Resonant Plasmon-Assisted Tunneling in a Double Quantum Dot Coupled to a Quantum Hall Plasmon Resonator.

Edge magnetoplasmon is an emergent chiral bosonic mode promising for studying electronic quantum optics. While the plasmon transport has been investigated with various techniques for decades, its coupling to a mesoscopic device remained unexplored. Here, we demonstrate the coupling between a single plasmon mode in a quantum Hall plasmon resonator and a double quantum dot (DQD). Resonant plasmon-assisted tunneling is observed in the DQD through absorbing or emitting plasmons stored in the resonator. By using the DQD as a spectrometer, the plasmon energy and the coupling strength are evaluated, which can be controlled by changing the electrostatic environment of the quantum Hall edge. The observed plasmon-electron coupling encourages us for studying strong coupling regimes of plasmonic cavity quantum electrodynamics.

Unconventional Temperature Dependence of the Anomalous Hall Effect in HgCr_{2}Se_{4}.

At sufficiently low temperatures, many quantum effects, such as weak localization, electron-electron interaction (EEI), and Kondo screening, can lead to pronounced corrections to the semiclassical electron transport. Although low temperature corrections to longitudinal resistivity, ordinary Hall resistivity, or anomalous Hall (AH) resistivity are often observed, the corrections to three of them have never been simultaneously detected in a single sample. Here, we report on the observation of sqrt[T]-type temperature dependences of the longitudinal, ordinary Hall and AH resistivities at temperatures down to at least 20 mK in n-type HgCr_{2}Se_{4}, a half-metallic ferromagnetic semiconductor that can reach extremely low carrier densities. For the samples with moderate disorder, the longitudinal and ordinary Hall conductivities can be satisfactorily described by the EEI theory developed by Altshuler et al., whereas the large corrections to AH conductivity are inconsistent with the existing theory, which predicts vanishing and finite corrections to AH conductivity for EEI and weak localization, respectively.

Evidence for Half-Metallicity in n-type HgCr2Se4.

High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport, and Andreev reflection spectroscopy. In the ferromagnetic ground state, the saturation magnetic moment of each unit cell corresponds to an integer number of electron spins (3 µB/Cr3+), and the Hall effect measurements suggest n-type charge carriers. Spin polarizations as high as 97% were obtained from fits of the differential conductance spectra of HgCr2Se4/Pb junctions with the modified Blonder-Tinkham-Klapwijk theory. The temperature and bias-voltage dependencies of the subgap conductance are consistent with recent theoretical calculations based on spin active scatterings at a superconductor-half-metal interface. Our results suggest that n-HgCr2Se4 is a half-metal, in agreement with theoretical calculations that also predict undoped HgCr2Se4 is a magnetic Weyl semimetal.

Observation of Anderson localization in ultrathin films of three-dimensional topological insulators.

Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here, we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi_{1-x}Sb_{x})_{2}Te_{3} films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually, positive magnetoconductivity emerges when the electron system becomes strongly localized. This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.

Quantized charge fractionalization at quantum Hall Y junctions in the disorder dominated regime.

Fractionalization is a phenomenon where an elementary excitation partitions into several pieces. This picture explains non-trivial transport through a junction of one-dimensional edge channels defined by topologically distinct quantum Hall states, for example, a hole-conjugate state at Landau-level filling factor ν = 2/3. Here we employ a time-resolved scheme to identify an elementary fractionalization process; injection of charge q from a non-interaction region into an interacting and scattering region of one-dimensional channels results in the formation of a collective excitation with charge (1-r)q by reflecting fractionalized charge rq. The fractionalization factors, r = 0.34 ± 0.03 for ν = 2/3 and r = 0.49 ± 0.03 for ν = 2, are consistent with the quantized values of 1/3 and 1/2, respectively, which are expected in the disorder dominated regime. The scheme can be used for generating and transporting fractionalized charges with a well-defined time course along a well-defined path.

Thickness Dependence of the Quantum Anomalous Hall Effect in Magnetic Topological Insulator Films.

The evolution of the quantum anomalous Hall effect with the thickness of Cr-doped (Bi,Sb)2 Te3 magnetic topological insulator films is studied, revealing how the effect is caused by the interplay of the surface states, band-bending, and ferromagnetic exchange energy. Homogeneity in ferromagnetism is found to be the key to high-temperature quantum anomalous Hall material.

Homoepitaxial SrTiO3(111) Film with High Dielectric Performance and Atomically Well-Defined Surface.

The six-fold symmetry possessed by the (111) surfaces of perovskite oxides allows the epitaxial growth of novel quantum materials such as topological insulators. The dielectric SrTiO3(111) thin film is an ideal buffer layer, providing the readily tunability of charge density in gate-controlled structures. But the high-quality film growth is challenging due to its strong surface polarity as well as the difficulty of obtaining the chemical stoichiometry. Here we show that the layer-by-layer growth of homoepitaxial SrTiO3(111) thin films can be achieved in molecular beam epitaxy method by keeping the growing surface reconstructed. And the cation stoichiometry is optimized precisely with the reflective high energy electron diffraction as the feedback signal that changes sensitively to the variation of metal concentration during growth. With atomically well-defined surfaces, the SrTiO3(111) films show high dielectric performance with the charge density modulated in the range of 2 × 10(13)/cm(2) with the back gate voltage lower than 0.2 V. Methods of further broadening the range are also discussed.