We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio $c/a$ shows a minimum between 2.0–2.9 GPa, indicating an enhanced $c$-axis bonding through $p_z$ band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition and are consistent with a recent theoretical proposal.

Using ${\mathrm{SrTiO}}_3$ nanocuboids as a model system, we show with aberration-corrected high resolution electron microscopy at sub-Å resolution that surface relaxations or reconstructions are present on the nanocuboids, depending on the synthetic process. Oleic acid synthesis, acetic acid synthesis, and microwave-assisted acetic acid synthesis result in a SrO termination, ${\mathrm{TiO}}_2$-rich reconstruction, and mixed termination, respectively. The experimental atomic positions are in better agreement with density functional theory calculations using an exact-exchange corrected PBEsol functional than the Perdew-Burke-Ernzerhof (PBE) functional.

We present a novel functional for spin-density-functional theory aiming at the description of noncollinear magnetic structures. The construction of the functional employs the spin-spiral-wave state of the uniform electron gas as reference system. We show that the functional depends on transverse gradients of the spin magnetization; i.e., in contrast with the widely used local spin density approximation, the functional is sensitive to local changes of the direction of the spin magnetization. As a consequence the exchange-correlation magnetic field is not parallel to the spin magnetization and a local spin torque is present in the ground state of the Kohn-Sham system. As a proof of principle, we apply the functional to a Chromium monolayer in the noncollinear 120°-Néel state.

A reversible structural transition is observed on Si(553)-Au by scanning tunneling microscopy, triggered by electrons injected from the tip into the surface. The periodicity of atomic chains near the step edges changes from the $1\times 3$ ground state to a $1\times 2$ excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first-principles density-functional calculations it is shown that the $1\times 2$ phase is created by temporary doping of the atom chains. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing lifetimes in the millisecond range.

We use angle-resolved photoemission spectroscopy and a new quantitative approach based on the partial density of states to study properties of seemingly disconnected portions of the Fermi surface (FS) that are present in the pseudogap state of cuprates called Fermi arcs. We find that the normal state FS collapses very abruptly into Fermi arcs at the pseudogap temperature ($T^{*}$). Surprisingly, the length of the Fermi arcs remains constant over an extended temperature range between $T^{*}$ and $T_{\mathrm{pair}}$, consistent with the presence of an ordered state below $T^{*}$. These arcs collapse again at the temperature below which pair formation occurs ($T_{\mathrm{pair}}$) either to a point or a very short arc, whose length is limited by our experimental resolution. The tips of the arcs span between points defining a set of wave vectors in momentum space, which are the fingerprints of the ordered state that causes the pseudogap.

We demonstrate using inelastic scanning tunneling spectroscopy and simulations based on density functional theory that the amplitude and sign of the magnetic anisotropy energy for a single Fe atom adsorbed onto the Pt(111) surface can be manipulated by modifying the adatom binding site. Since the magnitude of the measured anisotropy is remarkably small, up to an order of magnitude smaller than previously reported, electron-hole excitations are weak and thus the spin excitation exhibits long lived precessional lifetimes compared to the values found for the same adatom on noble metal surfaces.

We propose a simple first-principles method to describe the propagation of tightly bound excitons. By viewing the exciton as a composite object (an effective Frenkel exciton in Wannier orbitals), we define an exciton kinetic kernel to encapsulate the exciton propagation and decay for all binding energies. Applied to prototypical LiF, our approach produces three exciton bands, which we verified quantitatively via inelastic x-ray scattering. The proposed real-space picture is computationally inexpensive and thus enables study of the full exciton dynamics, even in the presence of surfaces and impurity scattering. It also provides an intuitive understanding to facilitate practical exciton engineering in semiconductors, strongly correlated oxides, and their nanostructures.

