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Weak localization in graphene is studied in inhomogeneous magnetic fields. To generate the inhomogeneous field, a thin film of type-II superconducting niobium is put in close proximity to graphene. A deviation from the ordinary quadratic weak localization behavior is observed at low fields. We attribute this to the inhomogeneous field caused by vortices in the superconductor. The deviation, which depends on the carrier concentration in graphene, can be tuned by the gate voltage. In addition, collective vortex motion, known as vortex avalanches, is observed through magnetoresistance measurements of graphene.

The electrolytic bubbling-assisted transfer of graphene from metal catalysts in chemical vapor deposition provides a high efficiency, low cost and environmental benign alternative to the traditional chemical etching method. Despite its high potential, the yield of bubbling delamination is yet low, mainly due to the induced pores in the graphene after the transfer. It is found that water and protons transported through the poly(methyl methacrylate) (PMMA) supporting layer play a critical role in pore formation. Once water and protons reach the PMMA-graphene interface before delamination is finished, the protons permeate the graphene and form trapped hydrogen bubbles between the graphene and the metal. The built-up gas pressure inside the bubbles is high enough to crack the PMMA/graphene sheet, thereby creating pores in the graphene. An optimized PMMA layer not only reduces trapped hydrogen bubble generation, but it is also mechanically stronger preventing cracking. This contributes significantly to the pore-free electrolytic bubbling-assisted delamination of graphene.

In this report electrical properties of the nanocomposite samples, prepared from graphene nanoplatelet (GnP) loaded low density polyethylene (LDPE) by extrusion and compression molding, were examined in order to elucidate the impacts of the nanoplatelets size and material’s manufacturing process. It is shown that the extrusion forces a strong anisotropy in material’s morphology. The graphene nanoplatelets become aligned along the flow direction. As compared to pure LDPE, a significant reductions of the through-plane low field electric conductivity is found in such samples. On the other hand, the samples produced by press molding exhibit slightly higher level of electric conductivity, which is connected to their less aligned microstructure and filler dispersion. For comparison results of measurements on LDPE-graphene monolayer sandwiches are also presented.

We have developed a bolometer with a suspended normal-metal absorber connected to superconducting leads via tunneling barriers. Such an absorber has reduced heat losses to the substrate, which greatly increases the responsivity of the bolometer to over 10(9) V/W at 75 mK when measured by dc Joule heating of the absorber. For high-frequency experiments, the bolometers have been integrated in planar twin-slot and log-periodic antennas. At 300GHz and 100 mK, the bolometer demonstrates the voltage and current response of 3 x 10(8) V/W and 1.1 x 10(4) A/W, respectively, corresponding to the quantum efficiency of similar to 15 electrons per photon. An effective thermalization of electrons in the absorber favors the high quantum efficiency. We also report on how the in-plane-and transverse magnetic fields influence the device characteristics.

It is experimentally shown that the initially shifted Dirac point in grapheme-on-dielectric devices can be brought to zero by illuminating the samples with UV light. This is much easier to accomplish compared to the common procedure of annealing at high temperature. Internal photoemission is concluded to be responsible for the observed effect.

A neutral particle detector is presented, in which the traditionally used target material, indium tin oxide (ITO), is replaced by graphene. The graphene-based detector enables collinear photodetachment measurements at a significantly shorter wavelength of light down to 230 nm compared to ITO-based detectors, which are limited at 335 nm. Moreover, the background signal from the photoelectric effect is drastically reduced when using graphene. The graphene based detector, reaching 1.7 eV further into the UV energy range, allows increased possibilities for photodetachment studies of negatively charged atoms, molecules, and clusters.A neutral particle detector is presented, in which the traditionally used target material, indium tin oxide (ITO), is replaced by graphene. The graphene-based detector enables collinear photodetachment measurements at a significantly shorter wavelength of light down to 230 nm compared to ITO-based detectors, which are limited at 335 nm. Moreover, the background signal from the photoelectric effect is drastically reduced when using graphene. The graphene based detector, reaching 1.7 eV further into the UV energy range, allows increased possibilities for photodetachment studies of negatively charged atoms, molecules, and clusters.

