Monolayer graphene embedded with a transition metal nitride (i.e., ${X\mathrm{N}}_4$) has been experimentally synthesized recently, where a transition metal atom together with four nitrogen atoms as a unit are embedded in graphene to form a stable planar single-atom-thick structure. We provide a systematic study on the structural, electronic, and topological properties of these ${X\mathrm{N}}_4$-embedded graphene structures by utilizing both first-principles calculations and a tight-binding model. We find that ${X\mathrm{N}}_4$-embedded graphene $(X=\text{Pt}, \mathrm{Ir}, \mathrm{Rh}, \mathrm{Os})$ can open topologically nontrivial band gaps that host two-dimensional ${\mathbb{Z}}_2$ topological insulators. We further show that the low-energy bands near the band gaps can be perfectly captured by a modified Kane-Mele model Hamiltonian. Our work not only provides concrete two-dimensional materials that are very rare to realize two-dimensional ${\mathbb{Z}}_2$ topological insulators, but also makes the graphene system to be realistic in hosting Kane-Mele type ${\mathbb{Z}}_2$ topological insulators.

Perovskite tantalates have become potential candidates for water splitting photocatalysts. Therefore, it is of importance to understand the behavior of the photoinduced excess charges in these materials. Herein, we investigate the formation of electron and hole polarons in ${\mathrm{NaTaO}}_3$ and ${\mathrm{KTaO}}_3$. We perform Perdew-Burke-Ernzerhof hybrid density functional $\mathrm{PBE}0\left(\alpha \right)$ calculations, in which we define the fraction $\alpha$ of the Fock exchange by enforcing the Koopmans' condition, to properly account for self-interaction corrections in these calculations. We find that the hole polaron mainly localizes on one oxygen site in both materials, leading to a structural distortion where two Ta–O bonds are elongated. The electron polaron, on the other hand, localizes within one atomic plane and exhibits a two-dimensional electron gas nature. Finally, we find that the strong localization of holes leads to a low hole mobility at room temperature $\sim 2.94\times {10}^{-6}{\mathrm{cm}}^2$/Vs and $\sim 1.87\times {10}^{-4}{\mathrm{cm}}^2$/Vs for ${\mathrm{KTaO}}_3$ and ${\mathrm{NaTaO}}_3$, respectively.

We used Raman spectroscopy to investigate phonon modes of single InP-InAs core-shell nanowire with a pure wurtzite phase to reveal the embedded strained state. Raman spectra show that the InP core exhibits tensile strain and the InAs shell exhibits compressive strain due to the radial heterostructure growth. Using different polarization excitation configurations, we identified distinct $A_1$ (TO) and $E_1$ (TO) modes, atomic vibrations along and perpendicular to the [0001] direction, respectively. We also identified $E_{2h}$ phonon modes in wurtzite InP and InAs, lattice vibration along with the [0001] direction, a fingerprint to distinguish a wurtzite phase from a zinc-blende phase. The Raman spectra changes reveal the resonance effects of InP and InAs between the $E_0+{\mathrm{\Delta }}_0$ energy level and the $E_1$ gap by varying excitation energy, which indicates ${\mathrm{\Gamma }}_{7\mathrm{c}}\text{−}{\mathrm{\Gamma }}_{6\mathrm{v}}$ transition consists of a split-off valence band and a band-gap transition along [0001] directions of the Brillouin zone. The polar patterns show the Raman $A_1$ (TO) mode of InP only predominates in the $x\left(yy\right)\overline{x}$ configuration, indicating the InP $A_1$ (TO) phonon is sensitive to excitation polarization, suggesting the resonant Raman spectroscopy with specific polarization offers a direct way for characterizing wurtzite InP. Our findings deepen the understanding of strain in wurtzite-phase core-shell nanowire, providing a significant reference for engineering strain in core-shell nanowire to control optical and electric properties.

