arXiv subjects

M. V. Sadovskii

Publications and source records attributed to M. V. Sadovskii.

At least 37 records · Page 2Linked to original sources

arXiv2017

Ginzburg - Landau expansion in strongly disordered attractive Anderson - Hubbard model

We have studied disordering effects on the coefficients of Ginzburg - Landau expansion in powers of superconducting order - parameter in attractive Anderson - Hubbard model within the generalized $DMFT+Σ$ approximation. We consider the wide region of attractive potentials $U$ from the weak coupling region, where superconductivity is described by BCS model, to the strong coupling region, where superconducting transition is related with Bose - Einstein condensation (BEC) of compact Cooper pairs formed at temperatures essentially larger than the temperature of superconducting transition, and the wide range of disorder - from weak to strong, where the system is in the vicinity of Anderson transition. In case of semi - elliptic bare density of states disorder influence upon the coefficients $A$ and $B$ before the square and the fourth power of the order - parameter is universal for any value of electron correlation and is related only to the general disorder widening of the bare band (generalized Anderson theorem). Such universality is absent for the gradient term expansion coefficient $C$. In the usual theory of "dirty" superconductors the $C$ coefficient drops with the growth of disorder. In the limit of strong disorder in BCS limit the coefficient $C$ is very sensitive to the effects of Anderson localization, which lead to its further drop with disorder growth up to the region of Anderson insulator. In the region of BCS - BEC crossover and in BEC limit the coefficient $C$ and all related physical properties are weakly dependent on disorder. In particular, this leads to relatively weak disorder dependence of both penetration depth and coherence lengths, as well as of related slope of the upper critical magnetic field at superconducting transition, in the region of very strong coupling.

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arXiv2017

On the origin of the shallow and "replica" bands in FeSe monolayer superconductors

We compare electronic structures of single FeSe layer films on SrTiO$_3$ substrate (FeSe/STO) and K$_x$Fe$_{2-y}$Se$_{2}$ superconductors obtained from extensive LDA and LDA+DMFT calculations with the results of ARPES experiments. It is demonstrated that correlation effects on Fe-3d states are sufficient in principle to explain the formation of the shallow electron -- like bands at the M(X)-point. However, in FeSe/STO these effects alone are apparently insufficient for the simultaneous elimination of the hole -- like Fermi surface around the $Γ$-point which is not observed in ARPES experiments. Detailed comparison of ARPES detected and calculated quasiparticle bands shows reasonable agreement between theory and experiment. Analysis of the bands with respect to their origin and orbital composition shows, that for FeSe/STO system the experimentally observed "replica" quasiparticle band at the M-point (usually attributed to forward scattering interactions with optical phonons in SrTiO$_3$ substrate) can be reasonably understood just as the LDA calculated Fe-3d$_{xy}$ band, renormalized by electronic correlations. The only manifestation of the substrate reduces to lifting the degeneracy between Fe-3d$_{xz}$ and Fe-3d$_{yz}$ bands in the vicinity of 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 LDA+DMFT calculations.

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arXiv2016

Ginzburg - Landau Expansion in BCS - BEC Crossover Region of Disordered Attractive Hubbard Model

We have studied disorder effects on the coefficients of Ginzburg - Landau (GL) expansion for attractive Hubbard model within the generalized DMFT+Sigma approximation for the wide region of the values of attractive potential U - from the weak-coupling limit, where superconductivity is described by BCS model, towards the strong coupling, where superconducting transition is related to Bose - Einstein condensation (BEC) of compact Cooper pairs. For the case of semi-elliptic initial density of states disorder influence on the coefficients A and B before the square and the fourth power of the order parameter is universal for at all values of electronic correlations and is related only to the widening of the initial conduction band (density of states) by disorder. Similar universal behavior is valid for superconducting critical temperature T_c (the generalized Anderson theorem) and specific heat discontinuity at the transition. This universality is absent for the coefficient C before the gradient term, which in accordance with the standard theory of "dirty" superconductors is strongly suppressed by disorder in the weak-coupling region, but can slightly grow in BCS - BEC crossover region, becoming almost independent of disorder in the strong coupling region. This leads to rather weak disorder dependence of the penetration depth and coherence length, as well as the slope of the upper critical magnetic field at T_c, in BCS - BEC crossover and strong coupling regions.

