Invited Talks
Corwin Booth - Lawrence Berkeley National Laboratory
Adventures in Ambiguity: Local Electronic and Atomic Structure Effects at the Edges of the Periodic Table
Adventures in Ambiguity: Local Electronic and Atomic Structure Effects at the Edges of the Periodic Table
There is a lot we don’t understand in condensed matter physics, especially when strong electron-electron interactions govern electronic, magnetic, and bonding properties. In the career I’ve had since I was a Ph.D. student in Bud’s lab in the early-to-mid 1990’s, I’ve studied such effects in heavier and heavier elements, moving from the transition metals to the rare earths and then to the actinides. In all of these regimes of the periodic table, the most fascinating behavior occurs when bonding gets ambiguous. I’ll discuss how various synchrotron x-ray techniques have clarified our understanding of correlated materials, including organometallic molecules based on Cp*2Yb(bipy), actinide intermetallics such as URu2Si2 and delta-Pu, and most recent work on Bk and Cf chelates, all while highlighting some important size effects.
Laura H. Greene - National High Magnetic Field Laboratory, Florida State University and the
Center for Emergent Superconductivity
High-Temperature Superconductivity: The Dark Energy of Condensed Matter
Center for Emergent Superconductivity
High-Temperature Superconductivity: The Dark Energy of Condensed Matter
The nearly 80-year-old correlated electron problem remains largely unsolved; with high-temperature superconductivity being one of dozens of families of unconventional superconductors that fall under that complex and fascinating umbrella. I believe we are on the brink of finding solutions, and argue this by drawing a fun analogy to LIGO.
D. N. C. Lin - Department of Astronomy and Astrophysics, University of California, Santa Cruz
Impacts of Ice on the Structure of Planetary Rings and the Formation of Planets: A Tribute to the Contributions of Physicist Frank Bridges in Astrophysical Contexts
Impacts of Ice on the Structure of Planetary Rings and the Formation of Planets: A Tribute to the Contributions of Physicist Frank Bridges in Astrophysical Contexts
Saturn's rings are primarily composed of ice particles. Collisions among them lead to energy dissipation and momentum transfer. Their experimentally-determined coefficient of restitution determines the thickness of the rings and regulate the structure of Saturn's rings. The laboratory-measured sticking probability and the cohesion strength also accounts for icy particles' size distribution. In the outer system, water-ice grains are often coated with organic molecules which can greatly enhance the binding energy between merger products and promote the growth of grains. Near the condensation fronts of various volatile ices, frost-covered surface of icy pebbles may be essential for the rapid emergence of planetesimals and the robust formation of super Earths.
M. Brian Maple - Department of Physics, University of California, San Diego
Novel Electronic Phases and Competing Interactions in the Heavy Fermion Compound URu2Si2
Novel Electronic Phases and Competing Interactions in the Heavy Fermion Compound URu2Si2
The heavy fermion compound URu2Si2 undergoes a second order transition at To = 17.5 K into an ordered phase whose identity has eluded researchers for three decades [1]. This so-called “hidden order” (HO) phase coexists with a type of unconventional superconductivity (SC) that is found below Tc ≈ 1.5 K. The features in the electrical resistivity, specific heat and magnetic susceptibility associated with the HO phase transition are reminiscent of a charge or spin density wave that forms a gap over about 40% of the Fermi surface below To, with the remainder of the Fermi surface gapped by the SC below Tc [2]. The compound URu2Si2 has been studied extensively using many experimental techniques (e.g., transport, thermal, magnetic, and spectroscopic measurements), and numerous theoretical models have been proposed to account for the HO phase [1]. Application of pressure depresses the Tc of the SC’ing phase and induces a transition from the HO phase to a large moment antiferromagnetic (LMAFM) phase at a critical pressure Pt ≈ 1.5 GPa [1, 3]. We have observed that substitution of isoelectronic Fe for Ru suppresses SC and induces a transition from the HO phase to the LMAFM phase, accompanied by a more than two-fold increase in the temperature of the HO/LMAFM phase boundary, similar to the behavior of pure URu2Si2 under pressure. It has been suggested that the HO-LMAFM phase transition in URu2-xFexSi2 is driven by “chemical pressure” Pch associated with the reduction of the unit cell volume upon substitution of smaller Fe atoms for Ru atoms [4]. Experiments on single crystals of URu2-xFexSi2 prepared in our laboratory reveal that the T-Pch phase diagram and variation of the U magnetic moment with Pch for URu2-xFexSi2 are consistent with the T-P phase diagram and evolution of the U magnetic moment with P in URu2Si2 [5]. Single crystals of URu2−xFexSi2 provide an opportunity to study the transition from the HO to the LMAFM phase at atmospheric pressure with techniques that cannot be readily performed on URu2Si2 under high pressure (e.g., ARPES, STM, neutron scattering, measurements in high magnetic fields, etc.). Such studies will yield information about the electronic structure of the HO and AFM phases in URu2Si2 and, hopefully, clues to the identity of the order parameter of the elusive HO phase. In this talk, we describe recent experiments on URu2-xFexSi2 single crystals in the HO and LMAFM phases.
