F.-T. Huang

Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3

We studied the defects of Bi2Se3 generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the BiSe antisite and partial Bi2-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction.

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious “Curie-Weiss” metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.

Large single crystal growth, transport property, and spectroscopic characterizations of three-dimensional Dirac semimetal Cd3As2.

The three dimensional (3D) Dirac semimetal is a new quantum state of matter that has attracted much attention recently in physics and material science. Here, we report on the growth of large plate-like single crystals of Cd3As2 in two major orientations by a self-selecting vapor growth (SSVG) method, and the optimum growth conditions have been experimentally determined. The crystalline imperfections and electrical properties of the crystals were examined with transmission electron microscopy (TEM), scanning tunneling microscopy (STM), and transport property measurements. This SSVG method makes it possible to control the as-grown crystal compositions with excess Cd or As leading to mobilities near 5–105 cm2V−1s−1. Zn-doping can effectively reduce the carrier density to reach the maximum residual resistivity ratio (RRRρ300K/ρ5K) of 7.6. A vacuum-cleaved single crystal has been investigated using angle-resolved photoemission spectroscopy (ARPES) to reveal a single Dirac cone near the center of the surface Brillouin zone with a binding energy of approximately 200 meV.

Plasmons dispersion and nonvertical interband transitions in single crystal Bi2Se3 investigated by electron energy-loss spectroscopy

Plasmons dispersion and nonvertical interband transitions in Bi2Se3 single crystals were investigated by electron energy-loss spectroscopy in conjunction with (scanning) transmission electron microscopy [(S)TEM-EELS]. Both volume plasmons (π plasmon at 7 eV and π + σ plasmon at 17 eV) and surface plasmons (∼5.5 and 10 eV) were demonstrated in STEM-EELS spectra and the corresponding spectral imaging in real space. In further EELS experiments in reciprocal space, the momentum-dependent spectra reveal very different dispersion behavior between π and π + σ plasmons, with π + σ plasmons showing a typical quadratic dependence, whereas the π plasmon exhibited a linear dispersion analogous to what was reported for graphene. Furthermore, a low energy excitation 0.7–1.6 eV was also observed which is attributed to direct nonvertical interband transitions

Electronic phase diagram of Li xCoO 2 revisited with potentiostatically deintercalated single crystals

Electronic phase diagram of LixCoO2 has been re-examined using potentiostatically de-intercalated single crystal samples. Stable phases of x ~ 0.87, 0.72, 0.53, 0.50, 0.43, and 0.33 were found and isolated for physical property studies. A-type and chain-type antiferromagnetic orderings have been suggested from magnetic susceptibility measurement results in x ~ 0.87 and 0.50 below ~ 10 K and 200 K, respectively, similar to those found in NaxCoO2 system. There is no Li vacancy superlattice ordering observed at room temperature for the electronically stable phase Li0.72CoO2 as revealed by synchrotron X-ray Laue diffraction. The peculiar magnetic anomaly near ~ 175 K as often found in powder samples of x ~ 0.46-0.78 cannot be isolated through this single crystal potentiostatic method, which supports the previously proposed explanation to be surface stabilized phase of significant thermal hysteresis and aging character.

Sodium ion ordering of Na0.77CoO2 under competing multivacancy cluster, superlattice, and domain formation

Hexagonal superlattice formed by sodium multivacancy-cluster ordering in Na0.77CoO2 has been proposed based on synchrotron x-ray Laue-diffraction study on electrochemically fine-tuned single crystals. The title compound sits closely to the proposed lower end of the miscibility gap of x0.77‐0.82 phase-separated range. The average sodium vacancy-cluster size is estimated to be 4.5 Na vacancies per layer within a large superlattice size of 19a19a3c. The exceptionally large Na vacancy-cluster size favors large twinned simple hexagonal superlattice of 19a, in competition with the smaller divacancy, trivacancy, and quadrivacancy clusters formed superlattices of 12a and 13a. Competing electronic correlations are revealed by the observed spin-glasslike magnetic hysteresis below 3 K and the twin domain, triple domain, and monodomain transformations during thermal cycling between 273 and 373 K.