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Imaging local magnetic fields produced by nano- and micrometer-scale superconductors has become a vital tool that can not only reveal crucial information on the vortex dynamics and order parameters of the superconducting materials but also visualize the working mechanism of superconducting devices made for quantum information. Here, we performed measurements of the magnetic field distributions of mesoscopic superconducting structures with various geometries by combining a thin ferromagnetic metal layer as a magneto-optical sensing element that responds to the environmental magnetic fields and a custom-made sensitive Sagnac interferometer. The sensitivity of the technique enables the observation of magnetic flux jumps due to individual vortex entries into nanostructured superconductors. In addition, with the control of incident power at a focused laser spot, we induce thermally driven movement of vortices that leaves a trace of a microscopic optical heating pattern. [ABSTRACT FROM AUTHOR]

Under the spell of BCS-electron–phonon theory [M. Tinkham, Introduction to Superconductivity, 2nd ed. (McGraw Hill, New York, 1996)], during the last 6 years experimentalists have purportedly discovered a plethora of high temperature conventional superconductors among pressurized hydrides [Pickard et al., Ann. Rev. Condens. Matter Phys. 11, 57 (2020) and R. F. Service, Science 373, 954 (2021)], and theorists have been busy predicting and explaining those findings [Lv et al., Matter Radiat. Extremes 5, 068101 (2020); Flores-Livas et al., Phys. Rep. 856, 1 (2020); and Boeri et al., J. Phys. Condens. Matter. (to be published)]. The alternative theory of hole superconductivity (see https://jorge.physics.ucsd.edu/hole.html for a list of references) predicts instead that no superconductivity can exist in these materials. In this Tutorial, I will first argue that, unclouded by the prejudice of BCS's validity, the existing experimental evidence for superconductivity in pressurized hydrides does not withstand scrutiny. Once it is established that superconductivity in pressurized hydrides is a myth and not a reality, the claim to validity of BCS-electron–phonon theory as a descriptor of superconductivity of real materials will be forever shattered, and an alternative theory will become imperative. I will explain the fundamentals of the theory of hole superconductivity, developed over the past 32 years [see https://jorge.physics.ucsd.edu/hole.html and J. E. Hirsch, Phys. Lett. A 134, 451 (1989)], and why it is compelling. Crucially, it explains the Meissner effect, that I argue the conventional theory does not. It applies to all superconducting materials and provides guidelines in the search for high temperature superconductors that are very different from those provided by BCS-electron–phonon theory. Light elements are predicted to be irrelevant to warm superconductivity because according to this theory the electron–phonon interaction plays no role in superconductivity. [ABSTRACT FROM AUTHOR]

In recent years, the field of flexible electronics has become one of the cross-disciplinary research hotspots, attracting worldwide attention and making rapid advances. So far, there has been plenty of research on the use of two-dimensional (2D) materials in flexible electronics, including graphene, transition metal dichalcogenide, and so on. In this work, we successfully prepared quasi-one-dimensional (Q1D) Nb2Pd0.73S5.97 superconductors on flexible paper by mechanical friction and systematically studied their physical properties at low temperatures. Superconductivity with transition temperature (Tc) ∼ 6.05 K by Meissner effect was observed in Nb2Pd0.73S5.97 wires coated on flexible paper, and a resistance drop at 4.80 K was confirmed in electrical transport measurements. The lower critical field (Hc1) of coated paper shows anisotropy effect under parallel and perpendicular magnetic fields, exhibiting a 2D-like feature, unlike the bulk Nb2Pd0.73S5.97 fibers. Our work provides a broader platform for the application of low-dimensional materials in flexible functional devices. [ABSTRACT FROM AUTHOR]

We show that a ferromagnetic (F) slab with the in-plane magnetization sandwiched between two superconducting (S) films experiences strong demagnetization effect due to the Meissner screening of the stray magnetic field by superconductors. In the extreme case, the transition of the S film from normal to the superconducting state can switch the demagnetization factor from 0 to 1, which is in a sharp contrast with the S/F bilayers where such transition affects the magnetic field inside the F film only slightly. The giant demagnetization effect is shown to be qualitatively robust against the decreasing superconducting film thickness and may provide a hint toward the explanation of the anomalously large ferromagnetic resonance frequency shift recently observed for the S/F/S structures [Golovchanskiy et al., Phys. Rev. Appl. 14, 024086 (2020)]. [ABSTRACT FROM AUTHOR]

