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APS Science 2020 (Vol. 1)
The U.S. Department of Energy's Advanced Photon Source (APS) is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. The APS occupies an 80-acre site on the Argonne campus, about 25 miles from downtown Chicago, Illinois. It shares a site with the Center for Nanoscale Materials and the Advanced Protein Characterization Facility.
Alternative approach to quantum imaginary time evolution
There is increasing interest in quantum algorithms (QAs) that are based on the imaginary time evolution (ITE), a successful classical numerical approach to obtain ground states. However, most of the proposals so far require heavy postprocessing computational steps on a classical computer, such as solving linear equations. Here we provide an alternative approach to implement ITE. A key feature in our approach is the use of an orthogonal basis set: the propagated state is efficiently expressed in terms of orthogonal basis states at every step of the evolution. We argue that the number of basis states needed at those steps to achieve an accurate solution can be kept on the order of n , the number of qubits, by controlling the precision (number of significant digits) and the imaginary time increment. The number of quantum gates per imaginary time step is estimated to be polynomial in n . Additionally, while in many QAs the locality of the Hamiltonian is a key assumption, in our algorithm this restriction is not required. This characteristic of our algorithm renders it useful for studying highly nonlocal systems, such as the occupation-representation nuclear shell model. Here, we illustrate our algorithm through numerical implementation on an IBM quantum simulator.
New Laboratory and Astrophysical Probes of Physics Beyond the Standard Model (Final Technical Report)
The first of these directions centers on new ideas in dark matter detection. As experimental scrutiny narrows the window for new physics at the weak scale, it has become apparent that the solution to the dark matter mystery may not reside there. Theoretical developments such as the hidden sector/valley paradigm have, at the same time, shown that compelling theories of dark matter reside below the weak scale. Searching for dark matter at lower mass scales requires looking beyond the current paradigm of dark matter direct detection based on nuclear recoils. It is the goal of this project to provide new ideas to guide the experimental program looking for light dark matter. I am actively proposing ideas for detecting dark matter as light as a meV, well beyond the current focus of GeV-TeV scale dark matter. Some of the ideas I have proposed, such as superconducting, superfluid, and polar material targets, are actively being developed into experiments. My group will provide the relevant calculations to determine dark matter reach, providing crucial input to experiment on which targets should be developed. We will also provide an effective field theory framework to understand which types of experiments are most sensitive to each interaction type. The second of these directions is on probes of dark matter substructure as a mean to constrain, or observe, models of dark matter. The standard ΛCDM paradigm assumes that the dark matter density perturbations are adiabatic, scale invariant, and produced during inflation. However, many standard models of dark matter will produce modifications of this prediction, such as axion models with symmetry broken below the inflationary scale. Remarkably, we have limited direct measurements of the dark matter clumpiness at mass scales below dwarf galaxies. This allows for the possibility of observing modifications from vanilla ΛCDM due to particle dynamics. In many cases, simulations of dark matter structure in the presence of non-scale invariant fluctu- ations are understudied or completely lacking. I plan to develop both theoretical predictions for small scale structure in models with additional matter power on small scales, as well as observa- tional probes of small scale halos or clumps. One idea I have been recently focused on is pulsar timing, though my group will pursue a variety of lensing and astrometric probes. Lastly, the most risky direction involves spacetime fluctuations from quantum gravity. I showed that if one assumes that metric fluctuations in the Minkowski vacuum, at a surface separating a region in and out of causal contact (which we call a “horizon”), are determined by standard thermodynamic considerations at horizon, these metric fluctuations are large enough to observe in an interferometer with similar sensitivity to LIGO. In a follow-up paper we showed that these assumption holds for the vacuum in AdS/CFT. In future work, I plan to connect this work to recent soft graviton results, to frame this work in a concrete Randall-Sundrum model, and to work out concrete phenomenological predictions from the model. This work unquestionably takes a less trodden path, and works well as part of a well-rounded portfolio of less and more risky ideas. Particle physics is currently at a juncture which requires bold exploration of qualitatively new directions. This proposal outlines some of my plans over the coming years in these directions, leaving room also for surprises.
