We study the low-energy properties and, in particular, the magnetization process of a spin-1/2 Heisenberg $J_1-J_2$ sawtooth and frustrated chain (also known as a zigzag ladder) with a spatially modulated $g$ factor. We treat the problem both analytically and numerically while keeping the $J_2/J_1$ ratio generic. Numerically, we use complete and Lanczos diagonalization as well as the infinite time-evolving block decimation method. Analytically, we employ (non-)Abelian bosonization. Additionally, for the sawtooth chain, we provide an analytical description in terms of flat bands and localized magnons. By considering a specific pattern for the $g$-factor modulation for both models, we show that a small inhomogeneity significantly enhances a magnetization plateau at half saturation. For the magnetization of the frustrated chain, we show the destruction of one-third of the full saturation plateau in favor of the creation of a plateau at half saturation. For large values of the inhomogeneity parameter, the existence of an additional plateau at zero magnetization is possible. Here and at higher magnetic fields, the system is locked in the half-saturation plateau, never reaching full saturation.

We investigate charge-state coherence dynamics in nonequilibrium transport through a double-dot Aharonov-Bohm interferometer. On the basis of accurate numerical evaluations, we identity an interesting many-particle mechanism, the Coulomb blockade assisted Rabi interference, as an interplay between interdot Coulomb interaction and environment-assisted interdot tunneling and Rabi resonance. This many-particle multiple-interference mechanism has distinct signatures in the transport current noise spectrum, with the emerging Fano characteristics sensitive to the Aharonov-Bohm phase.

We perform a comprehensive theoretical study of the pressure-induced evolution of the electronic structure, magnetic state, and phase stability of the late transition metal monoxides MnO, FeO, CoO, and NiO using a fully charge self-consistent $\mathrm{DFT}+\mathrm{dynamical}$ mean-field theory method. Our results reveal that the pressure-induced Mott insulator-to-metal phase transition in MnO-NiO is accompanied by a simultaneous collapse of local magnetic moments and lattice volume, implying a complex interplay between chemical bonding and electronic correlations. We compute the pressure-induced evolution of relative weights of the different valence states and spin-state configurations. Employing the concept of fluctuating valence in a correlated solid, we demonstrate that in MnO, FeO, and CoO a Mott insulator-metal transition and collapse of the local moments is accompanied by a sharp crossover of the spin-state and valence configurations. Our microscopic explanation of the magnetic collapse differs from the accepted picture and points out a remarkable dynamical coexistence (frustration) of the high-, intermediate-, and low-spin states. In particular, in MnO, the magnetic collapse is found to be driven by the appearance of the intermediate-spin state (IS), competing with the low-spin (LS) state; in FeO, we observe a conventional high-spin to low-spin (HS-LS) crossover. Most interestingly, in CoO, we obtain a remarkable (dynamical) coexistence of the HS and LS states, i.e., a HS-LS frustration, up to high pressure. Our results demonstrate the importance of quantum fluctuations of the valence and spin states for the understanding of quantum criticality of the Mott transitions.

The electronic structure and phonon properties of LaNiSi, LaPtSi, and LaPtGe have been studied using the first-principles density functional calculations. The electronic density of states near Fermi level of these compounds are dominated by the transition metal $d$ orbitals and Si/Ge $p$ orbitals, forming six sheets of Fermi surfaces. The asymmetric spin-orbit coupling lifts the spin degeneracies and creates four topological Weyl nodal rings around $X$ points, protected by the nonsymmorphic symmetry and time-reversal symmetry. The bare-electron susceptibility does not show a prominent feature, and electron-phonon coupling is sufficient to explain the observed superconductivity.

