We present the first application of the recently developed Basis Light-Front Quantization (BLFQ) method to self-bound systems in quantum field theory, using the positronium system as a test case. Within the BLFQ framework, we develop a two-body effective interaction, operating only in the lowest Fock sector, that implements photon exchange, neglecting fermion self-energy effects. We then solve for the mass spectrum of this interaction at the unphysical coupling α = 0.3. The resulting spectrum is in good agreement with the expected Bohr spectrum of non-relativistic quantum mechanics. We examine in detail the dependence of the results on the regulators of the theory.

We present77 Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, Ts. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe. PACS numbers:Much attention in recent research on iron-based superconductivity (SC) has been paid to understanding the nature of the electronic nematic phase, which breaks rotational symmetry while preserving time-reversal symmetry [1,2]. In the archetypical "122" compounds AFe 2 As 2 (A=Ca, Sr, Ba) [3,4], the nematic phase is closely tied to the stripe-type antiferromagnetic (AFM) phase in the phase diagram, suggesting a magnetic origin for the nematic state. Among the Fe-based SCs, FeSe is known to be an exception. At ambient pressure, FeSe undergoes a transition to the nematic phase at a bulk structural phase transition temperature T s ∼ 90 K, as well as to SC below T c ∼ 8 K, but has no stripe-type AFM ordered phase. Under pressure (p), T s is suppressed to ∼20 K at p ∼1.7 GPa [5][6][7] and an AFM ordered state emerges above ∼0.8 GPa [8][9][10][11]. In addition, T c is enhanced from 8 K at ambient pressure to ∼37 K at p ∼ 6 GPa [12]. The decrease of T s (p) and increase of T N (p) under pressure suggests competition between nematic and magnetic orders. Furthermore, NMR measurements [13,14] showed Korringa behavior above T s , consistent with an uncorrelated Fermi liquid, while AFM spin fluctuations (SFs) were found to be strongly enhanced only below T s . These observations suggested that SFs are not the driver for nematic order and therefore pointed to an orbital mechanism for the nematicity [14]. An orbital mechanism was also suggested by Raman spectroscopy [15].In contrast, several recent studies have suggested cooperation between nematicity and magnetism in FeSe. High-energy x-ray diffraction measurements [7] found that the orthorhombic distortion is enhanced in the magnetic state at p = 1.5 GPa. Furthermore, above 1.7 GPa T s (p) and T N (p) were found to coincide as a simultaneous first-order magneto-structural transition. These observations are consistent with a spin-driven mechanism for nematic order in FeSe. Similarly, inelastic neutron scattering (INS) measurements at ambient pressure [16,17] showed that commensurate stripe-type AFM SFs are in fact present well above T s , which could possibly drive the nematic transition. These SFs were not seen by NMR [13,14] due to a spin gap above ∼ 90 K. In addition, 77 Se-NMR data under pressure [18] revealed a first-order transition to a stripe-type magnetic ordered state, and suggested a magnetic driven nematicity. Therefore, the o...

We present 77 Se-NMR measurements on FeSe1−xSx samples with sulfur content x = 0, 9, 15 and 29%. Twinned nematic domains are observed in the NMR spectrum for all samples except x = 29%. The NMR spin-lattice relaxation rate shows that antiferromagnetic (AFM) fluctuations are initially enhanced between x = 0% and x = 9%, but are strongly suppressed for higher x values. The observed behavior of the AFM fluctuations parallels the superconducting transition temperature Tc in these materials, providing strong evidence for the primary importance of AFM fluctuations for superconductivity, despite the presence of nematic quantum criticality in the FeSe1−xSx system.