Control of the vortex flow in microchannel arrays produced in YBCO films by heavy-ion lithography

High-energy heavy-ion lithography is a powerful tool for tuning both structural and electromagnetic properties of high temperature superconductors by inducing nanometer scale defects confined in micron scale patterns. We show how the vortex dynamics in YBCO thin films patterned by heavy-ion lithography can be controlled and potentially exploited for device applications. Both local critical temperature and local critical currents are effectively tailored by the imposed irradiation geometry. The direct visualization of the real-time dynamics of the magnetic pattern is achieved by the magneto-optical imaging technique, while confined vortex flow is revealed by the simultaneous measurement of the electrical resistance both along and perpendicular (Hall resistance) to the direction of the applied current. It is shown that, for microchannel arrays inclined with respect to the transport current flow, the direction of vortex motion is solely determined by the imposed irradiation pattern geometry, in a well-defined temperature range, for a given applied current.     

Raman microprobe technique diagnostic of YBCO films produced by laser ablation

Summary We have used a Raman microprobe technique for the characterisation and the diagnostic of YBCO superconducting thin films deposited by Pulsed-Laser Ablation (PLA) on MgO insulating substrates. Using polarisation analysis associated with sample rotations we developed a method for films orientation determination without any request for absolute calibration of the Raman spectra. The use of a bidimensional multichannel detector (OMA 4) allowed an overall detection time of 40 minutes. Each spectrum (shift range from 100 cm⁻¹ to 700 cm⁻¹) takes about 600 seconds. The results of this detection were used to determine the oxygen content, from the position of the Raman mode at 500 cm⁻¹ Homogeneity was checked with the spatial resolution allowed by the dimension of the focused laser beam (10 μm). Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

超薄金属膜在太赫兹波段的光学特性研究

超薄金属膜在太赫兹波段的探测器、反射镜、波导器件以及太赫兹量子级联激光器中得到了广泛应用。超薄金属膜的光学常数不仅是这些器件设计中不可缺少的参数,而且是开发新型光电材料的一个重要依据。文章运用太赫兹差分时域光谱技术对超薄金属铬、镍和钛膜的光学特性研究,获得其在太赫兹波段的折射率和消光系数,并根据菲涅尔公式计算入射介质为高阻GaAs时,GaAs/Metal界面的反射谱,三种金属在0.3～1.5THz的波段范围内的平均反射率均超过80%。研究超薄金属膜在太赫兹波段的反射特性,为设计性能优良的太赫兹辐射源、探测器及太赫兹光学元件奠定基础。     

The analysis of superconducting thin films modified by AFM lithography with a spectroscopic imaging technique

A practical application of nanolithography using atomic force microscopy (AFM) was accomplished in fabricating superconducting flux flow transistors (SFFTs). It was found that it is essential to oxidize a superconducting thin film, grown on LaAlO₃ substrates by a thermal CVD process, by an applied bias voltage between a conducting AFM tip and the films, since I/V characteristics of the device were mainly controlled by the modified gate area in the SFFT. After AFM lithography, the critical current of an YBCO thin film was found to be degraded. Raman lines in the modified YBCO film were observed at 340, 502, and 632 cm⁻¹ in Ar the laser system and 142, 225, and 585 cm⁻¹ in the He-Ne laser system. Raman fluorescence images were also produced by mapping the Raman peaks. A strain image of the peak at 142 cm⁻¹ was most clear, which means that a surface of the YBCO thin film was changed into the YBa₂Cu₃O₆ insulator. AFM nanolithography enables us to fabricate a channel between a source and a drain in SFFT in order to get I/V characteristics.     

电子束蒸发制备YBCO超导薄膜研究

采用电子束沉积制备YBCO超导薄膜,研究了760℃—840℃的不同退火温度下高温热处理对YBCO薄膜双轴织构、表面形貌及超导性能的影响。超导临界电流密度测试、X射线衍射(XRD)和扫描电镜(SEM)的结果表明,退火温度在在800℃时,YBCO薄膜具有良好的织构和平整致密的表面形貌,在77K自场下的临界电流密度J可达4.2×106/cm2。     

Light absorption by island metal films in the infrared range

This study reports on theoretical investigations of the electric and magnetic absorption of small metallic particles as functions of their shape, with the wavelength of incident e/m radiation assumed to be much greater than the size of the particles. A general expression for the absorption cross-section is derived for the cases when both the bulk and surface electron scattering are dominant. In the IR-region the absorption cross-section is shown to be extremely sensitive to the shape of particles; for particles of a constant volume, yet different in shape, it can change by several orders of magnitude. It is also shown that under dominant surface scattering the optical conductivity of an asymmetric particle is described by a tensor whose principal values are calculated for the ellipsoidal shape. The expression obtained for the electric and magnetic absorption renders ground to infer that the ratio of the contributions due to these absorption mechanisms greatly depends on the shape of particles and polarization of light.     

