506 research outputs found
The National Security Agency\u27s Domestic Spying Program: Framing the Debate
On Friday, December 16, 2005, the New York Times reported that President George W. Bush had secretly authorized the National Security Agency (NSA) to conduct warrantless surveillance of Americans\u27 telephone and e-mail communications as part of an effort to obtain intelligence about future terrorist activity.\u27 The Times report was based on leaks of classified information, presumably by NSA officials concerned about the legality of the program. The Times reported that at the President\u27s request it had delayed publication of the story for more than a year.
The Indiana Law Journal reprinted four documents that, taken together, set forth the basic arguments concerning the lawfulness of the secret NSA surveillance program. The debate outlined by the four documents raises important issues about statutory interpretation in the face of claims of constitutional conflict, executive power during times of war, fundamental privacy rights of Americans, and ultimately, the rule of law in the war on terror
Plasmon-enhanced electron-phonon coupling in Dirac surface states of the thin-film topological insulator Bi2Se3
Raman measurements of a Fano-type surface phonon mode associated with Dirac
surface states (SS) in Bi2Se3 topological insulator thin films allowed an
unambiguous determination of the electron-phonon coupling strength in Dirac SS
as a function of film thickness ranging from 2 to 40 nm. A non-monotonic
enhancement of the electron-phonon coupling strength with maximum for the 8 -
10 nm thick films was observed. The non-monotonicity is suggested to originate
from plasmon-phonon coupling which enhances electron-phonon coupling when free
carrier density in Dirac SS increases with decreasing film thickness and
becomes suppressed for thinnest films when anharmonic coupling between in-plane
and out-of-plane phonon modes occurs. The observed about four-fold enhancement
of electron-phonon coupling in Dirac SS of the 8 - 10 nm thick Bi2Se3 films
with respect to the bulk samples may provide new insights into the origin of
superconductivity in this-type materials and their applications
Effect of Mn doping on ultrafast carrier dynamics in thin films of the topological insulator Bi2Se3
Transient reflectivity (TR) measured at laser photon energy 1.51 eV from the
indirectly intersurface coupled topological insulator Bi2-xMnxSe3 films (12 nm
thick) revealed a strong dependence of the rise-time and initial decay-time
constants on photoexcited carrier density and Mn content. In undoped samples (x
= 0), these time constants are exclusively governed by electron-electron and
electron-phonon scattering, respectively, whereas in films with x = 0.013 -
0.27 ultrafast carrier dynamics are completely controlled by photoexcited
electron trapping by ionized Mn2+ acceptors and their dimers. The shortest
decay-time (~0.75 ps) measured for the film with x = 0.27 suggests a great
potential of Mn-doped Bi2Se3 films for applications in high-speed
optoelectronic devices. Using Raman spectroscopy exploiting similar laser
photon energy (1.58 eV), we demonstrate that due to indirect intersurface
coupling in the films, the photoexcited electron trapping in the bulk enhances
the electron-phonon interaction strength in Dirac surface states
Preparation, characterization, and electrical properties of epitaxial NbO2 thin film lateral devices
Epitaxial NbO2 (110) films, 20 nm thick, were grown by pulsed laser
deposition on Al2O3 (0001) substrates. The Ar/O2 total pressure during growth
was varied to demonstrate the gradual transformation between NbO2 and Nb2O5
phases, which was verified using x-ray diffraction, x-ray photoelectron
spectroscopy, and optical absorption measurements. Electric resistance
threshold switching characteristics were studied in a lateral geometry using
interdigitated Pt top electrodes in order to preserve the epitaxial crystalline
quality of the films. Volatile and reversible transitions between high and low
resistance states were observed in epitaxial NbO2 films, while irreversible
transitions were found in case of Nb2O5 phase. Electric field pulsed current
measurements confirmed thermally-induced threshold switching.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in Journal of Physics D: Applied Physics. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at http://dx.doi.org/10.1088/0022-3727/48/33/33530
Acoustic phonon dynamics in thin-films of the topological insulator Bi2Se3
Transient reflectivity traces measured for nanometer-sized films of the
topological insulator Bi2Se3 revealed GHz-range oscillations driven within the
relaxation of hot carriers photoexcited with ultrashort laser pulses of 1.51 eV
photon energy. These oscillations have been suggested to result from acoustic
phonon dynamics, including coherent longitudinal acoustic phonons in the form
of standing acoustic waves. An increase of oscillation frequency from ~35 to
~70 GHz with decreasing film thickness from 40 to 15 nm was attributed to the
interplay between two different regimes employing traveling-acoustic-waves for
films thicker than 40 nm and the film bulk acoustic wave resonator (FBAWR)
modes for films thinner than 40 nm. The amplitude of oscillations decays
rapidly for films below 15 nm thick when the indirect intersurface coupling in
Bi2Se3 films switches the FBAWR regime to that of the Lamb wave excitation. The
frequency range of coherent longitudinal acoustic phonons is in good agreement
with elastic properties of Bi2Se3
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