148 research outputs found
Valence and magnetic ordering in intermediate valence compounds : TmSe versus SmB6
The intermediate valent systems TmSe and SmB6 have been investigated up to 16
and 18 GPa by ac microcalorimetry with a pressure (p) tuning realized in situ
at low temperature. For TmSe, the transition from an antiferromagnetic
insulator for p<3 GPa to an antiferromagnetic metal at higher pressure has been
confirmed. A drastic change in the p variation of the Neel temperature (Tn) is
observed at 3 GPa. In the metallic phase (p>3 GPa), Tn is found to increase
linearly with p. A similar linear p increase of Tn is observed for the
quasitrivalent compound TmS which is at ambiant pressure equivalent to TmSe at
p=7 GPa. In the case of SmB6 long range magnetism has been detected above p=8
GPa, i.e. at a pressure slightly higher than the pressure of the insulator to
metal transition. However a homogeneous magnetic phase occurs only above 10
GPa. The magnetic and electronic properties are related to the renormalization
of the 4f wavefunction either to the divalent or the trivalent configurations.
As observed in SmS, long range magnetism in SmB6 occurs already far below the
pressure where a trivalent Sm3+ state will be reached. It seems possible, to
describe roughly the physical properties of the intermediate valence
equilibrium by assuming formulas for the Kondo lattice temperature depending on
the valence configuration. Comparison is also made with the appearance of long
range magnetism in cerium and ytterbium heavy fermion compounds.Comment: 22 pages including figure
Multiconfigurational nature of 5f orbitals in uranium and plutonium intermetallics
Uranium and plutonium's 5f electrons are tenuously poised between strongly
bonding with ligand spd-states and residing close to the nucleus. The unusual
properties of these elements and their compounds (eg. the six different
allotropes of elemental plutonium) are widely believed to depend on the related
attributes of f-orbital occupancy and delocalization, for which a quantitative
measure is lacking. By employing resonant x-ray emission spectroscopy (RXES)
and x-ray absorption near-edge structure (XANES) spectroscopy and making
comparisons to specific heat measurements, we demonstrate the presence of
multiconfigurational f-orbital states in the actinide elements U and Pu, and in
a wide range of uranium and plutonium intermetallic compounds. These results
provide a robust experimental basis for a new framework for understanding the
strongly-correlated behavior of actinide materials.Comment: 30 pages, concatenated article and supporting information, 10 figure
Evidence for suppressed metallicity on the surface of La2-xSrxCuO4 and Nd2-xCexCuO4
Hard X-ray Photoemission spectroscopy (PES) of copper core electronic states,
with a probing depth of 60 \AA, is used to show that the Zhang-Rice
singlet feature is present in LaCuO but is absent in NdCuO.
Hole- and electron doping in LaSrCuO (LSCO) and
NdCeCuO (NCCO) result in new well-screened features which are
missing in soft X-ray PES. Impurity Anderson model calculations establish
metallic screening as its origin, which is strongly suppressed within 15
of the surface. Complemented with X-ray absorption spectroscopy,
the small chemical-potential shift in core levels ( eV) are shown to
be consistent with modifications of valence and conduction band states spanning
the band gap ( eV) upon hole- and electron-doping in LSCO and NCCO.Comment: 4 pages, 4 figure
A determination of the pairing interaction in the high Tc cuprate superconductor Tl2Ba2CaCu2O8 (Tl2212)
We have measured the near-normal reflectance of Tl2Ba2CaCu2O8 (Tl2212) for
energies from 0.1 to 4.0 eV at room temperature and used a Kramers-Kronig
analysis to find the complex, frequency dependent dielectric function, from
which the optical conductivity was determined. Using
Thermal-Difference-Reflectance (TDR) Spectroscopy the reflectance of the sample
in the normal state just above the superconducting transition, and in the
superconducting state were then obtained. From these data we determined the
ratio of the superconducting- to normal-state optical conductivities. Mattis
and Bardeen had calculated this function within the BCS theory, where the gap
is a fixed energy-independent quantity. Taking into account the retarded nature
of the electron-phonon coupling results in a complex, energy dependent gap
causing deviations from the Mattis-Bardeen plot at energies where the phonon
coupling function is large. We find a typical deviation near the phonon
energies in Tl2212, and in addition, at 1.2 and 1.7eV. The phonon, and these
electronic terms can each be described by a coupling constant. None of which by
itself gives rise to a high transition temperature, but the combination does.
