Symmetries of Thirring Models on 3D Lattices

We review some recent developments about strongly interacting relativistic Fermi theories in three spacetime dimensions. These models realize the asymptotic safety scenario and are used to describe the low-energy properties of Dirac materials in condensed matter physics. We begin with a general discussion of the symmetries of multi-flavor Fermi systems in arbitrary dimensions. Then we review known results about the critical flavor number $N_\mathrm{crit}$ of Thirring models in three dimensions. Only models with flavor number below $N_\mathrm{crit}$ show a phase transition from a symmetry-broken strong-coupling phase to a symmetric weak-coupling phase. Recent simulations with chiral fermions show that $N_\mathrm{crit}$ is smaller than previously extracted with various non-perturbative methods. Our simulations with chiral SLAC fermions reveal that for four-component flavors $N_\mathrm{crit}=0.80(4)$. This means that all reducible Thirring models with $\Nr=1,2,3,\dots$ show no phase transition with order parameter. Instead we discover footprints of phase transitions without order parameter. These new transitions are probably smooth and could be used to relate the lattice Thirring models to Thirring models in the continuum. For a single irreducible flavor, we provide previously unpublished values for the critical couplings and critical exponents.

Stabilizing nanolasers via polarization lifetime tuning

We investigate the emission dynamics of mutually coupled nanolasers and predict ways to optimize their stability, i.e., maximize their locking range. We find that tuning the cavity lifetime to the same order of magnitude as the dephasing time of the microscopic polarization yields optimal operation conditions, which allow for wider tuning ranges than usually observed in conventional semiconductor lasers. The lasers are modeled by Maxwell–Bloch type class-C equations. For our analysis, we analytically determine the steady state solutions, analyze the symmetries of the system and numerically characterize the emission dynamics via the underlying bifurcation structure. The polarization lifetime is found to be a crucial parameter, which impacts the observed dynamics in the parameter space spanned by frequency detuning, coupling strength and coupling phase.

Out of rhythm: Compromised precision of theta-gamma coupling impairs associative memory in old age

Episodic memory declines with advancing adult age. This decline is particularly pronounced when associations between items and their contexts need to be formed. According to theories of neural communication, the precise coupling of gamma power to the phase of the theta rhythm supports associative memory formation. To investigate whether age differences in associative memory are related to compromised theta-gamma coupling, we took electroencephalographic (EEG) recordings during the encoding phase of an item-context association task. Fifty-eight younger and 55 older adults studied pictures of objects superimposed on background scenes. In a recognition test, objects were presented on old or new backgrounds, and participants responded if they had seen (1) the object and (2) the object-scene pair. Theta-gamma coupling supported pair memory formation in both age groups. Whereas pair memory was associated with coupling closer to the peak of the theta rhythm, item-only memory was associated with a deviation in phase angle relative to pair memory. Furthermore, a stable relation between coupling phase and pair memory performance demonstrated that coupling closer to the peak is beneficial for associative memory. Critically, older adults' lower pair memory was accompanied by a shift in coupling phase relative to younger adults. In concert, the present results are consistent with the hypothesis that decrements in the precision with which gamma power is coupled to the theta phase underlie the decline of associative memory in normal cognitive aging.