arXiv subjects

J. Geck

Publications and source records attributed to J. Geck.

At least 37 records · Page 2Linked to original sources

arXiv2014

Femtosecond Time-resolved MeV Electron Diffraction

We report the experimental demonstration of femtosecond electron diffraction using high-brightness MeV electron beams. High-quality, single-shot electron diffraction patterns for both polycrystalline aluminum and single-crystal 1T-TaS2 are obtained utilizing a 5 femto-Coulomb (~3x10^4 electrons) pulse of electrons at 2.8 MeV. The high quality of the electron diffraction patterns confirm that electron beam has a normalized emittance of ~50 nm-rad. The corresponding transverse and longitudinal coherence length are ~11 nm and ~2.5 nm, respectively. The timing jitter between the pump laser and probe electron beam was found to be ~ 100 fs (rms). The temporal resolution is demonstrated by observing the evolution of Bragg and superlattice peaks of 1T-TaS2 following an 800 nm optical pump and was found to be 130 fs. Our results demonstrate the advantages of MeV electron diffraction: such as longer coherent lengths, large scattering cross-section and larger signal-to-noise ratio, and the feasibility of ultimately realizing 10 fs time-resolved electron diffraction.

physics.ins-det↗

arXiv2013

Emergence of charge density wave domain walls above the superconducting dome in TiSe2

Superconductivity (SC) in so-called "unconventional superconductors" is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. To better understand this connection, we used high-pressure x-ray scattering to directly study the CDW order in the layered dichalcogenide TiSe2, which was previously shown to exhibit SC when the CDW is suppressed by pressure [1] or intercalation of Cu atoms [2]. We succeeded in suppressing the CDW fully to zero temperature, establishing for the first time the existence of a quantum critical point (QCP) at Pc = 5.1 +/- 0.2 GPa, which is more than 1 GPa beyond the end of the SC region. Unexpectedly, at P = 3 GPa we observed a reentrant, weakly first order, incommensurate phase, indicating the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in TiSe2 may not be connected to the QCP itself, but to the formation of CDW domain walls.

cond-mat.supr-con↗

arXiv2013

Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show -- with simulations and REXS on Mn-substituted Sr$_3$Ru$_2$O$_7$ -- that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that -- in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy -- randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques.

cond-mat.str-el↗

arXiv2013

Pressure dependence of the charge density wave in 1T-TaS2 and its relation to superconductivity

We present a state-of-the-art x-ray diffraction study of the charge density wave order in 1T-TaS2 as a function of temperature and pressure. Our results prove that the charge density wave, which we characterize in terms of wave vector, amplitude and the coherence length, indeed exists in the superconducting region of the phase diagram. The data further imply that the ordered charge density wave structure as a whole becomes superconducting at low temperatures, i. e, superconductivity and charge density wave coexist on a macroscopic scale in real space. This result is fundamentally different from a previously proposed separation of superconducting and insulating regions in real space and, instead, provides evidence that the superconducting and the charge density wave gap exist in separate regions of reciprocal space.

cond-mat.str-el↗

arXiv2013

Resonant X-Ray Scattering Measurements of a Spatial Modulation of the Cu 3d and O 2p Energies in Stripe-Ordered Cuprate Superconductors

A prevailing description of the stripe phase in underdoped cuprate superconductors is that the charge carriers (holes) phase segregate on a microscopic scale into hole rich and hole poor regions. We report resonant elastic x-ray scattering measurements of stripe-ordered La$_{1.475}$Nd$_{0.4}$Sr$_{0.125}$CuO$_4$ at the Cu $L$ and O $K$ absorption edges that identify an additional feature of stripe order. Analysis of the energy dependence of the scattering intensity reveals that the dominant signature of the stripe order is a spatial modulation in the energies of Cu 3d and O 2p states rather than the large modulation of the charge density (valence) envisioned in the common stripe paradigm. These energy shifts are interpreted as a spatial modulation of the electronic structure and may point to a valence-bond-solid interpretation of the stripe phase.

cond-mat.supr-con↗

arXiv2012

Observation of charge accumulation and onsite Coulomb repulsion at transition metal impurities in the iron pnictides

We report a combined valence band photoemission and Auger spectroscopy study of single crystalline Ca(Fe,Co)2As2 and Ba(Fe,TM)2As2 with TM=Ni or Cu. The valence band photoemission data show directly that the TM-states move to higher binding energies with increasing atomic number, contributing less and less to the states close to the Fermi level. Furthermore, the 3d8 final state of the LVV Auger decay, which is observed for Ni and Cu, unambiguously reveals the accumulation of charge at these impurities. We also show that the onsite Coulomb interaction on the impurity strongly increases when moving from Co over Ni to Cu. Our results quantify the impurity potentials and imply that the superconducting state is robust against impurity scattering.

