We report on magnetic resonance studies of high-density atomic hydrogen and deuterium in solid hydrogen matrices at temperatures below 1 K. Average concentrations of H atoms $\approx 3\times {10}^{19}{\mathrm{cm}}^{-3}$ are obtained in chemical tunneling reactions of isotope exchange with D atoms. The products of these reactions are closely located pairs of H atoms near ${\mathrm{D}}_2$ molecules with strong exchange interactions. We discovered a dynamic nuclear polarization effect on H atoms created by pumping the center of the H electron spin resonance spectrum, similar to the Overhauser effect in metals. Our results indicate that H atoms may be arranged inside molecular matrices at separations equivalent to local concentrations of $2.6\times {10}^{21}{\mathrm{cm}}^{-3}$. This opens up a way to build a metallic state of atomic hydrogen at zero pressure.

We present a combination of experimental data and modeling that explains some of the important characteristics of black silicon (BSi) developed in cryogenic reactive ion etching (RIE) processes, including static properties (dependence of resulting topography on process parameters) and dynamic aspects (evolution of topography with process time). We generate a phase diagram predicting the RIE parameter combinations giving rise to different BSi geometries and show that the topographic details of BSi explain the metamaterial characteristics that are responsible for its low reflectivity. In particular, the unique combination of needle and hole features of various heights and depths, which is captured by our model and confirmed by focused ion beam nanotomography, creates a uniquely smooth transition in refractive index. The model also correctly describes dynamical characteristics, such as the dependence of aspect ratio on process time, and the prediction of new etching fronts appearing at topographical saddle points during the incipient stages of BSi development—a phenomenon reported here for the first time.

Plastic deformation mediated by collective dislocation dynamics is investigated in the two-dimensional phase-field crystal model of sheared single crystals. We find that intermittent fluctuations in the dislocation population number accompany bursts in the plastic strain-rate fluctuations. Dislocation number fluctuations exhibit a power-law spectral density $1/f^2$ at high frequencies $f$. The probability distribution of number fluctuations becomes bimodal at low driving rates corresponding to a scenario where low density of defects alternates at irregular times with high populations of defects. We propose a simple stochastic model of dislocation reaction kinetics that is able to capture these statistical properties of the dislocation density fluctuations as a function of shear rate.

The electronic structure of the iron-based superconductor ${\mathrm{Ba}}_2{\mathrm{Ti}}_2{\mathrm{Fe}}_2{\mathrm{As}}_4\mathrm{O}$ ($T_c^{\text{onset}}=23.5\mathrm{K}$) has been investigated by using angle-resolved photoemission spectroscopy and combined local density approximation and dynamical mean field theory calculations. The electronic states near the Fermi level are dominated by both the Fe $3d$ and Ti $3d$ orbitals, indicating that the spacer layers separating different FeAs layers are also metallic. By counting the enclosed volumes of the Fermi surface sheets, we observe a large self-doping effect; i.e., 0.25 electrons per unit cell are transferred from the FeAs layer to the ${\mathrm{Ti}}_2{\mathrm{As}}_2\mathrm{O}$ layer, leaving the FeAs layer in a hole-doped state. This exotic behavior is successfully reproduced by our dynamical mean field calculations, in which the self-doping effect is attributed to the electronic correlations in the $3d$ shells. Our work provides an alternative route of effective doping without element substitution for iron-based superconductors.

The precession and damping of a collinear magnetization displaced from its equilibrium are well described by the Landau-Lifshitz-Gilbert equation. The theoretical and experimental complexity of noncollinear magnetizations is such that it is not known how the damping is modified by the noncollinearity. We use first-principles scattering theory to investigate transverse domain walls (DWs) of the important ferromagnetic alloy ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ and show that the damping depends not only on the magnetization texture but also on the specific dynamic modes of Bloch and Néel DWs in ways that were not theoretically predicted. Even in the highly disordered ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ alloy, the damping is found to be remarkably nonlocal.

At the interface of an $s$-wave superconductor and a three-dimensional topological insulator, Majorana zero modes and Majorana helical states have been proposed to exist respectively around magnetic vortices and geometrical edges. Here we first show that randomly distributed magnetic impurities at such an interface will induce bound states that broaden into impurity bands inside (but near the edges of) the superconducting gap, which remains open unless the impurity concentration is too high. Next we find that an increase in the superconducting gap suppresses both the oscillation magnitude and the period of the Ruderman-Kittel-Kasuya-Yosida interaction between two magnetic impurities. Within a mean-field approximation, the ferromagnetic Curie temperature is found to be essentially independent of the superconducting gap, an intriguing phenomenon due to a compensation effect between the short-range ferromagnetic and long-range antiferromagnetic interactions. The existence of robust superconductivity and persistent ferromagnetism at the interface allows realization of a novel topological phase transition from a nonchiral to a chiral superconducting state at sufficiently low temperatures, providing a new platform for topological quantum computation.

