We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$ diamond-like units can host high-temperature superconductivity. The layer appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co pairs form a square lattice. The low energy physics near Fermi energy can be described by an effective two-orbital model. Without considering interlayer couplings, the two orbitals are effectively decoupled. This electronic structure satisfies the “gene” character proposed for unconventional high-temperature superconductors. We predict that the leading superconducting pairing instability is driven from an extended $s$-wave ($s^\pm$) to a $d$-wave by hole doping, e.g., in Ba$_{1-x}$K$_x$Co$_2$N$_2$. This study provides a new platform to establish the superconducting mechanism of unconventional high-temperature superconductivity.

We report the discovery of superconductivity on the border of antiferromagnetic order in a quasi-one-dimensional material RbMn$_{6}$Bi$_{5}$ via measurements of resistivity and magnetic susceptibility under high pressures. Its phase diagram of temperature versus pressure resembles those of many magnetism-mediated superconducting systems. With increasing pressure, its antiferromagnetic ordering transition with $T_{\rm N} = 83$ K at ambient pressure is first enhanced moderately and then suppressed completely at a critical pressure of $P_{\rm c} \approx 13$ GPa, around which bulk superconductivity emerges and exhibits a dome-like $T_{\rm c}(P)$ with a maximal $T_{\rm c}^{\rm onset} \approx 9.5$ K at about 15 GPa. In addition, the superconducting state around $P_{\rm c}$ is characterized by a large upper critical field $\mu_{0}H_{\rm c2}(0)$ exceeding the Pauli paramagnetic limit, implying a possible unconventional paring mechanism. The present study, together with our recent work on KMn$_{6}$Bi$_{5}$ (the maximum $T_{\rm c}^{\rm onset} \approx 9.3$ K), makes $A$Mn$_{6}$Bi$_{5}$ ($A$ = alkali metal) a new family of Mn-based superconductors with relatively high $T_{\rm c}$.

Realization of Kondo lattice in superconducting van der Waals materials not only provides a unique opportunity for tuning the Kondo lattice behavior by electrical gating or intercalation, but also is helpful for further understanding the heavy fermion superconductivity. Here we report a low-temperature and vector-magnetic-field scanning tunneling microscopy and spectroscopy study on a superconducting compound (4Hb-TaS$_{2})$ with alternate stacking of 1T-TaS$_{2}$ and 1H-TaS$_{2}$ layers. We observe the quasi-two-dimensional superconductivity in the 1H-TaS$_{2}$ layer with anisotropic response to the in-plane and out-of-plane magnetic fields. In the 1T-TaS$_{2}$ layer, we detect the Kondo resonance peak that results from the Kondo screening of the unpaired electrons in the Star-of-David clusters. We also find that the intensity of the Kondo resonance peak is sensitive to its relative position with the Fermi level, and it can be significantly enhanced when it is further shifted towards the Fermi level by evaporating Pb atoms onto the 1T-TaS$_{2}$ surface. Our results not only are important for fully understanding the electronic properties of 4Hb-TaS$_{2}$, but also pave the way for creating tunable Kondo lattice in the superconducting van der Waals materials.

Megabar pressures are of crucial importance for cutting-edge studies of condensed matter physics and geophysics. With the development of diamond anvil cell (DAC), laboratory studies of high pressure have entered the megabar era for decades. However, it is still challenging to implement in situ magnetic sensing under ultrahigh pressures. In this work, we demonstrate optically detected magnetic resonance and coherent quantum control of diamond nitrogen-vacancy (NV) center, a promising quantum sensor inside the DAC, up to 1.4 Mbar. The pressure dependence of optical and spin properties of NV centers in diamond are quantified, and the evolution of an external magnetic field has been successfully tracked at about 80 GPa. These results shed new light on our understanding of diamond NV centers and pave the way for quantum sensing under extreme conditions.

