Name: Yuan Cao, Ph. D.
Affiliation: Harvard University
Position: Junior Fellow
Title: Strong coupling and spin-triplet superconductivity in magic-angle twisted trilayer graphene
Abstract:Moiré superlattices have recently become a playground for exploring correlated physics andsuperconductivity. Despite the presence of correlated effects in several moiré systems, magic-angle
twisted bilayer graphene has remained the only one with robust superconductivity. Here we present a new
moiré superconductor, magic-angle twisted trilayer graphene with dramatically richer properties andtunability. By exploring the entire phase space as a function of carrier density, electric and magnetic fields,
and temperature, we determine the systems tunable phase boundaries and reveal the intimate connection
between the superconducting state and the broken symmetry phase at two carriers per moiré unit cell.
The suppression and bounding of superconductivity at the Van Hove singularities is difficult to reconcile
with the weak-coupling BCS theory. Furthermore, we can tune the system to be in the ultra-strong
coupling regime close to the two-dimensional BCS-BEC crossover, where the Ginzburg-Landau coherence
length reaches the average inter-particle distance and T_BKT/T_F ratios are in excess of 0.1. Strikingly, thissystem also exhibits signatures of spin-triplet superconductivity. Firstly, we find its parallel criticalmagnetic field to be greatly exceeding the Pauli limit, which cannot be accounted for by the weak spinorbit coupling in this system. Moreover, we find evidence of reentrant superconductivity at high magnetic
fields in excess of 8 tesla, which is reminiscent of the phase diagrams of superfluid He-3 and some heavyfermion superconductors. Our system establishes a new generation of moiré platform where we caninvestigate strong coupling and possibly spin-triplet superconductivity with unprecedented tunability.

Two-fold symmetric superconductivity in few-layer NbSe2
Associate Prof. Vlad Pribiag
School of Physics and Astronomy, University of Minnesota – Twin Cities
Few-layer samples of transition metal dichalcogenides (TMDs) feature a wide array of properties
such as layer-dependent inversion symmetry, valley-contrasted Berry curvatures, and strong
spin-orbit coupling (SOC). Among the superconducting TMDs, NbSe2 is profoundly affected byIsing SOC. Ising SOC not only helps stabilize the superconducting state against large in-planemagnetic fields, but in conjunction with other forms of SOC, it could also give rise to exotic
superconducting states such as nodal topological superconductivity. This talk will discuss recent
transport measurements of few-layer NbSe2, and NbSe2/CrBr3 junctions, studied under in-plane
external magnetic fields. Surprisingly, although the crystal lattice has a three-fold symmetry, themagneto-resistance and critical field show a two-fold anisotropy, which is absent in the normalstate. We will discuss these results in the context of a competition between the conventional swave pairing instability characteristic of the bulk and a competing d- or p-wave instability thatemerges in the few-layer limit. These results [1] demonstrate the unconventional character ofsuperconducting pairing in NbSe2 and open the possibility for further discoveries, such as nontrivial topologies, in few-layer TMDs.
[1] Alex Hamill, Brett Heischmidt, Egon Sohn, Daniel Shaffer, Kan-Ting Tsai, Xi Zhang,
Xiaoxiang Xi, Alexey Suslov, Helmuth Berger, László Forró, Fiona J. Burnell, Jie Shan, Kin FaiMak, Rafael M. Fernandes, Ke Wang and Vlad S. Pribiag, Nature Physics 17, 949 (2021).

