We study the strength of the electron-phonon interaction on Fe single adatoms on MgO/Ag(100) based on many-body ab initio spin collinear calculations. In particular, we analyze the relative importance of the substrate and, among other results, we conclude that the interface electron state of Ag(100) plays a prominent role in determining the electron-phonon coupling of localized Fe electron states. The analysis of the hybridization of the adatom with the substrate reveals qualitative differences for even or odd coverages of MgO, affecting significantly the spectral structure and strength of the electron-phonon coupling. Our calculations indicate that the electron-phonon interaction is very strong for $\le$ 1 layers of MgO, while it is sharply suppressed for larger coverages, a trend that is consistent with recent experimental findings.

Dirac points are found to emerge due to the crossing of bands in the electronic structure of so-called sliding bilayer graphene. Group representation theory analysis corroborated with a tight-binding model for the $p_z$ orbitals is employed to demonstrate that the band crossings of energy dispersion curves at generic $\mathbf{k}$ points are guaranteed by the compatibility relations between the symmetries of eigenstates at the high-symmetry $\mathbf{k}$ points in the Brillouin zone. The Lifshitz transition picture and the transport properties of the systems are shown as consequences of the presence of Dirac points in governing the geometrical and topological properties of the Fermi energy surfaces.

We present results of $\mathit{k}·\mathit{p}$ calculations of the effective $g$ factor of holes confined in spherical, cube, and planar semiconductor nanocrystals (NCs). We use the six-band Luttinger model for semiconductors with the zinc-blende crystal structure and study the size dependence of the ${\mathrm{\Gamma }}_8$ top valence subband hole $g$ factor caused by the admixture of the spin-orbit split-off valence subband ${\mathrm{\Gamma }}_7$. We present semianalytical expressions for the hole $g$ factor which depends on the light- to heavy-hole effective mass ratio $\beta$ and on the ratio between spin-orbit energy splitting of valence band ${\mathrm{\Delta }}_{\text{SO}}$ and the hole quantization energy $E_h$. The admixture of ${\mathrm{\Gamma }}_7$ states is significant for small ${\mathrm{\Delta }}_{\text{SO}}/E_h$ and, in spherical and cube NCs, leads to a strong size dependence of the hole $g$ factor. In thin planar nanoplatelets (NPLs) with infinite or large lateral sizes, the dependence of the heavy-hole $g$ factor on NPL thickness is relatively weak. It is drastically enhanced and may become nonmonotonic in NPLs with finite in-plane sizes due to the additional hole states mixing. We discuss our results in comparison with published experimental data for CdSe- and InP-based spherical NCs and NPLs and point out the specificity of extracting hole $g$ factor from the data measured on excitons.

We predict the existence of long-lived and highly anisotropic plasmon modes in a two-dimensional molybdenum oxide dichloride ($\mathrm{MoO}{\mathrm{Cl}}_2$) monolayer. We show that along the $x$ direction (Γ-X), the plasmon modes are immune from the single-particle excitations as $q<0.255{\AA }^{\text{–}1}$ and the maximal energy (up to ∼2.5 eV) reaches the visible region. In the $y$ direction (Γ-Y) the intraband plasmons decay rapidly into electron-hole pairs as $q>0.071{\AA }^{\text{–}1}$, but the interband plasmons are robust in a wide range of $q$ $\left(0.048{\AA }^{\text{–}1}<q<0.5{\AA }^{\text{–}1}\right)$ with the energy varying slowly from 0.82 to 1.20 eV. We correlate these intriguing plasmons to the unique spatial symmetries of the Bloch electron wave functions near the Fermi level, which can sustain highly anisotropic plasmons and suppress the single-particle excitations. In view of the strippability of the $\mathrm{MoO}{\mathrm{Cl}}_2$ monolayer from the already existing bulk material, the directional plasmons in the visible region demonstrated in the $\mathrm{MoO}{\mathrm{Cl}}_2$ monolayer offer a tantalizing platform for nanoplasmonics.

The unoccupied electronic structure of the Re(0001) surface is investigated by spin- and angle-resolved inverse photoemission, experimentally and theoretically. The work is focused on the states around the center of the surface Brillouin zone $\overline{\mathrm{\Gamma }}$, where a hole pocket of a surface state with Rashba-type spin splitting is detected. Furthermore, we observe spin-dependent photon emission from unpolarized surface and bulk states at $\overline{\mathrm{\Gamma }}$. The size and sign of the spin asymmetry depends on experimental parameters such as the direction of the electron spin polarization and the photon detection angle. Maximum (zero) spin asymmetry is detected if the electron spin polarization and the plane of photon emission are perpendicular (parallel). The effect is traced back to spin-orbit-induced hybridization of the involved states.

