Strong Phonon Coupling Research Articles

Phonon Interaction in the Luminescence of Porous Silicon

It is found that the band width of a Gaussian-like luminescence spectrum of porous Si depends strongly on chemical etching time after anodization of crystalline Si. The narrowest luminescence band observed could be explained in terms of the radiative recombination of localized excitons with strong phonon coupling, while the broader luminescence band arises from the inhomogeneous broadening due to size distribution of Si nanostructures in the porous Si.

Evidence of resonant intersubband optical phonon scattering in quantum wires

Under certain biasing conditions, the intersubband energy separation in a quantum wire can match the optical phonon energy, thereby enhancing the scattering of charge carriers. We provide direct theoretical evidence that an anomalous negative differential transconductance (NDT) recently observed in an array of quantum wires at high temperatures is caused by resonant intersubband optical phonon scattering. Self-consistent 2D solutions of the Schrodinger and Poisson equations with a subsequent Monte Carlo analysis of intersubband scattering reveal a strong resonant polar optic phonon coupling between subbands at the gate bias indicated by the experimental data. The peak-to-valley ratio of the NDT exhibits a maximum near 170 K, thereby confirming the predictions of several theoretical investigations. The observation of resonant scattering is only possible because the quasi-1D form of Gauss's law shows an opposite trend to that for 2D systems in the dependence of the eigenenergy spectrum on gate bias.

Strong coupling of Rayleigh phonons to charge density waves in 1T-TaS 2

The dispersion curves of charge density wave (CDW)-induced phonons have been measured in 1T-TaS 2 by inelastic He atom scattering along both crystallographic and CDW symmetry directions. Evidence is given for a strong coupling of the CDW to Rayleigh waves, i.e. to quasi-transverse acoustic modes. A pseudocharge model analysis of lattice dynamics accounts for the CDW-induced splitting of the Raman-active modes originated from the folding of transverse acoustic modes.

Inelastic-neutron-scattering study of the cubic-to-tetragonal transition in K0.965Li0.035TaO3.

For over ten years the existence of a ferroelectric phase transition in ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_{3}$ has been the object of a strong controversy despite early Raman results which revealed a splitting of the soft ${\mathrm{TO}}_{1}$ mode at low temperature. We report here the results of an inelastic-neutron-scattering study of a 3.5 at. % Li crystal which show, for q\ensuremath{\ne}0, a corresponding splitting of the TA branch between a-polarized and c-polarized phonons and no condensation of the TA mode. The phonon assignment is based on measurements made in the presence of a bias electric field. These results confirm the existence of a cubic-to-tetragonal transition in ${\mathrm{K}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Li}}_{\mathit{x}}$${\mathrm{TaO}}_{3}$ even for relatively low concentrations. They reveal the order-disorder nature of this transition and the strong coupling of c-polarized TA phonons to the spontaneous polarization. A strong anisotropy of the dispersion surface is also shown to explain the asymmetric shape of the phonon peaks.

Intersubband optic phonon resonances in electrostatically confined quantum wires

We investigate the occurrence of resonant intersubband optic phonon scattering in an array of quantum wires at high temperatures. Self-consistent solutions of the Schrödinger and Poisson equations with a subsequent Monte Carlo analysis of intersubband scattering reveal a strong resonant polar optic phonon coupling between subbands at the gate bias indicated by the experimental data. The observation of resonant scattering is only possible because the quasi-1D form of Gauss’ law shows an opposite trend to that for 2D systems in the dependence of the eigenenergy spectrum on gate bias.

Raman study of the anomalous behavior of the 340-cm-1 CuO2 planar-oxygen phonon in Ni-doped YBa2Cu3O7- delta superconducting films.

The unusual softening of the ${\mathit{B}}_{1\mathit{g}}$-like phonon of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ is studied as a function of ${\mathit{T}}_{\mathit{c}}$ by substituting up to 6% nickel for Cu. The abrupt onset and small temperature range over which the softening occurs in undoped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are modified upon doping with the softening occurring well above ${\mathit{T}}_{\mathit{c}}$ and continuing smoothly to 10 K when ${\mathit{T}}_{\mathit{c}}$ is reduced to 71 K. The phonon linewidth in the doped films shows no anomalies, regardless of Ni concentration. In contrast, the self-energies of the ${\mathit{A}}_{\mathit{g}}$ modes associated with the plane copper [Cu(2)] and bridging oxygen (${\mathrm{O}}_{4}$) atoms reveal normal thermal behavior for all films. We conclude that an additional mechanism, besides strong coupling of phonons to superconducting electrons, contributes to the ${\mathit{B}}_{1\mathit{g}}$-phonon anomalies.

