Jungho Kim - Academia.edu (original) (raw)

Papers by Jungho Kim

Research paper thumbnail of Magnetic excitation spectra of Sr2IrO4 probed by resonant inelastic x-ray scattering: establishing links to cuprate superconductors

Physical review letters, 2012

We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2I... more We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments. The magnon dispersion in Sr2IrO4 is well-described by an antiferromagnetic Heisenberg model with an effective spin one-half on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This point is further supported by the observation of exciton modes in Sr2IrO4, whose dispersion is strongly renormalized by magnons, which can be understood by analogy to hole propagation in the background of antiferromagnetically ordered spins in the cuprates.

Research paper thumbnail of Dynamic Spin Correlations in the Honeycomb Lattice Na2IrO3 Measured by Resonant Inelastic x-Ray Scattering

Research paper thumbnail of Magnetic Excitations across the Metal-Insulator Transition in the Pyrochlore Iridate Eu_{2}Ir_{2}O_{7}

Physical review letters, Jan 27, 2018

We report a resonant inelastic x-ray scattering study of the magnetic excitation spectrum in a hi... more We report a resonant inelastic x-ray scattering study of the magnetic excitation spectrum in a highly insulating Eu_{2}Ir_{2}O_{7} single crystal that exhibits a metal-insulator transition at T_{MI}=111(7) K. A propagating magnon mode with a 20 meV bandwidth and a 28 meV magnon gap is found in the excitation spectrum at 7 K, which is expected in the all-in-all-out magnetically ordered state. This magnetic excitation exhibits substantial softening as the temperature is raised towards T_{MI} and turns into a highly damped excitation in the paramagnetic phase. Remarkably, the softening occurs throughout the whole Brillouin zone including the zone boundary. This observation is inconsistent with the magnon renormalization expected in a local moment system and indicates that the strength of the electron correlation in Eu_{2}Ir_{2}O_{7} is only moderate, so that electron itinerancy should be taken into account in describing its magnetism.

Research paper thumbnail of Performance of quartz- and sapphire-based double-crystal high-resolution (∼10 meV) RIXS monochromators under varying power loads

Journal of synchrotron radiation, 2018

In the context of a novel, high-resolution resonant inelastic X-ray scattering spectrometer, a fl... more In the context of a novel, high-resolution resonant inelastic X-ray scattering spectrometer, a flat-crystal-based quartz analyzer system has recently been demonstrated to provide an unprecedented intrinsic-energy resolution of 3.9 meV at the Ir L absorption edge (11.215 keV) [Kim et al. (2018) Sci. Rep. 8, 1958]. However, the overall instrument resolution was limited to 9.7 meV because of an 8.9 meV incident band pass, generated by the available high-resolution four-bounce Si(844) monochromator. In order to better match the potent resolving power of the novel analyzer with the energy band pass of the incident beam, a quartz(309)-based double-bounce, high-resolution monochromator was designed and implemented, expected to yield an overall instrument resolution of 6.0 meV. The choice of lower-symmetry quartz is very attractive because of its wealth of suitable near-backscattering reflections. However, it was found that during room-temperature operation typical levels of incident power,...

Research paper thumbnail of Quartz-based flat-crystal resonant inelastic x-ray scattering spectrometer with sub-10 meV energy resolution

Scientific reports, Jan 31, 2018

Continued improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS) spec... more Continued improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS) spectrometers is crucial for fulfilling the potential of this technique in the study of electron dynamics in materials of fundamental and technological importance. In particular, RIXS is the only alternative tool to inelastic neutron scattering capable of providing fully momentum resolved information on dynamic spin structures of magnetic materials, but is limited to systems whose magnetic excitation energy scales are comparable to the energy resolution. The state-of-the-art spherical diced crystal analyzer optics provides energy resolution as good as 25 meV but has already reached its theoretical limit. Here, we demonstrate a novel sub-10 meV RIXS spectrometer based on flat-crystal optics at the Ir-L3 absorption edge (11.215 keV) that achieves an analyzer energy resolution of 3.9 meV, very close to the theoretical value of 3.7 meV. In addition, the new spectrometer allows efficient polariza...

Research paper thumbnail of Direct experimental observation of the molecular J eff = 3/2 ground state in the lacunar spinel GaTa4Se8

Nature communications, Oct 4, 2017

Strong spin-orbit coupling lifts the degeneracy of t 2g orbitals in 5d transition-metal systems, ... more Strong spin-orbit coupling lifts the degeneracy of t 2g orbitals in 5d transition-metal systems, leaving a Kramers doublet and quartet with effective angular momentum of J eff = 1/2 and 3/2, respectively. These spin-orbit entangled states can host exotic quantum phases such as topological Mott state, unconventional superconductivity, and quantum spin liquid. The lacunar spinel GaTa4Se8 was theoretically predicted to form the molecular J eff = 3/2 ground state. Experimental verification of its existence is an important first step to exploring the consequences of the J eff = 3/2 state. Here, we report direct experimental evidence of the J eff = 3/2 state in GaTa4Se8 by means of excitation spectra of resonant inelastic X-ray scattering at the Ta L3 and L2 edges. We find that the excitations involving the J eff = 1/2 molecular orbital are absent only at the Ta L2 edge, manifesting the realization of the molecular J eff = 3/2 ground state in GaTa4Se8.The strong interaction between electr...

Research paper thumbnail of Toroidal silicon polarization analyzer for resonant inelastic x-ray scattering

Review of Scientific Instruments, 2016

Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitation... more Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitations in materials. Standard high energy RIXS measurements do not measure the polarization of the scattered x-rays, which is unfortunate since it carries information about the nature and symmetry of the excitations involved in the scattering process. Here we report the fabrication of thin Si-based polarization analyzers with a double-concave toroidal surface, useful for Ledge RIXS studies in heavier atoms such as the 5-d transition metals.

Research paper thumbnail of Engineering 1D Quantum Stripes from Superlattices of 2D Layered Materials

Advanced materials (Deerfield Beach, Fla.), 2017

Dimensional tunability from two dimensions to one dimension is demonstrated for the first time us... more Dimensional tunability from two dimensions to one dimension is demonstrated for the first time using an artificial superlattice method in synthesizing 1D stripes from 2D layered materials. The 1D confinement of layered Sr2 IrO4 induces distinct 1D quantum-confined electronic states, as observed from optical spectroscopy and resonant inelastic X-ray scattering. This 1D superlattice approach is generalizable to a wide range of layered materials.

Research paper thumbnail of Collimating Montel mirror as part of a multi-crystal analyzer system for resonant inelastic X-ray scattering

Journal of synchrotron radiation, 2016

Advances in resonant inelastic X-ray scattering (RIXS) have come in lockstep with improvements in... more Advances in resonant inelastic X-ray scattering (RIXS) have come in lockstep with improvements in energy resolution. Currently, the best energy resolution at the Ir L3-edge stands at ∼25 meV, which is achieved using a diced Si(844) spherical crystal analyzer. However, spherical analyzers are limited by their intrinsic reflection width. A novel analyzer system using multiple flat crystals provides a promising way to overcome this limitation. For the present design, an energy resolution at or below 10 meV was selected. Recognizing that the angular acceptance of flat crystals is severely limited, a collimating element is essential to achieve the necessary solid-angle acceptance. For this purpose, a laterally graded, parabolic, multilayer Montel mirror was designed for use at the Ir L3-absorption edge. It provides an acceptance larger than 10 mrad, collimating the reflected X-ray beam to smaller than 100 µrad, in both vertical and horizontal directions. The performance of this mirror wa...

Research paper thumbnail of Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4

Nature materials, Jun 1, 2016

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our ... more Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a flue...

