Research

An unassuming twist between two two-dimensional carbon crystals has taken the condensed matter physics community by storm. In an entirely novel and simple way it has opened up Pandora’s box of new options to realize interacting topology, superconductivity, magnetism and other many-body states of matter. The main principles that give rise to the plethora of quantum phases in magic angle twisted bilayer graphene (MATBG), namely the ability to use moiré superpotentials to engineer topologically non-trivial flat-bands, can be transferred to a much larger set of van der Waals heterostructures, which has led to the discovery of an even bigger multitude of exotic ground states.

1. Engineering strongly correlated topological phases in moiré materials

When two twisted graphene sheets are positioned one on top of another, a geometric interference pattern between the individual lattices emerges, the moiré pattern. It forms a triangular superlattice, whose lattice constant scales inversely proportional to the twist-angle θ. In momentum space, the Brillouin zones of the two individual graphene sheets are also rotated by θ, and two degenerate mini-Brillouin zones at the K and K' points arise. When θ is small, however, the Dirac cones in the individual layers strongly overlap and begin to hybridize. For a twist-angle close to the magic-angle of θ = 1.1 degrees, the Dirac cones flatten and converge to zero energy. This results in the occurrence of ultra-flat bands with a band-width of only about 10meV. As all the electrons in these bands have almost the same energy, the density of states (DOS) is extremely large, where big DOS peaks arise in the centers of the moiré lattice sites. 

The topological flat bands are the key starting point to understand the rich phenomenology of magic-angle twisted bilayer graphene (MATBG). The lack of kinetic energy of the electrons makes the Coulomb energy of the electron-electron interactions the dominant term in the Hamiltonian. The ultra-high DOS increases the electronic interactions in the system and favors the formation of strongly correlated phases. The combination of strong electron interactions and topology represents a long sought-after blend of properties, that enables the formation of entirely new electronic phases. Here, as a result, many complex quantum phases that are not present in single monolayer graphene were discovered, including correlated insulators, superconductors and a “strange metal” phase. In addition, the inherent topological property of the flat-bands, give rise to orbital magnets and Chern insulator states. The main principles that give rise to the plethora of quantum phases in MATBG, namely the ability to engineer topologically non-trivial flat-bands, can be transferred to a much larger set of vdW heterostructures, which has led to the discovery of an even bigger multitude of exotic ground states.

Figure 1: Emergence of topological flat-bands in MATBG, induced by the moiré super-potential.

Selected publications:

Superconductivity and strong correlations in moiré flat bands
L. Balents, C. R. Dean, D. K. Efetov and A. F. Young
Nature Physics, 16, 725 (2020) – (Focus/Perspective – Invited Review Article).
News : Nature Physics – Focus Session on Emergent Superconductivity;
Journal Club for Condensed Matter Physics : by Senthil Todadri;

The marvels of moiré materials
E. Y. Andrei, D. K. Efetov, P. Jarillo-Herrero, A. H. MacDonald, K. F. Mak, T. Senthil, E. Tutuc, A. Yazdani and A. F. Young
Nature Reviews Materials, 6, 201 (2021) – (Invited Viewpoint Article).
Editorial: Moiré magic three years on – Nature Reviews Materials, 6, 191 (2021);

Effektvolle Drehung
D. K. Efetov
Physik Journal, 03, 28 (2021) – (Invited Overview – main journal German Research Foundation).
Cover: Physik Journal;

2. High-quality assembly of 2D van der Waals hetero-structures and the Quantum Twisting Microscope

We use a van der Waals (vdW) co-lamination technique to assemble the devices. To characterize the so-created devices we apply a broad range of state-of-the-art techniques, like AFM, Raman, TEM, STEM, and work towards improving device yield and twist-angle homogeneity. As different interactions within the system compete, experimental sample details could hamper targeted control of individual parameters and disentangling the origin of different phases. The details of the experimental findings finely depend on the competition between all the different orders that are closely adjacent to each other, resulting in different phase diagrams for different samples. The ultimate challenge is to find more control and reproducibility over the experimental parameters, which then will allow to define a unified formalism that can explain all the possible phases, while also pointing out to universal features of the problem, which might appear in the strange metal or at higher temperature than the diverse many-body ground states.

For this purpose, it is key to improve assembly protocols and to achieve more control over the device parameters, e.q. to reduce twist-angle inhomogeneity, strain etc. in the fabrication process. On this end an automated, reproducible technique to prepare devices is urgently needed, which at the same time allows cleaning and straightening of the devices. Recent experimental progress of the Quantum Twisting Microscope (QTM) is so far the most promising technique that allows to assemble twisted devices in a more controlled way, which works on the principle of anchoring the different 2D layers on separated mechanical actuators. Our group has recently succeeded in the development of a working room-temperature QTM setup, based on a commercially available AFM system. We will continue to develop a low temperature version of this technique, which will in the future allow to obtain ultra-clean devices of MATBG and other systems of interest, and so to obtain reproducible phase-diagrams with well-known and controlled sample parameters.