Vacuum emission of hot electrons from insulating and semiconducting films

Monte Carlo calculations and vacuum emission experiments have been shown to be powerful tools to investigate the high field electronic transport in metal–insulator–semiconductor and metal–semiconductor–insulator–semiconductor layer devices. Direct proof of hot-electron acceleration in semiconducting ZnS:Mn electroluminescent layers by means of vacuum emission and energy analysis in a spherical grid analyzer has been given as it was earlier for insulating a layer of SiO2. The energy distributions show strong peaking in the low-energy region around 3 eV as well as a high-energy tail exceeding even 20 eV. The enhanced emission of low-energy electrons with increasing field F indicates impact valence band excitation and direct avalanching in thin insulating and semiconducting films of sufficient thickness, than it has been predicted by strong acoustic phonon coupling and intervalley scattering.

MULTIPHONON TRANSITION AND TEMPERATURE DEPEN-DENCE OF PHOTOLUMINESCENCE SPECTRA OF DYE J-AGGREGATE LANGMIUR-BLODGETT FILMS

In temperature range of 20K to 300K, we measuded photoluminescence spectra of heterogeneous J-aggregate Langmiur-Blodgett (LB) films made of three kinds of cyanine dye. With temperature reduced, while the intensity of J-peak, the zero phonon line increased, anomalous enhancement of the phonon sideband was observed. Analysis shows that J-aggregatc LB films are of characteristics like that of low dimensional crystal, they have strong phonon coupling, and nonradiative transition plays important role.

Narrow-Band Synchrotron Radiation Excitation of Soft X-Ray Emission Spectra

ABSTRACTMonochromatized synchrotron radiation has been employed as the excitation source for soft x-ray emission spectroscopy. In the present paper changes in the emission spectra that occur as the excitation energy is varied near the core-absorption threshold are discussed. In the case of crystalline silicon, strong variations are seen in the L2,3 emission for excitation energies up to 30 eV above threshold. These variations are shown to be dependent on the crystalline order of the material and can be interpreted in terms of restrictions on the crystal momentum that arise in an inelastic scattering description of the combined absorption and emission. On the other hand this description is less relevant to the excitation-energy dependence of ionic insulators, in which strong phonon coupling removes these restrictions on crystal momentum. In the insulators B2O3 and BN strong variations in the emission are observed at threshold, upon creation of a core exciton: the exciton affects the emission through its influence on the phonon coupling as well as on the initial and final-state screening.

Growth, optical, and optoelectronic properties of CdZnTe/ZnTe multiple quantum wells

Abstract The use of molecular beam epitaxy has allowed growth of CdZnTe/ZnTe quantum wells with optical properties that match those of well-established infrared III-V quantum well systems. The sharp room temperature exciton absorption peaks that these II to VI quantum wells exhibit have been used in image processing devices and electroabsorption modulators operating at visible wavelengths. The large exciton-binding energy, very large saturation intensity, and strong exciton-LO phonon coupling exert a major influence on both the device characteristics and the basic physical properties of these films. The progress in these quantum wells has been achieved despite complications in the growth that result from the large lattice mismatches and the proximity, for many CdZnTe compositions, to a miscibility gap.

Strong phonon coupling and the relaxation rate spectrum in glasses

The influence of strong phonon coupling on the relaxation rate of two-level systems (TLS) in glasses is investigated. A unitary transformation is employed to express the Hamiltonian in terms of dressed TLS operators corresponding to weakly interacting units. The relaxation rate of these TLS is calculated; for asymmetric TLS, it turns out to be strongly reduced in comparison with the conventional result. Nevertheless, the shape of the distribution of relaxation rates is hardly changed, an effect that renders the previous success of weak-coupling theories understandable. In the light of these new results it is argued that most of the experimentally visible TLS rate spectrum may be dynamically generated by strong phonon coupling.

