Session L21: Superconductivity: Mostly Mesoscopics & Low-d

Superconducting and Critical Current Properties of NiBi$_{3}$ Microfibers and Thin Films

We report the superconducting and critical current properties of thin films of NiBi$_{3}$ formed on the surface of carbon microfibers and on sapphire substrates. The NiBi$_{3}$ coated C-fibers were prepared by reacting 7-\textit{$\mu $}m diameter Ni-coated carbon fibers with Bi shot, and the thin films on sapphire were formed by exposing electron-beam deposited Ni films to Bi vapor. The fibers and films show T$_{c}$ = 4.25 K and T$_{c}$ = 4.35 K, respectively, which was slightly higher than that of bulk polycrystalline NiBi$_{3}$. The extrapolated upper critical fields of the fibers [H$_{c2}$(0) = 12 T] and films [H$_{c2}$(0) = 9 T] are higher than the reported data on polycrystalline samples. The temperature dependence of the critical current density ($J_{c})$ is well described by Ginzburg-Landau theory and gives an extrapolated value of 5.26 $\times $ 10$^{5}$ A/cm$^{2}$.   

Proximity effect in superconductor exchange spring-superconductor junction

It is known that in ferromagnet/superconductor hybrid system when the magnetizations of the ferromagnetic layer are inhomogeneous superconductivity is not necessarily suppressed by the ferromagnet due to the presence of the triplet superconducting pairing. In our current work we used exchange spring magnet to produce inhomogeneous noncollinear magnetic configuration in the ferromagnet. Exchange spring trilayer structure, soft/hard/soft, was fabricated between two superconducting Nb layers; Nb/Py/SmFe(SmCo)/Py/Nb. Magnetic property of the structure was characterized using Vibrating Sample Magnetometer and Magneto Optical Kerr Effect (MOKE). MOKE was used to measure the magnetizations of the top Py and the bottom Py separately. Transition temperature of the system was measured as a function of magnitude and direction of the external magnetic field.   

Results of Resonant Activation and Macroscopic Quantum Tunneling Experiments in Magnesium Diboride Thin Film Josephson Junctions

The Josephson junction is an experimental testbed widely used to study resonant activation and macroscopic quantum tunneling. These phenomena have been observed in junctions based on conventional low-temperature superconductors such as Nb and Al, and even in high-$T_{c}$, intrinsic superconductors. We report results of superconducting-to normal state switching experiments below 1 K using MgB$_{2}$-based Josephson heterojunctions with Pb and Nb counter-electrodes. Measurements were made with and without RF excitation. With microwaves, we see evidence of a resonant peak, in addition to the primary escape (from ground state) peak -- consistent with resonant activation. We also observe features suggestive of macroscopic quantum tunneling including peaks in the escape rate enhancements and an ``elbow'' in the graph of calculated escape temperatures $T_{esc}$ versus sample temperature.   

Carrier Transport in Heterojunction Nanocrystals Under Strain

We present a theory for carrier transport in semiconducting nanoscale heterostructures that emphasizes the effects of strain at the interface between two different crystal structures. An exactly solvable model shows that the interface region, or junction, acts as a scattering potential that facilitates charge separation. As a case study, we model a Type-II CdS/ZnSe heterostructure. After advancing a theory similar to that employed in model molecular conductance calculations, we calculate the electron and hole photocurrents and conductances, including non-linear effects, through the junction at steady-state.   

Progress towards measuring electron-electron interactions in persistent currents

The equilibrium persistent current (PC) in normal metal rings have been a challenge for both experimentalists and theorists. Specifically, the magnitude of the average PC's in normal metals has been a long-standing puzzle. Previous measurements of the average current ($\left\langle {I_{h/2e} } \right\rangle )$ were larger than theoretical predictions and indicated a diamagnetic sign. A possible explanation for these results is that they arise from the interplay of attractive electron-electron interactions within the metal (leading to the enhanced average PC) and trace magnetic impurities (which suppress the BCS superconductivity that would otherwise result from the attractive interactions). In this talk, I will discuss our progress towards measurements that intend to clarify the role that electron-electron interactions and magnetic impurities play in the persistent current.   

