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  Welcome to Hasan Group/Lab website! Our lab is focused on the search and discovery (new physics) of novel quantum phases of matter. Currently, we grow and control properties of quantum materials, perform first-principle theoretical calculations/predictions and develop advanced spectroscopic techniques and related tools that provide new insights into the emergent behavior of matter. We invite you to visit our labs (Laboratory for Topo. Quantum Matter and Spectroscopy: B7-B2-B9-B60 Jadwin Hall) in the department of physics at Princeton. Prof. Hasan, Eugene Higgins Professor of Physics (endowed chair professorship 7/17-) at Princeton University is also affiliated with Lawrence Berkeley National Laboratory in California; Berkeley Lab page.

Berkeley (News) "Work at Berkeley Lab's Adv.Light Source helped to spawn a revolution in materials research"

DFT-FP Theoretical Predictions and Discovery of Topological Materials

Feature article (News) at Proceedings of National Academy of Sciences

Physics Today's "Search & Discovery" (News)

Princeton (News) "Artificial Topological Quantum Matter opens new research directions"

Moore Foundation (News) "Engineering topological states opens new frontier in quantum materials"

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  Lab Theme: QFT, Topology & Emergence in the Condensed Matter Universe
Probe-Predict-Probe and Control Topological Structure of Quantum Matter

New Topological States of Matter & New Emergent Particles
Reading List (click here)
U.S. Dept of Energy support (DOE physicists at work)
Topo.Insulators in 2007, KITP Talk Proc.(2007)
PNAS Front Matter homepage (September, 2016): Topological Insulators
Proc. of National Academy of Sciences USA
PhysicsWorld (U.K.) homepage (August, 2016): Topo. Insulators (100s Science Series)
Physics Today: "Search & Discovery" News on Topological Insulators
PhysicsWorld (February, 2011); Physics Today (January, 2010)
Physics Today (U.S.A.) New funding for Quantum Systems (2013) ; IEEE Spectrum (2015)
A vast majority of our experimental works are based on
our own theoretical predictions of topological materials (link)

Massless Yet Real (2015); PhysicsWorld (March, 2016)
PhysicsWorld News (July, 2015) Weyl fermion semimetals & Fermi arcs
PhysicsWorld (March, 2016) Weyl fermion semimetals & Topo. Nodal Fermion Semimetals
Discovery of Weyl Fermions, Topological Fermi Arcs and Topological Nodal-Line States of Matter
(Key Developments in Physics: 2Physics, 2015)

PhysicsWorld multimedia interview: "What is a Topological Metal? (2016)
New Topological Phases of Matter: Platform for emergent Dirac, Majorana & Weyl fermions (2016)
PhysicsWorld (2016)

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Recent Publications :
(Topological Semimetals) Weyl semimetals, Fermi arcs and chiral anomaly


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Cover Image
Weyl semimetals, Fermi arcs and chiral anomaly
S. Jia, S.-Y. Xu, M. Z. Hasan
" Physicists have discovered a new topological phase of matter, the Weyl semimetal, whose surface features a non-closed Fermi surface whereas the low-energy quasiparticles in the bulk emerge as Weyl fermions. A brief review of these developments and perspectives on the next steps forward are presented."
Nature Materials 15, 1140-1144 (2016) (Cover Story)

Focus Issue on Topological Semimetals (2016)
 
 

Artificial quantum matter : 1D Topological Phases (2017)


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"A novel artificial condensed matter lattice and a new platform for one-dimensional topological phases"
I. Belopolski et.al.,
"A novel artificial condensed matter lattice and a new platform for one-dimensional topological phases"
Science Advances 3, e1501692 (2017)
 
 

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Signatures of the Adler-Bell-Jackiw chiral anomaly in a Weyl semimetal

C. Zhang, S.-Y. Xu, I. Belopolski, Z. Yuan, Z. Lin, B. Tong, N. Alidoust, C.-C. Lee, S.-M. Huang, H. Lin, M. Neupane, D. S. Sanchez, H. Zheng, G. Bian, J. Wang, C. Zhang, T. Neupert, M. Z. Hasan, S. Jia
Nature Communications 7:10735 (2016)
 
 
Also see, Weyl semimetals, Fermi arcs and chiral anomaly (mini-review)

Nature Materials 15, 1140-1144 (2016) (Cover Story)

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A strongly robust type II topological Weyl fermion semimetal state in Ta3S2

Guoqing Chang, Su-Yang Xu, Daniel S. Sanchez, Shin-Ming Huang, Chi-Cheng Lee, Tay-Rong Chang, Guang Bian, Hao Zheng, Ilya Belopolski, Nasser Alidoust, Horng-Tay Jeng, Arun Bansil, Hsin Lin, and M. Zahid Hasan
Science Adv. e1600295 (2016)
 
 

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  Spin polarization and texture of the Fermi arcs in the Weyl Fermion semimetal TaAs
S.-Y. Xu, I. Belopolski, D. S. Sanchez, M. Neupane, G. Chang, K. Yaji, Z. Yuan, C. Zhang, K. Kuroda, G. Bian, C. Guo, H. Lu, Y. Feng, T.-R. Chang, N. Alidoust, H. Zheng, C.-C. Lee, S.-M. Huang, C.-H. Hsu, Horng-Tay Jeng, A. Bansil, A. Alexandradinata, T. Neupert, T. Kondo, S. Shin, H. Lin, S. Jia, M Z. Hasan
Phys. Rev. Lett. 116, 096801 (2016)

 
 

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  Discovery of Weyl fermions named a 'breakthrough of the year' by Physics World
PhysicsWorld News: "Weyl fermions are spotted at long last"

News at Lawrence Berkeley National Laboratory (LBNL),

Physics Highlights of the Year 2015 (Physics - American Physical Society)

