Relation between the superconducting gap energy and the two-magnon raman peak energy in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta)

The relation between the electronic excitation and the magnetic excitation for the superconductivity in Bi(2)Sr(2)Ca(1-x)Y(x)Cu(2)O(8+delta) was investigated by wide-energy Raman spectroscopy. In the underdoping region the B(1g) scattering intensity is depleted below the two-magnon peak energy due to the charge-spin interactions. The depleted region decreases according to the decrease of the two-magnon peak energy, as the carrier concentration increases. This two-magnon peak energy also determines the B(1g) superconducting gap energy as 2Delta approximately alphaPlanck's over 2piomega(two-magnon) approximately J(eff) (alpha = 0.34-0.41) from under to overdoping hole concentration.     

Raman scattering from phonons and magnons in antiferromagnetic Fe3BO6

Raman scattering by phonons and by magnon pairs has been observed in Fe₃BO₆. Of the predicted 60 Raman-active modes, 39 have been identified and classified according to their symmetries. The two-magnon band shows a strong decrease in intensity with increasing temperature, and almost vanishes close to T_N = 508 K. The origin of this effect is attributed to the existence of a nearly dispersionless magnon branch.     

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).     

The Effect of the Electron-Phonon Interaction on Giant Magnetoresistance in Magnetic Multilayers

We built electron and phonon free energies and attempted to investigate the effect of the electron-phonon interaction on giant magnetoresistance in magnetic multilayers. Starting from a jellium-like model, we found that the electron-phonon interaction can have an important effect on the spin splitting of electronic energy band. The expression of giant magnetoresistance has been obtained by considering the spin splitting of electronic energy band,indicating that the effect of phonon could not be neglected. Numerical calculations using our approach demonstrate the agreement between experimental and theoretical values.     

Possible interplay between a two phonon mode and high energy magnetic excitations in BiFeO<Subscript>3</Subscript>

We have performed Raman measurements on high energy excitations in BiFeO₃ single crystals as a function of both temperature and laser excitation lines. A strong feature observed at 1250 cm^-1 in the Raman spectra has been previously assigned to two phonon overtone. This peak exhibits an unusual frequency shift with the laser lines and the temperature dependence of its Fano lineshape shows two singularities at 150 K and 200 K which can be related to magnetic excitations. In the same energy range, we have also identified the two-magnon excitation with a temperature dependence very similar to the one measured for the one-magnon modes.     

Hole-interface optical phonon relaxation rates with valence band-mixing effects

We theoretically investigate the hole-interface optical phonon scattering rates for a InGaAs-AlGaAs quantum well structure, taking into account the valence-band mixing. The dispersion relation and the electrostatic potentials for interface optical phonon modes are obtained based on the macroscopic dielectric continuum model. For the hole dispersion relation, the Luttinger-Kohn Hamiltonian is used. The hole-interface optical phonon interaction is evaluated by the Fermi's golden rule taking into account the Bloch overlap factor.Our results show that the hole-interface phonon scattering rates within the parabolic band approximation are different from those including valence band mixing effects. Especially, in the low energy region, the hole-interface phonon scattering rates within the parabolic band approximation are overestimated very significantly.     

Free electron laser study of the suppression of non-radiative scattering processes in semiconductors

A brief review is given of pump–probe studies of far infrared inter-sub-level relaxation between conduction band states in doped `quasi' quantum dots (created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well) and of mid-infrared (MIR) interband recombination in narrow gap semiconductors, using the free electron laser at FOM-Rijnhuizen (FELIX). In the former case, the longitudinal optic (LO) phonon scattering rate is shown to be suppressed by a factor of about 100 when the Landau level separation is off-resonance with the optical phonon energy; in the latter case, Auger recombination is shown to be substantially suppressed in the lead salts due to their `mirror' energy band structure.     

Observation of low‐wavenumber out‐of‐plane optical phonon in few‐layer graphene

Few‐layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low‐wavenumber Raman peak of ~120 cm⁻¹ and a phonon‐induced oscillation in the kinetic curve of electron–phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low‐wavenumber out‐of‐plane optical mode in the few‐layer graphene. The phonon band shows an asymmetric shape, a consequence of so‐called Breit‐Wigner‐Fano resonance, resulting from the coupling between the low‐wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low‐wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd.     

Free carrier absorption in a ferromagnetic semiconductor below Curie temperature

The free carrier absorption (FCA) in a non-degenerate ferromagnetic semiconductor due to electron-two-magnon scattering is calculated below the Curie temperature, assuming a wide conduction band model. It has been found that in EuO FCA due to two-magnon scattering is proportional to λ^2·1 (wavelength of radiation) and it has been noticed that for high conductivity samples, explanation of FCA requires the inclusion of ionized impurity scattering of electrons besides the two-magnon process.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

Acoustical phonon assisted two-photon excitation of excitonic molecules and resonant raman scattering in evaporated CuCl films

In evaporated CuCl films, Chase et al. have concluded the Bose-Einstein condensation of excitonic molecules for the origin of a sharp emission line which has been ascribed so far to the resonant Raman line in single crystals. A trace of the similar works on evaporated samples confirms that the sharp line shifts its energy with the change of the excitation photon energy; indicating the Raman character. When the excitation photon energy exceeds the resonant energy the Raman line shows broadening, and a new emission band appears which can be ascribed to the radiative annihilation of excitonic molecules generated by the two-photon absorption and one acoustical phonon emission.     

Low frequency excitations in multiferroic MnWO(4)

Dynamic anomalies have been found in the magnetically ordered phases of multiferroic MnWO(4) using polarized Raman scattering. Strong phonon damping is observed for several B(g) modes within the ferroelectric phase and has been attributed to spin-phonon interactions. Moreover, a new low frequency excitation was detected near 33?cm(-1) that grows in intensity on cooling into the antiferromagnetic phases. It is argued that this signal is most probably due to a two-magnon process.     

Damping and modal structure of patterned magnetic nanoarrays

A number of studies have shown that ferromagnetic resonance linewidths in magnetic thin films contain a significant contribution due to processes, such as inhomogeneous broadening and two-magnon scattering followed by decay to the phonon thermal bath. We present a classical theoretical approach that permits the calculation of such processes for arrays of arbitrarily shaped magnetic nanoparticles and, by comparison with experimental results, demonstrates that the linewidth broadening and damping are dominated by inhomogenities that strongly depend on the equilibrium state of magnetization and can be used to identify the latter.     

Interaction of longitudinal phonons with discrete breather in strained graphene

We numerically analyze the interaction of small-amplitude phonon waves with standing gap discrete breather (DB) in strained graphene. To make the system support gap DB, strain is applied to create a gap in the phonon spectrum. We only focus on the in-plane phonons and DB, so the issue is investigated under a quasi-one-dimensional setup. It is found that, for the longitudinal sound waves having frequencies below 6 THz, DB is transparent and thus no radiation of energy from DB takes place; whereas for those sound waves with higher frequencies within the acoustic (optical) phonon band, phonon is mainly transmitted (reflected) by DB, and concomitantly, DB radiates its energy when interacting with phonons. The latter case is supported by the fact that, the sum of the transmitted and reflected phonon energy densities is noticeably higher than that of the incident wave. Our results here may provide insight into energy transport in graphene when the spatially localized nonlinear vibration modes are presented.     

一种计算半导体力常数的简单途径——对GaAs(111)-2×2表面声子研究的应用

本文利用成键轨道近似简化电子能带结构能量的计算,导出了任意杂化下半导体力常数的解析表达式。通过对一些材料声子色散曲线的计算和实验结果的比较,表明该方法是简单有效的。作者将其应用于GaAs(111)-2×2表面声子的研究,得到了该表面的强局域声子模的色散曲线。 关键词：     

Raman Spectroscopic Analysis of Perovskite-Structured Alkali Metal Fluorides Containing Nickel and Copper Ions

ABSTRACT

The mixed metal fluorides containing alkali metals have a range of important applications in optical and electronic devices. Raman spectrums of two such fluorides were examined. Raman spectrum of KCuF₃ at 300 K exhibited bands at 261, 295, 363, 468, 519, and 549 cm^?1, indicating site symmetry (orthorhombic) lower than the tetragonal symmetry as observed from the powder X-ray diffraction pattern. Cubic KNiF₃ showed bands at 410, 468, and 657 cm^?1. The first two bands were attributed to the second-order phonon scattering, and the band at 657 cm^?1 was assigned to two-magnon peak.     

Interplay between phonon confinement effect and anharmonicity in silicon nanowires

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.     

Oxyfluoroborate host glass for upconversion application: phonon energy calculation

Reducing the glass phonon energy is an essential procedure to achieve high efficient radiative upconversion process. The degree of covalence of chemical bonds is responsible for the high oscillator strength of intracenter transitions in rare-earth ions. So, conversion covalent to ionic glass character is proposed as a structure-sensitive criterion that controls the phonon energy of the glasses. A series of oxyfluoro aluminum-borate host glasses used for upconversion application is prepared by the conventional melt-quenching technique. Through lithium oxide substitution by lithium fluoride, the ionic-covalent property of Li⁺ ion successes to regulate the band gap energies of the studied glasses. Furthermore, a new method to determine the glass phonon energy is offered.     

Broad-Band FMR Linewidth of Co_2MnSi Thin Films with Low Damping Factor:The Role of Two-Magnon Scattering

The low Gilbert damping factor,which is usually measured by ferromagnetic resonance,is crucial in spintronic applications.Two-magnon scattering occurs when the orthogonality of the ferromagnetic resonance mode and other degenerate spin wave modes was broken by magnetic anisotropy,voids,second phase,surface defects,etc.,which is important in analysis of ferromagnetic resonance linewidth.Direct fitting to linewidth with Gilbert damping is advisable only when the measured linewidth is a linear function of measuring frequency in a broad band measurement.We observe the nonlinear ferromagnetic resonance linewidth of Co_2MnSi thin films with respect to measuring frequency in broad band measurement.Experimental data could be well fitted with the model including two-magnon scattering with no fixed parameters.The fitting results show that two-magnon scattering results in the nonlinear linewidth behavior,and the Gilbert damping factor is much smaller than reported ones,indicating that our Co_2 MnSi films are more suitable for the applications of spin transfer torque.     

Theory of two-magnon dissipation in disordered ferromagnets I. Relation to sample statistics

The theory of two-magnon dissipation in disordered ferromagnetic materials is developed. Fluctuations of one-ion anisotropy energy due to the disorder are assumed to be the source of the linewidth broadening (relaxation of magnons by two-magnon scattering). Ordinary two-magnon theory is reconsidered on the basis of the sample statistics. The main idea of it is to describe the macroscopic properties of materials by means of quantities, whose relative fluctuations over the ensemble of configurations are negligibly small for sufficiently large sample. The energies of magnons — modes with well defined wave vectork — and their lifetimes are shown to be such variables. Further, possible models for a two-component ferromagnet are considered in the approximation of high external field. The energies of magnons in mixed crystal are obtained as linear combinations of those in parent crystals. Finally, the dependence of magnon lifetimes on the ordering is separated from the dependence on the magnitude of anisotropy variations.