Coherent transmission of superconducting carriers through a ∼2 <i>μ</i> m polar semiconductor

Abstract

Abstract Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor–normal metal–superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μ m with a critical magnetic field B c ≈ 8 T at 1.6 K and T c ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG ( μ n &gt; 10 4 cm 2 V −1 s −1 ). The current and voltage probes were superconducting tungsten–gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current ( I c ) large enough such that the Josephson coupling parameter <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="italic">ℏ</mml:mi> <mml:msub> <mml:mrow> <mml:mi>I</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">ek</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>B</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> <mml:mo>≫</mml:mo> <mml:mn>1</mml:mn> </mml:math> . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a T c typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston–Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures.