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Research Highlights


Magnetic field dependance of the basal-plane superconducting anisotropy in YBa2Cu4O8 from small-angle neutron scattering measurements of the vortex lattice 


J.S. White, C.J. Bowell, A.S. Cameron, R.W. Heslop, J. Mesot, J.L. Gavilano, S. Strassle, L. Machler, R. Khasanov, C.D. Dewhurst, J. Karpinski, E.M. Forgan
We report a study of the basal-plane anisotropy of the superfluid density in underdoped YBa2Cu4O8 (Y124), showing the effects of both the CuO2 planes and the fully occupied CuO chains. From small-angle neutron scattering measurements of the vortex lattice, we can infer the superconducting (SC) properties for a temperature (T) range T = 1.5 K to Tc and magnetic induction B from 0.1 to 6 T. We find that the superfluid density along a has a simple d-wave T dependence. However, along b (the chain direction) the superfluid density falls much more rapidly with T and also with increasing field. This strongly suggests the suppression of proximity-effect-induced superconductivity in the CuO chains. In addition, our results do not support a common framework for the low-field in-plane SC response in Y124 and related YBa2Cu3O7, and also indicate that any magnetic field induced charge-density-wave order in Y124 exists only for fields above 6 T.
Phys. Rev. B 89 2014 (Jan.), DOI: 10.1103/PhysRevB.89.024501
Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2xSrxCuO4 
J. Chang, M. Månsson, S. Pailhe`s, T. Claesson, O.J. Lipscombe, S.M. Hayden, L. Patthey, O. Tjernberg & J. Mesot
High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space. The quasiparticle scattering and residue behave differently along the Fermi surface and hence the Kadowaki–Wood’s relation is not obeyed. This kind of Fermi liquid breakdown may apply to a wide range of strongly correlated metal systems where spin fluctuations are present. 
Nature Communications 2013 (Oct.), DOI: 10.1038/ncomms3559


Surface and bulk electronic structure of the strongly correlated system SmB6 and implications for a topological Kondo insulator


N. Xu, X. Shi, P. K. Biswas, C. E. Matt, R. S. Dhaka, Y. Huang, N. C. Plumb, M. Radović, J. H. Dil, E. Pomjakushina, K. Conder, A. Amato, Z. Salman, D. McK. Paul, J. Mesot, H. Ding, and M. Shi


Recent theoretical calculations and experimental results suggest that the strongly correlated material SmB6 may be a realization of a topological Kondo insulator. We have performed an angle-resolved photoemission spectroscopy study on SmB6 in order to elucidate elements of the electronic structure relevant to the possible occurrence of a topological Kondo insulator state. The obtained electronic structure in the whole three-dimensional momentum space reveals one electron-like 5d bulk band centered at the X point of the bulk Brillouin zone that is hybridized with strongly correlated f electrons, as well as the opening of a Kondo band gap (ΔB ∼ 20 meV) at low temperature. In addition, we observe electron-like bands forming three Fermi surfaces at the center Γ⎯⎯⎯ point and boundary X⎯⎯⎯ point of the surface Brillouin zone. These bands are not expected from calculations of the bulk electronic structure, and their observed dispersion characteristics are consistent with surface states. Our results suggest that the unusual low-temperature transport behavior of SmB6 is likely to be related to the pronounced surface states sitting inside the band hybridization gap and/or the presence of a topological Kondo insulating state.
Phys. Rev. B 88 2013 (Sep.), DOI: 10.1103/PhysRevB.88.121102
Evolution from a Nodeless Gap to dx2−y2-Wave in Underdoped La2−xSrxCuO4
E. Razzoli, G. Drachuck, A. Keren, M. Radovic, N. C. Plumb, J. Chang, Y.-B. Huang, H. Ding, J. Mesot, M. Shi
Using angle-resolved photoemission spectroscopy (ARPES), it is revealed that the low-energy electronic excitation spectra of highly underdoped superconducting and nonsuperconducting La2−xSrxCuO4 cuprates are gapped along the entire underlying Fermi surface at low temperatures. We show how the gap function evolves to a dx2−y2 form with increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates. Our results provide essential information for uncovering the symmetry of the order parameter(s) in strongly underdoped cuprates, which is a prerequisite for understanding the pairing mechanism and how superconductivity emerges from a Mott insulator.
Phys. Rev. Lett. 110 2013 (Jan.), DOI: 10.1103/PhysRevLett.110.047004
Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67


