A helium ion microscope is used to produce nanoscale patterns on different regions of a graphene device. The patterns consist of uniformly spaced strips with varying spacing in each region. Measurements of the longitudinal magnetoconductivity in each region at different temperatures and carrier densities reveal a transition from metallic to insulating regimes as the density of defects increases. We use the weak localization theory and Mott's theory for disordered two-dimensional systems to analyze the conductivity as it crosses the threshold value of $4e^2/h$.

Recent shock compression experiments produced clear evidence of a new carbon phase, but a full structural identification has remained elusive. Here we establish by ab initio calculations a body-centered cubic carbon phase in $Ia\overline{3}d\left(O_h^{10}\right)$ symmetry, which contains twelve atoms in its primitive cell, thus termed BC12, and comprises all-$s{p}^3$ six-membered rings. This structural configuration places BC12 carbon in the same bonding type as cubic diamond, and its stability is verified by phonon mode analysis. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak found in shock compression experiments. Electronic band and density of states calculations reveal that BC12 is a semiconductor with a direct band gap of $\sim 2.97\mathrm{eV}$. These results provide a solid foundation for further exploration of this new carbon allotrope.

The microscopic transport and the macroscopic flow behavior of Zr-(Co,Ni)-Al melts are systemically investigated using containerless processing techniques. A remarkable decrease of the Co, Ni self-diffusion coefficient and increase of the melt viscosity upon alloying Al, are observed. In contrast to many other metallic glass-forming liquids, the average packing fraction of the melt derived from the measured macroscopic density decreases. Our study indicates that chemical interactions of Al with transition metal atoms play an important role in slowing down liquid dynamics of metal melts, which also contribute to their improved glass-forming ability.

We study the nonlinear dynamics of exciton polaritons in an incoherently pumped semiconductor microcavity with an embedded weak-contrast lattice and coupled to an exciton reservoir. We elucidate fundamental features of nonequilibrium exciton-polariton condensates trapped in one-dimensional periodic potentials both in the vicinity of zero momentum (so-called “zero-momentum state”) and at the boundaries of Brillouin zone (“$\pi$ states”). Within the framework of the mean-field theory, we identify different regimes of both relaxation and oscillatory dynamics governed by superpositions of Bloch eigenstates of coherent exciton polaritons within the periodic lattice. In particular, we theoretically demonstrate stable macroscopic oscillations, akin to nonlinear Josephson oscillations, between different spectral components of the polariton condensate in the momenta space. We discuss a strong influence of dissipative effects and the feedback induced by the inhomogeneity of the incoherent reservoir on the dynamics of coherent polaritons.

We experimentally investigate spin-orbit torques and spin pumping in NiFe/Pt bilayers with direct and interrupted interfaces. The dampinglike and fieldlike torques are simultaneously measured with spin-torque ferromagnetic resonance tuned by a dc-bias current, whereas spin pumping is measured electrically through the inverse spin-Hall effect using a microwave cavity. Insertion of an atomically thin Cu dusting layer at the interface reduces the dampinglike torque, fieldlike torque, and spin pumping by nearly the same factor of $\approx 1.4$. This finding confirms that the observed spin-orbit torques predominantly arise from diffusive transport of spin current generated by the spin-Hall effect. We also find that spin-current scattering at the NiFe/Pt interface contributes to additional enhancement in magnetization damping that is distinct from spin pumping.

An austenite-martensite transition was observed in a 100-nm-thick ${\mathrm{Ni}}_{51.6}{\mathrm{Mn}}_{32.9}{\mathrm{Sn}}_{15.5}$ film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of $M_S\approx 260$ K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment $m_l/m_S$ can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of $B=3$ T reverses the change of the electronic structure of Mn and reduces the ratio of $m_l/m_S$ from 13.5 to $\approx 1$ % indicating a field-induced reverse martensitic transition.

Absorption of microwaves by metallic conductors is typically inefficient, albeit naturally broadband, due to the huge impedance mismatch between metal and free space. Reducing metal to ultrathin profile may improve absorption efficiency, but a maximal 50% absorption limit induced by the field continuity exists. Here, we experimentally show that broadband, perfect (100%) absorption of microwaves can be realized in a single layer of ultrathin conductive film when illuminated coherently by two oppositely directed incident beams. Our experiments keep the field continuity and simultaneously break the 50% limit. Inheriting the intrinsic broadband feature of metals, complete absorption is observed to be frequency independent in microwave experiments from 6 to 18 GHz. Remarkably, this occurs in films with thicknesses that are at the extreme subwavelength scales, $\sim \lambda /10000$ or less. Our work proposes a way to achieve total electromagnetic wave absorption in an ultrawide spectrum of radio waves and microwaves with a simple conductive film.

