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REVIEW 4 major objections 5 minor 85 references

The two-particle density matrix of a Luttinger liquid

T0 review · 4 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash

Pith's one-line read The two-particle density matrix of a Luttinger liquid has a closed finite-size form governed by a second exponent, lambda=(K^{-1}-K)/2, which controls off-diagonal correlations and emerges only beyond the one-particle level.

desk verdict A genuinely useful closed-form 2-RDM for a Luttinger liquid with a plausible new chiral exponent, but the exponential cutoff ansatz and thin DMRG validation leave the quantitative claim conditional. read the letter →

arxiv 2607.14402 v1 pith:VD3MQ5HL submitted 2026-07-15 cond-mat.str-el

classification cond-mat.str-el PACS 71.10.Pm
keywords Luttingerliquidtwo-particlereduceddensitymatrixbosonizationfinite-sizescalingcharge-densitywavep-wavepairingDMRGultravioletcutoff
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper derives a closed, finite-size expression for the equal-time two-particle reduced density matrix of spinless fermions in a Tomonaga-Luttinger liquid. The result shows that two-particle correlations are governed not only by the familiar exponent gamma^2=(K+K^{-1}-2)/2 but also by a second exponent lambda=(K^{-1}-K)/2 that encodes correlations between left- and right-moving fermions and controls the off-diagonal structure. A sympathetic reader would care because two-particle coherence is the first level of the density-matrix hierarchy inaccessible to single-particle observables, and analytic two-particle density matrices are rare even in one dimension. After fixing a single ultraviolet cutoff from the one-particle density matrix, the expression quantitatively reproduces DMRG data for an interacting lattice chain, giving access to density correlations, the static structure factor, and the orbital character of CDW and pairing correlations.

What carries the argument

The central mechanism is constructive bosonization with an explicit ultraviolet cutoff. The fermion field is decomposed into right and left movers, and the two-particle correlator is reduced to exponentials of boson commutators and Gaussian expectations; the load-bearing objects are the chord functions d_epsilon(x) and the two interaction exponents defined through the regularized momentum sums, cosh^2 theta_q + sinh^2 theta_q - 1 ≈ gamma^2 e^{-epsilon|q|} and -2sinh theta_q cosh theta_q ≈ lambda e^{-epsilon|q|}. The resulting expression Eq. (34) combines six chiral contributions into three real terms, with gamma^2 in all three and lambda only in the two anomalous terms, and the diagonal limi

What would settle it

Perform exact diagonalization or DMRG on a finite chain with a next-nearest-neighbor or long-range interaction, fit epsilon from the 1-RDM, and compare Eq. (34) to the numerically obtained 2-RDM elements: a systematic deviation beyond the short-distance scale would show that the single-exponential regulator ansatz is insufficient. Alternatively, measure the 2k_F decay of g2(r) in a cold-atom realization: if the exponent differs from 2K, the lambda structure is not the full story.

Watch

Extended reading notes

Core claim

For spinless fermions on a ring with periodic boundary conditions, the paper establishes Eq. (34): a closed finite-size expression for the equal-time two-particle reduced density matrix, built from chord functions d_epsilon(x)=(L/pi)sin(pi(x+i epsilon)/L). The normal (same-chirality) boson correlators contribute the exponent gamma^2=(K+K^{-1}-2)/2, while the anomalous (opposite-chirality) correlators contribute a second exponent lambda=(K^{-1}-K)/2, which is absent from the 1-RDM and can be negative for attractive interactions. Because the expression retains an explicit ultraviolet cutoff epsilon, it can be matched to a microscopic lattice model: with epsilon fixed from the 1-RDM, the analyt

Load-bearing premise

The derivation assumes that a single exponential ultraviolet cutoff epsilon, fitted to the one-particle density matrix, also reproduces the full momentum dependence of the normal and anomalous two-particle correlators; if the true q-dependence of the couplings differs from e^{-epsilon|q|}, the quantitative agreement would fail even though the universal exponents remain correct.

