All talks are in Room
B-131, except when otherwise noted. Regular seminar time is Friday
1:30PM. Follow the links to see the schedule in past semesters.
Alexandre Abanov
(
Stony Brook University
):
Allowed charge transfers between coherent conductors driven by a time-dependent
scatterer
We derive constraints on the statistics of the charge transfer between two conductors in the model
of arbitrary time-dependent instant scattering of non-interacting fermions at zero temperature. The
constraints are formulated in terms of analytic properties of the generating function: its zeroes
must lie on the negative real axis. This result generalizes existing studies for scattering by a time-
independent scatterer under time-dependent bias voltage.
Host: Allen
September 28, Friday, 1:30PM
Philip B. Allen
(
Stony Brook University
):
What happens to the geometric (Berry) phase
when band degeneracy is lifted by spin-orbit interaction?
Energy bands e(n,k) in k-space have lines where bands are accidentally
degenerate (e(1,k*)=e(2,k*).) These will be illustrated for the case of
aluminum metal. If a state is adiabatically evolved on a circuit C in
k-space which surrounds such a line k*, the wavefunction changes sign.
According to Berry, this is a geometric phase g(C). These affect the de
Haas-van Alphen oscillations, for example. Spin-orbit interaction lifts
accidental band degeneracy. The geometric phase cannot survive completely
unchanged. The answer depends on how the spin is fixed during adiabatic
evolution. For spin fixed along the internal spin-orbit field, g(C)
decreases to zero as the circuit collapses around the line of lifted
degeneracy. For spin fixed along a perpendicular axis, g(C) is unchanged
from the value of plus/minus p.
Host: Allen
October 5, Friday, 1:30PM
Peter Khalifah
(
Stony Brook University, Chemistry Dept.
):
What *actually* happens to the geometric (Berry) phase when band degeneracy is lifted by spin-orbit interaction.
One manner in which the Berry phase manifests itself in condensed matter
physics is in measurements of the Hall effect. In addition to the Hall response
which is inversely proportional to the carrier concentration ("ordinary Hall effect"),
there is a second component of the Hall effect which arises in response to
the magnetization of a sample, and in fact can persist at zero applied field in
a magnetized sample. Although the presence of this "anomalous" Hall effect
has been recognized for over a century, quantitative agreement with theory
has only recently been achieved thanks to theoretical frameworks which account
for Berry phase effects. I will discuss recent experimental results of investigations
of the anomalous Hall effect in solid solutions between the itinerant ferromagnet
SrRuO3 and the itinerant paramagnet CaRuO33, as well as providing a broader
introduction into the quantum phase transition in this system. I will also provide
a survery of some recent results on the orbital ordering transition in La4Ru2O10,
the charge ordering transition of Fe2BO4.
Host: Allen
October 12, Friday, 11:30PM
Simon Deleonibus
(
CEA/LETI, Grenoble, France
):
CMOS at the Nanoelectronics Era
After a presentation of the nanosciences activities in the Grenoble area
and, more precisely, at CEA-LETI on the MINATEC campus, I will adress the
topics related to electronic nanodevices architectures we developping in my
laboratory.
Host: Likharev/Luryi
October 26, Friday, 1:30PM
Arsen Subashiev
(
ECE Stony Brook
):
Energy resolution of semiconductor gamma detectors
The basics of semiconductor gamma detectors and statistical limitations for their energy resolution will be reviewed.
We demonstrate that the statistics of the cascade of impact ionizations is to a large extent similar to a generalized
random parking problem, so that multi-channel energy branching can be viewed as a parking from a multi-size mixture of
arriving cars. We present exact analytical results for the fill factor fluctuations in case of the random parking from
a two-size car flow, confirmed by Monte-Carlo modeling. Qualitative implications for semiconductor detectors and Monte
Carlo results will be discussed. The work was done in collaboration with Serge Luryi.