We demonstrate that a sizable chiral charge pumping can be achieved at room temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect, which cannot be attributed to the ordinary spin pumping, reveals itself in the creation of a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations (spin waves) in an adjacent out-of-plane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken spatial inversion symmetry in the studied system. The strength of effect shows a non-monotonous dependence on the spin-wave frequency, in agreement with the proposed theoretical model., 8 pages, 5 figures

Direct formation of large-area carbon thin films on gallium nitride by chemical vapor deposition without metallic catalysts is demonstrated. A high flow of ammonia is used to stabilize the surface of the GaN (0001)/sapphire substrate during the deposition at 950°C. Various characterization methods verify that the synthesized thin films are largely sp2 bonded, macroscopically uniform, and electrically conducting. The carbon thin films possess optical transparencies comparable to that of exfoliated graphene. This paper offers a viable route toward the use of carbon-based materials for future transparent electrodes in III-nitride optoelectronics, such as GaN-based light emitting diodes and laser diodes.

A systematic study of the Cu-catalyzed chemical vapor deposition of graphene under extremely low partial pressure is carried out. A carbon precursor supply of just PCH4 ~ 0.009 mbar during the deposition favors the formation of large-area uniform monolayer graphene verified by Raman spectra. A diluted HNO3 solution is used to remove Cu before transferring graphene onto SiO2/Si substrates or carbon grids. The graphene can be made suspended over a ~12 μm distance, indicating its good mechanical properties. Electron transport measurements show the graphene sheet resistance of ~0.6 kΩ/□ at zero gate voltage. The mobilities of electrons and holes are ~1800 cm2/Vs at 4.2 K and ~1200 cm2/Vs at room temperature.

A family of highly sensitive devices based on a graphene nanobridge and superconducting electrodes has been developed, manufactured, and examined. These devices can be used to create a graphene-based integral receiver. A cold-electron bolometer prototype with superconductor-insulator-normal metal tunnel junctions has been studied. Its response to a change in the temperature and external microwave radiation has been measured. A superconducting quantum interferometer with a graphene strip as a weak coupling between superconducting electrodes has been examined. The corresponding modulation of the voltage by a magnetic field at a given current has been measured. The effect of the gate voltage on the resistance of graphene has been analyzed for these samples. To confirm that graphene is single-layer, measurements with the reference samples were performed in high magnetic fields, displaying the half-integer quantum Hall effect.

This work suggests a simple method to flatten thick layers (similar to 50-200 mu m) of SU-8 photoresist on small substrates (similar to 1 cm). The method sidesteps the edge beads problem and enables photolithography patterns up to the substrates' edges. Moreover, the method can be used even for substrates with large aspect ratios when the spin coating is largely impossible.

We use small Bi2212 mesas for in situ detection of THz radiation from the large mesas at high bias at which Joule heating is important. We numerically analyze the heat balance in the emitter mesas taking into account anisotropy and temperature dependencies of all parameters involved in the heat transport. The mesa temperature is spatially non-uniform and can exceed Tc in the middle part of the mesa. The non-uniform temperature distribution can be important for synchronization of radiation from the intrinsic Josephson junctions.

The key first step in developing bacterial infections related to implants and medical devices is the attachment of planktonic bacterial cells, and subsequent formation of biofilms. Herein, it is reported that graphene, a 2D carbon-based material, can be effectively used to prevent bacterial attachment. The key parameter for this effect is the orientation of graphene with respect to the coated surface. Chemical vapor deposition (CVD) graphene, deposited horizontally on the surface, exhibits no antibacterial effect. By contrast, an array of graphene flakes grown perpendicularly to the surface by a plasma-enhanced CVD (PECVD) process prevent biofilm formation. Electron microscopy reveals that the exposed edges of vertically aligned graphene flakes penetrate the bacterial membrane and drain the cytosolic content. Bacteria are not able to develop resistance to this killing mechanism during multiple exposures. By keeping the height of the vertical graphene coating between 60 and 100 nm, the coating is able to effectively kill bacteria, while being completely harmless to mammalian cells.

