New Way to Achieve Ultrahigh Reduced Transition Temperature in Atomic Fermi Gases

Researchers have suggested a new strategy through which ultrahigh reduced transition temperature of up to T_c/T_F ~ 1 can be achieved in two-component atomic Fermi gases.

A two-component atomic Fermi gas is a system that imitates high T_c superconductors through a tunable pairing interaction strength, utilizing mixed dimensions in which one component is in three-dimensional (3D) free space, while the other remains in a deep one-dimensional (1D) optical lattice with a huge lattice spacing.

Quantum atomic Fermi gases have served as a perfect model for investigating the physics of high T_c superconductivity, such as the fundamental pairing mechanism as well as the strange pseudogap phenomena, which have been at the core of the debate in the field of high T_c superconductivity.

Now, theorists from Sun Yat-Sen University, Zhejiang University, Zhejiang University of Technology, and also the Synergetic Innovation Center of Quantum Information and Quantum Physics have reported that the reduced superfluid transition temperature, T_c/T_F, can be made very high by making the lattice spacing large in a mixed dimensional setting. This temperature can be made higher than that for any identified systems.

The study has been reported in SCIENCE CHINA Physics, Mechanics & Astronomy.

Achieving a high transition temperature T_c—one that is preferably up to room temperature (approximately 300 K) and above—has been a long-term objective in the field of superconductivity. This is just a small fraction because the usual electron kinetic energy, denoted by its Fermi temperature T_F, is of the order of 10,000 K in a solid.

Traditional superconductors in alloys and metals exhibit a transition temperature of a few Kelvins, typically less than 20 K. The high T_c cuprate superconductors, which were identified in 1986, have a T_c of up to 95 K at the optimal oxygen doping concentration under ambient pressure. But they have a relatively low T_F of up to 164 K under high pressure because of powerful electron correlations, which drive the reduced temperature T_c/T_F up to around 0.05.

The ratio does not surpass this value for other groups of superconductors, such as monolayer FeSe/SrTiO₃ superconductors, organic superconductors, heavy fermion superconductors, iron-based superconductors, the recently identified a magic-angle twisted bilayer graphene, and also H₂S—the T_c record holder—under high pressure.

It is possible to achieve a higher ratio in ultracold atomic Fermi gases, with T_F/T_c up to 0.218 in the BEC limit in the 3D homogeneous example. Using the improved local Fermi energy at the trap center, this ratio can be additionally increased to 0.518 in a harmonic trap in the BEC limit. This improvement led to the concept of utilizing mixed dimensions to adjust the Fermi energy as a function of lattice spacing.

The concept of improving T_F/T_c through mixed dimensions is shown in the first image. One of the two pairing components, known as spin-down atoms, continues to stay in 3D free space, taking up an isotropic Fermi sphere in momentum space.

By contrast, the spin-up atoms are prone to a deep 1D optical lattice potential (in the Z-direction) with a huge lattice spacing d and a huge band gap, allowing the fermions to occupy a thin disk of radius k_F and thickness 2p/d. This causes the Fermi energy E_F of the spin-up atoms to increase with an increase in d.

The lattice could be so deep that the spin-up atoms are actually localized in their respective lattice locations without the presence of the pairing interaction. When the pairing interaction is subsequently switched on and adjusted to be large through Feshbach resonance, it causes the Fermi sphere of the spin-down atoms to deform into a disk to correspond with that of the spin-up atoms.

Consequently, both components presently achieve a large Fermi energy (when compared to the non-interacting 3D value, that is, E_F) and thereby resulted in a high T_F/T_c. The authors pointed out that although the spin-up atoms are extremely localized without the presence of pairing interaction, the atomic pairs obtain high mobility as a result of the spin-down components.

This is slightly analogous to the example of superconductivity in a flat band, where each electron is localized while Cooper pairs achieve mobility through the pairing interaction. The E_F/k_F ratio largely governs the improvement of T_F/T_c.

The ratio achieves about unity for k_Fd = 55, which is higher than any recognized systems. (It must be noted that E_F=k_BT_F=ħ2K2F/2m, where k_B stands for the Boltzmann constant, ħ is the Planck constant h divided by 2p, m stands for the mass of atoms, and E_F and k_F are the Fermi energy and Fermi momentum, respectively, for the system without the presence of pairing interaction and lattice potential).

As highlighted by the authors, the above concept of improving T_c/T_F is not dependent on certain details of their theory and operates equally well for competing theories. Once a correct Feshbach resonance is known, the suggested scheme can be achieved experimentally with isotopic atoms like ¹⁶³Dy and ¹⁶¹Dy.

Ultracold atomic systems have been extensively investigated for their ability of quantum engineering and quantum simulation.

A major objective of the atomic Fermi gas community is to replicate and help to figure out the mechanism of high T_c superconductivity. A correct interpretation will definitely be crucial in the quest for designing novel higher T_c superconductors.

The study was supported by the National Natural Science Foundation of China (Grant Nos. 11274267, and 11774309), the National Basic Research Program of China (Grant No. 2012CB927404), and the Natural Science Foundation of Zhejiang Province of China (Grant No. LZ13A040001).