Junctions of one-dimensional systems are of great interest to the development of synthetic materials that harbor topological phases. We study a junction of three gapless spin-1 chains described by the $\mathrm{SU}{\left(2\right)}_2$ Wess-Zumino-Wittten model and coupled by exchange and chiral three-spin interactions. We show that a chiral fixed point appears as a special point on the transition line separating two regimes described by open boundary conditions, corresponding to decoupled chains and the formation of a boundary spin singlet state. Along this transition line, the junction behaves as a tunable spin circulator as the spin conductance varies continuously with the coupling constant of a marginal boundary operator. Since the spectrum of the junction contains fractional excitations such as Majorana fermions, in this paper, we set the stage for network constructions of non-Abelian chiral spin liquids.

The cascade of electronic phases in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ raises the prospect to disentangle their mutual interactions in a clean, strongly interacting kagome lattice. When the kagome planes are stacked into a crystal, its electronic dimensionality encodes how much of the kagome physics and its topological aspects survive. The layered structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ reflects in Brillouin-zone-sized quasi-two-dimensional Fermi surfaces and significant transport anisotropy. Yet here we demonstrate that ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ is a three-dimensional (3D) metal within the charge density wave (CDW) state. Small 3D pockets play a crucial role in its low-temperature magneto- and quantum transport. Their emergence at $T_{\mathrm{CDW}}\approx 93\text{K}$ results in an anomalous sudden increase of the in-plane magnetoresistance by four orders of magnitude. The presence of these 3D pockets is further confirmed by quantum oscillations under in-plane magnetic fields, demonstrating their closed nature. These results emphasize the impact of interlayer coupling on the kagome physics in 3D materials.

We study a simple model of interacting bosons on a $d$-dimensional cubic lattice whose dynamics conserves both total boson number and total boson dipole moment. This model provides a simple framework in which several remarkable consequences of dipole conservation can be explored. As a function of chemical potential and hopping strength, the model can be tuned between gapped Mott insulating phases and various types of gapless condensates. The condensed phase realized at large hopping strengths, which we dub a Bose-Einstein insulator, is particularly interesting: despite having a Bose condensate, it is insulating, and despite being an insulator, it is compressible.

We observe third-harmonic generation (THG) and fifth-harmonic generation (FHG) of free holes in the valence band of bulk Si:B at cryogenic temperature upon irradiation with intense terahertz pulses from a free-electron laser, polarized along the $\mathrm{\Gamma }\text{−}X$ direction. The intensities of both signals increase as a function of pulse energy following power laws with respective exponents of 4.2 and 6.2, larger than the exponents of 3 and 5 expected for ${\chi }^{\left(3\right)}$ and ${\chi }^{\left(5\right)}$ processes with constant susceptibilities and a fixed number of holes. The larger values are attributed to the increase in the density of mobile holes, which results from impact ionization of boron dopants by thermally activated holes in the electric field of the terahertz pulses as already observed in our studies of THG in Si:B reported in Phys. Rev. B 102, 075205 (2020). We apply Monte Carlo simulations of the nonlinear heavy- and light-hole field response, coupled with a finite-difference time-domain treatment of the pump-pulse propagation in the sample, which reproduce the experimental THG:FHG intensity ratio reasonably well. An analysis of the local response demonstrates that, in our pump regime, the harmonic generation is dominated by the band anharmonicity as opposed to the energy-dependent momentum scattering rates or interband scattering. Comparison with the results of a simple one-dimensional model and scrutiny of the three-dimensional band structure shows that one must account for the extent of the carrier distribution transverse to the $\mathrm{\Gamma }\text{−}X$ axis as the anharmonicity grows rapidly away from this axis.