Quantum Sensing of MoiréMagnetism
Xiaodong Xu
Department of Physics, Department of Materials Science and Engieering, University ofWashington, Seattle WA
Moirésuperlattices of twisted non-magnetic two-dimensional (2D) materials have proven to behighly controllable platforms for engineering exotic correlated and topological states. In this talk,I will present emerging magnetic textures in small-angle twisted 2D magnet chromium triiodide(CrI3). Employing single-spin quantum magnetometry, we directly visualize nanoscale magnetic
domains and periodic patterns, a signature of moirémagnetism, and gain quantitative information
on domain size and magnetization. In twisted bilayer CrI3, we observe the coexistence of
antiferromagnetic (AFM) and ferromagnetic (FM) domains with disorder-like spatial patterns
pinned by the underlying stacking structure. Remarkably, in twisted double trilayer CrI3, AFM andFM domains with periodic patterns appear, in good agreement with the calculated spatial magnetic
structures arising from the local stacking-dependent interlayer exchange interactions in CrI3 moirésuperlattices. Our results highlight magnetic moirésuperlattices as a new platform for exploring
emergent magnetic phases.

Interplay between spin-orbit coupling and magnetismat a magnetic van der Waals interface
Masaki Nakano
QPEC and Department of Applied Physics, University of Tokyo, Tokyo, JapanRIKEN Center for Emergent Matter Science (CEMS), Wako, Japannakano@ap.t.u-tokyo.ac.jp / mnakano@riken.jp

Two-dimensional (2D) materials and their integrated superstructures provide emergent
phenomena associated with reduced dimensionality, modified lattice symmetry, andenhanced proximity coupling. A remarkable example is Ising superconductivity emerging
in a non-centrosymmetric 2D superconductor with broken in-plane inversion symmetry
as represented by 2D NbSe2, where strong Zeeman-type spin-orbit coupling (SOC) locks
the orientation of the spins of Cooper pairs to the out-of-plane direction, providing exotic
non-BCS superconducting states. On the other hand, an interplay between such unique
Zeeman-type SOC and magnetism has been largely unexplored.
In this presentation, we will show our recent results on the transport properties of themagnetic van der Waals heterostructures based on 2D NbSe2 and a newly-developed 2D
Heisenberg ferromagnet V5Se8 [1], and discuss the unique proximity and inverse
proximity effects mediated by Zeeman-type SOC [2, 3]. We will in particular focus on
the specific regime, where the number of the V5Se8 layer was decreased down to the 2Dlimit so that the transport properties are dominated by NbSe2 [3]. Very interestingly, we
found that the sign of the anomalous Hall effect (AHE) of those samples were positive atthe lowest temperature, which is opposite to those of the V5Se8 individual films. We also
found that the AHE signal of those samples was enhanced with the in-plane magnetic
fields, suggesting an additional contribution to the AHE signal except magnetization. We
verify by band structure calculations that those unprecedented behavior could be wellunderstood by accepting the idea that NbSe2 is in a ferromagnetic/ferrovalley state, wherecharacteristic Zeeman-type SOC plays an essential role.
References:[1] M. Nakano et al., Nano Lett. 19, 8806 (2019).
[2] H. Matsuoka et al., Nano Lett. 21, 1807 (2021).
[3] H. Matsuoka et al., submitted.

Electrically tunable homo bilayer moiré semiconductor probed by exciton spectroscopy
Yuya Shimazaki
Center for Emergent Matter Science, RIKEN, Saitama, Japan

