arXiv subjects

Ferenc Simon

Publications and source records attributed to Ferenc Simon.

At least 37 records · Page 2Linked to original sources

arXiv2013

A unified theory of spin-relaxation due to spin-orbit coupling in metals and semiconductors

Spintronics is an emerging paradigm with the aim to replace conventional electronics by using electron spins as information carriers. Its utility relies on the magnitude of the spin-relaxation, which is dominated by spin- orbit coupling (SOC). Yet, SOC induced spin-relaxation in metals and semiconductors is discussed for the seemingly orthogonal cases when inversion symmetry is retained or broken by the so-called Elliott-Yafet and Dyakonov-Perel spin-relaxation mechanisms, respectively. We unify the two theories on general grounds for a generic two-band system containing intra- and inter-band SOC. While the previously known limiting cases are recovered, we also identify parameter domains when a crossover occurs between them, i.e. when an inversion symmetry broken state evolves from a Dyakonov-Perel to an Elliott-Yafet type of spin-relaxation and conversely for a state with inversional symmetry. This provides an ultimate link between the two mechanisms of spin-relaxation.

cond-mat.str-el↗

arXiv2013

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 ESR 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.

cond-mat.mtrl-sci↗

arXiv2013

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.

cond-mat.mtrl-sci↗

arXiv2012

Floquet topological insulators

Topological insulators represent unique phases of matter with insulating bulk and conducting edge or surface states, immune to small perturbations such as backscattering due to disorder. This stems from their peculiar band structure, which provides topological protections. While conventional tools (pressure, doping etc.) to modify the band structure are available, time periodic perturbations can provide tunability by adding time as an extra dimension enhanced to the problem. In this short review, we outline the recent research on topological insulators in non equilibrium situations. Firstly, we introduce briefly the Floquet formalism that allows to describe steady states of the electronic system with an effective time-independent Hamiltonian. Secondly, we summarize recent theoretical work on how light irradiation drives semi-metallic graphene or a trivial semiconducting system into a topological phase. Finally, we show how photons can be used to probe topological edge or surface states.

cond-mat.mes-hall↗

arXiv2012

Testing the Elliott-Yafet spin-relaxation mechanism in KC8; a model system of biased graphene

Temperature dependent electron spin resonance (ESR) measurements are reported on stage 1 potassium doped graphite, a model system of biased graphene. The ESR linewidth is nearly isotropic and although the g-factor has a sizeable anisotropy, its majority is shown to arise due to macroscopic magnetization. Albeit the homogeneous ESR linewidth shows an unusual, non-linear temperature dependence, it appears to be proportional to the resistivity which is a quadratic function of the temperature. These observations suggests the validity of the Elliott-Yafet relaxation mechanism in KC8 and allows to place KC8 on the empirical Beuneu-Monod plot among ordinary elemental metals.

cond-mat.mtrl-sci↗

arXiv2012

Optically engineering the topological properties of a spin Hall insulator

Time-periodic perturbations can be used to engineer topological properties of matter by altering the Floquet band structure. This is demonstrated for a spin Hall insulator in the presence of monochromatic circularly polarized light. The inherent spin structure of the edge state is influenced by the Zeeman coupling and not by the orbital effect. The photocurrent (and the magnetization along the edge) develops a finite, helicity dependent expectation value and turns from dissipationless to dissipative with increasing radiation frequency, signalling a change in the topological properties. The connection with Thouless' charge pumping and non-equilibrium Zitterbewegung is discussed, together with possible experiments.

cond-mat.mes-hall↗

arXiv2011

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 (SWCNTs). 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.

cond-mat.mtrl-sci↗

arXiv2011

Adaptation of a commercial Raman spectrometer for multiline and broadband laser operation

A commercial single laser line Raman spectrometer is modified to accommodate multiline and tunable dye lasers, thus combining the high sensitivity of such single monochromator systems with broadband operation. Such instruments rely on high-throughput interference filters that perform both beam alignment and Rayleigh filtering. Our setup separates the dual task of the built-in monochromator into two independent elements: a beam splitter and a long pass filter. Filter rotation shifts the transmission passband, effectively expanding the range of operation. Rotation of the filters has a negligible effect on the optical path, allowing broadband operation and stray light rejection down to 70-150 1/cm. Operation is demonstrated on single-walled carbon nanotubes, for which the setup was optimized.

cond-mat.mtrl-sci↗

arXiv2011

A broadband and high throughput single-monochromator Raman spectrometer; application for single-wall carbon nanotubes

