P. ToulemondeInstitut Néel , CNRS and UGA, F-38042 Grenoble, France.

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2

� Institut Néel, CNRS and UGA, Grenoble, France:S. Karlsson, P. Strobel, M. Raba, P. Rodière, A. Sulpice, F. Gay, L. Doussoulin

� SPSMS, CEA-INAC and UGA, Grenoble, France: C. Marcenat

� ESRF, Grenoble, France:V. Svitlyk, V. Dmitriev, D. Chernyshov, F. Cova, G. Garbarino, M. Mezouar

� ILL, Grenoble, France:Th. Hansen

Collaborations

� KIT, Karlsruhe, Germany

Haghighirad A.A.

� IMPMC, CNRS, UniversitéP&M. Curie, Paris, France:B. Lebert, S. Klotz

� LPS, Orsay, France:V. Balédent

� SOLEIL, Orsay, France:J. P. Rueff

0 30 60 90 120 150 180 210 240 270 3000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

FeSe crystal grown by CVTR(280K)/R(10K)=18.5

R(O

hm)

Temperature(K)

Ts=90K

Tc=8.7K

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� Our study on FeSe crystals grown

by chemical vapor transport:

1) X-ray diffraction / HP-lowT

2) Neutron diffraction / HP-lowT

3) X-ray Emission (XES) & X-ray Absorption

spectroscopies (XAS) / HP-lowT

� Conclusion & Perspectives

Outline

� Introduction/motivations

FeSe

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4

G. Garbarino, P. Toulemonde, M. Nunez-Regueiro et al. EPL 86 (2009)

Medvedev et al. Nat. Mat. (2009).

Huge increase of Tc under pressure: + 1.7-3.5K/GPa !

Introduction/motivations

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5

→ A priori: Superconductivity NOT issued from a

(long-range AFM ordered) magnetic phase

→ Non continuous Tc change

Samples: single crystals grown in a NaCl/KCl flux.

SrFe2As2

« Ortho-I » (Cmma)

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FeSe: P,T phase diagram: more complex than expected!!!

→ Non continuous Tc change→ Pressure induced magnetic phase!

Sun et al. Nat.Comm.7 (2016).

Resistivity/HP

Bendele et al. PRL 104 (2010); PRB 85 (2012).

Muons

spectroscopy

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7

Shanghai, China

Wang, Xue et al. Chin. Phy. Lett. 29 (2012)

Beijing, China

Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO 3

STM :Tc ∼∼∼∼ 65-77K

Transport:

ARPES:

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� Our study on FeSe crystals grown by chemical vapor transport:

1) X-ray diffraction / HP-lowT

2) Neutron diffraction / HP-lowT

3) X-ray Emission (XES) & X-ray Absorption spectroscopies (XAS) /

HP-lowT

� Conclusion & Perspectives

Outline

� Introduction/motivations

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9

HP(-lowT) X-ray diffraction study (ESRF)

ESRF, id27 high pressure beamline

Diamond anvil cellPTM= helium

Sample: single crystal grown by CVT.

Svitlyk et al. Phys. Rev. B 96, 014520 (2017).

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• at HP & 300K

Rapid degradation of the crystalline

quality of the HP ortho-II (Pbnm)

FeSe phase.

Some reciprocal layers of FeSe

LowP tetrag. (P4/nmm) FeSe

phase; yellow grids = pseudo-tetragonal

lattice .

HP ortho-II (Pbnm) FeSe phase; reciprocal lattice vectors indicated

by red arrows; bluearrow marks a second d-segregated domain.

Svitlyk et al. PRB 96, 014520 (2017).

hk0

→ transition between 6.6 GPaand 8.1 GPa @ 300K

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11

coexistence of the HP ortho-II FeSe phase (reciprocal lattice vectors indicated by red arrows) and the LP (Cmma) FeSe

phase

Strong decrease in the concentration of the LP

superconducting phase above 6.9GPa.

• at HP & 20 K → transition ortho-I → ortho-II between 6.2 GPa and 6.9 GPa

@ 20K

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Böhmer et al. arxiv/condmat: 1803.09449

� Confirmation of the ortho-I → HP ortho-II phasetransition occurring at 7.7GPa (at 60K) & 8 GPa (300K) in FeSe single crystals:

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13

Two sets of antiparallel displacements of both Fe (red spheres) and

Se (blue spheres)sublattices at 90°

(displacive ��� phonon

field).

