We demonstrate the existence of an insulating phase in the three-legged Hubbard ladder at two-thirds filling. In this phase chargons are bound because the physics within a unit cell favors the formation of triplets. The resultant moments lead to a ground state in the Haldane phase, a symmetry protected topological state of matter. In this purely fermionic model, reflection is protecting but time-reversal and dihedral symmetries are not, in contrast to spin models.

We investigate the ground-state nature of the transverse field Ising model on the $J_1\text{−}{J}_2$ square lattice at the highly frustrated point $J_2/J_1=0.5$. At zero field, the model has an exponentially large degenerate classical ground state, which can be affected by quantum fluctuations for nonzero field toward a unique quantum ground state. We consider two types of quantum fluctuations, harmonic ones by using linear spin-wave theory (LSWT) with single-spin-flip excitations above a long-range magnetically ordered background and anharmonic fluctuations, by employing a cluster-operator approach (COA) with multispin cluster-type fluctuations above a nonmagnetic cluster-ordered background. Our findings reveal that the harmonic fluctuations of LSWT fail to lift the extensive degeneracy as well as signaling a violation of the Hellmann-Feynman theorem. However, the string-type anharmonic fluctuations of COA are able to lift the degeneracy toward a string valence-bond-solid (VBS) state, which is obtained from an effective theory consistent with the Hellmann-Feynman theorem as well. Our results are further confirmed by implementing numerical tree tensor network simulation. The emergent nonmagnetic string VBS phase is gapped and breaks lattice rotational symmetry with only twofold degeneracy, which bears a continuous quantum phase transition at $\mathrm{\Gamma }/J_1\cong 0.50$ to the quantum paramagnet phase of high fields. The critical behavior is characterized by $\nu \cong 1.0$ and $\gamma \cong 0.33$ exponents.

Using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) spectroscopy, we evaluated the orbital magnetic moments of itinerant ferromagnet $\mathrm{SrRu}{\mathrm{O}}_3$ (SRO) epitaxial thin films with interfacially controlled magnetic anisotropy. We found that the orbital moment is closely correlated with the SRO's magnetic anisotropy, which can be controlled by interfacially engineering the $\mathrm{Ru}{\mathrm{O}}_6$ octahedral rotations. For the monoclinic film with magnetization along the direction 45° from the out-of-plane direction, the orbital moment is $\approx -0.1{\mu }_{\mathrm{B}}/\mathrm{Ru}$ along the magnetic easy axis direction and is aligned antiparallel to the direction of the spin magnetic moments. For the tetragonal film with in-plane magnetization, on the other hand, the out-of-plane component of the orbital moment is as small as $\approx -0.04{\mu }_{\mathrm{B}}/\mathrm{Ru}$, accounting for the film's in-plane magnetic anisotropy. Our results highlight that the magnetic anisotropy of SRO can be controlled by engineering the orbital magnetic moment through the octahedral distortions.

The martensitic start temperature $M_{\text{s}}$ is one of the key characteristics of shape memory materials. High-temperature shape memory alloys are a special class of materials where transformation temperatures between the martensite and austenite phase above 373 K are desirable. For the design of new high-temperature shape memory alloys it is therefore important to understand and predict the dependence of $M_{\text{s}}$ on the composition of the material. Using density functional theory in combination with the quasiharmonic Debye model, we evaluate the different contributions to the free energy to determine the transition temperature $T_0$ over a wide range of compositions in Ti-Ta alloys. Our approach provides physical insight into the various contributions that explain the strong composition dependence of $M_{\text{s}}$ that is observed experimentally. Based on our calculations, we identify the relative phase stability at $T=0$ K and the vibrational entropy difference between the involved phases as critical parameters to predict changes in $T_0$. We propose a simple, one-dimensional descriptor to estimate the transition temperature that can be used in the identification of new alloys suitable for high-temperature shape memory applications.

