The impact of leading collective electronic fluctuations on a free energy of a prototype 1D model for molecular systems is considered within the recently developed fluctuating local field (FLF) approach. The FLF method is a nonperturbative extension of a mean-field theory, where a self-consistent effective constant field is replaced by a fluctuating one. Integrating the fluctuating field out numerically exactly allows one to account for collective electronic fluctuations mediated by this field without any assumptions on their magnitude, degree of nonlinearity, etc. Using a half-filled Hubbard ring as a benchmark system, we find that the FLF method noticeably improves a mean-field estimation for the free energy, in particular below the mean-field Neél temperature. We further demonstrate that the mean-field result can be even more improved introducing a multimode FLF scheme that additionally takes into account subleading fluctuations. Possible applications for the thermodynamics of real molecules are also discussed.

Two-dimensional (2D) layered materials have attracted tremendous interest from the perspective of basic physics and technological applications in the last decade. Especially, the artificially assembled van der Waals (vdW) heterostructures and twisted 2D materials bring out fascinating properties, and render promising applications possible by engineering the stacking order. Here, based on first-principles calculations, we explored the interplay between stacking effect and electron-electron correlation effects in the layered vdW material of bulk $1T\text{−}{\mathrm{NbS}}_2$ with a 2D charge density wave (CDW) order. Without considering the Coulomb correlation effects, two energetically favorable out-of-plane stacking configurations are identified: one is a metallic phase with a single-layer stacking pattern, another is a band insulator with a paired-bilayer stacking configuration. Even if the Coulomb correlation effects are taken into account, the two energetic favorable stacking orders are still far more stable than other stacking orders. Furthermore, increasing the Coulomb interactions, the paired-bilayer stacking configuration transforms from nonmagnetic band insulator to antiferromagnetic insulator, while the single-layer stacked $1T\text{−}{\mathrm{NbS}}_2$ undergoes a Slater-Mott metal-insulator transition, which indicates the non-negligible role of electron-electron correlation interactions. In addition, the electronic structure and magnetic ground state change drastically among different stacking configurations, providing a platform to tune the electronic structures and interlayer magnetic interactions. In contrast to the widely accepted scenario of Mott localization as the driving force behind the gap formation in the CDW phase of layered transition-metal dichalcogenides, our results not only highlight the crucial role of stacking order in the electronic structures of $1T\text{−}{\mathrm{NbS}}_2$, but also shed fresh light on the distinct effects of Coulomb interactions on different stacking arrangements.

We propose a method to address the existence of topological edge modes in one-dimensional (1D) nonlinear lattices, by deforming the edge modes of linearized models into solutions of the fully nonlinear system. For sufficiently large nonlinearites, the energy of the modified edge modes may eventually shift out of the gap, leading to their disappearance. We identify a class of nonlinearities satisfying a generalized chiral symmetry where this mechanism is forbidden, and the nonlinear edge states are protected by a topological order parameter. Different behaviors of the edge modes are then found and explained by the interplay between the nature of the nonlinarities and the topology of the linearized models.

An intermediate state in atomic diffusion processes in the $\mathrm{O}(2\times 2)$ layer on Ru(0001) is resolved with spiral high-speed scanning tunneling microscopy (STM). The diffusion of atomic oxygen in the adlayer has been studied by density functional theory and STM. Transition state theory proposes a migration pathway for the diffusion in the oxygen adlayer. With spiral scan geometries—a new approach to high-speed STM—the oxygen vacancy mobility on the highly covered Ru(0001) surface is determined to be in the range of 0.1 to 1 Hz. Experimental evidence for the intermediate state along the oxygen diffusion pathway is provided in real space and real time.