HP (MnP-type, Pnma or Pbnm)

ortho-II structure of FeSe

composed of chains of face shared FeSe6

octahedra alongthe a axis.

• Transformation mechanism from LP Cmma (ortho-I) → HP Pbnm(ortho-II) phase

Result of combined structure distortions by the ��

� phonon field and additional ���� (~0.3)

strain. Red lines: hexagonal NiAs-type

lattice (�~ 3; dashed lines: (MnP-type) ortho-II

structure cell.

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14

0 1 2 3 4 5 6 7 85.08

5.10

5.12

5.14

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

5.34

AF

M p

hase

Tet

rago

nal p

hase

a &

b-a

xis

(Å)

P (GPa)

T=50K b-axis (Å) our data a-axis (Å) our data b-axis Kothapalli et al. a-axis Kothapalli et al. b-axis Millican et al. a-axis Millican et al.

Ort

horh

ombi

c ph

ase

15.6 15.8 16.0 16.2 16.4 16.6 16.8 17.0 17.2 17.4 17.6 17.8

0

5000

10000

15000

20000

2 theta (deg.)

inte

nsity

(a.

u.)

0

2000

4000

6000

8000

10000

12000

14000

(401

)(0

41)

(400

)

inte

nsity

(a.

u.)

7.03GPa

6.66GPa

5.85GPa

5.04GPa

4.58GPa

4.08GPa3.62GPa

3.10GPa

2.49GPa2.04GPa1.80GPa1.55GPa1.27GPa1.07GPa0.92GPa0.65GPa0.40GPa

0.25GPa

6.90GPa

6.23GPa

5.04GPa4.35GPa

4.09GPa

3.65GPa

3.11GPa

2.69GPa

2.21GPa

1.67GPa1.27GPa

0.95GPa0.81GPa0.49GPa

0.18GPa

(040

)

→ Analysis using Crysalis software or using directly specific (400), (040) and (331) Bragg peaks of the orthorhombic lattice.

• at HP & 50 K: Analysis of structural parameters in the low P region (P<8GPa)

ort

ho-I

Tetr

a.AFM

-phas

e (o

rtho-I

*)

50 K 20 K

ort

ho-I

AFM

-phas

eO

rtho-I

*

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Terashima et al.JPSJ 84, 063701 (2015).

Knöner et al. PRB 91, 174510 (2015): PTM = He.

50K

50K

transport/HP

transport/HP

0 1 2 3 4 5 6 7 85.08

5.10

5.12

5.14

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

5.34

AF

M p

hase

Tet

rago

nal p

hase

a &

b-a

xis

(Å)

P (GPa)

T=50K b-axis (Å) our data a-axis (Å) our data b-axis Kothapalli et al. a-axis Kothapalli et al. b-axis Millican et al. a-axis Millican et al.

Ort

horh

ombi

c ph

ase

• at 50K: ortho-I (Cmma)→ tetra. at 0.9-1 GPain agreement with transport measurements

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Bendele et al. PRL 104 (2010); PRB 85 (2012).

1st evidence of an induced AFM

phase emerging at 1GPa.

50K

2GPa

� Muons spectroscopy

0 1 2 3 4 5 6 7 85.08

5.10

5.12

5.14

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

5.34

AF

M p

hase

Tet

rago

nal p

hase

a &

b-a

xis

(Å)

P (GPa)

T=50K b-axis (Å) our data a-axis (Å) our data b-axis Kothapalli et al. a-axis Kothapalli et al. b-axis Millican et al. a-axis Millican et al.

Ort

horh

ombi

c ph

ase

• at 50K: tetra. → ortho-I* at 1.9 GPa

in agreement with µSR results* maybe monoclinic supplementary distortion in the AFM phase, because

(-331)ortho and (331)ortho become non equivalent at P>1.9GPa.

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• in agreement with Millican et al., SSC 149 (2009), for P<0.9GPa (neutron powder diffraction; PTM= helium) and with Kothapalli et al.Nat. Comm. 7 (2016) for P<3.1GPa.

