X-ray scattering and electrical resistivity measurements were performed on ${\mathrm{GdNiC}}_2$ and ${\mathrm{TbNiC}}_2$. We found a set of satellite peaks characterized by ${\mathit{q}}_1=(0.5,\eta ,0)$ below $T_1$, at which the resistivity shows a sharp inflection, suggesting the charge density wave (CDW) formation. The value of $\eta$ decreases with decreasing temperature below $T_1$, and then a transition to a commensurate phase with ${\mathit{q}}_{1\mathrm{C}}=(0.5,0.5,0)$ takes place. The diffuse scattering observed above $T_1$ indicates the presence of soft phonon modes associated with CDW instabilities at ${\mathit{q}}_1$ and ${\mathit{q}}_2=(0.5,0.5,0.5)$. The long-range order given by ${\mathit{q}}_2$ is developed in addition to that given by ${\mathit{q}}_{1\mathrm{C}}$ in ${\mathrm{TbNiC}}_2$, while the short-range correlation with ${\mathit{q}}_2$ persists even at 6 K in ${\mathrm{GdNiC}}_2$. The amplitude of the ${\mathit{q}}_{1\mathrm{C}}$ lattice modulation is anomalously reduced below an antiferromagnetic transition temperature $T_{\mathrm{N}}$ in ${\mathrm{GdNiC}}_2$. In contrast, the ${\mathit{q}}_2$ order vanishes below $T_{\mathrm{N}}$ in ${\mathrm{TbNiC}}_2$. We demonstrate that $R{\mathrm{NiC}}_2$ ($R$ = rare earth) compounds exhibit similarities with respect to their CDW phenomena, and discuss the effects of magnetic transitions on CDWs. We offer a possible displacement pattern of the modulated structure characterized by ${\mathit{q}}_{1\mathrm{C}}$ and ${\mathit{q}}_2$ in terms of frustration.

Three-dimensional gapless semiconductors with quadratic band touching, such as HgTe, $\alpha$-Sn, or ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, are believed to display a non-Fermi-liquid ground state due to long-range electron-electron interaction. We argue that this state is inherently unstable towards spontaneous formation of a (topological) excitonic insulator. The instability can be parameterized by a critical fermion number $N_{\mathrm{c}}$. For $N<N_{\mathrm{c}}$ the rotational symmetry is spontaneously broken, the system develops a gap in the spectrum, and features a finite nematic order parameter. To leading order in the $1/N$ expansion and in the static approximation, the analogy with the problem of dynamical mass generation in (2+1)-dimensional quantum electrodynamics yields $N_{\mathrm{c}}=16/\left[3\pi (\pi -2)\right]$. Taking the important dynamical screening effects into account, we find that $N_{\mathrm{c}}\ge 2.6\left(2\right)$ and therefore safely above the physical value of $N=1$. Some experimental consequences of the nematic ground state are discussed.

The general trends in the behavior of the band gaps in short period superlattices (SLs) composed from InAlN and GaN layers and grown along the wurtzite $c$ axis have been analyzed for different alloy compositions and several thicknesses of quantum wells and barriers. The composition dependence of the gaps is compared to that of the quaternary alloys $\mathrm{I}{\mathrm{n}}_y\mathrm{A}{\mathrm{l}}_{1-x-y}\mathrm{G}{\mathrm{a}}_x\mathrm{N}$ with the same effective contents of In, Al, and Ga as the SLs. The differences in the gaps are explained mainly by the built-in electric fields in the SLs caused by spontaneous and piezoelectric polarizations. Other factors, such as internal strain caused by lattice mismatch between wells and barriers and wave function hybridization, are taken into account. A simplified approach to determine the built-in electric fields directly from the dielectric properties of the constituent materials has been applied and verified by comparison to ab initio calculations for selected SLs. The calculated band gaps are compared with existing experimental data.

