Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X12: HTSC: Neutron Scattering and Magnetism |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Al Ehrlich, Naval Research Laboratory Room: LACC 402A |
Friday, March 25, 2005 8:00AM - 8:12AM |
X12.00001: Experimental phase boundary between quantum critical and quantum disordered regime in hole-doped La$_2$CuO$_4$ Ying Chen, Wei Bao, Emilio Lorenzo, Anne Stunault, John Sarrao, Sungil Park, Yiming Qiu The long-range antiferromagnetic order in La$_2$CuO$_4$ is
suppressed
by hole doping at $x_c\approx 3\%$ of Li dopant concentration.
This allows experimental
investigation of the spin fluctuations near the quantum critical
point
of a 2-dimensional $S$=1/2 antiferromagnet. According to
quantum critical theory, the
quantum critical (QC) $E/T$ scaling is expected to break down
and crosses over
to a constant energy quantum disordered (QD) scaling at low
temperatures when the doping
$x>x_c$. In this single
crystal inelastic cold neutron scattering study on spin dynamics
of
La$_2$Cu$_{1-x}$Li$_{x}$O$_4$ (0.04$ |
Friday, March 25, 2005 8:12AM - 8:24AM |
X12.00002: Electronic competing phases and their magnetic field dependence in electron-doped nonsuperconducting and superconducting Pr$_{1.88}$LaCe$_{0.12}$CuO$_4$ H.J. Kang, Pengcheng Dai, H.A. Mook, D.N. Argyriou, V. Sikolenko, J.W. Lynn, Y. Kurita, Seiki Komiya, Yoichi Ando We use neutron scattering to study the evolution of the static antiferromagnetic order of electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ (PLCCO) from an antiferromagnet to superconductors. When the superconductivity appears in PLCCO, a quasi-two- dimensional (2D) spin-density-wave (SDW) order is also induced, and both superconductivity and SDW coexist with the residual 3D antiferromagnetic (AF) phase. The SDW and 3D AF orders disappear when PLCCO becomes optimal doping with highest $T_c$. A $c$-axis aligned magnetic field enhances the quasi-2D SDW order at (0.5,0.5,0) in underdoped PLCCO, but has no effect on the 3D AF order in nonsuperconducting and superconducting samples. Since the same field along the $ab$-plane has no field effect on (0.5,0.5,0) and impurity positions, we conclude that the $c$-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X12.00003: Field Effect on the Superconducting Magnetic Gap in Nd$_{2-x}$Ce$_x$CuO$_4$ E.M. Motoyama, P.K. Mang, G. Yu, D. Petitgrand, O.P. Vajk, M. Greven Superconductivity in the cuprates occurs in close proximity to antiferromagnetic phases, and determining the nature of these phases is essential in arriving at a satisfactory understanding of these complex oxides. A unique opportunity exists in the prototypical electron-doped compound Nd$_{2-x}$Ce$_x$CuO$_4$ (NCCO), for which the upper critical field is relatively small ($\sim 10$ T). Although a magnetic field enhances the elastic response at the antiferromagnetic position $(\pi,\pi)$, it has been shown [1] that this effect is the paramagnetic response of the epitaxial secondary phase (Nd,Ce)$_2$O$_3$, and not due to the weakening of superconductivity. Our recent inelastic neutron scattering results demonstrate a real field effect on the magnetic excitations at $(\pi,\pi)$. It is known that in NCCO, these excitations become gapped below the superconducting critical temperature $T_c$ [2]. Here, we report on the effect of a magnetic field on this gap. In addition to the field-effect results, we report on the doping-dependence of two-dimensional antiferromagnetic correlations in non-superconducting oxygen-reduced NCCO, and compare the results to previous studies on as-grown NCCO [3]. [1] P.K. Mang et al, Nature 426, 139 (2003); Phys. Rev. B 70, 094507 (2004). [2] K. Yamada et al., Phys. Rev. Lett. 90, 137004 (2003). [3] P.K. Mang et al, Phys. Rev. Lett. 93, 027002 (2004). [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X12.00004: Neutron Scattering Studies of the Copper Oxide