Recent theoretical considerations have demonstrated that free-standing graphene doped with alkali metals ($A{\mathrm{C}}_x$) supports strong Dirac and weak acoustic plasmons. Here we show that when $A{\mathrm{C}}_x$ is deposited on a metal surface the effective Coulomb screening produced by the substrate can completely wash out these collective modes. However, even tiny detachment of the $A{\mathrm{C}}_x$ layer from the metal surface recovers the $A{\mathrm{C}}_x$ plasmonic properties. Especially the acoustic plasmon intensity is strongly enhanced in comparison to a free-standing case. We further provide the physical background of these intriguing phenomena. The studied systems consist of lithium- and cesium-doped graphene layers deposited on the Ir(111) surface.

Heterostructures formed between two different van der Waals materials enable interactions and functionalities absent in each component. In this work we show that vicinity to an atomically thin ${\mathrm{WSe}}_2$ sheet dramatically impacts the energies of the symmetry-broken low Landau levels of bilayer graphene, possibly due to Coulomb screening. We present a systematic study of the magnetic field and electrical displacement field dependences of the Landau level gaps at filling factor $\nu =1$, 2, 3, and compare to boron nitride encapsulated pristine bilayer graphene. The exchange-dominated energy splitting between the $N=0$ and 1 orbital wave functions is significantly enhanced, which leads to a modified phase diagram at filling factor $\nu =0$ and larger energy gaps at $\nu =1$ and 3 in ${\mathrm{WSe}}_2$/bilayer graphene heterostructures. The exchange-enhanced spin gap at $\nu =2$, on the other hand, is reduced by approximately twofold. Our results demonstrate a possible way to engineer quantum Hall phenomena via van der Waals heterostructures.

We proposed a formula for the ${\mathbb{Z}}_2$ invariant for topological insulators, which remains valid without translational invariance. Our formula is a local expression, in the sense that the contributions mainly come from quantities near a point. Using almost commute matrices, we proposed a method to approximate this invariant with local information. The validity of the formula and the approximation method is proved.

Dirac points in two-dimensional (2D) materials have been a fascinating subject of research. Recently, it has been theoretically predicted that Dirac points may also be stabilized in 2D magnetic systems. However, it remains a challenge to identify concrete 2D materials which host such magnetic Dirac points. Here, based on first-principles calculations and theoretical analysis, we propose a stable 2D material, the monolayer ${\mathrm{TaCoTe}}_2$, as an antiferromagnetic (AFM) 2D Dirac material. We show that it has an AFM ground state with an out-of-plane Néel vector. It hosts a pair of 2D AFM Dirac points on the Fermi level in the absence of spin-orbit coupling (SOC). When the SOC is considered, a small gap is opened at the original Dirac points. Meanwhile, another pair of Dirac points appear on the Brillouin zone boundary below the Fermi level, which are robust under SOC and have a type-II dispersion. Such a type-II AFM Dirac point has not been observed before. We further show that the location of this Dirac point as well as its dispersion type can be controlled by tuning the Néel vector orientation.

Based on accurate x-ray structure analysis of $\mathrm{Gd}{\mathrm{B}}_6$ over the temperature range 85–300 K it has been shown that anomalously strong charge carrier scattering in the quantum diffusion regime of charge transport in this compound arises due to the formation of (i) dynamically coupled $\mathrm{G}{\mathrm{d}}^{3+}$ pairs of about 3.3 Å in size and with energy of quasilocal oscillations ∼7–8 meV, and due to (ii) dynamic charge stripes along the [001] direction of the cubic lattice. It has been shown that the anharmonic approximation is appropriate when analyzing the static and dynamic components of the atomic displacement parameters of gadolinium. The barrier height of double-well potential of $\mathrm{G}{\mathrm{d}}^{3+}$ ions was determined both from low-temperature heat capacity measurements and from the electron density distribution reconstructed from x-ray data.

Spinel ${\mathrm{Li}}_x{\mathrm{Mn}}_2{\mathrm{O}}_4$ is a key cathode material that is used extensively in commercial Li-ion batteries. A challenge with this material has been that the capacity of the battery fades with cycling, an effect that can be traced to the presence of an antiferromagnetic insulator phase in the fully lithiated ${\mathrm{LiMn}}_2{\mathrm{O}}_4$ (LMO) and the associated charge disproportionation that drives distortions of the ${\mathrm{MnO}}_6$ octahedra. Here, by combining x-ray magnetic Compton scattering experiments with parallel first-principles computations, we show that the antiferromagnetic phase of LMO is surrounded by a robust ferrimagnetic metallic phase, which becomes stable when even a small amount of Li is removed from or added to the charge-ordered LMO. In this surprising ferrimagnetic state, charge-ordering and octahedral distortions are found to be strongly suppressed. We identify the nature of the ferrimagnetic orbitals involved through theoretical and experimental analyses of the magnetic Compton scattering spectra.