Thermal dissociation of free and acceptor-bound quasi-two-dimensional positive trions is investigated by measuring the temperature dependence of the integrated emission intensity in magnetic fields up to 17 T in high quality ${\mathrm{GaAs}/\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells. Three distinct dissociation processes are observed for the well-resolved hole cyclotron replicas (“shake-up”) of positive trions bound to neutral acceptors in the spin-doublet state ($\mathrm{SU}–A^0{X}_d^{+}$). To demonstrate that the hole involved in the shake-up process is not bound by the Coulomb interaction to the charged $A^0{X}^{+}$ complex, we calculate the valence Landau levels using the Luttinger model beyond the axial approximation. The calculated value of the hole cyclotron energy agrees well with the experimental data for the energy separation of the $A^0{X}^{+}$ and $\mathrm{SU}–A^0{X}^{+}$ lines, determined from the emission spectra. At low temperatures, below 6 K, the dominant dissociation results in a free hole and an exciton bound to the neutral acceptor in the spin-singlet or -triplet state, ($A^0{X}_d^{+}\to A^0{X}_s+h$ or $A^0{X}_t+h$). At higher temperatures, above 9 K, the dissociation into the free positive trion and the neutral acceptor ($A^0{X}_d^{+}\to A^0+X^{+}$) predominates. From the temperature evolution of the integrated emission of the free trion lines ($X^{+}$) we evaluate the transition energy between the two triplet trion states, the dark one ($X_{td}^{+}$) and the bright one ($X_{tb}^{+}$). The ionization energies of all detected dissociation processes are compared with the spectral positions of the relevant radiative recombination lines from which excellent quantitative agreement is achieved.

We study localized plasmonic excitations in bilayer graphene (BLG) nanodisks, comparing AA-stacked and AB-stacked BLG and contrasting the results to the case of two monolayers without electronic hybridization. The electrodynamic response of the BLG electron gas is described in terms of a spatially homogeneous surface conductivity, and an efficient alternative two-dimensional electrostatic approach is employed to carry out all the numerical calculations of plasmon resonances. Due to unique electronic band structures, the resonance frequency of the traditional dipolar plasmonic mode in the AA-stacked BLG nanodisk is roughly doping independent in the low-doping regime, while the mode is highly damped as the Fermi level approaches the interlayer hopping energy $\gamma$ associated with tunneling of electrons between the two layers. In addition to the traditional dipolar mode, we find that the AB-stacked BLG nanodisk also hosts a new plasmonic mode with energy larger than $\gamma$. This mode can be tuned by either the doping level or structural size, and, furthermore, this mode can dominate the plasmonic response for realistic structural conditions.

We report modifications of the ferroelectric and electrocaloric properties of ${\mathrm{BaTiO}}_3$ by defects. For this purpose, we have combined ab initio based molecular dynamics simulations with a simple model for defects. We find that different kinds of defects modify the ferroelectric transition temperatures and polarization, reduce the thermal hysteresis of the transition, and are no obstacle for a large caloric response. For a locally reduced polarization, the ferroelectric transition temperature and the adiabatic response are slightly reduced. For polar defects, an intriguing picture emerges. The transition temperature is increased by polar defects and the temperature range of the large caloric response is broadened. Even more remarkable, we find an inverse caloric effect in a broad temperature range.

We study numerically the effects of classical magnetic impurities on the Anderson metal-insulator transition. We find that a small concentration of Heisenberg impurities enhances the critical disorder amplitude $W_{\mathrm{c}}$ with increasing exchange coupling strength $J$. The resulting scaling with $J$ is analyzed which supports an anomalous scaling prediction by Wegner due to the combined breaking of time-reversal and spin-rotational symmetry. Moreover, we find that the presence of magnetic impurities lowers the critical correlation length exponent $\nu$ and enhances the multifractality parameter ${\alpha }_0$. The new value of $\nu$ improves the agreement with the value measured in experiments on the metal-insulator transition (MIT) in doped semiconductors like phosphor-doped silicon, where a finite density of magnetic moments is known to exist in the vicinity of the MIT. The results are obtained by a finite-size scaling analysis of the geometric mean of the local density of states which is calculated by means of the kernel polynomial method. We establish this combination of numerical techniques as a method to obtain critical properties of disordered systems quantitatively.

We construct a generalized transfer matrix corresponding to noninteracting tight-binding lattice models, which can subsequently be used to compute the bulk bands as well as the edge states. Crucially, our formalism works even in cases where the hopping matrix is noninvertible. Following Hatsugai [Phys. Rev. Lett. 71, 3697 (1993)], we explicitly construct the energy Riemann surfaces associated with the band structure for a specific class of systems which includes systems such as Chern insulator, Dirac semimetal, and graphene. The edge states can then be interpreted as noncontractible loops, with the winding number equal to the bulk Chern number. For these systems, the transfer matrix is symplectic, and hence we also describe the windings associated with the edge states on $\mathrm{Sp}(2,\mathbb{R})$ and interpret the corresponding winding number as a Maslov index.

