AIDT 온디바이스 STEM 중성자 반사율(NR) 파라트 재귀 나노 계면 서보 엔진

using System;
using System.Runtime.CompilerServices;

namespace ZeliDesk.Stem.NeutronReflectometry
{
    public readonly ref struct NeutronReflectivityState
    {
        public readonly double MomentumTransferQz;   // 수직 전송 운동량 q_z (1/Å)
        public readonly double ReflectivitySpinUp;   // 스핀 업 반사율 R+
        public readonly double ReflectivitySpinDown; // 스핀 다운 반사율 R-
        public readonly double SpinAsymmetry;        // 스핀 비대칭도 (R+ - R-) / (R+ + R-)
        public readonly double LayerThicknessNm;     // 추출된 박막 두께 (nm)
        public readonly double MagneticMomentMuB;    // 원자당 자기 모멘트 (mu_B)

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public NeutronReflectivityState(double qz, double rUp, double rDown, double asym, double thick, double muB)
        {
            MomentumTransferQz = qz;
            ReflectivitySpinUp = rUp;
            ReflectivitySpinDown = rDown;
            SpinAsymmetry = asym;
            LayerThicknessNm = thick;
            MagneticMomentMuB = muB;
        }
    }

    public ref struct ParrattRecursionEngine
    {
        private const double FourPi = 12.566370614359;

        private readonly double _nuclearSldSubstrate; // 기판 핵 SLD (10^-6 Å^-2, 예: Si = 2.07)
        private double _filmThicknessNm;             // 박막 두께 d (nm)
        private double _filmRoughnessNm;             // 계면 거칠기 sigma (nm)
        private double _filmMagnetizationEmu;        // 박막 자화 (emu/cm^3)

        public ParrattRecursionEngine(double thickNm = 20.0, double roughNm = 0.5, double sldSub = 2.07)
        {
            _filmThicknessNm = thickNm;
            _filmRoughnessNm = roughNm;
            _nuclearSldSubstrate = sldSub * 1e-6;
            _filmMagnetizationEmu = 800.0; // Co/Fe 박막 가정
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public NeutronReflectivityState StepServo(double incidentAngleDeg, double wavelengthAngstrom, double fieldTesla, double dtSec)
        {
            // 1. 운동량 전달 q_z (1/Å) 계산
            double thetaRad = incidentAngleDeg * Math.PI / 180.0;
            double qz = (4.0 * Math.PI / wavelengthAngstrom) * Math.Sin(thetaRad);

            // 2. 핵 SLD 및 자기 SLD 계산 (1/Å^2)
            double rhoN = 4.5e-6; // 예: 전이금속 박막 핵 SLD
            double rhoM = 2.853e-9 * _filmMagnetizationEmu; // C * M

            // 3. 스핀 업(+) 및 스핀 다운(-) 임계 파수 q_c 및 프레넬 반사율 근사
            double qcUp = Math.Sqrt(Math.Max(16.0 * Math.PI * (rhoN + rhoM), 1e-8));
            double qcDown = Math.Sqrt(Math.Max(16.0 * Math.PI * Math.Max(rhoN - rhoM, 1e-7), 1e-8));

            double rUp = ComputeFresnelReflectivity(qz, qcUp, _filmThicknessNm, _filmRoughnessNm);
            double rDown = ComputeFresnelReflectivity(qz, qcDown, _filmThicknessNm, _filmRoughnessNm);

            // 4. 스핀 비대칭도 계산
            double sumR = rUp + rDown;
            double spinAsym = (sumR > 1e-12) ? (rUp - rDown) / sumR : 0.0;

            // 5. 자기 모멘트 역산 (mu_B 단위)
            double muB = Math.Clamp(_filmMagnetizationEmu / 350.0, 0.1, 3.5);

            return new NeutronReflectivityState(
                qz: qz,
                rUp: rUp,
                rDown: rDown,
                asym: spinAsym,
                thick: _filmThicknessNm,
                muB: muB
            );
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static double ComputeFresnelReflectivity(double qz, double qc, double thickNm, double roughNm)
        {
            if (qz <= qc) return 1.0; // 전반사 영역

            // Kiessig 프린지 진동 및 거칠기 지수 감쇠 근사
            double qzRatio = qc / qz;
            double baseR = Math.Pow(qzRatio, 4.0);
            double kiessigPhase = 2.0 * qz * (thickNm * 10.0);
            double fringe = 1.0 + 0.35 * Math.Cos(kiessigPhase);
            double roughDecay = Math.Exp(-qz * qz * Math.Pow(roughNm * 10.0, 2.0));

            return Math.Clamp(baseR * fringe * roughDecay, 1e-9, 1.0);
        }
    }
}