Friction between two objects can be understood by the making, stretching, and breaking of thousands of atomic-scale asperities. We have probed single atoms in a nonisotropic surface [the H-terminated Si(100) surface] with a lateral force microscope operating in noncontact mode. We show that these forces are measurably different, depending upon the direction. Experimentally, these differences are observable in both the line profiles and the maximum stiffnesses. Density functional theory calculations show a concerted motion of the whole Si dimer during the tip-sample interaction. These results demonstrate that on an asperity-by-asperity basis, the surface atomic structure plays a strong role in the directional dependence of friction.

${\mathrm{Li}}_x{\mathrm{CoO}}_2$ exhibits intriguing electronic properties due to a strong electron correlation and complex interplay between Co and Li ions. However, fundamental understanding of the nanoscale distribution of Li ions and its effect on the electronic properties remains unclear. We use scanning tunneling microscopy and density functional theory to elucidate the degree of ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ surface electronic state modification that can be achieved by Li ordering. The surface Li ions are highly mobile and preferentially form a ($1\times 1$) hexagonal lattice, whereas the surface ${\mathrm{CoO}}_2$ layer shows metallic and insulating phases, indicating the coexistence of ordered and disordered Li ions in the subsurface layer. These results provide evidence of novel electronic properties produced by spatially inhomogeneous Li-ordering patterns.

We report the momentum-resolved measurement of a two-dimensional electron gas at the ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.

We generalize the exact strong-interaction limit of the exchange-correlation energy of Kohn-Sham density functional theory to open systems with fluctuating particle numbers. When used in the self-consistent Kohn-Sham procedure on strongly interacting systems, this functional yields exact features crucial for important applications such as quantum transport. In particular, the steplike structure of the highest-occupied Kohn-Sham eigenvalue is very well captured, with accurate quantitative agreement with exact many-body chemical potentials. While it can be shown that a sharp derivative discontinuity is present only in the infinitely strongly correlated limit, at finite correlation regimes we observe a slightly smoothened discontinuity, with qualitative and quantitative features that improve with increasing correlation. From the fundamental point of view, our results obtain the derivative discontinuity without making the assumptions used in its standard derivation, offering independent support for its existence.

A quantum point contact was used to observe single-electron fluctuations of a quantum dot in a GaAs heterostructure. The resulting random telegraph signals (RTS) contain statistical information about the electron spin state if the tunneling dynamics are spin dependent. We develop a statistical method to extract information about spin-dependent dynamics from RTS and use it to demonstrate that these dynamics can be studied in the thermal energy regime. The tunneling rates of each spin state are independently measured in a finite external magnetic field. We confirm previous findings of a decrease in overall tunneling rates for the spin excited state compared to the ground state as an external magnetic field is increased.

We derive ab inito exchange parameters for general noncollinear magnetic configurations, in terms of a multiple scattering formalism. We show that the general exchange formula has an anisotropiclike term even in the absence of spin-orbit coupling, and that this term is large, for instance, for collinear configuration in bcc Fe, whereas for fcc Ni it is quite small. We demonstrate that keeping this term leads to what one should consider a biquadratic effective spin Hamiltonian even in the case of collinear arrangement. In noncollinear systems this term results in new tensor elements that are important for exchange interactions at finite temperatures, but they have less importance at low temperature. To illustrate our results in practice, we calculate for bcc Fe magnon spectra obtained from configuration-dependent exchange parameters, where the configurations are determined by finite-temperature effects. Our theory results in the same quantitative results as the finite-temperature neutron scattering experiments.

We report a direct measurement of the low-frequency optical conductivity of large-area single-crystal herbertsmithite, a promising spin-liquid candidate material, by means of terahertz time-domain spectroscopy. In the spectral range below 1.4 THz, we observe a contribution to the real part of the in-plane conductivity ${\sigma }_{ab}(\omega )$ from the spin degree of freedom. This spin-induced conductivity exhibits a power-law dependence on frequency ${\sigma }_{ab}(\omega )\sim {\omega }^{\beta }$ with $\beta \approx 1.4$. Our observation is consistent with the theoretically predicted low-frequency conductivity arising from an emergent gauge field of a gapless U(1) Dirac spin liquid.

