Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F45: Normal State Properties of Superconductors |
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Sponsoring Units: DCMP Chair: Andrew May, Oak Ridge National Lab Room: LACC 505 |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F45.00001: Magnetic Field-Temperature Scaling in the Magnetotransport Properties of Unconventional Superconductors. James Analitis , Ian Hayes , Nikola Maksimovic The transport proeprties of unconventional superconductors and other quantum critical metals deviates strongly from the conventional theory of transport in metals. The dramatic T-linear temperature dependence observed in cuprate, iron-pnicitde and heavy fermion superconductors is thought to be a signature of quantum critical physics and is often used as evidence for the proximity to a quantum critical point. We discover that this same physics may be manifest in the magneto-transport properties of these materials. This talk focuses on a study of the longitudinal and transverse magnetoresistance in unconventional superconductors near putative quantum critical points. We reveal a new manifestation of this physics, leading to an unusual scaling relationship in field and temperature. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F45.00002: Optical conductivity of a two-dimensional metal near a quantum-critical point: \\
the status of the ``extended Drude formula'' Andrey Chubukov , Dmitrii Maslov The optical conductivity of a metal near a quantum critical point (QCP) is expected to depend on frequency not only via the scattering time but also via the effective mass, which acquires a singular frequency dependence near a QCP. On the other hand, the quasiparticle residue $Z$, no matter how singular, does not appear in the conductivity as the latter probes quasiparticles rather than bare electrons. In local theories of QCPs, however, the ratio of band and renormalized masses, $m^*/m_b$, coincides with $1/Z$, and it is not straightforward to separate the two quantities.In this work, we use a direct diagrammatic approach and compute the optical conductivity, $\sigma' (\Omega)$, |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F45.00003: Normal-State Optical Conductivity of the Cuprates from Tightly Bound Pre-Formed Pairs Long Zou , Wei Ku , Zhi Wang We demonstrate that the main low-energy structures of the normal-state optical conductivity of the cuprates can be described naturally from the picture of tightly bound pre-formed pairs. Specifically, we show that the observed onset at ~100meV (and its long tail extending toward zero energy) originate from inter-band excitations near the van Hove singularities of the pre-formed pairs. Without using any free parameter, our results reproduce not only the correct energy of the onset, but also the non-monotonic trends of its temperature and doping dependence. Our study reveals the bosonic nature of the low-energy carriers and provides a strong support for the picture of tightly bound preformed pairs in describing the high-temperature superconductivity and other low-energy physics of the cuprates. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F45.00004: Nematic quantum-critical fluctuations near the pseudogap critical point in high-T_{c} cuprate superconductors Kousuke Ishida , Suguru Hosoi , Yuki Teramoto , Tomohiro Usui , Yuta Mizukami , Kenji Itaka , Yuji Matsuda , Takao Watanabe , Takasada Shibauchi In cuprate high-T_{c} superconductors, enigmatic pseudogap state is considered to be a clue to its pairing mechanism. Recently electronic nematicity, characterized by four-fold rotational symmetry breaking in CuO_{2} plane, has emerged as a key signature inside pseudogap region. Here we report on elastoresistance measurements in Pb-substituted Bi_{2}Sr_{2}CaCu_{2}O_{8+δ}, which allow us to evaluate the nematic susceptibility from the change in in-plane resistivity induced by uniaxial strain. The nematic susceptibility shows Curie-Weiss-like enhancement toward pseudogap temperature T^{*}, evidencing a second-order phase transition with rotational symmetry breaking. Near pseudogap critical point the Weiss temperature changes its sign and the magnitude of nematic susceptibility is strongly enhanced. This indicates the nematic quantum-critical fluctuations in cuprate superconductors. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F45.00005: Singular Density Fluctuations in the Strange Metal Phase of a Copper-Oxide Superconductor Ali Husain , Matteo Mitrano , Sean Vig , Anshul Kogar , Melinda Rak , Samantha Rubeck , Jõrg Schmalian , Bruno Uchoa , John Schneeloch , Ruidan Zhong , Genda Gu , Peter Abbamonte The “strange metal” is a poorly understood phase of matter characterized by a lack of well-defined quasi-particles, violating a core tenet of Fermi Liquid theory. In ordinary metals, the interactions between quasi-particles give rise to propagating collective charge excitations; however, the analogous excitations of the strange metal are unknown. Here, we show measurements of the charge susceptibility χ(q,ω) across the phase diagram of the strange metal, Bi_{2.1}Sr_{1.9}CaCu_{2}O_{8+x} (Bi-2212) using momentum-resolved inelastic electron scattering (M-EELS). Surprisingly, we find an energy- and momentum-independent continuum of scale-invariant charge fluctuations extending to 1eV, instead of dispersing plasmons. This continuum is temperature-independent at optimal doping, but undergoes a significant redistribution of spectral weight at low temperatures away from optimal doping. Our results indicate that the strange metal phase arises from an anomalous type of charge dynamics, especially sensitive to doping, that is not described by any known theory of quantum matter. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F45.00006: Shot noise in La_{2-x}Sr_{x}CuO_{4} tunnel junctions Panpan Zhou , Anthony Bollinger , Xi He , Ivan Bozovic , Douglas Natelson We report shot noise measurement in La_{2-x}Sr_{x}CuO_{4} (LSCO) tunnel junctions with La_{2-x}Sr_{x}CuO_{4} (LCO) tunnel barriers. The LSCO tunnel junctions are grown via molecular beam epitaxy, with different dopings of LSCO ranging from underdoped to optimally doped. Shot noise, which originates from the discrete nature of charge carriers, carries information about the quasiparticles’ effective charge, and correlations in tunneling events. LSCO is a strongly correlated material where the electron-electron interaction is important. LSCO’s normal state is a “strange metal” that may exhibit unconventional quasiparticles at optimal doping, or a pseudogap at underdoping, while LCO is the parent antiferromagnetic Mott insulator. Shot noise specta in LSCO at various temperatures and dopings could be useful in revealing the quasiparticle evolution in different phases of cuprates and provide insights to help understand the non-Fermi liquid behavior in the normal state of these correlated systems. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F45.00007: Fluctuating charge-order correlations in the cuprate superconductor N2-xCexCuO4 Eduardo Da Silva Neto , Matteo Minola , Biqiong Yu , Wojciech Tabis , Martin Bluschke , Hakuto Suzuki , Yangmu Li , Guichuan Yu , Kurt KUMMER , Nicholas Brookes , Matthieu le Tacon , Martin Greven , Bernhard Keimer , Andrea Damascelli Charge order has now been observed in several cuprate high-temperature superconductors. However, it remains unclear if the charge order is purely static or whether it also features dynamic correlations. We report a resonant inelastic x-ray scattering (RIXS) experiment on the electron-doped cuprate Nd_{2-x}Ce_{x}CuO_{4} (NCCO) that demonstrates the existence of charge order fluctuations. We dissect the characteristic energy scales associated with the charge order peak in momentum-space and discuss its connection to magnetic fluctuations. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F45.00008: Suppression of electronic specific heat in the low-temperature normal state of cuprates: a possible scenario from coupling to superconducting fluctuations Laimei Nie , Greg Bentsen , Erez Berg Recent measurements on the model cuprate YBCO revealed that the electronic specific heat in the high-field normal state is significantly smaller than the value anticipated from the full band Fermi surface, despite the fact that the state displays properties expected from a conventional Fermi liquid. To explore theoretically the possible mechanism of this entropy depletion, we study how the superconducting fluctuations (a residual of the superconducting order in zero field) may play a key role in the process. We propose a low-energy effective field theory consisting of electrons coupled to bosonic superconducting fluctuations, and investigate the temperature dependence of the specific heat by using a controlled large-N approach. We also discuss some alternative explanations to the puzzle of missing specific heat in the low-temperature normal state of YBCO. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F45.00009: Charge and current orders in the cuprates: implications from spin-fermion model with overlapping hot spots. Pavel Volkov , Konstantin Efetov Experiments carried over the last years on the underdoped cuprates suggest the presence of a variety of symmetry-breaking phenomena in the pseudogap phase. Charge-density waves, breaking of C_{4} rotational symmetry as well as time-reversal symmetry breaking have all been observed in several cuprate families. In this regard, theoretical models where multiple non-superconducting orders emerge are of particular interest. We consider the spin-fermion model in the regime where adjacent 'hot spots' on the Fermi surface overlap and merge due to the finite antiferromagnetic correlation length. Focusing on the particle-hole instabilities we obtain a rich phase diagram with the chemical potential and the Fermi surface curvature in the antinodal regions being the control parameters. We find evidence for d-wave Pomeranchuk instability, d-form factor charge density wave as well as commensurate and incommensurate staggered bond current phases. The latter are found to be promoted by the curvature. Considering the appropriate parameter range for the hole-doped cuprates, we discuss the possible relation of these results to the pseudogap phase. |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F45.00010: Metal-insulator transition and enhanced superconducting pairing fluctuation in copper oxides induced by apex displacements Swagata Acharya , Cedric Weber , Evgeny Plekhanov , Dimitar Pashov , Arghya Taraphder , Mark van Schilfgaarde In this work, we use a recently developed first principles method to explore how |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F45.00011: Effective model for the magnon-mediated attraction in the parent cuprates Mona Berciu , Mirko Moeller , Clemens Adolphs , George Sawatzky Using a recently developed method to study magnon-mediated interactions between two holes doped in a cuprate parent compound, we identify four different types of processes which lead to magnon-mediated attraction and show that one of them is dominant. We introduce an effective model for this interaction, allowing us to estimate its coupling strength in the parent cuprates and the critical coupling strength needed to form a bound state. We find that the two values are of the same order of magnitude, indicating that holes in the parent cuprates are close to being bound into pre-formed pairs. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F45.00012: Linear Magnetoresistance in a Vortex Gas Samuel Lederer , Senthil Todadri Recent experiments on the high-T_{C} cuprate LSCO have revealed a substantial regime of doping, temperature, and magnetic field in which the magnetoresistance is linear in the applied field. This magnetoresistance adds on top of the T-linear resistivity, suggesting an interpretation, via Matthiessen's rule, as a field-dependent contribution to the quasiparticle scattering rate. We consider the possibility that the normal state in these experiments can be described as a vortex gas, in which local pairing correlations are modulated by vortices of finite but long lifetime, and inter-vortex interactions are negligible. In such a state, the vortex contribution to the electronic scattering rate (and therefore the resistivity) is proportional to the density of vortices, i.e. the magnetic field. We calculate the transport scattering rate in Born approximation for a simple model of a vortex gas with parameters relevant to LSCO, and find linear magnetoresistance with a coefficient in semi-quantitative agreement with experiments. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F45.00013: Violation of f-sum Rule with Generalized Kinetic Energy Kridsanaphong Limtragool , Philip Phillips Motivated by the normal state of the cuprates in which the integrated spectral weight of the optical conductivity or optical sum increases faster than a linear function of the particle density, we derive an f-sum rule for a system in which the kinetic energy operator in the Hamiltonian is a general function of the momentum squared. Such a kinetic energy arises in scale invariant theories and can be derived within the context of holography. Our derivation of the f-sum rule is based on the gauge couplings of a non-local Lagrangian in which the kinetic operator is a fractional Laplacian of order α. We find that the f-sum rule in this case deviates from the standard linear dependence on the particle density. We find two regimes. At high temperatures and low densities, the optical sum is proportional to nT^{(α-1)/α} where T is the temperature. At low temperatures and high densities, the optical sum is proportional to n^{1+2(α-1)/d} with d being the number of spatial dimensions. The result in the low temperature and high density limit, when α < 1, can be used to qualitatively explain the behavior of the effective number of charge carriers in the cuprates at various doping concentrations. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F45.00014: Quantum oscillations order in boson+fermion dimer models Garry Goldstein , Nigel Cooper , Claudio Chamon , Claudio Castelnovo We study the quantum oscillations for the cuprate superconductors in the underdoped pseudogap regime. We represent the system as a fermion+boson dimer model. We reduce the problem to an effective problem of free fermions hopping in an external field (which is not identical to a static magnetic field). We find the semiclassical equations of motion for these fermions. We find that the dimers undergo quantum oscillations in a strong magnetic field with the oscillation magnitude obeying the Lifshitz Kosevich formula. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F45.00015: The Theory of the Bose Metal Anthony Hegg , Wei Ku The existence of finite conductivity in low temperature Bosonic systems has long been suspected due to the behavior of, for example, the Cuprates. Several attempts to develop a theory for such behavior have been made, but all have either effectively required arbitrarily many free parameters or resulted in highly unstable states upon including interactions or disorder. Starting with a single-band dispersion exhibiting a line degeneracy in the ground state, we turn on weak quartic interactions and calculate the resulting many-body ground state and low energy dispersion. We find that not only does the degeneracy persist but the resulting system has finite conductivity at finite temperature and an absence of the Meissner effect. Adding weak on-site bounded uniform disorder does not destroy the degeneracy or the resulting behavior. This work constitutes the first and only few-parameter theory for metallic behavior of low temperature Bosons that is robust against interactions and disorder. Therefore, it is the best known minimal model for the Bose metal, and it identifies the metallic behavior as arising from the simple existence of a line degeneracy minimum in the band structure of a system of Bosons. |
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