Due to their similarities to metastable zinc-blende half-metals, we systematically examined the half-Heusler compounds $\beta \text{-LiMn}Z$ ($Z=\text{N},\text{P}$ and Si) for their electronic, magnetic, and stability properties at optimized lattice constants and strained lattice constants that exhibit half-metallic properties. We also report the other phases of the half-Heusler structure ($\alpha$ and $\gamma$ phases), but they are unlikely to be grown. The magnetic moments of these stable Li-based compounds are expected to reach as high as $4{\mu }_{\mathrm{B}}$ per unit cell when $Z=\text{Si}$ and $5{\mu }_{\mathrm{B}}$ per unit cell when $Z=\text{N}$ and P; however, the antiferromagnetic spin configuration is energetically favored when $Z$ is a pnictogen. $\beta$-LiMnSi at a lattice constant 14% larger than its equilibrium lattice constant is a promising half-metal due to its large magnetic moment, large gap, and vibrational stability. The modified Slater-Pauling rule for these compounds is determined. Finally, we investigated a plausible method for developing half-metallic $\mathrm{Li}{}_x\mathrm{Mn}Z$ at equilibrium by tuning $x$, but this type of alloying introduces local structural changes that preclude half-metallicity.

A comprehensive description of the exchange bias phenomenon in an antiferromagnetic/ferromagnetic IrMn(10 nm)/NiFe(5 nm) continuous film and in arrays of square dots with different sizes (1000, 500, and 300 nm) is presented, which elucidates the temperature dependence of the exchange field $H_{\mathrm{ex}}$ and coercivity $H_C$, in conjunction with spatial confinement effects. To achieve this goal, samples prepared by electron beam lithography and lift-off using dc sputtering were subjected to structural investigations by electron microscopy techniques and to magnetic study, through SQUID and magneto-optic magnetometry measurements coupled to micromagnetic calculations. In particular, we have observed that at $T=300\mathrm{K}$ $H_{\mathrm{ex}}$ decreases by reducing the size of the dots and it is absent in the smallest ones, whereas the opposite trend is visible at $T=10\mathrm{K}$ ($H_{\mathrm{ex}}\sim 1140\mathrm{Oe}$ in the dots of $300\mathrm{nm}$). The exchange bias mechanism and its thermal evolution have been explained through an exhaustive phenomenological model, which joins spatial confinement effects with other crucial items concerning the pinning antiferromagnetic phase: the magnetothermal stability of the nanograins forming the IrMn layer (mean size $\sim 10\mathrm{nm}$), assumed as essentially noninteracting from the magnetic point of view; the proven existence of a structurally disordered IrMn region at the interface between the NiFe phase and the bulk of the IrMn layer, with a magnetic glassy nature; and the stabilization of a low-temperature $(T<100\mathrm{K})$ frozen collective regime of the IrMn interfacial spins, implying the appearance of a length of magnetic correlation among them.

The interplay between frustration and quantum fluctuation in magnetic systems is known to be the origin of many exotic states in condensed matter physics. In this paper, we consider a frustrated four-leg spin tube under a magnetic field. This system is a prototype to study the emergence of a nonmagnetic ground state factorizable into local states and the associated order parameter without quantum fluctuation that appears in a wide variety of frustrated systems. The one-dimensional nature of the system allows us to apply various techniques: a path-integral formulation based on the notion of order by disorder, strong-coupling analysis where magnetic excitations are gapped, and density-matrix renormalization group. All methods point toward an interesting property of the ground state in the magnetization plateaus, namely, a quantized value of relative magnetizations between different sublattices (spin imbalance). The ground-state properties can be understood in terms of a direct product of local states on each rung, which is the exact ground state on certain plateaus in the strong-coupling limit.

