We rigorously prove that in nearly arbitrary quantum spin chains with power-law-distributed random fields, namely such that the probability of a field exceeding $h$ scales as $h^{-\alpha }$, it is impossible for any operator evolving in the Heisenberg picture to spread with dynamical exponent less than $1/\alpha$. In particular, ballistic growth is impossible for $\alpha <1$, diffusive growth is impossible for $\alpha <1/2$, and any finite dynamical exponent becomes impossible for sufficiently small $\alpha$. This result thus establishes a wide family of models in which the disorder provably prevents conventional transport. We express the result as a tightening of Lieb-Robinson bounds due to random fields—the proof modifies the standard derivation such that strong fields appear as effective weak interactions and then makes use of analogous recent results for random-bond spin chains.

Localized excitons in two-dimensional (2D) materials are considered promising sources of single photons on demand. The photon indistinguishability as a key figure of merit for quantum information processing is strongly influenced by the coupling of charge excitations to lattice vibrations of the surrounding semiconductor material. Here, we quantify the impact of exciton–acoustic-phonon interaction and cavity quantum electrodynamics effects on photon indistinguishability in a Hong-Ou-Mandel setup by solving fully quantum-mechanical equations for a coupled quantum dot-cavity-phonon system including non-Markovian effects. We find a strong reduction of indistinguishability compared to 3D systems due to increased exciton-phonon coupling efficiency. Moreover, we show that the coherence properties of photons are significantly influenced by the finite phonon lifetime in the surrounding material giving rise to pure dephasing. If these limitations are overcome, localized excitons in 2D semiconductors can become a new avenue for quantum light sources.

Transition metal dichalcogenides have attracted significant interest due to their rich physi cal properties, including charge density waves and superconductivity. They feature a characteristic sandwich structure, with a transition metal layer between two chalcogen layers. Recently, a quintuple-layer material, ${\mathrm{Nb}}_2{\mathrm{S}}_3$, was successfully synthesized, consisting of two Nb layers sandwiched between three S layers. In this study, we use first-principles calculations to investigate the electron-phonon coupling (EPC) and superconducting properties of the quintuple-layer $M_2{X}_3$ (M = Nb and Ta; X = S and Se). Our results show that all four $M_2{X}_3$ monolayers are dynamically and thermally stable, exhibiting no imaginary modes in their phonon dispersions. They also display semimetallic electronic band structures with small Fermi surfaces around the $\mathrm{\Gamma }$ and K points. EPC calculations reveal that the superconductivity transition temperatures $\left(T_c\right)$ are 4.4, 2.0, 2.4, and 0.1 K for ${\mathrm{Nb}}_2{\mathrm{S}}_3$, ${\mathrm{Nb}}_2{\mathrm{Se}}_3$, ${\mathrm{Ta}}_2{\mathrm{S}}_3$, and ${\mathrm{Ta}}_2{\mathrm{Se}}_3$, respectively. These values are approximately inversely proportional to the relative atomic masses of the constituent elements, a trend also observed in ${MX}_2$ (M = Nb and Ta; X = S and Se). Furthermore, we propose that the $T_c$ can be enhanced by slightly shifting the Fermi energy to the van Hove singularity. This work provides an effective strategy for tailoring the physical properties of the transition metal chalcogenides and is expected to inspire further theoretical and experimental studies on this intriguing class of materials.

Twisted multilayer systems, encompassing materials like twisted bilayer graphene (TBG), twisted trilayer graphene, and twisted bilayer transition-metal dichalcogenides, have garnered significant attention in condensed-matter physics. Despite this interest, a comprehensive description of their electronic structure, especially in dealing with incommensurate wave functions, poses a persistent challenge. Here, we introduce a unified theoretical framework for efficiently describing the electronic structure covering both spectra and wave functions in twisted multilayer systems, accommodating both commensurate and incommensurate scenarios. Our analysis reveals that physical observables in these systems can be systematically decomposed into contributions from individual layers and their respective $\mathit{k}$ points, even in the presence of intricate interlayer coupling. This decomposition facilitates the computation of wave-function-related quantities relevant to response characteristics beyond spectra. We propose a local low-rank approximated Hamiltonian in the truncated Hilbert space that can achieve any desired accuracy, offering numerical efficiency compared with the previous large-scale calculations. The validity of our theory is confirmed through computations of spectra and optical conductivity for moiré TBG, and demonstrating its applicability in incommensurate quasicrystalline TBG. Our findings provide a generic approach to studying the electronic structure of twisted multilayer systems and pave the way for future research.

