When matter undergoes a phase transition from one state to another, usually a change in symmetry is observed, as some of the symmetries exhibited are said to be spontaneously broken. The superconducting phase transition in the underdoped high-T c superconductors is rather unusual, in that it is not a mean-field transition as other superconducting transitions are. Instead, it is observed that a pseudo-gap in the electronic excitation spectrum appears at temperatures T * higher than T c , while phase coherence, and superconductivity, are established at T c (Refs. 1, 2). One would then wish to understand if T * is just a crossover, controlled by fluctuations in order which will set in at the lower T c (Refs. 3, 4), or whether some symmetry is spontaneously broken at T * (Refs. 5-10). Here, using angle-resolved photoemission with circularly polarized light, we find that, in the pseudogap state, left-circularly polarized photons give a different photocurrent than right-circularly polarized 2 photons, and therefore the state below T * is rather unusual, in that it breaks time reversal symmetry 11 . This observation of a phase transition at T* provides the answer to a major mystery of the phase diagram of the cuprates. The appearance of the anomalies below T* must be related to the order parameter that sets in at this characteristic temperature .We have investigated the time reversal invariance of the electronic states by ARPES, which becomes sensitive to this symmetry by the use of circularly polarized photons 11 .The measured ARPES intensity I α ∝ |M α | 2 , where the matrix element M α = 〈p|O α |ψ(k)〉, describes the ejection of the electron from an initial state |ψ(k)〉 to a final state |p〉, and the dipole operator O α contains the vector potential of α = L (left) or α = R (right) circularly polarized photons. The experimental setup is shown in Fig. 1. It consists of a plane grating monochromator beamline at the Aladdin synchrotron as a source of linearly polarized photons, quadruple reflection polarizer 12 , refocusing mirror, and the experimental chamber.It is crucial in this experiment, which is essentially measuring absolute intensity changes, to minimize beam movement, as extraneous intensity changes can occur from such movements.We therefore monitor the beam position, as shown in Fig. 1, finding a small residual beam movement of 150 µm, which is compensated for by adjusting the experimental chamber position as the polarizer is rotated. In the experiment one wishes to maximize the product TP 2 , where T denotes the transmission and P the polarization. In our experiment, this is accomplished with P = 86%, as shown in Fig. 1d. Although case (b) is the one that interests us, we first describe case (a), as this is a large effect, and needs to be correctly accounted for in the analysis of the data 13-15 . In Fig. 2a we it was found experimentally 16 , and confirmed here, that it is a symmetry direction for the CuO 2 planar electronic states). This is seen in Fig. 2e, where we plot spectra obtained at point M 1 that i...

The application of graphene in electronic devices requires large scale epitaxial growth. The presence of the substrate, however, usually reduces the charge carrier mobility considerably. We show that it is possible to decouple the partially sp 3 -hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp 2 -hybridized graphene layer with improved electronic properties.Electrons in graphene -sp 2 -bonded carbon atoms arranged in a honeycomb lattice -behave like massless Dirac particles and exhibit an extremely high carrier mobility [1]. So far, the only feasible route towards large scale production of graphene is epitaxial growth on a substrate. The presence of the substrate will, however, influence the electronic properties of the graphene layer. To preserve its unique properties it is desirable to decouple the graphene layer from the substrate. Here we present a new approach for the growth of highly decoupled epitaxial graphene on a silicon carbide substrate. By decoupling the strongly interacting, partially sp 3hybridized first graphitic layer (commonly referred to as zero layer (ZL) [2]) from the SiC(0001) substrate by gold intercalation, we obtain a completely sp 2 -hybridized graphene layer with improved electronic properties as confirmed by angleresolved photoemission spectroscopy (ARPES), scanning tunneling microscopy (STM) and Raman spectroscopy.There are essentially two ways for large scale epitaxial growth of graphene on a substrate: by cracking organic molecules on catalytic metal surfaces [3][4][5][6][7] or by thermal graphitization of SiC [2,[8][9][10][11]. Unfortunately, the presence of the substrate alters the electronic properties of the graphene layer on the surface and reduces the carrier mobility. Even though it has been shown that the graphene layer can be decoupled from a metallic substrate [6,[12][13][14] the system remains unsuitable for device applications. This problem can be solved by decoupling the graphene layer from a semiconducting SiC substrate [15].On both the silicon and the carbon terminated face of a SiC substrate, graphene is commonly grown by thermal graphitization in ultra high vacuum (UHV). When annealing the substrate at elevated temperatures Si atoms leave the surface whereas the C atoms remain and form carbon layers. On SiC(0001), the so-called C-face, the weak graphene-tosubstrate interaction results in the growth of rotationally disordered multilayer graphene and a precise thickness control becomes difficult [16]. On the other hand, the rotational disorder decouples the graphene layers so that the transport properties resemble those of isolated graphene sheets with room temperature mobilities in excess of 200,000 cm 2 /Vs [17].On SiC(0001), i. e. the Si-face, the comparatively strong graphene-to-substrate interaction results in uniform, long-range ordered layer-by-layer growth. The first carbon layer (=ZL) grown on the Si-face is partially sp 3 -hybridized to the substrate, wh...

