Lieb–Liniger model

The Lieb–Liniger model describes a gas of particles moving in one dimension and satisfying Bose–Einstein statistics.

Sometimes it is called a one dimensional Bose gas with delta interaction. It also can be considered as quantum non-linear Schrödinger equation.

Definition and solution of the model

There are $N$ boson particles with coordinates $x$ on the line $[0,L]$, with periodic boundary conditions. Thus, a state of the N-body system must be described by a wave function $\psi (x_{1},x_{2},\dots ,x_{j},\dots ,x_{N})$ that remains unchanged under permutation of any two particles (permutation symmetry), i.e., $\psi (\dots ,x_{i},\dots ,x_{j},\dots )=\psi (\dots ,x_{j},\dots ,x_{i},\dots )$ for all $i\neq j$ and $\psi$ satisfies $\psi (\dots ,x_{j}=0,\dots )=\psi (\dots ,x_{j}=L,\dots )$ for all $j$. The Hamiltonian, in appropriate units, is

$H=-\sum _{j=1}^{N}\partial ^{2}/\partial x_{j}^{2}+2c\sum _{1\leq i<j\leq N}\delta (x_{i}-x_{j})\ ,$

where $\delta$ is the Dirac delta function. The constant $c\geq 0$ denotes its strength. The delta function gives rise to a boundary condition when two coordinates, say $x_{1}$ and $x_{2}$ are equal; this condition is that as $x_{2}\searrow x_{1}$, the derivative satisfies $\left.\left({\frac {\partial }{\partial x_{2}}}-{\frac {\partial }{\partial x_{1}}}\right)\psi (x_{1},x_{2})\right|_{x_{2}=x_{1}+}=c\psi (x_{1}=x_{2})$. The hard core limit $c=\infty$ is known as the Tonks–Girardeau gas.

Schrödinger's time independent equation, $H\psi =E\psi$ is solved by explicit construction of $\psi$. Since $\psi$ is symmetric it is completely determined by its values in the simplex ${\mathcal {R}}$, defined by the condition that $0\leq x_{1}\leq x_{2}\leq \dots ,\leq x_{N}\leq L$.

$\psi (x_{1},\dots ,x_{N})=\sum _{P}a(P)\exp \left(i\sum _{j=1}^{N}k_{Pj}x_{j}\right)$

where the sum is over all $N!$ permutations, $P$, of the integers $1,2,\dots ,N$, and $P$ maps $1,2,\dots ,N$ to $P_{1},P_{2},\dots ,P_{N}$. The coefficients $a(P)$, as well as the $k$'s are determined by the condition $H\psi =E\psi$, and this leads to

$E=\sum _{j=1}^{N}\,k_{j}^{2}$

$a(P)=\prod _{1\leq i<j\leq N}\left(1+{\frac {ic}{k_{Pi}-k_{Pj}}}\right)\,.$

These equations determine $\psi$ in terms of the $k$'s. These lead to $N$ equations:

$L\,k_{j}=2\pi I_{j}\ -2\sum _{i=1}^{N}\arctan \left({\frac {k_{j}-k_{i}}{c}}\right)\qquad \qquad {\text{for }}j=1,\,\dots ,\,N\ ,$

where $I_{1}<I_{2}<\cdots <I_{N}$ are integers when $N$ is odd and, when $N$ is even, they take values $\pm {\frac {1}{2}},\pm {\frac {3}{2}},\dots$ . For the ground state the $I$'s satisfy

$I_{j+1}-I_{j}=1,\quad {\rm {for}}\ 1\leq j<N\qquad {\text{and }}I_{1}=-I_{N}.$

Thermodynamic limit

Fig. 1: The ground state energy, from. See text.

External links

• See also Elliott H. Lieb (2008), Scholarpedia, 3(12):8712.[1]