Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D15: Invited Session: Quantum Criticality at the Superconductor-Insulator Transition and the Higgs Mode |
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Sponsoring Units: DCMP Chair: Subir Sachdev, Harvard University Room: 008B |
Monday, March 2, 2015 2:30PM - 3:06PM |
D15.00001: Dynamical Conductivity near the Superconductor to Insulator Critical Point Invited Speaker: Assa Auerbach I review our results [1--5] for the dynamical complex conductivity near the two dimensional superconductor to insulator transition. The relativistic field theory is relevant to ordered Jospehson junction arrays and bosons trapped in an optical lattice. In the superconducting phase, the real conductivity reveals the Higgs mode mass as a threshold, with a weak $\omega^5$ sub-gap tail. The imaginary conductivity measures the critically vanishing superfluid density. In the insulator phase, the real conductivity has a sharp threshold at the Mott (charge) gap, and the imaginary conductivity goes as $-C \omega$, where the capacitance $C$ measures the inverse of the {\em vortex superfluid density}, which vanishes at the critical point. For layered, short coherence length (bosonic), superconductors, the Higgs threshold is raised by the inter-plane plasmon energy. [1] N. H. Lindner and A. Auerbach, Phys. Rev. B81, 054512, (2010); [2] D. Podolsky, A. Auerbach, D. P. Arovas, Phys. Rev. B 84, 174522 (2011); [3] S. Gazit, D. Podolsky, and A. Auerbach, Phys. Rev. Lett. 110, 140401 (2013); [4] S. Gazit, D. Podolsky, A. Auerbach, and D. P. Arovas, Phys. Rev. B 88, 235108 (2013); [5] S. Gazit, D. Podolsky, A. Auerbach, arXiv:1407.1055, Phys. Rev. Lett. in press. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:42PM |
D15.00002: Quantum critical dynamics without quasiparticles Invited Speaker: William Witczak-Krempa Understanding the real-time behavior of quantum systems without long-lived excitations (quasiparticles) constitutes a challenging problem. I'll discuss recent progress concerning the dynamics of quantum critical systems, in which quantum fluctuations destroy quasiparticles. I'll show concrete results stemming from an interdisciplinary approach combining field theory, quantum Monte Carlo simulations, and the holographic gauge/gravity duality (AdS/CFT). In particular, experimental predictions will be made regarding the charge response near the superfluid-insulator quantum critical transition of bosons on a 2d lattice. Extensions to other observables and universality classes will be discussed.\\[4pt] [1] W. Witczak-Krempa, E. S. Sorensen, S. Sachdev, Nature Physics 10, 361 (2014)\\[0pt] [2] E. Katz, S. Sachdev, E. S. Sorensen, W. Witczak-Krempa, arXiv:1409:3841 [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 4:18PM |
D15.00003: Evolution of the electronic spectral function and dynamical conductivity across the disorder-tuned superconductor-insulator transition Invited Speaker: Nandini Trivedi I will discuss the behavior of the single particle electronic spectral function, the bosonic (pair) spectral function $P(\omega)$, and the dynamical conductivity $\sigma(\omega)$ across the superconductor-insulator transition (SIT) calculated using quantum Monte Carlo simulations [1]. The transition is driven by tuning the charging energy relative to the Josephson coupling or by varying the degree of disorder. We identify a prominent Higgs mode in the superconductor, and characteristic energy scales in the insulator, that vanish at the transition due to enhanced quantum phase fluctuations, despite the persistence of a robust fermionic gap across the SIT [2]. Disorder leads to increased absorption at low frequencies compared to the SIT in a clean system. Disorder also expands the quantum critical region, due to a change in the universality class, with an underlying T=0 critical point. Obtaining the conductivity at the transition has been problematical because of analytic continuation of numerical data. We propose a well-defined integrated low-frequency conductivity that can be reliably estimated and discuss its universality.\\[4pt] [1] ``Dynamical conductivity across the disorder-tuned superconductor-insulator transition,'' M. Swanson, Y. L. Loh, M. Randeria, and N. Trivedi, Phys. Rev. X \textbf{4}, 021007 (2014).\\[0pt] [2] ``Single- and two-particle energy gaps across the disorder-driven superconductor-insulator transition,'' K. Bouadim, Y. L. Loh, M. Randeria, and N. Trivedi, Nat. Phys. \textbf{7}, 884 2011). [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:54PM |
D15.00004: Emergence of nanoscale inhomogeneity and finite frequency superfluid response in disordered superconductors Invited Speaker: Pratap Raychaudhuri The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using a combination of low-temperature scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking the superconducting state as a function of temperature we show that these superconducting domains persist across the bulk superconducting transition, T$_{c}$, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system. These results along with complementary measurements of the superfluid stiffness at microwave frequencies underpins the importance of phase fluctuations in strongly disordered s-wave superconductors. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:30PM |
D15.00005: The Higgs Mode in Disordered Superconductors Close to a Quantum Phase Transition Invited Speaker: Aviad Frydman The Higgs theory, which generates mass for elementary particles, was inspired by screening of magnetic fields in superconductors. The same theory also predicts an amplitude mode whose high-energy-physics analogue is the famous Higgs particle. It is somewhat disappointing that in superconductors, the Higgs-amplitude mode has not yet been observed, partially because it can rapidly decay into unpaired electrons. Nevertheless, recent theories show that if the Higgs mass could be softened below the pairing gap it should be visible in two dimensions. Such conditions can be met by tuning a superconducting film towards a quantum critical point (QCP). I will report on spectroscopic studies in the terahertz frequency regime of thin superconducting films for which the superconductor to insulator transition (SIT) is tuned by disorder. Tunneling spectroscopy determines the pairing gap $2\Delta$ which remains finite on both sides of the SIT. In contrast, the threshold frequency for dynamical conductivity, which in BCS theory is associated with the gap, vanishes critically toward the SIT. The excess optical spectral weight below $2\Delta$ is identified as an unambiguous observation of the Higgs mode in a superconductor. [Preview Abstract] |
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