Using density functional theory calculations, we establish that the half-metallicity of bulk ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ survives down to 1 nm thickness in ${\mathrm{BaTiO}}_3/{\mathrm{Ba}}_2{\mathrm{FeReO}}_6/{\mathrm{BaTiO}}_3$ heterostructures grown along the (001) and (111) directions. The confinement of the two-dimensional (2D) electron gas in this quantum well structure arises from the suppressed hybridization between Re/Fe $d$ states and unoccupied Ti $d$ states, and it is further strengthened by polar fields for the (111) direction. This mechanism, distinct from the polar catastrophe, leads to an order of magnitude stronger confinement of the 2D electron gas than that at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. We further show low-energy bands of (111) heterostructure display nontrivial topological character. Our work opens up the possibility of realizing ultrathin spintronic devices.

We investigate the orthorhombic phase $\left(T_d\right)$ of the layered transition-metal dichalcogenide ${\mathrm{MoTe}}_2$ as a Weyl semimetal candidate. ${\mathrm{MoTe}}_2$ exhibits four pairs of Weyl points lying slightly above $\left(\sim 6\mathrm{meV}\right)$ the Fermi energy in the bulk band structure. Different from its cousin ${\mathrm{WTe}}_2$, which was recently predicted to be a type-II Weyl semimetal, the spacing between each pair of Weyl points is found to be as large as 4% of the reciprocal lattice in ${\mathrm{MoTe}}_2$ (six times larger than that of ${\mathrm{WTe}}_2$). When projected onto the surface, the Weyl points are connected by Fermi arcs, which can be easily accessed by angle-resolved photoemission spectroscopy due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive ${\mathrm{MoTe}}_2$ from a type-II to a type-I Weyl semimetal.

Ground-state diffusion Monte Carlo is used to investigate the binding energies and intercarrier radial probability distributions of excitons, trions, and biexcitons in a variety of two-dimensional transition-metal dichalcogenide materials. We compare these results to approximate variational calculations, as well as to analogous Monte Carlo calculations performed with simplified carrier interaction potentials. Our results highlight the successes and failures of approximate approaches as well as the physical features that determine the stability of small carrier complexes in monolayer transition-metal dichalcogenide materials. Lastly, we discuss points of agreement and disagreement with recent experiments.

Monolayer graphene at neutrality in the quantum Hall regime has many competing ground states with various types of ordering. The outcome of this competition is modified by the presence of the sample boundaries. In this paper we use a Hartree-Fock treatment of the electronic correlations allowing for space-dependent ordering. The armchair edge influence is modeled by a simple perturbative effective magnetic field in valley space. We find that all phases found in the bulk of the sample, ferromagnetic, canted antiferromagnetic, charge-density wave, and Kekulé distortion, are smoothly connected to a Kekulé-distorted edge. The single-particle excitations are computed taking into account the spatial variation of the order parameters. An eventual metal-insulator transition as a function of the Zeeman energy is not simply related to the type of bulk order.

We demonstrate theoretically and by pump-probe picosecond acoustics experiments the simultaneous confinement of light and gigahertz sound in Tamm plasmon resonators, formed by depositing a thin layer of Au onto a GaAs/AlGaAs Bragg reflector. The cavity has InGaAs quantum dots (QDs) embedded at the maximum of the confined optical field in the first GaAs layer. The different sound generation and detection mechanisms are theoretically analyzed. It is shown that the Au layer absorption and the resonant excitation of the QDs are the more efficient light-sound transducers for the coupling of near-infrared light with the confined acoustic modes, while the displacement of the interfaces is the main back-action mechanism at these energies. The prospects for the compact realization of optomechanical resonators based on Tamm plasmon cavities are discussed.

We study tunneling of charge carriers in single- and bilayer graphene. We propose an explanation for nonzero “magic angles” with 100% transmission for the case of symmetric potential barrier, as well as for their almost-survival for slightly asymmetric barrier in the bilayer graphene known previously from numerical simulations. Most importantly, we demonstrate that these magic angles are not protected in the case of bilayer and give an explicit example of a barrier with very small electron transmission probability for any angles. This means that one can lock charge carriers by a p-n-p (or n-p-n) junction without opening energy gap. This creates new opportunities for the construction of graphene transistors.

