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The evolution of micro/nanostructure in metals subjected to high-pressure torsion (HPT) still need to be explained theoretically although experimental datasets are growing persistently. A major problem associated with the understanding of HPT is the synergetic effect of several competing processes that alter the material structure. In this study, we propose a piecewise model to analyze material hardness and true strain data during the HPT procedure. The model is built on two postulates: (a) the hardness vs true strain dependence is a sum of two piecewise power-law functions (each of these functions describes an unique micro/nanostructural stage of the deformation) and (b) each piecewise function has free-fitting strain breakpoints, which limit the strain range in which one mechanism predominantly determines the micro/nanostructure. The model was applied to analyze the HPT data for pure polycrystalline iron, AISI 1020 steel, and AISI 13B20 steel to reveal the distinctive strain breakpoints and power-law exponents. In the result, we found that deduced power-law exponents for AISI 1020 and AISI 13B20 steels are remarkably close to each other within full strain range.

Dissipative-free electric current flow is one of the most fascinating and practically important property of superconductors. Theoretical consideration of the charge carriers flow in infinitely long rectangular slab of superconductor in the absence of external magnetic field (so called, self-field) is based on an assumption that the charge carriers have rectilinear trajectories in the direction of the current flow whereas the current density and magnetic flux density are decaying towards superconducting slab with London penetration depth as characteristic length. Here, we calculate charge particle trajectories (as single electron/hole, as Cooper pair) at self-field conditions and find that charge carriers do not follow intuitive rectilinear trajectories along the slab surface, but instead ones have meander shape trajectories which cross the whole thickness of the slab. Moreover, if the particle velocity is below some value, the charge moves in opposite direction to nominal current flow. This disturbance of the canonical magnetic flux density distribution and backward movement of Cooper pairs can be entire mechanism for power dissipation in superconductors., Comment: 11 pages, 6 figures

The evolution of metals micro/nano-structure upon severe plastic deformation (SPD) is still far to be theoretically explained, while experimental datasets are persistently growing for several decades. Major problem associated with understanding of SPD is related to a fact that the latter is a synergetic product of several competing physical effects which alter the material micro/nano-structure. In attempt to find deformational boundaries, where predominantly one mechanism determines the micro/nano-structure, in this paper we propose a continuous piecewise model for the analysis of experiments on material hardness vs strain of SPD processed materials. The novelty of this approach lies in its ability to find, as free-fitting parameters, the strain breakpoints which separate different micro/nano-structure modes generated upon SPD process. The model is applied to analyse experimental data for polycrystalline samples of pure iron and two distinctive strain breakpoints are revealed with good accuracies. This finding is in a good agreement with our earlier results on TEM microscopy studies on pure iron polycrystals after SPD treatment.

Several research groups have reported on the observation of superconductivity in bilayer graphene structures where single atomic layers of graphene are stacked and then twisted at angles $\theta$ forming Moir\'e superlattices. The characterization of the superconducting state in these 2D materials is an ongoing task. Here we investigate the pairing symmetry of bilayer graphene Moir\'e superlattices twisted at $\theta$ = 1.05{\deg}, 1.10{\deg} and 1.16{\deg} for carrier doping states varied in the range of n=0.5-1.5 10^(12) cm^(-2) (where superconductivity can be realized) by analyzing the temperature dependence of the upper critical field Bc2(T) and the self-field critical current Jc(sf,T) within currently available models for single- and two-band s-, d-, p- and d+id-wave gap symmetries. Extracted superconducting parameters show that only s-wave and a specific kind of p-wave symmetries are likely to be dominant in bilayer graphene Moir\'e superlattices. More experimental data is required to distinguish between the s- and remaining p-wave symmetries as well as the suspected two-band superconductivity in these 2D superlattices., Comment: 38 pages, 7 figures, 3 tables, Supplementary Information (5 figures, 2 tables)

Since the milestone experimental discovery by Drozdov et al( 2015 Nature 525 73–6) who reported the observation of near-room-temperature (NRT) superconductivity in highly-compressed sulphur hydride, the quest for room-temperature superconductivity is primarily focused on highly-compressed materials. Extreme conditions and space confinement inside a diamond anvil cell (DAC) dramatically limits the number of experimental techniques which can be applied to study highly-compressed superconductors. For this reason, the development of new approaches to characterize materials at extreme conditions is one of the central topics in the field of NRT superconductivity. In this paper, we describe an approach to categorize highly-compressed superconductors, including NRT superconductors, as unconventional superconductors. The primary idea for the classification is based on the empirical finding of Uemura (1997 Physica C 282–7 197) who showed that all unconventional superconductors have the ratio of the superconducting transition temperature, T c, to the Fermi temperature, T F, within a range of 0.01 ≤ T c/T F ≤ 0.05. To deduce the Fermi temperature in highly-compressed superconductors, we utilize temperature dependence of the upper critical field and the resistance data (which both can be more or less routinely measured for highly-compressed superconductors) and reported results by first principles calculations for these materials. We demonstrate the application of the approach for highly-compressed oxygen, sulphur, lithium, and recently discovered yttrium superhydride polymorphs, YHn( n = 4,6,7,9) (Troyan et al( 2019 arXiv:1908.01534) and Kong et al( 2019 arXiv:1909.10482)). We also show the application of the approach for the newly discovered uncompressed Nd2-xSrxNiO2 nickelate superconductor.

