Experimental Signals Research Articles

Finite-momentum instability of a dynamical axion insulator

Due to the chiral anomaly, Weyl semimetals can exhibit a signature topological magnetoelectric response known as an axion term which is determined by the microscopic band structure. In the presence of strong interactions Weyl fermions may form a chiral condensate, with the intrinsic dynamics and fluctuations of the associated condensate phase producing a dynamical contribution to the axion response. Here we show that an imbalance in the density of right- and left-handed electrons drives an instability of the chiral condensate towards finite momentum and leads to strong fluctuations in the axion response. We derive a long-wavelength theory of Lifschitz type governing the dynamics of the Goldstone mode and use this to characterize its associated spatial fluctuations, which manifest as an inhomogeneous anomalous Hall effect. We show that these fluctuations produce signatures in inelastic light-scattering experiments across a broad spectrum of frequencies, and can be used to determine the structure factor for the axionic collective mode.

Stabilizing Fluctuating Spin-Triplet Superconductivity in Graphene via Induced Spin-Orbit Coupling.

A recent experiment showed that a proximity-induced Ising spin-orbit coupling enhances the spin-triplet superconductivity in Bernal bilayer graphene. Here, we show that, due to the nearly perfect spin rotation symmetry of graphene, the fluctuations of the spin orientation of the triplet order parameter suppress the superconducting transition to nearly zero temperature. Our analysis shows that both an Ising spin-orbit coupling and an in-plane magnetic field can eliminate these low-lying fluctuations and can greatly enhance the transition temperature, consistent with the recent experiment. Our model also suggests the possible existence of a phase at small anisotropy and magnetic field which exhibits quasilong-range ordered spin-singlet charge 4e superconductivity, even while the triplet 2e superconducting order only exhibits short-ranged correlations. Finally, we discuss relevant experimental signatures.

NLO QCD corrections to off-shell top–antitop production with semi-leptonic decays at lepton colliders

The study of top-quark properties will be a central aspect of the physics programme of any future lepton collider. In this article, we investigate the production of top-quark pairs in the semi-leptonic decay channel in {textrm{e}}^{+}textrm{e}^{-} collisions, whose experimental signature is one charged lepton, jets, and missing energy. We present for the first time fiducial cross sections and differential distributions at next-to-leading-order accuracy in QCD for the full off-shell process. We find that the QCD corrections for the considered process are strongly dependent on the beam energies and range from few per cent up to more than 100% (near threshold and above 1,text {TeV} ). We focus, in particular, on two scenarios: one close to threshold (365,text {GeV} ), dominated by top-pair production, and one at the TeV scale (1.5,text {TeV} ), for which irreducible-background contributions become relevant. An assessment of polarised-beam effects is also provided.

Simultaneous CTEQ-TEA extraction of PDFs and SMEFT parameters from jet and toverline{t} data

Recasting phenomenological Lagrangians in terms of SM effective field theory (SMEFT) provides a valuable means of connecting potential BSM physics at momenta well above the electroweak scale to experimental signatures at lower energies. In this work we jointly fit the Wilson coefficients of SMEFT operators as well as the PDFs in an extension of the CT18 global analysis framework, obtaining self-consistent constraints to possible BSM physics effects. Global fits are boosted with machine-learning techniques in the form of neural networks to ensure efficient scans of the full PDF+SMEFT parameter space. We focus on several operators relevant for top-quark pair and jet production at hadron colliders and obtain constraints on the Wilson coefficients with Lagrange Multiplier scans. We find mild correlations between the extracted Wilson coefficients, PDFs, and other QCD parameters, and see indications that these correlations may become more prominent in future analyses based on data of higher precision. This work serves as a new platform for joint analyses of SM and BSM physics based on the CTEQ-TEA framework.

Raman spectroscopy of bimagnon and trimagnon excitations and rotonlike points in a distorted triangular lattice antiferromagnet

We investigate the experimental signatures of Raman spectroscopy of bi- and tri-magnon excitations in the distorted triangular lattice antiferromagnets alpha-LCr2O4 (L=Sr, Ca). We utilize spin wave theory to analyze the nearly 120 degree spin-3/2 spiral ordered antiferromagnetic ground state to compute the single-magnon density of states, single-magnon dispersion, and bimagnon and trimagnon Raman spectra (polarized and unpolarized). It is found that Raman scattering is capable of capturing the effect of the rotonlike M and M' points on the bimagnon Raman spectrum. Our calculation confirms the connection between single-magnon rotonlike excitation energy and bimagnon Raman excitation spectrum observed experimentally. The roton energy minimum in momentum space is half of the energy of a bimagnon excitation signal. The experimental magnetic Raman scattering result displays two peaks which have a Raman shift of 15 meV and 40 meV, respectively. Theoretical modeling and analysis of the experimental spectrum of alpha-SrCr2O4 within our distorted Heisenberg Hamiltonian lattice suggests that the low-energy peak at 15 meV is associated with the bimagnon excitation, whereas the high-energy peak around 40 meV is primarily a trimagnon excitation. Based on our fitting procedure we propose a new set of magnetic interaction parameters for alpha-SrCr2O4. These parameters reproduce not only the experimental Raman spectrum, but also the inelastic neutron scattering response (including capturing high energy magnon branches). We also compute the unpolarized bimagnon and trimagnon Raman spectra for alpha-CaCr2O4. Furhtermore, we found that the polarization sensitivity of Raman spectrum can be utilized to distinguish the bi- and tri-magnon excitation channels.

Long-lived particle phenomenology in the 2HDM+a model

Higgs decays displaced from the primary interaction vertex represent a striking experimental signature that is actively searched for by the ATLAS, CMS and LHCb collaborations. We point out that signals of this type appear in the context of the 2HDM+a model if the mixing angle θ of the two CP-odd weak spin-0 eigenstates is tiny and the dark matter (DM) sector is either decoupled or kinematically inaccessible. Utilising two suitable benchmark scenarios, we determine the constraints on the parameter space of the 2HDM+a model that are set by the existing LHC searches for long-lived particles (LLPs) in Higgs decays. We find that depending on the precise mass spectrum of the spin-0 states, mixing angles θ in the ballpark of a few 10−8 to 10−5 can be excluded based on LHC Run II data. This finding emphasises the unique role that searches for displaced signatures can play in constraining the parameter space of the 2HDM+a model. The ensuing DM phenomenology is also discussed. In particular, we show that parameter choices leading to an interesting LLP phenomenology can simultaneously explain the DM abundance observed in today’s Universe.

Unifying semiclassics and quantum perturbation theory at nonlinear order

Nonlinear electrical response permits a unique window into effects of band structure geometry. It can be calculated either starting from a Boltzmann approach for small frequencies, or using Kubo’s formula for resonances at finite frequency. However, a precise connection between both approaches has not been established. Focusing on the second order nonlinear response, here we show how the semiclassical limit can be recovered from perturbation theory in the velocity gauge, provided that finite quasiparticle lifetimes are taken into account. We find that matrix elements related to the band geometry combine in this limit to produce the semiclassical nonlinear conductivity. We demonstrate the power of the new formalism by deriving a quantum contribution to the nonlinear conductivity which is of order \tau^{-1}τ−1 in the relaxation time \tauτ, which is principally inaccessible within the Boltzmann approach. We outline which steps can be generalized to higher orders in the applied perturbation, and comment about potential experimental signatures of our results.

Spin-Momentum Locking and Ultrafast Spin-Charge Conversion in Ultrathin Epitaxial Bi1 - x Sbx Topological Insulator.

The helicity of three-dimensional (3D) topological insulator surface states has drawn significant attention in spintronics owing to spin-momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice-versa, through the Rashba-Edelstein effect. However, experimental signatures of these surface states to the spin-charge conversion are extremely difficult to disentangle from bulk state contributions. Here, spin- and angle-resolved photo-emission spectroscopy, and time-resolved THz emission spectroscopy are combined to categorically demonstrate that spin-charge conversion arises mainly from the surface state in Bi1 - x Sbx ultrathin films, down to few nanometers where confinement effects emerge. This large conversion efficiency is correlated, typically at the level of the bulk spin Hall effect from heavy metals, to the complex Fermi surface obtained from theoretical calculations of the inverse Rashba-Edelstein response. Both surface state robustness and sizeable conversion efficiency in epitaxial Bi1 - x Sbx thin films bring new perspectives for ultra-low power magnetic random-access memories and broadband THzgeneration.

