Quasiparticle field

Consider again non-relativistic fermions. Their BCS spectrum (for homogeneous systems) is isotropic when the polarizing field drives apart the Fermi surfaces of spin-up and down fermions the phase space overlap is lost, the pair correlations are suppressed, and eventually disappear at the Chandrasekhar-Clogston limit. The LOFF phase allows for a finite center-of-mass momentum of Cooper pairs Q and the quasiparticle spectrum is of the form... 

At this time, the fastest growing area in the field of nanophysics is in the studies of buckyballs and nanotubes. After the discovery [33] of the Qo molecule, many properties of the molecule and solids formed from the molecule were explored. The doped C6o crystals showed interesting behavior, including superconductivity. [34] The standard model, including the GW quasiparticle theory, was used [35] successfully to explore the energy band structure, and the superconducting properties appear to be consistent with the BCS theory. [36]... 

Results on the thermal conductivity in the superconducting mixed state have been reported for LuNi2B2C for temperatures down to 70 mK (7c/200) and magnetic fields from H — 0 to above Hc2 = 70 kOe (Boaknin et al. 2001, see fig. 24). It was found that as soon as vortices enter the sample, the thermal conductivity at T - 0 grows rapidly, showing unambiguously that delocalized quasiparticles are present even at the... 

Figure IV 2 6. ojH) below and above Tc = 89 K. A fit at 55 K to a superposition of Cooper pair (dashed line) and quasiparticle (dash-dotted line) contributions to magneto-conductivity is indicated (see text). Insert Zero field oc vs T/Tc extracted from ac(H,T) for a Bi-2212 crystal for J = 0.05 A/cm2 (full circles) and J = 0.1 A/cm2 (triangles), and obtained from the I- V curves for a mesa (empty circles). Both fit a T2 dependence (dashed line) up to T = y...

Figure IV 2 10. Solid triangles show temperature dependence of the out-of-plane quasiparticle conductivity ac (a) and in-plane quasiparticle conductivity Gab (b). Below Tc they are extracted from the JFF experiment on BSCCO long stack [17] and above Tc they represent the normal state conductivities of whiskers measured independently on samples from the same batch. Open circles correspond to the oc data from Ref. 14, obtained on small mesas in zero field, open squares correspond to 14.4 GHz microwave data for cab from Ref. 18 obtained on epitaxial films. Solid lines in both plots are just guides for the eye. Insert in (a) shows the low temperature part of ac(T) plotted versus T2...

The absorption spectrum is proportional to the imaginary part of the macroscopic dielectric function. Adopting the same level of approximation that we have introduced to obtain GW quasiparticle energies, i.e. neglecting the vertex correction by putting T = 55, we get the so called random phase approximation (RPA) for the dielectric matrix. Within this approximation, neglecting local field effects, the response to a longitudinal field, for q 0, is ... 

FIGURE 52 Vortex lattice in YNi2B2C imaged by plotting the local quasiparticle density of states at 0.46 K in magnetic fields H applied along the c-axis. (a) m)H = 0 07 T (b) 0.3 T (after Nishimori et al., 2004). 

Note, in general case the effective Hamiltonian contains also the Coulomb interaction between doped holes and the interaction of quasiparticles via the phonon field. We dropped these terms here, because they do not contribute directly to the spin susceptibility. The hopping matrix element is Uj = tfj e rE /hcot [ + < SiSj >] where r . is the bare hopping inte-... 

The physics of free carriers is dominated by the Fermi surface. Looking at the linearized spectrum of Fig. 2, one realizes that electron-hole or electron-electron excitations involving quasiparticles (electrons or holes) on each side of the Fermi surface are gapless. This greatly influences the response functions to external fields. Let an external field Fa(q) couple to the operator Oa(q), where... 

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