arXiv subjects

Michael S. Fuhrer

Publications and source records attributed to Michael S. Fuhrer.

At least 91 records · Page 5Linked to original sources

arXiv2015

Observation of topological transition of Fermi surface from a spindle-torus to a torus in large bulk Rashba spin-split BiTeCl

The recently observed large Rashba-type spin splitting in the BiTeX (X = I, Br, Cl) bulk states due to the absence of inversion asymmetry and large charge polarity enables observation of the transition in Fermi surface topology from spindle-torus to torus with varying the carrier density. These BiTeX systems with high spin-orbit energy scales offer an ideal platform for achieving practical spintronic applications and realizing non-trivial phenomena such as topological superconductivity and Majorana fermions. Here we use Shubnikov-de Haas oscillations to investigate the electronic structure of the bulk conduction band of BiTeCl single crystals with different carrier densities. We observe the topological transition of the Fermi surface (FS) from a spindle-torus to a torus. The Landau level fan diagram reveals the expected non-trivial π Berry phase for both the inner and outer FSs. Angle-dependent oscillation measurements reveal three-dimensional FS topology when the Fermi level lies in the vicinity of the Dirac point. All the observations are consistent with large Rashba spin-orbit splitting in the bulk conduction band.

cond-mat.mes-hall↗

arXiv2014

Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene

Terahertz (THz) radiation has uses from security to medicine; however, sensitive room-temperature detection of THz is notoriously difficult. The hot-electron photothermoelectric effect in graphene is a promising detection mechanism: photoexcited carriers rapidly thermalize due to strong electron-electron interactions, but lose energy to the lattice more slowly. The electron temperature gradient drives electron diffusion, and asymmetry due to local gating or dissimilar contact metals produces a net current via the thermoelectric effect. Here we demonstrate a graphene thermoelectric THz photodetector with sensitivity exceeding 10 V/W (700 V/W) at room temperature and noise equivalent power less than 1100 pW/Hz^1/2 (20 pW/Hz^1/2), referenced to the incident (absorbed) power. This implies a performance which is competitive with the best room-temperature THz detectors for an optimally coupled device, while time-resolved measurements indicate that our graphene detector is eight to nine orders of magnitude faster than those. A simple model of the response, including contact asymmetries (resistance, work function and Fermi-energy pinning) reproduces the qualitative features of the data, and indicates that orders-of-magnitude sensitivity improvements are possible.

cond-mat.mes-hall↗

arXiv2014

Tuning bulk and surface conduction in topological Kondo insulator SmB6

Bulk and surface state contributions to the electrical resistance of single-crystal samples of the topological Kondo insulator compound SmB6 are investigated as a function of crystal thickness and surface charge density, the latter tuned by ionic liquid gating with electrodes patterned in a Corbino disk geometry on a single surface. By separately tuning bulk and surface conduction channels, we show conclusive evidence for a model with an insulating bulk and metallic surface states, with a crossover temperature that depends solely on the relative contributions of each conduction channel. The surface conductance, on the order of 100 e^2/h and electron-like, exhibits a field-effect mobility of 133 cm^2/V/s and a large carrier density of ~2x10^{14}/cm^2, in good agreement with recent photoemission results. With the ability to gate-modulate surface conduction by more than 25%, this approach provides promise for both fundamental and applied studies of gate-tuned devices structured on bulk crystal samples.

cond-mat.str-el↗

arXiv2014

Approaching the Limits of Transparency and Conductivity in Graphitic Materials through Lithium Intercalation

Various bandstructure engineering methods have been studied to improve the performance of graphitic transparent conductors; however none demonstrated an increase of optical transmittance in the visible range. Here we measure in situ optical transmittance spectra and electrical transport properties of ultrathin-graphite (3-60 graphene layers) simultaneously via electrochemical lithiation/delithiation. Upon intercalation we observe an increase of both optical transmittance (up to twofold) and electrical conductivity (up to two orders of magnitude), strikingly different from other materials. Transmission as high as 91.7% with a sheet resistance of 3.0 Ω per square is achieved for 19-layer LiC6, which corresponds to a figure of merit σ_dc/σ_opt = 1400, significantly higher than any other continuous transparent electrodes. The unconventional modification of ultrathin-graphite optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques enable the investigation of other aspects of intercalation in nanostructures.