Editorial extensions

If this is right

  • Two-particle observables of a Luttinger liquid can now be computed on a ring at finite size without fitting amplitudes: the only nonuniversal parameter is the cutoff epsilon.
  • The static structure factor follows from the diagonal of the 2-RDM and reproduces the linear small-momentum behavior s(q) ~ K|q|/2k_F, connecting universal scaling to measurable scattering response.
  • Off-diagonal coherences carry information about the orbital symmetry of ordered correlations: an even, node-free particle-hole eigenvector signals an s-wave CDW, while an antisymmetric particle-particle eigenvector signals p-wave pairing.
  • Because the expression is finite-size and cutoff-regularized, it can be used directly to compute lattice observables such as the ground-state energy of the J-V chain.
  • The new exponent lambda enters only for n>=2 density matrices, so the 2-RDM contains information about interactions that is not present in the 1-RDM, and the method is stated to generalize to higher n.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The subleading 1/N contributions to particle-partition Rényi entropies for n>1, which the paper connects to the 2-RDM structure, could be explicitly traced to lambda; computing the n-RDM cumulant would make this quantitative.
  • The same constructive bosonization should yield closed n-particle density matrices, and the coefficients of lambda in those expressions could be checked against exact diagonalization of small chains.
  • A time-dependent version of Eq. (34) after an interaction quench is a natural extension; the two-particle coherence growth predicted by it could be compared with the existing quench results for the 1-RDM.
  • The assumption of a single exponential regulator epsilon could be tested by applying the formula to models with long-range interactions, where the momentum dependence of the couplings deviates from e^{-epsilon|q|}; any quantitative failure would pinpoint where the regulator ansatz breaks.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

4 major / 5 minor

Summary. The paper derives a closed, finite-size expression for the equal-time two-particle reduced density matrix of a spinless Tomonaga-Luttinger liquid on a ring, using constructive bosonization with an explicit ultraviolet cutoff. The central result, Eq. (34), combines three chiral terms controlled by the familiar exponent γ²=(K+K^{-1}-2)/2 and a second exponent λ=(K^{-1}-K)/2 that arises from anomalous right-left correlators. The diagonal limit gives the density-density correlation function and static structure factor; off-diagonal elements are analyzed for CDW and p-wave pairing signatures. After fixing one interaction cutoff ϵ from the 1-RDM, the authors compare Eq. (34) with DMRG data for the J-V chain and report quantitative agreement. The appendices contain detailed bosonization algebra, limits, and a data/code availability statement.

Significance. If the quantitative match holds, this is a valuable analytic benchmark for two-particle correlations in interacting one-dimensional systems, going beyond the well-known 1-RDM. The derivation is substantial, reproduces the free-fermion limit and diagonal limits, and identifies λ as a genuinely new structural exponent. The paper also provides reproducible code and data, which strengthens the contribution. However, the central quantitative claim rests on an unproven exponential regulator ansatz and on the transfer of a single fitted parameter from the normal to the anomalous channel; the numerical support is narrow. These issues are addressable but currently leave the precision of Eq. (34) less certain than the universal power laws it contains.

