Chiral Anomaly Research Articles

Giant Negative Magnetoresistance beyond Chiral Anomaly in Topological Material YCuAs2.

The large negative magnetoresistance (MR) effect, which usually emerges in various magnetic systems, is a technologically important property for spintronics. Recently, the so-called "chiral anomaly" in topological semimetals offers an alternative to generate a considerable negative MR effect without utilizing magnetism. However, it requires that the applied magnetic field must be strictly along the electric current direction, which sets a strong limit for practical applications. Here, a giant negative MR effect is discovered with a value of up to -40% in 9 T at 2 K in the nonmagnetic Dirac material YCuAs2 , which is not restricted to the specific configuration for applied magnetic fields. Based on magnetic susceptibility and NMR measurements, the giant negative MR effect is tightly connected with the unexpected spin-dependent scattering from vacancy-induced local moments, which is also beyond the classical Kondo effect. The present work not only offers an alternative route for spintronics based on nonmagnetic topological materials, but also helps to further understand the negative MR effect in topological materials.

Possible Signatures of Chiral Anomaly in the Magnetoresistance of a Quasi‐2‐Dimensional Electron Gas at the Interface of LaVO3 and KTaO3

AbstractIn a Dirac semimetal, charges flow between two Weyl nodes when electric and magnetic fields (E||B) are parallel to each other manifesting interesting physical properties such as negative longitudinal magnetoresistance (NLMR), planar Hall effect (PHE). A co‐existence of weak antilocalization (WAL) is observed with large NLMR and an unusual Hall resistance with (B||E) configuration, at the conducting interface of LaVO3 (LVO) and KTaO3 (KTO). The full‐width half maxima of the depth of the conducting channel at the interface is estimated to be around 10 nm by using spectroscopy techniques of photoluminescence and time‐correlated single‐photon counting: suggesting a quasi‐2‐dimensional nature of the conducting electrons. The PHE exhibits oscillatory behavior as a function of the angle between E and B. Very similar temperature dependence of NLMR and PHE suggest a strong correlation among them.

Universality in anomaly flow

Abstract Universality in anomaly flow by an Aharonov–Bohm phase θH is shown in the flat M4 × (S1/Z2) spacetime and in the Randall–Sundrum (RS) warped space. We analyze the SU(2) gauge theory with doublet fermions. With orbifold boundary conditions the U(1) part of the gauge symmetry remains unbroken at θH = 0 and π. Chiral anomalies smoothly vary with θH in the RS space. It is shown that the anomaly coefficients associated with this anomaly flow are expressed in terms of the values of the wave functions of the gauge fields at the UV and IR branes in the RS space. The anomaly coefficients depend on θH, the warp factor of the RS space, and the orbifold boundary conditions for fermions, but not on the bulk mass parameters of fermions.

Torsional anomaly in Kerr black hole and quantization in Einstein–Cartan gravity

Recently, presence of gravitational and axial anomalies in Riemann–Cartan (RC) spacetime [Garcia de Andrade, Class. Quantum Gravity 38 (2021)] indicates that topological densities expressed in terms of torsion may be very useful in understanding the physics involved. Pontryagin and Euler density may be presented in terms of torsion in teleparallelism. In this paper, computation of these topological torsional densities in Kerr black holes is given. The geometric quantisation of torsion is also discussed in terms of the teleparallel metric. Actually, the recent work of Del Grosso and Poplawski [arXiv: 2107.06112] showed that the torsion quantization appears when torsion is the generator of momentum quantum mechanical commutator. Moreover, Poplawski et al. showed that use of quantum torsion in quantum electrodynamic (QED) may avoid the divergences in Feynmann integrals, process called, a torsion regularization. Here, we compute the momenta components in terms of teleparallel Kerr black hole torsion generator, like a Casimir tensor. The quantum generator, while not yet related by torsion, is seen to be naturally associated to axial torsion skew-symmetric in the teleparallel geometry. An important fact is that from torsion quantization, torsion chirality appears naturally in the Einstein–Cartan spin-torsion analogy, where now spin-angular momentum of the black hole is connected to torsion. Torsional chiral anomalies are shown to diverge at black hole singularity, and then they cannot be canceled at singularity.

