Continuum Of Excitations Research Articles

Possible Collective Spin Excitation in the Spin-Triplet Superconducting State of Sr2RuO4: Multi-Band Theory

We investigate possible collective spin excitation by calculating the dynamical spin susceptibility in the spin-triplet superconducting state of Sr 2 RuO 4 . The effect of the fluctuating triplet-pairing condensate on the spin susceptibility is taken into account. The momentum and band dependences of superconducting gap structure are determined microscopically by solving the linearized Eliashberg equation for a realistic three-band Hubbard model within the perturbation theory in Coulomb interaction, and the evolution of gap magnitude below the transition temperature is determined by the standard BCS gap equation. The triplet pairing symmetry is assumed to be the most promising form: \(\mathbf{d}(\mathbf{k})\sim(k_{x}+\mathrm{i}k_{y})\hat{z}\). For small (maybe realistic) anisotropy of pairing interaction in spin space, we expect a possible low-energy spin-wave mode as a peak structure in the imaginary part of the dynamical spin susceptibility, isolated from the spin excitation continuum. The present work ...

Threshold effects in the ionization of atoms by positron impact

We employ different theoretical models, both classical and quantum-mechanical, to explore the recoil-ion momentum distribution in positron–atom collisions. We pay special attention to the vicinity of the kinematical threshold between ionization and positronium formation. We demonstrate that it is intertwined by dynamical constraints to the formation of highly excited and low-lying continuum electron–positron states. Finally we discuss how the study of recoil-ion momentum distribution, which is characteristic of a reaction microscopy technique, might represent an alternative approach to the standard spectroscopy of electrons and positrons.

Phonon Confinement and Impurity Doping in Silicon Nanowires Synthesized by Laser Ablation

The effect of phonon confinement and impurity doping in silicon nanowires (SiNWs) synthesized by laser ablation were investigated. The diameter of SiNWs was controlled by the synthesis parameters during laser ablation and the subsequent thermal oxidation. Thermal oxidation increases the thickness of the SiNWs’ surface oxide layer, resulting in a decrease in their crystalline Si core diameter. This effect causes a downshift and asymmetric broadening of the Si optical phonon peak due to phonon confinement. Boron doping was also performed during the growth of SiNWs. Local vibrational modes of boron (B) in silicon nanowires (SiNWs) synthesized by laser ablation were observed at about 618 and 640 cm–1 by Raman scattering measurements. Fano broadening due to coupling between discrete optical phonons and the continuum of interband hole excitations was also observed in the Si optical phonon peak. These results prove that B atoms were doped in the SiNWs.

Laser-induced vibrational excitation and desorption in the Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems

The Cs/Cu(1 1 1) and Na/Cu(1 1 1) systems exhibit a transient excited electronic state localized on the adsorbate. Photo-excitation of this state triggers a motion of the alkali adsorbate away from the surface, leading to vibrational excitation of the adsorbate and possibly to desorption. A theoretical study of these photo-induced processes in the case of an exciting fs laser pulse is reported, based on a time-dependent approach of the adsorbate motion. The mean energy transfer from the laser photon energy to the adsorbate motion is shown to be weak, about 1% of the photon energy. Correspondingly, the vibrational excitation to high lying levels is very weak as well as the desorption process. The initial electronic state of the photo-induced process belongs to a continuum and vibrational excitation and desorption are found to vary rapidly with the energy of the initial electronic state. Initial vibrational excitation of the alkali adsorbate is also found to efficiently favour the desorption process, leading to a drastic variation of the desorption probability with the vibrational temperature of the adsorbate. The present results for the two systems are discussed and compared, in connection with available experimental data on these systems and on similar ones.

Bosonization approach to charge and spin dynamics of one-dimensional spin-12fermions with band curvature in a clean quantum wire

