Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V38: Magnets That Can Superconduct |
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice University Room: LACC 513 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V38.00001: Scaling Behavior of the Longitudinal and Transverse Transport in Quasi One-Dimensional Organic Conductors Martin Dressel, Konstantin Petukhov, Belal Salameh, Pedro Zornoza, Thierry Giamarchi We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{\prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below $T=70$~K, the dc resistivity follows a power-law with $\rho_a$ and $\rho_{b^{\prime}}$ proportional to $T^2$ while $\rho_{c^*}\propto T$. Above $T = 100$~K the exponents extracted from the data for the $a$ and $c^*$ axes are consistent with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about $100$~K. The $b^\prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{\prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $\rho_a/\rho_{b^{\prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200~K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V38.00002: Quantum phase transitions and non-uniform phases in low-dimensional superconductors Wei Zhang, Carlos Sa de Melo We discuss the possibility of coexistence of spin density wave (SDW) or antiferromagnetism (AF) and triplet superconductivity (TS) as a particular example of a broad class of systems where the interplay of magnetism and superconductivity is important. We focus on the case of quasi-one-dimensional metals, where it is known experimentally that SDW (AF) is in close vicinity to TS in the temperature versus pressure phase diagram. We use the functional integral method to derive an effective field theory describing the interplay between SDW (AF) and TS orders. Furthermore, we establish conditions for the existence of two quantum critical points (QCP's) in the pressure versus temperature phase diagram. The first QCP is between a pure SDW (AF) phase and a non-uniform phase (NUP) consisting of alternating insulating SDW (AF) and TS stripes. The second QCP is between the NUP and a pure TS phase. The NUP exists between the two QCP's and is favored over a narrow range of pressure. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V38.00003: Using metamagnetism to uncover 5f2 doublets in U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$, and implications for Ordering Neil Harrison, Alejandro Silhanek, Cristian Batista, Marcelo Jaime, Hiroshi Amitsuka, John Mydosh The specific heat of U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ (with 0 $<$ x $<$ 4\%) at high magnetic fields isfound to be very comparable to that of CeRu$_2$Si$_2$, and can be fit to a simple resonance model, with the resonance split away from the Fermi energy by the Zeeman interaction. The combined success of simulations to reproduce the experimental data and similarities with the well-defined Kondo lattice system CeRu$_2$Si$_2$ provide strong indications of a low energy magnetic doublet for U, which is expected to be in the $5f_2$ configuration. In contrast to CeRu$_2$Si$_2$ , which shows no sign of ordering in the vicinity of the metamagnetic quantum critical point, U(Ru$_{1-x}$Rh$_x$)$_2$Si$_2$ exhibits numerous ordered phases as a function of Rh-doping and magnetic field, which are the likely consequence of stronger couples between $5f$ electros than $4f$ electrons. We discuss the possibility of magnetic and electric quadrupolar ordering. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V38.00004: NMR/NQR and disorder effects in URu$_2$Si$_2$ O.O. Bernal, M.E. Moroz, K. Ishida, H. Murakawa, A.P. Reyes, P.L. Kuhns, D.E. MacLaughlin, H.G. Lukefarh, J.A. Mydosh, T.J. Gortenmulder, H. Amitsuka NMR experiments at ambient pressure in URu$_2$Si$_2$ demonstrate a linewidth enhancement effect at the hidden order transition temperature ($T_0$). We find that larger amounts of sample disorder appear to induce larger linewidth enhancement at $T_0$. We will present recent NMR and NQR experiments in oriented powder and single crystal samples of URu$_2$Si$_2$ and discuss the measurements with emphasis on a possible connection between linewidth/disorder effects and hidden order. Both NQR and NMR spectra as functions of temperature and NMR for different magnetic field strengths and orientations will be reported. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V38.00005: Spin-antisymmetric Landau-Pomeranchuk instability: hidden order parameter in URu$_2$Si$_2$ Lijun Zhu, C. M. Varma We show that the state arising from the Landau-Pomeranchuk instability in spin-antisymmetric channels exhibits long-range correlations of the mutual orientation of spins without any order of single-spin operators. Such a state is expected to show a divergence in the non-linear magnetic susceptibility at the phase transition to a broken spin-orbit rotational state. This conjecture is explicitly shown to be true in mean-field calculations in a simple model suitable for URu$_2$Si$_2$. The observed specific heat, the small change in the linear susceptibility, the divergence in the non-linear susceptibility and the change in the transition temperature with an applied uniform magnetic field are quantitatively explained with one parameter. Aspects of the observed anomalous nuclear relaxation in the hidden order parameter phase are also explained. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V38.00006: Magnetic fluctuations of filled skutterudites emerging in the transition region between singlet and triplet states Takashi Hotta In order to clarify magnetic properties of filled skutterudites, we investigate the Anderson model including seven $f$ orbitals hybridized with $a_{\rm u}$ conduction band. By using the numerical renormalization group method, we evaluate magnetic susceptibility and entropy of $f$ electron for $n$=1$\sim$13, where $n$ is local $f$-electron number. Then, we find that $f$-electron states are clearly distinguished as itinerant $\Gamma_7$ and localized $\Gamma_8$ in the filled skutterudite structure. For $n$=2 corresponding to Pr-based filled skutterudites, even if the ground state is $\Gamma_1$ singlet, there remain significant magnetic fluctuations from $\Gamma_4^{(2)}$ triplet state with small excitation energy. We envision a scenario that unconventional superconductivity is induced by such magnetic fluctuations in a limited region in which singlet and triplet states are interchanged. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V38.00007: Heavy Fermion Behavior, Crystalline Electric Field Effects, and Weak Ferromagnetism in SmOs$_4$Sb$_{12}$ W.M. Yuhasz, N.A. Frederick, P.-C. Ho, N.P. Butch, B.J. Taylor, T.A. Sayles, M.B. Maple, J.B. Betts, A.H. Lacerda, P. Rogl, G. Giester The filled skutterudite compound SmOs$_4$Sb$_{12}$ was prepared in single crystal form and characterized. The SmOs$_4$Sb$_{12}$ crystals have the LaFe$_4$P$_{12}$-type structure with lattice parameter $a = 9.3085$ \AA. Specific heat measurements indicate a large electronic specific heat coefficient of $\approx 880$ mJ/mol K$^{2}$, from which an enhanced effective mass $m^{*} \approx 170m_{e}$ is estimated. The specific heat data also suggest crystalline electric field (CEF) splitting of the Sm$^{3+} J = 5/2$ multiplet into a $\Gamma_{7}$ doublet ground state and a $\Gamma_{8}$ quartet excited state separated by $\sim 37$ K. Electrical resistivity $\rho$(T) measurements reveal a decrease in $\rho$(T) below $\sim 50$ K that is consistent with CEF splitting of $\sim 33$ K between a $\Gamma_{7}$ doublet ground state and $\Gamma_{8}$ quartet excited state. Specific heat and magnetic susceptibility measurements display a possible weak ferromagnetic transition at $\sim 2.6$ K, which could be an intrinsic property of SmOs$_4$Sb$_{12}$ or possibly due to an unknown impurity phase. This research was supported by the U.S. DOE (DE-FG02-04ER46105), the NSF (DMR-0335173), and the NEDO International Joint Research Program. Work at the NHMFL-LANL was supported by the NSF, the State of FL and the US DOE. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V38.00008: The effect of Nd substitution on superconductivity of PrOs$_4$Sb$_{12}$ P.-C. Ho, N.P. Butch, T. Yanagisawa, W.M. Yuhasz, N.A. Frederick, T.A. Sayles, D.P. Arovas, M.B. Maple, J.B. Betts, A.H. Lacerda The filled skutterudite compound PrOs$_4$Sb$_{12}$ is an unconventional heavy fermion superconductor with $T_c$=1.85K. Our recent study shows that NdOs$_4$Sb$_{12}$ is a heavy fermion ferromagnet ($T_{FM}\sim$1K) with possible spin wave excitations. Previous experimental evidence suggests that the superconducting pairing in PrOs$_4$Sb$_{12}$ is spin-triplet, which, when considered along with the low $T_{FM}$ of NdOs$_4$Sb$_{12}$ implies that PrOs$_4$Sb$_{12}$ may be near a ferromagnetic quantum critical point. We have measured zero-field ac magnetic susceptibility and electrical resistivity between 0T and 8T for the (Pr$_{1-x}$Nd$_x$)Os$_4$Sb$_{12}$ system with $x$=0.1, 0.2, 0.4, 0.6, 0.8. The superconductivity persists to $x$=0.4 and the ferromagnetism appears above $x$=0.6. In the normal state of samples with x=0.1, 0.2, 0.4, the features related to the antiferroquadrupolar phase are clearly observed, in contrast to the disappearance of the features above x=0.05 in the Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$ system. The phase diagrams of $T_{c}$ and $T_{FM}$-Nd concentration $x$ and $H$-$T$ will be presented. Funded by U.S. DOE and NSF; NHMFL by NSF, FL, and DOE. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V38.00009: Exciton mediated triplet superconductivity in Th system PrOs4Sb12 Mikito Koga We investigate a possibility of triplet superconductivity mediated by crystal-field excitations in a skutterudite PrOs$_4$Sb$_{12}$. The dispersive excitation modes (excitons) are caused by an effective exchange interaction due to the conduction electrons coupled with the Pr $f^2$ states. The Cooper pairing interaction reflects the point group symmetry of the Pr site rather than the global symmetry of the Fermi surface. The $T_h$ symmetry (without fourfold symmetric axes) of the skutterudite crystals is a key to stability of the triplet Cooper pairing. We attempt to explain a possible superconducting state with broken time-reversal symmetry and a multiple superconducting phase in a magnetic field, recently reported by experiments. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V38.00010: Low temperature normal and superconducting state properties of lightly doped PrOs$_{4}$Sb$_{12}$ N.A. Frederick, S.K. Kim, T.A. Sayles, P.-C. Ho, N.P. Butch, M.B. Maple The compound PrOs$_{4}$Sb$_{12}$ is a heavy fermion superconductor, the first one based on Pr, with a superconducting transition temperature $T_{c} = 1.85$ K. It has many interesting properties at low temperatures: multiple superconducting phases, apparently with nodes in the energy gap; possible time reversal symmetry breaking; and a High Field Ordered Phase (HFOP), believed to be due to antiferroquadrupolar ordering, in the normal state above $4.5$ T. When Ru is substituted for Os to form Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$, the multiple superconducting transitions observed in the specific heat seem to disappear, the gap structure may change, and the HFOP is drastically reduced. However, these previous results only included a minimum Ru substitution of $x = 0.05$. In this talk, we will present new measurements of the specific heat, magnetic susceptibility, and electrical resistivity of Pr(Os$_{1-x}$Ru$_{x}$)$_{4}$Sb$_{12}$ for $x \leq 0.05$ at low temperatures and high magnetic fields. This research was supported by the U.S. DOE (DE-FG02-04ER-46105) and the NSF (DMR-03-35173). [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V38.00011: Neutron Diffraction Study on Stoner Gap in the Superconducting Ferromagnet UGe$_2$ Naofumi Aso, Yoshiya Uwatoko, Kazuma Hirota, Gaku Motoyama, Seiko Ban, Noriaki K. Sato, Yoshiya Homma, Yoshinobu Shiokawa We report the neutron diffraction study on the temperature dependence of Bragg peak intensities ($I_B$) in the superconducting ferromagnet UGe$_2$. When the temperature is lowered, $I_B(T)$ steeply increases below a characteristic temperature ($T_X$) in the ferromagnetic phase. We have found that it can be explained in terms of a simple Stoner model, and that the so-called Stoner gap $\Delta$ decreases with increasing pressure and collapses to zero around a critical pressure ($P_X \sim $ 1.2 GPa) where the superconducting transtion temperature displays a maximum. We speculate that a transition from the paramagnetic to perfect polarized state takes place at $T_X$~and $P_X$~in heavy electron bands. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V38.00012: Coexistence of ferromagnetism and superconductivity near Andriy Nevidomskyy A microscopic mean-field theory of the phase coexistence between ferromagnetism and superconductivity in the nearly ferromagnetic itinerant electron system is constructed, while incorporating a realistic mechanism for superconducting pairing due to the exchange of critical spin fluctuations. The self-consistent solution of the resulting equations determines the superconducting transition temperature which is shown to depend strongly on the value of exchange splitting. The effect of phase crossover from isotropic (Heisenberg-like) to uniaxial (Ising-like) spin fluctuations near the quantum critical point is analysed and the resulting phase diagram is obtained. This scenario is then applied to the case of itinerant ferromagnet UGe$_2$, which allows us to explain successfully certain features of the experimentally observed phase diagram. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V38.00013: The symmetry of the superconducting order parameter in PuCoGa$_5$ R.H. Heffner, G.D. Morris, E.D. Bauer, L.A. Morales, J.S. Sarrao, M.J. Fluss, D.E. MacLaughlin, L. Shu, J.E. Anderson The symmetry of the superconducting order parameter in single- crystalline PuCoGa$_5$ ($T_{\rm c} = 18.5$~K) is investigated via zero- and transverse- field muon spin relaxation ($\mu$SR) measurements, probing the possible existence of orbital and/or spin moments (time reversal-symmetry violation TRV) associated with the superconducting phase and the in-plane magnetic-field penetration depth $\lambda(T)$ in the mixed state, respectively. We find no evidence for TRV, and show that $\Delta\lambda(T) = \lambda(T) - \lambda(0) \propto T$ for $T/T_ {\rm c} \leq 0.5$. Taken together these measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state. [Preview Abstract] |
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