$\mathrm{SCAN}+\mathrm{rVV}10$ has been demonstrated to be a versatile van der Waals (vdW) density functional that delivers good predictions of both energetic and structural properties for many types of bonding. Recently, the ${\mathrm{r}}^2\mathrm{SCAN}$ functional was devised as a revised form of SCAN with improved numerical stability. In this work, we refit the rVV10 functional to optimize the ${\mathrm{r}}^2\mathrm{SCAN}+\mathrm{rVV}10$ vdW density functional and test its performance for molecular interactions and layered materials. Our molecular tests demonstrate that ${\mathrm{r}}^2\mathrm{SCAN}+\mathrm{rVV}10$ outperforms its predecessor $\mathrm{SCAN}+\mathrm{rVV}10$ in both efficiency (numerical stability) and accuracy. This good performance is also found in lattice-constant predictions. In comparison with benchmark results from higher-level theories or experiments, ${\mathrm{r}}^2\mathrm{SCAN}+\mathrm{rVV}10$ yields excellent interlayer binding energies and phonon dispersions for layered materials.

We study the energetic stability and the local electronic structure of the general twisted trilayer graphene (TTG) with the top and bottom layers rotated with respect to the middle layer, respectively, by $\theta$ and ${\theta }^{\prime }$. Approximate supercells of the moiré-of-moiré superlattices with $\theta$ and ${\theta }^{\prime }$ within $1^{\circ }–2^{\circ }$ are established to describe the structural and electronic properties of relaxed TTG with the periodic boundary condition. Full relaxation demonstrates that the commensurate TTG with $\theta ={\theta }^{\prime }$ has the local minimum total energy ($E_{\text{tot}}$) at a fixed $\theta$, while $E_{\text{tot}}$ first reaches a local maximum and begins to drop with decreasing ${\theta }^{\prime }$ for ${\theta }^{\prime }<\theta$. Some regions exhibit enhanced in-plane relaxation in the top and bottom layers, but suppressed relaxation in the middle layer and form a hexagonal network with the moiré-of-moiré lengthscale. The stacking configurations with the atoms in the three layers vertically aligned at the origin of the relaxed TTG supercells at $\theta$ around $1.6^{\circ }$ and ${\theta }^{\prime }$ around $1.4^{\circ }$ have a high density of states (DOS) near the Fermi level ($E_F$), which can reach that of the mirror symmetric TTG with equal twist angles of about $1.7^{\circ }$. In contrast, some other stackings can have rather low DOS around $E_F$. The significant stacking dependence of DOS for some TTG supercells demonstrates that the local electronic structure of TTG can exhibit strong spatial inhomogeneity when the twist angles are slightly away from those of the small supercells with large variations of DOS among different stackings. Moreover, the structural relaxation of TTG plays a crucial role in the high DOS and its strong stacking dependence.

We studied the behavior of nontrivial Dirac fermion states in the antiferromagnetic metal SmBi using angle-resolved photoemission spectroscopy (ARPES). The experimental results exhibit multiple Fermi pockets around the $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ points along with a band inversion in the spectrum along the $\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ line consistent with the density functional theory results. In addition, ARPES data reveal Dirac cones at the $\overline{\mathrm{\Gamma }}$ and $\overline{M}$ points within the energy gap of the bulk bands. The Dirac cone at $\overline{M}$ exhibits a distinct Dirac point and is intense in the high-photon-energy data, while the Dirac cone at $\overline{\mathrm{\Gamma }}$ is intense at low photon energies. Employing ultrahigh-resolution ARPES, we discover destruction of a Fermi surface constituted by the surface states across the Neél temperature of 9 K. Interestingly, the Dirac cone at $\overline{\mathrm{\Gamma }}$ is found to be gapped at 15 K, and the behavior remains similar across the magnetic transition. These results reveal complex momentum-dependent gap formation and Fermi surface destruction across the magnetic transition in an exotic correlated topological material; the interplay between magnetism and topology in this system calls for ideas beyond the existing theoretical models.