0 1 2 3 4 5 6

5.14

5.16

5.18

5.20

5.22

5.24

5.26

5.28

5.30

5.32

5.34

Ort

horh

ombi

c ph

ase

a &

b-a

xis

(Å)

P (GPa)

T=20K b-axis our data a-axis our data b-axis Kothapalli et al. at 19K a-axis Kothapalli et al. at 19K

AF

M p

hase

0 1 2 3 4 5 6

5.15

5.20

5.25

5.30

5.35

5.40

5.45

Ort

horh

ombi

c ph

ase

(Cm

ma)

T=20K c-axis lattice volume

P (GPa)

c-ax

is (

Å)

68

70

72

74

76

78

Latti

ce v

olum

e (Å

3 ) in

the

tetr

agon

al c

ell

AF

M p

hase

• at HP & 20 K: lattice parameters (P<8GPa)

• at 20K both analyses detect only the ortho-I (Cmma ) → ortho-I (monoclinic?) AFM phase at 2GPa (anomaly in c-axis & volume variation).

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� Previous study: powder XRD/HP study @ 16K

• Sample: 31% of hexag. (NiAs) impurity

• Anomalies at 2GPa (Se height/Fe-Se

bond length) and 8 GPa (c-axis)

• At 16K transition at P > 7.5 GPa → hexag.

(NiAs) HP phase (P63/mmc)

• Hexag. (NiAs) → ortho (Pbnm) transition

detected at 9-12GPaMargadonna et al. PRB 80, 064506 (2009).

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� Raman spectroscopy

• Collapse of critical nematic fluctuations

between 1.6 and 3 GPa

• Anomalous softening of phonons below

2 GPa

P. Massat, Y. Gallais, P. Toulemonde et al. Phys. Rev. Lett. 121 (2018) 077001.

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0 1 2 3 4 5 6 7 80.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

Tet

ra. p

hase

AF

M p

hase

Ort

horh

ombi

c ph

ase

δ=(b

-a)/

(a+

b)

P (GPa)

orthorhombicity at 50K orthorhombicity at 20K• Orthorhombicity (defined as

δ = (b - a)/(b + a) parameters ratio)

o of the ortho-I phase at superconducting, i.e. for P > 1.7-2GPa, (20 K, blue)

o and non-superconducting, i.e. in particular in the AFM phase for P> 2GPa, (50 K, red) temperatures.

• at HP & lowT: orthorhombicity (P<8GPa)

→ Competition between superconductivityand magnetism: superconductivity tends to reduce the lattice distortion while magnetism increases it.

21

In agreement with Böhmer et al. arxiv/condmat: 1803.09449Orthorhombicity tends to decrease when Tc increases from 3 to 6.4GPa (when SC coexists with magnetism).

• at HP & lowT: orthorhombicity (P<8GPa)

0 1 2 3 4 5 6 7 80.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

Tet

ra. p

hase

AF

M p

hase

Ort

horh

ombi

c ph

ase

δ=(b

-a)/

(a+

b)

P (GPa)

orthorhombicity at 50K orthorhombicity at 20K

Pressure-dependence of the zero-T limit of the ortho. distortion

→ competition between superconductivityand magnetism:

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Large filled circles at the 20, 50 and 300 K isotherms = our workblack – tetragonal phase, grey – “ortho-I” phase, green – AFM phase, red – “ortho-II” phase, blue – a mixture of “ortho-I” and “ortho-II” phases, cyan – possible onset of the formation of the “ortho-II” phase; * - our unpublished data based on transport measurements below 1.3 GPa.

• Updated P,T phase diagram

Svitlyk et al. PRB 96, 014520 (2017).

Sun et al. Nat.Comm.7 (2016).

50K

50K

XRD & Mössbauer/HP

transport/HP

• Comparison with 2016 literature

Kothapalli et al., Nat.Comm. 7

(2016).

ILL, D20 instrument, Grenoble, France

Paris-Edinburgh large volume cellPTM= 4:1 deuterated methanol : ethanol mixture;

TiZr gasketSample: grinded single crystal grown by CVT.

λ=2.41 or 1.30 Å2 experiments:

(1) FeSe powder pressurized by c-BN anvils(2) FeSe presurized by sintered diamond anvils

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HP-low T neutron diffraction study

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24

10 20 30 40 50 60 70 80 90 100 1100

200

400

600

800

1000

1200

1400

1600

1800

(211

)

(200

)(1

12)

(111

)

(001

)

(101

)(1

11)

Pb

(P m

arke

r)

inte

nsity

(a.

u)

2 theta (deg.)

