arXiv subjects

P. Entel

Publications and source records attributed to P. Entel.

At least 19 recordsLinked to original sources

arXiv2011

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

We use first principles density functional theory to investigate the softening of polar phonon modes in rutile TiO$_2$ under tensile (110)-oriented strain. We show that the system becomes unstable against a ferroelectric distortion with polarization along (110) for experimentally accessible strain values. The resulting polarization, estimated from the Born effective charges, even exceeds the bulk polarization of BaTiO$_3$. Our calculations demonstrate the different strain dependence of polar modes polarized along (110) and (001) directions, and we discuss the possibility of strain engineering the polarization direction, and the resulting dielectric and piezoelectric response, in thin films of TiO$_2$ grown on suitable substrates.

cond-mat.mtrl-sci↗

arXiv2009

Surface energies of stoichiometric FePt and CoPt alloys and their implications for nanoparticle morphologies

We have calculated surface energies and surface magnetic order of various low-indexed surfaces of monoatomic Fe, Co, and Pt, and binary, ordered FePt, CoPt, and MnPt using density functional theory. Our results for the binary systems indicate that elemental, Pt-covered surfaces are preferred over Fe- and Co-covered and mixed surfaces of the same orientation. The lowest energy orientation for mixed surfaces is the highly coordinated (111) surface. We find Pt-covered (111) surfaces, which can be realized in the L11 structure only, to be lower in energy by about 400 meV/atom compared to the mixed L10 (111) surface. We conclude that this low surface energy stabilizes the L11 structure in small nanoparticles, which is suppressed in bulk alloys, but has been recently synthesized as thin film for CoPt. From the interplay of surface and bulk energies, equilibrium shapes of single-crystalline ordered nanoparticles and crossover sizes between the different orderings can be estimated.

cond-mat.mtrl-sci↗

arXiv2009

Ab initio study of the interface properties of Fe/GaAs(110)

We have investigated the initial growth of Fe on GaAs(110) by means of density functional theory. In contrast to the conventionally used (001)-surface the (110)-surface does not reconstruct. Therefore, a flat interface and small diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for spintronic applications. Since experimentally, the actual quality of the interface seems to depend on the growth conditions, e.g., on the flux rate, we simulate the effect of different flux rates by different Fe coverages of the semiconductor surface. Systems with low coverages are highly diffusive. With increasing amount of Fe, i.e., higher flux rates, a flat interface becomes more stable. The magnetic structure strongly depends on the Fe coverage but no quenching of the magnetic moments is observed in our calculations.

cond-mat.mtrl-sci↗

arXiv2008

Diagrammatic theory for Anderson Impurity Model. Stationary property of the thermodynamic potential

A diagrammatic theory around atomic limit is proposed for normal state of Anderson Impurity Model. The new diagram method is based on the ordinary Wick's theorem for conduction electrons and a generalized Wick's theorem for gtrongly correlated impurity electrons. This last theorem coincides with the definition of Kubo cumulants. For the mean value of the evolution operator a linked cluster theorem is proved and a Dyson's type equations for one-particle propagators are established. The main element of these equations is the correlation function which contains the spin, charge and pairing fluctuations of the system. The thermodynamic potential of the system is expressed through one-particle renormalized Green's functions and the corelation function. The stationary property of the thermodynamic potential is established with respect to the changes of correlation function.

cond-mat.str-el↗

arXiv2007

Molecular dynamics simulations of the dipolar-induced formation of magnetic nanochains and nanorings

Iron, cobalt and nickel nanoparticles, grown in the gas phase, are known to arrange in chains and bracelet-like rings due to the long-range dipolar interaction between the ferromagnetic (or super-paramagnetic) particles. We investigate the dynamics and thermodynamics of such magnetic dipolar nanoparticles for low densities using molecular dynamics simulations and analyze the influence of temperature and external magnetic fields on two- and three-dimensional systems. The obtained phase diagrams can be understood by using simple energetic arguments.

