半导体中电子-空穴对的相干态、复合辐射和噪声压缩

本文讨论了在光的相干激发下,半导体中电子-空穴对的玻色近似和SU(2)相干态的产生,讨论了两种情形下电子-空穴对噪声特性和复合辐射特性,给出了电子-空穴对的二阶相干函数和分布函数,说明了玻色近似相当于SU(2)群到谐振子群的收缩。 关键词：     

Classification of unconventional electron-hole condensates

Heavy Fermion metals with their very anisotropic quasiparticle states may support unconventional electron-hole (Peierls) pairing in addition to unconventional two electron (Cooper) pairs in the superconducting phase. For two different nesting Fermi surface models the possible types of electron hole condensates are classified according to the symmetries of their order parameters. This is performed within a continuum representation for the electronic states near the van Hove saddle point singularities. The quasiparticle bands and the unitary transformation to Bloch states in the condensed phase are derived for the two Fermi surface models with one and two independent nesting vectors respectively. Emphasis is put on the investigation of electron-hole condensed phases with 2Q-modulated structure. It is shown that in the continuum approximation the gap equations are all equivalent and the critical field curve is calculated in the rigid band model.     

Core excitons in the soft X-ray region: Solid argon

A theoretical calculation of core excitons is performed for the L_II,III soft X-ray threshold of solid argon at ～ 245 eV. The binding energies and the relative transition amplitudes for the lowest allowed exciton states are computed by formulating the problem in terms of Wannier functions and solving the resulting integral equation in the one-site approximation. The results obtained allow to locate the onset of interband transitions at an energy a few eV above previous theoretical determinations. Therefore, sharp structure previously interpreted in terms of conduction band density of states is attributed to discrete excitonic transitions, as strongly suggested by the close analogy with the atomic absorption spectrum. A comparison with the fundamental excitonic absorption in the vacuum ultra violet region is carried out in terms of the ratio of the electron-hole exchange interaction to the spin-orbit splitting of the hole states.     

Exciton States in a Gaussian Confining Potential Well

We consider the problem of an electron-hole pair in a Gaussian confining potential well. This problem is treated within the effective-mass approximation framework using the method of numerical matrix diagonalization. The energy levels of the low-lying states are calculated as a function of the electron-hole effective mass ratio and the size of the confining potential.     

Exciton States in a Gaussian Confining Potential Well

We consider the problem of an electron-hole pair in a Gaussian confining potential well. This problem is treated within the effective-mass approximation framework using the method of numerical matrix diagonalization. The energy levels of the low-lying states are calculated as a function of the electron-hole effective mass ratio and the size of the confining potential.     

Ultrarelativistic electron-hole pairing in graphene bilayer

We consider the ground state of an electron-hole graphene bilayer composed of two independently-doped graphene layers when a condensate of spatially separated electron-hole pairs is formed. In the weak coupling regime the pairing affects only the conduction band of the electron-doped layer and the valence band of the hole-doped layer, thus the ground state is similar to an ordinary BCS condensate. At strong coupling, an ultrarelativistic character of the electron dynamics reveals itself and the bands which are remote from Fermi surfaces (valence band of electron-doped layer and conduction band of hole-doped layer) are also affected by the pairing. Analysis of the instability of the unpaired state shows that s-wave pairing with band-diagonal condensate structure, described by two gaps, is preferable. The relative phase of the gaps is fixed, however at weak coupling this fixation diminishes allowing gapped and soliton-like excitations. The coupled self-consistent gap equations for these two gaps are solved at zero temperature in the constant-gap approximation and in the approximation of a separable potential. It is shown that, if the characteristic width of the pairing region is of the order of magnitude of the chemical potential, then the value of the gap in the spectrum is not much different from the BCS estimation. However if the pairing region is wider, then the gap value can be much larger and depends exponentially on its energy width.     

Electronic properties of silicon with ultrasmall germanium clusters

The electronic states of silicon with a periodic array of spherical germanium clusters are studied within the pseudopotential approach. The effects of quantum confinement in the energies and wave functions of the localized cluster states are analyzed. It is demonstrated that clusters up to 2.4 nm in size produce one localized s state whose energy monotonically shifts deep into the silicon band gap as the cluster size increases. The wave function of the cluster level corresponds to the single-valley approximation of the effective-mass method. In the approximation of an abruptly discontinuous potential at the heterointerface, the quantities calculated using the effective-mass method for clusters containing more than 200 Ge atoms are close to those obtained by the pseudopotential method. For smaller clusters, it is necessary to take into account the smooth potential at the interface.     

Internal-motion dependence of self-trapping of a Wannier exciton

The phase diagram of a Wannier exciton in the phonon fields is presented for 1s, 2s and 2p states of the internal (relative) motion on the basis of the adiabatic approximation. Differences in self-trapping among these states are revealed for an exciton with strong electron-hole Coulomb binding.     

Calculation of the dielectric function for highly excited polar semiconductors

The optical dielectric function of an electron-hole plasma in highly excited polar semiconductors with a direct band gap is calculated by solving a Bethe-Salpeter equation for the polarization function. The screening of the Coulomb potential is treated in a damped phonon-plasmon-pole approximation. We include the complex dynamical self-energy corrections as well as electron-hole correlations.Project of the Sonderforschungsbereich Frankfurt/Darmstadt, financed by special funds of the Deutsche Forschungsgemeinschaft     

New Energy Spectra Structure of Polaron in Trans-Polyacetylene

In the weakly coupled electron-phonon systems, the existing theory pointed out that the energy spectra of polaron include four electronic bound states. Our work shows that, due to the non-nearest neighbor hopping interactions, the electron-hole symmetry of the energy band structure implied by SSH model is broken, and the numbers of the bound electronic states are changed. For a negative charged polaron, one new bound state is found near the bottom of conduction band, and the original two bound states below the bottom of the valence band and above the top of the conduction band disappear. For a positive charged polaron, five bound states have been found: one of them is an additional bound state at the top of the conduction band, the others are just the states found in the SSH model. Besides, the energy gap 2Δ is slightly shifted by turning on the long-range hopping interactions.     

Theoretical characterization of carrier compensation in P-doped diamond

Based on the electron-hole recombination ratio (the number of electron-hole recombination in unit time and volume), we have examined several P complexes surrounded with vacancy (V) or H to explore the effect of carrier compensation on the electronic properties in P-doped diamond by first-principle calculations. Our calculated results show that the monovacancy complex P-V-H is not a valid recombination center in P-doped diamond, in which case electron cannot be recombined and thus donor cannot be compensated. However, the level in the band gap introduced by the divacancy complex P-2V-2H is a valid recombination center, which accelerates the electron-hole recombination at high ratio. For the trivacancy complex P-3V, three levels are introduced near the middle of the band gap, which may serve as more valid recombination centers than others. In this case, the electron-hole recombination ratio enhances successively, namely, the compensator density increases continuously too. In addition, the electronic properties of the P-related complexes in negative charge states are similar with those of neutral charge states. The study may explain well the experimental results and be useful for the further experiment research.     

An analytic expression for the exchange-correlation energy obtained with a simple plasmon-pole model

An analytic approximation for the shifted band gap and for the exchange-correlation energy of excited semiconductors at low temperatures is derived from a suitable plasmon-pole model. Both quantities behave tessentially as the one-fourth power of the density. Formulas for the ground-state energy and density of electron-hole liquids are also obtained.     

Ratio problem in single carbon nanotube fluorescence spectroscopy

The electronic band gaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show significant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron-hole interaction in the photoexcited states. The one-dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge. Analytic expression for the energies and line shapes of higher subband excitons are derived, and a comparison with experiment is used to extract the value of the screened electron-hole interaction.     

Electron and hole spectra of silicon quantum dots

In the framework of perturbation theory, the first several one-particle energies and wave functions for electrons and holes (six for each) in spherical silicon quantum dots are obtained in the envelope function approximation (kp method). It is shown that the model of an isotropic dispersion relation with the mean reciprocal effective mass is applicable for the ground state of holes in the valence band. Anisotropy of the dispersion relation, which takes place for bulk semiconductors, becomes significant for the electron ground state in the conduction band as well as for all excited (both electron and hole) states.     

