Mössbauer study of iron-based superconductors

A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2

1 Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland

2 Department of Solid State Physics, Faculty of Physics and Applied Computer Science,AGH University of Science and Technology, Cracow, Poland

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ICAME 2013International Conference on the Applications of the Mössbauer Effect 1-6 September 2013, Opatija, Croatia

Contents

Introduction to the iron-based superconductors

Mössbauer spectroscopy results for:

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AFe2As2 (A = Ca, Ba, Eu) – “122” parent compounds A. Błachowski, K. Ruebenbauer, J. Żukrowski et al., Phys. Rev. B 83, 134410 (2011)

CaFe2-xCoxAs2 ; Ba1-xRbxFe2As2 ; EuFe2-xCoxAs2 – “122” superconductors A. Błachowski, K. Ruebenbauer, J. Żukrowski et al., Phys. Rev. B 84, 174503 (2011) A. Błachowski, K. Ruebenbauer, J. Żukrowski et al., Acta Phys. Pol. A 121, 726 (2012)

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Fe1+xTe – “11” parent compound A. Błachowski, K. Ruebenbauer et al., J. Phys.: Condens. Matter 24, 386006 (2012)

FeSe – “11” superconductor A. Błachowski, K. Ruebenbauer, J. Żukrowski et al., J. Alloys Comp. 494, 1 (2010)

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FeAs – grand-parent compound A. Błachowski, K. Ruebenbauer, J. Żukrowski et al., J. Alloys Comp. 582, 167 (2014)

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Conclusions

Superconductivity in the non-magnetic state of iron under pressure K. Shimizu et al. Nature 412, 316 (2001)

hcp Fe becomes superconductor

at temperatures below 2 K and at pressures between 15 and 30 GPa

Journal of American Chemical SocietyReceived January 2008, Published online February 2008

Up to now the maximum superconducting critical temperature of iron-based superconductors

is 56 K

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Tsc max = 56 K 47 K 18 K 15 K

Fe-based Superconducting Familiespnictogens: P, As, Sb chalcogens: S, Se, Te

1111 122 111 11

LnO(F)FeAs AFe2As2 AFeAs FeTe(Se,S)

Ln = La, Ce, Pr, Nd, Sm, Gd … A = Ca, Sr, Ba, Eu, K A = Li , Na

Layered Structure of Fe-based Superconductors

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Parent Compounds

Doped Compounds

Superconductors

BaFe2As2

Ba1-xKxFe2As2

BaFe2-xCoxAs2

BaFe2As2-xPx

Phase DiagramHoles, electrons or isovalent doping

Spin density wave (SDW)

magnetic order

SDW

Spin density wave (SDW) – simple non-interlaced picture

] )12( sin[ )(1

12

N

nn qxnhqxB

h2n-1 – amplitudes of subsequent harmonics

q – wave number of SDW

x – relative position of the resonant nucleus along propagation direction of the stationary SDW

perpendicular

longitudinal

commensurate or incommensurate

Spin density wave (SDW) seen by Mössbauer Spectroscopy

] )12( sin[ )(1

12

N

nn qxnhqxB

h2n-1 – amplitudes of subsequent harmonics

q – wave number of SDW

x – relative position of the resonant nucleus along propagation direction of SDW

SDW hyperfine field distribution 57Fe Mössbauer spectrum

”122” family of Fe-based superconductors

BaFe2As2 (parent)

TSDW = 136 K Ba0.7Rb0.3Fe2As2

(superconductor)

Tsc = 37 K

57Fe Mössbauer spectra

NM non-magnetic

Shape of SDW

SDW is suppressed by doping

CaFe2As2 (parent)

TSDW = 175 KCaFe1.92Co0.08As2

(superconductor)

Tsc = 20 KResistivity measurements:

It seems that

magnetism and superconductivity coexist (?).

Mössbauer measurements:

Superconductivity has filamentary character

and occurs

in the regions free of 3d magnetic moments.

EuFe2As2

critical exponent 0 ≈ 0.125 universality class (1, 2)↓

one dimension in the spin space (Ising model) and

two dimensions in the real space (magnetic planes)

Root mean square amplitude of SDW

EuFe2-xCoxAs2

57Fe Mössbauer spectra

TSDW = 190 K

TSDW = 150 K

TSDW = 100 K

traces of SDW at 80 K

lack of SDW

TN (Eu) = 19 K

Eu2+ Transferred Field on 57Fe

filamentary superconductivity

superconductor

superconductor

superconductor

EuFe2-xCoxAs2

151Eu Mössbauer spectra

Eu(3+)

Eu(2+)

EuFe2As2

TSDW (Fe) = 190 KTN (Eu) = 19 K

Parent

Superconductor Tsc = 9.5 K

Over-doped

Eu2+ orders magnetically regardless of the Co-substitution level. Eu2+ moments rotate from a-axis to c-axis. Eu2+ magnetism and superconductivity coexist.

