Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on Pb/Ni and Al/(Co-Pd) proximity-effect bilayers show unambiguous evidence for the "inverse proximity effect," in which the ferromagnet (F) induces a finite magnetization in the superconducting (S) layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light's optical penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al..
In this letter, we have studied the effects of proximity of a superconductor to a normal metal. The system, represented by a bilayer attractive Hubbard model, is investigated using layer-dynamical mean field theory and iterated perturbation theory for superconductivity as an impurity solver. The bilayer system comprises a superconducting and a normal metallic layer, connected by an inter-planar hopping ($t_\perp$). It is found that superconductivity is induced in the normal layer for small $t_\perp$. With increasing inter-layer hopping, the bilayer system undergoes two transitions: a first ord…
Here we report on a feasibility study aiming to explore the potential of Polarized Neutron Reflectometry (PNR) for detecting the inverse proximity effect in a single superconducting/ferromagnetic bilayer. Experiments, conducted on the V(40nm)/Fe(1nm) S/F bilayer, have shown that experimental spin asymmetry measured at T = 0.5TC is shifted towards higher Q values compared to the curve measured at T = 1.5TC. Such a shift can be described by the appearance in superconducting vanadium of magnetic sub-layer with thickness of 7 nm and magnetization of +0.8 kG.
A resonant enhancement of the neutron standing waves is proposed to use in order to increase the magnetic neutron scattering from a "superconductor/ferromagnet"(S/F) bilayer. The model calculations show that usage of this effect allows to increase the magnetic scattering intensity by factor of hundreds. Aspects related to the growth procedure (order of deposition, roughness of the layers etc) as well as experimental conditions (resolution, polarization of the neutron beam, background etc) are also discussed. Collected experimental data for the S/F heterostructure Cu(32nm)/V(40nm)/Fe(1nm)/MgO c…
Polarized neutron reflectometry is used to study the magnetic proximity effect in a superconductor/ferromagnet (SC/FM) system of composition Cu(32nm)/V(40nm)/Fe(1nm)/MgO. In contrast to previous studies, here a single SC/FM bilayer, is studied and multilayer artefacts are excluded. The necessary signal enhancement is achieved by waveguide resonance, i.e. preparing the V(40nm)/Fe(1nm) SC/FM bilayer sandwiched by the highly reflective MgO substrate and Cu top layer, respectively . A new magnetic state of the system was observed at temperatures below 0.7 TC. manifested in a systematic change in t…
Conventional proximity effect in bilayers of superconducting underdoped $La_{1.88}Sr_{0.12}CuO_4$ islands coated with non superconducting overdoped $La_{1.65}Sr_{0.35}CuO_4$
Scanning tunneling spectroscopy measurements conducted on epitaxially grown bilayers of half-metallic ferromagnetic La2/3Ca1/3MnO3 (LCMO) on superconducting (SC) (100)YBa2Cu3O7-delta (YBCO) reveal long-range penetration of superconducting order into the LCMO. This anomalous proximity effect manifests itself in the tunneling spectra measured on the LCMO layer as gaps and zero bias conductance peaks. Remarkably, these proximity-induced spectral features were observed for bilayers with LCMO thickness of up to 30 nm, an order of magnitude larger than the expected ferromagnetic coherence length in …
The effect of impurities which induce local interlayer tunneling in bilayer exciton condensates is discussed. We show that a localized single fermion bound state emerges inside the gap for any strength of impurity scattering and calculate the dependence of the impurity state energy and wave function on the potential strength. We show that such an impurity induced single fermion state enhances the interlayer coherence around it, and is similar to the superconducting proximity effect. As a direct consequence of these single impurity states, we predict that a finite concentration of such impuriti…
In a search for a simple proximity system of a topological insulator and a superconductor for studying the role of surface versus bulk effects by gating, we report here on a first step toward this goal, namely the choice of such a system and its characterization. We chose to work with thin film bilayers of grainy 5 nm thick NbN films as the superconductor, overlayed with 20 nm thick topological layer of $\rm Bi_2Se_3$ and compare the transport results to those obtained on a 5 nm thick reference NbN film on the same wafer. Bilayers with ex-situ and in-situ prepared $\rm NbN-Bi_2Se_3$ interfaces…
