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
Slide 3
Dirac Hamiltonian with time-reversal/parity-breaking terms
Breaks time-reversal Breaks parity
Slide 4
Axial anomaly = = non-conservation of Weyl fermion number BUT:
number of states is fixed on the lattice???
Slide 5
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
Slide 6
Enhancement of electric conductivity along magnetic field
Intuitive explanation: no backscattering for 1D Weyl fermions
Slide 7
Slide 8
A lot of confusion in HIC physics Table-top experiments are
easier?
Slide 9
Weyl points survive ChSB!!!
Slide 10
Pyrochlore Iridates [Wan et al.2010] Strong SO coupling
(f-element) Strong SO coupling (f-element) Magnetic ordering
Magnetic ordering Stack of TIs/OIs [Burkov,Balents2011] Surface
states of TI Spin splitting Iridium: Rarest/strongest elements
Consumption on earth: 3t/year Tunneling amplitudes Magnetic
doping/TR breaking essential
Slide 11
How to split energies of Weyl nodes? [Halasz,Balents 2012]
Stack of TIs/OIs Stack of TIs/OIs Break inversion by voltage Break
inversion by voltage Or break both T/P Or break both T/P Chirality
pumping [Parameswaran et al.13] OR: photons with circular
polarization
Slide 12
Take simplest model of TIs: Wilson-Dirac fermions Take simplest
model of TIs: Wilson-Dirac fermions Model magnetic doping/parity
breaking terms by local terms in the Hamiltonian Model magnetic
doping/parity breaking terms by local terms in the Hamiltonian
Hypercubic symmetry broken by b Hypercubic symmetry broken by b
Vacuum energy is decreased for both b and A Vacuum energy is
decreased for both b and A
Slide 13
Wilson-Dirac with chiral chemical potential: No chiral symmetry
No chiral symmetry No unique way to introduce A No unique way to
introduce A Save as many symmetries as possible [Yamamoto10] Save
as many symmetries as possible [Yamamoto10] Counting
Zitterbewegung, not worldline wrapping
Slide 14
One flavor of Wilson-Dirac fermions One flavor of Wilson-Dirac
fermions Instantaneous interactions (relevant for condmat)
Instantaneous interactions (relevant for condmat) Time-reversal
invariance: no magnetic interactions Time-reversal invariance: no
magnetic interactions Kramers degeneracy in spectrum: Complex
conjugate pairs Complex conjugate pairs Paired real eigenvalues
Paired real eigenvalues External magnetic field causes sign
problem! External magnetic field causes sign problem! Determinant
is always positive!!! Determinant is always positive!!! Chiral
chemical potential: still T-invariance!!! Chiral chemical
potential: still T-invariance!!! Simulations possible with Rational
HMC Simulations possible with Rational HMC
Slide 15
Weyl Hamiltonian in momentum space: Full set of operators for
2x2 hamiltonian Any perturbation (transl. invariant) = just shift
of the Weyl point = 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:
Classical regime: neglect spin flips = off-diagonal terms in a
k Classical action (a p ) 11 looks like a field of Abelian monopole
in momentum space Berry flux Berry flux Topological invariant!!!
Fermion doubling theorem: In compact Brillouin zone only pairs of
monopole/anti-monopole
Slide 18
What are surface states of a Weyl semimetal? Boundary Brillouin
zone Boundary Brillouin zone Projection of the Dirac point
Projection of the Dirac point k x (), k y () curve in BBZ k x (), k
y () curve in BBZ 2D Bloch Hamiltonian 2D Bloch Hamiltonian Toric
BZ Toric BZ Chern-Symons Chern-Symons = total number of Weyl points
= total number of Weyl points inside the cylinder inside the
cylinder h(, k z ) is a topological Chern insulator h(, k z ) is a
topological Chern insulator Zero boundary mode at some Zero
boundary mode at some
Slide 19
Collective motion of chiral fermions High-energy physics:
High-energy physics: Quark-gluon plasma Quark-gluon plasma Hadronic
matter Hadronic matter Leptons/neutrinos in Early Universe
Leptons/neutrinos in Early Universe Condensed matter physics:
Condensed matter physics: Weyl semimetals Weyl semimetals
Topological insulators Topological insulators
Slide 20
Classical conservation laws for chiral fermions Energy and
momentum Energy and momentum Angular momentum Angular momentum
Electric charge No. of left-handed Electric charge No. of
left-handed Axial charge No. of right-handed Axial charge No. of
right-handed Hydrodynamics: Hydrodynamics: Conservation laws
Conservation laws Constitutive relations Constitutive relations
Axial charge violates parity New parity-violating transport
coefficients
Slide 21
Lets 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
Strong constraints on parity-violating transport coefficients
parity-violating transport coefficients [Son, Surowka 2009] [Son,
Surowka 2009] Non-dissipativity of anomalous transport
Non-dissipativity of anomalous transport
[Banerjee,Jensen,Landsteiner2012]
[Banerjee,Jensen,Landsteiner2012]
Slide 22
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!!!
