Chiral Anomaly Research Articles

Decay law of magnetic turbulence with helicity balanced by chiral fermions

In plasmas composed of massless electrically charged fermions, chirality can be interchanged with magnetic helicity while preserving the total chirality through the quantum chiral anomaly. The decay of turbulent energy in plasmas such as those in the early Universe and compact stars is usually controlled by certain conservation laws. In the case of zero total chirality, when the magnetic helicity density balances with the appropriately scaled chiral chemical potential to zero, the total chirality no longer determines the decay. We propose that in such a case, an adaptation to the Hosking integral, which is conserved in nonhelical magnetically dominated turbulence, controls the decay in turbulence with helicity balanced by chiral fermions. We show, using a high resolution numerical simulation, that this is indeed the case. The magnetic energy density decays and the correlation length increases with time just like in nonhelical turbulence with vanishing chiral chemical potential. But here, the magnetic helicity density is nearly maximum and shows a scaling with time t proportional to t−2/3. This is unrelated to the t−2/3 decay of magnetic energy in fully helical magnetic turbulence. The modulus of the chiral chemical potential decays in the same fashion. This is much slower than the exponential decay previously expected in theories of asymmetric baryon production from the hypermagnetic helicity decay after axion inflation.Received 1 February 2023Accepted 4 April 2023DOI:https://doi.org/10.1103/PhysRevResearch.5.L022028Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by Bibsam.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAstrophysical fluid dynamicsCosmologyMagnetohydrodynamic turbulenceGravitation, Cosmology & AstrophysicsPlasma PhysicsFluid Dynamics

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.

Light-nuclei production in pp and pA collisions in the baryon canonical ensemble approach

The increase in yields of light nuclei with charged particle multiplicity, as reported by the ALICE collaboration at CERN in p-p and p-Pb collisions at the LHC energy is investigated in the thermal hadron resonance gas model. The model is extended to account for exact baryon number conservation. The focus is on the production of protons, deuterons, $^3$He, and $^3_\Lambda$H. A very good description of proton and deuteron yields is obtained as a function of charged particle multiplicity in the mid-rapidity region using the same fixed temperature as in central Pb-Pb collisions. The yields of light nuclei $^3$He and $^3_\Lambda$H though qualitatively explained as a function of multiplicity, are lower than the model expectation. One of the possible reasons could be that for $^{3}$He and $^{3}_{\Lambda}$H, the chemical equilibrium is not yet reached at small multiplicities.

Field-induced chiral transitions in the Weyl semimetal TaP

The chiral anomaly effect has been regarded as a hallmark of Weyl semimetals. While most research focuses on the change of electrical conductivity due to the chiral charge-pumping effect, these transport measurements are accompanied by other effects such as weak antilocalization which are difficult to exclude. Also, the magnetic field intensity at which the energy dispersion of the Landau level starts to display the chiral anomaly effect is hard to identify. Here, we report the evolutionary process of the energy dispersion of the zeroth Landau level under increasing magnetic fields, and we elucidate the relationship between the chiral Landau level and the Fermi level via magneto-optical measurements in Weyl semimetal TaP. Two series of Landau-level transitions have been extracted and fitted well to a massless Weyl fermion model and a classical semiconductor model, respectively. The field-induced chiral Landau-level transition is obtained from a comparison between the spectra under Voigt geometry and Faraday geometry in an increasing magnetic field. The chiral nature of the zeroth Landau level is found to enhance with the increase of magnetic field and clearly manifests in the magneto-optical spectrum above 13 T. The state transition of the zeroth Landau level can be explained by the strength of the field-induced chiral anomaly effect and the position of the Fermi level. These magneto-optical results unveil the chiral Landau-level transitions in TaP, which serves as an ideal material platform to analyze the chiral anomaly effect.

Evidence of magnetic fluctuation induced Weyl semimetal state in the antiferromagnetic topological insulator Mn(Bi1−xSbx)2Te4

