On the harmonic oscillator with nonzero periodic mass

Summary Following previous papers of Leach, and Wille and Vennik, we consider various new expressions for the nonvanishing periodic mass parameter of the time-dependent harmonic oscillator in a Pérot-Fabry cavity in contact with an atomic reservior. The authors of this paper have agreed to not receive the proofs for correction     

Developments of solid-state spectroscopy in Pavia: from first steps to newest achievements

Summary In this paper some significant aspects of the evolution of solid-state spectroscopy in Pavia are briefly reviewed in their basic lines starting from the '50s. As an example of historical trends and scientific developments, the authors describe and discuss two experiments on low-dimensional systems which, over a period of nearly three decades, represent a coherent approach to spectroscopic disciplines. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Optical properties of an interacting large polaron gas

The normal state conductivity, , of a system of interacting large polarons is calculated within the Random Phase approximation and some numerical results are presented. The behaviour of the optical absorption as a function of the charge carrier density and of the temperature is analyzed for different values of the electron-phonon coupling constant. It is shown that exhibits features similar to those observed in the infrared spectra of the cuprates. Received 27 January 1999     

A bound polaron in a spherical quantum dot

The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998     

Magnetic transition and polaron crossover in a two-site single polaron model including double exchange interaction

A two-site double exchange model with a single polaron is studied using a perturbation expansion based on the modified Lang-Firsov transformation. The antiferromagnetic to ferromagnetic transition and the crossover from small to large polaron are investigated for different values of the antiferromagnetic interaction (J) between the core spins and the hopping (t) of the itinerant electron. Effect of the external magnetic field on the small to large polaron crossover and on the polaronic kinetic energy are studied. When the magnetic transition and the small to large polaron crossover coincide for some suitable range of J/t, the magnetic field has very pronounced effect on the dynamics of polarons. Received 1 June 2000     

Ground state description of bound polarons in parabolic quantum wires

The Feynman-Haken variational path integral theory is, for the first time, generalized to calculate the ground-state energy of an electron coupled simultaneously to a Coulomb potential and to a longitudinal-optical (LO) phonon field in parabolic quantum wires. It is shown that the polaronic correction to the ground-state energy is more sensitive to the electron-phonon coupling constant than the Coulomb binding parameter and monotonically stronger as the effective wire radius decreases. We apply our calculations to several semiconductor quantum wires and find that the polaronic correction can be considerably large. Received 16 November 1998     

On the question of possible experimental observation of Anderson localization of the neutron

A possible experiment is discussed, for the observation of Anderson localization of the neutron. The localized state may be formed in the process of inelastic downscattering of thermal or cold neutrons in a highly disordered substance with low neutron capture and upscattering cross sections. The lifetime of trapped (localized) neutrons in the sample is measured by counting the upscattered neutrons with a neutron counter surrounding the sample. Estimations of experimental parameters relevant to such an experiment are given. Received: 12 May 1997 / Revised: 12 September 1997 / Accepted: 16 September 1997     

Energy loss of 18 keV electrons in gaseous T and quench condensed D films

Measurements of the energy loss of fast electrons at an energy of 18 keV have been performed on molecules of hydrogen isotopes, gaseous T ₂ and frozen D ₂ . Whereas in the case of gaseous T ₂ the values of total inelastic cross-section ( cm² for E = 18.6 keV), average energy loss ( eV) and peak position of the energy loss spectra ( eV) agree well with the expectations, the corresponding values for quench condensed D ₂ differ significantly from the ones for gaseous T ₂ . We observe a significant lower total inelastic cross-section ( cm², for E = 18.6 keV) larger average energy loss ( eV) and higher peak position ( eV). These differences may be interpreted in terms of changes of the final state spectrum. A CI calculation for a D ₂ cluster shows indeed a clear shift of the excited states in agreement with the observation. Received 24 August 1999     

High-resolution resonant photoemission study of the 4f states in a typical Kondo system: CePd3

