Magnetic-field-induced electronic instability of Weyl-like fermions in compressed black phosphorus

Revealing the role of Coulomb interaction in topological semimetals with Dirac/Weyl-like band dispersion shapes a new frontier in condensed matter physics.Topological node-line semimetals(TNLSMs),anticipated as a fertile ground for exploring electronic correlation effects due to the anisotropy associated with their node-line structure,have recently attracted considerable attention.In this study,we report an experimental observation for correlation effects in TNLSMs realized by black phosphorus(BP)under hydrostatic pressure.By performing a combination of nuclear magnetic resonance measurements and band calculations on compressed BP,a magnetic-field-induced electronic instability of Weyl-like fermions is identified under an external magnetic field parallel to the so-called nodal ring in the reciprocal space.Anomalous spin fluctuations serving as the fingerprint of electronic instability are observed at low temperatures,and they are observed to maximize at approximately 1.0 GPa.This study presents compressed BP as a realistic material platform for exploring the rich physics in strongly coupled Weyl-like fermions.

Fluctuating magnetic droplets immersed in a sea of quantum spin liquid

The search of quantum spin liquid(QSL),an exotic magnetic state with strongly fluctuating and highly entangled spins down to zero temperature,is a main theme in current condensed matter physics.However,there is no smoking gun evidence for deconfined spinons in any QSL candidate so far.The disorders and competing exchange interactions may prevent the formation of an ideal QSL state on frustrated spin lattices.Here we report comprehensive and systematic measurements of the magnetic susceptibility,ultralow-temperature specific heat,muon spin relaxation(μSR),nuclear magnetic resonance(NMR),and thermal conductivity for NaYbSe2 single crystals,in which Yb3+ions with effective spin-1/2 form a perfect triangular lattice.All these complementary techniques find no evidence of long-range magnetic order down to their respective base temperatures.Instead,specific heat,μSR,and NMR measurements suggest the coexistence of quasi-static and dynamic spins in NaYbSe2.The scattering from these quasi-static spins may cause the absence of magnetic thermal conductivity.Thus,we propose a scenario of fluctuating ferrimagnetic droplets immersed in a sea of QSL.This may be quite common on the way pursuing an ideal QSL,and provides a brand new platform to study how a QSL state survives impurities and coexists with other magnetically ordered states.

Orbital ordering and fluctuations in a kagome superconductor CsV_(3)Sb_(5)

Recently,competing electronic instabilities,including superconductivity and density-wave-like order,have been discovered in vanadium-based kagome metals AV_(3)Sb_(5)(A=K,Rb,Cs)with a nontrivial band topology.This finding stimulates considerable interest to study the interplay of these competing electronic orders and possible exotic excitations in the superconducting state.Here,we performed51V and133Cs nuclear magnetic resonance(NMR)measurements on a CsV_(3)Sb_(5)single crystal to clarify the nature of density-wave-like transition in these kagome superconductors.A first-order structural transition is unambiguously revealed below T_(s)~94 K by observing the sudden splitting of Knight shift in^(51)V NMR spectrum.Moreover,combined with^(133)Cs NMR spectrum,the present result confirms a three-dimensional structural modulation.By further analyzing the anisotropy of Knight shift and 1/T_(1)T at^(51)V nuclei,we proposed that the orbital order is the primary electronic order induced by the firstorder structural transition,which is supported by further analysis on electric field gradient at^(51)V nuclei.In addition,the evidence for possible orbital fluctuations is also revealed above T_(s).The present work sheds light on a rich orbital physics in kagome superconductors AV_(3)Sb_(5).