A set of new excitonic recombinations is observed in arsenic-implanted diamond. It is composed of two groups of emissions at 5.355/5.361 eV and at 5.215/5.220/5.227 eV. They are respectively attributed to the no-phonon and transverse-optical phonon-assisted recombinations of excitons bound to neutral arsenic donors. From the Haynes rule, an ionization energy of 0.41 eV is deduced for arsenic in diamond, which shows that arsenic is a shallower donor than phosphorus (0.6 eV), in agreement with theory.

We investigate the structural properties of Ge nanostructures selectively grown on Si. Defect-free nanostructures with a lateral size of 100 nm and surrounded by a thick (∼20 times larger than the coherent-film limit) Ge layer are achieved as demonstrated by transmission electron microscopy. As demonstrated by modeling based on elasticity theory solved by finite element methods, the peculiar combination of morphology and chemical composition of the nanostructures allows for a very efficient elastic relaxation of the heteroepitaxial strain. We demonstrate that, despite the relatively large size of the nanostructures, even a single dislocation would raise the energy of the system. A direct comparison between the strain field predicted by modeling and measured by energy-dispersive synchrotron-radiation grazing incidence x-ray diffraction shows substantial agreement.

We report a study of the basal-plane anisotropy of the superfluid density in underdoped YBa${}_2$Cu${}_4$O${}_8$ (Y124), showing the effects of both the CuO${}_2$ planes and the fully occupied CuO chains. From small-angle neutron scattering measurements of the vortex lattice, we can infer the superconducting (SC) properties for a temperature ($T$) range $T$ = 1.5 K to $T_{\text{c}}$ and magnetic induction $B$ from 0.1 to 6 T. We find that the superfluid density along a has a simple $d$-wave $T$ dependence. However, along b (the chain direction) the superfluid density falls much more rapidly with $T$ and also with increasing field. This strongly suggests the suppression of proximity-effect-induced superconductivity in the CuO chains. In addition, our results do not support a common framework for the low-field in-plane SC response in Y124 and related YBa${}_2$Cu${}_3$O${}_7$, and also indicate that any magnetic field induced charge-density-wave order in Y124 exists only for fields above 6 T.

Based on the first-principles band structure calculations, we investigate the effects of hydrostatic pressure on the conventional insulator (CI) Sb${}_2$Se${}_3$ and predict that it undergoes a topological quantum phase transition from a CI to a nontrivial topological insulator at a critical pressure value. The pressure-induced topological quantum phase transition is confirmed by calculating the evolution of the bulk energy gap as a function of pressure, the inversion of energy band structure, and the $Z_2$ topological invariant, as well as the existence of the Dirac-like topological surface states. Our predictions can be tested by both spectroscopy and transport experiments.

We study the fluorescence emission from single dye molecules in coplanar plasmonic cavities composed of a thin gold film surrounded by two in-plane surface plasmon Bragg mirrors. We first discuss the effect of the presence of Bragg mirrors on the radiation diagram of surface plasmon coupled emission. Then, we investigate the role of the planar cavity size by single-molecule fluorescence lifetime imaging. Experimental data are compared to numerical simulations of the decay rates calculated as a function of the molecule orientation and position within the cavity. The creation of new decay channels by coupling to the cavity modes is also discussed. We measure a plasmonic Purcell factor up to five, attributed to the enhancement of the radiative rate.

We developed a quantitative approach for the determination of molecular arrangement and electronic structure in anisotropic organic ultrathin films based on the measurement of polarized reflectivity at the carbon K-edge. The reflectivity spectra were fitted to a parameterized model calculation. The method was applied to a self-assembled monolayer of 1,4-benzenedimethanethiol on gold. To simulate reflectivity, the organic anisotropic film was described by a dielectric tensor, obtained by ab initio calculations for the single molecule and suitable rotations to describe the molecular organization in film domains. Film structure was obtained though the best fit of the simulation to the experiment. Results were consistent with a monolayer-thick film composed of domains of molecules with in-plane isotropic distribution of orientations. In each domain, molecules adopted a standing configuration, with a tilt of 28° relative to the substrate normal. Information on the modification of the molecular electronic states due to chemical bonding was derived.

