# nIXS $L_{2,3}$

Besides low energy transitions nIXS can be used as a core level spectroscopy technique. One then measures resonances with non-resonant inelastic x-ray scattering .

This tutorial uses Radial wave functions in order to calculate the length of q dependence. You can download them in a zip file here nio_data.zip. Please unpack this file and make sure to have the folders NiO_Experiment and NiO_Radial in the same folder as you do the calculations.

The input script:

NIXS_L23.Quanty
-- using inelastic x-ray scattering one can not only measure low energy excitations,
-- but equally well core to core transitions. This allows one to probe for example
-- 3p to 3d transitions using octupole operators.

-- We use the definitions of all operators and basis orbitals as defined in the file
-- include and can afterwards directly continue by creating the Hamiltonian
-- and calculating the spectra

dofile("Include.Quanty")

-- The parameters and scheme needed is the same as the one used for XAS

-- We follow the energy definitions as introduced in the group of G.A. Sawatzky (Groningen)
-- J. Zaanen, G.A. Sawatzky, and J.W. Allen PRL 55, 418 (1985)
-- for parameters of specific materials see
-- A.E. Bockquet et al. PRB 55, 1161 (1996)
-- After some initial discussion the energies U and Delta refer to the center of a configuration
-- The L^10 d^n   configuration has an energy 0
-- The L^9  d^n+1 configuration has an energy Delta
-- The L^8  d^n+2 configuration has an energy 2*Delta+Udd
--
-- If we relate this to the onsite energy of the L and d orbitals we find
-- 10 eL +  n    ed + n(n-1)     U/2 == 0
--  9 eL + (n+1) ed + (n+1)n     U/2 == Delta
--  8 eL + (n+2) ed + (n+1)(n+2) U/2 == 2*Delta+U
-- 3 equations with 2 unknowns, but with interdependence yield:
-- ed = (10*Delta-nd*(19+nd)*U/2)/(10+nd)
-- eL = nd*((1+nd)*Udd/2-Delta)/(10+nd)
--
-- For the final state we/they defined
-- The 2p^5 L^10 d^n+1 configuration has an energy 0
-- The 2p^5 L^9  d^n+2 configuration has an energy Delta + Udd - Upd
-- The 2p^5 L^8  d^n+3 configuration has an energy 2*Delta + 3*Udd - 2*Upd
--
-- If we relate this to the onsite energy of the p and d orbitals we find
-- 6 ep + 10 eL +  n    ed + n(n-1)     Udd/2 + 6 n     Upd == 0
-- 6 ep +  9 eL + (n+1) ed + (n+1)n     Udd/2 + 6 (n+1) Upd == Delta
-- 6 ep +  8 eL + (n+2) ed + (n+1)(n+2) Udd/2 + 6 (n+2) Upd == 2*Delta+Udd
-- 5 ep + 10 eL + (n+1) ed + (n+1)(n)   Udd/2 + 5 (n+1) Upd == 0
-- 5 ep +  9 eL + (n+2) ed + (n+2)(n+1) Udd/2 + 5 (n+2) Upd == Delta+Udd-Upd
-- 5 ep +  8 eL + (n+3) ed + (n+3)(n+2) Udd/2 + 5 (n+3) Upd == 2*Delta+3*Udd-2*Upd
-- 6 equations with 3 unknowns, but with interdependence yield:
-- epfinal = (10*Delta + (1+nd)*(nd*Udd/2-(10+nd)*Upd) / (16+nd)
-- edfinal = (10*Delta - nd*(31+nd)*Udd/2-90*Upd) / (16+nd)
-- eLfinal = ((1+nd)*(nd*Udd/2+6*Upd)-(6+nd)*Delta) / (16+nd)
--
--
--
-- note that ed-ep = Delta - nd * U and not Delta
-- note furthermore that ep and ed here are defined for the onsite energy if the system had
-- locally nd electrons in the d-shell. In DFT or Hartree Fock the d occupation is in the end not
-- nd and thus the onsite energy of the Kohn-Sham orbitals is not equal to ep and ed in model
-- calculations.
--
-- note furthermore that ep and eL actually should be different for most systems. We happily ignore this fact
--
-- We normally take U and Delta as experimentally determined parameters

-- number of electrons (formal valence)
nd = 8
-- parameters from experiment (core level PES)
Udd     =  7.3
Upd     =  8.5
Delta   =  4.7
-- parameters obtained from DFT (PRB 85, 165113 (2012))
F2dd    = 11.14
F4dd    =  6.87
F2pd    =  6.67
G1pd    =  4.92
G3pd    =  2.80
tenDq   =  0.56
tenDqL  =  1.44
Veg     =  2.06
Vt2g    =  1.21
zeta_3d =  0.081
zeta_2p = 11.51
Bz      =  0.000001
H112    =  0

ed      = (10*Delta-nd*(19+nd)*Udd/2)/(10+nd)
eL      = nd*((1+nd)*Udd/2-Delta)/(10+nd)

epfinal = (10*Delta + (1+nd)*(nd*Udd/2-(10+nd)*Upd)) / (16+nd)
edfinal = (10*Delta - nd*(31+nd)*Udd/2-90*Upd) / (16+nd)
eLfinal = ((1+nd)*(nd*Udd/2+6*Upd) - (6+nd)*Delta) / (16+nd)

