Interactions between particles in quantum many-body systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge-neutrality point, which can form a strongly coupled Dirac fluid. This charge-neutral plasma of quasi-relativistic fermions is expected to exhibit a substantial enhancement of the thermal conductivity, thanks to decoupling of charge and heat currents within hydrodynamics. Employing high-sensitivity Johnson noise thermometry, we report an order of magnitude increase in the thermal conductivity and the breakdown of the Wiedemann-Franz law in the thermally populated charge-neutral plasma in graphene. This result is a signature of the Dirac fluid and constitutes direct evidence of collective motion in a quantum electronic fluid.

Abstract:We develop a general hydrodynamic framework for computing direct current thermal and electric transport in a strongly interacting finite temperature quantum system near a Lorentzinvariant quantum critical point. Our framework is non-perturbative in the strength of long wavelength fluctuations in the background charge density of the electronic fluid, and requires the rate of electron-electron scattering to be faster than the rate of electron-impurity scattering. We use this formalism to compute transport coefficients in the Dirac fluid in clean samples of graphene near the charge neutrality point, and find results insensitive to long range Coulomb interactions. Numerical results are compared to recent experimental data on thermal and electrical conductivity in the Dirac fluid in graphene and substantially improved quantitative agreement over existing hydrodynamic theories is found. We comment on the interplay between the Dirac fluid and acoustic and optical phonons, and qualitatively explain experimentally observed effects. Our work paves the way for quantitative contact between experimentally realized condensed matter systems and the wide body of high energy inspired theories on transport in interacting many-body quantum systems.

We propose to use graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum from visible to radio frequencies. Our approach takes advantage of the exceptionally low electronic heat capacity of monolayer graphene and its constricted thermal conductance to its phonon degrees of freedom. Such a system could provide high-sensitivity photon detection required for research areas including quantum information processing and radio astronomy. As an example, we present our device concepts for GJJ single-photon detectors in both the microwave and infrared regimes. The dark count rate and intrinsic quantum efficiency are computed based on parameters from a measured GJJ, demonstrating feasibility within existing technologies.

We develop a high frequency, wide bandwidth radiometer operating at room temperature, which augments the traditional technique of Johnson noise thermometry for nanoscale thermal transport studies. Employing low noise amplifiers and an analog multiplier operating at 2 GHz, auto-and cross-correlated Johnson noise measurements are performed in the temperature range of 3 to 300 K, achieving a sensitivity of 5.5 mK (110 ppm) in 1 second of integration time. This setup allows us to measure the thermal conductance of a boron nitride encapsulated monolayer graphene device over a wide temperature range. Our data shows a high power law (T ∼4 ) deviation from the Wiedemann-Franz law above T∼100 K.

Guided modes in anisotropic two-dimensional van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550-700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98-2.12 and 1.45-2.12, respectively. Our approach of using near-field scanning optical microscopy allows for direct study of interaction between light and two-dimensional van der Waals materials and heterostructures. KEYWORDS: 2D materials, near-field scanning optical microscopy, optical constants, hexagonal boron nitride Two-dimensional (2D) materials have recently garnered significant interest due to their high electrical mobility, atomic-level flatness, and large exciton binding energies which enable versatile nanophotonics and optoelectronics applications [1-23]. However, the understanding of the interaction of visible light surface plasmon polaritons with 2D materials and more complex van der Waals (vdW) heterostructures [5-7] is still in its infancy [9, 23]. In this work, we report a nearfield scanning optical microscopy (NSOM) [25-28] study of highly crystalline hexagonal boron nitride (h-BN) [21] mechanically-exfoliated flakes at visible wavelengths, demonstrating the direct observation of (i) transverse electric (TE) waveguide modes supported by h-BN on SiO2 and (ii) the interaction of transverse magnetic (TM) surface plasmon polariton (SPP) modes supported by silver (Ag) with a thin h-BN flake. From NSOM scans, we estimate the intrinsically anisotropic optical dielectric constants ( = ≠ ) of h-BN, which is an arduous task by conventionalmethods such as ellipsometry [24]. Our technique can be extended to other vdW solids and heterostructures, where we anticipate the study of guided modes coupled to 2D materials to be a useful tool in exploring rich physics of surface polaritons [9-14], plexcitons (SPPs-excitons) [29], gate-tunable [17, 30, 31], layer number dependent optical properties [13], in-plane anisotropy [15, 23], and selective circular dichroism [18, 19].