Exciton Lifetime and Optical Line Width Profile via Exciton-Phonon Interactions: Theory and First-Principles Calculations for Monolayer MoS2.

Exciton dynamics dictates the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron-phonon and many-electron interactions. We develop and apply here a first-principles approach to exciton dynamics resulting from exciton-phonon coupling in monolayer MoS2 and reveal the highly selective nature of exciton-phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest-energy bright A exciton. Moreover, we show that optical absorption processes rigorously require a second-order perturbation theory approach, with photon and phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such a treatment, thus far neglected in first-principles studies, gives rise to off-diagonal exciton-phonon self-energy, which is critical for the description of dephasing mechanisms and yields exciton line widths in excellent agreement with experiment.

Acoustic signature analysis of a bladeless blower

There has always been a concern about acoustic emissions in traditional machinery. For example, in the transportation and power generation sectors, the market demands efficient, compact, lightweight, and low-cost devices with very low noise levels. In such a challenging context, as opposed to conventional turbomachinery, Tesla type bladeless fluid machine has the advantage of reducing the overall noise emissions level.The originality of this paper lies in its experimental acoustic signature characterization of a Tesla type bladeless blower, which has never been done before, nor is there any comparison to conventional blowers. A tailored experimental setup is described in order to evaluate the acoustic behavior of Tesla blower prototypes at different rotational speeds and distances. To achieve an accurate acoustic characterization, several measurement methods, as well as post-processing techniques, have been evaluated and compared.The results of this study have been compared with those obtained by conventional blowers, while considering the same tip speeds at different distances. A Tesla blower does not exhibit the blade passage phenomenon, as opposed to a conventional blower. A significant contribution of acoustic energy can thus be avoided which is due to its characteristic frequency and its harmonic components, which instead are significant in conventional turbomachinery overall acoustic noise.As a result of this innovative investigation, it has been determined that the air Tesla blower is significantly quieter than conventional dynamic machines, thus paving the way to novel applications for bladeless machinery, especially in those cases aimed at achieving a heightened level of acoustic comfort.

Signatures of room-temperature magnetic glassiness in zinc ferrite epitaxial thin films

Zinc ferrite (${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$) epitaxial thin films were grown by reactive magnetron sputtering on ${\mathrm{MgAl}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ and ${\mathrm{Al}}_{\text{2}}{\mathrm{O}}_{\text{3}}$ substrates varying a range of preparation parameters. The resulting structural and magnetic properties were investigated using a range of experimental techniques confirming epitaxial growth of ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ with the nominal stoichiometric composition and long-range magnetic order at and above room temperature. The main preparation parameter influencing the temperature ${T}_{f}$ of the bifurcation between $M(T)$ curves under field-cooled and zero-field-cooled conditions was found to be the growth rate of the films, while growth temperature or the Ar:${\mathrm{O}}_{2}$ ratio did not systematically influence ${T}_{f}$. Furthermore ${T}_{f}$ was found to be systematically higher for ${\mathrm{MgAl}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ as substrate and ${T}_{f}$ extends to above room temperature. While in some samples ${T}_{f}$ seems to be more likely correlated with superparamagnetism, the highest ${T}_{f}$ occurs in ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ epitaxial films where experimental signatures of magnetic glassiness can be found. Element-selective x-ray magnetic circular dichroism measurements aim at associating the magnetic glassiness with the occurrence of a different valence state and lattice site incorporation of Fe pointing to a complex interplay of various competing magnetic interactions in ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$.

Andreev processes in mesoscopic multiterminal graphene Josephson junctions

There is growing interest in using multi-terminal Josephson junctions (MTJJs) as a platform to artificially emulate topological phases and to investigate complex superconducting mechanisms such as quartet and multiplet Cooper pairings. Current experimental signatures in MTJJs have led to conflicting interpretations of the salient features. In this work, we report a collaborative experimental and theoretical investigation of graphene-based four-terminal Josephson junctions. We observe resonant features in the differential resistance maps that resemble those ascribed to multiplet Cooper pairings. To understand these features, we model our junctions using a circuit network of coupled two-terminal resistively and capacitively shunted junctions (RCSJs). Under appropriate bias current, the model predicts that a current flowing between two diagonal terminals in a four-terminal geometry may be represented as a sinusoidal function of a weighted sum of the superconducting phases. We show that starting from a semi-classical model with diffusive current-phase relations, the MTJJ effectively emulates a general form of the expected current-phase relation for multiplet Cooper pairings. Our study therefore suggests that differential resistance measurements alone are insufficient to conclusively distinguish resonant Andreev reflection processes from semi-classical circuit-network effects.

