Research Interests
(1) Electron tunneling
Since the discovery of electron tunneling phenomena by Giaever and Esaki in around 1960, electron tunneling has become a powerful technique to study solids. The predication of Cooper pair tunneling in superconductors by Josephson in 1962 and the invention of scanning tunneling microscope in 1981 have enriched this research field.
Electron tunneling is a quantum effect, which allows the wave function of an electron to extend to the whole space including the area across a barrier. As a result, an electron, even with kinetic energy lower than the barrier height, can “tunnel” through the barrier and form a current.
Electron tunneling is usually studied on a tunnel junction formed by two electrodes separated by a very thin layer (a few monolayers) of dielectric material, i.e., the barrier. Since the electrode materials can be normal metals, superconductors, semiconductors, half-metals, and the barrier can be dielectric, ferroelectric, piezoelectric, ferromagnetic, semiconductor, etc., fascinating properties of all kinds of tunnel junctions have made this field attractive and prosperous.
A central problem of condensed matter physics is to find the ground state and excitation states of the material under investigation. Electron tunneling is an accurate probe (energy resolution 3-5 kT) to measure many fundamental physical properties, such as density of states, energy gap of a superconductor, effective phonon spectrum.
Electron tunneling also finds its applications in electronics. In less than ten years, the research of tunneling magnetoresistance (TMR, a spin-dependent tunneling phenomenon between two ferromagnetic electrodes, sensitive to magnetic field), has innovated today’s hard drive read heads. Due to TMR’s high sensitivity to magnetic field, the capacity of a hard drive has increased dramatically. Non-volatile memory based on TMR is also under development and will be available to the market shortly.
I am interested in fabricating electron tunneling devices and using them to study superconductors, ferromagnetics, ferroelectronics, and other artificial materials. Applications based on tunneling effect are also among my interests.
(2) Physics of superconductivity
Superconductors are fascinating materials that lose their dc resistance completely at temperatures below transition temperature (Tc). When superconducting, they also exclude magnetic field completely (Type I) or contain an array of magnetic flux quanta (Type II). Because no power is dissipated if a constant current less than the critical current is flowing through a superconductor, magnets wound by superconducting wires are widely used to generate strong magnetic fields (1-20 T), e.g. in nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), the Large Hadron Collider (LHC), and superconducting maglev trains. However superconducting applications are largely limited to extremely low working temperatures, at or below 77 K, the boiling point of liquid nitrogen under 1 atm.
Discovered in 1911, superconductivity has been known for almost 100 years. The Bardeen-Cooper-Shrieffer (BCS) theory is one of the most successful theories in condensed matter physics. It explains the conventional phonon-mediated superconductivity remarkably well, and its predictions to many properties usually have an accuracy of a few percent. However, the advent of high-Tc superconductors in 1986, the recently discovered iron-pnictides, and other exotic superconductors have posed great challenges to both theorists and experimentalists. Lack of an accepted theory, discovering new superconductors, especially those with even higher Tc, only relies on serendipity.
MgB2, a middle-Tc (40K) superconductor, was discovered totally unexpected. Despite that MgB2 has been available to people for decades, its superconductivity was not known until 2001 by low-temperature resistance measurement. Though its superconducting mechanism can be explained by the BCS theory, MgB2 has extraordinary properties including the two-band superconductivity and the highest Tc among phonon-mediated superconductors. Using electron tunneling, we have studied the two-band superconductivity in MgB2. Many interesting physical properties associated with this uniqueness have been revealed.
The research on superconductivity will continue to be one of central topics in condensed matter physics. Novel superconductors, with extraordinary properties, will continue to be discovered in the foreseeable future.
(3) Superconducting devices and applications
Superconductivity is a manifestation of quantum effects in macroscopic scale. Superconducting devices may excel classical devices in low-noise, high-speed, and high-sensitivity applications. Josephson junction is the core element of superconducting electronics. Its power consumption is close to the quantum limit and 100 times lower than contemporary semiconducting devices. Superconducting quantum interference devices (SQUIDs) are the most sensitive magnetic flux sensor in the world, sensitive enough to detect the weak magnetic field generated by human brain activities. Superconducting digital electronics with 40 GHz clock frequency have been developed. In contrast with semiconducting electronics based on Si encountering bottleneck of intolerable heat generated at clock frequency higher than a few GHz, superconducting circuits based on Nb are steadily increasing their running speed.