An in-plane magnetic field applied to an Ising superconductor converts spin-singlet Cooper pairs to spin-triplet ones. In this work, we study a Josephson junction formed by two Ising superconductors that are proximitized by ferromagnetic (FM) layers. This leads to highly tunable spin-triplet pairing correlations which allow to modulate the charge and spin supercurrents through the in-plane magnetic exchange fields. For a junction with a nonmagnetic barrier, the charge current is switchable by changing the relative alignment of the in-plane exchange fields, and a $\pi$ state can be realized. Furthermore, the charge and spin current-phase relations display a ${\varphi }_0$-junction behavior for a strongly spin-polarized FM barrier.

Experimental progress finally reached the metallic solid hydrogen phase, which was predicted by Wigner and Huntington over 80 years ago. However, the different structures in the phase diagram are still being debated due to the difficulty of diffraction experiments for high-pressured hydrogen. The determination of crystal structures under extreme condition is both of the basic condensed matter physics, and in planetary science: the behavior of giant gaseous planets (e.g., Jupiter and Saturn) strongly depends on the properties of inner high-pressured hydrogen. This work describes possible structures appearing under high pressures of $400\sim 600$ GPa. We applied a structural search using particle swarm optimization with density functional theory (DFT) to propose several candidate structures. For these structures, we performed fixed-node diffusion Monte Carlo simulations combined with DFT zero-point energy corrections to confirm their relative stability. We found $P{2}_1/c\text{−}8$ as a promising candidate structure for the ${\mathrm{H}}_2$-PRE phase. $P{2}_1/c\text{−}8$ is predicted the most stable at 400 and 500 GPa. $P{2}_1/c\text{−}8$ reproduces qualitatively the IR spectrum peaks observed in the ${\mathrm{H}}_2$-PRE phase.

Using a realistic cesium fluogermanate $\left({\mathrm{Cs}}_2{\mathrm{GeF}}_6\right)$ material as an example, we discuss the symmetry conditions for the co-occurrence of nodal-box and nodal-chain phonons via symmetry analysis and prove that the above-mentioned topological phonons can be realized in ${\mathrm{Cs}}_2{\mathrm{GeF}}_6$ via first-principles calculations. The nodal-box phonons in the ${\mathrm{Cs}}_2{\mathrm{GeF}}_6$ material are formed from the bowtie-shaped nodal lines in the [110], [101], [011], $\left[\overline{1}10\right]$, $\left[0\overline{1}1\right]$, and $\left[10\overline{1}\right]$ planes. The nodal-chain states in the phonon dispersion of ${\mathrm{Cs}}_2{\mathrm{GeF}}_6$ run along all three directions, forming a network structure. Moreover, the topologically nontrivial signatures of the nodal-box and nodal-chain phonons were determined. The phonon surface states related to the nodal-box and nodal-chain phonons are obvious, which is expected to benefit experimental detection. This work shows that the nodal-box phonons can exist in existing materials and reveals the possibility of the co-occurrence of nodal-box and nodal-chain phonons in one material.

We investigate the response of a doped topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with spin-triplet nematic superconductivity to external magnetization. We calculate the Zeeman part of the magnetic susceptibility for nematic and chiral superconducting phases near $T_c$ in the Ginzburg-Landau formalism. The superconducting order parameter from the $E_u$ representation has nontrivial coupling with the transversal Zeeman field that results in a paramagnetic response to magnetization. The topology of a Fermi surface has a strong influence on magnetic susceptibility. The Lifshitz transition from a closed to open Fermi surface eventually leads to a phase transition from the nematic to chiral phase. At the transition point, magnetic susceptibility diverges. Also, we study the effects of the electron-electron interaction on the competition between nematic and chiral phases. We find that in a real system, electron-electron interaction can drive the nematic to chiral phase only in the vicinity of the phase transition. We compare our results with the existing experimental data.

