14,533 research outputs found

    Extending the reach of uncertainty quantification in nuclear theory

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    The theory of the strong interaction—quantum chromodynamics (QCD)—is unsuited to practical calculations of nuclear observables and approximate models for nuclear interaction potentials are required. In contrast to phenomenological models, chiral effective field theories (χEFTs) of QCD grant a handle on the theoretical uncertainty arising from the truncation of the chiral expansion. Uncertainties in χEFT are preferably quantified using Bayesian inference, but quantifying reliable posterior predictive distributions for nuclear observables presents several challenges. First, χEFT is parametrized by unknown low-energy constants (LECs) whose values must be inferred from low-energy data of nuclear structure and reaction observables. There are 31 LECs at fourth order in Weinberg power counting, leading to a high-dimensional inference problem which I approach by developing an advanced sampling protocol using Hamiltonian Monte Carlo (HMC). This allows me to quantify LEC posteriors up to and including fourth chiral order. Second, the χEFT truncation error is correlated across independent variables such as scattering energies and angles; I model correlations using a Gaussian process. Third, the computational cost of computing few- and many-nucleon observables typically precludes their direct use in Bayesian parameter estimation as each observable must be computed in excess of 100,000 times during HMC sampling. The one exception is nucleon-nucleon scattering observables, but even these incur a substantial computational cost in the present applications. I sidestep such issues using eigenvector-continuation emulators, which accurately mimic exact calculations while dramatically reducing the computational cost. Equipped with Bayesian posteriors for the LECs, and a model for the truncation error, I explore the predictive ability of χEFT, presenting the results as the probability distributions they always were

    A consistent quantum field theory from dimensional reduction

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    We incorporate the concept of dimensional reduction at high energies within the perturbative formulation of quantum field theory. In this new framework, space and momentum integrations are modified by a weighting function incorporating an effective mass energy associated with the dimensional reduction scale. We quantize the theory within canonical formalism. We then show that it can be made finite in perturbation theory, free of renormalon ambiguities, and with better analytic behavior for infinitesimal coupling constant compared to standard quantum field theory. The new approach reproduces the known results at low energies. One key feature of this class of models is that the coupling constant always reaches a fixed point in the ultraviolet region, making the models ultra-violet complete.Comment: to appear in J. Phys. A Math. Theo

    KYT2022 Finnish Research Programme on Nuclear Waste Management 2019–2022 : Final Report

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    KYT2022 (Finnish Research Programme on Nuclear Waste Management 2019–2022), organised by the Ministry of Economic Affairs and Employment, was a national research programme with the objective to ensure that the authorities have sufficient levels of nuclear expertise and preparedness that are needed for safety of nuclear waste management. The starting point for public research programs on nuclear safety is that they create the conditions for maintaining the knowledge required for the continued safe and economic use of nuclear energy, developing new know-how and participating in international collaboration. The content of the KYT2022 research programme was composed of nationally important research topics, which are the safety, feasibility and acceptability of nuclear waste management. KYT2022 research programme also functioned as a discussion and information-sharing forum for the authorities, those responsible for nuclear waste management and the research organizations, which helped to make use of the limited research resources. The programme aimed to develop national research infrastructure, ensure the continuing availability of expertise, produce high-level scientific research and increase general knowledge of nuclear waste management

    Magnon-magnon interaction in monolayer MnBi2_2Te4_4

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    MnBi2_2Te4_4, the first confirmed intrinsic antiferromagnetic topological insulator, have attracted more and more attention in recent years. Here we investigate the energy correction and lifetime of magnons in MnBi2_2Te4_4 caused by magnon-magnon interaction. Firstly, a first-principles calculation was performed to get the parameters of the magnetic Hamiltonian of MnBi2_2Te4_4. Then the perturbation method of many-body Green's function is applied and the 1st-order self-energy [Σ(1)(k)\Sigma^{(1)}({\bf k})] and 2nd-order self-energy [Σ(2)(k,εk)\Sigma^{(2)}({\bf k},\varepsilon_{\bf k})] of magnon are obtained. Numerical computation shows that the correction from both Σ(1)(k)\Sigma^{(1)}({\bf k}) and Σ(2)(k,εk)\Sigma^{(2)}({\bf k},\varepsilon_{\bf k}) are strongly dependent on momentum and temperature, the energy renormalization near Brillouin zone (BZ) boundary is obviously stronger than that near BZ centre. We also find that some dip structures occur in renormalized magnon spectrum near K\rm K and M\rm M points, and these dip structures should be attributed to Σ(2)(k,εk)\Sigma^{(2)}({\bf k},\varepsilon_{\bf k}).Comment: 7 pages, 7 figure

