Péter Szirmai

Synthesis of Homogeneous Manganese-Doped Titanium Oxide Nanotubes from Titanate Precursors

We report a novel synthesis route of homogeneously manganese-doped TiO2 nanotubes in a broad concentration range. The scroll-type trititanate (H2Ti3O7) nanotubes prepared by hydrothermal synthesis were used as precursors. Mn2+ ions were introduced by an ion exchange method resulting MnxH2–xTi3O7. In a subsequent heat treatment, they were transformed into MnyTi1–yO2, where y = x/(3 + x). The state and the local environment of the Mn2+ ions in the precursor and final products were studied by the electron spin resonance (ESR) technique. It was found that the Mn2+ ions occupy two positions: the first having an almost perfect cubic symmetry while the other is in a strongly distorted octahedral site. The ratio of the two Mn2+ sites is independent of the doping level and amounts to 15:85 in MnxH2–xTi3O7 and to 5:95 in MnyTi1–yO2. SQUID magnetometry does not show long-range magnetic order in the homogeneously Mn2+-doped nanotubes.

A detailed analysis of the Raman spectra in superconducting boron doped nanocrystalline diamond

The light scattering properties of superconducting (Tc  ≈ 3.8 K) heavily boron doped nanocrystalline diamond has been investigated by Raman spectroscopy using visible excitations. Fano type interference of the zone-center phonon line and the electronic continuum was identified. Lineshape analysis reveals Fano lineshapes with a significant asymmetry (q ≈ −2). An anomalous wavelength dependence and small value of the Raman scattering amplitude is observed in agreement with previous studies.

Density of states deduced from ESR measurements on low‐dimensional nanostructures; benchmarks to identify the ESR signals of graphene and SWCNTs

Electron spin resonance (ESR) spectroscopy is an important tool to characterize the ground state of conduction electrons and to measure their spin-relaxation times. Observing ESR of the itinerant electrons is thus of great importance in graphene and in single-wall carbon nanotubes. Often, the identification of CESR signal is based on two facts: the apparent asymmetry of the ESR signal (known as a Dysonian lineshape) and on the temperature independence of the ESR signal intensity. We argue that these are insufficient as benchmarks and instead the ESR signal intensity (when calibrated against an intensity reference) yields an accurate characterization. We detail the method to obtain the density of states from an ESR signal, which can be compared with theoretical estimates. We demonstrate the success of the method for K doped graphite powder. We give a benchmark for the observation of ESR in graphene.

Observation of conduction electron spin resonance in boron-doped diamond

We observe the electron spin resonance of conduction electrons in boron-doped (6400 ppm) superconducting diamond (Tc=3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent electron spin resonance signal intensity and its magnitude, which is in good agreement with that expected from the density of states through the Pauli spin susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature, which can be understood in the framework of the Elliott-Yafet theory of spin relaxation. An anomalous and yet unexplained relation is observed between the g-factor, CESR linewidth, and the resistivity using the empirical Elliott-Yafet relation.

Doped carbon nanotubes as a model system of biased graphene

Albeit difficult to access experimentally, the density of states (DOS) is a key parameter in solid-state systems, which governs several important phenomena including transport, magnetism, thermal, and thermoelectric properties. We study DOS in an ensemble of potassium intercalated single-wall carbon nanotubes and show, using electron spin resonance spectroscopy, that a sizable number of electron states are present, which gives rise to a Fermi-liquid behavior in this material. A comparison between theoretical and the experimental DOS indicates that it does not display significant correlation effects, even though the pristine nanotube material shows a Luttinger-liquid behavior. We argue that the carbon nanotube ensemble essentially maps out the whole Brillouin zone of graphene, thus it acts as a model system of biased graphene.

Transport, magnetic and vibrational properties of chemically exfoliated few-layer graphene

We study the vibrational, magnetic and transport properties of Few Layer Graphene (FLG) using Raman and electron spin resonance spectroscopy and microwave conductivity measurements. FLG samples were produced using wet chemical exfoliation with different post-processing, namely ultrasound treatment, shear mixing, and magnetic stirring. Raman spectroscopy shows a low intensity D mode which attests a high sample quality. The G mode is present at 1580 cm(-1) as expected for graphene. The 2D mode consists of 2 components with varying intensities among the different samples. This is assigned to the presence of single and few layer graphene in the samples. Electron Spin Resonance (ESR) spectroscopy shows a main line in all types of materials with a width of about 1 mT and a g-factor in the range of 2.005-2.010. Paramagnetic defect centers with a uniaxial g-factor anisotropy are identified, which shows that these are related to the local sp2 bonds of the material. All kinds of investigated FLGs have a temperature dependent resistance which is compatible with a small gap semiconductor. The difference in resistance is related to the different grain size of the samples

Magnetotransport studies of Superconducting Pr

We report a study of the electrical transport properties of single crystals of Pr(4)Fe(2)As(2)Te(1-)xO(4), a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohlers law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic c axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of Pr4Fe2As2Te1-xO4. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

_4

We report a study of the electrical transport properties of single crystals of Pr(4)Fe(2)As(2)Te(1-)xO(4), a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohlers law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic c axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of Pr4Fe2As2Te1-xO4. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

Fe

We report a study of the electrical transport properties of single crystals of Pr(4)Fe(2)As(2)Te(1-)xO(4), a recently discovered iron-based superconductor. Resistivity, Hall effect, and magnetoresistance are measured in a broad temperature range revealing the role of electrons as dominant charge carriers. The significant temperature dependence of the Hall coefficient and the violation of Kohlers law indicate multiband effects in this compound. The upper critical field and the magnetic anisotropy are investigated in fields up to 16 T, applied parallel and perpendicular to the crystallographic c axis. Hydrostatic pressure up to 2 GPa linearly increases the critical temperature and the resistivity residual ratio. A simple two-band model is used to describe the transport and magnetic properties of Pr4Fe2As2Te1-xO4. The model can successfully explain the strongly temperature-dependent negative Hall coefficient and the high magnetic anisotropy, assuming that the mobility of electrons is higher than that of holes.

_2

We report electron spin resonance (ESR) measurements on stage-I potassium intercalated graphite (KC8). Angular dependent measurements show that the spin-lattice relaxation time is longer when the magnetic field is perpendicular to the graphene layer as compared to when the magnetic field is in the plane. This anisotropy is analyzed in the framework of the Elliott-Yafet theory of spin-relaxation in metals. The analysis considers an anisotropic spin-orbit Hamiltonian and the first order perturbative treatment of Elliott is reproduced for this model Hamiltonian. The result provides an experimental input for the first-principles theories of spin-orbit interaction in layered carbon and thus to a better understanding of spin-relaxation phenomena in graphene and in other layered materials as well.