Fractional quantum Hall states are the most prominent example of states with topological order, hosting excitations with fractionalized charge. Recent experiments in twisted ${\text{MoTe}}_2$ and graphene-based heterostructures provided evidence of fractional quantum anomalous Hall (FQAH) states, which spontaneously break time-reversal symmetry and persist even without an external magnetic field. Understanding the unique properties of these states requires the characterization of their low-energy excitations. To that end, we construct a parton theory for the energy- and momentum-resolved single-particle spectral function of FQAH states. We explicitly consider several experimentally observed filling fractions as well as a composite Fermi liquid in the half-filled Chern band. Charge fractionalization manifests itself in nearly momentum-independent spectra with a characteristic series of peaks determined from the filling fraction. The parton description qualitatively captures our numerical exact diagonalization results. Additionally, we discuss how the finite bandwidth of the Chern band and the nonideal quantum geometry affect the fractionalized excitations. Our work demonstrates that the energy- and momentum-resolved electronic single-particle spectral function provides a valuable tool to characterize fractionalized excitations of FQAH states in moiré lattices.

The interplay of unitary evolution and projective measurements is a modern interest in the study of many-body entanglement. On the one hand, the competition between these two processes leads to the recently discovered measurement-induced phase transition (MIPT). On the other hand, measurement-based quantum computation (MBQC) is a well-known computational paradigm where measurements simulate unitary evolution by utilizing the entanglement of special resources such as the two-dimensional (2D) cluster state. The entanglement properties enabling MBQC may be attributed to symmetry-protected topological (SPT) orders, particularly subsystem-symmetry-protected topological (SSPT) orders. It was recently found that the one-dimensional cluster state may be associated with an SPT phase in random circuits respecting a global ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ symmetry, and furthermore that all phase transitions in this scenario belong to the same universality class. As resources with greater computational power feature greater symmetry, it is fruitful to investigate further any relationship between levels of symmetry in MIPTs and MBQC. In this paper we investigate MIPTs on a torus with three levels of symmetry-respecting unitary evolution interspersed by measurements. Although we find two area-law phases and one volume-law phase with distinct entanglement structures for each ensemble, the phase transition from the volume-law phase to the area-law phase associated with the 2D SSPT cluster state has variable correlation length exponent $\nu$. Whereas $\nu \approx 0.90$ for unconstrained Clifford unitaries and $\nu \approx 0.83$ for globally symmetric Cliffords, subsystem-symmetric Cliffords feature a much smaller value $\nu \approx 0.38$. We discuss how these distinct $\nu$'s quantify spacetime response scales where quantum information is manipulated by single-qubit measurements as in MBQC.

We investigate competition and cooperation of magnetic frustration and the Kondo effect in the $J_1\text{−}{J}_2$ Kondo lattice model on the square lattice at zero temperature. In this model, the frustrated interactions $J_1,J_2$ between the localized spins stabilize spin nematic orders, while the Kondo coupling favors local spin singlets. Using the slave boson mean field approximation, we find that the spin nematic order remains stable against small Kondo coupling, and the localized spins and the conduction electrons are effectively decoupled. On the other hand, a standard Fermi liquid state is formed for sufficiently strong Kondo interactions. Furthermore, in an intermediate region with moderate Kondo coupling, the spin nematic order and the Kondo effect coexist, and superconducting pairing of the conduction electrons is induced by the spinon pairing. We discuss the ground state phase diagram and nature of the quantum phase transitions between the different superconducting states.

Polarization-resolved reflectance (R), absorption, and photoreflectance spectroscopy reveal several excitonlike resonances around the band gap of bulk $1T^{\prime }\text{−}{\mathrm{ReSe}}_2$ at low temperatures. Some show strong linear polarization anisotropy, with oscillator strength maximized for polarizations roughly parallel to the $a$-axis or $b$-axis. Electronic structure calculations based on density functional theory, which include electron-hole Coulomb interaction via the $GW+$Bethe-Salpeter equation, show that bulk $1T^{\prime }\text{−}{\mathrm{ReSe}}_2$ has a direct band gap at the $Z$-point of the Brillouin zone, with a single valence and conduction band, each being doubly spin degenerate. We find two prominent sets of excitons that arise around the $Z$-point. Each set has four excitons that are spin-orbit-coupling-induced mixtures of triplet and singlet states, all of which acquire finite oscillator strength. By comparison with calculated polarization-resolved exciton absorption spectra, we identify origins of five resonances and the band gap. The binding energy $E_b$ of the two dominant lowest energy excitons was estimated to be $\sim 110$ meV and $\sim 86$ meV. However, despite the large $E_b$, the exciton resonances in R and associated photoluminescence (PL) emission both decay by 200 K ($k_{B}T\sim 17.3$ meV). The combination of Raman, temperature-dependent R and PL spectroscopy suggests that an unusually strong exciton–optical-phonon interaction leads to this rapid thermal decay of excitons in $1T^{\prime }\text{−}{\mathrm{ReSe}}_2$.

