arXiv subjects

F. Simon

Publications and source records attributed to F. Simon.

At least 91 records · Page 5Linked to original sources

arXiv2008

Generalized Elliott-Yafet theory of electron spin relaxation in metals: the origin of the anomalous electron spin life-time in MgB2

The temperature dependence of the electron spin relaxation time in MgB2 is anomalous as it does not follow the temperature dependence of the resistivity above 150 K, it has a maximum around 400 K, and it decreases for higher temperatures. This violates the well established Elliot-Yafet theory of electron spin relaxation in metals. We show that the anomaly occurs when the quasi-particle scattering rate (in energy units) becomes comparable to the energy difference between the conduction- and a neighboring band. We find that the anomalous behavior is related to the unique band structure of MgB$_2$ and the large electron-phonon coupling. The saturating spin-lattice relaxation can be regarded as the spin transport analogue of the Ioffe-Regel criterion of electron transport.

cond-mat.str-el↗

arXiv2008

New Prototype Multi-gap Resistive Plate Chambers with Long Strips

A new kind of Multi-gap Resistive Plate Chamber (MRPC) has been built for the large-area Muon Telescope Detector (MTD) for the STAR experiment at RHIC. These long read-out strip MRPCs (LMRPCs) have an active area of 87.0 x 17.0 cm2 and ten 250 um-thick gas gaps arranged as a double stack. Each read-out strip is 2.5 cm wide and 90 cm long. The signals are read-out at both ends of each strip. Cosmic ray tests indicate a time resolution of ~70 ps and a detection efficiency of greater than 95%. Beam tests performed at T963 at Fermilab indicate a time resolution of 60-70 ps and a spatial resolution of ~1 cm along the strip direction.

physics.ins-det↗

arXiv2008

Electron spin resonance signal of Luttinger liquids and single-wall carbon nanotubes

A comprehensive theory of electron spin resonance (ESR) for a Luttinger liquid (LL) state of correlated metals is presented. The ESR measurables such as the signal intensity and the line-width are calculated in the framework of Luttinger liquid theory with broken spin rotational symmetry as a function of magnetic field and temperature. We obtain a significant temperature dependent homogeneous line-broadening which is related to the spin symmetry breaking and the electron-electron interaction. The result crosses over smoothly to the ESR of itinerant electrons in the non-interacting limit. These findings explain the absence of the long-sought ESR signal of itinerant electrons in single-wall carbon nanotubes when considering realistic experimental conditions.

cond-mat.str-el↗

arXiv2008

Enhanced thermal stability and spin-lattice relaxation rate of N@C60 inside carbon nanotubes

We studied the temperature stability of the endohedral fullerene molecule, N@C60, inside single-wall carbon nanotubes using electron spin resonance spectroscopy. We found that the nitrogen escapes at higher temperatures in the encapsulated material as compared to its pristine, crystalline form. The temperature dependent spin-lattice relaxation time, T_1, of the encapsulated molecule is significantly shorter than that of the crystalline material, which is explained by the interaction of the nitrogen spin with the conduction electrons of the nanotubes.

cond-mat.mtrl-sci↗

arXiv2008

Mg5C60: A stable two dimensional conducting polymer

We present a study on the structural, spectroscopic, conducting and magnetic properties of Mg5C60, a two dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. Infrared and Raman studies suggest the formation of single bonds between fulleride ions and possibly Mg - C60 covalent bonds. Mg5C60 is a metal at ambient temperature as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder.

cond-mat.str-el↗

arXiv2007

Development of Tracking Detectors with industrially produced GEM Foils

The planned tracking upgrade of the STAR experiment at RHIC includes a large-area GEM tracker used to determine the charge sign of electrons and positrons produced from W+(-) decays. For such a large-scale project commercial availability of GEM foils is necessary. We report first results obtained with a triple GEM detector using GEM foils produced by Tech-Etch Inc. of Plymouth, MA, USA. Measurements of gain uniformity, long-term stability as well as measurements of the energy resolution for X-Rays are compared to results obtained with an identical detector using GEM foils produced at CERN. A quality assurance procedure based on optical tests using an automated high-resolution scanner has been established, allowing a study of the correlation of the observed behavior of the detector and the geometrical properties of the GEM foils. Detectors based on Tech-Etch and CERN produced foils both show good uniformity of the gain over the active area and stable gain after an initial charge-up period, making them well suited for precision tracking applications.

physics.ins-det↗

arXiv2007

Inter-shell interaction in double walled carbon nanotubes: charge transfer and orbital mixing