 
 
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Highly-Cited Researcher (top 1%): Thomson-Reuters (2014, 2015)
Journal publications in 2003-2013
News link
 
 
 
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Topologically protected Single Dirac Cone (Spin-Textured)
Nature Physics N&V

 
 


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"Discovery of single-Dirac-cone Topological Insulators"
ALS-SPECTRUM
Lawrence Berkeley National Laboratory
 
 
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Atomic Scale Visualization of Quantum Interference on a Weyl Semimetal Surface by Scanning Tunneling Microscopy/Spectroscopy

H. Zheng, S.-Y. Xu, G. Bian, C. Guo, G. Chang, D. S. Sanchez, I. Belopolski, C.-C. Lee, S.-M. Huang, X. Zhang, R. Sankar, N. Alidoust, T.-R. Chang, F. Wu, T. Neupert, F. Chou, H.-T. Jeng, N. Yao, A. Bansil, S. Jia, H. Lin, M. Z. Hasan
ACS Nano 10, 1378 (2016, submitted Oct 2015)

 
 

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New funding for quantum systems

 
 
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Engineering electronic structure of a 2D topological insulator Bi(111) bilayer on Sb nanofilms by quantum confinement effect

G. Bian, Z. F. Wang, X. Wang, C. Xu, S.-Y. Xu, T. Miller, M. Z. Hasan, F. Liu, and T.-C. Chiang
ACS Nano 10, 3859 (2016)

 
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Signatures of Fermi Arcs in the Quasiparticle Interferences of the Weyl Semimetals TaAs and NbP
G. Chang, S.-Y. Xu, H. Zheng et al., Phys. Rev. Lett. 116, 066601 (2016)


 
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New type of Weyl semimetal with quadratic double Weyl fermions in SrSi2
S.-M. Huang, S.-Y. Xu, I. Belopolski et al., PNAS 113, 1180 (2015)

 
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Criteria for Directly Detecting Topological Fermi Arcs in Weyl Semimetals
I. Belopolski, S.-Y. Xu, D. S. Sanchez et al. Phys. Rev. Lett. 116, 066802 (2016)

 
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Prediction of an arc-tunable Weyl Fermion metallic state in MoxW1-xTe2
T.-R. Chang, S.-Y. Xu, G. Chang et al. Nature Commun. 7:10639 (2016)

 
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Discovery of a new type of Weyl semimetal state
I. Belopolski et al., Nature Commun. (2016)

 
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Unconventional transformation of spin Dirac phase across a topological quantum phase transition
S.-Y. Xu, M. Neupane, I. Belopolski et al. Nature Commun. 6:6870 (2015)

 
 
Topological Nodal-line Fermions and Semimetals
 
 
Topological Nodal-Line Fermions in Spin-Orbit Metal PbTaSe2

G. Bian, T.-R. Chang, R. Sankar, S.-Y. Xu, H. Zheng, T. Neupert, C.-K. Chiu, S.-M. Huang, G. Chang, I. Belopolski, D. S. Sanchez, M. Neupane, N. Alidoust, C. Liu, B. Wang, H.-T. Jeng, A. Bansil, F. Chou, H. Lin, and M. Z. Hasan
Nature Commun. 7:10556 (2016, submitted Nov 2015)
 
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  NbSe2  

Topological electronic structure in half-Heusler topological insulators.
Published in W. Al-Sawai, Hsin Lin, R. S. Markiewicz, L. A. Wray, Y. Xia, Su-Yang Xu, M. Z. Hasan, A. Bansil, et al.,

Phys. Rev. B 82, 125208 (2010)
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Room-temperature magnetic topological Weyl fermion and nodal line semimetal states in half-metallic Heusler Co2TiX (X=Si, Ge, or Sn).

Guoqing Chang, Su-Yang Xu, Hao Zheng, Bahadur Singh, Chuang-Han Hsu, Guang Bian, Nasser Alidoust, Ilya Belopolski, Daniel S. Sanchez, Songtian Zhang, Hsin Lin & M. Zahid Hasan

Scientific Reports 6, 38839 (2016).
 
Pump-Probe Ultrafast Spectroscopy

 
  Gigantic surface life-time of an intrinsic topological insulator revealed via time-resolved (pump-probe) ARPES
M. Neupane, S.-Y. Xu, Y. Ishida, S. Jia, B. M. Fregoso, C. Liu, I. Belopolski, G. Bian, N. Alidoust, T. Durakiewicz,
V. Galitski, S. Shin, R. J. Cava, and M. Z. Hasan
Phys. Rev. Lett. 115, 116801 (2015)

 
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Discovery of Weyl Fermion Semimetals: TaAs, NbAs, NbP, TaP, SrSi2, Ta3S2, LaAlGe
 
 
Topological electronic structure and Weyl semimetal in the TlBiSe2 class of semiconductors.
B. Singh, A.h Sharma, H. Lin, M. Z. Hasan, R. Prasad, and A. Bansil
Physical Review B 86, 115208 (2012)

Theoretical Prediction: A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class
S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, G. Bian, M. Neupane, C. Zhang, S. Jia, A. Bansil, H. Lin, M. Z. Hasan
Nature Commun. 6:7373 (2015) (submitted Nov 2014)
Also see: Phys. Rev. X 5, 011029 (2015) (submitted Jan 2015)

Experimental Discovery: Discovery of a Weyl Fermion semimetal and topological Fermi arcs
S.-Y. Xu, I. Belopolski, N. Alidoust, M. Neupane, G. Bian, C. Zhang, R. Sankar, G. Chang, Z. Yuan, C.-C. Lee, S.-M. Huang, H. Zheng, J. Ma, D. S. Sanchez, B. Wang, A. Bansil, F. Chou, P. P. Shibayev, H. Lin, S. Jia, and M. Z. Hasan
Science 349, 613 (2015) (submitted Feb. 2015)

PhysicsWorld News: "Weyl fermions are spotted at long last"

APS-Physics (American Physical Society): "Weyl Fermions"

Science News: "Elusive particle shows up in semimetal"

Nature Materials News: "Weyl fermions: Massless yet real"

Nature Physics News: "After a Weyl"

ChemistryWorld News: "Elusive fermion found at long last"

Physics (American Physical Society): "Where the Weyl Things Are"

Discovery of a Weyl Fermion state with Fermi arcs in niobium arsenide
S.-Y. Xu, N. Alidoust, I. Belopolski, Z. Yuan, G. Bian, T.-R. Chang, H. Zheng, V. Strocov, D. S. Sanchez, G. Chang, C. Zhang, D. Mou, Y. Wu, L. Huang, C.-C. Lee, S.-M. Huang, B. Wang, A. Bansil, H.-T. Jeng, T. Neupert, A. Kaminski, H. Lin, S. Jia, and M. Z. Hasan
Nature Physics 11, 748 (2015)

NaturePhysics: "Discovery of a Weyl Fermion state"

Nature Physics News: "After a Weyl"

Experimental discovery of a topological Weyl semimetal state in TaP
S.-Y. Xu, I. Belopolski, D. S. Sanchez, C. Guo, G. Chang, C. Zhang, G. Bian, Z. Yuan, H. Lu, Yi. Feng, T.-R. Chang, P. P. Shibayev, M. L. Prokopovych, N. Alidoust, H. Zheng, C.-C. Lee, S.-M. Huang, R. Sankar, F. Chou, C.-H. Hsu, H.-T. Jeng, A, Bansil, T. Neupert, V. N. Strocov, H. Lin, S. Jia, M. Z. Hasan
Science Advances 1, 1501092 (2015)

Fermi arc topology and interconnectivity in Weyl fermion semimetals TaAs, TaP, NbAs, and NbP
C.-C. Lee, S.-Y. Xu, S.-M. Huang, D. S. Sanchez, I. Belopolski, G. Chang, G. Bian, N. Alidoust, H. Zheng, M. Neupane,B. Wang, A. Bansil, M. Z. Hasan, and H. Lin
Phys. Rev. B 92, 235104 (2015)

Signatures of Fermi Arcs in the Quasiparticle Interferences of the Weyl Semimetals TaAs and NbP
G. Chang, S.-Y. Xu, H. Zheng, C.-C. Lee, S.-M. Huang, I. Belopolski, D. S. Sanchez, G. Bian, N. Alidoust, T.-R. Chang, C.-H. Hsu, H.-T. Jeng, A. Bansil, H. Lin, M. Z. Hasan
Phys. Rev. Lett. 116, 066601 (2016)

Criteria for Directly Detecting Topological Fermi Arcs in Weyl Semimetals
I. Belopolski, S.-Y. Xu, D. S. Sanchez, G. Chang, C. Guo, M. Neupane, H. Zheng, C.-C. Lee, S.-M. Huang, G. Bian, N. Alidoust, T.-R. Chang, B. Wang, X. Zhang, A. Bansil, H.-T. Jeng, H. Lin, S. Jia, M. Z. Hasan
Phys. Rev. Lett. 116, 066802 (2016)

New type of Weyl semimetal with quadratic double Weyl fermions
S.-M. Huang, S.-Y. Xu, I. Belopolski, C.-C. Lee, G. Chang, B. Wang, N. Alidoust, M. Neupane, H. Zheng, D. Sanchez, A. Bansil, G. Bian, H. Lin, and M. Z. Hasan
Proc. of National Academy of Sciences USA 113, 1180 (2015)

Prediction of an arc-tunable Weyl Fermion metallic state in MoxW1-xTe2
T.-R. Chang, S.-Y. Xu, G. Chang, C.-C. Lee, S.-M. Huang, B. Wang, G. Bian, H. Zheng, D. S. Sanchez, I. Belopolski, N. Alidoust, M. Neupane, A. Bansil, H.-T. Jeng, H. Lin, M. Zahid Hasan
Nature Commun. 7:10639 (2016)

Signatures of the Adler-Bell-Jackiw chiral anomaly in a Weyl semimetal
C. Zhang, S.-Y. Xu, I. Belopolski, Z. Yuan, Z. Lin, B. Tong, N. Alidoust, C.-C. Lee, S.-M. Huang, H. Lin, M. Neupane, D. S. Sanchez, H. Zheng, G. Bian, J. Wang, C. Zhang, T. Neupert, M. Z. Hasan, S. Jia
Nature Commun. 7:10735 (2016)

Spin polarization and texture of the Fermi arcs in the Weyl Fermion semimetal TaAs
S.-Y. Xu, I. Belopolski, D. S. Sanchez, M. Neupane, G. Chang, K. Yaji, Z. Yuan, C. Zhang, K. Kuroda, G. Bian, C. Guo, H. Lu, Y. Feng, T.-R. Chang, N. Alidoust, H. Zheng, C.-C. Lee, S.-M. Huang, C.-H. Hsu, Horng-Tay Jeng, A. Bansil, A. Alexandradinata, T. Neupert, T. Kondo, S. Shin, H. Lin, S. Jia, M Z. Hasan
Phys. Rev. Lett. 116, 096801 (2016)

