We present theoretical modeling of the quantum Hall phase and chiral edge states in a gyroelectric photonic crystal. The theoretical framework represented here is based on a coupled dipole method, which is a direct counterpart of the tight-binding approximation. We confirm that gyrotropic coupling produces a band gap characterized by a nonzero Chern number. Gapless edge states appear at an interface between a gyroeletric photonic crystal and a vacuum. At a domain wall between media of positive and negative gyrotropy, topological edge states that are analogous to the Jackiw-Rebbi states are observed. Propagation of the chiral edge states is also simulated in real space under excitation of an external dipole source. This analysis provides a toy model to describe the quantum Hall phase in a purely optical manner, without using a mapping or an analogy of the electron wave function in the quantum Hall effect.

We present an extension of a framework for simulating single quasiparticle or collective excitations on top of strongly correlated quantum many-body ground states using infinite projected entangled pair states, a tensor network ansatz for two-dimensional wave functions in the thermodynamic limit. Our approach performs a systematic summation of locally perturbed states in order to obtain excited eigenstates localized in momentum space, using the corner transfer matrix method, and generalizes the framework to arbitrary unit cell sizes, the implementation of global Abelian symmetries and fermionic systems. Results for several test cases are presented, including the transverse Ising model, the spin-$1/2$ Heisenberg model, and a free fermionic model, to demonstrate the capability of the method to accurately capture dispersions. We also provide insight into the nature of excitations at the $k=(\pi ,0)$ point of the Heisenberg model.

Magnetization plateaus are some of the most striking manifestations of frustration in low-dimensional spin systems. We present numerical studies of magnetization plateaus in the fascinating spin-1/2 skewed ladder system obtained by alternately fusing five- and seven-membered rings. This system exhibits three significant plateaus at $m=1/4$, 1/2, and 3/4, consistent with the Oshikawa-Yamanaka-Affleck condition. Our numerical as well as perturbative analysis shows that the ground state can be approximated by three weakly coupled singlet dimers and two free spins, in the absence of a magnetic field. With increasing applied magnetic field, the dimers progressively become triplets with large energy gaps to excited states, giving rise to stable magnetization plateaus. Finite-temperature studies show that $m=1/4$ and 1/2 plateaus are robust and survive thermal fluctuations while the $m=3/4$ plateau shrinks rapidly due to thermal noise. The cusps at the ends of a plateau follow the algebraic square-root dependence on $B$.

Two-dimensional (2D) band crossing semimetals (BCSMs) could be used to build a range of novel nanoscale devices, such as superlenses and transistors. We find that symmorphic symmetry can protect a new type of robust 2D BCSM, unlike the previously proposed 2D essential BCSMs protected by nonsymmorphic symmetry [Young and Kane, Phys. Rev. Lett. 115, 126803 (2015)]. This type of symmorphic symmetry-protected (SSP) 2D essential BCSM cannot be pair annihilated without destroying the crystalline symmetries as opposed to the 2D BCSM caused by the accidental band crossing. Through group-theory analysis, we find that 2D SSP BCSMs can only exist at the $K$(K′) point of the Brillouin zone of four-layer groups and identify nonmagnetic 2D $\mathrm{Fe}{\mathrm{B}}_2$ as a candidate. Interestingly, nonmagnetic 2D SSP BCSMs can host a single pair of band crossing points (BCPs), whereas nonmagnetic three-dimensional Weyl semimetals have, at least, two pairs of band crossing Weyl points. It is found that the single pair of BCPs is robust against any kind of strain. Furthermore, our calculation suggests that essential 2D SSP BCSMs can be used to realize electric-field control of spin texture and, thus, are promising candidates for spintronic devices.

A cornerstone in the operation of numerous detecting and sensing quantum setups is the sharp change in the response once a condition ceases to be effective; this regime is typically realized by perfectly cloaking a nanoinclusion from the propagating matter waves. Such a purpose is found well-served by optimally sized nanocylinders, embedded into a suitable quantum texture, that efficiently hide themselves from obliquely impinging electron beams. The optimal designs exhibit high selectivity for particle energies while being wideband with respect to incidence angles; these features render them deployable in various quantum devices from furtive sensors and detectors to quantum switches and matter-wave filters.

