Jonathan Lang | Argonne National Laboratory (original) (raw)

Papers by Jonathan Lang

Physical Review B, 2002

We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in ... more We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in which nearest-neighbor positions are virtually unchanged. Experimental spectra and multiple-scattering calculations are reported at the major absorption edges in the zinc-blende and wurtzite modifications of ZnS. We demonstrate that d-like unoccupied bands are preferentially affected, as only Ledge absorption is sensitive to this structural transition. The results allow near-edge x-ray absorption spectroscopy to be evaluated as a detection method for crystal structure modifications in nano-scale systems.

Physical Review Letters - PHYS REV LETT, 2007

We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5... more We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5} edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu{sub 0.95}FeSb is a ferrimagnet in which the Fe 3d moment and the Eu{sup 2+} 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L edge XAS, we find that about 13% of the Eu have a formal valence of 3{sup +}. We ascribe the origin of ferrimagnetism at a relatively high transition temperature T{sub c} of 85 K in Eu{sub 0.95}FeSb to f-electron interaction with the nearly ferromagnetic [FeSb]{sup 2.2-} host lattice.

Review of Scientific Instruments, 2009

We present a portable pulsed-magnet system for x-ray studies of materials in high magnetic fields... more We present a portable pulsed-magnet system for x-ray studies of materials in high magnetic fields (up to 30 T). The apparatus consists of a split-pair of minicoils cooled on a closed-cycle cryostat, which is used for x-ray diffraction studies with applied field normal to the scattering plane. A second independent closed-cycle cryostat is used for cooling the sample to near liquid helium temperatures. Pulsed magnetic fields (∼1 ms in total duration) are generated by discharging a configurable capacitor bank into the magnet coils. Time-resolved scattering data are collected using a combination of a fast single-photon counting detector, a multichannel scaler, and a high-resolution digital storage oscilloscope. The capabilities of this instrument are used to study a geometrically frustrated system revealing strong magnetostrictive effects in the spin-liquid state.

Physical Review B, 2008

The spin-and charge-density-wave order parameters of the itinerant antiferromagnet chromium are m... more The spin-and charge-density-wave order parameters of the itinerant antiferromagnet chromium are measured directly with non-resonant x-ray diffraction as the system is driven towards its quantum critical point with high pressure using a diamond anvil cell. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge, while the evolution of the incommensurate ordering vector provides important insight into the difference between pressure and chemical doping as means of driving quantum phase transitions. Measurement of the charge density wave over more than two orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly-coupled, BCS-like ground state. Evidence for the coexistence of this weakly-coupled ground state with high-energy excitations and pseudogap formation above the ordering temperature in chromium, the charge-ordered perovskite manganites, and the blue bronzes, among other such systems, raises fundamental questions about the distinctions between weak and strong coupling.

Physical Review B, 2005

Resonant and nonresonant magnetic x-ray scattering studies of GdCo 2 Ge 2 were performed to deter... more Resonant and nonresonant magnetic x-ray scattering studies of GdCo 2 Ge 2 were performed to determine its magnetic structure at low temperature. This compound orders in an incommensurate antiferromagnetic ͑AF͒ structure characterized by a propagation wave vector = ͑0 0 z ͒. The value of z is temperature dependent and approaches 0.930 reciprocal lattice units well below T N = 33.25 K. A peak corresponding to 3 z was also observed, indicating either a squaring up of the magnetic structure or the presence of a noncollinear amplitude modulated structure below T N. Fitting the angular dependence of the magnetic scattering integrated intensities to the relevant resonant and nonresonant scattering cross sections revealed that the moment direction lies primarily in the tetragonal basal plane. Scattering measurements at the Co K-edge failed to detect any resonant signal, consistent with the absence of a magnetic moment on the Co sites.

SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state.... more SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state. It serves as an experimental realization of the Shastry-Sutherland model for interacting S=1/2 dimers. The ratio of the intra and inter-dimer exchange in this compound is close to a quantum critical point, where the ground state transforms from a gapped, non-magnetic state to a gapless long-range ordered antiferromagnetic state as a function of the ratio of the strength of the magnetic interactions. We use synchrotron x-ray diffraction in a diamond anvil cell to investigate the pressure-driven quantum phase transition in high-quality single crystals of SrCu2(BO3). We will present the evolution of both the magnetic and structural properties up to pressures of 5 GPa.

