Marginal stability is the notion that stability is achieved, but only barely so. This property constrains the ensemble of configurations explored at low temperature in a variety of systems, including spin, electron, and structural glasses. A key feature of marginal states is a (saturated) pseudogap in the distribution of soft excitations. We examine how such pseudogaps appear dynamically by studying the Sherrington-Kirkpatrick (SK) spin glass. After revisiting and correcting the multi-spin-flip criterion for local stability, we show that stationarity along the hysteresis loop requires soft spins to be frustrated among each other, with a correlation diverging as $C(\lambda )\sim 1/\lambda$, where $\lambda$ is the stability of the more stable spin. We explain how this arises spontaneously in a marginal system and develop an analogy between the spin dynamics in the SK model and random walks in two dimensions. We discuss analogous frustrations among soft excitations in short range glasses and how to detect them experimentally. We also show how these findings apply to hard sphere packings.

Two-dimensional (2D) atom lattices provide model setups with Coulomb correlations that induce competing ground states. Here, SiC emerges as a wide-gap substrate with reduced screening. We report the first artificial high-$Z$ atom lattice on SiC(0001) by Sn adatoms, based on experimental realization and theoretical modeling. Density-functional theory of our triangular structure model closely reproduces the scanning tunneling microscopy. Photoemission data show a deeply gapped state ($\sim 2\mathrm{eV}$ gap), and, based on our calculations including dynamic mean-field theory, we argue that this reflects a pronounced Mott-insulating scenario. We also find indications that the system is susceptible to antiferromagnetic superstructures. Such artificial lattices on SiC(0001) thus offer a novel platform for coexisting Coulomb correlations and spin-orbit coupling, with bearing for unusual magnetic phases and proposed topological quantum states of matter.

We devise a cold-atom approach to realizing a broad range of bilinear quantum magnets. Our scheme is based on off-resonant single-photon excitation of Rydberg $P$ states (Rydberg dressing), whose strong interactions are shown to yield controllable $XYZ$ interactions between effective spins, represented by different atomic ground states. The distinctive features of Förster-resonant Rydberg atom interactions are exploited to enhance the effectiveness of Rydberg dressing and, thereby, yield large spin interactions that greatly exceed the corresponding decoherence rates. We illustrate the concept on a spin-1 chain implemented with cold rubidium atoms, and demonstrate that this permits the dynamical preparation of topological magnetic phases. Generally, the described approach provides a viable route to exploring quantum magnetism with dynamically tunable (an)isotropic interactions as well as variable space and spin dimensions in cold-atom experiments.

Precise measurements are performed on spectral line shapes of spontaneous Rayleigh-Brillouin scattering in mixtures of the noble gases Ar and Kr, with He. Admixture of a light He atomic fraction results in marked changes of the spectra, although in all experiments He is merely a spectator atom: it affects the relaxation of density fluctuations of the heavy constituent, but its contribution to the scattered light intensity is negligibly small. The results are compared to a theory for the spectral line shape without adjustable parameters, yielding excellent agreement for the case of binary monatomic gases, signifying a step towards modeling and understanding of light scattering in more complex molecular media.

Edge intrinsic rotation was investigated in Ohmic $L$-mode discharges on the Tokamak à Configuration Variable, scanning the major radial position of the $X$ point, $R_X$. Edge rotation decreased linearly with increasing $R_X$, vanishing or becoming countercurrent for an outboard $X$ point, in agreement with theoretical expectations. The core rotation profile shifted fairly rigidly with the edge rotation, changing the central rotation speed by more than a factor of two. Core rotation reversals had little effect on the edge rotation velocity. Edge rotation was modestly more countercurrent in unfavorable than favorable $\nabla B$ shots.

