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Conformal invariance in the FK-representation of the quantum Ising model and convergence of the interface to the \(\mathrm{SLE}_{16/3}\)

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We study the interface in the FK-representation of the 1D quantum Ising model and show that in the limit, it converges to the \(\mathrm{SLE}_{16/3}\) curve.

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This research is supported by the NCCR SwissMAP, the ERC AG COMPASP, and the Swiss NSF. The author is thankful to his advisers Hugo Duminil-Copin and Stanislav Smirnov for their help and constant support during his Ph.D. He is also grateful to the referee for his careful reading and useful suggestions.

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Correspondence to Jhih-Huang Li.

Appendix: Computation of residues

Appendix: Computation of residues

For a non-negative integer k and an integer m, define the following meromorphic function on \(\mathbb {C}\),

$$\begin{aligned} g_{k, m} (z) := \frac{1}{z} \left( \frac{1}{z+1} + \frac{1}{z-1} \right) ^k \left( \frac{z+1}{z-1} \right) ^{2m} = \frac{2^k z^{k-1}}{(z-1)^{k+2m} (z+1)^{k-2m}}. \end{aligned}$$

We notice that the possible singularities are at 0, 1 and \(-1\).

Lemma A.1

When \(k=0\), we have \({{\mathrm{\mathrm{Res}}}}(g_{k, m}, 1) = {{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) = 0\) for all integer m.


We fix \(k=0\). When \(m = 0\), the result is trivial. For \(m \geqslant 1\), the function \(g_{k, m}\) does not have any pole at \(-1\), so it is clear that \({{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) = 0\). The residue of \(g_{k, m}(z)\) at \(z = 1\) is the residue of \(g_{k, m}(y+1)\) at \(y = 0\). We have

$$\begin{aligned} g_{k, m} (y+1)&= \frac{1}{y+1} \left( 1 + \frac{2}{y} \right) ^{2m} \\&= \left[ \sum _{k \geqslant 0} (-y)^k \right] \left[ \sum _{l=0}^{2m} {2m \atopwithdelims ()l} \left( \frac{2}{y} \right) ^l \right] , \end{aligned}$$

where the coefficient of \(\frac{1}{y}\) is given by

$$\begin{aligned} \sum _{l=1}^{2m} {2m \atopwithdelims ()l} 2^l (-1)^{l-1} = - [(1-2)^{2m} - 1] = 0. \end{aligned}$$

When m is negative, the proof is similar. \(\square \)

Lemma A.2

For \(k \geqslant 1\), we have \({{\mathrm{\mathrm{Res}}}}(g_{k, m}, 1) + {{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) = 0\).


When \(k \geqslant 1\), the singularity at 0 is removable. We observe that \(|g_{k, m} (z)|\) behaves like \(|z|^{-k-1} \leqslant |z|^{-2}\) when |z| is large, giving

$$\begin{aligned} \lim _{R \rightarrow \infty } \frac{1}{2 \pi {{\mathrm{\mathrm{i}}}}} \int _{\partial B(0, R)} g_{k, m} (z) \mathrm{d}z = 0. \end{aligned}$$

Moreover, when \(R > 1\), we have

$$\begin{aligned} \frac{1}{2 \pi {{\mathrm{\mathrm{i}}}}} \int _{\partial B(0, R)} g_{k, m} (z) \mathrm{d}z = {{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) + {{\mathrm{\mathrm{Res}}}}(g_{k, m}, 1). \end{aligned}$$

Thus, we get the desired result. \(\square \)

Lemma A.3

For \(k \geqslant 1\) and \(k \leqslant 2 |m|\), \({{\mathrm{\mathrm{Res}}}}(g_{k, m}, 1) = {{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) = 0\).


