We report the observation of magnetic domains in the exotic, antiferromagnetically ordered all-in-all-out state of ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$, induced by spin canting. The all-in-all-out state can be realized by Ising-like spins on a pyrochlore lattice and is established in ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ below 0.31 K for external magnetic fields up to 0.14 T. Two different spin arrangements can fulfill this configuration which leads to the possibility of magnetic domains. The all-in-all-out domain structure can be controlled by an external magnetic field applied parallel to the [111] direction. This is a result of different spin canting mechanisms for the two all-in-all-out configurations for such a direction of the magnetic field. The change of the domain structure is observed through a hysteresis in the magnetic susceptibility. No hysteresis occurs, however, in the case the external magnetic field is applied along [100].

Several technical issues and challenges are identified and investigated for the planar tunneling spectroscopy of the topological Kondo insulator ${\mathrm{SmB}}_6$. Contrasting behaviors of the tunnel junctions prepared in two different ways are analyzed and explained in detail. The conventional approach based on an ${\mathrm{AlO}}_x$ tunnel barrier results in unsatisfactory results due to the interdiffusion between ${\mathrm{SmB}}_6$ and deposited Al. On the contrary, plasma oxidation of ${\mathrm{SmB}}_6$ crystals produces high-quality tunnel barriers on both (001) and (011) surfaces. Resultant conductance spectra are highly reproducible with clear signatures for the predicted surface Dirac fermions and the bulk hybridization gap as well. The surface states are identified to reside on two or one distinguishable Dirac cone(s) on the (001) and (011) surface, respectively, in good agreement with the recent literature. However, their topological protection is found to be limited within the low energy region due to their inevitable interaction with the bulk excitations, called spin excitons, consistent with a recent theoretical prediction. Implications of our findings on other physical properties in ${\mathrm{SmB}}_6$ and also other correlated topological materials are remarked.

The interplay between angular and quantum magnetoresistance oscillations in quasi-two-dimensional metals leads to the angular oscillations of the amplitude of quantum oscillations. This effect becomes pronounced in high magnetic field, where the simple factorization of the angular and quantum oscillations is not valid. The amplitude of quantum magnetoresistance oscillations is reduced at the Yamaji angles, i.e., at the maxima of the angular magnetoresistance oscillations. These angular beats of the amplitude of quantum oscillations resemble and may be confused with the spin-zero effect, coming from the Zeeman splitting. The proposed effect of “false spin zeros” becomes stronger in the presence of incoherent channels of interlayer electron transport and can be used to separate the different contributions to the Dingle temperature and to check for violations of the standard factorization of angular and quantum magnetoresistance oscillations.

The unique host-guest structure of the type-II silicon (Si) clathrate offers tunable electronic structures by doping guest atoms or molecules to the ${\mathrm{Si}}_{28}$ cages. Here we investigate the possibility of inducing intermediate bands (IBs) by Cu and Ag atoms employing first-principles calculations based on the density functional theory. Our analyses reveal that one or two isolated Cu/Ag atoms around the cage center are required to obtain IBs useful for photovoltaics; however, further clustering is likely to occur, which removes IBs and converts these Si clathrates into metal. Specifically, the formation of Cu and Ag clusters is mainly determined by local thermodynamics and local kinetics, respectively. All the Cu-clathrate structures presenting IBs are not energetically favorable, making Cu inappropriate for IB solar cells, whereas clathrates with one or two Ag atoms inside the cage that have IBs are thermodynamically stable, but the subsequent aggregation to form 3Ag or 4Ag clusters will destroy IBs. Thus, preventing clustering is crucial to realize IBs in Si clathrates by doping noble-metal atoms.

We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity.

Precise knowledge of a quantum system's Hamiltonian is a critical pre-requisite for its use in many quantum information technologies. Here, we report a method for the precise characterization of the nonsecular part of the excited-state Hamiltonian of an electronic-nuclear spin system in diamond. The method relies on the investigation of the dynamic nuclear polarization mediated by the electronic spin, which is currently exploited as a primary tool for initializing nuclear qubits and performing enhanced nuclear magnetic resonance. By measuring the temporal evolution of the population of the ground-state hyperfine levels of a nitrogen-vacancy center, we obtain the first direct estimation of the excited-state transverse hyperfine coupling between its electronic and nitrogen nuclear spin. Our method could also be applied to other electron-nuclear spin systems, such as those related to defects in silicon carbide.