Research paper thumbnail of Momentum-resolved Resonant and Nonresonant Inelastic X-ray Scattering at the Advanced Photon Source

Synchrotron Radiation News, 2009

The study of electronic excitations by inelastic X-ray scattering (IXS) has a rich history. Very ... more The study of electronic excitations by inelastic X-ray scattering (IXS) has a rich history. Very early IXS work, for example, provided seminal demonstrations of the validity of relativistic kinematics and the quantum hypothesis [1] and of Fermi-Dirac statistics [2]. While there have been many important results in the interim [3], it has been the development of the third generation light

Research paper thumbnail of Indirect probing of defects in unipolar resistive switching NiO x thin films by Ni K-edge resonant inelastic X-ray scattering

Applied Physics Express, 2015

ABSTRACT We present observations of the Ni K-edge resonant inelastic X-ray scattering (RIXS) in N... more ABSTRACT We present observations of the Ni K-edge resonant inelastic X-ray scattering (RIXS) in NiO x thin films showing unipolar resistive switching (RS). The RIXS spectra of RS NiO x thin films can be described in terms of crystal field (dd) and charge transfer (CT) excitations. We found distorted dd excitations in the films' pristine state before electroforming, and identical excitations for high and low resistance states after electroforming. This suggests that the RS property of NiO x thin film is related to defects in pristine NiO x films, and RS occurs in local nanosized spots too small to be detected by RIXS.

Research paper thumbnail of Competition between charge order and superconductivity in La2-xBaxCuO4 (x~ 1/8)

Understanding the role of stripe physics in cuprate superconductors is believed to be essential i... more Understanding the role of stripe physics in cuprate superconductors is believed to be essential in elucidating the superconducting mechanism of the cuprates. Despite the fundamental importance of charge ordering in the cuprates, a comprehensive examination of the relationship between charge stripes and superconductivity is still lacking. We have carried out a detailed investigation of temperature and magnetic field dependence of

Research paper thumbnail of Development of a graphite polarization analyzer for resonant inelastic x-ray scattering

The Review of scientific instruments, 2011

Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excita... more Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excitations in correlated electron systems. Current RIXS spectrometers measure the changes in energy and momentum of the photons scattered by the sample. A powerful extension of the RIXS technique is the measurement of the polarization state of the scattered photons which contains information about the symmetry of the excitations. This long-desired addition has been elusive because of significant technical challenges. This paper reports the development of a new diffraction-based polarization analyzer which discriminates between linear polarization components of the scattered photons. The double concave surface of the polarization analyzer was designed as a good compromise between energy resolution and throughput. Such a device was fabricated using highly oriented pyrolytic graphite for measurements at the Cu K-edge incident energy. Preliminary measurements on a CuGeO(3) sample are presented.

Research paper thumbnail of Magnetic field dependence of charge stripe order in La2-x Bax Cu O4 (x ≈ 1 8)

Physical Review B - Condensed Matter and Materials Physics, 2008

We have carried out a detailed investigation of the magnetic field dependence of charge ordering ... more We have carried out a detailed investigation of the magnetic field dependence of charge ordering in La2−xBaxCuO4 (x ≈ 1/8) utilizing high-resolution x-ray scattering. We find that the charge order correlation length increases as the magnetic field greater than ∼ 5 T is applied in the superconducting phase (T=2 K). The observed unusual field dependence of the charge order correlation length suggests that the static charge stripe order competes with the superconducting ground state in this sample.

Research paper thumbnail of Magnetic nature of the 500 meV peak in La 2-x Sr x CuO 4 observed with resonant inelastic x-ray scattering at the Cu K-edge

Physical Review B - Condensed Matter and Materials Physics, 2010

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak obs... more We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at q = (π, 0) in resonant inelastic x-ray scattering (RIXS) experiments on La2CuO4. The intensity of this peak persists above the Néel temperature (TN =320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon B1g Raman peak in La2CuO4, and with suitable rescaling, agrees with the Raman peak shifts in EuBa2Cu3O6 and K2NiF4. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

Research paper thumbnail of Hard X-ray Resonant Inelastic X-ray Scattering at the Advanced Photon Source

Synchrotron Radiation News, 2012

This article may be used for research, teaching, and private study purposes. Any substantial or s... more This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Research paper thumbnail of Spin-State Transition in the Fe Pnictides