Chiral anomaly in Euler fluid and Beltrami flow

We show that barotropic flows of a perfect, charged, classical fluid exhibit an anomaly analogous to the chiral anomaly known in quantum field theories with Dirac fermions. A prominent effect of the chiral anomaly is the transport electric current in the fluid at equilibrium with the chiral reservoir. We find that it is also a property of celebrated Beltrami flows — stationary solutions of the Euler equation with an extensive helicity.

Probing cosmic strings by reconstructing polarization rotation of the cosmic microwave background

Cosmic birefringence — the rotation of the polarization of the cosmic microwave background (CMB) photons as they travel to the Earth — is a smoking gun for axion-like particles (ALPs) that interact with the photon. It has recently been suggested that topological defects in the ALP field called cosmic strings can cause polarization rotation in quantized amounts that are proportional to the electromagnetic chiral anomaly coefficient \U0001d49c, which holds direct information about physics at very high energies. In this work, we study the detectability of a random network of cosmic strings through estimating rotation using quadratic estimators (QEs). We show that the QE derived from the maximum likelihood estimator is equivalent to the recently proposed global-minimum-variance QE; the classic Hu-Okamoto QE equals the global-minimum-variance QE under special conditions, but is otherwise still nearly globally optimal. We calculate the sensitivity of QEs to cosmic birefringence from string networks, for the Planck satellite mission, as well as for third- and fourth-generation ground-based CMB experiments. Using published binned rotation power spectrum derived from the Planck 2015 polarization data, we obtain a constraint \U0001d49c2 ξ0 < 0.93 at the 95% confidence level, where ξ0 is the total length of strings in units of the Hubble scale per Hubble volume, for a phenomenological but reasonable string network model describing a continuous distribution of string sizes. Looking forward, experiments such as the Simons Observatory and CMB-S4 will either discover or falsify the existence of an ALP string network for the theoretically plausible parameter space \U0001d49c ξ0 ≳ 0.01.

Coexistence of Kondo effect and Weyl semimetallic states in Mn-doped MnxVAl3 compounds

The strong correlation effect in Weyl semimetal is a critical issue in condensed matter physics. Recently, the Kondo effect in Weyl semimetal was theoretically proposed but not yet experimentally realized. Here we suggest a coexistence of the Weyl semimetal and Kondo effect in disordered Mn-doped MnxVAl3. Dilute Mn-doping in type-II Dirac semimetal VAl3 increases the chemical potential so that Dirac point is close to the Fermi energy and lifts band degeneracy, leading to the Weyl semimetal phase transition. We observed a Kondo effect, confirmed by the resistivity minimum at TK = 40 K, and logarithmic increase of electrical resistivity, magnetic susceptibility, and specific heat divided by temperature with a significant Sommerfeld coefficient at low temperature. The angle-resolved magnetoresistance has revealed the negative longitudinal magnetoresistance below Kondo temperature due to chiral anomaly in Mn-doped MnxVAl3. At low temperature below Kondo temperature (T ≤ TK), the exchange interaction by RKKY interaction in MnxVAl3 breaks time-reversal symmetry even in Kondo screening, resulting in the topological phase transition from Dirac to Weyl semimetal. This research shows the coexistence of the Kondo effect and Weyl semimetallic state as well as the temperature-induced topological phase transition.

Chiral anomaly in noncentrosymmetric systems induced by spin-orbit coupling

The chiral anomaly may be realized in condensed matter systems with pairs of Weyl points. Here we show that the chiral anomaly can be realized in diverse noncentrosymmetric systems even without Weyl point pairs when spin-orbit coupling induces nonzero Berry curvature flux through Fermi surfaces. This motivates the condensed matter chiral anomaly to be interpreted as a Fermi surface property rather than a Weyl point property. The spin-orbit-coupling-induced anomaly reproduces the well-known charge transport properties of the chiral anomaly such as the negative longitudinal magnetoresistance and the planar Hall effect in Weyl semimetals. Since it is of spin-orbit coupling origin, it also affects the spin transport and gives rise to anomaly-induced longitudinal spin currents and the magnetic spin Hall effect, which are absent in conventional Weyl semimetals.

