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Comment on “Condensation of Excitons in a Trap”

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  • Comment on Condensation of Excitons in a TrapDirk Semkat,* Siegfried Sobkowiak, Gunter Manzke, and Heinrich Stolz

    Institut fur Physik, Universitat Rostock, D-18051 Rostock, Germany

    Condensation of excitons is still a fascinating topic of solidstate physics. In a recent Letter Condensation ofExcitons in a Trap,1 High et al. claim to have observed acondensed state in a system of indirect excitons in doublequantum well structures within an electrostatic potential trap.As in every BoseEinstein condensate, spontaneous coherenceof matter waves should emerge in the exciton system. Thiscoherence is transferred to the decay luminescence and thusshould be observable in the light emission from the excitoncloud. Indeed, the authors have observed in a series ofexperiments a pronounced increase of the coherence of thelight emitted from the excitons either by lowering thetemperature or by increasing the power of the laser excitingthe excitons (compare Figures 3d,e, 4, and 5 of ref 1).The coherence properties of the emitted light were measured

    with the well-known technique of shift interferometry.2 Heretwo images of the same object, shifted by a small amount of ,are superimposed, and by varying the phase delay between thetwo light paths, interference fringes are generated (see Figure 2of ref 1). To determine the interference contrast, the authorsuse the simple formula C = (I12 I1 I2)/2(I1I2)1/2. Furthersupport for the interpretation of the occurrence of acondensate is derived by the authors from a simple idealBoson model for the excitons captured in a harmonic trap fromthe critical temperature Tc = (6

    1/2)/()2d with 2d =(xy)

    1/2 being the 2d oscillator frequency. With theassumptions for the trap oscillator frequencies the authorsgive, this indeed would lead to a critical temperature for BECfor N 3 103 excitons in the trap of 2 K.As will be shown in this comment, there are several

    objections against the statements in ref 1 based on a rigoroustheory of shift interferometry as well as on the thermodynamicproperties of a dense interacting exciton gas in a potential trap.Theory of Shift Interferometry. We start with a single

    point emitter at position xo,yo in the object plane at d1, whichis imaged by a lens. The amplitude of the light eld at a point(xi,yi) in the image plane at d2 is then given by

    3

    = +

    +

    + + +

    E x yMd

    ikd M

    ikMd

    x y E x y

    ikd

    x y P x Mx y My

    ( , ) exp[ (1 1/ )]

    exp2

    ( ) ( , )

    exp2

    ( ) ( , )

    I i i

    i i

    d i i

    12 1

    1

    2 2O o o

    1o2

    o2

    2 o o(1)

    with EO(x,y) denoting the eld amplitude of the emitter andP2d(x,y) the amplitude point spread function (PSF) of the lens.M = d1/d2 is the system magnication.In shift interferometry we superimpose on this image that of

    an identical object shifted by in, for example, the x directionand with an additional phase . Its eld is given by

    = + +

    +

    +

    + +

    E x yMd

    ikd M i

    ikMd

    x y E x y

    ikd

    x y

    P x Mx y My

    ( , , ) exp[ (1 1/ ) ]

    exp2

    ( ) ( , )

    exp2

    (( ) )

    ( , )

    I i i

    i i

    d i i

    12 1

    1

    2 2O o o

    1o

    2o2

    2 o o (2)

    Using the notation of ref 1, the interference pattern of asingle point emitter is given by

    = | + | = + +I x y E x y E x y I I I( , , ) ( , ) ( , , )i i I i i I i i122

    1 2 inter

    (3)

    While I1 and I2 are the images of the two shifted objects, theinterference term is given by

    = *

    | |

    + +

    * + +

    I E x y E x y

    E x yikdx i

    P x Mx y My

    P x Mx y My

    2Re[ ( , ) ( , , )]

    2 ( , ) Re exp

    ( , )

    ( , )

    I i i i i

    d i i

    d i i

    inter

    O o o2

    1o

    2 o o

    2 o o(4)

    In the following, we are interested only in the case of anincoherently emitting cloud of thermal excitons. Then we canapproximate the rst-order eld correlation functionGO(x, y, x, y) = EO(x, y)E*O(x, y), which represents themutual coherence function of the emitter4 by

    = G x y x y I x y x y x x y y( , , , ) ( , , , ) ( ) ( )O O (5)with IO = |EO(xo, yo)|

    2 representing the intensity distribution ofthe emitter. Therefore, we obtain the interference pattern of thetotal emission by integrating eq 4 over the whole emitter. Thisgives the following expression

    + +

    * + +

    I x y I x yikdx i

    P x Mx y My

    P x Mx y My x y

    ( , ) 2 ( , )Re exp

    ( , )

    ( , ) d d

    i i

    d i i

    d i i

    inter O o o1

    o

    2 o o

    2 o o o o(6)

