arXiv subjects

Qian Wang

Publications and source records attributed to Qian Wang.

At least 631 records · Page 35Linked to original sources

arXiv2006

Subtle Effects of a nonmagnetic impurity on the sub-gap spectrum of a d-wave superconductor in the Fulde-Ferrell-Larkin-Ovchinnikov state

The effect of a nonmagnetic impurity on the subgap electronic spectrum in a d-wave superconductor in the Fulde-Ferrell-Larkin-Ovchinnikov state is investigated. In a pure system, the spatial variation of the order parameter (OP) of the FFLO state forms a two-dimensional lattice, and there exist two types of sub-gap quasi-particle states. Our numerical iterations indicate that an OP saddle point, and slightly less so energetically, an OP extremum favor to be pinned at the impurity site. The local density of states and the fixed-bias map reveal the subtle effects of an impurity on the subgap energy spectrum, because the OP changes sign in both the (relative) momentum space and the real space.

cond-mat.supr-con↗

arXiv2006

Local tunneling spectroscopy as signatures of the Fulde-Ferrell-Larkin-Ovchinnikov state in s- and d-wave Superconductors

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SC) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter (OP) is found to have lower free energy in s-SC, a square lattice solution appears to be energetically more favorable in the case of d-SC. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SC exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

cond-mat.supr-con↗

arXiv2006

Thermally and Molecularly Stimulated Relaxation of Hot Phonons in Suspended Carbon Nanotubes

The high-bias electrical transport properties of suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at various temperatures in vacuum, in various gases and when coated with molecular solids. It is revealed that non-equilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules surrounding suspended SWNTs assist the relaxation of hot phonons and afford enhanced current flow along nanotubes. Molecular solids of carbon dioxide frozen onto suspended SWNTs quench the non-equilibrium phonon effect. The discovery of strong environmental effects on high current transport in nanotubes is important to high performance nanoelectronics applications of 1D nanowires in general.

cond-mat.mes-hall↗

arXiv2005

Thermal Conductance of an Individual Single-Wall Carbon Nanotube above Room Temperature

The thermal properties of a suspended metallic single-wall carbon nanotube (SWNT) are extracted from its high-bias (I-V) electrical characteristics over the 300-800 K temperature range, achieved by Joule self-heating. The thermal conductance is approximately 2.4 nW/K and the thermal conductivity is nearly 3500 W/m/K at room temperature for a SWNT of length 2.6 um and diameter 1.7 nm. A subtle decrease in thermal conductivity steeper than 1/T is observed at the upper end of the temperature range, which is attributed to second order three-phonon scattering between two acoustic modes and one optical mode. We discuss sources of uncertainty and propose a simple analytical model for the SWNT thermal conductivity including length and temperature dependence.

cond-mat.mes-hall↗

arXiv2005

Ultra-High Yield Growth of Vertical Single-Walled Carbon Nanotubes: Hidden Roles of Hydrogen and Oxygen

An oxygen assisted hydrocarbon chemical vapor deposition (CVD) method is developed to afford large-scale highly reproducible ultra high-yield growth of vertical single-walled carbon nanotubes (SWNT). It is revealed that reactive hydrogen (H)-species, inevitable in hydrocarbon-based growth, are damaging to the formation of sp2-like SWNTs. The addition of oxygen scavenges H-species and provides a powerful control over the C/H ratio to favor SWNT growth. The revelation of the roles played by hydrogen and oxygen leads to a unified and universal optimum growth condition for SWNTs. Further, a versatile method is developed to form vertical SWNT films on any substrate, lifting a major substrate-type limitation for aligned SWNTs.

cond-mat.mtrl-sci↗

arXiv2005

Electron Transport in Very Clean, As-Grown Suspended Carbon Nanotubes

Single walled carbon nanotubes (SWNT) have displayed a wealth of quantum transport phenomena thus far. Defect free, unperturbed SWNTs with wellbehaved or tunable metal contacts are important to probing the intrinsic electrical properties of nanotubes. Meeting these conditions experimentally is nontrivial due to numerous disorder and randomizing factors. Here we show that ~ 1 um long fully suspended SWNTs grown-in-place between metal contacts afford SWNT devices exhibiting well-defined characteristics over much wider energy ranges than nanotubes pinned on substrates. Various low temperature transport regimes in true-metallic, small and large bandgap semiconducting nanotubes are observed including quantum states shell-filling, -splitting and -crossing in magnetic fields for medium conductance devices. The clean transport data reveals a correlation between the contact junction resistance and the various transport regimes in SWNT devices. Further, we show that electrical transport data can be used to probe the band structures of nanotubes including nonlinear band dispersion.

