Session U8: Focus Session: Superconductivity and Magnetism

Odd-frequency pairing in superconducting heterostructures .

We present a general theory of the proximity effect in junctions between unconventional superconductors and diffusive normal metals (DN) or ferromagnets (DF). We consider all possible symmetry classes in a superconductor allowed by the Pauli principle: even-frequency spin-singlet even-parity state, even-frequency spin-triplet odd-parity state, odd-frequency spin-triplet even-parity state and odd-frequency spin-singlet odd-parity state. For each of the above states, symmetry and spectral properties of the induced pair amplitude in the DN (DF) are determined. The cases of junctions with spin-singlet s- and d-wave superconductors and spin-triplet p-wave superconductors are adressed in detail. We discuss the interplay between the proximity effect and midgap Andreev bound states arising at interfaces in unconventional (d- or p-wave) junctions. The most striking property is the odd-frequency symmetry of the pairing amplitude induced in DN (DF) in contacts with p-wave superconductors. This leads to zero-energy singularity in the density of states and to anomalous screening of an external magnetic field. Peculiarities of Josephson effect in d- or p-wave junctions are discussed. Experiments are suggested to detect an order parameter symmetry using heterostructures with unconventional superconductors.   

Phase Diagram and Thermodynamic Properties of Layered Ferromagnet-Superconductor Nanostructures

We present results obtained by exact, fully self-consistent numerical solution of the microscopic Bogoliubov-DeGennes equations for Superconductor/Ferromagnet (S/F) nanostructures in the clean limit. We discuss the thermodynamics of SFS trilayers, including the complete phase diagram over experimentally relevant parameter range. Both the first order $0$ to $\pi$ transition and the second order transition to the normal state at $T_c$ are included. We find excellent agreement between theoretical and experimental results for $T_c$. Other relevant quantities such as DOS, magnetization, and conductivity will also be discussed.   

Transition Temperature Shifts in Asymmetric FSF Multilayers

Ferromagnet-Superconductor-Ferromagnet multilayers, where the superconductor thickness is on the order of the coherence length, exhibit shifts in the superconducting $T_c$ when the relative magnetization of the ferromagnetic layers is changed. However, experimental work has produced shifts orders of magnitude smaller than theoretically predicted. We investigate this discrepancy by examining multilayers where the thickness of the two ferromagnetic layers is varied. Our work indicates that differences between the two superconductor-ferromagnet interfaces may play a role in explaining the discrepancy, and demonstrates how the use of hard vs. soft ferromagnets can effect these devices.   

Conductance spectra of ferromagnetic superconductors

Recent findings of superconductors that simultaneously exhibit multiple spontaneously broken symmetries, such as ferromagnetic order or lack of an inversion center and even combinations of such broken symmetries, have led to much theoretical and experimental research. Ferromagnetic superconductors represent a marriage of two physical phenomena that conventionally have been considered virtually incompatible. We here study quantum transport in a junction consisting of a ferromagnetic metal and a non-unitary p-wave ferromagnetic superconductor. Considering several different possible pairing symmetries, our results show how the magnitude of the superconducting gaps may be inferred from the conductance spectra, in addition to their relative orientation in momentum-space. Also, we investigate how the strength of the magnetic exchange energies on both sides of the junction affect these spectra.   

Derivation of the Ginzburg-Landau equations of a ferromagnetic $p$-wave superconductor

We derive a Ginzburg-Landau free energy for a $p$-wave ferromagnetic superconductor. The starting point is a microscopic Hamiltonian including a spin generalised BCS term and a Heisenberg exchange term. We find that coexistence of magnetisation and superconductivity depends on the sign of the energy-gradient of the DOS at Fermi level. We also compute the tunnelling contribution to the Ginzburg-Landau free energy, and find expressions for the spin-currents and Josephson currents across a tunnelling junction separating two ferromagnetic $p$-wave superconductors.   

Nearly ferromagnetic superconductors

The coexistence of ferromagnetism and superconductivity has received substantial attention over the years [1]. Here we report on a theory for the electromagnetic properties of superconductors in the paramagnetic phase near a ferromagnetic instability [2]. Using a generalized Ginzburg-Landau theory, we have found that the magnetic flux expulsion capability of the superconductor gets {\it stronger} as the normal-state magnetic susceptibility increases. The temperature dependencies of the London penetration depth, the critical fields, and the critical current are all strongly affected by ferromagnetic fluctuations. For the critical current we find a temperature exponent $\alpha \approx 2$ over an appreciable temperature range. The extent to which proximity to magnetic criticality may be a viable explanation for recent observations in MgCNi microfibers, which find $\alpha \approx 2$ [3], is discussed. \medskip\par\noindent [1] E.I Blount and C.M. Varma, Phys. Rev. Lett. {\bf 42}, 1079 (1979); D.E. Moncton et al., Phys. Rev. Lett. {\bf 45}, 2060 (1980); J.W. Lynn et al., Phys. Rev. Lett. {\bf 46}, 368 (1981). \par\noindent [2] Qi Li et al., Phys. Rev. B {\bf 74}, 134505 (2006). \par\noindent [3] A.P. Young et al., Phys. Rev. B {\bf 70}, 064508 (2004).   

