Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D2: HTSC: New Developments |
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Sponsoring Units: DCMP Chair: Chandra Varma, University of California, Riverdside Room: Spirit of Pittsburgh Ballroom BC |
Monday, March 16, 2009 2:30PM - 3:06PM |
D2.00001: New magnetic neutron scattering results for the high-T$_{c}$ superconductors HgBa$_{2}$CuO$_{4+\delta }$and Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ Invited Speaker: We have succeeded in growing sizable single crystals of HgBa$_{2}$CuO$_{4+\delta }$, the single-layer hole-doped compound with the highest superconducting transition temperature [1]. Careful characterization demonstrates the high quality of our crystals [2]. Using polarized neutron diffraction, we find an unusual magnetic order in the pseudogap phase [3]. Together with prior results for YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, this observation constitutes a demonstration of the universal existence of such a state. Our inelastic neutron scattering measurements reveal that the antiferromagnetic resonance occurs at a rather high energy in HgBa$_{2}$CuO$_{4+\delta }$ and, hence, that the resonance energy is not universally related to T$_{c}$ [4]. Finally, our results for the antiferromagnetic excitations in Nd$_{2-x}$Ce$_{x}$CuO$_{4+\delta }$ provide new insight into magnetic energy scales and the resonance of the electron-doped side of the phase diagram [5]. \\[4pt] [1] X. Zhao et al., Adv. Mater. \textbf{18}, 3243 (2006). \\[0pt] [2] N. Barisic et al., Phys. Rev. B \textbf{78}, 054518 (2008). \\[0pt] [3] Y. Li et al., Nature \textbf{455}, 372 (2008). \\[0pt] [4] G. Yu et al., arXiv:0810.5759. \\[0pt] [5] G. Yu et al., arXiv:0803.3250. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:42PM |
D2.00002: Quantum critical fluctuations in cuprates and d-wave superconductivity through their coupling to fermions Invited Speaker: The phase diagram of the Cuprates is a collection of anomalies that has challenged our understanding of quantum many body physics. An organizing principle proposed to unify the experimental observations is the existence of a quantum critical point near optimal doping separating a phase which has broken time reversal and a renormalized Fermi liquid. I will discuss recent theoretical developments related to the nature of the fluctuations near the quantum critical point. The long wavelength theory of the time reversal violating state belongs to the dissipative 2DXY universality class. The fluctuation spectrum, at the quantum critical point, is local in space and power law in time, precisely of the form observed in the marginal fermi liquid phase near optimal doping. The fluctuations couple to the local angular momentum of the fermions to give a momentum dependence to the coupling which leads directly to pairing attraction in the d-wave channel. \newline Theory of superconductivity in the cuprates, Vivek Aji, Arkady Shekhter and Chandra Varma, arXiv:0807.3741 \newline Theory of the quantum critical fluctuations in cuprate superconductors, Vivek Aji and Chandra Varma, Physical Review Letters, \textbf{99}, 067003 (2007) [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 4:18PM |
D2.00003: Search for a thermodynamic evidence of a phase transition in the pseudogap state of YBa$_{2}$Cu$_{3}$O$_{6+x}$ Invited Speaker: Recent polarized neutrons diffraction experiments have evidenced a symmetry breaking in the underdoped phase of superconducting cuprates [1,2,3]. This symmetry breaking takes place below a temperature T$_{MAG}$ which increases when the number of charge carriers in the superconducting planes is decreased. We present here magnetic susceptibility measurements in YBa$_{2}$Cu$_{3}$O$_{6+x}$ ceramics. We have measured the magnetization of about twenty samples with different oxygen contents under 1T magnetic field. In some of them, we have observed an anomaly in the temperature derivative of the susceptibility at a temperature T$_{1}$ which seems to be in good agreement to the temperature T$_{MAG}$. We show here the resulting phase diagram for underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$. These findings raise the question of a thermodynamic evidence for a phase transition in relation to the symmetry breaking observed by neutrons.\\[4pt] [1] B. Fauque et al, Phys. Rev. Letters 96, 197001 (2006). \\[0pt] [2] H. A. Mook et al, Phys. Rev. B 78 ,020506(2008) \\[0pt] [3] Y. Li, Nature 455, 372 (2008) [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:54PM |
D2.00004: Competition between the pseudogap and superconductivity in cuprates Invited Speaker: The relationship between the pseudogap and superconductivity is one of the central issues in physics of cuprates. By studying the spectral weights associated with pseudogap and superconductivity by angle resolved photoemission spectroscopy (ARPES) we found that there is a direct correlation between the loss of the low energy spectral weight due to the opening of the pseudogap and a decrease of the spectral weight associated with superconductivity as a function of momentum and doping. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space, where the superconducting gap is largest. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:30PM |
D2.00005: Nanoscale Imaging of the Pair Formation in High-Temperature Superconductors Invited Speaker: In the quest for a microscopic theory for the superconductivity in cuprates, one hotly debated issue is the temperature at which Cooper pairs first form. Do pairs form at the critical temperature Tc or do they form at higher temperatures lacking phase rigidity? To answer the question, we have developed new techniques, based on the scanning tunneling microscope, to visualize the process of the pair formation on the atomic scale. The magnitude of the low-temperature superconducting gap measured in Bi-2212 shows a large nanoscale spatial variation. These superconducting gaps evolve smoothly with temperature and close locally over a range of temperatures above the superconducting transition temperature Tc [1]. Our results provide evidence that pairing first occurs in nanoscale regions above the bulk superconducting transition temperature. Using the ability to track the same atomic position while changing the temperature, we have examined the evolution of the electronic states from well below Tc to above the temperature at which the pairs first form. Our technique allows us to investigate another fundamental question on the pairing mechanism: Is pairing mediated by a bosonic excitation, as in conventional BCS superconductors, or is pairing with d-wave symmetry an unavoidable consequence of the strong Coulomb repulsion in these compounds? We quantitatively analyze the temperature evolution of the gap and the local electron-boson coupling for various atomic sites with different pairing strengths [2]. We observe that the gap magnitude variation is not determined by the electron-boson coupling but instead it is strongly correlated to variations present in the normal (ungapped) electronic states. \\[4pt] [1] Gomes KK, Pasupathy AN, Pushp A, Ono S, Ando Y, Yazdani A, Nature 447, 569 (2007). \\[0pt] [2] Pasupathy AN, Pushp A, Gomes KK, Parker CV, Wen J, Xu Z, Gu G, Ono S, Ando Y, Yazdani A, Science 320, 196 (2008). [Preview Abstract] |
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