arXiv subjects

M. Huth

Publications and source records attributed to M. Huth.

31 records · Page 2Linked to original sources

arXiv2016

Fabrication of FeSi and Fe3Si compounds by electron beam induced mixing of [Fe/Si]2 and [Fe3/Si]2 multilayers grown by focused electron beam induced deposition

Fe-Si binary compounds have been fabricated by focused electron beam induced deposition by the alternating use of iron pentacarbonyl, Fe(CO)5, and neopentasilane, Si5H12 as precursor gases. The fabrication procedure consisted in preparing multilayer structures which were treated by low-energy electron irradiation and annealing to induce atomic species intermixing. In this way we are able to fabricate FeSi and Fe3Si binary compounds from [Fe=Si]2 and [Fe3=Si]2 multilayers, as shown by transmission electron microscopy investigations. This fabrication procedure is useful to obtain nanostructured binary alloys from precursors which compete for adsorption sites during growth and, therefore, cannot be used simultaneously.

cond-mat.mes-hall↗

arXiv2015

Direct writing of CoFe alloy nanostructures by focused electron beam induced deposition from a heteronuclear precursor

Recently, focused electron beam induced deposition has been employed to prepare functional magnetic nanostructures with potential in nanomagnetic logic and sensing applications by using homonuclear precursor gases like Fe(CO)5 or Co2(CO)8. Here we show that an extension towards the fabrication of bi-metallic compounds is possible by using a single-source heteronuclear precursor gas. We have grown CoFe alloy magnetic nanostructures from the HFeCo3(CO)12 metal carbonyl precursor. The compositional analysis indicate that the samples contain about 80 at% of metal and 10 at% of carbon and oxygen. Four-probe magnetotransport measurements are carried out on nanowires of various sizes down to a width of 50 nm, for which the room temperature resistivity of 43~$μΩ$cm is found. Micro-Hall magnetometry reveals that 50~nm$\times$250~nm nanobars of the material are ferromagnetic up to the highest measured temperature of 250 K. Finally, the TEM microstructural investigation shows that the deposits consist of a bcc Co-Fe phase mixed with a FeCo2O4 spinel oxide phase with nanograins of about 5 nm diameter.

cond-mat.mes-hall↗

arXiv2013

Long-range proximity-induced superconductivity in polycrystalline Co nanowires

We report an experimental study of proximity effect-induced superconductivity in crystalline Cu and Co nanowires and a nanogranular Co nanowire structure in contact with a superconducting W floating electrode which we call inducer. The nanowires were grown by electrochemical deposition in heavy-ion-track etched polycarbonate templates. The nanogranular Co structure was fabricated by focused electron beam induced deposition (FEBID), while the amorphous W inducer was obtained by focused ion beam induced deposition (FIBID). For electrical resistance measurements up to three pairs of Pt voltage leads were deposited by FIBID at different distances beside the inner inducer electrode, thus allowing us to probe the proximity effect over a length of 2-12 $μ$m. Relative $R(T)$ drops of the same order of magnitude have been observed for the Co and Cu nanowires when sweeping the temperature below 5.2 K ($T_c$ of the FIBID-deposited W inducer). By contrast, relative $R(T)$ drops were found to be an order of magnitude smaller for the nanogranular Co nanowire structure. Our analysis of the resistance data shows that the superconducting proximity length in crystalline Cu and Co is about 1 $μ$m at low temperatures, attesting to a long-range proximity effect in the case of ferromagnetic Co. Moreover, this long-range proximity effect has been revealed to be insusceptible to magnetic fields up to 11 T, which is indicative of spin-triplet pairing. At the same time, in the nanogranular Co structure proximity-induced superconductivity is strongly suppressed due to the dominating Cooper pair scattering caused by the intrinsic microstructure of the FEBID deposit.

cond-mat.supr-con↗

arXiv2011

Magnetotransport properties of iron microwires fabricated by focused electron beam induced autocatalytic growth

We have prepared iron microwires in a combination of focused electron beam induced deposition (FEBID) and autocatalytic growth from the iron pentacarbonyl, Fe(CO)5, precursor gas under UHV conditions. The electrical transport properties of the microwires were investigated and it was found that the temperature dependence of the longitudinal resistivity (rhoxx) shows a typical metallic behaviour with a room temperature value of about 88 microΩ cm. In order to investigate the magnetotransport properties we have measured the isothermal Hall-resistivities in the range between 4.2 K and 260 K. From these measurements positive values for the ordinary and the anomalous Hall coefficients were derived. The relation between anomalous Hall resistivity (rhoAN) and longitudinal resistivity is quadratic, rhoAN rho^2 xx, revealing an intrinsic origin of the anomalous Hall effect. Finally, at low temperature in the transversal geometry a negative magnetoresistance of about 0.2 % was measured.

