Coexistence of ferromagnetism and superconductivity in normal mental/superconductor/ferromagnet structures

We extend the Blonder, Tinkham and Klapwijk theory to the study of the inverse proximity effects in the normal mental/superconductor/ferromagnet structures. In the superconducting film, there are the gapless superconductivity and the spin-dependent density of states both within and without the energy gap. It indicates an appearance of the inverse-proximity-effect-induced ferromagnetism and a coexistence of ferromagnetism and superconductivity near the interface. The influence of exchange energy in the ferromagnet and barrier strength at the superconductor/ferromagnet interface on the inverse proximity effects is discussed.

A Novel Large Moment Antiferromagnetic Order in K_(0.8)Fe_(1.6)Se_(2) Superconductor

The discovery of cuprate high TC superconductors has inspired the search for unconventional superconductors in magnetic materials.A successful recipe has been to suppress long−range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point.We report an exception to this rule in the recently discovered potassium iron selenide.The superconducting composition is identified as the iron vacancy ordered K_(0.83)(2)Fe_(1.64)(1)Se_(2) with T_(C) above 30 K.A novel large moment 3.31μB/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature TN≈559 K for a bulk superconductor.Staggeringly polarized electronic density of states is thus suspected,which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.

Spin-Polarized Tunneling Spectroscopy and Shot Noise in Ferromagnet／f-WaveSuperconductor Junctions

The tunneling spectroscopy and shot noise in ferromagnet/insulator/triplet-superconductor (FM/I/triplet- SC) structures are studied by taking into account the roughness interracial barrier and exchange splitting in the FM. For the triplet-SG of Sr_2RuO_4,we consider two-dimensional f-wave order parameter symmetries having nodes within the RuO_2 plane,which reasonably describe both thermodynamic and thermal conductivity data.It is shown that the ferromagnetic exchange splitting gives rise to a decrease in the differential conductance,the average current,and the shot noise power,while the noise power-to-current ratio is increased;the interface roughness is found to lead to a decrease in the differential conductance and the average current,and an increase in the noise power-to-current ratio.

Alternating-current losses in two-layer superconducting cables consisting of second-generation superconductors coated by U-shaped ferromagnetic materials

Alternating-current losses in a two-layer superconducting cable, each layer being composed of 15 closely-spaced rectangular wires made up of second-generation superconductors when the ends of wires are coated by either a non-magnetic or strong ferromagnetic material having a U profile is numerically investigated. Computations are carried out through the finite-element method. The alternating-current losses do not increase significantly if the relative permeability of the coating is increased three orders of magnitude, provided that the current amplitude is less than half of the critical current in a superconducting wire. However, the losses are much higher for ferromagnetic coating if the amplitude of the applied current oscillating at 50 Hz is close to the critical current. The ferromagnetic coating is seen to accumulate the magnetic field lines normally on its surfaces, while the field lines are parallel to the long axes of the wires, leading to more significant flux penetration in the coated regions. This facilitates a uniform low-loss current flow in the uncoated regions of the wires. In contrast, coating with a non-magnetic material gives rise to a considerably smaller current flow in the uncoated regions, whereas the low-loss flow is maintained in the coated regions. Moreover, the current flows in opposite directions in the coated and uncoated regions, where the direction in each region is converse for the two materials.

Strong coupling between localized 5f moments and itinerant quasiparticles in the ferromagnetic superconductor UGe2

The heavy fermion physics arises from the complex interplay of nearly localized 4f/5f electrons and itinerant band- like ones, yielding heavy quasiparticles with an effective mass about 100 times （or more） of the bare electrons, Recently, experimental and theoretical investigations point out a localized and delocalized dual nature in actinide compounds, where itinerant quasiparticles account for the unconventional superconductivity in the vicinity of a magnetic instability. Here we report the strong coupling between localized 5f moments and itinerant quasiparticles in the ferromagnetic superconductor UGe2. The coupling is nearly antiferromagnetic. As embedded in the ferromagnetic matrix of localized 5f moments below Tc ~ 52 K, this coupling leads to short-range dynamic correlations of heavy quasiparticles, characterized by fluctuations of magnetic clusters. Those cluster-like spins of itinerant quasiparticles show a broad hump of magnetization at Tx≈28 K, which is typical for the spin-glass freezing. Thus, our results present the direct observation of itinerant quasiparticles coexisting with localized 5f moments by conventional magnetic measurements, providing a new route into the coexistence between ferromagnetism and superconductivity in heavy fermion systems.

Andreev Tunneling Through a Ferromagnet／Quantum－Dot／Superconductor System

We study Andreev tunneling through a ferromagnet/quantum-dot (QD)/superconductor system. By usingnonequilibrum Green function method, the averaged occupation of electrons in QD and the Andreev tunneling currentare studied. Comparing to the norma-metal/quantum-dot/superconductor, the system shows significant changes: (i)The averaged occupations of spin-up and spin-down electrons are not equal. (ii) With the increase of the polarizationof ferromagnetic lead, the Andreev reflection current decreases. (iii) However, even the ferromagnetic lead reaches fullpolarization, the averaged occupation of spin-down electrons is not zero. The physics of these changes is discussed.