The evolution of metals micro/nano-structure upon severe plastic deformation (SPD) is still far to be theoretically explained, while experimental datasets are persistently growing. Major problem associated with understanding of SPD is a synergetic effect of several competing processes which alter material structure. In this paper we propose a model to reveal, as free-fitting parameters, strain breakpoints where predominantly one mechanism determines the micro/nano-structure in SPD materials from material hardness vs true strain experimental data. The model is applied to analyse SPD data for pure polycrystalline iron and two distinctive strain breakpoints have been revealed.

Milestone experimental discovery of superconductivity above 200 K in highly-compressed sulphur hydride by Drozdov (Nature 525, 73 (2015)) sparked experimental and theoretical investigations of metallic hydrides. Since then, a dozen of superconducting binary and ternary polyhydrides have been discovered. For instance, there are three superconducting polyhydrides of thorium: Th4H15, ThH9, and ThH10 and four polyhydrides of yttrium: YH4, YH6, YH7, YH9. In addition to binary and ternary hydrogen-based metallic compounds, recently Eremets (arXiv:2109.11104) reported on the metallization of hydrogen, which exhibits a phase transition into metallic hydrogen phase III at P>330 GPa and T, Comment: 30 pages, 12 figures

Since pivotal experimental discovery of the near-room-temperature superconductivity (NRTS) in highly-compressed sulphur hydride by Drozdov et al (2015 Nature 525 73-76), more than a dozen of binary and of ternary hydrogen-rich phases exhibited superconducting transition above 100 K have been discovered to date. There is a widely accepted theoretical point of view that primary mechanism governing the emergence of superconductivity in hydrogen-rich phases is the electron-phonon pairing. However, our recent analysis of experimental temperature dependent resistance in $H_{3}S$, $LaH_{x}$, $PrH_{9}$ and $BaH_{12}$ (arXiv: 2104.14145) showed that these compounds exhibit the dominance of non-electron-phonon charge carrier interaction and, thus, it is unlikely that the electron-phonon pairing is the primary mechanism for the emergence of superconductivity in these materials. Here we use the same approach to reveal charge carrier interaction in highly-compressed lithium, black phosphorous, sulfur, and silane. We found that all these superconductors exhibit the dominance of non-electron-phonon charge carrier interaction. This explains the failure of high-Tc values predicted for these materials by the first-principles calculations which utilized the electron-phonon pairing as the mechanism for the emergence of superconductivity in these materials. Our result implies that alternative pairing mechanisms (i.e., electron-magnon, electron-polaron, electron-electron, etc.) should be tested within first-principles calculations approach as possible mechanisms for the emergence of superconductivity in highly-compressed superconductors., 13 pages, 4 figures

The discovery of superconductivity at 203 K in highly compressed sulphur hydride validates the ideas put forward by Ashcroft 50 years ago and galvanises the quest for room-temperature superconductivity. But at such temperatures, thermal fluctuations might be expected to break up Cooper pairs. For example, in the cuprates, fluctuations reduce T c by 30% or more below the mean-field value. Similar effects are found in iron pnictides. Here, we ask: how does superconductivity survive in sulphur hydride at such high temperatures? To answer this, we examine the superfluid density which is the key parameter for quantifying fluctuations in both amplitude and phase. We show that dimensionality plays a key role in suppressing or enhancing thermal fluctuations to the benefit of hydrogen sulphide and the detriment of its more layered 2D competitors. We find that the temperature scale for phase fluctuations, T φ , in superconducting H3S exceeds 1200 K, and therefore, these are irrelevant at 200 K. But the amplitude fluctuation temperature scale, T amp, at around 300 K is much lower and this has important implications for the ongoing quest for room temperature superconductivity. Appealing to the way in which superfluid density, T φ , T amp and T c scale with each other it seems that room temperature superconductivity is nearly ruled out, but perhaps not quite. It will require 3D systems with a large Fermi velocity to achieve this goal.

