We report the experimental and theoretical characterization of the angular-dependent spin dynamics in arrays of ferromagnetic nanodisks arranged on a honeycomb lattice. The magnetic field and microwave frequency dependence, measured by broadband ferromagnetic resonance, reveal a rich spectrum of modes that is strongly affected by the microstate of the network. Based on symmetry arguments with respect to the external field, we show that certain parts of the ferromagnetic network contribute to the detected signal. A comparison of the experimental data with micromagnetic simulations reveals that different subsections of the lattice predominantly contribute to the high-frequency response of the array. This is confirmed by optical characterizations using microfocused Brillouin light scattering. Furthermore, we find indications that nucleation and annihilation of vortexlike magnetization configurations in the low-field range affect the dynamics, which is different from clusters of ferromagnetic nanoellipses. Our work opens up new perspectives for designing magnonic devices that combine geometric frustration in gyrotropic vortex crystals at low frequencies with magnonic crystals at high frequencies.

A silicon micromechanical torsional oscillator is used to measure the hysteresis loops of two manganite ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ nanotubes at different temperatures, applying an external field along its main axes. These structures are composed of nanograins with a ferromagnetic core surrounded by a dead layer. Micromagnetic calculations based on the stochastic Landau-Lifshitz-Gilbert equation are performed to validate a simple model that allows for quantitatively describing the ferromagnetic behavior of the system. Further simulations are used to analyze the experimental data more in depth and to calculate the coercive field, the saturation and remanent magnetizations, and the effective magnetic volume for single nanotubes, over a wide temperature range.

The $\gamma \text{−}\mathrm{U}-\mathrm{Mo}$ alloys have been studied by means of ab initio molecular-dynamic simulations at 900 K as a function of the Mo concentration (0%, 25%, 50%, 75%, and 100%). Using the temperature-dependent effective potential method with the symmetry-imposed force constant extension, we obtained the vibrational, thermodynamic, and elastic properties of U–Mo, including anharmonicity and disorder. We show that the addition of Mo and temperature effects are responsible for the stabilization of the $\gamma$ phase of U–Mo alloys in the range of 0–25 % of Mo concentration above 900 K. The vibrational entropy is found to be very large compared to its value in other alloys, about half of the configurational entropy counterpart, and to act against the thermodynamic stabilization of the solid solution. The vibrational and thermodynamic properties are strongly impacted by the disorder, due to the large differences between mass and interatomic bonds of uranium and molybdenum. The increase of elastic shear modulus as a function of the Mo concentration also indicates a mechanical stabilization of the solid solution and validates the thermodynamic findings.

In this work, we study the entanglement and topological properties of an extended flat-band Creutz ladder by considering a compacted localized state (CLS). Based on the CLS picture, we find a multiple flat-band extension from the conventional two flat-band Creutz ladder. A simple vertical interchain coupling leads to a four complete flat-band system and creates an additive $\pi$-flux pattern on the Creutz ladder. Interestingly, the strong coupling induces a topological phase transition where the distribution of CLSs is modified: upper and lower flat-band CLSs are paired up. This pairing leads to the destruction of the CLS entanglement and, hence, to a vanishing edge mode (i.e., the breakdown of the nontrivial topological phase). Finally, we study the localization dynamics induced by the presence of complete flat bands in this extended flat-band system.

(MgFe)O is a solid solution with ferrous iron undergoing high to low spin state (HS-LS) crossover under high pressure. The exact state of the material in the region of the crossover is still a mystery as domains with different spin states may coexist over a wide pressure range without changing the crystal structure neither in terms of symmetry nor in terms of the atomic positions. At the conditions of the crossover, (MgFe)O is a special type of microscopic disorder system. We explore the influences of: (a) stress-strain relations in a diamond-anvil cell, (b) time-relaxation processes, and (c) the crossover itself on the characteristic features of a single-crystal ${\left(111\right)}_{hkl}$ Bragg spot across the transformation. Using high-resolution x-ray diffraction as a method for studies of unconventional processes at the conditions of suppressed diffusion, we detect and discuss subtle changes in the ${\left(111\right)}_{hkl}$ Bragg spot projections which we measure and analyze as a function of pressure. We report changes in the spot size which can be correlated with the HS-LS relative abundance. In addition, we report the formation of structural defects in a single crystal as an intrinsic material response to the HS-LS crossover. These static defects are formed and accumulated during the compression within the crossover pressure range.

