A wide variety of experimental results and theoretical investigations in recent years have convincingly demonstrated that several transition metal oxides and other materials have dominant states that ...are not spatially homogeneous. This occurs in cases in which several physical interactions--spin, charge, lattice, and/or orbital--are simultaneously active. This phenomenon causes interesting effects, such as colossal magnetoresistance, and it also appears crucial to understand the high-temperature superconductors. The spontaneous emergence of electronic nanometer-scale structures in transition metal oxides, and the existence of many competing states, are properties often associated with complex matter where nonlinearities dominate, such as soft materials and biological systems. This electronic complexity could have potential consequences for applications of correlated electronic materials, because not only charge (semiconducting electronic), or charge and spin (spintronics) are of relevance, but in addition the lattice and orbital degrees of freedom are active, leading to giant responses to small perturbations. Moreover, several metallic and insulating phases compete, increasing the potential for novel behavior.
High-temperature superconductivity in the iron-based materials emerges from, or sometimes coexists with, their metallic or insulating parent compound states. This is surprising, as these undoped ...states exhibit dramatically different antiferromagnetic spin arrangements and Néel temperatures. Although there is a general consensus that magnetic interactions are important for superconductivity, much remains unknown concerning the microscopic origin of the magnetic states. In this review, we summarize the progress in this area, focusing on recent experimental and theoretical results, and their microscopic implications. We conclude that the parent compounds are in a state that is more complex than that implied by a simple Fermi surface nesting scenario, and a dual description including both itinerant and localized degrees of freedom is needed to properly describe these fascinating materials. PUBLICATION 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, both the spin-lattice and orbital-lattice couplings are needed. The interplay between their respective coupling strengths regulates the separation between the structural and Néel transition temperatures. Experimental results for the temperature dependence of the resistivity anisotropy and the angle-resolved photoemission orbital spectral weight are reproduced by the present numerical simulations.
When Oxides Meet Face to Face Dagotto, Elbio
Science (American Association for the Advancement of Science),
11/2007, Letnik:
318, Številka:
5853
Journal Article
Recenzirano
New devices may be possible once we understand the interface between oxide materials such as superconductors and ferromagnets.
ABSTRACT
The key physical property of multiferroic materials is the existence of coupling between magnetism and polarization, i.e. magnetoelectricity. The origin and manifestations of ...magnetoelectricity can be very different in the available plethora of multiferroic systems, with multiple possible mechanisms hidden behind the phenomena. In this review, we describe the fundamental physics that causes magnetoelectricity from a theoretical viewpoint. The present review will focus on mainstream physical mechanisms in both single-phase multiferroics and magnetoelectric heterostructures. The most recent tendencies addressing possible new magnetoelectric mechanisms will also be briefly outlined.
An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using ...asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the n-type interface side than at the p-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change > 90% of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods.
Abstract
Planar Josephson junctions provide a versatile platform, alternative to the nanowire-based geometry, for the generation of the Majorana bound states, due to the additional phase tunability ...of the topological superconductivity. The proximity induction of chiral magnetism and superconductivity in a two-dimensional electron gas showed remarkable promise to manipulate topological superconductivity. Here, we consider a Josephson junction involving a skyrmion crystal and show that the chiral magnetism of the skyrmions can create and control the Majorana bound states without the requirement of an intrinsic Rashba spin-orbit coupling. Interestingly, the Majorana bound states in our geometry are realized robustly at zero phase difference at the junction. The skyrmion radius, being externally tunable by a magnetic field or a magnetic anisotropy, brings a unique control feature for the Majorana bound states.
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 (LaNiO3)n/(LaMnO3)n superlattices using a hybrid tightbinding model. In the LaNiO3 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 byGibert et al. Nat.Mater. 11, 195 (2012).
Three orbital model for the iron-based superconductors Daghofer, Maria; Nicholson, Andrew; Moreo, Adriana ...
Physical review. B, Condensed matter and materials physics,
01/2010, Letnik:
81, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The theoretical need to study the properties of the Fe-based high-Tc superconductors using reliable manybody techniques has highlighted the importance of determining what is the minimum number of ...orbital degrees of freedom that will capture the physics of these materials. While the shape of the Fermi surface FS obtained with the local-density approximation LDA can be reproduced by a two-orbital model, it has been argued that the bands that cross the chemical potential result from the strong hybridization of three of the Fe 3d orbitals. For this reason, a three orbital Hamiltonian for LaOFeAs obtained with the Slater-Koster formalism by considering the hybridization of the As p orbitals with the Fe dxz, dyz, and dxy orbitals is discussed here. This model reproduces qualitatively the FS shape and orbital composition obtained by LDA calculations for undoped LaOFeAs when four electrons per Fe are considered. Within a mean-field approximation, its magnetic and orbital properties in the undoped case are here described for intermediate values of J/U. Increasing the Coulomb repulsion U at zero temperature, four different regimes are obtained: 1 paramagnetic, 2 magnetic ,0 spin order, 3 the same ,0 spin order but now including orbital order, and finally 4 a magnetic and orbital ordered insulator. The spin-singlet pairing operators allowed by the lattice and orbital symmetries are also constructed. It is found that for pairs of electrons involving up to diagonal nearest-neighbors sites, the only fully gapped and purely intraband spin-singlet pairing operator is given by k= fkdk,, d k,, with fk=1 or cos kx cos ky which would arise only if the electrons in all different orbitals couple with equal strength to the source of pairing.
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