# Open Access Publications

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Open access publications by faculty, postdocs, and graduate students in the Department of Physics and Astronomy.

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Item Higgs inflation, seesaw physics and fermion dark matter(Elsevier Science BV, 2015-06-03) Okada, Nobuchika; Shafi, Qaisar; NobuchikaOkada, QaisarShafi; Shafi, QaisarWe present an inflationary model in which the Standard Model Higgs doublet field with non-minimal coupling to gravity drives inflation, and the effective Higgs potential is stabilized by new physics which includes a dark matter particle and right-handed neutrinos for the seesaw mechanism. All of the new particles are fermions, so that the Higgs doublet is the unique inflaton candidate. With central values for the masses of the top quark and the Higgs boson, the renormalization group improved Higgs potential is employed to yield the scalar spectral index ns 0.968, the tensor-to-scalar ratio r 0.003, and the running of the spectral index α=dns/d lnk −5.2 ×10−4for the number of e-folds N0=60(ns 0.962, r 0.004, and α −7.5 ×10−4for N0=50). The fairly low value of r 0.003predicted in this class of models means that the ongoing space and land based experiments are not expected to observe gravity waves generated during inflation.Item Determining the band alignment of TbAs:GaAs and TbAs:In0.53Ga0.47As(American Institute of Physics, 2015-09-10) Bomberger, Cory C.; Vanderhoef, Laura R.; Rahman, Abdur; Shah, Deesha; Chase, D. Bruce; Taylor, Antoinette J.; Azad, Abul K.; Doty, Matthew F.; Zide, Joshua M. O.; Cory C. Bomberger, Laura R. Vanderhoef, Abdur Rahman, Deesha Shah, D. Bruce Chase, Antoinette J. Taylor, Abul K. Azad, Matthew F. Doty, and Joshua M. O. Zide; Bomberger, Cory C.; Vanderhoef, Laura R.; Chase, D. Bruce; Doty, Matthew F.; Zide, Joshua M. O.We propose and systematically justify a band structure for TbAs nanoparticles in GaAs and In0.53Ga0.47As host matrices. Fluence-dependent optical-pump terahertz-probe measurements suggest the TbAs nanoparticles have a band gap and provide information on the carrier dynamics, which are determined by the band alignment. Spectrophotometry measurements provide the energy of optical transitions in the nanocomposite systems and reveal a large blue shift in the absorption energy when the host matrix is changed from In0.53Ga0.47As to GaAs. Finally, Hall data provides the approximate Fermi level in each system. From this data, we deduce that the TbAs:GaAs system forms a type I (straddling) heterojunction and the TbAs:In0.53Ga0.47As system forms a type II (staggered) heterojunction.Item Primordial monopoles, proton decay, gravity waves and GUT inflation(Elsevier B.V., 2015-11-18) Seno˘guz, Vedat Nefer; Shafi, Qaisar; Vedat Nefer¸Seno˘guz, Qaisar Shafi; Shafi, QaisarWe consider non-supersymmetric GUT inflation models in which intermediate mass monopoles may survive inflation because of the restricted number of e-foldings experienced by the accompanying symmetry breaking. Thus, an observable flux of primordial magnetic monopoles, comparable to or a few orders below the Parker limitmay be present in the galaxy. The mass scale associated with the intermediate symmetry breaking is 1013GeVfor an observable flux level, with the corresponding monopoles an order of magnitude or so heavier. Examples based on SO(10)and E6yield such intermediate mass monopoles carrying respectively two and three units of Dirac magnetic charge. For GUT inflation driven by a gauge singlet scalar field with a Coleman–Weinberg or Higgs potential, compatibility with the Planck measurement of the scalar spectral index yields a Hubble constant (during horizon exit of cosmological scales) H∼7–9 ×1013GeV, with the tensor to scalar ratio rpredicted to be 0.02. Proton lifetime estimates for decays mediated by the superheavy gauge bosons are also provided.Item Band structure and dispersion engineering of strongly coupled plasmon-phonon-polaritons in graphene-integrated structures(The Optical Society, 2016-01-19) Liu, Feng; Zhan, Tianrong; Zhu, Alexander Y.; Yi, Fei; Shi, Wangzhou; Feng Liu, Tianrong Zhan, Alexander