arXiv subjects

I. Eremin

Publications and source records attributed to I. Eremin.

68 records · Page 4Linked to original sources

arXiv2004

Fermi-liquid based theory for the in-plane magnetic anisotropy in untwinned high-T$_c$ superconductors

Using a generalized RPA-type theory we calculate the in-plane anisotropy of the magnetic excitations in hole-doped high-$T_c$ superconductors. Extending our earlier Fermi-liquid based studies on the resonance peak by inclusion of orthorhombicity we still find two-dimensional spin excitations, however, being strongly anisotropic. This reflects the underlying anisotropy of the hopping matrix elements and of the resultant superconducting gap function. We compare our calculations with new experimental data on {\it fully untwinned} ${YBa}_2{Cu}_3{O}_{6.85}$ and find good agreement. Our results are in contrast to earlier interpretations on the in-plane anisotropy in terms of stripes (H. Mook {\it et al.}, Nature {\bf 404}, 729 (2000)), but reveal a conventional solution to this important problem.

cond-mat.supr-con↗

arXiv2004

Possible isotope effect on the resonance peak formation in high-T$_c$ cuprates

Starting from the three-band $p-d$ Hubbard Hamiltonian we derive an effective $t-J$ model including electron-phonon interaction of quasiparticles with optical phonons. Within the effective Hamiltonian we analyze the influence of electronic correlations and electron-phonon interaction on the dynamical spin susceptibility in layered cuprates. We find a huge isotope effect on the resonance peak in the magnetic spin susceptibility, ${Im}χ({\bf q},ω)$, seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account beyond random phase approximation(RPA) scheme. We find at optimal doping the isotope coeffiecient $α_{res} \approx 0.35$ which can be further tested experimentally.

cond-mat.supr-con↗

arXiv2004

Unconventional superconductivity and magnetism in Sr$_2$RuO$_4$ and related materials

We review the normal and superconducting state properties of the unconventional triplet superconductor Sr$_2$RuO$_4$ with an emphasis on the analysis of the magnetic susceptibility and the role played by strong electronic correlations. In particular, we show that the magnetic activity arises from the itinerant electrons in the Ru $d$-orbitals and a strong magnetic anisotropy occurs ($χ^{+-} < χ^{zz}$) due to spin-orbit coupling. The latter results mainly from different values of the $g$-factor for the transverse and longitudinal components of the spin susceptibility (i.e. the matrix elements differ). Most importantly, this anisotropy and the presence of incommensurate antiferromagnetic and ferromagnetic fluctuations have strong consequences for the symmetry of the superconducting order parameter. In particular, reviewing spin fluctuation-induced Cooper-pairing scenario in application to Sr$_2$RuO$_4$ we show how p-wave Cooper-pairing with line nodes between neighboring RuO$_2$-planes may occur. We also discuss the open issues in Sr$_2$RuO$_4$ like the influence of magnetic and non-magnetic impurities on the superconducting and normal state of Sr$_2$RuO$_4$. It is clear that the physics of triplet superconductivity in Sr$_2$RuO$_4$ is still far from being understood completely and remains to be analyzed more in more detail. It is of interest to apply the theory also to superconductivity in heavy-fermion systems exhibiting spin fluctuations.

cond-mat.supr-con↗

arXiv2003

Evidence for the formation of magnetic moments in the cuprate superconductor Hg$_{0.8}$Cu$_{0.2}$Ba$_2$Ca$_2$Cu$_3$O$_{8+δ}$ below $T_c$ seen by NQR

We report pure zero field nuclear magnetic resonance (NQR) measurements on the optimally doped three layer high-$ T_{c} $-compounds HgBaCaCuO and HgBaCaCuO(F) with $T_c$ 134 K. Above $ T_{c}$ two Cu NQR line pairs are observed in the spectra corresponding to the two inequivalent Cu lattice sites. Below $ T_{c}$ the Cu NQR spectra show additional lines leading to the extreme broadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. The spin-lattice relaxation curves follow a triple exponential function with coefficients depend onto the saturation time (number of saturation pulses), whereas the spin-spin relaxation curve is described by a single exponential function. From the spin-lattice relaxation we deduced a complete removal of the Kramers degeneracy of the Cu quadrupole indicating that the additional lines are due to a Zeemann splitting of the $^{63/65}$Cu lines due to the spontaneous formation of magnetic moments within the CuO layers. Below 140 K, the spectra are well fitted by a number of 6 $^{63/65}$Cu line pairs. From the number of the Cu lines, the position of the lines relative to each other and the complete removal of the Kramers degeneracy we deduced an orientation of the magnetic moments parallel to the symmetry axis of the electric field gradient tensor with magnitudes of the order of 1000 G. We also discuss the possible microscopic origin of the observed internal magnetic fields.

