lowest state with k=(0,0) has only a
slightly higher energy. These results indicate that this simple two-orbital
model may be useful to capture some subtle aspects of the hole-doped pnictides
since calculations for the five-orbital model have unveiled a hole pocket
centered at M (k=(\pi,\pi)) in the unfolded Brillouin zone.
11/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The resistivity anisotropy unveiled in the study of detwinned single crystals of the undoped 122 pnictides is here studied using the two-dimensional, three-orbital Hubbard model in the mean-field approximation. Calculating the Drude weight in the x and y directions at zero temperature for a Q=(π,0) magnetically ordered state, the conductance along the antiferromagnetic direction is shown to be larger than along the ferromagnetic direction. This effect is caused by the suppression of the dyz orbital at the Fermi surface, but additional insight based on the momentum dependence of the transitions induced by the current operator is provided. It is shown that the effective suppression of the interorbital hopping dxy and dyz along the y direction is the main cause of the anisotropy.
Phys. Rev. B. 10/2011; 84(13).
-
[show abstract]
[hide abstract]
ABSTRACT: A microscopic model Hamiltonian for the ferroelectric field effect is
introduced for the study of oxide heterostructures with ferroelectric
components. The long-range Coulomb interaction is incorporated as an
electrostatic potential, solved self-consistently together with the charge
distribution. A generic double-exchange system is used as the conducting
channel, epitaxially attached to the ferroelectric gate. The observed
ferroelectric screening effect, namely the charge accumulation/depletion near
the interface, is shown to drive interfacial phase transitions that give rise
to robust magnetoelectric responses and bipolar resistive switching, in
qualitative agreement with previous density functional theory calculations. The
model can be easily adapted to other materials by modifying the Hamiltonian of
the conducting channel, and it is useful in simulating ferroelectric field
effect devices particularly those involving strongly correlated electronic
components where ab-initio techniques are difficult to apply.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: In this report, we study in detail the competitor to the FM metallic state at
electronic density $x=1/4$ in the CMR regime using the two-orbital
double-exchange model with Jahn-Teller lattice distortions on two-dimensional
clusters, employing a very careful large-scale cooling down process in the
Monte Carlo simulations to avoid being trapped in metastable states. Our
investigations show that this competing insulator has a very unexpected complex
structure, involving diagonal stripes with alternating regions with FM and
CE-like order. The level of complexity of this new state even surpasses that of
the recently unveiled spin-orthogonal-stripe states and their associated high
degeneracy. This new state complements the long-standing scenario of phase
separation, since the alternating FM-CE pattern appears even in the present
study which is carried out in the clean limit. The present and recent
investigations are also in agreement with the many "glassy" characteristics of
the CMR state found experimentally, due to the high degeneracy of the
insulating states involved in the process. Results for the spin-structure
factor of the new states are also here provided to facilitate the analysis of
neutron scattering experiments for these materials.
09/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The resistivity anisotropy unveiled in the study of detwinned single crystals
of the undoped 122 pnictides is here studied using the two-dimensional
three-orbital Hubbard model in the mean-field approximation. Calculating the
Drude weight in the x and y directions at zero temperature for a Q=(\pi,0)
magnetically ordered state, the conductance along the antiferromagnetic
direction is shown to be larger than along the ferromagnetic direction. This
effect is caused by the suppression of the d_{yz} orbital at the Fermi surface,
but additional insight based on the momentum dependence of the transitions
induced by the current operator is provided. It is shown that the effective
suppression of the inter-orbital hopping d_{xy} and d_{yz} along the y
direction is the main cause of the anisotropy.
08/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Motivated by the recent discovery of Fe-based superconductors close to an antiferromagnetic insulator in the experimental phase diagram, here the five-orbital Hubbard model (without lattice distortions) is studied using the real-space Hartree-Fock approximation, employing a 10×10 Fe cluster with Fe vacancies in a √5×√5 pattern. Varying the Hubbard and Hund couplings, and at electronic density n=6.0, the phase diagram contains an insulating state with the same spin pattern as observed experimentally, involving 2×2 ferromagnetic plaquettes coupled with one another antiferromagnetically. The presence of local ferromagnetic tendencies is in qualitative agreement with Lanczos results for the three-orbital model also reported here. The magnetic moment ∼3μB/Fe is in good agreement with experiments. Several other phases are also stabilized in the phase diagram, in agreement with recent calculations using phenomenological models.
Phys. Rev. B. 08/2011; 84(14).
