A theory of dissipative nonlinear conductivity, $σ_1(ω,H)$, of s-wave superconductors under strong electromagnetic fields at low temperatures is proposed. Closed-form expressions for $σ_1(H)$ and the surface resistance $R_s(ω,H)$ are obtained in the nonequilibrium dirty limit for which $σ_1(H)$ has a significant minimum as a function of a low-frequency $(\hbarω\ll k_BT)$ magnetic field $H$. The calculated microwave suppression of $R_s(H)$ is in good agreement with recent experiments on alloyed Nb resonator cavities. It is shown that superimposed dc and ac fields, $H=H_0+H_a\cosωt$, can be used to reduce ac dissipation in thin film nanostructures by tuning $σ_1(H_0)$ with the dc field.
Microwave Conductivity of a High Purity d-wave Superconductor
尖状行为的微波电导率观察到在清洁的ortho-II YBPDMATHABMM铜$_{3}$ O $_{6.50}$在低温和低频率的杂质散射率的频率依赖性与一个可忽略的小值在零频率直接相关。在弱散射极限下，电导率随频率线性下降。在涡旋状态，假设一个随机（高斯）分布的旋涡，我们表明，磁场深刻地改变了杂质散射率，现在获得一个有限的零频率值。因此，我们预测在低频下的微波电导率的德鲁德线形状。
The paper reports on finding the effect of a strong change in the microwave losses in an HTS-based coplanar waveguide (CPW) at certain values of the input power Pin and direct current Idc. CPW on the basis of 150 nm thick YBa2Cu3O7-δ epitaxial film on a single crystal MgO substrate was studied experimentally. A sharp and reversible transition of the CPW into a strongly dissipative state at the certain meanings of Pin and Idc depending on temperature was observed. Apparently the effect can be explained by self-heating of HTS structure caused by magnetic flux flow under the joint influence of MW…
Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors
最近的微波电导率测量的解孪晶，高纯度，稍微过掺杂的YBa $_{2}$ Cu $_{3}$ O $_{6.993}$晶体揭示了一个线性的温度依赖性和近Drude线的形状的温度之间的1和20 K和频率范围从1到75 GHz。先前的理论工作表明，d波超导体中点缺陷（杂质）散射的简单模型与这些结果不一致。因此，有人建议，散射扩展缺陷，如孪晶界残留物，从退孪过程中留下的，也可能是重要的。我们计算的自洽玻恩近似（包括顶点校正）的d-波超导体中存在的散射从扩展的线性缺陷的自能和微波电导率。我们发现，在实验相关的限制（$Ω, 1/τ\ll T \ll Δ_{0}$），由此产生的微波电导率有一个线性的温度依赖性和一个近Drude频率依赖性，符合实验。
We report investigations of molybdenum nitride (MoN) thin films with different thickness and disorder and with superconducting transition temperature 9.89 K $\ge{T_c}\ge$ 2.78 K. Using terahertz frequency-domain spectroscopy we explore the normal and superconducting charge carrier dynamics for frequencies covering the range from 3 to 38 cm$^{-1}$ (0.1 to 1.1 THz). The superconducting energy scales, i.e. the critical temperature $T_c$, the pairing energy $Δ$, and the superfluid stiffness $J$, and the superfluid density $n_s$ can be well described within the Bardeen-Cooper-Schrieffer theory for …
Microwave conductivity in the ferropnictides with specific application to Ba$_{1-x}$K$_x$Fe$_2$As$_2$
计算了具有$s^\pm$带隙对称性的两带超导体的微波电导率。非弹性散射近似包括在BCS模型增强的温度相关的准粒子散射率假设，然而，是频率无关的。的可能性，S波间隙的一个或另一个的电子或空穴口袋是各向异性的探索，包括费米表面上的间隙节点和没有间隙的情况下。我们的BCS结果与在两种流体模型（TFM）中得到的结果的比较，以及与铜酸盐的情况下，间隙具有d-波对称性和Ba $_{1-x}$ K $_x$ Fe $_2$ As $_2$的实验结果。目前可用的微波电导率数据在这种材料中提供了强有力的证据，大的各向异性的电子袋s波间隙。虽然最佳拟合有利于费米表面上具有节点的间隙，但这与一些但不是所有的穿透深度测量结果不一致，这些测量结果有利于无节点间隙，热导率和核磁共振数据也是如此。
Ultralow-dissipative conductivity by Dirac fermions in BaFe$_2$As$_2$
报道了BaFe $_{2}$ As $_{2}$在太赫兹（THz）频率范围内与狄拉克锥有关的复电导率的反常行为.在自旋密度波（SDW）转变温度以上，电导率谱遵循Drude模型。在SDW状态下，与根据Drude模型预期的情况相比，复电导率的虚部$σ_2$被抑制。真正的部分，$σ_1$，表现出几乎像德鲁德一样的行为。这种行为（即，$σ_1$几乎没有变化和$σ_2$的降低）可以被认为是额外电导率的增加，而在Drude型电导率中没有任何耗散。这种超低耗散电导率的起源被认为是由于狄拉克锥内的准粒子的电导率贡献。也就是说，我们可以通过BaFe $_2$ As $_2$的电导谱，清晰而直接地观察到狄拉克费米子的动力学过程。
Inverse Faraday effect (IFE) in superconductors is proposed, where a static magnetization is generated under the influence of a circularly polarized microwave field. Classical modeling of the IFE explicitly provides superconducting gyration coefficient in terms of its complex conductivity. IFE is then considered as a source of nonlinearity and gyrotropy even at a low-power microwave regime giving rise to a spectrum of phenomena and applications. Microwave-induced gyroelectric conductivity, Hall effect, microwave birefringence, flux quantization and vortex state are predicted and quantitatively…
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 …
Conventional BCS superconductors are expected to exhibit a conductivity with vanishing dissipation with decreasing temperature. While bulk physical properties measurements indicate PdPb$_{2}$ is a conventional superconductor with a $T_c$ of 3.0 K, measurements of surface impedance through the microwave cavity perturbation technique indicate a large, non-vanishing dissipative component below $T_c$ that is at odds with conventional superconductivity. We demonstrate PdPb$_2$ to be a possible topological superconductor with a fully gapped bulk and a dissipative Majorana fluid surface.
