LU20113
LU20113
Breaking of Lorentz invariance caused by the interplay between spin-orbit interaction and
transverse phonon modes in quantum wires
by D. V. Efremov, Weyner Ccuiro, Luis E. F. Foà Torres, et al.
Samindranath Mitra
Senior Editor
Physical Review Letters
Email: prl@aps.org
1
• Memo: Guidelines to referees for Physical Review Letters
2
Breaking of Lorentz invariance caused by the interplay between spin-orbit interaction and
transverse phonon modes in quantum wires
mass, αso denotes the Rashba spin-orbit coupling strength, where v = vF /g is the electronic velocity and vF is Fermi
⃗σ represents the Pauli matrices which describe the electron’s velocity. For short-range interaction V (x − x′ ) = V δ(x − x′ ),
spin, and U (y, z) is the spatially dependent confining po- g is given by g = (1 + V /(πvF ))−1/2 . For simplicity we
tential. It’s important to note that the electric field E ⃗ = assume g = 1. The full expression can be easily recovered.
−∇U (y, z), representing the gradient of the potential, is non- The mechanical degrees of freedom is given by the elastic
vanishing due to the absence of inversion symmetry in the sys- response of the wire on the transversal strain. We study a one-
tem. dimensional suspended double clamped beam of mass density
Following [31] we assume the external potential as ρm and stiffness T extending along the x direction. We as-
sume that y0 (x) = z0 (x) = 0 corresponds to equilibrium.
U (y, z) = by y 2 /2 + bz z 2 /2 + cyz. (2)
The mechanical part is related to the strain in the beam, which
By a special choice of the potential U (y, z) it is possible to is associated with small transverse displacements along the y
bring the single-particle Hamiltonian Eq. (1) into a quadratic and z axes from the equilibrium position: uy (x) = y(x) − y0
form, similar to the model of N electrons moving in a syn- and uz (x) = z(x)−z0 . The corresponding energy depends on
thetic magnetic field and described by the Hamiltonian h = the gradient of the displacements, and its behavior is captured
1/(2m)(p̂x − Ax )2 . One should note, however, that this syn- by the Hamiltonian:
thetic magnetic field is not breaking time-reversal symmetry
2 !
and therefore preserves Kramers’ degeneracy. In our problem, πζ2
X Z
T ∂uζ
it is realized with the following conditions Hu = dx + (6)
2ρm 2 ∂x
p ζ=y,z
2 −1 2 )−2 − 4c2 ,
2by,z = (mαso ) ± (mαso (3)
where c is kept as a free parameter. Under this condition the Here πζ (x) are the conjugated momenta of uζ (x), ζ =
Hamiltonian takes the following form: y, z, satisfying the commutation relations [uζ (x), πζ ′ (x′ )] =
iδ(x − x′ )δζζ ′ .
1 2
Ĥ = (p̂x + mαso [(bz z+cy)σy −(by y+cz)σz ]) . (4) Effective action.– Using the canonical transformation we
2m get the action from the Hamiltonian. It takes the form:
To demonstrate the basic properties of the dynamical model
with a synthetic magnetic field, we choose √ by = bz = c = S = SLL + Su + SLL−u
2 −1
(2mα√ so ) . Rotation in the spin (σ y − σz ) 2 →√ σz , (σy + Z ∞
v X β
Z
σz ) 2 → 2 → y, dx v −2 (∂τ ϕα )2 +(∂x ϕα )2
σ
√ y and the coordinate space (z + y)/ SLL = dτ
(y − z)/ 2 → z simplify the equation to the form: 2πg α=± 0 −∞
2 Z β Z ∞
1
1 T X
dx vs−2 (∂τ uη )2 + (∂x uη )2
Ĥ = p̂x + σz y . (5) Su = dτ
2m αso 2 η=y,z 0 −∞
Z ∞
iαso −1 β
Z
With this the electronic part of the Hamiltonian can be writ-
SLL−u = − dτ dx(∂τ ϕ+ − ∂τ ϕ− )uy .
ten as π 0 −∞
Z 2 (7)
1 X 1
He = dxΨ†α (x) p̂x + uy σαz Ψα (x)
2m αso
α,β=± vs is the phonon sound velocity and β = 1/T . Here we
Z
1 would like to note that the last term corresponds to the cou-
+ dxdx′ V (x − x′ )ρ(x)ρ(x′ ),
2 pling of the transverse displacement to the spinor current
e
where α = ±1 are the eigenstate indexes of the diagonal- J = mcα so
(∂t ϕ+ − ∂t ϕ− ), which emerges due to deviation
ized Hamiltonian and uy (x) = y(x) − y0 is y component of the beam from the equilibrium. It is worth to introduce new
of the transverse displacements from the equilibrium posi- fields ϕc + ϕs :
tion and y0 is the equilibrium position (see Fig. 1). The
1 1
second term
P describes the density-density interaction. Here ϕc = √ (ϕ+ + ϕ− ), ϕs = √ (ϕ+ − ϕ− )
ρ(x) = α=± Ψ†α (x)Ψα (x) and V (x − x′ ) is the electron- 2 2
electron interaction. We note that there are other possible
consequences of spin-orbit interaction and its interplay with which describe the charge and spin degrees of freedom respec-
electron-electron interactions such as those considered in [32] tively.
