In analogy to various fermions of electrons in topological semimetals, topological mechanical states with two types of bosons, Dirac and Weyl bosons, were reported in some macroscopic systems of kHz frequency, and those with a type of doubly-Weyl phonons in atomic vibrational framework of THz frequency of solid crystals were recently predicted. Here, through first-principles calculations, we have reported that the phonon spectra of the WC-type TiS, ZrSe, and HfTe commonly host the unique triply degenerate nodal points (TDNPs) and single two-component Weyl points (WPs) in THz frequency. Quasiparticle excitations near TDNPs of phonons are three-component bosons, beyond the conventional and known classifications of Dirac, Weyl, and doubly-Weyl phonons. Moreover, we have found that both TiS and ZrSe have five pairs of type-I Weyl phonons and a pair of type-II Weyl phonons, whereas HfTe only has four pairs of type-I Weyl phonons. They carry nonzero topological charges. On the ($10\overline{1}0$) crystal surfaces, we observe topological protected surface arc states connecting two WPs with opposite charges, which host modes that propagate nearly in one direction on the surface.

We present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active $A_{1g}$ and $B_{1g}$ phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20 K of all observed phonon energies is governed by the lattice contraction. Below 20 K the phonon energies increase by 0.5–1 ${\mathrm{cm}}^{-1}$, thus indicating putative short range magnetic order. Along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.

Spin-wave resonances in pelletized powder of bismuth orthoferrite (${\mathrm{BiFeO}}_3$) were studied in transmission spectroscopy at frequencies of 0.1–0.75 THz and in the temperature range of 300–650 K. A vector network analyzer with frequency extenders was used in conjunction with temperature scans. We extended the experimental coverage of spin-wave resonances in ${\mathrm{BiFeO}}_3$ up to almost the Néel temperature. We observed that ${\mathrm{\Psi }}_1^{\left(1\right)}$ and ${\mathrm{\Psi }}_1^{\left(2\right)}$ modes are degenerate around 420 K due to the temperature dependences of the magnetic anisotropy and the Dzyaloshinskii-Moriya interaction.

We haved studied the ground state of the classical kagome antiferromagnet ${\mathrm{NaBa}}_2{\mathrm{Mn}}_3{\mathrm{F}}_{11}$. Strong magnetic Bragg peaks observed for $d$ spacings shorter than 6.0 Å were indexed by the propagation vector of ${\mathit{k}}_0=(0,0,0)$. Additional peaks with weak intensities in the $d$-spacing range above 8.0 Å were indexed by the incommensurate vector of ${\mathit{k}}_1=[0.3209\left(2\right),0.3209\left(2\right),0]$ and ${\mathit{k}}_2=[0.3338\left(4\right),0.3338\left(4\right),0]$. Magnetic structure analysis unveils a ${120}^{\circ }$ structure with the tail-chase geometry having ${\mathit{k}}_0$ modulated by the incommensurate vector. A classical calculation of the Heisenberg kagome antiferromagnet with antiferromagnetic second-neighbor interaction, for which the ground state a ${\mathit{k}}_0{120}^{\circ }$ degenerated structure, reveals that the magnetic dipole-dipole (MDD) interaction including up to the fourth neighbor terms selects the tail-chase structure. The observed modulation of the tail-chase structure is attributed to a small perturbation such as the long-range MDD interaction or the interlayer interaction.

We ask whether a local Hamiltonian with a featureless (fully gapped and nondegenerate) ground state could exist in certain quantum spin systems. We address this question by mapping the vicinity of certain quantum critical point (or gapless phase) of the $d$-dimensional spin system under study to the boundary of a $(d+1)$-dimensional bulk state, and the lattice symmetry of the spin system acts as an onsite symmetry in the field theory that describes both the selected critical point of the spin system and the corresponding boundary state of the $(d+1)$-dimensional bulk. If the symmetry action of the field theory is nonanomalous, i.e., the corresponding bulk state is a trivial state instead of a bosonic symmetry-protected topological (SPT) state, then a featureless ground state of the spin system is allowed; if the corresponding bulk state is indeed a nontrivial SPT state, then it likely excludes the existence of a featureless ground state of the spin system. From this perspective, we identify the spin systems with $\mathrm{SU}(N$) and $\mathrm{SO}(N$) symmetries on one-, two-, and three-dimensional lattices that permit a featureless ground state. We also verify our conclusions by other methods, including an explicit construction of these featureless spin states.

