Inverse Proximity Effect Research Articles

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Self-consistent evaluation of proximity and inverse proximity effects with pair-breaking in diffusive superconducting–normal metal junctions

Inverse-proximity effect and 2D superconductivity of Ta/Au bilayer thin films

This study investigated the inverse-proximity effect (IPE) and two-dimensional (2D) superconductivity in α-phase Ta films and Ta/Au bilayers. These bilayers were deposited onto silicon substrates using a direct current (DC) sputtering system. The findings reveal that higher substrate temperatures and increased film thickness facilitate the formation of α-Ta up to a temperature limit of 300°C, resulting in higher Tc around 3.5–3.7 K. The fundamental superconductivity of α-Ta thin films is similar to that of a conventional s-wave BCS superconductor. The IPE in the Ta/Au bilayer films was observed, where the Tc increases and decreases with the Au overlayer thickness. This behavior was quantitatively described by considering a model accounting for the reduced superconducting fluctuations induced by the highly conductive Au layer. Furthermore, the presence of 2D superconductivity in the Ta film and Ta/Au bilayer was examined through critical field analysis, vortex motions, and Berezinskii-Kosterlitz-Thouless (BKT) transition detection. Finally, the results were summarized in an H-T phase diagram illustrating the presence of IPE and 2D superconductivity in a Ta/Au bilayer. This study provides additional insights into 2D superconductivity within normal/superconductor (N/S) bilayers, laying the groundwork for future research to optimize film properties to align more closely with theoretical expectations.

Evidence of the inverse proximity effect in tunnel magnetic josephson junctions

Magnetic Josephson junctions (MJJs) are a special class of hybrid systems where antagonistic correlations coexist, thus providing a key for advances in weak superconductivity, superconducting spintronics, and quantum computation. So far, the memory properties of MJJs have been mostly investigated in view of digital electronics and for spintronic devices at liquid-helium temperature. At the operating temperature of quantum circuits, a magnetic order can rise in a superconductor (S) at the S/ferromagnet (F) interface, i.e., the inverse proximity effect (IPE), thus leading to a significant modification of the magnetic field patterns in MJJs. In this work, we have carried out a comparative investigation of the magnetic behavior of tunnel MJJs with a strong ferromagnetic layer inserted in the layout of both Nb and Al JJs, respectively. The comparative analysis validates the crucial role of the temperature, the fundamental scaling energies of S/F coupling systems, and the transparency of the S/F interface. This investigation points out that the IPE is a key aspect to consider when designing tunnel MJJs operating well below 4 K and thus in the perspective of hybrid superconducting quantum architectures.

Nanoscale spin ordering and spin screening effects in tunnel ferromagnetic Josephson junctions

Magnetic Josephson junctions (MJJs) have emerged as a prominent playground to explore the interplay between superconductivity and ferromagnetism. A series of fascinating experiments have revealed striking phenomena at the superconductor/ferromagnet (S/F) interface, pointing to tunable phase transitions and to the generation of unconventional spin-triplet correlations. Here, we show that the Josephson effect, being sensitive to phase space variation on the nanoscale, allows a direct observation of the spin polarization of the S/F interface. By measuring the temperature dependence of the Josephson magnetic field patterns of tunnel MJJs with strong and thin F-layer, we demonstrate an induced nanoscale spin order in S along the superconducting coherence length at S/F interface, i.e., the inverse proximity effect, with the first evidence of full spin screening at very low temperatures, as expected by the theory. A comprehensive phase diagram for spin nanoscale ordering regimes at S/F interfaces in MJJs has been derived in terms of the magnetic moment induced in the S-layer. Our findings contribute to drive the design and the tailoring of S/F interfaces also in view of potential applications in quantum computing.

