铁磁超导态/绝缘层/自旋三重态p波超导体结的直流Josephson电流

由Bogoliubov-de Gennes方程得到铁磁超导共存态(FS)的自洽方程，利用推广的Furusaki-Tsukada的电流公式计算了铁磁超导态/绝缘层/自旋三重态p波超导体(FS/I/p)结的直流Josephson电流随结的温度、相位差以及FS中磁交换能、结界面的势垒散射强度的变化关系.研究表明：FS中磁交换能、结界面的势垒散射均抑制FS/I/p结的直流Josephson电流.当自旋三重态超导体具有p_x波配对势时，自旋三重态超导体结的直流Josephson电流随结两侧相位差的振荡周期是π. 关键词： 铁磁超导态 自旋三重态超导体 p波超导体 直流Josephson电流     

铁磁-超导隧道结中粗糙界面散射对直流Josephson电流的影响

通过求解Bogoliubov—de Gennes(BdG)方程得到铁磁超导共存态(FS)的自洽方程，在考虑结界面的粗糙情形下，由推广的Furusaki—Tsukada(FT)的电流公式计算了铁磁超导态／绝缘层／s波超导体(FS／I／S)结的直流Josephson电流，讨论了FS/I／S结的直流Josephson临界电流随磁交换能、温度的变化情况．研究表明：铁磁超导态中磁交换能和结界面的粗糙势垒散射均对FS／I／S结的直流Josephson电流有抑制作用．     

s/FI/d超导结的直流Josephson电流

在 s波超导体 /铁磁绝缘层 / d波超导体 Josephson结 (s/ FI/ d)中 ,考虑结界面铁磁绝缘层的磁散射和粗糙散射情况下 ,运用 Bd G方程和 FT的电流公式计算准粒子的输运系数及 s/ FI/ d结的直流 Josephson电流与温度 T、结两侧的相位差之间的关系。研究表明 :结界面的磁散射和粗造散射均抑制结中准粒子的 Andreev反射 ,降低了流过 s/ FI/ d结的直流 Josephson电流 ,直流Josephson电流 I随温度 T、相位差φ的变化曲线强烈地依赖于 d波超导体的晶轴方位     

反铁磁自旋波对高温超导体的电阻特性的影响

针对含铜氧化物超导体，本引用一个传导电子与局域电子间的自旋耦合来描述Ｃｕ－Ｏ平面上巡游空穴与铜离子上的局域空穴之间的相互作用。采用自旋波近似，使该自旋耦合表现为巡游载流子与自旋波之间的相互作用，其形式与电－声相互作用类似，据此可得到因自旋波对载流子的散射引起的电阻与温度的依赖关系，其性质与电－声散射电阻相同。同时还引入具有自旋波子吸收或发射的层间遂穿散射过程，讨论了垂直方向上的电阻。所得结果与实     

自旋与轨道耦合效应影响下的p波超导体的直流Josephson电流

在Bogoliubov-de Gennes(BdG)方程的基础上,研究了考虑自旋与轨道耦合效应下s波超导体/绝缘层/p波超导体结的直流Josephson效应.研究表明:p波超导体结的直流Josephson电流与温度、结的相位差的关系强烈地依赖于p波超导体的配对势形式,结界面的自旋与轨道耦合散射对s/I/p波的结直流Josephson电流有抑制作用也有增强作用.     

s波超导体绝缘层dx2-y2波超导体结的直流Josephson电流

在s波超导体绝缘层dx2-y2波超导体结(sId)中,考虑到结界面粗糙散射,运用BogoliubovdeGennes(BdG)方程和FurusakiTsukada(FT)电流公式,计算超导结中的准粒子传输系数和直流Josephson电流.结果表明:sId超导结的直流Josephson电流随温度以及结两侧的相位差变化的关系曲线强烈地依赖于d波超导体的晶轴方位;结界面的粗糙散射对Josephson电流有抑制作用 关键词： s/I/d超导结 dx2-y2波超导体 直流Josephson电流     

用红外光激励电流法研究非晶半导体带隙态分布

本文提出了用红外光激励电流法研究非晶半导体带隙态的新方法。当一本征脉冲光照射样品时,大量非平衡载流子陷落到带隙态上;经过延迟时间t_d,用红外光激励残存的非平衡载流子,能引起红外光电导的过冲,过冲量与延迟时间t_d成幂次关系。基于多次陷落模型分析红外光电导过冲与温度及时间t_d的关系,可以得到带隙态分布的细节。 关键词：     

自旋三重态超导体结的直流Josephson电流

在s波超导体/绝缘层/自旋三重态超导体结中,计算了直流Josephson电流随结的温度、相位差以及绝缘层势垒强度的变化关系.当自旋三重态超导体具有px波配对势时,自旋三重态超导体结的直流Josephson电流随结两侧相位差的振荡周期是π,与结的相位差的关系为sin(2).结界面的绝缘层势垒散射均抑制结的直流Josephson电流.     

具有铁磁绝缘层超导体结的直流Josephson电流

在s波超导体/铁磁绝缘层/s波超导体Josephson结（S/FI/S）中，考虑结界面铁磁绝缘层的磁散射和粗糙散射情况下，运用Bogoliubov-de(BdG)方程和Furusaki-Tsukada(FT)的电流公式计算准粒子的输运系数及S/FI/S结的直流Josephson电流与温度T，结两侧的相位差之间的关系，研究表明：结界面的磁散射和粗糙散射均抑制结中准粒子的Andreev反射，降低了流过S/FI/S结的直流Josephson电流。     

多晶样品SmxC60的电子输运性质研究

通过电阻和磁阻的测量，研究了多晶样品Sm_xC₆₀的电子输运性质．实验发现：Sm_xC₆₀的输运机制随ｘ的不同而发生变化．SmC₆₀的电阻率 温度关系可用涨落导致的隧道穿透模型解释，而Sm_2.75C₆₀的输运性质则可由弱局域化和电子-电子相互作用解释．Sm_2.75C₆₀的磁阻数据能用弱局域化和电子-电子相互作用模型很好地拟合，而SmC₆₀的磁阻数据则不能． 关键词：     

非平衡辐射响应与准二维高温超导体系的磁通涡旋激发

从实验上揭示了高温超导体YBa₂Cu₃O_{7-δ(YBCO)颗粒膜具有准二维结构的输运特征以及超导弱连接行为特征.在此基础上,将超导颗粒膜样品等效为准二维Josephson弱连接阵列系统,讨论了电流引起的二维超导体系中磁通“涡旋-反涡旋”束缚对的拆对激发问题,并给出了自由涡旋分布n(T,I)的分析表达式.将颗粒膜样品的非平衡辐射响应测量结果与电流激发自由涡旋分布n(T,I)相比较,两者具有相似的分布特征.该结果表明,高温超导体颗粒膜 关键词：}     

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.     

Scanning tunneling spectroscopy of a superlattice superconductor: Bi2Sr2CaCu2O8

It is clear that many of the cuprate superconductors are truly superlattices, composed of sheets whose individual superconducting critical temperatures may approach bulk values. Such a cuprate is Bi₂Sr₂CaCu₂O₈, often referred to as BSCCO-2212. Scanning tunneling spectroscopy (vacuum tunneling) applied toa-bBiO cleavage planes ofT_c≈90 K BSCCO single crystals under liquid helium simultaneously provides topography and local dI/dVspectra (superconducting density of states: DOS). The spectra, which are similar to those obtained from angle-resolved photoemission spectroscopy, confirm a large gap parameter Δ(x,y) on the uppermost layer. The dI/dVspectra do not unequivocally select order parameter symmetry, but are probably consistent with d-wave or anisotropic s-wave states. Spatial variations of Δ on a 100 Å scale are attributed to variation in BiO metallicity, originating in oxygen stoichiometry variations. A model is presented to explain the different dI/dVspectra which are seen, and associated with different local oxygen concentrations. This model, based on the superconducting proximity effect, assumes that in some regions the BiO uppermost layer is insulating and in other regions it is metallic, in the latter case induced superconductivity by proximity to the CuO₂planes. Our STM measurements appear to sample only the uppermost half cell of the crystal, and contain no obvious superlattice features. Recent measurements have confirmed Josephson radiation from voltage biasedc-axis pillars of BSCCO. From the point of view of the present work, the superconducting systems which weakly couple along thec-direction to create Josephson junctions are probably half-cell slabs of height 15.4 Å, each containing two CuO₂and two BiO layers, which act as single composite electrodes for the Josephson junctions.     

Planar graphene Josephson coupling via van der Waals superconducting contacts

We report on the fabrication and transport characteristics of van der Waals (vdW)-contacted planar Josephson junctions. In a device, two pieces of cleaved 2H-NbSe₂ superconducting flakes and a monolayer graphene sheet serve as the superconducting electrodes and the normal-conducting spacer, respectively. A stack of NbSe₂?graphene?hexagonal-boron-nitride (hBN) heterostructure with clean and flat interfaces was prepared by a dry transfer technique. The outermost hBN layer protected the NbSe₂?graphene?NbSe₂ Josephson junction from chemical contamination during the fabrication processes. The Josephson coupling was confirmed by a periodic modulation of the junction critical current $I_{\text{c}}$ in a perpendicular magnetic field. The temperature dependence of $I_{\text{c}}$ showed long and diffusive Josephson coupling characteristics. The temperature dependence of the superconducting gap, obtained from the multiple Andreev reflection features, followed the Bardeen?Cooper?Schrieffer (BCS) prediction.     

