Using high-resolution photoemission spectroscopy on in situ potassium surface-doped para-terphenyl crystals, we uncover a low-energy spectral gap of 12 meV. This spectral gap displays a strong temperature-dependent gap-filling behavior and persists up to 120 K, which is consistent with the onset temperature of a weak steplike diamagnetic transition found in this material. Among a few potential candidates for the origin of the observed gaps, we argue that electron pairing is a plausible origin—a possibility consistent with fluctuating Cooper pairs forming near 120 K.

Most interesting examples of violations of the Mott-Ioffe-Regel (MIR) resistivity limit are found in materials with strong electronic correlations that are not well understood by theory. We demonstrate that first principles theory can predict the experimentally observed frequency dependence of the optical conductivity for a novel class of metals where the residual resistivity is near or above the MIR limit, which we define as a “bad metal.” The predicted optical conductivity of a NiCoCr alloy is in good agreement with experiment. It is demonstrated that the width of the Drude peak describing the low-frequency part of optical conductivity is comparable to the Fermi energy. The latter, together with a mean free path comparable to the interatomic distance, indicates the absence of well-defined quasiparticles. In contrast to traditional bad metals with strong electron-electron interactions, both the high resistivity and the large width of the Drude peak in these alloys result from strong scattering on disordered atomic potentials that can be understood using modern density functionals.

Electron-doped $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ has been the subject of much recent attention because of its similarities with hole-doped $\mathrm{L}{\mathrm{a}}_2\mathrm{Cu}{\mathrm{O}}_4$. However, direct evidence of its superconductivity has not yet been observed, which has led to various unresolved issues related to this material. In this study, a 14-nm-thick La-doped $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ film was grown on a (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrate. We found that the fourfold component of the in-plane anisotropic magnetoresistance disappears and is replaced by a twofold symmetry as the temperature increases. Variable-temperature x-ray diffraction and spatially resolved optical second-harmonic generation experiments eliminate the possibility of crystal structural distortion. The temperature dependence of direct current magnetization measurements indicates that a change in the spin structure may account for the symmetry breaking, similar to the nematic ordering of iron-based and other superconductors. Remarkably, the transition temperature of the symmetry breaking is well consistent with the previously reported onset (50 K) of the low-energy gap in K-doped $\mathrm{S}{\mathrm{r}}_2\mathrm{Ir}{\mathrm{O}}_4$ observed by scanning tunneling spectroscopy. Our results provide a perspective on these materials that helps elucidate the nature of magnetic interactions and explores the expected superconductivity of iridates.

Energy levels of nitrogen-vacancy centers in diamond were investigated using optically detected magnetic-resonance spectroscopy near the electronic ground-state level anticrossing (GSLAC) at an axial magnetic field around 102.4 mT in diamond samples with a nitrogen concentration of 1 and 200 ppm. By applying radio waves in the frequency ranges from 0 to 40 MHz and from 5.6 to 5.9 GHz, we observed transitions that involve energy levels mixed by the hyperfine interaction. We developed a theoretical model that describes the level mixing, transition energies, and transition strengths between the ground-state sublevels, including the coupling to the nuclear spin of the NV centers' ${}^{14}\mathrm{N}$ and ${}^{13}\mathrm{C}$ atoms. The calculations were combined with the experimental results by fitting the ODMR spectral lines based on a theoretical model, which yielded information about the polarization of nuclear spins. This study is important for the optimization of experimental conditions in GSLAC-based applications, e.g., microwave-free magnetometry and microwave-free nuclear-magnetic-resonance probes.

Recent experimental studies have found that magnetic impurities deposited on superconducting monolayer ${\mathrm{NbSe}}_2$ generate coupled Yu-Shiba-Rusinov bound states. Here we consider ferromagnetic chains of impurities which induce a Yu-Shiba-Rusinov band and harbor Majorana bound states at the chain edges. We show that these topological phases are stabilized by strong Ising spin-orbit coupling in the monolayer and examine the conditions under which Majorana phases appear as a function of distance between impurities, impurity spin projection, orientation of chains on the surface of the monolayer, and strength of magnetic exchange energy between impurity and superconductor.

A graphene bilayer shows an unusual magnetoelectric response whose magnitude is controlled by the valley-isospin density, making it possible to link magnetoelectric behavior to valleytronics. Complementary to previous studies, we consider the effect of static homogeneous electric and magnetic fields that are oriented parallel to the bilayer's plane. Starting from a tight-binding description and using quasidegenerate perturbation theory, the low-energy Hamiltonian is derived, including all relevant magnetoelectric terms whose prefactors are expressed in terms of tight-binding parameters. We confirm the existence of an expected axion-type pseudoscalar term, which turns out to have the same sign and about twice the magnitude of the previously obtained out-of-plane counterpart. Additionally, small anisotropic corrections to the magnetoelectric tensor are found that are fundamentally related to the skew interlayer hopping parameter ${\gamma }_4$. We discuss possible ways to identify magnetoelectric effects by distinctive features in the optical conductivity.