Recent nuclear magnetic resonance measurements on isotope engineered double walled carbon nanotubes (DWCNTs) surprisingly suggest a uniformly metallic character of all nanotubes, which can only be explained by the interaction between the layers. Here we study the inter-shell interaction in DWCNTs by density functional theory and inter-molecular Hückel model. We find charge transfer between the layers using both methods. We show that not only does the charge transfer appear already at the fundamental level of the inter-molecular Hückel model, but also that the spatial distribution of the change in the electron density is well described already at this level of theory. We find that the charge transfer between the walls is on the order of 0.001 e/atom and that the inner tube is always negatively charged. We also observe orbital mixing between the states of the layers. We find that these two effects combined can in some cases lead to a semiconductor--to--metal transition of the double walled tube, but not necessarily in all cases.

cond-mat.mtrl-sci↗

arXiv2007

Conduction electron spin-lattice relaxation time in the MgB2 superconductor

The spin-lattice relaxation time, $T_{1}$, of conduction electrons is measured as a function of temperature and magnetic field in MgB$_2$. The method is based on the detection of the $z$ component of the conduction electron magnetization under electron spin resonance conditions with amplitude modulated microwave excitation. Measurement of $T_{1}$ below $T_c$ at 0.32 T allows to disentangle contributions from the two Fermi surfaces of MgB$_{2}$ as this field restores normal state on the Fermi surface part with $π$ symmetry only.

cond-mat.supr-con↗

arXiv2006

Spin excitations in the antiferromagnet NaNiO2

In NaNiO2, Ni3+ ions form a quasi two dimensional triangular lattice of S = 1=2 spins. The magnetic order observed below 20K has been described as an A type antiferromagnet with ferro- magnetic layers weakly coupled antiferromagnetically. We studied the magnetic excitations with the electron spin resonance for frequencies 1-20 cm-1, in magnetic fields up to 14 T. The bulk of the results are interpreted in terms of a phenomenological model involving bi-axial anisotropy for the spins: a strong easy-plane term, and a weaker anisotropy within the plane. The direction of the easy plane is constrained by the collective Jahn-Teller distortion occurring in this material at 480 K.

cond-mat.str-el↗

arXiv2006

Magnetic fullerenes inside single-wall carbon nanotubes

C59N magnetic fullerenes were formed inside single-wall carbon nanotubes by vacuum annealing functionalized C59N molecules encapsulated inside the tubes. A hindered, anisotropic rotation of C59N was deduced from the temperature dependence of the electron spin resonance spectra near room temperature. Shortening of spin-lattice relaxation time, T_1, of C59N indicates a reversible charge transfer toward the host nanotubes above $\sim 350$ K. Bound C59N-C60 heterodimers are formed at lower temperatures when C60 is co-encapsulated with the functionalized C59N. In the 10-300 K range, T_1 of the heterodimer shows a relaxation dominated by the conduction electrons on the nanotubes.

cond-mat.str-el↗

arXiv2006

Encapsulating C59N azafullerene derivatives inside single-wall carbon nanotubes

Filling of single-wall carbon nanotubes with C59N azafullerene derivatives is reported from toluene solvent at ambient temperature. The filling is characterized by high resolution transmission electron microscopy and Raman spectroscopy. The filling efficiency is the same as for C60 fullerenes and the tube-azafullerene interaction is similar to the tube-C60 interaction. Vacuum annealing of the encapsulated azafullerene results in the growth of inner tubes, however no spectroscopic signature of nitrogen built in the inner walls is detected.

cond-mat.mtrl-sci↗

arXiv2005

NMR Evidence for Gapped Spin Excitations in Metallic Carbon Nanotubes

We report on the spin dynamics of 13C isotope enriched inner-walls in double-wall carbon nanotubes (DWCNT) using 13C nuclear magnetic resonance (NMR). Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the innerwall nanotubes detected by NMR. In the high temperature regime (T > 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T < 150 K, we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below 20 K. The data clearly indicates the formation of a gap in the spin excitation spectrum, where the gap value 2 Delta = 40 K (= 3.7 meV) is H independent.

cond-mat.str-el↗

arXiv2005

Fine Structure of the Radial Breathing Mode in Double-Wall Carbon Nanotubes

The analysis of the Raman scattering cross section of the radial breathing modes of double-wall carbon nanotubes allowed to determine the optical transitions of the inner tubes. The Raman lines are found to cluster into species with similar resonance behavior. The lowest components of the clusters correspond well to SDS wrapped HiPco tubes. Each cluster represents one particular inner tube inside different outer tubes and each member of the clusters represents one well defined pair of inner and outer tubes. The number of components in one cluster increases with decreasing of the inner tube diameter and can be as high as 14.

