Hidden Fermi surface in Kx_xFe2−y_{2-y}Se2_2: LDA+DMFT study

In this paper we provide theoretical LDA+DMFT support of recent ARPES experimental observation of the so called hidden hole like band and corresponding hidden Fermi surface sheet near $\Gamma$-point in the K$_{0.62}$Fe$_{1.7}$Se$_2$ compound. To some extent this is a solution to the long-standing riddle of Fermi surface absence around $\Gamma$-point in the K$_x$Fe$_{2-y}$Se$_2$ class of iron chalcogenide superconductors. In accordance with the experimental data Fermi surface was found near the $\Gamma$-point within LDA+DMFT calculations. Based on the LDA+DMFT analysis in this paper it is shown that the largest of the experimental Fermi surface sheets is actually formed by a hybrid Fe-3d($xy,xz,yz$) quasiparticle band. It is also shown that the Fermi surface is not a simple circle as DFT-LDA predicts, but has (according to the LDA+DMFT) a more complicated `propeller'-like structure due to correlations and multiorbital nature of the K$_x$Fe$_{2-y}$Se$_2$ materials. While the smallest experimental Fermi surface around $\Gamma$-point is in some sense fictitious, since it is formed by the summation of the intensities of the spectral function associated with `propeller' loupes and is not connected to any of quasiparticle bands.Comment: 4 pages, 4 figure

The pulse and monochromatic light stimulation of semiconductor quantum wells

The light reflectance and absorbance are calculated for a quantum well (QW) the width of which is comparable with the light wave length. The difference of the refraction coefficients of the quantum well and barriers is taken into account. The stimulating pulse form is arbitrary. An existence of two closely situated discrete excitation energy levels is supposed. Such energy level pare may correspond to two magnetopolaron states in a quantizing magnetic field perpendicular to the QW plane. The relationship of the radiative and non-radiative damping is arbitrary. The final results does not use the approximation of the weak Coulomb interaction of electrons and holes.Comment: 10 pages, 1 figur

Influence of a polaron dispersion and excitonic effect on a magnetopolaron energy spectrum in a quantum well

It has been shown, that a magnetopolaron discrete energy spectrum is realized in the optical experiments where exciting light is directed perpendicularly to a quantum well (QW) plane. The phonon dispersion as well as Coulomb attraction can only shift the magnetopolaron discrete energy levels and broad some of them.Comment: 8 pages, 4 figures with caption

The Kubo-type Formula for Conductivity of Spatially Inhomogeneous Systems

The expressions for average densities of currents and charges induced by a weak electromagnetic field in spatially inhomogeneous systems are obtained. The case of finite temperatures is considered. It is shown that average values are separated into "basic" and "additional" parts. The former depends on electric fields, and the latter depends on derivatives of electric fields on coordinates. Semiconductor quantum wells, wires or dots may be considered as spatially inhomogeneous systems.Comment: 5 page

Resonant Transmission of a Light Pulse through a Quantum Well

Reflectance, transmittance and absorbance of a symmetric light pulse, the carrying frequency of which is close to the frequency of interband transitions in a quantum well, are calculated. Energy levels of the quantum well are assumed discrete, and two closely located excited levels are taken into account. The theory is applicable for the quantum wells of arbitrary widths when the size quantization is preserved. A distinction of refraction indices of barriers and quantum well is taken into account. In such a case, some additional reflection from the quantum well borders appears which changes essentially a shape of the reflected pulse in comparison to homogeneous medium. The reflection from the borders disappears at some definite ratios of the carrying frequency of the stimulating pulse and quantum well width.Comment: 8 pages, 8 figure

Influence of the magnetopolaron effect on light reflection and absorption by a wide semiconductor quantum well

Light reflection and absorption spectra by a semiconductor quantum well (QW) , which width is comparable to a light wave length of stimulating radiation, are calculated. A resonance with two close located exited levels is considered. These levels can arise due to splitting of an energy level of an electron-hole pair (EHP) due to magnetopolaron effect, if the QW is in a quantizing magnetic field directed perpendicularly to the QW plane. It is shown that unlike a case of narrow QWs light reflection and absorption depend on a QW width $d$. The theory is applicable at any ratio of radiative and non-radiative broadenings of electronic excitations

