Publications (14)14.74 Total impact
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Article: Change of pairing symmetry in the iron-based superconductor KFe2As2
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ABSTRACT: The pairing mechanism in iron-based superconductors is the subject of ongoing debate. Proximity to an antiferromagnetic phase suggests that pairing is mediated by spin fluctuations, but orbital fluctuations have also been invoked. The former typically favour a pairing state of extended s-wave symmetry with a gap that changes sign between electron and hole Fermi surfaces (s+-), while the latter yield a standard s-wave state without sign change (s++). Here we show that applying pressure to KFe2As2 induces a change of pairing state. The critical temperature Tc decreases with pressure initially, and then suddenly increases, above a critical pressure Pc. The constancy of the Hall coefficient through Pc rules out a change in the Fermi surface. There is compelling evidence that the pairing state below Pc is d-wave, from bulk measurements at ambient pressure. Above Pc, the high sensitivity to disorder argues for a particular kind of s+- state. The change from d-wave to s-wave is likely to proceed via an unusual s + id state that breaks time-reversal symmetry. The proximity of two distinct pairing states found here experimentally is natural given the near degeneracy of d-wave and s+- states found theoretically. These findings make a compelling case for spin-fluctuation-mediated superconductivity in this key iron-arsenide material.03/2013; -
Article: Decrease of upper critical field with underdoping in cuprate superconductors
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ABSTRACT: The transition temperature Tc of cuprate superconductors falls when the doping p is reduced below a certain optimal value. It is unclear whether this fall is due to strong phase fluctuations or to a decrease in the pairing gap. Different interpretations of photoemission data disagree on the evolution of the pairing gap and different estimates of the upper critical field Hc2 are in sharp contradiction. Here we resolve this contradiction by showing that superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via the Nernst effect, have a characteristic field scale that falls with underdoping. The critical field Hc2 dips at p = 0.11, showing that superconductivity is weak where stripe order is strong. In the archetypal cuprate superconductor YBCO, Hc2 extracted from other measurements has the same doping dependence, also with a minimum at p = 0.11, again where stripe order is present. We conclude that competing states such as stripe order weaken superconductivity and this, rather than phase fluctuations, causes Tc to fall as cuprates become underdoped.08/2012; -
Article: From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2
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ABSTRACT: The nature of the pairing state in iron-based superconductors is the subject of much debate. Here we argue that in one material, the stoichiometric iron pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state, characterized by symmetry-imposed vertical line nodes in the superconducting gap. This evidence is reviewed, with a focus on thermal conductivity and the strong impact of impurity scattering on the critical temperature Tc. We then compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the pairing symmetry is s-wave and the Tc is ten times higher. The transition from d-wave to s-wave within the same crystal structure provides a rare opportunity to investigate the connection between band structure and pairing mechanism. We also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which the pairing symmetry is probably not d-wave, but more likely s-wave with accidental line nodes.07/2012; -
Article: New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122
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ABSTRACT: The electrical resistivity rho of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured in applied pressures up to 2.6 GPa for four underdoped samples, with x = 0.16, 0.18, 0.19 and 0.21. The antiferromagnetic ordering temperature T_N, detected as a sharp anomaly in rho(T), decreases linearly with pressure. At pressures above around 1.0 GPa, a second sharp anomaly is detected at a lower temperature T_0, which rises with pressure. We attribute this second anomaly to the onset of a phase that causes a reconstruction of the Fermi surface. This new phase expands with increasing x and it competes with superconductivity. We discuss the possibility that a second spin-density wave orders at T_0, with a Q vector distinct from that of the spin-density wave that sets in at T_N.05/2012; -
Article: Universal heat conduction in the iron-arsenide superconductor KFe2As2 : Evidence of a d-wave state
