The Bragg diffraction of neutrons and x rays are well suited to the task of determining the distribution of magnetization in crystals. Applications of the two techniques proceed by contrasting observed intensities with intensities calculated with a specific model, and changing the model as need be to achieve satisfactory agreement. An all-in–all-out (AIAO) magnetic configuration of magnetic dipoles on a cubic face-centered lattice with networks of corner-sharing tetrahedra is often mentioned in the context of pyrochlore oxides, for example, but the corresponding neutron and x-ray-diffraction patterns appear to not have been calculated. Our results for patterns of Bragg spots from an AIAO magnetic configuration defined by a magnetic space group are symmetry informed and yield exact reflection conditions. Specifically, a long-range order of magnetic dipoles is forbidden in our model. Bulk properties arise from higher-order multipoles that include quadrupoles and octupoles. Bragg spots that exclude all magnetic multipoles other than an octupole have been discovered, and they can be observed by both neutron diffraction and resonant x-ray diffraction. All magnetic multipoles allowed in diffraction by cerium ions $\left(4f^1\right)$ are presented in terms of coefficients in a well-documented and unusual magnetic ground state. Symmetry of the cerium site in the cubic structure constrains the coefficients. Our scattering amplitudes have an application in both neutron- and x-ray diffraction experiments on ${\mathrm{Ce}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$, for example, and searches for the sought-after cerium octupole. Also presented for future use is a result for the total, energy-integrated magnetic neutron-scattering intensity by a powder sample.

We investigate the possibility and implications of interlayer coherence in electrically isolated superconducting bilayers. We find that in a mean-field approximation bilayers can have superconducting and excitonic order simultaneously if repulsive interactions between layers are sufficiently strong. The excitonic order implies interlayer phase coherence and can be studied in a representation of symmetric and antisymmetric bilayer states. When both orders are present we find several solutions of the mean-field equations with different values of the the symmetric and antisymmetric state pair amplitudes. The mixed state necessarily has nonzero pair amplitudes for electrons in different layers in spite of the repulsive interlayer interactions, and these are responsible for spatially indirect Andreev reflection processes in which an incoming electron in one layer can be reflected as a hole in the opposite layer. We evaluate layer diagonal and off-diagonal current-voltage relationships that can be used to identify this state experimentally.

We study the effects of quasidisorder and Anderson disorder on a two-dimensional topological superconductor with an applied external magnetic field. The cases of a $p$-wave superconductor and a noncentrosymmetric superconductor with mixed $p$- and $s$-wave pairings and the Rashba spin-orbit coupling are studied. We show that, for a perpendicular magnetic field, the introduction of quasidisorder leads to the appearance of topological phases in new regions, characterized by an integer value of the Chern number. For a parallel magnetic field, we identify regimes with the appearance of new Majorana flat bands and also new unidirectional Majorana edge states, as quasidisorder is introduced. We show that the Majorana flat bands have a quantized Berry phase of $\pi$ and identify it as a topological invariant. Two topological transitions are identified and the values of the critical exponents $z$ and $\nu$ are obtained. The fractal nature of the eigenstates is discussed both for Anderson and Aubry-André disorders.

The odd-frequency superconducting state appears generally in hybrid structures consisting of conventional superconductors and other materials, and features electrons that form temporally nonlocal Cooper pairs. The quasiclassical theory of superconductivity has been extensively used to model such systems, finding in many cases excellent agreement with experimental measurements. Therefore, it is of interest to study effective models of odd-frequency superconductivity to predict new phenomena associated with this form of unconventional pairing. We establish necessary criteria that the quasiclassical Green's functions in odd-frequency superconducting systems in the dirty limit must satisfy in order to be physically reasonable, including conservation of spectral weight. We show that it is possible to write down effective models, which satisfy all the above-mentioned criteria, but which still predict different behavior when it comes to the density of states and the magnetic response of the superconductor. For instance, an effective model for the odd-frequency anomalous Green's function that gives a conserved spectral weight can yield either a peaked or gapped density of states at the Fermi energy, and exhibit conventional, zero, or unconventional Meissner response. This finding demonstrates the importance of carefully considering the properties of effective models describing odd-frequency superconductivity in order to obtain physically reasonable results.

