Zero-energy Bound States Research Articles

Atomic line defects and zero-energy end states in monolayer Fe(Te,Se) high-temperature superconductors

Majorana zero-energy bound states (ZEBSs) have been proposed to exist at the ends of one-dimensional Rashba nanowires proximity-coupled to an s-wave superconductor in an external magnetic field induced Zeeman field. Such hybrid structures have been a central platform in the search for non-Abelian Majorana zero modes (MZMs) toward fault-tolerant topological quantum computing. Here we report the discovery of ZEBSs simultaneously appearing at each end of a one-dimensional atomic line defect in monolayer iron-based high-temperature superconductor FeTe0.5Se0.5 films grown on SrTiO3(001) substrates. The spectroscopic properties of the ZEBSs, including the temperature and tunneling barrier dependences, as well as their fusion induced by coupling on line defects of different lengths are found to be robust and consistent with those of the MZMs. These observations suggest a realization of topological Shockley defects at the ends of an atomic line defect in a two-dimensional s-wave superconductor that can host a Kramers pair of MZMs protected by time-reversal symmetry along the chain. Our findings reveal an unprecedented class of topological line defect excitations in two-dimensional superconductor FeTe0.5Se0.5 monolayer films and offer an advantageous platform for generating topological zero-energy excitations at higher operating temperatures, in a single material, and under zero external magnetic field.

Macroscopic quantum tunneling and quasiparticle-tunneling blockade effect ins-wave/d-wave hybrid junctions

We have theoretically investigated macroscopic quantum tunneling (MQT) and the influence of nodal quasiparticles and zero energy bound states (ZES) on MQT in s-wave/ d-wave hybrid Josephson junctions. In contrast to d-wave/d-wave junctions, the low-energy quasiparticle dissipation resulting from nodal quasiparticles and ZES is suppressed due to a quasiparticle-tunneling blockade effect in an isotropic s-wave superconductor. Therefore, the inherent dissipation in these junctions is found to be very weak. We have also investigated MQT in a realistic s-wave/d-wave (Nb/Au/YBCO) junction in which Ohmic dissipation in a shunt resistance is stronger than the inherent dissipation and find that MQT is observable within the current experimental technology. This result suggests high potential of s-wave/d-wave hybrid junctions for applications in quantum information devices.

Theory of macroscopic quantum tunneling in Nb/Au/YBCO Josephson junctions

We have theoretically investigated macroscopic quantum tunneling (MQT) in s-wave/d-wave (Nb/Au/YBCO) Josephson junctions, and the influence of the nodal-quasiparticle and the zero energy bound states (ZES) on MQT. In contrast to d-wave/d-wave junctions, low-energy quasiparticle excitations resulting from nodal-quasiparticles and ZES are suppressed due to the finite isotropic gap of the s-wave superconductor. Therefore, the inherent dissipation of these junctions is weak. This result suggests the high potential of s-wave/d-wave hybrid junctions for applications in quantum information devices.

Theory of macroscopic quantum tunnelling and dissipation inhigh-Tc Josephson junctions

We have investigated macroscopic quantum tunnelling (MQT) in in-planehigh-Tc superconductor Josephson junctions and the influence of the nodal-quasiparticle and zeroenergy bound states (ZES) on MQT. We have shown that the presence of ZES at theinterface between the insulator and the superconductor leads to strong Ohmic quasiparticledissipation. Therefore, the MQT rate is noticeably suppressed in comparison withc-axis junctions in which ZES are completely absent.

Theory of macroscopic quantum tunneling in high-Tc cuprate

To reveal macroscopic quantum tunneling (MQT) in high-Tc superconductor Josephson junctions is an important issue since there is a possibility to fabricate a superconducting quantum bit by use of high Tc junctions. Using the functional integral and the instanton theory, we analytically obtain the MQT rate (the inverse lifetime of the metastable state) for the c-axis twist Josephson junctions. In the case of the zero twist angle, the system shows the super-Ohmic dissipation due to the presence of the nodal quasiparticle tunneling. Therefore, the MQT rate is suppressed compared with the finite twist angle cases. Furthermore, the effect of the zero energy bound states (ZES) on the MQT in the in-plane junctions is theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/superconductor interface leads to a strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably inhibited compared with the c-axis junctions in which the ZES are completely absent.

Effect of zero-energy bound states on macroscopic quantum tunneling in high-Tcsuperconductor junctions

The macroscopic quantum tunneling (MQT) in the current biased high-Tc superconductor Josephson junctions and the effect of the zero energy bound states (ZES) on the MQT are theoretically investigated. We obtained the analytical formula of the MQT rate and showed that the presence of the ZES at the normal/superconductor interface leads to a strong Ohmic quasiparticle dissipation. Therefore, the MQT rate is noticeably inhibited in compared with the c-axis junctions in which the ZES are completely absent.

Quasiparticle energy spectrum in ferromagnetic Josephson weak links

The quasiparticles energy spectrum in clean ferromagnetic weak links between conventional superconductors is calculated. Large peaks in density of states (DOS), due to a special case of the Andreev reflection at the ferromagnetic barrier, correspond to spin-split bound states. Their energies are obtained as a function of the barrier thickness, exchange field strength, and of the macroscopic phase difference φ at the link, related to the Josephson current. In the ground state, φ can be 0 or π, depending on the ferromagnetic barrier influence. Conditions for the appearance of the zero-energy bound states (ZES) and for the spin polarized ground state (SPGS) are obtained analytically. It is shown that ZES appear only outside the weak link ground state.