Highlights on Some Experimental Progress of FeSe Xingjiang ZHOU 2014/10/08

Bulk FeSe Superconductor

F.-C. Hsu et al., PNAS 105 (2008) 14262; S. Medvedev et al., Nat. Mater. 8 (2009) 630.

N. Katayama et al., J. Phys. Soc. Jpn. 79 (2010)113702.



Simple crystal structure;



In FeSe, Tc can be enhanced from 8.5K to 36.7K at high pressure;



In Fe(Se x Te 1-x ), Tc can reach 14.5K.

8-Layer

FeSe Films Grown on Graphene

~30-Layer 2-Layer

 C. L. Song et al., Phys. Rev. B 84 (2011) 020503(R).

C. L. Song et al., Science 332 (2011) 1410.

Single-layer FeSe/Graphene is non superconducting above 2.2K;

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Tc increases with the number of layers in FeSe/Graphene films, reaching 8K for 8 layers with a superconducting gap of 2.1 meV;



V-shaped STS curve in the superconducting state indicating a node in the gap function.

FeSe Films Grown on SrTiO

3

(001)

Single-layer, T c ~ 80 K?

20.1meV

9.5meV

Double-layer, semiconducting

Q. Y. Wang et al., Chinese Phys. Lett. 29 (2012) 037402.



Single-layer FeSe/SrTiO 3 film shows a STS peak at ~20 meV, corresponding to an estimated Tc~80 K if it is superconducting;

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Double-layer FeSe/SrTiO 3 film is semiconducting.

Distinct Electronic Structure of Superconducting FeSe/SrTiO

Cut 1

3

Films

Cut 2 D. F. Liu et al., Nature Communications 3 (2012) 931.

Distinct electronic structure: no Fermi surface near

G

, only electron-like Fermi surface near M.

Phase Diagram and Indication of Tc~65K in Single-Layer FeSe/SrTiO 3 Film Tc~65 K 2

D/

kTc=6~7

S. L. He et al., Nature Materials 12, 605 (2013).



There are two phases present in single-layer FeSe/SrTiO 3 with different electronic structure (N phase and S phase); film



By annealing in vacuum, N phase can transform into S phase;



Indication of Tc~65 K observed.

Layer-Dependent Electronic Structure in FeSe/SrTiO 3 Films

S. Y. Tan et al., Nature Materials 12, 634 (2013).



For two-layer or multiple layer FeSe/SrTiO 3 that SDW state is formed; films, it is reported



The SDW temperature decreases with increasing number of FeSe layers.

Substrate Strain-Dependent Electronic Structure and Superconductivity of Single-layer FeSe Film

FeSe BR : SL-FeSe/Nb:BaTiO 3 /KTaO 3 a=3.76 A heterostructures, relaxed; FeSe BU : SL-FeSe/Nb:BaTiO 3 /KTaO 3 a=3.99 A heterostructures, unrelaxed; FeSe SX : SL FeSe/Nb:SrTiO 3 /KTaO 3 a=3.99 A heterostructures; FeSe S : FeSe/Nb:SrTiO 3 a=3.906 A ; R. Peng et al., Nature Communications 5 (2014) 5044.



The lattice constant changes from 3.76 A to 3.99 A, Tc shows little change near (70+_5)K;



The superconductivity is not sensitive to strain of the substrate.

Replica Bands and Electron-Phonon Coupling in Single-layer FeSe/SrTiO 3 Film

J. J. Lee et al., arXiv: 1312.2633.



Replica bands are observed with a separation of ~100 meV;



It is proposed this is due to electron coupling with STO phonon.

Transport and Magnetic Measurements on Single-Layer FeSe/SrTiO 3 Films Resistivity Magnetism

W. H. Zhanget al., Chinese Physics Letters 31 (2014) 017401.

Both transport and magnetic measurements indicate single-layer FeSe/SrTiO 3 is superconducting although the Tc is not as high as that from ARPES.

arXiv: 1406.3435.

Schematic of 4 point probe (4PP

）

transport measurement setup.

J. F. Ge et al., arXiv: 1406.3435.

Temperature and external magnetic field dependence of electrical transport property of a single-layer film of FeSe

11T 3K J. F. Ge et al., arXiv: 1406.3435.

4PP transport measurement on Bi2212 crystal (Tc~90K) with measurement configurations of C1423 (a) and C1234 (b), respectively.

J. F. Ge et al., arXiv: 1406.3435.

Temperature dependence of electrical transport property of another FeSe/STO sample measured with the 100-μm-tip-separation 4PP fixed at one measuring location while temperature was increasing.

J. F. Ge et al., arXiv: 1406.3435.