The recent observation of cyclotron resonance in optimally doped La2-xSrxCuO4 using time-domain THz spectroscopy in high magnetic field has given new possibilities for the study of cuprate superconductors. One can measure the cyclotron mass in the more disordered cuprates possessing short scattering times, therefore expanding the study to materials and dopings in which quantum oscillations have not been observed. Here we present the measurement of the carrier mass of the hole-doped cuprate La2-xSrxCuO4 across a range of dopings spanning from the slightly underdoped (p = 0.13) to highly overdoped (p = 0.26), near the termination of the superconducting dome. These results reveal a systematic increase of mc with doping, up to values greater than 13 times the bare electron mass. This is in contrast with the masses extracted from the heat capacity, which show a peak near the pseudogap critical point p* and/or Lifshitz transition. The cyclotron frequency is linear in field up to 31 T for all dopings, giving no evidence for field-induced Fermi surface reconstructions. The cyclotron mass is found to be positive for all dopings, but with a magnitude systematically below the heat-capacity mass for under and optimally doped samples, while exceeding it for overdoped samples. Among other aspects, these results are surprising as photoemission reveals a Lifshitz transition in the middle of our doping range and the sign of the cyclotron mass determined from a finite-frequency resonance is, in conventional theories, a topological quantity only sensitive to whether or not the Fermi surface is closed around holes or electrons. We see no sign of a divergence of the mass near p* nor near the Lifshitz transition, showing that any singularity, if it exists, is not strong enough to affect the cyclotron mass.

Using time-domain terahertz spectroscopy in pulsed magnetic fields up to 31 T, we measure the terahertz optical conductivity in an optimally-doped thin film of the high temperature superconducting cuprate La$_{1.84}$Sr$_{0.16}$CuO$_4$. We observe systematic changes in the circularly-polarized complex optical conductivity that are consistent with cyclotron absorption of p-type charge carriers characterized by a cyclotron mass of $4.9\pm 0.8$ $m_{\rm e}$, and a scattering rate that increases with magnetic field. These results open the door to studies aimed at characterizing the degree to which electron-electron interactions influence carrier masses in cuprate superconductors.

The ground-state properties of correlated electron systems can be extraordinarily sensitive to external stimuli, offering abundant platforms for functional materials. Using the multi-messenger combination of atomic force microscopy, cryogenic scanning near-field optical microscopy, magnetic force microscopy and ultrafast laser excitation, we demonstrate both ‘writing’ and ‘erasing’ of a metastable ferromagnetic metal phase in strained films of La2/3Ca1/3MnO3 (LCMO) with nanometre-resolved finesse. By tracking both optical conductivity and magnetism at the nanoscale, we reveal how strain-coupling underlies the dynamic growth, spontaneous nanotexture and first-order melting transition of this hidden photoinduced metal. Our first-principles calculations reveal that epitaxially engineered Jahn–Teller distortion can stabilize nearly degenerate antiferromagnetic insulator and ferromagnetic metal phases. We propose a Ginzburg–Landau description to rationalize the co-active interplay of strain, lattice distortions and magnetism nano-resolved here in strained LCMO, thus guiding future functional engineering of epitaxial oxides into the regime of phase-programmable materials. A multi-messenger combination of atomic force microscopy, scanning near-field optical microscopy and magnetic force microscopy demonstrates a strain-modulated photoinduced ferromagnetic metallic state in La2/3Ca1/3MnO3.

Terahertz spectroscopy of the c‐axis Josephson plasma resonance (JPR) in high‐temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La2−xSrxCuO4 single crystal (Tc = 32 K) is investigated. The metasurface resonance frequency (ωMM), determined by the SRR geometry, is swept through the JPR frequency (ωJPR = 1.53 THz) using a series of interchangeable tapes applied to the same single crystal. Terahertz reflectivity measurements on the resulting hybrid superconducting metamaterials (HSMMs) reveal anticrossing behavior characteristic of strong coupling. The experimental results, validated with numerical simulations, indicate a normalized Rabi frequency of ΩR = 0.29. Further, it is shown that HSMMs with ωMM > ωJPR provide a route to couple to hyperbolic waveguide modes in c‐axis cuprate samples. This work informs future possibilities for optimizing the coupling strength of HSMMs and investigating nonlinear superconductivity under high field terahertz excitation.

We report a nanoinfrared (IR) imaging study of the localized plasmon resonance modes of graphene nanoribbons (GNRs) using a scattering-type scanning near-field optical microscope (s-SNOM). By comparing the imaging data of GNRs that are aligned parallel and perpendicular to the in-plane component of the excitation laser field, we observed symmetric and asymmetric plasmonic interference fringes, respectively. Theoretical analysis indicates that the asymmetric fringes are formed due to the interplay between the localized surface plasmon resonance (SPR) mode excited by the GNRs and the propagative surface plasmon polariton (SPP) mode launched by the s-SNOM tip. With rigorous simulations, we reproduce the observed fringe patterns and address quantitatively the role of the s-SNOM tip on both the SPR and SPP modes. Furthermore, we have seen real-space signatures of both the dipole and higher-order SPR modes by varying the ribbon width.