Y. Zhu, Fei Yi, and Wangzhou Shi; Zhan, TianrongWe theoretically investigate the polaritonic band structure and dispersion properties of graphene using transfer matrix methods, with strongly coupled graphene plasmons (GPs) and molecular infrared vibrations as a representative example. Two common geometrical con- figurations are considered: graphene coupled subwavelength dielectric grating (GSWDG) and graphene nanoribbons (GNR). By exploiting the dispersion and the band structure, we show the possibility of tailoring desired polaritonic behavior in each of the two configurations. We compare the strength of coupling occurring in both structures and find that the interaction is stronger in GNR than that of GSWDG structure as a result of the stronger field confinement of the edge modes. The band structure and dispersion analysis not only sheds light on the physics of the hybridized polariton formation but also offers insight into tailoring the optical response of graphene light-matter interactions for numerous applications, such as biomolecular sensing and detection.Item Bulk Mn-Al-C permanent magnets prepared by various techniques(AIP Publishing, 2016-03-01) Madugundo, Rajasekhar; Koylu-Alkan, Ozlem; Hadjipanayis, George C.; Rajasekhar Madugundo, Ozlem Koylu-Alkan, and George C. Hadjipanayis; Madugundo, Rajasekhar; Koylu-Alkan, Ozlem; Hadjipanayis, George C.Bulk Mn-Al-C magnets have been prepared by hot-compaction, microwave sintering and hot-deformation. Powders of Mn53.5Al44.5C2 alloy in the ε-phase produced by high energy ball milling have been used as precursor for the hot-compacted and microwave sintered magnets. Hot-deformed magnets were produced from alloy pieces in the τ-phase. The hot-compacted magnet exhibits magnetization, remanence and coercivity of 50 emu/g, 28 emu/g and 3.3 kOe, respectively. Microwave sintered magnet shows a maximum magnetization of 94 emu/g, remanence of 30 emu/g and coercivity of 1.1 kOe. The best magnetic properties are obtained in hot-deformed magnets with magnetization, remanence, coercivity and energy product of 82 emu/g, 50 emu/g, 2.2 kOe and 1.8 MGOe, respectively. Hot-deformed magnets exhibit texture with the highest degree of texture obtained 0.26. It is found that the pressure applied during compaction/deformation favors coercivity.Item The spectral temperature of optically thick outflows with application to light echo spectra from η Carinae’s giant eruption(Oxford University Press on behalf of the Royal Astronomical Society, 2016-07-07) Owocki, Stanley P.; Shaviv, Nir J.; Stanley P. Owocki and Nir J. Shaviv; Owocki, Stanley P.The detection by Rest et al. of light echoes from η Carinae has provided important new observational constraints on the nature of its 1840s era giant eruption. Spectra of the echoes suggest a relatively cool spectral temperature of about 5500 K, lower than the lower limit of about 7000 K suggested in the optically thick wind-outflow analysis of Davidson. This has led to a debate about the viability of this steady wind model relative to alternative, explosive scenarios. Here we present an updated analysis of the wind-outflow model using newer low- temperature opacity tabulations and accounting for the stronger mass-loss implied by the > 10 M mass now inferred for the Homunculus. A major conclusion is that, because of the sharp drop in opacity due to recombination loss of free electrons for T < 6500 K, a low temperature of about 5000 K is compatible with, and indeed expected from, a wind with the extreme mass-loss inferred for the eruption. Within a spherical grey model in radiative equilibrium, we derive spectral energy distributions for various assumptions for the opacity variation of the wind, providing a basis for comparisons with observed light echo spectra. The scaling results here are also potentially relevant for other highly optically thick outflows, including those from classical novae, giant eruptions of luminous blue variables and supernovae Type IIn precursors. A broader issue therefore remains whether the complex, variable features observed from such eruptions are better understood in terms of a steady or explosive paradigm, or