We demonstrate the initialization of a single quantum-dot hole spin with high fidelity (lower bound $>97\%$), on picosecond time scales, and without the need for magnetic fields. Using the initialization scheme based on rapid electric-field ionization of a resonantly excited exciton, this is achieved by employing a self-assembled quantum dot with a low conduction-to-valence band offset ratio, allowing control of the relative electron and hole tunneling rates over three orders of magnitude. This large difference in tunneling rates could permit spin-storage efficiencies $>99.5\%$ by fast-switching to a low electric-field condition. Our results may provide a practical route towards ultrafast high-fidelity initialization of individual quantum-dot hole spins for the implementation of quantum error correction in a scalable spin-based quantum computer.

We study bond and spin correlations of the nearest-neighbor resonating valence bond (RVB) wave function for a SU(2) symmetric $S=1/2$ antiferromagnet on the honeycomb lattice. We find that spin correlations in this wave function are short ranged, while the bond energy correlation function takes on an oscillatory power-law form $D\left(\vec{r}\right)\sim cos(\mathbf{Q}·\vec{r})/{\left|\vec{r}\right|}^{{\eta }_w\left(2\right)}$, where $\mathbf{Q}=(2\pi /3,-2\pi /3)$ is the wave vector corresponding to “columnar” valence-bond solid order on the honeycomb lattice, and ${\eta }_w\left(2\right)\approx 1.49\left(3\right)$. We use a recently introduced large-$g$ expansion approach to relate bond-energy correlators of the $\mathrm{SU}\left(g\right)$ wave function to dimer correlations of an interacting fully packed dimer model with a three-dimer interaction of strength $V\left(g\right)=-ln(1+1/g^2)$. Putting $g=2$, we find numerically that the dimer correlation function $D^d\left(\vec{r}\right)$ of this dimer model has power-law behavior $D^d\left(\vec{r}\right)\sim cos(\mathbf{Q}·\vec{r})/{\left|\vec{r}\right|}^{{\eta }_d\left(2\right)}$ with ${\eta }_d\left(2\right)\approx 1.520\left(15\right)$, in rather good agreement with the wave function results. We also study the same quantities for $g=3,4,10$ and find that the bond-energy correlations in the $\mathrm{SU}\left(g\right)$ wave function are consistently well reproduced by the corresponding dimer correlations in the interacting dimer model.

We construct an exactly soluble Hamiltonian on the $D=3$ cubic lattice, whose ground state is a topological phase of bosons protected by time-reversal symmetry, i.e., a symmetry-protected topological (SPT) phase. In this model, excitations with anyonic statistics are shown to exist at the surface but not in the bulk. The statistics of these surface anyons is explicitly computed and shown to be identical to the three-fermion ${\mathbb{Z}}_2$ model, a variant of ${\mathbb{Z}}_2$ topological order which cannot be realized in a purely $D=2$ system with time-reversal symmetry. Thus the model realizes a novel surface termination for three-dimensional (3D) SPT phases, that of a fully symmetric gapped surface with topological order. The 3D phase found here was previously proposed from a field theoretic analysis but is outside the group cohomology classification that appears to capture all SPT phases in lower dimensions. Such phases may potentially be realized in spin-orbit-coupled magnetic insulators, which evade magnetic ordering. Our construction utilizes the Walker-Wang prescription to create a 3D confined phase with surface anyons, which can be extended to other topological phases.

We study the effects of strong coupling of a localized state charge to one-dimensional electronic channels out of equilibrium. While the state of this charge and the coupling strengths determine the scattering phase shifts in the channels, the nonequilibrium partitioning noise induces the tunneling transitions to the localized state. The strong coupling leads to a nonperturbative backaction effect which is manifested in the orthogonality catastrophe and the Fermi-edge singularity in the transition rates. We predict an unusually pronounced manifestation of the non-Gaussian component of noise that breaks the charge symmetry, resulting in a nontrivial shape, and a shift of the position of the tunneling resonance.