The role of defects in van der Waals heterostructures made of graphene and hexagonal boron nitride (h-BN) is studied using a combination of ab initio and model calculations. Despite the weak van der Waals interaction between layers, defects residing in h-BN, such as carbon impurities and antisite defects, reveal a hybridization with graphene $p_{\mathrm{z}}$ states, leading to midgap state formation. The induced midgap states modify the transport properties of graphene and can be reproduced by means of a simple effective tight-binding model. In contrast to carbon defects, it is found that oxygen defects do not strongly hybridize with graphene's low-energy states. Instead, oxygen drastically modifies the band gap of graphene, which emerges in a commensurate stacking on h-BN lattices.

The scaling of antiferromagnetic ordering temperature of corundum-type chromia films has been investigated. Néel temperature $T_N$ was determined from the effect of perpendicular exchange bias on the magnetization of a weakly-coupled adjacent ferromagnet. For a thick-film case, the validity of detection is confirmed by a susceptibility measurement. Detection of $T_N$ was possible down to 1-nm-thin chromia films. The scaling of ordering temperature with thickness was studied using different buffering materials and compared with Monte-Carlo simulations. The spin-correlation length and the corresponding critical exponent were estimated, and they were consistent between experimental and simulation results. The spin-correlation length is an order of magnitude less than cubic antiferromagnets. We propose that the difference is from the change of number of exchange-coupling links in the two crystal systems.

We study the magnetic properties of the adatom systems on a semiconductor surface Si(111):{C,Si,Sn,Pb}-$(\sqrt{3}\times \sqrt{3})$. On the basis of all-electron density functional theory calculations we construct effective low-energy models taking into account spin-orbit coupling and electronic correlations. The Hartree-Fock simulations for the unit cell with nine correlated orbitals put forward insulating ground states with the noncollinear ${120}^{\circ }$-Néel (for C, Si, Sn monolayer coverages) and ${120}^{\circ }$-row-wise (for Pb adatom) antiferromagnetic orderings. The corresponding spin Hamiltonians with anisotropic exchange interactions are derived by means of the superexchange theory and the calculated Dzyaloshinskii-Moriya interactions in the systems with Sn and Pb adatoms are revealed to be very strong and compatible with the isotropic exchange couplings. To simulate the excited magnetic states we solve the constructed spin models by means of the Monte Carlo method, where at low temperatures and zero magnetic field we observe complex spin spiral patterns in Sn/Si(111) and Pb/Si(111). On this basis the formation of antiferromagnetic skyrmion lattice states at high magnetic fields in the adatom $sp$ electron systems is discussed.

We report on the observation of nonlinear magnetoelectric effects at room temperature due to a dc current in the ferrimagnetic $M$-type strontium hexaferrite platelets. Utilizing microwave measurement techniques and data on the shift in magnetic mode frequencies, it was found that a dc current along the hexagonal $c$ axis resulted in a significant decrease in the saturation magnetization and an increase in the uniaxial magnetocrystalline anisotropy field. These changes in the magnetic order parameters were directly proportional to the square of applied electric field and were found to be much higher than variations due to Joule heating. A phenomenological theory that takes into account the current-induced magnetobielectric (MBE) effects is proposed. Expressions for coupling coefficients for MBE effects have been obtained and have been calculated from the variations in magnetic order parameters. The electric field $E$ (or current) tuning of the magnetic modes in $\mathrm{Sr}M$ reported here is orders of magnitude stronger than strain mediated $E$ tuning of magnetic resonance in hexaferrite-ferroelectric composites. The nonlinear magnetoelectric effects in hexaferrite, therefore, open up an avenue for the realization of $E$-tunable broadband microwave and millimeter wave ferrite signal processing devices such as resonators and filters.

