Cyclotron Mass Research Articles

Dirac cones reshaped by interaction effects in suspended graphene

Graphene’s linear dispersion relation makes its charge carriers behave as if they were massless. However, near the Dirac point where graphene’s valence and conduction bands meet, electron–electron interactions cause this relation to diverge, such that it becomes strongly nonlinear and the effective carrier velocity doubles. In graphene, electron–electron interactions are expected to play a significant role, as the screening length diverges at the charge neutrality point and the conventional Landau theory that enables us to map a strongly interacting electronic liquid into a gas of non-interacting fermions is no longer applicable1,2. This should result in considerable changes in graphene’s linear spectrum, and even more dramatic scenarios, including the opening of an energy gap, have also been proposed3,4,5. Experimental evidence for such spectral changes is scarce, such that the strongest is probably a 20% difference between the Fermi velocities vF found in graphene and carbon nanotubes6. Here we report measurements of the cyclotron mass in suspended graphene for carrier concentrations n varying over three orders of magnitude. In contrast to the single-particle picture, the real spectrum of graphene is profoundly nonlinear near the neutrality point, and vF describing its slope increases by a factor of more than two and can reach ≈3×106 m s−1 at n<1010 cm−2. No gap is found at energies even as close to the Dirac point as ∼0.1 meV. The observed spectral changes are well described by the renormalization group approach, which yields corrections logarithmic in n.

Formation of molecular hydrogen from protonated 9,10-dihydroanthracene: Is the ejected H 2 rotationally and vibrationally excited?

The infrared multiple-photon dissociation (IRMPD) spectrum of protonated 9,10-dihydroanthracene ([DHA+H] +, m/ z 181) has been recorded using an infrared free electron laser. Protonation was accomplished by electrospray ionization with subsequent mass-selection and trapping in a Fourier transform ion cyclotron mass spectrometer. IR-induced fragment ions were observed at m/ z 179, 166, and 165. Density functional calculations (B3LYP/6-311++G(d,p)) of the infrared spectra of the two possible protonated isomers of DHA showed that the observed IRMPD spectrum is best fit to a mixture of the two isomers. Potential energy surfaces for the loss of atomic and molecular hydrogen from the aliphatic carbons of [DHA+H] + have been calculated. The lowest energy barriers are for loss of H 2. After H 2 ejection, stabilization of the remaining fragment occurs by hydrogen migration from one of the aliphatic carbons to the bare ejection site. In all cases the stabilized fragment is computed to be 9-hydroanthracene. The IRMPD spectrum of the m/ z 179 fragment has been recorded and is shown to correspond closely both to the calculated and previously recorded IRMPD spectrum of ionic 9-hydroanthracene. The highly asymmetric transition state conformation of the to-be-formed H 2 and the remaining fragment is highly suggestive of rotational, vibrational, and, possibly, translational excitation of the ejected H 2. Evidence for such excitation from astronomical observations that show the close proximity of PAHs and H 2 in certain interstellar objects and that show H 2 rotational excitation, which has been difficult to explain via either collisional activation or UV pumping, is reviewed.

Cytokine-inducing lipoteichoic acids of the allergy-protective bacterium Lactococcus lactis G121 do not activate via Toll-like receptor 2

It was established in a mouse model that the cowshed Gram-positive bacterium Lactococcus lactis G121 modulates the immune system resulting in allergy protection. However, the molecules and mechanisms involved in this process have not been elucidated yet. Lipoteichoic acids (LTAs) represent one major cell envelope component of Gram-positive bacteria that is considered a pathogen-associated molecular pattern. In the investigations presented here, the isolation as well as the structural and functional analyses of the LTA of L. lactis G121 were performed. Extraction with butan-1-ol and purification by hydrophobic interaction chromatography yielded pure LTA. Structural investigations included chemical analytical methods, nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization Fourier-transformed ion cyclotron mass spectrometry. LTA comprised a heterogeneous mixture of molecules composed of a 1,3-linked poly(glycerol phosphate) backbone which was randomly substituted at C-2 by D-alanine and α-D-galactopyranose. The lipid anchor constituents were kojibiose linked to a heterogeneous diglyceride comprising in total six different fatty acid compositions. This LTA preparation possesses Toll-like receptor 2- (TLR2) and TLR4-independent cytokine-inducing activities in human mononuclear cells.