Tuning effective pairing potential through atomic scale control of superconductor heterostructures

Previous experiments showed that superconductivity persists in ultrathin films of conventional superconductors, even in films that are only 1-2 atomic layers thick. However, it has also been implied that the interface and substrate can strongly influence the electronic properties in the quasi two-dimensional regime. In this work, we fabricated a heterostructure with a normal metal layer (Ag here) placed in between a superconducting film (Pb here) and an insulating substrate, and measured its superconductivity with scanning tunneling spectroscopy subsequently. The most striking observation is the requirement of an overlayer (L$_{Pb})$ to recover the superconductivity, while the recovery thickness is linearly dependent on the underlayer thickness. Considering the renormalization of pairing interaction strength for Cooper pair in the hybrid structure, the ``effective attractive potential'' model was developed in this work. It could serve as a model with predictive power to describe the general behavior of superconductor heterostructures. Moreover, the physical origin of some discrepancies in the reported transition temperature as a function of film thickness is partly elucidated by this work as well.   

Area-dependence of spin-triplet supercurrent in ferromagnetic Josephson junctions

Spin-triplet supercurrents in strong ferromagnetic Josephson junctions were reported by several groups in 2010. At the same time, the 0-$\pi $ current-phase relationship of the spin-triplet supercurrent was predicted to be controllable by the magnetization orientations of different ferromagnetic layers. Our junctions contain a series of ferromagnetic layers consisting of a synthetic antiferromagnet Co/Ru/Co sandwiched between two thin magnetic layers such as PdNi or Ni [1]. When looking along the direction of current flow, one should obtain 0 junctions if the rotation direction of magnetizations is the same from one to the next, and $\pi $ junctions when the opposite rotation direction is the case. Since our magnetic layers have multiple domains in the virgin state, we should expect 0 and $\pi $ phases to alternate randomly in different locations in the junctions. The critical current in the virgin state should scale with the square-root of the junction area. After aligning the outer ferromagnetic layers in the same direction with an external field, the current-phase relation should be uniform across the whole junction area and the critical current should be proportional to the junction area. We will present data confirming this expectation for the magnetized state, whereas the situation for the virgin state is presently unclear. \\[4pt] [1] T.S. Khaire, M.A. Khasawneh, W.P. Pratt Jr and N.O. Birge, \textit{Phys. Rev. Lett.} 104 137002 (2010).   

Magnetic-field dependence of energy levels of superconducting nano-scale mettalic grains with strong spin-orbit scattering

We study the Zeeman splitting of discrete energy levels of superconducting nano-scale metallic grains whose single-electron dynamics is chaotic [1]. In the absence of spin-orbit scattering the Zeeman splitting of a single-electron level is trivial; it is the same for all levels and linear in magnetic field. Spin-orbit coupling suppresses this splitting, induces level-to-level fluctuations and non-linear corrections to the energies. We investigate the combined effect of pairing correlations, which lead to superconductivity in the bulk limit, and spin-orbit scattering on the many-electron energy levels in a weak magnetic field. In particular, we focus our studies on the linear (g-factor) and quadratic (zero-field level curvature) corrections and their mesoscopic fluctuations. The single-electron part of the Hamiltonian follows the statistics of the Gaussian symplectic ensemble of random matrix theory, which is applicable in the limit of strong spin-orbit scattering and a large dimensionless Thouless conductance. The interaction is given by a BCS-like pairing term and the magnetic field coupling is described by a Zeeman term. [1] K. Nesterov and Y. Alhassid, to be published.   

Temperature induced epitaxial strain and superconductivity in (Cu,C)Ba$_{2}$CuO$_{4+\delta }$ thin films

We have studied the effects of substrate temperature in establishing superconductivity in carbon incorporated thin films of Infinite Layered (IL) BaCuO$_{2+\delta }$. Carbon in the form of CO$_{3}^{2-}$ group modulates the IL structure into a superstructure with doubled out of plane lattice parameter, `$c$'. The superstructure, (Cu,C)Ba$_{2}$CuO$_{4+\delta }$ (Cu-1201) shows superconductivity within a narrow window of `$c$' lattice parameter varying between 8.28 {\AA} and 8.33 {\AA}. The structural analysis of these thin films using reciprocal space maps (RSMs) shows a pseudomorphic growth with an in-plane lattice parameter, `$a$', of 3.90 {\AA}, similar to that of the SrTiO$_{3}$ substrate. Growth of these films under compressive strain is obtained at substrate temperatures varying between 530 \r{ }C and 560 \r{ }C. At deposition temperatures less than 500 \r{ }C, films with a relaxed in-plane lattice parameter of 4.00 {\AA} were obtained which were non-superconducting. Deviations in the substrate temperature led to the coexistence of strained and relaxed phases. Thus the elongation along $c$ -- axis is compensated by the compression along $a $-- axis with increasing substrate temperature. Flexibility of tuning the in-plane mismatch between substrate and film also rule out the use of buffer layers. Such structural changes result in the change of bond lengths and subsequently rearrange the number of charge carriers in the CuO$_{2}$ planes. Optimum number of charge carriers lead to superconductivity.   

