One of the intrinsic drift velocity limit of the quantum Hall effect is the
collective magneto-exciton (ME) instability. It has been demonstrated in
bilayer graphene (BLG) using noise measurements. We reproduce this experiment
in monolayer graphene (MLG), and show that the same mechanism carries a direct
relativistic signature on the breakdown velocity. Based on theoretical
calculations of MLG- and BLG-ME spectra, we show that Doppler-induced
instabilities manifest for a ME phase velocity determined by a universal value
of the ME conductivity, set by the Hall conductance.

Strong electric field annihilation by particle--antiparticle pair creation, also known as the Schwinger effect, is a non-perturbative prediction of quantum electrodynamics. Its experimental demonstration remains elusive, as threshold electric fields are extremely strong and beyond current reach. Here, we propose a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac fermions with an approximate electron--hole symmetry. Using transport measurements, we report on universal one-dimensional Schwinger conductance at the pinchoff of ballistic graphene transistors. Strong pinchoff electric fields are concentrated within approximately 1thinspacemum of the transistor's drain and induce Schwinger electron--hole pair creation at saturation. This effect precedes a collective instability towards an ohmic Zener regime, which is rejected at twice the pinchoff voltage in long devices. These observations advance our understanding of current saturation limits in ballistic graphene and provide a direction for further quantum electrodynamic experiments in the laboratory.

Electroluminescence, a non-thermal radiative process, is ubiquitous in
semi-conductors and insulators but fundamentally precluded in metals. We show
here that this restriction can be circumvented in high-quality graphene. By
investigating the radiative emission of semi-metallic graphene field-effect
transistors over a broad spectral range, spanning the near- and mid-infrared,
we demonstrate direct far-field electroluminescence from hBN-encapsulated
graphene in the mid-infrared under large bias in ambient conditions. Through a
series of test experiments ruling out its incandescence origin, we determine
that the electroluminescent signal results from the electrical pumping produced
by interband tunneling. We show that the mid-infrared electroluminescence is
spectrally shaped by a natural quarter-wave resonance of the heterostructure.
This work invites a reassessment of the use of metals and semi-metals as
non-equilibrium light emitters, and the exploration of their intriguing
specificities in terms of carrier injection and relaxation, as well as emission
tunability and switching speed.

Raman spectroscopy is widely used to assess the quality of 2D materials thin
films. This report focuses on $rmPtSe_2$, a noble transition metal
dichalcogenide which has the remarkable property to transit from a
semi-conductor to a semi-metal with increasing layer number. While
polycrystalline $rmPtSe_2$ can be grown with various cristalline qualities,
getting insight into the monocrystalline intrinsic properties remains
challenging. We report on the study of exfoliated 1 to 10 layers $rmPtSe_2$
by Raman spectroscopy, featuring record linewidth. The clear Raman signatures
allow layer-thickness identification and provides a reference metrics to assess
crystal quality of grown films.

Platinum diselenide (PtSe2) is a promising two-dimensional (2D) material for
the terahertz (THz) range as, unlike other transition metal dichalcogenides
(TMDs), its bandgap can be uniquely tuned from a semiconductor in the
near-infrared to a semimetal with the number of atomic layers. This gives the
material unique THz photonic properties that can be layer-engineered. Here, we
demonstrate that a controlled THz nonlinearity - tuned from monolayer to bulk
PtSe2 - can be realised in wafer size polycrystalline PtSe2 through the
generation of ultrafast photocurrents and the engineering of the bandstructure
valleys. This is combined with the PtSe2 layer interaction with the substrate
for a broken material centro-symmetry permitting a second order nonlinearity.
Further, we show layer-dependent circular dichroism, where the sign of the
ultrafast currents and hence the phase of the emitted THz pulse can be
controlled through the excitation of different bandstructure valleys. In
particular, we show that a semimetal has a strong dichroism that is absent in
the monolayer and few layer semiconducting limit. The microscopic origins of
this TMD bandstructure engineering is highlighted through detailed DFT
simulations and show that circular dichroism can be controlled when PtSe2
becomes a semimetal and when the K-valleys can be excited. As well as showing
that PtSe2 is a promising material for THz generation through layer controlled
optical nonlinearities, this work opens up new class of circular dichroism
materials beyond the monolayer limit that has been the case of traditional
TMDs, and impacting a range of domains from THz valleytronics, THz spintronics
to harmonic generation.

