Models of superconductivity in unconventional materials can be experimentally differentiated by the predictions they make for the symmetries of the superconducting order parameter. In the case of the ...heavy-fermion superconductor UPt3, a key question is whether its multiple superconducting phases preserve or break time-reversal symmetry (TRS). We tested for asymmetry in the phase shift between left and right circularly polarized light reflected from a single crystal of UPt3 at normal incidence and found that this so-called polar Kerr effect appears only below the lower of the two zero-field superconducting transition temperatures. Our results provide evidence for broken TRS in the low-temperature superconducting phase of UPt3, implying a complex two-component order parameter for superconductivity in this system.
According to conventional wisdom, the extraordinary properties of the cuprate high-temperature superconductors arise from doping a strongly correlated antiferromagnetic insulator. The highly ...overdoped cuprates-whose doping lies beyond the dome of superconductivity-are considered to be conventional Fermi liquid metals. We report the emergence of itinerant ferromagnetic order below 4 kelvin for doping beyond the superconducting dome in thin films of electron-doped La
Ce
CuO
(LCCO). The existence of this ferromagnetic order is evidenced by negative, anisotropic, and hysteretic magnetoresistance, hysteretic magnetization, and the polar Kerr effect, all of which are standard signatures of itinerant ferromagnetism in metals. This surprising result suggests that the overdoped cuprates are strongly influenced by electron correlations.
Constraining symmetryMost superconductors have only one transition point and, below a certain temperature, their electrical resistance drops to zero. In very rare cases, another superconducting ...transition appears at a lower temperature. By measuring its specific heat, Hayes et al. reveal that this two-step superconductivity occurs in the compound uranium ditelluride. Complementary optical measurements indicated the breaking of time reversal symmetry, constraining the possible symmetries of the order parameter in this material.Science, abb0272, this issue p. 797An unconventional superconducting state was recently discovered in uranium ditelluride (UTe2), in which spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy-fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggests that a distinctive set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a nonzero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe2 is characterized by a two-component order parameter that breaks time-reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe2.
Three-dimensional topological insulators are bulk insulators with Z
topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally ...resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi
X
(X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.
The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at ...temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum—dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.
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Measurements of the polar Kerr effect using a zero-area-loop Sagnac magnetometer on Pb/Ni and Al/(Co-Pd) proximity-effect bilayers show unambiguous evidence for the "inverse proximity effect," in ...which the ferromagnet induces a finite magnetization in the superconducting layer. To avoid probing the magnetic effects in the ferromagnet, the superconducting layer was prepared much thicker than the light's optical-penetration depth. The sign and size of the effect, as well as its temperature dependence agree with recent predictions by Bergeret et al. Phys. Rev. B 69, 174504 (2004)10.1103/PhysRevB.69.174504.
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Scanning tunneling spectroscopy studies on high-quality Bi2Te3 crystals exhibit perfect correspondence to angle-resolved photoemission spectroscopy data, hence enabling identification of different ...regimes measured in the local density of states (LDOS). Oscillations of LDOS near a step are analyzed. Within the main part of the surface band oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear, dispersing with a wave vector that may result from a hexagonal warping term.
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The polar Kerr effect in the high-T_(c) superconductor YBa2Cu3O6+x was measured at zero magnetic field with high precision using a cyogenic Sagnac fiber interferometer. We observed nonzero Kerr ...rotations of order approximately 1 microrad appearing near the pseudogap temperature T(*) and marking what appears to be a true phase transition. Anomalous magnetic behavior in magnetic-field training of the effect suggests that time reversal symmetry is already broken above room temperature.
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STM imaging of impurity resonances on Bi2Se3 Alpichshev, Zhanybek; Biswas, Rudro R; Balatsky, Alexander V ...
Physical review letters,
2012-May-18, Volume:
108, Issue:
20
Journal Article
Peer reviewed
Open access
In this Letter we present detailed study of the density of states near defects in Bi2Se3. In particular, we present data on the commonly found triangular defects in this system. While we do not find ...any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory R. R. Biswas and A. V. Balatsky, Phys. Rev. B 81, 233405(R) (2010) once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators.
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