We consider one-dimensional (1D) spin-1/2 fermions in a clean quantum wire, with forward scattering interactions and a non-linear single-particle spectrum, $\xi_k = v|k| + k^2/2m$ where $v$ is the Fermi velocity and $1/m$ is the band-curvature. We calculate the dynamical structure factor (DSF) of the model at small wave-vector $q$ with the help of the bosonization technique. For spinless fermions, we show that, starting from the single-parametric spectrum: $\om = u |q|$, bosonization emulates the 2-parametric excitation spectrum: $\om = u |q| \pm q^2/2m^*$, where $m^*$ decreases with increasing repulsive interactions. Moreover, away from the excitation-cone, {\it i.e.} $\om \gg u |q|$, bosonization yields the 2-pair excitation continuum of the DSF. For spinful fermions, we show that the spin-charge coupling (SCC) due to band-curvature affects charge and spin DSF in an asymmetric way. For the charge DSF, SCC manifests as a two-peak structure: a charge peak at $\om = u_\rho |q|$ but also a spin peak at $\om = u_\sigma |q|$, as charge fluctuations may decay via chargeless spin-singlet excitations. For the magnetic DSF, SCC manifests as a continuous transfer of magnetic spectral weight to frequencies $\om > u_\sigma |q|$, as spin fluctuations decay via pairs of chargeless spin and spinless charge-neutral excitations.

Elementary electronic excitations in graphene nanoribbons

We analyze the collective mode spectrum of graphene nanoribbons within the random phase approximation. In the undoped case, only metallic armchair nanoribbons support a propagating plasmon mode. Landau damping of this mode is shown to be suppressed through the chirality of the single particle wave functions. We argue that undoped zigzag nanoribbons should not support plasmon excitations because of a broad continuum of particle-hole excitations associated with surface states, into which collective modes may decay. Doped nanoribbons have properties similar to those of semiconductor nanowires, including a plasmon mode dispersing as $q\sqrt{\ensuremath{-}\mathrm{ln}\phantom{\rule{0.2em}{0ex}}qW}$ and a static dielectric response that is divergent at $q=2{k}_{F}$.

Photoexcited Fano interaction in laser-etched silicon nanostructures

Photoexcitation dependent Raman studies on the optical phonon mode in silicon nanostructures (Si NS) prepared by laser-induced etching are done here. The increase in the asymmetry of the Raman spectra on the increasing laser power density is attributed to Fano interference between discrete optical phonons and continuum of electronic excitations in the few nanometer size nanoparticles made by laser-induced etching. No such changes are observed for the same laser power density in the crystalline silicon sample or ion-implanted silicon sample followed by laser annealing. A broad photoluminescence spectrum from Si NS contains multiple peak behavior, which reveals the presence of continuum of electronic states in the Si NS.

Collective excitations of Dirac electrons in a graphene layer with spin-orbit interactions

Coulomb screening and excitation spectra of electrons in a graphene layer with spin-orbit interaction (SOI) is studied in the random phase approximation. The SOI opens a gap between the valence and conduction bands of the semi-metal Dirac system and reshapes the bottom and top of the bands. As a result, we have observed a dramatic change in the long-wavelength dielectric function of the system. An undamped plasmon mode emerges from the inter-band electron-hole excitation continuum edge and vanishes or merges with a Landau damped mode on the edge of the intra-band electron-hole continuum. The characteristic collective excitation of the Dirac gas is recovered in a system with high carrier density or for a large wave vector.

Density functional theory for nuclei and hypernuclei

Nuclear matter and finite nuclei at extreme isospin are studied using the DDRH field theory. A relativistic energy density functional with density dependent meson–baryon vertex functionals, derived from the Dirac–Brueckner theory, is derived. Dynamical correlations from pairing interactions and core polarization are found to increase strongly when approaching the driplines. The above particle threshold core polarization gives rise to narrow Fano Resonances as a new mode of continuum excitation. This is accompanied by a strong reduction of mean-field configurations. Pygmy dipole resonances are found to provide a clear signature for neutron skins.

Optical investigation of the metal-insulator transition in FeSb2

We present a comprehensive optical study of the narrow gap $FeSb_2$ semiconductor. From the optical reflectivity, measured from the far infrared up to the ultraviolet spectral range, we extract the complete absorption spectrum, represented by the real part $\sigma_1(\omega)$ of the complex optical conductivity. With decreasing temperature below 80 K, we find a progressive depletion of $\sigma_1(\omega)$ below $E_g\sim 280$ cm$^{-1}$, the semiconducting optical gap. The suppressed (Drude) spectral weight within the gap is transferred at energies $\omega>E_g$ and also partially piles up over a continuum of excitations extending in the spectral range between zero and $E_g$. Moreover, the interaction of one phonon mode with this continuum leads to an asymmetric phonon shape. Even though several analogies between $FeSb_2$ and $FeSi$ were claimed and a Kondo-insulator scenario was also invoked for both systems, our data on $FeSb_2$ differ in several aspects from those of $FeSi$. The relevance of our findings with respect to the Kondo insulator description will be addressed.