Figure 2: (a) Typical MATBG device structure. (b) Home-build QTM. (c) Dynamically control of the twist-angle between stacks of 2D materials, and simultaneous conductance measurments. It enables to directly probe the band-structure of the so-created materials by electronic tunneling spectroscopy.

Selected publications:

The quantum twisting microscope
A. Inbar, J. Birkbeck, J. Xiao, T. Taniguchi, K. Watanabe, B. Yan, Y. Oreg, Ady Stern, E. Berg and S. Ilani
Nature, 614, 682 (2023).
News : Nature News;

3. Low-temperature transport experiments

The so-developed devices are extensively studied with electronic transport experiments at mK (<10mK) temperatures and high magnetic fields (>35T). The simultaneous occurrence of all these phases and the ability to tune between these by simply applying a voltage, has positioned MATBG as one of the richest and most tunable materials platforms in condensed matter physics.  Here of particular interest to us are the novel physical concepts that this young field has enabled, such as flat-band superconductivity, quantum geometry, topological heavy Fermions, fractional Chern insulators (FCI) and non-abelian Para-fermions.

Figure 3: (a) Atomic force microscopy image of a typical gated MATBG transport device. (b) Dillution refrigerator setup for electronic measurements below a temperature of T<35mK. (c) A typical low-temperature phase-diagram of MATBG, that shows co-existing superconducting (SC), correlated insulator (CI) and correlated Chern insulator (CCI) states.

Selected publications:

Superconductors, orbital magnets, and correlated states in magic angle bilayer graphene
X. Lu, P. Stepanov, W. Yang, M. Xie, A. M. Ali, I. Das, C. Urgell, K. Watanabe, T. Taniguchi, G. Zhang, A. Bachtold, A. MacDonald and D. K. Efetov
Nature, 574, 653 (2019).
News : The New York Times; Le Monde; La Vanguardia; Physics Today;
Journal Club for Condensed Matter Physics : by Mike Zaletel;

Untying the insulating and superconducting orders in magic-angle graphene
P. Stepanov, I. Das, X. Lu, A. Fahimniya, K. Watanabe, T. Taniguchi, F. H. L. Koppens, J. Lischner, L. Levitov and D. K. Efetov
Nature, 583, 375–378 (2020).
News : Nature : News and Views;

Symmetry-broken Chern insulators and Rashba-like Landau-level crossings in magic-angle bilayer graphene
I. Das*, X. Lu*, J. Herzog-Arbeitman, Z.-D. Song, K. Watanabe, T. Taniguchi, B. A. Bernevig and D. K. Efetov
Nature Physics , 17, 710 (2021).
News : PhysOrg;

Quantum-critical behavior in magic-angle twisted bilayer graphene
A. Jaoui, I. Das, G. Di Battista, J. Díez-Mérida, X. Lu, K. Watanabe, T. Taniguchi, H. Ishizuka, L. Levitov and D. K. Efetov
Nature Physics, 18, 633 (2022).
News : Nature Physics News & Views; Journal Club of Condensed Matter Physics;

Dirac spectroscopy and strongly correlated phases in twisted trilayer graphene
C. Shen, P. J. Ledwith, K. Watanabe, T. Taniguchi, E. Khalaf, A. Vishwanath and D. K. Efetov
Nature Materials, 22, 336 (2023).
News : Nature Materials News & Views;

4. Instrument development – novel thermal and heat-capacity probes of 2D materials

As device resistance drops to zero in a superconductor, conventional transport measurements provide only limited information about the nature of its state. Here thermal conductivity is a direct probe which is capable to study the orientation and symmetry of the nodes in the gap function, and specific heat measurements provide insight into the internal energy of the system and allow to extract the entire low-density excitations close to the Fermi level. Both experimental probes were instrumental in the understanding of unconventional superconductivity in various materials, but these properties are unmeasurably small in nano-scale materials with these traditional techniques. These limitations have their origin in the typically large background contribution of the phonon bath, the lack of suitable thermometers, and the ultra-fast thermal relaxation times in ultra-small objects.

To overcome these challenges we have recently adopted a Johnson noise thermometry technique to graphene devices, which allows to measure its electronic temperature T_e employing the Johnson-Niquist theorem. Here the noise power of thermally agitated carriers is resonantly transmitted through a LC impedance matched circuit to a quantum limited RF amplifier, and read out through a heterodyne measurement scheme. We have shown that it is possible to read out T_e with a sensitivity of 1mK and a time resolution of pico-seconds, by using ultra-fast laser heating. 

Figure 4: (a) Schematic of the electron heat-capacity measurement scheme for graphene devices, which is based on a Johnson noise thermometry and ultra-fast pulsed laser heating. (b) Electronic thermal conductivity measurement vs. temperature of a graphene device. (c) Extraction of the thermal relaxation time of graphene electrons using ultra-fast laser pulses.