One focus of nanoelectronics research is to exploit the physical limits in size and energy efficiency. Here, we demonstrate a device in the form of a fully metallic atomic-scale transistor based on a lead (Pb) single-atom quantum point contact. The atomic configuration of the point contact determines the conductance of the Pb atomic-scale transistor. The conductance multiplicity of the Pb single-atom transistor has been confirmed by performing switching between an electrically nonconducting “off-state” and conducting “on-states” at $1{\mathrm{G}}_0({\mathrm{G}}_0=2e^2/h$, where $e$ is the electron charge, and $h$ Planck's constant), $2.0{\mathrm{G}}_0$, $3.0{\mathrm{G}}_0$, $1.5{\mathrm{G}}_0$, $2.4{\mathrm{G}}_0$, $2.7{\mathrm{G}}_0$, $2.8{\mathrm{G}}_0$, and $5.4{\mathrm{G}}_0$, respectively. Our density-functional calculations for various ideal Pb single-atom contacts explain the atomic-configuration-related conductance multiplicity of the Pb single-atom transistor. The performance of the Pb single-atom transistors indicates that both the signatures of atomic valence and conductance quantization play roles in electron transport and bistable reconfiguration. The bistable reconfiguration of the electrode tips is an underlying mechanism in the switching of the Pb atomic-scale transistors. The absolute value of the electrochemical potential applied to the gate electrode is less than 30 mV. This merit suggests Pb [besides silver (Ag)] atomic-scale transistors as potential candidates for the development of electronic circuits with low power consumption. The dimension of the switching unit in the Pb single-atom transistor is in the range of 1 nm, which is much smaller than the projected scaling limit of the gate lengths in silicon transistors (5 nm). Therefore, the metallic single-atom transistors may provide perspectives for electronic applications beyond silicon.

Efficient heat dissipation, which is featured by high thermal conductivity, is one of the crucial issues for the reliability and stability of nanodevices. However, due to the generally fast $1/T$ decrease of thermal conductivity with temperature increase, the efficiency of heat dissipation quickly drops down at an elevated temperature caused by the increase of work load in electronic devices. To this end, pursuing semiconductor materials that possess large thermal conductivity at high temperature, i.e., slower decrease of thermal conductivity with temperature increase than the traditional $\kappa \sim 1/T$ relation, is extremely important to the development of disruptive nanoelectronics. Recently, monolayer gallium nitride (GaN) with a planar honeycomb structure emerges as a promising new two-dimensional material with great potential for applications in nano- and optoelectronics. Here, we report that, despite the commonly established $1/T$ relation of thermal conductivity in plenty of materials, monolayer GaN exhibits anomalous behavior that the thermal conductivity almost decreases linearly over a wide temperature range above 300 K, deviating largely from the traditional $\kappa \sim 1/T$ law. The thermal conductivity at high temperature is much larger than the expected thermal conductivity that follows the general $\kappa \sim 1/T$ trend, which would be beneficial for applications of monolayer GaN in nano- and optoelectronics in terms of efficient heat dissipation. We perform detailed analysis on the mechanisms underlying the anomalously temperature-dependent thermal conductivity of monolayer GaN in the framework of Boltzmann transport theory and further get insight from the view of electronic structure. Beyond that, we also propose two required conditions for materials that would exhibit similar anomalous temperature dependence of thermal conductivity: large difference in atom mass (huge phonon band gap) and electronegativity (LO-TO splitting due to strong polarization of bond). Our study offers fundamental understanding of phonon transport in monolayer GaN, and the insight gained from this study is of great significance for the design and search of materials superior for applications in nano- and optoelectronics in terms of high-performance thermal management.

We study the phase diagram of the two-dimensional repulsive Hubbard model with spin-dependent anisotropic hopping at half filling. The system develops Ising antiferromagnetic long-range order already at infinitesimal repulsive interaction strength in the ground state. Outside the perturbative regime, unbiased predictions for the critical temperatures of the Ising antiferromagnet are made for representative interaction values by a variety of state-of-the-art quantum Monte Carlo methods, including the diagrammatic Monte Carlo, continuous-time determinantal Monte Carlo, and path-integral Monte Carlo methods. Our findings are relevant to ultracold atom experiments in the $p$ orbital or with spin-dependent optical lattices.

We investigate the stability of conducting and insulating phases in multichannel Luttinger liquids with respect to embedding a single impurity. We devise a general approach for finding critical exponents of the conductance in the limits of both weak and strong scattering. In contrast to the one-channel Luttinger liquid, the system state in certain parametric regions depends on the scattering strength which results in the emergence of a bistability. Focusing on the two-channel liquid, the method developed here enables us to provide a generic analysis of phase boundaries governed by the most relevant (i.e., not necessarily single-particle) scattering mechanism. The present approach is applicable to channels of different nature as in fermion-boson mixtures, or to identical ones as on the opposite edges of a topological insulator. We show that interaction per se cannot provide protection in the particular case of topological insulators realized in narrow Hall bars.

