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Perovskite oxides are already widely used in industry and have huge potential for novel device applications thanks to the rich physical behaviour displayed in these materials. The key to the functional electronic properties exhibited by perovskites is often the so-called Jahn-Teller distortion. For applications, an electrical control of the Jahn-Teller distortions, which is so far out of reach, would therefore be highly desirable. Based on universal symmetry arguments, we determine new lattice mode couplings that can provide exactly this paradigm, and exemplify the effect from first-principles calculations. The proposed mechanism is completely general, however for illustrative purposes, we demonstrate the concept on vanadium based perovskites where we reveal an unprecedented orbital ordering and Jahn-Teller induced ferroelectricity. Thanks to the intimate coupling between Jahn-Teller distortions and electronic degrees of freedom, the electric field control of Jahn-Teller distortions is of general relevance and may find broad interest in various functional devices., Comment: The first two authors contributed equally

Architectural constructs are engineered to impart desirable mechanical properties facilitating bridges spanning a thousand meters and buildings nearly 1 km in height. However, do the same 'engineering-rules' translate to the nanoscale, where the architectural features are less than 0.0001 mm in size? Here, we calculate the mechanical properties of a porous ceramic functional material, ceria, as a function of its nanoarchitecture using molecular dynamics simulation and predict its yield strength to be almost two orders of magnitude higher than the parent bulk material. In particular, we generate models of nanoporous ceria with either a hexagonal or cubic array of one-dimensional pores and simulate their responses to mechanical load. We find that the mechanical properties are critically dependent upon the orientation between the crystal structure (symmetry, direction) and the pore structure (symmetry, direction). This journal is

We study a quantum Hall bilayer system of bosons at total filling factor $\nu = 1$, and study the phase that results from short ranged pair-tunneling combined with short ranged interlayer interactions. We introduce two exactly solvable model Hamiltonians which both yield the coupled Moore-Read state [Phys.~Rev.~Lett.~{\bf 108}, 256809 (2012)] as a ground state, when projected onto fixed particle numbers in each layer. One of these Hamiltonians describes a gapped topological phase while the other is gapless. However, on introduction of a pair tunneling term, the second system becomes gapped and develops the same topological order as the gapped Hamiltonian. Supported by the exact solution of the full zero-energy quasihole spectrum and a conformal field theory approach, we develop an intuitive picture of this system as two coupled composite fermion superconductors. In this language, pair tunneling provides a Josephson coupling of the superconducting phases of the two layers, and gaps out the Goldstone mode associated with particle transport between the layers. In particular, this implies that quasiparticles are confined between the layers. In the bulk, the resulting phase has the topological order of the Halperin 220 phase with $U(1)_2\times U(1)_2$ topological order, but it is realized in the symmetric/antisymmetric-basis of the layer index. Consequently, the edge spectrum at a fixed particle number reveals an unexpected $U(1)_4 \times U(1)$ structure., Comment: 20 pages, 4 figures, 3 tables

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated an enormous activity due to the variety of interesting effects it gives rise to. A case in point is the discovery of the electron gas at the interface between LaAlO3 and SrTiO3, which has since been shown to be quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite these findings, the origin of the two-dimensional electron gas is highly debated and several possible mechanisms remain. Here we review the main proposed mechanisms and attempt to model expected effects in a quantitative way with the ambition of better constraining what effects can/cannot explain the observed phenomenology. We do it in the framework of a phenomenological model for understanding electronic and/or redox screening of the chemical charge in oxide heterostructures. We also discuss the effect of intermixing, both conserving and non-conserving the total stoichiometry., 34 pages, 9 figures

Calcium phosphate based biomaterials are extensively used in the context of tissue engineering: small changes in composition can lead to significant changes in properties allowing their use in a wide range of applications. Samples of composition (Al(2)O(3))(x)(Na(2)O)(0.11-x)(CaO)(0.445)(P(2)O(5))(0.445), where x = 0, 0.03, 0.05, and 0.08, were prepared by melt quenching. The atomic-scale structure has been studied using neutron diffraction and solid state (27)Al MAS NMR, and these data have been rationalised with the determined density of the final glass product. With increasing aluminium concentration the density increases initially, but beyond about 3 mol. % Al(2)O(3) the density starts to decrease. Neutron diffraction data show a concomitant change in the aluminium speciation, which is confirmed by (27)Al MAS NMR studies. The NMR data reveal that aluminium is present in 4, 5, and 6-fold coordination and that the relative concentrations of these environments change with increasing aluminium concentration. Materials containing aluminium in 6-fold coordination tend to have higher densities than analogous materials with the aluminium found in 4-fold coordination. Thus, the density changes may readily be explained in terms of an increase in the relative concentration of 4-coordinated aluminium at the expense of 6-fold aluminium as the Al(2)O(3) content is increased beyond 3 mol. %.