Aps March Meeting Abstracts, Mar 1, 1997

The temperature dependence (T=10-300K) of circular magnetic x-ray dichroism at the rare earth L3 ... more The temperature dependence (T=10-300K) of circular magnetic x-ray dichroism at the rare earth L3 edge of REFe2 compounds (RE=Ho-Lu) has been studied. The magnetization of the rare earth 4f and 5d states has been separately determined by measuring the magnitude of the quadrupolar and dipolar features in the dichroic spectra. The 5d moment is found to scale with the RE 4f moment in all compounds except Lu, in which it scales with the Fe moment. An explanation of this and other observed features will be discussed. fˇill footnoterule ^*Work at the Advanced Photon Source is supported by USDOE-BES under contract No. W-31-109-ENG-38. Ames Laboratory is operated for USDOE by Iowa State Univ. under contract No. W-7405-ENG-82.

Physical Review B Condensed Matter and Materials Physics, 2008

We have observed a Fermi-surface (FS) induced lattice modulation in a YBa2Cu3O7-x superconductor ... more We have observed a Fermi-surface (FS) induced lattice modulation in a YBa2Cu3O7-x superconductor with a wave vector along CuO chains; i.e., q1=(0,δ,0) . The value of δ˜0.21 is twice the Fermi wave vector (2kF) along b∗ connecting nearly nested FS “ridges.” The q1 modulation exists only within O-vacancy-ordered islands [characterized by q0=((1)/(4),0,0) ] and persists well above and below Tc . Our results are consistent with the presence of a FS-induced charge-density wave.

We have fabricated and characterized the performance of a monolithic Ge Bragg-Laue phase retarder... more We have fabricated and characterized the performance of a monolithic Ge Bragg-Laue phase retarder capable of simultaneously producing both right-and left-handed circularly polarized x rays. The energy range of operation of the phase retarder is between 50 and 100 keV making it well suited to the measurement of spin-dependent Compton profiles within the impulse approximation, primarily because of the increased momentum resolution and larger Compton scattering cross section available at these higher incident energies. Although the phase retarder was optimized for operation at 86 keV, it can produce highly circularly polarized x rays over a substantial energy range. The performance of the phase retarder was tested via magnetic Compton scattering measurements on an Fe sample at the undulator A of the Advanced Photon Source. It was found to perform well in terms of flux and degree of circular polarization thereby greatly reducing the data collection times required for this inherently weak scattering process.

[ Research paper thumbnail of Direct observation of surface-driven twisted state in an Fe-terminated [Gd(50 �Fe(35 �multilayer](https://a.academia-assets.com/images/blank-paper.jpg)

The magnetic phase diagram of ferrimagnetic Gd/Fe multilayers has long been known to exhibit coll... more The magnetic phase diagram of ferrimagnetic Gd/Fe multilayers has long been known to exhibit collinear and twisted magnetic configurations that result from a delicate balance between inter/intra-layer exchange and Zeeman energies. At low applied fields exchange dominates and collinear structures are favored, the majority component (Gd) aligning with the field while the minority component (Fe) is constrained anti-parallel by interlayer exchange. At high applied fields a twisted phase arises when it becomes advantageous to reduce Zeeman energy losses in the anti-parallel minority component by a twist which reduces its projected moment along the field direction. It has also been proposed(LePage and Camley, Phys. Rev. Lett. 65), 1152 (1990) that when the multilayer is terminated with the minority component a transition into a ``surface'' twisted phase occurs at a lower field than that of its ``bulk'' twist counterpart. Using x-ray magnetic circular dichroism (XMCD) we have, for the first time, unambiguously determined the existence of this surface twisted phase.

Thesis Iowa State University 1993 Source Dissertation Abstracts International Volume 54 09 Section B Page 4750, Dec 30, 1993

