We report the static and dynamical properties of a newly discovered $S=2$ kagome antiferromagnet ${\mathrm{CsMn}}_3{\mathrm{F}}_6{\left({\mathrm{SeO}}_3\right)}_2$. By combining dc and ac magnetic susceptibilities, specific heat, electron spin resonance (ESR), and muon spin relaxation ($\mu \mathrm{SR}$), we identify two characteristic temperatures $T^{*}=8$ K and $T=|{\mathrm{\Theta }}_{\text{CW}}|=49\left(1\right)$ K, thereby three distinct regimes: a paramagnetic, a cooperative paramagnetic, and a quasistatic ordered state. At high temperature ($T>|{\mathrm{\Theta }}_{\text{CW}}|$), the ac susceptibility and ESR linewidth show a power-law dependence, reflecting short-range spin-spin correlations of the paramagnetic ${\mathrm{Mn}}^{3+}$ ions. In the cooperative paramagnetic regime ($T^{*}<T<\left|{\mathrm{\Theta }}_{\text{CW}}\right|$), the ESR signals evince the development of dichotomic spin correlations, which is interpreted in terms of the distinct timescales of in-plane and out-of-plane spin fluctuations. Remarkably, we observe a broad maximum at the characteristic temperature scale of $T_{\text{cl}}=20$ K ($\approx |J_{\text{cl}}|$) in the specific heat and ESR results, suggesting short-range ordering. At low temperatures below $T^{*}$, the $\mu \mathrm{SR}$ data suggest the presence of dynamically fluctuating fields with partially frozen moments, consistent with the absence of long-range order evidenced by specific heat and magnetic susceptibility data. A magnetic behavior that depends on the chosen time window points to the presence of multiple timescales and temporally anisotropic spin correlations, as predicted for a classical kagome antiferromagnet.

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin system and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, an observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two platinum strips and investigate the influence of the Pt strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 and a $100\mathrm{n}\mathrm{m}$ thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

One of the key effects which is predicted to arise in superconductors without a center of inversion is the mixing of singlet and triplet order parameters, which are no longer good quantum numbers on their own due to parity. We have probed the gap structure in the noncentrosymmetric superconductor ${\mathrm{Ru}}_7{\mathrm{B}}_3$, through small-angle neutron diffraction from the vortex lattice, in order to search for the proposed mixed order parameter. We find that the measured temperature dependence of the vortex-lattice form factor is well characterized by a model constructed to describe the effects of broken inversion symmetry on the superconducting state, indicating the presence of a mixed singlet-triplet gap and confirming the theoretical predictions.

The cesium halides ($\mathrm{Cs}X$) are ionic high-symmetry compounds, which at first would seem like well-understood systems. However, recent studies have shown that using the simple Perdew-Burke-Ernzerhof (PBE) functional in density-functional theory (DFT) calculations, $\mathrm{Cs}X$ materials do not adopt their namesake structure. Furthermore, peculiar low thermal conductivities have been observed experimentally in both CsCl and CsI at room temperature, and the origin has been linked to low-temperature anharmonicity derived from different types of experiments. In the case of CsCl the anharmonicity was observed from x-ray diffraction as an octahedral nuclear probability density function (nPDF), which, in contrast to expectations, becomes spherical at elevated temperature. Here, we study the nPDF of CsBr and CsI from single-crystal x-ray diffraction to compare with the findings of CsCl. It is shown that the aspherical features become less pronounced when substituting for a heavier halide. From periodic DFT calculations on CsCl, CsBr, and CsI probing the potential-energy surfaces this can be explained by progressively more similar masses upon substitution linked with Pauli repulsion. The apparent disappearance of the anharmonic features in CsCl with increasing temperatures can be understood as relatively larger population of acoustic phonons compared to the optical phonons following the Bose-Einstein distribution function. Finally, it is shown that theory can reproduce the correct equilibrium structures as well as phonon dispersions comparable to experimental values when adding a functional form of van der Waals interactions to a PBE DFT calculation.

Half-Heusler compounds are a very large class of materials that have received attention as promising high-temperature thermoelectric materials due to their favorable electrical transport behavior and other properties beneficial for device fabrication. A particular challenge is that while half-Heusler compounds show an exceptionally large range of thermal conductivities, these are still generally higher than other state-of-the-art thermoelectric materials. Here, we investigate PCdNa, which was reported to be a material that may form in the half-Heusler structure and have an unusually low thermal conductivity. This is in order to understand its very low thermal conductivity and elucidate its electronic properties in relation to thermoelectricity. We find that the low thermal conductivity of 2.6 W/mK at 300 K is a consequence of underbonding of the Cd ions within the network formed by the P. This underbonding leads to anharmonicity and relatively low frequency phonons and may be regarded as a generalized effective rattling phenomenon. The electronic structure shows a high valence band degeneracy, which we find leads to favorable good electrical transport properties. The combination of the low lattice thermal conductivity and favorable electrical transport properties results in high $p$-type thermoelectric performance. The figure of merit is predicted to reach $ZT=3.3$ at 900 K for optimum $p$-type doping. This suggests that PCdNa is a promising $p$-type high-temperature half-Heusler thermoelectric material.