We investigate the possibility of controlling the magnetic phase transition of the heterointerface between a half-doped manganite ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ and a multiferroic ${\mathrm{BiFeO}}_3$ (BFO) through magnetoelectric coupling. Using macroscopic magnetometry and element-selective x-ray magnetic circular dichroism at the Mn and Fe $L$ edges, we discover that the ferroelectric polarization of BFO controls simultaneously the magnetization of BFO and ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ (LCMO). X-ray absorption spectra at the oxygen $K$ edge and linear dichroism at the Mn $L$ edge suggest that the interfacial coupling is mainly derived from the superexchange between Mn and Fe $t_{2g}$ spins. The combination of x-ray absorption spectroscopy and mean-field theory calculations reveals that the $d$-electron modulation of Mn cations changes the magnetic coupling in LCMO, which controls the enhanced canted moments of interfacial BFO via the interfacial coupling. Our results demonstrate that the competition between ferromagnetic and antiferromagnetic instability can be modulated by an electric field at the heterointerface, providing another pathway for the electrical field control of magnetism.

The interaction between two associating hydrophobic particles has traditionally been explained in terms of the release of entropically frustrated hydration shell water molecules. However, this picture cannot account for the kinetics of hydrophobic association and is therefore not capable of providing a microscopic description of the hydrophobic interaction (HI). Here, Monte Carlo simulations of a pair of molecular-scale apolar solutes in aqueous solution reveal the critical role of collective fluctuations in the hydrogen bond (HB) network for the microscopic picture of the HI. The main contribution to the HI is the relaxation of solute-water translational correlations. The existence of a heat capacity maximum at the desolvation barrier is shown to arise from softening of non-HB water fluctuations and the relaxation of many-body correlations in the labile HB network. The microscopic event governing the kinetics of hydrophobic association has turned out to be a relatively large critical collective fluctuation in hydration water displacing a substantial fraction of HB clusters from the inner to the outer region of the first hydration shell.

We present a novel numerical approach to track the response of a quantum system to an external perturbation that is progressively switched on. The method is applied, within the framework of the density matrix renormalization group technique, to track current-carrying states of interacting fermions in one dimension and in the presence of an Aharonov-Bohm magnetic flux. This protocol allows us to access highly excited states. We also discuss the connection with the entanglement entropy of these excited states.

Localized Langmuir waves are commonly observed in space plasmas and are a potential source of radio waves. Using electric field data from STEREO, it is shown that these localized Langmuir waves are eigenmodes of density wells estimated independently. An analytic model is developed for the eigenmode frequencies. The inferred depths and widths of the density wells typically only allow the zeroth-order Langmuir eigenmode to form, explaining the preponderance of single-peaked waveforms. More complicated waveforms are shown to be consistent with single eigenmode solutions of more complicated density profiles. The inferred depth of the density well increases with Langmuir wave intensity, consistent with the ponderomotive force but not wave packet collapse.

We study high-energy neutrino production in collimated jets inside progenitors of gamma-ray bursts (GRBs) and supernovae, considering both collimation and internal shocks. We obtain simple, useful constraints, using the often overlooked point that shock acceleration of particles is ineffective at radiation-mediated shocks. Classical GRBs may be too powerful to produce high-energy neutrinos inside stars, which is consistent with IceCube nondetections. We find that ultralong GRBs avoid such constraints and detecting the TeV signal will support giant progenitors. Predictions for low-power GRB classes including low-luminosity GRBs can be consistent with the astrophysical neutrino background IceCube may detect, with a spectral steepening around PeV. The models can be tested with future GRB monitors.

When the Higgs field is nonminimally coupled to gravity, there exists a family of spherically symmetric particlelike solutions to the field equations. These monopoles are the only globally regular and asymptotically flat distributions with finite energy of the Higgs field around compact objects. Moreover, spontaneous scalarization is strongly amplified for specific values of their mass and compactness.

Large-$N$ volume independence in circle-compactified QCD with adjoint Weyl fermions implies the absence of any phase transitions as the radius is dialed to arbitrarily small values. This class of theories is believed to possess a Hagedorn density of hadronic states. It turns out that these properties are in apparent tension with each other, because a Hagedorn density of states typically implies a phase transition at some finite radius. This tension is resolved if there are degeneracies between the spectra of bosonic and fermionic states, as happens in the $N_f=1$ supersymmetric case. Resolution of the tension for $N_f>1$ then suggests the emergence of a fermionic symmetry at large $N$, where there is no supersymmetry. We can escape the Coleman-Mandula theorem since the $N=\infty$ theory is free, with a trivial $S$ matrix. We show an example of such a spectral degeneracy in a nonsupersymmetric toy example which has a Hagedorn spectrum.

We compute the one-loop electroweak Sudakov corrections to the production process $Z(\nu \overline{\nu })+n$ jets, with $n=1$, 2, 3, in $pp$ collisions at the LHC. It represents the main irreducible background to new physics searches at the energy frontier. The results are obtained at the leading and next-to-leading logarithmic accuracy by implementing the general algorithm of Denner and Pozzorini in the event generator for multiparton processes alpgen. For the standard selection cuts used by the ATLAS and CMS Collaborations, we show that the Sudakov corrections to the relevant observables can grow up to $-40\%$ at $\sqrt{s}=14\mathrm{TeV}$. We also include the contribution due to undetected real radiation of massive gauge bosons, to show to what extent the partial cancellation with the large negative virtual corrections takes place in realistic event selections.

