Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
TT 3.2 Mo 09:45 H18
Spectral Properties of One-Dimensional Heisenberg Antiferromagnets
— •Kai P. Schmidt, Alexander Reischl, Sébastien
Dusuel, and Götz S. Uhrig — Zülpicherstr. 77, D-50937 Köln, Germany
Spectral properties of the dimerized and frustrated S = 1/2 spin chain
are calculated by particle-conserving continuous unitary transformations
(P-CUT’s). A detailed analysis of spectral weights and spectral densities
for various excitation processes will be given. The results are relevant
for inelastic neutron scattering (INS), Raman spectroscopy and infrared
(IR) spectroscopy. The use of triplons (elementary triplets) as elementary
excitations gives a new perspective for the description of low dimensional
quantum spin liquids.
[1] K.P. Schmidt and G.S. Uhrig, Phys. Rev. Lett. 90, 227204 (2003)
[2] K.P. Schmidt, C. Knetter and G.S. Uhrig, cond-mat/0307678
TT 3.3 Mo 10:00 H18
Dimer Korrelationen im dimerisierten eindimensionalen S = 1/2
Heisenberg-Modell — •Carsten Aits und Ute Löw — Universität
zu Köln, Institut für Theoretische Physik, Zülpicher Str. 77, 50937 Köln
Wir untersuchen das Verhalten der Dimer-Korrelationen im eindimensionalen
S = 1/2 Heisenberg-Modell mit und ohne statische Dimerisierung.
Dazu werden numerische Resultate aus exakter Diagonalisierung
und Quanten-Monte-Carlo analysiert. Im nicht dimerisierten Fall werden
die Ergebnisse mit Aussagen aus der konformen Feldtheorie verglichen.
Darüberhinaus wird in dimerisierten Ketten der Zusammenhang
zwischen dem statischen Strukturfaktor und dem Skalenverhalten von
Grundzustandsenergie bzw. Anregungslücke diskutiert.
TT 3.4 Mo 10:15 H18
Theory of spin waves in diluted-magnetic-semiconductor quantum
wells — •Diego Frustaglia 1,2 , Jürgen König 3,2 , and Allan
MacDonald 4 — 1 Scuola Normale Superiore, Pisa, Italy — 2 Institut
für Theoretische Festkörperphysik, Universität Karlsruhe, Germany —
3 Institut für Theoretische Physik III, Ruhr-Universität Bochum, Germany
— 4 Department of Physics, University of Texas, Austin, USA
We present a theory of collective spin excitations in diluted magnetic
semiconductor quantum wells, in which local magnetic moments are magnetically
coupled via a quasi-two-dimensional electron or hole gas. In
contrast to bulk systems, a multiplet of spin-wave branches is predicted.
We discuss the dispersion of these collective excitations and the profiles
of the corresponding modes for different magnetic-ion doping concentrations
and free-carrier densities. In the case of a ferromagnetic state with
a partially spin-polarized two-dimensional electron gas, we find that the
Goldstone collective mode has anomalous k 4 dispersion and that for symmetric
quantum wells odd parity modes do not disperse at all. Moreover,
we further discuss the gap in the collective excitation spectrum which
appears when spin-orbit interactions are included.
TT 3.5 Mo 10:30 H18
Magnetocaloric effect in one-dimensional quantum spin models
— •A. Honecker 1 and M.E. Zhitomirsky 2 — 1 Universität Hannover,
Institut für Theoretische Physik, Appelstraße 2, 30167 Hannover
— 2 Commissariat à l’Énergie Atomique, DMS/DRFMC/SPSMS, 38054
Grenoble, Cedex 9 France
An external magnetic field induces changes in the entropy of a quantum
spin system at finite temperatures which can be rather large in
particular if the spin system is geometrically frustrated. This implies
a magnetocaloric effect, i.e. a change in temperature during an adiabatic
(de)magnetization process. Here we study several examples of onedimensional
spin models with low spin quantum number (S = 1/2 and 1)
employing exact diagonalization and the Jordan-Wigner transformation.
During an adiabatic (de)magnetization process the temperature drops
in the vicinity of a field-induced zero-temperature quantum phase transition.
Comparing different levels of frustration, we find that more frustrated
systems cool down to lower temperatures. Our study demonstrates
that frustrated quantum spin systems are promisimng alternative refrigerant
materials for low-temperature magnetic refrigeration.