The interplay between superconductivity, spin-orbit coupling, and Zeeman or exchange field, is studied theoretically in two different setups: a single wire in which all these fields coexist, and a double wire system in which superconducting pairing and the spin-dependent fields are spatially separated. We first explore a magnetoelectric effect, namely the appearance of anomalous charge supercurrents. We determine the conditions under which such currents are allowed by symmetry and express them in terms of the SU(2) electric and magnetic fields. In leading order in the strength of the fields we find that in the single wire setup such currents may appear only when the Zeeman field has both, a longitudinal and transverse component with respect to the spin-orbit field. In contrast, in the two wire setup a parallel component to the SOC can generate the anomalous current, which is allowed by symmetry. We confirm these findings by calculating explicitly the current in both setups together with the self-consistent superconducting order parameter. The latter shows in the ground state a spatial modulation of the phase that leads to currents that compensate the anomalous current, such that in both cases the ground state corresponds to a total zero-current state. However, in the two wire setup this zero-current state consists of two finite currents flowing in each of the wires in opposite direction.

We theoretically find that a finite-size Fulde-Ferrell (FF) superconductor (which is characterized by spatially nonuniform ground state $\mathrm{\Psi }\sim \text{exp}(-i{\mathbf{q}}_{FF}\mathbf{r})$ and $\left|\mathrm{\Psi }\right|\left(r\right)=\text{const}$ in the bulk case, where $\mathrm{\Psi }$ is a superconducting order parameter) has paramagnetic Meissner, vortex, and “onion” ground states with $\left|\mathrm{\Psi }\right|\left(r\right)\ne \text{const}$. These states are realized due to boundary effect when the lateral size of a superconductor $L\sim 1/q_{FF}$. We argue that predicted states could be observed in a thin disk/square made of a superconductor–ferromagnet–normal metal trilayer with $L\simeq 150–600\text{nm}$.

Topological phases of matter have been used as a fertile realm of intensive discussions about fermionic fractionalization. In this work, we study the effects of anisotropic superconducting correlations in the fermionic fractionalization on the topological phases. We consider a hybrid version of the SSH and Kitaev models with an anisotropic superconducting order parameter to investigate the unusual states with zero energy that emerges in a finite chain. To obtain these zero-energy solutions, we built a chain with a well-defined domain wall at the middle of the chain. Our solutions indicate an interesting dynamic between the zero-energy state around the domain wall and the superconducting correlation parameters. Finally, we find that the presence of an isolated Majorana at the ends of the chain is strongly dependent on the existence of the solitonic excitation in the middle of the chain.

The evolution of the electronic structures of strongly correlated insulators with doping has long been a central fundamental question in condensed matter physics; it is also of great practical relevance for applications. We have studied the evolution of NiO under hole and electron doping at low doping levels such that the system remains insulating using high-quality thin film and a wide range of experimental and theoretical methods. The evolution is in both cases very smooth with dopant concentration. The band gap is asymmetric under electron and hole doping, consistent with a charge-transfer insulator picture, and is reduced faster under hole doping than under electron doping. For both electron and hole doping, occupied states are introduced at the top of the valence band. The formation of deep donor levels under electron doping and the inability to pin otherwise empty states near the conduction-band edge are indicative that local electron addition and removal energies are dominated by a Mott-like Hubbard $U$ interaction even though the global band gap is predominantly a charge-transfer-type gap.