perhaps a balance of these idealizations.Item An ‘analytic dynamical magnetosphere’ formalism for X-ray and optical emission from slowly rotating magnetic massive stars(Oxford University Press on behalf of the Royal Astronomical Society., 2016-08-01) Owocki, Stanley P.; ud-Doula, Asif; Sundqvist, Jon O.; Petit, Veronique; Cohen, David H.; Townsend, Richard H. D.; Stanley P. Owocki, Asif ud-Doula, Jon O. Sundqvist, Veronique Petit, David H. Cohen, and Richard H. D. Townsend; Owocki, Stanley P.Slowly rotating magnetic massive stars develop ‘dynamical magnetospheres’ (DMs), characterized by trapping of stellar wind outflow in closed magnetic loops, shock heating from collision of the upflow from opposite loop footpoints, and subsequent gravitational infall of radiatively cooled material. In 2D and 3D magnetohydrodynamic (MHD) simulations, the interplay among these three components is spatially complex and temporally variable, making it difficult to derive observational signatures and discern their overall scaling trends. Within a simplified, steady-state analysis based on overall conservation principles, we present here an ‘analytic dynamical magnetosphere’ (ADM) model that provides explicit formulae for density, temperature, and flow speed in each of these three components – wind outflow, hot post-shock gas, and cooled inflow – as a function of colatitude and radius within the closed (presumed dipole) field lines of the magnetosphere. We compare these scalings with time-averaged results from MHD simulations, and provide initial examples of application of this ADM model for deriving two key observational diagnostics, namely hydrogen H α emission line profiles from the cooled infall, and X-ray emission from the hot post-shock gas. We conclude with a discussion of key issues and advantages in applying this ADM formalism towards derivation of a broader set of observational diagnostics and scaling trends for massive stars with such dynamical magnetospheres.Item Non-minimal quartic inflation in supersymmetric SO(10)(Elsevier Science, 2016-12-16) Leontaris, George K.; Okada, Nobuchika; Shafi, Qaisar; George K. Leontaris, Nobuchika Okada, Qaisar Shafi; Shafi, QaisarWe describe how quartic (λφ4) inflation with non-minimal coupling to gravity is realized in realistic supersymmetric SO(10)models. In a well-motivated example the 16 −16Higgs multiplets, which break SO(10)to SU(5)and yield masses for the right-handed neutrinos, provide the inflaton field φ. Thus, leptogenesis is a natural outcome in this class of SO(10)models. Moreover, the adjoint (45-plet) Higgs also acquires a GUT scale value during inflation so that the monopole problem is evaded. The scalar spectral index nsis in good agreement with the observations and r, the tensor to scalar ratio, is predicted for realistic values of GUT parameters to be of order 10−3–10−2.Item A subwavelength resolution microwave/6.3 GHz camera based on a metamaterial absorber(Nature Publishing Group, 2017-01-10) Xie, Yunsong; Fan, Xin; Chen, Yunpeng; Wilson ., Jeffrey D; Simons, Rainee N; Xiao, John Q.; Yunsong Xie; Xin Fan; Yunpeng Chen; Jeffrey D.Wilson; Rainee N. Simons; John Q. Xiao; Xie, Yunsong; Chen, Yunpeng; Xiao, John QThe design, fabrication and characterization of a novel metamaterial absorber based camera with subwavelength spatial resolution are investigated. The proposed camera is featured with simple and lightweight design, easy portability, low cost, high resolution and sensitivity, and minimal image interference or distortion to the original field distribution. The imaging capability of the proposed camera was characterized in both near field and far field ranges. The experimental and simulated near field images both reveal that the camera produces qualitatively accurate images with negligible distortion to the original field distribution. The far field demonstration was done by coupling the designed camera with a microwave convex lens. The far field results further demonstrate that the camera can capture quantitatively accurate electromagnetic wave distribution in the diffraction limit. The proposed camera can be used in application such as non-destructive image and