Recently, Troyan et al (2019 arXiv:1908.01534) and Kong et al (2019 arXiv:1909.10482) extended near-room-temperature superconductors family by new yttrium superhydride polymorphs, YHn (n = 4,6,7,9), which exhibit superconducting transition temperatures in the range of Tc = 210-243 K at pressure of P = 160-255 GPa. In this paper, temperature dependent upper critical field data, Bc2(T), for highly-compressed mixture of YH4+YH6 phases (reported by Kong et al 2019 arXiv:1909.10482) is analysed to deduce the ratio of Tc to the Fermi temperature, TF. Our analysis shows that in all considered scenarios the YH4+YH6 mixture has the ratio 0.01 ≤ Tc/TF ≤ 0.04. As the result, YH4+YH6 falls in the unconventional superconductors band in the Uemura plot. It is also found that the characteristic temperature of the order parameter amplitude fluctuations, Tfluc, in the YH4+YH6 mixture is only several percent above observed Tc, and thus the superconducting transition in yttrium superhydride polymorphs is fundamentally limited by thermodynamics fluctuations.

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th4H15-Th4D15 superhydrides exhibit superconductivity and have no isotope effect. The latter is fundamental contradiction with the concept of electron-phonon mediated superconductivity of Bardeen-Cooper-Schrieffer (BCS) theory. Soon after this work, Stritzker and Buckel (1972 Zeitschrift für Physik A Hadrons and nuclei 257 1-8) reported that superconductors in PdHx-PdDx system exhibit reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding on PdHx-PdDx-PdTx system. Recent interest to hydrogen- and deuterium-rich superconductors is based on the discovery of near-room-temperature superconductivity in highly-compressed H3S (Drozdov et al. 2015 Nature 525 73) and LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001). To date, there is no clarity about isotope effect in H3S-D3S system, because thorough examination of available experimental data reported by Drozdov et al (2015 Nature 525 73) shows that H3S-D3S system perhaps has reverse isotope effect. In attempt to reaffirm/disprove our primary idea that the mechanism for near-room-temperature superconductivity in hydrogen-rich superconductors is not BCS electron-phonon interaction, we analyse the upper critical field data, Bc2(T), in Th4H15-Th4D15 phases (Satterthwaite and Toepke 1970 Phys. Rev. Lett. 25 741) and two recently discovered high-pressure hydrogen-rich phases of ThH9 and ThH10 (Semenok et al 2019 arXiv:1902.10206). As the result, it is found that all known to date thorium super-hydrides/deuterides are unconventional superconductors which have Tc/TF ratios within a range of 0.008 < Tc/TF < 0.120, where Tc is the superconducting transition temperature and TF is the Fermi temperature.

Recently, several research groups have reported on anomalous enhancement of the self-field critical currents, Ic(sf,T), at low temperatures in superconductor/Dirac-cone material/superconductor (S/DCM/S) junctions. Some papers attributed the enhancement to the low-energy Andreev bound states arising from winding of the electronic wave function around DCM. In this paper, Ic(sf,T) in S/DCM/S junctions have been analyzed by two approaches: modified Ambegaokar-Baratoff and ballistic Titov-Beenakker models. It is shown that the ballistic model is an inadequate tool to analyze experimental data from S/DCM/S junctions. The primary mechanism for limiting superconducting current in S/DCM/S junctions is different from the conventional view that the latter is the maximum value within the order parameter phase variation. Thus, there is a need to develop a new model for self-field critical currents in S/DCM/S systems.

The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters strongly indicate that NdFeAs(O,F) is a bulk p-wave superconductor. Further investigation revealed that single atomic layer FeSe also shows p-wave pairing. In an attempt to generalize these findings, we re-examined the whole inventory of superfluid density measurements in iron-based superconductors show quite generally that most of the iron-based superconductors are p-wave superconductors., Comment: 35 pages, 15 figures

Satterthwaite and Toepke (1970 Phys. Rev. Lett. 25 741) discovered that Th4H15-Th4D15 superhydrides are superconducting but exhibit no isotope effect. As the isotope effect is a fundamental prediction of electron-phonon mediated superconductivity described by Bardeen, Cooper, and Schrieffer (BCS) its absence alludes to some other mechanism. Soon after this work, Stritzker and Buckel (1972 Zeitschrift f��r Physik A Hadrons and nuclei 257 1-8) reported that superconductors in the PdHx-PdDx system exhibit the reverse isotope effect. Yussouff et al (1995 Solid State Communications 94 549) extended this finding in PdHx-PdDx-PdTx systems. Renewed interest in hydrogen- and deuterium-rich superconductors is driven by the discovery of near-room-temperature superconductivity in highly-compressed H3S (Drozdov et al. 2015 Nature 525 73) and LaH10 (Somayazulu et al 2019 Phys. Rev. Lett. 122 027001) as well as two recently discovered high-pressure hydrogen-rich phases of ThH9 and ThH10 (Semenok et al 2019 Materials Today, DOI: 10.1016/j.mattod.2019.10.005). We conclude that all known thorium super-hydrides/deuterides, to date, are unconventional superconductors - along with the heavy fermions, fullerenes, pnictides, cuprates, where we find they have Tc/Tf ratios within a range of of 0.008 < Tc/Tf < 0.120, where Tc is the superconducting transition temperature and Tf is the Fermi temperature., 15 pages, 3 figures, 57 references

