arXiv subjects

Ting Yu

Publications and source records attributed to Ting Yu.

At least 163 records · Page 9Linked to original sources

arXiv2011

Wafer-scale graphene/ferroelectric hybrid devices for low-voltage electronics

Preparing graphene and its derivatives on functional substrates may open enormous opportunities for exploring the intrinsic electronic properties and new functionalities of graphene. However, efforts in replacing SiO$_{2}$ have been greatly hampered by a very low sample yield of the exfoliation and related transferring methods. Here, we report a new route in exploring new graphene physics and functionalities by transferring large-scale chemical vapor deposition single-layer and bilayer graphene to functional substrates. Using ferroelectric Pb(Zr$_{0.3}$Ti$_{0.7}$)O$_{3}$ (PZT), we demonstrate ultra-low voltage operation of graphene field effect transistors within $\pm1$ V with maximum doping exceeding $10^{13}\,\mathrm{cm^{-2}}$ and on-off ratios larger than 10 times. After polarizing PZT, switching of graphene field effect transistors are characterized by pronounced resistance hysteresis, suitable for ultra-fast non-volatile electronics.

cond-mat.mes-hall↗

arXiv2010

Electronic Structure and Structural Evolutions of Hydrogenated Graphene Probed by Raman Spectroscopy

The electronic structure and structural evolution of hydrogenated graphene are investigated by Raman spectroscopy with multiple excitations. The excitation energy dependent saturation effect on the ratio of integrated intensities of D and G modes (ID/IG) is revealed and further developed as a quick method for estimation of inter-defect distance and defect density in hydrogenated graphene. At low hydrogen coverage, the chemisorbed H atoms behave like defects in sp2 C=C matrix; while for a high hydrogen coverage, the sp3 C-H bonds become coalescent clusters, resulting in confinement effect on the sp2 C domains. Electronic structure changes caused by varying hydrogen coverage are evidenced by excitation energy dependent red shift of D and 2D bands. Our results provide a useful guide for developing applications of hydrogenated graphene, as well as using Raman spectroscopy as quick metrology of the defect density in further exploring other kinds of graphene derivatives.

cond-mat.mtrl-sci↗

arXiv2010

Large Scale Synthesis of Bi-layer Graphene in Strongly Coupled Stacking Order

Large scale synthesis of single layer graphene (SLG) by chemical vapor deposition (CVD) has received a lot of attention recently. However, CVD synthesis of AB stacked bi-layer graphene (BLG) is still a challenging work. Here we report synthesis of BLG homogeneously in large area by thermal CVD. The 2D Raman band of CVD BLG splits into four components, suggesting splitting of electronic bands due to strong interlayer coupling. The splitting of electronic bands in CVD BLG is further evidenced by the study of near infrared (NIR) absorption and carrier dynamics probed by transient absorption spectroscopy. Ultraviolet photoelectron spectroscopy invesigation also indiates CVD BLG possesses different electronic structures from those of CVD SLG. The growth mechanism of BLG is found to be related to catalystic activity of copper (Cu)surface, which is determined by purity of Cu foils employed in CVD process. Our work showsthat strongly coupled or even AB stacked BLG can be grown on Cu foils in large scale, which isof particular importance for device applications based on their split electronic bands

cond-mat.mtrl-sci↗

arXiv2010

Non-Markovian Relaxation of a Three-Level System: Quantum Trajectory Approach

The non-Markovian dynamics of a three-level quantum system coupled to a bosonic environment is a difficult problem due to the lack of an exact dynamic equation such as a master equation. We present for the first time an exact quantum trajectory approach to a dissipative three-level model. We have established a convolutionless stochastic Schrödinger equation called time-local quantum state diffusion (QSD) equation without any approximations, in particular, without Markov approximation. Our exact time-local QSD equation opens a new avenue for exploring quantum dynamics for a higher dimensional quantum system coupled to a non-Markovian environment.

quant-ph↗

arXiv2010

Gold on graphene as a substrate for surface enhanced Raman scattering study

In this paper, we report our study on gold (Au) films with different thicknesses deposited on single layer graphene (SLG) as surface enhanced Raman scattering (SERS) substrates for the characterization of rhodamine (R6G) molecules. We find that an Au film with a thickness of ~7 nm deposited on SLG is an ideal substrate for SERS, giving the strongest Raman signals for the molecules and the weakest photoluminescence (PL) background. While Au films effectively enhance both the Raman and PL signals of molecules, SLG effectively quenches the PL signals from the Au film and molecules. The former is due to the electromagnetic mechanism involved while the latter is due to the strong resonance energy transfer from Au to SLG. Hence, the combination of Au films and SLG can be widely used in the characterization of low concentration molecules with relatively weak Raman signals.

