Papers by Michael Sadovskii
arXiv (Cornell University), Oct 14, 2005
Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize th... more Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize the dynamical-mean field theory (DMFT) by including into the DMFT equations dependence on correlation length of pseudogap fluctuations via additional (momentum dependent) self-energy Σ k . This self -energy describes non-local dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+Σ k approach is used for the numerical solution of the weakly doped oneband Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W U (U -value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT+Σ k show a pseudogap formation near the Fermi level of the quasiparticle band. We also briefly discuss effects of random impurity scattering. Finally we demonstrate the qualitative picture of Fermi surface "destruction" due to pseudogap fluctuations and formation of "Fermi arcs" which agrees well with ARPES observations.
![Research paper thumbnail of Electronic and magnetic properties of the new iron-based superconductor [Li1 − x Fe x OH]FeSe](https://attachments.academia-assets.com/123411078/thumbnails/1.jpg)
JETP Letters, 2015
We present the results of paramagnetic LDA band structure calculations: band dispersions, densiti... more 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 TC =10K. To study magnetic properties of [Li0.8Fe0.2OH]FeSe system we have performed LSDA calculations for √ 5 × √ 5 superlattice and found ferromagnetism within the Li4Fe(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.
We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations ... more We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations some length scale via a (momentum dependent) ``external'' self-energy \Sigma_k. This external self-energy describes non-local dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+\Sigma_k approach is used for the numerical solution of the weakly doped one--band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem in this generalized DMFT+\Sigma_k is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W<=U (U - value of local Coulomb interaction) are conside...
Uspekhi Fizicheskih Nauk, 2012
This review discusses the generalization of dynamical mean-éeld theory (DMFT) for strongly correl... more This review discusses the generalization of dynamical mean-éeld theory (DMFT) for strongly correlated electronic systems to include additional interactions necessary for the correct description of physical effects in such systems. Speciécally, the additional interactions include: (1) the interaction of electrons with antiferromagnetic (or charge) order parameter êuctuations in high-temperature superconductors leading to the formation of a pseudogap state, (2) scattering on static disorder and its role in the general picture of the Anderson ë Hubbard metal ë insulator transition, (3) electron ë phonon interaction and the features of electronic spectra in strongly correlated systems. The proposed DMFT+S approach incorporates the above interactions by introducing into the general DMFT scheme an additional (generally momentum-dependent) self-energy S, which is calculated in a self-consistent way without violating the general structure of the DMFT iteration cycle. The paper formulates a general calculation scheme for both one-particle (spectral functions and densities of states) and two-particle (optical conductivity) properties. The problem of pseudogap formation is examined, including the Fermi arc formation and the partial destruction of the Fermi surface; the metal ë insulator transition in the disordered Anderson ë Hubbard model; and the general picture of kink formation in the electronic spectra of strongly correlated systems. A generalization of the DMFT+S approach to realistic materials with strong electron-electron correlations is presented based on the LDA+DMFT method. The general scheme of LDA+DMFT method is reviewed, as are some of its applications to real systems. The generalized LDA+DMFT+S approach is employed to calculate pseudogap states in electron-and hole-doped HTSC cuprates. Comparisons with angle-resolved photoemission spectroscopy (ARPES) results are presented.
Physical Review B, 2009
The electron-doped Pr2-xCexCuO4 (PCCO) compound in the pseudogap regime (x ≈ 0.15) was investigat... more The electron-doped Pr2-xCexCuO4 (PCCO) compound in the pseudogap regime (x ≈ 0.15) was investigated using angle-resolved photoemission spectroscopy (ARPES) and the generalized dynamical mean-field theory (DMFT) with the k-dependent self-energy (LDA+DMFT+Σ k ). Model parameters (hopping integral values and local Coulomb interaction strength) for the effective oneband Hubbard model were calculated by the local density approximation (LDA) with numerical renormalization group method (NRG) employed as an "impurity solver" in DMFT computations. An "external" k-dependent self-energy Σ k was used to describe interaction of correlated conducting electrons with short-range antiferromagnetic (AFM) pseudogap fluctuations. Both experimental and theoretical spectral functions and Fermi surfaces (FS) were obtained and compared demonstrating good semiquantitative agreement. For both experiment and theory normal state spectra of nearly optimally doped PCCO show clear evidence for a pseudogap state with AFM-like nature. Namely, folding of quasiparticle bands as well as presence of the "hot spots" and "Fermi arcs" were observed.
