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[Advances in Imaging and Electron Physics] || superconductors and magnetic electron lenses
摘要： The use of superconductors seems a logical step if one wants to make iron-free magnetic lenses or to reduce the dimensions of conventional lenses since, when decreasing the coil size, the current density is increased. In the 1960s and 1970s, research mainly concentrated on the applicability of superconductors to magnetic electron lenses, and took place on a relatively large scale. This is reviewed in Section 2, after a general introduction into superconductivity in Section 1. Superconducting lenses and microscopes did not become popular due to their inconvenient operation and the lack of interest in high-voltage electron microscopy. High-voltage microscopy has been one of the main reasons for working on strong magnetic lenses and, consequently, on the utilization of superconductors. In 1986, with the discovery of high-temperature superconductivity, discussion on the applicability of superconductors to magnetic electron lenses was reopened. In the past, one of the most serious disadvantages in operating superconducting lenses had been related to the use of liquid helium refrigeration, so high-T c superconductors might be employed to overcome this problem, because their cooling demands are much more relaxed. However, especially during the first years of high-Tc superconductivity, despite their high operating temperatures, the materials themselves seemed extremely unfriendly, as they were brittle, sensitive to water, unstable, and difficult to produce. Fortunately, most of these disadvantages have now disappeared and the discussion concerning their utilization assumes a more fundamental character in the sense that most of the properties of the high-Tc materials are known, though a sound theoretical basis has not yet been defined. The properties of high-T c superconductors are the subject of Section 3. The question whether high-Tc superconductors are more appropriate for applications in particle optics than their classical counterparts was considered to be an interesting research topic. Therefore, this work was started as a feasibility study to the use of high-temperature superconductors in particle optics. Most short-term applications in this field were expected to exploit the high current density of these materials at temperatures above liquid helium, so this work concentrated on magnetic lenses as one of the most straightforward high current density applications. Since conventional iron circuit lenses are already used to their limits, as set by the saturation of the ferromagnetic circuit, significant improvements are to be expected only from iron-free lenses or highly saturated pole piece lenses. Their performance is restricted by the current density allowed in the windings and, further, for the iron-free lens, by the attainable mechanical tolerances, since, in the absence of iron, a lack of axial symmetry in the windings directly results in parasitic aberrations. For making small iron free lenses, high-Tc thin films are potentially interesting candidates, as they possess a high current density and can be patterned very accurately using lithographic techniques. Advantages to be expected from thin film lenses are smaller dimensions and better optical properties. An overview of fabrication techniques for high-Tc thin films is given in Section 4. The first attempt to make a coil in a superconducting thin film, using co-evaporated YBa2Cu3O7?x films, is the subject of Section 5. A thin film has to be patterned with some form of spiral in order to obtain a coil. Therefore, thin film lenses basically do not possess full axial symmetry. The relationship between the geometry of a flat coil and its optical properties is given in Section 6. The geometry of a feasible thin film lens element, along with its corresponding optical properties, is given in Section 7 while the technology used to fabricate this lens element is the subject of Section 8. Finally, based upon the work presented here, Section 9 treats the potential applicability of high-Tc superconductors in instruments that employ particle beams.
关键词： magnetic electron lenses,High-Tc superconductors,thin films,capacitive alignment,lens design,YBa2Cu3O7?x,superconductivity,electron microscopy
更新于2025-09-23 15:23:52
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The effect of size and aspect ratio on the trapped field properties of single grain, Y–Ba–Cu–O bulk superconductors
摘要： Bulk, single grain (RE)Ba2Cu3O7?δ [(RE)BCO, where RE is a rare earth element or yttrium] high temperature superconductors exhibit significant potential for use in a variety of engineering applications due to their ability to trap large magnetic fields, which can be up to ten times greater than those generated by conventional, iron-based magnets. Limitations on the maximum size to which single grains can be grown, however, are a major obstacle to the further development of these materials. Indeed, multiple samples are often required to achieve the required superconducting properties in particular applications. The geometry of bulk (RE)BCO single grain samples plays an important role in determining the superconducting properties of a given technical arrangement. In order to gain a better understanding of the full application potential of bulk single grain superconductors, three relatively long, cylindrical YBCO single grains of different diameters were fabricated and their trapped field and total trapped flux measured at 77 K as a function of sample height. The effects of size and aspect ratio of YBCO single grains on these key applied properties have been investigated experimentally and the results compared qualitatively with the predictions of an established theoretical model. Conclusions based on the trapped field measurements on a variety of single grain samples are presented in this study and the possibilities of using assemblies of smaller samples for engineering devices, in particular, are discussed.