Coexistence of a Triplet Nodal Order Parameter and a Singlet Order Parameter at Co / CoO / In contacts

We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to a tunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at low bias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductance peaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wave pairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find that the values of the s-wave and p-wave gaps follow the BCS temperature dependance and that the amplitude of the s-wave fluid increases with the barrier strength.   

Exploring triplet superconductivity by controlling the magnetic non-collinearity

Recent theories predict spin-triplet superconductivity at the interface between a singlet superconductor (SC) and a ferromagnet (FM) with \textit{inhomogeneous} magnetization [1]. Magnetic non-collinearity is a crucial but not quantitatively controlled parameter in most experiments inferring triplet superconductivity [2]. In this work, we use Nb as the SC, and an epitaxial exchange spring Py/Sm-Co bilayer with in-plane uniaxial anisotropy in Sm-Co layer as the FM. Due to the interfacial exchange coupling, a \textit{tunable} noncollinear spin spiral can be achieved by controlling the external field. At a fixed temperature within the superconducting transition, starting from a collinear magnetic configuration, as the spin spiral winding angle \textit{$\phi $} increases, the superconducting critical current ($I_{c})$ first increases. There is an optimized winding angle \textit{$\phi $}$_{o}$, which maximizes $I_{c}$, after which $I_{c}$ decreases with increasing\textit{ $\phi $}. This \textit{non-monotonic} $I_{c}$(\textit{$\phi $}) dependence cannot be explained by the short range proximity effect alone and suggests triplet pairing. More importantly, combining micromagnetic simulations with magnetoresistance measurements, we have determined the~magnetic non-collinearity and correlated it \textit{quantitatively} with the superconducting transport results. Our findings demonstrate the superconducting proximity effect can be tuned by manipulating the magnetic non-collinearity in a \textit{single} sample.\\[0pt] [1] F. S. Bergeret et al., PRL \textbf{86}, 4096. [2] M. Eschrig, Physics Today \textbf{64}, 43.   

The Images of vortex penetration into superconducting MoGe plates observed by scanning SQUID microscope

The amorphous superconducting film is very preferential as a good model in studying nanostructured superconductors because it has weakened pinning centers compared to other superconductors. The MoGe films have been deposited by a DC sputtering apparatus using a Mo$_{80}$Ge$_{20}$ target and a number of small MoGe plates were fabricated with the aid of photolithography. The vortex distribution in a MoGe circle has been investigated by means of a scanning SQUID microscope. We found that vortices form different configurations including shell structures, which evolves with the increase in applied magnetic field. Observed results are compared to theoretical studies for vortices in mesoscopic circles.   

Image processing of scanning SQUID microscope for observing superconducting nanostructures

A newly developed image processing technique for the scanning superconducting quantum interference device (SQUID) microscope is presented and its application to several measurements on superconducting nanostructures, including networks and dots, is discussed. This method is based on the detailed analysis of the structures characteristic to the measurement apparatus, such as the shape and position of the pickup coil. We take account of the Meissner effect in the coil body and its influences on the obtained image are carefully removed. The separation between the coil and the sample is also considered. Actually applying this method, we present images of the superconducting networks and dots, which show clear improvement from the raw images. The results are also discussed from physical point of view.   

Fluctuating pancake vortices revealed by dissipation of Josephson vortex lattice

In strongly anisotropic layered superconductors in tilted magnetic fields the Josephson vortex lattice coexists with the lattice of pancake vortices. Due to the interaction between them, the dissipation of the Josephson-vortex lattice occurs to be very sensitive to the presence of the pancake vortices. If the c-axis magnetic field is smaller then the corresponding lower critical field the pancake stacks are not formed but the individual pancakes may exist in the fluctuational regime either near surface in large-size samples or in the central region for small-size mesas. We calculate the contribution of such fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex lattice and compare the theoretical result with measurements on small mesas fabricated out of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. A fingerprint of fluctuating pancakes is characteristic exponential dependence of the c-axis conductivity observed experimentally. Our results provide strong evidence of the existence of the fluctuating pancakes and their influence on the Josephson-vortex-lattice dissipation.