Coherent acoustic phonon oscillations in semiconductor multiple quantum wells with piezoelectric fields

Large coherent acoustic phonon oscillations were demonstrated using InGaN/GaN multiple quantum wells with piezoelectric fields. With UV femtosecond pulse excitation, photogenerated carriers screened the piezoelectric field and initiated the displacive coherent phonon oscillations. The specific phonon frequency was selected by the coupling between the periodic carrier distribution and the corresponding acoustic phonon mode. The induced acoustic phonon oscillation resulted in piezoelectric field modulation and then caused absorption variation through the quantum confined Franz-Keldysh effect. The wave vector uncertainty due to the finite sample width was found to determine the observed dephasing time.     

COHERENT OPTICAL PHONON EXCITATION BY FEMTOSECOND LASER PULSE IN YBa2Cu3O7-δ THIN FILMS

With a correlation of nonequilibrium carriers relaxation and coherent phonons displacive excitation, the coherent optical phonon oscillations in YBa₂Cu₃O_7-δ thin films excited by femtosecond laser pulse are simulated theoretically. It is revealed that as the oxygen concentration decreases, the coherent phonon oscillations become easier to be observed due to the decrease of the local coupling between the carriers and the lattice vibrations in the CuO₂ plane.     

The influence of surfaces and interfaces on coherent phonons in semiconductors

Experiments have shown that ultrafast optical excitation of semiconductors can produce oscillating changes in the optical properties of the material. The frequency of the oscillations in transmission or reflection usually matches one of the phonon modes, typically theq = 0 optical mode. These oscillations are known as coherent phonons. We discuss the role of surfaces and interfaces on the coherent phonon signal. We show that: (1) the coherent phonon signal can be used as a probe of the surface depletion field and (2) multiple interfaces as in a superlattice, can drastically alter the coherent phonon spectrum: screening of the modes in the superlattices is reduced and acoustic modes can now be excited.     

Transient Dynamics of Super Bloch Oscillations of a 1D Holstein Polaron under the Influence of an External AC Electric Field

Theoretical formalism for DC‐field polaron dynamics is extended to the dynamics of a 1D Holstein polaron in an external AC electric field using multiple Davydov trial states. Effects of carrier–phonon coupling on detuned and resonant scenarios are investigated for both phase and nonzero phase. For slightly off‐resonant or detuned cases, a beat between the usual Bloch oscillations and an AC driving force results in super Bloch oscillations, that is, rescaled Bloch oscillations in both the spatial and the temporal dimension. Super Bloch oscillations are damped by carrier–phonon coupling. For resonant cases, if the carrier is created on two nearest‐neighboring sites, the carrier wave packet spreads with small‐amplitude oscillations. Adding carrier–phonon coupling localizes the carrier wave packet. If an initial broad Gaussian wave packet is adopted, the centroid of the carrier wave packet moves with a certain velocity and with its shape unchanged. Adding carrier–phonon coupling broadens the carrier wave packet and slows down the carrier movement. Our findings may help provide guiding principles on how to manipulate the dynamics of the super Bloch oscillations of carriers in semiconductor superlattice and optical lattices by modifying DC and AC field strengths, AC phases, and detuning parameters.     

Ultrafast photoinduced melting of a spin-Peierls phase in an organic charge-transfer compound, K-tetracyanoquinodimethane

Ultrafast photoinduced phase transition in a spin-Peierls (SP) system of K-tetracyanoquinodimethane (K-TCNQ) was studied by femtosecond (fs) reflection spectroscopy. Photocarriers destabilize the SP phase, resulting in a decrease in molecular dimerization within 400 fs. Such a melting of the SP phase drives three kinds of coherent oscillations. By comparing the oscillations with the Raman bands activated by the dimerization, we show that the oscillation of 20 cm-1 is due to an LO phonon, and it plays an important role for the stabilization of the SP phase.     

Minibands of p-type δ-doping superlattices in GaAs

A microscopic theory is presented for high-field miniband transport in a two-dimensional superlattice. The energy transfer to the lateral electron motion is taken into account as well as scattering on polar optical phonons. Oscillatory current anomalies appear when the optical phonon frequency is a multiple of the Bloch frequency. The current oscillations, which are due to Wannier–Stark localization, are much more pronounced in a two-dimensional than in a three-dimensional system with a superlattice structure in one direction.     

