Coherent phonon excitation in bismuth

Ultrafast time-resolved reflectivity of a bismuth thin film evaporated on a silicon substrate is measured to investigate coherent phonons in bismuth. The reflectivity result is analyzed by a linear chirp approximation to obtain the time dependent frequencies of coherent phonons. Not only the optical modes are detected, which are generated by a combination of impulsive stimulated Raman scattering and displacive excitation of coherent phonon, acoustic phonon modes are also observed, which are emitted by the A_1g optical phonon.     

Quasiparticle recombination and 2Δ-phonon-trapping in superconducting tunneling junctions

The experimental recombination lifetime τ_eff of quasiparticles in superconducting films in general exceeds the intrinsic recombination lifetime τ _R by phonon trapping. On the basis of geometric acoustic propagation and reabsorption of phonons emitted in quasiparticle recombination, τ_eff is calculated as a function of film thicknessd taking into account longitudinal and transverse phonon reabsorption, bulk loss processes and acoustical phonon transmission into the substrate. With increasing thicknessd three characteristic ranges are found: range 1 with film thicknessd small compared to the phonon reabsorption mean free path Λ _w range 2 withd larger than Λ _w and dominating boundary losses, and range 3, also withd larger than Λ _w but with dominating bulk losses. For very smalld the relation between τ_eff and τ _R , the intrinsic recombination lifetime, contains only the limiting angle of total reflection of phonons within the superconducting film. Therefore, τ _R can be directly obtained by τ _eff measurements and from the sound velocities of the film-substrate system. Range 2 is characterized by a linear dependence of τ _eff ond. In this range it is not possible to obtain τ _R from τ _eff measurements, however, τ _eff allows a determination of the phonon boundary transmission. Range 3 shows no thickness dependence of τ _eff ond in the limit of larged values. In this range a further method for obtaining τ _R from τ _eff values is suggested.     

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF₂, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.     

Tunneling-states model and the propagation characteristics of surface phonons in amorphous solids at low temperatures

The propagation characteristics of the surface phonon in an amorphous film are theoretically investigated at low temperatures based on the tunneling-states model. It is shown that the attenuation rate of the surface phonon is proportional to ω tanh ?ω/2k_BT and the relative velocity variation to ln (T/T₀) for the thickness of the amorphous film comparable to or larger than the surface phonon wavelength. Below 3K, our result can well account for recent measurements of the relative velocity variation by Hartemann et al.     

A microscopic theory for the central peak in SrTiO3

A microscopic theory for the central peak at the R point of SrTiO₃ and similar systems is proposed. A phonon resonance is formed by two phonons near the R point. The central resonance arises from the decay of the soft zone boundary phonon into the resonance and a zone boundary phonon. The theory applies also to other non piezoelectric and non pyroelectric soft mode systems.     

Effect of microstructure on thermoelectric properties of CSD-grown Bi2Sr2Co2Oy thin films

Three Bi₂Sr₂Co₂O_y thin films with different microstructures have been prepared by chemical solution deposition on LaAlO₃(001) through varying the annealing temperature. With the decrease in the annealing temperature, both the size and c-axis alignment degree of grains in the film decrease as well, leading to an increase in the film resistivity. In addition, the decrease in the annealing temperature also results in a slight increase in the seebeck coefficient due to the enhanced energy filtering effect of small-grain film. The nanostructured Bi₂Sr₂Co₂O_y film with the average grain size of about 100 nm shows a power factor comparable to that of the films with larger grains. Since the thermal conductivity of the nanostrcutured films can be depressed due to the enhanced phonon scattering by grain boundary, a higher figure of merit is expected in Bi₂Sr₂Co₂O_y thin film with grains in nanometer size.     

Superconducting tunnel junctions as quantitative detector of intense thermal phonon radiation

Thermal phonons are emitted from a pulse-heated constantan film into an a-cut sapphire (kept at 1.2 K) and are detected quantitatively by recording the current-voltage characteristic of the tunnel junction during the relatively strong phonon irradiation. These measurements are made with the maximum of the Planck distribution of the emitted phonons varied in the frequency range from 0.3 to 1.5 THz. These momentary diode characteristics coincide within experimental error withd c-characteristics at higher substrate temperatures. Assuming thermal equilibrium between electrons and phonons in the tin film and assuming the validity of the acoustic-mismatch model, a comparison can be made between observed phonon power and theoretically expected phonon power. Good agreement is achieved if the influence of the acoustic cut-off frequencies of the tin detector material is taken into account.Supported by Deutsche Forschungsgemeinschaft     

第一性原理计算MgB2薄膜拉伸对超导电性的影响

应用全势线性响应线性糕模轨道方法计算MgB₂的电子能带结构、声子谱及电声子耦合常数,并讨论MgB₂的超导电性.通过比较MgB₂薄膜双轴拉伸前后超导电性的变化可以看出，随着a轴晶格常数增大和c轴晶格常数减小,声子谱中硼的E_2g声子频率显著下降，使得电声子耦合强度λ和声子对数平均频率ω_ln增强,提高了MgB₂薄 关键词： 超导电性 能带结构 声子频率 电声子耦合     

Surface radiative recombination in GaAs with surphon participation

Surface photoluminescence spectra of GaAs-Si₃N₄ (Ge₃N₄) systems excited by a He-Ne laser were investigated in the 4.2–50 K temperature range. Two phonon replicas (PR) of the zero phonon surface emission band at 1.477 eV were observed. This permits a calculation of the surface phonon energy (33 ± 0.5 meV). The PR behaviour agrees with Hopfield's theory on the basis of which information is given on the dynamic characteristics of the lattice at the surface, the Bohr electron radius of the radiative centre localized on the surface, etc. From the mean number of emitted surface phonons (surphons), the Franck-Condon shift, and the widening of the surface emission band, we conclude that there is an intensification of electron-phonon interactions at the surface in comparison with the bulk. The band of near-edge surface luminescence is discussed, which we associate with a manifestation of collective effects at the surface.     

Micro-Raman spectroscopic investigation of NiSi films formed on BF<Subscript>2</Subscript><Superscript>+</Superscript>-, B<Superscript>+</Superscript>- and non-implanted (100)Si substrates

The micro-Raman spectroscopic technique was used to investigate vibrational properties of NiSi thin films formed on three different (100)Si substrates: non-implanted, 20 keV BF₂⁺-implanted, and 20 keV B⁺-implanted. Raman measurements were also performed on NiSi powder to identify various phonon modes associated with different selection rules of group theory. It was found that the Raman peaks for NiSi thin films formed on the BF₂⁺-implanted substrate were broader and shifted to lower frequencies compared to those for films formed on the other substrates. The broadening of the Raman peaks for these films, which also exhibit much improved thermal stability, is attributed to the small grains that probably result from the segregation of fluorine to grain boundaries and interfaces. It is further proposed that grain boundary segregation influences the stress in the silicide film, resulting in shifts in phonon peak positions. PACS 78.30.Am; 74.25.Kc; 68.35.Dv; 68.55.Ln; 66.30.Jt     

TA1[110] phonon dispersion and martensitic phase transition in ordered alloys Fe3Pt

In a previous neutron scattering study, we had observed that the TA phonon softening in L1₂-ordered ferromagnetic Fe₇₂Pt₂₈ Invar is pronounced at the zone boundary M-point and leads to an antiferrodistortive phase transition at low temperatures. Here, we report on similar neutron scattering investigations on two ordered crystals with higher Fe content to investigate the relation between the TA phonon softening and the martensitic transformation, which occurs in Fe-rich ordered Fe-Pt. We find that the TA phonon softening, especially at the M-point zone boundary, does not depend on the composition of the investigated crystals. In Fe_74.5Pt_25.5, however, the antiferrodistortive phase transition temperature is enhanced due to tetragonal strain preceding the martensitic transition. In Fe₇₇Pt₂₃ a precursor driven premartensitic phase transition is not observed. The structure of the martensite is, however, influenced by the soft mode lattice instability of the austenite. This would explain the origin of structural details found previously for Fe₃Pt thermoelastic martensite. Received 18 January 1999 and Received in final form 11 March 1999     

Structural studies and Raman spectroscopy of forbidden zone boundary phonons in Ni‐doped ZnO ceramics

We present X‐ray diffraction and Raman spectroscopy studies of Ni‐doped ZnO (Zn_1−xNi_xO, x = 0.0, 0.03, 0.06, and 0.10) ceramics prepared by solid‐state reaction technique. The presence of the secondary phase along with the wurtzite phase is observed in Ni‐doped ZnO samples. The E₂(low) optical phonon mode is seen to be shifted to a lower wavenumber with Ni incorporation in ZnO and is explained on the basis of force‐constant variation of ZnO bond with Ni incorporation. A zone boundary phonon is observed in Ni‐doped samples at ∼130 cm⁻¹ which is normally forbidden in the first‐order Raman scattering of ZnO. Antiferromagnetic ordering between Ni atoms via spin‐orbit mechanism at low temperatures (100 K) is held responsible for the observed zone boundary phonon. Copyright © 2008 John Wiley & Sons, Ltd.     

Anomalies of the phonon properties in multiferroic BiFeO3 thin films near the magnetic phase transition temperature

Size, substrate, doping and magnetic field effects on the phonon properties in multiferroic BiFeO₃ thin films are studied based on a microscopic model. We obtain an anomaly near the magnetic phase transition temperature TN which can be attributed to the magnetoelectric nature of BiFeO₃ and strong anharmonic spin-phonon interaction. It is shown that due to crystal lattice distortion for dopants with ionic radius smaller than that of the host ions the phonon energy decreases (for example Tb or Ti), whereas for the opposite case (larger radius of the doping ions, for example Co or Ni) it increases. The phonon damping is always enhanced compared to the undoped thin film.     

Effective phonon scattering and enhancement of thermoelectric performance in Ga-excess Bi0.4Sb1.6Te3 compounds

We investigate the thermoelectric properties on Ga-excess p-type Ga_xBi_0.4Sb_1.6Te₃ compounds. Even though the random distribution of Ga-doping increases electrical resistivity giving rise to the decrease of power factor, the significant decrease of lattice thermal conductivity by the excess Ga-doping induces significant enhancement of ZT value (1.13 at 350 K) for the Ga x = 0.03 doped compound. From the X-ray diffraction and elemental mapping by energy dispersive X-ray spectroscopy, we observed Sb and Ga phase separation leading to the phonon scattering. The Sb precipitation implies atomic defect in the matrix which can induce short wavelength phonon scattering. The synergetic phonon scatterings from various scattering sources such as point defect, alloy scattering, and grain boundary phonon scattering have an important role in the enhancement of thermoelectric performance.     

Band-edge photoluminescence of PbI2

Photoluminescence spectra of the hexagonal layered semiconductor PbI₂ have been measured at 4.2°K on various single crystalline specimens. They are interpreted in relation with the anisotropic polariton model. The emitted radiation is assumed to originate from free and bound exciton recombination, with and without phonon emission. A peak polarized E ∥ c about 4 meV above the excitonic line is related to the extraordinary polariton mode.     

Enhanced Fröhlich interaction of semiconductor cuprous oxide films determined by temperature‐dependent Raman scattering and spectral transmittance

Anomalous low temperature behaviors in cuprous oxide (Cu₂O) film grown on quartz substrate have been investigated by temperature‐dependent Raman and transmittance spectra. The longitudinal optical components of two Γ₁₅‐ phonon modes become sharper and more intense at a low temperature. It can be found that the highest‐order electronic transition located at 6.4 eV exhibits a minimum transmittance near 200 K. Correspondingly, the variations from phonon intensity ratios reveal obvious anomalies with the decreasing temperature, indicating the existence of strong electron–phonon coupling mediated by Fröhlich interaction in the Cu₂O films below the temperature of 200 K. Copyright © 2012 John Wiley & Sons, Ltd.     

纳米陶瓷中限域效应与界面效应对热电性能影响的理论研究

采用玻尓兹曼输运方程与密度泛函计算相结合的方法, 理论研究了限域效应与界面效应对SrTiO₃纳米陶瓷热电性能的影响. 理论计算的结果表明, 室温下纳米陶瓷的热电优值较单晶有了大幅度提高, 可达到0.8. 热电优值的提高主要来源于晶粒内的声子限域效应与电子在晶界处的界面能量过滤效应, 电子的限域效应与声子的界面散射效应起到了辅助作用. 本文结论也可推广至其他材料, 因此对高性能热电纳米陶瓷的设计将起到积极的作用.     

Coherent phonon generation and detection in ultrathin SrTiO3 grown directly on silicon

Time‐resolved two color pump‐probe polarization spectroscopy was performed at room temperature on SrTiO₃ films grown directly on Si with film thickness varying from 2 nm to 7.8 nm. An E‐symmetry mode with a characteristic frequency of 0.2 THz is impulsively generated and measured in these coherently strained tetragonal phase SrTiO₃ thin films. A superimposed exponentially decaying signal observed indicates the possible relaxational hopping of Ti ion between double potential wells. The dependence of the coherent phonon signal on pump and probe laser polarization helps to identify the symmetry of the phonon modes.     

Phonon anomalies in mixed valent CeSn3

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of CeSn₃ and LaSn₃. Close to the zone boundary in the [111] symmetry direction pronounced differences were observed between the measured phonon frequencies of these two compounds.     

Ultrafast switching of hard X‐rays

A new concept for shortening hard X‐ray pulses emitted from a third‐generation synchrotron source down to few picoseconds is presented. The device, called the PicoSwitch, exploits the dynamics of coherent acoustic phonons in a photo‐excited thin film. A characterization of the structure demonstrates switching times of ≤ 5 ps and a peak reflectivity of ～10^?3. The device is tested in a real synchrotron‐based pump–probe experiment and reveals features of coherent phonon propagation in a second thin film sample, thus demonstrating the potential to significantly improve the temporal resolution at existing synchrotron facilities.