A one-third monolayer of the heavy metals Sn and Pb deposited on semiconductor substrates can lead to a $\sqrt{3}\times \sqrt{3}$ surface reconstruction, constituting an exciting triangular lattice material platform. A long history of experiments identified charge-ordered and magnetic ground states. These discoveries were accompanied by a decades-long debate of whether electron correlations or other effects involving phonons are the driving force of the symmetry-broken states. The most recent discovery of superconductivity in boron-doped Sn/Si(111) with a $T_c$ between 5 and 9 K led to a renewed excitement. Here we revisit the electronic and phononic properties of Sn and Pb adatom triangular lattices on Si(111) and SiC(0001). For all materials, we compute relativistic band structures using density functional theory (DFT)$+U$, where $U$ is only applied to the substrate atoms in order to adjust the band gap to match the experimental value; as a consequence, some of the resulting tight-binding parameters of the metallic surface band substantially differ compared to previous studies. Remarkably, for Pb/SiC(0001), we predict Rashba spin-orbit coupling as large as 45% of the nearest-neighbor hopping energy. In addition, we compute the phonon spectra and electron-phonon coupling constants for all materials, and for Pb/Si(111) even relativistically, although the inclusion of spin-orbit coupling has surprisingly little effect on the electron-phonon coupling constant. We conclude that the resulting couplings are too weak to account for electron-phonon-mediated superconductivity in any of these materials.

We theoretically investigate the quantum transport properties of three-dimensional spin-1 chiral fermion systems with a curved trivial band. In the multiband system with two distinct characters—a linear Dirac band and a quadratic trivial band—the hybridization induced by impurity effects leads to pronounced energy and temperature dependences in the electrical conductivity. We show that the conductivity is suppressed by the trivial band in the low-energy regime near the threefold degenerate point and enhanced in the band-crossing point of the Dirac and trivial bands. These results are derived using the self-consistent Born approximation within the framework of linear response theory.

The viscous flow of electrons in a narrow channel requires both strong electron-electron interactions and no-slip boundary conditions. However, introducing obstacles within the liquid can significantly increase flow resistance and, as a result, amplify the effects of viscosity. Even in samples with smooth walls, the presence of an obstacle can strongly alter electron behavior, leading to pronounced hydrodynamic effects. We investigated transport in mesoscopic samples containing a disordered array of obstacles. In contrast to samples without obstacles, which do not show a decrease in resistivity with rising temperature, samples with obstacles exhibit a significant resistivity reduction as the temperature increases (the Gurzhi effect). By measuring the negative magnetoresistance, we extracted shear viscosity and other parameters through comparison with theoretical predictions. Consequently, narrow-channel samples with a disordered obstacle array provide a valuable platform for studying hydrodynamic electron flow independently of boundary conditions.

We demonstrate that the optical response of twisted photonic bilayers, photonic counterparts of van der Waals structures, is sensitive to both spin angular momentum (SAM) and orbital angular momentum (OAM) of light. A beam of unpolarized light with zero angular momentum acquires SAM in transmission and OAM in reflection. The developed analytical theory and numerical calculations show that the SAM and OAM arise from distinct microscopic mechanisms and depend differently on the interlayer distance. The predicted phenomena do not require light absorption and are caused by the photon-helicity-dependent light diffraction by the moiré pattern, which inevitably occurs in the twisted structure, and the SAM-OAM conversion processes. We also reveal strong SAM and OAM in the moiré-diffracted beams. Our findings uncover a profound connection between the emergent SAM and OAM in twisted photonic systems offering new possibilities for angular-momentum-resolved light-matter interactions.

