Superconductivity of niobium nitride single crystals after implantation of carbon and nitrogen

Single crystals of δ-NbN_0.85 with a superconducting transition temperature T_c of 14.3 K were implanted with nitrogen and carbon ions at room temperature and subsequently annealed at high temperatures. Implantation was also performed at high substrate temperatures. After implantation at about 920°C maximum T_c-values of 16.5 and 17.8 K were obtained with N- and C-ions respectively. Disorder observed after room temperature implantation consisted of displaced Nb-atoms which could not be completely annealed in an isochromous annealing process up to 1000°C. For annealing temperatures above 1100°C nitrogen diffusion out of the implanted layers resulted in a reduction of T_c.     

Growth and properties of Li,Ta modified (K,Na)NbO3 lead‐free piezoelectric single crystals

Li, Ta modified (K,Na)NbO₃ single crystals with the size of 18 mm × 18 mm × 10 mm were successfully grown by top‐seeded solution growth method, with orthorhombic–tetra‐gonal phase transition temperature ～79 °C and Curie temperature ～276 °C. The electromechanical coupling factors k₃₃ and k_t were found to be ～88% and ～65%, respectively. The piezoelectric coefficient d₃₃ for the [001]_c poled crystals reached 255 pC/N. In addition, the electromechanical coupling factor exhibited high stability over the temperature range of –50 °C to 70 °C, making these lead free crystals good candidates for electromechanical applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sputtered magnesium diboride thin films: Growth conditions and surface morphology

Magnesium diboride (MgB₂) thin films were deposited on C-plane sapphire substrates by sputtering pure B and Mg targets at different substrate temperatures, and were followed by in situ annealing. A systematic study about the effects of the various growth and annealing parameters on the physical properties of MgB₂ thin films showed that the substrate temperature is the most critical factor that determines the superconducting transition temperature (T_c), while annealing plays a minor role. There was no superconducting transition in the thin films grown at room temperature without post-annealing. The highest T_c of the samples grown at room temperature after the optimized annealing was 22 K. As the temperature of the substrate (T_s) increased, T_c rose. However, the maximum T_s was limited due to the low magnesium sticking coefficient and thus the T_c value was limited as well. The highest T_c, 29 K, was obtained for the sample deposited at 180 °C, annealed at 620 °C, and was subsequently annealed a second time at 800 °C. Three-dimensional (3D) AFM images clearly demonstrated that the thin films with no transition, or very low T_c, did not have the well-developed MgB₂ grains while the films with higher T_c displayed the well-developed grains and smooth surface. Although the T_c of sputtered MgB₂ films in the current work is lower than that for the bulk and ex situ annealed thin films, this work presents an important step towards the fabrication of MgB₂ heterostructures using rather simple physical vapor deposition method such as sputtering.     

Pulsed laser deposition conditions and superconductivity of FeSe thin films

We report the effects of growth conditions on the superconducting properties of FeSe films epitaxially grown on LaAlO₃ substrates by pulsed laser deposition (PLD). Customary materials characterization techniques [X-ray diffraction (XRD), in-situ X-ray photoelectron spectroscopy (XPS), in-situ ultra-violet photoelectron spectroscopy (UPS), and scanning electron microscopy (SEM)] revealed the films had a c-axis oriented tetragonal structure with lattice constants dependent on the growth temperature (varied from 100 to 600°C). The standard four-point probe method was used to measure the resistivity and superconducting transitions. Films grown at 400–550°C showed a clear superconducting onset but no zero resistance down to 2 K. The highest superconducting onset temperature (T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}}) of 8 K was observed in films grown at 500°C and the onset temperature was clearly correlated to the ratio of the lattice constants (c/a). As the thickness of the FeSe films increased from 27 nm to 480 nm, T_c^onsetT_{\mathrm{c}}^{\mathrm{onset}} also increased as the strain in the system was relaxed.     

Superconductivity of Bi2 (Sr0.9Ca0.1)2 CuO y single crystals

Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y single crystals were grown by the self-flux method. The temperature dependence of resistance and ac susceptibility measurements showed that the as-grown crystal is semiconductive between 4.2 K and 300 K. However, after annealing in air for more than 20 days the crystal became superconducting with a T _c of 66 K obtained from the R-T curve. The ac susceptibility data also showed a strong superconducting transition at 71 K. X-ray diffraction indicated that the structure of the as-grown and the annealed crystal both are orthorhombic. The T _c of 71 K of the Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y crystal may be due to the increase of oxygen content in the crystal after the long-time annealing in air. It is suggested that the superconductivity of the Ca-doped 2201 phase of the Bi-based system, which up to now was found to have a T _c=7–22 K, is similar to the 2201 phase of the Tl-based system.     

