改进铜铌溅射型QWR超导腔性能的探讨

通过直流偏压二级溅射方法，在无氧铜腔体表面溅射一层铌膜，研制了铜铌溅射型射频超导1/4波长谐振腔(quarter wave resonator,QWR)，该腔主要用于重离子的加速，是北京放射性核束装置中后加速部分的预研项目.目前国际上很多实验室都在研究进一步提高铜铌溅射型QWR超导腔的性能，通过多种方法的实验研究，发现在无氧铜衬底与铌膜之间加入一层氮 化铌（NbN）薄膜，可以使得表面铌膜的超导温度转变点由原来的8.8K提高到了接近9.6K ，该方法有可能成为提高QWR腔加速性能的重要途径，目前进一步研究正 关键词： 溅射 QWR超导腔 氮化铌NbN 超导温度     

Degradation of the second critical magnetic field in multilayered composites with nanodimensional layers of niobium and Nb-Ti superconducting alloy

Multilayered tapes containing layers of superconducting niobium alloys with 30 and 31 wt % Ti and separated by niobium layers were investigated. The layers were ∼140 to ∼10 nm thick. Effective pinning of superconducting vortex filaments was observed on the interlayer Nb-NbTi interfaces. It was established that second critical magnetic field H _c2 decreased as the layer thickness decreased. With thin layers, its magnitude depended on the orientation of rolling the tape with respect to the external magnetic field. Results are explained by the proximity effect.     

Interlayer magnetotransport study in electron-doped Sm2−x Ce x CuO4−δ

Vortex and pseudogap states in electron-doped Sm_2−x Ce _x CuO_4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On cooling, the c-axis resistivity ρ_c of the mesa structures reveals a semiconductive upturn above T_c, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρ_c(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap, whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi₂Sr₂ CaCu₂ O_8+y.     

Reflectivity oscillations in superconducting superlattices Nb/SiO2

The reflectivity of superconducting superlattices Nb/SiO₂ was experimentally studied at room temperature in a spectral range 450-5000 cm^-1. It was found that the reflectivity as a function of SiO₂ layer thickness oscillates with a period 3.5 Å. This value coincides with the period of oscillations of the superconducting transition temperature found earlier on these superlattices. We suppose that in both cases the oscillations are due to changes of electron state density. So it is possible to considerably influence the superconducting transition temperature of a superconducting film by making sandwich-like structures using thin dielectric layers.     

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature T_c of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers d_s is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature T_c at the reduction of the thickness d_s is detected and is attributed to the 0-π transition.     

Magnetic patterns and flux pinning in Pd0.99Fe0.01-Nb hybrid structures

The magnetic properties of hybrid thin-film Pd_0.99Fe_0.01-Nb structures are studied by a magneto-optical technique. It is shown that, below 14 K, the samples exhibit the ferromagnetic ordering corresponding to the formation of weakly coupled ferromagnetic nanoclusters. In the clusters, the effective spin polarization of Fe ions is about 4μ_B, corresponding to that in the bulk Pd₃Fe alloy. The proximity of the ferromagnetic layer does not suppress the superconductivity in niobium. It does not affect the superconducting transition temperature but leads to an enhanced pinning and results in an increase in the critical current by about 30%. This behavior agrees well with the existence of the nanocluster structure in the ferromagnetic film.     

Electronic transport properties of hafnium/zirconium multilayers

We have investigated the electrical resistivity, superconducting transition temperature, and upper critical field as a function of layer thickness in hafnium/zirconium ( ) metallic superlattices. These films have equal Hf and Zr layer thicknesses (d_h and d_z, respectively). We have studied a series of samples with modulation wavelength λ = d_h + d_z ranging from 20 to 250 Å. All films show a metallic type of resistivity, with remarkably little difference in both room temperature and liquid helium resistivities. All structures undergo a transition to a superconducting state, with the transition temperature remaining nearly constant across the series. This indicates that the interface region of these structures is of rather high quality. Upper critical fields both parallel and perpendicular to the sample plane were determined. Because of the relatively large superconducting coherence length, these films behave essentially three-dimenstionally throughout the range of λ studied. However, we also observe a somewhat anomalous behavior in the ratio of the parallel to perpendicular critical fields near the transition temperature, the origin of which is not yet known.     