J. Chang, E. Blackburn, A. T. Holmes, N. B. Christensen, J. Larsen, J. Mesot, Ruixing Liang, D. A. Bonn,W. N. Hardy, A.Watenphul, M. v. Zimmermann, E. M. Forgan and S. M. Hayden
Superconductivity often emerges in the proximity of, or in competition with, symmetry-breaking ground states such as antiferromagnetism or charge density waves1–5 (CDW). A number of materials in the cuprate family, which includes the high transition-temperature (high-Tc) superconductors, show spin and charge density wave order5–7. Thus a fundamental question is to what extent do these ordered states exist for compositions close to optimal for superconductivity. Here we use high-energy X-ray diffraction to show that a CDW develops at zero field in the normal state of superconducting YBa2Cu3O6:67 (Tc D 67 K). This sample has a hole doping of 0.12 per copper and a well-ordered oxygen chain superstructure8. Below Tc, the application of a magnetic field suppresses superconductivity and enhances the CDW.
Hence, the CDW and superconductivity in this typical high-Tmaterial are competing orders with similar energy scales, and the high-Tc superconductivity forms from a pre-existing CDW environment. Our results provide a mechanism for the formation of small Fermi surface pockets9, which explain the negative Hall and Seebeck effects10,11 and the ‘Tc plateau’12 in this material when underdoped.
Nature Physics 2012 (Oct.), DOI.10.1038/NPHYS2456




Bulk Electronic Structure of Superconducting LaRu2P2 Single Crystals Measured by Soft-X-Ray Angle-Resolved Photoemission Spectroscopy

E. Razzoli, M. Kobayashi, V. N. Strocov, B. Delley, Z. Bukowski, J. Karpinski, N. C. Plumb, M. Radovic, J. Chang,  T. Schmitt,  L. Patthey,  J. Mesot,  and M. Shi 


We present a soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichio metric pnictide superconductor LaRu2P2. The observed electronic structure is in good agreement withdensity functional theory (DFT) calculations. However, it is significantly different from its counterpart inhigh-temperature superconducting Fe pnictides. In particular, the bandwidth renormalization present in the Fe pnictides is negligible in LaRu2P2 even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu2P2 has a different origin with respect to the iron pnictides. Finally, we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.

 Phys. Rev. Lett. 108 (2012) 257005.
Spin density wave induced disordering of the vortex lattice in superconducting La2-xSrxCuO4 
J. Chang, J. S. White, M. Laver, C. J. Bowell, S. P. Brown, A. T. Holmes, L. Maechler, S. Straessle, R. Gilardi, S. Gerber, T. Kurosawa, N. Momono, M. Oda, M. Ido, O. J. Lipscombe, S. M. Hayden, C. D. Dewhurst, R. Vavrin, J. Gavilano, J. Kohlbrecher, E. M. Forgan, and J. Mesot
We use small-angle neutron scattering to study the superconducting vortex lattice in La2xSrx CuO4 as a function of doping and magnetic field. We show that near optimally doping the vortex lattice coordination and the superconducting coherence length ξ are controlled by a Van Hove singularity crossing the Fermi level near the Brillouin zone boundary. The vortex lattice properties change dramatically as a spin-density-wave instability is approached upon underdoping. The Bragg glass paradigm provides a good description of this regime and suggests that spin-density-wave order acts as a source of disorder on the vortex lattice. 
Phys. Rev. B. 85 134520 (2012) 

Ortho-II band folding in YBa2Cu3O7-d films revealed by angle-resolved photoemission

