Light scattering by magnetorotons of collective excitations in the fractional quantum Hall regime

Light scattering by magnetorotons of collective excitations in the fractional quantum Hall regime

Physica E 12 (2002) 55 – 58 www.elsevier.com/locate/physe Light scattering by magnetorotons of collective excitations in the fractional quantum Hall...

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Physica E 12 (2002) 55 – 58

www.elsevier.com/locate/physe

Light scattering by magnetorotons of collective excitations in the fractional quantum Hall regime Moonsoo Kanga; b; c; ∗ , A. Pinczukb; c , I. Dujovneb; c , B.S. Dennisc , L.N. Pfei1erc , K.W. Westc a Department

of Physics, Washington State University, P.O. Box 642814, Pullman, WA 99164-2814, USA of Physics and Applied Physics, Columbia University, New York, NY 10027, USA c Bell Labs, Lucent Technologies, Murray Hill, NJ 07974, USA

b Departments

Abstract Magnetorotons in the dispersion curves of collective gap excitation modes of fractional quantum Hall liquids are measured by resonant inelastic light scattering. Two deep magnetoroton minima are observed at  = 25 , while a single deep minimum is resolved at  = 13 . The results support Chern–Simons and composite fermion calculations that predict multiple roton minima for states with  ¿ 13 . ? 2002 Elsevier Science B.V. All rights reserved. PACS: 73.20.Mf; 73.40.Hm; 78.30.−j Keywords: Inelastic light scattering; Fractional quantum Hall e1ect; Collective excitation

The states of the fractional quantum Hall e1ect (FQHE) are incompressible quantum liquids with behavior guided by fundamental interactions [1]. Condensation of 2D electron systems into quantum liquids is manifested in distinctive low-lying collective excitation modes built from neutral quasiparticle–quasihole pairs within the same Landau level [2– 4]. The energy (!) vs. wave vector (q) dispersion relations of the modes display characteristic features such as magnetoroton minima in the dispersion of charge-density gap excitations. The roton minima, caused by excitonic bindings in the neutral  pairs, occur at wave vectors q ≈ 1=l0 , where l0 = ˜c=eB is the magnetic length. Analytical studies and numerical calculations have ∗

Corresponding author. Department of Physics, Washington State University, PO Box 642814, Pullman, WA 99164-2814, USA. Fax: (509) 335-7816. E-mail address: m [email protected] (M. Kang).

explored the low-lying collective modes of FQH liquids within composite fermion (CF) and Chern– Simons (CS) frameworks [5 –9]. Experiments that probe these collective modes o1er crucial tests of theories of the quantum liquid. We report resonant inelastic light scattering measurements of the low-lying charge-density excitations at Glling factors  = 25 and 13 . The spectra are interpreted within a framework in which collective excitation modes are described by energy vs. wave vector dispersion relations. The major consequence of weak residual disorder in the quantum Hall states is the activation of modes with wave vectors ql0 & 1 that are larger than the light scattering wave vector k [10,11]. We Gnd that spectra at  = 25 display signiGcant di1erences from those at  = 13 . At  = 13 there is a single deep magnetoroton minimum and at  = 25 we identify an additional roton minimum. There are also marked di1erences in the spectral line shapes of modes with

1386-9477/02/$ - see front matter ? 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 2 4 1 - 7

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SW

ν = 1/3

ω L (m eV ) 1520.20

IN TE N S ITY (A .U .)

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n=5.5x10 cm B =6.93T T=50m K

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1520.48

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E nergy (m eV ) Fig. 1. Resonant inelastic light scattering spectra at  = 13 . SW denotes the long wavelength spin wave excitation at the Zeeman energy EZ = gB BT . Dotted lines indicate collective excitations of the FQH state.

q ≈ 0 that are explained as another manifestation of the multiple roton structure at  = 25 . We studied the high-quality 2D electron system in single GaAs quantum wells (SQW) of widths d=250– L Electron densities are in the range of n = 2:4 × 330 A. 1010 –1:2 × 1011 cm−2 . The low-temperature and low Geld electron mobilities are  & 5 × 106 cm2 =V s. Experimental details have been reported elsewhere [12–16]. Fig. 1 shows inelasic light scattering spectra at = 13 FQH state. The marked dependences of the intensities on !L are characteristic of resonant light scattering measurements. The peaks labeled SW are due to long wavelength spin wave excitations of the spin polarized 2D electron system [12–15]. There exist three other peaks interpreted as collective excitations of the FQH state due to their characteristic temperature and Glling factor dependences. The sharp peak at the highest energy (0:92 meV, 0.081 Ec ; Ec = e2 =l0 ) is the charge-density gap mode of the  = 13 state at long wavelengths [12–16]. The two modes at lower energies are assigned to peaks in the DOS of charge-density excitations. SpeciGc assignments of these observed modes are readily made by comparing the experimental results with the calculated dispersion curves shown in Fig. 2(a). The solid

