Publications

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We report on a scalable electrostatic process to transfer epitaxial graphene to arbitrary glass substrates, including Pyrex and Zerodur. This transfer process could enable wafer-level integration of graphene with structured and electronically-active substrates such as MEMS and CMOS. We will describe the electrostatic transfer method and will compare the properties of the transferred graphene with nominally-equivalent 'as-grown' epitaxial graphene on SiC. The electronic properties of the graphene will be measured using magnetoresistive, four-probe, and graphene field effect transistor geometries [1]. To begin, high-quality epitaxial graphene (mobility 14,000 cm2/Vs and domains >100 {micro}m2) is grown on SiC in an argon-mediated environment [2,3]. The electrostatic transfer then takes place through the application of a large electric field between the donor graphene sample (anode) and the heated acceptor glass substrate (cathode). Using this electrostatic technique, both patterned few-layer graphene from SiC(000-1) and chip-scale monolayer graphene from SiC(0001) are transferred to Pyrex and Zerodur substrates. Subsequent examination of the transferred graphene by Raman spectroscopy confirms that the graphene can be transferred without inducing defects. Furthermore, the strain inherent in epitaxial graphene on SiC(0001) is found to be partially relaxed after the transfer to the glass substrates.

We wish to present in this report experimental results from a one-year Senior Council Tier-1 LDRD project that focused on understanding the physics of a possible non-Abelian fractional quantum Hall effect state. We first give a general introduction to the quantum Hall effect, and then present the experimental results on the edge-state transport in a special fractional quantum Hall effect state at Landau level filling {nu} = 5/2 - a possible non-Abelian quantum Hall state. This state has been at the center of current basic research due to its potential applications in fault-resistant topological quantum computation. We will also describe the semiconductor 'Hall-bar' devices we used in this project. Electron physics in low dimensional systems has been one of the most exciting fields in condensed matter physics for many years. This is especially true of quantum Hall effect (QHE) physics, which has seen its intellectual wealth applied in and has influenced many seemingly unrelated fields, such as the black hole physics, where a fractional QHE-like phase has been identified. Two Nobel prizes have been awarded for discoveries of quantum Hall effects: in 1985 to von Klitzing for the discovery of integer QHE, and in 1998 to Tsui, Stormer, and Laughlin for the discovery of fractional QHE. Today, QH physics remains one of the most vibrant research fields, and many unexpected novel quantum states continue to be discovered and to surprise us, such as utilizing an exotic, non-Abelian FQHE state at {nu} = 5/2 for fault resistant topological computation. Below we give a briefly introduction of the quantum Hall physics.

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We report the observation of an even-denominator fractional quantum Hall state at {nu}=1/4 in a high quality, wide GaAs quantum well. The sample has a quantum well width of 50 nm and an electron density of n{sub e}=2.55 x 10{sup 11} cm{sup -2}. We have performed transport measurements at T{approx}35 mK in magnetic fields up to 45 T. When the sample is perpendicular to the applied magnetic field, the diagonal resistance displays a kink at {nu}=1/4. Upon tilting the sample to an angle of {theta}=20.3{sup o} a clear fractional quantum Hall state emerges at {nu}=1/4 with a plateau in the Hall resistance and a strong minimum in the diagonal resistance.

