Ultrafast manipulation of mirror domain walls in a charge density wave

Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS$_2$. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photo-injected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light

Evidence for topological defects in a photoinduced phase transition

Upon excitation with an intense ultrafast laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways dissimilar from those in thermal equilibrium. Determining the mechanism underlying these photo-induced phase transitions (PIPTs) has been a long-standing issue in the study of condensed matter systems. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) material, LaTe$_3$. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a quick ($\sim\,$1$\,$ps) recovery of the CDW amplitude is followed by a slower reestablishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order (LRO) is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other PIPTs by identifying the generation of defects as a governing mechanism

X-ray Absorption and Emission Spectroscopy Study of the Effect of Doping on the Low Energy Electronic Structure of PrFeAsO1-[delta]

We report electronic density of states measurements of oxygen vacated PrFeAsO using soft X-ray absorption and emission spectroscopy. The electronic density of states is observed to undergo doping dependent shifts. Oxygen X-ray absorption and emission show long-range intermixing of oxygen p states. Mean field theory and local density approximations give a good description of the measured oxygen and iron spectra. The near Fermi-level iron spectral weight shows a systematic doping dependence. Concomitant changes in the unoccupied iron-arsenic hybridized spectral features reveal that Fe-As bonding is involved in the process of electron addition near the Fermi-level. By combining x-ray emission and absorption spectra, we observe an increase in the Fe density of states at the Fermi-level as the doping decreases. This doping dependent electronic behavior indicates the possibility of a magnetic instability in the undoped compound. Our data overall imply that PrFeAsO1-[delta] has weak to intermediate electron correlations

Design and construction of a compact, high-repetition-rate ultrafast electron diffraction instrument

We present the design and performance of a compact ultrafast electron diffraction instrument. The diffractometer provides a means of examining time-resolved ultrafast dynamical properties of solids. The system's utilization is discussed in terms of instrument parameters and diffraction data from selected condensed matter samples. The difractometer's performance is highlighted in terms of detection sensitivity, instrumental temporal resolution, and the electron beam transverse coherence length. Following specific details of the construction, we present a practical discussion of parameters such as repetition rate and provide advice on general construction approaches for laboratory-based, keV ultrafast electron diffractometers. In addition, design guidance for constructing a compact electron gun source that is well-suited for studying diffraction from hard condensed matter is given. A unique data acquisition scheme, utilizing high laser repetition rates, is presented.Nanyang Technological UniversityPublished versionThis work was supported by the U.S. Department of Energy, BES DMSE (building of the experimental setup, data taking, and analysis), and the Gordon and Betty Moore Foundation’s EPiQS Initiative Grant No. GBMF9459 (instrumentation). L.J.W. was supported by the Nanyang Assistant Professorship start-up grant. T.R. received funding from the Humboldt Postdoctoral Fellowship. B.F. would like to thank the MIT Physics Department for generous support. B.F. acknowledges support from the Welch Foundation (Grant No. L-E001-19921203) and the Texas Center for Superconductivity at the University of Houston (TcSUH)

Smectic-A and smectic-C phases and phase transitions in 8̅ S5 liquid-crystal-aerosil gels

High-resolution x-ray scattering studies of the nonpolar thermotropic liquid crystal 4-n-pentylphenylthiol-4′-n-octyloxybenzoate (8̅ S5) in aerosil gel nanonetworks reveal that the aerosil-induced disorder significantly alters both the nematic to smectic-A and smectic-A to smectic-C phase transitions. The limiting 8̅ S5 smectic-A correlation length follows a power-law dependence on the aerosil density in quantitative agreement with the limiting lengths measured previously in other smectic-A liquid crystal gels. The smectic-A to smectic-C liquid crystalline phase transition is altered fundamentally by the presence of the aerosil gel. The onset of the smectic-C phase remains relatively sharp but there is an extended coexistence region where smectic-A and smectic-C domains can exist.United States. Dept. of Energy. Office of Basic Energy Sciences (Contract No. DE-ACO2-05CH11231)University of California, Berkele

Combining time-resolved optical (TOS), electronic (trARPES) and structural (UED) probes on the class of rare earth tritellurides RTe₃

The combination of EUV based time-resolved Angle-Resolved-Photo-Electron-Spectroscopy (trARPES), Ultrafast-Electron-Diffraction (UED) and Transient-Optical-Spectroscopy (TOS) facilitates a comprehensive study and all-embracing analysis of correlated dynamics, exemplified on the system of Charge-Density-Waves (CDW’s) in rare earth tritellurides (RTe3)

Evidence for topological defects in a photoinduced phase transition

Upon excitation with an intense ultrafast laser pulse, a symmetry-broken ground state can undergo a non-equilibrium phase transition through pathways dissimilar from those in thermal equilibrium. Determining the mechanism underlying these photo-induced phase transitions (PIPTs) has been a long-standing issue in the study of condensed matter systems. To this end, we investigate the light-induced melting of a unidirectional charge density wave (CDW) material, LaTe$_3$. Using a suite of time-resolved probes, we independently track the amplitude and phase dynamics of the CDW. We find that a quick ($\sim\,$1$\,$ps) recovery of the CDW amplitude is followed by a slower reestablishment of phase coherence. This longer timescale is dictated by the presence of topological defects: long-range order (LRO) is inhibited and is only restored when the defects annihilate. Our results provide a framework for understanding other PIPTs by identifying the generation of defects as a governing mechanism