Based on first principles total energy calculations, we investigate geometric, electronic, and spin structures in two-dimensional polyacene-based hydrocarbon networks of $s{p}^2$ and $s{p}^3$ C atoms. Polyacenes connecting adjacent $s{p}^3$ atoms form kagome networks. The networks are stable and retain their covalent topologies up to 2000 K. They possess kagome flatbands at or near the Fermi level, depending on the size and structure of the polyacene adjoining the $s{p}^3$ C atoms. We find that the spin states of the two-dimensional covalent hydrocarbon network are changeable from antiferromagnetic to ferromagnetic by increasing the length of the $s{p}^2$ C region.

Using a combination of magnetometry and magnetotransport techniques, we studied temperature and magnetic-field behavior of magnetization in Py/Gd heterostructures. It was shown quantitatively that proximity with Py enhances magnetic order of Gd. Micromagnetic simulations demonstrate that a spin-flop transition observed in a Py/Gd bilayer is due to exchange-spring rotation of magnetization in the Gd layer. Transport measurements show that the magnetoresistance of a ${[\mathrm{Py}\left(2\mathrm{nm}\right)/\mathrm{Gd}\left(2\mathrm{nm}\right)]}_{25}$ multilayer changes sign at the compensation temperature and below 20 K. The positive magnetoresistance above the compensation temperature can be attributed to an in-plane domain wall, which appears because of the structural inhomogeneity of the film over its thickness. By measuring the angular dependence of resistance, we are able to determine the angle between magnetizations in the multilayer and the magnetic field at different temperatures. The measurements reveal that, due to a change in the chemical thickness profile, a noncollinear magnetization configuration is only stable in magnetic fields above 10 kOe.

We present a modification of the magnetization and an electronic reconfiguration due to interfacial coupling between strain relaxed ${\mathrm{La}}_{0.66}{\mathrm{Ca}}_{0.34}{\mathrm{MnO}}_3$ and ultrathin ${\mathrm{BaTiO}}_3$ films using the magnetocaloric effect and photoemission electron microscopy at the Mn, Ti $L_{2,3}$ edges. The addition of a top ${\mathrm{BaTiO}}_3$ layer leads to strain-induced enhancement of the magnetocaloric effect in ${\mathrm{La}}_{0.66}{\mathrm{Ca}}_{0.34}{\mathrm{MnO}}_3$, due to the structural transitions in ${\mathrm{BaTiO}}_3$, and to room temperature ferromagnetism due to electronic reconfiguration at the interface.

It has recently been shown that there exists a class of stable gapless spin liquids in 3+1 dimensions described by higher-rank tensor U(1) gauge fields, giving rise to an emergent tensor electromagnetism. The tensor gauge field of these theories couples naturally to subdimensional particles (such as fractons), which are restricted by gauge invariance to move only along lower-dimensional subspaces of the system. Here we work out some of the basic generalized electromagnetic properties of subdimensional particles coupled to tensor electromagnetism, such as generalized electrostatic fields, potential formulations, Lorentz forces, Maxwell equations, and Biot-Savart laws. Some concepts from conventional electromagnetism will carry over directly, while others require significant modification.

We study the effects of spatial dispersion in disordered noncentrosymmetric metals. Specifically, we consider the kinetic magnetoelectric effect, and natural optical activity of metals, as well as the so-called dynamic chiral magnetic effect as a particular case of the latter. These effects stem from the magnetic moments of quasiparticles near the Fermi surface. We identify new disorder-induced contributions to these magnetic moments that come from the skew-scattering and side-jump processes, familiar from the theory of anomalous Hall effect. We show that at low frequencies the spatial dispersion of the conductivity tensor comes mainly either from the skew scattering or intrinsic contribution, and there is always a region of frequencies in which the intrinsic mechanism dominates. Our results imply that in clean three-dimensional metals, current-induced magnetization is in general determined by impurity skew scattering, rather than intrinsic contributions. Intrinsic effects are expected to dominate in cubic enantiomorphic crystals with point groups $T$ and $O$, and in polycrystalline samples, regardless of their mobility.

We use the Kotliar-Ruckenstein slave-boson formalism to study the temperature dependence of paramagnetic phases of the one-band Hubbard model for a variety of band structures. We calculate the Fermi liquid quasiparticle spectral weight $Z$ and identify the temperature at which it decreases significantly to a crossover to a bad metal region. Near the Mott metal-insulator transition, this coherence temperature $T_{\mathrm{coh}}$ is much lower than the Fermi temperature of the uncorrelated Fermi gas, as is observed in a broad range of strongly correlated electron materials. After a proper rescaling of temperature and interaction, we find a universal behavior that is independent of the band structure of the system. We obtain the temperature-interaction phase diagram as function of doping, and we compare the temperature dependence of the double occupancy, entropy, and charge compressibility with previous results obtained with dynamical mean-field theory. We analyze the stability of the method by calculating the charge compressibility.