Nickel and cobalt are both known to stimulate the hypoxia-inducible factor-1 (HIF-1α), thus significantly improving blood vessel formation in tissue engineering applications. We have manufactured nickel and cobalt doped bioactive glasses to act as a controlled delivery mechanism of these ions. The resultant structural consequences have been investigated using the methods of isotopic and isomorphic substitution applied to neutron diffraction. The structural sites present will be intimately related to their release properties in physiological fluids such as plasma and saliva, and hence the bioactivity of the material. Detailed structural knowledge is therefore a prerequisite for optimising material design. Results show that nickel and cobalt adopt a mixed structural role within these bioactive glasses occupying both network-forming (tetrahedral) and network-modifying (5-fold) geometries. Two thirds of the Ni (or Co) occupies a five-fold geometry with the remaining third in a tetrahedral environment. A direct comparison of the primary structural correlations (e.g. Si–O, Ca–O, Na–O and O–Si–O) between the archetypal 45S5 Bioglass® and the Ni and Co glasses studied here reveal no significant differences. This indicates that the addition of Ni (or Co) will have no adverse effects on the existing structure, and thus on in vitro/in vivo dissolution rates and therefore bioactivity of these glasses.

The aim was to determine the most effective calcium precursor for synthesis of sol-gel hybrids and for improving homogeneity of sol-gel bioactive glasses. Sol-gel derived bioactive calcium silicate glasses are one of the most promising materials for bone regeneration. Inorganic/organic hybrid materials, which are synthesized by incorporating a polymer into the sol-gel process, have also recently been produced to improve toughness. Calcium nitrate is conventionally used as the calcium source, but it has several disadvantages. Calcium nitrate causes inhomogeneity by forming calcium-rich regions, and it requires high temperature treatment (400 °C) for calcium to be incorporated into the silicate network. Nitrates are also toxic and need to be burnt off. Calcium nitrate therefore cannot be used in the synthesis of hybrids as the highest temperature used in the process is typically 40-60 °C. Therefore, a different precursor is needed that can incorporate calcium into the silica network and enhance the homogeneity of the glasses at low (room) temperature. In this work, calcium methoxyethoxide (CME) was used to synthesize sol-gel bioactive glasses with a range of final processing temperatures from 60 to 800 °C. Comparison is made between the use of CME and calcium chloride and calcium nitrate. Using advanced probe techniques, the temperature at which Ca is incorporated into the network was identified for 70S30C (70 mol % SiO(2), 30 mol % CaO) for each of the calcium precursors. When CaCl(2) was used, the Ca did not seem to enter the network at any of the temperatures used. In contrast, Ca from CME entered the silica network at room temperature, as confirmed by X-ray diffraction, (29)Si magic angle spinning nuclear magnetic resonance spectroscopy, and dissolution studies. CME should be used in preference to calcium salts for hybrid synthesis and may improve homogeneity of sol-gel glasses.

The atomic-scale structure of Bioglass and the effect of substituting lithium for sodium within these glasses have been investigated using neutron diffraction and solid state magic angle spinning (MAS) NMR. Applying an effective isomorphic substitution difference function to the neutron diffraction data has enabled the Na–O and Li–O nearest-neighbour correlations to be isolated from the overlapping Ca–O, O–(P)–O and O–(Si)–O correlations. These results reveal that Na and Li behave in a similar manner within the glassy matrix and do not disrupt the short range order of the network former. Residual differences are attributed solely to the variation in ionic radius between the two species. Successful simplification of the 2 < r (A) < 3 region via the difference method has enabled all the nearest neighbour correlations to be deconvolved. The diffraction data provides the first direct experimental evidence of split Na–O nearest-neighbour correlations in these melt quench bioactive glasses, and an analogous splitting of the Li–O correlations. The observed correlations are attributed to the metal ions bonded either to bridging or to non-bridging oxygen atoms. 23Na triple quantum MAS (3QMAS) NMR data corroborates the split Na–O correlations. The structural sites present will be intimately related to the release properties of the glass system in physiological fluids such as plasma and saliva, and hence to the bioactivity of the material. Detailed structural knowledge is therefore a prerequisite for optimizing material design.

We show how the phases of interacting particles in topological flat bands, known as fractional Chern insulators, can be adiabatically connected to incompressible fractional quantum Hall liquids in the lowest Landau-level of an externally applied magnetic field. Unlike previous evidence suggesting the similarity of these systems, our approach enables a formal proof of the equality of their topological orders, and furthermore this proof robustly extends to the thermodynamic limit. We achieve this result using the hybrid Wannier orbital basis proposed by Qi [Phys. Rev. Lett. 107, 126803 (2011)] in order to construct interpolation Hamiltonians that provide continuous deformations between the two models. We illustrate the validity of our approach for the groundstate of bosons in the half filled Chern band of the Haldane model, showing that it is adiabatically connected to the $\nu=1/2$ Laughlin state of bosons in the continuum fractional quantum Hall problem., Comment: 5 pages, 4 figures; v3: minor changes, as in final published version

We study correlated phases occurring in the flat lowest band of the dice lattice model at flux density one half. We discuss how to realize the dice lattice model, also referred to as the T_3 lattice, in cold atomic gases. We construct the projection of the model to the lowest dice band, which yields a Hubbard-Hamiltonian with interaction-assisted hopping processes. We solve this model for bosons in two limits. In the limit of large density, we use Gross-Pitaevskii mean-field theory to reveal time-reversal symmetry breaking vortex lattice phases. At low density, we use exact diagonalization to identify three stable phases at fractional filling factors \nu of the lowest band, including a classical crystal at \nu=1/3, a supersolid state at \nu=1/2 and a Mott insulator at \nu=1., Comment: 5 pages, 4 figures; v2: final published version, title changed