F0dd    = Udd + (F2dd+F4dd) * 2/63
F0pd    = Upd + (1/15)*G1pd + (3/70)*G3pd

Hamiltonian =  F0dd*OppF0_3d + F2dd*OppF2_3d + F4dd*OppF4_3d + zeta_3d*Oppldots_3d + Bz*(2*OppSz_3d + OppLz_3d) + H112 * (OppSx_3d+OppSy_3d+2*OppSz_3d)/sqrt(6) + tenDq*OpptenDq_3d + tenDqL*OpptenDq_Ld + Veg * OppVeg + Vt2g * OppVt2g + ed * OppN_3d + eL * OppN_Ld

XASHamiltonian =  F0dd*OppF0_3d + F2dd*OppF2_3d + F4dd*OppF4_3d + zeta_3d*Oppldots_3d + Bz*(2*OppSz_3d + OppLz_3d)+ H112 * (OppSx_3d+OppSy_3d+2*OppSz_3d)/sqrt(6) + tenDq*OpptenDq_3d + tenDqL*OpptenDq_Ld + Veg * OppVeg + Vt2g * OppVt2g + edfinal * OppN_3d + eLfinal * OppN_Ld + epfinal * OppN_2p + zeta_2p * Oppcldots + F0pd * OppUpdF0 + F2pd * OppUpdF2 + G1pd * OppUpdG1 + G3pd * OppUpdG3

-- we now can create the lowest Npsi eigenstates:
Npsi=3
-- in order to make sure we have a filling of 8 electrons we need to define some restrictions
StartRestrictions = {NF, NB, {"000000 00 1111111111 0000000000",8,8}, {"111111 11 0000000000 1111111111",18,18}}

psiList = Eigensystem(Hamiltonian, StartRestrictions, Npsi)
oppList={Hamiltonian, OppSsqr, OppLsqr, OppJsqr, OppSx_3d, OppLx_3d, OppSy_3d, OppLy_3d, OppSz_3d, OppLz_3d, Oppldots_3d, OppF2_3d, OppF4_3d, OppNeg_3d, OppNt2g_3d, OppNeg_Ld, OppNt2g_Ld, OppN_3d}

-- print of some expectation values
print("  #    <E>      <S^2>    <L^2>    <J^2>    <S_x^3d> <L_x^3d> <S_y^3d> <L_y^3d> <S_z^3d> <L_z^3d> <l.s>    <F>   <F>   <Neg^3d> <Nt2g^3d><Neg^Ld> <Nt2g^Ld><N^3d>");
for i = 1,#psiList do
io.write(string.format("%3i ",i))
for j = 1,#oppList do
expectationvalue = Chop(psiList[i]*oppList[j]*psiList[i])
io.write(string.format("%8.3f ",expectationvalue))
end
io.write("\n")
end

-- in order to calculate nIXS we need to determine the intensity ratio for the different multipole intensities
-- ( see PRL 99, 257401 (2007) for the formalism )
-- in short the A^2 interaction is expanded on spherical harmonics and Bessel functions
-- The 3d Wannier functions are expanded on spherical harmonics and a radial wave function
-- For the radial wave-function we calculate <R(r) | j_k(q r) | R(r)>
-- which defines the transition strength for the multipole of order k

-- The radial functions here are calculated for a Ni 2+ atom and stored in the folder NiO_Radial
-- more sophisticated methods can be used

-- order of functions
-- r	1S	2S	2P	3S	3P	3D
Rnl = {}
for line in file:lines() do
RnlLine={}
for i in string.gmatch(line, "%S+") do
table.insert(RnlLine,i)
end
table.insert(Rnl,RnlLine)
end

-- some constants
a0      =  0.52917721092
Rydberg = 13.60569253
Hartree = 2*Rydberg

-- pd transitions from 2p (index 4 in Rnl) to 3d (index 7 in Rnl)
-- <R(r) | j_k(q r) | R(r)>
function RjRpd (q)
Rj1R = 0
Rj3R = 0
dr = Rnl-Rnl
r0 = Rnl-2*dr
for ir = 2, #Rnl, 1 do
r = r0 + ir * dr
Rj1R = Rj1R + Rnl[ir] * math.SphericalBesselJ(1,q*r) * Rnl[ir] * dr
Rj3R = Rj3R + Rnl[ir] * math.SphericalBesselJ(3,q*r) * Rnl[ir] * dr
end
return Rj1R, Rj3R
end

-- the angular part is given as C(theta_q, phi_q)^* C(theta_r, phi_r)
-- which is a potential expanded on spherical harmonics
function ExpandOnClm(k,theta,phi,scale)
ret={}
for m=-k, k, 1 do
table.insert(ret,{k,m,scale * math.SphericalHarmonicC(k,m,theta,phi)})
end
return ret
end