Using the electron beam of a scanning electron microscope as an external current source with tunable energy, we investigate the transport properties of high-energy electrons injected from vacuum into the metal layer of Pt/Cu/Si Schottky junctions. When the injection energy is varied between 1 and 30 keV, the current transmitted into the semiconductor increases by several orders of magnitude and reaches values orders of magnitude larger than the current injected from vacuum. Inspecting the energy dependence of the transmitted current we identify two transport regimes. In the limit of low injection energies and thick metal films, the transport is dominated by the formation and propagation of a secondary electron distribution in the metal layer. However, when the injection energy is sufficiently large and the metal layer sufficiently thin, electrons are transmitted into the semiconductor with negligible energy loss, i.e., the metal layer becomes essentially transparent. The transmitted current is then dominated by impact ionization in the semiconductor. When the metal layer of the Schottky junction is relatively thick and the injection energy of a few keV typically, the transmitted current increases abruptly. The origin of this abrupt change is interpreted as a combined effect of a quasiballistic electron transport in the metal layer and a sudden variation of the density of states in the semiconductor substrate.

Magnetic helix (MH) structure can be a role model for future spintronic devices. Utilizing the advantage of constructing possible magnetic configurations, in this work we investigate spintronic behavior in a helical geometry with finite magnetic ordering. The interplay between short- and long-range hopping of electrons yields many nontrivial features which are thoroughly studied. Quite interestingly, we see that the MH exhibits the strong chiral-induced spin-selectivity effect, like what is observed in chiral molecules. Finally, to make the model more realistic, we also examine the effect of helical dynamics. All the results are valid for a wide range of physical parameters, which prove the robustness of our analysis.

Last year, we reported a perturbative theory of the Casimir-Lifshitz torque between planar biaxially anisotropic materials in the retarded limit [Thiyam et al., Phys. Rev. Lett. 120, 131601 (2018)], which is applied here to study the change in sign and magnitude of the torque with separation distance in biaxial black phosphorus having excess charge carriers. The study is carried out both in vacuum and in a background fluid medium. The presence of extra charge carriers and that of an intervening fluid medium are both found to promote enhancement of the magnitude of the torque between identical slabs. The degree of enhancement of the magnitude of torque increases not only with an increased carrier concentration but also with separation distance. In the nonidentical case when different planes of anisotropic black phosphorus face each other, owing to the nonmonotonic characteristic of the sign-reversal effect of the torque, the enhancement by carrier addition and intervening medium also becomes nonmonotonic with distance. In the presence of a background medium, the nonmonotonic degree of enhancement of the torque with distance is observed even between identical slabs.

We synthesized a kagome fluoride ${\mathrm{Cs}}_2{\mathrm{LiTi}}_3{\mathrm{F}}_{12}$ with $S=1/2$ spins, and studied magnetic properties of the compound. The temperature dependence of the magnetic susceptibility indicates that it has dominant antiferromagnetic interactions and that it has no magnetic order down to 2 K. We found two magnetization plateaus in its magnetization process approximately at 1/3 and 0.8 ${\mu }_{\mathrm{B}}$ per Ti. The monoclinic crystal structure gives four inequivalent nearest-neighbor exchange interactions. Our density functional theory calculations suggest that three of them are antiferromagnetic and one of them is weakly ferromagnetic, resulting in a magnetic system composed of antiferromagnetically coupled linear chains and $\mathrm{\Delta }$ chains. This explains the observed suppression of magnetic order. Numerical diagonalization gives a magnetization curve in good agreement with the experimental results.

We theoretically show that the Kitaev interaction generates a novel class of spin texture in the excitation spectrum of the antiferromagnetic insulator found in the Kitaev-Heisenberg-$\mathrm{\Gamma }$ model. In conducting electronic systems, there is a series of vortex type of spin texture along the Fermi surface induced by Rashba and Dresselhaus spin-orbit couplings. Such spin textures are rarely found in magnetic insulators since there had been no systematic ways to control the kinetics of its quasiparticle called magnon using a magnetic field or spatially asymmetric exchange couplings. Here, we propose a general framework to explore such spin textures in arbitrary insulating antiferromagnets. We introduce an analytical method to transform any complicated Hamiltonian to the simple representation based on pseudospin degrees of freedom, which couples to the momentum-dependent fictitious “Zeeman field.” The direction of the pseudospin on a Bloch sphere describes the degree of contributions from the two magnetic sublattices to the spin moment carried by the magnon. There, the “Zeeman field” determines the direction of the pseudospin and thus becomes the control parameter of the spin texture, which is explicitly described by the original model parameters. The framework enabled us to clarify the uncovered aspect of the Kitaev interaction, and further provides a tool to easily design or explore materials with intriguing magnetic properties. Since these spin textures can be a source of a pure spin current, the Kitaev materials $A_2{\mathrm{PrO}}_3$ ($A=\mathrm{Li}$, Na) shall become a potential platform of power-saving spintronics devices.

