We propose methods for extracting limits on the strength of $P$-odd interactions of pseudoscalar and pseudovector cosmic fields with electrons, protons, and neutrons, by exploiting the static and dynamic parity-nonconserving amplitudes and electric dipole moments they induce in atoms. Candidates for such fields are dark matter (including axions) and dark energy, as well as several more exotic sources described by Lorentz-violating standard model extensions. Atomic calculations are performed for H, Li, Na, K, Rb, Cs, ${\mathrm{Ba}}^{+}$, Tl, Dy, Fr, and ${\mathrm{Ra}}^{+}$. From these calculations and existing measurements in Dy, Cs, and Tl, we constrain the interaction strengths of the parity-violating static pseudovector cosmic field to be $7\times 10^{-15}\mathrm{GeV}$ with an electron, and $3\times 10^{-8}\mathrm{GeV}$ with a proton.

We develop ambitwistor string theories for four dimensions to obtain new formulas for tree-level gauge and gravity amplitudes with arbitrary amounts of supersymmetry. Ambitwistor space is the space of complex null geodesics in complexified Minkowski space, and in contrast to earlier ambitwistor strings, we use twistors rather than vectors to represent this space. Although superficially similar to the original twistor string theories of Witten, Berkovits, and Skinner, these theories differ in the assignment of world sheet spins of the fields, rely on both twistor and dual twistor representatives for the vertex operators, and use the ambitwistor procedure for calculating correlation functions. Our models are much more flexible, no longer requiring maximal supersymmetry, and the resulting formulas for amplitudes are simpler, having substantially reduced moduli. These are supported on the solutions to the scattering equations refined according to helicity and can be checked by comparison with corresponding formulas of Witten and of Cachazo and Skinner.

Using the Penning trap mass spectrometer TITAN, we performed the first direct mass measurements of ${}^{20,21}\mathrm{Mg}$, isotopes that are the most proton-rich members of the $A=20$ and $A=21$ isospin multiplets. These measurements were possible through the use of a unique ion-guide laser ion source, a development that suppressed isobaric contamination by 6 orders of magnitude. Compared to the latest atomic mass evaluation, we find that the mass of ${}^{21}\mathrm{Mg}$ is in good agreement but that the mass of ${}^{20}\mathrm{Mg}$ deviates by $3\sigma$. These measurements reduce the uncertainties in the masses of ${}^{20,21}\mathrm{Mg}$ by 15 and 22 times, respectively, resulting in a significant departure from the expected behavior of the isobaric multiplet mass equation in both the $A=20$ and $A=21$ multiplets. This presents a challenge to shell model calculations using either the isospin nonconserving universal $sd$ USDA and USDB Hamiltonians or isospin nonconserving interactions based on chiral two- and three-nucleon forces.

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal ${\mathrm{Mn}}_2\mathrm{RhSn}$ Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, ${\mathrm{Mn}}_2\mathrm{RhSn}$ is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

Precise identification of parameters governing quantum processes is a critical task for quantum information and communication technologies. In this Letter, we consider a setting where system evolution is determined by a parametrized Hamiltonian, and the task is to estimate these parameters from temporal records of a restricted set of system observables (time traces). Based on the notion of system realization from linear systems theory, we develop a constructive algorithm that provides estimates of the unknown parameters directly from these time traces. We illustrate the algorithm and its robustness to measurement noise by applying it to a one-dimensional spin chain model with variable couplings.

X-ray photoabsorption from intense short pulses by a molecule triggers complicated electron and subsequently ion dynamics, leading to photoelectron spectra, which are difficult to interpret. Illuminating fullerenes offers a way to separate out the electron dynamics since the cage structure confines spatially the origin of photo- and Auger electrons. Together with the sequential nature of the photoprocesses at intensities available at x-ray free-electron lasers, this allows for a remarkably detailed interpretation of the photoelectron spectra, as we will demonstrate. The general features derived can serve as a paradigm for less well-defined situations in other large molecules or clusters.

We identify the atomic structure of the $\mathrm{Au}/\mathrm{Si}(111)\text{−}(5\times 2)$ surface by using density functional theory calculations. With seven Au atoms per unit cell, our model forms a bona fide ($5\times 2$) atomic structure, which is energetically favored over the leading model of Erwin et al. [Phys. Rev. B 80, 155409 (2009)], and well reproduces the $Y$-shaped and $V$-shaped ($5\times 2$) STM images. This surface is metallic with a prominent half filled band of surface states, mostly localized around the Au-chain area. The correct identification of the atomic and band structure of the clean surface further clarifies the adsorption structure of Si adatoms and the physical origin of the intriguing metal-to-insulator transition driven by Si adatoms.

