arXiv subjects

P. Xu

Publications and source records attributed to P. Xu.

At least 37 records · Page 2Linked to original sources

arXiv2015

New Scanning Tunneling Microscopy Technique Enables Systematic Study of the Unique Electronic Transition from Graphite to Graphene

A series of measurements using a novel technique called electrostatic-manipulation scanning tunneling microscopy were performed on a highly-oriented pyrolytic graphite (HOPG) surface. The electrostatic interaction between the STM tip and the sample can be tuned to produce both reversible and irreversible large-scale vertical movement of the HOPG surface. Under this influence, atomic-resolution STM images reveal that a continuous electronic reconstruction transition from a triangular symmetry, where only alternate atoms are imaged, to a honeycomb structure can be systematically controlled. First-principles calculations reveal that this transition can be related to vertical displacements of the top layer of graphite relative to the bulk. Detailed analysis of the band structure predicts that a transition from parabolic to linear bands occurs after a 0.09 nm displacement of the top layer.

cond-mat.mes-hall↗

arXiv2015

Broad frequency and amplitude control of vibration in freestanding graphene via scanning tunneling microscopy with calculated dynamic pseudo-magnetic fields

A technique to locally generate mechanical vibrations in freestanding graphene using scanning tunneling microscopy (STM) is presented. The frequency of the mechanical vibrations is tuned over nearly four decades and is centered around 10 Hz. The amplitude of the vibrations also changes over nearly three decades centered on 1 nm. The oscillating motion is generated in two ways: first, by scanning the STM tip on the surface and second, by scanning the bias voltage on the STM tip. The frequency and amplitude of the displaced freestanding graphene is quantitatively transformed to the frequency and strength of the locally generated pseudo-magnetic field for our specific geometry.

cond-mat.mes-hall↗

arXiv2015

Graphene Manipulation on 4H-SiC(0001) Using Scanning Tunneling Microscopy

Atomic-scale topography of epitaxial multilayer graphene grown on 4H-SiC(0001) was investigated using scanning tunneling microscopy (STM). Bunched nano-ridges ten times smaller than previously recorded were observed throughout the surface, the morphology of which was systematically altered using a relatively new technique called electrostatic-manipulation scanning tunneling microscopy. Transformed graphene formations sometimes spontaneously returned to their original morphology, while others permanently changed. Using an electrostatic model, we calculate that a force up to ~5 nN was exerted by the STM tip, and an energy of around 10 eV was required to alter the geometry of a ~100 X 200 nm^2 area.

cond-mat.mes-hall↗

arXiv2015

Atomic-Scale Movement Induced in Nano-Ridges by Scanning Tunneling Microscopy on Epitaxial Graphene Grown on 4H-SiC(0001)

Nanoscale ridges in epitaxial multilayer graphene grown on the silicon face of 4 degree off-cut 4H-SiC (0001) were found using scanning tunneling microscopy (STM). These nano-ridges are only 0.1 nm high and 25-50 nm wide, making them much smaller than previously reported ridges. Atomic-resolution STM was performed near and on top of the nano-ridges using a dual scanning technique in which forward and reverse images are simultaneously recorded. An apparent 100% enlarged graphene lattice constant is observed along the leading edge of the image for both directions. Horizontal movement of the graphene, due to both an electrostatic attraction to the STM tip and weak bonding to the substrate, is thought to contribute to the results.

cond-mat.mes-hall↗

arXiv2015

Role of bias voltage and tunneling current in the perpendicular displacements of freestanding graphene via scanning tunneling microscopy

Systematic displacement measurements of freestanding graphene as a function of applied bias voltage and tunneling current setpoint using scanning tunneling microscopy (STM) are presented. When the bias voltage is increased the graphene approaches the STM tip, while, on the other hand, when the tunneling current is increased the graphene contracts from the STM tip. To understand the role of the bias voltage, we quantitatively model the attractive force between the tip and the sample using electrostatics. For the tunneling current, we qualitatively model the contraction of the graphene using entropic concepts. These complementary results enhance the understanding of each other and highlight peculiarities of the system.

cond-mat.mes-hall↗

arXiv2015

Etch-stop method for reliably fabricating sharp yet mechanically stable scanning tunneling microscope tips

An extension of the direct-current, double-lamella drop-off technique for electrochemically etching tungsten scanning-tunneling-microscope tips is presented. The key fabrication step introduced here is the use of an etch stop as a simple but accurate way to optimize the contact area between the etchant and the wire. By restricting the etching process, the final cone angle of the tips can be made sharp and mechanically stable without a lot of finesse from the STM tip maker.

cond-mat.mes-hall↗

arXiv2015

Competing scanning tunneling microscope tip-interlayer interactions for twisted multilayer graphene on the a-plane SiC surface