Anisotropic Pauli depairing effects and field-induced nodal excitations in superconductors without inversion symmetry

Superconductors without inversion symmetry has been currently attracting much interest. Here, we investigate theoretically a novel effect in the vortex state realized when the magnitude of the spin-orbit splitting due to the inversion symmetry breaking (ISB) is of the same order as the superconducting gap. Such a situation with the small ISB may be relevant to a superconductor without inversion symmetry Y$_2$C$_3$, which was recently discovered by Akimitsu et al. We show that in this case the Pauli depairing effect is anisotropic in the momentum space, and thus induces nodal excitations even for s-wave superconductors. We calculate the density of states and the specific heat coefficient on the basis of the Eilenberger's quasiclassical method, and compare the results with experiments for Y$_2$C$_3$.   

Superconducting properties and the Fermi surface in noncentrosymmetric CeRhSi$_3$

CeRhSi$_3$ is a recently-discovered noncentrosymmetric superconductor [Kimura \textit{et al}., PRL \textbf{95}, 247004 (2005)]. At ambient pressure $P$, it orders antiferromagnetically below $T_N$ = 1.6~K. $T_N$ decreases with $P$ above $\sim$8 kbar, and disappears somewhere above 20 kbar. Superconductivity is observed above $\sim$12 kbar. We have performed measurements of ac susceptibility and the de Haas-van Alphen effect (dHvA) with the field in the $c$ direction up to $P$ = 29.5 kbar. Remarkably high upper critical fields $B_{c2}$ are observed: e.g., $B_{c2}$ = 17.5 T at 0.46 K for $P$ =29.5 kbar, where the superconducting transition temperature is only 1.1 K. The Fermi surface continuously evolves from $P$ = 0 to 29.5 kbar, and the effective masses decrease with $P$. We argue that these are consistent with theoretical scenarios ascribing antiferromagnetism to spin-density-wave formation. Analyses of dHvA oscillations in the mixed state seem to suggest an anisotropic superconducting energy gap.   

Surface states in non-centrosymmetric superconductors

Since the discovery of superconductivity in CePt$_3$Si, there is a strong interest in superconducting materials without center of inversion. Lack of inversion symmetry results in strong spin-orbit interactions that lifts the spin degeneracy and leads to the mixing of `singlet' and `triplet' pairing channels. We consider surface bound states of a superconductor with bulk spin-orbit interactions described by a Rashba term, $\alpha (\hat{\bf z} \times {\bf k}) \cdot \mbox{\boldmath{$\sigma$}}$. We find that the scattering of the quasiparticle off the interface strongly mixes the two bands of opposite helicity. We analyze the properties of the surface states that appear for different surface orientations, investigate their spin structure and their possible signatures in tunneling experiments.   

Magnetic penetration depth in noncentrosymmetric Re$_3$W

The magnetic penetration depth is one of the most fundamental characteristics of a superconductor. We report measurements of temperature dependence of the penetration depth $\lambda$ in Re$_3$W- a superconductor without inversion symmetry. The penetration depth was extracted from dc magnetic susceptibility, measured on aligned quenched powder in epoxy using a SQUID magnetometer. At present, based on the low-temperature behavior of the superfluid density 1/$\lambda^2$, we see no evidence of unconventional behavior, i.e we see a fully-gapped state. Higher resolution data at low temperatures are needed to decide the case. ORNL is managed by UT-Battelle, LLC for USDOE under contract DE-AC05-00OR22725   

Helical and stripe vortex phases in non-centrosymmetric superconductors

When magnetic fields are applied to non-centrosymmetric superconductors, helical or stripe vortex phases are formed. We develop a quasiclassical microscopic theory for these phases. We will study the resulting phase diagrams and physical properties of these phases as a function of the relative density of states of the two spin-split bands and with varying Zeeman-field strength. We apply these results to CePt3Si, CeRhSi3, CeIrSi3, KOs2O6, Li2Pt3B and Li2Pd3B.   

Superconducting properties of noncentrosymmetric Mg$_{10}$Ir$_{19}$B$_{16}$

The magnetic, electrical transport and specific heat properties of the new ternary boride superconductor Mg$_{10}$Ir$_{19}$B$_{16}$ (T$_{C}$ = 5K) have been investigated. The polycrystalline Mg$_{10}$Ir$_{19}$B$_{16}$ sample was synthesized by reaction of Mg and Ir metal powders with amorphous boron powder. The material has a noncentrosymmetric crystal structure (I-43m) and therefore the superconducting properties are a subject of great interest.