Solid-state group seminars archive
1.3.2016 15h (cajna soba) Robert Triebl (Graz),
Breakdown of the bulk boundary correspondence in a strongly
correlated model Hamiltonian
In this talk I will give a short introduction to topology in general
and to the influence of strong correlations on a topological model
Hamiltonian in particular. The
first part will be about the definition of topological invariants, how
to compute them and what changes in case of strong interactions. Then
I will turn to the
Kane-Mele-Hubbard model, in particular focusing on the competition
between topological and magnetic order. Both bulk and ribbon Green's
functions have been calculated
using the variational cluster approach, employing a two-site dynamical
impurity approximation (DIA). The resulting invariants are compared to
the existence of gapless
edge states of the ribbon, where we use a site-dependent
antiferromagnetic Weiss field on the ribbon. It turns out that
spontaneous symmetry breaking occurs locally at
the edges already at much smaller interactions than in the middle of
the ribbon, leading to a gap in the edge spectral function. As a
consequence, the topological
invariant defined in the bulk may not correspond to the existence of
gapless edge states since time reversal invariance is locally broken
only at the edges. Eventually,
I will compare the results to Mean-Field approximations using in-plane
and out-of-plane magnetic moments, showing that the direction has a
huge influence on the correct
topological description.
Full abstract (.pdf)
2.10.2015 11h (seminarska soba) Ian Affleck (UBC),
From string theory to quantum dots experiments
Part of the string theory revolution of the 1980's was the development
of conformal field theory. While its applicability as a theory of
everything is still unclear, it led to
solutions of models of conduction electrons interacting with magnetic
impurities that predict remarkable behavior. A gated semi-conductor
device was tailored to finally
provide an indisputable experimental realization of this theory. I
will give a non-technical review of both theory and experiments.
Full abstract (.pdf)
22.9.2015 15h00 (seminarska soba) Alaska Subedi (MPI Hamburg),
Theory of nonlinear phononics for coherent light-control of solids
The use of light to control the structural and electronic properties
of solids is an area of great interest for both basic research and
potential applications. In this talk, I will present our recent work
on a microscopic theory for ultrafast control of solids with
high-intensity mid-infrared pulses. Our theory
predicts the dynamical path taken by the crystal lattice using first
principles calculations of the energy surface and classical equations
of motion, as well as symmetry considerations. We identify two classes
of dynamics. In the perturbative regime, displacements along the
normal mode coordinate of the symmetry preserving Raman active mode
can be achieved by cubic anharmonicities. This explains the
light-induced insulator-to-metal transition in manganites observed
experimentally. Furthermore, we predict a new regime in which
ultra-fast instabilities that break crystal symmetry can be
induced. This non-perturbative effect involves a quartic anharmonic
coupling and occurs above a critical threshold below which the
nonlinear dynamics of the driven mode displays softening and dynamical
stabilization.
Full abstract (.pdf)
8.9.2015 15h00 (cajna soba F1) Takami Tohyama (Dep. of Applied
physics, Tokio University of Science),
A numerical method to compute optical conductivity based on the
pump-probe simulations and its application to the
hall-filled one-dimensional extended Hubbard model
A numerical method to calculate optical conductivity based on a
pump-probe setup is discussed [1]. Its validity and limits
are demonstrated both in equilibrium and out of equilibrium. By
employing either a step-like or a Gaussian-like probing
vector potential, it is found that in nonequilibrium, the method can
be related to the linear response theory [2] or a
different generalized Kubo formula [3], respectively. The observation
reveals the probe-pulse dependence of the optical
conductivity in nonequilibrium The numerical method is applied to
nonequilibrium optical responses in the various phases of
the hall-filled one-dimensional extended Hubbard model [4,5].
[1] C. Shao, T. Tohyama, H.-G. Luo, and H. Lu, arXiv:1507.01200
[2] Z. Lenarcic, D. Golez, J. Bonca, and P. Prelovsek, Phys. Rev. B
89, 125123 (2014).
[3] G. De Filippis, V. Cataudella, E. A. Nowadnick, T. P. Devereaux,
A. S. Mishchenko, and N. Nagaosa, Phys. Rev. Lett. 109,
176402 (2012).
[4] H. Lu, C. Shao, J. Bonca, D. Manske, and T. Tohyama, Phys. Rev. B
91, 245117 (2015).
[5] N. Bittner, T. Tohyama, D. Manske, in preparation.
