The doping dependent phase diagram of the iron pnictide systems displays diverse electronic ground states including unconventional superconductivity and magnetic ordering. From previous bulk measurements, it was argued that the superconducting phase of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ might be described within a two band formalism where superconductivity emerges with significantly different magnitudes of the pairing amplitude in the different bands. We have performed point contact Andreev reflection spectroscopy on the optimally doped system $(x=0.4)$ where we found features that misleadingly mimic the signature of multiple gap amplitudes with large energy difference, when the point contacts are away from the ballistic regime. Closer to the ballistic regime, we found two types of spectra. In one type, a single superconducting gap with unusual broadening was found. The broadening might be due to the presence of multiple gap amplitudes with small energy spacing. The other kind of spectra displayed spectral features at $\sim 30$ meV in the normal state that gradually diminished with increasing temperature and eventually disappeared at 140 K, the spin density wave transition temperature of parent ${\mathrm{BaFe}}_2{\mathrm{As}}_2$. We attribute the 30 meV spectral feature to a characteristic electron-magnon interaction energy scale in the system.

Magnetic skyrmions have garnered attention for their potential roles in spintronic applications, such as information carriers in computation, data storage, and nano-oscillators due to their small size, topological stability, and the requirement of small electric currents to manipulate them. Two key challenges in harnessing skyrmions are the stabilization requirement through a strong out-of-plane field, and the skyrmion Hall effect (SkHE). Here, we present a systematic model study of skyrmions in ferromagnetic/antiferromagnetic (FM/AFM) multilayer structures by employing both atomistic Monte Carlo and atomistic spin dynamics simulations. We demonstrate that skyrmions stabilized by exchange bias have superior stability to field-stabilized skyrmions due to the formation of a magnetic imprint within the AFM layer. Additionally, stacking two skyrmion hosting FM layers between two AFM layers suppresses the SkHE and enables the transport of AFM-coupled skyrmions with high velocity in the order of a few km/s. This proposed multilayer configuration could serve as a pathway to overcome existing limitations in the development of skyrmion-based devices, and the insights obtained through this study contribute significantly to the broader understanding of topological spin textures in magnetic materials.

A recently discovered kagome antiferromagnet ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{19}{\mathrm{Cl}}_8$ has attracted significant interest due to its unique kagome lattice structure and magnetic properties. The kagome lattice has three types of exchange interactions: one hexagonal coupling and two different triangular couplings. Previous studies have shown that its ground state is significantly different from that predicted for the undistorted kagome lattice, forming a coplanar spin state with a commensurate magnetic wave vector $\mathbf{Q}=(1/3,1/3)$. Two separate studies have proposed distinct sets of exchange interaction parameters for this compound. In this study we investigate the ground state of the spin-1/2 Heisenberg kagome model with three types of nearest-neighbor exchange interactions under a magnetic field by exact diagonalization using the Lanczos method. We reveal that clear magnetization plateaus at $M/M_{\mathrm{sat}}=1/3$, 5/9, and 7/9 are present under both parameter sets, which are identified as magnon crystal states based on their spin structures. Our findings suggest that these plateaus could potentially be experimentally confirmed with magnetization measurements on ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{19}{\mathrm{Cl}}_8$ under a magnetic field of approximately 300 T, achievable with state-of-the-art magnetic field generators. In order to get a deeper understanding of magnetism of ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{19}{\mathrm{Cl}}_8$, we perform additional calculations by varying these interactions. Consequently, we discover additional plateau phases at $M/M_{\mathrm{sat}}=1/3$, 5/9, and 7/9, each distinctly different from the magnon crystal states.

The interplay of spin and lattice fluctuations in two-dimensional magnets without inversion symmetry is investigated. We find a general form for the magnetoelastic coupling between magnons and existing chiral phonons based on the symmetries of the crystalline lattice. We show that in hexagonal lattices, the coupling of magnons and chiral phonons derives from an anisotropic exchange spin model containing topological phases of magnons. Using the Heisenberg-Kitaev-$\mathrm{\Gamma }$ model, we show how magnon-polaron edge states with circular polarization arise from this interaction. Our findings exploit the polarization degrees of freedom in spin-lattice systems, thus setting the ground for the transfer of angular momentum between chiral phonons and magnons.

