171 research outputs found
Evolution of the thermodynamic properties and inelastic neutron scattering intensities for spin-1/2 antiferromagnetic quantum rings
This study examines the increasing complexity in the magnetic properties of
small = 3, 4, 5, 6 spin-1/2 quantum rings. Using an exact diagonalization
of the isotropic Heisenberg Hamiltonian with nearest and next-nearest neighbor
interactions, the energy eigenstates, magnetic specific heat capacity, magnetic
susceptibility, and inelastic neutron scattering structure factors are
determined for variable next-nearest neighbor interactions. Here, it is shown
that the presence of a complex spin-mixing, multiple ground states, and
non-zero ground states greatly complicate the spin Hamiltonian. Overall, the
energy eigenstates and structure factor intensities are presented in closed
form, while the thermodynamic properties detail the effect of a crossing
interaction in the rings. The goal of this work is to provide insight into the
evolution of the magnetic properties and spin excitations within these systems.Comment: 10 pages, 6 figures, 3 tables, Submitted for publicatio
Evolution of magnetic Dirac bosons in a honeycomb lattice
We examine the presence and evolution of magnetic Dirac nodes in the
Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the
collinear phase diagram as well as the change in the spin dynamics with various
exchange interactions. We show that the ferromagnetic structure produces
bosonic Dirac and Weyl points due to the competition between superexchange
interactions. Furthermore, it is shown that the criteria for magnetic Dirac
nodes are coupled to the magnetic structure and not the overall crystal
symmetry, where the breaking of inversion symmetry greatly affects the
antiferromagnetic configurations. The tunability of the nodal points through
variation of the exchange parameters leads to the possibility of controlling
Dirac symmetries through an external manipulation of the orbital interactions.Comment: 9 pages, 7 figures, Submitted for publicatio
Two-band superconductivity in doped SrTiO films and interfaces
We investigate the possibility of multi-band superconductivity in SrTiO
films and interfaces using a two-dimensional two-band model. In the undoped
compound, one of the bands is occupied whereas the other is empty. As the
chemical potential shifts due to doping by negative charge carriers or
application of an electric field, the second band becomes occupied, giving rise
to a strong enhancement of the transition temperature and a sharp feature in
the gap functions, which is manifested in the local density of states spectrum.
By comparing our results with tunneling experiments in Nb-doped SrTiO, we
find that intra-band pairing dominates over inter-band pairing, unlike other
known multi-band superconductors. Given the similarities with the value of the
transition temperature and with the band structure of LaAlO/SrTiO
heterostructures, we speculate that the superconductivity observed in
SrTiO interfaces may be similar in nature to that of bulk SrTiO,
involving multiple bands with distinct electronic occupations.Comment: revised expanded versio
Induced Ferromagnetism at BiFeO3/YBa2Cu3O7 Interfaces
Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from
high-temperature superconductivity and giant magnetoresistance to magnetism and
ferroelectricity. In addition, when TMOs are interfaced with each other, new
functionalities can arise, which are absent in individual components. Here, we
report results from first-principles calculations on the magnetism at the
BiFeO3/YBa2Cu3O7 interfaces. By comparing the total energy for various magnetic
spin configurations inside BiFeO3, we are able to show that a metallic
ferromagnetism is induced near the interface. We further develop an interface
exchange-coupling model and place the extracted exchange coupling interaction
strengths, from the first-principles calculations, into a resultant generic
phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the
presence of a hysteresis loop in field-dependent magnetization data. The
emergence of interfacial ferromagnetism should have implications to electronic
and transport properties.Comment: 13 pages, 4 figure
Electric field effect on superconductivity at complex oxide interfaces
We examine the enhancement of the interfacial superconductivity between
LaAlO and SrTiO by an effective electric field. Through the
breaking of inversion symmetry at the interface, we show that a term coupling
the superfluid density and an electric field can augment the superconductivity
transition temperature. Microscopically, we show that an electric field can
also produce changes in the carrier density by relating the measured
capacitance to the density of states. Through the electron-phonon induced
interaction in bulk SrTiO, we estimate the transition temperature.Comment: 7 Pages, Submitted to Physical Revie
Induced polarization at a paraelectric/superconducting interface
We examine the modified electronic states at the interface between
superconducting and ferro(para)-electric heterostructures. We find that
electric polarization and superconducting order parameters can be
significantly modified due to coupling through linear terms brought about by
explicit symmetry breaking at the interface. Using an effective action and a
Ginzburg-Landau formalism, we show that an interaction term linear in the
electric polarization will modify the superconducting order parameter at
the interface. This also produces modulation of a ferroelectric polarization.
It is shown that a paraelectric-superconductor interaction will produce an
interface-induced ferroelectric polarization.Comment: 4 pages, 3 figures, Submitted to Phys. Rev.
Investigation of the obscure spin state of Ti-doped CdSe
Using computational and experimental techniques, we examine the nature of the 2+ oxidation of Ti-doped CdSe. Through stoichiometry and confirmed through magnetization measurements, the weakly-doped material of Cd1-xTixSe (x = 0.0043) shows the presence of a robust spin-1 magnetic state of Ti, which is indicative of a 2+ oxidation state. Given the obscure nature of the Ti2+ state, we investigate the electronic and magnetic states using density functional theory. Using a generalized gradient approximation with an onsite potential, we determine the electronic structure, magnetic moment density, and optical properties for a supercell of CdSe with an ultra-low concentration of Ti. We find that, in order to reproduce the magnetic moment of spin-1, an onsite potential of 4-6 eV must be in included in the calculation. Furthermore, the electronic structure and density of states shows the presence of a Ti-d impurity band above the Fermi level and a weakly metallic state for a U = 0 eV. However, the evolution of the electronic properties as a function of the Hubbard U shows that the Ti-d drop below the Fermi around 4 eV with the onset of a semiconducting state. The impurity then mixes with the lower valence bands and produces the 2+ state for the Ti atom
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