The structural and magnetic properties of the noncentrosymmetric $\mathrm{E}{\mathrm{u}}_2\mathrm{P}{\mathrm{d}}_2\mathrm{Sn}$ compound have been investigated by synchrotron x-ray and neutron powder diffraction and ${}^{151}\mathrm{Eu}$ Mössbauer spectroscopy. No structural transition was observed in the temperature range 4–290 K. The magnetic phase that develops below $\sim 13.8\left(5\right)$ K is characterized by an incommensurate cycloidal ordering in the ac plane of the Eu substructure. This magnetic structure shows significant analogies to the structure observed in $\mathrm{EuNiG}{\mathrm{e}}_3$, possibly indicating the occurrence of a skyrmion lattice for $\mathrm{E}{\mathrm{u}}_2\mathrm{P}{\mathrm{d}}_2\mathrm{Sn}$ as well.

Motivated by the classical spin-nematic state observed in the breathing pyrochlore spinel ${\mathrm{LiGa}}_{0.95}{\mathrm{In}}_{0.05}{\mathrm{Cr}}_4{\mathrm{O}}_8$, we theoretically discuss spin dynamics in models of spin-lattice coupling in these materials. Semiclassical dynamical simulations successfully recover the key features of inelastic neutron-scattering experiments on ${\mathrm{LiGa}}_{0.95}{\mathrm{In}}_{0.05}{\mathrm{Cr}}_4{\mathrm{O}}_8$: a broad finite-energy peak alongside a continuum of scattering near the (200) wave vector that extends from the elastic line to high energies. To interpret this result, we generalize linear-spin-wave theory for conventionally ordered magnets to the disordered spin-ice-like ground states expected for moderate spin-lattice coupling, which reproduces the numerical simulation results quantitatively. In particular, we find that the inelastic peak is well explained by collective modes confined to ferromagnetic loops of the underlying nematic order. In addition, we find a sharp, linearly dispersing mode in the dynamic structure factor, which originates in long-wavelength fluctuations of the nematic director. We believe identifying this mode will be an interesting target for future experiments on these materials. We also outline potential future applications of our methods to both pyrochlore spinels and other spin-nematic systems.

We construct domain-wall skyrmion chains and domain-wall bimerons in chiral magnets with an out-of-plane easy-axis anisotropy and without a Zeeman term coupling to a magnetic field. Domain-wall skyrmions are skyrmions trapped inside a domain wall, and they are present in the ferromagnetic (FM) phase of a chiral magnet with an out-of-plane easy-axis anisotropy. In this paper, we explore the stability of domain-wall skyrmions in the FM phase and in a chiral soliton lattice (CSL) or spiral phase, which is a periodic array of domain walls and antidomain walls arranged in an alternating manner. In the FM phase, the worldline of a domain-wall skyrmion is bent to form a cusp at the position of the skyrmion. We describe such a cusp using both an analytic method and numerical solutions, and find a good agreement between them for small Dzyaloshinskii-Moriya interactions. We show that the cusp grows toward the phase boundary with the CSL, and eventually diverges at the boundary. Second, if we put one skyrmion trapped inside a domain wall in a CSL, it decays into a pair of merons by a reconnection of the domain wall and its adjacent antidomain wall. Third, if we put skyrmions and antiskyrmions alternately in domain walls and antidomain walls, respectively, such a chain is stable.

We report muon-spin rotation ($\mu \mathrm{SR}$) measurements of the temperature dependence of the absolute value of the magnetic penetration depth and the magnetic field dependence of the vortex core size in the mixed state of the nonsymmorphic superconductor ${\mathrm{LaNiGa}}_2$. The temperature dependence of the superfluid density is shown to be well described by a two-band model with strong interband coupling. Consistent with a strong coupling of the superconducting condensates in two different bands, we show that the field dependence of the vortex core size resembles that of a single-band superconductor. Our results lend support to the proposal that ${\mathrm{LaNiGa}}_2$ is a fully gapped, internally antisymmetric, nonunitary spin-triplet superconductor.

The charge density wave (CDW) or nematicity has been found to coexist with superconductivity in many systems. Thus, it is interesting that the superconducting transition temperature $T_c$ in the doped ${\mathrm{BaNi}}_2{\mathrm{As}}_2$ system can be enhanced up to six times as the CDW or nematicity in the undoped compound is suppressed. Here we show that the transverse acoustic phonons of ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{Ni}}_2{\mathrm{As}}_2$ are strongly damped in a wide doping range and over the whole $Q$ range, which excludes its origin from either CDW or nematicity. The damping of TA phonons can be understood as large electron-phonon coupling and possible strong hybridization between acoustic and optical phonons as shown by the first-principle calculations. The superconductivity can be quantitatively reproduced by the change of the electron-phonon coupling constant calculated by the McMillan equation in the BCS framework, which suggests that no quantum fluctuations of any order is needed to promote the superconductivity. On the contrary, the change of $T_c$ in this system should be understood as the sixfold suppression of superconductivity in undoped compounds.

