The discovery of ($4\times 4$) silicene formation on Ag(111) raised the question whether silicene maintains its Dirac fermion character, similar to graphene, on a supporting substrate. Previous photoemission studies indicated that the π band forms Dirac cones near the Fermi energy, while theoretical investigations found it shifted at deeper binding energy. By means of angle-resolved photoemission spectroscopy and density-functional theory calculations we show instead that the π-symmetry states lose their local character and the Dirac cone fades out. The formation of an interface state of free-electron-like Ag origin is found to account for spectral features that were theoretically and experimentally attributed to silicene bands of π character.

We theoretically analyze the possibility to confine electrons in single-layer graphene with the help of metallic gates, via the evaluation of the density of states of such a gate-defined quantum dot in the presence of a ring-shaped metallic contact. The possibility to electrostatically confine electrons in a gate-defined “quantum dot” with finite-carrier density, surrounded by an undoped graphene sheet, strongly depends on the integrability of the electron dynamics in the quantum dot. With the present calculations, we can quantitatively compare confinement in dots with integrable and chaotic dynamics, and verify the prediction that the Berry phase associated with the pseudospin leads to partial confinement in situations where no confinement is expected according to the arguments relying on the classical dynamics only.

Atomic structure and electronic transport properties of Cu-doped topological insulator ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ (${\mathrm{Cu}}_x$${\mathrm{Bi}}_2$${\mathrm{Se}}_3$) films were investigated by surface x-ray diffraction and magnetotransport measurements. We revealed that by depositing Cu on the pristine ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ films at room temperature the van der Waals gap between the quintuple layers was expanded to intercalate the Cu atoms. The gap size became maximum in the doping range of $0.12\le x\le 0.35$, in the same manner as the bulk superconducting ${\mathrm{Cu}}_x$${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ crystal. The optimally doped ($x=0.12$) film was in $n$ type and the carrier concentration was on the order of ${10}^{20}$ ${\mathrm{cm}}^{-3}$; that agrees with the superconducting bulk crystals and is one order of magnitude larger than the pristine ${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ films. The film exhibited the weak antilocalization behavior as observed in the pristine films, indicating that the strong spin-orbit coupling is maintained against the Cu doping. The room-temperature doping did not lead to the possible superconducting transition down to 0.8 K, suggesting that in addition to the electron doping, another feature, that may exist in the inhomogeneous bulk ${\mathrm{Cu}}_x$${\mathrm{Bi}}_2$${\mathrm{Se}}_3$ crystals synthesized at much higher temperatures, is responsible for the superconductivity.

We theoretically study the nuclear spin dynamics driven by electron transport and hyperfine interaction in an electrically defined double quantum dot in the Pauli-blockade regime. We derive a master-equation-based framework and show that the coupled electron-nuclear system displays an instability towards the buildup of large nuclear spin polarization gradients in the two quantum dots. In the presence of such inhomogeneous magnetic fields, a quantum interference effect in the collective hyperfine coupling results in sizable nuclear spin entanglement between the two quantum dots in the steady state of the evolution. We investigate this effect using analytical and numerical techniques, and demonstrate its robustness under various types of imperfections.

The dependence of the magnetic field sensitive current on the orientation of the magnetic field has been investigated in organic semiconductor devices where the active layer consists of the poly(p-phenylene vinylene) derivative “Super Yellow.” Previous work on ${\mathrm{Alq}}_3$ suggested that the anisotropy was caused either by anisotropic spin-spin interactions or by anisotropic hyperfine fields, but no discrimination could be made. In the present work, the anisotropy at the hyperfine field scale is best explained by dipolar coupling between the spin of polarons. In addition, a high field anisotropy is found with an opposite sign, different angle, and voltage dependence. Spin density matrix calculations were performed of polaron pair interactions for the low field effect, and a $\Delta g$-mechanism, triplet-polaron, or triplet-triplet interaction for the high field effect. The simulations confirm that the low field anisotropy can indeed be explained by dipolar coupling. However, the proposed models can not entirely account for the high field anisotropy. These results show that, although contemporary models can account for (anisotropic) magnetic field effects in organic semiconductors at low field scales, more experimental and theoretical research of high field effects is highly desirable.

