arXiv subjects

Louis Taillefer

Publications and source records attributed to Louis Taillefer.

At least 55 records · Page 3Linked to original sources

arXiv2016

Thermal Conductivity of the Iron-Based Superconductor FeSe : Nodeless Gap with Strong Two-Band Character

The thermal conductivity of the iron-based superconductor FeSe was measured at temperatures down to 50 mK in magnetic fields up to 17 T. In zero magnetic field, the electronic residual linear term in the T = 0 limit, κ_0/T, is vanishingly small. Application of a magnetic field H causes no increase in κ_0/T initially. Those two observations show that there are no zero-energy quasiparticles that carry heat and therefore no nodes in the superconducting gap of FeSe. The full field dependence of κ_0/T has the classic shape of a two-band superconductor, such as MgB2: it rises exponentially at very low field, with a characteristic field H* << Hc2, and then more slowly up to the upper critical field Hc2. This shows that the superconducting gap is very small on one of the pockets in the Fermi surface of FeSe.

cond-mat.supr-con↗

arXiv2016

Heat transport study of field-tuned quantum criticality in CeIrIn5

The in-plane electrical resistivity, rho, and thermal conductivity, kappa, of the heavy-fermion superconductor CeIrIn5 were measured down to 40mK in magnetic fields up to 11 T applied along the c axis. For all fields above Hc2=4T of filamentary superconductivity, we find that the ratio of heat and charge conductivities in the T to 0 limit obeys the Wiedemann-Franz law, kappa/T=L0/rho, where L0 = 2.45*10^-8 WOhmK^-2 is the Sommerfeld value of the Lorenz number. The temperature-dependent parts of both the electrical and thermal resistivity,w = T/L0 kappa, follow the functional dependence expected for the Fermi liquid theory of metals with rho - rho_0 = AT^2, w - w_0 = BT^2, with rho0 = w0 and B ~ 2A. The coefficient B does not show a significant field dependence even upon approaching Hc2 = 0.4 T of the bulk superconducting state. The weak response to the magnetic field is in stark contrast with the behavior found in the closely related CeCoIn5, in which the field-tuned quantum critical point coincides with Hc2. The value of the electron-electron mass enhancement, as judged by the A and B coefficients, is about one order of magnitude reduced in CeIrIn5 as compared to CeCoIn5 (in spite of the fact that the zero field gamma_0 in CeIrIn5 is twice as large as gamma_0 in CeCoIn5), which suggests that the material is significantly farther away from the magnetic quantum critical point at bulk Hc2 and at all of the studied fields. A suppressed Kadowaki-Woods ratio in CeIrIn5 compared to CeCoIn5 suggests a notably more localized nature of f electrons in the compound.

cond-mat.str-el↗

arXiv2016

Doping evolution of the superconducting gap structure in the underdoped iron arsenide Ba1-xKxFe2As2 revealed by thermal conductivity

The thermal conductivity kappa of the iron-arsenide superconductor Ba1-xKxFe2As2 was measured for heat currents parallel and perpendicular to the tetragonal c axis at temperatures down to 50 mK and in magnetic fields up to 15 T. Measurements were performed on samples with compositions ranging from optimal doping (x = 0.34; Tc = 39 K) down to dopings deep into the region where antiferromagnetic order coexists with superconductivity (x = 0.16; Tc = 7 K). In zero field, there is no residual linear term in kappa(T) as T goes to 0 at any doping, whether for in-plane or inter-plane transport. This shows that there are no nodes in the superconducting gap. However, as x decreases into the range of coexistence with antiferromagnetism, the residual linear term grows more and more rapidly with applied magnetic field. This shows that the superconducting energy gap develops minima at certain locations on the Fermi surface and these minima deepen with decreasing x. We propose that the minima in the gap structure arise when the Fermi surface of Ba1-xKxFe2As2 is reconstructed by the antiferromagnetic order.