As an important member of transition-metal borides, tungsten diboride (i.e., ${\mathrm{WB}}_{2+x}$) contains complex atomic vacancies with the originally assigned hexagonal symmetries, exhibiting many fascinating properties such as superconductivity and superior hardness. However, due to the difficulties in exploring atomic vacancies of transition-metal borides, the actual structure and composition of ${\mathrm{WB}}_{2+x}$ have been long-standing unsettled issues, which have impeded in-depth understanding of its structural stability and origins of such extraordinary properties. Here, we report a systematic investigation of the crystal structure of high-pressure synthesized ${\mathrm{WB}}_{2+x}$ samples by combination of state-of-the-art diffraction techniques and microstructural observations, leading to the discovery of an unusual coexistence of both the ordered and disordered atomic vacancies in ${\mathrm{WB}}_{2+x}$ with a broken hexagonal symmetry that can be well described by a dual-phase model involving hP12-${\mathrm{W}}_{0.70}{\mathrm{B}}_{1.73}$ and P1-${\mathrm{W}}_{0.75}{\mathrm{B}}_{1.64}$. Superposition of thin sample layers with disordered vacancies along the [001] or [110] direction is revealed to produce ordered vacancies, rationalizing the observed size-dependent symmetry breaking. These findings not only provide solid foundations for studying the phase stability and properties of this boride but also give powerful insights into how the intricate atomic vacancies can influence crystal structures of transition-metal borides.

By applying the projector to the filled lattice eigenstates on a specific position, or applying the local electron annihilation operator on the many-body ground state, one can construct a quantum state localized around a specific position in a solid. The overlap of two such local states at slightly different positions defines a quantum metric in real space, which manifests even in systems as simple as particles in a box. For continuous systems like electron gas, this metric weighted by the density gives the momentum variance of electrons, which is readily measurable by ARPES. The presence of disorder curves the real space manifold and gives rise to various differential geometrical quantities like Riemann tensor and Ricci scalar, indicating the possibility of engineering differential geometrical properties by disorder, as demonstrated by lattice models of 2D metals and topological insulators.

The complexity of simulating the out-of-equilibrium evolution of local operators in the Heisenberg picture is governed by the operator entanglement, which grows linearly in time for generic nonintegrable systems, leading to an exponential increase in computational resources. A promising approach to simplify this challenge involves discarding parts of the operator and focusing on a subspace formed by “light” Pauli strings—strings with few Pauli matrices—as proposed by Rakovszki et al. [Phys. Rev. B 105, 075131 (2022)]. In this work, we investigate whether this strategy can be applied to quenches starting from homogeneous product states. For ergodic dynamics, these initial states grant access to a wide range of equilibration temperatures. By concentrating on the desired matrix elements and retaining only the portion of the operator that contains Pauli strings parallel to the initial state, we uncover a complex scenario. In some cases, the light Pauli strings suffice to describe the dynamics, enabling efficient simulation with current algorithms. However, in other cases, heavier strings become necessary, pushing computational demands beyond our current capabilities. We analyze this behavior by introducing a measure of complexity, the operator weight entropy, which we compute for different operators across most points on the Bloch sphere.

Correlated quantum many-body states can be created and controlled by the dissipative protocols. Among these, particle number-conserving protocols are particularly appealing due to their ability to stabilize topologically nontrivial phases. Is there any fundamental limitation to their performance? We address this question by examining a general class of models involving a two-band fermion system subjected to dissipation designed to transfer fermions from the upper band to the lower band. By construction, these models have a guaranteed steady state—a dark state—with a completely filled lower band and an empty upper band. In the limit of weak dissipation, we derive equations governing the long-wavelength and long-time dynamics of the fermion densities and analyze them numerically. These equations belong to the Fisher-Kolmogorov-Petrovsky-Piskunov reaction-diffusion universality class. Our analysis reveals that the engineered dark state is generically unstable, giving way to a new steady state with a finite density of particles in the upper band. We also estimate the minimum system sizes required to observe this instability in finite systems. Our results suggest that number-conserving dissipative protocols may not be a reliable universal tool for stabilizing dark states.

The barlowite $\left[{\mathrm{Cu}}_4{\left(\mathrm{OD}\right)}_6\mathrm{FBr}\right]$ family of materials is characterized by antiferromagnetic spin-1/2 kagome layers and are believed to host interesting ground states of quantum magnetism. Substituting the interlayer sites with zinc ions yields Zn-barlowite (${\mathrm{Zn}}_x{\mathrm{Cu}}_{4-x}{\left(\mathrm{OD}\right)}_6\mathrm{FBr}, x\approx 0.80$) which is a strong candidate to have a quantum spin liquid (QSL) state. Experimentally distinguishing between valence bond crystal (VBC) and QSL states in the absence of magnetic ordering can prove challenging as both states exhibit similar average spin-spin correlations. Using high-resolution inelastic x-ray scattering with 1.4 meV energy resolution, we probe acoustic phonons in these structurally similar materials at various temperatures. Our measurements uncover robust differences possibly driven by different spin-phonon physics in VBC and QSL states, particularly at the $M$-point Brillouin zone boundary. Over a large $T=[15,200]\mathrm{K}$ temperature range, where neither material undergoes magnetic ordering nor a structural transition, we observe phonon softening in Zn-barlowite in contrast to a hardening of the same phonon in barlowite. These results indicate that acoustic phonons in the barlowite system can be used to study and possibly distinguish between QSL and disordered VBC states in quantum magnets.

