Mixing of topological states with superconductivity could result in topological superconductivity with the elusive Majorana fermions potentially applicable in fault-tolerant quantum computing. One possible candidate considered for realization of topological superconductivity is thin bismuth films on Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212). Here, we present angle-resolved and core-level photoemission spectroscopy studies of thin Bi films grown {\it in-situ} on as-grown Bi2212 that show the absence of proximity effect. We find that the electron transfer from the film to the substrate and the resulting severe underdoping of Bi2212 at the interface is a likely origin for the absence of proximity effect. We also propose a possible way of preventing a total loss of proximity effect in this system. Our results offer a better and more universal understanding of the film/cuprate interface and resolve many issues related to the proximity effect.
We observe proximity-induced superconductivity in the \textit{in situ} prepared heterostructures constructed by topological insulator Bi$_{2}$Te$_{3}$ thin films and high-temperature cuprate superconductors Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$. The superconducting gap maximum is about 7.6 meV on the surface of Bi$_{2}$Te$_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show a clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the pr…
We show how in principle a coherent coupling between two superconductors of opposite parity can be realized in a three-layer oxide heterostructure. Due to strong intraionic spin-orbit coupling in the middle layer, singlet Cooper pairs are converted into triplet ones and vice versa. This results in a large enhancement of the triplet superconductivity, persisting well above the native triplet critical temperature.
A nodeless superconducting (SC) gap was reported in a recent scanning tunneling spectroscopy experiment of a copper-oxide monolayer grown on the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) substrate [Y. Zhong {\it et al.}, Sci. Bull. {\bf 61}, 1239 (2016)], which is in stark contrast to the nodal d-wave pairing gap in the bulk cuprates. Motivated by this experiment, we first show with first-principles calculations that the tetragonal CuO (T-CuO) monolayer on the Bi2212 substrate is more stable than the commonly postulated CuO$_{2}$ structure. The T-CuO monolayer is composed of two CuO$_2$ layers sh…
We investigate the proximity effect in a heterostructure of the topological insulator (TI) \BiSe\ deposited on the HTSC material BSCCO. The latter is described by the one-band Hubbard model and is treated with cluster dynamical mean field theory (CDMFT), the TI layers being included via the CDMFT self-consistency loop. The penetration of superconductivity into the TI depends on the position of the Fermi level with respect to the TI gap. We illustrate the back action of the TI layer on the HTSC layer, in particular the gradual disappearance of Mott physics with increasing tunneling amplitude.
We theoretically study the proximity effect and the inverse proximity effect in a topological insulator/iron-based superconductor heterostructure based on the microscopic model. The superconducting order parameter is self-consistently calculated. Its magnitude decreases when the coupling of these two systems increases. The induced pairing order parameter exhibits negative and positive values, while the negative pairing near the $Γ=(0,0)$ point is dominant. This parameter has twofold symmetry and includes an $s$-wave component and a $d$-wave component. The magnitude of the induced pairing order…
We present a detailed analysis on the hopping between monolayer CuO$_2$ and bulk CuO$_2$ plane in the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ substrate. With a two-band model, we demonstrate that the nodeless gap can only exist when the hole concentration in monolayer CuO$_2$ plane is very large or very small. We argued that the possible phase separation may play important role in the recent experimental observation of nodeless gap.