${\mathrm{FeSe}}_{0.45}{\mathrm{Te}}_{0.55}$ (FeSeTe) has recently emerged as a promising candidate to host topological superconductivity, with a Dirac surface state and signatures of Majorana bound states in vortex cores. However, correlations strongly renormalize the bands compared to electronic structure calculations, and there is no evidence for the expected bulk band inversion. We present here a comprehensive angle resolved photoemission (ARPES) study of FeSeTe as a function of photon energies ranging from 15–100 eV. We find that although the top of the bulk valence band shows essentially no $k_z$ dispersion, its normalized intensity exhibits a periodic variation with $k_z$. We show, using ARPES selection rules, that the intensity oscillation is a signature of band inversion indicating a change in the parity going from $\mathrm{\Gamma }$ to $Z$. We also present a simple realistic tight-binding model which gives insight into ARPES observations. Thus we provide direct evidence for a topologically nontrivial bulk band structure that supports protected surface states.

The condensation of spin-orbit-induced excitons in $t_{2g}^4$ electronic systems is attracting considerable attention. At large Hubbard $U$, antiferromagnetism was proposed to emerge from the Bose-Einstein Condensation (BEC) of triplons ($J_{\text{eff}}=1$). Here, we show that even at intermediate $U$ regimes, the spin-orbit exciton condensation is possible leading also to staggered magnetic order. The canonical electron-hole excitations (excitons) transform into local triplon excitations at large $U$, and this BEC strong coupling regime is smoothly connected to the intermediate $U$ excitonic insulator region. We solved the degenerate three-orbital Hubbard model with spin-orbit coupling ($\lambda$) in one dimension using the density matrix renormalization group, while in two dimensions we use the Hartree-Fock approximation (HFA). Employing these techniques, we provide the full $\lambda$ versus $U$ phase diagrams for both one- and two-dimensional lattices. Our main result is that at intermediate Hubbard $U$, increasing $\lambda$ at fixed $U$ the system transitions from an incommensurate spin-density-wave metal to a Bardeen-Cooper-Schrieffer (BCS) excitonic insulator, with coherence length $r_{\mathrm{coh}}$ of $\mathcal{O}\left(a\right)$ and $\mathcal{O}\left(10a\right)$ in $1d$ and $2d$, respectively, with $a$ being the lattice spacing. Further increasing $\lambda$, the system eventually crosses over to the BEC limit (with $r_{\mathrm{coh}}\ll a$).

We show that the presence of a side-attached state strongly modifies the transmission through a one-dimensional double-barrier system in the window of wave vectors around the Fano antiresonance. Specifically, the interplay between the Fano interference and the size quantization inside the structure gives rise to narrow resonant peaks in the transmission coefficient. The height of the peaks may become close to unity (perfect transmission) even for an asymmetric setup with strong barriers, where the transmission coefficient in the absence of the Fano state is strongly suppressed at all other wave vectors. Thus, the two types of interference phenomena, each by itself leading to the suppression of the transmission, conspire in a peculiar way to produce the transparency enhancement.

CeCoSi is an intermetallic antiferromagnet with a very unusual temperature-pressure phase diagram: at ambient pressure it orders below $T_{\text{N}}=8.8\mathrm{K}$, while application of hydrostatic pressure induces a new magnetically ordered phase with exceptionally high transition temperature of $\sim 40\mathrm{K}$ at 1.5 GPa. We studied the magnetic properties and the pressure-induced magnetic phase of CeCoSi by means of elastic and inelastic neutron scattering (INS) and heat capacity measurements. At ambient pressure CeCoSi orders into a simple commensurate AFM structure with a reduced ordered moment of only $m_{\mathrm{Ce}}=0.37\left(6\right){\mu }_{\mathrm{B}}$. Specific heat and low-energy INS indicate a significant gap in the low-energy magnon excitation spectrum in the antiferromagnetic phase, with the CEF excitations located above 10 meV. Hydrostatic pressure gradually shifts the energy of the magnon band towards higher energies and the temperature dependence of the magnons measured at 1.5 GPa is consistent with the phase diagram. Moreover, the CEF excitations are also drastically modified under pressure.

