Eric Nilsson

Pauli blocking in Fermi liquids imposes strong phase-space constraints on quasiparticle lifetimes, leading to a well-known quadratic-in-temperature decay rate of quasiparticle modes at low temperatures. In two-dimensional systems, however, even longer-lived modes are predicted (dubbed “odd-parity” modes) that involve a collective deformation of the Fermi distribution. Here, we present an efficient method to evaluate the full spectrum of relaxational eigenmodes of a Fermi liquid within kinetic theory. We employ this method to study the experimentally relevant case of a Fermi liquid with screened Coulomb interactions and map out the decay rates of quasiparticle modes beyond the asymptotic low-temperature limit up to the Fermi temperature, thus covering the entire temperature range of typical experiments. We confirm the existence of anomalously long-lived odd-parity modes and provide a comprehensive classification and detailed analysis of the relaxation spectrum. In particular, we find that (i) the odd-parity effect in the decay rates extends to temperatures as large as T = 0.15 T_F, (ii) there is only a small number of long-lived oddparity modes, with an infinite number of remaining modes that show standard Fermi-liquid scaling, and (iii) the ratio between the odd- and even-parity lifetimes is tunable with the Coulomb interaction strength, in addition to temperature, which reflects a difference in the microscopic relaxation mechanism of the modes. Our findings provide a comprehensive description of the nonequilibrium relaxation behavior of twodimensional electron gases and bridge a significant gap in our understanding of these systems.

We study DC and AC thermoelectric and magnetotransport in 2D quantum critical theories with strong translational symmetry breaking due to a varying chemical potential lattice with zero average \barμ=0. The combination of quantum criticality and the absence of the average natural scale implies that such systems have idiosyncratic signatures that may apply more generally when the variance in the lattice potential far exceeds the average or for strong translational symmetry breaking in general. We model such theories holographically through near-extremal AdS black holes. We find that these systems (a) become \emphbetter conductors. In a 2D lattice, this can be explained by currents flowing around obstacles; (b) exhibit bad-metal electrical transport with Drude-like thermal transport, though it is not Drude, and, notably, (c) display an approximately B-linear longitudinal magnetoresistance at large fields, similar to Effective Medium Theory. We comment on how these results may apply when \barμ≠0.

Today’s novel materials can display many interesting phases. Two-dimensional materials with strong electron-electron interactions allow the electrons to enter a hydrodynamic regime at intermediate temperatures. This thesis presents a method for an exact description of the quasiparticle distribution in terms of kinetic theory, valid beyond the asymptotic low-temperature regime used in perturbative approaches. This is used to obtain of the full mode spectrum of the interacting electron gas. At low temperatures, the existence of long-lived modes of odd parity hint at the existence of a new transport regime in between the limits of ballistic and hydrodynamic flow. The method is also used to determine the shear viscosity of the electron liquid beyond the low temperature limit. If the coupling becomes strong enough, it invalidates the quasiparticle picture, which undermines many established methods within many-body physics. This happens in the strange metal phase of high-temperature superconductors, where the holographic duality – providing a description of a strongly coupled theory in terms of a weakly coupled gravitational theory – serves as one of the few ways to study the strongly coupled physics. Recent experiments on strange metals show an incoherent plasmon at small momenta, in qualitative agreement with previous holographic models of bulk plasmons. However, the relevant experiments also couple to collective surface excitations, which hitherto has not been considered. This thesis also presents a model for surface plasmon polaritons using the holographic duality, improving the theoretical description of plasmons in strongly correlated materials.