NP Lab at Caltech is interested in understanding fundamental quantum electronic phenomena in novel materials and nanoscale devices that potentially have applications in quantum nanoscience. These engineered systems also provide a playground for exploring exotic electronic states at (sub-) nano length scales. Presently, our main focus is on devices based on two-dimensional materials that only a few atoms thick. These promising new materials and related van der Waals heterostructures provide a platform for studying a variety of highly unique topological and correlated electronic quantum states. For further information about current research topics contact Stevan.


Van der Waals heterostructures


Two-dimensional materials (2D) offer unlimited possibilities for designing novel structures to explore electronic correlations and topological phenomena. Besides their importance in fundamental condensed matter physics, these structures have great potential for future quantum science applications. Broadly, our current structures of interest include:

  1. Twisted graphene bilayers. Two graphene layers joint together into a bilayer can exhibit highly unusual correlated quantum electronic states of matter when their rotational misalignment angle is close to 1.1 degrees. These structures have been shown to exhibit various correlated insulating, topological, and superconducting states. Our group has been actively exploring these novel quantum states using scanning tunneling microscopy and quantum transport techniques.
  2. Twisted graphene multilayers. More recently, it was shown that strong correlation states are present not only in graphene bilayers but also in structures that consist of three, four, five and likely more layers of graphene. All these structures host unusual superconducting states and exhibit topological and correlated phenomena.
  3. 2D Heterostructures. Besides graphene, many other two-dimensional (2D) materials exhibit unusual electronic properties. For example, tungsten ditelluride is a quantum spin Hall insulator in a monolayer form; moire structures based on tungsten diselenide and other transition metal dichalcogenides exhibit strongly correlated behavior; and various 2D materials are proposed to exhibit unusual magnetic quantum states of matter. We aim to explore basic properties of these states and possibly find ways in utilizing them for quantum science applications.

Related publications:

Hierarchy of Symmetry Breaking Correlated Phases in Twisted Bilayer Graphene

Robert Polski, Yiran Zhang, Yang Peng, Harpreet Singh Arora, Youngjoon Choi, Hyunjin Kim, Kenji Watanabe, Takashi Taniguchi, Gil Refael, Felix von Oppen, Stevan Nadj-Perge

ArXiv: 2205.05225 (2022).

Unsupervised learning of two-component nematicity from STM data on magic angle bilayer graphene

William Taranto, Samuel Lederer, Youngjoon Choi, Pavel Izmailov, Andrew Gordon Wilson, Stevan Nadj-Perge, and Eun-Ah Kim

ArXiv: 2203.04449 (2022).

Enhanced superconductivity in spin–orbit proximitized bilayer graphene

Yiran Zhang, Robert Polski, Alex Thomson, Étienne Lantagne-Hurtubise, Cyprian Lewandowski, Haoxin Zhou, Kenji Watanabe, Takashi Taniguchi, Jason Alicea, Stevan Nadj-Perge

Nature 613, 268–273 (2023).

Andreev reflection spectroscopy in strongly paired superconductors

Cyprian Lewandowski, Étienne Lantagne-Hurtubise, Alex Thomson, Stevan Nadj-Perge, and Jason Alicea

Phys. Rev. B 107, L020502 (2023) Editor's Suggestion.

Promotion of Superconductivity in Magic-Angle Graphene Multilayers

Y. Zhang, R. Polski, C. Lewandowski, A. Thomson, Y. Peng, Y. Choi, H. Kim, K. Watanabe, T. Taniguchi, G. Refael, F. v. Oppen, J. Alicea, and S. Nadj-Perge

Science 377, 1538-1543 (2022).

Evidence for unconventional superconductivity in twisted trilayer graphene

H. Kim, Y. Choi, C. Lewandowski, A. Thomson, Y. Zhang, R. Polski, K. Watanabe, T. Taniguchi, J. Alicea, and S. Nadj-Perge

Nature 606, 494–500 (2022).

Gate-defined wires in twisted bilayer graphene: from electrical detection of inter-valley coherence to internally engineered Majorana modes

A. Thomson, I. Sorensen, S. Nadj-Perge, J. Alicea

Phys. Rev. B 105, L081405 (2022).

Does filling-dependent band renormalization aid pairing in twisted bilayer graphene?

C. Lewandowski, S. Nadj-Perge, and D. Chowdhury

npj Quantum Materials 6, 82 (2021).

Interaction-driven Band Flattening and Correlated Phases in Twisted Bilayer Graphene

Y. Choi, H. Kim, C. Lewandowski, Y. Peng, A. Thomson, R. Polski, Y. Zhang, K. Watanabe, T. Taniguchi, J. Alicea, and S. Nadj-Perge

Nature Physics 17, 1375–1381 (2021).

Correlation-driven topological phases in magic-angle twisted bilayer graphene

Y. Choi, H. Kim, Y. Peng, A. Thomson, C. Lewandowski, R. Polski, Y. Zhang, H. S. Arora, K. Watanabe, T. Taniguchi, J. Alicea and S. Nadj-Perge,

Nature 589, 536–541 (2021).

Superconducivity in metalic twisted bilayer graphene stabilized by WSe2

Harpreet Singh Arora*, Robert Polski*, Yiran Zhang*, Alex Thomson, Youngjoon Choi, Hyunjin Kim, Zhong Lin, Ilham Zaky Wilson, Xiaodong Xu, Jiun-Haw Chu, Kenji Watanabe, Takashi Taniguchi, Jason Alicea, Stevan Nadj-Perge

Nature 583, 379–384 (2020).