Metal-organic frameworks (MOFs) have shown big potential in energy storage and thermal management, which are closely related to their thermal properties. It has been believed for some time that the vibrational mean free path in MOFs is relatively small and the resulting thermal conductivity has weak size dependence. By studying one of the typical MOFs, i.e., ${\mathrm{Cu}}_3{\left(\mathrm{BTC}\right)}_2$ where BTC is benzene-1,3,5-tricarboxylate, also referred to as HKUST-1, we computationally prove that the mean free path of these small wave vectors or equivalently low-frequency vibrations in HKUST-1 can be as large as ∼120 nm and the corresponding thermal conductivity is strongly size dependent, which is attributed to its good crystallinity. We further find that these long mean free path phonons are strongly scattered by the adsorbed water molecules and the thermal conductivity is decreased by ∼51.3% when the water molecules are adsorbed. Consequently, the thermal conductivity of HKUST-1 with water molecules is weakly size dependent. Two pathways for the thermal energy exchange, e.g., the phonons and the water molecules, exist in HKUST-1 with water molecules. The thermal conductivity of the adsorbed HKUST-1 is found to decrease and then increase with the quantity of the adsorbates owing to the competition between the two thermal pathways. Our study here provides a fundamental understanding on the thermal transport mechanisms in the metal-organic framework with consideration of adsorbates, which offers useful guidance for thermal management design in these thermal-related applications such as adsorption-driven heat pumps.

Topological insulators (TIs) hold promise as a platform for spintronics applications due to the fascinating spin-momentum locking (SML) of the surface states. One particular interest lies in using TIs as spin-polarized sources for spintronics structures. Here, we propose the band tilt induced nonlinear Nernst effect (NLNE) in TIs as a clean and efficient route to generate high-performance spin polarization (SP). We show that in the presence of SML and hexagonal warping effect a finite band tilt can induce an imbalance of two spin carriers and effectively give rise to spin-polarized NLNE current in TIs. In our scheme, both the spin current and charge current regime can be achieved under the thermal drive. The obtainable SP can be efficiently tuned in either a smooth or rapid way, exhibiting highly flexible tunability. In addition, near-unity SP can be generated within a wide range of tunable parameters, which is also found to be robust against temperature. Therefore, our work provides a mechanism to realize controllable room-temperature high-degree SP based on TIs, which is of essential importance for future spintronics applications.

Monolayer transition metal dichalcogenides (TMDs) host a variety of optically excited quasiparticle species that stem from two-dimensional confinement combined with relatively large carrier effective masses and reduced dielectric screening. The magnetic response of these quasiparticles gives information on their spin and valley configurations, nuanced carrier interactions, and insight into the underlying band structure. Recently, there have been several reports of $2s/3s$ charged excitons in TMDs, but very little is still known about their response to external magnetic fields. Using photoluminescence excitation spectroscopy, we observe the presence of the $2s$ charged exciton and report its response to an applied magnetic field. We benchmark this response against the neutral exciton and find that both the $2s$ neutral and charged excitons exhibit similar behavior with $g$ factors of $g_{X_0^{2s}}=-5.20\pm 0.11$ and $g_{X_{-}^{2s}}=-4.98\pm 0.11$, respectively.

Recent discoveries of charge order and electronic nematic order in the iron-based superconductors and cuprates have pointed towards the possibility of nematic and charge fluctuations playing a role in the enhancement of superconductivity. The ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Ni}}_2{\mathrm{As}}_2$ system, closely related in structure to the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system, has recently been shown to exhibit both types of ordering without the presence of any magnetic order. We report single-crystal x-ray diffraction experiments on ${\mathrm{Ba}}_{0.35}{\mathrm{Sr}}_{0.65}{\mathrm{Ni}}_2{\mathrm{As}}_2$, providing evidence that the previously reported incommensurate charge order with wave-vector ${(0,0.28,0)}_{\mathrm{tet}}$ in the tetragonal state of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ vanishes by 65% Sr substitution together with nematic order. Our measurements suggest that the nematic and incommensurate charge orders are closely tied in the tetragonal state and show that the ${(0,0.33,0)}_{\mathrm{tri}}$ charge ordering in the triclinic phase of ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ evolves to become ${(0,0.5,0)}_{\mathrm{tri}}$ charge ordering at $x=0.65$ before vanishing at $x=0.71$.