In this study, we investigated the magnetic properties of ${\mathrm{Na}}_{0.5}{\mathrm{Yb}}_{0.5}{\mathrm{WO}}_4$, a hyperhoneycomb lattice-based material with a scheelite structure characterized by intrinsic antisite-disorder between sodium (Na) and ytterbium (Yb) ions. Through detailed magnetization measurements down to 400 mK, we found no evidence of long-range magnetic ordering. Analysis of the inverse susceptibility data using a modified Curie-Weiss law suggests that the ground state is best described as a Kramers doublet with $J_{\mathrm{eff}}$ = 1/2. Analysis of the field dependence of the Schottky anomaly evident in the heat capacity data indicates a small finite energy gap in zero field, $\mathrm{\Delta }\left(0\right)\sim 0.5\mathrm{K}$. Our findings were corroborated by muon spin relaxation ($\mu \mathrm{SR}$) measurements, which indicated the presence of a dynamic magnetic state along with an Orbach process involving crystal electric field (CEF) levels below 20 K. Additionally, field-dependent thermal conductivity measurements displayed anomalies that, together with the magnetization and heat capacity data, suggest the excitation of the spins to higher energy levels for magnetic fields $H\ge$ 2 T. Finally, a combination of density functional theory (DFT) and Monte Carlo simulations revealed that a combination of geometrical frustration and competing exchange interactions are responsible for the quasiparamagnetic ground state observed in this system.

The intertwining of competing degrees of freedom, anisotropy, and frustration induced strong quantum fluctuations offers an ideal ground for realizing exotic quantum phenomena in the rare-earth based kagome lattice. Herein, we report the synthesis, structure, thermodynamic, muon spin relaxation (μSR), nuclear magnetic resonance (NMR), and inelastic neutron scattering (INS) studies of a frustrated quantum magnet ${\mathrm{Nd}}_3{\mathrm{BWO}}_9$ (NBWO), wherein $\mathrm{N}{\mathrm{d}}^{3+}$ ions constitute a distorted kagome lattice. The INS experiments on NBWO allow us to establish a detailed crystal electric field (CEF) spectrum. The magnetic susceptibility reveals the presence of two energy scales in agreement with the INS results, wherein the higher-energy state is dominated by the thermal population of CEF excitations. The lowest Kramers ground-state doublet is well separated from the excited state, suggesting that the compound realizes a low-energy $J_{\mathrm{eff}}=\frac{1}{2}$ state at low temperatures. The low-energy state is witnessed via thermodynamic results that reveal an anomaly at 0.3 K typical of a phase transition, which is attributed to the presence of complex magnetic ordering phenomena. The broad maximum in the specific heat well above 0.3 K indicates the presence of short-range spin correlations. The isothermal magnetization reveals a field-induced $\frac{1}{3}$ magnetization plateau at low temperatures. μSR relaxation rate experiments, on the other hand, neither show the signature of a phase transition nor spin freezing down to 34 mK. The zero-field μSR relaxation rate is governed by an Orbach process and reveals the presence of fluctuating moments owing to the depopulation of crystal field levels, reflected as a constant value of the relaxation rate in the temperature range 0.04 ≤ T ≤ 10 K. NMR results indicate the presence of fluctuating $\mathrm{N}{\mathrm{d}}^{3+}$ moments down to 1.8 K, consistent with μSR experiments. Our comprehensive results reveal that a field-induced quantum phenomenon is at play, exemplifying the proximity effect of competing magnetic states and the coexistence of static and fluctuating moments along with short-range spin correlations in this frustrated kagome magnet. The broad rare-earth ${\mathrm{RE}}_3{\mathrm{BWO}}_9$ family of frustrated kagome magnets is a promising candidate for hosting exotic quantum states driven by spin-orbit coupling and frustration.

