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Weyl semimetals Pavel Buividovich (Regensburg)

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Weyl semimetals TODO: - discuss different kinds of WSM, calculate the spectrum in the presence of mass Fermi arc states - Explain why anomalous response of conductivity is a signature of WSM

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**Simplest model of Weyl semimetals**

Dirac Hamiltonian with time-reversal/parity-breaking terms Breaks time-reversal Breaks parity

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**Nielsen, Ninomiya and Dirac/Weyl semimetals**

Axial anomaly on the lattice? Axial anomaly = = non-conservation of Weyl fermion number BUT: number of states is fixed on the lattice???

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**Nielsen, Ninomiya and Dirac/Weyl semimetals**

Weyl points separated in momentum space In compact BZ, equal number of right/left handed Weyl points Axial anomaly = flow of charges from/to left/right Weyl point

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**Nielsen-Ninomiya and Dirac/Weyl semimetals**

Enhancement of electric conductivity along magnetic field Intuitive explanation: no backscattering for 1D Weyl fermions

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**Nielsen-Ninomiya and Dirac/Weyl semimetals**

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**Field-theory motivation**

A lot of confusion in HIC physics… Table-top experiments are easier?

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**Weyl points survive ChSB!!!**

Weyl semimetals Weyl points survive ChSB!!!

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**Weyl semimetals: realizations**

Pyrochlore Iridates [Wan et al.’2010] Strong SO coupling (f-element) Magnetic ordering Stack of TI’s/OI’s [Burkov,Balents’2011] Surface states of TI Spin splitting Tunneling amplitudes Iridium: Rarest/strongest elements Consumption on earth: 3t/year Magnetic doping/TR breaking essential

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**Weyl semimetals with μA**

How to split energies of Weyl nodes? [Halasz,Balents ’2012] Stack of TI’s/OI’s Break inversion by voltage Or break both T/P Chirality pumping [Parameswaran et al.’13] Electromagnetic instability of μA [Akamatsu,Yamamoto’13] Chiral kinetic theory (see below) Classical EM field Linear response theory Unstable EM field mode μA => magnetic helicity OR: photons with circular polarization

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**Lattice model of WSM Take simplest model of TIs: Wilson-Dirac fermions**

Model magnetic doping/parity breaking terms by local terms in the Hamiltonian Hypercubic symmetry broken by b Vacuum energy is decreased for both b and μA

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**Weyl semimetals: no sign problem!**

Wilson-Dirac with chiral chemical potential: No chiral symmetry No unique way to introduce μA Save as many symmetries as possible [Yamamoto‘10] Counting Zitterbewegung, not worldline wrapping

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**Weyl semimetals+μA : no sign problem!**

One flavor of Wilson-Dirac fermions Instantaneous interactions (relevant for condmat) Time-reversal invariance: no magnetic interactions Kramers degeneracy in spectrum: Complex conjugate pairs Paired real eigenvalues External magnetic field causes sign problem! Determinant is always positive!!! Chiral chemical potential: still T-invariance!!! Simulations possible with Rational HMC

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**Topological stability of Weyl points**

Weyl Hamiltonian in momentum space: Full set of operators for 2x2 hamiltonian Any perturbation (transl. invariant) = just shift of the Weyl point Weyl point are topologically stable Only “annihilate” with Weyl point of another chirality E.g. ChSB by mass term:

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**Weyl points as monopoles in momentum space**

Free Weyl Hamiltonian: Unitary matrix of eigenstates: Associated non-Abelian gauge field:

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**Weyl points as monopoles in momentum space**

Classical regime: neglect spin flips = off-diagonal terms in ak Classical action (ap)11 looks like a field of Abelian monopole in momentum space Berry flux Topological invariant!!! Fermion doubling theorem: In compact Brillouin zone only pairs of monopole/anti-monopole

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**Fermi arcs [Wan,Turner,Vishwanath,Savrasov’2010]**

What are surface states of a Weyl semimetal? Boundary Brillouin zone Projection of the Dirac point kx(θ), ky(θ) – curve in BBZ 2D Bloch Hamiltonian Toric BZ Chern-Symons = total number of Weyl points inside the cylinder h(θ, kz) is a topological Chern insulator Zero boundary mode at some θ

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**Why anomalous transport?**

Collective motion of chiral fermions High-energy physics: Quark-gluon plasma Hadronic matter Leptons/neutrinos in Early Universe Condensed matter physics: Weyl semimetals Topological insulators

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**Hydrodynamic approach**

Classical conservation laws for chiral fermions Energy and momentum Angular momentum Electric charge No. of left-handed Axial charge No. of right-handed Hydrodynamics: Conservation laws Constitutive relations Axial charge violates parity New parity-violating transport coefficients

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**Hydrodynamic approach**

Let’s try to incorporate Quantum Anomaly into Classical Hydrodynamics Now require positivity of entropy production… BUT: anomaly term can lead to any sign of dS/dt!!! Strong constraints on parity-violating transport coefficients [Son, Surowka ‘ 2009] Non-dissipativity of anomalous transport [Banerjee,Jensen,Landsteiner’2012]

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**Anomalous transport: CME, CSE, CVE**

Chiral Magnetic Effect [Kharzeev, Warringa, Fukushima] Chiral Separation Effect [Son, Zhitnitsky] Chiral Vortical Effect [Erdmenger et al., Teryaev, Banerjee et al.] Flow vorticity Origin in quantum anomaly!!!

