Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
with the theoretical I3(h) given in terms of the excitation amplitude
h gives direct access to h(pcirc). We derived very similar characteristic
parameters from HTS resonators with extremely different geometries.
They suggest a divergence of Z(H) which can be identified with the critical
magnetic field Hc2 of the superconductor. From comparison of the
theoretical function Z(pcirc) with the experimental surface impedance
ZS(pcirc) = RS(pcirc) + iXS(pcirc) of a HTS disk resonator we deduce
Z(pcirc) ∝ RS(pcirc) identifying the rf power induced dissipation increase
as dominant IMD source.
TT 8.39 Mo 14:30 Poster A
Unusual electronic and magnetic properties of intermetallic antiperovskites
— •Claire Loison and Helge Rosner — Max Planck
Institut für Chemische Physik fester Stoffe
In the last years, antiperovskites XYT3 (X=Mg,Cu,La..., Y=B,C,N...
and T a transition metal) have received considerable attention because
of many unusual physical properties caused by different competing
interactions. Examples are the recently discovered superconductor
MgCNi3 (superconductivity vs. ferromagnetism) or the non-collinear
magnet CuNMn3. Here, we present a systematic study for a series of antiperovskites
using density functional theory (DFT) electronic structure
calculations within the local spin density approximation (LSDA). To investigate
the role of possible strong Coulomb repulsion we applied as well
LSDA +U. The focus of our study are the effects of pressure and doping
on the electronic properties and magnetism and the search for possible
instabilities and phase transitions.
TT 8.40 Mo 14:30 Poster A
Photo-induced Excitation of Cooper Pairs in Superconductors
— •Jamal Berakdar and Konstantin Kouzakov — Max-Planck-
Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany
In this work we study the photoinduced Cooper pair excitation in a superconductor.
It is shown [1] that the spectra of the excited electron pair
carry direct information on the energy and angular pair correlation. We
suggest an experimental set-up for the verification of the theoretical predictions
and provide, on the basis of the Bardeen-Cooper-Schrieffer theory
for superconductivity, numerical illustrations of electron-pair spectra.
[1] K. Kouzakov, J. Berakdar, Phys. Rev. Lett (2003) in press.
TT 8.41 Mo 14:30 Poster A
Optical conductivity and approximate conservation laws in dwave
superconductors — •Philip Howell 1 and Achim Rosch 2
— 1 Institut für Theorie der Kondensierten Materie, Universität Karlsruhe,
76128 Karlsruhe — 2 Institut für Theoretische Physik, Universität
zu Köln, 50937 Köln
Recent measurements on high-temperature superconductors show that
the optical conductivity depends strongly on temperature. It has been
pointed out [1] that inelastic quasiparticle–quasiparticle scattering in a
d-wave superconductor suffers kinematical constraints: momentum conservation
leads to an ‘Umklapp gap’ and hence exponential behaviour
of the conductivity. Here we argue that there are additional generalised
momenta [2] which are almost conserved. The decay of the resulting slow
modes determines the optical conductivity at low temperatures, and we
investigate their influence via a memory matrix approach.
[1] M. B. Walker and M. F. Smith, Phys. Rev. B 61, 11285 (2000)
[2] A. Rosch and N. Andrei, Phys. Rev. Lett. 85, 1092 (2000)
TT 8.42 Mo 14:30 Poster A
Raman study of charge ordering instabilities in La2−xSrxCuO4
— •Leonardo Tassini 1 , Francesca Venturini 1 , Rudi Hackl 1 ,
Qing-ming Zhang 2 , N. Kikugawa 3 , and Tojitsu Fujita 3 —
1 Walther-Meissner-Institut, Bayerische Akademie der Wissenschaften,
D-85748 Garching — 2 Department of Physics, Nanjing University,
Nanjing 210093, P.R. China — 3 ADSM, Hiroshima University,
Higashi-Hiroshima 739-8526, Japan
The electronic Raman effect has been studied in differently doped single
crystals of La2−xSrxCuO4 (LSCO). At low energies and temperatures
new structures indicating a very high conductivity are found for which
there is no evidence in Bi2212 and Y123. In the same temperature range
a related low-energy mode is found in the optical conductivity. For the
selection rules and for similarities with ladder compounds we conclude
that the features are an indication of incipient charge ordering and 1D
conduction. The correlation length of these stripes is at least of the order
of the electronic mean free path in LSCO. The direction of the 1D
structures is along the diagonals and the principle axes for x = 0.02 and
x = 0.10, respectively.
TT 8.43 Mo 14:30 Poster A
Photoemission and X-ray absorption study of La and Pb doped
Bi-2201 — •M. Schneider, R. Mitdank, R.-St. Unger, T.
