The magnetic susceptibility of a disordered two-dimensional system with strong Rashba spin-orbit interaction is calculated in a self-consistent Born approximation. In an ideal system, the response exhibits a delta-function singularity toward the diamagnetic direction at the energy where the band crossing takes place in an inner Weyl band. It is essentially paramagnetic below that energy except for the system with a certain value of $g$ factor, while it becomes the same as that in a system free from the spin-orbit interaction above the energy. It turns out that effects of disorder are not sensitive to kinds of scatterers such as short- and long-range. Explicit numerical results are presented in the case of dominant charged-impurity scattering. The delta-function susceptibility is broadened by disorder but remains appreciable in the case of strong disorder.

We investigate the thermoelectric properties of a quantum dot coupled to ferromagnetic and superconducting electrodes. The combination of spin polarized tunneling at the ferromagnetic–quantum dot interface and the application of an external magnetic field that Zeeman splits the dot energy level leads to large values of the thermopower (Seebeck coefficient). Importantly, the thermopower can be tuned with an external gate voltage connected to the dot. We compute the figure of merit that measures the efficiency of thermoelectric conversion and find that it attains high values. We discuss the different contributions from Andreev reflection processes and quasiparticle tunneling into and out of the superconducting contact. Furthermore, we obtain dramatic variations of both the magnetothermopower and the spin Seebeck effect, which suggest that in our device spin currents can be controlled with temperature gradients only.

Ab initio plane-wave pseudopotential calculations of electronic and vibrational properties have been carried out for the ternary phosphides $\mathrm{Ba}{M}_2{\mathrm{P}}_2$ $(M=\text{Ni},\text{Rh}\text{ and Ir})$ with a ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure. The calculated electronic results show the metallic character of $\mathrm{Ba}{M}_2{\mathrm{P}}_2$, and the plots of total and partial density of states of $\mathrm{Ba}{M}_2{\mathrm{P}}_2$ exhibit strong hybridization between the $d$ states of the $M$ atom and the $p$ states of the P atom below the Fermi energy. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges for these compounds are presented and discussed. The Eliashberg spectral function for these compounds has been calculated by using a linear response approach based on the density functional theory. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter $\left(\lambda \right)$ is determined to be 0.61 for ${\mathrm{BaNi}}_2{\mathrm{P}}_2$, 0.55 for ${\mathrm{BaIr}}_2{\mathrm{P}}_2$, and 0.43 for ${\mathrm{BaRh}}_2{\mathrm{P}}_2$. Using the calculated values of $\lambda$ and the logarithmically averaged phonon frequency ${\omega }_{ln}$ the superconducting critical temperature $\left(T_c\right)$ values for ${\mathrm{BaNi}}_2{\mathrm{P}}_2,{\mathrm{BaIr}}_2{\mathrm{P}}_2$, and ${\mathrm{BaRh}}_2{\mathrm{P}}_2$ are obtained to be 2.80, 1.97, and 0.70 K, respectively, which compare very well with their experimental values of 3.0, 2.1, and 1.0 K.

Ferromagnesite, an iron-bearing carbonate stable up to 100–115 GPa, is believed to be the major carbon carrier in the earth's lower mantle and play a key role in the earth's deep carbon cycle. In this paper, we use the local density approximation plus self-consistent Hubbard $U$ (LDA+$U_{\mathrm{sc}}$) method to study the iron spin crossover in ferromagnesite with a wide range of iron concentration (12.5–100%). Our calculation shows that this mineral undergoes a crossover from the high-spin (HS) ($S=2$) to the low-spin (LS) ($S=0$) state at around 45–50 GPa, regardless of the iron concentration. The intermediate-spin ($S=1$) state is energetically unfavorable and not involved in spin crossover. The anomalous changes of volume, density, and bulk modulus accompanying the spin crossover obtained in our calculation are in great agreement with experiments. Our calculation also predicts that an abrupt change of the iron nuclear quadrupole splitting, from $\gtrsim 2.8$ mm/s to $\lesssim 0.3$ mm/s, can be observed in Mössbauer spectra at 45–50 GPa as a signature of the HS-LS crossover.