We present a study of fully epitaxial Fe/Pt bilayers by means of ferromagnetic resonance (FMR) techniques. The dependence of the resonance field $H_{\mathrm{FMR}}$, the Gilbert damping parameter $\alpha$, and the FMR linewidth on the Pt thickness is presented and compared with reference layers. Since spin pumping is extremely sensitive to the interface properties, a corresponding study is of large importance. By studying fully epitaxial systems, the existence of an almost perfectly single-crystalline ordered interface is ensured. The measured effects, such as the dependence of the resonant field $H_{\mathrm{FMR}}$ on Pt thickness, are thus related to the physics of spin pumping and not to interface disorder effects. The damping parameter $\alpha$ has been extracted from the slope of the linewidth dependence on the FMR frequency. By measuring the enhancement of $\alpha$ with Pt a value for the effective spin mixing conductance of $g_{\mathrm{eff}}^{\uparrow \downarrow }=(4.9\pm 0.5)\times {10}^{19}{\mathrm{m}}^{-2}$ is obtained. We observe an opposite behavior in the dependence of $H_{\mathrm{FMR}}$ when compared with reports on YIG/Pt. In addition, the critical role of the proper choice of a reference layer for the estimation of this value is discussed.

Extensive first-principle molecular dynamics simulations are performed to study the phase behavior of metastable liquid Si at negative pressure. Our results show that the high-density liquid (HDL) and HDL-vapor spinodals indeed form a continuous reentrant curve and the liquid-liquid critical point seems to just coincide with its minimum. The line of density maxima also has a strong tendency to pass through this minimum. The phase behaviour of metastable liquid Si therefore tends to be a critical-point-free scenario rather than a second-critical-point one based on SW potential.

We report broadband spin-wave spectroscopy on kagome artificial spin ice (ASI) made of large arrays of interconnected ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanobars. Spectra taken in saturated and disordered states exhibit a series of resonances with characteristic magnetic field dependencies. Making use of micromagnetic simulations, we identify resonances that reflect the spin-solid-state and monopole-antimonopole pairs on Dirac strings. The latter resonances allow for the generation of highly charged vertices in ASIs via microwave-assisted switching. Our findings open further perspectives for fundamental studies on ASIs and their usage in reprogrammable magnonics.

The static dielectric constant of liquid water is computed using classical force field based molecular dynamics simulation at fixed electric displacement $\mathbf{D}$. The method to constrain the electric displacement is the finite-temperature classical variant of the constant $\mathbf{D}$ method developed by Stengel, Spaldin, and Vanderbilt [Nat. Phys. 5, 304 (2009)]. There is also a modification of this scheme imposing fixed values of the macroscopic field $\mathbf{E}$. The method is applied to the popular SPC/E model of liquid water. We compare four different estimates of the dielectric constant, two obtained from fluctuations of the polarization at $\mathbf{D}=0$ and $\mathbf{E}=0$ and two from the variation of polarization with finite $\mathbf{D}$ and $\mathbf{E}$. It is found that all four estimates agree when properly converged. The computational effort to achieve convergence varies, however, with constant $\mathbf{D}$ calculations being substantially more efficient. We attribute this difference to the much shorter relaxation time of longitudinal polarization compared to transverse polarization accelerating constant $\mathbf{D}$ calculations.

Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ within the local-density approximation (LDA) of the density-functional theory, and its combination with dynamical mean-field theory (DMFT). To compare these calculations, we measure magnetic and thermodynamic properties as well as x-ray magnetic circular dichroism and the photoemission spectrum of single-crystal ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. In addition, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. These results establish the importance of electronic correlations in this ferromagnet.

In this paper, we present a detailed theoretical investigation of the electronic and magnetic properties of ferromagnetic slabs and clusters deposited on ${\mathrm{SrTiO}}_3$ via first-principles calculations, with a particular emphasis on the magnetocrystalline anisotropy (MCA). We found that in the case of Fe ultrathin films deposited on ${\mathrm{SrTiO}}_3$ the effect of the interface is to quench the MCA whereas for Co we observe a spin reorientation from in-plane to out-of-plane as compared to the free surface. We also find a strong enhancement of MCA for small clusters upon deposition on a ${\mathrm{SrTiO}}_3$ substrate. The origin of this enhancement of MCA is attributed to the hybridization between the substrate and the $d$ orbitals of the cluster extending in-plane for Fe and out-of-plane for Co. As a consequence, we predict that the Fe nanocrystals (even rather small) should be magnetically stable and are thus good potential candidates for magnetic storage devices.

