Lead halide perovskite semiconductors are soft, polar materials. The strong driving force for polaron formation (the dielectric electron-phonon coupling) is balanced by the light band effective masses, leading to a strongly-interacting large polaron. A first-principles prediction of mobility would help understand the fundamental mobility limits. Theories of mobility need to consider the polaron (rather than free-carrier) state due to the strong interactions. In this material we expect that at room temperature polar-optical phonon mode scattering will dominate and so limit mobility. We calculate the temperature-dependent polaron mobility of hybrid halide perovskites by variationally solving the Feynman polaron model with the finite-temperature free energies of Ōsaka. This model considers a simplified effective-mass band structure interacting with a continuum dielectric of characteristic response frequency. We parametrize the model fully from electronic-structure calculations. In methylammonium lead iodide at $300\mathrm{K}$ we predict electron and hole mobilities of 133 and $94{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively. These are in acceptable agreement with single-crystal measurements, suggesting that the intrinsic limit of the polaron charge carrier state has been reached. Repercussions for hot-electron photoexcited states are discussed. As well as mobility, the model also exposes the dynamic structure of the polaron. This can be used to interpret impedance measurements of the charge-carrier state. We provide the phonon-drag mass renormalization and scattering time constants. These could be used as parameters for larger-scale device models and band-structure dependent mobility simulations.

Charge transport is crucial to the performance of hybrid halide perovskite solar cells. A theoretical model based on large polarons is proposed to elucidate charge transport properties in the perovskites. Critical new physical insights are incorporated into the model, including the recognitions that acoustic phonons as opposed to optical phonons are responsible for the scattering of the polarons; these acoustic phonons are fully excited due to the “softness” of the perovskites, and the temperature-dependent dielectric function underlies the temperature dependence of charge carrier mobility. This work resolves key controversies in literature and forms a starting point for more rigorous first-principles predictions of charge transport.

We achieve an explicit construction of the lowest Landau level (LLL) projected wave functions for composite fermions in the periodic (torus) geometry. To this end, we first demonstrate how the vortex attachment of the composite fermion (CF) theory can be accomplished in the torus geometry to produce the “unprojected” wave functions satisfying the correct (quasi)periodic boundary conditions. We then consider two methods for projecting these wave functions into the LLL. The direct projection produces valid wave functions but can be implemented only for very small systems. The more powerful and more useful projection method of Jain and Kamilla fails in the torus geometry because it does not preserve the periodic boundary conditions and thus takes us out of the original Hilbert space. We have succeeded in constructing a modified projection method that is consistent with both the periodic boundary conditions and the general structure of the CF theory. This method is valid for a large class of states of composite fermions, called “proper states,” which includes the incompressible ground states at electron filling factors $\nu =\frac{n}{2pn+1}$, their charged and neutral excitations, and also the quasidegenerate ground states at arbitrary filling factors of the form $\nu =\frac{{\nu }^{*}}{2p{\nu }^{*}+1}$, where $n$ and $p$ are integers and ${\nu }^{*}$ is the CF filling factor. Comparison with exact results known for small systems for the ground and excited states at filling factors $\nu =1/3$, 2/5, and 3/7 demonstrates our LLL-projected wave functions to be extremely accurate representations of the actual Coulomb eigenstates. Our construction enables the study of large systems of composite fermions on the torus, thereby opening the possibility of investigating numerous interesting questions and phenomena.

We report direct and unambiguous evidence of the existence of inner semiconducting tube (ISCT) photoluminescence (PL) from measurements performed on four individual freestanding index-identified double-walled carbon nanotubes (DWNTs). Based on thorough Rayleigh scattering, Raman scattering, and PL experiments, we are able to demonstrate that the ISCT PL is observed with a quantum yield estimated to be a few ${10}^{-6}$ independent of the semiconducting or metallic nature of the outer tube. This result is mainly attributed to ultrafast exciton transfer from the inner to outer tube. Furthermore, by carrying out PL excitation experiments on the (14,1)@(15,12) DWNT, we show that the ISCT PL can be detected through the optical excitation of the outer tube, indicating that the exciton transfer can also occur in the opposite way.