We explore a scheme to investigate the tunable magnon antibunching characteristics of the conventional magnon blockade (CMB) and the unconventional magnon blockade (UMB) in a hybrid ferromagnet-superconductor system. The magnon excited in the yttrium iron garnet is indirectly coupled to two qubits by eliminating two mutually perpendicular microwave cavity modes. In our paper, we relax the magnon-qubits detuning constraint. In two cases with and without magnon-qubits detuning, it is found that in the strong coupling regime CMB and UMB coexist in our system, and they can be regulated simultaneously by varying the magnon-qubits coupling strength ratio and the qubits detuning ratio. In the weak coupling regime, there exists only the UMB in the system. Without magnon-qubits detuning, the UMB shows a nonmonotonic dependence on the magnon-qubits coupling strength ratio. With magnon-qubits detuning, for the UMB, the magnon bunching effect and antibunching effect can be converted mutually by adjusting the magnon-qubits coupling strength ratio and the qubits detuning ratio. Moreover, changing the magnon-qubits coupling strength ratio and the qubits detuning ratio has a significant impact on the antibunching effect against the thermal magnon excitation. Our research results provide a theoretical scheme for the manipulation of magnon antibunching characteristics, and have important practical significance for the construction of a single magnon source.

Magnesium based alloys are promising solid materials for hydrogen storage. However, it is a real challenge to synthesise hydrogen storage materials with high hydrogen storage capacity and low dehydrogenation temperature. Here, we have performed extensively structural searches for ternary magnesium-based hydrogen storage compounds of ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_n$ with n ranging from 16 to 25 by calypso method and first-principles calculations. We readily identified the experimentally observed ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_{16}$ hydride and uncovered a stable stoichiometry of ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_{19}$ with high hydrogen storage capacity of 6.7 wt% and low dehydrogenation temperature of 273 K. The remarkable decrease of the hydrogen release temperature is attributed to the atomic rearrangements in ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_{19}$, which forms H-H pairs and has weakened metal-hydrogen chemical bonds compared to the stable ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_{16}$ and $\mathrm{Mg}{\mathrm{H}}_2$ compounds. Our calculations show that ${\mathrm{Mg}}_7\mathrm{Nb}{\mathrm{H}}_{16}$ undergoes a structural phase transition from its $P\overline{4}2m$ phase to a $Fm\overline{3}m$ phase at 75 GPa, and the $Fm\overline{3}m$ phase is a potential polyhydride superconductor. The present findings offer insights for understanding the hydrogen storage and release of Mg-Nb-H ternary magnesium-based hydrogen storage compounds, which open avenues for the design and synthesis of magnesium-based hydrogen storage material.

The Shastry-Sutherland model and its generalizations have been shown to capture emergent complex magnetic properties from geometric frustration in several quasi-two-dimensional quantum magnets. Using an $sd$ exchange model, we show here that metallic Shastry-Sutherland magnets can exhibit a topological Hall effect driven by magnetic skyrmions under realistic conditions. The magnetic properties are modeled with competing symmetric Heisenberg and asymmetric Dzyaloshinskii-Moriya exchange interactions, while a coupling between the spins of the itinerant electrons and the localized moments describes the magnetotransport behavior. Our results, employing complementary Monte Carlo simulations and a novel machine learning analysis to investigate the magnetic phases, provide evidence for field-driven skyrmion crystal formation for an extended range of Hamiltonian parameters. By constructing an effective tight-binding model of conduction electrons coupled to the skyrmion lattice, we clearly demonstrate the appearance of the topological Hall effect. We further elaborate on the effects of finite temperatures on both magnetic and magnetotransport properties.

