In this paper, we show that magnetic skyrmion nucleation can be controlled using ${\mathrm{Ga}}^{+}$ ion irradiation, which manipulates the magnetic interface effects (in particular, the magnetic anisotropy and Dzyaloshinskii-Moriya interaction) that govern the stability and energy cost of skyrmions in thin-film systems. We systematically and quantitatively investigate what effect these changes have on the nucleation of magnetic skyrmions. Our results indicate that the energy cost of skyrmion nucleation can be reduced up to $26\%$ in the studied dose range and that it scales approximately linearly with the square root of the domain wall energy density. Moreover, the total number of nucleated skyrmions in irradiated devices after nucleation was found to depend linearly on the ion dose and could be doubled compared to nonirradiated devices. These results show that ion irradiation cannot only be used to enable local nucleation of skyrmions but also allows for fine control of the threshold and efficiency of the nucleation process.

The model glass former glycerol and its aqueous mixtures were investigated with terahertz-time-domain spectroscopy (THz-TDS) in the frequency range 0.3–3.0 THz at temperatures from 80 to 305 K. It was shown that the infrared absorption coefficient measured with THz-TDS can be theoretically related to the reduced Raman intensity ($\propto \alpha /{\omega }^2$) and the reduced density of states ($\propto \alpha /{\omega }^3$) and the agreement with experimental results confirms this. The data were further used to investigate the behavior of model glasses in the harmonic (below the glass transition temperature $T_{\text{g}}$), anharmonic (above $T_{\text{g}}$), and liquid regimes. The onset temperature of molecular relaxation as measured by the infrared active dipoles, $T_{\text{g}}$, was found to correlate with the onset of anharmonic effects, leading to an apparent shift of the boson peak and obscuring it at elevated temperatures. The influence of clustered and unclustered water on the dynamics, the boson peak, and the vibrational dynamics was also investigated. A change in structural dynamics was observed at a water concentration of approximately 5 wt %, corresponding to a transition from isolated water molecules distributed homogeneously throughout the sample to the presence of small water clusters and an increased number of water-water hydrogen bonds which lower the barriers on the potential energy surface.

It has recently been demonstrated that it is possible to open a gap in a magnetic Weyl semimetal, while preserving the chiral anomaly along with the charge conservation and translational symmetries, which all protect the gapless nodes in a weakly interacting semimetal. The resulting state was shown to be a nontrivial generalization of a non-Abelian fractional quantum Hall liquid to three dimensions. Here we point out that a second fractional quantum Hall state exists in this case. This state has exactly the same electrical and thermal Hall responses as the first, but a distinct (fracton) topological order. Moreover, the existence of this second fractional quantum Hall state necessarily implies a gapless phase, which has identical topological response to a noninteracting Weyl semimetal, but is distinct from it. This may be viewed as a generalization (in a weaker form) of the known duality between a noninteracting two-dimensional Dirac fermion and ${\mathrm{QED}}_3$ to $3+1$ dimensions. In addition we discuss a $(3+1)$-dimensional topologically ordered state, obtained by gapping a nodal line semimetal without breaking symmetries.

We report on a combined experimental and theoretical study on ${\mathrm{CrI}}_3$ single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr $3d$ orbital splitting ($dd$ excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The $dd$ excitation energies are similar on the two sides of the ferromagnetic transition temperature, $T_C\sim 61$ K, although MD in RIXS is predominant at 0.4 T magnetic field below $T_C$. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr $3d$ orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr $3d$ intraorbital $dd$ excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr $3d$ hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in ${\mathrm{O}}_h$ and ${\mathrm{D}}_{3d}$ symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.