Noncollinear and noncoplanar magnetic orders lead to unusual electronic structures and transport properties. We investigate two types of multiple-$Q$ magnetically ordered states and a topological phase transition between them in two dimensions. One is a coplanar but noncollinear double-$Q$ state on a square lattice, which is a semimetal accommodating massless Dirac electrons. The other is a noncoplanar triple-$Q$ state on a triangular lattice, which is a Chern insulator showing the quantum anomalous Hall effect. We discuss the peculiar electronic structures in these two multiple-$Q$ states in a unified way on the basis of the Kondo lattice model, which suggests a quantum phase transition between the two states in a continuous change of lattice geometry between the square and triangular lattices. We systematically examine the possibility of such a transition by using the mean-field approximation for the ground state of the periodic Anderson model. After clarifying the parameter region in which the double-$Q$ (triple-$Q$) state is stabilized on the square (triangular) lattice, we show that a continuous topological phase transition indeed takes place between the double-$Q$ Dirac semimetal and the triple-$Q$ Chern insulator on the square-to-triangular lattice. The nature of the transition is discussed by the topologically protected edge states as well as the bulk magnetic and electronic properties. The results indicate that unusual critical phenomena may occur at finite temperature related with multiple-$Q$ chiral spin-liquid states.

Based on Mie scattering theory, we derive the complete dynamic polarizability tensor for circular, azimuthally symmetric cylinders excited by an arbitrary field distribution, and provide compact expressions for all of its elements. Our results comprise fully dynamic cylinder polarizabilities, improving existing approximate models that use averaged electric or magnetic current lines to describe the scattering response of moderately thin cylinders. We show that the derived polarizability tensor satisfies reciprocity and passivity relations, and analyze its response under different conditions, varying the excitation angle, material properties, and cylinder radius. Interestingly, magnetoelectric effects are shown to arise at oblique incidence, even in the case of centrosymmetric achiral thin cylinders, associated with a weak form of spatial dispersion. This finding is particularly relevant for the proper modeling of individual cylinders and arrays of them, as in the case of metamaterials. We expect this work to find applications in antenna and metamaterial design, and to improve the physical understanding of the wave interaction and spatial dispersion in artificial materials composed of elongated inclusions such as wire media.

The quantum confinement in a typical quantum dot (QD) is determined primarily by the nanosystem's dimensions and average composition. We demonstrate, however, that excitonic properties of natural QDs formed in the InAs/GaAs wetting layer are governed predominantly by effects of random fluctuations of the lattice composition. It is shown that the biexciton binding energy is a very sensitive function of the lattice randomness with a nearly flat dependence on the exciton energy. The large variation in different random realizations of a QD structure is shown to lead in some cases to the reversal of the order of excitonic lines. Results of theoretical calculations correspond to statistical properties of neutral excitons and biexcitons as well as trions confined to single natural QDs studied in our microspectroscopic measurements. We observe substantial variation of the biexciton and trion binding energies as well as a correlation of the trion and the biexciton energies. The transition from the negative to the positive binding energy of the trion is also observed, which strongly supports the attribution of the observed trion to the positively charged exciton.

Majorana qubits offer a promising way to store and manipulate quantum information by encoding it into the state of Majorana zero modes. As the information is stored in a topological property of the system, local noise cannot lead to decoherence. Manipulation of the information is achieved by braiding the zero modes. The measurement, however, is challenging as the information is well hidden and thus inherently hard to access. Here, we discuss a setup for measuring the state of a Majorana qubit by employing standard tools of microwave engineering. The basic physical effect that we employ is the fact that a voltage-biased Josephson junction hosting Majorana fermions allows photons to be emitted and absorbed at half the Josephson frequency. We show that in the dispersive regime, our setup allows us to perform a quantum nondemolition measurement and to reach the quantum limit. An appealing feature of our setup is that the interaction of the Majorana qubit with the measurement device can be turned on and off at will by changing the dc bias of the junction.

We show that oxidation of phosphorene can lead to the formation of a new family of planar (two-dimensional) and tubular (one-dimensional) oxides and suboxides, most of them insulating. This confers to black phosphorus a native oxide that can be used as barrier material and protective layer. Furthermore, the bandgap of phosphorene oxides depends on the oxygen concentration, suggesting that controlled oxidation can be used as a means to engineer the bandgap. For the oxygen saturated composition, ${\mathrm{P}}_2{\mathrm{O}}_5$, both the planar and tubular phases have a large bandgap energy of about 8.5 eV and are transparent in the near UV. These two forms of phosphorene oxides are predicted to have the same formation enthalpy as ${\mathrm{O}}^{\prime }\text{−}{\mathrm{P}}_2{\mathrm{O}}_5$, the most stable of the previously known forms of phosphorus pentoxide.