cond-mat.mtrl-sci↗

arXiv2014

Thickness and growth-condition dependence of \emph{in-situ} mobility and carrier density of epitaxial thin-film Bi$_2$Se$_3$

Bismuth selenide Bi$_2$Se$_3$ was grown by molecular beam epitaxy while carrier density and mobility were measured directly \emph{in situ} as a function of film thickness. Carrier density shows high interface n-doping (1.5 x 10$^{13}$ cm$^{-2}$) at the onset of film conduction, and bulk dopant density of $\sim$5 x 10$^{18}$ cm$^{-3}$, roughly independent of growth temperature profile. Mobility depends more strongly on the growth temperature and is related to the crystalline quality of the samples quantified by \emph{ex-situ} AFM measurements. These results indicate that Bi$_2$Se$_3$ as prepared by widely employed parameters is \emph{n}-doped before exposure to atmosphere, the doping is largely interfacial in origin, and dopants are not the limiting disorder in present Bi$_2$Se$_3$ films.

cond-mat.mtrl-sci↗

arXiv2014

Disorder induced magnetoresistance in a two dimensional electron system

We predict and demonstrate that a disorder induced carrier density inhomogeneity causes magnetoresistance (MR) in a two-dimensional electron system. Our experiments on graphene show a quadratic MR persisting far from the charge neutrality point. Effective medium calculations show that for charged impurity disorder, the low-field MR is a universal function of the ratio of carrier density to fluctuations in carrier density, a power-law when this ratio is large, in excellent agreement with experiment. The MR is generic and should occur in other materials with large carrier density inhomogeneity.

cond-mat.mes-hall↗

arXiv2013

Carbon Impurities on Graphene Synthesized by Chemical Vapor Deposition on Platinum

We report nanocrystalline carbon impurities coexisting with graphene synthesized via chemical vapor deposition (CVD) on platinum. We observe micron-size island-like impurity layers, which can easily be mistaken for second graphene layers in optical microscopy or secondary electron microscopy (SEM). The island orientation depends on the crystalline orientation of the Pt, as shown by electron backscatter diffraction(EBSD), indicating growth of amorphous carbon below graphene. Dark-field TEM indicates that in addition to uniform single-crystal graphene, our sample is decorated with nanocrystalline carbon impurities with a spatially inhomogeneous distribution. Raman spectra show a large D peak, however electrical characterization shows high mobility (~6,000 cm2/Vs), indicating a limitation for Raman spectroscopy in characterizing the electronic quality of graphene.

cond-mat.mes-hall↗

arXiv2013

Measurement of mobility in dual-gated MoS2 transistors

Atomically thin semiconducting MoS2 is of great interest for high-performance flexible electronic and optoelectronic devices. Initial measurements using back-gated field-effect transistor (FET) structures on SiO2 yielded mobility of 1-50 cm^2/Vs for few-layer MoS2[1, 2]. Initial measurements using back-gated field-effect transistor (FET) structures on SiO2 yielded mobility of 1-50 cm^2/Vs for few-layer MoS2[1, 2]. However, greatly increased mobility - as high as 900 cm^2/Vs - was recently reported for monolayer MoS2 by Radisavljevic, et al.[Ref. 3; see also Refs. 4-6], and for multilayer MoS2 by others[7, 8], in devices covered by a high-kappa dielectric layer and a metal top gate. Similar increases were reported for electrolyte top-gated MoS2[9]. A puzzling common aspect of these reports is that the double-gated devices consistently show much higher mobility when measured using the weakly-coupled back gate (typically 300 nm of SiO2) than using the strongly-coupled top gate. While this difference might be attributed to a different quality of the top and bottom surfaces, the large discrepancy motivates close examination of the measurement techniques to rule out spurious effects. Notably, it has been shown that the mobility may be greatly overestimated in this geometry[10, 11]. There is good evidence that this is the case here[4-9]; below we discuss Radisavljevic, et al.[4] as an illustrative example.