Antiferromagnetic materials are exciting quantum materials with rich physics and great potential for applications. On the other hand, an accurate and efficient theoretical method is highly demanded for determining critical transition temperatures, Néel temperatures, of antiferromagnetic materials. The powerful graph neural networks (GNNs) that succeed in predicting material properties lose their advantage in predicting magnetic properties due to the small dataset of magnetic materials, while conventional machine learning models heavily depend on the quality of material descriptors. We propose a new strategy to extract high-level material representations by utilizing self-supervised training of GNNs on large-scale unlabeled datasets. According to the dimensional reduction analysis, we find that the learned knowledge about elements and magnetism transfers to the generated atomic vector representations. Compared with popular manually constructed descriptors and crystal graph convolutional neural networks, self-supervised material representations can help us to obtain a more accurate and efficient model for Néel temperatures, and the trained model can successfully predict high Néel temperature antiferromagnetic materials. Our self-supervised GNN may serve as a universal pre-training framework for various material properties.

We performed high-pressure transport studies on the flat-band Kagome compounds, Pd$_3$P$_2$(S$_{1-x}$Se$_x$)$_8$ ($x=0$, 0.25), with a diamond anvil cell. For both compounds, the resistivity exhibits an insulating behavior with pressure up to 17 GPa. With pressure above 20 GPa, a metallic behavior is observed at high temperatures in Pd$_3$P$_2$S$_8$, and superconductivity emerges at low temperatures. The onset temperature of superconducting transition $T_{\rm C}$ rises monotonically from 2 K to 4.8 K and does not saturate with pressure up to 43 GPa. For the Se-doped compound Pd$_3$P$_2$(S$_{0.75}$Se$_{0.25}$)$_8$, the $T_{\rm C}$ is about 1.5 K higher than that of the undoped one over the whole pressure range, and reaches 6.4 K at 43 GPa. The upper critical field with field applied along the $c$ axis at typical pressures is about 50$\%$ of the Pauli limit, suggesting a 3D superconductivity. The Hall coefficient in the metallic phase is low and exhibits a peaked behavior at about 30 K, which suggests either a multi-band electronic structure or an electron correlation effect in the system.

We study a gate-tunable superconducting qubit (gatemon) based on a thin InAs-Al hybrid nanowire. Using a gate voltage to control its Josephson energy, the gatemon can reach the strong coupling regime to a microwave cavity. In the dispersive regime, we extract the energy relaxation time $T_1\sim0.56$ µs and the dephasing time $T_2^* \sim0.38$ µs. Since thin InAs-Al nanowires can have fewer or single sub-band occupation and recent transport experiment shows the existence of nearly quantized zero-bias conductance peaks, our result holds relevancy for detecting Majorana zero modes in thin InAs-Al nanowires using circuit quantum electrodynamics.

Recently, by intercalating organic ions into bulk FeSe superconductors, two kinds of layered FeSe-based superconductors [(TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe] with superconducting transition temperatures ($T_{\rm c}$) above 40 K have been discovered. Due to the large interlayer distance ($\sim $15 Å), these new layered superconductors have a large resistivity anisotropy analogous to bismuth-based cuprate superconductors. Moreover, remarkable pseudogap behavior well above $T_{\rm c}$ is revealed by nuclear magnetic resonance (NMR) measurements on $^{77}$Se nuclei, suggesting a preformed pairing scenario similar to that of cuprates. Here, we report another new kind of organic-ion-intercalated FeSe superconductor, (PY)$_{x}$FeSe, with a reduced interlayer distance ($\sim $10 Å) compared to (TBA)$_{x}$FeSe and (CTA)$_{x}$FeSe. By performing $^{77}$Se NMR and transport measurements, we observe a similar pseudogap behavior well above $T_{\rm c}$ of $\sim $40 K and a large resistivity anisotropy of $\sim$$10^{\boldsymbol{4}}$ in (PY)$_{x}$FeSe. All these facts strongly support a universal pseudogap behavior in these layered FeSe-based superconductors with quasi-two-dimensional electronic structures.