Spectral and temporal correlations determine the majority of pulse properties, and a high degree of coherence is needed for minimizing the pulse length. However, there is no simple way to quantify these correlations experimentally, and nonlinear methods are often required. In this paper, we confirm an earlier proposed experiment [Koivurova et al., Opt. Lett. 44, 522 (2019)] that can accurately estimate the spectral degree of coherence of arbitrary nonstationary fields. The method is entirely linear and can retrieve the quasicoherent contribution of the spectral correlation function. In particular, the method can be used to measure the overall degree of spectral coherence in a single-shot manner. We first establish the theoretical framework behind the method and experimentally test it for a bulk-generated supercontinuum. Our experimental results are in good agreement with the theory and confirm our earlier numerical findings [Halder et al., Photon. Res. 7, 1345 (2019)]. Moreover, the results yield insight into supercontinuum generation in bulk material.

We numerically determine the robustness of the lasing edge modes in a spin-torque oscillator array that realizes the non-Hermitian Su-Schrieffer-Heeger model. Previous studies found that the linearized dynamics can enter a topological regime in which the edge mode is driven into auto-oscillation, while the bulk dynamics are suppressed. Here we investigate the full nonlinear and finite-temperature dynamics, whose understanding is essential for spin-torque oscillators-based applications. Our analysis shows that the lasing edge mode dynamics persist in the nonlinear domain for a broad range of parameters and temperatures. We investigate the effects of perturbations relevant to experimental implementations and discuss which ones might be detrimental to the stability of the lasing edge mode. Finally, we map our model onto a photonic model. Our analysis has the potential to shed light onto the dynamics of a plethora of non-Hermitian systems with nonlinearities.

Since the proposal of monopole Cooper pairing by Li and Haldane [Phys. Rev. Lett. 120, 067003 (2018)], considerable research efforts have been dedicated to the study of Cooper pairing order parameters constrained (or obstructed) by the nontrivial normal-state band topology at Fermi surfaces in 3D systems. In the current work, we generalize the topologically obstructed pairings between Chern states (like the monopole Cooper pairing) by proposing Euler obstructed Cooper pairing in 3D systems. The Euler obstructed Cooper pairing widely exists between two Fermi surfaces with nontrivial band topology characterized by nonzero Euler numbers; such Fermi surfaces can exist in 3D $\mathcal{P}\mathcal{T}$-protected spinless-Dirac/nodal-line semimetals with negligible spin-orbit coupling, where $\mathcal{P}\mathcal{T}$ is the space-time inversion symmetry. An Euler obstructed pairing channel must have pairing nodes on the pairing-relevant Fermi surfaces, and the total winding number of the pairing nodes is determined by the sum or difference of the Euler numbers on the Fermi surfaces. In particular, we find that when the normal state is time-reversal invariant and the pairing is weak, a sufficiently dominant Euler obstructed pairing channel with zero total momentum leads to nodal superconductivity. If the Fermi surface splitting is small, the resultant nodal superconductor hosts hinge Majorana zero modes. The possible dominance of the Euler obstructed pairing channel near the superconducting transition and the robustness of the hinge Majorana zero modes against disorder are explicitly demonstrated using effective or tight-binding models. Our work presents the first class of higher-order nodal superconductivity originating from the topologically obstructed Cooper pairing.

$\alpha -{\mathrm{RuCl}}_3$ is a promising candidate material for the Kitaev spin liquid state where the half quantization of the thermal Hall effect, suggesting a topological character, has been observed. Here we propose a more direct signature of a chiral Majorana edge mode which emerges in a universal scaling of the Drude weight of the edge spin Seebeck effect in the Kitaev model. Moreover, the absence of backscatterings in the chiral edge mode results in the generation of a dissipationless spin current in spite of an extremely short spin correlation length close to a lattice constant in the bulk. This result is not only experimentally observable, but also opens a way towards spintronics application of Kitaev materials.

The Mott transition in honeycomb compounds with significant spin-orbit coupling is explored. At finite temperatures we identify a novel semimetallic phase located between a topological insulator and a topological Mott insulator. This semimetal is characterized by having no charge gap, no quasiparticles, and gapped spin excitations with nontrivial topological properties. While charge conduction is incoherent involving mean-free paths violating the Mott-Ioffe-Regel limit, spin excitations can be transported ballistically along the edges giving rise to a quantized spin conductance. Such bad topological semimetal could be searched for close to the Mott transition in certain twisted bilayers of transition metal dicalchogenides.

