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Register a new userOnset of metallic ferromagnetism in a doped spin-orbital chain
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Starting from a spin-orbital model for doped manganites, we investigate a competition between ferromagnetic and antiferromagnetic order in a one-dimensional model at finite temperature. The magnetic and orbital order at half filling support each other and depend on a small antiferromagnetic superexchange between t_{2g} spins and on an alternating Jahn-Teller potential. The crossover to a metallic ferromagnetic phase found at finite doping is partly suppressed by the Jahn-Teller potential which may localize e_g electrons.
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We develop a quantitatively predictive theory for impurity-band ferromagnetism in the low-doping regime of GaMnAs and compare with experimental measurements of a series of samples whose compositions span the transition from paramagnetic insulating to ferromagnetic conducting behavior. The theoretical Curie temperatures depend sensitively on the local fluctuations in the Mn-hole binding energy, which originates from disorder in the Mn distribution as well as the presence of As antisite defects. The experimentally-determined hopping energy at the Curie temperature is roughly constant over a series of samples whose conductivities vary more than 10^4 and whose hole concentrations vary more than 10^2. Thus in this regime the hopping energy is an excellent predictor of the Curie temperature for a sample, in agreement with the theory.
Using {it ab initio} density functional theory, here we systematically study the monolayer MoOCl$_2$ with a $4d^2$ electronic configuration. Our main results is that an orbital-selective Peierls phase (OSPP) develops in MoOCl$_2$, resulting in the dimerization of the Mo chain along the $b$-axis. Specifically, the Mo-$d_{xy}$ orbitals form robust molecular-orbital states inducing localized $d_{xy}$ singlet dimers, while the Mo-$d_{xz/yz}$ orbitals remain delocalized and itinerant. Our study shows that MoOCl$_2$ is globally metallic, with the Mo-$d_{xy}$ orbital bonding-antibonding splittings opening a gap and the Mo-$d_{xz/yz}$ orbitals contributing to the metallic conductivity. Overall, the results resemble the recently much discussed orbital-selective Mott phase but with the localized band induced by a Peierls distortion instead of Hubbard interactions. Finally, we also qualitatively discuss the possibility of OSPP in the $3d^2$ configuration, as in CrOCl$_2$.
We present electronic structure calculations in combination with local and non-local many-body correlation effects for the half-metallic ferromagnet CrO$_2$. Finite-temperature Dynamical Mean Field Theory results show the existence of non-quasiparticle states, which were recently observed as almost currentless minority spin states near the Fermi energy in resonant scattering experients. At zero temperatures, Variational Cluster Approach calculations support the half-metallic nature of CrO$_2$ as seen in superconducting point contact spectroscopy. The combination of these two techniques allowed us to qualitatively describe the spin-polarization in CrO$_2$.
Present work demonstrates the formation of spin-orbital polarons in electron doped copper oxides, that arise due to doping-induced polarisation of the oxygen orbitals in the CuO$_2$ planes. The concept of such polarons is fundamentally different from previous interpretations. The novel aspect of spin-orbit polarons is best described by electrons becoming self-trapped in one-dimensional channels created by polarisation of the oxygen orbitals. The one-dimensional channels form elongated filaments with two possible orientations, along the diagonals of the elementary CuO$_2$ square plaquette. As the density of doped electrons increases multiple filaments are formed. These may condense into a single percollating filamentary phase. Alternatively, the filaments may cross perpendicularly to create an interconnected conducting quasi-one-dimensional web. At low electron doping the antiferromagnetic (AFM) state and the polaron web coexist. As the doping is increased the web of filaments modifies and transforms the AFM correlations leading to a series of quantum phase transitions - which affect the normal and superconducting state properties.
Heterostructures of mixed-valence manganites are still under intense scrutiny, due to the occurrence of exotic quantum phenomena linked to electronic correlation and interfacial composition. For instance, if two anti-ferromagnetic insulators as LaMnO$_3$ and SrMnO$_3$ are grown in a (001)-oriented superlattice, a half-metallic ferromagnet may form, provided that the thickness is sufficiently small to allow tunneling across interfaces. In this article, we employ electronic structure calculations to show that all the layers of a (111)-oriented LaMnO$_3$|SrMnO$_3$ superlattice retain a half-metallic ferromagnetic character for a much larger thickness than in its (001) counterpart. This behavior is shown to be linked to the charge transfer across the interface, favored by the octahedral connectivity between the layers. This also results in a symmetry-induced quenching of the Jahn-Teller distortions, which are replaced by breathing modes. The latter are coupled to charge and spin oscillations, whose components have a pure e g character. Most interestingly, the magnetization reaches its maximum value inside the LaMnO$_3$ region and not at the interface, which is fundamentally different from what observed for the (001) orientation. The analysis of the inter-atomic exchange coupling shows that the magnetic order arises from the double-exchange mechanism, despite competing interactions inside the SrMnO$_3$ region. Finally, the van Vleck distortions and the spin oscillations are found to be crucially affected by the variation of Hunds exchange and charge doping, which allows us to speculate that our system behaves as a Hunds metal, creating an interesting connection between manganites and nickelates.
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