Charge equilibration between quantum Hall edge states can be studied to reveal the geometric structure of edge channels not only in the integer quantum Hall (IQH) regime but also in the fractional quantum Hall (FQH) regime, particularly for hole-conjugate states. Here we report on a systematic study of charge equilibration in both IQH and FQH regimes by using a generalized Hall bar, in which a quantum Hall state is nested in another quantum Hall state with different Landau filling factors. This provides a feasible way to evaluate equilibration in various conditions even in the presence of scattering in the bulk region. The validity of the analysis is tested in the IQH regime by confirming consistency with previous works. In the FQH regime, we find that the equilibration length for counterpropagating $\delta \nu =1$ and $\delta \nu =-1/3$ channels along a hole-conjugate state at Landau filling factor $\nu =2/3$ is much shorter than that for copropagating $\delta \nu =1$ and $\delta \nu =1/3$ channels along a particle state at $\nu =4/3$. The difference can be associated with the distinct geometric structures of the edge channels. Our analysis with generalized Hall-bar devices would be useful in studying edge equilibration and edge structures.

We study tunable polaritons in monolayers of silicene, germanene, and stanene ($\mathrm{Xenes}$) via an external electric field in an open optical microcavity whose length can be adjusted. An external electric field applied perpendicular to the plane of the $\mathrm{Xene}$ monolayer simultaneously changes the band gap and the exciton binding energy, while the variable length of the open microcavity allows one to keep the exciton and cavity photon modes in resonance. First, the Schrödinger equation for an electron and hole in an $\mathrm{Xene}$ monolayer is solved, yielding the eigenergies and eigenfunctions of the exciton as a function of the external electric field. The dependence of the polaritonic properties, such as the Rabi splitting, on the external electric field and on the cavity length, is analyzed. The Berezinskii-Kosterlitz-Thouless (BKT) transition temperature of polaritons is calculated as a function of the external electric field. We analyze and present the conditions for a room-temperature superfluid of lower polaritons by simultaneously maximizing the Rabi splitting and BKT transition temperature.

The nonlocal van der Waals density functional (vdW-DF) has had tremendous success since its inception in 2004 due to its constraint-based formalism that is rigorously derived from a many-body starting point. However, while vdW-DF can describe binding energies and structures for van der Waals complexes and mixed systems with good accuracy, one long-standing criticism—also since its inception—has been that the $C_6$ coefficients that derive from the vdW-DF framework are largely inaccurate and can be wrong by more than a factor of 2. It has long been thought that this failure to describe the $C_6$ coefficients is a conceptual flaw of the underlying plasmon framework used to derive vdW-DF. We prove here that this is not the case and that accurate $C_6$ coefficients can be obtained without sacrificing the accuracy at binding separations from a modified framework that is fully consistent with the constraints and design philosophy of the original vdW-DF formulation. Our design exploits a degree of freedom in the plasmon-dispersion model ${\omega }_{\mathbf{q}}$, modifying the strength of the long-range van der Waals interaction and the crossover from long to short separations, with additional parameters tuned to reference systems. Testing the new formulation for a range of different systems, we not only confirm the greatly improved description of $C_6$ coefficients, but we also find excellent performance for molecular dimers and other systems. The importance of this development is not necessarily that particular aspects such as $C_6$ coefficients or binding energies are improved, but rather that our finding opens the door for further conceptual developments of an entirely unexplored direction within the exact same constrained-based nonlocal framework that made vdW-DF so successful in the first place.

Angle-resolved secondary electron emission spectra were analyzed for studying the electron transmission properties of graphene when it is tightly bound to a metallic substrate and when it is decoupled from the latter. As respective model systems, graphene/Ni(111) and graphene/Au/Ni(111) were considered. We discovered two types of spectral structures linked with the transmission properties of graphene: bands related to scattering resonances and to umklapp reflection. These findings are interpreted based on an ab initio theory of the scattering of the outgoing electrons by the graphene overlayers. The experimental secondary electron emission maps are in good agreement with the calculations, indicating that such approach can be successfully applied for studying the electron transmission properties of a variety of two-dimensional materials.