Two-dimensional (2D) piezoelectric materials utilizing the unique electronic characteristic were widely needed for microelectronic sensors and energy conversion devices. However, the currently reported 2D piezoelectric films face thorny challenges in practical application due to lack of the mechanical durability or the strong vertical piezoelectric effect. Herein, based on first-principles calculations, we theoretically simulate the Janus semiconductor nanosheets, namely, $\mathrm{F}\text{−}M\text{−}H$ ($M=\mathrm{B}$, Al, and Ga) monolayers. The result shows that they have admirable dynamical and thermal stabilities to achieve strong mechanical durability and possess the desired vertical piezoelectric effect by applying uniaxial strain in the $z$-axis direction vertical to the nanosheet surface. For the three materials, F-B-H has the strongest stability and the most significant vertical piezoelectric effect. The out-of-plane piezoelectric stress coefficient $e_{31}$ of F-B-H is higher than other reported 2D piezoelectric SnOSe films by two orders of magnitude. Furthermore, we found an interesting relationship among these three nanosheets: the higher the electronegativity difference ratio, the higher the piezoelectric stress coefficient. Finally, this work not only deepens the understanding of 2D piezoelectric materials, but also provides a platform for fabricating cutting-edge piezoelectric equipment such as wearable electronic, medical blood pressure detectors and robotic bionic skin tactile sensors.

Zigzag nanoribbons of monolayer graphenelike two-dimensional materials host spontaneous edge magnetism at the zigzag terminations, whose configuration controls the band gap. In this paper, the edge magnetism of zigzag nanoribbons of bilayer hexagonal crystals is studied. The specific models of bilayer graphene and bilayer silicene are both studied. As the gated voltage increases, the total energy level, magnetic structure, and band structures of the ground state and the first quasistable excited state are tuned. For certain regions of the gated voltage, the band gaps of spin up and down are opened and closed, respectively, so the systems are in the spin-polarized metallic phase. The spin-polarized conducting edge band of the bilayer nanoribbons could be applied for gate tunable spintronic devices.

We investigate magnetic and charge correlations in graphene by using the formulation of extended dynamical mean-field theory (E-DMFT) for two-sublattice systems. First, we map the average nonlocal interaction onto the effective static interaction between different sublattices, which is treated together with the local interaction within an effective two-orbital local model. The remaining part of the nonlocal interaction is considered by introducing an effective retarded interaction within the E-DMFT approach. The nonlocal susceptibilities in the charge and spin channel are further evaluated in the ladder approximation. We verify the applicability of the proposed method to describe the effect of uniformly screened long-range Coulomb potential $\propto 1/r$, as well as screened realistic long-range electron interactions [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)] in graphene. We show that the developed approach describes a competition of semimetal, spin density wave (SDW), and charge-density-wave (CDW) correlations. The obtained phase diagram is in a good agreement with recent results of the functional renormalization group (fRG) for finite large graphene nanoflakes and scaling analysis of quantum Monte Carlo data on finite clusters. Similarly to the previously obtained results within the fRG approach, the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential.

Recent research predicted stable two-dimensional pentagraphene in $s{p}^2\text{−}s{p}^3$ bonding states and three-dimensional pentadiamond (BC14) in all-$s{p}^3$ bonding states, both comprising five-membered carbon-ring networks and exhibiting semiconducting character with sizable electronic band gaps. Here we identify by ab initio calculations an all-$s{p}^2$ carbon allotrope, also comprising the unusual five-membered rings as the basic structural units, with an eight-atom monoclinic primitive cell in $C2/m$ ($C_{2h}^3$) symmetry, termed mpg-${\mathrm{C}}_8$ pentagraphite. Total-energy calculations show that mpg-${\mathrm{C}}_8$ is more stable than or comparable to the previously reported all-$s{p}^2$ rh6 polybenzene and all-$s{p}^3$ BC14 pentadiamond. Electronic band calculations reveal that mpg-${\mathrm{C}}_8$ is a topological nodal-line semimetal protected by the inversion ($P$) and time reversal ($T$) symmetry, comprising two nodal rings centered at the $\mathrm{\Gamma }$ and $M$ points in the bulk first Brillouin zone and one projected surface flat band around the Fermi level on its (010) surface. These results establish a distinct type of carbon phase with nontrivial topological properties and offer insights into its outstanding electronic properties.

