Gauge fields in graphene

We theoretically calculate the phonon scattering limited electron mobility in extrinsic (i.e., gated or doped with a tunable and finite carrier density) two-dimensional graphene layers as a function of temperature $(T)$ and carrier density $(n)$. We find a temperature-dependent phonon-limited resistivity ${\ensuremath{\rho}}_{\mathit{ph}}(T)$ to be linear in temperature for $T\ensuremath{\gtrsim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the room-temperature intrinsic mobility reaching the values of above ${10}^{5}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$. We comment on the low-temperature Bloch--Gr\"uneisen behavior where ${\ensuremath{\rho}}_{\mathit{ph}}(T)\ensuremath{\sim}{T}^{4}$ for unscreened electron-phonon coupling.

Acoustic phonon scattering limited carrier mobility in two-dimensional extrinsic graphene

When two sheets of graphene are stacked at a small twist angle, the resulting flat superlattice minibands are expected to strongly enhance electron-electron interactions. Here, we present evidence that near three-quarters ([Formula: see text]) filling of the conduction miniband, these enhanced interactions drive the twisted bilayer graphene into a ferromagnetic state. In a narrow density range around an apparent insulating state at [Formula: see text], we observe emergent ferromagnetic hysteresis, with a giant anomalous Hall (AH) effect as large as 10.4 kilohms and indications of chiral edge states. Notably, the magnetization of the sample can be reversed by applying a small direct current. Although the AH resistance is not quantized, and dissipation is present, our measurements suggest that the system may be an incipient Chern insulator.

https://science.sciencemag.org/content/sci/early/2019/07/24/science.aaw3780.full.pdf

Emergent ferromagnetism near three-quarters filling in twisted bilayer graphene

Recent advances in Raman spectroscopy for characterizing graphene, graphite, and carbon nanotubes are reviewed comparatively We first discuss the first-order and the double-resonance (DR) second-order Raman scattering mechanisms in graphene, which give rise to the most prominent Raman features Then, we review phonon-softening phenomena in Raman spectra as a function of gate voltage, which is known as the Kohn anomaly Finally, we review exciton-specific phenomena in the resonance Raman spectra of single-wall carbon nanotubes (SWNTs) Raman spectroscopy of SWNTs has been especially useful for understanding many fundamental properties of all sp2 carbons, given SWNTs can be either semiconducting or metallic depending on their geometric structure, which is denoted by two integers (n,m)

Characterizing Graphene, Graphite, and Carbon Nanotubes by Raman Spectroscopy

Material properties emerge from phenomena on scales ranging from Angstroms to millimeters, and only a multiscale treatment can provide a complete understanding. Materials researchers must therefore understand fundamental concepts and techniques from different fields, and these are presented in a comprehensive and integrated fashion for the first time in this book. Incorporating continuum mechanics, quantum mechanics, statistical mechanics, atomistic simulations and multiscale techniques, the book explains many of the key theoretical ideas behind multiscale modeling. Classical topics are blended with new techniques to demonstrate the connections between different fields and highlight current research trends. Example applications drawn from modern research on the thermo-mechanical properties of crystalline solids are used as a unifying focus throughout the text. Together with its companion book, Continuum Mechanics and Thermodynamics (Cambridge University Press, 2011), this work presents the complete fundamentals of materials modeling for graduate students and researchers in physics, materials science, chemistry and engineering.

Modeling Materials: Continuum, Atomistic and Multiscale Techniques

A model for the temperature variations of the thermoelectric power (TEP) of acceptor graphite intercalation compounds (GIC's) is presented. At low temperatures, the TEP in GIC's increases monotonically with $T$, then levels off above \ensuremath{\sim}200 K in striking contrast to that of pristine graphite. The diffusion contribution to the TEP is proportional to $T$ and its magnitude is small as compared with that of the observed data. This observed behavior is attributed to the phonon drag effect. In the temperature region where the TEP is weakly temperature dependent, phonon relaxation is mainly controlled by the Rayleigh scattering due to point defects. The resultant TEP, which is composed of the phonon drag and diffusion terms, leads to a nearly $T$-independent value. Since the cross-sectional diameter of the Fermi surface in GIC's is larger than that of pristine graphite, the relaxation rate of the Rayleigh scattering, which is given by $\frac{1}{{t}_{I}(q)}=f{q}^{3}$, becomes very large at high temperatures ($Tg100$ K). At low temperatures, where the boundary scattering plays a dominant role, the TEP is proportional to ${T}^{3}$. Detailed calculations are carried out by solving the phonon-carrier-coupled Boltzmann equation.