Cyclotron resonance in a two-dimensional semimetal based on a HgTe quantum well

Microwave cyclotron resonance of electrons and holes at the metal-to-semimetal transition in HgTe quantum wells with an inversed band structure has been investigated. The resonance has been studied by measuring microwave photoresistance in the frequency range of 35–170 GHz. The effective cyclotron masses of electrons and holes have been determined. A shift of the cyclotron resonance of the two-dimensional electrons at the metal-to-semimetal transition possibly caused by plasma effects in the two-dimensional semimetal has been discovered.

Targeted and Non-Targeted Boron Complex Formation followed by Electrospray Fourier Transform Ion Cyclotron Mass Spectrometry: A Novel Approach for Identifying Boron Esters with Natural Organic Matter

The formation of boron esters was investigated in peat-soluble humified materials with a detailed molecular-level description of boron-organic interactions. Thousands of individually baseline separated signals were obtained from the analysis of natural organic matter of peat samples, using Fourier transform ion cyclotron resonance mass spectrometry. This technique offers unsurpassed isotope-specific mass resolution that can lead to precise molecular formula assignments by means of mathematical data analysis and visualisation techniques, such as mass defect (Kendrick) or elemental ratio (van Krevelen) plots. The analysis of potential boron binding structures within the sample of natural organic matter was described based on prior results. Herein, we describe an algorithm that can be used to effectively distinguish and filter complexes through data obtained from boron-enriched systems with highly intricate mass spectra, such as natural organic matter.

Millikelvin de Haas–van Alphen and magnetotransport studies of graphite

Recent studies of the electronic properties of graphite have produced conflicting results regarding the positions of the different carrier types within the Brillouin zone, and the possible presence of Dirac fermions. In this paper we report a comprehensive study of the de Haas--van Alphen, Shubnikov--de Haas, and Hall effects in a sample of highly orientated pyrolytic graphite, at temperatures in the range 30$ $mK to 4$ $K and magnetic fields up to 12$ $T. The transport measurements confirm the Brillouin-zone locations of the different carrier types assigned by Schroeder, Dresselhaus, and Javan, Phys. Rev. Lett. 20, 1292 (1968): electrons are at the $K$ point, and holes are near the $H$ points. We extract the cyclotron masses and scattering times for both carrier types from the temperature- and magnetic-field-dependences of the magneto-oscillations. Our results indicate that the holes experience stronger scattering and hence have lower mobility than the electrons. We utilize phase-frequency analysis and intercept analysis of the $1/B$ positions of magneto-oscillation extrema to identify the nature of the carriers in graphite, whether they are Dirac or normal (Schr\"odinger) fermions. These analyses indicate normal holes and electrons of indeterminate nature.

First Observation of Quantum Oscillations in the Ferromagnetic Superconductor UCoGe

We succeeded in growing high quality single crystals of the ferromagnetic superconductor UCoGe and measured the magnetoresistance at fields up to 34T. The Shubnikov-de Haas signal was observed for the first time in a U-111 system (UTGe, UTSi, T: transition metal). A small pocket Fermi surface (F~1kT) with large cyclotron effective mass 25m0 was detected at high fields above 22T, implying that UCoGe is a low carrier system accompanyed with heavy quasi-particles. The observed frequency decreases with increasing fields, indicating that the volume of detected Fermi surface changes nonlinearly with field. The cyclotron mass also decreases, which is consistent with the decrease of the A coefficient of resistivity.

Fermi surface of the electron-doped cuprate superconductor Nd2−xCexCuO4 probed by high-field magnetotransport

We report on the study of the Fermi surface of the electron-doped cuprate superconductor Nd2−xCexCuO4 by measuring the interlayer magnetoresistance as a function of the strength and orientation of the applied magnetic field. We performed experiments in both steady and pulsed magnetic fields on high-quality single crystals with Ce concentrations of x=0.13–0.17. In the overdoped regime of x>0.15, we found both semiclassical angle-dependent magnetoresistance oscillations (AMROs) and Shubnikov–de Haas (SdH) oscillations. The combined AMROs and SdH data clearly show that the appearance of fast SdH oscillations in strongly overdoped samples is caused by magnetic breakdown. This observation provides clear evidence for a reconstructed multiply connected Fermi surface up to the very end of the overdoped regime at x≃0.17. The strength of the superlattice potential responsible for the reconstructed Fermi surface is found to decrease with increasing doping level and likely vanishes at the same carrier concentration as superconductivity, suggesting a close relation between translational symmetry breaking and superconducting pairing. A detailed analysis of the high-resolution SdH data allowed us to determine the effective cyclotron mass and Dingle temperature, as well as to estimate the magnetic breakdown field in the overdoped regime.

