We report the electronic structure reconstruction of ${\mathrm{Ca}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ in the low temperature collapsed tetragonal (CT) phase observed by angle-resolved photoemission spectroscopy. Different from $\mathrm{Ca}({\mathrm{Fe}}_{1-x}{\mathrm{Rh}}_x{)}_2{\mathrm{As}}_2$ and the annealed ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ where all hole Fermi surfaces are absent in their CT phases, the cylindrical hole Fermi surface still persists in the CT phase of ${\mathrm{Ca}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Furthermore, we found at least three well separated electronlike bands around the zone corner in the CT phase of ${\mathrm{Ca}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$, which are more dispersive than the electronlike bands in the high temperature tetragonal phase. Based on these observations, we propose that the weakening of correlations (as indicated by the reduced effective mass), rather than the lack of Fermi surface nesting, might be responsible for the absence of magnetic ordering and superconductivity in the CT phase.

The electronic structure of nearly optimally doped novel superconductor $\mathrm{La}{\mathrm{O}}_{1-x}{\mathrm{F}}_x\mathrm{Bi}{\mathrm{S}}_2(x=0.46)$ was investigated using angle-resolved photoemission spectroscopy (ARPES). We clearly observed band dispersions from 2 to 6 eV binding energy and near the Fermi level $\left(E_{\mathrm{F}}\right)$, which are well reproduced by first-principles calculations when the spin-orbit coupling is taken into account. The ARPES intensity map near $E_{\mathrm{F}}$ shows a squarelike distribution around the $\Gamma \left(Z\right)$ point in addition to electronlike Fermi-surface (FS) sheets around the $X\left(R\right)$ point, indicating that FS of $\mathrm{La}{\mathrm{O}}_{0.54}{\mathrm{F}}_{0.46}\mathrm{Bi}{\mathrm{S}}_2$ is in close proximity to the theoretically predicted topological change.

We present Hall-effect measurements on MnSi/Si(111) epilayers and find an anomalous Hall contribution that is significantly smaller than that in bulk crystals, which enables the observation of an additional contribution to the anomalous signal previously overlooked in MnSi. Our measurements indicate the signal is not due to skyrmions in MnSi thin films, which are absent in out-of-plane fields, but rather are the result of scattering from the cone phase. The absence of magnetic contrast in the transmission electron microscopy (TEM) measurements is consistent with this interpretation [T. L. Monchesky et al., Phys. Rev. Lett. 112, 059701 (2014)]. We provide a method to model TEM images of skyrmion lattices to determine the conditions necessary for their observation in other B20 epilayers with an anisotropy that is favorable to their formation.

A comprehensive understanding of strain coupling across heterointerfaces and its impact on physical properties of oxide heterostructures is important for elucidating the mechanisms of certain novel physical phenomena occurring at heterointerfaces, such as magnetoelectric coupling, tunneling electroresistance effects, and strain-driven exchange bias. Using the $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{Pb}\left(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}\right){\mathrm{O}}_3\text{−}\mathrm{Pb}\left(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}\right){\mathrm{O}}_3\text{−}\mathrm{PbTi}{\mathrm{O}}_3$ (PINT) multiferroic heterostructure as a model system, we systematically investigated the influences of interface strain coupling on the electronic transport and magnetic properties as well as the electronic phase separation of charge-ordered $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ thin films through electric-field-induced ferroelectric domain switching. Upon the irreversible initial poling of the PMNT substrate, the induced in-plane compressive strain $({\varepsilon }_{xx\left(\mathrm{film}\right)}=-0.045\%)$ causes a decrease in $T_{\mathrm{CO}}(\Delta T_{\mathrm{CO}}=180\mathrm{K})$ and resistance $[{(\Delta R/R)}_{\mathrm{strain}}\sim -99.4\%]$, resulting in a gauge factor ${(\Delta R/R)}_{\mathrm{strain}}/{\varepsilon }_{xx\left(\mathrm{film}\right)}\sim 220800\%$. Such a large strain-tunability of resistance is unprecedented and magnetic-field tunable. This, together with the strain-tunable magnetoresistance (MR) and magnetization of the films, demonstrates strong coupling between the strain and the magnetic field. Further analysis indicates that this coupling is essentially mediated by the electronic phase separation, whose relative strength could be monitored by measuring ${(\Delta R/R)}_{\mathrm{strain}}$ against magnetic field and temperature. By combining 180° ferroelectric domain switching and x-ray diffraction and transport measurements, we identify that this electric-field modulation of the physical properties is strain-mediated but not interface charge-mediated. In addition, we observed that the non-180° ferroelastic domain switching-induced in-plane tensile strain $({\varepsilon }_{xx\left(\mathrm{film}\right)}=0.1\%)$ induces a large increase in the resistance (up to ∼87.4%) and $T_{\text{CO}}$ and a drop in MR, signaling the stabilizing of the charge-ordered phase. Our findings provide further insight into the strain effect and essential physics of perovskite manganites, particularly the electronic phase separation.

The critical theories for the topological phase transitions of integer quantum Hall states to a trivial insulating state with the same symmetry can be obtained by calculating the ground state entanglement spectrum under a symmetric checkerboard bipartition. In contrast to the gapless edge excitations under the left-right bipartition, a quantum network with bulk gapless excitations naturally emerges without fine tuning. On a large finite lattice, the resulting critical theory for the $\nu =1$ state is the (2 + 1)-dimensional relativistic quantum field theory characterized by a single Dirac cone spectrum and a pair of fractionalized zero-energy states, while for the $\nu =2$ state the critical theory exhibits a parabolic spectrum and no sign of fractionalization in the zero-energy states. A triangular correspondence is established among the bulk topological theory, gapless edge theory, and the critical theory via the ground state entanglement spectrum.

We theoretically analyze the process of charge recombination in the planar Mott-Hubbard insulators with the aim to explain the short picosecond-range lifetimes of photoexcited carriers, experimentally studied via pump-probe experiments on the undoped cuprates. The recombination mechanism consists of two essential ingredients: the formation of a metastable $s$-type bound holon-doublon pair, i.e., the Mott exciton, and the decay of such an excitonic state via the multimagnon emission. In spite of the large gap that requires many bosons to be emitted, the latter process is fast due to a large exchange scale and strong charge-spin coupling in planar systems. As the starting microscopic model we consider the single-band Hubbard model and then a more realistic three-band model for cuprates, both leading to the same minimal one. The decay rate of the exciton is evaluated numerically via the Fermi golden rule, having consistency also with the direct time-evolution calculation. The decay rate reveals exponential dependence on the ratio of the Mott-Hubbard gap and the exchange coupling, the result qualitatively reproduced also within a toy exciton-boson model.

We investigate the magnetic properties and Mott transition in the Hubbard model for weakly coupled chains on the anisotropic triangular lattice. Taking into account ${120}^{\circ }$ Néel and collinear orderings, the magnetic phase diagram is studied at zero temperature and half-filling by the variational cluster approximation. We found that, when the on-site Coulomb repulsion $U$ is relatively large, the nonmagnetic insulator, which is a candidate of the spin liquid state, is realized for a wide range of interchain hopping from the quasi-two-dimensional to the almost one-dimensional regime. When the interchain hopping is relatively large, this nonmagnetic insulator becomes a magnetic state as $U$ decreases. For rather small interchain hopping, this nonmagnetic insulator changes to a paramagnetic metal as $U$ decreases, thus purely paramagnetic metal-insulator transition (Mott transition) takes place. Implications of our results for ${\mathrm{Cs}}_2{\mathrm{CuBr}}_4$ and ${\mathrm{Cs}}_2{\mathrm{CuCl}}_4$ are discussed.

We have systematically studied the thermodynamic properties of a two-dimensional half-filled $\mathrm{SU}\left(2N\right)$ Hubbard model on a square lattice by using the determinant quantum Monte Carlo method. The entropy-temperature relation, the isoentropy curve, and the probability distribution of the on-site occupation number are calculated in both SU(4) and SU(6) cases, which exhibit prominent features of the Pomeranchuk effect. We analyze these thermodynamic behaviors based on energy scales in the density and spin channels. In the density channel, the interaction strength that marks the crossover from the weak to strong interaction regimes increases with the number of fermion components. In the spin channel, increasing the number of fermion components enhances quantum spin fluctuations, which is shown in the simulations of uniform spin susceptibilities and antiferromagnetic structure factors.

Using the dynamical mean-field approximation we investigate the formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multicritical point. Rapid but continuous transition between ferromagnetic and nonmagnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential.

We propose to study the lattice symmetry protection of Majorana zero bound modes in topological crystalline superconductors (SCs). With an induced $s\text{−}\mathrm{wave}$ superconductivity in the $\left(001\right)$ surface of the topological crystalline insulator ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$, which has a $C_4$ rotational symmetry, we show a class of two-dimensional topological SCs with four Majorana modes obtained in each vortex core, while only two of them are protected by the cyclic symmetry. Furthermore, applying an in-plane external field can break the fourfold symmetry and lifts the Majorana modes to finite energy states in general. Surprisingly, we show that even the $C_4$ symmetry is broken; two Majorana modes are restored exactly one time whenever the in-plane field varies $\pi /2$, i.e., $1/4$-cycle in the direction. This phenomenon has a profound connection to the fourfold cyclic symmetry of the original crystalline SC and uniquely demonstrates the lattice-symmetry protection of the Majorana modes. We further generalize these results to the system with generic $C_{2N}$ symmetry, and show that the symmetry class of the topological crystalline SC can be demonstrated by the $2N$ times of restoration of two Majorana modes when the direction of the external symmetry-breaking field varies one cycle.

The effect of a magnetic field on the electroluminescence of organic light emitting devices originates from the hyperfine interaction between the electron/hole polarons and the hydrogen nuclei of the host molecules. In this paper, we present an analytical theory of magnetoelectroluminescence for organic semiconductors. To be specific, we focus on bilayer heterostructure devices. In the case we are considering, light generation at the interface of the donor and acceptor layers results from the formation and recombination of exciplexes. The spin physics is described by a stochastic Liouville equation for the electron/hole spin density matrix. By finding the steady-state analytical solution using Bloch-Wangsness-Redfield theory, we explore how the singlet/triplet exciplex ratio is affected by the hyperfine interaction strength and by the external magnetic field. To validate the theory, spectrally resolved electroluminescence experiments on BPhen/m-MTDATA devices are analyzed. With increasing emission wavelength, the width of the magnetic field modulation curve of the electroluminescence increases while its depth decreases. These observations are consistent with the model.

Quantum transport and spintronics regimes are studied in $p$- and $n$-doped atomic layers of hexagonal transition-metal dichalcogenides (TMDCs), subject to the interplay between the valley structure and spin-orbit coupling. We find how spin relaxation of carriers depends on their areal density and show that it vanishes for holes near the band edge, leading to the density-independent spin-diffusion length, and we develop a theory of weak localization/antilocalization, describing the crossovers between the orthogonal, double-unitary, and symplectic regimes of quantum transport in TMDCs.

We demonstrate that a topological invariant term in graphene (e.g., spin-orbit coupling or interlayer coupling induced trigonal warping) can lead to a strong nonlinear optical response. We present a full quantum-mechanical analysis of the third-order nonlinear optical conductivity of the monolayer graphene with a finite Rashba spin-orbit coupling. For energy of the incident photon below the maximum height of the trigonal warping and the Rashba coupling strength exceeding a critical value the nonlinear conductance is enhanced dramatically in comparison with that in the absence of the topological term. Due to the trigonal warping effect and the topologically inequivalent change of the geometry of the energy band, the strong nonlinear effect persists, as long as the incident photon energy is limited below the energy maximum defined by the spin-orbit coupling.