2D material moiré heterostructure constitute a novel platform to study many body physics of electronsand excitons. Transport study of twisted bilayer graphene revealed rich exotic phase of electrons includingfrom Mott insulator state to superconductivity. On the other hand, optical spectroscopy study revealed theeffect of moiré lattice potential on excitons in transition metal dichalcogenide (TMD) heterostructure, but thecorrelated phase of electrons has been elusive.
In this talk, I will present optical spectroscopy study of homo bilayer TMD moiré heterostructure, andreport the observation of strongly correlated incompressible electronic state [1]. We utilized a boron nitride(hBN) encapsulated MoSe2 / hBN (1L) / MoSe2 heterostructure with top and bottom gates, which enable
independent control of the chemical potential and the electric field. Tightly bound excitons in TMD interact
with Fermi sea carriers by forming exciton-polaron, and we
utilized the energy shift of exciton-polaron resonance todetect layer resolved charge configuration by optical means.
In low electron density regime, we found periodic chemical
potential dependence of carrier filling behavior indicating
existence of moiré subband structure (Fig. 1). Tuning theelectric field at specific filling around one electron / moiré
unit cell results in abrupt interlayer charge transfer,evidencing the emergence of strongly correlated
incompressible electronic state. In this incompressible state,
we also observe emergence of new excitonic resonancesfrom umklapp scattering, which is evidencing theemergence of periodic charge distribution [2].
Exciton spectroscopy also shed light on potential Fig.1 Layer resolved periodic electron filling behavior
landscape of moiré lattice, and revealed moiré site
revealed by excitonic charge sensing.
dependent tunnel coupling of hole carriers (Fig. 2) [3].
Furthermore, quantum tunneling of holes together withelectrical biasing assist hybridization of optically excitedstates, enabled to observe exciton-hole Feshbachresonance (Fig. 2) [3] and interlayer - intralayer hybridexcitons [1].
This highly tunable twisted MoSe2 homo bilayer
system separated by a monolayer hBN barrier with weakmoiré potential, provides a promising platform forinvestigating strongly correlated Mott-Wigner physics andBose-Fermi mixture.
References[1] Y. Shimazaki et al. Nature 580, 472 (2020).
[2] Y. Shimazaki et al. Phys. Rev. X 11, 021027 (2021).
[3] I. Schwartz, Y. Shimazaki et al., Science 374, 336(2021). (equal contribution)

Fig.2 Optically detected moiré site (hole) hybridizationand electrically tunable exciton-hole Feshbach resonances.

Exotic quantum phenomena in two‐dimensionalsuperconductors
Ding Zhang
Department of Physics, Tsinghua University
Two‐dimensional (2D) superconductivity has been a vibrant field. Recent discoveries
include the Ising superconductivity, the quantum Griffiths singularity, as well as thehotly debated notion of the anomalous metal phase. In this talk, we will present ourrecent studies on a few representative 2D superconductors. In the first part, we report
our investigation of few‐layer stanene—ultrathin gray tin (111). Few‐layer stanene not
only exhibits two‐dimensional superconductivity but also topologically non‐trivial
band structures [1]. Furthermore, they host enhanced in‐plane upper critical fields
that greatly exceed the conventional limit. We propose that this anomalous behavior
stems from a novel type of Ising pairing [2]. In the second part, we study twisted high‐Tc Josephson junction. The junctions are realized by artificially stacking twoBi2Sr2CaCu2O8+x (BSCCO) thin flakes on top of each other. We demonstrate that theJosephson current solely arises from the coupling between the twisted double copper‐oxide planes. Notably, the Josephson coupling strength reaches the same level fordifferent twist angles, indicative of an s‐wave pairing symmetry [3]. Finally, we present
our on‐going efforts on electric‐field controlled lithium intercalations. This techniqueallows us to control or induce superconductivity. Our recent experiments on titanium
diselenide will be discussed [4].

[1] M. Liao et al., Nat. Phys. 14, 344 (2018).
[2] J. Falson et al., Science 367, 1454 (2020).
[3] Y. Zhu et al., Phys. Rev. X 11, 031011 (2021).
[4] M. Liao et al., Nat. Commun.12, 5342 (2021).

Designer electronic states in van der Waals heterostructures
Shawulienu Kezilebieke1, Viliam Vaňo1, Mohammad Amini1, Somesh C. Ganguli1,
Md Nurul Huda1, Markus Aapro1, Orlando J. Silveira1, Guangze Chen1, Szczepan
Głodzik2, Adam S. Foster1, Teemu Ojanen3, Jose L. Lado1, Peter Liljeroth1,*1

Department of Applied Physics, Aalto University, Finland
Institute of Physics, M. Curie-Skłodowska University, Lublin, Poland
3
Computational Physics Laboratory, Tampere University, Tampere, Finland* Email: peter.liljeroth@aalto.fi2

