Mott polaritons in cavity-coupled quantum materials

We show that strong electron-electron interactions in cavity-coupled quantum materials can enable collectively enhanced light-matter interactions with ultrastrong effective coupling strengths. As a paradigmatic example we consider a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant electron-cavity interactions result in the formation of a quasi-continuum of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and scales with $g_{\text{eff}}\propto\sqrt{2L}$, where $L$ is the number of electronic sites, and the maximal achievable value for $g_{\text{eff}}$ is determined by the volume of the unit cell of the crystal. We find that $g_{\text{eff}}$ for existing quantum materials can by far exceed the width of the first excited Hubbard band. This effect can be experimentally observed via measurements of the optical conductivity and does not require ultra-strong coupling on the single-electron level. Quantum correlations in the electronic ground state as well as the microscopic nature of the light-matter interaction enhance the collective light-matter interaction compared to an ensemble of independent two-level atoms interacting with a cavity mode.Comment: 11 pages, 4 figures. arXiv admin note: text overlap with arXiv:1806.0675

Anomalous spin-charge separation in a driven Hubbard system

Spin-charge separation (SCS) is a striking manifestation of strong correlations in low-dimensional quantum systems, whereby a fermion splits into separate spin and charge excitations that travel at different speeds. Here, we demonstrate that periodic driving enables control over SCS in a Hubbard system near half-filling. In one dimension, we predict analytically an exotic regime where charge travels slower than spin and can even become 'frozen', in agreement with numerical calculations. In two dimensions, the driving slows both charge and spin, and leads to complex interferences between single-particle and pair-hopping processes.Comment: arXiv admin note: text overlap with arXiv:2002.0231

Placing the poor while keeping the rich in their place

A central objective of modern US housing policy is deconcentrating poverty through "housing mobility programs" that move poor families into middle class neighborhoods. Pursuing these policies too aggressively risks inducing middle class flight, but being too cautious squanders the opportunity to help more poor families. This paper presents a stylized dynamicoptimization model that captures this tension. With base-caseparameter values, cost considerations limit mobility programs before flight becomes excessive. However, for modest departures reflecting stronger flight tendencies and/or weaker destination neighborhoods, other outcomes emerge. In particular, we find state-dependence and multiple equilibria, including both de-populated and oversized outcomes. For certain sets of parameters there exists a Skiba point that separates initial conditions for which the optimal strategy leads to substantial flight and depopulation from those for which the optimal strategy retains or even expands the middle class population. These results suggest the value of estimating middle-class neighborhoods' "carrying capacity" for absorbing mobility program placements and further modeling of dynamic response.housing policy, multiple equilibria, negative externality, optimal control, segregation, separation, Skiba point

Ballistic transport in graphene antidot lattices

Graphene samples can have a very high carrier mobility if influences from the substrate and the environment are minimized. Embedding a graphene sheet into a heterostructure with hexagonal boron nitride (hBN) on both sides was shown to be a particularly efficient way of achieving a high bulk mobility. Nanopatterning graphene can add extra damage and drastically reduce sample mobility by edge disorder. Preparing etched graphene nanostructures on top of an hBN substrate instead of SiO2 is no remedy, as transport characteristics are still dominated by edge roughness. Here we show that etching fully encapsulated graphene on the nanoscale is more gentle and the high mobility can be preserved. To this end, we prepared graphene antidot lattices where we observe magnetotransport features stemming from ballistic transport. Due to the short lattice period in our samples we can also explore the boundary between the classical and the quantum transport regime

Thermal Cyclotron Reprocessing of Gammy-Ray Bursts - Theory and Model Spectra

We examine the generation of infrared, optical, and ultraviolet flashes from single, magnetized neutron stars are experiencing of gamma-ray burst. Cyclotron reprocessing of energetic gamma-ray burst photons in the neutron star magnetosphere is assumed to be the underlying mechanism reponsible for the display at longer wavelengths, and thermal equilibrium is assumed in order to calculate electron distribution function. It is shown that thesea good approximations for a wide range of conditions expected in neutron star magnetospheres. The thermal cycoltron model proves capable fo generating ptical outbursts similar to bright historical events. althrough opitcal transients most likely would be much fainter. For a wide range of conditions the model predicts bright, nondelayed flashes, extending in some cases even beyond the ultraviolet. Since the emission at long wavelengths is correlated with the gammar-rays down to time scales small compared with the burst duration, time-averaged spectra are calculated corresponding to the time-averaged gamma-ray burst spectrum. For flashes that do not exhibit a spectral turnover in the optical region, Lopt α Bas with α ~ 3/4, so that optical transients could be used to constrain the magnetic field strength and distance of gamma-ray burst sources. The long-wavelength fluxes for the recently discovered soft repearing source SGR 1806-20 are also estimated