We present a high sensitivity single-monochromator Raman spectrometer which allows operation with a tunable laser source. The instrument is based on the modification of a commercial Raman spectrometer; such instruments operate with holographic Rayleigh filters which also act as a laser mirrors and are usually considered as inherently narrow-band. In our design, the two tasks are separated and the filter can be freely rotated without much effect on the light alignment. Since rotation shifts the filter passband, this modification allows tunable operation with efficient stray light filtering down to 150 1/cm. The design is optimized for single-wall carbon nanotubes, for which the performance is demonstrated using a tunable dye-laser source. The spectrometer thus combines the high sensitivity with the broadband characteristics of usual triple monochromator systems.

cond-mat.mtrl-sci↗

arXiv2009

Unusual hyperfine interaction of Dirac electrons and NMR spectroscopy in graphene

Theory of nuclear magnetic resonance (NMR) in graphene is presented. The canonical form of the electron-nucleus hyperfine interaction is strongly modified by the linear electronic dispersion. The NMR shift and spin-lattice relaxation time are calculated as function of temperature, chemical potential, and magnetic field and three distinct regimes are identified: Fermi-, Dirac-gas, and extreme quantum limit behaviors. A critical spectrometer assessment shows that NMR is within reach for fully 13C enriched graphene of reasonable size.

cond-mat.mes-hall↗

arXiv2009

Electron spin dynamics in strongly correlated metals

The temperature dependence of the electron spin life-time, T_1 and the g-factor are anomalous in alkali fullerides (K,Rb)_3C_60, which cannot be explained by the canonical Elliott-Yafet theory. These materials are archetypes of strongly correlated and narrow band metals. We introduce the concept of "complex electron spin resonance frequency shift" to treat these measurables in a unified manner within the Kubo formalism. The theory is applicable for metals with nearly degenerate conduction bands and large momentum scattering even with an anomalous temperature dependence and sizeable residual value.

cond-mat.str-el↗

arXiv2008

Thickness dependent magnetic anisotropy of ultrathin LCMO epitaxial thin films

The magnetic properties of La0.7Ca0.3MnO3 (LCMO) manganite thin films were studied with magnetometry and ferromagnetic resonance as a function of film thickness. They maintain the colossal magnetoresistance behavior with a pronounced metal-insulator transition around 150-200 K, except for the very thinnest films studied (3 nm). Nevertheless, LCMO films as thin as 3 nm remain ferromagnetic, without a decrease in saturation magnetization, indicating an absence of dead-layers, although below approx. 6 nm the films remain insulating at low temperature. Magnetization hysteresis loops reveal that the magnetic easy axes lie in the plane of the film for thicknesses in the range of 4-15 nm. Ferromagnetic resonance studies confirm that the easy axes are in-plane, and find a biaxial symmetry in-plane with two, perpendicular easy axes. The directions of the easy axes with respect to the crystallographic directions of the cubic SrTiO3 substrate differ by 45 degrees in 4 nm and 15 nm thick LCMO films.

cond-mat.str-el↗

arXiv2007

Spin gap and Luttinger liquid description of the NMR relaxation in carbon nanotubes

Recent NMR experiments by Singer et al. [Singer et al. Phys. Rev. Lett. 95, 236403 (2005).] showed a deviation from Fermi-liquid behavior in carbon nanotubes with an energy gap evident at low temperatures. Here, a comprehensive theory for the magnetic field and temperature dependent NMR 13C spin-lattice relaxation is given in the framework of the Tomonaga-Luttinger liquid. The low temperature properties are governed by a gapped relaxation due to a spin gap (~ 30K), which crosses over smoothly to the Luttinger liquid behaviour with increasing temperature.

cond-mat.str-el↗

arXiv2007

Recent advances in the internal functionalization of carbon nanotubes: synthesis, optical, and magnetic resonance studies

