We consider the optical and transport properties in a model two-dimensional Hamiltonian which describes the merging of two Dirac points. At low energy, in the presence of an energy gap parameter $\mathrm{\Delta }$, there are two distinct Dirac points with linear dispersion, these are connected by a saddle point at higher energy. As $\mathrm{\Delta }$ goes to zero, the two Dirac points merge and the resulting dispersion exhibits semi-Dirac behavior which is quadratic in the $x$ direction (“nonrelativistic”) and linear the $y$ direction (“relativistic”). In the clean limit for each direction $\left(x,y\right)$, the contribution of the intraband and interband optical transitions are both given by universal functions of photon energy $\mathrm{\Omega }$ and chemical potential $\mu$ normalized to the energy gap. We provide analytic formulas for both small and large $\mathrm{\Omega }/2\mathrm{\Delta }$ and $\mu /\mathrm{\Delta }$ limits. These define, respectively, Dirac and semi-Dirac-like regions. For $\mathrm{\Omega }/2\mathrm{\Delta }$ and $\mu /\mathrm{\Delta }$ of order one, there are deviations from these asymptotic behaviors. Considering optics and also transport, such as dc conductivity, thermal conductivity and the Lorenz number, such deviations provide signatures of the evolution from the Dirac to the semi-Dirac regime as the gap $\mathrm{\Delta }$ is varied.

Adsorption of the molecule CO on metallic surfaces is an important unsolved problem in Kohn-Sham density functional theory (KS-DFT). We present a detailed study of carbon monoxide adsorption on fcc (111) surfaces of $3d, 4d$, and $5d$ metals using nonempirical semilocal density functionals for the exchange-correlation energy: the local-density approximation (LDA), two generalized gradient approximations or GGAs [Perdew-Burke-Ernzerhof (PBE) and PBE for solids (PBEsol)], and a meta-GGA [strongly constrained and appropriately normed (SCAN) functional]. The typical error pattern (as found earlier for free molecules and for free transition-metal surfaces), in which results improve from LDA to PBE or PBEsol to SCAN, due to the satisfaction of more exact constraints, is not found here. Instead, for CO adsorption on transition-metal surfaces, we find that, while SCAN overbinds much less than LDA, it overbinds slightly more than PBE. Moreover, the tested functionals often predict the wrong adsorption site, as first pointed out for LDA and GGA in the CO/Pt (111) puzzle. This abnormal pattern leads us to suspect that the errors of PBE and SCAN for this problem are density-driven self-interaction errors associated with incorrect charge transfer between molecule and metal surface. We point out that, by the variational principle, overbinding by an approximate functional would be reduced if that functional were applied not to its self-consistent density for the adsorbed system but to an exact or more correct density for that system. Finally, we show for CO on Pt(111) that the site preference is corrected and the adsorption energy is improved for the PBE functional by using not the self-consistent PBE density but a $\text{PBE}+U$ density. The resulting correction to the PBE total energy is much larger for the adsorbed system than for its desorbed components, showing that the error is in the density of the adsorbed system. This seems to solve the Feibelman 2001 CO/Pt(111) puzzle, in principle if not fully in practice.

We study the wake potential produced by an external charged particle that moves parallel to various ${\mathrm{sy}}_1-{\mathrm{Al}}_2{\mathrm{O}}_3-{\mathrm{sy}}_2$ sandwich-like composites, where the system ${\mathrm{sy}}_i$ (with $i=1,2$) may be vacuum, pristine graphene, or doped graphene. The effective dielectric function of the composites is obtained using three complementary methods for graphene's electronic response, based on the massless Dirac fermions (MDF) method, the extended hydrodynamic (eHD) model, and the ab initio approach. Three velocity regimes are explored with respect to the threshold for excitations of the Dirac plasmon in graphene, given by its Fermi velocity $v_F$. In the low-velocity regime (below $v_F$), only the transverse optical (TO) phonons in the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer contribute to the wake potential in the surface with ${\mathrm{sy}}_2$ (which is nearest to the charged particle), in a manner that is only sensitive to the composition of that system: if ${\mathrm{sy}}_2$ is vacuum, the TO phonons give rise to intense oscillations in the wake potential, which are strongly suppressed if ${\mathrm{sy}}_2$ is pristine or doped graphene. For intermediate velocities (above $v_F$), the hybridized plasmon–TO phonon modes on both surfaces contribute to the wake potential in the surface with ${\mathrm{sy}}_2$, with the most dominant contribution coming from the hybridized Dirac-like plasmonic modes. In the high-velocity regime (well above $v_F$), the highest-lying hybridized Dirac plasmon gives the dominant contribution to the wake potential, which exhibits a typical $\mathsf{V}$-shaped wave-front pattern that lags behind the charged particle. It is found that the MDF method agrees very well with the results of the ab initio method for small and intermediate velocities. However, in the high-velocity regime, the high-energy $\pi$ plasmon in graphene introduces new features in the wake potential in the form of fast oscillations, just behind the charged particle. Those oscillations in the wake potential are well described by both the eHD and the ab initio method, proving that the $\pi$ plasmon indeed behaves as a collective mode.

