How an electron-hole pair escapes the Coulomb potential at a donor-acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast time scales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor-vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.

The magnetic behavior of polycrystalline samples of ${\mathrm{Er}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ and ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ pyrochlores is studied by magnetization measurements and neutron diffraction. Both compounds undergo a magnetic transition at 140 and 130 K, respectively, associated with an ordering of the Ir sublattice, signaled by thermomagnetic hysteresis. In ${\mathrm{Tb}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, we show that the Ir molecular field leads the Tb magnetic moments to order below 40 K in the all-in–all-out magnetic arrangement. No sign of magnetic long-range order on the Er sublattice is evidenced in ${\mathrm{Er}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ down to 0.6 K where a spin freezing is detected. These contrasting behaviors result from the competition between the Ir molecular field and the different single-ion anisotropy of the rare-earth elements on which it is acting. Additionally, this strongly supports the all-in–all-out iridium magnetic order.

We demonstrate that a ${\mathrm{C}}_{60}$ overlayer enhances the perpendicular magnetic anisotropy of a Co thin film, inducing an inverse spin reorientation transition from in plane to out of plane. The driving force is the ${\mathrm{C}}_{60}/\mathrm{Co}$ interfacial magnetic anisotropy that we have measured quantitatively in situ as a function of the ${\mathrm{C}}_{60}$ coverage. Comparison with state-of-the-art ab initio calculations show that this interfacial anisotropy mainly arises from the local hybridization between ${\mathrm{C}}_{60}$ $p_z$ and Co $d_{z^2}$ orbitals. By generalizing these arguments, we also demonstrate that the hybridization of ${\mathrm{C}}_{60}$ with a Fe(110) surface decreases the perpendicular magnetic anisotropy. These results open the way to tailor the interfacial magnetic anisotropy in organic-material–ferromagnet systems.

The giant acceleration (GA) of diffusion is a universal phenomenon predicted by the theoretical analysis given by Reimann et al. [Phys. Rev. Lett. 87, 010602 (2001)]. Here we apply the theory of the GA of diffusion to a single-molecule experiment on a rotary motor protein, ${\mathrm{F}}_1$, which is a component of ${\mathrm{F}}_{\mathrm{o}}{\mathrm{F}}_1$ adenosine triphosphate synthase. We discuss the energetic properties of ${\mathrm{F}}_1$ and identify a high energy barrier of the rotary potential to be $20k_{B}T$, with the condition that the adenosine diphosphates are tightly bound to the ${\mathrm{F}}_1$ catalytic sites. To conclude, the GA of diffusion is useful for measuring energy barriers in nonequilibrium and single-molecule experiments.

Remarkable advancements in coherence and control fidelity have been achieved in recent years with cryogenic solid-state qubits. Nonetheless, thermalizing such devices to their milliKelvin environments has remained a long-standing fundamental and technical challenge. In this context, we present a systematic study of the first-excited-state population in a 3D transmon superconducting qubit mounted in a dilution refrigerator with a variable temperature. Using a modified version of the protocol developed by Geerlings et al., we observe the excited-state population to be consistent with a Maxwell-Boltzmann distribution, i.e., a qubit in thermal equilibrium with the refrigerator, over the temperature range 35–150 mK. Below 35 mK, the excited-state population saturates at approximately 0.1%. We verified this result using a flux qubit with ten times stronger coupling to its readout resonator. We conclude that these qubits have effective temperature $T_{\text{eff}}=35\mathrm{mK}$. Assuming $T_{\text{eff}}$ is due solely to hot quasiparticles, the inferred qubit lifetime is $108\mu \mathrm{s}$ and in plausible agreement with the measured $80\mu \mathrm{s}$.

Atmospheric ${\mathrm{CO}}_2$ concentrations are being closely monitored by remote sensing experiments which rely on knowing line intensities with an uncertainty of 0.5% or better. Most available laboratory measurements have uncertainties much larger than this. We report a joint experimental and theoretical study providing rotation-vibration line intensities with the required accuracy. The ab initio calculations are extendible to all atmospherically important bands of ${\mathrm{CO}}_2$ and to its isotologues. As such, they will form the basis for detailed ${\mathrm{CO}}_2$ spectroscopic line lists for future studies.

