Band engineering in magnetic kagome material has gained interest due to the emergence of novel quantum magnetism that arises from the interplay between topology and electron correlations. Here, we report a detailed study on the crystal structure, magnetization, and electronic structure of ${\mathrm{Fe}}_{1-y}{\mathrm{Co}}_y\mathrm{Sn}$ as a function of composition ($y$). The kagome lattice of ${\mathrm{Fe}}_{1-y}{\mathrm{Co}}_y\mathrm{Sn}$ is found to shrink with the increase in $y$. A decrease in the antiferromagnetic transition temperature ($T_N$) and changes in spin orientation is observed in the magnetization for $y>$ 0. Core-level analysis showed a systematic decrease in the local moment of Fe and an increase in the local moment of Co with increasing $y$. Valence-band broadening and shifting of the bands away from the Fermi level ($E_F$) in Co-doped compositions are associated with enhanced hybridization and the correlation effects that give rise to localized and quasilocalized states near $E_F$. The band structure of FeSn (001) and ${\mathrm{Fe}}_{0.95}{\mathrm{Co}}_{0.05}\mathrm{Sn}$ (001) surfaces along the $\overline{K}\text{−}\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ direction showed evidence of quasilocalized and localized flat bands. We find that there is an interplay between the Coulomb correlation and hybridization in Co-doped composition that gives rise to the broadening of the flat band and a shift of the Dirac point away from $E_F$ and in $k$-space. The decrease in $T_N$ with increasing $y$ is associated with the correlation effects, which broaden the flat band and shift the Dirac point away from $E_F$, while the shift of the Dirac point in $k$-space is a signature of change in the magnetocrystalline anisotropy due to spin orientation.

To clarify the microscopic origin of chirality-induced optical effect, we develop an analytical method that extracts the chiral part of the Hamiltonian of molecular electronic states. We demonstrate this method in a model chiral molecule consisting of two helically stacked $N$-sided regular polygons, and compare it with numerical calculation for chiral discrimination via high harmonic generation (HHG) of the same molecule. The discrimination signal here is the Kuhn $g$ factor, the difference between the harmonic intensity from the bicircular laser field and that from its reflected laser field normalized by their average. The $g$ factor is a pseudoscalar quantity that reflects the chirality of the molecule. As a result, we find that the $g$ factor becomes large over a wide range of harmonic orders making HHG suitable for chiral discrimination. We further find that to increase the difference of harmonic intensity from the above two fields, the unnormalized $g$ factor, the increase of capacity to generate the longitudinal dipole moment is more advantageous than maximizing the transverse-to-longitudinal conversion efficiency via the optimization of molecular chirality. We speculate this criteria may be extended to other optical and current-induced processes relevant to the chiral molecules and materials.

The two-dimensional triangular-lattice antiferromagnet (TLAF) is a textbook example of frustrated magnetic systems. Despite its simplicity, the TLAF model exhibits a highly rich and complex magnetic phase diagram, featuring numerous distinct ground states that can be stabilized through frustrated next-nearest-neighbor couplings or anisotropy. In this paper, we report low-temperature magnetic properties of the TLAF material ${\mathrm{CsCeSe}}_2$. The inelastic neutron scattering (INS) together with specific heat measurements and density functional theory calculations of crystalline electric field suggest that the ground state of Ce ions is a Kramers doublet with strong easy-plane anisotropy. Elastic neutron scattering measurements demonstrate the presence of stripe-$yz$ magnetic order that develops below $T_{\mathrm{N}}=0.35\mathrm{K}$, with the zero-field ordered moment of $m_{\mathrm{Ce}}\approx 0.65{\mu }_{\mathrm{B}}$. Application of magnetic field first increases the ordering temperature by about 20% at the intermediate field region and eventually suppresses the stripe order in favor of the field-polarized ferromagnetic state via a continuous quantum phase transition (QPT). The field-induced response demonstrates sizable anisotropy for different in-plane directions, $\mathbf{B}\parallel \mathbf{a}$ and $\mathbf{B}\perp \mathbf{a}$, which indicates the presence of bond-dependent coupling in the spin Hamiltonian. We further show theoretically that the presence of anisotropic bond-dependent interactions can change the universality class of QPT for $\mathbf{B}\parallel \mathbf{a}$ and $\mathbf{B}\perp \mathbf{a}$.

The presence of quantum noises inherent to real physical systems can strongly impact the physics in hybrid quantum circuits with local random unitaries and midcircuit measurements. The quantum noises with a size-independent occurring probability can lead to the disappearance of a measurement-induced entanglement phase transition and the emergence of a single area-law phase. In this work, we investigate the effects of quantum noises with size-dependent probabilities $q=p/L^{\alpha }$, where $\alpha$ represents the scaling exponent. We have identified a noise-induced entanglement phase transition from a volume law to a power (area) law in the presence (absence) of measurements as $p$ increases when $\alpha =1$. With the help of an effective statistical model, we reveal that the phase transition is of first order arising from the competition between two types of spin configurations and shares the same analytical understanding as the noise-induced coding transition. This unified picture further deepens the understanding of the connection between entanglement behavior and the capacity of information protection. When $\alpha \ne 1$, one spin configuration always dominates regardless of $p$ and thus the phase transition disappears. Moreover, we highlight the difference between the effects of size-dependent bulk noise and boundary noises. We validate our analytical predictions with extensive numerical results from stabilizer circuit simulations.

The higher dimensionality in the crystal fields of the ${\mathrm{Ce}}_{2}M{\mathrm{In}}_8$ ($M=\mathrm{Rh},\mathrm{Ir}$) compounds and its interplay with hybridization and disorder are key ingredients to understand the complex phase diagrams by this family, which have been explored extensively by macroscopic techniques. Here, we present an investigation of the crystal-electric field schemes of ${\mathrm{Ce}}_2{\mathrm{Rh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_8$ using x-ray absorption spectroscopy. Our full multiplet calculations for the $4f^1$ configuration of ${\mathrm{Ce}}^{3+}$ to describe the temperature-dependent linear dichroism in ${\mathrm{Ce}}_{2}M{\mathrm{In}}_8$ are consistent with a ${\mathrm{\Gamma }}_7^1=\sqrt{1-{\alpha }^2}·|\mp \frac{3}{2}\rangle -\left|\alpha \right|·|\pm \frac{5}{2}\rangle$ ground state containing a predominant $|\pm 3/2\rangle$ contribution that increases further with $x$. This enhancement is believed to favor superconductivity in Ce-based heavy fermion materials, observed in previous results in the $\mathrm{Ce}M{\mathrm{In}}_5$ family. Our recent observations shed light on the unexpected emergence of the ambient-pressure superconducting dome in the center of the composition phase diagram and its subsequent suppression on the Ir-rich side due to the early onset of fluctuations associated with the structurally more disordered state, inferred from previous neutron magnetic diffraction experiments.

We theoretically investigate the entanglement between two well-separated quantum emitters (QEs) mediated by dark-gap-plasmon waveguides. Both the plasmon-QE coupling strength and the energy transfer efficiency between QEs are crucial for entanglement based on analytical results. To overcome the low energy transfer efficiency brought by common plasmonic waveguides, we construct a simple dark-gap-plasmon waveguide. Such a structure consists of two parallel silver nanowires, where dark plasmon modes are formed in the gap between the two nanowires. The leaky (or radiative) feature of propagating plasmons is highly suppressed, which results in the quality factors of the dark plasmon modes reaching the material limit. Thus, the energy transfer efficiency can reach more than 94% with a distance even over ten working wavelengths ($>10\mu \mathrm{m}$). Meanwhile, by introducing a tip dimer structure in the nanowire gap, the plasmon-QE coupling can reach a strong coupling regime with realistic parameters. The influences from the geometric parameters of the system are also considered. The energy transfer efficiency is robust to the geometric variations due to the dark feature of the plasmon responses.

Topological insulator/superconductor two-dimensional heterostructures are promising candidates for realizing topological superconductivity and Majorana modes. In these systems, a vortex pinned by a prefabricated antidot in the superconductor can host Majorana zero-energy modes (MZMs), which are exotic quasiparticles that may enable quantum information processing. However, a major challenge is to design devices that can manipulate the information encoded in these MZMs. One of the key factors is to create small and clean antidots, so the MZMs, localized in the vortex core, have a large gap to other excitations. If the antidot is too large or too disordered, the level spacing for the subgap vortex states may become smaller than temperature. In this paper, we numerically investigate the effects of disorder, chemical potential, and antidot size on the subgap vortex spectrum, using a two-dimensional effective model of the topological insulator surface. Our model allows us to simulate large system sizes with vortices up to $1.8\mu \mathrm{m}$ in diameter (with a $6\mathrm{nm}$ lattice constant). We also compare our disorder model with the transport data from existing experiments. We find that the spectral gap can exhibit a nonmonotonic behavior as a function of disorder strength, and that it can be tuned by applying a gate voltage.