Tunable zero-energy Dirac and Luttinger nodes in a two-dimensional topological superconductor

Cooper pairing in ultrathin films of topological insulators, induced intrinsically or by proximity effect, can produce an energetically favorable spin-triplet superconducting state. The spin–orbit coupling acts as an SU(2) gauge field and stimulates the formation of a spin-current vortex lattice in this superconducting state. Here we study the Bogoliubov quasiparticles in such a state and find that the quasiparticle spectrum consists of a number of Dirac nodes pinned to zero energy by the particle-hole symmetry. Some nodes are ‘accidental’ and move through the first Brillouin zone along high-symmetry directions as the order parameter magnitude or the strength of the spin–orbit coupling are varied. At special parameter values, nodes forming neutral quadruplets merge and become gapped out, temporarily producing a quadratic band-touching spectrum. All these features are tunable by controlling the order parameter magnitude via a gate voltage in a heterostructure device. In addition to analyzing the spectrum at the mean-field level, we briefly discuss a few experimental signatures of this spectrum.

Machine learning approach for predicting impedance signatures of construction steel structures in various tensile pull actions

The detection of structural damage based on the electromechanical impedance technique is proven to have a greater sensitivity to structures based on baseline impedance signature data. These baseline signature data are also used to quantify impedance signatures statistically. However, it poses significant drawbacks when implemented on existing infrastructures and early strength gain monitoring. This study investigates and aims to utilize various machine learning techniques as effective regression approaches to predict impedance signatures of construction steel for different tensile load actions at lower ranges. In recent times, machine learning approaches have been applied to diversified domains for efficient prediction and forecasting. Various machine learning algorithms were used in this study, i.e., logistic regression, ridge regression, lasso regression, k nearest neighbors (KNN), decision tree, support vector machine (SVM), and ensemble techniques such as random forest, XGBoost, AdaBoost. The experimental admittance signatures used to build these models were collected using three different multi-piezo configurations, i.e., surface bonded, clamped, and metal wire based, to monitor the tensile pull action of construction steel rebar. The models were developed using optimized hyperparameters for better model tuning. Stratified splitting was applied to validate the model. All these models were evaluated, and their efficiencies were compared using model evaluation metrics like root mean squared error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE). It was found that all the ensemble machine-learning models outperformed all other models. The MAPE value is superior for all the ML models, indicating over 90% accuracy in predicting futuristic EMI signals. This work significantly contributes to structural health monitoring (SHM) for predicting non-bonded and reusable piezo impedance data. These findings will help researchers to predict the multi-sensing impedance data, especially for complex structures, and utilize them for the futuristic health monitoring of infrastructure systems.

Isospin-Symmetry Breaking within the Nuclear Shell Model: Present Status and Developments

The paper reviews the recent progress in the description of isospin-symmetry breaking within the nuclear shell model and applications to actual problems related to the structure and decay of exotic neutron-deficient nuclei and nuclei along the N=Z line, where N is the neutron number and Z the atomic number. The review recalls the fundamentals of the isospin formalism for two-nucleon and many-nucleon systems, including quantum numbers, the spectrum’s structure and selection rules for weak and electromagnetic transitions; and at the end, summarizes experimental signatures of isospin-symmetry breaking effects, which motivated efforts towards the creation of a relevant theoretical framework to describe those phenomena. The main approaches to construct accurate isospin-nonconserving Hamiltonians within the shell model are briefly described and recent advances in the description of the structure and (isospin-forbidden) decay modes of neutron-deficient nuclei are highlighted. The paper reviews major implications of the developed theoretical tools to (i) the fundamental interaction studies on nuclear decays and (ii) the estimation of the rates of nuclear reactions that are important for nuclear astrophysics. The shell model is shown to be one of the most suitable approaches to describing isospin-symmetry breaking in nuclear states at low energies. Further efforts in extending and refining the description to larger model spaces, and in developing first-principle theories to deal with isospin-symmetry breaking in many-nucleon systems, seem to be indispensable steps towards our better understanding of nuclear properties in the precision era.

