Publications

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Skyrmion jellyfish in driven chiral magnets

Nina del Ser, Vivek Lohani

SciPost Phys. 15, 065 (2023) · published 17 August 2023 | · pdf

Chiral magnets can host topological particles known as skyrmions which carry an exactly quantised topological charge $Q=-1$. In the presence of an oscillating magnetic field $B_1(t)$, a single skyrmion embedded in a ferromagnetic background will start to move with constant velocity v$_{trans}$. The mechanism behind this motion is similar to the one used by a jellyfish when it swims through water. We show that the skyrmion's motion is a universal phenomenon, arising in any magnetic system with translational modes. By projecting the equation of motion onto the skyrmion's translational modes and going to quadratic order in $B_1(t)$, we obtain an analytical expression for v$_{trans}$ as a function of the system's linear response. The linear response and consequently v$_{trans}$ are influenced by the skyrmion's internal modes and scattering states, as well as by the ferromagnetic background's Kittel mode. The direction and speed of v$_{trans}$ can be controlled by changing the polarisation, frequency and phase of the driving field $B_1(t)$. For systems with small Gilbert damping parameter $\alpha$, we identify two distinct physical mechanisms used by the skyrmion to move. At low driving frequencies, the skyrmion's motion is driven by friction, and $v_{trans}\sim\alpha$, whereas at higher frequencies above the ferromagnetic gap the skyrmion moves by magnon emission, and $v_{trans}$ becomes independent of $\alpha$.

2 citations
A tail of eternal inflation

Timothy Cohen, Daniel Green, Akhil Premkumar

SciPost Phys. 14, 109 (2023) · published 12 May 2023 | · pdf

Non-trivial inflaton self-interactions can yield calculable signatures of primordial non-Gaussianity that are measurable in cosmic surveys. We calculate the non-Gaussian corrections to Stochastic Inflation within the framework of Soft de Sitter Effective Theory, from which we derive the associated probability distribution for the scalar fluctuations. As a consequence of this new result, we show that the phase transition to slow-roll eternal inflation is often incalculable in these models. Instead, this transition is sensitive to the non-Gaussian tail of the distribution of scalar fluctuations, which probes physics inside the horizon, potentially beyond the cutoff scale of the Effective Field Theory of Inflation. We delineate the parameter space consistent with current observations and weak coupling at horizon crossing in which the large fluctuations relevant for eternal inflation can only be determined by appealing to a UV completion. We also argue that this breakdown of the perturbative description is required for the de Sitter entropy to reflect the number of de Sitter microstates.

2 citations
Developments since Kira 2.0

Fabian Lange, Philipp Maierhöfer, Johann Usovitsch

SciPost Phys. Proc. 7, 017 (2022) · published 21 June 2022 | · pdf

Last year we released version 2.0 of the Feynman integral reduction program Kira. In this contribution we first report on changes and new features since then and, secondly, on new features for upcoming releases.

2 citations
Non-perturbative topological string theory on compact Calabi-Yau 3-folds

Jie Gu, Amir-Kian Kashani-Poor, Albrecht Klemm, Marcos Mariño

SciPost Phys. 16, 079 (2024) · published 19 March 2024 | · pdf

We obtain analytic and numerical results for the non-perturbative amplitudes of topological string theory on arbitrary, compact Calabi-Yau manifolds. Our approach is based on the theory of resurgence and extends previous special results to the more general case. In particular, we obtain explicit trans-series solutions of the holomorphic anomaly equations. Our results predict the all orders, large genus asymptotics of the topological string free energies, which we test in detail against high genus perturbative series obtained recently in the compact case. We also provide additional evidence that the Stokes constants appearing in the resurgent structure are closely related to integer BPS invariants.