major comments (4)
  1. [Section III, Eqs. (25) and (28)] The exponential regulator ansatz cosh²θ_q+sinh²θ_q−1 ≈ γ² e^{-ϵ|q|} and −2 sinhθ_q coshθ_q ≈ λ e^{-ϵ|q|} is introduced without derivation from the microscopic J-V interaction. The 1-RDM, Eq. (33), depends only on γ² and ϵ; therefore fitting ϵ to the 1-RDM constrains only the normal channel. Equation (34) then applies the same ϵ to the anomalous channel that controls λ. This transfer is load-bearing for the quantitative claim. Please provide evidence that the single-ϵ ansatz is valid, for example by testing λ-sensitive cuts of the 2-RDM in the attractive regime (K>1, where λ<0 and the sign of the anomalous contribution changes), or by extracting ϵ independently from the 1-RDM and from a λ-sensitive DMRG observable and showing consistency.
  2. [Section V.B, Figs. 11-12] The full 2-RDM benchmark is presented for a single interaction value, V/J=-0.5, and a single system size, L=50. The text claims agreement 'throughout the Luttinger liquid regime', but no repulsive (K<1) 2-RDM comparison is shown, and no error bars or quantitative discrepancy measure are reported. Please extend the benchmark to at least one repulsive case (and ideally one close to the K→1/2 CDW boundary), and report a concrete measure of the agreement, such as the maximum or mean absolute deviation between DMRG and Eq. (34).
  3. [Section V.C, Fig. 13] In the structure-factor calculation, the text states: 'We extract the interaction cutoff ϵ from a fit to g2.' Since g2 is the diagonal limit of the 2-RDM, this makes the agreement of s(q) a fit to the same observable, not an independent test of Eq. (34). This also conflicts with the abstract's claim that the cutoff is fixed from the one-particle density matrix. Please present the structure factor using the 1-RDM-fitted ϵ of Fig. 11, and assess the sensitivity of the result to the advertised trace correction of g2.
  4. [Section IV.D, Eq. (49)-(50)] The orbital-character analysis of the CDW coherence contains a gap. Equation (50) states that the residual δρ(ξ) equals the dominant eigenvector φ(ξ) of the particle-hole block, but the rank-one assumption is not demonstrated; the text asserts it is confirmed by a shift of the reference bond, but no data or figure is shown. Since this section makes a concrete physical claim (site-centered s-wave CDW), please provide the supporting comparison or mark the assertion as an observation from Eq. (34) rather than a proven property.
minor comments (5)
  1. [Section IV.B, Eq. (42)] The text refers to 'Eq. (45)' when quoting the free-fermion result; the cross-reference should point to the actual free-fermion expression, and the numbering around Eqs. (43)-(45) should be checked.
  2. [Section IV] The choice ϵ=1 for most analytical plots is presented without explanation. Since ϵ is later tied to microscopic parameters, a brief comment on the expected dependence of the plotted features on ϵ would improve interpretability.
  3. [Section IV.D, Eq. (50)] The asymptotic arrow notation in Eq. (50) is garbled ('− − − − − − →'). Use a standard symbol such as '→' with the stated limit.
  4. [Section VI] Typo: 'Tomonage-Luttinger' should be 'Tomonaga-Luttinger'. Also, the author affiliation line contains raw LaTeX ('F¨ ur') that should be rendered properly.
  5. [Fig. 13] The inset labels N=10 and N=25 with L=50 correspond to fillings 1/5 and 1/2; state this explicitly in the caption or main text to avoid ambiguity.

Circularity Check

1 steps flagged · score 2.0 of 10

Central 2-RDM derivation is self-contained; the main benchmark is a genuine out-of-sample test. Minor circularity appears only in the structure-factor application, where the cutoff is fit to g2 and s(q) is then recomputed from that same g2.

  1. fitted input called prediction [Section V.C.1 (Static Structure Factor), Eqs. (54)-(55), Fig. 13]
    "We compute the structure factor for filling fractions 1/2 and 1/5 from the analytical expression, Eq. (34), by discretizing it on the L lattice sites using that g2(r)=ρ2(r,0;0,r)/n20 for r>0. We extract the interaction cutoff ϵ from a fit to g2."

    In this application, s(q) is defined as the Fourier transform of the very same g2 used for the fit: s(q)=1+n0 Σ [g2(j)-1] e^{-iqj} (Eq. 55). Fitting the single cutoff ϵ to g2 at the same parameters and then presenting s(q) computed from that g2 as 'excellent agreement' with DMRG is a consistency check of the discretization and transform, not an independent prediction of the two-particle density matrix. This does not affect the central λ claim, and K is independently known from Bethe ansatz, so the circularity is partial and confined to validation.

