The quantum phase transitions induced by a magnetic field are theoretically studied in a frustrated two-leg spin ladder. Using the density-matrix renormalization-group method, we find some magnetic phase transitions and plateaus in two different cases of strong and weak rung couplings. With the strong rung coupling, the three magnetization plateaus are found at 1/3, 1/2, and 2/3 due to the frustration. Those can be understood in terms of a quasispinon reconstructed from the singlet and the triplets of spins on a rung. The plateau at 1/2 corresponds to the valence bond solid of the quasispinons, while the plateaus at 1/3 and 2/3 can be associated with the array of quasispinons such as the soliton lattice. This is different from the usual Bose-Einstein-condensation picture of triplons. Our results will be useful in understanding magnetization curves in ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$.

We study the quantum electrodynamics of two-dimensional (2D) and three-dimensional (3D) Dirac semimetals by means of a self-consistent resolution of the Schwinger-Dyson equations, aiming to obtain the respective phase diagrams in terms of the relative strength of the Coulomb interaction and the number $N$ of Dirac fermions. In this framework, 2D Dirac semimetals have just a strong-coupling instability characterized by exciton condensation (and dynamical generation of mass) that we find at a critical coupling well above previous theoretical estimates, thus explaining the absence of that instability in free-standing graphene samples. On the other hand, we show that 3D Dirac semimetals have a richer phase diagram, with a strong-coupling instability leading to dynamical mass generation up to $N=4$ and a line of critical points for larger values of $N$ characterized by the vanishing of the electron quasiparticle weight in the low-energy limit. Such a critical behavior signals the transition to a strongly correlated liquid, characterized by noninteger scaling dimensions that imply the absence of a pole in the electron propagator and are the signature of non-Fermi-liquid behavior with no stable electron quasiparticles.

$\gamma$-triclinic and $\alpha$-monoclinic polymorphs of ${\mathrm{CoV}}_2{\mathrm{O}}_6$ are two of the few known transition-metal ion-based materials that display stepped $\frac{1}{3}$ magnetization plateaus at low temperatures. Neutron diffraction [M. Markkula et al., Phys. Rev. B 86, 134401 (2012)], x-ray dichroism [N. Hollmann et al., Phys. Rev. B 89, 201101(R) (2014)], and dielectric measurements [J. Singh et al., J. Mater. Chem. 22, 6436 (2012)] have shown a coupling between orbital, magnetic, and structural orders in ${\mathrm{CoV}}_2{\mathrm{O}}_6$. We apply neutron inelastic scattering to investigate this coupling by measuring the spin-orbit transitions in both $\alpha$ and $\gamma$ polymorphs. We find the spin exchange and anisotropy in monoclinic $\alpha -{\mathrm{CoV}}_2{\mathrm{O}}_6$ to be weak in comparison with the spin-orbit coupling $\lambda$ and estimate an upper limit of $|J/\lambda |\sim 0.05$. However, the spin exchange is larger in the triclinic polymorph and we suggest the excitations are predominately two dimensional. The local compression of the octahedra surrounding the ${\mathrm{Co}}^{2+}$ ion results in a direct coupling between higher-energy orbital levels, the magnetic ground state, and elastic strain. ${\mathrm{CoV}}_2{\mathrm{O}}_6$ is therefore an example where the local distortion along with the spin-orbit coupling provides a means of intertwining structural and magnetic properties. We finish the paper by investigating the low-energy magnetic fluctuations within the ground-state doublet and report a magnetic excitation that is independent of the local crystalline electric field. We characterize the temperature and momentum dependence of these excitations and discuss possible connections to the magnetization plateaus.

Ferroelectric materials possess a spontaneous electric polarization and may be utilized in various technological applications ranging from nonvolatile memories to solar cells and light polarizers. Recently, hexagonal rare-earth manganites, $h-R\mathrm{Mn}{\mathrm{O}}_3$ ($R$ is a rare-earth ion), have attracted considerable interest due to their intricate multiferroic properties and improper ferroelectricity characterized by a sizable remnant polarization and high Curie temperature. Here we demonstrate that these compounds can serve as very efficient photovoltaic materials and, in addition, possess remarkable optical anisotropy properties. Using first-principles methods based on density functional theory and considering $h-\mathrm{TbMn}{\mathrm{O}}_3$ as a representative manganite, we predict a strong light absorption of this material in the solar spectrum range, resulting in a maximum light-to-electricity energy conversion efficiency of up to 33%. We also predict an extraordinary optical linear dichroism and linear birefringence properties of $h-\mathrm{TbMn}{\mathrm{O}}_3$ in a broad range of optical frequencies. These results uncover the unexplored potential of hexagonal rare-earth manganites to serve as photovoltaics in solar cells and as absorptive and birefringent light polarizers.