相位补偿算法对提高太赫兹雷达距离像分辨率的研究

介绍了0.2 THz频率步进雷达系统以及获得一维距离像的方法,并利用0.2 THz雷达对角反射器进行距离像分辨率实验,分析了频率步进信号相位不一致对一维距离像以及分辨率的影响,提出了回波相位补偿的方法.经过相位补偿后,目标距离像分辨率和信噪比都显著提高,分辨率达到了厘米量级.仿真和实验结果表明,宽带太赫兹频率步进雷达经过相位补偿,可以对目标进行高分辨率成像,从而为太赫兹雷达二维和三维成像奠定了基础.     

组氨酸和精氨酸的太赫兹光谱研究

采用太赫兹时域光谱(THz-TDS)测试和理论模拟相结合的方法，研究了组氨酸和精氨酸在THz波段的光谱特性.THz-TDS测试的有效光谱范围为0.2—2.8 THz，在该波段得到样品的特征吸收峰分别位于0.88，1.64，2.23 THz(组氨酸)和0.99，1.47，2.60 THz(精氨酸)；运用Gaussian03半经验理论PM3和AM1算法，计算了两种分子在0.1—10.0 THz波段的振动吸收谱，结果表明它们在该波段均具有多个特征吸收，其中在0.2—2.8 THz波段的吸收峰位与实验吸收峰位相互对应并且符合较好；给出了与光谱特征吸收对应的分子振转模型，为认识分子对THz波的响应机制提供了帮助，也为分子鉴别及更宽有效光谱区的实验测试研究提供了科学依据. 关键词： 太赫兹(THz) 半经验理论 THz时域光谱 氨基酸     

Room-temperature deposition of crystalline patterned ZnO films by confined dewetting lithography

In this work patterned ZnO films were prepared at room-temperature by deposition of ∼5 nm size ZnO nanoparticles using confined dewetting lithography, a process which induces their assembly, by drying a drop of ZnO colloidal dispersion between a floating template and the substrate. Crystalline ZnO nanoparticles exhibit a strong visible (525 nm) light emission upon UV excitation (λ = 350 nm). The resulting films were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM). The method described herein presents a simple and low cost method to prepare crystalline ZnO films with geometric patterns without additional annealing. Such transparent conducting films are attractive for applications like light emitting diodes (LEDs). As the process is carried out at room temperature, the patterned crystalline ZnO films can even be deposited on flexible substrates.     

超导材料的发展与研究现状

新型超导材料一直是人类追求的目标。该文主要从超导材料的探索与发现、制备技术、基础研究面临的挑战等几个方面来探讨超导材料的发展与研究现状。     

Substrate dependent growth studies of MgB2 films by electrocrystallization

The comparative study of MgB₂ film growth on Al₂O₃ and glass substrate by electrocrystallization technique is discussed. The precursor magnesium films were deposited by vacuum evaporation method. These magnesium films were then used as electrode for the growth of MgB₂ films by electrocrystallization. The structural, morphological and superconducting properties of the electrocrystallized MgB₂ films on Al₂O₃ substrate were examined by using XRD, SEM and electrical resistivity measurement techniques and compared with that of MgB₂ films grown on glass substrate. The films deposited on Al₂O₃ substrates showed enhanced crystallinity and relatively higher T_c value compared to films deposited on glass substrates.     

Investigation of transmission resonances on the one-dimensional metallic cylindrical gratings in THz frequency range

In the experiments of THz wave transmitting through the metallic cylindrical gratings fabricated by sub-wavelength brass wires, this paper reports that the discrepancy in the sharp resonances occurred as the grating perpendicular or parallel to the electric vector are observed. A simulation based on the finite difference time domain (FDTD) indicated that the enhanced transmission through the grating is attributed to the combined effects of surface plasmons and cavity modes in the perpendicular condition, while the cavity modes dominate the resonant transmission under the other conditions. Additional experimental data and calculated results show that ～1 enhanced coupling efficiency can be realized in some THz frequency, which could be applied to the design and improvement of various optoelectronic devices, or detection of biological molecule and powder samples, etc.     

Effect of secondary electrons from latent tracks created in YBCO by swift heavy ion irradiation

Swift heavy ions (SHI) with electronic energy loss exceeding a value of 14.4 keVnm⁻¹ create amorphized latent tracks in YBCO type superconductors. In the low fluence regime of an ion beam where tracks do not overlap, a decrease of the superconducting transition temperature as probed through resistivity studies, is not expected due to availability of percolating current paths. The present study however shows T_c decrease by about 1–3 K in thin films of YBCO when irradiated by 250 MeVAg ions at 79 K at a fluence of 5×10¹⁰–1×10¹² ionscm⁻². The highest fluence used in the present study is three times less than the fluence where track overlapping becomes significant. The T_c tends to increase towards the preirradiation value on annealing the films at room temperature. To explain this unusual result, we consider the effect of ion irradiation in inducing materials modification not only through creation of amorphized latent tracks along the ion path, but also through creation of atomic disorder in the oxygen sublattice in the Cu–O chains of YBCO by the secondary electrons. These electrons are emitted radially from the tracks during the passage of the SHI. Considering the correlation between the charge state of copper and its oxygen coordination, we show in particular that the latter process is a consequence of the inelastic interaction of the SHI induced low-energy secondary electrons with the YBCO lattice, which result in chain oxygen disorder and T_c decrease.     

