Table of contents

Future high energy physics colliders could benefit from accelerator magnets based on high-temperature superconductors, which may reach magnetic fields of up to 45 T at 4.2 K, twice the field limit of the two Nb-based superconductors. Bi₂Sr₂CaCu₂O_8-x (Bi-2212) is the only high-T_c cuprate material available as a twisted, multifilamentary and isotropic round wire. However, it has been hitherto unclear how an accelerator magnet can be fabricated from Bi-2212 round wires and whether high field quality can be achieved. This paper reports on the first demonstration of high current Bi-2212 coils using Rutherford cable based on a canted-cosine-theta (CCT) design and an overpressure processing heat treatment. Two Bi-2212 CCT coils, BIN5a and BIN5b, were made from a nine-strand Rutherford cable. Their electromagnetic design is identical, but they were fabricated differently: both coils underwent heat treatment in their aluminum–bronze mandrels, but unlike BIN5a that was impregnated with epoxy in its reaction mandrel, the conductor of BIN5b was transferred to a 3D printed Accura Bluestone mandrel after the heat treatment, a process attempted here for the first time, and was not impregnated. BIN5a reached a peak current of 4.1 kA with a self-field of 1.34 T in the bore. This corresponds to a wire engineering current density (J_e) of 912 A mm⁻², which is two times that of BIN2-IL, a previous Bi-2212 CCT coil fabricated at LBNL, which used a six-around-one cable processed with the conventional 1 bar pressure melt processing. On the other hand, BIN5b reached 3.1 kA. The coils exhibited no quench training. All the quenches were thermal runaways that occurred at the same location. In addition, we report on the field quality and ramp-dependent hysteresis measurements taken during the test of BIN5a at 4.2 K. Overall, our results demonstrate that the CCT technology is a route that should be further investigated for making high field, potentially quench training free dipole magnets with Bi-2212 cables.

Recent progress in the development of superconducting nanowire single-photon detectors (SNSPD) has delivered excellent performance, and their increased adoption has had a great impact on a range of applications. One of the key characteristic of SNSPDs is their detection rate, which is typically higher than other types of free-running single-photon detectors. The maximum achievable rate is limited by the detector recovery time after a detection, which itself is linked to the superconducting material properties and to the geometry of the meandered SNSPD. Arrays of detectors biased individually can be used to solve this issue, but this approach significantly increases both the thermal load in the cryostat and the need for time processing of the many signals, and this scales unfavorably with a large number of detectors. One potential scalable approach to increase the detection rate of individual detectors further is based on parallelizing smaller meander sections. In this way, a single detection temporarily disables only one subsection of the whole active area, thereby leaving the overall detection efficiency mostly unaffected. In practice however, cross-talk between parallel nanowires typically leads to latching, which prevents high detection rates. Here we show how this problem can be avoided through a careful design of the whole SNSPD structure. Using the same electronic readout as with conventional SNSPDs and a single coaxial line, we demonstrate detection rates over 200 MHz without any latching, and a fibre-coupled SDE as high as 77%, and more than 50% average SDE per photon at 50 MHz detection rate under continuous wave illumination.

In recent years, the applications of high-field REBa₂CuO_y (REBCO) coils have remarkably progressed towards NMR MRIs, accelerators, and other such devices. In a REBCO coil, the magnetic field due to the screening current is generated in the direction opposite to the field by the transport current, thus reducing the magnetic field, deteriorating the field homogeneity, and affecting the time stability of the magnetic field. Recently, the additional force and stress due to the screening current (referred to as screening-current-induced stress) has become a topic of concern. The screening current results in non-uniform current distributions in the REBCO tape. Therefore, the distribution of electromagnetic force in REBCO coils differs from that during the designing of the coil, assuming that the current flow in the tape is uniform. Thus, there is the possibility that the existence of screening current is a serious problem in the mechanical design of REBCO coils. In this study, we evaluate the electromagnetic force and stress due to the screening current by using the developed numerical simulation code for the electromagnetics and stress. We discuss the winding tension, thermal and electromagnetic stresses, and mechanical strength structure of the REBCO coil.