Platinum is relatively reactive, compared to silver and copper, which prompted us to study the growth and structure of a thin insulating layer on Pt(111). We grew monolayer islands of NaCl and studied them with scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images of the islands revealed a square lattice which we confirmed, via density functional theory (DFT) calculations, to be the Cl anions, similar to other surfaces. Surprisingly, however, the AFM images appeared to only resolve approximately two-thirds of the Cl ions. DFT calculations showed that the adsorption heights of the Cl anions above the surface have a bimodal distribution in which they are either approximately 60 pm above the Na ionic plane or 40 pm lower. An electrostatic model shows that this structure reproduces the AFM observations.

We study the nonequilibrium response of a $4\times 3$ Hubbard model at $U=8$ under the influence of a short electric field pulse, with the main focus on multiple photon excitations and on the change of double occupancy after the pulse. The behavior mainly depends on the driving frequency of the electric field. The largest change of double occupancy occurs during the pulse. For frequencies below the Mott gap, we observe multiphoton excitations at large field intensities. For frequencies beyond the gap energy, there is a region where Auger recombination reduces the double occupancy after the pulse. Impact ionization (multiexciton generation), namely, a growing double occupancy after the pulse, occurs for frequencies larger than twice the Mott gap. From the Loschmidt amplitude, we compute the eigenstate spectrum of the quantum state after the pulse, observing multiple distinct photon excitation peaks, in line with expectations from a quasiparticle picture. We introduce a technique with which we analyze the time evolution of double occupancy in each peak individually. The long-term behavior of the double occupancy almost only depends on the absorbed energy, and we explore the connection of this property to the eigenstate thermalization hypothesis.

Due to ${\mathrm{UO}}_2$ being a strong electron-phonon interaction system, resonant Raman scattering is particularly helpful in exploring the relationship between its optical properties and electronic structure. Fröhlich interaction induced resonant 2LO(Γ) phonon Raman scattering exhibits excitation energy $\left(E_i=1.58–3.06\mathrm{eV}\right)$, temperature (77–873 K), and pressure (≤29 GPa) dependent weighted contributions of two resonances. The first (incoming) resonance occurs when the incident photon energy equals the energy of an optical transition positioned at δ ∼ 0.19 above the ∼2.1 eV band gap. The second (outgoing) resonance occurs when the scattered-photon energy is equal to that of the optical transition. The energy of this transition is proposedly related to the Brillouin zone Γ point electronic density of states, positioned to be slightly above the band gap, by recent density functional theory calculations. The interplay between the simultaneous temperature dependent effects of tuning the band gap and altering the lifetime broadening of the excited electronic state is elucidated and demonstrated to strongly depend on the excitation energy and its position with respect to the band gap. The core difference between pressure and temperature dependent Raman response is mostly attributed to the “resonant-suppressing” role played by excited-state lifetime shortening, which becomes increasingly dominant at increased temperatures.

Building on the results of [Faye et al., Phys. Rev. B 97, 235151 (2018)], which identified an antiferromagnetic (AFM) and Kondo singlet phases on the Kondo-Hubbard square lattice, we use the variational cluster approximation to investigate the competition between these phases on a two-dimensional triangular lattice with ${120}^o$ spin orientation. In addition to the AFM exchange interaction $J_{\perp }$ between the localized (impurity) and conduction (itinerant) electrons, our model includes the local repulsion $U$ of the conduction electrons and the Heisenberg interaction $J_H$ between the impurities. At half filling, we obtain the quantum phase diagrams in both planes $(J_{\perp },U{J}_{\perp })$ and $(J_{\perp },J_H)$. We identify a long-range, three-sublattice, spiral magnetic order which dominates the phase diagrams for small $J_{\perp }$ and moderate $U$, while a Kondo singlet phase becomes more stable at large $J_{\perp }$. The transition from the spiral magnetic order to the Kondo singlet phase is a second-order phase transition. In the $(J_{\perp },J_H)$ plane, we observe that the effect of $J_H$ is to reduce the Kondo singlet phase, giving more room to the spiral magnetic order phase. It also introduces some small magnetic oscillations of the spiral magnetic order parameter. At finite doping and when spiral magnetism is ignored, we find superconductivity with symmetry order-parameter $d+id$, which breaks time-reversal symmetry. The superconducting order parameter has a dome centered at around $5\%$ hole doping, and its amplitude decreases with increasing $J_{\perp }$. We show that spiral magnetism can coexist with $d+id$ state and that superconductivity is suppressed, indicating that these two phases are in competition.

