99 research outputs found
The effect of confinement and defects on the thermal stability of skyrmions
The stability of magnetic skyrmions against thermal fluctuations and external
perturbations is investigated within the framework of harmonic transition state
theory for magnetic degrees of freedom. The influence of confined geometry and
atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is
shown that a skyrmion on a track has lower activation energy for annihilation
and higher energy for nucleation if the size of the skyrmion is comparable with
the width of the track. Two mechanisms of skyrmion annihilation are considered:
inside the track and escape through the boundary. For both mechanisms, the
dependence of activation energy on the track width is calculated. Non-magnetic
defects are found to localize skyrmions in their neighborhood and strongly
decrease the activation energy for creation and annihilation. This is in
agreement with experimental measurements that have found nucleation of
skyrmions in presence of spin-polarized current preferably occurring near
structural defects
Ab-initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge
In order to search for the magnetic ground state of surface nanostructures we
extended first principles adiabatic spin dynamics to the case of fully
relativistic electron scattering. Our method relies on a constrained density
functional theory whereby the evolution of the orientations of the spin-moments
results from a semi-classical Landau-Lifshitz equation. This approach is
applied to a study of the ground state of a finite Co chain placed along a step
edge of a Pt(111) surface. As far as the ground state spin orientation is
concerned we obtain excellent agreement with the experiment. Furthermore we
observe noncollinearity of the atom-resolved spin and orbital moments. In terms
of magnetic force theorem calculations we also demonstrate how a reduction of
symmetry leads to the existence of canted magnetic states.Comment: 4 pages, ReVTeX + 3 figures (Encapsulated Postscript), submitted to
PR
Changing the Magnetic Configurations of Nanoclusters Atom-by-Atom
The Korringa-Kohn-Rostoker Green (KKR) function method for non-collinear
magnetic structures was applied on Mn and Cr ad-clusters deposited on the
Ni(111) surface. By considering various dimers, trimers and tetramers, a large
amount of collinear and non-collinear magnetic structures is obtained.
Typically all compact clusters have very small total moments, while the more
open structures exhibit sizeable total moments, which is a result of the
complex frustration mechanism in these systems. Thus, as the motion of a single
adatom changes the cluster structure from compact to open and vice versa, this
can be considered as a magnetic switch, which via the local exchange field of
the adatom allows to switch the cluster moment on and off, and which might be
useful for future nanosize information storage.Comment: 7 page
Energy surface and lifetime of magnetic skyrmions
The stability of skyrmions in various environments is estimated by analyzing
the multidimensional surface describing the energy of the system as a function
of the directions of the magnetic moments in the system. The energy is given by
a Heisenberg-like Hamiltonian that includes Dzyaloshinskii-Moriya interaction,
anisotropy and external magnetic field. Local minima on this surface correspond
to the ferromagnetic and skyrmion states. Minimum energy paths (MEP) between
the minima are calculated using the geodesic nudged elastic band method. The
maximum energy along an MEP corresponds to a first order saddle point on the
energy surface and gives an estimate of the activation energy for the magnetic
transition, such as creation and annihilation of a skyrmion. The
pre-exponential factor in the Arrhenius law for the rate, the so-called attempt
frequency, is estimated within harmonic transition state theory where the
eigenvalues of the Hessian at the saddle point and the local minima are used to
characterize the shape of the energy surface. For some degrees of freedom,
so-called 'zero modes', the energy of the system remains invariant. They need
to be treated separately and give rise to temperature dependence of the attempt
frequency. As an example application of this general theory, the lifetime of a
skyrmion in a track of finite width for a PdFe overlayer on a Ir(111) substrate
is calculated as a function of track width and external magnetic field. Also,
the effect of non-magnetic impurities is studied. Various MEPs for annihilation
inside a track, via the boundary of a track and at an impurity are presented.
The attempt frequency as well as the activation energy has been calculated for
each mechanism to estimate the transition rate as a function of temperature
Analysis technique for exceptional points in open quantum systems and QPT analogy for the appearance of irreversibility
We propose an analysis technique for the exceptional points (EPs) occurring
in the discrete spectrum of open quantum systems (OQS), using a semi-infinite
chain coupled to an endpoint impurity as a prototype. We outline our method to
locate the EPs in OQS, further obtaining an eigenvalue expansion in the
vicinity of the EPs that gives rise to characteristic exponents. We also report
the precise number of EPs occurring in an OQS with a continuum described by a
quadratic dispersion curve. In particular, the number of EPs occurring in a
bare discrete Hamiltonian of dimension is given by ; if this discrete Hamiltonian is then coupled to continuum
(or continua) to form an OQS, the interaction with the continuum generally
produces an enlarged discrete solution space that includes a greater number of
EPs, specifically , in which
is the number of (non-degenerate) continua to which the discrete sector is
attached. Finally, we offer a heuristic quantum phase transition analogy for
the emergence of the resonance (giving rise to irreversibility via exponential
decay) in which the decay width plays the role of the order parameter; the
associated critical exponent is then determined by the above eigenvalue
expansion.Comment: 16 pages, 7 figure
The second law and beyond in microscopic quantum setups
The Clausius inequality (CI) is one of the most versatile forms of the second
law. Although it was originally conceived for macroscopic steam engines, it is
also applicable to quantum single particle machines. Moreover, the CI is the
main connecting thread between classical microscopic thermodynamics and
nanoscopic quantum thermodynamics. In this chapter, we study three different
approaches for obtaining the CI. Each approach shows different aspects of the
CI. The goals of this chapter are: (i) To show the exact assumptions made in
various derivations of the CI. (ii) To elucidate the structure of the second
law and its origin. (iii) To discuss the possibilities each approach offers for
finding additional second-law like inequalities. (iv) To pose challenges
related to the second law in nanoscopic setups. In particular, we introduce and
briefly discuss the notions of exotic heat machines (X machines), and "lazy
demons".Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Recent Progress and
Outlook", (Springer International Publishing). v1 does not include references
to other book chapter
Thermodynamic principles and implementations of quantum machines
The efficiency of cyclic heat engines is limited by the Carnot bound. This
bound follows from the second law of thermodynamics and is attained by engines
that operate between two thermal baths under the reversibility condition
whereby the total entropy does not increase. By contrast, the efficiency of
engines powered by quantum non-thermal baths has been claimed to surpass the
thermodynamic Carnot bound. The key to understanding the performance of such
engines is a proper division of the energy supplied by the bath to the system
into heat and work, depending on the associated change in the system entropy
and ergotropy. Due to their hybrid character, the efficiency bound for quantum
engines powered by a non-thermal bath does not solely follow from the laws of
thermodynamics. Hence, the thermodynamic Carnot bound is inapplicable to such
hybrid engines. Yet, they do not violate the principles of thermodynamics.
An alternative means of boosting machine performance is the concept of
heat-to-work conversion catalysis by quantum non-linear (squeezed) pumping of
the piston mode. This enhancement is due to the increased ability of the
squeezed piston to store ergotropy. Since the catalyzed machine is fueled by
thermal baths, it adheres to the Carnot bound.
We conclude by arguing that it is not quantumness per se that improves the
machine performance, but rather the properties of the baths, the working fluid
and the piston that boost the ergotropy and minimize the wasted heat in both
the input and the output.Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Recent Progress and
Outlook", (Springer International Publishing
- …