We study the size-dependent exciton fine structure in monolayer black phosphorus quantum dots (BPQDs) deposited on different substrates (isolated, Si, and ${\mathrm{SiO}}_2$) using a combination of the tight-binding method to calculate single-particle states and the configuration interaction formalism to determine the excitonic spectrum. We demonstrate that the substrate plays a dramatic role in the excitonic gaps and excitonic spectrum of the QDs. For reasonably high dielectric constants (${\varepsilon }_{\text{sub}}\sim {\varepsilon }_{\text{Si}}=11.7{\varepsilon }_0$), the excitonic gap can be described by a single power law $E_X\left(R\right)=E_X^{\left(\text{bulk}\right)}+C/R^{\gamma }$. For low dielectric constants ${\varepsilon }_{\text{sub}}\le {\varepsilon }_{{\text{SiO}}_2}=3.9{\varepsilon }_0$, the size dependence of the excitonic gaps requires the sum of two power laws $E_X\left(R\right)=E_g^{\left(\text{bulk}\right)}+A/R^n-B/R^m$ to describe both strong and weak quantum confinement regimes, where $A, B, C, \gamma , n$, and $m$ are substrate-dependent parameters. We also predict that the exciton lifetimes exhibit a strong temperature dependence, ranging between 2–8 ns (Si substrate) and 3–11 ns (${\mathrm{SiO}}_2$ substrate) for QDs up 10 nm in size.

Two-dimensional group-V elemental materials have attracted widespread attention due to their nonzero band gap while displaying high electron mobility. Using first-principles calculations, we propose a series of new elemental bilayers with group-V elements (Bi, Sb, As). Our study reveals the dynamical stability of four-, six-, and eight-atom ring structures, demonstrating their possible coexistence in such bilayer systems. The proposed structures for Sb and As are large-gap semiconductors that are potentially interesting for applications in future nanodevices. The Bi structures have nontrivial topological properties with a direct nontrivial band gap. The nontrivial gap is shown to arise from a band inversion at the Brillouin zone center due to the strong intrinsic spin-orbit coupling in Bi atoms. Moreover, we demonstrate the possibility of tuning the properties of these materials by enhancing the ratio of six-atom rings to four- and eight-atom rings, which results in wider nontrivial band gaps and lower formation energies.

The diffraction of fast atoms at crystal surfaces is ideal for a detailed investigation of the surface electronic density. However, instead of sharp diffraction spots, most experiments show elongated streaks characteristic of inelastic diffraction. This paper describes these inelastic profiles in terms of individual inelastic collisions with surface atoms taking place along the projectile trajectory and leading to vibrational excitation of the local Debye oscillator. A quasielastic regime where only one inelastic event contributes is identified as well as a mixed quantum-classical regime where several inelastic collisions are involved. These regimes describe a smooth evolution of the scattering profiles from sharp spots to elongated streaks merging progressively into the classical diffusion regime.

We theoretically investigate a topological Kitaev chain connected to a double quantum-dot (QD) setup hybridized with metallic leads. In this system we observe the emergence of two striking phenomena: (i) a decrypted Majorana fermion (MF) qubit recorded over a single QD, which is detectable by means of conductance measurements due to the asymmetrical MF-qubit leaked state into the QDs; (ii) an encrypted qubit recorded in both QDs when the leakage is symmetrical. In such a regime, we have a cryptographylike manifestation, since the MF qubit becomes bound states in the continuum, which is not detectable in conductance experiments.

$\alpha -{\mathrm{RuCl}}_3$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4\mathrm{K}$ in applied magnetic fields up to $9\mathrm{T}$ for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to ${\mu }_0{H}_{\text{c}}\approx 6.9$ T. Above $H_{\text{c}}$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point at $H_{\text{c}}$; this is supported by universal scaling behavior near $H_{\text{c}}$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below 1 meV, above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a $J_1-K_1-{\mathrm{\Gamma }}_1-J_3$ honeycomb model.

We present a computational study of $2p$ core-level x-ray photoemission spectra of transition metal monoxides $M\mathrm{O}$ ($M=\text{Ni}$,Co,Mn) and sesquioxides $M_2{\mathrm{O}}_3$ ($M=\text{V}$,Cr,Fe) using a theoretical framework based on the local-density approximation $+$ dynamical mean-field theory. We find a very good description of the fine spectral features, which is a considerable improvement over the conventional cluster model. We analyze the role of nonlocal screening and its relationship to long-range magnetic order and lattice geometry. Our results reveal the potential of the present method for the analysis and interpretation of modern high-energy-resolution experiments.

In the presence of parallel electric and magnetic fields, the violation of a separate number conservation laws for the three-dimensional left- and right-handed Weyl fermions is known as the chiral anomaly. The recent discovery of Weyl and Dirac semimetals has paved the way for experimentally testing the effects of chiral anomaly via magnetotransport measurements, since chiral anomaly can lead to negative longitudinal magnetoresistance (LMR) while the transverse magnetoresistance remains positive. More recently, a type-II Weyl semimetal (WSM) phase has been proposed, where the nodal points possess a finite density of states due to the touching between electron and hole pockets. It has been suggested that the main difference between the two types of WSMs (type I and type II) is that in the latter, chiral-anomaly-induced negative LMR (positive longitudinal magnetoconductance) is strongly anisotropic, vanishing when the applied magnetic field is perpendicular to the direction of tilt of Weyl fermion cones in a type-II WSM. We analyze chiral anomaly in a type-II WSM in a quasiclassical Boltzmann framework, and find that the chiral-anomaly-induced positive longitudinal magnetoconductivity is present along any arbitrary direction. Thus, our results are pertinent for uncovering transport signatures of type-II WSMs in different candidate materials.