This paper proposes a universal microscopic model for the shallow confinement regime of single-electron tunneling devices. We consider particle escape from a quantum well generically emerging as a bifurcation in a smooth electrostatic potential and develop a set of analytic and numerical approximations for the ground-state tunneling and thermally activated escape rates. These approximations are applied to the problem of electron capture by a closing tunnel barrier where the competition between the closing speed and the escape rate defines a scaling relation for the capture fidelity. Effective one-dimensional cubic potential approximation leads to a universal form of this scaling relation in terms of device-independent dimensionless depth and speed parameters. Using temperature and magnetic-field dependence predictions, we show how to infer the energy scales of cubic longitudinal and quadratic transverse confinement. Finally, we derive an intrinsic quantum speed bound for adiabatic protection of the ground-state tunneling and show that the latter can potentially be exploited up to the breakdown of confinement with a practical speed limit set by reaching the quantum uncertainty of the barrier height before the onset of nonadiabatic excitation. These results contribute to mapping out the physical limits of single-electron quantum technologies for electrical metrology and sensing.

In this work, we develop an analytical framework to understand quantum friction across distinct stability regimes, providing approximate expressions for frictional forces both in the deep stable regime and near the critical threshold of instability. Our primary finding is analytical proof that, near the instability threshold, the quantum friction force diverges logarithmically. This result, verified through numerical simulations, sheds light on the behavior of frictional instabilities as the system approaches criticality. Our findings offer new insights into the role of instabilities, critical divergence, and temperature in frictional dynamics across quantum and classical regimes.

Establishing a rigorous proof of integrability or nonintegrability in quantum many-body systems remains one of the most challenging tasks, as it requires demonstrating the presence or absence of local conserved quantities and deciphering the complex dynamics of the system. In this paper, we introduce a graph-theoretical approach to address this issue. This framework not only provides a comprehensive method for analyzing the nonintegrability of quantum systems but also brings advantages for integrable systems by enabling the classification of local conserved quantities through a straightforward count of isolated loops in the associated graphs. As an illustrative example, we focus on the PXP model, a system widely believed to be nonintegrable, and rigorously confirm its nonintegrability by demonstrating the absence of local conserved quantities through graph-theoretical analysis. This proof for the PXP model conveys several key insights: It not only confirms the model's nonintegrability but also indicates that the quantum many-body scarring properties of the model is not associated with local conserved quantities. Our innovative approach is broadly applicable for proving (non-)integrability in other quantum many-body systems, significantly simplifying the process and offering numerous potential applications.

Simulation of spectra of x-ray absorption spectroscopy (XAS) at the $L$ edge is a well-established and reliable computational tool that, in combination with experimental measurements, reveals details about the local electronic structure and the chemical environment of transition metal ions (TMIs). For light TMIs, $2p$ XAS model Hamiltonian approaches (MHAs) are fast and accurate in computing spectra. Here, an alternative method using artificial neural networks (ANNs) is employed. This approach predicts x-ray absorption spectra at the $L$ edge of light transition metal ions for elements from Ti to Ni, including different oxidation states, from sets of physical input parameters. Computational performance and accuracy were optimized through a systematic exploration of ANN design and compared to MHA simulations. While the MHA remains more accurate due to the statistical nature of the developed machine learning method, the ANN-based prediction of $2p$ XAS spectra achieved good accuracy of more than 99%, in addition to a noticeable advantage in computational speed. This makes the method intrinsically more suitable for computational high-throughput setups as well as in support of data-intense experimental workflows. However, the extrapolation weakness of artificial neural networks remained for most cases and displayed a strong dependence on the TMI. For one TMI, an extrapolation accuracy of 78% was maintained.

