Two years ago a new anisotropic Hall state was discovered at half integer filling factors.(M. P. Lilly, K. B. Cooper, J. P. Eisenstein, L. N. Pfeiffer, K. W. West, Phys. Rev. Lett. \bf82), 394 (1999) This effect may indicate the existence of a striped phase, whose origin remains unclear. The transport anisotropy is aligned along the principle crystallographic [0øverline11] and [011] directions and proposed to be pinned to surface steps of overgrown wafers. In order to gain insight into this puzzling behavior we investigate the influence of slightly miscut substrates on the transport properties of a high mobility two-dimensional electron system. The surface morphology of plain and overgrown (100)-GaAs substrates with a miscut angle of 1^\circ, 1.5^\circ, 2^\circ towards the (110) and the (111) plane is characterized by AFM. Samples of (100)-substrates misoriented towards (111) reveal an overall higher mobility, which decreases with increasing miscut angle. Specimens misoriented towards (110) show a reduced mobility relative to (100)-references. Using a dilution refrigerator with an additional nuclear demagnetization unit we explore the anisotropic transport in these samples down to 1 mK.
[Y25.002] Thermopower in Half-Filled Landau Levels in a Square Quantum Well
JIAN ZHANG, CHANGLI YANG, R.R. DU (University of Utah), J.A. SIMMONS, J.L. RENO (Sandia National Labs)
Low temperature (50 mK) high magnetic field (up to 33T) magnetotransport in GaAs-AlGaAs square (width d=35nm) quantum wells (QWs) had revealed a sharp R_xx minimum at Landau level filling factor \nu= 1/2, indicating a strong deviation from the characteristic composite fermions transport typical for heterojunctions.(R.R.Du,Bull.Amer.Phys.Soc. 45,198 (2000).) To pursue the \nu= 1/2 state further, we have measured the thermopower S_xx in the same QW system. The QWs have a mobility \mu=3.5x10^6cm^2/vs and an electron density n=3.6x10^11/cm^2 . It is found that the S_xx at \nu= 1/2 exhibits a broad maximum down to T = 300 mK, apparently consistent with the results reported for heterojunctions. Measurements of thermopower at \nu= 1/2 and 3/2 at lower temperatures, T < 100 mK, as well as those at \nu=5/2, 7/2, 9/2, and 11/2 will be presented.
[Y25.003] Experimental observation of the fractional quantum Hall effect sequences \nu = p/(6p \pm 1) around \nu=1/6
W. Pan (Princeton University and NHMFL), H.L. Stormer (Columbia University and Bell Labs), D.C. Tsui (Princeton University), L.N. Pfeiffer, K.W. Baldwin, K.W. West (Bell Labs)
We report experimental observation of the two fractional
quantum Hall effect (FQHE) sequences, \nu = p/(6p \pm 1),
p = 1, 2, and 3, around \nu = 1/6 as well as the \nu =
2/15 fraction in an exceptionally high quality GaAs/AlGaAs
sample of density n \sim 1 \times 10^11 cm^-2.
Different from earlier specimens R_xx around \nu = 1/4
is well behaved and does not diverge as temperature (T) is
lowered. In fact, in R_xx versus B the
even-denominator fractions \nu = 3/2, 3/4, 1/2 and 1/4
fall on one straight line intercepting the origin. On the
other hand, the FQHE states \nu = 1/5, 2/11, 3/17, 3/19,
2/13 and 1/7 around \nu = 1/6 reside on top of a large
background, following the zero-resistance state at \nu =
1/5. Due to the sharply rising background as T is lowered,
the \nu = p/(6p \pm 1) FQHE sequences are only visible in
a temperature range between \sim 300mK and \sim 100mK.
When lowering T to \sim 80mK the sample becomes
essentially insulating for magnetic fields higher than \nu
= 1/5 and no further fractions are observable. The
observation of the two sequences \nu = p/(6p \pm 1)
indicates that the composite fermion model may also hold
around \nu = 1/6.
[Y25.004] Fractional Quantum Hall Effect in the Low 10^9/cm^2 Regime
J. Zhu, D. Simonian (Physics Department, Columbia University, New York), H. L. Stormer (Columbia University and Bell Labs), L. N. Pfeiffer, K. W. West (Bell Labs, Lucent Technologies, Murray Hill, New Jersey)
Recent advances in molecular beam epitaxy have made possible
high quality two-dimensional electron systems in GaAs
quantum wells with record low densities and very high
mobilities. We have investigated the transport properties of
these samples at ultra low temperatures of \sim12mK.
Through a combination of growth parameter control and
backgating, our samples cover a wide range of density in the
10^9/cm^2 and 10^8/cm^2 regime and have
mobilities up to 4x10^6cm^2/Vs. Samples in the
10^9/cm^2 regime show well developed fractional
quantum Hall sequences around filling factors 1/2 and 3/2.
Even a strong 1/5 is observed. The anomalous ordering in the
strength of some of the fractional minima clearly indicates
the involvement of different spin polarizations. An
unintentional parallel conduction channel shows an
unprecedentedly clean reentrant insulator-quantum
Hall-insulator behavior near 4T. These low density samples
are also very well suited for studies of the metal-insulator
transition. At zero magnetic field, a sharp rise in
resistance occurs around an electron density of
6x10^8/cm^2.
[Y25.005] On the Theory of a New Maximum Observed in ds Transport in Modulated Quantum Hall Systema Near \nu = 1/2
Natalya A. Zimbovskaya (The Hunter College of CUNY, New York, NY), Joseph L. Birman (The City College of CUNY, New York, NY)
We propose a theory for the new maximum recently observed by
Willett et al [Phys. Rev. Lett. 83, 2624 (1999)] in
the longitudinal magnetoresistance of a weakly modulated
two-dimensional electron gas (2DEG) near filling factor \nu
= 1/2 for the current driven along the modulation lines.
