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Register a new userSurface impedance measurements on Nb$_{3}$Sn at high magnetic fields
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Nb$_{3}$Sn is a superconductor of great relevance for perspective RF applications. We present for the first time surface impedance $Z_s$ measurements at 15 GHz and low RF field amplitude on Nb$_{3}$Sn in high magnetic fields up to 12 T, with the aim of increasing the knowledge of Nb$_{3}$Sn behavior in such conditions. $Z_s$ is a fundamental material parameter that directly gives useful information about the dissipative and reactive phenomena when the superconductor is subjected to high-frequency excitations. Therefore, we present an analysis of the measured $Z_s$ with the aim of extracting interesting data about pinning in Nb$_{3}$Sn at high frequencies. From $Z_s$ we extract the vortex motion complex resistivity to obtain the $r$-parameter and the depinning frequency $ u_p$ in high magnetic fields. The comparison of the results with the literature shows that the measured $ u_p$ on bulk Nb$_{3}$Sn is several times greater than that of pure Nb. This demonstrates how Nb$_{3}$Sn can be a good candidate for RF technological applications, also in high magnetic fields.
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The high frequency vortex motion in Nb$_3$Sn was analyzed in this work up to 12 T. We used a dielectric loaded resonator tuned at 15 GHz to evaluate the surface impedance $Z$ of a Nb$_3$Sn bulk sample (24.8 at.%Sn). From the field induced variation of $Z$, the high frequency vortex parameters (the pinning constant $k_p$, the depinning frequency $ u_p$ and the flux flow resistivity $rho_{ff}$) were obtained over a large temperature and field range; their field and temperature dependence were analyzed. Comparison with other superconducting materials shows that high frequency applications in strong magnetic fields are also feasible with Nb$_3$Sn. In the present work, we report the first measurements about the microwave response in Nb$_3$Sn in strong magnetic fields.
The vortex lattice (VL) in the mixed state of the stannide superconductor Yb$_{3}$Rh$_{4}$Sn$_{13}$ has been studied using small-angle neutron scattering (SANS). The field dependencies of the normalized longitudinal and transverse correlation lengths of the VL, $xi_L/a_0$ and $xi_T /a_0$, reveal two distinct anomalies that are associated with vortex-glass phases below $mu_0H_l$~$approx$~700~G and above $mu_{0}H_h$~$sim$~1.7~T ($a_0$ is the intervortex distance). At high fields, around 1.7~T, the longitudinal correlation decreases abruptly with increasing fields indicating a weakening (but not a complete destruction) of the VL due to a phase transition into a glassy phase, below $mu_{0}H_{c_2}$(1.8 K)~$approx$~2.5~T. $xi_L/a_0$ and $xi_T /a_0$, gradually decrease for decreasing fields of strengths less than 1~T and tend towards zero. The shear elastic modulus $c_{66}$ and the tilting elastic modulus $c_{44}$ vanish at a critical field $mu_0H_l$~$approx$~700~G, providing evidence for a disorder-induced transition into a vortex-glass. A ring of scattered intensity is observed for fields lower than 700~G, $i.e.$, $mu_{0}H_{c_1}$~=~135~G~$<$~$mu_{0}H$~$<$~700~G. This low-field phenomenon is of different nature than the one observed at high fields, where $xi_L/a_0$ but not $xi_T/a_0$, decreases abruptly to an intermediate value.
Tunneling spectroscopy was performed on Nb pieces prepared by the same processes used to etch and clean superconducting radio frequency (SRF) cavities. Air exposed, electropolished Nb exhibited a surface superconducting gap delta=1.55 meV, characteristic of clean, bulk Nb. However the tunneling density of states (DOS) was broadened significantly. The Nb pieces treated with the same mild baking used to improve the Q-slope in SRF cavities, reveal a sharper DOS. Good fits to the DOS were obtained using Shiba theory, suggesting that magnetic scattering of quasiparticles is the origin of the gapless surface superconductivity and a heretofore unrecognized contributor to the Q-slope problem of Nb SRF cavities.
Magnetic impurities affect the spectrum of excitations of a superconductor and thus influence its impedance. We concentrate on the dissipative part of the surface impedance. We investigate its dependence on frequency, the density and strength of magnetic impurities, and the density and temperature of quasiparticles. Even a small concentration of weak magnetic impurities significantly modifies the excitation spectrum in the vicinity of the BCS gap. Therefore, we give special attention to the absorption threshold behavior at zero temperature and to the low-frequency absorption by quasiparticles. The discrete energy states introduced at low density of magnetic impurities may serve as traps for nonequilibrium quasiparticles, reducing the absorption in some range of low radiation frequencies.
We study mechanisms of vortex nucleation in Nb$_3$Sn Superconducting RF (SRF) cavities using a combination of experimental, theoretical, and computational methods. Scanning transmission electron microscopy (STEM) image and energy dispersive spectroscopy (EDS) of some Nb$_3$Sn cavities show Sn segregation at grain boundaries in Nb$_3$Sn with Sn concentration as high as $sim$35 at.% and widths $sim$3 nm in chemical composition. Using ab initio calculations, we estimate the effect excess tin has on the local superconducting properties of the material. We model Sn segregation as a lowering of the local critical temperature. We then use time-dependent Ginzburg-Landau theory to understand the role of segregation on magnetic vortex nucleation. Our simulations indicate that the grain boundaries act as both nucleation sites for vortex penetration and pinning sites for vortices after nucleation. Depending on the magnitude of the applied field, vortices may remain pinned in the grain boundary or penetrate the grain itself. We estimate the superconducting losses due to vortices filling grain boundaries and compare with observed performance degradation with higher magnetic fields. We estimate that the quality factor may decrease by an order of magnitude ($10^{10}$ to $10^9$) at typical operating fields if 0.03% of the grain boundaries actively nucleate vortices. We additionally estimate the volume that would need to be filled with vortices to match experimental observations of cavity heating.
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