Investigation of charged defects is necessary to understand the properties of semiconductors. Whereas density functional theory calculations can accurately describe the relevant physical quantities, these calculations increase the computational loads substantially, which often limits the application of this method to large-scale systems. In this paper, we propose a different scheme of neural network potential (NNP) to analyze the point defect behavior in multiple charge states. The proposed scheme necessitates only minimal modifications to the conventional scheme. We demonstrated the prediction performance of the proposed NNP using wurzite-GaN with a nitrogen vacancy with charge states of 0, $1+$, $2+$, and $3+$. The proposed scheme accurately trained the total energies and atomic forces for all the charge states. Furthermore, it fairly reproduced the phonon band structures and thermodynamics properties of the defective structures. Based on the results of this paper, we expect that the proposed scheme can enable us to study more complicated defective systems and lead to breakthroughs in novel semiconductor applications.

We formulate a theory for resonances in the many-body localized (MBL) phase of disordered quantum spin chains in terms of local observables. A key result is to show that there are universal correlations between the matrix elements of local observables and the many-body level spectrum. This reveals how the matrix elements encode the energy scales associated with resonance, thereby allowing us to show that these energies are power-law-distributed. Using these results, we calculate analytically the distributions of local polarizations and of eigenstate fidelity susceptibilities. The first of these quantities characterizes the proximity of MBL systems to noninteracting ones, while the second highlights their extreme sensitivity to local perturbations. Our theoretical approach is to consider the effect of varying a local field, which induces a parametric dynamics of spectral properties. We corroborate our results numerically using exact diagonalization in finite systems.

We propose a scheme to realize bosonic fractional quantum Hall conductance in shaken honeycomb optical lattices. This scheme does not require a very flat band, and the necessary long-range interaction relies on $s$-wave scattering, which is common in many ultracold-atom experiments. By filling the lattice at 1/4 with identical bosons under Feshbach resonance, two degenerate many-body ground states share one Chern number of 1 and correspond exactly to the fractional quantum Hall conductance of 1/2. Meanwhile, we prove that the fractional quantum Hall state can be prepared by adiabatically turning on the lattice shaking, and the fractional conductance is robust in the shaken lattice. This provides an easy way to initialize and prepare the fractional quantum Hall states in ultracold-atom platforms, and it paves the way to investigate and simulate strongly correlated quantum matters with degenerate quantum gas.

We develop the coherent state representation of lattice vibrations to describe their interactions with charge carriers. In direct analogy to quantum optics, the coherent state representation leads from quantized lattice vibrations (phonons) naturally to a quasiclassical field limit, i.e., the deformation potential. To an electron, the deformation field is a sea of hills and valleys, as “real” as any external field, morphing and propagating at the sound speed, and growing in magnitude with temperature. In this disordered potential landscape, the charge carrier dynamics is treated nonperturbatively, preserving its coherence beyond single collision events. We show the coherent state picture agrees exactly with the conventional Fock state picture in perturbation theory. Furthermore, it goes beyond by revealing aspects that the conventional theory could not explain: transient localization even at high temperatures by charge carrier coherence effects, and band tails in the density of states due to the self-generated disorder (deformation) potential in a pure crystal. The coherent state paradigm of lattice vibrations supplies tools for probing important questions in condensed matter physics as in quantum optics.

Dissipative coupling of resonators arising from their cooperative dampings to a common reservoir induces intriguingly new physics such as energy level attraction. In this study, we report the nonlinear properties in a dissipatively coupled cavity magnonic system. A magnetic material yttrium iron garnet is placed at the magnetic field node of a Fabry-Perot-like microwave cavity such that the magnons and cavity photons are dissipatively coupled. Under high power excitation, a nonlinear effect is observed in the transmission spectra, showing bistable behaviors. The observed bistabilities are manifested as clockwise, counterclockwise, and butterfly-like hysteresis loops with different frequency detuning. The experimental results are well explained as a Duffing oscillator dissipatively coupled with a harmonic one and the required trigger condition for bistability could be determined quantitatively by the coupled oscillator model. Our results demonstrate that the magnon damping has been suppressed by the dissipative interaction, which thereby reduces the threshold for conventional magnon Kerr bistability. This work sheds light on potential applications in developing low power nonlinearity devices, enhanced anharmonicity sensors and for exploring the non-Hermitian physics of cavity magnonics in the nonlinear regime.

