Solvable models for 2+1D quantum critical points: Loop soups of 1+1D conformal field theories

The manuscript presents an ingenious model in 2+1 dimensions, showing some excellent physics, which is understood in great depth using concepts ranging from quantum conformal field theories to the statistical physics of loops.

The model is finely tuned, probably far from the real world.

In the manuscript, the authors design a model of classical loops on the honeycomb lattice, where the loops are decorated with quantum degrees of freedom. Cleverly choosing the quantum Hamiltonian for the loop variables, they achieve longer loops to have lower ground state energy. At zero temperature, they end up with a manifold of long loops (a snake state) traversing the system. The quantum model for a loop is considered quantum-critical at a quantum phase transition between two gapped phases. The authors then study correlations of the decorated loops, find algebraic decay, and explain the measured exponents originating in the interplay of the quantum correlation and that of a self-avoiding walk. They also examine finite temperature behavior and quantum entanglement and provide a nice but involved argument for the area law they observed. It will be interesting to see what happens if the loops become quantum, with off-diagonal matrix elements reconfiguring them, like in the quantum dimer/ice model.

This is a nicely written manuscript, with careful numerics and clever theory (detailed explanations are given in appendices). I would like to recommend the acceptance for publication.

I only have a few primarily cosmetic comments listed below.

- In Eq. (1), I believe that the H_dec is symmetric, H_dec(1,2) = H_dec(2,1). It is worth mentioning explicitly.

- Regarding the style: It would be helpful if acronyms were in capitals, e.g., "dofs".

- In Fig 2, could the author also show the energies as a function of 1/L - it would be more informative.

- I would like to mention yet another paper with decorated critical loops (I leave it to the authors to decide whether to mention it): D. Poilblanc et al., Phys. Rev. B 75, 220503(R) (2007).

From the point of view of theoretical physics, creativity

From the point of view of material science, connection to a material.

This is a very well written and instructive paper that
I enjoyed reading. I recommend publication in SciPos essentially as is,
except for minor corrections.

The idea of this paper is to construct quantum-critical points in
(2+1)-dimensional spacetime as follows.

Step 1:
The nearest-neighbor ferromagnetic Ising Hamiltonian on the triangular lattice is the first ingredient. It is defined to be a sum of commuting projectors, one for each directed nearest-neighbor bond (direct bond)
of the triangular lattice, such that each projector selects with its
eigenvalue zero two parallel spins,
while it selects with its eigenvalue +1 two anti-parallel spins,
along the spin-1/2 quantization axis.
The ground states of this Hamiltonian are the twofold-degenerate
and gapped ferromagnetic ground states.

Step 2:
For each direct bond from the triangular lattice,
one defines a dual bond by demanding that the latter
is orthogonal to the former and they intersect through their midpoints.
The set of dual bonds defines the dual lattice, a honeycomb lattice.
Each site of the dual lattice hosts decoration degrees of freedom (dofs)
chosen to be spin-1/2 degrees of freedom.

Step 3:
Each projector from Step 1
is multiplied by a Hermitean polynomial
for the two dofs assigned to the dual bond attached to the direct bond
on which the projector from step 1 acts.
This is the quantum many-body Hamiltonian (1) defined in this paper.

Step 4:
Hamiltonian (1) can be rewritten as a sum over commuting Hermitean operators
$H_{1D}(l)$
labeled by the element $l$ belonging to the set $\mathcal{L}$
made of all non-crossing loops of the dual lattice,
whereby $H_{1D}(l)$ for each non-crossing loop $l$
is assigned the sum over the polynomials from step 3
for all the dual bonds whose union defines
the non-crossing loop $l$ from the dual lattice.
The subscript ``1D'' emphasizes that $H_{1D}(l)$ is defined on a
one-dimensional lattice made of $|l|$ sites, whereby
$|l|$ denotes the cardinality of the loop $l$.

Step 5:
It is assumed that
(i) the ground-state energy
$E_{1D;GS}(l)$ of $H_{1D}(l)$
is negative for any $l$ and
(ii) $H_{1D}(l)$ realizes a conformal field theory (CFT)
in the thermodynamic limit $|l|\to\infty$, i.e.,
$
\frac{E_{1D;GS}(l)}{|l|}=-|\epsilon_{0}|-\frac{\pi c_{l}}{3||l^{2}}
+\mathcal{|l|^{-3}}
$
Here, $c_{l}$ is a number of order one in powers of $|l|$
that can be negative for some values of $|l|\,\mathrm{mod}\, n$
with $n$ some given integer. The leading correction
$-\frac{\pi c_{l}}{3|l|^{2}}$
is called the Casimir energy.

The results from this paper are:

To leading order in the ground-state energy,
the ground state of Hamiltonian (1) is microscopically degenerate
as any loop covering of the dual lattice
that visits all sites of the dual lattice,
the so called subset of fully packed non-crossing loops from
$\mathcal{L}$, delivers a ground state.

