Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U9: Fluctuations, Phase Transitions and the Polar Kerr Effect |
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Sponsoring Units: DCMP Chair: Jeff Tallon, Industrial Research Ltd., New Zealand Room: Morial Convention Center RO7 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U9.00001: Fluctuations, mean-field $T_c$ and energy gaps in cuprate superconductors Jeffery Tallon, James Storey, John Loram We have carried out an analysis of Gaussian fluctuations about $T_c$ in the specific heat of (Y,Ca)Ba$_2$Cu$_3$O$_{7-x}$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The analysis employs a full ARPES derived dispersion, including the pseudogap. This enables us to calculate the doping dependence of the mean-field transition temperatures, $T_c^{mf}$, in the absence of fluctuations. The values lie well above $T_c$ especially for lower doping where $T_c^{mf}$ is trending towards 180K. As a result, while the observed $T_c$ follows the well-known parabolic doping dependence, the values of $T_c^{mf}$ decrease monotonically with doping along with the superconducting gap parameter, $\Delta_0$, such that $2\Delta_0/k_BT_c = 5$. Using this result we offer explanations for a number of anomalous observations. The line $T_c^{mf}(p)$ should not however be confused with the $T^*(p)$ line which is associated with the pseudogap and falls abruptly to zero at critical doping ($p_ {crit}$=0.19 holes/Cu). [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U9.00002: Universal values for the static and dynamic critical exponents in thin-film and bulk crystalline YBCO C.J. Lobb, Su Li, Hua Xu, M.C. Sullivan, K. Segawa, Yoichi Ando, S.M. Anlage Many researchers have used scaling of current vs.\ voltage curves to study the normal-superconducting phase transition of the high-temperature superconductors, searching for the static and dynamic critical exponents; however, there is little consensus among experimentalists as to the values of the exponents. We have studied this phase transition in optimally-doped YBa$_2$Cu$_3$O$_{7-\delta}$ thin films and bulk crystals. We consistently find $z=1.5\pm0.2$ for the dynamic critical exponent in films (when finite-size effects are taken into consideration) and in crystals (where there are no finite size effects). We also find for the static critical exponent $\nu=0.68\pm0.1$ for crystals and $\nu=0.63 \pm 0.1$ for films. The failure to account for finite-thickness effects in thin films may account for the wide ranges of values for $\nu$ and $z$ previously reported in the literature. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U9.00003: Dynamic critical behavior of YBCO thin films/crystals Hua Xu, Su Li, Christopher Lobb, Steven Anlage The zero-field phase transition of high $T_c$ superconductors has been studied by a number of techniques. However transport properties (such as the conductivity) which probe the dynamics near $T_c$ are less explored, and a wide range of critical exponents have been reported experimentally. We studied fluctuation effects of YBa$_2$Cu$_3$O$_{7-\delta}$ thin films around $T_c$ by doing frequency-dependent microwave conductivity and DC nonlinearity measurements. The length scales involved in the measurements and their influence on the fluctuation conductivity have been examined systematically, and this helps to clarify the historical discrepancy between experimental results and scaling predictions. Our results give a dynamical scaling exponent $z=1.55\pm0.15$, which indicates the superconducting to normal phase transition of high-$T_c$ materials likely belongs to the model E-dynamics. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U9.00004: Two-Dimensional Superconducting Fluctuations in Stripe-Ordered La$_{2-x}$Ba$_{x}$CuO$_{4}$ Qiang Li, Markus Hucker, Genda Gu, Alexei Tsvelik, John Tranquada Recent spectroscopic observations of a d-wave-like gap in stripe-ordered La$_{2-x}$Ba$_{x}$CuO$_{4}$ with x=1/8 have led us to critically analyze the anisotropic transport and magnetization properties of this material. The data suggest that concomitant with the spin ordering is an electronic decoupling of the CuO$_{2}$ planes. We observe a transition (or crossover) to a state of two-dimensional (2D) fluctuating superconductivity. Thus, it appears that the stripe order in La$_{2-x}$Ba$_{x}$CuO$_{4}$ frustrates three-dimensional superconducting phase order, but is fully compatible with 2D superconductivity and an enhanced $T_{c}$ --[Ref. Q. Li, et al., PRL \textbf{99, }067001 (2007)] [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U9.00005: Disorder-induced quantum critical point in an anisotropic gap superconductor Victor Galitski We consider an inhomogeneous anisotropic gap superconductor in the vicinity of the quantum critical point, where the transition temperature is suppressed to zero by disorder. Starting with the BCS Hamiltonian, we derive the Ginzburg-Landau action for the superconducting order parameter. It is shown that the critical theory corresponds to the marginal case in two dimensions and is formally equivalent to the theory of an antiferromagnetic quantum critical point, which is a quantum critical theory with the dynamic critical exponent, z=2. This allows us to use a parquet method to calculate the non-perturbative effect of quantum superconducting fluctuations on thermodynamic properties. We also discuss mesoscopic disorder fluctuations, which lead to the spatial variations of the local pairing temperature and the formation of superconducting islands above the mean-field transition. This disorder-induced Griffiths phase is described by a network of superconducting islands and metallic regions with a strongly suppressed density of states due to superconducting fluctuations. We argue that the phenomena associated with mesoscopic disorder fluctuations may also be relevant to high-temperature superconductors, in particular, to recent STM experiments, where gap inhomogeneities have been explicitly observed. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U9.00006: Fermi arcs in phase fluctuating d-wave superconductors Ehud Altman, Erez Berg One of the most puzzling aspects of the high $T_c$ superconductors is the appearance of Fermi arcs in the normal state of the underdoped cuprate materials. These are loci of low energy excitations covering part of the fermi surface, that suddenly appear above $T_c$ instead of the nodal quasiparticles. Based on a semiclassical theory, we argue that partial Fermi surfaces arise naturally in a d-wave superconductor that is destroyed by thermal phase fluctuations. Specifically, we show that the electron spectral function develops a square root singularity at low frequencies for wave-vectors positioned on the bare Fermi surface. We predict a temperature dependence of the arc length that can partially account for results of recent angle resolved photo emission (ARPES) experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U9.00007: Vortex dynamics and Nernst effect in fluctuating superconductors Daniel Podolsky, Srinivas Raghu, Ashvin Vishwanath, David Huse We present a new method to study the Nernst effect and diamagetism of an extreme type II superconductor dominated by phase fluctuations. We work directly with vortex variables and our method allows us to tune vortex parameters (eg. core energy and number of vortex species). We find that diamagnetic response and transverse thermoelectric conductivity ($\alpha_{xy}$) persist well above the Kosterlitz-Thouless transition temperature, and become more pronounced as the vortex core energy is increased. However, they \textit{weaken} as the number of internal vortex states are increased. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U9.00008: Nernst effect in the phase-fluctuating superconductor InO$_x$ Panayotis Spathis, Herve Aubin, Alexandre Pourret, Cigdem Capan, Kamran Behnia We present a study of the Nernst effect in the amorphous superconductor InO$_x$. The low carrier density in this system implies a weak superfluid stiffness and consequently, strong phase fluctuations of the superconducting order parameter are expected. Measurements as function of temperature show that the Nernst signal evolves continuously through the superconducting transition as previously observed in underdoped cuprates. This contrasts with the abrupt jump expected at a BCS transition, as observed previously in Nb$_{0.15}$Si$_{0.85}$. In the last system, the Nernst signal due to vortices below T$_{c}$ and by Gaussian fluctuations above are clearly distinct [1]. The behavior of the ghost critical field in InO$_x$ points to a correlation length which does not diverge at the Cooper-pair forming temperature $T_c$, a temperature below which the amplitude fluctuations freeze, but phase fluctuations survive. [1] Pourret et al, Nature Physics 2, 683 (2006) [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U9.00009: Bosonic Magnetic Field Driven Superconductor-Insulator Transitions in Amorphous Nano-honeycomb Films M.D. Stewart, Jr., Aijun Yin, J.M. Xu, James M. Valles, Jr. We have observed multiple magnetic field driven superconductor-insulator transitions (SIT) in amorphous Bi films perforated with a nano-honeycomb (NHC) array of holes. The magneto-resistance across the SITs is periodic, with a period $H=H_M=h/2eS$, where $S$ is the area of a unit cell of holes. These transitions are, therefore, boson dominated. In constant field the temperature dependence of the resistance can be parameterized by $R(T)=R_0(H)\exp(T_0(H)/T)$ on both sides of the transition so that the evolution between the superconducting and insulating states is controlled by the vanishing of $T_0\to0$. We compare these data to the thickness driven transition in NHC films and the field driven transitions in unpatterned Bi films, other materials, and Josephson junction arrays. Our results suggest a structural source for similar behavior found in some materials and that despite the clear bosonic nature of the SITs, quasiparticle degrees of freedom likely also play an important part in the evolution of the SIT. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U9.00010: Excess Voltage Noises in the Superconducting Transition in Tin Films Hengsong Zhang, Fulin Zuo We report voltage noise studies in the superconducting transition of thin Tin (Sn) films. Voltage noises are measured as a function of temperature and applied current. The noise spectral power S$^{1/2}$ is peaked during the superconducting transition, with the peak temperature shifted downward from that of dR/dT. Comparison with the dc noise measurement shows the S$^{1/2}$ is much larger with ac current than dc. I-V characteristics and voltage noises are measured simultaneously to correlate the voltage noise with the vortex motion. The noise depends on the voltage with a characteristic $\sqrt V $ dependence for small V, suggesting shot noise nature for the excess noises. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U9.00011: Time-reversal symmetry breaking by a $(d+id)$ density-wave state in underdoped cuprate superconductors Victor Yakovenko, Sumanta Tewari, Chuanwei Zhang, Sankar Das Sarma It was proposed that the $id_{x^2-y^2}$ density-wave state (DDW) may be responsible for the pseudogap behavior in the underdoped cuprates. Here we show that the admixture of a small $d_{xy}$ component to the order parameter of the DDW state breaks macroscopic time reversal symmetry, leading to a non-zero polar Kerr effect. The $d_{xy}$ component breaks the symmetry between the counter-propagating orbital currents of the DDW state, which is ultimately responsible for the non-zero Kerr signal. From the results of the recent experiments by Xia \textit{et al.}, arXiv:0711.2494, we deduce that the amplitude of the $d_{xy}$ admixture is quite small compared to the ordered DDW component. \\ Reference: S. Tewari, C. Zhang, V. M. Yakovenko, S. Das Sarma, arXiv:0711.2329. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U9.00012: Kerr Effect in Superconductor. Sang Boo Nam A magnetic field H is to make the time reversal symmetry of the system be broken. Using the formulation [1], neglecting H dependence of $\Delta $(T) and for the pair cyclotron frequency $\Omega $ = (2e/2m)H/c less than the photon frequency 2$\pi $ c/$\lambda $, the Kerr angle is obtained as $\theta _{{\rm K}}$(T) = $\theta _{K}$(0)[$\Delta $(T)/$\Delta $(0)]tanh [$\Delta $(T)/2k$_{B}$T], where $\theta _{{\rm K}}$(0) = A $\lambda ^{3} \quad \Omega $ / (8 $\pi ^{3}$ c N $\lambda _{L}^{2})$, A = (3$\lambda $/4, L)/$\xi _ {BCS}$ in the (non-local, local) limit, with mean free path length L and BCS coherence length $\xi _{BCS}=\hbar $v$_{F}$/$\pi \Delta $(0). N = (n-1) n (n+1) with index of refraction n. For Sr2RuO4 [2], $\lambda $ = 1550 nm, L = 1 $\mu $m, v$_{F}$ = 100Km/s, n=3 and the London penetration depth length $\lambda _{L}$ = 3 $\mu $m [3], T$_{C}$ = 1.5 K . In the strong coupling case [4], $\Delta $(0) = 2T$_{C}$. The effective H is sum of the external applied and internal (by pair current) magnetic fields, to maintain the fluxoid quantization. After cooling a sample in the external magnetic field, turning it off, before warming a sample, is not necessary to make H vanish, since the pair current was set in a sample during cooling it. Then, H$_{C2}$ = 750 Gauss [3], in the normal vortex core, is considered as H. With all values of parameters given above, we obtain $\theta _ {K}$(0) = (44, 38) nrad in the (non-local, local) limit in satisfactory agreement with data of 65nrad [2]. The fluxoid quantization makes the Kerr angle same within a range of the external applied magnetic fields. [1] Nam, PR. \textbf{156}, 470, 487 (67). [2] Xia et al., PRL. \textbf{97}, 167002 (06). [3] Mackenzie et al., RMP.\textbf{ 75}, 657 (2003). [4] Nam, PL. \textbf {A193}, 111 (94); (E) ibid\textbf{. A197}, 458 (95). [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U9.00013: Possible Weak Ferromagnetism in Time Reversal Violating State of Underdoped Cuprates Vivek Aji, Chandra Varma Recent polar Kerr effect measurements on underdoped YBCO have provided evidence for time reversal symmetry breaking near the pseudogap temperature. These results are consistent wih the existance of a ferromagnetic moment of order 10\^{}(-4) Bohr magneton along the c-axis. We discuss the conditions for the possible occurrence of ferromagnetism with moments perpendicular to the Copper-Oxideplanes, accompanying the loop current orbital magnetic order, in the underdoped phase of Cuprates. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U9.00014: Electromagnetic response of a chiral p-wave superconductor Rahul Roy, John Berlinsky, Catherine Kallin We study the response of a time-reversal symmetry breaking $p_x + i p_y$ superconductor to an external electromagnetic wave and calculate the ac Hall conductivity, paying particular attention to gauge invariance, the effects of the topological Chern Simons term in the effective action and contributions from collective modes. We also consider the effects of a long range interaction such as the Coulomb interaction on the collective modes and the Hall conductivity. These results will also be discussed in the context of the recent Kerr rotation experiments on Strontium Ruthenate by Xia et al. [1] \newline \newline [1] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, \textit{Phys. Rev. Lett.} \textbf{97}, 167002 (2006) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U9.00015: Absence of magnetic field (B $\leq$ 33~T) induced effects in the mid-infrared properties of La$_{2-x}$Sr$_{x}$CuO$_4$ films with 0~$\leq$~x~$\leq$~0.16 S.V. Dordevic, L.W. Kohlman, A. Gozar, G. Logvenov, I. Bozovic, L.C. Tung, Y.-J. Wang We have performed magneto-transmission measurements on a series of La$_{2-x}$Sr$_{x}$CuO$_4$ films with magnetic fields up to 18 Tesla. Studied samples include doping levels x= 0, 0.1, 0.3, 0.45, 0.6, 0.8, 0.10. In addition, an optimally doped film (x= 0.16) was studied in magnetic fields as high as 33 Tesla, both below and above its superconducting critical temperature T$_c$=\,41\,K. In neither of the studied samples we could detect any field-induced changes of transmission in the mid- infrared energy range (between about 1000 and 3500 cm$^{-1}$). We discuss how these observations can enhance our current understanding of medium energy range excitations in the cuprates, and their relation to high temperature superconductivity. [Preview Abstract] |
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