We explore the physics of the disordered XYZ spin chain using two complementary numerical techniques: exact diagonalization (ED) on chains of up to 17 spins, and time-evolving block decimation (TEBD) on chains of up to 400 spins. Our principal findings are as follows. First, we verify that the clean XYZ spin chain shows ballistic energy transport for all parameter values that we investigated. Second, for weak disorder there is a stable diffusive region that persists up to a critical disorder strength that depends on the XY anisotropy. Third, for disorder strengths above this critical value, energy transport becomes increasingly subdiffusive. Fourth, the many-body localization transition moves to significantly higher disorder strengths as the XY anisotropy is increased. We discuss these results, and their relation to our current physical picture of subdiffusion in the approach to many-body localization.

We report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published results. In contrast to our experimental observations, the calculations predict a monotonic reduction of the ordered moment with pressure. We suggest that the robustness of the stripe-type antiferromagnetism is due to strong electron correlations not fully considered in the calculations.

Eigenstates of fully many-body localized (FMBL) systems are described by quasilocal operators ${\tau }_i^z$ (l-bits), which are conserved exactly under Hamiltonian time evolution. The algebra of the operators ${\tau }_i^z$ and ${\tau }_i^x$ associated with l-bits $\left({\mathbf{\tau }}_i\right)$ completely defines the eigenstates and the matrix elements of local operators between eigenstates at all energies. We develop a nonperturbative construction of the full set of l-bit algebras in the many-body localized phase for the canonical model of MBL. Our algorithm to construct the Pauli algebra of l-bits combines exact diagonalization and a tensor network algorithm developed for efficient diagonalization of large FMBL Hamiltonians. The distribution of localization lengths of the l-bits is evaluated in the MBL phase and used to characterize the MBL-to-thermal transition.

Low-dimensional frustrated antiferromagnet is a good model system to study exotic quantum physics. Here we report the observation of half magnetization plateau and ferroelectricity which emerge simultaneously in the bond-alternating skew-chain compounds $R_2{\mathrm{V}}_2{\mathrm{O}}_7(R=\mathrm{Ni},\text{Co})$ induced by high magnetic fields. The half plateau is stabilized in fields of 8–30 T (7–12 T) for $\mathrm{N}{\mathrm{i}}_2{\mathrm{V}}_2{\mathrm{O}}_7\left(\mathrm{C}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_7\right)$, whereas two magnetic field induced ferroelectricities are located below and above this plateau. The remarkable high-field ferroelectricity for Ni or Co compound is about $50\text{−}60\mu \mathrm{C}/{\mathrm{m}}^2$ for the polycrystalline sample. The resulting magnetic field-temperature $\left(H\text{−}T\right)$ phase diagrams are very complex and several distinct phase transitions are observed. The forced electric polarization by sweeping magnetic fields evidences a clear magnetoelectric coupling in $\mathrm{C}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ associated with the low-field ferroelectricity. Our magnetization data also reveal that $\mathrm{C}{\mathrm{o}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ produces an effective spin-1/2 behavior at the magnetic ground state. These experimental findings improve the knowledge to multiferroics and pave the way for exploring the quantum state of frustrated antiferromagnets.

Nonreciprocal spin waves can facilitate the realization of spin-wave logic devices. It has been demonstrated that the nonreciprocity can emerge when an external magnetic field is applied to chiral magnets whose spin structures crucially depend on an asymmetric exchange interaction, that is, the Dzyaloshinskii-Moriya interaction (DMI). Here we demonstrate that the nonreciprocity can arise even without the DMI. We demonstrate this idea for the chiral antiferromagnet ${\mathrm{Ba}}_2{\mathrm{NbFe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14}$ whose DMI is very small and chiral spin structure arises mainly from the competition between symmetric exchange interactions. We show that when an external magnetic field is applied, an asymmetric energy-gap shift occurs and the spin wave becomes nonreciprocal from the competition between symmetric exchange interactions and the external magnetic field.

