We present Raman spectroscopy measurements on single-and few-layer graphene flakes. Using a scanning confocal approach we collect spectral data with spatial resolution, which allows us to directly compare Raman images with scanning force micrographs. Single-layer graphene can be distinguished from double-and few-layer by the width of the D' line: the single peak for single-layer graphene splits into different peaks for the double-layer. These findings are explained using the double-resonant Raman model based on ab-initio calculations of the electronic structure and of the phonon dispersion. We investigate the D line intensity and find no defects within the flake. A finite D line response originating from the edges can be attributed either to defects or to the breakdown of translational symmetry. The interest in graphite has been revived in the last two decades with the advent of fullerenes 1 and carbon nanotubes. 2 However, only recently single-and few-layer graphene could be transferred to a substrate. 3 Transport measurements revealed a highly-tunable two-dimensional electron/hole gas of relativistic Dirac Fermions embedded in a solid-state environment. 4,5 Going to few-layer graphene, however, disturbs this unique system in such a way that the usual parabolic energy dispersion is recovered. The large structural anisotropy makes few-layer graphene therefore a promising candidate to study the rich physics at the crossover from bulk to purely twodimensional systems. Turning on the weak interlayer coupling while stacking a second layer onto a graphene sheet leads to a branching of the electronic bands and the phonon dispersion at the K point. Double-resonant Raman scattering 6 which depends on electronic and vibrational properties turns out to be an ingenious tool to probe the lifting of that specific degeneracy.We report on Raman mapping of single-and few-layer graphene flakes resting on a silicon oxide substrate. Lateral resolution of 400 nm allows to address neighboring sections with various layers of graphene down to a single graphene sheet, previously determined with the scanning force microscope (SFM). We find that the integrated G line signal is directly correlated with the thickness of the graphitic flake and is shifted upward in frequency for double-and single-layer graphene compared to bulk graphite. The mapping of the peak width of the D' line shows a strong contrast between single-and few-layer graphene. Such a pronounced sensitivity to the transition to the very last layer offers an optical and nondestructive method to unambiguously detect single-layer graphene. In addition, we locally resolve the structural quality of the flake by investigating the D band, which is related to elastic backscattering. The map of the integrated D line signal of a graphitic flake with doubleand single-layer sections shows that the inner part of the flake is quasi defect free, whereas edges and steps serves as scatterers. Finally, we explain the splitting of the D' line as a function of the number of graphene layers with...

Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. Remarkably, an isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers to 1.1 • , resulting in the appearance of gate-tunable superconducting and correlated insulating phases. Here, we demonstrate that in addition to the twist angle, the interlayer coupling can also be modified to precisely tune these phases. We establish the capability to induce superconductivity at a twist angle larger than 1.1 • -in which correlated phases are otherwise absent -by varying the interlayer spacing with hydrostatic pressure. Realizing devices with low disorder additionally reveals new details about the superconducting phase diagram and its relationship to the nearby insulator. Our results demonstrate twisted bilayer graphene to be a uniquely tunable platform for exploring novel correlated states. arXiv:1808.07865v2 [cond-mat.mes-hall]

The discovery of superconductivity at 203 K in H3S 1 brought attention back to conventional superconductors whose properties can be described by the Bardeen-Cooper-Schrieffer (BCS) and the Migdal-Eliashberg theories. These theories predict that high, and even room temperature superconductivity (RTSC) is possible in metals possessing certain favorable parameters such as lattice vibrations at high frequencies. However, these general theories do not suffice to predict real superconductors. New superconducting materials can be predicted now with the aid of first principles calculations based on Density Functional Theory (DFT). In particular, the calculations suggested a new family of hydrides possessing a clathrate structure, where the host atom (Ca, Y, La) is at the center of the cage formed by hydrogen atoms 2-4 . For LaH10 and YH10 superconductivity, with critical temperatures Tc ranging between 240 and 320 K is predicted at megabar pressures 3-6 . Here, we report superconductivity with a record Tc  250 K within the Fm3m structure of LaH10 at a pressure P  170 GPa. We proved the existence of superconductivity at 250 K through the observation of zero-resistance, isotope effect, and the decrease of Tc under an external magnetic field, which suggests an upper critical magnetic field of 120 T at zerotemperature. The pressure dependence of the transition temperatures Tc (P) has a maximum of 250-252 K at the pressure of about 170 GPa. This leap, by  50 K, from the previous Tc record of 203 K 1 indicates the real possibility of achieving RTSC (that is at 273 K) in the near future at high pressures and the perspective of conventional superconductivity at ambient pressure.

