It is generally believed that filling atoms randomly and uniformly distribute in caged crystals, such as skutterudite compounds. Here, we report first-principles and experimental discovery of a multiscale filling-fraction fluctuation in the $R{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ system. ${\mathrm{La}}_{0.8}{\mathrm{Ti}}_{0.1}{\mathrm{Ga}}_{0.1}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ spontaneously separates into La-rich and La-poor skutterudite phases, leading to multiscale strain field fluctuations. As a result, glasslike ultralow lattice thermal conductivity approaching the theoretical minimum is achieved, mainly due to strain field scattering of high-energy phonons. These findings reveal that an uneven distribution of filling atoms is efficient to further reduce the lattice thermal conductivity of caged crystals.

We model the accretion of gas onto a population of massive primordial black holes in the Milky Way and compare the predicted radio and x-ray emission with observational data. We show that, under conservative assumptions on the accretion process, the possibility that $\mathcal{O}(10)M_{\odot }$ primordial black holes can account for all of the dark matter in the Milky Way is excluded at $5\sigma$ by a comparison with a Very Large Array radio catalog at 1.4 GHz and at $\simeq 40\sigma$ by a comparison with a Chandra x-ray catalog (0.5–8 keV). We argue that this method can be used to identify such a population of primordial black holes with more sensitive future radio and x-ray surveys.

Block copolymer (BCP) melt assembly has been studied for decades, focusing largely on self-organized spatial patterns of periodically ordered segment density. Here, we demonstrate that underlying the well-known composition profiles (i.e., ordered lamella, cylinders, spheres, and networks) are generic and heterogeneous patterns of segment orientation that couple strongly to morphology, even in the absence of specific factors that promote intra or interchain segment alignment. We employ both self-consistent field theory and coarse-grained simulation methods to measure polar and nematic order parameters of segments in a freely jointed chain model of diblock melts. We show that BCP morphologies have a multizone texture, with segments predominantly aligned normal and parallel to interdomain interfaces in the respective brush and interfacial regions of the microdomain. Further, morphologies with anisotropically curved interfaces (i.e., cylinders and networks) exhibit biaxial order that is aligned to the principal curvature axes of the interface.

We explore the interplay between tunneling and interatomic interactions in the dynamics of a bosonic Josephson junction. We tune the scattering length of an atomic ${}^{39}\mathrm{K}$ Bose-Einstein condensate confined in a double-well trap to investigate regimes inaccessible to other superconducting or superfluid systems. In the limit of small-amplitude oscillations, we study the transition from Rabi to plasma oscillations by crossing over from attractive to repulsive interatomic interactions. We observe a critical slowing down in the oscillation frequency by increasing the strength of an attractive interaction up to the point of a quantum phase transition. With sufficiently large initial oscillation amplitude and repulsive interactions, the system enters the macroscopic quantum self-trapping regime, where we observe coherent undamped oscillations with a self-sustained average imbalance of the relative well population. The exquisite agreement between theory and experiments enables the observation of a broad range of many body coherent dynamical regimes driven by tunable tunneling energy, interactions and external forces, with applications spanning from atomtronics to quantum metrology.

We present a first model-independent calculation of $\pi \pi$ intermediate states in the hadronic-light-by-light (HLBL) contribution to the anomalous magnetic moment of the muon $(g-2{)}_{\mu }$ that goes beyond the scalar QED pion loop. To this end, we combine a recently developed dispersive description of the HLBL tensor with a partial-wave expansion and demonstrate that the known scalar-QED result is recovered after partial-wave resummation. Using dispersive fits to high-statistics data for the pion vector form factor, we provide an evaluation of the full pion box $a_{\mu }^{\pi \mathrm{box}}=-15.9(2)\times {10}^{-11}$. We then construct a suitable input for the ${\gamma }^{*}{\gamma }^{*}\to \pi \pi$ helicity partial waves, based on a pion-pole left-hand cut and show that for the dominant charged-pion contribution, this representation is consistent with the two-loop chiral prediction and the COMPASS measurement for the pion polarizability. This allows us to reliably estimate $S$-wave rescattering effects to the full pion box and leads to our final estimate for the sum of these two contributions $a_{\mu }^{\pi \mathrm{box}}+a_{\mu ,J=0}^{\pi \pi ,\pi \text{−}\text{pole}\mathrm{LHC}}=-24(1)\times {10}^{-11}$.

Recent ab initio lattice studies have found that the interactions between alpha particles (${}^4\mathrm{He}$ nuclei) are sensitive to seemingly minor details of the nucleon-nucleon force, such as interaction locality. In order to uncover the essential physics of this puzzling phenomenon without unnecessary complications, we study a simple model involving two-component fermions in one spatial dimension. We probe the interaction between two bound dimers for several different particle-particle interactions and measure an effective potential between the dimers using external point potentials which act as numerical tweezers. We find that the strength and range of the local part of the particle-particle interactions play a dominant role in shaping the interactions between the dimers and can even determine the overall sign of the effective potential.