The structure and superconductivity of $\mathrm{WT}{\mathrm{e}}_2$ under pressure are investigated using ab initio calculations combined with high-pressure synchrotron x-ray diffraction and Raman spectroscopy. We find that the emergence of superconductivity in $\mathrm{WT}{\mathrm{e}}_2$ under pressure can be attributed to the phase transition from ambient $T_d$ phase to the monoclinic $1T$′ structure phase at around 4–5 GPa, which is associated with a sliding of the $\mathrm{WT}{\mathrm{e}}_2$ layers, resulting in a critical point in the changes of Te-Te interlayer distance. This phase transition introduces an inversion center and eliminates the topological Weyl fermions in the $T_d$ structure. Electron-phonon coupling calculations predict a similar $T_c$ as the reported value, implying that $\mathrm{WT}{\mathrm{e}}_2$ might belong to conventional normal Bardeen-Cooper-Schrieffer superconductors.

Recent experiments on $\mathrm{G}{\mathrm{d}}^{3+}$ electron-spin resonance (ESR) in the filled skutterudite $\mathrm{C}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{d}}_x\mathrm{F}{\mathrm{e}}_4{\mathrm{P}}_{12}\left(x\approx 0.001\right)$, at temperatures where the host resistivity manifests a smooth insulator-metal crossover, provide evidence of the underlying Kondo physics associated with this system. At low temperatures (below $T\approx 160$ K), $\mathrm{C}{\mathrm{e}}_{1-x}\mathrm{G}{\mathrm{d}}_x\mathrm{F}{\mathrm{e}}_4{\mathrm{P}}_{12}$ behaves as a Kondo insulator with a relatively large hybridization gap, and the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR spectra display a fine structure with Lorentzian line shape, typical of insulating media. In this work, based on previous experiments performed by the same group, we argue that the electronic gap may be attributed to the large hybridization present in the coherent regime of a Kondo lattice. Moreover, mean-field calculations suggest that the electron-phonon interaction is fundamental at explaining such hybridization. The resulting electronic structure is strongly temperature dependent, and at $T^{*}\approx 160\mathrm{K}$ the system undergoes an insulator-to-metal transition induced by the withdrawal of $4f$ electrons from the Fermi volume, the system becoming metallic and nonmagnetic. The $\mathrm{G}{\mathrm{d}}^{3+}$ ESR fine structure coalesces into a single Dysonian resonance, as in metals. Our simulations suggest that exchange narrowing via the usual Korringa mechanism is not enough to describe the thermal behavior of the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR spectra in the entire temperature region (4.2–300 K). We propose that the temperature activated fluctuating valence of the Ce ions is the key ingredient that fully describes this unique temperature dependence of the $\mathrm{G}{\mathrm{d}}^{3+}$ ESR fine structure

Quantum Monte Carlo (QMC) algorithms have long relied on Jastrow factors to incorporate dynamic correlation into trial wave functions. While Jastrow-type wave functions have been widely employed in real-space algorithms, they have seen limited use in second-quantized QMC methods, particularly in projection methods that involve a stochastic evolution of the wave function in imaginary time. Here we propose a scheme for generating Jastrow-type correlated trial wave functions for auxiliary-field QMC methods. The method is based on decoupling the two-body Jastrow into one-body projectors coupled to auxiliary fields, which then operate on a single determinant to produce a multideterminant trial wave function. We demonstrate that intelligent sampling of the most significant determinants in this expansion can produce compact trial wave functions that reduce errors in the calculated energies. Our technique may be readily generalized to accommodate a wide range of two-body Jastrow factors and applied to a variety of model and chemical systems.

The two-dimensional two-orbital Hubbard model is studied with the use of a finite-size cluster world-line quantum Monte Carlo algorithm. This model is widely used for simulation of the band structure of FeAs clusters, which are structure elements of Fe-based high-temperature superconductors. The choice of a special basis set of hypersites allowed us to take into account four-fermion operator terms and to overcome partly the sign problem. Spectral functions and the density of states for various parameters of the model are obtained in the undoped and low-doped regimes. The correlated distortion of the spectral density with the change of doping is observed, and the applicability of the “hard-band” approximation in the doped regime is demonstrated. Profiles of the momentum distribution are obtained for the first Brillouin zone; they have pronounced jump near the Fermi level, which decreases with the growth of the strength of the interaction. The invariance of the Fermi surface with respect to the strength of the interaction is testified. Nesting is found in the case of electron and hole doping. Fermi-liquid parameters of the model are derived. The $Z$ factor grows sharply with the increasing of the level of doping and monotonously decreases with the growth of the strength of the interaction. Moreover, electron-hole doping asymmetry of the $Z$ factor is revealed. The non-Fermi-liquid behavior and the deviation from Luttinger theorem are observed.

