28 research outputs found
Ordering and multiple phase transitions in ultra-thin nickelate superlattices
We interpret via advanced ab initio calculations the multiple phase
transitions observed recently in ultra-thin LaNiO/LaAlO
superlattices. The ground state is insulating, charge-ordered, and
antiferromagnetic due to concurrent structural distortion and weak valency
disproportionation. We infer distinct transitions at 40 K and 150 K,
respectively, from antiferromagnetic order to moment disorder, and from
structurally-dimerized insulator to an undistorted metallic Pauli paramagnet
(exhibiting a cuprate-like Fermi surface). The results are in satisfactory
agreement with experiment.Comment: ~4 pages, 4 figures; further improvement during revie
Fermi-surface pockets in : A comparison of ab initio techniques
We study the Fermi surface of metallic, non-magnetic \textit{ortho}-II
YBaCuO using three different density-functional-based
band-structure techniques (GGA, GGA+U, PSIC). The calculated Fermi surface
exhibits no pockets in GGA+U and PSIC, a minor one in GGA. Upon shifting the
Fermi level in the vicinity of the calculated value, we instead observe several
pocket structures. We calculate their cross-sectional areas and cyclotron
masses. Overall, our calculations show no solid evidence of the existence of
electron-like --nor, in fact, of any-- Fermi surface pockets in this phase.
This suggests that the origin of the pockets should be sought for in other,
different phases.Comment: 7 pages, 5 figures, in print on PRB 79 (2009
Topology of SO(5)-monopoles and three-dimensional, stable Dirac semimetals
The band-touching points of stable, three-dimensional, Kramers-degenerate,
Dirac semimetals are singularities of a five-component, unit vector field and
non-Abelian, -Berry's connections, whose topological classification is
an important, open problem. We solve this problem by performing second homotopy
classification of Berry's connections. Using Abelian projected connections, the
generic planes, orthogonal to the direction of nodal separation, and lying
between two Dirac points are shown to be higher-order topological insulators,
which support quantized, chromo-magnetic flux or relative Chern number, and
gapped, edge states. The Dirac points are identified as a pair of
unit-strength, - monopole and anti-monopole, where the relative Chern
number jumps by . Using these bulk invariants, we determine the
topological universality class of different types of Dirac semimetals. We also
describe a universal recipe for computing quantized, non-Abelian flux for Dirac
materials from the windings of spectra of planar Wilson loops, displaying
-gauge invariance. With non-perturbative, analytical solutions of
surface-states, we show the absence of helical Fermi arcs, and predict the
fermiology and the spin-orbital textures. We also discuss the similarities and
important topological distinction between the surface-states Hamiltonian and
the generator of Polyakov loop of Berry's connections.Comment: 19 pages, 8 figure
Uncorrelated Bi off-centering and the insulator-to-metal transition in ruthenium A(2)Ru(2)O(7) pyrochlores
The study of insulator-to-metal transitions is of interest from the viewpoint of fundamental understanding of the underlying physics, and materials at the brink of such transitions possess useful functionality. Driving this transition through compositional tuning can help engineer useful material properties. Here we study the role of disorder in the form of cation off-centering on the compositionally-controlled insulator-to-metal transition in the solid solution oxide pyrochlore (Pr1-xBix)2Ru2O7. Prior work has established site disorder by the Bi3+ cations shifting incoherently away from their ideal crystallographic site in the Bi end-member pyrochlore as a consequence of stereochemical activity of the lone pair of electrons. However, less is known about the consequences of such off-centering in solid solutions and its role in determining the electronic ground state. Here we demonstrate through total scattering studies that even a small substitution of Bi on the pyrochlore A site leads to site disorder that enhances the average effective size of the A-site cation. This indirectly increases Ru-O-Ru covalency, which appears to play a crucial role in the crossover from insulating to metallic behavior in the solid solution. Further, density functional electronic structure calculations suggest the combination of primary and secondary (due to size) electronic effects of the lone pair-driven incoherent cation displacements drive the solid solution into a metallic state