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ABSTRACT: The thermal conductivity of the iron-arsenide superconductor KFe2As2 was measured down to 50 mK for a heat current parallel and perpendicular to the tetragonal c-axis. A residual linear term (RLT) at T=0 is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of the RLT in the plane is equal to that reported by Dong et al. [Phys. Rev. Lett. 104, 087005 (2010)] for a sample whose residual resistivity was ten times larger. This independence of the RLT on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate for suppressing Tc to zero; the magnitude of the RLT, and its dependence on current direction and on magnetic field; the temperature dependence of the thermal conductivity.01/2012; -
Article: Universal Heat Conduction in the Iron Arsenide Superconductor KFe_{2}As_{2}: Evidence of a d-Wave State
Physical Review Letters 01/2012; 109(8):087001. · 7.37 Impact Factor -
Article: Pressure-induced Fermi-surface reconstruction in the iron-arsenide superconductor Ba_{1−x}K_{x}Fe_{2}As_{2}: Evidence of a phase transition inside the antiferromagnetic phase
Physical Review B. 01/2012; 86(14):140502. -
Article: Doping-induced vertical line nodes in the superconducting gap of the iron arsenide K-Ba122 from directional thermal conductivity
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ABSTRACT: The thermal conductivity k of the iron-arsenide superconductor K-Ba122 was measured down to 50 mK in a magnetic field up to 15 T, for a heat current parallel and perpendicular to the tetragonal c axis. In the range from optimal doping (x ~ 0.4) down to x = 0.16, there is no residual linear term in k(T) at T = 0, showing that there are no nodes in the superconducting gap anywhere on the Fermi surface. Upon crossing below x = 0.16, a large residual linear term suddenly appears, signaling the onset of nodes in the superconducting gap, most likely vertical line nodes running along the c axis. We discuss two scenarios: 1) accidental nodes in an s-wave gap, resulting from a strong modulation of the gap around the Fermi surface, in which minima deepen rapidly with underdoping; 2) a phase transition from a nodeless s-wave state to a d-wave state, in which nodes are imposed by symmetry.05/2011; -
Article: Isotropic three-dimensional gap in the iron-arsenide superconductor LiFeAs from directional heat transport measurements
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ABSTRACT: The thermal conductivity k of the iron-arsenide superconductor LiFeAs (Tc ~ 18K) was measured in single crystals at temperatures down to T~50mK and in magnetic fields up to H=17T, very close to the upper critical field Hc2~18T. For both directions of the heat current, parallel and perpendicular to the tetragonal c-axis, a negligible residual linear term k/T is found as T ->0, revealing that there are no zero-energy quasiparticles in the superconducting state. The increase in k with magnetic field is the same for both current directions and it follows closely the dependence expected for an isotropic superconducting gap. There is no evidence of multi-band character, whereby the gap would be different on different Fermi-surface sheets. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc).04/2011; -
Article: Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 from c-axis heat transport measurements
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ABSTRACT: The thermal conductivity k of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 was measured down to 50 mK for a heat current parallel (k_c) and perpendicular (k_a) to the tetragonal c axis, for seven Co concentrations from underdoped to overdoped regions of the phase diagram (0.038 < x < 0.127). A residual linear term k_c0/T is observed in the T = 0 limit when the current is along the c axis, revealing the presence of nodes in the gap. Because the nodes appear as x moves away from the concentration of maximal T_c, they must be accidental, not imposed by symmetry, and are therefore compatible with an s_{+/-} state, for example. The fact that the in-plane residual linear term k_a0/T is negligible at all x implies that the nodes are located in regions of the Fermi surface that contribute strongly to c-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g. H_c2/4) excites quasiparticles that conduct heat along the a axis just as well as the nodal quasiparticles conduct along the c axis. This shows that the gap must be very small (but non-zero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong k dependence of the gap Delta(k) which produces nodes on a Fermi surface sheet with pronounced c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.04/2010; -
Article: Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe(1-x)Co(x))2As2: from isotropic to a strongly k-dependent gap structure.
