Dear Michele,

Defining the product operator c_up(x)*bdag(x) as a SITEOPERATOR also leads to an incorrect output. But defining it in the SITEBASIS does work, so I was able to compute the correlation function as needed.

Thank you for the tip.

Best regards, Michael

On Tue, 9 Feb 2016 10:13:05 +0100 Michele Dolfi dolfim@phys.ethz.ch wrote:

Dear Michael,

Could you try defining the operator c_up(x)*bdag(x) as a SITEOPERATOR and use it in the BONDOPERATOR? Or even defining it in the basis.

Best, Michele

-- ETH Zurich Michele Dolfi Institute for Theoretical Physics HIT G 32.4 Wolfgang-Pauli-Str. 27 8093 Zurich Switzerland

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...

On Feb 6, 2016, at 2:08 PM, Michael Pasek michael.pasek@univ-paris-diderot.fr wrote:

Dear all,

I am trying to compute the correlation of two bond operators as follows:

p['MEASURE_CORRELATIONS[custom op]'] = 'cppdag:cpp'

where cpp and cppdag are define in mymodel.xml as:

<BONDOPERATOR name="cpp" source="x" target="y"> c_up(x)*bdag(x)*c_up(y)*bdag(y) </BONDOPERATOR> <BONDOPERATOR name="cppdag" source="x" target="y"> b(x)*cdag_up(x)*b(y)*cdag_up(y) </BONDOPERATOR>

where bdag, b, cdag_up and c_up are the usual boson, fermion creation-annihilation operators. Disappointingly, in this case mps_optim only outputs a single number for the correlation measurement.

If I try instead to compute the correlation 'oppdag:opp' with

<BONDOPERATOR name="opp" source="x" target="y"> bdag(x)*c_up(y) </BONDOPERATOR> <BONDOPERATOR name="oppdag" source="x" target="y"> cdag_up(x)*b(y) </BONDOPERATOR>

... I get a 4x4 matrix (i,i+1,j,j+1) as expected. So the problem seems to come from computing the correlation of bond operators that are products of more than two site operators.

Is this measurement expected to fail with mps_optim, or am I doing something wrong?

Thank you for your help.

Michael