We have investigated the structural, magnetic and superconducting properties of [Nb(1.5nm)/Fe(x)]$_{10}$ superlattices deposited on a thick Nb(50nm) layer. Our investigation showed that the Nb(50nm) layer grows epitaxially at 800$^\circ$C on Al$_2$O$_3$(1$\bar{1}$02) substrate. Samples grown at this condition posses a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with $x<$ 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has properties of a weak ferromagnet. Proximity of this we…
We address the controversy over the proximity effect between topological materials and high T$_{c}$ superconductors. Junctions are produced between Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ and materials with different Fermi surfaces (Bi$_{2}$Te$_{3}$ \& graphite). Both cases reveal tunneling spectra consistent with Andreev reflection. This is confirmed by magnetic field that shifts features via the Doppler effect. This is modeled with a single parameter that accounts for tunneling into a screening supercurrent. Thus the tunneling involves Cooper pairs crossing the heterostructure, showing the Fermi…
We analyse the possibility of the appearance of spontaneous currents in proximated superconducting/normal metal (S/N) heterostructure when Cooper pairs penetrate into the normal metal from the superconductor. In particular, we calculate the free energy of the S/N structure. We show that whereas the free energy of the N film $F_{N}$ in the presence of the proximity effect increases compared to the normal state, the total free energy, which includes the boundary term $F_{B}$, decreases. The condensate current decreases $F_{N}$, but increases the total free energy making the current-carrying stat…
The effect of proximity of the magnetism of the Pr-based manganite (Pr0.6Sr0.4MnO3) on the superconductivity of Bi-based high-temperature superconductor (Bi1.75Pb0.25Sr2Ca2Cu3O10+d) was studied based on the results obtained from the magnetotransport and magnetization measurements. Decrease in the values of the upper critical field (HC2(0)) and an increase in the width of the superconducting transition (Delta TC) of Bi1.75Pb0.25Sr2Ca2Cu3O10+d were observed in proximity with the manganite. The combined effect of magnetic exchange interaction arising from the manganite, the leakage of Cooper-pair…
Proximity effect is used to generate effective topological superconductor by placing strong spin orbit interacting metals with superconducting materials, aiming to produce Majorana zero modes useful for topologically protected quantum computations. In recent experiments high $T_c$ material $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+δ}$ is put in contact with a few quintuple layer thick $\text{Bi}_2\text{Se}_3$, and conflicting experimental results are reported. We use standard mean field approach to study this heterostructure, and find it is unlikely to have large proximity induced superc…
To understand the recently observed high temperature superconductivity in the monolayer CuO$_2$ grown on the Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ substrates, we propose a two band model of the hybridized oxygen $p_x$ and $p_y$ orbitals with the proximity effect of the substrate. We demonstrate that both the nodal and nodeless d-wave superconducting states can be induced by the proximity effect, depending on the strengths of the pairing parameters.
Prediction of new superconducting bilayers heterostructures using quantum confinement and proximity effects
Politecnico di Torino（都灵理工大学）；National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)（国立核研究大学莫斯科工程物理学院）；University of Milan（米兰大学）；University of Göttingen（哥廷根大学）
Heterostructures made of itinerant ferromagnets and superconductors are studied. In contrast to most previous models, ferromagnetism is not enforced as an external Zeeman field but induced in a correlated single-band model (CSBM) that displays itinerant ferromagnetism as a mean-field ground state. This allows us to investigate the influence of an adjacent superconducting layer on the properties of the ferromagnet in a self-consistent Bogoliubov-de Gennes approach. The CSBM displays a variety features not present in the Zeeman exchange model that influence the behavior of order parameters close…
Triplet proximity effect in superconducting heterostructures with a half-metallic layer
本文提出了描述半金属层异质结构中超导邻近效应的Usadel理论。结果表明，半金属内部的全自旋极化引起了超导体（S）-铁磁体（F）-半金属（HM）三层中的巨三重态自旋阀效应，以及S/F/HM/F/S系统中的$φ_0$结的形成.此外，我们考虑了原子厚度的S/F/HM三层膜的精确可解模型，并证明了它再现了Usadel方法中发现的自旋阀效应的主要特征。我们的结果与最近关于${\rm MoGe/Cu/Ni/CrO_2}$杂化材料中自旋阀效应的实验数据定性一致[A. Singh等人，Rev. X 5，021019（2015）]。
Recently, it was experimentally realized that 2D superconducting transition metal dichalcogenides (TMD) such as gated MoS$_2$ and monolayer NbSe$_2$ have in-plane upper critical magnetic fields much higher than the Pauli limit. This is due to the so-called Ising spin-orbit coupling (SOC) of TMD which pins the electron spins along the out-of-plane directions and protects the Cooper pairs from in-plane magnetic fields. However, many TMD materials with extremely large Ising SOC, in the order of a few hundred meV, are not superconducting. In this work, we show that TMD materials can induce strong …
Motivated by the recent findings of unconventional superconductivity in $\mathrm{CoSi_2 / TiSi_2}$ heterostructures, we study the effect of interface induced Rashba spin orbit coupling on the conductance of a three terminal "T" shape superconducting device. We calculate the differential conductance for this device within the quasi-classical formalism that includes the mixing of triplet-singlet pairing due to the Rashba spin orbit coupling. We discuss our result in the light of the conductance spectra reported by Chiu {\it et al.} for $\mathrm{CoSi_2 / TiSi_2}$ heterostructures.