cond-mat.supr-con↗

arXiv2012

Resonant Elastic Soft X-Ray Scattering

Resonant (elastic) soft x-ray scattering (RSXS) offers a unique element, site, and valence specific probe to study spatial modulations of charge, spin, and orbital degrees of freedom in solids on the nanoscopic length scale. It cannot only be used to investigate single crystalline materials. This method also enables to examine electronic ordering phenomena in thin films and to zoom into electronic properties emerging at buried interfaces in artificial heterostructures. During the last 20 years, this technique, which combines x-ray scattering with x-ray absorption spectroscopy, has developed into a powerful probe to study electronic ordering phenomena in complex materials and furthermore delivers important information on the electronic structure of condensed matter. This review provides an introduction to the technique, covers the progress in experimental equipment, and gives a survey on recent RSXS studies of ordering in correlated electron systems and at interfaces.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic frustration, phase competition and the magneto-electric effect in NdFe3(BO3)4

We present an element selective resonant magnetic x-ray scattering study of NdFe3(BO3)4 as a function of temperature and applied magnetic field. Our measurements show that the magnetic order of the Nd sublattice is induced by the Fe spin order. When a magnetic field is applied parallel to the hexagonal basal plane, the helicoidal spin order is suppressed and a collinear ordering, where the moments are forced to align in a direction perpendicular to the applied magnetic field, is stabilized. This result excludes a non-collinear spin order as the origin of the magnetically induced electric polarization in this compound. Instead our data imply that magnetic frustration results in a phase competition, which is the origin of the magneto-electric response.

cond-mat.str-el↗

arXiv2012

Electronic confinement and ordering instabilities in colossal magnetoresistive bilayer manganites

We present angle-resolved photoemission studies of (La1-zPrz)2-2xSr1+2xMn2O7 with x=0.4 and z=0.1,0.2 and 0.4 along with density functional theory calculations and x-ray scattering data. Our results show that the bilayer splitting in the ferromagnetic metallic phase of these materials is small, if not completely absent. The charge carriers are therefore confined to a single MnO2-layer, which in turn results in a strongly nested Fermi surface. In addition to this, the spectral function also displays clear signatures of an electronic ordering instability well below the Fermi level. The increase of the corresponding interaction strength with z and its magnitude of ~400 meV make the coupling to a bare phonon highly unlikely. Instead we conclude that fluctuating order, involving electronic and lattice degrees of freedom, cause the observed renormalisation of the spectral features.

cond-mat.str-el↗

arXiv2012

Probing the role of Co substitution in the electronic structure of iron-pnictides

The role of Co substitution in the low-energy electronic structure of Ca(Fe$_{0.944}$Co$_{0.056}$)$_2$As$_2$ is investigated by resonant photoemission spectroscopy and density functional theory. The Co 3d-state center-of-mass is observed at 250 meV higher binding energy than Fe's, indicating that Co posses one extra valence electron, and that Fe and Co are in the same 2+ oxidation state. Yet, significant Co character is detected for the Bloch wavefunctions at the chemical potential, revealing that the Co 3d electrons are part of the Fermi sea determining the Fermi surface. This establishes the complex role of Co substitution in CaFe2As2, and the inadequacy of a rigid-band shift description.

cond-mat.supr-con↗

arXiv2011

Bond disorder and breakdown of ballistic heat transport in the spin-1/2 antiferromagnetic Heisenberg chain as seen in Ca-doped SrCuO2

We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disorder levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of kappa_mag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.

cond-mat.str-el↗

arXiv2011

Electronic phase transitions in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films revealed by resonant soft x-ray scattering

We report the study of magnetic and orbital order in Pr$_{0.5}$Ca$_{0.5}$MnO$_3$ epitaxial thin films grown on (LaAlO$_3$)$_{0.3}$-(SrAl$_{0.5}$Ta$_{0.5}$O$_3$)$_{0.7}$ (LSAT) (011)$_c$. In a new experimental approach, the polarization and energy dependence of resonant soft x-ray scattering are used to reveal significant modifications of the magnetic order in the film as compared to the bulk, namely (i) a different magnetic ordering wave vector, (ii) a different magnetic easy axis and (iii) an additional magnetic reordering transition at low temperatures. These observations indicate a strong impact of the epitaxial strain on the spin order, which is mediated by the orbital degrees of freedom and which provides a promising route to tune the magnetic properties of manganite films. Our results further demonstrate that resonant soft x-ray scattering is a very suitable technique to study the magnetism in thin films, to which neutron scattering cannot easily be applied due to the small sample volume.

cond-mat.str-el↗

arXiv2011

A Comparison of Stripe Modulations in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$

