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Non-Fermi liquid (NFL) physics can be realized in quantum dot devices where competing interactions frustrate the exact screening of dot spin or charge degrees of freedom. We show that a standard nanodevice architecture, involving a dot coupled to both a quantum box and metallic leads, can host an exotic SO(5) symmetry Kondo effect, with entangled dot and box charge and spin. This NFL state is surprisingly robust to breaking channel and spin symmetry, but destabilized by particle-hole asymmetry. By tuning gate voltages, the SO(5) state evolves continuously to a spin and then flavor two-channel Kondo state. The expected experimental conductance signatures are highlighted.
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We investigate a well studied system of a quantum dot coupled to a Coulomb box and leads, realizing a spin-flavor Kondo model. It exhibits a recently discovered non-Fermi liquid (NFL) behavior with emergent SO(5) symmetry. Here, through a detailed bosonization and refermionization solution, we push forward our previous work and provide a consistent and complete description of the various exotic properties and phase diagram. A unique NFL phase emerges from the presence of an uncoupled Majorana fermion from the flavor sector, whereas FL-like susceptibilities result from the gapping out of a pair of Majroana fermions from the spin and flavor sectors. Other properties, such as a $T^{3/2}$ scaling of the conductance, stability under channel or spin symmetry breaking and a re-appearance of NFL behavior upon breaking the particle-hole symmetry, are all accounted for by a renormalization group treatment of the refermionized Majorana model.
The interplay of interactions and disorder in two-dimensional (2D) electron systems has actively been studied for decades. The paradigmatic approach involves starting with a clean Fermi liquid and perturbing the system with both disorder and interactions. We instead start with a clean non-Fermi liquid near a 2D ferromagnetic quantum critical point and consider the effects of disorder. In contrast with the disordered Fermi liquid, we find that our model does not suffer from runaway flows to strong coupling and the system has a marginally stable fixed point with perfect conduction.
Landaus Fermi liquid theory is a cornerstone of quantum many body physics. At its heart is the adiabatic connection between the elementary excitations of an interacting fermion system and those of the same system with the interactions turned off. Recently, this tenet has been challenged with the finding of a non-Landau Fermi liquid, that is a strongly interacting Fermi liquid that cannot be adiabatically connected to a non-interacting system. In particular, a spin-1 two-channel Kondo impurity with single-ion magnetic anisotropy $D$ has a topological quantum phase transition at a critical value $D_c$: for $D < D_c$ the system behaves as an ordinary Fermi liquid with a large Fermi level spectral weight, while above $D_c$ the system is a non-Landau Fermi liquid with a pseudogap at the Fermi level, topologically characterized by a non-trivial Friedel sum rule with non-zero Luttinger integrals. Here, we develop a non-trivial extension of this new Fermi liquid theory to general multi-orbital problems with finite magnetic field and we reinterpret in a unified and consistent fashion several experimental studies of iron phthalocyanine molecules on Au(111) metal substrate that were previously described in disconnected and conflicting ways. The differential conductance measured using a scanning tunneling microscope (STM) shows a zero-bias dip that widens when the molecule is lifted from the surface and is transformed continuously into a peak under an applied magnetic field. Numerically solving a spin-1 impurity model with single-ion anisotropy for realistic parameter values, we robustly reproduce all these central features, allowing us to conclude that iron phthalocyanine molecules on Au(111) constitute the first confirmed experimental realization of a non-Landau Fermi liquid.
Direct coupling between gapless bosons and a Fermi surface results in the destruction of Landau quasiparticles and a breakdown of Fermi liquid theory. Such a non-Fermi liquid phase arises in spin-orbit coupled ferromagnets with spontaneously broken continuous symmetries due to strong coupling between rotational Goldstone modes and itinerant electrons. These systems provide an experimentally accessible context for studying non-Fermi liquid physics. Possible examples include low-density Rashba coupled electron gases, which have a natural tendency towards spontaneous ferromagnetism, or topological insulator surface states with proximity-induced ferromagnetism. Crucially, unlike the related case of a spontaneous nematic distortion of the Fermi surface, for which the non-Fermi liquid regime is expected to be masked by a superconducting dome, we show that the non-Fermi liquid phase in spin-orbit coupled ferromagnets is stable.
A Dirac-Fermi liquid (DFL)--a doped system with Dirac spectrum--is an important example of a non-Galilean-invariant Fermi liquid (FL). Real-life realizations of a DFL include, e.g., doped graphene, surface states of three-dimensional (3D) topological insulators, and 3D Dirac/Weyl metals. We study the optical conductivity of a DFL arising from intraband electron-electron scattering. It is shown that the effective current relaxation rate behaves as $1/tau_{J}propto left(omega^2+4pi^2 T^2right)left(3omega^2+8pi^2 T^2right)$ for $max{omega, T}ll mu$, where $mu$ is the chemical potential, with an additional logarithmic factor in two dimensions. In graphene, the quartic form of $1/tau_{J}$ competes with a small FL-like term, $proptoomega^2+4pi^2 T^2$, due to trigonal warping of the Fermi surface. We also calculated the dynamical charge susceptibility, $chi_mathrm{c}({bf q},omega)$, outside the particle-hole continua and to one-loop order in the dynamically screened Coulomb interaction. For a 2D DFL, the imaginary part of $chi_mathrm{c}({bf q},omega)$ scales as $q^2omegaln|omega|$ and $q^4/omega^3$ for frequencies larger and smaller than the plasmon frequency at given $q$, respectively. The small-$q$ limit of $mathrm{Im} chi_mathrm{c}({bf q},omega)$ reproduces our result for the conductivity via the Einstein relation.
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