The discovery of two-dimensional (2D) ferroelectric materials has prompted the study of thin ferroelectric heterostructures for advanced functional device applications. Based on the first-principles density functional theory, we investigate the twist-angle-dependent ferroelectricity and ferromagnetism of 2D ferroelectric ${\mathrm{In}}_2{\mathrm{Se}}_3$/ferromagnetic ${\mathrm{CrBr}}_3$ twisted moiré heterobilayers. We show that, as the twist angle increases, the stronger structural distortions induced by the moiré pattern cause more changes in charge transfers, thus consistently affecting the ferroelectricity and ferromagnetism of heterobilayers. We also show that structural distortions and their induced charge transfer determine the changing tendency of band edges and band gaps with respect to the twist angle. Our findings provide fundamental knowledge of the effects of the twist-angle-dependent moiré pattern on the ferroelectric and ferromagnetic heterobilayer and facilitate the development of ferroelectric magnetic devices.

Carbon nanodots (CNDs) are surface-passivated fluorescent nanomaterials which can be potentially applied for, e.g., full-color lighting and displays. Here, we present a pioneering study on magneto-optical properties of nitrogen-doped CNDs (N-CNDs) using terahertz (THz) time-domain spectroscopy in the presence of the magnetic fields from 0 to 8 T at a fixed temperature of 80 K. In the Faraday geometry and applying the optical polarization tests, we can measure the real and imaginary parts of the left- and right-handed circularly polarized magneto-optical conductances for a thin N-CND layer on a sapphire substrate, which can fit well with the magneto-optical Drude-Smith formula we previously generalized. By fitting the experimental data with the theoretical model, we can determine magneto-optically the key electronic parameters of N-CND layer, such as the electron density related coefficient $\gamma$, the electronic relaxation time $\tau$, the photon-induced electronic localization factor $\alpha$, and particularly, the effective electron mass $m^{*}$ obtained under noncyclotron resonance condition. The impact of the magnetic field on these parameters is investigated. We find that the presence of the magnetic field can result in smaller $\gamma$ and $\alpha$, heavier $m^{*}$, and longer $\tau$ in the N-CND layer. The outcomes of this work are valuable in providing us with a deeper understanding of the basic electronic properties of N-CNDs.

Altermagnetism has extended the magnetic landscape beyond conventional ferromagnetism and antiferromagnetism. Here, we report a general scheme to realize phase transitions between ferromagnetism and altermagnetism, as well as between antiferromagnetism and altermagnetism, in various two-dimensional magnetic systems. Specifically, we predict a series of $A_{2}M{X}_4$-related ($A=3d$ transition metals; $M=\mathrm{W}$ or Mo; $X=\mathrm{S}$, Se) materials that exhibit diverse magnetic ground states, including ferromagnetism, antiferromagnetism, and altermagnetism. Through interlayer sliding methods, interlayer exchange coupling in bilayer $A_{2}M{X}_4$ can be effectively controlled and switched between ferromagnetic and antiferromagnetic coupling, thus realizing an intrinsic phase transition between different magnetic orders, as exemplified by bilayer ${\mathrm{V}}_2\mathrm{Mo}{\mathrm{S}}_4$, $\mathrm{F}{\mathrm{e}}_2\mathrm{Mo}{\mathrm{S}}_4$, and $\mathrm{MnP}{\mathrm{S}}_3$. More interestingly, we show that layer-dependent phase transition from ferromagnetism to altermagnetism and from antiferromagnetism to altermagnetism can be tailored in the representative ${\mathrm{V}}_{2}M{X}_4$ and $\mathrm{F}{\mathrm{e}}_{2}M{X}_4$ due to interlayer exchange coupling breaks and preserves certain combined symmetries. Our results extend the concept of experimentally observed layer-dependent ferromagnetic-to-antiferromagnetic phase transitions, as seen in systems such as $\mathrm{Cr}{\mathrm{I}}_3$ and $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$. These findings highlight the facile transitions between the recorded magnetic phases, providing insights into the manifestation of altermagnetic properties in two-dimensional lattice, which opens avenues for designing new spintronic devices through layer effect.

