We study the dynamics of three-dimensional Fisher fronts in the presence of density fluctuations. To this end we simulate the Fisher equation subject to stochastic internal noise, and study how the front moves and roughens as a function of the number of particles in the system, $N$. Our results suggest that the macroscopic behavior of the system is driven by the microscopic dynamics at its leading edge where number fluctuations are dominated by rare events. Contrary to naive expectations, the strength of front fluctuations decays extremely slowly as $1/\log N$, inducing large-scale fluctuations which we find belong to the one-dimensional Kardar-Parisi-Zhang universality class of kinetically rough interfaces. Hence, we find that there is no weak-noise regime for Fisher fronts, even for realistic numbers of particles in macroscopic systems.

An extensively studied mechanism to create particle-antiparticle asymmetries is the out-of-equilibrium and $CP$ violating decay of a heavy particle. We, instead, examine how asymmetries can arise purely from $2\to 2$ annihilations rather than from the usual $1\to 2$ decays and inverse decays. We review the general conditions on the reaction rates that arise from $S$-matrix unitarity and $CPT$ invariance, and show how these are implemented in the context of a simple toy model. We formulate the Boltzmann equations for this model, and present an example solution.

Using a first-principles calculation within quantum chromodynamics, we are able to determine a pattern of $\text{strangeness}=1$ resonances that appear as complex singularities within coupled $\pi K\text{−}\eta K$ scattering amplitudes. We make use of numerical computation in the lattice discretized approach to the quantum field theory with light quark masses corresponding to $m_{\pi }\sim 400\mathrm{MeV}$ and at a single lattice spacing. The energy dependence of scattering amplitudes is extracted through their relationship to the discrete spectrum in a finite volume, which we map out in unprecedented detail.

We present variational Monte Carlo calculations of the neutron matter equation of state using chiral nuclear forces. The ground-state wave function of neutron matter, containing nonperturbative many-body correlations, is obtained from auxiliary-field quantum Monte Carlo simulations of up to about 340 neutrons interacting on a $10^3$ discretized lattice. The evolution Hamiltonian is chosen to be attractive and spin independent in order to avoid the fermion sign problem and is constructed to best reproduce broad features of the chiral nuclear force. This is facilitated by choosing a lattice spacing of 1.5 fm, corresponding to a momentum-space cutoff of $\mathrm{\Lambda }=414\mathrm{MeV}/c$, a resolution scale at which strongly repulsive features of nuclear two-body forces are suppressed. Differences between the evolution potential and the full chiral nuclear interaction (Entem and Machleidt $\mathrm{\Lambda }=414\mathrm{MeV}$ [L. Coraggio et al., Phys. Rev. C 87, 014322 (2013)]) are then treated perturbatively. Our results for the equation of state are compared to previous quantum Monte Carlo simulations that employed chiral two-body forces at next-to-next-to-leading order (N2LO). In addition, we include the effects of three-body forces at N2LO, which provide important repulsion at densities higher than $0.02{\mathrm{fm}}^{-3}$ as well as two-body forces at next-to-next-to-next-to-leading order.

We trap cold, ground state argon atoms in a deep optical dipole trap produced by a buildup cavity. The atoms, which are a general source for the sympathetic cooling of molecules, are loaded in the trap by quenching them from a cloud of laser-cooled metastable argon atoms. Although the ground state atoms cannot be directly probed, we detect them by observing the collisional loss of cotrapped metastable argon atoms and determine an elastic cross section. Using a type of parametric loss spectroscopy we also determine the polarizability of the metastable $4s[3/2{]}_2$ state to be $(7.3\pm 1.1)\times {10}^{-39}\mathrm{C}{\mathrm{m}}^2/\mathrm{V}$. Finally, Penning and associative losses of metastable atoms in the absence of light assisted collisions, are determined to be $(3.3\pm 0.8)\times {10}^{-10}{\mathrm{cm}}^3{\mathrm{s}}^{-1}$.

It has long been known that lasers can interact with relativistic electrons in magnetic undulators to imprint sinusoidal modulations that can be used to slice electrons into microbunches equally separated at the laser wavelength. Here we report on the first direct measurement of laser-induced microbunching of a relativistic electron beam with femtosecond resolution in the time domain. Using a modified zero-phasing technique to map the electron beam’s temporal structures into the energy space, we show that this method can be used to directly quantify the time and spectral content of coherent current modulations imprinted on the beam for harmonic and multicolor lasing applications in accelerator-based light sources.

We study fermionic superfluidity in strongly anisotropic optical lattices with attractive interactions utilizing the cluster dynamical mean-field theory method, and focusing in particular on the role of nonlocal quantum fluctuations. We show that nonlocal quantum fluctuations impact the BCS superfluid transition dramatically. Moreover, we show that exotic superfluid states with a delicate order parameter structure, such as the Fulde-Ferrell-Larkin-Ovchinnikov phase driven by spin population imbalance, can emerge even in the presence of such strong fluctuations.

