The charge-density-wave (CDW) phase in the layered transition-metal dichalcogenide ${\mathrm{VTe}}_2$ is strongly coupled to the band inversion involving vanadium and tellurium orbitals. In particular, this coupling leads to a selective disappearance of the Dirac-type states that characterize the normal phase, when the CDW phase sets in. Here, we investigate the broadband time-resolved reflectivity variations caused by collective and single-particle excitations in the CDW ground state of ${\mathrm{VTe}}_2$. With the aid of density functional perturbation theory simulations we unveil the presence of two collective amplitude modes of the CDW ground state. By applying a double-pulse excitation scheme, we show the possibility to manipulate these modes, gaining insights into the coupling between these two collective excitations and demonstrating a more efficient way to perturb the CDW phase in ${\mathrm{VTe}}_2$.

An improved particle swarm optimization algorithm is proposed, and its superiority over the standard particle swarm optimization algorithm is tested on two typical benchmark functions. By employing this algorithm to search for the magnetic ground states of the Hubbard model on the real-space square lattice with finite size based on the mean-field approximation, two new magnetic states, namely, the double-striped-type antiferromagnetic state and the triple antiferromagnetic state, are found. We further perform mean-field calculations in the thermodynamical limit to confirm that these two new magnetic states are not a result of a finite-size effect, where the properties of the double-striped-type antiferromagnetic state are also presented.

The Ising model is one of the simplest and most well-established concepts to simulate phase transformations in complex materials. However, its most central constant, the interaction strength $\mathit{J}$ between two nearest neighbors, is hard to obtain. Here we show how this basic constant can be determined with a fractal dimension analysis of measured domain structures. We apply this approach to vanadium dioxide, a strongly correlated material with a first-order insulator-to-metal phase transition with enigmatic properties. We obtain a nearest-neighbor interaction of 13.8 meV, a value close to the thermal energy at room temperature. Consequently, even far below the transition temperature, there are spontaneous local phase flips from the insulating into the metallic phase. These fluctuations explain several measured anomalies in $\mathrm{V}{\mathrm{O}}_2$, in particular the low thermal carrier activation energy and the finite conductivity of the insulating phase. As a method, our fractal dimension analysis links the Ising model to macroscopic material constants for almost any first-order phase transition.

Destructive interference between electron wavefunctions on the two-dimensional kagome lattice induces an electronic flat band, which could host a variety of interesting quantum states. Key to realize these proposals is to demonstrate the real-space localization of kagome flat-band electrons. The extent to which the complex structure of realistic materials counteract the localizing effect of destructive interference is hitherto unknown. Moreover, a detailed understanding of the real-space distribution of the electronic states of kagome flat bands has not been developed yet. We used scanning tunneling microscopy to visualize the kagome flat band at the surface of CoSn, a kagome metal. Consistent with results from model calculations, we find that the local density of states associated with the kagome flat bands exhibits a unique real-space distribution by which it can be distinguished from the local density of states of dispersive electron bands and trivially localized states, such as well-localized orbitals and surface resonances. Our results further show that these states exhibit an extremely small localization length of two to three angstroms concomitant with a strongly renormalized quasiparticle velocity $v\approx 1\times {10}^4\mathrm{m}/\mathrm{s}$, comparable to that of moiré superlattices. Our findings provide fundamental insight into the electronic properties of kagome metals and present a key step for future research on emergent many-body states in these systems.

We develop a self-consistent variant of the constrained path quantum Monte Carlo approach which ensures its independence of the trial wave function, and apply the method to compute ground-state correlations in the two-dimensional SU(3) Fermi-Hubbard Hamiltonian at $\frac{1}{3}$ filling, modeling fermions with three possible spin flavors moving on a square lattice with an average of one particle per site. We provide clear evidence of a quantum critical point separating a nonmagnetic uniform metallic phase from a regime where long-range “spin” order is present. This discovery of multiple successive transitions to magnetic states with regular, long-range alternation of the different flavors, whose symmetry changes as the interaction strength increases, significantly extends previous work in the Heisenberg limit to itinerant fermions. In addition to the rich quantum magnetism, this important physical system allows one to study integer filling and the associated Mott transition disentangled from nesting, in contrast to the usual SU(2) model, while preserving the square-lattice geometry. Our results also provide a significant step towards the interpretation of present and future experiments on fermionic alkaline-earth atoms, and other realizations of $\mathrm{SU}\left(N\right)$ physics.

The intertwined coupling among various many-body interactions is increasingly recognized as playing a key role in strongly correlated electron systems. However, understanding their relationship to physical properties is challenging due to the lack of a definitive experimental measure. Here, we report an analytical approach that utilizes machine learning to enable a higher level of evaluation of many-body interactions through a large amount of data from a single material using spatially resolved and angle-resolved photoemission spectroscopy. We demonstrate that various physical parameters, including the coupling strengths of electron-electron and electron-boson interactions, can be statistically evaluated, and the correlation between many-body interactions can also be accessed. Our approach thus provides a quantitative measure of the microscopic variables and serves as a linking bridge between them, holding great promise in disentangling the complex nature of strongly correlated materials where many-body interactions generally mutually interplay.

In this work we study the Kubo-Martin-Schwinger (KMS) relation in the Yang-Gaudin model of an interacting mobile impurity. We use the integrability of the model to compute the dynamic injection and ejection Green's functions at finite temperatures. We show that due to separability of the Hilbert space with an impurity, the ejection Green's functions in a canonical ensemble cannot be reduced to a single expectation value as per microcanonical picture. Instead, it involves a thermal average over contributions from different subspaces of the Hilbert space which, due to the integrability, are resolved using the so-called spin rapidity. It is then natural to consider the injection and ejection Green's functions within each subspace. By means of reformulating the original KMS condition as a Riemann-Hilbert problem, we analytically demonstrate that such Green's functions obey a refined analogous relation, which is finally corroborated by numerical evaluation.

The topological phases of monolayer jacutingaite (${\mathrm{Pt}}_2{\mathrm{HgSe}}_3$) under off-resonance high-frequency and high-intensity laser irradiation and staggered sublattice potential $V$ are investigated. The various steady-state phases are realized by an appropriate choice of the off-resonance circularly polarized laser field parameters and the staggered sublattice potential. By tuning the band gap through $V$ and/or laser intensity, a phase diagram is deduced and explained. Increasing the laser intensity at $V=0$ leads to a photoinduced quantum Hall insulator state with chiral edge currents, while increasing $V$ in the absence of laser field changes the phase from quantum spin Hall to quantum valley Hall phases. An electromagnetically induced, steady-state spin-polarized quantum Hall state is predicted at particular values of $V$ and the laser intensity relative to the spin-orbit coupling strength. The emergence of the topological phases is corroborated by calculating the Chern numbers of each band, and the Hall conductivity at zero temperature. Calculations of the Nernst coefficient give further evidence at nonzero temperatures. Along the phase boundaries, we observe steady-state single Dirac cone, spin-polarized, and spin-valley-polarized metal states which may be exploited in use of jacutingaites in spin and valleytronics.

We propose a method for detecting single chiral phonons that will enable their use as dark-matter detectors. We suggest metal–organic frameworks (MOFs) as detector materials, as their flexibility yields low-energy chiral phonons with measurable magnetic moments, and their anisotropy leads to directional sensitivity, which mitigates background contamination. To demonstrate our proposal, we calculate the phononic structure of the MOF ${\mathrm{InF}}_3(4,4^{\prime }$-bipyridine), and show that it has highly chiral acoustic phonons. Detection of such chiral phonons via their magnetic moments would dramatically lower the excitation energy threshold for dark matter detection to the energy of a single phonon. We show that single-phonon detection in a MOF would extend detector reach ten or more orders of magnitude below current limits, enabling exploration of a multitude of as-yet-unprobed dark matter candidates.

We use a formalism that describes electron ejections from graphenelike targets by dark matter (DM) scattering for general forms of scalar and spin-1/2 DM-electron interactions in combination with state-of-the-art density functional calculations to produce predictions and reach estimates for various possible carbon-based detector designs. Our results indicate the importance of a proper description of the target electronic structure. In addition, we find a strong dependence of the predicted observed signal for different DM candidate masses and interaction types on the detailed geometry and design of the detector. Combined with directional background vetoing, these dependencies will enable the identification of DM particle properties once a signal has been established.

We develop a formalism to describe electron ejections from graphenelike targets by dark matter (DM) scattering for general forms of scalar and spin-1/2 DM-electron interactions, and we compare their applicability and accuracy within the density functional theory (DFT) and tight-binding (TB) approaches. This formalism allows for accurate prediction of the daily modulation signal expected from DM in upcoming direct detection experiments employing graphene sheets as the target material. A key result is that the physics of the graphene sheet and that of the DM and the ejected electron factorize, allowing for the rate of ejections from all forms of DM to be obtained with a single graphene response function. We perform a comparison between the TB and DFT approaches to modeling the initial state electronic wave function within this framework, with DFT emerging as the more self-consistent and reliable choice due to the challenges in the embedding of an appropriate atomic contribution into the TB approach.