full rationale

The core derivation is not circular. The exponents γ²=(K+K^{-1}-2)/2 and λ=(K^{-1}-K)/2 are obtained analytically from the Bogoliubov angle via K=lim e^{2θ_q} (Eqs. 25-29), with K itself taken from Bethe ansatz/XXZ results (Eq. 52), independent of the fitted cutoff. The 1-RDM and 2-RDM are not the same object: the 2-RDM contains λ-dependent anomalous-channel terms that are absent from the 1-RDM, so fixing ϵ from the 1-RDM and then matching the full 2-RDM (Figs. 11-12) is an out-of-sample test rather than a parameter renaming. The exponential regulator ansatz (Eqs. 25 and 28) is an assumption, but not a circular one: it is a stated approximation for the q-dependence of the couplings, and the universal power laws and free-fermion limit provide independent anchors. Self-citations to Ref. [37] for the 1-RDM form are not load-bearing because that expression is also attributed to independent earlier work [34-36]. The only genuine circular step is in the structure-factor subsection, where ϵ is fit to g2 and s(q), being a linear transform of the same g2, is then presented as agreement with DMRG; this is a minor, non-central validation issue. Overall score 2 reflects this limited circularity while the central claim retains independent content.

Assumptions & free parameters 1 free parameters · 4 assumptions · 0 invented entities

The derivation uses standard bosonization plus one ad hoc exponential-cutoff ansatz and one fitted cutoff ϵ. No new particles, forces, or conserved quantities are introduced. The λ exponent is a combination of K and K^{-1}, not a new physical entity.

free parameters (1)
  • ϵ (interaction UV cutoff) = 0.81 for V/J=-0.5, L=50; also fitted to g2 for structure factor at other fillings
    The single nonuniversal input. It is obtained by fitting the analytic 1-RDM to DMRG data (Fig. 11) and then reused unchanged in the 2-RDM. The quantitative benchmark therefore depends on this calibration being transferable.
assumptions (4)
  • domain assumption Low-energy linearization of the dispersion around ±k_F; only right- and left-moving chiral fields Ψ± are retained.
    Section III, below Eq. (15). Standard Tomonaga-Luttinger liquid approximation; breaks outside the low-energy sector.
  • ad hoc to paper Couplings ω0(q), m(q), g2(q) are taken as linear in |q| with momentum-independent coefficients, and the UV regularization is the exponential ansatz of Eqs. (25) and (28).
    Eqs. (18)–(28). The exponential forms cosh²+sinh²−1 ≈ γ² e^{-ϵ|q|} and −2sinhθ_q coshθ_q ≈ λ e^{-ϵ|q|} are introduced as regulators without derivation from the J-V Hamiltonian. This is the load-bearing modeling choice that makes the momentum sums close.
  • standard math Bogoliubov transformation diagonalizes the quadratic bosonic Hamiltonian, and the ground state is the vacuum of the quasiparticle operators a_q.
    Section III, Eqs. (19)–(24). Standard bosonization; also used in the cited 1-RDM literature.
  • domain assumption For the J-V chain at half filling, K is obtained from the Bethe-ansatz/XXZ mapping K=π/(2 cos^{-1}(-V/2J)); at 1/5 filling, K is computed numerically from Ref. [31].
    Section V.A, Eq. (52). This connects the Luttinger parameter to the microscopic lattice model and is independent of the present derivation, but it relies on exact solvability of the XXZ chain.

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Pith. "Pith review of The two-particle density matrix of a Luttinger liquid." pith.science (2026). https://pith.science/paper/VD3MQ5HL

@misc{pith2026260714402,
  author       = {Pith},
  title        = {Pith review of: The two-particle density matrix of a Luttinger liquid},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/VD3MQ5HL}},
  note         = {Machine review of arXiv:2607.14402}
}
abstract

