Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
H. Schoeller, and G. Schön, Phys. Rev. Lett. 78, 4482 (1997)
[2] R. Schäfer et al., cond-mat/0205223; K. W. Lehnert et al., Phys. Rev.
Lett. 91, 106801 (2003).
TT 11.2 Di 09:45 H19
The Coulomb blockade in two island systems with high conductive
junctions — •Roland Schäfer, Bernhard Limbach, Peter
vom Stein, and Christoph Wallisser — Forschungszentrum Karlsruhe,
Institut für Festkörperphysik, Postfach 3640, 76021 Karlsruhe
We report measurements on single electron pumps consisting of two
metallic islands formed by three tunnel junctions in a row. We focus
on the linear response conductance as a function of gate voltages
and temperature of three samples with varying system parameters. In
all cases strong quantum fluctuation phenomena are observed by a
log (kBT/(2Eco))-reduction of the maximal conductance where Eco measures
the coupling strength between the islands. The samples display
a rich phenomenology culminating in a non-monotonic behavior of the
maximal conductance as a function of temperature.
TT 11.3 Di 10:00 H19
Coulomb blockade and Non-Fermi-liquid behavior in quantum
dots — •Frithjof Anders 1 , Eran Lebanon 2 , and Avraham
Schiller 2 — 1 Insititut für Theoretische Physik, Universität Bremen,
Postfach 330 440, D-28334 Bremen — 2 Racah Institute of Physics, The
Hebrew University, Jerusalem 91904, Israel
The non-Fermi-liquid properties of an ultrasmall quantum dot coupled
to a lead and to a quantum box are investigated using a new variant of
Wilson’s numerical renormalization group. Below the charging energy of
the quantum box, a second screening channel is dynamically generated.
Tuning the ratio of the tunneling amplitudes to the lead and box, we
find a two-channel Kondo fixed point for arbitrary Coulomb repulsion on
the dot, proving that the two-channel Kondo effect is far more generic
to this setting than the original scenario of Oreg and Goldhaber-Gordon.
At T = 0, a step-like structure is found in the conductance of a twolead
setting, the height of which depends on the dot occupancy. The
temperature scale below which the two-channel Kondo effect sets in is
greatly enhanced away from the local-moment regime, making this effect
accessible in realistic devices.
TT 11.4 Di 10:15 H19
Thermopower of metallic single electron devices — •Marko
Turek, Jens Siewert, and Klaus Richter — Institut für theoretische
Physik, Universität Regensburg, D-93040 Regensburg
While charge transport in single-electron devices has been thoroughly
investigated during the past decade, heat transport did not attract much
attention, apart from basic considerations, e.g. [1 − 3]. This is surprising
since heat transport exhibits interesting behavior as a function of
external gate voltages and, in particular, renders transport regimes accessible
which are difficult to study by means of charge transport. Here
we present new results for the linear thermopower as a function of the
applied gate voltage in single-electron transistors with normal leads and
normal-conducting islands, as well as for devices with superconducting
islands.
[1] C. W. J. Beenakker and A. A. M. Staring, Phys. Rev. B 46, 9667
(1992)
[2] Y. M. Blanter, C. Bruder, R. Fazio and H. Schoeller, Phys. Rev. B 55,
4069 (1997)
[3] D. Boese and R. Fazio, Europhys. Lett. 56, 576 (2001)
TT 11.5 Di 10:30 H19
Weak charge quantization and full current statistics — •Dmitry
Bagrets 1 and Yuli Nazarov 2 — 1 Institut für Theoretische
Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe, Germany
— 2 Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The
Netherlands
We evaluate the full statistics of the current via a Coulomb island that
is strongly coupled to the leads [1]. This strong coupling weakens the
Coulomb interaction. We show that at voltages below the inverse RCtime
in the circuit all effects of interaction can be incorporated into the
energy renormalization of transmission eigenvalues of the arbitrary scatterers
that connect the island and the leads. At moderate conductance
G of the island below some critical value Gc the above renormalization
results in the onset of an effective Coulomb blockade gap in the currentvoltage
dependence, the value of this gap being exponentially suppressed
as compared to the classical charging energy E = e 2 /2C of the island.