Stemmler, R. Müller, H. Dwelk, C. Janowitz, A. Krapf, and
R. Manzke — Institut für Physik, Humboldt-Universität zu Berlin,
Newtonstr. 15, 12489 Berlin
Besides to investigate the almost undisturbed CuO2-plane of the
cuprates, the additional advantage of the superconducting system
Bi2Sr2−xLaxCuO6+δ and Bi2−yPbySr2−xLaxCuO6+δ is to cover the total
doping range from the insulating up to the metallic phase by adjusting
the La content. By additional substitution of some percentage of Bi by
Pb in these samples also the structural about (5x1) superstructure of the
BiO-planes is almost suppressed. We present a systematic photoemission
(PES) and X-ray absorption (XAS) study of ceramic Bi-2201 samples
for the entire doping range. Due to the determination of the density of
states of both sides of the Fermi energy by PES and XAS, respectively, a
model for the doping mechanism of the CuO2-plane of the cuprates can
be discussed.
TT 8.44 Mo 14:30 Poster A
In-plane infrared conductivity of untwinned La2−xSrxCuO4 crystals
in the regime of stripe order. — •Michael Dumm 1,2 , Dimitri
Basov 2 , Seiki Komiya 3 , and Yoichi Ando 3 — 1 1. Physikalisches Institut,
Universität Stuttgart, 70550 Stuttgart — 2 Department of Physics,
University of California at San Diego, La Jolla, CA, USA — 3 Central Research
Institute of Electric Power Industry, Tokyo, Japan
Using infrared spectroscopy, we show that spin self-organization in
untwinned La1.97Sr0.03CuO4 and La1.96Sr0.04CuO4 single crystals has profound
consequences for the dynamical conductivity σ(ω). The electronic
response of CuO2 planes acquires significant anisotropy in the spinordered
state with enhancement of the conductivity along the direction
of the diagonal spin stripes by up to a factor of two. An examination
of the anisotropic response indicates that the diagonal spin texture in
weakly doped La2−xSrxCuO4 is also accompanied by the modulation of
charge density. The electronic response of the charge stripes is found to be
gapless consistent with the hypothesis of the metallic ground state. Our
experiments directly show that the striped ordered systems reveal new
degrees of freedom not present in ordinary one-dimensional conductors.
TT 8.45 Mo 14:30 Poster A
Nernst effect of stripe ordering La1.8−xEu0.2SrxCuO4 —
•E. Ahmed 1 , C. Hess 2 , B. Büchner 3 , U. Ammeral 4 , A.
Revcolevschi 4 , and B. Keimer 5 — 1 II. Physikalisches Institut,
RWTH Aachen, D-52056 Aachen — 2 Department of Condensed Matter
Physics, University of Geneva, Switzerland — 3 IFW Dresden, D-01171
Dresden — 4 Laboratoire de Physico-Chimie des Solides, Université
Paris-Sud, France — 5 MPI für Festkörperforschung, D-70506 Stuttgart
Nernst effect has been measured for a wide range of stripe ordering
La1.8−xEu0.2SrxCuO4 single crystals. We observe that compared to superconducting
La2−xSrxCuO4, the Nernst signal in the non-superconducting
stripe phase is suppressed. Nevertheless, at higher temperatures we find
a relatively large Nernst signal for a wide temperature range, similar to
the findings in La2−xSrxCuO4. We discuss possible origins of this unusual
large Nernst signal.
TT 8.46 Mo 14:30 Poster A
Temperature dependence of Bismuth-based HTSC as seen in
ARPES — •B. Müller 1 , L. Dudy 1 , H. Dwelk 1 , A. Krapf 1 , C.
Janowitz 1 , H. Höchst 2 , and R. Manzke 1 — 1 Humboldt Universität
zu Berlin, Institut für Physik, Newtonstr. 15, 12489 Berlin —
2 Synchrotron Radiation Center (SRC), Madison/Wisconsin (USA)
Our current investigations focus on the single-, double-, and triple-layer
cuprates Bi2201, Bi2223 and Bi2212, respectively. In ARPES measurements
of Bi2201 a two-peak structure can be observed, which is dependent
on the polarization of the incident light [1,2]. It persists above Tc
and vanishes at a higher temperature. There are hints that this temperature
corresponds to T ∗ , the temperature where the pseudogap closes.
Interestingly this two-peak structure is only visible along one CuO-bond
direction. The comparison with a pseudo-1d-model directs interpretations
of this feature towards a pseudo-1d-effect. The question is, if this
can also be shown for double- and triple-layer compounds.
[1] C. Janowitz, R. Müller, L. Dudy, A. Krapf, R. Manzke, C. Ast, H.