We report the discovery of a correlated insulator with a bulk gap at 2/3 filling in a geometrically frustrated Hubbard model that describes the low-energy physics of ${\mathrm{Mo}}_3{\mathrm{S}}_7{(\mathrm{dmit})}_3$. This is very different from the Mott insulator expected at half-filling. We show that the insulating phase, which persists even for very weak electron-electron interactions ($U$), is adiabatically connected to the Haldane phase and is consistent with experiments on ${\mathrm{Mo}}_3{\mathrm{S}}_7{(\mathrm{dmit})}_3$.

After nearly 20 years of study, the origin of the spin-liquid state in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ remains a challenge for experimentalists and theorists alike. To improve our understanding of the exotic magnetism in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, we synthesize a chemical pressure analog: ${\mathrm{Tb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$. Substitution of titanium by germanium results in a lattice contraction and enhanced exchange interactions. We characterize the magnetic ground state of ${\mathrm{Tb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ with specific heat, ac and dc magnetic susceptibility, and polarized neutron scattering measurements. Akin to ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, there is no long-range order in ${\mathrm{Tb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ down to 20 mK. The Weiss temperature of $-19.2(1)\mathrm{K}$, which is more negative than that of ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, supports the picture of stronger antiferromagnetic exchange. Polarized neutron scattering of ${\mathrm{Tb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ reveals that liquidlike correlations dominate in this system at 3.5 K. However, below 1 K, the liquidlike correlations give way to intense short-range ferromagnetic correlations with a length scale similar to the Tb-Tb nearest neighbor distance. Despite stronger antiferromagnetic exchange, the ground state of ${\mathrm{Tb}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ has ferromagnetic character, in stark contrast to the pressure-induced antiferromagnetic order observed in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$.

It is well known that a bosonic Mott insulator can be realized by condensing vortices of a boson condensate. Usually, a vortex becomes an antivortex (and vice versa) under time reversal symmetry, and the condensation of vortices results in a trivial Mott insulator. However, if each vortex or antivortex interacts with a spin trapped at its core, the time reversal transformation of the composite vortex operator will contain an extra minus sign. It turns out that such a composite vortex condensed state is a bosonic topological insulator (BTI) with gapless boundary excitations protected by $U(1)⋊Z_2^T$ symmetry. We point out that in BTI, an external $\pi$-flux monodromy defect carries a Kramers doublet. We propose lattice model Hamiltonians to realize the BTI phase, which might be implemented in cold atom systems or spin-1 solid state systems.

We study the behavior of the stationary velocity of a driven particle in an environment of mobile hard-core obstacles. Based on a lattice gas model, we demonstrate analytically that the drift velocity can exhibit a nonmonotonic dependence on the applied force, and show quantitatively that such negative differential mobility (NDM), observed in various physical contexts, is controlled by both the density and diffusion time scale of the obstacles. Our study unifies recent numerical and analytical results obtained in specific regimes, and makes it possible to determine analytically the region of the full parameter space where NDM occurs. These results suggest that NDM could be a generic feature of biased (or active) transport in crowded environments.

Amyloid fibrils are ubiquitous proteinaceous aggregates occurring in vivo and in vitro, with an invariant structural fingerprint at the molecular length scale. However, interpretation of their mesoscopic architectures is complicated by diverse observable polymorphic states. We here present a constitutive model for amyloid fibrils based on the minimization of the total energy per fibril. The model is benchmarked on real amyloid fibrils studied by atomic force microscopy. We use multistranded $\beta$-lactoglobulin amyloid fibrils as a model system exhibiting a rich polymorphism. The constitutive model quantitatively recapitulates the main mesoscopic topological features of amyloid fibrils, that is, the evolution of fibril periodicity as a function of the ionic strength of the solution and of the fibril width. A universal mesoscopic structural signature of the fibrils emerges from this picture, predicting a general, parameter-free law for the periodicity of the fibrils, that depends solely on the number of protofilaments per fibril. These predictions are validated experimentally and conclusively highlight the role of competing electrostatic and elastic contributions as the main players in the establishment of amyloid fibrils structure.

