Search

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model., Comment: 4 pages, 4 figures

We have investigated the influence of an increasing in-plane magnetic field on the states of half filling of Landau levels ( $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11/2$, 9/2, 7/2, and 5/2) of a two-dimensional electron system. In the electrically anisotropic phase at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}9/2$ and 11/2 an in-plane magnetic field of $\ensuremath{\sim}1--2\mathrm{T}$ overcomes its initial pinning to the crystal lattice and reorients this phase. In the initially isotropic phases at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ and 7/2 an in-plane magnetic field induces a strong electrical anisotropy. In all cases, for high in-plane fields the high-resistance axis is parallel to the direction of the in-plane field.

We investigate the properties of composite fermions with four attached flux quanta through tilted field experiments near Landau level filling factor nu = 3/4. The observed collapse of fractional quantum Hall gaps in the vicinity of this quarter-filling state can be comprehensively understood in terms of composite fermions with mass and spin. Remarkably, the effective mass and g-factor of these four flux quanta composite fermions around nu = 3/4 are very similar to those of two flux quanta composite fermions around nu = 3/2., Comment: 4 pages, 1 table, 3 postscript figures

We report a study of the cyclotron resonance (CR) transitions to and from the unusual $n=0$ Landau level (LL) in monolayer graphene. Unexpectedly, we find the CR transition energy exhibits large (up to 10%) and non-monotonic shifts as a function of the LL filling factor, with the energy being largest at half-filling of the $n=0$ level. The magnitude of these shifts, and their magnetic field dependence, suggests that an interaction-enhanced energy gap opens in the $n=0$ level at high magnetic fields. Such interaction effects normally have limited impact on the CR due to Kohn's theorem [W. Kohn, Phys. Rev. {\bf 123}, 1242 (1961)], which does not apply in graphene as a consequence of the underlying linear band structure., 4 pages, 4 figures. Version 2, edited for publication. Includes a number of edits for clarity; also added a paragraph contrasting our work w/ previous CR expts. in 2D Si and GaAs

We report on infrared spectroscopy of bilayer graphene integrated in gated structures. We observed a significant asymmetry in the optical conductivity upon electrostatic doping of electrons and holes. We show that this finding arises from a marked asymmetry between the valence and conduction bands, which is mainly due to the inequivalence of the two sublattices within the graphene layer. From the conductivity data, the energy difference of the two sublattices is determined., Comment: 4 pages, 3 figures

The resistivity of ultraclean suspended graphene is strongly temperature ($T$) dependent for $5lTl240\text{ }\text{ }\mathrm{K}$. At $T\ensuremath{\sim}5\text{ }\text{ }\mathrm{K}$ transport is near-ballistic in a device of $\ensuremath{\sim}2\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ dimension and a mobility $\ensuremath{\sim}170\text{ }000\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$. At large carrier density, $ng0.5\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$, the resistivity increases with increasing $T$ and is linear above 50 K, suggesting carrier scattering from acoustic phonons. At $T=240\text{ }\text{ }\mathrm{K}$ the mobility is $\ensuremath{\sim}120\text{ }000\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{V}\text{ }\mathrm{s}$, higher than in any known semiconductor. At the charge neutral point we observe a nonuniversal conductivity that decreases with decreasing $T$, consistent with a density inhomogeneity $l{10}^{8}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}2}$.

We present the first measurements of cyclotron resonance of electrons and holes in bilayer graphene. In magnetic fields up to B = 18 T we observe four distinct intraband transitions in both the conduction and valence bands. The transition energies are roughly linear in B between the lowest Landau levels, whereas they follow \sqrt{B} for the higher transitions. This highly unusual behavior represents a change from a parabolic to a linear energy dispersion. The density of states derived from our data generally agrees with the existing lowest order tight binding calculation for bilayer graphene. However in comparing data to theory, a single set of fitting parameters fails to describe the experimental results., Comment: to appear in Phys. Rev. Lett. Updated version with two added references and minor text editing