cond-mat.mtrl-sci↗

arXiv2007

Single-site Anderson Model. I Diagrammatic theory

The diagrammatic theory is proposed for the strongly correlated impurity Anderson model. The strongly correlated impurity electrons are hybridized with free conduction electrons. For this system the new diagrammatic approach is formulated. The linked cluster theorem for vacuum diagrams is proved and the Dyson type equations for electron propagators of both electron subsystems are established, together with such equations for mixed propagators. The approximations based on the summing the infinite series of diagrams are proposed, which close the system of equations and permit the investigation of the system's properties.

cond-mat.str-el↗

arXiv2007

Single-site Anderson model. II Perturbation theory of symmetric model

The strong electron correlations caused by Coulomb interaction of impurity electrons are taken into account. The infinite series of diagrams containing irreducible Green's functions are summed. For symmetric Anderson model we establish the antisymmetry property of the impurity Green's function, formulate the exact Dyson type equation for it, find the approximate correlation function $Z_σ(iω$) and solve the integral equation which determines the full propagator of the impurity electrons. Analytical continuation of the obtained Matsubara Green's function determines the retarded one and gives the possibility to find the spectral function of impurity electrons. The existence of two resonances of this function has been proved. The smooth behaviour was found near the Fermi surface. The two resonances situated symmetrical to the Fermi surface correspond to the energies of quantum transitions of the impurity electrons. The widths and heights of these resonances are established.

cond-mat.str-el↗

arXiv2007

A novel diagrammatic technique for the single-site Anderson model

A diagrammatic theory around the atomic limit is proposed for the single-impurity Anderson model in which the strongly correlated impurity electrons hybridize with free (uncorrelated) conduction electrons. Using this diagrammatic approach, we prove the existence of a linked cluster theorem for the vacuum diagrams and derive Dyson type of equations for the localized and conduction electrons and corresponding equations for the mixed propagators. The system of equations can be closed by summing the infinite series of ladder diagrams containing irreducible Green's functions. The result allows to discuss resonances associated with the quantum transitions at the impurity site.

cond-mat.str-el↗

arXiv2005

Influence of dipole interaction on lattice dynamics of crystalline ice

The Born effective charges of component atoms and phonon spectra of a tetrahedrally coordinated crystalline ice are calculated from the first principles method based on density functional theory within the generalized gradient approximation with the projected augmented wave method. Phonon dispersion relations in a 3x1x1 supercell were evaluated from Hellmann-Feynman forces with the direct method. This calculation is an additional work to the direct method in calculating the phonon spectra which does not take into account the polarization charges arising from dipole interaction of molecules of water in ice. The calculated Born effective polarization charges from linear response theory are supplied as the correction terms to the dynamical matrix in order to further investigate the LO-TO splitting of the polar modes of ice crystal at k=0 which has long been speculated for this system especially in the region between 28 and 37 meV both in the theoretical and experimental studies. Our results clearly show the evidence of splitting of longitudinal and transverse optic modes at the k=0-point in agreement with some experimental findings.

cond-mat.mtrl-sci↗

arXiv2005

Anomalous vibrational effects in non-magnetic and magnetic Heusler alloys

First-principles calculations are used in order to investigate phonon anomalies in non-magnetic and magnetic Heusler alloys. Phonon dispersions for several systems in their cubic L2$\mathrm{_1}$ structure were obtained along the [110] direction. We consider compounds which exhibit phonon instabilities and compare them with their stable counterparts. The analysis of the electronic structure allows us to identify the characteristic features leading to structural instabilities. The phonon dispersions of the unstable compounds show that, while the acoustic modes tend to soften, the optical modes disperse in a way which is significantly different from that of the stable structures. The optical modes that appear to disperse at anomalously low frequencies are Raman active, which is considered an indication of a stronger polarizability of the unstable systems. We show that phonon instability of the TA$_{2}$ mode in Heusler alloys is driven by interaction(repulsion) with the low energy optical vibrations. The optical modes show their unusual behavior due to covalent interactions which are additional bonding features incommensurate with the dominating metallicity in Heusler compounds.