Motivated by the recent synthesis of layered hexagonal aluminum nitride ($h$-AlN), we investigate its layer- and strain-dependent electronic and optical properties by using first-principles methods. Monolayer $h$-AlN is a wide-gap semiconductor, which makes it interesting especially for usage in optoelectronic applications. The optical spectra of 1-, 2-, 3-, and 4-layered $h$-AlN indicate that the prominent absorption takes place outside the visible-light regime. Within the ultraviolet range, absorption intensities increase with the number of layers, approaching the bulk case. On the other hand, the applied tensile strain gradually redshifts the optical spectra. The many-body effects lead to a blueshift of the optical spectra, while exciton binding is also observed for 2D $h$-AlN. The possibility of tuning the optoelectronic properties via thickness and/or strain opens doors to novel technological applications of this promising material.

We have analyzed the effects of the anisotropic energy bands of phosphorene on magnetoroton branches for electrons and holes in the two Landau levels close to the band edges. We have found that the fractional quantum Hall effect gap in the lowest (highest) Landau level in the conduction (valence) band is slightly larger than that for conventional semiconductor systems and therefore the effect should be experimentally observable in phosphorene. We also found that the magnetoroton mode for both electrons and holes consists of two branches with two minima due to the anisotropy. Most importantly, in the long-wavelength limit a second mode with upward dispersion, well separated from the magnetoroton mode was found to appear, that is entirely a consequence of the anisotropy in the system. These novel features of the collective mode, unique to phosphorene, can be observed in resonant inelastic light-scattering experiments.

Referring to the propulsive field of low-dimensional magnetism, the paper discusses the foundation and principles of exploitation of the spin line groups, thus establishing a base for full implementation of the symmetry in further studies of frustrated quasi-one-dimensional magnetics. The classification of the symmetry-allowed magnetic phases reveals full diversity of the complex helimagnets, tilted along a singled-out direction, as well as within the cross sections perpendicular to it. The neutron diffraction amplitudes are exhaustively calculated for all the possible spin arrangements, providing experimentally verifiable fingerprints of the particular order of the spin structures. A recent prediction of the gate-voltage-dependent phases of ${}^{13}\text{C}$ carbon nanotubes serves as an illustration of the introduced concepts and their applications in the ground state determination.

We present a low-temperature and high-pressure ultrasonic study of elastic properties of isotopic ${\mathrm{H}}_2\text{O-}{\mathrm{D}}_2\mathrm{O}$ solid solutions, comparing their properties with those of the isotopically pure ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ ices. Measurements were carried out for solid state amorphization (SSA) from 1h to high-density amorphous (HDA) ice upon compression up to 1.8 GPa at 77 K and for the temperature-induced $(77-190\mathrm{K})$ u-HDA (unrelaxed HDA) → e-HDA (expanded HDA) → low-density amorphous $\left(\mathrm{LDA}\right)\to 1\mathrm{c}$ cascade of ice transformations near room pressure. There are many similarities in the elasticity behaviour of ${\mathrm{H}}_2\mathrm{O},{\mathrm{D}}_2\mathrm{O}$, and ${\mathrm{H}}_2\text{O-}{\mathrm{D}}_2\mathrm{O}$ solid solutions, including the softening of the shear elastic modulus as a precursor of SSA and the $\mathrm{HDA}\to \mathrm{LDA}$ transition. We have found significant isotopic effects during H/D substitution, including elastic softening of ${\mathrm{H}}_2\mathrm{O}-{\mathrm{D}}_2\mathrm{O}$ solid solutions with respect to the isotopically pure ices in the case of the bulk moduli of ices 1c and 1h and for both bulk and shear elastic moduli of HDA ice at high pressures $(>1\mathrm{GPa})$. This softening is related to the configurational isotopic disorder in the solid solutions. At low pressures, the isotope concentration dependence of the elastic moduli of u-HDA ice changes remarkably and becomes monotonic with pronounced change of the bulk modulus $(\approx 20\%)$.

The superconducting state of the newly discovered superconductor ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, with a quasi-one-dimensional crystal structure $(T_{\mathbf{c}}\sim 6\text{K})$, is investigated using magnetization and muon-spin relaxation or rotation $\left(\mu \mathrm{SR}\right)$ measurements. Our analysis shows that the temperature dependence of the superfluid density obtained from transverse-field $\mu \mathrm{SR}$ measurements fits either to an isotropic $s$-wave character for the superconducting gap or to a $d$-wave model with line nodes. Furthermore, the goodness-of-fit $\left({\chi }^2\right)$ values indicate that our data fit better to the $d$-wave model $\left({\chi }^2\sim 1\right)$ than the $s$-wave model $\left({\chi }^2\sim 1.38\right)$. Therefore our $\mu \mathrm{SR}$ analysis is more consistent with having line nodes than being fully gapped, which is in agreement with the results of the penetration depth measured using a tunnel diode oscillator technique. Our zero-field $\mu \mathrm{SR}$ measurements do reveal very weak evidence of the spontaneous appearance of an internal magnetic field below the transition temperature, which might indicate that the superconducting state is not conventional. This observation suggests that the electrons are paired via unconventional channels such as spin fluctuations, as proposed on the basis of theoretical models of ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. Furthermore, from our transverse-field $\mu \mathrm{SR}$ study the magnetic penetration depth ${\lambda }_L$, superconducting carrier density $n_s$, and effective-mass enhancement $m^{*}$ have been estimated to be ${\lambda }_L\left(0\right)=432\left(4\right)$ nm, $n_s=2.7\times {10}^{27}\text{carriers}/{\mathrm{m}}^3$, and $m^{*}=1.75m_e$, respectively.

In 1976, Larkin and Ovchinnikov [Zh. Eksp. Teor. Fiz. 68, 1915 (1975) [Sov. Phys.–JETP 41, 960 (1976)]] predicted that vortex matter in superconductors driven by an electrical current can undergo an abrupt dynamic transition from a flux-flow regime to a more dissipative state at sufficiently high vortex velocities. Typically, this transition manifests itself as a large voltage jump at a particular current density, so-called instability current density $J^{*}$, which is smaller than the depairing current. By tuning the effective pinning strength in Al films, using an artificial periodic pinning array of triangular holes, we show that a unique and well-defined instability current density exists if the pinning is strong, whereas a series of multiple voltage transitions appear in the relatively weaker pinning regime. This behavior is consistent with time-dependent Ginzburg-Landau simulations, where the multiple-step transition can be unambiguously attributed to the progressive development of vortex chains and subsequently phase-slip lines. In addition, we explore experimentally the magnetic braking effects, caused by a thick Cu layer deposited on top of the superconductor, on the instabilities and the vortex ratchet effect.

Easy axis antiferromagnets usually exhibit a first-order spin-flop transition when the magnetic field is applied along the easy axis. Recently a colossal magnetoelectric effect was discovered in ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$, suggesting a continuous spin-flop transition across a narrow phase in this material [Oh et al., Nat. Commun. 5, 3201 (2014)]. Additional evidence is, however, desirable to verify this mechanism. Here we measure the infrared vibrational properties of ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ in high magnetic fields and demonstrate that the phonon anomalies are consistent with a second-order mechanism.

We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800 Oe at room temperature with a coercive field around 600 Oe. The room-temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_{\mathrm{eff}}=0.41\mathrm{mJ}/{\mathrm{m}}^2$ and $K_{\mathrm{eff}}=37\mathrm{kJ}/{\mathrm{m}}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of MnN. The maximum blocking temperature exceeds $325{}^{\circ }\mathrm{C}$, however the median blocking temperature in the limit of thick MnN is $160{}^{\circ }\mathrm{C}$. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof of principle we demonstrate a simple giant magnetoresistance stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.