We study the effect of a terahertz field-driven single cavity mode for ultrafast control of a fermion chain with dissipation-induced nonlinearity and quadratic coupling to an infrared-active phonon mode. Unlike the first-order phase transition in the nonequilibrium steady state of the system without a cavity or with a high-quality cavity, we find that a realistic dissipation in a low-quality driven cavity process prevents such a transition. Without realistic photon loss from the cavity, the transition survives only for the lower phonon-polariton branch with strong drive; a weak laser field fails to induce the phase transition and renders the polaritons symmetrical. The ability to control the phase transition is crucial for realizing strongly modulated steady states; we propose experimentally feasible regimes where this occurs.

Twistronics has heavily relied on two-dimensional van der Waals materials. However, recent experiments have shown that twisting ferroelectric perovskite oxides could also result in novel features, such as the occurrence of an unusual shear strain morphology and the emergence of polar textures consisting of interconnected lattices of vortex and antivortex. Yet very little is known on the analytical formula of this strain pattern and of the finite-temperature stability and control or even annihilation of these polar textures in twisted ferroelectric oxide layers. We reveal here an analytical formula for the inhomogeneous shear strain imposed on a barium titanate layer by the twisting angle, and further develop a numerical scheme based on an effective Hamiltonian and resulting in Moiré lattices. We numerically confirm that the vortex-antivortex lattice emerges from the coupling of this inhomogeneous strain with local dipoles through the flexoelectric effects. We also predict that the vortex-antivortex lattice can be controlled and even annihilated by means of in-plane electric fields via striking phenomena, and that the shear strain (which is controlled by the twist angle) can revert the electric toroidal moment of the vortices, as well as a new order parameter characterizing polar antivortices, in the Moiré lattice.

Altermagnets constitute a class of collinear compensated Néel ordered magnets that break time-reversal symmetry and feature spin-split band structures. Based on versatile microscopic models able to capture the altermagnetic sublattice degrees of freedom, we study characteristic local signatures of altermagnetism near disorder sites. We give a complete list of two-dimensional models that exhibit altermagnetism classified by their corresponding layer groups. Specifically, we calculate the local density of states in the vicinity of pointlike nonmagnetic impurities and expose its spatial dependence for two minimal models showcasing $d$-wave and $g$-wave altermagnetism. The momentum structure of the nodes ($d$-wave, $g$-wave, etc.) is directly imprinted on the total local density of states, thus measurable by scanning tunneling conductance experiments. This signature is present both in the spin-resolved as well as the spin-summed local density of states. We find a weaker response in the nonmagnetic state from the anisotropic crystal environment and uncover the importance of the sublattice degree of freedom to model altermagnets. We also study coexistence phases of altermagnetism and superconductivity and provide predictions for the local impurity response of in-gap bound states. The response of impurity bound states strongly enhances the distinct altermagnetic signature.

Altermagnetism became very popular because of unique features, namely coupling between magnetic properties and momentum of itinerant electrons. The particular model of the altermagnetic system of our interest has already been studied in recent publications in a different context: Phys. Rev. B 108, L140408 (2023). Here, we study the scattering process of an itinerant electron from the altermagnetic material on the electron localized in a GaAs quantum dot doped on the altermagnetic material. We found a spatially inhomogeneous distribution of quantum entanglement in the postscattering state. An interesting observation is the spatially anisotropic distribution of concurrence which depends on the values of the altermagnetic spin-orbit interaction constant. We also studied Rényi entropy and the effect of disorder in the system leading to randomness in the spin-orbit constant. Our main finding is that, due to the unique properties of an altermagnetic system, tuning the applied external magnetic field allows tailoring of the desired entangled state. Thus the scattering process, in essence, mimics the Hadamard-CNOT gate transformation, converting the initial disentangled state into the entangled state of Bell's state. In particular, we achieved more than $70\%$ fidelity between the postscattering and Bell's states.

