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Single-Mode Superconductivity in LaAlO3/SrTiO3 Nanostructures Joshua P. Veazey1, Guanglei Cheng1, Patrick Irvin1, Shicheng Lu1, Mengchen Huang1, Feng Bi1, Chung-Wung Bark2, Sangwoo Ryu2, Kwang-Hwan Cho2, Chang-Beom Eom2, and Jeremy Levy1 1 University of Pittsburgh, Pittsburgh, PA 15260, U.S.A. 2 University of Wisconsin-Madison, Madison, WI 53706, U.S.A.

ABSTRACT The properties of superconductors at the extreme limits of dimensionality are of fundamental interest. The interface of LaAlO3 and SrTiO3 hosts a quasi-two-dimensional superconductor below Tc≈200 mK. Here we report superconductivity in nanowire-shaped structures created at the LaAlO3/SrTiO3 interface using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3 (w Tc=200 mK.

Figure 1. (a) Schematic of five-terminal Hall bar device. (b) V-I curves and (c) differential resistance (dV/dI) curves showing superconductivity and its suppression at sufficiently high temperatures and magnetic fields.

Rather than a sudden voltage increase at a critical current Ic, the switching currents in these nanostructures have a finite widths (Figure 1(b)). Here, Ic is defined as the location of the

peak in dV/dI between superconducting and normal states. Finite widths in switching current are sometimes observed in S-c-S Josephson junctions (JJ), where S is a superconducting reservoir and c is a constriction in the superconductor [16]. Indeed, the products IcRN agree well with the S-c-S characterization (IcRN~kBTc/e), where kB is the Boltzmann constant [17]. In Device A, the peaks in dV/dI at I=Ic exhibit asymmetric intensity. It is possible that variations in the carrier density that arose along the nanowire during c-AFM fabrication are contributing to this asymmetric transport feature. In contrast, the dV/dI peaks are symmetric in Device B (Figure 2). The four-terminal resistance in the superconducting state exhibits a surprising dependence on the configuration of the voltage and current leads. For example, simply changing the current pathway changes RC by a factor of three in Device B (Figure 2). However, the voltage leads are fixed, so the voltage drop is measured across the same segment. In another configuration where current and voltage leads are swapped, RC drops by an order of magnitude (Figure 2). The normal state resistance RN is the same in all three configurations.

Figure 2. Differential resistance dV/dI plotted for three unique permutations of the current and voltage leads of Device B. The graphical representations of the nanostructure illustrate the leads employed. Arrows denote current pathway, and plus/minus labels denote voltage sensing leads. Exchanging one current lead with one voltage lead changes the superconducting resistance RC by more than a factor of 10 (red to blue). Switching the current path and fixing the voltage leads changes RC by a factor of 3 (red to green). (T=50 mK). In short, these transport signatures reflect the 1D nature of the sketched LaAlO3/S