In the context of tensor network states, we for the first time reformulate the corner transfer matrix renormalization group (CTMRG) method into a variational bilevel optimization algorithm. The solution of the optimization problem corresponds to the fixed-point environment pursued in the conventional CTMRG method, from which the partition function of a classical statistical model, represented by an infinite tensor network, can be efficiently evaluated. The validity of this variational idea is demonstrated by the high-precision calculation of the residual entropy of the dimer model, and is further verified by investigating several typical phase transitions in classical spin models, where the obtained critical points and critical exponents all agree with the best known results in literature. Its extension to three-dimensional tensor networks or quantum lattice models is straightforward, as also discussed briefly.

Superlattice potentials are theoretically predicted to modify the single-particle electronic structures. The resulting Coulomb-interaction-dominated low-energy physics would generate highly novel many-body phenomena. Here, by in situ tunneling spectroscopy, we show the signatures of superstructure-modulated correlated electron states in epitaxial bilayer graphene (BLG) on 6H-SiC(0001). As the carrier density is locally quasi-‘tuned’ by the superlattice potentials of a $6 \times 6$ interface reconstruction phase, the spectral-weight transfer occurs between the two broad peaks flanking the charge-neutral point. Such a detected non-rigid band shift beyond the single-particle band description implies the existence of correlation effects, probably attributed to the modified interlayer coupling in epitaxial BLG by the $6 \times 6$ reconstruction as in magic-angle BLG by the moiré potentials. Quantitative analysis suggests that the intrinsic interface reconstruction shows a high carrier tunability of $\sim $1/2 filling range, equivalent to the back gating by a voltage of $\sim $70 V in a typical gated BLG/SiO$_{2}$/Si device. The finding in interface-modulated epitaxial BLG with reconstruction phase extends the BLG platform with electron correlations beyond the magic-angle situation, and may stimulate further investigations on correlated states in graphene systems and other van der Waals materials.

The switchability between the two ferroelectric (FE) states of an FE material makes FEs widely used in memories and other electronic devices. However, for conventional FEs, its FE switching only occurs between the two FE states whose spatial inversion symmetry is broken. The search for FE materials is therefore subject to certain limitations. We propose a new type of FEs whose FE states still contain spatial inversion centers. The change in polarization of this new type of FEs originates from electronic transfer between two centrosymmetric FE states under an external electric field. Taking BaBiO$_{3}$ as an example, we show that charge-ordering systems can be a typical representative of this new type of FEs. Moreover, unlike traditional ferroelectrics, the change in polarization in this new type of FEs is quantum in nature with the direction dependent on the specific FE transition path. Our work therefore not only extends the concept of FEs but may also open up a new way to find multiferroics.

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced. The exciton condensation can result in exotic physics such as super-fluidity and insulating state. In charge density wave (CDW) state, 1T-TiSe$_{2}$ is one of the candidates that may host the exciton condensation. However, to envision its excitonic effect is still challenging, particularly at the two-dimensional limit, which is applicable to future devices. Here, we realize the epitaxial 1T-TiSe$_{2}$ bilayer, the two-dimensional limit for its $2 \times 2\times 2$ CDW order, to explore the exciton-associated effect. By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference, we discover an unexpected state residing below the conduction band and right within the CDW gap region. As corroborated by our theoretical analysis, this mysterious phenomenon is in good agreement with the electron-exciton coupling. Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Employing a comprehensive structure search and high-throughput first-principles calculation method on 1561 compounds, the present study reveals the phase diagram of Lu–H–N. In detail, the formation energy landscape of Lu–H–N is derived and utilized to assess the thermodynamic stability of each compound that is created via element substitution. The result indicates that there is no stable ternary structure in the Lu–H–N chemical system, however, metastable ternary structures, such as Lu$_{20}$H$_{2}$N$_{17}$ $(C2/m)$ and Lu$_{2}$H$_{2}$N ($P\bar{3}m1$), are observed to have small $E_{\rm hull}$ ($ < 100$ meV/atom). It is also found that the energy convex hull of the Lu–H–N system shifts its shape when applying hydrostatic pressure up to 10 GPa, and the external pressure stabilizes a couple of binary phases such as LuN$_{9}$ and Lu$_{10}$H$_{21}$. Additionally, interstitial voids in LuH$_{2}$ are observed, which may explain the formation of Lu$_{10}$H$_{21}$ and LuH$_{3-\delta}$N$_{\epsilon}$. To provide a basis for comparison, x-ray diffraction patterns and electronic structures of some compounds are also presented.