In the present paper, the magnetocaloric effect (MCE) of ${\mathrm{NdCo}}_{2-x}{\mathrm{Fe}}_x$ ($x=0$, 0.2, 0.4, 0.6) compounds was investigated by magnetization measurements. The temperature-dependent high-resolution synchrotron x-ray diffraction study shows a magnetostructural transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase below their Curie temperatures. Differential scanning calorimetry analysis shows the absence of thermal hysteresis, indicating the second-order nature of the magnetostructural phase transition in these compounds. The maximum values of magnetic entropy change ($\mathrm{\Delta }{S}_M$) and wide operating temperature (OT) are obtained under a field change of 5 T, which are 7.33 ${\mathrm{Jkg}}^{-1}{\mathrm{K}}^{-1}$, 6.45 ${\mathrm{Jkg}}^{-1}{\mathrm{K}}^{-1}$, 5.71 ${\mathrm{Jkg}}^{-1}{\mathrm{K}}^{-1}$, 4.70 ${\mathrm{Jkg}}^{-1}{\mathrm{K}}^{-1}$ and 78 K, 82 K, 85 K, 92 K for $x$ = 0, 0.2, 0.4, and 0.6, respectively. The corresponding values of relative cooling power (RCP) are 529.96 ${\mathrm{Jkg}}^{-1}$, 497.25 ${\mathrm{Jkg}}^{-1}$, 470.55 ${\mathrm{Jkg}}^{-1}$, and 428.31 ${\mathrm{Jkg}}^{-1}$. The observed wide OT range and large RCP values are comparable with Gd, ${\mathrm{Gd}}_5{\mathrm{Ge}}_2{\mathrm{Si}}_2$, and some rare-earth-based giant magnetocaloric materials, making this series of compounds suitable for magnetic refrigeration.

The Higgs mode in superconductors is a scalar mode without an electric or magnetic dipole moment. Thus, it is commonly believed that its excitation is restricted to a nonlinear two-photon Raman process. However, recent efforts have shown that a linear excitation in the presence of a supercurrent is possible, resulting in a resonant enhancement at $\mathrm{\Omega }=2\mathrm{\Delta }$ with the driving light frequency $\mathrm{\Omega }$ and the energy of the Higgs mode $2\mathrm{\Delta }$. This is in contrast to the usual $2\mathrm{\Omega }=2\mathrm{\Delta }$ resonance condition found in nonlinear third-harmonic generation experiments. In this Rapid Communication, we show that such a linear excitation can still be described as an effective Raman two-photon process, with one photon at $\omega =2\mathrm{\Delta }$ and one virtual photon at $\omega =0$ which represents the dc supercurrent. At the same time we demonstrate that a straightforward infrared activation with a single-photon excitation is negligible. Moreover, we give a general context to our theory, providing an explanation for how the excitation of the Higgs mode in both THz quench and drive experiments can be understood within a conventional difference-frequency generation or sum-frequency generation process, respectively. In such a picture, the observed resonance condition $\mathrm{\Omega }=2\mathrm{\Delta }$ is just a special case. With the same approach, we further discuss another recent experiment, where we find a suppression of odd-order higher harmonics in the presence of a dc supercurrent.

We investigate the correlated electrons in the magic-angle twisted bilayer graphene by using the slave-rotor mean-field theory. Owing to the extended figure of Wannier orbitals, we study the two-orbital cluster Hubbard model with spin-valley fourfold degeneracy, focusing around half-filling of valence bands below the neutrality point. The theory predicts multiple Mott insulator phases at fractional fillings not only for integer charges per moiré site, and it demonstrates that long-range electron hopping is highly suppressed because multiple-charge excitations are induced. Furthermore, the Kekule valence bond order is investigated and is found to extend the Mott insulator phases to occupy a finite doping region. Adjacent to Mott insulator phases, superconducting domes emerges by virtue of spin-valley fluctuations. This work has provided a primal understanding and interesting phenomena of the correlated system, and for its unique interaction the model might produce plenty of possibilities waiting to be explored.

