Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA key role for unusual spin dynamics in ferropnictides
333
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The 2008 discovery of superconducting ferropnictides with Tc~26K-56K introduced a new family of materials into the category of high Tc superconductors. The ongoing project of understanding the superconducting mechanism and pairing symmetry has already revealed a complicated and often contradictory underlying picture of the structural and magnetic properties. There is an almost unprecedented sensitivity of the calculated magnetism and Fermi surface to structural details that prohibits correspondence with experiment. Furthermore, experimental probes of the order parameter symmetry are in surprisingly strong disagreement, even considering the relative immaturity of the field. Here we outline all of the various and seemingly contradictory evidences, both theoretical and experimental, and show that they can be rectified if the system is assumed to be highly magnetic with a spin density wave that is well-defined but with magnetic twin and anti-phase boundaries that are dynamic on the time-scale of experiments. Under this assumption, we find that our calculations can accurately reproduce even very fine details of the structure, and a natural explanation for the temperature separation of structural and magnetic transitions is provided. Thus, our theory restores agreement between experiment and theory in crucial areas, making further cooperative progress possible on both fronts. We believe that fluctuating magnetic domains will be an essential component of unravelling the interplay between magnetic interactions and superconductivity in these newest high Tc superconductors.
rate research
Read More
Experiments on the iron-pnictide superconductors appear to show some materials where the ground state is fully gapped, and others where low-energy excitations dominate, possibly indicative of gap nodes. Within the framework of a 5-orbital spin fluctuation theory for these systems, we discuss how changes in the doping, the electronic structure or interaction parameters can tune the system from a fully gapped to nodal sign-changing gap with s-wave ($A_{1g}$) symmetry ($s^pm$). In particular we focus on the role of the hole pocket at the $(pi,pi)$ point of the unfolded Brillouin zone identified as crucial to the pairing by Kuroki {it et al.}, and show that its presence leads to additional nesting of hole and electron pockets which stabilizes the isotropic $s^pm$ state. The pockets contribution to the pairing can be tuned by doping, surface effects, and by changes in interaction parameters, which we examine. Analytic expressions for orbital pairing vertices calculated within the RPA fluctuation exchange approximation allow us to draw connections between aspects of electronic structure, interaction parameters, and the form of the superconducting gap.
In this work, magnetization dynamics in helical-ordered spin systems is studied theoretically and experimentally using magnetization measurements and ferromagnetic resonance measurement technique. EuFe2As2 is considered as the case study. Theoretically two types of resonance responses are examined for helical-ordered systems: the molecular field response and collective spin resonance modes. Both types of responses demonstrate clear dependence on helicity. It is demonstrated that combination of magnetization measurements and ferromagnetic resonance studies define the helicity completely. Experimentally for EuFe2As2 the molecular field response is observed. The defined helical angle is about 2pi/5.
Understanding the competition between superconductivity and other ordered states (such as antiferromagnetic or charge-density-wave (CDW) state) is a central issue in condensed matter physics. The recently discovered layered kagome metal AV3Sb5 (A = K, Rb, and Cs) provides us a new playground to study the interplay of superconductivity and CDW state by involving nontrivial topology of band structures. Here, we conduct high-pressure electrical transport and magnetic susceptibility measurements to study CsV3Sb5 with the highest Tc of 2.7 K in AV3Sb5 family. While the CDW transition is monotonically suppressed by pressure, superconductivity is enhanced with increasing pressure up to P1~0.7 GPa, then an unexpected suppression on superconductivity happens until pressure around 1.1 GPa, after that, Tc is enhanced with increasing pressure again. The CDW is completely suppressed at a critical pressure P2~2 GPa together with a maximum Tc of about 8 K. In contrast to a common dome-like behavior, the pressure-dependent Tc shows an unexpected double-peak behavior. The unusual suppression of Tc at P1 is concomitant with the rapidly damping of quantum oscillations, sudden enhancement of the residual resistivity and rapid decrease of magnetoresistance. Our discoveries indicate an unusual competition between superconductivity and CDW state in pressurized kagome lattice.
We study the symmetry of spin excitation spectra in 122-ferropnictide superconductors by comparing the results of first-principles calculations with inelastic neutron scattering (INS) measurements on BaFe1.85Co0.15As2 and BaFe1.91Ni0.09As2 samples that exhibit neither static magnetic phases nor structural phase transitions. In both the normal and superconducting (SC) states, the spectrum lacks the 42/m screw symmetry around the (1/2 1/2 L) axis that is implied by the I4/mmm space group. This is manifest both in the in-plane anisotropy of the normal- and SC-state spin dynamics and in the out-of-plane dispersion of the spin-resonance mode. We show that this effect originates from the higher symmetry of the magnetic Fe sublattice with respect to the crystal itself, hence the INS signal inherits the symmetry of the unfolded Brillouin zone (BZ) of the Fe sublattice. The in-plane anisotropy is temperature-independent and can be qualitatively reproduced in normal-state density-functional-theory calculations without invoking a symmetry-broken (nematic) ground state that was previously proposed as an explanation for this effect. Below the SC transition, the energy of the magnetic resonant mode Er, as well as its intensity and the SC spin gap inherit the normal-state intensity modulation along the out-of-plane direction L with a period twice larger than expected from the body-centered-tetragonal BZ symmetry. The amplitude of this modulation decreases at higher doping, providing an analogy to the splitting between even and odd resonant modes in bilayer cuprates. Combining our and previous data, we show that at odd L a universal linear relationship Er=4.3*kB*Tc holds for all studied Fe-based superconductors, independent of their carrier type. Its validity down to the lowest doping levels is consistent with weaker electron correlations in ferropnictides as compared to the underdoped cuprates.
The charge-density-wave (CDW) instability in the underdoped, pseudogap part of the cuprate phase diagram has been a major recent research focus, yet measurements of dynamic, energy-resolved CDW correlations are still in their infancy. We report a high-resolution resonant inelastic X-ray scattering (RIXS) study of the underdoped cuprate superconductor HgBa$_{2}$CuO$_{4+delta}$ ($T_c = 70$ K). At $T=250$ K, above the CDW order temperature $T_mathrm{CDW} approx 200$ K, we observe significant dynamic CDW correlations at about 40 meV. This energy scale is comparable to both the superconducting gap and the previously reported low-energy pseudogap. At $T = T_c$, a strong elastic CDW peak appears, but the dynamic correlations around 40 meV remain virtually unchanged. In addition, we observe a new feature: dynamic correlations at significantly higher energy, with a characteristic scale of about 160 meV. A similar scale was previously identified in other experiments as a high-energy pseudogap. The existence of three distinct features in the charge response is highly unusual for a CDW system, and suggests that charge order in the cuprates is closely related to the pseudogap phenomenon and more complex than previously thought. We further observe the paramagnon dispersion along [1,0], across the two-dimensional CDW wavevector $boldsymbol{q}_mathrm{CDW}$, which is consistent with magnetic excitations measured by inelastic neutron scattering. Unlike for some other cuprates, our results point to the absence of a discernible coupling between CDW and magnetic excitations.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
