15 research outputs found
Phase Diagram of the Half-Filled Ionic Hubbard Model
We study the phase diagram of the ionic Hubbard model (IHM) at half-filling
using dynamical mean field theory (DMFT), with two impurity solvers, namely,
iterated perturbation theory (IPT) and continuous time quantum Monte Carlo
(CTQMC). The physics of the IHM is governed by the competition between the
staggered potential and the on-site Hubbard U. In both the methods we
find that for a finite and at zero temperature, anti-ferromagnetic
(AFM) order sets in beyond a threshold via a first order phase
transition below which the system is a paramagnetic band insulator. Both the
methods show a clear evidence for a transition to a half-metal phase just after
the AFM order is turned on, followed by the formation of an AFM insulator on
further increasing U. We show that the results obtained within both the methods
have good qualitative and quantitative consistency in the intermediate to
strong coupling regime. On increasing the temperature, the AFM order is lost
via a first order phase transition at a transition temperature within both the methods, for weak to intermediate values of U/t. But
in the strongly correlated regime, where the effective low energy Hamiltonian
is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition
from the AFM phase to the paramagnetic phase, but the CTQMC does. As a result,
at any finite temperature T, DMFT+CTQMC shows a second phase transition (not
seen within DMFT+IPT) on increasing U beyond . At , when
the Neel temperature for the effective Heisenberg model becomes lower
than T, the AFM order is lost via a second order transition. In the
3-dimensonal parameter space of , there is a line of
tricritical points that separates the surfaces of first and second order phase
transitions.Comment: Revised versio
Magnetoresistance anomaly during the electrical triggering of a metal-insulator transition
Phase separation naturally occurs in a variety of magnetic materials and it
often has a major impact on both electric and magnetotransport properties. In
resistive switching systems, phase separation can be created on demand by
inducing local switching, which provides an opportunity to tune the electronic
and magnetic state of the device by applying voltage. Here we explore the
magnetotransport properties in the ferromagnetic oxide (La,Sr)MnO3 (LSMO)
during the electrical triggering of an intrinsic metal-insulator transition
(MIT) that produces volatile resistive switching. This switching occurs in a
characteristic spatial pattern, i.e., the formation of an insulating barrier
perpendicular to the current flow, enabling an electrically actuated
ferromagnetic-paramagnetic-ferromagnetic phase separation. At the threshold
voltage of the MIT triggering, both anisotropic and colossal magnetoresistances
exhibit anomalies including a large increase in magnitude and a sign flip.
Computational analysis revealed that these anomalies originate from the
coupling between the switching-induced phase separation state and the intrinsic
magnetoresistance of LSMO. This work demonstrates that driving the MIT material
into an out-of-equilibrium resistive switching state provides the means to
electrically control of the magnetotransport phenomena
Light induced decoupling of electronic and magnetic properties in manganites
The strongly correlated material La0.7Sr0.3MnO3 (LSMO) exhibits
metal-to-insulator and magnetic transition near room temperature. Although the
physical properties of LSMO can be manipulated by strain, chemical doping,
temperature, or magnetic field, they often require large external stimuli. To
include additional flexibility and tunability, we developed a hybrid
optoelectronic heterostructure that uses photocarrier injection from cadmium
sulfide (CdS) to an LSMO layer to change its electrical conductivity. LSMO
exhibits no significant optical response, however, the CdS/LSMO
heterostructures show an enhanced conductivity, with ~ 37 % resistance drop, at
the transition temperature under light stimuli. This enhanced conductivity in
response to light is comparable to the effect of a 9 T magnetic field in pure
LSMO. Surprisingly, the optical and magnetic responses of CdS/LSMO
heterostructures are decoupled and exhibit different effects when both stimuli
are applied. This unexpected behavior shows that heterostructuring strongly
correlated oxides may require a new understanding of the coupling of physical
properties across the transitions and provide the means to implement new
functionalities
Phase diagram of the half-filled ionic Hubbard model in the limit of strong correlations
We investigate the ionic Hubbard model (IHM) at half-filling in the limit of strong correlations and large ionic potential. The low-energy effective Hamiltonian in this limit, obtained by a similarity transformation, is a modified t-J model with effective second-neighbor hopping terms. We explore the possibilities of d-wave pairing and extended s-wave pairing superconducting (SC) phases on a two-dimensional square lattice at zero temperature within a Gutzwiller projected renormalized mean-field theory. In the sector of solutions that forbid spin-ordering, the system shows a finite nonzero d-wave as well as extended s-wave pairing amplitude for Delta similar to U >> t. The width of the superconducting phase in U - Delta regime shrinks with increase in U and Delta, though the extended s-wave pairing phase is higher in energy than the d-wave pairing superconducting phase. But in a spin-resolved renormalized mean-field calculation, which allows for an antiferromagnetic (AF) order along with the d-wave or extended s-wave pairing, the SC phase is no longer viable and the system shows a direct transition from an AF ordered phase to a paramagnetic band insulator. Except for a thin sliver of a half-metallic AF phase close to the AF transition point, most of the AF ordered phase is a Mott insulator. We benchmarked the AF Mott insulator to band insulator transition within the Gutzwiller projected renormalized mean-field theory against the dynamical mean-field theory solved using continuous time quantum Monte Carlo. Our work suggests that the ground-state phase diagram of the IHM at half-filling in the limit of extreme correlations does not have any SC phase. The SC phase seen in the paramagnetic sector is a metastable phase, being higher in energy than the AF Mott insulator phase
Triple-decker complexes comprising heterocyclic middle-deck with coinage metals
International audienceEarlier accounts of triple-decker complexes comprising main group elements and transition metals in the middle-deck, motivated us to synthesize triple-decker complexes containing coinage metals in the middle-deck. As a result, we have explored the reactivity of open-cage nido - [(Cp *M) 2 { mu-B 2 H 2 E 2 }], 1 -3 (Cp * = eta 5 -C 5 Me 5 , 1 : M = Co, E = S ; 2 : M = Co, E = Se; 3 : M = Rh, E = Se) with [CuBr(SMe 2 )]. All the reactions yielded triple-decker complexes, [(Cp *M) 2 { mu-B 2 H 2 E 2 CuBr}], 4 -6 ( 4 : M = Co, E = S ; 5 : M = Co, E = Se; 6 : M = Rh, E = Se) having [CuBr] in the middle-deck. The removal of the SMe 2 ligand resulted in the formation of complexes 4 -6 as a single product. These complexes are examples of triple-decker species having a planar 5-membered [B 2 E 2 Cu] (E = S or Se) middle deck, in which the Cu exists as Cu(I) with an elongated M-Cu bonding interaction. Synthesized complexes have been established by ESI-MS, multinuclear nuclear magnetic resonance (NMR), and IR spectroscopy. The solid-state structures of 5 and 6 were confirmed by single-crystal X-ray diffraction analyses. Density functional theory (DFT) analyses of these complexes have presented a high electron donation from the [B 2 E 2 ] (E = S or Se) fragment of the middle ring to the axial metals and a weak bonding interaction between group 9 metals and Cu