898 research outputs found
Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si/SiGe Quantum Wells
Exploiting the spin resonance of two-dimensional (2D) electrons in SiGe/Si
quantum wells we determine the carrier-density-dependence of the magnetic
susceptibility. Assuming weak interaction we evaluate the density of states at
the Fermi level D(E_F), and the screening wave vector, q_TF. Both are constant
at higher carrier densities n, as for an ideal 2D carrier gas. For n < 3e11
cm-2, they decrease and extrapolate to zero at n = 7e10 cm-2. Calculating the
mobility from q_TF yields good agreement with experimental values justifying
the approach. The decrease in D(E_F) is explained by potential fluctuations
which lead to tail states that make screening less efficient and - in a
positive feedback - cause an increase of the potential fluctuations. Even in
our high mobility samples the fluctuations exceed the electron-electron
interaction leading to the formation of puddles of mobile carriers with at
least 1 micrometer diameter.Comment: 4 pages, 3 figure
Rashba spin-orbit coupling and spin relaxation in silicon quantum wells
Silicon is a leading candidate material for spin-based devices, and
two-dimensional electron gases (2DEGs) formed in silicon heterostructures have
been proposed for both spin transport and quantum dot quantum computing
applications. The key parameter for these applications is the spin relaxation
time. Here we apply the theory of D'yakonov and Perel' (DP) to calculate the
electron spin resonance linewidth of a silicon 2DEG due to structural inversion
asymmetry for arbitrary static magnetic field direction at low temperatures. We
estimate the Rashba spin-orbit coupling coefficient in silicon quantum wells
and find the and times of the spins from this mechanism as a
function of momentum scattering time, magnetic field, and device-specific
parameters. We obtain agreement with existing data for the angular dependence
of the relaxation times and show that the magnitudes are consistent with the DP
mechanism. We suggest how to increase the relaxation times by appropriate
device design.Comment: Extended derivations and info, fixed typos and refs, updated figs and
data. Worth a re-downloa
Lattice dynamics reveals a local symmetry breaking in the emergent dipole phase of PbTe
Local symmetry breaking in complex materials is emerging as an important
contributor to materials properties but is inherently difficult to study. Here
we follow up an earlier structural observation of such a local symmetry broken
phase in the technologically important compound PbTe with a study of the
lattice dynamics using inelastic neutron scattering (INS). We show that the
lattice dynamics are responsive to the local symmetry broken phase, giving key
insights in the behavior of PbTe, but also revealing INS as a powerful tool for
studying local structure. The new result is the observation of the unexpected
appearance on warming of a new zone center phonon branch in PbTe. In a harmonic
solid the number of phonon branches is strictly determined by the contents and
symmetry of the unit cell. The appearance of the new mode indicates a crossover
to a dynamic lower symmetry structure with increasing temperature. No
structural transition is seen crystallographically but the appearance of the
new mode in inelastic neutron scattering coincides with the observation of
local Pb off-centering dipoles observed in the local structure. The observation
resembles relaxor ferroelectricity but since there are no inhomogeneous dopants
in pure PbTe this anomalous behavior is an intrinsic response of the system. We
call such an appearance of dipoles out of a non-dipolar ground-state
"emphanisis" meaning the appearance out of nothing. It cannot be explained
within the framework of conventional phase transition theories such as
soft-mode theory and challenges our basic understanding of the physics of
materials
Experimental probing of exchange interactions between localized spins in the dilute magnetic insulator (Ga,Mn)N
The sign, magnitude, and range of the exchange couplings between pairs of Mn
ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have
been grown by metalorganic vapor phase epitaxy and characterized by
secondary-ion mass spectroscopy; high-resolution transmission electron
microscopy with capabilities allowing for chemical analysis, including the
annular dark-field mode and electron energy loss spectroscopy; high-resolution
and synchrotron x-ray diffraction; synchrotron extended x-ray absorption
fine-structure; synchrotron x-ray absorption near-edge structure; infra-red
optics and electron spin resonance. The results of high resolution magnetic
measurements and their quantitative interpretation have allowed to verify a
series of ab initio predictions on the possibility of ferromagnetism in dilute
magnetic insulators and to demonstrate that the interaction changes from
ferromagnetic to antiferromagnetic when the charge state of the Mn ions is
reduced from 3+ to 2+.Comment: 12 pages, 14 figures; This version contains the detailed
characterization of the crystal structure as well as of the Mn distribution
and charge stat
Tin telluride: a weakly co-elastic metal
We report resonant ultrasound spectroscopy (RUS),
dilatometry/magnetostriction, magnetotransport, magnetization, specific heat,
and Sn M\"ossbauer spectroscopy measurements on SnTe and
SnCrTe. Hall measurements at K indicate that our
Bridgman-grown single crystals have a -type carrier concentration of cm and that our Cr-doped crystals have an -type
concentration of cm. Although our SnTe crystals are
diamagnetic over the temperature range , the Cr-doped crystals are room temperature ferromagnets with a Curie
temperature of 294 K. For each sample type, three-terminal capacitive
dilatometry measurements detect a subtle 0.5 micron distortion at K. Whereas our RUS measurements on SnTe show elastic hardening near the
structural transition, pointing to co-elastic behavior, similar measurements on
SnCrTe show a pronounced softening, pointing to
ferroelastic behavior. Effective Debye temperature, , values of SnTe
obtained from Sn M\"ossbauer studies show a hardening of phonons in the
range 60--115K ( = 162K) as compared with the 100--300K range
( = 150K). In addition, a precursor softening extending over
approximately 100 K anticipates this collapse at the critical temperature, and
quantitative analysis over three decades of its reduced modulus finds with , a value
indicating a three-dimensional softening of phonon branches at a temperature
K, considerably below . We suggest that the differences in
these two types of elastic behaviors lie in the absence of elastic domain wall
motion in the one case and their nucleation in the other
Theory of Electric Dipole Spin Resonance in a Parabolic Quantum Well
A theory of Electric Dipole Spin Resonance (EDSR), that is caused by various
mechanisms of spin-orbit coupling, is developed as applied to free electrons in
a parabolic quantum well. Choosing a parabolic shape of the well has allowed us
to find explicit expressions for the EDSR intensity and its dependence on the
magnetic field direction in terms of the basic parameters of the Hamiltonian.
By using these expressions, we have investigated and compared the effect of
specific mechanisms of spin orbit (SO) coupling and different polarizations of
ac electric field on the intensity of EDSR. Angular dependences of the EDSR
intensity are indicative of the relative contributions of the competing
mechanisms of SO coupling. Our results show that electrical manipulating
electron spins in quantum wells is generally highly efficient, especially by an
in-plane ac electric field.Comment: 45 pages 6 figur
EDUCACORPOHUMANO3D: JOGO SÉRIO PARA O ESTUDO DO CORPO HUMANO NO ENSINO FUNDAMENTAL
EDUCACORPOHUMANO3D: JOGO SÉRIO PARA O ESTUDO DO CORPO HUMANO NO ENSINO FUNDAMENTA
EDUCACORPOHUMANO3D: JOGO SÉRIO PARA O ESTUDO DO CORPO HUMANO NO ENSINO FUNDAMENTAL
EDUCACORPOHUMANO3D: JOGO SÉRIO PARA O ESTUDO DO CORPO HUMANO NO ENSINO FUNDAMENTA
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