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Disordered Supersolids in the Extended Bose-Hubbard Model.

The extended Bose-Hubbard model captures the essential properties of a wide variety of physical systems including ultracold atoms and molecules in optical lattices, Josephson junction arrays, and certain narrow band superconductors. It exhibits a rich phase diagram including a supersolid phase where a lattice solid coexists with a superfluid. We use quantum Monte Carlo to study the supersolid part of the phase diagram of the extended Bose-Hubbard model on the simple cubic lattice. We add disorder to the extended Bose-Hubbard model and find that the maximum critical temperature for the supersolid phase tends to be suppressed by disorder. But we also find a narrow parameter window in which the supersolid critical temperature is enhanced by disorder. Our results show that supersolids survive a moderate amount of spatial disorder and thermal fluctuations in the simple cubic lattice.

Structure of spin excitations in heavily electron-doped Li0.8Fe0.2ODFeSe superconductors.

Heavily electron-doped iron-selenide high-transition-temperature (high-T c) superconductors, which have no hole Fermi pockets, but have a notably high T c, have challenged the prevailing s ± pairing scenario originally proposed for iron pnictides containing both electron and hole pockets. The microscopic mechanism underlying the enhanced superconductivity in heavily electron-doped iron-selenide remains unclear. Here, we used neutron scattering to study the spin excitations of the heavily electron-doped iron-selenide material Li0.8Fe0.2ODFeSe (T c = 41 K). Our data revealed nearly ring-shaped magnetic resonant excitations surrounding (π, π) at ∼21 meV. As the energy increased, the spin excitations assumed a diamond shape, and they dispersed outward until the energy reached ∼60 meV and then inward at higher energies. The observed energy-dependent momentum structure and twisted dispersion of spin excitations near (π, π) are analogous to those of hole-doped cuprates in several aspects, thus implying that such spin excitations are essential for the remarkably high T c in these materials.The microscopic mechanism underlying an enhanced superconductivity in electron-doped iron selenide superconductor remains unclear. Here, Pan et al. report the spin excitations of Li0.8Fe0.2ODFeSe, revealing analogous momentum structure and dispersion to hole-doped cuprates.

Itinerant Antiferromagnetism in RuO_{2}.

Bulk rutile RuO_{2} has long been considered a Pauli paramagnet. Here we report that RuO_{2} exhibits a hitherto undetected lattice distortion below approximately 900 K. The distortion is accompanied by antiferromagnetic order up to at least 300 K with a small room temperature magnetic moment of approximately 0.05µ_{B} as evidenced by polarized neutron diffraction. Density functional theory plus U (DFT+U) calculations indicate that antiferromagnetism is favored even for small values of the Hubbard U of the order of 1 eV. The antiferromagnetism may be traced to a Fermi surface instability, lifting the band degeneracy imposed by the rutile crystal field. The combination of high Néel temperature and small itinerant moments make RuO_{2} unique among ruthenate compounds and among oxide materials in general.

Probing the Pairing Interaction and Multiple Bardasis-Schrieffer Modes Using Raman Spectroscopy.

In unconventional superconductors, understanding the form of the pairing interaction is the primary goal. In this regard, Raman spectroscopy is a very useful tool, as it identifies the ground state and also the subleading pairing channels by probing collective modes. Here, we propose a general theory for a multiband Raman response and identify new features in the spectrum that can provide a robust test for a pairing theory. We identify multiple Bardasis-Schrieffer type collective modes and connect the weights of these modes to the subleading gap structures within a microscopic pairing theory. While our conclusions are completely general, we apply our approach to interpret the specific case of B_{1g} Raman scattering in hole-doped BaFe_{2}As_{2}.

Pairing in a dry Fermi sea.

In the traditional Bardeen-Cooper-Schrieffer theory of superconductivity, the amplitude for the propagation of a pair of electrons with momentum k and -k has a log singularity as the temperature decreases. This so-called Cooper instability arises from the presence of an electron Fermi sea. It means that an attractive interaction, no matter how weak, will eventually lead to a pairing instability. However, in the pseudogap regime of the cuprate superconductors, where parts of the Fermi surface are destroyed, this log singularity is suppressed, raising the question of how pairing occurs in the absence of a Fermi sea. Here we report Hubbard model numerical results and the analysis of angular-resolved photoemission experiments on a cuprate superconductor. In contrast to the traditional theory, we find that in the pseudogap regime the pairing instability arises from an increase in the strength of the spin-fluctuation pairing interaction as the temperature decreases rather than the Cooper log instability.

Glide-plane symmetry and superconducting gap structure of iron-based superconductors.

We consider the effect of glide-plane symmetry of the Fe-pnictogen/chalcogen layer in Fe-based superconductors on pairing in spin fluctuation models. Recent theories have proposed that so-called η-pairing states with nonzero total momentum can be realized and possess exotic properties such as odd parity spin singlet symmetry and time-reversal symmetry breaking. Here we show that η pairing is inevitable when there is orbital weight at the Fermi level from orbitals with even and odd mirror reflection symmetry in z; however, by explicit calculation, we conclude that the gap function that appears in observable quantities is identical to that found in earlier, 1 Fe per unit cell pseudocrystal momentum calculations.

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides.

High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe2As2 parent compound modifies the low-energy spin excitations and their correlation with superconductivity (<50 meV) without affecting the high-energy spin excitations (>100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above T(c) can account for the superconducting condensation energy. These results suggest that high-T(c) superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.

Evolution of the superconducting state of Fe-based compounds with doping.

We introduce an effective low-energy pairing model for Fe-based superconductors with s- and d-wave interaction components and a small number of input parameters and use it to study the doping evolution of the symmetry and the structure of the superconducting gap. We argue that the model describes the entire variety of pairing states found so far in the Fe-based superconductors and allows one to understand the mechanism of the attraction in s(±) and d(x(2)-y(2)) channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.

