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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

A vibrating sample technique was used to study the elastic behavior of the magnetic vortex system in YBa2Cu3O7 single crystals. The setup consists of a system of two weakly coupled mechanical oscillators (transducer, sample), the frequency and Q of which depend sensitively on the frequencies of the two subsystems as well as the coupling between both. By sweeping a magnetic field at temperatures below the superconducting transition temperature Tc, we observe pronounced attenuation peaks at characteristic field strengths H1 and H2. These fields mark points of constant elasticity of the vortex ensemble. Since softening precedes the melting of the vortex lattice by approaching Hc1 as well as Hc2, the observed angular dependence of H1 and H2 is interpreted as due to reentrant softening as precursor to reentrant melting.

In a thin film of superconducting YBa 2 Cu 3 O 7 a surface acoustic wave (SAW) induces a DC voltage-drop close to the superconducting transition. While this effect is often ascribed to the acoustoelectric effect, the signal is shown to be composed of two parts: a component due to SAW-induced vortex-drag and a component that is related to harmonic interference effects.

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.

We report on pronounced anomalies of the absorption and dispersion of the sound-induced rf magnetic field in YBa2Cu3O7-δ. The experimental findings are highly indicative of a near-coincidence of the superconducting transition and an antiferrodistortive transition into either an antiferroelectric or improper ferroelectric state. To our knowledge the experimental technique presented here, so far is the only method which offers the possibility to show experimentally that both superconductivity and ferro-/antiferroelectricity may coexist. Furthermore, it is demonstrated that in YBa2Cu3O7-δ microtwinning plays an important role for the dielectric phase transition to become manifest. The possible relationship between our findings and high-Tc superconductivity is also discussed.

Electrical transport and specific heat properties of Nd_{1-x}Pb_{x}MnO_{3} single crystals for 0.15 < x 0.5 have been studied in low temperature regime. The resistivity in the ferromagnetic insulating (FMI) phase for x < 0.3 has an activated character. The dependence of the activation gap Delta on doping x has been determined and the critical concentration for the zero-temperature metal-insulator transition was determined as x_{c} ~ 0.33. For a metallic sample with x=0.42, a conventional electron-electron (e-e) scattering term proportional T^{2} is found in the low-temperature electrical resistivity, although the Kadowaki-Woods ratio is found to be much larger for this manganite than for a normal metal. For a metallic sample with x=0.5, a resistivity minimum is observed for x= 0.5. The effect is attributed to weak localization and can be described by a negative T^{1/2} weak-localization contribution to resistivity for a disordered three-dimensional electron system. The specific heat data have been fitted to contributions from free electrons (gamma), spin excitations (beta_{3/2}), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The value of gamma is larger than for normal metals, which is ascribed to magnetic ordering effects involving Nd. Also, the Schottky-like anomaly appears broadened and weakened suggesting inhomogeneous molecular fields at the Nd-sites., 14 pages, 8 figures

The characteristics of surface acoustic waves (SAW's) in M-plane GaN and AlN layers grown on γ-LiAlO 2 (100) are investigated. The SAW velocities in the (100) plane of bulk γ-LiAlO 2 are virtually identical along the [010]and [001] directions and are significantly larger than those in the M-plane bulk GaN. Despite these velocity relations, the SAW velocity in GaN/γ-LiAlO 2 heterostructures increases steadily along the GaN[1120] direction with increasing the SAW wavelength, whereas it decreases along the GaN [0001] direction. A guided Rayleigh mode is observed solely for the propagation along the GaN[1120] direction. These experimental results are compared with numerical simulations. Velocity bowing is indicated to govern the dispersion in the heterostructures.

We present ultrasonic experiments on the elastic behavior of the magnetic vortex-system in YBCO. After a brief review of the influence of pinning-sites on the H-T phase diagram, results from a recently developed low frequency acoustic experiment, a sampled continuous wave technique on a twin-free YBCO single crystal, are presented. This technique is related to vibrating reed experiments, but has the advantage of constant angle between sample and applied magnetic field, thereby keeping the flux density inside the sample unaltered. It is related to ac-resistivity measurements as well, because the interaction between the driving sound-wave and the vortices is mediated by the sound-induced motion of charge carriers, but it operates at much lower current densities.

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.

Ultrasonic attenuation and sound velocity were measured at frequencies ranging from 25 MHz to 305 MHz in a large single crystal of La/sub 2-x/Sr/sub x/CuO/sub 4/ with nominal x of about 0.15. The observed features in attenuation and velocity suggest a relaxation process of magnetic origin at about 27 K, and those at /spl sim/37.9 K originate in the superconducting transition and a possible structural instability slightly above it.

The resistive behavior of type-II superconductors in the superconducting state is largely influenced by the materials' ability to pin magnetic flux lines. While an externally applied magnetic field leads to an ordered flux-lattice in defect-free superconductors, spatial variations in the superconducting order parameter tend to deform the vortex system. This has immediate implications for the temperatureand field-dependence of the critical current density. Simple estimates of the balance between the intervortex interaction and the vortexpinsite interaction allow for qualitative predictions of the temperature- and field-dependent resistive behavior of type-II superconductors. A low frequency ultrasonic technique was developed, which was used to study the viscous properties of the vortex system.

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.

Although acoustic microscopy is well-known to be an effective nondestructive control of local elastic and structural properties in the bulk of both opaque and transparent materials, the applicability of commercial scanning acoustic microscopes (SAM) is limited to temperatures near ambient, because SAM calls for the use of on immersion liquid. On the other hand, since the discovery of high-Tc superconductivity, there is a rapidly growing interest in non-destructive lowtemperature examination of ceramics, crystals and devices made out of these materials. It is interesting to investigate other materials also in low-temperature region.

We report on ultrasonic investigations of the frequency dependent and temperature dependent sound velocity in C60/C70 compounds. Although the wellknown phase transitions at about 260K and 155K are clearly seen in our experiments, concerning the absolute values of the sound velocity our results differ markedly from those found by means of low frequency vibrating reed techniques on C60 single crystals. Reasons for this unexpected behaviour are discussed briefly.

We report on ultrasonic measurements on polycrystalline samples of the high-T c system YBa 2 Cu 3 O 7−δ (YBCO). It is shown that the influence of the dielectric anomalies above T c on the elastic properties depends sensitively on boundary conditions as e.g. grainsize and intergranular electrical conductivity.

In order to determine the dielectric properties of the bound charges (soft dipoles) in superconducting YBa 2 Cu 3 O 7-δ (δ

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 .

We report on ultrasonic experiments on YBa2Cu3O7. The formation of twins as a precursor of a first order phase transition is discussed.