We construct the phase diagram of the chiral magnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ in a dc magnetic field $\left(H_{\mathrm{dc}}\right)$ using ac magnetic susceptibility measurements. At $H_{\mathrm{dc}}=0$, the ac response at the transition from the helical magnetic (HM) state to the paramagnetic (PM) state consists of a giant third-order harmonic component $\left(M_{3\omega }\right)$ and a first-order harmonic component $\left(M_{1\omega }\right)$. By applying $H_{\mathrm{dc}}$ perpendicular to the $c$ axis, the HM state is transformed to the chiral soliton lattice (CSL) state, which is a superlattice tuned by $H_{\mathrm{dc}}$. The above giant $M_{3\omega }$ is markedly suppressed at small $H_{\mathrm{dc}}$. The CSL state is found to consist of CSL-1, with dominant helical texture and a poor ferromagnetic array, and CSL-2, with a large ferromagnetic array. The transition between CSL-1 and the PM state causes a linear magnetic response, the dominant component of which is the in-phase $M_{1\omega }$. With increasing temperature, CSL-2 is transformed into the forced ferromagnetic (FFM) state, and ultimately the PM state is reached. The transition between CSL-2 and the FFM state consists of a large $M_{3\omega }$ and large out-of-phase $M_{1\omega }$ as well as in-phase $M_{1\omega }$. The transition between the FMM and PM states also yields a linear magnetic response, like the CSL-1–PM-state transition. Five typical magnetic dynamics in the transitions among the HM state, CSL-1, CSL-2, FFM state, and PM state were identified according to the equivalent dynamical motion equation of a nonlinear spring model.

Atomically engineered oxide heterointerfaces show a range of novel phenomena not present in their bulk form, thus providing an additional knob to tune the functional properties across such interfaces. Here we show that for an oxide Schottky interface between metallic ${\mathrm{SrRuO}}_3$ and semiconducting Nb doped ${\mathrm{SrTiO}}_3$ (Nb:STO) the terminating surface of the substrate plays a crucial role in determining the electronic transport across it. Interestingly this is achieved by engineering a monolayer of a charge-neutral SrO layer across the Schottky interface and not by the insertion of charged layers at the interface, as has been demonstrated earlier. These changes in the energy-band line-up across a symmetric interface indicate the presence of different polar characters at the SrO and ${\mathrm{TiO}}_2$ terminations of the substrate. Owing to the presence of the same AO layer (SrO) in both ${\mathrm{SrRuO}}_3$ and Nb:STO, we propose an intermixing of Ru and Ti ions across the interface for the SrO terminated Nb:STO substrate. First-principles density functional theory calculations on these systems conform with our experimental findings, and indicate a resultant shift in the interfacial atomic plane stacking of ${\mathrm{SrRuO}}_3$ at the intermixed interface, leading to a modification of the electrostatic potential at the interface. The interdependence between the interfacial band alignment to the local atomic plane displacement across an intrinsically nonpolar oxide heterointerface is a key mechanism in realizing novel electronic properties in oxide based devices.

Monte Carlo simulations are a powerful tool for elucidating the properties of complex systems, but when applied to fermionic quantum systems, quantum Monte Carlo (QMC) algorithms suffer from the so-called “negative sign problem,” causing the computational effort to grow exponentially with problem size. Here we demonstrate that the fermion sign problem originates in topological properties of the configurations. Using the widely used determinantal approaches, that remove the trivial sign problem due to particle exchange, we prove that the negative sign of a configuration is a topological invariant—an imaginary time counterpart of the Aharonov-Anandan phase—and reduces to a Berry phase in the adiabatic limit. This provides an intriguing connection between QMC simulations and classification of topological states.

Boson condensation in topological quantum field theories (TQFT) has been previously investigated through the formalism of Frobenius algebras and the use of vertex lifting coefficients. While general, this formalism is physically opaque and computationally arduous: analyses of TQFT condensation are practically performed on a case by case basis and for very simple theories only, mostly not using the Frobenius algebra formalism. In this paper, we provide a way of treating boson condensation that is computationally efficient. With a minimal set of physical assumptions, such as commutativity of lifting and the definition of confined particles, we can prove a number of theorems linking Boson condensation in TQFT with chiral algebra extensions, and with the factorization of completely positive matrices over ${\mathbb{Z}}_{+}$. We present numerically efficient ways of obtaining a condensed theory fusion algebra and $S$ matrices; and we then use our formalism to prove several theorems for the $S$ and $T$ matrices of simple current condensation and of theories which upon condensation result in a low number of confined particles. We also show that our formalism easily reproduces results existent in the mathematical literature such as the noncondensability of five and ten layers of the Fibonacci TQFT.