Magnetization control of the critical current in a S-(S/F)-S superconducting switch

We show the control of the critical current Ic of a superconductor–superconductor/ferromagnet–superconductor [S-(S/F)-S] type switch by the magnetization state of the F. The inverse proximity effect of a S/F bilayer is used to define the weak link and hence the maximum critical current of the junction. The magnetization of the F lead is set by an external magnetic field. We show that the critical current in zero field depends on the remanent magnetization state via stray-field effects and thereby allows for a post-manufacturing control of the junction's properties, in particular the tunability between a maximal critical current and a full suppression of Ic without applying a persistent magnetic field. We propose a simplified model based on the relative orientation of magnetic domains that explains our findings qualitatively.

Ferromagnetic insulator induced inverse proximity effect in superconducting DoS

A ferromagnetic insulator proximate with a superconductor induces some spin asymmetry in superconducting correlations. Density of states (DoS) changes as a result of the induction of magnetization in superconductor (S) under the influence of the inverse proximity effect with the ferromagnetic insulator (FI). The study solves the Usadel equations numerically and calculates characteristic features of the density of states at the interface boundary and at the boundary with vacuum, caused by FI. The electron density of states was analyzed, showing significant singlet-to-triplet conversion, even for small values of the interface parameter (φ). In DoS, the zero bias peak tends to play a larger role than the spin-splitting of the BCS peaks. The features of the density of state obtained in this work can be measured experimentally.

Bipolar thermoelectricity in S/I/NS and S/I/SN superconducting tunnel junctions

Recent studies have shown the potential for bipolar thermoelectricity in superconducting tunnel junctions with asymmetric energy gaps. The thermoelectric performance of these systems is significantly impacted by the inverse proximity effects present in the normal-superconducting bilayer, which is utilized to adjust the gap asymmetry in the junction. Here, we identify the most effective bilayer configurations, and we find that directly tunnel-coupling the normal metal side of the cold bilayer with the hot superconductor is more advantageous compared to the scheme used in experiments. By utilizing quasiclassical equations, we examined the nonlinear thermoelectric junction performance as a function of the normal metal film thickness and the quality of the normal-superconducting interface within the bilayer, thereby determining the optimal design to observe and maximize this nonequilibrium effect. Our results offer a roadmap to achieve improved thermoelectric performance in superconducting tunnel junctions, with promising implications for a number of applications.

Proximity effect and inverse proximity effect in a topological-insulator/iron-based-superconductor heterostructure

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 $\Gamma=(0,0)$ 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 parameter has a maximal value at the coupling strength $t_p=0.3\approx 0.06$ eV. The spectral function and the local density of states are calculated and may be used to probe the proximity effect. We also discuss the feedback of the topological insulator to the iron-based superconductor layer. The normal-state Fermi surface is distorted by the coupling, and additional Fermi pockets are induced. An effective spin-orbit interaction term is induced. In the superconducting state, the previous fourfold symmetry of the order parameter is broken, and a $d$-wave component pairing term is also induced. Our main results can be well understood by analyzing the Fermi surfaces of the original systems.

The inverse proximity effect in tunnel heterostructure ferromagnetic–insulator–superconductor Co2CrAl–I–Pb

Ferromagnetic–insulator–superconductor Co2CrAl–I–Pb tunnel junctions were created and their current-voltage characteristics (CVCs) were studied. It has been established that they differ from the usual CVCs of tunnel junctions of a normal metal–insulator–superconductor. It is proposed to assume that the observed effect of changing the type of CVC is associated with the existence of the destruction of Cooper pairs due to the inverse effect of proximity between the Co2CrAl and Pb electrodes. The CVCs were calculated based on the approach of P. Fulde.

Peculiarities of the density of states in SN junctions

We study the density of states (DoS) ν(E) in a normal-metallic (N) film contacted by a bulk superconductor (S). We assume that the system is diffusive and the SN interface is transparent. In the limit of thin N layer (compared to the coherence length), we analytically find three different types of the DoS peculiarity at energy equal to the bulk superconducting order parameter Δ0. (i) In the absence of the inverse proximity effect, the peculiarity has the check-mark form with ν(Δ0)=0 as long as the thickness of the N layer is smaller than a critical value. (ii) When the inverse proximity effect comes into play, the check-mark is immediately elevated so that ν(Δ0)>0. (iii) Upon further increasing of the inverse proximity effect, ν(E) gradually evolves to the vertical peculiarity (with an infinite-derivative inflection point at E=Δ0). This crossover is controlled by a materials-matching parameter which depends on the relative degree of disorder in the S and N materials.

