# Aspects of the pseudo chiral magnetic effect in 2D Weyl-Dirac matter

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## Abstract

A connection is established between the continuum limit of the low-energy tight-binding description of graphene immersed in an in-plane magnetic field and the Chiral Magnetic Effect in Quantum Chromodynamics. A combination of mass gaps that explicitly breaks the equivalence of the Dirac cones, favoring an imbalance of pseudo-chiralities, is the essential ingredient to generate a non-dissipative electric current along the external field. Currents, number densities and condensates generated from this setup are investigated for different hierarchies of the energy scales involved.

## 1 Introduction

The chiral magnetic effect (CME), predicted in Refs. [1, 2] to occur in the quark gluon plasma produced in heavy ion collisions (HIC), has a deep connection with the vacuum structure of quantum chromodynamics (QCD), its topology and symmetries. The non-dissipative current produced by this mechanism points out in the direction of the magnetic field generated in non-central collisions and is a direct consequence of a chiral imbalance. Such imbalance can only occur in certain domains where the gauge field configurations are topologically non-trivial. Interactions of fermions with these fields result in a chirality flip.

The difficulty to extract information about the early stage in HIC – due to unknown electromagnetic properties of the medium, the out-of-equilibrium regime and the lack of effective transport descriptions – makes the CME so far an unique attempt to trace a mechanism that connects some peculiar quantum properties of QCD to a macroscopic observable. Besides contributing for a more complete picture of QCD, its confirmation would have implications in our understanding about the early universe and baryogenesis. Because of that, it has received considerable attention from the theoretical, experimental and lattice communities in high energy physics, but in spite of all efforts, a conclusive observation of the mechanism is still missing. In effect, results from the CMS collaboration [3] comparing angular correlation between Pb-Pb collisions and p-Pb collisions have challenged previous results from the STAR collaboration [4] that seemed to have observed the CME. In order to clarify this issue, new technology on background analysis is being carried out [5] and new observables have been proposed [5, 6]. Remarkably, the 2018 run of RHIC includes isobar collisions in order to disentangle the chiral magnetic effect from background sources [7].

In a different order of ideas, the fast-growing family of Weyl-Dirac materials that have been discovered in the last few years has allowed to test an analog of this mechanism in a condensed matter environment [8]. In this kind of materials, the complex interaction between the charge carriers and the background lattice can be effectively represented considering the former as quasiparticles that obey relativistic-like equations of motion, with the velocity of light replaced by the corresponding Fermi velocity. In this way, it is possible to define chirality for these electrons and to construct the analogy with QCD [9, 10, 11].

In search for inducing a chiral splitting of charge carriers in Weyl-Dirac materials, an experiment was proposed in Ref. [8] such that by applying a parallel electric and magnetic field to a (3+1)D sample of zirconium pentatelluride (\(\mathrm{ZrTe}_5\)), the observation of a negative magnetoresistence signals the presence of a chiral anomaly [12] and the generation of a non-dissipative electric current. This represents yet another novel avenue allowing for interdisciplinary investigation connecting condensed matter and high energy physics. Nevertheless, the Weyl-Dirac behavior of the charge carriers in \(\mathrm{ZrTe}_5\) is still controversial. While the authors of Ref. [8] established the *ultrarelativistic* behavior of charge carriers through angle-resolved photoemission spectroscopy (ARPES), further investigation has opened the possibility that this may not be actually the case after all [13]. A more detailed understanding of the mechanism in the context of condensed matter is still needed and realizations of CME in different materials are welcome. In effect, it has been posteriorly observed in other materials [14, 15, 16, 17, 18] and alternative mechanisms have been proposed in order to generate the CME in \((3+1)\)D Weyl semimetals [19] but no experimental realization has been achieved so far.

In this paper, we extend the work presented in Refs. [20, 21], where some of us proposed an electromagnetic analogue of the CME in Weyl-Dirac materials in two spatial dimensions motivated by graphene and referred to as the *pseudo-chiral magnetic effect* (PCME).

It is natural to search for this type of analogy because quantum electrodynamics in (2+1)D, dubbed as QED\(_3\), has been widely used as a toy model for QCD inasmuch as it describes confinement and chiral symmetry breaking [22, 23, 24, 25, 26]. This is due to the fact that at high temperature, any field theory can be dimensionally reduced and, on the other hand, a non-Abelian three dimensional gauge field theory *abelianizes* for a large number of flavors [27]. With the physical realization of graphene and other planar materials containing Dirac electrons, QED\(_3\) was promoted from being a toy model to really describe physical systems and analogues of high energy physics constructed on table-top experiments started to take place.