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arXiv2015

Expansion of the tetragonal magnetic phase with pressure in the iron-arsenide superconductor Ba{1-x}KxFe2As2

In the temperature-concentration phase diagram of most iron-based superconductors, antiferromagnetic order is gradually suppressed to zero at a critical point, and a dome of superconductivity forms around that point. The nature of the magnetic phase and its fluctuations is of fundamental importance for elucidating the pairing mechanism. In Ba{1-x}KxFe2As2 and Ba{1-x}NaxFe2As2, it has recently become clear that the usual stripe-like magnetic phase, of orthorhombic symmetry, gives way to a second magnetic phase, of tetragonal symmetry, near the critical point, between x = 0.24 and x = 0.28. Here we report measurements of the electrical resistivity of Ba{1-x}KxFe2As2 under applied hydrostatic pressures up to 2.75 GPa, for x = 0.22, 0.24 and 0.28. We track the onset of the tetragonal magnetic phase using the sharp anomaly it produces in the resistivity. In the temperature-concentration phase diagram of Ba{1-x}KxFe2As2, we find that pressure greatly expands the tetragonal magnetic phase, while the stripe-like phase shrinks. This raises the interesting possibility that the fluctuations of the former phase might be involved in the pairing mechanism responsible for the superconductivity.

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arXiv2015

Two types of nematicity in the phase diagram of the cuprate superconductor YBa$_2$Cu$_3$O$_y$

Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order and the pseudogap remains unclear. Here we use measurements of transport anisotropy in YBa$_2$Cu$_3$O$_y$ to distinguish two types of nematicity. The first is associated with short-range charge-density-wave modulations in a doping region near $p = 0.12$. It is detected in the Nernst coefficient, but not in the resistivity. The second type prevails at lower doping, where there are spin modulations but no charge modulations. In this case, the onset of in-plane anisotropy - detected in both the Nernst coefficient and the resistivity - follows a line in the temperature-doping phase diagram that tracks the pseudogap energy. We discuss two possible scenarios for the latter nematicity.

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arXiv2015

Change of carrier density at the pseudogap critical point of a cuprate superconductor

The pseudogap is a central puzzle of cuprate superconductors. Its connection to the Mott insulator at low doping $p$ remains ambiguous and its relation to the charge order that reconstructs the Fermi surface at intermediate $p$ is still unclear. Here we use measurements of the Hall coefficient in magnetic fields up to 88 T to show that Fermi-surface reconstruction by charge order in YBa$_2$Cu$_3$O$_y$ ends sharply at a critical doping $p = 0.16$, distinctly lower than the pseudogap critical point at $p^* = 0.19$. This shows that pseudogap and charge order are separate phenomena. We then find that the change of carrier density from $n = 1 + p$ in the conventional metal at high p to $n = p$ at low $p$ - a signature of the lightly doped cuprates - starts at $p^*$. This shows that pseudogap and antiferromagnetic Mott insulator are linked.

cond-mat.supr-con↗

arXiv2015

Onset field for Fermi-surface reconstruction in the cuprate superconductor YBCO

Quantum oscillations and negative Hall and Seebeck coefficients at low temperature and high magnetic field have shown the Fermi surface of underdoped cuprates to contain a small closed electron pocket. It is thought to result from a reconstruction by charge order, but whether it is the order seen by NMR and ultrasound above a threshold field or the short-range modulations seen by X-ray diffraction in zero field is unclear. Here we use measurements of the thermal Hall conductivity in YBCO to show that Fermi-surface reconstruction occurs only above a sharply defined onset field, equal to the transition field seen in ultrasound. This reveals that electrons do not experience long-range broken translational symmetry in the zero-field ground state, and hence in zero field there is no quantum critical point for the onset of charge order as a function of doping.

cond-mat.supr-con↗

arXiv2015

Universal V-shaped temperature-pressure phase diagram in the iron-based superconductors KFe2As2, RbFe2As2, and CsFe2As2

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue that this change is due to a transition from one pairing state to another, with different symmetries on either side of Pc. We discuss a possible link between scattering and pairing, and a scenario where a d-wave state favored by high-Q scattering at low pressure changes to a state with s+- symmetry favored by low-Q scattering at high pressure.

cond-mat.supr-con↗

arXiv2014

Nernst effect in the electron-doped cuprate superconductor PCCO: Superconducting fluctuations, upper critical field Hc2, and the origin of the Tc dome