Recent experimental studies have indicated novel superconducting proximity effects in thin-film heterostructures comprising ferromagnetic manganites and superconducting cuprates. To look for such effects microscopically, we performed scanning tunneling spectroscopy on La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ (LCMO/YBCO) bilayer thin films. $\emph{c}$-axis oriented films of varying thickness were grown on SrTiO$_{3}$ substrates using pulsed laser-ablated deposition. Heteroepitaxiality of the films was confirmed by cross-sectional transmission electron microscopy. Tunneling spectra wer…
The existence of coherent magnetic correlations in the normal phase of cuprate high-temperature superconductors has proven difficult to measure directly. Here we report on a study of ferromagnetic-superconductor bilayers of La2/3Ca1/3MnO3/YBa2Cu3O7 (LCMO/YBCO) with varying YBCO layer thicknesses. Using x-ray magnetic circular dichroism, we demonstrate that the ferromagnetic layer induces a Cu magnetic moment in the adjacent high-temperature superconductor. For thin samples, this moment exists at all temperatures below the Curie temperature of the LCMO layer. However, for a YBCO layer thicker t…
Recent scanning tunneling microscopy measurements of the proximity effect in Au/La$_{2-x}$Sr$_{x}$CuO$_{4}$ and La$_{1.55}$Sr$_{0.45}$CuO$_{4}$/La$_{2-x}$Sr$_{x}$CuO$_{4}$ bilayers showed a proximity-induced pseudogap [Yuli et al., Phys. Rev. Lett. {\bf 103}, 197003 (2009)]. We describe the proximity effect in mesoscopic superconductor/normal-metal bilayers by using the Bogoliubov-de Gennes equations for a tight-binding Hamiltonian with competing antiferromagnetic and d-wave superconductivity orders . The temperature dependent local density of states is calculated as a function of the distance…
We propose a novel heterostructure to achieve chiral topological superconductivity in 2D. A substrate with a large Rashba spin-orbit coupling energy is brought in proximity to a twisted bilayer of thin films exfoliated from a high-temperature cuprate superconductor. The combined system is then exposed to an out-of-plane magnetic field. The rare $d + id$ pairing symmetry expected to occur in such a system allows for nontrivial topology; specifically, in contrast to the case of the twisted bilayer in isolation, the substrate induces an odd Chern number. The resulting phase is characterized by th…
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）
The proximity coupled topological insulator / superconductor (TI/SC) bilayer system is a representative system to realize topological superconductivity. In order to better understand this unique state and design devices from the TI/SC bilayer, a comprehensive understanding of the microscopic properties of the bilayer is required. In this work, a microwave Meissner screening study, which exploits a high-precision microwave resonator technique, is conducted on the SmB6/YB6 thin film bilayers as an example TI/SC system. The study reveals spatially dependent electrodynamic screening response of th…
We study the proximity-induced superconducting state in a general bilayer -- conventional $s$-wave superconductor hybrid structure. For the bilayer we include a general parabolic dispersion, Rashba spin-orbit coupling, and finite layer tunneling as well as the possibility to apply a bias potential and a magnetic Zeeman field, in order to address experimentally relevant bilayer systems, ranging from topological insulator thin films to generic double quantum well systems. By extracting the proximity-induced anomalous Green's function in the bilayer we show on a very rich structure for the superc…
We realized point contact spectroscopy experiment on ferromagnet/superconductor bilayers. Differential conductance curves show several features that we explained within Bogoliubov-de Gennes formalism considering the presence of two interfaces in the normal-metal-tip/ferromagnet/superconductor device. We demonstrate that such configuration is suitable as local probe of the spin polarization and thickness of ferromagnetic layer, directly on bilayer areas. This is due to the high sensitivity of the Andreev surface states to the physical properties of the ferromagnetic interlayer.
We calculate the contribution of superconducting fluctuations to the mesoscopic persistent current of an ensemble of rings, each made of a superconducting layer in contact with a normal one, in the Cooper limit. The superconducting transition temperature of the bilayer decays very quickly with the increase of the relative width of the normal layer. In contrast, when the Thouless energy is larger than the temperature then the suppression of the persistent current with the increase of this relative width is much slower than that of the transition temperature. This effect is similar to that predi…
We study theoretically the magnetic screening properties of thin, diffusive superconductor/ferromagnet bilayers subject to a perpendicular magnetic field. We find that the effective penetration depth characterizing the magnetic response oscillates with the thickness of the ferromagnetic layer on the scale of the ferromagnetic coherence length.