Physical Review B, 2005
We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations ... more We generalize the dynamical-mean field (DMFT) approximation by including into the DMFT equations some length scale ξ via a momentum dependent "external" self-energy Σ k . This external self-energy describes non-local dynamical correlations induced by short-ranged collective SDWlike antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT+Σ k approach is used for the numerical solution of the weakly doped oneband Hubbard model with repulsive Coulomb interaction on a square lattice with nearest and next nearest neighbour hopping. The effective single impurity problem in this generalized DMFT+Σ k is solved by numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) doped Mott insulator and (ii) the case of bandwidth W U (U -value of local Coulomb interaction) are considered. Densities of states, spectral functions and ARPES spectra calculated within DMFT+Σ k show a pseudogap formation near the Fermi level of the quasiparticle band.
Physica C: Superconductivity, 2007
Pseudogap phenomena are observed for normal underdoped phase of different high-T c cuprates. Amon... more Pseudogap phenomena are observed for normal underdoped phase of different high-T c cuprates. Among others Bi 2 Sr 2 CaCu 2 O 8Àd (Bi2212) compound is one of the most studied experimentally [A.
Low Temperature Physics, 2006
Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize th... more Pseudogap physics in strongly correlated systems is essentially scale dependent. We generalize the dynamical mean-field theory (DMFT) by introducing into the DMFT equations dependence on the correlation length of pseudogap fluctuations via an additional (momentum-dependent) self-energy Σk. This self-energy describes nonlocal dynamical correlations induced by short-ranged collective SDW-like antiferromagnetic spin (or CDW-like charge) fluctuations. At high enough temperatures these fluctuations can be viewed as a quenched Gaussian random field with finite correlation length. This generalized DMFT +Σk approach is used for the numerical solution of the weakly doped one-band Hubbard model with repulsive Coulomb interaction on a square lattice with nearest- and next-nearest-neighbor hopping. The effective single impurity problem is solved by the numerical renormalization group (NRG). Both types of strongly correlated metals, namely (i) the doped Mott insulator and (ii) the case of bandwi...
Journal of Physics and Chemistry of Solids, 2008
Pseudogap regime for the prototype high-T c compounds hole doped Bi 2 Sr 2 CaCu 2 O 8-x (Bi2212) ... more Pseudogap regime for the prototype high-T c compounds hole doped Bi 2 Sr 2 CaCu 2 O 8-x (Bi2212) and electron doped Nd 2-x Ce x CuO 4 (NCCO) is described by means of novel generalized LDA+DMFT+Σ k approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Σ k . In the present case Σ k describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO 2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible "hot-spots" for NCCO while in Bi2212 there is just rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.
Journal of Physics and Chemistry of Solids, 2011
We discuss interaction of strongly correlated electrons (described within the Hubbard model solve... more We discuss interaction of strongly correlated electrons (described within the Hubbard model solved by dynamical mean-field theory (DMFT)) with Debye and Einstein phonons using recently developed DMFT+Σ computational scheme. Electron-phonon interaction (EPI) is analyzed in adiabatic approximation (assuming the validity of Migdal theorem), allowing the neglect of EPI vertex corrections. This approach is valid for EPI coupling constant λ < ε F /ω ph ∼ 10, where ε F is Fermi energy and ω ph is Debye or Einstein frequency. For moderate values of λ only small changes in the electronic density of states are observed in DMFT+Σ approximation for both weakly and strongly correlated metallic regimes. Metal-insulator (Mott) transition due to the increase of Hubbard interaction U is slightly inhibited by EPI. Our main aim is to discuss the interplay of "kinks" in electronic dispersion due to EPI and recently discovered kinks of electronic origin. For the certain region of model parameters coexistence of phonon "kinks" in electronic dispersion with purely electronic "kinks" is readily observed and we formulate some simple criteria of such coexistence. However, for most general combinations of model parameters phonon "kinks" make electronic "kinks" hardly observable. In the general case an increase of Hubbard interaction U rapidly suppresses the slope of electronic dispersion within the phonon "kink". These results are important for deeper understanding of the shape and evolution of electronic dispersions in strongly correlated systems such as copper oxides, where different kinds of "kinks" were recently observed in ARPES experiments.
