Spin Seebeck Effect as a Probe for Majorana Fermions in Kitaev Spin Liquids

Quantum entanglement in strongly correlated electron systems often leads to exotic elementary excitations. Quantum spin liquids (QSLs) provide a paradigmatic example, where the elementary excitations are described by fractional quasiparticles such as spinons. However, such fractional quasiparticles behave differently from electrons, making their experimental identification challenging. Here, we theoretically investigate the spin Seebeck effect, which is a thermoelectric response via a spin current, as an efficient probe of the fractional quasiparticles in QSLs, focusing on the Kitaev honeycomb model. By comprehensive studies using the real-time dynamics, the perturbation theory, and the linear spin-wave theory based on the tunnel spin-current theory, we find that the spin current is induced by thermal gradient in the Kitaev spin liquid, via the low-energy fractional Majorana excitations. This underscores the ability of Majorana fermions to carry spin current, despite lacking spin angular momentum. Furthermore, we find that the induced spin current changes its sign depending on the sign of the Kitaev interaction, indicating that the Majorana fermions contribute to the spin current with (up-)down-spin like nature when the exchange coupling is (anti)ferromagnetic. Thus, in contrast to the negative spin current already found in a one-dimensional QSL, our finding reveals that the spin Seebeck effect can exhibit either positive or negative signals, contingent upon the nature of fractional excitations in the QSLs. We also clarify contrasting field-angle dependence between the Kitaev spin liquid in the low-field limit and the high-field ferromagnetic state, which is useful for the experimental identification. Our finding suggests that the spin Seebeck effect could be used not only to detect fractional quasiparticles emerging in QSLs but also to generate and control them.Comment: 16 pages, 10 figure

Short-Range Structures of Amorphous and Liquid Iron and Pd_<0.8>-Si_<0.2> Alloy

A comparison has been made on the short-range structures of iron and Pd_-Si_ alloy between the amorphous and the liquid states. It is suggested that the short-range structure of amorphous Fe film has higher degree of ordering in comparison to liquid Fe. The atomic configuration in the nearest neighbour in amorphous Pd_-Si_ alloy is close to that in Pd_3Si crystal and is not completely reproduced by the Percus-Yevick Hard Sphere model with high packing fraction corresponding to instantaneous freezing of the liquid structure

Computational Investigation and Experimental Verification of Multiplicity Counting from the Continuous Signals of Fission Chambers

In a series of previous publications, we suggested an alternative method to the pulse-counting based multiplicity counting technique for the characterisation of special nuclear materialscollision number expansion. The new method uses the continuous signals of fission chambers, and the multiplicity rates, i.e. the singles, doubles and triples rates are extracted from the auto- and cross-covariances of one or more fission chambers. Until recently only the theory of the method was elaborated. The purpose of the work described in this report was to verify the method and investigate its performance and applicability through detailed simulations as well as with a dedicated experiment. Numerical simulations of the method were performed by a code specially developed for this study, and pilot measurements were performed at the critical assembly KUCA of the Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS). This report gives an account of both the work performed and the results of the study

High-resolution seismic reflection profiling across the surface rupture associated with the 2004 Mid-Niigata Prefecture earthquake, central Japan : data acquisition and processing

The 200.4 Mid-Niigata Prefecture earthquake (Mj 6.8) generated surface ruptures along the eastern rim of the Uonuma hills. To reveal the relationship between a seismogenic source fault and surface ruptures, shallow, high-resolution seismic reflection profiling was undertaken across the surface ruptures and the active faults. The seismic source was a mini-vibrator and seismic data were recorded by a digital telemetry system. The source and receiver interval was 10 m4 The seismic data were processed using conventional CMP seismic reflection methods. The resultant depth-converted seismic section portrays an emergent thrust beneath the surface rupture associated with the Mid-Niigata Prefecture earthquake

