Compared with twisted bilayer graphene, twisted double bilayer graphene (TDBG) provides another important platform to realize the moiré flat bands. In this paper, we first calculate the valley Chern number phase diagram of TDBG in the parameter space spanned by the twist angle and the interlayer electric potential. To include the effects of interactions, we then phenomenologically introduce the spin-splitting and valley-splitting. We find that when the valley splitting is larger than the bandwidth of the first conduction band so that a gap is opened and the spin splitting is relatively weak, the orbital Chern insulator emerges at half-filling, associated with a large orbital magnetization (OM). Further calculations suggest that there is no sign reversal of the OM when the Fermi energy goes from the bottom to the top of the half-filling gap, as the OM remains negative in both AB-AB stacking and AB-BA stacking. The implications of our results for the ongoing experiments are also discussed.

We investigate room-temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material suitable for wireless communication and imaging technologies. To make band structure calculations for such extended systems of coupled quantum dots possible, we developed a “linear combination of quantum dot orbitals” method, based on single quantum dot wave functions. Our results demonstrate strong vertical emission of coupled quantum dots, reduced phonon coupling, and in-plane scattering, enabling room-temperature lasing with significantly reduced threshold current densities.

We present a novel “linear combination of atomic orbitals”-type of approximation, enabling accurate electronic structure calculations for systems of up to 20 or more electronically coupled quantum dots. Using realistic single quantum dot wave functions as a basis to expand the eigenstates of the heterostructure, our method shows excellent agreement with full 8-band $\mathit{k}·\mathit{p}$ calculations, exemplarily chosen for our benchmarking comparison, with orders of magnitude reduction in computational time. We show that, to correctly predict the electronic properties of such stacks of coupled quantum dots, it is necessary to consider the strain distribution in the whole heterostructure. Edge effects determine the electronic structure for stacks of $\lesssim 10$ quantum dots, after which a homogeneous confinement region develops in the center. The overarching goal of our investigations is to design a stack of vertically coupled quantum dots with an intraband staircase potential suitable as an active material for a quantum-dot-based quantum cascade laser. Following a parameter study in the ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}\text{/}\mathrm{GaAs}$ material system, varying quantum dot size, material composition, and interdot coupling strength, we show that an intraband staircase potential of identical transitions can, in principle, be realized. A species library we generated for over 800 unique quantum dots provides easy access to the basis functions required for different realizations of heterostructures. In the associated paper [Mittelstädt et al., Phys. Rev. B 103, 115301 (2021)], we investigate room temperature lasing of a terahertz quantum cascade laser based on a two-quantum-dot unit cell superlattice.

In order to clarify the controversial issue of the topological nature in a mixed-valent Kondo system, ${\mathrm{SmB}}_6$, we have explored the surface states on the nonpolar (110) surface of ${\mathrm{SmB}}_6$, employing both angle-resolved photoemission spectroscopy (ARPES) experiment and ab initio density-functional theory (DFT) band calculations. Based on ARPES spectroscopic fingerprints and the DFT surface band structures, we ascribe the observed spectral weights at $\overline{X}$ and $\overline{Y}$ on the (110) surface Brillouin zone to topological surface states (TSSs) of “topological insulator (TI)” nature and of “topological crystalline insulator (TCI)” nature, respectively. With varying the chemical potential, the double Dirac cones of the TCI nature exhibit a Lifshitz transition of Fermi surfaces with intriguing spin textures. We have also examined the TSSs on the nearly nonpolar (111) surface of ${\mathrm{SmB}}_6$ in connection with a recently reported ARPES result and proposed a way to probe the Dirac points that are buried in the bulk-projected bands.

