The in-plane resistivity, in-plane absolute thermopower, and upper critical field measurements are reported for single-crystal samples of YNi 2 B 2 C and LuNi 2 B 2 C superconductors. The in-plane resistivity shows metallic behavior and varies approximately linearly with temperature near room temperature ͑RT͒ but shows nearly quadratic behavior in temperature at low temperatures. The YNi 2 B 2 C and LuNi 2 B 2 C single-crystal samples exhibit large transverse magnetoresistance ͑Ϸ6-8 % at 45 kOe͒ in the ab plane. The absolute thermopower S(T) is negative from RT to the superconducting transition temperature T c. Its magnitude at RT is a few times of the value for a typical good metal. S(T) is approximately linear in temperature between Ϸ150 K and RT. Extrapolation to Tϭ0 gives large intercepts ͑few V/K͒ for both samples suggesting the presence of a much larger ''knee'' than would be expected from electron-phonon interaction renormalization effects. The upper critical fields for H parallel and perpendicular to the c axis and the superconducting parameters derived from it do not show any anisotropy for the YNi 2 B 2 C single-crystal samples in agreement with magnetization and torque magnetometry measurements, but a small anisotropy is observed for the LuNi 2 B 2 C single crystals. The analysis shows that these are moderately strong-coupling type-II superconductors ͑similar to the A-15 com-pounds͒ with a value of the electron-phonon coupling parameter ͑0͒ approximately equal to 1.2 for YNi 2 B 2 C and 1.0 for LuNi 2 B 2 C, the Ginzburg-Landau coherence length ͑0͒ approximately equal to 70 Å, and H c2 (0)ϳ60-70 kOe. The temperature dependence of the upper critical field shows a positive curvature near T c in disagreement with the Werthamer, Helfand, Hohenberg, and Maki ͑WHHM͒ theory but in agreement with a recent solution of the Gor'kov equation using a basis formed by Landau levels ͑Bahcall͒; however, the data show a severe disagreement between the observed low-temperature behavior of H c2 (T) and that predicted either by WHHM or Bahcall's expressions. ͓S0163-1829͑97͒06413-8͔

The in-plane resistivity and magnetization measurements as a function of the magnitude and direction of the magnetic field and the temperature are reported for single-crystal samples of the HoNi 2 B 2 C magnetic superconductor. Features corresponding to several distinct magnetic phases and the coexistence of superconductivity with two of the magnetic phases are observed. Contrary to previous measurements for polycrystalline samples, reentrant superconductivity is not observed in the absence of a field for these samples. The measurements indicate an extremely rich interplay between superconductivity and different magnetic structures that can be influenced by field, temperature, and current. The results correlate quantitatively with and complement previous determinations of the magnetic phase diagram and qualitatively with determinations of the superconducting phases by measurements of the single-crystal magnetization and heat capacity. HoNi 2 B 2 C is highly anisotropic, and phase diagrams for the field along the ͑100͒ and ͑001͒ directions are presented.

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The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. Twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutual misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts as an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ less than about 3.5 o , corresponding to moiré-pattern periods D longer than 4 nm. However, they totally vanish for θ > 5.5 o (D < 2.5 nm). Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.Graphene's novel electronic properties are a consequence of its two-dimensional honeycomb lattice [1]. Its electronic spectra are relatively easy to be tuned because graphene is a single-atom-thick membrane of carbon [2][3][4]. Very recently, it was demonstrated that a layer of hexagonal boron nitride (hBN) in contact with graphene can generate a periodic potential felt by graphene and lead to profound changes in graphene's electronic spectrum [5][6][7][8][9][10]. This provides an effective route to control the electronic spectra and properties of graphene via the creation of van der Waals heterostructures [5][6][7][8][9][10]. Graphene placed on top of another graphene monolayer with stacked misorientation forms a unique two dimensional van der Waals structure, i.e., twisted graphene bilayer [11][12][13][14][15][16][17][18][19][20][21], in which the graphene-on-graphene moiré modifies the electronic spectra [16,17,19]. The period of the moiré pattern D is related to the rotation angle θ by D = a/[2sin(θ/2)] with a = 0.246 nm the lattice parameter of graphene. This unique layered structure exhibits many fascinating physical properties beyond that of graphene monolayer due to interlayer coupling [16][17][18][19][20]. For example, the quasiparticles in twisted graphene bilayer are expected to show tunable chirality and adjustable probability of chiral tunneling [20].At small rotation angles electronic spectra of twisted graphene bilayer have been experimentally demonstrated to follow the predictions of the continuum models [11] and show twist-induced van Hove singularities (VHSs) [13][14][15]22,23], which directly arise from the finite interlayer coupling. However, the VHSs were not always observed and several experiments indicate that the electronic properties of the twisted graphene bilayer resemble a single graphene sheet [13,14,[24][25][26][27]. Obviously, the

Point contacts are used to investigate the anisotropy of the superconducting energy gap in LuNi2B2C in the ab plane and along the c axis. It is shown that the experimental curves should be described assuming that the superconducting gap is non-uniformly distributed over the Fermi surface. The largest and the smallest gaps have been estimated by two-gap fitting models. It is found that the largest contribution to the point-contact conductivity in the c direction is made by a smaller gap and, in the ab plane by a larger gap. The deviation from the one-gap BCS model is pronounced in the temperature dependence of the gap in both directions. The temperature range, where the deviation occurs, is for the c direction approximately 1.5 times more than in the ab plane. The Γ parameter, allowing quantitatively estimate the gap anisotropy by one-gap fitting, in c direction is also about 1.5 times greater than in the ab plane. Since it is impossible to describe satisfactorily such gap distribution either by the one-or two-gap models, a continuous, dual-maxima model of gap distribution over the Fermi surface should be used to describe superconductivity in this material.