Superconductor La$_{1.925}$Ba$_{0.075}$CuO$_4$ Sarah Dunsiger, Yang Zhao, Bruce Gaulin, Hanna Dabkowska, William Buyers, Zahra Yamani The interplay between superconductivity and magnetism is a central issue in the study of the high T$_c$ cuprates. The first to be discovered, the Bednorz-M\"{u}ller materials have been much less extensively studied due to the difficulty of growing single crystals. We report recent neutron scattering studies of a large single crystal of La$_{1.925}$Ba$_{0.075}$CuO$_4$. Incommensurate elastic peaks, evidence of static spin stripe order, develop below 40 K and persist into the superconducting state (T$_c \sim 25$ K). Investigations of the low energy incommensurate spin fluctuations show the dynamical spin susceptibility increases as T$_c$ is approached from above, remaining roughly constant below T$_c$. A similar variation in the incommensurability indicates a temperature dependent stripe spacing. We compare our results with studies of La$_{1.875}$Ba$_{0.125}$CuO$_4$ in the normal state [1]. [1] M. Fujita et al, cond-mat/0403396 (2004). [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X12.00005: Impact of the reduction process on the long-range antiferromagnetism in Nd1.85Ce0.15CuO4 Pierre Richard, Mario Poirier, Serge Jandl In contrast to the hole-doped high-temperature superconductors, for which a Cu$^{2+}$ long-range antiferromagnetic order is suppressed at low doping, the antiferromagnetic order is more robust in the electron-doped 2-1-4 cuprates RE$_{2-x}$Ce$_{x}$CuO$_{4}$ (RE = Pr, Nd, Sm, Eu) and persists up to optimal doping. Recent neutron measurements have suggested that the Cu$^{2+}$ N\'{e}el temperature decreases after the reduction of the as-grown samples. This questions the competition between the antiferromagnetic and superconducting ground states, as well as the role of the reduction process which triggers superconductivity in these materials. A recent ultrasonic study have revealed ultrasonic anomalies around 4 K related to competing Nd$^{3+}$-Nd$^{3+}$ and Nd$^{3+}$-Cu$^{2+}$ interactions. We present an ultrasonic study of Nd$_{1.85}$Ce$_{0.15}$CuO$_{4}$ in the reduced and oxygenated states. While ultrasonic anomalies are found below 10 K in the oxygenated state, in both the C$_{66}$ elastic moduli and the corresponding attenuation $\alpha $, no anomaly is observed in the reduced and superconducting state. We attribute this effect to the suppression of the Cu$^{2+}$ long-range antiferromagnetic order due to the presence of oxygen vacancies in the CuO$_{2}$ planes of reduced samples. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X12.00006: Commensurable, incommensurable, or discommensurable spin arrangements in high-T$_c$ superconductors Erich P. Stoll, Peter F. Meier In the undoped parent compounds of high-T$_c$ superconductors the spins on neighboring copper atoms are antiferromagnetically ordered. Neutron diffraction measurements on the La$_{2-x}$Sr$_x$CuO$_4$ system show at low temperatures four peaks in the spin-fluctuation spectrum in the neighborhood of the antiferromagnetic ($\pi /a ,\pi /b$) point. The positions of these four peaks depend on the level of Sr doping. This implies that in the direct lattice the antiferromagnetic ordering exhibits incommensuration or discommensuration with fluctuations. In both cases the nearest neighbor correlation is the same on longer time scales where NMR experiments are performed, but the further distant correlations are different. We show with computer simulations how the far-field correlations break down and the AFM order parameter vanishes upon introducing disorder by dopant atoms. We simulate both in- and dis- commensurable spin arrangements and compare the results with NMR data. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X12.00007: Magnetic Neutron Scattering Study of Nd$_{1.85}$Ce$_{0.15}$Cu$_{1-y}$M$_{y}$O$_{4}$ (M = Zn, Fe, Ni) Single Crystals Inna Vishik, Guichuan Yu, Eugene Motoyama, Patrick Mang, Owen Vajk, Martin Greven In order to arrive at a deeper understanding of the interplay between superconductivity and magnetism in the high-temperature superconductors, it is of interest to study the effects of impurity-doping on the copper site. A large body of work along these lines exists for several hole-doped materials, yet relatively little is known about the effects of such impurities on the prototypical electron-doped material (Nd,Ce)$_{2}$CuO$_{4+\delta }$. Paramagnetic dopants (Ni,Fe) have been shown to lower T$_{c}$ much more abruptly than non-magnetic ones (Zn). We have extended some of the previous work done on polycrystalline systems by growing large single crystals of Nd$_{1.85}$Ce$_{0.15}$Cu$_{1-y}$M$_{y}$O$_{4}$ (M = Zn, Fe, Ni) using the traveling-solvent floating-zone technique. In particular, we report our initial results for the in-plane charge transport measurements, as well as neutron scattering measurements of the N\'{e}el temperature T$_{N}$ and the two-dimensional spin correlation lengths for temperatures above T$_{N}$. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X12.00008: Electronic competing phases and their magnetic field dependence in electron-doped nonsuperconducting and superconducting Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ Stephen D. Wilson, Pengcheng Dai, Wei Bao, Seung-Hun Lee, Y. Kurita, Seiki Komiya, Yoichi Ando We present inelastic neutron scattering studies of electron- doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$ (PLCCO). Previous elastic neutron scattering results have shown when superconductivity is first induced by annealing in PLCCO, a quasi-two-dimensional spin density wave also appears and coexists with three- dimensional (3D) antiferromagnetic order. As a function of increasing $T_c$, the Neel temperature of the 3D order decreases and vanishes when optimal superconductivity is achieved, suggesting the potential presence of a quantum critical point at zero temperature. We have performed careful inelastic neutron scattering experiments on a $T_{c}= 21$ K and $T_{N}= 40$ K sample of PLCCO. We find that magnetic excitations in this material scale according to the proximity of a quantum critical regime. We compare the results with hole-doped La$_2$CuO$_4$. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X12.00009: Anomalous critical behavior near the quantum critical point of a hole-doped La$_2$CuO$_4$ Wei Bao, Ying Chen, Emilio Lorenzo, Anne Stunault, John Sarrao, Sungil Park, Yiming Qiu In zero temperature quantum critical phenomena, classical thermal fluctuations are replaced by zero-point quantum critical phenomena and quantum mechanical generalization of the Landau-Ginzburg- Wilson paradigm has been a central topic in condensed matter physics. In Sr or Ba-doped La$_2$CuO$_4$, which is a member of high-Tc superconducting cuprates, the energy spectra of spin fluctuations in the neighborhood of the ($\pi,\pi$) antiferromagnetic Bragg point have been shown to follow the $E/T$ scaling. In the neutron scattering study on spin dynamics in La$_2$Cu$_{1-x}$Li$_{x}$O$_4$, we find that the critical exponent $a$ in the $E/T$ scaling changes from an expected $a$ $\approx$ 1 to an anomalous $a$ $\approx$ 0.65 and the scaling function becomes anomalous when the doping is reduced towards the antiferromagnetic quantum critical point, which has not been expected in the current theories.[1] Our results suggests extra physics which is beyond mapping the 2-dimensional quantum spin system to a 3 dimensional one in classical Landau-Ginzburg-Wilson theory. [1] Y. Chen {\it et al.