When two jets of fluid collide, they can “bounce” off each other, due to a thin film of air which keeps them separated. We describe the phenomenon of stable noncoalescence between two jets of the same fluid, colliding obliquely with each other. Using a simple experimental setup, we carry out a parametric study of the bouncing jets by varying the jet diameter, velocity, angle of inclination, and fluid viscosity, which suggests that the contact time of bouncing jets scales as the square root of the normal Weber number We. A dimensionless parameter $K=(\mathrm{We}\sqrt{\mathrm{Re}}/\sin \alpha {)}^{1/2}$, where Re is the normal Reynolds number and $\alpha$ the angle of inclination of the jets, quantitatively captures the transition of colliding jets from bouncing to coalescence. This parameter draws parallels between jet coalescence and droplet splashing and indicates that the transition is governed by a surface instability. Stable and continuous noncoalescence between fluid jets makes it a good platform for experimental studies of the interaction between fluid interfaces and the properties of the interfacial air films.

The effects of an electron cloud ($e$-cloud) on beam dynamics are one of the major factors limiting performances of high intensity positron, proton, and ion storage rings. In the electron-positron collider DAΦNE, namely, a horizontal beam instability due to the electron-cloud effect has been identified as one of the main limitations on the maximum stored positron beam current and as a source of beam quality deterioration. During the last machine shutdown in order to mitigate such instability, special electrodes have been inserted in all dipole and wiggler magnets of the positron ring. It has been the first installation all over the world of this type since long metallic electrodes have been installed in all arcs of the collider positron ring and are currently used during the machine operation in collision. This has allowed a number of unprecedented measurements ($e$-cloud instabilities growth rate, transverse beam size variation, tune shifts along the bunch train) where the $e$-cloud contribution is clearly evidenced by turning the electrodes on and off. In this Letter we briefly describe a novel design of the electrodes, while the main focus is on experimental measurements. Here we report all results that clearly indicate the effectiveness of the electrodes for $e$-cloud suppression.

We consider a 1D Bose gas with attractive interactions in a highly excited state containing no bound states. We show that in this so-called super Tonks-Girardeau gas, relaxation processes are suppressed, making the system metastable over long time scales. We compute dynamical correlation functions, revealing the structure of excitations, an enhancement of umklapp correlations and new branches due to intermediate bound states. These features give a clear signature of the super Tonks-Girardeau regime and can be used to experimentally identify it. We demonstrate that, despite its off-equilibrium nature, the system displays critical behavior: correlation functions are characterized by asymptotic power-law decay described by the Luttinger liquid framework.

A slave rotor—Hartree-Fock formalism is presented for studying the properties of the $p\mathrm{\text{−}}d$ model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the ${\mathrm{LaNiO}}_3$ and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

Epitaxial circuitry offers a revolution in silicon technology, with components that can be fabricated on atomic scales. We perform the first ab initio calculation of atomically thin epitaxial nanowires in silicon, investigating the fundamental electronic properties of wires two P atoms thick, similar to those produced this year by Weber et al. For the first time, we catch a glimpse of disorder-related effects in the wires—a prerequisite for understanding real fabricated systems. Interwire interactions are made negligible by including 40 ML of silicon in the vertical direction (and the equivalent horizontally). Accurate pictures of band splittings and the electronic density are presented, and for the first time the effective masses of electrons in such device components are calculated.

We report noninvasive single-charge detection of the full probability distribution $P_n$ of the initialization of a quantum dot with $n$ electrons for rapid decoupling from an electron reservoir. We analyze the data in the context of a model for sequential tunneling pinch-off, which has generic solutions corresponding to two opposing mechanisms. One limit considers sequential “freeze-out” of an adiabatically evolving grand canonical distribution, the other one is an athermal limit equivalent to the solution of a generalized decay cascade model. We identify the athermal capturing mechanism in our sample, testifying to the high precision of our combined theoretical and experimental methods. The distinction between the capturing mechanisms allows us to derive efficient experimental strategies for improving the initialization.