InAs quantum dots embedded in InP nanowires form an important platform for basic research studies, as well as for quantum dot applications. Notably, understanding of nanowire quantum dot spectral properties is essential in both cases. Therefore, in this work we use atomistic theory to study spectra of the single exciton $\left(X\right)$, the biexciton $\left(XX\right)$, the triexciton $\left(XXX\right)$, and the positively and negatively charged trions $(X^{+}$ and $X^{-})$ confined in these nanostructures. We focus on the role of vertical and lateral confinement, therefore, we systematically study a large family of quantum dots with different heights and diameters, and find the important role of correlations due to presence of higher states. We find that the order of excitonic binding energies is a characteristic feature of InAs/InP nanowire quantum dots being (ordered from negative to positive values): $X^{-},XX$, and $X^{+}$, with strongly bound $X^{-}$, rather weakly bound $XX$, and typically unbound $X^{+}$. Next, we determine the key role of alloy randomness due to intermixing, which turns out to especially important for larger quantum dot heights and phosphorous contents over 40%. In selected cases, the alloying can lead to an unbound biexciton, and can even reverse ordering of excitonic lines.

We report on Andreev reflections at clean $\mathrm{NbS}{\mathrm{e}}_2$-bilayer graphene junctions. The high transparency of the junction, which manifests as a large conductance enhancement of up to 1.8, enables us to see clear evidence of a proximity-induced superconducting gap in bilayer graphene and two Andreev reflections through a vertical $\mathrm{NbS}{\mathrm{e}}_2$-graphene and a lateral graphene-graphene junction, respectively. Quantum transport simulations capture the complexity of the experimental data and illuminate the impact of various microscopic parameters on the transmission of the junction. Our work establishes the practice and understanding of an all-van-der-Waals, high-performance superconducting junction. The realization of a highly transparent proximized graphene-graphene junction opens up possibilities to engineer emergent quantum phenomena.

Berry curvature, which arises from the asymmetry in a system, is a paramount physical property that results in interesting phenomena such as valley Hall effect. There have been several experimental observations of valley Hall effect in hBN encapsulated bilayer graphene systems as well as hBN/single-layer graphene heterostructures. Although the intent of encapsulating graphene with hBN is to improve the electronic properties of the graphene, the effect of the hBN layer in breaking the layer symmetry and inducing Berry curvature in these systems is not studied explicitly. In this study, we show that for the commensurate state of the heterostructure, the configuration, as well as the orientation of the hBN layer, has an immense effect on the polarity as well as the magnitude of the Berry curvature in bilayer graphene system as they break the layer symmetry. Also, the polarity and the magnitude of the Berry curvature can further be manipulated with the application of an out-of-plane electric field by tuning the layer asymmetry. Whereas, in hBN/single-layer graphene commensurate systems, the presence of an hBN layer breaks the sublattice symmetry and induces Berry curvature, whose magnitude and polarity depend on the configuration and the orientation of the hBN layer. Moreover, although the magnitude can be further tuned with the application of an out-of-plane electric field, the polarity is rather insensitive to the field as the sublattice asymmetry cannot be reversed with the applied field.

An atomically thin $\mathrm{Mo}{\mathrm{S}}_2$ layer is a direct bandgap semiconductor exhibiting strong electron-hole interaction due to the extreme quantum confinement and reduced screening of Coulomb interactions, which results in the formation of stable excitons at room temperature. Therefore, various excitonic properties of the $\mathrm{Mo}{\mathrm{S}}_2$ monolayer are extremely important in determining the strength of light-matter interactions including their radiative recombination lifetime and optoelectronic response. In this paper, we report a comprehensive study of the underlying annihilation mechanism of various types of exciton in the $\mathrm{Mo}{\mathrm{S}}_2$ monolayer using the transient absorption spectroscopy. We rigorously demonstrate that the Förster-type resonance energy transfer is the main annihilation mechanism of A and Bexcitons, while the multistep diffusion process is responsible for C-exciton annihilation, which is supported by critical scientific evidence.

In this paper, we address lattice heat transport in single-walled carbon nanotubes (CNTs) by a quantum mechanical calculation of three-phonon scattering rates in the framework of the Boltzmann transport equation (BTE) and classical molecular dynamics (MD) simulation. Under a consistent choice of an empirical, realistic atomic interaction potential, we compare the tube-length dependence of the lattice thermal conductivity (TC) at room temperature determined from an iterative solution of the BTE and from a nonequilibrium MD (NEMD) approach. Qualitatively similar trends are found in the limit of short tubes, where an extensive regime of ballistic heat transport prevailing in CNTs of lengths $L\lesssim 1\mu \mathrm{m}$ is independently confirmed. In the limit of long tubes, the BTE approach suggests a saturation of TC with tube length, whereas direct NEMD simulations of tubes extending up to $L=10\mu \mathrm{m}$ are demonstrated to be insufficient to settle the question of whether a fully diffusive heat transport regime and an intrinsic value of TC exist for CNTs. Noting that acoustic phonon lifetimes lie at the heart of a saturation of TC with tube length as per the BTE framework, we complement the quantum mechanical prediction of acoustic phonon lifetimes with an analysis of phonon modes in the framework of equilibrium MD. A normal mode analysis with an emphasis on long-wavelength acoustic modes corroborates the BTE prediction that heat transport in CNTs in the long tube limit is governed by the low attenuation rates of longitudinal and twisting phonons.

Flexural modes play an important role in mechanical, thermal, and electronic properties of two-dimensional materials. Graphene nanoelectromechanical systems have been found in wide applications and have attracted huge attention recently, where the molecular dynamics (MD) simulation has been an essential route for the investigation of the mechanical responses of the system. In this paper, based on the expressions and parameters of the reactive empirical bond-order potential for the carbon-carbon atomic interactions implemented in Large-scale Atomic/Molecular Massively Parallel Simulator, the linearized part of the force for each atom along the $z$ direction has been obtained, yielding the stiffness matrix of the graphene lattice. By diagonalizing the stiffness matrix, the flexural modes and their corresponding frequencies can be obtained. The results have been validated with MD simulations in circular and square graphene resonators with different sizes. Since the stiffness matrix for graphene resonators with arbitrary shapes can be obtained readily from our results, we expect broad applications where eigenfrequencies and flexural modes are needed in the analysis for the nanoscale resonators.

In spite of their importance for understanding phonon transport phenomena in thin films and polycrystalline solids, the effects of boundary roughness scattering on phonon specularity and coherence are poorly understood because there is no general method for predicting their dependence on phonon momentum, frequency, branch, and boundary morphology. Using the recently formulated atomistic $S$-matrix method, we develop a theory of boundary roughness scattering to determine the mode-resolved phonon coherence and specularity parameters from the scattering amplitudes. To illustrate the theory, we apply it to phonon scattering in realistic nonsymmetric graphene grain boundary (GB) models derived from atomic structure predictions. The method is validated by comparing its predictions with frequency-resolved results from lattice dynamics-based calculations. We prove that incoherent scattering is almost perfectly diffusive. We show that phonon scattering at the graphene GB is not diffuse, although coherence and specularity are significantly reduced for long-wavelength flexural acoustic phonons. Our approach can be generalized to other atomistic boundary models.