Journal of Physics and Chemistry of Solids, 2011
LDA+DMFT+Σ k approach was applied to describe pseudogap phase of several prototype high-Tc compou... more LDA+DMFT+Σ k approach was applied to describe pseudogap phase of several prototype high-Tc compounds e.g. hole doped Bi 2 Sr 2 CaCu 2 O 8-δ (Bi2212) and La 2-x Sr x CuO 4 (LSCO) systems and electron doped Nd 2-x Ce x CuO 4 (NCCO) and Pr 2-x Ce x CuO 4 (PCCO), demonstrating qualitative difference of the Fermi surfaces (FS) for these systems. Namely for Bi2212 and LSCO the so called "hot-spots" (intersection of a bare FS and AFM Brillouin zone (BZ) boundary), where scattering on pseudogap fluctuations is most intensive were not observed. Instead here we have Fermi arcs with smeared FS close to the BZ boundary. However for NCCO and PCCO "hot-spots" are clearly visible. This qualitative difference is shown to have material specific origin. Good agreement with known ARPES data was demonstrated not only for FS maps but also for spectral function maps (quasiparticle bands including lifetime and interaction broadening).
Journal of Magnetism and Magnetic Materials, 2012
We present a brief review of the present day situation with studies of high-temperature supercond... more We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with T c > 30 K in A x Fe 2-x/2 Se 2 (A=K,Cs,Tl,) represents the major new step in the development of new concepts in the physics of Fe -based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122 -superconductors like BaFe 2 As 2 . It appears that electronic structure of new superconductors is rather different from that of FeAs 122 -systems. In particular, no nesting properties of electron and hole -like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the Γ point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (T N > 500 K), much exceeding superconducting T c makes these systems unique antiferromagnetic superconductors with highest T N observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature T c of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs -superconductors and in new FeSe -systems the value of T c correlates with the value of the total density of states (DOS) at the Fermi level.
Journal of Experimental and Theoretical Physics, 2013
We discuss the recently proposed LDA ′ +DMFT approach providing consistent parameter free treatme... more 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 exchangecorrelation 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 ′ +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.
Journal of Experimental and Theoretical Physics, 2013
We present detailed LDA ′ +DMFT investigation of doping dependence of correlation effects in nove... more We present detailed LDA ′ +DMFT investigation of doping dependence of correlation effects in novel K1-xFe2-ySe2 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 K0.76Fe1.72Se2 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 K1-xFe2-ySe2 system. Moreover in contrast to typical pnictides K1-xFe2-ySe2 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.
Journal of Experimental and Theoretical Physics, 2000
We analyze superconducting state (both s and d -wave) in a simple exactly solvable model of pseud... more We analyze superconducting state (both s and d -wave) in a simple exactly solvable model of pseudogap state, induced by short -range order fluctuations (e.g. antiferromagnetic), which is based upon model Fermi -surface with "hot patches". It is shown that superconducting energy gap averaged over these fluctuations is non -zero even for the temperatures larger than mean -field T c of superconducting transition in a sample as a whole. For temperatures T > T c superconductivity apparently exists within separate regions ("drops"). We study the spectral density and the density of states and demonstrate that superconductivity signals itself in these already for T > T c , while at T c itself nothing special happens from this point of view. These anomalies are in qualitative agreement with a number experiments on underdoped cuprates.