In this paper, we derive a ready-to-use symmetry-invariant expansion of the full-zone 30-band $k·p$ Hamiltonian for the $T_d$ point group. To find respective parameters, the band structures of III-V materials were calculated within a state-of-the-art density functional theory (DFT) approach and used next as targets to adjust the $k·p$ bands. A satisfactory agreement of the $k·p$ model with the DFT band structures, for all tested zinc-blende III-V semiconductors, has been achieved. Values of many of the parameters have not been known so far. We also compare the fitted $k·p$ parameters with the values calculated using momentum matrix elements obtained directly from DFT.

We explore the conservation of energy and momentum by solving Maxwell's equations for a charged particle passing through a single conductive sheet. By using a fully relativistic formulation of the problem, we evaluate the energy and momentum transfer from charged particle to electronic excitations localized in the sheet and to electromagnetic radiation emitted from the sheet in the far-field region. Using a suitable conductivity model for the sheet that can represent graphene or other two-dimensional (2D) material, our theory can elucidate the energy and momentum transfer involving the incident particle, plasmon polariton mode(s) in the 2D material, and the induced transition radiation from it.

We propose a spin transport induced by inertial motion. Our system is composed of two host media and a narrow vacuum gap in between. One of the hosts is sliding at a constant speed relative to the other. This mechanical motion causes the Doppler effect, which shifts the density of states and the nonequilibrium distribution function in the moving medium. Those shifts induce the difference in the distribution function between the two media, and they result in tunneling spin current. The spin current is calculated from the Schwinger-Keldysh formalism with a spin tunneling Hamiltonian. This scheme does not require temperature difference, voltage, or chemical potential.

Polarons originating from phonon displacement modulated hopping have relatively light masses and thus are of significant current interest as candidates for the bipolaron mechanism of high-temperature superconductivity [Sous et al., Phys. Rev. Lett. 121, 247001 (2018)]. We observe that the bond model, when the dominant coupling comes from atomic vibrations on lattice bonds, can be solved by efficient sign-free Monte Carlo methods based on the path-integral formulation of the particle sector in combination with either the (real-space) diagrammatic or Fock-path-integral representation of the phonon sector. We introduce the corresponding algorithms and provide illustrative results for singlet bipolarons in two dimensions. The data suggest that the route towards high-temperature superconductivity (if any) in the multiparametric space of the model lies between the Scylla of large size of moderately light bipolarons and the Charybdis of large mass of compact bipolarons. As a result, on-site repulsion is helping $s$-wave superconductivity in sharp contrast with existing expectations.

Persistent spin texture (PST) is an interesting phenomenon that a material maintains a uniform spin configuration in the momentum space. Recently, it was proposed that the PST can be enforced by the nonsymmorphic space group symmetry at certain high symmetry points in the Brillouin zone [Tao et al., Nat. Commun. 9, 2763 (2018)]. In this Letter, through group theory analysis, we report that the PST could occur in the whole Brillouin zone of a two-dimensional material, as long as it keeps $xy$ mirror (or glide-plane) symmetry but breaks the combination of inversion symmetry and time-reversal symmetry. By performing first-principles calculations, we demonstrate that this full-space persistent spin texture (FPST) can occur in both nonmagnetic material (e.g., monolayer ${\mathrm{MoS}}_2$) and magnetic material (e.g., superlattice ${\mathrm{LaGa}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_3$). Furthermore, our $\mathit{k}·\mathit{p}$ Hamiltonian analysis demonstrates that the FPST is robust for nondegenerate bands with an energy gap. The FPST proposed in this work may lead to promising spintronic applications as it may result in an extremely long spin-relaxation time.

