G. Baumgartner

ENERGY GAP IN SUPERCONDUCTING FULLERIDES : OPTICAL AND TUNNELING STUDIES

Tunneling and optical transmission studies have been performed on superconducting samples of Rb{sub 3}C{sub 60}. At temperatures much below the superconducting transition temperature {ital T}{sub c}, the energy gap is 2{Delta}=5.2{plus_minus}0.2 meV, corresponding to 2{Delta}/{ital k}{sub B}{ital T}{sub c}=4.2. The low temperature density of states and the temperature dependence of the optical conductivity resemble BCS behavior, although there is an enhanced {open_quote}{open_quote}normal state{close_quote}{close_quote} contribution. The results indicate that this fulleride material is an {ital s}-wave superconductor, but the superconductivity cannot be described in the weak coupling limit. {copyright} {ital 1996 The American Physical Society.}

Electronic properties of the

Electron Spin Resonance(ESR) at ambient and under high pressure and optical reflectivity measurements were used to study the consecutive phase transitions in Na2AC60 family of fulleride compounds. The high temperature fcc phase in all three A=K,Rb,Cs compounds are found to show semi-metallic behavior. For the first time a peculiar sequence of phase transitions from semi-metallic to metallic to superconducting phases are observed in Na2CsC60. The metallic properties of the polymeric phases in A=K,Rb were established.

Electronic properties of the Na2AC60 family (A=K, Rb, Cs)

Electron Spin Resonance(ESR) at ambient and under high pressure and optical reflectivity measurements were used to study the consecutive phase transitions in Na2AC60 family of fulleride compounds. The high temperature fcc phase in all three A=K,Rb,Cs compounds are found to show semi-metallic behavior. For the first time a peculiar sequence of phase transitions from semi-metallic to metallic to superconducting phases are observed in Na2CsC60. The metallic properties of the polymeric phases in A=K,Rb were established.

Frequency dependent ESR study of the magnetic phase transition in NH3K3C60

NH3K3C60 undergoes a magnetic ordering phase transition below 40K which is probably the reason that this system is not a superconductor at ambient pressure. We measured the magnetic field dependence (H=0.3, 2.7, 5.4 and 8.1 T) of the electron spin resonance of NH3K3C60. Below 40 K the resonance broadens and shifts as the magnetic order develops. The broadening and shift are much larger at 0.3 T than at higher fields, but do not follow the 1/H dependence expected for an antiferromagnetic order. The magnetic spin susceptibility measured by the ESR intensity changes only little, if at all, through the transition. At 9 GHz (0.3 T) results depend on the cooling rate from ambient temperatures to temperatures below 50K.

133Cs NMR study under pressure in linear polymers (CsC60)n

Alkali metal doped fullerides reveal a polymerized crystal structure with parallel chains of covalently bounded C60 molecules. The electronic structure shows a narrow p-electron band. The existence of strong electron-electron correlations in this fulleride is supported by the establishment of an antiferromagnetic ground state at low temperature. We have performed 133Cs-NMR relaxation time T1 measurements on (CsC60)n versus pressure and temperature. The salient and most unexpected result of our work is the fact that T1 is not sensitive to pressure (up to 9kbar) in the temperature range 100 and 300K unlike T1 measurements performed under pressure on 13C in the parent coumpound (RbC60)n. This finding allows to rule out a purely magnetic relaxation mechanism for the alkali atom. However, the relaxation becomes strongly pressure dependent at low temperature (T<100 K). In particular a new ground state (found to be non magnetic and possibly spin-Peierls) establishes above 5kbar with the concomitant activation of...

Electronic properties of the Na2AC60family (A=K, Rb, Cs)

Electron Spin Resonance(ESR) at ambient and under high pressure and optical reflectivity measurements were used to study the consecutive phase transitions in Na2AC60 family of fulleride compounds. The high temperature fcc phase in all three A=K,Rb,Cs compounds are found to show semi-metallic behavior. For the first time a peculiar sequence of phase transitions from semi-metallic to metallic to superconducting phases are observed in Na2CsC60. The metallic properties of the polymeric phases in A=K,Rb were established.

133Cs and 13C-NMR in CsC60 polymers under pressure

Abstract Alkali metal doped fullerides reveal a polymerized crystal structure with parallel chains of covalently bounded C 60 molecules. The electronic structure shows a narrow π-electron band. The existence of strong electron-electron correlations in this fulleride is supported by the establishment of an antiferromagnetic ground state at T N =30K. We report the measurements of 133 Cs and 13 C-NMR relaxation rates on CsC 60 polymers which have been performed versus pressure and temperature. Our results show that the hydrostatic pressure suppresses the antiferromagnetic order and stabilizes a spin-singlet long range order at 5kbar. A study of the quadrupolar spin-echo of 133 Cs performed at ambient pressure reveals a second order structural transition at T S =13.8K towards a spin-singlet ground state which coexists with the preexisting antiferromagnetic order.