YBCO Josephson junctions based on artificially generated biepitaxial grain boundary

Summary In this paper we report the in-plane fabrication and the characterization of artificially engineered biepitaxial grain boundaries (GB) obtained by partly interposing a MgO seed layer between a bare or even-buffered substrate and a CeO₂ thin layer. The main peculiarity of this technique can be summarized by the capability to locate and engineer a single 45°-tilted GB at any stage of the device preparation. The junctions, realized by patterning the grain boundary occurring in the overhanging YBCO film, show Josephson current modulation in a large temperature range with anI _c R _n value of about 200 μV at 4.2K, theR _n value being constant over the whole superconducting region. Under microwave irradiation, theI–V characteristics display several Shapiro steps while, according to the Resistively Shunted Junction (RSJ) behaviour, the step heights have the typical current biased junction dependence. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Characteristics of the Surface-Intrinsic Josephson Junction

During the fabrication of intrinsic Josephson junctions (IJJs) with Bi₂Sr₂CaCu₂O_8+δ (BSCCO) single crystals, the superconductivity of the surface Cu-O layer is degraded because of a deposited metal film on top of the stack. Thus, the characteristics of the surface junction consisting of the surface Cu-O double layers remarkably differ from those of the junctions deep in the stack, which will be referred to as ordinary IJJs. The electrical transport characteristics of the surface junction, such as I-V, I _c′-T, and R-T, show that the critical temperature T _c′ of the surface junction is always lower than that of ordinary IJJs, and that the change of its critical current I _c′ with temperature is different from that of ordinary IIJs. Furthermore, by shunting the surface junction resistively, we are able to observe the AC Josephson effect at 3-mm waveband. Translated from Chinese Journal of Low Temperature Physics, 2005, 27(2) (in Chinese)     

The Relation Between Barrier Structure and Current Uniformity in YBCO Josephson Junctions

Electromagnetic transport measurements were combined with high-resolution electron microscopy observations to study the relation between structure and local critical currents in YBa₂Cu₃O_7–x (YBCO) Josephson junctions. The spatial variation of the critical current J(x) along the length of the boundary for interface engineered Josephson junctions and bicrystal grain boundary Josephson junctions was determined using a phase retrieval algorithm. The current distribution solutions were found to be highly uniform along the length of interface engineered junctions in contrast to solutions for grain boundary junctions. The latter showed significant spatial oscillations in the critical current as well as areas along the boundary that carried no current. Microstructural evaluation of interface engineered junctions fabricated using identical processing parameters to the junctions used for transport measurements suggest that the uniform current distribution is controlled by a highly uniform barrier layer formed between the superconducting electrodes. Microstructural evaluation of grain boundary junctions similar to the junctions used for transport measurements show considerable variations of the grain boundary structure within a single junction.     

Solitons in Josephson junctions: An overview

The dynamics of the Josephson tunnel junction is approximately described by a perturbed sine-Gordon equation. The Josephson tunnel junction is thus a convenient experimental solid state device for the study of solitons and solitonlike phenomena. The physical manifestation of the soliton is a propagating magnetic flux quantum ( ₀=h/2e=2.064×10^–15 V sec). Basic properties of the soliton and its relation to observable experimental quantities (zero field steps, microwave radiation, etc.) are reviewed. Recent direct measurements of the actual soliton profile are also mentioned.     

高Tc超导薄膜约瑟夫逊结光探测研究

我们研制了具有约瑟夫逊效应的高Tc GdBa₂Cu₃O₇-薄膜双晶晶界结,对其交直流约瑟夫逊效应进行了观测,并用其进行光探测,用波长为0.6328μm的He-Ne激光器辐照双晶结结区,系统观测了双晶晶界结的光响应特性,得到的最好结果如下噪音等效功率NEP=1.9×10^-13W,归一化探测率D=53×109cmHz^1/2W^-1,响应率Rv=4.2×10⁷V/W,响应时间τ=4.35×10^-7s.     

用于电压基准的Nb/NbxSi1-x/Nb约瑟夫森单结的研制

现代可编程约瑟夫森电压基准的核心器件是约瑟夫森结阵.目前最具有优势的约瑟夫森结阵是 Nb/Nb_xSi_1-x/Nb材料的结阵. Nb/Nb_xSi_1-x/Nb材料的约瑟夫森结 具有三层薄膜的制作过程简便, Nb和Nb_xSi_1-x刻蚀工艺相同以及Nb_xSi_1-x 势垒层成分可调等优点.中国计量科学研究院设计制作了Nb/Nb_xSi_1-x/Nb约瑟夫森单结. 通过在4.2 K低温下对所做单结进行直流电流-电压特性测量,观测到了清晰的超导隧穿电流和 从零电压态向电压态的跳变,最后就测量结果进行了分析讨论.此项工作属于国内首个开展 Nb/Nb_xSi_1-x/Nb材料约瑟夫森单结研究的工作.     

