External Phonon Research Articles

Optomechanical effects in self-organization of a Bose–Einstein condensate in an optical cavity

The influence of mirror motion on the spatial self-organization of a Bose–Einstein condensate in an optical cavity is studied. We show that mirror dynamics tends to destroy the process of self-organization. An additional external phonon pump is shown to modify the critical photon pump needed to observe the onset of self-organization.

Implicit phonon shifts and thermodynamical properties of rigid carbon nanotube bunches

We calculate phonon shifts of external modes of a bunch of carbon nanotubes. The bunches form a 2-dimensional hexagonal arrangement of lattice with long molecules of carbon nanotubes. A simple model based on atom-atom potential has been used to calculate the implicit anharmonicity in the phonons of carbon nanotube bundles (also called ropes or bunches) having rigid tubes, with the assumption that under hydrostatic pressure only the inter-tube distance in the bunch varies. Various bulk and thermodynamic properties like thermal expansion, bulk modulus and the Gruneisen constants and external phonon shifts which naturally enter into the calculation are also described and compared with the available data. The specific heat capacity has also been calculated.

The quantum acoustomagnetoelectric field in a quantum well with a parabolic potential

The acoustomagnetoelectric (AME) field in a quantum well with a parabolic potential (QWPP) has been studied in the presence of an external magnetic field. The analytic expression for the AME field in the QWPP is obtained by using the quantum kinetic equation for the distribution function of electrons interacting with external phonons. The dependence of the AME field on the temperature T of the system, the wavenumber q of the acoustic wave and external magnetic field B for the specific AlAs/GaAs/AlAs is achieved by using a numerical method. The problem is considered for both cases: The weak magnetic field region and the quantized magnetic field region. The results are compared with those for normal bulk semiconductor and superlattices to show the differences, and we use the quantum theory to calculate the AME field in the QWPP.

Phonon coupling effect upon transport in nanoscale polymers

In an environmental coupled polymer, a variation of the conductivity is evaluated, which results from the external electron–phonon interaction coupling with the internal one. A quantized current appears under the external phonon coupling. The resonant tunnelling in the nanoscale polymer driven by the internal electron–phonon interaction is enhanced by the external phonon coupling. In addition, the external electron–phonon interaction softens the stiffness of the polymer.

A finite block spin model for permanent magnets

We propose a finite block spin phenomenology for permanent magnets in which we consider an average domain as a block spin. The permanent ferromagnetism arises in two ways: (1) the ferromagnetism that occurs inside a big block spin, i.e. the intrablock ferromagnetism, and the ferromagnetism between small block spins (SBSs) in a big block spin (BBS) which can be induced by collective ferromagnetic pairing between two SBSs mediated by temperature-irreversible bosonic strains; and (2) the ferromagnetism between BBSs, i.e. the interblock ferromagnetism. The coercivity originates from temperature-irreversible strains treated as external phonons.

Nonequilibrium phonon backaction on the current noise in atomic-sized junctions

We study backaction effects of phonon heating due to tunneling electrons on the current noise in atomic-sized junctions. Deriving a generalized kinetic approximation within the extended Keldysh Green's functions technique, we demonstrate the existence of a characteristic backaction contribution to the noise in case of low external phonon damping. We provide a physically intuitive interpretation of this contribution at large voltage in terms of slow fluctuations of the phonon occupation, and show that it generally gives a significant correction to the noise above the phonon emission threshold.

Real-time counting of single electron tunneling through a T-shaped double quantum dot system

Real-time detection of single electron tunneling through a T-shaped double quantum dot is simulated, based on a Monte Carlo scheme. The double dot is embedded in a dissipative environment and the presence of electrons on the double dot is detected with a nearby quantum point contact. We demonstrate directly the bunching behavior in electron transport, which leads eventually to a super-Poissonian noise. Particularly, in the context of full counting statistics, we investigate the essential difference between the dephasing mechanisms induced by the quantum point contact detection and the coupling to the external phonon bath. A number of intriguing noise features associated with various transport mechanisms are revealed.

Erratum: Identification of hydrogen bond modes in polarized Raman spectra of single crystals of α‐oxalic acid dihydrate

Polarized Raman spectra of single crystals of the α-polymorphs of protonated and deuterated oxalic acid dihydrate were recorded. The interpretation of the spectra is assisted by periodic DFT calculations using the CRYSTAL06 program and by comparison with the infrared spectra of the polycrystalline material. The agreement between the calculated and observed band wavenumbers is fair in the case of low-anharmonicity modes, but marked differences appear for the stretching modes that are strongly anharmonic. A very broad feature, extending between ∼2000 and 1200 cm−1, is attributed to OH stretching. Notable is the topping of this feature by distinct bands that can be attributed to CO stretching, H2O scissoring and COH bending coupled to CO stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the wavenumber limits of the broad OH stretching band, which suggests that the potential governing the proton dynamics is of the asymmetric double-minimum type with a very low barrier. The calculated normal coordinates show a strong participation of the bending modes of water molecules in almost all internal acid motions, as well as in the external phonons. Copyright © 2009 John Wiley & Sons, Ltd.

