Physics Commons^™

Relativistic Ultrafast Electron Diffraction At High Repetition Rates, K. M. Siddiqui, D. B. Durham, F. Cropp, F. Ji, S. Paiagua, C. Ophus, N. C. Andresen, L. Jin, J. Wu, S. Wang, X. Zhang, W. You, M. Murnane, Martin Centurion, X. Wang, D. S. Slaughter, R. A. Kaindl, P. Musumeci, A. M. Minor, D. Filippetto

Department of Physics and Astronomy: Faculty Publications

The ability to resolve the dynamics of matter on its native temporal and spatial scales constitutes a key challenge and convergent theme across chemistry, biology, and materials science. The last couple of decades have witnessed ultrafast electron diffraction (UED) emerge as one of the forefront techniques with the sensitivity to resolve atomic motions. Increasingly sophisticated UED instruments are being developed that are aimed at increasing the beam brightness in order to observe structural signatures, but so far they have been limited to low average current beams. Here, we present the technical design and capabilities of the HiRES (High Repetition-rate Electron …

Characterization Of Interlayer Laser Shock Peening During Fused Filament Fabrication Of Polylactic Acid (Pla), Fabien Denise

Department of Engineering Mechanics: Dissertations, Theses, and Student Research

The field of additive manufacturing (AM) has gained a significant amount of popularity due to the increasing need for more sustainable manufacturing techniques and the adaptive development of complex product geometries. The problem is that AM parts routinely exhibit flaws or weaknesses that affect functionality or performance. Over the years, surface treatments have been developed to compensate certain flaws or weaknesses in manufactured products. Combining surface treatments with the modularity of additive manufacturing could lead to more adaptable and creative improvements of product functions in the future. The current work evaluates the feasibility of pursuing a new research axis in …

Uv Dynamics Of Different Ring Molecules Studied By Ultrafast Electron Diffraction, Sajib Kumar Saha

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2024–

Understanding natural light-induced phenomena requires a direct viewing of atomic motion during structural evolution, which, in turn, facilitates controlling and manipulating these light-induced processes. Ultrafast Electron Diffraction (UED) is a structure-sensitive technique that can probe electronic and nuclear dynamics at sub-angstrom spatial and femtosecond time scales. UED has become a vital tool for studying photo-induced molecular dynamics and underlying science. Organic ring systems are prevalent in biology, materials, and pharmaceuticals. Their synthesis and transformation are fundamental in synthetic chemistry, influencing various fields. We used UV photons to photo-excite different cyclic molecules and investigated their photo-dynamics using gas phase UED. The …

Probing The Electroweak Phase Transition With Exotic Higgs Decays, Marcela Carena, Jonathan Kozaczuk, Zhen Liu, Tong Ou, Michael J. Ramsey-Musolf, Jessie Shelton, Yikun Wang, Ke-Pan Xie

Department of Physics and Astronomy: Faculty Publications

An essential goal of the Higgs physics program at the LHC and beyond is to explore the nature of the Higgs potential and shed light on the mechanism of electroweak symmetry breaking. An important class of models alter the thermal history of electroweak symmetry breaking from the predictions of the Standard Model (SM). This paper reviews the existence of a region of parameter space where a strong first-order electroweak phase transition is compatible with exotic decays of the SM-like Higgs boson. A dedicated search for exotic Higgs decays can actively explore this framework at the Large Hadron Collider (LHC), while …

Hydroxyapatite-Based Coatings On Silicon Wafers And Printed Zirconia, Antoine Chauvin

Department of Mechanical and Materials Engineering: Dissertations, Theses, and Student Research

Dental surgery needs a naturally attract implant design that can ensure both osseointegration and soft tissue integration. Hydroxyapatite (HAp), the main mineral constituent of dentine and tooth enamel, is commonly used as a coating component, notably for overlaying titanium– or ceramics–based implants. This thesis aims to investigate the behavior of a HAp-based coating, specifically designed to be compatible with a porous substrate. Coating layers are made by sol–gel dip coating by immersion of porous substrates made by additive manufacturing into solutions of HAp, having been mixed with polyethyleneimine (PEI), to improve the adhesion of HAp on the substrate. First, the …