Bethe-Salpeter equation with the sine-Gordon interaction

An attempt is made to study the interaction Hamiltonian,H _int =Gψ ²(x)U(φ(x)) in the Bethe-Salpeter framework for the confined states of theψ particles interactingvia the exchange of theU field, whereU(φ) = cos (gφ). An approximate solution of the eigenvalue problem is obtained in the instantaneous approximation by projecting the Wick-rotated Bethe-Salpeter equation onto the surface of a four-dimensional sphere and employing Hecke’s theorem in the weak-binding limit. We find that the spectrum of energies for the confined states,E =2m+B (B is the binding energy), is characterized byE ∼n ⁶, wheren is the principal quantum number.     

Effect of transparency on Josephson junction between s+g-wave superconductors

In this paper, a dc Josephson junction between borocarbide superconductors has been studied theoretically. The s+g-wave pairing symmetry which is observed in rare earth complex of borocarbides has a huge anisotropy and is an interesting form of unconventional superconductivity. We calculate the Josephson current in a superconductor--insulator--superconductor (SIS) Josephson junction with s+g-wave superconducting pairing symmetry. In our planar junction c-axis is parallel to an interface with finite transparency but ab-planes of two tetragonal superconductors are misorientated by angle α. We obtain that the Josephson current is strongly dependent on mis-orientation between the left and the right ab-planes. An insulator sandwiched between two superconductors which acts as a potential barrier is demonstrated by a transparency coefficient. The effects of the potential barrier and the mis-orientation on the current are studied analytically and numerically. Occurrence of 0--π transition in this s+g-wave junction is investigated in this paper. A comparison between d-wave Josephson junction and s+g-wave one is also made in the present paper.     

SrTiO3基片上Tl-2212双晶约瑟夫森结的动态特性及噪声影响研究

在SrTiO₃（STO）基片上制作了Tl-2212双晶约瑟夫森结,并对其进行了微波辐照下的I-V特性测试,观察到了夏皮罗台阶,符合约瑟夫森电压-频率关系.利用数值仿真对约瑟夫森结建立了RCSJ模型,仿真结果与实验数据符合较好,利用此模型深入研究了噪声对结动态特性的影响,解释了噪声影响下结的微波感应台阶幅度减小和极小值展宽现象,提出了有效噪声温度为工作温度和外部噪声的等效温度之和. 关键词： RCSJ模型 噪声 Tl-2212双晶约瑟夫森结     

The role of the surface depression of the pair potential on the Josephson and quasi-particle tunnelling in high-T c superconductors

Summary We discuss the influence of the intrinsic surface depression of the order parameter on the temperature dependence of the Josephson critical currentI _c(T) and, tentatively, on the quasi-particle tunnelling conductance in a superconductor with a very short coherence length. The comparison with the experiments shows that the presence of a surface-depressed pair potential can explain the large deviations of theI _c(T) from the ideal BCS behaviour that we recently observed in Bi₂Sr₂CaCu₂O_8+x Josephson break junctions and could mimic the presence of a broadening in the Superconductor-Insulator-Normal tunnelling conductance of the same high-T _c superconductor. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Synchronization of high-frequency vibrations of slipping phase centers in a tin whisker under microwave radiation

Current-voltage characteristics of a system with a variable number of slipping phase centers resulting from phase separation in a tin whisker under external microwave field with a frequency Ω/2π≅35–45 GHz have been studied experimentally. Emergence and disappearance of steps with zero slope in a whisker’s current-voltage characteristic at U _m/n =(m/n)U _Ω, where m and n are integers and U _Ω is determined by Josephson’s formula _ℏΩ=2eU _Ω, have been investigated. Microwave field generated by slipping phase centers is nonharmonic, and the system of slipping phase centers permits synchronization of internal oscillations at a microwave frequency by an external field with a frequency which is the n-th harmonic of internal oscillations. The estimated microwave power generated by a whisker is 10⁻⁸ W. Stimulation of superconductivity in a current-carrying whisker has been detected. Zh. éksp. Teor. Fiz. 113, 1364–1375 (April 1998)     

New model for systems of mesoscopic Josephson junctions

Quantum fluctuations of the phases of the order parameter in two-dimensional arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores containing superfluid helium, or Josephson junctions in which the average number of particles n ₀ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate operators is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for n ₀<5 and U<J (U is the characteristic interaction energy of the particles per granule and J is the Josephson coupling constant). Re-entrant superconductivity phenomena are discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 649–654 (25 November 1997)     

The gravity-induced phase shift detected by high-Tc Josephson junctions

We derive from the Kaluza-Klein theory a formula for the gravity-induced phase shift around a circuit loop, which amounts to the order of 10⁻⁶ We propose experiments to detect this phase shift by using the high-T _c d-wave Josephson junction, which is included in a cuprate superconductor circuit loop. By rotating the loop around the horizontal axis, the gravity-induced phase shift can be detected as a frequency shift. These settings can also be used in turn to determine the gravitational constant. This method will be sensitive and accurate.      