TT 3.6 Mo 10:45 H18
Non-dissipative thermal transport and crossover phenomena in
the gapped spin-1/2 XXZ chain — •Andreas Klümper 1 and
Kazumitsu Sakai 2 — 1 Universität Wuppertal, Theoretische Physik,
Gauß-Straße 20 — 2 Institute for solid state physics, University of Tokyo
We present exact results on the thermal conductivity of the onedimensional
spin-1/2 XXZ model in the gapless and the massive antiferromagnetic
as well as ferromagnetic regimes. The thermal Drude weight
is calculated by a lattice path integral formulation. Numerical results for
wide ranges of temperature and anisotropy as well as analytical results
in the low and high temperature limits are presented. At finite temperature,
the thermal Drude weight is finite and hence there is non-dissipative
thermal transport even in the massive regimes.
At low temperature, the thermal Drude weight behaves as Dth(T) ∼ T
in the gapless regime, and as Dth(T) ∼ e −δ/T / √ T in the gapped
regimes where δ is the one-spinon/one-magnon excitation gap in the
antiferromagnetic/ferromagnetic case. In the ferromagnetic regime we
observe a crossover in the low-temperature behaviour of the ratio of the
thermal Drude weight and the specific heat from Dth(T)/C(T) ≃ T to
Dth(T)/C(T) ≃ T 3/2 e −β(δm−δs) when changing from ∆ < 5/3 to ∆ > 5/3
(δm and δs are the magnon and the spinon gap).
TT 3.7 Mo 11:00 H18
Integral representations of correlation functions for the
anisotropic Heisenberg chain at finite temperatures — •Frank
Göhmann and Alexander Seel — Fachbereich C, Physik, Bergische
Universität Wuppertal, 42097 Wuppertal
We derived novel integral representations for correlation functions of
the anisotropic Heisenberg chain at finite values of the temperature and
the applied magnetic field. Our work is based on a combination of Bethe
ansatz techniques for the calculation of matrix elements with the quantum
transfer matrix approach.
TT 3.8 Mo 11:15 H18
Spin dynamics of 1D supersymmetric t-J model with 1/r2 interaction
— •Mitsuhiro Arikawa — Institut fuer Theoretische Physik
III, Universitaet Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart
We report on analytic results on dynamical spin structure factor
S(q, ω) for 1D supersymmetric t-J model with 1/r2 interaction. In this
model, the elementary excitations, spinons and holons appear as free
particle obeying fractional statistics. we presented that interpretation
of S(q, ω) requires both fractionalized and non-fractionalized electrons.
Our results constitutes the first analytical knowledge for S(q, ω) of lattice
electrons with no restriction on the density and the momentum-frequency
range. We discuss how these excitations appear in the dynamics in wide
range energy region.
TT 3.9 Mo 11:30 H18
Effects of proper crystal embedding for low dimensional spin
systems — •H. Rosner 1 , S.-L. Drechsler 2 , M. Knupfer 2 , J.
Málek 2 , H. Eschrig 2 , J. Richter 3 , A. Moskvin 4 , W. Ku 5 , and
W.E. Pickett 5 — 1 Max Planck Institut für Chemische Physik fester
Stoffe — 2 Institut für Festkörper- und Werkstoffforschung Dresden —
3 Universität Magdeburg — 4 Ural State University Ekatarinenburg, Russia
— 5 University of California Davis, USA
Many approaches for the description of low dimensional spin systems
consider their isolated structural units only. Here, we emphasize the importance
of the proper embedding of these structural units in the crystal.
Effects resulting from the crystal field and the inter-unit interaction
are studied in detail for several selected model compounds as Li2CuO2,
LiCu2O2, Cu(Si)GeO3 or CaCu2O3. Band structure calculation, exact
diagonalization and various spectroscopic, magnetic and thermodynamic
data are joined to suggest a novel consistent picture.
TT 3.10 Mo 11:45 H18
Thermal conductivity of spin-1/2 chains — •Achim Rosch 1 ,
Efrat Shimshoni 2 , and Natan Andrei 3 — 1 Institut für Theoretische
Physik, Universität zu Köln — 2 University of Haifa, Israel — 3 Rutgers
University, USA
We study [1] the low-temperature transport properties of clean onedimensional
spin-1/2 chains coupled to phonons. Due to the presence
of approximate conservation laws, the heat current decays very slowly
giving rise to an exponentially large heat conductivity, κ ∼ e T ∗ /T . As a
result of an interplay of Umklapp scattering and spinon-phonon coupling,
the characteristic energy scale T ∗ turns out to be of order ΘD/2, where
ΘD is the Debye energy, rather than the magnetic exchange interaction
J in agreement with recent measurements in SrCuO compounds. A large
magnetic field h strongly affects the heat transport by two distinct mechanisms.
First, it induces a linear spinon-phonon coupling, which alters the
nature of the T → 0 fixed point: the elementary excitations of the system