The x-ray absorption near-edge structure of graphene, graphene oxide, and diamond is studied by the recently developed real-space full potential multiple scattering (FPMS) theory with space-filling cells. It is shown how accurate potentials for FPMS can be generated from self-consistent charge densities obtained with other schemes, especially the projector augmented wave method. Compared to standard multiple scattering calculations in the muffin-tin approximation, FPMS gives much better agreement with experiment. The effects of various structural modifications on the graphene spectra are well reproduced. (1) Stacking of graphene layers increases the peak intensity in the higher energy region. (2) The spectrum of the C atom located at the edge of a graphene sheet shows a prominent pre-edge structure. (3) Adsorption of oxygen gives rise to the so-called interlayer-state peak. Moreover, O K-edge spectra of graphene oxide are calculated for three types of bonding, C-OH, C-O-C, and C-O, and the proportions of these bondings at $800{}^{\circ }\mathrm{C}$ are deduced by fitting them to the experimental spectrum.

We have observed the quantum Hall effect (QHE) and Shubnikov–de Haas (SdH) oscillations in highly disordered graphene at magnetic fields up to 65 T. Disorder was introduced by hydrogenation of graphene up to a ratio H/C $\approx 0.1\%$. The analysis of SdH oscillations and QHE indicates that the topological part of the Berry phase, proportional to the pseudospin winding number, is robust against introduction of disorder by hydrogenation in large-scale graphene.

Magneto-optical transitions between Landau levels can provide precise spectroscopic information on the electronic structure and excitation spectra of graphene, enabling probes of substrate and many-body effects. We calculate the magneto-optical conductivity of large-size graphene flakes using a tight-binding approach. Our method allows us to directly compare the magneto-optical response of an isolated graphene flake with one aligned on hexagonal boron nitride giving rise to a periodic superlattice potential. The substrate interaction induces band gaps away from the Dirac point. In the presence of a perpendicular magnetic field, Landau-level-like structures emerge from these zero-field band gaps. The energy dependence of these satellite structures is, however, not easily accessible by conventional probes of the density of states by varying the back-gate voltage. Here we propose the magneto-optical probing of the superlattice perturbed spectrum. Our simulation includes magneto-excitonic effects in first-order perturbation theory. Our approach yields a quantitative explanation of recently observed Landau-level-dependent renormalizations of the Fermi velocity.

Cracks generate the largest strain gradients that any material can withstand. Flexoelectricity (coupling between strain gradient and polarization) must therefore play an important role in fracture physics. Here we use a self-consistent continuum model to evidence two consequences of flexoelectricity in fracture: the resistance to fracture increases as structural size decreases, and it becomes asymmetric with respect to the sign of polarization. The latter phenomenon manifests itself in a range of intermediate sizes where piezo- and flexoelectricity compete. In ${\mathrm{BaTiO}}_3$ at room temperature, this range spans from 0.1 to 50 nm, a typical thickness range for epitaxial ferroelectric thin films.

We study the fragility of spin glasses to small temperature perturbations numerically using population annealing Monte Carlo. We apply thermal boundary conditions to a three-dimensional Edwards-Anderson Ising spin glass. In thermal boundary conditions all eight combinations of periodic versus antiperiodic boundary conditions in the three spatial directions are present, each appearing in the ensemble with its respective statistical weight determined by its free energy. We show that temperature chaos is revealed in the statistics of crossings in the free energy for different boundary conditions. By studying the energy difference between boundary conditions at free-energy crossings, we determine the domain-wall fractal dimension. Similarly, by studying the number of crossings, we determine the chaos exponent. Our results also show that computational hardness in spin glasses and the presence of chaos are closely related.

The lead-halide perovskites, including ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ undergoes a series of structural transitions associated with orientational order of the ${\mathrm{CH}}_3{\mathrm{NH}}_3$ (methylammonium) molecular cation and tilting of the ${\mathrm{PbBr}}_3$ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the ${\mathrm{PbBr}}_3$ framework and the quasistatic ${\mathrm{CH}}_3{\mathrm{NH}}_3$ dynamics at low energy transfers. At higher energy transfers, we observe a ${\mathrm{PbBr}}_6$ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to $\sim 50\text{–}100$ meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.

For organic host-guest systems with a low fraction of guest sites, i.e., the trap-limited case, field-induced detrapping of charge carriers is studied via master equation calculations under the assumption of Miller-Abrahams rates and two Gaussian distributions of uncorrelated energy levels. Among existing descriptions of carrier redistributions in the presence of an electric field, the effective temperature derived by F. Jansson, S. D. Baranovskii, F. Gebhard, and R. Österbacka [Phys. Rev. B 77, 195211 (2008)] for pure host materials shows the best agreement with the simulation results. The detrapping description based on carrier heating is extended to the case that the two material-specific hopping rate parameters ${\nu }_0$ (attempt frequency) and $\alpha$ (decay constant or inverse localization length of charge carriers) are different for host and guest sites.