beam direction tracer.Item Criticality-Enhanced Magnetocaloric Effect in Quantum Spin Chain Material Copper Nitrate(Nature Publishing Group, 2017-03-15) Xiang, Jun-Sen; Chen, Cong; Li, Wei; Sheng, Xian-Lei; Su, Na; Cheng, Zhao-Hua; Chen, Qiang; Chen, Zi-Yu; Jun-Sen Xiang, Cong Chen, Wei Li, Xian-Lei Sheng, Na Su, Zhao-Hua Cheng, Qiang Chen & Zi-Yu Chen; Sheng, Xian-LeiIn this work, a systematic study of Cu(NO3)2·2.5 H2O (copper nitrate hemipentahydrate, CN), an alternating Heisenberg antiferromagnetic chain model material, is performed with multi-technique approach including thermal tensor network (TTN) simulations, first-principles calculations, as well as magnetization measurements. Employing a cutting-edge TTN method developed in the present work, we verify the couplings J = 5.13 K, α = 0.23(1) and Landé factors g∥= 2.31, g⊥ = 2.14 in CN, with which the magnetothermal properties have been fitted strikingly well. Based on first-principles calculations, we reveal explicitly the spin chain scenario in CN by displaying the calculated electron density distributions, from which the distinct superexchange paths are visualized. On top of that, we investigated the magnetocaloric effect (MCE) in CN by calculating its isentropes and magnetic Grüneisen parameter. Prominent quantum criticality-enhanced MCE was uncovered near both critical fields of intermediate strengths as 2.87 and 4.08 T, respectively. We propose that CN is potentially a very promising quantum critical coolant.Item Robustness of quantized transport through edge states of finite length: Imaging current density in Floquet topological versus quantum spin and anomalous Hall insulators(Physical Review Research, 2020-09-17) Bajpai, Utkarsh; Ku, Mark J. H.; Nikolić, Branislav K.The theoretical analysis of topological insulators (TIs) has been traditionally focused on infinite homogeneous crystals with band gap in the bulk and nontrivial topology of their wave functions, or infinite wires whose boundaries host surface or edge metallic states. Such infinite-length edge states exhibit quantized conductance which is insensitive to edge disorder, as long as it does not break the underlying symmetry or introduce energy scale larger than the bulk gap. However, experimental devices contain finite-size topological region attached to normal metal (NM) leads, which poses a question about how precise is quantization of longitudinal conductance and how electrons transition from topologically trivial NM leads into the edge states. This particularly pressing issue for recently conjectured two-dimensional (2D) Floquet TI where electrons flow from time-independent NM leads into time-dependent edge states, the very recent experimental realization [J. W. McIver et al., Nat. Phys. 16, 38 (2020)] of Floquet TI using graphene irradiated by circularly polarized light did not exhibit either quantized longitudinal or Hall conductance. Here, we employ a charge-conserving solution for Floquet-nonequilibrium Green functions of irradiated graphene nanoribbon to compute longitudinal two-terminal conductance, as well as spatial profiles of local current density as electrons propagate from NM leads into the Floquet TI. For comparison, we also compute conductance of graphene-based realization of 2D quantum Hall, quantum anomalous Hall, and quantum spin Hall insulators. Although zero-temperature conductance within the gap of these three conventional time-independent 2D TIs of finite length exhibits small oscillations due to reflections at the NM-lead/2D-TI interface, it remains very close to perfectly quantized plateau at 2e2/h and completely insensitive to edge disorder. This is due to the fact that inside conventional TIs there is only edge local current density which circumvents any disorder. In contrast, in the case of Floquet TI both bulk and edge local current densities contribute equally to total current, which leads to longitudinal conductance below the expected quantized plateau that is further reduced by edge vacancies. We propose two experimental schemes to detect coexistence of bulk and edge current densities within Floquet TI: (i) drilling a nanopore in the interior of irradiated