arXiv subjects

D. Manske

Publications and source records attributed to D. Manske.

31 records · Page 2Linked to original sources

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

Phase diagram of underdoped cuprate superconductors: effect of Cooper-pair phase fluctuations

In underdoped cuprates fluctuations of the phase of the superconducting order parameter play a role due to the small superfluid density. We consider the effects of phase fluctuations assuming the exchange of spin fluctuations to be the predominant pairing interaction. Spin fluctuations are treated in the fluctuation-exchange approximation, while phase fluctuations are included by Berezinskii-Kosterlitz-Thouless theory. We calculate the stiffness against phase fluctuations, ns(omega)/m*, as a function of doping, temperature, and frequency, taking its renormalization by phase fluctuations into account. The results are compared with recent measurements of the high-frequency conductivity. Furthermore, we obtain the temperature T*, where the density of states at the Fermi energy starts to be suppressed, the temperature Tc*, where Cooper pairs form, and the superconducting transition temperature Tc, where their phase becomes coherent. We find a crossover from a phase-fluctuation-dominated regime with Tc proportinal to ns for underdoped cuprates to a BCS-like regime for overdoped materials.

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

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↗

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

Theoretical analysis of Cooper-pair phase fluctuations in underdoped cuprates: a spin-fluctuation exchange study

We study Cooper-pair phase fluctuations in cuprate superconductors for a spin fluctuation pairing interaction. Using an electronic theory we calculate in particular for the underdoped cuprate superconductors the superfluid density $n_s(T)$, the superconducting transition temperature $T_c(x) \propto n_s$ below which phase coherent Cooper-pairs occur, and $T_c^{*}(x)$ where the phase incoherent Cooper-pairs disappear. Also we present results for the penetration depth $λ(x,T)$ and for the weak pseudogap temperature $T^{*}(x)$ at which a gap structure occurs in the spectral density. A Meißner effect is obtained only for $T < T_c$. We find that phase fluctuations become increasingly important in the underdoped regime and lead to a reduction of $T_c$ in good agreement with the experimental situation.

cond-mat.supr-con↗

arXiv1999

Effect of a Normal-State Pseudogap on Optical Conductivity in Underdoped Cuprate Superconductors

We calculate the c-axis infrared conductivity $σ_c(ω)$ in underdoped cuprate superconductors for spinfluctuation exchange scattering within the CuO$_2$-planes including a phenomenological d-wave pseudogap of amplitude $E_g$. For temperatures decreasing below a temperature $T^* \sim E_g/2$, a gap for $ω< 2E_g$ develops in $σ_c(ω)$ in the incoherent (diffuse) transmission limit. The resistivity shows 'semiconducting' behavior, i.e. it increases for low temperatures above the constant behavior for $E_g=0$. We find that the pseudogap structure in the in-plane optical conductivity is about twice as big as in the interplane conductivity $σ_c(ω)$, in qualitative agreement with experiment. This is a consequence of the fact that the spinfluctuation exchange interaction is suppressed at low frequencies as a result of the opening of the pseudogap. While the c-axis conductivity in the underdoped regime is described best by incoherent transmission, in the overdoped regime coherent conductance gives a better description.

cond-mat.supr-con↗

arXiv1999

Theory for underdoped high-T_c superconductors: effects of phase fluctuations

In underdoped cuprates, T_c is thought to be determined by Cooper pair phase fluctuations because of the small superfluid density n_s. Experimentally, T_c is found to scale with n_s. The fluctuation-exchange approximation (FLEX) in its standard form fails to predict this behavior of T_c since it does not include phase fluctuations. We therefore extend the FLEX to include them selfconsistently. We present results for T_c[n_s,x], where x is the doping.

cond-mat.supr-con↗

arXiv1998

Collective Modes in High-Temperature Superconductors

The role of collective modes in various experiments on the cuprates is investigated. We calculate the neutron scattering, photoemission (ARPES), and Raman scattering intensities below $T_c$ within the fluctuation-exchange (FLEX) approximation for the two-dimensional Hubbard model. It is shown that the large peak in the dynamical spin susceptibility arises from a weakly damped spin-density-wave collective mode. This gives rise to a dip between the sharp low energy peak and the higher binding energy hump in the ARPES spectrum. Furthermore, we show that the collective mode of the amplitude fluctuation of the d-wave gap yields a broad peak above the pair-breaking threshold in the $B_{1g}$ Raman spectrum.

cond-mat.supr-con↗

arXiv1994

Exchange Enhancement of the Electron-Phonon Pair Interaction

The critical temperature of high-$T_c$ superconductors is determined, at least in part, by the electron-phonon coupling. We include the effect of an exchange interaction between the electrons and calculate the renormalization of the bare phonon frequencies and the electron-phonon verticies in a random phase approximation and obtain a strongly enhanced attractive phonon-induced electron-electron interaction. Using Fast Fourier Transform techniques, the weak-coupling selfconsistency equation for the order parameter is solved in the 2D first Brillouin zone for the Emery tight-binding band with different band fillings. The enhancement of $T_c$ arises primarily from the softening of the phonon frequencies rather than the vertex renormalization.

cond-mat↗