A Simple, Isotropic Depolarized Light Source, Keith Foreman, T. J. Gay

Department of Physics and Astronomy: Faculty Publications

Unpolarized light can be an important tool in optical experiments. Producing it, however, can prove to be a challenge. Natural sources of light that are commonly thought of as unpolarized are, in fact, either weakly polarized or not practical sources of light in a laboratory setting. Standard, commercially available light depolarizers produce unpolarized light only after the polarization state of the light across the diameter of the output beam has been averaged. Locally, such beams are highly polarized. In this work, we report a simple, low cost light depolarizer capable of producing light with a total polarization of less than …

Breakdown Of The Drift-Diffusion Model For Transverse Spin Transport In A Disordered Pt Film, Kirill D. Belashchenko, Giovanni G. Baez Flores, Wuzhang Fang, Alexey Kovalev, Mark Van Schilfgaarde, Paul M. Haney, Mark D. Stiles

Department of Physics and Astronomy: Faculty Publications

Spin-accumulation and spin-current profiles are calculated for a disordered Pt film subjected to an in-plane electric current within the nonequilibrium Green's function approach. In the bulklike region of the sample, this approach captures the intrinsic spin Hall effect found in other calculations. Near the surfaces, the results reveal qualitative differences with the results of the widely used spin-diffusion model, even when the boundary conditions are modified to try to account for them. One difference is that the effective spin-diffusion length for transverse spin transport is significantly different from its longitudinal counterpart and is instead similar to the mean-free path. This …

Majorana Bound States In A D-Wave Superconductor Planar Josephson Junction, Hamed Vakili, Moaz Ali, Mohamed Elekhtiar, Alexey Kovalev

Department of Physics and Astronomy: Faculty Publications

We study phase-controlled planar Josephson junctions comprising a two-dimensional electron gas with strong spin-orbit coupling and d-wave superconductors, which have an advantage of a high critical temperature. We show that a region between the two superconductors can be tuned into a topological state by the in-plane Zeeman field, and can host Majorana bound states. The phase diagram as a function of the Zeeman field, chemical potential, and the phase difference between superconductors exhibits the appearance of Majorana bound states for a wide range of parameters. We further investigate the behavior of the topological gap and its dependence on the …

Observation Of Τ Lepton Pair Production In Ultraperipheral Pb-Pb Collisions At √SNn=5.02 Tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

We present an observation of photon-photon production of τ lepton pairs in ultraperipheral lead-lead collisions. The measurement is based on a data sample with an integrated luminosity of 404 μb⁻¹ collected by the CMS experiment at a center-of-mass energy per nucleon pair of √s_NN = 5.02 TeV. The γγ → τ⁺τ⁻ process is observed for τ⁺τ⁻ events with a muon and three charged hadrons in the final state. The measured fiducial cross section is σ(γγ → τ⁺τ^-) = 4.8 ± 0.6(stat) ± 0.5(syst) μb, where the second …

Scaling Isolated-Attosecond-Pulse Duration By Controlling A Trajectory Parameter, Dian Peng, Jean Marcel Ngoko Djiokap

Department of Physics and Astronomy: Faculty Publications

Reaching an ever shorter duration of isolated attosecond pulses (IAPs) is an ongoing mission in attosecond science. Wide usage of long-wavelength driving lasers has greatly broadened bandwidths for generating IAPs. We propose a strategy to further exploit the ability of a long-wavelength laser in producing short IAPs. We introduce a scaling relationship between IAP duration and a trajectory parameter that is associated with the classical cutoff trajectory of high-order harmonic generation (HHG) in gases. This trajectory parameter can facilitate in shortening IAP duration by shaping laser waveform. We demonstrate the effectiveness of our methods with calculations of HHG from Ne …

Electrode Effect On Ferroelectricity In Free-Standing Membranes Of Pbzr0.2Ti0.8O3, Qiuchen Wu, Kun Wang, Alyssa Simpson, Yifei Hao, Jia Wang, Dawei Li, Xia Hong

