Gate-tunable superconducting weak link and quantum point contact spectroscopy on a strontium titanate surface

Abstract

Two-dimensional electron systems in gallium arsenide and graphene have enabled ground-breaking discoveries in condensed-matter physics, in part because they are easily modulated by voltages on nanopatterned gate electrodes. Electron systems at oxide interfaces hold a similarly large potential for fundamental studies of correlated electrons and novel device technologies 1-3 , but typically have carrier densities too large to control by conventional gating techniques. Here we present a quantum transport study of a superconducting strontium titanate (STO) interface, enabled by a combination of electrolyte 4-7 and metal-oxide gating. Our structure consists of two superconducting STO banks flanking a nanoscale STO weak link, which is tunable at low temperatures from insulating to superconducting behaviour by a local metallic gate. At low gate voltages, our device behaves as a quantum point contact that exhibits a minimum conductance plateau of e 2 /h in zero applied magnetic field, half the expected value for spin-degenerate electrons, but consistent with predictions 8-12 and experimental signatures 13-18 of a magnetically ordered ground state. The quantum point contact mediates tunnelling between normal and superconducting regions, enabling lateral tunnelling spectroscopy of the local superconducting state. Our work provides a generic scheme for quantum transport studies of STO and other surface electron liquids.Transition metal oxides exhibit electronic ground states not seen in conventional semiconductors 2,3 . For instance, the interface between lanthanum aluminate and strontium titanate 19 -two non-magnetic band insulators-hosts superconductivity 20 and magnetism [13][14][15][16][17][18] . Rashba spin-orbit coupling at conductive STO interfaces 21 has led to predictions of unconventional superconducting states 10 , and of helical wires and Majorana fermions in nanoscale STO channels 11 . Furthermore, nanoscale lateral control of carriers could allow fabrication of gate-tunable, single-material Josephson junctions and superconducting quantum interference devices, as well as mesoscopic devices. However, few experiments on nanostructures in STO systems exist: superconductivity has been demonstrated in submicron regions tunable with a global back gate 22 , and channels sketched by a nanoscale tip show superconducting features 23 . In this work, we demonstrate the first realization of an all-STO, gate-tunable superconducting weak link. We tune our device to the ballistic 1D limit and perform superconducting spectroscopy that agrees with BCS weak-coupling expectations and previous work 24 .Undoped STO is a band insulator. Inducing a two-dimensional (2D) electron density near 10 13 cm −2 creates a metal, and several times that density creates an optimally doped superconducting state 4 . To modulate the density on this scale, we use the electric double-layer transistor technique [4][5][6][7] . A schematic of our sample is shown in Fig. 1a. A single crystal of TiO 2 -terminated (100) SrTiO 3 is patterned with ohmic conta...

Accumulation, inversion, and depletion layers inSrTiO3

“…we recover the previous accumulation layer result, Eq. (15). For low electron charge density σ e/d 2 and small field D 0 D c we get…”

“…The electric field D c at which the transition from linear to nonlinear dielectric response occurs can be found from equating Eqs. (15) and (16). This gives…”

“…At larger x we reach the electron gas with a larger width given by Eqs. (15) or (16) for linear and nonlinear dielectric responses respectively. In this case, the electron gas resides at large distances and our considerations of the previous section are valid.…”

Metallic ground state in an ion-gated two-dimensional superconductor

“…The electric-doublelayer transistor (EDLT), which is composed of the interface between crystalline solids and electrolytes, can be a good candidate to realize such a clean system, since the conduction carriers are induced electrostatically at the atomically flat surfaces without introducing extrinsic disorder. In addition, the EDLT has the greatest advantage of their applicability to a wide range of materials, as exemplified by gate-induced superconductivity in three-dimensional SrTiO3 (3,11,12), KTaO3 (13), quasi-2D layered ZrNCl (14), transition metal dichalcogenides (15)(16)(17) and cuprates (18)(19)(20)(21)(22). Here, we report comprehensive transport studies on a ZrNCl-EDLT, which provide evidence of 2D superconductivity in this system based on several types of analyses.…”

Electron gas induced in SrTiO3

“…The induction D c at which the transition from linear to nonlinear dielectric response occurs can be found from equating Eqs. (15) and (16). This gives…”