Host: Allen
November 2, Friday, 1:30PM
George Jackeli
(
University of Stuttgart
):
Dimer phases in quantum antiferromagnets with orbital degeneracy
We study and solve the ground-state problem of a microscopic model for a family
of orbitally degenerate quantum magnets. The orbital degrees of freedom are
assumed to have directional character and are represented by static Potts-like
variables. In the limit of vanishing Hund's coupling, the ground-state manifold
of such a model is spanned by the hard-core dimer (spin singlet) coverings of
the lattice. The extensive degeneracy of dimer coverings is lifted at a finite
Hund's coupling through an order-out-of-disorder mechanism by virtual triplet
excitations. The relevance of our results to several experimentally studied
systems is discussed.
Host: Abanov
November 9, Friday, 1:30PM
Oleg Starykh
(
University of Utah
):
Spinons and triplons in a spatially anisotropic triangular lattice antiferromagnet.
The Triangular lattice spin-1/2 Heisenberg AntiFerromagnet (TAF) is a prototypical
model of frustrated quantum magnetism. While it is believed to exhibit long-range order
in the isotropic limit, changes such as spatial anisotropy can alter the delicate balance
amongst competing ground states. I will describe the static and dynamic properties of the
spatially anisotropic TAF, with inter-chain diagonal exchange
J' much weaker than the intrachain exchange J.
Despite retaining magnetic long range order in its ground state,
spatially anisotropic TAF exhibits very unusual dynamic response (broad continuum)
in a wide range of energy and momentum. I will show that the spectrum of excitations from
moderate to high energies is composed of incoherent continuum, arising from the
one-dimensional spinons of individual spin chains, and a sharp dispersing peak,
due to coherently propagating `triplon' bound states of two spinons.
I will present a direct and parameter-free comparison of the calculated dynamical
spin correlations with inelastic neutron measurements on Cs2CuCl4.
Host: Abanov
November 16, Friday, 1:30PM
Laszlo Forro
(
EPFL, Lausanne
):
From nanostructures to biomaterials
Host: Mihaly
November 30, Friday, 1:30PM Postponed
Andrei Shytov
(
BNL
):
Atomic Collapse in Graphene
Intercalating graphene with charged impurities, it is possible to model the
physics of supercritical heavy relativistic atoms (Z>137). To demonstrate
this, we consider Dirac-like quasiparticles interacting with a Coulomb
impurity. We find an infinite family of localized electronic states, when the
impurity charge exceeds a critical value Zc=1. These states are related to
collapsing relativistic trajectories that descend on the point charge. Klein
tunneling couples the localized states to Dirac continuum. Signatures of
this "atomic collapse" regime in STM measurements and transport properties
are discussed.
Host: Abanov
December 7, Friday, 1:30PM
Fabio Franchini
(
ICTP, Trieste
):
Quantum Entropy for the 1-D XY model
Entanglement in the ground state of the XY model on the infinite chain can be measured by
the von Neumann entropy or by the Renyi entropy of a block of neighboring spins. We study
a double scaling limit: the size of the block is much larger than 1 but much smaller than
the length of the whole chain. The entropy of the block has an asymptotic limit in the
gapped regimes. We calculate this limiting entropy analytically and study it as a function
of the anisotropy and of the magnetic field. We identify the minima of the limiting entropy
at product states and its divergences at the quantum phase transitions. We find that the
curves of constant entropy are ellipses and hyperbolas and that they all meet at one point.
Depending on the approach to this point, the entropy can take any value between 0 and
infinity: in the vicinity of this point small changes in the parameters cause large changes
of the entropy. The analytical expressions of the entropy we derived show interesting
symmetry properties under the effect of Modular Transformations. I'll argue that this
symmetry belongs to the underlying model (and not just to this correlator) by providing
explicit expressions for the zero-temperature partition function of the model.
Host: Abanov
December 14, Friday, 1:30PM
Andrei Shytov
(
BNL
):
Atomic Collapse in Graphene
Intercalating graphene with charged impurities, it is possible to model the
physics of supercritical heavy relativistic atoms (Z>137). To demonstrate
this, we consider Dirac-like quasiparticles interacting with a Coulomb
impurity. We find an infinite family of localized electronic states, when the
impurity charge exceeds a critical value Zc=1. These states are related to
collapsing relativistic trajectories that descend on the point charge. Klein
tunneling couples the localized states to Dirac continuum. Signatures of
this "atomic collapse" regime in STM measurements and transport properties
are discussed.
Host: Abanov