Showing papers by "László Forró published in 2000"

Infrared and Raman studies of the Verwey transition in magnetite

Abstract: We present infrared and Raman measurements of magnetite (Fe3O4). This material is known to undergo a metal-insulator and a structural transition (Verwey transition) at T-V = 120 K. The structural aspect of the Verwey transition is disclosed by the appearance of additional infrared-active and Raman-active phonons. The frequencies of the infrared-active phonons show no significant singularities at the transition whereas their linewidths increase. The frequency and linewidth of the Raman-active phonon at 670 cm(-1) change abruptly at the transition. For T

The Electrochemical Carbon Nanotube Field-Effect Transistor

Abstract: We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy shifts of order +/- 1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage the NTs are hole doped in air with E_F ? 0.3-0.5 eV, corresponding to a doping level of ? 10^{13} cm^{-2}. Hole-doping increases in the electrolyte. This hole doping (oxidation) is most likely caused by the adsorption of oxygen in air and cations in the electrolyte.

Optical and dc conductivity study of potassium-doped single-walled carbon nanotube films

Abstract: We have performed dc transport and optical conductivity measurements on purified and potassium doped thick films of single walled carbon nanotubes. The pristine sample is characterized by a metallic (Drude) response of the optical properties despite the non-metallic behavior in the dc resistivity measurements. Even though the spectral weight of the metallic component in the electrodynamic response increases with doping, in agreement with the trend of the dc transport properties, there is a quantitative mismatch between the optical $[{\ensuremath{\sigma}}_{1}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\omega}}0)]$ and dc transport. Comparing the dc and optical conductivity measurements we conclude that potassium doping influences differently the non-metallic tube-tube contact regions and the intrinsic on-tube transport.

Beyond Gedanken Experiments

Abstract: For future nanomechanical devices to be realized, sophisticated ways of manipulating small structures must be developed, and fundamental elements of macroscopic machines, such as switches and bearings, must be reproduced in the nanoscale. ForrA³ highlights the nanotube nanobearings and nanoswitches reported by Cumings and Zettl and concludes that multiwall nanotubes are ideal starting materials for future nanodevices.

Carrier relaxation, pseudogap, and superconducting gap in high-T c cuprates: A Raman scattering study

Abstract: We describe the results of electronic Raman-scattering experiments in differently doped single crystals of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}({\mathrm{Ca}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}){\mathrm{Cu}}_{2}{\mathrm{O}}_{8}.$ The data in antiferromagnetic insulating samples suggest that at least the low-energy parts of the spectra of metallic samples originate predominantly from excitations of free carriers. We therefore propose an analysis of the data in terms of a memory function approach which has been introduced earlier for the current response. Dynamical scattering rates $\ensuremath{\Gamma}(\ensuremath{\omega})=1/\ensuremath{\tau}(\ensuremath{\omega})$ and mass-enhancement factors $1+\ensuremath{\lambda}(\ensuremath{\omega}{)=m}^{*}(\ensuremath{\omega})/m$ of the carriers are obtained. It is found that a strong polarization dependence of the carrier lifetime develops towards low doping. In ${B}_{2g} (\mathrm{xy})$ symmetry selecting predominantly electrons with momenta along the diagonals of the ${\mathrm{CuO}}_{2}$ planes the Raman data compare well with the results obtained from dc and dynamical transport. In ${B}_{1g} {(x}^{2}\ensuremath{-}{y}^{2})$ symmetry projecting out momenta along the Cu-O bonds the dc scattering rates of underdoped materials become temperature independent and considerably larger than in ${B}_{2g}$ symmetry. This increasing anisotropy is accompanied by a loss of spectral weight in ${B}_{2g}$ symmetry in the range between the superconducting transition at ${T}_{c}$ and a characteristic temperature ${T}^{*}$ of the order of room temperature which compares well with the pseudogap temperature found in other experiments. The energy range affected by the pseudogap is doping and temperature independent. The integrated spectral loss is approximately 25% in underdoped samples and becomes much weaker towards higher carrier concentration. In underdoped samples, superconductivity-related features in the spectra can be observed only in ${B}_{2g}$ symmetry. The peak frequencies scale with ${T}_{c}.$ We do not find a direct relation between the pseudogap and the superconducting gap.

Printing Gel-like Catalysts for the Directed Growth of Multiwall Carbon Nanotubes

Abstract: Microcontact printing is used to transfer an Fe(III)-containing gel-like catalyst precursor from a hydrophilized elastomeric stamp to a substrate. The catalytic pattern activates the growth of multiwall carbon nanotubes using chemical vapor deposition of acetylene. Our results show that the choice of the catalyst is of extreme importance. Most of the aqueous and ethanolic Fe(III) inks used give rise to drying effects on the stamp surface, which lead to the formation of islands of the catalyst within the pattern. To avoid these shortcomings, we developed a catalyst precursor, which has better performance on the stamp and as a catalyst on the substrate. Simple aging of the ethanolic Fe(III) ink results in a polymerized gel-like catalyst, which can be printed homogeneously on the substrate with excellent contrast. Changing the concentration of the catalyst in the ink allows the density of the carbon nanotubes in the film to be tuned. A scanning anode field emission microscope was used to investigate the micr...

Fabrication of metal nanowire using carbon nanotube as a mask

Abstract: We report on the fabrication of metal nanowires on an insulating substrate using carbon nanotubes as a new kind of mask material. By irradiating Ar+ ions of 300 eV energy on a nanotube-coated Au/Ti thin layer on a SiO2 substrate, Au/Ti nanowires were successfully formed just underneath the nanotube, indicating that the carbon nanotubes had acted as a good mask against the argon ion bombardment. The Au/Ti wire of a few nanometers in width was frequently observed among the wires of various widths. After the formation of the Au/Ti nanowires, the carbon nanotube on the metal nanowire could be removed by atomic force microscope.