In this work, we have studied the phonon properties of multilayered graphene with the use of molecular dynamics simulations and the $k$-space velocity autocorrelation sequence method, for which we provide a theoretical proof. We calculate the phonon dispersion curves, densities of states, and lifetimes $\tau$ of few-layered graphene consisting of 1–5 layers and graphite. $\mathrm{\Gamma }$-point phonon energies and lifetimes are investigated for different temperatures ranging from 80 to 1000 K. The study focuses on the impact of the interlayer interaction and temperature on the energies and lifetimes of the $\mathrm{\Gamma }$-point phonons, as well as the type of interlayer potential used. For the latter we used the Kolmogorov-Crespi and the Lennard-Jones potentials. We have found that the number of layers $N$ has little effect on the intralayer ZO and $G$ mode energies and greater effect on the interlayer shear and breathing modes, while $\tau$ is generally affected by $N$ for all modes, except for the layer shear mode. The influence of $N$ on the lifetimes was also found to be independent of the type of potential used. For the Raman-active $G$ phonon, our calculations show that the lifetime increases with $N$ and that this increase is directly connected to the strength of the interlayer coupling and how this is modeled.

Transition metal dihalides (${MX}_2, M$= transition metal, $X$= halide) have attracted much attention recently due to their intriguing low-dimensional magnetic properties. Particular focus has been placed in this family in the context of multiferroicity—a common occurrence in ${MX}_2$ compounds that adopt noncollinear magnetic structures. One example of helimagnetic multiferroic material in the dihalide family is represented by ${\mathrm{NiBr}}_2$. Here, we study the evolution of the electronic structure and magnetic properties of this material under pressure using first-principles calculations combined with Monte Carlo simulations. Our results indicate there is significant magnetic frustration in ${\mathrm{NiBr}}_2$ due to the competing interactions arising from its underlying triangular lattice. This magnetic frustration increases with pressure and is at the origin of the helimagnetic order. Further, pressure causes a sizable increase in the interlayer interactions. Our Monte Carlo simulations show that a large (threefold) increase in the helimagnetic transition temperature can be achieved at pressures of around 15 GPa. This indicates that hydrostatic pressure can indeed be used as a tuning knob to increase the magnetic transition temperature of ${\mathrm{NiBr}}_2$.

We present the optical activity induced by the orbital magnetic moment in metals and superconductors using Green's function formalization. Usually, an apparent singularity of the optical activity vanishes in the normal state; however, we show that it remains finite in the superconducting state and is related to the superconducting Edelstein effect, ensuring the missing area measurement. Finally, we calculate the optical activity in a model Hamiltonian mimicking doped transition metal dichalcogenides to investigate its characteristic spectrum, which reveals a peak shift caused by the interband Fermi surface, its relation with the orbital magnetic moment, and the change of the spectrum in a superconductor at low frequency. We also analyze the optical rotation to discuss the possibility of observing the optical activity in experiments.

Biased bilayer graphene (BBG) is an important system for studies of excitonic effects in graphene-based systems, with its easily tunable band gap. This band gap is governed by an external gate voltage, allowing one to tune the optical response of the system. In this paper, we study the excitonic linear and nonlinear optical responses of Bernal-stacked BBG as a function of the gate voltage, both for in-plane (IP) and out-of-plane (OOP) directions. Based on a semianalytical model of the electronic structure of BBG describing the influence of gate voltage on excitonic binding energies, we focus our discussion on both the IP and OOP excitonic responses. Both linear and second harmonic generation nonlinear responses are shown to be very sensitive to the gate voltage, as both the interband momentum matrix elements and the band gap of the system will vary greatly with bias potential.