Identification of hydrogen bond modes in polarized Raman spectra of single crystals of α‐oxalic acid dihydrate

AbstractPolarized Raman spectra of single crystals of the α‐polymorphs of protonated and deuterated oxalic acid dihydrate were recorded. The interpretation of the spectra is assisted by periodic DFT calculations using the CRYSTAL06 program and by comparison with the infrared spectra of the polycrystalline material. The agreement between the calculated and observed band wavenumbers is fair in the case of low‐anharmonicity modes, but marked differences appear for the stretching modes that are strongly anharmonic. A very broad feature, extending between ∼2000 and 1200 cm−1, is attributed to OH stretching. Notable is the topping of this feature by distinct bands that can be attributed to CO stretching, H2O scissoring and COH bending coupled to CO stretching. The assignments are supported by isotope effects. However, deuteration does not notably affect the wavenumber limits of the broad OH stretching band, which suggests that the potential governing the proton dynamics is of the asymmetric double‐minimum type with a very low barrier. The calculated normal coordinates show a strong participation of the bending modes of water molecules in almost all internal acid motions, as well as in the external phonons. Copyright © 2009 John Wiley & Sons, Ltd.

Interface formation of Mg with DiMePTCDI studied by Raman spectroscopy

Metal/organic interfaces play an important role in the performance of organic based devices. In this work the interface formation between Mg and N,N′-DiMethyl-3,4,9,10-Perylene Tetra Carboxylic DiImide (DiMePTCDI) films grown on sulphur-passivated GaAs (100) substrates is investigated in situ by Raman spectroscopy. The Raman spectra are taken in a backscattering geometry at room temperature under resonance conditions with the 488 nm (2.54 eV) Ar+ laser line. They reveal that when Mg is deposited onto a 15 nm DiMePTCDI layer the external phonon modes are preserved up to large metal coverages. Since these modes are a fingerprint of the molecular crystal, their preservation indicates a low diffusion of Mg into the DiMePTCDI layer. Concerning the internal molecular modes, the Mg deposition induces a breakdown of selection rules which is proposed to originate from a dynamical charge transfer between the DiMePTCDI molecules and the metal. Above 0.3 nm nominal thickness of Mg the line-shape of the molecular breathing mode at 221 cm–1 develops an asymmetric tail towards higher frequencies. A similar effect is observed for the bands that occur at 1291 cm–1 and 1606 cm–1 but the asymmetry appears at the low frequency side. This line-shape asymmetry is likely to be related to a Fano resonant coupling between the molecular vibrational modes and the electronic continuum of states of metallic clusters formed above 0.3 nm Mg coverage. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vibrational study and lattice dynamics of disordered NaBi(WO4)2

Polarized Raman and IR spectra were obtained for NaBi(WO4)2 crystal at ambient temperature. Assignment of the observed bands to the respective internal and external phonons was proposed on the basis of lattice dynamics calculations. The spectra showed also the appearance of additional Raman and IR bands which are not predicted from group theory for the scheelite I41/a structure. It is argued that these bands result from symmetry breakdown induced by cation disorder. Copyright © 2004 John Wiley & Sons, Ltd.

Lattice dynamics of Ca4GdO(BO3)3

AbstractPolarized Raman and IR spectra were obtained for Ca4GdO(BO3)3 crystal at ambient temperature. The assignment of the observed bands to the respective internal and external phonons is proposed on the basis of lattice dynamics calculations. This study also shows that significant LO–TO splitting exists for some internal modes. Copyright © 2004 John Wiley & Sons, Ltd.

Coherent phonon wavepackets in quasi-1D organic molecular crystals

Phonon modes which are strongly coupled to the lowest optical transition of the quasi-one-dimensional organic crystal of MePTCDI ( N- N′-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved as they appear as coherent wavepackets in femtosecond pump–probe experiments. We observe both coherent oscillations of intramolecular vibrational wavepackets (internal phonons) and, for the first time in a quasi-one-dimensional organic crystal, lower-energetic modulations which are related to coherent lattice phonons (external phonons). The phononic wavepacket motion is attributed to the electronic ground-state potential surface. The coherence decay of all observed vibrations is in the range of some picoseconds.