The kagome lattice has gained intense attention as a prototypal system for hosting a range of exotic quantum states. Within this family of kagome-based materials, ${R\mathrm{Mn}}_6{\mathrm{Sn}}_6$ compounds (R=rare-earth element) are notable for fostering intricate interactions among band topology, electron correlation, and magnetism. Recently, exchange-bias (EB) effects were identified in kagome systems, marked by a shifted hysteresis loop after cooling the sample in an external magnetic field, suggesting its potential application in spintronics due to enhanced magnetic state stability. There is ample evidence of EB observed in bulk systems that are not limited to heterostructures, but instead host multiple coexisting magnetic phases, such as antiferromagnetic and ferromagnetic. In this work, we discover that EB arises in certain ${R\mathrm{Mn}}_6{\mathrm{Sn}}_6$ materials possessing a perpendicular magnetic anisotropy component and a spin-glass cluster. The existence of two different switching mechanisms for each branch of the hysteresis loop gives rise to the presence of EB. In particular, we unveil the role of the spin-glass cluster in the unconventional switching process. Our work offers an in-depth understanding of new magnetic properties of kagome magnets and sets up a potential platform for designing stable magnetic devices.

${\mathrm{ErCo}}_2$ is a compound which has recently attracted attention as a potential magnetic refrigeration material for hydrogen liquefaction. The interest in the material is due to the large magnetocaloric effect it exhibits, the tunability of the transition temperature, and the recent discovery of the tunability of the order of the transition by small substitutions of transition metals [X. Tang et al., Nat. Commun. 13, 1817 (2022)]. The reason for the change in the order of the transition is not yet understood. In this study the microscopic magnetic correlations and crystal lattice distortions in ${\mathrm{ErCo}}_{2-x}T{}_x$ ($T$=Fe and Ni) are investigated using neutron and synchrotron x-ray diffraction, as well as AC susceptibility measurements. The results indicate that in contrast to the previous study, the cubic to rhombohedral structure change was not suppressed, although the way the unit cell expansion occurred did change. No significant changes were observed in the magnetic long-range order and crystal lattice symmetry, which still formed a ferrimagnetic and rhombohedral crystal structure below the phase transition temperature $T_{\text{C}}$ for all substitutions as well as pure ${\mathrm{ErCo}}_2$. However, an increase in magnetic short-range correlations above $T_{\text{C}}$, which is strongly correlated with the entropy change and the dynamic AC susceptibility, was detected only in the substituted samples. Unlike pure ${\mathrm{ErCo}}_2$, the nucleation of the local ferrimagnetic clusters above $T_{\text{C}}$ occurs in the substituted samples, which significantly reduces the energy barrier between the higher-temperature cubic phase and the lower-temperature rhombohedral phase. We conclude that the reduction of the energy barrier due to the development of short-range order induced by chemical substitution at temperatures above $T_{\text{C}}$ is possibly the origin of the significant change in the nature of the phase transition in ${\mathrm{ErCo}}_2$.

We consider multiband BCS superconductors that exhibit time-reversal symmetry and uniform pairing and analyze their dynamic density and spin structure factors using linear-response theory within the mean-field BCS-BEC crossover framework at zero temperature. Our results for the multiorbital Hubbard model satisfy the associated $f$-sum rules in several limits. In particular, in the strong-coupling limit, they coincide with those of a weakly interacting Bose gas of Cooper pairs, where the low-energy collective Goldstone modes serve as Bogoliubov phonons. We further reveal that the quantum-geometric origin of the low-energy structure factors, along with related observables such as the superfluid-weight tensor and the effective-mass tensor of Cooper pairs, can be traced all the way back to the effective-mass theorem for Bloch bands in this limit. As an illustration, we investigate the pyrochlore-Hubbard model numerically and demonstrate that the Goldstone modes are the only relevant collective degrees of freedom in the flat-band regime.

We study the interplay between repulsive interaction and superfluidity in flat band systems. We consider spatially indirect excitons in the Lieb lattice bilayer as an example. We show that due to the presence of repulsive interaction, the excitons may form an incoherent fluid. With an increase of the quantum metric, the exciton fluid experiences a transition into the superfluid phase. This transition may be captured by a model of the Josephson junction array, which is beyond the mean field description.

We investigate the interaction between microwave photons and magnons in a system of two magnetic spheres within a microwave cavity, subjected to time-periodic driving fields. Through Floquet engineering, we demonstrate on-demand control over the cavity-magnon coupled dynamics, enabling tunable nonlocal magnon-magnon coupling mediated by the cavity mode. This approach facilitates reversible bright-dark mode conversion, overcoming the limitations of static coupling schemes and paving the way for coherent control of diverse systems within microwave cavities.