The Nernst effect was measured in the electron-doped cuprate superconductor Pr2-xCexCuO4 (PCCO) at four concentrations, from underdoped (x=0.13) to overdoped (x=0.17), for a wide range of temperatures above the critical temperature Tc. A magnetic field H up to 15 T was used to reliably access the normal-state quasiparticle contribution to the Nernst signal, Nqp, which is subtracted from the total signal, N, to obtain the superconducting contribution, Nsc. As a function of H, Nsc peaks at a field H* whose temperature dependence obeys Hc2* ln(T/Tc), as it does in a conventional superconductor like Nb1-xSix. The doping dependence of the characteristic field scale Hc2* - shown to be closely related to the upper critical field Hc2 - tracks the dome-like dependence of Tc, showing that superconductivity is weakened below the quantum critical point where the Fermi surface is reconstructed, presumably by the onset of antiferromagnetic order. Our data at all dopings are quantitatively consistent with the theory of Gaussian superconducting fluctuations, eliminating the need to invoke unusual vortex-like excitations above Tc, and ruling out phase fluctuations as the mechanism for the fall of Tc with underdoping. We compare the properties of PCCO with those of hole-doped cuprates and conclude that the domes of Tc and Hc2 vs doping in the latter materials are also controlled predominantly by phase competition rather than phase fluctuations.

cond-mat.supr-con↗

arXiv2014

Interplane resistivity of underdoped single crystals (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, $0 \leq x<0.34$

Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was measured in hole-doped iron-arsenide superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ over a doping range from parent compound to optimal doping $T_c\approx 38~K$, $0\leq x \leq 0.34$. Measurements were undertaken on high-quality single crystals grown from FeAs flux. The coupled magnetic/structural transition at $T_{SM}$ leads to clear accelerated decrease of $ρ_c(T)$ on cooling in samples with $T_c <$26~K ($x <0.25$). This decrease in hole-doped material is in notable contrast to an increase in $ρ_c(T)$ in the electron-doped Ba(Fe$_{1-x}$Co$_x$)Fe $_2$As$_2$ and iso-electron substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$. The $T_{SM}$ decreases very sharply with doping, dropping from $T_s$=71~K to zero on increase of $T_c$ from approximately 25 to 27~K. The $ρ_c(T)$ becomes $T$-linear close to optimal doping. The broad crossover maximum in $ρ_c(T)$, found in the parent BaFe$_2$As$_2$ at around $T_{max} \sim$200~K, shifts to higher temperature $\sim$250~K with doping $x$=0.34. The maximum shows clear correlation with the broad crossover feature found in the temperature-dependent in-plane resistivity $ρ_a(T)$. The doping evolution of $T_{max}$ in (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ is in notable contrast with both rapid suppression of $T_{max}$ found in Ba(Fe$_{1-x}TM_x$)$_2$As$_2$ ($TM$=Co,Rh,Ni,Pd) and its rapid increase BaFe$_2$(As$_{1-x}$P$_x$)$_2$. This observation suggest that pseudogap features are much stronger in hole-doped than in electron-doped iron-based superconductors, revealing significant electron-hole doping asymmetry similar to the cuprates. This paper replaces: cond-mat:1106.0533.

cond-mat.supr-con↗

arXiv2014

Sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2: A possible link between inelastic scattering and pairing symmetry

We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor CsFe2As2, similar to that discovered recently in KFe2As2 [Tafti et al., Nat. Phys. 9, 349 (2013)]. As in KFe2As2, we observe no change in the Hall coefficient at the zero temperature limit, again ruling out a Lifshitz transition across the critical pressure Pc. We interpret the Tc reversal in the two materials as a phase transition from one pairing state to another, tuned by pressure, and investigate what parameters control this transition. Comparing samples of different residual resistivity, we find that a 6-fold increase in impurity scattering does not shift Pc. From a study of X-ray diffraction on KFe2As2 under pressure, we report the pressure dependence of lattice constants and As-Fe-As bond angle. The pressure dependence of these lattice parameters suggests that Pc should be significantly higher in CsFe2As2 than in KFe2As2, but we find on the contrary that Pc is lower in CsFe2As2. Resistivity measurements under pressure reveal a change of regime across Pc, suggesting a possible link between inelastic scattering and pairing symmetry.