We express the recently introduced real-time diagrammatic quantum Monte Carlo [Phys. Rev. B 91, 245154 (2015)] in the Larkin-Ovchinnikov basis in Keldysh space. Based on a perturbation expansion in the local interaction $U$, the special form of the interaction vertex allows us to write diagrammatic rules in which vacuum Feynman diagrams directly vanish: This reproduces the main property of the previous algorithm, without the cost of the exponential sum over Keldysh indices. In an importance sampling procedure, this implies that only interaction times in the vicinity of the measurement time contribute, and such an algorithm can directly address the long-time limit needed in the study of steady states in out-of-equilibrium systems. We then implement and discuss different variants of Monte Carlo algorithms in the Larkin-Ovchinnikov basis. A sign problem reappears, showing that the cancellation of vacuum diagrams has no direct impact on it.

The discovery and design of materials with large thermal conductivities (${\kappa }_L$) is critical to address future heat management challenges, particularly as devices shrink to the nanoscale. This requires developing novel physical insights into the microscropic interactions and behaviors of lattice vibrations. Here, we use ab initio phonon Boltzmann transport calculations to derive fundamental understanding of lattice thermal transport in two-dimensional (2D) monolayer hexagonal boron-based compounds, h-BX ($X=\mathrm{N}$, P, As, Sb). Monolayer h-BAs, in particular, possesses structural and dispersion features similar to bulk cubic BAs and 2D graphene, which govern their ultrahigh room temperature ${\kappa }_L$ (1300 W/m K and 2000–4000 W/m K, respectively), yet here combine to give significantly lower ${\kappa }_L$ for monolayer h-BAs (400 W/m K at room temperature). This work explores this discrepancy, and thermal transport in the monolayer h-BX systems in general, via comparison of the microscopic mechanisms that govern phonon transport. In particular, we present calculations of phonon dispersions, velocities, scattering phase space and rates, and ${\kappa }_L$ of h-BX monolayers as a function of temperature, size, defects, and other fundamental parameters. From these calculations, we make predictions of the thermal conductivities of h-BX monolayers, and more generally develop deeper fundamental understanding of phonon thermal transport in 2D and bulk materials.

We propose a microscopic theory of terahertz (THz) radiation generation on metal gratings under the action of femtosecond laser pulses. In contrast to previous models, only low-frequency currents inside the metal are considered without involving electron emission and acceleration. The presented model generalizes thermal mechanism of THz response for the case of structured surfaces, when temperature gradients inside the electron gas can be enhanced by excitation and absorption of surface plasmons. Performed numerical calculations reproduce specific experimental features like delayed character of THz response and low conversion efficiency when the grating depth is too large.

Intercalation of lithium in few-layer phosphorene has aroused great concern because of the prospects of few-layer phosphorene as an anode material for high-performance lithium-ion batteries (LIBs). In this paper, we adopt first-principles calculations to explore in detail the effects of stacking structure of bilayer black phosphorene (BBP) on the adsorption and diffusion of lithium in BBP. Our calculated results indicate that Li energetically prefers to intercalate within rather than adsorb outside BBP, and it exists in a cationic state. Moreover, the stacking structure of BBP has very important effects on Li adsorption, the open-circuit voltage, and Li diffusion in BBP. Additionally, Li diffusion in BBP exhibits directional preference, with diffusing along the zigzag direction being easier. The analyses of the potential distribution indicate that the puckered honeycomb structure of monolayer phosphorene and the stacking structure of BBP greatly cause low-potential channels along the zigzag direction, which directly lead to the directional preference in Li diffusion. More interestingly, our results suggest that BBP with AC stacking is the most beneficial for Li adsorption and diffusion. These findings suggest that changing the stacking structure of BBP is one possible way to rationally design the anode material in high charge/discharge rate LIBs based on BBP.

