Although progress with twisted graphene nano-devices is boosting the superconductivity that is the consequence of their Moiré flat electronic bands, the immense choice for future development is an obstacle for their optimisation. We report here that soft-chemistry deintercalation of KC8 breaks down graphite stacking generating a strong disorder that includes stacking twists and variable local doping. We obtain a bulk graphite whose individual crystallites have different stackings with arbitrary twists and doping, scanning in the same sample a huge number of stacking configurations. We perform magnetisation measurements on batches with different synthesis conditions. The disorder weakens the huge diamagnetism of graphite, revealing several phase transitions. A tiny “ferromagnetic-like” magnetisation appears with Curie temperatures T0∼ 450 K, that has to be subtracted from the measured magnetisation. Depending on sample synthesis, anomalies towards diamagnetic states appear at Tc∼ 110 K (3 samples), ∼ 240 K (4 samples), ∼ 320 K (2 samples). Electrical resistivity measurements yield anomalies for the Tc∼ 240 K transition, with one sample showing a 90% drop. We discuss the possibility that these (diamagnetic and resistive) anomalies could be due to superconductivity. As the amount of these phases is small by construction, fine-tuning of the process will be necessary to increase their fraction and select the desired phase.

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe2 van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have Néel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe2 compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases.

The efficiency of the superconducting radio frequency cavities composed of Nb required the deposition of thickness-controlled multilayer coatings of superconductor-insulator-superconductor (S-I-S) on the internal surfaces of the cavities. Herein, we report the plasma-enhanced atomic layer deposition of carbon-free NbN (50 mu m thick), followed by a thermal treatment, to obtain the superconducting layer in the S-I-S structure. Using (tert-butylimido)-tris(diethylamino)-niobium as the niobium precursor and H-2 and NH3 plasma as reactive gasses, the deposition and annealing parameters were optimized by studying their effects on the film properties (crystallinity, density, and composition). We demonstrated that the superconducting critical temperature (Tc) can be improved after thermal annealing up to 13.8 K, a value compatible with the targeted application. In addition to the expected densified layers and increased grain size, we observed a partial transformation of the oxide present in the as-deposited layer into niobium oxynitride, which could indicate the origin of the improvement of the superconducting properties. With low carbon and oxygen impurity concentrations in the films, this study contributes to the understanding of the relationship between the structure, composition, and superconductivity in NbN.

We have performed neutron diffraction measurements of tetragonal Sr2CrO4 to study the crystallographic structure as a function of temperature and the magnetic properties developed below . We have measured the precise positions of the oxygen atoms of the CrO6 octahedra and observed that, with decreasing temperature, the octahedra pass from elongated to almost ideal, contradicting the intuitive idea of a Jahn-Teller effect. DFT calculations explain this apparent anomalous behaviour by establishing that energy level degeneracy only exists in a highly elongated octahedra. Our study agrees with the idea of a weak orbital ordering transition around 150 K followed by an itinerant antiferromagnetic ordering at 110 K with a wave vector determined both from neutron powder diffraction and theory.

We report on the discovery of a Ba-based chromate obtained by high pressure-high temperature treatment of the low pressure orthorhombic Ba2CrO4 phase. By combining transmission electron microscopy and powder X-ray diffraction measurements, we have determined its crystallographic structure. This new Cr-oxide has a cubic lattice with a = 13.3106(6) Å built from a three-dimensional network of two Cr sites, Cr1 and Cr2, both in octahedral environments, with face sharing between Cr1 and Cr2 octahedra and corner-sharing between two Cr1 octahedra. The resulting chemical composition Ba19Cr12O45 and bond valence sum analysis suggest a possible charge disproportion between Cr4+ in the Cr1 site and Cr5+ in the Cr2 site. Finally analysis of magnetization measurements indicates antiferromagnetic correlations between Cr cations and also points toward a probable charge disproportion between Cr sites.

2014 The study of the magnetization « irreversibility line » of a Bi2 -xPbxSr2Ca2Cu3O10 compound with a critical temperature Tc of 110 K is presented. At low temperatures, we observe H* ~ eT/T0 and close to Tc, H* ~ (1 T/Tc)3/2. The reversible magnetization as a function of the magnetic field H obeys the laws calculated for the Abrikosov flux lattice of an ideal J. Phys. France 50 (1989) 2857-2868 15 SEPTEMBRE 1989, Classification Physics Abstracts 74.30C 74.60G 74.50 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198900500180285700

We report on the structure and physical properties of a bidimensional chromate, Sr4Cr3O10, the n=3 member of the Ruddlesden-Popper Srn+1CrnO3n+1 series. For the first time, using complementary x-ray powder diffraction and electron diffraction data, we have solved its layered crystallographic structure. Our study shows also that this high pressure phase is insulating and antiferromagnetic below TN=280K, a similar behavior already observed for n=1, 2 and n=+∞ members.

