Gotham-Metro Condensed Matter Meeting

Agenda

Speaker Abstracts

Keynote Presentations

The Life and Death of a Drop: Topological Transitions and Singularities in Fluids

Sidney Nagel, Department of Physics, University of Chicago

The exhilarating spray from waves crashing into the shore, the distressing sound of a faucet leaking in the night, and the indispensable role of bubbles dissolving gas into the oceans are but a few examples of the ubiquitous presence and profound importance of drop formation and splashing in our lives. They are also examples of a liquid changing its topology. Although part of our common everyday experience, these transitions are far from understood and reveal delightful and profound surprises upon careful investigation. For example in droplet fission, the fluid forms a neck that becomes vanishingly thin at the point of breakup. This topological transition is thus accompanied by a dynamic singularity in which physical properties such as pressure diverge. Singularities of this sort often organize the overall dynamical evolution of nonlinear systems. I will first discuss the role of singularities in the breakup of drops. I will then discuss the fate of the drop after it falls and eventually splashes against a solid surface.

Imaging Electron Flow, Interference, and Interactions in High-Mobility Two-Dimensional Electron Gases

David Goldhaber-Gordon, Center for Probing the Nanoscale, Stanford University

Because of their extremely low levels of disorder, GaAs-based heterostructures at low-temperature can host nearly ideal two-dimensional electron gases (2DEGs), which exhibit a variety of remarkable electronic states.  I will discuss how one can use scanning gate microscopy (SGM) to image the flow of electron beams injected into a 2DEG, and will focus on the role of electron-electron scattering in deflecting such beams and dephasing the electrons in those beams. Electron-electron (e-e) scattering is the dominant source of dephasing in clean conductors at low-temperature.  One can adjust the e-e scattering rate by injecting electrons above or below the Fermi energy of the 2DEG. Using newly-observed interference effects -- and their destruction when electrons are injected at high energy -- my group has spatially measured the e-e scattering rate.  Additionally, from SGM images of electron flow at high injection energy we have found evidence for a highly non-equilibrium distribution of electrons in a localized region of 2DEG near the injection point.  We are able to measure the e-e scattering rate between injected electrons and this non-equilibrium distribution.

Short Talks

Irreversible Rearrangements, Correlated Domains and Local Structure in Aging Glasses

Peter Yunker, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia

Bidisperse colloidal suspensions of temperature-sensitive microgel spheres were quenched from liquid to glass states by a rapid temperature drop, and then the glass was permitted to age. Irreversible rearrangements, events that dramatically change a particle's local environment, were observed to be closely related to dynamic heterogeneity. The rate of these irreversible events decreased during aging and the the number of particles required to move as part of these irreversible rearrangements increased. Thus, the slowing dynamics of aging were governed by growing, correlated domains of particles. Additionally, short-range order developed, and a spatial decay length scale associated with orientational order was found to grow during aging.

Flow Visualization and Flow Cytometry with Holographic Video Microscopy

Fook Chiong Cheong, Bo Sun, Rémi Dreyfus, Jesse Amato-Grill, Ke Xiao, Lisa Dixon and David G. Grier, Department of Physics and Center for Soft Matter Research, New York University

The video stream captured by an in-line holographic microscope can be analyzed on a frame-by-frame basis to track individual colloidal particles' three-dimensional motions with nanometer resolution, and simultaneously to measure their sizes and refractive indexes. Through a combination of hardware acceleration and software optimization, this analysis can be carried out in near real time with off-the-shelf instrumentation. An efficient particle identification algorithm automates initial position estimation with sufficient accuracy to enable unattended holographic tracking and characterization. This technique's resolution for particle size is fine enough to detect molecular-scale coatings on the surfaces of colloidal spheres, without requiring staining or fluorescent labeling. We demonstrate this approach to label-free holographic flow cytometry by detecting the binding of avidin to biotinylated polystyrene spheres.

