Spectroscopic modes provide the most sensitive probe of the very weak interactions responsible for the properties of the long-wavelength cycloid in the multiferroic phase of BiFeO3 below TN ≈ 640 K. Three of the four modes measured by THz and Raman spectroscopies were recently identified using a simple microscopic model. While a Dzyaloshinskii-Moriya (DM) interaction D along [−1, 2, −1] induces the cycloid with wavevector (2π/a)(0.5 + δ, 0.5, 0.5 − δ) (δ ≈ 0.0045), easy-axis anisotropy K along the [1, 1, 1] direction of the electric polarization P induces higher harmonics of the cycloid, which split the Ψ1 modes at 2.49 and 2.67 meV and activate the Φ2 mode at 3.38 meV. However, that model could not explain the observed low-frequency mode at about 2.17 meV. We now demonstrate that an additional DM interaction D ′ along [1, 1, 1] not only produces the observed weak ferromagnetic moment of the high-field phase above 18 T but also activates the spectroscopic matrix elements of the nearly-degenerate, low-frequency Ψ0 and Φ1 modes, although their scattering intensities remain extremely weak. Even in the absence of easy-axis anisotropy, D ′ produces cycloidal harmonics that split Ψ1 and activate Φ2. However, the observed mode frequencies and selection rules require that both D ′ and K are nonzero. This work also resolves an earlier disagreement between spectroscopic and inelastic neutron-scattering measurements.

In this paper we report results for magnetic observables of finite spin clusters composed of S = 1/2 ions. We consider clusters of two, three and four spins in distinct spatial arrangements, with isotropic Heisenberg interactions of various strengths between ion pairs. In addition to the complete set of energy eigenvalues and eigenvectors, specific heat and magnetic susceptibility, we also quote results for the single crystal and powder average inelastic neutron scattering structure factors. Examples of the application of these results to experimental systems are also discussed.

We investigate the possibility of multi-band superconductivity in SrTiO3 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 SrTiO3, 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 LaAlO3/SrTiO3 heterostructures, we speculate that the superconductivity observed in SrTiO3 interfaces may be similar in nature to that of bulk SrTiO3, involving multiple bands with distinct electronic occupations.

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS.