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  • 1.
    Aperis, Alex
    et al.
    Department of Physics, National Technical University of Athens, Athens, Greece.
    Kotetes, P.
    Varelogiannis, G.
    Oppeneer, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Small-q phonon-mediated unconventional superconductivity in the iron pnictides2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 9, p. 092505-Article in journal (Refereed)
    Abstract [en]

    We report self-consistent calculations of the gap symmetry for iron-based high-temperature superconductors using realistic small-q phonon-mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless s± state that was first encountered in a spin-fluctuation mechanism picture. Nodal s± as well as other gap structures such as dx2−y2, s±+dx2−y2, and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase-sensitive experiments reporting a gap changing sign, attributed previously only to a nonphononic mechanism, and isotope effect measurements proving the involvement of phonons in the pairing.

  • 2.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio theory of magnetic-field-induced odd-frequency two-band superconductivity in MgB22015In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 92, no 5, article id 054516Article in journal (Refereed)
  • 3.
    Aperis, Alex
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Multiband full-bandwidth anisotropic Eliashberg theory of interfacial electron-phonon coupling and high-Tc superconductivity in FeSe/SrTiO32018In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Phys. Rev. B, Vol. 97, p. 060501(R)-Article in journal (Refereed)
  • 4.
    Aperis, Alexandros
    et al.
    Department of Physics, National Technical University of Athens, GR-15780 Athens, Greece.
    Varelogiannis, Georgios
    Littlewood, Peter B
    Magnetic-field-induced pattern of coexisting condensates in the superconducting state of CeCoIn52010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, P, Vol. 104, no 21, p. 216403-Article in journal (Refereed)
    Abstract [en]

    CeCoIn5 is an anomalous superconductor which exhibits a high-magnetic-field phase that consists of a modulated magnetic coupling together with persistent superconducting order. Here we use a generic microscopic model to argue that this state is a pattern of coexisting condensates: a d-wave singlet superconducting (SC) state, a staggered π-triplet SC state, and a spin density wave (SDW). Our microscopic picture allows a calculation of the phase diagram, and physical consequences including NMR. We interpret the appearance of the SDW order in the Q phase as being induced by odd-triplet pairing.

  • 5.
    Bekaert, J.
    et al.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Partoens, B.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Milosevic, M. V.
    Department of Physics, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB42018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 97, no 1, article id 014503Article in journal (Refereed)
    Abstract [en]

    We present an advanced method to study spin fluctuations in superconductors quantitatively and entirely fromfirst principles. This method can be generally applied to materials where electron-phonon coupling and spinfluctuations coexist. We employ it here to examine the recently synthesized superconductor iron tetraboride(FeB4) with experimentalTc∼2.4K[H.Gouet al.,Phys.Rev.Lett.111,157002(2013)]. We prove thatFeB4is particularly prone to ferromagnetic spin fluctuations due to the presence of iron, resulting in a largeStoner interaction strength,I=1.5 eV, as calculated from first principles. The other important factor is itsFermi surface that consists of three separate sheets, among which two are nested ellipsoids. The resultingsusceptibility has a ferromagnetic peak aroundq=0, from which we calculated the repulsive interaction betweenCooper pair electrons using the random phase approximation. Subsequently, we combined the electron-phononinteraction calculated from first principles with the spin fluctuation interaction in fully anisotropic Eliashbergtheory calculations. We show that the resulting superconducting gap spectrum is conventional, yet very stronglydepleted due to coupling to the spin fluctuations. The critical temperature decreases from Tc=41 K, if they arenot taken into account, toTc=1.7 K, in good agreement with the experimental value.

  • 6. Bekaert, J.
    et al.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Partoens, B.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Milošević, M. V.
    Evolution of multigap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB22017In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 96, article id 094510Article in journal (Refereed)
  • 7.
    Bekaert, J.
    et al.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Bignardi, L.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.; Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    van Abswoude, P.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.
    Mattevi, C.
    IOM CNR, Lab TASC, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.; Imperial Coll London, Dept Mat, Exhibit Rd, London SW7 2AZ, England.
    Gorovikov, S.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.; Canadian Light Source Inc, 44 Innovat Blvd, Saskatoon, SK S7N 2V3, Canada.
    Petaccia, L.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Goldoni, A.
    Elettra Sincrotrone Trieste, Str Statale 14 Km 163-5, I-34149 Trieste, Italy.
    Partoens, B.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Peeters, F. M.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Milošević, M. V.
    Univ Antwerp, Dept Phys, Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
    Rudolf, P.
    Univ Groningen, Zernike Inst Adv Mat, Nijenborgh 4, NL-9747 AG Groningen, Netherlands.
    Cepek, C.
    IOM CNR, Lab TASC, Str Statale 14 Km 163-5, I-34149 Trieste, Italy .
    Free surfaces recast superconductivity in few-monolayer MgB2: Combined first-principles and ARPES demonstration2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, no 1, article id 14458Article in journal (Refereed)
    Abstract [en]

    Two-dimensional materials are known to harbour properties very different from those of their bulk counterparts. Recent years have seen the rise of atomically thin superconductors, with a caveat that superconductivity is strongly depleted unless enhanced by specific substrates, intercalants or adatoms. Surprisingly, the role in superconductivity of electronic states originating from simple free surfaces of two-dimensional materials has remained elusive to date. Here, based on first-principles calculations, anisotropic Eliashberg theory, and angle-resolved photoemission spectroscopy (ARPES), we show that surface states in few-monolayer MgB2 make a major contribution to the superconducting gap spectrum and density of states, clearly distinct from the widely known, bulk-like σ- and π-gaps. As a proof of principle, we predict and measure the gap opening on the magnesium-based surface band up to a critical temperature as high as ~30 K for merely six monolayers thick MgB2. These findings establish free surfaces as an unavoidable ingredient in understanding and further tailoring of superconductivity in atomically thin materials.

  • 8.
    Bekaert, J.
    et al.
    Univ Antwerp, Dept Phys, Condensed Matter Theory Grp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Vercauteren, S.
    Univ Antwerp, Dept Phys, Condensed Matter Theory Grp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Komendova, Lucia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Univ Antwerp, Dept Phys, Condensed Matter Theory Grp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Prozorov, R.
    Iowa State Univ, Dept Phys & Astron, Ames, IA 50011 USA.;Iowa State Univ, Ames Lab, Ames, IA 50011 USA..
    Partoens, B.
    Univ Antwerp, Dept Phys, Condensed Matter Theory Grp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Milosevic, M. V.
    Univ Antwerp, Dept Phys, Condensed Matter Theory Grp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium..
    Anisotropic type-I superconductivity and anomalous superfluid density in OsB22016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 14, article id 144506Article in journal (Refereed)
    Abstract [en]

    We present a microscopic study of superconductivity in OsB2, and discuss the origin and characteristic length scales of the superconducting state. From first-principles we show that OsB2 is characterized by three different Fermi sheets, and we prove that this fermiology complies with recent quantum-oscillation experiments. Using the found microscopic properties, and experimental data from the literature, we employ Ginzburg-Landau relations to reveal that OsB2 is a distinctly type-I superconductor with a very low Ginzburg-Landau parameter kappa-a rare property among compound materials. We show that the found coherence length and penetration depth corroborate the measured thermodynamic critical field. Moreover, our calculation of the superconducting gap structure using anisotropic Eliashberg theory and ab initio calculated electron-phonon interaction as input reveals a single but anisotropic gap. The calculated gap spectrum is shown to give an excellent account for the unconventional behavior of the superfluid density of OsB2 measured in experiments as a function of temperature. This reveals that gap anisotropy can explain such behavior, observed in several compounds, which was previously attributed solely to a two-gap nature of superconductivity.

  • 9. Hosen, M. Mofazzel
    et al.
    Dhakal, Gyanendra
    Dimitri, Klauss
    Maldonado, Pablo
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kabir, Firoza
    Sims, Christopher
    Riseborough, Peter
    Oppeneer, Peter M.
    Kaczorowski, Dariusz
    Durakiewicz, Tomasz
    Neupane, Madhab
    Discovery of topological nodal-line fermionic phase in a magnetic material GdSbTe2018In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 8, no 1Article in journal (Refereed)
    Abstract [en]

    Topological Dirac semimetals with accidental band touching between conduction and valence bands protected by time reversal and inversion symmetry are at the frontier of modern condensed matter research. A majority of discovered topological semimetals are nonmagnetic and conserve time reversal symmetry. Here we report the experimental discovery of an antiferromagnetic topological nodal-line semimetallic state in GdSbTe using angle-resolved photoemission spectroscopy. Our systematic study reveals the detailed electronic structure of the paramagnetic state of antiferromagnetic GdSbTe. We observe the presence of multiple Fermi surface pockets including a diamond-shape, and small circular pockets around the zone center and high symmetry X points of the Brillouin zone (BZ), respectively. Furthermore, we observe the presence of a Dirac-like state at the X point of the BZ and the effect of magnetism along the nodal-line direction. Interestingly, our experimental data show a robust  Dirac-like state both below and above the magnetic transition temperature (TN  = 13 K). Having a relatively high transition temperature, GdSbTe provides an archetypical platform to study the interaction between magnetism and topological states of matter.