Mixed valence rare-earth samarium compounds $\mathrm{Sm}X$ ($X=\text{Se,Te}$) have been recently proposed as candidate materials for use in high-speed, low-power digital switches driven by stress induced changes of resistivity. At room temperature these materials exhibit a pressure driven insulator-to-metal transition with resistivity decreasing by up to seven orders of magnitude over a small pressure range. Thus, the application of only a few GPa's to the piezoresistor ($\mathrm{Sm}X$) allows the switching device to perform complex logic. Here we study from first principles the electronic properties of these compounds under uniaxial strain and discuss the implications for carrier transport. Based on changes in the band structure and a model we show that the piezoresistive response is mostly governed by the reduction of band gap with strain. Furthermore, the piezoresistive reponse becomes optimal when the Fermi level is pinned near the localized valence band. The piezoresistive effect under uniaxial strain, which must be taken into account in thin films and other systems with reduced dimensionality, is also studied. Under uniaxial strain we find that the piezoresistive response can be substantially larger than in the isotropic case. Analysis of the complex band structure of SmSe yields a tunneling length of the order of 1 nm. This suggest that the conduction mechanism governing the piezoresistive effect in bulk, i.e., thermal promotion of electrons, should still be dominant in few-nanometer-thick films.

In topological phases in $2+1$ dimensions, anyons fall into representations of quantum group symmetries. As proposed in our work [Hung and Wan, Int. J. Mod. Phys. B 28, 1450172 (2014)], the physics of a symmetry enriched phase can be extracted by the mathematics of (hidden) quantum group symmetry breaking of a “parent phase.” This offers a unified framework and classification of the symmetry enriched (topological) phases, including symmetry protected trivial phases as well. In this paper, we extend our investigation to the case where the “parent” phases are non-Abelian topological phases. We show explicitly how one can obtain the topological data and symmetry transformations of the symmetry enriched phases from that of the “parent” non-Abelian phase. Two examples are computed: (1) the $\text{Ising}\times \overline{\text{Ising}}$ phase breaks into the ${\mathbb{Z}}_2$ toric code with ${\mathbb{Z}}_2$ global symmetry; (2) the $\mathrm{SU}{\left(2\right)}_8$ phase breaks into the chiral Fibonacci $\times$ Fibonacci phase with a ${\mathbb{Z}}_2$ symmetry, a first non-Abelian example of symmetry enriched topological phase beyond the gauge-theory construction.

The internal structures of thin films, particularly interfaces between different materials, are critical to system properties and performance across many disciplines, but characterization of buried interface topography is often unfeasible. In this work, we demonstrate that grazing resonant soft x-ray scattering (GRSoXS), a technique measuring diffusely scattered soft x rays from grazing incidence, can reveal the statistical topography of buried thin-film interfaces. By controlling and predicting the x-ray electric field intensity throughout the depth of the film and simultaneously the scattering contrast between materials, we are able to unambiguously identify the microstructure at different interfaces of a model polymer bilayer system. We additionally demonstrate the use of GRSoXS to selectively measure the topography of the surface and buried polymer-polymer interface in an organic thin-film transistor, revealing different microstructure and markedly differing evolution upon annealing. In such systems, where only indirect control of interface topography is possible, accurate measurement of the structure of interfaces for feedback is critically important. While we demonstrate the method here using organic materials, we also show that the technique is readily extendable to any thin-film system with elemental or chemical contrasts exploitable at absorption edges.

We present resonant Raman scattering experiments on nanotube samples enriched in metallic armchair single-wall carbon nanotubes (SWCNTs). We establish the transverse optical $\left({\mathcal{A}}_{\mathrm{TO}}\right)$ phonon frequency for the (5,5) through (10,10) armchair species, ranging in diameter from 0.68 to 1.36 nm. The frequencies show a strong diameter dependence similar to that previously observed in semiconducting nanotubes. We show that the ${\mathcal{A}}_{\mathrm{TO}}$ frequencies in armchair SWCNTs are dramatically upshifted from those of semiconducting SWCNTs. Furthermore, using electrochemical doping, we demonstrated that the ${\mathcal{A}}_{\mathrm{TO}}$ frequencies in armchair SWCNTs are independent of the position of the Fermi level. These results suggest that the upshift is a result of a Kohn anomaly involving a forward-scattering mechanism of electrons close to the Fermi level. This is in contrast to the well-known Kohn anomaly that dominates the downshift of the ${\mathcal{A}}_{\mathrm{LO}}$ and ${\mathcal{E}}_{2g}$ phonons in nonarmchair metallic SWCNTs and graphene, respectively.

