Chiral Anomaly Research Articles

Robustness of semimetallic transport properties of TaAs against off-stoichiometric disorder

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Helicity conservation in perfect electromagnetic and chiral fluids

We derive the total helicity conservation law for a perfect electromagnetic relativistic fluid. As the conservation equation contains the derivative of the magnetic helicity, it can be reshaped as having the same form as the chiral anomaly equation if the fluid is isentropic. We also take the non-relativistic limit of the helicity conservation law, and check the agreement with the Abanov-Wiegmann equation at zero temperature, but we provide further corrections in the more general case. We then consider chiral fluids, when the chiral anomaly equation has to be incorporated in the hydrodynamical equations, together with other chiral transport effects which exist in the presence of a chiral imbalance. We finally study how the chiral imbalance modifies the helicity conservation law.

Chiral Transport Phenomena and Compact Stars

I will review the main chiral transport phemomena arising in systems made up of (almost) massless fermions associated to the quantum chiral anomaly. These quantum effects might have relevant implications in compact stars, and I will review some relevant works that reveal so. I will also show how a conservation law that has the same form of the chiral anomaly also emerge in perfect classical fluids, which expresses a conservation law of magnetic, fluid and mixed helicities for isentropic fluids, and why this should also be relevant in compact stars.

On the Search for a Gravitational Chiral Anomaly Outside Curved Spacetime

On the Search for a Gravitational Chiral Anomaly Outside Curved Spacetime

Anomalies, the Dark Universe and Matter-Antimatter asymmetry

I review a (3+1)-dimensional, string-inspired cosmological model with gravitational anomalies (of Chern-Simons (CS) type) at early epochs, and a totally-antisymmetric torsion, dual to a massless axion-like field (“gravitational axion”), which couples to the CS term. Under appropriate conditions, primordial gravitational waves can condense, leading to a condensate of the CS anomaly term. As a consequence, one obtains inflation in this theory, of running-vacuum-model (RVM) type, without the need for external inflatons. At the end of the inflationary era, chiral fermionic matter is generated, whose gravitational anomalies cancel the primordial ones. On the other hand, chiral anomalies of gauge type, which are also generated by the chiral matter, remain present during the post-inflationary epochs and become responsible for the generation of a non-perturbative mass for the torsion-related gravitational axion, which, in this way, might play the rôle of a Dark Matter component of geometrical origin. Moreover, in this model, stringy non-perturbative effects during the RVM inflationary phase generate periodic structures for the potential of axion-like particles that arise due to compactification, and co-exist with the gravitational axions. Such periodic potential modulations may lead to an enhanced production of primordial black holes during inflation, which in turn affects the profile of the generated gravitational waves during the radiation era, with potentially observable consequences. This model also entails an unconventional mechanism for Leptogenesis, due to Lorentz-violating backgrounds of the gravitational axions that are generated during inflation, as a consequence of the anomaly condensates, and remain undiluted in the radiation era.

Nonlinear current response and electric quantum oscillations in the Dirac semimetal Cd3As2

Chiral anomaly is a distinct quantum anomaly associated with chiral fermions in Dirac or Weyl semimetals. The use of negative magnetoresistance (negative MR) as a signature for this anomaly remains contentious, as trivial mechanisms such as current jetting and weak localization can also induce negative MR. In this study, we report a novel nonlinear behavior of the chiral anomaly in the longitudinal direction, which we observed by applying parallel current and magnetic field to the Dirac semimetal Cd3As2. This nonlinear characteristic peaks at an intermediate magnetic field of approximately 5 T, displaying a resistance-increasing property concomitant with strengthening of the current source. Through angle-dependence experiments, we were able to rule out trivial factors, such as thermal effects, geometric artifacts, and anisotropy. Furthermore, additional electric quantum oscillations were observed when the direct current (DC) was applied as high as 300 μA. Such an unusual phenomenon is ascribed to the formation of quantized levels due to Bloch oscillation in the high DC regime, suggesting that an oscillatory density distribution may arise as the electric field increases. The non-Ohmic electric quantum oscillations open a new avenue for exploring chiral anomaly and other nontrivial topological properties, which is also one of the salient features of nonequilibrium steady states in condensed matter physics.

Quasicrystals in QCD

We study the ground state of the low energy dense QCD with the assumption of chiral condensates of quarks. Under an external magnetic field, mesons could form soliton lattices via the chiral anomaly. For such scenarios, we present a unified description of pions and η meson with a U(2) field in the framework of the chiral perturbation theory. Our result shows the ground state is a mixture of the magnetized domain walls formed by neutral pion π0 and η meson when they coexist. The winding number of the ground state would alter according to the strength of the magnetic field. When the magnetic field is strong or the chemical potential is large, the proportion of the mixture is determined by the decay constants and the contributions to the anomalous action of π0 and η meson. The resulting configuration is either a mixed soliton lattice or a quasicrystal which could be dubbed a “chiral soliton quasicrystal”.

Fermi-Arc Metals.

We predict a novel metallic state of matter that emerges in a Weyl-semimetal superstructure withspatially varying Weyl-node positions. In the new state, the Weyl nodes are stretched into extended, anisotropic Fermi surfaces, which can be understood as being built from Fermi arclike states. This "Fermi-arc metal" exhibits the chiral anomaly of the parental Weyl semimetal. However, unlike in the parental Weyl semimetal, in the Fermi-arc metal the "ultraquantum state," in which the anomalous chiral Landau level is the only state at the Fermi energy, is already reached for a finite energy window at zero magnetic field. Thedominance of the ultraquantum state implies a universal low-field ballistic magnetoconductance and the absence of quantum oscillations, making the Fermi surface "invisible" to de Haas-van Alphen and Shubnikov-de Haas effects, although it signifies its presence in other response properties.

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.