We report on an angle-resolved photoemission investigation of the valence states and chiral properties of a nonchirally oriented phase of tartaric acid deposited on a $\mathrm{Cu}(110)$ surface, observed with circularly polarized light. The two optical enantiomers $R,R$ and $S,S$ of tartaric acid, separately deposited, produce (40,23) overlayers which show a large dichroic effect and enantiomeric behavior all over the valence energies. The dichroic effects are displayed by native chiral molecular states and molecule-copper interface states. Density-functional theory calculations of the site-resolved density of states analyze the formation of hybrid states at the tartaric acid-copper interface and suggest that an observed interface state acquires chirality on binding.

We investigate chiral superconductivity which occurs in the electronic nematic state. A vortex state in a $c-\text{axis}$ magnetic field is studied on the basis of the two-component Ginzburg-Landau model for nematic-chiral superconductors. It is shown that various vortex lattice structures are stabilized by nontrivial cooperation of nematicity and chirality in superconductors. In particular, the vortex lattice structural transition occurs when a square anisotropy parameter $\nu$ is positive (negative) and the nematicity is induced along the [110] axis ([100] axis). We discuss nematic-chiral superconductivity in ${\text{URu}}_2{\text{Si}}_2$, ${\text{Sr}}_2{\text{RuO}}_4$, and ${\text{UPt}}_3$. An experimental test for the examination of nematic order and chiral superconductivity is proposed.

We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for $\mathrm{MgB}{}_2$, where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the $c$ direction. We obtained images of flux vortices from a $\mathrm{MgB}{}_2$ single crystal cut in the $ac$ plane by focused ion beam milling and tilted to ${45}^{\circ }$ with respect to the electron beam about the crystallographic $a$ axis. A new method was developed to simulate these images that accounted for vortices with a nonzero core in a thin, anisotropic superconductor and a simplex algorithm was used to make a quantitative comparison between the images and simulations to measure the penetration depths and coherence lengths. This gave penetration depths ${\Lambda }_{ab}=100\pm 35$ nm and ${\Lambda }_c=120\pm 15$ nm at 10.8 K in a field of 4.8 mT. The large error in ${\Lambda }_{ab}$ is a consequence of tilting the sample about $a$ and had it been tilted about $c$, the errors on ${\Lambda }_{ab}$ and ${\Lambda }_c$ would be reversed. Thus obtaining the most precise values requires taking images of the flux lattice with the sample tilted in more than one direction. In a previous paper [J. C. Loudon et al., Phys. Rev. B 87, 144515 (2013)], we obtained a more precise value for ${\Lambda }_{ab}$ using a sample cut in the $ab$ plane. Using this value gives ${\Lambda }_{ab}=107\pm 8$ nm, ${\Lambda }_c=120\pm 15$ nm, ${\xi }_{ab}=39\pm 11$ nm, and ${\xi }_c=35\pm 10$ nm, which agree well with measurements made using other techniques. The experiment required two days to conduct and does not require large-scale facilities. It was performed on a very small sample, $30\times 15 \mu \mathrm{m}$ and 200-nm thick, so this method could prove useful for superconductors where only small single crystals are available, as is the case for some iron-based superconductors.

We study the robustness against nonmagnetic impurities in the topological superconductor with point nodes, focusing on an effective model of ${\text{Cu}}_x{\text{Bi}}_2{\text{Se}}_3$. We find that the topological superconductivity with point nodes is not fragile against nonmagnetic impurities, although the superconductivity with nodes in past studies is usually fragile. Exchanging the role of spin with the one of orbital, and vice versa, we find that in the “dual” space the topological superconductor with point nodes is regarded as the intraorbital spin-singlet $s$-wave one. From the viewpoint of the dual space, we deduce that the point-node state is not fragile against nonmagnetic impurity, when the orbital imbalance in the normal states is small. Since the spin imbalance is induced by the Zeeman magnetic field, we shall name this key quantity for the impurity effects the Zeeman “orbital” field. The numerical calculations support that the deduction is correct. If the Zeeman orbital field is small, the topological superconductivity is not fragile in dirty materials, even with nodes. Thus, the topological superconductors cannot be simply regarded as one of the conventional unconventional superconductors.