Recently ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$ has been reported as a spin-orbital entangled quantum spin liquid (QSL) [K. Kitagawa et al., Nature (London) 554, 341 (2018)], albeit its connection to Kitaev QSL has not been yet identified. To unveil the related Kitaev physics, we perform Raman spectroscopy studies on single-crystalline ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$ samples. We implement a soft chemical replacement of ${\mathrm{Li}}^{+}$ with ${\mathrm{H}}^{+}$ from $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ single crystals to synthesize the single crystal samples of the iridate second-generation ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$. The unusual phonon broadening in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$ implies the random positions of the substituted ${\mathrm{H}}^{+}$. As well as the lattice dynamics, Raman spectroscopy can also be used to diagnose the QSL state since the magnetic Raman continuum arises from a process involving pairs of fractionalized Majorana fermionic excitation in a pure Kitaev model. We observe a broad dome-shaped magnetic continuum in ${\mathrm{H}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$, in line with theoretical expectations for the two-spin process in the Kitaev QSL. The possible scenario of the structural disordered magnet for the magnetic Raman continuum is also discussed.

Electronic correlation has been shown to play an important role in generating plasmons unconventionally in strongly correlated electron systems. In this work, we calculate the band structure, complex dielectric function, reflectivity, and loss function of ${\mathrm{La}}_2{\mathrm{CuO}}_4$ in its insulating and antiferromagnetic phase through first-principle calculations. We find strong evidence of unconventional plasmons that are generated by hybridizations and enhanced by on-site Coulomb correlations. Interestingly, these unconventional plasmons, predominantly located in the Cu-O planes, are driven by hybridizations between $\mathrm{La}5p$/$\mathrm{Cu}3d$ and out-of-plane ${\mathrm{O}}_{\mathrm{z}}2p$. On the other hand, in-plane oxygen ${\mathrm{O}}_{\mathrm{xy}}2p$ induces conventional plasmons. This nonuniform hybridization scheme creates an anisotropic complex dielectric function and loss function, which can be influenced by conventional plasmons, unconventional plasmons, or a mixed state between both. Our result shows that the interplay of hybridizations, particularly involving oxygen, and on-site Coulomb correlations plays an important role in determining the properties of unconventional plasmons in strongly correlated ${\mathrm{La}}_2{\mathrm{CuO}}_4$.

Recently, the experimental realization of a quantum point contact (QPC) between helical edges has been accomplished. We predict that such a QPC, in the presence of a time periodic applied electric field, is characterized by a topological quantum phase transition in the Floquet spectrum. Crucially, the fact that the QPC is naturally attached to helical leads allows for the unambiguous detection of the topological Floquet quantum phase transition by means of multiterminal transport experiments. Moreover, such experiments enable the characterization of the topological bound states associated with the transition.

Weyl semimetals are topological materials that provide a condensed-matter realization of the chiral anomaly. A positive longitudinal magnetoconductance quadratic in magnetic field has been promoted as a diagnostic for this anomaly. By solving the Boltzmann equation analytically, we show that the magnetoconductance can become negative in the experimentally relevant semiclassical regime of weak magnetic fields. This effect is due to the simultaneous presence of the Berry phase and the orbital magnetic moment of carriers and occurs for sufficiently strong intervalley scattering.

We examine the metal-insulator transition in a half-filled Hubbard model of electrons with random and all-to-all hopping and exchange, and an on-site nonrandom repulsion, the Hubbard $U$. We argue that recent numerical results of Cha et al. [arXiv:2002.07181] can be understood in terms of a deconfined critical point between a disordered Fermi liquid and an insulating spin glass. We find a deconfined critical point in a previously proposed large $M$ theory, which generalizes the SU(2) spin symmetry to $\mathrm{SU}\left(M\right)$ and obtains exponents for the electron and spin correlators which agree with those of Cha et al. This critical point is expected to have the maximal quantum Lyapunov exponent. We also present a renormalization group analysis, and argue for the presence of an additional metallic spin glass phase at half-filling and small $U$.

We derive general rigorous results relating revivals in the dynamics of quantum many-body systems to the entanglement properties of energy eigenstates. For a $D$-dimensional lattice system of $N$ sites initialized in a low-entangled and short-range correlated state, our results show that a perfect revival of the state after a time of at most $O\left(\mathrm{poly}\right(N\left)\right)$ implies the existence of at least $O(\sqrt{N}/{log}^{2D}\left(N\right))$ “quantum many-body scars”: energy eigenstates with energies placed in an equally spaced ladder and with Rényi entanglement entropy of at most $O(log(N\left)\right)+O\left(\right|\partial A\left|\right)$ for any region $A$ of the lattice. This shows that quantum many-body scars are a necessary consequence of revivals, independent of particularities of the Hamiltonian leading to them. We also present results for approximate revivals and for revivals of expectation values of observables and prove that the duration of revivals of states has to become vanishingly short with increasing system size.