Skyrmions with multiple helicity or topology in centrosymmetric crystals are intriguing magnetic-domain objects due to their diverse dynamics under external stimuli. Here we illustrate how the two key gradients of magnetocrystalline anisotropy (MCA) and spin reorientation transition (SRT) affect the skyrmion formation and topology by Cu substitution in the biskyrmion-host MnNiGa alloy. The MCA and SRT are simultaneously tuned in a large scope, while the original high Curie temperature ($T_C$) is retained. Detailed neutron-scattering studies revealed the construction of a noncollinear canted magnetic structure below the SRT temperature ($T_{\mathrm{SR}}$), which effectively correlates the SRT with the evolution of the MCA, as well as the exchange interaction. The Cu substitution raises the $T_{\mathrm{SR}}$ to merge with the $T_{\mathrm{C}}$, and meanwhile, reduces the $c$-axis anisotropy. Lorentz transmission electron microscopy revealed the formation of stacked biskyrmions from above room temperature to lower temperatures in $\mathrm{MnN}{\mathrm{i}}_{1-x}\mathrm{C}{\mathrm{u}}_x\mathrm{Ga}\left(x=0–0.3\right)$ in the presence of proper MCA. Micromagnetic simulations further confirmed the great effect of uniaxial anisotropy on the stabilization of biskyrmions. Our work has helped clarify the evolution of magnetic structures and their correlation to the SRT, providing an account of the effect of MCA and exchange interaction on the biskyrmion formation.

A neutron powder diffraction experiment has been performed on the quasicrystal approximant ${\mathrm{Au}}_{72}{\mathrm{Al}}_{14}{\mathrm{Tb}}_{14}$, a body-centered-cubic crystal of icosahedral spin clusters. The long-range antiferromagnetic order was confirmed at the transition temperature $T_{\mathrm{N}}=10.4$ K. The magnetic structure consists of noncoplanar whirling spins on the icosahedral clusters, arranged in an antiferroic manner. A simple icosahedral spin-cluster model with uniaxial anisotropy accounts well the whirling spin order as well as the in-field metamagnetic transition, indicating that the icosahedral symmetry is essential.

Magnetic transitions in a single crystal of the orthoferrite ${\mathrm{ErFeO}}_3$ have been studied between 2 and 300 K using conventional superconducting quantum interference device measurements as well as pulsed high magnetic fields of up to 58 T. Spontaneous spin reorientations, as well as temperature and magnetic field induced spin reorientations, have been observed. At spin-reorientation transition temperatures, the ${\mathrm{Fe}}^{3+}$ sublattice exhibits a ${\mathrm{\Gamma }}_4\to {\mathrm{\Gamma }}_2$ transition where the magnetic coupling between the net magnetic moment of ${\mathrm{Fe}}^{3+}$ $3d$ and ${\mathrm{Er}}^{3+}$ $4f$ is antiferromagnetic, and the temperature dependence of the magnetization suggests that the order of magnitude of the interaction is $J_{\text{Er-Er}}<J_{\text{Er-Fe}}<J_{\mathrm{Fe}\text{−}\mathrm{Fe}}$. However, the magnetic coupling varies widely with temperature and/or magnetic field, which indicates that the ${\mathrm{Er}}^{3+}$ sublattice moment cannot be simply explained by the partial polarization of the paramagnetic spins due to the combination of the molecular field from the net ${\mathrm{Fe}}^{3+}$ moment and external applied magnetic fields. It is found that in a sufficiently high magnetic field, the magnetic interactions between the ${\mathrm{Fe}}^{3+}$ sublattice and the ${\mathrm{Er}}^{3+}$ sublattice are destroyed. Based on the detailed magnetization measurements, the complete high-field magnetic phase diagram has been obtained, providing an important framework for future investigations into the complicated magnetic interactions and magnetoelectric couplings in rare earth orthoferrites.

Iridium oxides have provided a playground to study novel phases originating from spin-orbit coupling and electron-electron interactions. Among them, the $d$-wave singlet superconductor was proposed for electron-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, containing two Ir atoms in a unit cell due to the staggered rotation of oxygen octahedra about the $c$ axis. It was also noted that such oxygen octahedra rotation affects electronic transports. Here we study the role of octahedra tilting away from the $c$ axis, in determining superconducting pairing symmetry. We show that the octahedra tilting changes the large Fermi surface to a Dirac point, which strongly suppresses the conventional $d$-wave pairing. Furthermore, it also promotes effective spin-triplet interactions in the strong Hubbard interaction limit, leading to a transition from the even-parity to odd-parity superconducting phase. Thus, tuning octahedra distortions can be used as a tool to engineer a spin-triplet superconductor in strongly correlated systems with strong spin-orbit coupling.

