In this thesis, we will study aspects of two phases close to criticality arising in solid state systems with strong interactions between electrons. In the first part, we study finite temperature transport in a non-Fermi-liquid phase arising from a nodal semimetal with long-range interactions – the so-called Luttinger-Abrikosov-Beneslavskii phase. We are particularly interested in calculating the finite temperature shear viscosity of the phase and find that it is consistent with a bound proposed in the context of gauge-gravity duality. In the second part of the thesis, we study a minimal model of nematic fluctuations in the high-temperature superconductor iron selenide. Nematic fluctuations arising from a quantum critical point have been proposed to explain the phenomenology of several high-temperature superconductors. In a numerical simulation using determinant quantum Monte Carlo methods, we find no direct evidence of a nematic critical point. However, we still observe a wide region of superconductivity correlated with nematic fluctuations as well as an unusual antiferro-quadrupole order.
The pseudogap regime of the cuprate high-temperature superconductors is characterized by a variety of competing orders, the nature of which are still widely debated. Recent experiments have provided evidence for electron nematic order, in which the electron fluid breaks rotational symmetry while preserving translational invariance. Raman spectroscopy, with its ability to symmetry resolve low energy excitations, is a unique tool that can be used to assess nematic fluctuations and nematic ordering tendencies. Here we compare results from determinant quantum Monte Carlo simulations of the Hubbard model to experimental results from Raman spectroscopy in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$, which show a prominent increase in the
B1g response around 10% hole doping as the temperature decreases, indicative of a rise in nematic fluctuations at low energy. Our results support a picture of nematic fluctuations with
B1g symmetry occurring in underdoped cuprates, which may arise from melted stripes at elevated temperatures.
We present a numerically exact solution for the BCS Hamiltonian at any temperature, including the degrees of freedom associated with classical phase, as well as amplitude fluctuations via a Monte Carlo integration. This allows for an investigation over the whole range of couplings: from weak attraction, as in the well-known BCS limit, to the mainly unexplored strong-coupling regime of pronounced phase fluctuations. In the latter, two characteristic temperatures T(*) and T(c), associated with short- and long-range ordering, respectively, can be identified in a mean-field-motivated Hamiltonian. T(*) at the same time corresponds to the opening of a gap in the excitation spectrum. In addition to introducing a novel procedure to study strongly coupled d-wave superconductors, our results indicate that classical phase fluctuations are not sufficient to explain the pseudogap features of high-temperature superconductors.