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Register a new userDebye mechanism of giant microwave absorption in superconductors
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We discuss a mechanism of microwave absorption in conventional superconductors which is similar to the Debye absorption mechanism in molecular gases. The contribution of this mechanism to the emph{ac} conductivity is proportional to the inelastic quasiparticle relaxation time $tau_mathrm{mathrm{in}}$ rather than the elastic one $tau_{mathrm{el}}$ and therefore it can be much larger than the conventional one. The Debye contribution to the linear conductivity arises only in the presence of a emph{dc} supercurrent in the system and its magnitude depends strongly on the orientation of the microwave field relative to the supercurrent. The Debye contribution to the nonlinear conductivity exists even in the absence of emph{dc} supercurrent. Since it is proportional to $tau_{mathrm{in}}$ the nonlinear threshold is anomalously low. Microwave absorption measurements may provide direct information about $tau_mathrm{in}$ in superconductors.
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Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB$_2$ and K$_3$C$_{60}$) for magnetic fields as small as a few $%$ of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K$_3$C$_{60}$ in the framework of the Coffey--Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant.
We discuss a new mechanism of microwave absorption in s- and d-wave superconductors, which arises in the presence of a dc supercurrent in the system. It produces a contribution to the ac conductivity that is proportional to the inelastic quasiparticle relaxation time. This contribution also determines the supercurrent dependence of the conductivity. It may significantly exceed the conventional contribution because in typical superconductors the inelastic relaxation time is several orders of magnitude longer than the elastic one. We show that the aforementioned contribution to the conductivity may be expressed in terms of the single particle density of states in superconductors in the presence of a dc supercurrent. Our results may enable determination of the inelastic relaxation time in superconductors from microwave absorption measurements.
We describe a mechanism by which the longitudinal thermal conductivity $kappa_{xx}$, measured in an in-plane magnetic field, oscillates as a function of field angle in layered nodal superconductors. These oscillations occur when the spin-orbit splitting at the nodes is larger than the nodal scattering rate, and are complementary to vortex-induced oscillations identified previously. In sufficiently anisotropic materials, the spin-orbit mechanism may be dominant. As a particular application, we focus on the cuprate high-temperature superconductor YBa$_2$Cu$_3$O$_{6+x}$. This material belongs to the class of Rashba bilayers, in which individual CuO$_2$ layers lack inversion symmetry although the crystal itself is globally centrosymmetric. We show that spin-orbit coupling endows $kappa_{xx}/T$ with a characteristic dependence on magnetic field angle that should be easily detected experimentally, and argue that for underdoped samples the spin-orbit contribution is larger than the vortex contribution. A key advantage of the magneto-thermal conductivity is that it is a bulk probe of spin-orbit physics, and therefore not sensitive to inversion breaking at surfaces.
Ultra low-loss microwave materials are crucial for enhancing quantum coherence and scalability of superconducting qubits. Van der Waals (vdW) heterostructure is an attractive platform for quantum devices due to the single-crystal structure of the constituent two-dimensional (2D) layered materials and the lack of dangling bonds at their atomically sharp interfaces. However, new fabrication and characterization techniques are required to determine whether these structures can achieve low loss in the microwave regime. Here we report the fabrication of superconducting microwave resonators using NbSe$_2$ that achieve a quality factor $Q > 10^5$. This value sets an upper bound that corresponds to a resistance of $leq 192 muOmega$ when considering the additional loss introduced by integrating NbSe$_2$ into a standard transmon circuit. This work demonstrates the compatibility of 2D layered materials with high-quality microwave quantum devices.
We investigate pairing mechanism in multiband superconductors. To put our feet on firm ground, unbiased renormalization group analysis is carried out for iron-based superconductors. It is quite remarkable that, after integrating out quantum fluctuations, the renormalization-group flows agree exceedingly well with a mean-field Hamiltonian where interband pair hopping plays an essential role. Through interband pair hopping, electrons can overcome the repulsive interaction between them and form resonating Cooper pairs between different bands. Unlike the conventional superconductors, the pairing mechanism in multiband superconductors is resonating pair hopping between different bands, just like the resonating chemical bonds in benzene. The effective mean-field Hamiltonian spots a small parameter dictating the critical temperature and also explains how interband pair hopping always enahnces spin fluctuations at the nesting momentum connecting the Fermi surfaces. In short, no attractive glue is needed and resonating interband pair hopping is the key to Cooper pair formation in unconventional superconductors. Implications to cuprates and related issues are also discussed at the end.
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