Monolayers of transition-metal dichalcogenides (TMDs) hold great promise as future nanoelectronic and optoelectronic devices. An essential feature for achieving high device performance is the use of suitable supporting substrates, which can affect the electronic and optical properties of these two-dimensional (2D) materials. Here, we perform many-body GW calculations using the SternheimerGW method to investigate the quasiparticle band structure of monolayer ${\mathrm{MoS}}_2$ subject to an effective dielectric screening model, which is meant to approximately describe substrate polarization in real device applications. We show that, within this model, the dielectric screening has a sizable effect on the quasiparticle band gap; for example, the gap renormalization is as large as 250 meV for ${\mathrm{MoS}}_2$ with model screening corresponding to ${\mathrm{SiO}}_2$. Within the ${\mathrm{G}}_0{\mathrm{W}}_0$ approximation, we also find that the inclusion of the effective screening induces a direct band gap, in contrast to the unscreened monolayer. We also find that the dielectric screening induces an enhancement of the carrier effective masses by as much as 27% for holes, shifts plasmon satellites, and redistributes quasiparticle weight. Our results highlight the importance of the dielectric environment in the design of 2D TMD-based devices.

Understanding vanishing transmission in Fano resonances in quantum systems and metamaterials and perfect and ultralow transmission in disordered media has advanced the knowledge and applications of wave interactions. Here, we use analytic theory and numerical simulations to understand and control the transmission and transmission time in complex systems by deforming a medium and adjusting the level of gain or loss. Unlike the zeros of the scattering matrix, the position and motion of the zeros of the determinant of the transmission matrix (TM) in the complex plane of frequency and field decay rate have robust topological properties. In systems without loss or gain, the transmission zeros appear either singly on the real axis or as conjugate pairs in the complex plane. As the structure is modified, two single zeros and a complex conjugate pair of zeros may interconvert when they meet at a square root singularity in the rate of change of the distance between the transmission zeros in the complex plane with sample deformation. The transmission time is the spectral derivative of the argument of the determinant of the TM. It is a sum over Lorentzian functions associated with the resonances of the medium, which is the density of states, and with the zeros of the TM. Transmission vanishes, and the transmission time diverges as zeros are brought near the real axis. Monitoring the transmission and transmission time when two zeros are close may open new possibilities for ultrasensitive detection.

We study a class of topological materials which in their momentum-space band structure exhibit threefold degeneracies known as triple points. Focusing specifically on $\mathcal{P}\mathcal{T}$-symmetric crystalline solids with negligible spin-orbit coupling, we find that such triple points can be stabilized by little groups containing a three-, four-, or sixfold rotation axis, and we develop a classification of all possible triple points as type A vs type B according to the absence vs presence of attached nodal-line arcs. Furthermore, by employing the recently discovered non-Abelian band topology, we argue that a rotation-symmetry-breaking strain transforms type-A triple points into multiband nodal links. Although multiband nodal-line compositions were previously theoretically conceived and related to topological monopole charges, a practical condensed-matter platform for their manipulation and inspection has hitherto been missing. By reviewing the known triple-point materials with weak spin-orbit coupling and by performing first-principles calculations to predict new ones, we identify suitable candidates for the realization of multiband nodal links in applied strain. In particular, we report that an ideal compound to study this phenomenon is ${\mathrm{Li}}_2\mathrm{Na}\mathrm{N}$, in which the conversion of triple points to multiband nodal links facilitates a largely tunable density of states and optical conductivity with doping and strain, respectively.

An important step in understanding the exotic electronic, vibrational, and optical properties of the moiré lattices is the inclusion of the effects of structural relaxation of the unrelaxed moiré lattices. Here, we propose novel structures for twisted bilayer of transition metal dichalcogenides. For $\theta \gtrsim 58.4^{\circ }$, we show a dramatic reconstruction of the moiré lattices, leading to a trimerization of the unfavorable stackings. We show that the development of curved domain walls due to the threefold symmetry of the stacking energy landscape is responsible for such lattice reconstruction. Furthermore, we show that the lattice reconstruction notably changes the electronic band structure. This includes the occurrence of flat bands near the edges of the conduction as well as valence bands, with the valence band maximum, in particular, corresponding to localized states enclosed by the trimer. We also find possibilities for other complicated, entropy stabilized, lattice reconstructed structures.

Based on the exact muffin-tin orbitals (EMTOs), we developed a first-principles method to calculate the current operators and investigated the anomalous Hall effect in bcc Fe as an example, with which we successfully separated the skew scattering contribution from the side jump and intrinsic contributions by fitting the scaling law with the introduction of sparse impurities. By investigating the temperature dependence of the anomalous Hall effect in bulk Fe, we predicted a fluctuated anomalous Hall angle as a function of temperature when considering only phonons, which, in the future, can be measured in experiments by suppressing magnon excitation, e.g., by applying a high external magnetic field.

