$\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{B}{\mathrm{a}}_{0.1}\mathrm{Mn}{\mathrm{O}}_3$ is a ferromagnetic insulator in its bulk form, but exhibits metallicity in thin-film form. It has a wide potential in a range of spintronic-related applications, and hence it is critical to understand thickness-dependent electronic structure in thin films as well as substrate/film interface effects. Here, using electrical and in situ photoemission spectroscopy measurements, we report the electronic structure and interface band profile of high-quality layer-by-layer-grown $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{B}{\mathrm{a}}_{0.1}\mathrm{Mn}{\mathrm{O}}_3$ on single-crystal $\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$ substrates. A transition from insulating-to-conducting was observed with increasing $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{B}{\mathrm{a}}_{0.1}\mathrm{Mn}{\mathrm{O}}_3$ thickness, which was explained by the determined interface band diagram of $\mathrm{L}{\mathrm{a}}_{0.9}\mathrm{B}{\mathrm{a}}_{0.1}\mathrm{Mn}{\mathrm{O}}_3$/$\mathrm{Nb}:\mathrm{SrTi}{\mathrm{O}}_3$, where a type II heterojunction was formed.

The production of spin-polarized currents in pristine carbon nanotubes with Rashba spin-orbit interactions has been shown to be very sensitive to the symmetry of the tubes and the geometry of the setup. Here we analyze the role of defects on the spin quantum conductances of metallic carbon nanotubes due to an external electric field. We show that localized defects, such as adsorbed hydrogen atoms or pentagon-heptagon pairs, increase the Rashba spin-polarized current. Moreover, this enhancement takes place for energies closer to the Fermi energy as compared to the response of pristine tubes. Such increments can be even larger when several equally spaced defects are introduced into the system. We explore different arrangements of defects, showing that for certain geometries there are flips of the spin-polarized current and even transport suppression. Our results indicate that spin valve devices at the nanoscale may be achieved via defect engineering in carbon nanotubes.

As Moore's law approaches its end, two-dimensional (2D) materials are intensely studied for their potentials as one of the “More than Moore’ (MM) devices. However, the ultimate performance limits and the optimal design parameters for such devices are still unknown. One common problem for the 2D-material-based device is the relative weak on-current. In this study, two-dimensional Schottky-barrier field-effect transistors (SBFETs) consisting of in-plane heterojunctions of 1T metallic-phase and 2H semiconducting-phase transition-metal dichalcogenides (TMDs) are studied following the recent experimental synthesis of such devices at a much larger scale. Our ab initio simulation reveals the ultimate performance limits of such devices and offers suggestions for better TMD materials. Our study shows that the Schottky-barrier heights (SBHs) of the in-plane 1T/2H contacts are smaller than the SBHs of out-of-plane contacts, and the contact coupling is also stronger in the in-plane contact. Due to the atomic thickness of the monolayer TMD, the average subthreshold swing of the in-plane TMD-SBFETs is found to be close to the limit of 60 mV/dec, and smaller than that of the out-of-plane TMD-SBFET device. Different TMDs are considered and it is found that the in-plane $\mathrm{WT}{\mathrm{e}}_2\text{−}\mathrm{SBFET}$ provides the best performance and can satisfy the performance requirement of the sub-10-nm high-performance transistor outlined by the International Technology Roadmap for Semiconductors, and thus could be developed into a viable sub-10-nm MM device in the future.