The magnetic structure of LiFeAs${}_2$O${}_7$ (monoclinic, space group C2) has been solved using neutron powder diffraction. This compound presents an antiferromagnetic behavior characterized by a long-range ordering observed in the neutron diffraction patterns below the Néel temperature ($T_N$ = 35 K). The magnetic structure is found to be incommensurate with respect to the nuclear structure, the magnetic peaks being indexed with a propagation vector k = (0.709, 0, 0.155). The magnetic moments form a general spiral (helical-cycloidal) arrangement with a constant magnetic moment of 4.21 μ. The magnetic structure is discussed in terms of super-super exchange interactions involving two oxygen atoms belonging to an AsO${}_4$ tetrahedron, and compared with the magnetic structure of the di-phosphate analogue LiFeP${}_2$O${}_7$. The presence of triangular super-super exchange paths is believed to be at the origin of this incommensurate magnetic structure. The potential of LiFeAs${}_2$O${}_7$ as a possible multiferroic material is discussed.

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high-temperature superconductor ${\text{YBa}}_2{\text{Cu}}_3{O}_{6+x}$ ($x=0.75$, 0.6, 0.55, and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa${}_2$${\text{Cu}}_3$O${}_{6.99}$, additional Raman-active modes appear upon cooling below $\sim$170–200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering thus has the potential to explore the evolution of this state under extreme conditions.

The question of whether spatially inhomogeneous hopping in the two dimensional Hubbard model can lead to enhancement of superconductivity has been tackled by a number of authors in the context of the checkerboard Hubbard model (CHM). We address the effects of disorder on superconducting properties of the CHM by using exact diagonalization calculations for both potential and hopping disorder. We characterize the superconducting tendencies of the model by focusing on the pair-binding energy, the spin gap, and $d$-wave pairing order parameter. We find that superconducting tendencies, particularly the pair-binding energy, are more robust to disorder when there is inhomogeneous hopping than for the uniform Hubbard model. We also study all possible staggered potentials for an eight-site CHM cluster and relate the behavior of these configurations to the disordered system.

We report the successful synthesis of single crystals of the layered iridate (Na${}_{1-x}$Li${}_x$)${}_2$IrO${}_3$, $0\le x\le 0.9$, and a thorough study of its structural, magnetic, thermal, and transport properties. This compound allows a controlled interpolation between Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$, while maintaining the quantum magnetism of the honeycomb Ir${}^{4+}$ planes. The measured phase diagram demonstrates a suppression of the Néel temperature $T_N$ at an intermediate $x$, indicating that the magnetic orders in Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$ are distinct. X-ray data show that for $x\approx 0.7$, when $T_N$ is suppressed the most, the honeycomb structure is least distorted, leading to the speculation that at this intermediate doping of the material is closest to the spin liquid that has been sought after in Na${}_2$IrO${}_3$ and Li${}_2$IrO${}_3$. By analyzing our magnetic data with a single-ion theoretical model we also show that the trigonal splitting on the Ir${}^{4+}$ ions changes sign from Na${}_2$IrO${}_3$ to Li${}_2$IrO${}_3$.

Using neutron diffraction and the pair density function analysis, the local atomic structure of the three-dimensional ${\text{Bi}}_2$Se${}_{3-x}$Te${}_x$ ($x=0$, 1, 2, and 3) topological insulator is investigated. The substitution of Te for Se in Bi${}_2$Se${}_{3-x}$Te${}_x$ ($x=0$, 1, 2, and 3) is not random and its preferred site is at the edges of the quintuple layer. This generates a local strain due to the atom size mismatch between Se and Te. The site preference is surprising given that the Bi to chalcogen bonds are strongest when the ions are at the edges than in the middle layer. The (Se/Te) atoms in the middle sublayer of the quintuple are coupled more softly to the Bi atoms than those of the edges and have lower Debye temperatures. This suggests that the atomic properties within the quintuple layer are different than those at the edges. Additionally, the results from band structure and density of state calculations are reported to show the dependence of doping and temperature.

