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Surfaces and interfaces offer new possibilities for tailoring the many-body interactions that dominate the electrical and thermal properties of transition metal oxides. Here, we use the prototypical two-dimensional electron liquid (2DEL) at the SrTiO3(001) surface to reveal a remarkably complex evolution of electron–phonon coupling with the tunable carrier density of this system. At low density, where superconductivity is found in the analogous 2DEL at the LaAlO3/SrTiO3interface, our angle-resolved photoemission data show replica bands separated by 100 meV from the main bands. This is a hallmark of a coherent polaronic liquid and implies long-range coupling to a single longitudinal optical phonon branch. In the overdoped regime the preferential coupling to this branch decreases and the 2DEL undergoes a crossover to a more conventional metallic state with weaker short-range electron–phonon interaction. These results place constraints on the theoretical description of superconductivity and allow a unified understanding of the transport properties in SrTiO3-based 2DELs.

The doping dependence as well as the oxygen and copper isotope effects on the pseudogap temperature T* were investigated by neutron spectroscopic experiments of the relaxation rate of crystal-field excitations in rare-earth based Y- and La-type high-temperature superconductors. We found three essential results from our truly bulk-sensitive experiments: (1) For all doping levels we had T* > Tc, even in the optimally doped and overdoped regimes. (2) In bilayer high-Tccompounds we observed huge oxygen and copper isotope effects on T*, which give evidence that the pseudogap formation is governed by lattice modes involving both the oxygen and copper ions. In single layer high-Tccompounds, on the other hand, no shift of T* was found for the copper isotope substitution. (3) Our results obtained for all doping levels support the unusual temperature dependence of the gap function in the pseudogap region reported for underdoped Bi-type compounds, i.e., a breakup of the Fermi surface into disconnected arcs in the temperature range Tc< T< T*.

Abstract:: The copper isotope effect (63Cu vs. 65Cu) on the relaxation rate of crystal-field excitations in the slightly underdoped high-temperature superconductor HoBa2Cu4O8 has been investigated by inelastic neutron scattering. For the 63Cu compound there is clear evidence for the opening of an electronic gap in the normal state at T * ≈ 160 K far above T c = 79.0 K. Upon substitution of 63Cu by 65Cu, T c decreases marginally to 78.6 K, whereas T * is increased to about 185 K. This large copper isotope shift Δ T * ( Cu ) = T * ( 65 Cu ) - T * ( 63 Cu ) ≈ 25 K - together with the corresponding oxygen isotope shift Δ T * ( O ) = T * ( 18 O ) - T * ( 16 O ) ≈ 50 K found in an earlier investigation - suggests that phonons or lattice fluctuations involving both the copper and the oxygen ions are important for the pairing mechanism in high-T c materials.

We have performed a systematic high energy (7–11 meV) and momentum (0.01 A˚−1) resolution angle-resolved photoemission spectroscopy (ARPES) on superstructure-free Bi1.80Pb0.38Sr2.01CuO6−δ with various dopings. All the samples from overdoped Tc = 0K to underdoped Tc = 18K possess a large hole-like Fermi surface centered at X point, whose volume shows a systematic evolution with doping. The superconducting gap exhibits a dx2−y2-like anisotropy with a typical gap value of 10–15 meV. By comparing with Bi2Sr2CaCu2O8, we found that the size of superconducting gap and the ARPES spectral feature near EF are well scaled with the maximum Tc (Tcmax). This suggests that the superconducting properties are characterized by Tcmax irrespective of the number of CuO2 layers or the structural modulation in BiO layer.

The temperature dependence of the relaxation rate of crystal-field excitations in the high-Tcsuperconductors HoBa2Cu4O8and HoBa2Cu3Ox(6.4 < x< 7) was investigated by inelastic neutron scattering techniques. The data show clear evidence for the opening of an electronic gap in the normal state at T* > Tcin the underdoped regime as well as for slightly overdoped samples. For HoBa2Cu4O8T* increases from 170 to 220 K upon oxygen isotope substitution (16O vs 18O). This huge isotope shift (which is absent in NMR and NQR experiments) suggests that the mechanism leading to an isotope effect on the pseudogap has to involve a time scale in the range 10−8≫ τ > 10−13s.

We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at(π, π). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(kx) − cos(ky) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.

Abstract: Neutron spectroscopy is a powerful tool to determine unambiguously the crystal-field (CF) potential in rare-earth (R) based high-T c superconducting materials. This technique provides detailed information on the electronic ground state of the R ions which is important to understand the thermodynamic magnetic properties as well as the observed coexistence between superconductivity and long-range magnetic ordering of the R ion sublattice at low temperatures. Moreover, the decay of the antiferromagnetic state of the parent compound as well as the evolution of the superconducting state upon doping can be directly and quantitatively monitored. It is found that the observed CF spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation which means that the superconductivity is an inhomogeneous materials property. Since the linewidths of CF transitions directly probe the static electronic susceptibility, we discuss temperature-dependent experiments of the relaxation rate of CF excitations in both optimally doped and underdoped regimes. It is shown that there is clear evidence for the opening of an electronic gap in the normal state of underdoped superconductors. Furthermore, the relaxation behavior appears to be extremely dependent upon the energy at which the static susceptibility is being probed. The main observed features can be reproduced by considering a strongly anisotropic gap function.