In view of the extensive use of hexagonal boron nitride (hBN) in 2D material electronics, it becomes important to refine its dielectric characterization in terms of low-field permittivity and high-field strength and conductivity up to the breakdown voltage. The present study aims at filling this gap using DC and RF transport in two Au-hBN-Au capacitor series of variable thickness in the 10–100 nm range, made of large high-pressure, high-temperature (HPHT) crystals and a polymer derivative ceramics (PDC) crystals. We deduce an out-of-plane low field dielectric constant ϵ ∥ = 3.4 ± 0.2 consistent with the theoretical prediction of Ohba et al, that narrows down the generally accepted window ϵ ∥ = 3–4. The DC-current leakage at high-field is found to obey the Frenkel-Pool law for thermally-activated trap-assisted electron transport with a dynamic dielectric constant ϵ ∥ ≃ 3.1 and a trap energy Φ B ≃ 1.3 eV, that is comparable with standard technologically relevant dielectrics.

High-mobility hexagonal boron nitride (hBN)/graphene/hBN heterostructures are able to reach intrinsic limits of transport. Here, we investigate optoelectronic mixing, which is a demanding function combining efficient photodetection and fast carrier dynamics. Using such a heterostructure embedded in a coplanar waveguide, we obtain a record conversion efficiency of about −40 dB for frequencies up to 65thinspaceGHz. This performance is obtained at high doping in the photobolometric regime. We provide a microscopic model of the photodetection, which accurately describes the experimental observations, allows the assessment of the intrinsic limits of our device, and paves the way for device optimization by revealing the different mechanisms at play.

Abstract Engineering of the cooling mechanism is of primary importance for the development of nanoelectronics. While radiation cooling is rather inefficient in current electronic devices, the strong anisotropy of 2D materials allows for enhanced efficiency because their hyperbolic electromagnetic dispersion near phonon resonances allows them to sustain much larger (≈105) number of radiating channels. In this review, radiation cooling in 2D materials is addressed. The hyperbolic dispersion of electromagnetic waves is presented, and how the spontaneous fluctuations in current in a 2D electronic channel can radiate thermal energy in its hyperbolic surrounding medium is described. Both the regime of thermal current fluctuations and out-of-equilibrium current fluctuations can be described within the same framework leading to superPlanckian thermal emission and electroluminescent cooling. A recent experimental investigation on graphene-on-hBN transistors using electronic noise thermometry is discussed. In high mobility semimetal-like graphene at large bias, a steady-state out-of-equilibrium situation is caused by Zener tunneling of electrons, opening a route for electroluminescence of hyperbolic electromagnetic modes. Electroluminescent cooling is particularly prominent once the Zener tunneling regime is reached: observed cooling powers are nine orders of magnitude larger than in conventional LEDs.

The current understanding of physical principles governing electronic transport in graphene field effect transistors (GFETs) has reached a level where we can model quite accurately device operation and predict intrinsic frequency limits of performance. In this work, we use this knowledge to analyze DC and RF transport properties of bottom-gated graphene on boron nitride field effect transistors exhibiting pronounced velocity saturation by substrate hyperbolic phonon polariton scattering, including Dirac pinch-off effect. We predict and demonstrate a maximum oscillation frequency exceeding 20   GHz . We discuss the intrinsic 0.1   THz limit of GFETs and envision plasma resonance transistors as an alternative for sub-THz narrow-band detection.

Electrostatic gating is pervasive in materials science, yet its effects on the electronic band structure of materials has never been revealed directly by angle-resolved photoemission spectroscopy (ARPES), the technique of choice to noninvasively probe the electronic band structure of a material. By means of a state-of-the-art ARPES setup with submicron spatial resolution, we have investigated a heterostructure composed of Bernal-stacked bilayer graphene (BLG) on hexagonal boron nitride and deposited on a graphite flake. By voltage biasing the latter, the electric field effect is directly visualized on the valence band as well as on the carbon 1s core level of BLG. The band gap opening of BLG submitted to a transverse electric field is discussed and the importance of intra layer screening is put forward. Our results pave the way for new studies that will use momentum-resolved electronic structure information to gain insight on the physics of materials submitted to the electric field effect.

The engineering of cooling mechanisms is a bottleneck in nanoelectronics. Thermal exchanges in diffusive graphene are mostly driven by defect-assisted acoustic phonon scattering, but the case of high-mobility graphene on hexagonal boron nitride (hBN) is radically different, with a prominent contribution of remote phonons from the substrate. Bilayer graphene on a hBN transistor with a local gate is driven in a regime where almost perfect current saturation is achieved by compensation of the decrease in the carrier density and Zener--Klein tunnelling (ZKT) at high bias. Using noise thermometry, we show that the ZKT triggers a new cooling pathway due to the emission of hyperbolic phonon polaritons in hBN by out-of-equilibrium electron--hole pairs beyond the super-Planckian regime. The combination of ZKT transport and hyperbolic phonon polariton cooling renders graphene on BN transistors a valuable nanotechnology for power devices and RF electronics.