Mildred S. Dresselhaus

Bio: Mildred S. Dresselhaus is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Carbon nanotube & Raman spectroscopy. The author has an hindex of 136, co-authored 762 publications receiving 112525 citations. Previous affiliations of Mildred S. Dresselhaus include University of California, Los Angeles & Universidade Federal de Minas Gerais.

Femtosecond dynamics studies of coherent phonon generation

Abstract: Recent femtosecond time-resolved pump-probe experiments have detected large amplitude oscillations due to coherent phonon generation in the reflection (R) from various materials). These oscillations show only vibrational modes with A1 symmetry, even though other Raman-active modes of comparable Raman cross section exist. Experimental results showing the properties of this phenomena and a model that explains many of the experimental phenomena are presented.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Commensurate-Incommensurate and Melting Transitions in Bromine-Intercalated Single Crystal Kish Graphite

Abstract: We summarize the results of an extensive x-ray scattering study of the phase transitions in intercalated graphite-Br2 compounds. The experiments are aimed at understanding the intraplanar and interplanar correlations of the intercalate bromine as a function of temperature. Using in-situ high resolution x-ray scattering techniques, we have studied stage-4 graphite-Br2 under equilibrium conditions, where the intercalant is in the commensurate phase, incommensurate phase, and fluid phase. We demonstrate that the transitions between these phases are model examples of phase transitions in quasi two-dimensional systems. The coherently ordered in-plane bromine regions exceed 10000 A in size in both commensurate and incommensurate phases. A commensurate-incommensurate transition in the 7–fold direction is observed from a centered (√3 × 7) phase to a stripe domain phase. The domain wall density exhibits a power-law behavior in temperature with an exponent of 0.50 ± 0.02. We have also observed power-law lineshapes for bromine superlattice peaks in the incommensurate phase due to the rigorous absence of true long range order. The intercalate layer appears to exhibit an anisotropic melting transition from an incommensurate solid to a novel fluid phase.

Raman Spectra from One Carbon Nanotube

Abstract: The use of Raman spectroscopy as a characterization tool for individual single wall carbon nanotubes is briefly reviewed. New physical phenomena occurring at the single nanotube level are discussed, with special emphasis given to the use of resonance Raman scattering for the structural determination of (n, m) for individual nanotubes, based on diameter and chirality dependent phenomena associated with the radial breathing mode, the G-band and the G[prime]-band features. Examples are given to show how single nanotube spectroscopy provides insight into the use of Raman spectroscopy for the characterization of nanotube bundles and for the study of new physical phenomena occurring at the single nanotube level.

Shubnikov—de Haas measurements in alkali-metal—graphite intercalation compounds

Abstract: Shubnikov\char22{}de Haas (SdH) oscillations are reported for well-characterized (single stage) encapsulated potassium (stages $n=4,5,8$) and rubidium ($n=2,3,5,8$) graphite intercalation compounds. Shapes of the Fermi surface (FS) are deduced from the dependence of the FS cross sections on the angle between the $c$-axis of the sample and $\stackrel{\ensuremath{\rightarrow}}{H}$. The temperature dependence ($1.4lT\ensuremath{\le}25$ K) of the amplitudes of the SdH oscillations has been studied to find cyclotron effective masses for specific FS cross sections. A simple phenomenological energy-band model, based on the $\ensuremath{\pi}$ bands of pristine graphite and $c$-axis zone folding, is used to calculate SdH frequencies as a function of Fermi energy and is applied to interpret the stage- and intercalant-dependent experimental SdH frequencies and effective masses. The good agreement between the observed and predicted effective masses and the FS cross sections confirms the general validity of this model.

Electric Field Effect in Atomically Thin Carbon Films

Abstract: We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in concentrations up to 10 13 per square centimeter and with room-temperature mobilities of ∼10,000 square centimeters per volt-second can be induced by applying gate voltage.

Helical microtubules of graphitic carbon

Abstract: THE synthesis of molecular carbon structures in the form of C60 and other fullerenes1 has stimulated intense interest in the structures accessible to graphitic carbon sheets. Here I report the preparation of a new type of finite carbon structure consisting of needle-like tubes. Produced using an arc-discharge evaporation method similar to that used for fullerene synthesis, the needles grow at the negative end of the electrode used for the arc discharge. Electron microscopy reveals that each needle comprises coaxial tubes of graphitic sheets, ranging in number from 2 up to about 50. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis. The helical pitch varies from needle to needle and from tube to tube within a single needle. It appears that this helical structure may aid the growth process. The formation of these needles, ranging from a few to a few tens of nanometres in diameter, suggests that engineering of carbon structures should be possible on scales considerably greater than those relevant to the fullerenes. On 7 November 1991, Sumio Iijima announced in Nature the preparation of nanometre-size, needle-like tubes of carbon — now familiar as 'nanotubes'. Used in microelectronic circuitry and microscopy, and as a tool to test quantum mechanics and model biological systems, nanotubes seem to have unlimited potential.

The rise of graphene

Abstract: Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

The electronic properties of graphene

Abstract: This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.