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Adeno-associated viruses (AAVs) comprise an area of rapidly growing interest due to their ability to act as a gene delivery vehicle in novel gene therapy strategies and vaccine development. Peptide mapping is a common technique in the biopharmaceutical industry to confirm the correct sequence, product purity, post-translational modifications (PTMs), and stability. However, conventional peptide mapping is time-consuming and has proven difficult to reproduce with viral capsids because of their high structural stability and the suboptimal localization of trypsin cleavage sites in the AAV protein sequences. In this study, we present an optimized peptide mapping-based workflow that provides thorough characterization within 1 day. This workflow is also highly reproducible due to its simplicity having very few steps and is easy to perform proteolytic digestion utilizing thermally stable pepsin, which is active at 70 °C in acidic conditions. The acidic conditions of the peptic digestions drive viral capsid denaturation and improve cleavage site accessibility. We characterized the efficiency and ease of digestion through peptide mapping of the AAV2 viral capsid protein. Using nanoflow liquid chromatography coupled with tandem mass spectrometry, we achieved 100% sequence coverage of the low-abundance VP1 capsid protein with a digestion process taking only 10 min to prepare and 45 min to complete the digestion.

Using ion-implanted $^{8}\mathrm{Li} \ensuremath{\beta}$-detected NMR, we study the evolution of the correlated metallic state of ${\mathrm{LaNiO}}_{3}$ in a series of ${\mathrm{LaNiO}}_{3}/{\mathrm{LaAlO}}_{3}$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of $6.55\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ reveal two equal amplitude components: one with metallic ($T$ linear) $1/{T}_{1}$ and a second with a more complex $T$ dependence. The metallic character of the slow relaxing component is only weakly affected by the ${\mathrm{LaNiO}}_{3}$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation.

Significance Chemical substitution is widely used to modify the charge-carrier concentration (“doping”) in complex quantum materials, but the influence of the associated structural disorder on the electronic phase behavior remains poorly understood. We synthesized thin films of the high-temperature superconductor L a 2 − x C a x C u O 4 with minimal structural disorder and characterized their doping levels through measurements of the optical conductivity. We find that superconductivity with T c = 15 to 20 K is stable up to much higher doping levels than previously found for analogous compounds with stronger disorder. The results imply that doping-induced disorder is the leading cause of the degradation of superconductivity for large carrier concentration, and they open up a previously inaccessible regime of the phase diagram of high-temperature superconductors to experimental investigation.

We present a comprehensive study on the silver bismuthate ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$, synthesized under high-pressure, high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present x-ray photoelectron spectroscopy results showing two different bismuth states and x-ray absorption spectroscopy results on the oxygen $K$ edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$. We estimate a band gap of $\ensuremath{\sim}1.25$ eV for ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the nonsymmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$ is expected to host Weyl nodal chains, one of which is located $\ensuremath{\sim}0.5$ eV below the Fermi level. Furthermore, we highlight similarities between ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$ and the well-known disproportionated bismuthate ${\mathrm{BaBiO}}_{3}$, including breathing phonon modes with similar energy. In both compounds, hybridization of Bi $6s$ and O $2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba $5p$ bands in ${\mathrm{BaBiO}}_{3}$, the Ag $4d$ bands in ${\mathrm{Ag}}_{2}{\mathrm{BiO}}_{3}$ extend up to the Fermi level.

In high-energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e., the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channels for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc., Interaction between Cooper pairs and other collective excitations may reveal important information about the pairing mechanism. Here, the authors observe a universal jump in the phase of the driven Higgs oscillations in cuprate thin films, indicating the presence of a coupled collective mode, as well as a nonvanishing Higgs-like response at high temperatures, suggesting a potential nonzero pairing amplitude above Tc.

We present an experimental study on microwave illuminated high mobility MgZnO/ZnO based two-dimensional electron systems with different electron densities and, hence, varying Coulomb interaction strength. The photoresponse of the low-temperature dc resistance in perpendicular magnetic field is examined in low and high density samples over a broad range of illumination frequencies. In low density samples a response due to cyclotron resonance (CR) absorption dominates, while high density samples exhibit pronounced microwave-induced resistance oscillations (MIRO). Microwave transmission experiments serve as a complementary means of detecting the CR over the entire range of electron densities and as a reference for the band mass unrenormalized by interactions. Both CR and MIRO-associated features in the resistance permit extraction of the effective mass of electrons but yield two distinct values. The conventional cyclotron mass representing center-of-mass dynamics exhibits no change with density and coincides with the band electron mass of bulk ZnO, while MIRO mass reveals a systematic increase with lowering electron density consistent with renormalization expected in interacting Fermi liquids.