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arXiv2016

High Temperature Superconductivity in FeSe Monolayers

This review discusses the main experiments and theoretical views related to observation of high-temperature superconductivity in intercalated FeSe compounds and single layer films of FeSe on substrates like SrTiO_3. We consider in detail the electronic structure of these systems, both theoretical calculations of this structure at hand and their correspondence with ARPES experiments. It is stressed that electronic spectrum of these systems is qualitatively different from typical picture of the spectrum in well studied FeAs superconductors and the related problems of theoretical description of spectrum formation are also discussed. We also discuss the possible mechanisms of Cooper pairing in monolayers of FeSe and problems appearing here. As single layer films of FeSe on SrTiO_3 can be represented as typical Ginzburg "sandwiches", we analyze the possibility of rising the critical temperature of superconducting transition T_c due to different variants of "excitonic" mechanism of superconductivity. It is shown, that in its classic variant (as proposed for such systems by Allender, Bray and Bardeen) this mechanism is unable to explain the observed values of T_c, but situation is different when we consider instead of "excitons" the optical phonons in SrTiO_3 (with energy of the order of 100 meV). We consider both the simplest model of T_c enhancement due to interaction with such phonons and more specific models with dominant "forward" scattering, which allow to understand the growth of T_c as compared with the case of bulk FeSe and intercalated FeSe systems. We also discuss the problems connected with antiadiabatic nature of superconductivity due to such mechanism.

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arXiv2016

Electronic structure of FeSe monolayer superconductors

We review a variety of theoretical and experimental results concerning electronic band structure of superconducting materials based on FeSe monolayers. Three type of systems are analyzed: intercalated FeSe systems A_xFe_2Se_{2-x}S_x and [Li_{1-x}Fe_xOH]FeSe as well as the single FeSe layer films on SrTiO_3 substrate. We present the results of detailed first principle electronic band structure calculations for these systems together with comparison with some experimental ARPES data. The electronic structure of these systems is rather different from that of typical FeAs superconductors, which is quite significant for possible microscopic mechanism of superconductivity. This is reflected in the absence of hole pockets of the Fermi surface at Γ-point in Brillouin zone, so that there are no "nesting" properties of different Fermi surface pockets. LDA+DMFT calculations show that correlation effects on Fe-3d states in the single FeSe layer are not that strong as in most of FeAs systems. As a result, at present there is no theoretical understanding of the formation of rather "shallow" electronic bands at M points. LDA calculations show that the main difference in electronic structure of FeSe monolayer on SrTiO_3 substrate from isolated FeSe layer is the presence of the band of O-2p surface states of TiO_2 layer on the Fermi level together with Fe-3d states, which may be important for understanding the enhanced T_c values in this system. We briefly discuss the implications of our results for microscopic models of superconductivity.

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arXiv2015

Attractive Hubbard Model: Homogeneous Ginzburg - Landau Expansion and Disorder

We derive Ginzburg - Landau (GL) expansion in disordered attractive Hubbard model within the combined Nozieres - Schmitt-Rink and DMFT+Sigma approximation. Restricting ourselves to the case of homogeneous expansion, we analyze disorder dependence of GL expansion coefficients on disorder for the wide range of attractive potentials U, from weak BCS coupling region to the strong coupling limit, where superconductivity is described by Bose - Einstein condensation (BEC) of preformed Cooper pairs. We show, that for the case of semi - elliptic "bare" density of states of conduction band, disorder influence on GL coefficients A and B before quadratic and fourth -- order terms of the order parameter, as well as on the specific heat discontinuity at superconducting transition, is of universal nature at any strength of attractive interaction and is related only to the general widening of the conduction band by disorder. In general, disorder growth increases the values of coefficients A and B, leading either to the suppression of specific heat discontinuity (in the weak coupling limit), or to its significant growth (in the strong coupling region). However, this behavior actually confirms the validity of the generalized Anderson theorem, as disorder dependence of superconducting critical temperature T_c, is also controlled only by disorder widening of conduction band (density of states).