Coherent phonons in quasi-one-dimensional organic crystals

Coherent phonon oscillations in quasi-1D organic crystals of MePTCDI (N-N′-dimethylperylene-3,4,9,10-dicarboximide) are directly time-resolved by femtosecond pump–probe experiments. We observe both oscillations caused by intra-molecular vibrations (internal phonons) and, for the first time in a quasi-1D organic system, modulations which are related to coherent lattice phonons (external phonons). From the observed phonon coherence of a few picoseconds, we assign the observed phonon oscillations to the electronic ground state potential.

Coherent external and internal phonons in quasi-one-dimensional organic molecular crystals.

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N'-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.

Controlling spatially indirect exciton condensate in coupled quantum wells by external fields and phonon laser

Abstract A new method for controlling and optical observing spatially and momentum space indirect exciton condensate in coupled quantum wells is proposed. The method is based on exciton dispersion engineering by external fields. Using external fields one can, at any instant, provide resonant enhancement in PL and acoustic phonon emission by tuning ‘dark’ condensate into the resonance of recombination via direct exciton level. In addition, the development of a phonon laser is a possibility.

Pressure-Induced Phase Transition of One-Dimensional Semiconductor C5H10NH2PbI3

We studied Raman scattering under hydrostatic pressure for the one-dimensional semiconductor C5H10NH2PbI3 up to 2 GPa, and observed a structural phase transition at 0.4 GPa. Raman modes which originate from 6-ring and H–N–C vibrations of C5H10NH2+ molecules show abrupt changes in their peak positions and linewidths at 0.4 GPa. Narrowing of these modes at the transition pressure is explained by ordering of organic molecules. External phonon modes in the range of 20–200 cm—1 also change abruptly at about 0.4 GPa. The results indicate that this pressure-induced structural phase transition is governed by rotational and orientational ordering of C5H10NH2+ molecules.

Vibrational characteristics of the double oxygen bridge in the NaIn(WO4)2 and NaSc(WO4)2 tungstates with wolframite structure

The normal coordinate analysis of the tungsten–oxygen core in NaIn(WO4)2 and NaSc(WO4)2 crystals was performed. The Urey–Bradley force field and potential energy distribution (PED) were applied in the internal and external phonon calculations for the W4O22Na2In2 molecular system. The dynamics of the asymmetric WO2W oxygen bridge as well as terminal WO bonds were analyzed and discussed. The vibrational characteristics of the oxygen double bridge bond, i.e. vibration energy, symmetry, force constants, PED, atomic displacements, direction of the transition dipole moments and mean square amplitudes were obtained and discussed. The theoretical considerations were based on the polarized IR and Raman spectra of the materials studied.

Infrared and infrared-visible sum frequency generation spectroscopic response of harmonic monolayer vibrons: Homogeneous profile

We determine the profile of the vibrational band of an ordered monolayer adsorbed on a clean surface corresponding to the infrared spectrum and to the resonant infrared-visible sum frequency generation spectrum. The theoretical model is based on the renormalization of the monolayer and substrate Hamiltonians. The harmonic dynamics of the effective vibrons characterizing the collective internal vibrations of the admolecules is written in terms of a complex dynamical matrix whose elements can be expressed as correlation functions of the external libron and phonon modes. The dephasing broadening is obtained by solving a master equation for the time evolution of the vibron modes while the external dynamics of the layer is described by using molecular dynamics simulation. An application to the calculation of the profile of the vibrational band of the low temperature (2×1) CO monolayer adsorbed on NaCl(100) is performed without any adjustable parameter by considering a well established semiempirical potential to describe the molecule-surface and molecule-molecule interactions. The comparison with experimental spectra shows that the homogeneous phase relaxation due to the coupling between the vibrons and the monolayer phonons-librons accounts for the most part of the peak width and interprets the temperature dependence of this width. Extension of the calculations to the interpretation of the orientationally disordered phase obtained for CO at higher temperature is done which corroborates very well the temperature behavior of the monolayer structure and profile.

Structure and intermolecular phonons study of solid C 70

The low-temperature orientationally ordered crystalline phase of fullerene C 70 is investigated in the approximation of intermolecular Lennard-Jones (12-6) potential. A calculation of the vibrational spectrum for solid C 70 in the field of external phonon modes is carried out. The sound velocities were found in the longwave approximation on the perturbation theory. The obtained results are in satisfactory agreement with available experimental data on the X-ray diffraction, Raman spectra and elastic properties.