The previous lemma tells us that it is enough to show that the residue is zero at either 1 or \(-1\). If m is positive, we notice that \(g_{k, m}(z)\) does not have any pole at \(-1\), thus the residue at \(-1\) is zero. If m is negative, the residue at 1 is zero. \(\square \)

Lemma A.4

More generally, for all \(k \in 2 \mathbb {N}\),

$$\begin{aligned} {{\mathrm{\mathrm{Res}}}}(g_{k, m}, 1) = {{\mathrm{\mathrm{Res}}}}(g_{k, m}, -1) = 0. \end{aligned}$$


Assume that \(k = 2l > m \geqslant 0\) for a positive integer l. Look at the residue of \(g_{k, m} (z)\) around \(z = 1\) is equivalent to looking at the residue of \(g_{k, m} (y+1)\) around \(y=0\),

$$\begin{aligned} {{\mathrm{\mathrm{Res}}}}(g_{k, m}(z), z=1) = {{\mathrm{\mathrm{Res}}}}(g_{k, m}(y+1), y=0). \end{aligned}$$

We have the following equivalent relations

$$\begin{aligned}&{{\mathrm{\mathrm{Res}}}}(g_{2l, m}(y+1), y=0) = 0 \nonumber \\&\quad \Leftrightarrow {{\mathrm{\mathrm{Res}}}}\left( \frac{(y+1)^{2l-1}}{y^{2l+2m}(y+2)^{2l-2m}}, y=0 \right) = 0 \nonumber \\&\quad \Leftrightarrow \frac{(1+y)^{2l-1}}{(1+\frac{y}{2})^{2l-2m}} [y^{2l+2m-1}]= 0 \end{aligned}$$

where the notation \(R(y) [y^k]\) gives the coefficient of \(y^k\) in the Laurent series of R(y).

We expand the rational fraction to evaluate this coefficient by applying the three following identities,

$$\begin{aligned} {{2m-2l \atopwithdelims ()2m+p}}&= (-1)^{2m+p} {{2l+p-1 \atopwithdelims ()2m+p}} \end{aligned}$$
$$\begin{aligned} {{2l+p-1 \atopwithdelims ()2m+p}}&= \sum _{q=0}^{2l-1} {{p \atopwithdelims ()q}} {{2l-1 \atopwithdelims ()2m+p-q}}, \end{aligned}$$
$$\begin{aligned} \sum _{k=0}^n {{n \atopwithdelims ()k}} {{ k \atopwithdelims ()r}} (-x)^k&= (-x)^r (1-x)^{n-r} {{n \atopwithdelims ()r}}. \end{aligned}$$

Equation (A.2) comes from the general definition of binomial coefficients, and in our case, we have \(2m-2l < 0\). Equation (A.3) is an easy combinatorial identity and Eq. (A.4) a simple expansion. Then, the left-hand side of Eq. (A.1) equals

$$\begin{aligned}&\sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} \left( \frac{1}{2} \right) ^{2l+2m-1-p} {{2m-2l \atopwithdelims ()2l+2m-1-p}} \\ (p \leadsto 2l-1-p)&\quad = \sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} \left( \frac{1}{2} \right) ^{2m+p} {{2m-2l \atopwithdelims ()2m+p}} \\&\quad = \sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} \left( -\frac{1}{2} \right) ^{2m+p} {{2l+p-1 \atopwithdelims ()2m+p}} \\ (\text{ Equation } ~(A.3))&\quad = \sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} \left( -\frac{1}{2} \right) ^{2m+p} \sum _{q=0}^{2l-1} {{p \atopwithdelims ()q}} {{2l-1 \atopwithdelims ()2m+p-q}} \\ (q \leadsto p-q)&\quad = \sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} \left( -\frac{1}{2} \right) ^{2m+p} \sum _{q=0}^{2l-1} {{p \atopwithdelims ()q}} {{2l-1 \atopwithdelims ()2m+q}} \\&\quad = \left( \frac{1}{2} \right) ^{2m} \sum _{q=0}^{2l-1} {{2l-1 \atopwithdelims ()2m+q}} \sum _{p=0}^{2l-1} {{2l-1 \atopwithdelims ()p}} {{p \atopwithdelims ()q}} \left( -\frac{1}{2} \right) ^p \\ (\text{ Equation } ~(A.4))&\quad = \left( \frac{1}{2} \right) ^{2m} \sum _{q=0}^{2l-1} {{2l-1 \atopwithdelims ()2m+q}} \left( -\frac{1}{2} \right) ^q \left( \frac{1}{2} \right) ^{2l-1-q} {{2l-1 \atopwithdelims ()q}} \\&\quad = \left( \frac{1}{2} \right) ^{2m+2l-1} \sum _{q=0}^{2l-1} (-1)^q {{2l-1 \atopwithdelims ()2m+q}} {{2l-1 \atopwithdelims ()2l-1-q}} =0 \end{aligned}$$

where the sum in the last line is the coefficient in front of \(x^{2m+2l-1}\) in \((1-x)^{2l-1}(1+x)^{2l-1} = (1-x^2)^{2l-1}\), which is zero because the polynomial only contains monomials of even degree. \(\square \)

Proposition A.5

For \(\zeta = m + {{\mathrm{\mathrm{i}}}}t \in \mathbb {L}_1\) with \(m \in \frac{1}{2} \mathbb {Z}\) and \(t \in \mathbb {R}\), let \(f_\zeta \) as defined in (3.22). Consider C the path defined in Proposition 3.18. Then,

$$\begin{aligned} \int _C f_\zeta (z) \mathrm{d}z = 0. \end{aligned}$$


We will expand the exponential into series and show that this integral is zero for all terms. We can do this because the series converges uniformly on all compacts to the exponential function. Therefore, it makes sense to exchange the integral and the series. After expanding, we get

$$\begin{aligned} f_\zeta (z) = \sum _{k \geqslant 0} \frac{(2 {{\mathrm{\mathrm{i}}}}t)^k}{k!} g_{k, m} (z) \end{aligned}$$

and the residue theorem along with Lemmas A.1 and A.2 allows us to conclude. \(\square \)

The previous proposition is important because it shows that the Green’s function as defined by (3.23) does not depend on the lift of the logarithm.

Proposition A.6

Around \(\zeta = m \in \mathbb {Z}\backslash \{ 0 \}\), the function G is \(\mathcal {C}^{2|m|}\).


To show this, we need to check that for all \(k \leqslant 2 |m|\), we have

$$\begin{aligned} \int _C g_{k, m}(z) \ln _1(z) \mathrm{d}z = \int _C g_{k, m}(z) \ln _2(z) \mathrm{d}z \end{aligned}$$

where \(\ln _i\) is chosen such that \(\ln _i(1) - \ln _i(-1) = (-1)^i {{\mathrm{\mathrm{i}}}}\pi \). Here, \(\ln _1\) corresponds to the logarithm chosen in the upper half-plan and \(\ln _2\) in the lower half-plane. From Proposition A.5, we can fix a lift of the logarithm such that \(\ln _1 - \ln _2\) is non zero around 1 and \(-1\), for example \(\ln _1 (1) = 2 {{\mathrm{\mathrm{i}}}}\pi \), \(\ln _1 (-1) = 3 {{\mathrm{\mathrm{i}}}}\pi \), \(\ln _2 (1) = 0\) and \(\ln _2(-1) = -{{\mathrm{\mathrm{i}}}}\pi \).

Let \(I_1\) be the integral on the left-hand side and \(I_2\) be the one on the right-hand side. Since \(\ln _1 - \ln _2\) is non zero at 1 and \(-1\), we can write

$$\begin{aligned} I_1 - I_2&= 2 {{\mathrm{\mathrm{i}}}}\pi [ 2 {{\mathrm{\mathrm{i}}}}\pi {{\mathrm{\mathrm{Res}}}}(g_{k, m} (z), z = 1) + 4 {{\mathrm{\mathrm{i}}}}\pi {{\mathrm{\mathrm{Res}}}}(g_{k, m} (z), z = -1) ] = 0 \end{aligned}$$

due to Lemma A.3. \(\square \)

Proposition A.6 provides us with the regularity of Green’s function; in particular, the second condition giving in Sect. 3.4.1 is satisfied. This comes to complete the proof of Proposition 3.18.

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Li, J. Conformal invariance in the FK-representation of the quantum Ising model and convergence of the interface to the \(\mathrm{SLE}_{16/3}\). Probab. Theory Relat. Fields 173, 87–156 (2019). https://doi.org/10.1007/s00440-018-0831-3

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Mathematics Subject Classification

  • 82B20
  • 82B27
  • 81T40