Quantum critical systems derive their finite temperature properties from the influence of a zero temperature quantum phase transition.1 The paradigm is essential for understanding unconventional high-T c superconductors and the nonFermi liquid properties of heavy fermion compounds. However, the microscopic origins of quantum phase transitions in complex materials are often debated.Here we demonstrate experimentally, with support from numerical renormalization group calculations, a universal crossover from quantum critical non-Fermi liquid behavior to distinct Fermi liquid ground states in a highly controllable quantum dot device. Our device realizes the non-Fermi liquid two-channel Kondo state, 2, 3 based on a spin-1/2 impurity exchange-coupled equally to two independent electronic reservoirs. 4 Arbitrarily small detuning of the exchange couplings results in conventional screening of the spin by the more strongly coupled channel for energies below a Fermi liquid scale T * . We extract a quadratic dependence of T * on gate voltage close to criticality and validate an asymptotically exact description of the universal crossover between strongly correlated non-Fermi liquid and Fermi liquid states. 5, 6A conventional second-order quantum phase transition (QPT) features quantum mechan- (FL) scale that vanishes at the quantum critical point (QCP); away from the QCP, a crossover from non-FL to FL behavior is observed at low energies. A diverging effective mass m * at the QCP, seen in both materials, signifies the absence of quasiparticles at the Fermi surface. 8In many heavy fermion materials and in high-T c superconductors, the relevant degrees of freedom and the effective Hamiltonian can be controversial. We aim to understand quantitatively a second-order QPT outside the usual order-parameter-fluctuation description.Quantum dots provide an experimental framework for realizing known quantum impurityHamiltonians that can feature tunable second-order QPTs. 9, 10 However, QCPs are challenging to reach even in engineered systems, since perturbations that steer away from quantum criticality may be inherently uncontrolled, as in two-impurity Kondo experiments to date. 11-13At the QCP of a two-channel Kondo (2CK) system, a single overscreened spin yields a non-FL state with no quasiparticles (i.e. only collective excitations) at the Fermi surface.An order parameter is typically not invoked; rather, the critical behavior is owing to the single spin. A FL scale T * results from several relevant perturbations: Zeeman splitting, difference in exchange couplings, and charge transfer between the two channels. Requiring that all these perturbations be small would seem to diminish prospects for observing the QCP in bulk systems. Nonetheless, two-channel Kondo physics has been invoked to explain experiments on heavy fermion materials 14-16 and two-level tunneling centers. 17-19 A 2CK state has been predicted 2 and observed 3 in a quantum dot tunnel-coupled to a "metallic grain," an electron reservoir big enough to have a small lev...

Abstract:We establish the existence of stable and metastable stationary black hole bound states at finite temperature and chemical potentials in global and planar fourdimensional asymptotically anti-de Sitter space. We determine a number of features of their holographic duals and argue they represent structural glasses. We map out their thermodynamic landscape in the probe approximation, and show their relaxation dynamics exhibits logarithmic aging, with aging rates determined by the distribution of barriers.

Incorporating ferromagnetic dopants into three-dimensional topological insulator thin films has recently led to the realisation of the quantum anomalous Hall effect. These materials are of great interest since they may support electrical currents that flow without resistance, even at zero magnetic field. To date, the quantum anomalous Hall effect has been investigated using low-frequency transport measurements. However, transport results can be difficult to interpret due to the presence of parallel conductive paths, or because additional non-chiral edge channels may exist. Here we move beyond transport measurements by probing the microwave response of a magnetised disk of Cr-(Bi,Sb)2Te3. We identify features associated with chiral edge plasmons, a signature that robust edge channels are intrinsic to this material system. Our results provide a measure of the velocity of edge excitations without contacting the sample, and pave the way for an on-chip circuit element of practical importance: the zero-field microwave circulator.

The physical properties of a material tuned to the cusp between two distinct ground states can be quite exotic, and unlike those in either of the neighboring phases [1,2]. The prospect of capturing such behavior in a simple model is tantalizing; for example, the interplay between heavy fermion physics and magnetic ordering in certain materials is often rationalized in terms of the quantum phase transition in the two-impurity Kondo model [3,4]. However, this model is oversimplified for the purpose: its quantum critical point does not reflect the distinctive properties of a magnetic lattice surrounded by mobile electrons [5,6]. In this work, we study a tunable nanoelectronic circuit comprising two coupled charge-Kondo quantum islands, realizing a new model which captures the essence of competition between local and collective screening of magnetic moments. This may have relevance for materials in which collective many-body effects drive lattice coherence [7]- [9]. We tune our device to a novel quantum critical point, and show experimentally that deviations as we tune away from this point match non-trivial predictions from the model. This work on the crucial role of inter-island interactions is a necessary first step in scaling up such circuits from individual sites to networks or lattices.

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