    Received: July 6, 2012Published: July 23, 2012

    Letter

    pubs.acs.org/NanoLett

    2012 American Chemical Society 5055 dx.doi.org/10.1021/nl302504h | Nano Lett. 2012, 12, 50555057

  • Equation 6 is the central relation for shift interferometry. Itshows that the interference pattern depends not only on thePSF but also on the intensity distribution of the emittingsource!To obtain quantitative results, we need the PSF of the actual

    imaging setup, which fortunately can be obtained from Figure2S of the supporting material of ref 1. For our purposes, thepoint spread function can be approximated quite well by asimple Gaussian,5

    = +

    P x y P

    x y( , ) expd

    p2 0

    2 2

    2(7)

    Noting that the intensity distribution shown in Figure 2S of ref1 is given by the square of the PSF, eq 7, we obtain p = 2.0 m.Representing the intensity distribution of the exciton cloud

    also by a Gaussian with dierent halfwidths in x- and y-directions as

    =

    I x y I

    x y( , ) exp

    x yO 0

    2 2

    (8)

    the interference pattern can be calculated analytically.Fortunately, due to the choice of the PSF, the nal resultfactorizes into a product of x- and y-dependent functions, sothat for a shift in x-direction, the result for the interferencecontrast C(x, ) = (I12 I1 I2)/2(I1I2)1/2 depends only on and x.As a rst result we demonstrate in Figure 1 that, contrary to

    the statement in ref 1, the image of a point source (full red line)

    is dierent from the interference visibility function; that is, thecontrast as a function of shift of an incoherent source is muchlarger than the optical resolution. Therefore, Figure 1S of ref 1seems to be incorrect, and the scale of upper and lower abscissamust dier by a factor of 1.4. By comparing Figures 1b and 2cof ref 1, one can deduce that it must be the shifts given in ref 1(in the following: B) that have to be scaled by V = 1.4 to getconsistency ( = VB).The interference contrast at x = 0 is shown in Figure 2 as a

    function of the source size for dierent shifts. Looking moreclosely at the curve for B = /V = 4 m, our theory predicts acontrast of 0.07 at source sizes above 6 m HWHM inagreement with the experiment. Reducing the source size, the

    contrast increases reaching values of about 0.20.3 for sizes of2 m similar to those measured experimentally (Figure 3e of ref1). This shows that our theory is able to reproduce theexperimental results of ref 1 almost quantitatively without thenecessity of a coherent condensate.Thermodynamics. To analyze the thermodynamics of the

    exciton gas in the trap, we use a HartreeFockBogoliubovPopov (HFBP) theory (for an overview, see ref 6) which wehave recently applied to excitons in bulk Cu2O.

    7 We use a localdensity approximation well-justied by the extension of the trap(Figure 1 in ref 1) which is, even in the narrower y-direction,still large compared to typical length scales, for example, the 2dexcitonic Bohr radius.Applying the HFBP theory in local density approximation to

    the 2d case, the densities of thermally excited nT and ofcondensed excitons nc (in ThomasFermi approximation) inan external potential Vext form a coupled system of equations,

    = +

    n

    d k k rE k r

    n E k r

    E k r

    (r)(2 )

    ( , )( , )

    ( ( , ))12

    12

    ( ( , ))

    T2

    2 B

    2

    3

    (9)

    =

    nU

    V r U n r

    V r U n r

    (r)1[ ( ) 2 ( )]

    ( ( ) 2 ( ))

    c

    0ext 0

    T

    ext 0T

    (10)

    Here nB is the Bose distribution, and the quasiparticle energy Eis given by (compare7)

    = E k r k r U n r( , ) ( , ) ( ( ))2 0 c 23 (11)with

    = + +k r k M V r U n r( , ) /2 ( ) 2 ( )2 2 ext 03 (12)The total density is given by n = nT + nc, is the globalchemical potential, and U0 is the strength of the interexcitonicinteraction. We extract the latter parameter from theexperimentally given energy shift of E = 1.3 meV at n = 3 1010 cm2 by E = 2U0n leading to U0 = 2.2 eVm

    2. The trappotential was taken directly from Figure 1c and d of ref 1.In Figure 3, the critical particle number Ncrit for Bose

    Einstein condensation according to eqs 912 is presented independence on the temperature by the black curve. The dotted

    Figure 1. Comparison of the relative intensity of a point source (fullred line) with the interference visibility function (blue dashed line) fora PSF given by eq 7 with P = 2.0 m and a source size x = 6 m (seeeq 8).

    Figure 2. Contrast at x = 0 for dierent eective shifts B = /V as afunction of the HWHM of the exciton cloud, which is given in theinset as a function of the size parameter x (see eq 8). To allow a directcomparison with the results of ref 1. the contrast was scaled by 0.65,which is the maximum contrast in the experiments.

    Nano Letters Letter

    dx.doi.org/10.1021/nl302504h | Nano Lett. 2012, 12, 505550575056

  • part of the line denotes the region, where the particle numberexceeds the capacity of the trap. Additionally, correspondingcurves are shown for the weakly interacting case (U0/100;dashed blue