Using the natural orbitals renormalization group, we studied the problem of a localized spin-$\frac{1}{2}$ impurity coupled to two helical liquids via the Kondo interaction in a quantum spin Hall insulator, based on the Kane-Mele model defined in a finite zigzag graphene nanoribbon. We investigated the influence of the Kondo couplings with the helical liquids on both the static and dynamic properties of the ground state. The number and distinct spatial structures of the active natural orbitals (ANOs), which play essential roles in constructing the ground-state wave function, were first analyzed. Our numerical results indicate that two ANOs emerge, equal to the number of helical liquids. Specifically, at the coupling symmetry point, both ANOs are fully active with their spatial structures being, respectively, constituted by the different helical liquids. In comparison, when deviating from the symmetry point, only one ANO remains fully active, which is dominantly constructed by the helical liquid with the larger Kondo coupling. Local screening of the impurity, described by the impurity spin polarization and susceptibility, was further studied. It shows that at the coupling symmetry point, the impurity is maximally polarized and the spin susceptibility reaches the maximum. On the contrary, the impurity tends to be screened without polarization when the Kondo couplings deviate well from the symmetry point. The Kondo screening cloud, manifested by the spin correlation between the impurity and the conduction electrons, was finally explored. It is demonstrated that the Kondo cloud is mainly formed by the helical liquid with the larger Kondo coupling to the impurity. On the other hand, the spin-orbital coupling breaks the symmetry in spatial distribution of the spin correlation, leading to anisotropy in the Kondo cloud.

Single crystals of ${\mathrm{LaMn}}_{0.86}{\mathrm{Sb}}_2$ were synthesized via the flux method. Band structure calculation indicates the existence of Dirac dispersion below the Fermi level as expected for the square-net-based materials. Magnetization in combination with neutron diffraction measurements suggests that the magnetic structure below $T_{\mathrm{N}}=146 \mathrm{K}$ can be described by a G-type antiferromagnetic structure with a weak spin canting. A spin-flop transition was observed when external magnetic fields were applied parallel to the $c$ axis. Negative magnetoresistance is observed at low temperatures, likely originating from the spin alignment effect. Our results indicate that ${\mathrm{LaMn}}_{1-x}{\mathrm{Sb}}_2$ has Dirac dispersion and a vacancy tunable magnetism, which could be a potential platform for studying the interplay of magnetism, charge transport, and possible topological band.

We present a method to accelerate the numerical evaluation of spatial integrals of Feynman diagrams when expressed on the real frequency axis. This can be realized through use of a renormalized perturbation expansion with a constant but complex renormalization shift. The complex shift acts as a regularization parameter for the numerical integration of otherwise sharp functions. This results in an exponential speed up of stochastic numerical integration at the expense of evaluating additional counterterm diagrams. We provide proof of concept calculations within a difficult limit of the half-filled two-dimensional Hubbard model on a square lattice.

In this study, we examine the statistics of level spectra in a noninteracting electron gas confined to a Sierpiński carpet lattice. These lattices are constructed using two types of the $\mathit{self}$ and $gene$ patterns, and they are categorized by the area-perimeter scaling law. The singularly continuous spectra, along with the nearest level spacing distribution and gap-ratio distribution, reveal a critical phase different from both extended and localized phases. This critical phase differs from the behavior near the Anderson model's metal-insulator transition. The Wigner-like conjecture is confirmed for both lattice classes, indicating Gaussian orthogonal symmetry. A similar observation was made in a quasiperiodic lattice [Phys. Rev. Lett. 80, 3996 (1998)]. The self-similar nature of fractals also leads to level clustering behavior.

The nonlocal response of plasmonic materials and nanostructures is often described within a hydrodynamic approach, which is based on the Euler-Drude equation. In this paper, we reconsider this approach within an extension proposed by Halevi [Phys. Rev. B 51, 7497 (1995)]. After discussing the impact of this extended model on the propagation of longitudinal volume modes, we reevaluate within this framework the Mie scattering coefficients for a cylinder and the corresponding plasmon-polariton resonances. Our analysis reveals a nonlocal, collisional, and size-dependent damping term, which influences the resonances in the extinction spectrum. A transfer of the Halevi model into the time domain allows to identify a contribution to the current, which shares similarities with Cattaneo-kind diffusive-wavelike dynamics. After a comparison to other approaches commonly used in the literature, we implement the Halevi model into the discontinuous-Galerkin time-domain finite-element Maxwell solver and identify an oscillatory contribution to the current. Such an implementation of the Halevi model in time domain is of particular importance for applications in nanoplasmonics where nanogap structures and other nanoscale features have to be modeled efficiently and accurately.