-- define nIXS transition operators
function TnIXS_pd(q, theta, phi)
Rj1R, Rj3R = RjRpd(q)
k=1
A1 = ExpandOnClm(k, theta, phi, I^k*(2*k+1)*Rj1R)
T1 = NewOperator("CF", NF, IndexUp_3d, IndexDn_3d, IndexUp_2p, IndexDn_2p, A1)
k=3
A3 = ExpandOnClm(k, theta, phi, I^k*(2*k+1)*Rj3R)
T3 = NewOperator("CF", NF, IndexUp_3d, IndexDn_3d, IndexUp_2p, IndexDn_2p, A3)
T = T1+T3
T.Chop()
return T
end

-- q in units per a0 (if you want in units per A take 5*a0 to have a q of 5 per A)
q=9.0

print("for q=",q," per a0 (",q / a0," per A) The ratio of k=1 and k=3 transition strength is:", RjRpd(q))

-- define some transition operators
qtheta=0
qphi=0
Tq001 = TnIXS_pd(q,qtheta,qphi)

qtheta=Pi/2
qphi=Pi/4
Tq110 = TnIXS_pd(q,qtheta,qphi)

qtheta=math.acos(math.sqrt(1/3))
qphi=Pi/4
Tq111 = TnIXS_pd(q,qtheta,qphi)

qtheta=math.acos(math.sqrt(9/14))
qphi=math.acos(math.sqrt(1/5))
Tq123 = TnIXS_pd(q,qtheta,qphi)

-- calculate the spectra
nIXSSpectra = CreateSpectra(XASHamiltonian, {Tq001, Tq110, Tq111, Tq123}, psiList, {{"Emin",-10}, {"Emax",20}, {"NE",6000}, {"Gamma",1.0}})

-- print the spectra to a file
nIXSSpectra.Print({{"file","NiOnIXS_L23.dat"}});

-- a gnuplot script to make the plots
gnuplotInput = [[
set autoscale
set xtic auto
set ytic auto
set style line  1 lt 1 lw 1 lc rgb "#FF0000"
set style line  2 lt 1 lw 1 lc rgb "#0000FF"
set style line  3 lt 1 lw 1 lc rgb "#00C000"
set style line  4 lt 1 lw 1 lc rgb "#000000"
set style line  5 lt 1 lw 3 lc rgb "#808080"

set xlabel "E (eV)" font "Times,12"
set ylabel "Intensity (arb. units)" font "Times,12"

set out 'NiOnIXS_L23.ps'
set size 1.0, 0.3
set terminal postscript portrait enhanced color  "Times" 8

energyshift=857.6

plot "NiOnIXS_L23.dat" using ($1+energyshift):(-$9  -$11 -$13 +0.16) title '011' with lines ls  2,\
"NiOnIXS_L23.dat" using ($1+energyshift):(-$15 -$17 -$19 +0.11) title '111' with lines ls  3,\
"NiOnIXS_L23.dat" using ($1+energyshift):(-$21 -$23 -$25 +0.06) title '123' with lines ls  4,\
"NiOnIXS_L23.dat" using ($1+energyshift):(-$3   -$5 -$7 +0.01) title '001' with lines ls  1

]]

-- write the gnuplot script to a file
file = io.open("NiOnIXS_L23.gnuplot", "w")
file:write(gnuplotInput)
file:close()

-- call gnuplot to execute the script
os.execute("gnuplot NiOnIXS_L23.gnuplot")
-- transform to pdf and eps
os.execute("ps2pdf NiOnIXS_L23.ps  ; ps2eps NiOnIXS_L23.ps  ;  mv NiOnIXS_L23.eps temp.eps  ; eps2eps temp.eps NiOnIXS_L23.eps  ; rm temp.eps")

The spectrum produced:

 nIXS for NiO looking at a $2p$ to $3d$ excitation The output is to standard out:

  #    <E>      <S^2>    <L^2>    <J^2>    <S_x^3d> <L_x^3d> <S_y^3d> <L_y^3d> <S_z^3d> <L_z^3d> <l.s>    <F>   <F>   <Neg^3d> <Nt2g^3d><Neg^Ld> <Nt2g^Ld><N^3d>
1   -3.395    1.999   12.000   15.147    0.000    0.000    0.000    0.000   -0.905   -0.280   -0.319   -1.043   -0.925    2.189    5.989    3.823    6.000    8.178
2   -3.395    1.999   12.000   15.147    0.000    0.000    0.000    0.000   -0.000   -0.000   -0.319   -1.043   -0.925    2.189    5.989    3.823    6.000    8.178
3   -3.395    1.999   12.000   15.147    0.000    0.000    0.000    0.000    0.905    0.280   -0.319   -1.043   -0.925    2.189    5.989    3.823    6.000    8.178
for q=	9	 per a0 (	17.007535121086	 per A) The ratio of k=1 and k=3 transition strength is:	0.081284239649905	0.04426369559805

\lstinputlisting[style=output]{../../Example_and_Testing/History/include/Tutorials/40_NiO_Ligand_Field/49_NIXS_L23.out}

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