A series of FRET-based β-arrestin2 biosensors are used to study the dynamics and conformational changes that occur when β-arrestin2 binds to and dissociates from the β2-adrenergic receptor in living cells; results show that after β-arrestin2 dissociates from the β2-adrenergic receptor, it stays at the cell membrane in an active conformation for a while, indicating that β-arrestin is able to signal in a G-protein-coupled receptor (GPCR)-free state. Much has been learnt about the structure of G-protein-coupled receptors (GPCRs), ubiquitous mediators of the actions of many hormones, neurotransmitters and drugs, but little is known about how the conformations of these membrane proteins change when they interact with proteins that regulate their function. Susanne Nuber et al. used a series of FRET-based β-arrestin2 biosensors to study the dynamics of the conformational changes that take place in β-arrestin2 when it binds to and eventually dissociates from the β-adrenergic receptor in living cells. They find that after dissociation, β-arrestin2 remains at the cell membrane in an active conformation at least initially, indicating that β-arrestin is able to signal in a GPCR-free state. Mi-Hye Lee et al. used a series of intramolecular FlAsH BRET reporters to monitor conformational changes in β-arrestin2 in the presence of six different GPCRs. They found that different GPCRs and ligands produce β-arrestin2 conformational signatures that correlate with the stability of the receptor–arrestin complex; this explains how different GPCRs can use a common effector for different purposes. (β-)Arrestins are important regulators of G-protein-coupled receptors (GPCRs)1,2,3. They bind to active, phosphorylated GPCRs and thereby shut off ‘classical’ signalling to G proteins3,4, trigger internalization of GPCRs via interaction with the clathrin machinery5,6,7 and mediate signalling via ‘non-classical’ pathways1,2. In addition to two visual arrestins that bind to rod and cone photoreceptors (termed arrestin1 and arrestin4), there are only two (non-visual) β-arrestin proteins (β-arrestin1 and β-arrestin2, also termed arrestin2 and arrestin3), which regulate hundreds of different (non-visual) GPCRs. Binding of these proteins to GPCRs usually requires the active form of the receptors plus their phosphorylation by G-protein-coupled receptor kinases (GRKs)1,3,4. The binding of receptors or their carboxy terminus as well as certain truncations induce active conformations of (β-)arrestins that have recently been solved by X-ray crystallography8,9,10. Here we investigate both the interaction of β-arrestin with GPCRs, and the β-arrestin conformational changes in real time and in living human cells, using a series of fluorescence resonance energy transfer (FRET)-based β-arrestin2 biosensors. We observe receptor-specific patterns of conformational changes in β-arrestin2 that occur rapidly after the receptor–β-arrestin2 interaction. After agonist removal, these changes persist for longer than the direct receptor interaction. Our data indicate a rapid, receptor-type-specific, two-step binding and activation process between GPCRs and β-arrestins. They further indicate that β-arrestins remain active after dissociation from receptors, allowing them to remain at the cell surface and presumably signal independently. Thus, GPCRs trigger a rapid, receptor-specific activation/deactivation cycle of β-arrestins, which permits their active signalling.

We report on the spectroscopic observation of a quantized electronic fine structure near the Fermi energy in thin Fe films grown on W(110). The quantum well states are detected down to binding energies of similar to 10 meV by angle-resolved photoelectron spectroscopy. The band dispersion of these states is found to feature a pronounced anisotropy within the surface plane: It is free-electron-like along the (Gamma H) over bar direction while it becomes heavy along (Gamma N) over bar. Density functional theory calculations identify the observed states to have both majority and minority spin character and indicate that the large anisotropy can be dependent on the number of Fe layers and coupling to the substrate.

