arXiv subjects

Anna Grünebohm

Publications and source records attributed to Anna Grünebohm.

30 records · Page 2Linked to original sources

arXiv2017

The electrocaloric effect in BaTiO$_3$ at all three ferroelectric transitions: anisotropy and inverse caloric effects

We study the electrocaloric (EC) effect in bulk BaTiO$_3$ (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can in principle be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudo-cubic direction.

cond-mat.mtrl-sci↗

arXiv2016

Influence of defects on ferroelectric and electrocaloric properties of BaTiO$_3$

We report modifications of the ferroelectric and electrocaloric properties of BaTiO$_3$ by defects. For this purpose, we have combined \textit{ab initio}-based molecular dynamics simulations with a simple model for defects. We find that different kinds of defects modify the ferroelectric transition temperatures and polarization, reduce the thermal hysteresis of the transition and are no obstacle for a large caloric response. For a locally reduced polarization the ferroelectric transition temperature and the adiabatic response are slightly reduced. For polar defects an intriguing picture emerges. The transition temperature is increased by polar defects and the temperature range of the large caloric response is broadened. Even more remarkable, we find an inverse caloric effect in a broad temperature range.

cond-mat.mtrl-sci↗

arXiv2016

On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys

We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni,Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co, and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with complicated spin structure and vanishing magnetization which so far has been overlooked in simulations.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the caloric response of BaTiO$_3$ by tensile epitaxial strain

We investigate the effect of epitaxial strain on the electrocaloric effect (ECE) in BaTiO$_3$ by means of ab initio based molecular dynamics simulations. We show that tensile strain can be used to optimize the operation range for ferroic cooling. Strain in the range of $\leq 1$ % can be used to shift the operation temperature by several hundreds of Kelvin both to higher and lower temperatures, depending on the direction of the external field. In addition, the transformation between multi-domain and mono-domain states, induced by an in-plane electric field, results in an additional peak of the adiabatic temperature change at lower temperatures, and a broad temperature interval where the caloric response scales linearly with the applied field strength, even up to very high fields.

cond-mat.mtrl-sci↗

arXiv2016

Positive and negative electrocaloric effect in BaTiO$_3$ in the presence of defect dipoles

The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO$_3$ is studied by means of lattice-based Monte-Carlo simulations. A Ginzburg-Landau type effective Hamiltonian is used. Oxygen vacancy-acceptor associates are described by fixed defect dipoles with orientation parallel or anti-parallel to the external field. By a combination of canonical and microcanoncial simulations the ECE is directly evaluated. Our results show that in the case of anti-parallel defect dipoles the ECE can be positive or negative depending on the density of defect dipoles. Moreover, a transition from a negative to positive ECE can be observed from a certain density of anti-parallel dipoles on when the external field increases. These transitions are due to the delicate interplay of internal and external fields, and are explained by the domain structure evolution and related field-induced entropy changes. The results are compared to those obtained by MD simulations employing an {\it{ab initio}} based effective Hamiltonian, and a good qualitative agreement is found. In addition, a novel electrocaloric cycle, which makes use of the negative ECE and defect dipoles, is proposed to enhance the cooling effect.

physics.comp-ph↗

arXiv2015

Ab initio phase diagram of BaTiO$_3$ under epitaxial strain revisited

We revisit the phase diagram of BaTiO$_3$ under biaxial strain using a first principles-based effective Hamiltonian approach. We show that, in addition to the tetragonal ($c$), quasi-rhombohedral ($r$), and quasi-orthorhombic ($aa$) ferroelectric phases, that have been discussed previously, there are temperature and strain regions, in particular under tensile strain, where the system decomposes into multi-domain structures. In such cases, the strained system, at least on a local level, recovers the same phase sequence as the unclamped bulk material. Furthermore, we extend these results from the case of "uniform" biaxial strain to the situation where the two in-plane lattice constants are strained differently and show that similar considerations apply in this case.

cond-mat.mtrl-sci↗

arXiv2015

First principles-based calculation of the electrocaloric effect in BaTiO$_3$: comparison between direct and indirect methods

We use molecular dynamics simulations for a first principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO$_3$, the adiabatic temperature change $ΔT$ and the isothermal entropy change $ΔS$, for different electric field strengths. We compare direct and indirect methods to obtain $ΔT$ and $ΔS$, and we confirm that both methods indeed lead to identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating $ΔT$ using the direct method within a molecular dynamics approach.

cond-mat.mtrl-sci↗

arXiv2014

First-principles calculation of the instability leading to giant inverse magnetocaloric efects

The structural and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The \textit{ab initio} calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting $d$-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the \textit{ab initio} magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

cond-mat.mtrl-sci↗

arXiv2012

First-principles investigation of incipient ferroelectric trends of rutile TiO$_2$ in bulk and at the (110) surface

The ferroelectric trends of rutile (TiO$_2$) in bulk and at the (110) surface are investigated by means of ab initio density functional theory. We discuss the underlying mechanism of the incipient ferroelectric behavior of rutile in terms of Born effective charges, which we decompose in individual contributions by means of maximally localized Wannier functions. We show that a ferroelectric phase can be stabilized for a variety of different lattice distortions which all enlarge the shortest Ti-O distance, even if the longer apical Ti-O bond is simultaneously shortened. At the (110) surface, the ferroelectric trends are modified compared to the bulk, but nevertheless ferroelectric phases with large polarization even in the topmost surface layer can be stabilized by uniaxial strain.

cond-mat.mtrl-sci↗

arXiv2011

First-principles study of the influence of (110) strain on the ferroelectric trends of TiO$_2$

We investigate the impact of uniaxial strain on atomic shifts, dipolar interactions, polarization and electric permittivity in TiO$_2$ (rutile) by using two different implementations of density functional theory. It is shown that calculations using the Vienna ab inito simulation package (VASP) and the plane-wave self-consistent field method (PWscf) yield qualitatively the same atomic relaxations and ferroelectric trends under strain. The phonon dispersion curves of unstrained and strained TiO$_2$ (rutile) obtained by employing the linear response method confirm previous calculations of the giant LO-TO splitting and the appearance of soft polar modes. A second order phase transition into a ferroelectric phase with polarization along (110) appears under expansive strain in (110) direction.

cond-mat.mtrl-sci↗

arXiv2006

Experimental study of pedestrian flow through a bottleneck

In this work the results of a bottleneck experiment with pedestrians are presented in the form of total times, fluxes, specific fluxes, and time gaps. A main aim was to find the dependence of these values from the bottleneck width. The results show a linear decline of the specific flux with increasing width as long as only one person at a time can pass, and a constant value for larger bottleneck widths. Differences between small (one person at a time) and wide bottlenecks (two persons at a time) were also found in the distribution of time gaps.

physics.soc-ph↗

arXiv2006

Experimental study of pedestrian counterflow in a corridor

In this work the results of a pedestrian counterflow experiment in a corridor of a width of 2 meter are presented. 67 participants were divided into two groups with varying relative and absolute size and walked in opposite direction through a corridor. The video footage taken from the experiment was evaluated for passing times, walking speeds, fluxes and lane-formation including symmetry breaking. The results include comparatively large fluxes and speeds as well as a maximal asymmetry between left- and right-hand traffic. The sum of flow and counterflow in any case turns out to be larger than the flow in all situations without counterflow.

cond-mat.stat-mech↗