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The interplay between various symmetries and electronic bands topology is one of the core issues for topological quantum materials. Spontaneous magnetism, which leads to the breaking of time-reversal symmetry, has been proven to be a powerful approach to trigger various exotic topological phases. In this Letter, utilizing the combination of angle-resolved photoemission spectroscopy, magneto-optical Kerr effect microscopy, and first-principles calculations, we present the direct evidence on the realization of the long-sought spontaneous ferromagnetism induced topological transition in soft ferromagnetic EuB_{6}. Explicitly, we reveal the topological transition is from Z_{2}=1 topological insulator in paramagnetic state to χ=1 magnetic topological semimetal in low temperature ferromagnetic state. Our results demonstrate that the simple band structure near the Fermi level and rich topological phases make EuB_{6} an ideal platform to study the topological phase physics.

The magnetic Weyl semimetal $\mathrm{C}{\mathrm{o}}_{3}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{S}}_{2}$ exhibits large anomalous Hall effect (AHE) due to its nontrivial band topology with enhanced Berry curvature. Here we investigate the pressure effect on the AHE of $\mathrm{C}{\mathrm{o}}_{3}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{S}}_{2}$ up to 12 GPa with a palm cubic anvil cell apparatus and first-principles calculations simulation. We find that both the ferromagnetic transition temperature and the AHE are suppressed monotonically upon the application of high pressure. Data analyses revealed that in the investigated pressure range the intrinsic mechanism due to Berry curvature dominates the AHE as reflected by the validation of ${\ensuremath{\rho}}_{xy}^{A}\ensuremath{\propto}{\ensuremath{\rho}}_{xx}^{2}$. However, both the anomalous Hall conductivity and anomalous Hall angle are reduced gradually into the regime for conventional ferromagnetic metals with trivial band topology. Combined with theoretical calculations, our results indicate that the distance between Weyl points with opposite chirality in $\mathrm{C}{\mathrm{o}}_{3}\mathrm{S}{\mathrm{n}}_{2}{\mathrm{S}}_{2}$ is substantially reduced accompanying the suppression of ferromagnetism by pressure, thus providing an experimental route to tune the AHE of magnetic Weyl semimetals via modifying the nontrivial band topology.

While condensed matter systems host both fermionic and bosonic quasiparticles, reliably predicting and empirically verifying topological states is only mature for Fermionic electronic structures, leaving topological Bosonic excitations sporadically explored. This is unfortunate, as Bosonic systems such as phonons offer the opportunity to assess spinless band structures where nodal lines can be realized without invoking special additional symetries to protect against spin-orbit coupling. Here we combine first-principles calculations and meV-resolution inelastic x-ray scattering to demonstrate the first realization of parity-time reversal symmetry protected helical nodal lines in the phonon spectrum of MoB_{2}. This structure is unique to phononic systems as the spin-orbit coupling present in electronic systems tends to lift the degeneracy away from high-symmetry locations. Our study establishes a protocol to accurately identify topological Bosonic excitations, opening a new route to explore exotic topological states in crystalline materials.

Two-dimensional topological insulators with a large bulk band gap are promising for experimental studies of quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap two-dimensional topological insulator candidates, none of them have been experimentally demonstrated to have a full gap, which is crucial for quantum spin Hall effect. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that ZrTe_{5} crystal hosts a large full gap of ∼100 meV on the surface and a nearly constant density of states within the entire gap at the monolayer step edge. These features are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.

Initial growth stage evaluation of high quality ZnSe films deposited on a glass substrate was investigated. The self-limiting monolayer epitaxial process was used to pregrowth the buffer layer for a zinc selenide (ZnSe) film deposited. After alternating depositions for several cycles, the growth mode was changed to the molecular beam deposition mode under growth conditions. Films deposited at substrate temperatures of 250–350 °C and Se/Zn beam equivalent pressure ratios of 0.77–1.87 were investigated. The crystal structure and the preferred orientation of as-grown ZnSe films were examined using x-ray diffraction patterns. The optical properties of the ZnSe films were revealed by photoluminescence spectra. The characteristics of the ZnSe films with and without a buffer layer were compared and discussed in detail. Finally, the results demonstrate how the quality of ZnSe film can be improved on glass substrates for application to various devices.

Using a monoenergetic positron beam, annealing study of the Al/n-GaSb system was performed by monitoring the Doppler broadening of the annihilation radiation as a function of the positron implanting energy. The S-parameter against positron energy data was successfully fitted by a three-layer model (Al/interface/GaSb). The annealing out of the open volume defects in the polycrystalline Al layer was revealed by the decrease in the S-parameter and the increase in the effective diffusion length of the Al layer. For the as-deposited samples, a ∼ 5 nm interfacial region with S-parameter larger than those of the Al overlayer and the bulk was identified. After the 400◦C annealing, this interfacial region extends to over 40 nm and its S-parameter dramatically drops. This is possibly due to the new phase formation at the interface. Annealing behaviors of SB and L+,B of the GaSb bulk showed the annealing out of positron traps (possibly the VGa-related defect) at 250 ◦C. However, a further annealing at 400◦C induces the formation of positron traps, which are possibly of another kind of VGa-related defect and the positron shallow trap GaSb antisite.

