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Friction in ordered atomistic layers plays a central role in various nanoscale systems ranging from nanomachines to biological systems. It governs transport properties, wear and dissipation. Defects and incommensurate lattice constants markedly change these properties. Recently, experimental systems have become accessible to probe the dynamics of nanofriction. Here, we present a model system consisting of laser-cooled ions in which nanofriction and transport processes in self-organized systems with back action can be studied with atomic resolution. We show that in a system with local defects resulting in incommensurate layers, there is a transition from sticking to sliding with Aubry-type signatures. We demonstrate spectroscopic measurements of the soft vibrational mode driving this transition and a measurement of the order parameter. We show numerically that both exhibit critical scaling near the transition point. Our studies demonstrate a simple, well-controlled system in which friction in self-organized structures can be studied from classical- to quantum-regimes.

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Trapped-ion optical clocks are capable of achieving systematic fractional frequency uncertainties of 10 −18 and possibly below. However, the stability of current ion clocks is fundamentally limited by the weak signal of single-ion interrogation. We present an operational, scalable platform for extending clock spectroscopy to arrays of Coulomb crystals consisting of several tens of ions, while allowing systematic shifts as low as 10 −19 . Using a newly developed technique, we observe 3D excess micromotion amplitudes inside a Coulomb crystal with atomic spatial resolution and sub-nanometer amplitude uncertainties. We show that in ion Coulomb crystals of 400 µm and 2 mm length, time dilation shifts of In + ions due to micromotion can be close to 1 × 10 −19 and below 10 −18 , respectively. In previous ion traps, excess micromotion would have dominated the uncertainty budget for spectroscopy of even a few ions. By minimizing its contribution and providing a means to quantify it, this work opens up the path to precision spectroscopy in many-body ion systems, enabling entanglement-enhanced ion clocks and providing a well-controlled, strongly coupled quantum system.

Dynamics of carrier recombination and localization in AlGaN quantum wells studied by time-resolved transmission spectroscopy Appl. Phys. Lett. 95, 091910 (2009); 10.1063/1.3222972Photoluminescence study of carrier dynamics and recombination in a strained InGaAsP/InP multiple-quantumwell structure Localization effects on the optical properties of GaAs 1Àx Bi x /GaAs single quantum wells (SQWs), with Bi contents ranging from x ¼ 1.1% to 6.0%, are investigated using continuous-wave and time-resolved photoluminescence. The temperature-and excitation density dependence of the PL spectra are systematically studied, and the carrier recombination mechanisms are analyzed. At low temperatures, the time-integrated PL emission is dominated by the recombination of localized electron-hole pairs due to the varying content and clustering of Bi in the alloy. The extracted energy scales fluctuate tremendously when the Bi content is varied with a weak tendency to increase with Bi content. Relatively low energy scales are found for the SQW with x ¼ 5.5%, which makes it a potential candidate for long-wavelength optoelectronic devices. V C 2013 AIP Publishing LLC.