Our understanding of how photons couple to different degrees of freedom in solids forms the bedrock of ultrafast physics and materials sciences. In this review, the emergent ultrafast dynamics in condensed matter at the attosecond timescale have been intensively discussed. In particular, the focus is put on recent developments of attosecond dynamics of charge, exciton, and magnetism. New concepts and indispensable role of interactions among multiple degrees of freedom in solids are highlighted. Applications of attosecond electronic metrology and future prospects toward attosecond dynamics in condensed matter are further discussed. These pioneering studies promise future development of advanced attosecond science and technology such as attosecond lasers, laser medical engineering, and ultrafast electronic devices.

As a typical hole-doped cuprate superconductor, Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$(Bi2212) carrier doping is mostly determined by its oxygen content. Traditional doping methods can regulate its doping level within the range of hole doping. Here we report the first application of CaH$_{2}$ annealing method in regulating the doping level of Bi2212. By continuously controlling the anneal time, a series of differently doped samples can be obtained. The combined experimental results of x-ray diffraction, scanning transmission electron microscopy, resistance and Hall measurements demonstrate that the CaH$_{2}$ induced topochemical reaction can effectively change the oxygen content of Bi2212 within a very wide range, even switching from hole doping to electron doping. We also found evidence of a low-$T_{\rm c}$ superconducting phase in the electron doping side.

The simple kagome-lattice band structure possesses Dirac cones, flat band, and saddle point with van Hove singularities in the electronic density of states, facilitating the emergence of various electronic orders. Here we report a titanium-based kagome metal CsTi$_{3}$Bi$_{5}$ where titanium atoms form a kagome network, resembling its isostructural compound CsV$_{3}$Sb$_{5}$. Thermodynamic properties including the magnetization, resistance, and heat capacity reveal the conventional Fermi liquid behavior in the kagome metal CsTi$_{3}$Bi$_{5}$ and no signature of superconducting or charge density wave (CDW) transition anomaly down to 85 mK. Systematic angle-resolved photoemission spectroscopy measurements reveal multiple bands crossing the Fermi level, consistent with the first-principles calculations. The flat band formed by the destructive interference of hopping in the kagome lattice is observed directly. Compared to CsV$_{3}$Sb$_{5}$, the van Hove singularities are pushed far away above the Fermi level in CsTi$_{3}$Bi$_{5}$, in line with the absence of CDW. Furthermore, the first-principles calculations identify the nontrivial $\mathbb{Z}_2$ topological properties for those bands crossing the Fermi level, accompanied by several local band inversions. Our results suppose CsTi$_{3}$Bi$_{5}$ as a complementary platform to explore the superconductivity and nontrivial band topology.

As a newly developed method for fabricating Josephson junctions, a focused helium ion beam has the advantage of producing reliable and reproducible junctions. We fabricated Josephson junctions with a focused helium ion beam on our 50 nm YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ (YBCO) thin films. We focused on the junction with irradiation doses ranging from 100 to 300 ions/nm and demonstrated that the junction barrier can be modulated by the ion dose and that within this dose range, the junctions behave like superconductor–normal-conductor–superconductor junctions. The measurements of the $I$–$V$ characteristics, Fraunhofer diffraction pattern, and Shapiro steps of the junctions clearly show AC and DC Josephson effects. Our findings demonstrate high reproducibility of junction fabrication using a focused helium ion beam and suggest that commercial devices based on this nanotechnology could operate at liquid nitrogen temperatures.

