Understanding the dynamics of open quantum systems is intrinsically important to control and utilize quantum hardware in various applications. The generic behavior of purely dissipative open quantum many-body systems with $k$-local, normal dissipation processes can be investigated using random matrix theory, revealing different decay timescales of observables determined by their complexity, as shown by Wang et al. [K. Wang et al., Phys. Rev. Lett. 124, 100604 (2020)]. The time evolution of an open quantum system is entirely described by the eigenvalue spectrum of the Lindbladian, in which the hierarchy of decay timescales is reflected in the formation of well-separated eigenvalue clusters. Here, we extend the analysis of this spectrum to the presence of unitary dynamics using a random matrix model. In the case of strong dissipation, the unitary dynamics can be treated perturbatively and the $k$ locality of the Hamiltonian determines how susceptible the spectrum is to such a perturbation. For the physically most relevant case of (dissipative) two-body interactions, the correction in the first order of the perturbation vanishes, leading to the relative robustness of the spectral features, which in turn implies the same robustness of the corresponding separation of decay timescales. We find the perturbative approach for the spectrum to yield an excellent agreement with exact-diagonalization results. For weak dissipation, the spectrum flows into clusters except for outlier eigenmodes, which we identify to be the local symmetries of the Hamiltonian. We support our analytic findings by numerical simulations and show that our generic model applies to a nonrandom Heisenberg chain.

We performed ${}^{23}\mathrm{Na}$ NMR measurements on a single-domain crystal of the Kitaev material ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ with magnetic field applied along the crystalline $a$ axis. A positive Curie-Weiss constant is obtained from the NMR Knight shift, which suggests the existence of ferromagnetic exchange couplings. The antiferromagnetic ordering is found to be suppressed at a field of 1.9 T. Inside the ordered phase, our data reveal two additional phase transitions. At 1.9 T the spin-lattice relaxation rate $1{/}^{23}{T}_1$ establishes a quantum critical behavior at high temperatures. However, at low temperatures a gapped behavior is observed at the “critical” field, which suggests a weak first-order transition instead and a possible field-induced quantum spin liquid. Our results reveal complex microscopic interactions in the system that may help to search for possible quantum spin liquids.

We theoretically study the topological properties of magnons and the relevant magnon thermal Hall effect in trimerized Lieb lattice ferromagnets in the presence of next-nearest-neighbor Dzyaloshinskii-Moriya interactions. By calculating the magnon band structures and their Chern numbers with the linear spin-wave theory, we show that the system can undergo a phase transition between a magnonic topological insulator phase and a magnonic trivial insulator phase. The main results are presented in the form of topological phase diagrams, where the Chern numbers or magnon thermal Hall conductivity are shown as a function of the two lattice trimerization parameters. We find a sharp change of the thermal Hall conductivity across the critical point of phase transformations associated with topological nontrivial edge states. The behaviors reflect that the existence of trimerizations breaks the ${\mathbf{C}}_4$ rotational symmetry of the Lieb lattice. Finally, we show that our theoretical predictions could be experimentally realized in high-temperature cuprate superconductors or organic magnetic materials.

High-quality monoclinic ${\mathrm{Cr}}_3{\mathrm{Te}}_4$ crystals are synthesized by the chemical vapor transport method and confirmed by synchrotron powder x-ray diffraction. Critical behaviors of ${\mathrm{Cr}}_3{\mathrm{Te}}_4$ are studied by bulk isothermal magnetization measurements around its paramagnetic to ferromagnetic phase transition. From these data, we evaluate the modified Arrott plot, the Kouvel-Fisher plot, and critical isotherms, and estimate critical exponent values of $\beta \sim 0.3827, \gamma \sim 1.2119$, and $\delta \sim 4.16$, and the Curie temperature $T_C\approx 321$ K. After the scaling, the isothermal magnetization curves below and above the critical temperatures collapse into two independent universal branches which signifies the reliability of our critical exponent estimation. The estimated critical exponent values of ${\mathrm{Cr}}_3{\mathrm{Te}}_4$ do not match well with theoretically calculated values of any three-dimensional models.

Theory and experiments show that, with increasing pressure, the chemical bonds of methane rearrange, leading to the formation of complex polymers and then to dissociation. However, there is disagreement on the exact conditions where these changes take place. In this study, methane samples were precompressed in diamond-anvil cells and then shock compressed to pressures reaching 400 GPa, the highest pressures yet explored in methane. The results reveal a qualitative change in the Hugoniot curve at 80–150 GPa, which is interpreted as a signature of dissociation based on thermodynamic calculations and theoretical predictions.

