Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z33: Fluctuation Phenomena |
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Sponsoring Units: DCMP Chair: Ruslan Prozorov, Ames Laboratory Room: 403 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z33.00001: Fluctuations of the superconducting order parameter as an origin of the Nernst effect Karen Michaeli, Alexander Finkel'stein We show that the strong Nernst signal observed recently in amorphous superconducting films far above the critical temperature is caused by the fluctuations of the superconducting order parameter. We demonstrate a striking agreement between our theoretical calculations and the experimental data at various temperatures and magnetic fields. Besides, the Nernst effect is interesting not only in the context of superconductivity. We discuss some subtle issues in the theoretical study of thermal phenomena that we have encountered while calculating the Nernst coefficient. In particular, we explain how the Nernst theorem (the third law of thermodynamics) imposes a strict constrain on the magnitude of the Nernst effect. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z33.00002: Interstate switching induced by non-Gaussian noise Lora Billings, Mark Dykman, Ira Schwartz We consider the rate of switching between stable states of a dynamical system driven by a non-Gaussian noise. The problem of the switching barrier is reduced to a variational problem of finding a mechanical action for an auxiliary noise-free system. The emphasis of our analysis is placed on the generic system-independent features of fluctuations induced by Poisson noise. If the system is overdamped, Poisson noise leads to switching only for a certain polarity of pulses. This is qualitatively different from the noise effect on underdamped systems. We study the transition between these types of behavior with varying damping. For systems close to a bifurcation point, the barrier height displays a scaling dependence on the control parameter and on the noise parameters. We study parametric dependence for generic types of bifurcations, such as saddle-node and pitchfork bifurcations. Analytical results are compared with the results of detailed numerical simulations. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z33.00003: Josephson current noise above Tc in superconducting tunnel junctions Alex Levchenko Tunnel junction between two superconductors is considered in the vicinity of the critical temperature. Superconductive fluctuations above Tc give rise to the noise of the ac Josephson current although the current itself is zero in average. As a result of fluctuations, current noise spectrum is peaked at the Josephson frequency, which may be considered as precursor of superconductivity in the normal state. Temperature dependence and shape of the Josephson current noise resonance line is studied for various junction configurations. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z33.00004: Superfluid density anisotropy as a probe of electronic nematic order in cuprate superconductors Benjamin Phillabaum, Yen Lee Loh, Erica Carlson, Karin Dahmen We propose that hysteretic effects in superfluid density anisotropy may be used as a probe of electronic nematic order in cuprate superconductors. Stripes, a unidirectional, nanoscale modulation of electronic charge, are strongly affected by quenched disorder in two-dimensional and quasi-two-dimensional systems. While stripe orientations tend to lock to ma jor lattice directions, dopant disorder locally breaks rotational symmetry. In a host crystal with otherwise C4 rotational symmetry, stripe orientations in the presence of quenched disorder map to the random field Ising model. We use simulations of the random field Ising model to generate ensembles of local stripe orientations, and then further simulate the effects of such a pattern on the superfluid density within the XY model. We find clear hysteretic effects in the superfluid density anisotropy as a function of applied orienting field. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z33.00005: Current-driven quantum switch M.V. Milosevic, A. Kanda, S. Hatsumi Hatsumi, F.M. Peeters, Y. Ootuka As a key component of a ballistic quantum switch proposed by Mel'nikov and Vinokur [Nature {\bf 415}, 60 (2002)], we realize the current-driven {\it giant-vortex splitting} in a mesoscopic superconducting square, in a given perpendicular magnetic field. We also demonstrate the controllable {\it current-induced transitions} between the states with different angular momenta, which provide the stepwise behavior of the up-down sample conductance as a function of applied current (not field). Theoretical simulations using time-dependent Ginzburg-Landau theory are fully corroborated by transport measurements, where vortex states are monitored using the small-tunnel-junction technique. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z33.00006: In search for the superconducting spin-switch: Magnetization induced resistance switching effects in La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/Yba$_{2}$Cu$_{3}$O$_{7-\delta }$ bi- and trilayers Menno Veldhorst, Maarten van Zalk, Alexander Brinkman, Hans Hilgenkamp, Jan Aarts The influence of the magnetization on the superconducting $T_{c}$ in bi- and trilayers consisting of the half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ and the high-temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ is studied. Interfaces that are partly oriented within the crystallographic ab-plane are achieved by tilted epitaxial growth on SrTiO$_{3}$ (305) substrates. Sharp magnetization switching behavior is observed in the (305) oriented structures, due to the uniaxial magnetic anisotropy. At temperatures close to $T_{c}$, resistance jumps are induced by magnetization switching. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor, rather than spin-switch or spin-accumulation effects. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z33.00007: Theory of Low-Temperature Hall Effect in Stripe-Ordered Cuprates Jie Lin, Andrew Millis We investigate the effect of static anti-phase stripe order on the weak-field Hall effect of electrons with dispersion appropriate to the high T$_c$ cuprates. We first consider the cases where the magnitudes of the spin and charge stripe potentials are smaller than or of the same order as the bandwidth of electrons. In a model with only spin stripe potential, and at carrier concentrations appropriate to hole-doped cuprates, the calculated $R_H$ shows sign change as increasing the stripe potential, in semi-quantitative agreement with data. In a charge-stripe-potential-only model, $R_H$ increases as the charge stripe potential increases, with no sign change occurring. In a model with both stripe potentials, $R_H$ may be enhanced or may change sign. We also consider the case in which the magnitudes of the stripe potentials are much larger than the bandwidth, where analytical results can be obtained. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z33.00008: Emergence of Particle-Hole Symmetry near Optimal Doping in High-Temperature Copper Oxide Superconductors Shiladitya Chakraborty, Dimitrios Galanakis, Philip Phillips High-temperature copper oxide superconductors (cuprates) display unconventional physics when they are lightly doped whereas the standard theory of metals prevails in the opposite regime. For example, the thermoelectric power changes sign abruptly near optimal doping in a wide class of cuprates, a stark departure from the standard theory of metals in which the thermopower vanishes only when one electron exists per site. We show that this effect arises from proximity to a state in which particle-hole symmetry is dynamically generated. The operative mechanism is dynamical spectral weight transfer from states that lie at least 2eV away from the chemical potential. The emergence of this symmetry close to optimal doping points to pairing in the cuprates being driven by high-energy electronic states. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z33.00009: Exact two-band model of Cu-O planes with charge stripes and plaquettes Stellan \"Ostlund, Mats Granath A standard model of the Cu-O planes of high $T_c$ superconductors suggests the relevance of a three-band model of electrons with strong Coulomb repulsion. Particularly dominant is the copper site interaction and charge fluctuations are most strongly suppressed on these sites. We simplify this model further by {\it completely} suppressing the charge fluctuations on the copper sites and replacing the copper spins by a spin texture that couples to the local hopping. The resulting generic two-band model of electrons in is then studied for various spin textures on the copper atoms. For general values of the effective hopping parameters, the low energy eigenstates strongly favor charge stripe and plaquette ordering, with a complex multiply connected Fermi surface with the possibility of both pockets and open orbits coexisting. Particularly striking is the emergence of multiparticle ground states that are both delocalized and still effectively minimize nearest neighbor density correlations. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z33.00010: Two-order-parameter theory of magnetism and superconductivity Victor Galitski, Tudor Stanescu Starting with a microscopic interacting electron Hamiltonian, we derive a self-consistent two-order-parameter theory to describe a general case of competing or co-existing magnetic and superconducting instabilities. This is achieved by splitting the initial interaction in two different channels and weighting each channel with an auxiliary field with a non-linear constraint. The double Hubbard-Stratonovich transform leads to a model similar to that in gauge theories. We analyze the resulting theory and argue that generally the magnetic and superconducting fluctuations are equally important and should be treated on equal footing. We discuss the general criteria of the two transitions occurring at similar energy scales and discuss the relevance of these results to superconductivity in the heavy fermion compounds and possibly the cuprates. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z33.00011: Two band model for the cuprates Shiu Liu, Steven White We use a numerical canonical transformation approach to derive an effective two-band model for the hole-doped cuprates, which keeps both oxygen and copper orbitals but removes double occupancy from each. A similar model was considered previously by Frenkel, Gooding, Shraiman, and Siggia (PRB 41, number 1, page 350). We compare the numerically derived model with previously obtained analytical results. In addition to the usual hopping terms between oxygens $t_{pp}$ and Cu-Cu exchange terms $J_{dd}$, the model also includes a strong copper-oxygen exchange interaction $J_{pd}$ and a Kondo-like spin-flip oxygen-oxygen hopping term $K_{pdp}$. We use the density matrix renormalization group to study the charge, spin, and pairing properties of the derived model on ladder systems. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z33.00012: Classification of topological insulators and superconductors in three spatial dimensions Shinsei Ryu, Andreas Schnyder, Akira Furusaki, Andreas Ludwig We systematically study topological phases of insulators and superconductors (or superfluids) in 3D. We find that there exist 3D topologically non-trivial insulators or superconductors in five out of ten symmetry classes introduced in seminal work by Altland and Zirnbauer within the context of random matrix theory, more than a decade ago. One of these is the recently introduced $Z_2$ topological insulator in the symplectic (or spin-orbit) symmetry class. We show there exist precisely four more topological insulators. For these systems, all of which are time-reversal invariant in 3D, the space of insulating ground states satisfying certain discrete symmetry properties is partitioned into topological sectors that are separated by quantum phase transitions. Three of the above five topologically non-trivial phases can be realized as time-reversal invariant superconductors, and in these the different topological sectors are characterized by an integer winding number defined in momentum space. When such 3D topological insulators are terminated by a 2D surface, they support stable surface Dirac (Majorana) fermion modes. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z33.00013: Spin-Orbit Scattering and Quantum Metallicity in Ultra-Thin Be Films Philip Adams, Yimin Xiong, Amar Karki, David Young We compare and contrast the low temperature magnetotransport properties of ultra-thin, insulating, Be films with and without spin-orbit scattering (SOS). Beryllium films have very little intrinsic SOS, but by ``dusting'' them with sub-monolayer coverages of Au, one can introduce a well controlled SOS rate. Pure Be films with sheet resistance $R>R_Q$ exhibit a low-temperature negative magnetoresistance (MR) that saturates to the quantum resistance $R_Q=h/e^2$. This high-field {\it quantum metal} phase is believed to represent a new ground state of the system. In contrast, the corresponding negative MR in Be/Au films is greatly diminished, suggesting that, in the presence of strong SOS, the quantum metal phase can only be reached at field scales well beyond those typically available in a low temperature laboratory. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z33.00014: Quantum critical point and van Hove singularity in La$_{2-x-y}$Sr$_x$Nd$_y$CuO$_4$ Ben Mallett, Jeffery Tallon By means of Zn substitution, thermopower and magnetic measurements we locate, distinguish and track the evolution of the pseudogap critical point and the van Hove singularity as a function of Nd content and relate these to pressure dependent effects in the Nd-free compound. The results have generic implications for all HTS cuprates. [Preview Abstract] |
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