We report the interplay between two different topological phases in condensed matter physics, the magnetic chiral domain wall (DW), and the quantum anomalous Hall (QAH) effect. It is shown that the chiral DW driven by Dzyaloshinskii–Moriya interaction can divide the uniform domain into several zones where the neighboring zone possesses opposite quantized Hall conductance. The separated domain with a chiral edge state (CES) can be continuously modified by external magnetic field-induced domain expansion and thermal fluctuation, which gives rise to the reconfigurable QAH effect. More interestingly, we show that the position of CES can be tuned by spin current driven chiral DW motion. Several two-dimensional magnets with high Curie temperature and large topological band gaps are proposed for realizing these phenomena. The present work thus reveals the possibility of chiral DW controllable QAH effects.

We propose an extended BCS–Hubbard model and investigate its ground state phase diagram in an external magnetic field. By mapping the model onto a model of spinless fermions coupled with conserving $Z_2$ variables which are mimicked by pseudospins, the model is shown to be exactly solvable along the symmetric lines for an arbitrary on-site Hubbard interaction on the bipartite lattice. In the zero field limit, the ground states exhibit an antiferromagnetic order of pseudospins. In the large field limit, on the other hand, the pseudospins are fully polarized ordered. With the increase of the applied field, a first-order phase transition occurs between these kinds of phases when the on-site Coulomb interaction is less than a critical value $U_{\rm c}$. Above this critical $U_{\rm c}$, a novel intermediate phase emerges between the fully polarized and antiferromagnetic phases. The ground states in this phase are macroscopically degenerate, like in a spin ice, and the corresponding entropy scales linearly with the lattice size at zero temperature.

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized under high-pressure and high-temperature conditions using diamond anvil cell combined with in situ high-pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature $T_{\rm c}$ was found to be $\sim$ $30$ K at 197 GPa in the sample that was synthesized at the same pressure with $\sim$ $2000$ K heating. The transitions are shifted to low temperature upon applying magnetic fields that support the superconductivity nature. The upper critical field at zero temperature $\mu_{0}H_{\rm c2}$(0) of the superconducting phase is estimated to be $\sim$ $20$ T that corresponds to Ginzburg–Landau coherent length $\sim$ $40$ Å. Our results suggest that the superconductivity may arise from $I\bar{4}3d$ phase of TaH$_{3}$. It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transition metals.

We study the spin-$1/2$ two-dimensional Shastry–Sutherland spin model by exact diagonalization of clusters with periodic boundary conditions, developing an improved level spectroscopic technique using energy gaps between states with different quantum numbers. The crossing points of some of the relative (composite) gaps have much weaker finite-size drifts than the normally used gaps defined only with respect to the ground state, thus allowing precise determination of quantum critical points even with small clusters. Our results support the picture of a spin liquid phase intervening between the well-known plaquette-singlet and antiferromagnetic ground states, with phase boundaries in almost perfect agreement with a recent density matrix renormalization group study, where much larger cylindrical lattices were used [J. Yang et al., Phys. Rev. B 105, L060409 (2022)]. The method of using composite low-energy gaps to reduce scaling corrections has potentially broad applications in numerical studies of quantum critical phenomena.

The intercalated iron selenide (Li,Fe)OHFeSe has a strongly layered structure analogous to the quasi-two-dimensional (2D) bismuth cuprate superconductors, and exhibits both high-temperature ($T_{\rm c}$) and topological superconductivity. However, the issue of its superconductivity dimensionality has not yet been fully investigated so far. Here we report that the quasi-2D superconductivity features, including the high anisotropy $\gamma = 151$ and the associated quasi-2D vortices, are also revealed for (Li,Fe)OHFeSe, based on systematic experiments of the electrical transport and magnetization and model fittings. Thus, we establish a new vortex phase diagram for (Li,Fe)OHFeSe, which delineates an emergent quasi-2D vortex-liquid state, and a subsequent vortex-solid dimensional crossover from a pancake-like to a three-dimensional state with decreasing temperature and magnetic field. Furthermore, we find that all the quasi-2D characteristics revealed here for the high-$T_{\rm c}$ iron selenide superconductor are very similar to those reported for high-$T_{\rm c}$ bismuth cuprate superconductors.