Novel Phases and Dynamics of Multi-orbital Mott Insulators
The goal of this project is to gain theoretical insight into magnetism in the presence of spin-orbit coupling (SOC) for multi-orbital Mott insulators. Inspired by the exact solution of the Kitaev model that harbors a quantum spin liquid with novel excitations and struck by its relevance for materials with anisotropic orbital interactions, the PI will explore various new classes of 4d and 5d transition metal oxides. Starting with all electron Hamiltonians, the PI will derive minimal magnetic models. The aim will be to understand the role played by orbital frustration, even in the absence of any geometric frustration, in creating orbitally ordered as well as spin-orbital liquid phases. Orbital frustration arises primarily from the directional or anisotropic nature of d-orbitals in contrast to the isotropic nature of the spin degree of freedom. These models will be investigated by a variety of theoretical and numerical methods, including exact diagonalization, mean field theories, density matrix renormalization group and quantum Monte Carlo methods. Testable predictions for three experiments: nuclear magnetic resonance (NMR) and resonant x-ray scattering (RXS) to probe orbital ordering and pump-probe experiments to probe quantum dynamics will test the validity of these models for materials.
Dissipative superfluid hydrodynamics for the unitary Fermi gas
In this work we establish constraints on the temperature dependence of the shear viscosity η in the superfluid phase of a dilute Fermi gas in the unitary limit. Our results are based on analyzing experiments that measure the aspect ratio of a deformed cloud after release from an optical trap. Here, we discuss how to apply the two-fluid formalism to the unitary gas and provide a suitable parametrization of the equation of state. We show that in expansion experiments the difference between the normal and superfluid velocities remains small and can be treated as a perturbation. We find that expansion experiments favor a shear viscosity that decreases significantly in the superfluid regime. Using an exponential parametrization, we find η(T c /2T F ) ≲ 0.37[η(T c /T F )], where T c is the critical temperature and T F is the local Fermi temperature of the gas.
Analysis of the multicomponent relativistic Boltzmann equation for electron scattering in big bang nucleosynthesis
Big-bang nucleosynthesis (BBN) is valuable as a means to constrain the physics of the early universe and it is the only probe of the radiation-dominated epoch. A fundamental assumption in BBN is that the nuclear velocity distributions obey Maxwell-Boltzmann (MB) statistics as they do in stars. Specifically, the BBN epoch is characterized by a dilute baryon plasma for which the velocity distribution of nuclei is mainly determined by the dominant Coulomb elastic scattering with mildly relativistic electrons. One must therefore deduce the momentum distribution for reacting nuclei from the multicomponent relativistic Boltzmann equation. However, the full multicomponent relativistic Boltzmann equation has only recently been analyzed and its solution has only been worked out in special cases. Moreover, a variety of schemes have been proposed that introduce nonthermal components into the BBN environment which can alter the thermal distribution of reacting nuclei. Here, we construct the relativistic Boltzmann equation in the context of BBN. We also derive a Langevin model and perform relativistic Monte-Carlo simulations which clarify the baryon distribution during BBN and can be used to analyze any relaxation from a nonthermal injection. Further, we show by these analyses that the thermalization process leads to a nuclear distribution function that remains very close to MB statistics even during the most relativistic environment relevant to BBN. Hence, the predictions of standard BBN remain unchanged.
APS Science 2020 (Vol. 2)
The U.S. Department of Energy's Advanced Photn Source (APS) is one of the world’s most productive x-ray light source facilities. Each year, the APS provides high-brightness x-ray beams to a diverse community of more than 5,000 researchers in materials science, chemistry, condensed matter physics, the life and environmental sciences, and applied research. Researchers using the APS produce over 2,000 publications each year detailing impactful discoveries, and solve more vital biological protein structures than users of any other x-ray light source research facility. APS x-rays are ideally suited for explorations of materials and biological structures; elemental distribution; chemical, magnetic, electronic states; and a wide range of technologically important engineering systems from batteries to fuel injector sprays, all of which are the foundations of our nation’s economic, technological, and physical well-being. The APS occupies an 80-acre site on the Argonne campus, about 25 miles from downtown Chicago, Illinois. It shares a site with the Center for Nanoscale Materials and the Advanced Protein Characterization Facility.
