How 'spin ice' freezes.

The large degeneracy of states resulting from the geometrical frustration of competing interactions is an essential ingredient of important problems in fields as diverse as magnetism, protein folding and neural networks. As first explained by Pauling, geometrical frustration of proton positions is also responsible for the unusual low-temperature thermodynamics of ice and its measured 'ground state' entropy. Recent work has shown that the geometrical frustration of ice is mimicked by Dy2Ti2O7, a site-ordered magnetic material in which the spins reside on a lattice of corner-sharing tetrahedra where they form an unusual magnetic ground state known as 'spin ice'. Here we identify a cooperative spin-freezing transition leading to the spin-ice ground state in Dy2Ti2O7. This transition is associated with a very narrow range of relaxation times, and represents a new form of spin-freezing. The dynamics are analogous to those associated with the freezing of protons in ice, and they provide a means through which to study glass-like behaviour and the consequences of frustration in the limit of low disorder.     

Strongly linked current flow in polycrystalline forms of the superconductor MgB2

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.     

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.     

Non-Fermi-liquid behaviour in La4Ru6O19

Understanding the complexities of electronic and magnetic ground states in solids is one of the main goals of solid-state physics. Transition-metal oxides have proved to be particularly fruitful in this regard, especially for those materials with the perovskite structure, where the special characteristics of transition-metal-oxygen orbital hybridization determine their properties. Ruthenates have recently emerged as an important family of perovskites because of the unexpected evolution from high-temperature ferromagnetism in SrRuO3 to low-temperature superconductivity in Sr2RuO4 (refs 1, 2). Here we show that a ruthenate in a different structural family, La4Ru6O19, displays a number of highly unusual properties, most notably non-Fermi-liquid behaviour. The properties of La4Ru6O19 have no analogy among the thousands of previously characterized transition-metal oxides. Instead, they resemble those of CeCu6-xAux-a widely studied f-electron-based heavy fermion intermetallic compound that is often considered as providing the best example of non-Fermi-liquid behaviour. In the ruthenate, non-Fermi-liquid behaviour appears to arise from just the right balance between the interactions of localized electronic states derived from Ru-Ru bonding and delocalized states derived from Ru-O hybridization.     

Superconductivity in the non-oxide perovskite MgCNi3

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.     

Superconductivity phase diagram of Na(x)CoO2*1.3H2O

The microscopic origin of superconductivity in the high-transition-temperature (high-T(c)) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting T(c) on the degree of electronic band filling. The recent report of superconductivity near 4 K in the layered sodium cobalt oxyhydrate, Na(0.35)CoO2*1.3H2O, is of interest owing to both its triangular cobalt-oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting T(c) of this compound displays the same kind of behaviour on chemical doping that is observed in the high-T(c) copper oxides. Specifically, the optimal superconducting T(c) occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that Na(x)CoO2*1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.     

The suppression of star formation by powerful active galactic nuclei

The old, red stars that constitute the bulges of galaxies, and the massive black holes at their centres, are the relics of a period in cosmic history when galaxies formed stars at remarkable rates and active galactic nuclei (AGN) shone brightly as a result of accretion onto black holes. It is widely suspected, but unproved, that the tight correlation between the mass of the black hole and the mass of the stellar bulge results from the AGN quenching the surrounding star formation as it approaches its peak luminosity. X-rays trace emission from AGN unambiguously, whereas powerful star-forming galaxies are usually dust-obscured and are brightest at infrared and submillimetre wavelengths. Here we report submillimetre and X-ray observations that show that rapid star formation was common in the host galaxies of AGN when the Universe was 2-6 billion years old, but that the most vigorous star formation is not observed around black holes above an X-ray luminosity of 10(44) ergs per second. This suppression of star formation in the host galaxy of a powerful AGN is a key prediction of models in which the AGN drives an outflow, expelling the interstellar medium of its host and transforming the galaxy's properties in a brief period of cosmic time.     

Spin entropy as the likely source of enhanced thermopower in Na(x)Co2O4

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.     

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.