Journal of Experimental and Theoretical Physics, 2008
Material specific electronic band structure of the electron-doped high-Tc cuprate Nd1.85Ce0.15CuO... more Material specific electronic band structure of the electron-doped high-Tc cuprate Nd1.85Ce0.15CuO4 (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Σ k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Σ k , which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Σ k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi2Sr2CaCu2O 8-δ (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations ξ is larger for NCCO; (iii) pseudogap potential ∆ is stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Σ k and compared with experimental results, demonstrating satisfactory agreement.
Journal of Experimental and Theoretical Physics, 2002
We analyze the anomalies of superconducting state within a simple exactly solvable model of the p... more We analyze the anomalies of superconducting state within a simple exactly solvable model of the pseudogap state, induced by fluctuations of "dielectric" short range order, for the model of the Fermi surface with "hot" patches. The analysis is performed for the arbitrary values of the correlation length ξ corr of this short range order. It is shown that superconducting energy gap averaged over these fluctuations is non zero in a wide temperature range above T c -the temperature of homogeneous superconducting transition. This follows from the absence of self averaging of the gap over the random field of fluctuations. For temperatures T > T c superconductivity apparently appears in separate regions of space ("drops"). These effects become weaker for shorter correlation lengths ξ corr and the region of "drops" on the phase diagram becomes narrower and disappears for ξ corr → 0, however, for the finite values of ξ corr the complete self averaging is absent.
Journal of Experimental and Theoretical Physics, 2006
We analyze non -Fermi liquid (NFL) behavior of fluctuating gap model (FGM) of pseudogap behavior ... more We analyze non -Fermi liquid (NFL) behavior of fluctuating gap model (FGM) of pseudogap behavior in both 1D and 2D. We discuss in detail quasiparticle renormalization (Z -factor), demonstrating a kind of "marginal" Fermi liquid or Luttinger liquid behavior and topological stability of the "bare" Fermi surface (Luttinger theorem). In 2D case we discuss effective picture of Fermi surface "destruction" both in "hot spots" model of dielectric (AFM, CDW) pseudogap fluctuations, as well as for qualitatively different case of superconducting d -wave fluctuations, reflecting NFL spectral density behavior and similar to that observed in ARPES experiments on copper oxides.
Journal of Experimental and Theoretical Physics, 2012
We present theoretical study of electronic structure (spectral functions and Fermi surfaces) for ... more We present theoretical study of electronic structure (spectral functions and Fermi surfaces) for incommensurate pseudogap and charge density wave (CDW) and commensurate CDW phases of quasi two dimensional diselenides 2H-TaSe 2 and 2H-NbSe 2 . Incommensurate pseudogap regime is described within the scenario based on short-range order CDW fluctuations, considered within the static Gaussian random field model. In contrast e.g. to high-T c cuprates layered dichalcogenides have several different CDW scattering vectors and electronic spectrum with two bands at the Fermi level. To this end we present theoretical background for the description of multiple scattering processes within multiple bands electronic spectrum. Thus obtained theoretical spectral functions and Fermi surfaces are compared with recent ARPES experimental data, demonstrating rather good qualitative agreement.
Journal of Experimental and Theoretical Physics, 2010
Slightly underdoped high-Tc system La1.86Sr0.14CuO4 (LSCO) is studied by means of high energy hig... more Slightly underdoped high-Tc system La1.86Sr0.14CuO4 (LSCO) is studied by means of high energy high resolution angular resolved photoemission spectroscopy (ARPES) and combined computational scheme LDA+DMFT+Σ k . Corresponding one band Hubbard model is solved via dynamical meanfield theory (DMFT), while model parameters needed are obtained from first principles within local density approximation (LDA). An "external" k-dependent self-energy Σ k describes interaction of correlated electrons with antiferromagnetic (AFM) pseudogap fluctuations. Experimental and theoretical data clearly show "destruction" of the LSCO Fermi surface in the vicinity of the (π,0) point and formation of "Fermi arcs" in the nodal directions. ARPES energy distribution curves (EDC) as well as momentum distribution curves (MDC) demonstrate deviation of the quasiparticle band from the Fermi level around (π,0) point. The same behavior of spectral functions follows from theoretical calculations suggesting AFM origin of the pseudogap state.
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Papers by Michael Sadovskii