In situ high-pressure ultrasonic velocity measurements of $\mathrm{CaA}{\mathrm{l}}_2{\mathrm{O}}_4$ glass reveal abrupt irreversible discontinuities in the elastic wave velocities at ∼8–10 GPa. Total structure factor and pair distribution functions measured by synchrotron x-ray diffraction show a rapid change in the intermediate range structure attributed to a rearrangement of calcium ions over this narrow pressure region. Atomistic models obtained from molecular dynamics simulations reveal that this intermediate range structure is explained by a transition of Ca–O void radius distribution from a bimodal distribution with peaks at ∼2.1 and ∼2.4 Å to a single distribution centered at ∼2.1 Å. These abrupt structural changes involving the rapid increase in elastic wave velocity are markedly different to the continuous transformations observed in conventional network-forming glasses, such as $\mathrm{Si}{\mathrm{O}}_2$.

Disorder free many-body localization (MBL) can occur in interacting systems that can dynamically generate their own disorder. We address the thermal-MBL phase transition of two isotropic Heisenberg spin chains that are quasiperiodically coupled to each other. The spin chains are incommensurate and are coupled through a short-range exchange interaction of the $XXZ$ type that decays exponentially with the distance. Using exact diagonalization, matrix product states, and a density matrix renormalization group, we calculate the time evolution of the entanglement entropy at long times and extract the inverse participation ratio in the thermodynamic limit. We show that this system has a robust MBL phase. We establish the phase diagram with the onset of MBL as a function of the interchain exchange coupling and of the incommensuration between the spin chains. The Ising limit of the interchain interaction optimizes the stability of the MBL phase over a broad range of incommensurations above a given critical exchange coupling. Incorporation of interchain spin flips significantly enhances entanglement between the spin chains and produces delocalization, favoring a prethermal phase whose entanglement entropy grows logarithmically with time.

The complex interplay of spin frustration and quantum fluctuations in low-dimensional quantum materials leads to a variety of intriguing phenomena. This research focuses on a detailed analysis of the magnetic behavior exhibited by NdZnPO, a bilayer spin-1/2 triangular lattice antiferromagnet. The investigation employs magnetization, specific heat, and powder neutron scattering measurements. At zero field, a long-range magnetic order is observed at $T_{\mathrm{N}}=1.64\mathrm{K}$. Powder neutron diffraction experiments show the Ising-like magnetic moments along the $c$ axis, revealing a stripelike magnetic structure with magnetic propagation vector $(1/2,0,1/2)$. Application of a magnetic field along the $c$ axis suppresses the antiferromagnetic order, leading to a fully polarized ferromagnetic state above $B_{\mathrm{c}}=4.5\mathrm{T}$. This transition is accompanied by notable enhancements in the nuclear Schottky contribution. Moreover, the absence of spin frustration and expected field-induced plateaulike phases are remarkable observations. Detailed calculations of magnetic dipolar interactions revealed complex couplings reminiscent of a honeycomb lattice, suggesting the potential emergence of Kitaev-like physics within this system. This comprehensive study of the magnetic properties of NdZnPO highlights unresolved intricacies, underscoring the imperative for further exploration to unveil the underlying governing mechanisms.

Transition metal (TM) oxides with high oxidation state TM ions exhibit a variety of unconventional electronic and magnetic states owing to electron correlations effects combined with highly covalent TM–O bonding. Here, we have studied the pressure dependence of electronic state and magnetism of the ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4$-type iron(IV) oxide $\mathrm{S}{\mathrm{r}}_2\mathrm{Fe}{\mathrm{O}}_4$ up to 89 GPa by temperature and magnetic field dependent energy-domain synchrotron Mössbauer spectroscopy and derived a ($P,T$) magnetic phase diagram. Considering also previous resistance studies [Rozenberg et al., Phys. Rev. B 58, 10283 (1998)] several magnetic and electronic regimes with increasing pressure can be identified. Near 7 GPa, the insulating cycloidal antiferromagnetic low-$P$ state is transformed into a semiconducting ferromagnetic state and the magnetic ordering temperature $T_{\mathrm{m}}$ increases from 55 K at ambient pressure to about 100 K at 13 GPa. Between 18 and about 50 GPa the system is ferromagnetic and metallic (FMM) with a strong rise of $T_{\mathrm{m}}$ to above room temperature (RT). Contrary to a recent theoretical study [Kazemi-Moridani et al., Phys. Rev. B 109, 165146 (2024)], the FMM state is attributed to a high-spin $t_{2g}^3{e}_g^1$ electronic state with itinerant $e_{\mathrm{g}}$ coupled to more localized $t_{2\mathrm{g}}$ electrons. Between 50 and 89 GPa a doublet with large quadrupole splitting in the RT Mössbauer spectra indicates a partial high-spin to low-spin ($t_{2g}^4$) transition leading to a decrease in $T_{\mathrm{m}}$ again. The general features of the ($P,T$) phase diagram of $\mathrm{S}{\mathrm{r}}_2\mathrm{Fe}{\mathrm{O}}_4$ are comparable to those of other simple and $A$-site ordered iron(IV) perovskite-related oxides with the peculiarity that $\mathrm{S}{\mathrm{r}}_2\mathrm{Fe}{\mathrm{O}}_4$ adopts an insulating state without charge disproportionation of $\mathrm{F}{\mathrm{e}}^{4+}$ in the low-$P$ region. The high-pressure behavior of $\mathrm{S}{\mathrm{r}}_2\mathrm{Fe}{\mathrm{O}}_4$ and other iron(IV) oxides may be relevant for exploring the role of Hund's physics in multiorbital systems and contributing to the understanding of the electronic situation in unconventional superconductors such as $\mathrm{L}{\mathrm{a}}_3\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ and $\mathrm{S}{\mathrm{r}}_2\mathrm{Ru}{\mathrm{O}}_4$.

