1,045 research outputs found
Probing the anomalous dynamical phase in long-range quantum spin chains through Fisher-zero lines
Using the framework of infinite Matrix Product States, the existence of an
\textit{anomalous} dynamical phase for the transverse-field Ising chain with
sufficiently long-range interactions was first reported in [J.~C.~Halimeh and
V.~Zauner-Stauber, arXiv:1610:02019], where it was shown that
\textit{anomalous} cusps arise in the Loschmidt-echo return rate for
sufficiently small quenches within the ferromagnetic phase. In this work we
further probe the nature of the anomalous phase through calculating the
corresponding Fisher-zero lines in the complex time plane. We find that these
Fisher-zero lines exhibit a qualitative difference in their behavior, where,
unlike in the case of the regular phase, some of them terminate before
intersecting the imaginary axis, indicating the existence of smooth peaks in
the return rate preceding the cusps. Additionally, we discuss in detail the
infinite Matrix Product State time-evolution method used to calculate Fisher
zeros and the Loschmidt-echo return rate using the Matrix Product State
transfer matrix. Our work sheds further light on the nature of the anomalous
phase in the long-range transverse-field Ising chain, while the numerical
treatment presented can be applied to more general quantum spin chains.Comment: Journal article. 9 pages and 6 figures. Includes in part what used to
be supplemental material in arXiv:1610:0201
Quasiparticle origin of dynamical quantum phase transitions
Considering nonintegrable quantum Ising chains with exponentially decaying
interactions, we present matrix product state results that establish a
connection between low-energy quasiparticle excitations and the kind of
nonanalyticities in the Loschmidt return rate. When domain walls in the
spectrum of the quench Hamiltonian are energetically favored to be bound rather
than freely propagating, anomalous cusps appear in the return rate regardless
of the initial state. In the nearest-neighbor limit, domain walls are always
freely propagating, and anomalous cusps never appear. As a consequence, our
work illustrates that models in the same equilibrium universality class can
still exhibit fundamentally distinct out-of-equilibrium criticality. Our
results are accessible to current ultracold-atom and ion-trap experiments.Comment: 9 pages, 8 figures, accepted versio
Entanglement at the boundary of spin chains near a quantum critical point and in systems with boundary critical points
We analyze the entanglement properties of spins (qubits) attached to the
boundary of spin chains near quantum critical points, or to dissipative
environments, near a boundary critical point, such as Kondo-like systems or the
dissipative two level system. In the first case, we show that the properties of
the entanglement are significantly different from those for bulk spins. The
influence of the proximity to a transition is less marked at the boundary. In
the second case, our results indicate that the entanglement changes abruptly at
the point where coherent quantum oscillations cease to exist. The phase
transition modifies significantly less the entanglement.Comment: 5 pages, 4 figure
Flow equations for Hamiltonians: Contrasting different approaches by using a numerically solvable model
To contrast different generators for flow equations for Hamiltonians and to
discuss the dependence of physical quantities on unitarily equivalent, but
effectively different initial Hamiltonians, a numerically solvable model is
considered which is structurally similar to impurity models. By this we discuss
the question of optimization for the first time. A general truncation scheme is
established that produces good results for the Hamiltonian flow as well as for
the operator flow. Nevertheless, it is also pointed out that a systematic and
feasible scheme for the operator flow on the operator level is missing. For
this, an explicit analysis of the operator flow is given for the first time. We
observe that truncation of the series of the observable flow after the linear
or bilinear terms does not yield satisfactory results for the entire parameter
regime as - especially close to resonances - even high orders of the exact
series expansion carry considerable weight.Comment: 25 pages, 10 figure
Prethermalization and Persistent Order in the Absence of a Thermal Phase Transition
We numerically study the dynamics after a parameter quench in the
one-dimensional transverse-field Ising model with long-range interactions
( with distance ), for finite chains and also directly
in the thermodynamic limit. In nonequilibrium, i.e., before the system settles
into a thermal state, we find a long-lived regime that is characterized by a
prethermal value of the magnetization, which in general differs from its
thermal value. We find that the ferromagnetic phase is stabilized dynamically:
as a function of the quench parameter, the prethermal magnetization shows a
transition between a symmetry-broken and a symmetric phase, even for those
values of for which no finite-temperature transition occurs in
equilibrium. The dynamical critical point is shifted with respect to the
equilibrium one, and the shift is found to depend on as well as on the
quench parameters.Comment: 6 pages, 4 figure
Universal asymptotic behavior in flow equations of dissipative systems
Based on two dissipative models, universal asymptotic behavior of flow
equations for Hamiltonians is found and discussed. Universal asymptotic
behavior only depends on fundamental bath properties but not on initial system
parameters, and the integro-differential equations possess an universal
attractor. The asymptotic flow of the Hamiltonian can be characterized by a
non-local differential equation which only depends on one parameter -
independent of the dissipative system or truncation scheme. Since the fixed
point Hamiltonian is trivial, the physical information is completely
transferred to the transformation of the observables. This yields a more stable
flow which is crucial for the numerical evaluation of correlation functions.