The ability of ferroelectric materials to generate high voltage under shock compression is a fundamental physical effect that makes it possible to create miniature autonomous explosive-driven pulsed power systems. Shock-induced depolarization releases an electric charge at the electrodes of the ferroelectric element, and a high electric potential and a high electric field appear across the element. We performed systematic studies of the electric breakdown field, $E_{b}(d)$ , as a function of the ferroelectric element thickness, $d$ , for Pb(Zr0.95Ti0.05)O3 (PZT 95/5) and Pb(Zr0.52Ti0.48)O3 (PZT 52/48) ceramics compressed by transverse shock waves (shock front propagates perpendicular to the polarization vector) and established a relationship between these two values: $E_{b}(d) = \textrm {const}\cdot d^{-\xi }$ . This law was found to be true in a wide range of ferroelectric element thicknesses from 1 to 50 mm. This result makes it possible to predict ferroelectric generator (FEG) output voltages up to 500 kV and it forms the basis for the design of ultrahigh-voltage FEG systems.

The current distribution across the thickness of a current-carrying rectangular film in the Meissner state was established long ago by the London brothers. The distribution across the width is more complicated but was later shown to be highly non-uniform, diverging at the edges. Accordingly, the standard view for type II superconductors is that vortices enter at the edges and, with increasing current, are driven inwards until they self-annihilate at the centre, causing dissipation. This condition is presumed to define the critical current. However we have shown that, under self-field (no external field), the transport critical current is a London surface current where the surface current density equals the critical field divided by {\lambda}, across the entire width. The critical current distribution must therefore be uniform. Here we report studies of the current and field distribution across commercial YBa2Cu3O7 conductors and confirm the accepted non-uniform distribution at low current but demonstrate a radical crossover to a uniform distribution at critical current. This crossover ends discontinuously at a singularity and calculations quantitatively confirm these results in detail. The onset of self-field dissipation is, unexpectedly, thermodynamic in character and the implied vortex-free critical state seems to require new physics., Comment: 9 pages, 5 figures

Universal scaling behaviour in superconductors has significantly elucidated fluctuation and phase transition phenomena in these materials. However, universal behaviour for the most practical property, the critical current, was not contemplated because prevailing models invoke nucleation and migration of flux vortices. Such migration depends critically on pinning, and the detailed microstructure naturally differs from one material to another, even within a single material. Through microstructural engineering there have been ongoing improvements in the field-dependent critical current, thus illustrating its nonuniversal behaviour. But here we demonstrate the universal size scaling of the self-field critical current for any superconductor, of any symmetry, geometry or band multiplicity. Key to our analysis is the huge range of sample dimensions, from single-atomic-layer to mm-scale. These have widely variable microstructure with transition temperatures ranging from 1.2 K to the current record, 203 K. In all cases the critical current is governed by a fundamental surface current density limit given by the relevant critical field divided by the penetration depth., Comment: 15 pages, 6 figures

We report the effects of heavy-ion irradiation on critical currents in (Y,Dy)Ba 2 Cu 3 O 7-δ coated conductors made by a commercial metal organic deposition process. Irradiation with 185 MeV Au ions is investigated and contrasted with previous results on 74 MeV Ag ions. The change in critical current (I c ) is reported as a function of temperature (30-77 K), magnetic field (0-7 T) and magnetic-field angle (full angle dependence). Significant improvements are seen particularly at 30-40 K where I c can be enhanced over a wide angular range and the minimum of I c (θ) has been enhanced by up to 60%. This contrasts with the data for the commonly utilized regime of 77 K, 1 T where the I c enhancements are small or limited to a narrow angular range. This data also serves to illustrate the need to measure Ic in all of the temperature and field regimes of technical interest and to measure the complete field-angular dependence I c (θ).

Relaxor ferroelectric single crystals have triggered revolution in electromechanical systems due to their superior piezoelectric properties. Here the results are reported on experimental studies of energy harvested from (1-y-x)Pb(In1/2Nb1/2)O3–(y)Pb(Mg1/3Nb2/3)O3–(x)PbTiO3 (PIN-PMN-PT) crystals under high strain rate loading. Precise control of ferroelectric properties through composition, size and crystallographic orientation of domains made it possible to identify single crystals that release up to three times more electric charge density than that produced by PbZr0.52Ti0.48O3 (PZT 52/48) and PbZr0.95Ti0.05O3 (PZT 95/5) ferroelectric ceramics under identical loading conditions. The obtained results indicate that PIN-PMN-PT crystals became completely depolarized under 3.9 GPa compression. It was found that the energy density generated in the crystals during depolarization in the high voltage mode is four times higher than that for PZT 52/48 and 95/5. The obtained results promise new single crystal applications in ultrahigh-power transducers that are capable of producing hundreds kilovolt pulses and gigawatt-peak power microwave radiation.