We perform an extensive study of Sr3Cr2O7, the n = 2 member of the Ruddlesden-Popper Srn+1CrnO3n+1 system. An antiferromagnetic ordering is clearly visible in the magnetization and the specific heat, which yields a huge transition entropy, R ln(6). By neutron diffraction as a function of temperature we have determined the antiferromagnetic structure that coincides with the one obtained from density functional theory calculations. It is accompanied by anomalous asymmetric distortions of the CrO6 octahedra. Strong coupling and Lanczos calculations on a derived Kugel-Khomskii Hamiltonian yield a simultaneous orbital and moment ordering. Our results favor an exotic ordered phase of orbital singlets not originated by frustration.

Single filament tapes with different Ca contents Bi2.1Sr1.96Ca x Cu2.0O8+δ (x = 0.90, 0.95, 1.00, and 1.05) were prepared by powder in tube (PIT) process. The introduction of Ca nonstoichiometry caused systematic changes in the phase compositions of the final tapes, thus changing the chemical compositions of Bi-2212 phase. The increase of critical temperature T c from 81.2 to 84.2 K can be attributed to the carrier concentration change. The critical current density, J c and flux pinning properties were measured and evaluated at 4.2 K and high field (0–20 T) with the variation of Ca content. Obvious improvements of J c were obtained with the Ca content of x = 0.95 and 1.05 under perpendicular field, which implied that the chemical compositions changes of Ca could effectively lead to the formation of lattice defects, thus enhancing the flux pinning properties on the superconducting layers.

We have synthesized for the first time the metastable compound 1T-CrTe2. We have done its complete structural characterization and measured its magnetization, specific heat and electrical resistivity between 4 and 330 K. We have also performed detailed band structure calculations. We have found that it crystallizes in the CdI2 structure type and that its electrical resistance follows a metallic behaviour below room temperature. Its magnetization and specific heat curves show that the compound has a transition to a ferromagnetic state at TC = 310 K, with the magnetic moments ordered parallel to the basal plane. From the specific heat measurements and the ferromagnetic solutions obtained from our DFT calculations, we conclude that the ferromagnetism is of itinerant nature.

Shubnikov-de Haas (SdH) oscillations and upper critical magnetic field (H-c2) of the iron-based superconductor FeSe (T-c = 8.6K) have been studied by tunnel diode oscillator-based measurements in magnetic fields of up to 55T and temperatures down to 1.6K. Several Fourier components enter the SdH oscillations spectrum with frequencies definitely smaller than predicted by band structure calculations indicating band renormalization and reconstruction of the Fermi surface at low temperature, in line with previous ARPES data. The Werthamer-Helfand-Hohenberg model accounts for the temperature dependence of Hc2 for magnetic field applied both parallel (H parallel to ab) and perpendicular (H parallel to c) to the iron conducting plane, suggesting that one band mainly controls the superconducting properties in magnetic fields despite the multiband nature of the Fermi surface. Whereas Pauli pair breaking is negligible for H parallel to c, a Pauli paramagnetic contribution is evidenced for H parallel to ab with Maki parameter alpha = 2.1, corresponding to Pauli field H-P = 36.5T. Copyright (C) EPLA, 2015

Effects of extra Mg additions on the Nb–B interface and the superconducting properties of mono-core MgB2/Nb/Cu tapes were investigated. Four Mg1+xB2/Nb/Cu tape samples were prepared by standard in situ PIT method (x=0, 0.03, 0.05, 0.10). For maintaining the superconducting properties, the extra Mg is supposed to prevent the Nb–B diffusion, which is the main cause of decline in superconducting properties of Nb sheathed MgB2 tapes. And it has been proved that a suitable amount of Mg addition did play a positive role in slowing down the Nb–B diffusion to some degree on one hand. And the superconducting properties also show some improvements. While over sufficient Mg addition would accelerate the formation of the Nb–B diffusion layer on the other hand. According to the experimental results, the suitable amount of Mg addition should be between x=0.03 and 0.05 depending on the different sintering temperatures.

•Kilometre level, Nb reinforced MgB2 wires was fabricated by in situ PIT method.•Higher stress–strain properties were acquired in this Nb reinforced MgB2 wires.•Good homogeneity of Ic distribution was acquired in this MgB2 wires.

This paper aims at examining the bending mechanical properties of MgB2/Nb/Cu wires and tapes fabricated with in situ powder in tube (PIT) method. The bending characteristics of five different MgB2 wires and tapes including four wires with mono-, 6-, 12-, and 36-filamentary and a 6-filamentary tapes were tested. The critical current density (Ic) as a function of bending diameter was measured and compared for all the prepared samples. This study aims at giving a clear picture of the optimum bending conditions at which MgB2 wires will possess a high current carrying capacity for practical applications.

We chose high strength and high conductive Cu–Nb composite as strengthening core to improve the mechanical properties of 6-filament MgB2 wires. The Cu–Nb core become partially dispersion strengthened during the fabrications of the MgB2 wires. It has been found that this Cu–Nb composite offers good promise of increased strength while maintaining the superconducting properties of the MgB2 wire. The Young’s modulus of the best wire samples increased significantly to about 130 GPa, which is comparable to those of high strength ferromagnetic materials sheathed wires but without negative ferromagnetic effects. Those mechanical properties were enough to satisfy the low field application needs. The critical current I c also achieves 200 A (engineering critical current density, J ce above 1.30 × 104 A/cm2) at 20 K 1 T field. The 91-filament Cu–Nb composite core reinforced wires were fabricated by in situ Powder In Tube method.

