The boson peak (BP) is a universal feature in the Raman and inelastic scattering spectra of both disordered and crystalline materials. Here, through a set of atomistically resolved characterizations of metallic glasses, we uncover a robust inverse proportionality between the intensity of the boson peak and the activation energy of excitations in the potential energy landscape (PEL). Larger boson peaks are linked with shallower basins and lower activation barriers and, consequently, with emergent anharmonic sectors of the PEL. Numerical evidence from atomistic simulations indicates that THz atomic vibrations contributing the most to the BP in atomic glasses are strongly correlated with such emergent anharmonicity of the PEL, as evidenced through very large values of the atomic- and mode-resolved Grüneisen parameter found for the atomic vibrations that constitute the BP. These results provide a direct bridge between the vibrational spectrum and the topology of the PEL in amorphous solids.

Dirac cones with isotropic and linear dispersion, as in graphene, exhibit ultrahigh carrier mobility and offer an intriguing method to manipulate the behavior of particles. This concept has recently been extended to anisotropic Dirac cones with anisotropic dispersion in condensed matter, but their photonic counterparts are yet to be explored. Here, by introducing anisotropic coupling, we propose a Floquet lattice with a realistic photonic scheme that supports anisotropic Dirac cones and abundant topological phases. Under the highly anisotropic circumstances, the presence of anomalous Floquet insulators is demonstrated, where topological slow edge states are found along a specific direction. The group velocity of the edge states can be tailored continuously by adjusting the anisotropy of the lattice. These slow edge states are robust against disorders and can be harnessed for developing intriguing applications such as anisotropic devices, topological delay lines, and optical nonlinear devices.

The enhancement of interfacial Dzyaloshinskii-Moriya Interaction (DMI) in magnetic multilayers results in the stabilization of topological spin textures such as chiral domain walls and skyrmions. Here we report on the evaluation of interface-driven magnetic interactions in a uniquely designed multilayer where each magnetic layer of two antiferromagnetically coupled sublattices of $3d$ and $4f$ moments is sandwiched between the layers of β-tungsten and holmium whose spin Hall angles are large but opposite in sign. The atomic and magnetic periodicity of these multilayers is established by polarized neutron reflectivity measurements and the presence of a labyrinth domain spin texture of zero remanence with x-ray photoelectron microscopy. Measurements of the Hall resistivity $\left[{\rho }_{xy}\left(T,H\right)\right]$ together with static magnetization [$M$($T, H$)] over a broad range of temperature ($T$) and magnetic field ($H$) indicate impending compensation between $3d$ and $4f$ sublattices at $T>350\mathrm{K}$. These multilayers are characterized by a small (0.04%) but positive magnetoresistance indicative of interface enhanced scattering, and a large (40 nΩ m) and negative anomalous ${\rho }_{xy}\left(T,H\right)$ which results from a parallel alignment of $4f$ moments with the external magnetic field. No distinct scaling is seen between ${\rho }_{xy}\left(T,H\right), {\rho }_{xx}\left(T,H\right)$, and $M$($T, H$) at temperatures above 200 K where the magnetization develops out of plane anisotropy. The field scans of ${\rho }_{xy}$ at $T>200\mathrm{K}$ show a distinct cusp in the vicinity of magnetic saturation. These Hall data have been analyzed in the framework of a model where a distinct topological contribution to ${\rho }_{xy}$ rides over the anomalous Hall resistivities of the $3d$ and $4f$ magnetic sublattices. It is suggested that this apparent topological effect results from an interfacial DMI and dominates ${\rho }_{xy}\left(T,H\right)$ in the temperature regime where the $3d$ and $4f$ lattices are nearly compensated.