We investigate time-resolved terahertz conductivity of thin superconducting NbN films with various thicknesses upon their excitation by intense femtosecond laser pulses. The recovery dynamics following a complete destruction of the superconducting state occurs via a growth of superconducting islands in the normal-state environment. This is in contrast with previous observations of the recovery upon strong-field terahertz excitation [R. Matsunaga and R. Shimano, Phys. Rev. Lett. 109, 187002 (2012)]. We observe that the density of electronic states in the superconducting islands deviates from the BCS theory predictions on a subnanosecond time scale, while equilibrium terahertz conductivity spectra confirm the standard BCS behavior in the ground state.

We study the dynamics of a waveguide made of coupled resonators with a sinusoidal modulation of the resonance frequencies. We present a modulation scheme that achieves complete dynamic localization and is experimentally suitable for optical cavities. Furthermore, we highlight the importance of the way the modulation is turned on and off. One striking consequence is that, although a state returns to its starting amplitude at the end of every cycle, it can also be fully time-reversed if the modulation is turned off midcycle. Finally, we show that localization is always achieved when the modulation envelope is adiabatic with respect to the oscillation frequency. The results are experimentally feasible using existing integrated photonic technologies and are relevant to applications, such as tunable delay lines, dispersion compensation, and imaging.

Improving matrix-product state techniques based on the purification of the density matrix, we are able to accurately calculate the finite-temperature dynamic response of the infinite spin-1 $\text{XXZ}$ chain with single-ion anisotropy in the Haldane, large-$D$, and antiferromagnetic phases. Distinct thermally activated scattering processes make a significant contribution to the spectral weight in all cases. In the Haldane phase, intraband magnon scattering is prominent, and the on-site anisotropy causes the magnon to split into singlet and doublet branches. In the large-$D$ phase response, the intraband signal is separated from an exciton-antiexciton continuum. In the antiferromagnetic phase, holons are the lowest-lying excitations, with a gap that closes at the transition to the Haldane state. At finite temperatures, scattering between domain-wall excitations becomes especially important and strongly enhances the spectral weight for momentum transfer $\pi$.

A high-amplitude microwave magnetic field localized at the nanoscale is a desirable tool for various applications within the rapidly developing field of nanomagnetism. Here we drive magnetization precession by coherent phonons in a metal ferromagnetic nanograting and generate ac-magnetic induction with extremely high amplitude (up to 10 mT) and nanometer scale localization in the grating grooves. We trigger the magnetization precession by a laser pulse which excites localized surface acoustic waves. The developed technique has prospective uses in several areas of research and technology, including spatially resolved access to spin states for quantum technologies.

Ferroelectric-ferromagnetic multiferroic composites are of great interest both from the scientific and technological standpoints. The extent of coupling between polarization and magnetization in such two-phase systems depends on how efficiently the magnetostrictive and electrostrictive/piezoelectric strain gets transferred from one phase to the other. This challenge is most profound in the easy to make 0-3 ferroelectric-ferromagnetic particulate composites. Here we report a self-grown ferroelectric-ferromagnetic 0-3 particulate composite through controlled spontaneous precipitation of ferrimagnetic barium hexaferrite phase ($\mathrm{BaF}{\mathrm{e}}_{12}{\mathrm{O}}_{19}$) amid ferroelectric grains in the multiferroic alloy system $\mathrm{BiFe}{\mathrm{O}}_3-\mathrm{BaTi}{\mathrm{O}}_3$. We demonstrate that a composite specimen exhibiting merely ∼1% hexaferrite phase exhibits ∼34% increase in saturation polarization in a dc magnetic field of ∼10 kOe. Using modified Rayleigh analysis of the polarization field loop in the subcoercive field region we argue that the substantial enhancement in the ferroelectric switching is associated with the reduction in the barrier heights of the pinning centers of the ferroelectric-ferroelastic domain walls in the stress field generated by magnetostriction in the hexaferrite grains when the magnetic field is turned on. Our study proves that controlled precipitation of the magnetic phase is a good strategy for synthesis of 0-3 ferroelectric-ferromagnetic particulate multiferroic composite as it not only helps in ensuring a good electrical insulating character of the composite, enabling it to sustain high enough electric field for ferroelectric switching, but also the factors associated with the spontaneity of the precipitation process ensure efficient transfer of the magnetostrictive strain/stress to the surrounding ferroelectric matrix making domain wall motion easy.

