We report magnetotransport properties for epitaxial thin films of SrFeO${}_3$ and SrFe${}_{0.99}$Co${}_{0.01}$O${}_3$ with possible skyrmion-related spin textures. Resonant soft x-ray diffraction measurements revealed formation of helical spin structures for both samples ($Q//\langle 111\rangle$). From magnetotransport measurements we found several distinct helimagnetic phases with multiple/single $Q$ vectors. A steep suppression of Hall resistivity is observed above the critical field to reach the high-field conical state, indicating the presence of skyrmionlike topological spin textures at lower fields responsible for the topological Hall effect.

We report on the microscopic magnetic modeling of the spin-$\frac{1}{2}$ copper mineral malachite at ambient and elevated pressures. Despite the layered crystal structure of this mineral, the ambient-pressure susceptibility and magnetization data can be well described by an unfrustrated quasi-one-dimensional magnetic model. Weakly interacting antiferromagnetic alternating spin chains are responsible for a large spin gap of 120 K. Although the intradimer Cu–O–Cu bridging angles are considerably smaller than the interdimer angles, density functional theory (DFT) calculations revealed that the largest exchange coupling of 190 K operates within the structural dimers. The lack of the inversion symmetry in the exchange pathways gives rise to sizable Dzyaloshinskii-Moriya interactions which were estimated by full-relativistic DFT+$U$ calculations. Based on available high-pressure crystal structures, we investigate the exchange couplings under pressure and make predictions for the evolution of the spin gap. The calculations evidence that intradimer couplings are strongly pressure dependent and their evolution underlies the decrease of the spin gap under pressure. Finally, we assess the accuracy of hydrogen positions determined by structural relaxation within DFT and put forward this computational method as a viable alternative to elaborate experiments.

We study a quantum point contact in the fractional quantum Hall regime at Landau level filling factors $\nu =1/3$ and $5/2$. By using exact diagonalizations in the cylinder geometry, we identify the edge modes in the presence of a parabolic confining potential. By changing the sign of the potential, we can access both the tunneling through the bulk of the fluid and the tunneling between spatially separated droplets. This geometry is realized in the quantum point contact geometry for two-dimensional electron gases. In the case of the model Moore-Read Pfaffian state at filling factor $\nu =5/2$, we identify the conformal towers of many-body eigenstates including the non-Abelian sector. By a Monte-Carlo technique, we compute the various scaling exponents that characterize the edge modes. In the case of hard-core interactions whose ground states are exact model wave functions, we find equality of neutral and charged velocities, both bosonic and fermionic, for the Pfaffian state.

We suggested recently [V. Virkkala et al., Phys. Rev. B 88, 035204 (2013)] that the band-gap narrowing in dilute GaAs${}_{1-x}$N${}_x$ alloys can be explained to result from the broadening of the localized N states due to the N-N interaction along the zigzag chains in the $\langle$110$\rangle$ directions. In that study our tight-binding modeling based on first-principles density-functional calculations took into account the random distribution of N atoms in a natural way. In this work we extend our modeling to GaAs${}_{1-x}$Bi${}_x$ alloys. Our results indicate that Bi states mix with host material states. However, the states near the valence-band edge agglomerate along the zigzag chains originating from individual Bi atoms. This leads to Bi-Bi interactions in a random alloy broadening these states in energy and causing the band-gap narrowing.

The ground state of an acceptor-bound hole in a zinc-blende semiconductor is formed by four eigenstates of the total angular momentum, which is a vector sum of spin and orbital moment of the hole. As a result, the hyperfine interaction of the hole with lattice nuclei becomes anisotropic and coordinate dependent. We develop a theory of dynamic polarization of nuclear spins by the acceptor-bound hole, giving full account for its complex spin structure. The rate of hole-nuclear flip-flop transitions is shown to depend on the angle between the total angular momentum of the hole and the position vector of the nucleus with respect to the acceptor center. The resulting spatially inhomogeneous spin polarization of nuclei gives rise to nonequidistant spin splitting of the hole, which can be detected by methods of optical or microwave spectroscopy.