cond-mat.mtrl-sci↗

arXiv2005

Ab-initio study of structure and dynamics properties of crystalline ice

We investigated the structural and dynamical properties of a tetrahedrally coordinated crystalline ice from first principles based on density functional theory within the generalized gradient approximation with the projected augmented wave method. First, we report the structural behaviour of ice at finite temperatures based on the analysis of radial distribution functions obtained by molecular dynamics simulations. The results show how the ordering of the hydrogen bonding breaks down in the tetrahedral network of ice with entropy increase in agreement with the neutron diffraction data. We also calculated the phonon spectra of ice in a 3x1x1 supercell by using the direct method. So far, due to the direct method used in this calculation, the phonon spectra is obtained without taking into account the effect of polarization arising from dipole-dipole interactions of water molecules which is expected to yield the splitting of longitudinal and transverse optic modes at the Gamma-point. The calculated longitudinal acoustic velocities from the initial slopes of the acoustic mode is in a reasonable agreement with the neutron scatering data. The analysis of the vibrational density of states shows the existence of a boson peak at low energy of translational region a characteristic common to amorphous systems.

cond-mat.mtrl-sci↗

arXiv2005

Interaction of strongly correlated electrons and acoustical phonons

We investigate the interaction of correlated electrons with acoustical phonons using the extended Hubbard-Holstein model in which both, the electron-phonon interaction and the on-site Coulomb repulsion are considered to be strong. The Lang-Firsov canonical transformation allows to obtain mobile polarons for which a new diagram technique and generalized Wick's theorem is used. This allows to handle the Coulomb repulsion between the electrons emerged into a sea of phonon fields (\textit{phonon clouds}). The physics of emission and absorption of the collective phonon-field mode by the polarons is discussed in detail. Moreover, we have investigated the different behavior of optical and acoustical phonon clouds when propagating through the lattice. In the strong-coupling limit of the electron-phonon interaction, and in the normal as well as in the superconducting phase, chronological thermodynamical averages of products of acoustical phonon-cloud operators can be expressed by one-cloud operator averages. While the normal one-cloud propagator has the form of a Lorentzian, the anomalous one is of Gaussian form and considerably smaller. Therefore, the anomalous electron Green's functions can be considered to be more important than corresponding polarons functions, i.e., pairing of electrons without phonon-clouds is easier to achieve than pairing of polarons with such clouds.

cond-mat.str-el↗

arXiv2004

A critical discussion of calculated modulated structures, Fermi surface nesting and phonon softening in magnetic shape memory alloys Ni$_2$Mn(Ga, Ge, Al) and Co$_2$Mn(Ga, Ge)

A series of first principles calculations have been carried out in order to discuss electronic structure, phonon dynamics, structural instabilities and the nature of martensitic transformations of the Heusler alloys Ni$_2$Mn(Ga, Ge, Al) and Co$_2$Mn(Ga, Ge). The calculations show that besides electronic pecularities like Fermi--surface nesting, hybridizing optical and acoustic phonon modes are important for the stabilization of the modulated martensitic structures.

cond-mat.mtrl-sci↗

arXiv2004

Crystal Structures of Ni$_2$MnGa from Density Functional Calculations

The different crystal structures of ferromagnetic Ni$_2$MnGa have been calculated using density functional theory (DFT) with special emphasis on the modulated structures 10M and 14M. These are important for understanding the stability of Ni$_2$MnGa martensites and their functionality as shape-memory materials. The modulated structures have been optimized in the calculations and their properties are discussed in relation to the structures without modulation. The occurrence of the modulated structures is related to the soft TA$_2$ phonon mode observed in Ni$_2$MnGa. The latter is related to the specific nesting behavior of the Fermi surface in Ni$_2$MnGa. Particular shapes of the modulated structures are stabilized by the covalent interaction mediated by the \textit{p}-orbitals of Ga and \textit{d}-orbitals of Ni. The role of this interaction becomes clear seen when considering the phonon dispersion spectrum of Ni$_2$MnGa, where some characteristic anomalies occur in the coupling of acoustical vibrational modes and the optical modes of Ni.

