In scanning tunneling microscopy, orbital selectivity of the tunneling process can make the topographic image dependent on a tip-surface distance. We have found reproducible dependence of the images on the distance for a monatomic layer of iron nitride formed on a Cu(001) surface. Observed atomic images systematically change between a regular dot array and a dimerized structure depending on the tip-surface distance, which turns out to be the only relevant parameter in the image variation. An accompanied change in the weight of $\mathrm{Fe}\text{−}3d$ local density of states to a tunneling background was detected in $dI/dV$ spectra. These have been attributed to a shift in surface orbitals detected by the tip from the $d$ states to the $s/p$ states with increasing the tip-surface distance, consistent with an orbital assignment from first-principles calculations.

The dynamics of a microscopic cuprate model, namely, the two-dimensional Hubbard model, is studied with a cluster extension of the dynamical mean-field theory. We find a nontrivial structure of the frequency-dependent self-energies, which describes an unprecedented interplay between the pseudogap and superconductivity. We show that these properties are well described by quasiparticles hybridizing with (hidden) fermionic excitations, emergent from the strong electronic correlations. The hidden fermion enhances superconductivity via a mechanism distinct from a conventional boson-mediated pairing, and originates the normal-state pseudogap. Though the hidden fermion is elusive in experiments, it can solve many experimental puzzles.

We study a generic model for quorum-sensing bacteria in circular confinement. Every bacterium produces signaling molecules, the local concentration of which triggers a response when a certain threshold is reached. If this response lowers the motility, then an aggregation of bacteria occurs which differs fundamentally from standard motility-induced phase separation due to the long-ranged nature of the concentration of signal molecules. We analyze this phenomenon analytically and by numerical simulations employing two different protocols leading to stationary cluster and ring morphologies, respectively.

Coupled length and time scales determine the dynamic behavior of polymers and underlie their unique viscoelastic properties. To resolve the long-time dynamics it is imperative to determine which time and length scales must be correctly modeled. Here we probe the degree of coarse graining required to simultaneously retain significant atomistic details and access large length and time scales. The degree of coarse graining in turn sets the minimum length scale instrumental in defining polymer properties and dynamics. Using linear polyethylene as a model system, we probe how the coarse-graining scale affects the measured dynamics. Iterative Boltzmann inversion is used to derive coarse-grained potentials with 2–6 methylene groups per coarse-grained bead from a fully atomistic melt simulation. We show that atomistic detail is critical to capturing large-scale dynamics. Using these models we simulate polyethylene melts for times over $500\mu \mathrm{s}$ to study the viscoelastic properties of well-entangled polymer melts.

We study the finite-shear-rate rheology of disordered solids by means of molecular dynamics simulations in two dimensions. By systematically varying the damping strength $\zeta$ in the low-temperature limit, we identify two well-defined flow regimes, separated by a thin (temperature-dependent) crossover region. In the overdamped regime, the athermal rheology is governed by the competition between elastic forces and viscous forces, whose ratio gives the Weissenberg number $\mathrm{Wi}\propto \zeta \dot{\gamma }$; the macroscopic stress $\mathrm{\Sigma }$ follows the frequently encountered Herschel-Bulkley law $\mathrm{\Sigma }={\mathrm{\Sigma }}_0+k\sqrt{\mathrm{Wi}}$, with yield stress ${\mathrm{\Sigma }}_0>0$. In the underdamped (inertial) regime, dramatic changes in the rheology are observed for low damping: the flow curve becomes nonmonotonic. This change is not caused by longer-lived correlations in the particle dynamics at lower damping; instead, for weak dissipation, the sample heats up considerably due to, and in proportion to, the driving. By thermostating more or less underdamped systems, we are able to link quantitatively the rheology to the kinetic temperature and the shear rate, rescaled with Einstein’s vibration frequency.

The statistical properties of interacting strongly chaotic systems are investigated for varying interaction strength. In order to model tunable entangling interactions between such systems, we introduce a new class of random matrix transition ensembles. The nearest-neighbor-spacing distribution shows a very sensitive transition from Poisson statistics to those of random matrix theory as the interaction increases. The transition is universal and depends on a single scaling parameter only. We derive the analytic relationship between the model parameters and those of a bipartite system, with explicit results for coupled kicked rotors, a dynamical systems paradigm for interacting chaotic systems. With this relationship the spectral fluctuations for both are in perfect agreement. An accurate approximation of the nearest-neighbor-spacing distribution as a function of the transition parameter is derived using perturbation theory.