We investigate how to engineer an antisymmetric spin-split band structure under spin density waves with finite ordering wave vectors in centrosymmetric systems without the relativistic spin-orbit coupling. On the basis of a perturbative analysis for the spin-charge coupled model in centrosymmetric itinerant magnets, we show that nonzero chiral-type bilinear and biquadratic spin cross products in momentum space under the magnetic orderings are related to an antisymmetric spin polarization in the electronic band structure. We apply the derived formula to the single-$Q$ cycloidal spiral and double-$Q$ noncoplanar states including the meron-antimeron and skyrmion crystals. Our results present a clue to realize a giant antisymmetric spin splitting driven by magnetic phase transitions in the centrosymmetric lattice structures without the spin-orbit coupling.

In a recent paper by dos Santos Dias et al. [Phys. Rev. B 103, L140408 (2021)], a critique of earlier works analyzing low-energy spin Hamiltonians is put forth. To be precise, it is the large noncollinear contributions to the Dzyaloshinskii-Moriya interaction (DMI) that is the main concern of dos Santos Dias et al. In this Comment, we clarify the microscopic mechanisms for the large DMI that can be found in noncollinear magnets. Furthermore, we outline the complementary nature of the different parametrizations of a spin Hamiltonian, with strengths and weaknesses of both approaches. Specifically, we stress the physical insight in the interpretation of the DMI, when decomposed in microscopic electron and spin densities and currents.

In our previous Letter [Phys. Rev. B 103, L140408 (2021)], we presented a discussion of the fundamental physical properties of the interactions parametrizing atomistic spin models in connection to first-principles approaches that enable their calculation for a given material. This explained how some of those approaches can apparently lead to magnetic interactions that do not comply with the expected physical properties, such as Dzyaloshinskii-Moriya interactions which are nonchiral and independent of the spin-orbit interaction, and which we consequently termed “improper.” In the preceding Comment by Cardias et al., Phys. Rev. B 105, 026401 (2022), the authors present arguments based on the distinction between global and local approaches to the mapping of the magnetic energy using first-principles calculations to support their proposed nonchiral Dzyaloshinskii-Moriya interactions and their dismissal of our distinction between “proper” and “improper” magnetic interactions. In this Reply, we identify the missing step in the local approach to the mapping and explain how ignoring this step leads to the identification of magnetic interactions which do not comply with established physical principles and that we have previously termed “improper.”

Quantum bits based on Majorana zero modes are expected to be robust against certain noise types, and hence provide a quantum computing platform that is superior to conventional qubits. This robustness is not complete, though: imperfections can still lead to qubit decoherence and hence to information loss. In this work, we theoretically study Majorana-qubit dephasing in a minimal model: in a Kitaev chain with quasistatic disorder. Our approach, based on numerics as well as first-order nondegenerate perturbation theory, provides a conceptually simple physical picture and predicts Gaussian dephasing. We show that as system parameters are varied, the dephasing rate due to disorder oscillates out-of-phase with respect to the oscillating Majorana splitting of the clean system. In our model, first-order dephasing sweet spots are absent if disorder is uncorrelated. We describe the crossover between uncorrelated and highly correlated disorder, and show that dephasing measurements can be used to characterize the disorder correlation length. We expect that our results will be utilized for the design and interpretation of future Majorana-qubit experiments.

Quantum systems are often described by parameter-dependent Hamiltonians. Points in parameter space where two levels are degenerate can carry a topological charge. Here we theoretically study an interacting two-spin system where the degeneracy points form a nodal loop or a nodal surface in the magnetic parameter space, similarly to such structures discovered in the band structure of topological semimetals. Key results of our work are that (1) we determine the topological charge distribution along these degeneracy geometries and (2) we show that these nonpointlike degeneracy patterns can be obtained not only by fine-tuning but they can be stabilized by spatial symmetries. Since simple spin systems such as the one studied here are ubiquitous in condensed-matter setups, we expect that our findings, and the physical consequences of these nontrivial degeneracy geometries, are testable in experiments with quantum dots, molecular magnets, and adatoms on metallic surfaces.