Physical Review Letters, 2013

We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth d... more We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca1−xRExFe2As2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼0.9µB at T = 300 K to ∼0.45µB at T = 70 K. In the collapsed tetragonal (cT) phase of Nd-and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca1−xRExFe2As2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe 2+ ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

Research paper thumbnail of Crystal-Field Splitting and Correlation Effect on the Electronic Structure ofA2IrO3

Physical Review Letters, 2013

The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investiga... more The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to non-cubic crystal fields, derived from the splitting of j eff =3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j eff physics in A2IrO3. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na2IrO3 and Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr2IrO4.

Research paper thumbnail of Large Spin-Wave Energy Gap in the Bilayer IridateSr3Ir2O7: Evidence for Enhanced Dipolar Interactions Near the Mott Metal-Insulator Transition

Physical Review Letters, 2012

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orb... more Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudo-dipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudo-dipolar interactions, may emerge in small charge-gap iridates.

Research paper thumbnail of Magnetic excitation spectra of Sr2IrO4 probed by resonant inelastic x-ray scattering: establishing links to cuprate superconductors

Physical review letters, 2012

We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2I... more We used resonant inelastic x-ray scattering to reveal the nature of magnetic interactions in Sr2IrO4, a 5d transition-metal oxide with a spin-orbit entangled ground state and J(eff)=1/2 magnetic moments. The magnon dispersion in Sr2IrO4 is well-described by an antiferromagnetic Heisenberg model with an effective spin one-half on a square lattice, which renders the low-energy effective physics of Sr2IrO4 much akin to that in superconducting cuprates. This point is further supported by the observation of exciton modes in Sr2IrO4, whose dispersion is strongly renormalized by magnons, which can be understood by analogy to hole propagation in the background of antiferromagnetically ordered spins in the cuprates.

Research paper thumbnail of Dynamic Spin Correlations in the Honeycomb Lattice Na2IrO3 Measured by Resonant Inelastic x-Ray Scattering

Research paper thumbnail of Magnetic Excitations across the Metal-Insulator Transition in the Pyrochlore Iridate Eu_{2}Ir_{2}O_{7}

Physical review letters, Jan 27, 2018

We report a resonant inelastic x-ray scattering study of the magnetic excitation spectrum in a hi... more We report a resonant inelastic x-ray scattering study of the magnetic excitation spectrum in a highly insulating Eu_{2}Ir_{2}O_{7} single crystal that exhibits a metal-insulator transition at T_{MI}=111(7) K. A propagating magnon mode with a 20 meV bandwidth and a 28 meV magnon gap is found in the excitation spectrum at 7 K, which is expected in the all-in-all-out magnetically ordered state. This magnetic excitation exhibits substantial softening as the temperature is raised towards T_{MI} and turns into a highly damped excitation in the paramagnetic phase. Remarkably, the softening occurs throughout the whole Brillouin zone including the zone boundary. This observation is inconsistent with the magnon renormalization expected in a local moment system and indicates that the strength of the electron correlation in Eu_{2}Ir_{2}O_{7} is only moderate, so that electron itinerancy should be taken into account in describing its magnetism.

Research paper thumbnail of Performance of quartz- and sapphire-based double-crystal high-resolution (∼10 meV) RIXS monochromators under varying power loads

Journal of synchrotron radiation, 2018

In the context of a novel, high-resolution resonant inelastic X-ray scattering spectrometer, a fl... more In the context of a novel, high-resolution resonant inelastic X-ray scattering spectrometer, a flat-crystal-based quartz analyzer system has recently been demonstrated to provide an unprecedented intrinsic-energy resolution of 3.9 meV at the Ir L absorption edge (11.215 keV) [Kim et al. (2018) Sci. Rep. 8, 1958]. However, the overall instrument resolution was limited to 9.7 meV because of an 8.9 meV incident band pass, generated by the available high-resolution four-bounce Si(844) monochromator. In order to better match the potent resolving power of the novel analyzer with the energy band pass of the incident beam, a quartz(309)-based double-bounce, high-resolution monochromator was designed and implemented, expected to yield an overall instrument resolution of 6.0 meV. The choice of lower-symmetry quartz is very attractive because of its wealth of suitable near-backscattering reflections. However, it was found that during room-temperature operation typical levels of incident power,...