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

The physical properties of two-dimensional van der Waals (2D vdW) crystals depend sensitively on the interlayer coupling, which is intimately connected to the stacking arrangement and the interlayer spacing. For example, simply changing the twist angle between graphene layers can induce a variety of correlated electronic phases 1-8 , which can be controlled further in a continuous manner by applying hydrostatic pressure to decrease the interlayer spacing 3 . In the recently discovered 2D magnets 9,10 , theory suggests that the interlayer exchange coupling strongly depends on layer separation, while the stacking arrangement can even change the sign of the magnetic exchange, thus drastically modifying the ground state 11-15 . Here, we demonstrate pressure tuning of magnetic order in the 2D magnet CrI3. We probe the magnetic states using tunneling 16,17 and scanning magnetic circular dichroism microscopy measurements 10 . We find that the interlayer magnetic coupling can be more than doubled by hydrostatic pressure. In bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phases. In trilayer CrI3, pressure can create coexisting domains of three phases, one ferromagnetic and two distinct antiferromagnetic. The observed changes in magnetic order can be explained by changes in the stacking arrangement. Such coupling between stacking order and magnetism provides ample opportunities for designer magnetic phases and functionalities. discussion.

The kagome lattice based on 3d transition metals is a versatile platform for novel topological phases hosting symmetry-protected electronic excitations and exotic magnetic ground states. However, the paradigmatic states of the idealized two-dimensional (2D) kagome lattice -Dirac fermions and topological flat bands -have not been simultaneously observed, partly owing to the complex stacking structure of the kagome compounds studied to date. Here, we take the approach of examining FeSn, an antiferromagnetic single-layer kagome metal with spatially-decoupled kagome planes. Using polarization-and termination-dependent angleresolved photoemission spectroscopy (ARPES), we detect the momentum-space signatures of coexisting flat bands and Dirac fermions in the vicinity of the Fermi energy. Intriguingly, when complemented with bulk-sensitive de Haas-van Alphen (dHvA) measurements, our data reveal an even richer electronic structure that exhibits robust surface Dirac fermions on specific crystalline terminations. Through band structure calculations and matrix element simulations, we demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals under kagome symmetry, while the surface state realizes a rare example of fully spin-polarized 2D Dirac fermions when combined with spin-layer locking in FeSn. These results highlight FeSn as a prototypical host for the emergent excitations of the kagome lattice. The prospect to harness these excitations for novel topological phases and spintronic devices is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated electron physics.

In the quest to increase the critical temperature T c of cuprate superconductors, it is essential to identify the factors that limit the strength of superconductivity. The upper critical field H c2 is a fundamental measure of that strength, yet there is no agreement on its magnitude and doping dependence in cuprate superconductors. Here we show that the thermal conductivity can be used to directly detect H c2 in the cuprates YBa 2 Cu 3 O y , YBa 2 Cu 4 O 8 and Tl 2 Ba 2 CuO 6 þ d , allowing us to map out H c2 across the doping phase diagram. It exhibits two peaks, each located at a critical point where the Fermi surface of YBa 2 Cu 3 O y is known to undergo a transformation. Below the higher critical point, the condensation energy, obtained directly from H c2 , suffers a sudden 20-fold collapse. This reveals that phase competitionassociated with Fermi-surface reconstruction and charge-density-wave order-is a key limiting factor in the superconductivity of cuprates.

We report on Coulomb blockade and Coulomb diamond measurements on an etched, tunable single-layer graphene quantum dot. The device consisting of a graphene island connected via two narrow graphene constrictions is fully tunable by three lateral graphene gates. Coulomb blockade resonances are observed and from Coulomb diamond measurements a charging energy of ≈ 3.5 meV is extracted. For increasing temperatures we detect a peak broadening and a transmission increase of the nanostructured graphene barriers.