Despite recent breakthroughs in topological bands for ultracold atoms in 1D and 2D, it has been an open challenge to realize and observe 3D topological matter in an atomic system. While numerous schemes have been proposed, the experimental complexity and the characterization of the 3D band structure acted as a barrier against experimental groups achieving this outstanding goal. In this talk, we report the realization and observation of 3D nodal-line semimetal band with spin-orbit-coupled ultracold fermions. The 3D topological band structure is achieved by stacking 2D Dirac semimetal in the x-y plane along z direction in Raman-dressed optical lattices. To detect 3D topological phases, we developed kz resolved spin texture measurement technique based on emergent magnetic group symmetry in our system. By directly imaging spin texture in specific kz plane with different Zeeman splitting, 3D nodal lines can be reconstructed. The realization of topological band structure is also verified in quench dynamics by detecting band inversion lines, which are bulk counterparts of Fermi arc states. This technique can be broadly applied to characterizing 3D topological states with similar symmetries.
Despite recent breakthroughs in topological bands for ultracold atoms in 1D and 2D, it has been an open challenge to realize and observe 3D topological matter in an atomic system. While numerous schemes have been proposed, the experimental complexity and the characterization of the 3D band structure acted as a barrier against experimental groups achieving this outstanding goal. In this talk, we report the realization and observation of 3D nodal-line semimetal band with spin-orbit-coupled ultracold fermions. The 3D topological band structure is achieved by stacking 2D Dirac semimetal in the x-y plane along z direction in Raman-dressed optical lattices. To detect 3D topological phases, we developed kz resolved spin texture measurement technique based on emergent magnetic group symmetry in our system. By directly imaging spin texture in specific kz plane with different Zeeman splitting, 3D nodal lines can be reconstructed. The realization of topological band structure is also verified in quench dynamics by detecting band inversion lines, which are bulk counterparts of Fermi arc states. This technique can be broadly applied to characterizing 3D topological states with similar symmetries.
Exploring 3D topological matter with spin-orbit-coupled fermions in optical lattices1 CHENGDONG HE, BO SONG, Hong Kong University of Science and Technology, SEN NIU, LONG ZHANG, Peking University, ZEJIAN REN, ENTONG ZHAO, Hong Kong University of Science and Technology, XIONG-JUN LIU, Peking University, GYU-BOONG JO, Hong Kong University of Science and Technology — Ultracold atoms offer a promising testbed for the experimental study of topological matter. Various topological models have been recently realized in 1D and 2D atomic systems. While 3D topological matter with rich physics remains unexplored due to experimental complexity. Here we report, for the first time, the realization of a 3D nodal line semimetal in optical lattices with SO coupled ultracold fermions. The 3D topological band structure is achieved by stacking 2D Dirac semimetal in the x-y plane along z direction in Raman-dressed optical lattices. kz resolved spin texture measurement technique based on emergent magnetic group symmetry has been developed to detect 3D topological state. Spin texture in a specific kz plane can be directly imaged with different Zeeman splitting. 3D nodal lines can be reconstructed with this pseudo-tomography method. Band inversion lines, the bulk counterparts of Fermi arc states, can be extracted from quench dynamics, which reconfirmed the realization of topological phase. Our detection technique can be broadly applied to characterizing 3D topological states with similar symmetries, which provides a new possibility to study exotic quantum physics in higher dimensions.