Electron energy-loss spectroscopy in transmission is applied to investigate the dependence of the exciton energies on the momentum, $q$, in the bulk of $2H\text{−}{\mathrm{WSe}}_2$. The exciton dispersion $E\left(q\right)$ for momentum vectors parallel to the $\mathrm{\Gamma }\mathrm{K}$ direction and the $\mathrm{\Gamma }\mathrm{M}$ direction at room temperature as well as at 20 K close to the K point is investigated. In addition, the exciton spectra at room temperature are measured for larger momenta parallel to the $\mathrm{\Gamma }\mathrm{K}$ direction approaching the indirect gap excitation. The exciton effective mass can be determined from the measured dispersion curves as a function of temperature and in-plane direction. Our data reveal that the exciton effective mass for the excitation close to the K point is virtually isotropic and temperature independent. The indirect band-gap exciton is characterized by a significantly larger effective mass compared to the direct exciton close to the K point.

We study the transmission through a system of two diffusive slabs with mirror-symmetric disorder separated by an opaque barrier. Using a diagrammatic approach to calculating the conductance of the system, we show that interference between waves propagating along symmetrical paths leads to a renormalization of the barrier transmission and reflection coefficients. The renormalized coefficients are expressed in terms of the correlation function of interfering waves. The transport equation derived for the correlation function makes it possible to account for the violation of mirror symmetry of disorder and to describe the crossover between exact and broken symmetry. The developed formalism enables explaining all aspects of the enhanced transmission effect found in full wave numerical simulations by E. Cheron et al. [Phys. Rev. Lett. 122, 125501 (2019); Phys. Rev. B 102, 134201 (2020)], including its sensitivity to the presence of absorption losses and to various types of mirror symmetry violation. The results obtained for classical waves are extended to electron transport, for which the symmetry-induced dependence of the conductance on the applied magnetic field is determined.

We report temperature-dependent reflectivity spectra of the layered van der Waals magnet CrSBr in the far-infrared region. Polarization-dependent measurements resolve the vibrational modes along the $E\parallel a$ and $b$ axes and reveal clear structural anisotropy. While the $a$-axis phonons notably harden on cooling, the $b$-axis phonon frequencies are almost temperature independent. A phonon splitting due to the antiferromagnetic phase transition is observed for the 180 ${\mathrm{cm}}^{-1} a$-axis vibrational mode, accompanied by a phonon softening below $T_N$. Furthermore, an additional mode with strong magnetic characteristics at $\sim 360 {\mathrm{cm}}^{-1}$ is identified and attributed to the magnon excitation of CrSBr.

The narrow band-gap semiconductor elemental tellurium (Te) has a unique electronic structure due to strong spin-orbit splitting and a lack of inversion symmetry of its helical lattice. Using broadband extreme ultraviolet core-level transient absorption, we simultaneously measure the coherently coupled photoinduced carrier and lattice dynamics at the Te ${\mathrm{N}}_{4,5}$ edge initiated by a few-cycle NIR pulse. Ultrafast excitation of carriers leads to a coherently excited $A_1$ phonon oscillation and the generation of a hot carrier population distribution that oscillates in temperature, and the phonon excursion and hot carrier temperature are $\pi$ out of phase with respect to each other. The depths of modulation suggest a significant coupling between the electronic and lattice degrees of freedom in Te. A long-lived shift of the absorption edge suggests an excited state of Te in another equilibrium potential energy surface that lives on the order of the carrier recombination timescale. The observed phonon-induced oscillations of the hot carriers are supportive of a change in the metallicity, whereby Te becomes more metallic with increasing phonon-induced displacement. Additionally, near the Fermi level we observe an energy-dependent phase of the displacive excitation of the $A_1$ phonon mode. The discovery of coherent coupling between the lattice and hot carriers in Te provides the basis to investigate coherent interactions between spin and orbital degrees of freedom. The results spectrally and temporally resolve the correlation between photoexcited hot carriers and coherent lattice excitations, providing insight on the optical manipulation of the Te electronic structure at high carrier densities exceeding ${10}^{21}{\mathrm{cm}}^{-3}$.

