Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userPossible unconventional superconductivity in substituted BaFe$_{2}$As$_{2}$ revealed by magnetic pair-breaking studies
102
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The possible existence of a sign-changing gap symmetry in BaFe$_{2}$As$_{2}$-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe$_{1.9}M_{0.1}$As$_{2}$ ($M=$ Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for $M=$ Cu and Mn compounds, which display very low SC transition temperature ($T_{c}$) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of $T_{c}$ cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.
rate research
Read More
We report $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on the superconductor Rb$_{2}$Cr$_{3}$As$_{3}$ with a quasi one-dimensional crystal structure. Below $Tsim$ 100 K, the spin-lattice relaxation rate (1/$T_{1}$) divided by temperature, 1/$T_{1}T$, increases upon cooling down to $T_{rm c}$ = 4.8 K, showing a Curie-Weiss-like temperature dependence. The Knight shift also increases with decreasing temperature. These results suggest ferromagnetic spin fluctuation. In the superconducting state, 1/$T_{1}$ decreases rapidly below $T_{text{c}}$ without a Hebel-Slichter peak, and follows a $T^5$ variation below $Tsim$ 3 K, which point to unconventional superconductivity with point nodes in the gap function.
Fe-K$_{beta}$ X-ray emission spectroscopy measurements reveal an asymmetric doping dependence of the magnetic moments $mu_text{bare}$ in electron- and hole-doped BaFe$_{2}$As$_{2}$. At low temperature, $mu_text{bare}$ is nearly constant in hole-doped samples, whereas it decreases upon electron doping. Increasing temperature substantially enhances $mu_text{bare}$ in the hole-doped region, which is naturally explained by the theoretically predicted crossover into a spin-frozen state. Our measurements demonstrate the importance of Hunds coupling and electronic correlations, especially for hole-doped BaFe$_{2}$As$_{2}$, and the inadequacy of a fully localized or fully itinerant description of the 122 family of Fe pnictides.
Beyond the conventional electron pairing mediated by phonons, high-temperature superconductivity in cuprates is believed to stem from quantum spin liquid (QSL). The unconventional superconductivity by doping a spin liquid/Mott insulator, is a long-sought goal but a principal challenge in condensed matter physics because of the lack of an ideal QSL platform. Here we report the pressure induced metallization and possible unconventional superconductivity in $NaYbSe_{2}$, which belongs to a large and ideal family of triangular lattice spin liquid we revealed recently and is evidenced to possess a QSL ground state. The charge gap of NaYbSe2 is gradually reduced by applying pressures, and at ~20 GPa the crystal jumps into a superconducting (SC) phase with Tc ~ 5.8 K even before the insulating gap is completely closed. The metallization is confirmed by further high-pressure experiments but the sign of superconductivity is not well repeated. No symmetry breaking accompanies the SC transition, as indicated by X-ray diffraction and low-temperature Raman experiments under high pressures. This intrinsically connects QSL and SC phases, and suggests an unconventional superconductivity developed from QSL. We further observed the magnetic-field-tuned superconductor-insulator transition which is analogous to that found in the underdoped cuprate superconductor $La_{2-x}Sr_{x}CuO_{4}$. The study is expected to inspire interest in exploring new types of superconductors and sheds light into the intriguing physics from a spin liquid/Mott insulator to a superconductor.
We use polarized inelastic neutron scattering to study low-energy spin excitations and their spatial anisotropy in electron-overdoped superconducting BaFe$_{1.85}$Ni$_{0.15}$As$_{2}$ ($T_c=14$ K). In the normal state, the imaginary part of the dynamic susceptibility, $chi^{primeprime}(Q,omega)$, at the antiferromagnetic (AF) wave vector $Q=(0.5,0.5,1)$ increases linearly with energy for $Ele 13$ meV. Upon entering the superconducting state, a spin gap opens below $Eapprox 3$ meV and a broad neutron spin resonance appears at $Eapprox 7$ meV. Our careful neutron polarization analysis reveals that $chi^{primeprime}(Q,omega)$ is isotropic for the in-plane and out-of-plane components in both the normal and superconducting states. A comparison of these results with those of undoped BaFe$_2$As$_2$ and optimally electron-doped BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K) suggests that the spin anisotropy observed in BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ is likely due to its proximity to the undoped BaFe$_2$As$_2$. Therefore, the neutron spin resonance is isotropic in the overdoped regime, consistent with a singlet to triplet excitation.
There is intense controversy around the unconventional superconductivity in strontium ruthenate, where the various theoretical and experimental studies suggest diverse and mutually exclusive pairing symmetries. Currently, the investigation is solely focused on only one material, Sr2RuO4, and the field suffers from the lack of comparison targets. Here, employing a density functional theory based analysis, we show that the heterostructure composed of SrRuO3 and SrTiO3 is endowed with all the key characteristics of Sr2RuO4, and, in principle, can host superconductivity. Furthermore, we show that competing magnetic phases and associated frustration, naturally affecting the superconducting state, can be tuned by epitaxial strain engineering. This system thus offers an excellent platform for gaining more insight into superconductivity in ruthenates.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
