227 research outputs found
Oscillatory Magneto-Thermopower and Resonant Phonon Drag in a High-Mobility 2D Electron Gas
Experimental and theoretical evidence is presented for new low-magnetic-field
( kG) 1/B-oscillations in the thermoelectric power of a high-mobility
GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from
inter-Landau-Level resonances of acoustic phonons carrying a momentum equal to
twice the Fermi wavenumber at . Numerical calculations show that both 3D
and 2D phonons can contribute to this effect.Comment: 4 pages, 5 figure
Stellar Populations of the Sagittarius Dwarf Irregular Galaxy
We present deep BVRI CCD photometry of the stars in the dwarf irregular
galaxy SagDIG. The color-magnitude diagrams of the measured stars in SagDIG
show a blue plume which consists mostly of young stellar populations, and a
well-defined red giant branch (RGB). The foreground reddening of SagDIG is
estimated to be E(B-V)=0.06. The tip of the RGB is found to be at
I_(TRGB)=21.55 +/- 0.10 mag. From this the distance to this galaxy is estimated
to be d = 1.18 +/- 0.10 Mpc. This result, combined with its velocity
information, shows that it is a member of the Local Group. The mean metallicity
of the red giant branch is estimated to be [Fe/H] < -2.2 dex. This shows that
SagDIG is one of the most metal-poor galaxies. Total magnitudes of SagDIG (<
r_H (= 107 arcsec)) are derived to be B^T=13.99 mag, V^T=13.58 mag, R^T=13.19
mag, and I^T=12.88 mag, and the corresponding absolute magnitudes are
M_B=-11.62 mag, M_V=-11.97 mag, M_R=-12.33 mag, and M_I=-12.60 mag. Surface
brightness profiles of the central part of SagDIG are approximately fit by a
King model with a core concentration parameter c = log (r_t / r_c) ~ 0.6, and
those of the outer part follow an exponential law with a scale length of 37
arcsec. The central surface brightness is measured to be mu_B (0) = 24.21 mag
arcsec^(-2) and mu_V (0) =23.91 mag arcsec^(-2). The magnitudes and colors of
the brightest blue and red stars in SagDIG (BSG and RSG) are measured to be,
respectively, _BSG = 19.89 +/- 0.13 mag, _BSG = 0.08 +/- 0.07
mag, _RSG = 20.39 +/- 0.10 mag, and _RSG = 1.29 +/- 0.12 mag.
The corresponding absolute magnitudes are derived to be _BSG = -5.66
mag and _RSG = -5.16 mag, which are about one magnitude fainter than
those expected from conventional correlations with galaxy luminosity.Comment: 16 pages(AASLaTeX), 10 Postscript figures, Accepted for publication
in Astronomical Journal, 200
Stellar Populations and the Local Group Membership of the Dwarf Galaxy DDO 210
We present deep BVI CCD photometry of the stars in the dwarf galaxy DDO 210.
The color-magnitude diagrams of DDO 210 show a well-defined red giant branch
(RGB) and a blue plume. The tip of the RGB is found to be at I_TRGB = 20.95 +/-
0.10 mag. From this the distance to DDO 210 is estimated to be d = 950 +/- 50
kpc. The corresponding distance of DDO 210 to the center of the Local Group is
870 kpc, showing that it is a member of the Local Group. The mean metallicity
of the red giant branch stars is estimated to be [Fe/H] = -1.9 +/- 0.1 dex.
Integrated magnitudes of DDO 210 within the Holmberg radius (r_H=110 arcsec =
505 pc) are derived to be M_B=-10.6 +/- 0.1 mag and M_V=-10.9 +/- 0.1 mag. B
and V surface brightness profiles of DDO 210 are approximately consistent with
an exponential law with scale lengths r_s(B) = 161 pc and r_s(V) = 175 pc. The
brightest blue and red stars in DDO 210 (BSG and RSG) are found to be among the
faintest in the nearby galaxies with young stellar populations: _{BSG}
= -3.41 +/- 0.11 mag and _{RSG} = -4.69 +/- 0.13 mag. An enhancement of
the star formation rate in the recent past (several hundred Myrs) is observed
in the central region of DDO 210. The opposite trend is observed in the outer
region of the galaxy, suggesting a possible two-component structure of the kind
disk/halo found in spiral galaxies. The real nature of this two-component
structure must, however, be confirmed with more detailed observations.Comment: Latex file, 17 pages with 9 figures, uses emulateapj.sty To appear in
the AJ (in August 1999
Commensurability oscillations in the rf conductivity of unidirectional lateral superlattices: measurement of anisotropic conductivity by coplanar waveguide
We have measured the rf magnetoconductivity of unidirectional lateral
superlattices (ULSLs) by detecting the attenuation of microwave through a
coplanar waveguide placed on the surface. ULSL samples with the principal axis
of the modulation perpendicular (S_perp) and parallel (S_||) to the microwave
electric field are examined. For low microwave power, we observe expected
anisotropic behavior of the commensurability oscillations (CO), with CO in
samples S_perp and S_|| dominated by the diffusion and the collisional
contributions, respectively. Amplitude modulation of the Shubnikov-de Haas
oscillations is observed to be more prominent in sample S_||. The difference
between the two samples is washed out with the increase of the microwave power,
letting the diffusion contribution govern the CO in both samples. The failure
of the intended directional selectivity in the conductivity measured with high
microwave power is interpreted in terms of large-angle electron-phonon
scattering.Comment: 8 pages, 5 figure
Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems
In higher Landau levels (N>0) and around filling factors nu =4N+1, a
two-dimensional electron gas in a double-quantum-well system supports a stripe
groundstate in which the electron density in each well is spatially modulated.