The magnetization reversal process of hexagonal-close-packed (hcp) (0001) oriented Co and $\mathrm{C}{\mathrm{o}}_{90}\mathrm{R}{\mathrm{u}}_{10}$ thin films with perpendicular magnetic anisotropy (PMA) has been studied as a function of temperature and applied magnetic field angle. Room temperature pure cobalt exhibits two characteristic reversal mechanisms. For angles near in-plane field orientation, the magnetization reversal proceeds via instability of the uniform magnetic state, whereas in the vicinity of the out-of-plane (OP) orientation, magnetization inversion takes place by means of domain nucleation. Temperature dependent measurements enable the modification of the magnetocrystalline anisotropy and reveal a gradual disappearance of the domain nucleation process during magnetization reversal for elevated temperatures. Ultimately, this suppression of the domain nucleation process leads to the exclusive occurrence of uniform state instability reversal for all field orientations at sufficiently high temperature. Comparative magnetic measurements of $\mathrm{C}{\mathrm{o}}_{90}\mathrm{R}{\mathrm{u}}_{10}$ alloy samples allow the identification and confirmation of the high temperature remanent magnetization state of cobalt as an OP stripe domain state despite the reduction of magnetocrystalline anisotropy. Detailed micromagnetic simulations supplement the experimental results and corroborate the physical understanding of the temperature dependent behavior. Moreover, they enable a comprehensive identification of the complex energy balance in magnetic films with PMA, for which three different magnetic phases occur for sufficiently high anisotropy values, whose coexistence point is tricritical in nature.

By analog to Maxwell construction for the first-order phase transition, the pseudo Jahn-Teller polar distortion arises naturally once the local bond length of transition-metal oxygen octahedra is enhanced beyond the inflexion point of pair potential into the concave-down dominated region. This concept is applied to search for the new multiferroic compounds for which we specifically choose the $\left({\mathrm{BaMnO}}_3\right){}_1$/$\left({\mathrm{BaFeO}}_3\right){}_1$ superlattice as a candidate. The large Ba radius favors the polar distortion in a ${\mathrm{BaMnO}}_3$ layer, while the orbital-ordering-induced superexchange ferromagnetic coupling among Fe-Fe and double-exchange mediated ferromagnetic coupling among Fe-Mn ions stabilize the overall ferromagnetic insulator. A large magnetic moment of $7{\mu }_B$ per unit cell and electric polarization of $14.4\mu {\mathrm{C}\mathrm{cm}}^{-2}$ are obtained. Our study offers an important insight for designing robust multiferroic compounds in the future.

To shed light on the transport properties of electronic nematic phases, we investigate the anisotropic properties of the AC and DC conductivities. Based on the analytical properties of the former, we show that the anisotropy of the effective scattering rate behaves differently than the actual scattering rate anisotropy and even changes sign as a function of temperature. Similarly, the effective spectral weight acquires an anisotropy even when the plasma frequency is isotropic. These results are illustrated by an explicit calculation of the AC conductivity due to the interaction between electrons and spin fluctuations in the nematic phase of the iron-based superconductors and shown to be in agreement with recent experiments.

${\mathrm{Li}}_2{\mathrm{VOSiO}}_4$ and ${\mathrm{VOMoO}}_4$ have been proposed as realizations of the frustrated two-dimensional $J_1\text{−}{J}_2$ quantum Heisenberg model. In this work, in order to test this picture, we study their electronic and magnetic properties by using the local-density approximation + dynamical mean-field theory method. We calculate the magnetic linear response function starting from material-specific Hubbard models and systematically map our results onto those from generalized quantum Heisenberg models. We obtain the effective local magnetic moments and the associated magnetic exchange couplings, in particular the ratio $J_2/J_1$, a measure of the frustration degree, and the ratio $2J_{\perp }/(J_1+J_2)$, measuring the three-dimensionality degree. Our results support a weak frustration picture for both materials, with small but non-negligible long-range interplane couplings, leading to three-dimensional order at low temperature. Implications for the physics of the two systems are discussed.

