We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.

Four-terminal conductivity and Hall coefficient are measured in high-mobility rubrene single-crystal field-effect transistors as functions of gate electric field and external hydrostatic pressure up to 900 MPa. Estimating charge density directly from the measured Hall coefficient regardless of contraction in the gate dielectric layer, genuine pressure dependence of the mobility is extracted. It turned out that enhanced intermolecular charge transfer causes an increase in the mobility of the bandlike charge carriers by 20% GPa up to 600 MPa, which is typically one order of magnitude larger than that in inorganic semiconductors. We also found negative pressure coefficient for the mobility above 600 MPa, suggesting that the external hydrostatic force causes additional molecular displacement due to a specific molecular shape.

Ankylography is a new 3D imaging technique, which, under certain circumstances, enables reconstruction of a 3D object from a single sample orientation. Here, we provide a matrix rank analysis to explain the principle of ankylography. We then present an ankylography experiment on a microscale phase object using an optical laser. Coherent diffraction patterns were acquired from the phase object using a planar CCD detector and were projected onto a spherical shell. The 3D structure of the object was directly reconstructed from the spherical diffraction pattern. This work may potentially open the door to a new method for 3D imaging of phase objects in the visible light region. Finally, the extension of ankylography to more complicated and larger objects is suggested.

We investigate the dependency of all-optical magnetization switching on the properties of the exciting laser pulse by specifically tailoring all accessible laser parameters—pulse duration, wavelength, chirp, and bandwidth—over a wide range. Our results show that all-optical switching can be achieved with picosecond instead of femtosecond laser sources of various wavelengths, which considerably relaxes technological feasibility of this technique. The most striking implication is that, in contrast to all current knowledge, a strong photoinduced nonequilibrium in the electronic system is not necessary to achieve magnetization switching with light.

Magnetic resonance force microscopy can enable nanoscale imaging of spin lifetime. We report temperature-dependent measurements of the spin correlation time ${\tau }_m$ of the statistical fluctuations of the spin polarization—the spin noise—of ensembles containing $\sim$100 electron spins by this technique. Magnetomechanical relaxation due to spin-cantilever coupling was controlled and spurious mechanisms that can affect the spin correlation time of the microscopic signal were characterized. These measurements have ramifications for optimizing spin sensitivity, understanding local spin dynamics, and for nanoscale imaging.

An adapted simulation method is used to systematically study grain boundary motion at velocities and driving forces across more than five orders of magnitude. This analysis reveals that grain boundary migration can occur in two modes, depending upon the temperature ($T$) and applied driving force ($P$). At low $P$ and $T$, grain boundary motion is diffusional, exhibiting the kinetics of a thermally activated system controlled by grain boundary self-diffusion. At high $P$ and $T$, grain boundary migration exhibits the characteristic kinetic scaling behavior of a ballistic process. A rather broad transition range in both $P$ and $T$ lies between the regimes of diffusive and ballistic grain boundary motion, and is charted here in detail. The recognition and delineation of these two distinct modes of grain boundary migration also leads to the suggestion that many prior atomistic simulations might have probed a different kinetic regime of grain boundary motion (ballistic) as compared to that revealed in most experimental studies (diffusional).