Department of Physics and Astronomy: Faculty Publications

We report the effects of screening capacity, surface roughness, and interfacial epitaxy of the bottom electrodes on the polarization switching, domain wall (DW) roughness, and ferroelectric Curie temperature (T_C) of PbZr_0.2Ti_0.8O₃ (PZT)- based free-standing membranes. Singe crystalline 10−50 nm (001) PZT and PZT/La_0.67Sr_0.33MnO₃ (LSMO) membranes are prepared on Au, correlated oxide LSMO, and two-dimensional (2D) semiconductor MoS₂ base layers. Switching the polarization of PZT yields nonvolatile current modulation in the MoS₂ channel at room temperature, with an on/off ratio of up to 2 × 10^{5 …}

Angular Dependence Of The Magnetization Relaxation In Co/Pt Multilayers, Anil Adhikari, Bryce Herrington, Nhat Nguyen, Ruthi Zielinski, Ather Mahmood, S. Adenwalla, Robert Streubel

Department of Physics and Astronomy: Faculty Publications

We study the influence of defects in Co/Pt multilayers on the room-temperature magnetization reversal and relaxation mechanisms via angle-dependent magnetic viscosity and coercive field measurements. The data reveal a transition from pinning-dominated domain wall propagation to a sequence of pinning-dominated and uniform switching, with increasing tilt away from the normal direction. The leading role of the dendritic domain wall propagation in the nanogranular exchange-coupled films is corroborated by the scaling of relaxation times, the angular dependence of the coercive field, and Kerr microscopy.

Measurement Of The Dependence Of The Hadron Production Fraction Ratios F S/F U And F D/F U On B Meson Kinematic Variables In Proton-Proton Collisions At √S=13 Tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

The dependence of the ratio between the B⁰_s and B⁺ hadron production fractions, f_s/f_u, on the transverse momentum (p_T) and rapidity of the B mesons is studied using the decay channels B⁰_s → J/ψϕ and B⁺ → J/ψK⁺. The analysis uses a data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS experiment in 2018 and corresponding to an integrated luminosity of 61.6 fb⁻¹. The f_s/f_u ratio is observed to depend on the B …

Mass-Selected Ion–Molecule Cluster Beam Apparatus For Ultrafast Photofragmentation Studies, Xiaojun Wang, Mahmudul Hasan, Lin Fan, Yibo Wang, Hui Li, Daniel S. Slaughter, Martin Centurion

Department of Physics and Astronomy: Faculty Publications

We describe an apparatus for investigating the excited-state dissociation dynamics of mass-selected ion–molecule clusters by mass-resolving and detecting photofragment-ions and neutrals, in coincidence, using an ultrafast laser operating at high repetition rates. The apparatus comprises a source that generates ion–molecule clusters, a time-of-flight spectrometer, and a mass filter that selects the desired anions, and a linear-plus-quadratic reflectron mass spectrometer that discriminates the fragment anions after the femtosecond laser excites the clusters. The fragment neutrals and anions are then captured by two channeltron detectors. The apparatus performance is tested by measuring the photofragments: I⁻, CF₃I⁻, …

Measurement Of The Top Quark Mass Using A Profile Likelihood Approach With The Lepton + Jets Final States In Proton–Proton Collisions At √ S = 13tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

The mass of the top quark is measured in 36.3 fb⁻¹ of LHC proton–proton collision data collected with the CMS detector at √ s = 13 TeV. The measurement uses a sample of top quark pair candidate events containing one isolated electron or muon and at least four jets in the final state. For each event, the mass is reconstructed from a kinematic fit of the decay products to a top quark pair hypothesis. A profile likelihood method is applied using up to four observables per event to extract the top quark mass. The top quark mass is measured …

Measurement Of The Top Quark Mass Using A Profile Likelihood Approach With The Lepton + Jets Final States In Proton–Proton Collisions At √S = 13tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