To describe the PCME, in Refs. [20, 21] we constructed a Lagrangian for relativistic fermions in (2+1)D including some effective interactions that simulate a chiral imbalance. The result is an electric current generated in the direction of an external magnetic field applied in-plane. Here we extend this study investigating other relevant quantities: the number densities and condensates that we found to be present and the current in certain limiting cases of magnitude of the magnetic field as compared with temperature, mass gaps and chemical potentials.

The paper is organized as follows: in Sect. 2 we give a brief review of how Dirac fermion emerge from graphene and in this context we present our Lagrangian and describe our model. In Sect. 3 we discuss the propagator of a fermion in the presence of a magnetic field based on the Schwinger proper time method. In Sect. 4 we present our results on the currents, densities and condensates. In Sect. 5 we explain our model in terms of the Fermi liquid model, commonly applied in condensed matter physics. In Sect. 6 we make our final remarks. We complement the article with three appendixes.

## 2 The effective model

The essence of the CME lies on the fact that the vacuum of the gauge sector in QCD is actually a superposition of vacua including configurations with non-trivial topology, and the interaction of these gauge fields with quarks can cause a flip on their chirality. In that case, an imbalance between chiralities occurs and, inside a certain domain where the topological gauge fields act, one has a difference in the number of left- and right-quarks, whose abundance and sign depend on the details of the topology of the gauge fields. The large magnetic field present in non-central heavy-ion collisions triggers the CME by generating an electric current along its direction. Therefore, the chiral imbalance and the magnetic field constitute the two pillars of the CME.

Planar graphene-like materials provide elements we need for a robust analogy. This kind of materials have the remarkable property that the charge carriers behave as Dirac electrons, which means that they have a linear dispersion relation. Besides, the continuum limit of its tight-binding Hamiltonian (or Lagrangian) exhibit Time- and Space-inversion symmetries such that massless charge carriers are, in fact, chiral. Theoretically, it is possible to break these symmetries if we consider special currents in the Lagrangian, as will be clear in a moment.

*A*and

*B*which are Bravais lattices, in which the nearest neighbor of a carbon atom belonging to sub-lattice

*A*are all atoms in the sub-lattice

*B*and vice versa, as shown in Fig. 1. With the primitive vectors \({{\mathbf {a}}}_1=a(1/2,\sqrt{3}/2)\), and \({{\mathbf {a}}}_2=a(-1/2,\sqrt{3}/2)\), where

*a*denotes the interatomic distance, and correspondingly in reciprocal space \({{\mathbf {K}}}_1=2\pi /a(1,\sqrt{3}/3)\) and \({{\mathbf {K}}}_2=2\pi /a(-1,\sqrt{3}/3)\), from Bloch theorem it is straightforward to find that the energy-momentum dispersion relation is

*t*is a constant that encodes the magnitude of the interaction (the hopping parameter) and \(k_i\) correspond to the momentum components in the two spatial directions. This function is plotted in Fig. 2. One can notice that the valence and conduction band touch in six points, generating no gap. Those are high symmetry points in the Brillouin zone of which only two are inequivalent, the so-called Dirac points

*K*and \(K'\). Expanding the energy around these points, one can verify that the dispersion relation is linear, \(E = \hbar v_F k\), and we can think of charge carriers as massless Dirac electrons, with the speed of light replaced by the Fermi velocity. Therefore, the tight-binding description of monolayer graphene corresponds in the continuum to a massless version of the fermion sector of QED\(_3\), but photons are allowed to move in (3+1) dimensions. The influence of external electromagnetic fields as usual is added via minimal couplings.

*flavor*or pseudo-spin and the Dirac point (valley) index, respectively. Working in Weyl representation, the gamma matrices can be explicitly written:

Let us now discuss the role of the parity anomaly and how it connects to the aforementioned masses. The ABJ anomaly in (3+1)D can be interpreted as production of Weyl fermions in the presence of external electric and magnetic fields [29]. This was initially proposed in a nuclear physics context and in [30] this interpretation was extended to condensed matter systems. The authors show that the contribution to the anomalous electric current coming from fermions with different chiralities have opposite sign and, since it was previously shown that in any lattice theory with locality, chiral fermions appear in pairs of opposite chirality [31] – fermion doubling – the net current vanishes.