Nanocrystalline freestanding Pb nanowires ~200 nm in diameter were fabricated by electrodeposition into track etched polycarbonate membranes in order to study their superconducting properties. Their superconducting critical temperature, as determined by measuring the Meissner effect, was the same as for bulk Pb, but their critical field was greatly enhanced up to ~3000 Oe. By assuming the wires consisted of spherical superconducting grains, an estimated grain size r=60±25 nm was obtained from the magnetization measured as a function of the applied magnetic field at a fixed temperature. An independent estimate for r=47±12 nm, in good agreement with the previous one, was obtained from the magnetization measured as a function of temperature at a fixed applied magnetic field. Transmission electron microscopy was used to characterize grain size at the wire edges, where a grain size in agreement with the magnetic studies was observed. [ABSTRACT FROM AUTHOR]

Experiment and analytical calculations show that the demagnetizing field of a superconductor is a sensitive probe of quantities otherwise difficult to measure, such as the sample-probe distance in flux-density imaging experiments and the field of first flux penetration Hp. In particular, the ratio of the maximum field measured above the superconductor edge and the applied field can be determined unambiguously so as to define a linear 'geometric' susceptibility. The evolution of this susceptibility with field depends on the regime of flux penetration and can be used as a means to determine Hp and the effect of a parallel field component in magneto-optical imaging experiments. [ABSTRACT FROM AUTHOR]

The flux jump in bulk superconductors is accompanied by a rapid change in temperature and magnetic field, which can induce change in electromagnetic bodyforce and thermal stress. It is well known that bulk superconductors are brittle and have low mechanical strength, and thus, large electromagnetic bodyforce and thermal stress can cause damage of the bulk superconductor. In this paper, an electromagnetic-thermal-mechanical multi-physics model is adopted to compute the mechanical response of a bulk superconductor during flux jump in an external magnetic field. The results indicate that the flux jump in the bulk superconductors can also lead to the jump of the average electromagnetic force, temperature, stress, and strain. Meanwhile, it can be found that the flux jump can occur more easily with a faster change in the magnetic field, a lower ambient temperature, and a large-size superconductor. The results also show that the peak value of thermal strain is much larger than the strain generated by electromagnetic bodyforce during the flux jump. In addition, the change in strain has the same trend as that of the temperature. Thus, the strain may also be used to monitor the flux jump. [ABSTRACT FROM AUTHOR]

The intrinsic properties of superconductors enable the direct determination of the absolute Seebeck coefficient at low temperature due to the disappearance of the Seebeck effect to obey the Meissner effect. We report a precision absolute Seebeck coefficient measurement for the fine Pt sample determined using the high-Tc YBa2Cu3O7-x (YBCO) superconductor as a reference and an analysis of the measurement uncertainty. To make a precision measurement and aid in the verification of the uncertainty components, we developed a cryostat system that enables temperature control in a stable manner. The expected performance of the reference superconductor yielded a zero value well below Tc, which was validated by a superconductor-superconductor thermocouple experiment. Uncertainty analysis shows that the main limiting factor for this measurement is the accuracy of the temperature difference measurement using the resistance temperature sensors, along with its analog noise. We obtained values of S = 5.6 ± 0.2 µV/K with a relative expanded uncertainty of 3% at 80 K and precisely compared the Pt value with that determined by the high-Tc Bi2Sr2Ca2Cu3O8+δ (Bi-2223) superconductor, which has a higher Tc. We found that there was no difference between the Seebeck coefficient values obtained from the YBCO and Bi-2223 references up to its Tc within the expanded measurement uncertainties of 0.3 µV/K (2σ). These results provide accurate validation that the high-Tc superconductor is a useful reference up to the liquid nitrogen temperature. [ABSTRACT FROM AUTHOR]

We present a new measurement method which can be used to image the gap nodal structure of superconductors whose pairing symmetry is under debate. This technique utilizes a high quality factor microwave resonance involving the sample of interest. While supporting a circularly symmetric standing wave current pattern, the sample is perturbed by a scanned laser beam, creating a photoresponse that was previously shown to reveal the superconducting gap anisotropy. Simulation and the measurement of the photoresponse of an unpatterned Nb film show less than 8% anisotropy, as expected for a superconductor with a nearly isotropic energy gap along with expected systematic uncertainty. On the other hand, measurement of a YBa2Cu3O7−δ thin film shows a clear 4-fold symmetric image with ∼12.5% anisotropy, indicating the well-known 4-fold symmetric d x 2 − y 2 gap nodal structure in the ab-plane. The deduced gap nodal structure can be further cross-checked by low temperature surface impedance data, which are simultaneously measured. The important advantage of the presented method over the previous spiral resonator method is that it does not require a complicated lithographic patterning process which limits one from testing various kinds of materials due to photoresponse arising from patterning defects. This advantage of the presented technique, and the ability to measure unpatterned samples such as planar thin films and single crystals, enables one to survey the pairing symmetry of a wide variety of unconventional superconductors. [ABSTRACT FROM AUTHOR]