We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations. Received: 13 October 1997 / Accepted: 21 January 1998     

Polaronic effects on the energy levels of a double donor impurity in quantum wells in the presence of a magnetic field

In the presence of a magnetic field the Hamiltonian of the single or double polaron bound to a helium-type donor impurity in semiconductor quantum wells (QWs) are given in the case of positively charged donor center and neutral donor center. The couplings of an electron and the impurity with various phonon modes are considered. The binding energy of the single and double bound polaron in Al_xlGa _1-xlAs/GaAs/Al_xrGa _1-xrAs QWs are calculated. The results show that for a thin well the cumulative effects of the electron-phonon coupling and the impurity-phonon coupling can contribute appreciably to the binding energy in the case of ionized donor. In the case of neutral donor the contribution of polaronic effects are not very important, however the magnetic field significantly modifies the binding energy of the double donor. The comparison between the binding energies in the case of the impurity placed at the quantum well center and at the quantum well edge is also given. Received 16 February 1999     

The electronic structure of from high energy spectroscopy

We report the results of a detailed study of the occupied and unoccupied electronic structure of dimers of the new heterofullerene by means of photoemission and electron energy-loss spectroscopy. A close similarity is found between the electronic structures of pristine and with an additional broadening of the spectra in the former due to the distortion of the fullerene cage caused both by dimerization and the chemical substitution. Both the occupied and unoccupied electronic states, as well as the interband transitions between them, attest to the high degree of molecular character retained in the solid state. Comparison of the shake-up structures in the and X-ray photoemission spectra confirm that the highest lying occupied states in the heterofullerene have a strong degree of N character, whereas the lowest lying unoccupied states have mainly C character. We also present the optical conductivity of the heterofullerene (derived from the loss function), which shows an optical gap of 1.4 eV, some 0.4 eV smaller than that of . Received: 25 August 1997 / Revised: 22 September 1997 / Accepted: 16 October 1997     

Study by Mössbauer spectroscopy of an iron meteorite

Summary An iron meteorite has been studied by backscattering M?ssbauer spectroscopy. The internal magnetic field is found to exceed significantly that of pure iron.     

Small polaron formation in many-particle states of the Hubbard-Holstein model: The one-dimensional case

We investigate polaron formation in a many-electron system in the presence of a local repulsion sufficiently strong to prevent local-bipolaron formation. Specifically, we consider a Hubbard-Holstein model of interacting electrons coupled to dispersionless phonons of frequency . Numerically solving the model in a small one-dimensional cluster, we find that in the nearly adiabatic case , the necessary and sufficient condition for the polaronic regime to occur is that the energy gain in the atomic (i.e., extremely localized) regime overcomes the energy of the purely electronic system . In the antiadiabatic case, , polaron formation is instead driven by the condition of a large ionic displacement (g being the electron-phonon coupling). Dynamical properties of the model in the weak and moderately strong coupling regimes are also analyzed. Received 15 February 1999     

Melting of a Wigner crystal in an ionic dielectric

The melting of a Wigner Crystal of electrons placed into a host polar material is examined as a function of the density and the temperature. When the coupling to the longitudinal optical modes of the host medium is turned on, the WC is progressively transformed into a polaronic Wigner crystal. We estimate the critical density for crystal melting at zero temperature using the Lindeman criterion. We show that above a certain critical value of the Fr?hlich electron-phonon coupling, the melting towards a quantum liquid of polarons is not possible, and the insulator-to-metal transition is driven by the ionization of the polarons (polaron dissociation). The phase diagram at finite temperature is obtained by making use of the same Lindeman criterion. Results are also provided in the case of an anisotropic electron band mass, showing that the scenario of polaron dissociation can be relevant in anisotropic compounds such as the superconducting cuprates at rather moderate e-ph couplings. Received 13 August 1999     

Charge-carrier density collapse in La0.67Ca0.33MnO3and La0.67Sr0.33MnO3 epitaxial thin films