Stochasticity in gene expression can give rise to fluctuations in protein levels and lead to phenotypic variation across a population of genetically identical cells. Recent experiments indicate that bursting and feedback mechanisms play important roles in controlling noise in gene expression and phenotypic variation. A quantitative understanding of the impact of these factors requires analysis of the corresponding stochastic models. However, for stochastic models of gene expression with feedback and bursting, exact analytical results for protein distributions have not been obtained so far. Here, we analyze a model of gene expression with bursting and feedback regulation and obtain exact results for the corresponding protein steady-state distribution. The results obtained provide new insights into the role of bursting and feedback in noise regulation and optimization. Furthermore, for a specific choice of parameters, the system studied maps on to a two-state biochemical switch driven by a bursty input noise source. The analytical results derived provide quantitative insights into diverse cellular processes involving noise in gene expression and biochemical switching.

We investigate the distribution of errors on a computationally useful entangled state generated via the repeated emission from an emitter undergoing strongly non-Markovian evolution. For emitter-environment coupling of pure-dephasing form, we show that the probability that a particular patten of errors occurs has a bound of Markovian form, and thus, accuracy threshold theorems based on Markovian models should be just as effective. Beyond the pure-dephasing assumption, though complicated error structures can arise, they can still be qualitatively bounded by a Markovian error model.

We examine the nonlinear structure of gravitationally collapsed objects that form in our simulations of wavelike cold dark matter, described by the Schrödinger-Poisson (SP) equation with a particle mass $\sim 10^{-22}\mathrm{eV}$. A distinct gravitationally self-bound solitonic core is found at the center of every halo, with a profile quite different from cores modeled in the warm or self-interacting dark matter scenarios. Furthermore, we show that each solitonic core is surrounded by an extended halo composed of large fluctuating dark matter granules which modulate the halo density on a scale comparable to the diameter of the solitonic core. The scaling symmetry of the SP equation and the uncertainty principle tightly relate the core mass to the halo specific energy, which, in the context of cosmological structure formation, leads to a simple scaling between core mass ($M_c$) and halo mass ($M_h$), $M_c\propto a^{-1/2}{M}_h^{1/3}$, where $a$ is the cosmic scale factor. We verify this scaling relation by (i) examining the internal structure of a statistical sample of virialized halos that form in our 3D cosmological simulations and by (ii) merging multiple solitons to create individual virialized objects. Sufficient simulation resolution is achieved by adaptive mesh refinement and graphic processing units acceleration. From this scaling relation, present dwarf satellite galaxies are predicted to have kiloparsec-sized cores and a minimum mass of $\sim 10^8{M}_{\odot }$, capable of solving the small-scale controversies in the cold dark matter model. Moreover, galaxies of $2\times 10^{12}{M}_{\odot }$ at $z=8$ should have massive solitonic cores of $\sim 2\times 10^9{M}_{\odot }$ within $\sim 60\mathrm{pc}$. Such cores can provide a favorable local environment for funneling the gas that leads to the prompt formation of early stellar spheroids and quasars.

Deformation, a key concept in our understanding of heavy nuclei, is based on a mean-field description that breaks the rotational invariance of the nuclear many-body Hamiltonian. We present a method to analyze nuclear deformations at finite temperature in a framework that preserves rotational invariance. The auxiliary-field Monte Carlo method is used to generate a statistical ensemble and calculate the probability distribution associated with the quadrupole operator. Applying the technique to nuclei in the rare-earth region, we identify model-independent signatures of deformation and find that deformation effects persist to temperatures higher than the spherical-to-deformed shape phase-transition temperature of mean-field theory.

We optimize chiral interactions at next-to-next-to leading order to observables in two- and three-nucleon systems and compute Gamow-Teller transitions in ${}^{14}\mathrm{C}$ and ${}^{22,24}\mathrm{O}$ using consistent two-body currents. We compute spectra of the daughter nuclei ${}^{14}\mathrm{N}$ and ${}^{22,24}\mathrm{F}$ via an isospin-breaking coupled-cluster technique, with several predictions. The two-body currents reduce the Ikeda sum rule, corresponding to a quenching factor $q^2\approx 0.84–0.92$ of the axial-vector coupling. The half-life of ${}^{14}\mathrm{C}$ depends on the energy of the first excited $1^{+}$ state, the three-nucleon force, and the two-body current.