We present a calculation of the giant dipole resonance in ${}^{16}\mathrm{O}$ based on a nucleon-nucleon ($NN$) interaction from chiral effective field theory that reproduces $NN$ scattering data with high accuracy. By merging the Lorentz integral transform and the coupled-cluster methods, we extend the previous theoretical limits for breakup observables in light nuclei with mass numbers ($A\le 7$) and address the collective giant dipole resonance of ${}^{16}\mathrm{O}$. We successfully benchmark the new approach against virtually exact results from the hyperspherical harmonics method in ${}^4\mathrm{He}$. Our results for ${}^{16}\mathrm{O}$ reproduce the position and the total strength (bremsstrahlung sum rule) of the dipole response very well. When compared to the cross section from photoabsorption experiments, the theoretical curve exhibits a smeared form of the peak. The tail region between 40 and 100 MeV is reproduced within uncertainties.

Clusters consisting of small molecules containing hydrogen do eject fast protons when illuminated by short x-ray pulses. A suitable overall charging of the cluster controlled by the x-ray intensity induces electron migration from the surface to the bulk leading to efficient segregation of the protons and to a globally hindered explosion of the heavy atoms even outside the screened volume. We investigate this peculiar effect systematically along the isoelectronic sequence of methane over ammonia and water to the atomic limit of neon as a reference. In contrast to core-shell systems where the outer shell is sacrificed to reduce radiation damage, the intricate proton dynamics of hydride clusters allows one to keep the entire backbone of heavy atoms intact.

We observe narrow band pairs of time-correlated photons of wavelengths 776 and 795 nm from nondegenerate four-wave mixing in a laser-cooled atomic ensemble of ${}^{87}\mathrm{Rb}$ using a cascade decay scheme. Coupling the photon pairs into single mode fibers, we observe an instantaneous rate of 7700 pairs per second with silicon avalanche photodetectors, and an optical bandwidth below 30 MHz. Detection events exhibit a strong correlation in time [$g^{(2)}(\tau =0)\approx 5800$] and a high coupling efficiency indicated by a pair-to-single ratio of 23%. The violation of the Cauchy-Schwarz inequality by a factor of $8.4\times {10}^6$ indicates a strong nonclassical correlation between the generated fields, while a Hanbury Brown–Twiss experiment in the individual photons reveals their thermal nature. The comparison between the measured frequency bandwidth and $1/e$ decay time of $g^{(2)}$ indicates a transform-limited spectrum of the photon pairs. The narrow bandwidth and brightness of our source makes it ideal for interacting with atomic ensembles in quantum communication protocols.

A new, more complete view of the mechanical energy budget for Rayleigh-Bénard convection is developed and examined using three-dimensional numerical simulations at large Rayleigh numbers and Prandtl number of 1. The driving role of available potential energy is highlighted. The relative magnitudes of different energy conversions or pathways change significantly over the range of Rayleigh numbers $\mathrm{Ra}\sim {10}^7–{10}^{13}$. At $\mathrm{Ra}<{10}^7$ small-scale turbulent motions are energized directly from available potential energy via turbulent buoyancy flux and kinetic energy is dissipated at comparable rates by both the large- and small-scale motions. In contrast, at $\mathrm{Ra}\ge {10}^{10}$ most of the available potential energy goes into kinetic energy of the large-scale flow, which undergoes shear instabilities that sustain small-scale turbulence. The irreversible mixing is largely confined to the unstable boundary layer, its rate exactly equal to the generation of available potential energy by the boundary fluxes, and mixing efficiency is 50%.

We numerically demonstrate the feasibility of kinematic fast dynamos for a class of time-periodic axisymmetric flows of conducting fluid confined inside a sphere. The novelty of our work is in considering the realistic flows, which are self-consistently determined from the Navier-Stokes equation with specified boundary driving. Such flows can be achieved in a new plasma experiment, whose spherical boundary is capable of differential driving of plasma flows in the azimuthal direction. We show that magnetic fields are self-excited over a range of flow parameters such as amplitude and frequency of flow oscillations, fluid Reynolds (Re) and magnetic Reynolds (Rm) numbers. In the limit of large Rm, the growth rates of the excited magnetic fields are of the order of the advective time scales and practically independent of Rm, which is an indication of the fast dynamo.