When the Casimir energy is negative,
the degeneracy of the fully packed non-crossing loops is partially
lifted by selecting fully packed non-crossing loops made of
the shortest allowed closed loops. This gives as the ground state
an hexagonal solid that breaks spontaneously translation symmetry
and supports a gap.

When the Casimir energy is positive,
the degeneracy of the fully packed non-crossing loops is
partially lifted by selecting all non-crossing loop that
visits every sites (a snake). In the thermodynamic limit,
this gives rise to a (2+1)-dimensional quantum critical theory
with unusual scaling properties, as shown analytically
and numerical in the rest of the paper.

Several examples of
$H_{1D}(l)$
such that they can be tuned to (1+1)-dimensional criticality
with a positive Casimir energy are given.
In turn, the (2+1)-dimensional lattice Hamiltonian (1)
displays (2+1)-dimensional quantum criticality.

Zero temperature correlation functions are studied analytically and
numerically so as to extract scaling exponents. It is found that
the scaling exponent at the (2+1)-dimensional quantum critical
for the equal-time two-point correlation function of a
primary field from the (1+1)-dimensional quantum criticality
associated to $H_{1D}(l)$ is twice the value of the scaling exponent
of this primary field.

Finite temperature correlation functions are shown to be
sensitive to the order of limit $L\to\infty$ and $T\to0$ with $L$
the linear size of the triangular lattice and $T$ the temperature.
When $L\to\infty$ before $T\to0$,
$1/T$ plays the role of an infrared cutoff.
When $T\to0$ before $L\to\infty$, order by disorder selects
a stripe phase from $T>0$ to a critical temperature at which
a first-order transition into a snake phase takes place.

Whereas the two-point spatial correlation functions
of the primary fields of $H_{1D}(l)$
at the (2+1)-dimensional quantum critical point share
the same scaling exponents as those of $H_{1D}(l)$,
the bipartite entanglement of the two-dimensional soup of
(1+1)-dimensional CFTs obeys a non-local area law.

Comments:

1) This is a pure theory paper
in which a lattice Hamiltonian is proposed and studied.
The connection to the "real world'' is presented in the third
paragraph. Would the preprint
arXiv:2311.05004 [pdf, other] cond-mat.stat-mech
Fluctuation-induced spin nematic order in magnetic charge-ice
Authors: A. Hemmatzade, K. Essafi, M. Taillefumier, M. Müller,
T. Fennell, P. M. Derlet
not realize some three-dimensional version of this manuscript?

2) Why is there a summation over Ising configurations
on the right-hand side of Eq. (2)?

3) Typo:
A pair of parenthesis is missing on the right-hand side of Eq.\ (5).

4) In the first paragraph of the discussion of non-vanishing temperature,
there are three scaling relations, whereby the first two is supposed
to imply the third. Was there not a typo in one of the first two
scaling relations?

5) Typo:
"... the chain length module some integer, ..."
-> ^
"... the chain length modulo some integer, ..."
^

I read this interesting paper with great interest. The authors consider a system of two types of degrees of freedom: a system A which is an antiferromagnetic Ising model on the triangular lattice at zero temperature (a well known fully frustrated system with an extensive grind state degeneracy) and a system B which is to a quantum system of spin-1/2 spins defined o the dual (hexagonal) lattice. The authors study several models with the property that the two systems couple only across the domain walls of system A (which enumerate the macroscopic ground state degeneracy). In this fashion the domain walls become "decorated" by the degrees of freedom of system B. The result is an ensemble of domain walls with quantum mechanical degrees of freedom residing on them. They investigate the nature of the phases of this interesting system. They find that different types of local interactions lead to either a phase in which the loops ("decorated domain walls") are small (a "snake phase") or a phase in which they are long and for stripe like patterns. As far as I can tell the models that they considered the decorated loops do not interact with each other although are self-avoiding. However interactions between loops can alter the physics. For instance one can imagine that short-range interactions would lead to a crystal-type phase of short loops. Phases of this type were found in quantum dimer models which have some similarities with these models. I want to note that the arguments of the sign of the Casimir term are correct for loops which are much larger than the lattice spacing . In fact studies on the effects of boundary conditions in 1+1 dimensional CFTs by Cardy and others found that the sign could indeed be changed upon twisting the boundary conditions. The dependence of the coefficient on the length of loop is equivalent to a change of boundary conditions in the CFT. On the other hand, the Casimir term is not a well defined concept for finite loops. In this sense these arguments hold only in phases in which the loops are long. I am not sure how do the authors deal with the smaller loops, which are present. I want to comment that questions of this type were raise in the context of stripe phases resulting from doping a Mott insulator (e.g. in the work of Kivelson et al, Nature 393, 550, 1998) and later papers in the early 2000s on "smectic" or "sliding" Luttinger liquids.
In summary, I find this paper interesting and the material suitable for publication. However before I give my full endorsement I would like to know the response of the authors to the questions that I raised.