Simplified Effective Model Research Articles

Majorana zero modes in fermionic wires coupled by Aharonov-Bohm cages

We devise a number-conserving scheme for the realization of Majorana zero modes in an interacting fermionic ladder coupled by Aharonov-Bohm cages. The latter provide an efficient mechanism to cancel single-particle hopping by destructive interference. The crucial parity symmetry in each wire is thus encoded in the geometry of the setup, in particular, its translation invariance. A generic nearest-neighbor interaction generates the desired correlated hopping of pairs. We exhibit the presence of an extended topological region in parameter space, first in a simplified effective model via bosonization techniques, and subsequently in a larger parameter regime with matrix-product-states numerical simulations. We demonstrate the adiabatic connection to previous models, including exactly solvable ones, and we briefly comment on possible experimental realizations in synthetic quantum platforms, like cold atomic samples.

Signatures of a flavor changing Z′ boson in Bq → γZ′

Rare B meson decays offer an opportunity to probe a light hidden Z′ boson. In this work we explore a new channel Bq→γZ′ (q=d,s) followed by a cascade decay of Z′ into an invisible (neutrino or dark matter) or charged lepton pair ℓ+ℓ− (ℓ=e,μ). The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with Z′. For the first time, we calculate BR(Bq→γZ′) at the leading power of 1/mb and 1/Eγ. Confronting with the strong constraints from semi-invisible decays of B meson, we find that the branching ratio for Bd→invisible+γ can be larger than its Standard Model background prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for Bd→e+e−γ can also be sizable when the corresponding flavor violating Z′ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of Bd→μ+μ−γ and Bs→ℓ+ℓ−γ are severely constrained by the experimental measurements.

Impact of crowders on the morphology of bacterial chromosomes

Inspired by recent experiments on the effects of cytosolic crowders on the organization of bacterial chromosomes, we consider a “feather-boa” type model chromosome in the presence of non-additive crowders, encapsulated within a cylindrical cell. We observe spontaneous emergence of complementary helicity of the confined polymer and crowders. This feature is reproduced within a simplified effective model of the chromosome. This latter model further establishes the occurrence of longitudinal and transverse spatial segregation transitions between the chromosome and crowders upon increasing the crowder size.

LHC dark matter signals from vector resonances and top partners

Extensions of the Standard Model which address the hierarchy problem and dark matter (DM) often contain top partners and additional resonances at the TeV scale. We explore the phenomenology of a simplified effective model with a vector resonance $Z'$, a fermionic vector-like coloured partner of the top quark $T'$ as well as a scalar DM candidate $\phi$ and provide publicly available implementations in CalcHEP and MadGraph. We study the $pp \to Z' \to T'\overline{T'} \to t\bar{t}\,\phi\phi$ process at the LHC and find that it plays an important role in addition to the $T'\overline{T'}$ production via strong interactions. It turns out that the presence of the $Z'$ can provide a dominant contribution to the $t\bar{t}+E_T^{\text{miss}}$ signature without conflicting with existing bounds from $Z'$ searches in di-jet and di-lepton final states. We find that through this process, the LHC is already probing DM masses up to about 900 GeV and top partner masses up to about 1.5 TeV, thus exceeding the current bounds from QCD production alone almost by a factor of two for both particles.

Ground-state phase diagram of the one-dimensional t−J model with pair hopping terms

The $t$-$J$ model is a standard model of strongly correlated electrons, often studied in the context of high-$T_c$ superconductivity. However, most studies of this model neglect three-site terms, which appear at the same order as the superexchange $J$. As these terms correspond to pair-hopping, they are expected to play an important role in the physics of superconductivity when doped sufficiently far from half-filling. In this paper we present a density matrix renormalisation group study of the one-dimensional $t$-$J$ model with the pair hopping terms included. We demonstrate that that these additional terms radically change the one-dimensional ground state phase diagram, extending the superconducting region at low fillings, while at larger fillings, superconductivity is completely suppressed. We explain this effect by introducing a simplified effective model of repulsive hardcore bosons.

Top quark and neutrino composite Higgs bosons

In the context of top-quark condensation models, the top-quark alone is too light to saturate the correct value of the electroweak scale by its condensate. Within the seesaw scenario the neutrinos can have their Dirac masses large enough so that their condensates can provide significant contribution to the value of the electroweak scale. We address the question of a phenomenological feasibility of the top-quark and neutrino condensation conspiracy against the electroweak symmetry. Mandatory is to reproduce the masses of electroweak gauge bosons, the top-quark mass and the recently observed scalar mass of $125\,\mathrm{GeV}$ and to satisfy the upper limits on absolute value of active neutrino masses. To accomplish that we design a reasonably simplified effective model with two composite Higgs doublets. Additionally, we work with a general number $N$ of right-handed neutrino flavor triplets participating on the seesaw mechanism. There are no experimental constraints limiting this number. The upper limit is set by the model itself. Provided that the condensation scale is of order $10^{17-18}\,\mathrm{GeV}$ and the number of right-handed neutrinos is ${\cal O}(100-1000)$, the model predicts masses of additional Higgs bosons below $250\,\mathrm{GeV}$ and a suppression of the top-quark Yukawa coupling to the $125\,\mathrm{GeV}$ particle at the $\sim60\,%$ level of the Standard model value.

Random fan-out state induced by site-random interlayer couplings

We study the low-temperature properties of a classical Heisenberg model with site-random interlayer couplings on the cubic lattice. This model is introduced as a simplified effective model of Sr(Fe${}_{1\ensuremath{-}x}$Mn${}_{x}$)O${}_{2}$, which was recently synthesized. In this material, when $x=0.3$, $(\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\pi})$ and $(\ensuremath{\pi}\ensuremath{\pi}0)$ mixed ordering is observed by neutron diffraction measurements. Using Monte Carlo simulations, we find an exotic bulk spin structure that explains the experimentally obtained results. We name this spin structure the ``random fan-out state.'' The mean-field calculations provide an intuitive understanding of this phase being induced by the site-random interlayer couplings. Since Rietveld analysis assuming the random fan-out state agrees well with the neutron diffraction pattern of Sr(Fe${}_{0.7}$Mn${}_{0.3}$)O${}_{2}$, we conclude that the random fan-out state is reasonable for the spin-ordering pattern of Sr(Fe${}_{0.7}$Mn${}_{0.3}$)O${}_{2}$ at the low-temperature phase.

Electrical Manipulation of Spin States in a Single Electrostatically Gated Transition-Metal Complex

We demonstrate an electrically controlled high-spin (S = 5/2) to low-spin (S = 1/2) transition in a three-terminal device incorporating a single Mn(2+) ion coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom. Adding a single electron thus stabilizes the low-spin configuration and the corresponding sequential tunnelling current is suppressed by spin-blockade. From low-temperature inelastic cotunneling spectroscopy, we infer the magnetic excitation spectrum of the molecule and uncover also a strongly gate-dependent singlet-triplet splitting on the low-spin side. The measured bias-spectroscopy is shown to be consistent with an exact diagonalization of the Mn-complex, and an interpretation of the data is given in terms of a simplified effective model.

Simplified model for effect of Ge doping on silica fibre acoustic properties

A simplified model for the effect of Ge doping on the longitudinal acoustic modulus and acoustic damping coefficient of silica fibres is presented. The model provides the longitudinal acoustic velocity, Brillouin spectral width, and Brillouin gain coefficient as a function of the Ge concentration. The model only requires knowledge of high-level, easily-measurable physical properties of the bulk fictive materials. The model and experimental data were found to be in excellent agreement.