Scanning tunneling microscopy (STM) images are obtained for the first time on few layer and twisted multilayer epitaxial graphene states synthesized on n+ 6H-SiC a-plane non-polar surface. The twisted graphene is determined to have a rotation angle of 5.4° between the top two layers, by comparing moiré patterns from stick and ball models of bilayer graphene to experimentally obtained images. Furthermore, the experimental moiré pattern shows dynamic behavior, continuously shuffling between two stable surface arrangements one bond length apart. The moiré pattern shifts by more than 1 nm, making it easy to observe with STM. Explanation of this dynamic behavior is attributed to electrostatic interactions between the STM tip and the graphene sample.

cond-mat.mes-hall↗

arXiv2015

Graphene Ripples as a Realization of a Two-Dimensional Ising Model: A Scanning Tunneling Microscope Study

Ripples in pristine freestanding graphene naturally orient themselves in an array that is alternately curved-up and curved-down; maintaining an average height of zero. Using scanning tunneling microscopy (STM) to apply a local force, the graphene sheet will reversibly rise and fall in height until the height reaches 60-70 percent of its maximum at which point a sudden, permanent jump occurs. We successfully model the ripples as a spin-half Ising magnetic system, where the height of the graphene is the spin. The permanent jump in height, controlled by the tunneling current, is found to be equivalent to an antiferromagnetic-to-ferromagnetic phase transition. The thermal load underneath the STM tip alters the local tension and is identified as the responsible mechanism for the phase transition. Four universal critical exponents are measured from our STM data, and the model provides insight into the statistical role of graphenes unusual negative thermal expansion coefficient.

cond-mat.mes-hall↗

arXiv2014

Multilayer graphene, Moiré patterns, grain boundaries and defects identified by scanning tunneling microscopy on the m-plane, non-polar surface of SiC

Epitaxial graphene is grown on a non-polar n+ 6H-SiC m-plane substrate and studied using atomic scale scanning tunneling microscopy. Multilayer graphene is found throughout the surface and exhibits rotational disorder. Moiré patterns of different spatial periodicities are found, and we found that as the wavelength increases, so does the amplitude of the modulations. This relationship reveals information about the interplay between the energy required to bend graphene and the interaction energy, i.e. van der Waals energy, with the graphene layer below. Our experiments are supported by theoretical calculations which predict that the membrane topographical amplitude scales with the Moiré pattern wavelength, L as L^-1 + αL^-2.

cond-mat.mes-hall↗

arXiv2014

Thermal mirror buckling in freestanding graphene locally controlled by scanning tunneling microscopy

Knowledge of and control over the curvature of ripples in freestanding graphene are desirable for fabricating and designing flexible electronic devices, and recent progress in these pursuits has been achieved using several advanced techniques such as scanning tunneling microscopy. The electrostatic forces induced through a bias voltage (or gate voltage) were used to manipulate the interaction of freestanding graphene with a tip (substrate). Such forces can cause large movements and sudden changes in curvature through mirror buckling. Here we explore an alternative mechanism, thermal load, to control the curvature of graphene. We demonstrate thermal mirror buckling of graphene by scanning tunneling microscopy and large-scale molecular dynamic simulations. The negative thermal expansion coefficient of graphene is an essential ingredient in explaining the observed effects. This new control mechanism represents a fundamental advance in understanding the influence of temperature gradients on the dynamics of freestanding graphene and future applications with electro-thermal-mechanical nanodevices.

cond-mat.mes-hall↗

arXiv2014

Unusual ultralow frequency fluctuations in freestanding graphene

Intrinsic ripples in freestanding graphene have been exceedingly difficult to study. Individual ripple geometry was recently imaged using scanning tunneling microscopy, but these measurements are limited to static configurations. Thermally-activated flexural phonon modes should generate dynamic changes in curvature. Here we show how to track the vertical movement of a one-square-angstrom region of freestanding graphene using scanning tunneling microscopy, thereby allowing measurement of the out-of-plane time trajectory and fluctuations over long time periods. We also present a model from elasticity theory to explain the very-low-frequency oscillations. Unexpectedly, we sometimes detect a sudden colossal jump, which we interpret as due to mirror buckling. This innovative technique provides a much needed atomic-scale probe for the time-dependent behavior of intrinsic ripples. The discovery of this novel progenitor represents a fundamental advance in the use of scanning tunneling microscopy, which together with the application of a thermal load provides a low-frequency nano-resonator.

cond-mat.mes-hall↗

arXiv2014

Integrated Quantum Controlled-NOT Gate Based on Dielectric-Loaded Surface Plasmon Polariton Waveguide

It has been proved that surface plasmon polariton (SPP) can well conserve and transmit the quantum nature of entangled photons. Therefore, further utilization and manipulation of such quantum nature of SPP in a plasmonic chip will be the next task for scientists in this field. In quantum logic circuits, the controlled-NOT (CNOT) gate is the key building block. Here, we implement the first plasmonic quantum CNOT gate with several-micrometer footprint by utilizing a single polarization-dependent beam-splitter (PDBS) fabricated on the dielectric-loaded SPP waveguide (DLSPPW). The quantum logic function of the CNOT gate is characterized by the truth table with an average fidelity of. Its entangling ability to transform a separable state into an entangled state is demonstrated with the visibilities of and for non-orthogonal bases. The DLSPPW based CNOT gate is considered to have good integratability and scalability, which will pave a new way for quantum information science.