Complex spectra of dissipative quantum systems may exhibit degeneracies known as exceptional points (EPs). At these points the systems' dynamics may undergo drastic changes. Phenomena associated with EPs and their applications have been extensively studied in relation to various experimental platforms, including, i.e., the superconducting circuits. While most of the studies focus on EPs appearing due to the variation of the system's physical parameters, we focus on EPs emerging in the full counting statistics of the system. We consider a monitored three-level system and find multiple EPs in the Lindbladian eigenvalues considered as functions of a counting field. These “hidden” EPs are not accessible without the insertion of the counting field into the Linbladian, i.e., if only the density matrix of the system is studied. Nevertheless, we show that the hidden EPs are accessible experimentally. We demonstrate that these EPs signify transitions between different topological classes which are rigorously characterized in terms of the braid theory. Furthermore, we identify dynamical observables affected by these transitions and demonstrate how experimentally measured quantum jump distributions can be used to spot transitions between different topological regimes. Additionally, we establish a duality between the conventional Lindbladian EPs (zero counting field) and some of the “hidden” ones. Our findings allow for easier experimental observations of the EP transitions, normally concealed by the Lindbladian steady state, without applying postselection schemes. These results can be directly generalized to any monitored open system as long as it is described within the Lindbladian formalism.

We study the dynamics of out-of-time-ordered correlators (OTOCs) and the entanglement of entropy as quantitative measures of information propagation in disordered many-body systems exhibiting Floquet time-crystal (FTC) phases. We find that an OTOC spreads in the FTC with different characteristic timescales due to the existence of a preferred “quasiprotected” direction, denoted as the $\ell$-bit direction, along which the spins stabilize their period-doubling magnetization for exponentially long times. While orthogonal to this direction the OTOC thermalizes as an usual many-body localization time-independent system (at stroboscopic times), along the $\ell$-bit direction the system features a more complex structure. The scrambling appears as a combination of an initially frozen dynamics (while in the stable period-doubling magnetization time window) and a later logarithmic slow growth (over its decoherence regime) until full thermalization. Interestingly, in the late-time regime, since the wave front propagation of correlations has already settled through the whole chain, scrambling occurs at the same rate regardless of the distance between the spins, thus resulting in an overall envelopelike structure of all OTOCs, independent of their distance, merging into a single growth. In addition, the entanglement entropy shows a logarithmic growth over all time, reflecting the slow dynamics up to a thermal volume-law saturation.

The search for room-temperature ferromagnetic two-dimensional materials is crucial for the development of future functional spintronic devices. In this study, we identify ${\beta }^{\prime }$-FeB as a two-dimensional pentacoordinate boron structure that energetically outperforms previously reported FeB structures. First-principles calculations reveal that ${\beta }^{\prime }$-FeB is a ferromagnetic metal with a Curie temperature exceeding room temperature. Additionally, ${\beta }^{\prime }$-FeB exhibits substantial resistance to external strain, maintaining its ferromagnetic ground state across a biaxial strain range of –5% to 5%. Furthermore, it exhibits superior thermodynamic, dynamic, and mechanical stability. These findings suggest ${\beta }^{\prime }$-FeB holds promising application prospects in spintronic devices.

In this work, we carried out a theoretical analysis of the dispersion relation $\omega \left(k\right)$ for spin waves in a layered structure comprising two ferromagnetic layers and a nonmagnetic spacer sandwiched between them. The spin-wave configuration considered is that of the Damon-Eshbach magnetostatic surface spin wave, characterized by a static magnetic field applied in the structure plane and perpendicular to the direction of wave propagation. We assumed that, in addition to the conventional ferromagnetic interlayer Heisenberg exchange via the Ruderman-Kittel-Kasuya-Yosida interaction, the ferromagnetic layers are also coupled by the recently discovered interlayer Dzyaloshinskii-Moriya interaction (IL-DMI). We found that the presence of IL-DMI modifies the eigenfrequencies of both fundamental modes of the structure—acoustic and optical. The acoustic mode experiences a downward shift in frequency and a reduction in slope. Conversely, the optical mode is shifted upward and develops a significant positive slope, whereas it remains dispersionless across most of the wave-number range accessible with Brillouin light-scattering spectroscopy in the absence of IL-DMI. Numerical calculations of the dispersion relation demonstrate that it will be possible to experimentally identify the presence of IL-DMI in such layered structures using Brillouin light scattering. Additionally, the value of the Dzyaloshinskii constant for the ferromagnetic/nonmagnetic/ferromagnetic interface pair can be extracted from the measurement results.

