Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y45: Graphene Terahertz Optics and Strain Engineering |
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Chair: Xinghan Cai, University of Maryland Room: Mile High Ballroom 4D |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y45.00001: Strain-induced time-reversal odd superconductivity in graphene Vladimir Juricic, Bitan Roy I will discuss the possibility of realizing a time-reversal-symmetry breaking superconducting state that exhibits an $f+is$ pairing symmetry in strained graphene [1]. Although the underlying attractive interactions need to be sufficiently strong and comparable in pristine graphene to support such pairing state, I will argue that strain can be conducive for its formation even for weak interactions. I will show that quantum-critical behavior near the transition is controlled by a fermionic multicritical point, characterized by various critical exponents computed in the framework of an $\epsilon$-expansion near four spacetime dimensions. I will then discuss the scaling of the superconducting gap with the strain-induced axial pseudo-magnetic field. Furthermore, a vortex in this mixed superconducting state hosts a pair of Majorana fermions supporting a quartet of insulating and superconducting orders, among which quantum spin Hall topological insulator. Finally, I will mention some experimental signatures of this $f+is$ time-reversal odd superconductor. These findings suggest that strained graphene could provide a platform for the realization of exotic superconducting states of Dirac fermions.\\[4pt] [1] B. Roy and V. Juricic, arXiv: 1309.0507. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y45.00002: Strain engineering on graphene directly from discrete atomic positions Salvador Barraza-Lopez, Alejandro Pacheco Sanjuan, Zhengfei Wang, Mihajlo Vanevic Graphene's strain engineering can be considered as a ``multiscale'' theory where effective Dirac fermions (Quantum Mechanics) couple with the Classical Mechanics and Geometry of an elastic membrane. Since the seminal work by Suzuura and Ando, the coupling to mechanics and geometry has been given in terms of continuum elasticity theory in the harmonic regime and relies on Riemannian, continuum geometry (e.g., [1-4]). Given an atomistic conformation is known, we express the coupling among effective Dirac fermions and mechanics directly onto the atomistic lattice and without reference to a continuum media; i.e., we couple effective Dirac fermions with atomistic mechanics. The approach has a solid mathematical underpinning known as Discrete Differential Geometry (DDG) [5]. We will provide a number of specific insights from this atomic-originated framework [6,7]. 1. H. Suzuura and T. Ando, \underline {PRB 65, 235412 (2002)}; 2. V. M. Pereira and A. H. Castro Neto, \underline {PRL 103, 046801 (2009)}. 3. F. Guinea, and M. I. Katsnelson, and A. K. Geim, \underline {Nat. Phys. 6, 30 (2010)}. 4. M. A. H. Vozmediano, and M. I. Katsnelson, and F. Guinea, Phys. Rep. 496, 109 (2010); 5. A. I. Bobenko, P. Schroder, J. M. Sullivan, and G. M. Ziegler, eds., Discrete Differential Geometry. Springer. 1st Ed.; 6. \underline {J. V. Sloan}, \underline {A. A. Pacheco Sanjuan}, \underline {Z. Wang}, \underline {C. Horvath}, and S. Barraza-Lopez. PRB 87, 155436 (2013); 7. S. Barraza-Lopez, A. A. Pacheco-Sanjuan, Z. Wang, and M. Vanevic. Solid State Comm 166, 70 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y45.00003: Suspended graphene device under strain Fen Guan, Bent Nielsen, Xu Du It has been theoretically proposed that strain can induce changes in band structure and electrical transport properties of graphene. While some evidences have been reported on spectroscopy measurements, transport study on strained graphene has been limited and is mostly based on non-suspended graphene, where the detrimental effect of the substrates may smear out the intrinsic response. To overcome this problem, we report the fabrication of suspended monolayer and bilayer graphene devices on flexible substrates. By bending the substrate and measuring the transport characteristics of graphene as a function of temperature and gate voltage, these devices allow study of the intrinsic properties of the materials under strain. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y45.00004: Manipulation of electronic states in triangular graphene quantum dots using optical selection rules Eleftheria Kavousanaki, Keshav Dani Triangular graphene quantum dots with zigzag edges have been known to exhibit half-filled Fermi edge states with a non-zero ground state magnetic moment. Using the tight binding model, we study the optical selection rules for these structures with and without an external magnetic field and demonstrate that only transitions between states of specific rotational symmetry are allowed in the case of excitation with circularly polarized light. Using these rules, we analyze the optical absorption spectra of quantum dots with either zigzag or armchair edges at zero and nonzero magnetic field, discuss their differences, and show that they allow for the manipulation of the pseudo magnetic properties of these dots using optical pulses. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y45.00005: Is silicon dioxide essential to make graphene visible? Case studies of graphene-substrate interaction Chia-Hao Chen, Hung Wei Shiu, Lo Yueh Lo Yueh, Hung-Ying Chen, Shangjr Gwo Making exfoliated graphene flakes visible is the key to successfully study the fundamental properties of graphene. Conventionally, this can be achieved by placing the graphene flakes on top of silicon substrate with 300 nm SiO2, but this silicon dioxide layer may cause substrate charging effect. We therefore started to ask ourselves, is silicon dioxide the only material to make graphene visible? Moreover, a recent study has successfully demonstrated a working GaN LEDs with CVD-synthesized multi-layer graphene as conduction electrodes. However, the energy coupling between graphene and GaN is still unclear. To fully utilize the advantage of graphene as transparent electrode, a further understanding of the electronic structure between graphene and the substrate is an urgent task. To answer those questions, we employed theoretical simulation using a model based on Fresnel's law, to calculate the optical contrast of single-layer graphene on various substrate structures. Based on the results, we grew those particular substrates to test the graphene visibility. The graphene flakes and thickness were verified by optical microscope and micro-Raman spectroscopy. The graphene-substrate interactions were then studied by scanning photoelectron microscopy. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y45.00006: Two-Dimensional Optoelectronic Graphene Nanoprobes for Neural Nerwork Tu Hong, Kristina Kitko, Rui Wang, Qi Zhang, Yaqiong Xu Brain is the most complex network created by nature, with billions of neurons connected by trillions of synapses through sophisticated wiring patterns and countless modulatory mechanisms. Current methods to study the neuronal process, either by electrophysiology or optical imaging, have significant limitations on throughput and sensitivity. Here, we use graphene, a monolayer of carbon atoms, as a two-dimensional nanoprobe for neural network. Scanning photocurrent measurement is applied to detect the local integration of electrical and chemical signals in mammalian neurons. Such interface between nanoscale electronic device and biological system provides not only ultra-high sensitivity, but also sub-millisecond temporal resolution, owing to the high carrier mobility of graphene. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y45.00007: Photo Induced Fluorescence Enhancement and Correlated FTIR of Single Layer Graphene Oxide Charudatta Galande, Sibel Ebru Yalcin, Akhilesh Singh, Gautam Gupta, Rajesh Kappera, Andrew M. Dattelbaum, Manish Chhowalla, Stephen K. Doorn, Pulickel M. Ajayan, Aditya D. Mohite Ultrafast recombinations of photo-excited electron-hole pairs and low absorption in graphene have prevented its use for several low light applications. However, graphene oxide (GO) is a wide band gap material with emission in the visible spectrum. For optoelectronic applications, it is desired to have a material with good optical absorption and electrical transport. We report the in-situ photo induced observation of functional groups in progressively reduced GO due to the presence of intercalated water. We perform correlated fluorescence and FTIR spectroscopy on an individual GO flake and we assign the formation of a specific functional group(s) to the observed increase in the fluorescence intensity. This provides insights into tuning the band structure of graphene via controlled oxidation for relevant applications. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y45.00008: Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene Xinghan Cai, Andrei Sushkov, Ryan Suess, Mohammad Jadidi, Gregory Jenkins, Luke Nyakiti, Rachael Myers-Ward, Virginia Wheeler, Charles Eddy, Jr., Jun Yan, D. Kurt Gaskill, Thomas Murphy, H. Dennis Drew, Michael Fuhrer Due to the weak electron-phonon coupling and strong electron-electron interaction in graphene, the thermoelectric effect provides a highly sensitive detection mechanism for heat absorbed in the electronic system. We present here a bi-metal contacted graphene thermoelectric THz photodetector with sensitivity exceeding 100 V/W at room temperature and noise equivalent power less than 100 pW/Hz$^{1/2}$, competitive with the best room-temperature THz detectors, while time-resolved measurements indicate our graphene detector is eight to nine orders of magnitude faster. We also measured the thermoelectric response to Joule heating, and compare to the thermoelectric response due to optical excitation in the near infrared and at THz frequencies. A simple model of the response, including contact asymmetries reproduces the qualitative features of the data. We also suggest that orders-of-magnitude sensitivity improvements are possible by using local gates to define graphene pn-junctions. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y45.00009: Photon Induced Dynamic THz Conductivity Change in Graphene Sufei Shi, Tom Tang, Bo Zeng, Long Ju, Feng Wang The linear dispersion relation in graphene gives rise to large and highly tunable conductivity at THz regime, which makes graphene a promising candidate for new optoelectronic devices. We use optical pump THz probe spectroscopy to investigate photon induced conductivity change in graphene in time domain, and show that the THz response sensitively depends on the initial doping of graphene. This study sheds light on the carrier relaxation in graphene after optical excitation and provides valuable information for designing future graphene-based opto-electronic device.~ [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y45.00010: Doping-dependent THz photoconductivity in large-area graphene Alex Frenzel, Chun Hung Lui, Yong Cheol Shin, Jing Kong, Nuh Gedik We have performed a systematic investigation of the transient terahertz photoconductivity of large-area CVD graphene following femtosecond optical excitation as a function of electrically-tuned carrier density. We observe a dramatic change in the transient response as the photoconductivity changes from positive to negative when the Fermi level is tuned from the charge neutrality point to the electron or hole doped regime. This effect is discussed within the context of the Drude model for free carriers, taking into account the elevated electron and phonon temperatures in photoexcited graphene. Our results demonstrate that previous conflicting measurements of terahertz photoconductivity in epitaxial and CVD graphene arise primarily from their different doping levels. Additionally, our measurements provide a link between ultrafast optical experiments and DC photocurrent measurements. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y45.00011: Manipulation of the polarization of Terahertz wave in subwavelength regime via hybridizing pseudo-spin polarized gaphenes and metal hole array Xiao Xiao, Che-Ting Chan, Weijia Wen In this presentation, we show that the subwavelength quarter wave plate and half wave plate in terahertz regime can be realized in a metal hole array (MHA) sandwiched by two stacks of pseudo-spin polarized graphenes (PSPGs). When the gaps of the two PSPGs are the same, the hybrid resonances in the system can convert the linearly polarized incident light into the circularly polarized transmitted light; On the other hand, when the gaps of the two PSPGs are opposite in sign, the polarization of the reflected light can be rotated by $90^{\circ}$. Interestingly, when the PSPGs is tuned in the quantum Hall regime, the fine structure constant can relate with the resonant frequency and the geometrical parameters of MHA directly. The rich properties of the system guarantee its potential applications in THz technologies. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y45.00012: Terahertz time-domain spectroscopy of large-area graphene on various substrates Iwao Kawayama, Makoto Ohshiro, Yuki Sano, Hironaru Murakami, James Allred, Minjie Wang, Junichiro Kono, Robert Vajtai, Pulickel Ajayan, Masayoshi Tonouchi The advent of large-area graphene samples has opened up tremendous new opportunities for terahertz and infrared optoelectronic devices as well as for fundamental studies of low-energy excitations in graphene in the terahertz frequency range. While there have been extensive studies on the strong influence of the supporting substrate and the local molecular environment on the optical and DC transport properties of graphene, no systematic studies exist on their effects on graphene's terahertz properties. In this study, we a terahertz time-domain spectroscopy study of large-area graphene mono-layers on various terahertz-transparent substrates (e.g., InAs, InP, GaAs, MgO, and polypropylene). We found that the terahertz optical conductivity spectrum shows qualitatively different behaviors, depending on the substrate, which can be understood through substrate-induced doping and defects as well as interaction with substrate phnons. In addition, we observed that the effects of adsorbed gas molecules on the terahertz conductivity also vary, depending on the substrate. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y45.00013: THz investigations of graphene-complex-oxide heterostructures Giriraj Jnawali, Lu Chen, Patrick Irvin, Jeremy Levy, Sangwoo Ryu, Chang-Beom Eom, Fereshte Ghahari, Jayakanth Ravichandran, Philip Kim The unique and multifaceted properties of graphene have fascinated scientists and engineers for a decade now. A new frontier in research concerns properties of graphene in the THz-IR region, where the collective excitation of graphene 2D electron gas (2DEG) into plasmonic waves has proven the salient feature.\footnote{L. Ju, \textit{et al.}, Nat. Nanotechnol. \textbf{6}, 630 (2011)} Complex oxide heterostructures (e.g., LaAlO$_{3}$/SrTiO$_{3}$, LAO/STO) also support a 2DEG with high carrier densities and expected plasmonic behavior. A unique feature of the LAO/STO system is the ability to control the electron density with nanoscale precision.\footnote{C. Cen, \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)} In addition, a method for sourcing and detecting broadband THz emission from LAO/STO nanojunctions has been recently demonstrated.\footnote{Y. Ma, \textit{et al.}, Nano Lett. \textbf{13}, 2284 (2013)} Here we describe initial efforts to investigate the THz properties of graphene-complex oxide (GCO) heterostructures. We envision that the proposed graphene plasmonic devices in the GCO will help to lay the foundation for a host of powerful THz-IR technologies for signal processing, imaging, spectroscopy and chemical sensing. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y45.00014: Theoretical modeling of the terahertz response of ultrafast photoexcited charge carriers in graphene Avinash Rustagi, Christopher J. Stanton We have formulated a semi-classical model to capture the terahertz response of photoexcited charge carriers in graphene. The model involves the time evolution of the initial carrier distribution function excited by a femtosecond laser pulse by solving the Boltzmann equation within the relaxation time approximation in presence of an in-plane DC electric field. We solve for the time dependent average velocity using the distribution function obtained from the Boltzmann equation. The time derivative of this average velocity is proportional to the terahertz signal measured in experiments. We also consider the contribution of virtual carriers to the terahertz signal. This model can also be applied to systems with a gapped graphene-like dispersion. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y45.00015: Transport in graphene exposed to a strong electromagnetic field Sergey Syzranov, Yaroslav Rodionov, Kliment Kugel, Franco Nori We study quasiparticle dynamics in graphene exposed to a linearly-polarized electromagnetic wave of very large intensity. We demonstrate that low-energy transport in such system can be described by an effective time-independent Hamiltonian, characterized by multiple Dirac points in the first Brillouin zone. Around each Dirac point the spectrum is anisotropic: the velocity along the polarization of the radiation significantly exceeds the velocity in the perpendicular direction. Moreover, in some of the points the transverse velocity oscillates as a function of the radiation intensity. These features of the quasiparticle spectrum manifest themselves in the conductance of graphene-based junctions in the regime of strong irradiation. For instance, we find that the conductance of a graphene p-n junction depends on the polarization as $G(\theta)\propto|\sin\theta|^{3/2}$, where $\theta$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity. [Preview Abstract] |
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