Electron-hole Lattice Research Articles

Elementary Excitation and Local Polarization in Layered High-Tc Cuprates

We explain the superconducting isotope effect in layered cuprates based on a 2D electron-hole lattice model, where electrons and holes are localized to stabilize local polarization mainly caused by the ion displacement. Upon doping carriers, high-T c superconductivity is assumed to be realized through the interaction with the elementary excitation of the 2D electron-hole lattice oscillation.

Elementary Excitation and Local Polarization in Layered High-Tc Cuprates

We explain the superconducting isotope effect in layered cuprates based on a 2D electron-hole lattice model, where electrons and holes are localized to stabilize local polarization mainly caused by the ion displacement. Upon doping carriers, high- T c superconductivity is assumed to be realized through the interaction with the elementary excitation of the 2D electronhole lattice oscillation.

Structural modulation in perovskite-like cuprates

The structural modulation of perovskite-like layered cuprates can be explained if a two-dimensional (2D) electron-hole lattice can be realized on the layer boundary where the electrons and holes are transferred on the boundary plane to stabilize local polarization mainly caused by the ion displacement in the c-axis direction. If the charge-transfer energy is small enough compared with 1 eV, it is found that the total energy of the 2D square lattice of a doubly occupied state is lower than that of a singly occupied one.

Microdomain caused by 2D electron system and modulated structure in incommensurate phase

The domain structure is discussed taking account of the neutralization charges on the crystal surface. The incommensurate state is discussed caused by the microdomain neutralized by single electron as a limit of the number of the neutralization charges that makes the 2D electron (or hole) lattice. We investigate also vibration modes of the electron-hole lattice in the 2D system.

2D electron-ion system on crystal surface in polarization reversal, incommensurate state and surface phonon

The influence of the 2D electron-ion system on the crystal surface, regarding that it neutralizes the polarization of the crystal which promotes polarization reversal, is discussed. In some region of polarization the 2D electron (or ion) system constructs the electron (or ion) lattice system, which modulates the structure of the polarization background. The origin and the period of the incommensurate (IC) state can be explained by the relation D 2 P s = e ( D is the electron lattice constant and P s is the spontaneous polarization). Various vibration modes of the electron-hole lattice is discussed. It is considered that the electron-phonon coupling in the 2D electron-hole lattice is important in microwave dispersion and light scattering. The role of the layered 2D electron-hole system in a high- T c superconductor is also speculated.

Stability of the 2D electron-hole lattice in the modulated structure of Bi based high- Tc superconductors

To explain the modulated structure of the displacive type in Bi based high- T c superconductors, we previously proposed a 2D electron-hole lattice model. It is assumed that the 2D electron-hole lattice is realized to neutralize local polarization charges mainly caused by the displacement of Bi and O ions on the BiO plane along the c-axis, and that the modulation period in the b-axis corresponds to twice the b lattice constant of the 2D electron-hole lattice. By examining the electronic state of the 2D electron-hole lattice on the BiO plane, we find that the modulation period at room temperature is approximately given by 25–50 Å, which well satisfies the typical experimental value.

Relationship between Tc and the modulated structure in Bi-based compounds based on a 2D electron-hole lattice model

We explain the relationship between the critical temperature T c and the modulated structure in the Bi-based compounds on the basis of a 2D electron-hole lattice model. The 2D electron-hole lattice is constructed on the BiO (and possibly on the CuO) planes to neutralize local polarization charges mainly caused by the displacement of the Bi (Cu) ions along the c-axis. Assuming that the superconductivity is realized by the exchange of the “phonons” of the 2D electron-hole lattice oscillation, we find that the optimum T c follows the relation of T c ( opt) ∝ D −2, where D, one side of the 2D electron-hole lattice, is related to the local polarization P loc through the neutralizing condition D 2 P loc = e ( e:elementary charge). We examine the validity of this relation for various Bi-based compounds.