Josephson current versus potential strength of the interface in ferromagnetic superconductors

Based on the scattering theory, we calculate the Josephson current in a junction between two ferromagnetic super-conductors as a function of the interface potential z. We consider the ferromagnetic superconductor （FS） in three different Cooper pairing states： spin singlet s-wave pairing （SWP） state, spin triplet opposite spin pairing （OSP） state, and spin triplet equal spin pairing （ESP） state. We find that the critical Josephson current as a function of z shows clear differences among the SWP, OSP, and ESP states. The obtained results can be used as a useful tool for determining the pair symmetry of the ferromagnetic superconductors.

Andreev Tunneling Through a Ferromagnet/Quantum-Dot/Superconductor System

We study Andreev tunneling through a ferromagnet/quantum-dot (QD)/superconductor system. By usingnonequilibrum Green function method, the averaged occupation of electrons in QD and the Andreev tunneling currentare studied. Comparing to the norma-metal/quantum-dot/superconductor, the system shows significant changes: (i)The averaged occupations of spin-up and spin-down electrons are not equal. (ii) With the increase of the polarizationof ferromagnetic lead, the Andreev reflection current decreases. (iii) However, even the ferromagnetic lead reaches fullpolarization, the averaged occupation of spin-down electrons is not zero. The physics of these changes is discussed.

Tunneling Conductance in Quantum-Wire/Ferromagnetic-Insulator/d-Wave Superconductor Junction

We have studied the quasiparticle transport in quantum-wire /ferromagnetic-insulator/d wave super- conductor Junction （q/FI/d） in the framework of the Blonder-Tinkham-Klapwijk model. We calculate the tunneling conductance in q/FI/d as a function of the bias voltage at zero temperature and finite temperature based on Bogoliubov- de Gennes equations. Different from the case in normal-metal/insulator/d wave superconductor Junctions, the zero-bias conductance peaks vanish for the single-mode case. The tunneling conductance spectra depend on the magnitude of the exchange interaction at the ferromagnetic-insulator.

Josephson Current in Superconductor-Ferromagnet／Insulator／d-Wave Superconductor Junctions

Solving the Bogoliubov-de Gennes equation, the energy levels of bound statesare obtained in the ferromagnetic superconductor. The Josephson currents in a ferromagneticsuperconductor/Insulator/d-wave superconductor junction are calculated as a function of the exchangefield, temperature, and insulating barrier strength. It is found that the Josephson criticalcurrent is always suppressed by the presence of exchange Geld h and depends on crystalline axisorientation of d-wave superconductor.

dc Josephson Effect in s-Wave Superconductor/Ferromagnet Insulator/p-Wave Superconductor Junctions

The Josephson currents in s-wave superconductor/ferromagnet insulator/p-wave superconductor（s/FI/p） junctions are calculated as a function of temperature and the phase taking into account the roughness scattering effect at interface. The phase dependence of the Josephson current I （φ） between s-wave and px-wave superconductor is predicted to be sin（2φ）. The ferromagnet scattering effect, the barrier strength, and the roughness strength at interface suppress the dc currents in s/FI/p junction.

Phase-field simulation of superconductor vortex clustering in the vicinity of ferromagnetic domain bifurcations

Superconductors and ferromagnets are highly non-compatible materials due to the natures of their respective electronic states.But when artificially brought together,they develop interesting characteristics,one of which,vortex clustering,is discussed here in this paper.Phase-field and micromagnetic simulations are performed to investigate the superconductor and ferromagnet bilayer,respectively.The ferromagnet with uniaxial anisotropy is observed to develop the maze domain,whereas the superconductor subjected to the influence of the ferromagnetic stray field displays a vortex pattern.Clustered vortices in superconductors at certain locations are observed to be precisely located over magnetic domain bifurcations.The enhanced out-of-plane stray field at bifurcations around the curved domain walls and the convergent Lorentz force due to screening currents in superconductor are attributed to the formation of clusters at bifurcation sites.Segregation of the inter-vortex spacing between straight and bifurcated domain is clearly observed.More importantly,inter-vortex spacing is predicted to serve as a precise tool to map local ferromagnet domain shapes.

Shot Noise in Ferromagnetic Superconductor Tunnel Junctions

In this paper, the superconducting order parameter and the energy spectrum of the Bogoliubov excitations are obtained from the Bogoliubov-de Gennes （BdG） equation for a ferromagnetic superconductor （FS）. Taking into account the rough interface scattering effect, we calculate the shot noise and the differential conductance of the normalmetal insulator ferromagnetic superconductor junction. It is shown that the exchange energy Eh in FS can lead to splitting of the differential shot noise peaks and the conductance peaks. The energy difference between the two splitting peaks is equal to 2Eh. The rough interface scattering strength results in descent of conductance peaks and the shot noise-to-current ratio but increases the shot noise.