GaAs Neutron Response Functions and Radiation Damage Metrics
The radiation effects community needs clear, well-documented, neutron energy-dependent responses that can be used in assessing radiation-induced material damage to GaAs semiconductors and for correlating observed radiation-induced changes in the GaAs electronic properties with computed damage metrics. In support of the objective, this document provides: a) a clearly defined set of relevant neutron response functions for use in dosimetry applications; b) clear mathematical expressions for the defined response functions; and c) updated quantitative values for the energy- dependent response functions that reflect the best current nuclear data and modelling. This document recaps the legacy response functions. It then surveys the latest nuclear data and updates the recommended response function to support current GaAs damage studies. A detailed tabulation for six of the energy-dependent response functions is provided in an Appendix.
High-current electron sources for Superconducting Radiofrequency injectors
Accelerators are finding increasing use in numerous fields such as medicine, industry accelerator-based nuclear energy programs and others. A recent survey reveals only 20 high-energy accelerators worldwide serve elementary-particle physics while thousands of accelerators are routinely used in hospitals and industry (packaging, food sterilization, etc.).
Electron spin coherence on a solid neon surface
We report a single electron floating on the surface of a condensed noble-gas liquid or solid can act as a spin qubit with ultralong coherence time, thanks to the extraordinary purity of such systems. Previous studies suggest that the electron spin coherence time on a superfluid helium (He) surface can exceed 100 s. In this paper, we present theoretical studies of the electron spin coherence on a solid neon (Ne) surface, motivated by our recent experimental realization of single-electron charge qubit on solid Ne. The major spin decoherence mechanisms investigated include the fluctuating Ne diamagnetic susceptibility due to thermal phonons, the fluctuating thermal current in normal metal electrodes, and the quasi-statically fluctuating nuclear spins of the 21 Ne ensemble. We find that at a typical experimental temperature about 10 mK in a fully superconducting device, the electron spin decoherence is dominated by the third mechanism via electron–nuclear spin–spin interaction. For natural Ne with 2700 ppm abundance of 21 Ne, the estimated inhomogeneous dephasing time ${T}_{2}^{*}$ is around 0.16 ms, already better than most semiconductor quantum-dot spin qubits. For commercially available, isotopically purified Ne with 1 ppm of 21 Ne, ${T}_{2}^{*}$ can be 0.43 s. Under the application of Hahn echoes, the coherence time T 2 can be improved to 30 ms for natural Ne and 81 s for purified Ne. Therefore, the single-electron spin qubits on solid Ne can serve as promising new spin qubits.
Spin Waves in Dirac Semimetal Ca 0.6 Sr 0.4 MnSb 2 Investigated with Neutrons by the Diffraction Method
The tunability of Dirac semimetals with antiferromagnetic Mn layers is of great interest. The observed sign change of interlayer magnetic coupling between CaMnBi 2 and SrMnBi 2 suggests ionic substitution as a potential tuning mechanism. If so, novel behavior near the compensation point could be expected. Here, to explore this, we study a mixed-cation analog, Ca 0.6 Sr 0.4 MnSb 2 , where Bi is replaced by Sb. Conventional inelastic neutron scattering is impractical due to the small crystal size (m ≈ 0.28 g) available for compositional studies; however, we find that using a neutron diffractometer with a wide-angle area detector we can obtain a good quality spin-wave signal, which is shaped by energy-momentum conservation and retains spectroscopic information even without direct energy analysis. Spin-wave modeling reveals an interlayer coupling quantitatively similar to SrMnSb 2 , indicating it is not directly tuned by ionic size and that the sign change in Bi-based compounds likely arises from the observed change in lattice symmetry. Beyond this key insight, our results present an efficient method for parametric and compositional studies of spin dynamics in small crystals.
Estimates of Quantum Tunneling Effects for Hydrogen Diffusion in PuO 2
We detail the estimation of activation energies and quantum nuclear vibrational tunneling effects for hydrogen diffusion in PuO 2 based on Density Functional Theory calculations and a quantum double well approximation. We find that results are relatively insensitive to choice of exchange correlation functional. In addition, the representation of spin in the system and use of an extended Hubbard U correction has only a small effect on hydrogen point defect formation energies when the PuO 2 lattice is held fixed at the experimental density. We then compute approximate activation energies for transitions between hydrogen interstitial sites seeded by a semi-empirical quantum model and determine the quantum tunneling enhancement relative to classical kinetic rates. Our model indicates that diffusion rates in H/PuO 2 systems could be enhanced by more than one order of magnitude at ambient conditions and that these effects persist at high temperature. The method we propose here can be used as a fast screening tool for assessing possible quantum nuclear vibrational effects in any number of condensed phase materials and surfaces, where hydrogen hopping tends to follow well defined minimum energy pathways.
