arXiv subjects

Elbio Dagotto

Publications and source records attributed to Elbio Dagotto.

At least 163 records · Page 9Linked to original sources

arXiv2011

Ab Initio Study of the Intrinsic Exchange Bias at the SrRuO$_3$/SrMnO$_3$ Interface

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 SrRuO$_3$/SrMnO$_3$ 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.

cond-mat.mtrl-sci↗

arXiv2011

Pairing Symmetries of a Hole-Doped Extended Two-Orbital Model for the Pnictides

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=(π,π) 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=(π,π)) in the unfolded Brillouin zone.

cond-mat.supr-con↗

arXiv2011

Multiferroic Properties of CaMn$_7$O$_{12}$

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.

cond-mat.mtrl-sci↗

arXiv2011

Microscopic model for the ferroelectric field effect in oxide heterostructures

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.

cond-mat.mtrl-sci↗

arXiv2011

Study of the magnetic state of K$_{0.8}$Fe$_{1.6}$Se$_2$ using the five-orbital Hubbard model in the Hartree-Fock approximation

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 10x10 Fe cluster with Fe vacancies in a $\sqrt{5}x\sqrt{5}$ 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 2x2 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 ~ 3$μ_B$/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.

cond-mat.supr-con↗

arXiv2011

Optical Conductivity Anisotropy in the Undoped Three-Orbital Hubbard Model for the Pnictides

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 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.

cond-mat.supr-con↗

arXiv2011

Role of Degeneracy, Hybridization, and Nesting in the Properties of Multi-Orbital Systems

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 (π, 0) and (0, π). 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.

cond-mat.supr-con↗

arXiv2011

Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides

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.

cond-mat.supr-con↗

arXiv2011

Emergent dimensional reduction of the spin sector in a model for narrow-band manganites

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.

cond-mat.str-el↗

arXiv2011

Charge Stripes in the Two-Orbital Hubbard Model for Pnictides

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.

cond-mat.supr-con↗

arXiv2010

Neutron and ARPES Constraints on the Couplings of the Multiorbital Hubbard Model for the Pnictides

The results of neutron scattering and angle-resolved photoemission experiments for the Fe-pnictide parent compounds, and their metallic nature, are shown to impose severe constraints on the range of values that can be considered "realistic" for the intraorbital Hubbard repulsion U and Hund coupling J in multiorbital Hubbard models treated in the mean-field approximation. Phase diagrams for three- and five-orbital models are here provided, and the physically realistic regime of couplings is highlighted, to guide future theoretical work into the proper region of parameters of Hubbard models. In addition, using the random phase approximation, the pairing tendencies in these realistic coupling regions are investigated. It is shown that the dominant spin-singlet pairing channels in these coupling regimes correspond to nodal superconductivity, with strong competition between several states that belong to different irreducible representations. This is compatible with experimental bulk measurements that have reported the existence of nodes in several Fe-pnictide compounds.

cond-mat.supr-con↗

arXiv2010

Unveiling First Order CMR Transitions in the Two-Orbital Model for Manganites

Large scale Monte Carlo simulation results for the two-orbital model for manganites, including Jahn-Teller lattice distortions, are here presented. 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.

cond-mat.str-el↗

arXiv2010

Orbital-weight redistribution triggered by spin order in the pnictides

The one-particle spectral function and its orbital composition are investigated in a three-orbital model for the undoped parent compounds of the iron-based superconductors. In the realistic parameter regime, where results best fit experimental data, it is observed that the magnetization in the xz and yz orbitals are markedly different and the Fermi surface presents mostly xz character, as recently observed in photoemission experiments [T. Shimojima et al., Phys. Rev. Lett. 104, 057002 (2010)]. Since the ferro-orbital order in this regime is at most a few percent, these results are mainly driven by the magnetic order. An analogous analysis for a five-orbital model leads to similar conclusions.

cond-mat.supr-con↗

arXiv2010

Electron-lattice coupling and partial nesting as the origin of Fermi-Arcs in manganites