Using Resonant Inelastic X-Ray Scattering (RIXS) we find that the d-to-d
excitations of the cuprate ion in Tl2212 fall at the same energies as the dips
in the Mattis-Bardeen plot. We conclude that the high superconducting
transition temperature of the cuprates is due to the sum of the phonon
interaction, and interactions with the Cu-ion d-shell.Comment: Preprint submitted to Physica C. Paper presented at M2S-HTSC-VIII,
Dresden, Germany, July 9-14,200
Magnetic anisotropy at the buried CoO/Fe interface
Interfaces between antiferromagnetic CoO and ferromagnetic Fe are typically characterized by the development of Fe oxides. Recently, it was shown that the use of a proper ultra-thin Co buffer layer prevents the formation of Fe oxides [Brambilla et al., Appl. Surf. Sci. 362, 374 (2016)]. In the present work, we investigate the magnetic properties of such an interface, and we find evidence for an in-plane uniaxial magnetic anisotropy, which is characterized by a multijump reversal behavior in the magnetization hysteresis loops. X-ray photoemission spectroscopy and element-sensitive hysteresis loops reveal that the occurrence of such an anisotropy is a phenomenon developing at the very interface
Strain induced phase separation in La0.7Sr0.3MnO3 thin films.
La0.7Sr0.3MnO3 thin films having different thicknesses were grown by pulsed laser deposition with in situ
reflection high energy electron diffraction diagnostics on LaAlO3 substrates. The mismatch between film and
substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than
30 unit cells. Accordingly, the ratio between the out-of-plane and the in-plane lattice parameter c/a varies
between 1.06 fully strained and 1.03 partially relaxed. In-plane compressive strain favors the stabilization of
the 3z2−r2 orbitals chain-type antiferromagnetic phase, thus giving rise to a sizeable x-ray absorption linear
dichroism signal. The shape of the linear dichroism depends weakly on the c/a ratio, while its intensity
strongly increases with c/a. At the same time, the metal-insulator transition temperature shifts from about
360 K towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the
30 unit cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the MnDE doubleexchange
metallic contribution with decreasing the thickness, which becomes negligible in the 30 unit cells
thick film. The experimental results demonstrate a strain driven competition between two stable phases: the
orbital ordered chain-type insulating antiferromagnetic and the orbital disordered metallic ferromagnetic. For
intermediate values of the epitaxial strain the local minimum state of the system lies in a gap region between
the two stable phases. Such a region has glassy characteristics with coexisting clusters of the two phases. The
strain is used as a driving force to span the glassy region
Coherent phonons and the interplay between charge density wave and Mott phases in 1-TaSe
1-TaSe is host to coexisting strongly-correlated phases including
charge density waves (CDWs) and an unusual Mott transition at low temperature.
Here, we investigate coherent phonon oscillations in 1-TaSe using a
combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES)
and time-resolved reflectivity (TRR). Perturbation by a femtosecond laser pulse
triggers a modulation of the valence band binding energy at the -point,
related to the Mott gap, that is consistent with the in-plane CDW amplitude
mode frequency. By contrast, TRR measurements show a modulation of the
differential reflectivity comprised of multiple frequencies belonging to the
distorted CDW lattice modes. Comparison of the temperature dependence of
coherent and spontaneous phonons across the CDW transition shows that the
amplitude mode intensity is more easily suppressed during perturbation of the
CDW state by the optical excitation compared to other modes. Our results
clearly identify the relationship of the in-plane CDW amplitude mode with the
Mott phase in 1-TaSe and highlight the importance of lattice degrees
of freedom.Comment: 7 pages, 4 figures, supplemental materia
Structural, Electronic and Magnetic Properties of a Few Nanometer-Thick Superconducting NdBa₂Cu₃O₇ Films
Abstract: Epitaxial films of high critical temperature (Tc) cuprate superconductors preserve
their transport properties even when their thickness is reduced to a few nanometers. However,
when approaching the single crystalline unit cell (u.c.) of thickness, Tc decreases and eventually,
superconductivity is lost. Strain originating from the mismatch with the substrate, electronic
reconstruction at the interface and alteration of the chemical composition and of doping can be
the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu L3 edge to study
the crystal field and spin excitations of NdBa2Cu3O7x ultrathin films grown on SrTiO3, comparing
1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the
in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with
respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the
hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen
deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy
and x-ray diffraction
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