The mass of the top quark is measured in 36.3 fb⁻¹ of LHC proton–proton collision data collected with the CMS detector at √ s = 13 TeV. The measurement uses a sample of top quark pair candidate events containing one isolated electron or muon and at least four jets in the final state. For each event, the mass is reconstructed from a kinematic fit of the decay products to a top quark pair hypothesis. A profile likelihood method is applied using up to four observables per event to extract the top quark mass. The top quark mass is measured …

Large Anomalous Hall, Nernst Effect And Topological Phases In The 3d-4d/5d Based Oxide Double Perovskites, Kartik Samanta, Jonathan Noky, Iñigo Robredo, Juergen Kuebler, Maia G. Vergniory, Claudia Felser

Department of Physics and Astronomy: Faculty Publications

Magnetism and spin-orbit coupling are two fundamental and interconnected properties of oxide materials, that can give rise to various topological transport phenomena, including anomalous Hall and anomalous Nernst effects. These transport responses can be significantly enhanced by designing an electronic structure with a large Berry curvature. In this context, rocksalt-ordered double perovskites (DP), denoted as A₂BB’O₆, with two distinct transition metal sites are very powerful platforms for exploration and research. In this work, we present a comprehensive study based on the intrinsic anomalous transport in cubic and tetragonal stable DP compounds with 3d-4d/5d elements. Our findings …

Near-Threshold Collisional Dynamics In The E−E+P System, H. B. Ambalampitiya, J. Stallbaumer, Ilya I. Fabrikant, I. Kalinkin, D. V. Fursa, A. S. Kadyrov, I. Bray

Department of Physics and Astronomy: Faculty Publications

We study e⁺-H(n) and Ps(n)-p collisions near the three-body breakup threshold and thresholds for the charge-transfer processes.We show that classical trajectoryMonte Carlo (CTMC) simulations for the three-body breakup agree reasonably well in this energy region with quantum-mechanical convergent close-coupling (CCC) calculations even if the initial hydrogen atom or positronium atom is in the ground state. The threshold behavior of the three-body breakup cross section in e⁺-H(1s) and Ps(1s)-p collisions agrees with the Wannier law with Klar’s exponent and obeys the classical scaling laws, although some deviation …

Reconstruction Of Decays To Merged Photons Using End-To-End Deep Learning With Domain Continuation In The Cms Detector, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

A novel technique based on machine learning is introduced to reconstruct the decays of highly Lorentz-boosted particles. Using an end-to-end deep learning strategy, the technique bypasses existing rule-based particle reconstruction methods typically used in high energy physics analyses. It uses minimally processed detector data as input and directly outputs particle properties of interest. The new technique is demonstrated for the reconstruction of the invariant mass of particles decaying in the CMS detector. The decay of a hypothetical scalar particle A into two photons, A → γγ, is chosen as a benchmark decay. Lorentz boosts γ_L = 60–600 are …

Search For Exotic Higgs Boson Decays H → Aa → 4Γ With Events Containing Two Merged Diphotons In Proton-Proton Collisions At √S=13 Tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

We present the first direct search for exotic Higgs boson decays H → AA, A → γγ in events with two photonlike objects. The hypothetical particle A is a low-mass spin-0 particle decaying promptly to a merged diphoton reconstructed as a single photonlike object. We analyze the data collected by the CMS experiment at √s = 13 TeV corresponding to an integrated luminosity of 136 fb⁻¹. No excess above the estimated background is found. We set upper limits on the branching fraction β(H → AA → 4γ) of (0.9–3.3) × 10⁻³ at 95% confidence level …

Ferromagnetic Resonators Synthesized By Metal-Organic Decomposition Epitaxy, Nhat Nguyen, Bryce Herrington, Kayetan Chorazewicz, Szu-Fan (Paul) Wang, Ruthi Zielinski, John Turner, Paul D. Ashby, Ufuk Kilic, Eva Schubert, Mathias Schubert, Ronald A. Parrott, Allen A. Sweet, Robert Streubel

Department of Physics and Astronomy: Faculty Publications

Metal-organic decomposition epitaxy is an economical wet-chemical approach suitable to synthesize high-quality low-spin-damping films for resonator and oscillator applications. This work reports the temperature dependence of ferromagnetic resonances and associated structural and magnetic quantities of yttrium iron garnet nanofilms that coincide with single-crystal values. Despite imperfections originating from wet-chemical deposition and spin coating, the quality factor for out-of-plane and in-plane resonances approaches 600 and 1000, respectively, at room temperature and 40 GHz. These values increase with temperature and are 100 times larger than those offered by commercial devices based on complementary metal-oxide semiconductor voltage-controlled oscillators at comparable production costs.

