Session Y23: Superconductivity: Proximity Effects

Supercurrent-Induced Magnetization Dynamics

We investigate supercurrent-induced magnetization dynamics in a Josephson junction with two misaligned ferromagnetic layers, and demonstrate a variety of effects by solving numerically the Landau-Lifshitz-Gilbert equation. In particular, we demonstrate the possibility to obtain supercurrent-induced magnetization switching for an experimentally feasible set of parameters, and clarify the favorable condition for the realization of magnetization reversal. These results constitute a superconducting analogue to conventional current-induced magnetization dynamics and indicate how spin-triplet supercurrents may be utilized for practical purposes in spintronics.   

Superfluid Densities in Superconducting/Ferromagnetic (Nb/NiV/Nb) Heterostructures

Superfluid density measurements allow us to probe the superconducting structure of thin films below T$_{c}$ with remarkable detail. They yield information not only of the inherent robustness of the superconducting state, but also about the homogeneity of the sample and possible ``hidden'' transitions at temperatures lower than the initial T$_{c}$. For this reason multiple transitions in superconducting heterostructures are revealed to us. We use superfluid density measurements on Nb/Ni$_{0.95}$V$_{0.05}$/Nb trilayers to study the interplay between two superconducting films separated by the destructive proximity effects of a ferromagnet. We show there are trilayers with strong coupling, which produces a single transition, that become decoupled to the point of separation into two transitions as the ferromagnetic layer thickness increases. We discuss the difficulties in observing the second transition in $\sigma _{1}$, while obvious in $\lambda ^{-2}$.   

Magnetic-state-controlled proximity effect across high-T$_{C}$ superconductor/ferromagnetic interfaces.

We have investigated the electronic density of states of a ferromagnet (F: a Co/Pt superlattice) in contact with a c-axis YBCO film. This was done by measuring the current-perpendicular-to-plane differential conductance across vertical junctions of area down to 6 $\mu $m$^{2}$, which were fabricated using optical lithography and ion etching. We have found salient features of the leakage of the superconducting order parameter into the F layer, such as a zero-bias conductance peak which can be modulated by the magnetic state of the ferromagnet. We discuss the possibility of triplet superconducting correlations induced in the F layer as the origin of this behavior.   

Long-Range Superconducting Proximity Effect in Template-Fabricated Single-Crystal Nanowires

We study a superconducting proximity effect observed in single-crystal nanowires of Zn, Sn, and Pb of length up to 60 $\mu $m. These nanowires are electrochemically deposited into the pores of anodic aluminum oxide membranes and polycarbonate membranes. Using an \textit{in situ} self-contacting method, single nanowires are electrically contacted on both ends to a pair of macroscopic film electrodes of Au, Sn, or Pb pre-fabricated on both surfaces of the membranes. Superconductivity in the nanowires is strongly suppressed when Au electrodes are used. When electrodes having higher superconducting transition temperatures are used, the nanowires become superconducting at the transition temperatures of the electrodes. Microscopy analyses of the structure and the chemical composition of the nanowires will be presented. Measurements of sample resistance and $I-V$ characteristics at various temperatures and magnetic fields will also be presented. Furthermore, the effects of the length, the diameter, and the residual resistance ratio of the nanowires on the proximity induced superconductivity will be analyzed and discussed.   

Radiative Interband Transition of Cooper Pairs in a Semiconductor

Interactions of photons and superconductors have been a hot topic for superconducting (SC) qubit operations. The relevant photon energies were limited below the superconducting gap of superconductors, that is, microwave frequencies. The possibility of electron Cooper-pair interactions with photons with much higher energies was discussed theoretically [1]. In this talk we will demonstrate that Cooper pairs penetrated into a semiconductor from an adjacent superconductor by the proximity effect play a major role in interband radiative recombinations in the semiconductor experimentally. SC Nb electrodes were formed on an InGaAs/InP light emitting diode (LED) and electroluminescence (EL) around 1.55um was observed from a slit formed on the surface Nb electrode. EL was drastically enhanced below the Nb SC critical temperature (T$_{c})$ of $\sim $8K [2]. The reduction of radiative recombination lifetime consistent with the observed EL enhancement was observed below T$_{c}$[3]. These results are well explained with the theory [1]. We will discuss the possibility of generating entangled photon pairs based on this new scheme. [1] Y. Asano et al., PRL 103 (2009) 187001. [2] Y. Hayashi et al., Appl. Phys. Express 1 (2008) 011701. [3] I. Suemune et al., APEX 3 (2010) 054001.   

Robustness of Majorana modes and minigaps in a spin-orbit-coupled semiconductor-superconductor heterostructure

We study the robustness of Majorana zero energy modes and minigaps of quasiparticle excitations in a vortex by numerically solving Bogoliubov-deGennes equations in a heterostructure composed of an \textit{s} -wave superconductor, a spin-orbit-coupled semiconductor thin film, and a magnetic insulator. This heterostructure was proposed recently as a platform for observing non-Abelian statistics and performing topological quantum computation. The dependence of the Majorana zero energy states and the minigaps on various physics parameters (Zeeman field, chemical potential, spin-orbit coupling strength) is characterized. We find the minigaps depend strongly on the spin-orbit coupling strength. In certain parameter region, the minigaps are linearly proportional to the \textit{s}-wave superconducting pairing gap $\Delta_{s}$, which is very different from the $\Delta_{s}^{2}$ dependence in a regular \textit{s-} or \textit{p}-wave superconductor. We characterize the zero energy chiral edge state at the boundary and calculate the STM signal in the vortex core that shows a pronounced zero energy peak. We show that the Majorana zero energy states are robust in the presence of various types of impurities. We find the existence of impurity potential may increase the minigaps and thus benefit topological quantum computation.   

Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure

A half-metal is by definition spin-polarized at its Fermi level and therefore was conventionally thought to have little proximity effect to an $s$-wave superconductor. Here we show that if there is spin-orbit coupling at the interface between a single-band half-metal and an $s$-wave superconductor, $p_x + ip_y$ superconductivity would be induced on the half-metal. This can give us topological superconductor with a single chiral Majorana edge state. We show that two atomic layers of CrO$_2$ or CrTe gives us the single-band half-metal and is thus a candidate material for realizing this physics.   

Superconducting proximity effects of Pb nano-islands

Superconductivity in systems with spatial dimensions smaller than the coherence length has been the subject of intense interest for decades. We systematically address how superconducting nano-islands interact each other via a detailed scanning tunneling microscopy/spectroscopy (STM/STS). By measuring the spatial mapping of the local superconducting gap, an intriguing lateral proximity effect is observed in an island containing regions of different thicknesses and different superconducting strength which shows a gradual but evident change of local superconductivity at the thickness boundary. This must be due to a lateral proximity effect caused by the tunneling of Cooper pairs with different binding energies across the boundary. We were also able to experimentally determine a proximity length. When an island is smaller than the proximity length, it is found that superconductivity within the island is rather uniform, indicating the rigidity of the order parameter on the scale of proximity length.   

S -N-S junction formed by graphene with lead (Pb) contacts

We fabricate lead (Pb) contacts to graphene that allow us to observe supercurrent in the Pb-graphene-Pb structure up to temperatures of $\sim $3K. The measured critical current is much smaller than a naive expectation based on calculations for a superconductor-insulator-superconductor (S-I-S) junction. Hysteresis is seen in the switching current despite the fact that the junction is made to be overdamped. The behavior of the Pb-graphene-Pb structure is qualitatively explained by considering it as an S-N-S junction.   

Switching and retrapping behavior in graphene proximity-effect superconducting junctions

Since the pioneering work by R. Holm and W.Meissner [Z. Physik.~86, 787 (1933)], who observed zero resistance in SNS pressed contacts, many manifestations of the superconducting proximity effect have been reported. Recently it was shown that when closely spaced superconducting leads are placed on graphene, the proximity effect is induced and a supercurrent can flow between the electrodes. Here we fabricate graphene proximity-effect junctions (GPJ) and compare them to Josephson junctions (JJ). As the bias current is increased to near the critical current, a thermal escape from the washboard potential can occur driving the junction into the runaway voltage state. ~The standard deviation of the switching current is measured as a function of temperature and compared to the thermal and quantum escape models for JJs. ~We find that the temperature dependence of the standard deviation of switching currents of graphene proximity junctions is qualitatively different from the well-studied behavior of the insulator-based JJs. Possible reasons will be discussed.   

Interface Engineering of Thin Film Superconductor Heterostructures

Thin film superconductivity is a subject with great scientific and technological importance. The previous works demonstrated that the superconductivity exits in extreme two-dimensional lead film with a thickness of only two atomic layers. Most strikingly, the Tc is only slightly suppressed from the bulk value. However, when the film is pseudomorphically strained, the Tc is suppressed further, implying the importance of the interface. In this work we explore thin film superconductivity in a new direction by engineering superconductor/normal metal heterostructures with atomically flat interface. Using in-situ scanning tunneling microscopy/spectroscopy, we explore the superconductivity of the Pb/Ag heterostructure by independently tuning the thicknesses of the atomically flat Ag films and superconducting Pb films respectively. The intriguing role of the Ag thin films on the superconductivity of Pb thin films will be reported.   

Tunneling Measurements of the Exchange Field in Superconducting Al-EuS Bilayers

We present tunneling density of states measurements of the proximity-induced exchange field in superconducting Al-EuS bilayers. By depositing thin Al films onto an insulating EuS layer we demonstrate that one can induce an exchange field of several tesla in the superconducting Al. Contrary to expectations, this exchange field is a strong function of the applied field below 2 T. This applied-field dependence is not associated with the alignment of domains in the EuS, but instead appears to be an intrinsic effect. In addition, we show that the exchange field decreases significantly with increasing temperature below 1 K.   

Exploring the Parameter Space of the Anti-Proximity Effect

The anti-proximity effect, in which the superconductivity of a nanowire is weakened by the superconductivity of measuring bulk electrodes, has been reported in Zn nanowires (Tian et. al., PRL 95, 076802 (2005), Chen et. al., PRL 103, 127002 (2009)). The mechanism of this effect is not completely understood. We have studied the anti-proximity effect in Al nanowires in a variety of configurations. The effect is found to manifest only when the critical temperature (T$_{c})$ of the nanowire is greater than the T$_{c}$ of its bulk form. The range of the effect is found to be $\sim $ 1$\mu $m. The effect is seen in single nanowires in the absence of a magnetic field establishing that the effect depends on the nature of the measuring electrodes and is not caused by an external magnetic field. The anti-proximity effect has also been seen with the bulk superconductor not connected to the measurement circuitry.