The anomalous Hall effect (AHE) in $L{1}_0{\mathrm{FePd}}_{1-x}{\mathrm{Pt}}_x$ alloy films is studied both experimentally and theoretically. We find that the intrinsic contribution (${\sigma }_{\mathrm{AH}}^{\mathrm{int}}$) to the AHE can be significantly increased, whereas the extrinsic side-jump contribution (${\sigma }_{\mathrm{AH}}^{\mathrm{sj}}$) can be continuously reduced from being slightly larger than ${\sigma }_{\mathrm{AH}}^{\mathrm{int}}$ in $L{1}_0$ FePd to being much smaller than ${\sigma }_{\mathrm{AH}}^{\mathrm{int}}$ in $L{1}_0$ FePt, by increasing the Pt composition $x$. We show that this chemical composition tuning of the intrinsic contribution is afforded by the stronger spin-orbit coupling strength on the $\mathrm{Pd}/\mathrm{Pt}$ site when the lighter Pd atoms are replaced by the heavier Pt atoms. Our results provide a means of manipulating the competing AHE mechanisms in ferromagnetic alloys for fully understanding the AHE and also for technological applications of ferromagnetic alloys.

The electronic properties of graphene can be manipulated via mechanical deformations, which opens prospects for both studying the Dirac fermions in new regimes and for new device applications. Certain natural configurations of strain generate large nearly uniform pseudomagnetic fields, which have opposite signs in the two valleys, and give rise to flat spin- and valley-degenerate pseudo-Landau levels (PLLs). Here we consider the effect of the Coulomb interactions in strained graphene with a uniform pseudomagnetic field. We show that the spin or valley degeneracies of the PLLs get lifted by the interactions, giving rise to topological insulator states. In particular, when a nonzero PLL is quarter or three-quarter filled, an anomalous quantum Hall state spontaneously breaking time-reversal symmetry emerges. At half-filled PLLs, a weak spin-orbital interaction stabilizes the time-reversal-symmetric quantum spin-Hall state. These many-body states are characterized by the quantized conductance and persist to a high temperature scale set by the Coulomb interactions, which we estimate to be a few hundreds Kelvin at moderate strain values. At fractional fillings, fractional quantum Hall states breaking valley symmetry emerge. These results suggest a new route to realizing robust topological states in mesoscopic graphene.

A first-principles-based approach is developed to simulate dynamical properties, including complex permittivity and permeability in the GHz–THz range, of multiferroics at finite temperatures. It includes both structural degrees of freedom and magnetic moments as dynamic variables in Newtonian and Landau-Lifshitz-Gilbert (LLG) equations within molecular dynamics, respectively, with the couplings between these variables being incorporated. The use of a damping coefficient and of the fluctuation field in the LLG equations is required to obtain equilibrated magnetic properties at any temperature. No electromagnon is found in the spin-canted structure of ${\mathrm{BiFeO}}_3$. On the other hand, two magnons with very different frequencies are predicted via the use of this method. The smallest-in-frequency magnon corresponds to oscillations of the weak ferromagnetic vector in the basal plane being perpendicular to the polarization while the second magnon corresponds to magnetic dipoles going in and out of this basal plane. The large value of the frequency of this second magnon is caused by static couplings between magnetic dipoles with electric dipoles and oxygen octahedra tiltings.

We study a frustrated quantum Ising model relevant for ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ that consists of a triangular lattice of weakly coupled ferromagnetic chains. According to our quantum Monte Carlo simulations, the chains become ferromagnetic and form a three-sublattice “up-up-down” structure for $T\le T_{\mathrm{CI}}$. In contrast, long-wavelength spin-density-wave microphases are stabilized along the chains for $T_{\mathrm{CI}}<T<T_c$. Our mean field solutions reveal a quasicontinuous temperature dependence of the modulation wavelength, implying the existence of metastable states that explain the very slow dynamics observed in ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$. We also discuss implications of microphases for the related multiferroic compounds ${\mathrm{Ca}}_3{\mathrm{CoMnO}}_6$ and ${\mathrm{Lu}}_2{\mathrm{MnCoO}}_6$.

We have determined the full magnetic dispersion relations of multiferroic ${\mathrm{BiFeO}}_3$. In particular, two excitation gaps originating from magnetic anisotropies have been clearly observed. The direct observation of the gaps enables us to accurately determine the Dzyaloshinskii-Moriya (DM) interaction and the single ion anisotropy. The DM interaction supports a sizable magnetoelectric coupling in this compound.