Groups of beating flagella or cilia often synchronize so that neighboring filaments have identical frequencies and phases. A prime example is provided by the unicellular biflagellate Chlamydomonas reinhardtii, which typically displays synchronous in-phase beating in a low-Reynolds number version of breaststroke swimming. We report the discovery that ptx1, a flagellar-dominance mutant of C. reinhardtii, can exhibit synchronization in precise antiphase, as in the freestyle swimming stroke. High-speed imaging shows that ptx1 flagella switch stochastically between in-phase and antiphase states, and that the latter has a distinct waveform and significantly higher frequency, both of which are strikingly similar to those found during phase slips that stochastically interrupt in-phase beating of the wild-type. Possible mechanisms underlying these observations are discussed.

We introduce a generic bosonic model exemplifying that (spin) Meissner currents can persist in insulating phases of matter. We consider two species of interacting bosons on a lattice. Our model exhibits separation of charge (total density) and spin (relative density): the charge sector is gapped in a bosonic Mott insulator phase with total density one, while the spin sector remains superfluid due to interspecies conversion. Coupling the spin sector to the gauge fields yields a spin Meissner effect reflecting the long-range spin superfluid coherence. We investigate the resulting phase diagram and describe other possible spin phases of matter in the Mott regime possessing chiral currents as well as a spin-density wave phase. The model presented here is realizable in Josephson junction arrays and in cold atom experiments.

We study the collapse of rapidly rotating supermassive stars that may have formed in the early Universe. By self-consistently simulating the dynamics from the onset of collapse using three-dimensional general-relativistic hydrodynamics with fully dynamical spacetime evolution, we show that seed perturbations in the progenitor can lead to the formation of a system of two high-spin supermassive black holes, which inspiral and merge under the emission of powerful gravitational radiation that could be observed at redshifts $z\gtrsim 10$ with the DECIGO or Big Bang Observer gravitational-wave observatories, assuming supermassive stars in the mass range ${10}^4–{10}^6{M}_{\odot }$. The remnant is rapidly spinning with dimensionless spin $a^{*}=0.9$. The surrounding accretion disk contains $\sim 10\%$ of the initial mass.

A measurement of the $CP$ asymmetry in $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$ decays is presented using $pp$ collision data, corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$, recorded by the LHCb experiment during 2011 at a center-of-mass energy of 7 TeV. The measurement is performed in seven bins of ${\mu }^{+}{\mu }^{-}$ invariant mass squared in the range $0.05<q^2<22.00{\mathrm{GeV}}^2/c^4$, excluding the $J/\psi$ and $\psi (2S)$ resonance regions. Production and detection asymmetries are corrected for using the $B^{+}\to J/\psi K^{+}$ decay as a control mode. Averaged over all the bins, the $CP$ asymmetry is found to be ${\mathcal{A}}_{CP}=0.000\pm 0.033(\mathrm{stat})\pm 0.005(\mathrm{syst})\pm 0.007$ ($J/\psi K$), where the third uncertainty is due to the $CP$ asymmetry of the control mode. This is consistent with the standard model prediction.

The $N$-representability problem for noncollinear spin-polarized densities was left open in the pioneering work of von Barth and Hedin [J. Phys. C 5, 1629 (1972)] setting up the Kohn-Sham density-functional theory for magnetic compounds. In this Letter, we demonstrate that, contrarily to the nonpolarized case, the sets of pure and mixed state $N$-representable densities are different in general. We provide a simple characterization of the latter by means of easily checkable necessary and sufficient conditions on the components ${\rho }^{\alpha \beta }(\mathbf{r})$ of the spin-polarized density.

Propagation of a viscous fluid beneath an elastic sheet is controlled by local dynamics at the peeling front, in close analogy with the capillary-driven spreading of drops over a precursor film. Here we identify propagation laws for a generic elastic peeling problem in the distinct limits of peeling by bending and peeling by pulling, and apply our results to the radial spread of a fluid blister over a thin prewetting film. For the case of small deformations relative to the sheet thickness, peeling is driven by bending, leading to radial growth as $t^{7/22}$. Experimental results reproduce both the spreading behavior and the bending wave at the front. For large deformations relative to the sheet thickness, stretching of the blister cap and the consequent tension can drive peeling either by bending or by pulling at the front, both leading to radial growth as $t^{3/8}$. In this regime, detailed predictions give excellent agreement and explanation of previous experimental measurements of spread in the pulling regime in an elastic Hele-Shaw cell.