We report $c$-axis transport studies on magnetic topological insulators $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$. We performed systematic $c$-axis magnetoresistivity measurements under high magnetic fields (up to 35 T) on several representative samples. We find that the lightly hole- and lightly electron-doped samples, while both having the same order of magnitude of carrier density and similar spin-flop transitions, exhibit sharp contrast in electronic anisotropy and transport mechanism. The electronic anisotropy is remarkably enhanced for the lightly hole-doped sample relative to pristine $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ but not for the lightly electron-doped sample. The lightly electron-doped sample displays a giant negative longitudinal magnetoresistivity (LMR) induced by the spin-valve effect at the spin-flop transition field, whereas the lightly hole-doped sample exhibits remarkable negative LMR consistent with the chiral anomaly behavior of a Weyl semimetal. Furthermore, we find the large negative LMR of the lightly hole-doped sample extends to a wide temperature range above the N\'eel temperature $({T}_{\mathrm{N}})$ where the magnetoconductivity is proportional to ${B}^{2}$. This fact, together with the short-range intralayer ferromagnetic correlation revealed in isothermal magnetization measurements, suggests the possible presence of the Weyl state above ${T}_{\mathrm{N}}$. These results demonstrate that in the $c$-axis magnetotransport of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$, the spin scattering is dominant in the lightly electron-doped sample but overwhelmed by the chiral anomaly effect in the lightly hole-doped sample due to the presence of the Weyl state. These findings extend the understanding of the transport properties of $\mathrm{Mn}{({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x})}_{2}{\mathrm{Te}}_{4}$.

Planar Hall effect in the quasi-two-dimensional topological semimetal candidate In0.93TaSe2

Here, we report the systematic study on the planar transport properties of the quasi-two-dimensional (quasi-2D) topological nodal-line semimetal candidate In0.93TaSe2. When rotating magnetic field in the plane, the anisotropic longitudinal resistance and planar Hall resistance are clearly observed and can be well described by the theoretical formulation of the planar Hall effect (PHE). Further analysis demonstrates that the anisotropic orbital resistance rather than the topological-nontrivial chiral anomaly plays a dominant role on the PHE in In0.93TaSe2. Our study provides another platform for understanding the mechanism of PHE, which may also be valuable for future planar Hall sensors based on quasi-2D materials.

Integrable sigma models on Riemann surfaces

We consider quantum aspects of a class of generalized Gross-Neveu models, which in special cases reduce to sigma models. We show that, in the case of gauged models, an admissible gauge is $A_\mu=0$, which is a direct analogue of the conformal gauge in string models. Chiral anomalies are a gauge counterpart of the Weyl anomaly, and are required to vanish. Topological effects on the worldsheet lead to an integration over moduli spaces of connections on a Riemann surface. This is an initial step in studying the effects of worldsheet geometry and topology in integrable sigma models.

Correlation-driven organic 3D topological insulator with relativistic fermions

Exploring new topological phenomena and functionalities induced by strong electron correlation has been a central issue in modern condensed-matter physics. One example is a topological insulator (TI) state and its functionality driven by the Coulomb repulsion rather than a spin-orbit coupling. Here, we report a ‘correlation-driven’ TI state realized in an organic zero-gap system α-(BETS)2I3. The topological surface state and chiral anomaly are observed in temperature and field dependences of resistance, indicating a three-dimensional TI state at low temperatures. Moreover, we observe a topological phase switching between the TI state and non-equilibrium Dirac semimetal state by a dc current, which is a unique functionality of a correlation-driven TI state. Our findings demonstrate that correlation-driven TIs are promising candidates not only for practical electronic devices but also as a field for discovering new topological phenomena and phases.

Anomalous hydrodynamics with triangular point group in 2+1 dimensions

We present a theory of hydrodynamics for a vector U(1) charge in 2+1 dimensions, whose rotational symmetry is broken to the point group of an equilateral triangle. We show that it is possible for this U(1) to have a chiral anomaly. The hydrodynamic consequence of this anomaly is the introduction of a ballistic contribution to the dispersion relation for the hydrodynamic modes. We simulate classical Markov chains and find compelling numerical evidence for the anomalous hydrodynamic universality class. Generalizations of our theory to other symmetry groups are also discussed.

Negative magnetoresistance in Dirac semimetal Cd3As2 with in-plane magnetic field perpendicular to current

Topological insulators and semimetals have exotic surface and bulk states with massless Dirac or Weyl fermions, demonstrating microscopic transport phenomenon based on relativistic theory. Chiral anomaly induced negative magnetoresistance (negative MR) under parallel magnetic field and current has been used as a probable evidence of Weyl fermions in recent years. Here we report a novel negative MR result with mutually perpendicular in-plane magnetic field and current in Cd3As2 nanowires. The negative MR has a considerable value of −16% around 1.5 K and could persist to room temperature of 300 K with value of −1%. The gate tuning and angle dependence of the negative MR demonstrate the mechanism of the observed negative MR is different from the chiral anomaly. Percolating current paths induced by charge puddles and disorder might be involved to produce such considerable negative MR. Our results indicate the negative MR effect in topological semimetals involves synergistic effects of many mechanisms besides chiral anomaly.

