Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moiré superlattice. Such superlattices are known ...to host bulk valley currents at tiny angles (α≈0.3°) and flat bands at magic angles (α≈1°). We show that tuning the twist angle to α*≈0.8° generates flat bands away from charge neutrality with a triangular superlattice periodicity. When doped with ±6 electrons per moiré cell, these bands are half-filled and electronic interactions produce a symmetry-broken ground state (Stoner instability) with spin-polarized regions that order ferromagnetically. Application of an interlayer electric field breaks inversion symmetry and introduces valley-dependent dispersion that quenches the magnetic order. With these results, we propose a solid-state platform that realizes electrically tunable strong correlations.
We present an analytic strong-coupling approach to the phase diagram and elementary excitations of the Jaynes-Cummings-Hubbard model describing a superfluid-insulator transition of polaritons in an ...array of coupled QED cavities. In the Mott phase, we find four modes corresponding to particle or hole excitations with lower and upper polaritons, respectively. Simple formulas are derived for the dispersion and spectral weights within a strong-coupling random-phase approximation (RPA). The phase boundary is calculated beyond RPA by including the leading correction due to quantum fluctuations.
Measuring the ac magnetic response of a type II superconductor provides valuable information on the pinning landscape (pinscape) of the material. We use strong pinning theory to derive a microscopic ...expression for the Campbell length λ(C), the penetration depth of the ac signal. We show that λ(C) is determined by the jump in the pinning force, in contrast to the critical current j(c), which involves the jump in pinning energy. We demonstrate that the Campbell lengths generically differ for zero-field-cooled and field-cooled samples and predict that hysteretic behavior can appear in the latter situation. We compare our findings with new experimental data and show the potential of this technique in providing information on the material's pinscape.
We present an analytic slave-boson approach to calculate the elementary excitations of the Jaynes-Cummings-Hubbard model (JCHM) describing strongly correlated polaritons on a lattice in various ...quantum optical systems. In the superfluid phase near the Mott transition we find a gapless, linear Goldstone mode and a gapped amplitude mode corresponding to phase and density fluctuations, respectively. The sound velocity of the Goldstone mode develops a peculiar anomaly as a function of detuning at low densities, which persists into the weakly interacting regime of a polariton BEC.
Pinning and thermal creep determine the response of numerous systems containing superstructures, e.g., vortices in type II superconductors, domain walls in ferroics, or dislocations in metals. The ...combination of drive and thermal fluctuations leads to the superstructure's depinning, and its velocity v determines the electric, magnetic, or mechanical response. It is commonly believed that pinning and creep collapse above the critical drive Fc, entailing a sharp rise in the velocity v. We challenge this perception by studying the effects of thermal fluctuations within the framework of strong vortex pinning in type II superconductors. In fact, we show that pinning and thermal creep persist far beyond the critical force. The resulting force-velocity characteristic largely maintains its zero-temperature shape, and thermal creep manifests itself by a downward renormalization of the critical drive. Such characteristics are in agreement with Coulomb's law of dry friction, and they have been often observed in experiments.
Abstract
Phase estimation algorithms are key protocols in quantum information processing. Besides applications in quantum computing, they can also be employed in metrology as they allow for fast ...extraction of information stored in the quantum state of a system. Here, we implement two suitably modified phase estimation procedures, the Kitaev and the semiclassical Fourier-transform algorithms, using an artificial atom realized with a superconducting transmon circuit. We demonstrate that both algorithms yield a flux sensitivity exceeding the classical shot-noise limit of the device, allowing one to approach the Heisenberg limit. Our experiment paves the way for the use of superconducting qubits as metrological devices which are potentially able to outperform the best existing flux sensors with a sensitivity enhanced by few orders of magnitude.
Type-II superconductors owe their magnetic and transport properties to vortex pinning, the immobilization of flux quanta through material inhomogeneities or defects. Characterizing the potential ...energy landscape for vortices, the pinning landscape (or short, pinscape), is of great technological importance. Aside from measurement of the critical current density j sub()cand of creep rates S, the ac magnetic response provides valuable information on the pinscape which is different from that obtained through j sub()cor S, with the Campbell penetration depth lambda sub(C) defining a characteristic quantity well accessible in an experiment. Here, we derive a microscopic expression for the Campbell penetration depth lambda sub(C) using strong-pinning theory. Our results explain the dependence of lambda sub(C) on the state preparation of the vortex system and the appearance of hysteretic response. Analyzing different pinning models, metallic or insulating inclusions, as well as delta T sub()cand delta scriptl pinning, we discuss the behavior of the Campbell length for different vortex-state preparations within the phenomenological H-T phase diagram and compare our results with recent experiments.
We determine the current-voltage characteristic of type-II superconductors in the presence of strong pinning centers. Focusing on a small density of defects, we derive a generic form for the ...characteristic with a linear flux-flow branch shifted by the critical current (excess-current characteristic). The details near onset, a hysteretic jump (for κ>>1) or a smooth velocity turn-on (κ→1), depend on the Labusch parameter κ characterizing the pinning centers. Pushing the single-pin analysis into the weak pinning domain, we reproduce the collective pinning results for the critical current.
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