We revisit the issue of superconductivity at the quantum-critical point (QCP) between a 2D paramagnet and a spin-density-wave metal with ordering momentum ($\pi$, $\pi$). This problem is highly nontrivial because the system at criticality displays a non-Fermi-liquid behavior and because the effective coupling constant $\lambda$ for the pairing is generally of order one, even when the actual interaction is smaller than fermionic bandwidth. Previous study [M. A. Metlitski and S. Sachdev, Phys. Rev. B 82, 075128 (2010)] has found that the renormalizations of the pairing vertex are stronger than in BCS theory and hold in powers of ${log}^2(1/T)$. We analyze the full gap equation and argue that summing up of the leading logarithms does not lead to a pairing instability. Yet, we show that superconductivity has no threshold and appears even if $\lambda$ is set to be small, because subleading logarithmical renormalizations diverge and give rise to a BCS-like result $\log 1/T_c\propto 1/\lambda$. We argue that the analogy with BCS is not accidental as at small $\lambda$ superconductivity at a QCP predominantly comes from fermions that retain Fermi-liquid behavior at criticality. We compute $T_c$ for the actual $\lambda \sim O(1)$, and find that both Fermi-liquid and non-Fermi-liquid fermions contribute to the pairing.

From magnetic, specific heat, ${}^{170}\mathrm{Yb}$ Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, we show that below the first order transition at 0.15 K all of the ${\mathrm{Yb}}^{3+}$ ions are long-range magnetically ordered and each has a moment of $1.1{\mu }_{\mathrm{B}}$ which lies at $\simeq 10°$ to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers $1/2$ effective spins.

We investigate the order parameter dynamics of the stripe-ordered nickelate, ${\mathrm{La}}_{1.75}{\mathrm{Sr}}_{0.25}{\mathrm{NiO}}_4$, using time-resolved resonant x-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters’ amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer reorientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the nonequilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

We present and analyze a new approach for the generation of atomic spin-squeezed states. Our method involves the collective coupling of an atomic ensemble to a decaying mode of an open optical cavity. We demonstrate the existence of a collective atomic dark state, decoupled from the radiation field. By explicitly constructing this state we find that it can feature spin squeezing bounded only by the Heisenberg limit. We show that such dark states can be deterministically prepared via dissipative means, thus turning dissipation into a resource for entanglement. The scaling of the phase sensitivity taking realistic imperfections into account is discussed.

The rectification of unbiased fluctuations, also known as the ratchet effect, is normally obtained under statistical nonequilibrium conditions. Here we propose a new ratchet mechanism where a thermal bath solicits the random rotation of an asymmetric wheel, which is also subject to Coulomb friction due to solid-on-solid contacts. Numerical simulations and analytical calculations demonstrate a net drift induced by friction. If the thermal bath is replaced by a granular gas, the well-known granular ratchet effect also intervenes, becoming dominant at high collision rates. For our chosen wheel shape the granular effect acts in the opposite direction with respect to the friction-induced torque, resulting in the inversion of the ratchet direction as the collision rate increases. We have realized a new granular ratchet experiment where both these ratchet effects are observed, as well as the predicted inversion at their crossover. Our discovery paves the way to the realization of micro and submicrometer Brownian motors in an equilibrium fluid, based purely upon nanofriction.

It is shown that the tension between recent neutrino oscillation experiments, favoring sterile neutrinos with masses of the order of 1 eV, and cosmological data which impose stringent constraints on neutrino masses from the free streaming suppression of density fluctuations, can be resolved in models of the present accelerated expansion of the Universe based on $f(R)$ gravity.

Planar $\mathcal{N}=4$ supersymmetric Yang-Mills theory appears to be integrable. While this allows one to find this theory’s exact spectrum, integrability has hitherto been of no direct use for scattering amplitudes. To remedy this, we deform all scattering amplitudes by a spectral parameter. The deformed tree-level four-point function turns out to be essentially the one-loop $R$ matrix of the integrable $\mathcal{N}=4$ spin chain satisfying the Yang-Baxter equation. Deformed on-shell three-point functions yield novel three-leg $R$ matrices satisfying bootstrap equations. Finally, we supply initial evidence that the spectral parameter might find its use as a novel symmetry-respecting regulator replacing dimensional regularization. Its physical meaning is a local deformation of particle helicity, a fact which might be useful for a much larger class of nonintegrable four-dimensional field theories.

Lattice QCD at finite density suffers from a severe sign problem, which has so far prohibited simulations of the cold and dense regime. Here we study the onset of nuclear matter employing a three-dimensional effective theory derived by combined strong coupling and hopping expansions, which is valid for heavy but dynamical quarks and has a mild sign problem only. Its numerical evaluations agree between a standard Metropolis and complex Langevin algorithm, where the latter is free of the sign problem. Our continuum extrapolated data approach a first order phase transition at ${\mu }_B\approx m_B$ as the temperature approaches zero. An excellent description of the data is achieved by an analytic solution in the strong coupling limit.

