Talks

Hall effect is one of the most studied phenomena in condensed matter physics. In its linear form, it relates the electrical potential difference perpendicular to the electrical current via a direct proportionality. Beyond the linear response regime, a recent theoretical proposal pointed to a novel form of Hall effect induced by a Berry curvature dipole, where the electric current in the nonlinear regime is always orthogonal to the local electric field and. The proposed nonlinear Hall effect unlocks an outstanding experimental challenge to properly probe, analyze, and understand the mechanism underlying transport response in the nonlinear regime. In this talk, we report a new scheme to measure and analyze nonlinear transport response from Bernal bilayer graphene. Based on angle-resolved transport measurement, we extract the nonlinear conductivity tensor and identify a non-zero component that corresponds to the nonlinear Hall effect. By examining the evolution of the conductivity tensor ...

Fractional quantum Hall (FQH) phases emerge due to strong electronic interactions and are characterized by anyonic quasiparticles, each distinguished by unique topological parameters, fractional charge, and statistics. In contrast, the integer quantum Hall (IQH) effects can be understood from the band topology of non-interacting electrons. In this talk, I will report a surprising super-universality of the critical behavior across all FQH  and IQH transitions. Contrary to the anticipated state-dependent critical exponents, our findings reveal the same critical scaling exponent $\kappa = 0.41 \pm 0.02$ and localization length exponent $\gamma = 2.4 \pm 0.2$ for fractional and integer quantum Hall transitions. From these, we extract the value of the dynamical exponent $z\approx 1$. We have achieved this in ultra-high mobility trilayer graphene devices with a metallic screening layer close to the conduction channels. The observation of these global critical exponents across various quantum...

The low-frequency fluctuations, or noise, in electrical resistance not only set a performance benchmark in devices, but also form a sensitive tool to probe non-trivial electronic phases and band structure in solids. Here we report the measurement of such noise in the electrical resistance in twisted bilayer graphene (tBLG), where the layers are misoriented close to the magic angle (θ ∼ 1 degree). At high temperatures (T >∼ 60 − 70 K), the power spectral density (PSD) of the fluctuations inside the low-energy moir ́e bands is predominantly ∝ 1/f, where f is the frequency, being generally lowest close to the magic angle, and can be well-explained within the conventional Mc. Whorter model of the ‘1/f noise’ with trap-assisted density-mobility fluctuations. At low T (

2D van der Waals materials-based heterostructures have led to new devices for fundamental science and applications. Superconducting Josephson devices based on 2D materials offer unique opportunities to engineer new functionality for quantum technology. I will present results from two classes of materials. First, proximitized graphene-based Josephson junctions lead to a quantum noise-limited parametric amplifier with performance comparable to best discrete amplifiers in this class [1]. Gate tunability of the center frequency of the amplifier, rather than flux, offers key advantages. An extension of graphene Josephson architecture to make state-of-the-art bolometers leveraging graphene's low specific heat, and I will present initial results. Second, twisted van der Waals heterostructures based on high Tc superconductor Bi2Sr2CaCu2O8+δ lead to the realization of a high-temperature Josephson diode [2] for the first time. Such Josephson diodes offer an opportunity to engineer the current ph...

Illuminating a material with light can reveal both interesting aspects of electronic and lattice degrees of freedom, as well as drive phase and topological transitions in the material itself. In this talk, I will focus on three distinct responses of a material to light: (1) Nonlinear phononic control of magnetism in bilayer CrI3, MnBi2Te4, and MnSb2Te4. (2) The non-linear photogalvanic response of Weyl semimetals with tilted cones and chiral charge up to 4 (the largest allowed in a lattice model), as well as the topological superconductor candidate 4Hb-TaS2, and (3) The coupling of phonons to electronic degrees of freedom to produce chiral phonons with large g-factors of order 1, which can be measured with Raman scattering. For nonlinear phononic control of magnetism, I will show how intense THz light can be used to transiently modify magnetic exchange constants, including their sign. In the case of the non-linear current response of Weyl systems, I will review how the quantum geometry...