Full abstract (.pdf)
23.6.2015 15h00 (cajna soba F1) Ambroz Kregar (F1-IJS),
Single-qubit transformations of spin-charge states of electron on
a ring in presence of Rashba
coupling
Quantum dots in semiconductor heterojunctions are one of the most
suitable candidates for realization of quantum computer. Their
advantages are long spin coherence times of electrons in
semiconductor, well developed technology of their production and
simple scalability, to only name a few. Traditionally, magnetic field
is used to manipulate spin, but since it is difficult to use it to
address single quantum dot, we seek different ways to control it. In
our work we theoretically study the possibility of using only electric
field to manipulate spin-charge states in quantum dot. The system of
interest is an electron in ring shaped quantum dot, entrapped with
external gate potential, enabling a controlled movement of the
electron around the ring. By considering the potential as parabolic,
we can analytically solve the Schrdinger equation with time dependent
position of poten- tial and adiabatically changing Rashba coupling.
Spin degenerate ground states of harmonic oscillator, interpreted as
qubit states, are transformed with unitary transformation, which can
be presented as a rotation of points on Bloch sphere. We show that the
axis of rotation depends on strength of Rashba coupling while the
angle of rotation is controlled by the shift in position of the
electron. Proper selection of translations of electron with suitable
Rashba coupling enables an arbitrary single-qubit transformation on
the time scale of 10 ns, allowing a few thousand transformation before
spin coherence is lost.
Full abstract (.pdf)
27.1.2015: Zala Lenarcic (F1-IJS), Charge recombination in excited
one-dimensional organic Mott insulators
Recent femtosecond pump-probe experiments on Mott-Hubbard insulators
reveal charge recombination, which is in picosecond range, much faster than
in clean band- gap semiconductors. I will present our proposal for the
mechanism that explain the recombination in effectively one-dimensional
organic salt ET-F 2TCNQ. I will show that fast recombination processes can
be explained even quantitatively assuming that charge energy is transmitted
to molecular vibrations. As suggested by experiments it is assumed that
effectively positive (holon) and negative charge excitations (doublon) are
bound in an exciton. Based on a model that ensure the existence of exciton
and couples the charge to vibrations we can express the recombination rate
analytically. At a reasonable coupling to vibrations the observed
recombination rate is reproduced. I will comment also the sub-leading effect
of spin background, which is for one-dimensional systems decoupled from
charge.
Full abstract (.pdf)
3.2.2015: Ziga Osolin (F1-IJS), Fine structure of spectra in
antiferromagnetic phase of the Kondo lattice model
We study the antiferromagnetic phase of the Kondo lattice model on
bipartite lattices at half-filling using the dynamical mean-field
theory with numerical renormalization group as the impurity solver,
focusing on the detailed structure of the spectral function,
self-energy, and optical conductivity. We discuss the deviations from
the simple hybridization picture, which adequately describes the
overall band structure of the system (four quasiparticle branches in
the reduced Brillouin zone), but neglects all effects of the
inelastic-scattering processes. These lead to additional structure
inside the bands, in particular asymmetric resonances or dips that
become more pronounced in the strong-coupling regime close to the
antiferromagnet-paramagnetic Kondo insulator quantum phase
transition.
Full abstract (.pdf)
27.2.2015: Jure Kokalj (F1-IJS), Effects of strong electronic
correlations on thermal expansion
Many strongly correlated electron systems show poorly understood
anomalies in the thermal expansion coefficient as well as in other
properties related to lattice
softening, e.g., in the sound velocity. In this talk I will present our work
towards understanding these properties by focusing on electronic effects in
the prototype
strongly correlated material; organic charge transfer salts. I will start
with the motivation from different experiments and continue with the
introduction of a
general theoretical framework. Then I will move on to the electronic
contribution for strongly correlated electron model (Hubbard on anisotropic
triangular lattice)
and finish with the discussion of results and future challenges.
