Coherent phonons in a Bi2Se3 film generated by an intense single-cycle THz pulse
We report an observation of coherent phonons of E-g(1), E-u(1), A(1g)(1), and E-g(2) symmetry generated in a single-crystal film of Bi2Se3 by an intense single-cycle THz pulse. The atomic vibrations reveal themselves through periodic modulation of the refractive index of the film. The largest signal is detected at the frequency of 4.05 THz that corresponds to the E-g(2) mode. The generation of E-g(2) phonons is interpreted as resonant excitation of the Raman mode by the second harmonic of THz-driven nonlinear E-u(1) oscillator, the fundamental frequency of which (2.05 THz) is approximately half that of E-g(2). The origin of nonlinearity in this case is cubic lattice anharmonicity, while generation of E-g(1) (1.1 THz) and A(1g)(1) (2.25 THz) phonons is a manifestation of quartic anharmonicity enhanced by the occasional combination relations between phonon frequencies in Bi2Se3.
The procedure of measurements and the formulas for calculating the sample's reflection (R) and transmission (T) coefficients with the use of a passive terahertz spectrometer are presented. The passive spectrometer comprises of cold and room-temperature chambers interconnected via a multimode waveguide (light pipe). Spectral selective liquid helium-cooled detector is placed at the cold side, and a specular disk shutter is positioned near the room-temperature end of the light pipe. The metered coefficients R and T can be calculated as a result of the two consecutive synchronous detection measurements — with the specimen placed before and after the shutter. Passive terahertz spectrometry based on the selective cooled detectors is a good alternative to active one when the use of a sophisticated teraherz radiation source is difficult or undesirable.
Selective excitation of coherent high-amplitude vibrations of atoms in a solid can induce exotic nonequilibrium states, in which the character of interactions between electronic, magnetic and lattice degrees of freedom is considerably altered and the underlying symmetries are broken. Here we use intense single-cycle terahertz pulses to drive coherently the dipole-active E1u phonon mode of a Bi2Se3 crystal. As a result, several Raman-active modes are simultaneously excited in a nonlinear process, while one of them, having the E2g symmetry, experiences dynamical splitting during the first two picoseconds after excitation. The corresponding angular scattering pattern is modified indicating the coexistence of two phonon modes characteristic of a nonequilibrium state with a lower crystal symmetry. We observe also a short-lived frequency splitting of the original E2g mode that immediately after excitation amounts to ∼25% of the unperturbed value. This transient state relaxes with a characteristic time of ∼1 ps, that is close to the decay time of the squared amplitude of the resonantly excited infrared-active E1u mode. We discuss possible mechanisms of the dynamical splitting: nonlinear lattice deformation caused by the intense E1u vibrations and excitation of anisotropic electronic distribution due to nonlinear electron-phonon interaction. Our data also contain an evidence in favor of the sum-frequency Raman mechanism of generation of the coherent E2g phonons in Bi2Se3 excited by terahertz pulses.
The ground state of He–HCO+ is investigated using a recently developed double resonance technique, consisting of a rotational transition followed by a vibrational transition into a dissociative state. In order to derive precise predictions for the rotational states, the high resolution infrared predissociation spectroscopy of the v1 C–H stretching mode is revisited. Eleven pure rotational transitions are measured via the double resonance method. A least squares fit of these transitions to a standard linear rotor Hamiltonian reveals that the semirigid rotor model cannot fully describe the loosely bound He–HCO+ complex. The novel double resonance technique is compared with other action spectroscopic schemes, and some potential future applications are presented.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
This volume presents new results in the study and optimization of information transmission models in telecommunication networks using different approaches, mainly based on theiries of queueing systems and queueing networks .
The paper provides a number of proposed draft operational guidelines for technology measurement and includes a number of tentative technology definitions to be used for statistical purposes, principles for identification and classification of potentially growing technology areas, suggestions on the survey strategies and indicators. These are the key components of an internationally harmonized framework for collecting and interpreting technology data that would need to be further developed through a broader consultation process. A summary of definitions of technology already available in OECD manuals and the stocktaking results are provided in the Annex section.