arXiv subjects

M. L. Kulic

Publications and source records attributed to M. L. Kulic.

14 recordsLinked to original sources

arXiv2016

The electron-phonon interaction with forward scattering peak is a relevant approach to high Tc superconductivity in FeSe films on SrTiO3 and TiO2

The theory of EPI with strong forward scattering peak (FSP)[1-3] is recently applied in [4-5] in studying high Tc(~100 K) in a FeSe grown on SrTiO3 [4] and TiO2 [6]. The EPI is due to a long-range dipolar electric field created by the high-energy oxygen vibrations (E0~90 meV) at the interface [4-5]. Besides obtaining new results we also correct some misleading results from the recent literature. We show that the mean-field critical temperature Tc0 is an interplay between the maximal pairing potential and the FSP-width qc. For Tc0~100 K the gap (G) is G~16 meV in agreement with ARPES experiments. We find that in leading order Tc0 is mass-independent and there is small oxygen isotope effect in next to leading order. In clean systems Tc0 for s-wave and d-wave pairing is degenerate but both are affected by non-magnetic impurities. The non-magnetic impurities are pair-weakening in the s-channel and pair-breaking in the d-channel. The normal state self-energy at the Fermi surface gives rise to the ARPES quasiparticle band at E=0 and to a replica band at Er=-E0(1+l(m))^1/2, respectively. The EPI coupling l(m), which enters the self-energy, is mass-dependent - the fact overlooked in the literature, makes at low energies the slope of the self-energy mass-dependent. The smallness of the oxygen isotope effect in Tc0 and its presence in the self-energy in FeSe films on SrTiO3 and TiO2 is a smoking-gun experiment for the application of the EPI-FSP theory to these systems. The EPI-FSP theory predicts a large number of low-laying pairing states (above the ground state) thus causing internal pair fluctuations. The latter reduce Tc0 additionally, by creating a pseudogap state for Tc<T<Tc0. Possibilities to increase Tc0 by designing novel structures are discussed in the framework of the EPI-FSP theory.

cond-mat.supr-con↗

arXiv2010

Time of life as it is in LiFeAs

The time of life of fermionic quasiparticles, the distribution of which in the momentum-energy space can be measured by angle resolved photoemission (ARPES), is the first quantity to look for fingerprints of interaction responsible for the superconducting pairing. Such an approach has been recently used for superconducting cuprates, but its direct application to pnictides was not possible due to essential three-dimensionality of the electronic band structure and magnetic ordering. Here, we report the investigation of the quasiparticle lifetime in LiFeAs, a non-magnetic stoichiometric superconductor with a well separated two-dimensional band. We have found two energy scales: the lower one contains clear fingerprints of optical phonon modes while the higher scale indicates a presence of strong electron-electron interaction. The result suggests that LiFeAs is a phonon mediated superconductor with strongly enhanced electronic density of states at the Fermi level.

cond-mat.supr-con↗

arXiv2010

Bosonic Spectral Function and The Electron-Phonon Interaction in HTSC Cuprates

In Part I we discuss accumulating experimental evidence related to the structure and origin of the bosonic spectral function in high-temperature superconducting (HTSC) cuprates at and near optimal doping. Some global properties of the spectral function, such as number and positions of peaks, are extracted by combining optics, neutron scattering, ARPES and tunnelling measurements. These methods give convincing evidence for strong electron-phonon interaction (EPI) with the coupling constant between 1-3 in cuprates near optimal doping. Here we clarify how these results are in favor of the Eliashberg-like theory for HTSC cuprates near optimal doping. In Part II we discuss some theoretical ingredients - such as strong EPI, strong correlations - which are necessary to explain the experimental results related to the mechanism of d-wave pairing in optimally doped cuprates. These comprise the Migdal-Eliashberg theory for EPI in strongly correlated systems which give rise to the forward scattering peak. The latter is further supported by the weakly screened Madelung interaction in the ionic-metallic structure of layered cuprates. In this approach EPI is responsible for the strength of pairing while the residual Coulomb interaction (by including spin fluctuations) triggers the d-wave pairing.

cond-mat.supr-con↗

arXiv2008

Conventional Superconductivity in Fe-Based Pnictides: the Relevance of Intra-Band Electron-Boson Scattering

