Berry-phase effects in spin systems lead to the suppression of tunneling effects when different tunneling paths interfere destructively. Such effects have been seen in several single-molecule magnets (SMMs) through measurements of magnetization dynamics, where the experimental signal may arise from the contributions of numerous energy levels. Here we present experimental measurements of Berry-phase interference effects that are determined through electron-spin resonance on a fourfold symmetric SMM. Specifically, we measure transitions between tunnel-split excited states in the ${\mathrm{Ni}}_4$ SMM in the presence of a transverse field in the hard plane of the crystalline sample. By using a home-built rotation apparatus, the direction of the sample can be changed in situ so that the field direction can be swept through the entire hard plane of the sample. When the field is in certain directions in the plane, we observe a splitting of the transition, a hallmark of Berry-phase interference. The experimental results are well reproduced by numerical simulations, and fitting of the data provides information about the effects of dipolar interactions and sample misalignment.

We consider electron transport in a model of a spinless superconductor described by a Kitaev-type lattice Hamiltonian where the electron interactions are modeled through a superconducting pairing term. The superconductor is sandwiched between two normal metals that are kept at different temperatures and chemical potentials and that are themselves modeled as noninteracting spinless fermions. For this setup we compute the exact steady-state properties of the system using the quantum-Langevin-equation approach. Closed-form exact expressions for particle current, energy current, and other two-point correlations are obtained in the Landauer-type forms and involve two nonequilibrium Green's functions. The current expressions are found to be the sum of three terms having simple physical interpretations. We then discuss a numerical approach where we construct the time evolution of the two-point correlators of the system from the eigenspectrum of the complete quadratic Hamiltonian describing the system and leads. By starting from an initial state corresponding to the leads in thermal equilibrium and the system in an arbitrary state, the long-time solution for the correlations, before recurrence times, gives us steady-state properties. We use this independent numerical method for verifying the results of the exact solution. We also investigate analytically the presence of high-energy bound states and obtain expressions for their contributions to two-point correlators. As applications of our general formalism we present results on thermal conductance and on the conductance of a Kitaev chain with next-nearest-neighbor interactions which allows topological phases with different winding numbers.

The electronic and crystal structures of $\left({\mathrm{Na}}_{1-x}{\mathrm{Ca}}_x\right){\mathrm{Cr}}_2{\mathrm{O}}_4$ have been studied in detail by combining photoelectron spectroscopy (PES), x-ray absorption spectroscopy (XAS), and x-ray diffraction (XRD). The PES results suggest a gap opening at the Fermi level with decreasing temperature and/or increasing the Ca concentration. The XAS spectra at the O $K$-absorption edge suggest a slight increase of the O $2p$–Cr $3d$ hybridization at low temperatures in ${\mathrm{NaCr}}_2{\mathrm{O}}_4$ and ${\mathrm{Na}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{Cr}}_2{\mathrm{O}}_4$, which corresponds to the appearance of the antiferromagnetic order. However, XRD showed no corresponding structural transition. The experimental results were compared with spin-resolved density functional theory (DFT) calculations. In ${\mathrm{Na}}_{1-x}{\mathrm{Ca}}_x{\mathrm{Cr}}_2{\mathrm{O}}_4$ the pre-edge intensity of the XAS spectra at the O $K$-absorption edge is strongly suppressed with increasing $x$, in accordance with the gap opening by Ca doping. This observation is consistent with the DFT calculation, where the density of states just above and below the Fermi level diminishes as the electron is doped into ${\mathrm{NaCr}}_2{\mathrm{O}}_4$.

Existing quantum Monte Carlo studies have investigated the properties of fermions on a Lieb (${\mathrm{CuO}}_2$) lattice interacting with on-site or near-neighbor electron-electron coupling. Attention has focused on the interplay of such interactions with the macroscopic degeneracy of local zero-energy modes, from which Bloch states can be formed to produce a flat band in which energy is independent of momentum. The resulting high density of states, in combination with the Stoner criterion, suggests that there should be pronounced instabilities to ordered phases. Indeed, a theorem by Lieb rigorously establishes the existence of ferrimagnetic order. Here we study the charge density wave phases induced by electron-phonon coupling on the Lieb lattice, as opposed to previous work on electron-electron interactions. Our key result is the demonstration of charge density wave phases at one third and two thirds fillings, characterized by long-range density-density correlations between doubly occupied sites on the minority or majority sublattices, and an accompanying gap. We also compute the transition temperature to the ordered phase as a function of the electron-phonon coupling.

Jacutingate, a recently discovered Brazilian naturally occurring mineral, has been shown to be an experimental realization of the Kane-Mele topological model. In this paper we present a class of materials, $M_{2}N{X}_3$ ($M=\mathrm{Ni}$, Pt, and Pd; $N=\mathrm{Zn}$, Cd, and Hg; and $X=\mathrm{S}$, Se, and Te), sharing jacutingaite's key features, i.e., high stability and a topological phase. By employing first-principles calculations we extensively characterize the energetic stability of this class while showing a common occurrence of the Kane-Mele topological phase. Here we present Pt-based materials surpassing jacutingaite's impressive topological gap and lower exfoliation barrier while retaining its stability.

