In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface stat... more In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metalliclike nonquantized conductivity, which exhibits a weak antilocalizationlike cusp at low magnetic fields and gives way to a nonsaturating linear magnetoresistance at large fields. We explain these results by considering the disordered network of electron-and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity using both numerical simulations and a theoretical scaling argument.
Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven... more Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic layers/domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offer...
Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven... more Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperaturedependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strainmodulated magnetic domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics.
Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero exte... more Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero external magnetic field. Recently, QAHE has been experimentally observed in thin films of Cr-doped (Bi,Sb) 2 Te 3 topological insulator grown on SrTiO 3 (111) substrate by molecular beam epitaxy. The QAHE in Cr-doped (Bi,Sb) 2 Te 3 films is found to be easily destroyed by slight variations in sample chemical composition, film thickness, substrate condition and capping layer. We have systematically investigated the influence of these parameters on the magnetism and anomalous Hall effect of Cr-doped (Bi,Sb) 2 Te 3 films. The crucial factors preventing QAHE are discussed based on the results. This work is helpful for a detailed understanding of QAHE and for routine preparation of QAHE samples for further studies.
As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrica... more 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 sub-picosecond regime. Here we use an X-ray free-electron laser to perform time-resolved diffraction to investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory (DFT) and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insight into instantaneous topological phases on ultrafast timescales.
Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electroni... more Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure. Rapid, low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs. Here we show that wafer-sized Bi 2 Te 3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO 2 /Si substrates by magnetron cosputtering. The SiO 2 /Si substrates enable us to electrically tune (Bi 1–x Sbx )2 Te 3 and Cr-doped (Bi 1–x Sbx )2 Te 3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states, such as the quantum anomalous Hall effect (QAHE). This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.
We find that in the ferromagnetic semiconductor EuS, near its Curie temperature, a single band-ed... more We find that in the ferromagnetic semiconductor EuS, near its Curie temperature, a single band-edge photon generates a spin polaron (SP), whose magnetic moment approaches 20,000 Bohr magnetons. This is much larger than the supergiant photoinduced SPs in antiferromagnetic europium chalcogenides, reported previously. The larger SP in ferromagnetic EuS, and still larger expected for EuO, is explained by a larger Bohr radius of the photoexcited electron state, which encircles and polarizes a greater number of lattice spins. However, because the wave function of the photoexcited electron spreads over a greater volume, the photoexcited electron's exchange interaction with individual lattice spins weakens, which makes the SP more easily quenched thermally.
Defining the magnetic anisotropy for in-plane or out-of-plane easy axis in ferrimagnetic insulato... more Defining the magnetic anisotropy for in-plane or out-of-plane easy axis in ferrimagnetic insulators films by controlling the strain, while maintaining high-quality surfaces, is desirable for spintronic and magnonic applications. We investigate ways to tune the anisotropy of amorphous sputtered ultrathin thulium iron garnet (TIG) films, and thus tailor their magnetic properties by the thickness (7.5 to 60 nm), substrate choice (GGG and SGGG), and crystallization process. We correlate morphological and structural properties with the magnetic anisotropy of post-growth annealed films. 30 nm thick films annealed at 600 °C show compressive strain favoring an in-plane magnetic anisotropy (IPMA), whereas films annealed above 800 °C are under a tensile strain leading to a perpendicular magnetic anisotropy (PMA). Air-annealed films present a high degree of crystallinity and magnetization saturation close to the bulk value. These results lead to successful fabrication of trilayers TIG/Au/TIG, with coupling between the TIG layers depending on Au thickness. These results will facilitate the use of TIG to create various in situ clean hybrid structures for fundamental interface exchange studies, and towards the development of complex devices. Moreover, the sputtering technique is advantageous as it can be easily scaled up for industrial applications.
