We report a combined experimental and theoretical x-ray magnetic circular dichroism (XMCD) spectroscopy study at the Ir-$L_{2,3}$ edges on the ${\mathrm{Ir}}^{5+}$ ions of the layered hybrid solid state oxide ${\mathrm{Sr}}_2{\mathrm{Co}}_{0.5}{\mathrm{Ir}}_{0.5}{\mathrm{O}}_4$ with the ${\mathrm{K}}_2{\mathrm{NiF}}_4$ structure. From theoretical simulation of the experimental Ir-$L_{2,3}$ XMCD spectrum, we found a deviation from a pure $J_{\mathrm{eff}}=0$ ground state with an anisotropic orbital-to-spin moment ratio ($L_x/2S_x=0.43$ and $L_z/2S_z=0.78$). This deviation is mainly due to multiplet interactions being not small compared to the cubic crystal field and due to the presence of a large tetragonal crystal field associated with the crystal structure. Nevertheless, our calculations show that the energy gap between the singlet ground state and the triplet excited state is still large and that the magnetic properties of the ${\mathrm{Ir}}^{5+}$ ions can be well described in terms of singlet Van Vleck paramagnetism.

Magnetic excitations in the multiferroic ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$ are measured by combination of unpolarized and polarized neutron-scattering techniques. The unpolarized neutron spectrum reveals an anisotropic $\mathbf{Q}$ dependence of the intensity of the electromagnon mode at 4 meV. The analysis of the polarized neutron spectra identifies the directions of the magnetic fluctuations of the observed modes. The determined modes are consistent with the calculation by the extended spin-wave theory. A couple of transverse modes and a single longitudinal mode are involved in the magnetic excitation of ${\mathrm{Ba}}_2{\mathrm{CoGe}}_2{\mathrm{O}}_7$. The longitudinal fluctuation takes on a main role in the electromagnon mode.

We present a comprehensive study of the magnetization dynamics and phase evolution in the chiral helimagnet ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$, which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for the first five harmonic components, $M_{\mathrm{n}\omega }\left(H\right)\left(n=1–5\right)$, using a phase sensitive measurement over a frequency range, $f=11–10000\mathrm{Hz}$. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains, which relax on macroscopic time scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. We experimentally resolve the tricritical point at $T_{\mathrm{TCP}}$ in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$.

A zero-temperature magnetic-field-driven superconductor to insulator transition (SIT) in quasi-two-dimensional superconductors is expected to occur when the applied magnetic field crosses a certain critical value [S. L. Sondhi, S. M. Girvin, J. P. Carini, and D. Shahar, Rev. Mod. Phys. 69, 315 (1997); A. M. Goldman, Int. J. Mod. Phys. B 24, 4081 (2010)]. A fundamental question is whether this transition is due to the localization of Cooper pairs or due to the destruction of them. Here we address this question by studying the SIT in amorphous WSi. Transport measurements reveal the localization of Cooper pairs at a quantum critical field $B_c^1$ (Bose insulator), with a product of the correlation length and dynamical exponents $z\nu \sim 4/3$ near the quantum critical point. Beyond $B_c^1$, superconducting fluctuations still persist at finite temperatures. Above a second critical field $B_c^2>B_c^1$, the Cooper pairs are destroyed and the film becomes a Fermi insulator. The different phases all merge at a tricritical point at finite temperatures with $z\nu =2/3$. Our results suggest a sequential superconductor to Bose insulator to Fermi insulator phase transition, which differs from the conventional scenario involving a single quantum critical point.

We introduce a stochastic excitation method for calculating the dispersion relation for waves propagating in periodic media or along waveguides and subject to material loss or radiation damping. Instead of looking for an explicit or implicit functional relation between frequency $\omega$ and wave number $k$, as is usually done, we consider a mapping of the resolvent set in the dispersion space $(\omega ,k)$. Bands appear as the trace of Lorentzian responses containing local information on propagation loss in both time and space domains. For illustration purposes, the method is applied to a lossy sonic crystal, a radiating surface phononic crystal, and a radiating optical waveguide. The resolvent band structure can be obtained for any system described by a time-harmonic wave equation.

