[Monthly Seminar] (April 12, 2024, 11:00~12:30) Joonho Jang & Keehoon Kim
- 극한양자기능물질연구센터
- Hit181
- 2024-04-03
April 12, 2024, 11:00~12:30, Samsung Library 7F Creative Learning Room, SKKU
[Talk 1]
Exploring engineered interacting quantum systems in multilayer graphene superlattices
Joonho Jang (Seoul National University, Korea)
A Bilayer of semiconducting 2D electronic systems has long been a versatile platform to study electronic correlation with tunable interlayer tunneling, Coulomb interactions and layer imbalance. In the natural graphite bilayer, Bernal-stacked bilayer graphene (BBG), the Landau level gives rise to an intimate connection between the valley and layer. Adding a moiré superlattice potential enriches the BBG physics with the formation of topological minibands, potentially leading to tunable exotic quantum transports. Further increasing the number of layers is expected to rapidly expand the possible phase space one can explore to tune the interplay between the electronic correlation and band topology.
In this talk, I will present our recent magneto-transport measurements of a high-quality bilayer graphene-hexagonal boron nitride (hBN) heterostructure. The zero-degree alignment between the bilayer graphene and hBN generates a strong moiré superlattice potential for the electrons in BBG and the resulting Landau fan diagram of longitudinal and Hall resistance displays a Hofstadter butterfly pattern with an unprecedented level of details. Our work demonstrates that the intricate relationship between valley and layer degrees of freedom controls the topology of moiré-induced bands, significantly influencing the energetics of interacting quantum phases in the BBG superlattice. We further observe signatures of field-induced correlated insulators and clear fractional quantization of interaction driven topological quantum phases, such as fractional Chern insulators. Finally, I will discuss the important considerations in utilizing multilayer graphene heterostructures as ideal platforms to study the delicate interplay between topology and electron correlation.
[Talk 2]
Characteristics of superconducting gaps and nematic charge bond orders in Cs(V1-xTix)3Sb5
Keehoon Kim (Seoul National University, Korea)
CsV3Sb5 exhibits several emergent phenomena, including superconductivity (TC ~ 3.2 K), charge-density wave (TCDW ~ 98 K), and nematicity (Tnem ~ 36 K). Despite tremendous investigations for several years, the nature of the superconductivity and multiple electronic orders, as well as the relationship among them, remain elusive. For instance, previous studies show that phase diagrams of both pressurizing and Sn doping of CsV3Sb5 exhibit double superconducting domes under the charge-density wave order, but its origin is not clear. In this study, we have investigated nematic susceptibility and thermal conductivity in a broad doping and temperature range in high-quality single crystals of Cs(V1-xTix)3Sb5 (x = 0 – 0.06) where a double-dome superconducting phase diagram is realized. In most doping ranges, the nematic susceptibility exhibits the Curie‒Weiss behavior above Tnem and Ti doping systematically suppresses the Curie‒Weiss temperature from ~30 K for x = 0 to ~4 K for x=0.0075, resulting in a sign change at x = ~0.009, where the first superconducting dome exists. Furthermore, the Curie constant reaches a maximum at x = 0.01, suggesting a drastic enhancement of the nematic susceptibility near a putative nematic quantum critical point (NQCP) at x = ~0.009. We have also found that the thermal conductivity of samples within the first superconducting dome exhibited anomalous responses to the external fields, including the gapless superconducting state, whereas typical two-gap superconducting behavior was observed at the sample for x=0.06 located within the second dome. These observations suggest the presence of the unusual superconducting phase within the first dome of the phase diagram.
[1] Y. Sur at al Nat. Commun. 14, 3899 (2023)
[2] K. Nam et al. in preparation (2024)
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