We present a study of phonon modes and electron-phonon coupling in superconductor WP using angle-resolved polarized Raman spectroscopy. Seven out of twelve Raman-active modes are detected, with frequencies in good accordance by the first-principles calculations. We analyze parallel- and cross-polarized configurations of ${\mathrm{A}}_{\mathrm{g}}$ and ${\mathrm{B}}_{3\mathrm{g}}$ phonon modes with polarization- and angle-resolved Raman measurements. For the observed seven modes, the total electron-phonon coupling parameter $\lambda$ is only about 0.07, which is too small to achieve superconductivity. Our results elucidate that the low-frequency phonon modes play a key role in the superconductivity within the Bardeen-Cooper-Schrieffer theory framework.

It is important to study the properties of high quality single crystal in order to resolve the issue of an interesting material in which certain debatable fundamental properties exist. However, it is unfortunate that a sizable single crystal for experimental measurements is not always available. NbN is one of the examples; it has attracted scientific and engineering interest due to its diverse physical properties and a variety of structural phases. Until now superconductivity is only observed in cubic $\delta$- and tetragonal $\gamma$-NbN but not in hexagonal $\varepsilon$-NbN. Recently, Zou et al. reported the observation of superconductivity with $T_c\sim 11.6\mathrm{K}$ in a hexagonal $\varepsilon$-NbN based on the measurement on a multiphase powder specimen. In order to resolve the issue, the work used the electron backscattering diffraction technique to characterize phases of micron-size NbN crystals from commercial powders and measure their transport properties. Our results unambiguously confirm that the hexagonal $\varepsilon$-NbN phase is not superconducting.

Higher-order topological insulators have aroused massive attention due to their fancy topological properties. Artificial metamaterials, with their exquisite fabrications and measurements, offer an ideal approach to investigate topological properties in classical systems. Therein lower-dimensional corner states obtained in the three-dimensional (3D) Su-Schrieffer-Heeger (SSH) model and the octupole insulator have been extensively explored in acoustic systems. However, the existing acoustic negative couplings in the quantized multipole topological phases are mostly formed by shifting the connecting waveguides between every two resonators, which support a restricted dipole resonance mode. Here we establish subwavelength acoustic third-order topological insulators based on the theoretical frame of acoustic transmission lines and provide an innovative way for freely engineering acoustic negative couplings that work in arbitrary resonance modes. Furthermore, topological invariant calculations and numerical simulations in analogous acoustic networks validate the occurrence of topological corner states in both the 3D SSH model and octupole insulator. We foresee that the proposed methodology will broaden future avenues for studying atypical topological quantum effects with negative couplings in the classical macroscale context.

We investigate the spread of correlations in a one-dimensional lattice system with high on-site energy disorder and long-range couplings with a power-law dependence on the distance ($\propto r^{-\mu }$). The increase in correlation between the initially quenched node and a given node exhibits three phases: quadratic in time, linear in time, and saturation. No further evolution is observed in the long time regime. We find an approximate solution of the model valid in the limit of strong disorder and reproduce the results of numerical simulations with analytical formulas. We also find the time needed to reach a given correlation value as a measure of the propagation speed. Because of the triple-phase evolution of the correlation function, the propagation changes its time dependence. In the particular case of $\mu =1$, the propagation starts as a ballistic motion and then, at a certain crossover time, turns into standard diffusion.

A novel combined unitary and symmetric group approach is used to study the spin-$\frac{1}{2}$ Heisenberg model and related Fermionic systems in a total spin-adapted representation, using a linearly-parameterised Ansatz for the many-body wave function. We show that a more compact ground-state wave function representation—indicated by a larger leading ground-state coefficient—is obtained when combining the symmetric group ${\mathcal{S}}_n$, in the form of permutations of the underlying lattice site ordering, with the cumulative spin coupling based on the unitary group $\mathrm{U}\left(n\right)$. In one-dimensional systems the observed compression of the wave function is reminiscent of block-spin renormalization group approaches, and allows us to study larger lattices (here taken up to 80 sites) with the spin-adapted full configuration interaction quantum Monte Carlo method, which benefits from the sparsity of the Hamiltonian matrix and the corresponding sampled eigenstates that emerge from the reordering. We find that in an optimal lattice ordering the configuration state function with highest weight already captures with high accuracy the spin-spin correlation function of the exact ground-state wave function. This feature is found for more general lattice models, such as the Hubbard model, and ab initio quantum chemical models, exemplified by one-dimensional hydrogen chains. We also provide numerical evidence that the optimal lattice ordering for the unitary group approach is not generally equivalent to the optimal ordering obtained for methods based on matrix-product states, such as the density-matrix renormalization group approach.