Metasurfaces are ultrathin planar arrays of carefully tailored subwavelength particles that enable agile and flexible manipulation of the impinging waves. Originally introduced in optics, their application to acoustic waves has recently opened exciting opportunities for exotic sound control. In conventional acoustic inclusions, the interactions with the impinging pressure and velocity are decoupled, limiting the functionalities that arrays of them can achieve. While the coupling between these two quantities in symmetry-breaking inclusions, known as Willis coupling, has been discussed for several years, only recently has it been realized that these phenomena can become nonperturbative in suitably tailored resonant scatterers. Here, we explore the opportunities that these Willis meta-atoms open in the context of acoustic metasurfaces, offering additional knobs to manipulate and tailor sound. The general response of Willis metasurface is analytically derived, yielding fundamental bounds and optimal surface responses enabling full control of the impinging acoustic wave front.

The characteristics of confined magnetoelastic waves in nanoscale ferromagnetic magnetostrictive waveguides have been investigated by a combination of analytical and numerical calculations. The presence of both magnetostriction and inverse magnetostriction leads to the coupling between confined spin waves and elastic Lamb waves. Numerical simulations of the coupled system have been used to extract the dispersion relations of the magnetoelastic waves as well as their mode profiles.

Thermodynamics of the Kitaev honeycomb magnet $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ is studied for different directions of in-plane magnetic field using measurements of the magnetic Grüneisen parameter ${\mathrm{\Gamma }}_B$ and specific heat $C$. We identify two critical fields $B_c^{\mathrm{AF}1}$ and $B_c^{\mathrm{AF}2}$ corresponding, respectively, to a transition between two magnetically ordered states and the loss of magnetic order toward a quantum paramagnetic state. The $B_c^{\mathrm{AF}2}$ phase boundary reveals a narrow region of magnetic fields where inverse melting of the ordered phase may occur. No additional transitions are detected above $B_c^{\mathrm{AF}2}$ for any direction of the in-plane field, although a shoulder anomaly in ${\mathrm{\Gamma }}_B$ is observed systematically at $8–10\mathrm{T}$ . Large field-induced entropy effects imply additional low-energy excitations at low fields and/or strongly field-dependent phonon entropies. Our results establish universal features of $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ in high magnetic fields and challenge the presence of a field-induced Kitaev spin liquid in this material.

The structural and electronic properties of the rutile-type oxide $\beta -\mathrm{Pb}{\mathrm{O}}_2$ (plattnerite) are studied by neutron and synchrotron radiation diffraction and first-principles density functional theory (DFT) calculations. Both diffraction measurements and DFT calculations show that $\beta -\mathrm{Pb}{\mathrm{O}}_2$ has a $\mathrm{Ca}{\mathrm{Cl}}_2$-type orthorhombic structure (space group $Pnnm$) instead of the widely accepted $\beta -\mathrm{Pb}{\mathrm{O}}_2$ rutile-type tetragonal structure (space group $P{4}_2/mnm$). This symmetry lowering in $\beta -\mathrm{Pb}{\mathrm{O}}_2$ is a robust effect observed at ambient pressure at temperatures between 100 and 400 K. The orthorhombic symmetry rules out the possibility of a semimetallic symmetry-protected state in $\beta -\mathrm{Pb}{\mathrm{O}}_2$. Both diffraction measurements and DFT calculations show an anisotropy of thermal expansion, atomic vibrations, and elastic constants of $\beta -\mathrm{Pb}{\mathrm{O}}_2$ along the [100] and [010] directions.

We use field-cycling-assisted dynamic nuclear polarization and continuous radio-frequency (RF) driving over a broad spectral range to demonstrate magnetic-field-dependent activation of nuclear spin transport from strongly hyperfine-coupled ${}^{13}\mathrm{C}$ sites in diamond. We interpret our observations with the help of a theoretical framework where nuclear spin interactions are mediated by electron spins. In particular, we build on the results from a four-spin toy model to show how otherwise localized nuclear spins must thermalize as they are brought in contact with a larger ancilla spin network. Further, by probing the system response to a variable driving field amplitude, we witness stark changes in the RF-absorption spectrum, which we interpret as partly due to contributions from heterogeneous multispin sets, whose zero-quantum transitions become RF active thanks to the hybrid electron-nuclear nature of the system. These findings could prove relevant in applications to dynamic nuclear polarization, spin-based quantum information processing, and nanoscale sensing.

