Publications

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THz higher-order topological photonics in Ge-on-Si heterostructures

Ian Colombo, Pietro Minazzi, Emiliano Bonera, Fabio Pezzoli, Jacopo Pedrini

SciPost Phys. Core 7, 039 (2024) · published 4 July 2024 | · pdf

We design germanium-based higher-order topological cavities for terahertz applications by breaking the symmetry of a two-dimensional photonic crystal following the Su-Schrieffer-Heeger model. Calculations demonstrate the parity inversion of the electric field in differently deformed unit cells. The interface between domains of opposite topology presents edge and corner modes. The former are chiral, locking light propagation to its helicity. The latter prove that Ge-based structures can be used as high-order topological photonic crystals. These findings can accelerate the development of Si-photonic components working in a spectral range of high technological interest.
Ultraperipheral collisions and low-x physics

Tuomas Lappi

SciPost Phys. Proc. 8, 003 (2022) · published 11 July 2022 | · pdf

Ultraperipheral collisions at the LHC and RHIC offer the highest currently available energy for photon-nucleon and photon-nucleus collisions. Thus they are a valuable tool for studying the gluonic structure of hadrons and nuclei at small x. We discuss recent theoretical work towards understanding such exclusive processes at NLO accuracy in QCD perturbation theory. These theoretical advances are also immediately relevant for understanding the physics of deep inelastic scattering at small x. We also discuss experimental results in ultraperipheral collisions, most prominently for exclusive vector meson production.
The mass of the $\pi^+$

M. Daum , P.-R. Kettle

SciPost Phys. Proc. 5, 011 (2021) · published 6 September 2021 | · pdf

At the Paul Scherrer Institute, the most precise pion mass value was obtained from a precision determination of the muon momentum, $p_{\mu^+}$, in pion decay at rest, $\pi^+ \rightarrow \mu^+ + \nu_{\mu}$. We find $p_{\mu^+} = (29.792\,00 \pm 0.000\,11$ MeV/c and $m_{\pi^+}$ = (139.570\,21 $\pm$ 0.000\,14) MeV/c$^2$. This value is more precise, however, in agreement with the recent compilation of the Particle Data Group for $m_{\pi^-}$. The agreement of $m_{\pi^+}$ with the most precise and recent single measurement, $m_{\pi^-}$ = (139.570\,77 $\pm$ 0.000\,18) MeV/c$^2$ is only fair (2.43 $\sigma$). From our $p_{\mu^+}$ value and using the above mentioned $m_{\pi^-}$ mass and the mass of the muon, $m_{\mu^+} = (105.658\,374\,5 \pm 0.000\,002\,4)$ MeV/c$^2$ we obtain an upper limit for the muon neutrino mass with 90 \%confidence, $m_{\mu_{\nu}} \leq 230$ keV/c$^2$. One also obtains a new quantitative measure of CPT invariance in the pion sector: $(m_{\pi^+} - m_{\pi^-})/m_{\pi}(\mbox{av}) = (-2.9 \pm 2.0)\cdot 10^{-6}$, an improvement by two orders of magnitude.
Stress correlations in near-crystalline packings

Roshan Maharana, Kabir Ramola

SciPost Phys. 17, 012 (2024) · published 15 July 2024 | · pdf

We derive exact results for stress correlations in near-crystalline systems in two and three dimensions. We study energy minimized configurations of particles interacting through Harmonic as well as Lennard-Jones potentials, for varying degrees of microscopic disorder and quenched forces on grains. Our findings demonstrate that the macroscopic elastic properties of such near-crystalline packings remain unchanged within a certain disorder threshold, yet they can be influenced by various factors, including packing density, pressure, and the strength of inter-particle interactions. We show that the stress correlations in such systems display anisotropic behavior at large lengthscales and are significantly influenced by the pre-stress of the system. The anisotropic nature of these correlations remains unaffected as we increase the strength of the disorder. Additionally, we derive the large lengthscale behavior for the change in the local stress components that shows a $1/r^d$ radial decay for the case of particle size disorder and a $1/r^{d-1}$ behavior for quenched forces introduced into a crystalline network. Finally, we verify our theoretical results numerically using energy-minimised static particle configurations.
Electron addition spectral functions of low-density polaron liquids