Unoccupied electronic structure and signatures of topological Fermi arcs in the Weyl semimetal candidate MoxW1-xTe2
I. Belopolski, S.-Y. Xu, Y. Ishida, X. Pan, P. Yu, D. S. Sanchez, M. Neupane, N. Alidoust, G. Chang, T.-R. Chang, Y. Wu, G. Bian, H. Zheng, S.-M. Huang, C.-C. Lee, D. Mou, L. Huang, Y. Song, B. Wang, G. Wang, Y.-W. Yeh, N. Yao, J. Rault, P. Lefevre, F. Bertran, H.-T. Jeng, T. Kondo, A. Kaminski, H. Lin, Z. Liu, F. Song, S. Shin, M. Z. Hasan
Phys. Rev. B (2016)

Observation of Topological Nodal Fermion Semimetal Phase in ZrSiS
M. Neupane, I. Belopolski, M. M. Hosen, D. S. Sanchez, R. Sankar, M. Szlawska, S.-Y. Xu, K. Dimitri, N. Dhakal, P. Maldonado, P. M. Oppeneer, D. Kaczorowski, F. Chou, M. Z. Hasan, T. Durakiewicz
Phys. Rev. B (2016)

A strongly robust type-II Weyl fermion semimetal state in Ta3S2
G. Chang, S.-Y. Xu, D. S. Sanchez, S.-M. Huang, C.-C. Lee, T.-R. Chang, H. Zheng, G. Bian, I. Belopolski, N. Alidoust, H.-T. Jeng, A. Bansil, H. Lin, M. Z. Hasan
Science Advances (2016)

Magnetic and noncentrosymmetric Weyl fermion semimetals in the RAlX family of compounds (R=rare earth, Al=aluminium, X=Si, Ge)
G. Chang, B. Singh, S.-Y. Xu, G. Bian, S.-M. Huang, C.-H. Hsu, I. Belopolski, N. Alidoust, D. S. Sanchez, H. Zheng, H. Lu, X. Zhang, Y. Bian, T.-R. Chang, H.-T. Jeng, A. Bansil, H. Hsu, S.g Jia, T.s Neupert, H. Lin, M. Z. Hasan
arXiv:1604.02124

Discovery of Lorentz-violating Weyl fermion semimetal state in LaAlGe materials
S.-Y. Xu, N. Alidoust, G. Chang, H. Lu, B. Singh, I. Belopolski, D. Sanchez, X. Zhang, G. Bian, H. Zheng, M.-A. Husanu, Y. Bian, S.-M. Huang, C.-H. Hsu, T.-R. Chang, H.-T. Jeng, A. Bansil, V. N. Strocov, H. Lin, S. Jia, M. Z. Hasan
arXiv:1603.07318

Discovery of a new type of topological Weyl fermion semimetal state
I. Belopolski, D. S. Sanchez et.al., Nature Commun. 7, 13643 (2016)

Phase Transitions in Weyl Semimetal Tantalum Monophosphide
C. Zhang, Z. Lin, C. Guo, S.-Y. Xu, C.-C. Lee, H. Lu, S.-M. Huang, G. Chang, C.-H. Hsu, H. Lin, L. Li, C. Zhang, T. Neupert, M. Z. Hasan, J, Wang, S. Jia
arXiv:1507.06301 (2015)

Room-temperature magnetic topological Weyl fermion and nodal line semimetal states in half-metallic Heusler Co2TiX (X=Si, Ge, or Sn)
G. Chang, S.-Y. Xu, H. Zheng, B. Singh, C.-H. Hsu, I. Belopolski, D. S. Sanchez, G. Bian, N. Alidoust, H. Lin, M. Z. Hasan
Scientific Reports 6, 38839 (2016)


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  Topological Weyl Semimetal and Topological Fermi Arcs  
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Discovery of a Weyl Fermion semimetal and topological Fermi arcs.
Published in S.-Y. Xu, I. Belopolski, N. Alidoust, et al., Science 349, 613 (2015).

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A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class.
Published in S.-M. Huang, S.-Y. Xu, I. Belopolski, et al., Nature Commun. 6:7373 (2015).

 
  DS  

Discovery of a Weyl fermion state with Fermi arcs in niobium arsenide.
Published in S.-Y. Xu, N. Alidoust, I. Belopolski, et al., Nature Physics 11, 748 (2015).

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Experimental discovery of a topological Weyl semimetal state in TaP.
Published in S.-Y. Xu, I. Belopolski, D. S. Sanchezet al., Science Advances 1, 1501092 (2015).

 
Topological Semimetal (Double Weyl Semimetal)
 
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Observation of Fermi Arc Surface States in a Topological Metal.
Published in S.-Y. Xu, C. Liu, S. Kushwaha, et al., Science 347, 294 (2015).

 
2D Topological Superconductor based on Topological Insulators
 
  NbSe2  
 
Spin and angle resolved photoemission (spin-ARPES) measurements on Bi2Se3/NbSe2 heterostructure demonstrate topological superconductivity and helical Cooper pairing via proximity effect.
Published in S.-Y. Xu, N. Alidoust, I. Belopolski et al., Nature Physics 10, 943 (2014).

 
  NbSe2  
 
Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator.
Published in Y. Xu, I. Miotkowski, C. Liu, J. Tian, H. Nam, N. Alidoust, J. Hu, C.-K. Shih, M. Z. Hasan and Y. P. Chen,
Nature Physics 10, 956 (2014).

 
  NbSe2  
 
Observation of a three-dimensional topological Dirac semimetal phase in high-mobility Cd3As2.
Published in M. Neupane, S.-Y. Xu, R. Sankar, et al., Nature Commun. 5, 3786 (2014).

 
  CeB6  
 
Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films.
Published in M. Neupane, A. Richardella, J. Sanchez-Barriga, et al., Nature Commun. 5, 3841 (2014).

