By minimizing, in the $L_2$ norm, the difference between the left- and the right-hand sides of the time-dependent Schrödinger equation, the variational principle of McLachlan (McLVP) [A. McLachlan, Mol. Phys. 8, 39 (1964)] provides a powerful tool for the generation of equations of motion. If the trial wave function is the Slater determinant, McLVP produces a temporally and spatially nonlocal exchange potential [V. U. Nazarov, Phys. Rev. B 87, 165125 (2013)]. We study the performance of the corresponding wave-vector and frequency-dependent exchange kernel $f_x^h(q,\omega )$ of the homogeneous electron liquid. While the McLVP-based $f_x^h(q,\omega )$ lacks correlations by construction, we find that it accurately accounts for exchange, reproducing features in the quantum Monte Carlo data, which the known constraint-based kernels miss. We argue that the complementary use of the McLVP- and the constraint-based exchange-correlation kernels will enhance the performance of the linear-response time-dependent density functional theory of the electron liquid.

We explore the time evolution of a topological system when the system undergoes a sudden quantum quench within the same nontrivial phase. Using Haldane's honeycomb model as an example, we show that equilibrium states in a topological phase can be distinguished by geometrical features, such as the characteristic momentum at which the half-occupied edge modes cross, the associated edge-mode velocity, and the winding vector about which the normalized pseudospin magnetic field winds along a great circle on the Bloch sphere. We generalize these geometrical quantities for nonequilibrium states and use them to visualize the quench dynamics of the topological system. In general, we find the prequench equilibrium state relaxes to the postquench equilibrium state in an oscillatory fashion, whose amplitude decay as $t^{1/2}$. In the process, however, the characteristic winding vector of the nonequilibrium system can evolve to regimes that are not reachable with equilibrium states.

Understanding how an antiferromagnetic Mott insulator develops into a paramagnetic metal and/or superconductor by carrier doping is a fundamental problem in high-$T_{\mathrm{c}}$ cuprate superconductor. The impact of the chemical potential $\mu$ controlled by doping has been extensively investigated to solve this issue. The chemical potential shift $\mathrm{\Delta }\mu$ in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ (LSCO) exhibits the different behavior from the other high-$T_{\mathrm{c}}$ cuprates. In this work, we have re-examined $\mathrm{\Delta }\mu$ of LSCO showing the anomalous behavior by using the hard and soft x-ray photoemission spectroscopies (HAXPES and SXPES). We found that $\mathrm{\Delta }\mu$ reveals downward behavior by small hole doping in HAXPES and SXPES, which is distinct from the anomalous pinning behavior over a wide doping range reported previously in LSCO but is consistent with the weak $\mathrm{\Delta }\mu$ pinning in the vicinity of the Mott insulator as in other hole-doped cuprates. Our spectroscopic findings demonstrate that the previously considered strong pinning effect in LSCO does not occur. Based on our results, we compare the bulk $\mathrm{\Delta }\mu$ from HAXPES with that estimated from the thermodynamic quantity and the three-dimensional tight-binding calculation and discuss the mechanism of $\mathrm{\Delta }\mu$ with doping.

The recent discovery of unconventional surface-state pairs, which give rise to Fermi arcs and spin textures, in antiferromagnetically ordered rare-earth monopnictides attracted the interest in these materials. We use angle-resolved photoemission spectroscopy measurements in conjunction with density functional theory calculations to investigate the evolution of the electronic structure of GdBi and DyBi. We find that new surface states, including a Dirac cone, emerge in the antiferromagnetic (AFM) state. However, they are located along a direction in momentum space that is different than what was found in NdSb, NdBi, and CeBi. The observed changes in the electronic structure are consistent with the presence of AFM-II-A type order.

Antiferromagnetic topological insulators (AFM TIs) are predicted to exhibit exotic physical properties such as gigantic optical and topological magnetoelectric responses. While a key to achieving such phenomena relies on how to break the symmetry protecting the Dirac-cone surface state (SS) and acquire the mass of Dirac fermions, the mechanism has yet to be clarified. To address this issue, we carried out microfocused angle-resolved photoemission spectroscopy for GdBi hosting the type-II AFM order, and uncovered the stripe-type $2\times 1$ reconstruction of the Fermi surface associated with the AFM band folding. Intriguingly, in contrast to NdBi with the type-I AFM order displaying the surface-selective Dirac-fermion mass, GdBi shows massless behavior irrespective of AFM domains due to the robust topological protection. These results strongly suggest a crucial role of the $\mathrm{\Theta }{\mathit{T}}_{\mathrm{D}}$ (time-reversal and translational) symmetry to create the Dirac-fermion mass in AFM TIs.

