196 research outputs found
Creating stable Floquet-Weyl semimetals by laser-driving of 3D Dirac materials
Tuning and stabilising topological states, such as Weyl semimetals, Dirac
semimetals, or topological insulators, is emerging as one of the major topics
in materials science. Periodic driving of many-body systems offers a platform
to design Floquet states of matter with tunable electronic properties on
ultrafast time scales. Here we show by first principles calculations how
femtosecond laser pulses with circularly polarised light can be used to switch
between Weyl semimetal, Dirac semimetal, and topological insulator states in a
prototypical 3D Dirac material, NaBi. Our findings are general and apply to
any 3D Dirac semimetal. We discuss the concept of time-dependent bands and
steering of Floquet-Weyl points (Floquet-WPs), and demonstrate how light can
enhance topological protection against lattice perturbations. Our work has
potential practical implications for the ultrafast switching of materials
properties, like optical band gaps or anomalous magnetoresistance. Moreover, we
introduce Floquet time-dependent density functional theory (Floquet-TDDFT) as a
general and robust first principles method for predictive Floquet engineering
of topological states of matter.Comment: 21 pages, 4 figure
Renormalization algorithm with graph enhancement
We introduce a class of variational states to describe quantum many-body
systems. This class generalizes matrix product states which underly the
density-matrix renormalization group approach by combining them with weighted
graph states. States within this class may (i) possess arbitrarily long-ranged
two-point correlations, (ii) exhibit an arbitrary degree of block entanglement
entropy up to a volume law, (iii) may be taken translationally invariant, while
at the same time (iv) local properties and two-point correlations can be
computed efficiently. This new variational class of states can be thought of as
being prepared from matrix product states, followed by commuting unitaries on
arbitrary constituents, hence truly generalizing both matrix product and
weighted graph states. We use this class of states to formulate a
renormalization algorithm with graph enhancement (RAGE) and present numerical
examples demonstrating that improvements over density-matrix renormalization
group simulations can be achieved in the simulation of ground states and
quantum algorithms. Further generalizations, e.g., to higher spatial
dimensions, are outlined.Comment: 4 pages, 1 figur
Entanglement entropy of two disjoint intervals in c=1 theories
We study the scaling of the Renyi entanglement entropy of two disjoint blocks
of critical lattice models described by conformal field theories with central
charge c=1. We provide the analytic conformal field theory result for the
second order Renyi entropy for a free boson compactified on an orbifold
describing the scaling limit of the Ashkin-Teller (AT) model on the self-dual
line. We have checked this prediction in cluster Monte Carlo simulations of the
classical two dimensional AT model. We have also performed extensive numerical
simulations of the anisotropic Heisenberg quantum spin-chain with tree-tensor
network techniques that allowed to obtain the reduced density matrices of
disjoint blocks of the spin-chain and to check the correctness of the
predictions for Renyi and entanglement entropies from conformal field theory.
In order to match these predictions, we have extrapolated the numerical results
by properly taking into account the corrections induced by the finite length of
the blocks to the leading scaling behavior.Comment: 37 pages, 23 figure
Optical Phonon Lasing in Semiconductor Double Quantum Dots
We propose optical phonon lasing for a double quantum dot (DQD) fabricated in
a semiconductor substrate. We show that the DQD is weakly coupled to only two
LO phonon modes that act as a natural cavity. The lasing occurs for pumping the
DQD via electronic tunneling at rates much higher than the phonon decay rate,
whereas an antibunching of phonon emission is observed in the opposite regime
of slow tunneling. Both effects disappear with an effective thermalization
induced by the Franck-Condon effect in a DQD fabricated in a carbon nanotube
with a strong electron-phonon coupling.Comment: 8 pages, 4 figure
Completeness of classical spin models and universal quantum computation
We study mappings between distinct classical spin systems that leave the
partition function invariant. As recently shown in [Phys. Rev. Lett. 100,
110501 (2008)], the partition function of the 2D square lattice Ising model in
the presence of an inhomogeneous magnetic field, can specialize to the
partition function of any Ising system on an arbitrary graph. In this sense the
2D Ising model is said to be "complete". However, in order to obtain the above
result, the coupling strengths on the 2D lattice must assume complex values,
and thus do not allow for a physical interpretation. Here we show how a
complete model with real -and, hence, "physical"- couplings can be obtained if
the 3D Ising model is considered. We furthermore show how to map general
q-state systems with possibly many-body interactions to the 2D Ising model with
complex parameters, and give completeness results for these models with real
parameters. We also demonstrate that the computational overhead in these
constructions is in all relevant cases polynomial. These results are proved by
invoking a recently found cross-connection between statistical mechanics and
quantum information theory, where partition functions are expressed as quantum
mechanical amplitudes. Within this framework, there exists a natural
correspondence between many-body quantum states that allow universal quantum
computation via local measurements only, and complete classical spin systems.Comment: 43 pages, 28 figure
Performance of local orbital basis sets in the self-consistent Sternheimer method for dielectric matrices of extended systems
We present a systematic study of the performance of numerical pseudo-atomic
orbital basis sets in the calculation of dielectric matrices of extended
systems using the self-consistent Sternheimer approach of [F. Giustino et al.,
Phys. Rev. B 81 (11), 115105 (2010)]. In order to cover a range of systems,
from more insulating to more metallic character, we discuss results for the
three semiconductors diamond, silicon, and germanium. Dielectric matrices
calculated using our method fall within 1-3% of reference planewaves
calculations, demonstrating that this method is promising. We find that
polarization orbitals are critical for achieving good agreement with planewaves
calculations, and that only a few additional \zeta 's are required for
obtaining converged results, provided the split norm is properly optimized. Our
present work establishes the validity of local orbital basis sets and the
self-consistent Sternheimer approach for the calculation of dielectric matrices
in extended systems, and prepares the ground for future studies of electronic
excitations using these methods.Comment: 10 pages, 8 figure
Concatenated tensor network states
We introduce the concept of concatenated tensor networks to efficiently
describe quantum states. We show that the corresponding concatenated tensor
network states can efficiently describe time evolution and possess arbitrary
block-wise entanglement and long-ranged correlations. We illustrate the
approach for the enhancement of matrix product states, i.e. 1D tensor networks,
where we replace each of the matrices of the original matrix product state with
another 1D tensor network. This procedure yields a 2D tensor network, which
includes -- already for tensor dimension two -- all states that can be prepared
by circuits of polynomially many (possibly non-unitary) two-qubit quantum
operations, as well as states resulting from time evolution with respect to
Hamiltonians with short-ranged interactions. We investigate the possibility to
efficiently extract information from these states, which serves as the basic
step in a variational optimization procedure. To this aim we utilize known
exact and approximate methods for 2D tensor networks and demonstrate some
improvements thereof, which are also applicable e.g. in the context of 2D
projected entangled pair states. We generalize the approach to higher
dimensional- and tree tensor networks.Comment: 16 pages, 4 figure
Tensor network techniques for the computation of dynamical observables in 1D quantum spin systems
We analyze the recently developed folding algorithm [Phys. Rev. Lett. 102,
240603 (2009)] to simulate the dynamics of infinite quantum spin chains, and
relate its performance to the kind of entanglement produced under the evolution
of product states. We benchmark the accomplishments of this technique with
respect to alternative strategies using Ising Hamiltonians with transverse and
parallel fields, as well as XY models. Additionally, we evaluate its ability to
find ground and thermal equilibrium states.Comment: 33 pages, 22 figure
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