Weak localization in disordered spin-1 chiral fermions

We theoretically investigate the quantum interference theory of magnetotransport of the three-component or spin-1 chiral fermions, which possess two linear Dirac bands and a flat band. For isotropic scalar impurities, the correction of conductivity from the coherent backscatter and non-coherent backscatter contributions cancel out in the intravalley scattering, leading to a weak localization correction to the Drude conductivity from the intervalley scattering. For the anisotropic impurities, the above cancelation is removed, we find the approximative quantum interference conductivity in the weak anisotropy case. The contributions from the chiral anomaly and classical Lorentz force are also discussed. Our work reveals some intriguing and detectable transport signatures of the novel spin-1 chiral fermions.

Confined chiral dynamo action in torsional dilatonic DWs from CME discontinuities

In this paper, spin polarized domain walls in Einstein–Cartan (EC) gravity have been recently investigated [L. C. Garcia de Andrade, Einstein–Cartan non-supersymmetric dynamo walls, Ann. Phys. 432 (2021).], where the spin polarizes along the direction orthogonal to planar domain wall. In this case, dynamo effects may appear on the wall. Moreover, dilatonic domain walls (DDWs) have shown to be responsible for inflating DWs in EC gravity, where DWs were non-magnetized [L. Garcia de Andrade, Axial and chiral anomalies in Riemann–Cartan spacetime black holes and electrodynamics, Class. Quantum Grav. 16 (1999) 2097–2103.]. In this paper, our goal is to show that when we consider a magnetized DW in the EC–Maxwell cosmology, the dilaton potential in the form of a DDW potential expressed in terms of torsion and the magnetic energy density may drive dynamo effects. Analytical solution of the dynamo equation shows that depending on the handness of DDW metric, the magnetic fields are amplified by dynamo action. From the coupled system of ECM and dilaton field equations, we find a relation between the magnetic energy density of [Formula: see text] and the DDW torsion of [Formula: see text] which leads to a thin DDW thickness of [Formula: see text]. For a galactic magnetic field, energy density of [Formula: see text] leads to a thickness of [Formula: see text], a much thinner DDW. For galactic dynamo amplification, the ratio r between the magnetic energy density and photon energy density is around [Formula: see text]. From this ratio, a relation between photon mass density and torsion is obtained. Chiral Magnetic effect (CME), torsion and magnetic discontinuities along the wall are investigated. Magnetic field discontinuity across the wall allows us to understand why the magnetic fields at DDW must be located along the wall since the orthogonal magnetic field may vanish.

Measurements of Proton High-Order Cumulants in sqrt[s_{NN}]=3 GeV Au+Au Collisions and Implications for the QCD Critical Point.

We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at sqrt[s_{NN}]=3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (p_{T}) within -0.5<y<0 and 0.4<p_{T}<2.0 GeV/c. In the most central 0%-5% collisions, a proton cumulant ratio is measured to be C_{4}/C_{2}=-0.85±0.09 (stat)±0.82 (syst), which is 2σ below the Poisson baseline with respect to both the statistical and systematic uncertainties. The hadronic transport UrQMD model reproduces our C_{4}/C_{2} in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C_{4}/C_{2} is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3GeV.

Heat kernel, spectral functions and anomalies in Weyl semimetals

Being motivated by applications to the physics of Weyl semimetals we study spectral geometry of Dirac operator with an abelian gauge field and an axial vector field. We impose chiral bag boundary conditions with variable chiral phase θ on the fermions. We establish main properties of the spectral functions which ensure applicability of the ζ function regularization and of the usual heat kernel formulae for chiral and parity anomalies. We develop computational methods, including a perturbation expansion for the heat kernel. We show that the terms in both anomalies which include electromagnetic potential are independent of θ.

Constraining Non-Dissipative Transport Coefficients in Global Equilibrium

The fluid in global equilibrium must fulfill some constraints. These constraints can be derived from quantum statistical theory or kinetic theory. In this work, we show how these constraints can be applied to determine the non-dissipative transport coefficients for chiral systems along with the energy-momentum conservation, chiral anomaly for charge current and trace anomaly in the energy-momentum tensor.

