Folding a sheet of paper along a curve can lead to structures seen in decorative art and utilitarian packing boxes. Here we present a theory for the simplest such structure: an annular circular strip that is folded along a central circular curve to form a three-dimensional buckled structure driven by geometrical frustration. We quantify this shape in terms of the radius of the circle, the dihedral angle of the fold, and the mechanical properties of the sheet of paper and the fold itself. When the sheet is isometrically deformed everywhere except along the fold itself, stiff folds result in creases with constant curvature and oscillatory torsion. However, relatively softer folds inherit the broken symmetry of the buckled shape with oscillatory curvature and torsion. Our asymptotic analysis of the isometrically deformed state is corroborated by numerical simulations that allow us to generalize our analysis to study structures with multiple curved creases.

Alfvénic modes are observed due to neutral beam injection for the first time in a reversed field pinch plasma. Modeling of the beam deposition and slowing down shows that the velocity and radial localization are high. This allows instability drive from inverse Landau damping of a bump-on-tail in the parallel distribution function or from free energy in the fast ion density gradient. Mode switching from a lower frequency toroidal mode number $n=5$ mode that scales with beam injection velocity to a higher frequency $n=4$ mode with Alfvénic scaling is observed.

Collisionless magnetic reconnection in high-temperature plasmas has been widely studied through fluid-based models. Here, we present results of fluid simulation implementing new equations of state for guide-field reconnection. The new fluid closure accurately accounts for the anisotropic electron pressure that builds in the reconnection region due to electric and magnetic trapping of electrons. In contrast to previous fluid models, our fluid simulation reproduces the detailed reconnection region as observed in fully kinetic simulations. We hereby demonstrate that the new fluid closure self-consistently captures all the physics relevant to the structure of the reconnection region, providing a gateway to a renewed and deeper theoretical understanding of reconnection in weakly collisional regimes.

We report on the creation of ultracold ${}^{84}\mathrm{Sr}_2$ molecules in the electronic ground state. The molecules are formed from atom pairs on sites of an optical lattice using stimulated Raman adiabatic passage (STIRAP). We achieve a transfer efficiency of 30% and obtain $4\times {10}^4$ molecules with full control over the external and internal quantum state. STIRAP is performed near the narrow ${}^{1}S_0\mathrm{\text{−}}{}^{3}P_1$ intercombination transition, using a vibrational level of the $1(0_u^{+})$ potential as an intermediate state. In preparation of our molecule association scheme, we have determined the binding energies of the last vibrational levels of the $1(0_u^{+})$, $1(1_u)$ excited-state and the $X{}^1\Sigma _g^{+}$ ground-state potentials. Our work overcomes the previous limitation of STIRAP schemes to systems with magnetic Feshbach resonances, thereby establishing a route that is applicable to many systems beyond alkali-metal dimers.

We have produced large samples of stable ultracold ${}^{88}\mathrm{Sr}_2$ molecules in the electronic ground state in an optical lattice. The fast, all-optical method of molecule creation involves a near-intercombination-line photoassociation pulse followed by spontaneous emission with a near-unity Franck-Condon factor. The detection uses excitation to a weakly bound electronically excited vibrational level corresponding to a very large dimer and yields a high-$Q$ molecular vibronic resonance. This is the first of two steps needed to create deeply bound ${}^{88}\mathrm{Sr}_2$ for frequency metrology and ultracold chemistry.

We construct equations for the growth kinetics of structural glass within mode-coupling theory, through a nonstationary variant of the three-density correlator defined by G. Biroli et al. [Phys. Rev. Lett. 97, 195701 (2006)]. We solve a schematic form of the resulting equations to obtain the coarsening of the three-point correlator ${\chi }_3(t,t_w)$ as a function of waiting time $t_w$. For a quench into the glass, we find that ${\chi }_3$ attains a peak value $\sim t_w^{0.5}$ at $t-t_w\sim t_w^{0.8}$, providing a theoretical basis for the numerical observations of Parisi [J. Phys. Chem. B 103, 4128 (1999)] and Kob and Barrat [Phys. Rev. Lett. 78, 4581 (1997)]. The aging is not “simple”: the $t_w$ dependence cannot be attributed to an evolving effective temperature.

