23 research outputs found
Emergence of the persistent spin helix in semiconductor quantum wells
According to Noethers theorem, for every symmetry in nature there is a
corresponding conservation law. For example, invariance with respect to spatial
translation corresponds to conservation of momentum. In another well-known
example, invariance with respect to rotation of the electrons spin, or SU(2)
symmetry, leads to conservation of spin polarization. For electrons in a solid,
this symmetry is ordinarily broken by spin-orbit coupling, allowing spin
angular momentum to flow to orbital angular momentum. However, it has recently
been predicted that SU(2) can be achieved in a two-dimensional electron gas,
despite the presence of spin-orbit coupling. The corresponding conserved
quantities include the amplitude and phase of a helical spin density wave
termed the persistent spin helix. SU(2) is realized, in principle, when the
strength of two dominant spin-orbit interactions, the Rashba (strength
parameterized by \alpha) and linear Dresselhaus (\beta_1), are equal. This
symmetry is predicted to be robust against all forms of spin-independent
scattering, including electron-electron interactions, but is broken by the
cubic Dresselhaus term (\beta_3) and spin-dependent scattering. When these
terms are negligible, the distance over which spin information can propagate is
predicted to diverge as \alpha approaches \beta_1. Here we observe
experimentally the emergence of the persistent spin helix in GaAs quantum wells
by independently tuning \alpha and \beta_1. Using transient spin-grating
spectroscopy, we find a spin-lifetime enhancement of two orders of magnitude
near the symmetry point.........Comment: Will be published in Nature on April 2, 200
Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets
Chemical reactions and biological processes are often governed by the
structure and transport dynamics of the interface between two liquid phases.
Despite their importance, our microscopic understanding of liquid-liquid
interfaces has been severely hindered by difficulty in accessing the interface
through the bulk liquid. Here we demonstrate a method for generating large-area
liquid-liquid interfaces within free-flowing liquid sheets, which we call
liquid heterostructures. These sheets can be made thin enough to transmit
photons from across the spectrum, which also minimizes the amount of bulk
liquid relative to the interface and makes them ideal targets for a wide range
of spectroscopies and scattering experiments. The sheets are produced with a
microfluidic nozzle that impinges two converging jets of one liquid onto two
sides of a third jet of another liquid. The hydrodynamic forces provided by the
colliding jets both produce a multilayered laminar liquid sheet with the
central jet is flattened in the middle. Infrared microscopy, white light
reflectivity, and imaging ellipsometry measurements demonstrate that the buried
layer has a tunable thickness and displays well-defined liquid-liquid
interfaces, and that the inner layer can be thinner than 100 nm.Comment: 30 pages, 8 figures, 1 table. Supplement: 19 pages, 8 figure
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Comparing serial X-ray crystallography and microcrystal electron diffraction (MicroED) as methods for routine structure determination from small macromolecular crystals.
Innovative new crystallographic methods are facilitating structural studies from ever smaller crystals of biological macromolecules. In particular, serial X-ray crystallography and microcrystal electron diffraction (MicroED) have emerged as useful methods for obtaining structural information from crystals on the nanometre to micrometre scale. Despite the utility of these methods, their implementation can often be difficult, as they present many challenges that are not encountered in traditional macromolecular crystallography experiments. Here, XFEL serial crystallography experiments and MicroED experiments using batch-grown microcrystals of the enzyme cyclophilin A are described. The results provide a roadmap for researchers hoping to design macromolecular microcrystallography experiments, and they highlight the strengths and weaknesses of the two methods. Specifically, we focus on how the different physical conditions imposed by the sample-preparation and delivery methods required for each type of experiment affect the crystal structure of the enzyme
Microfluidic liquid sheets as large-area targets for high repetition XFELs
The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined
The room temperature crystal structure of a bacterial phytochrome determined by serial femtosecond crystallography
Phytochromes are a family of photoreceptors that control light responses of plants, fungi and bacteria. A sequence of structural changes, which is not yet fully understood, leads to activation of an output domain. Time-resolved serial femtosecond crystallography (SFX) can potentially shine light on these conformational changes. Here we report the room temperature crystal structure of the chromophore-binding domains of the Deinococcus radiodurans phytochrome at 2.1 angstrom resolution. The structure was obtained by serial femtosecond X-ray crystallography from microcrystals at an X-ray free electron laser. We find overall good agreement compared to a crystal structure at 1.35 angstrom resolution derived from conventional crystallography at cryogenic temperatures, which we also report here. The thioether linkage between chromophore and protein is subject to positional ambiguity at the synchrotron, but is fully resolved with SFX. The study paves the way for time-resolved structural investigations of the phytochrome photocycle with time-resolved SFX.Peer reviewe
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Emergence of the Persistent Spin Helix in Semiconductor Quantum Wells
According to Noether's theorem, for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electron's spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin-orbit (SO) coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be recovered in a two-dimensional electron gas (2DEG), despite the presence of SO coupling. The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the 'persistent spin helix' (PSH). SU(2) is restored, in principle, when the strength of two dominant SO interactions, the Rashba ({alpha}) and linear Dresselhaus ({beta}{sub 1}), are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron-electron interactions, but is broken by the cubic Dresselhaus term ({beta}{sub 3}) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as {alpha} {yields} {beta}{sub 1}. Here we observe experimentally the emergence of the PSH in GaAs quantum wells (QW's) by independently tuning {alpha} and {beta}{sub 1}. Using transient spin-grating spectroscopy (TSG), we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant SO terms, identifying {beta}{sub 3} as the main SU(2) violating term in our samples. The tunable suppression of spin-relaxation demonstrated in this work is well-suited for application to spintronics
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Generation and characterization of ultrathin free-flowing liquid sheets.
The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 μm down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources