Electronic structure and possible pseudogap behavior in iron based superconductors

Starting from the simplified analytic model of electronic spectrum of iron-pnictogen (chalcogen) high-temperature superconductors close to the Fermi level, we discuss the influence of antiferromagneting (AFM) scattering both for stoichiometric case and the region of possible short-range order AFM fluctuations in doped compounds. Qualitative picture of the evolution of electronic spectrum and Fermi surfaces (FS) for different dopings is presented, with the aim of comparison with existing and future ARPES experiments. Both electron and hole dopings are considered and possible pseudogap behavior connected with partial FS “destruction” is demonstrated, explaining some recent experiments.     

Electronic structure and nonlinear optical properties of a two-dimensional hexagonal quantum dot

In this work electronic structure, the linear and the third-order nonlinear refractive index changes as well as optical absorption coefficients of a two-dimensional hexagonal quantum dot are investigated. Energy eigenvalues and eigenfunctions of the system are calculated by the matrix diagonalization technique, and optical properties are also obtained using the compact density matrix approach. As our results indicate, both the dot size and the confinement potential have a great influence on the intersubband energy intervals, the transition probability and consequently, the linear and the third-order nonlinear refractive index changes and optical absorption coefficients.     

Electronic structure for sliding charge density waves

We present a model for the electron system in NbSe₃ based on its quasi one-dimensional metallic properties. In a one-dimensional metal phonon drag of 2K_F-phonons takes place at temperatures higher than θ_D, since the phonon-electron scattering rate τ^?1_ph?el is greater than the phonon-phonon rate τ^?1_ph?ph. this situation is in contrast to the situation in three dimensional metals, where phonon drag takes place only at very low temperatures. Our model explains the transport properties of the material including the electrical conductivity anistropy, the conductivity in a strong electric field, and the Hall effect data.     

Electronic structure of hexagonal quantum—disc clusters

Within the effective-mass approximation,we investigate the electronic structure of hexagonal quantum-disc clusters using the finite element method.With an increasing amount of quantum dots in the cluster,the electronic energy levels quickly expand into mini-bands.each consisting of discrete,unevenly distributed energy levels,The corresponding electronic eigenfunctions are linear combinations of the electron orbits in each quantum dot.The spatial symmetry of the combination is the same as the electronic eigenfunctioin of a single quantum dot.     

Charge density waves in layered compounds

The metallic layered compounds of the transition metal dichalcogenide type show unusual deviations from simple metallic behavior. These deviations are due to Charge Density Wave instabilities that are driven by the Fermi surface. The Charge Density Wave is a coupled periodic distortion of the conduction electron density and the crystal lattice. The effects of the CDW on the physical properties are qualitatively discussed. The basic driving forces of the CDW are most likely responsible for structural changes in a wide variety of materials, such as high temperature superconductors and long period superlattice alloys.     

Charge density waves in a relaxation semiconductor

The response of a relaxation inhomogeneous semiconductor (specimen with an injecting contact and/or an inversion layer) containing an impurity with positive correlation energy, to a sudden increase in the electrical field and/or bias lighting is investigated. Under definite conditions the charge-carrier plasma turns out to be unstable relative to charge density wave excitation; their frequencies and damping decrements are found. Under the influence of waves of spatial overcharging of centers with positive correlation energy an increase is possible in the imaginary part of the impedance, as is the appearance of an oscillating frequency dependence of the imaginary and real parts of the impedance. The time dependence of the response is examined, and both exponential and power-law damping is possible depending on the kind of charge carrier forming the wave.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 54–62, June, 1987.     

Holographic lithography of a two-dimensional hexagonal structure: Effect of beam polarization

A comparison is made between the theoretical determination of the interference contrast function with fabricated two-dimensional hexagonal structures of submicron lattice constant. Experiments were performed using a siloxane based hybrid organic-inorganic material and a holographic lithography method.Thin hybrid structured layers were fabricated and characterized for different conditions of beam polarizations between 0° and 90°. As a result, the photo patterning appears to be strongly dependent on beam polarization in accordance with theoretical predictions.     

Electronic structure of two-dimensional semiconductor systems

The motion of two-dimensional carriers in semiconductor heterojunctions, quantum wells and superlattices is discussed, with emphasis on subband dispersion parallel to the interfaces and on quantization in a perpendicular magnetic field. The envelope function approximation is shown to provide an efficient and accurate method for the calculation of electronic states. It is shown that for states derived from degenerate or coupled bulk bands (valence bands, k . p-coupled conduction and valence bands in narrow-gap semiconductors) striking non-parabolicities of the subband dispersion and non-linearities of the Landau levels versus magnetic field occur. Results for GaAs-AlGaAs and InAs-GaSb systems are presented and compared with experiment.     

Dispersion properties of two-dimensional phonon crystals with a hexagonal structure

Acoustic and optical properties of two-dimensional phonon crystals with a hexagonal symmetry are described. Differential-difference equations describing plane oscillations of a two-dimensional lattice of material structures are derived in a harmonic approximation, and dispersion dependences of acoustic and optical phonons are calculated. Branches of optical oscillations and intervals of forbidden frequencies are shown to appear when a lattice consists of two types of atoms.     

Charge density waves in PbTe in strong magnetic fields

It is found that in a strong magnetic field the parameters of PbTe allow for a Peierls-type transition at a few degrees Kelvin.     