We study the transition of $\nu =1/3$ and $2/5$ fractional quantum Hall states of the honeycomb Hofstadter model as we tune to a two-orbital moiré superlattice Hamiltonian, motivated by the flat bands of twisted bilayer graphene in a perpendicular magnetic field. In doing so, we address the extent to which these states survive in moiré systems and analyze the nature of the transition. Through the use of a Peierls substitution, we determine the Landau-level splitting for the moiré Hamiltonian, and study the structure of the Chern bands for a range of magnetic flux per plaquette. We identify topological flat bands in the spectrum at low energies, with numerically tractable lattice geometries that can support the fractional quantum Hall effect. As we tune the model, we find that the orbital-polarized $\nu =1/3$ and $2/5$ states corresponding to the honeycomb Hofstadter model survive up to $\approx 30\%$ of typical moiré superlattice parameters, beyond which they transition into an insulating phase. We present evidence for this through density matrix renormalization group calculations on an infinite cylinder, by verifying the charge pumping, spectral flow, entanglement scaling, and conformal field theory edge counting. We conclude that fractional quantum Hall states from the Hofstadter model can persist up to hopping amplitudes of the same order as those typical for moiré superlattice Hamiltonians, which implies generally that fractional states for moiré superstructures can be discerned simply by analyzing the dominant terms in their effective Hamiltonians.

We reveal the unexpected role of the material inhomogeneity in unifying the formulation of intrinsic thermal and thermoelectric transport as well as magnetization currents. The smooth inhomogeneity leads to the position dependent local band dispersion and phase-space Berry curvature, enabling a general and rapid access to transport and magnetization currents displaying the momentum-space Berry curvature physics. Our theory does not invoke the boundary current, the thermodynamic approach to magnetization or any mechanical counterpart of statistical forces. By introducing a fictitious inhomogeneity, it applies to homogeneous samples as well, promoting the inhomogeneity to be a basic trick in semiclassical transport theories. Such a trick works regardless of the driving force of transport, e.g., temperature gradient, in contrast to the trick of fictitious gravitational field in quantum transport theories. We thus include more general mechanical driving forces and establish the Mott relation between the resulting transport thermal and electric currents, whereas this relation for these two currents was previously only known when an electric field is the driving force.

We measured the temperature dependence of low-frequency Raman spectra in ${\mathrm{Li}}_2{\mathrm{RuO}}_3$ and observed multipeak quasielastic scattering in the Ru honeycomb polarizations below and above the magnetostructural transition temperature. We attribute this scattering to the fluctuations of the energy density in the spin system. High-frequency electronic light scattering was observed at $2150{\text{cm}}^{-1}$. Its intensity increased significantly below the transition temperature, confirming substantial modification of the electronic structure due to removal of degeneracy in the $t_{2g}$ manifold of ${\mathrm{Ru}}^{4+}$ ions.

Recently, Haldane's conjecture about SU(2) chains has been generalized to $\mathrm{SU}\left(n\right)$ chains in the symmetric representations. For a rank-$p$ representation, a gapless phase is predicted when $p$ and $n$ are coprime; otherwise, a finite energy gap is present above the ground state. In this work we offer an intuitive explanation of this behavior based on fractional topological excitations, which are able to generate a mass gap except when $p$ and $n$ have no common divisor. This is a generalization of an older work in SU(2), which explains the generation of the Haldane gap in terms of merons in the O(3) nonlinear $\sigma$ model.

The carrier mobility and dynamics of nonequilibrium carriers in Eu-doped GaN have been determined by above-band-gap, optically excited, time-resolved terahertz spectroscopy and compared with that of undoped GaN. The carrier mobility of GaN:Eu was an order of magnitude smaller than that of a similarly grown undoped GaN layer. Two time constants were observed in the carrier relaxation dynamics for GaN:Eu. A fast one (2.2 ps), which we related to an efficient nonradiative carrier trap, and a slower one (∼85 ps) related to Eu-related traps. Ultrafast photoluminescence spectroscopy shows that Eu-related emission is observed within 100 ps after a pulsed excitation. These values set an upper value to the energy-transfer time from the Eu-related trap to $\mathrm{E}{\mathrm{u}}^{3+}$ ion and underlie the high efficiency of the excitation of $\mathrm{E}{\mathrm{u}}^{3+}$ ions in GaN.