The dynamics of a quantum phase transition is inextricably woven with the formation of excitations, as a result of critical slowing down in the neighborhood of the critical point. We design a transitionless quantum driving through a quantum critical point, allowing one to access the ground state of the broken-symmetry phase by a finite-rate quench of the control parameter. The method is illustrated in the one-dimensional quantum Ising model in a transverse field. Driving through the critical point is assisted by an auxiliary Hamiltonian, for which the interplay between the range of the interaction and the modes where excitations are suppressed is elucidated.

BiTeI has a layered and noncentrosymmetric structure where strong spin-orbit interaction leads to a giant Rashba spin splitting in the bulk bands. We present direct measurements of the bulk band structure obtained with soft x-ray angle-resolved photoemission (ARPES), revealing the three-dimensional Fermi surface. The observed spindle torus shape bears the potential for a topological transition in the bulk by hole doping. Moreover, the bulk electronic structure is clearly disentangled from the two-dimensional surface electronic structure by means of high-resolution and spin-resolved ARPES measurements in the ultraviolet regime. All findings are supported by ab initio calculations.

One-dimensional quasiperiodic systems, such as the Harper model and the Fibonacci quasicrystal, have long been the focus of extensive theoretical and experimental research. Recently, the Harper model was found to be topologically nontrivial. Here, we derive a general model that embodies a continuous deformation between these seemingly unrelated models. We show that this deformation does not close any bulk gaps, and thus prove that these models are in fact topologically equivalent. Remarkably, they are equivalent regardless of whether the quasiperiodicity appears as an on-site or hopping modulation. This proves that these different models share the same boundary phenomena and explains past measurements. We generalize this equivalence to any Fibonacci-like quasicrystal, i.e., a cut and project in any irrational angle.

We report a bound state of the one-dimensional two-particle (bosonic or fermionic) Hubbard model with an impurity potential. This state has the Bethe-ansatz form, although the model is nonintegrable. Moreover, for a wide region in parameter space, its energy is located in the continuum band. A remarkable advantage of this state with respect to similar states in other systems is the simple analytical form of the wave function and eigenvalue. This state can be tuned in and out of the continuum continuously.

We report the discovery of weak topological insulators by ab initio calculations in a honeycomb lattice. We propose a structure with an odd number of layers in the primitive unit cell as a prerequisite for forming weak topological insulators. Here, the single-layered KHgSb is the most suitable candidate for its large bulk energy gap of 0.24 eV. Its side surface hosts metallic surface states, forming two anisotropic Dirac cones. Although the stacking of even-layered structures leads to trivial insulators, the structures can host a quantum spin Hall layer with a large bulk gap, if an additional single layer exists as a stacking fault in the crystal. The reported honeycomb compounds can serve as prototypes to aid in the finding of new weak topological insulators in layered small-gap semiconductors.

We demonstrate that fermion-boson models with nonlocal interactions can be simulated at finite band filling with the continuous-time quantum Monte Carlo method. We apply this method to explore the influence of the electron-phonon interaction range for a half-filled band in one dimension, covering the full range from the Holstein to the Fröhlich regime. The phase diagram contains metallic, Peierls, and phase-separated regions. Nonlocal interactions suppress the Peierls instability, and thereby lead to almost degenerate power-law exponents for charge and pairing correlations.

Spins transverse to the magnetization of a ferromagnet only survive over a short distance. We develop a drift-diffusion approach that captures the main features of transverse spin effects in systems with arbitrary spin textures (e.g., vortices and domain walls) and generalizes the Valet-Fert theory. In addition to the standard characteristic lengths (mean free path for majority and minority electrons, and spin diffusion length), the theory introduces two length scales, the transverse spin coherence length ${\ell }_{\perp }$ and the (Larmor) spin precession length ${\ell }_L$. We show how ${\ell }_L$ and ${\ell }_{\perp }$ can be extracted from ab initio calculations or measured with giant magnetoresistance experiments. In long (adiabatic) domain walls, we provide an analytic formula that expresses the so-called “nonadiabatic” (or fieldlike) torque in terms of these length scales. However, this nonadiabatic torque is no longer a simple material parameter but depends on the actual spin texture: in thin ($<10\mathrm{nm}$) domain walls, we observe very significant deviations from the adiabatic limit.