Electronic properties of carbon nanohorns studied by ESR

Abstract: The magnetic properties of carbon nanohorns were investigated by electron spin resonance. Two decoupled electronic systems were found. The first one, which has a spin susceptibility linearly varying with temperature, was tentatively attributed to two-dimensional graphenelike sheets. The second contribution, showing a Curie-like behavior with a non-negligible Pauli part, was ascribed to the interior of the structures of coagulated nanohorns. Surprisingly, this second line shows a strong suppression of susceptibility at 17 K. Due to a concomitant line broadening and g shift below 17 K, this transition was assigned to antiferromagnetic pairing of localized spins. The low spin concentration is implying that a kind of magnetism, mediated by the free spin carriers, has to be invoked to explain the phenomenon.

Doping-dependent electronic structure of cuprates studied using angle-scanned photoemission

Abstract: Full kk-maps of the electronic structure near the Fermi level of dierently doped cuprates measured with angle-scanned photoelectron spectroscopy are presented. The valence band maximum of the antiferromagnetic insulator Sr2CuO2Cl2, which is taken as a representative of an undoped cuprate, and the Fermi surfaces of overdoped, optimally doped and underdoped Bi2Sr2CaCu2O8+ high-temperature superconductors are mapped in the normal state. The results conrm the existence of large Luttinger Fermi surfaces at high doping with a Fermi surface volume proportional to (1 +x), where x is the hole concentration. At very low doping, however, we nd that this assumption based on Luttinger's theorem is not fullled. This implies a change in the topology of the Fermi surface. Furthermore the intensity of the shadow bands observed on the Fermi surface of Bi2Sr2CaCu2O8+ as a function of the doping is discussed. PACS. 79.60.-i Photoemission and photoelectron spectra { 74.25.Jb Electronic structure { 71.18.+y Fermi surface: calculations and measurements; eective mass, g factor

Superfluid and normal-fluid densities in the high-Tc superconductors

Abstract: In clean metallic superconductors, 100% of the mobile carriers participate in the condensate, so that the London penetration depth (which measures the electromagnetic screening by the superconductor) indicates charge densities comparable to those inferred from the free-carrier plasma frequency. In the cuprates, this is not the case, even though penetration depth measurements have shown a good correlation between superfluid density and superconducting transition temperature in the underdoped-to-optimally-doped part of the phase diagram. Optical measurements, which permit independent determination of the total doping-induced spectral weight and the superfluid density, show that in optimally doped materials only about 20% of the doping-induced spectral weight joins the superfluid. The rest remains in finite-frequency, midinfrared absorption. In underdoped materials, the superfluid fraction is even smaller. This result implies extremely strong coupling for these superconductors.

Alternative pseudogap scenario: Spectroscopic analogies between underdoped and disordered Bi2Sr2CaCu2O8¿x

Abstract: Disorder has a strong influence on the spectral properties of the Bi2Sr2CaCu2O8+x high-T-c superconductor, both in the normal and in the superconducting state. High-resolution photoemission reveals a progressive quasiparticle suppression with increasing disorder in electron-irradiated optimally doped single crystals. The spectral Line shapes of the disordered samples show striking analogies with those of underdoped samples specifically the widely discussed pseudogap at the chemical potential. Disorder provides therefore an alternative mechanism for spectral weight suppression near the Fermi surface, possibly coexisting and competing with other mechanisms. Since doping unavoidably introduces disorder, these results require a reconsideration of the relative importance of intrinsic effects vs disorder in high temperature superconductivity.

Magnetic resonance in the antiferromagnetic and normal state of NH 3 K 3 C 60

Abstract: We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin-susceptibilty measured by ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.

Superconducting gap and pseudogap in Bi-2212

Abstract: We present results of Raman scattering experiments in differently doped Bi 2 Sr 2 (Ca x Y 1− x )Cu 2 O 8+ δ (Bi-2212) single crystals. Below T c the spectra show pair–breaking features in the whole doping range reflecting a d x 2 – y 2 order parameter. In the normal state between T c and T ∗ ≃200 K we find signatures for a pseudogap in B 2g symmetry which disappear above optimal doping.

Slater Transition in the Pyrochlore Cd2Os2O7

Abstract: Cd2Os2O7 crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in Cd2Os2O7 using X-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. Cd2Os2O7 orders magnetically at the MIT. The magnetization data is consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on Cd2Os2O7. These calculations indicate that Cd2Os2O7 is metallic, with a sharp peak in the density of states at the Fermi energy. We intepret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition-unlike a Mott transition-is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below $T_{MIT}$.

Alkali specific effects in superconducting fullerides: the observation of a high temperature insulating phase in Na_2CsC_60

Abstract: Electron Spin Resonance and optical reflectivity measurements demonstrate a metal-insulator transition in Na_2CsC_60 as the system passes from the low temperature simple cubic to the high temperature {\it fcc} structure above 300 K The non-conducting electronic state is especially unexpected in view of the metallic character of other, apparently isostructural fullerides, like K_3C_60 The occurence of this phase in Na_2CsC_60 suggests that alkali specific effects can not be neglected in the description of the electronic properties of alkali doped fullerides We discuss the origin of the insulating state and the relevance of our results for the anomaly observed in the magnitude of the superconducting transition temperature of Na_2AC_60 fullerides