Pressure effects on the superconducting transition of ytterbium doped Ce0.6Yb0.4FeAsO0.9F0.1

The Effect of pressure on the superconducting transition temperature of Yb doped Ce_0.6Yb_0.4FeAsO_0.9F_0.1 has been investigated for the first time using resistivity and magnetization studies. Increase in chemical pressure by substitution of smaller Yb³⁺ ions in place of Ce³⁺ ions results in a significant enhancement of T_c from 38 K (Yb free) to 47 K (40% Yb). Enhancement in T_c with external pressure has been observed for this compound up to a maximum value of T_c = 48.7 K at 1 GPa, beyond which T_c starts decreasing monotonously. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydrostatic pressure effect on Tc of new BiS2‐based Bi4O4S3 and NdO0.5F0.5BiS2 layered superconductors

We investigate the external hydrostatic pressure effect on the superconducting transition temperature (T_c) of new layered superconductors Bi₄O₄S₃ and NdO_0.5F_0.5BiS₂. Though the T_c is found to have a moderate decrease from 4.8 K to 4.3 K (dT_c^onset/dP = –0.28 K/GPa) for Bi₄O₄S₃ superconductor, the same increases from 4.6 K to 5 K (dT_c^onset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO_0.5F_0.5BiS₂ superconductor. The variation of T_c in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High temperature multiple transitions in bismuth-based superconductors

We report observation of possible superconducting transitions at 133 K, 117K and 105 K in Bi-Sr-Ca-Cu-O bulk samples which attain zero resistance state above the liquid nitrogen temperature (T_c(0)) = 80K). The transition at 105 K is prominent and shows a significant drop in the sample resistance. X-ray data depict characteristic superconducting peak at 20 = 5–6°. All the specimens show Meissner effect when cooled in liquid nitrogen.     

FeTe as a candidate material for new iron-based superconductor

Tetragonal FeSe is a superconductor with a transition temperature T_c of 8 K and shows a huge enhancement of T_c with applying pressure. Tetragonal FeTe has a structure very analogous to superconducting FeSe, but does not show superconducting transition. We investigated the pressure effect of resistivity on FeTe. The resistivity at room temperature decreased with increasing pressure. An anomaly in resistivity around 80 K shifted towards a lower temperature with increasing pressure.     

Pressure dependence of the superconducting transition temperature of the stage-2 potassium mercurographitide intercalation compound

Measurements of the pressure (P) dependence of the superconducting transition temperature T_c of stage-two KHgC₈ are reported. T_c is found to decrease with applied pressure from a room pressure value of 1.85K at a rate dT_c/dP=-6.5 × 10^-5K/bar, similar to typical superconducting elements such as Sn. No superconductivity was detected for stage-one KHgC₄ or K_0.5Hg_0.5 amalgam to a limiting temperature T = 1.3K and a limiting pressure P = 22 kbar. These results are discussed in reference to the possible occurence of structural and charge density wave transitions in these materials and recent theoretical models of superconducting graphire intercalation compounds.     

Electrical properties of thin films of Eu2O3 substituted compounds

Europium oxide (Eu₂O₃) substituted compound has been prepared by solid-solid reaction of the powders of Eu₂O₃, BaCO₃ and CuO at 950°C for 16 hours. The thin films have been deposited by high vacuum evaporation technique (vacuum ≈ 10⁻⁶ torr). The variation of current (I) with voltage (V) at room temperature (RT) i.e. 294 K and in ice (273 K) are found to be linear. The variation of electrical resistivity (ρ) with temperature (T) by heating the sample above RT has been determined. Resistivity is found to decrease with increase in temperature. Further the variation of electrical resistivity (ρ) with temperature (T) from 77 K, liquid nitrogen temperature (LNT), to 270 K has also been determined. It is observed that resistivity suddenly becomes zero at around 87 K. Thus the prepared material has superconducting properties with superconducting transition temperature, T _c at 87 K.      

