Rectified voltage induced by a microwave field in a confined two-dimensional electron gas with a mesoscopic static vortex

We investigate the effect of a microwave field on a confined two dimensional electron gas which contains an insulating region comparable to the Fermi wavelength. The insulating region causes the electron wave function to vanish in that region. We describe the insulating region as a static vortex. The vortex carries a flux which is determined by vanishing of the charge density of the electronic fluid due to the insulating region. The sign of the vorticity for a hole is opposite to the vorticity for adding additional electrons. The vorticity gives rise to non-commuting kinetic momenta. The two dimensional electron gas is described as fluid with a density which obeys the Fermi-Dirac statistics. The presence of the confinement potential gives rise to vanishing kinetic momenta in the vicinity of the classical turning points. As a result, the Cartesian coordinate do not commute and gives rise to a Hall current which in the presence of a modified Fermi-Surface caused by the microwave field results in a rectified voltage. Using a Bosonized formulation of the two dimensional gas in the presence of insulating regions allows us to compute the rectified current. The proposed theory may explain the experimental results recently reported by J. Zhang et al.

The topology of the ground state wave function plays a crucial role in determining the physical properties of a many-particle system. These properties are revealed through the quantization rules. It is known that Fermions and Bosons obey different quantization rules, while the quantized Hall conductance [1] and the value of the spin-Hall conductivity are a result of non-commuting Cartesian coordinates [2]. Similarly the phenomena of quantum pumping observed in one-dimensional electronic systems [3][4][5] is a result of a space-time cycle and can be expressed in the language of non-commuting frequency ω = i∂ t and coordinate x = i∂ k as shown in ref. [6] .
Recently, the phenomena of rectification current I r (V ) = [I(V ) + I(−V )]/2 has been proposed as a DC response to a low-frequency AC square voltage resulted from a strong 2k F scattering in a one dimensional Luttinger liquid [12].
In a recent experiment [7], a two-dimensional electron gas (2DEG) GaAs with three insulating antidots has been considered. A microwave field has been applied, and a DC voltage has been measured. The experiment has been performed with and without a magnetic field.
The major result which occurs in the absence of the magnetic field is a change in sign of the rectified voltage when the microwave frequency varies from 1.46 GHz to 17.41 GHz. This behavior can be understood as being caused by the antidots, which create obstacles for the electrons.
We report in this letter a proposal for rectification. In section A we present a theory which show that rectification can be viewed as a result of non-commuting coordinates. In section B we present a qualitative model for rectification, namely the presence of vanishing wave function is described by a vortex which induces non-commuting kinetic momenta. The sign the vorticity is determined by the vanishing of the electronic density. The electronic fluid can be seen as a hard core boson which carry flux, the removal of charge caused by the insulating region is equivalent to a decrease of flux with respect the flux of the uniform fluid. Including in addition a confining potential we obtain regions where the momentum vanishes. The combined effect non-commuting kinetic momenta and confinement gives rise to non-commuting cartesian coordinates. In section C we use the Bosonization method to construct a quantitative theory which gives rise to a set of equations of motion. Constructing an iterative solution of this equations reveals the phenomena of rectifications explained in sections A and B.

A-Rectifications due to non-commuting coordinates
Due to the existence of the obstacles, the wave function of the electron vanishes in the domain of the obstacles. This will give rise to a change in the wave function , |ψ >→ is the unitary transformation (induced by the obstacle) and the coordinate coordinate representation becomes, [1,2,15]. An interesting situation occurs when the wave function |Φ > has zero's [1,2,15] or points of degeneracy [16] in the momentum space .This gives rise to non -commuting coordinates [1,2,16]. As a result we will have a situation where the the commutator [r 1 , r 2 ] of the coordinates is non zero. (1) Using the one particle hamiltonian h = E( K, r) in the presence of an external electric field with the commutators [r i ( K), Heisenberg equations of motions , This equations are identical with the one obtained in ref. [15 ] where E( K, r) is the single particle energy being in the semi-classical approximation and E 2 (t) the external electric field. As a result of the external electric field E 2 (t) changes the velocity changes according to eq.2. Using the interaction picture we find, V 2 (t) is the voltage caused by the external field E 2 (t). The Fermi Dirac occupation in the presence of the electric field is used to sum over all the single particle states . We obtain the current density J 1 (r) in the i = 1, The result obtained in the last equation follows directly from the non-commuting coordinates given by Ω( K) = 0. The current in eq. 1 depends on . We expand the non equilibrium density ρ( K, r) to first order in V 2 (t) we obtain the final form of the rectified current. Ω( K) has dimensions of a frequency and can be replaced with the help of the Larmor's theorem, by an ef f ective magnetic field Ω( K) = e 2mc B ef f ( K). This allows us to replace eq. 4 by the formula.