Magnetization ($\mathit{M}$) precessions driven by ultrafast laser-induced nonthermal effects are observed in undoped yttrium iron garnet (YIG) films of (100) and (111) orientations using pump-probe time-resolved magneto-optical Kerr effect. The $\mathit{M}$ precessions show a strong dependence on the polarization direction of linearly polarized pump pulses of 400 nm. In contrast, we can barely observe any $\mathit{M}$ precession using circularly polarized pump pulses, which indicates that the inverse Faraday effect is negligible. For the case of linear pump polarization, a phenomenological model is introduced, based on the modulation of $\mathit{M}$ via a modulation of fourth-rank susceptibility tensors by a laser pulse. This allows one to distinguish the contributions of the inverse Cotton-Mouton effect (ICME) from those of the photoinduced magnetic anisotropy (PMA). Using the formula derived from the phenomenological model, we perform the fitting of the polarization-direction-dependent precession phase and amplitude in (100)- and (111)-oriented YIG films. The fitting results reveal that the $\mathit{M}$-precession excitation originates from a combination effect of ICME and PMA, but the ICME plays the dominant role.

Motivated by the success of experimental manipulation of the band structure through biaxial strain in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ thin film grown on a mismatched substrate, we investigate theoretically the effects of biaxial strain on the electronic instabilities, such as superconductivity (SC) and spin-density wave (SDW), by functional renormalization group. According to the experiment, the positive strain (from lattice expansion) causes charge transfer to the $\gamma$-band and consequently Lifshitz reconstruction of the Fermi surface. Our theoretical calculations show that within a limited range of positive strain, a $p$-wave superconducting order is realized with enhanced transition temperature. However, as the strain is increased further, the system develops into the SDW state well before the Lifshitz transition is reached. We also consider the effect of negative strains (from lattice constriction). As the strain increases, there is a transition from $p$-wave SC state to nodal $s$-wave SC state. The theoretical results are discussed in comparison to experiment and can be checked by further experiments. Since the essential role of the (positive) biaxial strain is to tune the system toward Van Hove filling, we predict that the superconducting transition temperature may also rise rapidly if the thin film is on a fixed substrate but slightly gated toward higher filling.

Motivated by the prediction of surface Kondo breakdown in topological Kondo insulators, we investigated the valence and magnetism of ${\mathrm{SmB}}_6$ with x-ray absorption and magnetic circular dichroism at the Sm $M_{4,5}$ and $L_2$ edges with surface and bulk sensitivity, respectively. A higher Sm valence state at the surface and in the bulk under pressure indicates the surface- and pressure-induced bulk Kondo breakdown. We confirmed the different magnetization origin from the surface ($f$ electron) to the bulk ($d$ electron) and ascertained the direct relationship between the disappearance of the Sm $L_2$-edge x-ray magnetic circular dichroism signal and the closure of the $d-f$ hybridization gap in ${\mathrm{SmB}}_6$ bulk under pressure, paving the way for further investigations of the Kondo insulators. Above $P_c=8.5$ GPa, a more localized $4f$ state with higher valence and the disappearing of the $5d$ magnetism are observed, suggesting the closure of the hybridization gap in the bulk.

We present a detailed quantum oscillatory study on the Dirac type-II semimetallic candidates ${\mathrm{PdTe}}_2$ and ${\mathrm{PtTe}}_2$ via the temperature and the angular dependence of the de Haas–van Alphen and Shubnikov–de Haas effects. In high-quality single crystals of both compounds, i.e., displaying carrier mobilities between ${10}^3$ and ${10}^4{\mathrm{cm}}^2$/Vs, we observed a large nonsaturating magnetoresistivity which in ${\mathrm{PtTe}}_2$ at a temperature $T=1.3$ K leads to an increase in the resistivity up to ($5\times {10}^4$)% under a magnetic field ${\mu }_{0}H=62$ T. These high mobilities correlate with their light effective masses in the range of 0.04 to 1 bare electron mass according to our measurements. For ${\mathrm{PdTe}}_2$ the experimentally determined Fermi surface cross-sectional areas show excellent agreement with those resulting from band structure calculations. Surprisingly, this is not the case for ${\mathrm{PtTe}}_2$, whose agreement between calculations and experiments is relatively poor even when electronic correlations are included in the calculations. Therefore, our study provides strong support for the existence of a Dirac type-II node in ${\mathrm{PdTe}}_2$ and probably also for ${\mathrm{PtTe}}_2$. Band structure calculations indicate that the topologically nontrivial bands of ${\mathrm{PtTe}}_2$ do not cross the Fermi level ${\varepsilon }_F$. In contrast, for ${\mathrm{PdTe}}_2$ the Dirac type-II cone does intersect ${\varepsilon }_F$, although our calculations also indicate that the associated cyclotron orbit on the Fermi surface is located in a distinct $k_z$ plane with respect to that of the Dirac type-II node. Therefore, it should yield a trivial Berry phase.