We investigate ferrimagnetic domain-wall dynamics induced by circularly polarized spin waves theoretically and numerically. We find that the direction of domain-wall motion depends on both the circular polarization of spin waves and the sign of net spin density of the ferrimagnet. Below the angular momentum compensation point, left- circularly (right-circularly) polarized spin waves push a domain wall towards (away from) the spin-wave source. Above the angular momentum compensation point, on the other hand, the direction of domain-wall motion is reversed. This bidirectional motion originates from the fact that the sign of spin-wave-induced magnonic torque depends on the circular polarization and the subsequent response of the domain wall to the magnonic torque is governed by the net spin density. Our finding provides a way to utilize a spin wave as a versatile driving force for bidirectional domain-wall motion.

We explore the effects of sample dimensionality on vortex pinning in a type-II, low-$T_C, s$-wave superconductor, NbN, in the presence of a perpendicular magnetic field, $H$. We find significant differences in the phase diagrams in the magnetic field–temperature plane between three-dimensional (3D) and 2D NbN films. The differences are most striking close to the normal-superconductor phase transition. We establish that these variances have their origin in the differing pinning properties in two different dimensions. We obtain the pinning strength quantitatively in both the dimensions from two independent transport measurements performed in two different regimes of vortex motion: (i) thermally assisted flux-flow regime and (ii) flux flow regime. Both the measurements consistently show that both the pinning potential and the zero-field free-energy barrier to depinning in the 3D superconductor are at least an order of magnitude stronger than that in the 2D superconductor. Further, we probed the dynamics of pinning in both 2D and 3D superconductor through voltage fluctuation spectroscopy. We find that the mechanism of vortex pinning-depinning is qualitatively similar for the 3D and 2D superconductors. The voltage-fluctuations arising from vortex motion are found to be correlated only in the 2D superconductor. We establish this to be due to the presence of long-range phase fluctuations near the Berezinskii-Kosterlitz-Thouless–type superconducting transition in 2D superconductors.

We investigated the structural, electronic, magnetic, and phonon properties of ${\mathrm{Pu}}_{3}M$, where $M$ = Al, Ga, In, and Tl, using first-principles calculations with the spin-orbit coupling taken into consideration. All four compounds are found to contain transverse phonons along [011] and [111] that remarkably soften at the boundaries of the first Brillouin zone, corresponding to the $\mathrm{M}$ and $\mathrm{R}$ points. Through detailed analyses of the vibrational modes and bond strengths, we reveal that all four types of $M$ atoms have softer bonding with neighboring Pu than Pu-Pu bond, which accounts for the phonon softening and may shed light on the $\delta$-stabilization of Pu by trivalent metal doping. In addition, we find that, due to the missing $p\text{−}d$ hybridization, ${\mathrm{Pu}}_3\mathrm{Al}$ behaves quite differently from the other three ${\mathrm{Pu}}_{3}M$ compounds. For example, ${\mathrm{Pu}}_3\mathrm{Al}$ has an unexpected equilibrium volume and high-frequency disentangled optical branches of phonons.

The low-temperature magnetic properties and magnetic structure of fully dehydrated black dioptase ${\mathrm{Cu}}_6{\mathrm{Si}}_6{\mathrm{O}}_{18}$ have been studied by single-crystal neutron diffraction and magnetization measurements in magnetic fields up to 30 T. The intrachain $J_c$ and interchain $J_{ab}$ interactions as well as the anisotropy of the exchange coupling $J_c$ have been determined using inelastic neutron-scattering techniques. Zero-field antiferromagnetic order at $T_{\mathrm{N}}=6.7$ K can be described by a commensurate propagation vector of $\mathbf{k}=(0,0,3/2)$ with respect to the hexagonal $R\overline{3}$:H unit cell. The Cu magnetic moments are aligned antiferromagnetically along the $c$ axis with about ${12}^{\circ }$ tilt and are coupled ferromagnetically between the chains. The high-field magnetization measurements provide strong evidence for the presence of a spin-flop phase above 8 T. We found that $J_c$ in black dioptase is significantly increased, while $J_{ab}$ is much weaker compared to its counterpart, green dioptase ${\mathrm{Cu}}_6[{\mathrm{Si}}_6{\mathrm{O}}_{18}] 6{\mathrm{H}}_2\mathrm{O}$. We suggest that black dioptase behaves like a nearly ideal $S=1/2$ antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement $J_{ab}/J_c\sim 0.02$.

