Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micrometer-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulfides and hBN are found to exhibit consistently high carrier mobilities of about 60 000 cm(2) V(-1) s(-1). In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide, and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ∼1000 cm(2) V(-1) s(-1). We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN, and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.

We convert the surface of Sm O 2 3 pieces to SmB 6 films by means of Mg-assisted boronization. Sm O 2 3 lumps and MgB 2 powder are sealed into a quartz ampule in vacuum. By utilizing thermally decomposed Mg from MgB 2 as a catalyst, SmB 6 films are produced at temperatures as low as 700 °C. The fabrication method hence enables low-temperature synthesis of SmB 6 films without using hazardous substances. We evaluate the structural properties of the films using x-ray diffraction and Raman spectroscopy.

We report on the gating in GaAs-(Al,Ga)As heterostructures using the electrical double layer of ionic liquids. We employ tetrabutylammonium bis(trifluoromethylsulfonyl)imide for the ionic liquid, which has a melting point of 92 • C. As the 'ionic liquid' is solid at room temperature, the device operation is more stable compared to when an aqueous electrolyte is used. Hysteretic behavior appeared in the variation of the resistance when the negative gate bias exceeded ∼ −3 V, which is considered to be the electrochemical window of the ionic liquid. Temporary gate degradation occurred when the positive gate bias was larger than 8 V. We attribute these characteristics to the generation of space charge and its slow dissipation in the ionic liquid. The gating ceases to function at low temperatures with a transition range of 100-200 K due to the freezing of ions.

The interaction of surface acoustic waves (SAW) with the magnetic vortex system in YBa2Cu3O7 is investigated. A 100nm YBCO film is deposited on a piezoelectric substrate and structured for electrical 4-point measurements. Interdigital transducers are fabricated on the same substrate. When applying an external magnetic field perpendicular to the film surface a SAW-induced dc-voltage perpendicular to the acoustic sound-path is observed. This is interpreted as consequence of directed dragging of vortices by the SAW induced dynamic pinning structure [1]. The piezoacoustic wave acts as a conveyor for mobile flux quanta. Additional ac-dc-conversion as result of the nonlinear current-voltage characteristics close to the superconducting transition temperature can be resolved and separated. In order to observe the sound-induced vortex motion directly, the flux distribution is analyzed magnetooptically. Magnetooptic imaging allows for time resolved analysis of flux distribution. Quantitative analysis of changes in the magnetization distribution when acoustic driving fields are applied is carried out. The influence of piezoacoustic waves on the pinning properties and sound-induced depinning is discussed. [1] F.