For those of you considering a career in physics, we are pleased to have,
exclusively on this website, a handy brochure on what a physicist does all day.
Thanks to the author, Megan Buchanan.

Jamil Tahir-Kheli, PhD

Publications

Staff Scientist, Applied Physics
California Institute of Technology
Beckman Institute 139-74
Pasadena, CA 91125 USA
626-395-8148

You can write to me by clicking here.


I entered Oxford to read Maths when I was 15 and received my Physics PhD from Caltech in 1992. My thesis work was on high temperature cuprate superconductivity. In particular, I studied the standard three-band Hubbard models. After spending one year after my degree doing a postdoc and thinking a lot about the experimental data, I became convinced that there was possibly a missing electronic component necessary to understand the hightc mechanism. Given there was not much funding available to persue a different direction in cuprates, I left Caltech.

I consulted in industry for 8 years before returning to the group in 2001. This was probably the best thing that happened to me. I was lucky enough to work on some fun projects during this time that opened my eyes to solving practical problems. Some of the projects I worked on are:

  1. financial modelling of a $2.5 billion US stock portfolio,
  2. modelling the fluid flow of a novel medical device that led to FDA approval of the product,
  3. signal processing for Synthetic Aperature Radar (SAR).
During this time, I worked closely with Dr. Jason Perry to understand if an additional electronic component made sense for the cuprates. We were able to argue that the missing orbital was out-of-plane and arose through application of the current state-of-the-art density functionals used in chemistry (B3LYP). The important difference between B3LYP and prior functionals applied to the cuprates was the inclusion of exact Hartree-Fock exchange (so-called hybrid DFT functionals). The functionals were developed in the early 90's and were not available during the late 80's when the band structure calculations of the cuprates were performed.

I returned to the Goddard group in 2001. We were able to show that out-of-plane character arose from doping in LaSrCuO. We published a PRB paper in July 2007 that incorporates out-of-plane character into a theory for the cuprates.


My current research interests include:

  1. High Temperature Superconductivity. We found:

    1. A mechanism to increase the Tc in cuprates to room-temperature. (arxiv)

    2. The universal doping values of the cuprate phase diagram (~0.05 hole/CuO2 plane = transition to superconductivity, ~0.16 = optimal Tc, ~0.27 = end of superconductivity). (With no adjustable parameters.) (paper)

    3. The so-called "41 meV" neutron resonance peak in the superconducting phase has recently been explained as arising from the finite correlation length arising from the AF clusters. (With no adjustable parameters.) (paper)

    4. The doping dependent dispersionless STM incommensurability. This is due to the size of the metallic regions. (With no adjustable parameters.) (paper)

    5. The universal room-temperature thermopower curve as arising from the sum of the contributions of the electrons in the metallic swath plus the induced magnon drag in the antiferromagnetic (AF) regions. (With exactly one adjustable parameter.) (paper)

    6. A counting explanation for the doping dependent pseudogap (paper).

    7. The newest result is that the temperature and doping dependence of the resistivity has been shown to arise from exactly the same atomic-scale inhomogeneity. We find that phonon scattering (not magnons) causes the resistivity.

      The paper is here.

      The main result of the paper is a plot of the low and high temperature linear resistivity slopes of La2-xSrxCuO4 as a function of doping.

      The data is from Figure 3 in Hussey, N. E. et al. Dichotomy in the T-linear resisitivty in hole-doped cuprates. Phil. Trans. Roy. Soc. A 369, 1626-1639 (2011).

      What is most interesting about all these results is that they are explained merely by counting four-Cu-site plaquettes.

      The essential point is that doping in cuprates creates out-of-the-plane hole orbitals that delocalize over a four-Cu-site region surrounding the dopant. When these four-Cu-site regions percolate through the crystal, the standard Cu x2y2/O psigma band is formed inside the percolating swath.

      I believe this is the missing electronic component necessary to understand these materials.

    8. We have recently found the Oxygen phonon mode that leads to the D-Wave superconducting gap and the Tc-Dome. Seminar, "Understanding Superconductivity in Cuprates," presented on 29 June 2015 at Caltech.

