Talks

1T-TaS2 exhibits several resistivity phases due to the modulation of charge density wave (CDW). The fact that such phase transition can be driven electrically has attracted a lot of attention in the recent past toward active-metal based electronics. However, the bias-driven resistivity switching is not very large (less than five-fold), and an enhancement in the same will highly impact such phase transition devices. One aspect that is often overlooked is that such phase transition is also accompanied by a significant change in the local temperature due to the low thermal conductivity of 1T-TaS2. In this work, such electrically driven phase transition induced temperature change is exploited to promote carriers over a thermionic barrier in a 1T-TaS2/2H-TaSe2/2H-MoS2 T-Junction, achieving a 964-fold abrupt switching in the current through the MoS2 channel. The device is highly reconfigurable and exhibits an abrupt reduction in current as well when the biasing configuration changes. The res...

In recent years, tuning electrical polarization by magnetic field and vice versa in multiferroic and magnetoelectric materials received much attention. Impedance spectroscopy is frequently used to probe magnetic-field dependence of capacitance in these materials. However, impedance spectroscopy has been rarely used to study metallic or semiconducting oxides above 1 MHz and below 10 GHz. In this talk, I will show selected applications of the impedance spectroscopy to electrically detect spin resonance, domain magnetization processes and spin/charge conversion. Extension of this technique to 2D materials will also be outlined.

Cuscuton field theory is an extension of general relativity that does not introduce additional propagating degrees of freedom, or violate relativistic causality. We construct a general geometric description of the cuscuton field theory by introducing curvature corrections to both the volume (potential) and the surface (kinetic) terms in the original cuscuton action. Our assumptions involve a stack of spacelike branes, separated by 4-dimensional bulks. We conjecture that the cuscuton, initially a discrete field, becomes continuous in the limit, there are many such transitions. From this we derive an effective action for the cuscuton theory and show that at the quadratic level our theory propagates only the two tensorial degrees of freedom.

I will first introduce screened modified gravity theories and then discuss the chameleon mechanism. Light scalars can be produced from the sun and detected on earth. I will discuss the production of chameleons, including novel production channels, and discuss potential detection in helioscopes.

Layered nickelates have been of interest since the early days of high-Tc superconducting (SC) cuprates as possible additional representants of unconventional superconductors. But only in 2019, a stable SC phase has been identified in thin-films of Sr-doped NdNiO2 with a Tc∼ 20 K [1]. Further SC members from this class of low-valence nickelates have been spotted afterwards. And just when the debate about the similarity between SC cuprates and nickelates, both with akin 3d9−x formal transition-metal valence, was at its zenith, a SC bilayer nickelate of formal 3d8−x valence was detected at high pressure with Tc∼ 80 K in spring 2023 [2]. Interestingly, according to our theoretical investigations [3,4] all these SC nickelates have a multiorbital Ni-eg flat-band scenario in common.

In this talk, it will be shown that an advanced combination of density functional theory (DFT) and dynamical mean-field theory (DMFT) provides unique access to this novel playground of high-Tc nickelate superco...

Graphene moiré superlattices host van Hove singularities appear at low energies, which are malleable with progressive band filling, leading to a sequence of Lifshitz transitions and resets observable in Hall measurements. However, at zero magnetic fields, transport measurements in the linear response regime have limited sensitivity to the band's topology. Here, we probe these unique features in twisted bilayer graphene at zero magnetic field using second-order transport measurements. We demonstrate that the nonlinear responses, induced by the Berry curvature dipole and extrinsic scattering processes, intricately map the Fermi surface reconstructions at various partial fillings of the band. Importantly, our study confirms that the applied magnetic field does not induce or stabilize the probed transitions, highlighting these features as intrinsic to the moiré bands. Additionally, we show the tunability of the Berry curvature dipole and extrinsic scattering process with an out-of-plane ...

Van der Waals (vdW) heterostructures of two-dimensional materials offer an unprecedented playground to combine materials with different electronic properties, without the constraints of lattice matching associated with epitaxial growth. Recent years have witnessed the emergence of interlayer twist as a new parameter that control the electronic properties of vdW heterostructures. This presentation will provide an overview of experimental techniques to control interlayer twist, with an emphasis on twist-controlled double layers. We show that interlayer tunnelling serves as unique tool to probe interlayer coherence in twist-aligned, closely spaced double layers where interaction leads to a coherent superposition of electronic states in individual layers, with Josephson junction-like tunnelling characteristics robust to temperature, and layer density detuning.

There has been a lot of recent interest in heterostructures of van der Waals materials, with the easy exfoliation of each layer allowing for novel structures to be constructed. In the hierarchy of interactions, the van der Waals interactions are the weakest, so finding unconventional phenomena merely by changing small details of how these materials are stacked seems puzzling. In this talk I will consider a family of materials, and show how rotating one layer with respect to the other leads to unconventional behavior [1-4].

Although d+id’ superconductors are topological, the Majorana zero modes therein occur in pairs. This talk will present the Fu-Kane setup between a three-dimensional topological insulator and a d+id’ superconductor to show that an unpaired Majorana Zero mode can be realized in the core of the Abrikosov vortex. The d+id superconducting order can be realized in High-Tc superconductors, thus these Majorana modes can persist up to unprecedented high temperatures.