Anisotropy of infrared optical absorption in layered superconductors

We analyze the problem of infrared optical absorption in a clean layered London superconductor in the vicinity of the gap =2. We conclude that absorption of light with wave vectorqc is enhanced over ordinary Drude absorption (qc) due to resonance absorption (Landau damping). Experimental absorption studies with qc might therefore improve chances to observe a superconducting gap in the high-T _c superconductors.     

Influence of infrared radiation on the excitonic absorption edge of CuCl

We report changes in the excitonic absorption edge of CuCl caused by intense CO₂ laser radiation at 10.6 m, a wavelength which lies in the infrared (ir) transparency region of CuCl. With an ir intensity of 0.4 GW/cm² we observe a 100% absorption increase for the Kr⁺ laser probe wavelength of 406.7nm. The effect scales linearly with ir intensity but does not depend on relative polarization. We explain the effect by laser field induced electroabsorption of the exciton. The magnitude of the effect is closely related to electroabsorption induced by static external fields and by internal electric fields from optical phonons.     

Layered superconductors with anisotropic energy gap: specific heat and infrared absorption

New oxide superconductors with layered structure are expected to have anisotropic energy gap in the generalized BCS pairing theory. The gap parameter can be quite different for perpendicular to the plane of the layers as compared tok parallel to layer planes. Because of short coherence lengths ξ, quite small compared to the normal state carrier meanfree pathl, the effect of these anisotropies do not average out, as in many of the conventional superconductors. For a proper comparison of experimental results with the correct predictions of the pairing theory, a formulation is developed to obtain important physical quantities like specific heat and infrared absorption in the superconducting state of such anisotropic systems. This includes a brief account of the pairing theory generalized to layered crystals with arbitrary number of layers per unit cell, not necessarily equidistant. In an explicit model for the anisotropy of the gap parameter ink-space, with a simple form for the nonspherical Fermi-surface, it is shown that the low-temperature specific heat can have even a linear or a power-law temperature-dependence in the superconducting state. Even if the gap parameter does not vanish anywhere, its smeared-out exponential temperature-dependence may be difficult to be distinguished experimentally from a power-law behaviour. Similarly, it is shown that in the case of appreciable anisotropy, infrared absorption can take place much below the in-plane gap parameter , wherek _t is the wavevector in the plane of the layers.     

Holographic conductivity of holographic superconductors with higher-order corrections

We analytically and numerically disclose the effects of the higher-order correction terms in the gravity and in the gauge field on the properties of s-wave holographic superconductors. On the gravity side, we consider the higher curvature Gauss–Bonnet corrections and on the gauge field side, we add a quadratic correction term to the Maxwell Lagrangian. We show that, for this system, one can still obtain an analytical relation between the critical temperature and the charge density. We also calculate the critical exponent and the condensation value both analytically and numerically. We use a variational method, based on the Sturm–Liouville eigenvalue problem for our analytical study, as well as a numerical shooting method in order to compare with our analytical results. For a fixed value of the Gauss–Bonnet parameter, we observe that the critical temperature decreases with increasing the nonlinearity of the gauge field. This implies that the nonlinear correction term to the Maxwell electrodynamics makes the condensation harder. We also study the holographic conductivity of the system and disclose the effects of the Gauss–Bonnet and nonlinear parameters \(\alpha \) and b on the superconducting gap. We observe that, for various values of \(\alpha \) and b, the real part of the conductivity is proportional to the frequency per temperature, \(\omega /T\), as the frequency is large enough. Besides, the conductivity has a minimum in the imaginary part which is shifted toward greater frequency with decreasing temperature.     

Optoelectronic transport mechanism from subband infrared absorption and tunneling regeneration

The main problems of conventional multi-quantum well infrared photodetectors (QWIPs) were discussed. In order to overcome the limitations of the conventional QWIPs, such as small photocurrent, high dark current and low response speed, novel QWIPs in which photocurrent increases with the number of well were proposed. The novel structure with several wells were calculated and analyzed in detail, and successfully fabricated. The dark current lower than conventional QWIPs by about one order of magnitude was obtained, well in agreement with theoretical value. I–V characteristics of the novel QWIPs with six wells has been presented, and six related negative differential resistance regions were observed at positive bias. The absorption photocurrents of the novel QWIPs at 77 K were found to increase with well numbers, confirming the mechanism of the new structure. Furthermore, the transportation of the optoelectronic and some other problems of the QWIPs were discussed.     

Effect of magnetostriction on the transport current in granulated type-II superconductors

The effect of magnetostriction on the dependence of the critical current density on the magnetic field applied normally to the current in granulated superconductors is investigated. In sufficiently high magnetic fields (H ? H _c1), magnetostriction suppresses the current-carrying capacity of the material. It is shown that the effect of magnetostriction on the transport current is sensitive to variation of the effective parameters of the Josephson medium, specifically, the grain bond strength and ratio between the sizes of grains constituting the junction.     

Influence of Sr doping on the free carrier absorption in high-T c superconductors La2−x SrxCuO4

The reflectance spectra of polycrystalline La_2–x Sr _x CuO₄ samples were investigated in the energy range between 50 meV and 4 eV in dependence of the Sr content. The spectra are attributed to free carrier absorption of the Drude type, superimposed by optical phonon excitations below 0.1 eV and intrinsic absorption above 1 eV. From the influence of Sr doping onto the plasma energy it is deduced that La_2–x Sr _x CuO₄ is ap-type conductor with a maximum carrier concentration of 2.0×10²¹ cm^–3 forx=0.15. The results are interpreted in terms of a Hubbard model with an empty upper and ap-doped lower Hubbard band with a width of 1.9 eV.     