We demonstrate and distinguish experimentally the existence of a special type of Fano resonances at $\mathbf{k}\approx \mathbf{0}$ in a macroscopic two-dimensional photonic crystal slab. We fabricate a square lattice array of holes in a silicon nitride layer and perform an angular resolved spectral analysis of the various Fano resonances. We elucidate their radiation behavior using temporal coupled-mode theory and symmetry considerations. The unique simplicity of this system whereby an ultralong lifetime delocalized electromagnetic field can exist above the surface and consequently easily interact with added matter, provides exciting new opportunities for the study of light and matter interaction.

Analytical solutions to the time-dependent Schrödinger equation describing a driven two-level system are invaluable to many areas of physics, but they are also extremely rare. Here, we present a simple algorithm that generates an unlimited number of exact analytical solutions. We show that a general single-axis driving term and its corresponding evolution operator are determined by a single real function which is constrained only by a certain inequality and initial conditions. Any function satisfying these constraints yields an exact analytical solution. We demonstrate this method by presenting several new exact solutions to the time-dependent Schrödinger equation. Our general method and many of the new solutions we present are particularly relevant to qubit control in quantum computing applications.

In this Letter we study the entanglement of a quantum radiation field interacting with a dielectric medium. In particular, we describe the quantum mixed state of a field interacting with a dielectric through plasma and Drude models and show that these generate very different entanglement behavior, as manifested in the entanglement entropy of the field. We also present a formula for a “Casimir” entanglement entropy, i.e., the distance dependence of the field entropy. Finally, we study a toy model of the interaction between two plates. In this model, the field entanglement entropy is divergent; however, as in the Casimir effect, its distance-dependent part is finite, and the field matter entanglement is reduced when the objects are far.

Quantum vortices in the color-flavor locked phase of QCD have bosonic degrees of freedom, called the orientational zero modes, localized on them. We show that the orientational zero modes are electromagnetically charged. As a result, a vortex in the color-flavor locked phase nontrivially interacts with photons. We show that a lattice of vortices acts as a polarizer of photons with wavelengths larger than some critical length.

The projection method is used to demonstrate the existence of positron attachment to three doubly excited states of helium. The $e^{+}\mathrm{He}(2s^2{}^{1}S^e)\mathrm{deg}$, $e^{+}\mathrm{He}(3s^2{}^{1}S^e)$, and the $e^{+}\mathrm{He}(2s2p{}^{3}P^o$) states have binding energies of 0.447, 0.256, and 0.486 eV, respectively. These energies were computed with the stochastic variational method and the configuration interaction method. These states will exist as resonances in the $e^{+}$-He continuum, and the $e^{+}\mathrm{He}(2s^2{}^{1}S^e)$ state could be detectable in the $e^{+}+\mathrm{He}$ collision spectrum. A resonance width of 0.068 eV was computed for the $e^{+}\mathrm{He}(2s^2{}^{1}S^e)$ state by using the complex rotation method. The existence of a series of $e^{+}\mathrm{He}({ns}^2{}^{1}S^e)$ resonances associated with the $\mathrm{He}({ns}^2)$ double Rydberg series is also predicted, and an explicit calculation demonstrating the existence of the $e^{+}\mathrm{He}(3s^2{}^{1}S^e)$ state is reported.

In cavity optomechanics, nanomechanical motion couples to a localized optical mode. The regime of single-photon strong coupling is reached when the optical shift induced by a single phonon becomes comparable to the cavity linewidth. We consider a setup in this regime comprising two optical modes and one mechanical mode. For mechanical frequencies nearly resonant to the optical level splitting, we find the photon-phonon and the photon-photon interactions to be significantly enhanced. In addition to dispersive phonon detection in a novel regime, this offers the prospect of optomechanical photon measurement. We study these quantum nondemolition detection processes using both analytical and numerical approaches.

Efficient continuous-wave four-wave mixing by using three different fundamental wavelengths with individual detunings to resonances of the nonlinear medium is shown. Up to $6\mu \mathrm{W}$ of vacuum ultraviolet light at 121 nm can be generated, which corresponds to an increase of three orders of magnitude in efficiency. This opens the field of quantum information processing by Rydberg entanglement of trapped ions.

A peaked ion temperature profile is observed inside the magnetic island during mode locking after the back transition from $H$ mode to $L$ mode in JT-60U. The thermal diffusivity evaluated inside the magnetic island is $0.1{\mathrm{m}}^2/\mathrm{s}$, which is much smaller than that outside the magnetic island by an order of magnitude. The present experiment gives clear evidence that ion heat transport inside a magnetic island can bifurcate and the transport level can be suppressed to the very low level associated with the strong flow shear at the boundary.