Manganese-substitution-doped iron nitride ${\text{MnFe}}_3\text{N}$ holds great promise for applications in high-density magnetic recording and spintronic devices. However, existing theory contradicts experimental results on the structural and magnetic stability of ${\text{MnFe}}_3\text{N}$, and the underlying mechanisms remain elusive. Here we demonstrate by first-principles calculations that the ferromagnetic state with enhanced magnetization in ${\text{MnFe}}_3\text{N}$ is driven by the electron correlation effect not previously considered. We further reveal a large nonlinear shear plasticity, which produces an unexpectedly high shear strength in ${\text{MnFe}}_3\text{N}$ despite its initial ductile nature near the equilibrium structure. Moreover, we identify strong lattice anharmonicity that plays a pivotal role in stabilizing ${\text{MnFe}}_3\text{N}$ under high pressures at room temperature. These remarkable properties stem from the intriguing bonding nature of the parent compound ${\text{Fe}}_4\text{N}$. Our results explain experimental results and offer insights into the fundamental mechanisms for the superior magnetic and mechanical properties of ${\text{MnFe}}_3\text{N}$.

We revisited, by means of numerical simulations, the one-dimensional bond diluted Levy Ising spin glasses outside the limit of validity of mean-field theories. In these models the probability that two spins at distance $r$ interact (via disordered interactions, $J_{ij}=\pm 1)$ decays as $r^{-\rho }$. We have estimated, using finite size scaling techniques, the infinite volume correlation length and spin glass susceptibility for $\rho =5/3$ and $\rho =9/5$. We have obtained strong evidence for divergences of the previous observables at a nonzero critical temperature. We discuss the behavior of the critical exponents, especially when approaching the value $\rho =2$, corresponding to a critical threshold beyond which the model has no phase transition. Finally, we numerically study the model right at the threshold value $\rho =2$.

Nematicphases, breaking spontaneously rotational symmetry, provide for ubiquitously observed states of matter in both classical and quantum systems. These nematic states may be further classified by their $N$-fold rotational invariance described by cyclic groups $C_N$ in 2 + 1D. Starting from the space groups of underlying 2D crystals, we present a general classification scheme incorporating $C_N$ nematic phases that arise from dislocation-mediated melting and discuss the conventional tensor order parameters. By coupling the $O\left(2\right)$ matter fields to the $Z_N$ lattice gauge theory, a unified $O\left(2\right)/Z_N$ lattice gauge theory is constructed in order to describe all these nematic phases. This lattice gauge theory is shown to reproduce the $C_N$ nematic-isotropic liquid phase transitions and contains an additional deconfined phase. Finally, using our $O\left(2\right)/Z_N$ gauge-theory framework, we discuss phase transitions between different $C_N$ nematics.

We formulate a theoretical approach of surface second-harmonic generation from semiconductor surfaces based on the length gauge and the electron density operator. Within the independent particle approximation, the nonlinear second-order surface susceptibility tensor ${\chi }^{\mathrm{a}\mathrm{b}\mathrm{c}}(-2\omega ;\omega ,\omega )$ is calculated, including in one unique formulation (i) the scissors correction, needed to have the correct value of the energy band gap, (ii) the contribution of the nonlocal part of the pseudopotentials, routinely used in ab initio band-structure calculations, and (iii) the derivation for the inclusion of the cut function, used to extract the surface response. The first two contributions are described by spatially nonlocal quantum-mechanical operators and are fully taken into account in the present formulation. As a test case of the approach, we calculate ${\chi }^{xxx}(-2\omega ;\omega ,\omega )$ for the clean $\mathrm{Si}\left(001\right)2\times 1$ reconstructed surface. The effects of the scissors correction and of the nonlocal part of the pseudopotentials are discussed in surface nonlinear optics. The scissors correction shifts the spectrum to higher energies though the shifting is not rigid and mixes the $1\omega$ and $2\omega$ resonances, and has a strong influence in the line shape. The effects of the nonlocal part of the pseudopotentials keeps the same line shape of $|{\chi }_{2\times 1}^{xxx}(-2\omega ;\omega ,\omega )|$, but reduces its value by 15%–20%. Therefore the inclusion of the three aforementioned contributions is very important and makes our scheme unprecedented and opens the possibility to study surface second-harmonic generation with more versatility and providing more accurate results.