In the opposite case, G > Gc, the Coulomb blockade does not develop
and the interaction correction to transport saturates at low voltages below
the Thouless energy of the island. Remarkably, our renormalization
group approach agrees with the previous equilibrium instanton calculations
[2], i.e. both methods give the same energy scale for the effective
charging energy.
[1] D.A. Bagrets and Yu. V. Nazarov, cond-matt/0304347
[2] Yuli V. Nazarov, Phys. Rev. Lett. 82, 1245 (1999).
TT 11.6 Di 10:45 H19
Negative differential resistance due to the resonance coupling of
a quantum-dot dimer — •Shidong Wang 1,2 , Zouzou Sun 1 , Nelson
Cue 1 , and Xiangrong Wang 1 — 1 Department of Physics, Hong
Kong University of science and technolgy, HKSAR, China — 2 Institut
für Theoretische Physik, Universität Regensburg, 93040 Regensburg
We investigate the electron tunneling through a coupled quantum-dot
dimer (a two quantum-dot system) under a dc bias. We find a new mechanism
for the negative differential resistance (NDR) in a system with more
than one quantum dots. We show that a peak in the I-V curves, that is,
a NDR, occurs at low temperature when two discrete electronic states in
the two quantum dots are aligned with each other. We call this coupling
of two states the resonance coupling. We study also the dependence of
the height and width of the I-V peak on the dot-dot coupling, e-e interaction,
and the temperature. We find that the peak disappears due to
thermal smearing effects at high temperature. We expect that this new
NDR is useful in STM experiments. We hope that a better understanding
of this resonance coupling effect may enhance the STM as a powerful
probe not just for a regular surface, but also for a cluster structure.
TT 11.7 Di 11:00 H19
Nonlinear Transport through Quantum Wires: Functional
Renormalization Group in Nonequilibrium — •Severin
Jakobs 1 , Volker Meden 2 , Herbert Schoeller 1 , and Kurt
Schönhammer 2 — 1 Institut für theoretische Physik A, RWTH Aachen,
D-52056 Aachen, Germany — 2 Institut für Theoretische Physik,
Universität Göttingen, Bunsenstr. 9, D-37073 Göttingen, Germany
We study nonlinear transport at finite temperature through a quantum
wire with impurities. The wire is modelled by a one-dimensional
quantum lattice including nearest neighbour interaction. It is coupled
via tunneling barriers to two reservoirs to which a finite bias voltage is
applied. For calculating the nonequilibrium properties we generalize a
functional renormalization group method (applied recently to the equilibrium
case [1]) to the nonequilibrium situation by utilizing real-time
Feynman diagrams. This enables us to handle weak as well as strong tunneling
and arbitrary potentials on the wire. The coulomb interaction is
treated within perturbative renormalization group, i.e. only the Hartree-
Fock type of diagrams are included. For a single impurity in an infinite
quantum wire we recover the exponents for the current vs. temperature
and for the current vs. bias voltage predicted from bosonization [2]. For
a finite wire however, the interplay of tunneling barriers and impurity
produces exponents depending on the position of the impurity.
[1] V.Meden et al., Phys.Rev.B 65, 045318 (2002)
[2] C.L.Kane, M.P.A.Fisher, Phys.Rev.B 46, 15233 (1992)
TT 11.8 Di 11:15 H19
Electron transport through interacting quantum dots —
•Andrei Zaikin 1 and Dmitri Golubev 1,2 — 1 Institut ür Nanotechnologie,
Forschungszentrum Karlsruhe, 76021 Karlsruhe — 2 Institut für
Theoretische Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe
We present a detailed theoretical investigation of the effect of Coulomb
interactions on electron transport through quantum dots and double
barrier structures connected to a voltage source via an arbitrary linear
impedance. Combining real-time path integral techniques with the scattering
matrix approach we derive the effective action and evaluate the
current-voltage characteristics of quantum dots with large conductances.
Our analysis reveals a reach variety of different regimes which we specify
in detail for the case of chaotic quantum dots. At sufficiently low energies
the interaction correction to the current depends logarithmically on
temperature and voltage. We identify two different logarithmic regimes
with the crossover between them occurring at energies of order of the
inverse dwell time of electrons in the dot. We also analyze the frequencydependent
shot noise in chaotic quantum dots and elucidate its direct
relation to interaction effects in mesoscopic electron transport.