cond-mat.soft↗

arXiv2005

Highly unperturbed inner tubes in CVD grown double-wall carbon nanotubes

The synthesis of double-wall carbon nanotubes (DWCNTs) with highly unperturbed inner shells is reported using the catalytic vapor deposition method. Temperature dependent and high resolution Raman measurements show an enhanced phonon life-time of the inner tubes with respect to the outer ones and similar diameter SWCNTs. This proves that the inner tubes are unperturbed similar to the inner tubes in peapod-grown DWCNTs. The presence of the outer tube is argued to protect the inner tube from interaction with impurities and also to stabilize the growth of defect free inner tubes. The current material underlines the application potential of DWCNTs.

cond-mat.mtrl-sci↗

arXiv2004

Highly 13C isotope enriched azafullerene, C59N, for nuclear spin labelling

Synthesis of highly $^{13}$C isotope enriched azafullerene, C$_{59}$N embedded in C$_{60}$ is reported. $^{13}$C enriched fullerenes, produced with the Krätschmer-Huffmann process, were subject to a N$_{2}$ discharge that produces C$_{59}$N with a low probability. Raman spectroscopy indicates a homogeneous $^{13}$C distribution. Electron spin resonance measurement (ESR) proves that the C$_{59}$N concentration, 0.2 %, is similar as in non-enriched fullerenes. The ESR spectrum is simulated accurately with the known $^{14}$N and $^{13}$C hyperfine coupling constants. The material enables the nuclear spin-labelling of heterofullerene complexes with a potential for biological applications. It might also find applications as a building element for quantum computation.

cond-mat.mtrl-sci↗

arXiv2004

Interaction between concentric Tubes in DWCNTs

A detailed investigation of the Raman response of the inner tube radial breathing modes (RBMs) in double-wall carbon nanotubes is reported. It revealed that the number of observed RBMs is two to three times larger than the number of possible tubes in the studied frequency range. This unexpected increase in Raman lines is attributed to a splitting of the inner tube response. It is shown to originate from the possibility that one type of inner tube may form in different types of outer tubes and the fact that the inner tube RBM frequency depends on the diameter of the enclosing tube. Finally, a comparison of the inner tube RBMs and the RBMs of tubes in bundles gave clear evidence that the interaction in a bundle is stronger than the interaction between inner and outer tubes.

cond-mat.mtrl-sci↗

arXiv2004

Isotope engineering in carbon nanotube systems

We report on single-wall carbon nanotube (SWCNT) specific $^{13}$C isotope enrichment. The high temperature annealing of isotope enriched fullerenes encapsulated in SWCNTs yields double-wall carbon nanotubes (DWCNTs) with a high isotope enrichment of the inner wall. The vibrational spectra evidences that no carbon exchange occurs between the two walls. The method facilitates the identification of the Raman signal of the outer and inner tubes. Nuclear magnetic resonance proves the significant contrast of the isotope enriched SWCNTs as compared to other carbon phases, and provides information on the electronic properties of the small diameter inner tubes of the DWCNTs.

cond-mat.mtrl-sci↗

arXiv2004

The Raman response of double wall carbon nanotubes

Raman spectroscopy on carbon nanotubes (CNT) yields a rich variety of information owing to the close interplay between electronic and vibrational properties. In this paper, we review the properties of double wall carbon nanotubes (DWCNTs). In particular, it is shown that SWCNT encapsulating C$_{60}$, so-called peapods, are transformed into DWCNTs when subject to a high temperature treatment. The inner tubes are grown in a catalyst free environment and do not suffer from impurities or defects that are usually encountered for as-grown SWCNTs or DWCNTs. As a consequence, the inner tubes are grown with a high degree of perfection as deduced from the unusually narrow radial breathing mode (RBM) lines. This apostrophizes the interior of the SWCNTs as a nano-clean room. The mechanism of the inner nanotube production from C$_{60}$ is discussed. We also report recent studies aimed at the simplification and industrial scaling up of the DWCNT production process utilizing a low temperature peapod synthesis method. A splitting of the RBMs of inner tubes is observed. This is related to the interaction between the two shells of the DWCNTs as the same inner tube type can be encapsulated in different outer ones. The sharp appearance of the inner tube RBMs allows a reliable assignment of the tube modes to (n,m) indexes and thus provides a precise determination of the relation between the tube diameter and the RBM frequencies.

cond-mat.mtrl-sci↗