Two-particle coherence is the first level of the reduced-density-matrix hierarchy that contains correlations inaccessible to single-particle observables, yet analytic two-particle density matrices are rare even in one dimension. We derive a closed, finite-size expression for the equal-time two-particle reduced density matrix of spinless fermions in a Tomonaga-Luttinger liquid using constructive bosonization with an explicit ultraviolet cutoff. In addition to the familiar Luttinger parameter $K$-dependent exponent $\gamma^2=(K+K^{-1}-2)/2$ which governs the spatial decay of matrix elements, the result exposes a second exponent, $\lambda=(K^{-1}-K)/2$, which encodes correlations between opposite chiralities and controls the off-diagonal structure. The diagonal limit of the two-particle reduced density matrix yields density correlations and the static structure factor, while its coherences resolve algebraic $2k_F$ charge-density-wave correlations for repulsion and odd-parity p-wave pairing correlations for attraction. After fixing the cutoff from the one-particle density matrix, the analytic result quantitatively reproduces density matrix renormalization group calculations of the interacting J-V chain within the Luttinger liquid regime. The result connects universal Luttinger liquid scaling with observables in finite microscopic systems.

Figures

Figures reproduced from arXiv: 2607.14402 by the authors.

Figure 1
Figure 1. FIG. 1. Graphical depiction of the two-body density of a [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Dependence of the two interaction-dependent expo [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. FIG. 4. The density-density correlation function [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figures from the paper (10 more)
Figure 5
Figure 5. Figure 5: FIG. 5. The 2-RDM of the Luttinger model on a ring for [PITH_FULL_IMAGE:figures/full_fig_p008_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. The second cumulant of the Luttinger model for [PITH_FULL_IMAGE:figures/full_fig_p009_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. The effects of interactions as measured with respect [PITH_FULL_IMAGE:figures/full_fig_p010_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Orbital character of the CDW from the unnormal [PITH_FULL_IMAGE:figures/full_fig_p011_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Heat map of [PITH_FULL_IMAGE:figures/full_fig_p011_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Illustration of the phase diagram for the [PITH_FULL_IMAGE:figures/full_fig_p012_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. We plot the one-body reduced density matrix [PITH_FULL_IMAGE:figures/full_fig_p012_11.png]
Figure 12
Figure 12. Figure 12: FIG. 12. Panel (a) shows a heat map of the full correlation [PITH_FULL_IMAGE:figures/full_fig_p013_12.png]
Figure 13
Figure 13. Figure 13: FIG. 13. Structure factor [PITH_FULL_IMAGE:figures/full_fig_p014_13.png]
Figure 14
Figure 14. Figure 14: FIG. 14. The ground state energy [PITH_FULL_IMAGE:figures/full_fig_p014_14.png]

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Works this paper leans on

85 extracted references · 1 canonical work pages

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    From the diagonal limit of the 2-RDM, Eq

    Onset of CDW correlations A CDW is a condensate of particle-hole pairs at wavevector 2kF , and it can be diagnosed in the diago- nal (density–density) sector. From the diagonal limit of the 2-RDM, Eq. (42), the 2k F component of the pair correlation function decays as a power law fixed by the Luttinger parameter [cf. Eq. (44)], C(r)≡ g2(r)−1 2kF = (n0ϵ)2K...

  2. [2]

    and thus the resulting 2-RDM can be obtained from⟨Ψ †(x′ 2)ˆn(x1)Ψ(x2)⟩by anticommuting the field op- erators. The intersection between hyperplanesx ′ 1 =x 1 and x′ 2 =x 2 gives rise to the diagonal elements of the 2-RDM,ρ 2(x2, x1;x 1, x2), while the intersection between hyperplanesx ′ 2 =x 1 andx ′ 1 =x 2 generates a negative copy of the diagonal elemen...

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    Orbital character The density-density limit establishes that phase with algebraically enhanced CDW correlations forms, but not which one: the condensing particle-hole pair may be on- site (s-wave) or bond-centered (p-wave), and these are distinguished only by the off-diagonal sector [38, 42]. We therefore introduce the off-diagonal coherence ρ(ξ) = D Ψ†(0...

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    Pairing correlations We now examine the attractive regime,K >1, where the system should exhibit a superconducting instability withp-wave pairing [31]. Although true long-range order is suppressed in one dimension due to strong phase fluc- tuations, bosonization predicts an algebraic decay of the pair correlator,ρ 2 ∝ |x|−2/K , consistent with quasi-long- ...