In this and the two following papers, we present the results of a combined study by density-functional band theory and angle-resolved photoemission spectroscopy (ARPES) of lithium purple bronze, ${\mathrm{Li}}_{1x}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$. This material is particularly notable for its unusually robust quasi-one-dimensional (quasi-1D) behavior. The band structure, in a large energy window around the Fermi energy, is basically two-dimensional and formed by three Mo $t_{2g}$-like extended Wannier orbitals (WOs), each one giving rise to a 1D band running at a ${120}^{\circ }$ angle to the two others. A structural “dimerization” from $\mathbf{c}/2$ to $\mathbf{c}$ gaps the $xz$ and $yz$ bands while leaving the $xy$ bands metallic in the gap but resonantly coupled to the gap edges and, hence, to the two other directions. The resulting complex shape of the quasi-1D Fermi surface (FS), verified by our ARPES, thus depends strongly on the Fermi energy position in the gap, implying a great sensitivity to Li stoichiometry of properties dependent on the FS, such as FS nesting or superconductivity. The theory is verified in detail by the recognition and application of an ARPES selection rule that enables the separation in ARPES spectra of the two barely split $xy$ bands and the observation of their complex split FS. The strong resonances prevent either a two-band tight-binding model or a related real-space ladder picture from giving a valid description of the low-energy electronic structure. Down to a temperature of 6 K we find no evidence for a theoretically expected downward renormalization of perpendicular single-particle hopping due to LL fluctuations in the quasi-1D chains. This paper I introduces the material, motivates our study, summarizes the $N\mathrm{th}$-order muffin-tin orbital (NMTO) method that we use, analyzes the crystal structure and the basic electronic structure, and presents our NMTO calculation of the $t_{2g}$ low-energy WOs and the resulting tight-binding Hamiltonian for the six lowest energy bands, only the four lowest being occupied. Thus this paper sets the theoretical framework and nomenclature for the following two papers.

This is the second paper of a series of three papers presenting a combined study by band theory and angle-resolved photoemission spectroscopy (ARPES) of lithium purple bronze. The $t_{2g}$ Wannier orbitals (WOs) and resulting six-band tight-binding (TB) Hamiltonian found in paper I are used here to develop a theory of the ARPES intensity variations, including a selection rule whose validity relies on the smallness of and the cancellation between the displacement- and inversion-dimerizations of the zigzag chains (ribbons) in regions of the final-state wave vector $\mathit{\kappa }$. We then present the ARPES results for the band structure of the four occupied $t_{2g}$ bands (gapped $xz$, $yz$, and split metallic $xy$). A detailed comparison to the theory validates the selection rule. We present the Fermi surface as seen directly in the raw ARPES data, both parallel and perpendicular (using photon-energy dependence) to the sample surface, and show that the selection rule can enable separation of the barely split and highly quasi-one-dimensional $xy$ bands. We adjust the energy of the $xy$ WO energy by 0.1 eV ($\approx 1/4$ of the gap) with respect to that of the gapped $xz$ and $yz$ WOs and, in a second step, fine-tune merely seven out of the more than 40 TB parameters to achieve an excellent fit to the ARPES bands lying more than 0.15 eV below the Fermi level. So doing then also gives nearly perfect agreement closer to the Fermi level.

This is the third paper of a series of three papers presenting a combined study by band theory and angle-resolved photoemission spectroscopy (ARPES) of lithium purple bronze. The first paper laid the foundation for the theory, and the second paper discussed a general comparison between theory and experiment, including deriving an ARPES selection rule. The present paper III focuses in detail on the two metallic, quasi-one-dimensional (quasi-1D) $xy$-like bands left in the 0.4 eV dimerization gap between the $xz$ and $yz$ valence and conduction (V&C) bands. The hybridizations with the latter change the perpendicular dispersions of — and splitting between — the resulting $\widetilde{xy}$ bands. The edges of the V (C) bands, in particular, push resonance peaks up (down) in the $\widetilde{xy}$ bands which are now described by a two-band Hamiltonian whose two first terms consist of the pure $xy$ block of the six-band tight-binding (TB) Hamiltonian and whose four following terms describe the resonant coupling to (i.e., indirect hopping via) the V&C bands. The two-band Hamiltonian extends the selection rule derived in the previous paper to the hybridized $\widetilde{xy}$ bands, which enables extracting the split quasi-1D Fermi surface (FS) from the raw ARPES data. The complex shape of the FS, verified in detail by our ARPES, depends strongly on the Fermi-energy position in the gap, implying a great sensitivity to Li stoichiometry of properties dependent on the FS, such as FS nesting or superconductivity. The strong resonances prevent either a two-band TB model or a related real-space ladder picture from giving a valid description of the low-energy electronic structure. Down to a temperature of $6\mathrm{K}$, we find no evidence for a theoretically expected downward renormalization of perpendicular single-particle hopping due to LL fluctuations in the quasi-1D chains.