We study various aspects of proximity effects in $F/S$ (Ferromagnet/Superconductor) bilayers, where $F$ has a spiral magnetic texture such as that found in Holmium, Erbium and other materials, and $S$ is a conventional s-wave superconductor. We numerically solve the Bogoliubov-de Gennes (BdG) equations self-consistently and use the solutions to compute quantities relevant to the proximity effects in these bilayers. We obtain the relation between the superconducting transition temperature $T_c$ and the thicknesses $d_F$ of the magnetic layer by solving the linearized BdG equations. We find that…
The magnetization in a superconductor induced due to the inverse proximity effect is investigated in hybrid bilayers containing a superconductor and a ferromagnetic insulator or a strongly spin-polarized ferromagnetic metal. The study is performed within a quasiclassical Green function framework, wherein Usadel equations are solved with boundary conditions appropriate for strongly spin-polarized ferromagnetic materials. A comparison with recent experimental data is presented. The singlet to triplet conversion of the superconducting correlations as a result of the proximity effect with a ferrom…
We investigate ferromagnetic resonance (FMR) modulation in $d$-wave superconductor (SC)/ferromagnetic insulator (FI) bilayer systems theoretically. The modulation of the Gilbert damping in these systems reflects the existence of nodes in the $d$-wave SC and shows power-law decay characteristics within the low-temperature and low-frequency limit. Our results indicate the effectiveness of use of spin pumping as a probe technique to determine the symmetry of unconventional SCs with high sensitivity for nanoscale thin films.
Superconductor/ferromagnet proximity effect has been studied for Pb/Co$_2$Cr$_{1-x}$Fe$_x$Al bilayers. Different substrate temperatures allowed us to prepare the Heusler alloy Co$_2$Cr$_{1-x}$Fe$_x$Al films with different degree of spin polarization of conduction band of the Heusler layer. We obtain a strong correlation between the dependence of the superconducting transition temperature on the Pb-layer thickness at fixed thickness of the Heusler layer and the degree of spin polarization in the ferromagnetic layer.
The theoretical description of the Fulde-Ferrell-Larkin-Ovchinnikov like state establishing in nanostructered bilayers of ferromagnetic (F) and superconducting (S) material leads to critical temperature oscillations and reentrant superconductivity as the F-layer thickness gradually increases. The experimental realization of these phenomena is an important prerequisite for the fabrication of the Ferromagnet/Superconductor/Ferromagnet core structure of the superconducting spin-valve. A switching of the spin-valve is only expected if such non-monotonic critical temperature behavior is observed in…
We use local and global magnetometry measurements to study the influence of magnetic domain width w on the domain-induced vortex pinning in superconducting/ferromagnetic bilayers, built of a Nb film and a ferromagnetic Co/Pt multilayer with perpendicular magnetic anisotropy, with an insulating layer to eliminate proximity effect. The quasi-periodic domain patterns with different and systematically adjustable width w, as acquired by a special demagnetization procedure, exert tunable vortex pinning on a superconducting layer. The largest enhancement of vortex pinning, by a factor of more than 10…
We study peculiarities of proximity effect in clean superconductor -- ferromagnet structures caused by either spatial or momentum dependence of the exchange field. Even a small modulation of the exchange field along the quasiparticle trajectories is shown to provide a long range contribution to the supercurrent due to the specific interference of particle- and hole- like wave functions. The momentum dependence of the exchange field caused by the spin -- orbit interaction results in the long -- range superconducting correlations even in the absence of ferromagnetic domain structure and can expl…
Spin dynamics in a superconductor / ferromagnet proximity system
测量了在Nb衬底上溅射的Ni_（80）Fe_（20）薄膜的铁磁共振。通过改变Nb的温度和厚度，探讨了在这些异质结构中超导性对整个铁磁层的作用。发现Nb的超导转变以下的自旋输运性质的变化表现在Ni 80 Fe 20层中的铁磁体的共振的锐化，或在有效的吉尔伯特阻尼系数的降低。这种动态邻近效应与这些系统中的低频研究相反，在这些系统中，超导体的效应仅限于铁磁体中的一个小区域。我们解释这方面的自旋泵模型。
A ferromagnetic insulator attached to a superconductor is known to induce an exchange splitting of the Bardeen-Cooper-Schrieffer (BCS) singularity by a magnitude proportional to the magnetization, and penetrating into the superconductor to a depth comparable with the superconducting coherence length. We study this long-range magnetic proximity effect in EuS/Al bilayers and find that the exchange splitting of the BCS peaks is present already in the unpolarized state of the ferromagnetic insulator (EuS), and is being further enhanced when magnetizing the sample by a magnetic field. The measureme…