Finite-size corrections for charged defect supercell calculations typically consist of image-charge and potential alignment corrections. Regarding the image-charge correction, Freysoldt, Neugebauer, and Van de Walle (FNV) recently proposed a scheme that constructs the correction energy a posteriori through alignment of the defect-induced potential to a model charge potential [C. Freysoldt et al., Phys. Rev. Lett. 102, 016402 (2009)]. This, however, still has two shortcomings in practice. First, it uses a planar-averaged electrostatic potential for determining the potential offset, which can not be readily applied to defects with large atomic relaxation. Second, Coulomb interaction is screened by a macroscopic scalar dielectric constant, which can bring forth large errors for defects in layered and low-dimensional structures. In this study, we use the atomic site potential as a potential marker, and extend the FNV scheme by estimating long-range Coulomb interactions with a point charge model in an anisotropic medium. We also revisit the conventional potential alignment and show that it is unnecessary for correcting defect formation energies after the image-charge correction is properly applied. A systematic assessment of the accuracy of the extended FNV scheme is performed for defects and impurities in diverse materials: $\beta$-Li${}_2$TiO${}_3$, ZnO, MgO, Al${}_2$O${}_3$, HfO${}_2$, cubic and hexagonal BN, Si, GaAs, and diamond. Defect formation energies with $-6$ to $+3$ charges calculated using supercells containing around 100 atoms are successfully corrected even after atomic relaxation within 0.2 eV compared to those in the dilute limit.

Spin-dependent relaxation and recombination processes in $\gamma$-irradiated $n$-type Czochralski-grown silicon are studied using continuous wave (cw) and pulsed electrically detected magnetic resonance (EDMR). Two processes involving the SL1 center, the neutral excited triplet state of the oxygen-vacancy complex, are observed which can be separated by their different dynamics. One of the processes is the relaxation of the excited SL1 state to the ground state of the oxygen-vacancy complex, the other a charge transfer between ${}^{31}$P donors and SL1 centers forming close pairs, as indicated by electrically detected electron double resonance. For both processes, the recombination dynamics is studied with pulsed EDMR techniques. We demonstrate the feasibility of true zero-field cw and pulsed EDMR for spin-1 systems and use this to measure the lifetimes of the different spin states of SL1 also at vanishing external magnetic field.

We solve a long-standing experimental discrepancy of NH${}_3$BH${}_3$, which—as a molecule—has a threefold rotational axis, but in its crystallized form at room temperature shows a fourfold symmetry around the same axis, creating a geometric incompatibility. To explain this peculiar experimental result, we study the dynamics of this system with ab initio Car-Parrinello molecular dynamics and nudged-elastic band simulations. We find that rotations, rather than spatial static disorder, at angular velocities of 2 rev/ps—a time scale too small to be resolved by standard experimental techniques—are responsible for the fourfold symmetry.

Multiplicity of magnetization dynamics in a thin insulating ferromagnet deposited on a topological insulator (TI) is studied as a function of electron flow at the interface. The spin current of TI surface electrons induces magnetization precession in the magnet, which in turn modifies the TI spin polarization. In contrast to the conventional linear analysis, the self-consistent nature of the interaction leads to auto-oscillation, magnetization reversal, or magnetic deviation under a dc bias, all of which are accompanied by the strong anomalous Hall effect. With a minimal energy requirement as low as tens of aJ, the proposed mechanism offers a highly efficient alternative to the spin transfer torque or spin-Hall based approaches.

We report spherical neutron polarimetry (SNP) and discuss the spin-flip scattering cross sections as well as the chiral fraction $\eta$ close to the helimagnetic transition in MnSi. For our study we have developed a miniaturized SNP device that allows fast data collection when used in small angle scattering geometry with an area detector. Critical spin-flip scattering is found to be governed by chiral paramagnons that soften on a sphere in momentum space. Carefully accounting for the incoherent spin-flip background, we find that the resulting chiral fraction $\eta$ decreases gradually above the helimagnetic transition reflecting a strongly renormalized chiral correlation length with a temperature dependence in excellent quantitative agreement with the Brazovskii theory for a fluctuation-induced first order transition.

We consider a classical model of charges $\pm q$ on a pyrochlore lattice in the presence of long-range Coulomb interactions. This model first appeared in the early literature on charge order in magnetite [P. W. Anderson, Phys. Rev. 102, 1008 (1956)]. In the limit where the interactions become short ranged, the model has a ground state with an extensive entropy and dipolar charge-charge correlations. When long-range interactions are introduced, the exact degeneracy is broken. We study the thermodynamics of the model and show the presence of a correlated charge liquid within a temperature window in which the physics is well described as a liquid of screened charged defects. The structure factor in this phase, which has smeared pinch points at the reciprocal lattice points, may be used to detect charge ice experimentally. In addition, the model exhibits fractionally charged excitations $\pm q/2$ which are shown to interact via a $1/r$ potential. At lower temperatures, the model exhibits a transition to a long-range ordered phase. We are able to treat the Coulombic charge ice model and the dipolar spin ice model on an equal footing by mapping both to a constrained charge model on the diamond lattice. We find that states of the two ice models are related by a staggering field which is reflected in the energetics of these two models. From this perspective, we can understand the origin of the spin ice and charge ice ground states as coming from a dipolar model on a diamond lattice. We study the properties of charge ice in an external electric field, finding that the correlated liquid is robust to the presence of a field in contrast to the case of spin ice in a magnetic field. Finally, we comment on the transport properties of Coulombic charge ice in the correlated liquid phase.

