Discovery of segmented Fermi surface induced by Cooper pair momentum on a hybrid material platform

In a new report now published in Science, primary authors Zhen Zhu, Michal Papaj, and an international research team in physics, materials science, and condensed matter at the Jiao Tong University, China, Massachusetts Institute of Technology, U.S., and the Chinese Academy of Sciences discovered the Fermi surface of supercurrent-induced quasiparticles in a superconducting system for the first time. This discovery comes 50 years after the initial theoretical prediction was made by physicist Peter Fulde and revealed the impact of the finite Cooper pair momentum on the quasiparticle spectrum. In condensed matter physics, Cooper pairs are a pair of electrons with opposite spins loosely bound due to electron-lattice interactions. Superconductivity is based on their condensation to Bosonic states at low temperatures. The interplay of superconductivity and magnetic fields leads to the phenomenon of a 'segmented Fermi surface." A leading author of this work, MIT Professor of Physics Liang Fu, outlined the significance of this discovery.

Supercurrent flow in a superconductor

Physicists assume that a sufficiently large supercurrent can close the energy gap in a superconductor and create gapless quasiparticles via the Doppler shift of quasiparticle energy. This is facilitated by the finite momentum of Cooper pairs in the presence of supercurrent flow in a superconductor, where the shift in Cooper pair momentum can result in a Doppler shift. In this work, Zhu et al. used quasiparticle interference to image the field-controlled Fermi surface of bismuth telluride (Bi₂Te₃) thin films proximitized by the superconductor niobium diselenide (NbSe₂). A small applied in-plane magnetic field induced a screening supercurrent, which led to finite momentum pairing on the topological surface states of Bi₂Te₃. The scientists identified distinct interference patterns to indicate a gapless superconducting state with a segmented Fermi surface to reveal the strong impact of the finite Cooper pair momentum on the quasiparticle spectrum. We asked Liang Fu at the Department of Physics at MIT, who led this research, to describe the significance of their work and how they arrived at this breakthrough, 50 years after its theoretical prediction. "It started with a purposeful search that, with some luck, led to a pleasant surprise" he said. "For a while I had the idea of creating zero-energy quasiparticles by applying a parallel magnetic field to a superconductor coated with a layer of topological insulator. These quasiparticles could be used to construct Majorana fermions and topological qubits."

Using a hybrid material platform to detect supercurrent-induced quasiparticles

During the study, the team showed how the Cooper pair momentum resulted in a Doppler shift of the energy. The Doppler shift then changed the energy of the quasiparticles that moved parallel or antiparallel to the supercurrent in the setup, without affecting quasiparticles moving in the perpendicular direction, to result in anisotropic quasiparticle energy dissipation. In condensed matter physics, a Bogoliubov quasiparticle or Bogoliubon is a quasiparticle that occurs within superconductors with elementary excitation above the ground state. To detect segmented Fermi surface of Bogoliubov quasiparticles, they used spectroscopic methods with both energy and momentum resolution. Typically, a supercurrent often larger than the de-pairing current is required to close the superconducting gap. To overcome the difficulty of this approach, Zhu et al. used an alternative approach—a hybrid material platform.

Liang Fu further described their platform: "My collaborator Jinfeng Jia, a world expert of scanning tunneling microscopy, was willing to give it a try [to detect the quasiparticles]. It turns out this material platform worked out beautifully, with a surprisingly small magnetic field—much smaller than originally expected." Instead of passing a transport current directly through a superconductor, the team applied a small external magnetic field to produce a screening current near the surface through the Meissner effect. The platform of choice contained a thin film of Bi₂Te_3, a quintessential topological insulator grown via molecular beam epitaxy on top of a bulk crystal NbSe₂—an S-wave superconductor. This setup provided an ideal platform to observe the formation of segmented Fermi surface. Liang Fu further described the nuance of this strategy to detect quasiparticles. "We later realized that the quasiparticles we saw are actually produced by the screening current flowing on the surface of the superconductor. The possibility of supercurrent-induced quasiparticles was first predicted by Peter Fulde over 50 years ago, but it was previously thought that to observe this effect requires a current so large as to destroy superconductivity. In hindsight, our experiment succeeded because this effect is amplified in the topological insulator film (bismuth telluride) that is proximitized by the superconductor (niobium diselenide)."

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Characterizing the material platform.

Zhu et al. then presented the topography of the thin film and obtained the atomic resolution of the Bi₂Te₃ lattice to demonstrate its high quality. Near the Fermi level they observed a hard, U-shaped superconducting energy gap at zero magnetic field. The team then applied an in-plane magnetic field to the thin film to investigate its gapless superconducting state by measuring the differential conductance. The observed in-gap features were a consequence of supercurrent induced quasiparticles. The team calculated the evolution of spectral function at energies around the Fermi level with the field. When asked about the impact of preceding research work that influenced this outcome, Liang Fu noted that "a lot influenced the outcome. I have worked for a long time on theory of topological superconductors and collaborated with Jinfeng Jia on related experiments. This kind of close collaboration between theory and experiment is essential to our success. Of course, the early theoretical works by Peter Fulde and Grigory Volovik are very important: They helped us see the big picture. The existence of zero-energy quasiparticles we discovered is by no means limited to proximitized topological insulators." The team thereafter detected the segmented Fermi surface directly within the momentum space by scanning the constant energy local density of states across the whole region of interest to obtain quasiparticle interference patterns.

Outlook

In this way, Zhen Zhu, Michal Papaj and colleagues led by Liang Fu revealed the strong impact of Cooper pair momentum caused by screening supercurrent on quasiparticle energy dissipation. The observation of segmented Fermi surface of Bogoliubov quasiparticles opens the way for further scanning tunneling microscopy studies of pair density wave and the formation of Cooper pairs with non-zero total momentum. When asked about the practical impact of these research outcomes, Liang Fu said, "It is too early to tell, but I can think of some possibilities. The quasiparticles we discovered could provide a potentially advantageous way to make topological qubits, which was my original motivation. These quasiparticles move freely and transport heat in a superconductor which is otherwise a poor thermal conductor. So, it seems to me that the ability of producing quasiparticles from electrical current may also enable a heat transistor controlling the flow of heat electrically."