Modulated structure and superconductivity in 2D electron-hole lattice system

The close relationship between modulated structures and superconductivity in high-Tc oxide superconductors is explained by using a two-dimensional (2D) electron-hole lattice. The 2D electron-hole lattice is constructed on the crystal surface or between adjacent layers by neutralizing polarization charges caused by a local polarization along thec-axis. The local polarization in the oxide superconductors is often accompanied by a modulated structure, with the modulation period corresponding to the length of one side of the 2D electron-hole lattice,D. Assuming that the Cooper pairs can be formed through the exchange of the “phonons” of the 2D electron-hole lattice oscillation, we show that both the optimum carrier density and the peak value of the transition temperature are approximately proportional toD−3/2 andD−2, respectively, and that for a typical value ofD ∼ 15 A, they are estimated to be on the order of 1014/cm2 and 100 K, respectively.

Role of 2D Electron-Hole Lattice System in Modulated Structure of High-Tc Superconductor Bi2-xPbxSr2CaCu2O8+δ

The role of a 2D electron-hole lattice system in the modulated structure of Bi2-xPbxSr2CaCu2O8+δ (Pb-doped Bi-2212) is discussed. The 2D electron-hole lattice system is constructed by neutralizing local polarization charges on the crystal surface or between two adjacent layers. Assuming that the Pb type of modulation is obtained by reversing two neighboring polarizations in the Bi type of modulation in pairs, we explain the following experimental findings: (1) the ratio of the modulation period of the Pb type structure, k2, to that of the Bi type structure, k1, gives discrete values, such as k2/k1=3/2, 5/3, and 2; (2) as x increases, k2 tends to increase and approximately follows the relationship k2-k1\\proptx; (3) when the sample is irradiated with an intense electron beam, the modulation of the Pb type transforms into that of the Bi type. The physical meaning of the above assumption is discussed.

Role of Modulated Structure in High-Tc Superconductors

A modulated structure in many high-Tc superconductors is discussed by using a localized 2D electron-hole lattice system. Assuming that the formation of Cooper pairs mediated by the 2D electron-hole lattice oscillation is possible, the transition temperature and the carrier surface density are estimated to be on the order of 100 K and 1014/cm2, respectively.

Long-Wavelength Shift in the Operation of Lightly Doped Semiconductor Lasers

The observation of a long-wavelength shift in the peak of the photoemission spectra of lightly doped InP and vapor-expitaxial GaAs is reported. It is shown that this long-wavelength shift of the band-edge emission is due to electron-hole-lattice (EHL) interactions. The samples of this work are volume photoexcited in a low-loss high-Q compound optical cavity which permits the observation of recombination radiation at energies E ≥ Eg, as well as at energies E ≤ Eg.

Threshold Requirements and Carrier Interaction Effects in GaAs Platelet Lasers (77°K)

The experimentally observed thresholds for laser operation of optically pumped GaAs platelets are presented. The measurements were performed on uniformly doped samples of p-type, n-type, and compensated crystal (doping range 1014/cm3≤n, p≤1020/cm3). The observed thresholds are considerably lower than previously measured or predicted and exhibit no essential dependence upon impurity concentration. The 16-meV shift of laser photon energy from bandgap in lightly doped GaAs is ascribed, as before, to electron-hole-lattice (EHL) interactions. Based upon the results of platelet laser threshold data and photon energy data, a comparison is made of the relative strength of the various EHL interaction mechanisms. If Coulomb interaction and dielectric screening can be ignored, over a certain doping range a remarkable fit of the photon energy data is afforded by the electron-electron interaction mechanism.

THE LASER TRANSITION AND PHOTON ENERGY OF GaAs IN THE LIGHTLY-DOPED LIMIT

Data are presented showing that lightly doped or relatively pure GaAs lases basically by a band-to-band recombination transition at a photon energy at threshold of 1.495 eV ≈ Eg − 0.015 eV (∼77°K). The ∼0.015-eV energy reduction from bandgap, which is greater than the exciton binding energy of 0.0034 eV (∼77°K) and unlike the exciton energy is dependent upon carrier concentration, occurs because of many-body, electron-hole-lattice (EHL) interactions.