We present a detailed study of the electronic structure of europium nitride (EuN), comparing spectroscopic data to the results of advanced electronic structure calculations. We demonstrate the epitaxial growth of EuN films, and show that in contrast to other rare-earth nitrides successful growth of EuN requires an activated nitrogen source. Synchrotron-based x-ray spectroscopy shows that the samples contain predominantly Eu${}^{3+}$, but with a small and varying quantity of Eu${}^{2+}$ that we associate with defects, most likely nitrogen vacancies. X-ray absorption and x-ray emission spectroscopies (XAS and XES) at the nitrogen K edge are compared to several different theoretical models, namely, local spin density functional theory with Hubbard $U$ corrections ($\text{LSDA}+U$), dynamic mean field theory (DMFT) in the Hubbard-I approximation, and quasiparticle self-consistent $GW$ (QS$GW$) calculations. The DMFT and QS$GW$ models capture the density of conduction band states better than does $\text{LSDA}+U$. Only the Hubbard-I model contains a correct description of the Eu $4f$ atomic multiplets and locates their energies relative to the band states, and we see some evidence in XAS for hybridization between the conduction band and the lowest-lying ${}^{8}S$ multiplet. The Hubbard-I model is also in good agreement with purely atomic multiplet calculations for the Eu M-edge XAS. $\text{LSDA}+U$ and DMFT calculations find a metallic ground state, while QS$GW$ results predict a direct band gap at X for EuN of about 0.9 eV that matches closely an absorption edge seen in optical transmittance at 0.9 eV, and a smaller indirect gap. Overall, the combination of theoretical methods and spectroscopies provides insights into the complex nature of the electronic structure of this material. The results imply that EuN is a narrow-band-gap semiconductor that lies close to the metal-insulator boundary, where the close proximity to the Fermi level of an empty Eu $4f$ multiplet raises the possibility of tuning both the magnetic and electronic states in this system.

We develop a model that predicts two threshold pump intensities in optically pumped organic semiconductor lasers (OSLs); one for pulsed lasing, $I_{PS}$, and another for continuous-wave (CW) lasing, $I_{CW}$. The theory predicts a decrease in $I_{CW}$ from 32 kW/cm${}^2$, or well above the damage threshold, to 2.2 kW/cm${}^2$, for a laser employing 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran–doped tris(8-hydroxyquinoline) aluminum if the triplets can be effectively removed from the emissive guest. Based on this analysis, we demonstrate that the lasing duration can be extended to nearly 100 μs, ultimately limited by degradation of the lasing medium when a “triplet manager” molecule, 9,10-di(naphtha-2-yl)anthracene, is blended into the gain region of an otherwise conventional distributed feedback OSL. The triplet manager facilitates radiative singlet transfer while suppressing nonradiative triplet transfer to the emitter molecule, thus reducing the triplet-induced losses. Our theory conclusively shows that these lasers have entered the CW lasing regime.

An efficient computational methodology is used to explore charge transport properties in chemically modified (and randomly disordered) graphene-based materials. The Hamiltonians of various complex forms of graphene are constructed using tight-binding models enriched by first-principles calculations. These atomistic models are further implemented into a real-space order-$N$ Kubo-Greenwood approach, giving access to the main transport length scales (mean free paths, localization lengths) as a function of defect density and charge carrier energy. An extensive investigation is performed for epoxide impurities with specific discussions on both the existence of a minimum semiclassical conductivity and a crossover between weak to strong localization regime. The 2D generalization of the Thouless relationship linking transport length scales is here illustrated based on a realistic disorder model.

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortex lines along the $c$ axis moving parallel to the $ab$ plane, and to vortex lines in the $ab$ plane moving perpendicular to and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter ${\varepsilon }_{\xi }$. This allows for the extraction of ${\varepsilon }_{\xi }$ at magnetic fields far below the upper critical field $B_{c2}$. We find that increasing the magnetic field decreases the anisotropy of the coherence length.

Temperature- and pressure-dependent electrical resistivity [($\rho T$,$P$)] studies have been performed on BaMn${}_2$As${}_2$ single crystal in the 4.2–300 K range up to 8.2 GPa in order to investigate the evolution of its ground state properties. ρ($T$) data show negative coefficients of resistivity up to a pressure of 3.2 GPa. At a pressure of ∼4.5 GPa an insulator-to-metal transition is seen to occur at ∼36 K, as indicated by a change in the temperature coefficient of the ρ($T$). However, at a pressure of ∼5.8 GPa the sample is metallic in the entire temperature range. X-ray diffraction studies performed as a function of pressure, at room temperature, also show an anomaly in the pressure versus volume curve around $P$ ≈ 5 GPa without a change in crystal structure, indicative of an electronic transition in support of the resistivity results. In addition to metallization, a clear precipitous drop in ρ($T$) is seen at ∼17 K for $P$ ≥ 5.8 GPa.

