Electronic transitions of N-O related shallow donors in silicon doped with nitrogen and oxygen have been studied under uniaxial stress by Fourier transform spectroscopy in the far infrared. In first approximation, the stress dependence of the hydrogeniclike transitions is described by ground and excited state wave functions belonging to the $T_2$ representation of the tetrahedral group. Small splittings of the lines originate from the lifting of orientational degeneracy of low-symmetry centers. Nonlinear stress responses of some of the transitions are attributed to interactions with higher-lying $1s$ states. Full analysis of the two most prominent shallow donors N-O-3 and N-O-5 in terms of selection rules and line intensities under polarized light prove that the electronic states are perturbed by a defect potential of apparent $C_{2\text{v}}$ symmetry, related to centers with an alignment along the $⟨110⟩$ and a twofold symmetry axis along the $⟨001⟩$ lattice direction. The donor wave function itself can be constructed from Bloch states belonging to a single pair of conduction band minima. It is concluded that present theoretical models for the microstructure of these complexes must be reconsidered, taking into account that any deviations from $C_{2\text{v}}$ symmetry must be small and have no measurable effect on the stress pattern.

We report electron paramagnetic resonance (EPR) experiments of phosphorus donors in isotopically controlled silicon single crystals. By varying the concentration of the ${}^{29}\text{S}\text{i}$ isotope, $f$, from 0.075% to 99.2%, we systematically study the effect of the environmental nuclear spins on the donor-electron spin. We find excellent agreement between experiment and theory for decoherence times due to nuclear-induced spectral diffusion, clarifying that the nuclear-induced decoherence is dominant in the range of $f$ studied. We also observe that the EPR linewidth shows a transition from the square-root dependence to the linear dependence on $f$, in agreement with theoretical predictions.

We show that a clear experimental test can distinguish between geminate and nongeminate recombination in low mobility semiconductors. For the particular case of the organic solar cell, the relative contribution of geminate recombination can be determined by measuring transient photoconductivity versus applied voltage. Measurements carried out at room temperature and 200 K on bulk heterojunction organic solar cells fabricated with two different semiconducting polymers show that neither exhibits significant geminate recombination.

We present a first principles study of an unusual heterostructure, an atomic-scale checkerboard of ${\text{BiFeO}}_3{\text{-BiMnO}}_3$, and compare its properties to the two bulk constituent materials, ${\text{BiFeO}}_3$ and ${\text{BiMnO}}_3$. The “nanocheckerboard” is found to have a multiferroic ground state with the desired properties of each constituent: polar and ferrimagnetic due to ${\text{BiFeO}}_3$ and ${\text{BiMnO}}_3$, respectively. The effect of B-site cation ordering on magnetic ordering in the ${\text{BiFeO}}_3{\text{-BiMnO}}_3$ system is studied. The checkerboard geometry is seen to give rise to a magnetostructural effect that is neither present in the bulk constituent materials, nor in the layered ${\text{BiFeO}}_3{\text{-BiMnO}}_3$ superlattice.

A polycrystalline test structure comprising a 5 nm cobalt and a 10 nm nickel/iron layer separated by a silicon layer ranging from 1.5 to 4 nm prepared by thermal evaporation has been investigated by resonant magnetic reflectivity measurements of horizontally polarized light in the extreme ultraviolet spectral range. By exploiting the transversal magneto-optical Kerr effect at the $M$ absorption edges of cobalt and nickel (59.5 eV and 66.5 eV) a magnetic contrast as large as 80% for cobalt and 25% for nickel can be obtained near a Brewster angle of about $45°$. Angle- and energy-dependent scans of the magnetic asymmetry as well as element-selective, magneto-optical loops of the hysteresis were recorded against the thickness of the interlayer, reflecting the switching behavior of the individual ferromagnetic layers as a function of the interlayer coupling.

The study of strongly correlated quantum gases in two dimensions has important ramifications for understanding many intriguing phenomena in solid materials, such as high-$T_c$ superconductivity and the fractional quantum-Hall effect. However, theoretical methods are plagued by the existence of significant quantum fluctuations. Here, we present two- and three-body exact solutions for both fermions and bosons trapped in a two-dimensional harmonic potential with an arbitrary $s$-wave scattering length. These few-particle solutions link in a natural way to the high-temperature properties of many-particle systems via a quantum virial expansion. As a concrete example, using the energy spectrum of few fermions, we calculate the second and third virial coefficients of a strongly interacting Fermi gas in two dimensions, and consequently investigate its high-temperature thermodynamics. Our thermodynamic results may be useful for ongoing experiments on two-dimensional Fermi gases. These exact results also provide an unbiased benchmark for quantum Monte Carlo simulations of two-dimensional Fermi gases at high temperatures.