A systematic investigation of the finite-temperature phonon dispersion, including its smearing and temperature effects, is carried out using autocorrelation function method together with first-principles molecular dynamics method along the [001] direction of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$, as an illustrating example of entropy-stabilized structures. A unique anharmonicity in the cubic phase mainly derived from the interactions between titanium and oxygen atoms is revealed, which provides extremely strong damping and smearing to longitudinal optical phonons but no discernible effect on the shift of phonon energies. The anharmonicity gives rise to a nearly constant density distribution in a cubic region around the equilibrium position of the relative motion of titanium atoms with respect to oxygen atoms. These results may help to gain a further insight into complex interactions in entropy-stabilized structures and provide an essential benchmark reference to the development of the promising machine-learning based molecular dynamics methods for the investigation of phonon properties.

We develop a truncated Hamiltonian method to investigate the dynamics of the $(1+1)$-dimensional ${\varphi }^4$ theory following quantum quenches. The results are compared to two different semiclassical approaches, the self-consistent Gaussian approximation and the truncated Wigner approximation, and used to determine the range of validity of these widely used approaches. We show that the self-consistent approximation is strongly limited in comparison to the truncated Hamiltonian method which for larger cutoffs is practically exact for the parameter range studied. We find that the self-consistent approximation is only valid when the effective mass is in the vicinity of the renormalized mass. Similarly to the self-consistent approximation, the truncated Wigner approximation (TWA) is not able to capture the correct mass renormalization, and breaks down for strong enough interactions where the bare mass becomes negative. We attribute the failure of TWA to the presence of a classical symmetry-broken fixed point. Aside from establishing the truncated Hamiltonian approach as a powerful tool for studying the dynamics of the ${\varphi }^4$ model, our results on the limitation of semiclassical approximations are expected to be relevant for modeling the dynamics of other quantum field theories.

Achieving strong coupling between light and matter excitations in hybrid systems is a benchmark for the implementation of quantum technologies. We recently proposed (Bittencourt, Liberal, and Viola-Kusminskiy, arXiv:2110.02984) that strong single-particle coupling between magnons and light can be realized in a magnetized epsilon-near-zero (ENZ) medium, in which magneto-optical effects are enhanced. Here we present a detailed derivation of the magnon-photon coupling Hamiltonian in dispersive media both for degenerate and nondegenerate optical modes, and show the enhancement of the coupling near the ENZ frequency. Moreover, we show that the coupling of magnons to plane-wave nondegenerate Voigt modes vanishes at specific frequencies due to polarization selection rules tuned by dispersion. Finally, we present specific results using a Lorentz dispersion model. Our results pave the way for the design of dispersive optomagnonic systems, providing a general theoretical framework for describing and engineering ENZ-based optomagnonic systems.

