Nadprzewodniki na bazie żelaza FeSe i LiFeP – badania metodą spektroskopii mössbauerowskiej oraz pomiary magnetyczne A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, J. Przewoźnik2, J. Marzec3, K. Wojciechowski4, Z.M. Stadnik 5, U.D. Wdowik6 1 Zakład Spektroskopii Mössbauerowskiej, Instytut Fizyki, Uniwersytet Pedagogiczny, Kraków 2 Katedra Fizyki Ciała Stałego, Wydział Fizyki i Informatyki Stosowanej, Akademia Górniczo-Hutnicza, Kraków 3 Katedra Energetyki Wodorowej, Wydział Energetyki i Paliw, 4 Katedra Chemii Nieorganicznej, Wydział Inżynierii Materiałowej i Ceramiki, 5 Department of Physics, University of Ottawa, Ottawa, Canada 6 Zakład Zastosowań Informatyki, Instytut Techniki,

Superconducting Materials

Fe-based Superconducting Families LaFeAsOF BaFe2As2 LiFeAs FeSe 1111 122 111 11 TC max = 56K 38K 25K 15K

Fe-Se phase diagram The following phases form close to the FeSe stoichiometry: 1) tetragonal P4/nmm structure similar to PbO, called β-FeSe (or α-FeSe) 2) hexagonal P63/mmc structure similar to NiAs, called δ-FeSe 3) hexagonal phase Fe7Se8 with two different kinds of order, i.e., 3c (α-Fe7Se8) or 4c (β-Fe7Se8) A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 100 K, and this phase is superconducting with Tc ≈ 8 K.

Crystal structure of -FeSe Aim of this contribution is to answer two questions concerned with tetragonal/orthorhombic FeSe: 1) is there electron spin density (magnetic moment) on Fe ? 2) is there change of electron density on Fe nucleus during transition from P4/nmm to Cmma structure ?

Fe1.05Se

Fe1.05Se P4/nmm a = 3.7720(1) Å c = 5.5248(1) Å

Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe - point A - spin rotation in hexagonal phase - region B - magnetic anomaly correlated with transition between orthorhombic and tetragonal phases - point C - transition to the superconducting state

Change in electron density  on Fe nucleus S = +0.006 mm/s tetragonal phase transition orthorhombic Change in isomer shift S ↓ Change in electron density  on Fe nucleus S = +0.006 mm/s ρ = –0.02 electron/a.u.3 orthorhombic orthorhombic and superconducting

Quadrupole splitting Δ does not change tetragonal T (K) S (mm/s) Δ (mm/s)  (mm/s) 120 0.5476(3) 0.287(1) 0.206(1) 105 0.5529(3) 0.203(1) 90 0.5594(3) 0.286(1) 0.198(1) 75 0.5622(3) 0.211(1) 4.2 0.5640(4) 0.295(1) 0.222(1) phase transition Quadrupole splitting Δ does not change it means that local arrangement of Se atoms around Fe atom does not change during phase transition orthorhombic orthorhombic orthorhombic and superconducting

Hyperfine magnetic field is equal to applied external magnetic field. Mössbauer spectra obtained in external magnetic field aligned with γ-ray beam Hyperfine magnetic field is equal to applied external magnetic field. Principal component of the electric field gradient (EFG) on Fe nucleus was found as negative.

Fermi level is marked by the vertical line. Total electron spin density versus energy for the Cmma phase at null pressure Spin-up and spin-down states are plotted separately in red and green colors, respectively. Fermi level is marked by the vertical line. This is obviously non-magnetic metallic system.

Phonon dispersion relations at null pressure and for the ground state

PHONON DYNAMICS IN TETRAGONAL/ORTHORHOMBIC PHASE Total density of the phonon states versus pressure for the orthorhombic phase (DOS) V. Ksenofontov, G. Wortmann, A.I. Chumakov et al., Density of Phonon States in Superconducting FeSe as a Function of Temperature and Pressure, arXiv:1004.2007

Electron spin density versus energy for the hexagonal phase A transition from the ferromagnetic insulating state to the metallic state with very small magnetic moment at high hydrostatic pressure Energy gap and magnetic moment in the hexagonal phase

LiFeP P4/nmm a = 3.698(1) Å c = 6.030(2) Å

Magnetization measured in ZFC mode

Magnetic hysteresis obtained at 2 K and 20 K

Magnetization measured in sweep mode

Mössbauer spectra of LiFeP T (K) S (mm/s) Δ (mm/s)  (mm/s) RT 0.247(1) 0.101(1) 0.172(1) 77 0.356(1) 0.112(2) 0.224(1) 4.2 0.364(1) 0.119(3) 0.227(2) [FeP4] tetrahedron coordination

Conclusions FeSe 1. There is no magnetic moment on iron atoms in the P4/nmm and Cmma phases. 2. The electron density on iron nucleus is lowered by 0.02 electron/a.u.3 at 105K during transition from P4/nmm to Cmma phase. LiFeP 3. There is no magnetic order in the superconducting LiFeP.