The discovery of quantum spin Hall materials with huge bulk gaps in experiment, such as bismuthene, provides a versatile platform for topological devices. We propose a topological quantum dot (QD) device in bismuthene ribbon in which two planar magnetization areas separate the sample into a QD and two leads. At zero temperature, peaks of differential conductance emerge, demonstrating the discrete energy levels from the confined topological edge states. The key parameters of the QD, the tunneling coupling strength with the leads and the discrete energy levels, can be controlled by the planar magnetization and the sample size. Especially, different from the conventional QD, we find that the angle between two planar magnetization orientations provides an effective way to manipulate the discrete energy levels. Combining the numerical calculation and the theoretical analysis, we identify that such manipulation originates from the unique quantum confinement effect of the topological edge states. Based on such a mechanism, we find the spin transport properties of QDs can also be controlled.

Following the discovery of topological insulators (TIs), topological Dirac/Weyl semimetal has attracted much recent interest. A prevailing mechanism for the formation of Weyl points is by breaking time-reversal symmetry (TRS) or spatial inversion symmetry of a Dirac point. Here we demonstrate a generic formation mechanism for Weyl points by breaking TRS of a three-dimensional (3D) TI featured with highly degenerate 3D flat bands (FBs). It is in direct contrast to the conventional view that breaking TRS of a 2D/3D TI leads to a Chern insulator exhibiting quantum anomalous Hall effect. Based on a tight-binding model of pyrochlore lattice, we show that this unusual 3D-FB-enabled Weyl state may contain only a minimum of two Weyl points. Furthermore, using first-principles calculations, we identify this Weyl state in a real material ${\mathrm{Sn}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$. The main features of the resulting Weyl points are analyzed with respect to symmetry, topological invariant and surface state. Our finding sheds new light on our fundamental understanding of topological physics and significantly extends the scope of Weyl semimetals to attract immediate experimental interest.

The success of graphene and its emerging Dirac physics has stimulated the quest for versatile and tunable electronic properties in atomically thin systems, leading to the discovery of various chemical classes of two-dimensional (2D) compounds. In particular, honeycomb lattices of group-IV elements, such as silicene and germanene, have been found experimentally. Whether it is a necessity of synthesis or a desired feature for application purposes, most 2D materials demand a supporting substrate. In this Rapid Communication, by combining ab initio simulations with multiorbital functional renormalization group analysis of Fermi surface instabilities, we highlight the constructive impact of substrates to enable the realization of exotic electronic quantum states of matter, where the buckling emerges as the decisive material parameter adjustable by the commensuration. At the example of germanene deposited on ${\mathrm{MoS}}_2$, an experimentally characterized superstructure, we find that the coupling between the monolayer and the substrate, together with the buckled hexagonal geometry, conspire to provide a highly suited scenario for unconventional triplet superconductivity.

The high-$T_c$ superconductor, $\mathrm{FeT}{\mathrm{e}}_{0.55}\mathrm{S}{\mathrm{e}}_{0.45}$, has recently been shown to support a surface state with topological character. Here we use low-energy laser-based angle-resolved photoemission spectroscopy with variable light polarization, including both linear and circular polarizations, to reexamine the same material and the related $\mathrm{FeT}{\mathrm{e}}_{0.7}\mathrm{S}{\mathrm{e}}_{0.3}$, with a larger Te concentration. In both cases, we observe the presence of a surface state displaying linear dispersion in a cone-like configuration. The use of circular polarization confirms the presence of a helical spin structure. These experimental studies are compared with theoretical studies that account for the local magnetic effects related to the paramagnetism observed in this system in the normal state. In contrast to previous studies, we find that including the magnetic contributions is necessary to bring the chemical potential of the calculated electronic band structure naturally into alignment with the experimental observations.

We study theoretically low-temperature phases of a recently synthesized compound ${\mathrm{Sr}}_2{\mathrm{CoO}}_3\mathrm{F}$ under pressure. The analysis combining LDA+DMFT and a strong-coupling effective model points to the existence of not only normal paramagnetic and antiferromagnetic regimes, but also a spin-state ordered phase in a certain range of applied pressure and low temperature. This order is characterized by a checkerboard arrangement of different spin states of cobalt atoms in the lattice.