Single pair of Weyl nodes in the spin-canted structure of EuCd2As2

Time reversal symmetry breaking Weyl semimetals are unique among Weyl materials in allowing the minimal number of Weyl points, thus offering the clearest signatures of the associated physics. Here we present neutron diffraction, density-functional theory, and transport measurement results which indicate that ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$, under ambient field, strain, and pressure, is such a material with a single pair of Weyl points. Our work reveals a magnetic structure (magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$) with Eu moments pointing along the [210] direction in plane and canted $\ensuremath{\sim}$ ${30}^{\ensuremath{\circ}}$ out of plane. Density functional theory calculations using this structure show that the observed canting drastically alters the relevant electronic bands, relative to the in-plane order, leading to a single set of well-defined Weyl points. Furthermore, we find the canting angle can tune the distance of the Weyl points above the Fermi level, with the smallest distance at low canting angles. Finally, transport measurements of the anomalous Hall effect and longitudinal magnetoresistance exhibit properties indicative of a chiral anomaly, thus supporting the neutron scattering and DFT results suggesting ${\mathrm{EuCd}}_{2}{\mathrm{As}}_{2}$ is close to the ideal situation of the Weyl hydrogen atom.

Possible permanent Dirac- to Weyl-semimetal phase transition by ion implantation

Three-dimensional (3D) topological semimetals (TSMs) are a new class of Dirac materials that can be viewed as 3D graphene and are referred to as Dirac semimetals (DSMs) or Weyl semimetals (WSMs) depending on whether time reversal symmetry and/or inversion symmetry are protected, respectively. Despite some interesting results on Dirac- to Weyl-semimetal phase transitions under conditions of low temperature or strong magnetic field (B), all of them are reversible phenomena. Here, we report for the first time a possible permanent transition in a single TSM by ion implantation. A Dirac- to Weyl-semimetal phase transition in a Bi0.96Sb0.04 DSM results from inversion-symmetry breaking induced by implantation with nonmagnetic Au ions for implant fluences (ϕG) ≥ 3.2 × 1016 Au cm−2. This phenomenon is evidenced by the ϕG-dependent behavior of the Raman spectra and quantum-oscillation parameters extracted from magnetoresistance (MR) measurements, which show abrupt changes at ϕG ≥ 3.2 × 1016 Au cm−2. The verification of the transition is further supported by observations of negative MR in the longitudinal B // electric field orientation, indicating the existence of a chiral anomaly in Weyl fermions induced by implantation with nonmagnetic Au ions. In contrast, implantation with magnetic Mn ions exhibits no such particular behavior. Our findings demonstrate the first realization of a possible permanent DSM-to-WSM phase transition in a single material by the simple approach of implantation using nonmagnetic elements.

PtTe2-based terahertz photodetector integrated with an interdigital antenna

Two-dimensional materials such as graphene and black phosphorus have attracted considerable attention owing to their peculiar properties and unique applications, serve also as potential candidates for terahertz (THz) photodetection. Recent works have shown that the responsivity and sensitivity of photodetectors have been improved largely. However, there are still a few bottlenecks imposed by the fundamental limits of optical properties that impede further improvement of THz photodetection. With the continuous exploration of two-dimensional materials, especially the topological quantum states of the Dirac semimetals, which possesses uniquely non-linear optical effects, chiral anomaly and other quantization phenomena, provide a new platform for the study of fast and efficient THz photodetection. In this work, we experimentally demonstrated THz photodetectors based on Dirac semimetal-PtTe2 integrated with an interdigital antenna, which gives rise to high responsivity and fast response time in the meantime at room temperature. The achieved photocurrent responsivity is over 62 mA/W (0.12 THz) in the self-powered mode, with the noise equivalent power (NEP) is less than 60 pW/Hz0.5 (0.12 THz), and the performance has been significantly improved by applying finite voltage. Our results provide a new approach for developing high-performance THz photodetectors based on type-II Dirac semimetals.

