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Rapid developments in time-resolved optical spectroscopy have led to renewed interest in the nonequilibrium state of superconductors and other highly correlated electron materials. In these experiments, the nonequilibrium state is prepared by the absorption of short (less than 100 fs) laser pulses, typically in the near-infrared, that perturb the density and energy distribution of quasiparticles. The evolution of the nonequilibrium state is probed by time resolving the changes in the optical response functions of the medium that take place after photoexcitation. Ultimately, the goal of such experiments is to understand not only the nonequilibrium state, but to shed light on the still poorly understood equilibrium properties of these materials. We report nonequilibrium experiments that have revealed aspects of the cup rates that have been inaccessible by other techniques. Namely, the diffusion and recombination coefficients of quasiparticles have been measured in both YBa2Cu3O6.5 and Bi2Sr2CaCu2O8+x using time-resolved optical spectroscopy. Dependence of these measurements on doping, temperature and laser intensity is also obtained. To study the recombination of quasiparticles, we measure the change in reflectivity Delta R which is directly proportional to the nonequilibrium quasiparticle density created by the laser. From the intensity dependence, we estimate beta, the inelastic scattering coefficient and gamma_th thermal equilibrium quasiparticle decay rate. We also present the dependence of recombination measurements on doping in Bi2Sr2CaCu2O8+x. Going from underdoped to overdoped regime, the sign of Delta R changes from positive to negative right at the optimal doping. This is accompanied by a change in dynamics. The decay of Delta R stops being intensity dependent exactly at the optimal doping. We provide possible interpretations of these two observations. To study the propagation of quasiparticles, we interfered two laser pulses to introduce a spatially periodic density of quasiparticles. Probing the evolution of the initialdensity through space and time yielded the quasiparticle diffusion coefficient, and both inelastic and elastic scattering rates. Measured diffusion coefficient suggests that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone.

Ultrafast electron thermalization—the process leading to carrier multiplication via impact ionization and hot-carrier luminescence —occurs when optically excited electrons in a material undergo rapid electron–electron scattering to redistribute excess energy and reach electronic thermal equilibrium. Owing to extremely short time and length scales, the measurement and manipulation of electron thermalization in nanoscale devices remains challenging even with the most advanced ultrafast laser techniques. Here, we overcome this challenge by leveraging the atomic thinness of two-dimensional van der Waals (vdW) materials to introduce a highly tunable electron transfer pathway that directly competes with electron thermalization. We realize this scheme in a graphene–boron nitride–graphene (G–BN–G) vdW heterostructure through which optically excited carriers are transported from one graphene layer to the other. By applying an interlayer bias voltage or varying the excitation photon energy, interlayer carrier transport can be controlled to occur faster or slower than the intralayer scattering events, thus effectively tuning the electron thermalization pathways in graphene. Our findings, which demonstrate a means to probe and directly modulate electron energy transport in nanoscale materials, represent a step towards designing and implementing optoelectronic and energy-harvesting devices with tailored microscopic properties., United States. Air Force Office of Scientific Research (FA9550-11-1-0225), National Science Foundation (U.S.) (DMR-1231319)

In the underdoped copper-oxides, high-temperature superconductivity condenses from a nonconventional metallic ”pseudogap” phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime. This CDW coexists and competes with superconductivity (SC) below the transition temperature T[subscript c], suggesting that these two orders are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, τ[subscript qp], as a function of temperature and magnetic field in underdoped HgBa[subscript 2]CuO[subscript 4+δ] (Hg-1201) and YBa[subscript 2]Cu[subscript 3]O[subscript 6+x] (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τ[subscript qp](T ) exhibits a local maximum in a small temperature window near T[subscript c] that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that T[subscript c] marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs., United States. Dept. of Energy. Office of Basic Energy Sciences (Contract No. DE-AC02-05CH11231)), Austrian Science Fund (FWF project P2798), United States. Dept. of Energy. Office of Basic Energy Sciences (Award No. DE-SC0006858)

When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI–FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces., Samsung Foundation of Culture (Scholarship), National Science Foundation (U.S.) (STC Center for Integrated Quantum Materials. Grant DMR-1231319), Gordon and Betty Moore Foundation (EPiQS Initiative Grant GBMF4540), National Science Foundation (U.S.). Division of Materials Research (Grant DMR-1207469), National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant DMR-0819762), United States. Dept. of Energy. Division of Materials Sciences and Engineering (Award DE-SC0006418), United States. Office of Naval Research (Grant N00014-13-1-0301)

Three-dimensional topological insulators (1-3) represent a new quantum phase of matter with spin-polarized surface states (4,5) that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission (4-8) and tunneling (9,10) spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions (11-13) and spin-polarized electrical currents (14,15). However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here we show that illuminating the topological insulator Bi(subscript 2)Se(subscript 3) with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light, and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators(11-15) and lead to novel opto-spintronic devices(16)., United States. Dept. of Energy (grant DE-FG02-08ER46521)