cond-mat.mes-hall↗

arXiv2005

Electrical Contacts to Carbon Nanotubes Down to 1nm in Diameter

Rhodium (Rh) is found similar to Palladium (Pd) in making near-ohmic electrical contacts to single-walled carbon nanotubes (SWNTs) with diameters d > ~ 1.6 nm. Non-negligible positive Schottky barriers (SBs) exist between Rh or Pd and semiconducting SWNTs (S-SWNTs) with d < ~ 1.6 nm. With Rh and Pd contacts, the characteristics of SWNT field-effect transistors (FETs) and SB heights at the contacts are largely predictable based on the SWNT diameters, without random variations among devices. Surprisingly, electrical contacts to metallic SWNTs (M-SWNTs) also appear to be diameter dependent especially for small SWNTs. Ohmic contacts are difficult for M-SWNTs with diameters < ~ 1.0 nm possibly due to tunnel barriers.

cond-mat.mes-hall↗

arXiv2005

Negative Differential Conductance and Hot Phonons in Suspended Nanotube Molecular Wires

Freely suspended metallic single-wall carbon nanotubes (SWNTs) exhibit reduced current carrying ability compared to those lying on substrates, and striking negative differential conductance (NDC) at low electric fields. Theoretical analysis reveals significant self-heating effects including electron scattering by hot non-equilibrium optical phonons. Electron transport characteristics under strong self-heating are exploited for the first time to probe the thermal conductivity of individual SWNTs (~ 3600 Wm-1K-1 at T=300 K) up to ~700 K, and reveal a 1/T dependence expected for Umklapp phonon scattering at high temperatures.

cond-mat.mes-hall↗

arXiv2005

Temperature and Gas-Environment Dependent Electron and Phonon Transport in Suspended Carbon Nanotubes Up to Electrical Breakdown

High bias electrical transport characteristics of freely suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at 250-400K in vacuum and various gases. Self-heating is exploited to examine the temperature dependence of phonon transport and optical phonon decay in SWNTs. The acoustic phonon thermal conductivity of a SWNT follows ~1/T at high temperatures. Non-equilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules assist the relaxation of hot optical phonons along the tube length and enable enhanced current flow. In vacuum, high bias breakdown of suspended SWNTs can occur via melting caused by electrically emitted hot optical phonons at a low acoustic phonon temperature.

cond-mat.mes-hall↗

arXiv2004

Suspended Carbon Nanotube Quantum Wires with Two Gates

Suspended single-walled carbon nanotube devices comprised of high quality electrical contacts and two electrostatic gates per device are obtained. Compared to nanotubes pinned on substrates, the suspended devices exhibit little hysteresis related to environmental factors and manifest as cleaner Fabry-Perot interferometers or single electron transistors. The high-field saturation currents in suspended nanotubes related to optical phonon or zone boundary phonon scattering are significantly lower due to the lack of efficient heat sinking. The multiple gate design may also facilitate future investigation of electromechanical properties of nanotube quantum systems.

cond-mat.mes-hall↗

arXiv2004

The Aharonov-Bohm Interference and Beating in Single-Walled Carbon Nanotube Interferometers

Relatively low magnetic fields applied parallel to the axis of a chiral single-walled carbon nanotube are found causing large modulations to the p-channel or valence band conductance of the nanotube in the Fabry-Perot interference regime. Beating in the Aharonov-Bohm type of interference between two field-induced non-degenerate sub-bands of spiraling electrons is responsible for the observed modulation with a pseudo period much smaller than that needed to reach the flux quantum F0 = h/e through the nanotube cross-section. We show that single-walled nanotubes represent the smallest cylinders exhibiting the Aharonov-Bohm effect with rich interference and beating phenomena arising from well-defined molecular orbitals reflective of the nanotube chirality.

cond-mat.mes-hall↗

arXiv2004

10 to 50 nm Long Quasi Ballistic Carbon Nanotube Devices Obtained Without Complex Lithography

A simple method combining photolithography and shadow (or angle) evaporation is developed to fabricate single-walled carbon nanotube (SWCNT) devices with tube lengths L~10-50 nm between metal contacts. Large numbers of such short devices are obtained without the need of complex tools such as electron beam lithography. Metallic SWCNTs with lengths ~ 10 nm, near the mean free path (mfp) of lop~15 nm for optical phonon scattering, exhibit near-ballistic transport at high biases and can carry unprecedented 100 mA currents per tube. Semiconducting SWCNT field-effect transistors (FETs) with ~ 50 nm channel lengths are routinely produced to achieve quasi-ballistic operations for molecular transistors. The results demonstrate highly length-scaled and high-performance interconnects and transistors realized with SWCNTs.

cond-mat.mtrl-sci↗

arXiv2004

Miniature Organic Transistors With Carbon Nanotubes as Quasi-One Dimensional Electrodes

