1,830 research outputs found
Metal-Insulator Transition of the LaAlO3-SrTiO3 Interface Electron System
We report on a metal-insulator transition in the LaAlO3-SrTiO3 interface
electron system, of which the carrier density is tuned by an electric gate
field. Below a critical carrier density n_c ranging from 0.5-1.5 * 10^13/cm^2,
LaAlO3-SrTiO3 interfaces, forming drain-source channels in field-effect devices
are non-ohmic. The differential resistance at zero channel bias diverges within
a 2% variation of the carrier density. Above n_c, the conductivity of the ohmic
channels has a metal-like temperature dependence, while below n_c conductivity
sets in only above a threshold electric field. For a given thickness of the
LaAlO3 layer, the conductivity follows a sigma_0 ~(n - n_c)/n_c characteristic.
The metal-insulator transition is found to be distinct from that of the
semiconductor 2D systems.Comment: 4 figure
Zero temperature optical conductivity of ultra-clean Fermi liquids and superconductors
We calculate the low-frequency optical conductivity sigma(w) of clean metals
and superconductors at zero temperature neglecting the effects of impurities
and phonons. In general, the frequency and temperature dependences of sigma
have very little in common. For small Fermi surfaces in three dimensions (but
not in 2D) we find for example that Re sigma(w>0)=const. for low w which
corresponds to a scattering rate Gamma proportional to w^2 even in the absence
of Umklapp scattering when there is no T^2 contribution to Gamma. In the main
part of the paper we discuss in detail the optical conductivity of d-wave
superconductors in 2D where Re sigma(w>0) \propto w^4 for the smallest
frequencies and the Umklapp processes typically set in smoothly above a finite
threshold w_0 smaller than twice the maximal gap Delta. In cases where the
nodes are located at (pi/2, pi/2), such that direct Umklapp scattering among
them is possible, one obtains Re sigma(w) \propto w^2.Comment: 7 pages, 3 figure
Current induced rotational torques in the skyrmion lattice phase of chiral magnets
In chiral magnets without inversion symmetry, the magnetic structure can form
a lattice of magnetic whirl lines, a two-dimensional skyrmion lattice,
stabilized by spin-orbit interactions in a small range of temperatures and
magnetic fields. The twist of the magnetization within this phase gives rise to
an efficient coupling of macroscopic magnetic domains to spin currents. We
analyze the resulting spin-transfer effects, and, in particular, focus on the
current induced rotation of the magnetic texture by an angle. Such a rotation
can arise from macroscopic temperature gradients in the system as has recently
been shown experimentally and theoretically. Here we investigate an alternative
mechanism, where small distortions of the skyrmion lattice and the transfer of
angular momentum to the underlying atomic lattice play the key role. We employ
the Landau-Lifshitz-Gilbert equation and adapt the Thiele method to derive an
effective equation of motion for the rotational degree of freedom. We discuss
the dependence of the rotation angle on the orientation of the applied magnetic
field and the distance to the phase transition.Comment: 11 pages, 6 figures; minor changes, published versio
Crystalline phases in chiral ferromagnets: Destabilization of helical order
In chiral ferromagnets, weak spin-orbit interactions twist the ferromagnetic
order into spirals, leading to helical order. We investigate an extended
Ginzburg-Landau theory of such systems where the helical order is destabilized
in favor of crystalline phases. These crystalline phases are based on periodic
arrangements of double-twist cylinders and are strongly reminiscent of blue
phases in liquid crystals. We discuss the relevance of such blue phases for the
phase diagram of the chiral ferromagnet MnSi.Comment: 6 pages, 5 figures (published version
Connecting Youth Through Multiple Pathways
Presents findings from a field scan of efforts to help vulnerable youth graduate from high school, what is working, what should be done, and what opportunities exist for Casey's involvement. Explores risk factors and alternative programs. Lists resources
Youth at High Risk of Disconnection
Updates the 2003 report "Connected by 25: Improving the Life Chances of the Country's Most Vulnerable 14-24 Year Olds" with demographic changes in disconnected and at-risk youth -- teens who are dropouts, mothers, in foster care, and/or incarcerated
Temperature-dependent changes in neuronal dynamics in a patient with an SCN1A mutation and hyperthermia induced seizures
Dravet syndrome is the prototype of SCN1A-mutation associated epilepsies. It is characterised by prolonged seizures, typically provoked by fever. We describe the evaluation of an SCN1A mutation in a child with early-onset temperature-sensitive seizures. The patient carries a heterozygous missense variant (c3818C > T; pAla1273Val) in the NaV1.1 brain sodium channel. We compared the functional effects of the variant vs. wild type NaV1.1 using patch clamp recordings from channels expressed in Chinese Hamster Ovary Cells at different temperatures (32, 37, and 40 °C). The variant channels produced a temperature-dependent destabilization of activation and fast inactivation. Implementing these empirical abnormalities in a computational model predicts a higher threshold for depolarization block in the variant, particularly at 40 °C, suggesting a failure to autoregulate at high-input states. These results reveal direct effects of abnormalities in NaV1.1 biophysical properties on neuronal dynamics. They illustrate the value of combining cellular measurements with computational models to integrate different observational scales (gene/channel to patient)
Helimagnon Bands as Universal Spin Excitations of Chiral Magnets
MnSi is a cubic compound with small magnetic anisotropy, which stabilizes a
helimagnetic spin spiral that reduces to a ferromagnetic and antiferromagnetic
state in the long- and short-wavelength limit, respectively. We report a
comprehensive inelastic neutron scattering study of the collective magnetic
excitations in the helimagnetic state of MnSi. In our study we observe a rich
variety of seemingly anomalous excitation spectra, as measured in well over
twenty different locations in reciprocal space. Using a model based on only
three parameters, namely the measured pitch of the helix, the measured
ferromagnetic spin wave stiffness and the amplitude of the signal, as the only
free variable, we can simultaneously account for \textit{all} of the measured
spectra in excellent quantitative agreement with experiment. Our study
identifies the formation of intense, strongly coupled bands of helimagnons as a
universal characteristic of systems with weak chiral interactions.Comment: 8 pages, 4 figures, references updated, introduction updated,
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