We show that spin angular momentum (SAM) of light is transferred to matter together with momentum and energy, and obtain the expression for the corresponding torque. This can lead to appearance of spin forces, which correspond to the discrepancy between the Lorentz and Einstein-Laub forces. In plasmonic metals these spin forces result in a plasmogalvanic phenomenon, which is pinning of the plasmon-induced electromotive force to an atomically thin layer at the metal interface.

Searching for two-dimensional (2D) organic Dirac materials, which have more adaptable practical applications compared with inorganic ones, is of great significance and has been ongoing. However, only two such materials with low Fermi velocity have been discovered so far. Herein, we report the design of an organic monolayer with ${\mathrm{C}}_4{\mathrm{N}}_3\mathrm{H}$ stoichiometry that possesses fascinating structure and good stability in its free-standing state. More importantly, we demonstrate that this monolayer is a semimetal with anisotropic Dirac cones and very high Fermi velocity. This Fermi velocity is roughly one order of magnitude larger than the largest velocity ever reported in 2D organic Dirac materials, and it is comparable to that in graphene. The Dirac states in this monolayer arise from the extended $\pi$-electron conjugation system formed by the overlapping $2p_z$ orbitals of carbon and nitrogen atoms. Our finding paves the way to a search for more 2D organic Dirac materials with high Fermi velocity.

We study the effects of a truncated band structure on the linear and nonlinear optical response of crystals using four methods. These are constructed by (i) choosing either the length or velocity gauge for the perturbation and (ii) computing the current density either directly or via the time derivative of the polarization density. In the infinite-band limit, the results of all four methods are identical, but basis truncation breaks their equivalence. In particular, certain response functions vanish identically and unphysical low-frequency divergences are observed for few-band models in the velocity gauge. Using a hexagonal boron nitride (hBN) monolayer as a case study, we analyze the problems associated with all methods and identify the optimal one. Our results show that the length-gauge calculations provide the fastest convergence rates as well as the most accurate spectra for any basis size and, moreover, that low-frequency divergences are eliminated.

With the use of nuclear-spin-free materials such as silicon and germanium, spin-based quantum bits (qubits) have evolved to become among the most coherent systems for quantum information processing. The new frontier for spin qubits has therefore shifted to the ubiquitous charge noise and spin-orbit interaction, which are limiting the coherence times and gate fidelities of solid-state qubits. In this paper we investigate superexchange, as a means of indirect exchange interaction between two single electron spin qubits, each embedded in a single semiconductor quantum dot (QD), mediated by an intermediate, empty QD. Our results suggest the existence of “supersweet spots”, in which the qubit operations implemented by superexchange interaction are simultaneously first-order-insensitive to charge noise and to errors due to spin-orbit interaction. The proposed spin-qubit architecture is scalable and within the manufacturing capabilities of semiconductor industry.

We report on the magnetic field dependence of half-integer quantized conductance plateaus (HQPs) in InAs quantum wires. We observe HQPs at zero applied magnetic field in InAs quantum wires fabricated from a high-quality InAs quantum well. The application of in-plane magnetic field causes Zeeman splitting of the HQP features, indicating that the origin of the observed HQP is not spontaneous spin polarization. Additionally we observe that the conductance of the split HQPs decreases gradually as the in-plane magnetic field increases. We finally assume electron-electron interaction as a possible mechanism to account for the zero-field HQPs and the anomalous field dependence.

