Session X47: Superconductivity: Response to Electromagnetic Fields-Surface Impedance, Optical, Raman, etc.

Live

Higgs spectroscopy, the study of Higgs bosons of a superconductor, is an emerging field in studying superconductivity. It is used to probe various superconductor properties. In the conventional theory, the Higgs mode only couples non-linearly to the external optical field via the band curvature and hence vanishes in flat band superconductors such as the twisted bilayer graphene at magic angle. In this work we show that the Berry curvature and the quantum metric of bands play a central role in the Higgs mode generation. They allow detection of Higgs bosons even when the conventional contribution from the band curvature vanishes. Furthermore, we show that the Higgs mode can couple to the external electromagnetic field linearly when mediated by the Berry connection. As a result, we predict the existence of a second harmonic generation, in addition to the well-known third harmonic generation. We apply our theory to the important case of twisted bilayer graphene and demonstrate that it has Higgs modes when superconductivity is realized in the flat band at the magic angle.   

Live

The new uranium-based superconductor UTe₂ attracts much interest as a paramagnetic analogue of ferromagnetic superconductors. The extremely high upper critical field along the magnetic hard axis, reentrant superconductivity, and only small reduction of the Knight shift in UTe₂ indicate the spin-triplet superconducting state likely mediated by ferromagnetic fluctuations. Furthermore, scanning tunneling spectroscopy and optical Kerr effect measurements suggest the time reversal symmetry breaking superconductivity with topologically nontrivial surface states. Focusing on the superconducting gap, previous experimental studies reported the gap structure with point nodes which is consistent with the spin-triplet superconductivity. However, whether the point nodes locate at the high-symmetry points or not is crucial to elucidate the superconducting properties, which is still unrevealed. Thus, we performed the magnetic penetration depth measurements using a tunnel diode oscillator with the ac magnetic field along each crystallographic axis. In addition, we also investigated the effects of weak dc magnetic field on the low-energy quasiparticle excitations. Based on our experimental results, we will discuss the possible superconducting gap functions in UTe₂.   

Live

Temperature-dependent anisotropy, γ_λ(T ) = λ_c/λ_ab, of the London penetration depth, λ, is often considered to be a hallmark of multi-gap superconductivity. Similarly, larger than 1.76 values of Δ(0)/T_c ratio are often considered to be due to strong coupling (Δ(0) is the gap magnitude at T=0). Here we use weak-coupling self-consistent theory applied to various types of the order parameter and show that all types of the temperature dependencies of γ_λ(T ) are possible in a single- gap scenario. On the other hand, if observed, temperature-dependent γ_λ(T ) imposes certain restrictions on the structure of the order parameter. For example, pure d−wave results in temperature-independent γ_λ even on anisotropic Fermi surface, whereas Maki’s s + g order parameter leads to a non-monotonic variation of γ_λ(T ). The description of the experimental evaluation of γ_λ(T ) from small-amplitude AC susceptibility will complete the discussion.   

Live

The microscopic origins of Superconducting Radio Frequency (SRF) cavity breakdown by surface defect are still not completely understood. To locally study the electrodynamics of superconductors, a near-field magnetic microwave microscope was built. We study the 3rd harmonic response as a function of rf field amplitude and temperature. In previous experiments on Nb, two different types of nonlinearity were observed, which we call Low-field and Periodic. The Low-field response can be explained by the intrinsic response of the sample due to vortex semiloops created by the magnetic writer probe. As for the Periodic case, the response can be linked to the Josephson effect at or near the surface. New results on a Nb₃Sn film show evidence for multiple superconducting transitions, probably because there are different Sn concentrations in the sample. In particular, there is a clear phase with a transition temperature of about 5.5 K. The most prominent nonlinear response appears to be due to the intrinsic Low-field mechanism. We summarize our results with the materials properties of Nb₃Sn, and compare to those previously obtained on Nb materials.   