Arrays of nanometre-thick Bi2212-intrinsic Josephson junctions (IJJ's) are studied in various geometries. The samples with only a few IJJ's allow for the intrinsic-tunnelling spectroscopy with minimum of Joule heating. The reproducible low-voltage peaks of the spectra probably stem from a superconducting gap which is half the usual size. We estimate the internal temperature in the IJJ stacks and analyze the importance of the self-heating for the macroscopic-quantum-tunnel ling experiments involving IJJ's.

We describe experiments which are only possible through an ultimate control of sample shape and dimensions down to nanometer scale whereby transport measurements can be done in various restricted geometries. We use photolithography patterning together with a flip-chip technique to isolate very thin (d similar to 100 nm) pieces of BiSr2CaCu2O8+delta (BSCCO) single crystals. Ar-ion milling allows us to further thin these crystals down to a few nanometers in a controlled way. With decreasing thickness below two to three unit cells, the superconducting transition temperature gradually decreases to zero and the in-plane resistivity increases to large values indicating the existence of a superconductor-insulator transition in these ultrathin single crystals. In a refined technique, a precise control of the etching depth from both sides of the crystal makes it possible to form stacks of intrinsic Josephson junctions (IJJs) inside the ultrathin single crystals. The stacks can be tailor-made to any microscopic height (0-9 nm < d, i.e. enclosing a specific number of IJJs (0-6). In certain geometries, by feeding current into the topmost CU2O4-layer of a mesa on the surface of a BSCCO single crystal, we measured the critical value of this current by detecting a sharp upturn or break in the current-voltage characteristics. From this, we estimate the sheet critical current density of a single CU2O4 plane to be similar to 0.3-0.7 A/cm at 4.5 K, corresponding to a bulk current density of similar to 2-5 MA/cm(2). These values are among the largest ever reported for BSCCO single crystals, thin-films and tapes.

Ultra-thin pieces of Bi2Sr2CaCu2O8+δ single crystals (d

With a cleaving technique and Ar-ion etching, we make ultra-thin bridges from Bi2Sr2CaCu2O8+x single crystals. Superconductivity can be observed in the bridges only a few unit cells thick. A superconductor–insulator transition occurs when the thickness is lower than 4.5 nm. The two-dimensional nature of the superconducting transition is examined in these ultra-thin samples. We see deviations from the usual Kosterlitz–Thouless-type behaviour and discuss a possible explanation.

In stacks of intrinsic Josephson junctions (IJJs) with lateral sizes of several microns, the current is non-uniform in many cases. In certain geometries a significant part of the current flows along the superconducting planes and can reach the critical value. The current-driven superconductivity breakdown within a single Cu2O4 plane can be seen as an extra branch structure of the c-axis current–voltage characteristics. This allows us to deduce the sheet critical current of a single Cu2O4 plane in different measurement configurations. The conditions for the observation of such a current-induced transition in different IJJ geometries are discussed.

Measurements of the c-axis tunneling characteristics across mesas patterned on the surfaces of Bi2212 single crystals are discussed. The measurements provide information on the magnetic phase diagram, vortex pinning and the interplanar pancake correlations at different temperatures and magnetic fields. The pancake correlations and/or pinning can be effectively controlled by: (i) carrier concentration (oxygen content), (ii) columnar defects via heavy-ion irradiation, (iii) changes of the tunneling-barriers' width via intercalation of inert molecules or pressure.