We propose an all-optical, three-dimensional electron pulse compression scheme in which Hermite–Gaussian optical modes are used to fashion a three-dimensional optical trap in the electron pulse's rest frame. We show that the correct choices of optical incidence angles are necessary for optimal compression. We obtain analytical expressions for the net impulse imparted by Hermite–Gaussian free-space modes of arbitrary order. Although we focus on electrons, our theory applies to any charged particle and any particle with non-zero polarizability in the Rayleigh regime. We verify our theory numerically using exact solutions to Maxwell's equations for first-order Hermite–Gaussian beams, demonstrating single-electron pulse compression factors of >10[superscript 2] in both longitudinal and transverse dimensions with experimentally realizable optical pulses. The proposed scheme is useful in ultrafast electron imaging for both single- and multi-electron pulse compression, and as a means of circumventing temporal distortions in magnetic lenses when focusing ultrashort electron pulses. Other applications include the creation of flat electron beams and ultrashort electron bunches for coherent terahertz emission., United States. Army Research Office (Contract No. W911NF-13-D-0001), United States. Defense Advanced Research Projects Agency (DARPA Young Faculty Award, Grant No. D13AP00050), Alexander von Humboldt-Stiftung, Singapore. Agency for Science, Technology and Research

Ultrafast electron diffraction (UED) is a powerful method for studying time-resolved structural changes. Currently, space-charge-induced temporal broadening prevents obtaining high-brightness electron pulses with sub-100 fs durations limiting the range of phenomena that can be studied with this technique. We review the state of the art of UED in this respect and propose a practical design for reflectron-based pulse compression that utilizes only electrostatic optics and has a tunable temporal focal point. Our simulation shows that this scheme is capable of compressing an electron pulse containing 100 000 electrons with 60:1 temporal compression ratio., United States. Dept. of Energy (Award DE-FG02-08ER46521), National Science Foundation (U.S.) (Materials Research Science and Engineering Center (MRSEC) Program, Award DMR-0819762)

When light is absorbed by a semiconductor, photoexcited charge carriers enhance the absorption of far-infrared radiation due to intraband transitions. We observe the opposite behavior in monolayer graphene, a zero-gap semiconductor with linear dispersion. By using time domain terahertz (THz) spectroscopy in conjunction with optical pump excitation, we observe a reduced absorption of THz radiation in photoexcited graphene. The measured spectral shape of the differential optical conductivity exhibits non-Drude behavior. We discuss several possible mechanisms that contribute to the observed low-frequency non-equilibrium optical response of graphene., United States. Dept. of Energy. Office of Basic Energy Sciences (Grant DE-SC0006423), National Science Foundation (U.S.). Graduate Research Fellowship Program, United States. Air Force Office of Scientific Research, United States. Office of Naval Research. Multidisciplinary University Research Initiative. Graphene Approaches to Terahertz Electronics, National Science Foundation (U.S.) (Award DMR-0819762), National Science Foundation (U.S.) (Grant ECS-0335765)

The phonon-mediated attractive interaction between carriers leads to the Cooper pair formation in conventional superconductors. Despite decades of research, the glue holding Cooper pairs in high-temperature superconducting cuprates is still controversial, and the same is true for the relative involvement of structural and electronic degrees of freedom. Ultrafast electron crystallography (UEC) offers, through observation of spatiotemporally resolved diffraction, the means for determining structural dynamics and the possible role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse excites the charge carriers, which relax through electron-electron and electron-phonon couplings, and the consequential structural distortion is followed diffracting fs electron pulses. In this paper, the recent findings obtained on cuprates are summarized. In particular, we discuss the strength and symmetry of the directional electron-phonon coupling in Bi(subscript 2)Sr(subscript 2)CaCu(subscript 20O(subscript 8+δ) (BSCCO), as well as the c-axis structural instability induced by near-infrared pulses in La9subscript 2)CuO(subscript 4) (LCO). The theoretical implications of these results are discussed with focus on the possibility of charge stripes being significant in accounting for the polarization anisotropy of BSCCO, and cohesion energy (Madelung) calculations being descriptive of the c-axis instability in LCO., National Science Foundation (U.S.), United States. Air Force Office of Scientific Research, National Science Foundation (U.S.) (Ambizione grant)

The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort midinfrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon-dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter., United States. Dept. of Energy (Award DE-FG02-08ER46521), United States. Dept. of Energy (Award DE-SC0006423), United States. Army Research Office (Grant W911NF-09-1-0170), United States. Dept. of Energy. Division of Materials Sciences and Engineering (Award DE-SC0006418)