We use density functional molecular dynamics (DFT-MD) to study the effect of finite-temperature exchange correlation (xc) in hydrogen. Using the Kohn-Sham approach, the xc energy of the system, $E_{x}c\left(r_s\right)$, is replaced by the xc free energy $f_{x}c(r_s,\mathrm{\Theta })$ within the local density approximation (LDA) based on parametrized path integral Monte Carlo data for the uniform electron gas (UEG) at warm dense matter (WDM) conditions. We observe insignificant changes in the equation of state (EOS) at the region of metal-insulator transition compared to the regular LDA form, whereas significant changes are observed for $T>10000\mathrm{K}$, i.e., in the important WDM regime. Thus our results further corroborate the need for temperature-dependent xc functionals for DFT simulations of WDM systems. Moreover, we present finite-temperature DFT results for the EOS of hydrogen in the electron liquid regime up to $r_s=14$ and find a drastic impact (about $20\%$) of thermal xc effects, which manifests at lower temperatures compared to WDM. We expect our results to be important for many applications beyond DFT, like quantum hydrodynamics and astrophysical models.

In this article, we propose a new two-dimensional ${\mathrm{CP}}_3$ material with distinguished properties from comprehensive first-principles calculations. It has excellent thermal, mechanical, and dynamical stabilities and promise for experimental fabrication. The low cleavage energy ($0.57\mathrm{J}/{\mathrm{m}}^2$) implies the plausibility of ${\mathrm{CP}}_3$ single-layer exfoliation from bulk structure. Other than exfoliation, we predict an alternative route to synthesize ${\mathrm{CP}}_3$ monolayer through C atoms doping into blue phosphorene. Moreover, ${\mathrm{CP}}_3$ monolayer displays metallic behavior and, most interestingly, its Fermi velocity is close to the graphene. A metal-semiconductor transition can be achieved by strain engineering. We also propose a new material through fluorination of ${\mathrm{CP}}_3$ monolayer where one fluorine atom is attached to each C and P atoms. The fluorinated ${\mathrm{CP}}_3$ monolayer also shows excellent stabilities and exhibits semiconducting properties with indirect band gap (2.32 eV) as well as excellent optical absorption (${10}^5{\mathrm{cm}}^{-1}$) in the entire UV-vis range. All these novel properties endow ${\mathrm{CP}}_3$ and its fluorinated derivative as a promising two-dimensional material for a wide range of applications, including electronics, optoelectronics, and photovoltaics.

We report anisotropic spin polarization of edge currents in ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ monolayer zigzag nanoribbons (ZNRs) deposited on either nonmagnetic or ferromagnetic insulator substrates. We employ an 11-band tight-binding model to calculate the electronic band structures of transition metal dichalcogenide (TMDC) monolayers and their corresponding nanoribbons in the presence of Rashba spin-orbit coupling (RSOC) and magnetic proximity effect produced by ferromagnetic substrate. We adopt the nonequilibrium Green's function method together with Landauer-Büttiker formalism to study the quantum transport behavior stemming from the edge states of ZNRs. We demonstrate that the spin-polarized edge current can be generated in both ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ ZNRs with RSOC. We find that the spin polarization spreads out in all three directions. This is in stark contrast to what occurs in zigzag graphene nanoribbons, for which the polarization only exists in the transverse direction (across the width of ribbons). In addition, the spin polarization direction strongly depends on the strength of the intrinsic SOC component. The interplay of Rashba and intrinsic SOC is crucial for the spin polarization of the currents in any spatial direction. For TMDCs with stronger intrinsic SOC such as in ${\mathrm{WS}}_2$ monolayer ZNRs, we observe that the spin polarization along the perpendicular direction to the plane of the ZNR can be as large as $90\%$. Moreover, the unusual anisotropy of the spin polarization can be further enhanced by the magnetic proximity effect. These results open up possibilities for the generation of tunable high-spin polarization currents in ZNRs without application of an external magnetic field.