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ABSTRACT: The temperature and magnetic field dependence of the in-plane thermal conductivity kappa of the iron-arsenide superconductor Ba(Fe(1-x)Co(x))2As2 was measured down to T approximately 50 mK and up to H = 15 T as a function of Co concentration x in the range 0.048 < or = x < or = 0.114. At H = 0, a negligible residual linear term in kappa/T as T-->0 at all x shows that the superconducting gap has no nodes in the ab plane anywhere in the phase diagram. However, while the slow H dependence of kappa(H) at T-->0 in the underdoped regime is consistent with a superconducting gap that is large everywhere on the Fermi surface, the rapid increase in kappa(H) observed in the overdoped regime shows that the gap acquires a deep minimum somewhere on the Fermi surface. Outside the antiferromagnetic-orthorhombic phase, the superconducting gap structure has a strongly k-dependent amplitude.Physical Review Letters 02/2010; 104(6):067002. · 7.37 Impact Factor -
Article: Quasiparticle Heat Transport in Ba$_{1-x}$K$_x$Fe$_2$As$_2$: Evidence for a k-dependent Superconducting Gap without Nodes
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ABSTRACT: The thermal conductivity $\kappa$ of the iron-arsenide superconductor Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($T_c \simeq$ 30 K) was measured in single crystals at temperatures down to $T \simeq 50$ mK ($\simeq T_c$/600) and in magnetic fields up to $H = 15$ T ($\simeq H_{c2}$/4). A negligible residual linear term in $\kappa/T$ as $T \to 0$ shows that there are no zero-energy quasiparticles in the superconducting state. This rules out the existence of line and in-plane point nodes in the superconducting gap, imposing strong constraints on the symmetry of the order parameter. It excludes d-wave symmetry, drawing a clear distinction between these superconductors and the high-$T_c$ cuprates. However, the fact that a magnetic field much smaller than $H_{c2}$ can induce a residual linear term indicates that the gap must be very small on part of the Fermi surface, whether from strong anisotropy or band dependence, or both.04/2009; -
Article: Quasiparticle heat transport in single-crystalline Ba_ {1− x} K_ {x} Fe_ {2} As_ {2}: Evidence for a k-dependent superconducting gap without nodes
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ABSTRACT: The thermal conductivity κ of the iron-arsenide superconductor Ba1−xKxFe2As2(Tc≃30 K) was measured in single crystals at temperatures down to T≃50 mK(≃Tc/600) and in magnetic fields up to H=15 T(≃Hc2/4). A negligible residual linear term in κ/T as T→0 shows that there are no zero-energy quasiparticles in the superconducting state. This rules out the existence of line and in-plane point nodes in the superconducting gap, imposing strong constraints on the symmetry of the order parameter. It excludes d-wave symmetry, drawing a clear distinction between these superconductors and the high-Tc cuprates. However, the fact that a magnetic field much smaller than Hc2 can induce a residual linear term indicates that the gap must be very small on part of the Fermi surface, whether from strong anisotropy or band dependence, or both.Phys. Rev. B. 80(14). -
Article: Nodes in the gap structure of the iron arsenide superconductor Ba (Fe_ {1− x} Co_ {x}) _ {2} As_ {2} from c-axis heat transport measurements
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ABSTRACT: The thermal conductivity κ of the iron-arsenide superconductor Ba(Fe1−xCox)2As2 was measured down to 50 mK for a heat current parallel (κc) and perpendicular (κa) to the tetragonal c axis for seven Co concentrations from underdoped to overdoped regions of the phase diagram (0.038≤x≤0.127). A residual linear term κc0/T is observed in the T→0 limit when the current is along the c axis, revealing the presence of nodes in the gap. Because the nodes appear as x moves away from the concentration of maximal Tc, they must be accidental, not imposed by symmetry, and are therefore compatible with an s± state, for example. The fact that the in-plane residual linear term κa0/T is negligible at all x implies that the nodes are located in regions of the Fermi surface that contribute strongly to c-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g., Hc2/4) excites quasiparticles that conduct heat along the a axis just as well as the nodal quasiparticles conduct along the c axis. This shows that the gap must be very small (but nonzero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong k dependence of the gap Δ(k) which produces nodes on a Fermi-surface sheet with pronounced c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.Phys. Rev. B. 82(6).
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2011
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Université de Sherbrooke
- Department of Physics
Sherbrooke, Quebec, Canada
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