With increasing temperature, metallic glasses (MGs) undergo first glass transition without pronounced structural change and then crystallization with distinct variation in structure and properties. The present study shows a structural change of short-range order induced by an electron-delocalization transition, along with an unusual large-volume shrinkage in Ce-based MGs. An $f$-electron localization-delocalization transition with thermal hysteresis is observed from the temperature dependence of x-ray absorption spectroscopy and resonant inelastic x-ray scattering spectra, indicating an inheritance of the $4f$ configuration of pure Ce. However, the delocalization transition becomes broadened due to the local structural heterogeneity and related fluctuation of $4f$ levels in the Ce-based MGs. The amorphous structure regulated $4f$ delocalization of Ce leads to bond shortening and abnormal structure change of the topological and chemical short-range orders. Due to the hierarchical bonding nature, the structure should change in a similar manner on different length scales (but not isostructurally like the Ce metal) in Ce-based MGs.

We introduce a local order metric (LOM) that measures the degree of order in the neighborhood of an atomic or molecular site in a condensed medium. The LOM maximizes the overlap between the spatial distribution of sites belonging to that neighborhood and the corresponding distribution in a suitable reference system. The LOM takes a value tending to zero for completely disordered environments and tending to one for environments that perfectly match the reference. The site-averaged LOM and its standard deviation define two scalar order parameters, $S$ and $\delta S$, that characterize with excellent resolution crystals, liquids, and amorphous materials. We show with molecular dynamics simulations that $S$, $\delta S$, and the LOM provide very insightful information in the study of structural transformations, such as those occurring when ice spontaneously nucleates from supercooled water or when a supercooled water sample becomes amorphous upon progressive cooling.

$\mathrm{MnC}{\mathrm{r}}_2{\mathrm{O}}_4$ that exhibits spin frustration and complex spiral spin order is of great interest from both fundamental as well as application-oriented perspectives. Unlike $\mathrm{CoC}{\mathrm{r}}_2{\mathrm{O}}_4$, whose ground state presents the coexistence of commensurate spiral spin order (CSSO) and ferroelectric order, $\mathrm{MnC}{\mathrm{r}}_2{\mathrm{O}}_4$ shows no multiferroicity. One reason is that the spiral spin order is highly sensitive to the oxygen concentration in $\mathrm{MnC}{\mathrm{r}}_2{\mathrm{O}}_4$. Here, we have successfully grown high-quality single-crystalline $\mathrm{MnC}{\mathrm{r}}_2{\mathrm{O}}_4$ by the chemical vapor transport method. We observe a first-order magnetic transition from the incommensurate spiral spin order (ICSSO) at 19.4 K to the CSSO at 17.4 K. This magnetic transition is verified by magnetization, specific heat, and magnetoelectric measurements, which also confirm that the ground state exhibits the coexistence of the CSSO and magnetoelectricity below 17.4 K. Interestingly, the temperature evolution of Raman spectra between 5.4 and 300 K suggests that the structure remains the same. We also find that the phase-transition temperature of the CSSO decreases as applied magnetic field increases up to 45 kOe.