A suggestion concerning the origin of the broad distribution of tunneling system relaxation rates in glasses

While it has been known for some time that the coupling between low-temperature tunneling degrees of freedom and phonons in glasses isstrong, this fact is rarely appreciated in theoretical descriptions based on the tunneling model. It is surprising, in view of this situation, how well these theories work. In this article, it is shown that the presence of strong phonon coupling may lead to a broad distribution of relaxation rates of the tunneling systems which is rather insensitive to the detailed shape ofP(Δ, λ), the distribution of asymmetry and overlap parameters Δ and λ. The (energy-)rate distribution $$\hat P$$ (E,r) turns out to be very similar to the one obtained in weak-coupling theory. However, whereas in the latter theoretical framework this is due to the flatness ofP(Δ, λ), it is proposed here that the experimentally visible part of $$\hat P$$ (E,r) isgenerated by the strong phonon coupling, i.e., it is to be considered adynamic effect. Appropriately normalishing the relaxation rate distribution, one can recover, within strong-coupling theory, many results of weak-coupling theory, whose undeniable success is thus rendered understandable.

Charge fluctuations in high- tc superconducting cuprates: A reformulation of the “symmetry dilemma” of the mean-field approximation

We investigate the strength and symmetry-dependence of the mean-square deviation of the electronic charge in a Cu 12O 17 10− cluster which has been used as a model for the CuO 2 plane of the recently discovered high- T c superconducting copper oxides. The charge fluctuations are derived in the mean-field (|φ SCF>) and the correlated ground state (|Ψ o>), respectively. The interatomic correlations in the cuprate cluster are described by modifying |φ SCF> obtained in a semi-empirical SCF formalism via the “local approach”. A comparison of the mean-field data and the correlated ones leads to a physically transparent alternative formulation of the “symmetry dilemma” of the restricted Hartree-Fock approximation. The mean-field errors in the simulation of the charge fluctuations are largest in the limit of highly symmetric wavefunctions. Electronic correlations lead here to a sizeable charge localization with the largest effect in the Cu 3d x2−y2 orbitals. Our results show that the high- T c superconducting oxides belong to the class of strongly correlated systems with fluctuating valences. They are furthermore characterized by a strong coupling of local phonons (breathing mode) to the redistribution of the hole density and to changes in electron correlations.

A TWO-BAND SUPERCONDUCTOR WITH A NARROW BAND NEAR THE FERMI LEVEL

The modern theory of conventional superconductivity is based upon the concept of quantum states with broken gauge symmetry providing non-zero values for the so-called "anomalous averages" [Formula: see text], [Formula: see text]. Being applied to the simplest Gorikov-like semiphenomenological electronic scenario for a metal superconductor, this idea (in the mean field approximation) leads towards a complete description of all properties of ordinary superconductors as a manifestation of macroscopic quantum phase coherence. Currently there exists already strong experimental evidence that the newly discovered high Tc superconductors exhibit the same quantum coherence effects including the highly characteristic quantity for these phenomena, namely the doubled electronic charge. On these grounds one can expect that the high Tc superconductors should also be described by "anomalous averages" of the same kind. The only question is whether Gorikov's scenario is still sufficient or we need a new scenario instead in order to grasp the whole situation. In the case of Gorikov's scenario one needs to explain how the strong electron-electron coupling (no matter of what origin) can be consistent with the system stability condition. Yet there is another argument in favour of a new scenario. The large variety of the new SC properties cannot be explained by a model with too limited number of physical parameters. In our dealing with the problem of a possible new scenario we have suggested that in order to understand the high T c superconductors one should not perhaps go so far as to doubt the one-particle approximation and to introduce the Hubbard-like models, RVB and so on. Taking into account the crystalline structure of perfect high T c superconductors we have assumed within the one-particle description the two overlapping bands approach which was proposed by Suhl, Mattias and Walker (USA) and Moskalenko (USSR) as early as in 1959. To obtain qualitatively new results we have suggested a sharp asymmetry between the two bands (one being practically a local energy level close to the Fermi level) and also a special structure of direct electron-electron interaction with dominating interband scattering of singlet electron pairs described by an amplitude g. As the final analysis has shown, an inevitable but comparatively small splitting of the two bands does not invalidate the results of the model. As to the peculiar structure of interaction we believe it might be explained through a strong electron-optical phonon coupling due to the Jahn-Teller-like instability manifested in tetragonal-orthorhombic transitions of HT c samples. The model just outlined can indeed yield high T c . In the mean field approximation (with two superconductivity order parameters) the order of magnitude of T c is given by T c ~ (|g|N(o)/2)2 EF where N(o) is the density of states in the wide band at the Fermi level and EF is a wide band width. There is an exponentially sharp decline of T c while overdoping or underdoping the system and there is also a broad plateau when the narrow band is filled partially (the heavy fermion situation). The other results of calculations including thermodynamics and also kinetic properties for the normal state do not contradict experiments except for the linear temperature dependence of the specific heat observed sometimes. But one must take into account that in reality all high T c superconductors are in fact disordered solutions, and it is necessary to consider the dirty alloy limit with Anderson's theorem being violated because of the narrow band existence. At the same time, as analysis shows, the mean field approximation is not good enough for the model (the small parameter is only ( log 2/|g|N(o))-1) and therefore one has to apply Green's functions technique in order to correct the results. There is also a possibility that the magnetic properties connected with localized spins (if there are any) must be taken into account as well.