Even though abundant research has focused on metamagnetic critical end points in itinerant magnets, this critical phenomenon for magnetic insulators is still to be explored extensively. ${\mathrm{DyVO}}_4$, a magnetic insulator exhibits a field induced first order metamagnetic transition (MMT), which has previously been investigated via thermodynamic measurements. In this work, we extend our investigations down to mK temperatures and probe the metamagnetic tricritical point (TCP) directly via the magnetocaloric effect (MCE). Heat capacity reveals an inimitable phase diagram where the second order antiferromagnetic phase transition terminates at the TCP, beyond which it continues as a line of first order MMT as the temperature is lowered towards 0 K. The sample temperature and magnetic Grüneisen parameter $\left({\mathrm{\Gamma }}_m\right)$ evaluated from MCE data show direct evidence of critical fluctuations and enhanced entropy in the vicinity of the TCP. These fluctuations are also supported by diverging susceptibility observed near the TCP. Critical analysis of magnetization suggests that the metamagnetic TCP in ${\mathrm{DyVO}}_4$ does not belong to any universal class. However, the extracted exponents agree with another insulating metamagnet, ${\mathrm{HoMnO}}_3$ [Phys. Rev. Lett. 110, 157202 (2013)], suggesting the presence of a unique universality class for metamagnetic critical systems.

The internal structure of doped holes in the Mott insulator may provide important insight into the physics of doped cuprates. Its observability via a single-particle probe by scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) is explored in this paper. Specifically, we study the single-particle spectral function based on a two-hole variational ground-state wave function [J.-Y. Zhao et al., Phys. Rev. X 12, 011062 (2022)] in the $t\text{−}J$ model. The latter as a strongly correlated state possesses a dichotomy of $d$-wave Cooper pairing and $s$-wave “twisted” hole pairing. This pairing structure will give rise to two branches of local spectral function at finite energies. The low-lying one corresponds to a nodal-like quasiparticle excitation and the higher branch is associated with the pair breaking of “twisted” quasiparticles, with the threshold energy resembling a pseudogap, which is consistent with the recent STS observation. It can be further extended into energy spectra in momentum space measurable by ARPES, where the low-energy dispersion is also shown to agree well with the quantum Monte Carlo numerical result for a single hole. It implies that the dominant pairing force arises from the twisted holes showing up in the high-energy branch. The effect of the next-nearest-neighbor hopping integral $t^{\prime }$ is also examined, which shows an interesting distinction between $t^{\prime }/t>0$ and $t^{\prime }/t\le 0$ with a dramatic shift of the low-lying excitation from the nodal region to the antinodal region, but with the high-energy branch remaining insensitive to $t^{\prime }$. Finally, a possible “orthogonality catastrophe” effect, namely, a “dark matter” component in the strongly correlated wave function that cannot be directly detected by the single-electron spectroscopy, is briefly discussed.

In this paper, we revisit the operator-state correspondence in the Majorana conformal field theory (CFT) with emphasis on its semion representation. Whereas the semion representation (or $Z_2$ extension of the chiral Ising CFT) gives a concise “Abelian” (or invertible) representation at the level of fusion rules and quantum states, there exists a subtlety when considering the chiral multipoint correlation function. In this sense, the operator-state correspondence in the semion sector of the fermionic theory inevitably contains a difficulty coming from its anomalous conformal dimension $1/16$ as a $Z_2$ symmetry operator. By analyzing the asymptotic behaviors of the existing correlation functions, we propose a nontrivial correspondence between the chiral conformal blocks and bulk correlation functions containing both order and disorder fields. One can generalize this understanding to $Z_N$ models or fractional supersymmetric models (for which there exist long-standing open problems). We expect that this may improve our understanding of the simple current extension of CFT, which commonly appears in the studies of topologically ordered systems.