Machine learning techniques are applied to make prediction models of the $G_0{W}_0$ band gaps for 270 inorganic compounds using Kohn-Sham (KS) band gaps, cohesive energy, crystalline volume per atom, and other fundamental information of constituent elements as predictors. Ordinary least squares regression (OLSR), least absolute shrinkage and selection operator, and nonlinear support vector regression (SVR) methods are applied with two levels of predictor sets. When the KS band gap by generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) or modified Becke-Johnson (mBJ) is used as a single predictor, the OLSR model predicts the $G_0{W}_0$ band gap of randomly selected test data with the root-mean-square error (RMSE) of 0.59 eV. When KS band gap by PBE and mBJ methods are used together with a set of predictors representing constituent elements and compounds, the RMSE decreases significantly. The best model by SVR yields the RMSE of 0.24 eV. Band gaps estimated in this way should be useful as predictors for virtual screening of a large set of materials.

We propose a method for analyzing two-dimensional symmetry-protected topological (SPT) wave functions using a correspondence with conformal field theories (CFTs) and integrable lattice models. This method generalizes the CFT approach for the fractional quantum Hall effect wherein the wave-function amplitude is written as a many-operator correlator in the CFT. Adopting a bottom-up approach, we start from various known microscopic wave functions of SPTs with discrete symmetries and show how the CFT description emerges at large scale, thereby revealing a deep connection between group cocycles and critical, sometimes integrable, models. We show that the CFT describing the bulk wave function is often also the one describing the entanglement spectrum, but not always. Using a plasma analogy, we also prove the existence of hidden quasi-long-range order for a large class of SPTs. Finally, we show how response to symmetry fluxes is easily described in terms of the CFT.

We report first-principles calculations of the energetic stability and electronic properties of metal-phthalocyanine (MPc) molecules (M = Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed on the $\delta$-doped Si(111)-$\mathrm{B}(\sqrt{3}\times \sqrt{3})$ reconstructed surface. (i) It can be seen that CrPc, MnPc, FePc, and CoPc are chemically anchored to the topmost Si atom. (ii) Contrastingly, the binding of the NiPc, CuPc, and ZnPc molecules to the $\mathrm{Si}\left(111\right)\text{−}\mathrm{B}(\sqrt{3}\times \sqrt{3})$ surface is exclusively ruled by van der Waals interactions, the main implication being that these molecules may diffuse and rearrange to form clusters and/or self-organized structures on this surface. The electronic structure calculations reveal that in point (i), owing to the formation of the metal-Si covalent bond, the net magnetic moment of the molecule is quenched by $1{\mu }_{\mathrm{B}}$, remaining unchanged in point (ii). In particular, the magnetic moment of CuPc $\left(1{\mu }_{\mathrm{B}}\right)$ is preserved after adsorption. Finally, we verify that the formation of ZnPc, CuPc, and NiPc molecular (self-assembled) arrangements on the Si(111)-$\mathrm{B}(\sqrt{3}\times \sqrt{3}$) surface is energetically favorable, in good agreement with recent experimental findings.

The search for half-metals and spin-gapless semiconductors has attracted extensive attention in material design for spintronics. Existing progress in such a search often requires peculiar atomistic lattice configuration and also lacks active control of the resulting electronic properties. Here we reveal that a boron nitride nanoribbon with a carbon-doped edge can be made a half-metal or a spin-gapless semiconductor in a programmable fashion. The mechanical strain serves as the on/off switches for functions of half-metal and spin-gapless semiconductor to occur. Our findings shed light on how the edge doping combined with strain engineering can affect electronic properties of two-dimensional materials.

A theoretical description of the charge transfer (CT) exciton across the donor-acceptor interface without the use of a completely localized hole (or electron) is a challenge in the field of organic solar cells. We calculate the total wave function of the CT exciton by solving an effective two-particle Schrödinger equation for the inhomogeneous dielectric interface. We formulate the magnitude of the CT and construct a minimal model of the CT exciton under the breakdown of inversion symmetry. We demonstrate that both a light hole mass and a hole localization along the normal to the dielectric interface are crucial to yield the CT exciton.

