M.-W. Chu

Probing bright and dark surface-plasmon modes in individual and coupled noble metal nanoparticles using an electron beam.

The rich structure of bright and dark surface-plasmon modes localized in individual and coupled gold nanoparticles is unveiled by electron-energy-loss spectroscopy performed in a scanning transmission electron microscope. Spatially resolved maps of surface-plasmon modes in the approximately 1.5-2.5 eV range (wavelengths approximately 500-800 nm), collected for individual nanorods, coupled nanorod dimers, and touching nanosphere dimers, are in excellent agreement with theory. Surface-plasmon maps constructed from the spatially and spectrally resolved energy-loss signals are shown to mimic rather well the near fields calculated for external illumination in the case of bright surface-plasmon modes (i.e., those coupling to external light). Dark surface-plasmon modes that cannot be excited by optical means are also found, and our electron probing technique provides further insight into their corresponding spatial distribution and symmetry, which are not accessible to any other existing techniques. Our results initiate the study of a whole set of new dark surface-plasmon modes that should become a source of new applications in sensing and microscopy but have escaped experimental scrutiny so far.

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.

Oxygen nonstoichiometry and the origin of Na ion ordering in P2-NaxCoO2

The impact of oxygen deficiency on physical properties of Na2/3CoO2-x has been investigated. From the combined thermogravimetric, magnetic susceptibility and synchrotron X-ray Laue diffraction studies, it is demonstrated that Na2/3CoO2 shows no superlattice ordering due to Na ions; however Na2/3CoO1.98, which has the same Co valence as that of Na0.71CoO2, shows nearly identical magnetic and transport properties and the same simple hexagonal superlattice ordering of sqrt(12)a. It is proposed that the Na ion ordering found in Na2/3CoO1.98 is identical to the ideal Na0.71CoO2 of large sqrt(12)a x sqrt(12)a x 3c superlattice but with additional Na vacancies which are bound to the oxygen defects at room temperature. We conclude that oxygen vacancies play a key role in stabilizing the superlattice structure and must be accounted for in its modeling.

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.

Nonmagnetic impurity perturbation to the quasi-two-dimensional quantum helimagnet LiCu2O2

A complete phase diagram of Zn substituted quantum quasi-two-dimensional helimagnet LiCu2O2 has been presented. Helical ordering transition temperature (T_h) of the original LiCu2O2 follows finite size scaling for less than ~ 5.5% Zn substitution, which implies the existence of finite helimagnetic domains with domain boundaries formed with nearly isolated spins. Higher Zn substitution > 5.5% quenches the long-range helical ordering and introduces an intriguing Zn level dependent magnetic phase transition with slight thermal hysteresis and a universal quadratic field dependence for T_c (Zn > 0.055,H). The magnetic coupling constants of nearest-neighbor (nn) J1 and next-nearest-neighbor (nnn) J2 (alpha=J2/J1) are extracted from high temperature series expansion (HTSE) fitting and N=16 finite chain exact diagonalization simulation. We have also provided evidence of direct correlation between long-range helical spin ordering and the magnitude of electric polarization in this spin driven multiferroic material.

Investigation of normal and superconducting states in noncentrosymmetric Re24Ti5

We report a detailed characterization of the noncentrosymmetric superconductor Re24Ti5 using powder x-ray diffraction (XRD), magnetic susceptibility, electrical resistivity, thermal conductivity, Seebeck coefficient, and specific heat measurements. Rietveld refinement of powder XRD data confirms that Re24Ti5 crystallizes in the -Mn structure. All measured quantities demonstrate a bulk superconducting transition at TcD 5:8 K. Our low-temperature specific heat data measured down to 0.5 K yield a Sommerfeld coefficient D 111:8 mJ mol 1 K 2 , which implies a high density of states at the Fermi level. Moreover, the electronic specific heat in the superconducting state was found to obey a typical s-wave expression, revealing a single gap1=kBD 10:6 K. This value gives a ratio of 21=kBTcD 3:68, higher than the value of 3.5 predicted from BCS theory. On this basis, we conclude that the noncentrosymmetric Re24Ti5 compound can be characterized as a moderately coupled BCS-type superconductor. Furthermore, the obtained parameters from the present study of Re24Ti5 were compared to those of the isostructural compound Re23:8Nb5:2, indicating the similarity between both systems. (Some figures may appear in colour only in the online journal)

Off-Stoichiometry Driven Carrier Density Variation at the Interface of LaAlO 3 /SrTiO 3

The interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted enormous interests due to its rich physical phenomena, such as metallic nature, magnetism and superconductivity. In this work, we report our experimental investigations on the influence of the LAO stoichiometry to the metallic interface. Taking advantage of the oxide molecular beam epitaxy (MBE) technique, a series of high quality LAO films with different nominal La/Al ratios and LAO thicknesses were grown on the TiO2-terminated STO substrates, where systematic variations of the LAO lattice constant and transport property were observed. In particular, the sheet density can be largely reduced by nearly an order of magnitude with merely about 20% increase in the nominal La/Al ratio. Our finding provides an effective method on tuning the electron density of the two-dimensional electron liquid (2DEL) at the LAO/STO interface.

Site occupancy and magnetic properties of pyrochlore-structured AgOs2O6

AgOs(2)O(6) prepared from ion-exchanged superconducting β-pyrochlore KOs(2)O(6) has been shown to be non-superconducting. Synchrotron x-ray structure refinement suggests that AgOs(2)O(6) has the Ag ion mostly occupying the low-symmetry 32e site in the [Formula: see text] space group of proper occupancy, which is different from the original major occupancy at the high-symmetry 8b site for KOs(2)O(6), and similar to non-superconducting Na(1.4)Os(2)O(6)·H(2)O. Magnetic susceptibility measurements found no magnetic ordering down to ~1.7 K. The trace amount of isolated spins suggests that the Ag could be neutral and lead to a pure Os(6+) valence state of zero spin in the newly prepared AgOs(2)O(6).

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.

Probing bright and dark surface plasmon modes in individual and coupled Au nanoparticles using a fast electron beam

Surface plasmons (SPs) are quantized collective plasma oscillations of conduction electrons propagating at the surface of metals. The excitations of SPs in the visible spectral regime dictate the color of noble metals. In noble metal nanoparticles (NPs) with only a few tens or hundreds of nanometers, the SP resonances are further tailored by the size, shape, and coupling of the NPs, determining their optical properties ranging from near-infrared to UV spectral regimes as demonstrated in nanoprisms, nanorings, nanostars, (coupled) nanorods, and (coupled/arrayed-) nanospheres.1–3 Using individual and/or coupled NPs with proper designed geometrical constraints, the SPs can, therefore, be tuned to the proximity of the laser energies (wavelengths) conveniently available. The associated resonant excitations have been shown to amplify the electric near-field of the SPs by astronomical orders of magnitudes, opening up the opportunities for several fascinating applications of the NPs such as guiding light beyond the subwavelength limit — photonics and plasmonics, surface enhanced Raman scattering, and scanning near-field optical microscopy (SNOM) with a spatial resolution around 20∼30 nm.4–6