0 GPa 0.1 GPa 1.22 GPa 2.24 GPa 3.14 GPa 4.21 GPa 6.23 GPa 7.48 GPa 7.91 GPa 8.47 GPa 8.49 GPa 9.13 GPa 10.36 GPa 10.90 GPa 11.8 GPa

(111

) C

-dia

mon

d an

vil

D20, ILL (l=2.41Å)

→ decrease of the sample signal with P→ Lattice contraction in agreement with the one obtained from XRD andPTM=He (at least up to 7GPa)→ Only a few % of ortho-II (at 10-12GPa) detected before signal lost!

0 1 2 3 4 5 6 7 8 9 10 11 1262

64

66

68

70

72

74

76

78

80

Latti

ce v

olum

e (Å

)

P(GPa)

volume at RT (from NPD, PTM=alcohol) volume at 300K (XRD, PTM=He)

NPD: from exp. number 2(loading with sintered diamond anvils)

• Compression at room temperature

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55 60 65 70 75 80 85 90 95 100 105200

300

400

500

600

(211

)(1

03)

Pb

Pb

(201

)

(200

)

(111

)

1.8GPa 300K 1.6GPa 6K

2 theta (deg.)

inte

nsity

(a.

u)

(112

)

200

400

600

800

1000

1200

inte

nsity

(a.

u.)

• Cooling at 1.8-1.6 GPa

→ non discernable ortho –I (Cmma) distortion at low T in our configuration (high flux)

P (GPa)

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26

• Cooling at 4-5GPa: to probe the pressure induced AFM phase

10 20 30 40 50 60 70 80 90 100 1100

5000

10000

15000

20000

25000

inte

nsity

(a.

u.)

2 theta (deg.)

Yobs Ycal Yobs-Ycal posr

P=4.8GPa, T=6K

FeSe (nuc)FeSe (mag)Pb

→ non discernable ortho (monoclinic) distortion at low T at both λ→ Rietveld fit: magnetic peaks (if any) hidden in the noise/background

→ Simulation with the single-stripe (colinear) AFM structure

with K = (0 1 ½) and m=0.75µB → m(Fe) probably < 0.5µB at 4-5GPa

K = (0 1 ½)

P (GPa)

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K = (0 1 ½)

10 15 20 25 30 35 40 45 50 55 60

5000

6000

7000

8000

9000

10000

11000

inte

nsity

(a.

u.)

2 theta (deg.)

Yobs Ycal Yobs-Ycal posr

P=4.8GPa, T=6K

FeSe (nuc)

FeSe (mag)

→ Rietveld fit: magnetic peaks (if any) hidden in the noise/background

→ Simulation with the single-stripe (collinear) AFM structure

with K = (0 1 ½) and m=0.75µB → m(Fe) probably < 0.5µB at 4-5GPa

• Cooling at 4-5GPa: to probe the pressure induced AFM phase

“collinear1”

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28

or

a

b

+ FM (I) or AFM (II) coupling along c-axis

� Long range AFM orders compatible with µ-SR study (R. Khasanov et al. Phys. Rev. B 95 (2017)):

• Cooling at 4-5GPa: to probe the pressure induced AFM phase

our data

CI or CII BI or BII

→ our suggested upper limit for m(Fe): 0.3µB for CI/II and 0.2µB for BI/II.

- Extrapolations from µ-SR (Pmax=1.9GPa)

predict a moment of 0.35µB at 4.5GPa, should be visible for the BI/II configuration in our data!→ CI/II structure with m(Fe)=0.3µB

= the most probable!

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- Update of the previous result obtained by NPD on FeSepowder/HP:

→ low m(Fe) in the AFM phase (1.5-6GPa) < 0.3µB

Bendele et al. PRB 85 (2012): from muons spectroscopy: m(Fe)

= 0.2µB at 2.4GPa and from NPD (PSI-SINQ) <0.5µB at

4.4GPa

• pressure induced AFM phase studied by NPD: Conclusion

Perspectives:

- neutron scattering on FeSe single crystal /HP-low T necessary to measure magnetic reflections

- NPD experiment at P>8-10 GPawith a larger sample signal required to reach the (ortho-II) Pbnm phase and probe its possible long range magnetic order

30

HP-low T XES & XAS studies

SOLEIL, Galaxies beamline, Saclay, France

B.W. Lebert et al. Phys. Rev. B 97, 180503(R) (2018)

10K

Diamond anvil cellPTM= silicon oil; CuBe gasket

T= 300K & 10K.Sample: few single crystals grown

by CVT.