In a related work [Phys. Rev. B 107, L121111 (2023)], we have shown that, in the two-impurity Anderson model with two hosts coupled by spin exchange in the most symmetric case, there are either two phase transitions or none. The phases comprise the conventional Kondo and RKKY regimes and a different one, interpreted as a Kondo-stabilized, metallic quantum spin liquid. Here, we analyze how various types of asymmetry affect this picture. We demonstrate that the transitions are robust against the coupling and particle-hole asymmetries, provided charge transfer is forbidden. This holds true despite the scattering phase shift at each impurity taking nonuniversal values. Finally, for an extended model including charge transfer between the hosts and a small Coulomb interaction at the host sites directly coupled to impurities, we show that the presence of charge transfer changes the phase transitions into crossovers. Provided the interhost hopping is sufficiently small, this leads to qualitatively the same physics at nonzero temperature. The relevance of this model for rare-earth atoms in a metallic host is discussed, and potential experimental setups for observing our findings are proposed.

A temperature gradient in chiral materials can generate a nonzero mean value of phonon angular momentum (PAM). We used the Kubo formula to investigate the contribution of both intraband and interband terms of PAM to the mean PAM. Interestingly, the interband term was found to be as important as the intraband term, indicating that the quantum transition between different phonon branches induced by a temperature gradient strongly affects locally atomic rotation. This discovery opens up an alternative mechanism for generating PAM and phonon magnetic moments.

Investigating electronic structures of two-dimensional van der Waals magnets is of great significance for the application in low-power nanodevices. The two-dimensional van der Waals ferromagnet ${\mathrm{CrGeTe}}_3$ has attracted much attention with chemically stable properties and relatively high Curie temperature. Here, we investigate the in-plane spin texture of ${\mathrm{CrGeTe}}_3$ in the ferromagnetic state by first-principles calculations, which reveal a helical-type spin texture that is attributed to surface symmetry breaking instead of spin-layer locking effect. Such spin texture is also supported by our spin-resolved photoemission spectroscopy experimental data. Our results provide a platform for exploring next-generation spintronic devices.

We study the nonequilibrium dynamics of a Luttinger liquid after a simultaneous quantum quench of the interaction and dissipative quench to the environment within the realm of the Lindblad equation. When the couplings to the environment satisfy detailed balance, the system is destined to thermalize, which we follow using bosonization. The thermodynamic entropy of the system, which also encodes information about the entanglement with the environment, either exhibits a maximum and minimum or grows monotonically before reaching its thermal value in a universal fashion. The single-particle density matrix reveals interesting features in the spatiotemporal “phase diagram,” including thermal, prethermal, and dissipation enhanced sudden quench Luttinger liquid behavior, as well as dissipation, enhanced Fermi liquid response.

We investigate the one-dimensional quantum ring constructed by the spin-orbit coupled material, in which the quantum spin-Hall Bernevig-Zhang (BZ) Hamiltonian and Rashba-Dirac (RD) type spin-orbit coupling are taken into account (called RD-BZ Hamiltonian in this paper). It is known that the curvature of the ring generates an out-of-plane effective magnetic field, acting as an internal Zeeman field. We find that only the BZ coupling can change the strength of the internal Zeeman field, which enables us to detect the effect of the internal Zeeman field. Furthermore, we find that the total angular momentum is conserved in the RD-BZ Hamiltonian, and the energy eigenvalue and wave function must be modified to fit the conserved quantity, which are ignored in the previous studies. The conductance without leads is discussed. Different from the previous results, we find that the conductance behaves like a beat phenomenon resulting from the interplay between the magnetic flux and Aharonov-Casher (AC) phase, and thus, it can oscillate without passing through the insulating state in some regimes of magnetic flux or AC phase. Importantly, we find that the conductance with integer and half-integer magnetic flux provides us a method to measure the AC phase. The cancellation of the internal Zeeman field due to the BZ coupling can be detected by using specific fractional magnetic flux. In the ring with nonvanishing RD and BZ couplings, the conductance can exhibit a quasiplateau near the small RD coupling. The increase in the strength of BZ coupling would result in wider quasiplateau in conductance, which implies that the ring could remain insulating state (or conducting state) regardless of the small change in RD coupling. The thermal average of spin and charge currents are calculated at low temperatures without impurities. Importantly, we find that the persistent spin and charge currents as a function of the magnetic flux exhibit nodelike lines, which amazingly are perpendicular to each other. As a consequence, the persistent spin current could be nonzero even when the charge current vanishes at nonzero magnetic flux. The result suggests a pure spin current in quantum rings, and its direction can be reversed by changing the magnetic flux. The increase in the BZ coupling would exhibit the plateaulike pure spin current.

