3,509 research outputs found
Magnetic exchange interaction between rare-earth and Mn ions in multiferroic hexagonal manganites
We report a study of magnetic dynamics in multiferroic hexagonal manganite
HoMnO3 by far-infrared spectroscopy. Low-temperature magnetic excitation
spectrum of HoMnO3 consists of magnetic-dipole transitions of Ho ions within
the crystal-field split J=8 manifold and of the triangular antiferromagnetic
resonance of Mn ions. We determine the effective spin Hamiltonian for the Ho
ion ground state. The magnetic-field splitting of the Mn antiferromagnetic
resonance allows us to measure the magnetic exchange coupling between the
rare-earth and Mn ions.Comment: accepted for publication in Physical Review Letter
Fabrication and Characterization of Topological Insulator BiSe Nanocrystals
In the recently discovered class of materials known as topological
insulators, the presence of strong spin-orbit coupling causes certain
topological invariants in the bulk to differ from their values in vacuum. The
sudden change of invariants at the interface results in metallic, time reversal
invariant surface states whose properties are useful for applications in
spintronics and quantum computation. However, a key challenge is to fabricate
these materials on the nanoscale appropriate for devices and probing the
surface. To this end we have produced 2 nm thick nanocrystals of the
topological insulator BiSe via mechanical exfoliation. For crystals
thinner than 10 nm we observe the emergence of an additional mode in the Raman
spectrum. The emergent mode intensity together with the other results presented
here provide a recipe for production and thickness characterization of
BiSe nanocrystals.Comment: 4 pages, 3 figures (accepted for publication in Applied Physics
Letters
Late positive complex in event-related potentials tracks memory signals when they are decision relevant.
The Late Positive Complex (LPC) is an Event-Related Potential (ERP) consistently observed in recognition-memory paradigms. In the present study, we investigated whether the LPC tracks the strength of multiple types of memory signals, and whether it does so in a decision dependent manner. For this purpose, we employed judgements of cumulative lifetime exposure to object concepts, and judgements of cumulative recent exposure (i.e., frequency judgements) in a study-test paradigm. A comparison of ERP signatures in relation to degree of prior exposure across the two memory tasks and the study phase revealed that the LPC tracks both types of memory signals, but only when they are relevant to the decision at hand. Another ERP component previously implicated in recognition memory, the FN400, showed a distinct pattern of activity across conditions that differed from the LPC; it tracked only recent exposure in a decision-dependent manner. Another similar ERP component typically linked to conceptual processing in past work, the N400, was sensitive to degree of recent and lifetime exposure, but it did not track them in a decision dependent manner. Finally, source localization analyses pointed to a potential source of the LPC in left ventral lateral parietal cortex, which also showed the decision-dependent effect. The current findings highlight the role of decision making in ERP markers of prior exposure in tasks other than those typically used in studies of recognition memory, and provides an initial link between the LPC and the previously suggested role of ventral lateral parietal cortex in memory judgements
Experimental observations of topologically guided water waves within non-hexagonal structures
We investigate symmetry-protected topological water waves within a strategically engineered square lattice system. Thus far, symmetry-protected topological modes in hexagonal systems have primarily been studied in electromagnetism and acoustics, i.e. dispersionless media. Herein, we show experimentally how crucial geometrical properties of square structures allow for topological transport that is ordinarily forbidden within conventional hexagonal structures. We perform numerical simulations that take into account the inherent dispersion within water waves and devise a topological insulator that supports symmetry-protected transport along the domain walls. Our measurements, viewed with a high-speed camera under stroboscopic illumination, unambiguously demonstrate the valley-locked transport of water waves within a non-hexagonal structure. Due to the tunability of the energy's directionality by geometry, our results could be used for developing highly-efficient energy harvesters, filters and beam-splitters within dispersive media
Phonon splitting and anomalous enhancement of infrared-active modes in BaFeAs
We present a comprehensive infrared spectroscopic study of lattice dynamics
in the pnictide parent compound BaFeAs. In the tetragonal structural
phase, we observe the two degenerate symmetry-allowed in-plane infrared active
phonon modes. Following the structural transition from the tetragonal to
orthorhombic phase, we observe splitting into four non-degenerate phonon modes
and a significant phonon strength enhancement. These detailed data allow us to
provide a physical explanation for the anomalous phonon strength enhancement as
the result of anisotropic conductivity due to Hund's coupling.Comment: 5 pages, 3 figures, 1 tabl
Interference Effects Due to Projectile Target Nucleus Scattering in Single Ionization of Hâ‚‚ by 75-keV Proton Impact
Doubly differential cross sections (DDCSs) for single ionization of molecular hydrogen by 75-keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. Interference structures are observed in the scattering angular dependence of the DDCSs, which disappear, however, at electron speeds near the projectile speed. The comparison to our calculations shows that the projectile-target nucleus interaction plays a central role. Furthermore, our data suggest that for a given scattering angle, ionization favors well-defined molecular orientations
Interference Effects Due to Projectile Target Nucleus Scattering in Single Ionization of Hâ‚‚ by 75-keV Proton Impact
Doubly differential cross sections (DDCSs) for single ionization of molecular hydrogen by 75-keV proton impact have been measured and calculated as a function of the projectile scattering angle and energy loss. Interference structures are observed in the scattering angular dependence of the DDCSs, which disappear, however, at electron speeds near the projectile speed. The comparison to our calculations shows that the projectile-target nucleus interaction plays a central role. Furthermore, our data suggest that for a given scattering angle, ionization favors well-defined molecular orientations
- …