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arXiv2015

DMFT+Sigma approach to disordered Hubbard model

We briefly review the generalized dynamical mean-field theory DMFT+Sigma treatment of both repulsive and attractive disordered Hubbard models. We examine the general problem of metal-insulator transition and the phase diagram in repulsive case, as well as BCS-BEC crossover region of attractive model, demonstrating certain universality of single - electron properties under disordering in both models. We also discuss and compare the results for the density of states and dynamic conductivity in both repulsive and attractive case and the generalized Anderson theorem behavior for superconducting critical temperature in disordered attractive case. A brief discussion of Ginzburg - Landau coefficients behavior under disordering in BCS-BEC crossover region is also presented.

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arXiv2015

Electronic structure of NaFeAs superconductor: LDA+DMFT calculations compared with ARPES experiment

We present the results of extended theoretical LDA+DMFT calculations for a new iron-pnictide high temperature superconductor NaFeAs compared with the recent high quality angle-resolved photoemission (ARPES) experiments on this system [1]. The universal manifestation of correlation effects in iron-pnictides is narrowing of conducting bands near the Fermi level. Our calculations demonstrate that for NaFeAs the effective mass is renormalized on average by a factor of the order of 3, in good agreement with ARPES data. This is essentially due to correlation effects on Fe-3d orbitals only and no additional interactions with with any kind of Boson modes, as suggested in [1], are necessary to describe the experiment. Also we show that ARPES data taken at about 160 eV beam energy most probably corresponds to $k_z=π$ Brillouin zone boundary, while ARPES data measured at about 80 eV beam energy rather represents $k_z=0$. Contributions of different Fe-3d orbitals into spectral function map are also discussed.

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arXiv2014

Attractive Hubbard model with disorder and the generalized Anderson theorem

Using the generalized DMFT+Sigma approach we have studied disorder influence on single-particle properties of the normal phase and superconducting transition temperature in attractive Hubbard model. The wide range of attractive potentials U was studied - from the weak coupling region, where both the instability of the normal phase and superconductivity are well described by BCS model, towards the strong coupling region, where superconducting transition is due to Bose-Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures much higher than the temperature of superconducting transition. We have studied two typical models of conduction band with semi-elliptic and flat densities of states, appropriate for three-dimensional and two-dimensional systems respectively. For semi-elliptic density of states disorder influence on all single-particle properties (e.g. density of states) is universal for arbitrary strength of electronic correlations and disorder and is due only to the general disorder widening of conduction band. In the case of flat density of states universality is absent in general case, but still the disorder influence is due mainly to band widening and universal behavior is restored for large enough disorder. Using the combination of DMFT+Sigma and Nozieres - Schmitt-Rink approximations we have studied disorder influence upon superconducting transition temperature T_c for the range of characteristic values of U and disorder, including the BCS-BEC crossover region and the limit of strong coupling. Disorder can either suppress T_c (in the weak coupling region) or significantly increase T_c (in strong coupling region). However in all cases the generalized Anderson theorem is valid and all changes of superconducting critical temperature are essentially due only to the general disorder widening of the conduction band.

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arXiv2014

Electronic and Magnetic Properties of the New Iron -- Based Superconductor [Li$_{1-x}$Fe$_x$OH]FeSe

We present the results of paramagnetic LDA band structure calculations: band dispersions, densities of states and Fermi surfaces, for the new iron based high-temperature superconductor LiOHFeSe. Main structural motif providing bands in the vicinity of the Fermi level is FeSe layer which is isostructural to the bulk FeSe prototype superconductor. The bands crossing the Fermi level and Fermi surfaces of the new compound are typical for other iron based superconductors. Experimentally it was shown that introduction of Fe ions into LiOH layer gives rise to ferromagnetic ordering of the Fe ions at T$_C$=10K. To study magnetic properties of [Li$_{0.8}$Fe$_{0.2}$OH]FeSe system we have performed LSDA calculations for $\sqrt 5 \times \sqrt 5$ superlattice and found ferromagnetism within the Li$_4$Fe(OH) layer. To estimate the Curie temperature we obtained Fe-Fe exchange interaction parameters for Heisenberg model from our LSDA calculations, leading to theoretical value of Curie temperature 10.4K in close agreement with experiment.

