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.

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic (AFM) and in-plane ferromagnetic (FM) wavevectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using 75 As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole-and electron-doped BaFe2As2 families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of Tc and the shape of the superconducting dome in these and other iron-pnictide families.PACS numbers: 74.70. Xa, 75.40.Gb The role of magnetic fluctuations in iron pnictide superconductors (SCs) has been extensively studied since their discovery. As the parent materials have antiferromagnetic (AFM) ground states, attention has been understandably focused on stripe-type AFM fluctuations, which are widely believed to give rise to the Cooper pairing in these systems. In the standard picture, carrier doping or pressure application results in suppression of the AFM order and the emergence of a SC state, with T c ranging from a few K to 56 K [1]. However, as of yet, there is no accepted theory for T c in these materials with which to explain the large variability in maximum T c between different iron arsenide families and the different shapes of the SC dome with electron and hole doping.Recent nuclear magnetic resonance (NMR) measurements on non-SC, paramagnetic (PM) SrCo 2 As 2 , the x = 1 member of the electron-doped Sr(Fe 1−x Co x ) 2 As 2 family, revealed strong ferromagnetic (FM) spin fluctuations in the Co layer coexisting with stripe-type AFM fluctuations [2,3]. Since stripe-type AFM fluctuations are a key ingredient to SC in the iron pnictides, this result suggested that FM fluctuations might compete with the stripe-type AFM fluctuations, suppressing SC in SrCo 2 As 2 . FM correlations were also observed in isostructural BaCo 2 As 2 [2, 4]. Similarly, CaCo 1.86 As 2 has an A-type AFM ground state with inplane FM order [5]. These results also raise the question of whether similar FM correlations exist generally in the SC A(Fe 1−x Co x ) 2 As 2 compounds, not just at the x = 1 edges of their phase diagrams.According to density functional theory calculations [6-10], the generalized static magnetic susceptibility χ(q) is enhanced at both the FM and stripe-type AFM wavevectors in all the iron-based SCs and parent compounds. Experimentally, the uniform χ(q = 0) of the parent compounds is enhanced by a factor of order five over band structure values, which is consistent with FM correlations [1]. Nevertheless, FM fluctuations have not been investigated microscopically, perhaps because low-energy FM fluctuations are difficult to observe via inelastic neutron scattering (INS). The peak in the inelastic structure factor at q = 0 coincides with the elastic Bragg diffractio...

We further analyze the holographic dipole-dipole scattering amplitude developed in G. Basar et al., Phys. Rev. D85, 105005 (2012) and A. Stoffers, I. Zahed, arXiv:1205.3223 [hep-ph]. Gribov diffusion at strong coupling yields the scattering amplitude in a confining background. We compare the holographic result for the differential cross section to diffractive proton-proton scattering data.

No abstract

No abstract

We present the results of 75 As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in KFe2As2 under pressure (p). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time (T1) and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature (T * ). T * is found to increase monotonically with pressure, consistent with increasing hybridization between localized 3d orbital-derived bands with the itinerant electron bands. No anomaly in T * is seen at the critical pressure pc = 1.8 GPa where a change of slope of the superconducting (SC) transition temperature Tc(p) has been observed. In contrast, Tc(p) seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR 1/T1 data, although such a correlation cannot be seen in the replacement effects of A in the AFe2As2 (A= K, Rb, Cs) family. In the superconducting state, two T1 components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short T1s indicates nearly gapless state below Tc. On the other hand, the temperature dependence of the long component 1/T1L implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in KFe2As2 under pressure.Recent NMR measurements have pointed out a possible d-electron heavy fermion behavior in the AFe 2 As 2