region of graphene will induce backscattering of bulk current density, thereby reducing longitudinal conductance by ∼28%; (ii) imaging of magnetic field produced by local current density using diamond nitrogen-vacancy centers.Item Scattering-induced and highly tunable by gate damping-like spin-orbit torque in graphene doubly proximitized by two-dimensional magnet Cr2Ge2Te6 and monolayer WS2(Physical Review Research, 2020-10-09) Zollner, Klaus; Petrović, Marko D.; Dolui, Kapildeb; Plecháč, Petr; Nikolić, Branislav K.; Fabian, JaroslavGraphene sandwiched between semiconducting monolayers of ferromagnet Cr2Ge2Te6 and transition-metal dichalcogenide WS2 acquires both spin-orbit (SO) coupling, of valley-Zeeman and Rashba types, and exchange coupling. Using first principles combined with quantum transport calculations, we predict that such doubly proximitized graphene within van der Waals heterostructure will exhibit SO torque driven by unpolarized charge current. This system lacks spin Hall current which is putatively considered as necessary for the efficient damping-like (DL) SO torque that plays a key role in magnetization switching. Instead, it demonstrates how a DL SO torque component can be generated solely by skew scattering off spin-independent potential barrier or impurities in purely two-dimensional electronic transport due to the presence of proximity SO coupling and its spin texture tilted out of plane. This leads to current-driven nonequilibrium spin density emerging in all spatial directions, whose cross product with proximity magnetization yields DL SO torque, unlike the ballistic regime with no scatterers in which only field-like (FL) SO torque appears. In contrast to SO torque on conventional metallic ferromagnets in contact with three-dimensional SO-coupled materials, the ratio of FL and DL components of SO torque can be tuned by more than an order of magnitude via combined top and back gates.Item Bimetal–organic frameworks derived tuneable Co nanoparticles embedded in porous nitrogen-doped carbon nanorods as high-performance electromagnetic wave absorption materials(Journal of Materials Chemistry C, 2021-05-04) Pan, Jiannan; Yang, Huadong; Hong, Qu; Wen, Hui-Min; Xiao, John Q.; Hu, JunThe in situ pyrolysis of metal–organic-frameworks (MOFs) is an effective strategy to prepare magnetic metal nanoparticle (NP) doped porous carbon materials. These composite materials have shown great potential as high performance electromagnetic wave (EMW) absorption materials. So far, the precise control of composite composition and structure has remained a major challenge in constructing highly porous composites with uniformly distributed NPs. In this work, we report a facile route to synthesize tuneable Co NPs embedded in porous nitrogen-doped carbon (Co/NC) nanorods through the direct thermolysis of the bimetal–organic framework (CoZn–ZIF) precursor. By adjusting the proportion of Co2+ in the MOF precursor, the content and distribution of Co NPs in the composite absorber change accordingly. When the molar ratio between Co2+ and Zn2+ is 3 : 1, the carbonized composites exhibit the largest external surface area and the best EMW absorption performance. With a filler mass loading of merely 15 wt%, the minimum reflection loss (RLmin) reaches −52.3 dB at 10.1 GHz with a thin layer thickness of 2.5 mm. The largest effective absorption bandwidth (EAB) of 5.0 GHz (11.1–16.1 GHz) is achieved in a 2.0 mm thick sample. The qualified bandwidth can be up to 14.5 GHz (3.5–18.0 GHz) with the integrated thickness from 1.0 mm to 5.5 mm. The enhanced conductive/magnetic losses, strong interfacial/dipolar/defect polarization, hierarchical pore structure and the geometric effect endow the Co/NC absorber with improved impedance matching and enhanced attenuation of EMW. This work provides a good direction for the future study of MOF-derived lightweight and efficient EMW absorbing materials.Item Electroweak monopoles and magnetic dumbbells in grand unified theories(Physical Review D, 2021-05-20) Lazarides, G.; Shafi, Q.We use the SU(5) model to show the presence in grand unified theories of an electroweak monopole and a magnetic