Using first-principles density functional calculations, we systematically investigate electronic structures and topological properties of layered materials $\mathrm{InNb}{X}_2$ ($X$=S, Se). In the absence of spin-orbit coupling (SOC), both compounds show nodal lines protected by mirror symmetry. Including SOC, the Dirac rings in ${\mathrm{InNbS}}_2$ split into two Weyl rings. This unique property is distinguished from other discovered nodal-line materials, which normally require the absence of SOC. On the other hand, SOC breaks the nodal lines in ${\mathrm{InNbSe}}_2$, and the compound becomes a type-II Weyl semimetal with 12 Weyl points in the Brillouin zone. Using a supercell slab calculation, we study the dispersion of Fermi arc surface states in ${\mathrm{InNbSe}}_2$; we also utilize a coherent potential approximation to probe their tolerance to the surface disorder effects. The quasi-two-dimensionality and the absence of toxic elements make these two compounds an ideal experimental platform for investigating novel properties of topological semimetals.

By comparing the in-plane and out-of-plane orbitally dependent magnetoresistance under a magnetic field of 9 T, the conductive layer thickness of the ${\mathrm{LaAlO}}_3$/${\mathrm{SrTiO}}_3$ interface is determined for temperatures between 45 and 300 K. As the influence of spin-orbit coupling cannot be ignored when considering the conductive layer thickness dependence at temperatures below 45 K, a lower magnetic field is employed. The results show that the thickness increases gradually and then remains constant as the temperature decreases. Such a tendency is well reproduced theoretically by considering the equilibrium between drift and diffusion of electrons near the interface. We discover that the thickness mainly depends on the relative permittivity of ${\mathrm{SrTiO}}_3$ and electron mobility, and it is independent of the sheet carrier density. Our investigations show that the dimensionality of the confined zone for electrons can be controlled and can be used as a reference for other interfacial systems in condensed matter physics.

We develop a new position-resolved band-unfolding method based on the density functional theory to clarify the single-electron energy spectrum of $(3\times 3)$ silicene on $\mathrm{Ag}\left(111\right)$ substrate. The position-resolved scheme enables us to clarify each contribution from each spatial region to the single-electron spectrum, which facilitates the chemical identification of each electron state. We find interface states which are distributed in the region of silicene and top two layers of the Ag substrate near the Fermi level and also below the Fermi level. The states are unique in silicene on a substrate in the sense that they are mixtures of Si and Ag orbitals. The obtained electronic structure near the Fermi level is interesting, featuring a hyperbolic-paraboloid-shaped energy band which leads to 12 Dirac-like cones at the boundary of the primitive Brillouin zone of $\mathrm{Ag}\left(111\right)$. Characteristics of measured photoemission spectra are satisfactorily explained by the obtained unfolded bands.

By efficient nanoscale plasma etching, the nitrogen-vacancy (NV) centers in diamond were brought to the sample surface step by step successfully. At each depth, we measured the ratios of spin coherence times before and after applying external spins on the surface, which reflected the contribution of external spins to the decoherence, and then investigated the relationships between depth and ratios. The values of ratios declined and then rose with the decreasing depth, which was attributed to the decoherence influenced by external spins, surface spins, discrete surface spin effects, and electric field noise. Moreover, our work experimentally and theoretically revealed a characteristic depth at which the NV center would experience relatively the strongest decoherence caused by external spins in consideration of inevitable surface spins. And the characteristic depth was found depending on the local environments of NV centers and the density of surface spins.