Temperature-induced anomalous magnetotransport in the Weyl semimetal Mn3Ge

The magnetic Weyl semimetallic state can lead to intriguing magnetotransport, such as chiral anomaly and the layered quantum Hall effect. Mn3X (X = Sn, Ge) is a noncollinear antiferromagnetic semimetal where a Weyl semimetallic state is stabilized by time-reversal symmetry breaking. Compared to the well-studied Mn3Sn, the Weyl fermion-induced magnetotransport in Mn3Ge has been merely studied. Here, we report an in-depth study on the magnetotransport in a microfabricated Mn3Ge single crystal from room temperature to 10 K. We reveal an anomalous anisotropic magnetoresistance with fourfold symmetry and a positive high-field longitudinal magnetoresistance below the critical temperature (160–170 K). The possible origin is the temperature-induced tilting of the Weyl nodes. Our study helps to understand the magnetotransport properties in the Weyl fermion system.

Decoupling intranode and internode scattering in Weyl fermions

A series of recent papers claimed that intranode scattering alone can contribute to positive longitudinal magnetoconductance (LMC) due to the chiral anomaly (CA) in Weyl semimetals (WSMs) in the quasiclassical limit. We revisit the problem of CA-induced LMC in WSMs in the quasiclassical limit and show that intranode scattering, by itself, does not result in enhancement of LMC. In the limit of zero internode scattering, chiral charge must remain conserved, which is shown to actually decrease LMC. Only in the presence of a finite nonzero internode scattering does one obtain a positive LMC due to nonconservation of the chiral charge. Even weak internode scattering suffices in generating positive LMC since it redistributes charges across both the nodes, although on a timescale larger than that of the intranode scattering. Our work is fundamental to correctly interpret the recent experiments on magnetoconductance in Weyl semimetals that claim to have observed chiral anomaly. Furthermore, our calculations reveal that, in contrast to recent works, in inhomogeneous WSMs strain-induced axial magnetic field ${B}_{5}$, by itself, leads to negative longitudinal magnetoconductance and a negative planar Hall conductance.

Topological order in interacting semimetals

It has recently been demonstrated that it is possible to open a gap in a magnetic Weyl semimetal, while preserving the chiral anomaly along with the charge conservation and translational symmetries, which all protect the gapless nodes in a weakly interacting semimetal. The resulting state was shown to be a nontrivial generalization of a non-Abelian fractional quantum Hall liquid to three dimensions. Here we point out that a second fractional quantum Hall state exists in this case. This state has exactly the same electrical and thermal Hall responses as the first, but a distinct (fracton) topological order. Moreover, the existence of this second fractional quantum Hall state necessarily implies a gapless phase, which has identical topological response to a noninteracting Weyl semimetal, but is distinct from it. This may be viewed as a generalization (in a weaker form) of the known duality between a noninteracting two-dimensional Dirac fermion and ${\mathrm{QED}}_{3}$ to $3+1$ dimensions. In addition we discuss a $(3+1)$-dimensional topologically ordered state, obtained by gapping a nodal line semimetal without breaking symmetries.

Event-by-event antideuteron multiplicity fluctuation in Pb+Pb collisions at [formula omitted] TeV

Using the nucleon coalescence model based on kinetic freeze-out nucleons from the hybrid model of MUSIC hydrodynamics and UrQMD hadronic transport model, we study the production of antideuteron and its event-by-event fluctuation in Pb+Pb collisions at sNN=5.02 TeV. We find a clear suppression of the antideuteron to antiproton yield ratio in peripheral collisions, which is in accordance with the measurements from the ALICE Collaboration. Also found is a Poissonian event-by-event fluctuation of the antideuteron multiplicity in all collision centralities, which is different from the prediction of a simple coalescence model calculation which assumes that all antiproton and antineutron pairs have the same probability to form an antideuteron. However, due to the lack of baryon number conservation at the particlization when the viscous fluid in the hydrodynamic stage of MUSIC is converted to a hadronic matter, our model fails to describe the net-proton number fluctuation and the negative correlation between the antiproton and antideuteron multiplicities at large pseudorapidity acceptance observed in ALICE experiments. Both can nevertheless be well reproduced after selecting a suitable subset of events to fit the measured net-proton number fluctuation.