Study of the bandgap renormalization in Ga-doped ZnO films by means of optical absorption under high pressure and photoelectron spectroscopy

In this paper we investigate the band gap renormalization in heavily Ga-doped ZnO thin films deposited by pulsed laser deposition on C$C$-plane sapphire and mica substrates. Thin films were studied by ultraviolet photoelectron spectroscopy and also by optical measurements under high pressure. The Fermi-level shift, as obtained from ultraviolet photoelectron experiments, exhibits a relatively small and positive shift (about 0.3 eV) with respect to the valence band for increasing electron concentrations up to 10²¹ cm⁻³. The optical gap exhibits a much larger increase (0.6 eV) for the same concentration range. Absorption measurements under pressure show that the pressure coefficient of the optical gap is correlated to the electron concentration in films, decreasing with increasing electron concentration. As a consequence, the contributions of band filling and band renormalization to the optical gap shift can be separated on the basis of the different pressure behavior of the physical parameters involved in each effect.     

用电泳技术制备YBCO高温超导厚膜

介绍了电泳技术制备YBCO高温超导厚膜的实验方法和YBCO高温超导厚膜的电学性质测量 ,讨论了在学生小型科研实验或设计实验中开展此实验的学时安排、注意事项和实验内容的扩展 .     

The effect of two-photon absorption and optical excitation area on the generation of THz radiation

It was found experimentally that the THz radiation throughput based on optical rectification is strongly related to the optical excitation size in a ZeTe emitter. The factors affecting the THz radiation throughput are investigated in detail both experimentally and theoretically. By taking into account optical rectification, diffraction and two-photon absorption effects, a theoretical model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the theoretical results and the experimental data under high excitation power. Both theoretical and experimental results demonstrate that there is a trade-off between these three effects for THz radiation, and that in order to increase the THz radiation throughput based on optical rectification, one should choose suitable excitation size under a given pump power.     

平面靶直流溅射制备YBCO高温超导薄膜研究

开展了平面靶溅射法制备YBa2Cu3O7-δ(YBCO)高温超导薄膜工艺研究,以达到提高沉积速率的目的。通过增加工作气体总压(Pt),采用基片旋转达到离轴溅射模式,有效地克服了传统平面靶直流溅射法中高能粒子轰击和负离子反溅射现象。在两英寸LaA lO3(LAO)基片上成功外延生长得到了微观结构良好、电学性能优越(临界电流密度Jc=2.3/2.0mA/cm2)的双面YBCO高温超导薄膜。     

Commensurability effects in magnetic properties of superconducting Nb thin films with periodic submicrometric pores

Pinning properties in 100 nm thick continuous and porous superconducting Nb films are examined by ac susceptibility and dc magnetization measurements. The Nb film was deposited on a smooth Si substrate, while the porous film, Nb_P, was deposited on an anodized Al oxide substrate. Pores or “antidots” 40 nm in diameter, 100 nm apart, form a triangular array. The porous film presents commensurate or matching field effects for applied magnetic fields where the magnetic flux threading each unit cell is an integer number of the flux quantum, where ac shielding capability and dc diamagnetic magnetization show an abrupt increase. The response to ac fields as a function of temperature and dc field provided a way to determine that Nb_P sample has higher pinning than the continuous one, and that T_C suppression due to fluxoid quantization is not relevant for the investigated temperature range.     

Practical method to estimate the standard deviation in absorption coefficients measured with THz time-domain spectroscopy

A model of standard deviation in the intensity spectrum of electric field observed with the terahertz time-domain spectroscopy (THz-TDS) is proposed to estimate the random error in the transmittance and absorption coefficients from a single or a few measurements of a sample. The proposed standard deviation which is derived on the basis of the statistical standard deviation and noise floor of intensity spectrum of reference fits well to the standard deviation of transmittance as well as absorption coefficient computed statistically. This study contributes the simple and computationally efficient method to demonstrate the accuracy in optical constants like imaginary part of refractive index and absorption coefficients measured using the THz-TDS.     

退火时间对MgB_2超导薄膜的影响

在多晶A l2O3衬底上,以B2H6作为硼源,化学气相沉积先驱B薄膜,采用Mg扩散方法,在不同退火时间条件下制备了MgB2超导薄膜。通过电阻-温度曲线测量、X射线衍射分析和扫描电子显微镜形貌观测方法,研究了退火时间对MgB2薄膜的超导特性、晶体结构、表面形貌的影响。