A. Carrington

University of Bristol, Bristol, England, United Kingdom

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Publications (126)468.72 Total impact

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    ABSTRACT: Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields, however, much less is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-sized single crystals of the triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and we combine magnetic torque, specific heat and transport data to construct the field-temperature phase diagram. The results are discussed in the context of a frustrated easy-axis Heisenberg model for the localized moments where intermediate applied magnetic fields are predicted to stabilize a magnetic supersolid phase. Deviations in the measured phase diagram from this model predictions are attributed to the role played by the itinerant electrons.
    Physical Review B 07/2014; 90:020401(R). · 3.66 Impact Factor
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    ABSTRACT: Fluctuations around an antiferromagnetic quantum critical point (QCP) are believed to lead to unconventional superconductivity and in some cases to high-temperature superconductivity. However, the exact mechanism by which this occurs remains poorly understood. The iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is perhaps the clearest example to date of a high temperature quantum critical superconductor, and so it is a particularly suitable system in which to study how the quantum critical fluctuations affect the superconducting state. Here we show that the proximity of the QCP yields unexpected anomalies in the superconducting critical fields. We find that both the lower and upper critical fields strongly violate the expectations from the conventional theory taking into account the observed mass enhancement near the QCP. These anomalous behaviours of the critical fields imply that the energy of superconducting vortices is enhanced, possibly due to a microscopic mixing of antiferromagnetism and superconductivity, suggesting that a highly unusual vortex state is realised in quantum critical superconductors.
    02/2014;
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    T. Shibauchi, A. Carrington, Y. Matsuda
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    ABSTRACT: 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 fermion compounds and most recently iron-pnictides. The existence of a QCP may offer a route to understand: the origin of their anomalous non-Fermi liquid properties, the microscopic coexistence between unconventional superconductivity and magnetic or some exotic order, and ultimately the mechanism of superconductivity itself. The isovalent substituted iron-pnictide BaFe$_2$(As$_{1-x}$P$_x$)$_2$ 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 BaFe$_2$(As$_{1-x}$P$_x$)$_2$ have provided the first clear and unambiguous evidence of a second order quantum phase transition lying beneath the superconducting dome.
    04/2013;
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    ABSTRACT: The cuprate superconductor YBa2Cu3O7−δ (Y123) has been synthesised for the first time in a spherical morphology. The use of carboxymethyl-dextran in a pre-formed, spherical morphology enables not only the engineering of this new macromorphological motif, but also crystallochemical control of the superconductor at the nano-scale.
    CrystEngComm 04/2013; 15(19):3763-3766. · 3.88 Impact Factor
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    ABSTRACT: We report a combined study of the specific heat and de Haas-van Alphen effect in the iron-pnictide superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$. Our data when combined with results for the magnetic penetration depth give compelling evidence for the existence of a quantum critical point (QCP) close to $x=0.30$ which affects the majority of the Fermi surface by enhancing the quasiparticle mass. The results show that the sharp peak in the inverse superfluid density seen in this system results from a strong increase in the quasiparticle mass at the QCP.
    Physical Review Letters 03/2013; 110(25). · 7.73 Impact Factor
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    ABSTRACT: The structure of the superconducting gap of the pncitide superconductors is an unresolved but crucial issue to understanding their mechanism of superconductivity. While some experiments and theories support a fully gapped s+/s- state, several experiments have revealed evidence for nodes in some families of pnictides. Detailed knowledge of the superconducting gap structure and how it varies between different families can be useful in helping to decide between microscopic theories. BaFe2(AsxP1-x)2 is a pnictide family where penetration depth and thermal conductivity measurements show evidence for nodes [1]. We have measured the specific heat of a single crystal of BaFe2(As0.7P0.3)2 (Tc˜29,) at low fields and as a function of applied field angle. The angle dependence of specific heat at low fields and low temperature is expected to show minima whenever it is along a nodal direction and can be used to differentiate between gap symmetries [2]. Our results show a clear angle dependent component consistent with the presence of nodes and we discuss the implications on the gap structure of BaFe2(As0.7P0.3)2.[4pt] [1] K. Hashimoto et al, Phys. Rev. B, 81, 220501R,(2010).[0pt] [2] A. B. Vorontsov et al, Phys. Rev. Lett. 105 ,187004 (2010)