We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse. Received 22 July 1999 and Received in final form 7 October 1999     

Band gap variation in laser irradiated polytypic crystals of cadmium iodide

Highly purified single crystals of cadmium iodide obtained through repeated zone refining have been subjected to laser beam exposure (Argon ion laser), both for various time durations and to various beam intensities, and then subjected to band gap determination by UV spectroscopy. The band gap has been found to decrease gradually with increase in the laser beam intensity, whereas its variation with increase in time of exposure shows an unusual behaviour such that it initially falls sharply followed by a gradual rise back to its original value. The results have been analysed and interpreted in terms of indirect band gap character of the material and an unusual variation in phonon frequency. The interpretation is well supported by X-ray diffraction and scanning electron microscopy (SEM) studies. Received: 26 May 1997 / Revised: 6 August 1997 / Accepted: 15 September 1997     

Effective electron Hamiltonian for a quasi one-dimensional system. The (TMTSF)2X and the (TMTTF)2X systems

In this paper we have introduced a variational approach to investigate the ground state of a model which includes both the Holstein electron-phonon interaction and the extended Hubbard electron-electron interaction. We have considered a variational state for the phonon subsystem which generalizes the previous used forms. This state allows to take into account the possibility of extended phonon mediated correlations. The effective electron Hamiltonian, which we have obtained, includes first and second neighbor electron-electron interaction terms. We have treated exactly, through a Lanczos method, this effective model in the one-dimensional case. We have applied our method to two Bechgaard salts and in these cases we have estimated the correlation parameters. We have shown that the introduction of electron-phonon interaction allows an estimate of the on site U and nearest-neighbor V Coulomb repulsion, which are in agreement with the experimental optical spectra of the above mentioned two compounds. Received: 30 October 1997 / Revised: 28 January 1998 / Accepted: 10 April 1998     

Interface polarons in a realistic heterojunction potential

The ground states of interface polarons in a realistic heterojunction potential are investigated by considering the bulk and the interface optical phonon influence. A self-consistent heterojunction potential is used and an LLP-like method is adopted to obtain the polaron effect. The numerical computation has been done for the Zn_1-xCd_xSe/ZnSe system to obtain the polaron ground state energy, self energy and effective mass parallel to the interface. A simplified coherent potential approximation is developed to obtain the parameters of the ternary mixed crystal and the energy band offset of the heterojunction. It is found that at small Cd concentration the bulk longitudinal optical phonons give the main contribution for lower areal electron densities, whereas the interface phonon contribution is dominant for higher areal electron densities. The interface polaron effect is weaker than the effect obtained by the three dimensional bulk phonon and by the two dimensional interface phonon models. Received 17 September 1998     

Canted antiferromagnetism and magnetoelastic coupling in metallic Ho0.1Ca0.9MnO3

Ho0.1Ca0.9MnO3 is a canted antiferromagnet with the magnetic space group Pn ' ma '. The magnetic structure is a superposition C x F y A z of the three types of order allowed in Pn ' ma '. In the Ca-rich corner of the system Ho _1-x Ca x MnO ₃ the title compound has a strong magnetoelastic distortion , the highest metallic conductivity and a ferromagnetic component F y close to the maximum in the series. Among the areas ab, bc, ca calculated from the lattice constants only ca shows a strong magnetoelastic effect below T N = 106 K. The x-, y-, z-spin components depend differently on the temperature. This gives rise to spin rotation which is particularly strong close to T N. MnO ₆ octahedra have short bond lengths with a temperature independent average . They are practically regular at room temperature and show a Jahn-Teller distortion of 3.5% in the magnetically ordered state. Above T N we find small polaron conductivity. The presence of the Jahn-Teller distortion due to the only small abundance (10%) of Mn ³⁺ in the t _2g ³ e g configuration is attributed to delocalised e g electrons. In the magnetically ordered state the averaged magnetic moment of Mn is reduced appreciably from the paramagnetic value due to spin disorder. Received 21 January 1999