By quantum Monte Carlo simulations of bosons in gapped honeycomb lattices, we show the existence of bosonic edge states. For a single layer honeycomb lattice, bosonic edge states can be controlled to appear, cross the gap, and merge into bulk states by an on-site potential applied on the outermost sites of the boundary. On a bilayer honeycomb lattice, A bosonic edge state traversing the gap at half filling is demonstrated. The topological origin of the bosonic edge states is discussed with pseudo Berry curvature. The results will simulate experimental studies of these exotic bosonic edge states with ultracold bosons trapped in honeycomb optical lattices.

We propose a series of simple two-dimensional (2D) lattice interacting fermion models that we demonstrate at low energy describe bosonic symmetry-protected topological (SPT) states and quantum phase transitions between them. This is because due to interaction, the fermions are gapped both at the boundary of the SPT states and at the bulk quantum phase transition, thus these models at low energy can be described completely by bosonic degrees of freedom. We show that the bulk of these models is described by a $\mathrm{Sp}(N$) principal chiral model with a topological $\mathrm{\Theta }$ term, whose boundary is described by a $\mathrm{Sp}(N$) principal chiral model with a Wess-Zumino-Witten term at level 1. The quantum phase transition between SPT states in the bulk is tuned by a particular interaction term, which corresponds to tuning $\mathrm{\Theta }$ in the field theory, and the phase transition occurs at $\mathrm{\Theta }=\pi$. The simplest version of these models with $N=1$ is equivalent to the familiar O(4) nonlinear sigma model (NLSM) with a topological term, whose boundary is a $(1+1)\text{D}$ conformal field theory with central charge $c=1$. After breaking the O(4) symmetry to its subgroups, this model can be viewed as bosonic SPT states with U(1), or $Z_2$ symmetries, etc. All of these fermion models, including the bulk quantum phase transitions, can be simulated with the determinant quantum Monte Carlo method without the sign problem. Recent numerical results strongly suggest that the quantum disordered phase of the O(4) NLSM with precisely $\mathrm{\Theta }=\pi$ is a stable $(2+1)\text{D}$ conformal field theory with gapless bosonic modes.

The material ${\mathrm{ZrTe}}_3$ is a well-known example of an incommensurate periodic lattice distortion (PLD) at low temperatures due to a charge density wave (CDW). Previous studies have found a sharp boundary as a function of pressure between CDW below 5 GPa and bulk superconductivity above this value. We present a study of low-temperature-high-pressure single crystal x-ray diffraction along with ab initio density functional theory calculations. The modulation vector $q_{\text{CDW}}$ is found to change smoothly with pressure until the PLD is lost. Fermi surface calculations reproduce these changes, but neither these nor the experimental crystal lattice structure show a particular step change at 5 GPa, thus leading to the conclusion that the CDW is lost accidentally by tipping the balance of CDW formation in the Fermi surface nesting that stabilizes it.

We argue that topological Kondo insulators can also have “intrinsic” topological order associated with fractionalized excitations on their surfaces. The hybridization between the local moments and conduction electrons can weaken near the surface, and this enables the local moments to form spin liquids. This coexists with the conduction electron surface states, realizing a surface fractionalized Fermi liquid. We present mean-field solutions of a Kondo-Heisenberg model in two spatial dimensions which display such surfaces.

In this work, we propose a systematic way of computing a low-rank globally adapted localized Tucker-tensor basis for solving the Kohn-Sham density functional theory (DFT) problem. In every iteration of the self-consistent field procedure of the Kohn-Sham DFT problem, we construct an additive separable approximation of the Kohn-Sham Hamiltonian. The Tucker-tensor basis is chosen such as to span the tensor product of the one-dimensional eigenspaces corresponding to each of the spatially separable Hamiltonians, and the localized Tucker-tensor basis is constructed from localized representations of these one-dimensional eigenspaces. This Tucker-tensor basis forms a complete basis, and is naturally adapted to the Kohn-Sham Hamiltonian. Further, the locality of this basis in real-space allows us to exploit reduced-order scaling algorithms for the solution of the discrete Kohn-Sham eigenvalue problem. In particular, we use Chebyshev filtering to compute the eigenspace of the Kohn-Sham Hamiltonian, and evaluate nonorthogonal localized wave functions spanning the Chebyshev filtered space, all represented in the Tucker-tensor basis. We thereby compute the electron-density and other quantities of interest, using a Fermi-operator expansion of the Hamiltonian projected onto the subspace spanned by the nonorthogonal localized wave functions. Numerical results on benchmark examples involving pseudopotential calculations suggest an exponential convergence of the ground-state energy with the Tucker rank. Interestingly, the rank of the Tucker-tensor basis required to obtain chemical accuracy is found to be only weakly dependent on the system size, which results in close to linear-scaling complexity for Kohn-Sham DFT calculations for both insulating and metallic systems. A comparative study has revealed significant computational efficiencies afforded by the proposed Tucker-tensor approach in comparison to a plane-wave basis.