We develop a quantum theory to investigate nonlinear effects using the lesser Green's function, through which we explore the circular photogalvanic effect (CPGE) in Weyl semimetals (WSMs) under strong magnetic fields. A simple analytical expression is derived for the injection conductivity. It demonstrates that the achiral Landau levels (LLs) can induce a chirality-dependent injection current, because of their topologically nontrivial wave functions. The CPGE in different frequency regions is dominated by distinct physical mechanisms. In the low-frequency regime, the CPGE can be significantly enhanced by magnetic fields, due to the chiral $n=0$ LL. At high frequencies, the injection conductivity saturates to the quantized value observed at zero magnetic field, reflecting the topological nature of the achiral LLs. Upon application of parallel electric and magnetic fields, the CPGE exhibits a quantum oscillation that is periodic in $1/B$. The periodic-in-$1/B$ quantum oscillation of the CPGE in WSMs can serve as a highly sensitive measurement of the chiral anomaly.

${\mathrm{TaTe}}_4$ is a quasi-one-dimensional tetrachalcogenide that exhibits a charge density wave (CDW) instability caused by a periodic lattice distortion. Recently, pressure-induced superconductivity has been achieved in this compound, revealing a competition between these different ground states and making ${\mathrm{TaTe}}_4$ very interesting for fundamental studies. Although ${\mathrm{TaTe}}_4$ exhibits CDW ordering below 475 K, transport experiments have reported metallic behavior with a resistivity plateau at temperatures lower than 10 K. In this paper, we study the electronic structure of ${\mathrm{TaTe}}_4$ using a combination of high-resolution angle-resolved photoemission spectroscopy and density functional calculations. Our results reveal the existence of the long-sought metallic states. These states exhibit mixed dimensionality, while some of them might have potential topological properties.

Germanene is one of the most promising two-dimensional materials beyond graphene. The electronic properties of germanene are outstanding. However, structural defects, which may appear during growth or processing, deteriorating the performance of germanene-based devices, are elusive. Here, we combine scanning tunneling microscopy/spectroscopy, Raman spectroscopy, and density functional theory calculations to investigate the defects in epitaxial ultraflat germanene on Cu(111). Three types of defects are found and their electronic properties are explored. Single germanium atom vacancy leads to three well-defined states in local density of states, whereas the mirror twin boundary induced four electronic states confirmed by theoretical calculations. Furthermore, strain induced wrinkles with enhanced electronic states occurred in ultraflat germanene. In addition, we synthesized bi- and trilayer germanene with significant different electronic properties, where a tiny band gap is opened due to the symmetry breaking.

The hot-carrier effect, a phenomenon where charge carriers generated by photon absorption remain energetic by not losing much energy, has been one of the leading strategies in increasing solar cell efficiency. Nanostructuring offers an effective approach to enhance the hot-carrier effect via the spatial confinement, as occurs in a nanowire structure. The recent experimental study by Esmaielpour et al. [ACS Appl. Nano Mater. 7, 2817 (2024)] reveals a fascinating nonmonotonic dependence of the hot-carrier effect in nanowire array on the diameter of the nanowire, contrary to what might be expected from quantum confinement alone. We show that this nonmonotonic behavior can be explained by a simple model for electron energy loss that involves two principal mechanisms. First, electron-phonon scattering, that increases with the nanowire diameter, leading to the hot-carrier effect that decreases with increasing diameter. Second, electron capture by a defect level within band gap, that is, a midgap state, that decreases with nanowire diameter, leading to a hot-carrier effect that increases with increasing diameter. The two mechanisms balance at a certain diameter corresponding to optimal hot-carrier effect. Our result offers a guideline to optimize the hot-carrier effect in nanowire solar cells and ultimately their efficiency by adjusting the dimensions and microstructural properties of nanowires.

Observation of filling factor 6/13 is one of the surprising fractional quantum Hall states in the second Landau level because, in contrast to the standard wisdom, the fractions $(\nu <1/2)$ with lower numerators, namely 4 and 5, have not yet been observed. We find that a state indeed forms at $\nu =6/13$ as an intermittent topological state between two prominent states at $\nu =1/2$ and $\nu =2/5$ with lower numerators. Also, we predict that a state forms at $\nu =5/13$ as an intermittent to $\nu =2/5$ and $\nu =3/8$. Our proposed wave functions for $\nu =6/13$ and $5/13$ have excellent overlaps with the corresponding exact ground state wave functions. The Chern-Simons coupling matrices deduced from the form of these wave functions are analyzed to predict the topological properties, which may be experimentally verified.