The fabrication of integrated quantum dot (QD)-optical microcavity systems is a requisite step for the realization of a wide range of nanophotonic experiments (and applications) that exploit the ability of QDs to emit nonclassical light, e.g., single photons. Thanks to their ∼20-nm positioning accuracy and to their proven potential for single-photon operation, the QDs obtained by spatially selective hydrogen irradiation of dilute-nitride semiconductors —such as Ga(AsN) and Ga(PN)—are uniquely suited for integration with photonic nanodevices. In the present work, we demonstrate the ability to deterministically integrate single, site-controlled Ga(AsN)/Ga(AsN):H QDs within a photonic crystal (PhC) cavity. The properties of the fabricated QD-PhC cavity systems are then probed by photon correlation—providing clear evidence of single-photon emission—and time-resolved microphotoluminescence spectroscopy. Detailed information on the dynamics of our integrated nanodevices can be inferred by comparing these experiments to the solutions of a rate-equations system, developed by taking into account all the main processes leading to the capture, relaxation, and recombination of carriers in and out of the QD. This allows us to follow the evolution of the relevant recombination rates in our system for varying energy detuning, Δ$E$, between the QD and the PhC cavity. When the QD exciton transition is nearly resonant with the cavity mode, a large (>tenfold) enhancement of the spontaneous emission rate is observed, in substantial agreement with Jaynes-Cummings (JC) theory. For intermediate detunings (Δ$E$ ∼ 1.5–3.5 meV), on the other hand, the observed enhancement is significantly larger than that predicted by JC theory, due to the important role played by acoustic phonons in mediating the QD-PhC cavity coupling in a solid-state environment. Apart from its fundamental interest, the observation of such phonon-mediated, broadband enhancement of light-matter interaction significantly relaxes the requirements for the realization of a large variety of cavity QED-based experiments and applications. These include many photonic devices for which the use of site-controlled Ga(AsN)/Ga(AsN):H QDs would be inherently advantageous, such as those based on the coupling between more than one QD and a single cavity mode (e.g., few-QD nanolasers and QD solids).

Manipulating the valley degree of freedom provides a novel paradigm for future valleytronics. Here we use first-principles approaches and the $kp$ model analyses to carry out a comparative study of ten van der Waals (vdW) heterostructures constructed by transition-metal dichalcogenides and recently discovered two-dimensional (2D) ferromagnetic (FM) insulators. Our calculations show that the $\mathrm{MoT}{\mathrm{e}}_2/\mathrm{CrB}{\mathrm{r}}_3$ heterostructure hosts a large valley splitting of $\sim 28.7\mathrm{meV}$, which is much larger than that observed in the $\mathrm{WS}{\mathrm{e}}_2/\mathrm{Cr}{\mathrm{I}}_3$ heterostructure $\left(\sim 3.5\mathrm{meV}\right)$. Specifically, the spin-orbital coupling can induce band inversions in $\mathrm{WS}{\mathrm{e}}_2/\mathrm{CrB}{\mathrm{r}}_3$ and $\mathrm{MoT}{\mathrm{e}}_2/\mathrm{CrB}{\mathrm{r}}_3$ heterostructures due to strong charge transfer across the interfaces of these two systems. Most strikingly, $\mathrm{WS}{\mathrm{e}}_2/\mathrm{CrB}{\mathrm{r}}_3$ is characterized as a valley-polarized quantum anomalous Hall effect system with Chern number $C=-1$ at the $K$ point while $C=0$ at the $K^{\prime }$ point. All those findings together with recent progress in 2D FM insulators point out an experimentally achievable scheme for exploration of abundant valley-polarized states in 2D FM vdW heterostructures.