Collective mode of homogeneous superfluid Fermi gases in the BEC-BCS crossover

We perform a detailed study of the collective mode across the whole BEC-BCS crossover in fermionic gases at zero temperature, covering the whole range of energy beyond the linear regime. This is done on the basis of the dynamical BCS model. We recover first the results of the linear regime in a simple form. Then specific attention is payed to the non linear part of the dispersion relation and its interplay with the continuum of single fermionic excitations. In particular we consider in detail the merging of collective mode into the continuum of single fermionic excitations. This occurs not only on the BCS side of the crossover, but also slightly beyond unitarity on the BEC side. Another remarkable feature is the very linear behaviour of the dispersion relation in the vicinity of unitarity almost up to merging with the continuum. Finally, while on the BEC side the mode is quite analogous to the Bogoliubov mode, a difference appear at high wavevectors. On the basis of our results we determine the Landau critical velocity in the BEC-BCS crossover which is found to be largest close to unitarity. Our investigation has revealed interesting qualitative features which would deserve experimental exploration as well as further theoretical studies by more sophisticated means.

Electron impact ionization cross-sections of plasma relevant and astrophysical silicon compounds: SiH 4, Si 2H 6, Si(CH 3) 4, SiO, SiO 2, SiN and SiS

Total cross-sections (TCSs) for collisions of electrons having energies from ∼10 to 2000 eV are calculated for atomic silicon and its compounds SiH 4, Si 2H 6, Si(CH 3) 4, SiO, SiO 2, SiN and SiS, important in plasma and astrophysical applications. In each case total inelastic cross-sections are determined in the complex potential formalism and are partitioned into discrete and continuum excitation contributions in order to derive total ionization cross-sections. The present total (complete) cross-sections and total ionization cross-sections are found to be in a good general agreement with the previous data available for Si, SiH 4, Si 2H 6 and Si(CH 3) 4. This paper also reports the first theoretical ionization cross-sections for new targets SiO, SiO 2, SiN, and SiS for which almost no work of this kind has appeared in literature so far.

Tails of the dynamical structure factor of 1D spinless fermions beyond the Tomonaga approximation

We consider one-dimensional (1D) interacting spinless fermions with a non-linear spectrum in a clean quantum wire (non-linear bosonization). We compute diagrammatically the 1D dynamical structure factor, $S(\om,q)$, beyond the Tomonaga approximation focusing on it's tails, $|\om| \gg vq$, {\it i.e.} the 2-pair excitation continuum due to forward scattering. Our methodology reveals three classes of diagrams: two "chiral" classes which bring divergent contributions in the limits $\om \to \pm vq$, {\it i.e.} near the single-pair excitation continuum, and a "mixed" class (so-called Aslamasov-Larkin or Altshuler-Shklovskii type diagrams) which is crucial for the f-sum rule to be satisfied. We relate our approach to the T=0 ones present in the literature. We also consider the $T\not=0$ case and show that the 2-pair excitation continuum dominates the single-pair one in the range: $|q|T/k_F \ll \om \mp vq \ll T$ (substantial for $q \ll k_F$). As applications we first derive the small-momentum optical conductivity due to forward scattering: $\sigma \sim 1/\om$ for $T \ll \om$ and $\sigma \sim T/\om^2$ for $T \gg \om$. Next, within the $2-$pair excitation continuum, we show that the attenuation rate of a coherent mode of dispersion $\Omega_q$ crosses over from $\gamma_q \propto \Omega_q (q/k_F)^2$, {\it e.g.} $\gamma_q \sim |q|^3$ for an acoustic mode, to $\gamma_q \propto T (q/k_F)^2$, independent of $\Omega_q$, as temperature increases. Finally, we show that the $2-$pair excitation continuum yields subleading curvature corrections to the electron-electron scattering rate: $\tau^{-1} \propto V^2 T + V^4 T^3/\eps_F^2$, where $V$ is the dimensionless strength of the interaction.