-
[show abstract]
[hide abstract]
ABSTRACT: To understand the role that degeneracy, hybridization, and nesting play in
the magnetic and pairing properties of multiorbital Hubbard models we here
study numerically two types of two- orbital models, both with hole-like and
electron-like Fermi surfaces (FS's) that are related by nesting vectors ({\pi},
0) and (0, {\pi}). In one case the bands that determine the FS's arise from
strongly hybridized degenerate dxz and dyz orbitals, while in the other the two
bands are determined by non-degenerate and non-hybridized s-like orbitals.
Using a variety of techniques, in the weak coupling regime it is shown that
only the model with hybridized bands develops metallic magnetic order, while
the other model exhibits an ordered excitonic orbital-transverse spin state
that is insulating and does not have a local magnetization. However, both
models display similar insulating magnetic stripe ordering in the strong
coupling limit. These results indicate that nesting is a necessary but not
sufficient condition for the development of ordered states with finite local
magnetization in multiorbital Hubbard systems; the additional ingredient
appears to be that the nested portions of the bands need to have the same
orbital flavor. This condition can be achieved via strong hybridization of the
orbitals in weak coupling or via the FS reconstruction induced by the Coulomb
interactions in the strong coupling regime. This effect also impacts the
pairing symmetry as demonstrated by the study of the dominant pairing channels
for the two models.
07/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We introduce and study an extended "t-U-J" two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (π,0)-(0,π) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A(1g) and B(2g) pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B(1g) states, suggesting that small changes in parameters may render any of the three channels stable.
Physical Review Letters 05/2011; 106(21):217002. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The two-orbital Hubbard model for the pnictides is studied numerically using
the real-space Hartree-Fock approximation on finite clusters. Upon electron
doping, states with a nonuniform distribution of charge are stabilized. The
observed patterns correspond to charge stripes oriented perpendicular to the
direction of the spin stripes of the undoped magnetic ground state. While these
charge striped states are robust when the undoped state has a Hubbard gap,
their existence when the intermediate-coupling magnetic metallic state of
pnictides is doped was also observed for particular model parameters. Results
for hole doping and implications for recent experiments that reported
electronic nematic states and spin incommensurability are also briefly
discussed.
03/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We introduce and study an extended "t-U-J" two-orbital model for the
pnictides that includes Heisenberg terms deduced from the strong coupling
expansion. Including these J terms explicitly allows us to enhance the strength
of the (pi, 0)-(0, pi) spin order which favors the presence of tightly bound
pairing states even in the small clusters that are here exactly diagonalized.
The A1g and B2g pairing symmetries are found to compete in the realistic
spin-ordered and metallic regime. The dynamical pairing susceptibility
additionally unveils low-lying B1g states, suggesting that small changes in
parameters may render any of the three channels stable.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: The widely used Double-Exchange model for manganites is shown to support
various "striped" phases at filling fractions 1/n (n=3, 4, 5...), in the
previously unexplored regime of narrow bandwidth and small Jahn-Teller
coupling. Working in two dimensions, our main result is that these stripes can
be individually spin flipped without a physically relevant change in the
energy, i.e., we find a large groundstate manifold with nearly degenerate
energies. The two-dimensional spin system thus displays an unexpected
dynamically generated dimensional reduction into decoupled one-dimensional
stripes, even though the electronic states remain two-dimensional. Relations of
our results with recent literature addressing compass models in quantum
computing are discussed.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We report that CaMn$_7$O$_{12}$ is a new magnetic multiferroic material. The
appearance of a ferroelectric polarization coinciding with the magnetic phase
transition ($\sim90$ K) suggests the presence of ferroelectricity induced by
magnetism, further confirmed by its strong magnetoelectric response. With
respect to other known magnetic multiferroics, CaMn$_7$O$_{12}$ displays
attractive multiferroic properties, such as a high ferroelectric critical
temperature and large polarization. More importantly, these results open a new
avenue to search for magnetic multiferroics in the catalogue of doped oxides.
01/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Large-scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are presented here. At hole density x=1/4 and in the vicinity of the region of competition between the ferromagnetic metallic and spin-charge-orbital ordered insulating phases, the colossal magnetoresistance (CMR) phenomenon is observed with a magnetoresistance ratio ∼10,000%. Our main result is that this CMR transition is found to be of first order in some portions of the phase diagram, in agreement with early results from neutron scattering, specific heat, and magnetization, thus solving a notorious discrepancy between experiments and previous theoretical studies. The first order characteristics of the transition survive, and are actually enhanced, when weak quenched disorder is introduced.
Physical Review Letters 08/2010; 105(9):097203. · 7.37 Impact Factor