Research paper thumbnail of Quartz-based flat-crystal resonant inelastic x-ray scattering spectrometer with sub-10 meV energy resolution

Scientific reports, Jan 31, 2018

Continued improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS) spec... more Continued improvement of the energy resolution of resonant inelastic x-ray scattering (RIXS) spectrometers is crucial for fulfilling the potential of this technique in the study of electron dynamics in materials of fundamental and technological importance. In particular, RIXS is the only alternative tool to inelastic neutron scattering capable of providing fully momentum resolved information on dynamic spin structures of magnetic materials, but is limited to systems whose magnetic excitation energy scales are comparable to the energy resolution. The state-of-the-art spherical diced crystal analyzer optics provides energy resolution as good as 25 meV but has already reached its theoretical limit. Here, we demonstrate a novel sub-10 meV RIXS spectrometer based on flat-crystal optics at the Ir-L3 absorption edge (11.215 keV) that achieves an analyzer energy resolution of 3.9 meV, very close to the theoretical value of 3.7 meV. In addition, the new spectrometer allows efficient polariza...

Research paper thumbnail of Direct experimental observation of the molecular J eff = 3/2 ground state in the lacunar spinel GaTa4Se8

Nature communications, Oct 4, 2017

Strong spin-orbit coupling lifts the degeneracy of t 2g orbitals in 5d transition-metal systems, ... more Strong spin-orbit coupling lifts the degeneracy of t 2g orbitals in 5d transition-metal systems, leaving a Kramers doublet and quartet with effective angular momentum of J eff = 1/2 and 3/2, respectively. These spin-orbit entangled states can host exotic quantum phases such as topological Mott state, unconventional superconductivity, and quantum spin liquid. The lacunar spinel GaTa4Se8 was theoretically predicted to form the molecular J eff = 3/2 ground state. Experimental verification of its existence is an important first step to exploring the consequences of the J eff = 3/2 state. Here, we report direct experimental evidence of the J eff = 3/2 state in GaTa4Se8 by means of excitation spectra of resonant inelastic X-ray scattering at the Ta L3 and L2 edges. We find that the excitations involving the J eff = 1/2 molecular orbital are absent only at the Ta L2 edge, manifesting the realization of the molecular J eff = 3/2 ground state in GaTa4Se8.The strong interaction between electr...

Research paper thumbnail of Toroidal silicon polarization analyzer for resonant inelastic x-ray scattering

Review of Scientific Instruments, 2016

Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitation... more Resonant Inelastic X-ray Scattering (RIXS) is a powerful probe for studying electronic excitations in materials. Standard high energy RIXS measurements do not measure the polarization of the scattered x-rays, which is unfortunate since it carries information about the nature and symmetry of the excitations involved in the scattering process. Here we report the fabrication of thin Si-based polarization analyzers with a double-concave toroidal surface, useful for Ledge RIXS studies in heavier atoms such as the 5-d transition metals.

Research paper thumbnail of Engineering 1D Quantum Stripes from Superlattices of 2D Layered Materials

Advanced materials (Deerfield Beach, Fla.), 2017

Dimensional tunability from two dimensions to one dimension is demonstrated for the first time us... more Dimensional tunability from two dimensions to one dimension is demonstrated for the first time using an artificial superlattice method in synthesizing 1D stripes from 2D layered materials. The 1D confinement of layered Sr2 IrO4 induces distinct 1D quantum-confined electronic states, as observed from optical spectroscopy and resonant inelastic X-ray scattering. This 1D superlattice approach is generalizable to a wide range of layered materials.