We revisit the Hall effect and magnetoresistivity by incorporating the anisotropic scattering caused by apical oxygen vacancies in overdoped La-based cuprates. The theoretical calculations within the Fermi liquid picture agree well with a handful of anomalous magnetotransport data, better than the results using an isotropic scattering rate alone. In particular, we obtain the upturn of Hall coefficient $R_H$ with decreasing temperature $T$, the initial drop of $R_H$ in magnetic field $B$ in all overdoped regimes, the linear resistivity $\rho$ versus $B$ near the van Hove doping level, the temperature dependence of the magnetoresistivity ratio, and the violation of Kohler's law. These results suggest that many of the anomalous transport behaviors in overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ could actually be understood within the Fermi liquid picture.

Recent years have witnessed the rising of halide perovskites for potential applications in photovoltaic and optoelectric devices. Due to the tilting motions of the octahedrons and rattling of the filling atoms, cubic halide perovskites show strong anharmonicity and imaginary frequencies in the phonon dispersion, which brings a great challenge to the prediction of carrier transports based on many-body theory. We have investigated the effect of quartic lattice anharmonicity on the carrier transports of cubic ${\mathrm{CsSnI}}_3$ and ${\mathrm{CsPbI}}_3$ in a comparative perspective by first-principles calculations. The hybrid functional of HSE06 was employed to get accurate band gaps and the self-consistent phonon method was used to renormalize interatomic force constants. Based on the electron-phonon Wannier interpolation and Boltzmann transport equation, the carrier mobilities and mode-resolved scattering rates were calculated and the dominant scattering channels were analyzed. Our results reveal that the mobility takes 595.9 and 84.5 ${\mathrm{cm}}^2/\mathrm{Vs}$ at room temperature for cubic ${\mathrm{CsSnI}}_3$ and ${\mathrm{CsPbI}}_3$, respectively, in good agreement with the experiment results. The longitudinal stretching mode of the Sn(Pb)-I bond with frequency of 120 ${\mathrm{cm}}^{-1}$ plays a dominant role in carrier scattering. In comparison, the acoustic modes and the rattling modes have a negligible effect on carrier scattering. Calculations of the band scattering rates show that deep valleys emerge at the band edges for both the highest valence band and the lowest conduction band, which is responsible for the slow relaxation of hot carriers. The strength of intraband electron-phonon coupling (EPC) between the band edge and the Sn(Pb)-I bond stretching mode is far larger than those of other modes, which leads to the dominant scattering in carrier transport. Compared with the harmonic approximation of atom interactions, the inclusion of quartic anharmonicity leads to the enhancement of atom interactions, which decreases the EPC strength and consequently increases the carrier mobilities.

Ferroelectric group-IV monochalcogenides $MX$ ($M=\mathrm{Sn}/\mathrm{Ge}$ and $X=\mathrm{S}/\mathrm{Se}$) are of particular importance for the new-generation photovoltaic devices due to their special in-plane spontaneous polarization resulting in a bulk photovoltaic effect. However, the spontaneous polarization and its related dynamical photoresponse under an ultrafast laser excitation are rarely studied via a noncontact all-optical method. Herein, the vertically grown SnS nanosheets have been synthesized by a physical vapor deposition (PVD) method and the corresponding spontaneous polarization is characterized via the terahertz (THz) emission spectroscopy. The primary THz emission mechanism is ascribed to the shift current, which is proved by the THz wave dependences on the pump light polarization angle and pump fluence. Interestingly, the shift current depends on the spontaneous polarization along the SnS nanosheet grown direction, according to the reversed polarity of the THz wave under opposite directional excitations. Furthermore, the amplitude and phase of the THz wave are modulated by the pump light polarization angle, resulting in the elliptical THz wave emission with tunable ellipticity and orientation angle. Our study has shed light on the shift current of ferroelectrics and controllable polarized THz wave generation, which provides application prospects for ferroelectric-based photovoltaics, nonlinear photonics, and THz polarized devices.

In this paper, we provide a systematically convergable and efficient numerical approach to simulate multitime correlation functions in the Mahan-Nozières-De Dominicis model, which crudely mimics the spectral properties of doped two-dimensional (2D) semiconductors such as monolayer transition metal dichalcogenides. We apply this approach to study the coherent 2D electronic spectra of the model. We show that several experimentally observed phenomena, such as peak asymmetry and coherent oscillations in the waiting-time dependence of the trion-exciton cross peaks of the 2D rephasing spectrum, emerge naturally in our approach. Additional features are also present which find no correspondence with experimentally expected behavior. We trace these features to the infinite hole mass property of the model. We use this understanding to construct an efficient approach which filters out configurations associated with the lack of exciton recoil, enabling the connection to previous work and providing a route to the construction of realistic 2D spectra over a broad doping range in 2D semiconductors.

Hole semiconductor nanowires (NW) are promising platforms to host spin qubits and Majorana bound states for topological qubits because of their strong spin-orbit interactions (SOI). The properties of these systems depend strongly on the design of the cross section and on strain, as well as on external electric and magnetic fields. In this paper, we analyze in detail the dependence of the SOI and $g$ factors on the orbital magnetic field. We focus on magnetic fields aligned along the axis of the NW, where orbital effects are enhanced and result in a renormalization of the effective $g$ factor up to $400\%$, even at small values of magnetic field. We provide an exact analytical solution for holes in Ge NWs and we derive an effective low-energy model that enables us to investigate the effect of electric fields applied perpendicular to the NW. We also discuss in detail the role of strain, growth direction, and high-energy valence bands in different architectures, including Ge/Si core/shell NWs, gate-defined one-dimensional channels in planar Ge, and curved Ge quantum wells. By comparing NWs with different growth directions, we find that the isotropic approximation is well justified. Curved Ge quantum wells feature large effective $g$ factors and SOI at low electric field, ideal for hosting Majorana bound states. In contrast, at strong electric field, these quantities are independent of the field, making hole spin qubits encoded in curved quantum wells to good approximation not susceptible to charge noise, and significantly boosting their coherence time.

One-dimensional Su-Schrieffer-Heeger (SSH) chains are one of the simplest topological models and have been studied extensively. The two-dimensional $\left(2\mathrm{D}\right)$ version has been confirmed to have a nontrivial topology. In this work, we further extend the $2\mathrm{D}$ SSH model by constructing different configurations, including all possible configurations with 4-site unit cells and another two complicated, but typical, configurations with 8- and 36-site unit cells, respectively. We calculate and analyze the electronic structures and topologies of these SSH models in detail and identify several rich and novel properties, such as topologically protected edge states, metallic chains with different shapes, and a bulk–edge separation in a metal system. In particular, a flat-band feature of topological edge states is obtained. By analyzing the spatial distribution of these edge states, we explain the origin of the flat edge bands and show that bonding squares play a crucial role in the formation of flat bands. Our study can be generalized to more configurations and higher dimensions, providing a basis for further theoretical and experimental explorations.

Dynamical quantum phase transitions (DQPTs) are a powerful concept of probing far-from-equilibrium criticality in quantum many-body systems. With the strong ongoing experimental drive to quantum simulate lattice gauge theories, it becomes important to investigate DQPTs in these models in order to better understand their far-from-equilibrium properties. In this work, we use infinite matrix product state techniques to study DQPTs in spin-$S U\left(1\right)$ quantum link models. Although we are able to reproduce literature results directly connecting DQPTs to a sign change in the dynamical order parameter in the case of $S=1/2$ for quenches starting in a vacuum initial state, we find that for different quench protocols or different values of the link spin length $S>1/2$ this direct connection is no longer present. In particular, we find that there is an abundance of different types of DQPTs not directly associated with any sign change of the order parameter. Our findings indicate that DQPTs are fundamentally different between the Wilson-Kogut-Susskind limit and its representation through the quantum link formalism.

