We have revisited the magnetic structure of manganese phosphorus trisulfide ${\text{MnPS}}_3$ using neutron diffraction and polarimetry. ${\text{MnPS}}_3$ undergoes a transition toward a collinear antiferromagnetic order at 78 K. The resulting magnetic point-group breaks both the time reversal and the space inversion thus allowing a linear magnetoelectric coupling. Neutron polarimetry was subsequently used to prove that this coupling provides a way to manipulate the antiferromagnetic domains simply by cooling the sample under crossed magnetic and electrical fields, in agreement with the nondiagonal form of the magnetoelectric tensor. In addition, this tensor has, in principle, an antisymmetric part that results in a toroidic moment and provides with a pure ferrotoroidic compound.

Transition voltage spectroscopy (TVS) has recently been introduced as a spectroscopic tool for molecular junctions where it offers the possibility to probe molecular level energies at relatively low bias voltages. In this work we perform extensive ab initio calculations of the nonlinear current-voltage relations for a broad class of single-molecule transport junctions in order to assess the applicability and limitations of TVS. We find, that in order to fully utilize TVS as a quantitative spectroscopic tool, it is important to consider asymmetries in the coupling of the molecule to the two electrodes. When this is taken properly into account, the relation between the transition voltage and the energy of the molecular orbital closest to the Fermi level closely follows the trend expected from a simple, analytical model.

A growing list of experiments show orthorhombic electronic anisotropy in the iron pnictides, in some cases at temperatures well above the spin-density-wave transition. These experiments include neutron scattering, resistivity and magnetoresistance measurements, and a variety of spectroscopies. We explore the idea that these anisotropies stem from a common underlying cause: orbital order manifest in an unequal occupation of $d_{xz}$ and $d_{yz}$ orbitals, arising from the coupled spin-orbital degrees of freedom. We emphasize the distinction between the total-orbital occupation (the integrated density of states), where the order parameter may be small and the orbital polarization near the Fermi level which can be more pronounced. We also discuss light-polarization studies of angle-resolved photoemission and demonstrate how x-ray absorption linear dichroism may be used as a method to detect an orbital-order parameter.

The effects of Coulomb interactions on the electronic properties of bilayer graphene nanoribbons (BGNs) covered by a gate electrode are studied theoretically. The electron-density distribution and the potential profile are calculated self-consistently within the Hartree approximation. A comparison to their single-particle counterparts reveals the effects of interactions and screening. Due to the finite width of the nanoribbon in combination with electronic repulsion, the gate-induced electrons tend to accumulate along the BGN edges where the potential assumes a sharp triangular shape. This has a profound effect on the energy gap between electron and hole bands, which depends nonmonotonously on the gate voltage and collapses at intermediate electric fields. We interpret this behavior in terms of interaction-induced warping of the energy dispersion.

We show through an exact numerical solution of the phonon Boltzmann equation that the lattice thermal conductivity of graphene is dominated by contributions from the out-of-plane or flexural phonon modes, previously thought to be negligible. We connect this unexpected result to the anomalously large density of states of flexural phonons compared to their in-plane counterparts and to a symmetry-based selection rule that significantly restricts anharmonic phonon-phonon scattering of the flexural modes. The result is found to hold in the presence of the ripples known to occur in graphene, phonon-isotopic impurity scattering, and rigidity of the flexural phonon branch arising from the long-wavelength coupling between flexural and in-plane modes. Finally, accurate inclusion of the momentum conserving Normal phonon-phonon scattering processes within the context of a full solution of the phonon Boltzmann equation are shown to be essential in accurately describing the graphene thermal conductivity, in contrast to the more commonly used relaxation time and long wavelength approximations.

We report a comprehensive study of the ultrafast optoelectronic response of a single self-assembled InGaAs/GaAs quantum dot embedded in a $n\text{−}i$-Schottky photodiode device. While manipulation of the artificial atom relies on two independently tunable picosecond pulse trains, sensitive all electrical readout is achieved via photocurrent. We apply our methods to probe the temporal evolution of the quantum dot absorption spectrum following coherent optical generation of a $s$-shell exciton. Our measurements reveal the biexciton absorption as well as previously unobserved $p$-shell transitions that appear in the presence of $s$-shell population. Furthermore, time-resolved measurements allow us to directly monitor the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also demonstrate the potential of our techniques for coherent quantum dot manipulations. Beginning with excitonic Rabi oscillations driven by a single pulse tuned to the neutral exciton we move on to demonstrate the coherent generation of the biexciton via a two-photon nonlinearity, before realizing conditional Rabi oscillations in the exciton-biexciton state manifold. The results provide significant potential for realizing and utilizing conditional coherent dynamics in quantum dot nanostructures for all optical, ultrafast quantum information processing.

We propose the implementation of an electronic Veselago lens on the conducting surface of a three-dimensional topological insulator (such as ${\text{Bi}}_2{\text{Te}}_3$). The negative refraction needed for such a flat lens results from the sign change in the curvature of the Fermi surface, changing from a circular to a snowflakelike shape across a sufficiently large electrostatic potential step. No interband transition (as in graphene) is needed. For this reason, and because the topological insulator provides protection against backscattering, the potential step is able to focus a broad range of incident angles. We calculate the quantum interference pattern produced by a point source, generalizing the analogous optical calculation to include the effect of a noncircular Fermi surface (having a nonzero conic constant).