which are beyond our present scope. Using the Fourier transformations of ϕα (x, τ ) and uζ (x, τ )
After linearization of the spectra close to the Fermi-level
and the standard procedure of bosonization the Hamilto- 1 X ikx−iωn τ
ϕα (x, τ ) = √ e ϕknα (8)
nian [1, 2] takes the following form: βL k,n
X v Z
−1
1 X ikx−iωn τ
σz uy )2 + g −1 (∂x ϕs )2 uζ (x, τ ) = √
He = dx g(∂x θs − αso e uknζ (9)
2π βL k,n
s=±
3
5
to the following conditions for stability of the system:
4
2
ω+
by − mαso (b2y + c2 ) > 0, 2
bz − mαso (b2z + c2 ) > 0
3
2
(by + bz )))2 < (bz − mαso
2
(b2z + c2 )) (12)
ω(k)
(c(1 − mαso
2 ω = vs k 2
ω=vk ×(by − mαso (b2y + c2 ))
1 ω-
0
We introduce the small transverse displacements as z−z0 =
0 1 2 3 4 r cos φ and y − y0 = r sin φ. Rotation in spin space we get
k
1 2
Ĥ = (p̂x + Λrσz ]) + Ũ (13)
Figure 2. Eigenmodes ω± (k) (solid lines). The dashed lines denote 2m
the bosonic mode for electron ω = vk, phonon ω = vs k.
with
Λ(φ) 1/2
= (by sin φ + c cos φ)2 + (bz cos φ + c sin φ)2
the Euclidean action can be written as .
(mαso )
X X X v ω2
S= n
2
+ k ϕ∗nk,α ϕnk,α
2
The effective potential is
2πg v
k ωn α=c,s
X X X T ω2 r2
+ n
+ k u∗nk,ζ unk,ζ
2
(10) Ũ = (by sin2 φ + bz cos2 φ + c sin(2φ))
2 v 2
s
2 (14)
k ωn ζ=y,z
√ r2 Λ2 (φ) K(φ)
X X X 2α−1 ωn − = r2
so
unk,y ϕ∗nk,s − u∗nk,y ϕnk,s 2m 2
+
ω
2π
k n ζ=y,z
with
Within this action we have four modes. Two modes are !2 1/2
decoupled. The decoupled modes are the transverse phonon b̃z + b̃y b̃z − b̃y
K(φ) = + + c̃2 cos(2φ − φ0 )
mode along z - direction with dispersion ω = vs k and the 2 2
charge mode of the Luttinger liquid with dispersion ω =
vk. The spin mode of the Luttinger liquid couples with − c2 mαso
2
,
the transverse phonon mode along the y direction. It gives (15)
2
a gapped transverse phonon mode with dispersion ω+ =
q 2c̃
1 2 2 2 2
2 where tan φ0 = and
2 (vs + v )k + Λ + ((vs2 + v 2 )k 2 + Λ2 ) − vs2 v 2 k 4 , b̃z −b̃y
−2 2
where Λ = 2πvF ραso and a gapless mode ω− = 2 2
1
q
2
b̃z = bz (1 − mαso bz ), b̃y = by (1 − mαso by )
2 2 2 2 (16)
2 (vs + v )k + Λ − ((vs2 + v 2 )k 2 + Λ2 ) − vs2 v 2 k 4 . 2
The gapless mode reflects the breaking of the Lorentz invari- c̃z = c(1 − mαso (bz + by ))
ance of the bosonic mode of the Luttinger liquid. Instead of
We choose the new axes along the direction 2φ = φ0 and
the linear dispersion, which corresponds to the Lorentz invari-
2φ = φ0 + π and y ′ and z ′ . The minimum of the effective
ance of the Luttinger liquid, the dispersion is ω− ∼ k 2 , which
potential corresponds to the direction of z ′ -axis. The effec-
corresponds to the Galilean invariance. The spectrum of the
tive external potential Eq.(14) depends on the transverse dis-
obtained quasiparticles are shown in Fig. 2
placements. It leads to the renormalization of the action of the
General case.– Previously, we examined an external poten-
mechanical degrees of the beam. If K(φ0 /2+π/2) = 0 is the
tial subjected to the constraint Eq. (3). Now we explore the
quantum critical point (see Fig. 3). The condition Eq. (3) cor-
effect the case without the constraint.
responds to K(φ0 /2+π/2) < 0. For K(φ0 /2+π/2) > 0 the
1 2
vacuum is trivial while for K(φ0 /2 + π/2) < 0 one needs to
Ĥ = (p̂x + mαso [(bz z+cy)σy −(by y+cz)σz ]) + Ũ (y, z) rederive the effective action with non-trivial vacuum uy0 ̸= 0.