Anisotropy in strongly correlated materials is a central parameter in determining the electronic ground state and is tuned through the local crystalline electric field. This is notably the case in the ${\mathrm{CeCo}}_x{\mathrm{Rh}}_{1-x}{\mathrm{In}}_5$ system where the ground-state wave function can provide the basis for antiferromagnetism and/or unconventional superconductivity. We develop a methodology to understand the local magnetic anisotropy and experimentally investigate with neutron spectroscopy applied to antiferromagnetic ($T_N=3.8\mathrm{K}$) ${\mathrm{CeRhIn}}_5$, which is isostructural to $d$-wave superconducting ($T_c=2.3\mathrm{K}$) ${\mathrm{CeCoIn}}_5$. Through diagonalizing the local crystal field Hamiltonian with discrete tetragonal $C_4$ point group symmetry and coupling these states with the random phase approximation, we find two distinct modes polarized along the crystallographic $c$ and $a-b$ planes, agreeing with experiment. The anisotropy and bandwidth, underlying the energy scale of these modes, are tuneable with a magnetic field which we use experimentally to separate in energy single and multiparticle excitations thereby demonstrating the instability of excitations polarized within the crystallographic $a-b$ plane in ${\mathrm{CeRhIn}}_5$. We compare this approach to a $S_{\mathrm{eff}}=\frac{1}{2}$ parametrizations and argue for the need to extend conventional SU(2) theories of magnetic excitations to utilize the multilevel nature of the underlying crystal-field basis states constrained by the local point-group $C_4$ symmetry.

We present a computational study of the parametric excitation of spin waves in thin-film disks with a mode-resolved approach. The method involves projecting out the time-dependent magnetization, computed using micromagnetics simulations, onto the spatial profile of the eigenmodes that are obtained from the linearization of the equations of motion. Unlike spectral analysis in the frequency domain, the projection allows for the analysis of transient mode dynamics under parametric excitation. We apply this method to parallel pumping of quantized spin wave modes in in-plane magnetized thin-film disks, where phenomena such as frequency pulling, mutual phase locking, and higher-order magnon scattering processes are identified.

We investigate the topological phases that appear in an orbital version of the Benalcazar-Bernevig-Hughes (BBH) model in the presence of conventional spin-singlet $s$-wave superconductivity and with the possibility of tuning an in-plane magnetic field. We chart out the phase diagram by considering different boundary conditions, with the topology of the individual phases further examined by considering both the Wannier and entanglement spectra, as well as the Majorana polarization. For weak to moderate values of magnetic field and superconducting pairing amplitude, we find a second-order topological superconductor phase with eight zero-energy corner modes. Further increasing field or pairing, half of the corner states can be turned into zero-energy edge-localized modes, thus forming what we name hybrid-order phase. Then, we find two different putative first-order topological phases, a nodal and a nodeless phase, both with zero-energy flat bands localized along mirror-symmetric open edges. For the nodal phase, the flat bands are, as expected, localized between the nodes in reciprocal space, while in the nodeless phase, the zero-energy boundary flat band instead spans the whole Brillouin zone and appears disjoint from the fully gapped bulk spectrum. As a consequence, this model present several unexpected phases with unusual surface states that can be tuned via an external magnetic field.