Nonequilibrium growth of Si-III-V or Si-II-VI alloys is a promising approach to obtaining optically more active Si-based materials. We propose a new class of nonisovalent $\mathrm{S}{\mathrm{i}}_2\mathrm{AlP}$ (or $\mathrm{S}{\mathrm{i}}_2\mathrm{ZnS}$) alloys in which the Al-P (or Zn-S) atomic chains are as densely packed as possible in the host Si matrix. As a hybrid of the lattice-matched parent phases, $\mathrm{S}{\mathrm{i}}_2\mathrm{AlP}$ (or $\mathrm{S}{\mathrm{i}}_2\mathrm{ZnS}$) provides an ideal material system with tunable local chemical orders around Si atoms within the same composition and structural motif. Here, using first-principles hybrid functional calculations, we discuss how the local chemical orders affect the electronic and optical properties of the nonisovalent alloys.

We investigated the dependence of the magnetic response from epitaxial Si-rich iron-silicide nanostructures on their geometry. By varying substrate orientation and deposition parameters, we altered the growth kinetics and the lattice matching conditions at the silicide/silicon interface. These affected the silicide nanoisland crystal structure, size, shape, and proximity due to spatial ordering and, consequently, their magnetic response in terms of shape and opening of the respective hysteresis loops. In particular, we demonstrated correlation between magnetic anisotropy, expressed as the hysteresis coercive field, and the nanoisland spatial length-to-width aspect ratio. This correlation is explained by the contribution of undercoordinated island edge atoms to the overall measured magnetic behavior of the nanoisland arrays. Further, the island self-ordering along periodic surface steps adds dipolar interactions between the otherwise superparamagnetic nanoislands, consequently resulting in a magnetic response resembling that of a superspin glass.

Cluster expansion (CE) is effective in modeling the stability of metallic alloys, but sometimes cluster expansions fail. Failures are often attributed to atomic relaxation in the DFT-calculated data, but there is no metric for quantifying the degree of relaxation. Additionally, numerical errors can also be responsible for slow CE convergence. We studied over one hundred different Hamiltonians and identified a heuristic, based on a normalized mean-squared displacement of atomic positions in a crystal, to determine if the effects of relaxation in CE data are too severe to build a reliable CE model. Using this heuristic, CE practitioners can determine a priori whether or not an alloy system can be reliably expanded in the cluster basis. We also examined the error distributions of the fitting data. We find no clear relationship between the type of error distribution and CE prediction ability, but there are clear correlations between CE formalism reliability, model complexity, and the number of significant terms in the model. Our results show that the size of the errors is much more important than their distribution.

For this paper, single-pulse ablation mechanisms of ultrafast laser pulses ($25\mathrm{ps}$) were studied for thin gold films ($65\mathrm{nm}$) on an array of substrates with varying physical properties. Using time-domain thermoreflectance, the interfacial properties of the thin-film systems are measured: in particular, the thermal boundary conductance. We find that an often used, and widely accepted relation describing threshold fluences of homogeneous bulk targets breaks down at the nanoscale. Rather than relying solely on the properties of the ablated Au film, the ablation threshold of these Au/substrate systems is found to be dependent on the measured thermal boundary conductance; we additionally find no discernible trend between the damage threshold and properties of the underlying substrate. These results are discussed in terms of diffusive thermal transport and the interfacial bond strength.

The Loschmidt echo, defined as the overlap between quantum wave function evolved with different Hamiltonians, quantifies the sensitivity of quantum dynamics to perturbations and is often used as a probe of quantum chaos. In this work we consider the behavior of the Loschmidt echo in the many-body localized phase, which is characterized by emergent local integrals of motion and provides a generic example of nonergodic dynamics. We demonstrate that the fluctuations of the Loschmidt echo decay as a power law in time in the many-body localized phase, in contrast to the exponential decay in few-body ergodic systems. We consider the spin-echo generalization of the Loschmidt echo and argue that the corresponding correlation function saturates to a finite value in localized systems. Slow, power-law decay of fluctuations of such spin-echo-type overlap is related to the operator spreading and is present only in the many-body localized phase, but not in a noninteracting Anderson insulator. While most of the previously considered probes of dephasing dynamics could be understood by approximating physical spin operators with local integrals of motion, the Loschmidt echo and its generalizations crucially depend on the full expansion of the physical operators via local integrals of motion operators, as well as operators which flip local integrals of motion. Hence these probes allow one to get insights into the relation between physical operators and local integrals of motion and access the operator spreading in the many-body localized phase.