| 1. |
- Anous, Tarek, et al.
(författare)
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A general definition of JTa — deformed QFTs
- 2021
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Ingår i: SciPost Physics. - 2542-4653. ; 10:4
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Tidskriftsartikel (refereegranskat)abstract
- We propose a general path-integral definition of two-dimensional quantum field theories deformed by an integrable, irrelevant vector operator constructed from the components of the stress tensor and those of a U (1) current. The deformed theory is obtained by coupling the original QFT to a flat dynamical gauge field and half a flat dynamical vielbein. The resulting partition function is shown to satisfy a geometric flow equation, which perfectly reproduces the flow equations for the deformed energy levels that were previously derived in the literature. The S-matrix of the deformed QFT differs from the original S-matrix only by an overall phase factor that depends on the charges and momenta of the external particles, thus supporting the conjecture that such QFTs are UV complete, although intrinsically non-local. For the special case of an integrable QFT, we check that this phase factor precisely reproduces the change in the finite-size spectrum via the Thermodynamic Bethe Ansatz equations.
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| 2. |
- Borla, Umberto, et al.
(författare)
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Gauging the Kitaev chain
- 2021
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Ingår i: SciPost Physics. - : SciPost Foundation. - 2542-4653. ; 10:6
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Tidskriftsartikel (refereegranskat)abstract
- We gauge the fermion parity symmetry of the Kitaev chain. While the bulk of the model becomes an Ising chain of gauge-invariant spins in a tilted field, near the boundaries the global fermion parity symmetry survives gauging, leading to local gauge-invariant Majorana operators. In the absence of vortices, the Higgs phase exhibits fermionic symmetry-protected topological (SPT) order distinct from the Kitaev chain. Moreover, the deconfined phase can be stable even in the presence of vortices. We also undertake a comprehensive study of a gently gauged model which interpolates between the ordinary and gauged Kitaev chains. This showcases rich quantum criticality and illuminates the topological nature of the Higgs phase. Even in the absence of superconducting terms, gauging leads to an SPT phase which is intrinsically gapless due to an emergent anomaly.
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| 3. |
- Edison, Alex, et al.
(författare)
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Gravity loop integrands from the ultraviolet
- 2021
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Ingår i: SciPost Physics. - : Stichting SciPost. - 2542-4653. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that loop integrands of (super-)gravity scattering amplitudes possess surprising properties in the ultraviolet (UV) region. In particular, we study the scaling of multi-particle unitarity cuts for asymptotically large momenta and expose an improved UV behavior of four-dimensional cuts through seven loops as compared to standard expectations. For N = 8 supergravity, we show that the improved large momentum scaling combined with the behavior of the integrand under BCFW deformations of external kinematics uniquely fixes the loop integrands in a number of non-trivial cases. In the integrand construction, all scaling conditions are homogeneous. Therefore, the only required information about the amplitude is its vanishing at particular points in momentum space. This homogeneous construction gives indirect evidence for a new geometric picture for graviton amplitudes similar to the one found for planar N = 4 super Yang-Mills theory. We also show how the behavior at infinity is related to the scaling of tree-level amplitudes under certain multi-line chiral shifts which can be used to construct new recursion relations.
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| 4. |
- Guica, Monica, et al.
(författare)
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Infinite pseudo-conformal symmetries of classical TT¯, JT¯ and JTa - deformed CFTs
- 2021
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Ingår i: SciPost Physics. - 2542-4653. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- We show that T (T) over bar, J (T) over bar and J T-a - deformed classical CFTs posses an infinite set of symmetries that take the form of a field-dependent generalization of two-dimensional conformal transformations. If, in addition, the seed CFTs possess an affine U (1) symmetry, we show that it also survives in the deformed theories, again in a field-dependent form. These symmetries can be understood as the infinitely-extended conformal and U (1) symmetries of the underlying two-dimensional CFT, seen through the prism of the dynamical coordinates that characterise each of these deformations. We also compute the Poisson bracket algebra of the associated conserved charges, using the Hamiltonian formalism. In the case of the J (T) over bar and J T-a deformations, we find two copies of a functional Witt - Kac-Moody algebra. In the case of the T (T) over bar deformation, we show that it is also possible to obtain two commuting copies of the Witt algebra.
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| 5. |
- Guica, Monica
(författare)
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Symmetries versus the spectrum of J¯T - deformed CFTs
- 2021
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Ingår i: SciPost Physics. - 2542-4653. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- It has been recently shown that classical J¯T - deformed CFTs possess an infinite-dimensional Witt-Ka\v{c}-Moody symmetry, generated by certain field-dependent coordinate and gauge transformations. On a cylinder, however, the equal spacing of the descendants' energies predicted by such a symmetry algebra is inconsistent with the known finite-size spectrum of J¯T - deformed CFTs. Also, the associated quantum symmetry generators do not have a proper action on the Hilbert space. In this article, we resolve this tension by finding a new set of (classical) conserved charges, whose action is consistent with semi-classical quantization, and which are related to the previous symmetry generators by a type of energy-dependent spectral flow. The previous inconsistency between the algebra and the spectrum is resolved because the energy operator does not belong to the spectrally flowed sector.
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| 6. |
- Guica, Monica, et al.
(författare)
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T¯T and the mirage of a bulk cutoff
- 2021
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Ingår i: SciPost Physics. - 2542-4653. ; 10:2
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Tidskriftsartikel (refereegranskat)abstract
- We use the variational principle approach to derive the large N holographic dictionary for two-dimensional T (T) over bar -deformed CFTs, for both signs of the deformation parameter. The resulting dual gravitational theory has mixed boundary conditions for the non dynamical graviton; the boundary conditions for matter fields are undeformed. When the matter fields are turned off and the deformation parameter is negative, the mixed boundary conditions for the metric at infinity can be reinterpreted on-shell as Dirichlet boundary conditions at finite bulk radius, in agreement with a previous proposal by McGough, Mezei and Verlinde. The holographic stress tensor of the deformed CFT is fixed by the variational principle, and in pure gravity it coincides with the Brown-York stress tensor on the radial bulk slice with a particular cosmological constant counterterm contribution. In presence of matter fields, the connection between the mixed boundary conditions and the radial bulk cutoff is lost. Only the former correctly reproduce the energy of the bulk configuration, as expected from the fact that a universal formula for the deformed energy can only depend on the universal asymptotics of the bulk solution, rather than the details of its interior. The asymptotic symmetry group associated with the mixed boundary conditions consists of two commuting copies of a state-dependent Virasoro algebra, with the same central extension as in the original CFT.
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| 7. |
- Hannukainen, Julia D., et al.
(författare)
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Electric manipulation of domain walls in magnetic Weyl semimetals via the axial anomaly
- 2021
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Ingår i: SciPost Physics. - : Stichting SciPost. - 2542-4653. ; 10:5
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Tidskriftsartikel (refereegranskat)abstract
- We show how the axial (chiral) anomaly induces a spin torque on the magnetization in magnetic Weyl semimetals. The anomaly produces an imbalance in left- and right-anded chirality carriers when non-orthogonal electric and magnetic fields are applied. Such imbalance generates a spin density which exerts a torque on the magnetization, the strength of which can be controlled by the intensity of the applied electric field. We show how this results in an electric control of the chirality of domain walls, as well as in an improvement of the domain wall dynamics, by delaying the onset of the Walker breakdown. The measurement of the electric field mediated changes in the domain wall chirality would constitute a direct proof of the axial anomaly. Additionally, we show how quantum fluctuations of electronic Fermi arc states bound to the domain wall naturally induce an effective magnetic anisotropy, allowing for high domain wall velocities even if the intrinsic anisotropy of the magnetic Weyl semimetal is small.
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| 8. |
- Köhler, Thomas, et al.