    03/2013;
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    ABSTRACT: We report de Haas-van Alphen (dHvA) measurements of the Fermi surface of the 111 iron based superconductor LiFeP with Tc 5 K. Comparison of our experimental results to density functional theory band-structure calculations show good agreement. As in other iron-based superconductors we find that the electron and hole bands are quasi-nested. The effective masses, determined individually for the different Fermi surface sheets (orbits) generally show significant enhancement. The smallest hole pocket sheet is an exception to this and shows a very small enhancement. This difference in the many body interaction suggest a suppression of electron-hole scattering for this sheet which may result from its different orbital character. This might be the reason why LiFeP has nodes in its superconducting gap whereas its sister compound LiFeAs does not.
    03/2013;
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    ABSTRACT: Iron based superconductors are one of many classes of material where superconductivity occurs in the vicinity of a magnetic quantum critical point (QCP). The degree to which the QCP drives or otherwise influences the high temperature superconductivity is however still a matter of debate. In this context it is useful to determine experimentally, the degree to which the quasiparticle effective mass diverges at the QCP and how this is reflected in various physical properties. Here we will report measurements of the specific heat γ and the de Haas-van Alphen effect which quantify these effects. Far from the QCP the enhancement of the mass as measured by γ, dHvA and the magnetic penetration depth λ are all consistent. However, very close to the QCP significant differences are found which likely result from finite temperature and/or multi-band effects.
    03/2013;
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    ABSTRACT: When a second-order magnetic phase transition is tuned to zero temperature by a nonthermal parameter, quantum fluctuations are critically enhanced, often leading to the emergence of unconventional superconductivity. In these "quantum critical" superconductors it has been widely reported that the normal-state properties above the superconducting transition temperature T(c) often exhibit anomalous non-Fermi liquid behaviors and enhanced electron correlations. However, the effect of these strong critical fluctuations on the superconducting condensate below T(c) is less well established. Here we report measurements of the magnetic penetration depth in heavy-fermion, iron-pnictide, and organic superconductors located close to antiferromagnetic quantum critical points, showing that the superfluid density in these nodal superconductors universally exhibits, unlike the expected T-linear dependence, an anomalous 3/2 power-law temperature dependence over a wide temperature range. We propose that this noninteger power law can be explained if a strong renormalization of effective Fermi velocity due to quantum fluctuations occurs only for momenta k close to the nodes in the superconducting energy gap Δ(k). We suggest that such "nodal criticality" may have an impact on low-energy properties of quantum critical superconductors.
    Proceedings of the National Academy of Sciences 02/2013; 110(9):3293-7. · 9.81 Impact Factor
  • Amalia I. Coldea, Daniel Braithwaite, Antony Carrington
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    ABSTRACT: Here we review measurements of the normal and superconducting state properties of iron-based superconductors using high magnetic fields. We discuss the various physical mechanisms that limit superconductivity in high fields, and the information on the superconducting state that can be extracted from the upper critical field, but also how thermal fluctuations affect its determination by resistivity and specific heat measurements. We also discuss measurements of the normal state electronic structure focusing on measurement of quantum oscillations, particularly the de Haas–van Alphen effect. These results have determined very accurately, the topology of the Fermi surface and the quasi-particle masses in a number of different iron-based superconductors, from the 1111, 122 and 111 families.RésuméNous passons en revue les mesures en fort champ magnétique des propriétés normales et supraconductrices des nouveaux composés supraconducteurs à base de fer. Nous rappelons les mécanismes qui limitent la supraconductivité à haut champ et discutons les informations sur la phase supraconductrice qui sont obtenues grâce à la mesure du champ critique supérieur, mais également les effets des fluctuations thermiques sur la détermination de celui-ci par des mesures de transport ou de chaleur spécifique. Nous discutons également la structure électronique de la phase normale, principalement par des mesures dʼoscillations quantiques comme lʼeffet de Haas–van Alphen. Ces résultats permettent de déterminer très précisément la topologie de la surface de Fermi et la masse des quasi-particules dans plusieurs supraconducteurs à base de fer des familles 1111, 122 et 111.
