Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y25: Superconductivity: Mainly HTSC Theory |
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Sponsoring Units: DCMP Chair: Igor Mazin, Naval Research Laboratory Room: D166 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y25.00001: The underdoped cuprates as fractionalized Fermi liquids Eun Gook Moon, Subir Sachdev We model the underdoped cuprates using fermions moving in a background with local antiferromagnetic order. The antiferromagnetic order fluctuates in orientation, but not in magnitude, so that there is no long-range antiferromagnetism, but a `topological' order survives. The normal state is described as a fractionalized Fermi liquid (FL*), with electron-like quasiparticles coupled to the fractionalized excitations of the fluctuating antiferromagnet. The FL* and its mother state, algebraic charge liquid, reveal interesting features in the underdoped cuprates such as shift of the Fermi pocket center from the magnetic Brillouin zone boundary . Also, with transition to superconductivity, the normal states can explain puzzling experiment data such as a nodal-anti-nodal `dichotomy' identifying characteristics of the two gaps. Implication of our model and extensions are discussed. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y25.00002: Collective Modes in the Loop Ordered Phase of Cuprates Yan He We show that the two branches of collective modes discovered recently in under-doped Cuprates with huge spectral weight are a necessary consequence of the loop-current state. Such a state has been shown in earlier experiments to be consistent with the symmetry of the order parameter competing with superconductivity in four families of Cuprates. We also predict a third branch of excitations and suggest techniques to discover it. Using parameters to fit the observed modes, we show that the direction of the effective moments in the ground state lies in a cone at an angle to the c-axis as observed in experiments. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y25.00003: Unconventional superconductivity in honeycomb lattice Yang Qi, Kai Sun, Zhengcheng Gu, Liang Fu Motivated by results of DMRG and tensor network simulations on doped $t$-$J$ model on honeycomb lattice, we study superconductivity of singlet and triplet pairing in this model. We show that a coexistence of singlet and triplet pairing superconductivity is induced by antiferromagnetic order near half-filling. The superconducting state we obtain is a topological superconductor. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y25.00004: Superconductivity as a Condensate of Collective Cooper Pairs Carlos Ramirez, Chumin Wang Along the last century, the fascinating phenomenon of superconductivity has recurrently been considered as a Bose-Einstein condensation (BEC) of Cooper pairs. However, creation and annihilation operators of the Cooper pairs do not satisfy the bosonic commutation relations [1] and then, the superconductivity theories based on the BEC have a weakness in their foundation. In this work, for the dilute limit we prove the bosonic nature of collective Cooper pairs (CCP), defined as linear combinations of Cooper pairs [2]. This bosonic nature is given rise from their diffuse character on the Cooper pairs, which allows the accumulation of many collective pairs at a single quantum state. Moreover, the superconducting ground state proposed by Bardeen, Cooper and Schrieffer (BCS) can be written in terms of these CCP, leading to a possible BEC theory of superconductivity. Finally, the energy spectra of CCP are calculated for a mixture of bosons and fermions, which permit to determine the condensation critical temperature as well as other thermodynamic properties of the CCP condensate. \\[4pt] [1] J. Bardeen, L.N. Cooper and J.R. Schrieffer, Phys. Rev. 108, 1175 (1957). \\[0pt] [2] C. Ramirez and C. Wang, Phys. Lett. A 373, 269 (2009). [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y25.00005: Self-consistent Eliashberg theory, $T_c$, and the gap function in electron-doped cuprates Dhananjay Dhokarh, Andrey Chubukov We consider normal state properties, the pairing instability temperature, and the structure of the pairing gap in electron-doped cuprates. We assume that the pairing is mediated by collective spin excitations, with antiferromagnetism emerging with the appearance of hot spots. We use a low-energy spin-fermion model and Eliashberg theory up to two-loop order. We justify ignoring vertex corrections by extending the model to $N >>1$ fermionic flavors, with $1/N$ playing the role of a small Eliashberg parameter. We argue, however, that it is still necessary to solve coupled integral equations for the frequency dependent fermionic and bosonic self-energies, both in the normal and superconducting state. Using the solution of the coupled equations, we find an onset of $d-$wave pairing at $T_c \sim 30$ K. To obtain the momentum and frequency dependent $d$-wave superconducting gap, $\Delta ({\vec k}_F, \omega_n)$, we derive and solve the non-linear gap equation. We find that $\Delta ({\vec k}_F, \omega_n)$ is a non-monotonic function of momentum along the Fermi surface, with its node along the zone diagonal and its maximum some distance away from it. We obtain $2\Delta_{\mathrm{max}} (T\rightarrow0) /T_c \sim 4$. We argue that the value of $T_c$, the non-monotonicity of the gap, and $2\Delta_{\textrm{max}}/T_c$ ratio are all in good agreement with the experimental data on electron-doped cuprates. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y25.00006: Spin-space entangled orbitals in a Hartree-Fock scheme predicting the AF and insulator properties of La2CuO4 Alejandro Cabo Montes de oca, Alejandro Cabo-Bizet Its is argued that spin-orbit entangled single particle states in a Hartree-Fock scheme can describe the insulator and antiferromagnetic nature of La2CuO4, as independent particle properties. Therefore, a currently considered as a Mott insulator material, is represented as a Slater one. This curious outcome is not strange if we consider that, strictly speaking, correlation quantities should be defined by the differences between the exact result and the ``best'' Hartree-Fock one. The discussion opens a road for understanding the connections between the successful phenomenological Mott picture and the First Principle (Slater) schemes of calculations. The results also furnish a simple framework for further studying the normal state properties of HTc superconductors. In particular, the microscopic structure of the antiferromagnetic order and the isolator size of the gap in La2CuO4 are both explained as coherent effects coming from the entangled ``spin-orbit'' structure of the single particle Hartree-Fock states. The possibility of the stability of the isolator gap when temperature rises up to the experimental Neel value is argued to be allowed by the same entanglement effect. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y25.00007: Anomalous Isotope Effect in Low and High Tc Superconductors: the contribution of the electronic structure G.L. Zhao Some of the low and high Tc superconductors exhibit an anomalous isotope effect, where the exponent ($\alpha )$ for the isotope effect is much smaller than $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $. We present first-principles calculations of the electronic structures of the selected superconductors, including Zirconium (Zr) and YBa$_{2}$Cu$_{3}$O$_{7 }$(YBCO). The characteristically narrow electron bands around the Fermi levels (E$_{f})$ in these materials suggest that the rapid variations of the densities of states around E$_{f}$, within the range of phonon energy, can have a noticeable effect on the total coupling matrix elements. Such effect may explain the anomalous isotope effect on Tc in these superconductors. The work is funded in part by NSF and the Air Force Office of Scientific Research. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y25.00008: Electronic Specific Heat and Dissipative Viscosity of Hole-Doped Cuprates Partha Goswami We investigate a d-density wave (DDW) mean field model Hamiltonian in the momentum space suitable for the hole-doped cuprates, such as YBCO, in the pseudo-gap phase to obtain the Fermi surface(FS)topologies, including the elastic scattering by disorder potential ($\vert $v$_{0}\vert )$. For the chemical potential $\mu =-$ 0.27 eV (at 10{\%} doping level), and $\vert $v$_{0}\vert \quad \ge \quad \vert $t$\vert $ (where $\vert $t$\vert $ = 0.25 eV is the first neighbor hopping), at zero/non-zero magnetic field (B) the FS on the first Brillouin zone is found to correspond to electron pockets around anti-nodal regions and barely visible patches around nodal regions. We next relate our findings regarding FS to the entropy per particle(S), the electronic specific heat C$_{el}$ and the dissipative viscosity ($\eta )$. The magneto-quantum oscillations in C$_{el}$ are shown to take place in the moderate disorder regime ($\vert $v$_{0}\vert \quad \sim $0.2 eV) only for B $\sim $ 40 T. For the density of viscosity $\eta $(\textbf{k}) on the first Brillouin zone, we find that whereas the negative contribution arises from the electron pockets in the anti-nodal region, the positive contributions are from the hole-pockets in the nodal region. The KSS bound ($\eta $/S $\ge $ h/4$\pi $k$_{B })$is easily satisfied for the moderately strong disorder potential. The viscosity is found to be proportional to the magnetic field up to B $\sim $ 50 T. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y25.00009: The stripe-like collective excitation in cuprates: A variational Monte-Carlo study Chung-Pin Chou, Ting-Kuo Lee In this study we report variational Monte-Carlo calculations of collective excitations for the extended $t-J$ model. We found a particular collective excitation involving modulation of charge, spin and pair field have a fairly small stiffness constant. These very easily excitable excitations are the same as the stripe-like states observed in our previous results for the $t- J$ model. This anomalous low rigidity from these low-lying collective excitations may provide a better understanding of the ubiquitous nature of the stripe states in cuprates. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y25.00010: Pairing theory of striped superconductivity Siegfried Graser, Arno Kampf, Thilo Kopp, Florian Loder Starting from a pairing Hamiltonian with an attractive interaction for electrons on nearest-neighbor sites on a square lattice we present a Hartree-Fock scheme which allows for spin and charge density order simultaneously with d-wave superconductivity. Specifically for filling 7/8 the stable groundstate solution is a striped superconductor with a stripe wavelength of eight lattice constants and $\pi$-shifted order parameters for d-wave pairing and antiferromagnetism. The superconducting state contains Cooper pairs with finite center of mass momenta ${\bf q}$ and $-{\bf q}$ corresponding to half the wavelength for the stripe pattern of the charge density. Despite the d-wave symmetry of the local pairing amplitude the striped superconductor is fully gapped. We characterize the striped superconducting state in real-space and in momentum space and discuss its possible relevance to La$_{1.875}$Ba$_{0.125}$CuO$_4$. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y25.00011: Pair Density Wave correlations in the Kondo-Heisenberg Model Eduardo Fradkin, Erez Berg, Steven Kivelson We show, using density matrix renormalization group calculations complemented by field theoretic arguments, that the spin gapped phase of the one dimensional Kondo-Heisenberg model exhibits quasi-long range superconducting correlations {\it only} at a non-zero momentum. The local correlations in this phase resemble those of the pair density wave state which was recently proposed to describe the phenomenology of the striped ordered high temperature superconductor {La$_{2-x}$Ba$_x$CuO$_4$}, in which the spin, charge, and superconducting orders are strongly intertwined. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y25.00012: Uniform and pair-Density-Wave SC states in asymmetric ladders Akbar Jaefari, Eduardo Fradkin We consider the problem of the superconducting state in ladder fermionic systems and focus on two possible types of condensates: a uniform (``d-wave'') state and a pair-density wave state (PDW). The uniform SC state is known to occur generically in symmetric hole-doped ladders. Recently it was shown that the PDW state occurs in the Kondo-Heisenberg chain on a broad range of parameters. The Kondo-Heisenberg chain is an extreme version of an asymmetric two-leg ladder. These facts suggest that there must be a quantum phase transition between these two states as a function of the relative doping of the two legs of a ladder. We investigate the nature of this quantum phase transition in the weak coupling limit, by taking advantage of bosonization methods available for 1D systems, and investigate the mechanism of this phase transition. We speculate on the relevance of these results to 2D systems. References: S. R. White, R. M. Noack and D. J. Scalapino: Phys. Rev. Lett 73 (1994) A.E. Sikkema, I. Affleck, and S.R. White, Phys. Rev. Lett. 79, 929 (1997) E. Berg, E. Fradkin, S. A. Kivelson, Phys. Rev. Lett. 105, 146403 (2010) O. Zachar and A. M. Tsvelik, Phys. Rev. B 64, 033103 (2001) [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y25.00013: Unconventional superconductivity nearby antiferromagnetism in quasi-1D conductors:the role of electron-phonon interaction Claude Bourbonnais, Hassan Bakrim The stabilization of unconventional superconductivity (SCd) close to a spin-density-wave state (SDW) under pressure in organic conductors like the Bechgaard salts points out the primary importance of the repulsive Coulomb term in the origin of these phases. However, the electron-(acoustic) phonon interaction is known to be finite in practice, as borne out for example by diffuse X-ray scattering experiments. The question then arises about the role of this coupling, if any, in the mechanism of interaction between SDW and SCd orders in such materials. In this work, we address this issue using the renormalization group method. This is done in the framework of the quasi-1D electron gas model with repulsive direct Coulomb terms and weak retarded electron-phonon interaction, which are treated on equal footing. The impact of electron-phonon interaction on the SDW and SCd instability lines of the phase diagram and on the strength of spin correlations in the normal phase are analyzed at arbitrary phonon frequency, and discussed in connection with experiments in organic superconductors like the Bechgaard salts. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y25.00014: Resonant alteration of supercurrent in guiding structures with complex de Gennes distance and its magnetic-field-induced restoration Oleg Olendski Properties of the superconducting 2D disk and 3D wire are calculated within the framework of linearized Ginzburg-Landau theory with the complex de Gennes distance $\Lambda$ in the boundary condition. As a result, the self-adjointness of the Hamiltonian is lost, its eigenvalues $E$ become complex too and the discrete bound states of the disk turn into the quasibound states with their lifetime defined by the eigeneneries imaginary parts $E_i$. Accordingly, the longitudinal supercurrent undergoes alteration with its attenuation/amplification being $E_i$-dependent too. It is shown that $E_i$ as a function of the de Gennes imaginary part $\Lambda_i$ exhibits a pronounced sharp extremum with its magnitude being the largest for the zero real part $\Lambda_r$ of the de Gennes distance. Increasing magnitude of $\Lambda_r$ quenches the $E_i-\Lambda_i$ resonance and at large $\Lambda_r$ the eigenenergies $E$ approach the asymptotic real values independent of the de Gennes length imaginary component. The extremum is also wiped out by the applied longitudinal uniform magnetic field. The finite lifetime of the disk quasibound states stems from the $\Lambda_i$-induced currents flowing through the superconductor boundary. The effect can be observed in the superconductors by applying to them the external electric field. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y25.00015: A real space study of the effect of disorder on superconductivity Shreemoyee Ganguly Our method of studying the effect of disorder on superconductivity is based on the augmented space formalism that goes beyond mean-field approximations for configuration averaging and effectively deals with the influence of configuration fluctuations of the neighbourood of an atom. In the regime of validity of Anderson's theorem our results for $s$- and $d$-wave dirty superconductors has excellent agreement with existing results. The formalism is extended and tested for random negative {\bf U} Hubbard model. Having verified the reliability of our method we use it to study environment dependent, inhomogeneous randomness in disordered superconducting systems. Our model can be easily extended to study multi-band systems which takes us a step closer to studying real materials. [Preview Abstract] |
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