Topological insulators with intrinsic magnetic ordering, potentially hosting rare quantum effects, recently attracted extensive attention. ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is the first established example. The Sb-doped variant $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ shows a great variety of electronic properties depending on the Sb content $x$, such as shifts in the Fermi level and the Néel temperature $T_{\mathrm{N}}$, and the change of the free charge carrier type from $n$ to $p$ type at high Sb substitution ratios. Here, we investigate the effect of magnetic ordering on the bulk electronic structure of $\mathrm{Mn}{\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_4$ with high Sb content $x=0.93$ by temperature-dependent reflectivity measurements over a broad frequency range. We observe anomalies in the optical response across $T_{\mathrm{N}}$ when the antiferromagnetic order sets, which suggests a coupling between the magnetic ordering and the electronic structure of the material.

Very recently, unconventional superconductivity has been observed in the double twisted trilayer graphene (TLG), where three monolayer graphene (MLG) is stacked on top of each other with two twist angles [J. M. Park et al., Nature (London) 590, 249 (2021); Z. Hao et al., Science 371, 1133 (2021); X. Zhang et al., Phys. Rev. Lett. 127, 166802 (2021)]. When some of the MLGs in the double twisted TLG are replaced by bilayer graphene (BLG), we get a family of double twisted moiré heterostructure, namely, double twisted few layer graphene (DTFLG). In this paper, we theoretically investigate the moiré band structures of the DTFLGs with diverse arrangements of MLG and BLG. We find that, depending on the relative rotation direction of the two twist angles (alternate or chiral twist) and the middle van der Waals (vdW) layer (MLG or BLG), a general (X + Y + Z)-DTFLG can be classified into four categories, i.e., (X + 1 + Z)-ATFLG, (X + 2 + Z)-ATFLG, (X + 1 + Z)-CTFLG, and (X + 2 + Z)-CTFLG, each of which has its own unique band structure. Here, X, Y, Z denote the three vdW layers, i.e., MLG or BLG. Interestingly, the (X + 1 + Z)-ATFLGs have a pair of perfect flat bands at the magic angle about $1.{54}^{\circ }$ coexisting with a pair of linear or parabolic bands, which is quite like the double twisted TLG. Meanwhile, when the twist angle is smaller than a magic angle $1.{70}^{\circ }$, the (X + 2 + Z)-CTFLGs can have two isolated narrow bands at $E_f$ with bandwidth less than 5 meV. The influence of the electric field and the topological features of the moiré bands have been studied as well. Our paper indicates that the DTFLGs, especially (X + 1 + Z)-ATFLG and (X + 2 + Z)-CTFLG, are promising platforms to study the moiré flat band induced correlation and topological effects.

Using the determinant quantum Monte Carlo method, we investigate the antiferromagnetic phase transition that is induced by the Zeeman field in a disordered interacting two-dimensional Dirac fermion system. At a fixed interaction strength $U$, the antiferromagnetic correlation is enhanced as the magnetic field increases and, when the magnetic field is larger than a $B_c\left(U\right)$, the antiferromagnetic correlation shall be suppressed by the increased magnetic field. The impact of Zeeman field $B$, Coulomb repulsion $U$, and disorder $\mathrm{\Delta }$ is not isolated. The intensity of magnetic field effect on the antiferromagnetic correlation shall be strongly suppressed by disorder. Differently, it will be promoted by weak interaction, but, when $U$ becomes larger than $U_c=4.5$, the increased interaction will suppress the intensity of this effect and here $U_c=4.5$ coincides with the critical strength inducing the metal-Mott insulator transition in a clean system. Moreover, at a fixed magnetic field $B$, strong interaction shall suppress the antiferromagnetic phase rather than promote it.

The evolution of the diamond (111) surface as it undergoes reconstruction and subsequent graphene formation is investigated with angle-resolved photoemission spectroscopy, low energy electron diffraction, and complementary density functional theory calculations. The process is examined starting at the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface reconstruction that occurs following detachment of the surface adatoms at 920 ${}^{\circ }\mathrm{C}$, and continues through to the liberation of the reconstructed surface atoms into a freestanding monolayer of epitaxial graphene at temperatures above 1000 ${}^{\circ }\mathrm{C}$. Our results show that the $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface is metallic as it has electronic states that intersect the Fermi level. This is in strong agreement with a symmetrically $\pi \text{-bonded}$ chain model and should contribute to resolving the controversies that exist in the literature surrounding the electronic nature of this surface. The graphene formed at higher temperatures exists above a newly formed $\mathrm{C}\left(111\right)\text{−}(2\times 1)$ surface and appears to have little substrate interaction as the Dirac point is observed at the Fermi level. Finally, we demonstrate that it is possible to hydrogen-terminate the underlying diamond surface by means of plasma processing without removing the graphene layer, forming a graphene-semiconductor interface. This could have particular relevance for doping the graphene formed on the diamond (111) surface via tuneable substrate interactions as a result of changing the terminating species at the diamond-graphene interface by plasma processing.