We report combined soft and hard x-ray scattering studies of the electronic and lattice modulations associated with stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$ and La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$. We find that the amplitude of both the electronic modulation of the hole density and the strain modulation of the lattice is significantly larger in La$_{1.875}$Ba$_{0.125}$CuO$_4$ than in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$ and is also better correlated. The in-plane correlation lengths are isotropic in each case; for La$_{1.875}$Ba$_{0.125}$CuO$_4$, $ξ^{hole}=255\pm 5$ Å whereas for La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$F, $ξ^{hole}=111\pm 7$ Å. We find that the modulations are temperature independent in La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the low temperature tetragonal phase. In contrast, in La$_{1.48}$Nd$_{0.4}$Sr$_{0.12}$CuO$_4$, the amplitude grows smoothly from zero, beginning 13 K below the LTT phase transition. We speculate that the reduced average tilt angle in La$_{1.875}$Ba$_{0.125}$CuO$_4$ results in reduced charge localization and incoherent pinning, leading to the longer correlation length and enhanced periodic modulation amplitude.

cond-mat.str-el↗

arXiv2011

Resonant elastic soft x-ray scattering in oxygen-ordered YBa_2Cu_3O_{6+delta}

Static charge-density wave (CDW) and spin-density wave (SDW) order has been convincingly observed in La-based cuprates for some time. However, more recently it has been suggested by quantum oscillation, transport and thermodynamic measurements that density wave order is generic to underdoped cuprates and plays a significant role in YBa_2Cu_3O_{6+delta} (YBCO). We use resonant soft x-ray scattering at the Cu L and O K edges to search for evidence of density wave order in Ortho-II and Ortho-VIII oxygen-ordered YBCO. We report a null result -- no evidence for static CDW order -- in both Ortho-II and Ortho-VIII ordered YBCO. While this does not rule out static CDW order in the CuO_2 planes of YBCO, these measurements place limits on the parameter space (temperature, magnetic field, scattering vector) in which static CDW order may exist. In addition, we present a detailed analysis of the energy and polarization dependence of the Ortho-II superstructure Bragg reflection [0.5 0 0] at the Cu L edge. The intensity of this peak, which is due to the valence modulations of Cu in the chain layer, is compared with calculations using atomic scattering form factors deduced from x-ray absorption measurements. The calculated energy and polarization dependence of the scattering intensity is shown to agree very well with the measurement, validating the approach and providing a framework for analyzing future resonant soft x-ray scattering measurements.

cond-mat.supr-con↗

arXiv2011

Observation of Electronic Ferroelectric Polarization in Multiferroic YMn$_2$O$_5$

We report the observation of a magnetic polarization of the O\,$2p$-states in YMn$_2$O$_5$ through the use of soft X-ray resonant scattering at the oxygen $K$-edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast to the temperature dependence observed at the Mn\,$L_3$-edge, which reflects the Mn magnetic order parameter. First principle calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O\,$2p$- and Mn\, 3d-states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions.

cond-mat.str-el↗

arXiv2011

Surface and bulk electronic structure of unconventional superconductor Sr_2RuO_4: unusual splitting of the beta-band

We present an angle resolved photoemission study of the surface and bulk electronic structure of the single layer ruthenate Sr$_2$RuO$_4$. As the early studies of its electronic structure by photoemission and scanning tunneling microscopy were confronted with a problem of surface reconstruction, surface aging was previously proposed as a possible remedy to access the bulk states. Here we suggest an alternative way by demonstrating that, in the case of Sr$_2$RuO$_4$, circularly polarised light can be used to disentangle the signals from the bulk and surface layers, thus opening the possibility of investigating many-body interactions both in bulk and surface bands. The proposed procedure results in improved momentum resolution, which enabled us to detect an unexpected splitting of the surface $β$ band. We propose that spin--orbit splitting might be responsible for this, and discuss possible relations of the newly observed surface feature to topological matter.

cond-mat.supr-con↗

arXiv2011

An in-vacuum diffractometer for resonant elastic soft x-ray scattering

We describe the design, construction and performance of a 4-circle in-vacuum diffractometer for resonant elastic soft x-ray scattering and reflectivity. The diffractometer, installed on the REIXS beamline at the Canadian Light Source, includes 9 in-vacuum motions driven by in-vacuum stepper motors and operates in ultra-high vacuum at base pressure of 2 x10^-10 Torr. Cooling to a base temperature of 18 K is provided with a closed-cycle cryostat. The diffractometer includes a choice of 3 photon detectors: a photodiode, a channeltron and a 2D sensitive channelplate detector. Along with variable slit and filter options, these detectors are suitable for studying a wide range of phenomena having both weak and strong diffraction signals. Example measurements of diffraction and reflectivity in Nd-doped (La,Sr)_2CuO_4 and thin film (Ga,Mn)As are shown.

cond-mat.str-el↗

arXiv2011

Signatures of electronic polarons in La$_{1-x}$Sr$_{1+x}$MnO$_4$ observed by electron energy-loss spectroscopy

The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with the onset of magnetic correlations for x = 0.5, implying a strong coupling of charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a signature of electronic polarons, which are created around doped holes, and that (ii) this low-energy excitation involves the charge transfer between manganese and oxygen. The finite dispersion of these excitations further indicates significant polaron-polaron interactions.

cond-mat.str-el↗