curve was scaled down, from the ideal 2D result [9], by a constant to facilitate the assignment of the observed modes. Open squares indicate results of calculations that incorporate the e1ects of Gnite thickness of the 2D electron system [17]. Solid squares indicate observed modes. The mode at the lowest energy (0:46 meV; 0:040Ec ) is assigned to the critical point at the deep magnetoroton minimum in the dispersion of gap excitations. The mode at 0:68 meV (0:060Ec ) is interpreted as a peak from the large DOS around large wave vectors (ql0 & 2) where the dispersion becomes Mat. These results suggest the existence of just one deep magnetoroton minimum at  = 13 . At  = 25 , we observe four collective excitation modes of the FQH state. These modes are assigned in a similar way in comparison with theoretical calculations. The highest energy mode is interpreted as the long wavelength gap mode of the = 25 state. The three modes at lower energies are assigned to three critical points in the dispersion of the modes [16]. A comparison of energies of measured modes (solid squares) and calculated critical points (open squares) in Fig. 2(b) indicates that at  = 25 there are additional critical points at wave vectors ql0 & 1. The result implies that in the dispersion of gap excitations at  = 25 there is one more magnetoroton minimum. Two lower en-

M. Kang et al. / Physica E 12 (2002) 55 – 58 0.12

(a) ν = 1/3

0.10 0.08 0.06

2

E nergy ( e / ε l0)

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0.08

(b) ν = 2/5 0.06

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q (1/ l0 ) Fig. 2. The dispersion of collective excitations at (a)  = 13 and (b)  = 25 . The solid curve was scaled down from an ideal 2D calculation [9] by a constant to help assigning the observed modes. Solid squares indicate observed mode energies with q assigned from the comparisons with calculations. Open squares indicates results of calculations that incorporate the e1ect of Gnite thickness [17].

ergy modes at 0.027 and 0:032Ec are assigned to two roton minima in the dispersion. The mode at 0:44Ec could be interpreted as the local maximum between two rotons or the q → ∞ mode. We could not make a more speciGc assignment because observation of critical points of the DOS determines mode energies but does not reveal mode wave vectors. The evidence of two rotons in the mode dispersion of the state with  = 25 supports key predictions of CS and CF theories of the FQH liquids [5 –8]. The di1erence in roton structures of collective excitations in the states at  = 25 and 13 can be visualized within the picture in which the FQHE is described as the integer QHE of composite fermions. In this picture, collective excitations of  = 13 and  = 25 states are conceived as excitations of spin-

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less  = 1 and  = 2 integer quantum Hall states of CF’s, respectively. The existence of roton minima in the dispersion of collective excitations can be understood from the insight on the structures of the wave functions in Landau levels. Magnetoroton minima are due to the attractive Coulomb interaction in a quasiparticle–quasihole pair of separation x = ql20 [2,18]. This interaction is maximized when the overlap of the wave functions of ground and excited states is largest. The existence of multiple rotons naturally comes from multiple nodes in wave functions of higher Landau levels. We note that the dispersion of inter-Landau level excitations at  = 1, like the collective mode dispersion at = 13 , has a single roton [2]. On the other hand, the roton structure at  = 25 could be similar to that of inter-Landau level excitations at  = 4 (or spinless  = 2), which displays multiple roton minima [19]. At  = 13 the energy of the q ≈ 0 mode (0:92 meV) is exactly twice the energy of the magnetoroton minimum (0:46 meV). This observation supports the early conjecture of Girvin and co-workers that the lowest long wavelength charge-density mode is a two-roton bound state [4]. Our measurements suggest that the binding energy of the two-roton state could be relatively small, probably less than the width of the roton mode in Figs. 1 and 2(a). Numerical studies show that the q ≈ 0 mode energy of single quasiparticle– quasihole pair excitation, as shown in Fig. 2(a), is substantially higher than twice the roton energy [9]. Current theory is not very speciGc about the character of the q ≈ 0 mode at = 25 . In the data of Fig. 2(b) the q ≈ 0 mode energy occurs in the range of twice the roton energies. This suggests that the q ≈ 0 mode here could also be related to excitations built of two-roton states. However, the existence of multiple rotons adds more possible combinations of multiple roton states for q ≈ 0 excitation, which may account for the broader line width at  = 25 . This is an aspect of collective excitations of states with  ¿ 13 that requires further experimental and theoretical study. In summary, we measured the energies of critical points in dispersion curves of collective gap excitations of the FQH states at  = 13 and 25 . These experiments are enabled by breakdown of wave vector conservation in resonant inelastic light scattering processes due to weak residual disorder. Our

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measurements provide evidence of two magnetoroton minima in the dispersion of gap modes at  = 25 and of a single minimum at  = 13 . The experiments conGrm key predictions of Chern–Simons and composite fermion theories. Measured energies of critical points and of long wavelength excitations also show good agreement with current theoretical evaluations of the FQH state collective excitations based on the composite fermion framework. We thank B.I. Halperin, J.K. Jain, P.M. Platzman and S.H. Simon for fruitful discussions. We are also grateful to V. Scarola, K. Park, and J.K. Jain for providing us many results prior to publication. References [1] R.B. Laughlin, Phys. Rev. Lett. 50 (1983) 1395. [2] C. Kallin, B.I. Halperin, Phys. Rev. B 30 (1984) 5655.

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