Molecular characterization of Type I kerogen from the Green River Formation using advanced NMR techniques in combination with electrospray ionization/ultrahigh resolution mass spectrometry

This study describes a new approach for characterizing high molecular weight compounds in Type I kerogen, involving both nuclear magnetic resonance (NMR) spectroscopy and Fourier transform ion cyclotron mass spectrometry (FTICR-MS). Kerogen isolated from the Mahogany zone of the Green River Formation was examined directly using high resolution magic angle spinning (HRMAS) NMR to obtain liquid-like multidimensional spectra. It was then successively extracted with n-pentane, dichloromethane and pyridine. Pyridine extraction was also performed for comparison with the successive extractions. Using solid-state NMR, we show that the sum of the successive extracts and the single pyridine extract are quantitatively representative of the unextracted kerogen. This suggests that a non-invasive characterization of Green River kerogen can be obtained by examining the soluble extracts, all of which were subjected to ESI-FTICR-MS to identify a wide series of compounds. Series of polar CHO, CHOS and CHON compounds between C 12 and C 50+ were found. In all the extracts the two homologous series of acids (C n H 2 n O 2 and C n H 2 n −2O 4) dominated. Collision-induced dissociation was also employed to identify the different functional groups comprising the different series. The CHO series contained carboxylic acid and alkoxyl groups, whereas the CHOS and CHON series contained sulfoxide groups and nitrile-type compounds. The results also show that pyridine extraction can be used either for screening a large series of samples or for the specific study of CHO compounds. However for a detailed and complete study of the different homologous series we recommend using the successive extraction protocol.

H2EJECTION FROM POLYCYCLIC AROMATIC HYDROCARBONS: INFRARED MULTIPHOTON DISSOCIATION STUDY OF PROTONATED ACENAPHTHENE AND 9,10-DIHYDROPHENANTHRENE

The infrared multiple-photon dissociation (IRMPD) spectra of protonated acenaphthene ([ACN+H]+) and 9,10-dihydrophenanthrene ([DHP+H]+) have been recorded using an infrared free electron laser after the compounds were protonated by electrospray ionization and trapped in a Fourier transform ion cyclotron mass spectrometer. In both compounds, the loss of two mass units is predominant. Density functional calculations (B3LYP/6–311++G(d,p)) of the infrared spectra of all possible protonated isomers of each species showed that the observed IRMPD spectra are best fit to the isomer with the largest proton affinity and lowest relative electronic energy. Potential energy surfaces of the most stable isomers of [ACN+H]+ and [DHP+H]+ have been calculated for H and H2 loss. The lowest energy barriers are for loss of H2, with predicted energies 4.28 and 4.15 eV, respectively. After H2 ejection, the adjacent aliphatic hydrogens migrate to the bare ejection site and stabilize the remaining fragment. Single H loss may occur from [ACN+H]+ but the energy required is higher. No single H loss is predicted from [DHP+H]+, only H migration around the carbon skeleton. The vibrational bands in the parent closed-shell protonated polycyclic aromatic hydrocarbons are compared to bands observed from the interstellar medium.

Terahertz spectroscopy of quantum-well narrow-bandgap HgTe/CdTe-based heterostructures

The energy spectra of quantum-well narrow-bandgap Hg1 − y Cd y Te/Cd x Hg1 − x Te heterostructures have been studied. The dependences of the effective cyclotron mass on the density (in classical magnetic fields) and the transition energy (in quantizing fields) have been obtained from the cyclotron resonance measurements. These dependences confirm the near-linear dispersion law for the electrons with small mass at the band bottom (the minimum cyclotron mass measured is 0.003 m 0). The interband photoconductivity of the CdHgTe-based structures with the long-wavelength photoresponse edge lower than 6 meV has been demonstrated.