The superconducting transition temperature of bulk samples of the niobium-germanium A15-phase after implantation of Ge,Si, O and Ar ions

Bulk material of Nb₃ (Ge_0.8Nb_0.2) with A15 structure and a superconducting transition temperature T_c of 6.5 K has been implanted with Ge, Si, Ar and O ions and subsequently annealed at high temperatures. After annealing between 700 and 750°C the Ge implanted samples showed a strong increase in T_c up to 16.2 K. With Si ions only a T_c of 13 K was obtained, with Ar and O ions T_c remained below 9 K. From X-ray measurements carried out on high T_c Ge implanted samples it could be concluded that the implanted surface layer grows up to a high degree epitaxially on the single crystallites of the bulk material. The lattice constant $\text{a}$₀ of the implanted film was reduced by 0.02 Å with respect to the bulk material. This reduction in $\text{a}$₀ is stronger than expected from the transition temperature of the implanted surface layer.     

Orthorhombic structure: a necessity in superconducting 1-2-3 compounds

X-ray and resistivity measurements on YBa₂Cu₃O_7−δ (1-2-3) samples show that for the same but low oxygen concentration,δ⋍0·55, no superconducting transition down to 4·2 K is observed for the tetragonal phase samples while the orthorhombic phase shows aT _c ∼ 31 K. The effect of oxygen concentration onT _c is isolated.T _c=91±1 K has, however, been observed continuously for the normal oxygen annealed samples,δ⋍0·07. The experimental results suggest strongly the necessity of the 1-2-3 compound to be in the orthorhombic phase for the superconducting mechanism to be operative.     

Fabrication of NdBa2Cu3O7−δ single crystals by the top-seeded solution-growth method in 1%, 21%, and 100% oxygen partial pressure atmosphere

Single crystals of NdBa₂Cu₃O_7−δ (Nd123) have been successfully grown by the top-seeded solution-growth (TSSG) method in 1%, 21% and 100% oxygen partial pressure atmosphere ((P(O₂) = 0.01 atm, P(O)₂) = 0.21 atm and P(O₂) = 1.00 atm). Ba---Cu---O solvent with a Ba to Cu ratio of 3:5 was used in a Nd₂O₃ crucible. Nd is supplied by the reaction between the molten solvent and the Nd₂O₃ crucible. Compositions of Nd123 single crystals grown in different oxygen partial pressure atmospheres were analyzed by inductivity coupled plasma atomic emission spectrometry (ICP-AES) and confirmed to be Nd:Ba:Cu = 1.01:1.97:3.00 for P(O₂) = 0.01 atm, Nd:Ba:Cu = 1.07:1.95:3.00 for P(O₂) = 0.21 atm and Nd:Ba:Cu = 1.10:1.90:3.00 for P(O₂) = 1.00 atm, respectively. The Nd123 single crystals grown in different oxygen partial pressure atmospheres were annealed in a pure oxygen gas flow, and the temperature dependence of the DC magnetization for these crystals was measured using a superconducting quantum interference device (SQUID) magnetometer. The Nd123 single crystal grown in P(O₂) = 0.01 atm, and annealed at 340°C for 200 h showed a steep superconductive transition at 96 K. On the other hand, the Nd123 crystal grown in P(O₂) = 0.21, 1.00 atm and, annealed at 340°C for 200 h exhibited a broad transition at 92 K for P(O₂) = 0.21 atm and at 88 K for P(O₂) = 1.00 atm, respectively. Therefore for Nd123 single crystal production with high quality superconductive characteristics, a low oxygen partial pressure atmosphere during crystal growth is found to be effective for minimizing the substitution of Nd ions into Ba sites.     

Raman analyses of co‐phasing and hysteresis behaviors in V2O3 thin film

We present the studies of the phase transition behaviors of V₂O₃ thin film using temperature‐dependent Raman scattering spectroscopy. Our results show that in both the cooling and heating processes of V₂O₃ thin film, the phase transition occurs gradually but not suddenly, contrary to that in single crystal. The coexistence of both the metal and insulator phases with co‐phasing ΔT_c larger than 30 K is observed in both the cooling and heating processes. We discuss that this large co‐phasing ΔT_c should be distinguished with the large hysteresis ΔT_h reported in nanostructures. In addition, our discussions indicate that co‐phasing ΔT_c and hysteresis ΔT_h would be mainly correlated with stress and defect states in sample, respectively. Furthermore, our Raman analyses suggest that stress would also induce phase transitions in V₂O₃, and the stress (pressure)‐induced phase transitions would behave differently comparing with the temperature‐induced transitions under normal pressure. Copyright © 2012 John Wiley & Sons, Ltd.     