Resonant infrared near-field optical spectroscopy provides a highly material-specific response with subwavelength lateral resolution of $\sim 10\mathrm{nm}$. Here, we report on the study of the near-field response of selected paraelectric and ferroelectric materials, i.e., ${\mathrm{SrTiO}}_3, {\mathrm{LiNbO}}_3$, and ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$, showing resonances in the wavelength range from 13.0 to $15.8\mu \mathrm{m}$. We investigate these materials using scattering scanning near-field optical microscopy in combination with a tunable mid-infrared free-electron laser. Fundamentally, we demonstrate that phonon-induced resonant near-field excitation is possible for both $p$- and $s$-polarized incident light, a fact that is of particular interest for the nanoscopic investigation of anisotropic and hyperbolic materials. Moreover, we exploit that near-field spectroscopy, as compared to far-field techniques, bears substantial advantages such as lower penetration depths, stronger confinement, and a high spatial resolution. The latter permits the investigation of minute material volumes, e.g., with nanoscale changes in crystallographic structure, which we prove here via near-field imaging of ferroelectric domain structures in ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ thin films.

In heavily $n$-doped graphene, when pushing the Fermi level to the vicinity of the Van Hove singularity, exotic electronic ground states are expected to occur driven by many-body interactions. These competing phases, such as chiral superconductivity, charge, or spin density waves, find their stability based on the amount of doping induced in the graphene layer. In this work we present a method for effectively tuning the doping level of graphene near the Van Hove singularity. Epitaxially grown buffer layer graphene on SiC(0001) is decoupled from the SiC substrate and strongly $n$ doped up to its Van Hove singularity via ytterbium intercalation. Upon annealing the Yb/graphene system at increasing temperatures, a topological transition at the Fermi level and a continuous upshift of the Dirac point are observed, indicating a decrease in charge carrier density. The intercalated Yb atoms adopt an ordered pattern while their concentration is reduced with annealing temperature. These variations significantly affect the charge transfer to the graphene layer and allow the systematic control of the doping level in the vicinity of the Van Hove singularity via one single experimental parameter. As such, the Yb intercalation technique can provide a reliable and convenient way of accessing different ordered ground states predicted in highly doped graphene.

Plasmonic superradiance originates from the plasmon mediated strong correlation that builds up between dipolar emitters coupled to a metal nanoparticle. This leads to a fast burst of emission so that plasmonic superradiance constitutes ultrafast and extremely bright optical nanosources of strong interest for integrated quantum nano-optics platforms. We elucidate the superradiance effect by establishing the dynamics of the system, including all features such as the orientation of the dipoles, their distance to the particle, and the number of active plasmon modes. We determine an optimal configuration for Purcell enhanced superradiance. We also show superradiance blockade at small distances.

Non-Hermiticity can vary the topology of system, induce topological phase transitions, and even invalidate the conventional bulk-boundary correspondence. Here, we show the introduction of non-Hermiticity without affecting the topological properties of the original chiral symmetric Hermitian systems. Conventional bulk-boundary correspondence holds; the topological phase transition and the (non)existence of edge states are unchanged even though the energy bands are inseparable due to non-Hermitian phase transitions. The Chern number for energy bands of the generalized non-Hermitian system in two dimensions is proved to be unchanged and favorably coincides with the simulated topological charge pumping. Our findings provide insights into the interplay between non-Hermiticity and topology. A topological phase transition independent of the non-Hermitian phase transition is a unique feature that is beneficial for future applications of non-Hermitian topological materials.