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only., Comment: 16 pages, 12 figures

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.

Atomically thin ferromagnetic and conducting electron systems are highly desired for spintronics, because they can be controlled with both magnetic and electric fields. We present (SrRuO3)1-(SrTiO3)5 superlattices and single-unit-cell-thick SrRuO3 samples that are capped with SrTiO3. We achieve samples of exceptional quality. In these samples, the electron systems comprise only a single RuO2 plane. We observe conductivity down to 50 mK, a ferromagnetic state with a Curie temperature of 25 K, and signals of magnetism persisting up to approximately 100 K., The version published at Phys. Rev. X (open access) contains a large amount of additional material compared to the version published here

Transition metal oxides undergo many different phase transitions when doped with charge carriers. Thanks to recent advances in the synthesis of metal-oxide heterostructures, doping can now be achieved through charge transfer across interfaces. This paper reports spectral ellipsometry measurements on multilayers of superconducting cuprates and ferromagnetic manganates and demonstrates that the optical spectra reveal fingerprints of interfacial charge transfer. Optical spectroscopy can thus serve as a nondestructive probe of the doping level at buried metal-oxide interfaces.

The almost multiferroic perovskite EuTiO3 (ETO) has been prepared as films on substrates of SrTiO3. For all prepared film thicknesses highly transparent insulating films with atomically flat surfaces and excellent orientation have been grown. They were characterized by X-ray diffraction, magnetic susceptibility and birefringence measurements and found to exhibit bulk properties, namely an antiferromagnetic transition at TN = 5.1 K and a structural transition at TS = 282 K. The latter could only be identified due to the high transparency of the samples since the optical band gap is of the order of 4.5 eV and larger than observed before for any bulk and thin film samples.

There is growing interest in the quasi-one-dimensional chalcogenide Ta${}_{2}$NiSe${}_{5}$ due to proposals that its ground state is an excitonic insulator. Here, the authors report a comparative spectroscopic study of Ta${}_{2}$NiSe${}_{5}$ and Ta${}_{2}$NiS${}_{5}$ single crystals and show that the excited states of Ta${}_{2}$NiSe${}_{5}$ can be described in terms of strongly overlapping exciton complexes along the Ta-Ni chains -- conditions highly conducive for the formation of an excitonic condensate. Their results offer a wealth of information on lattice and quasiparticle dynamics, as well as on excitons and their many-body interactions, that will support detailed assessments of the excitonic insulator hypothesis.

The excitonic insulator (EI) is an intriguing phase of condensed excitons undergoing a Bose-Einstein-Condensation (BEC)-type transition. A prominent candidate has been identified in Ta2NiSe5. Ultrafast spectroscopy allows tracing the coherent response of the EI condensate directly in the time domain. Probing the collective electronic response we can identify fingerprints for the Higgs-amplitude equivalent mode of the condensate. In addition we find a peculiar coupling of the EI phase to a low frequency phonon mode. We will discuss the transient response on multiple energies scales ranging from the exciton dynamics to the coherent THz response of the gap. Publisher PDF

The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices. By means of molecular beam epitaxy whole LaO-layers were periodically replaced through SrO-layers providing a charge reservoir, yet reducing the level of disorder typically present in doped cuprates to an absolute minimum. The induced superconductivity and its interplay with the antiferromagnetic order is studied by means of low-energy muSR. We find a quasi-2D superconducting state which couples to the antiferromagnetic order in a non-trivial way. Below the superconducting transition temperature, the magnetic volume fraction increases strongly. The reason could be a charge redistribution of the free carriers due to the opening of the superconducting gap which is possible due to the close proximity and low disorder between the different ordered regions.

We report the first determination of the in-plane complex optical conductivity of 1111 high-${T}_{c}$ superconducting iron oxypnictide single crystals PrFeAs(O,F) and thin films SmFeAs(O,F) by means of conventional and microfocused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission, we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at ${T}_{c}$ and experiences an anomalous $T$-linear softening and narrowing below ${T}^{*}\ensuremath{\approx}110\text{ }\text{ }\mathrm{K}\ensuremath{\gg}{T}_{c}$. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\approx}7$ in both materials, indicating strong pairing.