Reversible magnetic exchange springs can be formed in multilayer films, grown by molecular beam epitaxy. Here we demonstrate that small amounts of anisotropic ${\mathrm{ErFe}}_2$, placed in the middle of the ${\mathrm{YFe}}_2$ magnetic exchange springs, can bring about substantial changes. Results are presented for an Er-doped (110)-oriented multilayer film, at 100 K in fields of up to $\pm 14$ T. Using both Er and Dy-specific soft x-ray magnetic circular dichroism and micromagnetic modeling, it is demonstrated that Er doping gives rise to (i) noncollinear exchange spring states in zero field, (ii) magnetic exchange spring collapse, (iii) a marked increase in the number of different exchange spring states available to the system, and (iv) strikingly different Er and Dy magnetization loops. Full and partial loops are presented for both the in-plane hard and easy axes. The magnetization loops for Dy sublattice show that at least ten different exchange spring states are accessed during magnetic reversal. Magnetic switching scenarios, involving mixtures of Néel-like and Bloch-like domain walls, are presented and discussed.

We study the effect of Fe impurities in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ on the magnetic phase and topological insulating property using first-principles calculations. In particular, we investigate the ferromagnetic-antiferromagnetic phase transition and the energy gap variation of surface states in Fe-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$. We find that Fe-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ has a ferromagnetic phase at dilute doping regime by the interplay of the band inversion and intrinsic doping. For higher Fe concentration, $>1.7\mathrm{at}.\%, \mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ prefers the antiferromagnetic phase mediated by the superexchange interaction. We show that neighboring Fe impurities with antiferromagnetic ordering behave like nonmagnetic scattering centers that preserve the linear band dispersion and the in-plane spin texture of topological surface states. Our result indicates that interdependency of the magnetic phase and the band topology in transition-metal–doped topological insulators may tweak the electronic structure and topological surface states in peculiar ways.

We study the spin-fluctuation-mediated superconducting pairing gap in a weak-coupling approach to the Hubbard model for a two-dimensional square lattice in the paramagnetic state. Performing a comprehensive theoretical study of the phase diagram as a function of filling, we find that the superconducting gap exhibits transitions from $p$-wave at very low electron fillings to $d_{x^2-y^2}$-wave symmetry close to half filling in agreement with previous reports. At intermediate filling levels, different gap symmetries appear as a consequence of the changes in the Fermi surface topology and the associated structure of the spin susceptibility. In particular, the vicinity of a Van Hove singularity in the electronic structure close to the Fermi level has important consequences for the gap structure in favoring the otherwise subdominant triplet solution over the singlet $d$-wave solution. By solving the full gap equation, we find that the energetically favorable triplet solutions are chiral and break time-reversal symmetry. Finally, we also calculate the detailed angular gap structure of the quasiparticle spectrum, and show how spin-fluctuation-mediated pairing leads to significant deviations from the first harmonics both in the singlet $d_{x^2-y^2}$ gap as well as the chiral triplet gap solution.

First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient ${\mathrm{SrTiO}}_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a nonmagnetic double donor, manipulation of the doping conditions allows the stability of a single-donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices.

Evaluation and control of a polarization state of optical near fields are of tremendous advantage for locally probing intrinsic molecular orientations of highly anisotropic molecules with a plasmonic (metallic) nanoantenna. In this paper, we report on a physical mechanism of reading a dipole orientation at the apex of a rough cone-shaped gold tip illuminated with radially and azimuthally polarized light. In-plane and out-of-plane arrangement of nonlinear optical chromophores embedded into a glassy polymer is probed with tip-enhanced Raman scattering.

We present a study of the alloying series of the noncentrosymmetric $f$-electron intermetallic ${\mathrm{CePt}}_3{\mathrm{B}}_{1-x}{\mathrm{Si}}_x$ by means of muon spin rotation and relaxation measurements. In addition, we include a high-pressure magnetization investigation of the stoichiometric parent compound ${\mathrm{CePt}}_3\mathrm{B}$. From our data we establish the nature of the magnetic ground state properties of the series, derive the ordered magnetic moment as function of stoichiometry, and gain insight into the evolution of the symmetry of the ordered magnetic state with $x$. We thus can verify the notion that the behavior of the sample series can essentially be understood within the framework of the Doniach phase diagram. Further, our findings raise the issue of the role the Dzyaloshinskii-Moriya magnetic interaction plays in correlated electron materials and its effect on magnetic fluctuations in such materials.

We study the solutions of generic Hamiltonians exhibiting particle-hole mixing. We show that there exists a universal quantity that can describe locally the Majorana nature of a given state. This pseudo-spin-like two-component quantity is in fact a generalization of the Majorana polarization (MP) measure introduced by Sticlet, Bena, and Simon [Phys. Rev. Lett. 108, 096802 (2012)], which was applicable only for some models with specific spin and symmetry properties. We apply this to an open two-dimensional Kitaev system, as well as to a one-dimensional topological wire. We show that the MP characterization is a necessary and sufficient criterion to test whether a state is a Majorana or not, and use it to numerically determine the topological phase diagram.