We use scanning tunneling microscopy (STM) to study charge density wave (CDW) states in the rare-earth ditelluride, ${\mathrm{CeTe}}_2$. Our STM measurements surprisingly detect a unidirectional CDW with $q\sim 0.28a^{*}$, which differs from previous experimental and first-principles studies of the rare-earth ditellurides, and which is very close to what is found in experimental measurements of the related rare-earth tritellurides. Furthermore, in the vicinity of an extended subsurface defect, we find spatially-separated as well as spatially-coexisting unidirectional CDWs at the surface of ${\mathrm{CeTe}}_2$. We quantify the nanoscale strain and its variations induced by this defect, and establish a correlation between local lattice strain and the locally-established CDW states; this suggests that lattice strain plays an important role in determining the specific characteristics of the established CDW state. Our measurements probe the fundamental properties of a weakly-bound two-dimensional Te sheet, which experimental and theoretical work has previously established as the fundamental component driving much of the essential physics in both the rare-earth di- and tritelluride compounds.

Semiconductor quantum dots provide a spin-coupled system of an electron and nuclei via enhanced hyperfine interaction. We showed that the nuclear spin polarization in single quantum dots can have three stable branches under a longitudinal magnetic field. The states were accompanied by hysteresis loops around the boundaries of each branch with a change in the excitation condition. To explain these findings, we incorporated the electron spin relaxation caused by the nuclear spin fluctuation into the previously studied dynamic nuclear spin polarization mechanism. The model reproduces the features of nuclear spin polarization and the associated strong reduction in the observed electron spin polarization, and can refer to the tristability of nuclear spin polarization.

We investigate light transport in three-dimensional scattering systems generated according to subrandom sequences and demonstrate subdiffusive behavior typical of wave transport in disordered systems at the critical point for metal-insulator-transitions but in a wider range of parameters. Specifically, we solve the electromagnetic multiple scattering problem using the Green's matrix spectral theory for aperiodic systems based on Halton, Sobol, and stochastic Latin hypercube sequences. By studying the Thouless number and the level-spacing statistics of the electromagnetic resonances at different scattering densities, we demonstrate that light transport in deterministic Halton and Sobol structures exhibit multifractal behavior characterized by inverse power-law scaling of level-spacing statistics across a wide range of densities of dipolar scatterers. On the other hand, this scenario is absent in the stochastic Latin hypercube array, whose behavior instead resembles standard diffusion in uniform random media. Our findings establish a connection between subdiffusion and subrandom aperiodic order and provide a strategy to design three-dimensional structures with multifractal properties over a broad spectral range.

Layered misfit cobaltate ${\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]}_{0.62}\left[{\mathrm{CoO}}_2\right]$, which emerged as an important thermoelectric material [A. C. Masset et al. Phys. Rev. B 62, 166 (2000)], has been explored extensively in the last decade for the exact mechanism behind its high Seebeck coefficient. Its complex crystal and electronic structures have inhibited consensus among such investigations. This situation has arisen mainly due to difficulties in accurate identification of the chemical state, spin state, and site symmetries in its two subsystems (rocksalt $\left[{\mathrm{Ca}}_2{\mathrm{CoO}}_3\right]$ and triangular $\left[{\mathrm{CoO}}_2\right]$). By employing resonant photoemission spectroscopy and x-ray absorption spectroscopy along with charge transfer multiplet simulations (at the Co ions), we have successfully identified the site symmetries, valencies, and spin states of the Co in both layers. Our site-symmetry observations explain the experimental value of the high Seebeck coefficient and also confirm that the carriers hop within the rocksalt layer, which is in contrast to earlier reports where hopping within triangular ${\mathrm{CoO}}_2$ layer has been held responsible for the large Seebeck coefficient.