We report on exotic response properties in 3D time-reversal invariant Weyl semimetals with mirror symmetry. Despite having a vanishing anomalous Hall coefficient, we find that the momentum-space quadrupole moment formed by four Weyl nodes determines the coefficient of a mixed electromagnetic charge-stress response, in which momentum flows perpendicular to an applied electric field, and electric charge accumulates on certain types of lattice defects. This response is described by a mixed Chern-Simons-like term in 3 spatial dimensions, which couples a rank-2 gauge field to the usual electromagnetic gauge field. On certain 2D surfaces of the bulk 3D Weyl semimetal, we find what we will call rank-2 chiral fermions, with $\omega =k_x{k}_y$ dispersion. Such fermion states have a mixed charge-momentum anomaly which is canceled by the bulk of the 3D system.

Many-body interactions play a crucial role in quantum topological systems, being able to impact or alter the topological classifications of noninteracting fermion systems. In open quantum systems, where interactions with the environment cause dissipation and decoherence of the fermionic dynamics, the absence of Hermiticity in the subsystem Hamiltonian drastically reduces the stability of the topological phases of the corresponding closed systems. Here we investigate the nonperturbative effects induced by the environment on the prototype Su-Schrieffer-Heeger chain coupled to local harmonic oscillator baths through either intracell or intercell transfer integrals. Despite the common view, this type of coupling, if suitably engineered, can even induce a transition to topological phases. By using a world-line quantum Monte Carlo technique we determine the phase diagram of the model proving that the bimodality of the probability distribution of the polarization signals the emergence of the topological phase. We show that a qualitative description can be obtained by using an approach based on the cluster perturbation theory providing, in particular, a non-Hermitian Hamiltonian for the fermionic subsystem and insights on the dissipative dynamics. We prove that non-Hermitian effects disappear in the presence of classical heat baths.

We employ the large-scale quantum Monte Carlo simulations to investigate the Mott-insulating states of the half-filled SU(4) Hubbard model on the square lattice with a staggered-flux pattern. The noninteracting band structure that evolves from a nested Fermi surface at zero flux to isotropic Dirac cones at $\pi$ flux, exhibits anisotropic Dirac cones as the flux varies in between. Our simulations show transitions between the three phases of the Dirac semimetal, antiferromagnet, and valence-bond solid. A direct continuous transition between the antiferromagnetic phase and the valence-bond-solid phase is realized via varying the flux in the Mott regime. The simulated critical exponents remarkably agree with those of SU(4) $J\text{−}Q$ model. Inside the valence-bond-solid phase induced by the flux, the plaquette valence-bond state with vanishing single-particle gap is identified. At strong coupling, the valence-bond-solid phase disappears and the Mott-insulating state is always accompanied by antiferromagnetic ordering, regardless of the magnitude of the flux.

Quantum many-body systems with fractonic excitations can realize fascinating phases of matter. Here, we study the low-energy excitations of a constrained Bose-Hubbard model in one dimension, which conserves the center of mass or, equivalently, the dipole moment in addition to the particle number. This model is known to realize fractonic phases, including a dipole Mott insulator, a dipole Luttinger liquid, and a metastable dipole supersolid. We use tensor network methods to compute spectral functions from the dynamical response of the system and verify predictions from low-energy field theories of the corresponding ground-state phases. We demonstrate the existence of gapped excitations compatible with strong coupling results in a dipole Mott insulator, linear sound modes characteristic of a Luttinger liquid of dipoles, and soft quadratic modes at both zero and finite momenta in a supersolid state with charge density wave order and phase coherence at noninteger filling.