By combining the continuous matrix product state (cMPS) representation for quantum fields in the continuum with standard optimization techniques for matrix product states (MPS) on the lattice, we obtain an approximation $|\mathrm{\Psi }\rangle$, directly in the continuum, of the ground state of nonrelativistic quantum field theories. This construction works both for translation-invariant systems and in the more challenging context of inhomogeneous systems, as we demonstrate for an interacting bosonic field in a periodic potential. Given the continuum Hamiltonian $H$, we consider a sequence of discretized Hamiltonians ${\left\{H\left({\epsilon }_{\alpha }\right)\right\}}_{\alpha =1,2,\cdots ,p}$ on increasingly finer lattices with lattice spacing ${\epsilon }_1>{\epsilon }_2>\cdots >{\epsilon }_p$. We first use energy minimization to optimize an MPS approximation $\left|\mathrm{\Psi }\right({\epsilon }_1)\rangle$ for the ground state of $H\left({\epsilon }_1\right)$. Given the MPS $\left|\mathrm{\Psi }\right({\epsilon }_{\alpha })\rangle$ optimized for the ground state of $H\left({\epsilon }_{\alpha }\right)$, we use it to initialize the energy minimization for Hamiltonian $H\left({\epsilon }_{\alpha +1}\right)$, resulting in the optimized MPS $\left|\mathrm{\Psi }\right({\epsilon }_{\alpha +1})\rangle$. By iteration we produce an optimized MPS $\left|\mathrm{\Psi }\right({\epsilon }_p)\rangle$ for the ground state of $H\left({\epsilon }_p\right)$, from which we finally extract the cMPS approximation $|\mathrm{\Psi }\rangle$ for the ground state of $H$. Two key ingredients of our proposal are as follows: (i) a procedure to discretize $H$ into a lattice model where each site contains a two-dimensional vector space (spanned by vacuum $|0\rangle$ and one boson $|1\rangle$ states), and (ii) a procedure to map MPS representations from a coarser lattice to a finer lattice.

Measurements of dc magnetization $\left(M\right)$ and electrical resistivity $\left(\rho \right)$ have been carried out as a function of temperature $\left(T\right)$ for layered oxide ${\mathrm{Li}}_x{\mathrm{CoO}}_2 \left(0.51\le x\le 1.0\right)$ using single-crystal specimens. After slow cooling of the specimens down to 10 K, both of the $M\left(T\right)$ and $\rho \left(T\right)$ curves are found to exhibit a clear anomaly due to the occurrence of ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ charge ordering (CO) at $T_{\mathrm{S}}\sim 155\mathrm{K}$ for $0.6\le x\le 0.98$ (at $T_{\mathrm{S}}\sim 180–190\mathrm{K}$ for $0.5\le x\le 0.55)$. After rapid cooling of the specimens, additional anomalies are observed related to the onset of Li diffusion at $T_{\mathrm{F}1}\sim 370\mathrm{K}$ and/or $T_{\mathrm{F}2}=120–130\mathrm{K}$. Due to phase mixing with compositions of nearly ${\mathrm{LiCoO}}_2$ and ${\mathrm{Li}}_{2/3}{\mathrm{CoO}}_2$, the specimens with $0.7\lesssim x\lesssim 0.9$ show anomalies both at $T_{\mathrm{F}1}$ and $T_{\mathrm{F}2}$. For $0.6\lesssim x\lesssim 0.9$, the resistivity measured after rapid cooling is found to be fairly larger than that measured after slow cooling below $T_{\mathrm{S}}$. The enhanced resistivity can be explained by the scenario that disordered ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ arrangements, which have been observed and revealed to have an insulating electronic structure contrasting to the regular CO state in the previous scanning tunneling microscopy measurements [K. Iwaya et al., Phys. Rev. Lett. 111, 126104 (2013)], are realized due to the formation of an amorphouslike structure of Li ions after rapid cooling via interlayer Coulomb coupling. An electronic phase diagram for $0.5\le x\le 1.0$ is proposed.

We present a microscopic description of electronic reconstruction in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$, a system which undergoes a pressure-induced insulator-metal transition followed by a superconducting phase at 24 K. We stress the importance of multiorbital electron-electron interactions for a consistent understanding of its intrinsic Mott-insulating and pressurized, orbital-selective metallic normal states. We explain the first-order nature of the Mott transition, showing that it is driven by dynamical spectral weight transfer in response to changes in the on-site Coulomb interaction to bandwidth ratio. As a by-product of this analysis, we unearth how dynamical correlations underpin spectroscopy and resistivity responses, in good agreement with experiment. Upon electron/hole doping, carrier localization is found to persist because the chemical potential lies in a gap structure with vanishing states near the Fermi energy. We detail the implications of our microscopic analysis for the underlying physics which emerges in the normal state of a compressed ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ superconductor.

