arXiv subjects

D. Parker

Publications and source records attributed to D. Parker.

31 records · Page 2Linked to original sources

arXiv2008

Interband superconductivity: contrasts between BCS and Eliashberg theory

The newly discovered iron pnictide superconductors apparently present an unusual case of interband-channel pairing superconductivity. Here we show that, in the limit where the pairing occurs within the interband channel, several surprising effects occur quite naturally and generally: different density-of-states on the two bands lead to several unusual properties, including a gap ratio which behaves inversely to the ratio of density-of-states; the weak-coupling limit of the Eliashberg and the BCS theory, commonly taken as equivalent, in fact predict qualitatively different dependence of the $Δ_{1}/Δ_{2}$ and $Δ/T_{c}$ ratios on coupling constants. We show analytically that these effects follow directly from the interband character of superconductivity. Our results show that in the interband-only pairing model the maximal gap ratio is $\sqrt{N_{2}/N_{1}}$ as strong-coupling effects act only to reduce this ratio. This suggests that if the large experimentally reported gap ratios (up to a factor 2) are correct, the pairing mechanism must include more intraband interaction than is usually assumed.

cond-mat.supr-con↗

arXiv2008

NMR relaxation rate in superconducting pnictides: extended $s_{\pm}$ scenario

Recently, several measurements of the nuclear spin lattice relaxation rate $T_{1}^{-1}$ in the newly discovered superconducting Fe-pnictides have been reported. These measurements generally show no coherence peak below $T_{c}$ and indicate a low temperature power law behavior, the characteristics commonly taken as evidence of unconventional superconductivity with lines of nodes crossing the Fermi surface. In this work we show that (i) the lack of a coherence peak is fully consistent with the previously proposed nodeless extended $s_{\pm}$-wave symmetry of the order parameter (whether in the clean or dirty limit) and (ii) the low temperature power law behavior can be also explained in the framework of the same model, but requires going beyond the Born model.

cond-mat.supr-con↗

arXiv2008

Spin glass behavior in an interacting gamma-Fe2O3 nanoparticle system

In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, after subtraction of a superparamagnetic contribution from the superspin glass response functions. Finally, we discuss a possible origin of this superparamagnetic contribution in terms of dilute spin glass models.

cond-mat.dis-nn↗

arXiv2005

Upper critical field H_c2 in PrOs4Sb12

We study the upper critical field of the A and B phases in the triplet superconductor PrOs_{4}Sb_{12} within the p+h-wave superconductivity proposed recently for this material. The present result is compared with H_{c2}(t) and H*(t), the boundary between the A and B phase in PrOs_{4}Sb_{12}, reported earlier and with more recent data of H$_{c2}(t)$ for the single phase crystal. We find H_{c2}(t)'s for both the two phase crystal and the single phase crystal are described by the model for the A phase. From this fitting one can deduce the Fermi velocity as v=2.5 x 10^6 cm/s. On the other hand H_{c2}(t) for the B phase is found to be somewhat smaller than H*(t), which is rather puzzling.

cond-mat.supr-con↗

arXiv2005

New World of Gossamer Superconductivity

Since the discovery of the high-T$_{c}$ cuprate superconductor La$_{2-x}$BaCuO$_{4}$ in 1986 by Bednorz and Müller, controversy regarding the nature or origin of this remarkable superconductivity has continued. However, d-wave superconductivity in the hole-doped cuprates, arising due to the anti-paramagnon exchange, was established around 1994. More recently we have shown that the mean field theory, like the BCS theory of superconductivity and Landau's Fermi liquid theory are adequate to describe the cuprates. The keys for this development are the facts that a)the pseudogap phase is d-wave density wave (dDW) and that the high-T$_{c}$ cuprate superconductivity is gossamer (i.e. it exists in the presence of dDW).

cond-mat.supr-con↗

arXiv2005

Scaling Relations in the Vortex State of Nodal Superconductors

In contrast to multigap superconductors (e.g. MgB$_{2}$), the low-temperature properties of nodal superconductors are dominated by nodal excitations. Here we extend for a variety of nodal superocnductors the earlier work by Simon and Lee and Kübert and Hirschfeld. The scaling relations seen in the thermodynamics and the thermal conductivity will provide an unequivocal test of nodal superconductivity.

cond-mat.supr-con↗

arXiv2005

Gossamer Superconductivity, New Paradigm?

We shall review our recent works on d-wave density wave (dDW) and gossamer superconductivity (i.e. d-wave superconductivity in the presence of dDW) in high-T$_{c}$ cuprates and CeCoIn$_{5}$. a) We show that both the giant Nernst effect and the angle dependent magnetoresistance (ADMR) in the pseudogap phases of the cuprates and CeCoIn$_{5}$ are manifestations of dDW. b) The phase diagram of high-T$_{c}$ cuprates is understood in terms of mean field theory, which includes two order parameters $Δ_{1}$ and $Δ_{2}$, where one order parameter is from dDW and the other from d-wave superconductivity. c) In the optimally to the overdoped region we find the spatially periodic dDW, an analogue of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, becomes more stable. d) In the underdoped region where $Δ_{2}/Δ_{1} \ll 1$ the Uemera relation is obtained within the present model. We speculate that the gossamer superconductivity is at the heart of high-T$_{c}$ cuprate superconductors, the heavy-fermion superconductor CeCoIn$_{5}$ and the organic superconductors $κ$- (ET)$_{2}$Cu(NCS)$_{2}$ and (TMTSF)$_2$PF$_{6}$.

cond-mat.supr-con↗

arXiv2005

Topological defects in triplet superconductors UPt$_{3}$, Sr$_{2}$RuO$_{4}$, etc

After a brief introduction on nodal superconductors, we review the topological defects in triplet superconductors such as UPt$_{3}$, Sr$_{2}$RuO$_{4}$, etc. This is in part motivated by the surprising discovery of Ana Celia Mota and her colleagues that in some triplet superconductors the flux motion is completely impeded (the ideal pinning). Among topological defects the most prominent is Abrikosov's vortex with quantum flux $ϕ_{0}= \frac{hc}{2e}$. Abrikosov's vortex is universal and ubiquitous and seen in both conventional and unconventional superconductors by the Bitter decoration technique, small angle neutron scattering (SANS), scanning tunneling microscopy (STM), micromagnetometer and more recently by Lorentz electron micrograph. In order to interpret the experiment by Mota et al a variety of textures are proposed. In particular, in analogy to superfluid $^{3}$He-A the $\hat{\ell}$-soliton and $\hat{d}$-soliton play the prominent role. We review these notions and point out possible detection of these domain walls and half-quantum vortices in some triplet superconductors.

cond-mat.supr-con↗

arXiv2005

Scaling Relations in the Triplet Superconductor PrOs4Sb12

Scaling relations are one of the hallmarks of nodal superconductivity since they contain information characteristic for gapless order parameters. In this paper we derive the scaling relations for the thermodynamics and the thermal conductivity in the vortex state of the A and B phases of the skutterudite PrOs4Sb12. Experimental confirmation of these scaling relations can provide further support for anisotropic gap functions which were previously considered for this material.

cond-mat.supr-con↗

arXiv2005

BCS theory of nodal superconductors

This course has a dual purpose. First we review the successes of the weak-coupling BCS theory in describing new classes of superconductors discovered since 1979. They include the heavy-fermion superconductors, high-Tc cuprate superconductors, organic superconductors, Sr2RuO4, etc. Second, we present the quasiclassical approximation introduced by Volovik, which we extend to describe the thermodynamics and the thermal conductivity of the vortex state in nodal superconductors. This approach provides the most powerful tool to identify the symmetry of the energy gap function Delta(k) in these new superconductors.

cond-mat.supr-con↗

arXiv2004

Aging, rejuvenation and memory phenomena in spin glasses

In this paper, we review several important features of the out-of-equilibrium dynamics of spin glasses. Starting with the simplest experiments, we discuss the scaling laws used to describe the isothermal aging observed in spin glasses after a quench down to the low temperature phase. We report in particular new results on the sub-aging behaviour of spin glasses. We then discuss the rejuvenation and memory effects observed when a spin glass is submitted to temperature variations during aging, from the point of view of both energy landscape pictures and of real space pictures. We highlight the fact that both approaches point out the necessity of hierarchical processes involved in aging. Finally, we report an investigation of the effect of small temperature variations on aging in spin glass samples with various anisotropies which indicates that this hierarchy depends on the spin anisotropy.

cond-mat.dis-nn↗

arXiv2004

Gap Symmetry of Superconductivity in UPd2Al3

The angle dependent thermal conductivity of the heavy-fermion superconductor UPd$_2$Al$_3$ in the vortex state was recently measured by Watanabe et al. Here we analyze this data from two perspectives: universal heat conduction and the angle-dependence. We conclude that the superconducting gap function $Δ({\bf k})$ in UPd$_2$Al$_3$ has horizontal nodes and is given by $Δ({\bf k}) =Δ\cos(2χ)$, with $χ= ck_{z}$.

cond-mat.supr-con↗

arXiv2004

Panorama of Nodal Superconductors

Since 1979, many new classes of superconductors have been discovered, including heavy-fermion compounds, organic conductors, high-Tc cuprates, and Sr2RuO4. Most of these superconductors are unconventional and/or nodal. Therefore it is of central importance to determine the symmetry of the order parameter in each of these superconductors. In particular, the angular-controlled thermal conductivity in the vortex state provides a unique means of investigating the nodal structure of the superconducting energy gap when high-quality single crystals in the extremely clean limit are available. Using this method, Izawa et al have recently succeeded in identifying the energy gap symmetry of superconductivity in Sr2RuO4, CeCoIn5, kappa-(ET)2Cu(NCS)2, YNi2B2C, and PrOs4Sb12.

cond-mat.supr-con↗