The bandgap of insulating materials is renormalized in various ways by the electron-phonon interaction owing to the dynamical and quantum fluctuations of nuclei. These fluctuation effects are considered in the perturbative Allen-Heine-Cardona theory using the formulas for the Fan-Migdal and Debye-Waller terms. However, the material dependence is not clear in the formulas. Thus, in this study, we focus on the analytical form of the Debye-Waller term and find that the term can be reformulated using the momentum matrix. In addition, the optical selection rule is found to play a role. For diamond-type materials, the Debye-Waller term can be approximately decomposed into a product of the optical transition energy, the mean square displacement of nuclei, and the dipole transition probability. The decomposition can also be applied with an additional approximation to zinc-blende-type materials, as revealed by our first-principles calculation. The magnitudes of the Debye-Waller term of several materials can thus be estimated using basic physical quantities prior to performing the calculation of the electron-phonon interaction.

We present two two-body Hamiltonians that approximate the exact particle-hole Pfaffian wave function with their ground states for all the system sizes where this wave function has been numerically constructed to date. The approximate wave functions have high overlap with the original and reproduce well the low-lying entanglement spectrum and structure factor. The approximate generating Hamiltonians are obtained by an optimization procedure where three to four pseudopotentials are varied in the neighbourhood of second Landau level Coulomb interaction or of a noninteracting model. They belong to a finite region in the variational space of Hamiltonians where each point approximately generates the particle-hole Pfaffian. We diagonalize the identified Hamiltonians for up to 20 electrons and find that for them the particle-hole Pfaffian shift appears energetically more favorable. The possibility to interpret the data in terms of composite fermions is discussed.

We study three-dimensional time-reversal-invariant topological superconductivity in noncentrosymmetric materials such as RhSi, CoSi, and AlPt which host coupled multifold nodes energetically split by the spin-orbit coupling at the same time-reversal-invariant momentum (TRIM). The topological superconductivity arises from the $s_{+}\oplus s_{-}$ gap function, which is $\mathit{k}$ independent, but with opposite signs for the two nodes split at the same TRIM. We consider various electron-electron interactions in the tight-binding model for RhSi and find that the topological superconducting phase supporting a surface Majorana cone and topological nodal rings is favored in a wide range of interaction parameters.

In this Letter, the transmission properties of a nonperiodic array of slots arranged in the form of a Fibonacci sequence are investigated. By arranging the slots in this manner, an additional periodicity can be utilized, resulting in corresponding resonance features in the transmitted signal. By investigating the transmission response of a perforated metallic sheet over a broad frequency range (6–40 GHz), it is shown that this simple one-dimensional chain supports two periodicities, one due to the regular periodic separation and one due to average spacing—which is related to the golden ratio. This response replicates the resonant behavior of a two-dimensional periodic array with a single nonperiodic array also creating new families of diffraction lobes in the far-field region.

${\mathrm{BaZrO}}_3$ is reportedly one of few perovskites that retain an ideal cubic structure down to 0 K even though its ground state as predicted by first-principles calculations has remained contentious. Here, we combine electron diffraction with total neutron scattering measured on ceramic samples at cryogenic temperatures to demonstrate that below 80 K, this compound undergoes a structural change associated with the onset of correlated out of phase rotations of ${[\mathrm{ZrO}}_6]$ octahedra. The change is manifested in the appearance of weak but relatively sharp superlattice reflections at $\frac{1}{2}hkl$ $(h, k, l=\mathrm{odd})$, which are readily detectable by electron diffraction. Atomistic structural refinements from the total neutron-scattering data confirmed that these reflections are associated with octahedral tilting. The observed rotations are consistent with the $a^0{a}^0{c}^{–}$ type and feature a coherence length of about 3 nm. The average structure of ${\mathrm{BaZrO}}_3$, as seen by Bragg diffraction, remains cubic. According to the electron diffraction data, Nb-doped ${\mathrm{BaZrO}}_3$ undergoes a similar structural transition but, in this case, the tilting could not be recovered from the neutron scattering, suggesting that the distortions are smaller compared to those in the pure compound.