Session W03: 2D Materials: Superconductivity, Ferroelectricity, Density Waves, and Other Correlated States I

Integer and fractional quantum anomalous Hall effects in 2D semiconductor moiré superlattices

The emergence of topological moiré flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall effects without the need for an external magnetic field. These effects are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) effects. In this talk, I will mainly present electrical transport studies of moiré superlattices built on 2D transition metal dichalcogenide (TMDc) semiconductors. We have successfully achieved highly tunable topological phases in both TMDc heterobilayer and TMDc homobilayer moiré superlattices. Specifically, we have observed a robust IQAH effect and signatures of quantum spin Hall effect in AB-stacked WSe2/MoTe2. Furthermore, both the IQAH effect and the long-sought FQAH effect have been realized in twisted bilayer MoTe2. The band topology in TMDc moiré superlattices is highly tunable by external electric fields, which enable us to realize novel topological quantum phase transitions. Our studies pave the path for the investigation of fractionally charged excitations and anyonic statistics at zero magnetic field based on 2D moiré materials.

[1] F. Xu et al. PRX 13, 031037 (2023).

[2] T. Li et al. Nature 600, 641-646 (2021).

[3] T. Li et al. Nature 597, 350-354 (2021).   

Unconventional superconducting phase in NbSe2-based misfit layered superconductor

Two-dimensional (2D) superconductor is a good playground for investigating quantum transport. Recently, bulk 2D superconductors, where superconducting layers are well separated by non-superconducting block layers, has been intensively studied [1,2]. Among them, misfit layered compound, which consist of alternately stacked layers with incompatible unit cells [3], is a unique material, offering a new opportunity for exploring low dimensional superconductivity.

In this work, we report transport properties of misfit layered superconductor composed of NbSe₂. In this material, trilayer NbSe₂ layers are separated by PbSe block layers. Firstly, we found that the superconducting transition temperature is close to the exfoliated trilayer NbSe₂. Secondly, we observed an anomaly in temperature dependence of in-plane upper critical field, which suggests the unconventional superconductivity in low-temperature and high-magnetic-field region. Since this anomaly is absent in bulk and exfoliated trilayer NbSe₂ [4,5], we assume that PbSe block layers play crucial role in the emergence of unconventional superconducting phase in the present system.

[1] A. Devarakonda et al., Science 370, 231 (2020).

[2] P. Samuely et al., PRB 104, 224507 (2021).

[3] N. Ng and T. M. McQueen, APL Mater. 10, 100901 (2022).

[4] X. Xi et al., Nat. Phys. 12, 139 (2016).

[5] E. Sohn et al., Nat. Commun. 17, 504 (2018).   

Strong-coupling approaches to moiré superconductivity and competing orders

Superconductivity in the limit of a vanishing bandwidth in isolated bands is a classic example of a non-perturbative problem, where BCS theory does not apply. What sets the superconducting phase stiffness, and relatedly the transition temperature, in this limit is of both fundamental and practical interest. This question has become especially relevant with the discovery of superconductivity in moiré materials. I will present a non-perturbative framework to obtain the low-energy optical spectral weight for partially filled electronic flat bands with generic density-density interactions, and apply it to the problem of twisted bilayer graphene with screened Coulomb interactions at integer fillings to put upper bounds on the maximum superconducting transition temperature. I will also present numerically exact results obtained using sign-problem-free quantum Monte-Carlo methods for the interplay between superconductivity and various competing orders in models of interacting flat (non-)topological bands.   

Phonon-mediated nematic nodal superconductivity in twisted bilayer graphene

Recently, a topological heavy fermion model for the magic-angle twisted bilayer graphene has been proposed, which enlightens that in the heavy Fermi liquid (hFL) regime, the strong Coulomb repulsion is screened and gives way to, e.g., phonon induced, attractive interactions to mediate Cooper pairs. In the present work, we investigate superconducting instabilities of the fully symmetric hFL and various valley-ordered hFL's, where valley-polarized (VP), inter-valley coherent (IVC), and Kramer's inter-valley coherent (KIVC) orders are assumed respectively. As the kinetic energy of heavy electrons is small compared to interaction, a strong coupling picture applies - local Cooper pairs form at a higher energy scale and phase coherence developes at a relatively lower energy scale. Phase diagrams are numerically obtained for various parameters and match well with the strong coupling expansion. We find that the fully symmetric hFL favors the s-wave spin-singlet pairing that opens a full gap; while the valley-ordered hFL's - in a wide range of realistic parameters - can favor nematic nodal pairings, where the C_2zT symmetry is preserved but C_6z is spontaneously broken. In particular, in the VP and KIVC phases, the nematic pairing is a p-wave-like spin-singlet, and we mathematically prove that 2+4n (n∈Z) pairing nodes are enforced on the inner Fermi surface by the C_2zT symmetry and a π Berry phase. In the IVC phase, the nematic pairing is generally (s+d)-wave-like spin-singlet, and the existence of nodes is not topologically protected, but depends on interaction details.   