Exciton scattering from impurities and acoustic phonons in semiconducting carbon nanotubes is theoretically studied in an effective-mass approximation. A selection rule for transitions resulting from the scattering is obtained, showing that the ground-state exciton for the lowest-exciton band is never scattered from long-range impurities and deformation potentials near the band edge. This arises from the symmetry of nanotubes for the successive transformations of the charge conjugation and parity and for the reflection of the coordinate along the nanotube axis. Agreement of numerical results with recent experiments supports the validity of this selection rule.

We report experimental and theoretical studies of edge magnetoplasmon (EMP) transport in quantum Hall (QH) devices. We develop a model that allows us to calculate the transport coefficients of EMPs in QH devices with various geometries. In our model, a QH system is described as a chiral distributed-element (CDE) circuit, where the effects of Coulomb interaction are represented by an electrochemical capacitance distributed along unidirectional transmission lines. We measure the EMP transport coefficients through single- and coupled-edge channels, a quantum point contact, and single- and double-cavity structures. These measured transmission spectra can be reproduced well by simulations using the corresponding CDE circuits. By fitting the experimental results with the simulations, we deduce the circuit parameters that characterize the electrostatic environment around the edge channels in a realistic QH system. The observed gate-voltage dependences of the EMP transport properties in gate-defined structures are explained in terms of the gate tuning of the circuit parameters in CDE circuits.

Interlayer coupling effects between high mobility two-dimensional superconductors are studied in bilayer $\delta$-doped SrTiO${}_3$ heterostructures. By tuning the undoped SrTiO${}_3$ spacer layer between the dopant planes, clear tunable coupling is demonstrated in the variation of the sheet carrier density, Hall mobility, superconducting transition temperature, and the temperature and angle dependencies of the superconducting upper critical field. Systematic variation is found between one effective (merged) two-dimensional superconductor to two decoupled two-dimensional superconductors. In the intermediate coupled regime, a crossover is observed due to coupling arising from intersubband interactions.

We report on measurements of the low frequency current fluctuations in a tunnel junction between normal metal contacts placed at very low temperature, biased by a time-dependent positive voltage $V\left(t\right)=V(1+\cos 2\pi \nu t)$. We observe that the excess noise generated by the ac excitation exhibits oscillations as a function of the average voltage $V$ of the excitation, with a period given by $h\nu /e$ with $e$ the charge of a single electron. This is a direct consequence of the quantum nature of electricity in a normal conductor.

From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a nonquantized coefficient describes the electromagnetic response of the $(3+1)$-dimensional Weyl semimetal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the Chern-Simons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the Callan-Harvey mechanism. This provides a unique solution for the radiatively induced CPT-odd term in the electromagnetic polarization tensor of the Lorentz violating spinor electrodynamics, where the source of the Lorentz violation is a constant axial 4-vector term for the Dirac fermion. A direct linear response calculation also establishes anomalous thermal Hall effect and a Wiedemann-Franz law, but thermal Hall conductivity does not directly follow from the well known formula for the gravitational chiral anomaly.

We demonstrate the high-quality heteroepitaxy of ultrathin EuO films with bulklike ferromagnetism directly on MgO (001), an important step towards combining the spin-filter tunnel effect in magnetic insulators and symmetry-filter tunneling through single-crystalline MgO barriers. Despite a large compressive lattice mismatch, EuO grows fully relaxed on MgO (001) and adopts its bulk lattice parameter from the first monolayer on. This initial heteroepitaxial growth mode is discussed in terms of different electrostatic atomic configurations of the oxides interface. Single-crystalline EuO/MgO (001) thus can be envisioned as highly effective double spin-selective tunnel barriers for spintronics applications.

