Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3-the geometric ferroelectric with the greatest known planar rumpling-we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially-from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

The atomic and electronic structures of the misfit-layered thermoelectric oxide material, Ca3Co4O9, are investigated using detailed first principles computations performed within the framework of density functional theory (DFT) and its DFT+U extension to account for electron correlations. The structure of Ca3Co4O9, composed of two incommensurate subsystems, a distorted rocksalt-type Ca2CoO3 layer sandwiched between hexagonal CoO2 layers, is modeled by means of Fibonacci rational approximants with systematically increasing unit cells. We show that good agreement with photoemission and transport experiments can be obtained regarding the contribution of the two subsystems to states near the Fermi level, when electron correlations are taken into account with a Hubbard U . The size of the rational approximant plays a secondary role in the analysis; the relatively "small" structure of composition (Ca2CoO3)6(CoO2)10 represents a good model for investigating the atomic and electronic properties of Ca3Co4O9. Within the DFT+U formalism, the metallic conductivity of Ca3Co4O9 is shown to result from itinerant holes in the hexagonal CoO2 layers, in which the Co atoms are predicted to have a mixed valence of Co 4+ with ∼ 30% concentration and Co 3+ with ∼ 70% concentration, both in low-spin configurations. In most cases, the resulting electronic structures show very good agreement with available data from transport and magnetic measurements.

Ca3Co4O9 has a unique structure that leads to exceptionally high thermoelectric transport. Here we report the achievement of a 27% increase in the room-temperature in-plane Seebeck coefficient of Ca3Co4O9 thin films. We combine aberration-corrected Z-contrast imaging, atomic-column resolved electron energy-loss spectroscopy, and density-functional calculations to show that the increase is caused by stacking faults with Co4+-ions in a higher spin state compared to that of bulk Ca3Co4O9. The higher Seebeck coefficient makes the Ca3Co4O9 system suitable for many high temperature waste-heat-recovery applications.

The key challenge in superconductivity research is to go beyond the historical mode of discoverydriven research. We put forth a new strategy, which is to combine theoretical developments in the weak-coupling renormalization group approach with the experimental developments in lattice strain driven Fermi surface-engineering. For concreteness we theoretically investigate how superconducting tendencies will be affected by strain engineering of ruthenates' Fermi surface. We first demonstrate that our approach qualitatively reproduces recent experiments under uniaxial strain. We then note that order few % strain readily accessible to epitaxial thin films, can bring the Fermi surface close to van Hove singularity. Using the experimental observation of the change in the Fermi surface under biaxial epitaxial strain and ab-initio calculations, we predict Tc for triplet pairing to be maximized by getting close to the van Hove singularities without tuning on to the singularity. arXiv:1604.06661v1 [cond-mat.supr-con]