钛离子辐照对MgB2超导薄膜的载流能力和磁通钉扎能力的影响

利用混合物理化学气相沉积法(hybrid physical-chemical vapor deposition, HPCVD)可以制备出高性能的MgB₂超导薄膜, 再对薄膜进行钛(Ti)离子辐照处理.经过辐照处理后的样品被掺入了Ti元素, 与未处理的干净MgB₂样品相比,其超导转变温度没有出现大幅度的下降, 而在外加磁场下的临界电流密度得到了明显的提高,同时样品的上临界磁场也得到了提高. 在温度5 K, 外加垂直磁场为4 T的情况下, Ti离子辐照剂量为1× 10¹³/cm²的样品的临界电流密度达到了1.72× 10⁵ A/cm², 比干净的MgB₂要高出许多,而其超导转变温度仍能维持在39.9 K的较高水平.     

Superconducting properties of the atomically disordered MgB2 compound

The effect of disorder induced by neutron irradiation in a nuclear reactor (thermal neutron fluence 1×10¹⁹cm^?2) on the superconducting transition temperature T _c and the upper critical field H _c2 of polycrystalline MgB₂ samples was investigated. Despite the appreciable radiation-induced distortions (more than ten displacements per atom), the initial crystal structure (C32) was retained. The temperature T _c decreased from 38 to 5 K upon irradiation and was practically completely restored after the subsequent annealing at a temperature of 70°C. A weak change in the dH _c2/dT derivative upon irradiation is explained by the fact that the irradiated samples are described by the “pure” limit of the theory of disordered superconductors. The suppression of T _c upon disordering may be due to the isotropization of the originally anisotropic (or multicomponent) superconducting gap or to a decrease in the density of electronic states at the Fermi level.     

Nb3Sn正常-超导转变时的比热反常

在16.0°K—20.3°K之间测量了Nb₃Sn样品的热容量。Nb₃Sn在临界温度附近的比热跳跃值ΔC=2.21(±5％)焦耳/克分子·度。样品的临界温度T_c=17.88°K,转变宽度ΔT_c≈0.2°K。ΔC值利用热力学关系式确定了Nb₃Sn在0°K时的热力学临界场H₀=5300奥斯特。利用本文的结果和文献上关于热膨胀系数的跳跃值Δα及?T/?P值验证了热力学关系式。扼要地描述了比热测量装置.     

Magnetic properties of nanoparticle La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> under applied hydrostatic pressure

Pressure effects on magnetic properties of two La_0.7Ca_0.3MnO₃ nanoparticle samples with different mean particle sizes were investigated. Both the samples were prepared by the glycine-nitrate method: sample S—as-prepared (10 nm), and sample S₉₀₀—subsequently annealed at 900 °C for 2 h (50 nm). Magnetization measurements revealed remarkable differences in magnetic properties with the applied pressure up to 0.75 GPa: (i) for S sample, both transition temperatures, para-to-ferromagnetic T _C = 120 K and spin-glass-like transition T _f = 102 K, decrease with the pressure with the respective pressure coefficients dT _C/dP = −2.9 K/GPa and dT _f/dP = −4.4 K/GPa; (ii) for S₉₀₀ sample, para-to-ferromagnetic transition temperature T _C = 261 K increases with pressure with the pressure coefficient dT _C/dP = 14.8 K/GPa. At the same time, saturation magnetization M _S recorded at 10 K decreases/increases with pressure for S/S₉₀₀ sample, respectively. Explanation of these unusual pressure effects on the magnetism of sample S is proposed within the scenario of the combined contributions of two types of disorders present in the system: surface disorder introduced by the particle shell, and structural disorder of the particle core caused by the prominent Jahn–Teller distortion. Both disorders tend to vanish with the annealing of the system (i.e., with the nanoparticle growth), and so the behavior of the sample S₉₀₀ is similar to that previously observed for the bulk counterpart.     

Pressure-induced superconductivity in tetragonal NdBa2Cu3O6+x

The appearance of superconductivity and relaxation of the transition temperature to its equilibrium value T _c≈30 K over the course of five days have been observed in a tetragonal sample of NdBa₂Cu₃O_6.67 under 1 GPa pressure. The time dependence T _c(t) correlates with a decrease of the room-temperature electrical resistance R(t). The superconducting phase disappears 1.3 h after the pressure is removed. This behavior is explained by a redistribution of charge as a result of pressure-induced oxygen reordering in the CuO_x planes. A large effect of oxygen ordering on the transition temperature under pressure has been observed near the metal-insulator transition (the largest of all those measured in the 1-2-3 system), along with a nonlinear temperature dependence of T _c. Fiz. Tverd. Tela (St. Petersburg) 39, 1328–1334 (August 1997)