Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userSpin resonance peak in Fe-based superconductors with unequal gaps
95
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We study the spin resonance in superconducting state of iron-based materials within multiband models with two unequal gaps, $Delta_L$ and $Delta_S$, on different Fermi surface pockets. We show that due to the indirect nature of the gap entering the spin susceptibility at the nesting wave vector $mathbf{Q}$ the total gap $tildeDelta$ in the bare susceptibility is determined by the sum of gaps on two different Fermi surface sheets connected by $mathbf{Q}$. For the Fermi surface geometry characteristic to the most of iron pnictides and chalcogenides, the indirect gap is either $tildeDelta = Delta_L + Delta_S$ or $tildeDelta = 2Delta_L$. In the $s_{++}$ state, spin excitations below $tildeDelta$ are absent unless additional scattering mechanisms are assumed. The spin resonance appears in the $s_pm$ superconducting state at frequency $omega_R leq tildeDelta$. Comparison with available inelastic neutron scattering data confirms that what is seen is the true spin resonance and not a peak inherent to the $s_{++}$ state.
rate research
Read More
The spin resonance peak in the iron-based superconductors is observed in inelastic neutron scattering experiments and agrees well with predicted results for the extended s-wave ($s_pm$) gap symmetry. On the basis of four-band and three-orbital tight binding models we study the effect of nonmagnetic disorder on the resonance peak. Spin susceptibility is calculated in the random phase approximation with the renormalization of the quasiparticle self-energy due to the impurity scattering in the static Born approximation. We find that the spin resonance becomes broader with the increase of disorder and its energy shifts to higher frequencies. For the same amount of disorder the spin response in the $s_pm$ state is still distinct from that of the $s_{++}$ state.
The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 leq x leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 leq y leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $Delta H(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{rm SDW})$ and the normal metallic regime $(T > T_{rm SDW})$ are clearly distinguished. At $T > T_{rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x approx 0.2$ or at $y approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.
We consider the spin response within the five-orbital model for iron-based superconductors and study two cases: equal and unequal gaps in different bands. In the first case, the spin resonance peak in the superconducting state appears below the characteristic energy scale determined by the gap magnitude, $2Delta_L$. In the second case, the energy scale corresponds to the sum of smaller and larger gap magnitudes, $Delta_L + Delta_S$. Increasing the values of the Hubbard interaction and the Hunds exchange, we observe a shift of the spin resonance energy to lower frequencies.
Checkerboard patterns have been proposed in order to explain STM experiments on the cuprates BSCCO and Na-CCOC. However the presence of these patterns has not been confirmed by a bulk probe such as neutron scattering. In particular, simple checkerboard patterns are inconsistent with neutron scattering data, in that they have low energy incommsensurate (IC) spin peaks rotated 45 degrees from the direction of the charge IC peaks. However, it is unclear whether other checkerboard patterns can solve the problem. In this paper, we have studied more complicated checkerboard patterns (modulated checkerboards) by using spin wave theory and analyzed noncollinear checkerboards as well. We find that the high energy response of the modulated checkerboards is inconsistent with neutron scattering results, since they fail to exhibit a resonance peak at (pi,pi), which has recently been shown to be a universal feature of cuprate superconductors. We further argue that the newly proposed noncollinear checkerboard also lacks a resonance peak. We thus conclude that to date no checkerboard pattern has been proposed which satisfies both the low energy constraints and the high energy constraints imposed by the current body of experimental data in cuprate superconductors.
We analyze antiferromagnetism and superconductivity in novel $Fe-$based superconductors within the itinerant model of small electron and hole pockets near $(0,0)$ and $(pi,pi)$. We argue that the effective interactions in both channels logarithmically flow towards the same values at low energies, {it i.e.}, antiferromagnetism and superconductivity must be treated on equal footings. The magnetic instability comes first for equal sizes of the two pockets, but looses to superconductivity upon doping. The superconducting gap has no nodes, but changes sign between the two Fermi surfaces (extended s-wave symmetry). We argue that the $T$ dependencies of the spin susceptibility and NMR relaxation rate for such state are exponential only at very low $T$, and can be well fitted by power-laws over a wide $T$ range below $T_c$.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