Fragmentation of molecular nitrogen dimers $({\mathrm{N}}_2{)}_2$ induced by collision with low energy 90 keV ${\mathrm{Ar}}^{9+}$ ions is studied to evidence the influence of a molecular environment on the fragmentation dynamics of ${\mathrm{N}}_2$ cations. Following the capture of three or four electrons from the dimer, the three-body ${\mathrm{N}}_2^{+}+{\mathrm{N}}^{m+}+{\mathrm{N}}^{n+}$ [with $(m,n)=(1,1)$ or (1, 2)] fragmentation channels provide clean experimental cases where molecular fragmentation may occur in the presence of a neighbor molecular cation. The effect of the environment on the fragmentation dynamics within the dimer is investigated through the comparison of the kinetic energy release (KER) spectra for these three-body channels and for isolated ${\mathrm{N}}_2^{(m+n)+}$ monomer cations. The corresponding KER spectra exhibit energy shifts of the order of 10 eV, attributed to the deformation of the ${\mathrm{N}}^{m+}+{\mathrm{N}}^{n+}$ potential energy curves in the presence of the neighboring ${\mathrm{N}}_2^{+}$ cation. The KER structures remain unchanged, indicating that the primary collision process is not significantly affected by the presence of a neighbor molecule.

We study the generation of planar quantum squeezed (PQS) states by quantum nondemolition (QND) measurement of an ensemble of ${}^{87}\mathrm{Rb}$ atoms with a Poisson distributed atom number. Precise calibration of the QND measurement allows us to infer the conditional covariance matrix describing the $F_y$ and $F_z$ components of the PQS states, revealing the dual squeezing characteristic of PQS states. PQS states have been proposed for single-shot phase estimation without prior knowledge of the likely values of the phase. We show that for an arbitrary phase, the generated PQS states can give a metrological advantage of at least 3.1 dB relative to classical states. The PQS state also beats, for most phase angles, single-component-squeezed states generated by QND measurement with the same resources and atom number statistics. Using spin squeezing inequalities, we show that spin-spin entanglement is responsible for the metrological advantage.

One of the most fundamental problems in optomechanical cooling is how small the thermal phonon number of a mechanical oscillator can be achieved under the radiation pressure of a proper cavity field. Different from previous theoretical predictions, which were based on an optomechanical system’s time-independent steady states, we treat such cooling as a dynamical process of driving the mechanical oscillator from its initial thermal state, due to its thermal equilibrium with the environment, to a stabilized quantum state of higher purity. We find that the stabilized thermal phonon number left in the end actually depends on how fast the cooling process could be. The cooling speed is decided by an effective optomechanical coupling intensity, which constitutes an essential parameter for cooling, in addition to the sideband resolution parameter that has been considered in other theoretical studies. The limiting thermal phonon number that any cooling process cannot surpass exhibits a discontinuous jump across a certain value of the parameter.

We examine integrable turbulence (IT) in the framework of the defocusing cubic one-dimensional nonlinear Schrödinger equation. This is done theoretically and experimentally, by realizing an optical fiber experiment in which the defocusing Kerr nonlinearity strongly dominates linear dispersive effects. Using a dispersive-hydrodynamic approach, we show that the development of IT can be divided into two distinct stages, the initial, prebreaking stage being described by a system of interacting random Riemann waves. We explain the low-tailed statistics of the wave intensity in IT and show that the Riemann invariants of the asymptotic nonlinear geometric optics system represent the observable quantities that provide new insight into statistical features of the initial stage of the IT development by exhibiting stationary probability density functions.

We report the observation of superfluidlike mass flow through coin-shaped $8\mu \mathrm{m}$ thick solid ${}^4\mathrm{He}$ samples sandwiched between superfluid leads. Mass flow is found from the melting pressure to at least 30 bar with a concomitant decrease in the onset temperature from 1 to 0.25 K. The mass-flow rate is found to be sample dependent and can be enhanced by thermal annealing. The flow rate decreases with temperature and decays nearly exponentially with the pressure of the samples. The dissipation associated with the mass flow decreases with temperature and becomes superfluidlike near 0.1 K. In contrast to earlier studies on centimeter-thick samples, we do not see a sharp cutoff in the mass-flow rate at low temperature.

Employing ab initio calculations we demonstrate that the complex structural modulations experimentally observed in ultrathin Fe films on Cu(001) originate from Fe bulk phases that arise under extreme deformations. Specifically, we show that the structural modulations correspond to the motifs observed when transforming fcc Fe to bcc Fe in the Pitsch orientation relationship [$(001{)}_{\mathrm{fcc}}||(1\overline{1}0{)}_{\mathrm{bcc}}$]. The observed structural equivalence between surface and unstable bulk structures naturally explains the experimentally reported magnetic and structural transitions when going from low (two to four MLs) to intermediate (four to ten MLs) film coverages.