For the purposes of making reliable first-principles predictions of defect energies in semiconductors, it is crucial to distinguish between effective-mass-like defects, which cannot be treated accurately with existing supercell methods, and deep defects, for which density functional theory calculations can yield reliable predictions of defect energy levels. The gallium antisite defect ${\mathrm{Ga}}_{As}$ is often associated with the 78/203 meV shallow double acceptor in Ga-rich gallium arsenide. Within a conceptual framework of level patterns, analyses of structure and spin stabilization can be used within a supercell approach to distinguish localized deep defect states from shallow acceptors such as ${\mathrm{B}}_{As}$. This systematic approach determines that the gallium antisite supercell results has signatures inconsistent with an effective mass state and cannot be the 78/203 shallow double acceptor. The properties of the Ga antisite in GaAs are described, total energy calculations that explicitly map onto asymptotic discrete localized bulk states predict that the Ga antisite is a deep double acceptor and has at least one deep donor state.

We demonstrated theoretically that a circularly polarized electromagnetic field substantially modifies electronic properties of a periodical chain of quantum rings. Particularly, the field opens band gaps in the electron energy spectrum of the chain, generates edge electron currents, and induces the Fano-like features in the electron transport through the finite chain. These effects create physical prerequisites for the development of optically controlled nanodevices based on a set of coupled quantum rings.

A new ice polymorph, called ice XVI, has recently been discovered experimentally by extracting the guest molecules from Ne hydrate. The ice and its filled form (clathrate hydrate) have a unique network topology which results in several interesting properties. Here we provide a theoretical method to calculate thermodynamic properties of a semiopen system in equilibrium with guest gas and thus occupancy of the guest can be varied with temperature and pressure. Experimental observations such as the disappearance of negative thermal expansivity and contraction of the host lattice upon encaging guest molecules are well reproduced, and those behaviors are elucidated in terms of the free energy of cage occupation and its temperature and pressure dependence. We propose an application of the method for preparing ice XVI to create metastable clathrate hydrates having intriguing properties with much lower occupancy of guest molecules than that at equilibrium, which otherwise cannot form.

This paper presents the results of an investigation of the magnetic and structural properties of ${\text{Mn}}_{1-x}{\text{Fe}}_x\text{As}$ compounds under hydrostatic pressure and chemical doping. The chemical doping was performed by using low Fe doping levels $(x=0$, 0.003, 0.006, 0.010, 0.015, and 0.018), which emulates the negative pressure effect on the crystal structure. The results of this approach were compared with the physical pressure effect (hydrostatic pressure from 0 to 2.2 kbar) on the ${\text{Mn}}_{0.997}{\text{Fe}}_{0.003}\text{As}$. Both approaches exhibit the same magnetic behaviors: the $T_C$ and saturation magnetization decrease as the pressure increases; for the highest pressure studied, an orthorhombic antiferromagnetic phase occurs below the critical temperature and coexists with the ferromagnetic hexagonal phase. The equivalence between hydrostatic pressure and chemical doping indicates that the Fe doping only causes structural deformation. In addition, we performed magnetic measurements at high temperature (up to 520 K) on the samples with $x=0$ and 0.003 in order to investigate the magnetic behavior above $T_C=310$ K. These results, along with structural characterization, clearly show that between $T_C$ and $T_t$ the system is a weak antiferromagnet with short-range order confined only in the $ab$ plane. Finally, using the low- and high-temperature data, the magnetic phase diagrams of the compound under hydrostatic pressure and chemical doping were redrawn.