*Talk(35mins)+ Q&A(10mins) 

Suk Bum Chung (University of Seoul)

Title: Superconductivity in the doped quantum paraelectrics Nb-doped SrTiO3

Abstract: Recent experiments on Nb-doped SrTiO3 have shown that the superconducting energy gap to the transition temperature ratio maintains the Bardeen-Cooper-Schrieffer (BCS) value throughout its superconducting dome. Motivated by these and related studies, we show that the Cooper pairing mediated by a single soft transverse-optical phonon is the most natural mechanism for such a superconducting dome given experimental constraints, and present the microscopic theory for this pairing mechanism.  Furthermore, we show that this mechanism is consistent with the T2 resistivity in the normal state. Lastly, we discuss what physical insights SrTiO3 provides for superconductivity in other quantum paraelectrics such as KTaO3.

Keun Su Kim (Yonsei University)

Title: Pseudogap in a crystalline insulator doped by disordered dopants

Abstract: The tunable band structure of two-dimensional quantum mater is not only interesting in itself, but also useful for the fundamental study of condensed matter physics. As example, surface chemical doping can be used to reduce the band gap of black phosphorus over the energy range greater than its intrinsic band gap [1]. This could be used to deliberately induce a topological phase transition to a Dirac semimetal phase protected by spacetime inversion symmetry [2]. It could also be used to trace the evolution of quantum phases (pseudospin) order across the topological phase transition [3]. In this talk, I will introduce our recent study on the band renormalizations and pseudogap of black phosphorus doped by alkali metals [4]. Using angle-resolved photoemission spectroscopy (ARPES), we found that the simple quadratic band dispersion of doped black phosphorus anomalously bends back towards zero wavenumber, which can be explained by Anderson-McMillan’s framework developed for the band structure of liquid (or disordered) metals in the 1960s. This is a natural consequence of resonance scattering by the potential of dopant ions with short-range order. The depth of scattering potential tuned by different kinds of alkali metal (Na, K, Rb, and Cs) allows us to classify the pseudogap of p-wave and d-wave resonance.

1. J. Kim et al., Science 349, 723 (2015).

2. J. Kim et al., Phys. Rev. Lett. 119, 226801 (2017).

3. S. W. Jung, S. H. Ryu et al., Nature Mater. 19, 277 (2020).

4. S. H. Ryu, M. Huh, D. Y. Park et al., Nature 596, 68 (2021).

Seung-Sup Lee (Seoul National University)

Title: Multipoint Correlation Functions: Spectral Representation and Numerical Evaluation

Abstract: Many observables in the experiments of condensed matter and ultracold atoms, such as the conductivity and inelastic scattering spectra, are theoretically described as multipoint correlation functions. However, the analytic structure of multipoint functions has been elusive, and their non-perturbative, accurate computations have been intractable. Here I will talk about our recent two papers that provide a breakthrough in solving these problems. We derived the spectral representations of general multipoint functions, which reveal their structure with great clarity [1]. And we developed a numerical renormalization group (NRG) method for evaluating the spectral representations for local multipoint functions, which can treat temperatures and frequencies—imaginary or real—of all magnitudes, from large to arbitrarily small ones [2]. I will present the numerical results of four-point vertex functions and resonant inelastic x-ray scattering (RIXS) spectra of quantum impurity systems.

[1] F. B. Kugler*, S.-S. B. Lee*, and J. von Delft, Phys. Rev. X 11, 041006 (2021).
   *: These authors contributed equally to this work.
[2] S.-S. B. Lee, F. B. Kugler, and J. von Delft, Phys. Rev. X 11, 041007 (2021).

Jieun Lee (Seoul National University)

Title: Layer-dependent and air-stable ferromagnetism in van der Waals CrPS4

Abstract: Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI3, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. In this talk, we report the observation of an air-stable and layer-dependent magnetism in van der Waals crystal, CrPS4, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, CrPS4 in 1L form is found to be ferromagnetic (FM) with an out-of-plane spin orientation. Furthermore, measurements on 1L to 5L samples show AFM and FM properties in even and odd layers, respectively, suggesting that the interlayer coupling is governed by robust antiferromagnetic exchange interactions. The observed magnetism in these crystals remains resilient even after the exposure to air in ambient conditions, showing potentials for the practical applications in van der Waals spintronics.

Youngkuk Kim (Sungkyunkwan University) 

Title: Monolayer kagome metals AV3Sb5 (A=K, Rb, and Cs)

Abstract: We present our first-principles and mean-field studies of monolayer kagome metals AV3Sb5 (A = K, Rb, and Cs). Recently, layered kagome AV3Sb5 has attracted much attention as a fertile platform for strong correlation and topology. We suggest that AV3Sb5 should be a more affluent playground when formed in the genuine two-dimensional monolayer. Notably, dimensionality significantly affects the van Hove singularities' numbers and types. Exotic correlated states consequently occur when including the extended Hubbard-type electron-electron interactions. We compute electronic phase diagrams, which comprise diverse novel orders, including doublets of charge density waves and s- and d-wave superconductivity. Enhanced tunability is achieved due to its two-dimensional nature, enabling access to diverse electronic states via strains and chemical substitution. Our finding suggests the advent of a novel platform for exploring exotic correlated phases in two dimensions.

Youngwook Kim (DGIST)

Title: Exotic quantum Hall phases in partially-filled Landau level of graphene and twisted bilayer graphene

Abstract: An important development in the field of the fractional quantum Hall effect was the proposal that the 5/2 state observed in the Landau level with orbital index n = 1 of two-dimensional electrons in a GaAs quantum well originates from a chiral p-wave paired state of composite fermions that are topological bound states of electrons and quantized vortices. The excitations of this state, which is theoretically described by a ‘Pfaffian’ wavefunction or its hole partner called the anti-Pfaffian are neither fermions nor bosons but Majorana quasiparticles obeying non-Abelian braid statistics. This has inspired ideas for fault-tolerant topological quantum computation and has also instigated a search for other states with exotic quasiparticles. In first part, we discuss experiments on monolayer graphene that show clear evidence for unexpected even denominator fractional quantum Hall physics in the n = 3 Landau level.1 We numerically investigate the known candidate states for the even denominator fractional quantum Hall effect, including the Pfaffian, the particle–hole symmetric Pfaffian and the 221-parton states, and conclude that, among these, the 221-parton appears a potentially suitable candidate to describe the experimentally observed state. Like the Pfaffian, this state is believed to harbour quasi-particles with non-Abelian braid statistics.

In second part, we show on the quantum Hall effect in non-magic angle twisted bilayer graphene.2,3 The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is observed. We interpret this as a signature of Coulomb interaction induced interlayer coherence at half filling of each layer. A density imbalance gives rise to reentrant behavior due to a phase transition from the interlayer coherent state to incompressible behavior caused by simultaneous condensation of both layers in different quantum Hall states. With increasing overall density, magnetic breakdown gains the upper hand. As a consequence of the enhanced interlayer tunneling, the interlayer coherent state and the phase transition vanish.

References

1. Y. Kim et al, Nat. Phys. 15, 154-158 (2019).

2. Y. Kim et al, Nano, Lett. 21, 4249-4254 (2021)

3. D. Kim† and B. Kang† et al, In preparation (2022)

Hyobin Yoo (Sogang University)

Title: Operando TEM investigation on the domain dynamics in 2D ferroelectrics

Abstract: Control of interlayer stacking angle in two-dimensional (2-D) van der Waals (vdW) heterostructure enables one to engineer the crystal symmetry to imprint novel functionality. By stacking two layers of transition metal dichalcogenides (TMD) with designed twist angle, one can break the inversion symmetry and thereby develop vertical electric polarization. The direction of the electric polarization can be switched electrically, suggesting that the twisted bilayer TMD can host ferroelectricity. Such ferroelectricity reported in twisted bilayer vdW system is distinguished from conventional ferroelectrics in that the lateral sliding of the constituent layers induces vertical electric polarizations. Due to the reduced dimension, the ferroelectric domains do not require forward growth along the third dimension, suggesting unconventional 2-D domain dynamics under an applied electric field.

Here we employ operando transmission electron microscopy (TEM) to investigate the domain dynamics in 2-D vdW ferroelectrics. Operando TEM technique enables one to examine the structural change in the environment that mimics the device operating condition. On a thin SiN based TEM compatible platform, we fabricated double capacitor structure on 2D vdW ferroelectrics. Electrical gating in double capacitor structure and real time observation of structural change in a simultaneous manner provides an insight onto the switching mechanism of the 2-D vdW ferroelectrics.