cond-mat.mtrl-sci↗

arXiv2011

High-$κ$ field-effect transistor with copper-phthalocyanine

The use of SrTiO$_3$ dielectrics as high-permittivity insulator in organic thin film field effect transistors (FET) is evaluated. Field-effect transistors with sputtered SrTiO$_3$ and copper-phthalocyanine (CuPc) as semiconducting layer were fabricated. The device preparation was performed in-situ in an ultra high vacuum chamber system. The dielectric in the transistors had a permittivity of up to 200 which led to low driving voltages of 3 V. The field effect transistors were p-type and reached mobilities of about $μ= 1.5\times 10^{-3}$ cm$^2$/Vs and an on/off ratio of $10^3$. These properties are compared to devices based on other dielectric materials.

cond-mat.mtrl-sci↗

arXiv2011

Tuning the electrical conductivity of Pt-containing granular metals by postgrowth electron irradiation

We have fabricated Pt-containing granular metals by focused electron beam induced deposition from the $(CH_3)_3CH_3C_5H_4Pt$ precursor gas. The granular metals are made of platinum nanocrystallites embedded in a carbonaceous matrix. We have exposed the as-grown nanocomposites to low energy electron beam irradiation and we have measured the electrical conductivity as a function of the irradiation dose. Postgrowth electron beam irradiation transforms the matrix microstructure and thus the strength of the tunneling coupling between Pt nanocrystallites. For as-grown samples (weak tunnel coupling regime) we find that the temperature dependence of the electrical conductivity follows the stretched exponential behavior characteristic of the correlated variable-range hopping transport regime. For briefly irradiated samples (strong tunnel coupling regime) the electrical conductivity is tuned across the metal-insulator transition. For long-time irradiated samples the electrical conductivity behaves like that of a metal. In order to further analyze changes of the microstructure as a function of the electron irradiation dose we have carried out transmission electron microscope (TEM), micro-Raman and atomic force microscopy (AFM) investigations. TEM pictures reveal that the crystallites' size of long-time irradiated samples is larger than that of as-grown samples. Furthermore we do not have evidence of microstructural changes in briefly irradiated samples. By means of micro-Raman we find that by increasing the irradiation dose the matrix changes following a graphitization trajectory between amorphous carbon and nanocrystalline graphite. Finally, by means of AFM measurements we observe a reduction of the volume of the samples with increasing irradiation time which we attribute to the removal of carbon molecules.

cond-mat.mtrl-sci↗

arXiv2010

A new charge-transfer complex in UHV co-deposited tetramethoxypyrene and tetracyanoquinodimethane

UHV-deposited films of the mixed phase of tetramethoxypyrene and tetracyanoquinodimethane (TMP1-TCNQ1) on gold have been studied using ultraviolet photoelectron spectroscopy (UPS), X-ray-diffraction (XRD), infrared (IR) spectroscopy and scanning tunnelling spectroscopy (STS). The formation of an intermolecular charge-transfer (CT) compound is evident from the appearance of new reflexes in XRD (d1= 0.894 nm, d2= 0.677 nm). A softening of the CN stretching vibration (red-shift by 7 cm-1) of TCNQ is visible in the IR spectra, being indicative of a CT of the order of 0.3e from TMP to TCNQ in the complex. Characteristic shifts of the electronic level positions occur in UPS and STS that are in reasonable agreement with the prediction of from DFT calculations (Gaussian03 with hybrid functional B3LYP). STS reveals a HOMO-LUMO gap of the CT complex of about 1.25 eV being much smaller than the gaps (>3.0 eV) of the pure moieties. The electron-injection and hole-injection barriers are 0.3 eV and 0.5 eV, respectively. Systematic differences in the positions of the HOMOs determined by UPS and STS are discussed in terms of the different information content of the two methods.

cond-mat.mtrl-sci↗

arXiv2009

Transient electrical conductivity of W-based electron beam induced deposits during growth, irradiation and exposure to air