We use a tensor network renormalization group method to study random $S=2$ antiferromagnetic Heisenberg chains with alternating bond strength distributions. In the absence of randomness, an imposed dimerization with bond alternation induces two quantum critical points between the $S=2$ Haldane phase, a partially dimerized phase, and a fully dimerized phase, depending on the strength of dimerization. These three phases, called ($\sigma ,4-\sigma$) = (2,2), (3,1), and (4,0) phases, are valence-bond solid (VBS) states characterized by $\sigma$ valence bonds (effective spin-1/2 singlet pairs) forming across even links and $4-\sigma$ valence bonds on odd links. Here we study the effects of bond randomness on the ground states of the dimerized spin chain, calculating disorder-averaged twist order parameters and spin correlations. We classify the types of random VBS phases depending on the strength of bond randomness $R$ and dimerization $D$ using the twist order parameter, which has a negative/positive sign for a VBS phase with odd/even $\sigma$. Our results demonstrate the existence of a multicritical point in the intermediate disorder regime with finite dimerization, where (2,2), (3,1), and (4,0) phases meet. This multicritical point is at the junction of three phase boundaries in the $R\text{−}D$ plane: the (2,2)-(3,1) and (3,1)-(4,0) boundaries that extend to zero randomness, and the (2,2)-(4,0) phase boundary that connects another multicritical point in the undimerized limit. The undimerized multicritical point separates a gapless Haldane phase and an infinite-randomness critical line with the diverging dynamic critical exponent in the large-$R$ limit at $D=0$. Furthermore, we identify the (3,1)-(4,0) phase boundary as an infinite-randomness critical line even at small $R$ and find the signature of infinite randomness at the (2,2)-(3,1) phase boundary only in the vicinity of the multicritical point.

The pursuit of elemental superconductors with high critical temperature $\left(T_c\right)$ holds immense scientific significance due to their relatively simple material composition, which not only aids in understanding the mechanisms of superconductivity but also facilitates further exploration of high-temperature superconductors in compound materials. Here, we report experimental findings of superconductivity for scandium (Sc) in the pressure range of 64 GPa to 261 GPa. As a result, the highest $T_c$ of 37.6 K observed in Sc-V phase at 243 GPa sets the current record among all elements. An estimate based on the Ginzburg-Landau model gives a zero-temperature upper critical magnetic field of 21.3 T with a coherence length of 39.3 Å. Current-voltage measurements show that the zero-temperature critical current and its density can reach 0.146 A and 70 A ${\mathrm{mm}}^{-2}$ of Sc-IV at 198 GPa, and 0.116 A and 50 A ${\mathrm{mm}}^{-2}$ of Sc-V at 250 GPa, respectively. Further theoretical calculations show that pressure-induced phonon softening plays a key role in increasing $T_c$ upon compression. Our current findings shed light on the understanding of high superconductivity in simple elements and provide insight into exploring high-$T_c$ elemental superconductors and Sc-based superconductors.

RhPb was initially recognized as one of CoSn-like compounds with $P6/mmm$ symmetry, containing an ideal kagome lattice of $d$-block atoms. However, theoretical calculations predict the realization of the phonon soft mode, which leads to the kagome lattice distortion and stabilization of the structure with $P\overline{6}2m$ symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with $P\overline{6}2m$ symmetry, i.e. phonon dispersions and surface Green's functions using the modern theoretical methods based on density functional theory. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzaglike edge of Pb honeycomb sublattice.

We devise a generic and experimentally accessible recipe to prepare boundary states of topological or nontopological quantum systems through an interplay between coherent Hamiltonian dynamics and local dissipation. Intuitively, our recipe harnesses the spatial structure of boundary states which vanish on sublattices where losses are suitably engineered. This yields unique nontrivial steady states that populate the targeted boundary states with infinite lifetimes while all other states are exponentially damped in time. Remarkably, applying loss only at one boundary can yield a unique steady state localized at the very same boundary. We detail our construction and rigorously derive full Liouvillian spectra and dissipative gaps in the presence of a spectral mirror symmetry for a one-dimensional Su-Schrieffer-Heeger model and a two-dimensional Chern insulator. We outline how our recipe extends to generic noninteracting systems.