Based on the Wilson's criterion metal/insulator, extended to materials with strong electronic correlations, we have identified a specific class of the materials, which is not associated with their usual classification into Mott-Hubbard and charge transfer dielectrics. The local symmetry of these materials leads to the disappearance of quasiparticle states (so-called first removal or first extra states) in the Hubbard gap. It is especially unusual for doped materials, in which quasiparticles, being charge carriers, can disappear or appear under external factors without the Mott transition being achieved. In this work, we introduce the so-called “quasiparticle transparency”, and provide specific experiments to identify materials with the low quasiparticle transparency. A number of examples of such materials with a spin crossover under high pressure, showing the Jahn-Teller nature, are considered.

We investigated photoinduced ultrafast transient dynamics in different magnetic phases in CeSb by means of time resolved magneto-optical spectroscopy. We observe distinctive coherent oscillations in the ground-state antiferromagnetic (AF) phase and the high-magnetic-field ferromagnetic (F) phase. While the AF-phase oscillation frequencies match the recent Raman scattering findings the F-phase oscillation frequency does not correspond to the previously observed magnetic excitation. The large spectroscopic factor, $g=3.94$, and optical polarization properties suggest that it corresponds to a previously undetected ${\mathrm{Ce}}^{3+}$ coherent crystal-field state excitation. The AF-phase oscillations show no magnetic-field dependence so their lattice origin cannot be entirely excluded. The nonoscillatory part of the transients is qualitatively similar in all investigated magnetic phases with a faster subpicosecond dynamics in the ferromagnetic and ferroparamagnetic phases and is attributed to differences in the electronic structure, which affect the photoexcited quasiparticle energy relaxation kinetics.

Tuning exciton-plasmon interaction in transition metal dichalcogenides coupled with noble metal nanostructures has emerged as a powerful technique with broad implications in the fields of cavity quantum electrodynamics, spintronics, valleytronics, etc. Here, we report a simple strategy to modulate recombination pathways in the emerging transition metal dichalcogenide alloy MoSSe coupled with Au nanoclusters by engineering the geometrical parameters of Au nanoclusters. Systematic investigations using photoluminescence spectroscopy were performed on Au/MoSSe hybrids under 488 nm excitation. Our results demonstrate Au size-dependent quenching to the subsequent enhancement of the MoSSe emission within the same hybrid system. First-principles calculations provide a microscopic understanding of the presence of the midgap state in the hybrid system leading to the quenching phenomenon. Additionally, the evolution of the Au nanoclusters' geometry induces local surface-plasmon resonance, which eventually enhances the photoluminescence intensities of the hybrid system, in agreement with theoretical calculations. Excitation with a laser of energy close to the emission of Au nanoclusters leads to a significant modification of the photoluminescence emission of the hybrids compared to the previous one. The modulation of the absorption and vibrational properties of the Au/MoSSe hybrid system further supports the findings from the photoluminescence studies. This comprehensive study provides valuable insights into the metal–transition metal dichalcogenide interface, thereby contributing to an improved understanding of the underlying mechanism. Furthermore, the demonstrated potential of this approach presents a promising avenue for the development of novel broadband optoelectronic designs.

Odd-frequency pairing is an unconventional type of Cooper pairing in superconductors related to the frequency dependence of the corresponding anomalous Green's function. We show by a combination of analytical and numerical methods that odd-frequency pairing is ubiquitously present in the current of Andreev-scattered particles across a junction formed by a normal metal (N) and a superconductor (S), even if the superconducting pairing is of conventional $s$-wave, spin-singlet type. We carefully analyze the conductance of NS junctions with different pairing symmetries ($s$ wave, $p$ wave, $d$ wave). In all cases, we identify a generic equal balance of even- and odd-frequency pairing to the contributions related to Andreev reflection. This analysis shows in retrospect that the presence of odd-frequency pairing in electric currents across NS junctions is rather the rule, not the exception. This insight stems from an alternative approach of analyzing the transport problem of hybrid structures. It is based on the Kubo-Greenwood formula with direct access to symmetries of the anomalous Green's functions characterizing the superconducting pairing. We expect that our predictions substantially enrich the interpretation of transport data across NS junctions in many material combinations.

Spin wave and domain wall are two of basic excitations in magnetic systems, and their interplay is dictated by the competitions between underlying magnetic interactions. In biaxial ferromagnets with easy-axis and hard-axis anisotropies perpendicular to each other, both the spin wave precession and the domain wall rotation are suppressed in the hard-axial direction. Here we investigate the domain wall motion driven by the spin waves in a biaxial ferromagnet using a wave packet approach. We show that the domain wall acquires a universal velocity given by the product of spin wave group velocity and the square of spin wave amplitude in the third ordinary-axial direction. Such a universal domain wall velocity is a synergy of the angular and linear momentum transfer mechanisms, with their weights controlled by the spin wave ellipticity.