When electrons are subject to a potential with two incommensurate periods, translational invariance is lost, and no periodic band structure is expected. However, model calculations based on nearly free one-dimensional electrons and experimental results from high-resolution photoemission spectroscopy on a quasi-one-dimensional material do show dispersing band states with signatures of both periodicities. Apparent band structures are generated by the nonuniform distribution of electronic spectral weight over the complex eigenvalue spectrum.

We have used s-and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the K-point by angular resolved photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is suppressed due to the interference of photoelectrons emitted from the two equivalent carbon atoms per unit cell of graphene's honeycomb lattice. Here we show that by rotating the polarization vector, we are able to illuminate this 'dark corridor' indicating that the present theoretical understanding is oversimplified. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low photon energy regime.Graphene, a single layer of sp 2 -bonded carbon atoms, is one of the paradigm two-dimensional (2D) electron systems existing today. It is renowned for its high crystalline quality, its extremely high carrier mobility [1][2][3] as well as its peculiar charge carriers that behave like massless Dirac particles [2,[4][5][6][7][8] due to its honeycomb lattice consisting of two equivalent triangular sublattices A and B (see Fig. 1a). This leads to the description of graphene's charge carriers in terms of spinor . Panel (c) shows a sketch of the experimental setup. ky corresponds to a rotation of the sample around φ. kx is the direction perpendicular to the paper plane, it corresponds to the dispersion direction in the 2D detector. For s(p)-polarized light the electric field vector lies perpendicular to the plane of incidence (in the plane of incidence) spanned by the sample normal and the direction of incidence of the light.wavefunctions in analogy to the Dirac equation for massless particles, where the 'spin' index indicates the sublattice rather than the real electron spin, hence the term 'pseudospin ' [6]. This pseudospin is responsible for graphene's many intriguing electronic properties. First of all, the difference in pseudospin of the two cosine-shaped bands originating from the two sublattices allows them to cross at the K-point of the 2D Brillouin zone (see Fig. 1b) where they form the conical band structure [9,10]. Second, due to the pseudospin the charge carriers accumulate a Berry phase of π on closed loop paths resulting in the absence of backscattering. This has been observed in both magnetotransport [11][12][13][14] as well as scanning tunneling spectroscopy experiments [15]. Furthermore, the pseudospin is responsible for the peculiar half-integer quantum Hall effect observed in graphene [4,5,16]. In addition, the conservation of the pseudospin upon passing a potential barrier is expected to result in perfect transparency of the barrier for graphene's charge carriers (Klein tunneling) [17]. The pseudospin concept has spawned ideas for different 'pseudospintronic' device proposals, like e.g. the pseudospin valve [18].The effect of the pseudospin is also observed in angleresolved photoemission spectroscopy (ARPES) experiments. Here, it is rather unwanted because it suppresses the photoemission intensity on part of th...

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