We present the study of the effect of both magnetic and potential scattering ($M$ and $U$) of magnetic impurities on the local electronic structures on a topological insulator. For single impurity, the local density of states (LDOS) shows distinct patterns of two resonance states with the positions depending on the relative strength of magnetic and potential scattering $M$ and $U$. However, in the usual case where $U$ is much larger than $M$, these two resonance states have very similar energy and become indistinguishable. Thus only one single peak is present, in analog with the experimental observations by scanning tunneling spectroscopy and providing a possible explanation for the recent debate among different experimental results. Interestingly, the effect of $M$ can be significantly magnified when the magnetic impurities are forming nanosized quantum corrals, the apparent different features in the LDOSs allow justifying the existence of $M$ despite its relative smaller strength compared with $U$. Remarkably, we find a one-to-one correspondence between the strength of $U,M$, and the energy positions of the quantum well states, giving a unique scheme to determine both $U$ and $M$, which are of great importance for future TI based novel device design.

Optical conductivity measurements are combined with density functional theory calculations in order to understand the electrodynamic response of the frustrated Mott insulators herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ and the closely related kagome-lattice compound ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{19}{\mathrm{Cl}}_8$. We identify these materials as charge-transfer rather than Mott-Hubbard insulators, similar to the high-$T_c$ cuprate parent compounds. The band edge is at 3.3 and 3.6 eV, respectively, establishing the insulating nature of these compounds. Inside the gap, we observe dipole-forbidden local electronic transitions between the Cu $3d$ orbitals in the range 1–2 eV. With the help of ab initio calculations we demonstrate that the electrodynamic response in these systems is directly related to the role of on-site Coulomb repulsion: While charge-transfer processes have their origin on transitions between the ligand band and the Cu $3d$ upper Hubbard band, local $d\text{−}d$ excitations remain rather unaffected by correlations.

Quasiparticle excitations of free electrons in condensed-matter physics, characterized by the dimensionality of the band crossing, can find their elementary-particle analogs in high-energy physics, such as Majorana, Weyl, and Dirac fermions, while crystalline symmetry allows more quasiparticle excitations and exotic fermions to emerge. Using symmetry analysis and ab initio calculations, we propose that the three-dimensional honeycomb crystal ${\mathrm{NaCu}}_3{\mathrm{Te}}_2$ hosts triply degenerate nodal points (TDNPs) residing at the Fermi level. Furthermore, in this system we find a tunable phase transition between a trivial insulator, a TDNP phase, and a weak topological insulator (TI), triggered by a symmetry-allowed perturbation and the spin-orbital coupling (SOC). Such a topological nontrivial ternary compound not only serves as a perfect candidate for studying three-component fermions, but also provides an excellent playground for understanding the topological phase transitions between TDNPs, TIs, and trivial insulators, which distinguishes this system from other TDNP candidates.

Topologically protected wave engineering in artificially structured media resides at the frontier of ongoing metamaterials research, which is inspired by quantum mechanics. Acoustic analogs of electronic topological insulators have recently led to a wealth of new opportunities in manipulating sound propagation by means of robust edge mode excitations through analogies drawn to exotic quantum states. A variety of artificial acoustic systems hosting topological edge states have been proposed analogous to the quantum Hall effect, topological insulators, and Floquet topological insulators in electronic systems. However, those systems were characterized by a fixed geometry and a very narrow frequency response, which severely hinders the exploration and design of useful applications. Here we establish acoustic multipolar pseudospin states as an engineering degree of freedom in time-reversal invariant flow-free phononic crystals and develop reconfigurable topological insulators through rotation of their meta-atoms and reshaping of the metamolecules. Specifically, we show how rotation forms man-made snowflakelike molecules, whose topological phase mimics pseudospin-down (pseudospin-up) dipolar and quadrupolar states, which are responsible for a plethora of robust edge confined properties and topological controlled refraction disobeying Snell's law.