On surface plasmon damping in metallic nanoparticles

Two possible mechanisms of damping of surface plasmon (SP) oscillations in metallic nanoparticles (MNPs), not connected with the electron–phonon interaction, are investigated theoretically: (a) radiation damping of SPs and (b) resonant coupling of SP oscillations with electronic transitions in the matrix. For the mechanism (a) it is shown that the radiation damping rate is proportional to the number of electrons in a MNP and therefore this channel of energy outflow from the MNP becomes essential for relatively large particles. The strong frequency and size dependence of the radiation damping rate obtained allows us to separate the contributions of radiative processes and the electron–phonon interaction to the energy leakage. The investigation of the mechanism (b) shows that the rate of energy leakage of SP oscillations from a MNP does not depend on particle size and is fully determined by the optical characteristics of the matrix. It is demonstrated that for very small MNPs of -–3 5nm size, where the strong three-dimensional size quantization effect suppresses the electron–phonon interaction, the resonance coupling in certain cases provides an effective energy outflow. PACS 78.67.Bf     

Oscillatory photoconductivity of gallium arsenide

The photoconductivity of GaAs in the energy range 1.5 –1.8 eV at 4.2 K exhibits two types of quantum oscillations: Landau oscillations for epitaxial material in high magnetic fields or LO phonon oscillations for high resistivity material. The purity and surface treatment of the samples seem to determine the type of oscillations observed.     

Electron-LO-phonon interaction in wurtzite GaN quantum wells under a magnetic field

We calculate the electron-LO-phonon relaxation rates in wurtzite GaN quantum wells in the presence of a magnetic field parallel to the growth direction. Using the dielectric continuum model (DCM), we are able to include contributions from both the interface and the quasi-confined phonon modes. The relaxation rate expression takes the phonon dispersion into account, and is applicable to all phonon modes. We find that the relaxation rates show strong oscillations as a function of the applied magnetic field. In relatively wide (8 nm) quantum wells, the inclusion of interface phonon mode decreases this oscillation amplitude. But in thin wells (5 nm), the interface phonon mode is of the same importance as the quasi-confined mode, and it strongly modifies the oscillation behavior.     

Relaxation of hot holes in p-Ge

Saturated absorption of infrared radiation by p-doped Ge is treated theoretically, predicting that both the saturation intensity and the width of the probe-saturator spectrum oscillate as functions of the saturator photon energy, ?ω. The maxima occur for ?ω = n?ω_LO, where n is an integer and ?ω_LO is the zone-center LO phonon energy. The results may a lso provide the closest measurement of hole cascade relaxation times to date. The oscillations depend on theoretical estimates that the LO phonon emissions occur substantially more rapidly than other decay processes. Hence failure to observe the oscillations would belie these estimates.     

Photoconductivity and photovoltaic excitation spectra and their wavelength derivative in Cu2O

Photoconductivity and photovoltaic excitation spectra, both direct and wavelength modulated, have been performed on monocrystalline Cu₂O from 6 to 300K. At low temperatures both types of direct spectra exhibit the excitonic structures already observed by other techniques such as absorption and reflectivity. The wavelength-modulated PC spectra reveal additional structures and in particular well-defined oscillations in the green fundamental continuum, related to photocarrier LO phonon interactions. The analysis of these results shows that the 149cm^?1 (18.5 meV) LO phonon is responsible for the observed periodic structures. The values found for the energy of the collector level (2.305 ±0.001 eV) and for the effective mass ratio $\text{(}\text{m}{}_{\mathrm{h}}\text{m}{}_{\mathrm{e}}\text{bsime; 12)}$, indicate that the oscillations are produced by the electrons excited from the heavy hole valence band Γ₈⁺ to the Γ₁⁺ conduction band. A study of the position and of the contrast of the phonon oscillations as functions of temperature illustrates the behaviour of the gap displacement with changes in temperature and throws some light onto the mechanisms responsible for the existence of such oscillations.     

Analytical model of transverse oscillations of graphene

A simple analytical model of transverse oscillations of graphene is constructed. The model is applicable to both free and stretched graphene monolayers. The dispersion relation for transverse oscillations of graphene and the corresponding phonon state density are determined.     

Singularity spectrum of the resolvent in a nonmarkovian master equation

The well-known standard nonmarkovian master equation is used to describe the damping of a harmonic macroscopic coordinate coupled to a thermally equilibrated Fermi gas. The time evolution of the phonon distribution is expressed in terms of a resolvent whose spectrum of singularities is computed and analyzed. It is seen that an apparent limitation of the model related to the appearance of undesired oscillations does not inhibit the expected behavior of the average phonon number and phonon number dispersion.     

The role of acoustic phonons for Rabi oscillations in semiconductor quantum dots

The damping of Rabi oscillations in quantum dots as well as the renormalization of the carrier-light coupling, due to the interaction with longitudinal acoustic phonons are studied as a function of temperature and laser pulse parameters. Numerical results are obtained by using a correlation expansion within the density matrix theory. The observed features like a non-monotonous dependence of the damping on the pulse duration are characteristic for the strongly non-Markovian nature of the phonon coupling in these systems. The results can be well interpreted on the level of a perturbation expansion in the carrier-phonon interaction. PACS 78.67.Hc; 63.20.Kr; 03.65.Yz     

Phonon anomalies due to collective stripe modes in high T(c) cuprates

Phonon anomalies observed in various high T(c) cuprates by neutron experiments are analyzed theoretically in terms of the stripe concept. The phonon self-energy correction is evaluated by taking into account the charge collective modes of stripes, giving rise to dispersion gap, or kink and shadow phonon modes at twice the wave number of spin stripe. These features coincide precisely with observations. The gapped branches of the phonon are found to be in-phase and out-of-phase oscillations relative to the charge collective mode.     

Phonon autoecho in bismuth and antimony single crystals

Phonon autoecho is observed upon pumping Bi and Sb semimetals with ultrashort high-energy laser pulses. The autoecho is manifested as a revival of reflection oscillations generated by an A_1g coherent phonon after their complete disappearance. The phenomenon of phonon autoecho offers decisive evidence of the nonclassical character of the state of the crystal lattice that is accomplished in pumping-probing experiments by femtosecond laser pulses.     

Electric field control of spin splitting in III–V semiconductor quantum dots without magnetic field

We provide an alternative means of electric field control for spin manipulation in the absence of magnetic fields by transporting quantum dots adiabatically in the plane of two-dimensional electron gas. We show that the spin splitting energy of moving quantum dots is possible due to the presence of quasi-Hamiltonian that might be implemented to make the next generation spintronic devices of post CMOS technology. Such spin splitting energy is highly dependent on the material properties of semiconductor. It turns out that this energy is in the range of meV and can be further enhanced with increasing pulse frequency. In particular, we show that quantum oscillations in phonon mediated spin-flip behaviors can be observed. We also confirm that no oscillations in spin-flip behaviors can be observed for the pure Rashba or pure Dresselhaus cases.     

Phonon spectrum of double-wall carbon nanotubes

The results of studying the phonon spectra of carbon nanotubes within the framework of the periodic-cluster model are presented. Special attention is paid to the phonon spectrum of double-wall nanotubes. It is shown that the spectrum is separated into two subbands corresponding to the acoustic and optical vibrational modes. A specific feature of the phonon dispersion curves is their “doublet” character. The spectrum has an acoustic mode corresponding to longitudinal oscillations of the two walls of a double-wall carbon nanotube with respect to each other.     

Photoconductivity of 2D electron systems in magnetic field

According to recent photoconductivity measurements in 2D electron semiconductor systems in magnetic fields normal to the 2D plane, the photoconductivity as a function of magnetic field exhibits oscillations in the region of fields much weaker than those necessary for the observation of the Shubnikov-de Haas effect. In this paper, the aforementioned oscillations are interpreted as a two-dimensional analogue of magnetophoton (phonon) oscillations studied in detail by different authors on 3D samples.