}, cond-matt/0408547 [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X12.00010: Quantitative analysis of nonmonotonic T$_{\mbox{c}} $ behavior in Nb/Co$_{60}$Fe$_{40}$, Nb/Ni, and Nb/Cu$_{40}$Ni$_{60}$ bilayers Jinho Kim, Jun Hyung Kwon, Yong-Joo Doh, Kookrin Char, Hyeonjin Doh, Han-Yong Choi We have studied the behavior of superconducting critical temperature T$_{\mbox{c}} $ in Nb/Co$_{60}$Fe$_{40}$, Nb/Ni, and Nb/Cu$_{40}$Ni$_{60}$ bilayers as a function of each ferromagnetic metal thickness d$_{F}$. The T$_{\mbox{c}} $'s of each bilayer show nonmonotonic behavior as a function of d$_{F}$ with a shallow dip feature. From the quantitative analysis based on Usadel formalism [1], we observed that the T$_{\mbox{c}} $ behavior of Nb/Co$_{60}$Fe$_{40}$ bilayers is in good agreement with the theoretical prediction with interface parameter $\gamma _{b }$= R$_{b}$A/$\rho _{f}\xi _{f}$ = 0.34 in the whole range of d$_{F}$. On the other hand, T$_{\mbox{c}} $ values of Nb/Ni and Nb/Cu$_{40}$Ni$_{60}$ bilayers are higher in the small d$_{F}$ regime than the theoretical calculation with $\gamma _{b }$= 0.7 for Nb/Ni bilayer and $\gamma _{b }$= 0.57 for Nb/Cu$_{40}$Ni$_{60}$ bilayer, respectively, although the theoretical calculations match the dip position and the saturation value of T$_{\mbox{c}} $ in the large d$_{F}$ regime. We propose that this discrepancy is due to the weakened magnetism resulting from the structural disorder in Nb/Ni bilayer and a change in relative composition between Ni and Cu in Cu$_{40}$Ni$_{60}$ ferromagnetic layer at its initial nucleation on Nb film. [1] H. Doh and H. Y. Choi, cond-mat/0407149 (2004). [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X12.00011: Anomalous Proximity Effect Nb/Al/CoFe(Ni, Cu$_{40}$Ni$_{60})$ trilayers Jun Hyung Kwon, Jinho Kim, Kyungmoon Kim, Kookrin Char, Hyeonjin Doh, Han-Yong Choi We have fabricated Nb/Al/CoFe(Ni, Cu$_{40}$Ni$_{60})$ trilayers to study the difference from proximity effect of Superconductor/Ferromagnet bilayers. Increasing the thickness of Al in S/N/F trilayer, we observed that Tc value of S/N/F trilayers increase sharply almost to the Tc of S/N bilayer until the Al thickness of 3nm. As Al thickness increases from 3nm to 180nm, Tc value of S/N/F decreases again, following those of the S/N data. Although trilayers have different initial Tc due to different exchange energy(CoFe, Ni, Cu$_{40}$Ni$_{60})$ when Al thickness is 0nm, the Tc of S/N/F trilayers become almost same when Al thickness is 3nm. In order to fit the Tc data of Nb/Al/CoFe trilayers as a function of Al thickness, we had to use a large $\gamma _b ^{F}$(= R$_{b}$A/$\rho _{f}\xi _{f})$ value of about 4. Increasing the thickness of ferromagnet in S/N/F Trilayer, we observed minimum Tc value and the difference in dip position depending on the ferromagnetic materials. In each case, however, each dip position was almost same as in the S/F bilayers although the magnitudes of the dip structures were smaller. We will discuss the implication of our findings of large boundary resistances and the possibility of triplet superconductivity. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X12.00012: Interface Induced Antiferromagnetism in High-Tc/Manganite Superlattices N. Haberkorn, J. Guimpel, W. Saldarriaga, E. Baca, M.E. G\'omez, L. Civale We study the magnetic properties of perovskite based superconducting High-Tc / ferromagnetic manganite superlattices. The zero field cooled hysteresis loops show the expected ferromagnetic behaviour. However, field cooled low temperature hysteresis loops show the unexpected presence of exchange bias, signature of the existence of ferromagnetic/antiferromagnetic (F/AF) interfaces. The origin of the AF material is probably associated to interface disorder and Mn valence shift towards Mn$^{4+}$. The blocking temperature is found thickness dependent and the exchange bias field is found inversely proportional to the F layer thickness, as expected. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X12.00013: Comparative study of physical properties of ferromagnetic-superconducting (FS) system for different pairing symmetries Hari Dahal, Jason Jackiewicz, Kevin Bedell We study the physical properties of a F-S system described by mean field model. We compare the results of two different models*. Both of the models are comprised of long range ferromagnetic order due to spontaneously broken spin rotation symmetry but have different pairing symmetries to deal with superconducting properties, namely singlet (s-wave) and triplet (p-wave) pairing. We will present a comparative study of differences in the physics due to the difference in the nature of the pairing symmetry. We will mainly focus on the nuclear relaxation rate. The results will be useful in understanding the nature of superconducting pairing in ferromagnetic compounds such as UGe2. *Karchev et al. (PRL, 86, 846(2001)) *Zhang et al. (Cond-matt/0306691 V1 27June 2003) [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X12.00014: Spin Susceptibility in the Superconducting State of Cuprates Tamas Mayer, Mikhail Eremin, Ilya Eremin, Peter Fritz Meier An analytic expression for the spin susceptibility in the superconducting state was derived taking into account strong correlation effects. Starting from a Hubbard model that also accounts for the repulsion between doped holes, the equations of motion were solved using a decoupling scheme. Since the commutator algebra of the model is different from that valid in the weak-coupling conventional Fermi liquid picture, the general expression for the susceptibility is quite different from the standard Pauli--Lindhard formula. We have then evaluated $\chi(q,\omega)$ using parameter values adopted to Fermi surfaces and neutron scattering data. The temperature dependence of the spin susceptibility is calculated for both s-- and d--wave pairing symmetry. The results are compared with NMR and NQR experiments on the spin--lattice and spin--spin relaxation rates as well as with features observed in neutron scattering experiments. [Preview Abstract] |
Friday, March 25, 2005 10:48AM - 11:00AM |
X12.00015: Excitation of spin waves in superconducting ferromagnets Vitaly Braude, Edouard Sonin Excitation of spin waves in a superconducting ferromagnetic slab is analyzed theoretically. We calculated the surface impedance for various orientations of the ferromagnetic spontaneous magnetization with respect to the sample surface. Threshold frequencies were found at which the impedance has singularities. We determine the form of the singularities and discuss their physical meaning. Our analysis shows striking differences between a superconducting ferromagnet and both a ferromagnetic metal and a nonmagnetic superconductor. Hence experimental investigation of spin wave modes can be an effective probe of unusual magnetic properties of superconducting ferromagnets, including unconventional superconductors with broken time-reversal symmetry. [Preview Abstract] |
|
X12.00016: Spin freezing in Sm$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ R.W. Giannetta, D.D. Lawrie, R. Prozorov, I. Hetel, P. Fournier Insulating Sm$_{2}$CuO$_{4}$ shows antiferromagnetic ordering of Sm spins at T$_{Neel}$ = 6 K. T$_{Neel }$ is insensitive to external fields. Penetration depth measurements in superconducting Sm$_{1.85}$Ce$_{0.15}$CuO$_{4-y}$ (T$_{C}$ = 23 K) reveal a sharp increase of diamagnetism below T$^{\ast }$= 4.2 K. A c-axis magnetic field rapidly suppresses T*. The strong field dependence of T* suggests a spin-freezing transition below which spin-flip scattering is reduced, the density of states becomes sharper and Meissner screening is enhanced.[1] We discuss the role of Cu $^{2+}$ spins in the observed behavior. Work at UIUC supported through NSF DMR 01-01872. Work at USC supported by the NSF/EPSCoR, Grant No. EPS-0296165. PF acknowledges the support of CIAR, CFI, NSERC, FQRNT and the Universite de Sherbrooke.[1] R. Prozorov et. al., Phys. Rev. Lett. \textbf{93}, 147001 (2004) [Preview Abstract] |
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