A high-temperature ferromagnetic topological insulating phase by proximity coupling.

Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

Ultrafast Carrier-Lattice Interactions and Interlayer Modulations of Bi2Se3 by X-ray Free-Electron Laser Diffraction.

As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrically and optically controllable magnetic and optoelectronic devices. Understanding the coupling with its topological phase requires studying the interactions of carriers with the lattice on time scales down to the subpicosecond regime. Here, we investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films by time-resolved diffraction using an X-ray free-electron laser. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Our theoretical calculations suggest that the band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insights into the topological phase control by light-induced structural change on ultrafast time scales.

Strong interfacial exchange field in the graphene/EuS heterostructure.

Exploiting 2D materials for spintronic applications can potentially realize next-generation devices featuring low power consumption and quantum operation capability. The magnetic exchange field (MEF) induced by an adjacent magnetic insulator enables efficient control of local spin generation and spin modulation in 2D devices without compromising the delicate material structures. Using graphene as a prototypical 2D system, we demonstrate that its coupling to the model magnetic insulator (EuS) produces a substantial MEF (>14 T) with the potential to reach hundreds of tesla, which leads to orders-of-magnitude enhancement of the spin signal originating from the Zeeman spin Hall effect. Furthermore, the new ferromagnetic ground state of Dirac electrons resulting from the strong MEF may give rise to quantized spin-polarized edge transport. The MEF effect shown in our graphene/EuS devices therefore provides a key functionality for future spin logic and memory devices based on emerging 2D materials in classical and quantum information processing.

Induced Superconductivity and Engineered Josephson Tunneling Devices in Epitaxial (111)-Oriented Gold/Vanadium Heterostructures.

We report a unique experimental approach to create topological superconductors by inducing superconductivity into epitaxial metallic thin film with strong spin-orbit coupling. Utilizing molecular beam epitaxy technique under ultrahigh vacuum conditions, we are able to achieve (111) oriented single phase of gold (Au) thin film grown on a well-oriented vanadium (V) s-wave superconductor film with clean interface. We obtained atomically smooth Au thin films with thicknesses even down to below a nanometer showing near-ideal surface quality. The as-grown V/Au bilayer heterostructure exhibits superconducting transition at around 3.9 K. Clear Josephson tunneling and Andreev reflection are observed in S-I-S tunnel junctions fabricated from the epitaxial bilayers. The barrier thickness dependent tunneling and the associated subharmonic gap structures (SGS) confirmed the induced superconductivity in Au (111), paving the way for engineering thin film heterostructures based on p-wave superconductivity and nano devices exploiting Majorana Fermions for quantum computing.

Exchange-coupling-induced symmetry breaking in topological insulators.

An exchange gap in the Dirac surface states of a topological insulator (TI) is necessary for observing the predicted unique features such as the topological magnetoelectric effect as well as to confine Majorana fermions. We experimentally demonstrate proximity-induced ferromagnetism in a TI, combining a ferromagnetic insulator EuS layer with Bi(2)Se(3), without introducing defects. By magnetic and magnetotransport studies, including anomalous Hall effect and magnetoresistance measurements, we show the emergence of a ferromagnetic phase in TI, a step forward in unveiling their exotic properties.

Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet.

When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI-FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.

Low-Dimensional Conduction Mechanisms in Highly Conductive and Transparent Conjugated Polymers.

Electronic conduction in conjugated polymers is of emerging technological interest for high-performance optoelectronic and thermoelectric devices. A completely new aspect and understanding of the conduction mechanism on conducting polymers is introduced, allowing the applicability of materials to be optimized. The charge-transport mechanism is explained by direct experimental evidence with a very well supported theoretical model.