Chiral magnets, which break both spatial inversion and time reversal symmetries, carry a potential for quadratic optical responses. Despite the possibility of enhanced and controlled responses through the magnetic degree of freedom, the systematic understanding remains yet to be developed. We here study nonlinear optical responses in a prototypical chiral magnetic state with a one-dimensional conical order by using the second-order response theory. We show that the photovoltaic effect and the second harmonic generation are induced by asymmetric modulation of the electronic band structure under the conical magnetic order, and the coefficients, including the sign, change drastically depending on the frequency of incident lights, the external magnetic field, and the strength of spin-charge coupling. We find that both effects can be enormously large compared to those in the conventional nonmagnetic materials. Our results would pave the way for next-generation optical electronic devices, such as unconventional solar cells and optical sensors, based on chiral magnets.

Time-resolved measurements provide a new way to disentangle complex interactions in quantum materials due to their different timescales. We used pump-probe Raman scattering to investigate the apical oxygen vibration in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$ under nonequilibrium conditions. Time dependence of the phonon population demonstrated strong electron-phonon coupling. Most importantly, the phonon shifts to a higher energy due to transient smearing of the Fermi surface in a remarkable agreement with diagrammatic theory. We also discuss insights into photoinduced superconductivity reported at lower doping that follow from these results.

Recent formal classifications of crystalline topological insulators predict that the combination of time-reversal and rotational symmetry gives rise to topological invariants beyond the ones known for other lattice symmetries. Although the classification proves their existence, it does not indicate a way of calculating the values of those invariants. Here we show that a specific set of concentric Wilson loops and line invariants yields the values of all topological invariants in two-dimensional systems with pure rotation symmetry in class AII. Examples of this analysis are given for specific models with twofold and threefold rotational symmetry. We find new invariants that relate to the presence of higher-order topology and corner charges.

Nodal line states in electronic systems can be classified based on the order of dispersion around a band degeneracy (i.e., linear, quadratic, or cubic) and the various slopes of crossing bands (i.e., type I, type II, critical type, and hybrid type). The concept of topology has been extended to phonons, promoting the birth of topological phonons. A series of nodal line phonons have been predicted in solid-state materials both theoretically and experimentally. However, the proposed nodal line phonons have a type-I or type-II linear phonon band dispersion. This study proves, based on first principles, that the hybrid-type linear nodal ring phonons and the higher-order quadratic nodal line (QNL) phonons coexist in one realistic solid-state material—AgZr with P4/nmm structure. Moreover, an effective Hamiltonian characterizing the hybrid nodal ring (HNR) phonons and the QNL phonons was established to explore the relevant physics. The AgZr material can be viewed as an ideal material to study the entanglement between HNR and QNL phonons.

We investigated microwave propagation in the chiral spin soliton lattice (CSL) phase of micrometer-sized crystals of the monoaxial chiral helimagnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$. An advantage of the CSL is that its periodicity can be reconfigured over a macroscopic length scale by means of an external magnetic field. Using a two-antenna microwave spectroscopy technique, we measured the anisotropic response of the transmitted microwaves via the spin dynamics of the CSL. When propagating along the direction parallel to the helical axis, the microwave amplitude increased up to a factor of twenty with decreasing the number of chiral soliton kinks. When the propagation direction was rotated by ${90}^{\circ }$ with regards to the helical axis, the microwave amplitude increased by one order of magnitude upon formation of the chiral helimagnetic order in the vicinity of zero magnetic field, exceeding that of the ferromagnetic phase above the critical field. Our findings open a novel route for controlling the characteristics of microwave propagation using noncollinear spin textures.

Magnetic skyrmionium is a skyrmionlike spin texture with nanoscale size and high mobility. It is a topologically trivial but dynamically stable structure, which can be used as a nonvolatile information carrier for next-generation spintronic storage and computing devices. Here we study the dynamics of a skyrmionium driven by the spin torque in a ferrimagnetic nanotrack. It is found that the direction of motion is jointly determined by the internal configuration of a skyrmionium and the spin polarization vector. Besides, the deformation of a skyrmionium induced by the intrinsic skyrmion Hall effect depends on both the magnitude of the driving force and the net angular momentum. The ferrimagnetic skyrmionium is most robust at the angular momentum compensation point, whose dynamics is quite similar to the skyrmionium in antiferromagnet. The skyrmion Hall effect is perfectly prohibited, where it is possible to observe the position of the skyrmionium by measuring the magnetization. Furthermore, the current-induced dynamics of a ferrimagnetic skyrmionium is compared with that of a ferromagnetic and antiferromagnetic skyrmionium. We also make a comparison between the motion of a ferrimagnetic skyrmionium and a skyrmion. Our results will open a new field of ferrimagnetic skyrmioniums for future development of ferrimagnetic spintronics devices.