  2. High Figure of Merit (ZT) Thermoelectrics

    1. In collaboration with the Heath group, we developed a new theory to explain the approximately 100-fold increase in the ZT of Silicon nanowires from ZT~0.14 for bulk Si to ZT~1 for a 10 x 20nm p-type wire. Science had a writeup (Sept. 7, 2007 vol. 317, pp 1318) about the work. Nature published our paper along with a News and Views article.

      My contribution was the theory where two important discoveries were made:

      1. First, we showed that phonon drag thermopower can reappear for small nanowires contrary to a generation of theory and experiment "proving" this is impossible.

      2. Second, the so-called "Slack's minimum thermal conductivity" for Si is not a lower bound when a three-dimensional to one-dimensional crossover for phonon modes occurs in the smallest nanowires.


EDUCATION:

Ph. D., Physics, California Institute of Technology, Pasadena, CA (1992)
M.S., Physics, California Institute of Technology, Pasadena, CA (1986)
M..A. Oxon (Honorary), Oxford University, Oxford, England (1993)
B.A. Honours, First Class, Mathematics, Oriel College, Oxford University, Oxford, England (1984)

PROFESSIONAL EXPERIENCE:

February 2013-present
Staff Scientist, Applied Physics, California Institute of Technology
2001-February 2013
Staff Scientist, Materials and Process Simulation Center, California Institute of Technology
1994-2001
Consultant, First Principles Research, Inc., Los Angeles, CA
1993-1994
Research Associate, First Quadrant, Inc., Pasadena, CA
1992-1993
Postdoctoral Fellow, California Institute of Technology, Pasadena, CA

PUBLICATIONS:

    Jamil Tahir-Kheli, "Latent Room-Temperature Tc in Cuprate Superconductors," (2017) arxiv.

    Jason Crowley, Jamil Tahir-Kheli, and William Goddard III, "Resolution of the Band Gap Prediction Problem for Materials Design," J. PHYS. CHEM. LETT. vol 7, 1198-1203 (2016). (JPCL)

    Jason Crowley, Jamil Tahir-Kheli, and William Goddard III, "Ab Initio Quantum Mechanics Simulations of Bi2Se3 and Bi2Te3 Topological Insulator Surfaces," J. PHYS. CHEM. LETT. vol 6, 3792-3796 (2015). (JPCL)

    Jamil Tahir-Kheli, Seminar, "Understanding Superconductivity in Cuprates," presented on 29 June 2015 at Caltech. Link to (Caltech YouTube Channel). Direct link to the video.

    Jamil Tahir-Kheli, "Resistance of High-Temperature Cuprate Superconductors," NEW J. PHYS. vol 15, 073020 (2013). (NJP)   arxiv. (2013).

    Jamil Tahir-Kheli and W. A. Goddard III, "Origin of the Pseudogap in High-Temperature Cuprate Superconductors," J. PHYS. CHEM. LETT. vol 2, 2326-2330 (2011). (JPCL)   (JPCL-Supplement)  (arxiv)     Caltech Press Release

    Y. Matsuda, Jamil Tahir-Kheli, and W. A. Goddard III, "Surface and Electronic Properties of Hydrogen Terminated Si [001]," J. PHYS. CHEM. C. vol 115, 12586-12591 (2011). (JPCC) (JPCC-Supplement)

    M. J. Cheng, R. J. Nielsen, J. Tahir-Kheli, and W. A. Goddard III, "The Magnetic and Electronic Structure of Vanadyl Pyrophosphate from Density Functional Theory," Phys. Chem. Chem. Phys., vol 13, 9831-9838 (2011). (PCCP)

    H. Xiao, Jamil Tahir-Kheli, and W. A. Goddard III, "Accurate Band Gaps for Semiconductors from Density Functional Theory," J. PHYS. CHEM. LETT. vol 2, 212-217 (2011). (JPCL)

    Y. Matsuda, Jamil Tahir-Kheli, and W. A. Goddard III, "Definitive Band Gaps for Single-Wall Carbon Nanotubes," J. PHYS. CHEM. LETT. vol 1, 2946-2950 (2010). (JPCL) (JPCL-Supplement)

    J. Tahir-Kheli and W. A. Goddard III, "Universal Properties of Cuprate Superconductors: Tc Phase Diagram, Room-Temperature Thermopower, Neutron Spin Resonance, and STM Incommensurability Explained in Terms of Chiral Plaquette Pairing," J. PHYS. CHEM. LETT. vol 1, 1290-1295 (2010). (JPCL) (JPCL-Supplement) (arxiv)

    J. Tahir-Kheli and W. A. Goddard III, "The Chiral Plaquette Polaron Paradigm (CPPP) for high temperature cuprate superconductors," CHEMICAL PHYSICS LETTERS vol 472, 153-165 (2009). (CPL Frontiers paper). This paper was on the cover.

