Oystein Fischer

Scanning tunneling spectroscopy of high-temperature superconductors

Tunneling spectroscopy has played a central role in the experimental verification of the microscopic theory of superconductivity in classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy and spectroscopy (STM and STS) on these compounds allowed the main difficulties to be overcome. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. The crucial efforts to gain control over the technique and to obtain reproducible results are first recalled. Then a discussion on how the STM and STS techniques have contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors is presented: the unusually large gap values and the absence of scaling with the critical temperature, the pseudogap and its relation to superconductivity, the unprecedented small size of the vortex cores and its influence on vortex matter, the unexpected electronic properties of the vortex cores, and the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states and in the vortex cores.

Magnetic ordering in the superconducting state of rare earth molybdenum sulphides, (RE)1.2Mo6S8 (RE = Tb, Dy and Er)

Abstract The resistance and ac susceptibility measurements on Tb 1.2 Mo 6 S 8 , Dy 1.2 Mo 6 S 8 and Er 1.2 Mo 6 S 8 revealed a magnetic ordering in their superconducting state at 0.9, 0.4 and 0.15 K, respectively. Some of the important features of the ordering in each compound are discussed with reference to their upper critical field curves.

Vortex imaging in the pi band of magnesium diboride.

We report scanning tunneling spectroscopy imaging of the vortex lattice in single crystalline MgB2. By tunneling parallel to the c axis, a single superconducting gap (Delta=2.2 meV) associated with the pi band is observed. The vortices in the pi band have a large core size compared to estimates based on H(c2) and show an absence of localized states in the core. Furthermore, superconductivity between the vortices is rapidly suppressed by an applied field. These results suggest that superconductivity in the pi band is, at least partially, induced by the intrinsically superconducting sigma band.

Ferroelectric Field Effect in Epitaxial Thin Film Oxide SrCuO2/Pb(Zr0.52Ti0.48)O3 Heterostructures

A ferroelectric field effect in epitaxial thin film SrCuO2/Pb(Zr0.52Ti0.48)O3 heterostructures was observed. A 3.5 percent change in the resistance of a 40 angstrom SrCuO2 layer (a parent high-temperature superconducting compound) was measured when the polarization field of the Pb(Zr0.52Ti0.48)O3 layer was reversed by the application of a pulse of small voltage (<5 volts). This effect, both reversible and nonvolatile, is attributed to the electric field-induced charge at the interface of SrCuO2 and Pb(Zr0.52Ti0.48)O3. This completely epitaxial thin film approach shows the possibility of making nonvolatile, low-voltage ferroelectric field effect devices for both applications and fundamental studies of field-induced doping in novel compounds like SrCuO2.

New pseudo-one-dimensional metals: M2Mo6Se6 (M = Na, In, K, TI), M2Mo6S6 (M = K, Rb, Cs), M2Mo6Te6 (M = In, TI)

Abstract We present a new series of ternary chalcogenides, derived from divalent molybdenum: M2Mo6X6. These compounds crystallize in a hexagonal lattice with a ≈ 9 A, c ≈ 4.5 A, and space group P6 3 m . The compounds are characterized by clusters (Mo3)1∞ in the form of linear chains, resulting from a linear condensation of Mo6 octahedral clusters. The (Mo3)1∞ clusters are well separated from each other, with the shortest MoMo intercluster distance larger than 6 A. The resulting pseudo-one-dimensional structures show remarkable anisotropy of physical properties.

A study of the solid solutions PbyMo6X8−xX′x and Mo6X8−xX′x (X, X′ = S, Se, Te) and their superconducting properties

Abstract The solid solutions Pb y Mo 6 X 8−x X′ x and Mo 6 X 8−x X′ x (X, X′ = S, Se, Te) are studied and it is shown that there are correlations between T c and certain structural parameters.

New compounds of the type Mo2Re4X8 (M = S, Se) containing octahedral Mo2Re4 clusters

We report the preparation and some properties of new compounds with the composition Mo2Re4X8 (X = S, Se) and Mo4Re2Te8. These compounds are isostructural to Mo6Se8 and present mixed octahedral clusters : Mo2Re4 or Mo4Re2. The solid solutions Mo2Re4S8−xSex (0 ⩽ x ⩽ 8) and Mo2Re4Se8−xTex (0 ⩽ x ⩽ 1.2) are semiconducting, whereas Mo4Re2Te8 is metallic and becomes superconducting at Tc = 3.55 K.

On the superconductivity of PbMo6S8 and the series MxPbMo6S8 and MxPb1−xMo6S8

Abstract We have studied the correlation between the lattice parameters and the superconducting critical temperature T c in PbMo 6 S 8 and the series M x -PbMo 6 S 8 and M x Pb 1− x Mo 6 S 8 ( M = Cu , Ag, Ba, Sn, Eu, La, Gd, Lu, Y, Th, U). A net correlation was found between the volume V H and T c when M is trivalent or tetravalent. We also found that the high T c modification of PbMo 6 S 8 has a maximal occupation of the lattice site centred about the origin of the rhombohedral lattice cell and that this can be obtained by insertion of trivalent and tetravalent elements.

A new pseudo-one-dimensional superconductor: Tℓ2Mo6Se6

Abstract The properties of a new superconducting pseudo-one-dimensional compound, Tl2Mo6Se6 are presented. The crystal structure of this material contains linear chains in the form of infinite stacks of Mo6 clusters. The anisotropy in the normal state resistivity is found to be of the order of 103 and the parallel upper critical field is 26 times higher than the perpendicular one.

Synthesis and electrical properties of new chalcogenide compounds containing mixed (Mo, Me)6 octahedral clusters (Me = Ru or Rh)

Abstract We report the synthesis and some properties of new compounds and some solid solutions containing mixed octahedral ( Me ) 6 clusters ( Me = Mo, Ru, Rh). The compound Mo 4 Ru 2 Se 8 is semiconducting whereas the limit compounds Mo 4 Ru 2 Te 8 and Mo 4.66 Rh 1.33 Te 8 are metallic. Superconductivity was found around the composition Mo 5 RuTe 8 .