The particle-hole (PH) symmetry of electrons is an exact symmetry of the electronic Hamiltonian confined to a specific Landau level, and its interplay with the formation of composite fermions has attracted much attention of late. We investigate an emergent symmetry in the fractional quantum Hall effect, namely, the PH symmetry of composite fermions, which relates states at composite fermion filling factors ${\nu }^{*}=n+\overline{\nu }$ and ${\nu }^{*}=n+1-\overline{\nu }$, where the integer $n$ is the $\mathrm{\Lambda }$-level index and $0\le \overline{\nu }\le 1$. Detailed calculations using the microscopic theory of composite fermions demonstrate the following for low-lying $\mathrm{\Lambda }$ levels (small $n$): (i) The two-body interaction between composite-fermion particles is very similar, apart from a constant additive term and an overall scale factor, to that between composite-fermion holes in the same $\mathrm{\Lambda }$ level; and (ii) the three-body interaction for composite fermions is an order of magnitude smaller than the two-body interaction. Taken together, these results imply an approximate PH symmetry for composite fermions in low $\mathrm{\Lambda }$ levels, which is also supported by exact-diagonalization studies and available experiments. This symmetry, which relates states at electron filling factors $\nu =\frac{n+\overline{\nu }}{2(n+\overline{\nu })\pm 1}$ and $\nu =\frac{n+1-\overline{\nu }}{2(n+1-\overline{\nu })\pm 1}$, is not present in the original Hamiltonian and owes its existence entirely to the formation of composite fermions. With increasing $\mathrm{\Lambda }$-level index, the two-body and three-body pseudopotentials become comparable, but at the same time they both diminish in magnitude, indicating that the interaction between composite fermions becomes weak as we approach $\nu =1/2$.

The conductivity and Hall effect were measured in CVD-grown monolayer graphene as a function of the gate voltage, $V_g$, at temperatures down to $T=2$ K and in magnetic fields up to $B=8$ T. The minimal conductivity was observed at positive $V_g$, which shows the position of the charge neutrality point, $V_{\mathrm{NP}}$. With decreasing $T$, $V_{\mathrm{NP}}$ first decreases, but stops to decrease at low $T$. The hysteresis of conductivity shows a similar behavior: it decreases with decreasing $T$ and disappears at low $T$. A significant asymmetry was observed at low density of charge carriers $\left|n\right|=(n,p)$: the mobility of holes was less than the mobility of electrons. The asymmetry decreases with increasing $\left|n\right|$. It was observed that the value of $\left|n\right|$ determined from the Hall effect is less than the full value induced by $V_g$. In strong perpendicular $B$, Shubnikov-de Haas (SdH) oscillations were observed in the longitudinal conductivity, ${\sigma }_{xx}$, together with half-integer quantum Hall plateaus. It was found that $\left|n\right|$ determined from SdH oscillations is equal to the full value induced by $V_g$ as opposed to the Hall effect. Explanatory models for all observed phenomena are discussed.

We employ an eigenpolarization model including the description of direction dependent excitonic effects for rendering critical point structures within the dielectric function tensor of monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ yielding a comprehensive analysis of generalized ellipsometry data obtained from 0.75–9 eV. The eigenpolarization model permits complete description of the dielectric response. We obtain, for single-electron and excitonic band-to-band transitions, anisotropic critical point model parameters including their polarization vectors within the monoclinic lattice. We compare our experimental analysis with results from density functional theory calculations performed using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional. We present and discuss the order of the fundamental direct band-to-band transitions and their polarization selection rules, the electron and hole effective mass parameters for the three lowest band-to-band transitions, and their excitonic contributions. We find that the effective masses for holes are highly anisotropic and correlate with the selection rules for the fundamental band-to-band transitions. The observed transitions are polarized close to the direction of the lowest hole effective mass for the valence band participating in the transition.