We study by high-resolution transmission electron microscopy the structural response of bilayer graphene to electron irradiation with energies below the knock-on damage threshold of graphene. We observe that one type of divacancy, which we refer to as the butterfly defect, is formed for radiation energies and doses for which no vacancies are formed in clean monolayer graphene. By using first principles calculations based on density-functional theory, we analyze two possible causes related with the presence of a second layer that could explain the observed phenomenon: an increase of the defect stability or a catalytic effect during its creation. For the former, the obtained formation energies of the defect in monolayer and bilayer systems show that the change in stability is negligible. For the latter, ab initio molecular dynamics simulations indicate that the threshold energy for direct expulsion does not decrease in bilayer graphene as compared with monolayer graphene, and we demonstrate the possibility of creating divacancies through catalyzed intermediate states below this threshold energy. The estimated cross section agrees with what is observed experimentally. Therefore, we show the possibility of a catalytic pathway for creating vacancies under electron radiation below the expulsion threshold energy.

We present first-principles calculations of silicene/graphene and germanene/graphene bilayers. Various supercell models are constructed in the calculations in order to reduce the strain of the lattice-mismatched bilayer systems. Our energetics analysis and electronic structure results suggest that graphene can be used as a substrate to synthesize monolayer silicene and germanene. Multiple phases of single crystalline silicene and germanene with different orientations relative to the substrate could coexist at room temperature. The weak interaction between the overlayer and the substrate preserves the low-buckled structure of silicene and germanene, as well as their linear energy bands. The gap induced by breaking the sublattice symmetry in silicene on graphene can be up to 57 meV.

We present a detailed study on the charge ordering transition in a GdBaCo${}_2$O${}_{5.0}$ system by combining high-resolution synchrotron powder/single-crystal diffraction with electron paramagnetic resonance experiments as a function of temperature. We found a second-order structural phase transition at $T_{\mathrm{CO}}$ = 247 K (Pmmm to Pmma) associated with the onset of long-range charge ordering. At $T_{\min }$ ≈ 1.2$T_{\mathrm{CO},}$ the electron paramagnetic resonance linewidth rapidly broadens, providing evidence of antiferromagnetic spin fluctuations. This likely indicates that, analogously to manganites, the long-range antiferromagnetic order in GdBaCo${}_2$O${}_{5.0}$ sets in at ≈$T_{\mathrm{CO}}$. Pair distribution function analysis of diffraction data revealed signatures of structural inhomogeneities at low temperature. By comparing the average and local bond valences, we found that above $T_{\mathrm{CO}}$ the local structure is consistent with a fully random occupation of Co${}^{2+}$ and Co${}^{3+}$ in a 1:1 ratio and with a complete charge ordering below $T_{\mathrm{CO}}$. Below $T$ ≈ 100 K the charge localization is partially melted at the local scale, suggesting a reentrant behavior of charge ordering. This result is supported by the weakening of superstructure reflections and the temperature evolution of electron paramagnetic resonance linewidth that is consistent with paramagnetic reentrant behavior reported in the GdBaCo${}_2$O${}_{5.5}$ parent compound.

Using inelastic neutron scattering, we have observed a quasi-one-dimensional dispersive magnetic excitation in the frustrated triangular-lattice spin-2 chain oxide Ca${}_3$Co${}_2$O${}_6$. At the lowest temperature ($T=1.5$ K), this magnon is characterized by a large zone-center spin gap of $\sim$27 meV, which we attribute to the large single-ion anisotropy, and disperses along the chain direction with a bandwidth of $\sim$3.5 meV. In the directions orthogonal to the chains, no measurable dispersion was found. With increasing temperature, the magnon dispersion shifts towards lower energies, yet persists up to at least 150 K, indicating that the ferromagnetic intrachain correlations survive up to six times higher temperatures than the long-range interchain antiferromagnetic order. The magnon dispersion can be well described within the predictions of linear spin-wave theory for a system of weakly coupled ferromagnetic chains with large single-ion anisotropy, enabling the direct quantitative determination of the magnetic exchange and anisotropy parameters.