Recent experiments showed that thinning gallium, iron selenide and 2H tantalum disulfide to single/several monoatomic layer(s) enhances their superconducting critical temperatures. Here, we characterize these superconductors by extracting the absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the transition temperature from their self-field critical currents. Our central finding is that the enhancement in transition temperature for these materials arises from the opening of an additional superconducting gap, while retaining a largely unchanged bulk superconducting gap. Literature data reveals that ultrathin niobium films similarly develop a second superconducting gap. Based on the available data, it seems that, for type-II superconductors, a new superconducting band appears when the film thickness becomes smaller than the out-of-plane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity., Comment: 43 pages, 12 figures

Our previous efforts were focused on designing, constructing, and investigating miniature shock-wave ferroelectric generators (FEGs) based on Pb(Zr0.52 Ti0.48)O3 poled ferroelectric ceramics (that is widely used in modern technology). In this paper, we extended our efforts by exploring a different type of ferroelectric materials, i.e., Pb(Zr0.95Ti0.05)O3 (PZT 95/5), for designing miniature autonomous shock-wave FEGs. The performance of autonomous ultrahigh-voltage FEGs based on transverse explosive shock depolarization of poled PZT 95/5 ferroelectric ceramics was studied. As a result of this paper, miniature generators (38 mm in diameter) that are capable of producing output voltage pulses with amplitudes exceeding 120 kV and pulsewidths of 3 μs were developed.

In this paper, we present a brief review of current studies of the electric-field-free gas breakdown phenomenon along with new experimental data. Possible mechanisms for this phenomenon in explosively driven generators are proposed.

We study the formation mechanism of nanoparticles in thin films of the superconductor YBa2Cu3O7-δ(YBCO). We form the films by metal-organic deposition (MOD) on buffered, textured metal substrates. Through the addition of Dy or Zr salts to the precursor solution we precipitate (Y,Dy)2O3and BaZrO3nanoparticles, uniformly distributed through the film thickness. By quenching samples during the film growth, we show the nanoparticles form in the precursor layer before YBCO growth. The size of the nanoparticles was quantitatively analysed by TEM. We found that Zr doping produces smaller nanoparticles than Dy doping.

We investigate the pinning force anisotropy for BSCCO and YBCO commercial wires at 77 K. The pinning force is measured to 3 T and for all field angles. We show that the magnitude of the pinning force maximum as a function of applied field angle follows known statistical functions. The position of the maximum with field also follows these functions. This is a step towards a Dew-Hughes type scaling covering field, field angle and temperature for HTS wire.

We have investigated the temperature and field dependence of the critical current for YBCO coated conductors fabricated by metal-organic deposition. Samples with Dy2O3 or BaZrO3 nanoparticle pinning centers were compared with undoped and Dy-substituted YBCO. While emphasis in development is often limited by practical constraints to measurements at 77 K and self-field, the majority of applications will require operation at much lower temperatures and moderate-to-high magnetic fields. We compare Ic(B,T) for temperatures in the range 20-77 K and magnetic fields up to 7 T. Pinning enhancements observed at 77 K and attributed to nanoparticle additions are less conspicuous at 20 K.

Recently, Pan et al (2017 J. Am. Chem. Soc. 139 4623) reported that randomly restacked chemically exfoliated monolayers of TaS2 have enhanced superconducting transition temperatures of up to T c = 3 K, compared with T c = 0.8 K for the bulk 2 H-TaS2 compound. Ma et al (2018 NPJ Quantum Mater. 3 34) measured the angular dependence of the upper critical field, B c2(θ), for this material and employed several models to fit the experimental data, namely the three-dimensional Ginzburg–Landau (3D GL) model (Blatter et al 1994 Rev. Mod. Phys. 66 1125–1388), the two-dimensional Tinkham (2D Tinkham) model (Harper and Tinkham 1968 Phys. Rev. 172 441–450), and the modified 3D GL (Ma et al 2018 NPJ Quantum Mater. 3 34) model. However, differences between experimentally measured B c2(θ) and theoretical model values are large, showing a great enhancement of experimental B c2(θ) over a wide range of angles. The same result was obtained for 1T'-MoS2 restacked nanosheets (Ma et al 2018 NPJ Quantum Mater. 3 34). Here we stress that the physical reason for enhanced superconductivity in these materials is the randomness in restacked monolayers (Pan et al 2017 J. Am. Chem. Soc. 139 4623; Ma et al 2018 NPJ Quantum Mater. 3 34). Based on this viewpoint (Pan et al 2017 J. Am. Chem. Soc. 139 4623; Ma et al 2018 NPJ Quantum Mater. 3 34), and despite the fact that the B c2(θ) of each individual monolayer will obey the 2D Tinkham model, the total B c2(θ) should mainly reflect the angular statistical distribution of the 2D nanosheets within the stack. Fits of the experimental B c2(θ) data using a statistical distribution model for the 2D nanosheets have excellent quality and the deduced parameters have meaningful values. We also propose that the angular dependences of the lower, B c1(θ), and thermodynamic, B c(θ), critical fields in randomly restacked 2D nanosheets should also obey statistical distribution models.

