We theoretically study the charge order and orbital magnetic properties of a new type of antiferromagnetic kagome metal FeGe. Based on first-principles density functional theory calculations, we study the electronic structures, Fermi-surface quantum fluctuations, as well as phonon properties of the antiferromagnetic kagome metal FeGe. It is found that charge density wave emerges in such a system due to a subtle cooperation between electron–electron interactions and electron–phonon couplings, which gives rise to an unusual scenario of interaction-triggered phonon instabilities, and eventually yields a charge density wave (CDW) state. We further show that, in the CDW phase, the ground-state current density distribution exhibits an intriguing star-of-David pattern, leading to flux density modulation. The orbital fluxes (or current loops) in this system emerge as a result of the subtle interplay between magnetism, lattice geometries, charge order, and spin-orbit coupling (SOC), which can be described by a simple, yet universal, tight-binding theory including a Kane–Mele-type SOC term and a magnetic exchange interaction. We further study the origin of the peculiar step-edge states in FeGe, which sheds light on the topological properties and correlation effects in this new type of kagome antiferromagnetic material.

Surface acoustic wave (SAW) is a powerful technique for investigating quantum phases appearing in two-dimensional electron systems. The electrons respond to the piezoelectric field of SAW through screening, attenuating its amplitude, and shifting its velocity, which is described by the relaxation model. In this work, we systematically study this interaction using orders of magnitude lower SAW amplitude than those in previous studies. At high magnetic fields, when electrons form highly correlated states such as the quantum Hall effect, we observe an anomalously large attenuation of SAW, while the acoustic speed remains considerably high, inconsistent with the conventional relaxation model. This anomaly exists only when the SAW power is sufficiently low.

SnO$_{2}$ films exhibit significant potential as cost-effective and high electron mobility substitutes for In$_{2}$O$_{3}$ films. In this study, Li is incorporated into the interstitial site of the SnO$_{2}$ lattice resulting in an exceptionally low resistivity of $2.028 \times 10^{-3}\,\Omega \cdot$cm along with a high carrier concentration of $1.398 \times 10^{20}$ cm$^{-3}$ and carrier mobility of 22.02 cm$^{2}$/V$\cdot$s. Intriguingly, Li$_{i}$ readily forms in amorphous structures but faces challenges in crystalline formations. Furthermore, it has been experimentally confirmed that Li$_{i}$ acts as a shallow donor in SnO$_{2}$ with an ionization energy $\Delta E_{\rm D1}$ of $-0.4$ eV, indicating spontaneous occurrence of Li$_{i}$ ionization.

Moiré superlattices in twisted two-dimensional materials have emerged as ideal platforms for engineering quantum phenomena, which are highly sensitive to twist angles, including both the global value and the spatial inhomogeneity. However, only a few methods provide spatial-resolved information for characterizing local twist angle distribution. Here we directly visualize the variations of local twist angles and angle-dependent evolutions of the quantum states in twisted bilayer graphene by scanning microwave impedance microscopy (sMIM). Spatially resolved sMIM measurements reveal a pronounced alteration in the local twist angle, approximately 0.3$^{\circ}$ over several micrometers in some cases. The variation occurs not only when crossing domain boundaries but also occasionally within individual domains. Additionally, the full-filling density of the flat band experiences a change of over $2 \times 10^{11}$ cm$^{-2}$ when crossing domain boundaries, aligning consistently with the twist angle inhomogeneity. Moreover, the correlated Chern insulators undergo variations in accordance with the twist angle, gradually weakening and eventually disappearing as the deviation from the magic angle increases. Our findings signify the crucial role of twist angles in shaping the distribution and existence of quantum states, establishing a foundational cornerstone for advancing the study of twisted two-dimensional materials.

Within the extended vector meson dominance model, we investigate the $e^+ e^- \to \varLambda^+_c \bar{\varLambda}^-_c$ reaction and the electromagnetic form factors of the charmed baryon $\varLambda_c^+$. The model parameters are determined by fitting them to the cross sections of the process $e^+e^-\rightarrow \varLambda_c^+ \bar{\varLambda}_c^-$ and the magnetic form factor $|G_{\scriptscriptstyle{\rm M}}|$ of $\varLambda^+_c$. By considering four charmonium-like states, called $\psi(4500)$, $\psi(4660)$, $\psi(4790)$, and $\psi(4900)$, we can well describe the current data on the $e^+ e^- \to \varLambda^+_c \bar{\varLambda}^-_c$ reaction from the reaction threshold up to $4.96$ GeV. In addition to the total cross sections and $|G_{\scriptscriptstyle{\rm M}}|$, the ratio $|G_{\scriptscriptstyle{\rm E}}/G_{\scriptscriptstyle{\rm M}}|$ and the effective form factor $|G_{\mathrm{eff}}|$ for $\varLambda^+_c$ are also calculated, and found that these calculations are consistent with the experimental data. Within the fitted model parameters, we have also estimated the charge radius of the charmed $\varLambda_c^+$ baryon.

Motivated by the recent discovery of high-temperature superconductivity in bilayer La$_3$Ni$_2$O$_7$ under pressure, we study its electronic properties and superconductivity due to strong electron correlation. Using the inversion symmetry, we decouple the low-energy electronic structure into block-diagonal symmetric and antisymmetric sectors. It is found that the antisymmetric sector can be reduced to a one-band system near half filling, while the symmetric bands occupied by about two electrons are heavily overdoped individually. Using the strong coupling mean field theory, we obtain strong superconducting pairing with $B_{\rm 1g}$ symmetry in the antisymmetric sector. We propose that due to the spin-orbital exchange coupling between the two sectors, $B_{\rm 1g}$ pairing is induced in the symmetric bands, which in turn boosts the pairing gap in the antisymmetric band and enhances the high-temperature superconductivity with a congruent d-wave symmetry in pressurized La$_3$Ni$_2$O$_7$.