We develop a density-matrix formalism in the length gauge to calculate the nonlinear response of intrinsic monolayer graphene at terahertz frequencies. Employing a tight-binding model, we find that the interplay of the interband and intraband dynamics leads to strong harmonic generation at moderate field amplitudes. In particular, we find that at low temperature, the reflected field of undoped suspended graphene exhibits a third harmonic amplitude that is 32% of the fundamental for an incident field of 100 V/cm. Moreover, we find that up to the seventh harmonic and beyond are generated.

Thermal conductivity of single layer phosphorene is investigated by combining density functional calculations and the Peierls-Boltzmann transport equation. Differing from isotropic and divergent thermal conductivities in two-dimensional graphene and ${\mathrm{MoS}}_2$, a compelling coexistence of size-dependent and size-independent thermal conductivities is discovered for single layer black phosphorus (BP) along zigzag (ZZ) and armchair (AM) directions, respectively. Additionally, thermal conductivities in single layer phosphorene are found to be highly anisotropic because of orientation dependent group velocities and phonon relaxation times; e.g., thermal conductivities at 300 K are 83.5 and 24.3 W/m K along ZZ and AM directions, respectively, for single layer BP with a size of $10\mu \text{m}$.

The low-energy electronic states of the iron-based superconductors are strongly affected by both spin-orbit coupling and, when present, by the nematic order. These two effects have different physical origins, yet they can lead to similar gap features in the electronic spectrum. Here we show how to disentangle them experimentally in the iron superconductors with one Fe plane per unit cell. Although the splitting of the low-energy doublet at the Brillouin zone center ($\Gamma$ point) can be due to either the spin-orbit coupling or the nematic order, or both, the degeneracy of each of the doublet states at the zone corner ($M$ point) is protected by the space-group symmetry even when spin-orbit coupling is taken into account. Therefore, any splitting at $M$ must be due to lowering of the crystal symmetry, such as due to the nematic order. We further analyze a microscopic tight-binding model with two different contributions to the nematic order: $d_{xz}/d_{yz}$ on-site energy anisotropy and the $d_{xy}$ hopping anisotropy. We find that a precise determination of the former, which has been widely used to characterize the nematic phase, requires a simultaneous measurement of the splittings of the $\Gamma$-point doublet and at the two low-energy $M$-point doublets. We also discuss the impact of twin domains and show how our results shed new light on ARPES measurements in the normal state of these materials.

Very recently, it was demonstrated that the carrier mobility of a molecular monolayer dioctylbenzothienobenzothiophene $\left({\mathrm{C}}_8\text{−}\mathrm{BTBT}\right)$ on boron nitride can reach $10\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$, the highest among the previously reported monolayer molecular field-effect transistors. Here we show that the high-quality single crystal of the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ monolayer may be the key origin of the record-high carrier mobility. We discover that the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ molecules prefer layer-by-layer growth on both hexagonal boron nitride and graphene. The flatness of these substrates substantially decreases the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ nucleation density and enables repeatable growth of large-area single crystal of the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ monolayer. Our experimental result indicates that only out-of-plane roughness greater than 0.6 nm of the substrates could induce disturbance in the crystal growth and consequently affect the charge transport. This information would be important in guiding the growth of high-quality epitaxy molecular film.