The dynamics of coupled quasiparticle excitations can be crucial for understanding and engineering the novel states of matter, e.g., two-dimensional (2D) magnetic materials. ${\mathrm{NiPS}}_3$ is one of the 2D van der Waals antiferromagnets which host strong correlations between different degrees of freedom, creating new opportunities for potential next-generation device applications. In this study, we have performed temperature, magnetic-field, and polarization-dependent Raman as well as magnetic susceptibility measurements on flakes of ${\mathrm{NiPS}}_3$. Our measurements reveal the presence of magnetic excitation and its coupling with phonons well above the Néel temperature $({\mathrm{T}}_{\mathrm{N}}\sim 155\mathrm{K})$, at least up to $4{\mathrm{T}}_{\mathrm{N}}$ which is corroborated by the evolution of magnetic correlations observed in the susceptibility data. This coupling is manifested as Fano anomaly in the phonon modes, explained based on energy overlap and symmetry matching of phonons and the magnetic excitation, as it renormalizes across the magnetic transition $\left({\mathrm{T}}_{\mathrm{N}}\right)$. This is further supported by the observations made in Zn-doped magnetically diluted ${\mathrm{NiPS}}_3$, suggesting the robustness of its high temperature $(T>{\mathrm{T}}_{\mathrm{N}})$ magnetic state.

The Gilbert damping parameter, which describes magnetization dynamics, is crucial for the performance of modern spintronic devices, affecting factors such as the switching speed and critical current density of magnetoresistive random access memory. Thus, the ability to engineer it on demand is pivotal for developing spintronic applications. In this paper, we systematically examine the Gilbert damping parameter of Fe films in contact with a black phosphoruslike bismuth monolayer using first-principles calculations. In these Bi/Fe heterostructures, we obtain a significantly enhanced Gilbert damping owing to strong interfacial spin-orbit couplings (SOCs). Interestingly, we find nonmonotonic thickness-dependent Gilbert damping anisotropy and attribute that to the competition between the interfacial SOC and the intrinsically anisotropic SOC of Fe films. We further demonstrate that these SOC effects lead to anisotropic band structures, which are responsible for the anisotropic Gilbert damping. In this paper, we provide a deep understanding of the anisotropic Gilbert damping and open avenues for exploring it in ferromagnetic heterostructures.

We report ferromagnetic ordering at $T_{\mathrm{C}}=1.3\mathrm{K}$ in the quasi-two-dimensional magnet ${\mathrm{EuAl}}_{12}{\mathrm{O}}_{19}$ with large spins $S=7/2$. This ferromagnetic state was characterized by magnetization and specific heat measurements and the experimental results were compared with classical Monte Carlo simulations. They reveal a strong single-ion anisotropy leading to a uniaxial spontaneous magnetization along the $c$ axis. Furthermore, the application of a magnetic field in the hard magnetization plane $ab$ reduces the Curie temperature down to $T=0.5\mathrm{K}$ at ${\mu }_{0}H\approx 0.4\mathrm{T}$ while preserving the second-order nature of the ferromagnetic transition.

The quasi-one-dimensional topological insulator ${\mathrm{TaSe}}_3$ offers an intriguing platform to study the intertwined exotic quantum order, such as superconductivity and charge-density-wave (CDW) order, and nontrivial topological states. Nevertheless, it remains unclear how robust the superconductivity in this quasi-one-dimensional material is and how close the (hidden) CDW exists in its phase diagram. In this work, we unveil a well-defined CDW transition in ${\mathrm{TaSe}}_3$ crystals by leveraging a hydrostatic pressure. Remarkably, it is found that the CDW transition temperature gradually strengthens as the pressure increases, and crosses the room temperature above $\sim 2\mathrm{GPa}$, which makes it promising in future applications in electronic devices. Furthermore, based on the pressure-dependent resistivity and Raman spectroscopy measurements by using a diamond anvil cell, we have extended the high-pressure study up to 50 GPa and verified a proposed phase transition around 6–10 GPa. However, no superconductivity is observed down to 2 K in the whole pressure range studied, suggesting the superconductivity is fragile in this quasi-one-dimensional transition-metal trichalcogenide.

Besides the Berezinskii-Kosterlitz-Thouless phase transition, the two-dimensional generalized XY model, identified by a generalization parameter $q$ (as proposed by Romano and Zagrebnov), can also support a first-order phase transition, starting from a critical value $q_c$. However, the value of $q_c$ at which this transition takes place is unknown. In this paper, we take two approaches to accurately determine the critical parameter $q_c$. Furthermore, we show that the model is characterized by three distinct regions concerning both first-order and Berezinskii-Kosterlitz-Thouless phase transitions. Finally, the underlying mechanism governing such transitions is presented, along with an estimation of the critical temperatures.