The realization of spin-based logical gates crucially depends on magnetically coupled spin qubits. Thus, understanding decoherence when spin qubits are in close proximity will become a roadblock to overcome. Herein, we propose a method free of fitting parameters to evaluate the qubit phase memory time $T_m$ in samples with high electron spin concentrations. The method is based on a model aimed to estimate magnetic nuclear decoherence [P. C. E. Stamp and I. S. Tupitsyn, Phys. Rev. B 69, 014401 (2004)]. It is applied to a ground-spin $J=8$ magnetic molecule 1 displaying atomic clock transitions, namely ${{\left[\mathrm{H}{\mathrm{o}}^{\mathrm{III}}{\left({\mathrm{W}}_5{\mathrm{O}}_{18}\right)}_2\right]}^9}^{-}$, which remarkably increase $T_m$ at unusually high electron-spin concentrations. Our approach unveils the causes that limit the coherence reached at the clock transitions in challenging systems such as 1, where recent models fail.

Transverse domain wall (TDW) dynamics in long and narrow spin valves with perpendicular current injection is theoretically investigated. We demonstrate that stable traveling-wave motion of TDWs with finite velocity survives for strong enough planar-transverse polarizers. For typical ferromagnetic materials (for example, Co) and achievable spin polarization ($P=0.6$), TDWs acquire a velocity of ${10}^3$ m/s under a current density below ${10}^7\mathrm{A}/{\mathrm{cm}}^2$. This efficiency is comparable with that of perpendicular polarizers. More importantly, in this case the wall has ultrahigh “differential mobility” around the onset of stable wall excitations. Our results open possibilities for developing magnetic nanodevices based on TDW propagation with low energy consumption. Also, analytics for parallel and perpendicular polarizers perfectly explains existing simulation findings. Finally, further boosting of TDWs by external uniform transverse magnetic fields is investigated and turns out to be efficient.

Nonequilibrium susceptibility in photoinduced Floquet states is studied. We analyze an electron system coupled with a heat bath in a time-periodic electric field. Spin/charge susceptibility is formulated on the basis of the Floquet Green's function method, and is calculated numerically in a wide range of amplitude and frequency of light. When the frequency is larger than the bandwidth, the susceptibility is enhanced due to the dynamical localization effect, and their peak positions in the momentum space are shifted by the Fermi surface deformation. In the case of the small frequency and amplitude, a multiple-peak structure emerges in the susceptibility, originating from the multiple Floquet bands which cross the Fermi level of the bath. To confirm those numerical results and provide the interpretation, a perturbative expression of the susceptibility is derived for small electric-field amplitude.

I study theoretically quadrupolar topological insulators under applied static electric field rotated along the crystal axis. I demonstrate that the energy spectrum of this structure is a Wannier-Stark ladder that naturally visualizes the quantization of nested Wilson loops for the bulk bands. This enables a direct distinction between the topological phase, possessing localized corner states, and the trivial phase, lacking the corner states. The crossover between the topological and trivial cases takes place via the nonadiabatic regime. These results may find applications in the characterization of rapidly emerging higher-order topological phases of light and matter.

Novel lateral optical forces acting on dipoles near surfaces have been investigated in the past few years: circularly polarized dipoles experience lateral optical forces when in proximity to a surface due to the recoil of directionally excited modes. Recent work shows that even linearly polarized dipoles may experience lateral forces when nonreciprocal substrates are used, due to the asymmetric propagation of modes in the surface. We theoretically show that a linearly polarized particle emitting in close proximity to a magneto-optical substrate may experience a lateral optical force even if the external magnetic field is normal to the surface plane. The polarization conversion of the magneto-optic material introduces a gradient in the quasistatic fields reflected from the surface, resulting in a lateral optical force, and also gives surface plasmons a hybrid polarization character, which alters the surface plasmon directional excitation from the dipole, resulting in an optical recoil force. We envisage potential applications in nanomechanical devices, since similar magnetoplasmonic architectures have already been developed experimentally.

We reveal a proximity effect between a topological band (Chern) insulator described by a Haldane model and spin-polarized Dirac particles of a graphene layer. Coupling weakly the two systems through a tunneling term in the bulk, the topological Chern insulator induces a gap and an opposite Chern number on the Dirac particles at half filling, resulting in a sign flip of the Berry curvature at one Dirac point. We study different aspects of the bulk-edge correspondence and present protocols to observe the evolution of the Berry curvature as well as two counterpropagating (protected) edge modes with different velocities. In the strong-coupling limit, the energy spectrum shows flat bands. Therefore we build a perturbation theory and address further the bulk-edge correspondence. We also show the occurrence of a topological insulating phase with Chern number one when only the lowest band is filled. We generalize the effect to Haldane bilayer systems with asymmetric Semenoff masses. Moreover, we propose an alternative definition of the topological invariant on the Bloch sphere.