We develop a consistent theory of the interlayer exciton-polaron formed in atomically thin bilayers. Coulomb attraction between an electron and a hole situated in the different layers results in their flexural deformation and provides an efficient mechanism of the exciton coupling with flexural phonons. We study the effect of layers tension on the polaron binding energy and effective mass leading to suppression of polaron formation by the tension both in the weak and strong coupling regimes. We also consider the role of the nonlinearity related to the interaction between the out- and in-plane lattice displacements and obtain the criterion of the layer sticking, where the exciton collapses due to the Coulomb attraction between the charge carriers.

Three-dimensional (3D) topological materials exhibit much richer phenomena than their lower-dimensional counterparts. Here, we propose self-localized topological states (i.e., topological solitons) in a 3D nonlinear photonic Chern insulator. Despite being in the bulk and self-localized in all 3D, the topological solitons at high-symmetry points $K$ and $K^{\prime }$ rotate in the same direction, due to the underlying topology. Specifically, under the saturable nonlinearity the solitons are stable over a broad frequency range. Our results highlight how topology and nonlinearity interact with each other and can be extended to other 3D topological systems.

We analytically study plasma solitary waves, or solitons, in a two-dimensional electron system placed in close proximity to and between two ideal metallic gates. As a rule, solitons are described using a perturbative approach applicable only in the weak nonlinearity regime. In contrast, we analyze solitons considering a nonperturbative model. This framework enables an exact analytical description of the soliton shape. Moreover, it can be achieved in the regime beyond weak nonlinearity—when the concentration deviation due to the soliton is of the order of the equilibrium concentration. We determine the conditions required for a soliton to exist and derive the relationship between its amplitude, width, and velocity. We believe that our results obtained for the given model can provide valuable insight into the physics of nonlinear waves.

Recently, the intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ has attracted enormous research interest due to the great success in realizing exotic topological quantum states, such as the quantum anomalous Hall effect, the axion insulator state, and high-Chern-number and high-temperature Chern insulator (CI) states. One key issue in this field is to effectively manipulate these states and control topological phase transitions. Here, by systematic angle-dependent transport measurements, we reveal a magnetization-tuned topological quantum phase transition from CI to magnetic insulator with gapped Dirac surface states in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ devices. Specifically, as the magnetic field is tilted away from the out-of-plane direction by ∼40–60 °, the Hall resistance deviates from the quantization value, and a colossal, anisotropic magnetoresistance is detected. Theoretical analyses based on modified Landauer-Büttiker formalism show that the field-tilt-driven switching from the ferromagnetic state to the canted antiferromagnetic state induces a topological quantum phase transition from CI to magnetic insulator with gapped Dirac surface states in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ devices. These findings reveal a kind of topological phase transition associated with magnetism and spin-orbit coupling. In this letter, we suggest that ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ is an ideal platform for exploring topological quantum phase transitions, and it provides an efficient means for modulating topological quantum states and topological quantum phase transitions.

The structural stability and electronic properties of N-doped and B-N-codoped cubic and hexagonal diamane (single-layer diamond) were investigated based on first-principles methods. The N atom tends to stay in the substitutional site rather than the interstitial site, and the most stable configuration is the structure where the N atom is located at the external substitutional site without H saturation. The B and N atoms codoped in diamane tend to gather to form a covalent bond, where N is saturated by H, since the stability of a lone pair of electrons in N is reduced by the B dopant. Moreover, B and N dopants can adjust the bandgap (0–3.52 eV) and reduce the formation energy (FE) of diamane to promote its synthesis. The formation energies of N-doped diamane are not sensitive to N distributions, and diamane containing a vertical B-N bond has the lowest FE due to the attraction between the N-dopant-induced electron and the B-dopant-induced hole.