Magnetic Field-Induced Fano Profiles in the Absorption Coefficient of Semiconductors

A strongly asymmetric, Fano-like resonance profile has been found for magneto-absorption in the states of hot free electron-hole pairs scattered by defects in bulk semiconductors. The renormalization of the absorption profile, compared to that expected from the bare density of states in a high magnetic field, follows from a quasi-one-dimensional character of electronic excitations. The results are valid for absorption by the electronic states of large Landau quantum number where the Coulomb interaction should play a minor role. The resonance shape is in a good qualitattive agreement with experimental observations

LDA'+DMFT Investigation of Electronic Structure of K{1-x}Fe{2-y}Se2 Superconductor

We investigate electronic structure of the new iron chalcogenide high temperature superconductor K{1-x}Fe{2-y}Se2 (hole doped case with x=0.24, y=0.28) in the normal phase using the novel LDA'+DMFT computational approach. We show that this iron chalcogenide is more correlated in a sense of bandwidth renormalization (energy scale compression by factor about 5 in the interval +/-1.5 eV), than typical iron pnictides (compression factor about 2), though the Coulomb interaction strength is almost the same in both families. Our results for spectral densities are in general agreement with recent ARPES data on this system. It is found that all Fe-3d(t2g) bands crossing the Fermi level have equal renormalization, in contrast to some previous interpretations. Electronic states at the Fermi level are of predominantly xy symmetry. Also we show that LDA'+DMFT results are in better agreement with experimental spectral function maps, than the results of conventional LDA+DMFT. Finally we make predictions for photoemission spectra lineshape for K{0.76}Fe{1.72}Se2.Comment: 5 pages, 4 figure

Electronic structure of FeSe monolayer superconductors: shallow bands and correlations

Electronic spectra of typical single FeSe layer superconductors obtained from ARPES data reveal several puzzles: what is the origin of shallow and the so called "replica" bands near M-point and why the hole-like Fermi surfaces near $\Gamma$-point are absent. Our extensive LDA+DMFT calculations show that correlation effects on Fe-3d states can almost quantitatively reproduce rather complicated band structure, which is observed in ARPES, in close vicinity of the Fermi level for FeSe/STO and K$_x$Fe$_{2-y}$Se$_{2}$. Rather unusual shallow electron-like bands around the M(X)-point in the Brillouin zone are well reproduced. However, in FeSe/STO correlation effects are apparently insufficient to eliminate the hole-like Fermi surfaces around the $\Gamma$-point, which are not observed in most ARPES experiments. Detailed analysis of the theoretical and experimental quasiparticle bands with respect to their origin and orbital composition is performed. It is shown that for FeSe/STO system the LDA calculated Fe-3d$_{xy}$ band, renormalized by electronic correlations within DMFT gives the quasiparticle band almost exactly in the energy region of the experimentally observed "replica" quasiparticle band at the M-point. For the case of K$_x$Fe$_{2-y}$Se$_{2}$ most bands observed in ARPES can also be understood as correlation renormalized Fe-3d LDA calculated bands, with overall semi-quantitative agreement with our LDA+DMFT calculations. Thus the shallow bands near the M-point are common feature for FeSe-based systems, not just FeSe/STO. We also present some simple estimates of "forward scattering" electron-optical phonon interaction at FeSe/STO interface, showing that it is apparently irrelevant for the formation of "replica" band in this system and significant increase of superconducting $T_c$.Comment: 15 pages, 16 figures, 1 table. arXiv admin note: text overlap with arXiv:1702.0094

An echo of an exciting light pulse in quantum wells

The non-sinusoidal character oscillations appear in the transmitted, reflected and absorbed light fluxes when light pulses irradiate a semiconductor quantum well (QW), containing a large number of the equidistant energy levels of electronic excitations. A damping echo of the exciting pulse appears through the time intervals $2\pi\hbar/\Delta E$ in the case of the very short light pulses $\gamma_l^{-1}\ll\hbar/\Delta E$.Comment: 3 pages, 2 figures with caption