J. D. Koralek

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 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 strengths of two dominant spin–orbit interactions, the Rashba (strength parameterized by α) and linear Dresselhaus (β1) interactions, 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 (β3) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as α approaches β1. Here we report experimental observation of the emergence of the persistent spin helix in GaAs quantum wells by independently tuning α and β1. Using transient spin-grating spectroscopy, 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 spin–orbit terms, identifying β3 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.

From a single-band metal to a high-temperature superconductor via two thermal phase transitions.

Three techniques are used to probe the pseudogap state of cuprate high-temperature superconductors. The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (Tc), entangled in an energy-momentum–dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

Femtosecond structural dynamics drives the trans/cis isomerization in photoactive yellow protein.

Visualizing a response to light Many biological processes depend on detecting and responding to light. The response is often mediated by a structural change in a protein that begins when absorption of a photon causes isomerization of a chromophore bound to the protein. Pande et al. used x-ray pulses emitted by a free electron laser source to conduct time-resolved serial femtosecond crystallography in the time range of 100 fs to 3 ms. This allowed for the real-time tracking of the trans-cis isomerization of the chromophore in photoactive yellow protein and the associated structural changes in the protein. Science, this issue p. 725 The trans-to-cis isomerization of a key chromophore is characterized on ultrafast time scales. A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes after photon absorption. The initial step is often the photoisomerization of a conjugated chromophore. Isomerization occurs on ultrafast time scales and is substantially influenced by the chromophore environment. Here we identify structural changes associated with the earliest steps in the trans-to-cis isomerization of the chromophore in photoactive yellow protein. Femtosecond hard x-ray pulses emitted by the Linac Coherent Light Source were used to conduct time-resolved serial femtosecond crystallography on photoactive yellow protein microcrystals over a time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

Laser Based Angle-Resolved Photoemission, the Sudden Approximation, and Quasiparticle-Like Spectral Peaks in Bi2Sr2CaCu2O8+δ

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi2Sr2CaCu2O8+delta. The low energy increases bulk sensitivity, reduces background, and improves resolution. With this we observe spectral peaks which are sharp on the scale of their binding energy - the clearest evidence yet for quasiparticles in the normal state. Crucial aspects of the data such as the dispersion, superconducting gaps, and the bosonic coupling kink and associated weight transfer are robust to a possible breakdown of the sudden approximation.

Tracking Cooper Pairs in a Cuprate Superconductor by Ultrafast Angle-Resolved Photoemission

Dissecting Cooper Pairs Angle-resolved photoemission spectroscopy (ARPES) is used in the study of the electronic structure of complex materials. Recently, time-resolved ARPES has become possible, where the state of the system is excited by a short “pump” pulse, and ARPES is performed using a second “probe” pulse applied after varying times. Smallwood et al. (p. 1137) used this technique to study the recombination of Cooper pairs—the fundamental charge carriers in superconductors—in a cuprate high-temperature superconductor. Time-resolved spectroscopy is used to probe the dynamics of electron pairing recovery in a high-temperature superconductor. In high-temperature superconductivity, the process that leads to the formation of Cooper pairs, the fundamental charge carriers in any superconductor, remains mysterious. We used a femtosecond laser pump pulse to perturb superconducting Bi2Sr2CaCu2O8+δ and studied subsequent dynamics using time- and angle-resolved photoemission and infrared reflectivity probes. Gap and quasiparticle population dynamics revealed marked dependencies on both excitation density and crystal momentum. Close to the d-wave nodes, the superconducting gap was sensitive to the pump intensity, and Cooper pairs recombined slowly. Far from the nodes, pumping affected the gap only weakly, and recombination processes were faster. These results demonstrate a new window into the dynamical processes that govern quasiparticle recombination and gap formation in cuprates.

Experimental setup for low-energy laser-based angle resolved photoemission spectroscopy

A laser-based angle resolved photoemission (ARPES) system utilizing 6 eV photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is described. This light source greatly increases the momentum resolution and photoelectron count rate, while reducing extrinsic background and surface sensitivity relative to higher energy light sources. In this review, the optical system is described, and special experimental considerations for low-energy ARPES are discussed. The calibration of the hemispherical electron analyzer for good low-energy angle-mode performance is also described. Finally, data from the heavily studied high T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212) is compared to the results from higher photon energies.

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 Å 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 Å 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.

Accurate theoretical fits to laser-excited photoemission spectra in the normal phase of high-temperature superconductors

Accurate theoretical fits to laser-excited photoemission spectra in the normal phase of high-temperature superconductors

Doppler velocimetry of spin propagation in a two-dimensional electron gas

An optical technique based on Doppler velocimetry reveals important aspects of the physics underlying the propagation of spin polarization in a two-dimensional electron gas. The spin mobility is shown to track the high electron mobility, but coherent spin precession is lost at temperatures near 150 K, posing a challenge for future spintronics devices.

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.