The electronic and spin structure of a graphene monolayer synthesized on Pt(111) has been investigated experimentally by angle- and spin-resolved photoemission with different polarizations of incident synchrotron radiation and using density functional theory calculations. It is shown that despite the observed total quasifreestanding character of the dispersion of the graphene $\pi$ state remarkable local distortions and breaks in the dispersions take place due to hybridization between the graphene $\pi$ and Pt $d$ states. Corresponding spin-dependent avoided-crossing effects lead to significant modification of the spin structure and cause an enhanced induced spin-orbit splitting of the graphene $\pi$ states near the Fermi level in the region of the $\overline{\mathrm{K}}$ point of the graphene Brillouin zone (BZ) with a magnitude of 80–200 meV depending on the direction in the BZ. Using $p$, $s$, and elliptical polarizations of the synchrotron radiation, the contributions of the graphene $\pi$ and Pt $d$ states were separated and their intersection at the Fermi level, which is important for effective spin injection between these states, was shown. Moreover, analysis of the data allows us to conclude that in the region of the Dirac point the spin structure of the system cannot be described by a Rashba splitting, and even a spin-orbit gap between lower and upper Dirac cones is observed.

Electrons in graphene have four flavors associated with low-energy spin and valley degrees of freedom. The fractional quantum Hall effect in graphene is dominated by long-range Coulomb interactions, which are invariant under rotations in spin-valley space. This SU(4) symmetry is spontaneously broken at most filling factors, and also weakly broken by atomic scale valley-dependent and valley-exchange interactions with coupling constants $g_z$ and $g_{\perp }$. In this paper, we demonstrate that when $g_z=-g_{\perp }$, an exact SO(5) symmetry survives which unifies the Néel spin order parameter of the antiferromagnetic state and the $XY$ valley order parameter of the Kekulé distortion state into a single five-component order parameter. The proximity of the highly insulating quantum Hall state observed in graphene at $\nu =0$ to an ideal SO(5) symmetric quantum Hall state remains an open experimental question. We illustrate the physics associated with this SO(5) symmetry by studying the multiplet structure and collective dynamics of filling factor $\nu =0$ quantum Hall states based on exact-diagonalization and low-energy effective theory approaches. This allows to illustrate how manifestations of the SO(5) symmetry would survive even when it is weakly broken.

In this Rapid Communication, we study the electron spin decoherence of single defects in silicon carbide (SiC) nuclear spin bath. We find that, although the natural abundance of ${}^{29}\mathrm{Si}$ ($p_{\mathrm{Si}}=4.7\%$) is about four times larger than that of ${}^{13}\mathrm{C}$ ($p_{\mathrm{C}}=1.1\%$), the electron spin coherence time of defect centers in SiC nuclear spin bath in a strong magnetic field ($B>300\mathrm{G}$) is longer than that of nitrogen-vacancy (NV) centers in ${}^{13}\mathrm{C}$ nuclear spin bath in diamond. In addition to the smaller gyromagnetic ratio of ${}^{29}\mathrm{Si}$, and the larger bond length in SiC lattice, a crucial reason for this counterintuitive result is the suppression of the heteronuclear-spin flip-flop process in a finite magnetic field. Our results show that electron spin of defect centers in SiC are excellent candidates for solid state spin qubit in quantum information processing.

Interfacial control of a polar-(rhombohedral) to-non-polar (orthorhombic) phase transition in (001)-oriented epitaxial $\mathrm{BiFe}{\mathrm{O}}_3\text{/}\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{m}}_x\right)\mathrm{Fe}{\mathrm{O}}_3$ superlattices is presented. We demonstrate controlling the composition at which a polar phase transformation takes place by tuning the strength of the interlayer interactions while holding the average composition constant. It is shown that the thickness of the superlattice layers has a strong influence on the interlayer polar coupling, which in turn changes the phase transition. For the shortest periods studied (layers 5- and 10-nm thick) the onset of the phase transition is suppressed along with a significant broadening (as a function of $\mathrm{S}{\mathrm{m}}^{3+}$ concentration) of an incommensurately modulated phase determined by two-dimensional x-ray diffraction mapping. Consequently, a ferroelectric character with robust polarization hysteresis and enhanced dielectric constant is observed even for substitution concentration of $\mathrm{S}{\mathrm{m}}^{3+}$ which would otherwise lead to a leaky paraelectric in single-layer $(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{S}{\mathrm{m}}_x)\mathrm{Fe}{\mathrm{O}}_3$ films. The experimental results are fully consistent with a mean-field thermodynamic theory which reveals that the strength of the interlayer coupling is strongly affected by the polar-polar interaction across the interface.