First-principles study of narrow single-walled GaN nanotubes

Structural and electronic properties of narrow single-walled GaN nanotubes with diameter from 0.30 to 0.55 nm are investigated using the density functional method with generalized-gradient approximation. The calculations of total energies predict that the most likely GaN nanotubes in our calculation are (2,2), (3,2) and (3,3) nanotubes. From a detailed analysis we find that these narrow single-walled GaN nanotubes are all semiconductors, of which the armchair and chiral tubes are indirect-band-gap semiconductors whereas the zigzag ones have a direct gap except for (4,0) tube. The indirect band gap of (4,0) tube can stem from band sequence change induced by curvature effect. Our results show that the π-π hybridization effect and the formation of benign buckling separations play a key role in the band sequence changes of (4,0) tube.     

Guided waves characteristics of multi-walled carbon nanotubes

The theoretical analysis of propagation of guided waves in the multi-walled carbon nanotubes is presented within the framework of the classical electrodynamics. Electronic excitations of each wall of the system are modeled as an infinitesimally thin cylindrical layer of the π-electrons, whose dynamics are described by means of the fluid theory. General expressions of dispersion relations are obtained for the electromagnetic wave with the transverse magnetic and transverse electric modes, respectively, by solving Maxwell and fluid equations with appropriate boundary conditions.     

Abe homotopy classification of topological excitations under the topological influence of vortices

Topological excitations are usually classified by the nth homotopy group πn. However, for topological excitations that coexist with vortices, there are cases in which an element of πn cannot properly describe the charge of a topological excitation due to the influence of the vortices. This is because an element of πn corresponding to the charge of a topological excitation may change when the topological excitation circumnavigates a vortex. This phenomenon is referred to as the action of π₁ on πn. In this paper, we show that topological excitations coexisting with vortices are classified by the Abe homotopy group κn. The nth Abe homotopy group κn is defined as a semi-direct product of π₁ and πn. In this framework, the action of π₁ on πn is understood as originating from noncommutativity between π₁ and πn. We show that a physical charge of a topological excitation can be described in terms of the conjugacy class of the Abe homotopy group. Moreover, the Abe homotopy group naturally describes vortex-pair creation and annihilation processes, which also influence topological excitations. We calculate the influence of vortices on topological excitations for the case in which the order parameter manifold is Sn/K, where Sn is an n-dimensional sphere and K is a discrete subgroup of SO(n+1). We show that the influence of vortices on a topological excitation exists only if n is even and K includes a nontrivial element of O(n)/SO(n).     

Low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain

In order to confirm the picture of domain-wall excitations in the hidden antiferromagnetic order of the Haldane phase, the structure of the low-lying excitations in the S = 1 antiferromagnetic Heisenberg chain is studied by a quantum Monte Carlo method. It is confirmed that there exists a finite energy gap between the first- and the second-excited states at k = π as well as between the ground state and the first-excited state at k = π. In the thermodynamic limit, the second-excited state at k = π is separated from the ground state by the gap which is three times as large as the Haldane gap. From the size dependences of the low-lying-excitation energies, the interactions between the elementary excitations in the excited states are concluded to be repulsive.     

Radial excitations ofp- and π-mesons and their strong decays

Radial (L=0) excitations ofp- and π-mesons are considered in the framework of the quark model. The experimentally observed statesρ′ (1220) andρ″ (1560) are identified with the first and second radial excitations of thep-meson. Within the relativistically generalized model of spontaneous production of a \(q\bar q\) pair from vacuum we explain the dominant (ρ′ →ωπ;ρ″ →ω2π) modes of strong decays of these resonances and find nondominant modes (ρ ′ 2π;ρ″ →ωπ) to be suppressed. An estimation is given for masses of analogous excitations of the pion and partial widths of their decays.     

