SLAC Director Chi-Chang Kao, a noted X-ray scientist, came to SLAC in 2010 to serve as associate laboratory director for the Stanford Synchrotron Radiation Lightsource. He became SLAC’s fifth director in November 2012.
Previously, Kao served for five years as chairperson of the National Synchrotron Light Source at Brookhaven National Laboratory in New York. He undertook major upgrades to the light source's scientific programs and experimental facilities while developing potential science programs for NSLS-II, one of the newest and most advanced synchrotron facilities in the world. His research focuses on X-ray physics, superconductivity, magnetic materials and the properties of materials under high pressure.
Kao earned a bachelor's degree in chemical engineering in 1980 from National Taiwan University and a doctorate in chemical engineering from Cornell University in 1988. He joined Brookhaven shortly afterward, working his way from NSLS postdoctoral research assistant to chair. Kao also served as an adjunct professor in the Department of Physics and Astronomy at Stony Brook University.
He was elected a fellow of the American Physical Society in 2006 and was named a fellow of the American Association for the Advancement of Science in 2010 for his many contributions to resonant elastic and inelastic X-ray scattering techniques and their application to materials physics, as well as for his leadership at the NSLS.
Professor, Photon Science Directorate
Senior Fellow, Precourt Institute for Energy
Ph.D., Cornell, Chemical Engineering (1988)
Characterization of photoinduced normal state through charge density wave in superconducting YBa2Cu3O6.67.
2022; 8 (6): eabk0832
The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic field study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa2Cu3O6.67 through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high magnetic field x-ray scattering. In the nonequilibrium state where people predict a quenched superconducting state based on the previous optical spectroscopies, we experimentally observed a similar analogy to the competition between superconductivity and CDW shown in the equilibrium state. We further observe that the broken pairing states in the superconducting CuO2 plane via the optical pump lead to nucleation of three-dimensional CDW precursor correlation. Ultimately, these findings provide a critical clue that the characteristics of the photoinduced normal state show a solid resemblance to those under magnetic fields in equilibrium conditions.
View details for DOI 10.1126/sciadv.abk0832
View details for PubMedID 35138893
Detection of the Chiral Spin Structure in Ferromagnetic SrRuO3 Thin Film
ACS APPLIED MATERIALS & INTERFACES
2020; 12 (33): 37757–63
SrRuO3 (SRO) thin films and their heterostructure have attracted much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft X-ray scattering (RSXS) at the oxygen K edge to investigate the spin configuration of a four-unit-cell SRO film that was grown epitaxially on a single-crystal SrTiO3. The RSXS signal under a magnetic field (∼0.4 tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.
View details for DOI 10.1021/acsami.0c10545
View details for Web of Science ID 000563074900095
View details for PubMedID 32696641
Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy.
2020; 10 (1): 16837
We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted 'probe-before-destroy' measurement strategy can be adopted for electronic-structure related XFEL experiments.
View details for DOI 10.1038/s41598-020-74003-1
View details for PubMedID 33033373
- A role for subducted super-hydrated kaolinite in Earth's deep water cycle NATURE GEOSCIENCE 2017; 10 (12): 947-+
- Pressure-Dependent Structural and Chemical Changes in a Metal-Organic Framework with One-Dimensional Pore Structure CHEMISTRY OF MATERIALS 2016; 28 (15): 5336-5341
- Two-Step Pressure-Induced Superhydration in Small Pore Natrolite with Divalent Extra-Framework Cations CHEMISTRY OF MATERIALS 2015; 27 (11): 3874-3880
Pressure-Induced Metathesis Reaction To Sequester Cs
ENVIRONMENTAL SCIENCE & TECHNOLOGY
2015; 49 (1): 513-519
We report here a pressure-driven metathesis reaction where Ag-exchanged natrolite (Ag16Al16Si24O80·16H2O, Ag-NAT) is pressurized in an aqueous CsI solution, resulting in the exchange of Ag(+) by Cs(+) in the natrolite framework forming Cs16Al16Si24O80·16H2O (Cs-NAT-I) and, above 0.5 GPa, its high-pressure polymorph (Cs-NAT-II). During the initial cation exchange, the precipitation of AgI occurs. Additional pressure and heat at 2 GPa and 160 °C transforms Cs-NAT-II to a pollucite-related, highly dense, and water-free triclinic phase with nominal composition CsAlSi2O6. At ambient temperature after pressure release, the Cs remains sequestered in a now monoclinic pollucite phase at close to 40 wt % and a favorably low Cs leaching rate under back-exchange conditions. This process thus efficiently combines the pressure-driven separation of Cs and I at ambient temperature with the subsequent sequestration of Cs under moderate pressures and temperatures in its preferred waste form suitable for long-term storage at ambient conditions. The zeolite pollucite CsAlSi2O6·H2O has been identified as a potential host material for nuclear waste remediation of anthropogenic (137)Cs due to its chemical and thermal stability, low leaching rate, and the large amount of Cs it can contain. The new water-free pollucite phase we characterize during our process will not display radiolysis of water during longterm storage while maintaining the Cs content and low leaching rate.