dumbbell (“meson”) made up of a monopole-antimonopole pair connected by a Z-magnetic flux tube. The monopole is associated with the spontaneous breaking of the weak SU(2)L gauge symmetry by the induced vacuum expectation value of a heavy scalar SU(2)L triplet with zero weak hypercharge contained in the adjoint Higgs 24-plet. This monopole carries a Coulomb magnetic charge of (3/4)(2π/e) as well as Z-magnetic charge, where 2π/e denotes the unit Dirac magnetic charge. Its total magnetic charge is √3/8(4π/e), which is in agreement with the Dirac quantization condition. The monopole weighs about 700 GeV, but because of the attached Z-magnetic tube it exists, together with the antimonopole, in a magnetic dumbbell configuration whose mass is expected to lie in the TeV range. The presence of these topological structures in SU(5) and SO(10) and in their supersymmetric extensions provides an exciting new avenue for testing these theories in high-energy colliders.Item Spintronics Meets Density Matrix Renormalization Group: Quantum Spin-Torque-Driven Nonclassical Magnetization Reversal and Dynamical Buildup of Long-Range Entanglement(Physical Review X, 2021-06-23) Petrović, Marko D.; Mondal, Priyanka; Feiguin, Adrian E.; Plecháč, Petr; Nikolić, Branislav K.We introduce the time-dependent density matrix renormalization group (tDMRG) as a solution to a long-standing problem in spintronics—how to describe spin-transfer torque (STT) between flowing spins of conduction electrons and localized spins within a magnetic material by treating the dynamics of both spin species fully quantum mechanically. In contrast to conventional Slonczewski-Berger STT, where the localized spins are viewed as classical vectors obeying the Landau-Lifshitz-Gilbert equation and where their STT-driven dynamics is initiated only when the spin polarization of flowing electrons and localized spins are noncollinear, quantum STT can occur when these vectors are collinear but antiparallel. Using tDMRG, we simulate the time evolution of a many-body quantum state of electrons and localized spins, where the former are injected as a spin-polarized current pulse while the latter comprise a quantum Heisenberg ferromagnetic metallic (FM) spin-1/2XXZ chain initially in the ground state with spin polarization antiparallel to that of injected electrons. The quantum STT reverses the direction of localized spins, but without rotation from the initial orientation, when the number of injected electrons exceeds the number of localized spins. Such nonclassical reversal, which is absent from Landau-Lifshitz-Gilbert dynamics, is strikingly inhomogeneous across the FM chain, and it can be accompanied by reduction of the magnetization associated with localized spins, even to zero at specific locations. This feature arises because quantum STT generates a highly entangled nonequilibrium many-body state of all flowing and localized spins, despite starting from the initially unentangled ground state of a mundane FM. Furthermore, the mutual information between localized spins at the FM edges remains nonzero even at infinite separation as the signature of dynamical buildup of long-range entanglement. The growth in time of entanglement entropy differentiates between the quantum and conventional (i.e., noncollinear) setups for STT, reaching a much larger asymptotic value in the former case.Item A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the main sequence to planets Get access Arrow(Monthly Notices of the Royal Astronomical Society, 2021-07-28) Leto, P.; Trigilio, C.; Krtička, J.; Fossati, L.; Ignace, R.; Shultz, M. E.; Buemi, C. S.; Cerrigone, L.; Umana, G.; Ingallinera, A.; Bordiu, C.; Pillitteri, I.; Bufano, F.; Oskinova, L. M.; Agliozzo, C.; Cavallaro, F.; Riggi, S.; Loru, S.; Todt, H.; Giarrusso, M.; Phillips, N. M.; Robrade, J.; Leone, F.In this paper, we present the analysis of incoherent non-thermal radio emission from a sample of hot magnetic stars, ranging from early-B to early-A spectral type. Spanning a wide range of stellar parameters and wind properties, these stars display a commonality in their radio emission which presents new challenges to the wind scenario