cond-mat.mes-hall↗

arXiv2012

High Mobility Ambipolar MoS2 Field-Effect Transistors: Substrate and Dielectric Effects

We fabricate MoS2 field effect transistors on both SiO2 and polymethyl methacrylate (PMMA) dielectrics and measure charge carrier mobility in a four-probe configuration. For multilayer MoS2 on SiO2, the mobility is 30-60 cm2/Vs, relatively independent of thickness (15-90 nm), and most devices exhibit unipolar n-type behavior. In contrast, multilayer MoS2 on PMMA shows mobility increasing with thickness, up to 470 cm2/Vs (electrons) and 480 cm2/Vs (holes) at thickness ~50 nm. The dependence of the mobility on thickness points to a long-range dielectric effect of the bulk MoS2 in increasing mobility.

cond-mat.mes-hall↗

arXiv2012

Coherent Topological Transport on the Surface of Bi2Se3

The two-dimensional (2D) surface state of the three-dimensional strong topological insulator (STI) is fundamentally distinct from other 2D electron systems in that the Fermi arc encircles an odd number of Dirac points. The TI surface is in the symplectic universality class and uniquely among 2D systems remains metallic and cannot be localized by (time-reversal symmetric) disorder. However, in finite-size samples inter-surface coupling can destroy the topological protection. The question arises: At what size can a thin TI sample be treated as having decoupled topological surface states? We show that weak anti-localization(WAL) is extraordinarily sensitive to sub-meV coupling between top and bottom topological surfaces, and the surfaces of a TI film may be coherently coupled even for thicknesses as large as 12 nm. For thicker films we observe the signature of a true 2D topological metal: perfect weak anti-localization in quantitative agreement with two decoupled surfaces in the symplectic symmetry class.

cond-mat.mes-hall↗

arXiv2012

Princess and the Pea at the nanoscale: Wrinkling and delamination of graphene on nanoparticles

Thin membranes exhibit complex responses to external forces or geometrical constraints. A familiar example is the wrinkling, exhibited by human skin, plant leaves, and fabrics, resulting from the relative ease of bending versus stretching. Here, we study the wrinkling of graphene, the thinnest and stiffest known membrane, deposited on a silica substrate decorated with silica nanoparticles. At small nanoparticle density monolayer graphene adheres to the substrate, detached only in small regions around the nanoparticles. With increasing nanoparticle density, we observe the formation of wrinkles which connect nanoparticles. Above a critical nanoparticle density, the wrinkles form a percolating network through the sample. As the graphene membrane is made thicker, global delamination from the substrate is observed. The observations can be well understood within a continuum elastic model and have important implications for strain-engineering the electronic properties of graphene.

cond-mat.mes-hall↗

arXiv2012

Intrinsic Electron-Phonon Resistivity in Bi2Se3 in the Topological Regime

We measure the temperature-dependent carrier density and resistivity of the topological surface state of thin exfoliated Bi2Se3 in the absence of bulk conduction. When the gate-tuned chemical potential is near or below the Dirac point the carrier density is strongly temperature dependent reflecting thermal activation from the nearby bulk valence band, while above the Dirac point, unipolar n-type surface conduction is observed with negligible thermal activation of bulk carriers. In this regime linear resistivity vs. temperature reflects intrinsic electron-acoustic phonon scattering. Quantitative comparison with a theoretical transport calculation including both phonon and disorder effects gives the ratio of deformation potential to Fermi velocity D/\hbarvF = 4.7 Å-1. This strong phonon scattering in the Bi2Se3 surface state gives intrinsic limits for the conductivity and charge carrier mobility at room temperature of ~550 μS per surface and ~10,000 cm2/Vs.

cond-mat.mtrl-sci↗

arXiv2012

Determination of the Optical Index for Few-Layer Graphene by Reflectivity Spectroscopy