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    An important property ofs(q) for experiments, which can be described by the Luttinger theory, is its small momen- tum behavior

    Static Structure Factor In this section, we consider the static structure factor, which is an experimentally accessible quantity defined via the density-density correlationsg 2 as [60] g2(i−j) = ⟨ninj⟩ n2 0 − δij n0 ,(54) s(q) = 1 +n 0 L−1X j=0 [g2(j)−1]e −iqj .(55) Here, the momentum takes valuesq= 2πn/Lwithn∈N. An important property ofs(q) for experimen...

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    D. A. Mazziotti, Structure of fermionic density matrices: Completen-representability conditions, Phys. Rev. Lett. 108, 263002 (2012)

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    Ground State Lattice Energy To demonstrate the utility of the constructive bosonization approach taken here, we show that con- tinuum expressions for the 1-RDM and 2-RDM can be leveraged (through the interaction cutoffϵ) to provide access to even short-range observables in the context of a microscopic model. We compute the two-body lattice ground state en...

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    +e ikF x′ 2 Ψ† +(x′ 2) ih e−ikF x′ 1 Ψ† −(x′

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    In order to satisfy the condition⟨0|Ψ †(x′ 2)Ψ†(x′ 1)Ψ(x1)Ψ(x2)|0⟩ ̸= 0, if a right or left-moving fermion is created, one must also be destroyed

    +e ikF x′ 1 Ψ† +(x′ 1) i × eikF x1 Ψ−(x1) +e −ikF x1 Ψ+(x1) eikF x2 Ψ−(x2) +e −ikF x2 Ψ+(x2) .(A1) Carrying out the multiplication, we obtain sixteen total terms; however, the majority will vanish. In order to satisfy the condition⟨0|Ψ †(x′ 2)Ψ†(x′ 1)Ψ(x1)Ψ(x2)|0⟩ ̸= 0, if a r...

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    +iϵ) |γ2 |sin π L ((x2 −x 1) +iϵ) |γ2 |sin π L ((x′ 2 −x 1) +iϵ) |γ2 |sin π L ((x′ 1 −x 2) +iϵ) |γ2 (A7) ⟨Ψ† α(x′ 2)Ψ† β(x′ 1)Ψβ(x1)Ψα(x2)⟩= 1 4L2 1 sin (π L (x′ 2 −x 1)) sin (π L (x′ 1 −x 2)) × |sin ( π L ((x′ 2 −x ′

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    Combining all non-zero terms (e.g.⟨Ψ † R(x′ 2)Ψ† R(x′ 1)ΨR(x1)ΨR(x2)⟩and⟨Ψ † L(x′ 2)Ψ† L(x′ 1)ΨL(x1)ΨL(x2)⟩), we obtain a final expression for the 2-RDM, Eq

    +iϵ))| λ|sin ( π L ((x2 −x 1) +iϵ))| λ |sin ( π L ((x′ 2 −x 2) +iϵ))| λ|sin ( π L ((x′ 1 −x 1) +iϵ))| λ × |sin iπ L ϵ|2γ2 |sin ( π L ((x′ 2 −x 1) +iϵ))| γ2 |sin ( π L ((x′ 1 −x 2) +iϵ))| γ2 (A8) ⟨Ψ† α(x′ 2)Ψ† β(x′ 1)Ψα(x1)Ψβ(x2)⟩=−⟨Ψ † α(x′ 2)Ψ† β(x′ 1)Ψβ(x2)Ψα(x1)⟩(A9) where ...

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    hϵhϵ,0 hϵ,−hϵ,+ γ2 −1 # + cos (2kF r−) 2π2h0,−

    Finally, to connect with the set of orthogonal hyperplanes we mentioned earlier in section IV, we definer ± = (r′ ±r)/2 = (x ′ 2 −x ′ 1 ±x 2 ∓x 1)/2. The appearance of at least one of the functionsh 0,− andh 0,+ in the denominator in each of the three terms brings our attentio...

