Abstract:
Based on the topology, the ground state is determined by the symmetry of the system. Controlling symmetry leads to a quantum phase transition, which is a key to accessing exotic quantum states. Through a superconducting-normal-superconducting Josephson junction (JJ), both time reversal symmetry and inversion symmetry (parity symmetry) can be investigated. Broken symmetry may lead to exotic quantum states with superconductivity. Typically, JJs maintain symmetry under a small magnetic field unless the Andreev channel between superconducting electrodes exhibits a specific order. In this talk, we study the PtTe2 Josephson junction proximitized by superconducting NbTi contacts. Supercurrent through PtTe2 shows a conventional Fraunhofer pattern under a perpendicular magnetic field while keeping the Parity-Time (PT) symmetry. However, both time and inversion symmetry are broken when applying an in-plane magnetic field along the current direction. Remarkably, inversion symmetry is restored by reversing the in-plane magnetic field and current directions, hinting at a Chiral Parity-Time invariant due to the PtTe2 channel. Therefore, an internal degree of freedom from crystal symmetry may provide additional symmetry to recover the PT invariance. By rotating the in-plane magnetic field, we observe two-fold degeneracy in the field-current direction.
Interestingly, the PT symmetry is strongly conserved at two singular points where the current direction is perpendicular to the in-plane magnetic field. The singular CPT-invariant points are retained up to an intermediate range of magnetic field(≈ 100 mT), resulting in two quantum critical points as a function of field angle with respect to the current. This symmetry control allows us to phase-bias the supercurrent under a small perpendicular magnetic field with additional control knobs of the in-plane magnetic field, leading to the symmetry-controlled Josephson diode effect (sJDE), creating a novel type control in Josephson current.