The structure symmetry in artificial spin ice systems plays important roles in the magnetization reversal process when subjected to an external magnetic field. We investigated the magnetization reversal processes with different field orientations in the deformed kagome spin ices with threefold symmetry broken using high-resolution Kerr microscopy. Distinct magnetization reversal processes were observed in the deformed kagome system when compared to those found in standard kagome structures. In addition to the previously documented Dirac-string-like and system-size avalanche-type reversals, we identified two unusual reversal behaviors: the armchairlike avalanche and the discrete magnetization reversal within a single bar. These behaviors can be elucidated through the modified energy hierarchy present in the deformed kagome spin ices. Furthermore, we observed that the magnetization reversal behaviors in deformed kagome ice can be manipulated by selecting different initial spin states. These findings contribute to a deeper comprehension of nonequilibrium critical point phenomena in artificial spin ice systems.

Magnetic phase transitions and structures of the spin-chain compound ${\mathrm{BaErFeO}}_4$ were investigated by measurements of magnetic properties (specific heat, magnetic susceptibility) and neutron diffraction. The lattice geometry of the orthorhombic crystal structure (space group Pnma) of the $\mathrm{Ba}R\mathrm{Fe}{\mathrm{O}}_4$ compounds ($R=\mathrm{Dy}–\mathrm{Yb}$, Y) supports frustrations which lead to multiple magnetic phase transitions with complex magnetic structures. ${\mathrm{BaErFeO}}_4$ undergoes three successive magnetic phase transitions at $T_{\mathrm{N}1}=49\mathrm{K}, T_{\mathrm{N}2}=33.4\mathrm{K}$, and $T_{\mathrm{N}3}=3.4\mathrm{K}$. In contrast with the previously investigated $\mathrm{Ba}R\mathrm{Fe}{\mathrm{O}}_4$ ($R=\mathrm{Y}$, Tm, Yb) compounds, all with incommensurate magnetic propagation vectors ${\mathbf{k}}_1=\left(0,0,k_z\right), {\mathrm{BaErFeO}}_4$ is the first member in this series that shows a phase transition from an incommensurate (${\mathbf{k}}_1$ below $T_{\mathrm{N}1}$) to a commensurate magnetic structure with ${\mathbf{k}}_2=(\frac{1}{2},0,\frac{1}{2})$ below $T_{\mathrm{N}2}$. In the crystal structure, all magnetic ions (Fe1, Fe2, Er1, and Er2) are part of chains propagating along the $b$ axis. Below $T_{\mathrm{N}1}$, strong antiferromagnetic (AFM) Fe-Fe spin-exchange coupling between square pyramidal (Fe1) and octahedral (Fe2) centers generates a collinear AFM structure with a constant size of the ordered Fe moments and a constant magnetic phase inside each chain of ${\mathrm{Fe}}^{3+}$ cations. Exchange coupling between the Fe chains is much weaker. At $T_{\mathrm{N}2}, 3d\text{−}4f$ exchange interactions induce an ordered moment at the ${\mathrm{Er}}^{3+}$ ions, which results in a change of the direction of the ordered Fe moments from the $b$ direction (above $T_{\mathrm{N}2}$) to inside the $ac$ plane (below $T_{\mathrm{N}2}$) and a change from an incommensurate (${\mathbf{k}}_1$ above $T_{\mathrm{N}2}$) to a commensurate (${\mathbf{k}}_2$ below $T_{\mathrm{N}2}$) AFM structure. Toward lower temperature, $4f\text{−}4f$ exchange interactions become stronger and create at $T_{\mathrm{N}3}$ a constant magnetic phase inside each chain of ${\mathrm{Er}}^{3+}$ cations. At $T_{\mathrm{N}2}$ and $T_{\mathrm{N}3}$, the magnetic susceptibility shows sharp decreases that coincide with large increases of the correlation length of the magnetic structure. The unique magnetic structures of ${\mathrm{BaErFeO}}_4$ are compared with those of other $\mathrm{Ba}R\mathrm{Fe}{\mathrm{O}}_4$ compounds by considering experimental and theoretical aspects.