Using Ginzburg-Landau theory and two-dimensional (2D) phase field simulations, we analyze the influence of flexoelectric coupling on the domain patterns in ferroelectrics. The phase field simulations predict that a high strength of the flexoelectric coupling leads to formation of a fine structure in domain patterns in ferroelectrics. The fine structure forms when the coupling strength exceeds a critical value and is related to local transition into an incommensurate phase. Depending on the parameters, a structure with stripe patterns with antiparallel polarizations or another one, not seen before, with two-dimensional arrays of alternating vortices is found. Complex domain configurations with coexisting phases and unusual domain walls between them are observed. Although the incommensurate phase does not form for weaker couplings, the influence of flexoelectricity on bulk domain patterns can still be significant. The results of the calculations are rationalized using an analytical model. Directions for the modulation wave vectors in the fine structure are found in the framework of a linear analysis, while the type of the structure—stripes or vortices—is determined by anharmonicity.

We report the manipulation of nitrogen vacancy (NV) spins in diamond when a nearby ferrimagnetic insulator, yttrium iron garnet, is driven into precession. The change in NV spin polarization, as measured by changes in photoluminescence, is comparable in magnitude to that from conventional optically detected magnetic resonance, but relies on a distinct mechanism as it occurs at a microwave frequency far away from the magnetic resonance frequency of the NV spin. This observation presents an approach to transferring ferromagnetic spin information into a paramagnet and then transducing the response into a robust optical signal. It also opens strategies for studying ferromagnetism and spin transport at the nanoscale.

The magnetization dynamics of both Ta/CoFeB/MgO and Ta/CoFeB/Ta films were investigated using an all-optical pump-probe method. The magnetic field strength and the applied field direction dependencies of the precession frequency and the relaxation time were explained well by the Landau-Lifshitz-Gilbert equation when taking the magnetic anisotropy distribution in the film into account. The thickness dependence of the $\alpha$ values obtained for both stacked films was also discussed. The $\alpha$ values increased linearly with increasing inverse CoFeB thickness ($t_{\mathrm{CoFeB}}$). The slope of the $\alpha$ vs $1/t_{\mathrm{CoFeB}}$ characteristic for Ta/CoFeB/MgO films was smaller than that for Ta/CoFeB/Ta films, implying that the enhancement of $\alpha$ was caused by the CoFeB/Ta interface. Comparison of the annealing temperature dependence of $\alpha$ and the perpendicular magnetic anisotropy constant $K_{\mathrm{u}}$ revealed no correlation between $\alpha$ and $K_{\mathrm{u}}$.

Interface states in a silicon/barrier junction break the silicon valley degeneracy near the interface, a desirable feature for some Si quantum electronics applications. Within a minimal multivalley tight-binding model in one dimension, we inspect here the spatial extent of these states into the Si and the barrier materials, as well as favorable conditions for its spontaneous formation. Our approach—based on Green's-function renormalization-decimation techniques—is asymptotically exact for the infinite chain and shows the formation of these states regardless of whether or not a confining electric field is applied. The renormalization language naturally leads to the central role played by the chemical bond of the atoms immediately across the interface. In the adopted decimation procedure, the convergence rate to a fixed point directly relates the valley splitting and the spread of the wave function, consequently connecting the splitting to geometrical experimental parameters such as the capacitance of a two-dimensional electron gas—explicitly calculated here. This should serve as a probe to identify such states as a mechanism for enhanced valley splitting.

We present a new technique for probing the properties of quantum fluids in restricted geometries. We have confined liquid ${}^4\mathrm{He}$ within microfluidic devices formed from glass wafers, in which one dimension is on the micrometer scale. Using an ultrasonic analog to Fabry-Pérot interferometry, we have measured the first sound of the confined liquid ${}^4\mathrm{He}$, which can be a probe of critical behavior near the lambda point ($T_{\lambda }$). All thermodynamic properties of liquid ${}^4\mathrm{He}$ can be derived from first-sound and heat capacity measurements, and although quite a bit of experimental work has been done on the latter, no measurement of first sound has been reported for a precisely confined geometry smaller than a few tens of micrometers. In this work, we report measurements of isobaric first sound in liquid ${}^4\mathrm{He}$ confined in cavities as small as $\sim$5 $\mu$m. Our experimental setup allows us to pressurize the liquid up to $\sim$25 bar without causing deformation of the confined geometry, a pressure which is about four times larger than previously reported with similar microfluidic devices. Our preliminary results indicate that one can possibly observe finite-size effects and verify scaling laws, by using similar devices with smaller confinement.