Single layer (SL) two-dimensional transition metal dichalcogenides (TMDs), such as $\mathrm{Mo}{\mathrm{S}}_2$, $\mathrm{Re}{\mathrm{S}}_2$, $\mathrm{WS}{\mathrm{e}}_2$, and $\mathrm{MoT}{\mathrm{e}}_2$ have now become the focus of intensive fundamental and applied research due to their intriguing and tunable physical properties. These materials exhibit a broad range of structural phases that can be induced via elastic strain, chemical doping, and electrostatic field effect. These transformations, in turn, can open and close the band gap of SL-TMDs, leading to metal-insulator transitions, and lead to emergence of more complex quantum phenomena. These considerations necessitate detailed understanding of the mesoscopic mechanisms of these structural phase transitions. Here we develop a Landau-type thermodynamic description of SL-TMDs on a sample of the $\mathrm{SL}\text{−}{\left(\mathrm{Mo}{\mathrm{S}}_2\right)}_{1-x}\text{−}{\left(\mathrm{Re}{\mathrm{S}}_2\right)}_x$ system and analyze the free energy surfaces, phase diagrams, and order parameter behavior. Our results predict the existence of multiple structural phases with two-, six- and 12-fold degenerated energy minima for in-plane and out-of-plane order parameters. This analysis suggests that out-of-plane ferroelectricity can exist in many of these phases, with the switchable polarization being proportional to the out-of-plane order parameter. We further predict that the domain walls in $\mathrm{SL}\text{−}{\left(\mathrm{Mo}{\mathrm{S}}_2\right)}_{1-x}\text{−}{\left(\mathrm{Re}{\mathrm{S}}_2\right)}_x$ should become conductive above a certain strain threshold.

We construct an analytic continuum model to describe the electronic structure and the electron-phonon interaction in twisted bilayer graphenes with arbitrary lattice deformation. Starting from the tight-binding model, we derive the interlayer Hamiltonian in the presence of general lattice displacement and obtain a long-wavelength continuum expression for smooth deformation. We show that the continuum model correctly describes the band structures of the lattice-relaxed twisted bilayer graphenes. We apply the formula to the phonon vibration and derive an explicit expression of the electron-phonon matrix elements between the moiré band states and the moiré phonon modes. By numerical calculation, we find that the electron-phonon coupling is significantly enhanced in low twist angles due to the superlattice hybridization. At the magic angle, in particular, the phonon-mediated electron-electron interaction is found to be comparable to the order of the inverse density of states, suggesting that the system is in the strong coupling regime.

Standard density functionals like Perdew-Burke-Ernzerhof (PBE) or Strongly Constrained and Appropriately Normed (SCAN) need a correction to account for long-range van der Waals (vdW) interaction. The damped Zaremba-Kohn model (dZK) [J. Tao, H. Tang, A. Patra, P. Bhattarai, and J. P. Perdew, Phys. Rev. B 97, 165403 (2018)] starts from a formula for the vdW interaction of a distant atom with a solid surface, both with known dielectric properties, damps this formula at short range, and then treats an adsorbed molecule or atomic layer as a collection of renormalized atoms. We extend this model to graphene adsorbed on semiconducting layered materials [bulk graphite and hBN, and multilayer transition metal dichalcogenides (TMD)] by including the $C_4$ asymptotic term and multiple electrostatic image effects due to the two surfaces of the substrate slabs in the vdW calculations. The resulting SCAN-vdW-dZK and PBE-vdW-dZK give approximately the same results for the systems considered here, in agreement with available reference values. The predicted binding energies are roughly 25% lower than those from SCAN+rVV10 (revised Vydrov and Van Voorhis 2010), and $\sim 15\%$ lower than those from PBE+rVV10. Since SCAN+rVV10 usually overbinds, the predicted binding energies by SCAN-vdW-dZK and PBE-vdW-dZK are expected to be closer to the true values. The predicted equilibrium binding distances from SCAN-vdW-dZK and PBE-vdW-dZK are slightly larger ($\sim 0.1\AA$) than those from SCAN+rVV10, and close to those from SCAN. The binding energy depends upon the number of substrate layers more strongly in vdW-dZK than in rVV10. The $C_4$-term contributions can be 40% of the total vdW interactions, and the $C_5$ term contributes about 10%. The effects of images and the back surfaces of slabs can contribute about 4–10%. The vdW interaction energy power laws from the vdW-dZK model for graphene adsorbed on multilayer $\mathrm{Mo}{\mathrm{S}}_2$ show slowly varying decay to the pairwise exponent −4 with increasing separation $D$, very similar to those obtained from the random-phase approximation and renormalization group approaches by Ambrosetti et al. Both the PBE-vdW-dZK and SCAN-vdW-dZK give a greater increase in interlayer binding energy when the TMD substrate changes from monolayer to four-layer for the graphene/TMD adsorption systems. This is consistent with the relevant electron-energy-loss spectroscopy experimental results showing increased dielectric response from the substrate with increasing substrate layer number.