Whistler mode chorus emissions with a characteristic frequency chirp are important magnetospheric waves, responsible for the acceleration of outer radiation belt electrons to relativistic energies and also for the scattering loss of these electrons into the atmosphere. Here, we report on the first laboratory experiment where whistler waves exhibiting fast frequency chirping have been artificially produced using a beam of energetic electrons launched into a cold plasma. Frequency chirps are only observed for a narrow range of plasma and beam parameters, and show a strong dependence on beam density, plasma density, and magnetic field gradient. Broadband whistler waves similar to magnetospheric hiss are also observed, and the parameter ranges for each emission are quantified.

We show that the critical velocity of a superfluid flow through a penetrable barrier coincides with the onset of the analog black-hole lasing effect. This dynamical instability is triggered by modes resonating in an effective cavity formed by two horizons enclosing the barrier. The location of the horizons is set by $v(x)=c(x)$, with $v(x),c(x)$ being the local fluid velocity and sound speed, respectively. We compute the critical velocity analytically and show that it is univocally determined by the configuration of the horizons. In the limit of broad barriers, the continuous spectrum at the origin of the Hawking-like radiation and of the Landau energetic instability is recovered.

Using x-ray photoelectron spectroscopy we observe the breaking of the strong interatomic bond in molecular CO at low temperature on a stepped Cu surface. Since the electronic structure of Cu does not allow for the splitting of CO at such low temperatures it suggests that there may be a less obvious pathway for the process. Through x-ray photoelectron spectroscopy we can clearly identify products associated with the dissociation of CO and the subsequent formation of stable graphitic carbon on the surface. However, the dissociation of CO can be inhibited when the stepped Cu surface is kept clean from surface carbon. These observations imply that the reaction is driven by the presence of small amounts of weakly bound carbon at the surface. Density-functional theory calculations confirm that carbon atoms on a stepped Cu surface indeed are the preferred adsorption sites for CO, which increases the stabilization of CO on the surface and weakens the C–O bond. This results in the breaking of the C–O bond at the step edge via the Boudouard reaction ($2{\mathrm{CO}}_{\text{ads}}\to {\mathrm{C}}_{\text{ads}}+{\mathrm{CO}}_2$) with a barrier of 0.71 eV.

Circularly polarized light opens a gap in the Dirac spectrum of graphene and topological insulator (TI) surfaces, thereby inducing a quantum Hall-like phase. We propose to detect the accompanying edge states and their current by the magnetic field they produce. The topological nature of the edge states is reflected in the mean orbital magnetization of the sample, which shows a universal linear dependence as a function of a generalized chemical potential—independent of the driving details and the properties of the material. The proposed protocol overcomes several typically encountered problems in the realization and measurement of Floquet phases, including the destructive effects of phonons and coupled electron baths and provides a way to occupy the induced edge states selectively. We estimate practical experimental parameters and conclude that the magnetization signature of the Floquet topological phase may be detectable with current techniques.

The superconducting properties of ${\mathrm{LaFeAsO}}_{1-x}{\mathrm{F}}_x$ under conditions of optimal electron doping are investigated upon the application of external pressure up to $\sim 23\mathrm{kbar}$. Measurements of muon-spin spectroscopy and dc magnetometry evidence a clear mutual independence between the critical temperature $T_c$ and the low-temperature saturation value for the ratio $n_s/m^{*}$ (superfluid density over effective band mass of Cooper pairs). Remarkably, a dramatic increase of $\sim 30\%$ is reported for $n_s/m^{*}$ at the maximum pressure value while $T_c$ is substantially unaffected in the whole accessed experimental window. We argue and demonstrate that the explanation for the observed results must take the effect of nonmagnetic impurities on multiband superconductivity into account. In particular, the unique possibility to modify the ratio between intraband and interband scattering rates by acting on structural parameters while keeping the amount of chemical disorder constant is a striking result of our proposed model.

A first-principles investigation into the magnetic ferroelectric ${\mathrm{PbTi}}_{1-x}{\mathrm{Co}}_x{\mathrm{O}}_3$ has revealed a bi-stable magnetic system with strong spin-lattice coupling. The local distortions induced by the low-spin to high-spin crossover are ferroelectric in nature, and are characterized by the displacement of the dopant ion with respect to the surrounding ${\mathrm{O}}_6$ octahedral cage. We demonstrate how this spin-lattice effect could mediate magnetoelectric coupling and possible electric field induced spin-crossover, indicating a promising route to voltage manipulation of isolated spins in a solid-state system.