We report the absorption of a diffusive spin current in a bulk-carrier compensated topological insulator: ${\mathrm{Sn}}_{0.02}-{\mathrm{Bi}}_{1.08}{\mathrm{Sb}}_{0.9}{\mathrm{Te}}_2\mathrm{S}$ (Sn-BSTS). By injecting spins into a Sn-BSTS crystal from a ferromagnetic metal using spin pumping, we found that the magnetic damping of the ferromagnetic layer is enhanced due to the absorption of the spin current in the surface and bulk states of the topological insulator. We found that the damping enhancement depends critically on temperature, which allows us to disentangle the spin absorption in the surface and bulk states, owing to the nearly perfect carrier compensation in the bulk part at low temperature. Our results show that the absorption of the spin current is dominated by the surface state of the Sn-BSTS crystal at low temperature, whereas the spin absorption in the surface state is comparable to that in the bulk state near room temperature. The coexistence of the spin absorption in the surface and bulk states has been demonstrated in two different systems, where the surface state is coupled with the dynamical magnetization through the diffusive spin current or exchange coupling. These results will be essential for quantitative understanding of the spin absorption and spin-charge conversion due to the spin-momentum locked surface state of topological insulators.

Motivated by the recent resonant x-ray scattering (RXS) and resonant inelastic x-ray scattering (RIXS) experiments for electron-doped cuprates, we study the charge excitation spectrum in a layered $t\text{−}J$ model with the long-range Coulomb interaction. We show that the spectrum is not dominated by a specific type of charge excitations, but by different kinds of charge fluctuations, and is characterized by a dual structure in the energy space. Low-energy charge excitations correspond to various types of bond-charge fluctuations driven by the exchange term ($J$ term), whereas high-energy charge excitations are due to usual on-site charge fluctuations and correspond to plasmon excitations above the particle-hole continuum. The interlayer coupling, which is frequently neglected in many theoretical studies, is particularly important to the high-energy charge excitations.

We report the engineering of spin-hedgehog crystals in thin films of the chiral magnet MnGe by tailoring the magnetic anisotropy. As evidenced by neutron scattering on films with different thicknesses and by varying a magnetic field, we can realize continuously deformable spin-hedgehog crystals, each of which is described as a superposition state of a different set of three spin spirals (a triple-$\mathit{q}$ state). The directions of the three propagation vectors $\mathit{q}$ vary systematically, gathering from the three orthogonal $\langle 100\rangle$ directions towards the film normal as the strength of the uniaxial magnetic anisotropy and/or the magnetic field applied along the film normal increase. The formation of triple-$\mathit{q}$ states coincides with the onset of topological Hall signals, that are ascribed to skew scattering by an emergent magnetic field originating in the nontrivial topology of spin hedgehogs. These findings highlight how nanoengineering of chiral magnets makes possible the rational design of unique topological spin textures.

In this paper, we examine, in terms of the classical nucleation theory, how the strengthening of the attractive intermolecular interactions influences the crystallization process for systems like Lennard-Jones at different isobaric conditions. For this purpose, we modify the standard Lennard-Jones potential, and as a result, we obtain three different systems characterized by various strengths of attractive potentials occurring between molecules, which are in direct relationship to the physical quantities describing molecules, e.g., its polarizability or dipole moment. Based on performed analysis, we demonstrate that the molecular attraction primarily impacts the thermodynamics of the interface between liquid and crystal. This is reflected in the behavior of nucleation and overall crystallization rates during compression of the system.

Effective spin torques can generate the Néel vector oscillations in antiferromagnets (AFMs). Here, it is theoretically shown that these torques applied at one end of a normal AFM strip can excite a helical type of spin wave in the strip whose properties are drastically different from characteristic spin waves. An analysis based on both a Néel vector dynamical equation and the micromagnetic simulation identifies the direction of magnetic anisotropy and the damping factor as the two key parameters determining the dynamics. Helical wave propagation requires the hard axis of the easy-plane AFM to be aligned with the traveling direction, while the damping limits its spatial extent. If the damping is neglected, the calculation leads to a uniform periodic domain wall structure. On the other hand, finite damping decelerates the helical wave rotation around the hard axis, ultimately causing stoppage of its propagation along the strip. With the group velocity staying close to spin-wave velocity at the wave front, the wavelength becomes correspondingly longer away from the excitation point. In a sufficiently short strip, a steady-state oscillation can be established whose frequency is controlled by the waveguide length as well as the excitation energy or torque.