In layered superconductors the order parameter may be modulated within the unit cell, leading to nontrivial modifications of the vortex core if the interlayer coherence length ${\xi }_c(T)$ is comparable to the interlayer spacing. In the iron pnictide SmFeAs(O,F) ($T_c\approx 50\mathrm{K}$) this occurs below a crossover temperature $T^{\star }\approx 41\mathrm{K}$, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around $T^{\star }$, hybrid vortices between these two characteristics appear. Only in this region around $T^{\star }$ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the $c$-axis critical current $j_c(H)$ periodic in $1/\sqrt{H}$ due to a delicate balance of intervortex forces and interaction with the layered potential. $j_c(H)$ shows pronounced maxima when a hexagonal vortex lattice is commensurate with the underlying crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy (${\gamma }_{\xi }\approx 3–5$).

We investigate by angle-resolved photoemission the electronic structure of in situ grown tetragonal CuO, a synthetic quasi-two-dimensional edge-sharing cuprate. We show that, in spite of the very different nature of the copper oxide layers, with twice as many Cu in the CuO layers of tetragonal CuO as compared to the ${\mathrm{CuO}}_2$ layers of the high-$T_c$ cuprates, the low-energy electronic excitations are surprisingly similar, with a Zhang-Rice singlet dispersing on weakly coupled cupratelike sublattices. This system should thus be considered as a member of the high-$T_c$ cuprate family, with, however, interesting differences due to the intralayer coupling between the cupratelike sublattices.

Signal propagation in the nonequilibrium evolution after quantum quenches has recently attracted much experimental and theoretical interest. A key question arising in this context is what principles, and which of the properties of the quench, determine the characteristic propagation velocity. Here we investigate such issues for a class of quench protocols in one of the central paradigms of interacting many-particle quantum systems, the spin-$1/2$ Heisenberg $XXZ$ chain. We consider quenches from a variety of initial thermal density matrices to the same final Hamiltonian using matrix product state methods. The spreading velocities are observed to vary substantially with the initial density matrix. However, we achieve a striking data collapse when the spreading velocity is considered to be a function of the excess energy. Using the fact that the $XXZ$ chain is integrable, we present an explanation of the observed velocities in terms of “excitations” in an appropriately defined generalized Gibbs ensemble.

One necessary criterion for the thermalization of a nonequilibrium quantum many-particle system is ergodicity. It is, however, not sufficient in cases where the asymptotic long-time state lies in a symmetry-broken phase but the initial state of nonequilibrium time evolution is fully symmetric with respect to this symmetry. In equilibrium, one particular symmetry-broken state is chosen as a result of an infinitesimal symmetry-breaking perturbation. From a dynamical point of view the question is: Can such an infinitesimal perturbation be sufficient for the system to establish a nonvanishing order during quantum real-time evolution? We study this question analytically for a minimal model system that can be associated with symmetry breaking, the ferromagnetic Kondo model. We show that after a quantum quench from a completely symmetric state the system is able to break its symmetry dynamically and discuss how these features can be observed experimentally.

The density of states $\varrho (E)$ of graphene is investigated numerically and within the self-consistent $T$-matrix approximation in the presence of vacancies within the tight binding model. The focus is on compensated disorder, where the concentration of vacancies $n_A$ and $n_B$ in both sublattices is the same. Formally, this model belongs to the chiral symmetry class BDI. The nonlinear sigma model predicts for BDI a Gade-type singularity $\varrho (E)\sim |E{|}^{-1}\exp [-|\log (E){|}^{-1/x}]$. Our numerical data are comparable to this result in a preasymptotic regime that gives way, however, at even lower energies to $\varrho (E)\sim E^{-1}|\log (E){|}^{-\tilde{x}}$, $1\le \tilde{x}<2$. We take this finding as evidence that, similar to the case of dirty $d$-wave superconductors, generic bipartite random hopping models may also exhibit unconventional (strong-coupling) fixed points for certain kinds of randomly placed scatterers if these are strong enough. Our research suggests that graphene with (effective) vacancy disorder is a physical representative of such systems.

We study quantum interference effects in a two-dimensional chiral metal (bipartite lattice) with vacancies. We demonstrate that randomly distributed vacancies constitute a peculiar type of chiral disorder leading to strong modifications of critical properties at zero energy as compared to those of conventional chiral metals. In particular, the average density of states diverges as $\rho \propto E^{-1}|\ln E{|}^{-3/2}$ and the correlation length $L_c\propto \sqrt{|\ln E|}$ in the limit $E\to 0$. When the average density of vacancies is different in the two sublattices, a finite concentration of zero modes emerges and a gap in the quasiclassical density of states opens around zero energy. Interference effects smear this gap, resulting in exponentially small tails at low energies.