We present a measurement-based scheme for performing braiding operations on Majorana zero modes in mesoscopic superconductor islands and for detecting their non-Abelian statistics without moving or hybridizing them. In our scheme for “braiding without braiding”, the topological qubit encoded in any pair of well-separated Majorana zero modes is read out from the transmission phase shift in electron teleportation through the island in the Coulomb-blockade regime. We propose experimental setups to measure the teleportation phase shift via conductance in an electron interferometer or persistent current in a closed loop.

Type-II Weyl semimetals are a novel gapless topological phase of matter discovered recently in 2015. Similar to normal (type-I) Weyl semimetals, type-II Weyl semimetals consist of isolated band touching points. However, unlike type-I Weyl semimetals which have a linear energy dispersion around the band touching points forming a three-dimensional (3D) Dirac cone, type-II Weyl semimetals have a tilted conelike structure around the band touching points. This leads to various novel physical properties that are different from type-I Weyl semimetals. In order to study further the properties of type-II Weyl semimetals and perhaps realize them for future applications, generating controllable type-II Weyl semimetals is desirable. In this paper, we propose a way to generate a type-II Weyl semimetal via a generalized Harper model interacting with a harmonic driving field. When the field is treated classically, we find that only type-I Weyl points emerge. However, by treating the field quantum mechanically, some of these type-I Weyl points may turn into type-II Weyl points. Moreover, by tuning the coupling strength, it is possible to control the tilt of the Weyl points and the energy difference between two Weyl points, which makes it possible to generate a pair of mixed Weyl points of type-I and type-II. We also discuss how to physically distinguish these two types of Weyl points in the framework of our model via the Landau level structures in the presence of an artificial magnetic field. The results are of general interest to quantum optics as well as ongoing studies of Floquet topological phases.

Recently, several experiments [K. K. Gomes et al., Nature (London) 483, 306 (2012); S. Wang et al., Phys. Rev. Lett. 113, 196803 (2014)] have illustrated that metal surface electrons can be manipulated to form a two-dimensional (2D) lattice by depositing a designer molecule lattice on a metal surface. This offers a promising new technique to construct artificial 2D electron lattices. Here we theoretically propose a molecule lattice pattern to realize an artificial Lieb lattice on a metal surface, which shows a flat electronic band due to the lattice geometry. We show that the localization of electrons in the flat band may be understood from the viewpoint of electron interference, which may be probed by measuring the local density of states with scanning tunneling microscopy. Our proposal may be readily implemented in experiment and may offer an ideal solid state platform to investigate the novel flat band physics of the Lieb lattice.

The magnetic properties of zigzag graphene nanoflakes (ZGNFs) are investigated within the framework of inhomogeneous dynamical mean-field theory. At half-filling and for realistic values of the local interaction, the ZGNF is in a fully compensated antiferromagnetic (AF) state, which is found to be robust against temperature fluctuations. Introducing charge carriers in the AF background drives the ZGNF metallic and stabilizes a magnetic state with a net uncompensated moment at low temperatures. The change in magnetism is ascribed to the delocalization of the doped holes in the proximity of the edges, which mediate ferromagnetic correlations between the localized magnetic moments. Depending on the hole concentration, the magnetic transition may display a pronounced hysteresis over a wide range of temperatures, indicating the coexistence of magnetic states with different symmetries. This suggests the possibility of achieving electrostatic control of the magnetic state of ZGNFs to realize a switchable spintronic device.

We present the results of a comprehensive magneto-optical characterization of single CdTe quantum dots containing a few ${\mathrm{Mn}}^{2+}$ ions. We find that some quantum dots exhibit an unexpected evolution of excitonic photoluminescence spectrum with the magnetic field. At a certain value of the magnetic field, specific for each quantum dot, each of the broad spectral lines related to the recombination of various excitonic complexes confined inside the dot transforms into a pair of narrow lines split by several meV. We interpret this abrupt change in the character of excitonic emission spectrum as a consequence of a transition from a nonpolarized state of the ${\mathrm{Mn}}^{2+}$ spins in a low field regime to a highly (almost fully) polarized state above the critical magnetic field. Various optical experiments, including polarization-resolved studies, investigation of different excitation regimes and time-resolved measurements corroborate this scenario. However, these measurements indicate also that the observed effect is not related or influenced by the photocreated charge carriers, but it is rather originating from unusual spin configuration in the cluster of ${\mathrm{Mn}}^{2+}$ ions.

We report simultaneous quasi-dc magnetotransport and high-frequency surface acoustic wave measurements on bilayer two-dimensional electron systems in GaAs. Near strong integer quantized Hall states, a strong magnetic-field-sweep hysteresis in the velocity of the acoustic waves is observed at low temperatures. This hysteresis indicates the presence of a metastable state with anomalously high conductivity in the interior of the sample. This nonequilibrium state is not revealed by conventional low-frequency transport measurements which are dominated by dissipationless transport at the edge of the two-dimensional system. We find that a field-cooling technique allows the equilibrium charge configuration within the interior of the sample to be established. A simple model for this behavior is discussed.