Flat electronic bands at the Fermi energy $\left(E_{\mathrm{F}}\right)$ can induce interaction-driven instabilities and further result in the emergence of a plethora of new quantum phases such as Mott insulators, ferromagnetism, fractional quantum Hall states even at high temperatures, and superconductivity. Except for flat bands in $f$-electron systems and special geometric lattices, however, the materials with quantum criticality induced by flat bands just at $E_{\mathrm{F}}$ remain elusive. Here, by using angle-resolved photoemission spectroscopy and band structure calculations, we present a comprehensive study of the low-energy electronic structure of a nonsuperconducting iron pnictide, $\mathrm{Ba}{\left({\mathrm{Fe}}_{1/3}{\mathrm{Co}}_{1/3}{\mathrm{Ni}}_{1/3}\right)}_2{\mathrm{As}}_2$, which is a quantum-critical $3d$ transition metal alloy. We demonstrate the existence of a dispersionless flat band of $t_{2g}$ orbitals just at $E_{\mathrm{F}}$ which is responsible for quantum critical behaviors. Our findings will promote studies of emergent physics induced by flat bands, such as non-Fermi-liquid behaviors and quantum critical phenomena in $3d$ transition metals.

Charge-transfer excitons feature a permanent dipole moment introduced by the spatial charge separation of electron and hole wave functions. This directly influences and qualitatively modifies the coherent nonlinear optical response monitored in high-quality (Ga, In)As/Ga(As, Sb) type-II heterostructures through an optical-pump optical-probe experiment. A microscopic analysis based on the semiconductor Bloch equations reveals that the spatial inhomogeneity native to such type-II heterostructures introduces already on the Hartree-Fock a finite coupling between excitons of opposite spins which is pivotal for optical Stark effect experiments. This result in a blueshift of the charge-transfer exciton when pumping below the resonance for both, co-circular and counter circular polarization configurations, contrary to well-known and observed shifts of spatially direct exciton resonances.

We report on the observation of Fermi-edge singularity in the optical absorption and emission spectra of high-quality degenerate Ga-doped ZnO (GZO) films from cryogenic to room temperature, as well as on the related stimulated emission under femtosecond laser pulse excitation. When the photogenerated electron-hole pair density $n_{\mathrm{p}}$ reaches a threshold, which is as low as $n_{\mathrm{p}}\approx {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$ at room temperature, the emission from the Fermi edge shows a transition from a spontaneous to a stimulated regime, accompanied by a blueshift in the emission maximum. Further increase in the excitation fluence not only leads to a substantial increase in emission intensity, but it also results in broad and redshifted spectra, accompanied by the shortening of decay times. The resulting stimulated emission can be interpreted in terms of the emission of a population-inverted electron-hole plasma (EHP) in the weakly excited degenerated GZO. The observed low-threshold EHP stimulated emission is consistent with the calculated gain spectra based on a quantum many-body theory.

We study the impact of spin-orbit coupling on the topological band properties of copper-substituted lead phosphate apatite using a combination of group-theoretical analysis and full-relativistic density-functional theory calculations. We characterize Weyl points at time-reversal invariant momenta and find that a band inversion due to spin-orbit coupling leads to additional Weyl points close to the Fermi edge at general momenta. To determine the position of the altogether 66 Weyl points in the Brillouin zone, we develop an algorithm that follows a Berry-curvature-derived vector field to its monopole: the Weyl point. The emerging surface Fermi arcs and their spin polarization reveal avoided crossings and a Fermi loop detached from the Weyl points.

Bulk-boundary correspondence establishes the connection between the topological properties of the bulk and the edge states when a boundary is present. However, the boundary can be tailored to tune the appearance of the edge states. Using a higher-order topological Hamiltonian, we engineered the appearance of a one-corner state that can be translated between corners with a periodic parameter. As a result, we established that this is a pumping mechanism. The one-corner state is characterized using the inverse participation ratio, and we establish that the state has a topological invariant associated. Consequently, the state is topologically protected. Finally, we explore the robustness to random disorder.

In this paper we perform real-time simulations for probing the non-Abelian fusion of a pair of Majorana zero modes (MZMs). The nontrivial fusion outcomes can be either a vacuum, or an unpaired fermion, which reflect the underlying non-Abelian statistics. The two possible outcomes can cause different charge variations in the nearby probing quantum dot (QD), while the charge occupation in the dot is detected by a quantum point contact. In particular, we find that gradual fusion and gradual coupling of the MZMs to the QD (in nearly adiabatic switching-on limit) provide a simpler detection scheme than sudden coupling after fusion to infer the coexistence of two fusion outcomes, by measuring the occupation probability of the QD. For the scheme of sudden coupling (after fusion), we propose and analyze continuous weak measurement for the quantum oscillations of the QD occupancy. From the power spectrum of the measurement currents, one can identify the characteristic frequencies and infer thus the coexistence of the fusion outcomes.