Most widely used practical superconductors (i.e., NbTi, Nb3Sn, Bi2Sr2Ca2Cu3O10+x ) are manufactured in a form of multigranular superconducting filaments embedded in a metallic matrix. The performance of these multifilamentary conduits at different physical conditions has been a topic of extended research over the last decades. In this paper we targeted to reveal the precise onset of the electric power dissipation in one of these composite superconductors, Bi2Sr2Ca2Cu3O10+x (so-called 1G HTS wire), at the conditions when external magnetic field, B appl, is applied in the maximum Lorentz force geometry. As we showed earlier (Talantsev et al 2017 AIP Advances 7 125230), at self-field conditions the transition to the dissipative state in 1G wire upon increasing the transport current, I, despite a multifilamentary wire design sharply steepens at a threshold current, I c,surfB, at which a simultaneous and abrupt crossover from a non-linear to a linear dependence of the perpendicular component of the local magnetic flux density, B surf(I), measured at the conduit surface occurs. We found that the same transition takes place in 1G HTS wire with an applied magnetic field, B appl, and thus, the definition of critical current in multifilamentary superconductors based on I c,surfB can be extended to in-field conditions for 1G HTS wire. We also studied effects of the flux trap and magnetic hysteresis upon increasing/cycling the transport current, I, in this conduit superconducting wire.

Comprehensive studies of explosively driven ferromagnetic generators (FMGs) have been performed. It has been experimentally established that the initial magnetostatic energy stored in the magnetic element of the FMG (determined by the magnet's maximum energy product (BH)max and its volume), not the residual magnetic flux density Br, is the principal parameter that determines the output energy and the amplitude of the signals produced by the generator. Systematic studies were carried out with miniature high-current FMGs containing Nd2Fe14B high-energy hard ferromagnetic elements having a wide range of sizes. Utilization of transverse-shock demagnetization of Nd2Fe14B magnets (shock wave propagates across the magnetization vector \mmb M) instead of longitudinal-shock demagnetization dramatically changed the design of the FMGs and reduced the mass of the explosives used by two orders of magnitude in comparison with that used in longitudinal FMGs. Data for the initial magnetic flux and shock-induced magnetic flux change in Nd2 Fe14B magnets are presented. It was shown that FMGs with a volume of 25 cm3 are capable of producing pulsed currents with amplitudes of up to 4.4 kA and rise times of 23 μs in the seed coil of a magnetic flux compression generator.

Systematic studies of the depolarization of Pb(Zr0.52 Ti0.48)O3 (PZT 52/48) poled piezoelectric ceramic samples were performed using thermal heating and explosive longitudinal-shock compression. The electric charge density that is released due to thermal heating at a rate of 0.9 K/min was consistent for all sample sizes (Q = 27.8 μC/cm2). The charge density that is released due to shock compression was practically equal to that released due to thermal heating. The PZT 52/48 samples were completely depolarized by longitudinal shock waves at shock front pressures of PSW = 1.5 ±0.2 GPa. The electric charge that the samples release can be transformed into pulsed power. Miniature explosive-driven ferroelectric generators containing PZT 52/48 ceramic elements with a volume of 0.35-3.5 cm3 are capable of converting the depolarization charge into pulses of high voltage with amplitudes of up to 22 kV and high power with a peak power of up to 0.35 MW within a time interval of 0.3-1.5 μs .

There is a growing problem to represent and analyse large experimental datasets in many emerging fields of science aside of traditional big data-based disciplines, i.e., elementary particles, genetics/genomics and geoscience. One of these emerging fields is applied superconductivity where recently a large, regularly up-dated, public database of critical currents of commercial superconductors was established. The size, dimensionality and resolution of this data makes current methods of display and analysis inadequate. As is often the case in physics and materials science, when dealing with any anisotropic properties, one measures the effects of rotations around a low symmetry axis, this is also the case in critical current measurements as found in applied superconductivity. In this paper we propose the use of polar projected images to map these much larger data sets into useful visualizations for analysis. Where we suggest the radial coordinate and the colour represent amplitudes of two measured parameters, and sample rotation angle is naturally mapped to the polar coordinate. We demonstrate the advantage of this projection for analysing, otherwise unwieldy large, critical current datasets, and naturally recover previously used empirical relations.