In this work, we perform a first-principle study to investigate the atomic and electronic structures of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ van der Waals heterostructure (vdWH) as well as its tunable electronic structure via interlayer coupling and an external perpendicular electric field. The ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH is structurally and thermodynamically stable at room temperature. Our results demonstrate that the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH exhibits a semiconducting characteristic with a direct band gap of 1.86/2.66 eV as given by the PBE/HSE06 calculation. This value of band gap conveniently lies in the visible light energy range, thus unraveling the strong optical absorption of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH in the technologically important visible light regime. The band edges of the ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH separately from the ${\mathrm{C}}_3{\mathrm{N}}_4$ and ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ layers, thus resulting in a type-II band alignment, which is highly desirable for achieving efficient electron-hole separation. Remarkably, the electronic structure and the band alignment types can be flexibly tuned between type-I and type-II by applying an external electric field, by changing the interlayer distance and by applying the in-plane strain. Our findings reveal the potential of ${\mathrm{C}}_3{\mathrm{N}}_4/{\mathrm{MoSi}}_2{\mathrm{N}}_4$ vdWH as a tunable hybrid material with strong potential in optoelectronic applications.

Starting from a low-energy effective Hamiltonian model, we theoretically calculate the dynamical optical conductivity and permittivity tensor of a magnetized graphene layer with Rashba spin-orbit coupling (SOC). Our results reveal a transverse Hall conductivity correlated with nonreciprocal longitudinal conductivity. Further analysis illustrates that for intermediate magnetization strengths, the relative magnitudes of the magnetization and SOC can be identified experimentally by two well-separated peaks in the dynamical optical response (both the longitudinal and transverse components) as a function of photon frequency. Moreover, the frequency-dependent permittivity tensor is obtained for a wide range of chemical potentials and magnetization strengths. Employing experimentally realistic parameter values, we calculate the circular dichroism of a representative device consisting of magnetized spin-orbit coupled graphene and a dielectric insulator layer, backed by a metallic plate. The results reveal that this device has different relative absorptivities for right-handed and left-handed circularly polarized electromagnetic waves. It is found that the magnetized spin-orbit coupled graphene supports strong handedness-switchings, effectively controlled by varying the chemical potential and magnetization strength with respect to the SOC strength.

Recently, the premelting and formation of ice due to the Casimir-Lifshitz interaction, proposed in early 1990s by Elbaum and Schick [Phys. Rev. Lett. 66, 1713 (1991)], have been generalized to diverse practical scenarios, yielding novel physical intuitions and possibilities of application for those phenomena. The properties of materials, in particular, the electrical permittivity and permeability, exert significant influence on the Casimir-Lifshitz energies and forces and hence on the corresponding premelting and formation of ice. To address these influences in detail and explore the resulting physics, here we revisit and extend the analyses of previous work with both the dielectric data utilized there and the latest dielectric functions for ice and cold water. For the four-layer cases considered by some of us, the existence of stable configurations depending on the initial conditions has been confirmed, and different types of stability corresponding to minima of the Casimir-Lifshitz free energy are demonstrated. As the new dielectric functions for ice and cold water deviate considerably from those used by Elbaum and Schick, their vital impact on three- and four-layer configurations is therefore being reconsidered.

The $1/f$ resistance noise has been measured in thin CuMn (13.5 at.%) spin-glass films. The temperature and frequency dependence have been analyzed in terms of the landscape of free-energy barriers. This analysis provides the full barrier distribution for various thin film thicknesses between 10 nm and 80 nm. The free-energy barrier height distribution's width and energy position have been determined. Contrary to previous models with fixed shape and energy, the free-energy landscape is described by a distribution of barriers that both shifts and changes shape as the temperature is reduced. The dependence of this distribution is in contrast with recent predictions. Using the fluctuation dissipation theorem, the $1/f$ measurements connect with ${\chi }^{″}$, displaying agreement with other direct measurements of the latter.