The quantum size effect has a significant impact on electrons such that it can even change their topologically protected properties. An example of this phenomenon is the gap opening in the topologically protected gapless surface state in finite-thickness topological-insulator films. However, much is not known about the quantum size effect in topological heterojunctions. In this paper, by calculating the single-particle spectrum of $\mathrm{Bi}/{\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x$ based on the well-known Liu-Allen model, we found that the strong quantum size effect in topological heterojunctions yields an unexpected band alignment. The wave functions permeate each other through the attached materials, and this occurs even in 80-nm-thick heterojunctions. Furthermore, we theoretically found that one of the two major spectra obtained from the Bi surface does not represent the true surface state of Bi. This finding may overturn the previous understanding of the topological surface state of Bi.

We derive the analytical solutions of surface states in a series of lattice models for three-dimensional topological insulators and their nontopological counterparts based on an ansatz. A restriction on the spin-flip matrices in nearest-neighbor hopping characterizes the series. This restriction affords the ansatz and favors analytical solvability of surface-state eigenvectors. Despite the restriction, the series retains sufficient designability to describe various types of surface states. We also describe how it can serve as a tractable tool for elucidating unique phenomena on topological surfaces.

Using mapping between topological defects in an easy-plane magnet and electrical charges, we study interplay between vorticity and spin currents. We demonstrate that the flow of vorticity is accompanied by the transverse spin current generation; an effect which can be termed as the spin Hall effect of vorticity. We study this effect across the BKT transition and establish the role of dissipation and spin nonconservation in the crossover from spin superfluidity to diffusive spin transport. Our results pave the way for low power computing devices relying on vorticity and spin flows that can propagate over long distances.

In antiferromagnetic (AFM) materials, localized self-sustaining states called solitons, namely, domain walls (DWs) and skyrmions, can be efficiently driven by currents and achieve velocities of several kilometers per second. These solitons are massive particles and therefore cannot travel faster than a limiting velocity akin to the speed of light for the material. The specifics of these high-velocity dynamics, in which solitons begin to display relativistic effects, have been well understood for the case of DWs (single-dimensional particles). Here, we perform an extensive and systematic atomistic study of both one-dimensional and two-dimensional soliton dynamics in chiral magnetic materials, both at and away from angular momentum compensation. We develop an elastic restorative force model to explain the dynamic skyrmion deformations, supported by simulation data. We claim that these deformations arise from a local imbalance of gyrotropic forces. We also present an outlook on the role of skyrmion compactness in their deformation patterns, velocity limits, as well as the absence of behaviors similar to ones observed in relativistic DWs. We claim that limits on skyrmion compactness also impede their ability to reach the velocity regime where relativistic effects begin to occur in rapidly moving DWs, due to the critical skyrmion breakdown behavior. These results could prove to be significant to the field of spintronics, as well as the potential applications of skyrmions for novel logic and memory devices.

Determining the nature of band gaps in transition-metal compounds remains challenging. We present a first-principles study on electronic and optical properties of CoO using hybrid functional pseudopotentials. We show that optical absorption spectrum can provide a clear fingerprint to distinguish between Mott-Hubbard, intermediate, and charge-transfer insulators. This discrimination is reflected by the qualitative difference in peak satellites due to unique interplay between $d\text{−}d$ and $p\text{−}d$ excitations, thus allowing identification from experimental data alone, unlike the existing methods that require additional theoretical interpretation. We conclude that the CoO is an intermediate rather than a Mott-Hubbard insulator as is initially believed.

We study spin-fluctuation mediated divergent superconducting fluctuations in a Luttinger liquid proximity-coupled to a spin chain. Our study provides insight into how spin fluctuations can induce superconductivity in a strongly correlated non-Fermi liquid with repulsive electronic interactions only. The electrons in the system are governed by the Extended Hubbard Hamiltonian and are coupled to a chain of localized spins modeled by the spin-$\frac{1}{2}XX$ Hamiltonian. Using a multichannel Luttinger liquid approach, we determine the phase diagram of the metal chain. We find that spin-polarized triplet superconducting correlations persist for repulsive electronic interactions for sufficiently large interchain couplings.