MgB2 is an attractive superconductor for application due to its comparatively high Tc (40K) and simple composition. We have developed a high-Jc MgB2/MgO/MgB2 sandwich-type Josephson junction technique that may apply to digital circuits working at above 20 K. Further improvement of the junction performance and design and implement of the digital circuits are under investigation.
(4) Graphene tunneling device for chemical sensing
Graphene has extraordinary properties compared to all the other known materials. With monolayer carbon atoms it has electron conductivity better than copper and mechanical strength better than steel. The Nobel Prize in Physics in 2010 was awarded for graphene being successfully fabricated in 2004, which underscored the revolution that graphene may bring to the near future. Graphene has an extremely high electron mobility, which means that the electrons transporting in graphene are subject to much less scattering compared to other materials. However, when external molecules are tethered on graphene surface, the electrons will be scattered more. This effect is very prominent since graphene has an infinite surface/bulk ratio and can be easily detected by transport measurement.
Electron tunneling spectroscopy using graphene tunneling device can be used to detect molecules, including biological molecules, e.g. DNAs, RNAs, amino acids, being detected in their original environment. The potential applications are in wide areas including food safety, national security, environment monitoring, etc.
Key Publications
1. Elias Galan, Daniel Cunnane, X. X. Xi, and Ke Chen, Sandwich-type MgB2/TiB2/MgB2 Josephson junctions, Superconductor Science and Technology 27, 065015 (2014). http://dx.doi.org/10.1088/0953-2048/27/6/065015
2. Daniel Cunnane, Elias Galan, Ke Chen, and X. X. Xi, Planar-type MgB2 SQUIDs utilizing a multilayer process, Applied Physics Letters 103, 212603 (2013). http://dx.doi.org/10.1063/1.4833022
3. Daniel Cunnane, Ke Chen, and X. X. Xi, Superconducting MgB2 rapid single flux quantum toggle flip flop circuit, Applied Physics Letters 102, 222601 (2013). http://dx.doi.org/10.1063/1.4809587
4. Daniel Cunnane, Chenggang Zhuang, Ke Chen, X. X. Xi, Jie Yong, and T. R. Lemberger, Penetration depth of MgB2 measured using Josephson junctions and SQUIDs, Applied Physics Letters 102, 072603 (2012).http://dx.doi.org/10.1063/1.4793194
5. Ke Chen, Daniel Cunnane, Yi Shen, X. X. Xi, Alan W. Kleinsasser, and John M. Rowell, Multiple Andreev reflection in MgB2/MgO/MgB2 Josephson junctions, Applied Physics Letters 100, 122601 (2012).
6. Ke Chen, Wenqing Dai, C.G. Zhuang, Qi Li, Steve Carabello, Joseph G. Lambert, Jerome T. Mlack, Roberto C. Ramos, and X. X. Xi, Momentum-dependent multiple gaps in magnesium diboride probed by electron tunnelling spectroscopy, Nature Communications 3:619 doi: 10.1038/ncomms1626 (2012).
7. Ke Chen, C. G. Zhuang, Qi Li, Y. Zhu, P. M. Voyles, X. Weng, J. M. Redwing, R. K. Singh, A. W. Kleinsasser, and X. X. Xi, High-Jc MgB2 Josephson junctions with operating temperature up to 40 K, Applied Physics Letters 96 (2010) 042506.
8. Ke Chen, Y. Cui, Qi Li, C. G. Zhuang, Zi-Kui Liu, and X. X. Xi, Study of MgB2/I/Pb tunnel junctions on MgO (211) substrates, Applied Physics Letters 93 (2008) 012502.
9. Y. Cui, Ke Chen, Qi Li, J. M. Rowell, and X. X. Xi, Degradation-free interface in MgB2/insulator/Pb Josephson tunnel junctions, Applied Physics Letters 89 (2006) 202513.
10. Ke Chen, Shane A. Cybart, and R. C. Dynes, Planar thin film YBa2Cu3O7Josephson junction pairs and arrays via nanolithography and ion damage, Applied Physics Letters 85 (2004) 2863-2865.