We present the results of low-temperature dc magnetization measurements performed on Y0.8U0.2Pd3 ... more We present the results of low-temperature dc magnetization measurements performed on Y0.8U0.2Pd3 and UCu4Ni samples for magnetic fields up to 90kOe. Both systems have been reported to show electrical, thermal and magnetic properties suggestive of the development of “non-Fermi liquid” (NFL) behavior at low temperatures. The origin of this behavior has been a matter of controversy for more than one
We report the first Raman scattering measurements of BaC_6, SrC_6, and YbC_6, which along with ne... more We report the first Raman scattering measurements of BaC_6, SrC_6, and YbC_6, which along with new data on CaC_6, permits a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC_6, SrC_6, YbC_6, and CaC_6. This is due to increasing charge in the π^* electronic band. Electron-phonon interaction effects between the in-plane carbon modes at ≈1500 and the π^* electrons cause a strong non-adiabatic renormalization. As charge is transferred into the π^* band these non-adiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.
The field of superconductivity in the class of materials known as graphite intercalation compound... more The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s. This paper recontextualizes the field in light of the discovery of superconductivity in CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how this relates to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.
ABSTRACT Elastic neutron scattering studies down to 40 mK on YbCu4Au and YbCu4Pd revealed an anti... more ABSTRACT Elastic neutron scattering studies down to 40 mK on YbCu4Au and YbCu4Pd revealed an antiferromagnetic ground state for the former, but ferromagnetism for the latter compound. Due to the Kondo interaction the ordered magnetic moments are considerably reduced with respect to those expected for the Γ7 ground state doublet.
ABSTRACT The low-temperature phase diagram of polycrystalline UCu5 has been determined from magne... more ABSTRACT The low-temperature phase diagram of polycrystalline UCu5 has been determined from magnetization and magnetoresistance measurements. This system undergoes two magnetic transitions at low temperatures. The lower-temperature phase transition at T2=1.16 K, which manifests itself as a sudden jump in the magnetization and electrical resistance with decreasing temperature, shows a temperature hysteresis of about 40 mK for fields up to 12 T. The magnetic properties of this low-temperature phase show irreversible and metastable behaviour, which becomes more dramatic at the lower fields. The critical temperature T2 is observed to increase proportionally with the square of the magnetic field. Our results are discussed in comparison with available specific heat, neutron scattering and nuclear magnetic resonance data, with the aim of determining the actual nature of the low-temperature magnetic phase transition.
We present the results of low-temperature dc magnetization measurements performed on Y0.8U0.2Pd3 ... more We present the results of low-temperature dc magnetization measurements performed on Y0.8U0.2Pd3 and UCu4Ni samples for magnetic fields up to 90kOe. Both systems have been reported to show electrical, thermal and magnetic properties suggestive of the development of “non-Fermi liquid” (NFL) behavior at low temperatures. The origin of this behavior has been a matter of controversy for more than one
We report the first Raman scattering measurements of BaC_6, SrC_6, and YbC_6, which along with ne... more We report the first Raman scattering measurements of BaC_6, SrC_6, and YbC_6, which along with new data on CaC_6, permits a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC_6, SrC_6, YbC_6, and CaC_6. This is due to increasing charge in the π^* electronic band. Electron-phonon interaction effects between the in-plane carbon modes at ≈1500 and the π^* electrons cause a strong non-adiabatic renormalization. As charge is transferred into the π^* band these non-adiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.
The field of superconductivity in the class of materials known as graphite intercalation compound... more The field of superconductivity in the class of materials known as graphite intercalation compounds has a history dating back to the 1960s. This paper recontextualizes the field in light of the discovery of superconductivity in CaC6 and YbC6 in 2005. In what follows, we outline the crystal structure and electronic structure of these and related compounds. We go on to experiments addressing the superconducting energy gap, lattice dynamics, pressure dependence, and how this relates to theoretical studies. The bulk of the evidence strongly supports a BCS superconducting state. However, important questions remain regarding which electronic states and phonon modes are most important for superconductivity and whether current theoretical techniques can fully describe the dependence of the superconducting transition temperature on pressure and chemical composition.
ABSTRACT Elastic neutron scattering studies down to 40 mK on YbCu4Au and YbCu4Pd revealed an anti... more ABSTRACT Elastic neutron scattering studies down to 40 mK on YbCu4Au and YbCu4Pd revealed an antiferromagnetic ground state for the former, but ferromagnetism for the latter compound. Due to the Kondo interaction the ordered magnetic moments are considerably reduced with respect to those expected for the Γ7 ground state doublet.
ABSTRACT The low-temperature phase diagram of polycrystalline UCu5 has been determined from magne... more ABSTRACT The low-temperature phase diagram of polycrystalline UCu5 has been determined from magnetization and magnetoresistance measurements. This system undergoes two magnetic transitions at low temperatures. The lower-temperature phase transition at T2=1.16 K, which manifests itself as a sudden jump in the magnetization and electrical resistance with decreasing temperature, shows a temperature hysteresis of about 40 mK for fields up to 12 T. The magnetic properties of this low-temperature phase show irreversible and metastable behaviour, which becomes more dramatic at the lower fields. The critical temperature T2 is observed to increase proportionally with the square of the magnetic field. Our results are discussed in comparison with available specific heat, neutron scattering and nuclear magnetic resonance data, with the aim of determining the actual nature of the low-temperature magnetic phase transition.
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Papers by Mark Ellerby