Live

Electromagnetic responses in superconductors provide valuable information on the pairing symmetry as well as physical quantities such as the superfluid density. However, at the superconducting gap energy scale, optical excitations of the Bogoliugov quasiparticles are forbidden in conventional Bardeen-Cooper-Schrieffer superconductors when momentum is conserved. Accordingly, far-infrared optical responses have been understood in the framework of a momentum-non-conserving theory by Mattis and Bardeen for over 60 years. In this talk, we establish the conditions for nonzero intrinsic momentum-conserving optical excitations. We find that multi-band effects are essential, additionally, one of the following three conditions is required: (i) inversion symmetry breaking, (ii) the emergence of the Bogoliubov Fermi surfaces, or (iii) simply finite spin-orbit coupling. We apply our theory to optical responses in FeSe.   

Live

Hyperbolic Cooper-pair polaritons (HCP) in cuprate superconductors are of fundamental interest due to their potential for providing insights into the nature of unconventional superconductivity. Here, we critically assess an experimental approach using near-field imaging to probe HCP in Bi₂Sr₂CaCu₂O_8+x (Bi-2212) in the presence of graphene surface plasmon polaritons (SPP). Our simulations show that HCP features that are inherently weak in the near-field can be strongly enhanced when coupled to graphene SPP in layered graphene/hexagonal boron nitride (hBN)/Bi-2212 heterostructures. This enhancement arises from the hybridization of SPP and HCP into new collective modes. Importantly, we show that the degree of hybridization can be selectively controlled by tuning the interlayer hBN thickness with atomic precision. Finally, we present IR and THz near-field measurements on the graphene/hBN/Bi-2212 heterostructure, which attest to the validity of our approach for probing HCP in Bi-2212 and in other families of cuprates.   

Live

Quantum matter with striking properties has recently been generated by driving materials with ultrafast laser pulses. However, observing these phases by following a single property as a function of time after photoexcitation provides a limited picture. We directly tracked hot phonons and transient heating in a high temperature superconductor YBa₂Cu₃O_6.9 by time-resolved Raman scattering with time-resolution of 220 fs. In addition, we used the hardening of the Raman-active apical oxygen phonon right after photoexcitation as an indirect probe of hot electrons, which equilibrated with hot phonons around 0.5 ps. Analysis based on the two-temperature model revealed temperature-independent electron-phonon coupling with λ ≈ 0.8 as well as significant anharmonicity that increased with temperature. It is essential to reduce this anharmonicity to generate longer lived nonequilibrium states in copper oxides. We will also discuss new insights into photoinduced superconductivity recently reported at lower doping that follow from these results.   

Live

Superconducting (SC) and charge-density-wave (CDW) states are pair condensates of electrons and of electrons and holes, respectively. In many quantum materials, these two states are found to coexist or compete. Transition-metal chalcogenides such as TaSe_2−xS_x [1] and ZrTe_3−xSe_x [2] exhibit both SC and CDW phases and may be viewed as model systems to study the interplay between these two types of order. In both systems, substitutional disorder enhances SC order and suppresses CDW order [1,2]. We use AC nonlinear magnetic response, a probe that was recently shown to be highly sensitive to SC fluctuations [3], to correlate the diamagnetic response above the superconducting transition temperature with the substitutional disorder. Our results indicate rather narrow SC fluctuation regimes, yet with unconventional temperature dependences of the diamagnetic signal. We discuss these findings in the context of recent work on oxide superconductors [3].

[1] L. Li et al., npj Quant. Mater. 2, 11 (2017)
[2] X. Zhu et al., Sci. Rep. 6, 26974 (2016)
[3] D. Pelc et al., Nat. Commun. 10, 2729 (2019)   

Live

BiCh₂-based superconductors are layered superconductors composed of an alternate stacking of the Bi-chalcogen superconducting layers and various types of blocking layers. The structure is similar to high-T_c cuprate and Fe-based superconductors, which have unconventional pairing mechanisms. Whether the mechanism of BiCh₂-based superconductivity is conventional or unconventional is still an open question, and some conflicting experimental results of superconducting gap structure have been reported. While the specific heat and thermal conductivity measurements imply a fully gapped s-wave superconducting state, angle-resolved photoemission and isotope effect studies point to an unconventional pairing mechanism, which is predicted theoretically. Moreover, two-fold-symmetric magnetoresistance for in-plane field was recently reported in LaO_0.5F_0.5BiSSe, which breaks the rotational symmetry of the tetragonal system. Here, we report on high-resolution measurements of the magnetic penetration depth in single crystals of LaO_0.5F_0.5BiSSe, which is a sensitive probe of low-energy quasiparticle excitations. By comparing with the results of previous studies, we will discuss the superconducting gap structure and the pairing mechanism in this system.   

