Dissipative quantum disordered models

This article reviews recent studies of mean-field and one dimensional quantum disordered spin systems coupled to different types of dissipative environments. The main issues discussed are: (i) The real-time dynamics in the glassy phase and how they compare to the behaviour of the same models in their classical limit. (ii) The phase transition separating the ordered -- glassy -- phase from the disordered phase that, for some long-range interactions, is of second order at high temperatures and of first order close to the quantum critical point (similarly to what has been observed in random dipolar magnets). (iii) The static properties of the Griffiths phase in random Ising chains. (iv) The dependence of all these properties on the environment. The analytic and numeric techniques used to derive these results are briefly mentioned.Comment: Contribution to the 12th International Conference on Recent Progress in Many-Body Theories, Santa Fe, New Mexico, USA, August 2004; 10 pages no fig

Low Temperature Symmetry of Pyrochlore Oxide Cd2Re2O7

We report the X-ray study for the pyrochlore oxide Cd2Re2O7. Two symmetry-lowering structural transitions were observed at Ts1=200K and Ts2=120K. The former is of the second order from the ideal cubic pyrochlore structure with space group Fd-3m to a tetragonally distorted structure with I-4m2, while the latter is of the first order likely to another tetragonal space group I4122. We discuss the feature of the lattice deformation.Comment: 4 pages, 4 figure

Estrogens promote misfolded proinsulin degradation to protect insulin production and delay diabetes

Summary: Conjugated estrogens (CE) delay the onset of type 2 diabetes (T2D) in postmenopausal women, but the mechanism is unclear. In T2D, the endoplasmic reticulum (ER) fails to promote proinsulin folding and, in failing to do so, promotes ER stress and β cell dysfunction. We show that CE prevent insulin-deficient diabetes in male and in female Akita mice using a model of misfolded proinsulin. CE stabilize the ER-associated protein degradation (ERAD) system and promote misfolded proinsulin proteasomal degradation. This involves activation of nuclear and membrane estrogen receptor-α (ERα), promoting transcriptional repression and proteasomal degradation of the ubiquitin-conjugating enzyme and ERAD degrader, UBC6e. The selective ERα modulator bazedoxifene mimics CE protection of β cells in females but not in males. : Estrogens prevent diabetes in women, but the mechanism is poorly understood. Xu et al. report that estrogens activate the endoplasmic-reticulum-associated protein degradation pathway, which promotes misfolded proinsulin degradation, suppresses endoplasmic reticulum stress, and protects insulin secretion in mice and in human pancreatic β cells. Keywords: estrogens, beta cell, islet, endoplasmic reticulum stress, proinsulin misfolding, diabetes, bazedoxifene, sex dimorphism, ERAD, SER

The role of ECL2 in CGRP receptor activation: a combined modelling and experimental approach

The calcitonin gene-related peptide (CGRP) receptor is a complex of a calcitonin receptor-like receptor (CLR), which is a family B G-protein-coupled receptor (GPCR) and receptor activity modifying protein 1. The role of the second extracellular loop (ECL2) of CLR in binding CGRP and coupling to Gs was investigated using a combination of mutagenesis and modelling. An alanine scan of residues 271–294 of CLR showed that the ability of CGRP to produce cAMP was impaired by point mutations at 13 residues; most of these also impaired the response to adrenomedullin (AM). These data were used to select probable ECL2-modelled conformations that are involved in agonist binding, allowing the identification of the likely contacts between the peptide and receptor. The implications of the most likely structures for receptor activation are discussed.</jats:p

Itinerant-Electron Magnet of the Pyrochlore Lattice: Indium-Doped YMn2Zn20

We report on a ternary intermetallic compound, "YMn2Zn20", comprising a pyrochlore lattice made of Mn atoms. A series of In-doped single crystals undergo no magnetic long-range order down to 0.4 K, in spite of the fact that the Mn atom carries a local magnetic moment at high temperatures, showing Curie-Weiss magnetism. However, In-rich crystals exhibit spin-glass transitions at approximately 10 K due to a disorder arising from the substitution, while, with decreasing In content, the spin-glass transition temperature is reduced to 1 K. Then, heat capacity divided by temperature approaches a large value of 280 mJ K-2 mol-1, suggesting a significantly large mass enhancement for conduction electrons. This heavy-fermion-like behavior is not induced by the Kondo effect as in ordinary f-electron compounds, but by an alternative mechanism related to the geometrical frustration on the pyrochlore lattice, as in (Y,Sc)Mn2 and LiV2O4, which may allow spin entropy to survive down to low temperatures and to couple with conduction electrons.Comment: 5 pages, 4 figures, J. Phys. Soc. Jpn., in pres

