High-temperature superconductivity in cuprates arises from an electronic state that remains poorly understood. We report the observation of a related electronic state in a noncuprate material, strontium iridate (Sr2IrO4), in which the distinct cuprate fermiology is largely reproduced. Upon surface electron doping through in situ deposition of alkali-metal atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments of zero-energy states, known as Fermi arcs, and a gap as large as 80 millielectron volts. Its evolution toward a normal metal phase with a closed Fermi surface as a function of doping and temperature parallels that in the cuprates. Our result suggests that Sr2IrO4 is a useful model system for comparison to the cuprates.

We used the Linac Coherent Light Source free-electron x-ray laser to probe the electronic structure of CO molecules as their chemisorption state on Ru(0001) changes upon exciting the substrate by using a femtosecond optical laser pulse. We observed electronic structure changes that are consistent with a weakening of the CO interaction with the substrate but without notable desorption. A large fraction of the molecules (30%) was trapped in a transient precursor state that would precede desorption. We calculated the free energy of the molecule as a function of the desorption reaction coordinate using density functional theory, including van der Waals interactions. Two distinct adsorption wells-chemisorbed and precursor state separated by an entropy barrier-explain the anomalously high prefactors often observed in desorption of molecules from metals.

We give experimental and theoretical evidence of the Rashba effect at the magnetic rare-earth metal surface Gd(0001). The Rashba effect is substantially enhanced and the Rashba parameter changes its sign when a metal-oxide surface layer is formed. The experimental observations are quantitatively described by ab initio calculations that give a detailed account of the near-surface charge density gradients causing the Rashba effect. Since the sign of the Rashba splitting depends on the magnetization direction, the findings open up new opportunities for the study of surface and interface magnetism.PACS numbers: 71.70. Ej, A key issue in condensed-matter research aiming at future spintronic devices [1] is to control and manipulate the electron spin in a two-dimensional electron gas (2DEG) of semiconductor systems without the need of applying an external magnetic field. Rashba had realized early on [2] that this can be achieved by an electric field which acts as a magnetic field in the rest frame of a moving electron. The interaction between the spin s of a moving electron of momentumhk with an electric field oriented along the z-axis e z is described by the Rashba HamiltonianThe Rashba parameter α R is proportional to the electric field and depends on the effective, material-dependent spin-orbit coupling (SOC) strength. In nonmagnetic systems the Rashba effect lifts the spin-degeneracy of the energy dispersion ǫ(k) of an electronic state, and the energy difference between ǫ ↑ (k) and ǫ ↓ (k) is called Rashba splitting ∆ǫ(k) = α R |k|. Even though spintronic research currently focuses on spin-polarized electrons in semiconductors [3,4], it is important to explore the Rashba effect in other material classes as well. A necessary condition for the Rashba effect to occur is the absence of inversion symmetry and, while in the proposed FET-type spin transistor [5] a gate voltage must be applied to break inversion symmetry of the 2DEG, this condition is naturally fulfilled by the structural inversion asymmetry (SIA) existing at any crystal surface or interface. Owing to SIA, electrons in a two-dimensional surface or interface state experience an effective crystal potential gradient perpendicular to their plane of propagation, hereby optimizing (e z × k) in Eq. (1). One should expect that the Rashba effect is a general surface and interface phenomenon, but up to now Rashba splittings have only been observed for surface states at Au(111) [6,7] and W(110) [8,9]. Recently relativistic density functional theory (DFT) calculations were able to reproduce the observed splitting of the Au sp-like surface state [10] and the analogy to a 2DEG has been pointed out [11]. Yet, it is still a challenging task to give a physical picture of the Rashba effect from the electronic structure point of view.This Letter presents the first experimental and theoretical evidence of a Rashba splitting of exchange-split two-dimensional electron states. Using the surface state of ferromagnetic Gd metal as example we report on the novel finding of a k-depe...

As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown1, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator–metal, or Verwey, transition has long remained inaccessible2, 3, 4, 5, 6, 7, 8. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase9. Here we investigate the Verwey transition with pump–probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator–metal transition. We find this to be a two-step process. After an initial 300 fs destruction of individual trimerons, phase separation occurs on a 1.5±0.2 ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics10

At low-temperatures (T < T N =110 K < T CO/OO =220 K), single-layer La 0.5 Sr 1.5 MnO 4 exhibits CE-type charge, spin and orbital order, characterized by in-plane "zig-zag" ferromagnetic chains. These chains are antiferromagnetically coupled with one another, in and out of plane [11,12,13]. Resonant soft Xray diffraction is directly sensitive to this spin and orbital order, when the incident photon energy is tuned to the 2p→3d transitions (Mn L 2,3 edges), and provides both momentum-dependent and spectroscopic information [14,15]. Figure 1 The temporal evolution of the integrated diffraction spot intensity at the magnetic (¼ ¼ ½) wave vector, obtained from the fits as described above, is reported in Figure 2(a). Diffraction was reduced by 8%, with a single time constant of 12.2 ps. For comparison, we display the significantly faster response response measured after excitation with 5-mJ/cm 2 pulses at 800-nm wavelength [23,24,25], which reveals a prompt collapse of magnetic order on the 250 fs time resolution of the experiment.This observation of different timescales is evidence that lattice driven magnetic disordering must follow a different physical path than for electronic excitation in the near infrared.In Figure 2 timescale and amplitude, with the orbital order only reduced by only 3% with a single-exponential decay time of 6.3 ps. We note that this lattice-driven orbital disordering is slower than was observed previously by time-dependent optical birefringence [19]. However, time dependent optical birefringence, proportional to the orbital order parameter squared in equilibrium [18], is a less direct method than the resonant x-ray diffraction used here.Throughout these dynamics, we see no transient change in the position and width of the scattered diffraction spots for either order and conclude that the correlation lengths are not perturbed. This is shown in Figure 2(c) where we exemplarily plot the transient width of the magnetic diffraction spot together with its peak position. The latter is constant within < 1×10 -5 (calculated standard deviation). , among which we find the Raman-active Jahn-Teller mode depicted in Fig. 3(c). Thus, according to the IRS model, rectification of the mid-infrared mode is able to relax the cooperative Jahn-Teller distortion, which has no infrared activity and thus cannot be driven directly by mid-infrared excitation. Importantly, the Jahn-Teller mode shown in Fig. 3(c) relaxes the splitting between crystal field levels and reduces the ordering of the orbitals. In turn, this weakens the exchange interaction that stabilizes the CE-type order and would thus lead to a smaller equilibrium magnetization, or to a lower equivalent Neel temperature.We stress that in contrast to the case of La 0.7 Sr 0.3 MnO 3 [27], in which the envelope of the infraredactive E u mode drives a low-frequency 1.2-THz rotational (E g ) mode impulsively, the Jahn-Teller A g mode has a higher frequency (15 THz) than the inverse 130-fs envelope of the infrared-active mode.Thus, in La 0.5 Sr 1.5 MnO 4 , the A...

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 µm in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.