Full abstract (.pdf)
26.3.2015: Tilen Cadez (University of Braga, University of Lisbone,
Portugal, Beijing Computational Research Center, and F1- IJS ), Intrinsic versus proximity induced zero energy modes in a
superconductor with a ferromagnetic adatom chain
Recently an experiment was reported, where localized zero energy modes
were observed in the system of a ferromagnetic adatoms on top of a
conventional superconductor
using scanning tunneling microscope. The results indicate the detection of
the Majorana zero energy states. After a short presentation of the
experiment I will
present a simple 1D system, Kitaev chain, where the unpaired Majorana
fermions are found at the end of a quantum wire. In the main
part I will present the Majorana zero energy modes (MZEM) that occur in a
conventional superconductor with a ferromagnetic adatom chain in the
presence of Rashba spin-orbit interaction. I will assume a classical adatom
magnetic moments and
consider both proximity induced and intrinsic superconducting order.By
exactly solving Bogoliubov de Gennes (BdG) equations in real space, I
identify parametric
regimes with the zero energy modes, which occur in both cases. In comparison
to proximity induced superconductivity, is the region with MZEM shifted from
the band
edge to smaller chemical potential. This is connected to the decrease of the
average gap function with increasing chemical potential. I will discuss
similarities and
differences between the MZEM for the two superconducting
orders.
Full abstract (.pdf)
27.3.2015: Jacek Herbrych (Heraklion, Grece), Laser controlled
magnetization within large anisotropy S = 1 chain
Time evolution of the magnetization within large anisotropy S = 1
Heisenberg chain and circularly polarized laser (rotating magnetic filed) is
studied numerically and
analytically. Results with constant laser frequency Omega= Omega0 are
interpreted in terms of absorption lines of electronic spin resonance
spectrum. It is also shown
that time dependent laser frequency Omega=Omega(t), the called chirping of
the laser, is
better protocol in order to get larger value of the magnetization or to
magnetize the system fast. Both of the protocols yield orders of magnitude
larger Mz for
Hamiltonian with D > J than for adequate setups for Halden-like systems D <
J. Furthermore, comparison of large anisotropy D results with with two level
toy model
give satisfactory agreement.
Full abstract (.pdf)
30.3.2015 (14:30, cajna soba): Federico Becca (SISSA), Variational
wave functions for correlated electron systems
I give a review of recent developments on the possibility to describe
strongly- interacting systems by variational wave functions obtained by
inserting electron-
electron correlation on top of Slater determinants. In this regard, both
Jastrow and backflow terms are considered.[1,2] The former one is the
generalization of the
Gutzwiller (soft) projection to include long-range density-density
correlations and is nowadays widely used; instead, backflow terms have been
defined and introduced
only quite recently in strongly-interacting lattice models and make it
possible to include electron-electron correlation in the Slater
determinant.[3]
I discuss the accuracy of this approach for both weak and strong couplings
and how it is possible to describe the metal-insulator transition, as well
as the Mott
insulator. Applications for the Hubbard model on frustrated lattices will be
presented in the following talk by L.F. Tocchio.[4,5]
[1] M. Capello, F. Becca, M. Fabrizio, S. Sorella, and E. Tosatti, Phys.
Rev. Lett. 94, 026406 (2005).
[2] L.F. Tocchio, F. Becca, A. Parola, and S. Sorella, Phys. Rev. B 78,
041101 (2008).
[3] L.F. Tocchio, F. Becca, and C. Gros, Phys. Rev. B 83, 195138 (2011).
[4] L.F. Tocchio, H. Feldner, F. Becca, R. Valenti, and C. Gros, Phys. Rev.
B 87, 035143 (2013).
[5] L.F. Tocchio, C. Gros, R. Valenti, F. Becca, Phys. Rev. B 89, 235107
(2014)
Full abstract (.pdf)
30.3.2015 (14:30, cajna soba): Luca Tochhio (SISSA) Spin liquids, collinear- and spiral-order phases in the
anisotropic triangular lattice,
We study the competition between magnetic and spin-liquid phases in the
Hubbard model on the anisotropic triangular lattice, which is described by
two hop- ping
parameters in different spatial directions, and is relevant for layered
organic charge-transfer salts and for the inorganic compounds Cs2CuBr4 and
Cs2CuCl4 [1,2]. By
using variational wave functions which include both Jastrow and back- flow
terms, we provide solid evidence that two spin-liquid phases are stabilized
in the strongly
correlated regime, while states with spiral magnetic order and a non trivial
pitch vector are found close to the isotropic point. Two different kinds of
collinear
orders are found in a wide region of the phase diagram close, respectively,
to the limits of square lattice and decoupled one-dimensional chains. We
also introduce
another family of organic charge-transfer salts where a fully anisotropic
triangular-lattice description produces importantly different results,
including a
significant lowering of the critical U of the spin-liquid phase.[3]
[1] Tocchio, Feldner, Becca, Valenti, Gros, PRB 87, 035143 (2013).
