Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Donnerstag
non-Gaussian behavior below 20K as n/nc decreases through 1. This has
been interpreted as the onset of a glassy freezing of the electronic system
across the transition.
TT 30.16 Do 14:30 Poster A
Structure, Magnetism and Transport in Titanates — •H. Roth,
N. Schittner, A. Elfilali, K. Kordonis, M. Cwik, A. Komarek,
M. Reuther, R. Müller, T. Lorenz, M. Braden und A. Freimuth
— II. Physikalisches Institut, Universität zu Köln, Zülpicher str.
77, 50939 Köln
The RTiO3 compounds (R = Rare Earth) are typical Mott-insulators
with ferro- or antiferromagnetic order depending on the radius of the
R 3+ -ion. These Mott-insulators can be driven to a metal by hole doping.
We have prepared single crystals of the systems La1−xSrxTiO3, LaTiO3+δ
and Y1−xCaxTiO3 and studied the magnetization and resistivity. The resistivity
of La1−xSrxTiO3 with 0.07 < x ≤ 0.2 show a T 2 behaviour
over an unusually large temperature range up to 300K. In contrast to
La1−xSrxTiO3, where the transition from antiferromagnetic to paramagnetic
behaviour occurs around a hole concentration of 6%/Ti, the antiferromagnetic
order in LaTiO3+δ is stable up to δ = 0.1, which formally
corresponds to a hole concentration of 20%/Ti. Both, oxygen- as well
as Sr-doping causes a systematic decrease of the unusual orthorhombic
splitting observed in the pure LaTiO3. YTiO3, the other end compound
of the RTiO3 series, is a ferromagnet with a magnetic moment of 0.9µB.
In Y1−xCaxTiO3 the ferromagnetic order is strongly suppressed with increasing
x and we find some indication for a sign change of the effective
magnetic coupling, which could lead to a canted ferromagnetic ground
state for 0.1 < x < 0.4. gefördert von der DFG durch den SFB 608
TT 30.17 Do 14:30 Poster A
Metal-insulator transition in quasi 2D systems — •K.
Morawetz — Institute of Physics, Chemnitz University of Technology,
09107 Chemnitz, Germany — Max-Planck-Institute for the Physics of
Complex Systems, Nöthnitzer Str. 38, 01187 Dresden, Germany
The conductivity in quasi two-dimensional systems is calculated using
the quantum kinetic equation. Weak localization correction as an
interference effect appears from the field dependence of the collision integral.
The approximation is applied to a system of quasi two-dimensional
electrons in hetero-junctions which interact with charged and neutral
impurities and the low temperature correction to the conductivity is calculated
analytically. It turns out that the dynamical screening due to
charged impurities leads to a linear temperature dependence, while the
scattering from neutral impurities leads to the usual Fermi-liquid behavior.
The experimental metal-insulator transition at low temperatures are
reproduced [1]. The effective mass of quasiparticle excitations in quasi
two-dimensional systems is calculated analytically. It is shown that the
effective mass increases sharply when the density approaches the critical
one of metal-insulator transition. This suggests a Mott-type of transition
rather than an Anderson-like transition [2].
[1] K. Morawetz, Phys. Rev. B 67 (2003) 115125
[2] K. Morawetz, cond-mat/0210168
TT 30.18 Do 14:30 Poster A
Electrical conductivity of pure and doped K0.3MoO3 single crystals
grown by various preparation methods — •S. Yue 1 , M.
Dressel 1 , C. Kuntscher 1 , S. van Smaalen 2 , F. Ritter 3 , and
W. Assmus 3 — 1 1. Physikalisches Institut, Universität Stuttgart, 70550
Stuttgart, Germany — 2 Laboratory of Crystallography, University of
Bayreuth, 95440 Bayreuth, Germany — 3 Physikalisches Institut, Universität
Frankfurt, 60054 Frankfurt, Germany
We have studied the temperature dependence of the nonlinear electrical
conductivity in a series of pure and doped K0.3MoO3 single crystals
obtained by different preparation methods (i.e., electrochemistry and
temperature-gradient flux techniques). All the pure samples reveal the resistivity
anomaly near 180 K and the nonlinear transport, resulting from
the Peierls transition and the sliding of the charge-density wave (CDW),
respectively. Doping with Rb and W smears out the sharp transition,
with the transition temperature decreased. Furthermore, the doping introduces
pinning centers which enhance the threshold field for CDW depinning.
We will also discuss the temperature dependence of the threshold
field for the various samples.
TT 30.19 Do 14:30 Poster A
One-dimensional fermions with static dimerisation: Zero-mode
bosonisation and self consistent harmonic approximation —
•Carmen Mocanu, Peter Schwab, Michael Dzierzawa, and Ulrich
Eckern — Universität Augsburg, Theoretische Physik II, 86135
Augsburg
We investigate one-dimensional fermions with static dimerisation,
where the static dimerisation is introduced as a modulation of the hopping
amplitude. As theoretical tools we use bosonisation in connection
with the self consistent harmonic approximation (SCHA), which is based
on a variational principle. The conventional SCHA Hamiltonian is constructed
by replacing the non linear terms by quadratic forms. The Klein
factors and zero-modes are ignored. Here, we develop an improved SCHA,
including the Klein factors and zero-modes and compare the results with
the conventional procedure.
TT 30.20 Do 14:30 Poster A
Magnetische Beiträge zum Wärmetransport in den quasi-1D
Spinketten von SrCuO2 — •Patrick Ribeiro 1 , Christian Hess 2 ,
Pascal Reutler 1 , Rudiger Klingeler 1 , Bernd Büchner 1 und
Georg Roth 3 — 1 IFW-Dresden, D-01171 Dresden — 2 Department of
Condensed Matter Physics, CH-1211 Geneva — 3 Institut für Kristallographie
der RWTH-Aachen, D-52056 Aachen
Wir präsentieren Ergebnisse der Wärmeleitfähigkeit des Spinkettensystems
Sr1−xCaxCuO2 . Diese Verbindung enthält in ihrer Struktur zwei
parallel laufende Cu-Ketten, die um einen halben Cu-Cu Abstand versetzt
sind. Wegen der dadurch entstehenden magnetischen Frustration
zwischen jeweils drei Cu-Spins bilden sich magnetisch nahezu isolierte
antiferromagnetische S=1/2 Spinketten, deren Intrakettenkopplung
J=2100K beträgt. Unsere Ergebnisse zeigen eine starke anisotropie der
thermischen Leitfähigkeit, die wir auf einen zusätzlichen magnonischen
Beitrag zur Wärmeleitfähigkeit entlang der Ketten zurückführen. Eine
Bestätigung dieser Interpretation ergibt sich aus Messungen der thermischen
Leitfähigkeit an Systemen, in denen Sr isovalent durch Ca substituiert
wurde, da diese Messungen eine genauere Separation von phononischen
und magnonischen Beiträgen zum Wärmetransport erlauben.
TT 30.21 Do 14:30 Poster A
Studies of a CDW system under hydrostatic pressure and high
magnetic fields: from field-induced CDW transitions to superconductivity
— •Dieter Andres, Mark Kartsovnik, Werner
Biberacher, and Karl Neumaier — Walther-Meissner-Institut, Bayerische
Akademie der Wissenschaften, Walther-Meissner-Str.8, 85748
Garching
In α-(BEDT-TTF)2KHg(SCN)4 a charge density wave (CDW) is
formed below about 8 K. This extremely low transition temperature
gives the unique possibility to study the B-T phase diagram in an
extended range. Moreover, hydrostatic pressure is known to easily tune
the nesting conditions of the CDW system.
We show how orbital and Pauli effects of magnetic field compete within
the CDW state leading to new quantization phenomena of successive
field-induced CDW transitions. Furthermore, a coexistence of superconductivity
and CDW in a certain pressure range will be discussed.
TT 30.22 Do 14:30 Poster A
Quantum Monte Carlo simulation for the conductance of onedimensional
quantum spin systems — •Kim Louis und Claudius
Gros — Universität des Saarlandes
Recently, the stochastic series expansion (SSE) has been proposed as
a powerful MC-method, which allows simulations at low T for quantumspin
systems. We show that the SSE allows to compute the magnetic
conductance for various one-dimensional spin systems without further
approximations. We consider various modifications of the anisotropic Heisenberg
chain. We recover the Kane-Fisher scaling for one impurity in a
Luttinger-liquid and study the influence of non-interacting leads for the
conductance of an interacting system.
TT 30.23 Do 14:30 Poster A
Thermal conductivity of the layered cuprates R2CuO4 with R =
Pr, Nd, Sm, Eu and Gd — •H. Anapa 1 , K. Berggold 1 , K. Kordonis
1 , M. Kriener 1 , J. Baier 1 , T. Lorenz 1 , A. Freimuth 1 , and
S. Barilo 2 — 1 II. Physikalisches Institut, Universität zu Köln, 50937
Köln — 2 Inst. of Sol. St. & Semicond. Phys., National Academy of Sciences,
Minsk