We propose that the properties of the capacity of entanglement (COE) in gapless systems can efficiently be investigated through the use of the distribution of eigenvalues of the reduced density matrix (RDM). The COE is defined as the fictitious heat capacity calculated from the entanglement spectrum. Its dependence on the fictitious temperature can reflect the low-temperature behavior of the physical heat capacity and thus provide a useful probe of gapless bulk or edge excitations of the system. Assuming a power-law scaling of the COE with an exponent $\alpha$ at low fictitious temperatures, we derive an analytical formula for the distribution function of the RDM eigenvalues. We numerically test the effectiveness of the formula in a relativistic free scalar boson in two spatial dimensions and find that the distribution function can detect the expected $\alpha =3$ scaling of the COE much more efficiently than the raw data of the COE. We also calculate the distribution function in the ground state of the half-filled Landau level with short-range interactions and find better agreement with the $\alpha =2/3$ formula than with the $\alpha =1$ one, which indicates a non-Fermi-liquid nature of the system.

We theoretically address spin chain analogs of the Kitaev quantum spin model on the honeycomb lattice. The emergent quantum spin-liquid phases or Anderson resonating valence-bond (RVB) states can be understood, as an effective model, in terms of $p$-wave superconductivity and Majorana fermions. We derive a generalized phase diagram for the two-leg ladder system with tunable interaction strengths between chains allowing us to vary the shape of the lattice (from square to honeycomb ribbon or brickwall ladder). We evaluate the winding number associated with possible emergent (topological) gapless modes at the edges. In the $A_z$ phase, as a result of the emergent ${\mathbf{Z}}_2$ gauge fields and $\pi$-flux ground state, one may build spin-1/2 (loop) qubit operators by analogy to the toric code. In addition, we show how the intermediate gapless $B$ phase evolves in the generalized ladder model. For the brick-wall ladder, the $B$ phase is reduced to one line, which is analyzed through perturbation theory in a rung tensor product states representation and bosonization. Finally, we show that doping with a few holes can result in the formation of hole pairs and leads to a mapping with the Su-Schrieffer-Heeger model in polyacetylene; a superconducting-insulating quantum phase transition for these hole pairs is accessible, as well as related topological properties.

We study the crossover between the sudden quench limit and the adiabatic dynamics of superconducting states in the attractive Hubbard model. We focus on the dynamics induced by the change of the attractive interaction during a finite ramp time. The ramp time is varied in order to track the evolution of the dynamical phase diagram from the sudden quench to the equilibrium limit. Two different dynamical regimes are realized for quenches towards weak and strong coupling interactions. At weak coupling the dynamics depends only on the energy injected into the system, whereas a dynamics retaining memory of the initial state takes place at strong coupling. We show that this is related to a sharp transition between a weak and a strong coupling quench dynamical regime, which defines the boundaries beyond which a dynamics independent from the initial state is recovered. Comparing the dynamics in the superconducting and nonsuperconducting phases, we argue that this is due to the lack of an adiabatic connection to the equilibrium ground state for nonequilibrium superconducting states in the strong coupling quench regime.

We study the effect of Hund's splitting of repulsive interactions on electronic phase transitions in the multiorbital topological crystalline insulator ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ when the chemical potential is tuned to the vicinity of low-lying type-II Van Hove singularities. Nontrivial Berry phases associated with the Bloch states impart momentum dependence to electron interactions in the relevant band. We use a multipatch parquet renormalization-group (RG) analysis to study the competition of different electronic phases, and we find that if the dominant fixed-point interactions correspond to antiparallel spin configurations, then a chiral $p$-wave Fulde-Ferrell-Larkin-Ovchinnikov state is favored, otherwise none of the commonly encountered electronic instabilities occurs within the one-loop parquet RG approach.