Nonequilibrium Effect in Ferromagnet-Insulator-Superconductor Tunneling Junction Currents

Nonequilibrium effect due to the imbalance in the number of the ? and ? spin electrons has been studied for the tunneling currents in the ferromagnet-insulator-superconductor (FIS) tunneling junctions within a phenomenological manner. It has been stated how the nonequilibrium effect should be observed in the spin-polarized quasiparticle tunneling currents, and pointed out that the detectable nonequilibrium effect could be found in the FIS tunneling junction at 77 K using HgBa2Ca2Cu3O8+? (Hg-1223) high-Tc superconductor rather than Bi2Sr2CaCu2O8+? (Bi-2212) one.

Zeeman effects on Josephson current in d-wave superconductor/d-wave superconductor junctions

This paper solves a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, and studies the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor（S）/insulating layer（I）/d-wave S junctions are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction to a great extent depend on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field under certain conditions can enhance the Josephson critical current in a d-wave S/I/d-wave S junction.

高压增氧T′相R_2CuO_(4+δ)(R=Nd～Tm)低温磁性研究

T′相R2CuO4稀土铜氧化合物由于尺度效应而产生弱铁磁性行为已经被人们关注,报导了通过高温高氧压(6GPa,1000℃)合成稀土T′相R2CuO4(R=Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er和Tm)化合物的结构和磁学性能。磁化率曲线显示,在低温下所有的高压增氧R2CuO4样品都出现新的低温弱铁磁性反常行为,转变温度在28K附近。新的低温弱铁磁性行为是由于CuO2面上微量氧空穴的掺入,使处于反铁磁有序CuO2面形成局域化的铁磁性团簇造成。实验证明新发现的低温弱铁磁性行为与尺度效应产生弱铁磁性行为属于完全不同的物理机制。结果还预示T′相R2CuO4稀土铜氧化合物很难通过空穴掺杂而实现超导。

正常金属调制层对超导隧道结微分电导的影响

在考虑绝缘层厚度的基础上,运用Bogoliubov-de Gennes(BdG)方程和Blonder-Tinkham-Klapwijk(BTK)理论模型,计算了含有正常金属调制层的铁磁金属/绝缘层/正常金属/s波超导隧道结(FINS结)中的准粒子输运系数和微分电导。计算结果表明,微分电导随正常金属调制层厚度的变化呈周期性振荡,振荡周期与外加偏压有关;当金属调制层厚度较薄时,邻近效应将导致在金属层中有铁磁性和超导性共存的可能性。

超导体的超导电性和反铁磁性共存的研究

在二维空穴掺杂t-t′-J-U模型和重整化平均场理论的框架下,用Gutzwiller方法研究了Gossamer超导体的超导电性和反铁磁性的共存,探讨了电子次近邻跃迁对共存的影响,发现在掺杂浓度δ≤0.1的欠掺杂区反铁磁序和超导序共存。随着电子次近邻跃迁的增大,反铁磁序得到增强,超导序参数在欠掺杂区域受到抑制,在过掺杂区明显地得到增强,导致反铁磁序和超导序共存区域变大。超导序和反铁磁序共存的状态比纯粹的超导态能量低,共存状态更稳定。

两类镜象电流对的磁场

从磁场矢势出发,研究了两类镜象电流对磁场的特点,得出了这两类镜象电流对在其镜平面上激发的磁场的矢势和磁感应强度与镜平面的关系,以及其量值是一电流元激发磁场相应量的两倍的结论。进而讨论了上述结论在磁场镜象法和对称电流系统(如直螺线管和螺绕管)磁场分析中的应用。

磁交换作用对铁磁/d_(x^2-y^2)+id_(xy)混合波超导结隧道谱的影响

考虑到铁磁层中的磁交换作用和界面势垒的散射效应,利用推广的Blonder-Tinkham-Klapwijk散射理论,计算了铁磁/dx2-y2+idxy混合波超导结中的准粒子输运系数和自旋极化的隧道谱.研究表明:(1)铁磁/dx2-y2+idxy混合波超导结中自旋极化的隧道谱能较好地解释La2/3Ba1/3MnO3/DyBa2Cu3O7隧道结中观察到的零偏压电导凹陷的实验结果;(2)在Δ2/Δ1=1的情况下,磁交换作用和界面势垒散射可压低峰值,且导致隧道谱逐渐演化出四个电导峰;(3)虚的Andreev反射过程对铁磁/dx2-y2+idxy混合波超导结的隧道谱有显著影响.