A tight-binding model for eg electrons coupled to Jahn-Teller lattice distortions is studied via unbiased Monte- Carlo simulations. By focusing on the periodicity of the Jahn Teller distortions, and the one-particle spectral function, our results clarify the physical origin of the Fermi-arcs phase observed in layered manganites. In a range of parameters where no broken symmetry phase exists, the nearly nested Fermi surface favors certain correlations between Jahn Teller distortions. The spectral weight near the Brillouin zone edge is suppressed, leading to the pseudogap in the density of states. We discuss the stability of this phase as a function of temperature and coupling strength for different hole dopings.

cond-mat.str-el↗

arXiv2010

Conducting Jahn-Teller Domain Walls in Undoped Manganites

We investigate the electronic properties of multi-domain configurations in models for undoped manganites by means of variational and Monte Carlo techniques. These materials display simultaneous Jahn-Teller distortions and magnetic ordering. We find that a band of electronic states appears associated with Jahn-Teller domain walls, and this band is localized in the direction perpendicular to the walls. The energy and width of this band depends on the conformational properties of the domain walls. At finite temperatures, the conductance along the domain walls, induced by the localized domain wall bands, is orders magnitude larger than in the bulk.

cond-mat.str-el↗

arXiv2010

Highly anisotropic resistivities in the double-exchange model for strained manganites

The highly anisotropic resistivities in strained manganites are theoretically studied using the two-orbital double-exchange model. At the nanoscale, the anisotropic double-exchange and Jahn-Teller distortions are found to be responsible for the robust anisotropic resistivities observed here via Monte Carlo simulations. An unbalanced in the population of orbitals caused by strain is responsible for these effects. In contrast, the anisotropic superexchange is found to be irrelevant to explain our results. Our model study suggests that highly anisotropic resistivities could be present in a wide range of strained manganites, even without (sub)micrometer-scale phase separation. In addition, our calculations also confirm the formation of anisotropic clusters in phase-separated manganites, which magnifies the anisotropic resistivities.

cond-mat.str-el↗

arXiv2010

Constraints Imposed by Symmetry on Pairing Operators for the Pnictides

Considering model Hamiltonians that respect the symmetry properties of the pnictides, it is argued that pairing interactions that couple electrons at different orbitals with an orbital-dependent pairing strength inevitably lead to interband pairing matrix elements, at least in some regions of the Brillouin zone. Such interband pairing has not been considered of relevance in multiorbital systems in previous investigations. It is also observed that if, instead, a purely intraband pairing interaction is postulated, this requires that the pairing operator has the form $Δ^+(k)=f(k) \sum_α d^+_{k,α,\uparrow}d^+_{-k, α,\downarrow}$ where $α$ labels the orbitals considered in the model and f(k) arises from the spatial location of the coupled electrons or holes. This means that the gaps at two different Fermi surfaces involving momenta $k_F$ and $k'_F$ can only differ by the ratio $f(k_F)/ f(k'_F)$ and that electrons in different orbitals must be subject to the same pairing attraction, thus requiring fine tuning. These results suggest that previously neglected interband pairing tendencies could actually be of relevance in a microscopic description of the pairing mechanism in the pnictides.

cond-mat.supr-con↗

arXiv2010

Real-time dynamics of particle-hole excitations in Mott insulator-metal junctions

Charge excitations in Mott insulators (MIs) are distinct from their band-insulator counterparts and they can provide a mechanism for energy harvesting in solar cells based on strongly correlated electronic materials. In this paper, we study the real-time dynamics of holon-doublon pairs in a MI connected to metallic leads using the time-dependent density matrix renormalization group method. The transfer of charge across the MI-metal interface is controlled by both the electron-electron interaction strength within the MI and the voltage difference between the leads. We find an overall enhancement of the charge transfer as compared to the case of a (noninteracting) band insulator-metal interface with a similar band gap. Moreover, the propagation of holon-doublon excitations within the MI dynamically changes the spin-spin correlations, introducing time-dependent phase shifts in the spin structure factor.

cond-mat.str-el↗