Selected publications:

Fast thermal relaxation in cavity-coupled graphene bolometers with a Johnson noise read-out
D. K. Efetov, R.-J. Shiue, Y. Gao, B. Skinner, E. Walsh, C. Choi, J. Zheng, C. Tan, G. Grosso, C. Peng, J. Hone, K. C. Fong and D. Englund
Nature Nanotechnology, 13, 797–801 (2018).
News : MIT News

Ultra-sensitive calorimetric measurements of the electronic heat capacity of graphene
M. A. Aamir*, J. N. Moore*, X. Lu, P. Seifert, D. Englund, K. C. Fong and D. K. Efetov
Nano Letters, 21, 12, 5330 (2021).

5. Quantum sensing

Remarkably, “moiré” materials are true 2D single crystals with ultra-high electronic quality, with several orders of magnitude lower electron density and heat capacity, and with orders of magnitude higher superconducting coupling strength than conventional superconductors. Since the exploitation of heating effects from single photons represents the main detection principle in modern single photon detectors, these attributes position 2D “moiré” materials as exceptional materials for quantum sensing applications. Successful demonstration of true mid-IR single photon detection is direly needed in such distant fields as radio-astronomy and medical imaging, and will lead to a strong improvement in the resolution of the cosmic background radiation and will enable efficient sensing of molecular species. We have recently succeeded in the demonstration of single photon detection with BSCCO high-temperature superconductors, operating at a record-high temperature T=20K, and now are also pushing MATBG single photon detectors to the mid-IR and even THz ranges.

Figure 5: (a) Schematics of a MATBG single photon detector. (b) Photovoltage time traces show "clicks" due to absorption of single photons. (c) Photon count rate PCR vs. the average incident photon number ⟨N_photon ⟩ shows linear scalling, as expected from single photon absorption events.

Selected publications:

Graphene-based Josephson junction microwave bolometer
G.-H. Lee, D. K. Efetov, L. Ranzani, E. Walsh, J. Crossno, T. A. Ohki, T. Taniguchi, K. Watanabe, Kim, D. Englund and K. C. Fong
Nature 586, 42–46 (2020).
News : PhysOrg; NanoWerk;

Josephson junction infrared single-photon detector
E. D. Walsh, W. Jung, G.-H. Lee, D. K. Efetov, B.-I. Wu, K.-F. Huang, T. A. Ohki, T. Taniguchi, K. Watanabe, P. Kim, D. Englund and K. C. Fong
Science, 372, 6540 (2021).
News : PhysOrg;

Two-dimensional cuprate nanodetector with single photon sensitivity at T = 20 K
R. Luque-Merino, P. Seifert, J. Duran Retamal, R. Mech, T. Taniguchi, K. Watanabe, K. Kadowaki, R. H. Hadfield, D. K. Efetov
2D Materials, 10, 2 (2023).
News : Nature Nanotechnology News & Views;

Current projects

• 2024-2031, DFG Leibniz Preis (GER), 2.500.000€.
• 2024-2027, DFG SPP2244 Priority program (GER), Ref: 535146365, 268.522€.
• 2023           DFG FUGG, 50% Dill. Fridge with vector magnet (GER), Ref: INST 86/2236-1, 418.500€.
• 2023           DFG FUGG, 50% Dill. Fridge with 14T magnet (GER), Ref: INST 86/2235-1, 414.000€.
• 2023-2027, SuperC, Keele Foundation Grant (Private Donation), 315.000€.
• 2023-2027, FLATS, EU EIC Pathfinder Grant (EU), Ref: 101099139, 656.750€.
• 2021-2026, Munich Quantum Valley Quantum Technology Park (MQV QTPE) (GER), Ref: 1705414, 8.831.894€.
• 2022-2025, Core-member Munich Center of Quantum Science and Technology (MCQST) (GER), Ref: 390814868, 77.000€/year.
• 2020-2025, SuperTwist, ERC Starting Grant (EU), Ref: 852927, 1.780.000€.

Past projects

• 2018-2022, 2D-SIPC, EU Horizon 2020 Quantum Flagship (EU), Ref: 820378, 530.000€. 
• 2018-2021, 2DSC, LaCaixa foundation Junior Leaders fellowship (SP), 305.700€.

Major collaborations

• Prof. A. B. Bernevig, theory of topology in twistronic materials, Princeton, USA
• Prof. L. Levitov, theory of scattering mechanisms in twistronic materials, MIT, USA
• Prof. A. MacDonald, theory of superconductivity in graphene, UT Austin, USA
• Prof. A. Vishvanath, theory of correlated states in twistronic materials, Harvard, USA
• Prof. P. Kim, thermal transport in magic angle graphene, Harvard, USA
• Prof. E. Zeldov, scanning probe of graphene, Weizmann, IL
• Prof. P. Roulleau, shot noise in graphene, CEA Paris, FR
• Prof. P. Hakkonen, impedance spectroscopy of superconductors, Aalto, FIN
• Prof. S. Ganichev, THz spectroscopy of moiré materials, Regensburg, GER
• Dr. K. C. Fong, microwave and quantum circuits, BBN Raytheon and Harvard, USA
• Dr. B. Piot, high magnetic field transport of hetero-structures, Grenoble, FR