Single-crystal materials with sufficiently low crystal symmetry and strong spin-orbit interactions can be used to generate novel forms of spin-orbit torques on adjacent ferromagnets, such as the out-of-plane antidamping torque previously observed in ${\mathrm{WTe}}_2/\mathrm{ferromagnet}$ heterostructures. Here, we present measurements of spin-orbit torques produced by the low-symmetry material $\beta -{\mathrm{MoTe}}_2$, which, unlike ${\mathrm{WTe}}_2$, retains bulk inversion symmetry. We measure spin-orbit torques on $\beta -{\mathrm{MoTe}}_2/\mathrm{Permalloy}$ heterostructures using spin-torque ferromagnetic resonance as a function of crystallographic alignment and ${\mathrm{MoTe}}_2$ thickness down to the monolayer limit. We observe an out-of-plane antidamping torque with a spin-torque conductivity as strong as 1/3 of that of ${\mathrm{WTe}}_2$, demonstrating that the breaking of bulk inversion symmetry in the spin-generation material is not a necessary requirement for producing an out-of-plane antidamping torque. We also measure an unexpected dependence on the thickness of the $\beta -{\mathrm{MoTe}}_2$—the out-of-plane antidamping torque is present in ${\mathrm{MoTe}}_2/\mathrm{Permalloy}$ heterostructures when the $\beta -{\mathrm{MoTe}}_2$ is a monolayer or trilayer thick, but goes to zero for devices with bilayer $\beta -{\mathrm{MoTe}}_2$.

A detailed magnetic phase diagram of FeGe membranes with fields applied out-of-plane is determined using small-angle soft x-ray elastic scattering. In addition to the well-established skyrmion and helical phase, we identify an additional ordered phase which partly coexists with the helical phase in a pocket beneath the skyrmion phase. Furthermore, an evolution of the modulation wave vector, $\mathbf{q}$, is observed when traversing the magnetic phase pocket. For the ordinary and the additional helical phase, $\mathbf{q}$ only varies with normalized field but not with temperature—in sharp contrast to the skyrmion phase, for which $\mathbf{q}$ is both field and temperature dependent. This study of the helical phases in thin FeGe membranes lays the foundation for their use in future magnetic storage and logic devices.

The Kubo-Greenwood (KG) formula is often used in conjunction with Kohn-Sham (KS) density functional theory (DFT) to compute the optical conductivity, particularly for warm dense matter. For applying the KG formula, all KS eigenstates and eigenvalues up to an energy cutoff are required and thus the approach becomes expensive, especially for high temperatures and large systems, scaling cubically with both system size and temperature. Here, we develop an approach to calculate the KS conductivity within the stochastic DFT framework, which requires knowledge only of the KS Hamiltonian but not its eigenstates and values. We show that the computational effort associated with the method scales linearly with system size and reduces in proportion to the temperature, unlike the cubic increase with traditional deterministic approaches. In addition, we find that the method allows an accurate description of the entire spectrum, including the high-frequency range, unlike the deterministic method which is compelled to introduce a high-frequency cutoff due to memory and computational time constraints. We apply the method to helium-hydrogen mixtures in the warm dense matter regime at temperatures of $\sim 60\mathrm{kK}$ and find that the system displays two conductivity phases, where a transition from nonmetal to metal occurs when hydrogen atoms constitute $\sim 0.3$ of the total atoms in the system.

Electron-phonon interactions (EPIs) are presumably detrimental for thermoelectric performance in semiconductors because they limit carrier mobility. Here we show that enhanced EPIs with strong energy dependence offer an intrinsic pathway to a significant increase in the Seebeck coefficient and the thermoelectric power factor, particularly in the context of two-dimensional (2D) graphene-like Dirac bands. The increase is realized by enabling electron energy filtering through preferential scattering of electron/hole carriers. We prove this concept by implementing first-principles computational methods with explicit treatment of EPIs for a 2D gapless ${\mathrm{MoS}}_2$ allotrope, which has both massless Dirac bands and a heavy-fermion state that acts as the filter. We determine that the optimal location of the heavy state and hence the onset of the filtering process is at the Dirac point. Our study opens an avenue for attaining ultrahigh power factors via engineering the EPIs in graphene-like semimetals or identifying new compounds that intrinsically possess the featured electronic structure.