Y. Sassa, M. Radovic, M. Mansson, E. Razzoli, X. Y. Cui, S. Pailhes, S. Guerrero,6 M. Shi, P. R. Willmott, F. Miletto Granozio, J. Mesot, M. R. Norman, and L. Patthey 
We present an angle-resolved photoelectron spectroscopy study of YBa2 Cu3 O7δ films in situ grown by pulsed  laser deposition. We have successfully produced underdoped surfaces with ordered oxygen vacancies within the CuO chains resulting in a clear ortho-II band folding of the Fermi surface. This indicates that order within the  CuO chains affects the electronic properties of the CuO2 planes. Our results highlight the importance of having not only the correct surface carrier concentration, but also a very well ordered and clean surface in order that  photoemission data on this compound be representative of the bulk. 
 Phys. Rev. B 83, 140511(R) (2011)
Localized and delocalized Ti 3dcarriers in LaAlO3 /SrTiO3 superlattices revealed by resonant inelastic x-ray scattering
Ke-Jin Zhou,Milan Radovic,Justine Schlappa,Vladimir Strocov,Ruggero Frison,Joel Mesot, Luc Patthey, and Thorsten Schmitt
The important source of interface conductivity in LaAlO3 /SrTiO3 heterostructures, the Ti 3d carriers, is probed  with resonant inelastic x-ray scattering at the Ti 2p3/2 edge of epitaxially grown superlattices. We reveal unam-  biguously the generation of both localized and delocalized Ti 3d carriers as a result of the built-up heterointerface. Furthermore, we determine that the interface Ti3+ O6 octahedra are orthorhombically distorted and quantify the crystal-field splitting energies. We argue that for as-grown superlattices, both types of Ti 3d carriers originate mainly from oxygen vacancies, whereas for fully oxidized samples they result from electronic reconstruction. 
Phys. Rev. B 83, 201402(R) (2011) 
The Fermi surface and band folding in La2xSrxCuO4, probed by angle-resolved photoemission
E. Razzoli, Y. Sassa, G. Drachuck, M. Månsson, A. Keren, M. Shay, M.H.Berntsen, O.Tjernberg, M.Radovic, J.Chang, S.Pailhès, N. Momono, M. Oda, M. Ido, O.J.Lipscombe, S.M. Hayden, L. Patthey, J. Mesot and M. Shi
A systematic angle-resolved photoemission study of the electronic structure of La2xSrxCuOin a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (π,π) folded band, which shows unusual doping dependence.The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field. 
New Journal of Physics 12 (2010) 125003

Two-Dimensional Orbital-Like Magnetic Order in the High-Temperature LaSrCuO  Superconductor

V. Baledent, B. Fauque, Y. Sidis, N. B. Christensen, S. Pailhes, K. Conder, E. Pomjakushina, J.Mesot, and P. Bourges 
In high-temperature copper oxide superconductors, a novel magnetic order associated with the  pseudogap phase has been identified in two different cuprate families over a wide region of temperature  and doping. We report here the observation below 120 K of a similar magnetic ordering in the archetypal  cuprate La2-xSrxCuO4 (LSCO) system for x≈0.085. In contrast with the previous reports, the magnetic  ordering in LSCO is only short range with an in-plane correlation length of 10 Å and is bidimensional (2D). Such a less pronounced order suggests an interaction with other electronic instabilities. In particular, LSCO also exhibits a strong tendency towards stripes ordering at the expense of the superconducting state.
Phys. Rev. Lett.  105, 027004 (2010) 
Fermi Surface and Order Parameter Driven Vortex Lattice Structure Transitions in Twin-Free YBa2Cu3O7 
J. S. White, V. Hinkov, R.W. Heslop, R. J. Lycett, E. M. Forgan, C. Bowell, S. Strassle, A. B. Abrahamsen,
M. Laver, C. D. Dewhurst, J. Kohlbrecher, J. L. Gavilano, J. Mesot, B. Keimer, and A. Erb
We report on small-angle neutron scattering studies of the intrinsic vortex lattice (VL) structure in detwinned YBa2Cu3Oat 2 K, and in fields up to 10.8 T. Because of the suppressed pinning to twindomain boundaries, a new distorted hexagonal VL structure phase is stabilized at intermediate fields. It is separated from a low-field hexagonal phase of different orientation and distortion by a first-order transition at 2.0(2) T that is probably driven by Fermi surface effects. We argue that another first-order transition at 6.7(2) T, into a rhombic structure with a distortion of opposite sign, marks a crossover from a regime where Fermi surface anisotropy is dominant, to one where the VL structure and distortion is controlled by the order-parameter anisotropy.
Phys. Rev. Lett. 102, 097001 (2009)


Collective Magnetic Excitations in the Spin Ladder Sr14Cu24O41 Measured Using High-Resolution Resonant Inelastic X-Ray Scattering 