A simple digital measurement system of 2-detector Doppler broadening is presented. We use a data acquisition (DAQ) card to digitalize the pulse from main amplifier, and use software to process the digitalized pulse. We demonstrate that DAQ card with software can replace some conventional analog electronic instruments, such as gate generator, delay and coincidence system. The two-dimensional correlation spectrum shows that this system is as good as a conventional analog measurement system. The system is more convenient and more reliable because of using little analog hardware.

We performed an angle-resolved photoemission spectroscopy study of the BiS2-based superconductor Nd(O,F)BiS2. Two small electron-like Fermi surfaces around X (pi, 0) are observed, which enclose 2.4% and 1.1% of the Brillouin zone area, respectively, corresponding to an electron doping of 7% per Bi site. The low-energy spectrum consists of a weakly-dispersing broad hump and a dispersive branch, which follows well the calculated band dispersion. This hump is drastically suppressed with increasing temperature, while the dispersive branch is essentially unaffected. The anomalous thermal effect indicates a highly interacting electronic state, in which the superconducting pairing develops.

A new method of subtracting the chance coincidence background is presented. The two-dimensional (2D) background spectrum is discussed in detail. We found that the Compton scattering boundary is not parallel to the axis in the 2D spectrum. The conclusion has been demonstrated by some experiments.

Slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation γ-ray energy spectra measured at a number of different incident positron energies is characterized by a line-shape parameter, S. It was found that the measured S parameters are sensitive to thin film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction is quite different from that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.

Using nuclear magnetic resonance techniques with a solution of cytosine molecules, we show an implementation of certain quantum logic gates (including NOT gate, square-root of NOT gate and controlled-NOT gate), which have central importance in quantum computing. In addition, experimental results show that nuclear magnetic resonance spectroscopy can efficiently measure the result of quantum computing without attendant wave-functions collapse.

The two-dimensional electron gas in δ-doped double quantum wells has been studied by using the Shubnikov–de Haas measurement. It was discovered that the δ-doping layer at the central barrier is able to prevent the electron coupling in the two wells and that the persistent photoconductivity can be reduced due to the thinner barrier. A δ-doped multiple quantum well structure for photodetector application was also proposed.

Positron lifetime spectroscopy has been used to study the vacancy type defects in undoped gallium antimonide. Temperature dependent positron trapping into the VGa-related defect having a characteristic lifetime of 310ps was observed in the as-grown sample. The lifetime data were well described by a model involving the thermal ionization (0/-) of the VGa-related defect and its ionization energy was found to be E(0/-)=83meV. For the electron irradiated sample, the VGa-related defect with lifetime of 310ps that was found in the non-irradiated samples was also identified. Moreover, another lifetime component (280ps) was only observed in the electron irradiated sample but not in the non-irradiated sample. It was also attributed to the VGa-related defect. The two identified VGa-related defects should have different microstructures because of their difference in characteristic lifetimes. The 280ps component remains thermally stable after the 500°C annealing while the 310ps component anneals at 300°C.

Positron lifetime spectroscopy was employed to study the as-electron-irradiated (10 MeV, 1×1018 cm-2) n-type 6H silicon carbide sample in the measuring temperature range of 15 K to 294 K. Isochronal annealing studies were also performed up to the temperature of 1373 K by carrying out the room temperature positron lifetime measurement. Negatively charged carbon vacancies and VCVSi divacancy were identified as the major vacancy type defects induced by the electron irradiation process. The concentration of the VCVSi divacancy was found to decrease dramatically after the 1973 K annealing.

A slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation -y-ray energy spectra, measured at a number of different incident positron energies were characterized a line-shape parameter “5”. It was found that the measured S parameters were sensitive to thin film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction was quit different from that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.

SmFeAsO1−xFx polycrystalline samples were first studied by positron annihilation lifetime spectroscopy and Doppler-broadening spectroscopy, combined with the calculated results of positron lifetime. The experimental results agree well with the calculated positron bulk lifetime in SmFeAsO and SmFeAsF crystals. The temperature dependence of S-parameter shows a remarkable difference between the parent and superconductor. An abrupt jump of S-parameter is detected around 150 K in parent sample, however, two different slopes were shown below and above superconducting transition. The linearity S-T plot determines one-type of defects through the superconducting transition which must play an important role in superconductivity.