Atomic fountain of laser-cooled Yb atoms for precision measurements

We demonstrate launching of laser-cooled Yb atoms in a cold atomic fountain. Atoms in a collimated thermal beam are first cooled and captured in a magneto-optical trap (MOT) operating on the strongly allowed S-1(0) -> P-1(1) transition at 399 nm (blue line). They are then transferred to a MOT on the weakly allowed S-1(0) -> P-3(1) transition at 556 nm (green line). Cold atoms from the green MOT are launched against gravity at a velocity of around 2.5 m/s using a pair of green beams. We trap more than 107 atoms in the blue MOT and transfer up to 70% into the green MOT. The temperature for the odd isotope Yb-171 is similar to 1 mK in the blue MOT, and reduces by a factor of 40 in the green MOT.

Magnetic trapping of Yb in the metastable P-3(2) state

We report magnetic trapping of Yb in the excited P-3(2) state. This state, with a lifetime of 15 s, could play an important role in studies ranging from optical clocks and quantum computation to the search for a permanent electric dipole moment. Yb atoms are first cooled and trapped in the ground state in a 399-nm magneto-optic trap. The cold atoms are then pumped into the excited state by driving the S-1(0) -> P-3(1) -> S-3(1) transition. Atoms in the P-3(2) state are magnetically trapped in a spherical quadrupole field with an axial gradient of 110 G/cm. We trap up to 10(6) atoms with a lifetime of 1.5 s.

Bose-Hubbard model in a strong effective magnetic field: Emergence of a chiral Mott insulator ground state

Motivated by experiments on Josephson junction arrays, and cold atoms in an optical lattice in a synthetic magnetic field, we study the ``fully frustrated'' Bose-Hubbard model with half a magnetic flux quantum per plaquette. We obtain the phase diagram of this model on a two-leg ladder at integer filling via the density matrix renormalization group approach, complemented by Monte Carlo simulations on an effective classical XY model. The ground state at intermediate correlations is consistently shown to be a chiral Mott insulator (CMI) with a gap to all excitations and staggered loop currents which spontaneously break time-reversal symmetry. We characterize the CMI state as a vortex supersolid or an indirect exciton condensate, and discuss various experimental implications.

Frustration in Optical Lattices of Interacting Bosons

The realization of optical lattices of cold atoms has opened up the possibility of engineering interacting lattice systems of bosons and fermions, stimulating a frenzy of research over the last decade. More recently, experimental techniques have been developed to apply synthetic gauge fields to these optical lattices. As a result, it has become possible to study quantum Hall physics and the effects of frustration in lattices of cold atoms. In this article we describe the combined effect of frustration and interactions on the superfluidity of bosons. By focussing on a frustrated ladder of interacting bosons, we show that the effect of frustration is for ``chiral'' order to develop, which manifests itself as an alternating pattern of circulating supercurrents. Remarkably, this order persists even when superfluidity is lost and the system enters a Mott phase giving rise to a novel chiral Mott insulator. We describe the combined physics of frustration and interactions by studying a fully frustrated one dimensional model of interacting bosons. The model is studied using mean-field theory, a direct quantum simulation and a higher dimensional classical theory in order to offer a full description of the different quantum phases contained in it and transitions between the different phases. In addition, we provide physical descriptions of the chiral Mott insulator as a vortex-anitvortex super solid and indirect excitonic condensate in addition to obtaining a variational wavefunction for it. We also briefly describe the chiral Mott states arising in other microscopic models.

Fermions in Synthetic Non-Abelian Gauge Fields

Quantum emulation property of the cold atoms has generated a lot of interest in studying systems with synthetic gauge fields. In this article, we describe the physics of two component Fermi gas in the presence of synthetic non-Abelian SU(2) gauge fields. Even for the non-interacting system with the gauge fields, there is an interesting change in the topology of the Fermi surface by tuning only the gauge field strength. When a trapping potential is used in conjunction with the gauge fields, the non-interacting system has the ability to produce novel Hamiltonians and show characteristic change in the density profile of the cloud. Without trap, the gauge fields act as an attractive interaction amplifier and for special kinds of gauge field configurations, there are two-body bound states for any attraction even in three dimensions. For a many body system, the gauge fields can induce a crossover from a weak superfluid to a strong superfluid with transition temperature as high as the Fermi temperature. The superfluid state obtained for a very large gauge field strength is a superfluid of new kind of bosons, called ``rashbons'', the properties of which are independent of its constituent two component fermions and are solely determined by the gauge field strength. We also discuss the collective excitations over the superfluid ground states and the experimental relevance of the physics.