The $k\!\cdot\! p$ method is significant in condensed matter physics for the compact and analytical Hamiltonian. In the presence of magnetic field, it is described by the effective Zeeman's coupling Hamiltonian with Landé $g$-factors. Here, we develop an open-source package VASP2KP (including two parts: vasp2mat and mat2kp) to compute $k\!\cdot\! p$ parameters and Landé $g$-factors directly from the wavefunctions provided by the density functional theory (DFT) as implemented in Vienna ab initio Simulation Package (VASP). First, we develop a VASP patch vasp2mat to compute matrix representations of the generalized momentum operator $\hat{\boldsymbol \pi}=\hat{\boldsymbol p}+\frac{1}{2mc^2}[\hat{{\boldsymbol s}}\times\nabla V({\boldsymbol r})]$, spin operator $\hat{\boldsymbol s}$, time reversal operator $\hat{T}$, and crystalline symmetry operators $\hat{R}$ on the DFT wavefunctions. Second, we develop a python code mat2kp to obtain the unitary transformation $U$ that rotates the degenerate DFT basis towards the standard basis, and then automatically compute the $k\!\cdot\! p$ parameters and $g$-factors. The theory and the methodology behind VASP2KP are described in detail. The matrix elements of the operators are derived comprehensively and computed correctly within the projector augmented wave method. We apply this package to some materials, e.g., Bi$_2$Se$_3$, Na$_3$Bi, Te, InAs and 1H-TMD monolayers. The obtained effective model's dispersions are in good agreement with the DFT data around the specific wave vector, and the $g$-factors are consistent with experimental data. The VASP2KP package is available at https://github.com/zjwang11/VASP2KP.

In a Dirac semimetal, the massless Dirac fermion has zero chirality, leading to surface states connected adiabatically to a topologically trivial surface state as well as vanishing anomalous Hall effect. Recently, it is predicted that in the nonrelativistic limit of certain collinear antiferromagnets, there exists a type of chiral “Dirac-like” fermion, whose dispersion manifests four-fold degenerate crossing points formed by spin-degenerate linear bands, with topologically protected Fermi arcs. Such an unconventional chiral fermion, protected by a hidden $SU(2)$ symmetry in the hierarchy of an enhanced crystallographic group, namely spin space group, is not experimentally verified yet. Here, by angle-resolved photoemission spectroscopy measurements, we reveal the surface origin of the electron pocket at the Fermi surface in collinear antiferromagnet CoNb$_{3}$S$_{6}$. Combining with neutron diffraction and first-principles calculations, we suggest a multidomain collinear antiferromagnetic configuration, rendering the existence of the Fermi-arc surface states induced by chiral Dirac-like fermions. Our work provides spectral evidence of the chiral Dirac-like fermion caused by particular spin symmetry in CoNb$_{3}$S$_{6}$, paving an avenue for exploring new emergent phenomena in antiferromagnets with unconventional quasiparticle excitations.

Predicting thermal conductance of complex networks poses a formidable challenge in the field of materials science and engineering. This challenge arises due to the intricate interplay between the parameters of network structure and thermal conductance, encompassing connectivity, network topology, network geometry, node inhomogeneity, and others. Our understanding of how these parameters specifically influence heat transfer performance remains limited. Deep learning offers a promising approach for addressing such complex problems. We find that the well-established convolutional neural network models AlexNet can predict the thermal conductance of complex network efficiently. Our approach further optimizes the calculation efficiency by reducing the image recognition in consideration that the thermal transfer is inherently encoded within the Laplacian matrix. Intriguingly, our findings reveal that adopting a simpler convolutional neural network architecture can achieve a comparable prediction accuracy while requiring less computational time. This result facilitates a more efficient solution for predicting the thermal conductance of complex networks and serves as a reference for machine learning algorithm in related domains.

High-$T_{\rm c}$ superconductivity with possible $T_{\rm c}\approx 80$ K has been reported in the single crystal of ${\rm La}_{3}{\rm Ni}_{2}{\rm O}_{7}$ under high pressure. Based on the electronic structure given by the density functional theory calculations, we propose an effective bi-layer model Hamiltonian including both $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbital electrons of the nickel cations. The main feature of the model is that the $3d_{z^{2}}$ electrons form inter-layer $\sigma$-bonding and anti-bonding bands via the apical oxygen anions between the two layers, while the $3d_{x^{2}-y^{2}}$ electrons hybridize with the $3d_{z^{2}}$ electrons within each NiO$_2$ plane. The chemical potential difference of these two orbital electrons ensures that the $3d_{z^{2}}$ orbitals are close to half-filling and the $3d_{x^{2}-y^{2}}$ orbitals are near quarter-filling. The strong on-site Hubbard repulsion of the $3d_{z^{2}}$ orbital electrons gives rise to an effective inter-layer antiferromagnetic spin super-exchange $J$. Applying pressure can self dope holes on the $3d_{z^{2}}$ orbitals with the same amount of electrons doped on the $3d_{x^{2}-y^{2}}$ orbitals. By performing numerical density-matrix renormalization group calculations on a minimum setup and focusing on the limit of large $J$ and small doping of $3d_{z^{2}}$ orbitals, we find the superconducting instability on both the $3d_{z^{2}}$ and $3d_{x^{2}-y^{2}}$ orbitals by calculating the equal-time spin singlet pair–pair correlation function. Our numerical results may provide useful insights in the high-$T_{\rm c}$ superconductivity in single crystal La$_3$Ni$_2$O$_7$ under high pressure.