Non-Hermitian topological phases have gained immense attention due to their potential to unlock novel features beyond Hermitian bounds. $\mathcal{P}\mathcal{T}$-symmetric (parity time-reversal symmetric) non-Hermitian models have been studied extensively over the past decade. In recent years, the topological properties of general non-Hermitian models, regardless of the balance between gains and losses, have also attracted vast attention. Here, we propose a non-Hermitian second-order topological (SOT) insulator that hosts gapless corner states on a two-dimensional quasicrystalline lattice (QL). We first construct a non-Hermitian extension of the Bernevig-Hughes-Zhang model on a QL generated by the Amman-Beenker tiling. This model has real spectra and supports helical edge states. Corner states emerge by adding a proper Wilson-mass term that gaps out the edge states. We propose two variations of the mass term that result in fascinating characteristics. In the first variation, we obtain a purely real spectra for the second-order topological phase. In the latter, we get a complex spectra with corner states localized at only two corners due to the higher-order non-Hermitian skin effect of the edge modes. Our findings pave a path to engineering exotic SOT phases where corner states can be localized at designated corners.

Magnetic skyrmions are vortexlike quasiparticles characterized by long lifetime and remarkable topological properties. That makes them a promising candidate for the role of information carriers in magnetic information storage and processing devices. Although considerable progress has been made in studying skyrmions in classical systems, little is known about the quantum case: quantum skyrmions cannot be directly observed by probing the local magnetization of the system, and the notion of topological protection is elusive in the quantum realm. Here we explore the potential robustness of quantum skyrmions in comparison to their classical counterparts. We theoretically analyze the dynamics of a quantum skyrmion subject to local projective measurements and demonstrate that the properties of the skyrmionic quantum state change very little upon external perturbations. We further show that by performing repetitive measurements on a quantum skyrmion, it can be completely stabilized through an analog of the quantum Zeno effect.

We study a duality transformation from the gauge-invariant subspace of a ${\mathbb{Z}}_N$ lattice gauge theory on a two-leg ladder geometry to an $N$-clock model on a single chain. The main feature of this mapping is the emergence of a longitudinal field in the clock model, whose value depends on the superselection sector of the gauge model, implying that the different sectors of the gauge theory can show quite different phase diagrams. To investigate this and see if confined phases might emerge, we perform a numerical analysis for $N=2,3,4$, using both exact diagonalization and the density matrix renormalization group algorithm.

Here, the magnetic susceptibility of a triangular-graphene-quantum-dot-like system was examined by using the determinant quantum Monte Carlo method. We focused on three zigzag edge quantum dots or rings, namely, the triangular graphene quantum ring, bilayer triangular graphene quantum dots, and bilayer triangular graphene quantum ring. The triangular-graphene-quantum-dot-like system exhibited robust edge ferromagnetic behavior, which was independent of size, monolayer or bilayer, or dot or ring shape, according to the numerical results. At half filling, the edge magnetic susceptibility is increased by on-site interactions, especially in the low-temperature region. Spintronics systems may benefit from use of this system due to its robust edge ferromagnetic behavior.

Models with topological flat bands (TFBs) have been proposed on which the fractional Chern insulators (FCIs) have been studied. To flatten the lowest energy band, some long-range hopping terms are considered in Chern insulator (CI) models. In this paper, we study a tight-binding model on a maple leaf lattice with staggered magnetic fluxes. When the nearest-neighbor hopping term with a suitable phase factor is considered, a TFB with a large flatness ratio emerges, suggesting that staggered magnetic fluxes can give rise to TFBs. By adding the next-nearest-neighbor (NNN) hopping term, there are two types of topological states at one-third filling, i.e., the CI and the higher-order topological insulator (HOTI) states. Simultaneously, we find the CI with a high Chern number can transition to the HOTI state by tuning the NNN hopping parameter. When we consider more distant-neighbor hopping terms, a TFB with Chern number two is found. We further investigate FCI states when hard-core bosons are taken into account in these TFB models.