We study the polarization optical properties of microcavities with embedded (110)-oriented quantum wells. The spin dynamics of exciton polaritons in such structures is governed by the interplay of the spin-orbit splitting of exciton states, which is odd in the in-plane momentum, and the longitudinal-transverse splitting of cavity modes, which is even in the momentum. We demonstrate the generation of polariton spin currents by a linearly polarized optical pump and analyze the arising polariton spin textures in the cavity plane. By tuning of the excitation spot size, which controls the polariton distribution in the momentum space, one obtains symmetric or asymmetric spin textures.

Recent experimental demonstration of a parallel $(\pi ,2\pi )$ single qubit rotation on excitons in two distant quantum dots [Nano Lett. 13, 4666 (2013)] is extended in numerical simulations to the design of pulses for more general quantum state control, demonstrating the feasibility of full SU(2) rotations of each exciton qubit. Our results show that simultaneous high-fidelity quantum control is achievable within the experimentally accessible parameter space for commercial Fourier-domain pulse shaping systems. The identification of a threshold of distinguishability for the two quantum dots (QDs) for achieving high-fidelity parallel rotations, corresponding to a difference in transition energies of $\sim 0.25\mathrm{meV}$, points to the possibility of controlling more than 10 QDs with a single shaped optical pulse.

The magnetotransport of highly mobile two-dimensional electrons is studied in GaAs single quantum wells with three occupied subbands. The bottoms of the lower two subbands have nearly the same energy while the bottom of the third subband has a much higher energy ($E_1\approx E_2\ll E_3$). Magnetointersubband oscillations (MISOs) between the $i\mathrm{th}$ and $j\mathrm{th}$ subbands are observed and obey the relation ${\mathrm{\Delta }}_{ij}=E_j-E_i=l\hslash {\omega }_c$, where ${\omega }_c$ is the cyclotron frequency and $l$ is an integer. The slight difference in the energies of the lower subbands produces noticeable interference effects in the magnetoresistance. In contrast to the case of two populated subbands, by analyzing the amplitude of each component of the MISOs separately, the quantum lifetime ${\tau }_q^{\left(i\right)}$ of electrons in the $i\mathrm{th}$ subband is extracted. The studies show that ${\tau }_q^{\left(1\right)}\approx {\tau }_q^{\left(2\right)}={\tau }_q^{(1,2)}$. The temperature variation of $1/{\tau }_q^{(1,2)}$ is found to be proportional to $T^2$, indicating that the electron-electron interaction is the dominant source of the decrease of the electron lifetime at high temperatures. Different temperature dependence of the quantum scattering rate is found in the third subband, $\delta (1/{\tau }_q^{\left(3\right)})\sim T$, which is consistent with the theory of sparsely populated subbands.

Electron spin resonance (ESR) was studied in an asymmetrically doped 16-nm AlAs quantum well grown in the [001] direction. Surprisingly the ESR was detectable even if the magnetic field was parallel to the surface of the two-dimensional system. This allowed us to investigate precisely the in-plane anisotropy of the electron $g$ factor. In the case of the magnetic field aligned along the [110] or $\left[1\overline{1}0\right]$ crystallographic axes only one ESR peak was observed, whereas it tended to split into two well-separated peaks when the in-plane component of the magnetic field was between these directions. This fact clearly indicates that two in-plane valleys at $X$ points of the Brillouin zone were occupied with electrons and each valley was characterized by the anisotropic $g$ factor with the [100], [010], and [001] principal axes. The principal $g$-factor values were extracted.

Dirac metals are characterized by the linear dispersion of fermionic quasiparticles, with the Dirac point hidden inside a Fermi surface. We study the magnetotransport in these materials using chiral kinetic theory to describe within the same framework both the negative magnetoresistance caused by the chiral magnetic effect and quantum oscillations in the magnetoresistance due to the existence of the Fermi surface. We discuss the relevance of obtained results to recent measurements on ${\mathrm{Cd}}_3{\mathrm{As}}_2$.