The c -axis intrinsic tunneling properties of Bi2212 and HgBr 2 –Bi2212 single crystals have been measured in the temperature range 4.2–250 K. The 7–15-unit-cell-high mesa structures on the surfaces of these single crystals were investigated. Clear superconductor–insulator–superconductor like tunneling curves were observed for current applied in the c -axis direction. The dynamic conductance shows both sharp peaks at the superconducting energy gap (SG), followed by dips and wide maxima at larger voltages. The maxima together with depressed conductance at zero voltage persist in the whole temperature range, illustrating the presence of the pseudogap in the quasiparticle excitation spectra, while the SG peaks decrease in voltage and lose their distinctiveness at T → T c . The intercalation of Bi2212 with HgBr 2 molecules results in a drastic increase of the c -axis lattice constant and resistivity ρ c . Despite this, the overall shape of the ρ c ( T )-dependence and the characteristic temperature T * at which an upturn in the ρ c ( T )-dependence sets in upon cooling do not change. This implies that T * is not affected by coupling between the CuO 2 bilayers.

From intrinsic tunneling spectroscopy, superconducting- and pseudo-gaps could be separated within a wide range of temperatures. We have found that the two gaps have different temperature ( T ) and magnetic field ( H ) dependencies. The superconducting gap closes both as T → T c and H → H c2 , while the pseudo-gap does not change neither with T , or H . This would speak in favor of two coexisting phenomena and against the precursor superconductivity scenario of the pseudo-gap.

Intrinsic tunneling spectroscopy in high magnetic field (H) is used for a direct test of superconducting features in the quasiparticle density of states of pure Bi2212 and intercalated HgBr2-Bi2212 high- T(c) superconductors. We were able to distinguish with great clarity two coexisting gaps: (i) the superconducting gap, which closes as H-->H(c2)(T), and (ii) the c-axis pseudogap, which does not change either with H or with T. Strikingly different H dependencies, together with previously observed different temperature dependencies of the two gaps, speak against a superconducting origin of the pseudogap.

Pre-contacted, small mesas can be etched into a single crystal of Bi-2212 high- T c superconductor to study tunneling between superconducting planes in the c -axis direction, in so called intrinsic Josephson junctions. These can be applied, for example, to study current transport in the c -axis direction under magnetic fields in order to map vortex phase transitions and the irreversibility line, vortex pinning by columnar defects from heavy ion irradiation, or the effects on the pseudogap and the superconducting ininteraction by changing the separations of the copper oxygen planes by intercalation or pressure.

We experimentally study and compare the perpendicular (c-axis) transport properties of layered high-${T}_{c}$ superconductor (HTSC) ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ and low-${T}_{c}$ (LTSC) Nb/Cu multilayers. Both for HTSC and LTSC samples similar anomalous features were observed. (i) The c-axis critical current ${I}_{c}$ is multiple valued as a function of temperature T and in-plane magnetic field H. (ii) ${I}_{c}$ exhibits extremely large fluctuations and the probability distribution ${P(I}_{c})$ has multiple maxima. (iii) The ${I}_{c}(H)$ patterns are aperiodic. (iv) In the dynamic state, the flux-flow branches in current-voltage characteristics consist of multiple closely spaced sub-branches and exhibit large fluctuations due to switching between the sub-branches. Experimental data are in qualitative agreement with numerical simulations for the stack of long strongly coupled Josephson junctions. All this is taken as an evidence for the existence of multiple quasiequilibrium Josephson fluxon modes in our samples. From the study of LTSC multilayers we have found one-to-one correspondence between the anomalous behavior and appearance of stacked Josephson junctions in the two-dimensional state.

The $c$-axis interlayer coupling of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ single crystals was changed by means of hydrostatic pressure $Pl1\mathrm{GPa}$ resulting in a drastic increase of the $c$-axis critical current ${I}_{c}$ (up to $270%{\mathrm{GPa}}^{\ensuremath{-}1}$) and decrease of the normal state resistance ${R}_{n}$ (down to $\ensuremath{-}60%{\mathrm{GPa}}^{\ensuremath{-}1}$). The superconducting critical temperature only slightly increases with a rate of $\ensuremath{\approx}2%--6%{\mathrm{GPa}}^{\ensuremath{-}1}$. The much stronger effect of pressure on ${I}_{c}$ and ${R}_{n}$ in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}{\mathrm{CuO}}_{6+\ensuremath{\delta}}$ implies that the CuO-interlayer coupling has little effect on ${T}_{c}$ in this compound.