Here, we investigate and analyze the electronic properties of ladder- and zigzag-patterned antiphase boundaries (APBs) in II-VI and III-V semiconductors based on first-principles calculations performed on ZnS and InP. From the band-structure analysis on these configurations, we evidence a direct correlation between Fermi levels positioning and the bond length in ladder-patterned APBs. The changes on the APB bond lengths and electronic properties from III-V to II-VI ladder APBs are discussed based on the charges and atoms electronegativity. We then show how the specific atomic configuration of the zigzag-patterned and ladder-patterned APBs differ from the point of view of force accommodation. As a result, ladder-patterned APBs are found to be much more sensitive to any change of stress or the chemical environment. We finally demonstrate that a small change in the APB bond length deeply modifies the band structure and optoelectronic properties of the systems (for both III-V and II-VI semiconductors), with possible $n$- and $p$-doping type inversion, thus opening the way towards APB-engineered photoelectric devices.

Rare-earth nitrides, such as gadolinium nitride (GdN), have great potential for spintronic devices due to their unique magnetic and electronic properties. GdN has a large magnetic moment, low coercitivity, and strong spin polarization suitable for spin transistors, magnetic memories, and spin-based quantum computing devices. Its large spin splitting of the optical band-gap functions as a spin filter that offers the means for spin-polarized current injection into metals, superconductors, topological insulators, two-dimensional layers, and other novel materials. As spintronics devices require thin films, a successful implementation of GdN demands a detailed investigation of the optical and magnetic properties in very thin films. With this objective, we investigate the dependence of the direct and indirect optical band gaps ($E_g$) of half-metallic GdN, using the trilayer structure AlN (10 nm)/GdN ($t$)/AlN (10 nm) for GdN film thickness $t$ ranging from 6 to 350 nm, in both paramagnetic (PM) and ferromagnetic (FM) phases. Our results show a band gap of 1.6 eV in the PM state, while in the FM state the band gap splits for the majority (0.8 eV) and minority (1.2 eV) spin states. As the GdN film becomes thinner, the spin-split magnitude increases by 60%, going from 0.290 to 0.460 eV. Our results point to methods for engineering GdN films for spintronic devices.

In Weyl semimetals, Weyl points act as monopoles and antimonopoles of the Berry curvature, with a monopole-antimonopole pair producing a net-zero Berry flux. When inversion symmetry is preserved, the two-dimensional (2D) planes that separate a monopole-antimonopole pair of Weyl points carry quantized Berry flux. In this work, we introduce a class of symmetry-protected Weyl semimetals which host monopole-antimonopole pairs of Weyl points that generate a dipolar Berry flux. Thus, both monopolar and dipolar Berry fluxes coexist in the Brillouin zone, which results in two distinct types of topologically nontrivial planes separating the Weyl points, carrying either a quantized monopolar or a quantized dipolar flux. We construct a topological invariant—the staggered Chern number—to measure the latter, and employ it to topologically distinguish between various Weyl points. Finally, through a minimal two-band model, we investigate physical signatures of bulk topology, including surface Fermi arcs, zero-energy hinge states, and response to insertion of a $\pi$-flux vortex.

Multiferroics are rare materials that exhibit an interaction of ferroelectricity and magnetism. One such multiferroic material is the $\mathrm{Mn}$-based mullite ${\mathrm{YMn}}_2{\mathrm{O}}_5$. ${\mathrm{YMn}}_2{\mathrm{O}}_5$ undergoes several low-temperature phases, and the origin of ferroelectricity in the commensurate phase remains open. Changes in the Mn spin configuration are believed to be the main driving force, which can be induced by magnetostriction caused by symmetric exchange, the antisymmetric inverse Dzyaloshinskii-Moriya interaction, or a combination of both. These mechanisms are accompanied by specific displacements of ions in the structure. The space group $Pbam\left(55\right)$ of the paraelectric phase does not allow polar displacements. Moreover, conventional structure analysis has been unsuccessful in refining the charge structure in a lower symmetric phase due to its limited sensitivity in resolving the expected positional displacements. To shed light on this controversial discussion, our goal was to resolve potential ionic displacements within a polar space group by employing the new resonantly suppressed diffraction method, which is highly sensitive to minuscule structural changes in the (sub)picometer range. In this paper, we present the first refined structure model of the commensurate phase in ${\mathrm{YMn}}_2{\mathrm{O}}_5$ using the lower symmetric space group $Pb{2}_{1}m$, allowing polarization in the $b$ direction. We observed a significant displacement of the Mn ions and the partial structure of oxygen, resulting in a calculated spontaneous polarization $P_S=(1.3\pm 0.4)\mathrm{mC}{\mathrm{m}}^{-2}$, which is in good agreement with our measured value $P_S=(0.88\pm 0.06)\mathrm{mC}{\mathrm{m}}^{-2}$. Importantly, we confirm that $P_S$ predominantly arises from an ionic contribution induced by magnetostriction. These results hold great interest not only for all multiferroic Mn-based mullites, but also for other multiferroic materials where ferroelectricity arises from their magnetic order. Furthermore, a precise understanding of the ionic movement induced by magnetism will aid in the material tuning process to enhance or create multiferroic properties.