Slide 23
1) Weyl semimetals/Top.insulators are crystals 2) Lattice is
the only practical non-perturbative regularization of gauge
theories First, lets consider axial anomaly on the lattice
Slide 24
Dimension of Weyl representation: 1 Dimension of Weyl
representation: 1 Dimension of Dirac representation: 2 Dimension of
Dirac representation: 2 Just one Pauli matrix = 1 Just one Pauli
matrix = 1 Weyl Hamiltonian in D=1+1 Three Dirac matrices: Three
Dirac matrices: Dirac Hamiltonian:
Slide 25
Slide 26
Axial anomaly = = non-conservation of Weyl fermion number BUT:
number of states is fixed on the lattice???
Slide 27
DOUBLERS Even number of Weyl points in the BZ Even number of
Weyl points in the BZ Sum of chiralities = 0 Sum of chiralities = 0
1D version of Fermion Doubling 1D minimally doubledfermions
Slide 28
Lets try real two-component fermions Two chiral Dirac fermions
Anomaly cancels between doublers Try to remove the doublers by
additional terms
Slide 29
A) B) C)D) A) B) D)C) In A) and B): In C) and D): B) Maximal
mixing of chirality at BZ boundaries!!! Now anomaly comes from the
Wilson term + All kinds of nasty renormalizations (1+1)D Wilson
fermions
Slide 30
AAAA -A-A-A-A Excess of right-moving particles Excess of
right-moving particles Excess of left-moving anti-particles Excess
of left-moving anti-particles Directed current Not surprising weve
broken parity Effect relevant for nanotubes
Slide 31
Fixed cutoff regularization: Shift of integration variable:
ZERO UV regularization ambiguity
Slide 32
Polarization tensor in 2D: [Chen,hep-th/9902199] Value at k 0
=0, k 3 =0: NOT DEFINED Value at k 0 =0, k 3 =0: NOT DEFINED
(without IR regulator) (without IR regulator) First k 3 0, then k 0
0 First k 3 0, then k 0 0 Otherwise zero Otherwise zero Final
answer: Proper regularization (vector current conserved):
Slide 33
Excess of right-moving particles Excess of right-moving
particles Excess of left-moving particles Excess of left-moving
particles Directed axial current, separation of chirality Effect
relevant for nanotubes AAAA AAAA
Slide 34
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
Slide 35
Slide 36
Finite volume: Degeneracy of every level = magnetic flux
Additional operators [Wiese,Al-Hasimi, 0807.0630]
Slide 37
Lowest Landau level = 1D Weyl fermion
Slide 38
Parallel uniform electric and magnetic fields The anomaly comes
only from LLL Higher Landau Levels do not contribute
Slide 39
Nielsen-Ninomiya picture: Minimally doubled fermions Minimally
doubled fermions Two Dirac cones in the Brillouin zone Two Dirac
cones in the Brillouin zone For Wilson-Dirac, For Wilson-Dirac,
anomaly again stems anomaly again stems from Wilson terms from
Wilson terms VALLEYTRONICS VALLEYTRONICS
Classical action and equations of motion with gauge fields
Streaming equations in phase space More consistent is the Wigner
formalism Anomaly = injection of particles at zero momentum (level
crossing)
Slide 42
Anomalous current-current correlators: Chiral Separation and
Chiral Magnetic Conductivities:
Slide 43
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 angle Funny Goldstones!!!
Slide 44
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
Slide 45
Graphene Nice and simple standard tight-binding model Nice and
simple standard tight-binding model Many interesting specific
questions Many interesting specific questions Field-theoretic
questions (almost) solved Field-theoretic questions (almost) solved
Topological insulators Many complicated tight-binding models Many
complicated tight-binding models Reduce to several typical examples
Reduce to several typical examples Topological classification and
universality of boundary states Topological classification and
universality of boundary states Stability w.r.t. interactions?
Topological Mott insulators? Stability w.r.t. interactions?
Topological Mott insulators? Weyl semimetals Many complicated
tight-binding models, physics of dirt Many complicated
tight-binding models, physics of dirt Simple models capture the
essence Simple models capture the essence Non-dissipative anomalous
transport Non-dissipative anomalous transport Exotic boundary
states Exotic boundary states Topological protection of Weyl points
Topological protection of Weyl points