Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
TT 10 FV-internes Symposium ”Quantum Phase Transitions”
Zeit: Dienstag 10:00–12:55 Raum: H18
Hauptvortrag TT 10.1 Di 10:00 H18
Local quantum criticality and non-Fermi liquid properties —
•Qimiao Si — Dept of Physics and Astronomy, Rice University, Houston,
USA
Quantum criticality not only provides a mechanism for non-Fermi liquid
behavior, it also seems to control large regions of the phase diagram
in strongly correlated electrons. This talk will be devoted to issues that
have originated from quantum critical heavy fermions.I will describe some
recent developments on local quantum criticality. Recent progresses have
been made, within a Kondo lattice model that is amenable to controlled
calculations. Extensive experimental results that pertain to this picture
have also been emerging, including inelastic neutron scattering, NMR,
Hall coefficient, and Grueneisen ratio. The summary of these results will
be followed by some discussions about their broader implications.
Fachvortrag TT 10.2 Di 10:35 H18
Singular Behavior Near a Quantum Critical Point:
YbRh2(Si,Ge)2 — •P. Gegenwart — Max-Planck-Institut
für Chemische Physik fester Stoffe, 01187 Dresden
By discussing resistive, calorimetric, dilatometric and ESR data on
YbRh2(Si1−xGex)2 with 0 ≤ x < 0.05 we argue that these materials represent
systems exhibiting a qualitatively new class of quantum critical
point that cannot be described by the frequently adopted itinerant (3D
or 2D) spin-density-wave scenario. Upon approaching the quantum critical
point we observe large (“unscreened”, i.e., not reduced by the Kondo
effect) magnetic Yb 3+ moments in the static bulk susceptibility and recently
in ESR, disparate behavior in the T-dependences of the electrical
resistivity and the electronic specific heat as well as a fractional scaling
exponent in the T-dependence of the thermal expansion. These findings
seem to be compatible with a local-moment scenario for the quantum
critical point.
In collaboration with J. Custers, J. Ferstl, C. Geibel, R. Küchler,
T. Lühmann, K. Neumaier, S. Paschen, J. Sichelschmidt, G. Sparn, F.
Steglich, Y. Tokiwa, H. Wilhelm, S. Wirth, P. Coleman, C. Pepin, and
Q. Si.
Fachvortrag TT 10.3 Di 11:05 H18
Non-Fermi-Liquid Phase of a Pure Itinerant-Electron Magnet
— •C. Pfleiderer — Physikalisches Institut, Universität Karlsruhe,
D-76128 Karlsuhe
Extensive studies of the itinerant-electron magnetism in the cubic transition
metal compound MnSi as function of pressure are reviewed, that
suggest a distinct transition from a weakly spin polarized Fermi liquid
ground state to a non-Fermi liquid (NFL) phase of exceptional stability
above pc = 14.6kbar [1,2]. In a recent neutron diffraction experiment [3]
sizable quasi-static magnetic moments have been found to exist far into
this NFL phase. These moments are organized in a highly unusual pattern
never seen before, yielding Bragg intensity on the surface of a tiny
sphere in reciprocal space implying partial order. The properties of MnSi
shed new light on the possible nature of inconsistencies of the metallic
state near magnetic quantum phase transitions observed in numerous
itinerant-electron magnets. In particular, they support the genuine
existence of metallic phases with partial order of the conduction elec-
trons, that are reminiscent of liquid crystals, as proposed for the high-Tc
cuprates and heavy-fermion compounds.
[1] C. Pfleiderer et al., Nature 414 (2001) 427.
[2] N. Doiron-Leyraud et al. Nature 425 (2003) 595.
[3] C. Pfleiderer et al., Nature in print.
*in collaboration with J. Flouquet, M. Garst, S. R. Julian, H. v.
Löhneysen, G. G. Lonzarich, L. Pintschovius, D. Reznik, A. Rosch, A.
N. Stepanov, C. Thessieu and M. Uhlarz.
11:35 Pause
Fachvortrag TT 10.4 Di 11:50 H18
Monte Carlo simulations of quantum phase transitions in dissipative
systems and ultra-cold atoms — •Matthias Troyer 1,2 ,
Philipp Werner 1 , Klaus Völker 3 , Fabien Alet 1,2 , Stefan Wessel
1 , and George G. Batrouni 4 — 1 Theoretische Physik, ETH zürich
— 2 Computational Laboratory, ETH Zürich — 3 University of Toronto
— 4 Université de Nice
The past decade has seen tremendous progress in quantum Monte
Carlo algorithms. Advanced classical simulation techniques have been
generalized to quantum system and further refined. This enables simulations
with an unprecedented accuracy, allowing precise investigations
of quantum phase transitions. Here we will focus on some recent results.
The first part of the talk will be about the effect of dissipation on
quantum phase transitions. While in classical simulations dynamics and
thermodynamics are decoupled and dissipation thus has no influence on
the universality class, the dynamics and thermodynamics are intrinsically
coupled in a quantum system and can change the nature of the phase
transition. We will present recent results on the phase diagrams and the
critical behavior of dissipative quantum Ising models in a transverse field
and preliminary results on quantum XY models. The second part of the
talk will be on effects of constrained geometries, as they are created by
an optical or magnetic trap on an ultra-cold atomic gas. We will demonstrate,
that, the constrained geometry and the inhomogeneity of the trap
lead to an absence of quantum critical behavior at the quantum phase
transition in these systems. This explains the absence of critical slowing
down in recent experiments,
Fachvortrag TT 10.5 Di 12:25 H18
Numerical Renormalization Group for Bosons and Fermions:
Impurity Quantum Phase Transitions — •Ralf Bulla — Theoretische
Physik III, Universität Augsburg
Wilson’s numerical renormalization group (NRG) has been very successful
in describing magnetic impurities in a fermionic bath. Here we
review some recent results on quantum phase transitions of impurity
models in a general environment, including examples of local criticality.
In this context, we present the generalization of the NRG to models
where the impurity couples to a bosonic bath. Application of the bosonic
NRG to the subohmic spin-boson model reveals a line of quantum critical
points terminating in the Kosterlitz-Thouless transition for the ohmic
case. Further extensions of this approach (such as the investigation of the
Bose-Fermi Kondo model within the extended DMFT) are discussed.
TT 11 Nanoelektronik I: Quantenpunkte, -drähte, -punktkontakte
Zeit: Dienstag 09:30–13:00 Raum: H19
TT 11.1 Di 09:30 H19
Single-electron effects and quantum fluctuations in metallic
multi-island geometries — •Björn Kubala 1,2 , Göran Johansson
2 , and Jürgen König 1 — 1 Institut für Theoretische Physik III,
Ruhr-Universität Bochum, D-44780 Bochum — 2 Institut für Theoretische
Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe
We have developed a method to model electron transport through a
complex network of single-electron transistors.
Based on a real-time transport theory [1], we automatically generate
all diagrams up to second order in tunnel coupling and calculate their
contributions to the current. This captures quantum fluctuations and
multi-channel Kondo physics for each island as well as the mutual influence
of tunneling effects on two neighbouring islands. The latter is
essential for a detailed analysis of recent experiments [2], where an SET,
capacitively coupled to a single electron box, was used as a sensitive
electrometer, measuring the charge state of the box. Here the backaction
between measurement device (SET) and system (box) is fully included
in our approach.
We will discuss the applicability of our method to different experimental
setups of metallic islands, coupled capacitively and/or by tunneling
barriers, and possible generalizations to the case of magnetic or superconducting
leads.
[1] H. Schoeller and G. Schön, Phys. Rev. B 50, 18 436 (1994); J. König,