Observation of high-temperature superconductivity in compressed sulfur hydrides has generated an irresistible wave of searches for new hydrogen-containing superconductors. We herein report the prediction of high-$T_c$ superconductivity in tellurium hydrides stabilized at megabar pressures identified by first-principles calculations in combination with a swarm structure search. Although tellurium is isoelectronic to sulfur or selenium, its heavier atomic mass and weaker electronegativity makes tellurium hydrides fundamentally distinct from sulfur or selenium hydrides in stoichiometries, structures, and chemical bondings. We identify three metallic stoichiometries of ${\mathrm{H}}_4\mathrm{Te}$, ${\mathrm{H}}_5{\mathrm{Te}}_2$, and ${\mathrm{HTe}}_3$, which are not predicted or known stable structures for sulfur or selenium hydrides. The two hydrogen-rich ${\mathrm{H}}_4\mathrm{Te}$ and ${\mathrm{H}}_5{\mathrm{Te}}_2$ phases are primarily ionic and contain exotic quasimolecular ${\mathrm{H}}_2$ and linear ${\mathrm{H}}_3$ units, respectively. Their high-$T_c$ (e.g., 104 K for ${\mathrm{H}}_4\mathrm{Te}$ at 170 GPa) superconductivity originates from the strong electron-phonon couplings associated with intermediate-frequency H-derived wagging and bending modes, a superconducting mechanism which differs substantially with those in sulfur or selenium hydrides where the high-frequency H-stretching vibrations make considerable contributions.

Exciton diffusion in organic materials provides the operational basis for functioning of such devices as organic solar cells and light-emitting diodes. Here we track the exciton diffusion process in organic semiconductors in real time with a novel technique based on femtosecond photoinduced absorption spectroscopy. Using vacuum-deposited ${\mathrm{C}}_{70}$ layers as a model system, we demonstrate an extremely high diffusion coefficient of $D\approx 3.5\times {10}^{-3}{\mathrm{cm}}^2/\mathrm{s}$ that originates from a surprisingly low energetic disorder of $<5\mathrm{meV}$. The experimental results are well described by the analytical model and supported by extensive Monte Carlo simulations. The proposed noninvasive time-of-flight technique is deemed as a powerful tool for further development of organic optoelectronic components, such as simple layered solar cells, light-emitting diodes, and electrically pumped lasers.

The influence of size differences, shape, mass, and persistent motion on phase separation in binary mixtures has been intensively studied. Here we focus on the exclusive role of diffusivity differences in binary mixtures of equal-sized particles. We find an effective attraction between the less diffusive particles, which are essentially caged in the surrounding species with the higher diffusion constant. This effect leads to phase separation for systems above a critical size: A single close-packed cluster made up of the less diffusive species emerges. Experiments for testing our predictions are outlined.

Investigating a theoretical model of the optical-field-induced current in dielectrics driven by strong few-cycle laser pulses, we propose an asymmetric conducting of the current by forming a heterojunction made of two distinct dielectrics with a low hole mass ($m_h^{*}\ll m_e^{*}$) and low electron mass ($m_e^{*}\ll m_h^{*}$), respectively. This proposition introduces the novel concept of a petahertz (${10}^{15}\mathrm{Hz}$) diode to rectify the current in the petahertz domain, which should be a key ingredient for the electric signal manipulation of future light-wave electronics. Further, we suggest the candidate dielectrics for the heterojunction.

Analyzing the noise in the momentum profiles of single realizations of one-dimensional Bose gases, we present the experimental measurement of the full momentum-space density correlations $⟨\delta n_p\delta n_{p^{\prime }}⟩$, which are related to the two-body momentum correlation function. Our data span the weakly interacting region of the phase diagram, going from the ideal Bose gas regime to the quasicondensate regime. We show experimentally that the bunching phenomenon, which manifests itself as super-Poissonian local fluctuations in momentum space, is present in all regimes. The quasicondensate regime is, however, characterized by the presence of negative correlations between different momenta, in contrast to the Bogolyubov theory for Bose condensates, predicting positive correlations between opposite momenta. Our data are in good agreement with ab initio calculations.

