No abstract

Remarkable softenings of long wavelength intersubband spin excitations of dilute electron double layers are observed at even integer quantum Hall states. These excitations in coupled GaAs double quantum wells were probed by resonant inelastic light scattering. Their softening is attributed to enhanced exchange vertex corrections (excitonic binding) in the quantum Hall states. The collapse of the spin-density mode with dS z 0 to an energy close to the Zeeman splitting suggests the existence of unstable spin-flip intersubband excitations with dS z 1. [S0031-9007(96)02143-6] PACS numbers: 73.20.Dx, 73.20.Mf, 78.30.Fs Studies of dispersive collective excitations in the integer and fractional quantum Hall regimes offer unique insights into fundamental effects and new phenomena in electron systems of reduced dimensionality. The presence of magnetoroton minima in the dispersions of charge-density excitations (CDE) is one of many characteristic manifestations of electron-electron interactions [1,2]. The minima occur at wave vectors q Ӎ 1͞l 0 , where l 0 ͑hc͞eB͒ 1͞2 is the magnetic length and B is the magnetic field. The roton minima result from vertex corrections due to exchange Coulomb interactions in the neutral quasiparticle-quasihole pairs of the excitations. These interactions are excitonic effects that reduce the collective mode energies.In electron double layers, the introduction of the additional degree of freedom associated with layer index creates remarkable new electron correlation effects. For example, at filling factor n 1 electron bilayers show intriguing and rich phase diagrams determined by the magnitude of the symmetric-antisymmetric gap D SAS , the distance d between the layers, and the in-plane magnetic field [3][4][5]. In magnetotransport experiments, these quantum phase transitions appear as the suppression of the n 1 state [4,6]. Several theoretical works have linked the disappearance of the quantum Hall state to a vertex-correction-driven collapse of the symmetric to antisymmetric intersubband CDE energy at wave vectors q Ӎ 1͞l 0 [7]. Although not yet directly observed, the predicted CDE instability highlights the major role of excitonic effects on the dispersive collective excitations of the electron gas in the quantum Hall regime.Inelastic light scattering experiments have previously shown that vertex corrections also have a major impact on the intersubband spin-density excitation (SDE) modes of electron bilayers in double quantum wells (DQW) [8]. For such excitations a different class of vertexcorrection-driven quantum phase transitions has been predicted to occur at zero magnetic field [9]. Here the signature of the instability is the collapse of the energy of the long wavelength (q 0) intersubband SDE mode of the symmetric to antisymmetric transitions.While this zero-field instability is not observed in GaAs DQW [10], inelastic light scattering measurements in perpendicular magnetic field have uncovered marked softenings of the intersubband SDE at even n quantum Hall states [11]. A recent H...

Below a critical filling factor (v c =0.2&) and critical temperature (Tc^XA K at 26 T) an additional line has been observed in the luminescence spectrum of a GaAs-Al x Gaix As heterojunction; it grows in intensity with decreasing v, dominating the spectrum at v < 17 • Its appearance is accompanied by a strong reduction in overall integrated intensity, while its relative intensity decreases sharply at v = K j, and f. The lack of correlation between v c and the disorder-related properties of the system indicates the intrinsic nature of the line which we propose signals the formation of a pinned Wigner solid.

Inelastic light scattering by low-energy spin-excitations reveals three distinct configurations of spin of electron double layers in gallium arsenide quantum wells at even-integer quantum Hall states. The transformations among these spin states appear as quantum phase transitions driven by the interplay between Coulomb interactions and Zeeman splittings. One of the transformations correlates with the emergence of a spin-flip intersubband excitation at vanishingly low energy and provides direct evidence of a link between quantum phase transitions and soft collective excitations in a two-dimensional electron system.

The growth of single layer graphene nanometer size domains by solid carbon source molecular beam epitaxy on hexagonal boron nitride (h-BN) flakes is demonstrated. Formation of single-layer graphene is clearly apparent in Raman spectra which display sharp optical phonon bands. Atomic-force microscope images and Raman maps reveal that the graphene grown depends on the surface morphology of the h-BN substrates. The growth is governed by the high mobility of the carbon atoms on the h-BN surface, in a manner that is consistent with van der Waals epitaxy. The successful growth of graphene layers depends on the substrate temperature, but is independent of the incident flux of carbon atoms. Studies of atomic layers of graphene attract enormous interest for their impact in fundamental science and for their potential to revolutionize applications in diverse areas such as electronics and optoelectronics [1,2]. Much of the exciting research has been reported on high quality graphene obtained by micromechanical exfoliation of graphite. Advances in fundamental and applied research and technology would be greatly enhanced by implementation of scalable fabrication methods. While chemical vapor deposition (CVD) creates the potential for production of large area graphene layers [3,4], device performance of CVD grown films remains low with mobilities as much as 10 times smaller than that measured in exfoliated devices. A promising alternative is the growth of graphene by molecular beam epitaxy (MBE). MBE growth of graphene on substrates patterned at the nanoscale, to give an example, could lead to fabrication of nanostructures with controlled doping and energy gaps [5,6,7,8]. The remarkable potential of MBE grown graphene has resulted in numerous challenging developments [9,10,11,12,13].Single crystal hexagonal boron nitride (h-BN) has been proposed as an ideal substrate for epitaxial growth of graphene by MBE [14,15]. The hexagonal lattice structure yields an atomically flat surface with lattice constant close to that of graphene (less than 2% mismatch). h-BN is also an inert large-band-gap insulator that can withstand very high temperatures. Further, it has been recently demonstrated that h-BN is an ideal substrate for electrical transport devices fabricated from exfoliated graphene flakes [16,17,18].In this letter we report the MBE growth of single layer graphene on single crystal h-BN flakes. Characterization of the MBE grown graphene layers by Raman-scattering spectroscopy and atomic-force microscope (AFM) imaging indicate that the graphene layers consist of nanoscale domains. The non-uniformity of the growth suggests that the individual characteristics of the h-BN flakes, such as surface morphology, may play a significant role. The maximum substrate temperature that can be reached before the SiO 2 substrate decomposes is a key parameter limiting the growth conditions in this work. The quality of the layers depends critically on the substrate temperature during growth. The best results are obtained at growth temperat...