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Register a new userDetailed study of the ac susceptibility of Sr2RuO4 in oriented magnetic fields
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We have investigated the ac susceptibility of the spin triplet superconductor Sr$_2$RuO$_4$ as a function of magnetic field in various directions at temperatures down to 60 mK. We have focused on the in-plane field configuration (polar angle $theta simeq 90^{circ}$), which is a prerequisite for inducing multiple superconducting phases in Sr$_2$RuO$_4$. We have found that the previous attribution of a pronounced feature in the ac susceptibility to the second superconducting transition itself is not in accord with recent measurements of the thermal conductivity or of the specific heat. We propose that the pronounced feature is a consequence of additional involvement of vortex pinning originating from the second superconducting transition.
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We studied the specific heat and thermal conductivity of the spin-triplet superconductor Sr2RuO4 at low temperatures and under oriented magnetic fields H. We resolved a double peak structure of the superconducting transition under magnetic field for the first time, which provides thermodynamic evidence for the existence of multiple superconducting phases. We also found a clear limiting of the upper critical field Hc2 for the field direction parallel to the RuO2 plane only within 2 degrees. The limiting of Hc2 occurs in the same H-T domain of the second superconducting phase; we suggest that the two phenomena have the same physical origin.
Recent observations [A.~Pustogow et al. Nature 574, 72 (2019)] of a drop of the $^{17}$O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr$_2$RuO$_4$ challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering to show that there is a $34 pm 6$ % drop in the magnetic susceptibility at the ruthenium site below the superconducting transition temperature. Measurements are made at lower fields $H sim tfrac{1}{3} H_{c2}$ than a previous study allowing the suppression to be observed. Our results are consistent with the recent NMR observations and rule out the chiral odd-parity $mathbf{d}=hat{mathbf{z}}(k_xpm ik_y)$ state. The observed susceptibility is consistent with several recent proposals including even-parity $B_{1g}$ and odd-parity helical states.
We report a study of the magnetization density in the mixed state of the unconventional superconductor S2RuO4. On entering the superconducting state we find no change in the magnitude or distribution of the induced moment for a magnetic field of 1 Tesla applied within the RuO2 planes. Our results are consistent with a spin-triplet Cooper pairing with spins lying in the basal plane. This is in contrast with similar experiments performed on conventional and high-Tc superconductors.
The eutectic system Sr2RuO4-Ru is referred to as the 3-K phase of the spin-triplet supeconductor Sr2RuO4 because of its enhanced superconducting transition temperature Tc of ~3 K. We have investigated the field-temperature (H-T) phase diagram of the 3-K phase for fields parallel and perpendicular to the ab-plane of Sr2RuO4, using out-of-plane resistivity measurements. We have found an upturn curvature in the Hc2(T) curve for H // c, and a rather gradual temperature dependence of Hc2 close to Tc for both H // ab and H // c. We have also investigated the dependence of Hc2 on the angle between the field and the ab-plane at several temperatures. Fitting the Ginzburg-Landau effective-mass model apparently fails to reproduce the angle dependence, particularly near H // c and at low temperatures. We propose that all of these charecteric features can be explained, at least in a qualitative fashion, on the basis of a theory by Sigrist and Monien that assumes surface superconductivity with a two-component order parameter occurring at the interface between Sr2RuO4 and Ru inclusions. This provides evidence of the chiral state postulated for the 1.5-K phase by several experiments.
By using Nuclear Magnetic Resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.
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