Microwave surface resistance measurements on the heavy fermion superconductor UPt3

The relative change of the microwave surface resistance has been measured as a function of temperature in the superconducting and normal states of UPt₃ using a cavity perturbation technique. With our frequency range of ∼9–38 GHz, the microwave photon energy (ℏω) is ∼20–95% of the estimated zero-temperature (BCS) energy gap (2Δ₀) of UPt₃. The surface resistance shows a change in slope as the signature of T_c and has a significantly weaker temperature dependence in the superconducting state than would be predicted by the Mattis–Bardeen theory. At higher frequencies, the slope tends to change by a smaller amount. No indication of collective modes at any of the frequencies was observed down to the lowest temperatures measured (∼0.1 K).     

Probing the nodal gap in the pressure-induced heavy fermion superconductor CeRhIn5

We report field-orientation specific heat studies of the pressure-induced heavy-fermion superconductor CeRhIn5. These experiments provide the momentum-dependent superconducting gap function for the first time in any pressure-induced superconductor. In the coexisting phase of superconductivity and antiferromagnetism, field rotation within the Ce-In plane reveals fourfold modulation in the density of states, which favors a d-wave order parameter and constrains a theory of the interplay between superconductivity and magnetism.     

A polarised neutron scattering study of magnetic fluctuations in the heavy fermion superconductor CeCu2Si2

Magnetic fluctuations in CeCu₂Si₂ were directly observed, both above the superconducting transition temperatureT _C as well as below. Spatial correlations of the magnetisation density belowT _C are weak, resulting in an essentially wavevector independent magnetic response. The intensity observed within an energy window of ±5 THz centred on =0 THz corresponds to 1µ per Ce atom. Higher energy excitations peaking between 7–10 THz were observed and are probably due to higher crystal field levels.     

Spin fluctuation in heavy fermion CeRu2Si2

Spin fluctuations of the archetypal heavy fermion compound CeRu2Si2 have been investigated by neutron scattering in an entire irreducible Brillouin zone. The dynamical susceptibility is remarkably well described by the self-consistent renormalization (SCR) theory of the spin fluctuation in a phenomenological way, proving the effectiveness of the theory. The present analysis using the SCR phenomenology has allowed us to determine 14 exchange constants, which show the long-range nature of the Ruderman-Kittel-Kasuya-Yosida interaction.     

Magnetic moments contribution to the specific heat and susceptibility of heavy fermion system

In non-magnetic heavy fermion systems, valence fluctuations prevent formation of long-range magnetic correlations. The RKKY exchange fields fluctuate due to f-electron transfers to the conduction band. To simulate these processes, we use the Ising-type model with fluctuating fields. The linear temperature dependence of the specific heat and temperature independence of susceptibility suggest that the f-electron contribution to these quantities can be considered as complementary to the one originating from Kondo resonance.     

Magnetostriction of monocrystalline URu2Si2

We report magnetostriction measurements on a monocrystalline sample of the heavy-fermion compound URu₂Si₂, below (at 4.2 K) and above (at 20 K) the Néel temperature. For a field direction along the tetragonal axis the magnetostriction is strongly anisotropic; the volume effect amounts to 1.4 × 10^-6T^-1 at 8 T and 4.2 K. For a field direction in the tetragonal plane the magnetostriction is nearly two orders of magnitude smaller, and less anisotropic.     

Flux line lattice melting and the formation of a coherent quasiparticle Bloch state in the ultraclean URu2Si2 superconductor

We find that in the ultraclean heavy-fermion superconductor URu(2)Si(2) (T_{c0}=1.45 K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at sub-Kelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.     

Structural properties and band structure of heavy fermion systems: CeCu2Si2 and LaCu2Si2

The structure of CeCu₂Si₂, isotypic with ThCr₂Si₂, has been refined in a single-crystal study. The atomic parameters were used in self-consistent LMTO band structure calculations for CeCu₂Si₂ and isostructural LaCu₂Si₂. The results are analysed in terms of energy levels, charge distribution and in particular Fermi surface properties. The Ce-4f levels are situated mainly aboveE _F , except near the Γ-point. The density-of-states atE _F is large and heavily concentrated around the Ce-4f band. This makes local Ce 4f fluctuations possible, while interaction with the rest of the band structure is small. The comparison with LaCu₂Si₂ shows that the additional Cef electron is partly promoted into the non-f bands. Large enhancement factors are required to bring band results into agreement with observed specific heat and magnetic critical field. Strong Fermi surface anisotropies are pointed out for both compounds suggesting new experiments on single crystals.     

Evidence of partial gap opening and Ce-site dilution effects in a heavy fermion compound CeNiGe2

We report the results of magnetisation, non-linear dc susceptibility, electrical transport, and heat capacity measurements on Y-substituted heavy fermion CeNiGe₂. Investigations are carried out on the compounds CeNiGe₂, Ce_0.9Y_0.1NiGe₂, Ce_0.8Y_0.2NiGe₂ and Ce_0.6Y_0.4NiGe₂. It is observed that with the increase in Y-concentration, the magnetic ordering temperature decreases. For CeNiGe₂, below ordering temperature Arrott plots suggest the presence of spin density wave (SDW). Third- and fifth-order dc susceptibility indicates magnetic instability which possibly leads to partial gap opening resulting in the observation of SDW. These observations are further investigated through resistivity and heat capacity measurements which also point toward partial gap opening in CeNiGe₂. Interestingly, with the increase in Y-substitution, it is noted that the gap opening is suppressed and also shifted towards lower temperature. Moreover, our investigations reveal absence of non-Fermi liquid behaviour or zero field quantum critical point even after 40% dilution of Ce-site.     

关于二硼化镁超导体比热的一点探索

在双带密度泛函理论的基础上,研究了双能隙超导体MgB2的超导态的比热跃变ΔC(T)与能隙的关系,指出了比热跃变的主要原因,并在转变温度Tc时将ΔC(T)/Tc的理论计算值与实验值作了对比,结果符合得很好。     

重电子金属CeCu(6-x)Nix低温电阻与比热的研究

测量了重电子金属CeCu6-xNix(x=0,005,01,015,02)01K—250K的低温电阻和5K—70K低温比热,发现样品电阻的极大值温度随着掺Ni含量的增大而急剧下降,这一现象反映少数与Ni邻近的Ce离子在极低温下磁矩的加强和整个Ce离子点阵对导电电子相干散射的减弱.与此相反,低温电子比热系数γ在较低温度下近于常数,而在8K附近因有效质量变大而明显上升,但γ明显上升的温度,对Ni的含量却不敏感,表明绝大部分Ce离子的状况并未受到影响 关键词： 重费米子系统 低温比热 低温电阻