cond-mat.mtrl-sci↗

arXiv2004

Role of shuffles and atomic disorder in Ni-Mn-Ga

We report results of \textit{ab-initio} calculations of the ferromagnetic Heusler alloy Ni-Mn-Ga. Particular emphasis is placed on the stability of the low temperature tetragonal structure with $c/a = 0.94$. This structure cannot be derived from the parent L2$_1$ structure by a simple homogeneous strain associated with the soft elastic constant $C'$. In order to stabilise the tetragonal phase, one has to take into account shuffles of atoms, which form a wave-like pattern of atomic displacements with a well defined period (modulation). While the modulation is related to the soft acoustic [110]-TA$_2$ phonon mode observed in Ni$_2$MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni$_2$MnGa. In addition, we have simulated an off-stoichiometric systems, in which 25 % of Mn atoms are replaced by Ni. The energy of this structure also exhibits a local minimum at $c/a = 0.94$. This allows us to conclude that both shuffles and atomic disorder stabilize the $c/a = 0.94$ structure. In both cases the stability seems to be associated with a dip in the minority-spin density of states (DOS) at the Fermi level, being related to the formation of hybrid states of Ni-\textit{d} and Ga-\textit{p} minority-spin orbitals.

cond-mat.mtrl-sci↗

arXiv2003

Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials

The vibrational density of states (VDOS) of nanoclusters and nanocrystalline materials are derived from molecular-dynamics simulations using empirical tight-binding potentials. The results show that the VDOS inside nanoclusters can be understood as that of the corresponding bulk system compressed by the capillary pressure. At the surface of the nanoparticles the VDOS exhibits a strong enhancement at low energies and shows structures similar to that found near flat crystalline surfaces. For the nanocrystalline materials an increased VDOS is found at high and low phonon energies, in agreement with experimental findings. The individual VDOS contributions from the grain centers, grain boundaries, and internal surfaces show that, in the nanocrystalline materials, the VDOS enhancements are mainly caused by the grain-boundary contributions and that surface atoms play only a minor role. Although capillary pressures are also present inside the grains of nanocrystalline materials, their effect on the VDOS is different than in the cluster case which is probably due to the inter-grain coupling of the modes via the grain-boundaries.

cond-mat.mtrl-sci↗

arXiv2003

A First-Principles Investigation of Phonon Softenings and Lattice Instabilities in the Shape-Memory System Ni$_{2}$MnGa

Ferromagnetic \nmg has unique magnetoelastic properties. These are investigated by detailed computational studies of the phonon dispersion curves for the non-modulated cubic \Ltw and tetragonal structures. For the \Ltw\ structure, a complete softening of the transverse acoustic mode has been found around the wave vector $\mathbf{q}=[1/3,1/3,0](2 π/a)$. The softening of this \TA{2} phonon mode leads to the premartensitic modulated super-structure observed experimentally. Further phonon anomalies, related to other structural transformations in \nmg, have also been found and examined. These anomalies appear to be due to the coupling of particular acoustic phonon modes and optical modes derived from Ni.

cond-mat.mtrl-sci↗

arXiv2003

Density functional simulation of small Fe nanoparticles

We calculate from first principles the electronic structure, relaxation and magnetic moments in small Fe particles, applying the numerical local orbitals method in combination with norm-conserving pseudopotentials. The accuracy of the method in describing elastic properties and magnetic phase diagrams is tested by comparing benchmark results for different phases of crystalline iron to those obtained by an all-electron method. Our calculations for the bipyramidal Fe_5 cluster qualitatively and quantitatively confirm previous plane-wave results that predicted a non-collinear magnetic structure. For larger bcc-related (Fe_35) and fcc-related (Fe_38, Fe_43, Fe_62) particles, a larger inward relaxation of outer shells has been found in all cases, accompanied by an increase of local magnetic moments on the surface to beyond 3 mu_B.

cond-mat.mtrl-sci↗