Furthermore, the low energy behavior of correlation functions is determined
analytically. The presented procedure can also be applied if relevant
perturbations are present as is demonstrated by evaluating dynamical
correlation functions for sub-Ohmic environments. It can further be generalized
to other dissipative systems.Comment: 15 pages, 9 figures; to appear in Phys. Rev.
Energy spectrum and Landau levels in bilayer graphene with spin-orbit interaction
We present a theoretical study of the bandstructure and Landau levels in
bilayer graphene at low energies in the presence of a transverse magnetic field
and Rashba spin-orbit interaction in the regime of negligible trigonal
distortion. Within an effective low energy approach (L\"owdin partitioning
theory) we derive an effective Hamiltonian for bilayer graphene that
incorporates the influence of the Zeeman effect, the Rashba spin-orbit
interaction, and inclusively, the role of the intrinsic spin-orbit interaction
on the same footing. Particular attention is spent to the energy spectrum and
Landau levels. Our modeling unveil the strong influence of the Rashba coupling
in the spin-splitting of the electron and hole bands. Graphene
bilayers with weak Rashba spin-orbit interaction show a spin-splitting linear
in momentum and proportional to , but scales inversely proportional
to the interlayer hopping energy . However, at robust spin-orbit
coupling the energy spectrum shows a strong warping behavior near
the Dirac points. We find the bias-induced gap in bilayer graphene to be
decreasing with increasing Rashba coupling, a behavior resembling a topological
insulator transition. We further predict an unexpected assymetric
spin-splitting and crossings of the Landau levels due to the interplay between
the Rashba interaction and the external bias voltage. Our results are of
relevance for interpreting magnetotransport and infrared cyclotron resonance
measurements, including also situations of comparatively weak spin-orbit
coupling.Comment: 25 pages, 5 figure
Effect of Holstein phonons on the optical conductivity of gapped graphene
We study the optical conductivity of a doped graphene when a sublattice
symmetry breaking is occurred in the presence of the electron-phonon
interaction. Our study is based on the Kubo formula that is established upon
the retarded self-energy. We report new features of both the real and imaginary
parts of the quasiparticle self-energy in the presence of a gap opening. We
find an analytical expression for the renormalized Fermi velocity of massive
Dirac Fermions over broad ranges of electron densities, gap values and the
electron-phonon coupling constants. Finally we conclude that the inclusion of
the renormalized Fermi energy and the band gap effects are indeed crucial to
get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.
Patterns of Tobacco Product Use in the US Population using the Population Assessment of Tobacco and Health Study
The U.S. Centers for Disease Control and Prevention (CDC) reported that, in the United States, tobacco product use is the prominent cause of avoidable disease, disability, and death in the year 2017.While tobacco use has severe public health consequences, it has been difficult to fully understand the behaviors surrounding this preventable public health challenge. Introduced in 2011, the Population Assessment of Tobacco and Health (PATH) Study’s purpose is to influence the Food and Drug Administration\u27s regulatory activities via the Family Smoking Prevention and Tobacco Control Act (TCA). The PATH study is a longitudinal cohort study examining tobacco use and behavior in adolescents and adults. The purpose of this dissertation is to examine use and behavior for tobacco products over time and examine both initiation and switching of products. In our analysis, we found that demographically, those who initiated or consistently used smokeless products over time were white and male while those who used traditional combustible products varied more in terms of racial makeup. With smokeless and traditional combustible products, most participants chose to use tobacco products with moderate nicotine levels, regardless of previous exposure in a prior study wave. There was very little evidence of product switching that resulted in increased tar/nicotine content differences among the population we studied. Younger participants were using electronic cigarettes in greater proportions than adults and as opposed to traditional combustible products. We also found that the rate of nicotine metabolism was not related to frequency of e-cigarette use or type of product. These results provide insight into ways in which prevention strategies can be targeted to groups that are more likely to initiate and stay using tobacco products
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