Thermal and Electrical Properties of gamma-NaxCoO2 (0.70 < x < 0.78)

We have performed specific heat and electric resistivity measurements of Na$_{x}$CoO$_{2}$ ($x=0.70$-0.78). Two anomalies have been observed in the specific heat data for $x=0.78$, corresponding to magnetic transitions at $T_{c}=22$ K and $T_{k}\simeq 9$ K reported previously. In the electrical resistivity, a steep decrease at $T_{c}$ and a bending-like variation at $T_{b}$(=120K for $x=0.78$) have been observed. Moreover, we have investigated the $x$-dependence of these parameters in detail. The physical properties of this system are very sensitive to $x$, and the inconsistent results of previous reports can be explained by a small difference in $x$. Furthermore, for a higher $x$ value, a phase separation into Na-rich and Na-poor domains occurs as we previously proposed, while for a lower $x$ value, from characteristic behaviors of the specific heat and the electrical resistivity at the low-temperature region, the system is expected to be in the vicinity of the magnetic instability which virtually exists below $x=0.70$.Comment: 4 pages (3 figures included) and an extra figure (gif), to be published in J. Phys. Soc. Jpn. 73 (9) with possible minor revision

Gravitational GUT Breaking and the GUT-Planck Hierarchy

It is shown that non-renormalizable gravitational interactions in the Higgs sector of supersymmetric grand unified theories (GUT's) can produce the breaking of the unifying gauge group $G$ at the GUT scale $M_{\rm GUT} \sim 10^{16}$~GeV. Such a breaking offers an attractive alternative to the traditional method where the superheavy GUT scale mass parameters are added ad hoc into the theory. The mechanism also offers a natural explanation for the closeness of the GUT breaking scale to the Planck scale. A study of the minimal SU(5) model endowed with this mechanism is presented and shown to be phenomenologically viable. A second model is examined where the Higgs doublets are kept naturally light as Goldstone modes. This latter model also achieves breaking of $G$ at $M_{\rm GUT}$ but cannot easily satisfy the current experimental proton decay bound.Comment: 11 pages, REVTeX, 1 figure included as an uuencoded Z-compressed PostScript file. Our Web page at http://physics.tamu.edu/~urano/research/gutplanck.html contains ready to print PostScript version (with figures) as well as color version of plot

Magnetic and superconducting properties of Cd2Re2O7: Cd NMR and Re NQR

We report Cd NMR and Re NQR studies on Cd2Re2O7, the first superconductor among pyrochlore oxides Tc=1 K. Re NQR spectrum at zero magnetic field below 100 K rules out any magnetic or charge order. The spin-lattice relaxation rate below Tc exhibits a pronounced coherence peak and behaves within the weak-coupling BCS theory with nearly isotropic energy gap. Cd NMR results point to moderate ferromagnetic enhancement at high temperatures followed by rapid decrease of the density of states below the structural transition temperature of 200 K.Comment: 4 pages, 4 figure

Quasi-One-Dimensional Spin Dynamics in dd-Electron Heavy-Fermion Metal Y1−x_{1-x}Scx_xMn2_2

Slow spin fluctuations ($\nu < 10^{12}$ s$^-1$) observed by the muon spin relaxation technique in Y$_{1-x}$Sc$_x$Mn$_2$ exhibits a power law dependence on temperature ($\nu \propto T^\alpha$), where the power converges asymptotically to unity ($\alpha\rightarrow 1$) as the system moves away from spin-glass instability with increasing Sc content $x$. This linear $T$ dependence, which is common to that observed in LiV$_2$O$_4$, is in line with the prediction of the "intersecting Hubbard chains" model for a metallic pyrochlore lattice, suggesting that the geometrical constraints to t2g bands specific to the pyrochlore structure serve as a basis of the $d$-electron heavy-fermion state.Comment: 5 pages, 4 figures, to appear in J. Phys. Soc. Jp