Correlation-driven metal-insulator phase transitions are supposed to be of second order. Many experimental observations, however, appear to disagree with this classification. Using generalized ellipsometry, we have studied prominent examples of two-dimensional charge-transfer salts with a pronounced charge-order transition: $\alpha \text{−}{\left(\text{BEDT}\text{−}\text{TTF}\right)}_2{\mathrm{I}}_3$ and its selenium-substituted analogs, where effective correlations are reduced. Mueller matrix spectroscopic and temperature-dependent ellipsometry allows us to determine the anisotropic dielectric functions of these molecular materials across their metal-insulator transitions. Applying an effective-medium approximation, the nature of the metal-insulator phase transition of $\alpha \text{−}{\left(\text{BEDT}\text{−}\text{TTF}\right)}_2{\mathrm{I}}_3$ and its selenium-substituted salts is uncovered. Our findings confirm phase coexistence near the transition in $\alpha \text{−}{\left(\text{BEDT}\text{−}\text{TTF}\right)}_2{\mathrm{I}}_3$, establishing its first-order nature, while the substituted salts exhibit a more gradual evolution, indicative of a continuous phase transition. Through this comprehensive approach, generalized ellipsometry unravels the nature of the correlation-driven metal-insulator transition.

Magnetic adatoms on superconductors induce Yu-Shiba-Rusinov (YSR) states, which are key to the design of low-dimensional correlated systems and topological superconductivity. Competing magnetic interactions and superconducting pairing lead to a rich phase diagram. Using a scanning tunneling microscope (STM), we position Fe atoms on $2H\text{−}{\mathrm{NbSe}}_2$ to build a dimer with an odd-parity ground state, i.e., a partially screened YSR channel with the hybridized states spanning the Fermi level. This ground state makes the dimer a promising precursor for a topological YSR chain. By adding one atom at a time, we track the formation of YSR bands. The lowest-energy band crosses the Fermi level, and we find strong site-dependent spectral variations especially at the chain's terminations. We attribute these features to quantum spin effects and ferromagnetic coupling influenced by the local chemical environment.

We present a comprehensive analysis of boundary phenomena in a spin-$\frac{1}{2}$ anisotropic Heisenberg chain (XXZ-$\frac{1}{2}$) in the gapped antiferromagnetic phase, with a particular focus on the interplay between fractionalized spin-$\frac{1}{4}$ edge modes and a coupled spin-$\frac{1}{2}$ impurity at the edge. Employing a combination of Bethe ansatz, exact diagonalization, and density matrix renormalization group methods, we explore the intricate phase diagram that emerges when the impurity is coupled either integrably or nonintegrably to the chain. For integrable antiferromagnetic impurity couplings, we identify two distinct phases: the Kondo phase, where the impurity is screened by a multiparticle Kondo effect, and the antiferromagnetic bound mode phase, where an exponentially localized bound state screens the impurity in the ground state. When coupled ferromagnetically while maintaining integrability, the impurity behaves as a free spin-$\frac{1}{2}$, leading to either a ferromagnetic bound mode phase, where the impurity remains free in the ground state but may be screened at higher energy excitations or an unscreened (or local moment) phase where impurity remains unscreened in every eigenstate, whereas for nonintegrable ferromagnetic coupling, the impurity is not free. In the case of nonintegrable antiferromagnetic coupling, a third phase emerges, characterized by midgap excitations with two degenerate states below the mass gap on top of the Kondo and antiferromagnetic bound mode phases, further enriching the phase diagram. Our findings highlight the nuanced behavior of boundary impurities in gapped antiferromagnetic systems, offering new insights into Kondo effects and impurity screening in the presence of fractionalized edge modes and bulk antiferromagnetic order.

We report on the synthesis of single crystals of the kagome compound ${\mathrm{UV}}_6{\mathrm{Sn}}_6$, and present the results of magnetization, electrical resistivity, heat capacity, x-ray, and neutron diffraction experiments to characterize the structure and magnetic properties. ${\mathrm{UV}}_6{\mathrm{Sn}}_6$ crystallizes in a large supercell of the ${\mathrm{HfFe}}_6{\mathrm{Ge}}_6$ parent structure with an hexagonal symmetry in which some of the U atoms are shifted by $c/2$ in an ordered fashion. Below $T_{N1}\approx 29\mathrm{K}$, an incommensurate magnetic structure with a temperature-dependent wave vector $(0,0,k_z)$ is observed. Below $T_{N2}=23.5\mathrm{K}$, the wave vector locks in to (0,0,0.5), forming an antiferromagnetic ground state. The U moments align along the $c$ axis retaining a large magnetic anisotropy. These findings highlight the role of the $5f$ orbitals from uranium in this structural family in driving both magnetic ordering and structural modulation and distinguish ${\mathrm{UV}}_6{\mathrm{Sn}}_6$ from its lanthanide-based analogs.