Self-interacting one-dimensional oscillators

Energy eigenvalues and 〈x ²〉 _n for the oscillators having potential energyV(x)=(ω ² x ²/2)+λ<x ^2r >x ^2s have been determined for various values ofλ, r, s andn using renormalized hypervirial-Padé scheme. In general, the results show an improvement over the findings of earlier workers. Variation of the evaluated quantities and of the renormalization parameter withλ, r, s andn has been discussed. In addition, this potential has been employed as an illustrative example of the applicability of alternative formalism of perturbation theory developed by Kim and Sukhatme (J. Phys. A25 647 (1992)).     

Critical current calculations for long 0-π Josephson junctions

A zigzag boundary between a $d_{x^2 - y^2}$ and an s-wave superconductor is believed to behave like a long Josephson junction with alternating sections of 0 and π symmetry. We calculate the field-dependent critical current of such a junction, using a simple model. The calculation involves discretizing the partial differential equation for the phase difference across a long 0-π junction. In this form, the equations describe a hybrid ladder of inductively coupled small 0 and π resistively and capacitively shunted Josephson junctions (RCSJ's). The calculated critical critical current density J_c(H_a) is maximum at non-zero applied magnetic field H_a, and depends strongly on the ratio of Josephson penetration depth λ_J to facet length L_f. If λ_J/L_f ≫1 and the number of facets is large, there is a broad range of H_a where J_c(H_a) is less than 2% of the maximum critical current density of a long 0 junction. All of these features are in qualitative agreement with recent experiments. In the limit λ_J/L_f →∞, our model reduces to a previously-obtained analytical superposition result for J_c(H_a). In the same limit, we also obtain an analytical expression for the effective field-dependent quality factor Q_J(H_a), finding that . We suggest that measuring the field-dependence of Q_J(H_a) would provide further evidence that this RCSJ model applies to a long 0-π junction between a d-wave and an s-wave superconductor.     

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

Summary An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO₃ bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT _c∼89 K and a high critical current densityJ _c∼3·10⁶ A/cm² atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ _c(T) shows a linear behaviour at small reduced temperatures and a depressedJ _c value close toT _c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Anisotropy of amorphous nanogranular composites CoNbTa-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> and CoFeB-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

A comparative investigation of the magnetic properties of amorphous nanogranular composites (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} and (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} has been performed in the subpercolation region at temperatures in the range 4.2–300 K. The thermomagnetic dependences in the range 4.2–300 K and the processes of magnetization reversal and remanent magnetization relaxation at liquid-helium temperatures have been studied. It has been established that the average anisotropy constants of amorphous nanograins are equal to 3.6–7.0 kJ/m³ for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites and 5–8 kJ/m³ for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites. The fundamental differences in the concentration dependences of the anisotropy constant K _eff and the coercive force H _C have been revealed for the two systems under investigation. It has been demonstrated that, as the concentration of the metal phase increases, the quantities K _eff and H _C increase for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites and decrease for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites.     

Spontaneous photorefractive effect in Sr1−x CaxTiO3 (x=0.014)

The temperature dependence of the principal values of the refractive index in Sr_1−x Ca_xTiO₃ (x=0.014) has been measured in the 17–275 K range under various conditions of sample illumination with 1.96 eV photons. The spontaneous photorefractive contribution δn ^ph to the temperature-induced variation of the refractive index of Sr_1−x Ca_xTiO₃, which appears after illumination of the sample in the ferrophase (transition temperature T _c=32 K) and persists in the paraphase under heating up to 150 K, has been separated. The photoinduced polarization has been estimated. Fiz. Tverd. Tela (St. Petersburg) 39, 711–713 (April 1997)     

Carrier mobility in nonstoichiometric n-Bi2Te3−x Sex solid solutions

A study has been made of the thermoelectric and galvanomagnetic properties of n-Bi₂Te_3−x Se_x solid solutions (x=0.3 and 0.36) in the temperature range 80–300 K. The lowest carrier concentrations, (0.8–1)×10¹⁸ cm⁻³, were obtained by displacing the solid solution from the stoichiometric to a Te-rich composition. At such carrier concentrations, the second subband in the conduction band of n-Bi₂Te_3−x Se_x is not filled, which results in a growth of mobility because of the absence of interband scattering, and brings about an increase of thermoelectric efficiency in the 80–120-K range. Fiz. Tverd. Tela (St. Petersburg) 39, 483–487 (March 1997)     

Performance of 2-hole YBCO rf SQUID in flux-locked-loop mode at 77 K

A 2-hole rf SQUID has been fabricated using naturally present grain boundary Josephson weaklinks in a microbridge of bulk Y-Ba-Cu-O superconductor. Periodic oscillations in voltage-flux characteristics have been observed up to 80.5 K. The spectral density of flux noise of the SQUID is 8 × 10⁻⁴ Φ₀/√Hz at 100 Hz and 77 K for open-loop mode. The SQUID has been successfully operated in flux-locked-loop mode at 77 K, demonstrating the feasiblity of the device for practical applications. In the flux-locked-loop mode the stability as well as flux noise of the SQUID has been found to improve considerably as compared to that in the open-loop mode. Various SQUID parameters have also been estimated and reported here.