Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B35: Superconductivity: Spin Properties |
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Sponsoring Units: DCMP Chair: Yaroslaw Bazaliy, University of South Carolina Room: 343 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B35.00001: Magnetic excitations in the high-T$_{\mathrm{c}}$ superconductor HgBa$_2$CuO$_{4+d}$ at low doping Chelsey Dorow, M.K. Chan, Y. Tang, G. Yu, Yuan Li, N. Barisic, J. Park, O. Sobolev, A. Teichert, Y. Sidis, P. Steffens, D. Abernathy, X. Zhao, P. Bourges, M. Greven We report on the observation of magnetic excitations in the very underdoped regime of the high-T$_{\mathrm{c}}$ superconductor HgBa$_{2}$CuO$_{4+d}$ (Hg1201). Our previous inelastic neutron scattering measurements of optimally doped (T$_{\mathrm{c}}$ $\approx $ 95 K) and moderately underdoped (T$_{\mathrm{c}} \approx $ 65 K) samples revealed two novel, weakly-dispersive magnetic excitation branches below the pseudogap temperature T* [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012)]. These excitations are associated with the translational symmetry preserving magnetic order previously established to be a universal property of the pseudogap phase [B. Fauqu\'{e} et al., Phys. Rev. Lett. 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008); Y. Li et al. Phys. Rev. B 84, 224508 (2011)]. In YBa$_{2}$Cu$_{3}$O$_{6+d}$, the strength of this order was found to decrease in very underdoped samples [V. Bal\'{e}dent et al. Phys. Rev. B 83, 104504 (2011)]. Indeed, we find no evidence of pseudogap excitations in very underdoped Hg1201 (T$_{\mathrm{c}} =$ 45 K), and instead we observe strong antiferromagnetic fluctuations over a large energy range (10 -150 meV). [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B35.00002: Unusual form factor of the novel pseudogap excitations in HgBa$_{2}$CuO$_{4+\delta}$ Mun Chan, C. Dorow, Y. Tang, G. Yu, M. Greven, N. Barisic, Y. Li, K. Hradil, R. Mole, P. Steffens, X. Zhao, Y. Sidis, P. Bourges Following the discovery of a universal novel magnetic order in the pseudogap phase of the cuprates [B. Fauqu\'{e} et al. PRL 96, 197001 (2006); Y. Li et al., Nature 455, 372 (2008)], our inelastic neutron scattering measurements of HgBa2CuO4$+\delta $ (Hg1201) revealed two weakly-dispersive excitation branches associated with this ordered state [Y. Li et al., Nature 468, 283 (2010); Y. Li et al., Nature Phys. 8, 404 (2012).]. The dependences of the mode intensities on the momentum transfer Q $=$ (HKL) (r.l.u.) are inconsistent with traditional magnetic or structural form factors. The intensity of the high-energy mode is zero when Q is parallel to the copper-oxygen planes (i.e., for L$=$0), peaks at L $=$ 8 (r.l.u.), and decreases again at large L. We observe the opposite behaviour for the low- energy mode, which is strongest when L$=$0. In combination with polarized inelastic neutron scattering results, this indicates possible dual magnetic and structural characteristics of the novel excitations. Work supported by DOE-BES. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B35.00003: Polarised neutron study of the ``even'' and ``odd'' magnetic excitations in YBa$_2$Cu$_3$O$_{6.9}$ Christopher Lester, Stephen Hayden, Jiri Kulda, David Cardwell, Nadendla Hari Babu On cooling through $T_c$, the spin excitation spectra of cuprate superconductors becomes dominated by the neutron spin resonance (NSR), a collective mode centred at $\mathbf{Q}_{AF}$. We have used polarized inelastic neutron scattering to measure the spin excitations of YBa$_2$Cu$_3$O$_{6.9}$ ($T_c=$93~K), unequivocally confirming the magnetic character of the NSR in both the odd and even channels. In the odd channel, the NSR is anisotropic in spin space, that is the out of plane ($c$) component of $\chi^{\prime\prime}(\mathbf{Q},\omega)$ is approximately 1.4 times larger than the in-plane ($a/b$) component. Conversely, the much weaker even channel resonance is isotropic to within experimental error, and the low energy response maintains a large gap (below $\sim$30~meV) in the normal state. While it is generally accepted that the NSR is ubiquitous in at least the hole-doped cuprates, recently two further collective modes have been observed in HgBa$_2$CuO$_{4+\delta}$. If these weakly-dispersive ``Ising-like'' modes were also universally present, then they might radically alter our view of the cuprate superconducting state. However, we find no evidence of this type of excitation in YBa$_2$Cu$_3$O$_{6.9}$, suggesting that these modes may in fact be unique to certain systems. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B35.00004: Effective $J_1$-$J_2$ model for the spin wave in the superconducting $\rm (Tl,Rb)_2Fe_4Se_5$ Songxue Chi, Feng Ye, Wei Bao, Andrei T. Savici, Matthew B. Stone, Randy S. Fishman, H.D. Wang, C.H. Dong, Minghu Fang Spin wave excitations in the superconducting state of $\rm (Tl,Rb)_2Fe_4Se_5$ were determined by inelastic neutron scattering measurements. Four doubly degenerate spin wave branches, one gapped acoustic and 3 optical, span an energy range of about 210 meV. The spin wave spectra were successfully described by a $J_1$-$J_2$ Heisenberg model which includes the in-plane nearest ($J_1$ and $J'_1$), next nearest neighbor ($J_2$ and $J'_2$) interactions within and between the 4-spin blocks, inter-plane interaction ($J_c$) and a single-ion anisotropy. The exchange coupling constants obtained indicate that the spin block order verges on a noncollinear in-plane-spin phase observed in Tl$_2$Fe$_4$Se$_5$. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B35.00005: Anisotropy of the Superconducting State in Sr$_2$RuO$_4$ M.R. Eskildsen, C. Rastovski, W.J. Gannon, C.D. Dewhurst, D. Peets, H. Takatsu, Y. Maeno Multiple experimental and theoretical studies provide compelling support for triplet pairing of electrons and an odd, $p$-wave order parameter symmetry in superconducting Sr$_2$RuO$_4$. However, seemingly contradictory experimental results have left important questions concerning the detailed structure and coupling of the orbital and spin parts of the order parameter in this compound unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr$_2$RuO$_4$ in order to measure the intrinsic anisotropy ($\Gamma_{ac}$) of the superconducting state between the the $c$ axis and the RuO basal plane. Up to fields of $1.2$ T and temperature of 800 mK, we found no variation of $\Gamma_{ac} \approx 60$. This is consistent with the Fermi velocity anisotropy on the $\beta$ Fermi-surface sheet, but greatly exceeds the upper critical field anisotropy $H_{c2}^{\perp c}/H_{c2}^{\parallel c} = 20$. This result poses significant constraints on the possible order parameter symmetry in Sr$_2$RuO$_4$. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B35.00006: Bulk Magnetization in the Superconducting State of UPt$_3$ William Gannon, William Halperin, Catherine Rastovski, Morten Eskildsen, Pengcheng Dai, Anne Stunault The unconventional superconductor UPt$_3$ has long been thought to have an odd parity orbital, and triplet spin state. An important signature of such a state is the temperature independence of the spin susceptibility across the superconducting transition temperature. Here, we report bulk measurements of the susceptibility of UPt$_3$ for magnetic fields along the crystal a-axis performed with polarized neutron diffraction. Temperature independence at all magnetic fields is observed, suggesting a spin triplet superconducting state for the entirety of the phase diagram, with equal spin pairs in the crystal basal plane. These results will be discussed in the context of existing theories for the superconducting state of this paradigm heavy fermion material [Graf et. al., PRB 62, 14393; Tsutsumi et. al., JPSJ 81, 074717 (2012)]. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B35.00007: Kerr effect studies of the heavy fermion superconductor URu$_2$Si$_2$ Elizabeth Schemm, Hovnatan Karapetyan, Eric Bauer, Aharon Kapitulnik In the heavy fermion metal URu$_2$Si$_2$, the very large entropy carried by the 5f electrons is released at $\sim 17.5$ K via a second-order phase transition to a ``hidden order" state. Below $\sim 1.5$ K superconductivity emerges with an as-yet unknown gap structure, adding to the mystery associated with this material. In this talk we present polar Kerr effect (PKE) measurements of URu$_2$Si$_2$ crystals using a Sagnac interferometer. PKE is sensitive to time-reversal symmetry (TRS) breaking since it measures the existence of an antisymmetric contribution to the real and imaginary parts of the frequency-dependent dielectric tensor. Such a contribution is necessarily absent if TRS is not broken in the material. We find a weak magnetic signal in the hidden order phase that seems to not influence superconductivity. The presence of a finite Kerr signal below $T_c$ provides strong evidence that time reversal symmetry is broken in the superconducting state. The relationship between the magnetic response in the hidden order phase and superconductivity is also studied. We further compare our results to other unconventional superconductors. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B35.00008: High resolution $^{17}$O Knight shift measurements of HgBa$_2$CuO$_{4+y}$ single crystals Andrew M. Mounce, Sangwon Oh, Jeongseop A. Lee, W.P. Halperin, A.P. Reyes, P.L. Kuhns, M. Chan, J. Li, D. Xia, X. Zhao, M. Greven The high superconducting transition temperature and the simple tetragonal structure of HgBa$_2$CuO$_{4+y}$ (Hg1201) makes this material an ideal candidate to study unconventional superconductivity in the cuprates[1]. Nuclear magnetic resonance has been performed on Hg1201 single crystals which have been annealed in an $^{17}$O atmosphere to achieve superconducting transition temperatures of underdoped 72 K and overdoped 76 K. Oxygen spectra are sufficiently narrow to resolve planar, apical, and dopant oxygen sites in addition to all satellite transitions of the planar and apical sites. The deconvolution of oxygen spin shifts into isotropic and axial shifts, for the underdoped crystal, shows temperature dependence in both the isotropic and axial components of the planar oxygen while the apical oxygen only has temperature dependence in the axial component. The rotational dependence of the apical oxygen shift does not indicate a predicted static local field component due to circulating orbital currents[2] which have been observed by neutron scattering.[3] [1] Barisic, N, PRB 78, 054518 (2008). [2] Lederer, S. and Kivelson, S. A., PRB 85, 155130 (2012). [3] Li, Y., et al, Nature 455, 372 (2008). [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B35.00009: NMR study of spin fluctuations and superconductivity in LaFeAsO$_{1-x}$H$_x$ Naoki Fujiwara, Ryosuke Sakurai, Soushi Iimura, Satoru Matsuishi, Hideo Hosono, Yoichi Yamakawa, Hiroshi Kontani We have performed NMR measurements in LaFeAsO$_{1-x}$H$_x$, an isomorphic compound of LaFeAsO$_{1-x}$F$_x$. LaFeAsO$_{1-x}$H$_x$ is most recently known for having double superconducting (SC) domes on H doping. LaFeAsO$_{1-x}$H$_x$ is an electron- doped system, and protons act as H$^{-1}$ as well as F$^{-1}$. The first SC dome is very similar between F and H doping, suggesting that H doping supplies the same amount of electrons as F doping. Interestingly, an excess amount of H up to x=0.5 can be replaced with O$^{2-}$. In the H-overdoped regime ($x > 0.2$), LaFeAsO$_{1-x}$H$_x$ undergoes the second superconducting state [1]. We measured the relaxation rate of LaFeAsO$_{1-x}$H$_x$ for x=0.2 and 0.4, and fond an anomalous electronic state; spin fluctuations measured from $1/T_1T$ is enhanced with increasing the doping level from $x=0.2$ to 0.4. The enhancement of spin fluctuations with increasing carrier doping is a new phenomenon that has not observed in LaFeAsO$_{1-x}$F$_x$ in which the upper limit of the doping level is at most $x=0.2$. We will discuss the phenomenon in relation to superconductivity.\\[4pt] [1] S. Iimura, $et. al.$, Nature Communications (2012) [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B35.00010: Electronic and Magnetic Properties of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ Studied by $^{55}$Mn and $^{75}$As-NMR S. Yeninas, A. Pandey, D.C. Johnston, Y. Furukawa BaMn$_{2}$As$_{2}$ (Mn$^{2+}$; S = 5/2) is a G-type antiferromagnetic (AF) semiconductor with N\'{e}el temperature $T_{N} \sim $ 625 K and a small band gap of $\sim $ 27 meV. Hole doping by substitution of Ba with K drives BaMn$_{2}$As$_{2}$ into a metallic state while maintaining the same AF spin structure with similar high $T_{N}$. In order to investigate hole doping effects on electronic and magnetic properties in Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2 }$from a microscopic point of view, we have conducted $^{55}$Mn and $^{75}$As-NMR spectra and spin-lattice relaxation measurements on single crystals of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ ($x$ = 0, 0.04, 0.4). The temperature ($T)$ dependence of 1/$T_{1}$ for $^{55}$Mn and $^{75}$As for the $x$=0 compound shows 1/$T_{1} \quad \sim \quad T ^{3}$ dependence for both nuclei, suggesting that 1/$T_{1}$ of the nuclei arises from interactions with magnon excitations in the local-moment AF state. On the other hand, the 1/$T_{1}$ of both nuclei is found to be proportional to $T$ (Korringa relation) in K-doped materials below $T_{N}$, which corresponds to the AF metallic state in Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B35.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:27PM - 1:39PM |