We theoretically investigate the enhancement of surface enhanced Raman spectroscopy (SERS) using hyperbolic stratified nanostructures and compare to metal nanoresonators. The photon Green function of each nanostructure within its environment is first obtained from a semianalytical modal theory, which is used in a quantum optics formalism of the molecule-nanostructure interaction to model the SERS spectrum. An intuitive methodology is presented for calculating the single-molecule enhancement factor (SMEF), which is also able to predict known experimental SERS enhancement factors of a gold nanodimer. We elucidate the important figures-of-merit of the enhancement and explore these for different designs. We find that the use of hyperbolic stratified materials can enhance the photonic local density of states (LDOS) by close to two times in comparison to pure metal nanostructures, when both designed to work at the same operating wavelengths. However, the increased LDOS is accompanied by higher electric field concentration within the lossy hyperbolic material, which leads to increased quenching that serves to reduce the overall detected SERS enhancement in the far field. For nanoresonators with resonant localized surface plasmon wavelengths in the near-infrared, the SMEF for the hyperbolic stratified nanostructure is approximately one order of magnitude lower than the pure metal counterpart. Conversely, we show that by detecting the Raman signal using a near-field probe, hyperbolic materials can provide an improvement in SERS enhancement compared to using pure metal nanostructures when the probe is sufficiently close ($<50\mathrm{nm}$) to the Raman active molecule at the plasmonic hotspot.

Unlike other two-dimensional (2D) isovalent materials, the 2D group IV monochalcogenides, $MX$ ($M=\mathrm{Si}$, Ge, Sn, and Pb; $X=\mathrm{S}$, Se, and Te), are found to be either in a black phosphorene-derived distorted NaCl-type ($d$-NaCl) structure or a recently predicted $Pma2$ structure. Both $M$ and $X$ atoms in the $d$-NaCl structure are threefold coordinated, whereas $M$ and $X$ in the $Pma2$ structure are fourfold and twofold coordinated, respectively. Using first-principles total energy and electronic structure calculations and a global structural search technique, we systematically investigated the mechanism underlying the polymorphism of the 2D group-IV monochalcogenides. Our analysis show that the relative stability of the two distinct crystallographic phases depends on the strength of the $M-M$ covalent bond and the electronegativity difference between the constituent elements $M$ and $X$. For small cations, the covalency plays more important role, whereas for large cations the Coulomb interaction becomes more dominant. Therefore, the $\mathrm{Si}X$ and $\mathrm{Ge}X$ compounds assume the $Pma2$ structure, whereas the $MX$ compounds with heavy cation elements ($M=\mathrm{Sn}$ and Pb) tend to adopt the $d$-NaCl structure.

Based on first-principle calculations and $k·p$ model analysis, we show that the quantum anomalous Hall (QAH) insulating phase can be realized in the functionalized hematite (or $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3)$ nanosheet, and the obtained topological gap can be as large as $\sim 300\mathrm{meV}$. The driving force of the topological phase is the strong interactions of localized Fe $3d$ electrons operating on the quadratic band crossing point of the noninteracting band structures. Such an interaction driven QAH insulator is different from the single particle band topology mechanism in the experimentally realized QAH insulator, the magnetic ion doped topological insulator film. Depending on the thickness of the nanosheet, a topological insulating state with helical-like or chiral edge states can be realized. Our work provides a realization of the interaction-driven QAH insulating state in a realistic material.

We study the two-step spin crossover in a double-layered elastic model based on transition metal complexes each taking high spin (HS) and low spin (LS) states. Here, only the simplest elastic interactions between adjacent molecules are considered, and the system is exposed to the external pressure within the framework of NPT-Monte Carlo method. As a certain amount of pressure is applied, the first order thermal transition between uniform HS and LS phases transforms to a two-step transition with an emergent intermediate spin (IS) phase, where the HS and LS molecules are paired face to face between layers and form diagonally striped clusters within the layer. The difference in the size of HS and LS molecules is reflected both in the elastic interactions and in the enthalpy, and the IS phase could gain the latter over the loss of the former by significantly reducing its volume. The present pressure effect is interpreted to the chemical one in double-layered transition metal materials, which actually reveals a variety of multistep spin crossover transitions relevant to our numerical result.