 
  MoS21  
 
Observation of monolayer valence band spin-orbit effect and induced quantum well states in MoX2
Published in Nasser Alidoust, Guang Bian, Su-Yang Xu, et al.,
Nature Commun. 5, 4673 (2014)

 
  TKI  
  TKI1  
 
First spectroscopic observation of topological surface states in a topological Kondo insulator SmB6.
Published in M. Neupane, N. Alidoust, S.-Y. Xu, et al., Nature Commun. 4, 2991 (2013).

 
  MoS21  
 
Magnetic Topological Insulators: Hedgehog spin texture and Berry's phase tuning in a magnetic topological insulator
Published in S.-Y. Xu, M. Neupane, C. Liu, et al.,
Nature Physics 8, 616 (2012).

 
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Observation of a topological crystalline insulator phase and topological phase transition in Pb1-xSnxTe
Published in Su-Yang Xu, Chang Liu, N. Alidoust, et al.,
Nature Commun. 3, 1192 (2012)

 
  NbSe2  

Half-Heusler ternary compounds as new multifunctional experimental platforms for topological quantum phenomena.
Published in H. Lin, L. A. Wray, Y. Xia, et al., Nature Materials 9, 546 (2010).

  NbSe2  

Observation of topological order in a superconducting doped topological insulator.
Published in L. A. Wray, S.-Y. Xu, Y. Xia, et al., Nature Physics 6, 855 (2010).

  TQPT  
 
Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator.
S.-Y. Xu, Y. Xia, L. A. Wray et al.;
Science 332, 560 (2011)

 
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A tunable topological insulator in the spin helical Dirac transport regime.
D. Hsieh, D. Qian, Y. Xia, et al.;
Nature 460, 1101 (2009)

 
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Berry's phase and quantization in topological insulators.
M. Zahid Hasan
Physics 3, 62 (2010)


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  TI1  
 
The first 3D topological insulator: Bi1-xSbx

D. Hsieh, D. Qian, Y. Xia, et al., Nature 452, 970 (2008)
D. Hsieh, Y. Xia, L. A. Wray, et al., Science 323, 919 (2009)


 
  NbSe2  
  NbSe2  
  Demonstration of a Fully Bulk Insulating (Intrinsic) Topological Insulator
 
 
Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator.
Published in Y. Xu, I. Miotkowski, C. Liu, J. Tian, H. Nam, N. Alidoust, J. Hu, C.-K. Shih, M. Z. Hasan and Y. P. Chen,
Nature Physics 10, 956 (2014).

 
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Protected helical Dirac fermions as a topological boundary mode
Nature Physics N&V

 
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Advanced Angle-Resolved Photoemission Spectroscopy (ARPES) and Quantum Topology
 
 

A novel experimental approach to topological quantum phenomena: Traditionally spectroscopic methods have been used to characterize electronic behavior in quantum matter whereas initial discoveries originated from transport methods. Our works in 3D topological insulators suggest that spectroscopic methods such as ARPES can be utilized to discover novel topological quantum phenomena (Science 323, 919 (2009), Nature 452, 970 (2008), Nature 460, 1106 (2009), Science 332, 560 (2011)). Previously topological quantum phenomena (quantum Hall like effects) were being probed mainly with transport methods pioneered by von Klitzing (1980). Following our demonstration of application of spin-ARPES, there are world-wide efforts to apply this technique and its derivatives to probe and study novel topological quantum phenomena in condensed matter systems.

Traditionally spectroscopic methods have been used to characterize electronic or spin behavior in quantum matter whereas initial discoveries originated from non-spectroscopic methods. My work focuses on the theme which I often like to call spectroscopy for discovering new states of quantum matter. Three dimensional topological insulators (3D-TI) (originally called "Topological Insulators" to distinguish them from 2D quantum Hall type effects and insulators) are the first example of topological order in the bulk solids (there is no genuine quantum Hall effect in three dimensions). They feature a "protected" metallic Dirac-like surface state (2DEG or planar "topological metal") where electron's spin and momentum are locked to each other and possess half the degrees of freedom present in an ordinary electron Fermi gas. Strong spin-orbit coupling leads to an insulating bulk and the surface states are protected by time reversal symmetry and belong to the Z2 class (see theory by Kane-Mele'05; Fu-Kane-Mele'07, Moore-Balents'07; precursor theory of TR-breaking 2D topological insulator by Haldane in 1988). In experiments, 3D Topological Insulators are an example of non-quantum-Hall-like topological matter experimentally discovered and reported (2007) around the same time, in parallel, as the spin Hall edge-states in Hg(Cd)Te, in 2007. Both the spin quantum Hall effect (Wurzburg team, Science 2007) and 3D Topological Insulator Surface States (my team, Nature 452, 970 (2008), submitted in 2007, "Search&Discovery" Physics Today and KITP 2007) were reported the same year 2007 (a few months apart) using two independent and unrelated experimental methods. Experimentally, these two are unrelated. The spin quantum Hall effect (QSHE) can be thought of as two copies of well-known IQH (integer quantum Hall) states put together in two dimensions. Since IQH state is a 2D topological insulator, spin quantum Hall effect is also a 2D topological insulator but time-reversal invariant (protected by Z2 invariant). On the other hand, the 3D Topological Insulators are a new and distinct state of matter which cannot be reduced to multiple copies of IQH and there is no spin Hall effect in 3D (the term "Topological Insulators" was originally used exclusively for the novel and unprecedented 3D state since there is no spin Hall like effect there). The 3D state is thus an example of non-quantum-Hall-like topological matter and the first realization of topologically ordered bulk solid in nature (Physics World 2011). Experimentally, 3D TI did not arise from quantum spin Hall effect. Additionally, transport measurements cannot provide a proof of Z2 topology either (Physics World 2011). All of the 2D topological insulator examples (IQH, FQH, QSH) including the fractional one (FQH) involving Coulomb interaction are understood in the standard picture of quantized electron orbits in a spin-independent or spin-dependent magnetic field, the 3D topological insulator defies such description and is a novel type of topological order which cannot be reduced to multiple copies of quantum-Hall-like states. In fact, the 3D topological insulator exists not only in zero magnetic field, they also differ from the 2D variety in three very important aspects: 1) they possess topologically protected 2D metallic surfaces (a new type of 2DEG) rather than the 1D edges, 2) they can work at room temperature (300K and beyond, largegap topological insulators) rather than cryogenic (mK) temperatures required for the QSH effects and,3) they occur in standard bulk semiconductors rather than at buried interfaces of ultraclean semiconductor heterostructures thus tolerate stronger disorder than the IQH-like states. The non-quantum-Hall-like (novel) character and the extremely rich physics (surface 2DEG, helical fermion gas) of the 3D state has led to a world-wide research interest in this topic in general. Also the novel experimental approach in studying topological quantum phenomena demonstrated by us are now being used by many other groups world-wide studying many other topological materials and phenomena.