We have investigated the electronic structure of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ using core-to-core resonant inelastic x-ray scattering. The experimental spectra can be well reproduced using ab initio density functional theory based multiplet ligand field theory calculations, thereby validating these calculations. We found that the low-energy, effective Ir ${\mathrm{t}}_{2\mathrm{g}}$ orbitals are practically degenerate in their crystal-field energy. We uncovered that ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and iridates in general are negative charge transfer systems with large covalency and a substantial oxygen ligand hole character in the Ir ${\mathrm{t}}_{2\mathrm{g}}$ Wannier orbitals. This has far reaching consequences, as not only the on-site crystal-field energies are determined by the long-range crystal structure, but, more significantly, magnetic exchange interactions will have long-range distance dependent anisotropies in the spin direction. These findings set constraints and show pathways for the design of $d^5$ materials that can host compasslike magnetic interactions.

Although nanolasers typically have low $Q$ factors and high lasing thresholds, they have been successfully implemented with various gain media. Intuitively, it seems that an increase in the gain coefficient would improve the characteristics of nanolasers. For a plasmonic nanolaser—in particular, a distributed feedback laser—we propose a self-consistent model that takes into account both spontaneous emission and the multimode character of laser generation to show that for a given pumping strength, the gain coefficient has an optimal value at which the radiation intensity is at a maximum and the radiation linewidth is at a minimum.

The structural phase transition in hexagonal ${\mathrm{BaMnO}}_3$ occurring at $T_{\mathrm{C}}=130$ K was studied in ceramic samples using electron and x-ray diffraction, second harmonic generation, as well as by dielectric and lattice dynamic spectroscopies. The low-temperature phase (space group $P{6}_{3}cm)$ is ferroelectric with a triplicated unit cell. The phase transition is driven by an optical soft mode from the Brillouin-zone boundary $[q=(\frac{1}{3},\frac{1}{3},0)]$; this mode activates in infrared and Raman spectra below $T_{\mathrm{C}}$ and it hardens according to the Cochran law. Upon cooling below $T_{\mathrm{C}}$, the permittivity exhibits an unusual linear increase with temperature; below 60 K, in turn, a frequency-dependent decrease is observed, which can be explained by slowing down of ferroelectric domain wall motions. Based on our data we could not distinguish whether the high-temperature phase is paraelectric or polar (space groups $P{6}_3/mmc$ or $P{6}_{3}mc$, respectively). Both variants of the phase transition to the ferroelectric phase are discussed based on the Landau theory. Electron paramagnetic resonance and magnetic susceptibility measurements reveal an onset of one-dimensional antiferromagnetic ordering below $\approx 220\mathrm{K}$ which develops fully near 140 K and, below $T_{\mathrm{N}}\approx 59$ K, it transforms into a three-dimensional antiferromagnetic order.

Manipulation of magnetic and electronic structures of graphene nanoflakes is of great technological importance. Here, we systematically study the magnetic and electronic phases of graphene nanoflakes within first-principles calculations. We illustrate the intricate shape and size dependence on the magnetic and electronic properties and further investigate the effects of carrier doping, which could be tuned by gate voltage. A crossover from the nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance. We identify this as originating from the armchair defects at the junctions of two sublattices on the edge. Electron or hole doping simultaneously influences the magnetic and electronic structures and triggers phase crossover. Beyond a critical doping, antiferromagnetic to ferromagnetic phase crossover is observed for the flakes without sublattice imbalance. In contrast, suppression of magnetism and a possible crossover from the magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous with magnetic phase changes, a semiconductor to (half) metal transition is observed upon carrier doping. Our findings should have important implications in graphene-based spintronics.

Magnetic anisotropy of spin models with pseudo-dipolar interactions is theoretically studied in the high-temperature paramagnetic phase. Using the high-temperature expansion, we show that the pseudo-dipolar interaction gives rise to a magnetic anisotropy which shows $\propto T^{-5}$ temperature dependence. This phenomenon arises from the pseudo-dipolar interaction and is distinct from the orbital effect, such as Van Vleck susceptibility. By an explicit calculation, it is shown that the second order in the high-temperature expansion prefers to point the spins along the bond direction. The theory is applied to the Heisenberg-Kitaev model on the honeycomb lattice and a cubic lattice model which is potentially relevant to perovskite oxides. The leading order for the magnetic anisotropy arises from the second order in high-temperature expansion, which contribute to a fourth-order anisotropic term in Landau theory. The result shows that the anisotropy from the pseudo-dipolar interaction gives rise to $\langle 100\rangle$ magnetic anisotropy. These results are potentially relevant to heavy-transition-metal oxides such as iridates. Experimental observation of the magnitude of anisotropic interactions using magnetic torque measurement is also discussed.