The polarized neutron imaging technique provides a non-invasive method of characterizing localized magnetic fields inside superconductors. However, complete understanding of the magnetic field distribution has yet to be realized experimentally due to the complexity of the interaction between neutron polarization and magnetic field. In this article, we show that a well-defined and controlled magnetic field through the neutron path contributes to simplify the data analysis and makes future quantitative polarized neutron imaging possible. This is demonstrated in a set of experiments that visualize the magnetic field distribution inside and around the superconductors. The experimental results demonstrate that proper guide field setup allows the visualization of the magnetic field expulsion at the surface of the superconductor in the zero-field cooling condition, as well as the magnetic field trapped inside the superconductor under field cooling condition. [ABSTRACT FROM AUTHOR]

Magnetization as a function of the magnetic field m(H) is studied in the T=2-300 K temperature range at magnetic fields H ≤ 30 kOe in semiconducting alloys (PbzSn1–z)0.84In0.16Te with a lead content of z=0.3; 0.4, which corresponds to the highest critical parameters of the superconducting transition. For the sample (Pb0.4Sn0.6) 0.84In0.16Te the critical temperature Tc≈ 3.8 K and the critical magnetic field Hc2= 22 kOe (T=2 K). At T3 K the peak-effect is no longer observed. The obtained data is used as a basis for discussing the possible nature of the peak-effect observed in (PbzSn1–z)0.84In0.16Te, z=0.3 and 0.4. Models based on an analysis of the system of pinning centers in the material, and the vortex structure dynamics under changing external parameters, are considered. [ABSTRACT FROM AUTHOR]

Experimental studies of the effect of methane condensation temperature on the value of the refractive index and density of the resulting thin films are reported. The main unit of the installation is a high-vacuum chamber, which routinely operates at 10-8-10-6 Torr. Measurements using a twobeam He-Ne laser interferometer in the vicinity of the methane phase transition temperature T=20.4K in the range of 14-32K were carried out. It has been shown that in the vicinity of T=20K the temperature dependence of the refractive index undergoes an abrupt decrease with decreasing temperature. It is assumed that this gap is the result of the phase transition from the orientational disordered phase (α-phase) to the partially ordered phase (β-phase) of solid methane. The calculations of the polarizability of the methane molecules in the solid phase at two values of the deposition temperature T=16K and T=30K were performed using the Lorentz-Lorenz equation. [ABSTRACT FROM AUTHOR]

The directional curl spaces are introduced, which consist of vector fields that have directional curl along the given unit length vector fields. The basic properties of these spaces are derived, including the trace theorem and the representation of the dual spaces. The directional curl spaces are used to analyze the asymptotic behavior of the Meissner states of strongly anisotropic superconductors, where the limiting energy functionals are non-coercive in the usual Sobolev spaces. [ABSTRACT FROM AUTHOR]

We study magnetic field penetration into a thin film made of a superconducting niobium. Imaging of magnetic field is performed by optically detecting magnetic resonances of negatively charged nitrogen-vacancy defects inside a single crystal diamond, which is attached to the niobium film under study. The experimental results are compared with theoretical predictions based on the critical state model, and good agreement is obtained. [ABSTRACT FROM AUTHOR]

Recently, it was discovered that the non-uniform Meissner current flowing around the pinning sites in the type-II superconductor induces the unconventional vortex-antivortex pairs with the nonquantized magnetic flux [Ge et al., Nat. Commun. 6, 6573 (2015)]. Here, we provide the theory of this phenomenon showing that the vortex-like structures originate from the perturbation of the current streamlines by the non-superconducting defect, which results in the generation of the localized magnetic field. The position and the shape of such vortex dipoles are shown to be very sensitive to the defect form. Thus, applying the external magnetic field or current to the superconductor and using, e.g., the high-resolution scanning Hall microscope to measure the stray magnetic field, one can plot the map containing the information about the position of the defects and their shape. [ABSTRACT FROM AUTHOR]