We point out that the Kitaev materials may not necessarily support Kitaev spin liquid. It is well known that having a Kitaev term in the spin interaction is not the sufficient condition for the Kitaev spin-liquid ground state. Many other spin liquids may be stabilized by the competing spin interactions of the systems. We thus explore the possibilities of non-Kitaev spin liquids in the honeycomb Kitaev materials. We carry out a systematic classification of gapped ${\mathbb{Z}}_2$ spin liquids using the Schwinger boson representation for the spin variables. The presence of strong spin-orbit coupling in the Kitaev materials brings new ingredients into the projective symmetry group classification of the non-Kitaev spin liquid. We predict the spectroscopic properties of these gapped non-Kitaev spin liquids. Moreover, among the gapped spin liquids that we discover, we identify the spin liquid whose spinon condensation leads to the zigzag magnetic order that was observed in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha \text{−}{\mathrm{RuCl}}_3$. We further discuss the possibility of gapped ${\mathbb{Z}}_2$ spin liquid and the deconfined quantum criticality from the zigzag magnetic order to spin dimerization in pressurized $\alpha \text{−}{\mathrm{RuCl}}_3$.

Motivated by symmetry-protected topological phases (SPTs) with both spatial symmetry (e.g., lattice rotation) and internal symmetry (e.g., spin rotation), we propose a class of exotic topological terms, which generalize the well-known Wen-Zee topological terms of quantum Hall systems [X.-G. Wen and A. Zee, Phys. Rev. Lett. 69, 953 (1992)]. These generalized Wen-Zee terms are expressed as wedge product of spin connection and usual gauge fields (1-form or higher) in various dimensions. In order to probe SPT orders, we externally insert symmetry twists such as domain walls of discrete internal symmetry and disclinations that are geometric defects with nontrivial Riemann curvature. Then, generalized Wen-Zee terms simply tell us how SPTs respond to those symmetry twists. Classifying these exotic topological terms thus leads to a complete classification and characterization of SPTs within the present framework. We also propose SPT low-energy field theories, from which generalized Wen-Zee terms are deduced as topological response actions. Following the Abstract of the Wen-Zee paper, our work enriches alternative possibilities of condensed-matter realization of unification of electromagnetism and gravity.

In this paper we study the ground-state properties of a ladder Hamiltonian with chiral $\text{SU}\left(2\right)$-invariant spin interactions, a possible first step toward the construction of truly two-dimensional nontrivial systems with chiral properties starting from quasi-one-dimensional ones. Our analysis uses a recent implementation by us of $\text{SU}\left(2\right)$ symmetry in tensor network algorithms, specifically for infinite density matrix renormalization group. After a preliminary analysis with Kadanoff coarse graining and exact diagonalization for a small-size system, we discuss its bosonization and recap the continuum limit of the model to show that it corresponds to a conformal field theory, in agreement with our numerical findings. In particular, the scaling of the entanglement entropy as well as finite-entanglement scaling data show that the ground-state properties match those of the universality class of a $c=1$ conformal field theory (CFT) in $(1+1)$ dimensions. We also study the algebraic decay of spin-spin and dimer-dimer correlation functions, as well as the algebraic convergence of the ground-state energy with the bond dimension, and the entanglement spectrum of half an infinite chain. Our results for the entanglement spectrum are remarkably similar to those of the spin-$\frac{1}{2}$ Heisenberg chain, which we take as a strong indication that both systems are described by the same CFT at low energies, i.e., an $\text{SU}{\left(2\right)}_1$ Wess-Zumino-Witten theory. Moreover, we explain in detail how to construct matrix product operators for $\text{SU}\left(2\right)$-invariant three-spin interactions, something that had not been addressed with sufficient depth in the literature.

We present an ab initio computational study of triplet exciton diffusion in four phosphorescent emitters commonly used in organic light-emitting diodes (OLEDs). By kinetic Monte Carlo simulations, triplet diffusion lengths are obtained for these emitters in neat films and as a guest in two different hosts. The triplet transfer rates governing the diffusion contain a transfer integral factor that includes both Förster and Dexter contributions and a Franck-Condon weighted density of vibrational states that includes the coupling to all intramolecular vibrations in a fully quantum mechanical way. We find that at guest concentrations around 10 mol% the Förster transfer contribution is most important. At larger concentrations of about 30–40 mol% the Dexter contribution becomes dominant. We show that obtaining the triplet transfer rates by the semiclassical Marcus theory yields diffusion lengths that are too short and that using a simple cubic lattice in combination with the often used Miller-Abrahams rates instead of using a real morphology with the ab initio rates leads to an underestimation of the diffusion lengths due to transfers down in energy that are too slow.