We provide numerical evidence that a p_{x}-i p_{y} paired Bonderson--Slingerland (BS) non-Abelian hierarchy state is a strong candidate for the observed nu=12/5 quantum Hall plateau. We confirm the existence of a gapped incompressible nu = 12/5 quantum Hall state with shift S=2 on the sphere, matching that of the BS state. The exact ground state of the Coulomb interaction at S=2 is shown to have large overlap with the BS trial wave function. Larger overlaps are obtained with BS-type wave functions that are hierarchical descendants of general p_{x}-i p_{y} weakly-paired states at nu=5/2. We perform a finite size scaling analysis of the ground state energies for nu=12/5 states at shifts corresponding to the BS (S=2) and 3-clustered Read-Rezayi (S=-2) universality classes. This analysis reveals very tight competition between these two non-Abelian topological orders., Comment: 5 pages, 2 figures; v2: minor corrections; v3: new title, revised exposition, inclusion of neutral gap and finite well-width results; v4: minor corrections

We use particle entanglement spectra to characterize bosonic quantum Hall states on lattices, motivated by recent studies of bosonic atoms on optical lattices. Unlike for the related problem of fractional Chern insulators, very good trial wavefunctions are known for fractional quantum Hall states on lattices. We focus on the entanglement spectra for the Laughlin state at $\nu=1/2$ for the non-Abelian Moore-Read state at $\nu=1$. We undertake a comparative study of these trial states to the corresponding groundstates of repulsive two-body or three-body contact interactions on the lattice. The magnitude of the entanglement gap is studied as a function of the interaction strength on the lattice, giving insights into the nature of Landau-level mixing. In addition, we compare the performance of the entanglement gap and overlaps with trial wavefunctions as possible indicators for the topological order in the system. We discuss how the entanglement spectra allow to detect competing phases such as a Bose-Einstein condensate., Comment: 12 pages, 9 figures

Against expectations, robust switchable ferroelectricity has been recently observed in ultrathin (1 nm) ferroelectric films exposed to air [V. Garcia $et$ $al.$, Nature {\bf 460}, 81 (2009)]. Based on first-principles calculations, we show that the system does not polarize unless charged defects or adsorbates form at the surface. We propose electrochemical processes as the most likely origin of this charge. The ferroelectric polarization of the film adapts to the bound charge generated on its surface by redox processes when poling the film. This, in turn, alters the band alignment at the bottom electrode interface, explaining the observed tunneling electroresistance. Our conclusions are supported by energetics calculated for varied electrochemical scenarios., Comment: Updated, 7 pages

We have employed large scale exact numerical diagonalization in Haldane spherical geometry in a comparative analysis of the correlated many-electron states in the half-filled low Landau levels of graphene and such conventional semiconductors as GaAs, including both spin and valley (i.e., pseudospin) degrees of freedom. We present evidence that the polarized Fermi sea of essentially non-interacting composite fermions remains stable against a pairing transition in both lowest Landau levels of graphene. However, it undergoes spontaneous depolarization, which in (ideal) graphene is unprotected for the lack of a single-particle pseudospin splitting. These results point to the absence of the non-Abelian Pfaffian phase in graphene.

The two-dimensional electron gas (2DEG) at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) has become one of the most fascinating and highly-debated oxide systems of recent times. Here we propose that a one-dimensional electron gas (1DEG) can be engineered at the step edges of the LAO/STO interface. These predictions are supported by first principles calculations and electrostatic modeling which elucidate the origin of the 1DEG as an electronic reconstruction to compensate a net surface charge in the step edge. The results suggest a novel route to increasing the functional density in these electronic interfaces., 4 pages, 3 figures

We studied charged excitations of the ν=5/2 fractional quantum Hall state allowing for spin depolarization. It is generally accepted that the ground state is a spin-polarized incompressible quantum liquid, adiabatically connected to the Pfaffian state, whose spin-polarized quasiholes (QHs) obey non-Abelian statistics. Using numerical diagonalization and taking account of non-zero well widths we demonstrated that at a sufficiently low Zeeman energy it is energetically favorable for pairs of charge e/4 QHs to bind into charge e/2 Skyrmions. We showed that Skyrmion formation is further promoted by disorder, and argue that this can lead to a depolarized ground state in realistic experimental situations. © 2011 American Institute of Physics.

We present evidence supporting the weakly paired Moore-Read phase in the half-filled second Landau level, focusing on some of the qualitative features of its excitations. Based on numerical studies, we show that systems with odd particle number at the flux N_\phi=2N-3 can be interpreted as a neutral fermion mode of one unpaired fermion, which is gapped. The mode is found to have two distinct minima, providing a signature that could be observed by photoluminescence. In the presence of two quasiparticles the same neutral fermion excitation is shown to be gapless, confirming expectations for non-Abelian statistics of the Ising model with degenerate fusion channels 1 and \psi.

