Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
vated by its proximity to a quantum phase transition, the isoelectronic
compound YbIr2Si2 has not yet been investigated. We have synthesized
polycrystalline and single crystalline samples of YbIr2Si2 and investigated
their physical properties. Depending on synthesis conditions the
samples take either body-centred (ThCr2Si2 structure- I-type) or primitive
tetragonal (CaBe2Ge2 structure- P-type) structure. P-type sample
shows AF order below 700 mK. I-type sample does not show any signature
of magnetic order down to 100 mK. The magnetic susceptibility for
this sample is highly anisotropic with effective moment close to that expected
for Yb 3+ ions. The resistivity is nearly temperature independent
at high temperature followed by a rapid decrease below 100 K similar to
Kondo lattice/heavy fermion system. The resistivity does not show any
anomaly due to either magnetic or superconducting transition down to
20 mK. Further, the resistivity at low temperature follows ρ(T) = ρ0 +
AT n with n =1.3 instead of n = 2 expected from Fermi Liquid theory.
The specific heat is also Non-Fermi-Liquid (NFL) type as C/T continues
to increase with decreasing T down to 350 mK where C/T = 0.6 J/(mol
K 2 ). Thus, I-type YbIr2Si2 is a new heavy fermion compound situated
close to the quantum critical point and exhibits NFL behaviour.
TT 7.8 Mo 16:30 H19
Investigation of the magnetic and the superconducting states of
CeCu2Si2 using large single crystal. — •H.S. Jeevan 1 , M. Deppe 1 ,
O. Stockert 1 , R. Borth 1 , E. Faulhaber 2 , C. Geibel 1 , and F.
Steglich 1 — 1 Max Planck Institute for Chemical Physics of Solids,
Dresden — 2 TU Dresden, IAPD, D-01062 Dresden, Germany.
The discovery of superconductivity in CeCu2Si2 in 1979 opened the
field of unconventional superconductivity in strongly correlated systems.
Later on, another unconventional phase, the A-phase, of presumably
magnetic character was found to compete with the superconducting phase
in CeCu2Si2. Despite more than 20 years of intensive research, the nature
of the superconducting phase and of the A-phase as well as the interac-
TT 8 Postersitzung I: Supraleitung
tion between both are very far from being understood. A major problem
is the lack of large single crystals with well defined physical properties.
This prevents e.g. a study of the magnetic structure and magnetic excitations
of this compound with neutron scattering, which would allow
a much deeper insight into the nature of these unconventional phases.
We have developed a self-flux growth method, which allows the growth
of large CeCu2Si2 single crystal with well defined properties. The ground
state can be tuned by changing growth parameters. We have investigated
the magnetic and the superconducting ground state using specific
heat, resistivity and susceptibility measurements. The preliminary neutron
scattering measurements allow us to determine for the first time
the magnetic structure of the A-phase. We shall analyse and discuss the
physical properties of these single crystals and their relation to growth
parameters.
TT 7.9 Mo 16:45 H19
Weak itinerant metamagnetism at the quantum phase transition
of ZrZn2 — •M. Uhlarz 1 , C. Pfleiderer 1 , and S. M. Hayden
2 — 1 Physikalisches Institut, Universität Karlsruhe, D-76128 Karlsuhe,
Germany — 2 H. H. Wills Physics Laboratory, University of Bristol,
Bristol, BS8 1TL, UK
It is generally assumed that the coexistence of band-ferromagnetism
with superconductivity, recently discovered in ZrZn2 [1], is connected
with the proximity of ZrZn2 to a quantum critical point. We have revisited
the question of quantum criticality in ZrZn2 in a detailed study of
the pressure dependence of the magnetisation of single crystals of ZrZn2
up to 21kbar at temperatures down to 1.5K and magnetic field up to
12T. In the vicinity of the critical pressure we observe evidence for weak
first order behaviour and itinerant metamagnetism. The evolution of the
magnetic state in ZrZn2 is compared with that observed in the superconducting
ferromagnets UGe2 and URhGe.
[1] C. Pfleiderer et al., Nature 412 (2001) 58.
Zeit: Montag 14:30–19:00 Raum: Poster A
TT 8.1 Mo 14:30 Poster A
In-situ deposition of MgB2 thin films by magnetron sputtering
and thermal evaporation — •R. Schneider, A.G. Zaitsev, J.