Charge density waves and bond order waves in a quarter filled extended Hubbard ladder

We investigate the phase diagram of a quarter filled Hubbard ladder with nearest-neighbor Coulomb repulsion using bosonization and renormalization group approach. Focusing on the strong-repulsion regime, we discuss the effect of an interchain exchange interaction J and interchain repulsion V on the possible ground states of the system and charge order configurations. Since the spin excitations always possess a gap, we find competing bond-order wave and charge density wave phases as possible ground states of the ladder model. We discuss the elementary excitations in these various phases and point an analogy between the excitations on some of these phases and those of a Kondo-Heisenberg insulator. We also study the order of the quantum phase transitions between the different ground states of the system. We obtain second order transitions in the Ising or SU(2)₂ universality class or first order transitions. We map the complete phase diagram in the J –V plane by integrating perturbative renormalization-group equations. Finally, we discuss the effect of doping away from half-filling and the effect of an applied magnetic field.     

Models of the pseudogap state of two-dimensional systems

We analyze several almost exactly solvable models of the electronic spectrum of two-dimensional systems with well-developed short-range-order dielectric (e.g., antiferromagnetic) or superconducting fluctuations that give rise to an anisotropic pseudogap state in certain segments of the Fermi surface. We develop a recurrence procedure for calculating the one-electron Green’s function that is equivalent to summing all Feynman diagrams. The procedure is based on an approximate ansatz for higher order terms in the perturbation series. We do detailed calculations of the spectral densities and the one-electron density of states. Finally, we analyze the limits of the adopted approximations and some important points concerning the substantiation of these approximations. Zh. éksp. Teor. Fiz. 115, 1765–1785 (May 1999)     

晶体相场方法研究二维六角相向正方相结构转变

应用双模PFC模型,计算二维PFC相图,模拟二维六角晶格向正方晶格的结构转变过程, 观察新相(二维正方相)的形核、长大特点,以及相结构转变的动力学特征. 结果表明:六角结构相向正方结构相的转变,正方相最易在六角相晶界处, 尤其是在三晶粒的交汇处首先生成正方相的晶核,之后是正方相逐渐通过吞噬六角相的边缘, 向六角相内部推进,并不断长大.对于结构转变生成的正方相晶粒,其晶粒取向几乎是随机的, 与原先六角相晶粒取向角没有明显的关系.正方相转变的面积分数随时间变化的动力学曲线 呈现典型的"S"形.由Avrami曲线可将相变曲线看成由两阶段组成. 计算模拟得到的Avrami曲线的第二阶段直线斜率K的范围在2.0和3.0之间, 与JMAK理论的指数n相符合. 关键词： 结构转变 晶体相场 相图 晶粒取向     

Photonic band structure in a two-dimensional hexagonal lattice of equilateral triangles

In this paper, the plane wave expansion method is used to calculate the photonic band structure (PBS) for the transverse electric (TE) polarization in a two-dimensional hexagonal lattice composed air holes with cross-sections in the shape of equilateral triangles embedded in a GaAs background. We consider that the dielectric constant of GaAs is dependent on both hydrostatic pressure and temperature. Further, an increase can be observed in the width of the photonic band gap (PBG) while the lengths of the sides of the equilateral triangles are increased. When the pressure increases in the presence of a constant temperature, the dielectric constant of GaAs decreases; further, the PBS exhibits a more noticeable shift toward regions of higher frequencies than that observed while the temperature is increased at a given pressure. We also observed that the PBG obtained by rotating the triangular holes by an angle θ increases when compared against the results obtained for the lattice of holes without performing any rotation. In addition, the results obtained using the supercell technique denote that the position and width of the PBG remain unchanged while removing a triangular hole from the structure, denoting the presence of a defective band within the PBG.     

Interaction effects and pseudogap in two-dimensional lateral tunnel junctions

Tunneling characteristics of a two-dimensional lateral tunnel junction are reported. A pseudogap on the order of Coulomb energy is detected in the tunneling density of states (TDOS) when two identical two-dimensional electron systems are laterally separated by a thin energy barrier. The Coulombic pseudogap remains robust well into the quantum Hall regime until it is overshadowed by the cyclotron gap in the TDOS. The pseudogap is modified by the in-plane magnetic field, demonstrating a nontrivial effect of the in-plane magnetic field on the electron-electron interaction.     

Non-fermi-liquid behavior of quasi-one-dimensional pseudogap materials

We study the spectral function of the pseudogap phase of quasi-one-dimensional charge density wave materials. Using a stochastic approach and emphasizing an exact treatment of non-Gaussian order parameter fluctuations we will go beyond a usual perturbative calculation. Our results give a good fit to angle-resolved photoemission spectroscopy data and explain the absence of the Fermi edge in charge density wave materials even above the Peierls transition, indicating non-Fermi-liquid behavior.     

Charge density waves in silicon inversion layers: Effect of four valley interaction terms

Kelly and Falicov (KF) have demonstrated that phonon mediated intervalley electronic exchange interactions can lead to a charge density wave type ground state in silicon (111) inversion layers. In their model only two valley interaction terms are included. There are four valley interaction terms which, in principle, are as large in magnitude as the two valley terms. We have investigated the effect of including the four-valley interaction terms on a number of Hartree-Fock solutions.