Live

The quantitative characterization of different sources of decoherence is an important step in the optimization of superconducting circuits. Here we present a systematic study of the effect of shielding on CPW resonators and we find that magnetic shielding as well as shielding from IR radiation are both crucial to obtain internal quality factors (Qi) greater than five million in the single photon excitation regime. We investigate samples from different materials (Nb, TiN, Al) and probe the nature of the microwave-frequency loss by varying the power and the temperature, deducing the efficacy of the shielding in terms by way of the power spectral densities of Qi and the resonance frequency. We compare the performance of multiple shielding configurations involving Cryoperm 10, Metglas 2714A and Sn plated Cu coated with IR absorbing epoxy.   

Live

Optical control of chirality in chiral superconductors bears the potential for future topological quantum
computing applications. When a chiral domain is written and erased by a laser spot, Majorana modes around the domain can be manipulated on ultrafast time scales. Here we study topological superconductors with two chiral order parameters coupled via light fields by a time-dependent real-space Ginzburg-Landau approach. Continuous optical driving, or the application of supercurrent, hybridizes the two chiral order parameters, allowing one to induce and control the superconducting state beyond equilibrium. We show that superconductivity can even be enhanced if the mutual coupling between the two order parameters is sufficiently strong. Furthermore, we demonstrate that short optical pulses with spot size larger than a critical one can overcome a counteracting diffusion effect and write, erase, or move chiral domains. These domains are found to be stable, which might enable optically programmable quantum computers in the future.   

Live

Since its theoretical prediction in 1948, the Casimir effect has continued to attract considerable interest in the scientific community, with most studies focusing on calculating Casimir forces, which are attractive in the normal configuration of a metallic cavity. The question of the actual energy within Casimir cavities – the “Casimir energy” – has been less studied theoretically, and few measurements have been performed to detect it. Here we report on the theoretical understanding of a recent experimental realization. The experiment measures the change in critical temperature of a superconductor when it is introduced in a cavity with a conducting plate, due to the Casimir energy created in the cavity. The experiment shows a null result, which we justify theoretically. We also study how the change in critical temperature varies with the presence of a magnetic field as the system is taken across the superconducting transition, so as to be able to predict what changes in the experiment could have led to an observable result.   

Live

Recent experiments on superconducting cavities have found that under large radio-frequency (RF) electromagnetic fields the quality factor can improve with increasing field amplitude. Linear theories of dissipation break down under these extreme conditions and are unable to explain this behavior. We numerically solve the Bogoliubov-de Gennes equations at the surface of a superconductor in a parallel magnetic field, finding that at large fields there are quasiparticle surface states with energies below the bulk value of the superconducting gap. As the field oscillates, such states emerge and disappear with every cycle. We consider the dissipation resulting from inelastic quasiparticle-phonon scattering into these states and investigate the ability of this mechanism to explain features of the experimental observations, including the field dependence of the quality factor.   

On Demand

Recently, advances in film synthesis methods have enabled a study of extremely overdoped La_2-xSr_xCuO₄. This has revealed a surprising behavior of the superfluid density as a function of doping and temperature, the explanation of which is vividly debated. One popular class of models posits electronic phase separation, where the superconducting phase fraction decreases with doping, while some competing phase (e.g. ferromagnetic) progressively takes over. A problem with this scenario is that all the way up to the dome edge the superconducting transition remains sharp, according to mutual inductance measurements. However, the physically relevant scale is the Pearl penetration depth, Λ_P, and this technique probes the sample on a length scale L that is much larger than Λ_P. In the present paper, we use local scanning SQUID measurements that probe the susceptibility of the sample on the scale L << Λ_P. Our SQUID maps show uniform landscapes of susceptibility and excellent overall agreement of the local penetration depth data with the bulk measurements. These results contribute an important piece to the puzzle of how high-temperature superconductivity vanishes on the overdoped side of the cuprates phase diagram.