We performed nuclear forward and inelastic scattering of synchrotron radiation by elemental Os utilizing the nuclear excited state of ${}^{187}\mathrm{Os}$ which is otherwise inaccessible using any practical radioactive decay process. The lifetime of the excited state, $3.06\left(8\right)\text{ns}$, and the energy of the transition, $9.778\left(3\right)\text{keV}$, are refined. The nuclear quadrupole moment of the excited state, $Q_{3/2}=1.46\left(10\right)\text{b}$, is determined. The density of phonon states for elemental Os, which is herein experimentally determined, suggests that the Os lattice is a model for the lattice dynamics of hcp-Fe. The combination of the low energy of the nuclear transition and the large nuclear mass leads to a high recoil free fraction, $f_{LM}=0.95\left(1\right)$, at room temperature, a large value that strongly supports the viability of nuclear resonance scattering as a reliable method to study electronic, magnetic, and elastic properties of Os compounds, including Os organometallics.

We perform a systematic investigation on the hexagon-singlet solid (HSS) states, which are a class of spin liquid candidates for the spin-1 kagome antiferromagnet. With the Schwinger boson representation, we show that all HSS states have exponentially decaying correlations and can be interpreted as a (special) subset of the resonating Affleck-Kennedy-Lieb-Tasaki (AKLT) loop states. We provide a compact tensor network representation of the HSS states, with which we are able to calculate physical observables efficiently. We find that the HSS states have vanishing topological entanglement entropy, suggesting the absence of intrinsic topological order. We also employ the HSS states to perform a variational study of the spin-1 kagome Heisenberg antiferromagnetic model. When we use a restricted HSS ansatz preserving lattice symmetry, the best variational energy per site is found to be $e_0=-1.3600$. In contrast, when allowing lattice symmetry breaking, we find a trimerized simplex valence-bond crystal with a lower energy, $e_0=-1.3871$.

Effects of the substitution of rare-earths $R=\mathrm{Y}$, Er, and Ho on the magnetic properties of ${\mathrm{Dy}}_{0.7}{\mathrm{R}}_{0.3}{\mathrm{CrO}}_3$ are reported here in order to probe the nature of magnetism and related exchange interactions in these materials. By fitting the temperature dependence of the magnetic susceptibility to a modified Curie-Weiss law, which includes a correction for the Dzyaloshinskii-Moriya (DM) interaction, the strengths of the symmetric and antisymmetric ${\mathrm{Cr}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ exchange interactions were determined. It was found that the rare-earth substitutions had a slight effect on the strength of the symmetric ${\mathrm{Cr}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ interaction (reflected in the slight changes in the Néel temperatures), while the antisymmetric ${\mathrm{Cr}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ interaction remained unchanged. Isothermal magnetic measurements of the samples at successive temperatures revealed a plateau in the temperature dependent magnetic coercivity data, which was explained by the magnetic properties of the substituted ions. It was found that the Y substitution led to the reduction of the strength of the magnetization and a larger peak value of magnetic coercivity as compared to that in pure ${\mathrm{DyCrO}}_3$. The observed increase in magnetic coercivity with Er and Ho substitution can be attributed to an increased ${\mathrm{R}}^{3+}\text{−}{\mathrm{Cr}}^{3+}$ interaction strength dominated by the DM mechanism.

We report a structural-magnetic investigation by x-ray absorption spectroscopy (XAS), neutron diffraction, dc susceptibility $\left({\chi }_{\text{dc}}\right)$, and electron spin resonance (ESR) of the 12R-type perovskite ${\text{BaTi}}_{1/2}{\text{Mn}}_{1/2}{\text{O}}_3$. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of ${\text{Mn}}^{4+}$ and ${\text{Ti}}^{4+}$ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and $X$-band (9.4 GHz) electron spin resonance, both in the temperature interval of $2\le T\le 1000$ K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants $J_a/k_B=200\left(2\right)$ K and $J_b/k_B=130\left(10\right)$ K, and orphan spins. Thus, ${\text{BaTi}}_{1/2}{\text{Mn}}_{1/2}{\text{O}}_3$ is proposed as a rare case of an intrinsically disordered $S=\frac{3}{2}$ spin gap system with a frustrated ground state.

We use bulk sensitive hard x-ray photoelectron spectroscopy to provide unambiguous evidence for a pseudogap in the density of states around the Fermi level in icosahedral Zn-Mg-Y and Zn-Mg-Dy quasicrystals, in agreement with our density functional theory calculations. The pseudogap in these Zn-based quasicrystals is less pronounced compared to the Al-based ones such as Al-Pd-Mn and Al-Cu-Fe [J. Nayak et al., Phys. Rev. Lett. 109, 216403 (2012)]. This observation is in agreement with transport studies that indicate a larger charge carrier concentration in the Zn-based quasicrystals. Compared to Zn-Mg-Dy, the pseudogap is somewhat deeper in Zn-Mg-Y. The larger width of the Mg $1s$ and Zn $2p$ core-level spectra in Zn-Mg-Y is explained by different configurations of the local atomic surrounding compared to Zn-Mg-Dy.

We report the electric transport study of the three-dimensional topological insulator ${\text{TlBiSe}}_2$. We propose an analysis procedure which determines the two-carrier transport properties in precision. Magnetotransport properties reveal a multicarrier conduction of high- and low-mobility electrons in the bulk, which is in qualitative agreement with angle-resolved photoemission results [K. Kuroda et al., Phys. Rev. Lett. 105, 146801 (2010)]. The temperature dependence of the Hall mobility is explained well with the conventional Bloch-Grüneisen formula and yields the Debye temperature ${\mathrm{\Theta }}_{\mathrm{D}}=113\pm 14$ K. The results indicate that the scattering of bulk electrons is dominated by acoustic phonons.