Luminescence and optical absorption due to radiation damage centers in silicon have been studied exhaustively for decades, but are receiving new interest for applications as emitters for integrated silicon photonic technologies. While a variety of other optical transitions have been found to be much sharper in enriched ${}^{28}\mathrm{Si}$ than in natural Si, due to the elimination of inhomogeneous isotopic broadening, this has not yet been investigated for radiation damage centers. We report results for the well-known G, W, and C damage centers in highly enriched ${}^{28}\mathrm{Si}$, with optical linewidth improvements in some cases of over two orders of magnitude, revealing previously hidden fine structure in the G-center emission and absorption. These results have direct implications for the linewidths to be expected from single-center emission, even in natural Si, and for models for the G-center structure. The advantages of ${}^{28}\mathrm{Si}$ can be readily extended to the study of other radiation damage centers in Si.

We investigate the quenching of the photoluminescence (PL) from the divacancy defect in $4H$-SiC consisting of a nearest-neighbor silicon and carbon vacancies. The quenching occurs only when the PL is excited below certain photon energies (thresholds), which differ for the four different inequivalent divacancy configurations in $4H$-SiC. An accurate theoretical ab initio calculation for the charge-transfer levels of the divacancy shows very good agreement between the position of the (0/−) level with respect to the conduction band for each divacancy configuration and the corresponding experimentally observed threshold, allowing us to associate the PL decay with conversion of the divacancy from neutral to negative charge state due to capture of electrons photoionized from other defects (traps) by the excitation. Electron paramagnetic resonance measurements are conducted in the dark and under excitation similar to that used in the PL experiments and shed light on the possible origin of traps in the different samples. A simple model built on this concept agrees well with the experimentally observed decay curves.

The control of phonon scattering by interfaces is critical to the manipulation of heat conduction in composite materials and semiconducting nanostructures. However, one of the factors limiting our understanding of elastic phonon scattering is the lack of a computationally efficient approach for describing the phenomenon in a manner that accounts for the atomistic configuration of the interface and the exact bulk phonon dispersion. Building on the atomistic Green's function (AGF) technique for ballistic phonon transport, we formulate an atomistic $S$-matrix method that treats bulk phonon modes as the scattering channels and can determine the numerically exact scattering amplitudes for individual two-phonon processes, enabling a highly detailed analysis of the phonon transmission and reflection spectrum as well as the directional dependence of the phonon scattering specularity. Explicit formulas for the individual phonon reflection, absorption, and transmission coefficients are given in our formulation. This AGF-based $S$-matrix approach is illustrated through the investigation of (1) phonon scattering at the junction between two isotopically different but structurally identical carbon nanotubes, and (2) phonon boundary scattering at the zigzag and armchair edges in graphene. In particular, we uncover the role of edge chirality on phonon scattering specularity and explain why specularity is reduced for the ideal armchair edge. The application of the method can shed new light on the relationship between phonon scattering and the atomistic structure of interfaces.

We classify all possible boundary conditions (BCs) for a Weyl material into two classes: (i) BC that mixes the spin projection but does not change the chirality attribute, and (ii) BC that mixes the chiralities. All BCs are parametrized with angular variables that can be regarded as mixing angles between spins or chiralities. Using the Green's function method, we show that these two BCs faithfully reproduce the Fermi arcs. The parameters are ultimately fixed by the orientation of Fermi arcs. We build on our classification and show that in the presence of a background magnetic field, only the second-type BC gives rise to nontrivial Landau orbitals.

Low-temperature illumination of a two-dimensional electron gas in GaAs quantum wells is known to greatly improve the quality of high-field magnetotransport. The improvement is known to occur even when the carrier density and mobility remain unchanged, but what exactly causes it remains unclear. Here, we investigate the effect of illumination on microwave photoresistance in low magnetic fields. We find that the amplitude of microwave-induced resistance oscillations grows dramatically after illumination. Dingle analysis reveals that this growth reflects a substantial increase in the single-particle (quantum) lifetime, which likely originates from the light-induced redistribution of charge enhancing the screening capability of the doping layers.