We develop variational matrix product state (MPS) methods with symmetries to determine dispersion relations of one-dimensional quantum lattices as a function of momentum and preset quantum number. We test our methods on the XXZ spin chain, the Hubbard model, and a nonintegrable extended Hubbard model and determine the excitation spectra with a precision similar to the one of the ground state. The formulation in terms of quantum numbers makes the topological nature of spinons and holons very explicit. In addition, the method also enables an easy and efficient direct calculation of the necessary magnetic field or chemical potential required for a certain ground state magnetization or particle density.

A detailed investigation of the charge-ordered charge-transfer salt $\delta -{(\mathrm{EDT}-\mathrm{TTF}-{\mathrm{CONMe}}_2)}_2\mathrm{Br}$ by thermal-expansion measurements and dielectric spectroscopy reveals three dynamic processes of relaxational character. The slowest one exhibits the characteristics of glassy freezing and is ascribed to the conformational dynamics of terminal ethylene groups of the organic molecules. Such a process was previously found for related charge-transfer salts where, however, the anions form polymerlike chains, in contrast to the spherical anions of the present material. Dielectric spectroscopy reveals two additional relaxational processes. The characteristics of the faster one are consistent with excitations of a one-dimensional Wigner lattice as recently observed in this material by infrared spectroscopy, which are also accompanied by conformational changes of the molecules. However, at low temperature the ethylene-group relaxation exhibits the cooperativity-induced dramatic slowing down that is typical for glassy freezing, while the defect-related Wigner-lattice excitation follows thermally activated behavior as expected for single-dipole relaxations.

By means of dynamical mean-field theory allowing for complete account of SU(2) rotational symmetry of interactions between spin-1/2 particles, we observe a strong effect of suppression of ferromagnetic order in the multiorbital Fermi-Hubbard model in comparison with a widely used restriction to density-density interactions. In the case of orbital degeneracy, we show that the suppression effect is the strongest in the two-orbital model (with effective spin $S_{\mathrm{eff}}=1$) and significantly decreases when considering three orbitals ($S_{\mathrm{eff}}=3/2$), thus magnetic ordering can effectively revive for the same range of parameters, in agreement with arguments based on vanishing of quantum fluctuations in the limit of classical spins ($S_{\mathrm{eff}}\to \infty$). We analyze a connection to the double-exchange model and observe high importance of spin-flip processes there as well.

We examine how, in the presence of an external magnetic field, the Goos-Hänchen shift for terahertz radiation reflected, at oblique incidence, off an antiferromagnet can show significantly enhanced values under the right circumstances. Such enhanced shifts, which have associated absorptions in the presence of damping, occur in one of the magnon reststrahl regions and are related to the excitation of surface resonances, which can be considered as extensions of surface polariton modes into the reflectivity regime. Although the enhancement of the Goos-Hänchen shift due to surface modes is well known in the attenuated total reflection configuration using a three-layer prism geometry, such effects have not previously been reported in direct reflection and only occur here due to the presence of the externally applied field. We confirm the effect using simulations for reflection off ${\mathrm{MnF}}_2$ at low temperature and show that only a small applied field is necessary to induce a significant enhancement along with an associated sharp absorption. For a larger field, the effects are less distinct.

We study a three-level quantum dot in the singly occupied cotunneling regime coupled via a generic tunneling matrix to several multichannel leads in equilibrium or nonequilibrium. Denoting the three possible states of the quantum dot by the quark flavors up ($u$), down ($d$), and strange ($s$), we derive an effective model where also each reservoir has three flavors labeled by $u, d$, and $s$ with an effective density of states polarized with respect to an eight-dimensional $F$ spin corresponding to the eight generators of $\text{SU}\left(3\right)$. In equilibrium we perform a standard poor man's scaling analysis and show that tunneling via virtual intermediate states induces flavor fluctuations on the dot which become $\text{SU}\left(3\right)$ symmetric at a characteristic and exponentially small low-energy scale $T_K$. Close to $T_K$ the system is described by a single isotropic Kondo coupling $J>0$ diverging at $T_K$. Using the numerical renormalization group, we study in detail the linear conductance and confirm the $\text{SU}\left(3\right)$-symmetric Kondo fixed point with universal conductance $G=3{sin}^2(\pi /3)\frac{e^2}{h}=2.25\frac{e^2}{h}$ for various tunneling setups by tuning the level spacings on the dot. We also identify regions of the level positions where the $\text{SU}\left(2\right)$ Kondo fixed point is obtained and find a rather complex dependence of the various Kondo temperatures as function of the gate voltage and the tunneling couplings. In contrast to the equilibrium case, we find in nonequilibrium that the fixed-point model is not $\text{SU}\left(3\right)$ symmetric but characterized by rotated $F$ spins for each reservoir with total vanishing sum. At large voltage we analyze the $F$-spin magnetization and the current in Fermi's golden rule as function of a longitudinal ($h_z$) and perpendicular ($h_{\perp }$) magnetic field for the isospin and the level spacing $\mathrm{\Delta }$ to the strange quark. As a smoking gun to detect the nonequilibrium fixed point we find that the curve of zero $F$-spin magnetization in $(h_z,h_{\perp },\mathrm{\Delta })$ space is a circle when projected onto the $(h_z,h_{\perp })$ plane. We propose that our findings can be generalized to the case of quantum dots with an arbitrary number $N$ of levels.