Various recent experimental data and especially the large Fe-isotope effect point against unconventional pairings, since the large intra-band impurity scattering is strongly pair-breaking for them. The strength of the inter-band impurity scattering in some single crystals may be strong and probably beyond the Born scattering limit. In that case the proposed s(+-) pairing (hole(h)- and electron(el)-gaps are of opposite signs) is suppressed but possibly not completely destroyed. The data imply that the intra-band pairing in the h- and in the el-band, which are inevitably due to some nonmagnetic el-boson interaction (EBI), must be taken into account. EBI is either due to phonons (EPI) or possibly due to excitons (EEI), or both are simultaneously operative. We discuss their interplay briefly. The large Fe-isotope effect favors the EPI and the s(+) pairing (the h- and el-gaps are in-phase).

cond-mat.supr-con↗

arXiv2008

Bosonic Spectral Function in HTSC Cuprates: Part I - Experimental Evidence for Strong Electron-Phonon Interaction

In Part I we discuss accumulating experimental evidence related to the structure and origin of the bosonic spectral function α^{2}F in high-temperature superconducting (HTSC) cuprates near optimal doping. Some global properties of α^{2}F such as number and positions of peaks, are extracted by combining optics, neutron scattering, ARPES and tunnelling measurements. These methods give convincing evidence for strong electron-phonon interaction (EPI) with 1<λ\lesssim 3 in cuprates near optimal doping. Here we clarify how these results are in favor of the Eliashberg-like theory for HTSC cuprates near optimal doping.We argue that the neglect of EPI in some previous studies of HTSC was based on a number of deceptive prejudices related to the strength of EPI, on some physical misconceptions and misleading interpretation of experimental results.

cond-mat.supr-con↗

arXiv2004

The "unusual" isotope shift in high-temperature superconductors can be explained by the usual theory of the electron-phonon interaction

We show that recent ARPES results on the "unusual" oxygen isotope shift in the real part of the self-energy in the optimally doped $Bi2212$ samples can be qualitatively (and semi-quantitatively) explained by the theory of the electron-phonon interaction (EPI) elaborated few decades ago. However, for a quantitative analysis of the ARPES spectra it is necessary to know the momentum dependence of the EPI, the Coulomb contribution at high energies and the background due to impurities and defects.

cond-mat.supr-con↗

arXiv2004

ARPES kink is a "smoking gun" for the theory of high-Tc superconductors: dominance of the electron-phonon interaction with forward scattering peak

The ARPES spectra in high-Tc superconductors show four distinctive features in the quasiparticle self-energy. All of them can be explained consistently by the theory in which the electron phonon interaction (EPI) with the forward scattering peak dominates over the Coulomb scattering. In particular, this theory explains why there is no shift of the nodal kink at 70 meV in the superconducting state, contrary to the clear shift of the anti-nodal singularity at 40 meV. The theory predicts a ``knee''-like structure of the imaginary part of the self-energy, which is phonon dominated for $ω\approx ω^{(70)}_{ph}$, and shows linear behavior for $ω> ω^{(70)}_{ph}$ - due to the Coulomb scattering. Recent ARPES spectra give that the EPI coupling constant is much larger than the Coulomb one. The dip-hump structure in the spectral function comes out naturally from the proposed theory.

cond-mat.supr-con↗

arXiv2003

Why is the ARPES anti-nodal singularity at 40 meV shifted in superconducting state of HTSC, but the kink at 70 meV is not?

The theoretical model for the quasiparticle self-energy in HTSC is proposed, which is based on the forward scattering peak in the electron-phonon (EPI) interaction. By assuming that EPI dominates, the model explains qualitatively and in a consistent way the recent ARPES results. The latter show a kink in the normal state quasiparticle energy at 70 meV in the nodal direction, which is (surprisingly) not shifted in the superconducting (SC) state, while the singularity at 40 meV in the anti-nodal direction is shifted by the SC gap. The model predicts a dip-hump structure in the spectral function, which is observed in ARPES.