We provide a comprehensive theory of magnetic phases in two-dimensional triangulene crystals, using both Hubbard model and density functional theory (DFT) calculations. We consider centrosymmetric and noncentrosymmetric triangulene crystals. In all cases DFT and the mean-field Hubbard model predict the emergence of broken-symmetry antiferromagnetic (ferrimagnetic) phases for the centrosymmetric (noncentrosymmetric) crystals. This includes the special case of the [4,4]triangulene crystal, whose noninteracting energy bands feature a gap with flat valence and conduction bands. We show how the lack of contrast between the local density of states of these bands, recently measured via scanning tunneling spectroscopy, is a natural consequence of a broken-symmetry Néel state that blocks intermolecular hybridization. Using random phase approximation, we also compute the spin wave spectrum of these crystals, including the recently synthesized [4,4]triangulene crystal. The results are in excellent agreement with the predictions of a Heisenberg spin model derived from multiconfiguration calculations for the unit cell. We conclude that experimental results are compatible with an antiferromagnetically ordered phase where each triangulene retains the spin predicted for the isolated species.

The relaxation dynamics of an open quantum system is determined by the competition between the coherent Hamiltonian dynamics of a system and the dissipative dynamics due to interactions with environments. It is therefore of fundamental interest to understand the transition from the coherent to incoherent regimes. We find that hitherto unrecognized quasiparticles—incoherentons—describe this coherent-to-incoherent transition in eigenmodes of a Liouvillian superoperator that governs the dynamics of an open quantum many-body system. Here, an incoherenton is defined as an interchain bound state in an auxiliary ladder system that represents the density matrix of a system. The Liouvillian eigenmodes are classified into groups with different decay rates that reflect the number of incoherentons involved therein. We also introduce a spectral gap—quantum coherence gap—that separates the different groups of eigenmodes. We demonstrate the existence of incoherentons in a lattice boson model subject to dephasing, and show that the quantum coherence gap closes when incoherentons are deconfined, which signals a dynamical transition from incoherent relaxation with exponential decay to coherent oscillatory relaxation. Furthermore, we discuss how the decoherence dynamics of quantum many-body systems can be understood in terms of the generation, localization, and diffusion of incoherentons.

I describe a concrete and efficient real-space renormalization approach that provides a unifying perspective on interface states in a wide class of Hermitian and non-Hermitian models, irrespective of whether they obey a traditional bulk-boundary principle or not. The emerging interface physics are governed by a flow of microscopic interface parameters, and the properties of interface states become linked to the fixed-point topology of this flow. In particular, the quantization condition of interface states converts identically into the question of the convergence to unstable fixed points. As its key merit, the approach can be directly applied to concrete models and utilized to design interfaces that induce states with desired properties, such as states with a predetermined and possibly symmetry-breaking energy. I develop the approach in general, and then demonstrate these features in various settings, including for the design of circular, triangular, and square-shaped complex dispersion bands and associated arcs at the edge of a two-dimensional system. Furthermore, I describe how this approach transfers to nonlinear settings, and demonstrate the efficiency, practicability, and consistency of this extension for a paradigmatic model of topological mode selection by distributed saturable gain and loss.

Recently, the physics of $J=3/2$ electrons on a honeycomb lattice has received attention with the suggestion of hosting an SU(8) Dirac semimetallic state. Motivated by this, in this work we provide a systematic study of the interplay of various hopping pathways and atomic spin-orbit coupling for the low-energy electrons in candidate $d^1$ transition-metal halides $M{X}_3$ $(M=\mathrm{Ti},\mathrm{Zr},\mathrm{Hf}; X=\mathrm{F},\mathrm{Cl},\mathrm{Br})$. By combining first-principles calculations and a minimal hopping Hamiltonian, we uncover the role of dominant direct metal-metal hopping on top of indirect metal-halide-metal hopping. This sets up a hierarchy of hopping pathways that centrally modify the SU(8) picture for the above materials. These hopping interactions, along with the spin-orbit coupling, lead to a plethora of exactly compensated metals instead of the SU(8) Dirac semimetal. Remarkably, the same can be understood as descendants of a topological insulator obtained by gapping out the SU(8) Dirac semimetallic phase. The resultant compensated metals have varied Fermi surface topology and are separated by Lifshitz phase transitions. We discuss the implications of the proximate Lifshitz transition, which may be accessed via strain, in the context of the relevant materials.