Research interests in recent years have expanded into quantum materials that display novel magnetism incorporating strong correlations, topological effects, and dimensional crossovers. Fe$_{3}$GeTe$_{2}$ represents such a two-dimensional van der Waals platform exhibiting itinerant ferromagnetism with many intriguing properties. Up to date, most electronic transport studies on Fe$_{3}$GeTe$_{2}$ have been limited to its anomalous Hall responses while the longitudinal counterpart (such as magnetoresistance) remains largely unexplored. Here, we report a few unusual transport behaviors on thin flakes of Fe$_{3}$GeTe$_{2}$. Upon cooling to the base temperature, the sample develops a resistivity upturn that shows a crossover from a marginally $-\ln T$ to a ${-}{T}^{1/2}$ dependence, followed by a lower-temperature deviation. Moreover, we observe a negative and non-saturating linear magnetoresistance when the magnetization is parallel or antiparallel to the external magnetic field. The slope of the linear magnetoresistance also shows a nonmonotonic temperature dependence. We deduce an anomalous contribution to the magnetoresistance at low temperatures with a scaling function proportional ${-HT}^{1/2}$, as well as a temperature-independent linear term. Possible mechanisms that could account for our observations are discussed.

Interplay of spin-orbit coupling (SOC) and electron correlation generates a bunch of emergent quantum phases and transitions, especially topological insulators and topological transitions. We find that electron correlation will induce extra large SOC in multi-orbital systems under atomic SOC and change ground state topological properties. Using the Hartree–Fock mean field theory, phase diagrams of $p_{x}/p_{y}$ orbital ionic Hubbard model on honeycomb lattice are well studied. In general, correction of strength of SOC $\delta \lambda \propto (U'-J)$. Due to breaking down of rotation symmetry, form of SOC on multi-orbital materials is also changed under correlation. If a non-interacting system is close to fermionic instability, spontaneous generalized SOC can also be found. Using renormalization group, SOC is leading instability close to quadratic band-crossing point. Mean fields at quadratic band-crossing point are also studied.

Light-induced ultrafast spin dynamics in materials is of great importance for developments of spintronics and magnetic storage technology. Recent progresses include ultrafast demagnetization, magnetic switching, and magnetic phase transitions, while the ultrafast generation of magnetism is hardly achieved. Here, a strong light-induced magnetization (up to $0.86\mu_{\scriptscriptstyle{\rm B}}$ per formula unit) is identified in non-magnetic monolayer molybdenum disulfide (MoS$_{2}$). With the state-of-the-art time-dependent density functional theory simulations, we demonstrate that the out-of-plane magnetization can be induced by circularly polarized laser, where chiral phonons play a vital role. The phonons strongly modulate spin-orbital interactions and promote electronic transitions between the two conduction band states, achieving an effective magnetic field $\sim$ $380$ T. Our study provides important insights into the ultrafast magnetization and spin-phonon coupling dynamics, facilitating effective light-controlled valleytronics and magnetism.