Pulsed ESR on Confined Electrons in a Silicon MOSFET

S. Shankar, A. M. Tyryshkin, Jianhua He, and S. A. Lyon, Department of Electrical Engineering, Princeton University

Electron spins confined in quantum dots in silicon heterostructures are believed to be good candidates to make qubits for quantum computing. However, little is known about the coherence of isolated electrons at the Si/SiO₂ interface. We present ensemble measurements of spin relaxation and coherence times, T₁ and T₂ for confined electrons in a Metal-Oxide-Silicon transistor (MOSFET) using pulsed electron spin resonance (ESR) at temperatures down to 350mK. Upon biasing our MOSFET below threshold, we observe an ESR signal arising from two-dimensional (2D) electrons weakly confined into natural quantum dots by the disorder at the Si/SiO₂ interface. These confined electrons have a density of about 1010cm-2, a trap depth of a few meV and a Curie susceptibility characteristic of independent, isolated electrons. Using pulsed ESR, we measure a T₂ of 2μs at 5K, rising to around 10μs at 1K. T₂ saturates at 10μs from 1K down to 350mK. In contrast, T1 rises rapidly as the temperature is lowered, from 6μs at 1K to 1.1ms at 350mK. At 350mK, biasing the MOSFET at threshold raises the electron density at the interface to about 10^₁₁cm^-2 and reduces T₁ to 15μs. Our measured T₁'s and T₂ 's are long for a 2D electron system; at 5K when biasing our MOSFET above threshold, we find that mobile electrons have coherence times of only 0.3μs . While the mechanism for spin relaxation and decoherence remain unknown, the long T₁ of 1.1ms at 350mK demonstrates that quantum dots in a Si/SiO₂ heterostructure satisfy a key metric for building a scalable quantum computing architecture.

Towards Quantum Metrology of Electrical Current in Josephson Tunnel Junction Circuits

Vladimir E. Manucharyan, Jens Koch, Nicholas Masluk, Leonid I. Glazman and Michel H. Devoret, Departments of Physics and Applied Physics, Yale University

Coulomb blockade effects in mesoscopic physics have so far always been observed in conjunction with the noise of random charge “offsets”, inherent to many solid-state systems. In the case of superconducting tunnel junction devices, such charge “offsets” severely limit the promise of single Cooper pair quantum circuits for metrology and quantum information applications. We present a novel superconducting artificial atom, the fluxonium, whose energy spectrum manifests the anharmonicity associated with single Cooper pair effects combined with total insensitivity to offset charges.

The Fluxonium Circuit Consists of a Small Capacitance Josephson Tunnel Junction Shunted by an Array of Larger Junctions to Form a Loop. An external magnetic field tunes the device. The junction parameters are chosen such that the resulting charge fluctuation across the small junction is approximately “1 electron” while its conjugate phase fluctuation is approximately “1 radian”. At the same time phase fluctuation across every array junction is much less than “1 radian”, so that offset charges on all circuit islands are screened. Such regime, not accessible by conventional charge/phase/flux qubits, makes fluxonium transition frequencies modestly dependent on the device parameters and flux bias (compared to flux qubits) while staying strongly anharmonic (compared to phase and transmon qubits). At integer half flux quantum, the spectrum forms the 3-level structure resembling that of the Rb or Cs atoms used in atomic clock experiments and presents opportunities for a qubit with simultaneously long coherence and high readout fidelity.

Our experiments on the fluxonium atom also establish that a series array of Josephson junctions of less than 20 micrometer in length possesses inductive microwave reactance in excess of the impedance quantum for Cooper pairs and resistance of less than 1 Ohm. A circuit element with such property is a prerequisite to the utilization of Bloch oscillations for the metrology of electrical current.

Orbital Magnetoelectric Polarizability in Insulators with Broken Time Reversal Symmetry

Sinisa Coh (1), David Vanderbilt (1), Andrei Malashevich (2) and Ivo Souza (2), (1)Department of Physics and Astronomy, Rutgers University, (2)Department of Physics, University of California, Berkeley

The electronic component of magnetoelectric (ME) effect is usually considered to be small as compared to the ionic and lattice effects. This is not true in the case of Z_2 insulators where the "axionic" component of orbital ME tensor is large and equals half of the quantum due to presence of time-reversal symmetry in the bulk. Unfortunately the fact that surfaces of these compounds are metallic render this large ME coupling not very useful. On the other hand, even normal insulators with broken time-reversal symmetry can have arbitrary values of axionic component of ME tensor and there seems to be no reason why this component should be small especially if that insulator is in some sense "close" to being Z_2 non-trivial.

Theory of Spin Transport in a Two Component Fermi Gas for Weak Interactions

Stefan S. Natu and Erich J. Mueller, Laboratory of Atomic and Solid State Physics, Cornell University

By deriving the equations that describe spin transport in a gas of two-component fermions, we provide an explanation for the observation of spin segregation reported by Du et al. (Ref. [1]). The experiment observed a time dependent separation of the spin density profiles of the two hyperfine states of 6 Li with a s-wave scattering length tuned near zero. The spin segregated density profiles are observed to persist over time scales two orders of magnitude larger than the axial trapping period. We show that the experimental results can be fully accounted for by a Hatree-Fock mean-field description of the two particle interactions, which gives rise to a collisionless Boltzmann transport equation for the spin density matrix. [1] X.Du, L.Luo, B.Clancy, and J.E. Thomas, Phys. Rev. Lett. 101 150401 (2008).