Electronic properties of a (Cu-Li)-related neutral complex defect with a bound exciton at 2.25 eV in GaP.

GaP diffused with Cu and Li contains a large number of neutral (``isoelectronic'') complexes, of which this paper is focused on one with a lowest electronic bound exciton (BE) line at \ensuremath{\approxeq}2.248 eV at 2 K. This lowest BE line is a spin triplet (S=1), which has a quite strong phonon coupling, so that the electronic line is not clearly observed. The true spectral dependence of the triplet photoluminescence (PL) emission could only be obtained via optical detection of magnetic resonance (ODMR). A singlet (S=0) transition with an electronic line at 2.2508 eV is observed in excitation spectra of ODMR or of PL. The angular dependence of ODMR data gives g values for the BE as ${g}_{x}$=${g}_{y}$=${g}_{z}$=2.02\ifmmode\pm\else\textpm\fi{}0.01, i.e., quite isotropic, and D values ${D}_{x}$=(-1.03\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ eV, ${D}_{y}$=(-0.23\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ eV, and ${D}_{z}$=(1.26\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ eV, where the axes x, y, and z diagonalizing the D tensor are defined as follows: x is 15\ifmmode^\circ\else\textdegree\fi{} off [1\ifmmode\bar\else\textasciimacron\fi{} 1\ifmmode\bar\else\textasciimacron\fi{} 2] towards [001], y\ensuremath{\parallel}[11\ifmmode\bar\else\textasciimacron\fi{}0], and z is 15\ifmmode^\circ\else\textdegree\fi{} off [111] towards [110] in the (11\ifmmode\bar\else\textasciimacron\fi{}0) plane. The small values of the D components are due to unusually weak contributions from spin-orbit coupling and electron-hole magnetic dipole-dipole interaction. The symmetry of the defect is ${C}_{1h}$, and it is suggested to be a bent configuration of three atoms (Cu and Li).

Optical properties of complex defects created by Ag diffusion in ZnTe.

Several complex defects in ZnTe created by Ag diffusion at a rather high doping level are studied by optical spectroscopy. In addition to the usual substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ acceptor bound exciton (BE) at 2.3737 eV, a new prominent BE, ${S}_{1}$, with a lowest energy of 2.3149 eV appears. This transition has a strong phonon coupling and corresponds to a neutral isoelectronic defect. Two electronic states at zero field are revealed by transmission data; a doublet at the lowest state (2.3149 eV) and a singlet at slightly higher energy (2.3155 eV). The electronic properties of the ${S}_{1}$ BE are revealed by optical data, including the magneto-optical Zeeman spectra. The electronic structure can be understood as a consequence of a strong compressional axial local crystal field in combination with an electron-hole exchange interaction. It is further concluded that both carriers are bound to the complex defect by an attractive central-cell potential. The identity of the ${S}_{1}$ defect as a pair of substitutional ${\mathrm{Ag}}_{\mathrm{Zn}}$ and interstitial ${\mathrm{Ag}}_{\mathrm{i}}$ in the 〈111〉 direction is consistent with the trigonal symmetry observed in magneto-optical data. In addition to the ${S}_{1}$ defect, several acceptorlike complex defects are created, of which ${S}_{2}$ is a BE with its lowest electronic line at 2.3486 eV, and ${S}_{3}$ similarly at 2.365 eV. These BE excitations give rise to several electronic levels both in the ground state (the acceptor hole) and in the excited state (the BE state). The complicated electronic structure can be explained by a combination of a low-symmetry crystal field and an exchange interaction. An identification of the defects ${S}_{2}$ and ${S}_{3}$ as composed of three atoms is suggested.