Antiferromagnetic resonance in a bulk $\alpha \text{−}\mathrm{MnTe}$ crystal is investigated using both frequency-domain and time-domain THz spectroscopy techniques. At low temperatures, an excitation at the photon energy of $(3.5\pm 0.1)$ meV is observed and identified as a magnon mode through its distinctive dependence on temperature and magnetic field. This behavior is reproduced using a simplified model for antiferromagnetic resonance in an easy-plane antiferromagnet. The results of our experiments, when compared with exchange constants established in the literature, allow us to extract the out-of-plane component of the single-ion magnetic anisotropy reaching $(40\pm 10)\mu \mathrm{eV}$.

We study superconductivitiy under light irradiation based on the Floquet-Magnus expansion in the high-frequency regime. We find that, in spin-singlet superconductors with spin-orbit coupling, triplet superconductivity can be induced in the first-order perturbation for dynamical gap functions and the second-order perturbation for static gap functions. We also show that, in unitary triplet superconductors with altermagnetism, nonunitary triplet superconductivity can emerge in the first-order perturbation for a dynamical gap function and in the second-order perturbation for static gap functions. These results indicate optical generation and control of triplet superconductivity.

We revisit the challenging problem of identifying the quantum spin liquid candidate in the spin-1/2 $J_1\text{−}{J}_2$ Heisenberg antiferromagnet on the square lattice. By integrating the Gutzwiller-guided density matrix renormalization group method with analytical analyses, we present clear evidence that the ground state is a ${\mathrm{Z}}_2$ Dirac spin liquid. This state can be efficiently described by a Gutzwiller-projected parton theory characterized by its projective symmetry group. To distinguish the difference between the projected ${\mathrm{Z}}_2$ and U(1) parton state, we investigate the chiral spin liquid ground states as topological orders by incorporating a $J_{\chi }$ term into the $J_1\text{−}{J}_2$ model and observe a transition from a ${\mathrm{Z}}_2$ chiral spin liquid to a $\mathrm{U}{\left(1\right)}_2$ chiral spin liquid as $J_{\chi }$ increases.

The investigation of the electronic structures on perovskite oxides using surface-sensitive spectroscopy techniques is often hindered by their “uncleavable” nature, typically requiring expensive and complex in situ experimental setups that integrate the capabilities of sample synthesis and spectroscopy measurement under ultrahigh vacuum condition. Here, we address this challenge by developing an ozone-annealing process that yields atomically flat surfaces on perovskite oxide thin films, making them suitable for high-resolution angle-resolved photoemission spectroscopy measurements. Using this method, we present a three-dimensional electronic structure study of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_3$ ($x=0$ and 0.175) thin films with unprecedented accuracy. The experimentally determined low-energy fermiology exhibits quantitative agreements with two-band tight-binding simulations, which is further validated by first-principles calculations considering the material's actual crystal structure. This work provides an accessible approach for ex situ ARPES measurements on perovskite oxides and other strongly correlated oxides, including the recently discovered high-$T_{\text{c}}$ nickelates.

By means of Raman scattering and infrared reflectivity spectroscopies under high pressure, we investigate the metallic phase induced by pressure at $P_{cr}=17.5$ GPa in the Mott insulator $\left({\mathrm{NaMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$ (NMO). Upon approaching $P_{cr}$, two signatures of the presence of mobile carriers become increasingly pronounced: a Fano-like lineshape of the Raman modes and a Drude-like peak in the optical conductivity spectra. Remarkably, all Raman and infrared modes display a continuous evolution with pressure up to 22 GPa, indicating that no structural distortion occurs at the transition owing to the very compact quadruple perovskite structure of NMO. We therefore propose a unique scenario of a second-order Mott phase transition where only the electronic degrees of freedom are active, which opens the possibility of probing the effective Hamiltonian of the transition disentangled from the lattice degrees of freedom.

We investigate the photocurrent induced by bicircular light (BCL) in materials, with a focus on its multistate geometric nature. BCL, a combination of left- and right-circularly polarized light, can generate both injection and shift currents, originating from the geometric properties of gauge-invariant shift vectors, quantum geometric tensors, and triple-phase products. Crucially, the real parts of the quantum geometric tensors and triple-phase products remain nonzero in centrosymmetric systems, facilitating photocurrent generation in contrast to the traditional shift current bulk photovoltaic effect. Using a diagrammatic approach, we systematically analyze the BCL-induced photocurrents and demonstrate the multistate geometric nature within a one-dimensional three-site Rice-Mele model. Our findings provide a quantum geometric understanding of BCL-induced photocurrents, underscoring the importance of considering multiband contributions in real materials.