There are growing numbers of experimental evidences that the self-field critical currents, [Formula: see text], are a new instructive tool to investigate fundamental properties of superconductors ranging from atomically thin films [M. Liao et al., Nat. Phys. 6 (2018), https://doi.org/10.1038/s41567-017-0031-6 ; E. F. Talantsev et al., 2D Mater. 4 (2017) 025072; A. Fete et al., Appl. Phys. Lett. 109 (2016) 192601] to millimeter-scale samples [E. F. Talantsev et al., Sci. Rep. 7 (2017) 10010]. The basic empirical equation which quantitatively accurately described experimental [Formula: see text] was proposed by Talantsev and Tallon [Nat. Commun. 6 (2015) 7820] and it was the relevant critical field (i.e. thermodynamic field, [Formula: see text], for type-I and lower critical field, [Formula: see text], for type-II superconductors) divided by the London penetration depth, [Formula: see text]. In this paper, we report new findings relating to this empirical equation. It is that the critical wavelength of the de Broglie wave, [Formula: see text], of the superconducting charge carrier which within a numerical pre-factor is equal to the largest of two characteristic lengths of Ginzburg–Landau theory, i.e. the coherence length, [Formula: see text], for type-I superconductors or the London penetration depth, [Formula: see text], for type-II superconductors. We also formulate a microscopic criterion for the onset of dissipative transport current flow: [Formula: see text], where [Formula: see text] is the charge carrier momentum, [Formula: see text] is Planck’s constant and the inequality sign “[Formula: see text]” is reserved for the dissipation-free flow.

Linear damage tracks are created in production-quality YBCO coated conductors by irradiation with 61–74 MeV Ag ions. The ion tracks are observed by transmission electron microscopy to be elongated but discontinuous. The in-field transport critical current ( I c ) is enhanced significantly for fields applied parallel to the irradiation direction with a broad peak appearing in the magnetic field-angle dependence of the critical current, coinciding with the irradiation direction. The zero-field I c is typically reduced somewhat, however annealing at 200 °C partially restores this and enhances the in-field I c even for field parallel to the irradiation direction. Lower energies tend to produce a weaker peak, but also retain the zero-field I c to a greater extent, consistent with a trend of greater discontinuity in the ion tracks.

Tin-based nanoparticles, proposed to be barium stannate, have been formed in YBCO films fabricated by metal-organic deposition, through modification of the precursor solution. These randomly-oriented 30-50 nm particles are dispersed throughout the film thickness, providing an enhancement in the isotropic flux pinning relative to undoped YBCO. For a low level of nanoparticle addition of 2 vol%, a flux-pinning force enhancement of up to 32% is achieved. We report field and field-angle dependence of the transport critical current, transmission electron microscopy, and X-ray diffraction of our YBCO films on RABiTS substrates, and compare with similar barium zirconate additions.

The improvement of critical current densities of superconducting wires is essential for commercial applications. Many approaches have been demonstrated to enhance flux pinning of YBa2Cu3O7−δ (YBCO) second-generation (2G) superconducting wires and therefore to increase critical current densities of the materials. In this work, we study the microstructures which lead to enhanced critical current density through flux pinning by nanoparticles in metal-organic deposited (MOD) YBCO films with Dy or Zr additions. Transmission electron microscopy (TEM) observation revealed that large densities of nanoparticles can be formed in YBCO films through altering the precursor stoichiometry and cation additions. Most of these nanoparticles are in the range of 10–50 nm, and are well dispersed, making them ideal for flux pinning in YBCO 2G wires. The effects on critical current of these nanoparticles acting as flux pinning centers are investigated.

We describe a method for forming a nanoparticle dispersion in YBa2Cu3O7 superconducting films grown by metal-organic deposition on RABiTS substrates. Two separate precursor modifications have been shown by transmission electron microscopy to give nanoparticles of (Y, Dy)2O3 and BaZrO3 with size distributions of 10–50 nm and 10–25 nm, respectively and that are well dispersed in the film. These particles are effective flux pinning centers and enhance the transport critical current by up to 100% at 77 K for a magnetic field of 1.5 T applied perpendicular to the tape. The processing required to grow these films is not greatly altered from that of unmodified YBCO and can be readily transferred to continuous processing of long-length wires.

We have formed BaZrO 3 nanoparticles in YBa 2 Cu 3 O 7− δ second-generation HTS wires through modification of the chemical solution precursor for metal-organic deposition. Transmission-electron microscopy shows the particles to be 10–25 nm in diameter, approximately spherical, and well dispersed through the film. The in-field critical current density is enhanced over a wide angular range compared to undoped wires.