cond-mat.supr-con↗

arXiv2001

Optical Properties of Heavy Fermion Systems with SDW Order

The dynamical conductivity $σ(ω)$, reflectivity $R(ω)$, and tunneling density of states $N(ω)$ of strongly correlated systems (like heavy fermions) with a spin-density wave (SDW) magnetic order are studied as a function of impurity scattering rate and temperature. The theory is generalized to include strong coupling effects in the SDW order. The results are discussed in the light of optical experiments on heavy-fermion SDW materials. With some modifications the proposed theory is applicable also to heavy fermions with localized antiferromagnetic (LAF) order.

cond-mat.supr-con↗

arXiv1998

Forward Electron-Phonon Scattering in Normal and Superconducting States

The sharp forward electron-phonon $(FEP)$ and impurity $(FIS)$ scattering change the normal and superconducting properties significantly. The pseudo-gap like features are present in the density of states for $ω<Ω$, where $Ω$ is the phonon frequency. The superconducting critical temperature $T_c$, due to the $FEP$ pairing, is linear with respect to the electron-phonon coupling constant. The $FIS$ impurities are pair weakening for $s-$ and $d-wave$ pairing.

cond-mat.supr-con↗

arXiv1997

Normal and superconducting state in the presence of forward electron-phonon and impurity scattering

Impurities with the pronounced forward scattering (FS impurities) change analytical properties of the quasiparticle Green's function substantially compared to the isotropic scattering. By assuming that the superconducting pairing is due to the forward E-P scattering (FEP pairing) it is shown that the critical temperature of clean systems T_{c0} depends linearly on the E-P coupling constant λand the isotope effect αis small. The FS impurities make α=1/2 in the dirty limit and affect in the same way the s- and d-wave FEP pairing. The FS impurities are pair-weakening in both pairing channels. The usual isotropic impurity scattering is pair-weakening for the s-wave and pair-breaking for the d-wave FEP pairing.

cond-mat.supr-con↗

arXiv1997

Magnetism and Superconductivity in (RE)Ni2B2C: The Case of TmNi2B2C

The recently reported coexistence of an oscillatory magnetic order with the wave vector Q=0.241 Å^{-1} and superconductivity in TmNi2B2C is analyzed theoretically. It is shown that the oscillatory magnetic order and superconductivity interact predominantly via the exchange interaction between localized moments (LM's) and conduction electrons, while the electromagnetic interaction between them is negligible. In the coexistence phase of the clean TmNi2B2C the quasiparticle spectrum should have a line of zeros at the Fermi surface, giving rise to the power law behavior of thermodynamic and transport properties. Two scenarios of the origin of the oscillatory magnetic order in TmNi2B2C are analyzed: a) due to superconductivity and b) independently on superconductivity. Experiments in magnetic field are proposed in order to choose between them.

cond-mat.supr-con↗

arXiv1997

Theory of Interplay of Nuclear Magnetism and Superconductivity in AuIn2

The recently reported coexistence of a magnetic order, with the critical temperature T_M=35 μ*K, and superconductivity, with the critical temperature T_S=207 m*K, in AuIn_2 is studied theoretically. It is shown that superconducting (S) electrons and localized nuclear magnetic moments (LM's) interact dominantly via the contact hyperfine (EX) interaction, giving rise to a spiral (or domain-like) magnetic order in superconducting phase. The electromagnetic interaction between LM's and S electrons is small compared to the EX one giving minor contribution to the formation of the oscillatory magnetic order. In clean samples (l>ξ_0) of AuIn$_2$ the oscillatory magnetic order should produce a line of nodes in the quasiparticle spectrum of S electrons giving rise to the power law behavior. The critical field H_c(T=0) in the coexistence phase is reduced by factor two with respect to its bare value.

cond-mat.supr-con↗

arXiv1997

Why is d-wave pairing in HTS robust in the presence of impurities?

In the recent theory of strong correlations by Kulic and Zeyher it has been shown that by lowering doping concentration a forward peak in the charge scattering channel is developed. Accordingly, near the optimal doping the nonmagnetic scattering is pronounced in the d-channel and its effect on d-wave pairing is reduced. As a consequence, d-wave pairing is robust against defects and impurities, the order parameter keeps its d-wave shape for any scattering rate and the density of states becomes finite at the Fermi surface. For large doping scattering anisotropy parameter is small and d-wave loses its robustness. The theory is generally formulated for the bi-layer model by including: 1) intra- and inter-plane pairing; 2) intra- and inter-plane impurities.

cond-mat.supr-con↗