The carbon vacancy in $4H$-SiC is a powerful minority carrier recombination center in as-grown material and a major cause of degradation of SiC-based devices. Despite the extensiveness and maturity of the literature regarding the characterization and modeling of the defect, many fundamental questions persist. Among them, we have the shaky connection of the EPR data to the electrical measurements lacking sublattice site resolution, the physical origin of the pseudo-Jahn-Teller effect, the reasoning for the observed sublattice dependence of the paramagnetic states, and the severe temperature dependence of some hyperfine signals, which cannot be accounted for by a thermally activated dynamic averaging between equivalent Jahn-Teller distorted structures. In this work, we address these problems by means of semilocal and hybrid density functional calculations. We start by inventorying a total of four different vacancy structures from the analysis of relative energies. Diamagnetic states have well defined low-energy structures, whereas paramagnetic states display metastability. The reasoning for the rich structural variety is traced back to the filling of electronic states which are shaped by a crystal-field-dependent (and therefore site-dependent) pseudo-Jahn-Teller effect. From calculated minimum energy paths for defect rotation and transformation mechanisms, combined with the calculated formation energies and electrical levels, we arrived at a configuration-coordinate diagram of the defect. The diagram provides us with a detailed first-principles picture of the defect when subject to thermal excitations. The calculated acceptor and donor transitions agree well with the binding energies of electrons emitted from the ${\mathrm{Z}}_{1/2}$ and ${\mathrm{EH}}_{6/7}$ traps, respectively. From the comparison of calculated and measured $U$-values, and correlating the site-dependent formation energies with the relative intensity of the DLTS peaks in as-grown material, we assign ${\mathrm{Z}}_1$ (${\mathrm{EH}}_6$) and ${\mathrm{Z}}_2$ (${\mathrm{EH}}_7$) signals to acceptor (donor) transitions of carbon vacancies located on the $h$ and $k$ sublattice sites, respectively.

The photoinduced martensitic (MT) transition and reverse transition in a shape memory alloy $\mathrm{M}{\mathrm{n}}_{50}\mathrm{N}{\mathrm{i}}_{40}\mathrm{S}{\mathrm{n}}_{10}$ have been examined by using high spatiotemporal resolution four-dimensional transmission electron microscopy (4D-TEM), and the experimental results clearly demonstrate that the MT transition and reverse transition in this Heusler alloy contain a variety of structural dynamic features at picosecond time scales. The 4D-TEM imaging and diffraction observations clearly show that MT transition and MT domain nucleation, which are related to cooperative atomic motions, occur at between 10 and 20 ps, depending on the thickness of the sample. Moreover, a strong coupling between the MT transition and lattice breathing mode is discovered in this system, which can result in a periodic structural oscillation between the MT phase and austenitic (AUS) phase. This allows us to directly observe the MT nucleation and domain wall motions in transient states using high spatiotemporal imaging. A careful analysis of the ultrafast images demonstrates the presence of remarkable transient states, which exhibit the essential features of MT nucleation, lattice symmetry breaking, and a rapid growth of MT plates. These results not only provide insights into the time-resolved structural dynamics and elementary mechanisms that govern the MT transition but also contribute to the development of a novel technique for future 4D-TEM investigations.

Recent inelastic neutron scattering (INS) measurements on FeSe and $\mathrm{Fe}({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$) have sparked intense debate over the nature of the ground state in these materials. Here we propose an effective bilinear-biquadratic spin model, which is shown to consistently describe the evolution of low-energy spin excitations in FeSe, both under applied pressure and upon Se/Te substitution. The phase diagram, studied using a combination of variational mean-field, flavor-wave calculations and density-matrix renormalization group (DMRG), exhibits a sequence of transitions between the columnar antiferromagnet common to the iron pnictides, the nonmagnetic ferroquadrupolar phase attributed to FeSe, and the double-stripe antiferromagnetic order known to exist in ${\mathrm{Fe}}_{1+y}\mathrm{Te}$. The calculated spin structure factor in these phases mimics closely that observed with INS in the $\mathrm{Fe}({\mathrm{Te}}_{1-x}{\mathrm{Se}}_x$) series. In addition to the experimentally established phases, the possibility of incommensurate magnetic order is also predicted.