The lattice dynamics of the filled skutterudite YbFe${}_4$Sb${}_{12}$ was studied by resonant ultrasound spectroscopy and an anomalous softening in the temperature dependence of the elastic constants at $\sim$50 K was observed. This anomaly can not be explained by the dynamics of the filler, in contrast to other filled skutterudites. We have further investigated the origin of this anomaly using macroscopic and microscopic measurements. A rearrangement of the spectral weight of the Yb phonon states was observed in the temperature dependence of the density of phonon states, obtained by inelastic neutron scattering. We suggest that the anomaly is due to a change of the Yb valence state and that the anomaly and the phonon spectral weight rearrangement have the same origin.

While NMR measurements of nuclear energy spectra are routinely used to characterize the static properties of quantum magnets, the dynamical information locked in NMR $1/T_1$ relaxation rates remains notoriously difficult to interpret. The difficulty arises from the fact that information about all possible low-energy spin excitations of the electrons, and their coupling to the nuclear moments, is folded into a single number $1/T_1$. Here, we develop a quantitative theory of the NMR $1/T_1$ relaxation rate in a collinear antiferromagnet, focusing on the specific example of BaFe${}_2$As${}_2$. One of the most striking features of magnetism in BaFe${}_2$As${}_2$ is a strong dependence of $1/T_1$ on the orientation of the applied magnetic field. By careful analysis of the coupling between the nuclear and electronic moments, we show how this anisotropy arises from the “filtering” of spin fluctuations by the form factor for transferred hyperfine interactions. This allows us to make convincing, quantitative fits to experimental $1/T_1$ data for BaFe${}_2$As${}_2$ for different field orientations. We go on to show how a quantitative, angle-dependent theory for the relaxation rate leads to new ways of measuring the dynamical parameters of magnetic systems, in particular, the spin-wave velocities.

We investigate the transport properties of a bilayer exciton condensate that is contacted by four superconducting leads. We focus on the equilibrium regime and investigate how the Josephson currents induced in the bilayer by phase biases applied to the superconducting electrodes are affected by the presence of an exciton condensate in the bulk of the system. As long as the distance between the superconducting electrodes is much larger than the exciton coherence length, the Josephson current depends only on the difference between the phase biases in the two layers. This result holds true in both short- and long-junction limits. We relate it to a new correlated four-particle Andreev process, which occurs at the superconductor/exciton-condensate interface. The system we investigate provides an implementation of the supercurrent mirror proposed by Kitaev as a viable way to realize topologically protected qubits.

The amplitude mode is a ubiquitous collective excitation in condensed-matter systems with broken continuous symmetry. It is expected in antiferromagnets, short coherence length superconductors, charge density waves, and lattice Bose condensates. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the amplitude mode can decay into low-energy Goldstone modes, its experimental visibility has been questioned. Here we show that the visibility depends on the symmetry of the measured susceptibility. The longitudinal susceptibility diverges at low frequency as $\mathsf{Im}{\chi }_{\sigma \sigma }\sim {\omega }^{-1}$ $(d=2)$ or $\log (1/|\omega \left|\right)$ $(d=3)$, which can completely obscure the amplitude peak. In contrast, the scalar susceptibility is suppressed by four extra powers of frequency, exposing the amplitude peak throughout the ordered phase. We discuss experimental setups for measuring the scalar susceptibility. The conductivity of the $O\left(2\right)$ theory (relativistic superfluid) is a scalar response and therefore exhibits suppressed absorption below the Higgs mass threshold, $\sigma \sim {\omega }^{2d+1}$. In layered, short coherence length superconductors, (relevant, e.g., to cuprates) this threshold is raised by the interlayer plasma frequency.