Ac-locking of thermally-induced sine-Gordon breathers

A complete framework for exciting and detecting thermally-induced, stabilized sine-Gordon breathers in ac-driven annular long Josephson junctions is developed. The formation of long-time stable breathers locked to the ac source occurs for a sufficiently high temperature. The latter emerges as a powerful control parameter, allowing for the remarkably stable localized modes to appear. Nonmonotonic behaviors of both the breather generation probability and the energy spatial correlations versus the thermal noise strength are found. The junction’s resistive switching characteristics provides a clear experimental signature of the breather.

Radiative flavor template at the LHC: g−2 and the W mass

The Standard Model of particle physics and its description of nature have been recently challenged by a series of precision measurements performed via different accelerator machines. Statistically significant anomalies emerged when measuring the muon magnetic momentum, and very recently when deducing the mass of the $\mathcal{W}$ boson. Here we consider a radiative extension of the Standard Model devised to be sufficiently versatile to reconcile the various experimental results while further predicting the existence of new bosons and fermions with a mass spectrum in the TeV energy scale. The resulting spectrum is, therefore, within the energy reach of the proton-proton collisions at the LHC experiments at CERN. The model investigated here allows us to interpolate between composite and elementary extensions of the Standard Model with an emphasis on a new modified Yukawa sector that is needed to accommodate the anomalies. Focusing on the radiative regime of the model, we introduce interesting search channels of immediate impact for the ATLAS and CMS experimental programs such as the associate production of Standard Model particles with either invisible or long-lived particles. We further show how to adapt earlier supersymmetry-motivated searchers of new physics to constrain the spectrum and couplings of the new scalars and fermions. Overall, the new physics template simultaneously accounts for the bulk of the observed experimental anomalies while suggesting a wide spectrum of experimental signatures relevant for the current LHC experiments.

Topological quantum phase transition of nickelocene on Cu(100)

Local quantum phase transitions driven by Kondo correlations have been theoretically proposed in several magnetic nanosystems; however, clear experimental signatures are scant. Modeling a nickelocene molecule on a Cu(100) substrate as a two-orbital Anderson impurity with single-ion anisotropy coupled to two conduction bands, we find that recent scanning tunneling spectra reveal the existence of a topological quantum phase transition from the usual local Fermi liquid with high zero-bias conductance to a non-Landau Fermi liquid, characterized by a non-trivial quantized Luttinger integral, with a small conductance. The effects of intermediate valence, finite temperature, and structural relaxation of the molecule position allow us to explain the different observed behaviors.

Fluctuating fractionalized spins in quasi-two-dimensional magnetic V0.85PS3

Quantum spin liquid (QSL), a state characterized by exotic low energy fractionalized excitations and statistics, is still elusive experimentally and may be gauged via indirect experimental signatures. A remnant of the QSL phase may reflect in the spin dynamics as well as quanta of lattice vibrations, i.e., phonons, via the strong coupling of phonons with underlying fractionalized excitations. Inelastic light scattering (Raman) studies on ${\mathrm{V}}_{1--x}\mathrm{P}{\mathrm{S}}_{3}$ single crystals evidence the spin fractionalization into Majorana fermions deep into the paramagnetic phase reflected in the emergence of a low frequency quasielastic response, along with a broad magnetic continuum marked by a crossover temperature ${T}^{*}\ensuremath{\sim}200$ K from a pure paramagnetic state to a fractionalized spin regime qualitatively gauged via dynamic Raman susceptibility. We found further evidence of anomalies in the phonons' self-energy parameters, in particular, phonon line broadening and line asymmetry evolution at this crossover temperature, attributed to the decaying of phonons into itinerant Majorana fermions. This anomalous scattering response is thus indicative of fluctuating fractionalized spins suggesting a phase proximate to the quantum spin liquid state in this quasi-two-dimensional magnetic system.

Fano interference between collective modes in cuprate high-Tc superconductors

Cuprate high-Tc superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency. In this study, we report a new type of Fano resonance manifested by the nonlinear terahertz response of cuprate high-Tc superconductors, where we resolve both the amplitude and phase signatures of the Fano resonance. Our extensive hole-doping and magnetic field dependent investigation suggests that the Fano resonance may arise from an interplay between the superconducting fluctuations and the charge density wave fluctuations, prompting future studies to look more closely into their dynamical interactions.