as originally conceived. It was thought that relativistic electrons, responsible for the radio emission, originate in current sheets formed, where the wind opens the magnetic field lines. However, the true mass-loss rates from the cooler stars are too small to explain the observed non-thermal broad-band radio spectra. Instead, we suggest the existence of a radiation belt located inside the inner magnetosphere, similar to that of Jupiter. Such a structure explains the overall indifference of the broad-band radio emissions on wind mass-loss rates. Further, correlating the radio luminosities from a larger sample of magnetic stars with their stellar parameters, the combined roles of rotation and magnetic properties have been empirically determined. Finally, our sample of early-type magnetic stars suggests a scaling relationship between the non-thermal radio luminosity and the electric voltage induced by the magnetosphere’s co-rotation, which appears to hold for a broader range of stellar types with dipole-dominated magnetospheres (like the cases of the planet Jupiter and the ultracool dwarf stars and brown dwarfs). We conclude that well-ordered and stable rotating magnetospheres share a common physical mechanism for supporting the generation of non-thermal electrons.Item SU(5) × U(1)x axion model with observable proton decay(Physical Review D, 2021-09-27) Okada, Nobuchika; Raut, Digesh; Shafi, QaisarWe propose a SU(5) x U(1)x X U(1)PQ model, where U(1)x is the generalization of the B − L (baryon minus lepton number) gauge symmetry and U(1)PQ is the global Peccei-Quinn (PQ) symmetry. There are four fermions families in 5 + 10 representations of SU(5), a mirror family in 5 + 10 representations, and three SU(5) singlet Majorana fermions. The U(1)x related anomalies all cancel in the presence of the Majorana neutrinos. The SU(5) symmetry is broken at MGUT ≃ (6–9) x 10^15 GeV and the proton lifetime τp is estimated to be well within the expected sensitivity of the future hyper-Kamiokande experiment, τp ≲ 1.3 × 10^35 years. The SU(5) breaking also triggers the breaking of the PQ symmetry, resulting in axion dark matter (DM), with the axion decay constant fa of order MGUT or somewhat larger. The CASPEr experiment can search for such an axion DM candidate. The Hubble parameter during inflation must be low, Hinf ≲ 10^9 GeV, in order to successfully resolve the axion domain wall, axion DM isocurvature and SU(5) monopole problems. With the identification of the U(1)x breaking Higgs field with the inflaton field, we implement inflection-point inflation, which is capable of realizing the desired value for Hinf. The vectorlike fermions in the model are essential for achieving successful unification of the SM gauge couplings as well as the phenomenological viability of both axion DM and inflation scenario.Item von Karman Correlation Similarity of the Turbulent Interplanetary Magnetic Field(Astrophysical Journal Letters, 2021-10-01) Roy, Sohom; Chhiber, R.; Dasso, S.; Ruiz, M. E.; Matthaeus, W. H.A major development underlying much of hydrodynamic turbulence theory is the similarity decay hypothesis due to von Karman and Howarth here extended empirically to magnetic field fluctuations in the solar wind. In similarity decay the second-order correlation experiences a continuous transformation based on a universal functional form and a rescaling of energy and characteristic length. Solar wind turbulence follows many principles adapted from classical fluid turbulence, but previously this similarity property has not been examined explicitly. Here we analyze an ensemble of magnetic correlation functions computed from Advanced Composition Explorer data at 1 au, and demonstrate explicitly that the two-point correlation functions undergo a collapse to a similarity form of the type anticipated from von Karman's hypothesis. This provides for the first time a firm empirical basis for employing the similarity decay hypothesis to the magnetic field, one of the primitive variables of magnetohydrodynamics, and one frequently more accessible from spacecraft instruments. This approach is of substantial utility in space turbulence data analysis, and