Research paper thumbnail of Collimating Montel mirror as part of a multi-crystal analyzer system for resonant inelastic X-ray scattering

Journal of synchrotron radiation, 2016

Advances in resonant inelastic X-ray scattering (RIXS) have come in lockstep with improvements in... more Advances in resonant inelastic X-ray scattering (RIXS) have come in lockstep with improvements in energy resolution. Currently, the best energy resolution at the Ir L3-edge stands at ∼25 meV, which is achieved using a diced Si(844) spherical crystal analyzer. However, spherical analyzers are limited by their intrinsic reflection width. A novel analyzer system using multiple flat crystals provides a promising way to overcome this limitation. For the present design, an energy resolution at or below 10 meV was selected. Recognizing that the angular acceptance of flat crystals is severely limited, a collimating element is essential to achieve the necessary solid-angle acceptance. For this purpose, a laterally graded, parabolic, multilayer Montel mirror was designed for use at the Ir L3-absorption edge. It provides an acceptance larger than 10 mrad, collimating the reflected X-ray beam to smaller than 100 µrad, in both vertical and horizontal directions. The performance of this mirror wa...

Research paper thumbnail of Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4

Nature materials, Jun 1, 2016

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our ... more Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a flue...

Research paper thumbnail of Momentum-resolved Resonant and Nonresonant Inelastic X-ray Scattering at the Advanced Photon Source

Synchrotron Radiation News, 2009

The study of electronic excitations by inelastic X-ray scattering (IXS) has a rich history. Very ... more The study of electronic excitations by inelastic X-ray scattering (IXS) has a rich history. Very early IXS work, for example, provided seminal demonstrations of the validity of relativistic kinematics and the quantum hypothesis [1] and of Fermi-Dirac statistics [2]. While there have been many important results in the interim [3], it has been the development of the third generation light

Research paper thumbnail of Indirect probing of defects in unipolar resistive switching NiO x thin films by Ni K-edge resonant inelastic X-ray scattering

Applied Physics Express, 2015

ABSTRACT We present observations of the Ni K-edge resonant inelastic X-ray scattering (RIXS) in N... more ABSTRACT We present observations of the Ni K-edge resonant inelastic X-ray scattering (RIXS) in NiO x thin films showing unipolar resistive switching (RS). The RIXS spectra of RS NiO x thin films can be described in terms of crystal field (dd) and charge transfer (CT) excitations. We found distorted dd excitations in the films&amp;#39; pristine state before electroforming, and identical excitations for high and low resistance states after electroforming. This suggests that the RS property of NiO x thin film is related to defects in pristine NiO x films, and RS occurs in local nanosized spots too small to be detected by RIXS.

Research paper thumbnail of Competition between charge order and superconductivity in La2-xBaxCuO4 (x~ 1/8)

Understanding the role of stripe physics in cuprate superconductors is believed to be essential i... more Understanding the role of stripe physics in cuprate superconductors is believed to be essential in elucidating the superconducting mechanism of the cuprates. Despite the fundamental importance of charge ordering in the cuprates, a comprehensive examination of the relationship between charge stripes and superconductivity is still lacking. We have carried out a detailed investigation of temperature and magnetic field dependence of

Research paper thumbnail of Development of a graphite polarization analyzer for resonant inelastic x-ray scattering

The Review of scientific instruments, 2011

Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excita... more Resonant inelastic x-ray scattering (RIXS) is a powerful technique for studying electronic excitations in correlated electron systems. Current RIXS spectrometers measure the changes in energy and momentum of the photons scattered by the sample. A powerful extension of the RIXS technique is the measurement of the polarization state of the scattered photons which contains information about the symmetry of the excitations. This long-desired addition has been elusive because of significant technical challenges. This paper reports the development of a new diffraction-based polarization analyzer which discriminates between linear polarization components of the scattered photons. The double concave surface of the polarization analyzer was designed as a good compromise between energy resolution and throughput. Such a device was fabricated using highly oriented pyrolytic graphite for measurements at the Cu K-edge incident energy. Preliminary measurements on a CuGeO(3) sample are presented.