Ecological consequences of bacterioplankton lifestyles: changes in concepts are needed

In recent years, microbial ecology has developed from a peripheral discipline into a central field of microbiology. This change in state and perception is mainly driven by a rapid development of methods applied in the manifold fields related to microbial ecology. In biogeochemistry, for example, the use of high-resolution techniques such as FT-ICR-MS (Fourier transform ion cyclotron mass spectroscopy) has uncovered an enormous diversity and complexity of natural organic matter produced or degraded microbially either in dissolved or particulate forms. On the other hand, the introduction of high-throughput sequencing methods, such as 454 pyrosequencing, in combination with advances in bioinformatics allows for studying the bacterial diversity in natural samples circumventing cultivation dependent approaches. These new molecular tools enable in depth studies on single-cell genomes, distinct populations or even metacommunities. In combination with metatranscriptome and proteome studies it is for the first time possible to simultaneously unravel the structure and function of complex communities in situ. These technique-derived findings have, on the one hand, dramatically increased our knowledge on the vast diversity and complexity of bacterial habitats and, on the other hand, on phylogentic diversity and physiological responses of natural bacterial communities to their environment. However, until now microbial ecology is lacking an ecologically relevant species definition and useful tools for the identification of ecologically coherent taxa. Studies on intra- and interspecies interactions even with higher organisms demonstrate that bacteria can rapidly adapt to temporal and spatial changes in their environment. Aquatic bacteria have optimized and dramatically expanded their living space by efficient exploitation of organic matter point sources such as particles/aggregates and higher organisms. Although it is evident that particles/aggregates and organisms such as phytoplankton are 'hotspots' for microbial growth and transformation processes, it has not affected sampling strategies of aquatic microbial ecologists, who often focus solely on the free-living bacterial fractions and a priori exclude higher organisms by non-representative water sampling. Therefore, aquatic microbial ecologists have largely overlooked the fact that many aquatic bacteria may possess a complex lifestyle and frequently alternate between a free-living and a surface-associated stage. Here, I propose that modern concepts in aquatic microbial ecology should take into account the high chemical diversity and spatio-temporal variability of the bacterial environment. Interactions of aquatic bacteria with surfaces including living organisms are the key to understanding their physiological adaptations and population dynamics, as well as their contribution to biogeochemical cycles. New sampling strategies and theoretical concepts are needed in aquatic microbial ecology to access the whole spectrum of bacterial lifestyles and their ecological and evolutionary consequences.

Temperature-pressure phase diagram and electronic properties of the organic metalκ−(BETS)2Mn[N(CN)2]3

The crystal and electronic band structures as well as the transport properties of the recently synthesized layered radical cation salt -BETS2MnNCN23 are investigated. It is shown that this salt is more anisotropic in the plane of conducting layers than other phases, a feature which noticeably influences its physical behavior. A phase transition resulting in the formation of an incommensurate superstructure below 102 K has been found. At temperature below 3 0Kam etal-insulator transition is observed. We argue that the transition is associated with electronic interactions rather than with structural changes. The T-P phase diagram has been studied under 4 He gas pressure up to 2.5 kbar. At moderately high pressure P0.5 kbar the metal-insulating transition is suppressed and the crystal becomes superconducting with Tc5.7 K. In the pressurized metallic state, Shubnikov-de Haas oscillations have been found, revealing a very small closed Fermi surface with a high-effective cyclotron mass influenced by many-body interactions.

Quasiclassical cyclotron resonance of Dirac fermions in highly doped graphene

Cyclotron resonance in highly doped graphene has been explored using infrared magnetotransmission. Contrary to previous work, which only focused on the magneto-optical properties of graphene in the quantum regime, here we study the quasi-classical response of this system. We show that it has a character of classical cyclotron resonance, with an energy which is linear in the applied magnetic field and with an effective cyclotron mass defined by the position of the Fermi level m = E_F/v_F^2.

Far-infrared cyclotron resonance and Faraday effect inBi2Se3

The complex Faraday angle and transmission for the topological insulator ${\text{Bi}}_{2}{\text{Se}}_{3}$ were measured in the far infrared in magnetic fields up to 8 T and analyzed in terms of a simple Drude-Lorentz magnetoplasma dielectric model. Bulk carriers dominate the optical response. The bulk carriers are electrons as we determined from the sign of the Faraday angle. We obtain the bulk band-edge cyclotron mass ${m}_{cb}=0.16\ifmmode\pm\else\textpm\fi{}0.01{m}_{e}$ and free-electron concentration in the ${10}^{17}$ range. Electron-phonon interaction effects were found to be weak.

