Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U10: Superconductivity: Magnetic Field Effects |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Neil Sullivan, University of Florida Room: Morial Convention Center RO8 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U10.00001: Peak Effect in Co$_{x}$NbSe$_{2}$ Single Crystals Maria Iavarone, R. Di Capua, G. Karapetrov, A. Koshelev, D. Rosenmann, H. Claus, W.K. Kwok We report a pronounced peak effect in the magnetization of Co$_{x}$NbSe$_{2}$ single crystals having critical temperatures ranging between 7.1 K and 5.0 K. Magnetization studies reveal that the magnetic irreversibility below the peak effect regime is higher in samples with lower concentration of Co while exhibits a nearly reversible magnetization over a wide range of magnetic field for samples with higher concentration of Co. However, in the peak effect regime the situation is different. The irreversibility is a non-monotonic function of the Co content, and therefore of the critical temperature of the sample. This behavior cannot be explained as a crossover between collective to single pinning regimes as suggested for NbSe$_{2}$, since this should be a monotonic function of number of pinning centers. Furthermore, we investigated the peak effect regime with low temperature STM at 4.2 K and 1.8 K. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U10.00002: Effect of the sample geometry on the intermediate state in mesoscopic 3D Type-I superconductors Golibjon Berdiyorov, Alexander Hernandez, Francois Peeters The intermediate state (IS) of type-I superconductors (SC) has recently became a topic of increasing interest [1,2]. Direct imaging of type-I SC reveals two distinct topologies of the intermediate state: flux tubes are formed upon magnetic field penetration and laminar patterns appear upon flux exit [2]. However, spheres and cones show no hysteresis with flux tubes dominating the IS [1]. In this work we investigate the effect of the sample topology on the formation of the flux patterns in mesoscopic type-I SC using the phenomenological Ginzburg-Landau theory. We carry out simulations on three-dimensional samples of different geometries. We show that in the samples with sharp boundaries (cubes and disks) laminar structures are mostly located along the boundary, whereas radial distribution of the flux patterns is obtained for cones and spheres. The effect of the edge defects on the observed structures will also be studied. [1] R.Prozorov, Phys. Rev. Lett. \textbf{98}, 257001 (2007). [2] M. Menghini et al., Phys. Rev. B \textbf{75}, 014529 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U10.00003: Pauli Paramagnetic Effects on the Vortices in Superconducting TmNi$_2$B$_2$C L. DeBeer-Schmitt, M.R. Eskildsen, M. Ichioka, K. Machida, N. Jenkins, C.D. Dewhurst, S.L. Bud'ko, P.C. Canfield The magnetic field distribution around the superconducting vortices in TmNi$_2$B$_2$C in the paramagnetic phase above $T_N$ was studied experimentally as well as theoretically. Using small-angle neutron scattering we imaged the vortex lattice (VL). Using the magnitude of the VL scattering vector, we obtain a direct measure of the magnetic induction, $B$, which is found to exceed $\mu_0 H$ at all fields up to $H_{c2}$. Measurements of the VL reflectivity yielded a form factor which remains essentially constant up to $\sim 0.6 H_{c2}$, above which it decreases rapidly but remains measurable up to the upper critical field. This field dependence of the form factor is in striking contrast to the usual exponential suppression. The measured form factor is well fitted by model based on the Eilenberger equations, extended to include paramagnetic effects due to the exchange interaction with the localized 4$f$ Tm moments. The model shows how the induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor. The results will be compared to our recent measurements of the VL form factor in CeCoIn$_5$, which also indicate strong paramagnetic effects. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U10.00004: Pauli-Limited Superconductors with Magnetic Fluctuations Robert Beaird, Anton Vorontsov, Ilya Vekhter We examine the temperature ($T$) and magnetic field ($B$) phase diagram of a paramagnetically-limited superconductor in the presence of fluctuating magnetic moments. We derive a Ginzburg- Landau free energy functional for the superconducting order from microscopics and include classical fluctuations of the magnetization. We consider the full wave vector ($q$) dependence of the expansion coefficients and examine the onset of both the uniform and spatially modulated superconducting phases along critical magnetic field $B_c(T)$. In the absence of fluctuations, both superconducting transitions are second order in 2D and meet at a tricritical point $B_c (T^\star)$. We find that including magnetic fluctuations introduces first order transitions into the superconducting phase. We find the tricritical points where these transitions meet and determine $B_c(T)$ and $q(T)$ as functions of the coupling between superconducting order and the fluctuating magnetization. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U10.00005: Evidence for Field-Induced Quantum Criticality in an Overdoped Cuprate Takasada Shibauchi, Lia Krusin-Elbaum, Masashi Hasegawa, Yuichi Kasahara, Ryuji Okazaki, Yuji Matsuda In current views, the putative quantum phase transitions in high-$T_{\rm c}$ superconductors are deemed driven by charge doping. Here we uncover an unanticipated transition from a non-Fermi- to a Fermi-liquid state driven by magnetic field in a highly overdoped Tl$_2$Ba$_2$CuO$_{6+x}$ with $T_{\rm c}\approx 15$~K. From the $c$-axis resistivity measured up to 45~T, we show that the Fermi-liquid $\rho_c = \rho_c(0)+ AT^2$ features, accompanied by a (quantum) field-linear magnetoresistance, appear above a temperature-dependent field $H_{\rm FL}$, which decreases linearly with decreasing temperature and points to a quantum critical point near the upper critical field $H_{\rm c2}(0)$. The observed field-induced quantum criticality with a power-law diverging $A(H)$ bears a striking resemblance to that of heavy-fermion superconductor CeCoIn$_5$, suggesting a common underlying physics in these strongly correlated electron systems. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U10.00006: Superconducting Vortices in CeCoIn$_5$: Beyond the Abrikosov-Ginzburg-Landau Paradigm A. D. Bianchi, M. Kenzelmann, J. Mesot, M. Zolliker, J. Kohlbrecher, L. DeBeer-Schmitt, M. R. Eskildsen, J. S. White, E. M. Forgan, Z. Fisk, R. Movshovich, E. D. Bauer, J. L. Sarrao, C. Petrovic We report on the magnetic field ($H$) dependence of the form factor $|F|^2$ of the vortex lattice (VL) in CeCoIn$_5$ obtained by small angle neutron scattering for $H$ applied along the crystallographic $c$-axis. Superconductivity (SC) in CeCoIn$_5$ has several unconventional aspects to it: The $d$-wave SC is in competition with antiferromagnetic order, as suggested by the presence of an magnetic QCP located at the upper critical field $H_{c2}$ determined by the Pauli effect. At both 50 and 500~mK we observe an $H$-independent $|F|^2$ up to 2~T. With further increasing $H$, $|F|^2$ continues to increase all the way up to $H_{c2}$. This finding is in contrast to that normally observed in type-II SC's, where $|F|^2$ decreases with increasing $H$. It suggests a departure from the Abrikosov-Ginzburg-Landau paradigm, where the properties of the vortex state can be described by the coherence length $\xi$ and the penetration depth $\lambda$. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U10.00007: Reversal of specific heat oscillations with planar magnetic field in 2D d-wave superconductors: Doppler shift beyond the nodal approximation. G.R. Boyd, P.J. Hirschfeld, Ilya Vekhter Experiments on several novel superconducting compounds found oscillations of the specific heat when an applied magnetic field is rotated with respect to the crystal axes. The results are commonly interpreted as arising from the nodes of an unconventional order parameter, but the identifications of nodal directions are sometimes controversial. While the semiclassical method predicted the minima for the field aligned with the nodes in the low-energy range, inclusion of vortex scattering was shown to lead to an inversion of the oscillation pattern at finite energy. Here we show with a simple model calculation that even within semiclassical approach with no scattering on the vortices, either minima or maxima can occur in thermodynamic quantities depending on the the temperature and the magnetic field scale. Using a d-wave BCS model and approximating the quasiparticle excitations semiclassically, we find a sign reversal of the field angle oscillations as temperature is increased. Absence of this effect in earlier work within the same approach is attributed to the use of the nodal approximation, which breaks down at higher energies. The result indicates that the inversion of oscillations is an intrinsic feature of unconventional superconductors. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U10.00008: Effect of Charge Carrier Density on the Vortex Regimes in Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystals P. Gyawali, V. Sandu, C.C. Almasan, B.J. Taylor, M.B. Maple We report the evolution of the vortex matter state in the temperature and field range where the second magnetization peak SMP is present by studying the magnetization and magnetic relaxation of a series of Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ ($x=0.13$, $T_{c}=82$ K; $x=0.34$, $T_{c}=50$ K; $x=0.47$, $T_{c}=34$ K) single crystals. Our study has shown that the main ingredient that controls the evolution of the vortex matter through the different regimes is the charge carrier density. The SMP is first enhanced and then suppressed as $Pr$ concentration increases. The reason for this behavior is the softening of the elastic moduli, which makes the vortex lattice less stable to defect invasion. Within the collective creep theory, we determined the apparent activation energy. Its evolution with current density has shown that the vortex system is predominantly elastically pinned below the SMP, while above there is a smooth crossover to a vortex regime most likely dominated by the proliferation of dislocations. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U10.00009: Skyrmion Flux Lattices and their $\mu$SR signature Qi Li, John Toner, Dietrich Belitz Recently, topological excitations known as skyrmions were predicted to exist in p-wave superconductors [1]. The elastic theory of an induced skyrmion lattice was developed in [2], and its melting curve was found to be qualitatively different from that for vortex lattices. Here we show that the muon spin resonance ($\mu$SR) signatures of the two types of lattices are also very different. $\mu$SR has been applied extensively to study the magnetic properties of vortex flux lattices [3]. The observable in this technique is the $\mu$SR line shape $n(B)$, which is the probability density that a muon experiences a local magnetic induction $B$. In a vortex lattice, for small $B$, $n(B) \propto \ln(1/B)/B$. By contrast, for a skyrmion lattice we predict $n(B) \propto B^ (-3/2)$. This difference provides another way to easily distinguish between vortex and skyrmion flux lattices, and can thus help to identify p-wave superconductors. [1] A. Knigavko, B. Rosenstein, and Y.F. Chen, Phys. Rev. B 60, 550 (1999). [2] Qi Li, John Toner, and D. Belitz, Phys.Rev. Lett. 98, 187002 (2007). [3] J. E. Sonier, J.H. Brewer, and R. F. Kiefl, Rev. Mod. Phys. 72, 769 (2000). [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U10.00010: Non-linear magnetization effects within the Kosterlitz-Thouless theory Lara Benfatto, Claudio Castellani, Thierry Giamarchi Recent experiments in cuprate superconductors have attracted the attention on the role of vortex fluctuations. Measurements of the field-induced magnetization showed that the correlation length diverge exponentially, as predicted within the Kosterlitz-Thouless (KT) theory. However, it is somehow puzzling thepersistence of strong non-linear magnetization effects at low field. Here we address this issue by means of a new theoretical approach to the KT transition at finite magnetic field, based on the sine-Gordon model. This approach is particularly useful in two respects. First, it leads to a straightforward definition of the field-induced magnetization as a function of the external magnetic field $H$ instead of the magnetic induction $B$, which is crucial to get a consistent description of the Meissner phase. Second, it allows us to identify the cross-over field $H_{cr}$ from linear to non-linear magnetization both below and above the transition. Above $T_{KT}$ $H_{cr}$ turns out to scale as the inverse correlation length, so that it decreases as the transition is approached. As a consequence, the fact that only the non-linear regime is accessible experimentally should be interpreted as a typical signature of the fast divergence of the correlation length within the KT theory. L.Benfatto, C.Castellani and T.Giamarchi, Phys. Rev. Lett. 99, 207002 (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U10.00011: Theory of diamagnetic response in layered superconductor above ``Tc" PeiJen Lin, DingPing Li, Baruch Rosenstein Recent work by Wang et al. (Phys. Rev. Lett. 95, 247002) on field induced diamagnetic properties above Tc attracted a lot of attention from both theoretical and experimental physicist. In this talk, we will show that the phenomenon can be understood using conventional Ginsburg Landau theory for anisotropic materials. Above Tc, where the thermal fluctuations are strong, the effective description based on GL becomes non trivial due to the important contribution of higher landau levels. In previous works, some progress was achieved when certain additional assumptions (such as Lowest Landau Level approximation ) were made. However, the validity of these assumptions is under debate. In our study of this system, we include the contribution of higher landau levels with nonperturbative method.Comparison will be made with Varlamov-Larkin results. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U10.00012: Theory of quantum magneto-oscillations in underdoped cuprate superconductors Sasha Alexandrov Magneto-oscillations in kinetic and magnetic response functions of a few underdoped cuprates are perhaps one of the most striking observations since many probes of underdoped cuprates clearly point to a non Fermi-liquid normal state. Their observation in the vortex state well below the upper critical field raises a doubt concerning their normal state origin. Here I propose an explanation of the magneto- oscillations as emerging from the quantum interference of the vortex lattice and checkerboard modulations of the electron density of states revealed by STM with atomic resolution in some cuprate superconductors. The checkerboard effectively pins the vortex lattice, when the period of the latter is commensurate with the period of the checkerboard. This condition yields $1/\sqrt{B}$ periodicity of the response functions, rather than $1/B$ periodicity of conventional normal state oscillations periodic versus inverse magnetic field $B$. Our solution of the Gross-Pitaevskii-type equation for composed charged bosons accounts for the d-wave symmetry of the order-parameter and its checkerboard modulations, and describes well changes in resonant frequency of the tunnel-diode oscillator circuit with YBa$_2$Cu$_4$O$_8$ and the oscillatory part of the Hall resistance in the mixed state of YBa$_2$Cu$_3$O$_{6.5}$. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U10.00013: Enhanced tunneling in a magnetic field Boris Ivlev As known, a probability of quantum tunneling through a static potential barrier $U(x)$ can be substantially reduced by a static magnetic field $H_{z}$. This happens due to increase of the effective barrier height caused by Landau's gauge potential in a magnetic field (the same potential results in Landau levels). There is an exponentially small current in the direction of tunneling, $x$. An underbarrier current in the direction perpendicular to tunneling, $y$, is not small. If the potential barrier $U(x,y)$ depends also on the coordinate $y$, a new unexpected scenario can occur. Now the partial de Broglie waves, generated under the barrier, are not collected to the current in the $y$ direction only but can be reflected by the potential $U(x,y)$. An interference of those underbarrier waves after reflections can result in a peak of the particle density at a classically allowed region close to the conventional exit point from under the barrier. At the certain magnetic field, $H_{z}=H_{R}$, the peak amplitude is not exponentially small (Euclidean resonance). The same phenomenon can occur in tunneling through nonstationary barriers and is expected for photon tunneling when a refractive index is slightly inhomogeneous in the tunnel region. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U10.00014: Current induced properties in bulk YBCO above the transition temperature George Zimmerman Simultaneous measurement of the electrical resistivity at high current densities and magnetic susceptibility of YBCO superconducting material reveals interesting behavior of the samples above the transition temperature. In addition to anomalies which appear at temperatures between 85K and 140K, the susceptibility seems to be affected by the electrical current up 200K. The electrical current, of density between 8 and 400 A/cm$^2$ seems to induce the behavior, with a lowered resistivity, which suggests a first order phase transition, possibly meta-stable, and persists despite the repeated cycling between 77K and 300K. The samples of bulk polycrystalline cylindrical rods 1.22 mm diameter and between 6cm and 20 cm long have a density of 5.4 gm/cm$^3$ and were prepared sintering and annealing from a YBCO powder. Most were 10 to 15 years old. At 77K most samples exhibit relaxation times of several minutes in their magnetic and resistive behavior. The details of the measurements as a function of temperature, current density, and low magnetic field, will be described along with possible implications as to the nature of the pseudo-gap and other competing theories. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700