Anisotropic magnetoresistances and intrinsic adiabatic transport features are generated on quantum Hall samples based on an (Al,Ga)As/GaAs heterostructure with alloyed Au/Ge/Ni contacts. We succeed to probe the microscopic origin of these transport features with a cryogenic scanning force microscope by measuring the local potential distribution within the two-dimensional electron system (2DES). These local measurements reveal the presence of an incompressible strip in front of contacts with insulating properties depending on the orientation of the contact/2DES interface line relatively to the crystal axes of the heterostructure. Such an observation gives another microscopic meaning to the term “nonideal contact” used in context with the Landauer-Büttiker formalism applied to the quantum Hall effect.

A magnetic phase transition is induced by magnetic field in a Haldane gap system ${\text{Y}}_2{\text{BaNi}}_{1-y}{\text{Mg}}_y{\text{O}}_5$, where nonmagnetic Mg dopants cut an infinite chain into finite segments randomly. The ordering is realized only for field direction parallel to the $c$ axis and above a critical field ${\mu }_0{H}_{\text{c}}\sim 2\text{T}$, which is much smaller than the Haldane gap of ${\text{Y}}_2{\text{BaNiO}}_5$ $\left(\sim 70\text{T}\right)$. The phenomenon is correlated with the field-induced energy-level crossing of the eigenstates for coupled edge spins at both ends of each segment. Although the crossing field depends on the length of the segments, macroscopic ordering is realized.

Proposed even-denominator fractional quantum Hall effect (FQHE) states suggest the possibility of excitations with non-Abelian braid statistics. Recent experiments on wide square quantum wells indicate even-denominator FQHE even under electrostatic tilt. We theoretically analyze these structures and develop a procedure to accurately test proposed quantum Hall wave functions. We find that tilted wells favor partial subband polarization to yield Abelian even-denominator states. Our results show that tilting quantum wells effectively engineers different interaction potentials allowing exploration of a wide variety of even-denominator states.

The state-resolved reaction probability of ${\text{CH}}_4$ on $\text{Pt}\left(110\right)\text{−}\left(1\times 2\right)$ was measured as a function of ${\text{CH}}_4$ translational energy for four vibrational eigenstates comprising different amounts of C-H stretch and bend excitation. Mode-specific reactivity is observed both between states from different polyads and between isoenergetic states belonging to the same polyad of ${\text{CH}}_4$. For the stretch/bend combination states, the vibrational efficacy of reaction activation is observed to be higher than for either pure C-H stretching or pure bending states, demonstrating a concerted role of stretch and bend excitation in C-H bond scission. This concerted role, reflected by the nonadditivity of the vibrational efficacies, is consistent with transition state structures found by ab initio calculations and indicates that current dynamical models of ${\text{CH}}_4$ chemisorption neglect an important degree of freedom by including only C-H stretching motion.

We investigate, experimentally and theoretically, the epitaxy of body-centered-cubic Fe on hexagonal GaN. For growth on the Ga-polar GaN(0001) surface we find the well-known Pitsch-Schrader orientation relationship between Fe and GaN. On the N-polar $\text{GaN}\left(000\overline{1}\right)$ surface we observe coexistence between the familiar Burgers orientation and a new orientation in which the Fe(001) plane is parallel to $\text{GaN}\left(000\overline{1}\right)$. This “cube-on-hexagon” orientation constitutes the high-symmetry link required for constructing a symmetry diagram for bcc/hcp systems in which all orientation relationships are connected by simple rotations.

We use the local-density approximation in combination with the dynamical mean-field theory to carry out a comparative investigation of a typical electron-doped and a typical hole-doped copper oxide, NCCO, and LSCO, respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge-transfer insulator transition. In NCCO the magnetic long-range order is essential to open a charge-transfer gap while Mott physics is responsible for the gap in LSCO. We highlight the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized x-ray absorption, and angle-resolved photoemission are presented and compared with experiments.

We have measured second-harmonic generation (SHG) from graphene and other graphitic films, from two layers to bulk graphite, at room temperature; all samples are mounted on a 300 nm oxide layer of a Si(001) substrate. With 800 nm, 150 fs fundamental pulses, the anisotropic response was recorded for combinations of $p$-, $s$-, and diagonally polarized fundamental and second-harmonic beams as the samples were rotated about their normal. Graphene samples display SHG signatures only slightly different from that of the bare substrate which shows SHG with fourfold rotational symmetry. All other layered systems show threefold symmetry, although the ratio of isotropic to anisotropic response varies with the number of layers. A model based on linear light propagation in layered media with interface dipole and bulk quadrupole SHG sources is presented for the analysis. We show that data from all layered samples can be understood in terms of well-known linear optical properties, the SHG response of the bare substrate and four independent, complex nonlinear dipole susceptibility tensor elements of the graphene/air interface.