In odd dimensions, it is known that the conservation laws for fermionic currents do not present the chiral anomaly, but it might still emerge abnormal currents for each species of fermions due to the parity anomaly [32]. In [33] the author proposes a (2+1)D analogue of [30] in a graphene-like system. Introducing a gap between the bands induced by a sublattice symmetry breaking, the author show that the contribution from the two inequivalent Dirac points cancel out, similarly to what happens in (3+1)D. The two Dirac points are therefore the analogues for chirality.

In our work we propose something similar, but we look for a Lagrangian structure where the masses for the two valleys are different and therefore for suitable values of the parameters the net current does not vanish. We obtain this by the combination of Haldane mass \(m_o\) and the mass \(m_e\), which carries an identical structure as the one Semenoff uses in [33]. The anomaly is therefore parametrized in \(m_\pm \) in the same way that the chiral anomaly is parametrized in \(\mu _5\) in the original chiral magnetic effect. The underlying Hamiltonian is therefore, equivalent to the one in [33] but with different masses for each Dirac point. Further, as we explain below, we promote the interaction of this system with an external magnetic field via minimal coupling and consider thermal effects.

To complete our analogy, we consider an external magnetic field aligned along the graphene membrane described by the vector potential \(A_3^{\mathrm {ext}}=By\), where *y* represents the second spatial coordinate along the graphene plane and we assume \(B>0\). This field is assumed to be classical and does not play a role in quantum corrections. Since it is external, in our setup \(A_\mu \) lives in a bulk rather than in a two-dimensions sheet. Other field theory approaches for QED in planar condensed matter systems have been presented previously considering an explicit treatment of the gauge sector, see for instance the Pseudo Quantum Electrodynamics (PQED) [34, 35, 36, 37] and the Reduced Quantum Electrodynamics (RQED) [38]. In the present work we do not deal with the Maxwell term and do not consider quantum corrections coming from the gauge sector and therefore we do not make use of PQED/RQED. However, in a work in progress we are currently investigating if within this more complete framework \(m_o\) can be dynamically generated by the presence of parallel electric and magnetic fields. One of us have checked some formal aspects of the theory in mixed dimensions [39, 40] in order to prepare for the numerical calculations and we expect to report soon. In our approach we just consider the fermion sector and the interaction with the classical field. We restrict the dynamical term \(\partial \psi /\partial x_3 =0\) but we preserve the fermion interaction with \(A_3\).

## 3 Propagator

*x*-direction. Notice that the Green function is non-local due to the term \(M_\pm \).

## 4 Currents, densities and condensates

*B*, temperature

*T*, chemical potential \(\mu \) as well as the charge carrier species gaps \(m_\pm \) with different hierarchies among these scales. The non-local nature of the Green function in Eq. (11) indicates that finite size effects are important to be considered. The relevant expectation value of the objects

*y*-direction. The setup is symmetric with respect to the

*x*-direction. The appropriate value can be considered then as the average in space, namely

*y*. A translation in our propagator can be written as \(A_3=By \rightarrow B(y-y_0)\). Note that this is in fact equivalent to shift the mass \(m_e \rightarrow m_e+eBy_0\) and therefore different integration limits yield to the same result as far as \(m_e\) is correctly shifted. In the absence of magnetic field this parameter corresponds to the gap generated by sublattice symmetry breaking obtained for example placing graphene over a boron nitride substrate. If a magnetic field is turned on, \(m_e\) plays the role of a bare mass that can absorb an eventual constant added to the gauge field and its physical value must be fixed by some observable like the electric current.

This is similar for instance to the procedure adopted in [44], where a constant uniform electric field generated from a static potential produces a non-local term, which is considered as a coordinate-dependent chemical potential and must be fixed by a suitable quantity like the number density.

All the relevant quantities can be expressed in a closed analytical form, instead of integrals in proper-time. We explore two regimes for the magnetic field with respect to the temperature and chemical potential. More specifically, the relevant scale to which we compare the field strength is \((\pi T)^2-\mu ^2\), namely, the quantity that needs to be positive to enable for a Wick rotation. Details of the asymptotic low- and high-magnetic field approximations are described in the appendices. The case of pure magnetic contribution for the condensates will be treated separately.

### 4.1 Currents

*eB*| is shown in Fig. 4. We can observe that the approximations are in good accordance with the numerical expression. It is interesting that in fact for strong magnetic field (low temperature) the dependence of the currents on the temperature is negligible.