We study theoretically the groundstates of two-dimensional Bose-Hubbard models which are frustrated by gauge fields. Motivated by recent proposals for the implementation of optically induced gauge potentials, we focus on the situation in which the imposed gauge fields give rise to a pattern of staggered fluxes, of magnitude $\alpha$ and alternating in sign along one of the principal axes. For $\alpha=1/2$ this model is equivalent to the case of uniform flux per plaquette $n_\phi=1/2$, which, in the hard-core limit, realizes the "fully frustrated" spin-1/2 XY model. We show that the mean-field groundstates of this frustrated Bose-Hubbard model typically break translational symmetry. We introduce a general numerical technique to detect broken symmetry condensates in exact diagonalization studies. Using this technique we show that, for all cases studied, the groundstate of the Bose-Hubbard model with staggered flux $\alpha$ is condensed, and we obtain quantitative determinations of the condensate fraction. We discuss the experimental consequences of our results. In particular, we explain the meaning of gauge-invariance in ultracold atom systems subject to optically induced gauge potentials, and show how the ability to imprint phase patterns prior to expansion can allow very useful additional information to be extracted from expansion images., Comment: 14 pages, 10 figures

We consider a two-dimensional model of noninteracting chains of spinless fermions weakly coupled via a small interchain hopping and a repulsive interchain interaction. The phase diagram of this model has a surprising feature: an abrupt change in the Fermi surface as the interaction is increased. We study in detail this metanematic transition and show that the well-known 2 1 2 -order Lifshitz transition is the critical end point of this first-order quantum phase transition. Furthermore, in the vicinity of the end point, the order parameter has a nonperturbative BCS-type form. We also study a competing crystallization transition in this model and derive the full phase diagram. This physics can be demonstrated experimentally in dipolar ultracold atomic or molecular gases. In the presence of a harmonic trap, it manifests itself as a sharp jump in the density profile. © 2010 The American Physical Society.

It is now well established that in two-dimensional chiral p-wave paired superfluids, the vortices carry zero-energy modes which obey non-abelian exchange statistics and can potentially be used for topological quantum computation. In such superfluids there may also exist other excitations below the bulk gap inside the cores of vortices. We study the properties of these subgap states, and argue that their presence affects the topological protection of the zero modes. In conventional superconductors where the chemical potential is of the order of the Fermi energy of a non-interacting Fermi gas, there is a large number of subgap states and the mini-gap towards the lowest of these states is a small fraction of the Fermi energy. It is therefore difficult to cool the system to below the mini-gap and at experimentally available temperatures, transitions between the subgap states, including the zero modes, will occur and can alter the quantum states of the zero-modes. We show that compound qubits involving the zero-modes and the parity of the occupation number of the subgap states on each vortex are still well defined. However, practical schemes taking into account all subgap states would nonetheless be difficult to achieve. We propose to avoid this difficulty by working in the regime of small chemical potential mu, near the transition to a strongly paired phase, where the number of subgap states is reduced. We develop the theory to describe this regime of strong pairing interactions and we show how the subgap states are ultimately absorbed into the bulk gap. Since the bulk gap vanishes as mu -> 0 there is an optimum value mu_c which maximises the combined gap. We propose cold atomic gases as candidate systems where the regime of strong interactions can be explored, and explicitly evaluate mu_c in a Feshbach resonant K-40 gas., 19 pages, 10 figures; v2: main text as published version, additional detail included as appendices

Based on the known physics of the excitonic superfluid or 111 state of the quantum Hall ��=1/2+1/2 bilayer, we create a simple trial wavefunction ansatz for constructing a low energy branch of (Goldstone) excitations by taking the overall ground state and boosting one layer with respect to the other. This ansatz works extremely well for any interlayer spacing. For small d this is simply the physics of the Goldstone mode, whereas for large d this is a reflection of composite fermion physics. We find hints that certain aspects of composite fermion physics persist to low d whereas certain aspects of Goldstone mode physics persist to high d. Using these results we show nonmonotonic behavior of the Goldstone mode velocity as a function of d., 6 pages, 4 figures

We report on exact-diagonalization studies of correlated many-electron states in the half-filled Landau levels of graphene, including pseudospin (valley) degeneracy. We demonstrate that the polarized Fermi sea of non-interacting composite fermions remains stable against a pairing transition in the lowest two Landau levels. However, it undergoes spontaneous depolarization, which is unprotected owing to the lack of single-particle pseudospin splitting. These results suggest the absence of the Pfaffian phase in graphene., Comment: 3 pages, 4 figures (revision: reference update)

Despite the numerous studies of bone mineral, there are still many questions regarding the exact structure and composition of the mineral phase, and how the mineral crystals become organised with respect to each other and the collagen matrix. Bone mineral is commonly formulated as hydroxyapatite, albeit with numerous substitutions, and has previously been studied by (31)P and (1)H NMR, which has given considerable insight into the complexity of the mineral structure. However, to date, there has been no report of an NMR investigation of the other major component of bone mineral, calcium, nor of common minority cations like sodium. Here, direct analysis of the local environment of calcium in two biological apatites, equine bone (HB) and bovine tooth (CT), was carried out using both (43)Ca solid state NMR and Ca K-edge X-ray absorption spectroscopy, revealing important structural information about the calcium coordination shell. The (43)Ca delta(iso) in HB and CT is found to correlate with the average Ca-O bond distance measured by Ca K-edge EXAFS, and the (43)Ca NMR linewidths show that there is a greater distribution in chemical bonding around calcium in HB and CT, compared to synthetic apatites. In the case of sodium, (23)Na MAS NMR, high resolution 3Q-MAS NMR, as well as (23)Na{(31)P} REDOR and (1)H{(23)Na} R(3)-HMQC correlation experiments give the first direct evidence that some sodium is located inside the apatite phase in HB and CT, but with a greater distribution of environments compared to a synthetic sodium substituted apatite (Na-HA).