Selectively tuning the properties of a material with multiple order parameters, such as polarization and magnetization, requires precise control of the crystal structure. Recently developed midinfrared THz laser techniques provide an efficient way to manipulate the structure of materials at an ultrafast rate. This paper presents a methodology to evaluate the possibility of picosecond multiple switching of order parameters in perovskite structure oxides using first-principles calculations. With multiferroic ${\mathrm{BiFeO}}_3$ as a model system, we demonstrate that the desired property can be tuned by an external laser pulse's perturbation with frequency 15.2 THz. The dynamics of phonon modes are illustrated, revealing that lasers enable switching of polarization and magnetization within 1 ps. The switching mechanisms can be attributed to the nonlinear coupling of the pumped infrared-active mode with the ferroelectric and antiferrodistortive modes. Our work provides an initiative for more detailed experimental investigations to explore light control orders and dynamic material design.

We report the anomalous breakdown in the scaling of the microscopic magnetic susceptibility—as measured via the ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) shift $K$—with the bulk magnetic susceptibility $\chi$ in the paramagnetic state of ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$. This anomaly occurs near $T_{\max }\sim 117$ K the maximum in $\chi \left(T\right)$ and is therefore associated with the onset of quasi-two-dimensional (quasi-2D) magnetic correlations. The spin-lattice relaxation rate divided by temperature ${\left(T_{1}T\right)}^{-1}$ in ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ exhibits broad peaklike behavior as a function of temperature, qualitatively following $\chi$, but displaying no evidence of critical slowing down above the Néel temperature $T_N$. In the magnetic state of ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, NMR spectra provide good evidence for ${60}^{\circ }$ rotation of stacking-fault-induced magnetic domains, as well as observation of the spin-flop transition that onsets at 4 T. The temperature-dependent critical behavior of the internal hyperfine field at the P site in ${\mathrm{Mn}}_2{\mathrm{P}}_2{\mathrm{S}}_6$ is consistent with previous measurements and the two-dimensional anisotropic Heisenberg model. In a sample of ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, we observe only two magnetically split resonances in the magnetic state, demonstrating that the multiple-peaked NMR spectra previously associated with ${60}^{\circ }$ rotation of stacking faults is sample-dependent. Finally, we report the observation of a spin-flop-induced splitting of the NMR spectra in ${\mathrm{Ni}}_2{\mathrm{P}}_2{\mathrm{S}}_6$, with an onset spin-flop field of ${\mu }_0{H}_{\mathrm{sf}}=14$ T.

The anisotropic-exchange spin-1/2 model on the triangular lattice has been used to describe the rare-earth chalcogenides, which may have exotic ground states. We investigate the quantum phase diagram of the model by using the projected entangled pair state method, and compare it to the classical phase diagram. Besides two stripe-ordered phases, and the ${120}^{\circ }$ phase, there is also a multi-Q phase. We identify the multi-Q phase as a $Z_2$ vortex state. No quantum spin liquid state is found in the phase diagram, contrary to the previous density matrix renormalization group calculations.

It is still debated whether the low-doping Fermi surface of cuprates is composed of hole pockets or of disconnected Fermi arcs. Results from cellular dynamical mean field theory (c-DMFT) support the Fermi arcs hypothesis by predicting corresponding Fermi arcs for the Hubbard model. Here, we introduce a simple parametrization of the self-energy, in the spirit of Yang-Rice-Zhang theory, and show that state of the art c-DMFT calculations cannot give a definitive answer to the question of Fermi arcs vs hole pockets and this, independent of the periodization (cumulant or Green's function) used to display spectral weights of the infinite lattice. Indeed, when our model is restricted to a cluster and periodized like in c-DMFT, only two adjustable parameters suffice to reproduce the qualitative details of the frequency and momentum dependence of the low energy c-DMFT spectral weight for both periodizations. In other words, even though our starting model has a Fermi surface composed of hole and electron pockets, it leads to Fermi arcs when restricted to a cluster and periodized like in c-DMFT. We provide a different periodization scheme, named compact tiling, to recover the hole and electron pockets of our starting noninteracting lattice model, suggesting that better periodization schemes might exist.