Realizing sizable Dzyaloshinskii-Moriya interaction (DMI) in intrinsic two-dimensional (2D) magnets without any manipulation will greatly enrich potential application of spintronics devices. The simplest and most desirable situation should be 2D magnets with intrinsic DMI and intrinsic chiral spin textures. Here, we propose to realize DMI by designing periodic ripple structures with different curvatures in low-dimensional magnets and demonstrate the concept in both one-dimensional ${\mathrm{CrBr}}_2$ and 2D ${\mathrm{MnSe}}_2$ magnets by using first-principles calculations. We find that DMIs in curved ${\mathrm{CrBr}}_2$ and ${\mathrm{MnSe}}_2$ can be efficiently controlled by varying the curvature $c$, where $c$ is defined as the ratio between the height $h$ and the length $l$ of the curved structure. Moreover, we unveil that the dependence of first-principles calculated DMI on curvature $c$ can be well described by the three-site Fert-Lévy model. At last, we uncover that field-free magnetic skyrmions can be realized in curved ${\mathrm{MnSe}}_2$ by using atomistic spin model simulations based on first-principles calculated magnetic parameters. The work will open an avenue for inducing DMI and chiral spin textures in simple 2D magnets via curvature.

We investigate the temperature dependence of $g$ factor in two-dimensional (2D) ferromagnet ${\mathrm{CrSiTe}}_3$ by combining three-dimensional vector-ferromagnetic resonance (FMR) experiments. It is shown from the angular dependence that the dramatic $g$ shift near critical temperature originates from the interplay of enhanced magnetic fluctuations and anisotropic spin interactions in 2D magnetism. The crossover from isotropic to anisotropic $g$ factor for $T\to T_c$ yields anomalously parallel pumped excitations. In the critical regime, the field suppression of critical fluctuations is associated with the field dependence of $g$ shift. Furthermore, the results of FMR $g$ shift are scaled by susceptibilities and spontaneous magnetization obtained from magnetometry measurements, which show good agreement with the universality class of the 2D Ising model.

Superconductivity was recently observed in boron-doped $(\sqrt{3}\times \sqrt{3})\mathrm{Sn}/\mathrm{Si}(111)$. The material can be described by an extended Hubbard model on a triangular lattice. Here, we use the random-phase approximation to investigate the charge and spin fluctuations as well as the superconducting properties of the system with respect to filling and the relative strength of the extended versus the on-site Hubbard interactions. Our calculations reveal that near half-filling and weak extended Hubbard interactions, the superconducting ground state exhibits chiral $d$-wave pairing. Far from half filling and for stronger nearest-neighbor Coulomb interactions, the system shows chiral $p$-wave (hole-doping) and $f$-wave (electron-doping) pairings. The dependence of the pairing symmetry on the extended Hubbard interactions suggests that charge fluctuations play an important role in the formation of Cooper pairs. Finally, the temperature dependence of the Knight shift is calculated for all observed superconducting textures and put forward as an experimental method to examine the symmetry of the superconducting gap function.

Unlocking the mystery of the strange metal state has become the focal point of high-$T_c$ research, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed “quantum supreme matter.” Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low-$T_c$ cuprate superconductor, ${\mathrm{Bi}}_{2-x}{\mathrm{Pb}}_x{\mathrm{Sr}}_{2-y}{\mathrm{La}}_y{\mathrm{CuO}}_{6+\delta }$ allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are twofold: (i) a low energy Drude response with Drude width on the order of temperature and (ii) a high energy conformal tail with a doping dependent power-law exponent. While the Drude weight evolves monotonically throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed “return to normalcy” is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width.