For next-generation accelerator magnets for fields beyond those achievable using Nb–Ti, Nb₃Sn is the most viable superconductor. The high luminosity upgrade for the Large Hadron Collider (HL-LHC) marks an important milestone as it will be the first project where Nb₃Sn magnets will be installed in an accelerator. Nb₃Sn is a brittle intermetallic, so magnet coils are typically wound from composite strands containing ductile precursors before heat treating the wire components to form Nb₃Sn. However, some mechanical assembly is still required after the coils have been heat-treated. In this paper, we present direct evidence of cracking of the brittle Nb₃Sn filaments in a prototype dipole that resulted in degraded magnet performance. The cracking of the Nb₃Sn, in this case, can be attributed to an issue with the collaring process that is required in the assembly of dipole accelerator magnets. Metallographic procedures were developed to visualize cracks present in the cables, along with quantitative image analysis for location-based crack analysis. We show that the stresses experienced in the damaged coil are above the critical damage stress of Nb₃Sn conductor, as evidenced by a measured Cu stabilizer hardness of 85 HV_0.1, which is higher than the Cu stabilizer hardness in a reference Nb₃Sn cable ten-stack that was subjected to a 210 MPa transverse compression. We also show that once the collaring procedure issue was rectified in a subsequent dipole, the Nb₃Sn filaments were found to be undamaged, and the Cu stabilizer hardness values were reduced to the expected levels. This paper provides a post-mortem verification pathway to analyze the damage, provides strand level mechanical properties, which could be beneficial for improving model prediction capabilities. This method could be applied beyond Nb₃Sn magnets to composite designs involving high work hardening materials.

Numerous experiments have shown that the loads applied to Nb₃Sn strands and cables can reduce their critical current. Experiments, performed on uniaxially loaded strands, allowed to define clear laws to describe the evolution of the critical surface as a function of the applied current, field, temperature and strain. It is, however, still unclear how these laws can be applied to superconducting magnets. The present paper proposes a methodology to estimate the critical current and temperature margin reduction on superconducting magnets due to stress on the superconducting material. The methodology is tested on the MQXF magnets, a quadrupole developed for the High Luminosity LHC project, and successfully validated by comparing computed strain with data from strain gauge measurements. Results suggested that, because of the stresses arising in winding during assembly, cool-down and powering, the current limit of the magnet is lower than the expected short sample limit, and that the most critical region does not coincide with the peak field location.

The magnetization in a superconductor induced due to the inverse proximity effect is investigated in hybrid bilayers containing a superconductor and a ferromagnetic insulator or a strongly spin-polarized ferromagnetic metal. The study is performed within a quasiclassical Green function framework, wherein Usadel equations are solved with boundary conditions appropriate for strongly spin-polarized ferromagnetic materials. A comparison with recent experimental data is presented. The singlet to triplet conversion of the superconducting correlations as a result of the proximity effect with a ferromagnet is studied.

Direct current tests performed in the past on the conductor samples of the toroidal field (TF) ITER coils revealed degradation of current sharing temperature, T_cs. The degradation progresses with repetitive electromagnetic loading, and also with thermal cycles between 4.5 K and room temperature. This feature was observed on short samples in SULTAN test facility (EPFL-SPC, Switzerland) as well as in TF Insert Coil tests in CSMC test facility (Naka, Japan). We present three independent observations suggesting that initiation of sample quench followed by a fast current discharge, which normally complements every I_c and T_cs test in both SULTAN and CSMC, enhances the T_cs degradation rate. The exact mechanism of this contribution to the degradation remains unidentified.

A common feature of commercially available conductors based on high-temperature superconducting compounds is the fluctuation of critical current along the length. Fortunately, the practice adopted by manufacturers nowadays is to supply the detailed I_c(x) data with the conductor. Compared to knowing just the average of critical current, this should also allow a much better prediction of the conductor performance. Statistical methods are suitable for this purpose in the case when the fluctuations are regular at the low end of critical current distribution. However, a different approach is necessary at the presence of 'weak spots' that drop out of any statistics. Because of the strong nonlinearity of the current–voltage curve, such a location could transform into a 'hot spot' at transporting direct current (DC), with an abrupt increase of temperature endangering the conductor operation. We present a set of analytical formulas including the prediction of the maximum DC that could be carried sustainably before the thermal runaway appears. It is necessary to know the cooling conditions as well as the properties of the conductor constituents and their architecture. A formula for the voltage appearing on a weak spot, and its dependence on the DC, is also proposed. For this purpose the result of previous theoretical work has been slightly modified after comparing it with numerical iterative computations and finite element modeling. We demonstrate that the derived model allows a powerful analysis of experimental data comprising an estimation of the weak spot parameters i.e. its critical current and the length of the defect zone.