When a parallel magnetic field is added in the plane of the wells, tunneling
between the wells acts as a spatially rotating effective Zeeman field coupled
to the ``pseudospins'' describing the well index of the electron states. For
small parallel fields, these pseudospins follow this rotation, but at larger
fields they do not, and a commensurate-incommensurate transition results.
Working in the Hartree-Fock approximation, we show that the combination of
stripes and commensuration in this system leads to a very rich phase diagram.
The parallel magnetic field is responsible for oscillations in the tunneling
matrix element that induce a complex sequence of transitions between
commensurate and incommensurate liquid or stripe states. The homogeneous and
stripe states we find can be distinguished by their collective excitations and
tunneling I-V, which we compute within the time-dependent Hartree-Fock
approximation.Comment: 23 pages including 8 eps figure
Magnetotunneling Between Two-dimensional Electron Gases in InAs-AlSb-GaSb Heterostructures
We have observed that the tunneling magnetoconductance between
two-dimensional (2D) electron gases formed at nominally identical InAs-AlSb
interfaces most often exhibits two sets of Shubnikov-de Haas oscillations with
almost the same frequency. This result is explained quantitatively with a model
of the conductance in which the 2D gases have different densities and can
tunnel between Landau levels with different quantum indices. When the epitaxial
growth conditions of the interfaces are optimized, the zero-bias
magnetoconductance shows a single set of oscillations, thus proving that the
asymmetry between the two electron gases can be eliminated.Comment: RevTeX format including 4 figures; submit for publicatio
Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation
A model to treat the anomalous Hall effect is developed. Based on the Kubo
formalism and on the Dirac equation, this model allows the simultaneous
calculation of the skew-scattering and side-jump contributions to the anomalous
Hall conductivity. The continuity and the consistency with the
weak-relativistic limit described by the Pauli Hamiltonian is shown. For both
approaches, Dirac and Pauli, the Feynman diagrams, which lead to the
skew-scattering and the side-jump contributions, are underlined. In order to
illustrate this method, we apply it to a particular case: a ferromagnetic bulk
compound in the limit of weak-scattering and free-electrons approximation.
Explicit expressions for the anomalous Hall conductivity for both
skew-scattering and side-jump mechanisms are obtained. Within this model, the
recently predicted ''spin Hall effect'' appears naturally
Silicon-based molecular electronics
Molecular electronics on silicon has distinct advantages over its metallic
counterpart. We describe a theoretical formalism for transport through
semiconductor-molecule heterostructures, combining a semi-empirical treatment
of the bulk silicon bandstructure with a first-principles description of the
molecular chemistry and its bonding with silicon. Using this method, we
demonstrate that the presence of a semiconducting band-edge can lead to a novel
molecular resonant tunneling diode (RTD) that shows negative differential
resistance (NDR) when the molecular levels are driven by an STM potential into
the semiconducting band-gap. The peaks appear for positive bias on a p-doped
and negative for an n-doped substrate. Charging in these devices is compromised
by the RTD action, allowing possible identification of several molecular
highest occupied (HOMO) and lowest unoccupied (LUMO) levels. Recent experiments
by Hersam et al. [1] support our theoretical predictions.Comment: Author list is reverse alphabetical. All authors contributed equally.
Email: rakshit/liangg/ ghosha/[email protected]
Exact Solution of the strong coupling t-V model with twisted boundary conditions
We present the solution of the one-dimensional t-V model with twisted
boundary conditions in the strong coupling limit, t<<V and show that this model
can be mapped onto the strong coupling Hubbard chain threaded by a fictitious
flux proportional to the total momentum of the charge carriers. The high energy
eigenstates are characterized by a factorization of degrees of freedom
associated with configurations of soliton and antisoliton domains and degrees
of freedom associated with the movement of ``holes'' through these domains. The
coexistence of solitons and antisolitons leads to a strange flux dependence of
the eigenvalues. We illustrate the use of this solution, deriving the full
frequency dependence of the optical conductivity at half-filling and zero
temperature.Comment: 11 pages, 1 figure; to be published in Physical Review
Atomic Tunneling from a STM/AFM tip: Dissipative Quantum Effects from Phonons
We study the effects of phonons on the tunneling of an atom between two
surfaces. In contrast to an atom tunneling in the bulk, the phonons couple very
strongly, and qualitatively change the tunneling behavior. This is the first
example of {\it ohmic} coupling from phonons for a two-state system. We propose
an experiment in which an atom tunnels from the tip of an STM, and show how its
behavior would be similar to the Macroscopic Quantum Coherence behavior
predicted for SQUIDS. The ability to tune and calculate many parameters would
lead to detailed tests of the standard theories. (For a general intro to this
work on the on the World-Wide-Web: http://www.lassp.cornell.edu. Click on
``Entertaining Science Done Here'' and ``Quantum Tunneling of Atoms'')Comment: 12 pages, ReVTex3.0, two figures (postscript). This is a
(substantially) revised version of cond-mat/9406043. More info (+ postscript
text) at : http://www.lassp.cornell.edu/ardlouis/publications.htm
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