The joint effects of Kekulé lattice distortions and Rashba-type spin-orbit coupling on the electronic properties of graphene are explored. We modeled the position dependence of the Rashba energy term in a manner that allows its seamless integration into the scheme introduced by Gamayun et al. [New J. Phys. 20, 023016 (2018)] to describe graphene with Kekulé lattice distortion. Particularly for the Kekulé-Y (Kek-Y) texture, the effective low-energy Dirac Hamiltonian contains a new spin-valley locking term in addition to the well-known Rashba-induced momentum-pseudospin and spin-pseudospin couplings and the Kekulé-induced momentum-valley coupling term. We report on the low-energy band structure and Landau level spectra of Rashba-spin-orbit-coupled Kek-Y graphene and propose an experimental scheme to discern between the presence of Rashba spin-orbit coupling, Kek-Y lattice distortion, and both based on doping-dependent magnetotransport measurements.

In some insulators, corner charges are fractionally quantized, due to the topological invariant called a filling anomaly. The previous theories of fractional corner charges have been mostly limited to two-dimensional systems. In three dimensions, only limited cases have been studied. In this study, we derive formulas for the filling anomaly and the corner charge in various crystals with all the tetrahedral and cubic space groups. We discuss that the quantized corner charge requires the crystal shapes to be vertex-transitive polyhedra. We show that the formula of the filling anomaly is universally given by the difference between electronic and ionic charges at the Wyckoff position $1a$. The fractional corner charges appear by equally distributing the filling anomaly to all the corners of the crystal. We also derive the $k$-space formulas for the fractional corner charge. In some cases, the corner charge is not determined solely from the irreps at high-symmetry $k$ points. In such cases, we introduce a ${\mathbb{Z}}_2$ topological invariant to determine the corner charge.

Exactly solvable Hamiltonians with spin liquid ground states have proven to be extremely useful, not only because they unambiguously demonstrate that these phases can arise in systems of interacting spins but also as a pedagogical illustration of the concept and as a controlled starting point for further theoretical analysis. However, adding dissipative couplings to the environment—an important aspect for the realization of these phases—generically spoils the exact solvability. We here present and study a Lindbladian, describing a square-lattice spin-liquid with dissipative coupling to the environment, that admits an exact solution in terms of Majorana fermions coupled to static ${\mathbb{Z}}_2$ gauge fields. This solution allows us to characterize the steady-state solutions as well as “quasiparticle” excitations within the Lindbladian spectrum. We uncover distinct types of quasiparticle excitations of the Lindbladian associated with parametrically different timescales governing the equilibration time of the expectation values of different classes of observables. Most notably, for small but nonzero dissipation, we find a separation into three different timescales associated with a three-step heating profile. On a more general level, our exactly solvable Lindbladian is expected to provide a starting point for a better understanding of the behavior of fractionalized systems under dissipative time evolution.

We present a theoretical study of an exciton-polariton annular microcavity with an additional quasiperiodic structure along the ring which is implemented in the form of a bicosine dependence. We demonstrate that for a sufficiently strong quasiperiodic modulation, the microcavity features a sharp mobility edge separating a cluster of localized states from the rest of the spectrum consisting of states extended over the whole ring. Localized modes can be excited using a resonant pump whose topological charge determines the phase distribution of excited patterns. Repulsive polariton interactions make the resonance peaks distinctively asymmetric and enable the formation of multistable states which feature the attractor-like dynamical behavior and hysteresis. We also demonstrate that the localized states can be realized in a biannular cavity that consists of two rings, each having periodic modulation, such that the periods of two modulations are different.

A combinatorial approach is employed to investigate the atomic and electronic structures of a metal vacancy in titanium carbide. It turns out that the usual relaxed geometry of the vacancy is just a metastable state representing a local energy minimum. Using ab initio calculations and by systematically searching through the configurational space of a Ti monovacancy, we identify a multitude of local minima with reconstructed geometry that are lower in energy. Among them, there is a planar configuration with two displaced carbons forming a dimer inside the vacancy. This structure has the optimal number and order of C–C bonds making it the global minimum. Further calculations show that this reconstructed geometry is also the ground state of metal vacancies in other carbides such as ZrC, HfC, and VC. The reconstructed metal vacancies are characterized by localized electron states due to the relatively short C–C bonds. The defect states lie just below the upper and lower valence bands. The existence of reconstructed vacancy configurations is essential for understanding the mechanism of metal self-diffusion in transition-metal carbides.