In previous studies, it has proved difficult to realize periodic graphene ripples with wavelengths of a few nanometers. Here we show that one-dimensional (1D) periodic graphene ripples with wavelengths from 2 nm to tens of nanometers can be implemented in the intrinsic areas of a continuous mosaic (locally N-doped) graphene monolayer by simultaneously using both the thermal strain engineering and the anisotropic surface stress of the Cu substrate. Our result indicates that the constraint imposed at the boundaries between the intrinsic and the N-doped regions play a vital role in creating these 1D ripples. We also demonstrate that the observed rippling modes are beyond the descriptions of continuum mechanics due to the decoupling of graphene’s bending and tensional deformations. Scanning tunneling spectroscopy measurements indicate that the nanorippling generates a periodic electronic superlattice and opens a zero-energy gap of about 130 meV in graphene. This result may pave a facile way for tailoring the structures and electronic properties of graphene.

We report results of exact diagonalization studies of the spin- and valley-polarized fractional quantum Hall effect in the $N=0$ and $N=1$ Landau levels in graphene. We use an effective model that incorporates Landau level mixing to lowest order in the parameter $\kappa =\mathbf{(}(e^2/\epsilon \ell )/(\hslash v_F/\ell )\mathbf{)}=(e^2/\epsilon v_F\hslash )$, which is magnetic field independent and can only be varied through the choice of substrate. We find Landau level mixing effects are negligible in the $N=0$ Landau level for $\kappa \lesssim 2$. In fact, the lowest Landau level projected Coulomb Hamiltonian is a better approximation to the real Hamiltonian for graphene than it is for semiconductor based quantum wells. Consequently, the principal fractional quantum Hall states are expected in the $N=0$ Landau level over this range of $\kappa$. In the $N=1$ Landau level, fractional quantum Hall states are expected for a smaller range of $\kappa$ and Landau level mixing strongly breaks particle-hole symmetry, producing qualitatively different results compared to the $N=0$ Landau level. At half filling of the $N=1$ Landau level, we predict the anti-Pfaffian state will occur for $\kappa \sim 0.25–0.75$.

A combination of scanning tunneling microscopy and spectroscopy and density functional theory is used to characterize excess electrons in ${\mathrm{TiO}}_2$ rutile and anatase, two prototypical materials with identical chemical composition but different crystal lattices. In rutile, excess electrons can localize at any lattice Ti atom, forming a small polaron, which can easily hop to neighboring sites. In contrast, electrons in anatase prefer a free-carrier state, and can only be trapped near oxygen vacancies or form shallow donor states bound to Nb dopants. The present study conclusively explains the differences between the two polymorphs and indicates that even small structural variations in the crystal lattice can lead to a very different behavior.

Using angle-resolved photoemission spectroscopy, we show that the recently discovered surface state on ${\mathrm{SrTiO}}_3$ consists of nondegenerate $t_{2g}$ states with different dimensional characters. While the $d_{xy}$ bands have quasi-2D dispersions with weak $k_z$ dependence, the lifted $d_{xz}/d_{yz}$ bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the $d_{xz}/d_{yz}$ Fermi surface components are essentially the same for different sample preparations. Irradiating ${\mathrm{SrTiO}}_3$ in ultrahigh vacuum is one method observed so far to induce the “universal” surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O $1s$ core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-$E_F$ electrons in the surface region.

A theory of dissipative nonlinear conductivity, ${\sigma }_1(\omega ,H)$, of $s$-wave superconductors under strong electromagnetic fields at low temperatures is proposed. Closed-form expressions for ${\sigma }_1(H)$ and the surface resistance $R_s(\omega ,H)$ are obtained in the nonequilibrium dirty limit for which ${\sigma }_1(H)$ has a significant minimum as a function of a low-frequency ($\hslash \omega \ll k_{B}T$) magnetic field $H$. The calculated microwave suppression of $R_s(H)$ is in good agreement with recent experiments on alloyed Nb resonator cavities. It is shown that superimposed dc and ac fields, $H=H_0+H_a\cos \omega t$, can be used to reduce ac dissipation in thin film nanostructures by tuning ${\sigma }_1(H_0)$ with the dc field.

We report on a single-molecule magnet where the spatial arrangement of three manganese ions and their spin-orbit coupling tensor orientations result in threefold angular modulations of the magnetization tunneling rates and quantum interference patterns that mimic the form of a three-leaf clover. Although expected in all quantum tunneling of magnetization resonances for a trigonal molecular symmetry, the threefold modulation only appears at resonances for which a longitudinal magnetic field is applied (i.e., resonance numbers $|k|>0$). A sixfold transverse field modulation observed at resonance $k=0$ manifests as a direct consequence of a threefold corrugation of the spin-orbit coupling energy landscape, creating an effective longitudinal field which varies the resonance condition in the presence of a transverse field. The observations allow for an association between the trigonal distortion of the local spin-orbit interactions and the spatial disposition of the constituent ions, a finding that can be extrapolated to other systems where spin-orbit coupling plays a significant role.