The microscopic spin-spin correlations in the two-dimensional (2D) layered spin-1 honeycomb lattice compound ${\mathrm{Na}}_2{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6$ have been investigated by neutron diffraction and inelastic neutron scattering (INS). In this paper, we reveal a magnetic phenomenon where the magnetic symmetry is controlled by the nonmagnetic Na layer, which is a unique feature for the studied compound ${\mathrm{Na}}_2{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6$ with respect to other Na-based layered compounds, especially $A_2{M}_{2}X{\mathrm{O}}_6$ or $A_3{M}_{2}X{\mathrm{O}}_6$ compounds. The honeycomb lattice of spin-1 ${\mathrm{Ni}}^{2+}$ ions, within the crystallographic $ab$ planes, are well separated (∼5.6 Å) along the $c$ axis by an intermediate Na layer whose crystal structure contains chiral nuclear density distributions of Na ions. The chirality of the alternating Na layers is opposite. Such alternating chirality of the Na layer dictates the magnetic periodicity along the $c$ axis where an up-up-down-down $\left(\uparrow \uparrow \downarrow \downarrow \right)$ spin arrangement of the in-plane zigzag antiferromagnetic (AFM) structure [characterized by the propagation vector $\mathbf{k}=(\frac{1}{2} \frac{1}{2} \frac{1}{2})$] is found. Our results, thus, provide a strong correlation between the magnetic moments in the transition metal layers and the Na-chiral order in the adjacent nonmagnetic Na layers. Additionally, the above-described commensurate (CM) zigzag AFM order state is found to coexist with an incommensurate (ICM) AFM state below the $T_{\mathrm{N}}\sim 27.5\mathrm{K}$. The ICM state is found to appear at much higher temperature ∼50 K and persists down to the lowest measured temperature of 1.7 K. Our reverse Monte Carlo analysis divulges a 2D magnetic correlations (within the $ab$ plane) of the ICM AFM state over the entire temperature range 1.7–50 K. Further, the spin Hamiltonian has been determined by carrying out INS experiments and subsequent linear spin-wave theory analysis which reveal the presence of competing in-plane exchange interactions up to the third nearest neighbors consistent with the zigzag AFM ground state, and weak interplanar interaction as well as a weak single-ion anisotropy. The values of the exchange constants yield that ${\mathrm{Na}}_2{\mathrm{Ni}}_2\mathrm{Te}{\mathrm{O}}_6$ is situated well inside the zigzag AFM phase (spans over a wide ranges of $J_2/J_1$ and $J_3/J_1$ values) in the theoretical phase diagram. In this paper, we thus provide a detailed microscopic understanding of the magnetic correlations and divulge the intertwining magnetostructural correlations.

The thickness limit is utilized to investigate the doping physics in ZnO, i.e., monolayer (ML) ZnO. First-principles study demonstrates that the $p/n$-type defects in ML ZnO still have doping asymmetry. Among the doping defect models widely studied in bulk ZnO, ${\mathrm{Li}}_{\text{Zn}}$ and ${\mathrm{Ga}}_{\text{Zn}}$ with ionization energies of 0.86 and 0.82 eV are the optimal $p$- and $n$-type doping defects in ML ZnO, respectively. Their ionization energies are comparable with those of relatively shallow defects in other ML semiconductors. However, the ${\mathrm{Li}}_{\text{Zn}}$ acceptor faces a severe issue in that ${\mathrm{Li}}_{\text{Zn}}$ is the metastable structure and will transform into the most stable Jahn-Teller-distorted structure (${\mathrm{Li}}_{\text{Zn}}^{\text{JT}}$) with increasing its ionization energy to 1.53 eV. Furthermore, our scanning tunneling microscopy simulations show even a little structural distortion of the doping defects can be easily detected with the appropriate positive bias voltage on a sample of ML ZnO. The present study reveals the $p/n$-type defects' properties in ML ZnO and offers a way to understand and directly identify defect behaviors in wide-band-gap semiconductors in their two-dimensional limit form.

We predict then produce superhard metallic compound ${\mathrm{TaB}}_2$ via exploring crystal orientation resolved stress-strain relations. First-principles calculations identify prominent strain stiffening that generates superhigh indentation strengths of 46–49 GPa in the (001) oriented ${\mathrm{TaB}}_2$ crystal; in sharp contrast, dynamic instability diminishes strain stiffening even causes softening, leading to notably lower strengths around 30 GPa in the (110) and (100) orientations. Ensuing experimental synthesis creates well crystallized and textured (001) oriented ${\mathrm{TaB}}_2$ that exhibits indentation hardness of 45.9 GPa and electrical resistivity of $1.71\times {10}^{-6}\mathrm{\Omega }\mathrm{m}$, validating key superhard and metallic benchmarks. The present findings showcase an enabling protocol of crystal configuration engineering for selective property optimization, opening a path for rational design and discovery of long-sought but hitherto scarcely produced superhard metallic materials among vast transition-metal compounds.