We present a renormalization group (RG) theory for the breakdown of Kondo screening in the Kondo lattice model (KLM) without pre-assumptions about the competition between Kondo effect and magnetic ordering or Fermi surface criticality. We show that the vertex between a single, local Kondo spin and the extended conduction electrons obtains RKKYinduced, non-local contributions in the in-and out-going coordinates of scattering electrons due to scattering at surrounding Kondo sites, but it remains local in the Kondo spin position. This enables the existence of a local Kondo screening scale TK(y) in the KLM, controlled by the effective RKKY coupling parameter y. TK(y) is determined by the RG flow of the local spin exchange coupling in the presence of the self-consistent spin response on surrounding Kondo sites. We show that TK(y) exhibits universal behavior and is suppressed by the antiferromagnetic RKKY coupling. Beyond a maximal RKKY parameter value ymax Kondo screening ceases to exist even without magnetic ordering. The theory opens up the possibility of describing quantum critical scenarios involving spin wave instabilities or local Kondo breakdown on the same footing.

We study, using high-resolution angle-resolved photoemission spectroscopy, the evolution of the electronic structure in URu2Si2 at the Γ, Z, and X high-symmetry points from the high-temperature Kondo-screened regime to the low-temperature hidden-order (HO) state. At all temperatures and symmetry points, we find structures resulting from the interaction between heavy and light bands related to the Kondo-lattice formation. At the X point, we directly measure a hybridization gap of 11 meV already open at temperatures above the ordered phase. Strikingly, we find that while the HO induces pronounced changes at Γ and Z, the hybridization gap at X does not change, indicating that the hidden-order parameter is anisotropic. Furthermore, at the Γ and Z points, we observe the opening of a gap in momentum in the HO state, and show that the associated electronic structure results from the hybridization of a light electron band with the Kondo-lattice bands characterizing the paramagnetic state.

We report on a quantitative investigation on the Rashba effect in the Shockley state of the model system Ag/Au(111) by means of angle-resolved photoemission spectroscopy with high-energy resolution. The spectra are discussed in detail in comparison with results of one-step photoemission calculations. The systematic overestimation in the wave-vector splitting present in all actual theoretical studies is explained in terms of an insufficient description of the surface potential. Also, we conclude that the coupling between the semi-infinite bulk and the surface potential mediated by the bulk reflection matrix $B$ is of major importance for a quantitative description of the Rashba effect.

We report on the results of a high-resolution angle-resolved photoemission study on the ordered surface alloy CePt(5). The temperature dependence of the spectra show the formation of the coherent low-energy heavy-fermion band near the Fermi level. These experimental data are supported by a multiband model calculation in the framework of the dynamical mean-field theory.

We report on a detailed analysis of the evolution and spatial localization of quantum-well states (QWSs) in Ag layers on a Au(111) substrate by means of high-resolution photoelectron spectroscopy combined with model calculations based on a simple particle-in-a-box picture, the phase accumulation model, and density functional theory-based slab-layer calculations. Due to the finite electron escape depth we could link the photoemission intensity of the QWS to the simulated charge-density distribution and therewith confirm the calculated localization of these states. The first QWS starts to be localized within the Ag film at layer thicknesses $>$7 ML.

The ‘hidden-order’ (HO) transition of URu2Si2 remains a puzzle after 25 years of research. Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we found that a hole-like band around Γ having its band maximum at E=−35meV, and previously thought to be a bulk band of the system, is indeed a surface state not related to the HO phase transition. Here we present our detailed investigations to assign that state to a surface feature, and discuss on the possible origins of this surface band.

We propose a new criterion for distinguishing the Hertz-Millis (HM) and the local quantum critical (LQC) mechanism in heavy-fermion systems with a magnetic quantum phase transition (QPT). The criterion is based on our finding that the complete spin screening of Kondo ions can be suppressed by the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling to the surrounding magnetic ions even without magnetic ordering and that, consequently, the signature of this suppression can be observed in spectroscopic measurements above the magnetic ordering temperature. We apply the criterion to high-resolution photoemission measurements on CeCu(6 - x)Au(x) and conclude that the QPT in this system is dominated by the LQC scenario.

A quantum phase transition in a heavy-fermion compound may destroy the Fermi-liquid ground state. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu(6-x)Au(x) by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale T(K) exhibits an abrupt step near the quantum critical Au concentration of x(c) = 0.1. We show theoretically that this step is expected from a highly nonlinear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for heavy-fermion quasiparticle breakdown at a quantum phase transition.