Oscillations of the Critical Temperature in a (Fe/Cr/Fe)/V/Fe Heterostructure

The superconducting and magnetic properties of the (Fe/Cr/Fe)/V/Fe layered system with variable thickness of the chromium layer have been experimentally and theoretically studied. The magnetic properties of the system have been studied by the ferromagnetic resonance method, and the superconducting transition temperature has been measured from the jump in the magnetic susceptibility. A wide variety of magnetic states are observed in the system; in particular, the structure of small domains can arise in the iron layer placed between vanadium and chromium. It has been shown experimentally that the critical temperature T_c of the superconducting transition undergoes nonmonotonic oscillations with a noticeable amplitude in the given system with the change in the thickness of the Cr layer. The proposed model based on the proximity effect theory makes it possible to relate these T_c oscillations to the features of the magnetic structure of the samples.     

氮化铌表面氧化物的XPS研究

本文用X射线光电子能谱(XPS)对室温下自然氧化和射频氧化的氮化铌薄膜表面进行了成分分析。指出:与在类似条件下纯铌膜的氧化不同,NbN膜的表面氧化物中不存在NbO和NbO₂,而是以Nb₂O₃作为从NbN到Nb₂O_5-y的过渡相。氮化铌在氧化过程中表现出一种抑制Nb₂O,NbO等低价传导性氧化物生成的能力,这对于用氧化物作势垒的超导隧道结的研制具有实际参考价值。 关键词：     

Fabrication of sub-micron Nb/Al-oxide/Nb sis junctions

I describe a process for fabricating high critical current density, submicron superconductor/insulator/superconductor (SIS) tunnel junctions, suitable for use as millimeter-wave or submillimeter-wave mixers. The superconducting electrodes are niobium; the insulating barrier is aluminum oxide. Standard optical photolithography is employed, with subsequent shrinkback of the photoresist mesa defining the device through reactive-ion etching in an oxygen plasma to enhance step-coverage by the insulating layer. Active areas as small as 0.5µm² have been made. I discuss two variations of the process, one starting from a small initial trilayer region defined by liftoff, and the other starting from a whole-wafer initial trilayer.     

Study of the magnetic superconductor,Y9Co7, at high pressure and high magnetic field

We have measured the electrical resistance and magnetic susceptibility of the magnetic superconductor, Y_gCo₇, at pressures up to 20 kbar and in magnetic fields up to 6T. We have found that pressure suppresses the magnetism resulting in a higher superconducting transition temperature and conclude that Y₉Co₇ is an itinerant ferromagnet, not a spin-glass. Pressure also sharpens the superconducting transition and increases the critical magnetic field, signifying that the long range ferromagnetic and superconducting order parameters co-exist but vary spatially. For pressures greater than 6 kbar, the magnetoresistance is always positive, further indicating the suppresion of magnetism by high pressure.     

La0.67Ca0.33MnO3薄膜皮秒光电效应

在1064 nm波长脉冲激光（脉宽25 ps）的照射下，钙钛矿氧化物薄膜La0.67Ca0.33MnO3/SrTiO3具有超快光电效应，对激光脉冲显示ps量级的响应时间，上升沿响应时间300 ps，半高宽700 ps，同时，对激光能量的响应灵敏度为500 mV/mJ。     

Superconducting and magnetic properties of Nb/Pd 1-xFex/Nb triple layers

The superconducting and magnetic properties of Nb/Pd_1-xFe_x/Nb triple layers with constant Nb layer thickness d_Nb=200 ? and different interlayer thicknesses 3 ?≤ d_PdFe ≤ ? are investigated. The thickness dependence of the magnetization and of the superconducting transition temperature shows that for small iron concentration x the Pd_1-xFe_x layer is likely to be in the paramagnetic state for very thin films whereas ferromagnetic order is established for x ≥ 0.13. The parallel critical field B_c2||(T){B_{c2||}}(T) exhibits a transition from two-dimensional (2D) behavior where the Nb films are coupled across the interlayer, towards a 2D behavior of decoupled Nb films with increasing d_PdFeand/or x. This transition allows a determination of the penetration depth x_F{\xi _F} of Cooper pairs into the Pd_1-xFe_x layer as a function of x. For samples with a ferromagnetic interlayer x_F{\xi _F} is found to be independent of x.     