View details for DOI 10.1021/es504659z
View details for Web of Science ID 000347589300059
View details for PubMedID 25515673
Atomically Engineered Metal Insulator Transition at the TiO2/LaAlO3 Heterointerface
2014; 14 (11): 6743-6746
We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO3 (001) from AlO2 to LaO, the room-temperature sheet conductance of anatase TiO2 films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency.
View details for DOI 10.1021/nl5039192
View details for Web of Science ID 000345723800113
View details for PubMedID 25343440
Irreversible xenon insertion into a small-pore zeolite at moderate pressures and temperatures
2014; 6 (9): 835-839
Pressure drastically alters the chemical and physical properties of materials and allows structural phase transitions and chemical reactions to occur that defy much of our understanding gained under ambient conditions. Particularly exciting is the high-pressure chemistry of xenon, which is known to react with hydrogen and ice at high pressures and form stable compounds. Here, we show that Ag16Al16Si24O8·16H2O (Ag-natrolite) irreversibly inserts xenon into its micropores at 1.7 GPa and 250 °C, while Ag(+) is reduced to metallic Ag and possibly oxidized to Ag(2+). In contrast to krypton, xenon is retained within the pores of this zeolite after pressure release and requires heat to desorb. This irreversible insertion and trapping of xenon in Ag-natrolite under moderate conditions sheds new light on chemical reactions that could account for the xenon deficiency relative to argon observed in terrestrial and Martian atmospheres.
View details for DOI 10.1038/NCHEM.1997
View details for Web of Science ID 000341373500022
View details for PubMedID 25143221
Super-Hydrated Zeolites: Pressure-Induced Hydration in Natrolites
CHEMISTRY-A EUROPEAN JOURNAL
2013; 19 (33): 10876-10883
High-pressure synchrotron X-ray powder diffraction studies of a series of alkali-metal-exchanged natrolites, A16Al16Si24O80·nH2O (A=Li, K, Na, Rb, and Cs and n=14, 16, 22, 24, 32), in the presence of water, reveal structural changes that far exceed what can be achieved by varying temperature and chemical composition. The degree of volume expansion caused by pressure-induced hydration (PIH) is inversely proportional to the non-framework cation radius. The expansion of the unit-cell volume through PIH is as large as 20.6% in Li-natrolite at 1.0 GPa and decreases to 6.7, 3.8, and 0.3% in Na-, K-, and Rb-natrolites, respectively. On the other hand, the onset pressure of PIH appears to increase with non-framework cation radius up to 2.0 GPa in Rb-natrolite. In Cs-natrolite, no PIH is observed but a new phase forms at 0.3 GPa with a 4.8% contracted unit cell and different cation-water configuration in the pores. In K-natrolite, the elliptical channel undergoes a unique overturn upon the formation of super-hydrated natrolite K16Al16Si24O80·32H2O at 1.0 GPa, a species that reverts back above 2.5 GPa as the potassium ions interchange their locations with those of water and migrate from the hinge to the center of the pores. Super-hydrated zeolites are new materials that offer numerous opportunities to expand and modify known chemical and physical properties by reversibly changing the composition and structure using pressure in the presence of water.