B-A model for non-commuting coordinates
We consider a two dimensional electron gas (2DEG) in the presence of a parabolic confining potential V c ( r). The 2DEG contains an insulting region of radius D caused by an infinite potential U I (r) ( in the experiment the insulating region this is caused by three antidots) see figure 1a.The effect of the insulating region of radius D causes the electronic wave function |ψ(r; R) > to vanish for | r − R| ≤ D. The spin of the electrons seems not to play any significant role,therefore we approximate the 2DEG by a spinless charge system.
Such a charged electronic system is equivalent to a hard core charged Boson . For Bosonic wave function has zero's which can be described as a vortex centered at r = R .
We will show that the following properties are essential in order to have non-commuting coordinates.
1-The vanishing of the wave function for | r − R| ≤ D is described by a vortex localized at R.

2-
The many particles will be described in term of a continuous Lagrange formulation [14] r( u, t). Here, r( u, t) is the continuous form of r α (t), where "α " denotes the particular particle, α = 1, 2, . . . , N with a density function ρ 0 ( r), which satisfies N/L 2 = ρ 0 ( u)d 2 u in two dimensions (L 2 is the two dimensional area). The coordinate r( u) and the momentum 3-The parabolic confining parabolic potential V c ( r) = mω 2 0 2 r 2 provides the confining length L F ,see figures 1a and 1b.
Using the conditions 1-3 we will show that the non-commuting coordinates emerge: -The vanishing of the wave f unction.
In the literature it was established that the vanishing of the Bosonic wave function gives rise to a multivalued phase and vorticity . See in particular the derivation given in ref. [9]. The vortex (the insulating region ) gives rise to non-commuting kinetic momenta , and the phase ∂θ( r; R) is caused by the localized vortex [8,9,10].This result is obtained in the following way: In the presence of a vortex the single particle operator is parametrize as follows:ψ( r; R) = | r− R| D e iθ( r, R) ψ( r) for | r− R| < D, andψ( r; R) = e iθ( r, R) ψ( r) for | r− R| > D. The field ψ( r) is a regular hard core boson field and θ( r; R) is a multivalued phase. As a result, the Hamiltonian h 0 = 2 2m K 2 + U I ( r) and the fieldψ( r; R) are replaced by the transformed Hamiltonian: The momentum K is replaced by the kinetic momentum , Π = K − ∂θ( r; R). The derivative of the multivalued phase θ( r; R) determines the vector potential A( r; R) = ∂θ( r; R).
whereB is an effective magnetic field due to the insulation region, which is defined as For the remaining part of this paper we will replace the delta function by a step function E( r, R) which takes the value of one for | r − R| < D and zero otherwise.
-T he many particle representation.
In the presence of hard core Bosons (spinless Fermions) the momenta is replaced by The static vortex describes the insulating region and modifies the momentum operator , Making use of this continuous formulation, we find a similar result as we have for the single particles [9].
Where E( u, R) = 1 for | u − R| < D and zero otherwise .
-T he conf ining potential.
The last ingredient of our theory is provided by the confining potential and the F ermi energy. Due to the confining potential the kinetic momentum has to vanishes for particles which have the coordinate close to the classical turning point (see figure 1b) . This lead to the following constraint problem for the kinetic momentum, and The kinetic momentum Π 1 (| u| ≈ L F ) and Π 2 (| u| ≈ L F ) form a second class constraints (according to Dirac's definition [11] the commutator of the constraints has to be non-zero given by eq.7.
We define the matrix [ Using the function E( u, R) (which replaces the delta function ) and the eqs.8,9 we obtain The overlapping conditions are given by the conditions: E( u, R) is equal to one for | u − R| < D and zero otherwise and δ(| r( u) − L F )| describes the condition of the classical turning points. Using eq.10 we find according to Dirac's second class constraints [11] the following new commutator [, ] D , For | r − R| < D we define a field Ω( u; R) trough the equation, This means that Ω( u; R) is approximated by Ω ∝ D 2 for | r − R| < D .
Eq. 11 shows that the presence of the momentum , with the constraints given by eqs. 8,9 gives rise to non-commuting coordinates Once we have the result that the coordinate do not commute we can use the analysis given in eq 4 (and the result for the current I 1 derived with the help of equation 4) to compute the rectified current, I 1 ∝ e 2 D 2 L 2 F V 2 2 (t) This results can be derived in directly using a modified Bosonization method with a non -commuting Kack Moody algebra [16,17].

C1-Bosonization for the 2DEG
We introduce a continuous formulation for the 2DEG many particles system. We replace the single particle Hamiltonian h total ( Π, r) by a many electron formulation [14]. We introduce a continuous representation, namely r( u, t). Here, r( u, t) is the continuous form of r α (t). The coordinate and the momentum obey r( u, t = 0) = u and P ( u, t = 0) = K The equilibrium Fermi-Dirac density is given by One of the useful description for many electrons in two dimensions is the Bosonization method. We will modify this method [16,17] in order to introduce the effect of the vortex field and the conf ining potential V c ( u).