In recent years, renewed interest has been given to alternatives to bulk Nb for use in superconducting radio frequency (SRF) cavities. Much of this research has involved the use of thin films of Nb or other alternative high T_c materials. Given the recent insights into the potential of single layer high ${{ - \kappa }}$ materials and the prospective performance improvements due to the use of multilayer (ML) superconductor–insulator–superconductor (SIS) film structures, NbN stands as a potential candidate for use in future accelerating cavities. Much of the research completed thus far has focused on the deposition of NbN onto single crystal substrates such as Si and MgO, or onto bulk Nb. In this study, the deposition of high T_c NbN thin films onto copper substrates, in the form of both single layers and as part of an ML SIS structure, using DC magnetron sputtering (DC MS) was explored. The effects of the deposition parameters on the film microstructure and superconducting properties of the NbN films as well as the challenges involved in depositing ML SIS films onto copper substrates are reported on. A maximum T_c = 16.1 K has been achieved for single layer NbN films, as determined by AC susceptometry measurements. Initial results for the SIS film structures, deposited onto copper in the form of Nb/AlN/NbN, have also shown an enhancement in the first magnetic flux entry field value above that achieved by a single Nb layer. This enhancement has been found to be highly dependent on the quality of the SIS film coatings.

Nb₃Sn is a promising next-generation material for superconducting radiofrequency cavities, with significant potential for both large scale and compact accelerator applications. However, so far, Nb₃Sn cavities have been limited to continuous wave accelerating fields <18 MV m⁻¹. In this paper, new results are presented with significantly higher fields, as high as 24 MV m⁻¹ in single cell cavities. Results are also presented from the first ever Nb₃Sn-coated 1.3 GHz 9-cell cavity, a full-scale demonstration on the cavity type used in production for the European XFEL and LCLS-II. Results are presented together with heat dissipation curves to emphasize the potential for industrial accelerator applications using cryocooler-based cooling systems. The cavities studied have an atypical shiny visual appearance, and microscopy studies of witness samples reveal significantly reduced surface roughness and smaller film thickness compared to typical Nb₃Sn films for superconducting cavities. Possible mechanisms for increased maximum field are discussed as well as implications for physics of RF superconductivity in the low coherence length regime. Outlook for continued development is presented.

Low-angle grain boundaries (GBs) constitute the most important current-limiting mechanism in the operation of biaxially textured YBa₂Cu₃O_7−d (YBCO)-coated conductors. Ca doping of YBCO is known to improve the critical current density J_c across the GB because of carrier doping by anisovalent Ca²⁺ substitution for Y³⁺ and the strain relief induced by Ca segregation at the GB cores; however, the reduction of the superconducting critical temperature T_c accompanying such doping is a marked drawback. Here we study the substitution of isovalent Nd³⁺ for Y³⁺ again using strain-driven segregation, in this case Nd³⁺, to improve J_c without incurring significant T_c reduction. Transport characteristics of low-angle GBs of 10% Nd-doped YBCO, Y_0.9Nd_0.1Ba₂Cu₃O_7−d, grown on single crystal and 6° and 9° [001] tilt symmetric bicrystal SrTiO₃ substrates are reported. It was found that J_c across the 6° GB recovers to the intra-grain J_c value in the 10% Nd-doped YBCO, while the 9° GB shows a modest J_c enhancement compared to the pure YBCO 9° GB without a significant T_c reduction. It is shown that the transparency of the GB could be enhanced without a large T_c reduction by the isovalent substitution of rare-earth ions, suggesting new opportunities for cation segregation engineering in YBCO by isovalent rare-earth substitution.