HighT c superconductivity in the itinerant ising model: Spin singlet pairing

We study the spin singlet superconductivity exhibited in an itinerant Ising model Hamiltonian. This Hamiltonian models the Cu–O layers in highT _c oxide superconductors. Electrons are itinerant through nearest neighbor hopping. An Ising term is introduced to describe the antiferromagnetic superexchange interaction between electrons nominally on nearest neighbor Cu sites. We discuss various symmetry states allowed by the model, and give detailed predictions of the superconducting energy gap, specific heat, susceptibility, andT _c variation with carrier concentration. Results are compared to experimental data on highT _c superconductors and reasonable agreement is obtained.     

Theory of novel properties of Josephson effect in anisotropic superconductors

Recent studies on high- T_csuperconductors aroused our new interest on the Josephson effects in anisotropic superconductors. In contrast to the conventional s-wave superconductors, there are two additional intrinsic effects originating from an internal phase of the pair potential. One is the sign change of the Josephson current depending on the direction of the motion of the Cooper pair. The other is the formation of localized states near the insulator. By taking account of these two effects, a general formula for the Josephson current is presented. Calculated results predict several anomalous properties including a strong enhancement of the Josephson current at low temperature. The influences of the spatial dependence of the pair potential on the Josephson current are also clarified.     

Properties of YBa2Cu3O7−δ/YBa2CoxCu3−xOy/YBa2Cu3O7−δ Josephson edge junctions with 0.1 x 0.3 and the effect of flux flow on their normal resistance

YBa₂Cu₃O_7−δ (YBCO) based SNS edge junctions with cobalt doped YBCO barriers were prepared and characterized. At 77 K, good junctions had RSJ-like I–V curves with excess current, magnetic suppression of I_c of about 50% or more, and clear microwave steps. The conductance values 1/R_N at 77 K of junctions with different barrier thickness and composition, were proportional to the junction areas A, but show little correlation with the thickness of the barriers t_B in the range of 15t_B100nm. The corresponding I_cR_N products were observed to scale as J^0.66±0.09_c, similar to what was found by others. At the same time, the measured values of R_N are much smaller than what is expected based on the dimensions of the junction and the resistivity of the barrier material. To explain all of this, we propose a model in which at high supercurrent densities, flux flow of Josephson vortices in the junction leads to R_N values which are lower than expected. This model predicts

Full-size image (1K)

, which fits the observed results very well.     

Josephson current in superconductor/ferromagnet/superconductor junctions

By using the Bogoliubov–de Gennes equation and the Nambu spinor Greens function approach, we have theoretically studied the dc Josephson current and the coupling phase state of superconductor/ferromagnet/superconductor (SC/FM/SC) junctions, where the FM is of weak ferromagnetism. From the behavior of the temperature-dependent dc Josephson current (I_c), we confirm that such SC/FM/SC junction may change from 0-phase to π-phase state with increasing the temperature (T), for particular parameters of the thickness and the strength of ferromagnetism of the FM interlayer. We attribute such changement to an extra phase difference between the two SCs. The results are qualitatively consistent with an experiment [Phys. Rev. Lett. 86 (2001) 2427], which shows a sharp cusp structure on the I_c−T curves of Nb/Cu_0.48Ni_0.52/Nb junction for specific thickness of the Cu_0.48Ni_0.52, indicating the junction changes from 0-phase state at high temperatures to π-phase state at low temperatures.     

THE ELECTRON-BEAM IRRADIATION ON THE HIGH TEMPERATURE SUPERCONDUCTING THIN FILMS: A NUMERICAL SIMULATION OF THE LOW TEMPERATURE SCANNING ELECTRON MICROSCOPY

Low-temperature scanning electron microscopy (LTSEM) is a promising measuring technique for probing the spatial distribution of the superconducting properties of the high-temperature superconducting (HTS) thin films. A theoretical analysis of the electron-beam irradiation on the HTS thin films in the LTSEM has been carried out. An inhomogeneously distributed grain array model has been applied in the analysis, and some numerical sim-ulations have been carried out on the electron-beam induced voltage (EIV) signals in the LTSEM experiments. The comparisons of our numerical results with the LTSEM experi-mental data indicate that it is quite reasonable to use a two-dimensional Josephson junction array for stimulating the inhomogeneous HTS thin film sample. Our numerical results also show that the EIV signals are influenced by the electron-beam power used in the LTSEM, and a reduction of the electron-beam power is suggested in order to eliminate the errors in estimating the local values of critical temperature T_c and critical current I_c by the sample temperature and the bias current at which the first EIV signal occurs.     

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)