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**Why anomalous transport on the lattice?**

1) Weyl semimetals/Top.insulators are crystals 2) Lattice is the only practical non-perturbative regularization of gauge theories First, let’s consider axial anomaly on the lattice

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**Warm-up: Dirac fermions in D=1+1**

Dimension of Weyl representation: 1 Dimension of Dirac representation: 2 Just one “Pauli matrix” = 1 Weyl Hamiltonian in D=1+1 Three Dirac matrices: Dirac Hamiltonian:

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**Warm-up: anomaly in D=1+1**

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**Axial anomaly on the lattice**

= non-conservation of Weyl fermion number BUT: number of states is fixed on the lattice???

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**Anomaly on the (1+1)D lattice**

1D minimally doubled fermions DOUBLERS Even number of Weyl points in the BZ Sum of “chiralities” = 0 1D version of Fermion Doubling

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**Anomaly on the (1+1)D lattice**

Let’s try “real” two-component fermions Two chiral “Dirac” fermions Anomaly cancels between doublers Try to remove the doublers by additional terms

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**Anomaly on the (1+1)D lattice**

(1+1)D Wilson fermions In A) and B): In C) and D): A) B) C) D) B) Maximal mixing of chirality at BZ boundaries!!! Now anomaly comes from the Wilson term + All kinds of nasty renormalizations… A) B) D) C)

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**Now, finally, transport: “CME” in D=1+1**

Excess of right-moving particles Excess of left-moving anti-particles Directed current Not surprising – we’ve broken parity Effect relevant for nanotubes

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**“CME” in D=1+1 Fixed cutoff regularization: Shift of integration**

variable: ZERO UV regularization ambiguity

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**Dimensional reduction: 2D axial anomaly**

Polarization tensor in 2D: Proper regularization (vector current conserved): [Chen,hep-th/ ] Final answer: Value at k0=0, k3=0: NOT DEFINED (without IR regulator) First k3 → 0, then k0 → 0 Otherwise zero

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**Directed axial current, separation of chirality**

“CSE” in D=1+1 μA μA Excess of right-moving particles Excess of left-moving particles Directed axial current, separation of chirality Effect relevant for nanotubes

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**Energy flux = momentum density**

“AME” or “CVE” for D=1+1 Single (1+1)D Weyl fermion at finite temperature T Energy flux = momentum density (1+1)D Weyl fermions, thermally excited states: constant energy flux/momentum density

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**Going to higher dimensions: Landau levels for Weyl fermions**

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**Going to higher dimensions: Landau levels for Weyl fermions**

Finite volume: Degeneracy of every level = magnetic flux Additional operators [Wiese,Al-Hasimi, ]

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**LLL, the Lowest Landau Level**

Lowest Landau level = 1D Weyl fermion

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**Anomaly in (3+1)D from (1+1)D**

Parallel uniform electric and magnetic fields The anomaly comes only from LLL Higher Landau Levels do not contribute

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**Anomaly on (3+1)D lattice**

Nielsen-Ninomiya picture: Minimally doubled fermions Two Dirac cones in the Brillouin zone For Wilson-Dirac, anomaly again stems from Wilson terms VALLEYTRONICS

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**Anomalous transport in (3+1)D from (1+1)D**

CME, Dirac fermions CSE, Dirac fermions “AME”, Weyl fermions

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**Chiral kinetic theory [Stephanov,Son]**

Classical action and equations of motion with gauge fields More consistent is the Wigner formalism Streaming equations in phase space Anomaly = injection of particles at zero momentum (level crossing)

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**CME and CSE in linear response theory**

Anomalous current-current correlators: Chiral Separation and Chiral Magnetic Conductivities:

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**Chiral symmetry breaking in WSM**

Mean-field free energy Partition function For ChSB (Dirac fermions) Unitary transformation of SP Hamiltonian Vacuum energy and Hubbard action are not changed b = spatially rotating condensate = space-dependent θ angle Funny Goldstones!!!

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**Electromagnetic response of WSM**

Anomaly: chiral rotation has nonzero Jacobian in E and B Additional term in the action Spatial shift of Weyl points: Anomalous Hall Effect: Energy shift of Weyl points But: WHAT HAPPENS IN GROUND STATE (PERIODIC EUCLIDE???) Chiral magnetic effect In covariant form

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**Topological insulators**

Summary Graphene Nice and simple “standard tight-binding model” Many interesting specific questions Field-theoretic questions (almost) solved Topological insulators Many complicated tight-binding models Reduce to several typical examples Topological classification and universality of boundary states Stability w.r.t. interactions? Topological Mott insulators? Weyl semimetals Many complicated tight-binding models, “physics of dirt” Simple models capture the essence Non-dissipative anomalous transport Exotic boundary states Topological protection of Weyl points

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