Weak antilocalization effect and triply degenerate state in Cu-doped CaAuAs

The effect of 50% Cu doping at the Au site in the topological Dirac semimetal CaAuAs is investigated through electronic band structure calculations, electrical resistivity, and magnetotransport measurements. Electronic structure calculations suggest a broken-symmetry-driven topological phase transition from the Dirac to triple-point state in CaAuAs via alloy engineering. The electrical resistivity of both the CaAuAs and ${\mathrm{CaAu}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ compounds shows metallic behavior. Nonsaturating quasilinear magnetoresistance (MR) behavior is observed in CaAuAs. On the other hand, MR of the doped compound shows a pronounced cusplike feature in the low-field regime. Such behavior of MR in ${\mathrm{CaAu}}_{0.5}{\mathrm{Cu}}_{0.5}\mathrm{As}$ is attributed to the weak antilocalization (WAL) effect. The WAL effect is analyzed using different theoretical models, including the semiclassical $\ensuremath{\sim}\sqrt{B}$ one which accounts for the three-dimensional WAL and the modified Hikami-Larkin-Nagaoka model. A strong WAL effect is also observed in the longitudinal MR, which is well described by the generalized Altshuler-Aronov model. Our study suggests that the WAL effect originates from weak disorder and the spin-orbit coupled bulk state. Interestingly, we have also observed the signature of chiral anomaly in longitudinal MR when both the current and field are applied along the $c$ axis. The Hall resistivity measurements indicate that the charge conduction mechanism in these compounds is dominated by the holes with a concentration $\ensuremath{\sim}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and mobility $\ensuremath{\sim}{10}^{2}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{4pt}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{4pt}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.

Competition of chiral soliton lattice and Abrikosov vortex lattice in QCD with isospin chemical potential

We investigate the thermodynamics of two-flavor quark matter in presence of nonzero isospin chemical potential and external magnetic field. It is known that at sufficiently high isospin chemical potential, charged pions undergo Bose–Einstein condensation (BEC). The condensate behaves as a superconductor, exhibiting Meissner effect in weak external magnetic fields. Stronger fields stress the superconducting state, turning it first into an Abrikosov lattice of vortices, and eventually destroying the condensate altogether. On the other hand, for sufficiently strong magnetic fields and low-to-moderate isospin chemical potential, the ground state of quantum chromodynamics (QCD) is expected to be a spatially modulated condensate of neutral pions, induced by the chiral anomaly: the chiral soliton lattice (CSL). We map the phase diagram of QCD as a function of isospin chemical potential and magnetic field in the part of the parameter space accessible to a low-energy effective field theory description of QCD. Our main result is an explicit account of the competition between the CSL and the Abrikosov vortex lattice. This is accomplished by adopting a fast numerical algorithm for finding the vortex lattice solution of the equation of motion and the corresponding Gibbs energy. We find that the Abrikosov vortex lattice phase persists in the phase diagram, separating the uniform charged pion BEC phase from the CSL phase. The precise layout of the phase diagram depends sensitively on the choice of the vacuum pion mass.

Evidence for the Chiral Anomaly in the Coherent Scattering of Kaons on Copper Nuclei

Evidence for the Chiral Anomaly in the Coherent Scattering of Kaons on Copper Nuclei

Freeze-in and freeze-out generation of lepton asymmetries after baryogenesis in the νMSM

The νMSM — an extension of the Standard Model by three relatively light singlet Majorana fermions N 1,2,3 — allows for the generation of lepton asymmetry which is several orders of magnitude larger than the observed baryon asymmetry of the Universe. The lepton asymmetry is produced in interactions of N 2,3 (with masses in the GeV region) at temperatures below the sphaleron freeze out T ≲ 130 GeV and can enhance the cosmological production of dark matter (DM) sterile neutrinos N 1 (with the mass of the keV scale) happening at T ∼ 200 MeV due to active-sterile neutrino mixing.This asymmetry can be generated in freeze-in, freeze-out, or later in decays of heavy neutral leptons.In this work, we address the question of the magnitude of the late-time asymmetry (LTA) generated by the heavy neutral leptons N 2,3 during their freeze-in and freeze-out, leaving the decays for later work. We study how much of this asymmetry can survive down to the lower temperatures relevant for the sterile neutrino DM creation. We find that this LTA could result in the production of a sizeable fraction of dark matter. We also examine a role played by magnetic fields and the Abelian chiral anomaly in the generation of LTA, not accounted for in the previous studies. We argue that the production of LTA can be increased significantly and make an estimate of the influence of this effect.