The maximum is superimposed upon a new resonance structure.
This occurs due to the geometric resonance of composite
fermion cyclotron orbits with the period of modulation of
the effective magnetic field. We propose here a
semiquantitative theory of the dc magnetotransport in a
modulated 2DEG near one half filling. Our analysis is based
on the Boltzmann transport equation and it enables us to
obtain this maximum which was neither observed nor predicted
previously.
[Y25.006] Theory of Spin-Flip Rotons in the Fractional Quantum Hall Effect
Sudhansu S. Mandal, J.K. Jain (The Pennsylvania State University)
Motivated by the recent discovery of mysterious new spin reversed collective excitations of the fractional quantum Hall effect by Kang, Pinczuk et al.,( M. Kang, A. Pinczuk, B.S. Dennis, M.A. Eriksson, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. 84), 546 (2000); Private Communication. we have theoretically studied the spin reversed excitations for the fully spin polarized quantum Hall states at filling factors 2/5 and 3/7. The expected spin wave mode and the other spin reversed modes are obtained by suitable orthonormalization among several spin reversed composite fermion (CF) states. We find that a spin reversed mode which involves a decrease in the CF Landau level index has significantly lower energy than the usual spin wave mode, except at small wave vectors, and its dispersion contains a roton minimum. We argue that the observed modes are the spin-flip rotons, and enumerate several testable consequences of our theory.
[Y25.007] Mixed States of Composite Fermions Carrying Two and Four Vortices
K. Park (Yale University), J.K. Jain (Penn State Univeristy)
There now exists preliminary experimental evidence for some
fractions, such as \nu = 4/11 and 5/13, that do not belong
to any of the sequences \nu=n/(2pn\pm 1), p and n
being integers. We propose that these states are mixed
states of composite fermions of different flavors, for
example, composite fermions carrying two and four vortices.
We also obtain an estimate of the lowest-excitation
dispersion curve as well as the transport gap; the gaps for
4/11 are smaller than those for 1/3 by approximately a
factor of 50.
[Y25.008] Proximity effect in fractional quantum Hall junctions
Nancy Sandler (Brandeis University)
Junctions between two-dimensional systems in different
fractional quantum Hall states or between fractional quantum
Hall states and normal metals present the possibility of
studying the spread of Laughlin correlations in real space.
In analogy with normal-metal/superconductor junctions, the
penetration of Laughlin correlations into the normal metal
side of a fractional quantum Hall junction is termed
``proximity effect''. This work addresses the existence of
such an effect for the particular case of a point contact
junction between a \nu =1/3 state and a normal metal
contact. The relation with the Kondo screening cloud is also
discussed.
[Y25.009] Effect of Disorder on the FQHE: Gaps and Magnetoexcitons
Ganpathy Murthy (University of Kentucky)
The effect of disorder on the FQHE gaps and ME dispersions is an outstanding problem. The Hamiltonian theory of Composite Fermions developed by R. Shankar and myself[1] is applied to this problem. A crucial point is that a disorder potential creates a random static density, which is felt by the CFs as a random gauge potential. Using standard diagrammatic techniques the disorder average is carried out self-consistently[2,3] including both the random gauge potential and potential scattering due to disorder. Results for gaps using the ``preferred" density and for the magnetoexcitons using a conserving approximation will be presented for a range of fractions.
1. R. Shankar and G. Murthy, Phys. Rev. Lett. 79, 4437 (1997).
2. C. Kallin and B. I. Halperin, Phys. Rev. B 31, 3635 (1985).
3. A. H. MacDonald, K. L. Liu, S. M. Girvin, and P. M.
Platzman, Phys. Rev. B 33, 4014 (1986).
[Y25.010] Study of inter-composite fermion interaction
Seung Yeop Lee, J.K. Jain, Vito W. Scarola (The Pennsylvania State University)
The interaction between composite fermions is much weaker than that between electrons, and most of the phenomenology of the fractional quantum Hall effect can be understood by neglecting it altogether. However, one may ask whether the weak inter-CF interaction has interesting consequences. We determine the inter-CF interaction by calculating the Haldane pseudopotentials for composite fermions and then constructing from them a real space effective interaction between composite fermions. Surprisingly, the inter-CF interaction is attractive at short distances. We investigate the possibility of new phases such as the bubble or the stripe phase in a Hartree-Fock approximation.
[Y25.011] Light scattering measurements of spin-excitations in the FQHE regime
I. Dujovne, M. Kang, A. Pinczuk (Columbia U. and Bell Labs.), B.S. Dennis (Bell Labs.), L.N. Pfeiffer (Bell Labs), K.W. West (Bell Labs.)
Inelastic light scattering methods access low-lying spin excitations in fractional quantum Hall states such as those with filling factors \nu=2/5, 3/7 and 4/9. In recent light scattering experiments we discovered that spectra of spin excitations of FQHE states display characteristic off-diagonal light polarization selection rules when they are excited in resonance with the lowest optical gap of the host GaAs quantum well. We take advantage of such clear selection rules to identify low-lying spin excitations. We find that at \nu=3/7 the lowest spin mode is below the energy of long wavelength spin waves. These results offer insights into quasiparticle excitations of the quantum fluid. We find agreement with composite fermion evaluations of low lying spin-reversed excitation modes(S. S. Mandal and J. K. Jain, Phys. Rev. Lett. submitted, (2000)).