The miniaturization of spintronic devices, specifically, nanoscale devices employing spintronics, has attracted intensive attention from a scientific as well as engineering perspective. In this paper, we study a non-Markovian effect on spin pumping to describe spin current generation driven by the magnetization of arbitrary precession frequency in a quantum dot attached to an electron lead. Although the Markovian approximation can be used when driving is sufficiently slow compared with relaxation times in electron tunneling, recent developments in nanospintronic devices show that we need to include non-Markovian effects. In contrast to the one-way-only nature of the spin current generation under Markovian dynamics, we find that non-Markovian dynamics exhibit a temporal backflow of spin, called spin backflow for brevity. We capture the phenomenon by introducing its quantifier, and show that the backflow reduces the amount of spin current significantly when the frequency exceeds the relaxation rate. This prevents an unphysical divergence of the spin current in the high-frequency limit that occurs under the Markovian approximation. We believe our analysis provides an understanding of the spin pumping particularly in regard to producing a more efficient spin current generation over shorter time scales by going beyond the conventional Markovian approximation.

Faraday rotation, being a dispersive effect, is commonly considered as the method of choice for nondestructive detection of spin states. Nevertheless Faraday rotation is inevitably accompanied by spin flips induced by Raman scattering, which compromises nondestructive detection. Here, we derive an explicit general relation relating the Faraday rotation and the spin-flip Raman scattering cross sections, from which precise criteria for nondestructive detection are established. It is shown that, even in ideal conditions, nondestructive measurement of a single spin can be achieved only in anisotropic media. Monolayers of transitions metal dichalcogenides are shown to be promising candidates for this purpose.

We study the fluctuation-induced interaction between two impurities in a weakly interacting one-dimensional Bose gas using the field-theoretical approach. At separations between impurities shorter and of the order of the healing length of the system, the induced interaction has a classical origin and behaves exponentially. At separations longer than the healing length, the interaction is of a quantum origin and scales as the third power of the inverse distance. Finite temperature destroys the quasi-long-range order of the Bose gas and, accordingly, the induced interaction becomes exponentially suppressed beyond the thermal length. We obtain analytical expressions for the induced interaction at zero and finite temperature that are valid at arbitrary distances. We discuss experimental realizations as well as possible formation of bound states of two impurities, known as bipolarons.

Interconnecting well-functioning, scalable stationary qubits and photonic qubits could substantially advance quantum communication applications and serve to link future quantum processors. Here, we present two protocols for transferring the state of a photonic qubit to a single-spin and to a two-spin qubit hosted in gate-defined quantum dots (GDQD). Both protocols are based on using a localized exciton as intermediary between the photonic and the spin qubit. We use effective Hamiltonian models to describe the hybrid systems formed by the exciton and the GDQDs and apply simple but realistic noise models to analyze the viability of the proposed protocols. Using realistic parameters, we find that the protocols can be completed with a success probability ranging between $84–96\%$.

The plasticity and $\alpha \to \omega$ transformation are important for the toughness and ductility of hcp titanium under shock loading. However, three questions remain outstanding: (i) what mechanisms govern the plasticity and transformation, (ii) how does the microstructure, i.e., grain boundaries (GBs) and crystallographic orientations, affect them, and (iii) does the transformation take place dependent on the plasticity, such as dislocation slips? We conduct large-scale nonequilibrium molecular dynamics simulations to study shock-induced plasticity and phase transformation in hexagonal columnar nanocrystalline Ti. Significant anisotropy and strong dependence on crystallographic orientation are presented during shock-induced plasticity and phase transformation. The shock first prompts “heterogeneous” dislocation slips and ${90}^{\circ }$ lattice reorientation, via coupling deformation twinning and slips. Then, the on-going plastic deformation induces a “heterogeneous” $\alpha \to \omega$ phase transformation at lower impact velocities or a “homogeneous” solid-state disordering at higher impact velocities. The phase transformation mostly obeys the TAO-1 pathways originated from GBs, while a few of them are governed by Silcock mechanisms within the grains. The TAO-1 and Silcock-governed transformations stem from the emission and propagation of basal-prismatic and prismatic stacking faults, respectively. At the release/tension stage, a $\omega \to \alpha$ transformation occurs, acting as the reversed process of the $\alpha \to \omega$ transformation at the compression stage. Meanwhile, structural recovery and spallation initiate in the extending tension area induced by the release fans. Serving as the nucleation of the plasticity, phase transformation, and spallation, GBs play the key role during the loading.