Research paper thumbnail of Magnetic field dependence of charge stripe order in La2-x Bax Cu O4 (x ≈ 1 8)

Physical Review B - Condensed Matter and Materials Physics, 2008

We have carried out a detailed investigation of the magnetic field dependence of charge ordering ... more We have carried out a detailed investigation of the magnetic field dependence of charge ordering in La2−xBaxCuO4 (x ≈ 1/8) utilizing high-resolution x-ray scattering. We find that the charge order correlation length increases as the magnetic field greater than ∼ 5 T is applied in the superconducting phase (T=2 K). The observed unusual field dependence of the charge order correlation length suggests that the static charge stripe order competes with the superconducting ground state in this sample.

Research paper thumbnail of Magnetic nature of the 500 meV peak in La 2-x Sr x CuO 4 observed with resonant inelastic x-ray scattering at the Cu K-edge

Physical Review B - Condensed Matter and Materials Physics, 2010

We present a comprehensive study of the temperature and doping dependence of the 500 meV peak obs... more We present a comprehensive study of the temperature and doping dependence of the 500 meV peak observed at q = (π, 0) in resonant inelastic x-ray scattering (RIXS) experiments on La2CuO4. The intensity of this peak persists above the Néel temperature (TN =320 K), but decreases gradually with increasing temperature, reaching zero at around T=500 K. The peak energy decreases with temperature in close quantitative accord with the behavior of the two-magnon B1g Raman peak in La2CuO4, and with suitable rescaling, agrees with the Raman peak shifts in EuBa2Cu3O6 and K2NiF4. The overall dispersion of this excitation in the Brillouin zone is found to be in agreement with theoretical calculations for a two-magnon excitation. Upon doping, the peak intensity decreases analogous to the Raman mode intensity and appears to track the doping dependence of the spin correlation length. Taken together, these observations strongly suggest that the 500 meV mode is magnetic in character and is likely a two-magnon excitation.

Research paper thumbnail of Hard X-ray Resonant Inelastic X-ray Scattering at the Advanced Photon Source

Synchrotron Radiation News, 2012

This article may be used for research, teaching, and private study purposes. Any substantial or s... more This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Research paper thumbnail of Spin-State Transition in the Fe Pnictides

Physical Review Letters, 2013

We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth d... more We report a Fe Kβ x-ray emission spectroscopy study of local magnetic moments in the rare-earth doped iron pnictide Ca1−xRExFe2As2 (RE=La, Pr, and Nd). In all samples studied the size of the Fe local moment is found to decrease significantly with temperature and goes from ∼0.9µB at T = 300 K to ∼0.45µB at T = 70 K. In the collapsed tetragonal (cT) phase of Nd-and Pr-doped samples (T<70K) the local moment is quenched, while the moment remains unchanged for the La-doped sample, which does not show lattice collapse. Our results show that Ca1−xRExFe2As2 (RE= Pr and Nd) exhibits a spin-state transition and provide direct evidence for a non-magnetic Fe 2+ ion in the cT-phase, as predicted by Yildirim. We argue that the gradual change of the the spin-state over a wide temperature range reveals the importance of multiorbital physics, in particular the competition between the crystal field split Fe 3d orbitals and the Hund's rule coupling.

Research paper thumbnail of Crystal-Field Splitting and Correlation Effect on the Electronic Structure ofA2IrO3

Physical Review Letters, 2013

The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investiga... more The electronic structure of the honeycomb lattice iridates Na2IrO3 and Li2IrO3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to non-cubic crystal fields, derived from the splitting of j eff =3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j eff physics in A2IrO3. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na2IrO3 and Li2IrO3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr2IrO4.

Research paper thumbnail of Large Spin-Wave Energy Gap in the Bilayer IridateSr3Ir2O7: Evidence for Enhanced Dipolar Interactions Near the Mott Metal-Insulator Transition

Physical Review Letters, 2012

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orb... more Using resonant inelastic x-ray scattering, we observe in the bilayer iridate Sr3Ir2O7, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of ≈92 meV for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of ≈70 meV and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate Sr2IrO4. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudo-dipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudo-dipolar interactions, may emerge in small charge-gap iridates.