While ${\mathrm{MoS}}_2$ and ${\mathrm{WS}}_2$ nanostructures gain an increasing importance in a number of recent technological applications, the control of their structure as a function of their size and their environment appears of prominent importance. In the present study which relies on first-principles simulations, we predict the dimerized ${1\mathrm{T}}^{\prime }$ structural phase to be the actual ground state of ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoSe}}_2$ zigzag nanoribbons of small width and monolayer thickness. We assign this result to the competition between edge energy—which favors the nonpolar ${1\mathrm{T}}^{\prime }$ edges over the polar 1H edges—and the energy of atoms in the center of the ribbons—which favors the 1H ground state of the infinite monolayers. A metal-to-semiconductor transition accompanies the structural transition. At variance, ${\mathrm{ZrS}}_2$ zigzag ribbons are predicted to display the 1T structure whatever their width. In compounds of major technological importance, such structural and electronic flexibility associated with polarity effects opens the possibility for controlling the ribbon type during synthesis.

We investigate the Rashba and Dresselhaus spin-orbit (SO) couplings in GaAs quantum wells in the range of well widths $w$ allowing for a transition of the electron occupancy from one to two subbands. By performing a detailed Poisson-Schrödinger self-consistent calculation, we determine all the intra- and intersubband Rashba (${\alpha }_1$, ${\alpha }_2$, $\eta$) and Dresselhaus (${\beta }_1$, ${\beta }_2$, $\Gamma$) coupling strengths. For relatively narrow wells with only one subband occupied, our results are consistent with the data of Koralek et al. [Nature (London) 458, 610 (2009)], i.e., the Rashba coupling ${\alpha }_1$ is essentially independent of $w$ in contrast to the decreasing linear Dresselhaus coefficient ${\beta }_1$. When we widen the well so that the second subband can also be populated, we observe that ${\alpha }_2$ decreases and ${\alpha }_1$ increases, both almost linearly with $w$. Interestingly, we find that in the parameter range studied (i.e., very asymmetric wells), ${\alpha }_2$ can attain zero and change its sign, while ${\alpha }_1$ is always positive. In this double-occupancy regime of $w$'s, ${\beta }_1$ is mostly constant and ${\beta }_2$ decreases with $w$ (similarly to ${\beta }_1$ for the single-occupancy regime). On the other hand, the intersubband Rashba coupling strength $\eta$ decreases with $w$, while the intersubband Dresselhaus $\Gamma$ remains almost constant. We also determine the persistent-spin-helix symmetry points, at which the Rashba and the renormalized (due to cubic corrections) linear Dresselhaus couplings in each subband are equal, as a function of the well width and doping asymmetry. Our results should stimulate experiments probing SO couplings in multisubband wells.

We study nonlinear modes in arrays of doped graphene nanodisks with Kerr-type nonlinear response in the presence of an external electric field. We introduce a theoretical model describing the evolution of the nanodisks' polarizations taking into account intrinsic graphene losses and dipole-dipole coupling between the graphene nanodisks. We reveal that this nonlinear system can support discrete dissipative scalar solitons of both longitudinal and transverse polarizations, as well as vector solitons composed of two mutually coupled polarization components. We demonstrate the formation of stable resting and moving localized modes in this discrete model under controlling guidance of the external driving field.

We present a simple device architecture that allows all-electrical detection of plasmons in a graphene waveguide. The key principle of our electrical plasmon detection scheme is the nonlinear nature of the hydrodynamic equations of motion that describe transport in graphene at room temperature and in a wide range of carrier densities. These nonlinearities yield a dc voltage in response to the oscillating field of a propagating plasmon. For illustrative purposes, we calculate the dc voltage arising from the propagation of the lowest-energy modes in a fully analytical fashion. Our device architecture for all-electrical plasmon detection paves the way for the integration of graphene plasmonic waveguides in electronic circuits.