The temperature-dependent complex dielectric functions of the MnV2O4 ferrimagnetic spinel were investigated at temperatures from 15 K to 300 K in the spectral range of 0.7–4.5 eV by using a spectroscopic ellipsometry. The imaginary part of the dielectric function exhibited an optical gap of approximately 0.8 eV and a broad peak around 1.8 eV. The origin of the peak was assigned as the charge excitation between neighboring V sites with the same spin orientation. The dielectric functions at 1.2 eV, at 1.8 eV, and at 2.5 eV were investigated by increasing the temperature in 0.4 K steps at the temperatures near the structural phase transition temperature (58 K) and the magnetic phase transition temperature (61 K). The temperature-scanned dielectric functions exhibited the sharp and clear signatures of phase transitions, however the temperature dependencies at the three photon energies were different. Analysis of the temperature depedence shows that the width of the spin-polarized transition between the vanadium Hubbard bands can be quite narrow and that the interpretation of multiple vanadium Hubbard bands with different orbital correlation cannot be appropriate for the origin of the full spectral band from the optical gap to an energy of about 3 eV.

This work was partly supported by JSPS KAKENHI Grants No. 24224010, No. 15H05852, and No. 17H01140.

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

We report the complex dielectric function along and perpendicular to the IrO2 planes in the layered perovskite Sr2IrO4 determined by spectroscopic ellipsometry in the spectral range from 12 meV to 6 eV. Thin high quality single crystals were stacked to measure the c-axis optical conductivity. In the phonon response we identified 10 infrared-active modes polarized within the basal plane and only four modes polarized along the c-axis, in full agreement with first-principle lattice dynamics calculations. We also observed a strong optical anisotropy in the near-infrared spectra arising from direct transitions between Ir 5d t2g Jeff = 1/2 and Jeff = 3/2 bands, which transition probability is highly suppressed for light polarized along the c-axis. The spectra are analyzed and discussed in terms of relativistic LSDA+U band structure calculations.

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 $\Gamma$ 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., Comment: Open access article available online at http://www.nature.com/articles/srep18620

We report on an investigation of the redistribution of interlayer coherence in the trilayer cuprate Bi2Sr2Ca2Cu3O10. The experiment is performed under the same apical-oxygen phonon excitation discussed in the past for the bilayer cuprate YBa2Cu3O6.5. In Bi2Sr2Ca2Cu3O10, we observe a similar spectral weight loss at the transverse plasma mode resonance seen in YBa2Cu3O6.5. However, this feature is not accompanied by light-enhanced interlayer coherence that was seen in YBa2Cu3O6+x, for which the transverse plasma mode is observed at equilibrium even in the normal state. These new observations offer new experimental perspective in the context of the physics of light-enhanced interlayer coupling in various cuprates., Comment: 16 pages, 4 figures

The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal lanthanum nickelate (LaNiO(3)) and the wide-gap insulator lanthanum aluminate (LaAlO(3)) with atomically precise layer sequences. We used optical ellipsometry and low-energy muon spin rotation to show that superlattices with LaNiO(3) as thin as two unit cells undergo a sequence of collective metal-insulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO(3) layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

We demonstrate the preparation of large, free-standing iron pnictide single crystals with a size up to 20×10×1 mm3 using solvents in zirconia crucibles under argon atmosphere. Transport and magnetic properties have been investigated to study the effect of potassium doping on the structural and superconducting property of the compounds. The spin density wave (SDW) anomaly at Ts∼138 K in BaFe2As2 single crystals from self-flux shifts to Ts∼85 K due to Sn solvent growth. We show evidence for an incorporation of Sn on the Fe site. The electrical resistivity data show a sharp superconducting transition temperature Tc∼38.5 K for a single crystal of Ba0.68K0.32Fe2As2. A nearly 100% shielding fraction and the bulk nature of the superconductivity for the single crystal are confirmed by magnetic susceptibility data. A sharp transition Tc∼25 K occurs for the single crystal of Sr0.85K0.15Fe2As2. There is direct evidence for a coexistence of the SDW and superconductivity in the low doping regime of Sr1−xKxFe2As2 single crystals. Structural implications of the doping effects as well as the coexistence of the two order parameters are discussed.

The vortex structure in the iron pnictide single crystals Ba 1– x K x Fe 2 As 2 and Sr 1– x K x Fe 2 As 2 of the 122 type and SmFeAsO 1– xFx of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.