We have investigated the physics of Bloch oscillations (BO) of electrons, engineered in high mobility quantum wells patterned into lateral periodic arrays of nanostructures, i.e. two-dimensional (2D) quantum dot superlattices (QDSLs). A BO occurs when an electron moves out of the Brillouin zone (BZ) in response to a DC electric field, passing back into the BZ on the opposite side. This results in quantum oscillations of the electron--i.e., a high frequency AC current in response to a DC voltage. Thus, engineering a BO will yield continuously electrically tunable high-frequency sources (and detectors) for sensor applications, and be a physics tour-de-force. More than a decade ago, Bloch oscillation (BO) was observed in a quantum well superlattice (QWSL) in short-pulse optical experiments. However, its potential as electrically biased high frequency source and detector so far has not been realized. This is partially due to fast damping of BO in QWSLs. In this project, we have investigated the possibility of improving the stability of BO by fabricating lateral superlattices of periodic coupled nanostructures, such as metal grid, quantum (anti)dots arrays, in high quality GaAs/Al{sub x}Ga{sub 1-x}As heterostructures. In these nanostructures, the lateral quantum confinement has been shown theoretically to suppress the optical-phonon scattering, believed to be the main mechanism for fast damping of BO in QWSLs. Over the last three years, we have made great progress toward demonstrating Bloch oscillations in QDSLs. In the first two years of this project, we studied the negative differential conductance and the Bloch radiation induced edge-magnetoplasmon resonance. Recently, in collaboration with Prof. Kono's group at Rice University, we investigated the time-domain THz magneto-spectroscopy measurements in QDSLs and two-dimensional electron systems. A surprising DC electrical field induced THz phase flip was observed. More measurements are planned to investigate this phenomenon. In addition to their potential device applications, periodic arrays of nanostructures have also exhibited interesting quantum phenomena, such as a possible transition from a quantum Hall ferromagnetic state to a quantum Hall spin glass state. It is our belief that this project has generated and will continue to make important impacts in basic science as well as in novel solid-state, high frequency electronic device applications.

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The silicon microelectronics industry is the technological driver of modern society. The whole industry is built upon one major invention--the solid-state transistor. It has become clear that the conventional transistor technology is approaching its limitations. Recent years have seen the advent of magnetoelectronics and spintronics with combined magnetism and solid state electronics via spin-dependent transport process. In these novel devices, both charge and spin degree freedoms can be manipulated by external means. This leads to novel electronic functionalities that will greatly enhance the speed of information processing and memory storage density. The challenge lying ahead is to understand the new device physics, and control magnetic phenomena at nanometer length scales and in reduced dimensions. To meet this goal, we proposed the silicon nanocrystal system, because: (1) It is compatible with existing silicon fabrication technologies; (2) It has shown strong quantum confinement effects, which can modify the electric and optical properties through directly modifying the band structure; and (3) the spin-orbital coupling in silicon is very small, and for isotopic pure {sup 28}Si, the nuclear spin is zero. These will help to reduce the spin-decoherence channels. In the past fiscal year, we have studied the growth mechanism of silicon-nanocrystals embedded in silicon dioxide, their photoluminescence properties, and the Si-nanocrystal's magnetic properties in the presence of Mn-ion doping. Our results may demonstrate the first evidence of possible ferromagnetic orders in Mn-ion implanted silicon nanocrystals, which can lead to ultra-fast information process and ultra-dense magnetic memory applications.

The magnetoresistance, R{sub xx}, at even-denominator fractional fillings, of an ultra high quality two-dimensional electron system at T {approx} 35 mK is observed to be strictly linear in magnetic field, B. While at 35 mK R{sub xx} is dominated by the integer and fractional quantum Hall states, at T {approx_equal} 1.2 K an almost perfect linear relationship between R{sub xx} and B emerges over the whole magnetic field range except for spikes at the integer quantum Hall states. This linear R{sub xx} cannot be understood within the Composite Fermion model, but can be explained through the existence of a density gradient in our sample.

We have investigated the valley splitting of two-dimensional electrons in high-quality Si/Si{sub 1-x}Ge{sub x} heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor {nu} = 3 ({Delta}{sub 3}) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear dependence of {Delta}{sub 3} on the electron density was observed, while the slope of these two configurations differs by more than a factor of 2. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of {Delta}{sub 3} before and after the coincidence.

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We have observed quantization of the diagonal resistance, Rxx, at the edges of several quantum Hall states. Each quantized Rxx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, Rxy. Peaks in Rxx occur at different positions in positive and negative magnetic fields. Practically all Rxx features can be explained quantitatively by a 1%/cm electron density gradient. Therefore, Rxx is determined by Rxy and unrelated to the diagonal resistivity ρxx. Our findings throw an unexpected light on the empirical resistivity rule for 2D systems. © 2005 The American Physical Society.