We develop a theory describing a parameter-based phase diagram to be associated with materials incorporating skipped-valence ions [Phys. Rev. Lett. 61, 2713 (1988)]. We use a recently developed tight-binding approach for the bismuthates to study the phase diagram exhibiting a crossover from a bond disproportionated (BD) to a charge disproportionated (CD) system in addition to the presence of a new metallic phase. We argue that three parameters determine the underlying physics of the BD-CD crossover when electron correlation effects are small: the hybridization between $\mathrm{O}-2p_{\sigma }$ and $\mathrm{Bi}-6s$ orbitals ($t_{sp\sigma }$), the charge-transfer energy between $\mathrm{Bi}-6s$ and $\mathrm{O}-{\mathrm{a}}_{1g}$ molecular orbitals ($\mathrm{\Delta }$), and the width of the oxygen sublattice band ($W$). In the BD system, we estimate the effective attractive interaction $U$ between the two holes on the same hybrid ${\mathrm{A}}_{1g}$ orbital composed of a linear combination of $\mathrm{O}-{\mathrm{a}}_{1g}$ and $\mathrm{Bi}-6s$ orbitals. Although we concentrate here on the example of the bismuthates, the basic ideas can be directly transferred to other perovskites with negative charge-transfer energy, like $Re{\mathrm{NiO}}_3$ ($Re$: rare-earth element), $\mathrm{Ca}\left(\mathrm{Sr}\right){\mathrm{FeO}}_3, {\mathrm{CsTIF}}_3$, and ${\mathrm{CsTlCl}}_3$.

The perovskite Co oxides ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{CoO}}_3$ exhibit at around $x\sim 0.5$ an unusual transition at temperature $T_{\mathrm{S}}$ [S. Tsubouchi et al. Phys. Rev. B 66, 052418 (2002)] with no space group change and no long-range magnetic order. We measured inelastic neutron scattering intensities of magnetic excitations $I(\mathit{Q},\omega$) for two single crystals of (${\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y{)}_{1-x}{\mathrm{Ca}}_x{\mathrm{CoO}}_3$ in which the $x$ region of the transition is widened by the Pr-site doping to study whether the recent proposal of the excitonic condensation model is relevant to this “hidden order transition” of the system. While ${\chi }^{\prime \prime }(\mathit{Q},\omega )$ seems to have characteristics of strongly correlated Co $3d$ electrons above $T_{\mathrm{S}}$, it abruptly exhibits a weak feature reminiscent of a pseudogaplike structure at $T_{\mathrm{S}}$ with decreasing $T$. The first peak of the Pr crystal-field excitations has also been observed at low temperatures. Here, using a model of the coexistence of the exciton condensed phase (EC phase) with the nearly or weakly ferromagnetic one, we show that the EC phase appears as the collective transition in ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{CoO}}_3$.

We study the two-dimensional $t-J$ model with second-neighbor hopping parameter $t^{\prime }$ and in a broad range of doping $\delta$ using a closed set of equations from the extremely correlated Fermi liquid theory. We obtain asymmetric energy distribution curves and symmetric momentum distribution curves of the spectral function, consistent with experimental data. We further explore the Fermi surface and local density of states for different parameter sets. Using the spectral function, we calculate the resistivity, Hall number, and spin susceptibility. The curvature change in the resistivity curves with varying $\delta$ is presented and connected to intensity loss in angle-resolved photoemission spectroscopy experiments. We also discuss the role of the superexchange $J$ in the spectral function and the resistivity in the optimal to overdoped density regimes.

Nonresonant inelastic x-ray scattering (NIXS) has been performed on single crystals of ${\mathrm{UO}}_2$ to study the directional dependence of higher-order-multipole scattering from the uranium $O_{4,5}$ edges (90–110 eV). By comparing the experimental results with theoretical calculations the symmetry of the ground state is confirmed directly as the crystal-field (CF) ${\mathrm{\Gamma }}_5$ triplet state within the $J=4$ manifold. The results demonstrate that the directional dichroism of the NIXS spectra is sensitive to the CF strength and establish NIXS as a tool for probing CF interactions quantitatively.

In this paper, we discuss the effect of a periodically driving circularly polarized laser beam in the high-frequency limit, on the band structure and thermal transport properties of type-I and type-II Weyl semimetals (WSMs). We develop the notion of an effective Fermi surface stemming from the time-averaged Floquet Hamiltonian and discuss its effects on the steady-state occupation numbers of electrons and holes in the linearized model. In order to compute the transport coefficients averaged over a period of the incident laser source, we employ the Kubo formalism for Floquet states and show that the Kubo formula for the conductivity tensor retains its well-known form with the difference that the eigenstates and energies are replaced by the Floquet states and their quasienergies. We find that for type-I WSMs the anomalous thermal Hall conductivity grows quadratically with the amplitude $A_0$ of the U(1) gauge field for low tilt, while the Nernst conductivity remains unaffected. For type-II WSMs, the Hall conductivity decreases nonlinearly with $A_0$ due to the contribution from the physical momentum cutoff, required to keep finite electron and hole pocket sizes, and the Nernst conductivity falls off logarithmically with $A_0^2$. These results may serve as a diagnostic for material characterization and transport parameter tunability in WSMs, which are currently the subject of a wide range of experiments.