The Liouvillian skin effect describes the boundary affinity of Liouvillian eignemodes that originates from the intrinsic non-Hermiticity of the Liouvillian superoperators. Dynamically, it manifests as directional flow in the transient dynamics, and the accumulation of population near open boundaries at long times. Intriguingly, similar dynamic phenomena exist in the well-known process of optical pumping, where the system is driven into a desired state (or a dark-state subspace) through the interplay of dissipation and optical drive. In this work, we show that typical optical pumping processes can indeed be understood in terms of the Liouvillian skin effect. By studying the Liouvillian spectra under different boundary conditions, we reveal that the Liouvillian spectra of the driven-dissipative pumping process sensitively depend on the boundary conditions in the state space, a signature that lies at the origin of the Liouvillian skin effect. Such a connection provides insights and practical means for designing efficient optical pumping schemes through engineering Liouvillian gaps under the open boundary condition. Based on these understandings, we show that the efficiency of a typical sideband cooling scheme for trapped ions can be dramatically enhanced by introducing counterintuitive dissipative channels. Our results provide a useful perspective for optical pumping, with interesting implications for state preparation and cooling.

Motivated by a recent experimental study on the quantum Ising magnet ${\text{K}}_2\text{Co}{\left({\text{SeO}}_3\right)}_2$ that presented spectroscopic evidence of zero-field supersolidity (Chen et al., arXiv:2402.15869), we simulate the excitation spectrum of the corresponding microscopic $XXZ$ model for the compound using the recently developed excitation ansatz for infinite projected entangled-pair states. We map out the ground state phase diagram and compute the dynamical spin structure factors across a range of magnetic field strengths, focusing especially on the two supersolid phases found near zero and saturation fields. Our simulated excitation spectra for the zero-field supersolid “Y” phase are in excellent agreement with the experimental data, recovering the low-energy branches and integer quantized excited energy levels ${\omega }_n=n{J}_{zz}$. Furthermore, we demonstrate the nonlocal multi-spin-flip features for modes at ${\omega }_2$, indicative of their multimagnon nature. Additionally, we identify characteristics of the high-field supersolid “$\mathrm{\Psi }$” phase in the simulated spectra, which should be compared with future experimental results.

Changing the macroscopic properties of quantum materials by optically activating collective lattice excitations has recently become a major trend in solid state physics. One of the most commonly employed light-matter interaction routes is the displacive mechanism. However, the fundamental contribution to this process remains elusive, as the effects of free-carrier density modification and raised effective electronic temperature have not been disentangled yet. Here we use time-resolved pump-probe spectroscopy to address this issue in the Rashba semiconductor BiTeI. Exploring the conventional regime of electronic interband transitions for different excitation wavelengths as well as the barely accessed regime of electronic intraband transitions, we answer this question regarding the displacive mechanism: the lattice modes are predominantly driven by the rise of the effective electronic temperature. In the intraband regime, which allows an increase of the effective carrier temperature while leaving the carrier density unaffected, the phonon coherence time does not display significant fluence-dependent variations. Our results thus reveal a pathway to displacive excitation of coherent phonons, free from additional scattering and dissipation mechanisms typically associated with an increase of the free-carrier density.

The compound ${\mathrm{SrNi}}_2{\mathrm{P}}_2$ is unique among the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ class since it exhibits a temperature-induced transition upon cooling from an uncollapsed tetragonal (ucT) state to a one-third-collapsed orthorhombic (tcO) state where one out of every three P-rows bond across the Sr layers. This compound is also known for exhibiting bulk superconductivity below 1.4 K at ambient pressure. In this paper, we report on the effects of Rh substitution in $\mathrm{Sr}{\left({\mathrm{Ni}}_{1-x}{\mathrm{Rh}}_x\right)}_2{\mathrm{P}}_2$ on the structural and superconducting properties. We studied the variation of the nearest P-P distances as a function of the Rh fraction at room temperature, as well as its temperature dependence for selected compositions. We find that increasing the Rh fraction leads to a decrease in the transition temperature between the ucT and tcO states, until a full suppression of the tcO state for $x\ge 0.166$. The superconducting transition first remains nearly insensitive to the Rh fraction, and then it increases to 2.3 K after the tcO state is fully suppressed. These results are summarized in a phase diagram, built upon the characterization by energy dispersive x-ray spectroscopy, x-ray diffraction, resistance, magnetization, and specific heat measurements done on crystalline samples with varying Rh content. The relationship between band structure, crystal structure, and superconductivity is discussed based on previously reported band structure calculations on ${\mathrm{SrRh}}_2{\mathrm{P}}_2$. Moreover, the effect of Rh fraction on the stress-induced structural transitions is also addressed by means of strain-stress studies done by uniaxial compression of single-crystalline micropillars of $\mathrm{Sr}{\left({\mathrm{Ni}}_{1-x}{\mathrm{Rh}}_x\right)}_2{\mathrm{P}}_2$.