for adopting von Karman-type heating rates in global and subgrid-scale dynamical modeling.Item Inner shell excitation by strong field laser rescattering: optimal laser conditions for high energy recollision(Journal of the Optical Society of America B, 2021-11-15) Kelley, L.; Germain, Z.; Jones, E. C.; Milliken, D.; Walker, Barry C.We address the challenge of finding the optimal laser intensity and wavelength to drive high-energy, strong field rescattering and report the maximum yields of K-shell and LI-shell hole creation. Surprisingly, our results show laser-driven rescattering is able to create inner shell holes in all atoms from lithium to uranium with the interaction spanning from the deep IR to x-ray free electron laser sources. The calculated peak rescattering follows a simple scaling with the atomic number and laser wavelength. The results show it is possible to describe the ideal laser intensity and wavelength for general high-energy laser rescattering processes.Item Quantum many-body states and Green's functions of nonequilibrium electron-magnon systems: Localized spin operators versus their mapping to Holstein-Primakoff bosons(Physical Review B, 2021-11-22) Bajpai, Utkarsh; Suresh, Abhin; Nikolić, Branislav K.It is well-known that operators of localized spins within a magnetic material satisfy neither fermionic nor bosonic commutation relations. Thus, to construct diagrammatic many-body perturbation theory requiring the Wick theorem, the spin operators are usually mapped to the bosonic ones with Holstein-Primakoff (HP) transformation being the most widely used in magnonics and spintronics literature. However, to make calculations tractable, the square root of operators in the HP transformation is expanded into a Taylor series truncated to some low order. This poses a question on the range of validity of the truncated HP transformation when describing nonequilibrium dynamics of localized spins interacting with each other or with conduction electron spins—a problem frequently encountered in numerous transport phenomena in magnonics and spintronics. Here we apply exact diagonalization techniques to a Hamiltonian of fermions (i.e., electrons) interacting with HP bosons versus a Hamiltonian of fermions interacting with the original localized spin operators to compare their many-body states and one-particle equilibrium and nonequilibrium Green's functions (GFs). We employ as a test bed a one-dimensional quantum Heisenberg ferromagnetic spin-S XXX chain of N≤7 sites, where S=1 or S=5/2, and the ferromagnet can be made metallic by allowing electrons to hop between the sites while interacting with the localized spins via sd exchange interaction. For these two different versions of the Hamiltonian of this model, we compare the structure of their ground states, time evolution of excited states, spectral functions computed from the retarded GF in equilibrium, and matrix elements of the lesser GF out of equilibrium. Interestingly, magnonic spectral function can be substantially modified by acquiring additional peaks due to quasibound states of electrons and magnons once the interaction between these subsystems is turned on. The Hamiltonian of fermions interacting with HP bosons gives an incorrect ground state and electronic spectral function unless a large number of terms are retained in the truncated HP transformation. Furthermore, tracking the nonequilibrium dynamics of localized spins over longer time intervals requires a progressively larger number of terms in truncated HP transformation, even if a small magnon density is excited initially, but the required number of terms is reduced when interaction with conduction electrons is turned on. Finally, we show that recently proposed [M. Vogl et al., Phys. Rev. Res. 2, 043243 (2020); J. König et al., SciPost Phys. 10, 007 (2021)] resummed HP transformation, where spin operators are expressed as polynomials in bosonic operators, resolves the trouble with truncated HP transformation while allowing us to derive an exact quantum many-body (manifestly Hermitian) Hamiltonian consisting of a finite and fixed number of boson-boson and electron-boson interacting terms.