Nonlinear electromagnetic stabilization by suprathermal pressure gradients found in specific regimes is shown to be a key factor in reducing tokamak microturbulence, augmenting significantly the thermal pressure electromagnetic stabilization. Based on nonlinear gyrokinetic simulations investigating a set of ion heat transport experiments on the JET tokamak, described by Mantica et al. [Phys. Rev. Lett. 107, 135004 (2011)], this result explains the experimentally observed ion heat flux and stiffness reduction. These findings are expected to improve the extrapolation of advanced tokamak scenarios to reactor relevant regimes.

A rotating dusty plasma apparatus was constructed to provide the possibility of experimental emulation of extremely high magnetic fields by means of the Coriolis force, observable in a corotating measurement frame. We present collective excitation spectra for different rotation rates with a magnetic induction equivalent of up to 3200 T. We identify the onset of magnetoplasmon-equivalent mode dispersion in the rotating macroscopic two-dimensional single-layer dusty plasma. The experimental results are supported by molecular dynamics simulations of 2D magnetized Yukawa systems.

The first observation of the $K$-shell photoabsorption edge of strongly coupled matter with an ion-ion coupling parameter of about 65 generated by intense x-ray radiation-driven shocks is reported. The soft x-ray radiation generated by laser interaction with a “dog bone” high-$Z$ hohlraum is used to ablate two thick CH layers, which cover a KCl sample, to create symmetrical inward shocks. While the two shocks impact at the central KCl sample, a highly compressed KCl is obtained with a density of 3–5 times solid density and a temperature of about 2–4 eV. The photoabsorption spectra of chlorine near the $K$-shell edge are measured with a crystal spectrometer using a short x-ray backlighter. The redshift of the $K$ edge up to 11.7 eV and broadening of 15.2 eV are obtained for the maximum compression. A comparison of the measured redshifts and broadenings with dense plasma calculations are made, and it indicates potential improvements in the theoretical description.

Using ab initio molecular dynamics simulations, we perform a comparative study of the defect accumulation process in silicon carbide (SiC) and zirconium carbide (ZrC). Interestingly, we find that the fcc Si sublattice in SiC spontaneously and gradually collapses following the continuous introduction of C Frenkel pairs (FPs). Above a critical amorphization dose of $\sim 0.33$ displacements per atom (dpa), the pair correlation function exhibits no long-range order. In contrast, the fcc Zr sublattice in ZrC remains structurally stable against C sublattice displacements up to the highest dose of 1.0 dpa considered. Consequently, ZrC cannot be amorphized by the accumulation of C FPs. We propose defect-induced mechanical instability as the key mechanism driving the amorphization of SiC under electron irradiation.

Inelastic neutron scattering measurements of paramagnetic ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ at $T=5\mathrm{K}$ reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of ${\mathbf{Q}}_{\mathrm{AFM}}=(1/2,1/2,1)$ and possess a large energy scale. These stripe spin fluctuations are similar to those found in $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by ${\mathbf{Q}}_{\mathrm{AFM}}$. ${\mathrm{SrCo}}_2{\mathrm{As}}_2$ has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride ${\mathrm{FeB}}_4$ were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in ${\mathrm{FeB}}_4$ is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

We study the spin-$1/2$ quantum Heisenberg antiferromagnet on a Bethe lattice diluted to the percolation threshold. Dilution creates areas of even or odd sublattice imbalance resulting in “dangling spins” [L. Wang and A. W. Sandvik, Phys. Rev. Lett. 97, 117204 (2006); Phys. Rev. B 81, 054417 (2010)]. These collectively act as “emergent” spin-$1/2$ degrees of freedom and are responsible for the creation of a set of low-lying “quasidegenerate states.” Using density matrix renormalization group calculations, we detect the presence and location of these emergent spins. We find an effective Hamiltonian of these emergent spins, with Heisenberg interactions that decay exponentially with the distance between them.