We consider work fluctuation theorems for isolated driven systems and the possibility to probe them in mesoscopic systems. In this context nonequilibrium fluctuation theorems (NFTs) relate work performed on a system as its Hamiltonian varies with time, to equilibrium data of the initial and final states. In a classical context the system energy can be directly measured, while a quantum implementation requires the incorporation of a work agent. Here, as a work agent we consider a dynamical single-coordinate object, which exchanges energy with the system. The work done on the system is defined as the energy reduction of the work agent, which requires an energy measurement of the agent (only) at the end of the process. To justify the applicability of the NFT we require the agent's trajectory to be weakly affected by the energy exchange with the system. We furthermore argue that the uncertainty in the energy measurements imposes an inherent quantum limitation on the validity of the NFT. We demonstrate our findings for a two-level system, and discuss applications to more complex mesoscopic systems.

The unexpected discovery of superconductivity and strong electron correlation in twisted bilayer graphene (TBG), a system containing only $sp$ electrons, is considered as one of the most intriguing developments in two-dimensional materials in recent years. The key feature is the emergent flat energy bands near the Fermi level, a favorable condition for novel many-body phases, at the so-called “magic angles”. The physical origin of these interesting flat bands has been elusive to date, hindering the construction of an effective theory for the unconventional electron correlation. In this paper, we have identified the importance of charge accumulation in the AA region of the moiré supercell and the most critical role of the Fermi ring in AA-stacked bilayer graphene. We show that the magic angles can be predicted by the moiré periodicity determined by the size of this Fermi ring. The resonant criterion in momentum space makes it possible to coherently combine states on the Fermi ring through scattering by the moiré potential, leading to flat bands near the Fermi level. We thus establish the physical origin of the magic angles in TBG and identify the characteristics of one-particle states associated with the flat bands for further many-body investigations.

The effect of $\mathrm{\Gamma }\text{−}\text{X}$ mixing strength on the energy levels fine structure of indirect in k-space excitons in an ensemble of (In,Al)As/AlAs quantum dots with type I band alignment was experimentally studied. Using the methods of optical orientation and optical alignment in a Faraday magnetic field, an increase in the anisotropic exchange splitting of exciton levels (from approximately 0.6 to 5 µeV) to the extent of $\mathrm{\Gamma }\text{−}\text{X}$ mixing was revealed. It was shown that the ratio of direct electronic states admixing to indirect ones depends on the size of the quantum dot. Moreover, the photoluminescence polarization properties of excitons change radically with increasing of the $\mathrm{\Gamma }$ states admixture to the X states: in the absence of a magnetic field, the optical orientation of excitons decreases from 18 to 3%, while the alignment of excitons is restored from 6 to 53%.

We show that in some transition metal dichalcogenides, minority regions of the cleaved sample surfaces show—unexpectedly and anomalously—a finite number of 2D electronic states instead of the expected 3D valence bands. In the case of ${\mathrm{NbS}}_2$, in addition to the typical spectrum associated with bulk 2Ha stacking, we also find minority regions with electronic structures consistent with few layers of 3R stacking. In ${\mathrm{MoS}}_2$, we find areas of both bulk 2Hc and 3R stackings, and regions exhibiting finite-layer quantization of both types. We further find evidence for a more exotic 4Ha stacking of ${\mathrm{MoS}}_2$, in which the valence band maximum is quasi-2D. The results highlight how variation of the interlayer stacking of van der Waals materials beyond the commonly reported bulk polytypes can yield novel electronic structures.

Twisted bilayer graphene near the magic angle is known to have a cascade of insulating phases at integer filling factors of the low-energy bands. In this Letter we address the nature of these phases through an unrestricted, large-scale Hartree-Fock calculation on the lattice that self-consistently accounts for all electronic bands. Using numerically unbiased methods, we show that Coulomb interactions produce ferromagnetic insulating states at integer fillings $\nu \in [-3,3]$ with maximal spin polarization $M_{\text{FM}}=4-\left|\nu \right|$. We find that the $\nu =0$ state is a pure ferromagnet, whereas all other insulating states are spin-valley polarized. At odd filling factors $\left|\nu \right|=1,3$ those states have a quantum anomalous Hall effect with Chern number $\mathcal{C}=1$. Except for the $\nu =0,-2$ states, all other integer fillings have insulating phases with additional sublattice symmetry breaking and antiferromagnetism in the remote bands. We map the metal-insulator transitions of these phases as a function of the effective dielectric constant. Our results establish the importance of large-scale lattice calculations to faithfully determine the ground states of twisted bilayer graphene at integer fillings.