Ground state of excitons in quantum-dot quantum-well nanoparticles： stochastic variational method

Within the framework of effective mass approximation, the ground state of excitons confined in spherical core-shell quantum-dot quantum-well (QDQW) nanoparticles is solved by using the stochastic variational method, in which the finite band offset and the heavy (light) hole exciton states are considered. The calculated 1s_e－1s_h transition energies for the chosen CdS/HgS/CdS QDQW samples are in good agreement with the experimental measurements. Moreover, some previous theoretical results are improved.     

Exciton states and interband optical transitions in ZnO/MgZnO quantum dots

Within the framework of the effective-mass approximation, the exciton states confined in wurtzite ZnO/MgZnO quantum dot (QD) are calculated using a variational procedure, including three-dimensional confinement of carriers in the QD and the strong built-in electric field effect due to the piezoelectricity and spontaneous polarizations. The exciton binding energy and the electron-hole recombination rate as functions of the height (or radius) of the QD are studied. Numerical results show that the strong built-in electric field leads to a remarkable electron-hole spatial separation, and this effect has a significant influence on the exciton states and optical properties of wurtzite ZnO/MgZnO QD.     

Validity of an algebraic-variational approach to the theory of collective motion for an exactly soluble shell model with R(5) symmetry

An algebraic-variational approach to the theory of collective motion previously applied in variant forms to pairing and monopole interaction models is here developed for an exactly soluble shell model Hamiltonian with R(5) symmetry. The spectrum of this class of Hamiltonian operators has previously been shown to represent a two-dimensional vibrator-rotator. The approximation scheme developed yields almost exact results up to the two-phonon level in the spherical region and goes over smoothly into a theory of the lowest states of the ground state rotational band in the deformed regime.     

Photoinduced phenomena in correlated electron systems with multi-degrees of freedom

Photo-induced electronic dynamics in strongly correlated electron system with spin-charge coupling are studied. Motivated from recent optical pump-probe experiments in perovskite manganites and cobaltites, the double-exchange system and the spin-state transition system are theoretically analyzed. First, we focus on the double-exchange interaction in photo-excited state. The time evolutions for the spin and charge structures are examined as functions of the pump photon amplitudes. In the weak-pumping case, the initial charge-ordered antiferromagnetic insulating state tends to become the ferromagnetic metallic state. On the other hand, in the strong-pumping case, the initial antiferromagnetic correlation and insulating character are enhanced after photoirradiation. This unexpected photo-excited state is detectable by the ultrafast optical measurement. Second, in the spin-state transition system, the two-band Hubbard model is analyzed by the time-dependent mean-field approximation. By introducing photons in the low-spin band insulator, high-spin states are generated through the electron-hole pair annihilation process which is governed by the electron band width. We also take into account the relativistic spin-orbit interaction, and compare the results with and without the spin-orbit interaction.     

Effects of the second and third neighbor hopping interactions on the electronic states of soliton in polyacetylene

We have studied the energy spectra and the electronic states of a soliton in the weakly coupled electron-phonon systems using an extension of SSH model that includes non-nearest neighbor hopping interactions. The results show that: (1) the electron-hole symmetry of the energy band structure implied by SSH model is broken, and the energy gap 2 increases. (2) for a negative charged soliton, only two bound states have been found, one of them is the midgap state, another is a new shallow state near the bottom of the conduction band; for a neutral soliton, all three bound states exist as in the SSH model, but their localizations are strengthened; for a positive charged soliton, four bound states have been found, one of which is an additional state near the top of the conduction band.     

Magnetic hyperfine field for zinc in nickel

Electron mobility in the narrow band solid with a cellular disorder is investigated by methods of the coherent potential theory. Numerical results for the tight-binding model with the Lorentzian distribution of site potentials are presented. The motion of the electron in the region of extended states is neither a well defined coherent band motion nor a simple Brownian diffusion. The statistical electron-hole correlation proves to be important and increases fast with the potential distribution width to band width ratio. The correlation also increases when approaching the mobility edges.