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Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micron-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 disulphides 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 ~ 1,000 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., 19 pages, 11 figures, 1 table including Supporting Information

Recently numerous experiments have aimed at verifying the existence of a melting transition in the vortex system of high-temperature superconductors. Direct experimental access to thermodynamic quantities of the vortex system is hard to obtain. On the other hand, the interpretation of experiments probing the vortex behaviour indirectly is also difficult. Transport measurements in single-crystal YBa2Cu3O7-δ by Kwok et al. (1994, Phys. Rev. Lett., 73, 2614) reveal a peak in the critical current density below H c2 (T). Their field-dependent data close to T c indicate that the pinning behaviour of the magnetic flux system undergoes several marked changes with increasing field. A simple model, based on the assumption of a strongly pinned disordered vortex phase at low and at high magnetic fields, separated by a depinned or weakly pinned ordered vortex phase could explain these and other experimental observations.

Owing to the fact that superconductivity can be described as a dielectric phenomenon, where the screening of the repulsive electron-electron interaction is accounted for by introducing a temperature-dependent dielectric function ϵ(ω, k), in this paper emphasis is put on a brief discussion of values of ϵ consistent with high superconducting transition temperatures in principle as well as on experimental techniques which allow determination of the temperature-dependence of the dielectric function ϵ(ω, k).

We report on ultrasonic investigations of the frequency dependent and temperature dependent sound velocity in pure C60 and in C60/C70-compounds. For both compositions of pressed bulk samples used here elastic anomalies owing to the wellknown transitions at about 260 K and 160 K are observed. Concerning the absolute values of the elastic stiffness our results differ markedly from those found by means of low frequency vibrating reed techniques on C60 single crystals.

A technique was developed which allows for surface acoustic wave (SAW) experiments up to 500 MHz in YBCO single crystals and films on piezoelectric as well as non-piezoelectric substrates. Experimental data on a granular YBCO film show a field dependent attenuation peak below T/sub c/ which is indicative of temperature dependent changes of the coupling of the magnetic vortex system to the crystal lattice and might be related to melting or depinning of the vortex lattice.

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.

We report on pronounced anomalies of the absorption and dispersion of the sound-induced rf magnetic field in superconducting YBa 2 Cu 3 O 7−δ ( δ K T K and for a near coincidence of the superconducting transition and a ferroelectric phase transition at about 90 K.

For oriented YBa2Cu3O7-δ powder samples we report for what we believe to be the first time on measurements of the temperature dependence of the magnetic penetration depth and of the lower critical magnetic field Bc1(T). The results are found to agree with theoretical expressions derived for weak-coupled BCS-type superconductors in the clean local limit. From the lower critical field values Bc1∣(0) = (340 ± 50)G and Bc1⊥(0) = (113 ± 30)G the magnitude of the anisotropic penetration depth has been determined yielding λ ∣ (0) = (1400 ± 200) A and λ ⊥ (0) = (6100 ± 1000) A .