cond-mat.supr-con↗

arXiv2014

Direct measurement of the upper critical field in a cuprate superconductor

The upper critical field Hc2 is a fundamental measure of the pairing strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. We have used thermal conductivity as a direct probe of Hc2 in the cuprates YBa2Cu3Oy and YBa2Cu4O8 to show that there is no vortex liquid at T = 0, allowing us to use high-field resistivity measurements to map out the doping dependence of Hc2 across the phase diagram. Hc2(p) exhibits two peaks, each located at a critical point where the Fermi surface undergoes a transformation. The condensation energy obtained directly from Hc2, and previous Hc1 data, undergoes a 20-fold collapse below the higher critical point. These data provide quantitative information on the impact of competing phases in suppressing superconductivity in cuprates.

cond-mat.supr-con↗

arXiv2013

Wiedemann-Franz law and non-vanishing temperature scale across the field-tuned quantum critical point of YbRh2Si2

The in-plane thermal conductivity kappa(T) and electrical resistivity rho(T) of the heavy-fermion metal YbRh2Si2 were measured down to 50 mK for magnetic fields H parallel and perpendicular to the tetragonal c axis, through the field-tuned quantum critical point, Hc, at which antiferromagnetic order ends. The thermal and electrical resistivities, w(T) and rho(T), show a linear temperature dependence below 1 K, typical of the non-Fermi liquid behavior found near antiferromagnetic quantum critical points, but this dependence does not persist down to T = 0. Below a characteristic temperature T* ~ 0.35 K, which depends weakly on H, w(T) and rho(T) both deviate downward and converge in the T = 0 limit. We propose that T* marks the onset of short-range magnetic correlations, persisting beyond Hc. By comparing samples of different purity, we conclude that the Wiedemann-Franz law holds in YbRh2Si2, even at Hc, implying that no fundamental breakdown of quasiparticle behavior occurs in this material. The overall phenomenology of heat and charge transport in YbRh2Si2 is similar to that observed in the heavy-fermion metal CeCoIn5, near its own field-tuned quantum critical point.

cond-mat.str-el↗

arXiv2013

Change of pairing symmetry in the iron-based superconductor KFe2As2

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.

cond-mat.supr-con↗

arXiv2013

Superconductivity in the noncentrosymmetric half-Heusler compound LuPtBi : A possible topological superconductor

We report superconductivity in the ternary half-Heusler compound LuPtBi, with Tc = 1.0 K and Hc2 = 1.6 T. The crystal structure of LuPtBi lacks inversion symmetry, hence the material is a noncentrosymmetric superconductor. Magnetotransport data show semimetallic behavior in the normal state, which is evidence for the importance of spin-orbit interaction. Theoretical calculations indicate that the strong spin-orbit interaction in LuPtBi should cause strong band inversion, making this material a promising candidate for 3D topological superconductivity.

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arXiv2012

Vortex Lattice Melting and Hc2 in underdoped YBa2Cu3Oy

Vortices in a type-II superconductor form a lattice structure that melts when the thermal displacement of the vortices is an appreciable fraction of the distance between vortices. In an anisotropic high-Tc superconductor, such as YBa2Cu3Oy, the magnetic field value where this melting occurs can be much lower than the mean-field critical field Hc2. We examine this melting transition in YBa2Cu3Oy with oxygen content y from 6.45 to 6.92, and fit the data to a theory of vortex-lattice melting. The quality of the fits indicates that the transition to a resistive state is indeed the vortex lattice melting transition, with the shape of the melting curves being consistent with the known change in penetration depth anisotropy from underdoped to optimally doped YBa2Cu3Oy. From the fits we extract Hc2(T = 0) as a function of hole doping. The unusual doping dependence of Hc2(T =0) points to some form of electronic order competing with superconductivity around 0.12 hole doping.

cond-mat.supr-con↗

arXiv2012

New Phase Induced by Pressure in the Iron-Arsenide Superconductor K-Ba122

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.

cond-mat.supr-con↗

arXiv2012

Decrease of upper critical field with underdoping in cuprate superconductors

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.

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