    Comptes Rendus Physique 01/2013; 14(1):94–105. · 1.82 Impact Factor
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    ABSTRACT: 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, λ(L). In two-dimensional systems, this ratio n/m* (~1/λ(L)(2)) determines the effective Fermi temperature, T(F). We report a sharp peak in the x-dependence of λ(L) at zero temperature in clean samples of BaFe(2)(As(1)(-x)P(x))(2) at the optimum composition x = 0.30, where the superconducting transition temperature T(c) reaches a maximum of 30 kelvin. This structure may arise from quantum fluctuations associated with a quantum critical point. The ratio of T(c)/T(F) at x = 0.30 is enhanced, implying a possible crossover toward the Bose-Einstein condensate limit driven by quantum criticality.
    Science 06/2012; 336(6088):1554-7. · 31.20 Impact Factor
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    ABSTRACT: The upper critical field H(c2) of purple bronze Li0:9Mo6O17 is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, H(c2) increases monotonically with decreasing temperature to a value 5 times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of H(c2) invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li0:9Mo6O17 is unconventional and possibly spin triplet.
    Physical Review Letters 05/2012; 108(18):187003. · 7.73 Impact Factor
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    ABSTRACT: The upper critical field $H_{c2}$ of purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ is found to exhibit a large anisotropy, in quantitative agreement with that expected from the observed electrical resistivity anisotropy. With the field aligned along the most conducting axis, $H_{c2}$ increases monotonically with decreasing temperature to a value five times larger than the estimated paramagnetic pair-breaking field. Theories for the enhancement of $H_{c2}$ invoking spin-orbit scattering or strong-coupling superconductivity are shown to be inadequate in explaining the observed behavior, suggesting that the pairing state in Li$_{0.9}$Mo$_6$O$_{17}$ is unconventional and possibly spin-triplet.
    03/2012;
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    ABSTRACT: Quantum oscillations are a powerful technique to establish with accuracy the three-dimensional topology of the Fermi surface and it has been successfully used in the study of iron-based superconductors. Here we report quantum oscillations in the 111 pnictide superconductor, LiFeAs, with T˜18K, by using highly sensitive torque magnetometry in high magnetic fields (up to 58T) and at very low temperatures (0.3-4.2K). We observe clearly three different orbits around 1.5kT, 2.4kT and 2.9kT. By comparing the angular dependence of the measured frequencies with the predictions given by the first principle band calculations we conclude that the observed orbits belong to the electronic bands. The values of the quasiparticle masses for these orbits are significantly enhanced as compared with the band masses (a factor 4-5) suggesting that either that electron-electron and/or electron-phonon correlations are significant. We will compare our data with available APRES data on the same material and discuss the effect of the spin-orbit coupling. The details of the Fermi surface of LiFeAs will be compared with other iron-based superconductors. This work was supported by EPSRC (UK), EuroMagNET II, KAKENHI from JSPS and National Science Foundation, State of Florida and the U.S. Department of Energy.
    02/2012;
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    ABSTRACT: The recently discovered layered heavy fermion superconductor Ce2PdIn8 (Tc = 0.7 K) has attracted a great interest because of the appearance of unconventional superconductivity near a quantum critical point. Here we report high-precision measurements of magnetic penetration depth λ down to 60 mK (˜0.1T/Tc) by using a tunnel diode oscillator operating at ˜13 MHz. A strong power-law temperature dependence of λ demonstrates low-energy excitations of quasiparticles, which is consistent with recent thermal conductivity measurements indicative of nodal superconductivity. The observed T^1.5 behavior at low temperatures, which is commonly seen in superconductors with line nodes near a magnetic quantum critical point such as CeCoIn5 or organics, can be attributed to the enhancement of effective mass towards zero temperature due to quantum magnetic fluctuations even in the superconducting state.