Electronic-vibrational coupling in single-molecule junctions may result in current-induced bond rupture and is thus an important mechanism for the stability of molecular junctions. We use the hierarchical quantum master equation method in combination with the quasiclassical Ehrenfest approach for the nuclear degrees of freedom to simulate current-induced bond rupture in single-molecule junctions. Employing generic models for molecular junctions with dissociative nuclear potentials, we analyze the underlying mechanisms. In particular, we investigate the dependence of the dissociation probability on the applied bias voltage and the molecule-lead coupling strength. The results show that an applied bias voltage can not only lead to dissociation of the molecular junction, but under certain conditions can also increase the stability of the molecule.

The pairing symmetry of the Hubbard Hamiltonian on a triangle lattice with a nearly flat low-energy band is studied with the determinant quantum Monte Carlo method (DQMC). We show that the low-temperature phase is insulating at half-filling, even for relatively weak interactions. The natures of the spin and pairing correlations upon doping are determined, and they exhibit an electron-hole asymmetry. Among the pairing symmetries allowed, we demonstrate that the dominating channels are $d$ wave, opening the possibility of condensation into an unconventional $d_{x^2-y^2}+i{d}_{xy}$ phase, which is characterized by an integer topological invariant and gapless edge states. The results are closely related to the correlated insulating phase and unconventional superconductivity discovered recently in twisted bilayer graphene.

Using first-principles calculations, we investigate the photogalvanic effect in the Weyl semimetal material TaAs. We find colossal photocurrents caused by the Weyl points in the band structure in a wide range of laser frequency. Our calculations reveal that the photocurrent is predominantly contributed by the three-band transition from the occupied Weyl band to the empty Weyl band via an intermediate band away from the Weyl cone, for excitations both by linearly and circularly polarized light. Therefore, it is essential to sum over all three-band transitions by considering a full set of Bloch bands (both Weyl bands and trivial bands) in the first-principles band structure while it does not suffice to only consider the two-band direct transition within a Weyl cone. The calculated photoconductivities are well consistent with recent experiment measurements. Our work provides a first-principles calculation on nonlinear optical phenomena of Weyl semimetals and provides a deeper understanding of the photogalvanic effects in complexed materials.

In the present Rapid Communication, we provide an easily accessible way to achieve the singlet-triplet Kondo effect in a hybrid system consisting of a quantum point contact (QPC) coupled to an electronic cavity. We show that by activating the coupling between the QPC and cavity, a zero-bias anomaly occurs in a low conductance regime, a coexistence of a zero-bias and finite-bias anomaly (FBA) in a medium conductance regime, and a FBA-only anomaly in a high conductance regime. The latter two observations are due to the singlet-triplet Kondo effect.

Owing to its large electric band gap, two-dimensional (2D) hexagonal boron nitride (hBN) is a very promising candidate for making atomically thin flexible devices. Further, its high mechanical strength makes it suitable as a reinforcing material in composites. It is thus of great interest to study the strain-induced phonon shift of mechanically exfoliated hBN of various thicknesses under tension. It is found that splitting of the $E_{2g}$ phonon to two subpeaks due to the lifting of degeneracy under uniaxial loading occurs at low strain level with average shift rates of $-8.4\left(1.0\right)\mathrm{c}{\mathrm{m}}^{-1}/\%$ and $-25.2\left(1.5\right)\mathrm{c}{\mathrm{m}}^{-1}/\%$ for the $E_{2g}^{+}$ and $E_{2g}^{-}$, respectively. These large redshifts of the $E_{2g}$ modes with strain are close to the values obtained from graphene and confirm the high stiffness of 2D hBN. Furthermore, the measured $E_{2g}$ mode Grüneisen parameter is about 1.88 (2), in agreement with the values obtained by classical molecular dynamics (MD) and ab initio MD simulations. The shift rates are similar for hBN with thickness of two to four layers, which is in contrast with graphene, indicating the effective load transfer between the hBN layers. The present results provide useful insights for the use of hBN in electronic devices, 2D heterostructures, and also as a reinforcing agent in composite materials.