We have studied the effect of substitution of Cr in metastable 1T -CrSe2 by Ti and V on its structural and magnetic properties. The structural transitions observed between 165-180 K in the pure material are stomped by the doping. The pure compound has a magnetization corresponding to an antiferromagnetic (AF) ground state. On Ti substitution, we observe an increase of the lattice constants and a gradual passage towards a ferromagnetic state, while V replacement maintains AF order up to our highest doping, xV = 0.5.With our experimental results and the help of first-principles calculations, we construct the phase diagram of the system.

The MgB2 wires were prepared by in-situ Powder-in-Tube (PIT) method with Nb/Cu compounds tube as the sheath and then were heat treated at various temperatures for 2 h in flowing argon. The resistivity (R-T), magnetic moment (M-T), transport critical current (Ic-B) and magneto optical (MO) properties of MgB2 wires were analyzed. The results show that there is a diffusion layer between MgB2 superconducting core and Nb barrier layer when the heat-treatment temperature is higher than 750 °C, and this diffusion layer obstructs the current transfer into MgB2 superconductor core from the Cu sheath. This indicates that the best heat-treatment temperature for the MgB2 wires with Nb as the barrier layer should be lower than 750 °C.

The effects of amorphous carbon doping on superconducting properties of MgB2/Nb/Cu wires fabricated by the in-situ powder-in-tube (PIT) method have been investigated. Using Nb as the barrier and Cu as the stabilizer, the MgB2 wires were fabricated by cold drawing. The wires of MgB2-xCx (x = 0.0, 0.05, 0.08, 0.10, 0.15) have been investigated to realize the effect of amorphous carbon doping on the phase formation, microstructure and superconducting properties. Characterization of MgB2/Nb/Cu wires was carried out using XRD, SQUID, SEM/EDS, and Ic measurements. XRD result shows that the high purity MgB2 phase is acquired after heat-treatment at 700 °C. The microstructure observation for wires shows that the MgB2/Nb/Cu wires have better grain connectivity after doping amorphous carbon, and Energy Dispersive X-ray (EDS) analysis confirms the uniform distribution of carbon in the MgB2 superconducting phase. The critical current density (Jc) of wires was measured in different magnetic fields by a standard four probes method, and the transport Jc values as high as 1.4 × 105 A/cm2 (4.2 K, 5 T) and 3.3 × 104 A/cm2 (4.2 K, 10 T) have been achieved.

The comparisons of strengthening abilities of Cu, Cu–Nb, and NbTi for 6-filamentary MgB1.92(TiC)0.08/Nb/Cu wires and the electrical properties of the best reinforced wire are investigated. The Cu–Nb reinforcing core is more appropriate for manufacturing MgB2 wires. It is observed that the Cu–Nb reinforced wire sintered at 750°C for 2h has the highest yield strength of 101.8MPa. With the sintering temperature increasing, the yield strength of the Cu–Nb and NbTi reinforced wires increased while the value slightly decreased in the case of Cu reinforced one in this study. According to our calculations, those mechanical properties are suitable for the application requirements of superconducting magnets in low field with the critical engineering current Ic of 197 and 205A (Jce exceeding 1.7×104 and 1.8×104A/cm2) before and after 70MPa axial load on the wire at 20K, 1.5T respectively. And within the working temperature range from 10K to 35K, the Cu–Nb reinforced wires show the lowest resistivity as well. The Cu–Nb reinforced long MgB1.92(TiC)0.08/Nb/Cu wires with unit length over 1km have already been manufactured using the similar process. The Cu, Cu–Nb, and NbTi reinforced 6-filament MgB1.92(TiC)0.08/Nb/Cu wires are fabricated by in situ PIT (Powder-in-Tube) method respectively.

The Nb–B diffusion behaviors and their effects on the superconducting properties of MgB2/Nb/Cu tapes were investigated. Two relevant samples of the Nb–B diffusion couples and monocore MgB2/Nb/Cu tapes were prepared with the same standard in situ PIT method, respectively. And both the samples were sintered at 650, 750, 850, and 950°C for 2 hours, respectively. It has been found that Nb–B interface is invisible in the diffusion couples sintered at low temperatures as 650°C, correspondingly the superconducting properties of MgB2 tapes are superior to those of the ones sintered at the other three temperatures. With the heating temperature increasing, Nb–B interface, mainly NbB2 with a little NbB, appears and becomes thicker gradually in the Nb–B diffusion couples. The corresponding superconducting properties of MgB2 tapes follow the similar patterns: within the range from 650°C to 950°C, the higher the sintering temperature, the poorer the superconducting performances, because the Nb–B diffusion in MgB2 tapes could cause the property degradations of MgB2/Nb tape. As extended to the actual fabrications of Nb sheathed MgB2 tapes or wires, suitable sintering temperature range should be from 650°C to 750°C for the desired performances.