Electronic Correlations in Twisted Bilayer Graphene near the Magic Angle

Youngjoon Choi, Jeannette Kemmer, Yang Peng, Alex Thomson, Harpreet Arora, Robert Polski, Yiran Zhang, Hechen Ren, Jason Alicea, Gil Refael, Felix von Oppen, Kenji Watanabe, Takashi Taniguchi, and Stevan Nadj-Perge

Nature Physics 15, 1174-1180 (2019).


Josephson \( \bf{\phi_0} \)-junctions

The Josephson effect describes supercurrent flowing through a junction connecting two superconducting leads by a thin barrier. This current is driven by a superconducting phase difference \( \phi \) between the leads. In the presence of chiral and time reversal symmetry of the Cooper pair tunneling process, the current is strictly zero when \( \phi \) vanishes. Only if these underlying symmetries are broken the supercurrent for \( \phi=0 \) may be finite. This corresponds to a ground state of the junction being offset by a phase \( \phi_0 \). We realized such junctions in nanowire quantum dot and probed the phase offsets using superconducting quantum interference device (see Figure on the left). This work is done in the group of Leo Kouwenhoven at QuTech.

Related publications:

Josephson \( \bf{\phi_0} \)-junction in nanowire quantum dots

D. B. Szombati, S. Nadj-Perge, D. Car, S. R. Plissard, E. P. A. M. Bakkers, and L. P. Kouwenhoven

Nature Physics doi:10.1038/nphys3742 (2016).

Two dimensional topological insulator InAs/GaSb

In two dimensional (2D) topological insulators band structure is inverted compared to the usual (trivial) insulators. Topological materials in 2D host helical boundary states which have various applications in the fields of spintronics and topological quantun information processing. We investigated heterostructure based on Indium Arsenide (InAs) and Gallium Antimonide (GaSb) layers (panel (a)) and found that in these heterostructures both trivial and topological phase can be accessed just by the means of electrostatic gating ((b) and (c)). This work has been performed in the group of Leo Kouwenhoven at QuTech.

Related publications:

Electric and Magnetic Tuning Between the Trivial and Topological Phases in InAs/GaSb Double Quantum Wells

F. Qu, A. J. A. Beukman, S. Nadj-Perge, M. Wimmer, B.-M. Nguyen, Wei Yi, J. Thorp, M. Sokolich, A. A. Kiselev, M. J. Manfra, C. M. Marcus, and L. P. Kouwenhoven

Phys. Rev. Lett. 115, 036803 (2015).

Majorana bound states in magnetic atomic chains on a surface of a superconductor

Majorana bound states are zero energy topological excitation appearing at boundaries of one-dimensional (1D) topological superconductors. In order to realize 1D topological superconductor, we have assembled iron atomic chains on the surface of superconducting lead. Figure on the left shows image of a chain obtained using scanning tunneling microscopy technique (STM). The inset shows spatially resolved conductance taken at zero voltage bias. The enhanced conductance near the end of the chain indicates the existence of zero-energy end mode consistent with the existence of Majorana modes in this system. This work has been performed in the group of Ali Yazdani at Princeton University.

Related publications:

Observation of Majorana fermions in ferromagnetic atomic chains on a superconductor

S. Nadj-Perge, I. K. Drozdov, J. Li, H. Chen, S. Jeon, J. Seo, A. H. MacDonald, B. A. Bernevig, and A. Yazdani

Science 346, 602 (2014).

Proposal for realizing Majorana fermions in chains of magnetic atoms on a superconductor

S. Nadj-Perge, I. K. Drozdov, B. A. Bernevig, and Ali Yazdani

Phys. Rev. B 88, 020407 (Rapid communication) (2013).

Quantum bits in semiconductor nanowires


In materials with strong spin-orbit interaction spin and orbital degree of freedom are strongly coupled. This effect can be used for spin manipulation by means of electric fields (as opposed to using magnetic fields which couple directly to spin). In small band gap semiconductors such as Indium Arsenide and Indium Antimonide the spin-orbit coupling is sufficiently strong to enable such spin manipulation on a very fast timescales (~5ns for pi/2 rotation). This work has been performed in the group of Leo Kouwenhoven at Delft University of Technology.

Figure on the left shows schematics of the voltage pulse sequence used for spin manipulation and readout (panels (a) and (b)). Panel (c) shows the spin precession (Rabi oscillations) data for various driving powers.

Related publications:

Fast spin-orbit qubit in an indium antimonide nanowire

J. W. G. van den Berg, S. Nadj-Perge, V. S. Pribiag, S. R. Plissard, E. P. A. M. Bakkers, S. M. Frolov, and L. P. Kouwenhoven

Physical Review Letters 110, 066806 (2013).

Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires

S. Nadj-Perge, V. S. Pribiag, J. W. G. van den Berg, K. Zuo, S. R. Plissard, E. P. A. M. Bakkers, S. M. Frolov, and L. P. Kouwenhoven

Physical Review Letters 108, 166801 (2012). (Editor suggestion)

Spin-orbit qubit in semiconductor nanowire

S. Nadj-Perge, S. M. Frolov, E. P. A. M. Bakkers and L. P. Kouwenhoven

Nature 468, 1084 (2010).