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Andreev Reflection Spectroscopy (ARS)


At a normal metal superconductor interface, when a normal current is injected into the superconductor, it must be converted into a supercurrent. Each electron must carry another electron with appropriate spin orientation to form a Cooper pair to go into the superconductor, consequently reflect a hole back into the normal metal. This is Andreev reflection, it depends on the availability of the electrons with the required spin orientation, and the Cooper pairs in the superconductor. Most phenomena in solids are intimately related to the spin of conduction electrons. Thus Andreev reflection spectroscopy can be utilized to explore many phenomena in Condensed Matter Physics.

 

Andreev Reflection

 
ARS
For a normal metal, one conduction electron can always find another electron with opposite spin to go into the superconductor as a Cooper pair. Thus Andreev reflection always occurs.
ARS
For a half metal, there is only one spin band available at the Fermi level, thus the conduction electron cannot find another electron with opposite spin to form a Cooper pair. Andreev reflection cannot occur.
 
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ARS: determine spin polarization

For ARS using a singlet SC, the AR is limited by the number of minority spins of the conduction lectrons, this can be utilized to measure spin polarization.
 
Selected publications:
 
Using structural disorder to enhance the magnetism and spin-polarization in FexSi1−x thin films for spintronics
J Karel, Y N Zhang, C Bordel, K H Stone, T. Y. Chen, C A Jenkins, David J Smith, J Hu, R Q Wu, S M Heald, J B Kortright and F Hellman,
Materials Research Express 1 026102 (2014).
Fabrication of highly spin-polarized Co2FeAl0.5Si0.5 thin-films
M. Vahidi, J. A. Gifford*, S. K. Zhang, S. Krishnamurthy, Z. G. Yu, L. Yu, M. Huang, C. Youngbull, T. Y. Chen, and N. Newman,
APL Materials 2, 046108 (2014).
Effect of three dimensional interface in determination of spin polarization using Andreev reflection spectroscopy
J. A. Gifford, C. N. Snider, J. Martinez, and T. Y. Chen,
J. Appl. Phys. 113, 17B105 (2013).
Unified formalism of Andreev reflection at a ferromagnet/superconductor interface
T. Y. Chen, Z. Tesanovic, and C. L. Chien
Phys. Rev. Lett., 109, 146602 (2012).
Pronounced effect of extra resistance in point contact Andreev reflection
T. Y. Chen, S. X. Huang, and C. L. Chien,
Phys. Rev. B, 81, 214444(2010).
Spin polarization of amorphous ferromagnet FeCoB determined by Andreev reflection
S. X. Huang, T. Y. Chen, and C. L. Chien,
Appl. Phys. Lett.92, 242509 (2008).
Enhanced Curie temperature and spin polarization in Mn4FeGe3
T. Y. Chen, C. L. Chien and C. Petrovic,
Appl. Phys. Lett. 91, 142505 (2007).
Composition controlled spin polarization in Co1-xFexS2 alloys
C. Leighton, M. Manno, A. Cady, J. W. Freeland, L. Wang, K. Umemoto, R. M. Wentzcovitch, T. Y. Chen, C. L. Chien, P. L. Kuhns, M. J. R. Hoch, A. P. Reyes, W. G. Moulton, E. D. Dahlberg, J. Checkelsky and J. Eckert,
J. Phys.: Condens. Matter19, 315219 (2007) (INVITED REVIEW).
Sulfur stoichiometry effects in highly spin polarized CoS2 single crystals
L. Wang, T. Y. Chen, C. L. Chien, and C. Leighton,
Appl. Phys. Lett. 88, 232509 (2006).
Composition controlled spin polarization in Co1-xFexS2: electronic, magnetic, and thermodynamic properties
L. Wang, T. Y. Chen, C. L. Chien, J. G. Checkelsky, J. C. Eckert, E. D. Dahlberg, K. Umemoto, R. M. Wentzcovitch, and C. Leighton,
Phys. Rev. B 73, 144402 (2006).
Co1-xFexS2: a tunable source of highly spin-polarized electrons
L. Wang, K. Umemoto, R.M. Wentzcovitch, T. Y. Chen, C.L. Chien, J.G. Checkelsky, J.C. Eckert, E.D. Dahlberg and C. Leighton,
Phys. Rev. Lett. 94, 056602 (2005).
Spin-dependent band structure effects and measurement of the spin polarization in the candidate half-metal CoS2
L. Wang, T. Y. Chen and C. Leighton,
Phys. Rev. B 69, 094412 (2004).
Magnetic, structural, and transport properties of the Heusler alloys Co2MnSi and NiMnSb
Lance Ritchie and Gang Xiao, Y. Ji, T. Y. Chen, C. L. Chien, Ming Zhang, Jinglan Chen, Zhuhong Liu, Guangheng Wu, and X. X. Zhang,
Phys. Rev. B 68, 104430 (2003).
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ARS: measure superconducting gap

Superconductivity is intimately related to the spin of electrons. In a superconductor, the spins of the two electrons are strongly correlated to form a Cooper pair. Electrons of a Cooper pair must have opposite spins in a singlet superconductor and they can have parallel spins in a triplet superconductor. The most essential property of a superconductor is the pairing mechanism, which can be revealed by the nature of the superconducting gap. And triplet superconductivity may be revealed by the spin orientations in the Cooper pair. ARS can be utilized to measure the superconducting gap and the spin of conduction electrons.

- Superconducting gap structures


- Triplet superconductivity


 
Selected publications:
Possible p-Wave Superconductivity in Epitaxial Bi/Ni Bilayers
X. X. Gong, H. X. Zhou, P. C. Xu, D. Yue, K. ZHU. X. F. Jin, H. Tian, G. J. Zhao, T. Y. Chen,
Chin. Phys. Lett. 32, 067402 (2105).
Determination of Superconducting Gap of SmFeAsFxO1-x Superconductors by Andreev Reflection Spectroscopy
T. Y. Chen, S. X. Huang, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien,
Physica C 469, 521 (2009) (INVITED REVRIEW).
A BCS-like gap in superconductor SmFeAsO0.85F0.15
T. Y. Chen, Z. Tesanovic, R. H. Liu, X. H. Chen, and C. L. Chien,
Nature 453, 1224 (2008).
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Administration of ARS

It is non-trivial to operate ARS experiment. We have an system which is optimized for ARS experiment at ASU.

Andreev reflection spectroscopy (ARS)

- Temperature: 1.5K - 400 K, 100 mK with extra option

- Vector magnetic field: 3D, 3.5T

- Magnetic field: 9T in z direction

- Two schemes for differential resistance measurement

- V/I and dV/dI measured at the same time

ARS system
 
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ARS: data analysis

To understand the ARS data, it is crucial to include many inevitable parasitic effects such as 3D interface and extra resistance. The effect can be different in different systems.
 
Selected publications:
Effect of three dimensional interface in determination of spin polarization using Andreev reflection spectroscopy
J. A. Gifford, C. N. Snider, J. Martinez, and T. Y. Chen,
J. Appl. Phys.113, 17B105 (2013).
Unified formalism of Andreev reflection at a ferromagnet/superconductor interface
T. Y. Chen, Z. Tesanovic, and C. L. Chien
Phys. Rev. Lett., 109, 146602 (2012).
Pronounced effect of extra resistance in point contact Andreev reflection
T. Y. Chen, S. X. Huang, and C. L. Chien,
Phys. Rev. B, 81, 214444(2010).
 
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Other effects of ARS

- Crossed Andreev reflection

- Andreev reflection of multiple interfaces

 
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