关键词： distribution of the trapped field,aspect ratio,assemblies,bulk superconductors,applications
更新于2025-09-23 15:22:29
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Measuring the Lower Critical Field of Superconductors Using Nitrogen-Vacancy Centers in Diamond Optical Magnetometry
摘要： The lower critical magnetic field, Hc1, of superconductors is measured by optical magnetometry using ensembles of nitrogen-vacancy centers in diamond. The technique is minimally invasive and allows accurate detection of the vector magnetic field with subgauss sensitivity and submicrometer spatial resolution. These capabilities are used for detailed characterization of the first vortex penetration into superconducting samples from the corners. Aided by the revised calculations of the effective demagnetization factors of actual cuboid-shaped samples, these measurements provide precise determination of Hc1 and the related absolute value of the London penetration depth, λ. We apply this method to three well-studied superconductors: optimally doped Ba(Fe1?xCox)2As2, stoichiometric CaKFe4As4, and the high-Tc cuprate YBa2Cu3O7?δ. Our results compared well with the values of λ obtained with other techniques, thus adding another noninvasive and sensitive method to measure these important parameters of superconductors.
关键词： optical magnetometry,nitrogen-vacancy centers,London penetration depth,superconductors,lower critical field
更新于2025-09-23 15:22:29
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Fanoa??Andreev and Fanoa??Majorana Correspondence in Quantum Dot Hybrid Structures
摘要： A correspondence on the single-electron transport properties between systems of different nature, a topological quantum system and a (conventional) non-topological one, is demonstrated. Specifically, the equivalence between the Fano resonances obtained in T-shaped double quantum dot system coupled to a superconducting lead (QD–QD–S) and a topological superconductor ring nanowire (QD–MBSs) is presented. The non-zero value of the Fano (anti)resonance in the conductance of the QD-MBSs systems is shown as a consequence of a complex Fano factor qM, which is equivalent to the complex Fano factor qS of the QD–QD–S. The complex nature of qS is understood as a sign of a phase introduced by the superconducting lead in the QD–QD–S. It is because of this phase that the correspondence between the QD–QD–S and the QD–MBSs is possible. It is believed that these results can help to reveal the importance of the Fano interference process toward the understanding of the role of the superconductor lead in the QD–QD–S system as well as giving a clear signature of the presence of MBS’s in the topological system.
关键词： Majorana bound states,proximity effects,Andreev reflection,(anti)fano resonances,topological superconductors
更新于2025-09-23 15:19:57
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Multi-field dependence responses and their micro-structure analysis in YBa <sub/>2</sub> Cu <sub/>3</sub> O <sub/>7a??x</sub> -based second-generation coated conductors with different protective layers by Nd-laser irradiations
摘要： High-temperature superconductors (HTS) are materials that hold great promise for future particle-accelerator plants. YBa2Cu3O7-x-based (YBCO) second-generation coated conductor coils can produce the intense field needed for ion confinement. Such coils can act as a radiation resistance magnet when exposed to radiation, and therefore, it is essential to better understand irradiation responses and defect creation in YBCO superconductors. The influence of laser or ion irradiation on the electrical characteristics of HTS has been studied previously; however, most results remain ambiguous and inconsistent. Degradation of or incremental changes in the critical current of the HTS tap are currently only observed and explained via irradiation dose. The relation between irradiation defects and macro-multi-field responses has yet to be established for YBCO tapes, for magnet design and operation. In this study, YBCO (2 G HTS) taps with Ag, Cu–Ag, and Cu protective cap layers were selected as targets of Nd-laser irradiation, at varying power. Irradiation defects were formed in YBCO taps, and irradiation dose was monitored as a function of superconducting properties and mechanical behaviors. The critical current and its N-values eventually degrade under intense Nd-laser irradiation, and the mechanical behavior of YBCO tapes shows nonlinear features after irradiation. Scanning electron microscopy and x-ray energy-dispersive spectrometry were applied to investigate the micro-defect changes that occurred in the taps after each Nd-laser irradiation dose. Micro-structural observations showed that the protective layers were critical for radiation resistance in YBCO taps. Furthermore, the macro-stress dependence of the critical current field was measured before and after Nd-laser irradiation to establish the relationship with micro-defect morphology by Nd-laser irradiation.