Although the production of YBa₂Cu₃O_7-δ (Y123) has been extensively reported, there is still a lack of information on the ideal heat treatment to produce this material in the form of one dimension nanostructures. Thus, by means of the Solution Blow Spinning technique, metals embedded in polymer fibers were prepared. These polymer composite fibers were fired and then investigated by thermogravimetric analysis. The maximum sintering temperatures of heat treatment were chosen in the interval 850 °C–925 °C for 1 h under oxygen flux. SEM images allowed us to determine the wire diameter as approximately 350 nm for all samples, as well as to map the evolution of the entangled wire morphology with the sintering temperature. XRD analysis indicated the presence of Y123 and secondary phases in all samples. Ac magnetic susceptibility and dc magnetization measurements demonstrated that the sample sintered at 925 °C/1 h is the one with the highest weak-link critical temperature and the largest diamagnetic response.

Bulk samples of magnesium diboride (MgB₂) doped with 0.5 wt% of the rare earth oxides (REOs) Nd₂O₃ and Dy₂O₃ (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB₂ doped with 0.5 wt% Dy₂O₃ (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T_c), magnetic and resistive measurements of upper critical field (B_c2) and irreversibility field (B_irr), as well as magnetic and transport measurements of critical current densities versus applied field (J_cm(B) and J_c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB₂ lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F_p) versus reduced field (b = B/B_irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F_p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F_p(b) was still further increased by a Dy₂O₃ additive-induced increase in B_irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy₂O₃ increases B_irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB₂ conductors.

This paper proposes a method for designing a miniaturized ultra-narrowband high-temperature superconducting (HTS) linear phase filter based on the resonator-shared cascaded quadruplet (CQ) units constructed by the quarter-wavelength microstrip line. Two types of CQ unit with different coupling configurations are utilized to introduce transmission zeros and realize the linear phase. The introduction of transmission zeros increases the transition band roll-off rate, and the in-band group delay characteristic is improved through the phase compensation. The number of resonators and the filter size further decrease by sharing a resonator between two adjacent CQ units. Finally, a ten-order compact ultra-narrowband linear phase HTS filter with three CQ units is designed to demonstrate the feasibility. The central frequency of the filter is 450 MHz with the fractional bandwidth of 2.22%, and the dimension is 38.7 mm × 12.5 mm (0.141λ_g0 × 0.047λ_g0, λ_g0 is the guided wavelength at the central frequency f₀). The measured results show an excellent performance in the insertion loss, the out-of-band rejection, the return loss, and the passband selectivity. Moreover, the bandwidth with a group delay ripple within 30 ns occupies 70% of the entire passband, highlighting the linear phase feature. In addition, the first parasitic passband is located at 3.8 times of the fundamental frequencies. The test results are in good agreement with the simulated ones.

The short-circuit levels have increased considerably in transmission and distribution systems in the last years. Fault current limiter (FCL) devices are a potential solution to this problem. Among several FCL topologies, this group has good experience in the use of superconducting fault current limiters (SFCL) to reduce the electrical current during short-circuits. The literature also presents studies of the saturated iron core superconducting fault current limiter (SIC-SFCL) topology employing mathematical modeling and prototypes design. Some of them have shown promising results, including the construction of pilot prototypes in medium and high voltage substations. The SIC-SFCL simulation studies presented optimal topologies that reduce the amount of ferromagnetic material used in the core and represent well the behavior of this limiter. The finite element method and the finite element analysis are suitable to model the SIC-SFCL. However, a more detailed study focusing on the optimization of the DC bias superconducting coil of the SIC-SFCL has not been presented in the literature yet. In this context, this work proposes a multi-objective optimization method using the Nelder–Mead algorithm to find an optimal geometry for the superconducting coil. In this optimization, the objectives functions are: to maximize the critical current density in the high-temperature superconductor (HTS), minimize the voltage drop in the copper winding, minimize the current through the DC biased superconducting winding, and minimize the price of the HTS superconducting winding. Before implementing the multi-objective optimization algorithm, we have tested a non-superconducting saturated iron core prototype and used the results to validate the simulation models. After that, we have replaced the DC copper winding with an HTS coil in the simulations and initiate the optimization process. Results show that constructing the DC bias superconducting coil using the minimum possible fill factor might not be the best choice.