    02/2012;
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    ABSTRACT: The iron-pnictides are highly unusual in that there appears to be considerable variation in the structure of the superconducting gap across the different materials. The 111 compounds, LiFeX (X=As, P) superconduct at ambient pressure in their undoped stoichiometric form. LiFeP ((Tc==5K) was found to have superconducting gap nodes whereas LiFeAs (Tc=17K) does not. Linking these differences in gap structure to Fermi surface features could provide a key test of microscopic theories which seek to explain superconductivity in iron pnictides. Here we report de Haas-van Alphen effect data which determine, almost completely, the bulk Fermi surface of LiFeP. The topology of the Fermi surface, which consists of quasi nested electron and hole sheets, is in good agreement with DFT band structure calculations when allowance for small band energy shifts is made. We find that one hole sheet has a anomalously small mass enhancement (compared to the others) which suggest it interacts weakly. This is probably because of its mixed orbital character rather than for any geometrical reason. We suggest that this could be the driver for node formation in this material.
    02/2012;
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    ABSTRACT: There is growing evidence that the superconducting gap structure is not universal in the iron-based superconductors. It is essential to determine experimentally what causes nodal and nodeless states. The 111 materials, LiFeAs and LiFeP provide a unique route to study this problem as both materials are superconducting, nonmagnetic, and importantly very clean, with long electronic mean-free paths. Here we report on high-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors, which reveal contrasting low-energy excitations of quasiparticles. In LiFeAs the low-temperature λ(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density &-2circ;, indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of ˜1.33,.
    02/2012;
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    ABSTRACT: We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T(c) ≈ 18 K and LiFeP with T(c) ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.
    Physical Review Letters 01/2012; 108(4):047002. · 7.73 Impact Factor
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    ABSTRACT: High-precision measurements of magnetic penetration depth λ in clean single crystals of LiFeAs and LiFeP superconductors reveal contrasting behaviors. In LiFeAs the low-temperature λ(T) shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a T-linear dependence of superfluid density infinity λ(-2), indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of ~1.33 Å.
    Physical Review Letters 01/2012; 108(4):047003. · 7.73 Impact Factor
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    A. Carrington
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    ABSTRACT: This paper reviews quantum oscillation studies of iron-pnictide superconductors and related materials. These measurements give unique information regarding the full three dimensional topology of the Fermi surfaces and the renormalisation of the quasi-particle masses. The review will cover measurements of the 122 arsenide end members, XFe_2As_2 (X = Ba, Sr, Ca) which have a spin density wave ground state, but will concentrate on the phosphide end members (LaFePO and XFe2P2) which have a paramagnetic ground state and a Fermi surface topology which is similar to the higher Tc superconducting iron-pnictides. All three of the 122 phosphides become superconducting when P is partially substituted by As, and for the BaFe2(As1-xPx)2 series de Haas-van Alphen oscillations are observable for 0:42 \leq x \leq 1 with Tc up to 25 K. The results show the changes in the Fermi surface topology and the increase in the mass renormalisation as the correlations which induce superconductivity develop.
    Reports on Progress in Physics 10/2011; 74(12). · 13.23 Impact Factor

Publication Stats

3k Citations
468.72 Total Impact Points

Institutions

  • 2000–2013
    • University of Bristol
      • School of Chemistry
      Bristol, England, United Kingdom
    • Universidade Federal de São Carlos
      • Departamento de Física (DF)
      São Carlos, Estado de Sao Paulo, Brazil
  • 2010–2012
    • Kyoto University
      • Department of Physics II
      Kyoto, Kyoto-fu, Japan
  • 1992–2006
    • University of Cambridge
      • Department of Physics: Cavendish Laboratory
      Cambridge, ENG, United Kingdom
  • 2000–2001
    • University of Leeds
      • School of Physics and Astronomy
      Leeds, ENG, United Kingdom
  • 1996–1999
    • University of Illinois, Urbana-Champaign
      • Department of Physics
      Urbana, Illinois, United States
  • 1994
    • Cea Leti
      Grenoble, Rhône-Alpes, France