Inverse proximity effect in a topological-insulator–superconductor hybrid system

The recent experiments on ultrathin superconductors (SCs), in proximity with some substrates, have unveiled the possible enhancement and suppression of superconducting order parameters; yet their mechanisms are still ambiguous. Here we examine the inverse proximity effect between an ultrathin SC and the surface state of a topological insulator. We show the effect of the renormalized electron-phonon ($e$-ph) coupling constant $\ensuremath{\lambda}$ on superconducting order parameters is determined by two important mechanisms: the modified density of states (DOS) $N({\ensuremath{\epsilon}}_{\mathrm{F}})$ from the occupied bands near the Fermi surfaces, and the renormalized interaction ${U}_{\mathrm{eff}}$ from the unoccupied bands. We demonstrate the enhanced order parameter near the bottom of the double-well bands with appropriate interface coupling due to the enhanced DOS; while in the strong coupling regime we observe the suppressed order parameter even with finite occupation in the Fermi surface, which is the result of the conjunct influence of the suppressed DOS and the renormalized ${U}_{\mathrm{eff}}$. This picture is applied to multiple SC layers, showing that the oscillation of order parameter can be well ascribed to the oscillation of an $e$-ph coupling constant, instead of solely the DOS. Our results not only provide a theoretical basis for tuning of the order parameters in the SC by chemical potential and interface coupling, but also provide some new insight into the order parameters in a large number of SC-substrate hybrid systems via the inverse proximity effect.

Nucleation of superconductivity in clean superconductor-ferromagnet hybrid structures with Rashba spin-orbit interaction

We study the influence of the inverse proximity effect on the nucleation of superconductivity in planar hybrid structures consisting of a thin superconducting film proximity coupled to a material with a strong exchange or Zeeman field and the Rashba spin-orbit interaction. Based on numerical simulations and analytical estimates within the framework of the Gor'kov equations, we find the superconducting transition temperature of the system and determine the spatial structure of the gap function. It is shown that the spin-orbit interaction partially compensates the pair-breaking effect of the exchange or Zeeman field and stabilizes modulated superconducting states, which can be traced to the Lifshitz invariant in the Ginzburg-Landau free-energy density. The suggested approach provides a microscopic justification of this invariant for a large class of superconducting hybrid systems and clarifies the validity range of the corresponding phenomenological models with terms linear in the spatial gradients of the superconducting order parameter.

Investigation of the Inverse Magnetic Hysteresis of the Josephson Supercurrent in Magnetic Josephson Junctions

In tunnel magnetic Josephson junctions (JJs), we have provided evidence of an inverse magnetic dependence of the critical current <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c(H)</i> , i.e., a shift of the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c(H)</i> curves in the opposite expected direction. We have established a self-consistent method to ascribe this unconventional feature to spin polarization phenomena at the S/F interface. Through a magnetic characterization of Superconductor (S) /Ferromagnet (F) /Superconductor (S) trilayers and a magnetostatic analysis of the devices, we show that the F-stray fields fail to determine the pattern inversion. The comparative analysis with the literature supports our thesis that this phenomenon can be explained in terms of inverse proximity effect and observed with a proper choice of materials and thickness of the ferromagnetic layer involved in the junction structure. More generally, our approach proves that a careful investigation of the magnetic field pattern in magnetic JJs can lead to the detection of novel effects arising at the S/F interface.

