BCS superconductors

Consider a BCS superconductor under the action of an external field that produces an asymmetry in the population of the fermions the effect of such... 

Takabayashi Y, Ganin AY, Jeglic P, Arcon D, Takano T, Iwasa Y, Ohishi Y, Takata M, Takeshita N, Prassides K, Rosseinsky MJ (2009) The disorder-free non-BCS superconductor CS3C60 emerges from an antiferromagnetic insulator parent state. Science 323 1585-1590... 

One of the most important features in the IR region of the spectrum should be the appearance of the superconducting energy gap. The higher transition temperature materials should display larger values for the gap if they are BCS superconductors. The gap observed for BaPbj.jjBijPg has been shown to agree with the BCS prediction (57). 

The theory above has been applied in a variety of realistic situations. The range includes ionic conductance in aqueous solutions and molten alkali chlorides, damped spin-wave behaviour in paramagnetic systems, stimulated emission of radiation in masers, the fractional quantum Hall effect and quantum correlations in high-Tc cuprates and other non-BCS superconductors [4, 5, 7, 8, 14, 30]. In the next section we will also make some comments on the problem of long-range transcorrelations of protons in DNA [31]. 

A model-independent conclusion The general behaviour of the in-plane paraconductivity is not affected, even up to T°, by doping. The SCF effects in L l eSr CuCU thin films seem to be not related to the pseudogap. t From the comparison of the measured paraconductivity with the ex-ended GGL approach i) The measured l c is a good mean-held critical temperature for the GGL approach, ii) Both the relaxation time of the SCF and the reduced temperature, e°, where the SCF vanish, are doping-independent and they take values close to those of BCS superconductors. This last result demands further studies. 

An analysis of the time dependent transient photo-reflectance gives us a good estimate of the effective pair recombination rate [8], The inset in Fig. 1(b) shows the average reflectance change as a function of the delay between pump and probe pulses at 6 K. In accordance with other BCS superconductors the effective recombination time is found to be a few nanoseconds. One important remark is that, within the experimental time resolution, no evidence of multiple decays is found. In fact, ultra-fast pump-probe measurements on MgB2 [12, 13] did not find any evidence for a double relaxation down to the ps regime. 

Newer superconductors are MgB2, a type-II BCS superconductor (TC = 39K) [62], "doped" FeSe (TC<27K) [63], and d-metal pnictide oxide superconductors LaFeAsO0.89F0.11 (TC = 26K) [64], Lao.sYosFeAsOo/,... 

What thus really makes one dimension so peculiar resides in the fact that the symmetry of the spectrum for the Cooper and Peierls instabilities refer to the same phase space of electronic states [108]. The two different kinds of pairing act as independent and simultaneous processes of the electron-electron scattering amplitude which interfere with and distort each other at all order of perturbation theory. What comes out of this interference is neither a BCS superconductor nor a Peierls/density-wave superstructure but a different instability of the Fermi liquid called a Luttinger liquid. 

Now due to the well-known isotope effect, the change of 7(. with the change of nuclear mass upon isotopic exchange, provides direct evidence for the phonon-mediated mechanism in the BCS superconductor. Since the discovery of high-7). materials, many effects have been shown in isotope effect studies. There exist small isotope exponents, a = — log 7)/log M, 0.12 and 0.04 for LSCO and YBCO7, respectively. The non-zero isotope exponent produces doubts about the exclusion of a phonon-mediated model although the discussion above supports strongly the spin-fluctuation-mediated model. 

The microwave absorption of the paired boson superconductor is calculated in the extremely local (or dirty) limit (6). We find that it is sharply different from that of the Mattis-Bardeen (8) behavior characteristic of a BCS superconductor. In particular, we find that the ratio os(<<>)/on is strongly temperature dependent and... 

Essential parts of the BCS theory can be taken over in the mean-field theory of the Peierls transition if one replaces the Debye energy tkoo in the superconductor by the Fermi energy Ep in the metal. Since Eplhrop, 10 -100, the Peierls phase transition temperatures are considerably higher than the critical temperatures of BCS superconductors, ii) The frequency of the phonons which are responsible for the Peierls transition has, for T > the temperature dependence... 

Many of the properties of the BCS superconductor are consequences of the formation of an energy gap Eg in the electron energy band at the Fermi energy ey when the temperature is reduced below Tc. The magnitude of this gap and Te are related by EglkaTc) < 4 where ks is Boltzman s constant. This energy gap arises from an effective attractive interaction between paired elec-... 

Further results from TF-p,SR studies on UPd2Al3 concern superconducting properties that are outside this review. We mention the approximately isotropic London penetration depth ( 5000A) and the ratio %/T 10 (T being the Fermi tenqierature) which is one order of magnitude larger than that for simple metallic BCS superconductors (W.D. Wu et al. 1994b). 

The specific heat at low temperatures consists of electronic, lattice, magnetic, and hyperfine contributions (see for example, Junod 1996). The electronic contribution in a metal due to the conduction elections is linear in temperature. In a BCS superconductor, the conduction electrons start to condense into Cooper pairs at and the density of unpaired electrons decreases exponentially below this temperature. Hence, at the temperatures of interest in this chapter (i.e., below 4K) the density of unpaired electrons is essentially zero as is expected for the electronic contribution. However, for RBa2Cu30j (R123x) a linear term of the order of y = 3-6 mJ mole was always observed (van der... 

At the time of writing it still remains unclear whether or not the ab-plane response of the electron-doped superconductor Nd2 ,CeyCu04 is also characterized by residual losses. Microwave measurements seem to indicate conventional electrodynamic response consistent with an s-wave gap (Wu et al. 1993). But this assertion was not supported by a FIR transmission experiment which did not show any of the expected features of the s-wave BCS superconductor (Choi et al. 1996). The conclusion of an isotropic BCS gap with a magnitude between 4.1 and 4.9A 7 (Stadlober et al. 1995) was inferred from Raman... 

As results accumulated, it was a matter of concern how the characteristic temperatures T, T() and Tc, as well as the gap energy A, relate to each other, and how they depend on the carrier density. Although controversies still remain, it is interesting to note that Tc does not scale either with T or Tq. On the other hand, the latter two temperatures seem to rise in the underdoped region, while falls. Another important indication is that the gap energy A remains rather constant as the temperature changes. Besides, A does not scale with Tc, which it does for a BCS superconductor as lA/k Tc = 3.52. 

Superconducting Properties. By means of flux quantization and Josephson tunneling experiments, it has been firmly established that superconductivity in the cuprate corrqtounds is based on electron pairs similar to that of conventional Bardeen-Cooper-Schreiffer (BCS) superconductors. However, there are currently widely... 

Onuki et al. (1987) have reported that the superconducting upper-critical field H (0) of polycrystalline URu2Si2 decreases at a rate of —8.4kOe/kbar (fig. 43c). For BCS superconductors, is related to the superconducting coherence length by... 

---