Spectral intensity distribution of trapped

To calculate static response properties of a many-body system, local density approximation (LDA) can be safely applied. But, to obtain dynamical response functions, the applicability of LDA is limited bacause dynamics of the system needs to be considered as well. To examine this in the context of cold atoms, we consider a system of non-interacting spin4 fermions confined by a harmonic trapping potential. We have calculated a very important response function, the spectral intensity distribution function (SIDF), both exactly and using LDA at zero temperature and compared with each other for different dimensions, trap frequencies and momenta. The behaviour of the SIDF at a particular momentum can be explained by noting the behaviour of the density of states (DoS) of the free system (without trap) in that particular dimension. The agreement between exact and LDA SIDFs becomes better with increase in dimensions and number of particles.

Cold beam of isotopically pure Yb atoms by deflection using 1D-optical molasses

We demonstrate the generation of an isotopically pure beam of laser-cooled Yb atoms by deflection using 1D-optical molasses. Atoms in a collimated thermal beam are first slowed using a Zeeman slower. They are then subjected to a pair of molasses beams inclined at 45(a similar to) with respect to the slowed atomic beam. The slowed atoms are deflected and probed at a distance of 160 mm. We demonstrate the selective deflection of the bosonic isotope Yb-174 and the fermionic isotope Yb-171. Using a transient measurement after the molasses beams are turned on, we find a longitudinal temperature of 41 mK.

Continuous beam of laser-cooled Yb atoms

We demonstrate the launching of laser-cooled Yb atoms in a continuous atomic beam. The continuous cold beam has significant advantages over the more-common pulsed fountain, which was also demonstrated by us recently. The cold beam is formed in the following steps: i) atoms from a thermal beam are first Zeeman-slowed to a small final velocity; ii) the slowed atoms are captured in a two-dimensional magneto-optic trap (2D-MOT); and iii) atoms are launched continuously in the vertical direction using two sets of moving-molasses beams, inclined at +/- 15 degrees to the vertical. The cooling transition used is the strongly allowed S-1(0) -> P-1(1) transition at 399 nm. We capture about 7x10(6) atoms in the 2D-MOT, and then launch them with a vertical velocity of 13m/s at a longitudinal temperature of 125(6) mK. Copyright (C) EPLA, 2013

X-ray diffraction and Mossbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mossbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (approximate to 10(15)/m(2)) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of 111 screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.

Dependence of metal Auger intensity ratios on the effective charge on metal atoms

Metal Auger line intensity ratios were shown by Rao and others to be directly related to the occupancy of valence states. It is now shown that these intensity ratios are more generally related to the effective charge on the metal atom. The Auger intensity ratios are also directly proportional to valence band intensities of metals. Correlations of the intensity ratios with Auger parameters have also been examined.

Structures of mannose-6-phosphate isomerase from Salmonella typhimurium bound to metal atoms and substrate: implications for catalytic mechanism

Mannose-6-phosphate isomerase (MPI) catalyzes the inter-conversion of mannose 6-phosphate and fructose 6-phosphate. X-ray crystal structures of MPI from Salmonella typhimurium in the apo form (with no metal bound) and in the holo form (with bound Zn2+) and two other structures with yttrium bound at an inhibitory site and complexed with Zn2+ and fructose 6-phosphate (F6P) were determined in order to gain insights into the structure and the isomerization mechanism. Isomerization involves acid/base catalysis with proton transfer between the C1 and C2 atoms of the substrate. His99, Lys132, His131 and Asp270 are close to the substrate and are likely to be the residues involved in proton transfer. The interactions observed at the active site suggest that the ring-opening step is probably catalyzed by His99 and Asp270. An active-site loop consisting of residues 130-133 undergoes conformational changes upon substrate binding. Zn2+ binding induces structural order in the loop consisting of residues 50-54. The metal atom appears to play a role in substrate binding and is probably also important for maintaining the architecture of the active site. Isomerization probably follows the previously suggested cis-enediol mechanism.