[2] Tocchio, Gros, Valenti, Becca, PRB 89, 235107 (2014).
[3] Jacko, Tocchio, Jeschke, Valenti, PRB 88, 155139 (2013).
Full abstract (.pdf)
7.4.2015 15h: Jaksa Vucicevic (University of Belgrade), Mott quantum criticality and bad metal behavior
Quantum critical scaling is an ubiquitous phenomenon accompanying
zero-temperature phase transitions, but its prominence in the context of
finite
temperature (first-order) transitions and connection with bad metal behavior
have not been clarified until recently. We solve the Hubbard model by means of
single-site dynamical mean-field theory (DMFT) to show that even though the
Mott metal-insulator transition here is of the first order, at sufficiently
high temperatures a typical quantum critical behavior is recovered. Our
results
display striking agreement with measurements on Kappa-organic materials, where
the system can be tuned through a finite temperature Mott transition by
varying
pressure. We also consider the doping driven Mott transition and find that the
associated quantum critical region matches perfectly the region of bad
metallic
behavior, where resistivity is around (and above) the Mott-Ioffe-Regel limit
and is linear in temperature. In this regime, the linearity and slope of the
temperature dependence of resistivity can be simply explained by the presence
of quantum critical scaling, and the full DMFT result is quantitatively
reproduced by a simple semi-phenomenological formula. The results are found to
be in agreement with the high temperature (200-1000K) behavior in cuprate
films, which we believe can be traced back to the presence of a hidden Mott
transition at zero temperature.
Full abstract (.pdf)
14.4.2015 14h15 (cajna soba) Taegeun Song (ICTP, Trieste), Quantum shuttling with
Lorentz back-action
Nanoelectromechanical (NEM) systems are attracting much interest not only
be- cause of its divers potential to the application for useful nano-devices
but also due to being an efficient tool for modern nano-electronics. In the
Coulomb block regime, a movable nano-sized conductor shows interesting
one-by-one elec- tron transfer phenomenon due to the finite energy pumping
originated from the Coulomb interaction, so called, the shuttling
process.[1] In this talk, I will intro- duce two shuttling devices which are
governed by Lorentz back-action: Kondo shuttling device[2] and
shuttle-promoted current switching device[3]. The Kondo- shuttling device, I
will discuss the dynamical properties and possibility of the experimental
realization of the dynamical Kondo-cloud probe by using high me- chanical
dissipation tunability. For current switching device, the perspective on
practical NEM application based on Lorentz back-action will be discussed.
[1] R. I. Shekhter, L. Y. Gorelik, I. V. Krive, M. N. Kiselev, A. V.
Parafilo, and M. Jonson, Nanoelectromechanical Systems, 1, 1 (2013).
[2] T. Song, M. N. Kiselev, K. Kikoin, R. I. Shekhter, and L. Y. Gorelik,
New J. of Phys. 16, 033043 (2014).
[3] T. Song, L. Y. Gorelik, R. I. Shekhter, M. N. Kiselev, and K. Kikoin.
arXiv:1502.07524 (2015).
9.6.2015 15h00 (seminarska soba za fiziko) Jan Kogoj (F1-IJS),
Non-equilibrium Optical Conductivity of the Holstein
Polaron
Time-resolved ultra-fast spectroscopy is a powerful experimental
technique, enabling the study of solids far from equilibrium. Recent
advances have pushed the
time resolution to extremely short time scales, much shorter than
typical phonon oscillation period. It is thus illusory to expect that
the system is in
(thermal) equilibrium the whole time during probing. It is of utter
importance that one knows which theories are applicable at which time
scale in order to
resolve a dilemma whether one should use strictly non-equilibrium
concepts or are (quasi) equilibrium theories justified as well.
We follow the time evolution of a Holstein polaron after a quantum
quench, imitating the photo-excitation in ultra-fast spectroscopy. We
observe that selected
properties indicate relaxation of the system from a highly excited
state towards a steady state, dependent only on the total energy of
the system. After fairly
short times, calculated optical conductivity matches quite well with
the thermal form with a well defined effective temperature, however,
different initial
states with equal total energy result in different effective
temperatures. The system under investigation therefore does not
thermalise in a strict sense despite
exhibiting many signatures characteristic of thermalisation.
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