### 4.2 Number densities

Notice that a very similar expression was derived in Ref. [45] Eq.(74), replacing \(eB\eta \) by a mass gap \(\Delta \). This is totally expected since the relevant value to calculate the currents, as can be checked in Eq.(25), is \(\eta =y+y_\pm \), where \(y_\pm =\frac{m_\pm }{eB}\). It means that the argument in the exponentials of the equation above is a function of \(m_\pm \) only, which plays precisely the same role as \(\Delta \): breaking sublattice symmetry.

The comparison between the strong and weak magnetic field limits and the full function is described in Fig. 5 for all parameters scaled with |*eB*|.

### 4.3 Condensates

*condensates*and not as a currents is because there is no dynamics along the

*z*-direction. In this sense, \(\sigma _3\) and \(\sigma _{35}\) behave like order parameters for the symmetry breaking generated by the mass gaps in the sense that an expansion around these values naturally give a finite gap in the corresponding Lagrangian. It is convenient to write these condensates as

### 4.4 Currents, densities and condensates at zero temperature

## 5 Chiral chemical potential

In QCD, the CME is described by a single parameter, the so-called chiral chemical potential, which is proportional to the time derivative of the CS term. To establish our analogy, we have described a 2D Weyl-Dirac material where the parity anomaly is manifested through the mass term \(m_\pm \) in the Lagrangian. But because we need the system to be filled with charge carriers, the chemical potential also plays a role in the dynamics. If we consider the fermion system of our material degenerated with respect to the energy gaps \(m_\pm \), we can explore what happens with the charge carriers, or holes, moving near the Fermi surface. There are two scenarios in this case, which we consider separately.

*antiparticles*can be omitted in the discussion because those states are already filled by assumption. Now, the Fermi momentum for each chirality is:

Let us emphasize once more that our construction is based on the symmetry breaking of Dirac Points which in those planar systems with relativistic fermions play the role of pseudo-chirality. As such, \(\mu _5\) encodes this asymmetry rather than being related to actual chirality.

## 6 Discussion and conclusions

In this article we have established an analogy of the CME for 2D Dirac-Weyl matter. We have considered a system of fermions with dynamics restricted to a plane which have two different mass gaps. One of those mass gaps is known to be related with the CS term in QED\(_3\). It is under investigation if the same holds for PQED or RQED. The other mass gap \(m_e\) breaks the equivalence between the two triangular sublattices of the underlying honeycomb array. When an external magnetic field is applied parallel to this system, the generation of an electric current along the direction of the magnetic field is formed. This corresponds to an analogue of the CME, but in terms of the pseudo-spin of the charge carriers of our system. We have further explored the formation of number density, chiral number density, axial current and condensates associated to the mass gaps.

The most remarkable relation and close connection of the CME and the effect discussed here is represented in Eq. (31), which was obtained in the strong magnetic field (low temperature) regime and is functionally the same obtained for the ordinary CME in QCD [2]. Let us stress that in the latter case, the phenomenon is believed to occur at high-temperature. The CME is described through a chiral chemical potential, while in our case we need two mass gaps. This can be understood considering that in graphene-like system first we need a mechanism that distinguishes the left and right pseudo-chiralities and further another one that breaks the balance.

Considering our Fermi system to be degenerate, it is possible to describe it as a Fermi liquid effective model, and the chiral imbalance can be seen in two ways: as quasi-particles with different pseudo-chiralities propagating at different Fermi velocities, or like a fluid whose imbalance is produced by a chiral chemical potential that depends on the difference of the mass gaps, as can be seen from Eq. (60). In both cases, the consequence is a shift in the Dirac points, hinting a deformation of the crystal structure. This observation suggests the fact that strained graphene is a good candidate to reproduce this effect.

It is interesting to remark that different arrangements of a graphene membrane can mimic many QCD situations. We want to explore in more detail the mass gap generation, including the possibility of generating it through external fields, which requires a proper treatment of the gauge sector. This idea is under scrutiny and results shall be presented elsewhere.

## Notes

### Acknowledgements

This work was supported by FONDECYT (Chile) under Grant numbers 1150847, 1130056, 1150471 and 1170107. AR is supported by Consejo Nacional de Ciencia y Tecnología (Mexico) Grant 256494. AJM is supported by BELSPO-Belgium and FAPESP under grant 2016/12705-7. We acknowledge the research group GI 172309 C *Cosmología y Partículas Elementales* at UBB and G. Krein at IFT-UNESP for the kind hospitality. AJM ackowledges D. Dudal for fruitful discussions.

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