**CURRENT RESEARCH INTERESTS**

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.

**CURRENT PROJECTS:**

**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:

- 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.
- 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.
- 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).

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).

**PAST PROJECTS:**

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

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**

Related publications:

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**

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).