Spin-orbit coupling of as large as a half eV for electrons in $5d$ orbitals often gives rise to the formation of spin-orbital entangled objects, with the effective total angular momentum $J_{\mathrm{eff}}$. Of particular interest are the $J_{\mathrm{eff}}=3/2$ states realized in $5d^1$ transition metal ions surrounded by an anion octahedron. The pure $J_{\mathrm{eff}}=3/2$ quartet does not have any magnetic dipolar moment $\left(\langle M\rangle =0\right)$ but hosts hidden pseudodipolar moments accompanied by charge quadrupoles and magnetic octupoles. $\mathrm{C}{\mathrm{s}}_2\mathrm{TaC}{\mathrm{l}}_6$ and $\mathrm{R}{\mathrm{b}}_2\mathrm{TaC}{\mathrm{l}}_6$ are correlated insulators with $5d^1\mathrm{T}{\mathrm{a}}^{4+}$ ions in a regular Cl octahedron. Here we demonstrate that these Ta chlorides indeed have a substantially suppressed effective magnetic dipolar moment of $\sim 0.2{\mu }_{\mathrm{B}}$. Two phase transitions are observed at low temperatures that are not pronounced in the magnetization but accompanied with large electronic entropy of ∼R ln4. We ascribe the two transitions to the ordering of hidden multipoles.

Honeycomb lattices (HLs) have been widely explored for realization of massless Dirac quasiparticles and intriguing topological properties in both the quantum and classical regimes, with elemental and artificial atoms as the corresponding building blocks. Here we provide a demonstration showing that when the fullerene molecules of ${\mathrm{C}}_{28}$ are used as prototypical building blocks, stable two-dimensional (2D) HLs can also be achieved, with the structural, electronic, and topological properties tuned via proper atom encapsulation. Specifically, whereas a closely packed structure is preferred for the ${\mathrm{C}}_{28}$ lattice, honeycomb structures with different spacial symmetries are energetically favored for both the Bi@${\mathrm{C}}_{28}$ and In@${\mathrm{C}}_{28}$ endohedral fullerenes. In particular, Bi@${\mathrm{C}}_{28}$-HL is revealed to be a quantum spin Hall insulator because of strong spin-orbit coupling effects in the $f$ molecular orbitals, while In@${\mathrm{C}}_{28}$-HL is a quantum valley Hall insulator resulting from mirror symmetry breaking. The present study offers superatom-based platforms for realizing quantum spin Hall and quantum valley Hall effects in 2D systems, with distinctly enhanced tunability and robustness against atomic-scale imperfections.

We show that the continuum limit of the tilted Dirac cone in materials such as $8-Pmmn$ borophene and layered organic conductor $\alpha$-(BEDT-${\mathrm{TTF})}_2{\mathrm{I}}_3$ corresponds to deformation of the Minkowski space-time of Dirac materials. From its Killing vectors we construct an emergent tilted-Lorentz ($t$-Lorentz) symmetry group for such systems. As an example of the $t$-Lorentz transformations we obtain the exact solution of the Landau bands for a crossed configuration of electric and magnetic fields. For any given tilt parameter $0\le \zeta <1$, if the ratio $\chi =v_F{B}_z/E_y$ of the crossed magnetic and electric fields satisfies $\chi \ge 1+\zeta$, one can always find appropriate $t$-boosts in both valleys labeled by $\tau =\pm 1$ in such a way that the electric field can be $t$-boosted away, whereby the resulting pure effective magnetic field $B_z^{\tau }$ governs the Landau level spectrum around each valley $\tau$. The effective magnetic field in one of the valleys is always larger than the applied perpendicular magnetic field. This amplification comes at the expense of of diminishing the effective field in the opposite valley and can be detected in various quantum oscillation phenomena in tilted Dirac cone systems. Tuning the ratio of electric and magnetic fields to ${\chi }_{\min }=1+\zeta$ leads to valley selective collapse of Landau levels. Our geometric description of the tilt in Dirac systems reveals an important connection between the tilt and an incipient “rotating source” when the tilt parameter can be made to depend on space-time in a certain way.