Alberto Nocera, Mona Berciu

SciPost Phys. 15, 110 (2023) · published 22 September 2023 | · pdf

Spectral functions are important quantities that contain a wealth of information about the quasiparticles of a system, and that can also be measured experimentally. For systems with electron-phonon coupling, good approximations for the spectral function are available only in the Migdal limit (at Fermi energies much larger than the typical phonon frequency, $E_F\gg \Omega$, requiring a large carrier concentration $x$) and in the single polaron limit (at $x=0$). Here we show that the region with $x\ll 1$ ($E_F <\Omega$) can also be reliably investigated with the Momentum Average (MA) variational approximation, which essentially describes the formation of a polaron above an inert Fermi sea. Specifically, we show that for the one-dimensional spinless Holstein model, the MA spectral functions compare favorably with those calculated using variationally exact density matrix renormalization group simulations (DMRG) evaluated directly in frequency-space, so long as $x<0.1$ and the adiabaticity ratio $\Omega/t>0.5$. Unlike in the Migdal limit, here 'polaronic physics' emerges already at moderate couplings. The relevance of these results for a spinful low-$x$ metal is also discussed.
Topological insulators and geometry of vector bundles

Alexander S. Sergeev

SciPost Phys. Lect. Notes 67 (2023) · published 12 April 2023 | · pdf

For a long time, band theory of solids has focused on the energy spectrum, or Hamiltonian eigenvalues. Recently, it was realized that the collection of eigenvectors also contains important physical information. The local geometry of eigenspaces determines the electric polarization, while their global twisting gives rise to the metallic surface states in topological insulators. These phenomena are central topics of the present notes. The shape of eigenspaces is also responsible for many intriguing physical analogies, which have their roots in the theory of vector bundles. We give an informal introduction to the geometry and topology of vector bundles and describe various physical models from this mathematical perspective.
Triplet character of 2D-fermion dimers arising from $s$-wave attraction via spin-orbit coupling and Zeeman splitting

Ulrich Ebling, Ulrich Zülicke, Joachim Brand

SciPost Phys. 12, 167 (2022) · published 19 May 2022 | · pdf

We theoretically study spin-$1/2$ fermions confined to two spatial dimensions and experiencing isotropic short-range attraction in the presence of both spin-orbit coupling and Zeeman spin splitting - a prototypical system for developing topological superfluidity in the many-body sector. Exact solutions for two-particle bound states are found to have a triplet contribution that dominates over the singlet part in an extended region of parameter space where the combined Zeeman- and center-of-mass-motion-induced spin-splitting energy is large. The triplet character of dimers is purest in the regime of weak $s$-wave interaction strength. Center-of-mass momentum is one of the parameters determining the existence of bound states, which we map out for both two- and one-dimensional types of spin-orbit coupling. Distinctive features emerging in the orbital part of the bound-state wave function, including but not limited to its $p$-wave character, provide observable signatures of unconventional pairing.
Decorated defect construction of gapless-SPT states

Linhao Li, Masaki Oshikawa, Yunqin Zheng

SciPost Phys. 17, 013 (2024) · published 15 July 2024 | · pdf

Symmetry protected topological (SPT) phases are one of the simplest, yet nontrivial, gapped systems that go beyond the Landau paradigm. In this work, we study an extension of the notion of SPT for gapless systems, namely, gapless symmetry protected topological states. We construct several simple gapless-SPT models using the decorated defect construction, which allow analytical understanding of non-trivial topological features including the symmetry charge under twisted boundary conditions, and boundary (quasi)-degeneracy under open boundary conditions. We also comment on the stability of the gapless-SPT models under symmetric perturbations, and apply small-scale exact diagonalization when direct analytic understanding is not available.
State diagrams to determine tree tensor network operators

Richard M. Milbradt, Qunsheng Huang, Christian B. Mendl

SciPost Phys. Core 7, 036 (2024) · published 19 June 2024 | · pdf

This work is concerned with tree tensor network operators (TTNOs) for representing quantum Hamiltonians. We first establish a mathematical framework connecting tree topologies with state diagrams. Based on these, we devise an algorithm for constructing a TTNO given a Hamiltonian. The algorithm exploits the tensor product structure of the Hamiltonian to add paths to a state diagram, while combining local operators if possible. We test the capabilities of our algorithm on random Hamiltonians for a given tree structure. Additionally, we construct explicit TTNOs for nearest neighbour interactions on a tree topology. Furthermore, we derive a bound on the bond dimension of tensor operators representing arbitrary interactions on trees. Finally, we consider an open quantum system in the form of a Heisenberg spin chain coupled to bosonic bath sites as a concrete example. We find that tree structures allow for lower bond dimensions of the Hamiltonian tensor network representation compared to a matrix product operator structure. This reduction is large enough to reduce the number of total tensor elements required as soon as the number of baths per spin reaches 3.
SUSY-QCD corrections to squark annihilation into gluons and light quarks

Luca Paolo Wiggering

SciPost Phys. Proc. 12, 047 (2023) · published 4 July 2023 | · pdf

We calculate the full one-loop SUSY-QCD corrections to stop annihilation into gluons and light quarks including the Sommerfeld enhancement effect and discuss their impact on the neutralino relic density in the MSSM.

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