The physical design of FOCUS, a new time-of-flight (TOF) spectrometer for cold neutrons located at the continuous spallation source SINQ at the Paul Scherrer Institute/CH is presented. The main purpose of the instrument is to provide a versatile TOF machine being suited for a large variety of applications. The concept of the instrument consists in the combination of a doubly focussing monochromator with a Fermi-chopper. The monochromator will be provided with variable curvatures in both horizontal and vertical direction. By variable distances between the main spectrometer components FOCUS will have the option to be operated either in time focussing or monochromatic focussing mode for quasielastic and inelastic scattering applications, respectively. By means of two interchangeable monochromators a continuous wavelength band ranging from 2Å up to 18Å will be accessible. Monte Car10 simulations for an ideal Pb-target at SINQ show that for 4Å and 6Å and in its time focussing mode FOCUS will yield comparable energy resolutions in connection with 70–90% of the intensity of the time focussing TOF spectrometer IN6 at the ILL.

The modifications of the crystallographic structure of PrNiO3 associated to the decrease of the metal insulator temperature TM-I with external pressure (Pmax ≈ 5 kbar) have been investigated by means of neutron powder diffraction. The most interesting finding is the unexpected evolution of the tilt angle π of the NiO6 octahedra which, in contrast to the results reported for other isostructural compounds, decreases under pressure. A simple charge-transfer scheme is used to relate the evolution of ≈ to the decrease of TM-I upon pressure.

By means of a quantitative analysis of extensive elastic neutron scattering experiments performed on theTc=78 K superconductor DyBa2Cu4O8 it is shown that two-dimensional (2D) antiferromagnetic Dy ordering in the (a,b)-plane remains stable in the whole temperature range fromTN=(1.10±0.02) K down to 7 mK. The magnetic difference pattern with good angular resolution fits well to the Warren equation for scattering on 2D systems with powder averaging. Consequently, the fitted sublattice magnetisation is found to have a 2D Ising character. Moreover, the Dy3+ crystal-field levels were determined by inelastic neutron scattering. From the resulting crystal-field parameters we calculate the sublattice magnetisation of Dy3+ at saturation to beµDy,CEF=6.0µB in good agreement withµDy,obs=(5.9±0.5)µB as measured by neutron diffraction.

Abstract: Neutron spectroscopy is a powerful tool to determine unambiguously the crystal-field (CF) potential in rare-earth (R) based high-T c superconducting materials. This technique provides detailed information on the electronic ground state of the R ions which is important to understand the thermodynamic magnetic properties as well as the observed coexistence between superconductivity and long-range magnetic ordering of the R ion sublattice at low temperatures. Moreover, the decay of the antiferromagnetic state of the parent compound as well as the evolution of the superconducting state upon doping can be directly and quantitatively monitored. It is found that the observed CF spectra separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for cluster formation. The onset of superconductivity can be shown to result from percolation which means that the superconductivity is an inhomogeneous materials property. Since the linewidths of CF transitions directly probe the static electronic susceptibility, we discuss temperature-dependent experiments of the relaxation rate of CF excitations in both optimally doped and underdoped regimes. It is shown that there is clear evidence for the opening of an electronic gap in the normal state of underdoped superconductors. Furthermore, the relaxation behavior appears to be extremely dependent upon the energy at which the static susceptibility is being probed. The main observed features can be reproduced by considering a strongly anisotropic gap function.

Neutron diffraction, neutron spectroscopy and magnetization measurements have been employed to study the structural, electronic, and magnetic behavior of eleven compounds with the general formula Nd1+yCavBa2-y-vCu3Ox (0?y?0.5; 0?v?0.25; 6?x?7). The structure turned out to react to oxygen reduction similar as other 123-compounds, yielding discontinuities close to the metal-insulator-transition and the well-known relations of bond lengths as a function ofTc. The crystalline electric field (CEF) interaction, splitting the 10-fold degenerate ground-state J-multiplet of the Nd3+-ions into five doublet states, was investigated by neutron spectroscopy. The derived CEF parameters have been used to determine changes in the electronic surroundings of the Nd3+ ions. In addition, with the help of the CEF parameters the thermodynamic magnetic properties of these compounds were calculated which turn out to be in good agreement with the experimental data.

Neutron scattering has been employed to study the crystalline electric field (CEF) interaction at the Ho3+ site in Bi2Sr2Ca0.5Ho0.5Cu2O8+x. The observed energy spectra exhibit a large number of broad but well-resolved CEF transitions between 1.2 and 73 meV, so that we have been able to unambiguously determine all nine CEF parameters required for the average orthorhombic symmetry. The unusually large line widths of the CEF transitions are shown to be related to the modulated structure. The CEF potential is essentially governed by the charge distribution of the CuO2 planes which turns out to be very similar as in HoBa2Cu3O7-x.