We study the dissipative Fermi-Hubbard model in the limit of weak tunneling and strong repulsive interactions, where each lattice site is tunnel-coupled to a Markovian fermionic bath. For cold baths at intermediate chemical potentials, the Mott insulator property remains stable and we find a fast relaxation of the particle number towards half filling. On longer time scales, we find that the antiferromagnetic order of the Mott-Néel ground state on bipartite lattices decays, even at zero temperature. For zero and nonzero temperatures, we quantify the different relaxation time scales by means of waiting time distributions, which can be derived from an effective (non-Hermitian) Hamiltonian and obtain fully analytic expressions for the Fermi-Hubbard model on a tetramer ring.

Multidimensional coherent spectroscopy (MDCS) is used to separate coherent and incoherent nonlinear contributions to the population-time dynamics in a GaAs-based semiconductor microcavity encapsulating a single InGaAs quantum well. In a three-pulse four-wave-mixing scheme, the second delay time is the population time that in MDCS probes excited-state coherences and population dynamics. Nonlinear optical interactions can mix these contributions, which are isolated here for the lower- and upper-exciton polariton through the self- and mutual-interaction features. Results show fast decays and oscillations arising from the coherent response, including a broad stripe along the absorption energy axis, and longer time mutual-interaction features that do not obey a simple population decay model. These results are qualitatively replicated by Bloch equation simulations for the $1s$ exciton strongly coupled to the intracavity field. The simulations allow for separation of coherent and incoherent Pauli-blocking and Coulomb interaction terms within the ${\chi }^{\left(3\right)}$-limit, and a direct comparison of each feature in one-quantum rephasing and zero-quantum spectra.

By means of first-principles density-functional theory calculations, we investigate the role of hydrostatic pressure in the electronic structure of the $T_d$ ($Pmn{2}_1$) and $1T^{″}$ ($Pm$) phases of the Weyl semimetal ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{Te}}_2$, which is a promising material for phase-change memory technology and superconductivity. We particularly focus on changes occurring in the distribution of the gapless Weyl points (WPs) within the 0 to 45 GPa pressure range. We further investigate the structural phase transition and lattice dynamics of the $T_d$ and $1T^{″}$ phases within the aforementioned pressure range. Our calculations suggest that both the $T_d$ and $1T^{″}$ phases of ${\mathrm{Mo}}_{0.5}{\mathrm{W}}_{0.5}{\mathrm{Te}}_2$ host four WPs in their full Brillouin zone at zero pressure. The total number of WPs increases to 44 (36) with increasing pressure via pair creation up to 20 (15) GPa for the $T_d$ ($1T^{″}$) phase, and beyond this pressure pair annihilation of WPs starts occurring, leaving only 16 WPs at 45 GPa in both phases. The enthalpy versus pressure data reveal that the $1T^{″}$ phase is more favorable below the critical pressure of 7.5 GPa; however, beyond this critical pressure the $T_d$ phase becomes enthalpically favorable. We also provide the calculated x-ray diffraction spectra along with the calculated Raman- and infrared-active phonon frequencies to facilitate the experimental identification of the studied phases.

We report a self-consistent quasinormal mode theory for nanometer scale electromagnetism where the possible nonlocal and quantum effects are treated through quantum surface responses. With Feibelman's frequency-dependent $d$ parameters to describe the quantum surface responses, we formulate the source-free Maxwell's equations into a generalized linear eigenvalue problem to define the quasinormal modes. We then construct an orthonormal relation for the modes and consequently unlock the powerful toolbox of modal analysis. The orthonormal relation is validated by the reconstruction of the full numerical results through modal contributions. Significant changes in the landscape of the modes are observed due to the incorporation of the quantum surface responses for a number of nanostructures. Our semianalytical modal analysis enables transparent physical interpretation of the spontaneous emission enhancement of a dipolar emitter as well as the near-field and far-field responses of plane-wave excitations in the nanostructures.

As a hallmark of a pure quantum effect, quantum entanglement has provided unconventional routes to characterize condensed matter systems. Here, from the perspective of quantum entanglement, we disclose exotic quantum physics in non-Hermitian quasicrystals. We study a class of experimentally realizable models for non-Hermitian quasicrystal chains, in which asymmetric hopping and complex potential coexist. We diagnose the global phase diagram by means of entanglement from both the real-space and momentum-space partitions. By measuring the entanglement entropy, we numerically determine the metal-insulator transition point. We combine real-space and momentum-space entanglement spectra to complementarily characterize the delocalization phase and the localization phase. Inspired by the entanglement spectrum, we further analytically prove that a duality exists between the two phase regions. The transition point is self-dual and exact, further validating the numerical result from diagonalizing non-Hermitian matrices. Finally, we identify the mobility edge by means of entanglement.