We investigate the effect of multiphoton emission on polarization-entangled photon pairs from a coherently driven quantum dot by comparing quantum state tomography and second-order autocorrelation measurements as a function of the excitation power. We observe that the relative (absolute) multiphoton emission probability is as low as $p_m=\left(5.6\pm 0.6\right)\times {10}^{-4}[p_2=\left(1.5\pm 0.3\right)\times {10}^{-4}]$ at the maximum source brightness, with a negligible effect on the degree of entanglement. In contrast with probabilistic sources of entangled photons, the multiphoton emission probability and the degree of entanglement remain practically unchanged against the excitation power over multiple Rabi cycles, while observing oscillations in the second-order autocorrelation function by more than one order of magnitude. Our results, explained by a model which links the second-order autocorrelation function to the actual multiphoton contribution in the two-photon density matrix, highlight that quantum dots can be regarded as a multipair-free source of entangled photons in the solid state.

In the present work, we explored the interplay between the lattice and magnetic degrees of freedom in a rare-earth nickelate ${\text{EuNiO}}_3$, by carrying out temperature-dependent structural analysis in conjunction with distortion mode analysis. The temperature-dependent powder synchrotron x-ray diffraction (SXRD) studies revealed the presence of an orthorhombic $Pbnm$ phase (tilt system $a_0^{-}{a}_0^{-}{c}_0^{+}$), with an elementary perovskite (pseudomonoclinic) cell, over the analyzed temperature range, i.e., 100–623 K. Further, we observed two distinct anomalies in the temperature-dependent evolution of pseudomonoclinic cell parameters $(c_p/a_p,$ $\gamma ,$ $V_{\text{mono}})$ around 463 K and 200 K corresponding to respective isosymmetric metal-insulator transition temperature $\left(T_{M\text{−}I}\right)$, and Neel temperature $\left(T_N\right)$ linked with a volume gain at low temperatures dictating a magnetoelastic coupling in the system. We show the existence of two distinct pseudomonoclinic phases, viz., ${\mathrm{Mono}}_{\mathrm{metal}}$ (T > $T_{M\text{−}I}$) and ${\mathrm{Mono}}_{\mathrm{insulator}}$ (T < $T_{M\text{−}I}$), where the latter is more distorted than the former. The transition from ${\mathrm{Mono}}_{\mathrm{metal}}$ to ${\mathrm{Mono}}_{\mathrm{insulator}}$ at $T_{M\text{−}I}$ is reminiscent of a phase transition from orthorhombic (metallic) to monoclinic (insulating) phase, observed in other members of the rare-earth family. In addition, $T_{M\text{−}I}$ and $T_N$ are clearly evident by the nonanalytical behavior of the condensed soft phonon modes amplitude corresponding to the zone boundary of the cubic Brillouin zone, viz., $X_5^{+}(q=0,1/2,0)$ and $R_5^{+}(q=1/2,1/2,1/2)$ as a function of temperature.

Chaotic transverse-field Ising model with measurements exhibits interesting purification dynamics. An ensemble of nonunitary dynamics of a chaotic many-body system with measurements exhibits a purification phase transition. We numerically find that the law of the increase dynamics of the purity changes by projective measurements in the model. In order to study this behavior in detail, we construct the formalism of the tripartite mutual information (TMI) for a nonunitary time evolution operator by using the state-channel map. The numerical result of the saturation value of the TMI indicates the existence of a measurement-induced phase transition. This implies the existence of two distinct phases, a mixed phase and purified phase. Furthermore, the real-space spread of the TMI is investigated to explore spatial patterns of information spreading. Even in the purified phase, the spatial pattern of the light-cone spread of quantum information is not deformed, but its density of information propagation is reduced on average by the projective measurements. We also find that this spatial pattern of the TMI distinguishes the chaotic and integrable regimes of the system.