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Register a new userScanning Tunneling Spectroscopy of the superconducting proximity effect in a diluted ferromagnetic alloy
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We studied the proximity effect between a superconductor (Nb) and a diluted ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density of states on top of the ferromagnetic layer, which thickness varies on each sample, with a very low temperature Scanning Tunneling Microscope. The measured spectra display a very high homogeneity. The analysis of the experimental data shows the need to take into account an additional scattering mechanism. By including in the Usadel equations the effect of the spin relaxation in the ferromagnetic alloy, we obtain a good description of the experimental data.
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Using a dual-mode STM-AFM microscope operating below 50mK we measured the Local Density of States (LDoS) along small normal wires connected at both ends to superconductors with different phases. We observe that a uniform minigap can develop in the whole normal wire and in the superconductors near the interfaces. The minigap depends periodically on the phase difference. The quasiclassical theory of superconductivity applied to a simplified 1D model geometry accounts well for the data.
Conventional spin-singlet superconductivity that deeply penetrates into ferromagnets is typically killed by the exchange interaction, which destroys the spin-singlet pairs. Under certain circumstances, however, superconductivity survives this interaction by adopting the pairing behavior of spin triplets. The necessary conditions for the emergence of triplet pairs are well-understood, owing to significant developments in theoretical frameworks and experiments. The long-term challenges to inducing superconductivity in magnetic semiconductors, however, involve difficulties in observing the finite supercurrent, even though the generation of superconductivity in host materials has been well-established and extensively examined. Here, we show the first evidence of proximity-induced superconductivity in a ferromagnetic semiconductor (In, Fe)As. The supercurrent reached a distance scale of $sim 1~mu$m, which is comparable to the proximity range in two-dimensional electrons at surfaces of pure InAs. Given the long range of its proximity effects and its response to magnetic fields, we conclude that spin-triplet pairing is dominant in proximity superconductivity. Therefore, this progress in ferromagnetic semiconductors is a breakthrough in semiconductor physics involving unconventional superconducting pairing.
The proximity effect (PE) between superconductor and confined electrons can induce the effective pairing phenomena of electrons in nanowire or quantum dot (QD). Through interpreting the PE as an exchange of virtually quasi-excitation in a largely gapped superconductor, we found that there exists another induced dynamic process. Unlike the effective pairing that mixes the QD electron states coherently, this extra process leads to dephasing of the QD. In a case study, the dephasing time is inversely proportional to the Coulomb interaction strength between two electrons in the QD. Further theoretical investigations imply that this dephasing effect can decrease the quality of the zero temperature mesoscopic electron transportation measurements by lowering and broadening the corresponding differential conductance peaks.
We study Andreev reflection in a ballistic one-dimensional channel coupled in parallel to a superconductor via a tunnel barrier of finite length $L$. The dependence of the low-energy Andreev reflection probability $R_A$ on $L$ reveals the existence of a characteristic length scale $xi_N$ beyond which $R_A(L)$ is enhanced up to unity despite the low interfacial transparency. The Andreev reflection enhancement is due to the strong mixing of particle and hole states that builds up in contacts exceeding the coherence length $xi_N$, leading to a small energy gap (minigap) in the density of states of the normal system. The role of the geometry of such hybrid contacts is discussed in the context of the experimental observation of zero-bias Andreev anomalies in the resistance of extended carbon nanotube/superconductor junctions in field effect transistor setups.
Using low-temperature scanning tunneling microscopy and spectroscopy, we have studied the proximity effect at the interfaces between superconducting Pb island structures and metallic Pb-induced striped-incommensurate phase formed on a Si(111) substrate. Our real-space observation revealed that the step structures on the two-dimensional metallic layer exhibit significant roles on the propagation of the superconducting pair correlation; the proximity effect is terminated by the steps, and in the confined area by the interface and the steps the effect is enhanced. The observed results are explained quantitatively with an elastic reflection of electrons at the step edges based on calculations with the quasi-classical Greens function formulation using Usadel equation.
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