Support of the US DOE, NNSA, and NSF is gratefully acknowledged.
References:
1. J. A. Mydosh and P. M. Oppeneer, Rev. Mod. Phys. 83, 1301 (2011).
2. M. B. Maple et al., Phys. Rev. Lett. 56, 185 (1986).
3. N. P. Butch, Phys. Rev. B 82, 060408 (2010).
4. N. Kanchanavatee et al., Phys. Rev. B 84, 245122 (2011).
5. S. Ran et al., Proc. Nat. Acad. Sci. 113, 13348 (2016).
Support of the US DOE, NNSA, and NSF is gratefully acknowledged.
References:
1. J. A. Mydosh and P. M. Oppeneer, Rev. Mod. Phys. 83, 1301 (2011).
2. M. B. Maple et al., Phys. Rev. Lett. 56, 185 (1986).
3. N. P. Butch, Phys. Rev. B 82, 060408 (2010).
4. N. Kanchanavatee et al., Phys. Rev. B 84, 245122 (2011).
5. S. Ran et al., Proc. Nat. Acad. Sci. 113, 13348 (2016).
John Neumeier - Physics Department, Montana State University
Measuring the Thermal Expansion of Solids
Measuring the Thermal Expansion of Solids
When sharing with others that I measure the thermal expansion of solids, their eyes tend to glaze over and they seek an exit strategy. I’ll try to convince you that this is a very “cool” experiment, first by describing the device and its construction, second by proving to you that it has revealed some entirely unexpected behavior in solids. The device is a tribute to simplicity and function, consisting of five parts carved from glass that, when assembled, can determine length changes as small as 1/10th of an atom’s diameter. Some examples of its application in the measurement of phase transitions (magnetic, superconducting, structural) and H2O ice will be shown.
ARthur ramirez- DEPARTMENT OF PHYSICS, UNIVERSITY OF California, Santa Cruz
Searching for a Quantum Spin Liquid
Searching for a Quantum Spin Liquid
The Quantum Spin Liquid (QSL) is a predicted phase of matter comprised of coherent quantum fluctuating spins. While QSL eigenstates are theoretically known, the materials recipe is less specific, beyond needing to suppress near-lying classical states. I will describe recent work which represents a new direction in materials-based searches for the QSL. Specifically, in a quasi-1D molecular copper salt, chosen for its specific values of anisotropic exchange energies, we control the interplay between spin interaction energy and Zeeman energy. We show that the main effect of applying a magnetic field is to shift entropy from the short range order region into the quantum critical point. This approach seems to present a large number of potential QSL candidate materials.
John J. Rehr - Department of Physics, University of Washington
Efficient Calculations of Vibrational Properties and Negative Thermal Expansion in Complex Materials*
Efficient Calculations of Vibrational Properties and Negative Thermal Expansion in Complex Materials*
Thermodynamic properties such as thermal vibrations and thermal expansion are essential to obtain a more complete understanding of complex materials. Formally, these properties can be obtained by minimizing the Helmholtz free energy F(T,V). However, practical calculations been computationally challenging, since they require accurate treatments of both electronic and ionic motion at finite temperature. In this talk we focus on an approach that permits efficient simulations of these properties in materials with large simulation cells [1]. This method was originally introduced to calculate crystallographic and x-ray absorption fine structure (XAFS) Debye-Waller factors from first principles, quantities which are important in the analysis and simulation of XAFS experiment [2]. The approach is based on a separation of the free energy into electronic and vibrational parts. The electronic part is obtained from density functional theory calculations of the total electronic energy and lattice constant. The vibrational free energy is calculated using Debye integrals in terms of the phonon densities of modes, which are obtained using a Lanczos recursion algorithm of the phonon-Green’s function, with a dynamical matrix for relaxed structures at each T and V. The improved efficiency of the method stems from the parallelization of the calculations of the dynamical matrix. As an illustration, we present results for negative thermal expansion in GaAs and ZrW2O8. We also briefly compare with alternative approaches, such as DFT/MD which has been applied to thermal properties in nanoscale systems [3] and thermoelectrics [4]. *Supported by DOE grant DE-FG02-03ER15476 with computer support from DOENERSC. [1] F. D. Vila and J. J. Rehr, APS March Meeting 2016, [http://meetings.aps.org/link/BAPS.2016.MAR.A22.3]. [2] F. D. Vila, J. J. Rehr, H. H. Rossner, and H. J. Krappe, Phys. Rev. B 76, 014301 (2007). [3] F. D. Vila, J. J. Rehr, J. Kas, R. G. Nuzzo, and A. I. Frenkel, Phys. Rev. B 78, 121404R (2008). [4] C. Carbogno, R. Ramprasad, and M. Scheffler, arXiv:1608.06917v2.