We have studied local structure of ${\text{IrTe}}_2$ by Ir $L$${}_3$-edge extended x-ray absorption fine structure (EXAFS) measurements as a function of temperature to investigate origin of the observed structural phase transition at $T_s\sim 270$ K. The EXAFS results show an appearance of longer Ir-Te bond length ($\Delta R\sim 0.05$ Å) at $T<T_s$. We have found Ir-Ir dimerization, characterized by distinct Ir-Ir bond lengths ($\Delta R\sim 0.13$ Å), existing both above and below $T_s$. The results suggest that the phase transition in IrTe${}_2$ should be an order-disorder-like transition of Ir-Ir dimers assisted by Ir-Te bond correlations, thus indicating important role of the interaction between the Ir $5d$ and Te $5p$ orbitals in this transition.

Progress in the emerging field of engineered quantum systems requires the development of devices that can act as quantum memories. The realization of such devices by doping solid-state cavities with paramagnetic ions imposes a tradeoff between ion concentration and cavity coherence time. Here, we investigate an alternative approach involving interactions between photons and naturally occurring impurity ions in ultrapure crystalline microwave cavities exhibiting exceptionally high quality factors. We implement a hybrid whispering gallery/electron spin resonance method to perform rigorous spectroscopy of an undoped single-crystal sapphire resonator over the frequency range 8–19 GHz, and at external applied DC magnetic fields up to 0.9 T. Measurements of high-purity sapphire cooled close to 100 mK reveal the presence of Fe${}^{3+}$, Cr${}^{3+}$, and V${}^{2+}$ impurities. A host of electron transitions are measured and identified, including the two-photon classically forbidden quadrupole transition ($\Delta m_s=2$) for Fe${}^{3+}$, as well as hyperfine transitions of V${}^{2+}$.

The magnetic penetration depth $\lambda$ as a function of temperature in Bi${}_4$O${}_4$S${}_3$ was studied by muon-spin-spectroscopy measurements. The superfluid density of Bi${}_4$O${}_4$S${}_3$ is found to be very low. The dependence of ${\lambda }^{-2}$ on temperature possibly suggests the existence of two $s$-wave-type energy gaps with the zero-temperature values of 0.93 (3) and 0.09 (4) meV. The upturn in the temperature dependence of the upper critical field close to $T_{\mathrm{c}}$ further supports multigap superconductivity in Bi${}_4$O${}_4$S${}_3$. The presence of two superconducting energy gaps is consistent with theoretical and other experimental studies. However, a single-gap $s$-wave model fit with a gap of 0.88 (2) meV cannot be ruled out completely. The value of $\lambda \left(T\right)$ at $T=0$ K is estimated to be $\lambda \left(0\right)=861\left(17\right)$ nm, one of the largest of all known layered superconductors, reflecting a very low superfluid density.

We study collective excitations in three- and four-band superconductors with interband frustration, which causes neither 0 nor $\pi$ interband phases in the superconducting state. Using a low-energy spin Hamiltonian originating from a multiband tight-binding model, we find that mass reduction of a Leggett mode occurs in a wide parameter region of this four-band system. As a limiting case, we have a massless Leggett mode. This massless mode is related to the fact that the mean-field energy does not depend on a relative phase of superconducting order parameters. In other words, we find a link of the massless mode with a degeneracy between a time-reversal-symmetry-breaking state (neither 0 nor $\pi$ phases) and a time-reversal-symmetric state (either 0 or $\pi$ phases). Therefore, the mass of collective modes characterizes well the time-reversal symmetry in frustrated multiband superconductors.