W-based granular metals have been prepared by electron beam induced deposition from the tungsten-hexacarbonyl W(CO)6 precursor. In situ electrical conductivity measurements have been performed to monitor the growth process and to investigate the behavior of the deposit under electron beam post irradiation and by exposure to air. During the first part of the growth process, the electrical conductivity grows non-linearly, independent of the electron beam parameters. This behavior is interpreted as the result of the increase of the W-particles diameter. Once the growth process is terminated, the electrical conductivity decreases with the logarithm of time, sigma ln(t). Temperature-dependent conductivity measurements of the deposits reveal that the electrical transport takes place by means of electron tunneling either between W-metal grains or between grains and trap sites in the matrix. After venting the electron microscope the electrical conductivity of the deposits shows a degradation behavior, which depends on the composition. Electron post-irradiation increases the electrical conductivity of the deposits.

cond-mat.mtrl-sci↗

arXiv2007

Criterion of multi-switching stability for magnetic nanoparticles

We present a procedure to study the switching and the stability of an array of magnetic nanoparticles in the dynamical regime. The procedure leads to the criterion of multi-switching stability to be satisfied in order to have stable switching. The criterion is used to compare various magnetic-field-induced switching schemes, either present in the literature or suggested in the present work. In particular, we perform micromagnetic simulations to study the magnetization trajectories and the stability of the magnetization after switching for nanoparticles of elliptical shape. We evaluate the stability of the switching as a function of the thickness of the particles and the rise and fall times of the magnetic pulses, both at zero and room temperature. Furthermore, we investigate the role of the dipolar interaction and its influence on the various switching schemes. We find that the criterion of multi-switching stability can be satisfied at room temperature and in the presence of dipolar interactions for pulses shaped according to CMOS specifications, for switching rates in the GHz regime.

cond-mat.mtrl-sci↗

arXiv2002

Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3

The optical properties of the heavy-fermion compound UPd$_2$Al$_3$ have been measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm$^{-1}$) at temperatures $2 {\rm K}<T< 300$ K. Below the coherence temperature $T^*\approx 50$ K, the hybridization gap opens around 10 meV. As the temperature decreases further ($T\leq 20$ K), a well pronounced pseudogap of approximately 0.2 meV develops in the optical response; we relate this to the antiferromagnetic ordering which occurs below $T_N\approx 14$ K. The frequency dependent mass and scattering rate give evidence that the enhancement of the effective mass mainly occurs below the energy which is associated to the magnetic correlations between the itinerant and localized 5f electrons. In addition to this correlation gap, we observe a narrow zero-frequency conductivity peak which at 2 K is less than 0.1 meV wide, and which contains only a fraction of the delocalized carriers. The analysis of the spectral weight infers a loss of kinetic energy associated with the superconducting transition.

cond-mat.str-el↗

arXiv2002

The Nature of Heavy Quasiparticles in Magnetically Ordered Heavy Fermions

The optical conductivity of the heavy fermions UPd2Al3 and UPt3 has been measured in the frequency range from 10 GHz to 1.2 THz (0.04 meV to 5 meV) at temperatures 1 K < T < 300 K. In both compounds a well pronounced pseudogap of less than a meV develops in the optical response at low temperatures; we relate this to the antiferromagnetic ordering. From the energy dependence of the effective electronic mass and scattering rate we derive the energies essential for the heavy quasiparticle. We find that the enhancement of the mass mainly occurs below the energy which is related to magnetic correlations between the local magnetic moments and the itinerant electrons. This implies that the magnetic order in these compounds is the pre-requisite to the formation of the heavy quasiparticle and eventually of superconductivity.

cond-mat.str-el↗

arXiv2000

Pseudogap in the Optical Spectra of UPd_2Al_3

The in-plane optical conductivity of UPd_2Al_3 was measured at temperatures $2 {\rm K}<T<300$ K in the spectral range from 1 cm^{-1} to 40 cm^{-1} (0.14 meV to 5 meV). As the temperature decreases below 25 K a well pronounced pseudogap of 0.2 meV develops in the optical response. In addition we observe a narrow conductivity peak at zero frequency which at 2 K is less than 1 cm^{-1} wide but which contains only a fraction of the delocalized carriers. The gap in the electronic excitations might be an inherent feature of the heavy fermioin ground state.

cond-mat↗

arXiv1997

The influence of crystal-field effects on the electronic transport properties of heavy-fermion systems: a semiphenomenological approach

The electronic transport properties of heavy-fermion systems were calculated based on a semiphenomenological approach to the lattice non-crossing approximation in the limit of infinite local correlations augmented by crystal-field effects. Within the scope of this calculation using the linearized Boltzmann theory in the relaxation time approximation the qualitative features of the temperature-dependent resistivity, the magnetoresistivity and the thermoelectric power can be successfully reproduced; this is exemplified by a comparison with experimental results on CeCu_2Si_2.

cond-mat.str-el↗