Using the Landau-Ginzburg-Devonshire theory and one component approximation, we examined the conditions of the soft acoustic phonon mode (A-mode) appearance in a ferroelectric (FE) depending on the magnitude of the flexoelectric coefficient $f$ and temperature $T$. If the flexocoefficient $f$ is equal to the temperature-dependent critical value $f_{}^{\mathrm{cr}}\left(T\right)$ at some temperature $T=T_{\mathrm{IC}}$, the A-mode frequency tends to zero at wave vector $k=k_0^{\mathrm{cr}}$, and the spontaneous polarization becomes spatially modulated in the temperature range $T<T_{\mathrm{IC}}$, where $T_{\mathrm{IC}}$ is turned into the transition temperature to the incommensurate spatially modulated phase (SMP). When $f>f_{}^{\mathrm{cr}}\left(T_{\mathrm{IC}}\right)$, the A-mode becomes zero for two wave vectors $k=k_{1,2}^{\mathrm{cr}}$, and does not exist in the range of wave vectors $k_1^{\mathrm{cr}}<k<k_2^{\mathrm{cr}}$, indicating the transition to SMP. The corresponding temperature dependence of the dielectric susceptibility is in agreement with experimental data in ferroics with SMPs. This gives us a background to predict flexocoupling-induced soft acoustic amplitudon-type modes in FEs with SMPs of type II. The available experimental results on neutron scattering in organic incommensurate FE betaine calcium chloride dihydrate are in semiquantitative agreement with our theoretical results.

In this paper, the magnetism of the single-crystal ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$, which exhibits a chiral magnetic soliton lattice (CSL) state, is investigated. The magnetization displays strong magnetic anisotropy when the field is applied perpendicularly and parallel to the $c$ axis in the low-field region ($H<H_S,H_S$ is the saturation field). The critical exponents of ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_2$ are obtained as $\beta =0.370\left(4\right),\gamma =1.380\left(2\right)$, and $\delta =4.853\left(6\right)$, which are close to the theoretical prediction of the three-dimensional Heisenberg model. Based on the scaling equation and the critical exponents, the $H-T$ phase diagram in the vicinity of the phase transition is constructed where two critical points are determined. One is a tricritical point which locates at the intersection across the CSL, the forced ferromagnetic, and the paramagnetic (PM) states. The other one is a critical point situated at the boundaries across the CSL, the helimagnetic, and the PM states.

Boron nitride polymorphs hold great promise for integration into electronic and optoelectronic devices requiring ultrawide band gaps. We use first-principles calculations to examine the prospects for $p$-type doping of hexagonal ($h-\mathrm{BN}$), wurtzite ($wz-\mathrm{BN}$), and cubic ($c-\mathrm{BN}$) boron nitride. Group-IV elements (C, Si) substituting on the N site result in a deep acceptor, as the atomic levels of the impurity species lie above the BN valence-band maximum. On the other hand, group-II elements (Be, Mg) substituting on the B site do not give impurity states in the band gap; however, these dopants lead to the formation of small hole polarons. The tendency for polaron formation is far more pronounced in $h-\mathrm{BN}$ compared to $wz-\mathrm{BN}$ or $c-\mathrm{BN}$. Despite forming small hole polarons, Be acceptors enable $p$-type doping, with ionization energies of 0.31 eV for $wz-\mathrm{BN}$ and 0.24 eV for $c-\mathrm{BN}$; these values are comparable to the Mg ionization energy in GaN.