Large And Tunable Magnetoresistance In Van Der Waals Ferromagnet/Semiconductor Junctions, Wenkai Zhu, Yingmei Zhu, Tong Zhou, Xianpeng Zhang, Hailong Lin, Qirui Cui, Faguang Yan, Ziao Wang, Yongcheng Deng, Hongxin Yang, Lixia Zhao, Igor Žutić, Kirill D. Belashchenko, Kaiyou Wang

Department of Physics and Astronomy: Faculty Publications

Magnetic tunnel junctions (MTJs) with conventional bulk ferromagnets separated by a nonmagnetic insulating layer are key building blocks in spintronics for magnetic sensors and memory. A radically different approach of using atomically-thin van der Waals (vdW) materials in MTJs is expected to boost their figure of merit, the tunneling magnetoresistance (TMR), while relaxing the lattice-matching requirements from the epitaxial growth and supporting high-quality integration of dissimilar materials with atomically-sharp interfaces. We report TMR up to 192% at 10 K in all-vdW Fe₃GeTe₂/GaSe/Fe₃GeTe₂ MTJs. Remarkably, instead of the usual insulating spacer, this large TMR …

Ferroelectric Hafnia Surface In Action, Xia Hong

Nebraska Center for Materials and Nanoscience: Faculty Publications

Piezoresponse microscopy and spectroscopy reveal the inextricable role of surface electrochemistry in stabilizing and controlling ferroelectricity in doped hafnia.

Doped hafnia (HfO₂), a relatively new member of the ferroelectric family, has challenged in many ways our conventional perception of ferroelectric oxides. It possesses extremely localized electric dipoles that are independently switchable,¹ making it immune to finite size effects — the loss of long-range dipole order in ferroic materials due to size scaling. While polycrystalline grains and microstructures can yield lower polarization and poorer cycling behavior in canonical ferroelectrics such as Pb(Zr,Ti)O₃ and BaTiO₃, in …

Observation Of The Rare Decay Of The Η Meson To Four Muons, A. Hayrapetyan

Department of Physics and Astronomy: Faculty Publications

A search for the rare η → μ⁺μ⁻μ⁺μ⁻ double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers during 2017 and 2018 and corresponding to an integrated luminosity of 101 fb⁻¹. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the η → μ⁺μ⁻ decay as normalization, the branching fraction β(η → μ⁺μ⁻μ⁺μ⁻) = [5.0 ± 0.8(stat) ± 0.7(syst) ± …

Observation Of Same-Sign Ww Production From Double Parton Scattering In Proton-Proton Collisions At √S=13 Tev, A. Tumasyan

Department of Physics and Astronomy: Faculty Publications

The first observation of the production of W^±W^± bosons from double parton scattering processes using same-sign electron-muon and dimuon events in proton-proton collisions is reported. The data sample corresponds to an integrated luminosity of 138 fb⁻¹ recorded at a center-of-mass energy of 13 TeV using the CMS detector at the CERN LHC. Multivariate discriminants are used to distinguish the signal process from the main backgrounds. A binned maximum likelihood fit is performed to extract the signal cross section. The measured cross section for production of same-sign W bosons decaying leptonically is 80.7 ± 11.2(stat)^+9.5_{-8.6 …}

An Improved Source Of Spin-Polarized Electrons Based On Spin Exchange In Optically Pumped Rubidium Vapor, K. J. Ahrendsen, Kenneth Wayne Trantham, D. Tupa, T. J. Gay