We provide a compact full description of multiboson correlation measurements of arbitrary order $N$ in passive linear interferometers with arbitrary input single-photon pure states. This allows us to physically analyze the novel problem of multiboson correlation sampling at the output of random linear interferometers. Our results also describe general multiboson correlation landscapes for an arbitrary number of input single photons and arbitrary interferometers. In particular, we use two different schemes to demonstrate, respectively, arbitrary-order quantum beat interference and 100% visibility entanglement correlations even for input photons distinguishable in their frequencies.

X-ray free-electron lasers (XFELs) are cutting-edge research tools that produce almost fully coherent radiation with high power and short-pulse length with applications in multiple science fields. There is a strong demand to achieve even shorter pulses and higher radiation powers than the ones obtained at state-of-the-art XFEL facilities. In this context we propose a novel method to generate terawatt-attosecond XFEL pulses, where an XFEL pulse is pushed through several short good-beam regions of the electron bunch. In addition to the elements of conventional XFEL facilities, the method uses only a multiple-slotted foil and small electron delays between undulator sections. Our scheme is thus simple, compact, and easy to implement both in already operating as well as future XFEL projects. We present numerical simulations that confirm the feasibility and validity of our proposal.

Nanoscale self-assembly is naturally subject to impediments at the nanoscale. The recently developed ability to follow processes at the molecular level forces us to resolve older, coarse-grained concepts in terms of their molecular mechanisms. In this Letter, we highlight one such example. We present evidence based on experimental and simulation data that one of the cornerstones of crystal growth theory, the Cabrera-Vermilyea model of step advancement in the presence of impurities, is based on incomplete physics. We demonstrate that the piercing of an impurity fence by elementary steps is not solely determined by the Gibbs-Thomson effect, as assumed by Cabrera-Vermilyea. Our data show that for conditions leading up to growth cessation, step retardation is dominated by the formation of critically sized fluctuations. The growth recovery of steps is counter to what is typically assumed, not instantaneous. Our observations on mesoscopic impurities for lysozyme expose a nucleation-dominated regime of growth that has not been hitherto considered, where the system alternates between zero and near-pure velocity. The time spent by the system in arrest is the nucleation induction time required for the step to amass a supercritical fluctuation that pierces the impurity fence.

We demonstrate that the ratchet effect—a radiation-induced direct current in periodically modulated structures with built-in asymmetry—is dramatically enhanced in the vicinity of the plasmonic resonances and has a nontrivial polarization dependence. For a circular polarization, the current component, perpendicular to the modulation direction, changes sign with the inversion of the radiation helicity. In the high-mobility structures, this component might increase by several orders of magnitude due to the plasmonic effects and exceed the current component in the modulation direction. Our theory also predicts that in the dirty systems, where the plasma resonances are suppressed, the ratchet current is controlled by the Maxwell relaxation.

We study the electronic properties of graphene with a finite concentration of vacancies or other resonant scatterers by a straightforward lattice quantum Monte Carlo calculation. Taking into account a realistic long-range Coulomb interaction, we calculate the distribution of the spin density associated with midgap states and demonstrate antiferromagnetic ordering. An energy gap is open due to interaction effects, both in the bare graphene spectrum and in the vacancy or impurity bands. In the case of a 5% concentration of resonant scatterers the latter gap is estimated to be 0.7 eV and 1.1 eV for graphene on boron nitride and freely suspended graphene, respectively.

We study optimally doped Bi-2212 ($T_c=96\mathrm{K}$) using femtosecond time- and angle-resolved photoelectron spectroscopy. Energy-resolved population lifetimes are extracted and compared with single-particle lifetimes measured by equilibrium photoemission. The population lifetimes deviate from the single-particle lifetimes in the low excitation limit by 1–2 orders of magnitude. Fundamental considerations of electron scattering unveil that these two lifetimes are in general distinct, yet for systems with only electron-phonon scattering they should converge in the low-temperature, low-fluence limit. The qualitative disparity in our data, even in this limit, suggests that scattering channels beyond electron-phonon interactions play a significant role in the electron dynamics of cuprate superconductors.