"Central Atoms" models for ternary silicate and alumino-silicate melts

The �Central Atoms� model presented by the authors in an earlier paper is extended to ternary silicate and alumino-silicate melts. The model is applied to the CaO-FeO-SiO2 and the CaO-Al2O3-SiO2 system. Use is made of the parameters from the relevant binaries only. The agreement between experimental and calculated isoactivity curves is good in all cases.

Nature of induction of tryptophan pyrrolase in cold exposure

The activity of hepatic tryptophan pyrrolase in rats exposed to cold increased rapidly and reached a maximum of three-fold at 8 h. On continued exposure up to 48 h stress, the activity partly decreased but remained at a level higher than the initial. Withdrawal from the cold stress reversed the change. Adrenalectomy or treatment with inhibitors of protein synthesis abolished the increase in the enzyme activity during cold stress indicating a possible involvement of corticosteroids and de novo protein synthesis. Treatment with drugs known to block autonomic nervous system failed to inhibit the cold-mediated increase in enzyme activity. The results suggest that the increase in enzyme activity obtained on cold exposure is mediated by corticosteroids and not by either indoleaklylamines or autonomic nervous system. The changes in the enzyme obtained under cold stress with respect to the overshoot phenomenon, relationship to the degree of stress and reversibility on withdrawal from the stress indicate the "adaptate" nature of the response.

Supersolid and solitonic phases in the one-dimensional extended Bose-Hubbard model

We report our findings on the quantum phase transitions in cold bosonic atoms in a one-dimensional optical lattice using the finite-size density-matrix renormalization-group method in the framework of the extended Bose-Hubbard model. We consider wide ranges of values for the filling factors and the nearest-neighbor interactions. At commensurate fillings, we obtain two different types of charge-density wave phases and a Mott insulator phase. However, departure from commensurate fillings yields the exotic supersolid phase where both the crystalline and the superfluid orders coexist. In addition, we obtain the signatures for the solitary waves and the superfluid phase.

Recognition of Interaction Interface Residues in Low-Resolution Structures of Protein Assemblies Solely from the Positions of C alpha Atoms

Background: The number of available structures of large multi-protein assemblies is quite small. Such structures provide phenomenal insights on the organization, mechanism of formation and functional properties of the assembly. Hence detailed analysis of such structures is highly rewarding. However, the common problem in such analyses is the low resolution of these structures. In the recent times a number of attempts that combine low resolution cryo-EM data with higher resolution structures determined using X-ray analysis or NMR or generated using comparative modeling have been reported. Even in such attempts the best result one arrives at is the very course idea about the assembly structure in terms of trace of the C alpha atoms which are modeled with modest accuracy. Methodology/Principal Findings: In this paper first we present an objective approach to identify potentially solvent exposed and buried residues solely from the position of C alpha atoms and amino acid sequence using residue type-dependent thresholds for accessible surface areas of C alpha. We extend the method further to recognize potential protein-protein interface residues. Conclusion/Significance: Our approach to identify buried and exposed residues solely from the positions of C alpha atoms resulted in an accuracy of 84%, sensitivity of 83-89% and specificity of 67-94% while recognition of interfacial residues corresponded to an accuracy of 94%, sensitivity of 70-96% and specificity of 58-94%. Interestingly, detailed analysis of cases of mismatch between recognition of interface residues from C alpha positions and all-atom models suggested that, recognition of interfacial residues using C alpha atoms only correspond better with intuitive notion of what is an interfacial residue. Our method should be useful in the objective analysis of structures of protein assemblies when positions of only C alpha positions are available as, for example, in the cases of integration of cryo-EM data and high resolution structures of the components of the assembly.