The $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ compound has been proposed as another example of a multigap superconductor [Imai et al., J. Phys. Soc. Jpn. 81, 113708 (2012)]. Here, we report on measurements of several important physical quantities capable of showing a presence of multiple energy gaps on our superconducting single crystals of $\beta \text{−}\mathrm{B}{\mathrm{i}}_2\mathrm{Pd}$ with the critical temperature $T_c$ close to 5 K. The calorimetric study via a sensitive ac technique shows a sharp anomaly at the superconducting transition, however only a single energy gap is detected. Also other characteristics inferred from calorimetric measurements as the field dependence of the Sommerfeld coefficient and the temperature and angular dependence of the upper critical magnetic field point unequivocally to standard single $s$-wave gap superconductivity. The Hall-probe magnetometry provides the same result from the analysis of the temperature dependence of the lower critical field. A single-gapped BCS density of states is detected by the scanning tunneling spectroscopy measurements. Then, the bulk as well as the surface sensitive probes evidence a standard conventional superconductivity in this system where the topologically protected surface states have been recently detected by angle-resolved photoemission spectroscopy [Sakano et al., Nat. Commun. 6, 8595 (2015).].

Proximity orbital and spin-orbital effects of graphene on monolayer transition-metal dichalcogenides (TMDCs) are investigated from first-principles. The Dirac band structure of graphene is found to lie within the semiconducting gap of TMDCs for sulfides and selenides, while it merges with the valence band for tellurides. In the former case, the proximity-induced staggered potential gaps and spin-orbit couplings (all on the meV scale) of the Dirac electrons are established by fitting to a phenomenological effective Hamiltonian. While graphene on ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WS}}_2$ has a topologically trivial band structure, graphene on ${\mathrm{WSe}}_2$ exhibits inverted bands. Using a realistic tight-binding model we find topologically protected helical edge states for graphene zigzag nanoribbons on ${\mathrm{WSe}}_2$, demonstrating the quantum spin Hall effect. This model also features “half-topological states,” which are protected against time-reversal disorder on one edge only.

After the discovery of the spaser (surface plasmon amplification by stimulated emission of radiation), first proposed by Bergman and Stockman in 2003, it has become possible to deliver optical energy beyond the diffraction limit and generate an intense source of an optical field. The spaser is a nanoplasmonic counterpart of a laser. One of the major advantages of the spaser is its size: A spaser is a truly nanoscopic device whose size can be made smaller than the skin depth of a material to a size as small as the nonlocality radius ($\sim 1$ nm). Recently, an electrically pumped graphene based nanospaser has been proposed that operates in the midinfrared region and utilizes a nanopatch of graphene as a source of plasmons and a quantum-well cascade as its gain medium. Here we propose an optically pumped nanospaser based on three-dimensional topological insulator (3D TI) materials, such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, that operates at an energy close to the bulk band-gap energy $\sim 0.3$ eV and uses the surface as a source for plasmons and its bulk as a gain medium. Population inversion is obtained in the bulk and the radiative energy of the exciton recombination is transferred to the surface plasmons of the same material to stimulate spasing action. This is truly a nanoscale spaser as it utilizes the same material for dual purposes. We show theoretically the possibility of achieving spasing with a 3D TI. As the spaser operates in the midinfrared spectral region, it can be a useful device for a number of applications, such as nanoscopy, nanolithography, nanospectroscopy, and semiclassical information processing.

We show how lattice quantum Monte Carlo can be applied to the electronic properties of carbon nanotubes in the presence of strong electron-electron correlations. We employ the path-integral formalism and use methods developed within the lattice QCD community for our numerical work. Our lattice Hamiltonian is closely related to the hexagonal Hubbard model augmented by a long-range electron-electron interaction. We apply our method to the single-quasiparticle spectrum of the (3,3) armchair nanotube configuration, and consider the effects of strong electron-electron correlations. Our approach is equally applicable to other nanotubes, as well as to other carbon nanostructures. We benchmark our Monte Carlo calculations against the two- and four-site Hubbard models, where a direct numerical solution is feasible.

We study the current-phase relation of a topological ring-shape Josephson junction, where the ring structure is defined by one-dimensional topological interface states constructed in a two-dimensional honeycomb-lattice system. We show that control of the potential difference between the two ring arms can lead to a ${\phi }_0$ Josephson junction. The physics origin is the superconducting electron- and holelike quasiparticles possessing a valley-dependent chirality and moving separately in the two ring arms. Our findings provide a purely electric way to consecutively manipulate the Josephson current-phase relation.