Quantum Materials may act as conductors, insulators, semiconductors or even as superconductors. E... more Quantum Materials may act as conductors, insulators, semiconductors or even as superconductors. Especially combinations of different quantum materials are of high interest to explore new phenomena and to build the foundation for future electronic devices at the nanometer scale. The properties of quantum materials differ strongly from the properties of their three dimensional bulk state. Our research on quantum materials is widely spread, reaching from defect formation in graphene to the characterization of hybrid quantum materials. We use low voltage aberration corrected electron microscopy to investigate chemical various quantum materials. In particular, we will present imaging and analysis of vapor deposition (CVD) graphene with added copper and mercury defects; graphene based hybrid structures, and FeSn Topological Insulators. The surface plasmon polaritons is a highly investigated field of research due to their high potential applications for sensors, information technologies, high-resolution imaging, to name a few. These collective delocalized electron oscillations are common at metal-dielectric interfaces. However, they also exist in highly doped semiconductors, conducting oxide systems or graphene, and in many other systems with high carrier mobility. This poses the question of whether such resonances can be observed at the insulator interfaces with these novel materials. Chalcogenides, the general name of sulfides, selenides, tellurides can show highly anisotropic crystal structures, that gives them exotic properties such as topological surface state, superconductivity or negative dielectric permittivity. It was already shown that Bi2Se3 supports Dirac plasmons, but the Dirac state is not the only reason for the existence of plasmon resonance in Bi2Se3. This material also has highly anisotropic dielectric properties allowing surface plasmon excitations, and it is not the only chalcogenide system that can support them. In this study we show electron energy loss spectroscopy and a finite-difference frequency-domain study for investigating Bi2Se3, and BaNbS (Figure 1.) with high spatial resolutions [1].
In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface stat... more In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metalliclike nonquantized conductivity, which exhibits a weak antilocalizationlike cusp at low magnetic fields and gives way to a nonsaturating linear magnetoresistance at large fields. We explain these results by considering the disordered network of electron-and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity using both numerical simulations and a theoretical scaling argument.
Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven... more Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic layers/domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offer...
Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven... more Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperaturedependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strainmodulated magnetic domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics.
Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero exte... more Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero external magnetic field. Recently, QAHE has been experimentally observed in thin films of Cr-doped (Bi,Sb) 2 Te 3 topological insulator grown on SrTiO 3 (111) substrate by molecular beam epitaxy. The QAHE in Cr-doped (Bi,Sb) 2 Te 3 films is found to be easily destroyed by slight variations in sample chemical composition, film thickness, substrate condition and capping layer. We have systematically investigated the influence of these parameters on the magnetism and anomalous Hall effect of Cr-doped (Bi,Sb) 2 Te 3 films. The crucial factors preventing QAHE are discussed based on the results. This work is helpful for a detailed understanding of QAHE and for routine preparation of QAHE samples for further studies.
As a 3D topological insulator, bismuth selenide (Bi2Se3) has potential applications for electrica... more 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 sub-picosecond regime. Here we use an X-ray free-electron laser to perform time-resolved diffraction to investigate the ultrafast carrier-induced lattice contractions and interlayer modulations in Bi2Se3 thin films. The lattice contraction depends on the carrier concentration and is followed by an interlayer expansion accompanied by oscillations. Using density functional theory (DFT) and the Lifshitz model, the initial contraction can be explained by van der Waals force modulation of the confined free carrier layers. Band inversion, related to a topological phase transition, is modulated by the expansion of the interlayer distance. These results provide insight into instantaneous topological phases on ultrafast timescales.
Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electroni... more Three-dimensional (3D) topological insulators (TIs) are candidate materials for various electronic and spintronic devices due to their strong spin-orbit coupling and unique surface electronic structure. Rapid, low-cost preparation of large-area TI thin films compatible with conventional semiconductor technology is the key to the practical applications of TIs. Here we show that wafer-sized Bi 2 Te 3 family TI and magnetic TI films with decent quality and well-controlled composition and properties can be prepared on amorphous SiO 2 /Si substrates by magnetron cosputtering. The SiO 2 /Si substrates enable us to electrically tune (Bi 1–x Sbx )2 Te 3 and Cr-doped (Bi 1–x Sbx )2 Te 3 TI films between p-type and n-type behavior and thus study the phenomena associated with topological surface states, such as the quantum anomalous Hall effect (QAHE). This work significantly facilitates the fabrication of TI-based devices for electronic and spintronic applications.