Ultracold atoms provide a controllable platform to explore topological matter induced by spin-orbit (SO) coupling. This thesis reports an experimental realization of topological matter with Ytterbium atoms implemented by a Raman (or SO coupling) lattice - an optical lattice dressed by SO coupling, where internal atomic states are coupled by Raman beams. Remarkably, the lattice simultaneously serves as one of the Raman beams. As the building block of the Raman lattice, SO coupling is first studied in a one-dimensional (1D) bulk system and manifested by two hallmarks, the locking between the atomic spin and momentum and the dephasing in the Rabi oscillation. SO coupling in a periodic lattice renders topological matter. 1D Raman lattice show-cases a novel symmetry-protected topological (SPT) phase beyond the tenfold Altland-Zirnbauer classification. SPT phases distinguished by the Z2 invariant are determined from spin-textures. We highlight textures not only in equilibrium, but also of far-from-equilibrium dynamics after a quench between distinct phases, reflecting the band topology. Higher dimensional Raman lattices are richer. A three-dimensional (3D) topological nodal line semimetal structure has been observed for the first time with ultracold atoms, and reconstructed by a well-designed (pseudo-)tomography - extraction of Dirac points on different momentum layers, owing to the emergence of a crystalline symmetry. Similarly, topological features, such as band inversion lines that are bulk counterparts of Fermi arc states and connect Dirac points, are also uncovered from quench dynamics. "More is different." The SU(N) symmetric spin-independent interaction is investigated by Tan's contact and collective modes. Enhanced by the (orbital) Feshbach resonance, the Raman lattice with tunable interactions can be applied to engineer elusive topological many-body phases, which is envisaged for unveiling the attractive topological superfuilds.
Weyl semimetals are three-dimensional (3D) gapless topological phases with Weyl cones in the bulk band. According to lattice theory, Weyl cones must come in pairs, with the minimum number of cones being two. A semimetal with only two Weyl cones is an ideal Weyl semimetal (IWSM). Here we report the experimental realization of an IWSM band by engineering 3D spin-orbit coupling for ultracold atoms. The topological Weyl points are clearly measured via the virtual slicing imaging technique in equilibrium and are further resolved in the quench dynamics. The realization of an IWSM band opens an avenue to investigate various exotic phenomena that are difficult to access in solids.
Abstract The gapless Weyl superfluid has been widely studied in the three-dimensional ultracold fermionic superfluid. In contrast to Weyl superfluid, there exists another kind of gapless superfluid with topologically protected nodal lines, which can be regarded as the superfluid counterpart of nodal line semimetal in the condensed matter physics, just as Weyl superfluid with Weyl semimetal. In this paper we study the ground states of the cold fermionic gases in cubic optical lattices with one-dimensional spin-orbit coupling and transverse Zeeman field and map out the topological phase diagram of the system. We demonstrate that in addition to a fully gapped topologically trivial phase, some different nodal line superfluid phases appear when the Zeeman field is adjusted. The presence of topologically stable nodal lines implies the dispersionless zero-energy flat band in a finite region of the surface Brillouin zone. Experimentally these nodal line superfluid states can be detected via the momentum-resolved radio-frequency spectroscopy. The nodal line topological superfluid provide fertile grounds for exploring exotic quantum matters in the context of ultracold atoms.
Weyl fermion, also referred to as pseudo-magnetic monopole in momentum space, is an undiscovered massless elementary particle with half-integer spin predicted according to relativistic quantum field theory. Motivated by the recent experimental observation of Weyl semimetal band in ultracold Bose gases with Raman-assisted 3D spin-orbit coupling, we investigate the properties and possible observation of Weyl fermions in the low-energy quasi-particle excitations of ultracold Fermi gases. Following a previous suggestion that the existing Raman lattice scheme can be readily generalized to fermionic systems, here we discuss the movement of the Weyl points in the Brillouin Zone, as well as the creation and annihilation of Weyl fermions by adjusting the effective Zeeman field. The relevant topological properties are also demonstrated by calculating the Chern number. Furthermore, we propose how to experimentally verify the existence of the Weyl fermions and the associated quantum phase transition via density profile measurements.
The recent realization of two-dimensional (2D) synthetic spin-orbit (SO) coupling opens a broad avenue to explore novel topological states for ultracold atoms. Here, we propose a scheme to realize chiral Fulde-Ferrell superfluid for ultracold fermions, and show a generic theory that the topology of chiral superfluid phases can be uniquely determined from the normal states. The main findings are twofold. First, a semimetal is driven by a type of 2D SO coupling, and can be tuned into massive Dirac fermion phase with or without inversion symmetry. Without inversion symmetry, the superfluid phase of nonzero pairing momentum is favored under an attractive interaction. Furthermore, we show a fundamental theorem that the Chern number of a 2D chiral superfluid can be obtained from the 1D invariants of Fermi surfaces, with which the chiral Fulde-Ferrell superfluid with a broad topological region is predicted. This generic theorem is also useful for condensed-matter physics and material science in the search for new topological superconductors.