Electrons inGaAs/Ga1−xAlxAssuperlattices: Spin and orbital states in a magnetic field

Spin and orbital properties of electrons in $\text{GaAs}/{\text{Ga}}_{1\ensuremath{-}x}{\text{Al}}_{x}\text{As}$ superlattices (SLs) in an external longitudinal and transverse magnetic field are considered. A five-level $\mathbf{P}\ensuremath{\cdot}\mathbf{p}$ model of the band structure is used, accounting for nonparabolicity and nonsphericity of the conduction band in III-V compounds as well as their inversion asymmetry. The electron-spin $g$ values are computed as functions of superlattice spacings. It is shown that, in the transverse-field configuration, the $g$ value is different for various electron locations in an SL. Transverse and longitudinal cyclotron masses are calculated and compared with available experimental data for various $\text{GaAs}/{\text{Ga}}_{1\ensuremath{-}x}{\text{Al}}_{x}\text{As}$ SLs. Finally, orbital electron states in short SLs in a transverse magnetic field are described for different field intensities. In addition to the standard Landau levels unexpected states are found whose energies have a strong quadraticlike field dependence. The origin of the states is investigated.

Lifshitz transition in underdoped cuprates

Recent studies show that quantum oscillations thought to be associated with a density wave reconstructed Fermi surface disappear at a critical value of the doping for YBa2Cu3O6+y, and the cyclotron mass diverges as the critical value is approached from the high doping side. We argue that the phenomenon is due to a Lifshitz transition where the pockets giving rise to the quantum oscillations connect to form an open (quasi-1d) Fermi surface. The estimated critical doping is close to that found by experiment, and the theory predicts a logarithmic divergence of the cyclotron mass with a coefficient comparable to that observed in experiment.

Illuminated darkness: Molecular signatures of Congo River dissolved organic matter and its photochemical alteration as revealed by ultrahigh precision mass spectrometry

Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole‐water (i.e., unfractionated) DOM was analyzed pre‐ and post‐irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT‐ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo‐resistant, (2) photo‐labile, and (3) photo‐produced. Photo‐resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with &gt; 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT‐ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic‐rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon‐like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo‐labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean.

Data Processing Algorithms for Analysis of High Resolution MSMS Spectra of Peptides with Complex Patterns of Posttranslational Modifications

The emergence of efficient fragmentation methods such as electron capture dissociation (ECD) and electron transfer dissociation (ETD) provides the opportunity for detailed structural characterization of heavily covalently modified large peptides and small proteins such as intact histones. Even with effective gas phase ion isolation so that a single molecular precursor ion is selected, the MSMS spectrum of a heavily modified peptide may reveal the presence of a mixture of peptides with the same amino acid sequence and the same total number of posttranslational modification (PTM) moieties (same PTM composition) but with different PTM configurations or site-specific occupancy isoforms. Currently available data analysis methods depend on a deisotoping procedure, which becomes less effective when spectra (fragmentation patterns) contain many overlapping isotopic distributions. Peptide database search engines can only identify the most abundant PTM configuration (PTM arrangement on different residues) in such mixtures. To identify all the PTM configurations present in these mixtures and to estimate their relative abundances, we extended our fragment assignment by visual assistance program to search for ions representing all possible configurations, subjected to the total PTM composition constraint. This resulted in the identification of PTM configurations supported by unique fragment ions, and their relative abundances were estimated by use of a non-negative least squares procedure.

Magneto-optics of InAs/GaSb superlattices

We investigate the optical and electrical properties of a series of InAs/GaSb superlattices (SLs) as a function of InAs layer width d, from 21 to 55 Å, with a fixed GaSb layer width of 24 Å, corresponding to SLs with the cutoff wavelengths between 4 and 19 μm. Since the higher electron mass in InAs/GaSb SLs than in mercury cadmium telluride should lead to lower photodiode tunneling currents, we also measured the cyclotron effective mass for a very long wavelength infrared design SLs. For d&amp;lt;40 Å, the SLs were p-type, with hole mobilities of approximately 8 000 cm2/V s. For a high mobility p-type sample no hole cyclotron resonance signal was detected. However, the SLs with d≥40 Å were n-type, with electron mobilities increasing from 865 to 6126 cm2/V s. Cyclotron resonance data on an n-type SL sample yielded an electron cyclotron mass of 0.068 m0, which is three times the InAs bulk value of 0.023 m0. The mass enhancement was only partially accounted for by conduction band nonparabolicity, based on our 8×8 envelope function calculation.