physics.optics↗

arXiv2014

Membrane amplitude and triaxial stress in twisted bilayer graphene deciphered using first-principles directed elasticity theory and scanning tunneling microscopy

Twisted graphene layers produce a moiré pattern (MP) structure with a predetermined wavelength for given twist angle. However, predicting the membrane corrugation amplitude for any angle other than pure AB-stacked or AA-stacked graphene is impossible using first-principles density functional theory (DFT) due to the large supercell. Here, within elasticity theory we define the MP structure as the minimum energy configuration, thereby leaving the height amplitude as the only unknown parameter. The latter is determined from DFT calculations for AB and AA stacked bilayer graphene in order to eliminate all fitting parameters. Excellent agreement with scanning tunneling microscopy (STM) results across multiple substrates is reported as function of twist angle.

cond-mat.mes-hall↗

arXiv2014

Reconfigurable on-chip entangled sources based on lithium-niobate waveguide circuits

Integrated quantum optics becomes a consequent tendency towards practical quantum information processing. Here, we report the on-chip generation and manipulation of photonic entanglement based on reconfigurable lithium niobate waveguide circuits. By introducing periodically poled structure into the waveguide interferometer, two individual photon-pair sources with controllable phase-shift are produced and cascaded by a quantum interference, resulting in a deterministically separated identical photon pair. The state is characterized by 92.9% visibility Hong-Ou-Mandel interference. Continuous morphing from two-photon separated state to bunched state is further demonstrated by on-chip control of electro-optic phase-shift. The photon flux reaches ~1.4*10^7 pairs nm-1 mW-1. Our work presents a scenario for on-chip engineering of different photon sources and paves a way to the fully integrated quantum technologies.

quant-ph↗

arXiv2014

Precision measurement of transverse velocity distribution of a strontium atomic beam

We measure the transverse velocity distribution in a thermal Sr atomic beam precisely by velocity-selective saturated fluorescence spectroscopy. The use of an ultrastable laser system and the narrow intercombination transition line of Sr atoms mean that the resolution of the measured velocity can reach 0.13 m/s, corresponding to 90$μK$ in energy units. The experimental results are in very good agreement with the results of theoretical calculations. Based on the spectroscopic techniques used here, the absolute frequency of the intercombination transition of $^{88}$Sr is measured using an optical-frequency comb generator referenced to the SI second through an H maser, and is given as 434 829 121 318(10) kHz.

physics.optics↗

arXiv2013

Compact engineering of path-entangled sources from a monolithic quadratic nonlinear photonic crystal

Photonic entangled states lie at the heart of quantum science for the demonstrations of quantum mechanics foundations and supply as a key resource for approaching various quantum technologies. An integrated realization of such states will certainly guarantee a high-degree of entanglement and improve the performance like portability, stability and miniaturization, hence becomes an inevitable tendency towards the integrated quantum optics. Here, we report the compact realization of steerable photonic path-entangled states from a monolithic quadratic nonlinear photonic crystal. The crystal acts as an inherent beam splitter to distribute photons into coherent spatial modes, producing the heralded single-photon even appealing beamlike two-photon path-entanglement, wherein the entanglement is characterized by quantum spatial beatings. Such multifunctional entangled source can be further extended to high-dimensional fashion and multi-photon level as well as involved with other degrees of freedom, which paves a desirable way to engineer miniaturized quantum light source.

physics.optics↗

arXiv2010

Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr2CuO2Cl2 insulator using resonant x-ray scattering:Evidence for extended interactions

Using high-resolution resonant inelastic x-ray scattering (RIXS), we performed a momentum-resolved study of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr_2CuCl_2O_2. We identify both a single-spin-wave feature and a multi-magnon continuum, and show that the X-ray polarization can be used to distinguish these two contributions in the cross-section. The spin-waves display a large (70 meV) dispersion between the zone-boundary points ($π$,0) and ($π$/2,$π$/2). Employing an extended $t$-$t'$-$t"$-$U$ one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions. We conclude that sizeable extended magnetic interactions are present in \scoc{} and probably important in all undoped cuprates.

cond-mat.str-el↗

arXiv2010

Occurrence of superconductivity when the metal-insulator transition is inhibited in $1T$-TaS${_2}$

When a Mott metal-insulator transition is inhibited by a small amount of disorder in the layered dichalcogenide 1T-TaS$_2$, an inhomogeneous superconducting state arises below T=2.1 K, and coexists with a nearly-commensurate charge-density-wave. By angle-resolved photoelectron spectroscopy (ARPES) we show that it emerges from a bad metal state with strongly damped quasiparticles. Superconductivity is almost entirely suppressed by an external magnetic field of 0.1 T.

cond-mat.str-el↗