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Resarch Overview

In addtion to charge, electrons also carry spin angular momentum, which has important applications in spintronics and can be utilized to explore fundamental science. One aim of our research is to fabricate nanostructures and devices to explore fundamental science and their applications exploiting spin of conduction electrons.

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.
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Selected publications:
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).

Magnetism: spintronics and quantum transport

- Highly spin-polarized materials

- Detection and generation of pure spin currents

- Magnetoresistive phenomena

- Manipulation of magnetism

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Selected publications:
Enhanced magnetoresistance induced by spin transfer torque in granular films with a magnetic field
T. Y. Chen, S. X. Huang, C. L. Chien and M. D. Stiles,
Phys. Rev. Lett. 96, 207203 (2006).
Current-driven switching in a single exchange-biased ferromagnetic layer
T. Y. Chen, Y. Ji, C. L. Chien and M. D. Stiles,
Phys. Rev. Lett. 93, 026601 (2004).
Switching by point-contact spin injection in a continuous film
T. Y. Chen, Y. Ji and C. L. Chien,
Appl. Phys. Lett. 84, 380 (2004).
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).
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).


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

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Selected publications:
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).

Emergent matters



- Geographical spatial pattern analysis based on fundamental Physics concepts

Spatial pattern analysis plays an important role in geography for understanding geographical phenomena, identifying causes, and predicting future trends. Traditional pattern analysis tools assess cluster or dispersed patterns of geographical features based on the distribution of nonspatial attributes. These metrics ignore the shape of spatial objects—a critical consideration. The study of shape analysis, on the other hand, measures the compactness, elongation, or convexity of an areal feature based merely on geometry, without considering patterns of its attribute distribution. This article reports our efforts in developing a new pattern analysis method called the normalized mass moment of inertia (NMMI) that integrates both shape and nonspatial attributes into the analysis of compactness patterns. The NMMI is based on a well-known concept in physics—the mass moment of inertia—and is capable of detecting the degree of concentration or diffusion of some continuous attribute on an areal feature.
NMMI: A Mass Compactness Measure for Spatial Pattern Analysis of Areal Features
Wenwen Li, Tingyong Chen, Elizabeth A.Wentz, and Chao Fan
Annals of the Association of American Geographers, 104 (6), 1116 (2014).