The normal and superconducting state c-axis transport properties of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ single crystals have been measured as a function of doping and magnetic fields parallel and perpendicular to the ${\mathrm{CuO}}_{2}$ planes, using mesa structures containing a small number of atomic unit-cell layers. For low current bias, the peak in the magnetic-field-dependent c-axis resistance is observed to scale approximately inversely with the zero-field critical current. The critical current ${I}_{c}$ is strongly dependent on temperature and field ${B}_{\ensuremath{\perp}}$ perpendicular to the ${\mathrm{CuO}}_{2}$ planes. This provides information on the magnetic phase diagram and the correlation of flux pancakes across adjacent pairs of superconducting ${\mathrm{CuO}}_{2}$ planes. Over an appreciable region of the phase diagram the critical current ${I}_{c}{(B}_{\ensuremath{\perp}})$ varies $\ensuremath{\sim}{1/B}_{\ensuremath{\perp}}^{\ensuremath{\mu}},$ with \ensuremath{\mu}\ensuremath{\approx}0.8 to 0.9. In a parallel field, ${I}_{c}{(B}_{\ensuremath{\parallel}})$ varies as $\mathrm{exp}[\ensuremath{-}{(B}_{\ensuremath{\parallel}}{/B}_{0})],$ with ${B}_{0}\ensuremath{\sim}2\mathrm{T},$ consistent with a model involving pinned Josephson vortices.

We used the hydrostatic pressure P

We have experimentally demonstrated that the c-axis magnetoresistance peak effect is determined by the subgap resistance of intrinsic Josephson junctions in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} (Bi2212). Given a Bi2212 single crystal, the effect may be predicted from the c-axis current-voltage (I-V) characteristics in {ital zero} magnetic field. At high magnetic field H, the I-V characteristics preserve their overall nonlinear shape at the transition temperature T{sub c}(H){le}T{sub c}(0) suggesting a smooth change of the vortex system with temperature. Observation of the gap feature below T{sub c}(0) at all {mu}{sub 0}H{le}7 T means that the upper critical field is not attained at these fields. {copyright} {ital 1997} {ital The American Physical Society}

The technology for making a specified (1–100) number of intrinsic Josephson junctions in B-2212 is reported. Measurements of the resistance during the fabrication process made it possible to avoid the in-plane contribution. The c -axis resistivity ϱ c was about 30 Ω cm at 90 K and it obeyed an exponential ϱ c ( T ) ∝ exp( Λ / T ), with Λ ≈43 meV, suggesting a thermally activated hopping mechanism of the c -axis transport. The c -axis gap parameter of an individual junction is about one half of that reported elsewhere and is strongly temperature dependent, suggesting proximity induced superconductivity of the intermediate BiO layers in Bi-2212.

Stacked series arrays of intrinsic Josephson tunnel junctions have been fabricated on the surfaces of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} single crystals using photolithography and Ar-ion milling together with {ital in} {ital situ} monitoring of the resulting current-voltage ({ital I}-{ital V}) characteristics. The number of junctions in the stack (along the {ital c} axis) is varied from 1{endash}5 to {approximately}200 in a controlled way. Both Josephson coupling and multiple branches with well developed gap features are seen in the {ital I}-{ital V} dependences of the arrays at {ital T}{approx_lt}90 K. The gap parameter {Delta}{sub {ital c}} is 10{endash}13 meV at 4.2 K, which is approximately half the value reported in the literature. The temperature dependence of {Delta}{sub {ital c}} deviates strongly from the BCS one. Proximity induced superconductivity of the Bi-O layers may possibly explain both the reduced gap parameter and its temperature dependence. {copyright} {ital 1996 The American Physical Society.}

Graphene, a true two-dimensional material with extraordinary mechanical- and electronic properties, is thought to be ideal for nanoelectromechanical systems (NEMS), like mass- and force sensors. Here, we present two different ways to fabricate suspended graphene for the intended use in future NEMS applications. The fabrication schemes do not require transfer of graphene from a catalyst where the graphene is grown on to another supporting substrate. The transfer is a source of several issues causing irreproducibility in large-scale production of graphene devices. We obtain suspended graphene membranes by locally removing the copper thin film on top of which the graphene is catalytically grown. The membranes are uniform and exhibit mechanical properties similar to those of exfoliated graphene. Also, suspended graphene beams with electrical interconnects are fabricated from non-catalytically grown graphene on SiO 2 . Both approaches represent the first steps towards transfer-free fabrication of suspended graphene for NEMS applications.

Joule heating in large Bi2Sr2CaCu2O8+delta mesas was numerically analyzed while taking into account typical thermal conductivities and their temperature dependences of all the materials involved in heat dissipation and removal. Such mesas are used in experiments on THz-range radiation. The analysis shows that the temperature increases with bias current and is distributed unevenly along the mesas. The temperature of the mesa's middle part can even exceed T-c at sufficiently high bias. The overall current-voltage characteristics are also calculated self-consistently, showing a negative differential conductance in a wide range of currents.

Intrinsic Josephson tunneling takes place between copper-oxide (CuO) planes of several anisotropic high-temperature superconductors, such as Bi2Sr2Ca Cu2O8 + δ (Bi2212). To study the intrinsic tunneling effects, mesas are usually patterned into the surfaces of Bi2212 single crystals. The mesa structure contains an array of typically 5–30 intrinsic Josephson junctions formed between adjacent superconducting CuO planes. The number of defects that could affect measurements is limited in the small volume of the mesa. Variations in the carrier concentration and introduction of columnar defects by heavy-ion irradiation can be used to control the correlations and pinning of magnetic vortex pancakes. The intrinsic tunneling provides information on the magnetic phase diagram of Abrikosov vortices, allows for direct measurements of the superconducting critical current of the individual CuO planes, permits for the observation of macroscopic quantum tunneling, allows for tunneling spectroscopy of phonons, the superconducting- and pseudo gaps, and shows coherent THz radiation of significant power.

A significant increase of the magnetic relaxation rate was observed in superconducting Bi-2212 single crystals at helium temperatures under the influence of visible light with an intensity 10−4−101 mW cm−2. To a first approximation, the change of the magnetic moment is proportional to the dose of incident light. The breaking of Cooper pairs under illumination, as well as interaction between the vortices and optical phonons produced during recombination of the quasiparticles, may be the reasons for the effect observed. The relaxation time of the hot electrons produced under the action of light was estimated to be about 5 × 10−8s.

We present measurements of the electrical resistivity, thermal conductivity, and Hall, Nernst, and Seebeck effects in the mixed state of single crystalline Bi2Sr2CaCu2Ox. It is shown that the sign of the Hall voltage changes twice as temperature decreases below Tc. From the Nernst effect we estimate the transport entropy Sφ to be about 10−10 erg/K cm. Sφ is equal to zero in the normal state, increases and passes through a maximum at the mixed state as expected. The temperature dependences of the thermoelectric power in magnetic fields are analogous to the resistive transition curves. These phenomena are discussed in terms of flux flow. The contribution of the flux flow to the thermal conductivity is estimated to be negligible. Lowering of the thermal conductivity at temperatures below Tc by a magnetic field is attributed to phonon scattering by the vortex lines.

We have investigated the current-voltage (I-V) characteristics of sub-µm sized mesas made on the surface of Bi2Sr2CaCu2O8 (Bi2212) single crystals. The mesas were fabricated using focused ion beam etching. The samples showed excellent I-V characteristics and their conductance-voltage (G-V) curves were measured from above the transition temperature down to 4.2 K. New ways of making annular mesas with diameters down to 0.5 µm were also investigated.