The transport and superconducting properties of Ba_{1-x}K_xFe_2As_2 single crystals with T_c = 31 K were studied. Both in-plane and out-of plane resistivity was measured by modified Montgomery method. The in-plane resistivity for all studied samples, obtained in the course of the same synthesis, is almost the same, unlike to the out-of plane resistivity, which differ considerably. We have found that the resistivity anisotropy \gamma=\rho_c /\rho_{ab} is almost temperature independent and lies in the range 10-30 for different samples. This, probably, indicates on the extrinsic nature of high out-of-plane resistivity, which may appear due to the presence of the flat defects along Fe-As layers in the samples. This statement is supported by comparatively small effective mass anisotropy, obtained from the upper critical field measurements, and from the observation of the so-called "Friedel transition", which indicates on the existence of some disorder in the samples in c-direction., Comment: 5 pages, 5 figures

Here we apply high resolution angle-resolved photoemission spectroscopy (ARPES) using a wide excitation energy range to probe the electronic structure and the Fermi surface topology of the Ba 1− x K x Fe 2 As 2 ( T c = 32 K) superconductor. We find significant deviations in the low energy band structure from that predicted in calculations. A set of Fermi surface sheets with unexpected topology is detected at the Brillouin zone boundary. At the X -symmetry point the Fermi surface is formed by a shallow electron-like pocket surrounded by four hole-like pockets elongated in Γ − X and Γ − Y directions.

We report on the growth of stoichiometric Sr$_2$IrO$_4$ single crystals, which allow us to unveil their intrinsic magnetic properties. The effect of different growth conditions has been investigated for crystals grown by the flux method. We find that the magnetic response depends very sensitively on the details of the growth conditions. We assess the defect concentration based on magnetization, X-ray diffraction, Raman scattering, and optical conductivity measurements. We find that samples with a low concentration of electronically active defects show much reduced in-gap spectral weight in the optical conductivity and a pronounced two-magnon peak in the Raman scattering spectrum. A prolonged exposure at high temperature during the growth leads to higher defect concentration likely due to creation of oxygen vacancies. We further demonstrate a systematic intergrowth of Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$ phases by varying the growth temperature. Our results thus emphasize that revealing the intrinsic magnetic properties of Sr$_2$IrO$_4$ and related materials requires a scrupulous control of the crystal growth process.

We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO$_3$ lattice associated with holes localized on both Mn$^{3+}$ cation and O$^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn$^{3+}$, O$^{2-}$ and La$^{3+}$ ions in LaMnO$_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO$_3$ lattice by matching the optical energies of La$^{4+}$ and O$^-$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn$^{4+}$ state in LaMnO$_3$ host lattice, but the energy of a hole in the O$^-$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn$^{4+}$ and O$^-$ in LaMnO$_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn$^{4+}$ and/or O$^-$ self-trapped holes in non-stoichiometric LaMnO$_3$ compound., Comment: 18 pages, 6 figures, it was presented partially at SCES-2001 conference in Ann Arbor, Michigan

Spectroscopic ellipsometry study of Na3Ir3O8 suggests thatelectron-phonon interactions play a prominent role in its electronicstructure.

The infrared reflectivity of a La_(0.67)Ca_(0.33)MnO_3 single crystal is studied over a broad range of temperatures (78–340 K), magnetic fields (0–16 T), and wave numbers (20–9000cm^(-1)). The optical conductivity gradually changes from a Drude-like behavior to a broad peak feature near 5000cm-1 in the ferromagnetic state below the Curie temperature T_C=307K. Various features of the optical conductivity bear striking resemblance to recent theoretical predictions based on the interplay between the double exchange interaction and the Jahn-Teller electron-phonon coupling. A large optical magnetoconductivity is observed near T_C.

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors $A$MnAs ($A=\mathrm{Li}$, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs, and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. Neutron and x-ray diffraction measurements on LiMnAs show a magnetic phase transition corresponding to the N\'eel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory calculations.