cond-mat.mtrl-sci↗

arXiv2010

Second-order Overtone and Combinational Raman Modes of Graphene Layers in the Range of 1690 cm-1 to 2150 cm-1

Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of second-order overtone and combinational Raman modes in an unexplored range of 1690-2150 cm-1 in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. Based on the double resonance theory, four dominant modes in this range have been assigned as 2oTO (M band), iTA+LO, iTO+LA and LO+LA. Differing to AB-stacked bilayer graphene or few layer graphene, the M band disappears in single layer graphene. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of M band whereas the substrate has no effect on this. Dispersive behaviors of these "new" Raman modes in graphene have been probed by the excitation energy dependent Raman spectroscopy. It is found that the appearance of the M band strictly relies on the AB stacking, which could be a fingerprint of AB-stacked bilayer graphene. This work expands the unique and powerful abilities of Raman spectroscopy on study of graphene and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit electronic band structure of graphene layers.

cond-mat.mtrl-sci↗

arXiv2010

Ultrafast carrier dynamics in pristine and FeCl3-intercalated bilayer graphene

Ultrafast carrier dynamics of pristine bilayer graphene (BLG) and bilayer graphene intercalated with FeCl3 (FeCl3-G), were studied using time-resolved transient differential reflection (delta R/R). Compared to BLG, the FeCl3-G data showed an opposite sign of delta R/R, a slower rise time, and a single (instead of double) exponential relaxation. We attribute these differences in dynamics to the down-shifting of the Fermi level in FeCl3-G, as well as the formation of numerous horizontal bands arising from the d-orbitals of Fe. Our work shows that intercalation can dramatically change the electronic structure of graphene, and its associated carrier dynamics.

cond-mat.mes-hall↗

arXiv2010

Qubit Entanglement Dephasing Dynamics Driven by a Bath of Spins

We study the entanglement dynamics for a two-spin system coupled to a spin environment of different configurations by z-x type interaction. The models considered in this paper are solved both analytically and numerically giving rise to some concise analytical expressions when certain approximations are properly made. Our purpose is to find how the initial states of the environment with different numbers of spins affect the decay or revival of the entanglement between central qubits. In Particular, it is found that the block-entangled environment could speed up the decay and revival of the qubit entanglement. Our results exhibit some interesting features that have not been found for a boson bath.

quant-ph↗

arXiv2010

Raman study on G mode of graphene for determination of edge orientation

We report a confocal Raman study on edges of single layer graphene. It is found that edge orientations could be identified by G mode besides D mode. We observe that G mode at edges of single layer graphene exhibits polar behaviors and different edges like zigzag- or armchair-dominated responses differently to the polarization of the incident laser. Moreover, G mode shows stiffening at zigzag-dominated edges, while it is softened at armchair- dominated ones. Our observations are in good agreement with recent theory (K. Sasaki, et al., J. Phys. Soc. Jpn. 79, 044603) and could be well explained by the unique properties of pseudospin at graphene edges, which lead to asymmetry of Raman active modes and non-adiabatic processes (Kohn Anomaly) at different types of edges. This work could be useful for further study on the properties of graphene edge and development of graphene-based devices.

cond-mat.mtrl-sci↗

arXiv2010

FeCl3 based Few-Layer Graphene Intercalation Compounds: Single Linear Dispersion Electronic Band Structure and Strong Charge Transfer Doping

Graphene has attracted great attentions since its first discovery in 2004. Various approaches have been proposed to control its physical and electronic properties. Here, we report that graphene based intercalation compounds is an efficient method to modify the electronic properties of few layer graphene (FLG). FeCl3 intercalated FLG were successfully prepared by two-zone vapor transport method. This is the first report on full intercalation for graphene samples. The features of the Raman G peak of such few-layer graphene intercalation compounds (FLGIC) are in good agreement with their full intercalation structures. The FLGIC presents single Lorentzian 2D peak, similar to that of single layer graphene, indicating the loss of electronic coupling between adjacent graphene layers. First principle calculations further reveal that the band structure of FLGIC is similar to single layer graphene but with strong doping effect due to the charge transfer from graphene to FeCl3. The successful fabrication of FLGIC opens a new way to modify properties of FLG for fundamental studies and future applications.

cond-mat.mtrl-sci↗

arXiv2010

Stacking Dependent Optical Conductivity of Bilayer Graphene

The optical conductivities of graphene layers are strongly dependent on their stacking orders. Our first-principle calculations show that while the optical conductivities of single layer graphene (SLG) and bilayer graphene (BLG) with Bernal stacking are almost frequency independent in the visible region, the optical conductivity of twisted bilayer graphene (TBG) is frequency dependent, giving rise to additional absorption features due to the band folding effect. Experimentally, we obtain from contrast spectra the optical conductivity profiles of BLG with different stacking geometries. Some TBG samples show additional features in their conductivity spectra in full agreement with our calculation results, while a few samples give universal conductivity values similar to that of SLG. We propose those variations of optical conductivity spectra of TBG samples originate from the difference between the commensurate and incommensurate stackings. Our results reveal that the optical conductivity measurements of graphene layers indeed provide an efficient way to select graphene films with desirable electronic and optical properties, which would great help the future application of those large scale misoriented graphene films in photonic devices.