As the dimensions of electronic devices approach those of molecules, the size, geometry and chemical composition of the contact electrodes play increasingly dominant roles in device functions. It is shown here that single-walled carbon nanotubes (SWNT) can be used as quasi one-dimensional (1D) electrodes to construct organic field effect transistors (FET) with molecular scale width (~2 nm) and channel length (1-3 nm). An important feature owing to the quasi 1D electrode geometry is the favorable gate electrostatics that allows for efficient switching of ultra-short organic channels. This affords room temperature conductance modulation by orders of magnitude for organic transistors that are only several-molecules in length, with switching characteristics superior to similar devices with lithographically patterned metal electrodes. With nanotubes, covalent carbon-carbon bonds could be utilized to form contacts to molecular materials. The unique geometrical, physical and chemical properties of carbon nanotube electrodes may lead to various interesting molecular devices.

cond-mat.mtrl-sci↗

arXiv2004

Surface Chemistry and Electrical Properties of Germanium Nanowires

Germanium nanowires with p- and n-dopants were synthesized by chemical vapor deposition and used to construct complementary field effect transistors . Electrical transport and x-ray photoelectron spectroscopy data are correlated to glean the effects of Ge surface chemistry to the electrical characteristics of GeNWs. Large hysteresis due to water molecules strongly bound to GeO2 on GeNWs is revealed. Different oxidation behavior and hysteresis characteristics and opposite band bending due to Fermi level pinning by interface states between Ge and surface oxides are observed for p- and n-type GeNWs. Vacuum annealing above 400C is used to remove surface oxides and eliminate hysteresis in GeNW FETs. High-k dielectric HfO2 films grown on clean GeNW surfaces by atomic layer deposition (ALD) using an alkylamide precursor is effective serving as the first layer of surface passivation. Lastly, the depletion length along the radial direction of nanowires is evaluated. The result suggests that surface effects could be dominant over the bulk properties of small diameter wires.

cond-mat.mtrl-sci↗

arXiv2004

Self-Aligned Ballistic Molecular Transistors and Electrically Parallel Nanotube Arrays

Carbon nanotube field-effect transistors with structures and properties near the scaling limit with short (down to 50 nm) channels, self aligned geometries, palladium electrodes with low contact resistance and high-k dielectric gate insulators are realized. Electrical transport in these miniature transistors is near ballistic up to high biases at both room and low temperatures. Atomic layer deposited (ALD) high-k films interact with nanotube sidewalls via van der Waals interactions without causing weak localization at 4 K. New fundamental understanding of ballistic transport, optical phonon scattering and potential interfacial scattering mechanisms in nanotubes are obtained.

cond-mat.other↗

arXiv2004

High-Field, Quasi-Ballistic Transport in Short Carbon Nanotubes

Single walled carbon nanotubes with Pd ohmic contacts and lengths ranging from several microns down to 10 nm are investigated by electron transport experiments and theory. The mean free path (mfp) for acoustic phonon scattering is estimated to be lap~300 nm, and that for optical phonon scattering is lop~15 nm. Transport through very short (~10 nm) nanotubes is free of significant acoustic and optical phonon scattering and thus ballistic and quasi-ballistic at the low and high bias voltage limits respectively. High currents of up to 70 uA can flow through a short nanotube. Possible mechanisms for the eventual electrical breakdown of short nanotubes at high fields are discussed. The results presented here have important implications to high performance nanotube transistors and interconnects.

cond-mat.mes-hall↗

arXiv2004

Advancements in Complementary Carbon Nanotube Field-Effect Transistors

High performance p- and n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) are obtained by using high and low work function metals, Pd and Al as source/drain (S/D) electrodes respectively. Ohmic contacts made to chemically intrinsic SWNTs, with no or small Schottky barriers (SB), afford high ON-state currents up to 20 uA per tube. The lack of significant Fermi-level pinning at the nanotube-metal interfaces allows for fine-tuning of the barrier heights for p-and n-channel conductions by changing the contact metals. The air-stable p- and n-FETs thus obtained can be used for complementary nanoelectronics, as demonstrated with the fabrication of an inverter. Other important issues regarding nanotube FETs including hysteresis, OFF-state leak currents, choice of nanotube diameter, and threshold voltage control are discussed.

cond-mat.mes-hall↗

arXiv2003

Carbon Nanotube Field-Effect Transistors With Integrated Ohmic Contacts and High-k Gate Dielectrics

High performance enhancement mode semiconducting carbon nanotube field-effect transistors (CNTFETs) are obtained by combining ohmic metal-tube contacts, high dielectric constant HfO2 films as gate insulators, and electrostatically doped nanotube segments as source/drain electrodes. The combination of these elements affords high ON currents, subthreshold swings of ~ 70-80 mV/decade, and allows for low OFF currents and suppressed ambipolar conduction. The doped source and drain approach resembles that of MOSFETs and can impart excellent OFF states to nanotube FETs under aggressive vertical scaling. This presents an important advantage over devices with metal source/drain, or devices commonly referred to as Schottky barrier FETs.

cond-mat.mes-hall↗