2m
Z β Z ∞
where T X
Su = dτ dx
2 −∞
2
mαso ζ=y ′ ,z ′ 0
(17)
Ũ (y, z) = U (y, z) − [(by y + cz)2 + (bz z + cy)2 ] (11)
K(φζ ) 2
2 × vs−2 (∂τ uz′ )2 + (∂x uz′ )2 + uz′ ,
T
Negative Ũ (y, z) leads to the shift of the position of the equi-
librium of the beam, uy,z (0) ̸= 0. A simple calculation lead where φy′ = φ0 /2 and φz′ = φ0 /2 + π/2.
4
(d
)
Figure 4. Correlations functions a) Im⟨u∗y (k, ω)uy (k, ω)⟩; b) ω 2 Im⟨u∗y (k, ω)uy (k, ω)⟩; c) Im⟨ϕ∗s (k, ω)ϕs (k, ω)⟩; d)
ω 2 Im⟨ϕ∗s (k, ω)ϕs (k, ω)⟩ at the the position of the quantum critical point. Plots e) - h) show the same correlations functions as in a)-c)
away from the quantum critical point (see the phase diagram on Fig. 3). The parameters are vs = 1, vF = 2, ρ = 1, Λ = 1. Parameter K = 0
for a) - d) and K = 4 for e) - h).
theoretically and experimentally, relates to the quantum noise coupled to the chiral mode at the edge.
associated with injecting charge or spin current in one wire In summary, we have analyzed a nano-mechanical system
and measuring the noise in the second. This effect is ex- consisting of a double-clamped suspended metallic nano-wire
pected to be analogous to the quantum noise observed in subject to an external dc electric field. The transverse flexu-
co-propagating edge modes in Integer Quantum Hall (IQH) ral elastic modes of the wire couple to the electronic charge
ν = 2 devices [33]. By adjusting the electric circuit, one can degrees of freedom, with the coupling strength controlled by
also model counter-propagating modes. The two-wire model the Rashba spin-orbit interaction in the wire. We derived the
serves as a minimal system for studying quantum interference hydrodynamic action of the model and demonstrated that it
effects controllable by interaction. is described by bosonic Gaussian theory. Our analysis re-
Nano-mechanically coupled Sliding Luttinger Liquids of- veals a quantum phase transition between two gapless phases
fer a promising avenue for investigating the interplay be- with trivial and non-trivial vacua, corresponding to straight
tween electronic and elastic degrees of freedom in nano- and bent equilibrium positions of the wire, respectively. We
mechanically driven Integer and Fractional Topological Insu- showed that the dispersion of the softest Goldstone mode tran-
lators. This model enables the study of 2D metal-insulator sitions from sound-like to quadratic at the quantum phase
quantum phase transitions (QPTs) in bulk and the emergence transition point. The breaking of Lorentz invariance at the
of gapless chiral modes at the edge. The simplest model de- QPT is attributed to the current-displacement character of the
scribing a transition to the Integer Topological Insulator state, boson-boson interaction. Our findings suggest that measur-
analogous to the transition to the IQH regime in a synthetic ing the back-action of the nano-mechanical system provides a
magnetic field [15], can be formulated by accounting for tun- highly sensitive and efficient spectroscopic tool for detecting
neling between neighboring wires and adjusting the chemical the QPT position, complementing quantum transport experi-
potential to the position of the bulk gap [31]. In the Frac- ments. We hope that our proposal may trigger new experi-
tional Topological Insulator state, the bulk gap opens due to ments with suspended nanowires.
inter-wire interaction effects [31]. This allows for the explo- We thank B.L. Altshuler, Ya. Blanter, A.Chubukov,
ration of various bulk instabilities driven by phonons and their Y.Gefen, Th. Giamarchi, D. Maslov, C. Mora, F. von Op-
influence on chiral edge modes. Additionally, elastic modes pen, S. Sachdev and J. Schmalian for fruitful discussions. The
associated with vibrations of a membrane in a Corbino disk work of M.N.K is conducted within the framework of the Tri-
geometry provide access to modeling 2D transverse phonons este Institute for Theoretical Quantum Technologies (TQT)
6
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