We analyze the high-temperature spin dynamics of quantum compass models by a moment expansion method. We point out that the evaluation of moments maps to the enumeration of paths in a branching process on the lattice. This mapping to a statistical mechanics combinatorics problem provides an elegant visualization of this analysis. We present results for the time-dependent spin autocorrelation function (which is of relevance to NMR experiments) for two compass models: the honeycomb (Kitaev) and two-dimensional compass lattice model. Furthermore, following this approach, we argue that in quantum compass models the spin correlations are generically strongly anisotropic and short ranged.

The magnetic susceptibility $\chi \left(T\right)$ of spin-$\frac{1}{2}$ chains is widely used to quantify exchange interactions, even though $\chi \left(T\right)$ is similar for different combinations of ferromagnetic $J_1$ between first neighbors and antiferromagnetic $J_2$ between second neighbors. We point out that the spin-specific heat $C\left(T\right)$ directly determines the ratio $\alpha =J_2/\left|J_1\right|$ of competing interactions. The $J_1-J_2$ model is used to fit the isothermal magnetization $M(T,H)$ and $C(T,H)$ of spin-$\frac{1}{2}$ Cu(II) chains in ${\mathrm{LiCuSbO}}_4$. By fixing $\alpha ,C\left(T\right)$ resolves the offsetting $J_1,\alpha$ combinations obtained from $M(T,H)$ in cuprates with frustrated spin chains.

Spin-orbit coupling (SOC) is found to be crucial for understanding the magnetic and electronic properties of $5d$ transition metal oxides. In $5d$ systems, with ${\mathrm{Ir}}^{5+}$ ions, where ideally a nonmagnetic $J=0$ ground state is expected to be stabilized in the presence of strong SOC, often spontaneous moments are generated due to hopping induced superexchange. This effect is more pronounced when the Ir atoms are close by, as in systems with ${\mathrm{Ir}}_2{\mathrm{O}}_9$ dimers in $6H{\mathrm{Ba}}_{3}M{\mathrm{Ir}}_2{\mathrm{O}}_9$ compounds where magnetism is an outcome of complex Ir-O-Ir exchange paths, and is strongly influenced by the presence of local distortions. We find that subtle variations in the local structure of ${\mathrm{Ba}}_{3}M{\mathrm{Ir}}_2{\mathrm{O}}_9$ ($M$ = Mg, Sr, and Ca) lead to markedly different magnetic properties. While SOC plays a pivotal role in explaining the insulating ground states of these systems, it is seen that ${\mathrm{Ba}}_3{\mathrm{MgIr}}_2{\mathrm{O}}_9$, having a $P{6}_3/mmc$ symmetry, does not order down to low temperature despite having antiferromagnetic exchange interactions, while ${\mathrm{Ba}}_3{\mathrm{CaIr}}_2{\mathrm{O}}_9$ shows weak dimer-like features and stabilizes in $C2/c^{\prime }$ magnetic configuration with no net moment, and ${\mathrm{Ba}}_3{\mathrm{SrIr}}_2{\mathrm{O}}_9$ possesses a ground state corresponding to the magnetic space group $C{2}^{\prime }/c^{\prime }$ and exhibits ferromagnet-like features.

In a recent experiment [P. Roushan et al., Nat. Phys. 13, 146 (2017)], a spin current in an architecture of three superconducting qubits was produced during a few microseconds by creating synthetic magnetic fields. The lifetime of the current was set by the typical dissipative mechanisms that occur in those systems. We propose a scheme for the generation of permanent currents, even in the presence of such imperfections, and scalable to larger system sizes. It relies on striking a subtle balance between multiple nonequilibrium drives and the dissipation mechanisms, in order to engineer and stimulate chiral excited states which can carry current.