The polar interface between LaAlO$_{3}$ and SrTiO$_{3}$ has shown promise as a field effect transistor, with reduced (nanoscale) feature sizes and potentially added functionality over conventional semiconductor systems. However, the mobility of the interfacial two-dimensional electron gas (2DEG) is lower than desirable. Therefore to progress, the highly debated origin of the 2DEG must be understood. Here we present a case for surface redox reactions as the origin of the 2DEG, in particular surface O vacancies, using a model supported by first principles calculations that describes the redox formation. In agreement with recent spectroscopic and transport measurements, we predict a stabilization of such redox processes (and hence Ti 3$d$ occupation) with film thickness beyond a critical value, which can be smaller than the critical thickness for 2D electronic conduction, since the surface defects generate trapping potentials that will affect the interface electron mobility. Several other recent experimental results, such as lack of core level broadening and shifts, find natural explanation. Pristine systems will likely require changed growth conditions or modified materials with a higher vacancy free energy., Comment: Revised version

We analyse an array of linearly extended monodomain dipoles forming square and kagome lattices. We find that its phase diagram contains two (distinct) finite-entropy kagome ice regimes - one disordered, one algebraic - as well as a low-temperature ordered phase. In the limit of the islands almost touching, we find a staircase of corresponding entropy plateaux, which is analytically captured by a theory based on magnetic charges. For the case of a modified square ice array, we show that the charges (`monopoles') are excitations experiencing two distinct Coulomb interactions: a magnetic `three-dimensional' one as well as a logarithmic `two dimensional' one of entropic origin.

We study charged excitations of the non-abelian Moore-Read liquid at filling factor nu=5/2, allowing for spin depolarization. Using a combination of numerical studies, and taking account of non-zero well widths, we find that at sufficiently low Zeeman energy it is energetically favourable for charge e/4 quasiholes to bind into "skyrmions" of charge e/2. We show that skyrmion formation is further promoted by disorder, and argue that this can lead to a depolarized nu=5/2 ground state in realistic experimental situations. We comment on the consequences for the activated transport., 4 pages, 3 figures

We study the groundstates of the Bose-Hubbard model in a uniform effective magnetic field, illustrating the physics of cold atomic gases on `rotating optical lattices'. Mapping the bosons to composite fermions leads to the prediction of quantum Hall fluids that have no counterpart in the continuum. We construct trial wavefunctions for these phases, and perform numerical tests of the predictions of the composite fermion model. Our results establish the existence of strongly correlated phases beyond those in the continuum limit, and provide evidence for a wider scope of the composite fermion approach beyond its application to the lowest Landau-level.

A class of melt quenched silicate glasses, containing calcium, phosphorus and alkali metals, and having the ability to promote bone regeneration and to fuse to living bone, is produced commercially as Bioglass. The changes in structure associated with reacting the bioglass with a body fluid simulant (a buffered Tris(hydroxymethyl)aminomethane growth medium solution or a blood plasma-like salt simulated body fluid) at 37°C have been studied using both high energy and grazing incidence x-ray diffraction. This has corroborated the generic conclusions of earlier studies based on the use of calcia–silica sol-gel glasses whilst highlighting the important differences associated with glass composition; the results also reveal the more subtle effects on reaction rates of the choice of body fluid simulant. The results also indicate the presence of tricalcium phosphate crystallites deposited onto the surface of the glass as a precursor to the growth of hydroxyapatite, and indicates that there is some preferred orientation to their growth.

We investigate, with the help of Monte-Carlo and exact-diagonalization calculations in the spherical geometry, several compressible and incompressible candidate wave functions for the recently observed quantum Hall state at the filling factor $��=1/4$ in a wide quantum well. The quantum well is modeled as a two-component system by retaining its two lowest subbands. We make a direct connection with the phenomenological effective-bilayer model, which is commonly used in the description of a wide quantum well, and we compare our findings with the established results at $��=1/2$ in the lowest Landau level. At $��=1/4$, the overlap calculations for the Halperin (5,5,3) and (7,7,1) states, the generalized Haldane-Rezayi state and the Moore-Read Pfaffian, suggest that the incompressible state is likely to be realized in the interplay between the Halperin (5,5,3) state and the Moore-Read Pfaffian. Our numerics shows the latter to be very susceptible to changes in the interaction coefficients, thus indicating that the observed state is of multicomponent nature., 14 pages, 8 figures; minor changes, accepted for publication in Phys. Rev. B