We present an analytic study, based on the tight-binding approximation, of strain effects on the electronic bandgap in single-layer black phosphorus. We obtain an expression for the variation of the bandgap induced by a general strain type that includes both tension in and out of the plane and shear, and use this to determine the most efficient strain direction for different strain types, along which the strongest bandgap manipulation can be achieved. We find that the strain direction that enables the maximum manipulation of the bandgap is not necessarily in the armchair or zigzag direction. Instead, to achieve the strongest bandgap modulation, the direction of the applied mechanical strain is dependent on the type of applied strain.

We use the symmetry-constrained low-energy effective Hamiltonian of iron-based superconductors to study the Raman scattering in the normal state of underdoped iron-based superconductors. The incoming and scattered Raman photons couple directly to orbital fluctuations and indirectly to the spin fluctuations. We computed both couplings within the same low-energy model. The symmetry-constrained Hamiltonian yields the coupling between the orbital and spin fluctuations of only the same symmetry type. Attraction in the $B_{2g}$ symmetry channel was assumed for the system to develop the subleading instability towards the discrete in-plane rotational symmetry breaking, referred to as Ising nematic transition. We find that upon approaching this instability, the Raman spectral function develops a quasielastic peak as a function of energy transferred by photons to the crystal. We attribute this low-energy $B_{2g}$ scattering to the critical slowdown associated with the buildup of nematic correlations.

Metamaterials are known to exhibit a variety of electromagnetic properties nonexisting in nature. We show that an all-dielectric (nonmagnetic) system consisting of deep-subwavelength, high-permittivity resonant spheres possesses effective negative magnetic permeability (dielectric permittivity being positive and small). Due to the symmetry of the electromagnetic wave equations in classical electrodynamics, localized “magnetic” plasmon resonances can be excited in a metasphere made of such metamaterial. This is theoretically demonstrated by the coupled-dipole approximation and numerically for real spheres, in full agreement with the exact analytical solution for the scattering process by the same metasphere with effective material properties predicted by effective medium theory. The emergence of this phenomenon as a function of structural order within the metastructures is also studied. Universal conditions enabling effective negative magnetic permeability relate subwavelength sphere permittivity and size with critical filling fraction. Our proposal paves the way towards (all-dielectric) magnetic plasmonics, with a wealth of fascinating applications.

We develop a method of constructing excited states in one-dimensional spin chains which are derived from the $\text{SU}{\left(2\right)}_1$ Wess-Zumino-Witten conformal field theory (CFT) using a parent Hamiltonian approach. The resulting systems are equivalent to the Haldane-Shastry model. In our ansatz, correlation functions between primary fields correspond to the ground state of the spin system, whereas excited states are obtained by insertion of descendant fields. Our construction is based on the current algebra of the CFT and emphasizes the close relation between the spectrum of the spin system and the underlying CFT. This general structure might imply that the method could be applied to a wider range of model systems.

We report on experimental evidence of the Berry phase accumulated by the charge-carrier wave function in single-domain nanowires made from a (Ga, Mn)(As, P) diluted ferromagnetic semiconductor layer. Its signature on the mesoscopic transport measurements is revealed as unusual patterns in the magnetoconductance that are clearly distinguished from the universal conductance fluctuations. We show that these patterns appear in a magnetic field region where the magnetization rotates coherently and are related to a change in the band-structure Berry phase as the magnetization direction changes. They should thus be considered a band-structure Berry phase fingerprint of the effective magnetic monopoles in the momentum space. We argue that this is an efficient method to vary the band structure in a controlled way and to probe it directly. Hence, (Ga, Mn)As appears to be a very interesting test bench for new concepts based on this geometrical phase.

The origin and influence of finite-size effects on the nonlinear dynamics of space charge stored by multilayer graphene on a ferroelectric and resistivity of graphene channel were analyzed. Here, we develop a self-consistent approach combining the solution of electrostatic problems with the nonlinear Landau-Khalatnikov equations for a ferroelectric. The size-dependent behaviors are governed by the relations between the thicknesses of multilayer graphene, ferroelectric film, and the dielectric layer. The appearance of charge and electroresistance hysteresis loops and their versatility stem from the interplay of polarization reversal dynamics and its incomplete screening in an alternating electric field. These features are mostly determined by the dielectric layer thickness. The derived analytical expressions for electric fields and space-charge-density distribution in a multilayer system enable knowledge-driven design of graphene-on-ferroelectric heterostructures with advanced performance. We further investigate the effects of spatially nonuniform ferroelectric domain structures on the graphene layers’ conductivity and predict its dramatic increase under the transition from multi- to single-domain state in a ferroelectric. This intriguing effect can open possibilities for the graphene-based sensors and explore the underlying physical mechanisms in the operation of graphene field-effect transistor with ferroelectric gating.