We report a hydrothermal route to remove interstitial excess Fe in non-superconducting iron chalcogenide Fe$_{1+\delta}$Se$_{1-x}$Te$_{x}$ single crystals. The extra-Fe-free ($\delta \sim 0$) FeSe$_{0.2}$Te$_{0.8}$ single crystal thus obtained shows bulk superconductivity at $T_{\rm c} \sim 13.8$ K, which is about 2 K higher than the FeSe$_{0.2}$Te$_{0.8}$ sample obtained by usual post-annealing process. The upper critical field $\mu_{0}H_{\rm c2}$ is estimated to be $\sim$ $42.5$ T, similar to the annealed FeSe$_{0.2}$Te$_{0.8}$. It is surprising to find that the hydrothermal FeSe$_{0.2}$Te$_{0.8}$ exhibits a remarkably small isothermal magnetization hysteresis loop at $T = 3$ K. This yields an extremely low critical current density $J_{\rm c} \sim 1.1\times 10^{2}$ A$\cdot$cm$^{-2}$ (over 100 times smaller than the annealed FeSe$_{0.2}$Te$_{0.8}$) and indicates more free vortices in the hydrothermal FeSe$_{0.2}$Te$_{0.8}$.

The interlayer hybridization (IH) of van der Waals (vdW) materials is thought to be mostly associated with the unignorable interlayer overlaps of wavefunctions ($t$) in real space. Here, we develop a more fundamental understanding of IH by introducing a new physical quantity, the IH admixture ratio $\alpha$. Consequently, an exotic strategy of IH engineering in energy space can be proposed, i.e., instead of changing $t$ as commonly used, $\alpha$ can be effectively tuned in energy space by changing the on-site energy difference (${2\varDelta}$) between neighboring-layer states. In practice, this is feasible via reshaping the electrostatic potential of the surface by deposing a dipolar overlayer, e.g., crystalline ice. Our first-principles calculations unveil that IH engineering via adjusting ${2\varDelta}$ can greatly tune interlayer optical transitions in transition-metal dichalcogenide bilayers, switch different types of Dirac surface states in Bi$_{2}$Se$_{3}$ thin films, and control magnetic phase transition of charge density waves in 1H/1T-TaS$_{2}$ bilayers, opening new opportunities to govern the fundamental optoelectronic, topological, and magnetic properties of vdW systems beyond the traditional interlayer distance or twisting engineering.

Density matrix renormalization group (DMRG) and its extensions in the form of matrix product states are arguably the choice for the study of one-dimensional quantum systems in the last three decades. However, due to the limited entanglement encoded in the wave-function ansatz, to maintain the accuracy of DMRG with the increase of the system size in the study of two-dimensional systems, exponentially increased resources are required, which limits the applicability of DMRG to only narrow systems. We introduce a new ansatz in which DMRG is augmented with disentanglers to encode area-law-like entanglement entropy (entanglement entropy supported in the new ansatz scales as $l$ for an $l \times l$ system). In the new method, the $O(D^3)$ low computational cost of DMRG is kept (with an overhead of $O(d^4)$ and $d$ the dimension of the physical degrees of freedom). We perform benchmark calculations with this approach on the two-dimensional transverse Ising and Heisenberg models. This new ansatz extends the power of DMRG in the study of two-dimensional quantum systems.

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$_7$.

Moiré superlattices have emerged as a highly controllable quantum platform for exploration of various fascinating phenomena, such as Mott insulator states, ferroelectric order, unconventional superconductivity and orbital ferromagnetism. Although remarkable progress has been achieved, current research in moiré physics has mainly focused on the single species properties, while the coupling between distinct moiré quantum phenomena remains elusive. Here we demonstrate, for the first time, the strong coupling between ferroelectricity and correlated states in a twisted quadrilayer MoS$_{2}$ moiré superlattice, where the twist angles are controlled in sequence to be $\sim$ $57^{\circ}$, $\sim$ $0^{\circ}$, and $\sim$ $-57 ^{\circ}$. Correlated insulator states are unambiguously established at moiré band filling factors $v = 1$, 2, 3 of twisted quadrilayer MoS$_{2}$. Remarkably, ferroelectric order can occur at correlated insulator states and disappears quickly as the moiré band filling deviates from the integer fillings, providing smoking gun evidences of the coupling between ferroelectricity and correlated states. Our results demonstrate the coupling between different moiré quantum properties and will hold great promise for new moiré physics and applications.