49 research outputs found
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
(SWCNT) and show using electron spin resonance spectroscopy that a sizeable
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
cm 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. ESR spectroscopy
shows a main line in all types of materials with a width of about mT and
and a -factor in the range of . Paramagnetic defect centers
with a uniaxial -factor anisotropy are identified, which shows that these
are related to the local sp 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
Multipurpose High Frequency Electron Spin Resonance Spectrometer for Condensed Matter Research
We describe a quasi-optical multifrequency ESR spectrometer operating in the
75-225 GHz range and optimized at 210 GHz for general use in condensed matter
physics, chemistry and biology. The quasi-optical bridge detects the change of
mm wave polarization at the ESR. A controllable reference arm maintains a mm
wave bias at the detector. The attained sensitivity of 2x10^10 spin/G/(Hz)1/2,
measured on a dilute Mn:MgO sample in a non-resonant probe head at 222.4 GHz
and 300 K, is comparable to commercial high sensitive X band spectrometers. The
spectrometer has a Fabry-Perot resonator based probe head to measure aqueous
solutions, and a probe head to measure magnetic field angular dependence of
single crystals. The spectrometer is robust and easy to use and may be operated
by undergraduate students. Its performance is demonstrated by examples from
various fields of condensed matter physics.Comment: submitted to Journal of Magnetic Resonanc
Role of the antisymmetric exchange in quantum spin liquids
The quantum critical state of organic quantum spin liquids (QSL) exhibits large sensitivity even to weak perturbations. For example, the antisymmetric exchange, the Dzyaloshinskii-Moriya (DM) interaction, which is present in all spin systems without inversion symmetry, could result in a phase transition from the quantum critical phase to an antiferromagnetic phase already at moderate magnetic fields. Using the combination of multi-frequency Electron Spin Resonance spectroscopy (ESR) in the 1-500 GHz frequency range and muon spin rotation (mSR), we studied the influence of the DM interaction in two-dimensional and quasi-one-dimensional organic QSL candidates.
In the triangular lattice QSL, k-(ET)2Ag2(CN)3 (J’/J=0.94, J=175 K), our ESR measurements found a static staggered moment of 6×10-3 mB at T=1.5 K and at B=15 T [1]. The magnetic field dependence of the ESR linewidth, which measures the spectral density of the antiferromagnetic fluctuations, proves that this staggered moment stems from the DM interaction (DM0=4 K) in a perfectly crystalline two-dimensional structure. In a new quasi-one-dimensional QSL candidate, (EDT-TTF-CONH2)2+BABCO-, which is a weak Mott insulator with a distorted triangular lattice (J’/J=3, J=360 K), our combined ESR and mSR study confirmed the absence of magnetic ordering down to 20 mK [2]. This remarkable observation is partially attributed to a unique structural motif of the (EDT-TTF-CONH2)2+BABCO- salt. Here, the (EDT-TTF-CONH2)2+ conducting layers are separated by the highly disordered BABCO- molecular rotors. Importantly, despite the presence of a sizable DM interaction (DM0=0.6 K), the staggered moment is smaller than 4×10-4 mB at T=1.5 K and B=15 T. The magnetic field dependence of the ESR linewidth does not show the effect of the DM interaction. Instead, the linear dependence is indicative of the presence of fast spin fluctuations, which is supported by longitudinal-field mSR measurements that reveal the spin excitations to possess one-dimensional diffusive character. The quenching of the effect of the DM interaction is explained by the strong disorder introduced by the anion layer.
Despite the fact that the magnitude of the DM interaction is 2 to 3 orders of magnitude weaker than the symmetric exchange, it can substantially alter the phase diagram of QSLs. Our work gives a novel explanation to the field-induced phase transitions, and it demonstrates that high-frequency ESR is a powerful technique to study the spin dynamics of QSLs
Strain- and Adsorption-Dependent Electronic States and Transport or Localization in Graphene
The chapter generalizes results on influence of uniaxial strain and
adsorption on the electron states and charge transport or localization in
graphene with different configurations of imperfections (point defects):
resonant (neutral) adsorbed atoms either oxygen- or hydrogen-containing
molecules or functional groups, vacancies or substitutional atoms, charged
impurity atoms or molecules, and distortions. To observe electronic properties
of graphene-admolecules system, we applied electron paramagnetic resonance
technique in a broad temperature range for graphene oxides as a good basis for
understanding the electrotransport properties of other active carbons. Applied
technique allowed observation of possible metal-insulator transition and
sorption pumping effect as well as discussion of results in relation to the
granular metal model. The electronic and transport properties are calculated
within the framework of the tight-binding model along with the Kubo-Greenwood
quantum-mechanical formalism. Depending on electron density and type of the
sites, the conductivity for correlated and ordered adsorbates is found to be
enhanced in dozens of times as compared to the cases of their random
distribution. In case of the uniaxially strained graphene, the presence of
point defects counteracts against or contributes to the band-gap opening
according to their configurations. The band-gap behaviour is found to be
nonmonotonic with strain in case of a simultaneous action of defect ordering
and zigzag deformation. The amount of localized charge carriers (spins) is
found to be correlated with the content of adsorbed centres responsible for the
formation of potential barriers and, in turn, for the localization effects.
Physical and chemical states of graphene edges, especially at a uniaxial strain
along one of them, play a crucial role in electrical transport phenomena in
graphene-based materials.Comment: 16 pages, 10 figure
Ultralong spin lifetime in light alkali atom doped graphene
Today's great challenges of energy and informational technologies are
addressed with a singular compound, the Li and Na doped few layer graphene. All
what is impossible for graphite (homogeneous and high level Na doping), and
unstable for single layer graphene, works very well for this structure. The
transformation of the Raman G line to a Fano lineshape and the emergence of
strong, metallic-like electron spin resonance (ESR) modes, attest the high
level of graphene doping in liquid ammonia for both kinds of alkali atoms. The
spin-relaxation time in our materials, deduced from the ESR line-width, is 6-8
ns, which is comparable to the longest values found in spin-transport
experiments on ultrahigh mobility graphene flakes. This could qualify our
material as promising candidate in spintronics devices. On the other hand, the
successful sodium doping, this being a highly abundant metal, could be an
encouraging alternative to lithium batteries.Comment: 10 pages, 5 figures+ Supplementary Material
Ultralong 100 ns spin relaxation time in graphite at room temperature
Graphite has been intensively studied, yet its electron spins dynamics remains an unresolved problem even 70 years after the first experiments. The central quantities, the longitudinal (T1) and transverse (T2) relaxation times were postulated to be equal, mirroring standard metals, but T1 has never been measured for graphite. Here, based on a detailed band structure calculation including spin-orbit coupling, we predict an unexpected behavior of the relaxation times. We find, based on saturation ESR measurements, that T1 is markedly different from T2. Spins injected with perpendicular polarization with respect to the graphene plane have an extraordinarily long lifetime of 100 ns at room temperature. This is ten times more than in the best graphene samples. The spin diffusion length across graphite planes is thus expected to be ultralong, on the scale of ~ 70 μm, suggesting that thin films of graphite — or multilayer AB graphene stacks — can be excellent platforms for spintronics applications compatible with 2D van der Waals technologies. Finally, we provide a qualitative account of the observed spin relaxation based on the anisotropic spin admixture of the Bloch states in graphite obtained from density functional theory calculation