Challenges in solving chiral hydrodynamics

We prove that ideal chiral hydrodynamics, as derived from chiral kinetic theory, is acausal and its initial-value problem is ill posed, both in the linearized case around a local equilibrium solution and also in the full nonlinear regime. Therefore, such theory cannot be used to determine how the chiral anomaly affects the hydrodynamic evolution. We show that these fundamental issues can be fixed by using different definitions (frames) for the hydrodynamic fields. This leads to a causal theory of ideal chiral hydrodynamics where the vorticity strength is constrained by the coefficient that encodes the anomaly.

Cumulants from short-range correlations and baryon number conservation at next-to-leading order

We calculate the baryon number cumulants within acceptance with short-range correlations and global baryon number conservation in terms of cumulants in the whole system without baryon conservation. We extract leading and next-to-leading order terms of the large baryon number limit approximation. Our results extend the findings of Refs. [1,2]. These approximations are checked to be very close to the exact results.

Gravitational chiral anomaly and anomalous transport for fields with spin 3/2

In a fluid with vorticity and acceleration, an axial current arises in the third order of gradient expansion, the corresponding phenomenon can be called the Kinematical vortical effect (KVE). It has recently been shown that the KVE while existing in the absence of gravitational fields, is nonetheless associated with effects in curved space-time, namely with the gravitational chiral quantum anomaly. In this paper, we find the KVE transport coefficients using the Zubarev quantum-statistical density operator for the Rarita-Schwinger-Adler theory, which includes massless fields with spins 3/2 and 1/2, and demonstrate the relationship with the gravitational anomaly. A prediction is made about the possible form of the transport coefficients for massless fields with arbitrary spin.

Large Topological Hall Effect and Spiral Magnetic Order in the Weyl Semimetal SmAlSi

Weyl electrons are intensely studied due to novel charge transport phenomena such as chiral anomaly, Fermi arcs, and photogalvanic effect. Recent theoretical works suggest that Weyl electrons can also participate in magnetic interactions, and the Weyl-mediated indirect exchange coupling between local moments is proposed as a new mechanism of spiral magnetism that involves chiral electrons. Despite reports of incommensurate and non-collinear magnetic ordering in Weyl semimetals, an actual spiral order has remained hitherto undetected. Here, we present evidence of Weyl-mediated spiral magnetism in SmAlSi from neutron diffraction, transport, and thermodynamic data. We show that the spiral order in SmAlSi results from the nesting between topologically non-trivial Fermi pockets and weak magnetocrystalline anisotropy, unlike related materials (Ce,Pr,Nd)AlSi, where a strong anisotropy prevents the spins from freely rotating. We map the magnetic phase diagram of SmAlSi and reveal an A-phase where topological magnetic excitations may exist. This is corroborated by the observation of a topological Hall effect within the A-phase.

RGESolver: a C++ library to perform renormalization group evolution in the Standard Model Effective Theory

Renormalization group evolution above the electroweak scale is a crucial ingredient in the phenomenology of the Standard Model Effective Theory. The RGESolver open-source C++ library performs the evolution at leading order for dimension-six operators in the most general flavour scenario (assuming lepton and baryon number conservation). Given its efficiency, RGESolver can be used to include the effects of renormalization group evolution in extensive phenomenological analyses in the framework of the Standard Model Effective Theory.

Gauge invariance and anomalies in condensed matter physics

This paper begins with a summary of a powerful formalism for the study of electronic states in condensed matter physics called “gauge theory of states/phases of matter.” The chiral anomaly, which plays quite a prominent role in that formalism, is recalled. I then sketch an application of the chiral anomaly in 1 + 1 dimensions to quantum wires. Subsequently, some elements of the quantum Hall effect in two-dimensional (2D) gapped (“incompressible”) electron liquids are reviewed. In particular, I discuss the role of anomalous chiral edge currents and of the anomaly inflow in 2D gapped electron liquids with explicitly or spontaneously broken time reversal, i.e., in Hall and Chern insulators. The topological Chern–Simons action yielding transport equations valid in the bulk of such systems and the associated anomalous edge action are derived. The results of a general classification of “Abelian” Hall insulators are outlined. After some remarks on induced Chern–Simons actions, I sketch results on certain 2D chiral photonic wave guides. I then continue with an analysis of chiral edge spin-currents and bulk response equations in time-reversal invariant 2D topological insulators of electron gases with spin–orbit interactions. The “chiral magnetic effect” in 3D systems and axion-electrodynamics are reviewed next. This prepares the ground for an outline of a general theory of 3D topological insulators, including “axionic insulators.” Some remarks on Weyl semi-metals, which exhibit the chiral magnetic effect, and on Mott transitions in 3D systems with dynamical axion-like degrees of freedom conclude this review.