The ${}^{181}\mathrm{Ta}$ quadrupole resonance [nuclear quadrupole resonance (NQR)] technique is utilized to investigate the microscopic magnetic properties of the Weyl semimetal TaP. We find three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I=7/2$ nuclear spin. A quadrupole coupling constant, ${\nu }_Q=19.250\mathrm{MHz}$, and an asymmetric parameter of the electric field gradient, $\eta =0.423$, are extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin-lattice relaxation rate ($1/T_{1}T$) is measured for the $f_2$ line ($\pm 5/2\leftrightarrow \pm 3/2$ transition). We find that there exist two regimes with quite different relaxation processes. Above $T^{*}\approx 30\mathrm{K}$, a pronounced $(1/T_{1}T)\propto T^2$ behavior is found, which is attributed to the magnetic excitations at the Weyl nodes with temperature-dependent orbital hyperfine coupling. Below $T^{*}$, the relaxation is mainly governed by a Korringa process with $1/T_{1}T=\text{const}$, accompanied by an additional $T^{-1/2}$-type dependence to fit our experimental data. We show that Ta NQR is a novel probe for the bulk Weyl fermions and their excitations.

We report on a metal-insulator transition (MIT) that is observed in an electron system at the ${\mathrm{SmTiO}}_3/{\mathrm{SrTiO}}_3$ interface. This MIT is characterized by an abrupt transition at a critical temperature, below which the resistance changes by more than an order of magnitude. The temperature of the transition systematically depends on the carrier density, which is tuned from $\sim 1\times {10}^{14}$ to $3\times {10}^{14}{\mathrm{cm}}^{-2}$ by changing the ${\mathrm{SmTiO}}_3$ thickness. An analysis of the transport properties shows non-Fermi-liquid behavior and mass enhancement as the carrier density is lowered. We compare the MIT characteristics with those of known MITs in other material systems and show that they are distinctly different in several aspects. We tentatively conclude that both long-range Coulomb interactions and the fixed charge at the polar interface are likely to play a role in this MIT. The strong dependence on the carrier density makes this MIT of interest for field-tunable devices.

In the vanadium oxyfluoride compound $({\mathrm{NH}}_4{)}_2[{\mathrm{C}}_7{\mathrm{H}}_{14}\mathrm{N}][{\mathrm{V}}_7{\mathrm{O}}_6{\mathrm{F}}_{18}]$ (DQVOF), the ${\mathrm{V}}^{4+}$ ($3d^1$, $S=1/2$) ions realize a unique, highly frustrated breathing kagome lattice composed of alternately sized, corner-sharing equilateral triangles. Here we present an ${}^{17}\mathrm{O}$ NMR study of DQVOF, which isolates the local susceptibility of the breathing kagome network. By a fit to series expansion, we extract the ratio of the interactions within the breathing kagome plane, $J_{\nabla }/J_{\mathrm{\Delta }}=0.55(4)$, and the mean antiferromagnetic interaction $\overline{J}=60(7)\mathrm{K}$. Spin lattice ($T_1$) measurements reveal an essentially gapless excitation spectrum with a maximum gap $\mathrm{\Delta }/\overline{J}=0.007(7)$. Our study provides new impetus for further theoretical investigations in order to establish whether the gapless spin liquid behavior displayed by DQVOF is intrinsic to its breathing kagome lattice or whether it is due to perturbations to this model, such as a residual coupling of the ${\mathrm{V}}^{4+}$ ions in the breathing kagome planes to the interlayer ${\mathrm{V}}^{3+}$ ($S=1$) spins.

Nuclear resonant x-ray diffraction in grazing incidence geometry is used to determine the lateral magnetic configuration in a one-dimensional lattice of ferromagnetic nanostripes. During magnetic reversal, strong nuclear superstructure diffraction peaks appear in addition to the electronic ones due to an antiferromagnetic order in the nanostripe lattice. We show that the analysis of the angular distribution together with the time dependence of the resonantly diffracted x rays reveals surface spin structures with very high sensitivity. This scattering technique provides unique access to laterally correlated spin configurations in magnetically ordered nanostructures and, in perspective, also to their dynamics.

We present a semiclassical analytic model for spherical core-shell surface plasmon lasers. Within this model, we drop the widely used one-mode approximations in favor of fully electromagnetic Mie theory. This allows for incorporation of realistic gain relaxation rates that so far are massively underestimated. Especially, higher order modes can undermine and even reverse the beneficial effects of the strong Purcell effect in such systems. Our model gives a clear view on gain and resonator requirements, as well as on the output characteristics that will help experimenters to design more efficient particle-based spasers.