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arXiv2014

Disorder Effects in BCS-BEC Crossover Region of Attractive Hubbard Model

We study the disorder effects upon superconducting transition temperature T_c and the number of local pairs in attractive Hubbard model within the combined Nozieres - Schmitt-Rink and DMFT+Σapproximations. We analyze the wide range of attractive interaction U, from the weak coupling region, where instability of the normal phase and superconductivity are well described by BCS model, to the limit of strong coupling, where superconducting transition is determined by Bose--Einstein condensation of compact Cooper pairs, forming at temperatures much higher than superconducting transition temperature. It is shown that disorder can either suppress T_c in the weak coupling limit, or significantly enhance T_c in the case of strong coupling. However, in all cases we actually prove the validity of generalized Anderson theorem, so that all changes of T_c are related to change of the effective bandwidth due to disorder. Similarly, disorder effects on the number of local pairs are only due to these band-widening effects.

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arXiv2014

Electronic structure of iron-based superconductors: from pnictides to chalcogenides and other similar systems

This review discusses and compares electronic spectra of new iron-based high-temperature superconductors (HTSC), as well as a number of chemically similar compounds. Particular attention is payed to iron chalcogenide K_{1-x}Fe_{2-y}Se_2, which is isostructural to BaFe_2As_2 (122) pnictide. It is shown, that Fermi surfaces of K_{1-x}Fe_{2-y}Se_2 are essentially different from those for pnictides. Using LDA+DMFT and LDA'+DMFT calculations we show, that electronic correlations in K_{1-x}Fe_{2-y}Se_2 influence the electronic structure much more significantly, than in the most studied 122 system. We also discuss the electronic structure of several multiple-band superconductors, chemically similar to iron-based HTSC, with relatively small values of $T_c$, such as SrPt_{2}As_{2}, APt_{3}P, (Sr,Ca)Pd_2As_2, and non superconducting compound BaFe_2Se_3. It is shown, that electronic structure of these systems is very different from previously studies iron pnictides and chalcogenides. The value of T_c in these systems can be understood within the simple BCS model.

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arXiv2014

Normal phase and superconducting instability in attractive Hubbard model: the DMFT(NRG) study

We study the normal (non-superconducting) phase of attractive Hubbard model within dynamical mean field theory (DMFT) using numerical renormalization group (NRG) as impurity solver. Wide range of attractive potentials $U$ is considered, from the weak-coupling limit, where superconducting instability is well described by BCS approximation, up to the strong-coupling region, where superconducting transition is described by Bose-condensation of compact Cooper pairs, which are formed at temperatures much exceeding superconducting transition temperature. We calculate density of states, spectral density and optical conductivity in the normal phase for this wide range of $U$, including the disorder effects. Also we present the results on superconducting instability of the normal state dependence on the attraction strength $U$ and the degree of disorder. Disorder influence on the critical temperature $T_c$ is rather weak, suggesting in fact the validity of Anderson theorem, with the account of the general widening of the conduction band due to disorder.

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arXiv2013

Comparative Study of Electronic structure of New Superconductors (Sr,Ca)Pd2As2 and related compound BaPd2As2

This paper presents the comparative study of LDA calculated electronic structure of new isostructural to iron based systems superconductors (Sr,Ca)Pd2As2 with Tc about 1K and similar but structurally different system BaPd2As2. Despite chemical formula looks similar to iron superconductors and even main structural motif is the same - layers of Fe square lattices, electronic structure of (Sr,Ca)Pd2As2 and BaPd2As2 differs from Fe(As,Se)-HTSC completely. All these systems have essentially three dimensional Fermi surfaces in contrast to Fe(As,Se) materials. The Fermi level is crossed by low intensive tails of Pd-4d and As-4p states. However (Sr,Ca)Pd2As2 and BaPd2As2 materials have rather well developed peaks of Pd-4d(x2-y2) band. Thus by doping of about 2 holes per unit cell one can increase density of states at the Fermi level by a factor about 2.5. Since experimentally these compounds were found to be simple BCS superconductors the hole doping may considerably increase Tc. LDA calculated total densities of states at the Fermi level for stoichiometric systems perfectly agree with experimental estimates signifying rather small role of electronic correlations.

