Search

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.

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

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

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

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.

Using the angle-resolved photoemission spectroscopy (ARPES) with resolution of all three components of electron momentum and electronic states symmetry, we explicate the electronic structure of hole-doped BaFe2As2, and show that widely discussed nesting and dimensionality of Fermi surface (FS) sheets have no immediate relation to the superconducting pairing in iron-based superconductors. Alternatively a clear correlation between the orbital character of the electronic states and their propensity to superconductivity is observed: The magnitude of the superconducting gap maximizes at 10 meV exclusively for iron 3d(xz, yz) orbitals, while for others drops to 3 meV. Presented results imply that the relation between superconducting and magnetostructural transitions goes beyond simple competition for FS, and demonstrate importance of orbital physics in iron superconductors.

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

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 optical properties of the multiband superconductors with an s± order parameter symmetry. By comparing results of our theory with experimental data on optical conductivity for Ba0.68K0.32Fe2As2 single crystals, we show that satisfactory description of the novel superconductors can be obtained only considering a strong electron-boson coupling. We reexamine the effect of disorder and demonstrate that multiband superconductors are more robust with respect to it than naively expected by simple analogy with paramagnetic impurities in single-band superconductors. Moreover, disorder may give rise to new effects, in particular to a phase transition s±→s++. We discuss how the systematic study of disorder impact on the density of states and the optical conductivity may provide information on the underlying order parameter structure

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.

Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap to the critical temperature, Tc. We present an extensive comparison of the gap ratios among many single- and multiband superconductors from simple metals to high-Tc cuprates and iron pnictides. Contrary to the recently suggested universality of this ratio in Fe-based superconductors, we find that the coupling in pnictides ranges from weak, near the BCS limit, to strong, as in cuprates, bridging the gap between these two extremes. Moreover, for Fe- and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and Tc, which is not found in conventional superconductors and therefore supports a common unconventional pairing mechanism in both families. An important consequence of this result for ferropnictides is that the separation in energy between the excitonic spin-resonance mode and the particle-hole continuum, which determines the resonance damping, no longer appears independent of Tc., 15 pages, 3 figures, 5 tables with an exhaustive overview of the published gap and spin-resonance measurements in Fe-based superconductors. New in V3: updated references. To be published in Phys. Rev. B

One of the central tenets of conventional theories of superconductivity, including most models proposed for the recently discovered iron-pnictide superconductors, is the notion that only electronic excitations with energies comparable to the superconducting energy gap are affected by the transition. Here we report the results of a comprehensive spectroscopic ellipsometry study of a high-quality crystal of superconducting $\textrm{Ba}_{0.68}\textrm{K}_{0.32}\textrm{Fe}_2\textrm{As}_2$ that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of $2.5\ \textrm{eV}$, two orders of magnitude above the superconducting gap energy $2\Delta\sim 20\ \textrm{meV}$. Based on density-functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi surface. We identify a related effect at the spin-density-wave transition in parent compounds of the 122 family. This suggests that As-p states deep below the Fermi level contribute to the formation of the superconducting and spin-density-wave states in the iron arsenides., Comment: 5 pages, 3 figures. Supplementary material: 6 pages, 3 figures (1 interactive)

Spectroscopic ellipsometry was used to study the dielectric function of ${\mathrm{LiCuVO}}_{4}$, a compound comprised of chains of edge-sharing ${\mathrm{CuO}}_{4}$ plaquettes, in the spectral range 0.75--6.5 eV at temperatures 7--300 K. For photon polarization along the chains, the data reveal a weak but well-resolved two-peak structure centered at 2.15 and 2.95 eV whose spectral weight is strongly enhanced upon cooling near the magnetic ordering temperature. We identify these features as an exciton doublet in the Mott-Hubbard gap that emerges as a consequence of the Coulomb interaction between electrons on nearest and next-nearest-neighbor sites along the chains. Our results and methodology can be used to address the role of the long-range Coulomb repulsion for compounds with doped copper-oxide chains and planes.

A comprehensive ellipsometric study was performed on ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ single crystals in the spectral range from 0.01 to 6.2 eV. Direct and indirect band gaps of 73 and 10 meV, respectively, were observed in FeSi at 7 K. One of four infrared-active phonons that is energetically close to the direct absorption edge is coupled both to the electrons and to the low-energy phonon. This is evident from asymmetry in the phonon line shape and a reduction in its frequency when the absorption edge shifts across the phonon energy due to the temperature dependence of the direct band gap. As the temperature increases, the indirect gap changes sign, which manifests as a transition from a semiconductor to a semimetal. The corresponding gain of the spectral weight at low energies was recovered within an energy range of several eV. The present findings strongly support the model indicating that ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$ can be well described in an itinerant picture, taking into account self-energy corrections.