Doping Dependence of Bilayer Resonant Spin Excitations in(Y,Ca)Ba2Cu3O6+x

Resonant magnetic modes with odd and even symmetries were studied by inelastic neutron scattering experiments in the bilayer high-Tc superconductor Y1-xCa+Ba2Cu3O6+y over a wide doping range. The threshold of the spin excitation continuum in the superconducting state, deduced from the energies and spectral weights of both modes, is compared with the superconducting d-wave gap, deduced from electronic Raman scattering in the B1g symmetry on the same samples. Above a critical doping level of delta approximately =0.19, both mode energies and the continuum threshold coincide. We find a simple scaling relationship between the characteristic energies and spectral weights of both modes, which indicates that the resonant modes are bound states in the superconducting energy gap, as predicted by the spin-exciton model of the resonant mode.

Dynamic probes of quantum spin chains with the Dzyaloshinskii-Moriya interaction

We consider the spin-$\frac{1}{2}$ anisotropic $XY$ chain in a transverse $(z)$ field with the Dzyaloshinskii-Moriya interaction directed along the $z$ axis in spin space to examine the effect of the Dzyaloshinskii-Moriya interaction on the $zz$, $xx$, and $yy$ dynamic structure factors. Using the Jordan-Wigner fermionization approach we analytically calculate the dynamic transverse spin structure factor. It is governed by a two-fermion excitation continuum. We analyze the effect of the Dzyaloshinskii-Moriya interaction on the two-fermion excitation continuum. Other dynamic structure factors which are governed by many-fermion excitations are calculated numerically. We discuss how the Dzyaloshinskii-Moriya interaction manifests itself in the dynamic properties of the quantum spin chain at various fields and temperatures.

Polarized Neutron Inelastic Scattering Study of Anisotropic Magnetic Fluctuations in Quasi-One-Dimensional Ising-like Antiferromagnet TlCoCl3

Polarized neutron inelastic scattering experiments have been carried out in the quasi-1D Ising-like antiferromagnet TlCoCl 3 . We observed the longitudinal magnetic fluctuation S z z ( Q , ω) for the spin-wave excitation continuum, which has not yet been observed in unpolarized neutron inelastic scattering experiments on the quasi-1D Ising-like antiferromagnets CsCoCl 3 and TlCoCl 3 , together with the transverse magnetic fluctuation S x x ( Q , ω). We compared both the obtained intensities of S x x ( Q , ω) and S z z ( Q , ω) with those calculated using the perturbation theory from the pure Ising limit by Ishimura and Shiba (IS), and a semi quantitative agreement was found.

Dynamics of quantum spin chains and multi-fermion excitation continua

We use the Jordan–Wigner representation to study dynamic quantities for the spin- 1 2 XX chain in a transverse magnetic field. We discuss in some detail the properties of the four-fermion excitation continuum which is probed by the dynamic trimer structure factor.

Carrier Doping of Silicon Nanowires Synthesized by Laser Ablation

ABSTRACTBoron (B) doped silicon nanowires (SiNWs) were synthesized by laser ablation. Local vibrational modes of B in SiNWs were observed by micro-Raman scattering measurements at room temperature. Broadening due to a coupling between the discrete optical phonon and a continuum of interband hole excitations was also observed in the Si optical phonon peak. This is called Fano broadening. These results prove that B atoms were doped in substitutional sites of crystalline Si core of SiNWs during laser ablation.

Core excited Fano-resonances in exotic nuclei

Fano-resonances are investigated as a new continuum excitation mode in exotic nuclei. By theoretical model calculations we show that the coupling of a single particle elastic channel to closed core-excited channels leads to sharp resonances in the low-energy continuum. A signature for such bound states embedded in the continuum (BSEC) are characteristic interference effects leading to asymmetric line shapes. Following the quasiparticle-core coupling model we consider the coupling of 1-QP (one-quasiparticle) and 3-QP components and find a number of long-living resonance structures close to the particle threshold. Results for 15C are compared with experimental data, showing that the experimentally observed spectral distribution and the interference pattern are in qualitative agreement with a BSEC interpretation.

Optically sectioned fluorescence lifetime imaging using a Nipkow disk microscope and a tunable ultrafast continuum excitation source

We demonstrate an optically sectioned fluorescence lifetime imaging microscope with a wide-field detector, using a convenient, continuously tunable (435-1150 nm) ultrafast source for fluorescence imaging applications that is derived from a visible supercontinuum generated in a microstructured fiber.