We report the evolution of electrical transport properties in insulating FeSe films with electron doping induced by the ionic liquid gating technique. Superconductivity never emerges in the strong insulators with variable-range hopping behavior but is shown to arise once the resistance of the normal state varies as $ln(1/T)$, indicating that this behavior corresponds to the minimal conducting character for developing superconductivity. Our work points toward granular metallicity for the $ln(1/T)$ behavior, suggesting that the emergence of superconductivity requires at least an insulating state containing metallic granules. Moreover, it unravels an electronic segregation in proximity to superconductor-insulator transition, which calls for a comprehensive understanding of this segregated phase.

Using multiple Andreev reflection spectroscopy, we studied superconductor–normal metal–superconductor semiballistic contacts with incoherent transport in overdoped pnictide superconductors ${\mathrm{BaFe}}_{1.86}{\mathrm{Ni}}_{0.14}{\mathrm{As}}_2$ with critical temperature $T_c\approx 12\mathrm{K}$. We directly determined an anisotropic large gap with the characteristic ratios $2{\mathrm{\Delta }}_L^{\mathrm{out}}\left(0\right)/k_B{T}_c\approx 6$ and $2{\mathrm{\Delta }}_L^{\mathrm{in}}\left(0\right)/k_B{T}_c\approx 4$, almost isotropic small gap with $2{\mathrm{\Delta }}_S\left(0\right)/k_B{T}_c\approx 2$, and the temperature dependence of the resolved superconducting order parameters. Above the large gap position $2{\mathrm{\Delta }}_L$, a fine $dI\left(V\right)/dV$ structure fully vanishing at $T_c$ and possibly caused by a resonant coupling with a bosonic mode was observed.

For a ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7/{\mathrm{Nd}}_{0.65}{\left({\mathrm{Ca}}_{0.7}{\mathrm{Sr}}_{0.3}\right)}_{0.35}{\mathrm{MnO}}_3$ (YBCO/NCSMO) superlattice, we studied with resonant elastic x-ray scattering (REXS) at the Cu $L_3$ edge how the copper sublattice charge density wave (Cu-CDW) order in YBCO is affected by a large magnetic field up to 6.9 T that weakens the CE-type antiferromagnetic (AF) and the charge/orbital (Mn-CO) orders of the manganite in favor of a ferromagnetic state. While a field of only 2 T induces a strong ferromagnetic moment in the manganite, we find that the Bragg peak of the Cu-CDW hardly changes up to 6 T. Moreover, as the magnetic field is further increased to 6.9 T, the Cu-CDW Bragg peak gets suddenly enhanced and broadened, whereas the ferromagnetic moment of the manganite is already saturated. The observed uncorrelated magnetic field dependence of the charge orders in the cuprate and manganite layers suggests that these orders are not directly coupled across the interface. We rather interpret our data in terms of an indirect coupling via the domain boundaries of the Mn-CO and the related disorder and lattice strain. This interpretation is supported by additional studies of the magnetoelectric response, which provide evidence for a crossover in the dynamics of the Mn-CO in the range between 6 and 7 T, from a low-field state with pinned domains to a high-field state with more mobile and flexible domain boundaries. We attribute the concomitant enhancement and broadening of the Cu-CDW Bragg peak to this crossover.

The light-induced femtosecond dynamics of the sublattice magnetizations in the antiferromagnetically ordered phase of the semiconductor $\alpha \text{−}\mathrm{MnTe}$ is investigated theoretically as a function of an external driving field. The electromagnetic field is coupled to optical modes and the concomitant atomic displacements modulate the Heisenberg exchange couplings. We derive the equations of motion for the time-dependent sublattice magnetization in spin wave theory and analyze the contributions from the driven magnon modes. The antiferromagnetic order parameter exhibits coherent longitudinal oscillations determined by the external driving frequency which decay due to dephasing. Including a phenomenological dissipative term to mimic spin-lattice relaxation processes leads to relaxation back to thermal equilibrium. We provide approximate analytic solutions of the resulting differential equations which allow us to understand the effect of the driving light pulse on the amplitude, frequency, and lifetime of the coherent spin dynamics.