关键词： micro-structure analysis,YBa2Cu3O7-x,critical current,High-temperature superconductors,Nd-laser irradiation
更新于2025-09-23 15:19:57
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revealed by spatially and angle-resolved photoemission spectroscopy
摘要： Nematicity, where rotational symmetry is broken while translational symmetry is conserved, is prevalent in high-temperature superconductors. In particular, nematic quantum critical point has been universally found near the optimum doping of the superconducting dome of several iron-based superconductor families. In such a regime, evidence for strong nematic ?uctuations have been observed. As the precursor to this order, nematic ?uctuations emerge before nematicity, providing favorable ground to study how nematic order modi?es the electronic structure in the absence of structural distortion. Here we use spatially resolved angle-resolved photoemission spectroscopy to investigate the correlation between the onset of nematic ?uctuations and electronic structure in an optimally doped BaFe2(As1?xPx )2 (x ～ 0.3) superconductor. We reveal a strong spatially varying anisotropy of the Fermi surface on a length scale of tens of microns with strong correlation between the changes in the hole and electron Fermi pockets, consistent with the variations expected in the presence of ?uctuating nematic order. These results provide direct evidence for spatial nematic ?uctuations in the optimal doping regime of iron-based superconductors.
关键词： iron-based superconductors,high-temperature superconductors,angle-resolved photoemission spectroscopy,nematicity,Fermi surface
更新于2025-09-19 17:13:59
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Oxygen in Complex Oxide Thin Films Grown by Pulsed Laser Deposition: a Perspective
摘要： For thin film synthesis of complex oxides, one of the most important issues has always been how to oxidise the material. For a technique like pulsed laser deposition, a key benefit is the relatively high oxygen background pressure one can operate at, and therefor oxidation should be relatively straightforward. However, understanding the microscopic oxidation mechanisms turns out to be rather difficult. In this perspective, we give a brief overview of the sources of oxidation for complex oxide thin films grown by pulsed laser deposition. While it is clear what these sources are, their role in the kinetics of the formation of the crystal structure and oxygen stoichiometry is not fully understood.
关键词： High Tc superconductors,Oxidation,Thin films,Complex oxides,Pulsed laser deposition
更新于2025-09-19 17:13:59
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Anomalous doping evolution of nodal dispersion revealed by <i>in situ</i> ARPES on continuously doped cuprates
摘要： We study the systematic doping evolution of nodal dispersions by in situ angle-resolved photoemission spectroscopy on the continuously doped surface of a high-temperature superconductor Bi2Sr2CaCu2O8+x and reveal that the nodal dispersion has three fundamentally different segments separated by two kinks, located at ～10 meV and roughly 70 meV, respectively. These three segments have different band velocities and different doping dependence. In particular, in the underdoped region the velocity of the high-energy segment increases monotonically as the doping level decreases and can even surpass the bare band velocity. We propose that electron fractionalization is a possible cause for this anomalous nodal dispersion and may even play a key role in the understanding of exotic properties of cuprates.
关键词： electron fractionalization,high-temperature superconductors,nodal dispersion,ARPES,cuprates
更新于2025-09-12 10:27:22
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[IEEE 2018 48th European Microwave Conference (EuMC) - Madrid (2018.9.23-2018.9.27)] 2018 48th European Microwave Conference (EuMC) - Dispersion of THz Modes Localized on Layered Superconductor Controlled by DC Magnetic Field
摘要： Being materials that support terahertz waves propagation, layered superconductors attract great attention of many researchers. The Josephson plasma modes localized on a slab of layered superconductor can possess an anomalous dispersion. The possibility of anomalous dispersion manipulation opens wide perspectives for applications. One of the tools that can flexibly change the electromagnetic properties of layered superconductors is the external DC magnetic field. The effect of the DC magnetic field on the spectrum of the localized modes is a subject of the present paper. We present the derivation and analysis of the dispersion relations and discuss some interesting corollaries such as resonant amplification of wave transmission induced by the internal excitation of the localized modes and the possibility of the internal reflection of the localized modes controlled by the external DC magnetic field.
关键词： high-temperature superconductors,electromagnetic fields,Terahertz metamaterials,Josephson junctions
更新于2025-09-12 10:27:22
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Visualization of a Berezinskii–Kosterlitz–Thouless Topological Phase Transition in a Josephson Medium: Detection of an Anomalous Temperature Dependence of the Magnetoresistance of YBa2Cu3O7–δ Granular High-Temperature Superconductors
摘要： The possibility of implementing a Berezinskii–Kosterlitz–Thouless topological phase transition in a Josephson medium of two-level high-temperature superconductors induced by a magnetic field has been studied. The effect of temperature and external magnetic field on the transport properties of the YBa2Cu3O7 – δ granular high-temperature superconductor considered as a model object is studied experimentally. For the first time, an anomalous behavior of the magnetoresistance isotherms is detected in a narrow range of temperature and external magnetic field indicating a topological phase transition in the Josephson medium of granular high-temperature superconductors.
关键词： YBa2Cu3O7 – δ,high-temperature superconductors,magnetoresistance,Josephson medium,Berezinskii–Kosterlitz–Thouless topological phase transition
更新于2025-09-10 09:29:36