We successfully synthesized a verdazyl-based complex, ${\left(p\text{−}\mathrm{Py}\text{−}\mathrm{V}\text{−}p\text{−}\mathrm{CN}\right)}_2\left[\mathrm{Cu}{\left(\mathrm{hfac}\right)}_2\right]$. Molecular orbital calculations reveal that three types of antiferromagnetic (AF) interactions are involved in the formation of a spin-$\frac{1}{2}$ distorted octagonal lattice composed of the verdazyl radical and Cu spins. The magnetic properties of the compound exhibited contributions from AF correlations and a phase transition to an AF ordered state at approximately $T_{\mathrm{N}}=2.5$ K. Below $T_{\mathrm{N}}$, we observed a $T^2$ dependence of the specific heat, indicating dominant two-dimensional AF correlations within the octagonal lattice. The magnetization curve in the low-temperature region exhibited a low-field linear increase, subsequent bending at $\frac{1}{3}$ magnetization, and high-field nonlinear increase. We reproduced the observed unique magnetic behavior through the numerical analysis based on the octagonal lattice. These results demonstrate that the present compound exhibits magnetic properties characteristic of octagonal lattice topology.

CeNiSn is a Kondo semimetal where a gap opens at low temperatures due to hybridization between $4f$ and conduction electrons, but a full insulating state fails to develop. Upon the insertion of hydrogen, long-range magnetic order is induced. Here we report zero-field muon spin relaxation and inelastic neutron scattering measurements of polycrystalline samples of the deuterides ${\mathrm{CeNiSnD}}_x$ ($x=1.0,1.8$). The muon spin relaxation results confirm magnetic ordering in the whole sample of CeNiSnD below around 4.7 K, while inelastic neutron scattering reveals two well-defined crystalline-electric field (CEF) excitations at around 13 and 34 meV in CeNiSnD, and 5 and 27 meV for ${\mathrm{CeNiSnD}}_{1.8}$. These results suggest that hydrogenation leads to the localization of the Ce-$4f$ electrons, giving rise to long-range magnetic order. We propose CEF level schemes for both systems, which predict a ground-state moment of $0.96{\mu }_{\mathrm{B}}$/Ce within the $ab$ plane for ${\mathrm{CeNiSnD}}_{1.8}$ and a saturated moment of $1.26{\mu }_{\mathrm{B}}$/Ce along the easy $c$ axis for CeNiSnD, that account for the observed magnetic properties.

A quantum phase transition from the miscible to the immiscible phase of a quasi-one-dimensional binary Bose-Einstein condensate is driven by ramping down the coupling amplitude of its two hyperfine states. It results in a random pattern of spatial domains where the symmetry is broken separated by defects. In distinction to previous studies [J. Sabbatini et al., Phys. Rev. Lett. 107, 230402 (2011), New J. Phys. 14, 095030 (2012)], we include nonzero detuning between the light field and the energy difference of the states, which provides a bias towards one of the states. Using the truncated Wigner method, we test the biased version of the quantum Kibble-Zurek mechanism [M. Rams et al., Phys. Rev. Lett. 123, 130603 (2019)] and observe a crossover to the adiabatic regime when the quench is sufficiently fast to dominate the effect of the bias. We verify a universal power law for the population imbalance in the nonadiabatic regime both at the critical point and by the end of the ramp. Shrinking and annihilation of domains of the unfavorable phase after the ramp, that is, already in the broken symmetry phase, enlarges the defect-free sections by the end of the ramp. The consequences of this phase-ordering effect can be captured by a phenomenological power law.