The magnetic phase diagram of a spin-$\frac{1}{2}$ chain antiferromagnet ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ is studied by an ultrasound phase-sensitive detection technique. The system is in the extreme proximity of the Luttinger-liquid quantum-critical point and we observe an unusually strong effect of magnetic field, which is very weak compared to the in-chain interaction, on the Néel ordering temperature. Inside the ordered phase, we detect an unexpected, field-induced continuous phase transition. The transition is accompanied by softening of magnetic excitation observed by electron-spin resonance, which in previous work [E. G. Sergeicheva et al., Phys. Rev. B 95, 020411(R) (2017)] was associated with a longitudinal (amplitude) mode of the order parameter. These results suggest a transition from a transverse collinear antiferromagnet to an amplitude-modulated spin-density-wave phase in a very weak magnetic field, which is unexpected for a system of weakly coupled Heisenberg spin-$\frac{1}{2}$ chains.

We apply the quasiparticle self-consistent $GW$ method (QSGW) to slab models of ionic materials—LiF, KF, NaCl, MgO, and CaO—under electric field. Then we obtain the optical dielectric constants ${\epsilon }_{\infty }\left(\mathrm{Slab}\right)$ from the differences of the slopes of the electrostatic potential in the bulk and vacuum regions. Calculated ${\epsilon }_{\infty }\left(\mathrm{Slab}\right)$ show very good agreement with experiments. For example, we have ${\epsilon }_{\infty }\left(\mathrm{Slab}\right)=2.91$ for MgO, in agreement with the experimental value ${\epsilon }_{\infty }\left(\mathrm{Experiment}\right)=2.96$. This is in contrast to ${\epsilon }_{\infty }\left(\mathrm{RPA}\right)=2.37$, which is calculated in the random-phase approximation for the bulk MgO in QSGW. After we explain the difference between the quasiparticle-based perturbation theory and the Green's-function-based perturbation theory, we interpret the large difference ${\epsilon }_{\infty }\left(\mathrm{Slab}\right)-{\epsilon }_{\infty }\left(\mathrm{RPA}\right)=2.91-2.37$ as the contribution from the vertex correction of the proper polarization, which determines the screened Coulomb interaction $W$. Our result encourages the theoretical development of the self-consistent $G_{0}W$ approximation along the line of QSGW self-consistency, as was performed by Shishkin, Marsman, and Kresse [Phys. Rev. Lett. 99, 246403 (2007)].

The rainbow state denotes a set of valence bond states organized concentrically around the center of a spin 1/2 chain. It is the ground state of an inhomogeneous XX Hamiltonian and presents a maximal violation of the area law of entanglement entropy. Here, we add a tunable exchange coupling constant at the center, $\gamma$, and show that it induces entanglement transitions of the ground state. At very strong inhomogeneity, the rainbow state survives for $0\le \gamma \le 1$, while outside that region the ground state is a product of dimers. In the weak inhomogeneity regime, the entanglement entropy satisfies a volume law, derived from CFT in curved space-time, with an effective central charge that depends on the inhomogeneity parameter and $\gamma$. In all regimes we have found that the entanglement properties are invariant under the transformation $\gamma ⟷1-\gamma$, whose fixed point $\gamma =1/2$ corresponds to the usual rainbow model. Finally, we study the robustness of nontrivial topological phases in the presence of the defect.