    C.Y. Yam, Y. Mo, F. Wang, X. Li, G.-H. Chen, X. Zheng, Y. Matsuda, J. Tahir-Kheli, and W. A. Goddard III, "Dynamic Admittance of Carbon nanotube-based molecular electronic devices and their equivalent electronic circuit," NANOTECHNOLOGY vol 19, 495203 (2008) paper.

    Akram I. Boukai, Yuri Bunimovich, Jamil Tahir-Kheli, Jen-Kan Yu, William A. Goddard III, and James R. Heath, "Silicon nanowires as highly efficient thermoelectric materials," NATURE 451, 168 (2008). (paper) (Supplementary information) (News and Views article in Nature) (Science article about) (Sept. 7, 2007 vol. 317, pp 1318)

    Jamil Tahir-Kheli and W.A. Goddard III, "Chiral plaquette polaron theory of cuprate superconductivity," PHYS. REV. B 76, 014514 (2007). (paper)

    Y.H. Kim, Jamil Tahir-Kheli, P.A. Schultz, and W.A. Goddard III, "First-principles approach to the charge-transport characteristic of monolayer molecular-electronics devices: Application to hexanedithiolate devices," PHYS. REV. B 73, 235419 (2006).

    Jamil Tahir-Kheli, M. Miyata, and W. A. Goddard III, "Dielectric breakdown in SiO2 via electric field induced attached hydrogen defects," MICROELECTRONIC ENGINEERING 80, 174 (2005).

    Y. Gilman, P.B. Allen, Jamil Tahir-Kheli, and W. A. Goddard III, "Numerical resistivity calculations for disordered three-dimensional metal models using tight-binding Hamiltonians," PHYS. REV. B 70, 224201 (2004).

    Jason K. Perry, Jamil Tahir-Kheli, and William A. Goddard III, "Ab Initio Evidence for the Formation of Impurity d(3z2-r2) Holes in Doped La2-xSrxCuO4," PHYS. REV. B 65, 144501 (2002).

    Jason K. Perry, Jamil Tahir-Kheli, and William A. Goddard III, "Antiferromagnetic Band Structure of La2CuO4: Becke-3-Lee-Yang-Parr Calculations," PHYS. REV. B 63, 144510 (2001).

    Jamil Tahir-Kheli, "The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations," J. PHYS. CHEM A. 104, 2432 (2000).

    Jason K. Perry and Jamil Tahir-Kheli, "Electronic Structure of La_(1.85)Sr_(0.15)CuO_4: Characterization of a Fermi Level Band Crossing," PHYS. REV. B 58, 12323 (1998).

    Jamil Tahir-Kheli, "Interband Pairing Theory of Superconductivity," PHYS. REV. B 58, 12307 (1998).

    Jamil Tahir-Kheli, "Inter-Band Pairing: Resolution of Observed S and D-Wave Tunneling with Isotropic S-Wave Pairing," in "Proceedings of the 10th Anniversary HTS Workshop on Physics, Materials and Applications," ed. B. Batlogg, C.W. Chu, W.K. Chu, D.U. Gubser, and K.A. Muller (World Scientific, New Jersey: 1996), 491-492.

    Jamil Tahir-Kheli and William A. Goddard III, "The Infinite Range Heisenberg Model and High Temperature Superconductivity," PHYS. REV. B 48, 13002 (1993).

    Jamil Tahir-Kheli and W. A. Goddard III, "Spinons and Holons in 1-D Three-Band Hubbard Models for High T_c Superconductors," PROC. NAT. ACAD. SCI. 90, 9959 (1993).

    Jamil Tahir-Kheli and William A. Goddard III, "Exact Solution to a Strongly Coupled Hubbard Model in 1-D for High T_c Superconductors," PHYS. REV. B 47, 1116 (1993).