A common approach to modeling dispersion interactions and overcoming the inaccurate description of long-range correlation effects in electronic structure calculations is the use of pairwise-additive potentials, as in the Tkatchenko-Scheffler [Phys. Rev. Lett. 102, 073005 (2009)] method. In previous work [H. Muhli et al., Phys. Rev. B 104, 054106 (2021)], we have shown how these are amenable to highly efficient atomistic simulation by machine learning their local parametrization. However, the atomic polarizability and the electron correlation energy have a complex and nonlocal many-body character and some of the dispersion effects in complex systems are not sufficiently described by these types of pairwise-additive potentials. Currently, one of the most widely used rigorous descriptions of the many-body effects is based on the many-body dispersion (MBD) model [A. Tkatchenko et al., Phys. Rev. Lett. 108, 236402 (2012)]. In this work, we show that the MBD model can also be locally parametrized to derive a local approximation for the highly nonlocal many-body effects. With this local parametrization, we develop an atomwise formulation of MBD that we refer to as linear MBD (lMBD), as this decomposition enables linear scaling with system size. This model provides a transparent and controllable approximation to the full MBD model with tunable convergence parameters for a fraction of the computational cost observed in electronic structure calculations with popular density-functional theory codes. We show that our model scales linearly with the number of atoms in the system and is easily parallelizable. Furthermore, we show how using the same machinery already established in previous work for predicting Hirshfeld volumes with machine learning enables access to large-scale simulations with MBD-level corrections.

We investigate a one-dimensional tight-binding lattice with asymmetrical couplings and various type of nonlinearities to study the nonlinear non-Hermitian skin effect. Our focus is on the exploration of nonlinear skin modes through a fixed-point perspective. The nonlinear interactions are shown to have no impact on the spectral region in a semi-infinite system; however, they induce considerable changes when boundaries are present. The spectrum under open boundary conditions is found not to be a subset of the corresponding spectrum under the semi-infinite boundary conditions. We identify distinctive features of nonlinear skin modes, such as degeneracy and power-energy discontinuity. Furthermore, we demonstrate that a family of localized modes that are neither skin nor scale-free localized modes is formed with the introduction of a coupling impurity. Additionally, we show that an impurity can induce discrete dark and antidark solitons.

We study the scattering of one-dimensional ferro- and antiferromagnetic magnons by a bond defect. The bond defect connects two side chains of spins with isotropic Heisenberg first-neighbor coupling. The bond defect accounts for an anisotropic Heisenberg coupling and an antisymmetric Dzyaloshinskii-Moriya interaction. Within a semiclassical approach, the stationary scattering theory determines the transmission spectra of magnon excitations as a function of the coupling parameters at the bond defect. We compare the spectral properties of ferro- and antiferromagnetic magnons. Full flat-band transmission occurs for a particular superposition of anisotropic and antisymmetric defect couplings. Further, we unveil a transparent mode of ferromagnetic magnons with a specific wave vector in a region of the defect's coupling parameter space, a feature absent for antiferromagnetic magnons.

Using atomistic simulations, we show that different types of skyrmion states called skyrmion molecular crystals and skyrmion superlattices can be realized on triangular substrates when there are two or three skyrmions per substrate minimum. We find that as a function of the magnetic field and substrate periodicity, a remarkably wide variety of ordered phases appear similar to those found in colloidal or Wigner molecular crystals, including ferromagnetic and herringbone states. The ability of the skyrmions to annihilate, deform, and change size gives rise to a variety of superlattice states in which a mixture of different skyrmion sizes and shapes produces bipartite or more complex lattice structures. Our results are relevant for skyrmions on structured triangular substrates or in magnetic arrays, or for skyrmions in moiré materials.

In $\mathrm{Cr}{X}_2$($X$ = Br, I), a Jahn-Teller effect distorts the octahedral configuration of the anions about Cr, resulting in a ribbon chain structure. We previously observed helimagnetism in ${\mathrm{CrI}}_2$ propagating along the ribbon chains, with an $80–{90}^{\circ }$ rotation per Cr ion. Via neutron scattering, we report that ${\mathrm{CrBr}}_2$ and the solid solution $\mathrm{CrIBr}$ are also helimagnetic, with Néel temperatures of 17 and 12 K, respectively, and signs of intermediate magnetic transitions in both compounds. The helical angle between spins on consecutive Cr ions along the ribbon chains increases with increasing Br substitution, from $89.7^{\circ }$ (${\mathrm{CrI}}_2$) to ${116}^{\circ }$ ($\mathrm{CrIBr}$) to ${147}^{\circ }$ (${\mathrm{CrBr}}_2$), possibly as a result of decreasing longer-range intrachain spin interactions with the increased electron localization of the lighter anions.