Recent theoretical studies suggest that superconductivity may be found in doped chromium pnictides with crystal structures similar to their iron counterparts. Here, we report a comprehensive study on the magnetic arsenides ${\mathrm{BaCr}}_2{\mathrm{As}}_2$ and ${\mathrm{BaCrFeAs}}_2$ (space group $I4/mmm)$, which are possible mother compounds with $d^4$ and $d^5$ electron configurations, respectively. DFT-based calculations of the electronic structure evidence metallic antiferromagnetic ground states for both compounds. By powder neutron diffraction, we confirm for ${\mathrm{BaCr}}_2{\mathrm{As}}_2$ a robust ordering in the antiferromagnetic $G$-type structure at $T_{\mathrm{N}}=580$ K with ${\mu }_{\mathrm{Cr}}=1.9{\mu }_{\mathrm{B}}$. Anomalies in the lattice parameters point to magnetostructural coupling effects. In ${\mathrm{BaCrFeAs}}_2$, the Cr and Fe atoms randomly occupy the transition-metal site and $G$-type order is found below 265 K with ${\mu }_{\mathrm{Cr}/\mathrm{Fe}}=1.1{\mu }_{\mathrm{B}}$. ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy demonstrates that only a small ordered moment is associated with the Fe atoms, in agreement with electronic structure calculations leading to ${\mu }_{\mathrm{Fe}}\sim 0$. The temperature dependence of the hyperfine field does not follow that of the total moments. Both compounds are metallic but show large enhancements of the linear specific heat. Electrical transport in ${\mathrm{BaCrFeAs}}_2$ is dominated by the atomic disorder and the partial magnetic disorder of Fe. Our results indicate that Néel-type order is unfavorable for Fe moments and thus it is destabilized with increasing Fe content.

We present a detailed analytical derivation of the spin wave (SW) dispersion relation in magnetic nanotubes with magnetization along the azimuthal direction. The obtained formula can be used to calculate the dispersion relation for any longitudinal and azimuthal mode. The obtained dispersion is asymmetric for all azimuthal modes traveling along the axial direction. As reported in our recent publication [Phys. Rev. Lett. 117, 227203 (2016)], the asymmetry is a curvature-induced effect originating from the dipole-dipole interaction. Here, we discuss the asymmetry of the dispersion for azimuthal modes by analyzing the SW asymmetry $\mathrm{\Delta }f\left(k_z\right)=f_n\left(k_z\right)-f_n(-k_z)$, where $f_n\left(k_z\right)$ is the eigenfrequency of a magnon with a longitudinal and azimuthal wave vectors, $k_z$ and $n$, respectively; and the dependence of the maximum asymmetry with the nanotube radius $R$. The analytical results are in perfect agreement with micromagnetic simulations. Furthermore, we show that the dispersion relation simplifies to the thin-film dispersion relation with in-plane magnetization when analyzing the three limiting cases: (i) $k_z=0$, (ii) $k_z\gg 1/R$, and (iii) $k_z\ll 1/R$. In the first case, for the zeroth-order modes the thin-film Kittel formula is obtained. For modes with higher order the dispersion relation for the Backward-Volume geometry is recovered. In the second case, for the zeroth-order mode the exchange dominated dispersion relation for SW in Damon-Esbach configuration is obtained. For the case $k_z\ll 1/R$, we find that the dispersion relation can be reduced to a formula similar to the Kalinikos-Slavin [J. Phys. C: Sol. State Phys. 19, 7013 (1986)] type.

Scanning force microscopy (SFM) and Raman spectroscopy allow the unraveling of charge doping and strain effects upon wetting and dewetting of a graphene-mica slit pore with water. SFM reveals a wetting monolayer of water, slightly thinner than a single layer of graphene. The Raman spectrum of the dry pore exhibits the $D^{\prime }$ peak of graphene, which practically disappears upon wetting, and recurs when the water layer dewets the pore. Based on the $2D$- and $G$-peak positions, the corresponding peak intensities, and the widths, we conclude that graphene on dry mica is charge-doped and variably strained. A monolayer of water in between graphene and mica removes the doping and reduces the strain. We attribute the $D^{\prime }$ peak to direct contact of the graphene with the ionic mica surface in dry conditions, and we conclude that a complete monolayer of water wetting the slit pore decouples the graphene from the mica substrate both mechanically and electronically.