B35.00012: High Energy Magnetic Excitations in overdoped high Temperature Superconductors M. Le Tacon, G. Ghiringhelli, D.C. Peets, M. Moretti-Sala, S. Blanco-Canosa, M. Minola, V. Hinkov, R. Liang, D. Bonn, W. Hardy, C.T. Lin, T. Schmitt, L. Braicovich, B. Keimer Motivated by the search for the mechanism of high-temperature superconductivity, an intense research effort has been focused on the evolution of the spin excitation spectrum upon doping from the AF insulating to the superconducting (SC) states of the cuprates. Taking advantage of the recent developments of RIXS, we have shown that high energy magnetic excitations with dispersions and spectral weights similar to those of magnons in AF cuprates exist up to optimal doping. In the overdoped region, the normal state appears in many aspects similar to a Fermi liquid, and the available data on the magnetic excitations is rather limited. Inelastic neutron scattering work by Lipscombe et al. revealed the persistence of magnetic excitations up to 160 meV in an overdoped LSCO. This surprising result motivates us to investigate further the high energy magnetic excitations using RIXS in Ca-doped YBCO and Tl2201 compounds. We show that the high energy part of the excitation spectrum is essentially unaffected with hole doping, and that excitations up to 300 meV survive even at doping levels at which SC vanishes. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B35.00013: Two Dimensional Incommensurate Spin Excitations and Lattice Fluctuations in $La_{2-x}Ba_{x}CuO_{4}$ J.J. Wagman, J.P. Carlo, G. van Gastel, Y. Zhao, A.B. Kallin, E. Mazurek, H.A. Dabkowska, A. Savicii, G.E. Granroth, Z. Yamani, Z. Tun, B.D. Gaulin 'Hour-glass' shaped dispersions of antiferromagnetic (AF) spin fluctuations are a robust feature common to many high temperature superconductors. In 214 cuprates, these phenomena are well known to display a strong dependence on the concentration of holes that are introduced into the copper oxide planes by doping. The incommensurability (IC) of the two dimensional magnetic order in this system is sensitive to hole concentration. Here, we present a series of neutron scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO), with $0.035 \leq x \leq 0.095$, a doping range that spans the transition from diagonal to parallel IC ordering wavevectors, and from non-superconducting to superconducting ground states. Our measurements map out the evolution of the spin excitations for energies below $\sim$ 50 meV, and focus on an enhancement in the scattered intensity centered in the 17-20 meV at the AF IC positions. This regime corresponds to the approximate crossing of very dispersive spin excitations and weakly dispersive low lying optic phonons in LBCO. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B35.00014: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 2:03PM - 2:15PM |
B35.00015: Spin Susceptibility Enhancement in Superconductors Ben Rosemeyer, Anton Vorontsov We calculate electronic vector-dependent spin susceptibility tensor, $\chi_{\alpha\beta}(\bf{q})$, in the superconducting state, for a 2D Fermi surface. We investigate dependence of $\chi_{\alpha\beta}(\bf{q})$ on: a) magnetic ordering wave vector $\bf{q}$; b) symmetry of the order parameter, $\Delta(\bf{k})$; c) temperature; and d) effects of external Zeeman field. We find that under certain conditions longitudinal and transverse components of the susceptibility in the superconducting state can be enhanced compared to the normal state value, indicating effective attraction between magnetically ordered and superconducting phases. In particular, $d$-wave superconductors at low temperatures in strong magnetic field show increase of $\chi$ for $q=2k_f-\delta q$ ($\delta q/k_f\approx0.05$) for near-nodal direction of $\bf{q}$. We relate such enhancement or lack thereof to behavior of low-energy excitations in the system. These findings may be relevant to materials where magnetic and superconducting phases are close neighbors, such as heavy fermion CeCoIn$_5$, or Fe-based superconductors. [Preview Abstract] |
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