Strong vertex corrections from weak disorder in 2D d-wave superconductors

We show that weak static random potentials have pronounced effects on the quasiparticle states of a 2Dd-wave superconductor close to a node. We prove that the vertex correction coming from the simplest crossed diagram is important even for a nonmagnetic potential. The leading frequency and momentum dependent logarithmic singularities in the self-energy are calculated exactly to second order in perturbation theory. The self-energy corrections lead to a modified low energy density of states which depends strongly on the type of random potential and which can be measured in experiments. There is an exceptional case for a potential with extremely local scatterers and opposite nodes separated by (, ) where an exact cancelation takes place eliminating the leading frequency dependent singularity in the simplest crossed diagram. A comparison of the perturbative results with a self-consistent CPA (coherent potential approximation) for the nonmagnetic disorder reveals qualitative differences in the self-energy at the smallest energies which are due to the neglectance of vertex corrections in CPA.     

Far infrared laser thermal spectroscopy of superconductors

An optically pumped far infrared laser provides fixed photon energies near the superconducting gap, making it possible to carry out experiments where the temperature is swept through T_c. Such thermal measurements in thin films of V₃Si show both known and unexplained features, with some of the latter perhaps due to a second gap. The thermal technique shows potential for the study of superconductors.Work supported by the U.S. Department of Energy under Contract No. DE-AS05-79ER10436.     

Magnetically modulated microwave absorption in cuprate superconductors

Magnetically modulated microwave absorption (MMMA) is a highly sensitive microprobe, which is increasingly being used to extract a variety of information about the flux dynamics in the high-transition-temperature superconductors. The correspondence between the information obtained from the MMMA and the directly monitored (without modulation) field-induced microwave absorption techniques is discussed. The validity of some of the commonly held views about the features that characterize the MMMA signal are examined.     

On the mechanisms of transport losses in high-temperature superconductors

Monte Carlo simulations of a high-temperature superconductor carrying an alternating transport current have been performed. The influence of defect concentration and external magnetic field on the transport energy losses is investigated. We show that there are two components of the losses in a superconductor carrying an alternating transport current: a hysteresis component related to the superconductor remagnetization by the self-current field and a dynamic component related to the steady flow of vortices-antivortices and their annihilation in the superconductor.     

Nonlinear fluctuation phenomena in the transport properties of superconductors

There exists a wide temperature region $(GiT < T - T_c < T\sqrt {Gi} )$ where the influence of fluctuations on the thermodynamic properties of superconductors can be taken into account in the linear (Gaussian) approximation, while their influence on the kinetic properties is strongly nonlinear. The Maki-Thompson contribution to the conductivity saturates in this region. However, the Aslamazov-Larkin contribution becomes even more singular. This enhancement is related to the fact that nonlinear effects increase the lifetime of fluctuating pairs. The pair breaking and energy relaxation processes can decrease the nonlinear effects.     

Impurity-induced infrared absorption in CsCl

The infrared absorption of CsCl doped with K⁺ and with Rb⁺ impurity ions have been calculated. A nearest-neighbour defect model with the central and the noncentral force constant changes has been employed. The obtained structure in the infrared absorption is seen to be in goog agreement with the experimental data.     

Multiphonon infrared absorption in silicon

Investigations have been carried out on silicon crystals, grown by float zone (FZ) and Czochralski (CZ) methods, of infrared absorption bands using a Fourier transform infrared spectrophotometer. Multiphonon bands are identified in the light of recent theoretical calculations based on the total energy of silicon crystal lattice. Theoretical results of Ihm et al.⁽¹⁾ and Yin and Cohen^(2,3) are found to be in good agreement with the experimental observations of multiphonon infrared bands.     

铁基超导体的输运性质

在铁基超导体中存在着多种有序态,例如电子向列相和自旋密度波等,从而呈现出丰富的物理现象.输运性质的测量能为认识铁基超导体的低能激发提供极为有用的信息.铁砷超导体由于其电子结构的多能带特性,其电阻率和霍尔系数与温度的关系出现多样性的变化,但在正常态并没有看到有类似铜氧化物超导体的赝能隙打开等奇异行为.在空穴型掺杂的铁基超导体中观测到霍尔系数在低温下变号,对应温区的电阻率上出现一个很宽的鼓包等,可能是从非相干到相干态的转变.热电势行为也表现出与铜氧化物超导体的明显差异,比如铁基超导体的正常态热电势的绝对值反而在最佳掺杂区是最大的,这也许跟强的带间散射有关.能斯特效应表明铁基超导体在Tc以上的超导位相涨落并不明显,与铜氧化物超导体存在明显差别.在铁基超导体上所显示出来的这些反常热电性质,并没有在类似结构的镍基超导体(如LaNiAsO)上观测到,镍基超导体表现得更像一个通常的金属.这些均说明铁基超导体的奇异输运性质与其高温超导电性存在内在的关联,这些因素是建立其超导机理时需要考虑进去的.     

Lattice absorption of Ge in the far infrared

The lattice absorption at low frequency (100 cm⁻¹) and low temperature (50 K) of Ge is very small, yet it may influence the performance of the p---Ge hot hole far-infrared laser. We have measured by temperature-tuned laser interferometry.     

Final-state-interactions in the infrared absorption of doped semiconductors

We calculate the profile of the infrared inter-valence-band absorption in p-doped non-polar semiconductor as modified by the final-state interaction of electronic excitations with optical phonons. The absorption coefficient vanishes at a light frequency equal to the frequency of the non-polar optical vibrations. The spectrum is asymmetric with respect to the absorption minimum. We show that the type of asymmetry depends on the position of the optical phonon energy relative to the electronic continuum. The results are compared to available data for p-doped Si.