Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U2: Electron Nematics |
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Sponsoring Units: DCMP Chair: Jan Zaanen, University of Leiden Room: Morial Convention Center LaLouisiane C |
Thursday, March 13, 2008 8:00AM - 8:36AM |
U2.00001: Transport anisotropy as a signature of electron nematicity Invited Speaker: Strong electron correlations often give rise to novel phenomena that are never found in ordinary materials. One of such phenomena is the emergence of electron nematicity (EN), which was theoretically proposed in 1998 to occur due to a combined effect of electron self- organization and quantum fluctuations. Experimentally, the EN was first discovered in 1999 in the two-dimensional electron gas (2DEG) at high Landau levels, where a clear transport anisotropy was found to grow upon lowering temperature in the mK region. In search for the signatures of the EN in cuprates, we have done extensive transport measurements of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and YBa$_2$Cu$_3$O$_y$ (YBCO) systems using high-quality single crystals. We discovered in 2001 that the in-plane resistivity anisotropy in untwinned single crystals of LSCO and YBCO in the lightly hole-doped region grows below $\sim$150 K with decreasing temperature, similar to the case in 2DEG, albeit the much higher temperature scale. In those samples, the easy transport axis was apparently dictated by the orthorhombic crystal structure; however, the orthorhombicity $\eta$ was only up to 1.5\%, while the resistivity anisotropy was up to a factor of 3, which was obviously too large for the small $\eta$. Furthermore, the anisotropy in YBCO was found to be {\it enhanced} with decreasing $y$ below $\sim$6.5 despite the {\it decreasing} $\eta$ until the crystal structure turns to tetragonal at $y \simeq$ 6.30. While this result gave strong evidence for the self-organized EN in high-$T_c$ cuprates, it was not completely conclusive because of the existence of the orthorhombicity that chooses the preferred direction; also, the lack of support from neutron scattering kept the skepticism remain. However, very recently, neutron scattering has finally found corroborating anisotropy in YBCO and convincing evidence for EN in a related oxide Sr$_3$Ru$_2$O$_7$ was obtained, which together strengthened the case for cuprates considerably. \\ \\ In collaboration with Kouji Segawa, Seiki Komiya, and A. N. Lavrov. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U2.00002: Theory of the nematic quantum critical point in a nodal superconductor Invited Speaker: In the last several years, experimental evidence has accumulated in a variety of highly correlated electronic systems of new quantum phases which (for purely electronic reasons) spontaneously break the rotational (point group) symmetry of the underlying crystal. Such electron ``nematic'' phases have been seen in quantum Hall systems[1], in the metamagnetic metal Sr$_3$Ru$_2$O$_7$[2], and more recently in magnetic neutron scattering studies of the high temperature superconductor, YBCO[3]. In the case of a high $T_c$ superconductor, the quantum dynamics of nematic order parameter naturally couples strongly to quasiparticle (qp) excitations. In this talk, I will discuss our recent results on the effects of the coupling between quantum critical nematic fluctuations and the nodal qp's of a d-wave superconductor in the vicinity of a putative quantum critical point inside the superconducting phase. We solve a model system with $N$ flavors of quasiparticles in the large $N$ limit[4]. To leading order in $1/N$, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strong scattering which critically broadens the quasiparticle peaks in the spectral function, except in the vicinity of ``the tips of the banana,'' where the qp's remain sharp. We will discuss the possible implications of our results to ARPES and STM experiments. \par [1] M.P. Lilly, K.B. Cooper, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, PRL {\bf 83}, 824 (1999). \par [2] R. A. Borzi and S. A. Grigera and J. Farrell and R. S. Perry and S. J. S. Lister and S. L. Lee and D. A. Tennant and Y. Maeno and A. P. Mackenzie, Science {\bf 315}, 214 (2007). \par [3] V. Hinkov, D. Haug, B. Fauqu\'e, P. Bourges, Y. Sidis, A. Ivanov, C. Bernhard, C. T. Lin, B. Keimer, unpublished. \par [4] E.-A. Kim, M. Lawler, P. Oreto, E. Fradkin, S. Kivelson, cond-mat/0705.4099. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U2.00003: Metamagnetic Nematic Phase of Sr$_{3}$Ru$_{2}$O$_{7}$ Invited Speaker: In this talk I will review our group's recent observations that a quantum phase with pronounced electrical transport anisotropies forms in the vicinity of a metamagnetic quantum critical point in Sr$_{3}$Ru$_{2}$O$_{7}$. The behaviour, which is strongly dependent on disorder and is only seen in the highest purity crystals, has phenomenological similarities with prior observations on two-dimensional electron gases in semiconductor devices [1,2]. Its appearance in bulk in Sr$_{3}$Ru$_{2}$O$_{7}$ has allowed us to perform a number of thermodynamic measurements, and also offers the promise of study using modern surface-based spectroscopies such as angle resolved photoemission and spectroscopic imaging scanning tunneling microscopy. References [1] For example M.P. Lilly \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 394 (1999); ibid \textbf{83}, 824 (1999) [2] W. Pan \textit{et al}., Phys. Rev. Lett. \textbf{83}, 820 (1999). Collaborators: S.A. Grigera$^{1}$, R.A. Borzi$^{1,2}$, A. Rost$^{1}$, J.F. Mercure$^{1}$, J. Farrell$^{1}$, R.S. Perry$^{3}$, A.G. Green$^{1}$, M. Allan$^{1}$, M. Wang$^{4}$, J. Lee$^{1}$, F. Baumberger$^{1}$, S.J.S Lister$^{1}$, S.L. Lee$^{1}$, J.C.S. Davis$^{1,4}$, Z.X. Shen$^{5}$, Y. Maeno$^{6}$. $^{1}$ University of St Andrews, Scotland $^{2 }$INFTA, La Plata, Argentina $^{3}$ University of Edinburgh, Scotland $^{4 }$Cornell University, USA $^{5}$ Stanford University, USA $^{6}$ Kyoto University, Japan [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U2.00004: Electronic Nematic Liquid in Correlated Systems Invited Speaker: It was proposed that the electronic nematic phase with a broken rotational symmetry is a generic ground state of a doped Mott insulator, and it has attracted much attention with the discovery of anisotropic quantum Hall phases in GaAs-heterostructures in large magnetic fields. A series of recent experiments on the bilayer ruthenate Sr$_3$Ru$_2$O$_7$ also suggest the existence of an anisotropic metallic phase which is uncovered by tuning the magnetic field. In this talk, I will show that the two consecutive metamagnetic transitions, a large residual resistivity, and an anisotropic magnetoresistance observed in the bilayer ruthenate can be understood via the electronic nematic order and its domains. I will also discuss an SO(4) invariance at the critical point between the electronic nematic and d-wave superconducting states, and its relation to the similar SO(4) invariance between the d-density wave and d-wave superconducting states. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U2.00005: Spins in cuprates near the edge of the superconducting phase - incoherent pairing, fluctuations on all timescales and observation of pseudogap energies. Invited Speaker: When doping is just enough to create the superconducting phase spins behave very differently from heavily doped cuprates. Nonetheless low doping reveals the key ingredients for the coherence of superconducting pairs. The ordered spins of the insulating antiferromagnet are replaced by a hedgehog phase with isotropically polarized spins, and a commensurate central mode with slow short-range spin correlations extending over four planar unit cells. A subcritical 3D enhancement is destroyed by temperature and energy. The spins follow not via the coherence of charge pairs but the structures caused by hole doping, so that glassy short range spin order occurs within both the superconducting and normal phase. The tumbling regions of spins exhibit a power-law spectrum similar to 1/f noise down to microeV energies. Excitations are overdamped with a millieV relaxation rate a thousand times faster, unlike the well-defined resonance familiar at large doping. Below 50 K the scale length is geometric and not linked by velocity to dynamic widths. As excited states are depopulated spin weight transfers to the central mode. At energies 300,000 times larger, decay of paramagnons into pseudogap states has been detected [1], a strong indication that it is the large density of high-energy spin states that provides the superconducting glue, rather than the resonance which is absent as a sharp spectral feature. \newline [1] C.Stock et al, Phys. Rev. B 75,172510 (2007). [Preview Abstract] |
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