Search Results (12 total)

We investigate the influence of the angular dependence of optically detected magnetophonon resonance (ODMPR) in n-type germanium in a tilted magnetic field. With the ODMPR conditions, qualitative features of the ODMPR effects are investigated according to the incident photon frequency, the strength of the magnetic field, the temperature, and the difference of Landau level indices as a function of the tilt angle to the applied magnetic field in the quantum limit condition, in which ħω_c≫k_BT is satisfied. In particular, anomalous behaviors of the ODMPR line shape, such as the splitting and the shift of ODMPR peaks, are discussed in detail.

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the critical statistics similar to that known in the Anderson metal-insulator transition. In the energy spectra, there exist three different regimes characterized by Wigner-Dyson, Poisson, and semi-Poisson distributions. This feature implies that the electron transport in incommensurate multiwalled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy.

We apply the frequency-dependent magnetoconductivity presented previously by one of the present authors not only to bulk nonpolar (n-Ge) and polar (n-GaAs) semiconductors but also to short-period Ge-based (nonpolar) and GaAs-based (polar) semiconductor superlattices. We also obtain the optically detected magnetophonon resonance (ODMPR) conditions and the energy range in which the relaxation rates are allowed. With the ODMPR conditions and the obtained energy range, qualitative features of the ODMPR effects are investigated according to the incident photon frequency and the strength of the applied magnetic field in the quantum limit condition, in which ħω_c≫k_BT are satisfied. In particular, anomalous behaviors of the ODMPR lineshape such as the splitting and the shift of ODMPR peaks are discussed. Furthermore, the appearance of the plateau between neighboring ODMPR peaks, the disappearance of ODMPR peaks, and changes in the ODMPR amplitude in the semiconductor superlattices are discussed in detail.

A comprehensive atomic model for the reconstructed surface of Si₃N₄ thin layer grown on Si(111) is presented. Kikuchi electron holography images clearly show the existence of adatoms on the Si₃N₄(0001)/Si(111)-(8×8) surface. Compared with the ab initio calculations, more than 30 symmetry-inequivalent atomic pairs in the outmost layers are successfully identified. Scanning tunneling microscopy (STM) images show diamond-shaped unit cells and nine adatoms in each cell. High-resolution STM images reveal extra features and are in good agreement with the partial charge density distribution obtained from total-energy calculations.

The muon anomalous magnetic moment has been measured in a new experiment at Brookhaven. Polarized muons were stored in a superferric ring, and the angular frequency difference, ω_a, between the spin precession and orbital frequencies was determined by measuring the time distribution of high-energy decay positrons. The ratio R of ω_a to the Larmor precession frequency of free protons, ω_p, in the storage-ring magnetic field was measured. We find R  =  3.707 220(48)×10^-3. With μ_μ/μ_p  =  3.183 345 47(47) this gives a_μ⁺  =  1 165 925(15)×10^-9 ( ±13 ppm), in good agreement with the previous CERN measurements for μ⁺ and μ^- and of approximately the same precision.

The formation of magnetic edge states along with corresponding classical trajectories is investigated for a magnetic quantum dot with inhomogeneous distributions of magnetic fields. The magnetic edge states are found to circulate either clockwise or counterclockwise along the boundary region of the quantum dot, depending on the number of missing flux quanta, and exhibit quite different properties, as compared to the conventional ones which are induced by electrostatic confinements in the quantum Hall system.