cond-mat.mtrl-sci↗

arXiv2009

Sudden Death of Entanglement

A new development in the dynamical behavior of elementary quantum systems is the surprising discovery that correlation between two quantum units of information called qubits can be degraded by environmental noise in a way not seen previously in studies of dissipation. This new route for dissipation attacks quantum entanglement, the essential resource for quantum information as well as the central feature in the Einstein-Podolsky-Rosen so-called paradox and in discussions of the fate of Schröinger's cat. The effect has been labeled ESD, which stands for early-stage disentanglement or, more frequently, entanglement sudden death. We review recent progress in studies focused on this phenomenon.

quant-ph↗

arXiv2009

Modulated Entanglement Evolution Via Correlated Noises

We study entanglement dynamics in the presence of correlated environmental noises. Specifically, we investigate the quantum entanglement dynamics of two spins in the presence of correlated classical white noises, deriving Markov master equation and obtaining explicit solutions for several interesting classes of initial states including Bell states and X form density matrices. We show how entanglement can be enhanced or reduced by the correlation between the two participating noises.

quant-ph↗

arXiv2009

Entanglement Evolution in a Non-Markovian Environment

We extend recent theoretical studies of entanglement dynamics in the presence of environmental noise, following the long-time interest of Krzysztof Wodkiewicz in the effects of stochastic models of noise on quantum optical coherences. We investigate the quantum entanglement dynamics of two spins in the presence of classical Ornstein-Uhlenbeck noise, obtaining exact solutions for evolution dynamics. We consider how entanglement can be affected by non-Markovian noise, and discuss several limiting cases.

quant-ph↗

arXiv2009

G band Raman double resonance in twisted bilayer graphene: an evidence of band splitting and folding

The stacking faults (deviates from Bernal) will break the translational symmetry of multilayer graphenes and modify their electronic and optical behaviors to the extent depending on the interlayer coupling strength. This paper addresses the stacking-induced band splitting and folding effect on the electronic band structure of twisted bilayer graphene. Based on the first-principles density functional theory study, we predict that the band folding effect of graphene layers may enable the G band Raman double resonance in the visible excitation range. Such prediction is confirmed experimentally with our Raman observation that the resonant energies of the resonant G mode are strongly dependent on the stacking geometry of graphene layers.

cond-mat.mtrl-sci↗

arXiv2009

Comment on "Raman spectra of misoriented bilayer graphene"

In a recent paper [Phys. Rev. B 78, 113407 (2008)], Poncharal et al. studied the Raman spectra of misoriented bilayer graphene. They found that the blueshift of 2D band of misoriented graphene relative to that of single layer graphene shows a strong dependence on the excitation laser energy. The blueshift increases with decreasing excitation energy. This finding contradicts our explanation of reduction of Fermi velocity of folded/misoriented graphene [Ni et al. Phys. Rev. B 77, 235403 (2008)]. In this comment, we present more experimental results from our group as well as from others to show that the blueshift is indeed only weakly dependent on excitation energy. We therefore suggest that our explanation of 2D blushift of folded graphene due to reduction of Fermi velocity is still valid.

cond-mat.mtrl-sci↗

arXiv2009

Probing charged impurities in suspended graphene using Raman spectroscopy

Charged impurity (CI) scattering is one of the dominant factors that affect the carrier mobility in graphene. In this paper, we use Raman spectroscopy to probe the charged impurities in suspended graphene. We find that the 2D band intensity is very sensitive to the CI concentration in graphene, while the G band intensity is not affected. The intensity ratio between the 2D and G bands, I2D/IG, of suspended graphene is much stronger compared to that of non-suspended graphene, due to the extremely weak CI scattering in the former. This finding is consistent with the ultra-high carrier mobility in suspended graphene observed in recent transport measurements. Our results also suggest that at low CI concentrations that are critical for device applications, the I2D/IG ratio is a better criterion in selecting high quality single layer graphene samples than is the G band blue shift.

cond-mat.mtrl-sci↗

arXiv2009

Entanglement sudden birth of two trapped ions interacting with a time-dependent laser field

We explore and develop the mathematics of the two multi-level ions. In particular, we describe some new features of quantum entanglement in two three-level trapped ions confined in a one-dimensional harmonic potential, allowing the instantaneous position of the center-of-mass motion of the ions to be explicitly time-dependent. By solving the exact dynamics of the system, we show how survivability of the quantum entanglement is determined by a specific choice of the initial state settings.

quant-ph↗