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• X-ray Emission Spectra at K-edge of Fe: Kβ1,3 (~7070eV) andKβ’ (~7060eV, sensitive to local magnetic moment)

→ XES reveals completely unexpected LS → HS transition under HP

--- FeS2 zero-spin reference

Kβ’ Kβ1,3

• PFY-XAS = Partial-fluorescence yield x-ray absorption spectroscopy→ Measure Kβ1,3 emission while scanning Fe K-edge

• Pre-peak follows ortho-I → ortho-II structural transition• Decreased hybridization

• Tetrahedral (Td) → octahedral (Oh) coordination• Increased Fe-Se bond lengths (5%)

• ortho-I → ortho-II structural transition associated with LS → HS

XES

sate

llit

e i

nte

nsit

y (

% o

f K

β)

XA

S «

peak A

» a

rea (

arb

. u

nit

s)

• XAS spectra simulations (FDMNES) - Includes spin-orbit coupling

- Fermi level found self-consistently

→ they confirm ortho-I phase is LSortho-II phase is HS

• Intensity of the ortho-II pre-edge peak not well reproduced (atomic positions slightly wrong?)

• ortho-II pre-edge shape most important, related to orbitals’ energy splitting & spin state, matches with the HS state

• XAS C-D feature tracks Tc increase?

• Proximity of ortho-II phase induces enhanced spin fluctuations in the LS ortho-I phase?

XES

sate

llit

e i

nte

nsit

y (

% o

f K

β)

Peak A

C-D

XA

S a

rea (

arb

. u

nit

s)

• Between 2 – 6.5GPa: a Tc (XAS C-D) plateau followed by a strong increase

• XAS C-D feature tracks Tc increase?• Proximity of ortho-II phase induces

enhanced spin fluctuations in the LS ortho-I phase?

• Between 2 – 6.5GPa: a plateau followed by a strong increase

Sun et al. PRL 118, 147004 (2017)

→ Strong increase of Hall coefficient at ~6.5GPa

evidencing strongly enhanced interband spin fluctuations in the high-Tc

phase

XES

sate

llit

e i

nte

nsit

y (

% o

f K

β)

Peak A

C-D

XA

S a

rea (

arb

. u

nit

s)

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36

Conclusions

� Use of high quality FeSe single crystals grown by CVT method in the 3 experiments

1) XRD / HP:- Superconductivity enhanced when orthorhombic distortion is reduced- Transformation mechanism from LP Cmma (ortho-I) → Pbnm HP (ortho-II) phase- Ortho-II phase already present at 7GPa & lowT, i.e. near the P(Tc=max)

2) NPD / HP:- Only a few % of ortho-II (at 10-12GPa) detected before signal lost!- Refinement of the previous result obtained by NPD on FeSe powder/HP: low m(Fe) in

the AFM phase (1.5-6GPa) < 0.3µB

3) XES & XAS / HP:- XES: gradual decrease of the Fe local magnetic moment, until 6-7 GPa, similar to

Kumar et al. (APL 2011) & Chen et al. (PRB 2011), then a sudden rise from 7 GPa to 10 GPa associated with the ortho-II phase, suggesting a LS → HS transition

- XAS: clear spectral change and decrease of the pre-edge region intensity at P>6GPa- XAS simulations confirm the octahedral site of Fe in the ortho-II phase above

6-7GPa and the HS state

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37

Perspectives� Neutron scattering experiment on FeSe single crystal/HP-lowT

[1st attempt: IN22/ILL Oct.2018]

� Study of S-substituted Fe(Se1-xSx) crystals where orthorhombic (nematic) phase is weaken [XRD experiments done for x=0.12 and 0.18: id15B/ESRF, June 2018]

- to systematically study the lattice distortion versus x(S)- to find a possible link with the AFM state or the SC state

Matsuura et al. Nat. Comm. 8 (2017)

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38

Thank you for your attention!