Recent experiments motivated by solar light harvesting applications have brought a renewed interest in $\mathrm{AgBi}{\mathrm{S}}_2$ as an environmentally friendly material with appealing photovoltaic properties. The lack of detailed knowledge on its bulk structural and electronic structure however inhibits further development of this material. Here we have investigated by first-principles quantum mechanical methods models of the two most commonly reported $\mathrm{AgBi}{\mathrm{S}}_2$ crystal structures, the room temperature matildite structure, and the metastable schapbachite. Density functional theory (DFT) based calculations using the Perdew-Burke-Ernzerhof exchange-correlation (xc) functional reveal that matildite can be 0.37 eV per $\mathrm{AgBi}{\mathrm{S}}_2$ stoichiometry unit more stable than a schapbachite structure in bulk, and that the latter, in its ordered form, may display a metallic electronic structure, precluding its use for solar light harvesting. This points out the fact that $\mathrm{AgBi}{\mathrm{S}}_2$ nanocrystals used in solar cells should present a structure based on matildite. Matildite is found to be an indirect gap semiconductor, with an estimated band gap of $\sim 1.5\mathrm{eV}$ according to DFT based calculations using the more accurate hybrid xc functionals. These reveal that hole effective mass is twice that of electron effective mass, with concomitant consequences for the generated exciton. Hybrid DFT calculations also show that matildite has a high dielectric constant pertinent to that of an ionic semiconductor and slightly higher than that of PbS, a material that has been extensively used in solar cells in its nanocrystalline form. The calculated Bohr exciton radius of 4.6 nm and the estimated absorption coefficient of ${10}^5\mathrm{c}{\mathrm{m}}^{-1}$ within the solar light spectrum are well in line with those experimentally reported in the literature.

The effect of Bi incorporation on the conduction band structure of Ga(AsBi) alloys is revealed by a direct estimation of the electron effective mass via cyclotron resonance absorption spectroscopy at THz frequencies in pulsed magnetic fields up to 65 T. A strong enhancement in the electron effective mass with increasing Bi content is reported, with a value of mass $\sim 40\%$ higher than that in GaAs for $\sim 1.7\%$ of Bi. This experimental evidence unambiguously indicates a Bi-induced perturbation of the host conduction band states and calls for a deep revision of the theoretical models describing dilute bismides currently proposed in the literature, the majority of which neglect or exclude that the incorporation of a small percentage of Bi may affect the conduction band states of the host material.

Time-resolved transmission and reflection measurements were performed for bulk GaN at room temperature to evaluate the energy of the first conduction band satellite valley. The measurements showed clear threshold-like spectra for transmission decay and reflection rise times. The thresholds were associated with the onset of the intervalley electron scattering. Transmission measurements with pump and probe pulses in the near infrared produced an intervalley energy of $0.97\pm 0.02$ eV. Ultraviolet pump and infrared probe reflection provided a similar value. Comparison of the threshold energies obtained in these experiments allowed estimating the hole effective mass in the upper valence band to be $1.4m_0$. Modeling of the reflection transients with rate equations has allowed estimating electron–LO (longitudinal optical) phonon scattering rates and the satellite valley effective mass.