Why Topological Surface States (TSS) are so exciting?

Topological Insulators (3DTI) is new and unprecedented and cannot be reduced to multiple copies of quantum Hall or spin Hall like states. Most topological states of matter are realized in two or lower dimensions (quantum Hall states, quantum spin Hall effect, non-Fermi liquid chains and wires, quantum spin-liquids etc.). Unlike all others, neither strong electron-electron interactions (necessary for quantum spin liquids), high magnetic fields and low temperatures (necessary for quantum Hall states), nor low dimensionality (needed for quantum Hall states, spin quantum Hall states (QSHE), and non-Fermi liquid spin chains) are needed for the 3D topological insulator. The theoretical and experimental discovery of the 3D TIs - the first example of topological order in bulk solids - has generated much experimental and theoretical efforts to understand and utilize all aspects of these quantum phenomena and the materials that exhibit them. One of the major challenges in going from quantum Hall-like 2D states to 3D topological insulators is to develop new experimental approaches/methods to precisely probe this novel form of topological-order since the standard tools and settings that work for IQH-state also work for QSH states. The method to probe 2D topological-order is exclusively with charge transport (pioneered by Von Klitzing in the 1980s), which either measures quantized transverse conductance plateaus in IQH systems or longitudinal conductance in quantum spin Hall (QSH) systems. In a 3D topological insulator, the boundary itself supports a two dimensional electron gas (2DEG) and transport is not (Z2) topologically quantized hence cannot directly probe the topological invariants ?o or the topological quantum numbers analogous to the Chern numbers of the IQH systems. This is unrelated to the fact that the present materials have some extrinsic or residual/impurity conductivity in their naturally grown bulk. In this paper, we review the birth of momentum- and spin-resolved spectroscopy as a new experimental approach and as a directly boundary sensitive method to study and prove topological-order in three-dimensions via the direct measurements of the topological invariants ?o that are associated with the Z2 topology of the spin-orbit band structure and opposite parity band inversions, which led to the experimental discovery of the first 3D topological insulator in Bi-based semiconductors.

Topological Surface States (A New Type of 2D Electron System) & Topological Insulators:

Experimentally demonstrated, a three dimensional topological insulator (3D-TI) features a protected two-dimensional electron gas on its surface. The high magnetic fields, low temperatures or low dimensionality are not necessary for retaining the topological protection or topological order of a macroscopic 2DEG on the surface of a topological insulator. Nature 452, 970 (2008), submitted in 2007, also KITP 2007 "Search&Discovery" Physics Today

Experimental Discovery : Topological Surface States - A New Type of 2D Electron Systems

Measurements of topological invariants {vo}:

Transport measurements that have been the key to probe topological order in conventional quantum Hall like systems cannot (even in theory!) be used to measure the topological quantum numbers of Z2 TIs (Fu-Kane's {vo}). We have shown a technique/method for the direct measurement of Z2 topological quantum numbers {vo} for the first time: Science 323, 919 (2009), later further expanded: Science 332, 560 (2011) see below for details.

Topological (Z2) Order is more directly manifested in the spin and momentum correlated motion of electrons on the surface. This leads to a new type of 2DEG where the electron's spin and linear momentum are one-to-one locked. Such a 2DEG only carries half of the total degrees of freedom of a conventional 2DEG and in the vicinity of the Kramers' point takes the form of a half Dirac gas: Nature 460, 1101 (2009) and further details in related materials Phys. Rev. Lett. (2009) News in Physics Today

Consequence of Z2 topological order:

Spin-Momentum locking and pi-Berry's phase lead to the absence of elastic backscattering on the surfaces. By combining spin-ARPES and tunneling (collaboration), we have demonstrated such absence of elastic backscattering: Nature 460, 1106 (2009) (STM+Spin-ARPES), Berry's phase was shown at Science 323, 919 (2009) Discovery of the next generation and "room temperature topological insulators": the Bi2Se3 class: The advantages of large band gap and simple spin-polarized Dirac cone topology of these spin-orbit insulators led to our observation of topological quantum phenomena at room temperatures without magnetic fields and without high purity semiconductors

Discovery of Single-Dirac-Cone TSS Bi2Se3 as a TI class:

N&V NatPhys (2009) KITP Proc. 2008 "The Hydrogen Atom of Topological Insulator" BiSb (2007) to Bi2Se3 KITP (2008) Nature Physics 5, 398 (2009) submitted in 2008 and (see above Nature (2009)) Phys. Rev. Lett. 103, 146401 (2009) further work on Bi2Te3 and Sb2Te3 in comparison with Bi2Se3 Phys. Rev. Lett. 105, 036404 (2010) Bi2Se3 related TIs such as spin-orbit BiTlSe2 class of materials

Given that the topological insulators are standard bulk semiconductors and their topological characteristics can survive to high temperatures, their novel properties could lead to many exciting applications. An exciting progress along this line is that the Bi2Se3 class of 3D TIs can be turned in to superconductors to form the host material for Majorana Fermions (Nature Physics 6, 855 (2010)). Also both integer and fractional quantum Hall effects have been reported in this Bi2Se3 class of materials by other groups indicating the high mobility of the topological surface state. These developments have unleashed a world-wide experimental effort to understand all aspects of electrical and spin properties leading to a nearly graphene-like revolution in physics (Physics World 2011).