We have experimentally studied the optical refractive index of few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a coherent supercontinuum laser source measured the reflectivity of an exfoliated graphene flake on a Si/SiO2 substrate, containing monolayer, bilayer and trilayer areas, as the wavelength of the laser was varied from 545nm to 710nm. The complex refractive index of few-layer graphene, n-ik, was extracted from the reflectivity contrast to the bare substrate and the Fresnel reflection theory. Since the SiO2 thickness enters to the modeling as a parameter, it was precisely measured at the location of the sample. It was found that a common constant optical index cannot explain the wavelength-dependent reflectivity data for single-, double- and three-layer graphene simultaneously, but rather each individual few-layer graphene possesses a unique optical index whose complex values were precisely and accurately determined from the experimental data.

cond-mat.mes-hall↗

arXiv2012

Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3

The newly-discovered three-dimensional strong topological insulators (STIs) exhibit topologically-protected Dirac surface states. While the STI surface state has been studied spectroscopically by e.g. photoemission and scanned probes, transport experiments have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (<10 nm), low-doped Bi2Se3 (\approx10^17/cm3) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with linear Hall resistivity and well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e^2/h per surface) and the residual (puddle) carrier density (0.4 x 10^12 cm^-2 to 4 x 10^12 cm^-2). From the measured carrier mobilities 320 cm^2/Vs to 1,500 cm^2/Vs, the charged impurity densities 0.5 x 10^13 cm^-2 to 2.3 x 10^13 cm^-2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1 x 10^13 cm^-2 to 3 x 10^13 cm^-2), identifying dopants as the charged impurities.

cond-mat.mes-hall↗

arXiv2012

Insulating behavior in ultra-thin bismuth selenide field effect transistors

Ultrathin (~3 quintuple layer) field-effect transistors (FETs) of topological insulator Bi2Se3 are prepared by mechanical exfoliation on 300nm SiO2/Si susbtrates. Temperature- and gate-voltage dependent conductance measurements show that ultrathin Bi2Se3 FETs are n-type, and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.

cond-mat.mes-hall↗

arXiv2012

Topological insulator quantum dot with tunable barriers

Thin (6-7 quintuple layer) topological insulator Bi2Se3 quantum dot devices are demonstrated using ultrathin (2~4 quintuple layer) Bi2Se3 regions to realize semiconducting barriers which may be tuned from Ohmic to tunneling conduction via gate voltage. Transport spectroscopy shows Coulomb blockade with large charging energy >5 meV, with additional features implying excited states.

cond-mat.mes-hall↗

arXiv2011

Correlated charged impurity scattering in graphene

Understanding disorder in graphene is essential for electronic applications; in contrast to conventional materials, the extraordinarily low electron-phonon scattering1, 2 in graphene implies that disorder3-7 dominates its resistivity even at room temperature. Charged impurities5, 8-10 have been identified as an important disorder type in graphene on SiO2 substrates11, 12, giving a nearly linear carrier-density-dependent conductivity σ(n), and producing electron and hole puddles13-15 which determine the magnitude of graphene's minimum conductivity σmin10. Correlations of charged impurities are known to be essential in achieving the highest mobilities in remotely-doped semiconductor heterostructures16-18, and are present to some degree in any impurity system at finite temperature. Here we show that even modest correlations in the position of charged impurities, realized by annealing potassium on graphene, can increase the mobility by more than a factor of four. The results are well understood theoretically19 considering an impurity correlation length which is temperature dependent but independent of impurity density. Impurity correlations also naturally explain the sub-linear σ(n) commonly observed in substrate-bound graphene devices2, 11, 12, 20.

cond-mat.mes-hall↗

arXiv2011

Raman and optical characterization of multilayer turbostratic graphene grown via chemical vapor deposition

We synthesize large-area graphene via atmospheric-pressure (AP) chemical vapor deposition (CVD) on copper, and transfer to SiO2 wafers. In contrast to low-pressure (LP) CVD on copper, optical contrast and atomic force microscopy measurements show AP-CVD graphene contains significant multi-layer areas. Raman spectroscopy always shows a single Lorentzian 2D peak, however systematic differences are observed in the 2D peak energy, width, and intensity for single- and multi-layer regions. We conclude that graphene multi-layers grown by AP-CVD on Cu are rotationally disordered.

cond-mat.mtrl-sci↗