The physics of oxide superlattices is considered for pristine (001) multilayers of the band insulators LaAlO3 and SrTiO3 with alternating p and n interfaces. First principles results and a model of capacitor plates offer a simple paradigm to understand their dielectric properties and the insulator to metal transition (IMT) at interfaces with increasing layer thickness. The charge at insulating interfaces is found to be as predicted from the formal ionic charges, not populations. Different relative layer thicknesses produce a spontaneous polarization of the system, and allow manipulation of the interfacial electron gas. Large piezoresistance effects can be obtained from the sensitivity of the IMT to lateral strain. Carrier densities are found to be ideal for exciton condensation., Comment: 5 pages, 4 pdf figures, 1 added figure and additional references and discussions

We provide numerical evidence for composite fermion pairing in quantum Hall bilayer systems at filling $��=1/2 + 1/2$ for intermediate spacing between the layers. We identify the phase as $p_x + i p_y$ pairing, and construct high accuracy trial wavefunctions to describe the groundstate on the sphere. For large distances between the layers, and for finite systems, a competing "Hund's rule" state, or composite fermion liquid, prevails for certain system sizes. We argue that for larger systems, the pairing phase will persist to larger layer spacing., 4 pages, 2 figures; v2: final version, as published in journal

We study a rapidly rotating gas of unpolarized spin-1/2 ultracold fermions in the two-dimensional regime when all atoms reside in the lowest Landau level. Due to the presence of the spin degree of freedom both s-wave and p-wave interactions are allowed at ultralow temperatures. We investigate the phase diagram of this system as a function of the filling factor in the lowest Landau level and in terms of the ratio between s- and p-wave interaction strengths. We show that the presence of attractive interactions induces a wide regime of phase separation with formation of maximally compact droplets that are either fully polarized or composed of spin-singlets. In the regime with no phase separation, we give evidence for fractional quantum Hall states. Most notably, we find two distinct singlet states at the filling nu =2/3 for different interactions. One of these states is accounted for by the composite fermion theory while the other one is a paired state for which we identify two competing descriptions with different topological structure. This paired state may be an Abelian liquid of composite spin-singlet Bose molecules with Laughlin correlations. Alternatively, it may be a known non-Abelian paired state, indicated by good overlaps with the corresponding trial wavefunction. By fine tuning of the scattering lengths it is possible to create the non-Abelian critical Haldane-Rezayi state for nu =1/2 and the permanent state of Moore and Read for nu =1. For purely repulsive interactions, we also find evidence for a gapped Halperin state at nu=2/5., 12 pages, 9 figs (best viewed in color), published version with additional evidence for a non-Abelian spin singlet state at filling nu=2/3

We study theoretically the low-temperature phases of a two-component atomic Fermi gas with attractive s-wave interactions under conditions of rapid rotation. We find that, in the extreme quantum limit, when all particles occupy the lowest Landau level, the normal state is unstable to the formation of "charge" density wave (CDW) order. At lower rotation rates, when many Landau levels are occupied, we show that the low-temperature phases can be supersolids, involving both CDW and superconducting order., 4 pages, 1 figure, uses feynmp.sty

We construct a family of BCS paired composite fermion wavefunctions that generalize, but remain in the same topological phase as, the Moore-Read Pfaffian state for the half-filled Landau level. It is shown that for a wide range of experimentally relevant inter-electron interactions the groundstate can be very accurately represented in this form., 4 pages, 2 figures

We study a frustrated dipolar array recently manufactured lithographically by Wang {\em et al.} [Nature {\bf 439}, 303 (2006)] in order to realize the square ice model in an artificial structure. We discuss models for thermodynamics and dynamics of this system. We show that an ice regime can be stabilized by small changes in the array geometry; a different magnetic state, kagome ice, can similarly be constructed. At low temperatures, the square ice regime is terminated by a thermodynamic ordering transition, which can be chosen to be ferro- or antiferromagnetic. We show that the arrays do not fully equilibrate experimentally, and identify a likely dynamical bottleneck., Comment: 4 pages, 4 figures; v2: minor modifications in typesetting, figures 1 and 3 updated

The method of Jain and Kamilla [PRB {\bf 55}, R4895 (1997)] allows numerical generation of composite fermion trial wavefunctions for large numbers of electrons in high magnetic fields at filling fractions of the form nu=p/(2mp+1) with m and p positive integers. In the current paper we generalize this method to the case where the composite fermions are in an effective (mean) field with opposite sign from the actual physical field, i.e. when p is negative. We examine both the ground state energies and the low energy neutral excitation spectra of these states. Using particle-hole symmetry we can confirm the correctness of our method by comparing results for the series m=1 with p>0 (previously calculated by others) to our results for the conjugate series m=1 with p 0, m=2 cases., 11 pages

The electronic structure of 26 Yb compounds is calculated with the ab initio self-interaction-corrected local-spin-density approximation. In this approach f electrons can be described as either localized or delocalized. Hence a divalent Yb ion is represented with a completely localized f14 shell, while a trivalent Yb ion is represented with a localized f13 shell with the remaining 14th f electron giving rise to a very narrow f resonance, which straddles the Fermi energy. The systems studied comprise the Yb monopnictides and monochalcogenides as well as a series of intermetallic compounds. Experimental equilibrium volumes are well reproduced. The results provide quantitative support to the experimental classification of Yb compounds in terms of effective valencies.