The magnetoelectric effect is an appealing property displayed by most compounds in the corundum-derived 429 series ($M_4{A}_2{\mathrm{O}}_9, A=\mathrm{Nb}$ or Ta). The exception is ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, crystallizing in the orthorhombic Pbcn space group: because of symmetry considerations, its magnetic space group forbids the occurrence of the magnetoelectric effect. ${\mathrm{Li}}_2{\mathrm{Ni}}_2{\mathrm{W}}_2{\mathrm{O}}_9$ crystallizes in the same space group as ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$; however, as half the ${\mathrm{Ni}}^{2+}$ cations are replaced by ${\mathrm{Li}}^{+}$ cations, the interactions between the remaining magnetic elements are drastically modified. In this paper, we present the results of magnetic bulk measurements, showing an antiferromagnetic ordering coupled with strong ferromagnetic interactions below $T_{\mathrm{N}}\approx 8\mathrm{K}$ in ${\mathrm{Li}}_2{\mathrm{Ni}}_2{\mathrm{W}}_2{\mathrm{O}}_9$. Through neutron diffraction, the magnetic structure of ${\mathrm{Li}}_2{\mathrm{Ni}}_2{\mathrm{W}}_2{\mathrm{O}}_9$ is solved, and it is confidently established that it presents the ${Pb}^{\prime }{c}^{\prime }{n}^{\prime }$ Shubnikov space group (#60.425), whose point symmetry $m^{\prime }{m}^{\prime }{m}^{\prime }$ allows the magnetoelectric effect. This could possibly open the door to a family of magnetoelectric, orthorhombic compounds.

Uncovering pathways to optically drive magnetic order-disorder transitions on ultrashort timescales can lead to the realization of novel out-of-equilibrium quantum phenomena. A long-sought pathway is to directly excite a highly nonthermal energy-momentum distribution of magnons, bypassing both charge and lattice degrees of freedom. However, this remains elusive owing to the weak coupling and large momentum mismatch between photons and magnons. Here we demonstrate strong parametric excitation of magnons across the entire Brillouin zone of the antiferromagnetic insulator ${\mathrm{Sr}}_2{\mathrm{Cu}}_3{\mathrm{O}}_4{\mathrm{Cl}}_2$ by periodically modulating the superexchange interaction with the electric field of light. The excitation efficiency is greatly enhanced by tuning to the van Hove singularity in the magnon spectrum, sufficient to transiently collapse the antiferromagnetic state using a pulsed laser field of ${10}^9$ V/m. The order parameter recovery timescale increases by over 1000 times as a function of excitation density, reflecting a crossover from high- to low-energy magnon dominated decay dynamics. This electric-field induced parametric magnon pumping mechanism is applicable to a broad range of magnetic insulators and opens up the possibility of dynamically engineering magnon distributions by design.

The pyrochlore osmate ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is a candidate material for a fragile $J=0$ local singlet ground state; however little is known regarding the single-ion behavior of either the Os or Ho ions. To address this we present polarized neutron powder diffraction (PNPD) and RIXS measurements that separately probe the local site behavior of the Os and Ho ions. The PNPD results are dominated by ${\mathrm{Ho}}^{3+}$ scattering and the analysis reveals local site susceptibility behavior consistent with spin-ice materials. Complementary unpolarized neutron powder diffraction shows an ordered spin-ice ground state in an applied magnetic field. To isolate the ${\mathrm{Os}}^{4+}$ single-ion behavior we present resonant inelastic x-ray scattering (RIXS) measurements at the osmium $L$ edge. Analysis of the RIXS spectra parametrizes the spin-orbit coupling (0.35 eV), Hund's coupling (0.27 eV), and trigonal site distortion $(-0.17 \mathrm{eV})$. The results are considered within the context of a $J=0$ model and possible departures from this through structural distortions, excitonic interactions, and $5d\text{−}4f$ interactions between the Os ion and the surrounding Ho lattice. The experimental methodology employed highlights the complementary information available in rare-earth-based $5d$ pyrochlores from distinct neutron and x-ray scattering techniques that allow for the isolation and determination of the behavior of the different ions.

Crystallographic anisotropy of the spin-dependent conductivity tensor can be exploited to generate transverse spin-polarized current in a ferromagnetic film. This ferromagnetic spin Hall effect is analogous to the spin-splitting effect in altermagnets and does not require spin-orbit coupling. First-principles screening of 41 noncubic ferromagnets revealed that many of them, when grown as a single crystal with tilted crystallographic axes, can exhibit large spin Hall angles comparable with the best available spin-orbit-driven spin Hall sources. Macroscopic spin Hall effect is possible for uniformly magnetized ferromagnetic films grown on some low-symmetry substrates with epitaxial relations that prevent cancellation of contributions from different orientation domains. Macroscopic response is also possible for any substrate if magnetocrystalline anisotropy is strong enough to lock the magnetization to the crystallographic axes in different orientation domains.