Mike Toney - Stanford Synchrotron Radiation Lightsource
EXAFS Applied to Solar Absorbers
EXAFS Applied to Solar Absorbers
Student Posters
Understanding Temperature and Light Degradation of Perovskite Films and Devices
Ghada Abdelmageed, Leila Jewell, Kaitlin Hellier, Lydia Seymour, Mark Tingwald, Frank Bridges, Jin Zhang, Sue A. Carter
University of California, Santa Cruz
Ghada Abdelmageed, Leila Jewell, Kaitlin Hellier, Lydia Seymour, Mark Tingwald, Frank Bridges, Jin Zhang, Sue A. Carter
University of California, Santa Cruz
Organometal halide perovskite materials such as methylammonium lead iodide (MAPbI3) have demonstrated unprecedented progress in the photovoltiacs research field, achieving a power conversion efficiency comparable with silicon in under 6 years. However, the instability of these materials in environmental elements such as light, oxygen, heat, and moisture threatens the possibility of industrialization. In our work, we utilize a variety of spectroscopy methods, including x-ray absorption fine structure (EXAFS), to study the long-term stability of MAPbI3 films and devices in different conditions and to understand possible degradation mechanisms.
Photothermal Deflection and Fourier Transform Infrared Spectroscopy of Organic PV Materials Undergoing Degradation
Kaitlin Hellier, Renee Sully, Jacob Kermish-Wells, Glenn Alers, Sue A Carter
University of California, Santa Cruz
Kaitlin Hellier, Renee Sully, Jacob Kermish-Wells, Glenn Alers, Sue A Carter
University of California, Santa Cruz
Encapsulation is a critical component for protecting photovoltaic cells from degradation; however, standard plastic encapsulation materials can degrade with exposure to the elements, leading to reduced power efficiency or destruction of the cells. In order to address this failure and extend current lifetimes, an increased understanding of degradation rates and mechanisms must be gained. Through UV-Vis absorption and FTIR, we begin work on the changes in the chemical structures of the materials and prepare to utilize the precision of photothermal deflection spectroscopy to observe extremely small changes in absorption at short time scales.
Analysis of Local Structure in Tetrahedrite using Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy
Valentin Urena Baltazar, Frank Bridges, Ph.D., and Cameron MacKeen, M.S.
Physics Department, University of California, Santa Cruz
Valentin Urena Baltazar, Frank Bridges, Ph.D., and Cameron MacKeen, M.S.
Physics Department, University of California, Santa Cruz
Tetrahedrites are a class of naturally occurring minerals with high thermoelectric efficiency. Thermoelectric materials are needed in devices aiming to convert heat to electricity directly via the Seebeck effect, and also for cooling without a refrigerant via the Peltier effect. We examine pure Cu12Sb4S13 and zinc doped Cu10Zn2Sb4S13 tetrahedrite to better understand the metal to semiconductor transition at T~90K in the pure tetrahedrite. Understanding why the transition is suppressed upon doping will shed light on the structural properties of this material. To probe the crystal structure, X-ray spectroscopy data of a sample of tetrahedrite were collected at the Stanford Synchrotron Radiation Lightsource (SSRL). The X-rays induce photoelectrons in the targeted atom whose wavefunction backscatters off neighboring atoms and interferes with itself, resulting in small oscillations in the Extended X-ray Absorption Fine Structure (EXAFS) region of the data. We use the Real Space X-ray Absorption Program (RSXAP) to analyze the EXAFS region of both the zinc and copper absorption edges. We expect to learn how the zinc atoms affect the local structure of the tetrahedrite and specifically how the displacement of copper atoms affects the thermoelectric efficiency of the material.
INSCRIBING REWRITEABLE GRAPHENE PN JUNCTIONS UNDER AMBIENT CONDITIONS
EBERTH A. QUEZADA, JOHN DAVENPORT, HECHIN CHEN, TAKASHI TANIGUCHI, KENJI WATANABE, JAIRO VELASCO JR.
PHYSICS DEPARTMENT, UNIVERSITY OF CALIFORNIA, SANTA CRUZ
EBERTH A. QUEZADA, JOHN DAVENPORT, HECHIN CHEN, TAKASHI TANIGUCHI, KENJI WATANABE, JAIRO VELASCO JR.
PHYSICS DEPARTMENT, UNIVERSITY OF CALIFORNIA, SANTA CRUZ
Heterostructures of graphene and hexagonal boron nitride (BN) are highly tunable platforms that enable the study of novel physical phenomena and technologically promising nanoelectronic devices. Recently, for such graphene/BN heterostructures, it has been shown that electric field excitation can be used to control charge-defect ensembles in the underlying BN. This enables nanoscale control of rewriteable graphene pn junctions. Notably, the fabrication of these pn junctions requires highly specialized conditions, such as ultra-high vacuum and cryogenic temperatures, thus limiting further exploration of these pn junctions. To address this issue, we have developed a new technique that uses an ambient atomic force microscope to inscribe rewriteable graphene pn junctions. We will discuss our latest experimental progress on the development of this technique.
Growth, Structural and Dielectric Study of NaMnF3 Thin Films on SrTiO3
Amit KC1,2, Pavel Borisov2, Vladimir V. Shvartsman3 and David Lederman1,2
1University of California, Santa Cruz, CA 95064, USA; 2West Virginia University, Morgantown, WV 26506-6315, USA; 3University of Duisburg-Essen, Universitätsstraße 15, 45141 Essen, Germany
Amit KC1,2, Pavel Borisov2, Vladimir V. Shvartsman3 and David Lederman1,2
1University of California, Santa Cruz, CA 95064, USA; 2West Virginia University, Morgantown, WV 26506-6315, USA; 3University of Duisburg-Essen, Universitätsstraße 15, 45141 Essen, Germany
Perovskite fluorides (ABF3) exhibit many interesting phenomena, e.g. dipolar and magnetic long-range order, superconductivity, as well as magnetoelectric coupling. Recently, the orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if a = c distortion of the bulk Pnma structure is imposed. Thus, in combination with the weak ferromagnetic order, this material is expected to be multiferroic. Here, we report the growth of epitaxial NaMnF3 thin films on SrTiO3 (001) single crystal substrates via Molecular Beam Epitaxy (MBE). The best films were smooth and single phase with four twin domains. In-plane magnetization measurement of the sample revealed an antiferromagnetic ordering, with a Neel temperature (TN) of 66K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics.
Calculating Boosted Higgs Boson Production as a Function of Couplings to Top Quarks, Bottom Quarks, and Vector Bosons
Jesus J. navarro and Jason Nielsen
Physics Department, University of California, Santa Cruz
Jesus J. navarro and Jason Nielsen
Physics Department, University of California, Santa Cruz
In 2012 the Higgs boson was discovered at the Large Hadron Collider (LHC) at CERN. Measurements of the Higgs boson’s mass and couplings to other particles are now a high priority for particle physicists. In particular, deviations from the expected couplings would appear as changes to the Higgs cross-section. It is therefore important to analyze the cross sections in terms of the coupling parameters. In this study, Higgs production rates in 13 TeV proton-proton collisions were calculated with a Monte Carlo-based program and event generator named VBFNLO. The dependence on the Higgs couplings to top quarks, bottom quarks, and vector bosons was parameterized with a polynomial fit. Because boosted Higgs production is expected to result in a cleaner signature for precision measurements, the fits were performed in both inclusive and boosted samples. Overall, these fits manifest the clear relations between the Higgs cross section and the couplings of the fundamental particles. Precision measurements that use this technique will require larger data sets. Larger samples can be obtained by colliding more intense beams in the LHC. The proposed HL-LHC (High Luminosity Large Hadron Collider) will feature 5-7 times more intense proton beams than the current LHC. The production of additional particles will require an upgraded pixel detector system containing smaller pixels. To read out the increased data set will require higher frequency of data transmission rates, up to 5Gbps. The wires that will be utilized to transmit this data must fulfill several criteria in order to be utilized in the pixel detector system; these include: high radiation tolerance and low mass. Thus tests were made on a hybrid set of data transmission wires at low voltage. Bit Error Rates(BER) were measured based on the length of Pseudo-Random- Bit-Sequence (PSRB) being used and the type of signal conditioning being applied on the wires.