The hollow inside of single-wall carbon nanotubes (SWCNT) provides a unique degree of freedom to investigate chemical reactions inside this confined environment and to study the tube properties. It is reviewed herein, how encapsulating fullerenes, magnetic fullerenes, $^{13}$C isotope enriched fullerenes and organic solvents inside SWCNTs enables to yield unprecedented insight into their electronic, optical, and interfacial properties and to study their growth. Encapsulated C$_{60}$ fullerenes are transformed to inner tubes by a high temperature annealing. The unique, low defect concentration of inner tubes makes them ideal to study the effect of diameter dependent treatments such as opening and closing of the tubes. The growth of inner tubes is achieved from $^{13}$C enriched encapsulated organic solvents, which shows that fullerenes do not have a distinguished role and it opens new perspectives to explore the in-the-tube chemistry. Encapsulation of magnetic fullerenes, such as N@C$_{60}$ and C$_{59}$N is demonstrated using ESR. Growth of inner tubes from $^{13}$C enriched fullerenes provides a unique isotope engineered heteronuclear system, where the outer tubes contain natural carbon and the inner walls are controllably $^{13}$C isotope enriched. The material enables to identify the vibrational modes of inner tubes which otherwise strongly overlap with the outer tube modes. The $^{13}$C NMR signal of the material is specific for the small diameter SWCNTs. Temperature and field dependent $^{13}$C $T_1$ studies show a uniform metallic-like electronic state for all inner tubes and a low energy, ~3 meV gap is observed that is assigned to a long sought Peierls transition.

cond-mat.mtrl-sci↗

arXiv2006

Universal temperature dependence of optical excitation life-time and band-gap in chirality assigned semiconducting single-wall carbon nanotubes

The temperature dependence of optical excitation life-time, Gamma, and transition energies, E_ii, were measured for bucky-papers of single-wall carbon nanotubes (SWCNTs) and inner tubes in double-wall carbon nanotubes (DWCNTs) using resonant Raman scattering. The temperature dependence of Gamma and E_ii is the same for both types of samples and is independent of tube chirality. The data proves that electron-phonon interaction is responsible for temperature dependence of E_ii(T). The temperature independent inhomogeneous contribution to Gamma is much larger in the SWCNT samples, which is explained by the different SWCNT environment in the two types of samples. Gamma of the inner tubes for the bucky-paper DWCNT sample is as low as \sim 30 meV, which is comparable to that found for individual SWCNTs.

cond-mat.mtrl-sci↗

arXiv2006

Growth of single wall carbon nanotubes from $^{13}$C isotope labelled organic solvents inside single wall carbon nanotube hosts

Exploring the synthesis of novel molecular nanostructures has been in the forefront of material research in the last decade. One of the most interesting nanostructures are single wall carbon nanotubes (SWCNTs). Their catalyst free growth, however, remains an elusive goal. Here, we present the growth of single wall carbon nanotubes from organic solvents such as benzene and toluene in a confined environment, inside a host SWCNT. The solvents encapsulated in SWCNTs are transformed to an inner tube when subject to a heat treatment under dynamic vacuum at 1270 $^{\circ}$C. We used isotope labeling of the different carbon sources to prove that the source of the inner tubes is indeed the solvent. Our results put constraints on the models explaining the inner tube growth and provides a simple alternative for the fullerene based inner tube growth. It also provides the possibility to study a completely new field of in-the-tube chemistry.

cond-mat.mtrl-sci↗

arXiv2004

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9 T superconducting magnet is described. The probehead contains a compact two-loop, one gap resonator and is embedded in the variable temperature insert of the magnet enabling measurements in the 0- 9 T magnetic field and 1.5-400 K\ temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the $g$ $\approx 2$ paramagnetic condition such as in strongly interacting spin systems. The low quality factor of the resonator allows time resolved experiments such as e.g. longitudinally detected ESR. We demonstrate the performance of the spectrometer on the MgB$_{2}$ superconductor and the RbC$_{60}$ conducting alkaline fulleride polymer.

cond-mat.other↗

arXiv2000

Magnetic Field Induced Low-Energy Spin Excitations in YBa2Cu4O8 Measured by High Field Gd3+ ESR

We have measured the spin susceptibility of the underdoped high temperature superconductor, YBa2Cu4O8 by Gd^{3+} electron spin resonance in single crystals and aligned powders at several magnetic fields between 3 and 15.4 T. At low temperatures and high fields, the spin susceptibility of the CuO2 planes is enhanced slightly in the $B\parallel c$ orientation with respect to the $B\perp c$ orientation. The enhancement in an applied field of 15.4 T ($\approx 0.15 H_{c2}$) at 16 K (0.2 $T_c$) is approximately 10 percent of the susceptibility measured at $T_c$. Such a small magnitude suggests that the second critical field of superconductivity, $H_{c2}\approx 100 T$, would not suppress the pseudogap. This work demonstrates the potential of high field ESR in single crystals for studying high $T_c$ superconductors.

cond-mat.supr-con↗