A consistent factorization theorem is presented in the framework of effective field theories. Conventional factorization suffers from infrared divergences in the soft and collinear parts. We present a factorization theorem in which the infrared divergences appear only in the parton distribution functions by carefully reorganizing collinear and soft parts. The central idea is extracting the soft contributions from the collinear part to avoid double counting. Combining it with the original soft part, an infrared-finite kernel is obtained. This factorization procedure can be applied to various high-energy scattering processes.

Long-lived isomers in ${}^{212}\mathrm{Bi}$ have been studied following ${}^{238}\mathrm{U}$ projectile fragmentation at 670 MeV per nucleon. The fragmentation products were injected as highly charged ions into a storage ring, giving access to masses and half-lives. While the excitation energy of the first isomer of ${}^{212}\mathrm{Bi}$ was confirmed, the second isomer was observed at 1478(30) keV, in contrast to the previously accepted value of $>1910\mathrm{keV}$. It was also found to have an extended Lorentz-corrected in-ring half-life $>30\min$, compared to 7.0(3) min for the neutral atom. Both the energy and half-life differences can be understood as being due a substantial, though previously unrecognized, internal decay branch for neutral atoms. Earlier shell-model calculations are now found to give good agreement with the isomer excitation energy. Furthermore, these and new calculations predict the existence of states at slightly higher energy that could facilitate isomer deexcitation studies.

We study Bose-Hubbard models on tight-binding, non-Bravais lattices, with a filling of one boson per unit cell—and thus fractional site filling. We discuss situations where no classical bosonic insulator, which is a product state of particles on independent sites, is admitted. Nevertheless, we show that it is possible to construct a quantum Mott insulator of bosons if a trivial band insulator of fermions is possible at the same filling. The ground state wave function is simply a permanent of exponentially localized Wannier orbitals. Such a Wannier permanent wave function is featureless in that it respects all lattice symmetries and is the unique ground state of a parent Hamiltonian that we construct. Motivated by the recent experimental demonstration of a kagome optical lattice of bosons, we study this lattice at $1/3$ site filling. Previous approaches to this problem have invariably produced either broken-symmetry states or topological order. Surprisingly, we demonstrate that a featureless insulator is a possible alternative and is the exact ground state of a local Hamiltonian. We briefly comment on the experimental relevance of our results to ultracold atoms as well as to $1/3$ magnetization plateaus for kagome spin models in an applied field.

We investigate theoretically how single molecule spectroscopy techniques can be used to perform fast and high resolution displacement detection and manipulation of nanomechanical oscillators, such as singly clamped carbon nanotubes. We analyze the possibility of real time displacement detection by the luminescence signal and of displacement fluctuations by the degree of second order coherence. Estimates of the electromechanical coupling constant indicate that intriguing regimes of strong backaction between the two-level system of a molecule and the oscillator can be realized.

We investigated the crystal structures of an ordered perovskite-type cobaltate, ${\mathrm{GdBaCo}}_2{\mathrm{O}}_{5+\delta }$ ($\delta <0.5$), at elevated temperatures by transmission electron microscopy. Above the magnetic ordering temperature, we observed a first-order structural phase transition between the low-temperature tetragonal $3a_p\times 3a_p$ and high-temperature orthorhombic $1a_p\times 2a_p$ superstructure phases (where $a_p$ is the perovskite-unit cell). Upon the application of a magnetic field, an incommensurate phase emerges around the structural phase-transition temperature, which indicates a magnetic-field-induced structural phase transition via no magnetic ordering in the ordered perovskite-type cobaltate.

The equilibrium properties of a minimal tiling model are investigated. The model has extensive ground state entropy, with each ground state having a quasiperiodic sequence of rows. It is found that the transition from the ground state to the high temperature disordered phase proceeds through a sequence of periodic arrangements of rows, in analogy with the commensurate-incommensurate transition. We show that the effective free energy of the model resembles the Frenkel-Kontorova Hamiltonian, but with temperature playing the role of the strength of the substrate potential, and with the competing lengths not explicitly present in the basic interactions.