Tuning superconductivity in topological materials has been identified as a promising pathway for achieving topological superconductivity, which could accelerate advancements in topological quantum computation. Ternary transition-metal chalcogenides ${ABX}_4$ (where $A/B$ = Zr, Hf, or Ti; $X$ = Te) have been theoretically predicted to be candidates for two-dimensional (2D) topological insulators. In this study we systematically investigate the crystal structure, electronic band properties, and pressure effects of one member of this family, namely, $\mathrm{ZrTiT}{\mathrm{e}}_4$. Our single-crystal x-ray diffraction studies reveal that $\mathrm{ZrTiT}{\mathrm{e}}_4$ adopts a trigonal structure with space group $P\overline{3}m1$ (no. 164), rather than the previously reported monoclinic ($P2/m$) structure. Band structure calculations, complemented by angle-resolved photoemission spectroscopy measurements, indicate that $\mathrm{ZrTiT}{\mathrm{e}}_4$ exhibits a topological semimetal nature. Notably, the application of high pressure induces superconductivity, with a maximum transition temperature ($T_{\mathrm{c}}$) of approximately 5.6 K at pressures around 11–14 GPa. Intriguingly, in the optimal pressure region of the dome-shaped phase diagram, the normal-state resistance shows the highest values, and a nearly linear temperature dependence of resistance is observed above $T_{\mathrm{c}}$. Overall, our results establish the ${ABX}_4$ family as a promising candidate for exploring the interaction between the nontrivial band topology and superconductivity.

We study the three-dimensional topological insulator ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ in its ultrathin limit, i.e., when the thickness is of the same order as the surface-state penetration depth. It is expected that in this limit a hybridization gap opens at the Dirac point, which gives rise to a quantum spin Hall (QSH) or insulating phase, depending on the material thickness. We fabricate ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$ Hall bars with a thicknesses of 6 and 9 nm and measure an insulating phase around the Dirac point for low bias and at sub-Kelvin temperatures only in samples fabricated from the 6 nm films, which indicates the presence of a hybridization gap. The effect of a perpendicular magnetic field on the hybridization gap is studied but remains partially unresolved. The results form an important step towards experimentally realizing the quantum spin Hall state via hybridization in ultrathin films of ${\left({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x\right)}_2{\mathrm{Te}}_3$, yet, they also expose a knowledge gap regarding transport measurements in these systems.

We propose the realization of interaction-driven insulators in periodically strained monolayer graphene and transition metal dichalcogenides (TMDs). Through extensive many-body exact diagonalization, we provide compelling evidence for various fractional Chern insulators (FCIs) in both strained monolayer graphene and TMDs, including the Laughlin states, Halperin states, and FCIs with tunable topological orders in Chern number $\left|\mathcal{C}\right|=2$ bands. We also discuss the relationship among band geometry, band filling, and spin polarization. Notably, by examining both the entanglement spectrum and many-body Chern number, we reveal a state with Laughlin-like topological order emerging in the $\left|\mathcal{C}\right|=2$ band, which challenges the existing theoretical understanding of high Chern number (high-$\mathcal{C}$) FCIs. These findings suggest that periodically strained monolayer graphene and TMDs provide promising platforms for engineering fractional Chern insulators and underscore the need for further investigation into high-$\mathcal{C}$ FCIs.

Size-dependent behavior of bismuth nanowire networks under high-pressure conditions was investigated at room temperature. Three-dimensional networks of freestanding interconnected bismuth nanowires with diameters between 34 and $85\mathrm{n}\mathrm{m}$ were synthesized by electrodeposition in ion-track etched membranes, along with microcrystals as bulk analogues. Both types of samples were simultaneously compressed in diamond anvil cells under hydrostatic conditions up to $19.5\mathrm{G}\mathrm{Pa}$. Synchrotron x-ray diffraction data reveal a shift of the Bi-I/-II, Bi-II/-III, and Bi-III/-V phase transition to higher-pressure values for decreasing nanowire diameter. In case of the thinnest wires, compression and decompression cycles revealed hysteresislike behavior of the shifts and a pronounced coexistence of the Bi-III and Bi-V phases upon compression. All samples exhibited bulklike compression behavior, as reflected in the evolution of their lattice parameter and bulk modulus. Only nanowires in the Bi-III phase showed a slightly reduced modulus compared to the corresponding bulk value. The systematic size dependence highlights the importance of excellent control on the geometrical parameters of the nanowires and consistent experimental conditions. The mechanical stability of the three-dimensional nanowire networks allowed the pressurization of samples of varying wire diameters under identical experimental conditions and facilitates systematic studies of size effects in nanomaterials with diameters as small as $10\mathrm{n}\mathrm{m}$.