While much is known about isoelectronic materials related to carbon nanostructures, such as boron-nitride layers and nanotubes, rather less is known about equivalent silicon-based materials. Following the recent discovery of phosphorene, here we discuss isoelectronic silicon-monosulfide monolayers. We describe a set of anisotropic structures that clearly have a high stability with respect to previously reported silicon-monosulfide monolayers. The source of the layer anisotropy is related to the presence of Si-S double chains linked by some Si-Si covalent bonds together with a remarkable spd hybridization on Si. The increased stability is related to silicon forming four bonds, including an additional double-bond-like Si-Si bond. The involvement of $d$ orbitals brings more variety to silicon-sulfide-based nanostructures that are isoelectronic to phosphorene, which could be relevant for future applications, adding extra degrees of freedom.

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to “freely” spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite-temperature ab initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long-range antiferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures, the motions are less correlated.

We present direct local-probe evidence for strongly hybridized nuclear-electronic spin states of an Ising ferromagnet ${\mathrm{LiHoF}}_4$ in a transverse magnetic field. The nuclear-electronic states are addressed via a magnetic resonance in the GHz frequency range using coplanar resonators and a vector network analyzer. The magnetic resonance spectrum is successfully traced over the entire field-temperature phase diagram, which is remarkably well reproduced by mean-field calculations. Our method can be directly applied to a broad class of materials containing rare-earth ions for probing the substantially mixed nature of the nuclear and electronic moments.

The dispersive magnetic excitations in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ have previously been well described within an isospin-1/2 Heisenberg model on a square lattice that revealed parallels with ${\mathrm{La}}_2{\mathrm{CuO}}_4$. Here we investigate the inelastic spectra of ${\mathrm{Sr}}_2{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_4$ with resonant inelastic x-ray scattering (RIXS) at the Ir $L_3$ edge. The results are well described using linear spin-wave theory within a similar Heisenberg model applicable to ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$; however, the disorder induced by the substitution of $20\%{\mathrm{Ir}}^{4+}$ ions for ${\mathrm{Ru}}^{4+}$ removes longer range exchange interactions. A large spin gap (40 meV) is measured indicating strong anisotropy from spin-orbit coupling that is manifest due to the altered magnetic structure in ${\mathrm{Sr}}_2{\mathrm{Ir}}_{0.8}{\mathrm{Ru}}_{0.2}{\mathrm{O}}_4$ with $c$-axis aligned moments compared to the basal plane moments in the parent. Collectively the results indicate the robustness of a Heisenberg model description even when the magnetic structure is altered and the $J_{\mathrm{eff}}=1/2$ moments are diluted.

Within one-dimensional disordered models of interacting fermions, we perform a numerical study of several dynamical density correlations, which can serve as hallmarks of the transition to the many-body localized state. The results confirm that density-wave correlations exhibit quite an abrupt change with increasing disorder, with a nonvanishing long-time value characteristic for the nonergodic phase. In addition, our results reveal a logarithmic variation of correlations in time in a wide time window, which we can bring in connection with the anomalous behavior of the dynamical conductivity near the transition. Our results support the view that the transition to many-body localization can be characterized by universal dynamical exponents.

Earlier Monte Carlo calculations on the dissipative two-dimensional XY model are extended in several directions. We study the phase diagram and the correlation functions when dissipation is very small, where it has properties of the classical 3D-XY transition, i.e., one with a dynamical critical exponent $z=1$. The transition changes from $z=1$ to the class of criticality with $z\to \infty$ driven by topological defects, discovered earlier, beyond a critical dissipation. We also find that the critical correlations have power-law singularities as a function of tuning the ratio of the kinetic energy to the potential energy for fixed large dissipation, as opposed to essential singularities on tuning dissipation keeping the former fixed. A phase with temporal disorder but spatial order of the Kosterlitz-Thouless form is also further investigated. We also present results for the transition when the allowed Caldeira-Leggett form of dissipation and the allowed form of dissipation coupling to the compact rotor variables are both included. The nature of the transition is then determined by the Caldeira-Leggett form.

We introduce a generalization of conventional lattice gauge theory to describe fracton topological phases, which are characterized by immobile, pointlike topological excitations, and subextensive topological degeneracy. We demonstrate a duality between fracton topological order and interacting spin systems with symmetries along extensive, lower-dimensional subsystems, which may be used to systematically search for and characterize fracton topological phases. Commutative algebra and elementary algebraic geometry provide an effective mathematical tool set for our results. Our work paves the way for identifying possible material realizations of fracton topological phases.

