arXiv subjects

H. Banerjee

Publications and source records attributed to H. Banerjee.

10 recordsLinked to original sources

arXiv2022

Oxygen Hole Formation Controls Stability in LiNiO$_2$ Cathodes: DFT Studies of Oxygen Loss and Singlet Oxygen Formation in Li-Ion Batteries

Ni-rich cathode materials achieve both high voltages and capacities in Li-ion batteries but are prone to structural instabilities and oxygen loss via the formation of singlet oxygen. Using ab initio molecular dynamics simulations, we observe spontaneous O$_2$ loss from the (012) surface of delithiated LiNiO$_2$, singlet oxygen forming in the process. We find that the origin of the instability lies in the pronounced oxidation of O during delithiation, i.e., O plays a central role in Ni O redox in LiNiO$_2$. For LiNiO$_2$, NiO$_2$, and the prototype rock salt NiO, density-functional theory and dynamical mean-field theory calculations based on maximally localised Wannier functions yield a Ni charge state of ca. +2, with O varying between -2 (NiO), -1.5 (LiNiO$_2$) and -1 (NiO$_2$). Predicted XAS Ni $K$ and O $K$-edge spectra are in excellent agreement with experimental XAS spectra, confirming the predicted charge states. The calculations also show that a high-voltage O $K$-edge feature at 531 eV previously assigned to lattice O-redox processes could alternatively arise from O-redox induced water intercalation and O-O dimer formation with lattice O at high states of charge. The O$_2$ surface loss route observed here consists of 2 surface O$^{.-}$ radicals combining to form a peroxide ion, which is oxidised to O$_2$, leaving behind 2 O vacancies and 2 O$^{2-}$ ions: effectively 4 O$^{.-}$ radicals disproportionate to O$_2$ and 2 O$^{2-}$ ions. The reaction liberates ca. 3 eV per O$_2$ molecule. Singlet oxygen formation is caused by the singlet ground state of the peroxide ion, with spin conservation dictating the preferential release of $^1$O$_2$, the strongly exergonic reaction providing the free energy required for the formation of $^1$O$_2$ in its excited state.

cond-mat.mtrl-sci↗

arXiv2008

U(1) Problem Revisited

In the anomaly equation for the singlet axial current the chiral limit of the quark mass term does not vanish but comprises contribution from fermion zero modes whose integral exactly cancels the topological charge arising from the Adler-Bell-Jackiw anomaly. This signals chiral symmetry and opens a window for restoring the status of Goldstone boson for the singlet $η^\prime$ without having to invoke the large $N_c$ limit in the underlying QCD. We construct the anomaly term in the effective action that incorporates the chiral symmetry property and yet accounts for the excess mass of $η^\prime$ only when chiral symmetry is broken explicitly by the quark masses. The anomaly term in the present scenario thus plays the role of a catalytic agent that enhances the mass of $η^\prime$ so that the singlet axial current obeys the popular PCAC condition.

hep-ph↗

arXiv2000

Is there still a strong CP problem?

The role of a chiral U(1) phase in the quark mass in QCD is analysed from first principles. In operator formulation, there is a parity symmetry and the phase can be removed by a change in the representation of the Dirac gamma matrices. Moreover, these properties are also realized in a Pauli-Villars regularized version of the theory. In the functional integral scenario, attempts to remove the chiral phase by a chiral transformation are thought to be obstructed by a nontrivial Jacobian arising from the fermion measure and the chiral phase may therefore seem to break parity. But if one starts from the regularized action with the chiral phase also present in the regulator mass term, the Jacobian for a combined chiral rotation of quarks and regulators is seen to be trivial and the phase can be removed by a combined chiral rotation. This amounts to a taming of the strong CP problem.

hep-ph↗

arXiv1999

Fermion decoupling and the Axial Anomaly on the Lattice

In the axial Ward identity of lattice QED we show that in the limit of infinite fermion mass m the pseudoscalar density term exactly cancels the Adler-Bell-Jackiw anomaly. Using this result we calculate the U(1) axial anomaly in a non-abelian gauge theory.