Circular magnetic x-ray dichroism (CMXD) has been used to probe the magnetic properties of rare e... more Circular magnetic x-ray dichroism (CMXD) has been used to probe the magnetic properties of rare earth compounds. This study has been undertaken in two distinct parts. First, the angular dependence of the CMXD signal has been examined in order to determine the multiple nature of the transitions involved. Second, the nature of the dichroic spectra of amorphous and crystalline GdFe_2 has been probed. CMXD offers a unique probe of the magnetic properties of amorphous materials providing information that is not available from any other technique. Previous CMXD measurements at the L edges of rare earth materials have demonstrated two distinct features, one above and one below the Fermi energy. Theoretical calculations have ascribed the two features to dipolar and quadrupolar transitions to the empty 5d and 4f states respectively. This study sought to determine the multipole nature of the transitions involved by measuring the angular dependence of the CMXD signal. Two experiments attempting to observe this angular dependence were undertaken, one at the L_3 edge of Er in Er _2Fe_{14}B, and another at the L_2 and L _3 edges of grain oriented REFe_2 compounds. All features in the CMXD spectra, within experimental error, were found to be consistent with dipolar transitions. We discuss some possible reasons for the apparent absence of the predicted quadrupolar angular dependence of the features below E_{rm f }.. The spin dependent absorption of circularly polarized x-rays at the K edge of Fe and the L_2 and L_3 edges Gd in amorphous and crystalline GdFe_2 has also been studied. Large differences in the magnitude of the dichroic signal are observed between the two samples. The application of recently derived sum rules indicated substantial quenching of the orbital moment in the amorphous sample. The results are compared to a theoretical spectrum for crystalline GdFe_2.

Using high-energy synchrotron x-ray scattering experiments, we present clear evidence of lattice ... more Using high-energy synchrotron x-ray scattering experiments, we present clear evidence of lattice modulations in twinned and detwinned single crystals of optimally doped YBa_2Cu_3O_6.92 superconductor. Diffuse superlattice peaks associated with a real-space priodicity of four unit cells (4a) along the shorter Cu-O-Cu bond direction are found at positions q_0=(1/4,0,0) (in units of frac2πa where a is the lattice parameter) away from the Bragg points in the momentum space. We observe the q0 peaks at room temperature, which grow significantly stronger well into the superconducting state. Furthermore, our observations of interplanar correlations unambiguously show that modulations occurring on the CuO2 planes are coupled to those on the CuO chains and BaO planes, respectively. Intriguingly, q0 is the same as the 4a periodicity observed previously in the local density of states near vortex cores and is the wave vector where discontinuity in phonon dispersions was observed in other high-temperature superconductors, respectively. In addition, the appearance of anisotropic diffuse scattering around Bragg points is indicative of the coexistence of lattice strain effects in this material.

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Aps Meeting Abstracts, Mar 1, 2002

The occurrence of a remarkably high Curie temperature ( ~ 400 K) in ferrimagnetic vanadium-tetrac... more The occurrence of a remarkably high Curie temperature ( ~ 400 K) in ferrimagnetic vanadium-tetracyanoethylene (V[TCNE]_x, x ~ 2) molecular magnet is of great interest. Understanding the chemical structure of this material is the first step towards solving the more difficult problem of magnetic ordering. Since samples grown as thin films are amorphous, the x-ray absorption fine structure (XAFS) technique is ideal for deriving the local structure around V-ions. Temperature-dependent XAFS measurements at the vanadium K-edge indicate a coordination of 4.1(4) nitrogen atoms at an average distance < R >=2.08(1) ÅThe V-N bonding is very strong with a characteristic Einstein temperature above 1000 K. This leads to a narrow distribution of V-N nearest neighbor distances with an rms variation of sigma ~ 0.03 ÅThe absorption data also indicates lack of centro-symmetry at the V-ion site, and preliminary evidence suggests a valence state for V-ion higher than 2.

Aps Meeting Abstracts, Mar 1, 2005

Superstructures characterized by q=(qx, 0, 0) are observed throughout the phase diagram of yttriu... more Superstructures characterized by q=(qx, 0, 0) are observed throughout the phase diagram of yttrium-barium cuprates (YBa2Cu3O6+x, YBCO); q decreases with doping from 12 (2-unit-cell) in the heavily un-derdoped compound to 15 in the overdoped material. A 4-unit-cell superstucture is stable in the vicinity of optimal doping. The superstructures in YBCO correspond to short-range ordered regions of coupled atomic displacements on neighboring CuO, BaO, and CuO2 planes, respectively. T-dependent measurements suggest that these ``nanodomains'' experience anharmonic thermal motion. These regions induce a long-range strain in the host, which manifests as ``bow-tie''-shape Huang diffuse scattering below ˜200 K. X- ray diffuse scattering results will be presented within the context of the oxygen ordering and the phase diagram.

Physical Review Letters, Nov 1, 2010

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIr... more The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ˜1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L2,3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.