First-principles calculations have been performed to understand the structural and electronic properties of wurtzite (wz) phase of cation mixed ${\mathrm{Cu}}_2{\mathrm{Cd}}_x{\mathrm{Zn}}_{1-x}{\mathrm{SnS}}_4$ (CCZTS) alloys which have a band gap that fits the requirement of a solar cell light absorber. We have used five different exchange correlations (XCs) to fix the structural parameters with high accuracy. The hybrid XC, which has been used to explain thermodynamical, electronic, and optical properties, shows very promising results. We find that (i) the CCZTS alloys are highly miscible with low formation enthalpies, (ii) the band gap of CCZTS alloys can be tuned in the range 1.56–1.39 eV as Cd concentration $\left(\mathit{x}\right)$ increases from 0.0 to 1.0 with a small bowing parameter $b=0.293$ eV, and (iii) the calculated band gap of CCZTS alloys decreases mainly due to the upshift of the valence band. Our predicted results are in agreement with the only available synthesized wz phase CCZTS data [Ramasamy et al., RSC Adv. 3, 1186 (2013)]. The dependence of the CCZTS alloys energy on growth temperature is also calculated. Our results could be very useful for growth of these materials.

Ordered submonolayers of adsorbate atoms on semiconductor surfaces constitute a playground for electronic correlation effects, which are tightly connected to the local atomic arrangement and the corresponding vibration eigenmodes. We report on a study of the vibration eigenmodes of Au-covered Si(111) surfaces with $(5\times 2)$ reconstruction using polarized Raman spectroscopy and first-principles calculations. Upon Au coverage, the vibration eigenmodes of the clean reconstructed Si(111)-$(7\times 7)$ surface are quenched and replaced by new eigenmodes, determined by the Au-$(5\times 2)$ reconstruction. Several polarization-dependent surface eigenmodes emerge in the spectral range from 25 to $120{\mathrm{cm}}^{-1}$, with the strongest ones at 29, 51, and $106{\mathrm{cm}}^{-1}$. In our first-principles calculations we have determined the vibration frequencies, the corresponding elongation patterns, and the Raman intensities for two different structure models currently discussed in the literature. The best agreement with the experimental results is achieved for a model with 0.7 monolayer coverage and seven Au atoms per unit cell, proposed by S. G. Kwon and M. H. Kang [Phys. Rev. Lett. 113, 086101 (2014)].

We investigate the nature of the ordering among magnetic adatoms, randomly deposited on the surface of topological insulators. Restricting ourselves to dilute impurity and weak coupling (between itinerant fermion and magnetic impurities) limit, we show that for arbitrary amount of chemical doping away from the apex of the surface Dirac cone the magnetic impurities tend to arrange themselves in a spin-density-wave pattern, with the periodicity approximately $\pi /k_F$, where $k_F$ is the Fermi wave vector, when magnetic moment for impurity adatoms is isotropic. However, when magnetic moment possesses strong Ising or easy-axis anisotropy, pursuing both analytical and numerical approaches we show that the ground state is ferromagnetic for low to moderate chemical doping, despite the fragmentation of the system into multiple ferromagnetic islands. For high doping away from the Dirac point as well, the system appears to fragment into many ferromagnetic islands, but the magnetization in these islands is randomly distributed. Such magnetic ordering with net zero magnetization is referred to here as ferromagnetic spin glass, which is separated from the pure ferromagnet state by a first order phase transition. We generalize our analysis for cubic topological insulators (supporting three Dirac cones on a surface) and demonstrate that the nature of magnetic orderings and the transition between them remains qualitatively the same. We also discuss the possible relevance of our analysis to recent experiments.

We consider a semi-infinite spatially dispersive dielectric with unequal transverse and longitudinal susceptibilities. The effect of the boundary is characterized by arbitrary reflection coefficients for polarization waves in the material that propagate to the surface. Specific values of these coefficients correspond to various additional boundary conditions (ABCs) for Maxwell's equations. We derive the electromagnetic reflection and transmission coefficients at the boundary and investigate their dependence on material parameters and ABCs. We also investigate the electromagnetic zero-point and thermal spectral energy density outside the dielectric. The nonlocal response removes the boundary divergence of the spectral energy density that is present in a local model. The spectral energy density shows a large dependence on the difference between the transverse and longitudinal susceptibilities, even at distances up to 10 nm from the boundary.