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2D materials and devices, twisted layers incl. Gothenburg Mesoscopic Lecture

The discovery, five years ago, by Pablo Jarillo-Herrero that two layers of graphene put on top of each other with a magic angle of 1.1 degrees will form a Moiré material with a periodicity much larger than the graphene unit cell, flat electron bands, resulting in a metallic state and tunable superconductivity at low temperature. Several other phases may occur as ferroelectricity, magnetism, and correlated insulator. The field is sometimes called ”twistronics”. Large research activity concerns not only graphene but also other two dimensional layers that form quantum Moiré matter.  Two-dimensional matter has been a fruitful research area with rich, controllable phenomena and important applications.

Pablo Jarillo-Herrero will give the 2023 Gothenburg Mesoscopic Lecture, sponsored by the Nobel Institute of Physics. It will be an introduction to a symposium, arranged together with the Area of Advance, Nano at Chalmers concerning properties and applications of 2D electron gases, particularly Moiré matter. Several international and national researchers will give reviews and details of a fashionable and fascinating field of research.


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The schedule may be subject to change. 

Thursday, 5 October

13:00-13:10 Registration, welcome

13:10-14:00 GOTHENBURG MESOSCOPIC LECTURE, Pablo Jarillo Herrero, MIT, The Magic of Moiré Quantum Matter
The understanding of strongly-interacting quantum matter has challenged physicists for decades. The discovery five years ago of correlated phases and superconductivity in magic angle twisted bilayer graphene has led to the emergence of a new materials platform to investigate strongly interacting physics, namely moiré quantum matter. These systems exhibit a plethora of quantum phases, such as correlated insulators, superconductivity, magnetism, ferroelectricity, and more. In this talk I will review some of the recent advances in the field, focusing on the newest generation of moiré quantum systems, where correlated physics, superconductivity, and other fascinating phases can be studied with unprecedented tunability. I will end the talk with an outlook of some exciting directions in this emerging field.

14:10-14:35 Samuel Lara Avila, Chalmers University of Technology, Growth and transfer of wafer-scale, single-crystal, monolayer and Bernal-stacked bilayer graphene
Epitaxial graphene on a silicon carbide substrate (epigraphene) is an attractive scalable technology for high-quality graphene electronics. Under suitable conditions, epigraphene can be produced as single crystal over entire silicon carbide (SiC) wafers (up to 4”). Epigraphene has revolutionized quantum resistance metrology, and it is nowadays the preferred embodiment of the primary electrical resistance standard (von Klitzing constant, RK = h/e2) as it allows for higher accuracy quantum Hall effect measurements in more relaxed measurement conditions. While many material-specific qualities of epigraphene derive from the graphene-SiC substrate interaction, it might be advantageous to have the capability to transfer the graphene layer onto arbitrary substrates. In this talk I will describe the efforts at Chalmers in transferring large-area graphene crystals (up to 3 mm x 3mm so far) onto arbitrary samples. The recent material characterization results, including surface science techniques and electron transport will be discussed during the presentation. The observed uniformity and reproducibility of transfer of millimeter-scale graphene single crystals make this an interesting material platform for basic and applied research.

14:35-15:00 Johannes Hofmann, University of Gothenburg, Hydrodynamic transport in interaction-dominated electron gases
Our conventional description of electron transport in metals is based on the interaction of electrons with impurities and phonons, while interactions between the electrons themselves usually do not play a role. However, in ultraclean two-dimensional materials these conventional mechanisms for electron relaxation are suppressed over a wide temperature range, and electron-electron interactions do become dominant. Such an interaction-dominated regime has recently been reported in experiments on materials like graphene or GaAs, which observe signatures of hydrodynamic transport where the electrons flow collectively like a fluid. I will present recent theoretical work to describe the interaction-dominated electron gas. In particular, I will present evidence for a new odd-parity transport regime, in which parity-even Fermi surface deformations decay rapidly but odd-parity modes are anomalously long-lived, and which exists at fairly large temperatures T≲0.1TF, putting this regime within the reach of current experiments.

15:00-15:30 Coffee break

15:30-16:15 Peter Böggild, DTU, Prison break: how 2D quantum materials may escape into reality
After two decades of astonishing physics discoveries emerging from high-quality exfoliated 2D materials and their heterostructures, the projections for future technologies are dizzying. There are, however, a number of serious technical and scientific challenges to be solved before 2D materials can effectively make it into real world applications. In this presentation I will discuss some of the most severe and interesting technological barriers, and what we know today about solutions and strategies to ultimately “break out” 2DQM from these metaphorical "prison walls".

16:20-16:45 Emil J Bergholtz, Stockholm University, Anyons and Quantum Geometry in Moiré Materials
The flatbands of Moiré materials provide a rich playground for the study of strongly correlated phases of matter. I will discuss the prediction and discovery of fractional Chern insulators with accompanying anyon excitations in these systems, as well as novel symmetry breaking competing states emerging as a consequence of the underlying quantum geometry of the flatbands.