The precise momentum dependence of the superconducting gap in the iron-arsenide superconductor ${\text{Ba}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{Fe}}_{2}{\text{As}}_{2}$ (BKFA) with ${T}_{c}=32\text{ }\text{K}$ was determined from angle-resolved photoemission spectroscopy (ARPES) via fitting the distribution of the quasiparticle density to a model. The model incorporates finite lifetime and experimental resolution effects, as well as accounts for peculiarities of BKFA electronic structure. We have found that the value of the superconducting gap is practically the same for the inner $\ensuremath{\Gamma}$ barrel, $\mathrm{X}$ pocket, and ``blade'' pocket, and equals 9 meV, while the gap on the outer $\ensuremath{\Gamma}$ barrel is estimated to be less than 4 meV, resulting in $2\ensuremath{\Delta}/{k}_{B}{T}_{c}=6.8$ for the large gap and $2\ensuremath{\Delta}/{k}_{B}{T}_{c}l3$ for the small gap. A large $(77\ifmmode\pm\else\textpm\fi{}3\text{ }%)$ nonsuperconducting component in the photoemission signal is observed below ${T}_{c}$. Details of gap extraction from ARPES data are discussed in Appendixes A,B.

The anisotropic complex dielectric response was accurately extracted from spectroscopic ellipsometry measurements at phonon frequencies for the three principal crystallographic directions of an orthorhombic $(Pbnm)$ ${\text{YTiO}}_{3}$ single crystal. We identify all 25 infrared-active phonon modes allowed by symmetry $7{B}_{1u}$, $9{B}_{2u}$, and $9{B}_{3u}$ polarized along the $c$, $b$, and $a$ axes, respectively. From a classical dispersion analysis of the complex dielectric functions $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$ and their inverses $\ensuremath{-}1/\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}(\ensuremath{\omega})$, we define the resonant frequencies, widths, and oscillator strengths of the transverse-optical (TO) and longitudinal-optical phonon modes. We calculate eigenfrequencies and eigenvectors of ${B}_{1u}$, ${B}_{2u}$, and ${B}_{3u}$ normal modes and suggest assignments of the TO phonon modes observed in our ellipsometry spectra by comparing their frequencies and oscillator strengths with those resulting from the present lattice-dynamics study. Based on these assignments, we estimate dynamical effective charges of the atoms in the ${\text{YTiO}}_{3}$ lattice. We find that in general, the dynamical effective charges in ${\text{YTiO}}_{3}$ lattice are typical for a family of perovskite oxides. By contrast to a ferroelectric ${\text{BaTiO}}_{3}$, the dynamical effective charge of oxygen related to a displacement along the $c$ axis does not show the anomalously large value. At the same time, the dynamical effective charges of Y and $ab$ plane oxygen exhibit anisotropy, indicating a strong hybridization along the $a$ axis.

Spectroscopic ellipsometry is used to determine the dielectric function of the superconducting LaFeAsO$_{0.9}$F$_{0.1}$ ($T_c$ = 27 K) and undoped LaFeAsO polycrystalline samples in the wide range 0.01-6.5 eV at temperatures 10 $\leq T \leq$ 350 K. The free charge carrier response in both samples is heavily damped with the effective carrier density as low as 0.040$\pm$0.005 electrons per unit cell. The spectral weight transfer in the undoped LaFeAsO associated with opening of the pseudogap at about 0.65 eV is restricted at energies below 2 eV. The spectra of superconducting LaFeAsO$_{0.9}$F$_{0.1}$ reveal a significant transfer of the spectral weight to a broad optical band above 4 eV with increasing temperature. Our data may imply that the electronic states near the Fermi surface are strongly renormalized due to electron-phonon and/or electron-electron interactions., 4 pages, 4 figures, units in Fig.2 added

We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.

