Electron-momentum dependence of electron-phonon coupling underlies dramatic phonon renormalization in YNi2B2C

Electron-phonon coupling, i.e., the scattering of lattice vibrations by electrons and vice versa, is ubiquitous in solids and can lead to emergent ground states such as superconductivity and charge-density wave order. A broad spectral phonon line shape is often interpreted as a marker of strong electron-phonon coupling associated with Fermi surface nesting, i.e., parallel sections of the Fermi surface connected by the phonon momentum. Alternatively broad phonons are known to arise from strong atomic lattice anharmonicity. Here, we show that strong phonon broadening can occur in the absence of both Fermi surface nesting and lattice anharmonicity, if electron-phonon coupling is strongly enhanced for specific values of electron-momentum, k. We use inelastic neutron scattering, soft x-ray angle-resolved photoemission spectroscopy measurements and ab-initio lattice dynamical and electronic band structure calculations to demonstrate this scenario in the highly anisotropic tetragonal electron-phonon superconductor YNi2B2C. This new scenario likely applies to a wide range of compounds.

Fermion and gluon spectral functions far from equilibrium

Motivated by the quark-gluon plasma, we develop a simulation method to obtain the spectral function of (Wilson) fermions non-perturbatively in a non-Abelian gauge theory with large gluon occupation numbers [1]. We apply our method to a non-Abelian plasma close to its non-thermal fixed point, i.e., in a far-from-equilibrium self-similar regime, and find mostly very good agreement with perturbative hard loop (HTL) calculations. For the first time, we extract the full momentum dependence of the damping rate of fermionic collective excitations and compare our results to recent non-perturbative extractions of gluonic spectral functions in two and three spatial dimensions [2, 3].

Hadron wave functions in high-energy scattering, form factors and strong decays: The effects of the Lorentz contracted form

In this paper, we study the class of the processes, where dynamics depends essentially on the properties of the hadron wave functions involved in the reactions. In this case, the momentum dependence of the form of the wave functions, imposed by the Lorentz invariance and in particular by the Lorentz contraction, can be tested in the experiment and may strongly influence the resulting cross-sections. One example of such observables is given by the hadron form factors in the case when the large [Formula: see text] behavior is mostly frozen, while the Lorentz contraction of the hadron wave functions is taken into account. Another example, considered earlier, is the strong hadron decay with high-energy emission. In this paper, we study the role of the Lorentz contraction in the high-energy hadron–hadron scattering process at large momentum transfer. For the [Formula: see text] and [Formula: see text] scattering at large [Formula: see text], it is shown that at small [Formula: see text], the picture of two exponential slopes in the differential cross-section, explained previously by the author, remains stable, while the backward scattering cross-section is strongly increased by the Lorentz contraction.

Dynamics of string-like states of a hole in a quantum antiferromagnet: A diagrammatic Monte Carlo simulation

The dynamics of a hole motion in a quantum antiferromagnet has been studied in the past three decade because of its relationship to models related to superconductivity in cuprates. The same problem has received significant attention because of its connection to very recent experiments of the dynamics of ultra-cold atoms in optical lattices where models of strongly correlated electrons can be simulated. In this paper we apply the diagrammatic Monte Carlo method to calculate the single-hole Green's function in the t-J model, where the $J$ term is linearized, in a wide range of imaginary-time with the aim to examine the polaron formation and in particular the details of the contribution of the so-called {\it string excitations} found in such recent experiments. We calculate the single-hole spectral function by analytic continuation from imaginary to real time and study the various aspects that constitute the string picture, such as, the energy-momentum dependence of the main quasiparticle peak and its residue, the {\it internal excitations} of the string which appear as multiple peaks in the spectral function as well as their momentum dependence. We find that the earlier analysis of the spectral function based on a mobile-hole connected with a string of overturn spins and the contribution of the internal string excitations as obtained from the non-crossing approximation is accurate.