We demonstrate that exciting possible realizations of quantum Floquet matter are within reach for modern silicon spin qubits based in quantum dots, most notably the discrete time crystal (DTC). This is significant given that spin qubits have fallen behind other qubit architectures in terms of size and control. However, silicon spin qubits are especially well suited to this task as the charge noise that usually foils gate operations can now be leveraged as an asset in this time-crystal realization. We illustrate differences between prethermal phenomena and true time-crystalline spatiotemporal order. We demonstrate that even for a spin chain of four qubits, rich regime structures can be established by observing signatures of the discrete time crystal and the Floquet symmetry-protected topological regime both distinct from the thermal regime. We also analyze the persistence of these signatures at longer chain lengths, showing that the DTC lifetime grows exponentially with the system length and that these signatures may even be detectable for chains as small as three qubits. We also discuss the effects of longer pulse durations and the effectiveness of pulse sequences for converting the exchange interaction to an Ising model. Our theoretical predictions are well suited for immediate experimental implementations using currently existing quantum dot spin qubit systems.

The central spin model has a wide applicability. It is ideally suited to describe a small quantum system, for instance a quantum bit, in contact to a bath of spins, e.g., nuclear spins or other small quantum systems in general. According to previous work [R. Röhrig et al., Phys. Rev. B 97, 165431 (2018)], a large bath of quantum spins can be described as a bath of quantum harmonic oscillators. But the resulting quantum model is still far from being straightforwardly solvable. Hence we consider a chain representation for the bosonic degrees of freedom to study how well a semi-classical truncated Wigner approximation of the effective model of harmonic oscillators (bTWA) works in comparison with other approximate and exact methods. We find that the bTWA works well for short times, but deviates from the results of other methods for long times. The general message is that the applicability of semi-classical approaches strongly depends on the variables in terms of which the model is formulated. Numerically, we examine the effect of the number of bath spins and of the truncation level, i.e., the chain length.

Curved magnetic architectures are key enablers of prospective magnetic devices with respect to size, functionality, and speed. By exploring geometry-governed magnetic interactions, curvilinear magnetism offers a number of intriguing effects in curved magnetic wires and curved magnetic films. The applicability of the current micromagnetic theory requires that the sample has a constant width and thickness, which does not correspond in many cases to the specificity of experimental sample preparation. Here, we put forth a self-consistent micromagnetic framework of the curvilinear magnetism of nanowires and narrow stripes with a spatially inhomogeneous cross section. The influence of the varying cross section is exploited and illustrated by an example of the simplest topological texture, which is a transversal head-to-head (tail-to-tail) domain wall. The cross-section gradient becomes a source of domain wall pinning which competes with the curvature gradient. Eigenfrequencies of the domain wall free oscillations at the pinning potential are determined by both curvature and cross-section gradients. Prospects for curvilinear magnonics and spintronics are discussed.

A controlled dynamical probe measurement of complex order parameter fluctuations of a system may reveal its massive collective excitations. Here, we design dynamical quench protocols to excite independently all ten midgap Higgs bosons in the isotropic Balian-Werthamer state of a spinful $p$-wave superfluid or superconductor. The analysis is based on microscopic equations of motion of an SO(5) pseudospin, an extension of the usual Bloch equation to a five-dimensional space. Key to these protocols is the realization of quenches that break the rotational symmetry of the kinetic energy and exploit the irreducible representation of the angular momentum $J=2$. For perturbative quenches, we find (nondecaying) periodic oscillations in time of these Higgs modes. We present experimental protocols for superconductors (superfluids), with the intention of unveiling the nature of their order parameters.