We explore the dynamics of entangled polymer chains embedded into nanocomposites. From primitive path analysis, highly entangled polymer chains are found to be significantly disentangled during increment of the volume fraction of spherical nonattractive nanoparticles (NPs) from 0 to 42%. A critical volume fraction, ${\varphi }_c=31\%$, is found to control the crossover from polymer chain entanglements to “NP entanglements.” While below ${\varphi }_c$, the polymer chain relaxation accelerates upon filling, above ${\varphi }_c$, the situation reverses: polymer dynamics becomes geometrically constrained upon adding NPs. Our findings provide a microscopic understanding of the dynamics of entangled polymer chains inside their composites, and offer an explanation for the unusual rheological properties of polymer composites.

Chromatin structure is dynamically organized by chromatin remodelers, motor protein complexes which move and remove nucleosomes. The regulation of remodeler action has recently been proposed to underlie a kinetic proofreading scheme which combines the recognition of histone-tail states and the ATP-dependent loosening of DNA around nucleosomes. Members of the ISWI-family of remodelers additionally recognize linker length between nucleosomes. Here, we show that the additional proofreading step involving linker length alone is sufficient to promote the formation of regular arrays of nucleosomes. ATP-dependent remodeling by bidirectional motors is shown to reinforce positioning as compared to statistical positioning.

The phenomenon of collective predation is analyzed by using a simple individual-based model reproducing spatial animal movements. Two groups of self-propelled organisms are simulated by using Vicseklike models including steric intragroup repulsion. Chase and escape are described by intergroups interactions, attraction (for predators) or repulsion (for preys) from nearest particles of the opposite group. The quantitative analysis of some relevant quantities (total catch time, lifetime distribution, predation rate) allows us to characterize many aspects of the predation phenomenon and gives insights into the study of efficient escape strategies. The reported findings could be of relevance for many basic and applied disciplines, from statistical physics, to ecology, and robotics.

Phase contrast microscopy has become ubiquitous in the field of biology, particularly in qualitative investigations of cellular morphology. However, the use of quantitative phase retrieval methods and their connection to cellular refractive index and dry mass density remain under utilized. This is due in part to the restriction of phase and cellular mass determination to custom built instruments, involved mathematical analysis, and prohibitive sample perturbations. We introduce tomographic bright field imaging, an accessible optical imaging technique enabling the three dimensional measurement of cellular refractive index and dry mass density using a standard transillumination optical microscope. The validity of the technique is demonstrated on polystyrene spheres. The technique is then applied to the measurement of the refractive index, dry mass, volume, and density of red blood cells. This optical technique enables a simple and robust means to perform quantitative investigations of engineered and biological specimens in three dimensions using standard optical microscopes.

We use the “magnetic tweezers” technique to show the structural transitions that the DNA undergoes in the force-torsion space. In particular, we focus on the regions corresponding to negative supercoiling. These regions are characterized by the formation of the so-called denaturation bubbles, which play an essential role in the replication and transcription of DNA. We experimentally map the region of the force-torsion space where the denaturation takes place. We observe that large fluctuations in DNA extension occur at one of the boundaries of this region, i.e., when the formation of denaturation bubbles and of plectonemes compete. To describe the experiments, we introduce a suitable extension of the classical model. The model correctly describes the position of the denaturation regions, the transition boundaries, and the measured values of the DNA extension fluctuations.

We model the robustness against random failure or an intentional attack of networks with an arbitrary large-scale structure. We construct a block-based model which incorporates—in a general fashion—both connectivity and interdependence links, as well as arbitrary degree distributions and block correlations. By optimizing the percolation properties of this general class of networks, we identify a simple core-periphery structure as the topology most robust against random failure. In such networks, a distinct and small “core” of nodes with higher degree is responsible for most of the connectivity, functioning as a central “backbone” of the system. This centralized topology remains the optimal structure when other constraints are imposed, such as a given fraction of interdependence links and fixed degree distributions. This distinguishes simple centralized topologies as the most likely to emerge, when robustness against failure is the dominant evolutionary force.

In the Dirac-sea picture, the physics of pair production and instability of the quantum electrodynamics vacuum in presence of an oscillating electric field resembles the phenomenon of interband transition of light waves in photonic superlattices induced by a geometric curvature. We realize a binary wave guide superlattice with a curved optical axis mimicking dynamical pair production induced by two counterpropagating ultrastrong laser pulses. Our optical analogue enables visualization of formation of electron-positron pair in physical space as splitting of a wave packet, originally representing an electron in the Dirac sea.