Model of Quantum Criticality in He3 bilayers Adsorbed on graphite

Recent experiments on He3 bilayers adsorbed on Graphite have shown striking quantum critical properties at the point where the first layer localizes. We model this system with the Anderson lattice plus inter-layer Coulomb repulsion in two dimensions. Assuming that quantum critical fluctuations come from a vanishing of the effective hybridization, we can reproduce several features of the system, including the apparent occurrence of two quantum critical points (QCP), the variation of the effective mass and coherence temperature with coverage.Comment: 4 pages, 2 figures, new version as published on PRL, journal reference and DOI adde

Kondo lattices with inequivalent local moments: Competitive vs. co-operative Kondo screening

While standard heavy fermion metals feature a single spin-1/2 local moment per unit cell, more complicated systems with multiple distinct local moments have been synthesized as well, with Ce_3Pd_20(Si,Ge)_6 being one example. Here, we discuss the physics of a Kondo lattice model with two local-moment sublattices, coupled with different Kondo couplings to conduction electrons. The phase diagram will be strongly modified from that of the standard Kondo lattice if the characteristic screening temperatures of the distinct moments are well separated. Therefore, we investigate the interplay between the two Kondo effects using a local self-energy approximation via slave bosons. We find that the two Kondo effects can either compete or co-operate depending on the conduction-band filling. In the regime of competition, small differences in the two Kondo couplings can lead to huge differences in the respective Kondo scales, due to non-trivial many-body effects. We also study the low-temperature properties of the collective heavy Fermi-liquid state and propose a connection to depleted Kondo lattice systems.Comment: 14 pages, 15 figure

Gr\"uneisen ratio at the Kondo breakdown quantum critical point

We show that the scenario of multi-scale Kondo breakdown quantum critical point (QCP) gives rise to a divergent Gr\"uneisen ratio with an anomalous exponent 0.7. In particular, we fit the experimental data of $YbRh_{2}(Si_{0.95}Ge_{0.05})_{2}$ for specific heat, thermal expansion, and Gr\"uneisen ratio based on our simple analytic expressions. A reasonable agreement between the experiment and theory is found for the temperature range between 0.4 K and 10 K. We discuss how the Gr\"uneisen ratio is a key measurement to discriminate between the Kondo breakdown and spin-density wave theories

Two-channel pseudogap Kondo and Anderson models: Quantum phase transitions and non-Fermi liquids

We discuss the two-channel Kondo problem with a pseudogap density of states, \rho(\w)\propto|\w|^r, of the bath fermions. Combining both analytical and numerical renormalization group techniques, we characterize the impurity phases and quantum phase transitions of the relevant Kondo and Anderson models. The line of stable points, corresponding to the overscreened non-Fermi liquid behavior of the metallic $r=0$ case, is replaced by a stable particle-hole symmetric intermediate-coupling fixed point for 0. For r>\rmax, this non-Fermi liquid phase disappears, and instead a critical fixed point with an emergent spin--channel symmetry appears, controlling the quantum phase transition between two phases with stable spin and channel moments, respectively. We propose low-energy field theories to describe the quantum phase transitions, all being formulated in fermionic variables. We employ epsilon expansion techniques to calculate critical properties near the critical dimensions $r=0$ and $r=1$, the latter being potentially relevant for two-channel Kondo impurities in neutral graphene. We find the analytical results to be in excellent agreement with those obtained from applying Wilson's numerical renormalization group technique.Comment: Added reference

Preformed heavy-electrons at the Quantum Critical Point in heavy fermion compounds

The existence of multiple energy scales is regarded as a signature of the Kondo breakdown mechanism for explaining the quantum critical behavior of certain heavy fermion compounds, like YbRh$_{2}$Si$_{2}$. The nature of the intermediate state between the heavy Fermi liquid and the quantum critical region, however, remains elusive. In this study we suggest an incoherent heavy-fermion scenario, where inelastic scattering with novel soft modes of the dynamical exponent $z = 3$ gives rise to non-Fermi liquid physics for thermodynamics and transport despite the formation of the heavy-fermion band. We discuss a crossover from $z = 3$ to $z = 1$ for quantum phase fluctuations

A phenomenological description on an incoherent Fermi liquid near optimal doping in high T_{c} cuprates

Marginal Fermi-liquid physics near optimal doping in high T_{c} cuprates has been explained within two competing scenarios such as the spin-fluctuation theory based on an itinerant picture and the slave-particle approach based on a localized picture. In this study we propose an alternative scenario for the anomalous transport within the context of the slave-particle approach. Although the marginal Fermi-liquid phenomenology was interpreted previously within deconfinement of the compact gauge theory, referred to as the strange metal phase, we start from confinement, introducing the Polyakov-loop parameter into an SU(2) gauge theory formulation of the t-J model. The Polyakov-loop parameter gives rise to incoherent electrons through the confinement of spinons and holons, which result from huge imaginary parts of self-energy corrections for spinons and holons. This confinement scenario serves a novel mechanism for the marginal Fermi-liquid transport in the respect that the scattering source has nothing to do with symmetry breaking. Furthermore, the incoherent Fermi-liquid state evolves into the Fermi liquid phase through crossover instead of an artificial second-order transition as temperature is lowered, where the crossover phenomenon does not result from the Anderson-Higgs mechanism but originate from an energy scale in the holon sector. We fit an experimental data for the electrical resistivity around the optimal doping and find a reasonable match between our theory and the experiment.Comment: 15 pages. 5 figure. Title has been changed. Final version publishec in J. Phys.: Condens. Matter 23 (2011) 49570

Orbital-selective Mott transitions: Heavy fermions and beyond

Quantum phase transitions in metals are often accompanied by violations of Fermi liquid behavior in the quantum critical regime. Particularly fascinating are transitions beyond the Landau-Ginzburg-Wilson concept of a local order parameter. The breakdown of the Kondo effect in heavy-fermion metals constitutes a prime example of such a transition. Here, the strongly correlated f electrons become localized and disappear from the Fermi surface, implying that the transition is equivalent to an orbital-selective Mott transition, as has been discussed for multi-band transition-metal oxides. In this article, available theoretical descriptions for orbital-selective Mott transitions will be reviewed, with an emphasis on conceptual aspects like the distinction between different low-temperature phases and the structure of the global phase diagram. Selected results for quantum critical properties will be listed as well. Finally, a brief overview is given on experiments which have been interpreted in terms of orbital-selective Mott physics.Comment: 29 pages, 4 figs, mini-review prepared for a special issue of JLT