We perform a quantum simulation of the Ising model with a transverse field using a collection of three trapped atomic ion spins. By adiabatically manipulating the Hamiltonian, we directly probe the ground state for a wide range of fields and form of the Ising couplings, leading to a phase diagram of magnetic order in this microscopic system. The technique is scalable to much larger numbers of trapped ion spins, where phase transitions approaching the thermodynamic limit can be studied in cases where theory becomes intractable.

We have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. Our results are compatible with hydrogen adsorbates inducing intervalley mixing by exerting a short-range scattering potential. The saturation coverages for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach $1.5\times {10}^4{\text{cm}}^2/\text{V}\text{s}$ in the absence of the hydrogen-adsorbing sites. This affinity to hydrogen is the signature of the most dominant type of native scatterers in graphene-based field-effect transistors on ${\text{SiO}}_2$.

The onset of antiferromagnetic order in ${\text{URu}}_2{\text{Si}}_2$ has been studied via neutron diffraction in a helium pressure medium, which most closely approximates hydrostatic conditions. The antiferromagnetic critical pressure is 0.80 GPa, considerably higher than values previously reported. Complementary electrical resistivity measurements imply that the hidden-order-antiferromagnetic bicritical point falls between 1.3 and 1.5 GPa. Moreover, the redefined pressure-temperature phase diagram suggests that the superconducting and antiferromagnetic phase boundaries actually meet at a common critical pressure at zero temperature.

Harmonically trapped ultracold atoms and ${}^4\text{H}\text{e}$ in nanopores provide different experimental realizations of bosons in one dimension, motivating the search for a more complete theoretical understanding of their low-energy properties. Worm algorithm path-integral quantum Monte Carlo results for interacting bosons restricted to the one dimensional continuum are compared to the finite temperature and system size predictions of Luttinger-liquid theory. For large system sizes at low temperature, excellent agreement is obtained after including the leading irrelevant interactions in the Hamiltonian which are determined explicitly.

We measured the electronic structure of an iron arsenic parent compound LaFeAsO using angle-resolved photoemission spectroscopy (ARPES). By comparing with a full-potential linear augmented plane wave calculation we show that the extra large $\Gamma$ hole pocket measured via ARPES comes from electronic structure at the sample surface. Based on this we discuss the strong-polarization dependence of the band structure and a temperature-dependent holelike band around the $M$ point. The two phenomena give additional evidences for the existence of the surface-driven electronic structure.

We describe the formation of one-dimensional lithium wires on a Cu(001) substrate, providing an atomic-scale description of the onset of metallization in this prototypical adsorption system. A combination of helium atom scattering and density-functional theory reveals pronounced changes in the electronic charge distribution on the formation of the $c\left(5\sqrt{2}\times \sqrt{2}\right)R45°$ Li/Cu(001) structure, as in-plane bonds are created. Charge donation from Li-substrate bonds is found to facilitate the formation of stable, bonded, and depolarized chains of Li adatoms that coexist with an interleaved phase of independent adatoms. The resultant overlayer has a commensurate charge distribution and lattice modulations but differs fundamentally from structurally similar charge-density wave systems.

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific-heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at $T_S\sim 142\text{K}$ and the onset of antiferromagnetic (AFM) Fe order below $T_N\sim 137\text{K}$. The Fe moments order AFM in the well-known stripelike structure in the $\left(ab\right)$ plane but change from AFM to ferromagnetic (FM) arrangement along the $c$ direction below $T^{\ast }\sim 15\text{K}$ accompanied by the onset of Nd AFM order below $T_{\text{Nd}}\sim 6\text{K}$ with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of $c$-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.