We report on the magnetic properties of single crystals of Ho- and Dy-based ${\left({\mathrm{Mo}}_{2/3}{R}_{1/3}\right)}_2\mathrm{AlC} i\text{−}\mathrm{MAX}$ phases. In these nanolamellar compounds, where planes of $R$ and Mo arranged in a skewed triangular lattice are separated by planes of Al and C, geometrical frustration and magnetic exchange interactions lead to complex magnetic properties. Temperature-dependent bulk magnetization, specific heat, and resistivity measurements reveal two magnetic phase transitions in Dy $i\text{−}\mathrm{MAX}$ (15 and 12 K) and only one in Ho $i\text{−}\mathrm{MAX}$ (8.5 K). Strong magnetic anisotropy and metamagnetic transitions with a step at $\frac{1}{3}$ of saturation moment along the crystal $a$ axis are observed in field-dependent bulk magnetization curves. X-ray magnetic circular dichroism measurements unveil induced moments on Mo and Al, and a quantitative estimation of the orbital and spin moments of Mo based on magneto-optical sum rules suggests an unusual interaction between the $R 4f$ and the Mo $4d$ magnetic moments. Magnetic structures are derived from neutron diffraction measurements, revealing a zero-field incommensurate amplitude modulated order in both compounds, followed by an antiferromagnetic equal-moments structure at lower temperature for Dy $i\text{−}\mathrm{MAX}$. The bulk magnetization $\frac{1}{3}$ step is found to be linked to the flip of one $R$ moment out of three within the planes. Detailed phase diagrams for Ho and Dy $i\text{−}\mathrm{MAX}$ are derived from these measurements.

The delta phase of Pu is stabilized by Ga doping, but the mechanism of this stabilization remains an open question. Density functional theory calculations focused on how Ga doping affects the phonons sheds some light on the phonons' contribution to the stabilization. The calculated phonon modes of Ga-doped delta phase Pu fall into two distinct types: localized, high frequency Ga-dominated phonon modes, and Pu-dominated modes at lower frequencies. Increasing the Ga concentration has an effect on the Pu-dominated phonon modes opposite to that of compression: higher-frequency modes soften, and lower-frequency modes stiffen. The latter provides an indication that the stabilization mechanism is not due to a thermodynamic contribution from the phonons. Furthermore, the stiffened phonon modes include candidate modes that describe possible pathways into low-temperature phases, suggesting that doping with Ga could impede such pathways.

Perpendicular synthetic ferrimagnet (pSFi) due to the low stray field and net magnetization is expected to replace single ferromagnet as a free layer to optimize the memory devices for high storage density and low-energy consumption. Here, we investigate the dependence of exchange-coupling strength on the thickness of the spacer Ru and current-induced magnetization reversal due to spin-orbit torque in $\mathrm{Ta}/\mathrm{Pt}/{\left[\mathrm{Pt}/\mathrm{Co}\right]}_2/\mathrm{Ru}/{\left[\mathrm{Co}/\mathrm{Pt}\right]}_4/\mathrm{Pt}$ structure with a perpendicular magnetic anisotropy. An oscillating interlayer exchange coupling as a function of Ru spacer layer thickness with a period of 1.16 nm is determined by combining the anomalous Hall effect and the polar magnetic-optical Kerr effect. Furthermore, current-controllable magnetization reversal experiments reveal that the magnetizations of top and bottom layers with antiferromagnetic coupling switch simultaneously due to the combination of spin-orbit torque generated from the two adjacent heavy-metal layers and the interlayer exchange torque. The SOT efficiency $\chi \sim 57.52\mathrm{Oe}/\left({10}^6\mathrm{A}/\mathrm{c}{\mathrm{m}}^2\right)$, corresponding to an effective spin Hall angle ${\xi }_{DL}\sim 0.68$, is estimated by analyzing current-dependent anomalous Hall resistance with our proposed quasistatic balance equation of magnetic moments. In addition to the advantage of minimizing stray field acting on the storage layer, the observed low switching current density and high spin-orbit torque efficiency suggest that the pSFi structure with a high thermal stability factor has great potential to realize the high-density and low-power consumption of nonvolatile magnetic memory devices.

We use density functional theory based ab initio calculations to investigate the structural, vibrational, magnetic, and electronic properties of the layered ferromagnet ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT) that has attracted attention for potential spintronic applications. We optimize the structure for a fixed $c/a$ ratio. Our results are in excellent agreement with experimental data on structure, phonons, and electronic properties. The use of van der Waals interactions and relativistic spin-orbit coupling yields accurate lattice constants and interlayer distances. Simultaneously, the adopted theoretical methods lead to a rigorous description of the vibrational normal modes, as well as the valence bands, that are in excellent agreement with Raman spectroscopy and angle-resolved photoemission spectroscopy spectra, respectively. The magnetic moment is slightly overestimated, and the magnetic anisotropy has the correct sign but is greater in magnitude than that in experiment. We find that inclusion of the on-site Coulomb repulsion on $\mathrm{Cr}\text{−}3d$ orbitals worsens agreement with experiment, in contrast to previous studies. Our results provide a path toward ab initio analysis of magnetic heterostructures based on CGT.