We report the complex dielectric function of high-quality $A{\text{Fe}}_{2}{\text{As}}_{2}$, ($A=\text{Ca}$, Sr, Ba) single crystals with ${T}_{\mathrm{N}}\ensuremath{\approx}150\phantom{\rule{4pt}{0ex}}\text{K}$, $200\phantom{\rule{4pt}{0ex}}\text{K}$, and $138\phantom{\rule{4pt}{0ex}}\text{K}$, respectively, determined by broadband spectroscopic ellipsometry at temperatures $10\ensuremath{\le}T\ensuremath{\le}300\phantom{\rule{4pt}{0ex}}\text{K}$ and wave numbers from $100\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ to $52000\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. In ${\text{CaFe}}_{2}{\text{As}}_{2}$ we identify the optical spin-density-wave gap $2{\ensuremath{\Delta}}_{\mathrm{SDW}}\ensuremath{\approx}1250\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. The $2{\ensuremath{\Delta}}_{\mathrm{SDW}}/({k}_{\mathrm{B}}{T}_{\mathrm{N}})$ ratio, characterizing the strength of the electron-electron coupling in the spin-density-wave state, amounts to $\ensuremath{\approx}12$ in ${\text{CaFe}}_{2}{\text{As}}_{2}$, significantly larger than the corresponding values for the ${\text{SrFe}}_{2}{\text{As}}_{2}$ and ${\text{BaFe}}_{2}{\text{As}}_{2}$ compounds: 8.7 and 5.3, respectively. We further show that, similarly to the Ba-based compound, two characteristic SDW energy gaps can be identified in the infrared-conductivity spectra of both ${\text{SrFe}}_{2}{\text{As}}_{2}$ and ${\text{CaFe}}_{2}{\text{As}}_{2}$ and investigate their detailed temperature dependence in all three materials. This analysis reveals the existence of an anomaly in ${\text{CaFe}}_{2}{\text{As}}_{2}$ at a temperature ${T}^{*}\ensuremath{\approx}80\phantom{\rule{4pt}{0ex}}\text{K}$, well below the N\'eel temperature of this compound, which implies weak coupling between the two SDW subsystems. The coupling between the two subsystems evolves to intermediate in the Sr-based and strong in the Ba-based material. The temperature dependence of the infrared phonons reveals clear anomalies at the corresponding N\'eel temperatures of the investigated compounds. In ${\text{CaFe}}_{2}{\text{As}}_{2}$, the phonons exhibit signatures of SDW fluctuations above ${T}_{N}$ and some evidence for anomalies at ${T}^{*}$. Investigation of all three materials in the visible spectral range reveals a spin-density-wave-induced suppression of two absorption bands systematically enhanced with decreasing atomic number of the intercalant. A dispersion analysis of the data in the entire spectral range clearly shows that ${\text{CaFe}}_{2}{\text{As}}_{2}$ is significantly more metallic than the other two compounds. Our results single out ${\text{CaFe}}_{2}{\text{As}}_{2}$ in the class of ${\text{ThCr}}_{2}{\text{Si}}_{2}$-type iron-based materials by demonstrating the existence of two weakly coupled and extremely metallic electronic subsystems.

We study the model high-temperature superconductor ${\mathrm{HgBa}}_{2}{\mathrm{CuO}}_{4+\ensuremath{\delta}}$ with electronic Raman scattering and optical ellipsometry over a wide doping range. The dependence of the resonant Raman cross section on the incident photon energy changes drastically as a function of doping, in a manner that corresponds to a rearrangement of the interband optical transitions seen with ellipsometry. This doping-dependent Raman resonance allows us to reconcile the apparent discrepancy between Raman and x-ray detection of magnetic fluctuations in superconducting cuprates. Intriguingly, the strongest variation occurs across the doping level where the antinodal superconducting gap appears to reach its maximum.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and quantum confinement of the valence electrons. The data are discussed in the light of recent theoretical predictions., Comment: 5 pages, 3 figures

We studied phase separation in the single-crystalline antiferromagnetic superconductor ${\mathrm{Rb}}_{2}{\mathrm{Fe}}_{4}{\mathrm{Se}}_{5}$ (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-$\ensuremath{\mu}\mathrm{SR}$). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. By means of LE-$\ensuremath{\mu}\mathrm{SR}$ we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