Gibbs ensemble Monte Carlo simulations are reported for the vapor- liquid phase coexistence of argon, krypton, and xenon. The calculations employ accurate two-body potentials in addition to contributions from three-body dispersion interactions resulting from third-order triple-dipole, dipole-dipole-quadrupole, dipole- quadrupole-quadrupole, quadrupole-quadrupole-quadrupole, and fourth- order triple- dipole terms. It is shown that vapor-liquid equilibria are affected substantially by three-body interactions. The addition of three-body interactions results in good overall agreement of theory with experimental data. In particular, the subcritical liquid- phase densities are predicted accurately. (C) 1999 American Institute of Physics. S0021- 9606(99)50728-9.

ECRYS 2005 International Workshop on Electronic Crystals; In Quantum Hall bilayers, at total filling factor one, a transition from a compressible phase with weak interlayer correlations to an incompressible phase with strong interlayer correlations is observed as the distance between the two layers is reduced. The transition between these two regimes can be understood using a trial wavefunction approach based on the composite particle picture.

By means of first-principles calculations, various properties of SrRuO3 are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO3 show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen-rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. This not only explains the ground state of SrRuO3 but also contributes to rationalize why most ABO3 perovskites exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties., Comment: Also see LabTalk (http://iopscience.iop.org/0953-8984/labtalk-article/55704) and HighLight of 2014 (http://iopscience.iop.org/0953-8984/page/Highlights-of-2014)

Rare-earth co-doping in inorganic materials has a long-held tradition of facilitating highly desirable optoelectronic properties for their application to the laser industry. This study concentrates specifically on rare-earth phosphate glasses, (R2O3)x(R'2O3)y(P2O5)(1-(x+y)), where (R, R') denotes (Ce, Er) or (La, Nd) co-doping and the total rare-earth composition corresponds to a range between metaphosphate, RP3O9, and ultraphosphate, RP5O14. Thereupon, the effects of rare-earth co-doping on the local structure are assessed at the atomic level. Pair-distribution function analysis of high-energy X-ray diffraction data (Q(max) = 28 Å(-1)) is employed to make this assessment. Results reveal a stark structural invariance to rare-earth co-doping which bears testament to the open-framework and rigid nature of these glasses. A range of desirable attributes of these glasses unfold from this finding; in particular, a structural simplicity that will enable facile molecular engineering of rare-earth phosphate glasses with 'dial-up' lasing properties. When considered together with other factors, this finding also demonstrates additional prospects for these co-doped rare-earth phosphate glasses in nuclear waste storage applications. This study also reveals, for the first time, the ability to distinguish between P-O and P[double bond, length as m-dash]O bonding in these rare-earth phosphate glasses from X-ray diffraction data in a fully quantitative manner. Complementary analysis of high-energy X-ray diffraction data on single rare-earth phosphate glasses of similar rare-earth composition to the co-doped materials is also presented in this context. In a technical sense, all high-energy X-ray diffraction data on these glasses are compared with analogous low-energy diffraction data; their salient differences reveal distinct advantages of high-energy X-ray diffraction data for the study of amorphous materials.

We have studied the electronic structure of the ordered intermetallic compound Ti-Fe with CsCl structure and a corresponding hypothetical random solid solution on a bcc and CsCl lattice using comparable first-principles self-consistent calculations. The differences of the one-electron spectra en- able us to comment on the nature of bonding and the absence of disorder in this interesting system.