(författare)
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Efficient and flexible approach to simulate low-dimensional quantum lattice models with large local Hilbert spaces
- 2021
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Ingår i: SciPost Physics. - : SCIPOST FOUNDATION. - 2542-4653. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- Quantum lattice models with large local Hilbert spaces emerge across various fields in quantum many-body physics. Problems such as the interplay between fermions and phonons, the BCS-BEC crossover of interacting bosons, or decoherence in quantum simulators have been extensively studied both theoretically and experimentally. In recent years, tensor network methods have become one of the most successful tools to treat such lattice systems numerically. Nevertheless, systems with large local Hilbert spaces remain challenging. Here, we introduce a mapping that allows to construct artificial U(1) symmetries for any type of lattice model. Exploiting the generated symmetries, numerical expenses that are related to the local degrees of freedom decrease significantly. This allows for an efficient treatment of systems with large local dimensions. Further exploring this mapping, we reveal an intimate connection between the Schmidt values of the corresponding matrix-product-state representation and the single-site reduced density matrix. Our findings motivate an intuitive physical picture of the truncations occurring in typical algorithms and we give bounds on the numerical complexity in comparison to standard methods that do not exploit such artificial symmetries. We demonstrate this new mapping, provide an implementation recipe for an existing code, and perform example calculations for the Holstein model at half filling. We studied systems with a very large number of lattice sites up to L = 501 while accounting for N-ph = 63 phonons per site with high precision in the CDW phase.
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| 9. |
- Lootens, Laurens, et al.
(författare)
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Matrix product operator symmetries and intertwiners in string-nets with domain walls
- 2021
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Ingår i: SciPost Physics. - : SCIPOST FOUNDATION. - 2542-4653. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- We provide a description of virtual non-local matrix product operator (MPO) symmetries in projected entangled pair state (PEPS) representations of string-net models. Given such a PEPS representation, we show that the consistency conditions of its MPO symmetries amount to a set of six coupled equations that can be identified with the pentagon equations of a bimodule category. This allows us to classify all equivalent PEPS representations and build MPO intertwiners between them, synthesising and generalising the wide variety of tensor network representations of topological phases. Furthermore, we use this generalisation to build explicit PEPS realisations of domain walls between different topological phases as constructed by Kitaev and Kong [Commun. Math. Phys. 313 (2012) 351-373]. While the prevailing abstract categorical approach is sufficient to describe the structure of topological phases, explicit tensor network representations are required to simulate these systems on a computer, such as needed for calculating thresholds of quantum error-correcting codes based on string-nets with boundaries. Finally, we show that all these string-net PEPS representations can be understood as specific instances of Turaev-Viro state-sum models of topological field theory on three-manifolds with a physical boundary, thereby putting these tensor network constructions on a mathematically rigorous footing.
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| 10. |
- Minahan, Joseph A., et al.
(författare)
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Conformal field theories on deformed spheres, anomalies, and supersymmetry
- 2021
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Ingår i: SciPost Physics. - : Stichting SciPost. - 2542-4653. ; 10:3
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Tidskriftsartikel (refereegranskat)abstract
- We study the free energy of four-dimensional CFTs on deformed spheres. For generic nonsupersymmetric CFTs only the coefficient of the logarithmic divergence in the free energy is physical, which is an extremum for the round sphere. We then specialize to N = 2 SCFTs where one can preserve some supersymmetry on a compact manifold by turning on appropriate background fields. For deformations of the round sphere the c anomaly receives corrections proportional to the supersymmetric completion of the (Weyl)(2) term, which we determine up to one constant by analyzing the scale dependence of various correlators in the stress-tensor multiplet. We further show that the double derivative of the free energy with respect to the marginal couplings is proportional to the two-point function of the bottom components of the marginal chiral multiplet placed at the two poles of the deformed sphere. We then use anomaly considerations and counter-terms to parametrize the finite part of the free energy which makes manifest its dependence on the Kahler potential. We demonstrate these results for a theory with a vector multiplet and a massless adjoint hypermultiplet using results from localization. Finally, by choosing a special value of the hypermultiplet mass where the free energy is independent of the deformation, we derive an infinite number of constraints between various integrated correlators in N = 4 super Yang-Mills with any gauge group and at all values of the coupling, extending previous results.
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