Through a thorough magnetotransport study of antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ (MBT) thick films, a positive linear magnetoresistance (LMR) with a two-dimensional (2D) character is found in high perpendicular magnetic fields and at temperatures up to at least 260 K. The nonlinear Hall effect further reveals the existence of high-mobility surface states in addition to the bulk states in MBT. We ascribe the 2D LMR to the high-mobility surface states of MBT, thus unveiling a transport signature of surface states in thick MBT films. A suppression of LMR near the Néel temperature of MBT is also noticed, which might suggest the gap opening of surface states due to the paramagnetic-antiferromagnetic phase transition of MBT. Besides these, the failure of the disorder and quantum LMR model in explaining the observed LMR indicates new physics must be invoked to understand this phenomenon.

We show that a quantum scar state, an atypical eigenstate breaking eigenstate thermalization hypothesis embedded in a many-body energy spectrum, can be constructed in flat band systems. The key idea of our construction is to make use of orthogonal compact localized states. We concretely discuss our construction scheme, taking a sawtooth flat lattice system as an example, and numerically demonstrate the presence of a quantum scar state. Examples of higher-dimensional systems are also addressed. Our construction method of quantum scar has broad applications to various flat band systems.

Topological phases of matter can support fractionalized quasiparticles localized at topological defects. The current understanding of these exotic excitations, based on the celebrated bulk-defect correspondence, typically relies on crude approximations where such defects are replaced by a static classical background coupled to the matter sector. In this work, we explore the strongly correlated nature of symmetry-protected topological defects by focusing on situations where such defects arise spontaneously as dynamical solitons in intertwined topological phases, where symmetry breaking coexists with topological symmetry protection. In particular, we focus on the ${\mathbb{Z}}_2$ Bose-Hubbard model, a one-dimensional chain of interacting bosons coupled to ${\mathbb{Z}}_2$ fields, and show how solitons with ${\mathbb{Z}}_n$ topological charges appear for particle/hole dopings about certain commensurate fillings, extending the results in D. González-Cuadra et al. [Phys. Rev. Lett. 125, 265301 (2020)] beyond half filling. We show that these defects host fractionalized bosonic quasiparticles, forming bound states that travel through the system unless externally pinned, and repel each other giving rise to a fractional soliton lattice for sufficiently high densities. Moreover, we uncover the topological origin of these fractional bound excitations through a pumping mechanism, where the quantization of the intersoliton transport allows us to establish a generalized bulk-defect correspondence. This in-depth analysis of dynamical topological defects bound to fractionalized quasiparticles, together with the possibility of implementing our model in cold-atomic experiments, paves the way for further exploration of exotic topological phenomena in strongly correlated systems.

We calculate and resolve with unprecedented detail the local density of states (DOS) and momentum-dependent spectral functions at zero temperature of one of the key models for strongly correlated electron materials, the degenerate two-orbital Kanamori-Hubbard model, by means of the dynamical mean-field theory, which uses the density matrix renormalization group as the impurity solver. When the system is hole doped and in the presence of a finite interorbital Coulomb interaction, we find the emergence of a novel holon-doublon in-gap subband which is split by the Hund's coupling. We also observe interesting features in the DOS, such as the splitting of the lower Hubbard band into a coherent narrowly dispersing peak around the Fermi energy, and another subband which evolves with the chemical potential. We characterize the main transitions giving rise to each subband by calculating the response functions of specific projected operators and by comparing with the energies in the atomic limit. The detailed results for the spectral functions found in this work pave the way to study with great precision the microscopic quantum behavior in correlated materials.

Bloch-waves scattering theory is applied to photoemission from Mg(0001) up to photon energies of 320 eV. The quality of various approximations to the photoemission final state is analyzed based on the Fourier decomposition of the ab initio time-reversed scattering states. The relative importance of bulk and surface photoeffect in a one-dimensional pseudopotential model and in a real crystal is studied. The crucial role of the lateral umklapp scattering at high energies is revealed, and its implications for the stationary as well as for the attosecond time-resolved photoemission are discussed. The fine structure of the photoemission intensity distribution at high energies observed in earlier experiments is reproduced and explained.

Motivated by the recent numerical simulations for doped $t\text{−}J$ model on the honeycomb lattice, we study superconductivity of singlet and triplet pairing on the honeycomb lattice Hubbard model. We show that a superconducting state with coexisting spin-singlet and spin-triplet pairings is induced by the antiferromagnetic order near half filling. The superconducting state we obtain has a topological phase transition that separates a topologically trivial state and a nontrivial state with Chern number two. Possible experimental realization of such a topological superconductivity is also discussed.