We have used Raman scattering to investigate the effects of intense laser pulses on the structure of resolidified graphite. Graphite was irradiated with 0.325–3.25-J/cm², 620-nm, 90-fs single-laser pulses causing it to melt and rapidly resolidify. Raman studies of the resolidified carbon in the crater show that the rapid annealing process (by pulses with energy fluences ≥0.82 J/cm²) causes a breakdown in the ordered layers of hexagonal carbon rings and disorder in the intraplanar spacing upon resolidification into a nanocrystalline material. The thickness of the nanocrystalline-graphite near-surface layer increases with increasing fluence. Residual planar structure of the resulting material is observed for the various pulse-energy values by comparing the narrow graphitic 1581-cm^-1 and the broad 1360-cm^-1 and 1600-cm^-1 vibrational bands. The interplanar structure of our nanocrystalline graphite is also studied quantitatively via the low-frequency shear mode at 42 cm^-1. The Raman spectrum of our glassy carbon is found to be well described by planar ordering approximately 2 to 3 layers in extent using a simple correlation function approach. Our results indicate a layered morphology is present in our nanocrystalline graphite, confirming a strong sp² bonding character.

A comprehensive report of femtosecond time-resolved reflectivity and transmission of graphite and diamond following optical excitation above critical melting fluences F_m of 0.13 and 0.63 J/cm², respectively, is presented. Normal- and oblique-incidence reflectivity has been measured with 100-fs resolution at wavelengths ranging from 700 to 310 nm. Within 1 ps following excitation above F_m, probe reflectance increases sharply at visible frequencies, remains nearly unchanged at near-ultraviolet frequencies, and depends weakly on excitation fluence. These optical changes are interpreted as an ultrafast melting transition from crystalline graphite or diamond to a common, more reflective liquid state. During the first picosecond following excitation, electron and lattice temperatures substantially equilibrate, and the lattice melts, before heat conducts out of the absorbing volume or the surface hydrodynamically expands. A Drude model of the reflectance spectrum 1 ps after excitation reveals a strongly damped plasma (plasma frequency-relaxation time product ω_pτ∼1), in contrast to liquid silicon (ω_pτ∼5). Inferred electron mean free paths approach the average interatomic spacing (2 Å), implying electron localization. Optically determined dc resistivities up to 625±75 μΩ cm agree with measurements at kilobar ambient pressure, but significantly exceed resistivities measured and calculated at low pressure. Thus, the attribution ‘‘metal’’ is questionable for fluid carbon under these conditions. The results demonstrate that femtosecond lasers can extend condensed-matter thermophysics measurements to temperature-pressure regimes inaccessible by other methods.

To search for spontaneous conversion of muonium to antimuonium with very low background, a new signature was implemented that required the coincident detection of the decay products of the antimuonium atom, the energetic e^- and the atomic e⁺. No conversion events were seen, which sets an improved upper limit of 6.5×10^-7 (90% C.L.) on the conversion probability. The corresponding limit on the coupling constant, using a V-A form for the conversion Hamiltonian, is G_MM¯<0.16G_F (90% C.L.), where G_F is the Fermi coupling constant.

We present a comprehensive report of pump-probe reflectivity and transmission measurements on highly oriented pyrolytic graphite with 50 fs time resolution. The experiments trace the generation, relaxation, and recombination of nonequilibrium carriers in a quasi-two-dimensional semimetallic solid over a wide range of experimental parameters. The fluence of excitation at hν=2.0 eV was varied between 10^-6 and 10^-2 J/cm², below the threshold for optical damage, while probe pulses in the photon energy range 1.5<hν<4.0 eV were used. On a subpicosecond time scale we observe a strong, initial, broadband absorption saturation caused by state filling by a hot, dense π-band electron population, which recovers with a fluence- and probe-wavelength-dependent time constant as the carriers cool and recombine in less than 1 ps. Later dynamics reflect the generation and diffusion of heat in the lattice, and are consistent with previous picosecond reflectivity measurements.

We report the first femtosecond time-resolved measurements of melting dynamics of graphite. A high-reflectivity phase, lasting less than 10 ps, appears when and only when the surface is photoexcited above a critical fluence of 0.13 J/cm² needed to produce surface damage. The wavelength, polarization, and fluence dependence of time-resolved reflectivity suggest formation of an initial monovalent metallic phase, which transforms within 30 ps to a low-reflectivity phase, similar to that observed in picosecond reflectivity experiments.