  • 10.
    Hosen, M. Mofazzel
    et al.
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Dimitri, Klauss
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Belopolski, Ilya
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Dhakal, Gyanendra
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Kabir, Firoza
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Sims, Christopher
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Hasan, M. Zahid
    Princeton Univ, Joseph Henry Lab, Princeton, NJ 08544 USA.
    Kaczorowski, Dariusz
    Polish Acad Sci, Inst Low Temp & Struct Res, PL-50950 Wroclaw, Poland.
    Durakiewicz, Tomasz
    Marie Curie Sklodowska Univ, Inst Phys, PL-20031 Lublin, Poland.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Neupane, Madhab
    Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA.
    Observation of gapless Dirac surface states in ZrGeTe2018In: Physical Review B, Vol. 97, no 12, article id 121103Article in journal (Refereed)
    Abstract [en]

    The experimental discovery of the topological Dirac semimetal establishes a platform to search for various exotic quantum phases in real materials. ZrSiS-type materials have recently emerged as topological nodal-line semimetals where gapped Dirac-like surface states are observed. Here, we present a systematic angle-resolved photoemission spectroscopy (ARPES) study of ZrGeTe, a nonsymmorphic symmetry protected Dirac semimetal. We observe twoDirac-like gapless surface states at the same <overline> X point of the Brillouin zone. Our theoretical analysis and first-principles calculations reveal that these are protected by crystalline symmetry. Hence, ZrGeTe appears as a rare example of a naturally fine tuned system where the interplay between symmorphic and nonsymmorphic symmetry leads to rich phenomenology and thus opens up opportunities to investigate the physics of Dirac semimetallic and topological insulating phases realized in a single material.

  • 11. Hosen, M. Mofazzel
    et al.
    Dimitri, Klauss
    Nandy, Ashis K.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sankar, Raman
    Dhakal, Gyanendra
    Maldonado, Pablo
    Kabir, Firoza
    Sims, Christopher
    Chou, Fangcheng
    Kaczorowski, Dariusz
    Durakiewicz, Tomasz
    Oppeneer, Peter M.
    Neupane, Madhab
    Distinct multiple fermionic states in a single topological metal2018In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, no 1Article in journal (Refereed)
    Abstract [en]

    Among the quantum materials that have recently gained interest are the topological insulators, wherein symmetry-protected surface states cross in reciprocal space, and the Dirac nodal-line semimetals, where bulk bands touch along a line in k-space. However, the existence of multiple fermion phases in a single material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic material Hf2Te2P and discover properties, which are unique in a single topological quantum material. We experimentally observe weak topological insulator surface states and our calculations suggest additional strong topological insulator surface states. Our first-principles calculations reveal a one-dimensional Dirac crossing—the surface Dirac-node arc—along a high-symmetry direction which is confirmed by our ARPES measurements. This novel state originates from the surface bands of a weak topological insulator and is therefore distinct from the well-known Fermi arcs in semimetals.

  • 12. Kotetes, P.
    et al.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Varelogiannis, G.
    Magnetic-field-induced chiral hidden order in URu2Si22014In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 94, no 32-33, p. 3789-3802Article in journal (Refereed)
    Abstract [en]

    Two of the most striking and yet unresolved manifestations of the hidden order (HO) in [GRAPHICS] are associated, on one hand, with the double-step metamagnetic transitions and, on the other, with the giant anomalous Nernst signal. Both are observed when a magnetic field is applied along the [GRAPHICS] -axis. Here, we provide for the first time a unified understanding of these puzzling phenomena and the related field-temperature ( [GRAPHICS] ) phase diagram. We demonstrate that the HO phase at finite fields can be explained with a chiral [GRAPHICS] spin density wave, assuming that the zero-field HO contains only the time-reversal symmetry preserving [GRAPHICS] component. We argue that the presence of the field-induced chiral HO can be reflected in a distinctive non-linear [GRAPHICS] dependence of the Kerr angle, when a Kerr experiment is conducted for finite fields. This fingerprint can be conclusive for the possible emergence of chirality in the HO.

  • 13. Livanas, G.
    et al.
    Aperis, Alex
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Kotetes, P.
    Varelogiannis, G.
    Nematicity from mixed S± + dx2-y2 states in iron-based superconductors2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 10, article id 104502Article in journal (Refereed)
    Abstract [en]

    We propose a novel mechanism for nematicity potentially relevant in some iron-based superconductors (SCs). We demonstrate that the mixed S +or-+dx 2-y 2 SC state is a physically accessible state and may emerge under generic conditions, lowering spontaneously the fourfold rotational symmetry C 4 to C 2. We provide a detailed study of the mixed S +or-+dx 2-y 2 nematic state including its behavior under a Zeeman field. As a fingerprint of this state, we predict at low temperatures a first-order Zeeman-field-induced transition from the mixed nematic SC phase to the pure dx 2-y 2 SC phase. The Zeeman field-temperature phase diagram for a nodeless mixed nematic state exhibits three distinct SC phases and a tetracritical point, remarkably reminiscent of the one observed in UPt 3. Our mechanism for nematicity may also be relevant for nonsuperconducting nematic states involving mixed S +or-+dx 2-y 2 spin/charge density waves.

1 - 13 of 13
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