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arXiv2013

Doping Dependence of Correlation Effects in K{1-x}Fe{2-y}Se2 Superconductor: LDA'+DMFT Investigation

We present detailed LDA'+DMFT investigation of doping dependence of correlation effects in novel K{1-x}Fe{2-y}Se2 superconductor. Calculations were performed at four different hole doping levels, starting from hypothetical stoichiometric composition with total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons towards the case of 26.52, which corresponds to chemical composition K{0.76}Fe{1.72}Se2 studied in recent ARPES experiments. In general case the increase of hole doping leads to quasiparticle bands in wide energy window +/-2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation induced compression of Fe-3d LDA' bandwidths stays almost the same and of the order of ~1.3 for all hole concentrations. However close to the Fermi level situation is more complicated. Here in the energy interval from -1.0 eV to 0.4 eV the bare Fe-3d LDA' bands are compressed by significantly larger renormalization factors up to 5 with hole doping increase, while the value of Coulomb interaction remains the same. This fact manifests the increase of correlation effects with hole doping in K{1-x}Fe{2-y}Se2 system. Moreover in contrast to typical pnictides K{1-x}Fe{2-y}Se2 does not have well defined quasiparticle bands on the Fermi leves but pseudogap like "dark" region instead. We also find that with the growth of hole doping Fe-3d orbitals of various symmetries are affected by correlations in a different way in different parts of Brillouin zone. To illustrate this we determine quasiparticle mass renormalization factors and energy shifts, which transform the bare Fe-3d LDA' bands of various symmetries into LDA'+DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy dependent self-energy effects and can be used to analyze the available ARPES data.

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arXiv2012

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.

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arXiv2012

Consistent LDA'+DMFT approach to electronic structure of transition metal oxides: charge transfer insulators and correlated metals

We discuss the recently proposed LDA'+DMFT approach providing consistent parameter free treatment of the so called double counting problem arising within the LDA+DMFT hybrid computational method for realistic strongly correlated materials. In this approach the local exchange-correlation portion of electron-electron interaction is excluded from self consistent LDA calculations for strongly correlated electronic shells, e.g. d-states of transition metal compounds. Then the corresponding double counting term in LDA+DMFT Hamiltonian is consistently set in the local Hartree (fully localized limit - FLL) form of the Hubbard model interaction term. We present the results of extensive LDA'+DMFT calculations of densities of states, spectral densities and optical conductivity for most typical representatives of two wide classes of strongly correlated systems in paramagnetic phase: charge transfer insulators (MnO, CoO and NiO) and strongly correlated metals (SrVO3 and Sr2RuO4). It is shown that for NiO and CoO systems LDA'+DMFT qualitatively improves the conventional LDA+DMFT results with FLL type of double counting, where CoO and NiO were obtained to be metals. We also include in our calculations transition metal 4s-states located near the Fermi level missed in previous LDA+DMFT studies of these monooxides. General agreement with optical and X-ray experiments is obtained. For strongly correlated metals LDA$^\prime$+DMFT results agree well with earlier LDA+DMFT calculations and existing experiments. However, in general LDA'+DMFT results give better quantitative agreement with experimental data for band gap sizes and oxygen states positions, as compared to the conventional LDA+DMFT.

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arXiv2012

Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt3P (A=Ca,Sr,La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling superconductivity, suggested by experimental data on specific heat of SrPt3P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease of superconducting transition temperature Tc from Sr to La compound can be explained by corresponding decrease of total density of states at the Fermi level N(E_F). The shape of the density of states near the Fermi level suggests that in SrPt3P electron doping (such as replacement Sr by La) decreases N(E_F) and Tc, while hole doping (e.g. partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E_F) and possibly Tc.

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