We have investigated the electronic structure of superconducting (SC) and nonsuperconducting (non-SC) K${}_x$Fe${}_{2-y}$Se${}_2$ using x-ray photoemission spectroscopy (XPS). The spectral shape of the Fe 2$p$ XPS is found to depend on the amount of Fe vacancies. The Fe 2$p_{3/2}$ peak of the SC and non-SC Fe-rich samples is accompanied by a shoulder structure on the lower binding energy side, which can be attributed to the metallic phase embedded in the Fe${}^{2+}$ insulating phase. The absence of the shoulder structure in the non-SC Fe-poor sample allows us to analyze the Fe 2$p$ spectra using a FeSe${}_4$ cluster model. The Fe 3$d$-Se 4$p$ charge-transfer energy of the Fe${}^{2+}$ insulating phase is found to be $\sim$2.3 eV which is smaller than the Fe 3$d$-Fe 3$d$ Coulomb interaction of $\sim$3.5 eV. This indicates that the Fe${}^{2+}$ insulating state is the charge-transfer type in the Zaanen-Sawatzky-Allen scheme. We also find a substantial change in the valence-band XPS as a function of Fe content and temperature. The metallic state at the Fermi level is seen in the SC and non-SC Fe-rich samples and tends to be enhanced with cooling in the SC sample.

Effects of impurities and disorder on transport properties by electronic quasiparticles in superconducting iron pnictides are theoretically considered. The most prominent new features compared to the case of pure material should appear at high enough impurity concentration when a specific narrow band of conducting quasiparticle states can develop within the superconducting gap, around the position of localized impurity level by a single impurity center. The predicted specific threshold effects in the frequency-dependent optical conductivity and temperature-dependent thermal conductivity and also in Seebeck and Peltier coefficients can have interesting potentialities for practical applications.

We investigate nonlinear thermoelectric transport through quantum impurity systems with strong on-site interactions. We show that the steady-state transport through interacting quantum impurities in contact with electron reservoirs at significantly different temperatures can be captured by an effective-equilibrium density matrix, expressed compactly in terms of the Lippmann-Schwinger operators of the system. In addition, the reservoirs can be maintained at arbitrary chemical potentials. The interplay between the temperature gradient and bias voltage gives rise to a nontrivial breaking of particle-hole symmetry in the strongly correlated regime, manifest in the Abrikosov-Suhl localized electron resonance. This purely many-body effect, which is in agreement with experimental results, is beyond the purview of mean-field arguments.

Understanding and manipulating spin polarization and transport in the vicinity of semiconductor-hosted defects is a problem of present technological and fundamental importance. Here, we use high-field magnetic resonance to monitor the relaxation dynamics of spin-3/2 nuclei in semi-insulating GaAs. Our experiments benefit from the conditions created in the limit of low illumination intensities, where intermittent occupation of the defect site by photoexcited electrons leads to electric field gradient fluctuations and concomitant spin relaxation of the neighboring quadrupolar nuclei. We find indication of a heterogeneous distribution of polarization, governed by different classes of defects activated by either weak or strong laser excitation. Upon application of a train of light pulses of variable repetition rate and on/off ratio, we uncover an intriguing regime of mesoscale nuclear spin diffusion restricted by long-range, nonuniform electric field gradients. Given the slow time scale governing nuclear spin evolution, such optically induced polarization patterns could be exploited as a contrast mechanism to expose dark lattice defects or localized charges with nanoscale resolution.

We present an alternative methodology for calculating the quasiparticle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (${\text{C}}_6{\text{H}}_6$) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates [pristine/doped graphene or (jellium) metal surface] reduces the quasiparticle highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active ($E_{1u}$) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as $\Gamma \approx 174$ meV for pristine graphene and $\Gamma \approx 362$ meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the $E_{1u}$ exciton decay rate.