Ferromagnetic resonance in a system of magnetic films with different Curie temperatures

The peculiarities of absorption of rf electromagnetic radiation (ferromagnetic resonance) in multilayer NiFe/Ni_0.65Cu_0.35(d)/CoFe structures in a wide temperature range are analyzed. It is shown that the type of interaction of the NiFe and CoFe ferromagnetic films via a “weak” ferromagnetic Ni_0.65Cu_0.35 interlayer changes from antiferromagnetic to ferromagnetic upon cooling and a decrease in interlayer thickness d. The detected temperature dependence of the interlayer interaction indicates the possibility of observation of a strong magnetocaloric effect in the structures under investigation.     

Magnetism in structures with ferromagnetic and superconducting layers

The influence of superconductivity on ferromagnetism in the layered Ta/V/Fe_1–x V _x /V/Fe_1–x V _x /Nb/Si structures consisting of ferromagnetic and superconducting layers is studied using polarized neutron reflection and scattering. It is experimentally shown that magnetic structures with linear sizes from 5 nm to 30 μm are formed in these layered structures at low temperatures. The magnetization of the magnetic structures is suppressed by superconductivity at temperatures below the superconducting transition temperatures in the V and Nb layers. The magnetic states of the structures are shown to undergo relaxation over a wide magnetic-field range, which is caused by changes in the states of clusters, domains, and Abrikosov vortices.     

Low-temperature deformation of nanocrystalline niobium

The strain characteristics of nanocrystalline niobium are measured in the temperature range 4.2–300 K. It is shown that the development of a strong local deformation with clearly delineated macroscopic slip bands occurs at 4.2 K and 10 K. The thermal effects at a stress jump observed upon transition of the sample (or a niobium strip placed close to the sample) from the superconducting state to the normal state are estimated. It is demonstrated that the temperature dependence of the yield point σ_s(T) can be divided into three portions: two portions (T<10 K and T>70 K) with a slight change in σ_s and the third portion with a strong dependence σ_s(T). The strain characteristics of polycrystals with nano-and larger-sized grains are compared with those of single crystals.     

Magnetic phase transitions in layered intermetallic compounds

Magnetic, magnetoelastic, and magnetotransport properties have been studied for the RMn₂Si₂ and RMn₆Sn₆ (R is a rare earth metal) intermetallic compounds with natural layered structure. The compounds exhibit wide variety of magnetic structures and magnetic phase transitions. Substitution of different R atoms allows us to modify the interatomic distances and interlayer exchange interactions thus providing the transition from antiferromagnetic to ferromagnetic state. Near the boundary of this transition the magnetic structures are very sensitive to the external field, temperature and pressure. The field-induced transitions are accompanied by considerable change in the sample size and resistivity. It has been shown that various magnetic structures and magnetic phase transitions observed in the layered compounds arise as a result of competition of the Mn–Mn and Mn–R exchange interactions.     

Parallel upper critical field of niobium/graphite bilayers

The influence of the superconducting proximity effect to the upper critical field (H_C) in niobium/graphite bilayers was studied and related to the ratio : and are the transition temperature of Nb/KG film and that of Nb film respectively. The thickness of niobium (Nb) film was controlled about 40 nm, and that of graphite (kish graphite: KG) film ranged from 120 nm to 140 nm. For making a specimen, Nb was deposited on a KG film and a quartz glass substrate simultaneously, and magnetic field was applied parallel to the Nb/KG interface. H_C of Nb/KG film () lowered from that of Nb film () at all temperatures within this work, and the value of ΔH_C defined as showed different temperature dependence among samples, depending on whether < or 1.00. This behavior of H_C suggests the additional existence of the interference of electrons in the clean KG film.     

Observation of a first-order transition in nonequilibrium superconducting lead films

We have observed a first order transition of a superconducting lead film due to quasiparticle injection through a tunnel barrier for the temperatures below T_λ using a double-tunnel-junction configuration. Comparison with the existing nonequilibrium model is discussed.     

Properties of electron-sensitive detectors with a Schottky barrier

A comparative study is performed of Schottky barrier structures based on Si and GaAs as photoelements and detectors of electrons with energies of 40 and 60 keV. It is shown that introduction of a dielectric interlayer between the metal and semiconductor 1.5–3 nm thick permits increase in i_c and V_x of the structures in both the photodetector and electron count regimes. A complex temperature dependence of structure properties in the electron count regime was found, depending on barrier metal thickness and electron energy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 76–81, April, 1991.