We report experimental confirmation of the ionization-grating-induced transient birefringence predicted by Wahlstrand and Milchberg [Opt. Lett. 36, 3822 (2011)] and discuss its impact on the higher-order Kerr effect interpretation by Loriot et al. of pump-probe transient birefringence measurements made at 800 nm [Opt. Express 17, 13429 (2009)]. Measurement of the transient birefringence in air at 400 nm shows a negative contribution to the index of refraction at zero delay for frequencies within the pump bandwidth, the degenerate case, and no negative contribution for frequencies exceeding the pump bandwidth, the nondegenerate case. Our findings suggest that a reevaluation of the higher-order Kerr effect hypothesis of Loriot et al. is necessary.

The low-temperature adsorption of isolated transition metal adatoms (Mn, Co, and Fe) onto hexagonal boron nitride monolayers on Rh(111) creates a bistable adsorption complex. The first state considerably weakens the hexagonal boron nitride- ($h$-BN-) substrate bond for 60 BN unit cells, leading to a highly symmetric ring in STM images, while the second state is imaged as a conventional adatom and leaves the BN-substrate interaction intact. We demonstrate reversible switching between the two states and, thus, controlled pinning and unpinning of the $h$-BN layer from the metal substrate. $I(z)$ and $d\ln I/dz$ curves are used to reveal the BN deformation in the ring state.

Topological phases of quantum matter defy characterization by conventional order parameters but can exhibit a quantized electromagnetic response and/or protected surface states. We examine such phenomena in a model for three-dimensional correlated complex oxides, the pyrochlore iridates. The model realizes interacting topological insulators, with and without time-reversal symmetry, and topological Weyl semimetals. We use cellular dynamical mean-field theory, a method that incorporates quantum many-body effects and allows us to evaluate the magnetoelectric topological response coefficient in correlated systems. This invariant is used to unravel the presence of an interacting axion insulator absent within a simple mean-field study. We corroborate our bulk results by studying the evolution of the topological boundary states in the presence of interactions. Consequences for experiments and for the search for correlated materials with symmetry-protected topological order are given.

We propose that real-space properties of the two-impurity Kondo model can be obtained from an effective spin model where two single-impurity Kondo spin chains are joined via an Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the two impurity spins. We then use a density matrix renormalization group approach, valid in all ranges of parameters, to study its features using two complementary quantum-entanglement measures, the negativity and the von Neumann entropy. This nonperturbative approach enables us to uncover the precise dependence of the spatial extent ${\xi }_K$ of the Kondo screening cloud with the Kondo and RKKY couplings. Our results reveal an exponential suppression of the Kondo temperature $T_K\sim 1/{\xi }_K$ with the size of the effective impurity spin in the limit of large ferromagnetic RKKY coupling, a striking display of “Kondo resonance narrowing” in the two-impurity Kondo model. We also show how the antiferromagnetic RKKY interaction produces an effective decoupling of the impurities from the bulk already for intermediate strengths of this interaction, and, furthermore, exhibit how the non-Fermi liquid quantum critical point is signaled in the quantum entanglement between various parts of the system.

We reveal a “high-energy anomaly” (HEA) in the band dispersion of the unconventional ruthenate superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, by means of high-resolution angle-resolved photoemission spectroscopy (ARPES) with tunable energy and polarization of incident photons. This observation provides another class of correlated materials exhibiting this anomaly beyond high-$T_c$ cuprates. We demonstrate that two distinct types of band renormalization associated with and without the HEA occur as a natural consequence of the energetics in the bandwidth and the energy scale of the HEA. Our results are well reproduced by a simple analytical form of the self-energy based on the Fermi-liquid theory, indicating that the HEA exists at a characteristic energy scale of the multielectron excitations. We propose that the HEA universally emerges if the systems have such a characteristic energy scale inside of the bandwidth.

On $\mathrm{Si}(111)\mathrm{\text{−}}(5\times 2)$-Au it is shown that metallic sections of quantum wires between two doping adatoms establish a local electronic structure which is primarily defined by the section length. Such confined doping is a direct consequence of reduced dimensionality and is not observed in higher dimensions. Within a chain segment, the effect of a spatially independent charge-carrier concentration is superimposed by a Coulomb-like interaction due to the positively charged dopants. This offers a natural explanation for the relatively broad photoemission features and the complex appearance in scanning tunneling microscopy and spectroscopy images.

We measure the dependence of qubit phase coherence and flux noise on inductor loop geometry. While wider inductor traces change neither the flux noise power spectrum nor the qubit dephasing time, increased inductance leads to a simultaneous increase in both. Using our new tomographic protocol for measuring low frequency flux noise, we make a direct comparison between the flux noise spectrum and qubit phase decay, finding agreement within 10% of theory.