A topological superconductor A promising path toward topological quantum computing involves exotic quasiparticles called the Majorana bound states (MBSs). MBSs have been observed in heterostructures that require careful nanofabrication, but the complexity of such systems makes further progress tricky. Zhang et al. identified a topological superconductor in which MBSs may be observed in a simpler way by looking into the cores of vortices induced by an external magnetic field. Using angle-resolved photoemission, the researchers found that the surface of the iron superconductor FeTe0.55Se0.45 satisfies the required conditions for topological superconductivity. Science, this issue p. 182 Angle-resolved photoemission spectroscopy indicates that FeTe0.55Se0.45 harbors Dirac-cone–type spin-helical surface states. Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1–xSex (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc. Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

Observation of topological superconductivity on the surface of iron-based superconductor

I review recent works on the symmetry and the structure of the superconducting gap in Fe-based superconductors and on the underlying pairing mechanism in these systems. The experimental data on superconductivity show very rich behavior, with potentially different symmetry of a superconducting state for different compositions of the same material. The variety of different pairing states raised the issue whether the physics of Fe-based superconductors is model-dependent or is universal, governed by a single underlying pairing mechanism. I argue that the physics is universal and that all pairing states obtained so far can be understood within the same universal pairing scenario and are well described by the effective low-energy model with a small number of input parameters.

Pairing mechanism in Fe-based superconductors

Charge-density-wave-driven electronic nematicity in a kagome superconductor

In this paper, recent progress on experimental analysis and theoretical models for non-local transport (non-Fickian fluxes in real space) is reviewed. The non-locality in the heat and momentum transport observed in the plasma, the departures from linear flux-gradient proportionality, and externally triggered non-local transport phenomena are described in both L-mode and improved-mode plasmas. Ongoing evaluation of ‘fast front’ and ‘intrinsically non-local’ models, and their success in comparisons with experimental data, are discussed

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Towards an emerging understanding of non-locality phenomena and non-local transport

Recent experimental and simulation work has substantially advanced the understanding of L-mode plasma edge turbulence and plasma flows and their mutual interaction across the L–H transition. Flow acceleration and E × B shear flow amplification via the turbulent Reynolds stress have been directly observed in multiple devices, using multi-tip probe arrays, Doppler backscattering, beam emission spectroscopy, and gas puff imaging diagnostics. L–H transitions characterized by limit-cycle oscillations (LCO) allow probing of the trigger dynamics and the synergy of turbulence-driven and pressure-gradient-driven flows with high spatio-temporal resolution. L-mode turbulent structures exhibit characteristic changes in topology (tilting) and temporal and radial correlation preceding the L–H transition. Long-range toroidal flow correlations increase preceding edge-transport-barrier formation. The energy transfer from the turbulence spectrum to large-scale axisymmetric flows has been quantified in L-LCO and fast L–H transitions in several devices. After formation of a transient barrier, the increasing ion pressure gradient (via the E × B flow shear associated with diamagnetic flow) sustains fluctuation suppression and secures the transition to H-mode. Heuristic models of the L–H trigger dynamics have progressed from 0D predator–prey models to 1D extended models, including neoclassical ion flow-damping and pressure-gradient evolution. Initial results from 2D and 3D reduced fluid models have been obtained for high-collisionality regimes.

https://iopscience.iop.org/article/10.1088/1741-4326/57/2/025003/pdf

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

The dynamic features of the low-intermediate--high-(L-I-H) confinement transitions on HL-2A tokamak are presented. Here we report the discovery of two types of limit cycles (dubbed type-$Y$ and type-$J$), which show opposite temporal ordering between the radial electric field and turbulence intensity. In type-$Y$, which appears first after an L-I transition, the turbulence grows first, followed by the localized electric field. In contrast, the electric field leads type-$J$. The turbulence-induced zonal flow and pressure-gradient-induced drift play essential roles in the two types of limit cycles, respectively. The condition of transition between types-$Y$ and -$J$ is studied in terms of the normalized radial electric field. An I-H transition is demonstrated to occur only from type-$J$.

Dynamics of low-intermediate-high-confinement transitions in toroidal plasmas.

Superconductivity arises from two distinct quantum phenomena: electron pairing and long-range phase coherence. In conventional superconductors, the two quantum phenomena generally take place simultaneously, while in the underdoped high- ${T}_{c}$ cuprate superconductors, the electron pairing occurs at higher temperature than the long-range phase coherence. Recently, whether electron pairing is also prior to long-range phase coherence in single-layer FeSe film on ${\mathrm{SrTiO}}_{3}$ substrate is under debate. Here, by measuring Knight shift and nuclear spin-lattice relaxation rate, we unambiguously reveal a pseudogap behavior below ${T}_{p}\ensuremath{\sim}60\text{ }\text{ }\mathrm{K}$ in two kinds of layered FeSe-based superconductors with quasi2D nature. In the pseudogap regime, a weak diamagnetic signal and a remarkable Nernst effect are also observed, which indicates that the observed pseudogap behavior is related to superconducting fluctuations. These works confirm that strong phase fluctuation is an important character in the 2D iron-based superconductors as widely observed in high-${T}_{c}$ cuprate superconductors.

Preformed Cooper Pairs in Layered FeSe-Based Superconductors.

A new quantum state of matter in an FeSe superconductor arises from the interplay between electron correlation and spin-orbit coupling, cementing this material family as a solid platform for exploring connections between these two fundamental interactions.

https://link.aps.org/pdf/10.1103/PhysRevX.10.011034

Spin-Orbital-Intertwined Nematic State in FeSe

Recently, competing electronic instabilities, including superconductivity and density-wave-like order, have been discovered in vanadium-based kagome metals AV3Sb5 (A = K, Rb, Cs) with a nontrivial band topology. This finding stimulates wide interests to study the interplay of these competing electronic orders and possible exotic excitations in the superconducting state. Here, in order to further clarify the nature of density-wave-like transition in these kagome superconductors, we performed 51V and 133Cs nuclear magnetic resonance (NMR) measurements on the CsV3Sb5 single crystal. A first-order phase transition associated with orbital ordering is revealed by observing a sudden splitting of orbital shift in 51V NMR spectrum at the structural transition temperature Ts ~ 94 K. In contrast, the quadrupole splitting from a charge-density-wave (CDW) order on 51V NMR spectrum only appears gradually below Ts with a typical second-order transition behavior, suggesting that the CDW order is a secondary electronic order. Moreover, combined with 133Cs NMR spectrum, the present result also confirms a three-dimensional structural modulation with a 2ax2ax2c period. Above Ts, the temperature-dependent Knight shift and nuclear spin-lattice relaxation rate (1/T1) further indicate the existence of remarkable magnetic fluctuations from vanadium 3d orbitals, which are suppressed due to orbital ordering below Ts. The present results strongly support that, besides CDW order, the previously claimed density-wave-like transition also involves a dominant orbital order, suggesting a rich orbital physics in these kagome superconductors.

L. P. Nie

Papers

Charge-density-wave-driven electronic nematicity in a kagome superconductor

Dynamics of low-intermediate-high-confinement transitions in toroidal plasmas.

Preformed Cooper Pairs in Layered FeSe-Based Superconductors.

Spin-Orbital-Intertwined Nematic State in FeSe

Orbital ordering and fluctuations in a kagome superconductor CsV3Sb5