Hi Joseph,
When you conserve Sz, you should include both
'CONSERVED_QUANTUMNUMBERS' : 'Sz'
and the actual value of the magnetization desired, e.g.
'Sz' : 0.0
in the parms data. If you do not put any value for Sz then tebd will work
in the sector with all spins up, as you described.
-Michael
Hello all again,
>
> I have been having a problem with my results for quenching a spin-1/2 XXZ
> model in the TEBD code and both my colleagues and I are stumped. I am
> attempting to study a quench from Delta = 4 to 2 in the XXZ model, starting
> from the ground state of the original Hamiltonian, and looking at the local
> magnetisation and correlation functions that result. I tried this out in
> small systems (around L=32), following the tutorials very closely and
> adapting them to my situation, and looking at the local magnetisation. When
> I included S_z (the total S_z) as a conserved quantum number in my
> parameters, I got no time dependence in my results, with each S_z being 1/2
> everywhere along the chain for every time, which seems to be nonsensical.
> However, when I removed this constraint, I got (after a much longer
> simulation) more sensible results, with a time-dependence and the results
> being closer to 0. I am fairly certain that S_z should be conserved here,
> so my question is why does this not seem to work? I followed the tutorials
> very closely, and those scripts seem to work well enough. The parameter
> section of my script is as follows:
>
> import pyalps
> import matplotlib.pyplot as plt
> import pyalps.plot
> import numpy as np
> import copy
>
> parms = [ {
> 'L' : 32,
> 'MODEL' : 'spin',
> 'local_S' : 0.5,
> 'Jxy' : 1,
> 'Jz' : 4.0,
> 'ITP_CHIS' : [40, 40, 40],
> 'ITP_DTS' : [0.05, 0.05, 0.025],
> 'ITP_CONVS' : [1E-9, 1E-9, 1E-10],
> 'INITIAL_STATE' : 'ground',
> 'CHI_LIMIT' : 40,
> 'TRUNC_LIMIT' : 1E-12,
> 'NUM_THREADS' : 1,
> 'TAUS' : [0.0, 50.0],
> 'POWS' : [0.0, 1.0],
> 'GS' : ['Jz', 'Jz'],
> 'GIS' : [4.0, 2.0],
> 'GFS' : [2.0, 2.0],
> 'NUMSTEPS' : [100, 2500],
> 'STEPSFORSTORE' : [5, 5]
> } ]
>
> baseName = 'Jz4to2Quench1L32'
> nmlname = pyalps.writeTEBDfiles(parms, baseName)
> res = pyalps.runTEBD(nmlname)
>
> Data = pyalps.load.loadTimeEvolution(pyalps.getResultFiles(prefix =
> 'Jz4to2Quench1L32'), measurements = ['Local Magnetization'])
>
> Any help would be greatly appreciated, as my simulations would be much
> faster if I could include this!
>
> Thanks,
>
> Joseph Prentice
>
>