Anisotropic microwave conductivity of cuprate superconductors in the presence of CuO chain induced impurities
在动能驱动的超导机制下，研究了邻位Ⅱ型$_2$的微波电导各向异性.邻位-Ⅱ $_2$ $_3$ O $_{6.50}$的特征在于填充和空$\hat{b}$轴CuO链的周期性交替。考虑CuO链诱导的扩展各向异性杂质散射，基于准粒子激发和散射过程的节块近似，再现了邻位Ⅱ型$_2$微波电导各向异性的主要特征，包括$\hat{a}$轴和$\hat{b}$轴微波电导的能量和温度依赖性的强度和线形.我们的研究结果也证实了$\hat{b}$轴CuO链诱导杂质是各向异性的主要来源。
Isolation of a system from its environment is often desirable, from precision measurements to control of individual quantum systems; however, dissipation can also be a useful resource. Remarkably, engineered dissipation enables the preparation of quantum states of atoms, ions or superconducting qubits as well as their stabilization. This is achieved by a suitably engineered coupling to a dissipative cold reservoir formed by electromagnetic modes. Similarly, in the field of cavity electro- and optomechanics, the control over mechanical oscillators utilizes the inherently cold, dissipative natur…
Microwave conductivity experiments can directly measure the quasiparticle scattering rate in the superconducting state. We show that this, combined with knowledge of the Fermi surface geometry, allows one to distinguish between closely related superconducting order parameters, e.g., d$_{x^2-y^2}$ and d$_{xy}$ superconductivity. We benchmark this method on YBa$_2$Cu$_3$O$_{7-δ}$ and, unsurprisingly, confirm that this is a d$_{x^2-y^2}$ superconductor. We then apply our method to $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which we discover is a d$_{xy}$ superconductor.
The morphological, transport and terahertz optical properties of DC magnetron sputtered granular molybdenum thin-films with nano-grains embedded in an amorphous matrix have been studied in the normal and superconducting states. The superconducting transition temperatures of these films are much higher than that of bulk molybdenum. The optical properties of these thin-films have been studied using terahertz time-domain spectroscopy. Their properties have been compared with the existing materials used for the development of radiation detectors. The resistivity of the films lies in >100 micro-Ohm…
We study the dissipative conductivity $σ_1$ of a dirty superconductor with a finite Dynes parameter $Γ$ under a dc-biased weak time-dependent field. The Usadel equation for the current-carrying state is solved to calculate the pair potential, penetration depth, supercurrent density, and quasiparticle spectrum. It is shown that, while the depairing current density $j_d$ for $Γ=0$ is coincident with the Kupriyanov-Lukichev theory, a finite $Γ$ decreases the superfluid density, resulting in a reduction of $j_d$. The broadening of the peaks of the quasiparticle density of states induced by a combi…
In-plane superfluid density and microwave conductivity of the organic superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br: evidence for d-wave pairing and resilient quasiparticles
We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of BaFe2As2 single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band $s_{\pm}$-wave Eliashberg approach, able to account for all the main …
We present a dual cut-off, dc-tunable low-pass microwave filter on a superconducting Nb microstrip with uniaxial asymmetric nanogrooves. The frequency response of the device was measured in the range $300$\,KHz to $14$\,GHz at different temperatures, magnetic fields, and dc current values. The microwave loss is most effectively reduced when the Abrikosov vortex lattice spatially matches the underlying washboard pinning landscape. The forward transmission coefficient $S_{21}(f)$ of the microstrip has a dc-tunable cut-off frequency $f_d$ which notably changes under dc bias reversal, due to the t…
FeSe is an iron-based superconductor of immense current interest due to the large enhancements of Tc that occur when it is pressurized or grown as a single layer on an insulating substrate. Here we report precision measurements of its superconducting electrodynamics, at frequencies of 202 and 658 MHz and at temperatures down to 0.1 K. The quasiparticle conductivity reveals a rapid collapse in scattering on entering the superconducting state that is strongly reminiscent of unconventional superconductors such as cuprates, organics and the heavy fermion material CeCoIn5. At the lowest temperature…
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 $τ_\mathrm{\mathrm{in}}$ rather than the elastic one $τ_{\mathrm{el}}$ 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 microwa…
Characterizing superconducting microwave resonators with highly dissipative elements is a technical challenge, but a requirement for implementing and understanding the operation of hybrid quantum devices involving dissipative elements, e.g. for thermal engineering and detection. We present experiments on $λ/4$ superconducting niobium coplanar waveguide (CPW) resonators, terminating at the antinode by a dissipative copper microstrip via aluminum leads, such that the resonator response is difficult to measure in a typical microwave environment. By measuring the transmission both above and below …