A quantum phase transition arises from competition between different ground states and is typically accessed by varying a single physical parameter near absolute zero temperature. The quantum anomalous Hall (QAH) effect with high Chern number $C$ has recently been achieved in magnetic topological insulator (TI) multilayers. In this work, we employ molecular beam epitaxy to synthesize a series of magnetic TI pentalayers by varying the thickness of the middle magnetic TI layer, designated as $m$ quintuple layers. Electrical transport measurements demonstrate a quantum phase transition between the $C=1$ and $C=2$ QAH states. For $m\le 1$ and $m\ge 2$, the sample exhibits the well-quantized $C=1$ and $C=2$ QAH states, respectively. For $1<m<2$, we observe a monotonic decrease in Hall resistance from $h/e^2$ to $h/2e^2$ with increasing $m$, accompanied by a peak in the longitudinal resistance. The quantum phase transition between the $C=1$ and $C=2$ QAH states is attributed to the weakening of the interlayer coupling between the top and bottom $C=1$ QAH layers. Our findings provide a scalable strategy for engineering QAH devices with a tunable Chern number. This approach enables precise control and enhanced functionality in chiral edge current based electronic devices.

We prove that the Ising models with transverse and longitudinal fields on the hypercubic lattices with dimensions higher than one have no local conserved quantities other than the Hamiltonian. This holds for any value of the longitudinal field, including zero, as far as the transverse field and the Ising interactions are nonzero. The conserved quantity considered here is “local” in a very weak sense: it can be written as a linear combination of operators whose side lengths of the supports in one direction do not exceed half the system size, while the side lengths in the other directions are arbitrary. We also prove that the above result holds even in the ladder system. Our results extend the recently developed technique of the proof of absence of local conserved quantities in one-dimensional systems to higher dimensions and to the ladder.

We report a comprehensive investigation of the physical properties of ${\mathrm{LuOs}}_3{\mathrm{B}}_2$, characterized by an ideal Os-based kagome lattice. Resistivity and magnetization measurements confirm the emergence of type-II bulk superconductivity with a critical temperature $T_{\text{c}}\approx 4.63$ K. The specific heat jump and the calculated electron-phonon coupling parameter support a moderately coupled superconducting state. Electron correlation effects are supported by the enhanced Wilson ratios. First-principles calculations reveal hallmark features of kagome band structure, including Dirac points, van Hove singularities, and quasiflat bands, primarily derived from the Os $d$ orbitals. The inclusion of spin-orbit coupling opens a gap at the Dirac points, significantly altering the electronic properties. Furthermore, the superconductivity and electronic properties of isomorphic compounds are discussed. This paper provides a thorough exploration of the superconducting and normal states of ${\mathrm{LuOs}}_3{\mathrm{B}}_2$, deepening the understanding of kagome superconductors.

Structural two-level systems (TLSs) ubiquitous in amorphous solids are dramatically sensitive to thermal cycling to about $20\mathrm{K}$ and then back to low temperature, a process upon which the excitation energy of most TLSs is significantly changed. Using Monte Carlo simulations we demonstrate that this phenomenon is not contained within the standard tunneling model, but is well explained by a model that includes an additional set of TLSs that are pseudogapped at low energies, yet possess a strong strain interaction through which they generate significant dynamical disorder upon thermal cycling. Our results provide additional support for the broad applicability of the Two-TLS model to amorphous solids at low temperatures, bringing us closer to a comprehensive understanding of the universal behavior of phonon attenuation in these materials. With regard to quantum superconducting circuits, our results suggest thermal cycling as a unique protocol to distinguish TLS noise from other noise sources. The possible relation of the Two-TLS model to ionizing radiation effects on long-time fluctuations in qubit relaxation times and on TLS scrambling is discussed.