C2-The Bosonization method in the absence of the insulating region and confining potential
In this section we will present the known [16,17] results for a two dimensional interacting metal in the absence of the vortex field and conf ining potential V c ( u). Our starting point is the Bosonized form of the 2DEG given in ref. [16,17].
Where Γ (s, s ′ ; − → r − − → r ′ ) is the Landau function for the two body interaction [17] and the notation :: represents the normal order with respect the Fermi Surface. [17]. According to ref. [16,17] the F.S. is described by, As a result the the F ERMI SURF ACE ( F.S.) excitations is given by , .The "normal" deformation to the F.S. is given by, n(s, u) is the normal to the F.S. as a function of the polar angle s and real space coordinate u.
Following refs. [16,17,18] we obtain the Bosonized hamiltonian for the many particles system in the presence of the potentials , V c ( u) and time dependent external potential U ext ( u, t) .
The new part of in the Bosonic hamiltonian is the presence of the confining V c ( u) = mω 2 0 2 u 2 and external potential U ext ( u, t). U ext ( u, t) is the external microwave radiation field, The commutation relations for the F ERMI SURF ACE in are given by the Kack Moody commutation relation [16,17 ]

C4-The Bosonization method in the presence of the insulating region and confining potential
This problem can be investigated using the hamiltonian given in eq.12 supplemented by the constraints conditions imposed by the vanishing density. described by a vortex. Using the results given in eq.11 one modifies the commutation relations. This modification can be viewed as Dirac's bracket due to second class constraints [11 ].
The result given by eq. 14 due to non-commuting coordinates [r 1 ( u), r 2 ( u ′ )] D = 0. We will compare this result with the one for a magnetic field B , δk || (s, − → u ) , δk || (s ′ , − → u ′ ) B given in ref. 17. The commutator for the two dimensional densities in the presence of a magnetic field B( u) perpendicular to the 2DEG has been obtained in ref. [17].
The modified commutator caused by the magnetic field (see eq. 27 in ref. 17) is: Where B( u) is the magnetic field and d dt(s) = −sin(s) ∂ ∂u 1 + cos(s) ∂ ∂u 2 is the derivative in the tangential direction perpendicular to the vector n(s) ( the normal to the Fermi surface) , n(s) · ∇ = cos(s) ∂ ∂u 1 + sin(s) ∂ ∂u 2 Using the analogy between the vortex field an the external magnetic field (eq. 15 ) we can represent equation 14 in terms of the parameters given in eq.11.
To conclude the commutator given in equation 16 allows to investigate the physics given in the hamiltonian 12. Using this formulation we will compute the rectified current.
where d dt(s) = −sin(s) ∂ ∂u 1 +cos(s) ∂ ∂u 2 is the derivative according to the tangential direction which is perpendicular to the vector n(s) with, n(s) · ∇ = cos(s) ∂ ∂u 1 + sin(s) ∂ ∂u 2 Using this commutation relations given in equation 16  and s = π + π 2 where V F (s, u) is the Fermi velocity. The effect of the vortex is to generates a change in the kinetic momentum perpendicular to the external electric field . This part is given by the last term. The last term is restricted to | u − R| < D and represents the vortex contribution .This term is maximum for the polar angles s = 0 and s = π. The maximum effect will be obtained in the region close to the classical turning point where the Fermi velocity obeys, V F (s, u) = m K 0 F (s, u) ≈ 0. The current density in the i = 1 direction is given by the polar integration of s ,[0 − 2π].
We introduce the dimensionless parameter γ =   obtained from eq.17 into the current density formula given by eq.18.
In order to provide a physical interpretation of our theory we will use physical parameters determined by the experiment. In the experiment the electronic density is n ≃ 10 15 m −2 this corresponds to a Fermi energy of E F ≃ 0.01eV , equivalent to a temperature of T F ≃ 120K and a Fermi wavelength of λ F ≃ 0.5×10 −7 m. For high mobility GaAs, the typical scattering time is τ ≃ 10 −11 sec, which corresponds to the mean free path l = v F τ . The ratio between the mean free path and the Fermi wave length obeys the condition, l λ F = v F τ λ F = hτ λ 2 F m ≃ 30. Therefore, we can neglect multiple scattering effects. When the thermal length is comparable with the size of the system L ≃ λ thermal = ( T F T ) 1/2 λ F , one obtains a ballistic system with negligible multiple scattering.
We make a single harmonic approximations (neglect terms which oscillate with frequencies 2ω 0 ,3ω 0 ...) We have used figure 3 in ref. [7] to extract the voltage changes as a function of the microwave field for a zero magnetic field. Figure 3 in ref. [7] shows clearly a change of sign when the microwave varies between 1.46 GHz to 34 GHz and vanishes at 17.41 GHz. In figure 2, we have plotted our results given by the formula G(ϕ) as a function of the microwave frequency with the rescaled experimental points (see the three points on our theory graph). As shown we find a good agreement of our theory with the experimental results once we choose ω 0 = 17.41 GHz. For frequencies which obeys 1.46 17.41 < ω ω 0 < 34 17.41 , we find good agreement with the experimental results. However, for low frequencies, our theory is inadequate and does not fit the experiment.
In conclusion, we can say that the origin of the rectification is the emergence of the non-commuting Cartesian coordinates and the non-commuting density excitations are a result of the vortex field accompanied the classical turning caused by the confining potential. Using the modified KacK Moody commutations rule for the density excitations we find that excitations with different polar angles s become coupled.
Using this theory we have explained the results of the experiment [7] in a region where the magnetic field was zero.