Bulk observables at 5.02 TeV using quasiparticle anisotropic hydrodynamics

We compare predictions of 3+1D quasiparticle anisotropic hydrodynamics (aHydroQP) for a large set of bulk observables with experimental data collected in 5.02 TeV Pb–Pb collisions. We make predictions for identified hadron spectra, identified hadron average transverse momentum, charged particle multiplicity as a function of pseudorapidity, the kaon-to-pion ($$K/\pi $$ K / π ) and proton-to-pion ($$p/\pi $$ p / π ) ratios, identified particle and charged particle elliptic flow, and HBT radii. We compare to data collected by the ALICE collaboration in 5.02 TeV Pb–Pb collisions. We find that, based on available data, these bulk observables are well described by aHydroQP with an assumed initial central temperature of $$T_0=630$$ T 0 = 630 MeV at $$\tau _0 = 0.25$$ τ 0 = 0.25 fm/c and a constant specific shear viscosity of $$\eta /s=0.159$$ η / s = 0.159 , which corresponds to a peak specific bulk viscosity of $$\zeta /s = 0.048$$ ζ / s = 0.048 . In particular, we find that the momentum dependence of the kaon-to-pion ($$K/\pi $$ K / π ) and proton-to-pion ($$p/\pi $$ p / π ) ratios reported recently by the ALICE collaboration are extremely well described by aHydroQP in the 0–5% centrality class.

Vector-meson production and vector meson dominance

We consider the fidelity of the vector meson dominance (VMD) assumption as an instrument for relating the electromagnetic vector-meson production reaction $$e + p \rightarrow e^\prime + V + p$$ e + p → e ′ + V + p to the purely hadronic process $$V + p \rightarrow V+p$$ V + p → V + p . Analyses of the photon vacuum polarisation and the photon-quark vertex reveal that such a VMD Ansatz might be reasonable for light vector-mesons. However, when the vector-mesons are described by momentum-dependent bound-state amplitudes, VMD fails for heavy vector-mesons: it cannot be used reliably to estimate either a photon-to-vector-meson transition strength or the momentum dependence of those integrands that would arise in calculations of the different reaction amplitudes. Consequently, for processes involving heavy mesons, the veracity of both cross-section estimates and conclusions based on the VMD assumption should be reviewed, e.g., those relating to hidden-charm pentaquark production and the origin of the proton mass.

The Role of Orbital Nesting in the Superconductivity of Iron-Based Superconductors

We analyze the magnetic excitations and the spin-mediated superconductivity in iron-based superconductors within a low energy model that operates in the band basis, but fully incorporates the orbital character of the spin excitations. We show how the orbital selectivity, encoded in our low energy description, simplifies substantially the analysis and allows for analytical treatments, while retaining all the main features of both spin excitations and gap functions computed using multiorbital models. Importantly, our analysis unveils the orbital matching between the hole and electron pockets as the key parameter to determine the momentum dependence and the hierarchy of the superconducting gaps, instead of the Fermi surface matching, as in the common nesting scenario.

Two-loop $$ \mathcal{O} $$((αt + αλ + ακ)2) corrections to the Higgs boson masses in the CP-violating NMSSM

We present our computation of the $$ \mathcal{O} $$ O ((αt + αλ + ακ)2) two-loop corrections to the Higgs boson masses of the CP-violating Next-to-Minimal Supersymmetric Standard Model (NMSSM) using the Feynman-diagrammatic approach in the gaugeless limit at vanishing external momentum. We choose a mixed $$ \overline{\mathrm{DR}} $$ DR ¯ -on-shell (OS) renormalisation scheme for the Higgs sector and apply both $$ \overline{\mathrm{DR}} $$ DR ¯ and OS renormalisation in the top/stop sector. For the treatment of the infrared divergences we apply and compare three different regularisation methods: the introduction of a regulator mass, the application of a small momentum expansion, and the inclusion of the full momentum dependence. Our new corrections have been implemented in the Fortran code NMSSMCALC that computes the Higgs mass spectrum of the CP-conserving and CP-violating NMSSM as well as the Higgs boson decays including the state-of-the-art higher-order corrections. Our numerical analysis shows that the newly computed corrections increase with rising λ and κ, remaining overall below about 3% compared to our previously computed $$ \mathcal{O} $$ O (αt(αt + αs)) corrections, in the region compatible with perturbativity below the GUT scale. The renormalisation scheme and scale dependence is of typical two-loop order. The impact of the CP-violating phases in the new corrections is small. We furthermore show that the Goldstone Boson Catastrophe due to the infrared divergences can be treated in a numerically efficient way by introducing a regulator mass that approximates the momentum-dependent results best for squared mass values in the permille range of the squared renormalisation scale. Our results mark another step forward in the program of increasing the precision in the NMSSM Higgs boson observables.