We test radiation detectors made from single-crystalline Bi2Sr2CaCu2O8+x flakes put on oxidized Si substrates. The 100-nm-thick flakes are lithographically patterned into 4 x 12 mu m(2) large rectangles embedded in thin-film log-spiral antennas. The SiO2 layer weakens the thermal link between the flakes and the bath. Two modes of radiation detection have been observed. For a bolometric type of sensors a responsivity of similar to 300 V/W and a noise equivalent power of 30 nW/root Hz has been deduced at 70 K. Much more sensitive is the non-bolometric device showing characteristics similar to a Golay-type detector while being at least a thousand times faster. Making smaller (sub-mu m) structures is expected to significantly improve the performance of these devices and makes them very competitive among other microwave and terahertz detectors.

We make stacks of intrinsic Josephson junctions (IJJs) imbedded in the bulk of very thin ($d\leq 100$~nm) $\mathrm{Bi_2Sr_2CaCu_2O_{8+x}}$ single crystals. By precisely controlling the etching depth during the double-sided fabrication process, the stacks can be reproducibly tailor-made to be of any microscopic height ($0-9 \mathrm{nm}, Comment: 3 pages, 4 figures, to be published in APL May. 29

Stacked series arrays of intrinsic Josephson (IJ) tunnel junctions have been fabricated on the surfaces of Bi2Sr2CaCu2O8+δ single crystals using photolithography and Ar-ion milling together with in situ monitoring of the resulting current–voltage (I–V) characteristics. The number of unit-cell-sized junctions in the stack (along the c-axis) may be varied from 1–5 to ∼200 in a controlled way. The c-axis resistivity ρc, estimated from the resistance of an individual tunnel junction is ≈30 Ω cm at 90 K. The temperature dependence ρc(T)∝ exp(Λ/T) with Λ≈43 meV, suggesting thermally activated hopping mechanism of transport in the c-axis direction.

By feeding current into the topmost Cu2O4 layer of a mesa etched into the surface of a Bi2Sr2CaCu2O8+x (BSCCO) single crystal, we measured its superconducting critical value from a sharp upturn or break in the current-voltage characteristics of the mesa. From this, we estimate the sheet critical current density of a single Cu2O4 plane to be similar to 0.3-0.7 A/cm at 4.5 K, corresponding to the bulk current density of 2-5 MA/cm(2). These values are among the largest ever measured for BSCCO single crystals, thin films and tapes, and we argue that they represent the true intrinsic values of the material.

We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped ${\mathrm{B}\mathrm{i}}_{2}({\mathrm{S}\mathrm{r}}_{2\ensuremath{-}x}{\mathrm{L}\mathrm{a}}_{x}){\mathrm{C}\mathrm{u}\mathrm{O}}_{6+\ensuremath{\delta}}$ ($\mathrm{B}\mathrm{i}2201\mathrm{\text{\ensuremath{-}}}{\mathrm{L}\mathrm{a}}_{x}$). Despite a difference of a factor of 3 in the optimal superconducting critical temperatures for $\mathrm{B}\mathrm{i}2201\mathrm{\text{\ensuremath{-}}}{\mathrm{L}\mathrm{a}}_{0.4}$ and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature ${T}^{*}\ensuremath{\approx}230--300\text{ }\mathrm{K}$ for both compounds. We find also that, in $\mathrm{B}\mathrm{i}2201\mathrm{\text{\ensuremath{-}}}{\mathrm{L}\mathrm{a}}_{x}$, pseudogap humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.

We present an experimental setup for fast detection of resonances of graphene microelectromechanical structures of different quality. The relatively simple technique used to read-out of the resonance frequency is the main advantage of the proposed system. The resolution is good enough to detect vibrations of the graphene resonator with the quality factor of ∼24 and resonance frequency of 104 MHz.