Minimal Kitaev chains composed of two semiconducting quantum dots coupled via a superconductor have emerged as a promising platform to realize and study Majorana bound states (MBSs), which appear for fine-tuned configurations. We propose a hybrid qubit based on a Josephson junction between two such double quantum dots (DQDs) embedded in a superconducting qubit geometry. The qubit makes use of the $4\pi$-Josephson effect in the Kitaev junction to create a subspace based on the even/odd fermionic parities of the two DQD arrays hosting MBSs. Deep in the transmon regime, we demonstrate that by performing circuit QED spectroscopy on such a hybrid Kitaev-transmon “Kitmon” qubit, one could observe distinct MBS features in perfect agreement with precise analytical predictions in terms of DQD parameters only. This agreement allows us to extract the Majorana polarization in the junction from the microwave response.

Fascinating new phases of matter can emerge from strong electron interactions in solids. In recent years, a new exotic class of many-body phases, described by generalized electromagnetism of symmetric rank-2 electric and magnetic fields and immobile charge excitations dubbed fractons, has attracted wide attention. Besides having interesting properties in their own right, the models with generalized electromagnetism are also closely related to gapped fracton quantum orders, new phases of dipole-covering systems, as well as quantum information and quantum gravity. However, experimental realization of the rank-2 U(1) gauge theory is still absent and even known practical experimental routes are scarce. In this work, we propose a scheme of coupled optical phonons and nematic degrees of freedom, as well as several concrete experimental platforms for their realizations. We show that these systems can realize the electrostatics sector of the rank-2 U(1) gauge theory. A great advantage of the proposed scheme is that it requires only the basic ingredients of phonon and nematic physics, and hence may be applicable to a wide range of experimental realizations from liquid crystals to electron orbitals.

We develop a hydrodynamic description of electron magnetotransport in conductors without Galilean invariance in the presence of a weak long-range disorder potential. We show that magnetoresistance becomes strong (of order 100%) at relatively small fields, at which the inverse square of the magnetic length becomes comparable to disorder-induced variations of the electron density. The mechanism responsible for this anomalously strong magnetoresistance can be traced to the appearance of magnetic friction force in liquids with nonvanishing intrinsic conductivity. We derive general results for the galvanomagnetic and thermomagnetic kinetic coefficients, and obtain their dependence on the intrinsic dissipative properties of the electron liquid and the correlation function of the disorder potential. We apply this theory to graphene close to charge neutrality and cover the crossover to a high-density regime.

The unique mechanical properties of auxetic materials are widely used in specialized fields, such as medicine, defense security, and aerospace. Here, a potential two-dimensional (2D) auxetic (CuI)${\mathrm{P}}_4{\mathrm{Se}}_4$ material is predicted by first-principles calculations. The 2D (CuI)${\mathrm{P}}_4{\mathrm{Se}}_4$ monolayer is a quasidirect-band-gap semiconductor with a relatively large band gap of 2.66 eV, which can be regulated by applying uniaxial strain. In particular, the unique coordinate structure of polymeric ${\mathrm{P}}_4{\mathrm{Se}}_4$ strands and ${\mathrm{Cu}}_2{\mathrm{I}}_2$ units endows the (CuI)${\mathrm{P}}_4{\mathrm{Se}}_4$ monolayer with fascinating mechanical flexibility and anisotropy. The calculated Young's modulus of 8.62 (41.81) N/m and ideal fracture strength 0.66 (1.35) N/m in the $x$ ($y$) direction are one to two orders of magnitude smaller than those of other previously reported 2D materials. More remarkably, the (CuI)${\mathrm{P}}_4{\mathrm{Se}}_4$ monolayer exhibits the strain-dependent near-zero Poisson's ratio (ZPR) and negative Poisson's ratio (NPR) behaviors. Both the in-plane and out-of-plane Poisson's ratios can translate from positive value to negative value under suitable in-plane uniaxial strain. Our calculations show that the in-plane NPR characteristic is mainly controlled by the distances and angles of the neighbor Cu atoms and the out-of-plane NPR behavior is mainly dominated by the rotation of ${\mathrm{Cu}}_2{\mathrm{I}}_2$ units. In addition, the electronic and geometric response analyses further determine that the difference of strain-response capabilities between ${\mathrm{P}}_4{\mathrm{Se}}_4$ strands and ${\mathrm{Cu}}_2{\mathrm{I}}_2$ units should be responsible for the strain-dependent mechanical properties. Our results reveal a 2D intrinsic near ZPR and NPR material (CuI)${\mathrm{P}}_4{\mathrm{Se}}_4$ and explore the origin of its strain-dependent mechanical behaviors.