Spontaneous emergence of Josephson junctions in homogeneous rings of single-crystal Sr2RuO4

Abstract

The chiral p-wave order parameter in Sr2RuO4 would make it a special case amongst the unconventional superconductors. A consequence of this symmetry is the possible existence of superconducting domains of opposite chirality. At the boundary of such domains, the locally supressed condensate can produce an intrinsic Josephson junction. Here, we provide evidence of such junctions using mesoscopic rings, structured from Sr2RuO4 single crystals. Our order parameter simulations predict such rings to host stable domain walls across their arms. This is verified with transport experiments on loops, with a sharp transition at 1.5 K, which show distinct critical current oscillations with periodicity corresponding to the flux quantum. In contrast, loops with broadened transitions at around 3 K are void of such junctions and show standard Little-Parks oscillations. Our analysis demonstrates the junctions are of intrinsic origin and makes a compelling case for the existence of superconducting domains. seems probable, domains or edge currents have not been observed directly. Indications for their existence, however, have been found in transport experiments, which utilize Ru inclusions to form proximity junctions between Sr2RuO4 and a conventional s-wave superconductor [10,11]. A complication in the physics of Sr2RuO4 is that breaking of the tetragonal crystal symmetry due to Ru inclusions or a uniaxial strain can induce a different superconducting state with an enhanced superconducting transition temperature ≈ 3 K [13,14]. Recent experiments suggest that this so-called 3-K phase may exhibit a non-chiral state with a single-component order parameter [15,16]. In this paper, we refer to the multicomponent phase with of around 1.5 K, associated with the pure bulk limit, as the "intrinsic phase" and the possible single-component phase, characterized by ≈ 3 K, as the "extrinsic phase".The vast majority of experiments in the past two decades have been limited to bulk crystals, typically hundreds of microns in dimension. This is partly due to the unavailability of superconducting Sr2RuO4 films. The chiral domains, however, are expected to be no more than a few microns in size [5,11]. Moreover, the time-dependent switching noise observed in transport measurements suggests the domains are mobile [10,11]. We note here that the role of chiral domains resulting in hysteretic behaviour has been discussed in the Bi-Ni bilayer system [12]. The arbitrary configuration of the domains introduces an element of uncertainty.On the other hand, the energy cost associated with a chiral domain wall (ChDW), grows per area [17]. It has been recently discussed that mesoscopic samples made of chiral p-wave superconductors could host multichiral states [18,19], where the two ± chiral components are divided into superconducting domains, separated by ChDWs. This makes mesoscopic structures a promising platform to verify and potentially control the domains.

A Review of Electronic Transport in Superconducting Sr2RuO4 Junctions

“…This indicates that the electrical transport properties of an SRO214 ring may depend on the ring geometry. To understand this, recently, Yasui et al [61] investigated SRO214 rings fabricated in two different geometries, as shown in Figure 3a,b. They observed SQUID-like oscillations in I c [58,59] at various different temperatures below T c , as shown in Figure 3c.…”

“…The colour maps show the Cooper-pair density |ψ| 2 .In the absence of a magnetic field, for a wider ring, the Meissner state is stable, and the chiral domain walls are stabilised by reducing the width of the ring wall. (Adopted with permission from[61] copyright NPG 2020).…”

Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca2RuO4

“…By using ultra-low currents (∽nA), we have confirmed that the insulating-state resistivity of the microscopic samples matches that of their mmsized counterparts, signaling the absence of strain. Moreover, in a previous work on the isostructural unconventional superconductor Sr 2 RuO 4 , we have prepared microscopic samples using equal methods and substrates 45 . These samples retained bulk properties to sizes below 200 nm, whereas, like Ca214, its transport properties are heavily dependent on strain [46][47][48] .…”

Reveal the nematic topological superconductivity from the anisotropic unconventional Josephson effect: Theory and application to doped Bi2Se3

“…The Little-Parks effect reflects the oscillation of the free energy of the superconductor as a function of the applied magnetic flux, resulting in a corresponding periodicity of the resistance near the superconducting transition temperature T c [79]. The minimum of the free energy [80,81], as well as the minimum of resistance, is achieved when the applied magnetic flux becomes (n + 1/2)Φ 0 for a superconducting loop containing an odd number of π-junctions [82][83][84][85], instead of nΦ 0 for a conventional s-wave superconducting loop. Here, n is an integer and Φ 0 = h/2e is the magnetic flux quantum with h Planck constant and e the elemental charge [86,87].…”