Department of Physics and Astronomy: Faculty Publications

We have improved a polarized electron source in which unpolarized electrons undergo collisions with a mixture of buffer gas molecules and optically spin-polarized Rb atoms. With a nitrogen buffer gas, the source reliably provides spin polarization between 15% and 25% with beam currents >4 μA. Vacuum pump upgrades mitigate problems caused by denatured diffusion pump oil, leading to longer run times. A new differential pumping scheme allows the use of higher buffer gas pressures up to 800 mTorr. With a new optics layout, the Rb polarization is continuously monitored by a probe laser and improved pump laser power provides more …

Search For Magnetoelectric Monopole Response In Cr2O3 Powder, Syed Qamar Abbas Shah, Ather Mahmood, Arun Parthasarathy, Christian H. Binek

Department of Physics and Astronomy: Faculty Publications

Powder samples have been suggested as a pathway to fabricate isotropic magnetoelectric (ME) materials which effectively only have a pseudoscalar or monopole ME response. We demonstrate that random distribution of ME grains alone does not warrant isotropic ME response because the activation of a nonvanishing ME response requires a ME field cooling protocol which tends to induce preferred axes.We investigate the evolution of ME susceptibility in powder chromia samples for various ME field cooling protocols both theoretically and experimentally. In particular, we work out the theoretical expressions for ME susceptibility for powder chromia in the framework of statistical mechanics where …

A Sensitivity Study Of Triboson Production Processes To Dimension-6 Eft Operators At The Lhc, R. Bellan, S. Bhattacharya, G. Boldrini, F. Cetorelli, P. Govoni, A. Massironi, A. Mecca, C. Tarricone, A. Vagnerini

Department of Physics and Astronomy: Faculty Publications

We present the first parton-level study of anomalous effects in triboson production in both fully and semi-leptonic channels in proton-proton collisions at 13TeV at the Large Hadron Collider (LHC). The sensitivity to anomalies induced by a minimal set of bosonic dimension-6 operators from the Warsaw basis is evaluated with specific analyses for each final state. A likelihood-based strategy is employed to assess the most sensitive kinematic observables per channel, where the contribution of Effective Field Theory operators is parameterized at either the linear or quadratic level. The impact of the mutual interference terms of pairs of operators on the sensitivity …

A Search For Decays Of The Higgs Boson To Invisible Particles In Events With A Top-Antitop Quark Pair Or A Vector Boson In Proton-Proton Collisions At √S = 13 Tev, Armen R. Tumasyan

Department of Physics and Astronomy: Faculty Publications

The first search for nonresonant production of Higgs boson pairs (HH) with one H decaying into four leptons and the other into a pair of b quarks is presented, using proton-proton collisions recorded at a center-of-mass energy of √s = 13 TeV by the CMS experiment. The analyzed data correspond to an integrated luminosity of 138 fb⁻¹. A 95% confidence level upper limit of 32.4 is set on the signal strength modifier μ, defined as the ratio of the observed HH production rate in the HH → ZZ*bb → 4ℓbb decay channel to the standard model …

Initial Experience With An Electron Flash Research Extension (Flex) For The Clinac System, Kyuhak Oh, Kyle J. Gallagher, Megan Hyun, Diane Schott, Sarah Wisnoskie, Yu Lei, Samuel Hendley, Jeffrey Wong, Shuo Wang, Brendan Graff, Christopher Jenkins, Frank Rutar, Md Ahmed, Joshua Mcneur, Jeffrey Taylor, Marty Schmidt, Lasitha Senadheera, Wendy Smith, Subodh M. Lele, Ran Dai, Dong Jianghu (James), Y Yan, Su-Min Zhou

Department of Physics and Astronomy: Faculty Publications

Purpose: Radiotherapy delivered at ultra-high-dose-rates (≥40 Gy/s), that is, FLASH, has the potential to effectively widen the therapeutic window and considerably improve the care of cancer patients. The underlying mechanism of the FLASH effect is not well understood, and commercial systems capable of delivering such dose rates are scarce. The purpose of this study was to perform the initial acceptance and commissioning tests of an electron FLASH research product for preclinical studies.

Methods: A linear accelerator (Clinac 23EX) was modified to include a nonclinical FLASH research extension (the Clinac-FLEX system) by Varian, a Siemens Healthineers company (Palo Alto, …