The discovery of topological semimetal phase in three-dimensional (3D) systems is a new breakthrough in topological material research. Dirac nodal-line semimetal is one of the three topological semimetal phases discovered so far; it is characterized by linear band crossing along a line/loop, contrasted with the linear band crossing at discrete momentum points in 3D Dirac and Weyl semimetals. The study of nodal-line semimetal is still at initial stage; only three material systems have been verified to host nodal line fermions until now, including PbTaSe2 [1], PtSn4 [2]and ZrSiS [3]. In this letter, we report evidence of nodal line fermions in ZrSiSe and ZrSiTe probed in de Haas-van Alphen (dHvA) quantum oscillations. Although ZrSiSe and ZrSiTe share similar layered structure with ZrSiS, our measurements of angular dependences of dHvA oscillations indicate the Fermi surface (FS) enclosing Dirac nodal line is of 2D character in ZiSiTe, in contrast with 3D-like FS in ZrSiSe and ZrSiS. Another important property revealed in our experiment is that the nodal line fermion density in ZrSi(S/Se) (~ 10 20 -10 21 cm -3 ) is much higher than the Dirac/Weyl fermion density of any known topological materials. In addition, we have demonstrated ZrSiSe and ZrSiTe single crystals can be thinned down to 2D atomic thin

*Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies have addressed the general operation of graphene-based photothermoelectric devices and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster timescale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 fs. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 fs laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity of between 500 and 1,500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications. Photovoltage generation through the photothermoelectric (PTE) effect occurs when light is focused at the interface of monolayer and bilayer graphene, or at the interface between regions of graphene with different Fermi energies E F (refs 1-6). In such graphene PTE devices-which operate over a large spectral range 7,8 that extends even into the far-infrared 9 -local heating of electrons by absorbed light, in combination with a difference in Seebeck coefficients between the two regions, gives rise to a PTE voltage V PTE = (S 2 − S 1 )(T el − T 0 ). Here, S 1 and S 2 are the Seebeck coefficients of regions 1 and 2, respectively, T el is the hot electron temperature after photoexcitation and electron heating, and T 0 is the temperature of the electrode heat sinks. The performance of PTE graphene devices is intimately connected to the dynamics of the photoexcited electrons and holes, which have mainly been studied in graphene samples through ultrafast optical pumpprobe measurements [10][11][12][13][14][15][16][17] . As shown in Fig. 1a, the dynamics start with (i) photoexcitation and electron-hole pair generation, followed by (ii) electron heating through carrier-carrier scattering, in competition with lattice heating, both of which take place on a sub-100 fs timescale, and finally (iii) electron cooling by thermal equilibration with the lattice, which takes place on a picosecond timescale. The effect of the picosecond cooling step (iii) on the switching speed of graphene devices has been studied using timeresolved photovoltage scanning experiments with ∼200 fs time resolution [18][19][20] . These studies showed that the picosecond electron cooling time limits the intrinsic photo-switching rate of these devices to a few hundred gigahertz, because faster switching would reduce the switching contrast, as the system does not have time to return to the ground state. Indeed, gigahertz switching speeds have been demonstrated in graphene-based devices [21][22][23]...

Many physical phenomena can be understood by single-particle physics; that is, treating particles as non-interacting entities. When this fails, many-body interactions lead to spontaneous symmetry breaking and phenomena such as fundamental particles' mass generation, superconductivity and magnetism. Competition between single-particle and many-body physics leads to rich phase diagrams. Here we show that rhombohedral-stacked trilayer graphene offers an exciting platform for studying such interplay, in which we observe a giant intrinsic gap B42 meV that can be partially suppressed by an interlayer potential, a parallel magnetic field or a critical temperature B36 K. Among the proposed correlated phases with spatial uniformity, our results are most consistent with a layer antiferromagnetic state with broken time reversal symmetry. These results reflect the interplay between externally induced and spontaneous symmetry breaking whose relative strengths are tunable by external fields, and provide insight into other low-dimensional systems.

Graphene is nature's thinnest elastic membrane, and its morphology has important impacts on its electrical, mechanical, and electromechanical properties. Here we report manipulation of the morphology of suspended graphene via electrostatic and thermal control. By measuring the out-of-plane deflection as a function of applied gate voltage and number of layers, we show that graphene adopts a parabolic profile at large gate voltages with inhomogeneous distribution of charge density and strain. Unclamped graphene sheets slide into the trench under tension; for doubly clamped devices, the results are well-accounted for by membrane deflection with effective Young's modulus E = 1.1 TPa. Upon cooling to 100 K, we observe buckling-induced ripples in the central portion and large upward buckling of the free edges, which arises from graphene's large negative thermal expansion coefficient.

ABA-stacked trilayer graphene is a unique 2D electron system with mirror reflection symmetry and unconventional quantum Hall effect. We present low-temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν = -8, -2, 2, 6, and 10, which is in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau level (LL). Under an out-of-plane electric field E(perpendicular), we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E(perpendicular) selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene.