According to Noether's theorem, 1 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. 2 The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the "persistent spin helix" (PSH) . 2 SU(2) is restored, in principle, when the strength of two dominant SO interactions, the Rashba 3 (α) and linear Dresselhaus 4 (β 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 (β 3 ) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as α → β 1 . Here we observe experimentally the emergence of the PSH in GaAs quantum wells (QW's) by independently tuning α and β 1 . Using transient spin-grating spectroscopy 5 (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 β 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. 6, 7 SLAC-PUB-13935 SIMES,

The nature of the pseudogap phase of cuprate high-temperature superconductors is one of the most important unsolved problems 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 observe the coincident onset at T * of a particle-hole asymmetric antinodal gap, a non-zero Kerr rotation, and a change in the relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum dependent fashion with the pre-existing pseudogap features.As complex oxides, cuprate superconductors belong to a class of materials which exhibit many broken-symmetry states; unravelling the relationship between superconductivity in the cuprates and other possible broken-symmetry states has been a major challenge of condensed matter physics. A possibly related issue concerns the nature of the pseudogap in the cuprates and its relationship with superconductivity. Angle-resolved photoemission spectroscopy (ARPES) studies have shown that the pseudogap develops below a temperature T * near the Brillouin zone boundary while preserving a gapless Fermi arc near the zone diagonal (1). A key issue is the extent to which the pseudogap is a consequence of superconducting fluctuations (2-5), which should exhibit a rough particle-hole symmetry, or another form of (incipient) order (6-12), which typically should induce particle-hole asymmetric spectral changes. Candidate orders include various forms of density wave, nematic or unconventional magnetic orders that break different combinations of lattice translational (6-8, 13-19), rotational (6, 9, 15, 17, 20-22), and time-reversal (7, 9, 23-26) symmetries.We have focused on crystals of nearly optimally-doped (OP) Pb 0.55 Bi 1.5 Sr 1.6 La 0.4 CuO 6+δ (PbBi2201, T c = 38 K, T * = 132 ± 8 K) (27), and combined the ARPES measurements of the evolution of the band structure over a wide range of temperature, momentum and energy, with high-precision measurements of the polar Kerr effect (PKE) and time-resolved reflectivity (TRR).Bi2201 was chosen to avoid the complications resulting from bilayer splitting and strong antinodal bosonic mode coupling inherent to Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) (1). Whereas ARPES is a surface probe, PKE enables us to monitor a bulk, thermodynamic (via the fluctuation-dissipation theorem) 2 property which has proven (28) to be a sensitive probe of the onset of a broken-symmetry state, and TRR gives complementary information on the bulk, near-equilibrium dynamics of the system. We will first analyze our ARPES data collected in different temperature regions. Above T * , PbBi2201 has a simple one-band band structure (right side of Fig. 1). For each cut in momentum space Fig. 1), the only distinct feature in the ...

A variety of organisms have evolved mechanisms to detect and respond to light, in which the response is mediated by protein structural changes following photon absorption. The initial step is often the photo-isomerization of a conjugated chromophore. Isomerization occurs on ultrafast timescales, 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 PYP microcrystals over the time range from 100 femtoseconds to 3 picoseconds to determine the structural dynamics of the photoisomerization reaction.

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 Bi(2)Sr(2)CaCu(2)O(8+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.

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Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.

A new low photon energy regime of angle resolved photoemission spectroscopy is accessed with lasers and used to study the superconductor Bi 2 Sr 2 CaCu 2 O 8+δ . 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. 74.72.Hs, 74.00.00, 74.25.Jb, 73.90.+f High T c superconductivity has been at the forefront of solid state physics research since its discovery in 19861 . Tunneling spectroscopy 2 3 and angle resolved photoemission spectroscopy (ARPES) 4 5 have been among the key techniques for studying the electronic structure of the cuprates in the quest to understand the many-body interactions responsible for high T c superconductivity. Unfortunately, both of these techniques are surface sensitive, making unclear their detailed applicability to bulk physics such as superconductivity. Here we introduce laser-based ARPES for studies of superconductors, which is expected to have significantly greater bulk sensitivity and which also offers superior energy and momentum resolution.The bulk-sensitivity of ARPES is limited by the electron mean free path in the solid, which depends on the electron kinetic energy in a roughly universal way 6 . Typically, there is a broad minimum of the mean free path in the 20-50 eV kinetic energy range with a sharp increase at lower energy and a slower increase at high energy, which is thought to hold true for Bi2212 7 . With the current interest in using ARPES to study bulk-physics such as superconductivity, the surface sensitivity has become a real hindrance. In order to improve the bulk-sensitivity of ARPES, one may go to very high photon energy such as a few thousand eV, though photoelectron cross sections decrease, and it becomes prohibitively difficult to obtain high energy and momentum resolution. Moving to low energy is thus a more attractive rout to increase bulk sensitivity, though there are some limitations to the extent of k-space that can be accessed.A critical question for ARPES, especially at low photon energy, is whether the sudden approximation, in which one assumes that the electron leaves the sample prior to relaxation of the created photo-hole, is valid 7 8 9 . If this is so, then the photoelectron spectrum should be directly proportional to the spectral function A(k,ω) which is in principle calculable using many-body techniques. This is consistent with a recent theoretical calculation which argues that the energy at which the adiabatic-sudden transition occurs should be a function of the type of excitation -in particular the transition is expected to occur at lower energies for more localized excitations 9 .We have built a high resolution ARPES system centered around a Scienta 10 SES...

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B, 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-N 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.