Realizing Majorana modes (MMs) in condensed-matter systems is of vast experimental and theoretical interests, and some signatures of MMs have been measured already. To facilitate future experimental observations and to explore further applications of MMs, generating many MMs at ease in an experimentally accessible manner has become one important issue. This task is achieved here in a one-dimensional $p$-wave superconductor system with the nearest- and next-nearest-neighbor interactions. In particular, a periodic modulation of some system parameters can induce an effective long-range interaction (as suggested by the Baker-Campbell-Hausdorff formula) and may recover time-reversal symmetry already broken in undriven cases. By exploiting these two independent mechanisms at once we have established a general method in generating many Floquet MMs via periodic driving.

We predict that long-range interactions give rise to anisotropy in the electrical resistivity of Weyl metals at low temperatures, where the electrical resistivity becomes much reduced when electric fields are applied to the direction of the momentum vector to connect two paired Weyl points. Performing the renormalization group analysis, we find that the distance between two Weyl points becomes enhanced logarithmically at low temperatures although the coupling constant of such interactions vanishes inverse-logarithmically. Considering the Adler-Bell-Jackiw anomaly, scattering between these two Weyl points becomes suppressed to increase electrical conductivity in the “longitudinal” direction, counter intuitive in the respect that interactions are expected to reduce metallicity. We also propose that the anomalous contribution in the Hall effect shows the logarithmic enhancement as a function of temperature, originating from the fact that the anomalous Hall coefficient turns out to be proportional to the distance between two paired Weyl points. Correlations with topological constraints allow unexpected and exotic transport properties.

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$ and its nonsuperconducting parent compound LaOBiS${}_2$. The crystal structures and vibrational modes have been examined. Bragg peaks from the superconducting sample exhibit pronounced broadening compared to those from the nonsuperconducting sample. In the inelastic measurements, a large difference in the high-energy phonon modes was observed upon F doping. Alternatively, the low-energy modes remain almost unchanged between nonsuperconducting and superconducting states either by F doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

Aberration-corrected scanning transmission electron microscopy is employed to investigate the local chemistry in the vicinity of a Si${}_{0.8}$Ge${}_{0.2}$/Si interface grown by molecular-beam epitaxy. Atomic-resolution high-angle annular dark field contrast reveals the presence of a nonuniform diffusion of Ge from the substrate into the strained Si thin film. On the basis of multislice calculations, a model is proposed to quantify the experimental contrast, showing that the Ge concentration in the thin film reaches about 4$\%$ at the interface and decreases monotonically on a typical length scale of 10 nm. Diffusion occurring during the growth process itself therefore appears as a major factor limiting the abruptness of interfaces in the Si-Ge system.

The gradient of the Casimir force between a Si-SiO${}_2$-graphene substrate and an Au-coated sphere is measured by means of a dynamic atomic force microscope operated in the frequency shift technique. It is shown that the presence of graphene leads to up to a $9\%$ increase in the force gradient at the shortest separation considered. This is in qualitative agreement with the predictions of Lifshitz theory using the dielectric permittivities of Si and SiO${}_2$ and the Dirac model of graphene.

Using the tight-binding approximation we calculated the diamagnetic susceptibility of graphene quantum dots (GQDs) of different geometrical shapes and characteristic sizes of 2–10 nm, when the magnetic properties are governed by the electron edge states. Two types of edge states can be discerned: the zero-energy states (ZESs), located exactly at the zero-energy Dirac point, and the dispersed edge states (DESs), with the energy close but not exactly equal to zero. DESs are responsible for a temperature-independent diamagnetic response, while ZESs provide a temperature-dependent spin paramagnetism. Hexagonal, circular, and randomly shaped GQDs contain mainly DESs, and, as a result, they are diamagnetic. The edge states of the triangular GQDs are of ZES type. These dots reveal the crossover between spin paramagnetism, dominating for small dots and at low temperatures, and orbital diamagnetism, dominating for large dots and at high temperatures.

We determine the Raman scattering efficiency of the $G$ and $2D$ peaks in graphene. Three substrates are used: silicon covered with 300 or 90 nm oxide, and calcium fluoride (CaF${}_2$). On Si/SiO${}_x$, the areas of the $G$ and $2D$ peak show a strong dependence on the substrate due to interference effects, while on CaF${}_2$ no significant dependence is detected. Unintentional doping is reduced by placing graphene on CaF${}_2$. We determine the Raman scattering efficiency by comparison with the 322 cm${}^{-1}$ peak area of CaF${}_2$. At 2.41 eV, the Raman efficiency of the $G$ peak is $\sim 200\times {10}^{-5}$ m${}^{-1}$Sr${}^{-1}$, and changes with the excitation energy to the power of 4. The $2D$ Raman efficiency is at least one order of magnitude higher than that of the $G$ peak, with a different excitation energy dependence.

One- and two-electron systems confined in single and coupled quantum dots defined within a nanowire with a finite radius are studied in the context of spin-orbit coupling effects. The anisotropy of the spin-orbit interaction is discussed in terms of the system geometry and orientation of the external magnetic field vector. We find that there are easy and hard spin-polarization axes, and in the quantum dot with strong lateral confinement electron spin becomes well defined in spite of the presence of spin-orbit coupling. We present an analytical solution for the one-dimensional limit and study its validity for nanowires of finite radii by comparing the results with a full three-dimensional calculation. The results are also compared with the recent measurements of the effective Landé factor and avoided crossing width anisotropy in InSb nanowire quantum dots [S. Nadj-Perge et al., Phys. Rev. Lett. 108, 166801 (2012)].

We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 to 300 GPa. On heating or pressurizing we find first-order phase transitions under the experimental conditions of the phase III-IV boundary. At even higher pressures, close to 350 GPa, we find a second phase transformation to the previously proposed Cmca-4. Our method enables, for the first time, a direct comparison of Raman vibrons between theory and experiment at finite temperature. This turns out to provide excellent discrimination between subtly different structures found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in high-temperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers, and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite-size effects.

The nature of intrinsic point defects, i.e., energetic, ferroelectric, and electronic properties of vacancies in ferroelectric PbTiO${}_3$, is studied using first-principles calculations based on the hybrid Hartree-Fock density functional, which correctly reproduces the band gap and thus provides the accurate defect electronic states. The oxygen vacancies are found to behave as double shallow donors and are thermodynamically stable over a wide range of Fermi levels under oxygen-poor conditions, while Pb vacancies are likely to be formed under oxygen-rich conditions and act as double shallow acceptors, which lead to $p$-type conductivity. The result shows good consistency with the relevant experimental observations. The ferroelectric distortion is disturbed anisotropically outward by oxygen vacancies and is characterized by localized defect electronic states contributed by $d$ orbitals. In contrast, an isotropically inward ferroelectric disturbance is found around Pb and Ti vacancies. Such a ferroelectric disturbance is strongly enhanced by the charging of vacancies, except for Pb vacancies that induce delocalized defect states.

We propose a quantum Monte Carlo (QMC) algorithm for nonequilibrium dynamics in a system with a parameter varying as a function of imaginary time. The method is based on successive applications of an evolving Hamiltonian to an initial state and delivers results for a whole range of the tuning parameter in a single run, allowing for access to both static and dynamic properties of the system. This approach reduces to the standard Schrödinger dynamics in imaginary time for quasi-adiabatic evolutions, i.e., including the leading nonadiabatic correction to the adiabatic limit. We here demonstrate this quasi-adiabatic QMC (QAQMC) method for linear ramps of the transverse-field Ising model across its quantum-critical point in one and two dimensions. The critical behavior can be described by generalized dynamic scaling. For the two-dimensional square-lattice system, we use the method to obtain a high-precision estimate of the quantum-critical point ${(h/J)}_c=3.04458\left(7\right)$, where $h$ is the transverse magnetic field and $J$ is the nearest-neighbor Ising coupling. The QAQMC method can also be used to extract the Berry curvature and the metric tensor.

The structural and magnetic properties of Fe octaethylporphyrin molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and x-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a $\sqrt{2}\times 2\sqrt{2}$R45${}^{\circ }$ reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT, we calculate the spin dipolar term $7T\left(\theta \right)$, which is also important in view of the magnetic anisotropy of the molecule. The measured x-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of $7T\left(\theta \right)$, e.g., on Cu(001), $7T\left(\theta \right)$ amounts to $-2.07$ ${\mu }_{\mathrm{B}}$ for normal incidence leading to a reduction of the effective spin moment $(m_s+7T\left(\theta \right))$. Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the x-ray absorption are clearly related to residual Cl ligands.

In order to understand the nature of the two-dimensional Bose-Einstein condensed (BEC) phase in BaCuSi${}_2$O${}_6$, we performed detailed ${}^{63}$Cu and ${}^{29}$Si NMR above the critical magnetic field, $H_{c1}=23.4$ T. The two different alternating layers present in the system have very different local magnetizations close to $H_{c1}$; one is very weak, and its size and field dependence are highly sensitive to the nature of inter-layer coupling. Its precise value could only be determined by “on-site” ${}^{63}$Cu NMR, and the data are fully reproduced by a model of interacting hard-core bosons in which the perfect frustration associated to tetragonal symmetry is slightly lifted, leading to the conclusion that the population of the less populated layers is not fully incoherent but must be partially condensed.

The dynamics of the demagnetization induced by an ultrashort laser pulse in GdCo and GdCoFe alloys is shown to be substantially different from the expected trend. We find that the intersublattice exchange interaction between Gd and Co(Fe) is used as an additional route of heat transfer during the demagnetization process and leads to a temporary cooling of the Co(Fe) spin subsystem. The observed results are described by a four-temperature model (4T model) in which electrons, lattice, and 4$f$ and 3$d$ spins represent four reservoirs of heat energy. In this model, the coupling between the electron and spin reservoirs is responsible for an ultrafast demagnetization, while the intersublattice exchange coupling gives rise to an immediate remagnetization and serves as a thermal link between the Co(Fe) and Gd spins.

The coupling between superconductivity and orthorhombic distortion is studied in vapor-grown FeSe single crystals using high-resolution thermal-expansion measurements. In contrast to the Ba122-based (Ba122) superconductors, we find that superconductivity does not reduce the orthorhombicity below $T_c$. Instead we find that superconductivity couples strongly to the in-plane area, which explains the large hydrostatic pressure effects. We discuss our results in light of the spin-nematic scenario and argue that FeSe has many features that are quite different from typical Fe-based superconductors.

The origin of the Kerr rotation observed in the pseudogap phase of cuprates has been the subject of much speculation. Recently, it has been proposed that this rotation might be due to chiral charge order. Here, I investigate whether such order can be observed by x-ray natural circular dichroism (XNCD). Several types of charge order were considered, and they can give rise to an XNCD signal depending on the stacking of the order along the $c$ axis.

We report on and characterize, via molecular dynamics studies, the evolution of the structure of Cu${}_{50}$Zr${}_{50}$ and Cu${}_{64}$Zr${}_{36}$ metallic glasses (MGs) as temperature is varied. Interestingly, a percolating icosahedral network appears in the Cu${}_{64}$Zr${}_{36}$ system as it is supercooled. This leads us to introduce a static length scale, which grows dramatically as this three-dimensional system approaches the glass transition. Amidst interpenetrating connections, noninterpenetrating connections between icosahedra are shown to become prevalent upon supercooling and to greatly enhance the connectivity of the MG's icosahedral network. Additionally, we characterize the chemical compositions of the icosahedral networks and their components. These findings demonstrate the importance of noninterpenetrating connections for facilitating extensive structural networks in Cu-Zr MGs, which in turn drive dynamical slowing in these materials.

Insertion of a thin 3$\mathit{\text{d}}$ ferromagnetic metal/alloy layer between the barrier layer and the perpendicularly magnetized ferromagnetic electrode is an effective method to enhance the magnetoresistance (MR) ratio in perpendicular magnetic tunnel junctions (p-MTJs). In the present paper we systematically studied the structural and magnetic properties as well as the spin-dependent transport in p-MTJs with a core structure MnGa/FM/MgO/CoFeB (FM $=$ Fe, Co), with the MnGa being the $\mathit{\text{L}}{1}_0$ MnGa alloy ($M{n}_{57}G{a}_{43}$, $M{n}_{62}G{a}_{38}$) and the $\mathit{\text{D}}{0}_{22}$ MnGa alloy ($M{n}_{70}G{a}_{30}$). The insertion of the Fe and Co layers enhances the MR ratio significantly as well as the MnGa composition dependence of the MR ratio. In addition, opposite magnetic properties and $\mathit{\text{MR}}$($\mathit{\text{H}}$) curves of MTJs with Fe and Co interlayers are observed, naturally suggesting the ferromagnetic and antiferromagnetic exchange coupling for MnGa/Fe(bcc) and MnGa/Co(bcc), respectively. By considering the exchange coupling between the FM and MnGa, we successfully simulated the $\mathit{MR}$($\mathit{\text{H}}$) curves of the samples with Fe and Co interlayers based on a simple model. Furthermore, the interlayer effect on the transport properties are discussed based on the temperature dependence of the MR ratio by using the magnon excitation model modified with impurity-induced hopping. It shows that the FM interlayer restrains the impurity induced hopping and the magnon excitation; and furthermore, the Co is more effective in restraining the impurity diffusion and magnon excitation as compared to Fe.

We propose a general approach to construct symmetry protected topological (SPT) states (i.e., the short-range entangled states with symmetry) in 2D spin/boson systems on lattice. In our approach, we fractionalize spins/bosons into different fermions, which occupy nontrivial Chern bands. After the Gutzwiller projection of the free fermion state obtained by filling the Chern bands, we can obtain SPT states on lattice. In particular, we constructed a U(1) SPT state of a spin-1 model, a SO(3) SPT state of a boson system with spin-1 bosons and spinless bosons, and a SU(2) SPT state of a spin-1/2 boson system. By applying the “spin gauge field” which directly couples to the spin density and spin current of $S^z$ components, we also calculate the quantum spin Hall conductance in each SPT state. The projective ground states can be further studied numerically in the future by variational Monte Carlo etc.

We have synthesized and characterized laser-deposited film samples of perovskite BaCrO${}_3$, a missing member of the perovskite-chromate family. The BaCrO${}_3$ films have a substantially larger lattice constant than other chromates, are insulating, and exhibit weak ferromagnetism. Employing first-principles density functional theory calculations, we attribute such weak ferromagnetism to a canted $C$-type antiferromagnetic spin order. Additionally, comparison with the other sister compounds CaCrO${}_3$ and SrCrO${}_3$ suggests an anomalous Mott transition where magnetism is independent of whether the compound is metallic or insulating.