Within the semiclassical Boltzmann transport theory, we perform a systematic first-principles calculation of the intrinsic resistivity of beryllium, a nodal-line semimetal. The Wannier interpolation technique to treat the electron-phonon ($e$-ph) interaction is employed to guarantee a high precision of the numerical results. Our numerical results of the intrinsic resistivity of beryllium agree quantitatively with experimental data in a large temperature range. We find that around each joint region between the electron and hole pockets, the Fermi surface of beryllium forms a pair of vertical facets (parallel to $c$ axis). Then, the nesting effect between such Fermi surface segments near inequivalent vertices of the hexagonal Brillouin zone can be realized by $e$-ph scattering with a relatively short phonon wavelength. Such a Fermi surface nesting effect plays the dominant role in the intrinsic resistivity. It is also the underlying mechanism for linear temperature dependence of the intrinsic resistivity from a very low critical temperature (200 K). In contrast, the contribution of the topological nontrivial states near the nodal line to the intrinsic resistivity is less important because only a few of such states appear in the vicinity of the Fermi surface due to the sizable dispersion of the nodal line.

Layered quasi-two-dimensional systems have garnered huge interest both in the advancement of technology and in understanding emergent physics such as unconventional superconductivity and topological phases. In particular, the study of topological properties in some bilayer systems like transition metal chalcogenides and iridates has been the point of attraction due to comparatively strong spin-orbit coupling of transition metal ions. In this paper, we analyze the topological phases induced by the interplay of electron correlation and spin-orbit coupling in different stacking orders of bilayer honeycomb lattice at quarter filling. Considering the two most common stacking orders, AA and Bernal (AB) stacking, we show that the stacking order plays a crucial role in the topological phase transitions of the bilayer interacting system. For the AA-stacking case, the system realizes quantum spin Hall insulator in the presence (absence) of time-reversal symmetry and magnetically ordered insulator. For the Bernal-stacking case, however, additional phases such as charge-ordered normal insulator or Chern insulator with both charge and magnetic order can be stabilized. Based on our analysis, we discuss the scope of experimental realization in bilayers of transition metal chalcogenides.

A semilocal exchange-correlation functional is proposed with the efficient prediction of the solid-state band gap. The underlying construction of the exchange functional is based on the modeling of the exchange hole and constructing the exchange energy functional. Being a meta-generalized gradient approximation (meta-GGA) level functional, it holds the key feature of the derivative discontinuity through the generalized Kohn-Sham (gKS) formalism. We validate our construction by demonstrating the functional performance for solid-state band gaps and comparing it with the other meta-GGA and hybrid functionals. It is shown that for the semiconductors having narrow, and moderate band gaps, as well as for layered materials the present functional performs as accurately as (or comparable to) the expensive hybrid functional, while for wide-band gap solids, it outperforms the hybrid functional. This indicates that the present functional holds promise for the multiscale modeling of materials with a very low computational cost. Also, the underlying construction is practically very useful as it can be easily implemented in any density functional platform that supports the gKS formalism.

Achieving low thermal conductivity and good electrical properties is a crucial condition for thermal energy harvesting materials. Nanostructuring offers a very powerful tool to address both requirements: in nanostructured materials, boundaries preferentially scatter phonons compared to electrons. The computational screening for low-thermal-conductivity nanostructures is typically limited to materials with simple crystal structures, such as silicon, because of the complexity arising from modeling branch- and wave-vector-dependent nanoscale heat transport. The phonon mean-free-path (MFP) dependent Boltzmann transport equation (MFP-BTE) approach is a model that overcomes this limitation. To illustrate this, we analyze thermal transport in 75 nanoporous half-Heusler compounds for different pore sizes. Our calculations demonstrate that, in most cases, the optimization of thermal transport in nanostructures should take into account both bulk thermal properties and geometry-dependent size effects, two aspects that are typically engineered separately. To enable efficient calculations within this paradigm we derive a model, based on the “gray” formulation of the BTE, that can decouple the influence of the geometry and the material on the effective thermal conductivity with relatively little loss in accuracy compared to the MFP-BTE. Our study motivates the need for a holistic approach to engineering thermal transport and provides a method for high-throughput low-thermal conductivity materials discovery.

