Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X2: Fulde-Ferrell-Larkin-Ovchinnikov Superconductivity in Heavy Fermion Compounds |
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Sponsoring Units: DCMP Chair: Stanley Tozer, NHMFL Room: LACC 151 |
Friday, March 25, 2005 8:00AM - 8:36AM |
X2.00001: Superconductivity magnetically enhanced by spin domains: The FFLO state Invited Speaker: We report angular dependent heat capacity, penetration depth, and magnetization measurements on CeCoIn$_{5}$, a clean and anisotropic strongly correlated superconductor. CeCoIn$_{5}$ has a number of unusual properties that vastly enhance the coupling of an external magnetic field to the electron spins (paramagnetic limit) over the traditional orbital coupling. We will show that the angle between an external magnetic field and the conducting planes determines the high-field superconducting ground state. Our results point to the existence of the Fulde-Ferrell-Larkin-Ovchinnikov state at fields above 10 tesla and temperatures below 300 mK with the field applied parallel to the conducting planes. The FFLO state, first predicted in 1964, consists of a periodic array of ferromagnetic walls of depaired electrons coexisting with superconducting regions. For small tilt angles, where there is an admixture of paramagnetic and orbital effects, the superconducting order parameter assumes higher Landau levels within the FFLO state. This is experimentally realized as a cascade of phase transitions in the magnetization. Finally, for an applied field at angles greater than approximately 15 degrees out of the planes, no FFLO signature is observed and a BCS-type superconducting state emerges. \par In collaboration with S.W. Tozer, T.P. Murphy, E.C. Palm, S.T. Hannahs, and D. Hall at the NHMFL, N.A. Fortune at Smith College, C.C. Agosta and C. Martin at Clark University, J.C. Cooley and J.L. Sarrao at LANL, and C. Petrovic at BNL. Funded by DOE {\#}DE-FG52-03NA00066, NSF {\#}DMR-0084173, and the State of Florida. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 9:12AM |
X2.00002: Pauli Limiting, First Order Superconducting Phase Transition, and a Possible Fulde-Ferrell-Larkin Ovchinnikov Inhomogeneous Superconducting State in CeCoIn$_5$ Invited Speaker: We report specific heat and thermal conductivity measurements of the heavy fermion superconductor CeCoIn$_{5}$ in the vicinity of the superconducting critical field H$_{c2}$, with magnetic field in the plane of this quasi-2D compound and at temperatures down to 50 mK. The superconducting phase transition changes from second to first order for field above 10 T, as evident from a sharp peak in specific heat and a jump in thermal conductivity, indicating the importance of the Pauli limiting effect in CeCoIn$_{5}$. In the same range of magnetic field we observe a second specific heat anomaly within the superconducting state. We interpret this anomaly as a signature of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting state. In addition, the thermal conductivity data as a function of field display a kink at a field H$_{k}$ below the superconducting critical field, which closely coincides with the low temperature anomaly in specific heat, tentatively identified with the appearance of the FFLO superconducting state. Our results indicate that the thermal conductivity is enhanced within the FFLO state, and call for further theoretical investigations of the real space structure of the order parameter (and in particular, the structure of vortices) and of the thermal transport within the inhomogeneous FFLO state. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:48AM |
X2.00003: Theoretical Study of Fulde-Ferrell-Larkin-Ovchinnikov State in Unconventional Superconductors Invited Speaker: In this talk I will report results of our theoretical studies on the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) State in unconventional superconductors. In particular, I will discuss (i) the possibilities of using phase-sensitive Josephson effect and exotic vortex lattice structure to detect the FFLO state, and measure the momentum of the order parameter; (ii) an exact solution that indicates the second order nature of the BCS-FFLO transition in the 1D limit; (iii) An analysis of fluctuation effects based on renormalization group, which suggests that the transition between the normal and FLLO states is a fluctuation-driven first order transition, even when mean-field theory suggests a second-order transition. Comparisons will the made with experiments whenever appropriate. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:24AM |