Controlling the superconducting layers in misfit layered chalcogenide superconductors

wo-dimensional (2D) superconductivity has been one of the top topics in condensed matter and the new materials as well as techniques have been proposed and studied intensively. Recent finding on intercalated transition metal dichalcogenides (TMD) show 2D behavior [1, 2]. Misfit layered chalcogenides are naturally grown crystals that host 2D electrons. The materials consist of cubic monochalcogenide insulating layer and hexagonal TMD layers that dominate the electronic states.  

In this work, we report the numbers of TMD layers can be modulated and the resultant modulation of electronic states are observed. We also successfully synthesized NbSe₂ trilayer series, which is expected to own very clean Ising superconducting layer in between the neighboring NbSe₂ layers.

 

 

[1] H. Zhang et al., Nat. Phys. 18, 1425 (2022).

[2] D. Zhao et al., Nat. Phys. (2023) (https://doi.org/10.1038/s41567-023-02202-4)   

Giant Eliashberg enhancement of superconductivity in flat bands

The enhancement of superconductivity under appropriate electromagnetic radiation due to redistribution of quasiparticles into a more favorable nonequilibrium configuration is called Eliashberg effect. Here, we investigate the Eliashberg effect in flat band superconductors. While the superconductivity in flat bands is established by non-trivial winding of wavefunction of the slow-moving electrons, the Eliashberg enhancement is completely determined by the density of states which for flat bands is peaked in a narrow range of energies. We show that this large density of states in flat band materials can mediate a drastic enhancement of superconductivity when the material is irradiated with a light of frequency under pair breaking limit and band width. Assuming BCS coupling, we illustrate this enhancement in flat bands of twisted bilayer graphene. This opens new possibilities of nonequilibrium phenomena driven by peaked density of states in flat band superconductors, e.g., superfluid density and supercurrent.   

Superfluid Stiffness of Td-MoTe2 at Microwave Frequencies

Superfluid stiffness is an intrinsic property that we leverage to reveal information about the pairing symmetry in superconductors. Robust cQED theory and technique allow us to measure the superfluid stiffness of MoTe₂ to parts per million precision as a function of temperature and power. Measurements show evidence of nodal superconductivity and a non-linear Meissner effect in T^d-MoTe₂. Furthermore, we develop a novel microwave pump-probe technique to study the recombination time, aiming to understand the Cooper-pairing mechanism of Weyl superconductivity in T^d-MoTe₂.   

Superconducting diode effect in strain-controlled trigonal superconductor PbTaSe2

Superconducting diode effect (SDE) is a switching between the superconducting and normal current in asymmetric superconductors [1-4]. Symmetry plays important roles in the emergence of SDE but the effect of crystal symmetry, particularly in the context of zero-magnetic field SDE, is still elusive. In this work, we report a strain-controlled SDE in a van der Waals layered trigonal superconductor PbTaSe₂. The SDE is demonstrated exclusively in a strained PbTaSe₂ device while it is absent in an unstrained device, implying the critical role of broken three-fold rotational symmetry induced by the strain. Furthermore, the zero-field or magnetic field-even (magnetic field-odd) SDE is demonstrated when the device is strained and current flows in the armchair (zigzag) direction. This current directional dependence is consistent with the crystal symmetry, providing a firm evidence for the intrinsic nature of the observed SDE.

[1] F. Ando et al., Nature 584, 373–376 (2020).

[2] N. F. Q. Yuan & L. Fu, PNAS 119, e2119548119 (2022).

[3] H. Wu et al., Nature 604, 653–656 (2022).

[4] K. Misaki & N. Nagaosa, PRB 103, 245302 (2021).   

Wafer-scale CVD graphene-based Josephson field-effect transistors with local top-gate tunability

Critical current (I_c) tunability with an electrostatic gate in superconductor-graphene-superconductor (SGS) junctions is essential for superconducting electronics based on Josephson field-effect transistor (JoFET) [1]. Previously, CVD graphene JoFETs were demonstrated using the global Si-wafer back-gate, see e.g. [2]. Here, we present tunable SGS junctions encapsulated with atomic layer deposition (ALD) grown Al₂O₃ dielectric and lithography-defined local top gates.

Starting with 6” CVD graphene on Si/SiO₂ wafers provided by Graphenea, our graphene devices are encapsulated with Al₂O₃, the top contacts are then evaporated using Ti/Al, the gate dielectric is grown using ALD, and finally the top gate is evaporated with Ti/Al. Using process optimization, we achieve contact resistances down to 300 Ω∙µm.

The resulting device arrays fabricated on 6” wafers consist of SGS junctions with length from 150 nm to 350 nm and width from 10 µm to 50 µm. Cooling down JoFETs with different dimensions in a dilution refrigerator, we observed repeatable superconducting proximity effect in all tested devices. We show the I_c tunability with top gate and analyze the temperature dependence of I_c. Our results constitute an important milestone toward scalable superconducting electronics based on graphene JoFETs [3].

[1] F. Wen et al., IEEE TED, vol. 66, no.12, pp. 5367-5374 (2019)

[2] T. Li et al., IEEE Trans. Appl. Supercond. vol. 29, no. 5, pp. 1-4 (2019)

[3] A. Generalov, K. Viisanen et al., in preparation (2023)