Long-range entanglement in quantum spin liquids (QSLs) leads to novel low-energy excitations with fractionalized quantum numbers and (in two dimensions) statistics. Experimental detection and manipulation of these excitations present a challenge particularly in view of diverse candidate magnets. A promising probe of fractionalization is their coupling to phonons. Here, we present Raman scattering results for the $S=1/2$ honeycomb iridate ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$, a candidate Kitaev QSL with fractionalized Majorana fermions and Ising flux excitations. We observe anomalous low-temperature frequency shift and linewidth broadening of the Raman intensities in addition to a broad magnetic continuum, both of which, as we derive, are naturally attributed to the phonon decaying into itinerant Majoranas. The dynamic Raman susceptibility marks a crossover from the QSL to a thermal paramagnet at $\sim 120$ K. The phonon anomalies below this temperature demonstrate a strong phonon-Majorana coupling. These results provide evidence of spin fractionalization in ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$.

Time series measurements [G. Shaw et al., Phys. Rev. B 85, 174517 (2012); B. Bag et al., Sci. Rep. 7, 5531 (2017)] in $2H\text{−}{\mathrm{NbS}}_2$ crystal had unraveled a drive-induced transition wherein the critical current $\left(I_c\right)$ changes from a low to a high $I_c$ jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current-voltage $\left(I\text{−}V\right)$ measurement cycles, we explore the statistical nature of observing the NDR (or a quasi-NDR in reversing $I$ measurements) transition in the free-flux-flow (FF) regime in a single crystal of $2H\text{−}{\mathrm{NbS}}_2$. Prior to the occurrence of the NDR transition, the pristine full $I\text{−}V$ curve exhibits a featureless smooth depinning from the low $I_c$ state. With subsequent current cycling, the NDR transition appears in the $I\text{−}V$ curve. Post-NDR, the full $I\text{−}V$ curve is seen to be noisy with depinning commencing from the higher $I_c$ state. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field, $\dot{B}$. The probability of observing the NDR transition in the FF regime is found to systematically increase with magnetic field $\left(B\right)$ in weak collective pinning regime. In the strong pinning regime, the said probability becomes field independent. Retaining of a nonzero probability for the occurrence of the NDR transition under all conditions, the observed new data shows that the $I\text{−}V$ branch with higher $I_c$ is the more stable compared to the lower $I_c$ branch. We show that the higher $I_c$ state, generated via the NDR transition, is unique and cannot be accessed via any conventional route, in particular, by preparing the static vortex state with a different thermomagnetic history. While the $I\text{−}V$ curves do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes of preparing the vortex state, the probability for observing an NDR transition has different $B$ dependences for the vortex matter prepared in the ZFC and FC modes. We find that the NDR transition occurs in a high dissipation regime, where the flow resistivity is well above the theoretical value expected in the FF regime. We understand our results on the basis of a rapid drop in vortex viscosity at high drives in $2H\text{−}{\mathrm{NbS}}_2$, which triggers a rapid increase in the vortex velocity and reorganization in the moving vortex matter leading to a dynamical unstable vortex flow. This dynamical instability leads to the NDR transition into a high entropy vortex state with high $I_c$.

Topological characterization of non-Hermitian band structures demands more than a straightforward generalization of the Hermitian cases. Even for one-dimensional tight-binding models with nonreciprocal hopping, the appearance of point gaps and the skin effect leads to the breakdown of the usual bulk-boundary correspondence. Luckily, the correspondence can be resurrected by introducing a winding number for the generalized Brillouin zone for systems with an even number of bands and chiral symmetry. Here, we analyze the topological phases of a nonreciprocal hopping model on the stub lattice, where one of the three bands remains flat. Due to the lack of chiral symmetry, the biorthogonal Zak phase is no longer quantized, invalidating the winding number as a topological index. Instead, we show that a $Z_2$ invariant can be defined from Majorana's stellar representation of the eigenstates on the Bloch sphere. The parity of the total azimuthal winding of the entire Majorana constellation correctly predicts the appearance of edge states between the bulk gaps. We further show that the system is not a square-root topological insulator, despite the fact that its parent Hamiltonian can be block diagonalized and related to a sawtooth lattice model. The analysis presented here may be generalized to understand other non-Hermitian systems with multiple bands.