We present intrinsic tunneling spectroscopy measurements on small Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesas. The tunnel conductance curves show both sharp peaks at the superconducting gap voltage and broad humps representing the $c$-axis pseudo-gap. The superconducting gap vanishes at $T_c$, while the pseudo-gap exists both above and below $T_c$. Our observation implies that the superconducting and pseudo-gaps represent different coexisting phenomena., Comment: 5 pages, 4 figures

The c-axis tunneling properties of both pristine Bi2212 and its HgBr$_2$ intercalate have been measured in the temperature range 4.2 - 250 K. Lithographically patterned 7-10 unit-cell heigh mesa structures on the surfaces of these single crystals were investigated. Clear SIS-like tunneling curves for current applied in the $\it c$-axis direction have been observed. The dynamic conductance d$I/$d$V(V)$ shows both sharp peaks corresponding to a superconducting gap edge and a dip feature beyond the gap, followed by a wide maximum, which persists up to a room temperature. Shape of the temperature dependence of the {\it c}-axis resistance does not change after the intercalation suggesting that a coupling between $\rm CuO_2$-bilayers has little effect on the pseudogap., Comment: 6 pages, 5 figures; presented at the Second Int Conf. New3Sc-1999 (Las Vegas, NV)

Modulation of the critical current across layers, Ic(H), of stacked Josephson junctions (SJJs) as a function of an applied magnetic field parallel to the junction planes is studied theoretically and experimentally for different junction lengths and coupling parameters. It is shown that the Ic(H) patterns of long SJJs are very complicated without periodicity in H. This is due to interaction between junctions in the stack. This, in turn, gives rise to the existence of multiple quasi-equilibrium Josephson fluxon modes and submodes which are different with respect to the symmetry of the phase and the fluxon sequence in SJJs. The critical current of long SJJs is multiple valued and is governed by switching between energetically close fluxon modes/submodes. Due to this, the probability distribution of the critical current may become wide and may consist of multiple maxima each representing a particular mode/submode. Experimentally, multiple branched Ic(H) patterns and multiple maxima in the Ic probability distribution were observed for Bi2Sr2CaCu2O8+x intrinsic SJJs, which are in a good agreement with numerical simulations and support the idea of having different quasi-equilibrium fluxon modes/submodes in intrinsic SJJs., 5 pages, 5 figures

In this paper, we report on a microwave characterization of the interface between Ti/Au contacts and chemical vapor deposition graphene using structures of Corbino geometry, with primary focus on extracting and modeling the capacitance associated with the contact region. It is found that with the current contact resistivity, ρc∼10^−6 Ωcm2, the contact capacitance, on the order Cc∼1 μF/cm2, has a negligible effect on microwave transmission through the contact below ∼100 GHz. Finally, a parallel plate capacitance model for the contact is presented.

Graphene contacts to p-GaN are considered as an alternative to indium–tin-oxide transparent electrodes in GaN based vertical-cavity surface-emitting lasers (VCSELs). Contact properties were investigated on light-emitting diode and p-GaN test structures, where dielectric apertures were used to eliminate the influence of the metal pads used to bias the contacts. Using single layer graphene we were able to operate light emitting diodes with current densities of 300 A/cm2. Addition of a second layer of graphene increased the maximum bias current to 1 kA/cm2. However, the contacts are non-linear and cannot withstand high current densities for a long time. The results are promising but further investigation and improvement is needed for graphene to be a viable alternative to indium–tin-oxide for blue VCSELs.

Graphene, an atomic monolayer of sp(2)-hybridised carbon atoms, is a promising material for future NEMS based on its remarkable electronic and mechanical properties. Through the rapid progress of chemical vapour deposition of large-scale, high-quality graphene, these applications seem to be close to reality. However, issues related to the graphene transfer process limit the reproducibility of such devices. In this Letter, the authors present two different approaches for fabricating suspended graphene devices without any transfer step, using both catalytically and non-catalytically grown graphene. The authors achieve high reproducibility in manufacturing flat and uniform suspended graphene beams. While good mechanical properties are observed, the electrical performance is still poor, requiring improvements.