Electronic structure of BN nanotubes with intrinsic defects NB and BN and isoelectronic substitutional impurities PN, AsN, SbN, InB, GaB, and AlB

The effect of intrinsic defects and isoelectronic substitutional impurities on the electronic structure of boron-nitride (BN) nanotubes is investigated using a linearized augmented cylindrical wave method and the local density functional and muffin-tin approximations for the electron potential. In this method, the electronic spectrum of a system is governed by a free movement of electrons in the interatomic space between cylindrical barriers and by a scattering of electrons from the atomic centers. Nanotubes with extended defects of substitution N_B of a boron atom by a nitrogen atom and, vice versa, nitrogen by boron B_N with one defect per one, two, and three unit cells are considered. It is shown that the presence of such defects significantly affects the band structure of the BN nanotubes. A defect band π(B, N) is formed in the optical gap, which reduces the width of the gap. The presence of impurities also affects the valence band: the widths of s, sp, and p_π bands change and the gap between s and sp bands is partially filled. A partial substitution of the N by P atoms leads to a decrease in the energy gap, to a separation of the Ds(P) band from the high-energy region of the s(B, N) band, as well as to the formation of the impurity Dπ(P) and Dπ*(P) bands, which form the valence-band top and conduction-band bottom in the doped system. The influence of partial substitution of N atoms by the As atom on the electronic structure of BN nanotubes is qualitatively similar to the case of phosphorus, but the optical gap becomes smaller. The optical gap of the BN tubule is virtually closed due to the effect of one Sb atom impurity per translational unit cell, in contrast to the weak indium-induced perturbation of the band structure of the BN nanotube. Introduction of the one In, Ga or Al atom per three unit cells of the (5, 5) BN nanotube results in 0.6 eV increase of the optical gap. The above effects can be detected by optical and photoelectron spectroscopy methods, as well as by measuring electrical properties of the pure and doped BN nanotubes. They can be used to design electronic devices based on BN nanotubes.     

Spin and phonon contribution to the superconductivity in La2−xSrxCuO4

Anisotropy of high-energy spin excitations and low-energy conductive electronic excitations in Raman spectra can be interpreted, if quantum spin fluctuation is weak. The low-energy states at (π/2, π/2) in the underdoped phase are composed of insulating states induced by the insulator-metal transition and the conductive states induced by electron-phonon coupling. The superconducting states are created in the latter states and the pairing symmetry is s or d(xy). On the other hand, the electronic states at (π, 0) in the overdoped phase are electron-spin coupled states and the pairing symmetry is d(x²−y²).     

The electronic spectrum of PN. A configuration interaction study

Potential energy curves were calculated for the ground state of PN and for all excited singlet and triplet states resulting from the 2π → 3π, 7σ → 3π, 2π → 8σ, and 7σ → 8σ orbital excitations. CI studies at 4 Å served to establish dissociation energies. Spectroscopic constants were calculated, and are in good agreement with those of the known X¹Σ⁺ and A¹Π states. Overall, their similarity with those observed for N₂ is striking. Various states considered to perturb the known excitations are discussed. The recently discovered second ¹Σ⁺ state is included.     

The origin of polarized blackbody radiation from resistively heated multiwalled carbon nanotubes

We observed very pronounced polarization of light emitted by highly aligned free-standing multiwall carbon nanotube (MWNT) sheet in axial direction which is turned to the perpendicular polarization when a number of layers are increased. The radiation spectrum of resistively heated MWNT sheet closely follows to the Plank's blackbody radiation distribution. The obtained polarization features can be described by a classical dielectric cylindrical shell model, taking into consideration the contribution of delocalized π-electrons (π surface plasmons). In absorption (emission) the optical transverse polarizability, which is much smaller than longitudinal one, is substantially suppressed by depolarization effect due to screening by induced charges. This phenomenon suggests very simple and precise method to estimate the alignment of nanotubes in bundles or large assemblies.     

Optical properties of semiconducting carbon nanotubes under axial magnetic field

The linear polarizability absorption spectra of semiconducting carbon nanotubes under axial magnetic field (B) have been calculated by the π-orbital tight-binding model and sum-over-state method. We have found that the optical spectra are split by the B-induced symmetry breaking and the amount of splitting increases with increase of magnetic field. Although the results are obtained within the noninteracting tight-binding model, the amount of splitting is still consistent with the experimental observation, offering a fast estimation of the B-induced splitting. Our numerical results also indicate that the splitting amounts of the second and third absorption peaks are close to that of the first one, which may be observed by the future experiments.     

Excited strange mesons and their decays in a chiral U(3)×U(3) Lagrangian

We bosonize the Nambu-Jona-Lasinio quark model with separable nonlocal interactions in order to derive a chiral U(3)×U(3) Lagrangian, containing, besides the usual meson fields, their first radial excitations. The spontaneous breaking of chiral symmetry is governed by the Nambu-Jona-Lasinio gap equation. The first radial excitations of the kaon, K*, and ? are described with the help of two form factors. The decays K*′ → ρK, K*′ → K*π, K*′ → Kπ, ?′ → K*K, ?′ → $\bar KK$ K, K′ → Kρ, K′ → K*π, and K′ → K2π are considered, and a qualitative agreement of our results with the experimental data is found.     

Investigation of the pion condensation threshold in finite nuclei

The threshold condition for pionic instabilities in finite nuclei is investigated by evaluating the response function for pion-like nuclear excitations (i.e. isovectorJ ^π =0^?, 1⁺, 2^?, ... excitations). The onset of pion condensation is then defined as the point where the response function for givenJ ^π becomes infinite at frequencyω=0. Using a momentum space representation for the response function which allows to establish clear connections to the nuclear matter case, we find that for angular momentaJ smaller than a maximum value which depends on the size of the nucleus, the threshold condition is almost the same as for infinite matter. We also study the systematics of this phenomenon as a function of nuclear mass number.     

Growing networks with preferential deletion and addition of edges

A preferential attachment model for a growing network incorporating the deletion of edges is studied and the expected asymptotic degree distribution is analyzed. At each time step t=1,2,…, with probability π₁>0 a new vertex with one edge attached to it is added to the network and the edge is connected to an existing vertex chosen proportionally to its degree, with probability π₂ a vertex is chosen proportionally to its degree and an edge is added between this vertex and a randomly chosen other vertex, and with probability π₃=1−π₁−π₂<1/2 a vertex is chosen proportionally to its degree and a random edge of this vertex is deleted. The model is intended to capture a situation where high-degree vertices are more dynamic than low-degree vertices in the sense that their connections tend to be changing. A recursion formula is derived for the expected asymptotic fraction p_k of vertices with degree k, and solving this recursion reveals that, for π₃<1/3, we have p_k∼k^{−(3−7π₃)/(1−3π₃)}, while, for π₃>1/3, the fraction p_k decays exponentially at rate (π₁+π₂)/2π₃. There is hence a non-trivial upper bound for how much deletion the network can incorporate without losing the power-law behavior of the degree distribution. The analytical results are supported by simulations.     

Shell model and octupole states in148Gd from in-beam experiments

Through (α, 4n) and (τ, 3n) reactions the high-spin states in the two-neutron nucleus¹⁴⁸Gd were populated up toI ^π=21? at 7.2 MeV, including numerous states above the yrast line. The¹⁴⁸Gd energy spectrum is interpreted in terms of the spherical shell model. Identification of the (νf _7/2 i _{1 3/2})_10? state gives the νi _13/2 single particle energy free of octupole admixtures as 2.1(1) MeV. Eight high-spin states between 1.2 and 3.7 MeV were identified as the couplings of the two valence-particles to the¹⁴⁶Gd octupole phonon, and three above-lying levels are assigned as double-octupole excitations including a 12⁺ state which decays by anE3-E3 stretched cascade. All these octupole levels can be quantitatively predicted from the one-particle x phonon spectrum of¹⁴⁷Gd. The high-spin states above 3 MeV are four-quasiparticle excitations ofπ ⁺¹ π ^?1 ν ² andπ ² ν ² type and their energies are in good accord with shell model estimates.     