The thermodynamic properties of the ferromagnetic perovskite YTiO$_3$ are investigated by thermal expansion, magnetostriction, specific heat, and magnetization measurements. The low-temperature spin-wave contribution to the specific heat, as well as an Arrott plot of the magnetization in the vicinity of the Curie temperature $T_C\simeq27$ K, are consistent with a three-dimensional Heisenberg model of ferromagnetism. However, a magnetic contribution to the thermal expansion persists well above $T_C$, which contrasts with typical three-dimensional Heisenberg ferromagnets, as shown by a comparison with the corresponding model system EuS. The pressure dependences of $T_C$ and of the spontaneous moment $M_s$ are extracted using thermodynamic relationships. They indicate that ferromagnetism is strengthened by uniaxial pressures $\mathbf{p}\parallel \mathbf{a}$ and is weakened by uniaxial pressures $\mathbf{p}\parallel \mathbf{b},\mathbf{c}$ and hydrostatic pressure. Our results show that the distortion along the $a$- and $b$-axes is further increased by the magnetic transition, confirming that ferromagnetism is favored by a large GdFeO$_3$-type distortion. The c-axis results however do not fit into this simple picture, which may be explained by an additional magnetoelastic effect, possibly related to a Jahn-Teller distortion., Comment: 12 pages, 13 figures

Single crystals of iron(IV) rich oxides $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ with controlled oxygen content $(0\ensuremath{\leqslant}\ensuremath{\delta}\ensuremath{\leqslant}0.31)$ have been studied by M\"ossbauer spectroscopy, magnetometry, magnetotransport measurements, Raman spectroscopy, and infrared ellipsometry in order to relate the large magnetoresistance (MR) effects in this system to phase composition and magnetic and charge order. It is shown that three different types of MR effects occur. In cubic $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3}$ $(\ensuremath{\delta}=0)$ a large negative MR of 25% at $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is associated with a hitherto unknown $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ magnetic transition and a subsequent drop in resistivity. The $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ transition appears in addition to the onset of helical ordering at $\ensuremath{\sim}130\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In crystals with vacancy-ordered tetragonal $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ as the majority phase $(\ensuremath{\delta}\ensuremath{\sim}0.15)$ a coincident charge-antiferromagnetic ordering transition near $70\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is considered as the origin of a negative giant MR effect of 90% at $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. A positive MR effect is observed in tetragonal and orthorhombic materials with increased oxygen deficiency $(\ensuremath{\delta}=0.19,0.23)$ which are insulating at low temperatures. Phase mixtures can result in a complex superposition of these different MR phenomena. The MR effects in $\mathrm{Sr}\mathrm{Fe}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ differ from those in manganites as no ferromagnetic states are involved.

We observe a large and anomalous oxygen isotope effect on the a-axis component of the far-infrared electronic response of detwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}^{16,18}{\mathrm{O}}_{6.9}.$ For ${}^{18}\mathrm{O}$ a pronounced low-energy electronic mode (LEM) appears around $240{\mathrm{cm}}^{\ensuremath{-}1}.$ This a-axis LEM exhibits a clear aging effect, after 1 y it is shifted to $190{\mathrm{cm}}^{\ensuremath{-}1}.$ For ${}^{16}\mathrm{O}$ we cannot resolve a corresponding a-axis LEM above $120{\mathrm{cm}}^{\ensuremath{-}1}.$ We interpret the LEM in terms of a collective electronic mode that is pinned by ``isotopic defects,'' i.e., by the residual ${}^{16}\mathrm{O}$ in the matrix of ${}^{18}\mathrm{O}.$

The electronic and magnetic properties of ${\mathrm{S}\mathrm{r}\mathrm{F}\mathrm{e}\mathrm{O}}_{3\mathrm{\text{\ensuremath{-}}}\ensuremath{\delta}}$ single crystals with controlled oxygen content ($0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.19$) have been studied systematically by susceptibility, transport, and spectroscopic techniques. An intimate correlation between the spin-charge ordering and the electronic transport behavior is found. Giant negative as well as positive magnetoresistance are observed.

The far-infrared (FIR) c axis conductivity of a Bi2223 crystal has been measured by ellipsometry. Below T(c) a strong absorption band develops near 500 cm(-1), corresponding to a transverse Josephson plasmon. The related increase in FIR spectral weight leads to a giant violation of the Ferrell-Glover-Tinkham sum rule. The gain in c axis kinetic energy accounts for a sizable part of the condensation energy. We also observe phonon anomalies which suggest that the Josephson currents lead to a drastic variation of the local electric field within the block of closely spaced CuO2 planes.