Fe1+xTe

x = 0.04 – 0.18x = 0.06 , 0.10 , 0.14 , 0.18

Magnetic-crystallographic phase diagram

S. Röler et al., Phys. Rev. B 84 174506 (2011)

x in Fe1+xTe

Parent CompoundFe1+yTe

Doped Compound → Superconductory ≈ 0

Fe1+yTe1-xSex Fe1+yTe1-xSx

K. Katayama et al., J. Phys. Soc. Japan 79 113702 (2010)

Fe1.06Te 57Fe Mössbauer spectrum SDW field distribution shape of SDW

regular (tetrahedral) Fe excess (interstitial) Fe SDW

Fe1.14Te

57Fe Mössbauer spectrum SDW field distribution shape of SDW

Three different kinds (surroundings) of excess (interstitial) Fe. Magnetism of the excess Fe and SDW disappear at the same transition temperature.

regular Fe - SDW

Fe1+xTe

x=0.06

x=0.10

x=0.14

x=0.18

65 K 4.2 Kshape of SDW

at 4.2 K

SDW is very sensitive to concentration of interstitial iron with relatively large localized magnetic moments.

Localized iron moments prevent superconductivity, so interstitial iron must be removed by doping and/or deintercalation to get superconducting material.

regular Fe (SDW) excess Fe

Fe1.01Se Tsc = 8 K

High (external) magnetic field Mössbauer spectroscopy

Hyperfine magnetic field is equal to applied external magnetic field- it means that there is no magnetic moment on the Fe atoms

tetragonal

orthorhombic

orthorhombicandsuperconductor

orthorhombic

structuraldistortion

sharp magnetic transition paramagnetic

region magnetic

region

SPIN SPIRAL

FeAs

Crystal structurePnma or Pna21 ?

Arrows show Pna21 – like distortion E.E. Rodriguez et al., PRB 8383, 134438 (2011)

Anisotropy of the hyperfine magnetic fields (spiral projections onto a-b plane) in FeAsLeft column shows [0 k+1/2 0] iron, right column shows [0 k 0] iron.

Ba and Bb - iron hyperfine field components along the a-axis and b-axis, respectively.

Orientation of the EFG and

hyperfine magnetic field in the main crystal axes

Average hyperfine fields <B> for

[0 k+1/2 0] and [0 k 0] irons.

Tc - transition temperature - static critical exponent

A. Błachowski et al., JALCOM 582, 167 (2014)

FeAs

Spectral shift S and

quadrupole coupling constant AQ versus temperature

for [0 k+1/2 0] iron and [0 k 0] iron.

Line at 72 K separate magnetically ordered region from paramagnetic region.

Relative recoilless fraction <f>/<f0> versus temperature

Green points correspond to magnetically ordered region. Red point is the normalization point.

Inset shows relative spectral area RSA plotted versus temperature.

. 1

RSA1 0

0

C

n

n

N

NN

C

Conclusions

Thank you very much for your attention!

AFe2As2 - parentsThe SDW magnetic order with universality class (1, 2) and with almost rectangular shape at saturation.

Ba1-xRbxFe2As2

The SDW vanishes upon doping leading to superconductivity.

CaFe2-xCoxAs2

Superconductivity has filamentary character and occurs in the regions free of 3d magnetic moments.

EuFe2-xCoxAs2

Localized 4f magnetic moments could order within the superconducting phase.

Fe1+xTeExcess (interstitial) iron with relatively large localized magnetic moment strongly influence on the ordering temperature, shape and amplitude of the SDW.

FeSeThere is no magnetic moment on iron in superconducting FeSe and it is PRESUMABLY the feature of all iron-based superconductors.

FeAsSpin spiral leads to the complex variation of the hyperfine field amplitude with the spin orientation (local magnetic moment) varying in the a-b plane. Pattern express symmetry of 3d electrons in the a-b plane with the significant distortion caused by the arsenic bonding p electrons.Strong coupling between magnetism and lattice dynamics i.e. strong phonon-magnon interaction.