The nature of the unconventional ordered phase occurring in ${\text{CeRu}}_2{\text{Al}}_{10}$ below $T_0=27\text{K}$ was investigated by neutron scattering. Powder-diffraction patterns show clear superstructure peaks corresponding to forbidden $\left(h+k\right)$-odd reflections of the $Cmcm$ space group. Inelastic neutron-scattering experiments further reveal a pronounced magnetic excitation developing in the ordered phase at an energy of 8 meV.

The low-temperature state of ${\text{CeRu}}_2{\text{Al}}_{10}$ has been studied by neutron powder diffraction and muon spin relaxation. By combining both techniques, we prove that the transition occurring at $T^{\ast }\sim 27\text{K}$, whose origin has been the subject of considerable debate, is unambiguously magnetic due to the ordering of the Ce sublattice. The magnetic structure with a propagation vector of $\mathbf{k}=\left(1,0,0\right)$ involves a collinear antiferromagnetic alignment of the Ce moments along the $c$ axis of the $Cmcm$ space group with a reduced moment of $0.34\left(2\right){\mu }_B$. No structural changes within the resolution limit have been detected below the transition temperature. However, the temperature dependence of the magnetic Bragg peaks and the muon precession frequency show an anomaly around $T_2\sim 12\text{K}$ indicating a possible second transition.

We have carried out neutron diffraction, muon spin relaxation $\left(\mu \text{SR}\right)$, and inelastic neutron scattering (INS) investigations on a polycrystalline sample of ${\text{CeOs}}_2{\text{Al}}_{10}$ to investigate the nature of the phase transition observed near 29 K in the resistivity and heat capacity. Our $\mu \text{SR}$ data clearly reveal coherent frequency oscillations below 28 K, indicating the presence of an internal field at the muon site, which confirms the long-range magnetic ordering of the Ce moment below 28 K. Upon cooling the sample below 15 K, unusual behavior of the temperature-dependent $\mu \text{SR}$ frequencies may indicate either a change in the muon site, consistent with the observation of superstructure reflections in electron diffraction, or a change in the ordered magnetic structure. Neutron diffraction data do not reveal any clear sign of either magnetic Bragg peaks or superlattice reflections. Furthermore, INS measurements clearly reveal the presence of a sharp inelastic excitation near 11 meV between 5 and 26 K, due to opening of a gap in the spin-excitation spectrum, which transforms into a broad response at and above 30 K. The magnitude of the spin gap (11 meV) as derived from the INS peak position agrees very well with the gap value as estimated from the bulk properties.

We report on measurements of supercurrents through the half-metallic ferromagnet ${\text{CrO}}_2$ grown on hexagonal ${\text{Al}}_2{\text{O}}_3$ (sapphire). The current was observed to flow over a distance of 700 nm between two superconducting amorphous ${\text{Mo}}_{70}{\text{Ge}}_{30}$ electrodes which were deposited on the ${\text{CrO}}_2$ film. The critical current $I_c$ increases as function of decreasing temperature. Upon applying an in-plane magnetic field, $I_c$ goes through a maximum at the rather high field of 80 mT. We believe this to be a long-range proximity effect in the ferromagnet, carried by odd-frequency pairing correlations.

A distinct approach for calculations of Dzyaloshinskii-Moriya interactions in molecules and crystals is proposed. It is based on the exact perturbation expansion of the total energy of weak ferromagnets in the canting angle with the only assumption of local Hubbard-type interactions. This scheme leads to a simple and transparent analytical expression for the Dzyaloshinskii-Moriya vector with a natural separation into spin and orbital contributions. The main problem was transferred to calculations of effective tight-binding parameters in the properly chosen basis including the spin-orbit coupling. Test calculations for ${\text{La}}_2{\text{CuO}}_4$ give the value of the canting angle in agreement with experimental data.

Electronic absorption, magnetic circular dichroism (MCD), photoconductivity, and valence-band x-ray photoelectron (XPS) spectroscopic measurements were performed on epitaxial ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ films to investigate the origin of the mid-gap band that appears upon introduction of ${\text{Mn}}^{2+}$ into the ZnO lattice. Absorption and MCD spectroscopies reveal ${\text{Mn}}^{2+}$-related intensity at energies below the first excitonic transition of ZnO, tailing well into the visible energy region, with an onset at $\sim 2.2\text{eV}$. Photoconductivity measurements show that excitation into this visible band generates mobile charge carriers, consistent with assignment as a ${\text{Mn}}^{2+/3+}$ photoionization transition. XPS measurements reveal the presence of occupied ${\text{Mn}}^{2+}$ levels just above the valence-band edge, supporting this assignment. MCD measurements additionally show a change in sign and large increase in magnitude of the excitonic Zeeman splitting in ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ relative to ZnO, suggesting that $sp\text{−}d$ exchange in ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is not as qualitatively different from those in other II-VI diluted magnetic semiconductors as has been suggested. The singular electronic structure feature of ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is the presence of this ${\text{Mn}}^{2+/3+}$ ionization level within the gap, and the influence of this level on other physical properties of ${\text{Zn}}_{1-x}{\text{Mn}}_x\text{O}$ is discussed.