cond-mat.supr-con↗

arXiv2003

Renormalization of the elementary excitations in hole- and electron-doped cuprates due to spin fluctuations

Extending our previous studies we present results for the doping-, momentum-, frequency-, and temperature- dependence of the kink-like change of the quasiparticle velocity resulting from the coupling to spin fluctuations. In the nodal direction a kink is found in both the normal and superconducting state while in the antinodal direction a kink occurs only below $T_c$ due to the opening of the superconducting gap. A pronounced kink is obtained only for hole-doped, but not for electron-doped cuprates and is characteristically different from what is expected due to electron-phonon interaction. We further demonstrate that the kink structure is intimately connected to the resonance peak seen in inelastic neutron scattering. Our results suggest similar effects in other unconventional superconductors like ${Sr}_2{RuO}_4$.

cond-mat.supr-con↗

arXiv2002

Electronic theory for the normal state spin dynamics in Sr$_2$RuO$_4$: anisotropy due to spin-orbit coupling

Using a three-band Hubbard Hamiltonian we calculate within the random-phase-approximation the spin susceptibility, $χ({\bf q},ω)$, and NMR spin-lattice relaxation rate, 1/T$_1$, in the normal state of the triplet superconductor Sr$_2$RuO$_4$ and obtain quantitative agreement with experimental data. Most importantly, we find that due to spin-orbit coupling the out-of-plane component of the spin susceptibility $χ^{zz}$ becomes at low temperatures two times larger than the in-plane one. As a consequence strong incommensurate antiferromagnetic fluctuations of the quasi-one-dimensional $xz$- and $yz$-bands point into the $z$-direction. Our results provide further evidence for the importance of spin fluctuations for triplet superconductivity in Sr$_2$RuO$_4$.

cond-mat.supr-con↗

arXiv2002

Theory for phonon-induced superconductivity in MgB$_2$

We analyze superonductivity in MgB$_2$ observed below $T_c=39$ K resulting from electron-phonon coupling involving a mode at $\hbar ω_1 = 24$ meV and most importantly the in-plane B-B $E_{2g}$ vibration at $\hbar ω_2=67$ meV. The quasiparticles originating from $π$- and $σ$-states couple strongly to the low-frequency mode and the $E_{2g}$-vibrations respectively. Using two-band Eliashberg theory, $λ_π = 1.4$ and $λ_σ = 0.7$, we calculate the gap functions $Δ^{i}(ω,0)$ ($i=π$, $σ$). Our results provide an explanation of recent tunneling experiments. We get $H^{ab}_{c_2}/H^{c}_{c_2} \approx 3.9$.

cond-mat.supr-con↗

arXiv2001

Network patterns and strength of orbital currents in layered cuprates

In a frame of the $t-J-G$ model we derive the microscopical expression for the circulating orbital currents in layered cuprates using the anomalous correlation functions. In agreement with $μ$-on spin relaxation ($μ$SR), nuclear quadrupolar resonance (NQR) and inelastic neutron scattering(INS) experiments in YBa$_2$Cu$_3$O$_{6+x}$ we successfully explain the order of magnitude and the monotonous increase of the {\it internal} magnetic fields resulting from these currents upon cooling. However, the jump in the intensity of the magnetic fields at T$_c$ reported recently seems to indicate a non-mean-field feature in the coexistence of current and superconducting states and the deviation of the extended charge density wave vector instability from its commensurate value {\bf Q}$\approx(π,π$) in accordance with the reported topology of the Fermi surface.

cond-mat.supr-con↗

arXiv2001

Analysis of the elementary excitations in high-T_c cuprates: explanation of the new energy scale observed by ARPES

Using the Hubbard Hamiltonian we analyze the energy- and momentum-dependence of the elementary excitations in high-T$_c$ superconductors resulting from the coupling to spin fluctuations. As a result of the energy dependence of the self-energy $Σ({\bf k}, ω)$, characteristic features occur in the spectral density explaining the 'kink' in recent ARPES experiments. We present results for the spectral density $A({\bf k}, ω)$ resulting from the crossover from Im $Σ({\bf k}, ω)\propto ω$ to Im $Σ({\bf k}, ω)\propto ω^2$, for the feedback of superconductivity on the excitations, and for the superconducting order parameter $Δ({\bf k}, ω)$. These results relate also to inelastic neutron scattering and tunneling experiments and shed important light on the essential ingredients a theory of the elementary excitations in the cuprates must contain.