The ferromagnetic semiconductor $\mathrm{B}{\mathrm{a}}_2\mathrm{NiOs}{\mathrm{O}}_6$ ($T_{\mathrm{mag}}\sim 100\mathrm{K}$) was synthesized at 6 GPa and 1500 °C. It crystallizes into a double perovskite structure [$Fm-3m$; $a=8.0428\left(1\right)\AA$], where the $\mathrm{N}{\mathrm{i}}^{2+}$ and $\mathrm{O}{\mathrm{s}}^{6+}$ ions are perfectly ordered at the perovskite $B$ site. We show that the spin-orbit coupling of $\mathrm{O}{\mathrm{s}}^{6+}$ plays an essential role in opening the charge gap. The magnetic state was investigated by density functional theory calculations and powder neutron diffraction. The latter revealed a collinear ferromagnetic order in a $>21\mathrm{kOe}$ magnetic field at 5 K. The ferromagnetic gapped state is fundamentally different from that of known dilute magnetic semiconductors such as (Ga,Mn)As and (Cd,Mn)Te ($T_{\mathrm{mag}}<180\mathrm{K}$), the spin-gapless semiconductor $\mathrm{M}{\mathrm{n}}_2\mathrm{CoAl}$ ($T_{\mathrm{mag}}\sim 720\mathrm{K}$), and the ferromagnetic insulators EuO ($T_{\mathrm{mag}}\sim 70\mathrm{K}$) and $\mathrm{B}{\mathrm{i}}_3\mathrm{C}{\mathrm{r}}_3{\mathrm{O}}_{11}$ ($T_{\mathrm{mag}}\sim 220\mathrm{K}$). It is also qualitatively different from known ferrimagnetic insulators and semiconductors, which are characterized by an antiparallel spin arrangement. Our finding of the ferromagnetic semiconductivity of $\mathrm{B}{\mathrm{a}}_2\mathrm{NiOs}{\mathrm{O}}_6$ should increase interest in the platinum group oxides, because this alternative class of materials should be useful in the development of spintronic, quantum magnetic, and related devices.

The canonical quantization procedure in dispersive and lossy media assumes the eigenstates of the system to be collective excitations of the electromagnetic field and reservoir degrees of freedom. In this paper, we show that in low loss limit, the collective plasmonic modes in a quasistatic approximation separate from the reservoir. As an example, we consider the localized surface plasmon on a spherical metallic nanoparticle in a vacuum and find the macroscopic longitudinal electric near field per plasmon in the cases of gold and silver. Using our canonical approach, we calculate the correction to the electric near field per plasmon obtained from phenomenological quantization. The canonical conjugated variable to the electric near field is determined.

Phase-field simulation (PFS) has revolutionized the understanding of domain structure and switching behavior in ferroelectric thin films and ceramics. Generally, PFS is based on the solution of (a set of) Landau-Ginzburg-Devonshire equations for a defined order parameter field(s) under physical boundary conditions (BCs) of fixed potential or charge. While well matched to the interfaces in bulk materials and devices, these BCs are generally not applicable to free ferroelectric surfaces. Here, we developed a self-consistent phase-field model with BCs based on electrochemical equilibria. We chose $\mathrm{Pb}\left(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}\right){\mathrm{O}}_3$ ultrathin film consisting of (001) oriented single tetragonal domain $\left(P_z\right)$ as a model system and systematically studied the effects of oxygen partial pressure, temperature, and surface ions on the ferroelectric state and compared it with the case of complete screening. We have further explored the polarization switching induced by the oxygen partial pressure and observed pronounced size effect induced by chemical screening. Our paper thus helps to understand the emergent phenomena in ferroelectric thin films brought about by the electrochemical ionic surface compensations.

We study theoretically and numerically spasing conditions and optical dynamics of a composite hyperbolic metamaterial interacting with gain molecules. By combining Maxwell-Bloch equations with Green's function approach, we calculate lasing frequency and threshold population inversion for various gain density in the gain layer. We demonstrate a high level of enhancement for photonic density of states in a spacious spectral range provided by hyperbolic metastructures. We find that direct dipole-dipole interactions between molecules in the gain layer has a negligible effect on spasing conditions. We identify a region of parameters in which spasing can occur considering these effects.

We report the study of Nernst effect in quasi-low-dimensional tungsten bronze ${\mathrm{P}}_4{\mathrm{W}}_{12}{\mathrm{O}}_{44}$ showing a sequence of Peierls instabilities. We demonstrate that both condensation of the electronic carriers in the charge density wave state and the existence of high-mobility electrons and holes originating from the small pockets remaining in the incompletely nested Fermi surface give rise to a Nernst effect of a magnitude similar to that observed in heavy fermion compounds.

We propose a one-parameter family of noninteracting lattice models for Weyl semimetals with four Weyl nodes and tunable Fermi arcs. These two-band model Hamiltonians are time-reversal symmetric with ${\mathrm{T}}^2=+1$, and tuning the parameter changes the connectivity of the Fermi arcs continuously without affecting the location and chiralities of the Weyl nodes in the bulk Brillouin zone. The bulk polarization and magnetization are shown to vary with this parameter, a dependence inaccessible to the low energy effective field theory.