The layered sodium cobalt oxyhydrate superconductor Na 0.3 CoO 2 ⋅1.4H 2 O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na 0.3 CoO 2 ⋅ Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 Å implies that the superconductivity turns on in this system between CoO 2 layer spacings of 6.9 and 9.9 Å at nominally constant chemical doping.

Synthesis and crystal structures are described for the compounds Ln 2 (Ti 2-x Ln x )O 7-x/2 , where Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu, and x ranges from 0 to 0.67. Rietveld refinements on

The third law of thermodynamics dictates that the entropy of a system in thermal equilibrium goes to zero as its temperature approaches absolute zero. In ice, however, a "zero point" or residual entropy can be measured -attributable to a high degeneracy in the energetically preferred positions of the hydrogen ions associated with the so-called "ice rules". 1,2 Remarkably, the spins in certain magnetic materials with the pyrochlore structure of corner-sharing tetrahedra, called "spin ice", have an equivalent degeneracy of energetically preferred states and also have been shown to display a zero point entropy. 3,4,5,6,7 Here we report that we have chemically altered Ho 2 Ti 2 O 7 spin ice by stuffing extra Ho magnetic moments into normally non-magnetic Ti sites surrounding the Ho tetrahedra. The resulting series, Ho 2 (Ti 2-x Ho x )O 7-x/2 , provides a unique opportunity to study the effects of increased connectivity between spins on a frustrated lattice. Surprisingly, the measured zero point entropy per spin appears unchanged by these excess spins, and the dynamic freezing of the spins is suppressed to lower temperatures. The results challenge our understanding of the spin ice state, and suggest a new avenue for using chemistry to study both ice-like frustration and the properties of the broad family of geometrically frustrated magnets based on the pyrochlore structure. 2 2The energy equivalence of different configurations for hydrogen ions in ice is an example of geometrical frustration. In general, geometrical frustration occurs when local interactions between a system's components cannot all be simultaneously satisfied due to their spatial arrangement on a regular lattice, leading to a large degeneracy of low energy states. Geometrical frustration in magnetic materials 8,9 has led to a wide range of novel physics and has received particular attention in the rare earth pyrochlore compounds. These materials have the generic formula R 2 M 2 O 7 , where the R 3+ and M 4+ cations form equivalent interpenetrating, equal size sublattices of corner-sharing tetrahedra (as shown in Fig. 1). 10 These systems display low temperature behavior ranging from exotic ordered states, 11,12,13,14 to spin liquids, in which the spins are correlated yet continue to fluctuate as T → 0, 15 to the spin ice state.Spin ice behavior has been observed in pyrochlores where the R site is occupied by either Dy or Ho magnetic cations, and non-magnetic Ti or Sn occupy the equivalent M sublattice. 5,6 The crystal field environment in this structure is such that each Ho or Dy spin must point along the line connecting the centers of its two tetrahedra (i.e., either directly toward or away from the centers of the tetrahedra). Coupled with the dipolar and exchange interactions, this anisotropy results in six energetically equivalent lowenergy states for each tetrahedron, in which two spins point in to and two spins point out from each tetrahedron, exactly analogous to the 'ice rules' for hydrogen positions in water ice. Neutron scattering and magneti...

Water ice and spin ice are important model systems in which theory can directly account for "zero point" entropy associated with quenched configurational disorder.Spin ice differs from water ice in the important respect that its fundamental constituents, the spins of the magnetic ions, can be removed through replacement with non-magnetic ions while keeping the lattice structure intact. In order to investigate the interplay of frustrated interactions and quenched disorder, we have performed systematic heat capacity measurements on spin ice materials which have been thus diluted up to 90%.Investigations of both Ho and Dy spin ices reveal that the zero point entropy depends non-monotonically on dilution and approaches the value of Rln2 in the limit of high dilution. The data are in good agreement with a generalization of Pauling's theory for the entropy of ice. 2Entropy is one of the most important concepts in thermodynamics, but it is rarely directly connected through experiment to its statistical definition rooted in the number of available states. One such connection is found in the case of water ice wherein each oxygen atom is bonded to four hydrogen atoms. The lowest energy state has two hydrogens located closer to the oxygen with the other two further away, establishing the so-called 'ice rules ' [1]. This arrangement allows for a high degeneracy of states, and a resultant 'zero point entropy' has been measured thermodynamically and can be associated with the disordered states frozen in place asA high degeneracy of states also leads to a range of exotic physics in geometrically frustrated magnets, including 'spin ice' materials [3,4,5,6] in which the low temperature behavior is closely analogous to that of water ice. In these materials, the magnetic ions (Ho 3+ or Dy 3+ ) occupy a pyrochlore lattice of corner-sharing tetrahedra, and the local crystal field environment causes the magnetic moments to point along the lines connecting the centers of two tetrahedra at low temperatures [5,6]. This strong single-ion Ising anisotropy, combined with dipolar and exchange interactions [7], results in a highly degenerate two-in/two-out spin configuration for the ground states of these materials [3,5] -locally equivalent to the situation for hydrogen atoms in water ice.Although numerical calculations [8] suggest the existence of a long range ordered state for these systems which would have no zero point entropy, the spins in these materials freeze into a non-equilibrium low temperature state with approximately the same zero point entropy as water ice [4,9 ], and no long range ordering has been observed experimentally in zero magnetic field. 3Here we examine experimentally how dilution of the spin ice lattice affects the zero point entropy over a broad range of dilution in both Ho and Dy spin ice materials --a measurement which would not be possible in water ice. We find that the zero point entropy of diluted spin ice depends non-monotonically on the level of dilution, with the zero point entropy per spin approaching th...