cond-mat.supr-con↗

arXiv2001

Electronic theory for superconductivity in Sr$_2$RuO$_4$: triplet pairing due to spin-fluctuation exchange

Using a two-dimensional Hubbard Hamiltonian for the three electronic bands crossing the Fermi level in Sr$_2$RuO$_4$ we calculate the band structure and spin susceptibility $χ({\bf q}, ω)$ in quantitative agreement with nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) experiments. The susceptibility has two peaks at {\bf Q}$_i = (2π/3, 2π/3)$ due to the nesting Fermi surface properties and at {\bf q}$_i = (0.6π, 0)$ due to the tendency towards ferromagnetism. Applying spin-fluctuation exchange theory as in layered cuprates we determine from $χ({\bf q}, ω)$, electronic dispersions, and Fermi surface topology that superconductivity in Sr$_2$RuO$_4$ consists of triplet pairing. Combining the Fermi surface topology and the results for $χ({\bf q}, ω)$ we can exclude $s-$ and $d-$wave symmetry for the superconducting order parameter. Furthermore, within our analysis and approximations we find that $f$-wave symmetry is slightly favored over p-wave symmetry due to the nesting properties of the Fermi surface.

cond-mat.supr-con↗

arXiv2001

Dynamical charge susceptibility in layered cuprates: the influence of screened inter-site Coulomb repulsion

The analytical expression for dynamical charge susceptibility in layered cuprates has been derived in the frame of singlet-correlated band model beyond random-phase-approximation (RPA) scheme. Our calculations performed near optimal doping regime show that there is a peak in real part of the charge susceptibility $χ({\bf q},ω)$ at {\bf Q} = ($π$, $π$) at strong enough inter-site Coulomb repulsion. Together with the strong maximum in the Im $χ({\bf Q},ω)$ at 15 meV it confirms the formation of low-energetic plasmons or charge fluctuations. This provides a jsutification that these excitations are important and together with a spin flcutuations can contribute to the Cooper pairing in layered cuprates. Analysing the charge susceptibilitiy with respect to an instability we obtain a new plasmon branch, $ω_{\bf q}$, along the Brillouin Zone. In particular, we have found that it goes to zero near {\bf Q}$_{CDW} \approx (2π/3, 2π/3)$.

cond-mat.supr-con↗

arXiv2000

Analysis of the resonance peak and magnetic coherence seen in inelastic neutron scattering of cuprate superconductors: a consistent picture with tunneling and conductivity data

Assuming the exchange of antiferromagnetic spin fluctuations as the Cooper pairing mechanism we calculate the doping dependence of the resonance peak seen in inelastic neutron scattering and the magnetic coherence effect. Most importantly, we find that the resonance peak in the magnetic susceptibility, ${Im}χ({\bf q},ω)$, appears {\it only} in the superconducting state, that it scales with $T_c$, and that magnetic coherence is a result of a d-wave order parameter. We further analyze the structure of ${Im}χ$ below $T_c$, the position of the peak at $ω_{res}$ and the consequences for photoemission, tunneling spectroscopy and the optical conductivity.

cond-mat.supr-con↗

arXiv2000

Theory for Electron-Doped Cuprate Superconductors: d-wave symmetry order parameter

Using as a model the Hubbard Hamiltonian we determine various basic properties of electron-doped cuprate superconductors like ${Nd}_{2-x}{Ce}_{x}{CuO}_{4}$ and ${Pr}_{2-x}{Ce}_{x}{CuO}_{4}$ for a spin-fluctuation-induced pairing mechanism. Most importantly we find a narrow range of superconductivity and like for hole-doped cuprates $d_{x^{2}-y^{2}}$ - symmetry for the superconducting order parameter. The superconducting transition temperatures $T_{c}(x)$ for various electron doping concentrations $x$ are calculated to be much smaller than for hole-doped cuprates due to the different Fermi surface and a flat band well below the Fermi level. Lattice disorder may sensitively distort the symmetry $d_{x^{2}-y^{2}}$ via electron-phonon interaction.

cond-mat.supr-con↗

arXiv1999

How large can be SDW and CDW apmlitudes in underdoped cuprates

Self-consistent calculation of spin (charge) density wave order (SDW/CDW) parameters have been performed for bilayered cuprates on the basis of a singlet correlated band model. Evolution of the Fermi surface in the strongly underdoped regime is described by using a two-band approach. The smooth development of the pseudogap formation temperature is explained from underdoped to overdoped states and the Fourier amplitudes (spin) and (charge) modulations have been calculated. We have found a maximum of the incommensurability for doping 0.09-0.11 holes per copper site.

cond-mat.supr-con↗