2 citations
Critical properties of a comb lattice

Natalia Chepiga, Steven R. White

SciPost Phys. 9, 013 (2020) · published 27 July 2020 | · pdf

In this paper we study the critical properties of the Heisenberg spin-1/2 model on a comb lattice -- a 1D backbone decorated with finite 1D chains -- the teeth. We address the problem numerically by a comb tensor network that duplicates the geometry of a lattice. We observe a fundamental difference between the states on a comb with even and odd number of sites per tooth, which resembles an even-odd effect in spin-1/2 ladders. The comb with odd teeth is always critical, not only along the teeth, but also along the backbone, which leads to a competition between two critical regimes in orthogonal directions. In addition, we show that in a weak-backbone limit the excitation energy scales as $1/(NL)$, and not as $1/N$ or $1/L$ typical for 1D systems. For even teeth in the weak backbone limit the system corresponds to a collection of decoupled critical chains of length $L$, while in the strong backbone limit, one spin from each tooth forms the backbone, so the effective length of a critical tooth is one site shorter, $L-1$. Surprisingly, these two regimes are connected via a state where a critical chain spans over two nearest neighbor teeth, with an effective length $2L$.

2 citations
Entanglement Dynamics of Random GUE Hamiltonians

Daniel Chernowitz, Vladimir Gritsev

SciPost Phys. 10, 071 (2021) · published 19 March 2021 | · pdf

In this work, we consider a model of a subsystem interacting with a reservoir and study dynamics of entanglement assuming that the overall time-evolution is governed by non-integrable Hamiltonians. We also compare to an ensemble of Integrable Hamiltonians. To do this, we make use of unitary invariant ensembles of random matrices with either Wigner-Dyson or Poissonian distributions of energy. Using the theory of Weingarten functions, we derive universal average time evolution of the reduced density matrix and the purity and compare these results with several physical Hamiltonians: randomized versions of the transverse field Ising and XXZ models, Spin Glass and, Central Spin and SYK model. The theory excels at describing the latter two. Along the way, we find general expressions for exponential $n$-point correlation functions in the gas of GUE eigenvalues.

2 citations
Forward Higgs production at NLO using Lipatov's high energy effective action

Martin Hentschinski

SciPost Phys. Proc. 8, 136 (2022) · published 14 July 2022 | · pdf

We use Lipatov's high energy effective action to determine the next-to-leading order corrections to Higgs production in the forward region. As a new element we provide a proper definition of the desired next-to-leading order coefficient within the high energy effective action framework, extending a previously proposed prescription. We further propose a subtraction mechanism to achieve for this coefficient a stable cancellation of real and virtual infra-red singularities in the presence of external off-shell legs and discuss aspects related to choice of a reference scale for high energy resummation.

2 citations
Pressure-induced transitions in FePS$_3$: Structural, magnetic and electronic properties

Shiyu Deng, Siyu Chen, Bartomeu Monserrat, Emilio Artacho, Siddharth S. Saxena

SciPost Phys. 15, 020 (2023) · published 20 July 2023 | · pdf

FePS$_3$ is a prototype van der Waals layered antiferromagnet and a Mott insulator under ambient conditions, which has been recently reported to go through a pressure-induced dimensionality crossover and an insulator-to-metal transition. These transitions also lead to the appearance of a novel magnetic metallic state. To further understand these emergent structural and physical properties, we have performed a first-principles study using van der Waals and Hubbard $U$ corrected density functional theory including a random structure search. Our computational study attempts to interpret the experimental coexistence of the low- and intermediate-pressure phases and we predict a novel high-pressure phase with distinctive dimensionality and different possible origins of metallicity.

2 citations
Four-body Faddeev-type calculation of the $\bar{K}NNN$ system: Preliminary results

Nina V. Shevchenko

SciPost Phys. Proc. 3, 041 (2020) · published 27 February 2020 | · pdf

The paper is devoted to the $\bar{K}NNN$ system, consisting of an antikaon and three nucleons. Four-body Faddeev-type AGS equations are being solved in order to find possible quasi-bound state in the system.

2 citations
On the discrepancy between the FOPT and CIPT approaches for hadronic tau spectral function moments

Andre H. Hoang, Christoph Regner

SciPost Phys. Proc. 7, 005 (2022) · published 20 June 2022 | · pdf

The discrepancy between the FOPT and CIPT approaches for hadronic tau spectral function moments constitutes the major theoretical uncertainty for strong coupling determinations from tau decay data. We show that the discrepancy can be analytically understood since the Borel representations -- which have been assumed to be identical for both approaches previously -- differ in the presence of IR renormalons. This implies that the OPE condensate corrections are different for both approaches and that the discrepancy may eventually be reconciled. In the talk we explain the difference and some mathematical aspects of the FOPT and CIPT Borel representations and show numerical results.

2 citations

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