Plasmon polaritons are hybrid excitations of light and mobile electrons that can confine the energy of long-wavelength radiation at the nanoscale. Plasmon polaritons may enable many enigmatic quantum effects, including lasing 1 , topological protection2,3 and dipole-forbidden absorption 4 . A necessary condition for realizing such phenomena is a long plasmonic lifetime, which is notoriously difficult to achieve for highly confined modes 5 . Plasmon polaritons in graphene—hybrids of Dirac quasiparticles and infrared photons—provide a platform for exploring light–matter interaction at the nanoscale6,7. However, plasmonic dissipation in graphene is substantial 8 and its fundamental limits remain undetermined. Here we use nanometre-scale infrared imaging to investigate propagating plasmon polaritons in high-mobility encapsulated graphene at cryogenic temperatures. In this regime, the propagation of plasmon polaritons is primarily restricted by the dielectric losses of the encapsulated layers, with a minor contribution from electron–phonon interactions. At liquid-nitrogen temperatures, the intrinsic plasmonic propagation length can exceed 10 micrometres, or 50 plasmonic wavelengths, thus setting a record for highly confined and tunable polariton modes. Our nanoscale imaging results reveal the physics of plasmonic dissipation and will be instrumental in mitigating such losses in heterostructure engineering applications. The fundamental limits to plasmon damping in graphene are determined using nanoscale infrared imaging at cryogenic temperatures, and plasmon polaritons are observed to propagate over 10 micrometres in high-mobility encapsulated graphene.

The explanation and control of phase transitions remain cornerstones of contemporary physics. Landau provided an invaluable insight into the thermodynamics of complex systems by formulating their phase transitions in terms of an order parameter. Within this formulation, continuous evolution of the order parameter away from zero classifies the phase transition as second-order, whereas a discontinuous change signals a first-order transition. Here we show that the temperature-tuned insulator–metal transition in the prototypical correlated electron system NdNiO3 defies this established binary classification. By harnessing a nanoscale optical probe of the local electronic conductivity, we reveal two physically distinct yet concurrent phase transitions in epitaxial NdNiO3 films. Whereas the sample bulk exhibits a first-order transition between metal and insulator phases, we resolve anomalous nanoscale domain walls in the insulating state that undergo a distinctly continuous insulator–metal transition, with hallmarks of second-order behaviour. We ascribe these domain walls to boundaries between antiferromagnetically ordered domains within the charge ordered bulk. The close correspondence of these observations to predictions from a Landau theory of coupled charge and magnetic orders highlights the importance of coupled order parameters in driving the complex phase transition in NdNiO3. A phase transition between metallic and insulating states is observed to simultaneously happen in two ways at once. The bulk of the sample shows an instantaneous jump in the conductivity, while 1D domain walls show a slow switching into a metallic state.

We investigated the ab-plane optical properties of single crystals of WTe2 for light polarized parallel and perpendicular to the W-chain axis over a broad range of frequency and temperature. At far-infrared frequencies, we observed a striking dependence of the reflectance edge on light polarization, corresponding to anisotropy of the carrier effective masses. We quantitatively studied the temperature dependence of the plasma frequency, revealing a modest increase of the effective mass anisotropy in the ab plane upon cooling. We also found strongly anisotropic interband transitions persisting to high photon energies. These results were analyzed by comparison with ab initio calculations. The calculated and measured plasma frequencies agree to within 10% for both polarizations, while the calculated interband conductivity shows excellent agreement with experiment.

Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)_2Te_3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that electron- and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.

We spectroscopically investigated the energy gap of the correlated antiferromagnetic insulator LaMnPO1-xFx (x = 0.0 and 0.04) as a function of temperature and pressure, separately, in conjunction with many-body electronic structure calculations. These results show that the electronic structure in all measured regimes is well described by a model that includes both Mott-Hubbard interactions and Hund's rule coupling. Moreover, we find that by applying external pressure, thereby reducing the effective Mott-Hubbard interaction and Hund's coupling, the energy gap in LaMnPO1 xFx can be fully closed, yielding a metallic state.

In a comprehensive study, we investigate the electronic scattering effects in EuFe2(As1-xPx)(2) by using Fourier-transform infrared spectroscopy. In spite of the fact that Eu2+ local moments order at around T-Eu similar to 20 K, the overall optical response is strikingly similar to the one of the well-known Ba-122 pnictides. The main difference lies within the suppression of the lower spin-density-wave gap feature. By analyzing our spectra with a multi-component model, we find that the high-energy feature around 0.7 eV - often associated with Hund's rule coupling is highly sensitive to the spin-density-wave ordering; this further confirms its direct relationship to the dynamics of itinerant carriers. The same model is also used to investigate the in-plane anisotropy of magnetically detwinned EuFe(2)As2 in the antiferromagnetically ordered state, yielding a higher Drude weight and lower scattering rate along the crystallographic a-axis. Finally, we analyze the development of the room-temperature spectra with isovalent phosphor substitution and highlight changes in the scattering rate of hole-like carriers induced by a Lifshitz transition. (C) 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure and Eliashberg calculations to show that nearly optimally doped NaFe 0.978 Co 0.022 As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.