In the high-magnetic-field low-disorder limit the ground state at v=1/5 Landau-level filling is an incompressible quantum limit. This is determined by observing vanishing resistivity p xx as the temperature T→0. At filling factors below v=1/5 as well as in a narrow region above v=1/5, p xx diverges exponentially as T→0. This contrasts the T dependence at any higher v. The quantum limit at v=1/5 is surrounded by a different phase. In as much as the exponential divergencies are indicative of an electron solid this solid phase is reentrant in a narrow region above v=1/5

We investigate the quantum Hall (QH) states near the charge-neutral Dirac point of a high mobility graphene sample in high magnetic fields. We find that the QH states at filling factors $\ensuremath{\nu}=\ifmmode\pm\else\textpm\fi{}1$ depend only on the perpendicular component of the field with respect to the graphene plane, indicating that they are not spin related. A nonlinear magnetic field dependence of the activation energy gap at filling factor $\ensuremath{\nu}=1$ suggests a many-body origin. We therefore propose that the $\ensuremath{\nu}=0$ and $\ifmmode\pm\else\textpm\fi{}1$ states arise from the lifting of the spin and sublattice degeneracy of the $n=0$ Landau level, respectively.

We report infrared studies of the Landau level (LL) transitions in single layer graphene. Our specimens are density tunable and show \textit{in situ} half-integer quantum Hall plateaus. Infrared transmission is measured in magnetic fields up to B=18 T at selected LL fillings. Resonances between hole LLs and electron LLs, as well as resonances between hole and electron LLs are resolved. Their transition energies are proportional to $\sqrt{B}$ and the deduced band velocity is $\tilde{c}\approx1.1\times10^6$ m/s. The lack of precise scaling between different LL transitions indicates considerable contributions of many-particle effects to the infrared transition energies., Comment: 4 pages, 3 figures, to appear in Phys. Rev. Lett

Magnetotransport measurements were performed in a ultra-high mobility GaAs/AlGaAs quantum well of density $\sim 3.0 \times 10^{11}$ $cm^{-2}$. The temperature dependence of the magnetoresistance $R_{xx}$ was studied in detail in the vicinity of $\nu={9/2}$. In particular, we discovered new minima in $R_{xx}$ at filling factor $\nu\simeq 4{1/5}$ and $4{4/5}$, but only at intermediate temperatures $80\lesssim T\lesssim 120$ mK. We interpret these as evidence for a fractional quantum Hall liquid forming in the N=2 Landau level and competing with bubble and Wigner crystal phases favored at lower temperatures. Our data suggest that a magnetically driven insulator-insulator quantum phase transition occurs between the bubble and Wigner crystal phases at T=0., Comment: Phys. Rev. Lett.93 266804 (2004)

We have investigated the behavior of electronic phases of the second Landau level under tilted magnetic fields. The fractional quantum Hall liquids at $\nu=$2+1/5 and 2+4/5 and the solid phases at $\nu=$2.30, 2.44, 2.57, and 2.70 are quickly destroyed with tilt. This behavior can be interpreted as a tilt driven localization of the 2+1/5 and 2+4/5 fractional quantum Hall liquids and a delocalization through melting of solid phases in the top Landau level, respectively. The evolution towards the classical Hall gas of the solid phases is suggestive of antiferromagnetic ordering.

At a very low-temperature of 9 mK, electrons in the second Landau level of an extremely high-mobility two-dimensional electron system exhibit a very complex electronic behavior. With a varying filling factor, quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters. We observe a fully developed $\ensuremath{\nu}=2+2/5$ state separated from the even-denominator $\ensuremath{\nu}=2+1/2$ state by an insulating phase and a $\ensuremath{\nu}=2+2/7$ and $\ensuremath{\nu}=2+1/5$ state surrounded by such phases. A developing plateau at $\ensuremath{\nu}=2+3/8$ points to the existence of other even-denominator states.

We study the scattering properties of an interface between a one-dimensional (1D) wire and a two-dimensional (2D) electron gas. Experiments were conducted in the highly controlled geometry provided by molecular bean epitaxy overgrowth onto the cleaved edge of a high quality GaAs /AlGaAs quantum well. Such structures allow for the creation of variable length 1D-2D coupling sections. We find ballistic 1D electron transport through these interaction regions with a mean free path as long as 6mgr;m. Our results explain the origin of the puzzling nonuniversal conductance quantization observed previously in such 1D wires.