In this article, the temperature-dependent complex Hall effect near the magnetization compensation temperature of CoGd alloy films is systematically studied. We find anomalies of Hall loops with characteristics similar to the topological Hall effect (THE), which is usually considered to indicate the occurrence of topological spin texture. However, the THE is ruled out by detailed magnetic and transport measurements on CoGd alloy films with different composition ratios. After considering the influence of the spin-flop effect and the inhomogeneity of the components naturally formed in the ferrimagnetic CoGd alloy, we constructed a phenomenological model that can explain the complex Hall effect near the magnetization compensation point without relying on the THE. The main result is helpful for explaining the microscopic magnetic properties and transport properties of rare-earth transition metal ferrimagnetic alloys near the compensation point.

Dilute alloys of rare earths have played a vital role in understanding magnetic phenomena. Here, we model the ground state of dilute $4f$ rare-earth impurities in light metals. When the $4f$ subshells are open (but not half-filled), the spin-orbit coupling imprints a rotational charge current of conduction electrons around rare-earth atoms. The sign and amplitude of the current oscillate similar to the Ruderman-Kittel-Kasuya-Yosida (RKKY) spin polarization. We compute the observable effect, namely, the Ørsted field generated by the current vortices and the Knight shift.

The fluctuations of massless Dirac fermion can not only turn a first-order bosonic phase transition (in the Landau sense) to a quantum critical point, but also work reversely to enhance the first-order transition itself, depending on the implementation of finite-size effects in the coupling corrections. Here, we report a case study of the latter by employing quantum Monte Carlo simulation upon a lattice model in which the bosonic part featuring the Landau-Devonshire first-order phase transition and Yukawa coupled to the Dirac fermions. We find that the parameter range for the first-order phase transition becomes larger as the Yukawa coupling increases, and the microscopic mechanism of this phenomena is revealed, at a quantitative level, as the interplay between the critical fluctuations and the finite-size effects. Moreover, the scaling behavior at the separation point between the first-order and the continuous phase transitions is found to belong to the chiral tricritical Gross-Neveu universality. Our results demonstrate that the interplay of massless Dirac fermions, critical fluctuations, and the finite-size effects could trigger a plethora of interesting phenomena, and therefore great care is called for when making generalizations.

We study the influence of the chiral anomaly of a time-reversal broken Weyl semimetal on Andreev reflection in a superconductor junction. It is found that the chiral chemical potential can lead to the switching between the retro Andreev reflection (RAR) and specular Andreev reflection (SAR), which appears in both intraband and interband electron-hole conversion, violating the common rule that the intraband conversion is the RAR while the interband one is the SAR. This switching effect is dependent on the tilt of the Weyl cones and the direction of the line connecting the two Weyl points. In addition, a perfect Andreev reflection is found at the incident angle, not perpendicular to the junction interface. We also discuss the zero-bias differential conductance which provides an observable signature of the switching effect, especially for the switching between RAR and SAR caused by band transition. These findings are useful in better understanding the Andreev reflection types and detecting the chiral anomaly.

A non-Hermitian generalization of the Su-Schrieffer-Heeger model driven by a periodic external potential is investigated, and its topological features are explored. We find that the bi-orthonormal geometric phase acts as a topological index, well capturing the presence/absence of the zero modes. The model is observed to display trivial and nontrivial insulator phases and a topologically nontrivial Möbius metallic phase. The driving field amplitude is shown to be a control parameter causing topological phase transitions in this model. While the system displays zero modes in the metallic phase apart from the nontrivial insulator phase, the metallic zero modes are not robust, as are the ones found in the insulating phase. We further find that zero modes' energy converges slowly to zero as a function of the number of dimers in the Möbius metallic phase compared to the nontrivial insulating phase.