The Lang-Firsov Hamiltonian, a well-known solvable model of interacting fermion-boson system with sideband features in the fermion spectral weight, is generalized to have the time-dependent fermion-boson coupling constant. We show how to derive the two-time Green's function for the time-dependent problem in the adiabatic limit, defined as the slow temporal variation of the coupling over the characteristic oscillator period. The idea we use in deriving the Green's function is akin to the use of instantaneous basis states in solving the adiabatic evolution problem in quantum mechanics. With such “adiabatic Green's function” at hand we analyze the transient behavior of the spectral weight as the coupling is gradually tuned to zero. Time-dependent generalization of a related model, the spin-boson Hamiltonian, is analyzed in the same way. In both cases the sidebands arising from the fermion-boson coupling can be seen to gradually lose their spectral weights over time. Connections of our solution to the two-dimensional Dirac electrons coupled to quantized photons are discussed.

We study the instabilities of a particle-hole symmetric Weyl metal with both electron and hole Fermi surfaces (FSs) around the Weyl points. For a repulsive interaction we find that the leading instability is towards a longitudinal spin-density-wave $\left({\mathrm{SDW}}_z\right)$ order. Besides, there exist three degenerate subleading instabilities: a charge-density-wave (CDW) instability, and two transverse spin-density-wave $\left({\mathrm{SDW}}_{x,y}\right)$ instabilities. For an attractive interaction the leading instabilities are towards two pair-density-wave (PDW) orders which pair the two FSs separately. Both the PDW and ${\mathrm{SDW}}_z$ order parameters fully gap out the FSs, while the CDW and ${\mathrm{SDW}}_{x,y}$ ones leave line nodes on both FSs. For the ${\mathrm{SDW}}_z$ and the PDW states, the surface Fermi arc in the metallic state evolves to a chiral Fermi line which passes the projection of the Weyl points and traverses the full momentum space. For the CDW state, the line node projects to a “drumhead” band localized on the surface, which can lead to a topological charge polarization. We verify the surface states by computing the angular-resolved photoemission spectroscopy data.

Several models of the charge transfer exciton (CTE) have been proposed to explain its dissociation at the donor-acceptor (DA) interface. However, the underlying physics is still under debate. Here, we derive a temperature $\left(T\right)$ dependent tight-binding model for an electron-hole pair at the DA interface. The main finding is the existence of the localization-delocalization transition at a critical $T$, which can explain the CTE dissociation. The present study highlights the combined effect of entropy (finite $T)$ and carrier delocalization in the CTE dissociation.

We show that the density of surface Shockley states of Ag(111) probed by the differential conductance $G\left(V\right)=dI/dV$ by a scanning-tunneling microscope (STM) can be enhanced significantly at certain energies and positions introducing simple arrays of Co or Ag atoms on the surface, in contrast to other noble-metal surfaces. Specifically we have studied resonators consisting of two parallel walls of five atoms deposited on the clean Ag(111) surface. A simple model in which the effect of the adatoms is taken into account by an attractive local potential and a small hybridization between surface and bulk at the position of the adatoms explains the main features of the observed $G\left(V\right)$ and allows us to extract the proportion of surface and bulk states sensed by the STM tip. These results might be relevant to engineer the surface spectral density of states, to study the effects of surface states on the Kondo effect, and to separate bulk and surface contributions in STM studies of topological surface states.

We propose a guideline for exploring substrates that stabilize the monolayer honeycomb structure of silicene and germanene while simultaneously preserving the Dirac states: in addition to having a strong binding energy to the monolayer, a suitable substrate should be a large-gap semiconductor with a proper work function such that the Dirac point lies in the gap and far from the substrate states when their bands align. We illustrate our idea by performing first-principles calculations for silicene and germanene on the Al-terminated (0001) surface of ${\mathrm{Al}}_2{\mathrm{O}}_3$. The overlaid monolayers on Al-terminated ${\mathrm{Al}}_2{\mathrm{O}}_3$(0001) retain the main structural profile of the low-buckled honeycomb structure via a binding energy comparable to the one between silicene and Ag(111). An unfolded band structure derived from the $k$-projection method reveals that a gapped Dirac cone is formed at the K point due to the structural distortion and the interaction with the substrate. The gaps of 0.4 and 0.3 eV, respectively, for the supported silicene and germanene suggest that they may have potential applications in nanoelectronics.