We explore the dielectric properties of graphene-like two-dimensional Kane-Mele topological insulators manifest in buckled honeycomb lattices (such as silicene and germanene). The effect of an on-site potential difference (${\Delta }_z$) between sublattices is given particular attention. We present the results for the real and imaginary parts of the dynamical polarization function. We show that these results display features of three regimes (topological insulator, valley-spin polarized metal, and trivial band insulator) and may be used to extract information on the strength of the intrinsic spin-orbit coupling. We study the inverse dielectric function and provide numerical results for the plasmon branch. We discuss the behavior of the plasmon as a function of sublattice potential difference and show that the behavior of the plasmon branch as ${\Delta }_z$ is varied is dependent on the location of the chemical potential with respect to the gaps. The static polarization is discussed and numerical results for the screening of a charged impurity are provided. We observe a beating phenomenon in the effective potential which is dependent on ${\Delta }_z$.

We discuss a recent resonant ultrasound spectroscopy (RUS) study of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\delta }$, which infers a line of phase transitions bounding the pseudogap phase and argue that this scenario is not supported by thermodynamic evidence. We show that the anomalies in RUS, heat capacity, and thermal expansion at the superconducting transition temperatures agree well, but there are large discrepancies between RUS and thermodynamic measurements at $T^{*}$ where the pseudogap phase transitions are purported to occur. Moreover, the frequency and temperature dependence of the RUS data for the crystal with $\delta =0.98$, interpreted in terms of critical slowing down near an electronic phase transition, is five orders of magnitude smaller than what is expected. For this crystal the RUS data near $T^{*}$ are more consistent with nonequilibrium effects, such as oxygen relaxation.

Recent work has indicated that the symmetry of the lead-free piezoelectric perovskite ${\mathrm{Na}}_{0.5}$${\mathrm{Bi}}_{0.5}$${\mathrm{TiO}}_3$ can be changed from monoclinic to rhombohedral through the application of an electric field, which may have implications for the study and design of piezoelectric materials close to a morphotropic phase boundary. We have examined high-quality, single-crystal ${\mathrm{Na}}_{0.5}$${\mathrm{Bi}}_{0.5}$${\mathrm{TiO}}_3$ using transmission electron microscopy and have used digital electron diffraction to observe the symmetry of defect-free regions of material on length scales of a few nanometers. This unequivocally demonstrates that the material is rhombohedral with space group $R$3$c$ on this length scale. We find that a model that allows disordered displacements of Bi atoms from their nominal sites in the $R$3$c$ symmetry, while retaining this symmetry on average, gives a very significant improvement in fit to simulations. We use conventional transmission electron microscopy to enumerate the different types of defects that are observed in other regions of the crystal and find a complex microstructure of antiphase boundaries, domain walls, and tetragonal platelets. Their interaction leads to the formation of very high densities of nanotwins. We show that these are expected to have a variable monoclinic Cc symmetry that is driven by the constraint of continuity of the crystal across a domain wall.

In the Hamiltonian approach to Chern bands one constructs an algebraically exact mapping which expresses the electron density projected to the Chern band ${\rho }_{\mathrm{FCB}}$ as a sum of Girvin-MacDonald-Platzman density operators ${\rho }_{\mathrm{GMP}}$. The ${\rho }_{\mathrm{GMP}}$ operators, which obey the magnetic translation algebra, are expressed in terms of auxiliary composite fermion (CF) variables, while preserving the algebra. This produces, in a natural way, a unique Hartree-Fock ground state for the CFs, which can be used as a springboard for various computations. In previous presentations we had not realized that this procedure, which works in any gauge, in principle, is greatly optimized by one family of gauges introduced by Y.-L. Wu, N. Regnault, and B. A. Bernevig [Phys. Rev. B 86, 085129 (2012); Phys. Rev. Lett. 110, 106802 (2013)] in their exact diagonalization program for Laughlin-like fractions. Here we explain in detail why their gauge choices also enable us to obtain better variational energies in our Hamiltonian approach. We illustrate the ideas with some results on the Haldane model, comparing our results to exact diagonalizations.