Thermoelectric power of graphite intercalation compounds

The measurements of the $c$-axis conductivity ${\ensuremath{\sigma}}_{c}$ at room temperature for acceptor graphite intercalation compounds typically show very small values of ${\ensuremath{\sigma}}_{c}$, ranging from \ensuremath{\sim}${10}^{\ensuremath{-}1}$ to \ensuremath{\sim}10 ${(\mathrm{\ensuremath{\Omega}}\mathrm{c}\mathrm{m})}^{\ensuremath{-}1}$. The formula ${\ensuremath{\sigma}}_{c}=\frac{N{e}^{2}{l}_{c}}{{m}_{c}{v}_{c}}$ leads to a mean-free path ${l}_{c}ll{10}^{\ensuremath{-}8}$ cm. This implies that the $c$-axis conduction is not a band conduction but a hopping process. The model for the phonon-assisted and impurity-assisted hopping conduction is presented and the diffusion thermoelectric power along the $c$ direction is also calculated. Reasonable orders of magnitude are obtained for these transport coefficients. The phonon-assisted hopping conductivity increases with temperature, while the impurity-assisted hopping conduction is temperature independent at low temperatures and decreases with temperature in the region where the phonon scattering process is predominant. The diffusion thermoelectric power ${S}_{c}$ is proportional to temperature and takes the form which is essentially equivalent to the one in metallic conductors.

c -axis conductivity and thermoelectric power in graphite intercalation compounds

In an earlier paper, we introduced the concepts of phonon-assisted and impurity-assisted hopping processes to explain the c-axis resistivity in graphite intercalation compounds (GIC's). A different theory was proposed by Shimamura. Both theories provide a qualitative explanation for the observed behaviors in low-stage compounds but they cannot account for the high-temperature behavior in high-stage compounds. In this paper two new mechanisms are introduced. One is the interaction of the carriers with the LO phonons polarized along the c axis. This is important in low-stage compounds with large charge transfer. The other mechanism is the scattering of the carriers by stacking faults, which is important in high-stage compounds. By considering the mechanisms proposed previously by us and by Shimamura together with the new mechanism, we obtain a qualitative explanation for the temperature and stage dependences of the c-axis resistivity in GIC's.

c-axis conduction in graphite intercalation compounds.

This chapter considers the methods by which the various types of fibers are produced. It is noted that ex-polymer fibers are produced by methods which lie in the area of applied polymer chemistry. Several reviews of their manufacture have been written. Therefore, only a brief description of their preparation is given here, enabling the reader to relate structural features to growth methods. The methods of growing CCVD filaments is considered in greater detail. The growth of the CCVD filaments in lengths of hundreds of millimeters has not been reviewed previously. It is emphasized that commercial carbon fibers are manufactured from polymers, and that one type (ex-PAN) dominates the commercial market, though low cost isotropic pitch fibers are also beginning to find wide application in high bulk, low performance applications. The present review of fiber growth would therefore seem to be out-of-balance with respect to current commercial utilization. However, the most interesting physical experiments have been carried out on the newer CCVD fibers, thereby providing the rationale for the balance we have chosen.

Synthesis of Graphite Fibers and Filaments

The magnetoresistance of graphite exhibits a sharp increase at a certain critical field ${H}_{c}$ (${H}_{c}\ensuremath{\ge}22$ T), where ${H}_{c}$ is sensitive to temperature. This anomaly was ascribed to the formation of an electronic charge density wave by Yoshioka and Fukuyama (YF). To obtain an expression of ${H}_{c}$ versus $T$, it is very important to estimate the screening constant of the Coulomb interaction beyond the Thomas-Fermi approximation which was employed in YF theory. By introducing the non---Thomas-Fermi term in the screening constant we can obtain an expression for ${H}_{c}$ versus $T$ which is in good agreement with the observed results. The Hamiltonian is diagonalized by using a decoupling procedure in the Hartree-Fock scheme. With the use of these results, the transverse magnetoresistance is calculated, and the calculation describes the qualitative features of the observed results.

Ko Sugihara

Papers

Thermoelectric power of graphite intercalation compounds

c -axis conductivity and thermoelectric power in graphite intercalation compounds

c-axis conduction in graphite intercalation compounds.

Synthesis of Graphite Fibers and Filaments

Charge-density wave and magnetoresistance anomaly in graphite