Probing electronic wave functions of polyatomic molecules is one of the major challenges in high-harmonic spectroscopy. The extremely nonlinear nature of the laser-molecule interaction couples the multiple degrees of freedom of the probed system. We combine two-dimensional control of the electron trajectories and vibrational control of the molecules to disentangle the two main steps in high-harmonic generation—ionization and recombination. We introduce a new measurement scheme, frequency-resolved optomolecular gating, which resolves the temporal amplitude and phase of the harmonic emission from excited molecules. Focusing on the study of vibrational motion in ${\mathrm{N}}_2{\mathrm{O}}_4$, we show that such advanced schemes provide a unique insight into the structural and dynamical properties of the underlying mechanism.

We revisit the $m{\alpha }^6(m/M)$ order corrections to the hyperfine splitting in the ${\mathrm{H}}_2^{+}$ ion and find a hitherto unrecognized second-order relativistic contribution associated with the vibrational motion of the nuclei. Inclusion of this correction term produces theoretical predictions which are in excellent agreement with experimental data [K. B. Jefferts, Phys. Rev. Lett. 23, 1476 (1969)], thereby concluding a nearly 50-year-long theoretical quest to explain the experimental results within their 1-ppm error. The agreement between the theory and experiment corroborates the proton structural properties as derived from the hyperfine structure of atomic hydrogen. Our work furthermore indicates that, for future improvements, a full three-body evaluation of the $m{\alpha }^6(m/M)$ correction term will be mandatory.

We propose an approach to quantum phase estimation that can attain precision near the Heisenberg limit without requiring single-particle-resolved state detection. We show that the “one-axis twisting” interaction, well known for generating spin squeezing in atomic ensembles, can also amplify the output signal of an entanglement-enhanced interferometer to facilitate readout. Applying this interaction-based readout to oversqueezed, non-Gaussian states yields a Heisenberg scaling in phase sensitivity, which persists in the presence of detection noise as large as the quantum projection noise of an unentangled ensemble. Even in dissipative implementations—e.g., employing light-mediated interactions in an optical cavity or Rydberg dressing—the method significantly relaxes the detection resolution required for spectroscopy beyond the standard quantum limit.

When an impurity interacts with a bath of phonons it forms a polaron. For increasing interaction strengths the mass of the polaron increases and it can become self-trapped. For impurity atoms inside an atomic Bose-Einstein condensate (BEC) the nature of this transition is not understood. While Feynman’s variational approach to the Fröhlich model predicts a sharp transition for light impurities, renormalization group studies always predict an extended intermediate-coupling region characterized by large phonon correlations. To investigate this intricate regime and to test polaron physics beyond the validity of the Fröhlich model we suggest a versatile experimental setup that allows us to tune both the mass of the impurity and its interactions with the BEC. The impurity is realized as a dark-state polariton (DSP) inside a quasi-two-dimensional BEC. We show that its interactions with the Bogoliubov phonons lead to photonic polarons, described by the Bogoliubov-Fröhlich Hamiltonian, and make theoretical predictions using an extension of a recently introduced renormalization group approach to Fröhlich polarons.

A solid wooden cube fragments into pieces as we sequentially drill holes through it randomly. This seemingly straightforward observation encompasses deep and nontrivial geometrical and probabilistic behavior that is discussed here. Combining numerical simulations and rigorous results, we find off-critical scale-free behavior and a continuous transition at a critical density of holes that significantly differs from classical percolation.

We study a three-dimensional plaquette spin model whose low temperature dynamics is glassy, due to localized defects and effective kinetic constraints. The thermodynamics of this system is smooth at all temperatures. We show that coupling it to a second system with a fixed (quenched) configuration leads to a phase transition, at finite coupling. The order parameter is the overlap between the copies, and the transition is between phases of low and high overlap. We find critical points whose properties are consistent with random-field Ising universality. We analyze the interfacial free energy cost between the high- and low-overlap states that coexist at (and below) the critical point, and we use this cost as the basis for a finite-size scaling analysis. We discuss these results in the context of mean-field and dynamical facilitation theories of the glass transition.