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The apparent metal-insulator transition is observed in a high-quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si/Si{sub 1-x}Ge{sub x} heterostructure with mobility {mu} = 1.9 x 10{sup 5} cm{sup 2}/V s at density n = 1.45 x 10{sup 11} cm{sup -2}. The critical density, at which the thermal coefficient of low T resistivity changes sign, is -0.32 x 10{sup 11} cm{sup -2}, a very low value obtained in Si-based 2D systems. The in-plane magnetoresistivity {rho}(B{sub ip}) was measured in the density range, 0.35 x 10{sup 11} < n < 1.45 x 10{sup 11} cm{sup -2}, where the 2DES shows the metallic-like behavior. It first increases and then saturates to a finite value {rho}(B{sub c}) for B{sub ip}>B{sub c} , with B{sub c} the full spin-polarization field. Surprisingly, {rho}(B{sub c})/{rho}(0)-1.8 for all the densities, even down to n = 0.35 x 10{sup 11} cm{sup -2}, only 10% higher than n{sub c}. This is different from that in clean Si metal-oxide-semiconductor field-effect transistors, where the enhancement is strongly density dependent and {rho}(B{sub c})/{rho}(0) appears to diverge as n {yields} n{sub c}. Finally, we show that in the fully spin-polarized regime, dependent on the 2DES density, the temperature dependence of {rho}(B{sub ip}) can be either metallic-like or insulating.

Magnetotransport properties are studied in a high-mobility 2DES in the strained Si quantum well. We observe around ν = 1/2 the two-flux composite fermion (CF2) series of the FQHE states at ν = 2/3, 3/5, 4/7, and at ν = 4/9, 2/5, 1/3. Of the CF series, the ν = 3/5 state is weaker than the nearby 4/7 state and the 3/7 state is missing, resembling the observation that the ν = 3 is weaker than the ν = 4 state. Our data indicate that the CF model still applies for the multivalley Si/SiGe system when taking into account the two-fold valley degeneracy. © World Scientific Publishing Company.

The high mobility of two dimensional electron system in the second Landau level was discussed. In the second level, the larger extent of the wave function as compared to the lowest LL and its additional zero allows for a much broader range of electron correlations to be favorable. An example of electron correlations encountered in the second LL is the even-denominator v=2+1/2 fractional quantum hall effect (FQHE) state. With a varying filling factor, it was observed that quantum liquids of different origins compete with several insulating phases leading to an irregular pattern in the transport parameters.

Cyclotron resonance at the microwave frequency is used to measure the band edge mass (m{sub b}) in the two-dimensional hole (2DH) system, confined in 30 nm quantum wells in the Al{sub 0.1}Ga{sub 0.9}As/GaAs/Al{sub 0.1}Ga{sub 0.9}As heterostructures. We find that for 2DH density p {le} 1.0 x 10{sup 10} cm{sup -2}, m{sub b} is nearly constant, {approx}0.35m{sub e}. It increases with increasing density, to {approx}0.5m{sub e} at p = 7.4 x 10{sup 10} cm{sup -2}.

In the second Landau level around {nu} = 5/2 filling of an extremely high quality 2D electron system and at temperatures T down to 9 mK we observe a very strong even-denominator fractional quantum Hall effect at Landau level filling {nu} = 5/2 and its energy gap is large and {Delta} {approx} 0.45 K. A clear FQHE state is seen at {nu} = 2+2/5, with well-quantized R{sub xy}. A novel, even denominator FQHE state at {nu} = 2+3/8 seems to develop, as deduced from the T-dependence of dR{sub xy}/dB. In addition, four fully developed re-entrant integral quantum Hall effect (RIQHE) states are also observed. At low temperatures, the wide RIQHE plateau around at {nu} = 2+2/7 is interrupted by a dip, indicating an additional reentrance. Finally, the tilted magnetic field experiment at an ultra-low temperature of 10 mK was carried out to examine the spin-polarization of the {nu} = 5/2 FQHE state.