Geerk, G. Linker, and F. Ratzel — Forschungszentrum Karlsruhe,
Institut für Festkörperphysik
We report on two approaches to the in-situ synthesis of superconducting
MgB2 thin films. In the first approach, Mg and B were simultaneously
sputtered from two separate planar targets. The substrate temperature
Ts was limited to a small range of 290 to 320 ◦ C. The resulting films on
sapphire substrates were c-axis textured with low growth quality. Their
transition temperature Tc reached a maximum of 24 K with a transition
width of 0.6 K. A short-time in-situ annealing at 600 ◦ C improved Tc to
28 K. In the second approach, the Mg sputter source was replaced by
a specially designed Mg evaporator. Due to this intense Mg source Ts
could be increased to 440 ◦ C, and Tc of the ”as-grown” films rose to 33
K. Short-time in-situ annealing after the film deposition enhanced Tc to
36 K. For these films we also measured a high critical current density of
15 MA/cm 2 at 6 K.
TT 8.2 Mo 14:30 Poster A
Preparation, Structure, and Properties of Hg,Re-Containing
High-Temperature Superconductor Thin Films — •Abouelwafa
Salem, Gerhard Jakob und Hermann Adrian — Institut für Physik,
Johannes Gutenberg–Universität, 55099
Fully textured (Hg0.9Re0.1)Ba2CaCu2O6+δ (HgRe-1212) thin films have
been prepared by pulsed laser deposition (PLD) and post-annealing.
The films exhibit sharp superconducting transitions at Tc= 124 K with
transition widths of ∆Tc ≃ 2 K. Conditions for reproducible film preparation
have been found. The resistive transitions have been investigated in
magnetic fields up to 8 T (parallel to the c-axis) and 10 T (perpendicular
to the c-axis). The activation energy of thermally activated flux-motion
has been determined for both magnetic field orientations. The Hall resistivity
(ρxy) has been measured in the ab-plane at 8 T. In the mixed state
a power-law behavior is observed, where ρxy scales to ρxx: ρxy = Aρ β xx,
with β = 1.4 ±0.1. Above 130 K the T 2 dependence of the cotangent of
the Hall angle is observed. Magnetization loops were measured at various
temperatures in a SQUID magnetometer. The corresponding critical cur-
rent densities Jc were calculated. The anisotropic properties of the vortex
state and the depinning field Bdp(θ) have been studied by varying the angle
θ between the field direction and the c-axis of the film. The measured
angular dependence Bdp(θ) shows a cusp for θ = 90 ◦ . The films exhibit
a rather low critical field anisotropy of 7.7. Under the same conditions
(Hg0.9Re0.1)Ba2Ca2Cu3O8+δ (HgRe-1223) thin film (Tc= 130 K) have been
prepared.
TT 8.3 Mo 14:30 Poster A
Novel consideration of the phase diagram of electron doped
high-temperature superductors — •Yoshiharu Krockenberger
1 , Michio Naito 1 , Akio Tsukada 1 , Hideki Yamamoto 1 ,
Lambert Alff 2 , and Rudolf Gross 2 — 1 NTT Basic Research
Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi-shi,
Kanagawa 243-0198, Japan — 2 Walther-Meissner-Institut, 85748
Garching, Germany
The common phase diagram of high-temperature superconductors
(HTS) including electron doped HTS was published first by Luke et al.[1].
Improved sample quality of hole doped cuprates supported and extended
this phase diagram. Using molecular beam epitaxy it is possible to grow
high quality single phase thin films of electron doped high-temperature
superconductors. Usually, for doping the T ′ -structure tetravalent Ce (e.g.
La2−xCexCuO4+δ) is used. Using trivalent ions instead of Ce 4+ , e.g. Tb 3+ ,
one would expect a Mott insulating and antiferromagnetic state. The caxis
dependence of the doping level as well as XPS measurements clearly
show that the Tb-cation is in the trivalent state. Furthermore, other
trivalent dopants such as Lu, Y, Dy, Eu, and Sm have been investigated
and all of them show the same result: superconductivity below about
20K. This result may shed new light on the usually used phase diagram
of electron doped high temperature superconductors.
[1] G. M. Luke et al., Phys. Rev. B 42, 7981 (1990)
TT 8.4 Mo 14:30 Poster A
Growth and superconducting properties of ternary rare earth
(Gd, Eu, Nd)Ba2Cu3Oy thin films — •C. Cai, J. Hänisch, L.
Fernandez, and B. Holzapfel — IFW Dresden, D-01171 Dresden,
Germany