We present the generalized quasiclassical theory of the long-range superconducting proximity effect in heterostructures with strong ferromagnets, where the exchange splitting is of the order of Fermi energy. In the ferromagnet the propagation of spin-triplet Cooper pairs residing on the spin-split Fermi surfaces is shown to be governed by the spin-dependent Abelian gauge field which results either from the spin-orbital coupling or from the magnetic texture. The additional gauge field enters into the quasiclassical equations in superposition with the usual electromagnetic vector potential and r…
High-temperature superconductors (HTS) are important for potential applications and for understanding the origin of strong correlations. Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (BSCCO), a van der Waals material, offers a platform to probe the physics down to a unit-cell. Guiding the flow of electrons by patterning 2DEGS and oxide heterostructures has brought new functionality and access to new science. Similarly, modifying superconductivity in HTS locally, on a small length scale, will be of immense interest for superconducting electronics. Here we report transport studies on few unit-cell thick BSCCO a…
Interfacial roughness and proximity effects in superconductor/ferromagnet CuNi/Nb heterostructures
本文报道了用磁控溅射法制备的Nb/Cu $_{60}$ Ni $_{40}$混合超导/铁磁体（S/F）双层膜的结构和电子性质。X射线和中子反射测量表明，沉积在Al $_2$ O $_3$（1 $\bar{1}$ 02）衬底上的异质结构的总界面粗糙度和不同界面粗糙度的垂直相关性都低于沉积在Si（111）衬底上的异质结构。互感实验研究了界面粗糙度对超导转变温度$T_C$的影响。这些测量结果表明，沉积在Al $_2$ O $_3$上的异质结构中的$\sim$比沉积在Si上的异质结构高$T_C$ 4%。我们将这种效应归因于S层中电子的更高平均自由程，这是由界面处的扩散散射的抑制引起的。然而，$T_C$对铁磁层厚度的依赖性在两个系统中没有显著差异，表明库珀对的透明性上的界面粗糙度的微弱影响。
Unconventional superconductors have been long sought for their potential applications in quantum technologies and devices. A key challenge impeding this effort is the difficulty associated with probing and characterizing candidate materials and establishing their order parameter. In this Letter, we present a platform that allows us to spectroscopically probe unconventional superconductivity in thin-layer materials via the proximity effect. We show that inducing an s-wave gap in a sample with an intrinsic d-wave instability leads to the formation of bound-states of quasiparticle pairs, which ma…
Motivated by the recent observations of nodeless superconductivity in the monolayer CuO$_{2}$ grown on the Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ substrates, we study the two-dimensional superconducting (SC) phases described by the two-dimensional $t$-$J$ model in proximity to an antiferromagnetic (AF) insulator. We found that (i) the nodal d-wave SC state can be driven via a continuous transition into a nodeless d-wave pairing state by the proximity induced AF field. (ii) The energetically favorable pairing states in the strong field regime have extended s-wave symmetry and can be nodal or nodel…
We demonstrate the transfer of the superconducting properties of NbTi---a large-gap high-critical-field superconductor---into an InAs heterostructure via a thin intermediate layer of epitaxial Al. Two device geometries, a Josephson junction and a gate-defined quantum point contact, are used to characterize interface transparency and the two-step proximity effect. In the Josephson junction, multiple Andreev reflection reveal near-unity transparency, with an induced gap $Δ^*=0.50~\mathrm{meV}$ and a critical temperature of $7.8~\mathrm{K}$. Tunneling spectroscopy yields a hard induced gap in the…
The AC conduction of epitaxially-grown SmB$_6$ thin films and superconducting heterostructures of SmB$_6$/YB$_6$ are investigated via time domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB$_6$ thin films, demonstrating the presence of surface states in SmB$_6$. While the observed reductions in the simultaneously-measured superconducting gap, transition temperature, and superfluid density of SmB$_6$/YB$_6$ heterostructures relative to bare YB$_6$ indicate the pe…
Proximity and Josephson effects in superconductor - antiferromagnetic Nb / γ-Fe50Mn50 heterostructures
我们研究了超导反铁磁双层膜中的邻近效应，并报道了第一个三层S/AF/S约瑟夫森结的制备和测量。无序的f.c.c.采用γ-Fe50Mn50合金作为AF，S为Nb。测量微米和亚微米尺度的结，并且$J_C (d_AF)$的缩放在AF中给出2.4 nm的相干长度，其与由于双层样品中$T_C$的抑制而导致的相干长度相关。FeMn的扩散常数为1.7 \times 10 $^{-4}$ m $^2$ s $^-1$，并得到了费米能级的态密度。交换偏置FeMn/Co双层证实了在该厚度范围内FeMn的AF性质。
Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/ spectroscopy, we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) and YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) along their $c$-axes on a scale shorter than the interlayer spacing. By tunneling into the (100) plane of a Bi2212 single crystal and a YBCO film, we observe both U-shaped tunneling spectra with extended flat zero-conducta…
Strong variations in superconducting critical temperatures in different families of the cuprate perovskites, even with similar hole doping in their copper-oxygen planes, suggest the importance of lattice modulation effects. The one-dimensional incommensurate lattice modulation (ILM) of Bi_2Sr_2CaCu_2O_{8+y}, with the average atomic positions perturbed beyond the unit cell, offers an ideal test ground for studying the interplay between superconductivity and the long-range incommensurate lattice fluctuations. Here we report Scanning nano X-ray Diffraction (SnXRD) imaging of incommensurate lattic…
Proximity effect induced intriguing superconductivity in van der Waals heterostructure of magnetic topological insulator and conventional superconductor
We introduce a two-dimensional Chern insulator in proximity to a $d$-wave pseudogap state of the high-T$_c$ superconducting material as an effective platform to realize the higher order topological system. The proximity-induced $d$-density-wave (DDW) order in the Chern insulator layer serves as an effective mass. The edge states will be fully gapped by this DDW order. In the real space, the sign of the DDW order parameter changes at the system corners due to the $d$-wave factor, leading to the gapless corner states, indicating that this system may be in a higher order topological state. The hi…
Superconductor-topological insulator (SC-TI) heterostructures were proposed to be a possible platform to realize and control Majorana zero-modes. Despite experimental signatures indicating their existence, univocal interpretation of the observed features demands theories including realistic electronic structures. To achieve this, we solve the Kohn-Sham-Dirac-Bogoliubov-de Gennes equations for ultrathin Bi$_2$Se$_3$ films on superconductor PdTe, within the fully relativistic Korringa-Kohn-Rostoker method, and investigate quasiparticle spectra as a function of chemical potential and film thickne…
We study the superconducting properties of the thin film BCS superconductor proximity coupled to a magnetically doped topological insulator(TI). Using the mean field theory, we show that Fulde-Ferrell(FF) pairing can be induced in the conventional superconductor by having inverse proximity effect(IPE). This occurs when the IPE of the TI to the superconductor is large enough that the normal band of the superconductor possesses a proximity induced spin-orbit coupling and magnetization. We find that the energetics of the different pairings are dependent on the coupling strength between the TI and…
We theoretically consider superconducting proximity effect, using the Bogoliubov-de Gennes theory, in heterostructure sandwich-type geometries involving a normal s-wave superconductor and a non-superconducting material with the proximity effect being driven by Cooper pairs tunneling from the superconducting slab to the non-superconducting slab. Applications of the superconducting proximity effect may rely on an induced spectral gap or induced pairing correlations without any spectral gap. We clarify that in a non-superconducting material the induced spectral gap and pairing correlations are in…
The introduction of superconductivity to the Dirac surface states of a topological insulator leads to a topological superconductor, which may support topological quantum computing through Majorana zero modes. The development of a scalable material platform is key to the realization of topological quantum computing. Here we report on the growth and properties of high-quality (Bi,Sb)2Te3/graphene/gallium heterostructures. Our synthetic approach enables atomically sharp layers at both hetero-interfaces, which in turn promotes proximity-induced superconductivity that originates in the gallium film…
This review discusses the electronic properties and the prospective research directions of superconductor-graphene heterostructures. The basic electronic properties of graphene are introduced to highlight the unique possibility of combining two seemingly unrelated physics, superconductivity and relativity. We then focus on graphene-based Josephson junctions, one of the most versatile superconducting quantum devices. The various theoretical methods that have been developed to describe graphene Josephson junctions are examined, together with their advantages and limitations, followed by a discus…