The influence of the partial Er for Y substitution on the crystal structure and magnetic properties of $\mathrm{YF}{\mathrm{e}}_2{\mathrm{D}}_{4.2}$ has been investigated by high field magnetization and neutron diffraction experiments. ${\mathrm{Y}}_{0.7}\mathrm{E}{\mathrm{r}}_{0.3}\mathrm{F}{\mathrm{e}}_2{\mathrm{D}}_{4.2}$ compound crystallizes in the same monoclinic structure as $\mathrm{YF}{\mathrm{e}}_2{\mathrm{D}}_{4.2}$ described in $Pc\left(P_1{c}_1\right)$ space group with D atoms located in 18 different tetrahedral interstitial sites. A cell volume contraction of 0.6% is observed upon Er substitution, inducing large modification of the magnetic properties. Electronic effect of D insertion as well as lowering of crystal symmetry are important factors determining the magnetic properties of Fe sublattice, which evolves towards more delocalized behavior and modifying the Er-Fe exchange interactions. In the ground state, the Er and Fe moments are arranged ferrimagnetically within the plane perpendicular to the monoclinic $b$ axis and with average moments $m_{\mathrm{Er}}=6.4\left(3\right){\mu }_B{\mathrm{Er}}^{-1}$ and ${\mathrm{m}}_{\mathrm{Fe}}=2.0\left(1\right){\mu }_B{\mathrm{Fe}}^{-1}$ at 10 K. Upon heating, $m_{\mathrm{Er}}$ decreases progressively until $T_{\mathrm{Er}}=55\mathrm{K}$. Between 55 K and 75 K, the Fe sublattice undergoes a first-order ferromagnetic-antiferromagnetic (FM-AFM) transition with a cell volume contraction due to the itinerant metamagnetic behavior of one Fe site. In the AFM structure, $m_{\mathrm{Fe}}$ decreases until the Néel temperature $T_{\mathrm{N}}=125\mathrm{K}$. At high field, two different types of field induced transitions are observed. The Er moments become parallel to the Fe one and saturates to the $\mathrm{E}{\mathrm{r}}^{3+}$ free ion value, leading to an unusual field induced FM arrangement at a transition field $B_{\mathrm{Trans}}$ of only 78 kG below 30 K. Then above ${T_{\mathrm{M}}}_0=66\mathrm{K}$, an AFM-FM transition of the Fe sublattice, accompanied by a cell volume increase is observed. $B_{\mathrm{Trans}}$ increases linearly versus temperature and with a larger $d{B}_{\mathrm{Trans}}/dT$ slope than for $\mathrm{YF}{\mathrm{e}}_2{\mathrm{D}}_{4.2}$. This has been explained by the additional contribution of Er induced moments above $B_{\mathrm{Trans}}$.

Continuous symmetries are believed to emerge at many quantum critical points in frustrated magnets. In this work, we study two candidates of this paradigm: the transverse-field frustrated Ising model (TFFIM) on the triangular and the honeycomb lattices. The former is the prototypical example of this paradigm, and the latter has recently been proposed as another realization. Our large-scale Monte Carlo simulation confirms that the quantum phase transition (QPT) in the triangular lattice TFFIM indeed hosts an emergent O(2) symmetry, but that in the honeycomb lattice TFFIM is a first-order QPT and does not have an emergent continuous symmetry. Furthermore, our analysis of the order-parameter histogram reveals that such different behavior originates from the irrelevance and relevance of anisotropic terms near the QPT in the low-energy effective theory of the two models. The comparison between theoretical analysis and numerical simulation in this work paves the way for scrutinizing investigation of emergent continuous symmetry at classical and quantum phase transitions.

We have performed scanning photoemission spectromicroscopy of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ across the metal-insulator transition at 123 K, which is accompanied by V $3d$ charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ single crystals, similar to Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$ in which electronic configurations of ${\mathrm{Cr}}^{3+}$ and ${\mathrm{V}}^{3+}$ are the same as those of ${\mathrm{V}}^{2+}$ and ${\mathrm{V}}^{3+}$ in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$. Typical domain size is $\sim 5\text{–}10\mu \mathrm{m}$ at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$ is different from that of Cr-doped ${\mathrm{V}}_2{\mathrm{O}}_3$, probably due to the charge degrees of freedom in ${\mathrm{BaV}}_{10}{\mathrm{O}}_{15}$.

In our work, we theoretically study the dynamics of a single excitation in a one-dimensional array of two-level systems, which are chirally coupled through a single mode waveguide. The chirality is achieved owing to a strong optical spin-locking effect, which in an ideal case gives perfect unidirectional excitation transport. We obtain a simple analytical solution for a single excitation dynamics in the Markovian limit, which directly shows the tolerance of the system with respect to the fluctuations of emitters position. We also show that the Dicke state, which is well known to be superradiant, has twice lower emission rate in the case of unidirectional quantum interaction. Our model is supported and verified with the numerical computations of quantum emitters coupled via surface plasmon modes in a metallic nanowire. The obtained results are based on a very general model and can be applied to any chirally coupled system that gives a new outlook on quantum transport in chiral nanophotonics.