$\left({\mathrm{Ga}}_{1-x},{\mathrm{Fe}}_x\right)\mathrm{Sb}$ is one of the promising ferromagnetic semiconductors for spintronic device applications because its Curie temperature $\left(T_{\mathrm{C}}\right)$ is above 300 K when the Fe concentration $x$ is equal to or higher than ∼0.20. However, the origin of the high $T_{\mathrm{C}}$ in (Ga,Fe)Sb remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and first-principles calculations to investigate the $x$ dependence of the Fe $3d$ states in $\left({\mathrm{Ga}}_{1-x},{\mathrm{Fe}}_x\right)\mathrm{Sb}$ ($x=0.05$, 0.15, and 0.25) thin films. The observed Fe $2p\text{−}3d$ RPES spectra reveal that the $\mathrm{Fe}\text{−}3d$ impurity band (IB) crossing the Fermi level becomes broader with increasing $x$, which is qualitatively consistent with the picture of double-exchange interaction. Comparison between the obtained $\mathrm{Fe}\text{−}3d$ partial density of states and the first-principles calculations suggests that the $\mathrm{Fe}\text{−}3d$ IB originates from the minority-spin ($\downarrow$) $e$ states. The results indicate that enhancement of the double-exchange interaction between $e_{\downarrow }$ electrons with increasing $x$ is the origin of the high $T_{\mathrm{C}}$ in (Ga,Fe)Sb.

Magneto-optical control of optical absorption spectra is theoretically investigated in two-dimensional (2D) dilute magnetic semiconductors such as monolayer transition metal dichalcogenides (TMDs) doped with magnetic ions. The underlying mechanism relies on efficient spin transfer between spin-polarized photoexcited carriers and localized magnetic ions via exchange scattering, and subsequent shifts in the electronic band structure induced by the resulting time-reversal symmetry breaking. A self-consistent model based on a rate equation is developed to analyze dynamical polarization of itinerant carrier spins and localized magnetic moments under circularly polarized optical excitation and the corresponding band modifications. The results illustrate that nonlinear effects such as optical bistability and self-opacity can indeed be achieved efficiently for a range of excitation power and frequency. In particular, the addition of magnetic dopants is shown to reduce the optical power required for the necessary band shifts by four orders of magnitude compared to that via the optical Stark effect in a nonmagnetic counterpart. Further investigation in a multidimensional parameter space elucidates the conditions for practical realization of the desired nonlinear effects in 2D TMD monolayers.

The two-dimensional F-model is an ice-rule-obeying model, with a low-temperature antiferroelectric state and high-temperature critical Coulomb phase. Polarization in the system is associated with topological defects in the form of system-spanning windings which makes it an ideal system on which to observe topological sector fluctuations, as have been discussed in the context of spin ice and Berezinskii-Kosterlitz-Thouless (BKT) systems. In particular, the F-model offers a useful counterpoint to the BKT transition in that winding defects are energetically suppressed in the ordered state, rather than dynamically suppressed in the critical phase. In this paper we develop Lieb and Baxter's historic solutions of the F-model to exactly calculate relevant properties, several apparently for the first time. We further calculate properties not amenable to exact solution by an approximate cavity method and by referring to established scaling results. Of particular relevance to topological sector fluctuations are the exact results for the applied field polarization and the “energetic susceptibility.” The latter is a both a measure of topological sector fluctuations and, surprisingly, in this case, a measure of the order parameter correlation exponent. In the high-temperature phase, the temperature tunes the density of topological defects and algebraic correlations, with the energetic susceptibility undergoing a jump to zero at the antiferroelectric ordering temperature, analogous to the “universal jump” in BKT systems. We discuss how these results are relevant to experimental systems, including to spin-ice thin films, and, unexpectedly, to three-dimensional dipolar spin ice and water ice, where we find that an analogous “universal jump” has previously been established in numerical studies. This unexpected result suggests a universal limit on the stability of perturbed Coulomb phases that is independent of dimension and of the order of the transition. Experimental results on water ice Ih are not inconsistent with this proposition. We complete the paper by relating our new results to experimental studies of artificial spin-ice arrays.

Powder neutron-diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the ${\mathrm{Sm}}_2{\mathrm{MnMnMn}}_{4-x}{\mathrm{Ti}}_x{\mathrm{O}}_{12} A$-site columnar ordered quadruple perovskite manganites ($x=1, x=2$, and $x=3$). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27, 62, and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the $x=1$ and $x=2$ samples, which indicates a considerable departure from mean-field behavior. This result is corroborated by large reductions in the theoretical ground-state magnetic moments observed across the series, which indicate the presence of spin fluctuations and/or disorder. We show that long-range magnetic order in the $x=3$ sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if it is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally, we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.

Realizing topological magnetism and its electric control are intensive topics in spintronics due to their promising applications in information storage and logic technologies. Here, we unveil that both can be achieved in two-dimensional (2D) magnetoelectric multiferroics. Using first-principles calculations, we show that a strong Dzyaloshinskii-Moriya interaction (DMI), which is the key ingredient for the formation of exotic chiral magnetism, could be induced in 2D multiferroics with vertical electric polarization via the Rashba effect. We verify that such a significant DMI can promote the stabilization of sub-10-nm skyrmions in 2D multiferroics with perpendicular magnetic anisotropy such as a CrN monolayer. In addition, the presence of both magnetization and electric polarization in 2D multiferroics provides us with the unique opportunity for the effective electric control of both the strength and chirality of the DMI and thereby the topological magnetism. As an example, we introduce four multiferroic skyrmions with different chiralities and polarities that can be manipulated by an external field.