Non-Uniform Density of Up States in First Neighbor Tight-Binding Model

Hello,

I am trying to calculate the density of up-spin particles in an hexagonal chain with 6 sites considering only first-neighbor tight binding interactions.

When I set the chain to have one electron-up only, the density is equally distributed between all 6 sites. However, when more electrons are added, the density is no longer uniform, regardless of the DMRG parameters I choose (maxm, niter, cutoff, noise and number of sweeps). This is very weird, as I don't see any reason the density shouldn't be the same in all sites.

Also, the results obtained can change depending on the position I choose to initialize the electrons. For instance, if I choose

state1.set(1,"Up");
state1.set(2,"Up");


the density of "up" states in each site is different than in the case

state1.set(1,"Up");
state1.set(6,"Up");


Here is an example of my complete code for the case with two electrons. Am I doing something wrong? Isn't the expected behavior that electrons should be evenly distributed between all sites?

#include "itensor/all.h"
#include <iostream>
#include <fstream>

using namespace itensor;

int main()
{

int N = 6;
double t = 1.0;
int nsweeps = 20;
int maxm = 200;
double cutoff = 1E-15;
int niter = 2;
double noise = 1E-5;

auto sites = Hubbard(N);

auto ampo = AutoMPO(sites);

//Tight-binding: first neighbors in the linear chain (sites 1-2 and 5-6)
for(int j = 1; j <= N-1; j+=4)
{
ampo += -t,"Cdagup",j,"Cup",j+1;
ampo += -t,"Cdagup",j+1,"Cup",j;
ampo += -t,"Cdagdn",j,"Cdn",j+1;
ampo += -t,"Cdagdn",j+1,"Cdn",j;
}

//Tight-binding: second neighbors in the linear chain (sites 1-3, 2-4, 3-5, 4-6)
for(int j=1; j <= N-2; j+=1)
{
ampo += -t,"Cdagup",j,"Cup",j+2;
ampo += -t,"Cdagup",j+2,"Cup",j;
ampo += -t,"Cdagdn",j,"Cdn",j+2;
ampo += -t,"Cdagdn",j+2,"Cdn",j;
}

//Use IQMPO or MPO to generate the hamiltonian
auto H = IQMPO(ampo);

//Define initial wavefunction MPS for IQMPS
auto state1 = InitState(sites);

state1.set(1,"Up");
state1.set(2,"0");
state1.set(3,"0");
state1.set(4,"0");
state1.set(5,"0");
state1.set(6,"Up");

//Create IQMPS
auto psi = IQMPS(state1);

//Define DMRG sweeps
auto sweeps = Sweeps(nsweeps);
sweeps.maxm() = maxm/20,maxm/10,maxm/2,maxm/2,maxm;
sweeps.cutoff() = cutoff;
sweeps.niter() = niter;
sweeps.noise() = noise;

auto energy = dmrg(psi,H,sweeps,{"Quiet",true});

//Calculating density in each site

std::ofstream file1;
file1.open ("nup.txt");

for(int ii = 1; ii <= N; ++ii)
{
auto Nup = sites.op("Nup",ii);
psi.position(ii);
ITensor ket = psi.A(ii);
ITensor bra = dag(prime(ket,Site));
auto nup = (bra*Nup*ket).real();
file1 << nup << std::endl;
}

file1.close();

return 0;
} //End of main


P.S.: Since I am working with an hexagon, I needed to number each site and displace them in a linear chain in order to conduct the calculations, which ended up creating second neighbor interactions in the linear chain. That is why the code includes first and second neighbors interactions for the linear chain. To make things clearer, here is how I chose to number the sites in the hexagon:

              3
/       \
1         5
|         |
2         6
\       /
4


Hi rmagaldi,
Thanks for the question. So I think I may have figured out what's going wrong. The good news is that it does not appear to be the DMRG optimization itself, but the definition of the Hamiltonian.

What I did to test this is to observe that your system is just a periodic ring, so one can make the Hamiltonian in a simpler way:

for(int j = 1; j <= N-1; j+=1)
{
ampo += -t,"Cdagup",j,"Cup",j+1;
ampo += -t,"Cdagup",j+1,"Cup",j;
ampo += -t,"Cdagdn",j,"Cdn",j+1;
ampo += -t,"Cdagdn",j+1,"Cdn",j;
}
ampo += +t,"Cdagup",1,"Cup",N;
ampo += +t,"Cdagup",N,"Cup",1;
ampo += +t,"Cdagdn",1,"Cdn",N;
ampo += +t,"Cdagdn",N,"Cdn",1;


This is just a nearest-neighbor hopping with an additional "long bond" connecting site 1 to site N. Note that the coupling has the opposite sign for the long bond. Choosing this sign gives a uniform density, regardless of the initial state. Choosing a -t coupling gives the behavior you reported.

So probably if one works through this carefully, the reason the +t is the correct coupling is because really one should think of the long bond as going from N to 1 with a -t, which is equivalent to going from 1 to N with a +t, due to the fact that C and Cdagger anti commute.

Hope that helps!

Miles

commented by (120 points)
Hello Miles,