In terms of the state-of-the-art first-principles computational methods combined with the numerical renormalization-group technique the spectroscopic properties of Co adatoms deposited on silicene are analyzed. By establishing an effective low-energy Hamiltonian based on first-principles calculations, we study the behavior of the local density of states of Co adatoms on external parameters, such as magnetic field and gating. It is shown that the Kondo resonance with a Kondo temperature of the order of a few kelvins can emerge by fine-tuning the chemical potential. The evolution and splitting of the Kondo peak with external magnetic field is also analyzed. Furthermore, it is shown that the spin polarization of an adatom's spectral function in the presence of magnetic field can be relatively large, and it is possible to tune the polarization and its sign by electrical means.

We present a scheme of interaction-induced topological band structures based on the spin anisotropy of exciton-polaritons in semiconductor microcavities. We predict theoretically that this scheme allows the engineering of topological gaps, without requiring a magnetic field or strong spin-orbit interaction (transverse electric-transverse magnetic splitting). Under nonresonant pumping we find that an initially topologically trivial system undergoes a topological transition upon the spontaneous breaking of phase symmetry associated with polariton condensation. Under either nonresonant or resonant coherent pumping we find that it is also possible to engineer a topological dispersion that is linear in wave vector—a property associated with polariton superfluidity.

We present a charge self-consistent scheme combining density functional and dynamical mean field theory, which uses Green's functions of multiple-scattering type. In this implementation, the many-body effects are incorporated into the Kohn-Sham iterative scheme without the need for the numerically ill-posed analytic continuation of the Green's function and of the self-energy, which was previously a bottleneck in multiple-scattering-type Green's function approaches. This is achieved by producing the Kohn-Sham Hamiltonian in the subspace of correlated partial waves and allows to formulate the Green's function directly on the Matsubara axis. The spectral moments of the Matsubara Green's function enable us to put together the real-space charge density, therefore, the charge self-consistency can be achieved. Our results for the spectral functions (density of states) and equation-of-state curves for transition-metal elements Fe, Ni, and FeAl compound agree very well with those of Hamiltonian-based LDA+DMFT implementations. The current implementation improves on numerical accuracy, compared to previous implementations where analytic continuation was required at each Kohn-Sham self-consistent step. A minimal effort aside from the multiple-scattering formulation is required, and the method can be generalized in several ways that are interesting for applications to real materials.

Scanning gate microscopy images from measurements made in the vicinity of quantum point contacts were originally interpreted in terms of current flow. Some recent work has analytically connected the local density of states to conductance changes in cases of perfect transmission, and at least qualitatively for a broader range of circumstances. In the present paper, we show analytically that in any time-reversal invariant system there are important deviations that are highly sensitive to imperfect transmission. Nevertheless, the unperturbed partial local density of states can be extracted from a weakly invasive scanning gate microscopy experiment, provided the quantum point contact is tuned anywhere on a conductance plateau. A perturbative treatment in the reflection coefficient shows just how sensitive this correspondence is to the departure from the quantized conductance value and reveals the necessity of local averaging over the tip position. It is also shown that the quality of the extracted partial local density of states decreases with increasing tip radius.

The temperature scaling of the micromagnetic Dzyaloshinsky-Moriya exchange interaction is calculated from saturated to vanishing magnetization. We use Green's function theory to derive the finite-temperature spin wave spectrum of ferromagnetic systems described by a classical atomistic spin model Hamiltonian with temperature-independent parameters. Within this model, we find universal expressions for the temperature scaling not only of the Dzyaloshinsky-Moriya interaction but also of the Heisenberg exchange stiffness and the single-ion anisotropy. In the spirit of multiscale models, we establish a clear connection between the atomistic interactions and the temperature-dependent coefficients in the spin wave spectrum and in the micromagnetic free-energy functional. We demonstrate that the corrections to mean-field theory or the random phase approximation for the temperature scaling of Dzyaloshinsky-Moriya and Heisenberg exchange interactions have very similar forms. In the presence of thermal fluctuations and Dzyaloshinsky-Moriya interaction an anisotropylike term emerges in the spin wave spectrum which, at low temperature, increases with temperature, in contrast to the decreasing single-ion anisotropy. We evaluate the accuracy of the theoretical method by comparing it to the spin wave spectrum calculated from Monte Carlo simulations.