We develop the effective medium theory (EMT) to calculate the effective “refractive index” of quantum dot array for electron waves in graphene. It can be used to design electron metamaterials which will control the flow of graphene electrons by achieving any desirable effective “refractive index.” Here the coherent-potential approximation is adopted to derive the analytic formulation of the effective “refractive index,” which has been successfully used in the EMT of electromagnetic metamaterials. The EMT is valid beyond the long-wavelength limit. The region of validity is identified by the comparison of the energy band structures obtained by the EMT and the rigorous multiple scattering theory. We also provide the expressions for effective “refractive indices” in the frame of the long-wavelength theory and the Maxwell-Garnett theory so that a simple estimation can be done in the long-wavelength limit. We perform the simulation of the electron density distribution around a metamaterial with circular cross-section composed of smaller quantum dots. The simulation results show excellent agreement with the equal-size effective circular potential, verifying the effectiveness of our theory to design electron metamaterials. Finally, we show that even though these quantum dots that make up the metamaterial are disordered, the electron density distribution is almost the same as that around the effective one again and the effective medium theory is still applicable.

Topological phases in quantum and classical systems have been of significant recent interest due to their fascinating physical properties. Floquet topological insulators represent one of the possible venues for engineering topological phases, yet they have been so far largely restricted to temporal modulation of Hermitian potentials. On the other hand, in many physical systems, including acoustic and optical systems, modulating loss or gain can be more straightforwardly achieved. On the other hand, non-Hermitian Floquet potentials have not been shown to yield any novel topological phases to date. It is therefore of great interest to explore time-modulated non-Hermitian potentials in periodic lattices, and the emergence of topological phases associated with them. Here we demonstrate that non-Hermitian Hamiltonians can indeed result in topological phases supporting nonreciprocal edge states propagating without dissipation, as well as regimes of dissipative and amplifying topological edge transport.

We develop a theory of the phonon-mediated Casimir interaction between two pointlike impurities which is based on the single-impurity scattering $T$-matrix approach. We show that the Casimir interaction at $T=0$ falls off as a power law with the distance between the impurities. We find that the power in the weak and unitary phonon-impurity scattering limits differs, and we relate the power law to the low-energy properties of the single-impurity scattering $T$ matrix. In addition, we consider the Casimir interaction at finite temperature and show that at finite temperatures the Casimir interaction becomes exponential at large distances.

Time dependence in photonic structures leads to a number of intriguing and useful phenomena such as optical isolation, topological states of light, and frequency conversion. Here we present a mechanism to achieve efficient and selective frequency conversion using a system of two time-modulated cavities. This setup allows us to fine-tune the conversion process by controlling important parameters such as the intercavity coupling and the external excitation frequency. Both symmetric and asymmetric (up- or down-conversion) outputs can be targeted at will. We describe the processes extensively, with for example a leading role for the dynamic modes of the coupled system, the Floquet modes.

Dynamical quantum phase transitions hold a deep connection to the underlying equilibrium physics of the quench Hamiltonian. In a recent study [J. C. Halimeh et al., arXiv:1810.07187] it has been numerically demonstrated that the appearance of anomalous cusps in the Loschmidt return rate coincides with the presence of bound domain walls in the spectrum of the quench Hamiltonian. Here we consider transverse-field Ising chains with power-law and exponentially decaying interactions, and show that by removing domain-wall coupling via a truncated Jordan-Wigner transformation onto a Kitaev chain with long-range hopping and pairing, anomalous dynamical criticality is no longer present. This indicates that bound domain walls are necessary for anomalous cusps to appear in the Loschmidt return rate. We also calculate the dynamical phase diagram of the Kitaev chain with long-range hopping and pairing, which in the case of power-law couplings is shown to exhibit rich dynamical criticality including a doubly critical dynamical phase.

A practical problem for memory applications involving perpendicularly magnetized magnetic tunnel junctions is the reliability of switching characteristics at high-bias voltage. Often it has been observed that at high bias, additional error processes are present that cause a decrease in switching probability upon further increase of bias voltage. We identify the main cause of such error-rise process through examination of switching statistics as a function of bias voltage and applied field, and the junction switching dynamics in real time. These experiments show a coincidental onset of error rise and the presence of a low-frequency microwave emission well below that dictated by the anisotropy field. We show that in a few-macrospin coupled numerical model, this is consistent with an interface region with concentrated perpendicular anisotropy, and where the magnetic moment has limited exchange coupling to the rest of the layers. These results point to the important role high-frequency interface magnetic moment dynamics play in determining the switching characteristics of these tunnel junction devices.