Transition metal dichalcogenides have been proven to be highly tunable and versatile materials that show promise in electronics applications. Chemical vapor deposition as a monolayer growth method is scalable for mass production and can reliably yield sample sizes larger than those from mechanical exfoliation. Characterizing defect concentrations of chemical vapor deposition grown ${\mathrm{MoS}}_2$ to rapidly evaluate sample quality is possible through Raman spectroscopy, though this method can prove difficult due to strain interference in the monolayer Raman scattering signals. In this paper, first-principles density functional theory phonon and mode-Grüneisen parameter calculations are compared with experimentally derived mode-Grüneisen values to better characterize the sample strain on chemical vapor deposition grown ${\mathrm{MoS}}_2$ monolayers. We show that mode-Grüneisen parameter computations performed assuming a uniaxial straining direction match more closely with experimental findings than calculations performed assuming uniform biaxial strain, suggesting that substrate-sample strain for the grown ${\mathrm{MoS}}_2$ monolayers is primarily uniaxial. In addition, uniaxial strain computations show a break in K and ${\mathrm{K}}^{\prime }$ reciprocal point symmetry, which accounts for a reduction in the intensity of double-resonant Raman processes observed in experimental data, reinforcing the assumption that strain on ${\mathrm{MoS}}_2$ monolayers fabricated with chemical vapor deposition is nonuniformly biaxial.

We investigate a higher-order topological insulator (HOTI) under strong nonlinearity, focusing on the existence and stability of high-amplitude corner states, which can find applications in optics, acoustics, elastodynamics, and other wave-based systems. Our study centers on a breathing kagome lattice composed of point masses and springs, known to exhibit edge and corner states in its linear regime. By introducing on-site cubic nonlinearity, we analyze its impact on both edge and corner states. The nonlinear continuation of the corner state unveils stable high-amplitude corner states within the lattice, featuring nonzero displacements at even sites from the corner—a characteristic absent in the linear limit. Interestingly, the nonlinear continuation of the edge state reveals its transformation into distinct families of high-amplitude corner states via two pitchfork bifurcations. While some states maintain stability, others become unstable through real instability and Neimark-Sacker bifurcation. These unstable corner states dissipate their energy into the edges and the bulk over an extended period, as corroborated by long-time dynamical simulations. Consequently, our study provides insights into achieving significant energy localization at the corners of HOTIs through various classes of nonlinear states.

Suppressed three-phonon scattering processes have been considered to be the direct cause of materials exhibiting significant higher-order four-phonon interactions. However, after calculating the phonon-phonon interactions of 128 half-Heusler materials by high throughput, we find that the acoustic phonon bandwidth dominates the three-phonon and four-phonon scattering channels and keeps them roughly in a co-increasing or decreasing behavior. The $aao$ and $aaa$ three-phonon scattering channels in half-Heusler materials are weakly affected by the acoustic-optical gap and acoustic bunched features, respectively, only when acoustic phonon bandwidths are close. Finally, we found that half-Heusler materials with smaller acoustic bandwidths tend to have a more pronounced four-phonon effect, although three-phonon scattering may not be significantly suppressed at this time. Our study gives insights into the high-order four-phonon effect in solids, which has guiding significance for designing thermal management materials.

We study the generation of collective orbital modes, their evolution, and the characteristic n onlinear optical response induced by them in a photoinduced orbital-ordered correlated oxide using real-time simulations based on an interacting multiband tight-binding (TB) model. The d-d optical transitions under femtoseconds light-pulse in an orbital-ordered state excite collective orbital modes, also known as “orbitons”. Consistently incorporating electronic interactions and the interplay between charge, spin, and lattice degrees of freedom in the TB-model provides a clearer understanding of how these factors influence the generation and evolution of collective orbital modes. The dynamics of Jahn-Teller vibrational modes in the photoinduced state modify the intersite orbital interaction, which further amplifies these orbital modes. In the presence of weak ferroelectricity, the excitation of collective orbital modes induces a strong THz oscillatory photocurrent, which is long-lived. This suggests an alternative way to experimentally detect low-energy collective modes through THz-emission studies in the photoinduced state. Our study also elucidates that quasiparticle dynamics in improper ferroelectric oxides can be exploited to achieve highly interesting and nontrivial optoelectronic properties.