Mapping the distribution of currents inside a superconductor is usually performed indirectly through imaging of the stray magnetic fields above the surface. Here, we show that by direct imaging of the Doppler shift contribution to the quasiparticle excitation spectrum in the superconductor using low temperature scanning tunneling microscopy, we obtain directly the distribution of supercurrents inside the superconductor. We demonstrate the technique at the example of superconductor/ ferromagnet hybrid structure that produces intricate current pattern consisting of combination Meissner shielding currents and Abrikosov vortex currents. [ABSTRACT FROM AUTHOR]

A new extensional rheometer limited in achievable strain only by sample rupture is presented. The rheometer is based on the Meissner design of two pairs of counter-rotating rollers pulling a sample of fixed length but is small enough to fit inside the oven of a standard rotational rheometer and thus is dubbed the Meissner Extensional Rheometry Accessory (MERA). The true strain rate is calculated by visually accessing the sample during deformation using a high-speed digital camera. Extensional experiments were performed on three materials representing a wide range expected rheological behavior, a styrene-butadiene rubber blend, a linear polystyrene, and a branched lowdensity polyethylene. The MERA was able to accurately replicate the results of the well-known Sentmanat extensional rheometer (SER) design in terms of onset of strain-hardening and absolute transient extensional viscosity values. However, due to its design, it was possible to achieve homogeneous extensional flow up to real Hencky strains in excess of 8, which corresponds to a linear stretch in excess of 3000. By comparison, the SER is limited to one drum revolution, which corresponds to a Hencky strain of 3.5-4.0 or a maximum linear stretch of approximately 50. Previously the highest Hencky deformations reported in the literature are for the filament stretching extensional rheometer and Rheometric Scientific RME apparatus and in both cases approach 7, which corresponds to a linear stretch of approximately 1000. [ABSTRACT FROM AUTHOR]

Cloaking of static magnetic fields by a finite thickness type-II superconductor tube being in the full critical state and surrounded by a coaxial paramagnet shell is studied. On the basis of exact solutions to the Maxwell equations, it is shown that, in addition to previous studies assuming the Meissner state of the superconductor constituent, perfect cloaking is still realizable at fields higher than the field of full flux penetration into the superconductor and for arbitrary geometrical parameters of both constituents. It is also proven that simultaneously the structure is fully undetectable under the cloaking conditions. Different from the case of the Meissner state, the cloaking properties in the application relevant critical state are realized, however, only at a certain field magnitude. [ABSTRACT FROM AUTHOR]

We investigate "hot" regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-μSR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth λL = 23±2 nm. In contrast, a cutout sample from the 120 °C baked cavity shows a much larger λ >100 nm and a depth dependent mean free path, likely due to gradient in vacancy concentration. We suggest that these vacancies can efficiently trap hydrogen and hence prevent the formation of hydrides responsible for rf losses in hot regions. [ABSTRACT FROM AUTHOR]

Although forces between magnets and superconductors have been profusely studied in many previous papers, little attention has been paid to calculations or measurements of the torque exerted. In this paper, we focus on the equilibrium orientation of a magnet when it is levitating over a superconductor and present measurements for different positions and configurations. We compare these measurements with values predicted by our proposed local model. Agreement between predicted values and experimental values demonstrates that the model is useful for calculating torques in addition to forces. The angle of equilibrium varies along the path over the superconducting cylinder. Therefore, shape and size determine the angle of equilibrium of the magnet. [ABSTRACT FROM AUTHOR]

In this work, we report observation of two superconducting transitions and their response to an external field in pulsed laser deposited YBa2Cu3O7/La0.7Sr0.3MnO3/YBa2Cu3O7 (Y-L-Y) hetero-epitaxial tri-layer. Magnetization data of Y-L-Y tri-layer in 'out-plane' geometry showed a single Meissner transition indicating superconductivity of the top and bottom YBa2Cu3O7 layers to be magnetically coupled. In contradiction, the magneto-transport data unusually showed two transition onsets Tcon1 and Tcon2 implying electrically decoupled superconductivity. The Tcon1 decreased systematically with field, whereas Tcon2 showed anomalous behavior with substantial increase in transition width due to the local stray field induced vortex states. [ABSTRACT FROM AUTHOR]

Partially suppressed Meissner effect and magnetic flux trapping in the bulk of poly-crystalline Pb samples with virtually zero demagnetization and large (cm3) volume containing non-superconducting parts could be visualized by polarized neutrons and quantified with respect to the shape and amount of the trapped flux in the intermediate state of lead. These measurements provided a unique look at the macroscopic Quantum mechanics effects and the coexistence of superconducting state, intermediate state, and normal conducting parts in the same sample, respectively. [ABSTRACT FROM AUTHOR]