X2.00004: Fulde-Ferrell-Larkin-Ovchinnikov state in quasi-low-dimensional superconductors Invited Speaker: The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO or LOFF) state, which has been predicted in clean type II superconductors, is favored in quasi-low-dimensional systems for the following two reasons. (1) The orbital pair-breaking effect can be suppressed by orienting magnetic field to any direction parallel to the conductive layer. For such a direction of the magnetic field, the upper critical field can reach a value near the Pauli paramagnetic limit, where the FFLO state may occur. (2) The FFLO state is stabilized by a Fermi-surface effect which is analogous to the Fermi surface nesting effect in spin density wave (SDW) and charge density wave (CDW). Such an effect is most pronounced in the nearly one-dimensional system, but the nesting condition for SDW and CDW is also perfectly satisfied in such a system. Hence, taking into account the competition with the SDW and CDW instabilities, quasi-two-dimensional superconductors would be the best candidates for the FFLO state to occur. In particular, quasi-two-dimensional heavy fermion superconductors would be favorable for the FFLO state, since the orbital pair-breaking effect is suppressed also due to the heavy effective mass in addition to the effects mentioned above. Some of the organic superconductors can also be good candidates. We also discuss the competition between the vortex state and the FFLO state in quasi-two-dimensional systems. For example, the FFLO state can be regarded as the vortex state with infinite Landau level index $n$. In the systems with sufficient three dimensionality, the center-of-mass momentum $\mathbf{q}$ of the FFLO state is oriented to the direction of the magnetic field as found in Gruenberg and Gunther's theory. In contrast, in the two-dimensional system in exactly parallel magnetic field, $\mathbf{q}$ is oriented to the direction for which the Fermi-surface effects are maximized. These two states in the opposite limits must be continuously connected by the vortex states with higher Landau level indexes. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 11:00AM |
X2.00005: Texture in the superconducting order parameter of CeCoIn$_5$; FFLO state as evidenced by ultrasound and NMR Invited Speaker: A myriad of fascinating properties have been proposed for unconventional superconductors in the presence of a strong magnetic field. Among the possible exotic superconducting (SC) phases, a spatially nonuniform SC state originating from the paramagnetism of conduction electrons has become a subject of particular interest after the pioneering work by Fulde and Ferrel and Larkin and Ovchinnikov (FFLO) in the mid-1960's. We present here ultrasound and NMR studies of the quasi-2D heavy-fermion superconductor CeCoIn$_5$ with extremely large Pauli paramagnetic susceptibility, which is believed to host a FFLO state. Ultrasound velocity measurements reveal an unusual structural transformation of the flux line lattice (FLL) in the vicinity of the upper critical field and with a magnetic field applied parallel to the $ab$ plane. The transition field coincides with that at which heat capacity measurements reveal a second order phase transition. The lowering of the sound velocity at the transition is consistent with the collapse of the FLL tilt modulus and a crossover to quasi two-dimensional FLL pinning. In the vicinity of the upper critical field , the $^{115}$In NMR spectrum also exhibits a dramatic change below $T^*(H)$ which well coincides with the position of reported anomalies in specific heat and ultrasound velocity. Below $T^*(H)$ a new resonance line appears at higher frequency, which can be attributed to the normal quasiparticle sheets formed in the SC regime. The NMR spectrum also indicate that the vortex core structure of CeCoIn$_5$ appears to be markedly different from that of ordinary superconductors. On the basis of these results, we were able to establish a clear evidence of the spatially inhomogeneous SC state at high field and low temperatures, precisely as expected in a FFLO state. In collaboration with T.Watanabe, Y.Kasahara, K.Izawa (Univ. of Tokyo), C.J. van der Beek(Ecole Polytechnique),K.Kakuyanagi, K.Kumagai (Hokkaido Univ.), M.Nohara, T.Hanaguri, H.Takagi,(Univ. of Tokyo) H.Shishido, R.Settai, and Y.Onuki(Osaka Univ.) [Preview Abstract] |
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