Among many two-dimensional (2D) high $T_C$ superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that ${\mathrm{Ba}\mathrm{C}}_6$, the superconductivity of which has been missing for many years so far among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at $T_C=65\mathrm{mK}$. By adding this finding as the additional experimental point, a complete figure displaying the relationship between $T_C$ and interlayer distance ($d$) for GICs is now provided, and their possible superconducting mechanisms raised so far are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs.

We demonstrate the first solid-state spin-wave optical quantum memory with on-demand read-out. Using the full atomic frequency comb scheme in a ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal, we store weak coherent pulses at the single-photon level with a signal-to-noise ratio $>10$. Narrow-band spectral filtering based on spectral hole burning in a second ${\mathrm{Pr}}^{3+}:{\mathrm{Y}}_2{\mathrm{SiO}}_5$ crystal is used to filter out the excess noise created by control pulses to reach an unconditional noise level of $(2.0\pm 0.3)\times {10}^{-3}$ photons per pulse. We also report spin-wave storage of photonic time-bin qubits with conditional fidelities higher than achievable by a measure and prepare strategy, demonstrating that the spin-wave memory operates in the quantum regime. This makes our device the first demonstration of a quantum memory for time-bin qubits, with on-demand read-out of the stored quantum information. These results represent an important step for the use of solid-state quantum memories in scalable quantum networks.

Long-lived quantum memories are essential components of a long-standing goal of remote distribution of entanglement in quantum networks. These can be realized by storing the quantum states of light as single-spin excitations in atomic ensembles. However, spin states are often subjected to different dephasing processes that limit the storage time, which in principle could be overcome using spin-echo techniques. Theoretical studies suggest this to be challenging due to unavoidable spontaneous emission noise in ensemble-based quantum memories. Here, we demonstrate spin-echo manipulation of a mean spin excitation of 1 in a large solid-state ensemble, generated through storage of a weak optical pulse. After a storage time of about 1 ms we optically read-out the spin excitation with a high signal-to-noise ratio. Our results pave the way for long-duration optical quantum storage using spin-echo techniques for any ensemble-based memory.

Fluctuations in nonequilibrium steady states generically lead to power law decay of correlations for conserved quantities. Embedded bodies which constrain fluctuations, in turn, experience fluctuation induced forces. We compute these forces for the simple case of parallel slabs in a driven diffusive system. Our model calculations show that the force falls off with slab separation $d$ as $k_{B}T/d$ (at temperature $T$, and in all spatial dimensions) but can be attractive or repulsive. Unlike the equilibrium Casimir force, the force amplitude is nonuniversal and explicitly depends on dynamics. The techniques introduced can be used to study pressure and fluctuation induced forces in a broad class of nonequilibrium systems.

We investigate the direct detection phenomenology of a class of dark matter (DM) models in which DM does not directly interact with nuclei, but rather, the products of its annihilation do. When these annihilation products are very light compared to the DM mass, the scattering in direct detection experiments is controlled by relativistic kinematics. This results in a distinctive recoil spectrum, a nonstandard and/or even absent annual modulation, and the ability to probe DM masses as low as a $\sim 10\mathrm{MeV}$. We use current LUX data to show that experimental sensitivity to thermal relic annihilation cross sections has already been reached in a class of models. Moreover, the compatibility of dark matter direct detection experiments can be compared directly in $E_{\min }$ space without making assumptions about DM astrophysics, mass, or scattering form factors. Lastly, when DM has direct couplings to nuclei, the limit from annihilation to relativistic particles in the Sun can be stronger than that of conventional nonrelativistic direct detection by more than 3 orders of magnitude for masses in a 2–7 GeV window.