Half-flux-quantum (HFQ) circuits store and propagate half-flux quanta. The basic circuit element is a 0-π SQUID, which is a superconducting quantum interference device with a conventional Josephson junction (0-junction) and a π-shifted ferromagnetic junction (π-junction). A 0-π SQUID achieves a small critical current in the absence of an external magnetic field, thus reducing power consumption. It is easy to set up 0-0-π SQUIDs with two 0-junctions and a π-junction which serves as a π phase-shifter. We simulated 0-0-π SQUID-based HFQ circuits driven by low bias voltages, referred to as LV-HFQ circuits. In these circuits, shunt resistors are not required for switching junctions because there is no hysteresis in the current–voltage characteristics of 0-0-π SQUIDs. We estimated the power consumption and maximum operating frequency of an HFQ Josephson transmission line based on 0-0-π SQUIDs. When operating at 43.5 GHz, the power dissipation of a single element composed of a 0-0-π SQUID and a bias resistor fell to about 0.165 nW when biased at 60 μV. The LV-HFQ circuit is potentially more power-efficient than all other currently available superconducting logic circuits.

High-temperature superconducting double-pancake (DP) coils wound by the no-insulation (NI) approach have been proved to have a high thermal stability and a self-protecting ability. This paper mainly studies the effect of a quench of one pancake coil on the electromagnetic-thermal-mechanical behaviors of an NI DP coil in the self field and the high field. An electromagnetic-thermal coupling quench model is used to calculate the distributions of current, temperature and electromagnetic field in the coil, and then a three-dimensional homogeneous mechanical model is built to analyze the changes in strain and stress during a quench by considering the distributions of thermal strain and Lorentz force of the coil. The results indicate that the obvious increase in circumferential current and radial current density in the bottom pancake coil is induced by a quench of the top pancake coil due to the electromagnetic coupling effect in the self field and the high field, and that the DP coil still has a negative coil voltage during a quench in different fields. Although the bottom pancake coil has a large circumferential current, the mechanical deformation of the DP coil during a quench is mainly caused by the temperature rise in the self field. The thermal expansion of the top pancake coil has a remarkable effect on the mechanical behaviors of the bottom pancake coil. Moreover, the DP coil has the same temperature rise and mechanisms of bypass current in the self field and the high field. However, the mechanical deformation of the DP coil is based on the combined effects of temperature rise and Lorentz force in the high field. It can be found that the values of the hoop and axial stresses are affected by a large electromagnetic stress.

High-voltage direct current transmission systems are expected to allow the transmission of huge volumes of electricity over long distances. The use of superconducting fault current limiters (SFCLs) based on second-generation (2G) high-temperature superconductor (HTS) coated conductors (CCs) is a promising solution to mitigate fault currents in DC transmission systems. To fabricate a SFCL whose size remains acceptable, which means minimizing the length of the HTS tape used, the tape must sustain a high electric field during the whole fault duration. In this paper, high performance commercial 2G HTS CCs from THEVA (more than 750 A/cm-width at 77 K in self-field), on which a 500 µm thick Hastelloy shunt was soldered, were tested by submitting them to faults of different amplitudes and durations. Measurements revealed that these HTS tapes could sustain any type of fault up to 100 V m⁻¹, lasting up to 50 ms. Three-dimensions finite element simulations were able to reproduce accurately the experiments by using the appropriate temperature dependence of the critical current density and power law index, and by accounting for the variations in the local critical current along the length of the HTS tapes.

Field decay rate is the key characteristic of superconducting magnets based on closed-loop coils. However, in Maglev trains or rotating machines, closed-loop magnets work in external AC fields and will exhibit an evidently accelerated field decay resulting from dynamic resistances, which are usually much larger than joint resistance. Nevertheless, there has not been a numerical model capable of systematically studying this behaviour, which is the main topic of this work. The field decay curves of a closed-loop high-temperature-superconducting (HTS) coil in various AC fields are simulated based on H-formulation. A non-uniform external field generated by armature coils is considered. Reasonable consistence is found between experimental and simulation results. In our numerical model, the impact of current relaxation, which is a historical challenge, is analysed and subsequently eliminated with acceptable precision. Our simulation results suggest that most proportion of the field decay rate is from the innermost and outermost turns. Based on this observation, a magnetic shielding pattern is designed to reduce the field decay rate efficiently. This work has provided magnet designers with an effective method to predict the field decay rate of closed-loop HTS coils in external AC fields, and explore various shielding designs.