We introduce a unary coding of bosonic occupation states based on the famous balls and walls counting for the number of configurations of $N$ indistinguishable particles on $L$ distinguishable sites. Each state is represented by an integer with a human readable bit string that has a compositional structure allowing for the efficient application of operators that locally modify the number of bosons. By exploiting translational and inversion symmetries, we identify a speedup factor of order $L$ over current methods when generating the basis states of bosonic lattice models. The unary coding is applied to a one-dimensional Bose-Hubbard Hamiltonian with up to $L=N=20$, and the time needed to generate the ground-state block is reduced to a fraction of the diagonalization time. For the ground state symmetry resolved entanglement, we demonstrate that variational approaches restricting the local bosonic Hilbert space could result in an error that scales with system size.

The local structure of high-pressure ${\mathrm{BaTiO}}_3$ has been interrogated by neutron total scattering methods up to pressures of 4.18 GPa at ambient temperature. Competitive refinements of cubic, tetragonal, and rhombohedral distortion modes against pair distribution functions indicate contrasting local structure behavior of temperature- and pressure-induced cubic ${\mathrm{BaTiO}}_3$. Suppression of the mode amplitude, isotropy of the order-parameter direction, and loss of sensitivity to correlated Ti displacements at high pressure all suggest a high-pressure local structure that is more consistent with the harmonic approximation rather than an order-disorder model which better describes high-temperature cubic ${\mathrm{BaTiO}}_3$ in the vicinity of the tetragonal phase transition.

A high-pressure study of the structural and thermal transport properties is carried out on one of the most efficient filled skutterudites, ${\mathrm{Yb}}_{0.3}{\mathrm{Co}}_4{\mathrm{Sb}}_{12}$, to understand the relatively low thermal conductivity behavior in this family. By combining x-ray diffraction and Raman scattering measurements, we detect a phase transition at around 12.4 GPa. The mode Grüneisen parameters of the observed phonon modes are obtained from the determined bulk modulus and the phonon frequency shifts with pressure. The strong anharmonicity in this material is demonstrated by the obtained large average Grüneisen parameter. We also find the depressed group velocity within the low-frequency range, the flat guest mode avoided crossing with the acoustic-phonon mode, and the significant contribution of optical phonons. The hybridization of the guest atom and host lattice and the related enhanced anharmonicity are suggested to account for the low lattice thermal conductivity in the studied system. These findings provide new insight into how phonon-phonon interactions lower lattice thermal conductivity in this important thermoelectric family.

As machine learning becomes increasingly important in science and engineering, it holds the promise to provide a universal approach applicable to various systems to investigate their crystalline phase transitions. Here, we build and train accurate artificial neural networks that can distinguish tiny energy difference, which is crucial to predict the crystalline phase transitions. Employing the trained artificial neural networks in Monte Carlo simulations as the surrogate energy function, we apply this approach to monochalcogenides, including bulk and two-dimensional monolayer SnTe and GeTe, investigating their crystalline phase transitions. The machine-learning approach, when viewed as providing a universal mathematical structure, can be transferred to the investigation of other materials when the training data set generated with ab initio methods are available. Moreover, the machine-learning approach resolves the difficulties associated with constructing the effective Hamiltonian for monochalcogenides, showing great potential with its accuracy and efficiency.

The shear-banding instability in quasistatically driven bulk metallic glasses emerges from collective dynamics, mediated by shear transformation zones and associated nonlocal elastic interactions. It is also phenomenologically known that sharp structural features of shear bands are typically correlated to the sharpness of the plastic yielding transition, being predominant in commonly studied alloys composed of multiple different elements, that have very different atomic radii. However, in the opposite limit where elements' radii are relatively similar, plastic yielding of bulk metallic glasses is highly dependent on compositional and ordering features. In particular, a known mechanism at play involves the formation of short-range order dominated by icosahedra-based clusters. Here, we report on atomistic simulations of multicomponent metallic glasses with different chemical compositions showing that the degree of strain localization is largely controlled by the interplay between composition-driven icosahedra-ordering and collectively-driven shear transformation zones. By altering compositions, strain localization ranges from diffuse homogenized patterns to singular crack-like features. We quantify the dynamical yielding transition by measuring the atoms' susceptibility to plastic rearrangements, strongly correlated to the local atomic structure. We find that the abundance of short-range ordering of icosahedra within rearranging zones increases glassy materials' capacity to delocalize strain. This could be understood on the basis of structural heterogeneities that are enhanced by the presence of local order. The kind of plastic yielding can be often qualitatively inferred by the commonly used compositional descriptor that characterizes element associations, the misfit parameter ${\delta }_a$, and also by uncommon ones, such as shear-band width and shear-band dynamics' correlation parameters.