We have demonstrated the feasibility of charging a capacitor bank to a high voltage using an autonomous ultra-compact explosively driven source of prime power. The prime power source is a longitudinally driven shock wave depolarization of a ferroelectric ceramic. The energy-carrying elements of the shock wave ferroelectric generators (FEGs) were poled Pb(Zr52Ti48)O3 polycrystalline ceramic disks with 0.35 cm3 volume. FEGs charged 9 nF, 18 nF, and 36 nF capacitor banks and provided pulsed-power with peak amplitudes up to 0.29 MW. The maximum efficiency of electric charge transfer from shocked Pb(Zr52Ti48)O3 elements to a capacitor bank was 46%. We demonstrated experimentally that the FEG-capacitor bank system can perform as an oscillatory circuit. A methodology was developed for numerical simulation of the operation of the FEG-capacitor bank system; the simulation results were in a good agreement with the experimental results.

The microstructure of metal-organic deposited YBa2Cu3O7−δ with dysprosium (Dy) additions has been investigated by transmission electron microscopy (TEM). Dy additions which increase the density of normal-state nanoparticles in the YBCO have been demonstrated to enhance the critical current densities in moderate magnetic fields. The influence of nanoparticles, stacking faults and other planar defects on flux pinning is discussed. We observed a high density of nanoparticles in the size range of 10–50 nm, which may act as flux pinning centres to enhance the critical current density of the material. Stacking faults and planar defects are observed which may also be effective flux pinning centres in YBCO samples with and without Dy addition.

A random walk model of the vortex paths in high temperature superconductors is investigated for different relative densities of c-axis, ab-plane and point pinning defects. The model suggests an origin for some of the more unusual features seen in the field angle dependence of the critical current. It also predicts greater noise in critical currents for fields parallel to the plane of the dominant pinning defects and anomalous behavior of n-values. The random walk model also suggests an alternative form for the expected field angle dependence of the upper critical fields in disordered layered superconductors.

For any practical superconductor the magnitude of the critical current density, $J_\textrm{c}$, is crucially important. It sets the upper limit for current in the conductor. Usually $J_\textrm{c}$ falls rapidly with increasing external magnetic field but even in zero external field the current flowing in the conductor generates a self-field which limits $J_\textrm{c}$. Here we show for thin films of thickness less than the London penetration depth, $\lambda$, this limiting $J_\textrm{c}$ adopts a universal value for all superconductors - metals, oxides, cuprates, pnictides, borocarbides and heavy Fermions. For type I superconductors, it is $H_{\textrm{c}}/\lambda$ where $H_\textrm{c}$ is the thermodynamic critical field. But surprisingly for type II superconductors we find the self-field $J_\textrm{c}$ is $H_{\textrm{c}1}/\lambda$ where $H_{\textrm{c}1}$ is the lower critical field. $J_\textrm{c}$ is thus fundamentally determined and this provides a simple means to extract absolute values of $\lambda(T)$ and, from its temperature dependence, the symmetry and magnitude of the superconducting gap., Comment: 8 pages, 6 figures

Ability of ferroelectric materials to generate high voltage under shock compression is fundamental physical effect that makes possible to create miniature autonomous explosive-driven pulsed power systems. As the result of shock induced depolarization, an electric charge is released at the electrodes of the ferroelectric element and a high electric potential and a high electric field appears across the element. We performed systematic studies of electric breakdown field, E b (d), as function of ferroelectric element thickness, d, for Pb(Zr 0.95 Ti 0.05 )O 3 (PZT 95/5) and Pb(Zr 0.52 Ti 0.48 )O 3 (PZT 52/48) ceramics compressed by transverse shock waves (shock front propagates across the polarization vector) and established a relationship between these two values: E b (d) = const·d−0.25. This law was found to be true in wide range of voltages from 4 to 150 kV and ferroelectric element thicknesses varied from 4.7 to 51 mm. This result makes it possible to predict the ferroelectric generator (FEG) output voltage and it forms the basis for design of ultrahigh voltage FEG systems.

Poled ferroelectrics are key components of autonomous explosive-driven pulsed power systems. Shock depolarization of ferroelectrics is a basic physical effect providing prime electrical power to autonomous systems. In this paper we report results of experimental studies of shock-induced and thermal-induced depolarization, and X-Ray diffraction of lead zirconate titanate ferroelectrics of two different compositions, PbZr 0.52 Ti 0.48 O 3 (PZT 52/48) and PbZr 0.95 Ti 0.05 O 3 (PZT 95/5). Specimens were shock loaded perpendicular to the polarization vector. The experimental results indicate that the shock induced depolarization mechanisms are different for these two compositions. Thus, the shock-induced charge released by PZT 52/48 is less than half of its remnant polarization. PZT 52/48 is transformed to a state with lower polarization, while PZT 95/5 under the same loading conditions undergoes a phase transition to a non-polar antiferroelectric phase and completely depolarized as a result of this phase transition.