The structure of siderite, ${\text{FeCO}}_3$, was determined to 56 GPa, beyond the spin-pairing transition of its iron $d$ electrons. ${\text{Fe}}^{2+}$ in the siderite structure is in the high-spin state at low pressures and transforms to the low-spin (LS) state over a narrow pressure range, 44 to 45 GPa, that is concomitant with a shrinkage of the octahedral bond distance by 4%, and a volume collapse of 10%. The structural rearrangements associated with the electronic transition are nearly isotropic in contrast with other properties of siderite, which mostly are highly anisotropic. Robust refinements of the crystal structure from single-crystal x-ray diffraction data were performed at small pressure intervals in order to accurately evaluate the variation in the interatomic distances and to define the geometry of the carbonate hosting ${\text{LS-Fe}}^{2+}$. Thermal vibrations are remarkably lowered in the LS-Sd as shown by atomic displacement parameters. The formation of like-spin domains at the transition shows a hysteresis of more than 3 GPa, compatible with a strong cooperative contribution of neighboring clusters to the transition.

We explain recent experimental observations on effective charge of edge states tunneling through a quantum point contact in the weak backscattering regime. We focus on the behavior of the excess noise and on the effective tunneling charge as a function of temperature and voltage. By introducing a minimal hierarchical model different filling factors, $\nu =p/\left(2p+1\right)$, in the Jain sequence are treated on equal footing, in presence also of nonuniversal interactions. The agreement found with the experiments for $\nu =2/3$ and $\nu =2/5$ reinforces the description of tunneling of bunching of quasiparticles at low energies and quantitatively defines the condition under which one expects to measure the fundamental quasiparticle charge. We propose high-order current cumulant measurement to cross-check the validity of the above scenario and to better clarify the peculiar temperature behavior of the effective charges measured in the experiments.

Low-temperature scanning tunneling microscopy and density-functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with $\left(2\times 1\right)$ periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the $\sim 200\text{meV}$ water-water H bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a $\left(4\times 2\right)$ superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen-honeycomb structure, which is metastable relative to the original $\left(2\times 1\right)$. This stable structure is not recovered until after heating to temperatures close to 260 K.

We use the time-resolved magneto-optical Kerr effect to measure the local temperature and heat-flow dynamics in ferromagnetic ${\text{SrRuO}}_3$ thin films. After heating by a pump pulse, the film temperature decays exponentially, indicating that the heat flow out of the film is limited by the film/substrate interface. We show that this behavior is consistent with an effective boundary resistance resulting from disequilibrium between the spin and phonon temperatures in the film.

We investigate bilayer graphene transport in the presence of electron-hole puddles induced by long-range charged impurities in the environment. We explain the insulating behavior observed in the temperature-dependent conductivity of low mobility bilayer graphene using an analytic statistical theory taking into account the non-mean-field nature of transport in the highly inhomogeneous density and potential landscape. We find that the puddles can induce, even far from the charge neutrality point, a coexisting metallic and insulating transport behavior due to the random local activation gap in the system.

We study electronic-transport phenomena in a Fabry-Perot interferometer in the fractional quantum Hall regime in two limits. We analyze the lowest-order interference pattern in a nearly open interferometer (weak-backscattering limit) and the temperature dependence of the Coulomb-blockade transmission peaks in a nearly closed interferometer (strong-backscattering limit). For both limits, we consider two series of fractional quantized Hall states, one with Abelian and one with non-Abelian quasiparticles. We show that the results obtained in the two limits give identical information about the quasiparticle statistics. Although the experimental signatures of the Abelian and non-Abelian states may be similar in some circumstances, we argue that the two cases may be distinguished due to the sensitivity of the Abelian states to local perturbations, to which the non-Abelian states are insensitive.

This work demonstrates a surface scanning method for the quantitative determination of the local pyroelectric coefficient in ferroelectric thin films. It allows generating a map of the pyroelectric response with very high spatial resolution. In domains of previously aligned dipole moments (achieved by tip-assisted poling) small heat fluctuations are achieved by laser diode excitation from the sample’s bottom side thus inducing changes in the surface potential due to the pyroelectric effect. Simultaneously, the surface potential variations are detected by scanning surface potential microscopy thus forming the base for the pyroelectric coefficient map. The potential of the method is demonstrated on the basis of ferroelectric semicrystalline copolymer thin films yielding local maxima of the pyroelectric coefficients up to $40\mu \text{C}/{\text{m}}^2\text{K}$. Another promising feature is the ability to visualize “screened” polarization thus enabling in-depth profiling of polarization distributions and domain formation and to study the composition dependence of ferroelectric nanodomains.