View details for DOI 10.1002/chem.201300591
View details for Web of Science ID 000322626500020
View details for PubMedID 23852613
- Role of Cation-Water Disorder during Cation Exchange in Small-Pore Zeolite Sodium Natrolite JOURNAL OF PHYSICAL CHEMISTRY C 2013; 117 (31): 16119-16126
Resolving Material-Specific Structures within Fe3O4 vertical bar gamma-Mn2O3 Core vertical bar Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering
2013; 7 (2): 921-931
Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles with a diameter of 8.2 ± 0.2 nm consist of a core with a composition near Fe₃O₄ surrounded by a (Mn(x)Fe(1-x))₃O₄ shell with a graded composition, ranging from x ≈ 0.40 at the inner shell toward x ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe₃O₄ cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.
View details for DOI 10.1021/nn303600e
View details for Web of Science ID 000315618700008
- Thermal Expansion of the Superhydrated Small-Pore Zeolite Natrolite JOURNAL OF PHYSICAL CHEMISTRY C 2012; 116 (5): 3286-3291
Immobilization of Large, Aliovalent Cations in the Small-Pore Zeolite K-Natrolite by Means of Pressure
ANGEWANDTE CHEMIE-INTERNATIONAL EDITION
2012; 51 (20): 4848-4851
High-pressure ion exchange of small-pore zeolite K-natrolite allows immobilization of nominally non-exchangeable aliovalent cations such as trivalent europium. A sample exchanged at 3.0(1) GPa and 250 °C contains about 4.7 Eu(III) ions per unit cell, which is equivalent to over 90 % of the K(+) cations being exchanged.
View details for DOI 10.1002/anie.201201045
View details for Web of Science ID 000303925200010
View details for PubMedID 22473627
Pressure- and Heat-Induced Insertion of CO2 into an Auxetic Small-Pore Zeolite
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2011; 133 (6): 1674-1677
When the small-pore zeolite natrolite is compressed at ca. 1.5 GPa and heated to ca. 110 °C in the presence of CO(2), the unit cell volume of natrolite expands by 6.8% and ca. 12 wt % of CO(2) is contained in the expanded elliptical channels. This CO(2) insertion into natrolite is found to be reversible upon pressure release.
View details for DOI 10.1021/ja109765d
View details for Web of Science ID 000287831800017
View details for PubMedID 21268577
- In-situ dehydration studies of fully K-,Rb-, and Cs-exchanged natrolites AMERICAN MINERALOGIST 2011; 96 (2-3): 393-401
Electronic Structure of Crystalline He-4 at High Pressures
PHYSICAL REVIEW LETTERS
2010; 105 (18)
Using inelastic x-ray scattering techniques, we have succeeded in probing the high-pressure electronic structure of helium at 300 K. Helium has the widest known valence-conduction band gap of all materials a property whose high-pressure response has been inaccessible to direct measurements. We observed a rich electron excitation spectrum, including a cutoff edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV. We determined the electronic dispersion along the Γ-M direction over two Brillouin zones, and provided a quantitative picture of the helium exciton beyond the simplified Wannier-Frenkel description.
View details for DOI 10.1103/PhysRevLett.105.186404
View details for Web of Science ID 000283652100003
View details for PubMedID 21231121
- High-pressure evolution of Fe2O3 electronic structure revealed by x-ray absorption PHYSICAL REVIEW B 2010; 82 (14)
Atomic-scale visualization of inertial dynamics
2005; 308 (5720): 392-395
The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.
View details for DOI 10.1126/science.1107996
View details for Web of Science ID 000228492000046
View details for PubMedID 15831753
Clocking femtosecond x rays
PHYSICAL REVIEW LETTERS
2005; 94 (11)
Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.
View details for DOI 10.1103/PhysRevLett.94.114801
View details for Web of Science ID 000227923200034
View details for PubMedID 15903864