16:50-17:30 Hélène Bouchiat, Université Paris-Saclay, Singular orbital magnetism in Graphene and moiré potential
A singular Landau orbital magnetism of graphene, with a sharp narrow diamagnetic peak at the Dirac point was already predicted in 1956 by McClure. It is now understood as a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions.
Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage–dependent magnetization of a single graphene layer. The signal exhibits a sharp diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. These measurements enable the investigation of orbital currents in 2D materials that cannot be detected in usual transport measurements. Among the predictions an intriguing orbital paramagnetism at saddle points of 2D materials is also expected.
In order to reveal this unusual orbital paramagnetism, we investigated graphene layers aligned with the hexagonal lattice of a boron nitride substrate, giving rise to a large period moiré potential acting on graphene charge carriers. Beside the sharp diamagnetic peak at the Dirac point, followed by de Haas-van Alphen oscillations at larger doping, we detect extra diamagnetic peaks at the satellite Dirac peaks of the moiré lattice. We also find paramagnetic peaks surrounding these satellite diamagnetic peaks related to van-Hove singularities in the density of states. These findings confirm the existence of paramagnetic orbital loops in 2D systems when the Fermi energy is tuned in the vicinity of saddle points.
J. Vallejo-Bustamante et al. Science 2021 and Physical Review letters 2023

17:40 Reception

Friday, 6 October

09:00-09:45 Dmitri Efetov, LMU. Munich, Topological properties of magic angle twisted bilayer graphene devices
Twist-angle engineering of 2D materials has led to the recent discoveries of novel many-body ground states in moiré systems such as correlated insulators, unconventional superconductivity, strange metals, orbital magnetism and topologically nontrivial phases. These systems are clean and tuneable, where all phases can coexist in a single device, which opens up enormous possibilities to address key questions about the nature of correlation induced superconductivity and topology, and allows to create entirely novel quantum phases with enhanced interactions. In this talk we will introduce some of the main concepts underlying these systems, concentrating on magic angle twisted bilayer graphene (MATBG) and show how symmetry-broken states emerge at all integer electron fillings [1]. We further will discuss recent experiments including screened interactions [2], Chern insulators [3], magnetic Josephson junctions [4], quantum criticality [5], re-entrant correlated insulators at high magnetic fields [6], Dirac spectroscopy of correlated states in magic angle trilayers and discuss some of the avenues for novel quantum sensing applications [8].

[1] Nature, 574, 653 (2019).
[2] Nature, 583, 375–378 (2020).
[3] Nature Physics, 17, 710 (2021).
[4] arXiv:2110.01067 (2021).
[5] Nature Physics, 18, 633 (2022).
[6] PRL 128, 217701 (2022).
[7] Nature Materials, in press (2022).
[8] Nano Letters, 22, 6465(2022).

09:55-10:20 Floriana Lombardi, Chalmers University of Technology, Manipulating the phase diagram of high critical temperature superconductors through substrate strain engineering.
Both High critical Temperate Superconductors (HTS) and magic angle twisted bilayer graphene (MATBG) exhibit complex phase diagrams with various correlated phases of matter. These phases include superconductivity, pseudogap, charge density wave, and a non-Fermi liquid "strange metal phase." This suggests that there might be underlying physics principles or mechanisms shared between these seemingly different systems. In MATBG the different phases can be studied with unprecedented tunability; HTS materials instead remain only very marginally tunable. This difference has represented a major obstacle to advance our understanding of HTS material and to unraveling the various correlated electronic phase contributing to the phase diagram. In this contribution I will elucidate our recent efforts focused on manipulating the phase diagram of YBCO thin films through substrate strain engineering. In nanometer-thick YBCO films, we have observed substantial alterations in the ground state: the Fermi surface undergoes a nematic transformation, the strange metal phase persists to significantly lower temperatures compared to single crystals, charge density waves are suppressed and the superconducting critical temperature enhanced. Moreover, in strongly underdoped nanometer-thick YBCO films, subjected to substrate strain, the strange metal phase becomes the precursor to an insulating phase before the onset of superconductivity, in complete analogy to the behavior observed in MATBG. Our findings demonstrate the promising prospect of employing substrate strain control to achieve an unparalleled degree of tunability across the various strongly correlated phases that populate the phase diagram of HTS.

10:20-10:50 Coffee

10:55-11:50 Klaus Ensslin, ETH Zurich, Quantum devices in twisted graphene
We review recent results on Josephson junctions and SQUIDs realized in magic angle twisted bilayer graphene. New results on Little-Parks oscillations in gate-defined rings will be discussed. We observe Aharonov-Bohm as well as Little-Parks oscillations in a single gate-defined ring device in which we can confine the superconducting or the normal conducting ring by a correlated or band insulator.

11:55-12:20 Saroj Dash, Chalmers University of Technology, Emergent Spin Phenomena in 2D Quantum Materials Heterostructures
Two-dimensional (2D) materials and their van der Waals heterostructures represent a new platform for realizing novel quantum and spin-based phenomena and device applications. While materials such as graphene are suitable for spin-polarized electron transport, magnets and materials with topological spin textures are useful for spin-polarized electron sources. Here, we utilized large-scale CVD graphene for spin interconnect and realized multifunctional spin logic operations at room temperature [1,2]. To generate spin polarization and their electrical control, we engineered 2D material heterostructures by combining the semiconductor MoS¬2 and topological materials (WTe2 and BiSbTeSe) with graphene to realize strong proximity-induced spin-orbit coupling [3,4,5] and proximity magnetism with Cr3GeTe2 [7]. Furthermore, room temperature van der Waals magnet based spin-valve [8] and spin-orbit torque devices were realized using Fe5GeTe2 and CoFeGeTe with co-existence of ferromagnetic and anti-ferromagnetic orders [9]. These findings open a new platform for realizing spintronic devices using all-2D heterostructure devices.

[1] Nature Communications 6, 6766 (2015).
[2] Physical Review Applied 18 (6), 064063 (2022).
[3] Nature Communications 8, 16093 (2017).
[4] Science Advances 4:eaat9349 (2018).
[5] Nature Communication 11, 3657 (2020).
[6] Advanced Materials 32, 2000818 (2020).
[7] 2D Materials 7 (1), 015026 (2019).
[8] Advanced Materials, 2209113 (2023).

12:25-12:50 Eva Olsson, Chalmers University of Technology, TBD

13:00           Lunch sandwich and end