The magnetic properties of iron-based superconductors $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ ($A=\mathrm{K}$, Cs, and Rb), which are characterized by the V-shaped dependence of the critical temperature (${T}_{\mathrm{c}}$) on pressure ($P$), were studied by means of the muon spin rotation/relaxation technique. In all three systems studied the magnetism was found to appear for pressures slightly below the critical one (${P}_{\mathrm{c}}$), i.e., at pressure where ${T}_{\mathrm{c}}(P)$ changes the slope. Rather than competing, magnetism and superconductivity in $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ are coexisting at the $P\ensuremath{\gtrsim}{P}_{\mathrm{c}}$ pressure region. Our results support the scenario of a transition from one pairing state to another, with different symmetries on either side of ${P}_{\mathrm{c}}$.
Recently discovered A-Fe-Se (A - alkali metal) materials have questioned the most popular theories of iron-based superconductors because of their unusual electronic structure [1]. Controversial photoemission data taken in the superconducting state [2-7] are in conflict with highly magnetic state seen by neutron-, muSR-spectroscopies and transport/thermodynamic probes [8-10]. These results lead to suggestions to consider all iron-based materials as originating from Mott-insulators or semiconductors, thus once again raising the question of close relation between the cuprates and Fe-based superconductors [e.g. 2]. Here we study electronic and magnetic properties of Rb0.77Fe1.61Se2 (Tc = 32.6 K) in normal and superconducting states by means of photoemission and muSR spectroscopies as well as band structure calculations. We demonstrate that the puzzling behavior of these novel materials is the result of separation into metallic (~12%) and insulating (~ 88%) phases. Only the former becomes superconducting and has a usual electronic structure of electron-doped FeSe-slabs. Our results thus imply that the antiferromagnetic insulating phase is just a byproduct of Rb-intercalation and its magnetic properties have hardly any relation to the superconductivity. Instead, we find that also in this, already third class of iron-based compounds, the key ingredient for superconductivity is a certain proximity of a van Hove singularity to the Fermi level. These findings set the direction for effective search of new superconducting materials.
We investigate magnetic ordering in metallic Ba(Fe${}_{1\ensuremath{-}x}$Mn${}_{x}$)${}_{2}$As${}_{2}$ and discuss the unusual magnetic phase, which was recently discovered for Mn concentrations $x>10$%. We argue that it can be understood as a Griffiths-type phase that forms above the quantum critical point associated with the suppression of the stripe-antiferromagnetic spin-density-wave (SDW) order in BaFe${}_{2}$As${}_{2}$ by the randomly introduced localized Mn moments acting as strong magnetic impurities. While the SDW transition at $x=0$, 2.5$%$, and 5$%$ remains equally sharp, in the $x=12$$%$ sample we observe an abrupt smearing of the antiferromagnetic transition in temperature and a considerable suppression of the spin gap in the magnetic excitation spectrum. According to our muon-spin-relaxation, nuclear magnetic resonance and neutron-scattering data, antiferromagnetically ordered rare regions start forming in the $x=12$$%$ sample significantly above the N\'eel temperature of the parent compound. Upon cooling, their volume grows continuously, leading to an increase in the magnetic Bragg intensity and to the gradual opening of a partial spin gap in the magnetic excitation spectrum. Using neutron Larmor diffraction, we also demonstrate that the magnetically ordered volume is characterized by a finite orthorhombic distortion, which could not be resolved in previous diffraction studies most probably due to its coexistence with the tetragonal phase and a microstrain-induced broadening of the Bragg reflections. We argue that Ba(Fe${}_{1\ensuremath{-}x}$Mn${}_{x}$)${}_{2}$As${}_{2}$ could represent an interesting model spin-glass system, in which localized magnetic moments are randomly embedded into a SDW metal with Fermi surface nesting.
<p>Anti-SEA/E-120 vs IL-2 day 1 3h. Concentration of induced IL-2 after Nap at day 1 of treatment. Moving median of 8 overlapping subgroups of patients with different baseline anti-SEA/E-120 (pmol/mL). Median{plus minus}SE is depicted.</p>
<p>Anti-SEA/E-120 vs exposure of Nap. Concentration of Nap after Nap at day 1 of treatment. Moving median of 8 overlapping subgroups of patients with different baseline anti-SEA/E-120 (pmol/mL). Median{plus minus}SE is depicted.</p>
We investigated the Abrikosov vortex lattice (VL) of a pure niobium single crystal with the muon spin rotation ($\ensuremath{\mu}$SR) technique. Analysis of the $\ensuremath{\mu}$SR data in the framework of the BCS-Gor'kov theory allowed us to determine microscopic parameters and the limitations of the theory. With decreasing temperature the field variation around the vortex cores deviates substantially from the predictions of the Ginzburg-Landau theory and adopts a pronounced conical shape. This is evidence of partial diffraction of Cooper pairs on the VL predicted by Delrieu for clean superconductors.
Using muon-spin rotation, we studied the in-plane (${\ensuremath{\lambda}}_{ab}$) and the out of plane (${\ensuremath{\lambda}}_{c}$) magnetic field penetration depth in ${\mathrm{SrFe}}_{1.75}{\mathrm{Co}}_{0.25}{\mathrm{As}}_{2}$ (${T}_{c}\ensuremath{\simeq}13.3\text{ }\text{ }\mathrm{K}$). The penetration depth anisotropy ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}={\ensuremath{\lambda}}_{c}/{\ensuremath{\lambda}}_{ab}$ increases from ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}\ensuremath{\simeq}2.1$ at ${T}_{c}$ to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume.
We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale ($\ensuremath{\sim}40\text{ }\text{ }\mathrm{meV}$), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in ${\mathrm{CuO}}_{2}$ planes of high-temperature superconductors, indicating they have a common origin.
The antiferromagnet and semimetal EuCd$_2$As$_2$ has recently attracted a lot of attention due to a wealth of topological phases arising from the interplay of topology and magnetism. In particular, the presence of a single pair of Weyl points is predicted for a ferromagnetic configuration of Eu spins along the $c$-axis in EuCd$_2$As$_2$. In the search for such phases, we investigate here the effects of hydrostatic pressure in EuCd$_2$As$_2$. For that, we present specific heat, transport and $\mu$SR measurements under hydrostatic pressure up to $\sim\,2.5\,$GPa, combined with {\it ab initio} density functional theory (DFT) calculations. Experimentally, we establish that the ground state of EuCd$_2$As$_2$ changes from in-plane antiferromagnetic (AFM$_{ab}$) to ferromagnetic at a critical pressure of $\,\approx\,$2\,GPa, which is likely characterized by the moments dominantly lying within the $ab$ plane (FM$_{ab}$). The AFM$_{ab}$-FM$_{ab}$ transition at such a relatively low pressure is supported by our DFT calculations. Furthermore, our experimental and theoretical results indicate that EuCd$_2$As$_2$ moves closer to the sought-for FM$_c$ state (moments $\parallel$ $c$) with increasing pressure further. We predict that a pressure of $\approx$\,23\,GPa will stabilize the FM$_c$ state, if Eu remains in a 2+ valence state. Thus, our work establishes hydrostatic pressure as a key tuning parameter that (i) allows for a continuous tuning between magnetic ground states in a single sample of EuCd$_2$As$_2$ and (ii) enables the exploration of the interplay between magnetism and topology and thereby motivates a series of future experiments on this magnetic Weyl semimetal.
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