R. J. Cava

Superconductivity near 70 K in a new family of layered copper oxides

A new family of high-temperature superconductors is described, with the general formula Pb2Sr2ACu3O8+δ. Although they have the planes of CuO5 square pyramids characteristic of the other copper-oxide superconductors, the new compounds belong to a distinct structural series, with wide scope for elemental substitution. Their unusual electronic configuration also gives new insight into the role of charge distribution among the structural building blocks in controlling superconductivity.

Large, non-saturating magnetoresistance in WTe2

Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors, in magnetic memory, and in hard drives at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.Magnetoresistance is the change of a material’s electrical resistance in response to an applied magnetic field. In addition to its intrinsic scientific interest, it is a technologically important property, placing it in “Pasteur’s quadrant” of res earch value: materials with large magnetorsistance have found use as magnetic sensors 1, in magnetic memory2, hard drives3, transistors4, and are the subject of frequent study in the field of spintronics5, 6. Here we report the observation of an extremely large one-dimensional posi tive magnetoresistance (XMR) in the layered transition metal dichalcogenide (TMD) WTe2; 452,700% at 4.5 Kelvin in a magnetic field of 14.7 Tesla, and 2.5 million% at 0.4 Kelvin in 45 Tesla, with no saturation. The XMR is highly anisotropic, maximized in the crystallographic direction where small pockets of holes and electrons are found in the electronic structure . The determination of the origin of this effect and the fabrication of nanostructures and devices based on the XMR of WTe2 will represent a significant new direction in the study and uses of magnetoresistivity.

Spin entropy as the likely source of enhanced thermopower in Na x Co 2 O 4

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 NaxCo2O4, 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.

Why does undoped FeSe become a high-Tc superconductor under pressure?

Unlike the parent phases of the iron-arsenide high-Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc approximately 9 K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to approximately 37 K. We investigate the electronic properties of FeSe using 77Se NMR to search for the key to the superconducting mechanism. We demonstrate that the electronic properties of FeSe are very similar to those of electron-doped FeAs superconductors, and that antiferromagnetic spin fluctuations are strongly enhanced near Tc. Furthermore, applied pressure enhances spin fluctuations. Our findings suggest a link between spin fluctuations and the superconducting mechanism in FeSe.

Observation of a topological crystalline insulator phase and topological phase transition in Pb 1− x Sn x Te

A topological insulator protected by time-reversal symmetry is realized via spinorbit interaction driven band inversion. The topological phase in the Bi1−xSbx system is due to an odd number of band inversions. A related spin-orbit system, the Pb1−xSnxTe, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the Pb1−xSnxTe class of materials which has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi1−xSbx. Our observation of the spin-polarized Dirac surface states in the inverted Pb1−xSnxTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi(1-x)Sb(x) system is due to an odd number of band inversions. A related spin-orbit system, the Pb(1-x)Sn(x)Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb(1-x)Sn(x)Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi(1-x)Sb(x). Our observation of the spin-polarized Dirac surface states in the inverted Pb(1-x)Sn(x)Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

Superconductivity in Weyl semimetal candidate MoTe2

Transition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.

Electronic structure basis for the extraordinary magnetoresistance in WTe2

The electronic structure basis of the extremely large magnetoresistance in layered nonmagnetic tungsten ditelluride has been investigated by angle-resolved photoelectron spectroscopy. Hole and electron pockets of approximately the same size were found at low temperatures, suggesting that carrier compensation should be considered the primary source of the effect. The material exhibits a highly anisotropic Fermi surface from which the pronounced anisotropy of the magnetoresistance follows. A change in the Fermi surface with temperature was found and a high-density-of-states band that may take over conduction at higher temperatures and cause the observed turn-on behavior of the magnetoresistance in WTe2 was identified.

Dielectric properties of Ta2O5–ZrO2 polycrystalline ceramics

The dielectric properties of (Ta2O5)1−x(ZrO2)x polycrystalline ceramics at 1 MHz and temperatures between 0 and 100 °C are reported for 0.0<x<0.25. The dielectric constant is moderately enhanced in the solid solution as a function of Zr content, and there is a significant decrease in the temperature coefficient of the dielectric constant (TCK). The annealing temperature of the ceramics is found to have a dramatic effect on TCK, associated with the appearance of the low temperature form of tantalum oxide at intermediate Zr concentrations, and Ta4Zr11O32 at high Zr concentrations: a bulk ceramic consisting of a mixture of the latter two phases was found to display a dielectric constant of ≈50 and a temperature coefficient of ≈−40 ppm/°C.

Static disorder from lone-pair electrons in Bi{sub 2-x}M{sub x}Ru{sub 2}O{sub 7-y} (M = Cu, Co; x = 0, 0.4) pyrochlores.

Abstract The crystal structures of Bi2Ru2O7 and Bi 1.6 M 0.4 Ru 2 O 7 (M= Cu,Co ) pyrochlores are determined by Rietveld refinement of neutron powder diffraction data collected between 300 and 12 K . An appreciable oxygen non-stoichiometry, with no vacancy ordering, is found only in undoped Bi2Ru2O7. In all three compounds, static displacive disorder of both the Bi and O′ sites is observed. This type of disorder has not been reported previously for Bi2Ru2O7, and is proposed to be a common feature of A2B2O7 pyrochlores having a lone electronic pair on the A-site cation. The electronic properties of Bi2Ru2O7 are discussed in terms of calculated electronic band structure, local ruthenium coordination, and the static bismuth displacement.

13)C NMR investigation of the superconductor MgCNi(3) up to 800 K.

We report (13)C NMR characterization of the new superconductor MgCNi(3) [T. He et al., Nature (London) 411, 54 (2001)]. We found that both the uniform spin susceptibility and the spin fluctuations show a strong enhancement with decreasing temperature, and saturate below approximately 50 K and approximately 20 K, respectively. The nuclear spin-lattice relaxation rate 1/(13)T(1) exhibits typical behavior for isotropic s-wave superconductivity with a coherence peak below T(c) = 7.0 K that grows with decreasing magnetic field.