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    Wick’s theorem We simplify the expression by introducing the relative coordinatesr=x 2 −x 1,r ′ =x ′ 2 −x ′ 1, and center-of-mass coordinatesR= x1+x2 2 ,R ′ = x′ 1+x′ 2 2 , with ∆R=R ′ −R. The two-body density matrix becomes: ⟨Ψ†(x′ 2)Ψ†(x′ 1)Ψ(x1)Ψ(x2)⟩= hγ2 ϵ hλ ϵ,0 π2hγ2 ϵ,...

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    Similarly, we can define the limits forX,Y, andW

    Calculating the limitx ′ 1 →x 1,x ′ 2 →x 2 andx 2 →x 1 Starting from ρ(x′ 2, x′ 1;x 1, x2) =⟨Ψ †(x′ 2)Ψ†(x′ 1)Ψ(x1)Ψ(x2)⟩ = |hϵ|γ2 |hϵ,0 |λ 2π2h0,+h0,−|hϵ,+hϵ,−|γ2 cos (kF (x′ 1 +x ′ 2 −x 1 −x 2))[h0,0 |hϵ,0 |δ +h 0,+|hϵ,+|δ −h 0,−|hϵ,−|δ] + |hϵ,0 |λ |hϵ,+|λ ⟨Ψ†(x′ 2)Ψ(x2)⟩⟨Ψ†...

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    +iϵ) δ sin π L ((x2 −x 1) +iϵ) δ − sin π L ((x′ 1 −x 2) +iϵ) δ sin π L ((x′ 2 −x 1) +iϵ) δi =− L2 π2 δ−1 δ 2 π L sin π L ((x2 −x 1) +iϵ) δ sin π L ((x′ 2 −x ′

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    +iϵ) δ−2 sin 2π L (x′ 2 −x ′ 1) − L2 π2 δ−1 δ 2 π L sin π L ((x′ 2 −x 1) +iϵ) δ sin π L ((x′ 1 −x 2) +iϵ) δ−2 sin 2π L (x′ 1 −x 2) .(C11) The derivative of the denominator is ∂ ∂x′ 1 sin (π L (x′ 1 −x 1)) sin (π L (x′ 2 −x 2)) = π L cos (π L (x′ 1 −x 1)) sin (π L (x′ 2 −x 2))....

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    +iϵ] λ sin π L [(x2 −x 1) +iϵ] λ sin iπ L ϵ 2γ2 × ( sin[kF (x′ 2 −x 2)] sin[kF (x′ 1 −x 1)] sin π L [(x′ 2 −x 2) +iϵ] −γ2 L2 sin π L (x′ 2 −x 2) sin π L (x′ 1 −x 1) sin π L [(x′ 1 −x 2) +iϵ] λ sin π L [(x′ 2 −x 1) +iϵ] λ × 1 sin π L [(x′ 1 −x 1) +iϵ] γ2 − sin[kF (x′ 2 −x 1)] s...

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    (C21) Finally, in limitx ′ 1 →x 1,x ′ 2 →x 2 andx 2 →x 1 we have lim x2→x1 lim x′ 2→x2 lim x′ 1→x1 Ψ†(x′

    Ψ(x1) Ψ(x2) =F 2(X2 −Y 2) +W 2 = sin π L [(x2 −x 1) +iϵ] 2λ−2γ2 2L 2 ( − sin π L [(x2 −x 1) +iϵ] 2δ − sin iπ L ϵ 2δ sin2 π L (x2 −x 1) + δ 2 sin π L [(x2 −x 1) +iϵ] 2δ−4h 2 sin π L [(x2 −x 1) +iϵ] 2 cos 2π L (x2 −x 1) −sin 2 2π L (x2 −x 1) i) +n 2 0 − sin iπ L ϵ 2δ sin2 kF (x2...

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    Ψ(x1) Ψ(x2) = lim x2→x1 [F2(X2 −Y 2) +W 2] =F 3(X3 −Y 3) +W 3 = 0. (C22)

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