hep-lat↗

arXiv1999

Chiral Anomalies In Field Theories

The role of the contribution from the fermion mass term in the axial vector Ward identity in generating the U(1) axial anomaly, both local and global, is elucidated. Gauge invariance requires the fermion to decouple from the gauge field if it is very heavy. This identifies the Adler-Bell-Jackiw (ABJ) anomaly with the asymptotic limit of the sign reversed mass term. In an instanton background, the chiral limit $(m = 0)$ of the mass term does not vanish but consists of contributions from fermion zero modes. Space time integral of these zero mode contributions exactly cancels, thanks to the Atiyah-Singer index theorem, the integral of the ABJ anomaly and suggests that the Jacobian for global U(1) chiral transformation is trivial even in an instanton background. This can be realised in the representation of the fermion partition function in a Weyl basis. The resolution of the strong CP problem is thus achieved in an axionless physical world. In chiral gauge theories the fermion partition function admits of a gauge invariant representation but only at the cost of locality. Implementation of fermion averaging of the gauge current with the invariant partition function yields the current whose covariant derivative is the covariant anomaly. With the covariant current as input one can derive an integrable current whose covariant derivative is the minimal consistent anomaly obeying the Wess-Zumino consistency condition. The distinction between the two currents disappears if either the covariant or the consistent anomaly vanishes. This is realised only if the fermion belongs to an anomaly-free representation of the gauge group.

hep-th↗

arXiv1999

Fermion decoupling and the axial anomaly on the lattice

By an explicit calculation of the continuum limit of the triangle graph amplitude in lattice QED we show that in the axial Ward identity the ABJ anomaly exactly cancels the pseudoscalar density term in the limit of infinite fermion mass $m$. The result, a reflection of decoupling of the heavy fermion, provides a convenient framework for computing the flavor-singlet or U(1) axial anomaly in non-Abelian gauge theories on lattice. Our calculations on the lattice are performed using Wilson fermions but the results are general.

hep-lat↗

arXiv1998

Lattice Fermions and the Chiral Anomaly

We show in the Wilson model that the contribution of the regular mass term to the four-divergence of the axial vector current in weak coupling perturbation theory is not zero in the chiral limit and is precisely the axial anomaly. Explicit breaking of chiral symmetry in the Wilson term is not relevant for the result. The ABJ anomaly is generated by the fermion mass term also with a chirally symmetric irrelevant term.

hep-lat↗

arXiv1996

Lattice Fermions and Chiral Symmetry

We propose a formulation of lattice fermions with one-sided differences that is hermitian, chirally symmetric (barring a bare mass term) and completely free of doubling. To obtain the axial anomaly in perturbation theory it was necessary to break chiral symmetry on the lattice only through a bare mass term for the physical fermion. The chiral limit may be taken once the continuum limit is reached. We comment on the role of the mass term with examples elsewhere in field theory.

hep-lat↗

arXiv1996

Fermions on lattice and chiral invariance

A model for lattice fermion is proposed which is, (i) free from doublers, (ii) hermitian, and (iii) chirally invariant. The price paid is the loss of hypercubic and reflection symmetries in the lattice action. Thanks to the $ε$-prescription, correlation functions are free from the ill effects due to the loss of these symmetries. In weak coupling approximation, the U(1) vector current of a gauge theory of lattice fermion in this model is conserved in the continuum limit. As for the U(1) axial vector current, one obtains the ABJ anomaly if the continuum limit is implemented before the chiral limit $m = 0$. The anomaly disappears, as in the Wilson model, if the order of the two limits is reversed.

hep-th↗

arXiv1995

Resolution of the strong CP and U(1) problems

Definition of the determinant of Euclidean Dirac operator in the nontrivial sector of gauge fields suffers from an inherent ambiguity. The popular Osterwalder-Schrader (OS) recipe for the conjugate Dirac field leads to the option of a vanishing determinant. We propose a novel representation for the conjugate field which depends linearly on the Dirac field and yields a nonvanishing determinant in the nontrivial sector. Physics, it appears, chooses this second option becuase the novel representation leads to a satisfactory resolution of two outstanding problems, the strong CP and U(1) problems, attributed to instanton effects.

hep-th↗