We study the intrinsic Hall conductivity of the ordinary and topological superconducting phases of a Rashba metal in a perpendicular Zeeman field. In this system, the normal metal breaks time reversal symmetry while the superconducting order parameter does not, in contrast to the chiral $p$-wave superconducting state predicted in the monolayer strontium ruthenate $\left({\mathrm{Sr}}_2{\mathrm{RuO}}_4\right)$ whose Hall conductivity has been studied extensively. We study the effects of intraband and interband pairing and find there is qualitatively larger change in the intrinsic Hall conductivity when there is interband pairing, with the change in magnitude linear in the pairing gap. We argue that interband pairing leads in general to higher energy costs for the topological phase compared to the topologically trivial phase and thus that the qualitative behavior of the intrinsic Hall conductivity with superconductivity in these systems could provide important clues about the nature of pairing in the superconducting phase and even some hints of whether it is topological or not.

We study the topologically protected Majorana zero modes induced by lattice dislocations in chiral topological superconductors. Dislocations provide a new way to realize Majorana modes at zero magnetic field. In particular, we study several different types of dislocations in the candidate material ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$ We also discuss the properties of linked dislocation lines and linked dislocation and flux lines. Various experimental consequences are predicted which provide an approach to determine the nature of the superconducting phase of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4.$

In high-purity GaN grown by hydride vapor phase epitaxy, the commonly observed yellow luminescence (YL) band gives way to a green luminescence (GL) band at high excitation intensity. We propose that the GL band with a maximum at 2.4 eV is caused by transitions of electrons from the conduction band to the 0/+ level of the isolated ${\mathrm{C}}_{\mathrm{N}}$ defect. The YL band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV and can be observed only for some high-purity samples. However, in less pure GaN samples, where no GL band is observed, another YL band with a maximum at 2.2 eV dominates the photoluminescence spectrum. The latter is attributed to the ${\mathrm{C}}_{\mathrm{N}}{\mathrm{O}}_{\mathrm{N}}$ complex.

We report a determinant Quantum Monte Carlo investigation which quantifies the behavior of the susceptibility and the entropy in the framework of the periodic Anderson model, focusing on the evolution with different degree of conduction electron $\left(c\right)$-local moment $\left(f\right)$ hybridization. These results capture the behavior observed in several experiments, including the universal behavior of the NMR Knight shift anomaly below the crossover temperature $T^{*}$. We find that $T^{*}$ is a measure of the onset of $c-f$ correlations and grows with increasing hybridization. These results suggest that the NMR Knight shift and spin-lattice relaxation rate measurements in non-Fermi-liquid materials are strongly influenced by the temperature dependence of the $c-f$ kinetic energy. Our results provide a microscopic basis for the phenomenological two-fluid model of Kondo lattice behavior, and its evolution with pressure and temperature.

We study theoretically the spin transport in a device in which the active layer is an organic film with numerous deep and widely dispersed in energy in-gap levels serving as traps. A carrier, diffusing between a magnetized injector and detector, spends a considerable portion of time on the traps. This feature of transport does not affect the giant magnetoresistance, which is sensitive only to the mutual orientation of magnetizations of the injector and detector. By contrast, the presence of traps strongly affects the sensitivity of the spin transport to an external magnetic field perpendicular to the magnetizations of the electrodes (the Hanle effect). Namely, the Hanle curve narrows dramatically and develops a flat background. The origin of such a narrowing is that the spin precession takes place during the entire time of the carrier motion between the electrodes, while the spin relaxation takes place only during diffusive motion between the subsequent traps. If the resulting width of the Hanle curve is smaller than the measurement resolution, observation of the Hanle peak becomes impossible. This would explain why the Hanle effect is missing in experiments on organic spin valves, where the giant magnetoresistance is unambiguously detected.

We are presenting a rapid and straightforward approach to determine the tip radius of sharp tungsten tips characterized by field ion microscopy. The utilization of certain features with well-known dimensions on the surface of these tips around the crystallographic [111] direction allows us to increase the accuracy of the radius measurement by almost one order of magnitude in comparison to standard methods. By employing a few reasonable approximations, it is possible to derive an analytical expression for the tip radius as a function of the observed feature size on the microchannel plate and some geometric parameters of the setup. Finally, we show that field ion microscopy images can be reconstructed on the atomic level by using a perfect hemisphere with the determined radius as a starting value and a low number of modifications in the topmost surface layers. In particular, this is useful for quantifying tip-sample interactions and characterizing material properties in atomic force microscopy.