Strain-induced high-temperature perovskite ferromagnetic insulator.

Ferromagnetic insulators are required for many new magnetic devices, such as dissipationless quantum-spintronic devices, magnetic tunneling junctions, etc. Ferromagnetic insulators with a high Curie temperature and a high-symmetry structure are critical integration with common single-crystalline oxide films or substrates. So far, the commonly used ferromagnetic insulators mostly possess low-symmetry structures associated with a poor growth quality and widespread properties. The few known high-symmetry materials either have extremely low Curie temperatures (≤16 K), or require chemical doping of an otherwise antiferromagnetic matrix. Here we present compelling evidence that the LaCoO3 single-crystalline thin film under tensile strain is a rare undoped perovskite ferromagnetic insulator with a remarkably high TC of up to 90 K. Both experiments and first-principles calculations demonstrate tensile-strain-induced ferromagnetism which does not exist in bulk LaCoO3 The ferromagnetism is strongest within a nearly stoichiometric structure, disappearing when the Co2+ defect concentration reaches about 10%. Significant impact of the research includes demonstration of a strain-induced high-temperature ferromagnetic insulator, successful elevation of the transition over the liquid-nitrogen temperature, and high potential for integration into large-area device fabrication processes.

Engineering the exchange bias and bias temperature by modulating the spin glassy state in single phase Bi9Fe5Ti3O27.

Exchange anisotropy is normally observed at ferromagnetic (FM)/anti-ferromagnetic (AFM) interfaces and also usually at low temperatures. An effective way to find new single-phase materials with strong exchange bias (EB) and high operation temperatures is required for making them useful in practical applications of future spintronic devices. In this work, the exchange bias behaviors of single phase Bi9Fe5Ti3O27 (BFT) were found able to be manipulated by altering its spin glassy state, when the material was intentionally prepared in the nanobelt format. A core-shell model has been proposed to explain the controllable behavior of both exchange bias and bias temperature, in which the AFM domain could be effectively reduced within the nanobelts when changing their particle sizes. A direct observation of two spin glassy behaviors corresponding to two distinct peaks in the zero field cooling (ZFC) curves manifests the validity of the proposed core-shell model.

Giant frequency tunability enabled by external magnetic and a gate electric fields in graphene devices.

Graphene possesses a unique Landau level system that is non-equidistantly spaced in energy, as thus a large amount of optical transitions may become possible. Here, by utilizing this unique feature, we propose a novel dual field method which combines both external magnetic field and gate electric field together to control the optical response of the graphene-based devices. The key principle of this method is to selectively allow different optical transitions in graphene among Landau levels via an electric gate tuning of the Fermi level. By applying this method to a graphene based amplitude modulator and through an implementation based on transfer matrix method, we numerically demonstrated the well characteristics of switchable modulation on four individual channels, a huge modulation depth up to 80 dB and an extremely low required energy of tuning Fermi level down to 10 meV. Such excellent frequency tunability and gate controlling ability of this dual field method may open up the potential for applications in active optoelectronics, spin optics, ultrafast optics and etc.

Nanoscale structural modulation and enhanced room-temperature multiferroic properties.

Availability of a single-phase multiferroic material functional at room temperature poses a big challenge, although it is very important to both fundamental physics and application development. Recently, layered Aurivillius oxide materials, one of the most promising candidates, have attracted considerable interest. In this work, we investigated the nanoscale structural evolution of the six-layer Bi7Fe(3-x)Co(x)Ti3O21 when substituting excessive Co. Nanoscale structural modulation (NSM) occurred at the boundaries when changing the material gradually from the originally designed six-layer nanoscale architecture down to five and then four, when increasing the Co content, inducing a previously unidentified analogous morphotropic transformation (AMT) effect. The AMT's net contribution to the enhanced intrinsic multiferroic properties at room temperature was confirmed by quantifying and deducting the contribution from the existing impurity phase using derivative thermo-magneto-gravimetry measurements (DTMG). Significantly, this new AMT effect may be caused by a possible coupling contribution from co-existing NSM phases, indicating a potential method for realizing multiferroic materials that function at room temperature.

Hybrid tandem solar cell enhanced by a metallic hole-array as the intermediate electrode.

A metallic hole-array structure was inserted into a tandem solar cell structure as an intermediate electrode, which allows a further fabrication of a novel and efficient hybrid organic-inorganic tandem solar cell. The inserted hole-array layer reflects the higher-energy photons back to the top cell, and transmits lower-energy photons to the bottom cell via the extraordinary optical transmission (EOT) effect. In this case light absorption in both top and bottom subcells can be simultaneously enhanced via both structural and material optimizations. Importantly, this new design could remove the constraints of requiring lattice-matching and current-matching between the used two cascaded subcells in a conventional tandem cell structure, and therefore, the tunnel junction could be no longer required. As an example, a novel PCBM/CIGS tandem cell was designed and investigated. A systematic modeling study was made on the structural parameter tuning, with the period ranging from a few hundreds nanometers to over one micrometer. Surface plasmon polaritons, magnetic plasmon polaritons, localized surface plasmons, and optical waveguide modes were found to participate in the EOT and the light absorption enhancement. Impressively, more than 40% integrated power enhancement can be achieved in a variable structural parameter range.

Enhanced photon absorption and carrier generation in nanowire solar cells.

Overall performance of a thin film solar cell is determined by the efficiency of converting photons to electrons through light absorption, carrier generation, and carrier collection. Recently, photon management has emerged as a powerful tool to further boost this conversion efficiency. Here we propose a novel nanograting solar cell design that achieves enhanced broadband light absorption and carrier generation in conjunction with the reduced use of active and non-earth-abundant materials. A test using this design for the short circuit current density in CuInxGa(1-x)Se2 (CIGS) thin film solar cells shows up to 250% enhancement when compared to the bare thin film cells. In addition, placing metal strips on top of the nanograting to act as the top electrode reduces the use of non-earth-abundant materials that is normally used as the transparent conducting materials. This novel solar cell design has the potential to become a new solar cell platform technology for various thin film solar cell systems.