We find that in the ferromagnetic semiconductor EuS, near its Curie temperature, a single band-ed... more We find that in the ferromagnetic semiconductor EuS, near its Curie temperature, a single band-edge photon generates a spin polaron (SP), whose magnetic moment approaches 20,000 Bohr magnetons. This is much larger than the supergiant photoinduced SPs in antiferromagnetic europium chalcogenides, reported previously. The larger SP in ferromagnetic EuS, and still larger expected for EuO, is explained by a larger Bohr radius of the photoexcited electron state, which encircles and polarizes a greater number of lattice spins. However, because the wave function of the photoexcited electron spreads over a greater volume, the photoexcited electron's exchange interaction with individual lattice spins weakens, which makes the SP more easily quenched thermally.
Defining the magnetic anisotropy for in-plane or out-of-plane easy axis in ferrimagnetic insulato... more Defining the magnetic anisotropy for in-plane or out-of-plane easy axis in ferrimagnetic insulators films by controlling the strain, while maintaining high-quality surfaces, is desirable for spintronic and magnonic applications. We investigate ways to tune the anisotropy of amorphous sputtered ultrathin thulium iron garnet (TIG) films, and thus tailor their magnetic properties by the thickness (7.5 to 60 nm), substrate choice (GGG and SGGG), and crystallization process. We correlate morphological and structural properties with the magnetic anisotropy of post-growth annealed films. 30 nm thick films annealed at 600 °C show compressive strain favoring an in-plane magnetic anisotropy (IPMA), whereas films annealed above 800 °C are under a tensile strain leading to a perpendicular magnetic anisotropy (PMA). Air-annealed films present a high degree of crystallinity and magnetization saturation close to the bulk value. These results lead to successful fabrication of trilayers TIG/Au/TIG, with coupling between the TIG layers depending on Au thickness. These results will facilitate the use of TIG to create various in situ clean hybrid structures for fundamental interface exchange studies, and towards the development of complex devices. Moreover, the sputtering technique is advantageous as it can be easily scaled up for industrial applications.
Quantum Materials may act as conductors, insulators, semiconductors or even as superconductors. E... more Quantum Materials may act as conductors, insulators, semiconductors or even as superconductors. Especially combinations of different quantum materials are of high interest to explore new phenomena and to build the foundation for future electronic devices at the nanometer scale. The properties of quantum materials differ strongly from the properties of their three dimensional bulk state. Our research on quantum materials is widely spread, reaching from defect formation in graphene to the characterization of hybrid quantum materials. We use low voltage aberration corrected electron microscopy to investigate chemical various quantum materials. In particular, we will present imaging and analysis of vapor deposition (CVD) graphene with added copper and mercury defects; graphene based hybrid structures, and FeSn Topological Insulators. The surface plasmon polaritons is a highly investigated field of research due to their high potential applications for sensors, information technologies, high-resolution imaging, to name a few. These collective delocalized electron oscillations are common at metal-dielectric interfaces. However, they also exist in highly doped semiconductors, conducting oxide systems or graphene, and in many other systems with high carrier mobility. This poses the question of whether such resonances can be observed at the insulator interfaces with these novel materials. Chalcogenides, the general name of sulfides, selenides, tellurides can show highly anisotropic crystal structures, that gives them exotic properties such as topological surface state, superconductivity or negative dielectric permittivity. It was already shown that Bi2Se3 supports Dirac plasmons, but the Dirac state is not the only reason for the existence of plasmon resonance in Bi2Se3. This material also has highly anisotropic dielectric properties allowing surface plasmon excitations, and it is not the only chalcogenide system that can support them. In this study we show electron energy loss spectroscopy and a finite-difference frequency-domain study for investigating Bi2Se3, and BaNbS (Figure 1.) with high spatial resolutions [1].
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Papers by Yunbo Ou