The rapid developments in the manipulation of two-dimensional monoatomic layers such as graphene or h-BN allow one to create heterostructures consisting of possibly many chemically different layers, stacked owing to van der Waals attraction. We propose a Frenkel-Kontorova model including a transverse degree of freedom in order to describe local deformations in these heterostructures. We study the case where two dissimilar monolayers are alternatively stacked, and find that stacking faults may emerge spontaneously for a large enough number of stacked layers as a result of the competition between adhesion and elastic energies. This symmetry-breaking transition should become of fundamental importance for the description of three-dimensional van der Waals heterostructures as soon as a precise control on the lattice orientation of the van der Waals layers is achieved.

We design from first principles a type of two-dimensional metal-organic framework (MOF) using phenalenyl-based ligands to exhibit a half-filled flat band of the kagome lattice, which is one of a family of lattices that show Lieb-Mielke-Tasaki's flat-band ferromagnetism. Among various MOFs, we find that trans-Au-THTAP (THTAP=trihydroxytriaminophenalenyl) has such an ideal band structure, where the Fermi energy is adjusted right at the flat band due to unpaired electrons of radical phenalenyl. The spin-orbit coupling opens a band gap giving a nonzero Chern number to the nearly flat band, as confirmed by the presence of the edge states in first-principles calculations and by fitting to the tight-binding model. This is a novel and realistic example of a system in which a nearly flat band is both ferromagnetic and topologically nontrivial.

Two-dimensional (2D) semi-Dirac materials are characterized by a quadratic dispersion in one direction and a linear dispersion along the orthogonal direction. We study the topological phase transition in such 2D systems in the presence of an electromagnetic field. We show that a Chern insulating state emerges in a semi-Dirac system with two gapless Dirac nodes in the presence of light. In particular, we show that the intensity of a circularly polarized light can be used as a knob to generate topological states with nonzero Chern number. In addition, for fixed intensity and frequency of the light, a semi-Dirac system with two gapped Dirac nodes with trivial band topology can reveal the topological transition as a function of polarization of the light.

The relationship between electromagnetic phase and power flow in metamaterials composed of periodic magnetodielectric inclusions is investigated. The distribution of Poynting vectors in $k$ space across multiple Brillouin zones suggests that backward phase is a result of field compression within the inclusions, and that left-handedness can be achieved without negative electric or magnetic susceptibility (i.e., without out-of-phase dipole moments). This hypothesis is verified by showing that a one-dimensional periodic array of magnetodielectric slabs can be designed to mimic the properties of an ideal (homogeneous) left-handed medium. Furthermore, using exact fields and without any field averaging, it is shown that the structure can be designed such that practically all power is carried in the fundamental left-handed spectral component. In the limit where the fundamental contains all the power, this periodic structure has the same spectral power signature as that of a homogeneous left-handed medium. It is shown that this congruence does not manifest when the inclusions are purely dielectric, nor does it manifest for magnetodielectric inclusions in two or three dimensions.

The magnetic orders in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha -{\text{RuCl}}_3$, i.e., honeycomb systems with strong spin-orbit coupling and correlations, have been recently described by models with the dominant Kitaev interactions. In this work, we discuss how the orientation of the magnetic order parameter is selected in this class of models. We show that while the order-by-disorder mechanism in the models with solely Kitaev anisotropies always selects cubic axes as easy axes for magnetic ordering, the additional effect of other small bond-dependent anisotropies, such as, e.g., $\mathrm{\Gamma }$ terms, leads to a deviation of the order parameter from the cubic directions. We show that both the zigzag ground state and the face-diagonal orientation of the magnetic moments in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ can be obtained within the $J_1-K_1-J_2-K_2-J_3$ model in the presence of perturbatively small $\mathrm{\Gamma }$ terms. We also show that the zigzag phase found in the nearest-neighbor Kitaev-Heisenberg model, relevant for $\alpha -{\mathrm{RuCl}}_3$, has some stability against the $\mathrm{\Gamma }$ term.