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Register a new userEvidence for time-reversal symmetry breaking in superconducting PrPt4Ge12
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Added by
Alexander Maisuradze
Publication date
2010
fields
Physics
and research's language is
English
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Zero and longitudinal field muon spin rotation (muSR) experiments were performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an anomalous superconductor and the weak and gradual change of Tc and of the related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12) suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.
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The single helical Fermi surface on the surface state of three-dimensional topological insulator Bi2Se3 is constrained by the time-reversal invariant bulk topology to possess a spin-singlet superconducting pairing symmetry. In fact, the Cu-doped, and pressure-tuned superconducting Bi2Se3 show no evidence of the time reversal symmetry breaking. We report on the detection of the time reversal symmetry (TRS) breaking in the topological superconductor Sr0.1Bi2Se3 , probed by zero-field (ZF) {mu}SR measurements. The TRS breaking provides strong evidence for the existence of spin-triplet pairing state. The temperature dependent super-fluid density deduced from transverse-field (TF) {mu}SR measurement yields nodeless superconductivity with low superconducting carrier density and penetration depth {lambda} = 1622(134) nm. From the microscopic theory of unconventional pairing, we find that such a fully gapped spin triplet pairing channel is promoted by the complex interplay between the structural hexagonal warping and higher order Dresselhaus spin-orbit coupling terms. Based on Ginzburg-Landau analysis, we delineate the mixing of singlet to triplet pairing symmetry as the chemical potential is tuned far above from the Dirac cone. Our observation of such spontaneous TRS breaking chiral superconductivity on a helical surface state, protected by the TRS invariant bulk topology, can open new avenues for interesting research and applications.
The ordinary magnetoresistance (MR) of doped semiconductors is positive and quadratic in a low magnetic field, B, as it should be in the framework of the Boltzmann kinetic theory or in the conventional hopping regime. We observe an unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic nor always positive. In a certain current direction MR is negative and linear in B in fields below a few tens of mT with a crossover to a positive MR at higher fields, while in a perpendicular current direction we observe a giant super-linear and positive MR. These extraordinary MRs are respectively explained by a hopping magneto-conductance via non-zero angular momentum orbitals, and by the magneto-conductance of inhomogeneous media. The linear orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in graphite. While some local paramagnetic centers could be responsible for the broken TRS, the observed large diamagnetism suggests a more intriguing mechanism of this breaking, involving superconducting clusters with unconventional (chiral) order parameters and spontaneously generated normal-state current loops in graphite.
We report muon spin relaxation measurements on the superconductor Sr2RuO4 that reveal the spontaneous appearance of an internal magnetic field below the transition temperature: the appearance of such a field indicates that the superconducting state in this material is characterized by the breaking of time-reversal symmetry. These results, combined with other symmetry considerations, suggest that superconductivity in Sr2RuO4 is of p-wave (odd-parity) type, analogous to superfluid 3He.
To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation ($mu$SR) studies of superconducting noncentrosymmetric Re$_{0.82}$Nb$_{0.18}$ ($T_c = 8.8$ K) and centrosymmetric Re ($T_c = 2.7$ K). In Re$_{0.82}$Nb$_{0.18}$, the low temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re$_{0.82}$Nb$_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $mu$SR below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric Re$T$ ($T$ = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg$_{10}$Ir$_{19}$B$_{16}$ and Nb$_{0.5}$Os$_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and Re$T$. We discuss the superconducting order parameter symmetries that are compatible with the observations.
Fascinating phenomena have been known to arise from the Dirac theory of relativistic quantum mechanics, which describes high energy particles having linear dispersion relations. Electrons in solids usually have non-relativistic dispersion relations but their quantum excitations can mimic relativistic effects. In topological insulators, electrons have both a linear dispersion relation, the Dirac behavior, on the surface and a non-relativistic energy dispersion in the bulk. Topological phases of matter have attracted much interest, particularly broken-symmetry phases in topological insulator materials. Here, we report by Nb doping that the topological insulator Bi2Se3 can be turned into a bulk type-II superconductor while the Dirac surface dispersion in the normal state is preserved. A macroscopic magnetic ordering appears below the superconducting critical temperature of 3.2 K indicating a spontaneous spin rotation symmetry breaking of the Nb magnetic moments. Even though such a magnetic order may appear at the edge of the superconductor, it is mediated by superconductivity and presents a novel phase of matter which gives rise to a zero-field Hall effect.
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