Air quality in China has been gradually improving because of recent emission control policies, but synoptic circulations in the planetary boundary layer have become increasingly conducive to haze formation. The effect of persistent synoptic circulation (PSC) on long-term variation in haze episodes remains to be fully elucidated. This study identified and analyzed PSC similarity and its influence on haze during the winter half year from 1961 to 2013. We analyzed sea level pressure and geopotential heights at the 850-hPa level by using correlation coefficients and mean Hamming distance to quantify PSC similarity. A total of 1,754 PSC events were identified, of which 236 and 167 PSC events of 868 and 625 days, respectively, were associated with six polluted types and three clean types, respectively. The variations in occurrence frequencies of polluted PSC events exhibited different trends in the past decades over eastern China: an increase in 1961-1979, no obvious change in 1980-1999, and a rapid increase since 2000. The durations of polluted and clean PSC events in the planetary boundary layer over eastern China have tended to become longer and shorter, respectively, over the past decades. Our analyses suggest that the reduction in Arctic sea ice in autumn may be favorable for less cold activities and more polluted PSC events with longer durations in eastern China, positively contributing to the formation and maintenance of persistent haze pollution. This work provides convincing evidence that the occurrence frequencies and durations of haze episodes increased because of the increase in pollution-related PSC events.

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We have characterized the magnetic and structural properties of the CdLn 2 Se 4 (Ln = Dy, Ho), and CdLn 2 S 4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all compounds to be normal spinels, possessing a geometrically frustrated sublattice of lanthanide atoms with no observable structural disorder. Fits to the high temperature magnetic susceptibilities indicate these materials to have effective antiferromagnetic interactions, with Curie-Weiss temperatures Θ W ~ -10 K, except CdYb 2 S 4 for which Θ W ~ -40 K. The absence of magnetic long range order or glassiness above T = 1.8 K strongly suggests that these materials are a new venue in which to study the effects of strong geometrical frustration, potentially as rich in new physical phenomena as that of the pyrochlore oxides. 1The geometry of the lanthanide ion sublattice in the pyrochlore structure is an array of corner sharing tetrahedra based on kagomé planes that connect via triangular layers. This arrangement of spins can show geometric frustration, such that no long range magnetic ordering occurs, even at low temperatures. 1,2 The magnetic behavior of the Ln 2 Ti 2 O 7 pyrochlores is widely studied, and, despite weak coupling between structural and magnetic degrees of freedom, the rare earth pyrochlores exhibit a particularly rich range of magnetic behavior due to a balance between dipole interactions, single ion anisotropy, and exchange couplings. 3,4,5,6,7,8,9 Many of these materials possess strong crystal fields causing highly anisotropic spin states. In Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 the spins' directionality and effective ferromagnetic interactions produce Ising-like spins leading to spin ice characteristics, where the orientations of the spins are analogous to the H positions in ice. 10,11,12,1314,15,16,17,18,19,20,21,22 We characterized the sample structures through x-ray powder diffraction data using CuKα radiation and a diffracted beam monochromater. Structural refinements were made using the Bruker AXS software package TOPAS 2.1 © operated with a Pseudo-Voight TCHZ fitting profile. Refined parameters include: zero corrections; sample displacement; scaling factors; cell dimensions; atomic positional coordinates; and 3 thermal parameters (B eq ). We measured the d.c. magnetic susceptibility with a SQUID magnetometer (Quantum Design MPMS) on cooling over T = 290 -2 K in an applied field of H = 0.1 T. We also performed measurements of a.c. magnetic susceptibility using the ACMS option for a Quantum Design PPMS cryostat. This instrument also gives d.c. magnetization measurements in fields up to 9 tesla using an extraction method.Curie-Weiss fits to the d.c. susceptibility data performed over T = 80 -270 K, with the exception of CdYb 2 S 4 , where fits were performed between T = 250 -350 K. The applied field dependence of the magnetization was measured at selected temperatures for all materials up to a field of 7 Tesla.Structure refinement shows all samples to be normal spinels with lattice parameters similar to those found previously 32,35,36 . ...

Understanding the evolution of heatwaves is important for their prediction, mitigation, and adaptation. While most studies focused on either their temporal variability at individual station (or grid point) or the spatial variation over fixed durations, their daily joint evolution over space and time remains largely unexplored. Here, we investigate multi‐dimensional characteristics of spatiotemporally contiguous heatwaves across China during 1961–2018 using observational data from a 3D perspective (i.e., latitude × longitude × time). Results obtained by the 3D identification show that large contiguous heatwaves in China exhibit distinct characteristics in different geographical locations and time periods. Heatwaves in northern areas prefer to move from west to east at a faster speed, persist longer, and have greater extent and intensity than southern heatwaves, which are generally originated from the east and move westward. It is also found that contiguous heatwaves are growing in frequency, magnitude, areal extent, duration, and traveling distance across China.