Plasmon hybridization in metallic nanotubes with a nonconcentric core

We develop a theory of plasmon excitations in a metallic nanotube with a nonconcentric core using the plasmon hybridization method. We apply the two-center cylindrical coordinate system for mathematical convenience and find an explicit form of the dispersion relation for surface plasmons, in terms of interaction between the bare plasmon modes of the individual surfaces of the nanotubes. We present numerical result displaying the effect of the offset distant d of the inner core from the nanotube center, when there is no angular momentum transfer, i.e., m=0. For large offsets, the plasmon shifts is strong, but weak for small offset.     

Photoproduction of neutral pions to discrete nuclear states

We study the photoproduction of neutral pions on nuclei to discrete final states at intermediate energies. Both photoproduction and distortion of the π⁰ are dominated by Δ-excitation, which requires a careful treatment of the Δ-nucleus dynamics. We use the Δ-hole approach, which has been used to describe a variety of pion- and photon-induced nuclear reactions. It is shown that π⁰ photoproduction is very sensitive to medium effects of Δ-propagation and can therefore be used to investigate the Δ-nucleus interaction. The validity of the distorted-wave impulse approximation for this reaction is examined. Results for low-lying excitations in ¹²C are compared with the available data.     

Investigation of 228Th in the (α, α'2n) reaction

The ²³⁰Th(α, α'2n)²²⁸Th reaction at E_α = 56 MeV was used to investigate states of moderately high spin in ²²⁸Th. Conversion electron and e^?-γ coincidence measurements were carried out, where the electrons were detected with an iron-free orange spectrometer. The ground state and low-lying K^π = 0^? rotational bands were observed up to I^π = 14⁺ and 13^?, respectively. The data are interpreted in terms of an ω-expansion for the ground-state rotational band, and an octupole vibrational band distorted by the Coriolis coupling to the K^π ? 1^? excitations for the K^π = 0^? band.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

New 0+ states in 146Sm from a (p,t) experiment and the particle-core coupling model

The reaction ¹⁴⁸Sm(p,t)¹⁴⁶Sm was studied at E_p = 26 MeV. Fifty-six excited states in the final N = 84 nucleus were observed below E_exc = 4.2 MeV and their angular distributions were measured at 8 angles. Spin and parity assignments are made for 37 of them, based on the determination of the transferred angular momentum L. Ten excited J^π = 0⁺ states were identified. The emerging excitation energies of the J^π = 0⁺ states and the monopole strength distribution in ¹⁴⁶Sm are compared with those observed in nearby nuclei and with results of calculations done in the framework of the Particle-Core Coupling Model. The main features of the experimental results are interpreted using a superposition of two types of configurations: two-particle states coupled to the collective excitations of the N = 82 core and two holes coupled to the collective excitations of the N = 86 core. Calculations show also that two-phonon⊗two-particle configurations are spread over many states and their identification becomes difficult.     

Rotations and intrinsic Kπ = 1+ excitations in deformed rare-earth nuclei

The K^π = 1⁺ intrinsic excitations in deformed rare-earth nuclei are studied in the framework of the random-phase approximation, which excludes the spurious state (zero-energy excitation) related with the rotation of the whole nucleus. We take the Nilsson-plus-pairing, quadrupole and spin-dependent (magnetic dipole or spin-quadrupole) force model. Cranking-model formulae for the moment of inertia and the collective gyromagnetic ratio are derived as the zero-energy limit of the lowest K^π = 1⁺ excitation. They are modified from the usual formulae due to the spin-dependent force. The M1 and E2 transitions from the K^π = 1⁺ excited states to the ground state of ¹⁷⁰Yb are also calculated. The M1 transition strengths are concentrated almost below 6 MeV, while the E2 ones are scattered even up to 12 MeV. Several levels around 12 MeV show rather strong E2 transitions of ΔN = 2 character.