The spin-ice materials Ho2Ti2O7 (HTO) and Dy2Ti2O7 (DTO) are part of a large family of compounds called magnetic pyrochlore oxides. Typically, the magnetism of these systems arise from the rare-earth ions RE3+ which sits at the vertices of a lattice of corner-sharing tetrahedra and couple with the degrees of freedom of the crystal leading to a wide spectrum of exotic phenomena. In spin-ices the magnetic moments of the individual RE3+ ions are large enough to let their mutual dipolar interactions be the leading factors for the thermodynamics. Moreover, the strength and the symmetries of their local crystalline environment are such that each ion behaves like a magnetic dipole with only two allowed configurations: it points either parallel or opposite to the ?111? axis joining the vertex where it sits to the centre of the tetrahedron. As a result, the ground state of the system is macroscopically degenerate because the ferromagnetic interactions between the 4 Ising-like spins in each tetrahedron cannot be satisfied simultaneously, and in turn the manifold of possible configurations minimising the energy (2 spins inward - 2 spins outward) increases with the size of the system. This exotic ground state is such that the Ising configurations of the dipoles map to the disordered vector-displacements of the protons in the water molecules of conventional ice; hence the name spin-ice. Violations of the (2in-2out) ice rules take the systems out of the ground state into more energetic configurations. More precisely, the flipping of a spin between two adjacent ground-state tetrahedra creates a local excitation (1in-3out in a tetrahedron, 3in-1out in the neighbouring one) that is made of two fractionalised opposite magnetic fluxes. Once the two fluxes are created they can separate and freely hop across the lattice as their motion does not involve ahy higher order violation of the ice-rules. The low temperature properties of spin-ice is found to depend heavily, if not exclusively, on the density and mobility of such flux-defects which effectively behave as magnetic monopoles mutually interacting via a Coulomb potential. Beside the success of many experiments which exploit the physics of the monopoles in their exclusively classical formulation, there has been an increasing curiosity about the microscopic mechanisms which dictate their propagation across the lattice. At present, the dynamics of the monopoles are still puzzling showing different responses under different probes and non-identical behaviours between the two compounds HTO and DTO. As the monopoles themselves consist of packed magnetic fluxes originating from the magnetic moments of the RE3+ ions, investigating the microscopical mechanisms underlying their motion requires revisiting the foundations of the classicality which emerges form the quantum substrate of the interactions of the magnetic ions. This is the subject of the work presented in this thesis. With particular focus on the interplay between the local crystal-symmetries and the mutual interactions between the RE3+ ions, the present study gives an accurate de- scription of the microscopic mechanisms which occur in the pyrochlore substrate in the presence of a monopole. The results suggest that the motion of the monopoles is achieved thanks to the spin-tunnelling of the RE3+ ions which account for the flipping of the Ising spins, necessary for the propagation of a monopole. A major improvement we add to the standard theory of spin ice, is the role of the exchange interactions that are overcome by the dipolar ones in the ice-state but posses a dynamical resilience which manifests in the presence of a monopole. Furthermore, the present study brings to light the articulated statistical structure of the kinematic spin-constraints which are expected to dictate the diffusion of the free monopoles and their response under dif- ferent probes also in conditions out of equilibrium. From a more general perspective, the mathematical and physical models developed during this work promise to be of in- terest also in other magnetic systems. Primarily, in the other pyrochlore oxides whose microscopical structure is akin the spin-ice one, secondarily, in other RE3+ compounds where the interplay between quantum and classical physics leads to the manifestation of unusual dynamical effects.

Employing a realistic tight-binding model describing the Fermi surface in the normal state of \(Sr_2RuO_4\) we map out magnetic field versus temperature phase diagrams for \(d_{x2−y2}(B_{1g})\) and \(d_{xz}+id_{yz}(E_g)\) pairing types. Both produce (i) a similar Knight shift suppression of ∼80% and (ii) a bicritical point at \(T=0.88K\) separating low field second order phase transitions from high field Pauli limiting first order transitions. We find, however, strikingly different phase behaviour within the high field Pauli limiting region. For \(d_{x2−y2}\) pairing symmetry an additional lower critical line of first order transitions is found (terminating in a critical point at \(T=0.09−0.22K\) depending on the choice of Hubbard U parameters) while for \(d_{xz}+id_{yz}\) no such additional high field phase transitions are found for any choice of Hubbard U. In conjunction with our earlier finding [{\it Physical Review B} {\bf 102} (23), 235203] for \(p\)-wave helical pairing of a still different high field phase structure (a lower critical field line meeting the upper critical field line exactly at the bicritical point), we suggest high field Pauli limiting phase structure as a possible route to distinguish pairing symmetries in this material.

Gallium-doped phosphate-based glasses (PBGs) have been recently shown to have antibacterial activity. However, the delivery of gallium ions from these glasses can be improved by altering the calcium ion concentration to control the degradation rate of the glasses. In the present study, the effect of increasing calcium content in novel gallium (Ga2O3)-doped PBGs on the susceptibility of Pseudomonas aeruginosa is examined. The lack of new antibiotics in development makes gallium-doped PBG potentially a highly promising new therapeutic agent. The results show that an increase in calcium content (14, 15 and 16 mol.% CaO) cause a decrease in degradation rate (17.6, 13.5 and 7.3 microg mm(-2) h(-1)), gallium ion release and antimicrobial activity against planktonic P. aeruginosa. The most potent glass composition (containing 14 mol.% CaO) was then evaluated for its ability to prevent the growth of biofilms of P. aeruginosa. Gallium release was found to reduce biofilm growth of P. aeruginosa with a maximum effect (0.86 log(10) CFU reduction compared to Ga2O3-free glasses) after 48 h. Analysis of the biofilms by confocal microscopy confirmed the anti-biofilm effect of these glasses as it showed both viable and non-viable bacteria on the glass surface. Results of the solubility and ion release studies show that this glass system is suitable for controlled delivery of Ga3+. 71Ga NMR and Ga K-edge XANES measurements indicate that the gallium is octahedrally coordinated by oxygen atoms in all samples. The results presented here suggest that PBGs may be useful in controlled drug delivery applications, to deliver gallium ions in order to prevent infections due to P. aeruginosa biofilms.

We review the recently developed bi-partite composite fermion model, in the context of so-called Pfaffian incompressible quantum liquid with fractional and non-Abelian quasiparticle statistics, a promising model for describing the correlated many-electron ground state responsible for fractional quantum Hall effect at the Landau level filling factor ? = 5/2. We use the concept of composite fermion partitions to demonstrate the emergence of an essential ingredient of the non-Abelian braid statistics – the topological degeneracy of spatially indistinguishable configurations of multiple widely separated (non-interacting) quasiparticles.