Van der Waals (vdW) heterostructures have emerged as a playground for engineering
exotic quantum states not found in naturally occurring materials. In these designersystems, the desired physics emerges through the controlled interactions between thedifferent layers. The use of vdW heterostructures also makes it possible to fabricate
device structures potentially allowing further control through e.g. electrostatic gating.
I will outline our recent results on realizing topological superconductivity [1,2] andartificial heavy fermion systems in vdW heterostructures [3]. We use molecular-beam
epitaxy (MBE) in ultra-high vacuum for the sample growth and low-temperaturescanning tunneling microscopy (STM) for characterization. Topologicalsuperconductivity can be realized by combining ferromagnetic CrBr3 on asuperconducting NbSe2 substrate [1,2]. This brings together out of plane
ferromagnetism, Rashba-type spin-orbit interactions and s-wave superconductivity,which are the necessary ingredients for topological superconductivity. On the other
hand, artificial heavy fermion systems can be realized by coupling localized magnetic
moments with mobile conduction electrons. We achieve this experimentally bybringing together 1T-TaS2, where a strong charge-density wave modulation results in
a formation of localized electronic states, and normal metal layer 1H-TaS2 [3]. These
examples highlight the versatility of vdW heterostructures in realizing quantum statesthat are difficult to find and control in naturally occurring materials.
[1]
[2]
[3]

S. Kezilebieke et al., Nature 588, 424 (2020).
S. Kezilebieke et al., arXiv: 2011.09760.
V. Vaňo et al,, arXiv:2103.11989.

Title: 2D Magnetic Materials probed by tunneling magetoresistance

Alberto Morpurgo (University of Geneva)
Abstract: In this talk, I will discuss our research on 2D magnetic materials and heterostructures.
After a short introduction of the physical systems and of the general questions investigated inthis domain of research, I will present results obtained in my group on atomically thinmultilayers of different ferromagnetic semiconductors such as CrI3, CrCl3 (layeredantiferromagnets), MnPS3 (antiferromagnetic within individual layers), and CrBr3(ferromagnetic semiconductors). We use atomically thin crystals of these materials to formtunnel barriers and we obtain information from the measurement of their tunneling resistanceas a function of temperature and magnetic field. Examples of observed phenomena and ofproperties that we extract include: i) a giant tunneling magnetoresistance in CrI3; ii) a thoroughcharacterization of the magnetic phase diagram of CrCl3 multilayers; iii) the observation of aspin-flop transition in MnPS3 persisting to the ultimate thickness of an individual monolayer,and iv) the demonstration that the tunneling magnetoresistance of ferromagnetic CrBr3
barriers depends on magnetic field and temperature only through the magnetization (from well
above to well below the Curie temperature). A key conclusion drawn from our work is thatmeasurements of the temperature and magnetic field dependence of the tunnelingmagnetoresistance allow precise information about the magnetic state of atomically thincrystals to be obtained, something impossible to do with most conventional experimentaltechniques, not sufficiently sensitive when used on such a small amount of material.

References- Z. Wang et al., Nature Communications 9, 2516 (2018) - Very Large Tunneling

Magnetoreistance in Layered Magnetic Semiconductor CrI3
- Z. Wang et al., Nature Nanotechnology 14, 1117 (2019), Determining the phase diagram of

atomically thin layered antiferromagnet CrCl3- M. Gibertini et al., Nature Nanotechnology 14, 408 (2019), Magnetic 2D materials andheterostructures- L. Thiel et al., Science 364, 973 (2019), Probing magnetism in 2D materials at the nanoscalewith single spin microscopy- G. Long et al., Nano Letters 20, 2452 (2020), Persistence of Magnetism in atomically thin

MnPS3 crystals- Z. Wang et al., arXiv :2106.13930, Magnetization dependent tunneling conductance offerromagnetic barriers