Pyrochlore iridates $A_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($A$ = rare earth elements, Y or Bi) hold great promise for realizing novel electronic and magnetic states owing to the interplay of spin-orbit coupling, electron correlation, and geometrical frustration. A prominent example is the formation of all-in/all-out (AIAO) antiferromagnetic order in the ${\mathrm{Ir}}^{4+}$ sublattice that comprises corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon, namely, a cooling-field-induced shift of magnetic hysteresis loop along the magnetization axis, and its possible origin in pyrochlore iridates with nonmagnetic Ir defects (e.g., ${\mathrm{Ir}}^{3+}$). In a simple model, we attribute the magnetic hysteresis loop to the formation of ferromagnetic (FM) droplets in the AIAO antiferromagnetic (AFM) background. The weak ferromagnetism originates from canted antiferromagnetic order of the ${\mathrm{Ir}}^{4+}$ moments surrounding each nonmagnetic Ir defect. The shift of hysteresis loop can be understood quantitatively based on an exchange-bias-like effect in which the moments at the shell of the FM droplets are pinned by the AIAO AFM background via mainly the Heisenberg ($J$) and Dzyaloshinsky-Moriya ($D$) interactions. The magnetic pinning is stable and robust against sweeping cycle and sweeping field up to 35 T, which is possibly related to the magnetic octupolar nature of the AIAO order.

The consequences of random nonmagnetic-ion dilution for the pyrochlore family ${\mathrm{Y}}_2$($M{}_{1-x}N{}_x$)${}_2{\mathrm{O}}_7$ ($M$ = magnetic ion, $N$ = nonmagnetic ion) have been investigated. As a first step, we experimentally examine the magnetic properties of ${\mathrm{Y}}_2{\mathrm{CrSbO}}_7$ ($x=0.5$), in which the magnetic sites (${\mathrm{Cr}}^{3+}$) are percolative. Although the effective Cr-Cr spin exchange is ferromagnetic, as evidenced by a positive Curie-Weiss temperature, ${\mathrm{\Theta }}_{\mathrm{CW}} \simeq 19.5 \mathrm{K}$, our high-resolution neutron powder diffraction measurements detect no sign of magnetic long-range order down to 2 K. In order to understand our observations, we construct a lattice model to numerically study the bond disorder introduced by the ionic size mismatch between $M$ and $N$, which reveals that the bond disorder percolates at $x_b \simeq 0.23$, explaining the absence of magnetic long-range order. This model could be applied to a series of frustrated magnets with a pyrochlore sublattice, for example, the spinel compound $\mathrm{Zn}({\mathrm{Cr}}_{1-x}{\mathrm{Ga}}_x$)${}_2{\mathrm{O}}_4$, wherein a Néel to spin glass phase transition occurs between $x=0.2$ and 0.25 [Lee et al., Phys. Rev. B 77, 014405 (2008)]. Our study stresses the non-negligible role of bond disorder on magnetic frustration, even in ferromagnets.

Two common approaches of studying theoretically the property of a superconductor are shown to have significant differences, when they are applied to the Larkin-Ovchinnikov state of Weyl metals. In the first approach the pairing term is restricted by a cutoff energy to the neighborhood of the Fermi surface, whereas in the second approach the pairing term is extended to the whole Brillouin zone. We explore their difference by considering two minimal models for the Weyl metal. For a model giving a single pair of Weyl pockets, both approaches give a partly-gapped (fully-gapped) bulk spectrum for small (large) pairing amplitude. However, for very small cutoff energy, a portion of the Fermi surface can be completely unaffected by the pairing term in the first approach. For the other model giving two pairs of Weyl pockets, while the bulk spectrum for the first approach can be fully gapped, the one from the second approach has a robust line node, and the surface states are also changed qualitatively by the pairing. We elucidate the above differences by topological arguments and analytical analyses. A factor common to both of the two models is the tilting of the Weyl cones which leads to asymmetric normal state band structure with respect to the Weyl nodes. For the Weyl metal with two pairs of Weyl pockets, the band folding leads to a double degeneracy in the effective model, which distinguishes the pairing of the second approach from all others.