Nonlinear Behaviour in Long Range Integrable Models with Spin

Manas Kulkarni(1,2), Fabio Franchini (3), Alexander G. Abanov (1), (1) Department of Physics and Astronomy, Stony Brook University, (2) Department of Condensed Matter Physics and Material Science, Brookhaven National Laboratory, (3) The Abdus Salam ICTP, Trieste, Italy

We study nonlinear aspects of long range integrable models with spin by going beyond the Luttinger Liquid theory. The interaction between spin and charge, for example, is a non-linear phenomenon which cannot be captured by conventional bosonization. We present here [1], the fully nonlinear dynamics of spin and charge in spin-Calogero model (sCM) a well-known integrable 1D model of quantum spin-1/2 particles interacting through inverse square interaction and exchange. Classical hydrodynamic equations of motion are written for this model in the regime where gradient corrections to the exact hydrodynamic formulation of the theory may be neglected. In this approximation, variables separate in terms of dressed Fermi momenta of the model. Hydrodynamic equations reduce to a set of decoupled Riemann-Hopf equations for the dressed Fermi momenta. We study the dynamics of some non-equilibrium spin-charge configurations for times smaller than the time-scale of gradient catastrophe. We then show [2] how this collective field description allows to calculate correlation functions that cannot be considered with conventional bosonization. In particular, we calculate the Emptiness Formation Probability and interestingly we find that the result can be casted in a form reminiscing of spin-charge separation, which should be violated for such a non-linear effect. We also highlight the connections between sCM, Haldane-Shastry model and λ = 2 spin-less Calogero model. Refs: [1] M. Kulkarni, F. Franchini, A. G. Abanov, Phys. Rev. B 80, 165105 (2009) [2] F. Franchini, M. Kulkarni, Nucl. Phys. B, 825, 320 (2010)

Optical Detection of Spins

Yunpu Li, Q Zhang, X Liu, D Pagliero, and Carlos Merites, Physics Department, CCNY

An important current trend in spin detection is the exploitation of optical detection with high sensitivity and spatial selectivity. By applying the optical pump-probe technique, the electron spin can be generated and monitored. Furthermore, nuclear spin polarization can be enhanced under optical irradiation and be detected locally. We measured the electron spin lifetime of several CdTe epilayer films by time resolved Kerr effect (TRKR), at room temperature down to 80K. As temperature went down, a monotonous increase of the spin lifetime of (100) films has been observed. And we also measured optical nuclear pumping of doped GaAs bulk sample at 4.4K, via time resolved Faraday effect (TRFR).

Quantum Interference and Klein Tunneling in Graphene Heterojunctions

Andrea Young and Philip Kim, Columbia University

The low energy effective theory of ideal graphene is one of massless, chiral particles, implying an absence of backscattering. I will discuss our recent observation of quantum conductance oscillations in extremely narrow graphene heterostructures where a resonant cavity is formed between two electrostatically created bipolar junctions. The experiment reveals that p-n junctions have a collimating effect on transmitted carriers, leading directly to the observation of resonant oscillations despite the largely diffusive carrier dynamics. The oscillatory part of the conductance is insensitive to electrons which scatter in the junctions, making them a novel probe of the ballistic physics of graphene at the Dirac point. Magnetooscillations show a phase shift at finite magnetic field characteristic of reflectionless scattering at normal incidence, or "Klein Tunneling".

Carbon Nanofilms on Diamond for Sensor Applications

Vivek Kumar, Alexander Zaitsev, Alexandra Bergman and Anshel Gorokhovsky, College of Staten Island, The City University of New York

Graphitization of diamond surface at high temperature in high vacuum and in inert atmosphere, combined with plasma etching was used to obtain conductive carbon nano-films on diamond substrates. Characterization of the films was done by electrical measurements, Raman spectroscopy and Atomic Force Microscopy. As-grown films were about 10 - 90 nm thick (depending upon the vacuum level and the temperature), which were further reduced layer-by-layer (nano-shaving) to a thickness of a few interatomic distances by controlled plasma treatment. The grown carbon nano-films were amorphous in nature and showed significant temperature and chemical sensitivity in the conductance

"Additional abstracts coming soon."

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