Unitary transformations are the most general input-output maps available in closed quantum systems. Good control protocols have been developed for qubits, but questions remain about the use of optimal control theory to design unitary maps in high-dimensional Hilbert spaces, and about the feasibility of their robust implementation in the laboratory. Here we design and implement unitary maps in a 16-dimensional Hilbert space associated with the $6S_{1/2}$ ground state of ${}^{133}\mathrm{Cs}$, achieving fidelities $>0.98$ with built-in robustness to static and dynamic perturbations. Our work has relevance for quantum information processing and provides a template for similar advances on other physical platforms.

QCD perturbation theory ignores the compact nature of the $SU(3)$ gauge group that gives rise to the periodic $\theta$ vacuum of the theory. We propose to modify the gluon propagator to reconcile perturbation theory with the anomalous Ward identities for the topological current in the $\theta$ vacuum. As a result, the gluon couples to the Veneziano ghost describing the tunneling transitions between different Chern-Simons sectors of the vacuum; we call the emerging gluon dressed by ghost loops a “glost.” We evaluate the glost propagator and find that it has the form $G(p)=(p^2+{\chi }_{\text{top}}/p^2{)}^{-1}$ where ${\chi }_{\text{top}}$ is the Yang-Mills topological susceptibility related to the ${\eta }^{\prime }$ mass by the Witten-Veneziano relation; this propagator describes the confinement of gluons at distances $\sim {\chi }_{\text{top}}^{-1/4}\simeq 1\mathrm{fm}$. The same functional form of the propagator was originally proposed by Gribov as a solution to the gauge copies problem that plagues perturbation theory. The resulting running coupling coincides with the perturbative one at $p^2\gg \sqrt{{\chi }_{\text{top}}}$, but in the infrared region either freezes (in pure Yang-Mills theory) or vanishes (in full QCD with light quarks), in accord with experimental evidence. Our scenario makes explicit the connection between confinement and topology of the QCD vacuum; we discuss the implications for spin physics, high energy scattering, and the physics of quark-gluon plasma.

We introduce a method to achieve the three-dimensional dynamic localization of light. We consider a dynamically modulated resonator lattice that has been previously shown to exhibit an effective gauge potential for photons. When such an effective gauge potential varies sinusoidally in time, dynamic localization of light can be achieved. Moreover, while previous works on such an effective gauge potential for photons were carried out in the regime where the rotating wave approximation is valid, the effect of dynamic localization persists even when the counterrotating term is taken into count.

We employ density functional theory plus dynamical mean field theory and identify the physical origin of why two layers of ${\mathrm{SrVO}}_3$ on a ${\mathrm{SrTiO}}_3$ substrate are insulating: the thin film geometry lifts the orbital degeneracy, which in turn triggers a first-order Mott-Hubbard transition. Two layers of ${\mathrm{SrVO}}_3$ are just at the verge of a Mott-Hubbard transition and hence ideally suited for technological applications of the Mott-Hubbard transition: the heterostructure is highly sensitive to strain, electric field, and temperature. A gate voltage can also switch between metal (on) and insulator (off), so that a transistor with ideal on-off switching properties is realized.

We analyze a $t_{2g}$ double-exchange system where the orbital directionality of the itinerant degrees of freedom is a key dynamical feature that self-adjusts in response to doping and leads to a phase diagram dominated by two classes of ground states with zigzag and checkerboard patterns. The prevalence of distinct orderings is tied to the formation of orbital molecules that in one-dimensional paths make insulating zigzag states kinetically more favorable than metallic stripes, thus allowing for a novel doping-induced metal-to-insulator transition. We find that the basic mechanism that controls the magnetic competition is the breaking of orbital directionality through structural distortions, and highlight the consequences of the interorbital Coulomb interaction.