Picosecond and nanosecond kinetic spectroscopic investigations of the relaxation and the solute—solvent reaction of electronically excited 3,5-dinitroanisole

We investigated the relaxation of electronically excited nitro aromatic compounds, in particular 3,5-dinitroanisole (3,5-DINA), by means of picosecond and nanosecond laser kinetic spectroscopy. Complete neglect of differential overlap/spectroscopic—configuration interaction calculations were performed to characterize their excited states. The gas phase UV spectrum of 3,5-DINA and the electron spin resonance spectrum of the anion serve the same purpose. It is shown by means of IR spectroscopy that 3,5-DINA in its ground state does not form hydrogen bonds. The envelope of the first absorption band of 3,5-DINA in liquid solutions covers a weak nπ* and a strong ππ* transition. In non-hydrogen-bonding solvents, excitation in this band populates a primary excited state S i which decays within 10 ps to S 0 and to T 0. The state T 0 (in CH 3CN) is converted within 780 ps to S 0. In hydrogen bonding solvents the decay is quite different. The conversion T 0 → S 0 and probably S i → S 0 is much slower, while the intersystem crossing S i → T 0 remains very fast. We conclude that in aprotic solvents both S i and T 0 are nπ* states in which the excitation is localized on the NO 2 groups, which are consequently distorted to a large extent relative to the ground state. The distortion is thought to cause strong phonon coupling in the radiationless transitions S i → S 0 and T 0 → S 0. Local vibrations in the NO 2 group, which modulate the overlap of lone pair orbitals on adjacent oxygen atoms, are efficient promoting modes in the radiationless decay and they make the rate constant much larger than in other cases involving local excitation of small chromophores, e.g. ketones. The primary state populated by excitation with our pulsed lasers is a ππ * chare transfer state. When 3,5-DINA is brought into this state in hydrogen bonding solvents, it forms a (single) hydrogen bond before any other types of relaxation. Thus S i and T 0 are both hydrogen-bonded delocalized ππ * states. The known efficient bimolecular photo-induced reactions of 3,5-DINA involve the formation of the hydrogen bond as the first step. A reinterpretation is given of the variation in the triplet lifetime of 3,5-DINA over three orders of magnitude with solvent composition. The thermal dissociation of the hydrogen bond to the NO 2 group is the rate-determining step in the decay of 3,5-DINA in hydrogen bonding solvents. Both the activation energy and the activation entropy of this reaction depend on the solvent structure.

Theory of order-disorder transitions for inhomogeneous mixed-valence systems

A simple alloy-analogy model for inhomogeneous semiconducting mixed-valence compounds is extended to include the coupling of the long-range charge-order order parameter to the lattice. For weak coupling to the lattice the order-disorder transition is of second order, while it is of first order in the case of strong phonon coupling. We calculate the entropy, the conductivity, and the thermal dilation close to the transition and discuss the results in view of recent experiments for ${\mathrm{Eu}}_{3}$${\mathrm{S}}_{4}$.

Strong electron-phonon interaction effects in modulated transient reflectance spectra of Ga.50In.50P

Time-resolved modulated reflection spectrum near the fundamental edge of MBE-grown direct-gap GaxIn1−xP (x = 0.5) has been studied by using sensitive picosecond excite-probe methods. The spectrum at 77K shows considerable structure in comparison with steady-state photoluminescence data. In particular, a strong optical phonon coupling effect has been observed in the transient spectra, suggestive of impurity intermediated bound polaron behavior.

Strong electron-phonon interaction effects in modulated transient reflectance spectra of Ga .50In .50P

Time-resolved modulated reflection spectrum below the fundamental edge of MBE-grown direct-gap Ga xIn 1-xP (x = 0.5) has been studied by using sensitive picosecond excite-probe methods. The spectrum at 77K shows considerable structure in comparison with previous steady-state photoluminescence data. In particular, a strong optical phonon coupling effect has been observed in the excitation spectrum, suggestive of an acceptor bound polaron.