${\mathrm{APtO}}_3$ (A = Ca, Sr, Ba) is a unique family in the complex transition metal oxides with the ${\mathrm{AMO}}_3$ formula. Most $3d$ and $4d{\mathrm{AMO}}_3$ compounds adopt the perovskite structure when synthesized at ambient or high pressure; however, ${\mathrm{APtO}}_3$ perovskites have not been reported, even if their geometric tolerance factor, $t$, falls in the range suitable for the perovskite structure. Herein, we studied the crystal structures of ${\mathrm{BaPtO}}_3$ by performing the in situ x-ray diffraction over a broad range in the pressure ($P$)–temperature ($T$) phase diagram. The perovskite ${\mathrm{BaPtO}}_3$ can be stabilized at $P>35$ GPa and $T>1100$ K. A variety of crystal structures, including the perovskite structure and its hexagonal polytypes, observed in the $P$–$T$ phase diagram of ${\mathrm{BaPtO}}_3$ match the structure predictions from the first-principles calculations reasonably well.

We present a systematic ab initio study of the temperature- and pressure-dependent thermoelastic properties of hcp beryllium within the quasiharmonic approximation (QHA). The accuracies of the zero static internal stress approximation (ZSISA) and of the volume-constrained ZSISA that are widely applied in ab initio thermodynamic calculations are quantified. Particularly, the effect of ZSISA on the calculation of $C_{11}$ and $C_{12}$ is compared with a numerical approach which minimizes the free energy with respect to the atomic positions at each strain. In beryllium, minor deviations are found within ZSISA, which gives elastic constants (ECs) in good agreement with the full free energy minimization. A substantial difference is found between the QHA and the quasistatic approximation (QSA), with the former closer to experiments. Within the QSA, we compare the ECs obtained by interpolating from a set of geometries along the “stress-pressure” isotherm at 0 K (within the constant-volume ZSISA) with a more general interpolation on a two-dimensional grid of crystal parameters, which allows the calculation of the ECs along the 0 kbar isobar. This paper provides a practical approach for the investigation of the thermoelastic properties of hcp metals at extreme conditions.

We have probed the atomic dynamics of two sodium silicate glasses in a wide wave vector range by means of x-ray photon correlation spectroscopy. The wide angle setup has been modified to cover a $Q$ range that extends to small angles, close to the range normally covered by the usual small-angle x-ray scattering configuration. During irradiation, the oxide samples undergo a transformation to a new amorphous state with different structural and dynamical properties. The observed dynamics before and after the structural transformation shows the presence of de Gennes narrowing. In the low wave vector region, the dynamics is affected by a ballisticlike motion at low absorbed doses, while the glass obtained after prolonged irradiation has a $Q$-independent relaxation time. This observation strengthens the interpretation of the new amorphous phase as a rejuvenated glass that lies in a higher-energy region of the potential landscape.

We explore the magnetoelastic dynamics in magnetic insulators exposed to a focused twisted laser beam, revealing the generation of twisted magnons and phonons through magnetoelastic coupling driven by orbital angular momentum transfer. The excited modes exhibit unique orbital quantum numbers, with coherently induced twisted dynamics mirroring the handedness of excitation pulse, i.e., the sign of orbital quantum number, while heating-induced dynamics remain unaffected by the pulse's orbital angular momentum. This approach provides precise spatial control of coherent spin dynamics in magnetic materials.

Antiperovskite-type compounds with noncollinear spin arrangements have drawn great interest due to their rich strong-correlation properties associated with unique magnetic structures. Pressure is an effective way to tune the spin ordering and provoke novel phenomena by adjusting their complex exchange integrals. Here, we manipulate the unique magnetic structures in the frustrated antiperovskite $\mathrm{M}{\mathrm{n}}_3\mathrm{GaN}$ and try to clarify the evolution of strong-correlation physical properties under a high-pressure field up to 12 GPa. High-pressure induces the transition from the magnetic ground state with a typical 120 ° spin arrangement to a 90 ° spin configuration, and further to a ferromagnetic state. The observed anomalies in the electronic conduction and thermal expansion curves corresponding to the magnetic phase transitions are also gradually suppressed by pressure. Based on the high-pressure experiments, the T-P magnetic phase diagram is derived and the pressure-induced linear shift of the transition temperatures can be well observed. A spin model based on the classical Heisenberg theory was constructed to clarify the nature of the pressure effects, which is ascribed to the enhancement of the crystal field of N and the Mn-N-Mn superexchange contribution. Our results provide insight into the stabilization of noncollinear spin order.

Cooper pairs formed by two electrons with different effective masses are common in multiband superconductors, pair density wave states, and other superconducting systems with multiple degrees of freedom. In this paper, we show that there are paramagnetic contributions to the superfluid stiffness in superconductors with different-mass Cooper pairs. This paramagnetic response is owing to the relative motion between two electrons with different mass. Additionally, we identify a particle-hole dichotomy effect related to this mass difference. We investigate the paramagnetic contributions based on the linear-response theory in two-band superconductors with interband pairings and in pair density wave states, respectively. Our results offer a different perspective of the electromagnetic superfluid stiffness in unconventional superconductors beyond the traditional BCS response.