We investigated the mechanical properties and superconducting characteristics of high-strength (Bi, Pb)₂Sr₂Ca₂Cu₃O$_{10+\delta}$ tapes reinforced by Ni-based alloy lamination over a wide range of temperature (30 K≤T ≤ 300 K), magnetic field ($B\parallel c\leq6\ \textrm{T}$), and uniaxial tensile strain (ε ≤ 0.6%) conditions. We found that the Young's modulus evaluated from the stress–strain curves increases gradually with decreasing temperature (120 GPa at 300 K and 142 GPa at 40 K), indicating the importance of low-T data for designing high-field magnets. The enhancement of the Young's modulus of the tape at such low temperatures is probably attributable to the use of a Ni-based alloy for reinforcement and is also reflected in the irreversible stress. The critical current, $I_{\textrm{c}}$, measured under various (T, B, ε) conditions decreases almost linearly with uniaxial tensile strain in the reversible region. We constructed a phenomenological model of $I_{\textrm{c}}(T,B,\varepsilon)$ that can roughly reproduce the observed $I_{\textrm{c}}$ under all the (T, B, ε) conditions we investigated. We expect that fine adjustment of the model parameters based on more detailed measurements will make it possible to estimate $I_{\textrm{c}}(T,B,\varepsilon)$ under arduous measurement conditions from data obtained under conditions where measurement is relatively easy.

High-temperature superconductors (HTS) and MgB₂ may potentially improve the usability of superconducting magnets dramatically owing to their large energy margin. When HTS and MgB₂ wires are used for magnets operated in the persistent current mode, such as in magnetic resonance imaging (MRI) scanners, the electric field generated in the wires must be lower than 10⁻¹⁰ V m⁻¹. In this paper, critical current density, J_c, defined at an electric field criterion of 10⁻¹⁰ V m⁻¹ is evaluated from a magnetisation decay measurement for state-of-the-art monofilamentary MgB₂ wires. By using the obtained J_c, a critical line of our multifilamentary MgB₂ wire is shown on the temperature-magnetic field plane. Here, the critical line is defined as a line on which an electric field of 10⁻¹⁰ V m⁻¹ is generated at a coil current density of 150 A mm⁻². The area inside the critical line is demonstrated to be large enough to fulfil the requirement of 1.5 T MRI scanners operated at 10K–15K. In addition, the iso energy-margin lines are shown on the temperature-magnetic field plane and compared with those of NbTi wires. The MgB₂ wire has an order of magnitude greater energy margin than the NbTi wires in most of the area inside the critical line. This suggests that the MgB₂ wire is highly unlikely to be quenched due to mechanical disturbances.

The competing orders arising in a two-dimensional limit are the key to understanding unconventional superconductivity. The monolayer NbSe₂ possessing a charge density wave (CDW) and superconducting orders provides a playground to study unconventional superconductivity. Here we fabricate an ultrathin 2H–NbSe₂ device based on the mechanical exfoliation method and our electrode transfer technique. Detailed four-lead and two-lead transport measurements are performed in the ultrathin sample. The superconducting transition (T_c) at 4.3 K and CDW transition at 70 K are found in the sample, consistent with the literature report for ultrathin 2H–NbSe₂. The superconducting gap (Δ) of ultrathin 2H–NbSe₂ is estimated by fitting the two-lead transport spectroscopy with the Blonder–Tinkham–Klapwijk (BTK) theory. We found that Δ(T) exhibits a BCS-like temperature dependence with Δ $\approx$ 1.1 meV at 0 K and $\Delta/k_BT_c\approx\,\textbf{3}$. Meanwhile the gap-like features persist in the normal state and become unresolvable with the increasing temperature and magnetic field, suggesting a possible precursor superconductivity similar to the cuprate superconductors. Our results provide a new insight into superconductivity in ultrathin 2H–NbSe₂.