J. Schlappa, T. Schmitt, F. Vernay, V. N. Strocov, V. Ilakovac, B. Thielemann, H. M. Rønnow, S. Vanishri,

A. Piazzalunga, X. Wang, L. Braicovich, G. Ghiringhelli, C. Marin, J. Mesot, B. Delley, and L. Patthey
We investigate magnetic excitations in the spin-ladder compound Sr14Cu24O41 using high-resolution Cu Ledge resonant inelastic x-ray scattering (RIXS). Our findings demonstrate that RIXS couples to two-triplon collective excitations. In contrast to inelastic neutron scattering, the RIXS cross section changes only moderately over the entire Brillouin zone, revealing high sensitivity also at small momentum transfers, allowing determination of the two-triplon energy gap as 100  30 meV. Our results are backed by calculations within an effective Hubbard model for a finite-size cluster, and confirm that optical selection rules are obeyed for excitations from this spherically symmetric quantum spin-liquid ground state.
Phys. Rev. Lett. 103, 047401 (2009)
Anomalous Magnetic Excitations of Cooperative Tetrahedral Spin Clusters
K. Prsa, H. M. Rønnow, O. Zaharko, N. B. Christensen, J. Jensen, J. Chang, S. Streule, M. Jimenez-Ruiz, H. Berger,  M. Prester,  and J. Mesot 
An inelastic neutron scattering study of Cu2Te2O5X(X 1/4 Cl, Br) shows strong dispersive modes with large energy gaps persisting far above TN, notably in Cu2Te2O5Br2. The anomalous features: a coexisting unusually weak Goldstone-like mode observed in Cu2Te2O5Cland the size of the energy gaps cannot be explained by existing theories, such as our mean-field or random-phase approximation. We argue that our findings represent a new general type of behavior due to intercluster quantum fluctuations and call for development of a new theoretical approach.
Phys. Rev. Lett. 102, 177202 (2009) 
Magnetic-Field-Induced Soft-Mode Quantum Phase Transition in the High-Temperature Superconductor La1.855Sr0.145CuO4: An Inelastic Neutron-Scattering Study 
J. Chang, N. B. Christensen, Ch. Niedermayer, K. Lefmann, H. M. Rønnow, D. F. McMorrow,
A. Schneidewind, P. Link, A. Hiess, M. Boehm, R. Mottl, S. Pailhes, N. Momono, M. Oda, M. Ido, and J. Mesot
Inelastic neutron-scattering experiments on the high-temperature superconductor La1:855Sr0:145CuOreveal a magnetic excitation gap  that decreases continuously upon application of a magnetic field perpendicular to the CuOplanes. The gap vanishes at the critical field required to induce long-range incommensurate antiferromagnetic order, providing compelling evidence for a field-induced soft-mode driven quantum phase transition.

Phys. Rev. Lett. 102, 177006 (2009)


Direct Observation of Magnon Fractionalization in the Quantum Spin Ladder

B. Thielemann, Ch. Ruegg, H. M. Rønnow, A. M. Lauchli, J.-S. Caux, B. Normand, D. Biner, K.W. Kramer,
H.-U. Gudel, J. Stahn, K. Habicht, K. Kiefer, M. Boehm, D. F. McMorrow, and J. Mesot
We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material <eth>C5H12N?2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.
Phys. Rev. Lett. 102, 107204 (2009)
Coherent d-Wave Superconducting Gap in Underdoped La2-xSrxCuO4 by Angle-Resolved Photoemission Spectroscopy  


M. Shi, J. Chang, S. Pailhes,  M. R. Norman, J. C. Campuzano, M. Mansson, T. Claesson, O. Tjernberg,
A. Bendounan, L. Patthey, N. Momono, M. Oda, M. Ido, C. Mudry, and J. Mesot
We present angle-resolved photoemission spectroscopy data on moderately underdoped La1:855Sr0:145CuOat temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1:895Sr0:105CuOalso show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.
Phys. Rev. Lett. 101, 047002 (2008) 

Superconducting Vortices in CeCoIn5: Toward the Pauli-Limiting Field

A. D. Bianchi, M. Kenzelmann, L. DeBeer-Schmitt, J. S. White, E. M. Forgan, J. Mesot, M. Zolliker, J. Kohlbrecher, R. Movshovich, E. D. Bauer, J. L. Sarrao, Z. Fisk, C. Petrovic, M. R. Eskildsen4
Many superconducting materials allow the penetration of magnetic fields in a mixed state in which the superfluid is threaded by a regular lattice of Abrikosov vortices, each carrying one quantum of magnetic flux. The phenomenological Ginzburg-Landau theory, based on the concept of characteristic length scales, has generally provided a good description of the Abrikosov vortex lattice state. We conducted neutron-scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor cerium-cobalt-indium (CeCoIn5) and found that this form factor increases with increasing field—opposite to the expectations within the Abrikosov-Ginzburg-Landau paradigm. We propose that the anomalous field dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and from the proximity of the superconducting state to a quantum critical point.

Science 319, 177 (2008)