Crystal structure and magnetism of actinide oxides: a review
In actinide systems, the 5f electrons experience a uniquely delicate balance of effects and interactions having similar energy scales, which are often difficult to properly disentangle. This interplay of factors such as the dual nature of 5f-states, strong electronic correlations, and strong spin–orbit coupling results in electronically unusual and intriguing behavior such as multi-k antiferromagnetic ordering, multipolar ordering, Mott-physics, mixed valence configurations, and more. Despite the inherent allure of their exotic properties, the exploratory science of even the more basic, binary systems like the actinide oxides has been limited due to their toxicity, radioactivity, and reactivity. In this article, we provide an overview of the available synthesis techniques for selected binary actinide oxides, including the actinide dioxides, sesquioxides, and a selection of higher oxides. For these oxides, here we also review and evaluate the current state of knowledge of their crystal structures and magnetic properties. In many aspects, substantial knowledge gaps exist in the current body of research on actinide oxides related to understanding their electronic ground states. Bridging these gaps is vital for improving not only a fundamental understanding of these systems but also of future nuclear technologies. To this end, we note the experimental techniques and necessary future investigations which may aid in better elucidating the nature of these fascinating systems.
Lattice and magnetic structure in the van der Waals antiferromagnet VBr 3
Here, we report a comprehensive investigation of the lattice and magnetic structure in van der Waals antiferromagnet VBr 3 , characterized by a BiI 3 -type structure at room temperature. Neutron diffraction experiments were performed on both polycrystalline and single-crystalline VBr 3 samples, revealing clear magnetic Bragg peaks emerging below the Néel temperature of T N =26.5 K. These magnetic Bragg peaks can be indexed by k=(0, 0.5, 1) in hexagonal notation. Our refinement analysis suggests that the antiferromagnetic order in VBr 3 manifests as a zigzag structure. Moreover, we observed peak splitting for nuclear Bragg peaks in the HK plane below the structure transition temperature of T S =90.4 K, indicating the breaking of threefold symmetry within the ab plane.
Broadband Fourier-Transform-Detected EPR at W-Band
This manuscript reports development of wideband Fourier transform (FT) electron paramagnetic resonance (EPR) detection capability at the uniquely high frequency of 94 GHz. Here, this technique is based on the HiPER spectrometer developed at the University of St. Andrews, into which we have integrated an arbitrary waveform generator (AWG) that is used to modulate the output from a solid-state multiplier chain prior to amplification, generating up to 1 kW microwave power with 1 GHz bandwidth. Benchmark experiments are presented for a standard TEMPOL radical, which comprises a 500 MHz broad EPR spectrum at 94 GHz. Using a single frequency-swept microwave pulse, efficient inversion of this spectrum is demonstrated, enabling frequency-dependent studies of the magnetization relaxation in the time-domain, again via broadband echo detection. From these measurements, an anisotropy in the spin–lattice relaxation time, T 1 , can be determined with ease for TEMPOL. In addition, we implement the FT detection scheme for multi-dimensional (electron–electron double resonance, or ELDOR) experiments, demonstrating the full capabilities of the HiPER spectrometer. As an example, we present FT-detected version of the ELDOR-detected nuclear magnetic resonance technique for the TEMPOL radical. We also describe potential applications of the new instrumentation for studies of gadolinium qubits.
Letter of Intent: Muonium R&D/Physics Program at the MTA
With the planned turn-on of the PIP-II 800 MeV superconducting proton linac, Fermilab will potentially become the world's best laboratory at which to carry out fundamental muon measurements, sensitive searches for symmetry violation, and precision tests of theory. In preparation, we propose to develop the techniques that will be needed. An R&D and physics program is proposed at the Fermilab MeV Test Area to use the existing 400 MeV Linac to demonstrate the efficient production of a slow muonium beam using $\mu^+$ stopped in a ~100 $\mu$m-thick layer of superfluid helium, and to use that beam to measure muonium gravity.
Coherent Control of a Nuclear Spin via Interactions with a Rare-Earth Ion in the Solid State
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