Alex Aperis

Small-q phonon-mediated unconventional superconductivity in the iron pnictides

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.

Evolution of multigap superconductivity in the atomically thin limit: Strain-enhanced three-gap superconductivity in monolayer MgB2

Evolution of multigap superconductivity in the atomically thin limit : Strain-enhanced three-gap superconductivity in monolayer MgB2

Nematicity from mixed S± + dx2-y2 states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.

Advanced first-principles theory of superconductivity including both lattice vibrations and spin fluctuations: The case of FeB4

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 ...

Distinct multiple fermionic states in a single topological metal

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.The existence of multiple topological phases in a single material, although theoretically possible, has not been verified. Here, the authors observe weak topological insulator surface states and a one-dimensional Dirac-node crossing surface state in a single metallic material Hf2Te2P.

Magnetic-field-induced chiral hidden order in URu2Si2

Two of the most striking and yet unresolved manifestations of the hidden order (HO) in 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 -axis. Here, we provide for the first time a unified understanding of these puzzling phenomena and the related field–temperature () phase diagram. We demonstrate that the HO phase at finite fields can be explained with a chiral spin density wave, assuming that the zero-field HO contains only the time-reversal symmetry preserving component. We argue that the presence of the field-induced chiral HO can be reflected in a distinctive non-linear 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.

Discovery of topological nodal-line fermionic phase in a magnetic material GdSbTe

M. Mofazzel Hosen, Gyanendra Dhakal, Klauss Dimitri, Pablo Maldonado, Alex Aperis, Firoza Kabir, Peter M. Oppeneer, Dariusz Kaczorowski, Tomasz Durakiewicz, 5 and Madhab Neupane* Department of Physics, University of Central Florida, Orlando, Florida 32816, USA Department of Physics and Astronomy, Uppsala University, P.O. Box 516, S-75120 Uppsala, Sweden Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland Condensed Matter and Magnet Science Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Institute of Physics, Maria Curie Sklodowska University, 20-031 Lublin, Poland (Dated: July 18, 2017)

Nematicity from mixed S +or-+dx 2-y 2 states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.

Nematicity from mixedS±+dx2−y2states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.

Nematicity from mixed states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.