We report the complex dielectric function of high-quality nearly-stoichiometric ${\mathrm{Rb}}_{2}{\mathrm{Fe}}_{4}{\mathrm{Se}}_{5}$ single crystals with ${T}_{\mathrm{c}}=32\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ determined by wide-band spectroscopic ellipsometry and time-domain transmission spectroscopy in the spectral range of $1\phantom{\rule{0.28em}{0ex}}\mathrm{meV}\ensuremath{\le}\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\le}6.5\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ at temperatures of $4\phantom{\rule{0.28em}{0ex}}\mathrm{K}\ensuremath{\le}T\ensuremath{\le}300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. This compound simultaneously displays a superconducting and a semiconducting optical response. It reveals a direct band gap of $\ensuremath{\approx}\phantom{\rule{-0.16em}{0ex}}0.45\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ determined by a set of spin-controlled interband transitions. Below 100 K, in the lowest terahertz spectral range, we observe a clear metallic response characterized by the negative dielectric permittivity ${\ensuremath{\varepsilon}}_{1}$ and bare (unscreened) ${\ensuremath{\omega}}_{\mathrm{pl}}\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$. At the superconducting transition, this metallic response exhibits a signature of a superconducting gap below 8 meV. Our findings suggest a coexistence of superconductivity and magnetism in this compound as two separate phases.

We report the results of a comprehensive spectroscopic ellipsometry study of ${\mathrm{NaCu}}_{2}{\mathrm{O}}_{2}$, a compound composed of chains of edge-sharing ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{4}$ plaquettes and planes of Cu${}^{1+}$ ions in a $\mathrm{O}$-${\mathrm{Cu}}^{1+}$-$\mathrm{O}$ dumbbell configuration, in the spectral range $0.75\text{--}6.5$ eV at temperatures $7\text{--}300$ K. The spectra of the dielectric function for light polarized parallel to the ${\mathrm{Cu}}^{1+}$ planes reveal a strong in-plane anisotropy of the interband excitations. Strong and sharp absorption bands peaked at 3.45 eV (3.7 eV) dominate the spectra for polarization along (perpendicular) to the ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ chains. They are superimposed on flat and featureless plateaux above the absorption edges at 2.25 eV (2.5 eV). Based on density-functional calculations, the anomalous absorption peaks can be assigned to transitions between bands formed by ${\mathrm{Cu}}^{1+}$ 3${d}_{xz}$(${d}_{yz}$) and ${\mathrm{Cu}}^{2+}$ 3${d}_{xy}$ orbitals, strongly hybridized with O $p$ states. The major contribution to the background response comes from transitions between ${\mathrm{Cu}}^{1+}$ 3${d}_{{z}^{2}}$ and 4${p}_{x}$(${p}_{y}$) bands. This assignment accounts for the measured in-plane anisotropy. The dielectric response along the ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ chains develops a weak two-peak structure centered at 2.1 and 2.65 eV upon cooling below $\ensuremath{\sim}$100 K, along with the appearance of spin correlations along the ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ chains. These features bear a striking resemblance to those observed in the single-valent ${\mathrm{Cu}}^{2+}{\mathrm{O}}_{2}$ chain compound ${\mathrm{LiCuVO}}_{4}$, which were identified as an exciton doublet associated with transitions to the upper Hubbard band that emerges as a consequence of the long-range Coulomb interaction between electrons on neighboring ${\mathrm{Cu}}^{2+}$ sites along the chains. An analysis of the spectral weights of these features yields the parameters characterizing the on-site and long-range Coulomb interactions.

We report the full complex dielectric function of high-purity Ba0.68K0.32Fe2As2 single crystals with Tc=38.5K determined by wideband spectroscopic ellipsometry at temperatures 10≤T≤300K. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies, 2ΔA≈6kBTc and 2ΔB≈2.2kBTc. The observed unusual suppression of the optical conductivity in the superconducting state at energies up to 14kBTc can be ascribed to spin-fluctuation–assisted processes in the clean limit of the strong-coupling regime.

We report temperature- and magnetic-field-dependent measurements of the dc resistivity and the far-infrared reflectivity (FIR) (photon energies {h_bar}{omega} = 50-700 cm{sup -1}) of superlattices comprising ten consecutive unit cells of the antiferromagnetic insulator CaMnO{sub 3}, and four to ten unit cells of the correlated paramagnetic metal CaRuO{sub 3}. Below the Neel temperature of CaMnO{sub 3}, the dc resistivity exhibits a logarithmic divergence upon cooling, which is associated with a large negative, isotropic magnetoresistance. The {omega} {yields} 0 extrapolation of the resistivity extracted from the FIR reflectivity, on the other hand, shows a much weaker temperature and field dependence. We attribute this behavior to scattering of itinerant charge carriers in CaRuO{sub 3} from sparse, spatially isolated magnetic defects at the CaMnO{sub 3}-CaRuO{sub 3} interfaces. This field-tunable 'transport bottleneck' effect may prove useful for functional metal-oxide devices.