Magnetic Symmetry Breaking:

Topological protection and degree of robustness: The Dirac cone materials are probed via the modification of surface potential: How robust the topological properties of a Topological Insulator surface are investigated (Nature Physics 7, 32 (2011)). This paper reported preliminary results regarding magntism, for a full detail see, Magnetic Topological Insulators : The effect of time-reversal symmetry leads to unconventional spin textures on the surface of a topological insulator. Hedgehog spin texture and Berry's phase tuning in a magnetic topological insulator was observed in our experiments; Nature Physics 7, 032 (2011) Coulomb/disorder/magnetic perturbation effects. Nature Physics 8, 616 (2012) for Magnetic symmetry breaking, for details see Phys. Rev. B (2012) A novel experimental approach to topological quantum phenomena: Traditionally spectroscopic methods have been used to characterize electronic behavior in quantum matter whereas initial discoveries originated from transport methods. Our works in 3D topological insulators suggest that spectroscopic methods such as ARPES can be utilized to discover novel topological quantum phenomena (Science 323, 919 (2009), Nature 452, 970 (2008), Nature 460, 1106 (2009), Science 332, 560 (2011)). Previously topological quantum phenomena (quantum Hall like effects) were being probed mainly with transport methods pioneered by von Klitzing (1980). Following our demonstration of application of spin-ARPES, there are world-wide efforts to apply this technique and its derivatives to probe and study novel topological quantum phenomena in condensed matter systems ["Search&Discovery" Physics Today 2009; Physics World 2011].

Superconducting Symmetry Breaking: Superconducting Topo. Insulators and Topological-Order:

Superconducting topological insulators can serve as Majorana platforms. Observation of topological order in a superconducting doped topological insulator demonstrates a platform for realizing Majorana fermions. Some of the doped topological insulators that superconduct at low temperatures may also turn out to be topological superconductors proposed in theory based on our experimental observations and spin-ARPES results Nature Physics 6, 855 (2010) and additional details in Phys. Rev. B (2012)

Bulk Insulating Topological Insulators:

Recently, highly bulk insulating topological insulators have also been realized where Dirac surface states contribute more than 70% of the total conduction channel (Preprint at Xiong et.al., arXiv:1101.1315v1 on Bi-based 3D TIs identified by us, Preprint at S.-Y. Xu et.al., arXiv:1007.5111v1 (2010) which led to our work reported at Phys. Rev. B (2012)). Also see works by other groups Phys. Rev. B (2010) [ our eariler ARPES on Bi2Te2Se at arXiv (2010)] Phys. Rev. B (2012) and Phys. Rev. B (2013) BTS family of topological insulators; arXiv (2010) Nature Commun. 04, 2991 (2013) (see the TKI section below for details)

Adiabatic Continuation Method for Predicting TI & Topological Phase Transition:

Working with Hsin Lin and others we demonstrated that first-principles based adiabatic continuation approach is a very powerful and efficient tool for constructing topological phase diagrams and locating non-trivial topological insulator materials. Applied to real materials our results demonstrated the efficacy of adiabatic continuation as a useful tool for exploring topologically nontrivial alloying systems and for identifying new topological insulators even when the underlying lattice does not possess inversion symmetry, and the approaches based on parity analysis of Fu-Kane are not viable. Nature Materials 9, 546 (2010) and more recently at Phys. Rev. B (2013)

Topological Phase transition and Texture inversion:

It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction driving the system through a topological quantum phase transition. We reported the observation of such a phase transition in a tunable spin-orbit system where the topological state formation is visualized. In the topological state, vortex-like polarization states are observed to exhibit 3D vectorial textures, which collectively feature a chirality transition as the spin-momentum locked electrons on the surface go through the zero carrier density point. Such phase transition and texture inversion can be the physical basis for observing fractional charge (±e/2) and other fractional topological phenomena. Science 332, 560 (2011) and in a related system, see Phys. Rev. Lett. 109, 186403 (2012)

Topological Crystalline Insulator (TCI) Phase:

Z2 topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi1?xSbx system is due to an odd number of band inversions. We experimentally investigated the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb1?xSnxTe class of materials that has been theoretically predicted (by Fu et.al.,) to exist in its end compound SnTe. Our observation of the spin-polarized Dirac surface states in the inverted Pb1?xSnxTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices. Nature Commun. 03, 1192 (2012) TCI-phase and BI to TCI Phase Transition Science 341, 1496 (2013) Dirac node formation and mass acquisition in a topological crystalline insulator (TCI)-phase, in collaboration with scanning tunneling spectroscopy (Madhavan) group at Boston and on orbital-texture physics, see Nature Physics (in press) (2014)

"3D Graphene", Topological 3D Dirac Semimetals:

Symmetry-broken three-dimensional (3D) topological Dirac semimetal systems with strong spin-orbit coupling can host many exotic Hall-like phenomena and Weyl fermion quantum transport. Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on Cd 3 As 2 , which has been predicted to be the parent material, from which many unusual topological phases can be derived. We observe a highly linear bulk band crossing to form a 3D dispersive Dirac cone projected at the Brillouin zone centre by studying the (001)-cleaved surface. Remarkably, an unusually high in-plane Fermi velocity up to 1.5 x 10^ 6 m/s is observed in our samples, where the mobility is known up to 40,000 cm 2/ V.s, suggesting that Cd3As2 can be a promising candidate as an anisotropic-hypercone (three-dimensional) high spin-orbit analogue of 3D graphene. Our discovery of the Dirac-like bulk topological semimetal phase in Cd3As2 opens the door for exploring higher dimensional spin-orbit Dirac physics in a real material. The 3D Dirac semimetals can be realized from topo. phase transition. Near the critical point a 3D Dirac version of Graphene is realized which was probed in our 2011 Science paper (more recently elaborated at our Nat.Com. paper): Science 332, 560 (2011) and more recently in other materials such as Cd3As2 (see below) Nature Commun. 05, 3786 (2014) topological 3D Dirac Semimetal Cd3As2 Preprint on Na3Bi (2013) topological 3D Dirac Semimetal Na3Bi

Ultrafast Time-resolved response of Topological Insulators:

The advent of topological insulators has made it possible to realize two-dimensional spin polarized gases of relativistic fermions with unprecedented properties in condensed matter. Their photoconductive control with ultrafast light pulses is of interest in optoelectronics. In collaboration with M. Marsi group (Paris) we probed the interplay of surface and bulk transient carrier dynamics in a photoexcited topological insulator Bi2Se3 and related materials. Currently, we are probing other exotic aspects of ultrafast response of bulk insulating TI materials (preprint) and correlated electron systems. Nature Commun. 05, 3786 (2014) Relativistic nanoscale Schottky barrier (with M. Marsi group) Preprint (2014) Exotic ultrafast response of TIs (our recent work) Nanoscale Ultra-Thin-Films/MBE of Topological Insulators and related spin-orbit films: Understanding the spin behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices. Using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we reported tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition and separately in magnetically doped thin-films. Nature Physics 8, 616 (2012) Magnetic Thin Films Nature Commun. 05, 3786 (2014) Tunelling and spin-texture evolution in films for potential devices. Nature Commun. (in press) (2014) Spin-orbit physics in molydiselenide films for potential devices.

Topological Kondo or Mixed-Valence Correlated Electron Systems:

Topological States can also arise in correlated electron systems such as Kondo or Mixed-Valence Insulators. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, we probed the surface electronic structure of the anomalous conductivity regime (sub 6K) in SmB6. We observe that the bulk bands exhibit a Kondo gap of 15 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. These states disappear as temperature is raised above 15K in correspondence with the complete disappearance of the 2D conductivity channels in SmB6. Our bulk and surface measurements carried out in the transport-anomaly-temperature regime (T 10K) are consistent with the first-principle predicted Fermi surface topology of a topological Kondo insulator phase in this material. Nature Commun. 04, 2991 (2013) Preprint on YbB6 (2014) An expt. algorithm for topo Kondo & mixed-val. insulators (2013) Physics World 2011



First five experimental papers on 3DTI (Topological Insulators)

Nature 452, 970 (2008); D.Hsieh, D.Qian, Y.Xia et.al., [April, '08] Submt.(2007)

Science 323, 919 (2009); D.Hsieh, Y.Xia, L.A.Wray et al., [February, '09] Submt.(2008)

Nature Physics 5, 398 (2009); Y.Xia, D.Qian, L.A.Wray, D.Hsieh et al., [May '09] Submt.(2008)

Extended version at Nature 460, 1101 (2009); D.Hsieh, Y.Xia, D.Qian et.al., Submt.(2009)

Phys.Rev.B 79, 195208 (2009); Y.Hor, A.Richardella, Y.Xia, D.Hsieh et.al., [May '09] Submt.(2009)

Science 325,178 (2009); Y.L.Chen, J.Analytis, . S.-C. Zhang et al., [Stanford] [June '09] Submt.(Mar. 2009)

 
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Review/Perspective Articles (Invited):



Topological Insulators, Topological Dirac semimetals, Topological Crystalline Insulators, and Topological Kondo Insulators

M. Z. Hasan, S.-Y. Xu, M. Neupane

Book Chapter in Topological Insulators, Fundamentals and Perspectives (John Wiley & Sons) (2015)

Topological Insulators, Helical Topological Superconductors and Weyl Fermion Semimetals

M. Z. Hasan, S.-Y. Xu and G. Bian

Phys. Scr. T164 014001 (2015)

Experimental Discoveries: Topological Surface States - A New Type of 2D Electrons Systems

M. Z. Hasan, S.-Y. Xu, D. Hsieh, L. Wray, Y. Xia

Book Chapter in Topological Insulators (Elsevier/Oxford) (2013)

Topological Quantization in Topological Insulators

M. Z. Hasan

Physics 3, 62 (2010)

Three-Dimensional Topological Insulators

M. Z. Hasan and J. E. Moore

Ann. Rev. Cond. Mat. Phys 2, 55 (2011)

Topological Insulators (Colloquium Article)

M. Z. Hasan and C. L. Kane

Rev. Mod. Phys 82, 3045 (2010)

Weyl semimetals, Fermi arcs and chiral anomaly
S. Jia, S.-Y. Xu, M. Z. Hasan
Nature Materials 15, 1140-1144 (2016) (Cover Image & Story)



  DS

Topologically protected Single Dirac Cone (Spin-Textured)
Nature Physics N&V

 

Discovery of Weyl Fermion Semimetals and Topological Fermi Arc States

M. Z. Hasan, S.-Y. Xu, I. Belopolski, S.-M. Huang

Ann. Review Cond. Mat. Phys 8, 289 (2017)