Li and Haldane conjectured and numerically substantiated that the entanglement spectrum of the reduced density matrix of ground states of time-reversal-breaking topological phases [fractional quantum Hall (FQH) states] contains information about the counting of their edge modes when the ground state is cut in two spatially distinct regions and one of the regions is traced out. We analytically substantiate this conjecture for a series of FQH states defined as unique zero modes of pseudopotential Hamiltonians by finding a one-to-one map between the thermodynamic limit counting of two different entanglement spectra: the particle entanglement spectrum (PES), whose counting of eigenvalues for each good quantum number is identical to the counting of bulk quasiholes (up to accidental zero eigenvalues of the reduced density matrix), and the orbital entanglement spectrum (OES), considered by Li and Haldane. By using a set of clustering operators that have their origin in conformal-field-theory (CFT) operator expansions, we show that the counting of the OES eigenvalues in the thermodynamic limit must be identical to the counting of quasiholes in the bulk. The latter equals the counting of edge modes at a hard-wall boundary placed on the sample. Our results can be interpreted as a bulk-edge correspondence in entanglement spectra. Moreover, we show that the counting of the PES and OES is identical even for CFT states that are likely bulk gapless, such as the Gaffnian wave function.

We consider spin-1 Haldane chains with single-ion anisotropy, which exists in known Haldane chain materials. We develop a perturbation theory in terms of anisotropy, where the magnon-magnon interaction is important even in the low-temperature limit. The exact two-particle form factor in the $O\left(3\right)$ nonlinear sigma model leads to quantitative predictions on several dynamical properties, including the dynamical structure factor and electron-spin-resonance frequency shift. These agree very well with numerical results, and with experimental data on the Haldane chain material Ni(C${}_5$H${}_{14}$N${}_2$)${}_2$N${}_3$(PF${}_6$).

We measure the statistical polarization of quadrupolar nuclear spins in a submicrometer ($0.6\mu {\mathrm{m}}^3$) particle of GaAs using magnetic resonance force microscopy. The crystalline sample is cut out of a GaAs wafer and attached to a micromechanical cantilever force sensor using a focused ion beam technique. Nuclear magnetic resonance is demonstrated on ensembles containing less than $5\times {10}^8$ nuclear spins and occupying a volume of around ${\left(300\text{nm}\right)}^3$ in GaAs with reduced volumes possible in future experiments. We discuss how the further reduction of this detection volume will bring the spin ensembles into a regime where random spin fluctuations, rather than Boltzmann polarization, dominate their dynamics. The detection of statistical polarization in GaAs therefore represents an important first step toward 3D magnetic resonance imaging of III-V structures on the nanometer scale.

Recently, much progress has been made in applying field theory methods, first developed to study x-ray edge singularities, to interacting one-dimensional systems in order to include band curvature effects and study edge singularities at arbitrary momentum. Finding experimental confirmations of this theory remains an open challenge. Here, we point out that spin chains with uniform Dzyaloshinskii-Moriya interactions provide an opportunity to test these theories since these interactions may be exactly eliminated by a gauge transformation that shifts the momentum. However, this requires an extension of these x-ray edge methods to the transverse spectral function of the $XXZ$ spin chain in a magnetic field, which we provide.

We report the full complex dielectric function of high-purity ${\text{Ba}}_{0.68}{\text{K}}_{0.32}{\text{Fe}}_2{\text{As}}_2$ single crystals with $T_{\mathrm{c}}=38.5\text{K}$ determined by wideband spectroscopic ellipsometry at temperatures $10\le T\le 300\text{K}$. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies, $2{\Delta }_{\mathrm{A}}\approx 6k_{\mathrm{B}}{T}_{\mathrm{c}}$ and $2{\Delta }_{\mathrm{B}}\approx 2.2k_{\mathrm{B}}{T}_{\mathrm{c}}$. The observed unusual suppression of the optical conductivity in the superconducting state at energies up to $14k_{\mathrm{B}}{T}_{\mathrm{c}}$ can be ascribed to spin-fluctuation–assisted processes in the clean limit of the strong-coupling regime.