Recently the interest in thermal counterflow of superfluid ${}^4\mathrm{He}$, the most extensively studied form of quantum turbulence, has been renewed. Particularly, an intense theoretical debate has arisen about what form, if any, of the so-called Vinen equation accurately captures the dynamics of vortex line density, $L$. We address this problem experimentally, in a 21 cm long channel of square $7\times 7{\mathrm{mm}}^2$ cross section. Based on large statistics of second-sound data measured in nonequilibrium square-wave modulated thermally induced counterflow we investigate the phase portrait of the general form of the governing dynamical equation and conclude that for sparse tangles $(L\lesssim {10}^5{\mathrm{cm}}^{-2})$ all proposed forms of this equation based on the concept of a homogeneous random tangle of quantized vortices provide equally adequate descriptions of the growth of $L$, while for dense tangles $(L>{10}^5{\mathrm{cm}}^{-2})$ none of them is satisfactory or able to account for the significant slow-down in tangle growth rate as the steady state is approached. We claim, however, that agreement with theory is recovered if the geometrical parameter $c_2$ introduced in numerical studies by K. W. Schwarz [Phys. Rev. B 38, 2398 (1988)] is allowed to vary with vortex line density which also greatly improves the prediction of the observed early decay rate.

${}^{121,123}\mathrm{Sb}$ nuclear quadrupole resonance (NQR) was applied to $\text{Fe}{\left({\text{Sb}}_{1-x}{\text{Te}}_x\right)}_2$ in the low doping regime $(x=0$, 0.01, and 0.05) as a microscopic zero field probe to study the evolution of $3d$ magnetism and the emergence of metallic behavior. Whereas the NQR spectra itself reflects the degree of local disorder via the width of the individual NQR lines, the spin lattice relaxation rate (SLRR) $1/T_1\left(T\right)$ probes the fluctuations at the Sb site. The fluctuations originate either from conduction electrons or from magnetic moments. In contrast to the semimetal ${\mathrm{FeSb}}_2$ with a clear signature of the charge and spin gap formation in $1/T_1\left(T\right)T[\sim exp/\left(\mathrm{\Delta }{k}_{B}T\right)]$, the 1% Te-doped system exhibits almost metallic conductivity and the SLRR nicely confirms that the gap is almost filled. A weak divergence of the SLRR coefficient $1/T_1\left(T\right)T\sim T^{-n}\sim T^{-0.2}$ points towards the presence of electronic correlations towards low temperatures. This is supported by the electronic specific heat coefficient $\gamma =(C_{\text{el}}/T)$ showing a power-law divergence $\gamma \left(T\right)\sim T^{-m}\sim {(1/T_{1}T)}^{1/2}\sim T^{-n/2}\sim C_{\text{el}}/T$ which is expected in the renormalized Landau Fermi liquid theory for correlated electrons. In contrast to that the 5% Te-doped sample exhibits a much larger divergence in the SLRR coefficient showing $1/T_1\left(T\right)T\sim T^{-0.72}$. According to the specific heat divergence a power law with $n=2m=0.56$ is expected for the SLRR. This dissimilarity originates from admixed critical magnetic fluctuations in the vicinity of antiferromagnetic long range order with $1/T_1\left(T\right)T\sim T^{-3/4}$ behavior. Furthermore Te-doped ${\mathrm{FeSb}}_2$ as a disordered paramagnetic metal might be a platform for the electronic Griffith phase scenario. NQR evidences a substantial asymmetric broadening of the ${}^{121,123}\mathrm{Sb}$ NQR spectrum for the 5% sample. This has a predominant electronic origin in agreement with the electronic Griffith phase and stems probably from an enhanced Sb-Te bond polarization and electronic density shift towards the Te atom inside Sb-Te dumbbell.

Using a dual representation of lattice fermion models that is based on spin-charge transformation and fermionization of the original description, I derive an algorithm for diagrammatic Monte Carlo simulation of strongly correlated systems. This scheme allows eliminating large expansion parameters, as well as large corrections to the density matrix that generally prevent diagrammatic methods from being efficient in this regime. As an example, I compute the filling factor for the Hubbard model at infinite on-site repulsion and compare the results to controllable data obtained from numerical linked-cluster expansion. I find excellent agreement between the two methods, as well as rapid convergence of the diagrammatic series. I also report results for the momentum distribution and kinetic energy of the electrons.