Specific-heat measurements for Er2Ba4Cu7O15−δshow a magnetic phase transition at TN=0.54 K for δ≈0.1 and at TN=0.50 K for δ≈0.7. While the specific heat data of Er2Ba4Cu7O14.92can be interpreted with an anisotropic 2D-Ising model, the specific heat data of Er2Ba4Cu7O14.30can only be understood by assuming two different types of magnetic clusters to be present in this sample. Such an interpretation is supported by our inelastic neutron data demonstrating the presence of at least two distinctly different surroundings for the rare-earth ions. Furthermore, the importance of dipolar interaction and superexchange for the observed magnetic ordering will be discussed.

The 10K crystal structures of high-Tc RBa2Cu3O7 (R=Y and rare earths) ceramic materials investigated by neutron diffraction were consistently analysed concerning systematic trends. Several interionic distances exhibit the well known lanthanide contraction behaviour, i.e., a linear relationship with the ionic radii of trivalent rare-earth ions. The only exceptions are associated with the apex oxygen O(1) ions: the chain copper Cu(1)-O(1) distances are constant, and the plane copper Cu(2)-O(1) distances are increasing across the rare-earth series. For R=Pr the 4f-CuO2 valence band hybridization effects are found to be highly anisotropic, since exclusively the interionic distances involving the b-direction (i.e, the chain direction) indicate that the Pr ions have an intermediate valence of about+3.4.

The discovery of high-Tc superconductivity in the perovskite-type copper-oxide compounds has given rise to a large amount of materials research. Of particular interest in determining the nature of the superconductivity is the effect of substituents at various sites in these compounds. It has been realized that the superconducting transition temperature is essentially unchanged upon replacing the Y and La ions by "magnetic" rare-earth (R) ions. This surprising observation is in contrast to conventional super-conductors, for which paramagnetic ions usually have a large detrimental effect on superconductivity. It is therefore important to achieve a detailed understanding of the low-energy electronic properties which define the magnetic ground state of the R ions. In particular, information on the crystalline-electric-field (CEF) interaction at the R site is highly desirable, for the following reasons: For many high-Tc compounds superconductivity and long-range three-dimensional magnetic ordering of the R ion sublattice coexist at low temperatures. An understanding of both the nature of the magnetic ordering and its apparent lack of influence on Tc requires a detailed knowledge of the CEF states of the R ions. In most high-Tc compounds, the R ions are situated close to the CuO2 planes where it is widely believed that the superconducting carriers are located, thus the CEF interaction at the R site constitutes an ideal probe of both the local symmetry and the charge distribution of the superconducting planes and thereby monitors directly changes of the carrier concentration induced, e.g., by oxygen nonstoichiometry, pressure and disordering effects. The temperature dependence of the line widths of CEF excitations provides information on the supercon ducting energy gap.

The modifications of the cystallographic and magnetic structures of PrNiO3 associated to the decrease of the metal—insulator transition temperature TM—I with external pressure have been investigated by means of neutron powder diffraction. The changes observed in the average NiO distances and NiONi superexchange angles at 4.7 kbar are similar in sign but different in magnitude from those reported for the application of an equivalent internal pressure. The low temperature phase (T < TM—I) is magnetic (and probably insulating) at 4.7 kbar.

Neutron spectroscopic data obtained for ErBa2Cu3Ox (6 ⩽ × ⩽ 7) and Pr2 − x CexCuO4 − δ (0 ⩽ × ⩽ 0.2) are discussed. Since the rare-earth ions in these compounds are situated in close proximity to the superconducting CuO2-planes, crystal-field spectroscopy constitutes an ideal local probe of the charge distribution and therefore monitors directly changes of the carrier concentration induced by doping. Clear evidence for cluster formation and percolative superconductivity is provided by the observed separation of crystal-field transitions — observed in hole- and electron-doped superconductors — into different local components whose spectral weights depend on the doping level.

We report inelastic neutron scattering (INS) measurements on strongly underdoped (Tc = 70K) Bi2Sr2CaCu2O8+δ single crystals. We observe the existence of a magnetic resonance, whose energy is centered around 34 meV, a value much lower than the one reported for the optimally doped compound. Furthermore, our result is consistent with the energy of the collective mode inferred from angle-resolved photoemission spectroscopy (ARPES).

Abstract: Inelastic neutron scattering experiments of the crystal-electric-field (CEF) splitting of the R3+-ions (R=Ho, Er, Tm) in the superconductor RNi211B2C have been performed in the paramagnetic as well as in the magnetically ordered state in order to deduce the CEF parameters which in turn determine the magnetic properties of these systems. Both the easy axis and the size of the zero-field moments are correctly reproduced by our calculations. Our CEF parameter set also reproduces the behaviour of specific heat and M/H measured for single crystals.

The lowest ground-state crystalline-electric-field transition in grain-aligned RBa2Cu3O7(R  Ho, Er) has been studied in detail by means of inelastic neutron scattering. The observed dispersion in energy is analyzed in terms of a generalized Heisenberg model including anisotropies in the coupling of spin operators. So far very detailed results are obtained for HoBa2Cu3O7.