Antony Carrington

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-Tc) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+δ in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-Tc materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Quantum oscillations in an overdoped high-Tc superconductor

The nature of the metallic phase in the high-transition-temperature (high-Tc) copper oxide superconductors, and its evolution with carrier concentration, has been a long-standing mystery. A central question is how coherent electronic states, or quasiparticles, emerge from the antiferromagnetic insulator with doping. Recent quantum oscillation experiments on lightly doped copper oxides have shown evidence for small pockets of Fermi surface, the formation of which has been associated with the opening of the pseudogap—an anisotropic gap in the normal state excitation spectrum of unknown origin. As the doping is increased, experiments suggest that the full Fermi surface is restored, although the doping level at which the pseudogap closes and the nature of the electronic ground state beyond this point have yet to be determined. Here we report the observation of quantum oscillations in the overdoped superconductor Tl2Ba2CuO6+δ that show the existence of a large Fermi surface of well-defined quasiparticles covering two-thirds of the Brillouin zone. These measurements confirm that, in overdoped superconducting copper oxides, coherence is established at all Fermi wavevectors, even near the zone boundary where the pseudogap is maximal and electronic interactions are strongest; they also firmly establish the applicability of a generalized Fermi-liquid picture on the overdoped side of the superconducting phase diagram.

Small Fermi Surface Pockets in Underdoped High Temperature Superconductors: Observation of Shubnikov-de Haas Oscillations in YBa2Cu4O8

A. F. Bangura, J. D. Fletcher, A. Carrington, J. Levallois, M. Nardone, B. Vignolle , P. J. Heard, N. Doiron-Leyraud, D. LeBoeuf, L. Taillefer, S. Adachi, C. Proust and N. E. Hussey H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, United Kingdom. Laboratoire National des Champs Magnétiques Pulsés, UMR CNRS-UPS-INSA 5147, Toulouse, France. Département de physique and RQMP, Université de Sherbrooke, Sherbrooke, J1K 2R1, Canada. and Superconducting Research Laboratory, International Superconductivity Center, Shinonome 1-10-13, Tokyo 135, Japan. (Dated: February 1, 2008)

Evidence for a Nodal-Line Superconducting State in LaFePO

In several iron-arsenide superconductors there is strong evidence for a fully gapped superconducting state consistent with either a conventional s-wave symmetry or an unusual s{+/-} state where the gap changes sign between the electron and hole Fermi-surface sheets. Here we report measurements of the penetration depth lambda(T) in very clean samples of the related iron-phosphide superconductor, LaFePO, at temperatures down to approximately 100 mK. We find that lambda(T) varies approximately linearly with T strongly suggesting the presence of gap nodes in this compound. Taken together with other data, this suggests the gap function is not universal for all pnictide superconductors.

Line nodes in the energy gap of superconducting BaFe2(As1-xPx)2 single crystals as seen via penetration depth and thermal conductivity

We report magnetic penetration depth and thermal conductiv ity data for high-quality single crystals of BaFe2(As1−xPx)2 (Tc = 30K) which provide strong evidence that this material has line nodes in its energy gap. This is distinctly different from the nodeless gap foun d for (Ba,K)Fe2As2 which has similarTc and phase diagram. Our results indicate that repulsive electronic in teractions play an essential role for Fe-based highTc superconductivity but that uniquely there are distinctly d ifferent pairing states, with and without nodes, which have comparable Tc.

A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2

A Spike Inside the Dome The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev) measured the penetration depth of the pnictide series BaFe2(As1−xPx)2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30. A quantum critical point may be lurking inside the superconducting dome of a pnictide series. In a superconductor, the ratio of the carrier density, n, to its effective mass, m*, is a fundamental property directly reflecting the length scale of the superfluid flow, the London penetration depth, λL. In two-dimensional systems, this ratio n/m* (~1/λL2) determines the effective Fermi temperature, TF. We report a sharp peak in the x-dependence of λL at zero temperature in clean samples of BaFe2(As1–xPx)2 at the optimum composition x = 0.30, where the superconducting transition temperature Tc reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of Tc/TF at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.

Fermi surface of superconducting LaFePO determined from quantum oscillations.

We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (T(c) approximately 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor approximately 2. The quasi-two-dimensional Fermi surface consists of nearly nested electron and hole pockets, suggesting proximity to a spin or charge density wave instability.

A Quantum Critical Point Lying Beneath the Superconducting Dome in Iron Pnictides

Whether a quantum critical point (QCP) lies beneath the superconducting dome has been a long-standing issue that remains unresolved in many classes of unconventional superconductors, notably cuprates, heavy fermions, and, most recently, iron pnictides. The existence of a QCP may offer a route to understanding the origin of unconventional superconductors’ anomalous non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some other exotic order, and, ultimately, the mechanism of superconductivity itself. The isovalent substituted iron pnictide BaFe2(As1−xPx)2 offers a new platform for the study of quantum criticality, providing a unique opportunity to study the evolution of the electronic properties in a wide range of the phase diagram. Recent experiments in BaFe2(As1−xPx)2 have provided the first clear and unambiguous evidence of a second-order quantum phase transition lying beneath the superconducting dome.

Evolution of the Fermi Surface of BaFe2(As1-xPx)2 on Entering the Superconducting Dome

Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx){2} as a function of isoelectric substitution (As/P) for 0.41<x<1 (T{c} up to 25 K). We find that the volumes of electron and hole Fermi surfaces shrink linearly with decreasing x. This shrinking is accompanied by a strong increase in the quasiparticle effective mass as x is tuned toward the maximum T{c}. These results are not explained by simple band structure calculations, and it is likely that these trends originate from the same many-body interactions which give rise to superconductivity.

de Haas – van Alphen effect in single crystal MgB 2

Abstract We present an experimental study of the de Haas–van Alphen effect in single crystals of MgB 2 , using a piezo-resistive torque technique. Three quasi-particle orbits were observed. Two originate from a single warped Fermi surface tube along the c direction, and the third from a cylindrical section of an in-plane honeycomb network. The quasi-particle effective masses on these orbits were determined and compared to band structure calculations. From this we deduce that the electron–phonon coupling strength λ , is a factor ∼3 larger for the c -axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.