Nuclear Reactor Physics Experiments

First published in 2010, Reprinted 2018Preface [iv]List of Contributors [vi]Introduction to Kyoto University Critical Assembly (KUCA)1. General Description of KUCA Facility [1]1-1 KUCA Facility [1]1-2 Solid-Moderated-Cores (A- and B-cores) [2]1-3 Light-Water-Moderated Core (C-core) [3]1-4 Pulsed Neutron Source [4]1-5 Control Room [5]2. Details of the Light-Water-Moderated Core (C-core) [5]2-1 Overall Structure [5]2-2 Core Tank and Grid Plate [6]2-3 Fuel Plate and Fuel Frame [6]2-4 Core Configuration [10]Chapter 1 Approach to Criticality1-1 Fission Chain Reaction, Neutron Multiplication, and Approach to Criticality [13]1-1-1 Fission Chain Reaction [13]1-1-2 Neutron Multiplication [15]1-1-3 Inverse Count Rate and Approach to Criticality [16]1-2 Experiments [18]1-2-1 Neutron Detectors [18]1-2-2 Actual Procedure of Experiments [18]1-2-3 Determination of Infinite Reflector Thickness [22]1-3 Discussion [24]1-4 Preparatory Report [25]1-4-1 Numerical Simulation of Approach to Criticality Experiment [25]1-4-2 Two-Energy-Group Diffusion Calculation of Reflected Reactor [29]Appendix 1 [33]1A. Analytical Solution of Two-Energy-Group Diffusion Equation [33]1B. Solution for Core Region [34]1C. Solution for Reflector Region [36]1D. Determination of Critical Core Size [38]1E. Neutron Flux Distribution [40]Chapter 2 Control Rod Calibration2-1 Purpose [43]2-2 Principle [44]2-2-1 Reactor Kinetic Equation [44]2-2-2 Positive Period Method [45]2-2-3 Control Rod Drop Method [48]2-2-4 Compensation Method [50]2-3 Experiments [50]2-3-1 Core Configuration [50]2-3-2 Period Method Experiment [52]2-3-3 Rod Drop Method Experiment [52]2-4 Discussion [52]2-5 Preparatory Report [53]Appendix 2 [55]2A. Neutron Lifetime [55]2B. Delayed Neutron Data and Basic Parameters of KUCA [55]2C. First-Order Perturbation Theory [56]2D. Control Rod Calibration Curve [58]Chapter 3 Measurement of Reaction Rate3-1 Purpose [61]3-2 Principle [62]3-2-1 Features of Neutron Activation Detector [62]3-2-2 Measurement of Neutron Flux Using Activation Detector [63]3-2-3 Measurement of Radioactivity Using Gold (Au) Activation Foil [67]3-2-4 Detection Efficiency [69]3-3 Activation Reaction Rate Contributed by Thermal Neutron Flux [70]3-3-1 Neutron Spectrum in Reactor Core [70]3-3-2 Activation Reaction Rate Contributed by Thermal Neutrons [72]3-4 Experiments [81]3-4-1 Core Configuration [81]3-4-2 Equipment and Irradiation of Gold Wires and Foils [81]3-4-3 Measurement of Radiation of Gold Wires and Foils [83]3-5 Discussion [88]3-6 Preparatory Report [90]Appendix 3 [95]3A. Activation Reaction Rate by 4πβ-γ Coincidence Method [95]3A-1 Principle of 4πβ-γ Coincidence Method [95]3A-2 Absolute Measurement by 4πβ-γ Coincidence Method [97]3B. Outline of the HPGe Detector [98]Chapter 4 Feynman-α Method4-1 Purpose [101]4-2 Variance-to-Mean Ratio in Multiplication System [102]4-2-1 Decay Constant α [103]4-2-2 Y Value Expressed by Reactivity [104]4-2-3 Y Value [104]4-2-4 Asymptotic Behavior of Y Value [104]4-2-5 Y Value in a Critical System by Delayed Neutrons [105]4-2-6 Relationship between Power and Y Value [106]4-3 Experiments [106]4-3-1 Experimental Equipment [106]4-3-2 Experimental Methods [108]4-3-3 Data Processing [109]4-4 Discussion [110]Appendix 4 [113]4A. Derivation of Equations for Feynman-α Method [113]4A-1 Steady State [116]4A-2 Consideration of Delayed Neutrons [118]4A-3 Initial Correlation Correction, Spatial Dependence, and Fission Counter [118]Chapter 5 Pulsed Neutron Source Method5-1 Purpose [119]5-2 Principle [119]5-3 Experimental Equipment [126]5-4 Experimental Methods [126]5-5 Data Processing [127]5-6 Discussion [128

Theory of Feynman-alpha technique with masking window for accelerator-driven systems

Recently, a modified Feynman-alpha technique for the subcritical system driven by periodically triggered neutron bursts was developed. One of the main features of this technique is utilization of a simple formula that is advantageous in evaluating the subcriticality. However, owing to the absence of the theory of this technique, this feature has not been fully investigated yet. In the present study, a theory of this technique is provided. Furthermore, the experimental conditions under which the simple formula works are discussed to apply this technique to the subcriticality monitor for the accelerator-driven system

Determination of prompt neutron decay constant by time-domain fluctuation analyses of detector current signals

The conventional time-domain reactor noise techniques analyse the number of neutron detector pulse signals, so that they sometimes encounter serious difficulties owing to the count-loss effect due to the dead time of detector systems. To avoid all the difficulties coming from the count-loss effect, a novel time-domain technique was recently proposed (P\ue1l and P\ue1zsit, 2015). This technique analyses the auto-covariance function of continuous current signals arising from ionization chambers such as the fission chamber, so that it is inherently insensitive to the count-loss effect. In the present study, a different time-domain technique that analyses the integral values of current signals is proposed. With regard to these two techniques, the experimental conditions under which they successfully measure the subcriticality through determination of the prompt neutron decay constant are clarified

Accuracy of reaction rates in the accelerator-driven system with 14MeV neutrons at the Kyoto University Critical Assembly

Reaction rate experiments on the accelerator-driven system (ADS) are conducted by combining a critical assembly of a solid-moderated and -reflected core with a pulsed neutron generator. Neutrons (14 MeV) generated from the accelerator are injected into a subcritical system and the reaction rates are measured by the foil activation method to obtain neutronic spectrum data. The numerical calculations are executed by MCNPX with ENDF/B-VI.8, JENDL-3.3 and JENDL/D-99 libraries to evaluate the reaction rates of activation foils set in the center of the core. For the ADS experiments with 14 MeV neutrons, the C/E values between the experiments and the calculations are found to be well within the relative difference of about 30% in all foils up to subcriticality 1.05%Δk/k. The reaction rates do not depend on the subcriticality level in cases of [115]In, [56]Fe (purity 99.99%), [27]Al, whereas subcriticality dependence is observed in [93]Nb. In the critical experiments carried out in the A, B and C cores, special mention should be made of the remarkable effect of the composition rate of [56]Fe material. Thus a remarkable improvement is observed in the accuracy of experimental and numerical reaction rates, demonstrating the importance of material impurity for subcritical experiments

Observation of ferrotoroidic domains in a metal

ISSN:1098-0121ISSN:0163-1829ISSN:1550-235XISSN:0556-2805ISSN:2469-9969ISSN:1095-3795ISSN:2469-995