Considerable evidence for proximity-induced triplet superconductivity on the ferromagnetic side of a superconductor-ferromagnet (S-F) interface now exists; however, the corresponding effect on the superconductor side has hardly been addressed. We have performed scanning tunneling spectroscopy measurements on NbN superconducting thin films proximity coupled to the half-metallic ferromagnet La2/3Ca1/3MnO3 (LCMO) as a function of magnetic field. We have found that at zero and low applied magnetic fields the tunneling spectra on NbN typically show an anomalous gap structure with suppressed coheren…
The spread of the Cooper pairs into the ferromagnet in proximity coupled superconductor - ferromagnet (SF) structures is shown to cause a strong inverse electromagnetic phenomenon, namely, the long-range transfer of the magnetic field from the ferromagnet to the superconductor. Contrary to the previously investigated inverse proximity effect resulting from the spin polarization of superconducting surface layer, the characteristic length of the above inverse electrodynamic effect is of the order of the London penetration depth, which usually much larger than the superconducting coherence length…
We examined the proximity effect in granular films made of Pb, a superconductor, and Ni, a ferromagnet, with various compositions. Slow decay of the critical temperature as a function of the relative volume concentration of Ni per sample was demonstrated by our measurements, followed by a saturation of T$_c$. Using an approximate theoretical description of our granular system in terms of a layered one, we show that our data can only be reasonably fitted by a multilayer model. This indicates the importance of the interplay between different ferromagnetic grains; when non-collinearly magnetized …
We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+δ}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $T<T_c$ and is easily suppressed by a magnetic field. The phenomenon is attributed to a novel type of the superconducting proximity effect of non-equilibrium electrons injected into the conduction band of the ferromagnetic insulator via a Fowler-Nordheim tunne…
Ultrathin $\rm Bi_2Se_3$-NbN bilayers comprise a simple proximity system of a topological insulator and an s-wave superconductor for studying gating effects on topological superconductors. Here we report on 3 nm thick NbN layers of weakly connected superconducting islands, overlayed with 10 nm thick $\rm Bi_2Se_3$ film which facilitates enhanced proximity coupling between them. Resistance versus temperature of the most resistive bilayers shows insulating behavior but with signs of superconductivity. We measured the magnetoresistance (MR) of these bilayers versus temperature with and without a …
We investigate inverse proximity effects in a spin-triplet superconductor (TSC) interfaced with a ferromagnet (FM), assuming different types of magnetic profiles and chiral or helical pairings. The region of the coexistence of spin-triplet superconductivity and magnetism is significantly influenced by the orientation and spatial extension of the magnetization with respect to the spin configuration of the Cooper pairs, resulting into clearcut anisotropy signatures. A characteristic mark of the inverse proximity effect arises in the induced spin-polarization at the TSC interface. This is unexpec…
We show that in superlattices with alternating superconducting (S) and ferromagnetic (F) layers the spontaneous magnetic field induced in the superconducting layers due to the electromagnetic proximity effect becomes dramatically enhanced compared to the previously studied S/F bilayers. The effect reveals itself for the in-plane orientation of the magnetic moments both for ferromagnetic and anti-ferromagnetic ordering of the moments in the F layers. In the finite size samples the magnetic field decays from the sample surface towards the bulk of the structure, and the decay length strongly depe…
We have observed the evolution of phase diagram and vortex pinning using a single ferromagnet/superconductor bilayer of [Co/Pt]8/Nb through a special demagnetization procedure. It induces a continuous and reversible change of the domain width with equal positive/negative domains enabling the observation of the predicted tunable phase diagram. The tunable domain pattern also systematically affects vortex pinning. We have determined the dependence of the activation energy of vortex pinning on domain width, temperature, and magnetic field.