    Footprint of a topological phase transition on the density of states

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    For a generalized Su-Schrieffer-Heeger model the energy zero is always critical and hyperbolic in the sense that all reduced transfer matrices commute and have their spectrum off the unit circle. Disorder driven topological phase transitions in this model are characterized by a vanishing Lyapunov exponent at the critical energy. It is shown that the integrated density of states away from a transition has a pseudogap with an explicitly computable H\"older exponent, while it has a characteristic divergence (Dyson spike) at the transition points. The proof is based on renewal theory for the Pr\"ufer phase dynamics and the optional stopping theorem for martingales of suitably constructed comparison processes.Comment: Title and introduction modified, but results essentially identically. Contains comparison with earlier contributions that were not cited in previous versio

    Quantum Integrability vs Experiments: Correlation Functions and Dynamical Structure Factors

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    Integrable Quantum Field Theories can be solved exactly using bootstrap techniques based on their elastic and factorisable S-matrix. While knowledge of the scattering amplitudes reveals the exact spectrum of particles and their on-shell dynamics, the expression of the matrix elements of the various operators allows the reconstruction of off-shell quantities such as two-point correlation functions with a high level of precision. In this review, we summarise results relevant to the contact point between theory and experiment providing a number of quantities that can be computed theoretically with great accuracy. We concentrate on universal amplitude ratios which can be determined from the measurement of generalised susceptibilities, and dynamical structure factors, which can be accessed experimentally e.g. via inelastic neutron scattering or nuclear magnetic resonance. Besides an overview of the subject and a summary of recent advances, we also present new results regarding generalised susceptibilities in the tricritical Ising universality class.Comment: 53 pages, 12 figures. arXiv admin note: text overlap with arXiv:2109.0976

    Full stack development toward a trapped ion logical qubit

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    Quantum error correction is a key step toward the construction of a large-scale quantum computer, by preventing small infidelities in quantum gates from accumulating over the course of an algorithm. Detecting and correcting errors is achieved by using multiple physical qubits to form a smaller number of robust logical qubits. The physical implementation of a logical qubit requires multiple qubits, on which high fidelity gates can be performed. The project aims to realize a logical qubit based on ions confined on a microfabricated surface trap. Each physical qubit will be a microwave dressed state qubit based on 171Yb+ ions. Gates are intended to be realized through RF and microwave radiation in combination with magnetic field gradients. The project vertically integrates software down to hardware compilation layers in order to deliver, in the near future, a fully functional small device demonstrator. This thesis presents novel results on multiple layers of a full stack quantum computer model. On the hardware level a robust quantum gate is studied and ion displacement over the X-junction geometry is demonstrated. The experimental organization is optimized through automation and compressed waveform data transmission. A new quantum assembly language purely dedicated to trapped ion quantum computers is introduced. The demonstrator is aimed at testing implementation of quantum error correction codes while preparing for larger scale iterations.Open Acces

    Quantum Phase transitions observed in a moving frame and emergent space-time near Quantum Phase transitions

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    The important role of Lorentz transformation (LT) and invariance (LI) in relativistic quantum field theory is well appreciated and understood. At a low velocity, the LT just reduces to the Galileo transformation (GT). While many body systems in materials or cold atom systems break the LI spontaneously or explicitly, therefore are non-relativistic. Unfortunately, the possible effects of GT in these non-relativistic quantum many-body systems were poorly understood. In this work, we explore its dramatic effects, especially near quantum phase transitions (QPT) which are among the most fantastic phenomena in Nature. We show that an observer in a different inertial frame may detect new quantum phases through novel quantum phase transitions. We demonstrate this new effect by studying one of the simplest QPTs: Superfluid (SF)-Mott transitions of interacting bosons in a square lattice observed in a frame moving with a constant velocity v⃗ \vec{v} relative to the underlaying lattice. These new effects are contrasted to the Doppler shifts in a relativistic quantum field theory, Unruh effects in an accelerating observer and non-relativistic AdSd+1/CFTd AdS_{d+1}/CFT_d correspondence at a low d d .The methods can be extended to study all the quantum and topological phase transitions, even some dynamic classical phase transition in a moving frame in any dimension. an effective way not only measure various intrinsic properties of the materials, tune various quantum and topological phases through phase transitions, but also probe the new emergent space-time structure near any QPT. As a byproduct, we also comment on the GT and GI in fractional Quantum Hall systems and the associated edge states.Comment: 60 PRB pages, 12 figures, Sec.IX-A and B contains part of arXiv:cond-mat/0512480 for the sake of self-contained. We dedicate this work to celebrate the 75th birthday of Moses Chan, a world class experimentalist and 60th birthday of Subir Sachdev, a world class theoris