Neutron scattering studies of spin excitations in hole-doped Ba(0.67)K(0.33)Fe(2)As(2) superconductor.

We report inelastic neutron scattering experiments on single crystals of superconducting Ba(0.67)K(0.33)Fe(2)As(2) (T(c) = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the Q(AFM) direction in momentum space, consistent with density functional theory predictions. On cooling below T(c), while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba(0.67)Ka(0.33)Fe(2)As(2) are more complicated than those suggested from angle resolved photoemission experiments.

Dynamic cluster quantum Monte Carlo simulations of a two-dimensional Hubbard model with stripelike charge-density-wave modulations: interplay between inhomogeneities and the superconducting state.

Using dynamic cluster quantum Monte Carlo simulations, we study the superconducting behavior of a 1/8 doped two-dimensional Hubbard model with imposed unidirectional stripelike charge-density-wave modulation. We find a significant increase of the pairing correlations and critical temperature relative to the homogeneous system when the modulation length scale is sufficiently large. With a separable form of the irreducible particle-particle vertex, we show that optimized superconductivity is obtained for a moderate modulation strength due to a delicate balance between the modulation enhanced pairing interaction, and a concomitant suppression of the bare particle-particle excitations by a modulation reduction of the quasiparticle weight.

Parquet approximation for the 4x4 Hubbard cluster.

We present a numerical solution of the parquet approximation, a conserving diagrammatic approach which is self-consistent at both the single-particle and the two-particle levels. The fully irreducible vertex is approximated by the bare interaction thus producing the simplest approximation that one can perform with the set of equations involved in the formalism. The method is applied to the Hubbard model on a half-filled 4x4 cluster. Results are compared to those obtained from determinant quantum Monte Carlo (DQMC), FLuctuation EXchange (FLEX), and self-consistent second-order approximation methods. This comparison shows a satisfactory agreement with DQMC and a significant improvement over the FLEX or the self-consistent second-order approximation.

RXRA gene variations influence Alzheimer's disease risk and cholesterol metabolism.

Cholesterol metabolism is altered in Alzheimer's disease (AD). The nuclear hormone receptor Retinoic X Receptor a (RXRa) is a member of the nuclear ligand-activated transcription factor family. RXRs are key regulators of cholesterol synthesis and thus cholesterol metabolism. We performed a systematic screen for gene variants in the RXRA gene. The effect of these gene variants on the risk of AD was investigated in 405 AD patients (mean age: 74.27 +/- 9.37 years; female 78.6%) and 347 controls (mean age: 73.26 +/- 8.37 years; female 57.2%). Furthermore, the influence of RXRA gene variants on CSF and plasma levels of cholesterol, lathosterol and 24S-hydroxycholesterol were evaluated. One of the identified seven SNPs in RXRA influenced AD risk in our single marker analysis (rs3132293: P= 0.006). Haplotype analysis identified a three-marker haplotype (TGC) consisting of rs3118570, rs1536475 and rs3132293, which decreased the risk of AD (P= 0.009). The single marker rs3132293 (P= 0.026) and the TGC haplotype (P= 0.026) influenced CSF lathosterol levels in non-demented controls, and cholesterol levels in the combined sample comprising AD patients and controls (Rs3132293: P= 0.050; TGC haplotype: P= 0.035). 24S-Hydroxycholesterol CSF and plasma levels were also influenced by rs3132293 (CSF: P= 0.004; plasma: P= 0.001) and the TGC haplotype (CSF: P= 0.004; plasma: P= 0.002); this effect was most pronounced in AD patients (rs3132293: CSF: P= 0.009, plasma: P= 0.002; TGC haplotype: CSF: P= 0.019, plasma: P= 0.005). Our results suggest that RXRA gene variants might act as risk factor for AD via an influence on cerebral cholesterol metabolism.

Dynamics of the pairing interaction in the hubbard and t-J models of high-temperature superconductors.

The question of whether one should speak of a "pairing glue" in the Hubbard and t-J models is basically a question about the dynamics of the pairing interaction. If the dynamics of the pairing interaction arises from virtual states, whose energies correspond to the Mott gap, and give rise to the exchange coupling J, the interaction is instantaneous on the relative time scales of interest. In this case, while one might speak of an "instantaneous glue," this interaction differs from the traditional picture of a retarded pairing interaction. However, as we will show, the dominant contribution to the pairing interaction for both of these models arises from energies reflecting the spectrum seen in the dynamic spin susceptibility. In this case, the basic interaction is retarded, and one speaks of a spin-fluctuation glue which mediates the d-wave pairing.

Structure of the pairing interaction in the two-dimensional Hubbard model.

Dynamic cluster Monte Carlo calculations for the doped two-dimensional Hubbard model are used to study the irreducible particle-particle vertex responsible for dx2-y2 pairing in this model. This vertex increases with increasing momentum transfer and decreases when the energy transfer exceeds a scale associated with the Q=(pi, pi) spin susceptibility. Using an exact decomposition of this vertex into a fully irreducible two-fermion vertex and charge and magnetic exchange channels, the dominant part of the effective pairing interaction is found to come from the magnetic, spin S=1 exchange channel.

Systematic study of d-wave superconductivity in the 2D repulsive Hubbard model.

The cluster size dependence of superconductivity in the conventional two-dimensional Hubbard model, commonly believed to describe high-temperature superconductors, is systematically studied using the dynamical cluster approximation and quantum Monte Carlo simulations as a cluster solver. Because of the nonlocality of the d-wave superconducting order parameter, the results on small clusters show large size and geometry effects. In large enough clusters, the results are independent of the cluster size and display a finite temperature instability to d-wave superconductivity.