Phases analogous to supersolids can be realized in spin systems. Here we have obtained, using a bond-operator representation, a mean-field theory and a generalized spin-wave theory, the phase diagram of a frustrated dimer-spin-$\frac{1}{2}$ system on a square lattice and study the collective excitation spectra, focusing on the supersolid (SS) state. In the phase diagram on a parameter space of the exchange interaction and magnetic field, we have identified not only the SS phase, but a phase that has no counterpart in the Bose-Hubbard model and this state becomes dominant at the region where the enhancement of SS occurs in the Bose-Hubbard model. We then investigate the excitation spectrum and spin-spin correlation, detectable by neutron-scattering experiments. We obtain an analytic expression for the spin-wave velocity, which agrees with hydrodynamic relations. The intensity of excitation modes in the spin-spin correlation function is calculated and their change in the supersolid and superfluid states is discussed.

An effective Hamiltonian approach is used to reveal the effects of the (biquadratic) magnetoelectric (ME) coupling between local electric dipoles and local magnetic moments on magnetic properties of antiferromagnetic multiferroics. In addition to showing that the Néel magnetic ordering temperature ($T_N$) can strongly depend on such coupling, the simulations also reveal that the perpendicular component of the magnetic susceptibility, ${\chi }_{M,\perp }$, can present some strong deviations from that of a typical antiferromagnet because of the ME coupling. Examples of systems for which $T_N$ is lower than the first-order paraelectric-to-ferroelectric transition temperature ($T_C$) are (1) a discontinuity of ${\chi }_{M,\perp }$ at $T_C$; (2) the inverse of the magnetic susceptibility not following anymore the traditional Curie law when the temperature ranges between $T_N$ and $T_C$; and (3) ${\chi }_{M,\perp }$ increasing with temperature, rather than forming a plateau, when the system is heated up to $T_N$. A Landau-type phenomenological model is further developed to reproduce and understand all these effects.

We propose an analog quantum simulator for the Holstein molecular-crystal model based on a superconducting circuit QED system in the dispersive regime. By varying the driving field on the superconducting resonators, one can access both the adiabatic and antiadiabatic regimes of this model, and the strong electron-phonon coupling required for small-polaron formation can be readily reached. We show that a small-polaron state of arbitrary quasimomentum can be generated by applying a microwave pulse to the resonators. We also show that significant squeezing in the resonator modes can be achieved in the polaron-crossover regime through a measurement-based scheme.

We have investigated the x-ray diffraction (XRD) structure, magnetic susceptibility, and heat capacity of GdCoO${}_3$ in a wide temperature range. A model of phase separation of the low-spin (LS) and high-spin (HS) states has been proposed based on the analysis of XRD peak shape anomalies in the temperature range 200–800 K. From magnetic measurements we separated the HS Co${}^{3+}$ contribution and fitted it with the temperature-dependent spin gap. We found a smooth LS-HS crossover at $T=800$ K. The possible contribution of the intermediate spin (IS) state to the thermodynamics is excluded by the calculation IS-LS excitation energy within the modified crystal-field approach. In the two-phase model, with HS/LS probabilities calculated from the found spin gap and the LS and HS volumes calculated by the DFT-GGA method, we were able to reproduce the temperature dependence of the unit-cell volume and thermal expansion. Thus, we conclude that in GdCoO${}_3$ the main mechanism of the lattice expansion is not the conventional lattice anharmonicity, but the HS/LS fluctuations. The electronic structure has been calculated by the LDA+GTB method. At zero temperature, we have obtained the charge-transfer insulator with the charge gap $E_g=0.5$ eV. The thermal population of the HS term results in the in-gap band formation inside the insulator gap and smooth insulator-metal transition at $T_{\text{IMT}}=780$ K. Heat-capacity measurements revealed a smooth maximum near the $T_{\text{IMT}}$.