    Theory and simulation of moiré graphene multilayers

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    Graphene has been hailed as a material which is going to revolutionise myriad technologies due to its extraordinary stability, mechanical strength yet flexibility, and remarkable transport properties. Furthermore, it was recently discovered that if two graphene layers are stacked and twisted relative to one another, referred to as twisted bilayer graphene (tBLG), correlated insulating states and superconductivity are observed, even though graphene does not intrinsically exhibit these properties. These phases only emerge at twist angles close to the "magic angle" of 1.1 degrees, and by tuning the temperature and doping level, the system can undergo electronic phase transitions between these states. I studied electron interactions and electronic screening in tBLG and other moiré graphene multilayers. In the absence of external and internal electronic screening, I found the on-site Hubbard parameter of the flat bands of tBLG scales linearly with twist angle. Upon considering internal screening, this linear scaling breaks down, where the Hubbard interaction energy decreases more rapidly towards the magic angle owing to increased screening. Moreover, external screening, from proximity to metallic gates which dope tBLG, was found to substantially affect these Hubbard interactions, owing to the moiré length scale of the magic-angle being comparable to the distance to these metallic gates. For a sufficiently small separation to these gates, I predicted that the correlated insulating states should be screened-out and the superconducting phase should be stabilised. Long-ranged Hartree interactions were found to induced doping-dependent band-flattening in tBLG that I predicted to increase the magic-angle range of tBLG. For moiré graphene multilayers, the role of these Hartree interactions were found to sensitively depend on the stacking sequence of the structure: systems with alternating twist angles have similar interaction-driven band flattening, but systems where there are also adjacent layers that are aligned have no such interaction-driven band flattening.Open Acces

    Computational investigations of thermoelectric properties of lead telluride, magnesium silicide, and magnesium stannide under high pressure and anisotropic stress

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    I present a comprehensive picture of the electrical transport properties for PbTe, Mg2Si, and Mg2Sn under high pressure and stress. The electronic band structure and thermoelectric properties of p- and n-doped lead telluride, PbTe, were calculated within the framework of density functional theory, as implemented in Wien2k, and Boltzmann transport theory, as implemented in the BoltzTraP code. Thermoelectric properties were calculated as a function of hydrostatic pressure, as well as uniaxial stress along several crystal directions. Signicant enhancements of the thermoelectric power factor can be obtained by using anisotropic stress, and the underlying changes in the electronic band structure are analysed. We identify two key eects in PbTe: redistribution of carriers between dierent carrier pockets, each of which has anisotropic transport properties; and reshaping of the carrier pockets themselves. Individual carrier pockets of PbTe have anisotropic transport properties. Stress along [111] is shown to redistribute dopant charges between the carrier pockets. Due to this eect, compressive [111] stress improves the thermoelectric power factor along [111] of p-PbTe, and tensile [111] stress improves the power factor along [111] of n-PbTe. Compressive stress along [001], which causes an intervalleytransfer-eect, is shown to reshape the valence band of PbTe, improving the p-type power factor along [001]. Along with specic results for PbTe, general rules are presented for identifying other materials which could have their thermoelectric power factor increased by stress along specic directions. Guided by these rules, the thermoelectric properties of magnesium silicide and magnesium stannide under strain are presented. The same key eects, intervalley-transfer and valley-reshaping, are observed. The thermoelectric power factors of Mg2Si, and Mg2Sn are shown to be greatly improved by anisotropic strain
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