Weyl Superfluidity in a Three-Dimensional Dipolar Fermi Gas

Abstract

Weyl superconductivity or superfluidity, a fascinating topological state of matter, features novel phenomena such as emergent Weyl fermionic excitations and anomalies. Here we report that an anisotropic Weyl superfluid state can arise as a low temperature stable phase in a 3D dipolar Fermi gas. A crucial ingredient of our model is a direction-dependent two-body effective attraction generated by a rotating external field. Experimental signatures are predicted for cold gases in radio-frequency spectroscopy. The finite temperature phase diagram of this system is studied and the transition temperature of the Weyl superfluidity is found to be within the experimental scope for atomic dipolar Fermi gases.Weyl superfluids or semimetals represent recent developments in generalizing topological phases from gapped to gapless systems (e.g., from topological insulators to semimetals), in condensed matter physics [1,2]. These Weyl states are characterized by the presence of two (or more) gapless Weyl points, which are topologically protected against small perturbations. The Weyl nodes lead to a variety of fascinating phenomena such as unusual surface states [3,4], Hall effects [5,6], and other transport features [7,8]. Finding electronic materials supporting Weyl states has attracted considerable interests [9]. There are many proposed potential candidate materials, such as the pyrochlore iridates [3], topological insulator multilayer structures [7,[10][11][12], as well as certain quasicrystals [13]. However, there is still no compelling experimental evidence for the observation of one. In the field of ultracold atoms, this phase was predicted to appear in spinorbit coupled Fermi gases [14,15]. This line of active research awaits for the future experimental breakthrough of synthesizing higher dimensional artificial spin-orbit coupling with controlled heating [16]. After all, the search for Weyl superconductors remains an open problem for both electronic and ultracold atomic systems.In this letter, we report the emergence of Weyl superfluidity in a 3D single-component dipolar Fermi gas with an effective attraction engineered by a rotating external field. Recently, degenerate dipolar Fermi gases witnessed rapid developments in both magnetic dipolar atoms (such as 167 Er [17,18] and 161 Dy [19,20] atoms) and polar molecules [21,22], stimulating tremendous interests in dipolar effects in many-body phases. The effects of the anisotropic dipolar interaction on the fermion many-body physics have been extensively investigated [23]. In particular, this provides the possibility of superfluid pairing between dipolar Fermi atoms in spinless or multicomponent systems [24][25][26][27] at low temperatures. For dipoles aligned parallel to the z direction, a p-wave superfluid state with the dominant p z symmetry was studied in a three-dimensional dipolar Fermi gas [28] and the competition between this superfluidity and nematic charge-density-wave (CDW) was also discussed [29]. For a dipolar Fermi gas confined in a 2D plane, superfluid states o...

Weyl Exceptional Rings in a Three-Dimensional Dissipative Cold Atomic Gas

“…One celebrated example in three dimensions is the zero-dimensional Weyl point [7][8][9][10][11][12][13][14][15][16][17][18] described by the Weyl Hamiltonian, which has been long sought-after in particle physics but only experimentally observed in condensed matter materials [19][20][21]. Such a Weyl point can be viewed as a magnetic monopole [22] in the momentum space and possesses a quantized Chern number on a surface enclosing the point.…”

Chirality-Dependent Hall Effect in Weyl Semimetals

“…Remarkably, when one of the two symmetries is broken, the critical point expands in the phase diagram and the Dirac point splits into pairs of Weyl points related by the unbroken symmetry. This emergent phase [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], dubbed as Weyl semimetal (WSM), is an appealing topological state of matter, with the Fermi surface being those Weyl points.In the simplest case when T symmetry is broken, a WSM at long wavelength only has one pair of Weyl points, which may be described by the HamiltonianHere v's are the Fermi velocities, σ's are Pauli matrices, and τ = ± denote the left-and right-handed Weyl fermions, which are required to come in pairs by the Nielsen-Ninomiya theorem [22]. τ bẑ are the positions of the pair of Weyl points in the Brillouin zone (BZ), and 2b 0 is their energy splitting, which vanishes when P symmetry is not broken.…”

“…Remarkably, when one of the two symmetries is broken, the critical point expands in the phase diagram and the Dirac point splits into pairs of Weyl points related by the unbroken symmetry. This emergent phase [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], dubbed as Weyl semimetal (WSM), is an appealing topological state of matter, with the Fermi surface being those Weyl points.…”

Realization and detection of Weyl semimetals and the chiral anomaly in cold atomic systems

“…We suggest that the measurements of LandauZener tunneling probabilities can be used to detect the Weyl semimetal phase and the possible exploration of chiral anomaly is discussed. Our scheme will lay the foundation for the realisation of even more exotic nodal topological phases such as Weyl superconductors [44,45] when attractive interactions between atoms are introduced. Note Added: At the finishing stage of this work, we noted that a scheme to realize Weyl semimetal phase by stacking 2D Harper systems in cubic lattices was proposed [46].…”