Intricate interplay between superconductivity and topological surface states of c axis oriented MoTe2

The interaction between superconductivity and spin-polarized surface states of topological materials provides an exciting platform for the research and development of proximity induced coupling effects, Majorana fermions, spin valves, spintronics, etc. and so on. In this work, the inverse proximity effect observed exactly at the super conducting transition temperature of indium (3.5 K) demonstrates the complex interplay between robust 2D spin-polarized surface states observed in our (002n) oriented MoTe2 nanolayer sheets with that of superconducting states. Interestingly, our phenomenological model based on the Werthamer-Helfand-Hohenberg (WHH) model and Ginzburg–Landau formalism, invoked to validate the experimental observations, indicates a competition between superconductivity and topological order, marked by a close correspondence between the temperature of crossover (Tcr = 2.45 K) of their respective length scales, ξ and Lφ, and the saturation temperature in resistivity.

Homointerface planar Josephson junction based on inverse proximity effect

The quality of a superconductor–normal metal–superconductor Josephson junction (JJ) depends crucially on the transparency of the superconductor–normal metal (S/N) interface. We demonstrate a technique for fabricating planar JJs with perfect S/N interfaces. The technique utilizes a strong inverse proximity effect discovered in Al/V5S8 bilayers, by which the Al layer is driven into the resistive state. The highly transparent S/N homointerface and the peculiar normal metal enable the flow of Josephson supercurrent across a 2.9 μm long weak link. Moreover, our JJ exhibits a giant critical current and a large product of the critical current and the normal state resistance.

Anomalous inverse proximity effect in unconventional superconductor junctions

We investigate the effects of Andreev bound states due to the unconventional pairing on the inverse proximity effect of ferromagnet/superconductor junctions. Utilizing quasiclassical Eilenberger theory, we obtain the magnetization penetrating into the superconductor. We show that in a wide parameter range the direction of the induced magnetization is determined by two factors: whether Andreev bound states are present at the junction interface and the sign of the spin-mixing angle. In particular, when Andreev bound states appear at the interface, the direction of the induced magnetization is opposite to that without Andreev bound states. We also clarify the conditions under which the inverted induced magnetization appears. Applying this novel effect helps distinguishing the pairing symmetry of a superconductor.

Magnetization in Superconductor–Ferromagnetic Metal Bilayers Induced by the Inverse Proximity Effect

Magnetization in Superconductor–Ferromagnetic Metal Bilayers Induced by the Inverse Proximity Effect

Meissner screening as a probe for inverse superconductor-ferromagnet proximity effects

We present experimental results on the observed flux screening in proximity coupled superconductor-ferromagnet thin film structures using Nb and Co as the superconductor and ferromagnet respectively. Using the low-energy muon-spin rotation technique to locally probe the magnetic flux density, we find that the addition of the ferromagnet (F) increases the total flux screening inside the superconductor. Two contributions can be distinguished. One is consistent with the predicted spin-polarization (or magnetic proximity) effect, while the other is in line with the recently emerged electromagnetic (EM) proximity models. Furthermore, we show that the addition of a few nanometers of a normal metallic layer between the Nb and the Co fully destroys the contribution due to electromagnetic proximity. This is unanticipated by the current theory models in which the magnetization in the F layer is assumed to be the only driving force for the EM effect and suggests the role of additional factors. Further experiments to explore the influence of the direction of the F magnetization also reveal deviations from theory. These findings are an important step forward in improving the theoretical description and understanding of proximity coupled systems.

Inverse proximity effects at spin-triplet superconductor-ferromagnet interface

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 unexpectedly stronger when the magnetic proximity is weaker, thus unveiling immediate detection signatures for spin-triplet pairs. We show that an anomalous magnetic proximity can occur at the interface between the itinerant ferromagnet, SrRuO$_3$, and the unconventional superconductor Sr$_2$RuO$_4$. Such scenario indicates the potential to design characteristic inverse proximity effects in experimentally available SrRuO$_3$-Sr$_2$RuO$_4$ heterostructures and to assess the occurrence of spin-triplet pairs in the highly debated superconducting phase of Sr$_2$RuO$_4$.

The inverse proximity effect in strong ferromagnet–superconductor structures

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 ferromagnet is studied.